Applied Science, Faculty of
Chemical and Biological Engineering, Department of
DSpace
UBCV
Shi, Xuetong
Doctor of Philosophy - PhD
University of British Columbia
Amid growing climate challenges and the need to decarbonize energy systems, the development of renewable materials has become a cornerstone of bio-innovation. Wood, with its intrinsic hierarchical porosity and mechanical robustness, emerges as a promising candidate for engineering multifunctional materials to meet energy and environmental demands. However, novel applications of wood require enhanced control over light absorption, hygroscopicity, and rigidity, necessitating advanced structural and chemical modifications. This thesis explores the potential of wood, specifically balsa and basswood, as a foundational material for sustainable energy management. The research targets solar energy conversion, thermal regulation, and rainfall energy harvesting through synergistic physical and chemical transformations. By integrating materials science, nanotechnology, and sustainable design principles, the work advances wood-based systems that bridge the gap between renewable feedstocks and high-performance energy technologies. Four interconnected studies illustrate this approach. First, a lightweight, anisotropic foam was engineered from balsa via controlled delignification, microfibrillation, and surface amorphization, resulting in tunable compressibility and enhanced thermal insulation. Second, to harness rainfall as an energy source, an all-wood triboelectric nanogenerator was fabricated through in situ lignin clustering and redistribution, improving both hydrophobicity and electrical output. Third, wood infused with a phase change material (PCM) was developed for solar-thermal energy conversion and storage, demonstrating efficient unidirectional heat transfer and stable cyclic performance for passive building temperature regulation. Lastly, lignin's inherent photothermal properties were valorized to design sustainable, high-efficiency solar absorbers integrated into wood–PCM composites. Overall, this thesis demonstrates how harmonizing wood's architecture with targeted modifications, such as lignin redistribution, anisotropic structuring, and surface functionalization, enables the development of cost-effective, scalable bioproducts for sustainable energy systems.
SUSTAINABLE ENERGY SOLUTIONS THROUGH PHYSICALLY AND CHEMICALLY MODIFIED WOOD by Xuetong Shi B.S., Beijing Forestry University, 2015 M.S., KTH and Aalto University, 2019 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Doctor of Philosophy in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Chemical and Biological Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) October 2025 © Xuetong Shi, 2025 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled: Sustainable Energy Solutions through Physically and Chemically Modified Wood submitted by Xuetong Shi in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemical and Biological Engineering Examining Committee: Orlando Rojas, Professor, Department of Chemical and Biological Engineering, UBC John Smith, Professor, Materials Engineering, UBC Supervisor Naoko Ellis, Professor, Department of Chemical and Biological Engineering, UBC Supervisory Committee Member Feng Jiang, Associate Professor, Department of Wood Science, UBC Supervisory Committee Member Chunping Dai, Associate Professor, Department of Wood Science, UBC University Examiner Lacey Samuels, Professor, Department of Botany, UBC University Examiner Gloria Oporto, Associate Professor, Chemical and Biomedical Engineering, West Virginia University External Examiner iii Abstract Amid growing climate challenges and the need to decarbonize energy systems, the development of renewable materials has become a cornerstone of bio-innovation. Wood, with its intrinsic hierarchical porosity and mechanical robustness, emerges as a promising candidate for engineering multifunctional materials to meet energy and environmental demands. However, novel applications of wood require enhanced control over light absorption, hygroscopicity, and rigidity, necessitating advanced structural and chemical modifications. This thesis explores the potential of wood, specifically balsa and basswood, as a foundational material for sustainable energy management. The research targets solar energy conversion, thermal regulation, and rainfall energy harvesting through synergistic physical and chemical transformations. By integrating materials science, nanotechnology, and sustainable design principles, the work advances wood-based systems that bridge the gap between renewable feedstocks and high-performance energy technologies. Four interconnected studies illustrate this approach. First, a lightweight, anisotropic foam was engineered from balsa via controlled delignification, microfibrillation, and surface amorphization, resulting in tunable compressibility and enhanced thermal insulation. Second, to harness rainfall as an energy source, an all-wood triboelectric nanogenerator was fabricated through in situ lignin clustering and redistribution, improving both hydrophobicity and electrical output. Third, wood infused with a phase change material (PCM) was developed for solar-thermal energy conversion and storage, demonstrating efficient unidirectional heat transfer and stable cyclic performance for passive building temperature regulation. Lastly, lignin's inherent iv photothermal properties were valorized to design sustainable, high-efficiency solar absorbers integrated into wood–PCM composites. Overall, this thesis demonstrates how harmonizing wood's architecture with targeted modifications, such as lignin redistribution, anisotropic structuring, and surface functionalization, enables the development of cost-effective, scalable bioproducts for sustainable energy systems. v Lay Summary Wood, with its wide availability, biodegradability, and unique multi-porous three-dimensional architecture, presents a promising platform for advanced material applications. In recent years, wood-based materials have attracted growing interest due to their potential in sustainable technologies. This thesis explores the utilization of wood for solar energy conversion, thermal energy storage, thermal insulation, and rainfall energy harvesting. The core objective is to investigate the structure–property–application relationships in wood by examining interfacial interactions, physical phenomena, and chemical transformations. Emphasis is placed on understanding the underlying mechanisms that govern performance and identifying effective strategies to tailor wood's properties for specific energy applications. By translating the natural advantages of wood into scalable energy solutions, this work advances the development of functionalized wood materials that exceed the capabilities of conventional wood. The findings contribute to a broader framework for renewable resource utilization, emphasizing efficiency and environmental compatibility in pursuit of global sustainability goals. vi Preface This thesis presents four strategies to develop functional wood materials with distinct structural features and applications, primarily focusing on sustainable energy conversion and utilization. The work integrates experimental design, data analysis and mechanistic exploration in five research papers that are part of this thesis under my authorship and the supervision of Professor Orlando Rojas. These works correspond to Chapters 3 through 6 and are outlined below: • Chapter 3 is based on the publication by Shi, X.; Bi, R.; Wan, Z.; Jiang, F.; Rojas, O. J. Solid wood modification toward anisotropic elastic and insulative foam-like materials. ACS Nano, 2024, 18(11), 7959. I was responsible for conceptualization, experimental design, characterization, data analysis, illustration, discussion, mechanism study, writing, and manuscript revision. Dr. Ran Bi contributed to methodological development, mechanism discussion, and manuscript editing. Prof. Orlando Rojas supported the project through conceptual input, supervision, discussion, funding, and manuscript revision. • Chapter 4 is based on the publication: Shi, X.; Bi, R.; Wan, Z.; Jiang, F.; Rojas, O. J. Native lignin migration and clustering in wood: Superhydrophobic, anti-mold, and tribo-negative layers for rain-driven electrification. Advanced Functional Materials, 2025, 2504381. I developed the material, experimental work, characterization, data analysis, illustrations, and manuscript drafting and revision. Dr. Yi Lu contributed to the original concept and supported the mechanistic analysis and manuscript revision. Prof. Orlando Rojas provided supervision, scientific discussion, funding acquisition, and editing. vii • Chapter 5 is based on the publication by Shi, X.; Meng, Y.; Bi, R.; Wan, Z.; Zhu, Y.; Rojas, O. J. Enabling unidirectional thermal conduction of wood-supported phase change material for photo-to-thermal energy conversion and heat regulation. Composites B, 2022, 245, 110231. In this work, I led the conceptualization, experimental design, characterization, formal data analysis, illustration, mechanism study, writing, and revision of the manuscript. Dr. Yang Meng contributed to the discussion and manuscript revision. Prof. Orlando Rojas provided supervision, scientific discussion, funding acquisition, and manuscript editing. • Chapter 6 is based on a manuscript currently under preparation, titled: Relocated Lignin Enables Efficient Photothermal Conversion in Wood-Stabilized PCM for Thermal Energy Storage. In this study, I carried out material preparation, experiments, characterization, data analysis, illustration, and manuscript preparation. Prof. Orlando Rojas contributed through supervision, discussion, funding acquisition, and manuscript editing. Generative AI tools, specifically OpenAI's ChatGPT, were used solely for language editing and grammar correction in this thesis. ChatGPT was employed to improve the clarity and correctness of English in Chapters 1, 2, 6, and 7. These chapters were written by the author and revised for language quality using the tool. No generative AI was used for content generation, data analysis, or the development of research ideas. Chapters 3, 4, and 5 are reproduced directly from the author’s previously published peer-reviewed journal articles and were not edited using generative AI tools. viii Table of Contents Abstract ......................................................................................................................................... iii Lay Summary ................................................................................................................................ v Preface ........................................................................................................................................... vi Table of Contents ....................................................................................................................... viii List of Tables .............................................................................................................................. xiii List of Figures ............................................................................................................................. xiv List of Abbreviations ................................................................................................................ xxii Acknowledgements ................................................................................................................... xxv Chapter 1: Introduction ............................................................................................................... 1 1.1 Background of Sustainable energy usage and conversions ........................................ 1 1.2 Sustainable strategies by wood .................................................................................. 5 1.2.1 Nanostructured wood ....................................................................................... 5 1.2.2 Wood structure and components ...................................................................... 8 Chapter 2: Wood Functionalization .......................................................................................... 13 2.1 Treatment methods of wood ..................................................................................... 13 2.2 Summarized emerging approaches of wood modification and functionality ........... 16 2.3 Literature review ...................................................................................................... 18 2.3.1 Functionalization: wood as a supporting material ......................................... 18 2.3.2 Structural design for materials with targeted mechanical performance ......... 21 ix 2.3.4 Wood-based composites and hybrids for energy management ...................... 24 2.3.5 Heat energy storage: phase change materials and combination with wood ... 28 2.4 Thesis outline and research objectives ..................................................................... 29 2.4.1 Research objectives ........................................................................................ 29 2.4.2 Thesis outline ................................................................................................. 34 Chapter 3: Compressible Wood Foam for Thermal Insulation ............................................. 36 3.1 Introduction .............................................................................................................. 36 3.2 Materials and methods ............................................................................................. 39 3.2.1 Materials ......................................................................................................... 39 3.2.2 Compressible wood foam ............................................................................... 39 3.2.3 Characterization ............................................................................................. 40 3.2.4 Molecular dynamics (MD) simulation ........................................................... 43 3.3 Results and discussion .............................................................................................. 44 3.3.1 Compressible wood foam ............................................................................... 44 3.3.2 Structural features .......................................................................................... 47 3.3.3 Impact of cellulose crystallinity on EWF structure ........................................ 52 3.3.4 Hydrophobization of EWF ............................................................................. 54 3.3.5 Thermal insulation ......................................................................................... 57 3.3.6 Flame retardancy ............................................................................................ 59 x 3.4 Conclusions .............................................................................................................. 61 Chapter 4: Hydrophobic Wood for Triboelectric Nanogenerator ......................................... 63 4.1 Introduction .............................................................................................................. 63 4.2 Materials and methods ............................................................................................. 66 4.2.1 Materials ......................................................................................................... 66 4.2.2 Chemical treatment of wood .......................................................................... 66 4.2.3 Wood with relocated lignin (Lig-wood) ........................................................ 67 4.2.4 Wood-based liquid-solid TENG system and electric performance ................ 69 4.3. Results and Discussion ............................................................................................ 70 4.3.1 Compositional features of Lig-wood ............................................................. 70 4.3.2 Morphology, structure and hydrophobicity of Lig-wood .............................. 75 4.3.3 Triboelectric Performance Through Surface Engineering ............................. 79 4.3.4 Role of lignin in S-L tribogeneration ............................................................. 83 4.3.5 Lig-wood performance under simulated rainfall conditions .......................... 86 4.3.6 Application perspectives ................................................................................ 89 4.4 Conclusions .............................................................................................................. 93 Chapter 5: Thermally Conductive Wood for Photothermal Conversion and Heat Regulation....................................................................................................................................................... 95 5.1 Introduction .............................................................................................................. 95 xi 5.2 Materials and methods ............................................................................................. 98 5.2.1 Materials ......................................................................................................... 98 5.2.2 Liquid exfoliation of boron nitride, BN ......................................................... 98 5.2.3 Delignified wood and LbL deposition ........................................................... 98 5.2.4 Impregnation with the PCM (PEG) ................................................................ 99 5.2.5 Structural, morphological, chemical, and thermal characterization ............... 99 5.3. Results and discussion ........................................................................................... 100 5.3.1 Wood modification and loading ................................................................... 100 5.3.2 Morphology of conductive BW, PW and PW@PEGs ................................. 103 5.3.3 PCM-loaded systems .................................................................................... 105 5.3.4 Thermal energy storage of PW@PEG ......................................................... 106 5.3.5 Thermal durability and stability ................................................................... 112 5.3.6 Light-to-thermal energy conversion ............................................................. 114 5.4 Conclusions ............................................................................................................ 119 Chapter 6: Wood-Supported Phase Change Material with Lignin Redistribution for Photothermal Conversion and Thermal Energy Storage...................................................... 120 6.1 Introduction ............................................................................................................ 120 6.2 Experiments ............................................................................................................ 122 6.2.1 Materials ....................................................................................................... 122 xii 6.2.2 Chemical treatment on wood ....................................................................... 123 6.2.3 Preparation of wood composite: PEG@TW ................................................ 123 6.2.4 Characterization ........................................................................................... 123 6.3 Results and Discussion ........................................................................................... 125 6.3.1 Wood modification and compositional analysis .......................................... 125 6.3.2 Improved hydrophobicity and humidity resistance ...................................... 128 6.3.3 Microstructure and morphology change ...................................................... 131 6.3.4 Thermal energy storage performance ........................................................... 134 6.3.5 Thermal stability and reversibility ............................................................... 139 6.3.6 Photothermal energy conversion and T regulation performance ................. 140 6.4 Conclusions ............................................................................................................ 143 Chapter 7: Conclusion and outlook ........................................................................................ 145 7.1 Main contributions of thesis ................................................................................... 145 7.2 Future perspectives ................................................................................................. 148 Bibliography .............................................................................................................................. 151 Appendix A ................................................................................................................................ 179 xiii List of Tables Table 3.1. Phase change properties and energy storage efficiency (F). ...................................... 108 Table 6.1. Phase change properties and energy storage efficiency (F) of PEG and its composites...................................................................................................................................................... 137 Table A1. The compositional analysis (HPLC) of balsa, delignified wood and EWF after the given treatment times. ........................................................................................................................... 181 xiv List of Figures Figure 1.1. Anisotropy and hieratical structure of wood. ............................................................... 7 Figure 1.2. Major biopolymers in wood and their chemical structures. ....................................... 12 Figure 2.1. Functionalization and fabrication approaches of functional wood. ............................ 18 Figure 2.2. Thesis objectives and research outline ....................................................................... 33 Figure 3.1. (a) Schematic illustration of the chemical treatment (oxidation-reduction) applied to balsa wood. (b) Elastic wood foam (EWF) subjected to loading and unloading cycles (1 kg). ... 45 Figure 3.2. (a) Degree of oxidation and yield of EWF for given oxidation time, 4 to 14h. (b) Relative content of cellulose, hemicellulose and lignin in natural wood, Holo-wood and EWF (with 8-h oxidation) obtained from compositional analysis. (c) Compression stress-strain curves of EWF obtained at given oxidation time. .................................................................................................. 47 Figure 3.3. Holo-Wood morphology observed in the (a1) transverse plane and (a2) tangential plane, highlighting the exposed fibrils in EWF. Morphology of EWF in the (b1) transverse plane and (b2) tangential plane. .............................................................................................................. 48 Figure 3.4. Illustrative schematics of amorphization at fiber surface yielding a more open fibrillar structure......................................................................................................................................... 49 Figure 3.5. (a-b) Stress-strain curves of Holo-Wood and EWF. (c) Stress-strain profiles upon loading/unloading tests (100 cycles). ............................................................................................ 50 Figure 3.6. (a) Compression stress retention of EWF. (b) Stress-strain curves of EWF and Cell-Wood (20 loading/unloading cycles) at a maximum applied strain of 40%. (c) Energy loss of a sample under compression (strain of 40%). .................................................................................. 51 xv Figure 3.7. (a) XRD curves of Holo-Wood and EWF. (b) 2D-XRD patterns of Holo-wood and EWF. ............................................................................................................................................. 52 Figure 3.8. (a) Molecular dynamic simulation model of Holo-Wood and EWF: front and side views. (b) Non-bonded interaction energy among inter- and intra- fibrils of Holo-wood and EWF, calculated based on the molecule dynamic simulation. (c) Number of hydrogen bonds among inter- and intra- fibrils of Holo-wood and EWF. .................................................................................... 54 Figure 3.9. SEM images of Si-EWF in the tangential plane ((a) low and (b) high magnification), and in the (c) transverse plane. ..................................................................................................... 55 Figure 3.10. (a) Mechanism of organosilanes coupling with EWF surfaces. (b-c) Static water contact angle measured on Si-EWF samples tested in the tangential plane (T) and cross section/ transverse plane (C). ..................................................................................................................... 55 Figure 3.11. (a) Strain changes at different relative humidity (0-95%) for Holo-Wood, EWF and Si-EWF under a stress-controlled (0.01 MPa) creep recovery test by dynamic mechanical analysis. Strain at different RH was recorded once the RH has been stabilized for 5 min. (b) Stress-strain curve of Si-EWF following 20 loading/unloading cycles at RH=60%, under an increased strain from 1% to 50% with a 5% strain/min increments. ...................................................................... 56 Figure 3.12. Thermal conductivity of Holo-Wood and EWF along both longitudinal and transverse directions (grey and blue area) and the thermal conductivities of the reported cellulosic aerogels........................................................................................................................................................ 58 Figure 3.13. (a) Thermal insulation experiments with unmodified balsa, EWF, Si-EWF and commercial PU foam (all: 20 x 20 x 50 mm) (the samples were placed on a 70 oC hot plate). (b) xvi Temperature evolution of EWF and PU foam recorded by an IR camera during 120 min. (c) Changes of the top surface temperature with time. ....................................................................... 59 Figure 3.14. Length and weight retention of unmodified balsa wood, EWF, Si-EWF when subjected to (a) vertical and (b) horizontal burning tests. (c) Appearance of materials after the flame source was removed in the horizontal test, within the given periods of time. (d) Schematic illustration of the flame retardancy mechanism. ........................................................................... 61 Figure 4.1. Schematic illustration of the process used to convert wood into a superhydrophobic TENG material. ............................................................................................................................. 71 Figure 4.2. Photo of unmodified wood and Lig-wood after deposition of a dyed water droplet. Included is also an SEM image revealing the microstructure of Lig-wood in its transverse plane, which indicated the formation of spherical lignin particles. ......................................................... 72 Figure 4.3. (a) FTIR compositional analysis for wood, Lig-wood and delignified wood (reference). (b) Compositional cellulose, hemicellulose and relative concentration. (c) Surface atom composition by XPS. (d) Mass yield of Lig-wood materials as a function of treatment time...... 74 Figure 4.4. Fluorescent images by confocal microscopy of unmodified wood and Lig-wood showing qualitatively the relative amounts of lignin and cellulose on their surfaces. The respective wood was dyed with Pricion Red MX-5B (red dye for cellulose) to generate images that distinguish cellulose from lignin (autofluorescence in green) - see maps including merged images (scale bar = 200 μm). ........................................................................................................................................ 76 Figure 4.5. Illustration of the structural and morphological changes of lignin during wood treatment. ...................................................................................................................................... 77 xvii Figure 4.6. SEM images corresponding to the morphology of (a1-a3) unmodified wood and (b1-b4) Lig-wood observed in the tangential plane view at different magnifications, as indicated. Note the particles on the surface of (b1-b4) that correspond to lignin clusters. .................................... 78 Figure 4.7. (a) AFM images of unmodified wood and (b) Lig-wood. (c) Static water contact angle measured within 5s on unmodified wood and illustration of the nature of the surface. (d) Surface change in Lig-wood and its static water contact angle in the tangential plane (measurements for 600 s and WCA figure of Lig-wood at 10 s and 500 s). (e) Rolling water angle of Lig-wood. ... 80 Figure 4.8. Illustration of water droplets contacting a solid surface, leading to contact electrification between a liquid and a solid phase. ....................................................................... 81 Figure 4.9. Digital photos acquired with a high-speed camera (900 fps) showing water droplets contacting the surface of (a) unmodified wood and (b) Lig-wood within 2s. (c) Illustration of the assembled L-S TENG in a single electrode mode. ....................................................................... 82 Figure 4.10. (a) Voltage and (b) current generated by unmodified wood and Lig-wood. (c) Average voltage and current produced by unmodified wood and Lig-wood following 5 measurements. (d) Cyclic voltage output corresponding to continuous water droplets falling for 2000 s on unmodified wood and (e) Lig-wood. ................................................................................................................ 86 Figure 4.11 The Lig-wood performance as L-S TENG under varying simulated rainfall conditions: (a) Effect of increased water falling rates on (b) voltage, (c) current and (d) charge. .................. 87 Figure 4.12 The Lig-wood performance as L-S TENG under varying simulated rainfall conditions: when water droplets (6-μL) were dropped from a 20-cm height at a surface tilt of 45o (with respect xviii to the horizontal plane). (a) Effect of droplet (6 μL) fall height at 6 rpm rate and Lig-wood surface tilt of 45o. (b) output voltage, (c) current, are (d) charge. ............................................................. 88 Figure 4.13 The Lig-wood performance as L-S TENG under varying simulated rainfall conditions: (a) Effect of Lig-wood tilt at water droplet (6 μL) fall rates of 6 rpm from 20-cm height: (b) voltage, (c) current, and (d) charge. ............................................................................................................ 89 Figure 4.14. (a) Schematic diagram of the Wood-TENG system (12 pieces of wood placed in parallel) to power electronic devices. (b) Illustration of the equivalent circuit model. (c) LED lights and timer turns on powered by Wood-TENG. (d) Charging performance of the wood-TENG with capacitors of various capacitance.................................................................................................. 90 Figure 4.15. (a) Illustration of the application of Wood-TENG as roof to collect rain energy for power generation. (b) Water contact on treated wood from different species (size bar = 2cm). (c) Water contact angles of treated wood before and after the water dripping for 1h. ....................... 91 Figure 4.16. Photos of native basswood (left) and Lig-wood (right) in contact with soil during given period of time (size bar = 1cm). .......................................................................................... 93 Figure 5.1. Schematic illustration of the procedure used for the synthesis of BW, PW and PW@PEG. PEI/BN LbL assembly on DW was carried out using 2, 5, 10 and 15 LbL cycles, yielding the respective BW. PPy was synthesized on BW to form the thermally conductive PW samples, which were further infiltrated with the phase change material, PEG. ......................... 102 Figure 5.2. SEM of the longitudinal section of (a) DW. (b) BW15, and (c) PW15. SEM of the radial morphology of (d) DW, (e) BW15 and (e1) BW15 at an increased magnification as well as xix (f) PW15. (g) Photos of DW (top left), BW15 (top right), PW15 (bottom left) and PW15@PEG (bottom right), (h) SEM of the radial morphology of PW15@PEG. .......................................... 104 Figure 5.3. (a) UV and (b) FTIR spectra of the respective samples, as indicated and (c) XRD of DW, BW10, PW15 and PW15@PEG. ....................................................................................... 106 Figure 5.4. Molecule interactions in PW@PEG and the PEG chain motion during phase transition...................................................................................................................................................... 108 Figure 5.5. DSC thermograms for PW@PEG upon (a) heating and (b) cooling. (c) The latent heat of heating and cooling in PW@PEGs. (d)Schematic illustration of the interactions and hydrogen bonding between the components. (e) Longitudinal thermal conductivity of the PWs modified with BN and DW. (f) Longitudinal thermal conductivity of PW@PEGs and pristine PEG. ............. 110 Figure 5.6. Thermal conductivity along the radial direction of (a) PWs modified with BN and DW, as well as those of (b) PW@PEGs and pristine PEG. ................................................................ 112 Figure 5.7. (a) Thermogravimetric analysis (TGA) of DW, PEG and PW@PEG for given BN modification cycles. Stability and recyclability of PW10@PEG before and after 50 consecutive cycles of heating and cooling as observed by (b) DSC; (c) FTIR and (d) XRD measurements. 