[JACS]Nanostructured-Nickel-Phosphide-as-an-Electr

NanostructuredNickelPhosphideasanElectrocatalystfortheHydrogenEvolutionReactionEricJ.Popczun,†JamesR.McKone,‡CarlosG.Read,†AdamJ.Biacchi,†AlexM.Wiltrout,†NathanS.Lewis,*,‡andRaymondE.Schaak*,††DepartmentofChemistryandMaterialsResearchInstitute,ThePennsylvaniaStateUniversity,UniversityPark,Pennsylvania16802,UnitedStates‡DivisionofChemistryandChemicalEngineering,CaliforniaInstituteofTechnology,Pasadena,California91125,UnitedStates*SSupportingInformationABSTRACT:Nanoparticlesofnickelphosphide(Ni2P)havebeeninvestigatedforelectrocatalyticactivityandstabilityforthehydrogenevolutionreaction(HER)inacidicsolutions,underwhichprotonexchangemembrane-basedelectrolysisisoperational.ThecatalyticallyactiveNi2PnanoparticleswerehollowandfacetedtoexposeahighdensityoftheNi2P(001)surface,whichhaspreviouslybeenpredictedbasedontheorytobeanactiveHERcatalyst.TheNi2PnanoparticleshadamongthehighestHERactivityofanynon-noblemetalelectro-catalystreportedtodate,producingH2(g)withnearlyquantitativefaradaicyield,whilealsoaffordingstabilityinaqueousacidicmedia.Theproductionofmolecularhydrogenbytheelectro-chemicalreductionofwaterisanimportantcomponentofseveraldevelopingclean-energytechnologies.1,2Thehydrogenevolutionreaction(HER)iseffectivelyfacilitatedbynoblemetalssuchasPt,whichgeneratelargecathodiccurrentdensitiesforthisreactionatlowoverpotentials.1,3,4Replace-mentofPtwithearth-abundantmetalswouldbedesirabletofacilitatetheglobalscalabilityofsuchpotentialclean-energytechnologies.Onenon-precious-metalalternativetoPtisMoS2,whichhashighHERactivityandexhibitsgoodstabilityinacidicsolutions.3MoBandMo2ChavealsobeenidentifiedasactiveHERcatalystsinbothacidicandalkalinesolutions.5,6Thefirst-rowmetalnickel,whichissignificantlymoreabundantthanMo,isoftenusedasanelectrocatalystfortheHER,withactiveelectrocatalystsproducedbyuseofalloyssuchasNi−Mo,7−9Ni−Mo−Zn,10Ni−Fe,9andNi−P.11,12TheseNi-basedcatalystsarenot,however,stableinacidicsolutions,inwhichprotonexchangemembrane-basedelectrolysisisfeasibleandoperational.AdditionofnitrogentoNi−Mo,toformNi−Mo−Ncomposites,hasbeenshowntosignificantlyimprovetheacidstabilityofsuchalloys,13butsuchsystemsstillshowsignificantlylowerHERactivityand/orstabilityinacidicsolutionsthannoblemetalssuchasPt.MoS2,anactiveearth-abundantHERelectrocatalyst,isalsohighlyactiveforhydrodesulfurization(HDS).3,14BothHDSandtheHERrelyonthecatalysttoreversiblybindH2,withH2dissociatingtoproduceH2SinHDSandwithprotonsboundtothecatalysttopromotetheformationofH2intheHER.15−17TheputativeactivesitesofMoS2forHDSaretheunder-coordinatededgesasopposedtothebasalplanes.14,18Similarly,fortheHER,themostactivesitesofMoS2arebelievedtobethe(101̅0)planesthatareexposedontheedges,ratherthanthe(0001)basalplanes.19,20ThesecommonalitiesbetweenthemechanismsandputativeactivesitesofMoS2forbothHDSandHERcatalysissuggestthatothermaterialsthatareknownHDScatalystsmayalsoprovideactiveelectrocatalystsfortheHER.Nickelphosphide(Ni2P),whichadoptsthehexagonalFe2Pstructure(Figure1A),21isawell-knownHDScatalyst,16,22andNi2PalsoproducesH2(g)viathewater−gasshiftreaction.23Additionally,densityfunctionaltheorycalculationshaveindicatedthattheNi2P(001)surface,whichhasexposedNiandPsites(Figure1B,C),exhibitsanensembleeffect,wherebyproton-acceptorandhydride-acceptorcentersarebothpresenttofacilitatecatalysisoftheHER.17Ni2P(001)hasalsobeenshowntohavestructuralandelectronicanalogiestotheactivesiteof[NiFe]hydrogenase,whichisahighlyactivebiologicalHERcatalyst.17Ni2P(001)hasfurthermorebeenidentifiedasapotentialHERcatalystthatcouldmergetheactivityof[NiFe]hydrogenasewiththethermostabilityofaheterogeneouscatalyst.17Motivatedbythesepredictions,hereinwereportReceived:April6,2013Published:June13,2013Figure1.CrystalstructureofNi2P:(A)fourunitcellsstackedontopofoneanother,withasingleunitcelloutlined,(B)top-downviewoftheNi2P(001)surface,and(C)atwo-dimensionalsliceofNi2P,showingthe(001)surfaceontop.(D)ExperimentalpowderXRDpatternfortheNi2Pnanoparticles(top),withthesimulatedpatternofNi2Pshownforcomparison(bottom).Communicationpubs.acs.org/JACS©2013AmericanChemicalSociety9267dx.doi.org/10.1021/ja403440e|J.Am.Chem.Soc.2013,135,9267−9270thatnanostructuredNi2PisanactiveHERelectrocatalystthatiscomprisedentirelyofinexpensiveandearth-abundantelementswhileexhibitingimprovedperformanceandstabilityrelativetoothercomparableearth-abundantelectrocatalysts.Ni2Pnanoparticlesweresynthesizedbyheatingnickel(II)acetylacetonate[Ni(acac)2]in1-octadecene,oleylamine,andtri-n-octylphosphine(TOP)to320°Cfor2h,coolingtoroomtemperature,washingwithethanolandhexanes,andisolatingbycentrifugation.24,25[Caution!High-temperaturedecompositionofaphosphinecanliberatephosphorus,sothisreactionshouldbeconsideredashighlycorrosiveandflammableandthereforeshouldonlybecarriedoutbyappropriatelytrainedpersonnel,underrigorouslyair-freeconditions.]Figure1DshowsthepowderX-raydiffraction(XRD)patternofthenanoparticles,confirmingtheformationinhighyieldofFe2P-typeNi2P.Scherreranalysisofthepeakwidthsindicatedanaveragegrainsizeof17nm.ConsistentwiththeScherreranalysis,transmissio