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Calcium-decorated boron-doped graphene for high-capacity hydrogen storage Beheshti Zavareh, Elham 2009-12-31

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Calcium-DecoratedBoron-Doped Graphene forHigh-Capacity HydrogenStoragebyElham Beheshti ZavarehB. Sc. (Electrical Engineering), Sharif University of Technology, 2006A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF APPLIED SCIENCEinThe Faculty of Graduate Studies(Electrical and Computer Engineering)THE UNIVERSITY OF BRITISH COLUMBIA(Vancouver)October, 2009©Elham Beheshti Zavareh 2009AbstractHydrogen has been viewed as a clean synthetic energy carrier that couldreplace fossilfuels, especially for transport applications. One bottleneck indeveloping a hydrogeneconomy is to find feasible and safe storage materialsthat can store hydrogen withhigh gravimetric and volumetric densities at ambient conditions.The U.S. department of energy has set a system target of 6 wt.% hydrogen storagedensity by 2010and 9 wt.% by 2015, which has not been met yet.In this thesis, hydrogen adsorption and storage in calcium-decoratedboron-dopedgraphene is studied by ab initio calculations usingdensity functional theory (DFT).We first consider pure graphene coated with calcium atomson both sides, supposingthat metal atoms are dispersed uniformlyon the surface with a calcium coverageof25%. We find that up to four hydrogenmolecules can bind to a Ca atom,whichresults in a storage capacity of 8.32 wt.%. Then,we address the issue of metal adsorbate clustering. Our calculations show that Ca clusteringtakes place on pristinegraphene because of the small binding energy of Cato graphene. One way to enhancethe metal adsorption strength onthe graphene plane is to dope graphenewith acceptors such as boron atoms. Weshow that upon boron doping with aconcentrationof 12 at.%, the clustering problemcould be prevented and the resultinggravimetriccapacity is 8.38 wt.% hydrogen.11ContentsAbstract iiContentsiiiList of TablesviList of FiguresviiAcknowledgmentsxDedicationxiStatement of Co-Authorshipxii1 Introduction11.1 Hydrogen fuel cell basics11.1.1 How a hydrogen fuel cell works21.1.2 Hydrogen as a future energy carrier2‘IIContents1.2 Hydrogen storage 51.2.1 Compressed gas and liquid hydrogen tanks 51.2.2 Materials-based hydrogen storage 6Hydrogen storage by metal hydrides 7Hydrogen adsorption on solids with large surface area . . .81.3 Hydrogen storage in carbon nanostructures101.3.1 The optimum conditions for adsorptive storageon carbon nanostructures131.3.2 Metal-coated carbon nanostructures131.4 Research objectives141.5 Methodology: first-principles calculations15Bibliography182 Calcium-Decorated Boron-Doped Graphenefor High-Capacity Hydrogen Storage: A First-Principles Study212.1 Introduction212.2 Computational details242.3 Results and discussion252.4 Conclusion35Bibliography36ivA6)jJOjO1fltLJSUO!SflpUOO‘A.rTumnguowooList of Tables2.1 Average adsorption energies of hydrogen molecules on Ca-decoratedpure graphene and the corresponding bond lengths for one to fouradsorbed H2 molecules per Ca atom. Both LDA (CA) and GGA(PBE) DFT-level calculation results are presented 282.2 Comparison of binding energies (in eV) and bond lengths (in A) obtained by LDA (SVWN5), OGA (PBE), and MP2 calculations on themolecular model of Coronene34viList of Figures1.1 Schematic model of a hydrogen fuel cell. At the anode side, hydrogenfuel is channeled through the cell. Then, a catalyst causes the hydrogen to split into hydrogen ions (Hj and electrons. At the next step,the electrolyte allows the hydrogen ions to pass between the anode andthe cathode. Consequently, electrons (which cannot passthrough theelectrolyte) pass through an external circuit to the cathode.At thecathode, channeled oxygen reacts with electrons and hydrogenions toform water as a waste product, which flows out of the cell31.2 Schematic model of hydrogen storage by metalhydrides. (a) First,the hydrogen molecules react with the surface ofthe metal and dissociate into separate H atoms. (b) Then, single hydrogenatoms adoptrandom locations in the metal lattice. (c) Finally, thehydrogen atomsform a regular arrangement in the lattice and make metallic, covalent,or ionic bonds with the metal8viiList of Figures1.3 Porous carbonaceous materials with high surface area. SEM imagesof the porous structure of (a) activated carbon nanotube (CNT).Reprinted with permission from [7]. Copyright © 2002 Elsevier Science Ltd. (b) foliated graphite sheets prepared upon 10 h of ultrasonic irradiation. Reprinted with permission from [8]. Copyright ©2004 Elsevier Science Ltd. (c) exfoliated graphite sheet. Reprintedwith permission from [9]. Copyright © 2001 Elsevier Science Ltd. (d)Schematic model of hydrogen storage by materialswith high porosity.In this scheme, hydrogen gas molecules accumulate atthe surface ofthe material, but they do not react chemically with thehost material. 91.4 Graphene as a 2D building material for carbonmaterials of all otherdimensionalities. It can be (a) rolled into 1D carbonnanotubes, (b)wrapped up into OD buckballs, or (c) stacked into3D graphite. . . . 122.1 (a) The optimized structure of a single Caatom adsorbed on the H siteof the (2 x 2) cell of graphene (0(2 x 2)) and thecalculated adsorptionenergy and bond length. (b) Double-sided adsorptionof Ca atoms onthe H site of graphene. (c) Single-sided adsorptionof Ca on a (4 x 4)cell of graphene (G(4 x 4)). (d) The calculatedenergy band structuresof bare and Ca-coated graphene folded to the (2x 2) cell. It is seenthat upon Ca adsorption, thelr*bands of graphene are occupied anddistorted. The Fermi energy EF is set tozero262.2 Optimized structures of Ca-decoratedpure graphene (Ca-G(2 x 2))with one to four H2 molecules, obtainedby LDA calculations. Insingle and double H2 adsorption, the adsorbedmolecules are parallelto graphene. The adsorbed 112molecules tend to tilt toward the Caatom upon adding the third and fourthhydrogen molecules28VuList of Figures2.3 The optimized structures, binding energies, bond lengths and energyband diagrams of a single Ca atom adsorbed on the H site of a (2 x 2)cell with PBC of (a) single B-doped, (b) pair B-doped, and (c) tripletB-doped configurations 302.4 Optimized structures of dimerized and separated Ca atoms adsorbedon pure (a) and B-doped (b) (4 x 4) cells of graphene. Our calculationsshow that in the B-doped structure the isolated configuration is energetically more favorable by 0.2 eV while in the pure configurationthe dimerized configuration is more stable by 0.4 eV312.5 Projected density of states (PDOS) of the Ca 3dorbitals and H2 uorbitals involved in the adsorption of hydrogen on a (a) Ca-decoratedpure graphene and (b) Ca-decorated triplet B-dopedgraphene. TheFermi energy EF is set to zero332.6 Structure of Ca attached to a pure coronenemolecule.This molecularmodel has previously been shown to provide anadequate representation of graphene for the purpose of binding energycalculations. . . . 34ixAcknowledgmentsI am most grateful to my advisor Dr. Peyman Servati for his guidance that he hasprovided throughout the duration of my M.Sc. It has been a privilege working withhim on such an interesting and challenging project.I owe particular thanks to Dr. Alireza Nojeh for the many productive meetings wehad, and whose penetrating questions taught me to question moredeeply and helpedme to accomplish this work.Thank you to all my friends and lab mates for their help and encouragement.Specialthanks are owed to my beloved parents, who have supported methroughout my yearsof education with their endless inspiration and guidance. Andfinally, I would liketo thank Soroush for his love and encouragementthat pushed me to attempt harderthan what I have ever done before.xDedicationI dedicate this work to my parents.xiStatement of Co-AuthorshipChapters 1 and 3 were written by the author. Chapter2 is a paper ready to besubmitted for publication, which is co-authored bymyself, Dr. Alireza Nojeh, andDr. Peyman Servati. I performed the simulation and analysis;I generated andprepared the figures; I wrote the first draft of thepaper, and participated in thecollaborative effort that led to the final version of thepaper.xiiChapter 1Introduction1.1 Hydrogen fuel cell basicsWith an ever increasing energy demand by the worldpopulation and limited fossilfuel resources, efficient energy generation and use have becomeurgent issues. Fuelcells may therefore play an