113 Figure 5.8. (a) Images acquired with an IR camera of natural wood, BW10, PW10, BW10@PEG and PW10@PEG after illumination for 600s. (b-c) Temporal changes at the surfaces of BW10, PW10, PW10@PEG and BW10@PEG. ..................................................................................... 115 Figure 5.9. (a)A wood model house equipped with a roof made from PW@PEG. (b) Illustration of the effect of PW@PEG enabling an enhanced photothermal conversion, solar energy storage and thermal regulation. Efficient photothermal conversion ability and thermal energy storage xx performance characterized by (c) a temperature increasing rate (wood with and without PPy), (e) temperature of PW@PEG roof model after illumination for 8 min and (f) after 15 min, when light is turned off. (d) Real time indoor temperature change in the model house covered with different roof materials when the model (with and without illumination under a simulated sunlight). (g) Mechanism of PPy-modified wood-supported PCM enabling efficient thermal energy absorption and release. (h) 12 cycles of heat absorption and release in PW@PEG while light is turned on and off. ............................................................................................................................................... 118 Figure 6.1. Schematic diagram of functional wood fabrication process. .................................... 126 Figure 6.2. (a) Wood samples after treatment in wet and dry state. (b) FTIR spectra and (c) XRD pattern of untreated wood and treated wood. .............................................................................. 127 Figure 6.3. (a) Water droplet on the traverse surface of Balsa wood before and after treatment. (b) Dynamic Vapor Sorption (DVS) curves of wood samples before and after treatment in a ramping relative humidity at a measurement of equilibrium mass content (%). (c) Hysteresis of wood in increasing humidity before and after treatment. ......................................................................... 130 Figure 6.4. Static water contact angle of treated wood changing with time in (a) transverse plane and (b) tangential plane. .............................................................................................................. 131 Figure 6.5. (a) Microstructure of tangential plane of native wood and (b) lumen structure at large magnification. (c) Morphology of the transverse plane of native wood and (d) its cell wall at larger magnification. (e) Microstructure of lignin-containing wood in its longitudinal direction at low and (f) high magnification. Morphology of the transverse plane of treated wood at (g) lower and (i, h) larger magnification. (g) and (i) are the cross sections of TW after cutting at the surface. (h) presents the cell wall of TW without surface cut before SEM imaging. .................................... 134 xxi Figure 6.6. (a) SEM image of cross section of wood composite and theoretical interactions between matrix and PCM. (b) FTIR spectrum of TW, PEG and wood composite. .................................. 136 Figure 6.7. DSC curves of PEG and PEG composites supported by wood with various treatment conditions during (a) heating and (b) cooling. ............................................................................ 137 Figure 6.8. (a) latent heat of heating and cooling of PEG and its composites. (b) TGA curves of PEG, treated wood and PEG@TW. ............................................................................................ 139 Figure 6.9. (a) Appearance of PEG and PEG@TW before and after heating. (b) PEG@TW after 100 heating/cooling consecutive cyclic DSC test. (c) The change of thermal energy storage property after 100 cycles. ............................................................................................................ 140 Figure 6.10 (a) Appearance of PEG encapsulated by native wood, delignified wood and treated wood in this study (scale bar=1cm). (b) UV-Vis-IR spectra of PEG@TW, PEG@DW and PEG@NW. (c) Illustration of samples exposed under a solar simulator for solar energy absorption and conversion analysis. (d) Temperature change with time for PEG@TW, PEG@DW and PEG@NW at 2 sun illumination. (e) IR images of PEG@TW during irradiation within 110s, 300s, 450s and 2000s. (f) Temperature change of PEG@TW within 5 cycles of heating/cooling when monitoring daytime and nighttime. ............................................................................................. 143 xxii List of Abbreviations 3D 3-dimentional AFM Atomic force microscopy BN Boron nitride C-plane Cross section CA Contact angle Cell-wood Wood containing only cellulose CrI Crystallinity index CNF Cellulose nano fibrils DI Distilled DMA Dynamic mechanical analysis DMAc/LiCl dimethylacetamide/lithium chloride DP Degree of polymerization DVS Dynamic vapor sorption DSC Differential scanning calorimetry EDX energy-dispersive X-ray spectroscopy EWF Elastic wood foam FTIR Fourier transform infrared spectroscopy Holo-wood Wood composed of holocellulose HPLC high performance liquid chromatography HW Hard wood IR Infrared LCC Lignin carbohydrate compound xxiii L direction Longitudinal direction LOI Limited oxygen index MDS Molecular dynamic simulation ML Middle lamella MTMS trimethoxy(methyl)silane Oxi-wood Oxidized wood PAM Polyacrylamide PCM Phase change materials PDMS Polydimethylsiloxane PEI Polyethyleneimine PEG Polyethylene glycol PINIPAM Poly(N-isopropylacrylamide)-polyacrylamide PMMA Poly(methyl methacrylate) PPy Polypyrrole PU Polyurethane PVA Polyvinyl alcohol PVP Polyvinylpyrrolidone RH Relative humidity SEM Scanning electron microscopy Si-EWF Silane coated elastic wood foam SSA Specific surface area SW Soft wood TENG Triboelectric nano generator xxiv TEMPO 2,2,6,6-tetramethylpiperidin-1-oxyl T-direction Transverse TGA Thermogravimetric analysis T-plane Tangential plane UV Ultraviolet UV-vis-IR Ultraviolet-visible-infrared light WAXS Wide angle X ray scattering WRV Water retention value XPS X-ray photoelectron spectroscopy XRD X-ray diffraction xxv Acknowledgements After four years of study, although time has flown by, I feel incredibly fortunate to have experienced a joyful and rewarding PhD journey. Along the way, I had the privilege of meeting, working with, and learning from many inspiring and professional individuals. As I reach the conclusion of this chapter, I would like to express my deepest gratitude to the following people and organizations: First and foremost, I am profoundly grateful to my supervisor, Professor Orlando Rojas, for his unwavering support, scientific, financial, and personal. I sincerely thank Orlando for accepting me as a PhD student at UBC after I completed my master's thesis under his supervision. This opportunity has been invaluable and shaped my entire doctoral experience. I was continually inspired by our one-on-one research meetings over the past four years, where his insightful guidance and creative brainstorming helped me grow intellectually. From him, I learned to improve experimental design, think critically and independently, and always keep the bigger picture in mind. Orlando's thoughtful advice and kind mentorship will continue to guide me far beyond my PhD. I am also thankful for the many opportunities he provided me to attend conferences and present my work, which allowed me to connect with colleagues, broaden my perspectives, and gain confidence as a researcher. I extend my heartfelt appreciation to Dr. Ran Bi and Dr. Yi Lu for their guidance on various research projects. Ran generously provided practical advice and technical support, and I greatly admire her expertise, patience, and positivity. Her encouragement has been especially meaningful during challenging moments in both research and life. I am also thankful to Yi, whose deep knowledge, innovative ideas, and clear thinking consistently inspired me during our xxvi meetings and casual discussions in the lab. I have learned so much from his approach to research, manuscript writing, and his strong work ethic. I am also grateful to Dr. Bruno Mattos and Dr. Yang Meng for their close collaboration and insightful discussions during the early stages of my PhD. Their guidance helped me transition smoothly from my master's studies and build a solid research foundation. Special thanks also go to Xin Shu, Zhangmin Wan, Dr. Ya Zhu, Bin Zhao, and Professor Feng Jiang, among others, for their collaboration and support. I thank Professor Naoko Ellis and Professor Feng Jiang for taking the time to review this thesis. I gratefully acknowledge financial support from the Canada Excellence Research Chair Program (CERC-2018-00006) and the Canada Foundation for Innovation (Project No. 38623). Many thanks to George Soong for ensuring that the PPC lab remains a safe, well-maintained, and welcoming environment. To my current and former colleagues in Orlando's research group (BiCMat)—Adam, Ayako, Bruno, Carolina, Coco, Daniel, Dong, Gabriel, Gio, Guang, Isabella, Jing, Jingqian, Joice, Julia, Junhua, Like, Long, Marina, Mario, Milad, Minke, Ming, Nelly, Oliver, Sameer, Siqi, Shasha, Svena, Xun, Yeedo, Yi, and others—thank you for creating such a friendly, fun, and supportive lab atmosphere. I also appreciate the help and collaboration from colleagues in other labs, including Dr. Yan Yu, Dr. Tao Zou, Dr. Yuhang Ye, Dr. Huaiyu Zhang, Dr. Liyang Liu, Dr. Yeling Zhu, Adel, Fernanda, Victoria, Ruby, Elizabeth, and many more, for their guidance, assistance, and instrument training. Finally, I would like to express my heartfelt appreciation to my family for their love and encouragement throughout my academic journey. My deepest thanks go to my parents, sister, xxvii and grandparents for their endless support, patience, and understanding. Their love has been a constant source of strength, especially as I pursued my studies far from home. I am especially thankful to my aunt, who cared for me during my time in Canada and made me feel at home—particularly during the holidays. Their continued support has been instrumental in helping me reach this milestone. Thank you all. 1 Chapter 1: Introduction 1.1 Background of Sustainable energy usage and conversions In recent decades, as issues of energy shortage are getting more and more severe, finding sustainable solutions has become increasingly important. Utilizing renewable or green energy sources can be an effective way to address these issues and reduce our heavy reliance on fossil-based materials and address the issues of carbon management. When we consider renewable natural resources1 such as solar energy, wind energy, geothermal energy, and energy from biomass, we find that they represent abundant and valuable reservoirs of untapped potential, offering critical opportunities for research and technological advancement. For instance, wind energy, a rapidly expanding resource, has seen significant development in recent years, with its capacity for electricity generation driving substantial growth in the wind power industry. Solar energy is vast and serves as the primary source for nearly all living organisms. Both light and heat are provided by the sun which have been harnessed for human use for centuries due to their inexhaustible nature.2 To use solar light radiation, scientists developed optical solar panels for photovoltaic generation and electricity supply.3 Traditional solar panels are normally made by silicon-based materials, which present advantageous properties of good durability, suitable flexibility, high reflectivity and optical accuracy. In recent years, from an environmentally friendly perspective, greener materials and production approaches are researched, aiming to achieve goals of desirable photoelectric conversion efficiency, low-processing coast, flexibility, certain transparency and biodegradability.4 Some efforts have been made to seek more 2 sustainable features.5-7 Solar heat can also be collected for different types of usages such as thermal power generation, water heaters, energy storage, photothermal energy etc. Unlike photovoltaic devices which require high transparency to enable desired light transmission, dark surface colors which characterized by high absorptance at solar wavelengths are preferred for photothermal energy device to maximize light absorption for efficient heat storage.8 Surface designs by incorporating micromorphology and nano-structural change at the surface have been studied and applied to enhance the interactions with solar light, maximizing light trapping for improved light and heat utilization.8, 9 Converting and collecting solar energy by photothermal conversion has shown dramatically increasing research interests given by the high conversion efficiency it can reach and its wide range of application areas including thermal energy storage,10 water purification,11, 12 solar steam generation,13 desalination, and sensors. To enhance such light irradiation trapping performance and conversion efficiency, dark-colored materials have been used as coatings of a substrate, such as carbon materials14 like carbon black, carbon nanotube, and reduced graphene oxide, due to their high thermal-conductivity and distinct electromagnetic confinement capability.12 Polymeric materials like polypyrrole (PPy),15 polyaniline (PAN),16 polydopamine, lignin,17 etc. have been studied for such purpose due to their chemical structure, which features substantial π-electron delocalization.12 This property makes them highly responsive to solar light in the visible and NIR regions, yielding high efficiency of light absorption. Thus, intermedia material substrate and materials designs including surface morphology modification, functional materials deposition at surface as well as treatment condition manipulation are critical points to confine, collect and convert solar energy for our use. 3 Besides, water is also an abundant resource in which energy can also be employed. For instance, hydropower can be used to produce electricity through hydrovoltaic technology. When water comes into direct contact or interaction with another material, it can generate electricity, which phenomenon is attributed to the production of streaming potential, evaporation potential, gradient-induced ion diffusion, and hydrovoltaic effects.18 In the global water cycle, water presents as various forms of gas, liquid and crystal during evaporation, steaming, rainfall, snowfall, and in river, ocean, and melting glaciers. Thus, hydrovoltaic effects can be utilized in various ways, such as the triboelectric nano energy (TENG) can be harvested from rainfall19 as well as by sea waves.20, 21 Hydrovoltaic energy can be stored and collected through water evaporation.22-24 Research has been engaged in seeking approaches to maximize the electrical output for using the blue energy effectively. For example, increasing surface roughness to obtain superhydrophobic material can largely enhance the output of the TENG system during the interaction of water droplets and materials.25 Modifying the microstructure and the surface feature of the material is crucial for achieving satisfied electricity power.26 Similarly, tuning the porosity and micromorphology of the material can influence interaction of moisture and the substrate and thus, the electric energy harvesting capacity of a moisture-driven energy generators can be optimized.27, 28 Therefore, to achieve a sufficient hydrovoltaic effect, proper contact is essential. Therefore, finding suitable intermediary material (contacting material) and optimizing the structural design during material fabrication are crucial. Biomass is a sustainably available resource that can be renewed through growth. Biomass sources can commonly be found among various plants and feedstocks, including grass, cotton, wheat, bamboo, wood, and hemp. They normally grow, form and renew by absorbing solar 4 energy to form carbohydrates and structures through photosynthesis and converting carbon dioxide into organic matter. This type of resource has been extensively utilized for energy production through ways such as heat and power generation, pyrolysis, gasification and digestion.29 Besides burning them directly as a biofuel (for cooking or keeping warm), using some of their desirable mechanical properties as construction materials, e.g. wood, has also been applied for centuries. To conclude, natural resources are abundant and come in different types and forms. People have utilized them for a long history, and scientists have researched ways to convert and harness these vast energy resources with high efficiency and to achieve potential solutions and alternatives to fossil-based energy. Employing suitable materials intermedia or substrate, applying optimal modification methods for materials design, optimizing surface properties are significantly important to harness energy from nature. Combining the use of different types of renewable energy resources, as introduced above, can also be a potential option to address current pressing energy issues and pursue a more sustainable energy utilization system, e.g. utilizing the unique structural properties of biomass to act as the material substrate for solar energy and hydropower harvesting. Studying the structural properties of biomass, finding the optimal structural feature to meet the energy conversion requirement is crucial. And this thesis will make an effort in focusing on one type of biomass-wood (specifically balsa, Ochroma pyramidale), and how balsa can be applied in the utilization of solar energy, rainfall energy and thermal energy. The main focus will be on how the morphological and compositional property of wood can contribute to the enhancement of utilization efficiency of different energy systems. 5 1.2 Sustainable strategies by wood 1.2.1 Nanostructured wood With the increasing need for resources and energy, there is a growing concern about sustainable development and natural resources usage. Wood, as a naturally abundant resource, has several advantages, including high mechanical strength, hydrophilicity, wide availability as a feedstock, ease of processing, and low cost.30, 31 Its interconnected structure consists of numerous hollow cells that are joint to form a matrix of interconnected channels. Wood can primarily be classified into two distinct categories: hardwood (HW) and softwood(SW).32 Gaining an understanding of the physical characteristics of softwood and hardwood xylem is crucial in comprehending its nature and associated products. For example, softwood is comprised of simple cell types, notably a significant proportion of longitudinal tracheids, along with a lesser amount of parenchyma cells (aligned longitudinally), rays (ray parenchyma and ray tracheids) and resin canals.33 Hardwood is composed of a more complex structure including parenchyma cells, vessels, fibers, ray cells and gum canals (in a few hardwoods).34 The presence of vessels is a distinguishing feature of hardwood (HW), while the bordered pits in tracheids serve as a key characteristic of softwood (SW). Given by this inherent structural property, wood possesses natural functionalities and capabilities such as conduction (transport), support, storage, and secretion.35 Throughout history, and for various purposes, wood has been cut and assembled into different shapes and used in diverse products including plywood, laminate and veneers.36 Wood has three principal directions (longitudinal, radial, and tangential), each offering different morphology, texture, and strength, making it a three-dimensional anisotropic material. When cut 6 along the longitudinal direction, the exposed surface of wood is the tangential plane, while cutting perpendicular to the longitudinal direction (L-direction) reveals the transverse plane (T-plane) or cross section, as shown in Figure 1.1. From the cross-section, we can observe that wood has a mesoporous structure, featuring a honeycomb-like arrangement formed by cell walls that create hollow spaces (see SEM image in Figure 1.1). This naturally porous structure gives wood a low density, making it desirable for engineering materials and serving as a natural template for composite development and other structural material functionalization. When we look into bulk wood, the cell wall (with a thickness of ca. 5 µm) is the lower-scale component.30 Cell walls are composed of primary and secondary walls and middle lamella (ML), as illustrated in Figure 1.1. The difference among different layers, except for thickness, is the chemistry/polymer distribution in each of the cell walls, for example, lignin, can be found predominantly in the ML. The secondary cell wall, which constitutes the largest volume of the cell wall (approximately 80%), consists of three layers, S1, S2, and S3, which significantly contributes to the mechanical strength of wood. These layers are composed of crystallized cellulose (up to 40%), branched hemicellulose (20-40%), and lignin.37 A detailed discussion of the chemical structure, properties, and functions of the compositional molecules will be provided in the next section. However, this observation suggests that wood possesses a unique hierarchical structure, with its fundamental characteristics revealed when examined at different scales. Along the longitudinal direction, wood's long, well-aligned fibers exhibit low tortuosity, contributing to favorable dimensional stability and reinforcing wood cells against breakage. In the tangential plane, fiber bundles connect to form tracheids, vessels, and other tissue elements. At a much smaller scale, individual fibrils at the micro/nano level (with widths of 3–5 nm and lengths of several microns) can be observed, which are the primary constituents of wood fibers. Nano-sized 7 fibrils are mainly composed of molecules in Å scale, namely cellulose. Micromorphology provides insight into the hierarchical structure of wood and reveals its potential for structural design by enabling functionalization at different scales. In recent years, functional wood development starts by removing some components, meanwhile aiming to keep the original structure of wood. Through a top-down approach, scaffolds that retain the natural wood structure have been studied by various delignification methods.38-41 The obtained, multi-porous structure and three-dimensional skeleton of wood provide great potential for sustainable applications,42 such as the replacement of plastic and energy-intensive man-made materials, as well as in the production of optoelectronics, sustainable energy storage, and water purification units.43 By selectively removing several components in natural wood, including lignin, hemicelluloses and extract, the porosity and specific surface area of wood can be improved which is favorable according to the intended applications.43 Figure 1.1. Anisotropy and hierarchical structure of wood. SEM images were taken from balsa wood by the author Xuetong Shi; illustrations of wood were sourced from BioRender; all elements were arranged using PowerPoint. 8 1.2.2 Wood structure and components As has been studied and discussed in the last section, wood is a hierarchical structure, consisting of a fibrous structure in different scales. Wood is a natural composite in which well-aligned nano/micro-fibrils work as the reinforcement region to provide stiffness, which part embedded into the amorphous matrix to maximize the overall mechanical performance of the wood (Figure 1.2). From a chemical composition perspective, wood fibers are primarily composed of two characteristic polymers, cellulose and hemicellulose, and a branched molecule called lignin. The combination of these three major components contributes significantly to the strength of wood.44 The composition and ratios of the three main components of wood-cellulose, hemicellulose and lignin-can vary across different wood species and types. Wood also contains other components, such as extractives, resins, and inorganic matter (ash), but these make up only a small portion. Thus, studying the chemical structure, functional groups, properties, and the basic chemical reactions of the main composition can allow us to apply modifications that can tune the bulk structure of wood and introduce potential multifunctional capabilities. Cellulose Cellulose is the reinforcing component of the cell wall. Its excellent mechanical properties are correlated to its structural features, which originates from its properties in the molecular scale. Cellulose is a highly crystalline linear polymer with a high degree of polymerization and crystallinity (Figure 1.2). The basic unit of cellulose is D-glucose, connected by β-(1→4)-glycosidic linkages to form glucan chains. These chains associate to form nanofibrils, which further aggregate into microfibrils containing crystalline regions. The formation of highly crystalline regions is due to the abundant and strong inter- and intramolecular hydrogen bonding 9 through the hydroxyl groups on C2, C3, and C6, both within the glucose plane between two repeating units in one chain and between two parallel cellulose chains. This crystalline structure gives cellulose its characteristic three primary crystal planes, (200), (110), and (1-10), which appear as peaks at 14.7°, 16.7°, and 22.1° in an XRD pattern.45 Due to the abundance of hydroxyl groups on its surface, when exposed during the separation of cellulose nano- or microfibrils from fiber bundles, cellulose exhibits unique colloidal properties, enabling it to form stable suspensions in aqueous systems.46, 47 In addition to these physical properties, the abundance of hydroxyl groups allows for chemical reactions that can modify and tailor the material's properties for specific applications. For example, TEMPO oxidation has become one of the most common, applicable, and effective treatments for cellulosic materials in recent decades.48, 49 It generates carboxyl groups at the hydroxyl position on C6 of the cellulose chain, increasing the material's solubility and colloidal stability in water by raising its charge density. Other chemical reactions such as periodate oxidation,50, 51 esterification,52, 53 etherification,54-56 silanization57 have all been applied and studied on the modification of hydroxyl groups on cellulose. More detailed reactions and functionalities will be discussed in the next section about wood functionalization. Hemicellulose Hemicellulose is a branched-chain amorphous polysaccharide of various sugar molecules that features a heterogeneous and a more complex structure with a variety of side chains.58 Similar to cellulose, it can also be found in the primary and secondary cell wall of wood and other plants. It strengthens cell walls by interacting with cellulose and sometimes lignin, as reflected in current models of the primary wall. Hemicelluloses are plant cell wall polysaccharides with β-(1→4)-10 linked backbones, including xyloglucans, xylans, mannans, glucomannans.59 Their structures and abundance differ across species and cell types. Hemicellulose is more amorphous compared to cellulose, with a more branched polymer chain and a lower degree of polymerization.60 As illustrated in Figure 1.2, the native form of hemicellulose is a flexible, linear, branched colloidal chain that is typically found surrounding crystalline cellulose chains and amorphous lignin complexes. This amorphous morphological property makes hemicelluloses partially soluble in water. Due to the abundance of hydroxyl groups on the surface of its xylan chains, hemicellulose can undergo reactions such as esterification, acetylation,61 and etherification62 to modify its original properties. For example, esterification of hemicellulose can improve its thermal stability, and this thermal property can be tuned by its degree of substitution.63 On the other hand, oxidation on C2 and C3 of hemicellulose backbone can make it even more hydrophilic and more water-soluble.64 Lignin In wood structure, lignin is an amorphous and highly branched molecule that can be found predominantly in the middle lamella and cell corners (also presents in primary and secondary cell wall). This compound has a phenylpropane structure composed of mainly three monomeric subunits: guaiacyl, syringyl and 4-hydroxyphenyl, linked by various types of bonds. Lignin links with cellulose and hemicellulose mainly through ether bonds, but also via other complex interactions with carbohydrates.65 It is generally recognized as the component responsible for the brownish color of wood. The brownish color of lignin is due to the presence of chromophore groups, such as quinoids, catechols, aromatic ketones, and conjugated carbonyls.66 Additionally, during the extraction and 11 fractionation of lignin from biomass, an even darker color can show up because of the condensation involving the conjugation and unsaturation of polyphenolic structures, which phenomenon is mainly correlated with the changes in pH. For a long time in the past, lignin was considered as a waste material, was often removed to utilize wood fibers and discarded due to its dark color in the paper industry. Recently, to use wood as a template material for various functionalities, lignin was also very likely to be removed. This is because the lignin naturally occurs in a highly branched form, which binds tightly to ordered cellulosic fibers within wood cell walls.65 When extracting any valuable components from wood through chemical treatment, lignin is the first part to be dissolved and separated from the complex network due to its amorphous molecular structure, no crystallinity, and reactive positions. Moreover, the removal of lignin creates more pores in the wood's original structure, which is beneficial for further chemical access, material interactions and specific functionalization. Thus, most functional wood processing starts from partially or completely removing the lignin. In this thesis we will first follow this protocol (applying delignification in Chapter 3 and Chapter 5) and also explore opportunities to reintegrate the extracted lignin into wood systems. Details will be discussed in Chapter 4 and Chapter 6. 12 Figure 1.2. Major biopolymers in wood and their chemical structures (partial illustrations were made by the author Xuetong Shi and Zhangmin Wan, partial illustrations were sourced from BioRender and arranged using PowerPoint.). 