The hydrogen adsorption on Zr-decorated LiB (001)_ A DFT stu

The hydrogen adsorption on Zr-decorated LiB (001)_ A DFT study

Vacuum110(2014)62e68

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Vacuum

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ThehydrogenadsorptiononZr-decoratedLiB(001):ADFTstudy

WeibinZhanga,ShaolinZhanga,ZhijunZhanga,c,LiliWangb,WoochulYanga,*

a

DepartmentofPhysics,DonggukUniversity,Seoul100-715,RepublicofKoreaInstituteofCalculation,CAEP,Mianyang621900,PRChinac

SchoolofMaterialsScienceandEngineering,ShanghaiUniversity,Shanghai,200072,PRChina

b

articleinfo

Articlehistory:

Received12May2014Receivedinrevisedform30July2014

Accepted6August2014

Availableonline27August2014Keywords:

DensityfunctionaltheoryLiB(001)

Zr-decoration

AdsorptionenergyHydrogenadsorption

abstract

TheadsorptionabilityofH2moleculesontheZr-decoratedLiB(001)2?2surfaceisinvestigatedusingdensityfunctionaltheory.ItwasdemonstratedthatZratomsarepreferentiallyadsorbedonthetopofLiatomoftheLiBsurface.TheformationofcovalentbondofZreBandionicbondofZreLiontheLiBsurfaceleadstoastableZr-decoratedLiBsurfacesforH2adsorption.TheZr-decoratedLiB(001)canadsorbuptofourH2moleculesatambienttemperatureandpressurewithaverageadsorptionenergyinarangefromà0.16toà0.30eV/H2,whichisclosetothetargetvaluespeci?edbyU.S.DepartmentofEnergyforcommercialapplications.Onthebasisofchargeandbondpopulationanalysis,theprojecteddensityofstatesandtheelectrondensitydistributionanalysis,itwasfoundthatZratomactsasabridgeinteractswithbothH2moleculesandLiBsurfaceatoms.TheinteractionbetweenH2andZr-decoratedLiBsurfaceisweakchemisorptions.Furthermore,weshowthattwoZratomscanbedecoratedontheLiB(001)2?2surfaceafterconsideringtheadsorptionenergyofZrandminimumatomicdistancebetweenZratoms.Thus,theZr-decoratedLiB(001)canadsorbupto8H2moleculesindicatingthatZr-decoratedLiB(001)systemmightbeapromisinghydrogenstoragematerial.

?2014ElsevierLtd.Allrightsreserved.

1.Introduction

Hydrogen-basedfuelsystemshavebeenconsideredtobeahighlyimportantresearchareaforfutureenergyschemesbecausehydrogenisanideal,environmentalfriendlyenergycarrier.How-ever,howtostorehydrogeneffectivelyandsafelyisstillapendentissueforthehydrogeneconomy.TheU.S.DepartmentofEnergy(DOE)'stargetforgravimetricstoragedensityis5.5wt%forcompetitiveconsumervehiclesthatusehydrogenasthefuel[1,2].Eachofthecurrentavailablehydrogenstoragemodalities,includingcompressedgaseoushydrogen,cryogenicliquidhydrogen,metalhydridesandcomplexhydrides[3e7],cannotmeetalltherequirementsforcommercialapplications.Despitemanyrecenttechnologicaldevelopmentsinthehydrogenbasedfuelsystems,itisstillanenormouschallengetohavesafeandef?cientreversiblehydrogenstoragesystemsatambientconditions.

Onepossiblewayistodevelopanef?cientandcontrollableadsorption/desorptionmaterialsystemforhydrogenstorage.Thus,manyattemptshavebeenmadetodevelopnewmaterials,suchas

*Correspondingauthor.Tel.:t82222603444;fax:t82222784519.E-mailaddress:wyang@dongguk.edu(W.

Yang).http://dx.doi.org/10.1016/j.vacuum.2014.08.007

0042-207X/?2014ElsevierLtd.Allrightsreserved.

NaAlH4,graphene,andboronnitridenanotubes[8e11],withhighhydrogenstoragecapacityinordertomeetthedemandofcom-mercialvehiclespoweredbyH2/O2protonexchangemembranefuelcells[12e14].Amongthesematerials,lithiumborohydride(LiBH4)isonekindofimportanthydrogenstoragematerials,whichhasintrinsicallyhighgravimetricandvolumetrichydrogenstoragecapacities(18.2wt%,121kg/m3)[13].LiBH4ismainlysynthesizedbyexposingLiBcompoundstohighhydrogenpressure[15e19].Friedrichs[15]andCetin[16]synthesizedLiBH4usingLiBpowderwithhighpressureof6?106Paandtemperatureof700??C.InToyoda'sexperiment[17],theLiB?lmsweredepositedbyusinganevaporationsystemwithshortglowdischargein40Pahydrogen.Theyobtainedhydrogenadsorptionrateof6.5?1020atommà2in28s.Also,othergroupsreportedthattheweightpercentoftheadsorptionH2wasobtainedtobe~8wt%fromtheLiB?lmsdepositedbyPLDmethodandfollowedbyheattreatments[18,19].Despitetheabovementionedefforts,theintrinsicLiBstillcannotfullysatisfythepracticalapplicationrequirementsbecauseonlyfewH2moleculescanbeadsorbedonthesurface,orbecauseahighpressure,temperatureorplasmaarerequiredforenhancingH2adsorption.Alternativemethodtoimprovethehydrogenstor-agecapacityisdecorating(ordoping)foreignatomsonthesurfaceofstoragematerials.Recently,itwasfoundthattheatoms,suchasAl,Ti,Zr,Li,Ni,CeandB,af?xedtocarbonnano-structuresor