important role in the nearfuture as power sources in thetransportation area, for portable electronic items orstationary devices for industrialand domestic use. Fuel cells convert chemical energydirectly into electrical energywith high efficiency and low emissionof pollutants. Theoretically, the maximumenergy efficiency of a fuel cell can be ashigh as 50-60%. twice as much as thatofthermal processes.The first primitive fuel cell was presentedby William Grove in 1839 [1]. He discoveredthe basic operation principle of fuel cells,which remains unchanged since thenevenin today’s modern fuel cell technology: a fuel cellis an electrochemical device thatcontinuously converts chemical energy into electricalenergy (and some heat) as longas the fuel and oxidant are supplied [2].Different types of fuel cells exist underactive development. The protonexchangemembrane fuel cell (PEMFC), also knownas polymer electrolyte membranefuel cell,is a type of fuel cell which typicallyoperates on pure hydrogenfuel. PEMFCs areconsidered to have high energy densityand quick start up time. Therefore,PEMECsare believed to be the best typeof fuel cell to eventually replacegasoline and dieselinternal combustion engines.11.1. Hydrogen fuel cell basics1.1.1 How a hydrogen fuel cell worksA whole family of fuel cells exists with different designs, but all these fuel cells workbased on the same basic principle. A schematic PEMFC and the way it operatesis illustrated in Figure 1.1. A fuel cell converts chemical energy to electrical energyand produces electricity from fuel and oxidant. The reactant fuel (i.e. hydrogen) ischanneled through the anode side of the cell and separated to electrons and protonsinthe presence of a catalyst which is typically a platinum alloy. The oxidant(usuallyoxygen from air) is channeled through the cathode side and is reducedto oxideions. In PEMFCs, fuel and oxidant react in the presenceof an electrolyte (a solidpolymer membrane) which is electronically insulating but conducts thepositivelycharged ions. Therefore, the protons from the anode side aretransported throughthe electrolyte to combine with the oxide ions at theother side and form water asa waste product. Eventually, the negativelycharged electrons (which cannot passthrough the electrolyte) must travel along an externalcircuit to the other side andgenerate electricity.Typically, the power generated by a fuel cell isbarely enough for most applications.Therefore, to produce the desired amount of electricity,individual fuel cells are combined in series or parallel circuits and form afuel cell stack. A fuel cell stack maybe comprised of a few or a huge number of single cells,which makes it ideal for awide variety of applications from laptop computers(50-100 Watts) to central powergenerations (1-200 MW or more) [3].1.1.2 Hydrogen as a future energycarrierHydrogen is the lightest element in the periodictable and its electron (valence electron) is accompanied by only one proton; thus,the hydrogen molecule has thehighestenergy gain per electron and energy-to-massratio of any chemical. The chemicalenergy per mass of hydrogen is 142 MJkg’,which is approximately three timesmore21.1. Hydrogen fuel cell basicsFigure 1.1: Schematic model of a hydrogenfuel cell. At the anode side, hydrogenfuel is channeled through the cell. Then, a catalystcauses the hydrogen to splitinto hydrogen ions (H) and electrons. At the next step,the electrolyte allows thehydrogen ions to pass between the anode and thecathode. Consequently, electrons(which cannot pass through the electrolyte)pass through an external circuit to thecathode. At the cathode, channeled oxygen reactswith electrons and hydrogen ionsto form water as a waste product, which flows outof the cell.ICatalyst31.1. Hydrogen fuel cell basicsthan that of other fuels (e.g. the equivalent value for liquid hydrocarbons is 47MJkg’) [4]. Hydrogen is a clean synthetic fuel: it is not comprised of carbon; it isnon-toxic; and its waste product by either thermal or electrochemical combustion isoniy water vapor.Hydrogen is the most abundant element on Earth, but it can barely be presentasmolecular hydrogen gas H2 (less than 1%); it combines readily with other elementssuch as oxygen or carbon. Thus, the majority of hydrogen is in I2O formandsome is in the form of hydrocarbons. In other words, the main source ofhydrogenis water, which is essentially an unlimited resource and its amountis easily morethan enough to sustain a hydrogen economy for widespread use. Theclean way toextract hydrogen from water is to use renewable energy sources suchas wind or solarpower (photovoltaic cells) or hydro-electric power. Hydrocarbons,the other sourceof hydrogen, such as methanol, ethanol, and natural gas canyield hydrogen by a fuelreforming process. Moreover, hydrogen can be extracted frombiomass technologies.The main problem with hydrogen as a fuelis that although its energy content byweight is very high, it has a very low energycontent by volume (8 MJ/liter for liquidhydrogen versus 32 MJ/liter for gasoline) [5];hydrogen is a gas at room temperatureand it takes an extremely large amount of space.Thus, to have on-board energystorage for mobile applications, hydrogen gas mustbe compressed in some way.Using today’s technology, a modern, safe carweighs 1.2-1.5 tons. This car is optimized to travel a range of 500 km burning 30-35liters of petrol in a combustionengine. A hydrogen fuel cell electric motorsystem would need about 5 kg of hydrogen to travel the same range(in case of a car fitted with a hydrogencombustionengine, 10 kg or more of hydrogen is needed).Here is the main problem: atroomtemperature and atmospheric pressure, 5 kg ofhydrogen occupies about 56,000liters,which corresponds to a balloon of 4.8 mdiameter. Consequently, using hydrogeninits gaseous form is not practical for avehicle and it is essential tofind a method tosqueeze the hydrogen volumedown by at least 1000 times [6].41.2. Hydrogen storage1.2 Hydrogen storageOne of the main challenges in developing fuel cell technology for mobile applicationsis to find a compact. light-weight hydrogen storage system. A desirable hydrogenstorage system needs to be capable of delivering hydrogen gas to a fuel cell at approximately room temperature and at a pressure not much greater than atmosphericpressure (1 bar). In 2002, the U.S. Department of Energy (DOE) set systemtargetsfor the hydrogen content of storage media: the DOE target is 6% by mass (or 6 wt.%,i.e. at least 6% of the storage system weight is usable H2 weight) by 2010,and then9 wt.% by 2015. So far, the targets have not been metwhile intensive research hasbeen conducted in this area.Hydrogen can be stored in different forms. Thegoal is to pack hydrogen gas asdensely as possible, i.e. to reach the highest volumetricdensity by using as littleadditional material as possible. Besides conventionalstorage methods, i.e. highpressure gas tanks and liquid hydrogen, hydrogencan be reversibly absorbed oradsorbed by certain solid materials, greatly reducingits volume. Materials-basedhydrogen storage includes physisorption of hydrogenon solids with a high specificsurface area and hydrogen intercalation in metalsand complex hydrides.1.2.1 Compressed gas and liquidhydrogen tanksHydrogen can be stored at high pressure or inliquefied form to reduce the volume.The energy density of hydrogen gas can beimproved by compressing hydrogenandstoring it at higher pressures. Classical high-pressuretanks made of steel can takeup to 300 bar (30 MPa)and be filled up to 200 bar.However this is still not apractical solution for on-boardhydrogen storage; to store 5 kg of hydrogen itrequiresa volume of 280 liters. Thepressure of the tank can be ashigh as 600 bar by newhigh-pressure tanks, which are made ofcarbon-fiber-reinforced materials. Toprevent‘A hydride is a chemical compound inwhich hydrogen is combined withanother element.51.2. Hydrogen storagethe reaction between hydrogen and polymer, another approach is to use hydrogen-inert aluminum tanks, which are strengthened with external carbon-fiber coating.These novel containers can store hydrogen up to 4 wt.