13 Chapter 2: Wood Functionalization 2.1 Treatment methods of wood Wood has been used in human life for centuries. Through an appropriate physicochemical treatment, the structure of wood's cell walls begins to change. Over time, wood has been structurally and compositionally redesigned for applications primarily in papermaking and construction. Treatment methods are mainly focused on altering the interactions of wood's main chemical components. In recent decades, more advanced functions have been explored through various treatments, though these strategies still largely target changes and interactions within these three key components. To access the biopolymers in the wood matrix, chemical treatments are used to swell and dissolve the wood's cell walls. During this process, the lignin is typically the first component to be dissolved and removed. To retain the three-dimensional structure of the wood, partial or complete delignification by various delignification methods38-41 is commonly employed and recognized as a classic approach. Removing lignin while preserving the mesoporous scaffold allows the exposure of cellulose microfibrils for further functionalization. Cooking wood under alkaline and sulfite conditions, using a mixture of sodium hydroxide (NaOH) and sodium sulfite (Na₂SO₃) with careful control, is one of the classic delignification method, as lignin can be cleaved and degraded during Kraft pulping.67, 68 In this process, an additional bleaching step using hydrogen peroxide (H₂O₂) is sometimes required to obtain white wood material. Besides treatment in a basic system, oxidation can also cleave lignin. For example, a sodium chlorite system can selectively remove lignin through oxidation (under acidic conditions) while retaining a large amount of polysaccharides in the wood structure, preserving its integrity and mechanical 14 properties.69 In the field of functional wood development, this method has become more and more commonly used, which can be found in many classic research studies.70-72 Additionally, peracetic acid is another system reported to have high selectivity for lignin removal in biomass,73, 74 By either sodium chlorite or peracetic acid, these methods selectively react with lignin, producing a decolorizing effect while preserving structural integrity. Instead of fully removing lignin, they directly modify its chromophore groups without targeting hemicellulose, unlike NaOH.75, 76 These methods generate nanoporosity, enabling further polymer impregnation and functionalization. Considering the environmental impact, greener solvents for delignification have been researched. Deep eutectic solvents (DESs) have attracted interest due to their beneficial features including low cost, biodegradability, recoverability and recycling ability,77 helping to reduce reliance on conventional solvents. A DES system used as a solvent for biomass typically consists of a mixture of two to three components, including a hydrogen-bond donor and a hydrogen-bond acceptor.78 Its performance in wood delignification can vary depending on the composition of the DES. For example, DES prepared from choline chloride and lactic acid (or malic acid and oxalic acid) demonstrated a softening effect on the wood structure during delignification.79 Choline chloride combined with oxalic acid exhibited a controlled hydrolysis effect on polysaccharides, with the wood structure largely retained and a higher residue yield 80 compared to conventional delignification methods mentioned earlier. This treatment can increase the specific surface area of wood by 19-fold. The combination of choline chloride and citric acid shows the ability of partially depolymerizing, modifying, and removing lignin as well as polysaccharides in their 15 amorphous state within the cell wall under this treatment.81 This makes the bulk wood softer and more pliable, which is beneficial for further treatment or functionalization. The dissolution and removal of lignin create more pores in the bulk wood, exposing the cellulosic fibers within the cell wall and making the cellulose more accessible for further treatment. However, due to the abundance of hydroxyl groups on the cellulose fibers and the strong inter- and intramolecular hydrogen bonds between cellulose chains, dissolving the remaining cell wall is still challenging. Various solvent systems have been studied to explore the dissolution mechanisms and their effects. For example, an aqueous NaOH solution is commonly used to swell the cell wall, as the hydroxyl groups on cellulosic fibers can become deprotonated in the alkaline environment, disrupting hydrogen bonding and increasing fiber accessibility (at concentrations of 5-35%). However, if the treatment is too harsh, cellulose chains begin to degrade due to alkaline hydrolysis.82, 83 Ionic liquid is also recognized as a solvent that can dissolve cellulose fibers. Prior to dissolving, cell wall started swelling first. For example, Chen et al. demonstrated that an ionic liquid-water mixture has a softening effect on the cell wall of delignified wood through a mild swelling process.84 Li et al. found that a mixture of ionic liquid and DMSO can partially dissolve the cell wall of delignified wood, leading to cellulose fiber regeneration and nanoscale spatial reassembly of wood biopolymers within the cell wall.85, 86 This process resulted in an increased specific surface area. Orema et al. studied pure ionic liquids without solvents and found that they can soften the cell wall and partially dissolve the surface of cellulose fibers.87 Similar to Li's work, the dimethylacetamide/lithium chloride (DMAc/LiCl) solution has also been shown to induce micro- and nanofibrillation, as well as fiber rearrangement effect within cell wall during the dissolution of delignified wood.88 16 As summarized above, various pretreatment and treatment methods, including physical strategies like swelling and chemical treatment including modifications of lignin and polysaccharides, can tailor porosity, surface chemical properties, structural properties, and functionalities. This top-down approach gradually removes specific components of wood while retaining different residues with various structural properties, allowing the wood to maintain its original shape and integrity due to its distinct hierarchical structure composed of entangled lignocellulose fibers. This compositional engineering enables the structural modification of wood at the nanoscale, leading to the development of advanced wood materials. This top-down strategy has been concluded and referred to as wood nanotechnology or nano wood engineering.89 Through various preparation and fabrication methods, the mesoporosity and surface functional groups of wood can be tuned as designed, allowing further functionalization of this natural substrate. This leads to the development of multifunctional wood with advanced properties, which can significantly expand the application fields by utilizing the sustainable wood materials. In the next section, functionality approaches will be introduced. 2.2 Summarized emerging approaches of wood modification and functionality Since wood is a native and excellent substrate material, and its fundamental components and chemistry have been well studied, it serves as an ideal starting material for various functionalization. For more advanced materials development, modification strategies and fabrication methods are typically related to the structural properties and wood chemistry, which can generally be categorized as either physical or chemical modifications. As summarized in Figure 2.1, various modification designs are based on the surface properties of wood, enabling the creation of value-added materials. For example, the incorporation of two-dimensional 17 materials and other inorganic particles can impart dielectric properties, conductivity, EMI shielding, and other functionalities to wood.90-92 Due to the inert chemical properties of most inorganic materials, their attachment and adhesion are primarily based on physical adsorption. Similarly, because of the anionic surface properties of delignified wood, cationic polymers and polyelectrolytes can also be incorporated via electrostatic interactions at the interface. Penetrating the wood scaffold with a guest polymer to prepare composite materials is a typical approach for producing transparent wood. The adhesion and interactions between two phases are mainly attributed to the physical interactions at the lumen surface as well as the capillary forces from the wood fibers. Additionally, densification is a commonly applied method to enhance the mechanical properties of wood without any chemical treatment or compositional changes. The properties of native wood can also be modified through chemical treatments. As mentioned earlier, these treatments typically target reactions with the major components and their surface chemistry (Figure 2.1). For example, polymers with specific functional groups can be grafted onto cellulose chains via the hydroxyl groups on delignified wood, carboxyl groups on TEMPO-oxidized wood, or other reactive sites, to realize the incorporation or deposition of the polymers via a chemical reaction. Similarly, polymers can also be synthesized on the wood substrate to introduce various properties through an in-situ polymerization approach.93 Additionally, the surface properties of wood can also be tuned through treatments targeting the exposed lignin and cellulose fibers. For example, compressible wood can be developed from stiff wood through partial dissolution or surface oxidation. Chemical modification involves the creation or breakage of covalent bonds and changes to the chemical structure of wood. Another notable example is TEMPO-oxidized wood, which alters the surface chemistry of cellulose fibers by generating 18 additional carboxyl groups.94 This partial chemical dissolution significantly enhances microporosity and increases the specific surface area. The commonly used wood modification strategies have been briefly summarized in this section. Detailed treatment methods and fabrication approaches will be introduced in the next section of the literature review, where more examples will be listed and explained based on various methods and applications. Figure 2.1. Functionalization and fabrication approaches of functional wood (partial illustrations elements were sourced from BioRender; the overall figure was arranged using PowerPoint). 2.3 Literature review 2.3.1 Functionalization: wood as a supporting material Wood, as a structural material, exhibits exceptional mechanical properties that make it an excellent engineering material. Through careful selective removal of specific components from wood, it is possible to retain its intrinsic structure and integrity. Ding et al. prepared a Janus wood membrane95 for controlled directional water transport by the partial hydrophobization on the delignified wood. This method preserves the unidirectional wood lumina while incorporating surface modifications and structural designs that facilitate one-directional water transfer. This 19 feature makes it suitable for fog harvesting applications. Liu et al.96 developed a wood membrane which enables photo-thermal conversion for solar steam generation. They successfully engineered the growth of CuFeSe2 nanoparticles (NPs) along the wood lumina, enabling the unidirectional transfer of water under the high solar thermal efficiency of 86.2%. Similarly, Chen et al. coated wood with Pd nanoparticles97 and CNT;13 meanwhile, Zhu et al.98 used carbonized mesoporous wood to extract water and showed a high light absorption and rapid water evaporation. Li et al.99 utilized the alignment of cellulose nanofibrils derived from wood to produce a structural nanowood with exceptional thermal insulation capabilities. This nanowood demonstrates anisotropic thermal properties, exhibiting an incredibly low thermal conductivity of 0.03 W/m·K in the radial direction. Inspired by the well-aligned structure, Hu's group100 made a whole-wood material with superior mechanical strength. By selectively removing certain components from the cell wall, followed by densification, wood with highly aligned cellulose nanofibers was produced, and demonstrated a stress resistance that is higher than the majority of structural metals and alloys. Combining with this densification procedure, coating with inorganic particles (e.g., boron nitride, h-BN) wood was enabled as thermally conductive flame-retardant material101. Taking advantage of the unique structure of wood, the developed materials showed heat insulation in the vertical plane. Likewise, bentonite72 was employed as a coating within the microchannels of the wood, which was subsequently densified. As a result, the wood's densely packed laminate structure exhibited pronounced flame retardancy. In conjunction with the densification process, nano-SiO2 particles were synthesized within wood, imparting hydrophobic properties99 to the material. Corrugated wood structures were used to produce 3D-molded wood 20 honeycomb structures.102 This latter approach preserved the inherent anisotropy of wood while improving the interaction between wood fibers, resulting in enhanced mechanical strength, which makes wood a promising candidate for applications in structural engineering. By similar strategies, including delignification and compression, radiative cooling wood103, 104 was produced and used to cool passively energy-efficient buildings. Apart from its engineering applications, the good mechanical properties of wood also make it a strong candidate for providing matrix support and strength in the development of flexible, gel-like materials. For example, as multifunctional hydrogels have recently become a focus of research, scientists have started using wood scaffolds as reinforcement structures instead of starting from cellulosic fibrils through a bottom-up self-assembly routine, which approach takes advantage of the mechanical property and reduced cost in processing feedstock. For instance, based on the study of Dong et al., a wood-PAM composite was prepared that overcomes the swelling behavior of native hydrogels because the wood matrix helps distribute the swelling stress, making the entire material resistant to swelling.105 Once the tough PAM was synthesized via in-situ polymerization in wood matrix, it's tough and flexible mechanical performance can make this type of wood-supported hydrogel for artificial mussel.106 Besides starting from penetration of monomers to obtain the gel phase through in-situ polymerization inside the cellulosic framework, direct impregnation of large molecules has also been researched under the vacuum condition. For example, polyvinyl alcohol (PVA) was also incorporated into delignified wood to fabricate flexible wood-based hydrogel, the rich interactions and hydrogen bonding between cellulosic matrix and PVA makes the material strong and stable.107 A thermo-responsive gel-like polymer, Poly(N-isopropylacrylamide)-polyacrylamide (PINIPAM), has also been 21 applied to prepare a transparent wood-gel device.108 Its robust mechanical properties and thermochromic properties given suggests that it is a good candidate for energy-saving windows applications. More sustainable polymers has been researched to act as the flexible phase, for instance, gelatin has been incorporated into such systems through vacuum impregnation, where its crosslinking enhances the mechanical properties of the gel.109 Similarly, an osteoconductive hydrogel was fabricated by incorporating sodium alginate for bone repair, in which system, precursor solution of Ca2+ ions can induce the ionic cross-linking to enhance its gel formation and mechanical property.110 Additional mineralization and deposition of particles in various shapes to wood-supported hydrogels, can induce other functions, such as parallel solar steam generation and hydrogen energy conversion.111 2.3.2 Structural design for materials with targeted mechanical performance Taking advantage of the natural lamellar structure of wood, compressible wood aerogels have been studied through different chemical treatment methods. Through a process involving partial dissolution of certain components from the cell wall, complete removal of hemicellulose, and subsequent cellulose regeneration, the wood lumens undergo wrinkling and gain enhanced flexibility. A series of structural changes contribute to the mechanical resilience observed in wood. For instance, Song et al.112 removed hemicellulose and lignin in fibers by NaOH and Na2SO3 treatment, resulting in the softening of the channels and the generation of nanopores within the wood structure. This treated wood membrane showed desired flexibility when it was rolled, twisted, bent and wrapped by hand. By a similar method, a mechanism was proposed that involved fibrillation-endowed super elasticity113. An excellent anti-fatigue property (after 10000 loading/unloading cycles) was demonstrated. In ETH, Prof. Burgert's group developed a flexible 22 wood-derived bio-scaffold by simply removing lignin through an oxidation system by hydrogen peroxide under acid conditions.38 The bulk properties of wood were preserved, but at same time, it allows the thick wood to be bent, twisted, and shaped as desired. After that, Burgert et al.114 studied the potential of applying this compressible wood as piezoelectric nanogenerator for sensing. Besides, cellulose regeneration through the partial dissolution by ionic liquid, wood tends to generate nanofibers and form interlocking network within the lumina. Similarly, an ionic liquid mixture was used to further dissolve delignified wood.85, 86 By carefully controlling the process, a dissolution of approximately 41% of crystalline cellulose was achieved, which effectively diffused into lumina. In these studies, Li et al. concluded that an interconnected regenerated network was formed within the wood structure which imparted mechanical resilience and anti-fatigue properties when subjected to compression. Prof. Li and her research group also found that further periodate oxidation of delignified wood, without removing hemicellulose, led to holocellulose degradation, which contributed to softening the wood skeleton.115 This process increased the nano- and submicron-sized pores in the cell wall and expanded the specific surface area from 1.3 m²/g to 26.7 m²/g. Wu et al. also studied the use of deep eutectic solvent treatment to prepare wood foams with a honeycomb structure,116 which allows for sustained compression while exhibiting good elasticity and durable compressibility. Based on this flexible template, further modifications have been made for value-added applications. Shi et al.15 developed a solar desalination wood which incorporated a coating on the compressible wood membrane with polypyrrole and subsequently carbonization. This modification allowed the membrane to achieve a remarkable solar conversion efficiency of 86%. The inherent elasticity of the membrane facilitated the efficient removal of water and ensured its 23 suitability for long-term cycling and repeated usage. Chen et al.117 simply carbonized the compressible wood aerogel and showed a desirable elasticity, which was maintained as a carbon sponge, which found uses in electronic strain sensors. Similar strategy118 was applied on flexible wooden carbon aerogels, which were studied for pressure sensing. By coating with organosilane119 (via chemical vapor deposition), compressible wood acquired superhydrophobic characteristics for uses in oil/water separation and oil absorption. Recently, Li et al. developed a stress sensor using a wood aerogel through deep cell wall metallization by removing cell wall components and efficiently loading copper.120 The lamellated, multilayered cell structure of the cellulose fibers in the wood scaffold benefited its compressibility and enabled stable performance during repeated cyclic use. On the other hand, instead of softening wood, wood can be transformed into super-strong materials with significantly enhanced strength compared to native wood. Prof. Hu's group developed a “super wood”100 that is 10 times stronger and 12 times tougher than untreated wood (along the fiber direction), making it a promising candidate for use in armor, buildings, and vehicles. The process involves delignification, heating, and densification. The superior mechanical properties are attributed to strong hydrogen bonding between exposed cellulose nanofibrils and the unidirectional fiber alignment. Increasing interfacial interactions is another effective strategy to improve the mechanical property of wood and to overcome fractures at weak points. Partial dissolution of the cell wall can promote fibril regeneration at interfaces, forming strong hydrogen bonds. Using this approach, a strong laminated “healed” wood with an excellent isotropic tensile strength of ~190 MPa was achieved through LiCl/DMAc treatment combined with densification.88 24 Similarly, adhesive-free strong wood can also be developed through induced self-crosslinking and self-condensation at the interface of laminations. Rojas et al. fabricated such fully bio-based glazes through surface oxidation, followed by densification and multilayered stacking, achieving an outstanding wet strength of approximately 128 MPa.121 Besides self-adhesion at the interface of two phases, the use of a green adhesive between laminations is another way to achieve the desired mechanical strength. A strong all-wood based plywood was prepared using the separated uncondensed lignin aqueous suspension as alternatives to traditional wood adhesives.122 The subsequent hot pressing allowed the softened lignin to fill the vessels and crosslink at the interface, forming strong interactions. Similarly, lignin nano particles have also been proven to work as an effective green adhesive for producing plywood with high strength.123 2.3.4 Wood-based composites and hybrids for energy management Lumina in wood show unique characteristics that make them advantageous to control transport properties. This fact has attracted researchers' interests to study the potential of utilizing wood's natural structure to transport electrons and ions. For example, Wu et al.124 used carbonized wood as a template to grow carbon nanotubes perpendicular to tracheids within lumina. With such properties, the fabricated material (a supercapacitor) reached an energy density of 39.8 Wh/kg. Similarly, by carbonization of wood with the incorporation of red phosphorus, a carbon-wood electrode was developed and examined as suitable application in sodium-ion batteries.125 Finally, 3-dimensional hierarchical structures have been developed as high-performance supercapacitor.126-129 The utilization of triboelectric energy has emerged as a promising research area. Wood-based Tribo-nanogenerators materials (TENG) have sparked research interest, primarily due to the 25 advantageous characteristics of bulk wood, such as its low cost, ease processing, and biodegradability. Bang et al. 130 made a wood TENG material by modifying wood with trichloro(1H,1H,2H,2H-perfluorooctyl) silane as a negative electrode. The power generated reached 90.1 V. Burgert's group131 found that MOF-coated wood exhibited superior tribopositive properties. By growing PDMS on the wood as a more tribonegative material, the wood-TENG system enabled the modulation of indoor lighting. Researchers in this group also studied wood samples treated with O2 and C4F8 plasma can benefit to the TENG electric output in both periodic contact-separation and single-electrode operation modes, reaching a maximum voltage of 227 V and a current of 4.8 µA132. A flexible transparent wood film (optical transmittance of 86 %) with high toughness (2.37 MJ/m3) and desired electric output was studied by Chu et al.133 They applied methacrylic anhydride to modify delignified wood firstly, through in-situ esterification modification, methyl groups and acrylate groups were exposed at wood surface. Afterwards, materials were treated with polymerizable deep eutectic solvents (PDES) to allow photo-polymerization and the formation of the covalent-noncovalent dual crosslinked functionalized wood. This modified tough methyl group containing wood film presented enhanced charge density transportation performance, and outstanding antibacterial performance. To use transparent wood as TENG devices, Wu et al. modified wood through delignification and penetration with UV-curable resin.134 They demonstrated that, due to the strong electron-donating groups in the resin, the energy output of the modified material was improved by 6.5 times compared to native wood. Thus, containing specific functional groups is crucial for improving electrical output. Prof. Berglund and his team modified wood veneer through sequential delignification, TEMPO oxidation, and model fluorination to create a flexible sensor,135 which sensor exhibited excellent piezo response, generating output voltages of 60 mV. 26 Prof. Nie and his team136 found that incorporating carbon nanotubes into highly porous treated wood resulted in selective ammonia sensitivity, with an ammonia-sensing response of 0.85 at 500 ppm, making it an effective sensor. Chen et al. also found that wood is a promising piezoelectric material for sensor.137 They explored compressible wood-based sensors across various scales and applications, including assembling a wood floor for low-power electronics. They also demonstrated a self-powered sensor system capable of detecting human movement when adhered to the skin. Besides the application in electronics, wood has been extensively investigated for light management purposes. Starting from 2016, transparent wood71, 138-146 has been developed by utilizing various methods such as impregnating delignified wood with a guest polymer that possesses a comparable refractive index. To obtain a high optical transmittance (84%-92%), groups in the University of Maryland and KTH have employed materials such as PVP, PMMA, pyridine, melamine formaldehyde and epoxy resins, which were infiltrated in the wood framework.138-146 They delved deep into studying how the structural factors, such as complete lignin removal, lignin bleaching, infiltration efficiency, and composite thickness, influenced the optical performance. The main goal in achieving high transparency has been to ensure compatibility between the guest material and wood, which entails minimizing the presence of air as much as possible. On the basis of such concept, recent achievements have been advanced in transparent and luminescent wood.147, 148 Fu et al. incorporated quantum properties in transparent wood fabrication systems and a laminated plywood showed load bearing and light-emitting possibilities.147 Wang et al.149 fabricated luminescent films with the addition of AIE-active groups which exhibited near full UV-blocking capability. Superior mechanical property, 27 transparency, UV-protection, anisotropic light scattering, and biodegradability were achieved. Prof. Zhou and his team at KTH developed transparent wood150 using a highly mesoporous wood xerogel template, enabling effective penetration of UV-curable acrylic resin, ethoxylated bisphenol A diacrylate (ABPE-10), into the wood cell walls. In this study, TEMPO oxidation of delignified wood opens the cell walls, facilitating efficient interactions between the polymer and the wood structure. Using thermoset precursor thiol-ene as the filling polymer, Prof. Berglund's group investigated the relationship between the haze properties of transparent wood and various lignin treatment methods.151 They found that removing the chromophores from lignin is effective in creating a cellulose template, and the high transparency is primarily due to improved interfacial adhesion between the two phases, which plays a more critical role than matching the refractive indices of the wood and polymer. Rather than incorporating a guest polymer, for obtaining a high light transmittance, optically transparent wood can also be developed by densifying wood fibers (with low lignin content) to close pores as much as possible. Fu et al. fabricated a flexible, transparent, film-like electronic device152 directly from wood by extensively removing lignin and hemicellulose, followed by densification, which enabled cell-wall collapsing and strong interfacial bonding between the aligned cellulose nanofibers. Similarly, to preserve the integrity of the wood while achieving strong mechanical properties, hemicellulose can be retained in wood-film preparation, and introduced chemical crosslinking can enhance intercellular bonding between holocellulose fibers. Subsequent thermal compression causes cell wall collapse, resulting in a transparent film with high optical transmittance of 71% and a haze of 85%.153 28 For thermal energy usage, wood has been considered as a candidate for supporting phase change materials. Natural wood, characterized by its porous structure, low density, and high loading content, over 80%, holds great promise as supporting material. The incorporation of phase change material will be introduced in the next session. 2.3.5 Heat energy storage: phase change materials and combination with wood A Phase Change Material (PCM) has the ability to store a large amount of heat during phase transition. PCMs can be employed in applications such as sustainable thermal energy storage, temperature regulation and thermal insulation.154-156 Thus wood-supported PCM composites is of interest in the energy usage area. Montanari157 developed transparent wood-PCM composites by the impregnation of PCM with partial PMMA addition in wood to form a shape stable composites. The same group used bio-based 1-dodecanol and limonene acrylate that were combined with DW and generated a fully-biobased PCM,158 which showed thermo-responsive optical transmittance. Yang159 also developed a transparent composite composed of thermochromic compounds and delignified wood which showed thermal insulation ability and enabled temperature regulation for prolonged time. Meng160 used delignified wood (SSA=19.2 ± 1.1 m2/g) to encapsulate PEG and reached 83.5% encapsulation efficiency. In this study the mechanism leading to the effectiveness of PCM adsorption was attributed to the increased hydrogen bonding between PCM and the wood matrix. Besides the hydrophilic compatibility, capillary forces of the tracheid contribute to the adsorption and further shape-stabilization. Similar mechanisms (physical adsorption, secondary interactions) are applied in other wood-supported PCM work.161 These studies highlighted the properties of wood-supported PCM, including mechanical strength, outstanding heat dissipation 29 capabilities, and proper phase transition temperature, all supporting the potential for outdoor building energy conservation and management. Overall, recent studies have shown the possibility and effectiveness of thermal energy storage by employing PCM. 2.4 Thesis outline and research objectives 2.4.1 Research objectives Wood has been used as a construction material for centuries, and its engineered forms continue to exhibit excellent mechanical and functional properties. However, beyond its conventional structural applications, there is growing interest in imparting multifunctionality to wood, particularly to enhance its role in harnessing natural energy sources. For example, wooden roofs in buildings are typically designed for shielding and insulation but rarely for active energy conversion. This presents an opportunity to develop value-added wood materials that can serve dual functions: as structural elements and as energy-harvesting components. A dark-colored, wood-based panel, for instance, could convert and store solar energy during sunny days. Similarly, in rainy climates, it is desirable that wooden roof materials be designed to harvest energy from rainfall. Enhancing wood with increased surface roughness and triboelectric properties enables interaction with rain droplets, enabling energy conversion through triboelectric nanogeneration (TENG). The main goal of this thesis is to tune the native properties of wood to develop multifunctional, sustainable wood-based materials for smart construction applications. Specifically, the work aims to add energy conversion capabilities to traditional construction wood, making it a more valuable and versatile material. Based on the understanding of wood’s natural properties and 30 insights from the literature, this thesis hypothesizes that the hierarchical and porous structure of natural wood provides a sustainable scaffold that, through targeted functionalization and additive integration, can be transformed into multifunctional composites capable of overcoming intrinsic limitations and achieving high performance in energy conversion and management. Various types of sustainable energy conversion and utilization derived from this concept are examined and discussed in detail in Chapters 3 through 6. This thesis utilizes natural wood as the primary structural scaffold. By tailoring the properties of its inherent components, especially those within the cell wall, through surface modification and the integration of functional additives, the research seeks to create advanced wood-based materials for diverse energy applications. These include solar energy conversion, thermal energy storage and insulation, and rainfall energy harvesting. The work addresses critical limitations of natural wood, such as low light absorption, hygroscopicity, and rigidity, by implementing advanced functionalization strategies. In this thesis, balsa is selected as the primary system and starting material for modification. Its unique natural characteristics are studied across three chapters. Balsa (Ochroma pyramidale) is a low-density (0.06–0.38 g/cm3), high-porosity (74–91%) hardwood distinguished by its vessel-rich anatomy and thin-walled fibers, particularly in low-density variants.162, 163 Its chemical composition is typical of hardwoods, consisting of approximately 50% cellulose and 25% lignin. This distinctive structure imparts exceptional physical properties, making balsa an ideal material for lightweight structural applications. Moreover, its well-established features of low density and high porosity make it an excellent substrate for foundational studies, as these characteristics 31 facilitate homogeneous treatment and reaction, enabling a clearer analysis of fundamental mechanisms in wood-based energy materials. Beyond the influence of porosity, when the objective shifted toward investigating wood surface properties independent of porosity, the study was expanded to include additional species. This broader approach aimed to evaluate the versatility and general applicability of the proposed treatments across different wood types. The potential of these treatments was examined in Chapter 4 through comparative studies involving basswood, fir, walnut, and oak. To support this development, this research investigates the fundamental structure, composition, and chemistry of wood, drawing on both contemporary and historical scientific studies. Controlled adjustment of wood components enables precise tuning of material properties. For instance, partial or complete lignin removal increases porosity, producing a more compliant structure that facilitates secondary polymer infiltration, topics explored in two chapters of this thesis. Conversely, selective lignin retention following hydrotropic treatment induces favorable surface morphological changes and enhances light absorption, also examined in two dedicated chapters. The overall approach begins with the modification of wood’s primary chemical constituents, namely lignin and hemicellulose, to achieve tailored structural and functional properties. The performance of the modified wood materials is systematically compared with that of native wood prior to treatment, with details presented and discussed in subsequent chapters. Four core innovations are introduced, emphasizing structural design, chemical modification, and lignin valorization to enhance the multifunctionality of wood for sustainable energy applications, 32 structural design, chemical modification, and lignin valorization to improve wood's multifunctionality. Collectively, this work demonstrates the versatility of wood as a sustainable platform for energy technologies. By combining its hierarchical structure with targeted modifications, the thesis contributes to the development of cost-effective bioproducts for solar, thermal, and rainfall energy systems—aligned with global sustainability goals. The specific research objectives include (Figure 2.2): • Modify solid wood through simple chemical treatments to tune mechanical properties for thermal insulation and energy dissipation. • Design and fabricate wood-based solid–liquid triboelectric nanogenerator (S-L TENG) systems as alternatives to fluorine-containing polymers for harvesting rainfall energy. • Develop an effective strategy to enhance thermal conductivity in wood composites for efficient thermal energy storage and desired photothermal conversion performance. • Establish a green, polymer-free approach to fabricate all-wood composites with high photothermal conversion efficiency. 