UBC Theses and Dissertations
Ionic diodes based on cellulose nanocrystals (CNC) and related rheological studies Keyvani, Parya
In this thesis, cellulose nanocrystals (CNC) fibers were chemically modified to produce anionic (pCNC) and cationic (nCNC) polyelectrolyte hydrogels to fabricate ionic diodes. In the beginning, the rheological behaviour of cationic and anionic CNC was studied and compared to that of pristine CNC. It was demonstrated that in the sonicated state, anionic and cationic CNC form hydrogen bonding, which notably contributes to interparticle forces and gel strengths. These structures between individual rods defeat the purpose of flocculation and ultimately leading to a more stable suspension. Moreover, enhanced rheological properties were observed in the case of nCNC in comparison with the pCNC and this may be due to the extensive formation of hydrogen bonding. In addition, the surface-modified cellulose nanocrystals were used to fabricate ionic diodes. Rectification behaviour from two oppositely charged hydrogels doped with cellulose nanocrystals with positive and negative surface charges was observed. It was found that the current−voltage characteristics of the CNC−hydrogel diode are influenced by several parameters including gel thickness, hydrogel concentration, applied voltage, and scanning frequency. Pronounced rectification ratio and high current densities in forward bias occurred as a result of the high surface area followed by a high charge density. Analyzing the experimental data, we demonstrated that unidirectional current response originated from an anisotropic distribution of counterions at the interface between the two gels doped with oppositely charged CNCs. Moreover, the physical mechanism is described quantitatively by an electrochemical model. We investigated and validated the proposed electrochemical mechanism by the Yamamoto-Doi model using experimental data. We demonstrated that the diode works via a physical mechanism that involves the electrochemical generation of hydroxyl ions and protons at the electrodes to create current. Exponential currents (J) in the forward bias were observed while J = A√(-V) in the backward bias, which is in agreement with predictions of the electrochemical model proposed by Yamamoto-Doi ¹. The results of this thesis can be directly utilized to fabricate biodegradable diodes of good, stable rectification performance. Also, this work provides insight on how to control ionic movement in ionic devices.
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