% [4].Using high pressure tanks has significant disadvantages: hydrogen cannot be usedin such a high pressure for the fuel cell and it is thus essential to have additionalpressure control. Furthermore, both compressing hydrogen and storing it at highpressure are highly dangerous and safety issues would become a serious concern.Condensing into liquid hydrogen results in hydrogen in an extremelyenergy-denseform. But the condensation temperature of hydrogen at atmospheric pressure(1bar) is -250 °C [6]. Therefore, the energy requirement for hydrogen liquefactionishigh; it results in a loss of about 30% of the generated energy for liquefaction [5].Additionally, the required cryogenic techniques are not feasible forordinary fuelstations.1.2.2 Materials-based hydrogen storageHydrogen molecules or atoms bound with otherelements in potential storage materials make it possible to have a storage mediumwith larger capacities (i.e. largerquantities of hydrogen in smaller volumes) at low pressureand near room temperature. There are two main classes of hydrogen storage inmaterials, correspondingto two different sorption mechanisms:physisorption and chemisorption. Whereas inphysisorption hydrogen molecules are weaklyadsorbed by the material andstoredon its surface, in chemisorption hydrogen moleculesare dissociated into hydrogenatoms and absorbed into the bulk materialthrough a chemical reaction.In material-based hydrogen storage, hydrogenneeds to be released by heat andlowering the pressure. Consequently, an efficienthydrogen storage medium has tobereversible, i.e. the uptake and release ofhydrogen need to take place at thepressure and temperature range attractive for mobilestorage (1-10 bar, 0-10000)[4].61.2. Hydrogen storageMoreover, having a high gravimetric capacity (wt.%) of hydrogen uptake is the mostcritical challenge for different media.Hydrogen storage by metal hydridesCertain metals and alloys (and also other chemical compounds) arecapable of reacting with hydrogen (chemisorption) and forming hydrides. This reactionresults in afast and reversible absorption of a large amount of hydrogen moleculesat practicallyaccessible temperatures. Figure 1.2 shows the stepsof hydride formation: H2 is dissociated at the surface of the metal; then, hydrogenatoms adopt random locationsin the host metal lattice; finally, hydrogen atoms makemetallic, covalent, or ionicbonds with the material in a regular arrangement.The metal lattice expands duringthis process and mostly loses its high symmetry.Many different hydrides of either elemental metals(e.g. Pd and Mg) or compoundshave been studied as appropriate hydrogenstorage materials. For instance,alloys derived from LaNi5 show some verypromising properties formingLaNi5H6.Hydrogen-volume density in mostof these materials is far abovethat of liquefiedhydrogen. However, because the hostmaterials are large elements theamount ofhydrogen stored in these materialshas not yet exceeded 4 wt.%; theproportion ofhydrogen in LaNi5H6is too low (about 1.3wt.%) and it is not practical for mobileapplications.71.2. Hydrogen storage•,j.j1,..,e•e:•J•.. .,.a b cFigure 1.2: Schematic model of hydrogen storage bymetal hydrides. (a) First, thehydrogen molecules react with the surface of the metal and dissociateinto separateH atoms. (b) Then, single hydrogen atoms adoptrandom locations in the metallattice. (c) Finally, the hydrogen atoms form a regulararrangement in the latticeand make metallic, covalent, or ionic bonds withthe metal.Many promising new ideas are being studiedwith the goal of increasing the hydrogenmass density in hydrides. New familiesof alloys are being explored based onsomemetals such as vanadium and titanium [4]. One approachis to achieve higher massdensity by using light metal elementssuch as calcium and magnesium. Butthereaction of forming hydrides with theselighter elements is extremelylow and isreversible only at very high temperatures.Hydrogen adsorption on solidswith large surface areaHydrogen can be adsorbed on the surface ofsolids via weak van derWaals interaction(physisorption). In the physisorptionscheme, light porous materialswith very large81.2. Hydrogen storagesurface area are potential candidates for hydrogen storage applications(Figure 1.3).In this scheme, it is desirable to make use of lightelements with porous structuresfrom the first row of the periodic table. Among theseelements, there has beenconsiderable interest in the possible use of carbon asthe hydrogen storage material.Figure 1.3: Porous carbonaceous materialswith high surface area. SEMimages ofthe porous structure of(a) activated carbon nanotube(CNT). Reprinted with permission from [7]. Copyright © 2002Elsevier Science Ltd. (b) foliatedgraphite sheetsprepared upon 10 h of ultrasonicirradiation. Reprinted withpermission from [8].Copyright © 2004 Elsevier ScienceLtd. (c) exfoliatedgraphite sheet. Reprintedwith permission from [9].Copyright © 2001 Elsevier ScienceLtd. (d) Schematicmodel of hydrogen storage bymaterials with high porosity.In this scheme, hydrogen gas molecules accumulateat the surface of thematerial, but they do notreactchemically with the hostmaterial.91.3. Hydrogen storage in carbon nanostructuresRecent developments of carbonaceous material synthesis have resultedin severalnew forms of carbon nanostructures such as fullerenes, carbon nanotubes, carbonnanofibers, and graphene sheets. There are hypotheses that thesecarbon nanomaterials may have extraordinarily high hydrogen storage capacities due totheir highsurface area and porous structure [10]. In the next section, wewill discuss differentcarbon nanostructures as the potential hydrogenstorage medium.1.3 Hydrogen storage in carbonnanostructuresThere are several different types of carbonmaterials with potential for hydrogenstorage. Early work in this area focused onthe hydrogen adsorption propertiesof activated carbon. Activated carbon (AC) is asynthetic carbon containing verysmall graphite crystallites and amorphous carbonwhich has been processed to makeit extremely porous. Thus, activated carboncould have a very large specific surfacearea2of up to 3000 m2g’ ,with pore diametersof less than 1 nm [11]. The hydrogenstorage properties of activated carbonshave been extensively studied.However,AC is not an efficient hydrogen storagemedium as only a small fractionof thepores are small enough to interactstrongly with hydrogen moleculesat ambientconditions. More than 50% of the total porevolume of AC are macroporeswhich arenot useful for ambient temperature hydrogenstorage[121.Thus conventional carbonadsorbents exhibit poor hydrogen uptakes,which has motivated thedevelopment ofnovel carbon nanomaterialswith high surface area and appropriatepore size thatare more specifically designed for hydrogenstorage.Graphite nanofibers (GNF)and multi-walled and single-walledcarbon nanotubes(MWNT and SWNT, respectively) areexamples of engineered carbonmaterials thathave recently been investigatedfor hydrogen storage. For the firsttime, in 1997Dillon et al. [13] reported excellent hydrogenstorage properties of sootscontainingSWNTs. The hydrogen storage capacitywas estimated to beabout 5 to 10 wt.% at2surface area is a property of solidsmeasured as the total surfacearea per unit of mass.101.3. Hydrogen storage in carbon nanostructures133 K and 40 kPa. Chambers et al. [14] (1998) reported their findings on various carbon nanostructures. They observed that at 11 MPa and room temperature, GNFswith herringbone structure could uptake hydrogen up to 67 wt.%. This extraordinarily high value caused an avalanche of research on hydrogen storage in CNTs.However, until today, nobody has been able to reproduce their results [15]. In anycase, all the reports with promising results for hydrogen uptake in GNF and CNTpoint to the necessity of high pressure or subambient temperature (or both).In contrast to these promising reports, Ritschel et al. [16] (2002) found a limited hydrogen uptake (less than 1 wt%) in different forms of carbon nanostructures (SWNT,MWNT and GNF). In agreement with their results, Hirscher et al. [17] found a lowhydrogen storage capacity in all forms of carbon nanostructures. They studied hydrogen storage on SWNT, MWNT, and GNF structures and found a small reversiblehydrogen uptake only for SWNTs. In their report in 2005 [18], theyobserved thatdifferent carbonaceous materials with different structures show similaradsorptionproperties. Moreover, a linear relation between hydrogen uptake and specificsurfacearea is obtained for all samples, which is independent of the nature andstructure ofthe carbon material. The best material with a specific surface areaof 2560 m2g’shows a storage capacity of 4.5 wt% at 77 K.As far as the specific surface area is concerned, a single layer ofgraphene may besuperior to other carbon nanostructures because both of itssides can be