33 Figure 2.2. Thesis objectives and research outline (partial illustration elements were illustrated by the author Xuetong Shi or taken from the autor’s publication in this thesis; illustrations elements of the house was sourced from BioRender; all elements were arranged using PowerPoint). 34 2.4.2 Thesis outline In line with the research objectives, this PhD thesis comprises four main experimental studies that utilize various wood species—primarily balsa (ochroma pyramidale), with additional studies involving basswood (tilia americana), fir (Abies), walnut (Juglans), and oak (Quercus) as the starting material. The aim is to develop four types of functional materials, including composites, foams, and slabs, for different sustainable energy applications. A key focus of this work is to investigate how physical and chemical treatments influence material performance, how to strategically design and fabricate these materials, and how microstructural transformations at the nanoscale contribute to their functional behavior. These aspects of wood nanotechnology are analyzed and discussed in detail throughout the thesis. The dissertation is organized into seven chapters: • Chapter 1 introduces the background and motivation for this research, emphasizing the global need for renewable energy technologies and the role of sustainable materials in addressing energy challenges. • Chapter 2 presents a comprehensive review of wood as a green material solution, outlining its hierarchical structure, the primary biopolymers (cellulose, hemicellulose, and lignin), and their relevance in functional applications. This chapter also reviews current progress in functional wood research, summarizes modification methods, and defines the objectives and scope of the thesis. Particular attention is given to selective component removal (e.g., lignin or hemicellulose) to produce delignified or lignin-rich wood as intermediate platforms for further functionalization. 35 • Chapter 3 introduces a novel, top-down approach to fabricate a flexible, compressible wood foam—referred to as elastic wood foam (EWF), in line with the related publication. This material exhibits tunable compressibility and surface properties, making it suitable for energy dissipation and thermal insulation. • Chapter 4 focuses on rainfall energy harvesting. A hydrophobic, all-wood triboelectric nanogenerator (TENG) was developed via in situ lignin redistribution. The modified surface enhances contact electrification with raindrops, resulting in improved triboelectric output. The chapter explores the structural characteristics, underlying mechanisms, and energy output of the TENG system. • Chapter 5 details the development of a photothermal wood composite through the deposition of polypyrrole onto delignified wood. This strategy significantly enhances solar light absorption, enabling efficient thermal storage and unidirectional heat regulation for passive building applications. • Chapter 6 continues the topic of solar-thermal conversion, as addressed in Chapter 4, but explores a more sustainable pathway by utilizing lignin already present in the wood, thus avoiding the use of fossil-fuel-based polymers. This chapter highlights the use of intrinsic lignin for photothermal enhancement in wood-stabilized phase change materials (PCMs). • Chapter 7 concludes the thesis by summarizing key findings and proposing future directions for advancing functional wood materials in energy applications. 36 Chapter 3: Compressible Wood Foam for Thermal Insulation 3.1 Introduction In this chapter, we address the challenge of tuning wood's structure to enhance its thermal insulation and energy dissipation properties through a novel structural modification strategy that differentiates from conventional methods. Although naturally stiff, wood can be transformed into a flexible, foam-like material with increased porosity, enhancing thermal insulation and enabling effective heat management. To this end, we investigate the feasibility of developing a wood-based flexible foam-like insulating material capable of energy absorption and structural failure prevention via cushioning effects. Lightweight materials, such as sponges, foams and aerogels, are of significance in diverse products.164, 165 Given sustainability and low carbon metrics, the utilization of renewable materials such as plant fibers has become attractive for the synthesis of these materials. For instance, foams generated from cellulose, chitin, alginates and other biopolymers166-168 take advantage of the their inherent structural features (alignment, porosity, etc.).169 Bio-based polymers that are widely available and feature low-density, renewability, biocompatibility170 are most desirable in applications associated with the fields of energy (absorption, thermal and sound absorption), health (wound dressings and biomedical scaffolds) and others (composites, packaging, sensing devices).171-175 The development of anisotropic foams has incentivized efforts 37 to form ordered and aligned structures that benefit from directional heat/electron transfer, mechanical compressibility, and energy adsorption.176-178 Controllable, highly-regular, anisotropic foams mainly use bottom-up approaches to assemble nano- and micro-scaled materials by means of ice-templating and unidirectional freeze-drying.179, 180 These techniques enable ice crystal growth along a preferred direction under a temperature gradient, yielding anisotropic 3D porous structures upon drying.181, 182 However, ice templating and cryo-processing (< -70oC) incur in material and energy costs, limiting large-scale production.183 Not surprisingly, recent efforts to synthesize lightweight materials have been focused on top-down strategies. One notable case involves wood, which is cost-effective and offers anisotropic morphology. Wood's multi-porous and three-dimensional structure, characterized by well-aligned fibrous channels,35 makes it an excellent scaffold for anisotropic foams. Typically, wood-based foams require lignin removal, leaving vertically aligned tracheid structures, primarily composed of holo-cellulose.184 Further steps such as partial dissolution and regeneration within the cell wall area applied.85, 86, 185 The resultant highly porous anisotropic foams present high specific surface area (>200 m2/g), which limit the mean free path of air transport, critical to thermal insulation. The reported efforts so far use non-derivatizing solvents such as ionic liquids and dimethylacetamide/lithium chloride (DMAc/LiCl). The obtained porous cellulosic materials are mechanically strong in all directions but lack flexibility which is a requirement of packaging materials designed for cushioning. Hence, anisotropic wood-base foams exhibiting insulative and compressibility are being sought after. Attempts to prepare elastic wood aerogels118, 186 require hemicellulose dissolution after delignification so that the cell walls become lamellar structures that exhibit flexibility and compressibility.113, 117, 119, 187 38 Unfortunately, the developments in the area imply a low material yield, given the required removal of non-cellulosic components. Following these efforts, multifunctional elastic wood foam-like materials have been shown to display hydrophobicity,119 electrical188 and magnetic189 conductivity as well as desalination capability.15 In an attempt to avoid the removal of hemicelluloses and induce flexibility after delignification, chemical crosslinkers190 and hydrophilic polymers191 have been applied, mostly relying on synthetic options. High yield, compressible wood foams obtained by top-down approaches require minimizing the dissolution of cell wall biopolymers, cellulose and hemicelluloses. Unfortunately, the methods reported so far are designed to remove lignin and hemicelluloses to generate a wave-like cell wall that bring flexibility to the final material. This is at the cost of a ca. 40% mass loss.119 Achieving a high mass yield requires hemicellulose retention in the structure and modulation of the stiffness of the holocellulose structure. In such endeavors, ordered cellulose can be converted to more amorphous structures by oxidation,50 hydrothermal treatment,192 etc., which promote mechanical compressibility. Herein, we introduce a strategy to produce lightweight and resilient wood-based foams that can be tailored for their compressibility. Balsa has a highly porous structure and low density; hence, it is a desirable starting wood species to study the effect of the modification to ensure the homogeneous chemical modification of the bulk material. Thus, high yield chemical treatment of balsa is introduced by an all-aqueous system that partially oxidizes the surface of holocellulose at a low temperature followed by reduction. The structural integrity of balsa is preserved (high total yield, 72.8%) and core-shell fibrillar structures are hypothesized to form at the microscale. High compressibility and flexibility result from a spring-like scaffold that comprises interconnected fibril networks. The cellulosic fibrils become exposed to interfaces 39 comprising multi-scaled pores combined with anisotropic arrangements that limit heat conduction. Full shape recovery after compression (up to 60% of the original dimension) is shown in the wood-derived foams. Optionally, the elastic wood foam can be easily hydrophobized with short-chained organosilanes leading to hydrophobicity and flame retardancy. 3.2 Materials and methods 3.2.1 Materials Balsa (Ochroma pyramidale) from Modulor (Germany) was cut into small specimens (30 x 30 x 5, 25 x 30 x 20 and 20 x 20 x 100 mm). Sodium chlorite (NaClO2, ≥90%), sodium periodate (>99%), sodium borohydride (99%), hydroxylamine hydrochloride (98%) and trimethoxy(methyl)silane were purchased from Sigma-Aldrich. 3.2.2 Compressible wood foam Balsa wood was delignified following a previous protocol160 yielding “Holo-Wood” (in reference to a material composed of both cellulose and hemicelluloses). As a reference, Holo-Wood was treated with 0.1M NaOH at 80 oC to remove the hemicelluloses and then freeze-dried, leading to “Cell-Wood”. Another set of samples used Holo-Wood (30 mm (length in cross section) x 30 mm (width in cross section) x 5 mm (thickness in tangential section)) to partially convert cellulose into its dialcohol form. This was done by sequential oxidation and reduction. The first step was carried out by immersion (4 to 14 h) in NaIO4 solution (10 g/L) at 50 oC, yielding partially dialdehyde Holo-Wood. Water was then used to wash excess chemicals until the electrical conductivity of the leachate was < 5 mS/cm. The obtained samples are thereafter referred to as Oxi-Woodx (with x indicating the oxidation time, in hours). The degree of oxidation was measured by titration of the aldehyde groups.193 The oxidized wood was then 40 reduced by immersion in NaBH4 solution (2g/L, 0.5 g/g wood) at room temperature during a given time (2 to 4 h). This step involved the reduction of the dialdehyde groups to prevent self-condensation, yielding dialcohol forms of cellulose. Following freeze drying, the samples became highly elastic, referred to as elastic wood foam or EWFx, with x referring to the oxidation time (4-14 h). Chemical treatment, including oxidation and reduction, were first conducted on small specimens with a size of 30 mm (cross section) x 30 mm (cross section) x 5 mm (tangential section) to measure the properties as a function of treatment time. Then cubic EWFs (25 x 30 x 20 mm) were treated for 12 h by using the same method, which was used in studying further properties, tests and treatments. Reference materials were used as a set of compressible samples obtained by wood delignification, as explained before, followed by removal of hemicelluloses (HC). Finally, the EWFx samples were optionally hydrophobized by chemical vapor deposition (CVD) with trimethoxy(methyl)silane (MTMS) at 80 oC. These samples are thereafter referred to as Si-EWF. 3.2.3 Characterization Scanning electron microscopy (SEM, Helios NanoLab 650) equipped with energy-dispersive X-ray spectroscopy (EDX) was used to observe the microstructures and elemental composition of the materials. For morphology, all samples were first freeze-dried after chemical treatment and then the dry samples were cut carefully with a blade to obtain a smooth surface. Both EWF and Si-EWF samples were cut and the inner surface in tangential plane was observed. 1D X-ray diffraction (1D-XRD) patterns of the powder samples were acquired in the 2θ range between 10o and 70o by using CuKα radiation while 2D XRD (transmittance mode) patterns of Holo-Wood were acquired using thin slices of samples cut along the grain direction (Bruker APEX DUO 41 with APEX II CCD Detector). The samples were scanned at 120 sec/frame using a detector distance of 60 mm at 2θ = 0°,  = 0°,  = 0°,  = 0°. The yield of EWF was measured on foams produced following different oxidation time (30 x 30 x 5 mm, 4 replicates) by the ratio of dry weight (after freeze drying) to the weight of untreated wood with same size. The degree of oxidation (DO) was measured on the oxidized wood samples (1 piece~0.2g, 3 replicates) with diffident oxidation time by the titration of carbonyl groups. Oxidized wood samples, each with an approximately 0.2g dry weight, were immersed in a 50-ml of 0.25 M hydroxylamine hydrochloride solution (with a pH =4). This solution was stirred for 2.5 h at room temperature for further titration. Then the pH was set back to 4 by using 0.01 M NaOH solution and the volume consumed was noted. Then DO was calculated from the base consumption.194 A stress-controlled dynamic mechanical analyzer (DMA Analyzer 850, TA instruments) was used to assess the mechanical properties of the elastic woods, EWFx (10 x 10 x 5 mm), by using 10 creep recovery cycles. The given sample was compressed using a stress of 0.01 MPa for a creep time of 5 min following a recovery time of 5 min. This protocol was applied repeatedly 10 times to study mechanical recovery (strain, compliance and recovery compliance). DMA measurements were also carried out on Holo-Wood, EWF and Si-EWF (10 x 10 x 5 mm) to assess the mechanical stability in air at given relative humidity (RH) after equilibration for 5 min. Stress-controlled creep recovery profiles were acquired at 0.01 MPa using 5-min humidity conditioning at 0% RH. Then the RH was set at 5% for 5 min followed by creep recovery at 0.01 MPa for 5 min. The produced strain was recorded in the 0 to 95% humidity range using stepwise increases of 5% RH. At RH=60%, 20 compression-release cycles were applied on Si-EWF using a rate-controlled strain from 1% to 50% (and vice versa) using 5% strain/min increments. The mechanical tests were carried out on EWF samples of different degrees of oxidation by using an 42 Instron 3345 Materials Testing System operating under compression mode with a 2-kN load cell, until reaching 60% compressive strain. The mechanical tests were also conducted under strain ramp mode with Cell-Wood, Holo-Wood and EWF samples to study their recovery and mechanical resilience. Compression (2-kN load, 5 mm/min displacement rate) was applied at strains set to 10, 20, 30, 40, 50 and 60% followed by unloading (5 mm/min until 0% compression). The EWF samples were compressed (5 mm/min displacement rate) from 0 to 50% strain and released (0%) at the same rate under a 2-kN load. The procedure was repeated during 100 consecutive cycles on EWF (10 x 10 x 20 mm) to determine the stability and retention of the mechanical properties. The thermal conductivity of the samples was measured with a C-Therm TCiTM Thermal Conductivity Analyzer and temperature maps produced upon heating were collected by using an IR camera (FLIR, E76). The water retention value (WRV) of samples (25 x 25 x 30 mm) was measured in duplicate following the Scandinavian test method SCAN-C 62:00 after soaking in DI water overnight. The wet samples were then centrifuged (3000 rpm) for 30 min and the weight recorded before and after oven drying. The WRV was calculated following the equation 𝑊𝑅𝑉(%) =𝑚𝑤𝑒𝑡−𝑚𝑜𝑑𝑚𝑜𝑑∗ 100% where mwet and mod are the weight of samples determined following dewatering under centrifugal field and after drying, respectively. The compositional analysis of the reference natural balsa, Holo-Wood and fabricated EWF samples were measured (in duplicate) following the Klason protocol using TAPPI standard method T222. All samples were freeze-dried firstly and milled into powder, then 0.2g of each sample were mixed with 3 mL of 72% H2SO4 for 2 hours. Then the solution was diluted with 112 mL of DI water and autoclaved at 121 °C for 1 hour. The sugar content was analyzed by a Dionex HPLC (ICS-6000) 43 and the acid-soluble lignin was analyzed by UV–Vis (SHIMADZU UV-2600). The non-acid soluble lignin was obtained by the dry weight of samples after filtration. The water contact angle of given surfaces was measured by using a Theta Flex 300-Pulsating Drop 200 Tensiometer using a sessile drop (5 ml). The limiting oxygen index (LOI) of Si-EWF (20 x 20 x 100 mm) was measured by a Limiting Oxygen Index analyzer (QINSUN, China) on 18 specimens following the ASTM D2863-2019 and ISO 4589-2-2006 standards. Flame retardancy tests were also carried out on unmodified wood, EWF and Si-EWF by using vertical and horizontal flame tests (UL-94 standard). Both vertical and horizontal tests were conducted in a laboratory fume hood. For vertical burning test, cuboid specimens (length of 100 mm ± 0.5 mm, width of 15 mm ± 0.5 mm and thickness of 13 mm ± 2 mm) were prepared and set vertically above a torch while keeping 20 mm separation between the specimen and the flame. The burning was kept for 10 s, for twice on each sample. For horizontal tests, specimens with the same dimensions were placed horizontally and a torch was set at 45 o to the end of the material (30 mm in distance was kept). When the torch was on, the flame propagation time was controlled for 30 s. Both vertical and horizontal tests were conducted using four replicates. 3.2.4 Molecular dynamics (MD) simulation Cellulose Iβ crystallites were built according to known crystallographic vectors195using the native cellulose crystallite built by the author's script (Figure A1e). The GLYCAM06 force field was utilized to describe the motion of unmodified cellulose.196 The modified cellulose configuration (after oxidation and reduction) was built by Avogadro, and the GAFF topology parameters were those generated by Sobtop.197 For an accurate description of the electrostatic charges of cellulose in Holo-Wood, restrained electrostatic potential (RESP) atomic charge was 44 computed based on the MKRESP (Merz-Kollman Restrained Electrostatic Potential) methodology.198 The cellulose chains (5-unit molecules) were firstly optimized by using B3LYP/Def2-SVP with D3 dispersion correction.199 During the period of configuration optimization, the backbone of the whole chain was fixed. Then, a single-point calculation was performed at B3LYP-D3/Def2-TZVP level 37 using Orca 5.0.200 RESP fitting of charges was performed using Multiwfn (version 3.8 dev).201 A TIP3P (transferable intermolecular potential 3P) water model202 was used to study cellulose interactions in aqueous medium. The system was built by using PACKMOL (version 20.3.5).203 Visualization and analysis of MD trajectories were carried out with the Visual Molecular Dynamics (VMD) package (version 1.9.3) and gmx utility.204 All the configurations were firstly energy-minimized by both steep descent and followed by conjugated gradient methods. The energy-minimization configurations were carried out with pre-equilibrium simulation using an NPT ensemble at 303.15 K and 1 bar pressure. Subsequently, simulations were carried out in NPT ensemble for another 100 ns. During all the simulations, the particle mesh Ewald (PME) approach was used to evaluate long-range electrostatic interactions.205 A real space cut-off of 1.2 nm and Lennard-Jones interaction cut-off of 1.2 nm were used.206 The Potential-Shift-Verlet scheme and force-switch function were used to modify the electrostatic and Lennard-Jones interactions to reach a smooth transition effect at the cut-off.207 3.3 Results and discussion 3.3.1 Compressible wood foam Wood shows mechanical strength at a low mass density, which enables the support of high loads without significant deformation. In this work, to endow flexibility, periodate oxidation was 45 applied to balsa, which reduced the structural rigidity of the fiber bundles after partial removal of lignin. The periodate oxidation converted the C2 and C3 secondary hydroxyls of cellulose's glucose ring into aldehydes,208 which were then reduced into primary hydroxyls (Figure 3.1a). As a result, some of the crystalline cellulose was converted into amorphous di-alcohol forms,209, 210 disrupting inter- and intra-molecular hydrogen bonding. In addition, the reduction of aldehyde groups into primary hydroxyls prevented self-crosslinking and formation of hemiacetal groups between vicinal aldehyds.209 Therefore, we hypothesize that upon oxidation an enhanced cellulose chain flexibility is transferred to the bulk structure, as discussed below. After this treatment, wood became flexible that it was easily compressed by hands (Figure 3.1b). Figure 3.1. (a) Schematic illustration of the chemical treatment (oxidation-reduction) applied to balsa wood. (b) Elastic wood foam (EWF) subjected to loading and unloading cycles (1 kg). The oxidation-reduction reaction is expected to also affect the heterogenous and more complex hemicellulose components.64, 211 We determined that periodate oxidation of hemicellulose was limited. However, we generally refer to dialcohol cellulose/hemicellulose as the form obtained after the oxidation-reduction treatment. The elementary fibers composed of disordered dialcohol cellulose/hemicellulose are mobile and flexible given the ring-opening reaction and partial 46 dissolution of the ordered backbone of holocellulose. The changes in the molecular structure of the treated balsa, herein referred to as elastic wood foam (EWF), endowed the solid material with excellent flexibility. As expected, the relative proportion of dialdehyde cellulose/hemicellulose (as measured by the degree of oxidation, DO) increased with treatment time (4 - 14 h), Figure 3.2a. Excess oxidation (>8 h) caused material and shape losses (Figure A1a), giving a large decrease in mass yield (Figure 3.2c). Delignification followed by oxidation-reduction, using oxidation for eight hours, led to a good balance between elasticity and total yield (72.8 %). The obtained EWF showed a fibrillar structure with highly anisotropic behavior (40% height change in the transverse direction and no significant deformation in the longitudinal direction under a 1-kg load, Figure 3.1b). Upon loading, EWF is easily compressed in its cross section but quickly recovers upon unloading. The system is reversibly recovered under cyclic loading in its transverse direction (compressed for 2 min with a 1-Kg weight), with no permanent deformation (Figure 3.1b). The residual hemicellulose content in EWF was 16%, Figure 3.2b and Table A1, much higher than in elastic wood reported so far (<5% residual hemicellulose).117, 119, 187 The fibrils were partially dissolved when a longer treatment was applied, reducing the holocellulose content in EWF compared to that of the wood material after delignification, herein termed Holo-Wood, Figure 3.2b and Table A1). The amorphization of cellulose and softening of the structure promoted mechanical flexibility. The stress-strain profiles indicate an increased compressibility and elasticity with the increase of degree of oxidation (oxidation time) (Figure 3.2c). EWF displayed recoverable strain (EWF8 sample under compression cycles), indicating suitable material resilience, mechanical stability and cyclability (DMA, loading/unloading tests, Figure A1c-d, Appendix). 47 Figure 3.2. (a) Degree of oxidation and yield of EWF for given oxidation time, 4 to 14h. (b) Relative content of cellulose, hemicellulose and lignin in natural wood, Holo-wood and EWF (with 8-h oxidation) obtained from compositional analysis. (c) Compression stress-strain curves of EWF obtained at given oxidation time. 3.3.2 Structural features Having discussed the basic characteristics of EWF, we next examine more in detail the relevant structure-properties relationships, which are first drawn by comparing the micromorphology of the wood samples before and after modification. Naturally, balsa has a thin cell wall, and the shape integrity can be retained in Holo-wood upon delignification. A neat and smooth lumina was observed in Holo-Wood (Figure 3.3a1-a2). By contrast, with further modification, EWF showed interconnected and entangled fibrillated surfaces (Figure34.3b1-b2, Figure A2a-f). The exposure of the fibrillar structures became more evident with the extent of chemical treatment, yielding a looser or disconnected cell wall network (Figure 3.3b1-b2). This is because oxidation exposes cellulosic chains in the elementary fibrils by partially dissolving the holocellulose. Upon chemical modification, partial dissolution at the surface softened the fibrils initially forming a closely packed cell wall (Figure 3.3b2), contributing to the reduction of the rigidity of the aligned fibrils and endowing mobility and flexibility.113 The amorphization of holocellulose 48 exposed the fibrillated nature of the cell walls, leading to interconnected networks in the lamellar structure (Figure A2a-f). Hence, abundant microfiber entanglement took place between pore spaces, which facilitated a cushioning effect when the material was under load. Figure 3.3. Holo-Wood morphology observed in the (a1) transverse plane and (a2) tangential plane, highlighting the exposed fibrils in EWF. Morphology of EWF in the (b1) transverse plane and (b2) tangential plane. Besides the partial dissolution of hemicelluloses, the oxidation-reduction reduced the apparent crystallinity of cellulose (discussed in section of crystallinity).212 As suggested by earlier reports, a core-shell structure might be formed locally on the surfaces of elementary fibrils, comprising amorphous dialcohol cellulose/hemicellulose and branched hemicellulose (shell structure) surrounding the more ordered cellulose core,194, 213 Figure 3.4. The changes that occur in the 49 surface regions combined with retention of an ordered cellulose backbone in the core provided integrity to the treated wood. This amorphization of holocellulose (molecular level) promotes flexibility in the bulk material while simultaneously maintaining its shape. Rich in porosity and thin cell walls, balsa is a good starting material for modification, e.g., to favor homogeneous reaction to produce the foam materials. Overall, the intertwined and interconnected fibrillar structure in balsa's lumina endowed EWF with flexibility and elasticity (Figure 3.3b1, b2 and Figure A2a-f). Simultaneously, the laminar structure and fiber alignment of wood was largely preserved, which contributed to mechanical strength in the axial direction (Figure A2g, h). Figure 3.4. Schematic illustration of amorphization at fiber surface yielding a more open fibrillar structure. The flexibility and elasticity of EWF were examined by mechanical compression tests. As shown in Figure 3.5a, a small stress (0.002 MPa) produced a large strain in EWF along transverse direction. By contrast, Holo-Wood was highly rigid: much larger stress (0.8 MPa to 2.3 MPa) were required to achieve similar deformation (strain of 10% to 60%), Figure 3.5a. Under cyclic loading/unloading (ramping strain of 0 to 60%, Figure 3.5b), the stress-strain curve of EWF was 50 nonlinear and showed a hysteresis loop at strains up to 60%, indicating a recoverable mechanical property and good elasticity during compression. The resilience and compression recovery stability of EWF was examined by applying 100 loading/unloading cycles at a strain of 50%. The results confirmed spring-like behavior (Figure 3.5c). Just as the property of typical energy-dissipative materials, EWF samples displayed an initial linear elastic region at ε<10%, Figure 3.5b and 3.5c. As the strain increased (medium range, 10%<ε<30%), EWF showed a plateau region, which involved cell-wall bending.117, 179 The compression profiles followed a densification region (ε >35%, with a sharp increased stress). An inflection point was noticed as the strain increased above 50%, where the relative increase of stress became less steep, caused by the partial tearing of cell walls.214 Overall, EWF presented typical compressible and resilient performance, with superior stress retention after cyclic loading/ unloading (Figure 3.5c). Figure 3.5. (a-b) Stress-strain curves of Holo-Wood and EWF. (c) Stress-strain profiles upon loading/unloading tests (100 cycles). After 100 cycles of loading/unloading, EWF presented a large stress retention, providing stable mechanical performance (Figure 3.6a). The methods available so far to prepare super elastic 51 wood have been generally based on the removal of most of the hemicelluloses, resulting in low yields.113, 117, 119 By applying the reported methods, a “Cell-Wood” sample was obtained and used as reference. As indicated in Figure 3.6b and 3.6c, the Cell-wood showed a larger energy loss in the first few cycles compared to EWF and the change following 20 cyclic tests (energy loss coefficient of 72% and 40% after cycle 1 and cycle 20) suggests a less degree of reversibility. As revealed in Figure 3.6b and 3.6c, compared with Cell-Wood, EWF presented minor stress-strain changes following 20 cycles of loading/unloading (energy loss coefficient of 63% and 41% in cycle 1 and cycle 20, respectively) and much lower energy losses, demonstrating more stable mechanical performance, better reversibility and good resilience after 20 compression cycles. Figure 3.6. (a) Compression stress retention of EWF. (b) Stress-strain curves of EWF and Cell-Wood (20 loading/unloading cycles) at a maximum applied strain of 40%. (c) Energy loss of a sample under compression (strain of 40%). Combined with the presence of hemicelluloses in Holo-Wood, the generated amorphous dialcohol cellulose/hemicellulose in EWF worked as a matrix and as a cushioning layer. Additionally, given the high hemicellulose retention in EWF, the free volume under shape 52 deformation was lower than that of Cell-Wood. This enabled a material that is durable, resilient and less prone to collapse compared to a skeleton composed of only cellulose. Finally, the residual hemicelluloses acted as adhesive of the cellulose scaffold60 adding to the stability and the recoverable deformation under loading/unloading cycles. 3.3.3 Impact of cellulose crystallinity on EWF structure We next examine the changes in crystallinity of EWF, which preserved the cellulose I polymorphism (see characteristic XRD peaks at 14.7o, 16.7o and 22.6o corresponding to the (1-10), (110) and (200) crystal planes),85 Figure 3.7a. The crystallinity of EWF was reduced, as judged by the height of the (200) peak and the signal from the disordered region (Figure 3.7a). The crystallinity of Holo-Wood and EWF were measured as 0.59 and 0.52, respectively, by fitting the curve using Rietveld method with TOPAS software. EWF showed lower reflections in the (200) and (110) planes, Figure 3.7b, indicating a more disordered system. This crystallinity reduction reflected changes in the configuration of cellulose and the contribution of amorphous di-alcohol cellulose. Figure 3.7. (a) XRD curves of Holo-Wood and EWF. (b) 2D-XRD patterns of Holo-wood and EWF. 53 Molecular dynamic simulation (MDS) was used to understand the structural changes of cellulose following the extent of oxidation (Figure A3c). Figure 3.8a displays cellulose structures of Holo-Wood and EWF in an aqueous environment. No deformation in the transverse crystallographic morphology was observed. Dialcohol cellulose exhibited a disordered configuration, Figure 3.8a, in agreement with a reduced crystallinity, XRD (Figure 3.7) and attributed to the surface oxidation that opened the original pyranose rings and reduced inter-chain hydrogen bonding. The exterior portion of cellulose in EWF was subjected to deformation, with dialcohol cellulose chains wrapping the more ordered cellulose, which we hypothesize as enhancing the flexibility of the material.215 The mechanical recovery of EWF depends on hydrogen bonding, electrostatic and van der Waals interactions.216 The hydrogen bonding and non-bonded interactions were quantified by considering both the external water phase and the internal environment, which encompassed the interactions between crystal plane interfaces (1-10)-(110) and (200)-(200) (Figure 3.8b and 3.8c). For instance, the overall non-bonding interactions upon dialcohol formation in EWF were improved compared to Holo-Wood (increased electrostatic interactions from -389532 to -440684 kJ/mol and a slight decrease of van der Waals forces, from -48238 to -42985 kJ/mol), Figure 3.8b and 3.8c. The improved interactions mainly included forces between two hydrophobic interfaces (200)-(200) and between the hydrophilic (1-10) plane with hydrophobic surfaces (110), Figure A6. In EWF, the more disordered structures presented high mobility, which effectively promoted interactions between opened glucopyranose chains at the exterior edges of the fibers, thus increasing electrostatic forces. EWF showed a similar number of H-bonds to Holo-Wood; however, there was a slight downward trend in this parameter primarily attributed 54 to a reduction in the heterogeneous interaction between (1-10) -(110) interfaces (Figure A6a). By opening the glucopyranose ring, the pristine H-bonds between (110) and (1-10) became disrupted and reconfigured, which slightly reduced H-bond formation. Figure 3.8. (a) Molecular dynamic simulation model of Holo-Wood and EWF: front and side views. (b) Non-bonded interaction energy among inter- and intra- fibrils of Holo-wood and EWF, calculated based on the molecule dynamic simulation. (c) Number of hydrogen bonds among inter- and intra- fibrils of Holo-wood and EWF. 3.3.4 Hydrophobization of EWF EWF showed a high-water absorption capacity, as expected (Figure A4). Hence, EWF was hydrophobized with a silane coupling agent (MTMS), which is effective to bind organic chains onto cellulose surfaces. For this purpose, we used chemical vapor deposition to obtain Si-EWF (Figure 3.9a-c). The surface roughness at the micro-scale was increased owing to the assembly of siloxane particles, both inside the lumina and on the surface of EWF (Figure 3.9a-c). 