W.Zhangetal./Vacuum110(2014)62e6863

complexmetalhydride,caneffectivelyadsorbhydrogenmolecules[8e11].TheH2mainlyinteractedwiththeBatom,andwetrytoknowtheinteractionbetweenBandHatoms.Asthe(001)surfacehavethelongestdistancebetweentheneighboringBatomamongallthelowindexsurfacesofLiB,the(001)surfaceischosen.WealsopreviouslyreportedthattheTidecorationonLiB(001)surfacecangreatlyimprovethehydrogenstoragecapacity[20,21].UptofourH2moleculescouldbeadsorbedontheTidecoratedLiB(001)2?2surface.However,inthissystemtheTiatomsonLiB(001)surfacetendtograduallymigrateintothebulksincetheTiatomsener-geticallyprefertooccupyinterstitialpositionsinthebulk[22].Theinter-diffusioneventuallyleadstothedegenerationofthehydrogenstoragecapacity.Thus,anewdecorator,whichcanstablyanchoronLiB(001)surfaceaswellasef?cientlyenhancetheH2moleculesadsorption,shouldbestronglysuggested.

Inthisstudy,theeffectofZrasasubstituteddecoratorontheH2storagecapacityofLiB(001)wasinvestigatedby?rstprinciplescalculation.TheadsorptionsitesofZratomsonLiB(001)2?2surface,theinteractionbetweenZrandLiB(001)surface,andtheadsorptionbehaviorofH2ontheZrdecoratedLiB(001)surfaceweresystematicallystudied.Ourcalculationrevealedthatupto8H2moleculescanbeadsorbedonthetwoZratomsdecorated2?2LiB(001)surface.Theadsorptionenergywasaroundà0.16toà0.30eV/H2,whichwasclosetothetargetadsorptionenergyofà0.20toà0.40eV/H2atambienttemperatureandmodestpressure.Theprojectedelectronicdensityofstates(PDOS)andtheelectrondensitydistributionofthesystemswerealsoconsideredtoexaminetheinteractionbetweenatomsintheadsorptionprocesses.

2.Computationaldetails

Firstprinciplescalculationsbasedonthedensityfunctionaltheory(DFT)andthegeneralizedgradientapproximation(GGA)withaplaneewavepseudo-potentialbasiswereperformedusingtheCASTEP(CambridgeSerialTotalEnergyPackage)[23]code.ThePerdewWang(1991)(PW91)[24]typegradient-correcteddensityfunctionalwasemployedfortheexchangecorrelationpotentialGeometryoptimizationwasperformedbytheBroydenFletcherGoldfarbShanno(BFGS)[25]routineandanultrasoftpseudo-potential[26].Theplanewaveexpansionwastruncatedatacut-offenergyof270eVandthe?rstBrillouin-zonewassampledwitha3?3?1k-pointgrid[27].Wealsotriedhigherk-pointandcut-offenergy,buttherewerenoextraordinaryimprovement.Testcalculationsshowedthattheresultswerenotsigni?cantlyaltereduponincreasingthek-point.Theconvergencetoleranceoftheen-ergywassetto10à5eV/atom,andthemaximumallowedforce,maximumstressanddisplacementwere0.05eV/?,0.05GPaand10à3?,respectively.Theseparameterswereappliedtooptimizetheunitcelluntilthedeviationwasnomorethan5%comparingwiththeunitcellobtainfromcrystallographicre?nementexperi-ments[28].Thiswasimportantforestablishingcon?denceinthefunctional(GGA)usedinthisstudy.Thisfunctionwaswell-knownforaccuratepredictionsofsurfacereactionsandchemisorptionenergiesinmetals,alloys,andothersimilarsystems.Therelaxedunitcellwasusedtoformsuper-cell,andthenthelowestenergysurfacewascleaved.

ThemodeledLiB(001)2?2surfacewasaslabofsixatomiclayers,whichwasseparatedfroma15?thickvacuumregion.Thethicknessofslabsandvacuumregionweresettobelargeenoughtoavoidtheinteractionbetweenthereplicas.H2wasoptimizedinalargeperiodiccubicboxwithacellparameterof10?10?10?3.ThebondlengthofHeHwasabout0.75?.Thesurfacewasdeco-ratedwithZrandrelaxedtoattainenergy-minimizedstablestructures.TheZrdecoratedLiB(001)surfacewassubsequently

usedtostudyadsorptionofthemolecularhydrogen.Forcompu-tationalef?ciency,thebottomthreelayersofLiB(001)were?xed,onlythetopthreelayers,andZrandH2wereallowedtorelaxuntiltheirresidualforceswerelessthan0.01eV?à1.

Theadsorptionenergy,Ea(Zr),oftheZratomdecoratedontheLiB(001)wasde?nedas

EZrT?h

mEEi.aeZrtLiBe001TàEmZrtLiBe001Tm

(1)

wheremindicatesthenumberofZratoms.EZr,ELiB(001),EmZrtLiB(001)werethetotalenergiesofafreeZratom,thepureLiB(001)andtheZr/LiB(001)hybridsystem,respectively.

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