readilyutilized for hydrogen adsorption.Graphene is a single planar sheet of sp2 bondedcarbon atoms packed into a two-dimensional (2D) honeycomb lattice, which isidentical to one isolated layer of thegraphite structure. Graphene can be wrapped up intoOD fullerenes, rolled into 1Dnanotubes or stacked into 3D graphite (Figure 1.4).For more than 70 years, graphenehas been studied extensively in theory for describingdifferent carbon materials. However, an isolated graphene layer was assumed to beunstable, leading to the formationof stable curved structures such ascarbon soot, fullerenes and nanotubes. Veryrecently (in 2004), Novoselov et al. [19] found a wayto isolate graphene by peeling it111.3. Hydrogen storage in carbon nanostructuresoff from graphite with Scotch tape. Using the density of graphite (p = 2267 kgm3)and assuming that all the atoms are in one single plane the maximum specific surfacearea of graphene isS1=1315 m2g1,or in case of considering both sides of graphene,S2=2630 m2g’. Therefore, graphene has a high specific surface area compared tothe outer surface area of nanotubes [15].Figure 1.4: Graphene as a 2D building materialfor carbon materials of all otherdimensionalities. It can be (a) rolled into1D carbon nanotubes, (b) wrapped upinto OD buckballs, or (c) stacked into 3Dgraphite.Ib121.3. Hydrogen storage in carbon nanostructures1.3.1 The optimum conditions for adsorptive storage oncarbon nariostructuresStorage materials should be able to reversibly adsorb/desorb hydrogen moleculesin a desirable range of temperature and pressure. The affinity of hydrogen to theadsorbent material should be strong enough to store a large amount of hydrogen gasat the charging pressure (about 30 bar), but at the same time weak enough to releasemost of the hydrogen at the discharge pressure (about 1.5 bar). Recently, Bhatia etal.[201have examined the optimum thermodynamic conditions for hydrogen storage.Based on their studies, the optimal adsorption energy for hydrogen should be in therange of 0.2-0.4 eV/H2 at room temperature. This range of binding energy is theintermediate between physisorbed and chemisorbed states.For carbon in any form, binding energies are too low for storing hydrogen at ambienttemperature. Both experimental and theoretical studies indicate that the interactionbetween hydrogen molecules and the carbon nanostructure is due tophysisorption.Brown et al. [21] showed experimentally that hydrogen physisorbson the curvedexterior surface of SWNTs with the binding energy of 6-40 meV at25 K. Also, thehydrogen binding energy is calculated to be in the range of 30-60 meVon SWNTs andgraphene sheets by theoretical studies [22, 23, 24, 25].Therefore, it is essential to finda way to enhance the binding energy on the carbonnanostructure surface. In recentyears, many studies have been conducted on functionalizedcarbon nanostructuressuch as metal-coated structures.1.3.2 Metal-coated carbon nanostructuresFor hydrogen storage, pristine carbon nanostructuresare chemically too inert to makestrong enough binding with hydrogen molecules atroom temperature. One approachto increase their chemical activity is using metaldecorated structures by adding alkali metals or transition metals (TMs) as thecoating material. In 1999, Chen et3Alkali metals constitute the first group of periodic table.131.4. Research objectivesal. [26] reported that alkali-metal-doped (lithium- or potassium-doped) carbon nanotubes are capable of high hydrogen uptake. However, the binding sites for thesemetals are still too weak. In recent years, light transition metals such as titaniumand scandium have attracted much attention as the coating material. It has beenshown theoretically that TM-coated fullerenes and nanotubes can store up to 8 wt.%hydrogen with a binding energy of 0.3 eV/H2 [27, 28]. In these studies, TM atomswere assumed to be uniformly distributed on the surface. However, TM atoms tendto form clusters on the surface of carbon nanostructures and it is thus verydifficultto achieve uniformly coated monolayers of TM atoms experimentally[29]. Consequently, the hydrogen storage capacity drops dramatically. Moreover, TM-coatedsystems are highly reactive, which leads to the dissociationof the first adsorbedhydrogen molecule.In 2008, Yoon et al. [30] found using theoreticalcalculations that the clustering canbe prevented in Ca-coated C60 systems. Calcium isan alkaline-earth metal, betweenthe too weak alkali and the too reactive transitionmetals, which has empty d orbitals.The strong binding of Ca results from the chargetransfer mechanism involving itsempty 3d orbitals. The Ca atom donates part of itscharge to C atoms, which givesrise to an electric field between the Ca atom andthe substrate. Due to this field,part of the donated charge is ba& donated to the empty 3dorbitals of the Ca atom.This charge redistribution mechanism enhancesthe polarization of the H2 moleculesand thus the binding energy of hydrogen is increased.1.4 Research objectivesThe objective of this research is to find thehydrogen storage capacity of calcium-decorated graphene using first-principlescalculations. In this work, we firstaddresscritical questions in determining the storagecapacity of a calcium-decoratedstructure: (i) What is the effective coverage ofcalcium atoms adsorbed on graphenethatcan be attained for single-sided or double-sidedadsorption? (ii) How many hydro141.5. Methodology: first-principles calculationsgen molecules can be adsorbed on each calcium atom? and (iii) What is the averagebinding energy of hydrogen?Next, we address the important issue of calcium stability on the graphene plane.In contrast to previous studies [31], we show that for dense coverage of calcium ongraphene, calcium aggregation takes place, which decreases the storage capacityofthe system. The next question that we address is how to prevent calcium clusteringon graphene.Yang et al. have recently (in February 2009) investigated the stability of calciumadsorbates on carbon nanotubes and defective graphene. In their theoreticalstudies,they considered graphene with pentagonal and octagonal defects.They found thatcalcium is stable on defective graphene. Later (in September 2009),Lee et al. [32]investigated calcium-decorated carbon nanotubes for hydrogen storage.They suggested that upon boron doping the calcium clustering problem couldbe prevented.They also showed that individual Ca-decorated B-doped carbonnanotubes with aconcentration of ‘-S-’ 6 at.% B doping can reach a gravimetriccapacity of 5 wt.%hydrogen. Here, we consider boron-doped graphene with differentconcentrations ofboron atoms and study the optimal doping concentrationfor a stable decoration ofCa on graphene.1.5 Methodology: first-principlescalculationsOur first-principles calculations were mainlyconducted at the density functionaltheory (DFT) level. The DFT calculations werecarried out by using SIESTA [33]andGaussian 03 [34] simulation packages. The detailsand parameters used in calculations are mentioned in chapter 2.SIESTA is based on DFT, making availableboth local density and generalizedgradient functionals. In SIESTA, coreelectrons are replaced bynorm-conservingpseudopotentials, which are factorized in theKleinman-Bylander nonlocalform [35].151.5. Methodology: first-principles calculationsThe pseudopotentials were constructed using the Troullier and Martins parametrization [36] to describe the ion-electron interaction. The one particle problem is thensolved using a linear combination of atomic orbitals (LCAO) basis set. The basisfunctions are flexible both in the radial shape of the orbitals and the sizeof the basis,but they go to zero beyond a certain radius. We have selected adouble-c singly polarized (DZP) basis set for all the species and also added diffusefunctions for C atomsto produce the band structure of graphene more accurately.Forces on the atoms andthe stress tensor are obtained from the Hellmann-Feynman theoremand are used forstructure relaxations. Systems are treated in a supercell scheme,which uses threelattice vectors to describe the periodicity. The exchange andcorrelation potentialwas treated using both the local density (LDA) and generalizedgradient (GGA) approximation functionals. The Ceperely-Alder [36]and Perdew-Burke-Ernzerhof [37]parametrizations were used for LDA and GGAfunctionals, respectively.First, the optimized atomic positions of different periodicstructures were obtainedby relaxing them using the conjugategradient algorithm. Then