55 Figure 3.9. SEM images of Si-EWF in the tangential plane ((a) low and (b) high magnification), and in the (c) transverse plane. At a high temperature and in the presence of moisture, MTMS reacted with the abundant hydroxyl groups of EWF217 and formed layered structures (Figure 3.10a, see also Si and O in EDX mapping in Figure A5). The formed siloxane provided Si-EWF with a super-hydrophobic character (Figure 3.10b) in the tangential plane (T) and Transverse plane/cross sections (C) and were long lasting (Figure 3.10b and 3.10c). Figure 3.10. (a) Mechanism of organosilanes coupling with EWF surfaces. (b-c) Static water contact angle measured on Si-EWF samples tested in the tangential plane (T) and cross section/ transverse plane (C). 56 According to the dynamic mechanical performance, humidity influenced the strain of Holo-Wood and EWF (Figure 3.11a). Compared to Si-EWF, Holo-wood and EWF with no hydrophobization treatment show a weaker tolerance to the wet environment, and present less stable mechanical performance when tested at increased relative humidity (RH=0-95%, 5% increment after RH stabilization for 5 min). A 10-25% strain change in EWF was observed when the sample was exposed to an atmosphere of >60% relative humidity. In a wet environment, the mechanical properties of Holo-Wood and EWF were compromised, as explained by the associated hygroscopic expansion and plasticization effects,218 most relevant to EWF which contains disordered cellulose and presents high moisture uptake.219 Significantly, Si-EWF tolerated moisture, showed better strain retention than EWF and displayed no significant strain changes (from 3 to 12%) at an increased humidity (RH from 60% to 95%) (Figure 3.11a). Moreover, Si-EWF presented a good resilience in dry and RH=60% conditions after 20 loading/unloading cycles (Figure 3.11b), enabling applications in moderate humidity conditions. Figure 3.11. (a) Strain changes at different relative humidity (0-95%) for Holo-Wood, EWF and Si-EWF under a stress-controlled (0.01 MPa) creep recovery test by dynamic mechanical analysis. Strain at different RH was recorded once the RH has been stabilized for 5 min. (b) Stress-strain 57 curve of Si-EWF following 20 loading/unloading cycles at RH=60%, under an increased strain from 1% to 50% with a 5% strain/min increments. 3.3.5 Thermal insulation Taking advantage of the inherent high porosity of balsa and the development of a fibrillated structure after modification, the elastic wood foam was expected to be a thermal insulative material. EWF presented a low thermal conductivity (k = 0.035 W/m.K in the longitudinal direction), lower than that of Holo-Wood, 0.043 W/m.K (Figure 3.12). The reduction in k is explained by the disruption of the pristine alignment220, 221 of fibers in the cell wall of EWF, which reduced solid conduction. The created spaces and pores along with individualized fibrils in the cell wall of EWF limited the solid conduction (constrained air transport).222 The structure with low heat transfer also showed a reduced solid heat convection throughout the material.223, 224 Additionally, the narrowed pore size in the cell wall minimized the contribution of gas conduction220, 225 due to the Knudsen effect, providing an overall reduced thermal conductivity. Compared with Holo-Wood, the structural changes in EWF lowered the thermal conduction along the transverse (T) direction. EWF exhibited better thermal insulation (k = 0.028 W/m.K in T direction), approaching the performance of super-insulating materials (note the thermal conductivity of air of 0.025 W/m.K).225 The better thermal insulation performance of EWF along the T direction is also associated with the solid heat transfer pathway that was interrupted by the gas trapped between the wood lumina, which hindered solid conduction. Similarly, thermal convection was limited within the interlaminar regions. Moreover, within lamella structure, the presence of multiple refraction and emission restricted thermal radiation.223 Compared with 58 aerogels derived from biomass,184, 226-230 EWF showed a competitive thermal insulation performance (Figure 3.12). Figure 3.12. Thermal conductivity of Holo-Wood and EWF along both longitudinal and transverse directions (grey and blue area) and the thermal conductivities of the reported cellulosic aerogels. Samples of unmodified balsa (20 x 20 x 50 mm), commercial PU foam rubber, EWF and Si-EWF were placed on a hotplate (70 oC) for 2h and the top surface temperature was monitored (Figure 3.13). EWF showed a slower temperature rise compared to the commercial PU foam (Figure 3.13b and 3.13c). PU foam reached 32 oC within 30 min while EWF temperature remained below 30 oC for 60 min (Figure 3.13c). After 30 min under heat, the bottom temperature of EWF increased to 60.9 oC while that of the PU foam was 66.9 oC (see side view of the foams, Figure 3.13b). Heating for 120 min led to an EWF surface temperature of 29.6 oC, 5 oC lower than that of the PU foam (34.6 oC). Similarly, Si-EWF exhibited a lower thermal 59 conduction as the top side temperature was limited to 31 oC after 120 min heating (3.5 oC lower than the PU foam) (Figure 3.13c). Both EWF and Si-EWF showed improved thermal insulation compared to unmodified balsa, which is attributed to the microstructures that resulted from the modification. The thermal insulative properties of EWF is suitable for packaging applications. Figure 3.13. (a) Thermal insulation experiments with unmodified balsa, EWF, Si-EWF and commercial PU foam (all: 20 x 20 x 50 mm) (the samples were placed on a 70 oC hot plate). (b) Temperature evolution of EWF and PU foam recorded by an IR camera during 120 min. (c) Changes of the top surface temperature with time. 3.3.6 Flame retardancy The flame retardancy of the Si-EWF was examined by measuring the limiting oxygen index (LOI) and conducting flame tests (burning samples vertically and horizontally following the UL-94 standard). The weight loss as well as the dimensional retention reflect the extent of ignition. Unmodified wood was easily ignited in air (LOI around 21%) and burned continuously, forming a surface char layer after ignition in the vertical test (Figure 3.14a). In both vertical and 60 horizontal burning experiments, EWF quickly ignited in air, with only little material retention (3.1 wt% and 2.8 wt%, respectively, Figure 3.14a-b). EWF experienced rapid and continuous combustion in air because of the high pore volume, leading to a continuous oxidation (oxygen supply by air encapsulation in the structure). Combustion inhibition and self-extinguishing effects were observed for EWF modified with MTMS (Si-EWF, LOI of 24.0 %). Si-EWF showed a large material retention, namely, 84.9 and 87.6 wt% in the vertical and horizontal burning tests, respectively (Figure 3.14a and 3.14b). Si-EWF exhibited a self-extinguishing effect when exposed to a flame for 60 s in the horizontal burning test (fire propagation was negated along the material, with 98.2% of its length retained, Figure 3.14b and 3.14c). In contrast, a visible flame was observed in untreated wood generating char within 120 s (Figure 3.14c). EWF burned violently and evolved into ashes while Si-EWF exhibited a significantly increased flame retardancy. The inorganic silica barrier was thermally stable when Si-EWF was ignited. Besides, the generated char layer restricted combustible gases from air and gases produced during degradation, which ultimately prevented flame spreading (Figure 3.14d). The combustion of silicon consumed oxygen, which stopped fire evolution. Moreover, water was produced during the combustion of silicon, which dissipated heat and limited the combustion.225 Si-EWF featured thermal stability (degradation at 300 oC, 25% ash content, Figure A8) and flame retardancy which is an advantage over EWF when considering packaging applications. In the future work, wood foams with high flame resistance should be characterized more thoroughly and include smoldering combustion tests. 61 Figure 3.14. Length and weight retention of unmodified balsa wood, EWF, Si-EWF when subjected to (a) vertical and (b) horizontal burning tests. (c) Appearance of materials after the flame source was removed in the horizontal test, within the given periods of time. (d) Schematic illustration of the flame retardancy mechanism. 3.4 Conclusions To conclude, a compressible, thermally insulative and superhydrophobic foam-like material was developed through consecutive oxidation and reduction of balsa. The surface modification and partial dissolution and the resultant disordered fibrillar structures obtained from wood endowed high mechanical and fatigue resilience. The amorphization of the nanofibrils and the loosening of the cell wall fibrils further facilitated a high anisotropic compressibility. A recoverable compressive strain of up to 60% and 98% stress retention were demonstrated after 100 loading/unloading cycles. Modification of EWF with MTMS led to super hydrophobicity (Si-62 EWF), with no negative effects on the mechanical performance of the material under humid conditions (RH=60%), as shown by the reversible compression strain after loading/unloading (20 cycles). Both EWF and Si-EWF exhibited excellent thermal insulation performance (lower thermal conductivity than the precursor wood and commercial PU foams). Modification with organosilanes endowed EWF with flame-retardancy and self-extinguishing capabilities. The wood-based foam is introduced as a sustainable alternative for insulation and packaging materials. For a broader application, the proposed method to produce flexible wood foams can be used for other wood species. 63 Chapter 4: Hydrophobic Wood for Triboelectric Nanogenerator 4.1 Introduction Wood has long been used as a construction material, particularly for roofing. If engineered to harvest and convert raindrop energy into usable forms for smart home systems, wood could serve as a high-value material, enabling sustainable energy utilization in rainy environments. In this chapter, we developed a fully wood-based material capable of generating electricity via the triboelectric effect during raindrop impact. The methodology for fabricating this material is comprehensively elaborated, and the underlying energy conversion mechanism (through triboelectric nanogenerators system) during mechanical tapping is systematically investigated. Triboelectric nanogenerators (TENGs) represent an innovative and sustainable energy technology capable of converting mechanical energy from motion, contact, friction, and vibrations—arising from human activity or environmental forces—into electricity.231, 232 Examples include energy harvested from footstep friction on floors,132, 233 raindrop impact on surfaces,234 and wave motion.235 Due to their ability to capture and store even minute mechanical movements, TENGs have garnered significant attention in applications such as self-powered sensors, flexible electronics, and blue energy harvesting.134, 236-238 However, achieving reliable and high-output electrical performance in TENGs has traditionally relied on petroleum-based polymers, particularly fluorinated polymers, raising environmental concerns and emphasizing the need for sustainable alternatives. 64 As a result, research has increasingly focused on developing functional materials derived from renewable and naturally abundant resources to replace conventional plastics in TENGs.239 Among these, wood stands out as an ideal candidate due to its durability, abundance, cost-effectiveness, and unique structural properties.89 Its favorable mechanical characteristics, well-established processing routes, and tunable surface chemistry make it an excellent substrate for green energy applications, particularly when modified through top-down approaches.43 Wood has been explored as a tribo-layer in solid-solid TENG systems, demonstrating potential for self-powered sensors and smart flooring.240, 241 Various strategies have been employed to develop wood-based TENG materials for energy harvesting, including chemical treatments and the incorporation of functional additives. However, its weak polarity places it mid-range in the triboelectric series, limiting its triboelectric performance.242 To address this, a few studies have investigated ways to enhance wood's polarizability and surface charge density via chemical modifications. For example, exposing hydroxyl and carboxyl groups from carbohydrates has been shown to increase tribo-positivity and improve electrical output.243-246 Additionally, integrating secondary tribo-materials with specific chemical functionalities has shifted wood's polarity, increasing charge generation.130, 131 Such strategies have led to the development of flexible and transparent wood materials suitable for TENG sensors, capitalizing on wood's mechanical strength and unique properties.134, 247, 248 Despite these advances, solid-solid wood-based TENGs face durability challenges due to its fibrous nature. Furthermore, existing wood-based TENGs often involve lignin removal, leading to electron loss and reinforcing dependence on fluorinated tribonegative layers (such as PTFE134), which hinders the development of environmentally friendly systems. Interestingly, lignin, an electron-rich, naturally abundant 65 component of wood, has been largely overlooked as a potential tribonegative enhancer. Lignin's phenolic rings enriched with π-electrons due to their conjugated aromatic system, benefiting triboelectric performance by introducing additional low-energy unoccupied electron states.248 Additionally, its phenolic hydroxyl groups contribute to contact electrification (CE).249 Chemical modifications, such as oxidation or the incorporation of electron-donating groups, can further increase lignin's polarity and charge density.250 Previous studies on cellulose nanopapers containing residual lignin suggest that lignin could serve as an alternative to PTFE as a tribonegative layer when paired with various polymeric or metallic tribo-materials.236 Furthermore, electron exchange in lignin can be modulated by adjusting its wettability and surface chemistry.251 Since surface roughness and porosity influence interfacial contact area in TENGs, optimizing these factors presents an opportunity to enhance contact electrification.252 Collectively, these findings highlight lignin's potential in wood-based TENGs, particularly as a key component in hydrophobic tribonegative layers for liquid-solid (L-S) systems. In L-S TENG systems, rainwater or seawater typically acts as a tribopositive material, generating electrical energy through contact electrification with a solid surface25, 253 However, current strategies for modifying surface morphology and wettability254-256 still rely heavily on fluorine-based tribonegative materials.257, 258 To date, the use of naturally available, low-cost, hydrophobic lignin as a tribonegative material for L-S TENGs has not been explored, and engineering lignin within wood for L-S TENG applications remains uncharted. 66 In this study, we present wood as a self-hydrophobizing and self-cleaning tribonegative material for L-S triboelectric nanogenerators, capable of harvesting energy from rain. Using basswood as a renewable precursor, we demonstrate that lignin migrates and regenerates in situ, leading to its relocation within the wood structure via a rapid solvent-exchange process. As a result, the material achieves water resistance and a significant enhancement in triboelectric performance. Unlike methods through mineralization or synthetic polymer deposition—which often require delignification—our approach is both efficient (ca. 30 min processing time) and sustainable. The resulting wood with relocated native lignin (Lig-wood) exhibits notable improvements in voltage output and durability, as demonstrated by continuous water droplet impact tests, showing markedly superior performance compared to unmodified wood. 4.2 Materials and methods 4.2.1 Materials Basswood (Tilia americana) boards (24”×4” ×1/16” in length × width × thickness) were purchased from McMaster Carr, Canada. p-toluenesulfonic acid monohydrate (p-TsOH, ACS reagent, ≥98.5%) was purchased from Sigma-Aldrich, Canada. Pricion Red MX-5B (Sigma-Aldrich, Canada) was used to prepare samples for confocal microscopy. 4.2.2 Chemical treatment of wood Basswood pieces (30 mm × 50 mm, in tangential plane and 3 mm in thickness in cross section) were immersed in 60% p-TsOH solution for 0.5, 1, 1.5 and 2h (using 1g dry mass of wood for 40 mL of the solvent), and heated at 80 °C in an oil bath. Following the p-TsOH pretreatment, the wood samples were thoroughly washed with deionized water to induce lignin regeneration. The 67 samples were then freeze-dried at –70 ºC and 0.01 mbar using a Labconco freeze dryer. Subsequently, the dried samples were heat-treated at 230 ºC for 2 hours with a temperature ramping rate of 5 ºC/min in the Muffle furnace (Thermo Scientific, Small Benchtop Muffle, Type FB1300). These final materials are referred to as Lig-wood. For sustainability considerations related to chemical recycling and recovery, the spent liquor can be processed to isolate solid lignin and recycle p-TsOH. For example, the black liquor can be collected in a dialysis tube and subjected to dialysis, after which the extracted lignin is recovered by centrifugation. The acid solvent can then be reused following the procedures reported in previous studies employing the same system.259, 260 4.2.3 Wood with relocated lignin (Lig-wood) Chemical compositional analysis was conducted by using Fourier transform infrared spectroscopy (FTIR) and High-Performance Liquid Chromatography (HPLC). FTIR measurements were done by using the IR spectrophotometer Spectrum 100 (Bruker), at a wavenumber range of 500 to 4000 cm-1. HPLC measurement was conducted to examine the content of polysaccharide and lignin in wood samples before and after treatment, following the Klason protocol using TAPPI standard method T222. Powder samples (0.2 g) were dissolved in 3 mL H2SO4 by mixing for 2h to reach the complete dissolution, triplicates were measured for each sample. Each dissolved material was diluted by the addition of 112 mL distilled water and then they were heated at 121 °C for 1h by an autoclave. Afterwards, solution was filtrated by funnel, the residue was collected as nonacid 68 soluble lignin, and the filtrate was collected for sugar analysis by Dionex HPLC (ICS-6000) and acid-soluble lignin analysis by UV–vis (SHIMADZU UV-2600). Wood samples before and after treatment were observed under a confocal laser scanning microscope (Leica DMRXE, Leica, Germany) to confirm the presence location and distribution of lignin, with the objective lens (4×). Laser 1 at a wavelength of 405 nm was used to detect and collect signals from cellulose dyed with Procion Red MX-5B, and Laser 2 at a wavelength of 473 nm was used to detect lignin due to its self-fluorescent properties. Both unmodified wood and Lig-Wood were dyed with a Procion Red MX-5B before observation. The microstructural morphology of freeze-dried wood samples before and after treatment were observed by Scanning Electron Microscope (SEM) in their transverse and tangential planes (Helios NanoLab 650 Focused Ion Beam SEM). The hygroscopic properties were characterized by dynamic vapor sorption and water contact angle measurement. Moisture adsorption isotherms were studied by the conduction of Dynamic water vapor sorption (DVS, Surface Measurement Systems) to study the hygroscopic properties of Lig-wood and unmodified wood. Samples were put in an N2 atmosphere, under a changing relative humidity (RH) condition (changing range of 0 to 100% RH, in 10% RH steps, 25±0.2 °C), the weight was measured to obtain the equilibrium moisture content. RH was controlled in a way that it maintained when the sample weight balanced within a rate of 0.002%/min over a 2-min period. The static and dynamic water contact angle (WCA) experiment was conducted by using Thetaflex 300, Biolin Scientific. A droplet of distilled water (6 µL) was dripped and static WCA was recorded for 800s starting from when water drop contact with surface of the material. 69 Dynamic WCA was recorded with time when samples substrate surface standing with a stable water droplet started rotating (36o/min) with a tilting angle from 0° to 60°. X-ray diffraction (XRD) patterns of the samples were measured in the 2θ range of 5°–80° with radiation of Copper K1 & K2 in a Bragg-Brentano Configuration mode by Bruker D8-Advance X-ray diffractometer. X-ray photoelectron spectroscopy (XPS) was performed on both unmodified wood and Lig-wood, by an electron spectrometer (ThermoFisher, Thermo Scientific K-Alpha+), under the atmosphere condition of 5x10-9 mbar, 15 KV, 15 mA and 1486.6 eV. Atomic force microscopy (AFM) images of unmodified wood and Lig-wood were acquired in air by a Jupiter XR AFM (Oxford Instruments) in tapping mode. The images of water contacting wood materials were taken by the high-speed camera (Phantom, mode Miro C211, lens: MVL16M23), that DI water was dropped with the droplet size of 3uL by controlling the connected pump. The videos were taken with the frame rate of 1800 fps and the images were captured from the videos. 4.2.4 Wood-based liquid-solid TENG system and electric performance Unmodified wood and Lig-wood were employed as tribo-negative materials in a single-electrode mode lateral-sliding triboelectric nanogenerator (L-S TENG). Copper (Cu) tape, used as the electrode, was adhered to the top surface of the wood materials, enabling water droplets to be introduced at the center of the wood surface during the experiment. Water droplets were dispensed using a peristaltic pump connected to a plastic dispensing tip with a Luer lock connection (tip diameter: 0.035 inches). The electrical signals, including voltage and current, were measured using a Keithley 6514 electrometer. These measurements were conducted by 70 connecting the wood materials to a resistor and the electrometer during controlled water-dropping experiments (Figure 4.9c). This study explored the potential of wood self-hydrophobization through native lignin, using basswood as the initial model due to its low density, uniform porosity, and widespread use as veneer. These attributes make basswood particularly suitable for laboratory-scale investigations of chemical penetration. Most experiments and characterizations were performed on basswood, with subsequent extensions to other wood species to demonstrate the versatility of the approach across both hardwoods and softwoods. To further evaluate the general applicability of the method, veneer samples of fir (Abies), walnut (Juglans), and oak (Quercus) were examined, representing a diverse selection of anatomical and compositional characteristics. Specimens of varying dimensions (fir: 50 mm × 30 mm × 1.5 mm; oak: 50 mm × 30 mm × 1 mm; walnut: 50 mm × 30 mm × 1.5 mm) were chemically treated with p-TsOH for 20 minutes, thoroughly rinsed with water, and freeze-dried. Subsequently, all samples underwent heat treatment to complete the preparation process. Partial oak samples after chemical treatment were hot-pressed followed with heat treatment, and 12 oak samples were connected for the application of raindrop energy harvesting study. 4.3. Results and Discussion 4.3.1 Compositional features of Lig-wood A self-hydrophobized, all-wood material (Lig-wood) was developed using a simple and rapid aqueous process involving migration and regeneration of native lignin into particles (Figure 4.1). 71 During this process, basswood was treated with p-TsOH, inducing dissolution and rapid changes in the lignin contained in the highly interconnected network of cellulose fibers. The dissolved lignin was then clustered into particles through an anti-solvent (water)-induced regeneration process. These lignin particles were rapidly relocated and uniformly redistributed across the outer surface of the wood matrix. Figure 4.1. Schematic illustration of the process used to convert wood into a hydrophobic TENG material. The obtained wood was freeze-dried, followed by a low-temperature (230 °C) treatment to remove exposed hydroxyl groups from the lignin surface while preserving the wood's integral structure.261 This process resulted in a re-lignified, self-hydrophobized wood, referred to as Lig-wood. As shown in Figure 4.2, Lig-wood exhibited excellent hydrophobicity, providing enhanced water repellency. This hydrophobicity was attributed to lignin particle migration, where lignin relocated from cell corners to the surface of the cell walls, forming uniformly distributed spherical morphologies that effectively coated the wood fibers. The darker brownish 72 appearance of Lig-wood further confirmed lignin regeneration and homogeneous redistribution within the wood structure. Figure 4.2. Photo of unmodified wood and Lig-wood after deposition of a dyed water droplet. Included is also an SEM image revealing the microstructure of Lig-wood in its transverse plane, which indicated the formation of spherical lignin particles. FTIR spectra (Figure 4.3a revealed characteristic peaks at 1592 and 1505 cm-1, corresponding to the aromatic ring skeleton of lignin,75, 160 verifying lignin retention during the process. Additionally, as basswood was treated with p-TsOH at increased treatment times, a reduction in hemicellulose content was observed, indicating carbohydrate hydrolysis262 (Figure 4.3b). The significant degradation of hemicellulose indicates that it was effectively and extensively hydrolyzed by the p-TsOH treatment system. Meanwhile, only a small portion of lignin was removed, suggesting partial lignin fractionation during the treatment. Importantly, no significant reduction in lignin content was detected across all Lig-wood samples, confirming the successful regeneration of lignin within the wood structure. The relative increase in lignin content determined from the chemical composition of samples obtained over prolonged treatment is primarily due to the reduction of carbohydrates. To ensure substantial lignin retention and facilitate the subsequent reassembly of lignin nanoparticles, an optimal treatment duration of 0.5 73 hours was identified. Accordingly, further experiments on triboelectric generation will focus on this specific treatment condition. To further investigate surface chemical composition changes, X-ray photoelectron spectroscopy (XPS) was performed before and after treatment. As shown in Figure 4.3c, the deconvoluted C–C peak (~284.8 eV) in Lig-wood decreased slightly from 60.1% (in unmodified wood) to 58.8%, reflecting the fractionation of lignin during p-TsOH treatment.263, 264 The reduction in O-C=O and O-C-O/C=O bonding indicates that the p-TsOH treatment selectively cleaved the ether and ester bonds linking lignin and carbohydrates. As a result, an increase in C–O groups was observed (Figure 4.3c), primarily originating from carbohydrates. due to the exposure of a greater number of hydroxyl groups at the termini of retained lignin on the fiber surface—groups that mainly originate from carbohydrates. This observation aligns with the severe hemicellulose degradation revealed by compositional analyses (Figure 4.3b). This increase likely resulted from the exposure of hydroxyl groups caused by fiber structure disruption through acid hydrolysis and hemicellulose cleavage during p-TsOH hydrotropic fractionation. The elevated C–OH content may facilitate noncovalent hydrogen bonding between cellulosic fibers and the regenerated lignin on their surfaces, leading to strong interfacial interactions and a more stable structure. Additionally, the observed reduction in C=O groups in Lig-wood suggests a decrease in hemicellulose content, as carboxyl groups are primarily derived from hemicellulose. Furthermore, an increase in the C–OH content and the decrease in C=O groups observed in the O1s region spectra exhibited the same elemental trend as seen in the C1s region. This further supported the reduction of ester linkages and the depletion of hemicelluloses particularly on the surface of Lig-wood. These findings are consistent with the results from HPLC compositional analysis. 74 The regenerated lignin remained anchored within the wood matrix through hydrogen bonding and van der Waals interactions. For samples subjected to prolonged acid treatment, hemicellulose hydrolysis led to degradation of the carbohydrate backbone, lowering material yield (Figure 4.3d). All Lig-wood samples demonstrated enhanced water resistance and reduced water absorption compared to unmodified wood under high relative humidity conditions. Based on these results, an optimal treatment time of 30 minutes was identified, balancing high yield (preserving cellulose while partially removing hemicellulose) with improved performance, as discussed in the following section. Figure 4.3. (a) FTIR compositional analysis for wood, Lig-wood and delignified wood (reference). (b) Compositional cellulose, hemicellulose and relative concentration. (c) Surface atom composition by XPS. (d) Mass yield of Lig-wood materials as a function of treatment time. 75 4.3.2 Morphology, structure and hydrophobicity of Lig-wood The morphological changes, microstructural features, and self-hydrophobization of Lig-wood were examined using confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). Following acid hydrotropic fractionation, basswood retained its bulk structure, with fiber alignment remaining clearly visible (4x magnification in confocal imaging). Lignin, which is self-fluorescent, became more prominent and exhibited a more uniform distribution over a larger surface area compared to its original localization in unmodified wood. This suggests that lignin, initially embedded within the wood matrix, migrated and became evenly redistributed onto the outer surfaces of the wood fibers (Figure 4.4). Dyed cellulose fibers appeared with muted coloration and darker reflectance under confocal microscope observation, likely due to the lignin coating on the surface. Additionally, the hydrophobic nature of Lig-wood limited water-soluble dye penetration, reducing dye absorption in the cellulose regions compared to unmodified wood. This further confirmed that the surface of Lig-wood was predominantly composed of hydrophobic lignin. Higher-magnification SEM imaging revealed particle-shaped regenerated lignin covering the wood lumen along the tangential plane of Lig-wood, resulting in a rougher surface morphology. This structure was distinctly different from the smooth, empty cell walls observed in unmodified wood (Figure 4.6 a1-a3). The self-lignification process was attributed to the use of p-TsOH as an acid hydrotropic agent, which functioned as a surfactant, facilitating the stretching of lignin molecules from the wood fiber bundle matrix via π–π stacking interactions262 (Figure 4.5). During this process, lignin was initially dissolved in p-TsOH, undergoing rapid fractionation upon heating at 80 °C. This step involved cleaving the bonds between lignin and carbohydrates 76 while largely preserving the β-O-4 linkages,264-266 thus maintaining the pristine lignin structure. Upon rapid quenching with water, lignin regenerated onto fiber surfaces, reassembling from the wood's internal structure to the outer surface. This method efficiently repurposed the indigenous components of the precursor material in a simple, single-step process, eliminating the need for conventional approaches, such as inorganic particle impregnation267, 268 or the separate addition of extracted lignins.269, 270 Figure 4.4. Fluorescent images by confocal microscopy of unmodified wood and Lig-wood showing qualitatively the relative amounts of lignin and cellulose on their surfaces. The respective wood was dyed with Pricion Red MX-5B (red dye for cellulose) to generate images that distinguish cellulose from lignin (autofluorescence in green) - see maps including merged images (scale bar = 200 μm). 77 Figure 4.5. Illustration of the structural and morphological changes of lignin during wood treatment. The integral wood structure remained largely intact, preserving its aligned channel architecture. However, minor micro-fibrillation and softening of the structure were observed (Figure 4.6b1), likely due to hemicellulose degradation during acid fractionation.265, 271, 272 Lignin depolymerization was largely inhibited in the hydrotropic system, allowing extracted lignin molecules to shrink and regenerate in water, forming nano-sized clustered particles on fiber surfaces (Figure 4.6). These lignin particles were well-distributed and firmly attached through various secondary interactions.265 The surface micromorphology was significantly modified, with increased surface roughness attributed to the deposition of lignin particles associated with carbohydrate-derived intermediates. This characteristic facilitated the reconstruction of wood precursors, simultaneously retaining both cellulose and lignin. Notably, the lignin particle formation process in our approach is fundamentally distinct from hydrothermal treatments, which typically extrude lignin from the cell wall but require high 78 pressure and temperature and often result in mechanical performance loss.273-277 Additionally, hydrothermal treatments are often not universally applicable across different batches and wood species when using reactor systems, making reaction control and quality consistency challenging.278 In contrast, our approach employed a recyclable aqueous system operating under atmospheric conditions, achieving lignin regeneration without relying on organic solvents279 or deep eutectic solvents (DES).280 Compared to DES systems, our method offered a sustainable alternative, significantly reducing both chemical and water consumption, particularly during the purification step, which often requires the removal of high concentrations of chemicals. Figure 4.6. SEM images corresponding to the morphology of (a1-a3) unmodified wood and (b1-b4) Lig-wood observed in the tangential plane view at different magnifications, as indicated. Note the particles on the surface of (b1-b4) that correspond to lignin clusters. 79 4.3.3 Triboelectric Performance Through Surface Engineering Previous TENG systems utilizing lignin-containing cellulose nanofibrils (LCNF) have demonstrated tribonegative performance, outperforming PTFE in triboelectric output.236 These findings support the potential of Lig-wood as a fully green and sustainable tribonegative material for liquid-solid triboelectric nanogenerators (L-S TENGs). By combining lignin retention, surface-exposed lignin, and water repellency, Lig-wood is suggested as an innovative and eco-friendly solution for sustainable energy harvesting. Atomic force microscopy (AFM) images of unmodified wood (Figure 4.7a) revealed a relatively smooth, well-aligned fibrous wood micromorphology with no significant surface roughness. The exposed hydroxyl groups on its surface readily interacted with water, contributing to their high hydrophilicity. In contrast, Lig-wood exhibited hydrophobic lignin particles, leading to localized micro-morphological changes and increased surface roughness (Figure 4.7b). Due to the abundant hydroxyl groups on the cellulosic fiber surface, unmodified wood rapidly absorbed water within 5 seconds, as evidenced by the sharp reduction in contact angle (CA) (Figure 4.7c). In contrast, Lig-wood demonstrated enhanced hydrophobicity, attributed to the presence of regenerated lignin particles and the reduction of hydrophilic hydroxyl groups on the surface (Figure 4.7c). Thanks to its unique surface morphology (Figures 4.6 and 4.7b), Lig-wood maintained a stable water contact angle (CA) of up to 149° on the tangential plane, with negligible reduction observed during a static CA measurement over 10 minutes (Figure 4.6d). The hydrophobic surface exhibited strong water repellency, preventing water droplets from effectively adhering to Lig-wood (see video in the Supporting Information). As a result, the actual water contact angle (WCA) was likely greater than the measured 149°. Additionally, the 80 hydrophobic surface facilitated efficient water droplet rolling along the surface, with a rolling angle of 20° (Figure 4.6e), promoting easy detachment. This effect is critical for enabling the contact-separation process. Figure 4.7. (a) AFM images of unmodified wood and (b) Lig-wood. (c) Static water contact angle measured within 5s on unmodified wood and illustration of the nature of the surface. (d) Surface change in Lig-wood and its static water contact angle in the tangential plane (measurements for 600 s and WCA figure of Lig-wood at 10 s and 500 s). (e) Rolling water angle of Lig-wood. When tribopositive water droplets landed on the Lig-wood surface, spontaneous negative charge generation occurred due to ion separation (H⁺ and OH⁻) and electron transfer281 from the liquid to the Lig-wood (Figure 4.8). As the water droplet moved along the surface, the expanding contact area further enhanced the contact electrification effect, increasing the negative polarity of the solid phase through charge accumulation. Once the droplet detached from the surface, the 81 triboelectric process was completed, and subsequent droplets generated new electrons upon impact. Due to differences in solid-water interactions, unmodified wood was unsuitable for long-term and high-power energy production, as water droplets adhered to and were absorbed into the wood structure. This led to undesirable charge dissipation and a reduced charge difference between the liquid and solid phases. In contrast, the hydrophobic Lig-wood surface showed a significant electron separation and transfer efficiency during rapid interactions. As a result, Lig-wood demonstrated superior cyclic performance, high durability, and effective energy generation capabilities, making it a promising material for liquid-solid triboelectric applications. Figure 4.8. Illustration of water droplets contacting a solid surface, leading to contact electrification between a liquid and a solid phase. To investigate the triboelectric properties of wood before and after treatment, the contact–separation dynamics upon interaction with water droplets were recorded. This was accomplished by dripping water onto the wood surface, which was integrated with a copper (Cu) electrode and operated in a single-electrode transistor-like configuration (Figure 4.9a). As expected, when a water droplet impacted on the unmodified wood surface, it spread rapidly, stagnated, and was fully absorbed within 5 seconds, due to the absence of intrinsic hydrophobic properties (high-speed 82 imaging, Figure 4.9a). In contrast, on Lig-wood, the droplet rolled off and rebounded quickly, leaving no residual trace (Figure 4.9b). Water rapidly slid off the electrode surface without adhering, indicating pronounced non-wetting behavior. This behavior highlights Lig-wood's superior hydrophobicity, which effectively prevents water accumulation, even with successive droplet impacts. As hydrophobic surface plays a significant role in enhancing the contact electrification effect upon interaction with water. Therefore, the observed complete contact–separation process in Lig-wood is crucial for effective triboelectric generation and overall power conversion efficiency. Figure 4.9. Digital photos acquired with a high-speed camera (900 fps) showing water droplets contacting the surface of (a) unmodified wood and (b) Lig-wood within 2s. (c) Illustration of the assembled L-S TENG in a single electrode mode. Both materials exhibited a typical triboelectric effect when used as the tribonegative layer in a single-electrode mode (Figure 4.9c), where ion and electron transfer occurred upon contact with water. However, unmodified wood generated a significantly lower electrical output (Figure 4.10a-c). This was attributed to droplet coalescence, where water droplets merged and stagnated on the surface, failing to fully rebound. Eventually, the droplets slid off the tilted surface due to 83 gravity (Figure 4.9a). This incomplete separation limited energy output, as charge transfer between the droplet and the friction layer only occurred during the contact, spreading, and separation phases. 4.3.4 Role of lignin in S-L tribogeneration Lig-wood exhibited a 7.5-fold increase in voltage and a 6-fold increase in current, underscoring the effectiveness of lignin structuring in enhancing triboelectric performance (Figures 4.10a-4.10c). To achieve maximum electrical output, it is essential for a water droplet to spread to its maximum diameter, fully retract, rebound, and ultimately leave the surface, rather than departing immediately upon impact.25, 255 Notably, the water droplet initially contacted the surface at a position above and in close proximity to the electrode (Figure 4.9b). As the droplet spread upon impact, it induced electrostatic charging and facilitated electron accumulation on the Lig-wood surface. It subsequently reached the electrode as it expanded to its maximum diameter during interaction with the wood. The resulting electric current was generated by the movement of the accumulated charges toward the attached electrode, reaching equilibrium once the droplet fully spread. At this stage, the maximum output was achieved, as the optimal spreading and retraction maximized the contact area, enhanced charge transfer efficiency, and intensified the contact electrification effect—allowing the electrode to collect the highest possible charge density (Figure 4.9b). Finally, the water droplet rebounded and completely detached from the surface, enabling a full separation step (Figure 4.9b). This process facilitates efficient charge transfer, in contrast to hydrophilic surfaces where adhesion impedes such separation. The complete departure of the droplet from the wood surface allowed the accumulated charges to equilibrate, 84 enabling charge transfer from the electrode to the ground and resulting in a negative current cycle. Unlike conventional hydrophobic surfaces, where water droplets bounce off after minimal contact, Lig-wood's hydrophobic lignin particles—covering the cellulose fibrous structure—allowed for efficient water removal while maintaining sufficient contact time. This balance maximized triboelectric performance, making Lig-wood an optimized friction layer for energy harvesting applications. The enhanced tribonegativity of Lig-wood, compared to unmodified wood, was attributed to its surface chemistry and lignin redistribution. It has been found that lignin plays an important role in the tribonegativity of lignocellulose materials236, owing to lignin's electrochemical properties and electron-withdrawing capabilities. The polar hydroxyl (-OH) groups in lignin enhance the electron-attracting properties of the surface upon contact with water. These exposed functional groups increase the electronegativity of the Lig-wood surface relative to water, thereby promoting electron withdrawal during interaction. The π-electron-rich aromatic groups exposed on the Lig-wood surface enhance electrostatic induction, create additional low-energy unoccupied electron states. The conjugated phenolic-ring structure in lignin, featuring unoccupied electron states, enables charge stabilization through electron delocalization across the π-system, thereby enhancing charge retention and increasing overall charge acquisition. By combining lignin's spherical morphology with its surface chemistry—rich in polar functional groups and aromatic structures—Lig-wood demonstrates a competitive output performance, comparable to that of fluorine-containing tribo-negative materials and low-cost waste-derived alternatives in liquid–85 solid (L–S) systems. These results highlight its strong potential as a sustainable tribo-negative material for rain-driven energy harvesting. In our case of Lig-wood, where lignin is retained, desirable dielectric properties emerge due to the preservation of β-O-4 linkages and the high molecular weight of lignin, which is largely maintained through hydrotropic fractionation.282 These properties facilitate charge storage and minimize dissipation, further enhancing energy output efficiency.283 Furthermore, the lignin particles on the Lig-wood surface increased the contact area and interaction with water droplets, significantly improving charge transfer efficiency compared to unmodified wood. The nano- and microscale morphology formed by migrated and clustered lignin increased the surface area, thereby enhancing charge generation through improved electrostatic induction at the droplet interface by providing more active sites for interaction. Thanks to these advantages, Lig-wood exhibited excellent tribonegative properties, with stable voltage and current outputs and long-term durability. It maintained consistent performance during a 2000-second test, simulating rain conditions, whereas unmodified wood produced minimal energy (Figures 4.10d-4.10e). The stable electrical output performance underscores the material's potential for long-term use. The durability is attributed to the well-preserved and dependable surface morphology, which maintains a high degree of lignin particle retention and 86 distribution on the treated wood. Figure 4.10. (a) Voltage and (b) current generated by unmodified wood and Lig-wood. (c) Average voltage and current produced by unmodified wood and Lig-wood following 5 measurements. (d) Cyclic voltage output corresponding to continuous water droplets falling for 2000 s on unmodified wood and (e) Lig-wood. 4.3.5 Lig-wood performance under simulated rainfall conditions After investigating triboelectric energy generation in the water-wood system (L-S TENG), we examined the influence of contact parameters, including droplet fall rate, impact height, and surface tilt, on output performance. For all experiments, a hydrophobic Lig-wood sample (30 mm × 50 mm, 2 mm thick) was used as the tribonegative layer. Water droplets were dispensed from varying heights (up to 20 cm) using a peristaltic pump, which controlled the falling rate at 2, 4, 6, 8, and 12 rpm. A 45° surface tilt was selected for power generation analyses (Figure 4.11a). 87 Droplet Fall Rate. The results showed that as droplet impinging rates increased, peak voltage and current outputs rose from 0.5 V and 2 nA at 2 rpm to 2.8 V and 5 nA at 12 rpm (Figures 4.11b–c). Faster droplet rates reduced contact time, enabling quicker charging and discharging, which saturated surface charges more efficiently and improved charge generation (Figure 4.11d). However, at rates exceeding 8 rpm, the enhancement in current and charge output was less pronounced (Figures 4.11c, d). This was attributed to the transition from discrete droplets to a continuous water flow, which disrupted the complete contact-separation process. Successive droplets impacted on the surface before the previous ones fully departed, limiting effective charging and discharging. To ensure stable and efficient TENG performance, an impinging rate of 6 rpm with a 45° tilt was selected for subsequent experiments examining the effects of droplet height (Figure 4.12a). Figure 4.11 The Lig-wood performance as L-S TENG under varying simulated rainfall conditions: (a) Effect of increased water falling rates on (b) voltage, (c) current and (d) charge. Droplet Height. The results indicated a general increase in voltage and current outputs with greater droplet heights, peaking at 16 cm, where outputs reached 3.8 V and 14 nA. Minimal differences were observed between 16 cm and 20 cm (Figures 4.12a–d). Higher falling distances resulted in greater impact forces, enhancing charge transfer and accumulation (Figure 4.12d). 88 The higher velocity and impact force of droplets at increased heights facilitated multiple and more efficient charge transfer events, improving output performance.284 The decrease in peak performance at higher dripping distances (>16 cm) is primarily attributed to the shape deformation of the droplet, which limits the effective contact area. Droplets falling from greater heights possess higher kinetic energy and impact speeds, often resulting in droplet fragmentation upon impact. As a result, smaller droplets make reduced contact with the material surface, which diminishes the contact electrification effect and lowers charge transfer efficiency due to less effective spreading and a smaller contact area. Figure 4.12 The Lig-wood performance as L-S TENG under varying simulated rainfall conditions: when water droplets (6-μL) were dropped from a 20-cm height at a surface tilt of 45o (with respect to the horizontal plane). (a) Effect of droplet (6 μL) fall height at 6 rpm rate and Lig-wood surface tilt of 45o. (b) output voltage, (c) current, are (d) charge. Surface Tilt. Surface tilt had minimal impact on triboelectric performance (Figure 4.13a). Both 15° and 45° tilt angles produced comparable peak voltage (1.8 V) and current (5 nA) outputs (Figures 4.13a-d). However, at smaller tilt angles, water droplets tended to accumulate on the surface, leading to incomplete charge transfer during the charging-discharging process and 89 reducing charge transport efficiency (Figure 4.13d). Conversely, at excessive tilt angles (e.g., 60°), droplets rolled off too quickly, reducing contact time and limiting the triboelectric effect (Figures 4.13a-d). Figure 4.13 The Lig-wood performance as L-S TENG under varying simulated rainfall conditions: (a) Effect of Lig-wood tilt at water droplet (6 μL) fall rates of 6 rpm from 20-cm height: (b) voltage, (c) current, and (d) charge. 4.3.6 Application perspectives The practical application, versatility, and long-term operational potential of Lig-wood as a triboelectric nanogenerator (TENG) for power generation were investigated in this section. A controlled dripping system was designed to simulate rainfall conditions, where a peristaltic pump regulated the water flow rate, and the dripping distance from the tips to Lig-wood samples (n = 12) was fixed at 30 cm (Figure 4.14a). At the onset of rainfall, an enhanced electrical output (~4 V, 60 nA) was generated upon contact with the wood surface. This improvement was attributed to the amplified power resulting from the parallel-series connection of 12 sample pieces (Figure 4.14a). The instantaneous energy produced could be effectively harnessed to power small electronic devices when the electrical signal was accumulated over time. During simulated rainfall, the alternating current (AC) generated by the TENG was rectified using a bridge 90 rectifier, converting it into direct current (DC), which was then stored in a 2.2 µF capacitor (Figure 4.14b). The equivalent circuit model (Figure 4.14b) illustrated this energy conversion process, enabling the charging and activation of electronic devices. Energy Storage and Device Activation. Low-power electronics, such as LED lights and timers, were successfully powered by the capacitor after approximately 5 minutes of continuous dripping (Figure 4.14c). Capacitors with capacitances ranging from 1 µF to 22 µF were tested, all of which demonstrated successful charging using the Lig-wood-based TENG (Figure 4.14d), achieving voltages of approximately 2 V within 10 minutes. These findings confirm that the Lig-wood-based TENG effectively converts raindrop kinetic energy into electrical power. When integrated into roofing systems, this technology offers a viable strategy for harvesting rainfall energy, providing a sustainable power source for small electronics, and highlighting its potential for smart home applications (Figure 4.15a). Figure 4.14. (a) Schematic diagram of the Wood-TENG system (12 pieces of wood placed in parallel) to power electronic devices. (b) Illustration of the equivalent circuit model. (c) LED lights and timer turns on powered by Wood-TENG. (d) Charging performance of the wood-TENG with capacitors of various capacitance. 91 Versatility Across Different Wood Species. To evaluate the versatility of the Lig-wood treatment, various softwood and hardwood species were tested. All treated samples exhibited significantly improved hydrophobicity compared to untreated samples. For instance, untreated fir, oak, and walnut veneers exhibited high wettability, with water droplets rapidly absorbed within seconds (Figure 4.15b). However, after treatment, these wood samples demonstrated significant improvements in water resistance (Figure 4.15b). This confirmed the universality of this treatment that can be applied to a large variety of species, yielding stable performance and overcoming the shortage of uneven property of treated species produced by conventional hydrothermal treatment. Figure 4.15. (a) Illustration of the application of Wood-TENG as roof to collect rain energy for power generation. (b) Water contact on treated wood from different species (size bar = 2cm). (c) Water contact angles of treated wood before and after the water dripping for 1h. As a representative case, oak exhibited an average water contact angle (WCA) of 149° (measured at eight distinct positions) after treatment (Figure 4.15c). Notably, after one hour of continuous water dripping under simulated rainfall, the material retained its hydrophobic properties, maintaining an average WCA of 145°. This minimal reduction in hydrophobicity 92 confirmed the material's reusability under rainy conditions and its potential for long-term durability. Mold Resistance and Long-Term Durability. To ensure long-term suitability for rain energy harvesting, Lig-wood was designed with anti-molding properties, enhancing its sustainability for outdoor applications. Comparative testing was conducted by placing unmodified wood and Lig-wood samples in a wet, natural soil environment. Within 20 days, untreated basswood showed visible stains, and by 100 days, significant mildew growth was observed (Figure 4.16). In contrast, Lig-wood remained unchanged in color, with no visible mold growth, even after 100 days of exposure (Figure 4.16). The mold resistance of Lig-wood is attributed to the high lignin retention on the wood surface, which contributes to antimicrobial properties. Lignin's polyphenolic structure inhibits microbial growth through direct interactions with bacteria. Additionally, the hydrophobicity of Lig-wood prevents water penetration, creating an unfavorable environment for bacterial and fungal growth285. Moreover, lignin present on the fiber surface inhibits enzymatic hydrolysis of cellulose and binds in a manner that is unproductive.286 The combined antimicrobial and water-resistant properties make Lig-wood a promising material for outdoor applications, particularly in rain energy harvesting systems, where long-term durability and resistance to environmental factors are crucial. 93 Figure 4.16. Photos of native basswood (left) and Lig-wood (right) in contact with soil during given period of time (size bar = 1cm). 4.4 Conclusions A self-hydrophobized all-wood material (Lig-wood) was successfully developed through a simple and efficient aqueous hydrotropic pretreatment. This process effectively regenerated and redistributed lignin, exposing lignin particles that enabled wood hydrophobization. The resulting Lig-wood exhibited a water contact angle (WCA) of 148° and a rolling angle of 20° on its tangential plane, facilitating efficient water separation upon contact. These properties make Lig-wood a promising tribonegative material for liquid-solid triboelectric nanogenerators (L-S TENGs). Upon contact with water droplets, Lig-wood generated electrical signals with a 7.5-fold increase in voltage compared to unmodified wood, demonstrating excellent long-term durability. Further triboelectric performance analysis revealed adaptability and efficiency across various operational parameters. Under simulated rainfall conditions, Lig-wood successfully powered disposable electronic devices by converting alternating signals into direct current. Additionally, Lig-wood anti-molding properties enabled by the relocation of native lignin on the surface, ensured long-term durability for outdoor applications. This first-of-its-kind, fully wood-based, 94 and environmentally friendly material presents significant potential for sustainable rain energy collection and energy-efficient construction technologies. 95 Chapter 5: Thermally Conductive Wood for Photothermal Conversion and Heat Regulation 5.1 Introduction Beyond its role in rainfall energy capture during rainy conditions, wood can be further functionalized as a structural material to harness solar light energy for smart home systems, enabling solar energy utilization in sunny environments. Its capacity to convert solar radiation into thermal energy and realizing thermal storage performance positions engineered wood as a high-value material, particularly in extreme cold areas. In this chapter, we present a strategy for functionalizing wood with the incorporation of phase change materials to integrate solar heat storage and temperature regulation capabilities. To enhance photothermal conversion efficiency, the pale native wood surface was modified with a polypyrrole deposition, which improves solar energy utilization. A comprehensive investigation of the structure-property relationships to highlight this functionalization is systematically discussed and studied. Phase Change Materials (PCMs) allow thermal energy storage (TES) and can be used to convert solar radiation, absorb excess environmental heat, as well as store and release thermal energy. Combined with the possibility for temperature regulation, PCM systems have attracted interest for sustainable thermal energy utilization.287-291 This is possible given the high thermal (latent) energy involved during phase transition. For their use in TES systems, PCMs should ideally display a high latent heat storage density and good chemical stability. For these purposes, 96 noncorrosive organic PCMs have been considered, including paraffins, fatty acids, fatty acid esters as well as organic polymers.292 All of these PCMs show great value for thermal management but display a low thermal conductivity (k) and are prone to leakage upon melting. These factors limit the efficiency of heat transfer, energy storage and release.293, 294 To address these issues, conductive fillers have been proposed, including carbon nanotubes,295 nanomagnetite,296 expanded graphite,297 graphene oxide,298 carbon black,299 and aluminum oxide nanoparticles.300 Unfortunately, poor compatibility and phase separation of mixtures of such fillers with PCMs lead to low performance and uneven heat distribution, especially after cycling operation.301, 302 Therefore, new strategies are required in designing PCM-based systems for long-term stability and uniform heat transfer. Shape-stabilization of PCMs by encapsulation in a supporting matrix303 is a possible route to reduce leakage. For this purpose, supporting matrices for TES have included metal-organic frameworks (MOFs),304 carbon aerogels,305, 306 mesoporous silica,307 and polymers. 308, 309 Meanwhile, environmental factors such as sustainability and biodegradability are of great significance. Hence, wood has been tested as a promising three-dimensional support for thermal management; for this purpose, an additional feature is wood's vertically aligned and interconnected hierarchical tubular fibers.93, 310 The capillary effects and surface interactions between the cellulosic porous scaffold and organic PCM contribute to leakage-proof encapsulation.311, 312 As such, this wood-stabilized PCM has been proposed for thermal energy storage.157, 160, 313-315 Unfortunately, the low thermal conductivity of wood limits effective heat storage and release.184 Though wood haven been utilized in PCM systems to approach better sustainability, a higher thermal conduction still needs to be realized to ensure an efficient 97 utilization of latent heat. Hence, the incorporation method, mechanism as well as the loading of thermal enhancer filler in the PCM composite system are subjects that need special consideration. Additionally, a process that is typically used in related efforts, i.e., lignin removal, significantly reduces light absorption in the visible and ultraviolet light spectrum. The poor photo-absorption ability of wood (or delignified wood) as well as that of the PCM reduce the conversion efficiency of solar irradiation and limit applications in the field of solar-to-thermal energy management.316 In this study, we address the above limitations by introducing a simple strategy that uses a wood-based PCM composite that exhibits high thermal conductivity and efficient light-to-thermal energy conversion. For this purpose, controlled wood delignification is used to increase the free volume for PCM infiltration while allowing high hemicellulose retention, to preserve the mechanical stability of wood.317 Meanwhile, hexagonal particles of boron nitride (BN) are incorporated as a low-cost filler to endow high thermal conductivity.318 This enables heat transfer pathways in an adiabatic process involving the 3D interconnected structure of wood facilitating heat energy storage and release. Simultaneously, an efficient photothermal energy conversion is added by in-situ polymerization of cationic pyrrole (producing polypyrrole, PPy), which enhances light absorption and electrical conductivity. BN and PPy were effectively grown inside wood by electrostatic assembly. Compared with methods such as wood carbonization319-322 or loading with Fe3O4 particles296 and MOFs, for instance, to capture and convert solar energy, our modification is realized by following simple steps and processing in aqueous media, at room temperature. In sum, light adsorption capacity and light-to-thermal energy conversion are demonstrated in a multifunctional wood-supported PCM composite. 98 5.2 Materials and methods 5.2.1 Materials Boards of balsa were purchased from Modulor, Berlin, Germany. Sodium chlorite (NaClO2, ≥90%), polyethylenimine (PEI, Mn=10000 by GPC, branched), pyrrole (≥98%), poly(ethylene glycol) (PEG, Mn=6000g/mol), iron(III) chloride hexahydrate (FeCl3 · 6H2O), boron nitride (powder, size around 1 μm, 98%), isopropanol(IPA, ≥98%) were purchased from Sigma-Aldrich. 5.2.2 Liquid exfoliation of boron nitride, BN BN powder (5 g) was mixed with 500 ml isopropanol for 14 h at 800 rpm (magnetic stirrer). Then the dispersion was sonicated in a water bath at 80oC for 10h. These steps were repeated 8 times to exfoliate the BN and to obtain a well-dispersed BN suspension. 5.2.3 Delignified wood and LbL deposition The balsa samples (with two different sizes, 20 x 20 x 5mm and 100 x 30 x 5 mm) were delignified by treatment with 1 wt% NaClO2 aqueous solution at 100 oC with the pH of the solution adjusted to 4.6, following a well-established protocol reported in the literature.75, 160 The resulting, delignified wood (DW) showed an off-white color. The smaller samples were primarily used for material characterization, while the larger ones were prepared for demonstration in house-roof applications and related photothermal conversion experiments. DW was modified by adsorption of boron nitride (herein termed as BN-modified wood, BW) following layer-by-layer (LbL) deposition. For this purpose, the samples of delignified wood (DW) were immersed overnight in cationic PEI (3 mg/ml) solution. The samples were then washed and immersed in a 10 mg/ml BN dispersion for 10 mins. The negatively charged BN 99 adsorbed on the cationic PEI forming a strongly adsorbed layer. The samples were then washed with de-ionized (DI) water. Some samples were immersed again in 3 mg/ml PEI solution for 10 min and rinsed with DI water. To form multilayers of PEI/BN, the process was repeated 2, 5, 10 and 15 times. Part of samples were freeze-dried for further use. The BW subjected to the above treatments are thereafter referred to as BW2, BW5, BW10 and BW15, where the numeral refers to the respective number of PEI/BN layers. Additional experiments were carried out with BW capped with PPy (termed PW). More specifically, the BW samples (BW2, BW5, BW10 and BW15) were immersed in a 1M pyrrole aqueous solution for 24h. A total amount of 1M FeCl3.6H2O was then added to initiate in-situ polymerization to produce PPy. After reaction for 24 h, PW samples were washed with DI water and freeze-dried for further use. These samples were named as PW2, PW5, PW10 and PW15. 5.2.4 Impregnation with the PCM (PEG) The prepared BW and PW wood samples were immersed in PEG using a vacuum oven (-0.1MPa, 80oC), following our earlier procedure.160 The BW samples filled with PEG are designated as BW2@PEG, BW5@PEG, BW10@PEG and BW15@PEG and those with PPy capping layer are referred to as PW2@PEG, PW5@PEG, PW10@PEG and PW15@PEG. 5.2.5 Structural, morphological, chemical, and thermal characterization The zeta potential of PEI, BN and pyrrole were determined by using a Malvern Zetasizer (Nano ZS90) analyzer. The morphologies of the wood samples were observed by scanning electron microscopy (SEM) using a Zeiss Sigma VP scanning electron microscope (Germany) and a Hitachi S2600N Variable Pressure SEM (Japan). The UV absorption was measured with a 100 Shimadzu (UV-2600 with ISR-2600Plus) Integrating Sphere. The thermal properties of the samples were studied by differential scanning calorimetry (TA instruments MT-DSC Q2000) operated at 5 °C/min heating/cooling rate, from 25 °C to 80 °C under a nitrogen atmosphere (50 ml/min flow rate). Two replicates of each group were tested following three heating-cooling cycles. The average values of six experiments were determined. Thermal conductivity was measured with a C-Therm TCiTM Thermal Conductivity Analyzer. The specific heat capacity was determined by using a TA instrument (DSC Q1000) with sapphire calibration and the thermal conductivity was reported at a temperature of 23 oC. Fifty consecutive thermal cycles were conducted from 20 °C to 80 °C (TA instrument DSC Q1000) at a heating/cooling rate of 5 °C/min under a nitrogen atmosphere (50ml/min flow rate). The thermal stability and degradation of the samples were studied by using thermogravimetric analysis on a TA instrument (TGA Q5500). The samples were heated from 30 °C to 650 °C at 10°C/min rate, under a 60 ml/min nitrogen gas flow. FTIR measurements were conducted for composition analysis using a PerkinElmer spectrometer (Spectrum Two FT-IR spectrometer) using the wavenumbers between 500 and 4000 cm-1 with a resolution of 32 cm-1. X-ray diffraction (XRD) patterns of the samples were measured in the 2θ range of 3o–80o with CoKα radiation on a Bruker D8 Advance Bragg-Brentano diffractometer. The photothermal energy conversion was determined by exposing the samples to an infrared lamp (120 V AC, 600W, US) and real-time monitoring of using of the temperature evolution using an IR (FLIR, T620) camera. 