we calculatedthebinding energies of metal atoms and hydrogenmolecules as follows:= EAB — (EA + EB)(1.1)In equation 1.1, all energies were calculatedfor the corresponding optimizedstructures. Hydrogen adsorption on the substratesin this study mainly originatesfromvan der Waals (vdW) interactions. The weak vdWinteraction is usually evaluated byusing the second-order perturbation theory.Therefore, we also benchmarkedour resuits with second-order Moller—Plessetperturbation theory (MP2) [38]and checkedthe reliability of our DFTcalculations. We have performed DFTand MP2 calculations using Gaussian 03 [34] simulation packagefor a molecular model of coroneneFor our calculations within Gaussian 03, Weused both a largebasis set and theintensive convergence criterion (i0eV for total energy) toobtain accurate results.4Coronene (C24H12)is a hydrocarbon consistingof six benzene rings pa.ssivatedwith H atoms161.5. Methodology: first-principles calculationsThe structural optimizations and binding energy calculations were performed usingdouble-zeta polarized 6-31G(d,p) and triple-zeta polarized 6-311G(d,p) basis sets,respectively. In our DFT calculations in Gaussian, GGA was described by the PBEfunctional. Also, LDA is described by the Slater exchange plus Vosko-Wilk-Nusaircorrelation (SVWN5) [39], which fits the Ceperely-Alder solution to thatof theuniform electron gas [40].17Bibliography[1] Grove, W. R. Phil. 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Rev.B 2009, 80, 115412.[33] Soler, J. M.; Artacho. E.; Gale, J. D.; Garcia,A.; Junquera, J.; Ordejón, P.;Sanchez-Portal, D. J. Phys.: Condens. Matter 2002, 14, 2745—2779.[34] Frisch, M. J. et al. Gaussian 03, Revision C.0f4Gaussian, Inc., Wallingford,CT, 2004.[35] Kleinman, L.; Bylander, D. M. Phys. Rev. Lett.1982, 48, 1425—1428.[36] Troullier, N.; Martins, J. L. Phys. Rev. B 1991, 43,1993—2006.[37] Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev.Lett. 1996, 77, 3865—3868.[38] Moller, C.; Plesset, M. S. Phys.Rev. 1934, 46, 618—622.[39] Vosko, S. H.; Wilk, L.;Nusair, M. Can. J. Phys. 1980, 58, 1200.[40] Ceperley, D. M.; Alder,B. J. Phys. Rev. Lett. 1980, 45, 566—569.20Chapter 2Calcium-Decorated Boron-DopedGraphene for High—CapacityHydrogen Storage: AFirst-Principles Study2.1 IntroductionHydrogen has been viewed as a clean energy carrier that could replace fossil fuels,particularly for transport applications such as hydrogen fuel cell vehicles. [1] One ofthe main challenges in developing hydrogen fuel cell technology formobile applications is to find a compact, safe and affordable storage system. A desirable systemwould be capable of storing hydrogen with high gravimetric and volumetricdensitiesat near room temperature and ambient pressure. The U.S.Department of Energy hasset a system target of 6 wt.% hydrogen storagedensity by 2010 and 9 wt.% by 2015.Furthermore, hydrogen recycling (hydrogen adsorption and desorption)should beperformed reversibly in a feasible range of temperature and pressure,which requiresthe optimal hydrogen adsorption energy of 0.2-0.4 eV/H2 [2, 3J.This range of energy is intermediate between the physisorbed and chemisorbed states.Many differentA version of this chapter will be submitted for publication.Beheshti-Zavareh, E., Nojeh, A.,and Servati, P. Calcium-Decorated Boron-Doped Graphene for High-CapacityHydrogen Storage:A First-Principles Study.212.1. Introductiontechniques such as using metal hydrides, liquefied hydrogen or high pressure tankshave been investigated for hydrogen storage, nevertheless, these techniques have several drawbacks such as low capacity, non-reversibility or safety problems. Recently,carbon nanomaterials with high specific surface area have been widely studied forhydrogen storage [4, 5, 6]. However, for carbon in any form, the interaction with H2is through weak van der Waals forces [7, 8, 9]. Therefore, binding energies are toolow for storing hydrogen at ambient conditions and it is essential to find a way toenhance the binding energy on the carbon nanostructure surface.In recent years, many studies have been devoted to functionalizing the surface of carbon nanostructures with transition metals (TMs) [10, 11, 12, 13] to provide possiblesystems for hydrogen storage applications. A TM atom interacts with the carbonsubstrate and hydrogen molecules through Dewar [14] and Kubas [15] interactions,respectively. The combination of these two interactions, in which the TM’s 3d orbitals are involved, enhances the binding energy of hydrogen to TM atoms. Yildirimet al. [11] and Zhao et al. [12] showed that TM-coated fullerenes and nanotubes canstore up to 8 wt.% hydrogen with the average binding energy of 0.3 eV/H2.In thesestudies, TM atoms were assumed to be uniformly distributed on the surface. However, because of their large cohesive energy, TM atoms tend tocluster on the surfaceof carbon nanostructures and it is thus difficult to achieveuniformly-coated mono-layers of TM atoms experimentally. Consequently, the achievable hydrogenstoragecapacity is low [10, 16, 17]. Moreover, TM-coated systems arehighly reactive, whichleads to the dissociation of the first adsorbed hydrogenmolecule [12].One possible way to have stable adsorbates withoutclustering tendency is to usemetals with relatively smaller cohesive energy such as alkali [18, 19] (AM) oralkalineearth metals (AEM). Whereas alkali atoms can be coateduniformly, the bindingsites for these systems are too weak and the nature ofthe bonding remains physisorption [20]. Calcium, the first AEM. atom with empty 3dorbitals, has recentlyemerged as a superior element to improve the storage capacityin carbon nanomaterials [21, 22, 23, 24]. Yoon et al. [21] have suggested Ca-coated C60fullerenes (Ca32C60)for high capacity hydrogen storage (2.7 H2 per Ca, whichresults in 8.4 wt.% capac222.1. Introductionity). Using density functional theory, they have shown that among AEM atoms, Beand Mg cannot be stabilized on C60 whereas Ca can decorate these materials throughthe electron donation/back-donation mechanism, which involves Ca’s empty 3d orbitals [21]. This mechanism stabilizes the chemical interaction between Ca and thecarbon nanomaterial. Moreover, Ca could bind with hydrogen more strongly thanthe AM atoms or earlier AEM atoms (i.e. Be and Mg) due to the hybridization of its3d orbitals with hydrogen’s a orbitals. However, structural stability is still an issuefor Ca-coated carbon nanomaterials. In fact, Ca has less tendency of clustering compared with TM atoms due to its smaller cohesive energy(1.8 eV) [25]. Nevertheless,Ca atoms aggregation takes place on graphitic materials as Ca’s binding energy onthe carbon absorbent is still lower than that of Ca bulk structure [23, 26].In this letter, we investigate hydrogen adsorption on calcium-decorated boron-dopedgraphene using first-principles calculations. Graphene, a monolayer of graphite. is aprecursor to C60 and carbon nanotubes and has been recently synthesized, showingunusual electronic and magnetic properties [27]. Using both sides of the plane,graphene has a high specific surface area (S=2630 m2g’), much higher than theouter surface area of nanotubes [28]. Assuming individually dispersed Caatoms ongraphene, we have shown that Ca atoms bonding to both sidesof a graphene planecan uptake 4 hydrogen molecules each, which resultsin a storage capacity of 8.32wt.