5.3. Results and discussion 5.3.1 Wood modification and loading 101 The procedure used for surface modification on balsa as well as the designated PCM composites is illustrated in Figure 3.1. Balsa (Ochroma pyramidale) is highly porous (the tracheid content is as high as 90%) and lightweight (± 87.7 mg/cm3). Delignification created large void volumes, not only from the cell wall but from the middle lamella and cell corners, which allowed extensive PCM infilling.160 After the removal of lignin, the residual hemicelluloses contributed to the anionic character of the delignified wood sample (-COO- surface groups).323 When immersed in water, the inner structures of wood displayed features typical of anionic hydrogels. The hydroxyl groups on the defective edges of the BN,324-326 was able to promote hydrogen bonding with PEI and cellulose in the BW samples. After fully infiltration of pyrrole, the addition of Fe3+ initiated the in-situ polymerization.327 In these PW samples, hydrogen bonding also occurred between PPy's amine groups and the hydroxyl groups of BN (Figure 5.1). 102 Figure 5.1. Schematic illustration of the procedure used for the synthesis of BW, PW and PW@PEG. PEI/BN LbL assembly on DW was carried out using 2, 5, 10 and 15 LbL cycles, yielding the respective BW. PPy was synthesized on BW to form the thermally conductive PW samples, which were further infiltrated with the phase change material, PEG. 103 5.3.2 Morphology of conductive BW, PW and PW@PEGs The wood structures were preserved after the LbL self-assembly (PEI/BN) and after deposition of PPy (BW, PW and PW@PEG, Figure 5.2a-g). The off-light yellow color of the BW samples (Figure 5.2e), developed upon PEI/BN immersion, was taken as qualitative indication of the effect of PEI adsorption. The efficient BN adsorption was confirmed by the observation of the mineral nanosheets attached on the wood tracheid surfaces, as evidenced in SEM images taken in the longitudinal and radial directions of PW15 after 15 adsorption cycles (Figure 5.2b, 5.2e and 5.2e1), which were contrasted with the neat, clean and smooth cell wall surfaces typical of DW (Figure 5.2a and 5.2d). To endow efficient light-to-thermal energy conversion, PPy was applied on the BW samples, given PPy light absorption capacity.93 The capping PPy covered the PEI/BN layers and changed the surface roughness in both the radial and longitudinal directions (Figure 5.2c and 5.2f). Compared with the surface of BW and neat DW (Figure 5.2a, 5.2b, 5.2d and 5.2e), the addition of PPy increased the roughness, which enhances the interfacial interactions with the infiltrated PEG.325 The surface features or protuberances observed in Figure 5.2c and 5.2f, provides better molecular contact in PW15, which increases the work of adhesion and encapsulation of PEG. 104 Figure 5.2. SEM of the longitudinal section of (a) DW. (b) BW15, and (c) PW15. SEM of the radial morphology of (d) DW, (e) BW15 and (e1) BW15 at an increased magnification as well as (f) PW15. (g) Photos of DW (top left), BW15 (top right), PW15 (bottom left) and PW15@PEG (bottom right), (h) SEM of the radial morphology of PW15@PEG. The rough surface of the PW samples facilitates multiple light scattering, absorption and energy harvesting.310 Additionally, the dark color imparted by PPy in PW and PW@PEG samples, enhances light absorption (Figure 5.3a). Besides its photothermal conversion effect, the PPy capping layer protects the thermally conductive boron nitride particles in the PW samples. The system guarantees light-capture by wood following simple and low-energy processing. 105 5.3.3 PCM-loaded systems For thermal energy storage, PEG was applied as a source of latent heat in PW@PEG. After the impregnation of PEG in PW via vacuum infiltration, the lumina were filled completely, as seen in the fully covered lumen shown in Figure 5.2h. For PW15, the appearance of peaks at 1548 cm-1 and 1200 cm-1 corresponded to the pyrrole ring C=C and C–N stretching vibrations,310 confirming the presence of PPy. The peaks around 800 cm-1 (B–N stretching vibration) and 1380cm-1 (B-N bending vibration)328 are indicative of the presence of BN in PW. On the spectrum of PW15, the characteristic peaks at 3337 cm-1, 1736 cm-1and 1032cm-1 are assigned to the stretching vibration of O-H, C=O and C–O valence vibrations of cellulose, respectively,160, 329 indicating that the chemical composition of wood was preserved after both physical adsorption and in-situ polymerization of PPy on the surface. However, the absorbance values corresponding to C=O and C–O were reduced and that of O-H was broadened in PW15, which imply the formation of hydrogen bonds after surface modification. The adsorption of BN and PPy rely on secondary interactions including electrostatic attraction between oppositely charged surfaces and hydrogen bonding. The spectrum of PW15@PEG (Figure 5.3b) showed the characteristic peaks of PEG and PW. No new peaks were observed in PW15@PEG, indicating that no chemical reactions took place. Meanwhile the reduced absorbance from 3300 cm-1 to 3600 cm-1 corresponded to the O-H stretching vibration typical of the formation of hydrogen bonds by the addition of PEG (Figure 5.3b). The chemical compatibility and structural stability of PW and PW@PEG were also confirmed by XRD analyses. The typical crystalline peaks of cellulose I at 2θ=14.9o, 16.5o, 22.5o and 34o, were assigned to the (101), (10-1), (020) and (040) crystal planes, respectively,330, 331 which remained in the PW samples, Figure 5.3c. No new 106 crystal peaks appeared in the PW samples, confirming the physicochemical stability of the systems with the given BN loading, Figure 5.3c. After the incorporation of PEG, Figure 5.3c, the characteristic peaks of PW15@PEG corresponded to the superposition of PW15 and PEG, confirming the preservation of the crystal structures of the initial materials and the chemical compatibility of the system. Figure 5.3. (a) UV and (b) FTIR spectra of the respective samples, as indicated and (c) XRD of DW, BW10, PW15 and PW15@PEG. 5.3.4 Thermal energy storage of PW@PEG The phase change and thermal energy storage properties of the samples are next discussed in terms of the phase transition temperature and latent heat, Table 5.1. The latent heats of melting (Hm) and crystallization (Hc) of neat PEG are 204.5 J/g and 187.7 J/g, respectively. Incorporated in a supporting matrix, wood, the PEG in PW@PEG showed no leakage and had a similar phase change performance compared to that of neat PEG. The results indicate a successful solution to the PCM leakage problem without compromising the thermal energy storage performance. 107 PEG infilling mainly relied on capillary forces acting in the lumina and the hydrogen bonding between PEG and the surface of PW. The amino groups of PPy and the hydroxyl groups of DW together with the edges of exfoliated BN participated in hydrogen bonding (Figure 5.4) and facilitated the interaction with PEG, achieving high encapsulation efficiency (FTIR, Figure 5.3). The PEG loaded in the PW samples showed a high phase change enthalpy (Hm =159.7 J/g) with 78.1% efficiency (F) in PW2@PEG. With the addition of BN, the enthalpy of melting and cooling as well as the encapsulation efficiency were reduced (Table 5.1), which is explained by the reduced space and pore volume available for PEG diffusion and the increased number of BN/PEI layers. Additionally, PEG chains were confined in the wood scaffold by surface interaction so that not all PEG undergoes solid to liquid transition, producing a loss of fractional enthalpy,332 as illustrated in Figure 5.4. Furthermore, the higher molecular contact area given by the rough, outermost surface of the wood framework constrains PEG chains near the cell wall, so that a higher adhesion energy and hydrogen bonding result in a less extended PEG molecules during phase change.304 Therefore, the reduced impregnation volume of PEG with the increased BN loading as well as the lower fraction of extended PEG chains produced a lower latent heat of melting (from 159.7J/g to 139.2J/g) and cooling (from 153.0 J/g to 133.9J/g). 108 Table 5.1. Phase change properties and energy storage efficiency (F). Sample Tm (oC) Hm (J/g) Tc (oC) Hc (J/g) F (%) PEG6000 62.4 204.5 40.21 187.7 100 PW2@PEG 62.2±0.5 159.7±7.3 41.7±0.3 153.0±4.5 78.1% PW5@PEG 62.2±0.9 154.3±10.9 40.6±1.0 149.5±6.9 75.4% PW10@PEG 61.7±0.3 156.4±12.5 42.7±0.5 149.6±11.6 76.5% PW15@PEG 61.9±1.0 139.2±13.2 42.4±1.0 133.9±11.4 68.1% Note: F (%) is the effective melting enthalpy content, which can be calculated as: 𝐹(%) =𝛥𝐻𝑚 of PWX@PEG𝛥𝐻𝑚 of PEG∗ 100% Figure 5.4. Molecule interactions in PW@PEG and the PEG chain motion during phase transition. Compared to the values of neat PEG, PW@PEG samples showed a reduced Tm and an increased Tc. The interconnected three-dimensional wood matrix provided sites for PEG to crystallize, serving as nucleating sites during cooling. The phase change temperature was slightly affected by BN loading (Figure 5.5a and 5.5b, Table 5.1): a reduced Tm and improved Tc were observed for samples PW2@PEG-PW15@PEG, as seen in the shift of peaks shown in the DSC profile. 109 This favorable effect of BN on phase transition relied on the enhanced thermal conductivity, which accelerated both the melting and crystallization processes. Additionally, upon melting (Figure 5.5b), the PEG in contact with BN engaged in effective heat absorption (given the enhanced heat transfer of BN) compared to the wider range of melting temperature of in pure PEG. With BN loading endowed a more effective heat absorption (shortened melting process) (Figure 5.5b). The PW@PEG structures are well maintained, presenting stable crystal structures and chemical properties depending on BN adsorption cycles (Figure 5.5c). The thermal conductivity of the PW and PW@PEG samples was measured and compared with the references (DW and PEG), Figure 5.5e and 5.5f. Generally, cellulosic materials are insulating333 and DW exhibited a low thermal conductivity (23 oC), ~0.029 W/(m⋅K) in the longitudinal direction. This value increased (0.086 W/(m⋅K) by the effect of BN, a figure that is ~3-fold higher compared to that of DW (Figure 5.5e). After the incorporation of PEG, the axial thermal conductivity was measured to be 0.758 W/(m⋅K), which is 4.41 times greater than the value of neat PEG (0.177 W/(m⋅K)) and 26-fold higher than that of DW. 110 Figure 5.5. DSC thermograms for PW@PEG upon (a) heating and (b) cooling. (c) The latent heat of heating and cooling in PW@PEGs. (d)Schematic illustration of the interactions and hydrogen bonding between the components. (e) Longitudinal thermal conductivity of the PWs modified with BN and DW. (f) Longitudinal thermal conductivity of PW@PEGs and pristine PEG. The improved thermal conductivity is explained by the effect of BN, and the increased k in PW@PEG, which scaled with BN loading. Additionally, the growth of BN along the anisotropic wood lumina ensures an effective unidirectional heat transfer in the longitudinal direction (note the lower k in radial direction for all samples with a maximum of 0.597 W/(m⋅K), Figure 5.6). This indicates improved thermal conduction via the LbL strategy. As reported previously,328, 334 BN has been broadly and effectively employed as a filler for heat management. In this study, BN was applied in a layer-by-layer format along the natural wood lumina providing unidirectional heat transfer pathways (as illustrated in Figure 5.6). Moreover, the enhanced k was attributed to the reduced thermal interface resistance,316 due to the improved adsorption and interfacial 111 interactions of PPy. Compared to the PWs and with the same amount of BN loading, the higher thermal conductivity in all PW@PEGs is explained by the removal of air and the closing of pores in PCM composites, given PEG infiltration. Compared with previously reported wood-stabilized PCM systems (Table 5.2), the thermal conductivity in the present study is the highest. Table 5.2. Comparison of thermal conductivity reported for related similar phase change composites. Wood-supported PCMs Thermal Conductivity (W/mK) refs Delignified wood/capric acid-palmitic acid 0.49 311 Wood powder/paraffin-PEG/graphite 0.32 314 Wood/PEG/SiO2 0.33 335 Wood powder/n-octadecane/SiO2/BN 0.73 315 Carbonized wood/PEG/Fe3O4 0.23 319 Carbonized wood/1-tetradecanol 0.67 322 PPy-modified wood/PEG/BN 0.76 This study 112 Figure 5.6. Thermal conductivity along the radial direction of (a) PWs modified with BN and DW, as well as those of (b) PW@PEGs and pristine PEG. 5.3.5 Thermal durability and stability The thermal resistance of DW was improved by the treatments (PW@PEG degradation occurred at higher temperature, Figure 5.7a). As depicted in Figure 5.7a, the PW@PEG samples with different BN loading exhibited similar decomposition: first partial degradation at around 250-350 oC and a second stage with degradation at ~400 oC. The BN material is thermally resistant334 as there are negligible pyrolysis losses at 650 oC (Figure 5.7a). A higher percentage of residual mass at 650oC was measured with BN loading, 4.74% and 7.98% for PW2@PEG and PW15@PEG, respectively. This increased char is attributed to the incorporated BN and the lower volume of PEG in the material. 113 Figure 5.7. (a) Thermogravimetric analysis (TGA) of DW, PEG and PW@PEG for given BN modification cycles. Stability and recyclability of PW10@PEG before and after 50 consecutive cycles of heating and cooling as observed by (b) DSC; (c) FTIR and (d) XRD measurements. The thermal degradation temperature of the PW@PEG samples was around 400oC, which indicated a good thermal stability, and a slightly delayed decomposition of PW@PEG with increased BN loading. Besides the thermal resistance of BN, interactions between PEG and the cellulose surface modified with PPy, BN and PEI contributed to the improved thermal stability. The effect of PPy as a protective barrier against the degradation of cellulose was also a factor. 114 The durability of PCM composites is an important consideration, i.e., to ensure stable performance and avoid phase separation or loss of properties. To examine the long-term applicability, PW10@PEG was consecutively heated and cooled for 50 cycles and the phase change performance as well as the compositional properties were examined. As shown in Figure 5.7b, there was no significant evolution of latent heat and the similar phase change performance indicated good thermal durability and repeatability. The partial enthalpy loss may be caused by the strong interactions between PEG and PW, which enabled segmental free-extended PEG chains.332 The compositional stability was studied by FTIR and XRD (Figure 5.7c and 5.7d), which indicated no significant variation in the profiles after 50 thermal cycles. The preservation of the crystal structure and negligible chemical degradation of the material indicated great stability and prospects for cyclic use as thermal energy storage system. The repeatable and cycling thermal and physiochemical performance support our observation of good compatibility of the system and the success fabrication of PW@PEG with the incorporation of BN. 5.3.6 Light-to-thermal energy conversion We examined the light absorption ability as well as the thermal performance of natural wood, PW10 and PW10@PEG (BW10 and BW10@PEG). Images obtained with an infrared camera were recorded with samples exposed to an infrared lamp (temperature−time profiles, Figure 5.8a-c). Consistently, a higher light absorption capacity and more efficient performance were observed for PW10 and PW10@PEG. With the addition of PPy, the surface temperature reached 70 oC within 50 s (Figure 5.8a-c). By contrast, natural wood and BW showed a limited temperature gain after much longer times, 600 s (Figure 5.8a). As shown in Figure 5.8b and 5.8c, PW and PW@PEG led to a significantly high temperature rise. This was not the case in the 115 absence of the PPy capping layer. The efficient light-to-thermal conversion performance is mainly attributed to the high efficiency of photon capture provided by PPy.336 Additionally, compared to natural wood the improved thermal conductivity of BW10 accelerated the rate of temperature raise (Figure 5.8a). Figure 5.8. (a) Images acquired with an IR camera of natural wood, BW10, PW10, BW10@PEG and PW10@PEG after illumination for 600s. (b-c) Temporal changes at the surfaces of BW10, PW10, PW10@PEG and BW10@PEG. The capping PPy layer was effective for light absorption and allowed light-to-thermal energy conversion. As shown in Figure 6b-c, both PW and PW10@PEG experienced a rapid temperature raise, within a few seconds, from 20oC to 55oC. It is noteworthy noting that 116 PW10@PEG experienced a plateau at around 60oC and the temperature was maintained for a period of time before the onset of change. This performance indicated the ability to regulate the temperature and to keep the temperature constant by avoiding abrupt heat evolution when PW@PEG underwent phase transition. The thermal energy storage plateau was also shown in the cooling process at 40-45oC interval, when the light was turned off (Figure 5.8b). This phenomenon was not obvious for BW10@PEG because the poor photothermal energy conversion. To examine the application of PW@PEG for temperature regulation, we used a wooden construction model (Figure 5.9a) and followed the real-time indoor temperature in the system containing a roof of natural wood, PW, DW@PEG and PW10@PEG and under infrared light. The working mechanism of the functionalized wood-based roof is illustrated in Figure 5.9b. The efficient photothermal conversion and rate of temperature increase (Figure 5.9c) of PW@PEG wooden roof leads to a temperature rise to 80 oC within 8 min illumination, as recorded by IR camera (Figure 5.9e). The absorbed heat in PW@PEG is released slowly and, therefore, maintains a warm environment for long periods of time (Figure 5.9f, T is maintained at ca. 40 oC after 15min without heating). The interior temperature is also examined to study the potential for temperature regulation. PW10@PEG underwent fast temperature raise and showed the longest heat preservation (Figure 5.9d), which is attributed to the PPy layer and the thermal energy storage capacity of the infilled PCM. In contrast, the inner room temperature rapidly decreased to 23oC with the wood and PW roof (Figure 5.9d): the absence of PEG prevented thermal energy absorption as latent heat. By contrast, the latent heat of PW10@PEG was released gradually, maintaining a warm indoor environment, given the improved thermal insulation. The fast light absorption property of PW and PW10@PEG confirmed that PPy acted as effective light-to-thermal energy component. In PW10@PEG, the interconnected 3D wood 117 network modified with BN and PPy served as a thermal diffusion pathway that enabled uniform PEG melting/crystallization, as illustrated in Figure 5.9g. As illustrated in Figure 5.9g, BN and PPy were uniformly distributed in the channels of the continuous three-dimensional wood network. This modification endowed heat conversion from light and a heat transfer pathway to enable uniform phase transition and effective thermal energy storage from solar irradiation. Using this material with thermal energy storage performance, the room temperature can be regulated by reducing the heat loss (Figure 5.9b, 5.9d). After 12 cycles of heating/cooling, PW@PEG exhibited reliable and repeatable thermal energy storage and heat regulation performance (Figure 5.9h). Thus, the proposed wood modification strategy guarantees an improved heat absorption and release performance during the phase change process. Solar to thermal energy conversion and storage in construction materials can be facilitated by the proposed wood modification, leading to energy saving and temperature regulation. 118 Figure 5.9. (a)A wood model house equipped with a roof made from PW@PEG. (b) Illustration of the effect of PW@PEG enabling an enhanced photothermal conversion, solar energy storage and thermal regulation. Efficient photothermal conversion ability and thermal energy storage performance characterized by (c) a temperature increasing rate (wood with and without PPy), (e) temperature of PW@PEG roof model after illumination for 8 min and (f) after 15 min, when light is turned off. (d) Real time indoor temperature change in the model house covered with different roof materials when the model (with and without illumination under a simulated sunlight). (g) 119 Mechanism of PPy-modified wood-supported PCM enabling efficient thermal energy absorption and release. (h) 12 cycles of heat absorption and release in PW@PEG while light is turned on and off. 5.4 Conclusions A functional wood-based PCM composite was introduced following a simple and low-energy process based on layer-by-layer assembly. BN was employed to enhance thermal conductivity and was effectively attached to the wood scaffold via hydrogen bonding and electrostatic interactions. A capping PPy layer enabled efficient light-to-thermal energy conversion. The distinct three-dimensional wood structure modified with self-assembled BN and PPy was effective in encapsulating PEG and preventing its leakage; simultaneously, the system endowed a heat transfer path that guaranteed uniform temperature distribution. The latent heat of melting of PW10@PEG reached up to 159.7 J/g for a 78.1 % PEG encapsulation efficiency. The thermal conductivity of PW was improved, about 3-times higher compared to DW. The PW10@PEG material showed enhanced thermal conductivity, up by 4.4 times compared with neat PEG and 26 times compared to DW. The thermal stability, durability and chemical reliability after 50 consecutive cycles of heating and cooling suggest PW@PEG as a potential candidate for light-to-thermal energy conversion and storage. 120 Chapter 6: Wood-Supported Phase Change Material with Lignin Redistribution for Photothermal Conversion and Thermal Energy Storage 6.1 Introduction Although previous investigations demonstrated the potential of wood-based materials for solar energy utilization, these systems largely relied on synthetic polymers to achieve the necessary light absorption—raising sustainability concerns. To address this limitation, the development of a fully bio-based alternative with enhanced photothermal conversion efficiency is essential. This chapter builds upon the wood–PCM composite fabrication framework established in Chapter 5 and introduces an environmentally friendly approach that leverages native lignin within the wood matrix. The thermal regulation properties imparted by the incorporated phase change material (PCM) are systematically evaluated, with a focus on optimizing solar energy management for roofing applications. Solar energy is an abundant and sustainable resource, offering a virtually unlimited supply of light and heat. Harnessing this energy has enabled the development of numerous technologies for applications such as electricity generation, space heating, and water purification. Among these, phase change materials (PCMs) have emerged as promising candidates for thermal energy 121 storage. PCMs absorb and release heat from sources like solar radiation and waste heat, storing energy in the form of latent heat during phase transitions.337-339 This thermal storage capability makes PCMs particularly advantageous for solar energy storage and conversion. A variety of PCMs have been explored for applications in heat management, such as water heaters, electronic devices,340 satellite thermal systems,341 construction materials,342 passive heating and cooling devices,343 and wearable sensors.344 However, the light color of many PCMs reduces their capacity to harness solar energy and leakage during the solid-liquid transition of PCMs limit their broader use. To address these challenges, various strategies have been developed to confine PCMs within structural materials. Wood offers a sustainable and effective solution for PCM encapsulation due to its naturally porous, interconnected lumen structure. These features facilitate the retention of PCMs through capillary forces, effectively minimizing leakage during phase transitions.157, 160, 345-349 Unfortunately, conventional wood-based strategies for PCM integration often involve lignin removal to increase the available pore volume. While this facilitates PCM loading, it also reduces light absorption due to the lighter color of delignified wood, ultimately diminishing the overall photothermal performance. To overcome this limitation, porous wood matrices have been carbonized or supplemented with light-absorbing materials. Dark-colored photothermal agents such as carbon black,350 graphene,351 Mxene,349, 352 polypyrrole,10 polydopamine353 have all been demonstrated to enhance solar energy absorption. However, both carbonization and the synthesis of functional 2D materials typically involve complex, high-cost processes. Additionally, carbonization compromises the mechanical integrity of wood, limiting its structural applicability. 122 One approach to achieving a fully bio-based photothermal system is to extract lignin from wood and subsequently reintroduce it as a natural light-absorbing component. Lignin possesses inherent photothermal properties due to its polyaromatic and -stacked structure, making it a viable photothermal inducer. However, this method involves complex processing steps and results in low mass yields, limiting its scalability.354,17. In this study, we propose an alternative, more sustainable strategy by using wood subjected to partial lignin dissolution, which enables effective solar energy conversion. This mild treatment is conducted in an aqueous environment under ambient pressure and selectively hydrolyzes hemicellulose while retaining both cellulose and a significant portion of native lignin. The preserved cellulose maintains the wood's anisotropic fiber structure, ensuring mechanical strength and dimensional stability to support the PCM. Meanwhile, the residual lignin enhances the material's hydrophobicity, facilitating efficient PCM absorption and providing strong encapsulation to prevent leakage. The resulting composite demonstrates excellent thermal energy storage capacity, structural stability, and cycling durability, maintaining performance over 100 heating/cooling cycles. 6.2 Experiments 6.2.1 Materials Balsa was purchased from Modulor, Germany. Chemicals including p-toluenesulfonic acid monohydrate (p-TsOH, ACS reagent, ≥98.5%) and poly (ethylene glycol) (PEG, Mn=6000g/mol) were purchased from Sigma-Aldrich. 123 6.2.2 Chemical treatment on wood Balsa (Ochroma pyramidale) pieces with the following dimensions: 20 mm × 20 mm (in the transverse plane) × 5 mm (in the tangential plane) were treated with 60% p-TsOH solution at 80 °C for 20 min, 40 min, and 60 min. Samples were washed several times until the surface chemicals were removed completely and then freeze-dried to obtain dry samples. All dry samples underwent thermal treatment in a muffle furnace, with the temperature increased from 25 °C to 240 °C at a heating rate of 6 °C/min and maintained at 240 °C for 2.5 hours. The resulting treated woods were labeled as TW20, TW40 and TW60 respectively. 6.2.3 Preparation of wood composite: PEG@TW Treated wood samples with different treatment periods were immersed in a PEG solution to prepare a PEG@TW composite. To ensure optimal impregnation efficiency, TW was vacuum impregnated with PEG at 80 °C under a vacuum pressure of -0.4 MPa. Samples were collected once equilibrium weight uptake was achieved (i.e., no further weight gain was observed). Residual PEG on the composites surfaces was removed using filter paper. Finally, composites exhibiting no PEG leakage at 80 °C were labeled as PEG@TW20, PEG@TW40 and PEG@TW60, based on their respective lignin-extraction periods (multiple samples were prepared for different characterizations). 6.2.4 Characterization Functional groups of lignin was identified by using IR spectrophotometer Spectrum 100 (Bruker, DE), at attenuated total reflectance (ATR) mode. Samples were measured using the wavenumbers between 500 and 4000 cm-1 with a resolution of 32 cm-1. X-ray diffraction (XRD) patterns of the samples were measured in the 2θ range of 5o–70o with Cu LFF HR tube in a 124 Bragg-Brentano Configuration mode by Malvern-Panalytical Empyrean 3. Untreated balsa wood pieces and TW40 were cut by scalpel, their cross section and tangential sections were observed by scanning electron microscope (SEM, Helios NanoLab 650). Water contact angles within a period of time were measured in two planes of the treated wood samples, by Thetaflex 300, Biolin Scientific. Dynamic water vapor sorption (DVS, Surface Measurement Systems) performance was measured to examine the moisture adsorption properties of TW and native wood. Samples were placed in an N2 atmosphere at a temperature of 25±0.2 °C, with the relative humidity (RH) gradually ramping from 0 to 100% RH (in 10% RH steps) and then dropping from 100%RH to 0 RH (in 10% RH steps). RH was controlled that it did not change until the sample weight balanced within a rate of 0.002%/min over a 2-min period. The thermal energy storage properties of the PCM composites were studied by using Differential Scanning Calorimetry (PerkinElmer, DSC 4000). A 5-10 mg samples were used for heating/cooling scanning from 25°C to 80oC for 2 cycles, with a rate of 5oC/min in a N2 flow (50ml/min). Measurements were conducted on 3 replicates for each sample and the average Tm, Tc, latent heat of melting and cooling were recorded. Thermal storage stability was studied by conducting 100 cycles of heating/cooling on PEG@TW40 at a rate of 5oC/min from 25oC to 80oC under a 50 ml/min N2 flow. The thermal degradation properties of treated wood and its PCM composites were studied by heating the samples in N2 (60 ml/min) at a temperature range of 30oC to 900oC with the rate of 10°C/min by thermogravimetric analysis (TGA Q5500). 125 The photothermal energy conversion performance of different PCM composites was measured by the illumination under a solar simulator (Sciencetech, SciSun-300) at a solar irradiation intensity of 2 kW/m2) and the temperature change profile was recorded by a digital thermometer (EXTECH 3 Channel Datalogging Thermometer, SD200), the temperature evolution of PEG@TW40 was also recorded by an infrared thermal camera (FLIR E76). The UV absorption was measured with a Shimadzu (UV-3600i Plus) Integrating Sphere. 6.3 Results and Discussion 6.3.1 Wood modification and compositional analysis Native lignin hydrotropic dissolution process facilitated its relocation within the wood matrix onto the surface of the cell walls (see schematic illustration, Figure 6.1a). Similar to our previous work, p-TsOH acts as a hydrotrope, enhancing the aqueous solubility of lignin through − stacking interactions. This hydrotropic effect facilitates the extraction of lignin from the wood matrix while avoiding the use of harsh organic solvents. The redistributed lignin imparted a significantly darker, brownish appearance to the treated wood, indicating the extraction and uniform deposition of lignin across the matrix (Figure 6.2a). Scanning electron microscopy (SEM) analysis confirmed the presence of densely condensed lignin layers on the cell walls. This morphological observation aligns with findings from our prior study on basswood, where the same principle was applied. 126 Figure 6.1. Schematic diagram of functional wood fabrication process. The chemical and structural properties of lignin-containing wood were characterized to evaluate lignin retention before and after fractionation, as well as the processing effects on the fiber matrix. FTIR spectra clearly showed the presence of characteristic peaks of lignin160 at 1592 cm-1, 1505 cm-1, and 1462 cm-1, confirming the preservation of lignin in the lignin-containing wood (Figure 6.2b). Notably, the treated wood exhibited chemical signatures nearly identical to untreated wood, showing similar characteristic functional groups with the native species, revealing the presence of lignin. This observation highlighted the effectiveness of our strategy, which demonstrated the inherent rearrangement of native wood components (e.g., lignin) to achieve uniform structural integration through a one-step process with much less chemical consumption compared to the deep eutectic solvent. 127 The hydrotropic fractionation introduced partial hydrolysis of polysaccharides within the wood matrix,262 yielding a lighter, more porous, yet structurally integral material (Figure 6.2a). A key indicator of polysaccharide loss was the reduced crystallinity of cellulose backbone which was presented by the lower diffraction peaks in the XRD patterns at 2θ = 16.1° and 22.3° (Figure 6.2c). Additionally, the cellulose crystallinity index (CrI), calculated using Equation 6.1, decreased progressively with extended fractionation time (CrI = 61.8% for native wood vs. 56.6%, 55.7%, and 55.3% for TW20, TW40, and TW60, respectively). Nevertheless, the hemicellulose hydrolysis and removal of partial polysaccharides by p-TsOH generated more void spaces within the wood structure, enhancing the loading of the phase material (PC). The lignin-containing wood substate guaranteed light absorbance capability. These structural modifications made the lignin-containing wood as a promising substrate for advanced PCM impregnation and therefore for heat storage and management applications. 