% for a dense coverage of Ca on a (2x 2) cell.To prevent metal aggregation and achieve a high-capacitystorage material, it isessential to enhance the adsorption energy of Ca on graphene byintroducing defectsor doping. Yang et al. [25] investigated the effect ofsurface configuration on thestability of Ca adsorbates and found that Ca atomsare stable on graphene withpentagonal or octagonal defects. Here, instead of consideringdefective graphene, weexplore boron-doped graphene as a solution for theclustering problem. The emptyPzorbital of boron acts as a strong charge acceptor andconsequently the strength ofCa adsorption on graphene could be enhanced.Boron could be easily incorporatedin the hexagonal structure of graphene withoutchanging the structure drasticallyand boron-doped graphene has been synthesizedexperimentally [29, 30].232.2. Computational details2.2 Computational detailsOur ab initio calculations were performed using density functional theory (DFT) andthe SIESTA software package [31]. The self-consistent DFT code is based on a numerical atomic basis set and the core electrons are replaced by norm-conserving pseudopotentials in their fully nonlocal form [32]. The pseudopotentials were constructedusing the Troullier and Martins parametrization [33] to describe the ion-electroninteraction. The exchange and correlation potential was treated using the localdensityapproximation (LDA) with Perdew-Zunger parametrization [34]. Furthermore,wecompared the LDA results with those of the generalized gradient approximation(GGA) with the Perdew-Burke-Ernzerhof [35] (PBE) functional. The useof any ofthese approximations with DFT calculations to describe van der Waalscontributionsis still controversial. However, previous studies showed that GGAunderestimates therelatively weak binding energies whereas LDA overestimatesthe interaction [36]. Toevaluate the reliability of our LDA and GGA calculations, wealso performed second-order Moller—Plesset (MP2) perturbation theory calculations [37]as will be detailedlater. We observed that LDA gives results very close tothose of MP2 for bothenergies and structures.The pseudopotential for Ca is constructed using apartial core correction [38], whichaccounts for the nonlinear interaction of the coreand valence densities in the calculations. We selected a double- singlypolarized (DZP) basis set and toobtain awell balanced basis, we used atomic orbitalswith the fixed common energyshift of50 ineV for all the species. We alsoadded a diffuse 3s orbital for C to producetheband structure of graphene with the descriptionof the electronic spectrum abovethe Fermi level the same as in planewave methods [39]. Moreover, for Ca,the 3psemicore state was included together with thevalence states.The charge density was projected onto a real-spacegrid with an equivalent kinetic energy cutoff of 2D0 Ry, which provideswell-converged results equivalent toplane wavecalculations. Adsorption of the calciumatoms and hydrogen moleculesis treatedwithin the supercell geometry with latticeparameters asc=bsc=4.92 A and CSG=25242.3. Results and discussionA for a (2 x 2) cell of graphene (asc=bsc=9.85 A for a (4 x 4) cell). The distancebetween the graphene plane and its images (cSc) is large enough to avoid any interactions between them. According to the Monkhorst—Pack scheme,[401the Brillouinzone was sampled by (31 x 31 x 1) and (15 x 15 x 1) special mesh points in K spacefor (2 x 2) and (4 x 4) graphene cells, respectively. Full structural optimizationswere carried out using the conjugate gradient (CC) algorithm until the maximumresidual forces were less than 0.02 eV/A and the total pressure of the system wassmaller than 0.1 kbar per unit cell. The convergence criterion for energy was chosenas 10 eV between CC steps.2.3 Results and discussionWe first consider pure graphene coated with Ca atoms. supposingthat metal atomsare dispersed uniformly on the surface. As illustrated inFigure 2.1, (2 x 2) and (4 x 4)cells of graphene with periodic boundary conditions are consideredto compare denseand sparse coverage of Ca on the surface, respectively. In a(2 x 2) cell (Figure 2. la),the average Ca-Ca distance is 4.96 A, which is largerthan that of bulk Ca (3.82A [25]) but still the Ca atoms have some interactions.However, a large distance of9.85 A in a (4 x 4) cell results in a negligibleinteraction between Ca atoms. Ourcalculations show that the denser Ca coverage in a (2x 2) cell is energetically morefavorable and stable than a (4 x 4) cell (see Eb values in—c) due to Ca’stendency to aggregate. We also investigate differentsites of adsorption and find thatthe H site (namely the hollow site above the centerof the hexagon) is the mostfavorable site for both single and double-sidedadsorption. The Ca atom adsorbedon the H site has a binding energy (Es) of 1.09and 1.2 eV for single and double-sidedcoverage, respectively. In the double-sided configuration,the binding energy of thesecond Ca atom is more than that of single-sidedadsorption by 0.11 eV, which is dueto the electric field produced bythe first Ca atom. Our results areconsistent withearlier research on Ca-decorated carbon nanostructures;Yoon et al. [21] found thatCa and Sr bind strongly on top of a hexagonalring of a 060 fullerene with a binding252.3. Results and discussionstrength of 1.3 eV, which is roughly half of that of Ti. By DFT computations,Yang et al. [25] showed that the binding energies of Ca adsorbates with the coverageof 50% on the (5,0) and (4,0) nanotubes are 1.62 and 1.94 eV, respectively. They alsofound that Ca binding energy on the nanotubes with the same chirality decreases asthe diameter of the tube increases (e.g. for the (7,0) tube, Eb is 1.32 eV).d2.14A(c)d=2.17AEb=1.2OeV Eb=1.OIeV.Figure 2.1: (a) The optimized structure of asingle Ca atom adsorbed on the Hsiteof the (2 x 2) cell of graphene (G(2 x 2))and the calculated adsorption energyandbond length. (b) Double-sided adsorptionof Ca atoms on the H siteof graphene.(c) Single-sided adsorption of Ca ona (4 x 4) cell of graphene (G(4x 4)). (d)The calculated energy bandstructures of bare and Ca-coated graphenefolded tothe (2 x 2) cell. It is seen that uponCa adsorption, thelr*bands of graphene areoccupied and distorted. The Fermi energy EFis set to zero.Similar to the binding mechanismof Ca on C60, Ca donatesits 4s electrons to(a)d=2.18A(b)Eb= 1.09eV2Ca-G(2x2)H siteCa-G(2x2)H site(d)>VCa-G(4x4).Hsite Ca•M r KMG(2x2)M r KMCa-G(2x2)262.3. Results and discussiongraphene easily due to its relatively low ionization potential, and the donated electrons partially fill the carbon7*states. As the Ca atom is brought close to thesurface, due to the formation of an electric field between Ca and the graphene plane,C atoms back-donate part of their received electrons to the available empty 3d orbitals, resulting in a strong hybridization between the carbonlr*and calcium d states.The resulting charge transfer (i.e. positive charge on the Ca atom), calculated byMulliken population analysis, is ‘-.-‘ 0.36 and ‘-.-‘ 0.40 electrons for singleand double—sided coating, respectively. The calculated band structures of bareand Ca-coatedgraphene show that carbon’s emptylr*bonds in graphene accommodate the transfered charge and accordingly get distorted (Figure 2.ld). The occupied7r*bondsbring up the Fermi energy level and consequently semimetallic graphenebecomesmetalized.Then, the double-sided adsorption of hydrogen molecules by theCa-coated (2 x 2)cell of graphene with periodic boundary conditionsis studied (Figure 2.2). Ourcalculations show that the resulting electric fieldbetween Ca and graphene is strongenough to attract up to 4 H2 molecules per Ca atom.When we add the fifth 112molecule, it cannot make a strong enough bondwith the Ca atom. The calculatedbinding energies of hydrogen molecules on grapheneare summarized in Table 2.1showing both LDA and GGA results, where Eb =EH2_hQSt-(EH+Eh08t).Here, allenergies are calculated for the corresponding optimizedstructures. The results showthat hydrogen adsorbs on Ca-coated graphenewith an average binding energyof0.43 eV and ‘- 0.16 eV per hydrogenaccording to LDA and GGA, respectively.Overall trends in the GGA calculations are found tobe similar to those obtainedusing LDA while the adsorption energies of H2molecules to the Ca atom are abouthalf of the LDA values. These results are in goodagreement withRef[211,wherethe calculated average H2 binding energy onGaG60 is 0.2 eV/H2according to GCAand 0.4 eV/H2 if LDA is employed. Theoptimized physical quantities showninTable 2.1, such as hydrogen binding energyand H-Ca bond length, do notchangesignificantly as the number of 112 moleculesincrease, unlike in early transitionmetal(i.e. Sc, Ti, and V) coatings, wherethe first hydrogen molecule isdissociated due to272.3. Results and discussionits strong interaction with the TM atom [11, 12]. Such optimal molecular hydrogenbinding energies in case of Ca make hydrogen adsorption and desorption feasible atambient conditions for all of the adsorbed hydrogens. Similar results were obtainedby Ataca et al. [24]. Adsorption of four hydrogen molecules on each Ca atomin the(2 x 2) cell of graphene results in a hydrogen storage capacity of 8.32 wt.%..4••• •••• IFigure 22: Optimized structures of Ca-decorated pure graphene(Ca-G(2 x 2)) withone to four 112 molecules, obtained by LDA calculations. Insingle and double H2adsorption, the adsorbed molecules are parallel tographene. The adsorbed H2molecules tend to tilt toward the Ca atom uponadding the third and fourth hydrogen molecules.Eb(eV/H2) d(H-Ca) (A)d(H-H) (A)LDA GGA LDA OGALDA GGACaG(2 x 2)H2 0.254 0.0752.29 2.42 0.81 0.83CaG(2 x 2)(H2) 0.413 0.1422.28 2.38 0.84 0.82CaG(2 x 2)(H2)3 0.609 0.2702.21 2.27 0.87 0.84CaG(2 x 2)(H2)4 0.456 0.1642.22 2.29 0.86 0.83Table 2.1: Average adsorption energiesof hydrogen molecules on Ca-decoratedpuregraphene and the corresponding bond lengths for oneto four adsorbed H2 moleculesper Ca atom. Both LDA (CA) andGGA (PBE) DFT-level calculationresults arepresented.In all the above calculations, Ca atomsare assumed to be uniformlydistributed282.3. Results and discussionon the graphene plane. However, the calculated adsorption energy of single-sided(double-sided) Ca atoms on (2 x 2) cell of graphene (with the average Ca-Ca distanceof 4.96 A on one side) is 1.09 eV (1.2 eV) which is smaller than the Ca cohesive energy(--‘1.8 eV). Thus metal aggregation could actually take place on pure graphenewhich decreases the storage capacity of system. To have a stable decoration of Ca ongraphene, the Ca-graphene adsorption energy should be enhanced to be more thanthe Ca cohesive energy in bulk Ca.One way to enhance the metal adsorption strength on graphene plane is todopegraphene with acceptors such as boron atoms. Our calculations showthat uponsubstitutional boron doping the clustering problem can be prevented.As shown inFigure 2.3, the adsorption energy of Ca is enhanced to 2.86,2.98, and 3.82 eV forsingle, pair, and triplet B-doped (2 x 2) cells (withsingle-sided Ca coating), respectively. These binding energies are much higher than thatof Ca’s cohesive energy.Corresponding band structures (Figure 2.3) showthat in B-doped configurations theit and ir bands are shifted above the Fermi level asthe emptyPzorbital of the boronatom acts as a strong charge accepting center. As aresult, the B-doping forms anelectron-deficient structure and this deficiency increasesas the number of B atomsincreases. Charge transfers from Ca atom to grapheneoccur more efficiently for theseelectron-deficient structures leading to stronger Ca-graphenebinding.292.3. Results and discussiond=2.17AEb=2.86eV/_t*:rB+G(2x2)‘T /‘‘\-2M F KMCa-B+G(2x2)M F KM2B+G(2x2)-__:-_M F KMCa.2B+G(2x2)(c)Ca0BOd= 1.96 AEb= 3.82eVM F KM3B+G(2 x2)M F KMCa-3B+G(2x2)(a) (b)d = 2.02AEb=2.98eV2L7/QC.4Figure 2.3: The optimized structures, binding energies,bond lengths and energyband diagrams of a single Ca atom adsorbedon the H site of a (2 x 2) cell with PBCof (a) single B-doped, (b) pair B-doped, and (c) tripletB-doped configurations.To provide a more explicit analysis ofmetal stability in B-doped structures,wecalculate the binding energy of a Ca dimerand two isolated Ca atoms on pureandB-doped (4 x 4) cell of graphene (Figure2.4). These calculations showthat thedimerized Ca atom on pure graphene isenergetically more favorable by- 0.4 eVas compared with the isolated case.In contrast, for the B-dopedstructure, the302.3. Results and discussionenergy of the isolated configuration is “. 0.2 eV lower than that of the dimerizedcase. Therefore, Ca can be individually dispersed on a single B-doped graphene. Incontrast to calcium, TM atoms (such as Sc,V and Ti) have been found to prefer thedimer form in single and pair B-doped graphene, while they form separated metalatoms in the triplet B-doped configuration [41]. It has been experimentally observedthat B doping concentration in graphene can be up to 20 at.% [29, 30].Thereofore,a triplet B-doped configuration, with the doping concentration of ‘- 38%,may notbe a stable structure. Consequently, we can say that solving the clusteringproblemin Ca-decorated graphene could be more practical than in TM-coatedgraphene.Figure 2.4: Optimized structures of dimerized and separatedCa atoms adsorbed onpure (a) and B-doped (b) (4 x 4) cells of graphene.Our calculations show that inthe B-doped structure the isolated configuration isenergetically more favorable by0.2 eV while in the pure configuration the dimerizedconfiguration is more stableby 0.4 eV.Now, we investigate hydrogen adsorptionon B-doped structures using LDAcalculations. For this study we consider one-sided Cacoating and compare binding energies(a)E= 0eV(b)E= 0.40eV312.3. Results and discussionof adsorbed H2 molecules on different boron-doped configurations. Here, the maximum number of adsorbed 112 per adsorbed Ca atom is still four for the (2 x 2) coveragesimilar to that of pure graphene. The average binding energies of the 112moleculeon single, pair, and triplet structures are 0.42, 0.39, and 0.38 eV/H2,respectively, which is close to the average Eb of H2 molecules on pure graphene(.-.‘0.43eV/H2). However, the binding energy of the first H2 molecule tothe Ca atom whichprefers to be parallel to the graphene layer is generally decreased by 0.1eV.The binding mechanism of ff2 on Ca coated graphene is analogousto that of H2 onTM coated graphene. Two binding mechanisms contribute to theH2 adsorption onCa which are polarization of the H2 molecule underthe electric field produced bythe Ca atom and hybridization of the Ca 3d orbitalswith the H2 a orbitals [23, 24].Figure 2.5 displays the projected density of states(PDOS) for the H2 moleculesand Ca 3d orbitals when one hydrogenmolecule is adsorbed on a pure andtripletB-doped graphene. We think that thelower binding energy of the first hydrogenmolecule on B-doped graphene can beattributed to less hybridization of H2 aandCa 3d orbitals below the Fermi level(Figure 2.5).The above calculations were performedusing DFT, and the accuracy of therelatively weak binding energies obtained with thismethod is questionable. AsTable 2.1 shows, binding energies calculatedwith LDA are more than those of GGAresults. To evaluate our DFT calculations,we performed MP2 simulations. MP2 isexpected to provide a good descriptionof dispersive interactions, whichis missingin DFT calculations. The comparison wascarried out with the Gaussian 03package [42] using the molecularmodel of coronene (six benzene ringspassivated withhydrogen) as a model for a graphene fragment(PBCs are not availablefor MP2 inGaussian)(Figure 2.6). This model haspreviously been shown toprovide an adequate representationof graphene for the purpose of bindingenergy calculations [43].The structural optimizationsand binding energy calculations wereperformed usingdouble-zeta polarized 6-31G(d,p) andtriple-zeta polarized 6-311G(d,p)basis sets,respectively. In our DFT calculationsin Gaussian, GGA is describedby the PBEfunctional. Also, LDA is described bythe Slater exchange plusVosko-Wilk-Nusair322.3. Results and discussioncorrelation (SVWN5) [44] which fits the Ceperely-Alder solution to the uniform electron gas [45]. The results are summarized in Table 2.2. It can beseen that LDAgives almost the same results as MP2 for both energies and strcutures.QFigure 2.5: Projected density of states(PDOS) of the Ca 3d orbitalsand H2 aorbitals involved in the adsorptionof hydrogen on a (a) Ca-decoratedpure grapheneand (b) Ca-decorated triplet B-dopedgraphene. The Fermi energy EF is set tozero.C(b)E-Ef(eV)332.3. Results and discussion(•CaHOFigure 2.6: Structure of Ca attached to a purecoronene molecule.This molecularmodel has previously been shown to provide anadequate representation ofgraphenefor the purpose of binding energy calculations.d(Ca-C) d(H-Ca) d(H-H) Eb(Ca-C) Eb(H-Ca)SVWN5 3.30 4.56 0.770.233 0.007PBE 4.08 6.81 0.750.007 0.002MP2 3.60 5.30 0.73 0.3820.005Table 2.2: Comparison of binding energies(in eV) and bond lengths (in A) obtainedby LDA (SVWN5), GGA(PBE), and MP2 calculations onthe molecular model ofCoronene.342.4. Conclusion2.4 ConclusionIn conclusion, we have studied the metal dispersion and hydrogen binding propertieson Ca-coated B-doped graphene. It is found that a stable decoration of Ca atoms ongraphene can be obtained upon substitutional boron doping. Our calculations showthat individual calcium atoms are stable on a single B-doped (2 x 2) cell of graphene(i.e. with a concentration of 12 at.% boron doping). The double-sided Ca-decoratedsingle B-doped graphene can reach a gravimetric capacity of 8.38 wt.%. We encouragean experimental research to synthesize these hydrogen storage nanomaterials thatmay operate at room temperature and ambient pressure.35Bibliography[11Schlapbach, L.; Züttel, A. Nature (London) 2001, 4L, 353—358.[2] Bhatia, S. K.; Myers, A. L. Langmuir 2006, 22, 1688—1700.[3] Lochan, R. C.; Head-Gordon, M. Phys. Chem. Chem.Phys. 2006, 8, 1357—1370.[41Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C.H.; Bethune, D. S.;Heben, M. J. Nature 1997, 886, 377—379.