𝐶𝑟𝐼 =𝐼(002)−𝐼𝑎𝑚𝐼(002)𝑥100% (Equation 6.1) Figure 6.2. (a) Wood samples after treatment in wet and dry state. (b) FTIR spectra and (c) XRD pattern of untreated wood and treated wood. 128 6.3.2 Improved hydrophobicity and humidity resistance During p-TsOH treatment, the lignin is extracted from the native lignocellulosic matrix and subsequently relocated to the surface of the wood cell walls. Thereafter, an antisolvent (i.e., water in this study) was introduced to quench the reaction system, inducing the precipitation and redistribution of lignin onto the wood cell wall surfaces as nanoparticles.261 This lignin reconfiguration conferred superhydrophobicity to the treated wood (Figure 6.3a), attributed to the hierarchical micro/nanostructured surface morphology generated by the lignin nanoparticles at the lumen and fiber interfaces. It is worth noting that such hydrophobicity was achieved without any additional chemical/polymer coatings relying only on the rearrangement of intrinsic wood components. The significant improvement in surface hydrophobicity endowed the treated wood with a largely improved resistance to atmospheric humidity, thereby expanding its potential applications in humid environment. Native wood, by contrast, presented hygroscopic behavior due to the abundance of hydroxyl groups in its cellulosic fibers355, which resulted in large mass gain with increasing relative humidity (RH) (Figure 6.3b). In the lignin-containing wood, a slower water adsorption behavior was observed, leading to a 10% reduction in equilibrium mass content (EMC) at RH=100%. Such humidity-resistant property can be explained by the surface condensation of hydrophobic lignin, which reduces water interaction sites. The partial removal of hemicellulose that contained large amorphous regions and high hygroscopicity due to its abundance of sorption-active hydroxyl groups 356 was also contributed to the lower EMC performance. 129 The water adsorption-desorption hysteresis loop is a key character to evaluate the moisture sorption performance in porous materials. Hysteresis occurred when the substrate had a strong affinity with humidity and cannot quickly release water molecules in the desorption process. Native wood demonstrated more severe hysteresis than the lignin-redistributed TW, particularly when RH levels exceed 30% (Figure 6.3c). This disparity was attributed to the greater abundance of hydrophilic adsorption sites in natural wood, such as hydroxyl groups in native polysaccharides and lignin. It was also generally recognized that the hemicellulose composition contributed to a softening effect in wood, which had an inherent hysteresis peak at around 70% RH.357 Thus, the shift of this hysteresis peak in TW to lower RH values indicated the partial removal of flexible and branched hemicellulose, thereby minimizing its plasticizing influence and reducing hygro-responsive softening. Furthermore, lignin fractionation during processing enhanced the molecular mobility of the substrates composed of saccharides in TW. Unlike native wood, where rigid lignin-polysaccharide networks restrict polymer chain movement, TW's more flexible matrix facilitated faster structural adaptation to humidity changes, ultimately diminishing hysteresis. Additionally, the hysteresis loop in TW started diminishing at lower RH levels compared to untreated wood. This behavior indicates early saturation of residual adsorption sites, corroborating the reduced number of water-binding groups (e.g., hydroxyls) post-treatment. 130 Figure 6.3. (a) Water droplet on the traverse surface of balsa wood before and after treatment. (b) Dynamic Vapor Sorption (DVS) curves of wood samples before and after treatment in a ramping relative humidity at a measurement of equilibrium mass content (%). (c) Hysteresis of wood in increasing humidity before and after treatment. Through lignin relocation and redistribution, the wood surface exhibited a significant enhancement in hydrophobicity across both transverse and tangential directions, as implied by water contact angles (WCA), which exceeded 120° (Figures 6.4a and 6.4b). This hydrophobicity was stable, with negligible reduction in WCA observed over time in both orientations. The improved hydrophobicity was attributed to both the increased surface roughness and the reduction of surface hydroxyl groups through heat treatment. The reassembly of hydrophobic lignin particles increased the surface roughness and heterogeneity, promoting the formation of entrapped air pockets at the wood cell wall interface. This related a Cassie-Baxter wetting regime, wherein air pockets trapped within the micro/nanostructured surface minimize liquid-solid contact, thereby amplifying hydrophobicity.254 The cross-sectional plane of the treated wood exhibited a higher water contact angle (WCA) (~156°, Figure 6.4a) compared to that in its tangential section (~123°, Figure 6.4b). This result can be explained by greater local roughness formed by exposed fibers of varying heights across 131 multiple scales. However, the inherent hydrophilicity of surface hydroxyl groups on the cellulose fibers limited its ability to fully inhibit water penetration. The removal of surface hydroxyl groups by further thermal treatment contributed to the hydrophobization effect from lignin particles. A significant increase in nanoscale surface roughness, induced by the relocation and redistribution of lignin nanoparticles, further amplified hydrophobicity. This nanoscale particle reassembly, combined with the intrinsic multiscale porosity and interconnected 3D architecture of the wood matrix, synergistically enhanced liquid filtration efficiency. Such structural roughness and hydrophobic features are advantageous for subsequent organic phase-change material (PCM) adsorption and encapsulation, as they facilitate uniform PCM infiltration while minimizing leakage. Figure 6.4. Static water contact angle of treated wood changing with time in (a) transverse plane and (b) tangential plane. 6.3.3 Microstructure and morphology change To investigate the origin of hydrophobicity, we conducted SEM imaging to test the microstructural change in lignin-redistributed wood (treated wood, TW). Untreated wood presented a relatively clean and smooth lumen structure (Figures 6.5a and 6.5b). In this scenario, the lignin is embedded and entangled within the polysaccharides to form a uniform 132 fiber structure, imparting structural rigidity to the cellulose framework and wood. From the view of its transverse plane, wood presented a highly porous, three-dimensional (3D) honeycomb-like architecture (Figure 6.5c), characterized by aligned, open channels that served as an optimal scaffold for polymer impregnation. At a higher magnification, the native cell wall appeared as a continuous and closed feature (Figure 6.5d), where reinforcement cellulosic fibers were embedded and tightly entangled with lignin. Wood surfaces showed a large morphological difference compared to the pristine surface after lignin redistribution. The wood scaffold was largely retained with the original structures (Figures 6.5e and 6.5g), despite minor structural fracturing caused by partial hemicellulose loss.262 During chemical treatment, lignin was extracted from the middle lamella and cell corners, yielding a more porous framework (see hollow middle lamella in Figure 6.5i). These newly formed voids served as volumetric reservoirs that facilitated PCM encapsulation. Simultaneously, dislocated lignin was redistributed uniformly across the wood matrix, resulting in a roughened surface along the cell walls (see lignin particles along two directions of the redistributed wood, Figure 6.5e-i). These nano-sized lignin particles, regenerated via solvent-exchange process, were observed in both the transverse planes and tangential planes of TW. These nanoparticles were redeposited and attached onto the wood skeleton firmly and uniformly, which was attributed to the hydrogen bonding and other interactions.263 The uniform deposition of redistributed lignin nanoparticles significantly enhances the hydrophobicity of TW (Figure 6.5h, wood surface without cutting before imaging), as corroborated by contact angle measurements. Following lignin redistribution, SEM imaging highlighted surface roughening and intrinsic content reassembly of the wood cell walls. These modifications synergistically 133 enhanced hydrophobicity by trapping air pockets (Cassie-Baxter state) while retaining the inherent 3D porosity necessary for functional applications such as PCM encapsulation. Most remarkably, the one-step lignin relocation and regeneration method achieved uniformity in lignin distribution across the entire wood matrix, including both internal lumina and external surfaces, Figure 6.5e-i. Such bulk homogeneity was exceptional with conventional two-step approaches (e.g., post-lignification by a secondary step of lignin coating), which often suffered from localized aggregation and inconsistent coverage. The exposed lignin was able to absorb light and gave enhanced photothermal conversion property by absorbing incident light, enabling the development of wood composites for advanced thermal energy applications.358, 359 In general, this our offered a simple and effective approach for wood multi-functionalization, by utilizing native lignin, it simultaneously achieved enhanced hydrophobicity, light absorption capacity and PCM compatibility which was the key attributed for sustainable energy storage and conversion systems. 134 Figure 6.5. (a) Microstructure of tangential plane of native wood and (b) lumen structure at large magnification. (c) Morphology of the transverse plane of native wood and (d) its cell wall at larger magnification. (e) Microstructure of lignin-containing wood in its longitudinal direction at low and (f) high magnification. Morphology of the transverse plane of treated wood at (g) lower and (i, h) larger magnification. (g) and (i) are the cross sections of TW after cutting at the surface. (h) presents the cell wall of TW without surface cut before SEM imaging. 6.3.4 Thermal energy storage performance PCM impregnation can be significantly enhanced by using lignin-containing TW which is attributed to the better miscibility of the hydrophobic wood phase and organic PCMs phase (e.g., PEG in this work). As shown in Figure 6.6a, PEG was fully impregnated into the free pores, middle lamella, and cell corners of TW, resulting in a flat and enclosed surface at the cross-135 sectional plane of the wood composite. This confirms that the lignin-containing TW prepared in this study served as an effective matrix for PEG incorporation. Interactions between PEG and the lignin-redistributed TW were mainly physical and non-covalent (H-bonding). No covalent bonds were formed between PEG and TW, as evidenced by the absence of new peaks in the FTIR spectrum (Figure 6.6b). The formation of hydrogen bonds contributed largely to stabilizing PEG to obtain PEG-wood composite, as illustrated in Figure 6.6a. The preserved wood lumina offered capillary force to entrap PEG polymers during the solid-liquid phase transition.17, 160 Additionally, the redistributed surface lignin facilitated the hydrophobic interactions with the organic oil phase. Therefore, these mechanisms effectively restricted PEG within the porous structure of the lignin-redistributed TW, ensuring excellent encapsulation efficiency and leak-proof performance.360 The rougher wood framework increased the molecular contact area, thereby constraining PEG chains near the cell wall. This resulted in higher surface adhesion energy361 and stronger interactions, preventing PEG leakage. This confirmed the produced lignin-relocated TW as an efficient encapsulation matrix and validated the successful incorporation of PEG using this treated wood. 136 Figure 6.6. (a) SEM image of cross section of wood composite and theoretical interactions between matrix and PCM. (b) FTIR spectrum of TW, PEG and wood composite. The encapsulation of PEG by using lignin-containing TW demonstrated good material integrity and stability. To ensure optimal thermal energy storage performance, the encapsulation efficiency of the composite was evaluated. Differential scanning calorimetry (DSC) analysis revealed that all PEG composites exhibited consistent thermal energy storage properties, closely matching the phase transition behavior of neat PEG (Figure 6.7a and 6.7b). For instance, PEG@TWs showed similar endothermic peaks compared to those of pure PEG during heat absorption, with a minimal deviation in melting point (Figure 6.7a and Table 6.1). The crystallization temperatures of PEG@TWs (37.1°C for PEG@TW20, 35.7°C for PEG@TW40 and 36.2°C for PEG@TW60, respectively) also gave approximately similar values to that of pure PEG during the exothermic process, indicating the stable phase change property reserved in composites (Figure 6.7b). 137 Table 6.1. Phase change properties and energy storage efficiency (F) of PEG and its composites. Sample Tm (°C) Hm (J/g) Tc (°C) Hc (J/g) F (%) PEG6000 63.6±0.9 197.4±5.7 37.6±2.0 195.4±3.4 100 PEG@TW20 62.8±1.1 133.5±3.6 37.1±0.5 134.5±4.9 67.6 PEG@TW40 62.3±1.7 154.9±7.2 35.7±0.9 154±7.1 78.5 PEG@TW60 63.1±1.5 167.5±1.1 36.2±1.6 166.9±4.3 84.8 Note: F (%) is the effective melting enthalpy content, which can be calculated as: 𝐹(%) =𝛥𝐻𝑚 of PEG@TWX𝛥𝐻𝑚 of PEG∗ 100% (Equation 6.2) Figure 6.7. DSC curves of PEG and PEG composites supported by wood with various treatment conditions during (a) heating and (b) cooling. For the performance of energy storage density (Figure 6.8a), PEG-wood composites exhibited progressively comparable latent heat storage capacity as PEG content increases, since the wood matrix itself lacked intrinsic phase change functionality. Specifically, a high PEG loading within wood substrate was desirable to enhance the thermal energy storage property (which required a 138 large pore volume of wood); while excessive PCM incorporation would lead to inevitable leakage during the melting process.160 Therefore, it's crucial to strike a balance between the high energy storage density and leakage-proof qualities for the composites performance. PEG@TWs supported by TW with increasing treatment time (i.e., more porous and higher PEG content), exhibited higher enthalpies of melting and cooling (Figures 6.7a-b and Table 6.1).262, 362 For example, PEG@TW20 demonstrated an encapsulation efficiency (F) (by Equation 6.2) of 67.6%, characterized by a melting enthalpy of 133.5 J/g and a crystallization enthalpy of 134.5 J/g, respectively. PEG@TW40 provided a much higher thermal energy storage density (F = 78.5%) with a melting enthalpy of 154.9 J/g and a crystallization enthalpy of 154 J/g. Furthermore, an encapsulation efficiency of 84.8% was attained in PEG@TW60 which went through with a more severe dissolution process (60 min) onto the supporting matrix-wood, yielding a high latent heat capacity (Hm =167.5 J/g and Hc=166.9 J/g) of thermal energy storage. This observed growing tendency of latent heat for PEG@PWs can be explained by the gradual removal of amorphous constituents (hemicellulose and lignin) and then the increased PEG volume to be incorporated in composites. However, extended treatment durations resulted in substantial material loss, compromising structural integrity and increasing the risk of PCM leakage. In this study, PEG@TW40 emerged as the most optimal stable material for efficient thermal energy storage and usage, balancing high PEG loading capacity with mechanical stability. The TGA results also confirmed that a high PEG fraction was obtained in PEG@TW composite (Figure 6.8b). Therefore, these results indicated that lignin-containing TW facilitated hydrophobic interactions with PEG's ethylene backbone, enhancing encapsulation efficiency. 139 Additionally, lignin contributed to the composite's thermal stability, ensuring robust performance under cyclic thermal conditions. Figure 6.8. (a) latent heat of heating and cooling of PEG and its composites. (b) TGA curves of PEG, treated wood and PEG@TW. 6.3.5 Thermal stability and reversibility The lignin-relocated TW demonstrated stable encapsulation capability for PEG, as evidenced by the absence of leakage during melting tests at 80°C. (Figure 6.9a). This addressed a key limitation in leakage of traditional PCMs, facilitating the broader material applicability. As had been discussed, the surface properties of the lignin-relocated TW—including capillary forces, hydrophobic interactions, and hydrogen bonding—collectively facilitated the effective adsorption of molten PEG, ensuring the formation of a form-stable PCM composite. This composite presented a stable and reversible thermal storage property over 100 cycles of heating/cooling (Figure 6.9b). There was no apparent change in thermal energy storage property that negligible loss in latent heat (5.4% for melting and 0.7% for cooling 0.7%) was noted (Figure 6.9c). These results indicated the exceptional durability of PEG@TW, highlighting it as a promising candidate for long-term thermal management applications. 140 Figure 6.9. (a) Appearance of PEG and PEG@TW before and after heating. (b) PEG@TW after 100 heating/cooling consecutive cyclic DSC test. (c) The change of thermal energy storage property after 100 cycles. 6.3.6 Photothermal energy conversion and T regulation performance To verify the superior photothermal conversion properties and heat storage performance attributed to the feature of modified structure with its conserved lignin, the light absorption capability of TW-supported PEG was compared with PCM composites incorporated into delignified wood (DW) and native wood (NW). Visually, PEG@TW exhibited a darker appearance than PEG@DW and PEG@NW, attributed to lignin fractionation and dislocation, which exposed lignin to the surface of the wood scaffold (Figure 6.10a). This enhanced π–π stacking within the lignin structure promoted non-radiative migration, thereby improving light capture efficiency.363 Observation in UV-Vis-IR spectra also confirmed the improved photonic absorption of PEG@TW that it demonstrated a higher absorption capability than the other two composites across the entire solar spectra from 200 to 2500 nm (Figure 6.10b). Additionally, an 141 apparent red shift was noted in the absorption of PEG@TW, suggesting an enhanced solar light utilization and photothermal conversion property in terms of the absorption in both visible and near-infrared reflectance bands from solar light. Then, three composites' samples were irradiated under a standard solar simulator at 2000W/m2, and the top surface temperatures of them were recorded to evaluate their photothermal conversion properties and to analyze how it correlated with different lignin content (Figure 6.10d). As expected, PEG@TW showed the fastest temperature ramp, reaching 45°C within 2 min and achieving its Tm at 60°C within 400s (Figures 6.10c and 6.10e). In contrast, PEG@DW (lignin-removed) and PEG@NW (pale-colored native wood) showed limited and slow heating, attaining 33°C and 53°C within 400 s, respectively (Figures 6.10c). Notably, composites lacking lignin fractionation failed to reach 60°C even after 35 minutes of irradiation. The lack of photonic capture features in these materials hindered their ability to reach the phase transition temperature and made them hard to initiate their intrinsic function of phase transition, resulting in insufficient latent heat storage, heat release and ineffective environmental temperature regulation when the light is turned off (Figure 6.10c). Distinctly, exposed and relocated lignin in PEG@TW enabled activated non-radiative migration (induced by π-π stacking from lignin molecular structures) and photothermal conversion17, 359, 364 (Figure 6.10d), absorbing and storing heat from solar energy effectively to regulate the temperature by releasing stored thermal energy gradually via crystallization (plateau in Figure 6.10c) when it was cooled. The ascribed performance was stable and repeatable, exhibiting steady latent heat storage and regulation features in 5 cyclic heating/cooling processes when exposed to solar simulator (Figure 6.10f). An 82% photothermal conversion efficiency was obtained by measuring the time corresponding to energy storage plateau following Equation 6.3, which guaranteed a sufficient solar energy utilization property. While native wood 142 (PEG@NW) maintained some light-trapping capacity due to its original lignin content, its efficiency was limited by inadequate pore space and insufficient heat storage from PEG from non-removal native wood composites. In contrast, the exposed lignin was able to absorb light and gave enhanced photothermal conversion property given by strong 𝜋-𝜋 stacking from the phenolic lignin structure, which assisted non-radiative migration and promote photothermal conversion.358, 359 Overall, the unique structural design of PEG@TW, applying lignin's inherent properties and optimized porosity, enabled efficient light capture and thermal storage, achieving promising solar energy conversion performance. 𝜃 =𝑚∗∆𝐻𝑆∗𝑃∗𝑡 (Equation 6.3) where m, ΔH, S and P meant weight, enthalpy of melting, surface area of PCM composites and power of the solar simulator respectively. “t” in equation represented the time spent on heat absorption plateau in T-t curves. 143 Figure 6.10 (a) Appearance of PEG encapsulated by native wood, delignified wood and treated wood in this study (scale bar=1cm). (b) UV-Vis-IR spectra of PEG@TW, PEG@DW and PEG@NW. (c) Temperature change with time for PEG@TW, PEG@DW and PEG@NW at 2 sun illumination. (d) Illustration of samples exposed under a solar simulator for solar energy absorption and conversion analysis. (e) IR images of PEG@TW during irradiation within 110s, 300s, 450s and 2000s. (f) Temperature change of PEG@TW within 5 cycles of heating/cooling when monitoring daytime and nighttime. 6.4 Conclusions In summary, this study presented the development of a phase change material (PCM) composite supported by chemically-modified wood. A simple and efficient aqueous treatment was employed to fabricate a lignin-relocated, three-dimensional porous wood matrix capable of encapsulating polyethylene glycol (PEG) for latent heat storage with excellent leakage 144 resistance. The redistributed lignin within the wood structure served as a sustainable, intrinsic photothermal converter, enabling efficient solar energy absorption, conversion, and utilization. The optimized PEG@TW composite achieved an encapsulation efficiency of 78.5% and exhibited a melting enthalpy of 154.9 J/g. By modulating the treatment duration, the thermal energy storage density of the composite could be precisely tuned. Lignin particles embedded in the wood framework facilitated hydrophobic interactions with the organic PCM, ensuring high PEG loading and enhanced storage capacity. Simultaneously, lignin contributed to increased solar absorption, achieving a photothermal conversion efficiency of 82%. The composite demonstrated excellent durability and thermal/chemical stability over 100 consecutive heating–cooling cycles, underscoring its potential as a robust candidate for long-term solar energy storage and thermal management applications. 145 Chapter 7: Conclusion and outlook 7.1 Main contributions of thesis The global transition toward sustainable energy systems calls for the development of innovative materials that combine high performance with environmental responsibility. This thesis addressed this imperative by reimagining wood, a naturally abundant, renewable, and carbon-sequestering resource, as a multifunctional platform for energy storage, conversion, and harvesting. Through systematic structural and chemical modifications, this work demonstrates how wood's inherent hierarchical architecture, tunable porosity, and biopolymer chemistry can be harnessed to create advanced materials that rival or surpass conventional synthetic alternatives. By integrating principles of bioinspired engineering, the functional wood systems developed herein reduce dependence on materials derived from fossil fuel while advancing carbon-negative sustainability goals. The key innovations presented across four core chapters are summarized below, highlighting their interconnected contributions to the field of sustainable energy technologies. Chapter 3 built on structural optimization, presenting a foam-like material derived from wood with multifunctional properties including anisotropic compressibility, thermal insulation, and flame retardancy. Through selective lignin removal and hemicellulose retention, a lamellar nanostructure was formed, enabling up to 60% compressive strain recovery, modulated by oxidation-induced changes in crystallinity. This tunable wood foam addressed the needs of 146 energy-dissipating packaging and thermal insulation, demonstrating how wood's cell wall architecture can be reengineered for mechanical and thermal functionality. Chapter 4 shifted focus to energy harvesting, detailing an all-wood triboelectric nanogenerator (TENG) that leveraged lignin relocation for rainfall energy conversion. A pretreatment strategy enabled the redistribution of lignin within the wood matrix, forming a superhydrophobic, tribo-negative surface. This redesigned interface enhanced charge transfer and powered low-voltage devices (e.g., LEDs, timers) under simulated rainfall. The system also demonstrated self-cleaning and anti-mold properties, ensuring outdoor durability. Notably, this approach eliminated the need for fluorinated polymers, exemplifying circular design by transforming lignin into a valuable functional component. Chapter 5 introduced a wood-supported phase change material (PCM) composite for passive thermal management and enhanced photothermal energy conversion. Leveraging the intrinsic anisotropy of wood, a green synthesis strategy was developed to embed polyethylene glycol (PEG) into a delignified wood framework modified with exfoliated boron nitride (BN) and polypyrrole (PPy). This hierarchical architecture markedly enhanced thermal conductivity and enabled efficient solar-to-thermal energy conversion. The resulting composite demonstrated excellent cycling stability, confirming its potential scalability for solar energy storage and directional heat regulation. Chapter 6 complemented the earlier solar thermal work by reinforcing lignin's role as an intrinsic photothermal agent in wood-PCM composites. Retaining lignin allowed for the exclusion of synthetic absorbers while maintaining high solar-to-thermal conversion efficiency. 147 Selective polysaccharide removal improved surface roughness and interfacial adhesion, enabling high PEG loading and effective heat encapsulation. This dual-function system, combining lignin's light absorption with PEG's latent heat storage, demonstrated the feasibility of closed-loop material systems that valorize wood’s native chemistry. Across the thesis, each chapter built upon the previous ones, progressing from multifunctional thermal management (Chapter 3) to energy harvesting (Chapter 4), solar thermal energy storage (Chapter 5) and fully integrated photothermal systems by a sustainable strategy (Chapter 6). This progression established a comprehensive framework for designing functional wood-based materials for energy applications. The contributions can be concluded into three main themes: • Simple fabrication: Environmentally friendly methods such as layer-by-layer assembly, lignin relocation, and vapor-phase hydrophobization minimized solvent use and chemical waste. • Multifunctionality: Materials were engineered to simultaneously address energy, mechanical, and environmental needs, as exemplified by flame-retardant foams and self-cleaning TENGs. • Circularity: The thesis advanced the valorization of lignin and hemicellulose, that components often discarded in conventional wood processing, thereby aligning with low-waste and circular bioeconomy principles. Together, these contributions position wood as a versatile, sustainable, and high-performance material platform for next-generation energy technologies. 148 7.2 Future perspectives This thesis contributed to redefining the role of wood in the energy landscape, transforming it from a traditionally passive structural material into an active, multifunctional platform. By aligning environmental sustainability with technological innovation, the developed materials offer viable pathways toward decarbonizing energy systems while promoting responsible resource use. While this work has demonstrated the feasibility of wood-based systems across a range of energy-related applications, further opportunities exist to advance their potential in green energy utilization, particularly in optimizing light management and enabling motion-initiated electricity generation. One promising direction involves leveraging the low light absorption of delignified wood, which lacks lignin's chromophoric groups, making it a suitable candidate for radiative cooling. When chemically treated, such wood can serve as a smart building material capable of regulating indoor temperatures and adaptively responding to environmental conditions. Function-specific modifications are possible: white wood for radiative cooling, black wood for photothermal conversion, and transparent wood which is an emerging substitute for traditional glass, for enhanced light transmission. Although transparent wood has been widely studied, future research should emphasize fully biobased systems, eliminating the need for synthetic polymers such as PMMA or epoxy resins. Another exciting avenue involves exploiting cellulose's intrinsic biodegradability and electrical insulation properties to create dielectric, wood-based flexible films. These films can function as tribo-positive materials in triboelectric nanogenerators (TENGs), enabling solid–solid TENG 149 systems capable of harvesting biomechanical energy, such as that generated by human motion, for portable sensing and wearable electronics. The sustainable energy applications of wood were investigated in this work, with a primary focus on highly porous species such as balsa and basswood. Their low density and uniform porosity facilitate laboratory-scale processing and homogeneous chemical reactions, making them ideal model systems for proof-of-concept demonstrations. However, a key challenge to be addressed in translating these laboratory successes into practical, scalable applications. Species such as balsa are scarce in northern regions, and their cultivation and transportation can compromise the carbon neutrality of the proposed energy systems. Future research should therefore focus on transitioning toward more abundant and regionally available high-density wood species such as oak, maple, and pine. While these woods offer greater accessibility and sustainability potential, their structural complexity and higher density pose significant processing challenges. Pretreatment strategies such as presoaking or mild chemical conditioning could improve reaction uniformity and material homogeneity; however, these additional steps also increase energy, water, and chemical consumption, potentially offsetting the environmental and economic benefits of large-scale implementation. Balancing performance enhancement with process sustainability will thus be essential in advancing wood-based energy materials toward practical and scalable applications. 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Overall, to reach a balance between mechanical compressibility and shape recovery, while keeping a high yield, the EWF8 sample (yield=72.8%, density=0.0702 g/cm3) was selected to further examine the mechanical stability and cyclability (DMA, 8 cyclic loading-unloading tests). EWF8 displayed recoverable strain and compliance profiles after 8 compression cycles, indicating a remarkable material resilience (Figure A1c-d). After the loading was released, it presented the same recoverable compliance level (blue area in Figure A1d) with the compliance under loading (yellow area), which supported a good mechanical recovery performance after 8 times of continuous loading/unloading. 181 Table A1. The compositional analysis of balsa, delignified wood and EWF after the given treatment times. Arabinan (%) Galactan (%) Xylan (%) Mannan (%) AIL (%) ASL (%) Ash (%) Cellulose (%) Hemi-cellulose (Ara+Gala+ Xyl+Man) (%) Wood 0.1 0.3 13.8 1.8 21.6 3.5 0.0 43.3 16.0 Holo-Wood 0.1 0.1 20.0 2.3 5.2 2.8 2.3 71.4 22.5 EWF4 0.0 -0.1 19.7 2.1 4.3 2.8 0.3 74.9 21.7 EWF6 0.0 -0.1 17.6 1.8 3.2 1.3 0.1 72.0 19.3 EWF8 0.0 -0.1 14.1 1.7 3.0 2.6 0.4 72.3 15.7 EWF14 0.0 -0.1 8.2 1.7 3.9 2.1 0.4 71.7 9.8 182 Figure A2. Fibrillated structure of EWF observed in its (a-d) tangential plane and (e-f) transverse plane using SEM under different magnifications. EWF is mechanically strong along axial direction showing no changes in size and shape (g) before and (h) after loading (1 kg). a b c d e f g h 183 Figure A3. Degree of oxidation of delignified wood after given treatment times. 184 Figure A4. Wettability of wood, EWF and Si-EWF within 60s with a dyed (Methyl blue) water droplet dropped on the top of their surface. The wettability in both transverse plane and tangential planes of the native wood, elastic wood foam as well as the hydrophobic Si-EWF are demonstrated in the pictures taken immediately and after 60s after the droplet applied. Transverse plane Tangential plane Wood EWF Si-EWF 0s 60s 0s 60s 185 Figure A5. EDAX images of Si-EWF scanning for (a1) Si, (a2) C and (a3) O elements in its longitudinal direction. (b) SEM of the Si-EWF and (c) elemental analysis report from EDAX. a1 a2 a3 b c 186 Figure A6. Water droplet rolling on the surface of Si-EWF in the longitudinal direction (a1) after 20 and (a2) 400 s. (b) Changes in the advancing and receding contact angle with time for Si-EWF tilted from 0o to 90o. a1 a2 b 187 Figure A7. (a) Number of hydrogen bonds of Holo-wood and EWF at the (100)-(100) and the (110)-(1-10) crystal planes calculated by molecule dynamic simulation. (b) Non-bonded interactions energy of Holo-wood and EWF in (100)-(100) and (110)-(1-10) crystal plane. Figure A8. Thermal stability of Si-EWF. a b
Thesis/Dissertation
eng
Chemical and Biological Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Rojas, Orlando J.
Sustainable energy solutions through physically and chemically modified wood
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