[5] Chambers, A.; Park, C.; Baker, R. T.K.; Rodriguez, N. M. J. Phys. Chem.B1998, 102, 4253—4256.[6] Liu, C.; Fan, Y. Y.; Liu, M.; Cong,H. T.; Cheng, H. M.; Dresseihaus,M. S.Science 1999, 286, 1127—1129.[7] Ritschel, M.; Uhlemann,M.; Gutfleisch, 0.; Leonhardt, A.;Graff, A.;Tschner, C.; Fink, J. Appi. Phys. Lett. 2002, 80,2985—2987.[8] Hirscher, M.; Becher, M.;Haluska, M.; Quintel, A.; Skakalova, V.;Choi, Y. M.;Dettlaff-Weglikowska, U.; Roth. S.; Stepanek,I.; Bernier, P.; Leonhardt,A.;Fink, J. J Alloys and Compd 2002, 330-332, 654—658.[9] Panella, B.; Hirscher, M.;Roth, S. Carbon 2005, 43, 2209—2214.[10] Zhang, Y.; Franklin, N. W.; Chen, R. J.;Dai, H. Chem. Phys. Lett. 2000,331,35—41.[11] Yildirim, T.; Ciraci, S. Phy.s. Rev.Lett. 2005, 94, 175501.36Chapter 2. Bibliography[121Zhao, Y.; Kim, Y.; Dillon, A. C.; Heben, M. J.; Zhang, S. B. Phys. Rev. Lett.2005, 94, 155504.[13] Durgun, E.; Ciraci, S.; Yildirim, T. Phys. Rev. B 2008, 77, 085405.[14] Mingos, D. M. P. J. Organomet. Chem. 2001, 635, 1—8.[15] Kubas, G. J. Ace. Chem. Res. 1988, 21, 120—128.[16] Sun,Q.;Wang,Q.;Jena, P.; Kawazoe, Y. J. Am. Chem. Soc. 2005, 127 14582.[17] Durgun, E.; Ciraci, S.; Zhou, W.; Yildirim, T. Phys. Rev. Lett. 2006, 97, 226102.[18] Chen, P.; Wu, X.; Lin, J.; Tan, K. L. Science 1999, 285, 91—93.[19] Yang, R. T. Carbon 2000, 38, 623—641.[20] Dag, S.; Ozturk, Y.; Ciraci, S.; Yildirim, T. Phys. Rev. B 2005, 72, 155404.[21] Yoon, M.; Yang, S.; Hicke, C.; Wang, E.; Geohegan, D.;Zhang, Z. Phys. Rev.Lett. 2008, 100, 206806.[22] Li, M.; Li, Y.; Zhou, Z.; Shen, P.; Chen, Z. NanoLett. 2009, 9, 1944—1948.[23] Lee, H.; Ihm, J.; Cohen, M. L.; Louie, S. G. Phys.Rev. B 2009, 80, 115412.[24] Ataca, C.; Aktürk, E.; Ciraci, S. Phys. Rev. B2009, 79, 041406.[25] Yang, X.; Zhang, R.Q.;Ni, J. Phys. Rev. B 2009, 79, 075431.[26] Sun, Y. Y.; Lee, K.; Kim, Y.; Zhang, S. B. Appi. Phys.Lett. 2009, 95, 033109.[27] Novoselov, K. S.; Geim, A. K.; Morozov, S. V.;Jiang, D.; Zhang, Y.;Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science2004, 306, 666—669.[28] Züttel, A.; Sudan, P.; Mauron, P.;Kiyobayashi, T.; Emmenegger, C.; Schiapbach, L. mt. J. Hydrogen Energy 2002, 27,203—212.37Chapter 2. Bibliography[29] Way, B. M.; Dahn, J. R.; Tiedje, T.; Myrtle, K.; Kasrai, M. Phys. Rev. B 1992,46, 1697.[30] Shirasaki. T.; Derré, A.; Ménétrier. M.; Tressaud, A.; Flandrois, S. Carbon2000, 38, 1461—1467.[31] Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.; Ordejón, P.;Sanchez-Portal, D. J. Phys.: Condens. Matter 2002, 14, 2745—2779.[32] Kleinman, L.; Bylander, D. M. Phys. Rev. Lett. 1982, 48, 1425—1428.[33] Troullier, N.; Martins, J. L. Phys. Rev. B 1991, 43,1993—2006.[34] Perdew, J. P.; Zunger, A. Phys. Rev. B 1981, 23, 5048—5079.[35] Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys.Rev. Lett. 1996, 77, 3865—3868.[36] Okamoto, Y.; Miyamoto, Y. J. Phys. Chem.B 2001, 105, 3470—3474.[37] Moller, C.; Plesset, M. S. Phys. Rev. 1934,46, 618—622.[38] Louie, S. G.; Froyen, S.; Cohen,M. L. Phys. Rev. B 1982, 26, 1738—1742.[39] Machón, M.; Reich, S.;Thomsen. C.; Sanchez-Portal, D.; Ordejón, P.Phys.Rev. B 2002, 66, 155410.[40] Monkhorst, H. J.; Pack, J. D. Phys. Rev.B 1976, 13, 5188—5191.[41] Kim, G.; Jhi, S. Phys. Rev. B 2008, 78, 085408.[42] Frisch, M. J. et al. Gaussian 03, Revision G.02,Gaussian, Inc., Wallingford,CT, 2004.[43] Heine, T.; Zhechkov, L.; Seifert, G. Phys. Chem.Chem. Phys. 2004, 6, 980—984.[44] Vosko, S. H.; Wilk, L.; Nusair, M. Can. J.Phys. 1980, 58, 1200.[45] Ceperley, D. M.; Alder, B. J. Phys. Rev.Lett. 1980, 45, 566—569.38Chapter 3Summary, Conclusions and FutureWorkHydrogen has been recognized as a highly appealing energy carrierfor renewableenergy because of its abundance and environmental friendliness. Toachieve economicfeasibility, hydrogen storage materials with high gravimetricand volumetric densitiesmust be developed. Furthermore, hydrogen recyclingshould be performed reversiblyunder near ambient conditions. The current state of the artis at an impasse inproviding a storage material that meets a storage capacityof 9 wt.% or more. Inrecent years, carbon nanomaterials have been widelystudied for their application inhydrogen storage [1, 2, 3, 4]. However, pristine carbonnanostructures are chemicallytoo inert to be useful for practical hydrogenstorage [5, 6].Recently, transition metal-decorated carbon nanostructureshave been proposed tosatisfy the above requirements [7, 8]. The metal-hydrogenbinding energy and ratiolook very promising with respect to the capacity andrelease temperature. However,the issues of structural stability and poor reversibilityin transition metal (TM)dispersions are major concerns in developing a practicalstorage material [9]. In2008, for the first time, Yoon etal. [10] suggested calcium-coated C60fullerenes forhigh capacity hydrogen storage.As discussed in Chapter 2, we exploredtheoretically hydrogen adsorption andstoragein calcium-decorated boron-dopedgraphene. Our results werecarried out usingdensity functional theory (DFT) andthe SIESTA simulation package.First, we39Chapter 3. Summary, Conclusions and Future Workconsidered the pristine graphene plane functionalized by calcium coating. Calciumatoms were assumed to be uniformly distributed on both sides of graphene withacoverage of 25%. Calcium strongly binds to graphene due to the chargetransferbetween Ca and the graphene plane. We found that a (2 x 2)cell of graphene (withperiodic boundary conditions) coated with 2 Ca atoms on both sides,can store upto eight hydrogen molecules with an average binding energy of ‘ 0.4 eV/H2.In thiscase, the corresponding gravimetric density is 8.32 wt.% hydrogen.In Chapter 2, we also addressed the important issue of metal clustering.Our energycalculations show that calcium atoms tend to form clusterson pristine graphenebecause of the small binding energy of Ca to graphene. Consequently,the hydrogenstorage capacity is decreased due to metal aggregation.One possible way to havea stable decoration of Ca atoms on graphene isto dope the substrate with boronatoms. A boron atom acts as a strong charge acceptingcenter and consequentlythe clustering could be prevented in B-doped configurations.Our calculations showthat individual calcium atoms are stableon a single B-doped (2x 2) cell of graphene(i.e. with a concentration of 12 at.% boron doping).The double-sided Ca-decoratedsingle B-doped graphene can reach a gravimetriccapacity of 8.38 wt.%.Our results advance our fundamental understandingof adsorption of hydrogen incalcium-decorated graphene and suggestnew routes to better storage systems. Obviously, investigating the effect of different parametersin our calculations is an immediate further work that caneasily be performed. In the first instance,we could explore the effect of distance between grapheneplanes. Here, we chose a large distanceof 25 A between graphene and its periodicimages to ensure negligibleinteraction between neighboring planes. In future work,we can study the effect of the interactionbetween two graphene planes on hydrogenadsorption and storage.Moreover, wecould investigate energy variations and reactionpaths for molecular adsorptionanddesorption of hydrogen on a Ca-decoratedgraphene to have a betterunderstandingof our storage system. We hope thatthese theoretical studies motivatean active lineof experimental efforts towards novelmaterials needed forhydrogen storage in thefuel cell technology.40Bibliography[1] Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C.H.; Bethune, D. S.;Heben, M. J. Nature 1997, 386, 377—379.[2] Züttel, A.; Sudan, P.; Mauron, P.; Kiyobayashi, T.;Emmenegger, C.; Schiapbach, L. mt. J. Hydrogen Energy 2002, 27, 203—212.[31Ritschel, M.; Uhlemann, M.; Gutfleisch, 0.; Leonhardt,A.; Graff, A.;Tischner, C.; Fink, J. Appi. Phys. Lett. 2002, 80, 2985—2987.[4] Chambers, A.; Park, C.; Baker, R. T. K.; Rodriguez,N. M. J. Phys. Chem. B1998, 102, 4253—4256.[5] Brown, C.; Yildirim, T.; Neumann, D.;Heben, M.; Dillon, T. G. A.; Alleman, J.;Fischer, J. Chem. Phys. Lett. 2000, 329, 311—316.[6] Dag, S.; Ozturk, Y.; Ciraci, S.; Yildirim,T. Phys. Rev. B 2005, 72, 155404.[7] Zhao, Y.; Kim, Y.; Dillon, A. C.;Heben, M. J.; Zhang, S. B. Phys. Rev.Lett.2005, 94, 155504.[8] Yildirim, T.; Ciraci, S. Phys. Rev. Lett.2005, 94, 175501.[9] Zhang, Y.; Franklin, N. W.; Chen, R. J.;Dai, H. Chem. Phys. Lett. 2000,331,35—41.[10] Yoon, M.; Yang, S.; Hicke, C.; Wang, E.; Geohegan,D.; Zhang, Z. Phys. Rev.Lett. 2008, 100, 206806.41


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