CopperSilverThinFilmswithMetastableMiscibilityforOxygenReductionElectrocatalysisinAlkalineElectrolytes

DrewHiggins,1,2MelissaWette,3BrennaM.Gibbons,2,3SamiraSiahrostami,1,2ChristopherHahn,1,2MariaEscudero-Escribano,1MaxGarcía-Melchor,1,2ZacharyUlissi,1,2RyanC.Davis,4ApurvaMehta,4BruceM.Clemens*,3JensK.Nørskov*,1,2ThomasF.Jaramillo*1,2

1 Department of Chemical Engineering, StanfordUniversity, 443ViaOrtegaWay, Stanford, California,94305,USA2SUNCATCenterforInterfaceScienceandCatalysis,SLACNationalAcceleratorLaboratory,2575SandHillRoad,MenloPark,California,94025,USA3 Department of Materials Science and Engineering, Stanford University, 496 Lomita Mall, Stanford,California,94305,USA4StanfordSynchrotronRadiationLightsource,SLACNationalAcceleratorLaboratory,2575SandHillRoad,MenloPark,California,94025,USACorrespondingauthoremails:jaramillo@stanford.edu;norskov@stanford.edu;bmc@stanford.edu

Abstract

Increasing the activity of Ag-based catalysts for the oxygen reduction reaction (ORR) is important forimproving the performance and economic outlook of alkaline-based fuel cell and metal-air batterytechnologies. In this work, we prepare CuAg thin films with controllable compositions using e-beamphysical vapour deposition. X-ray diffraction analysis indicates that this fabrication route yieldsmetastable miscibility between these two thermodynamically immiscible metals, with the thin filmsconsistingofaAg-richandaCu-richphase.Electrochemicaltestingin0.1MpotassiumhydroxideshowedsignificantORRactivityimprovementsfortheCuAgfilms.Onageometricbasis,themostactivethinfilm(Cu70Ag30)demonstrateda4-foldactivityimprovementversuspureAgat0.8Vvsthereversiblehydrogenelectrode.Furthermore,enhancedORRkineticsforCu-rich(>50at%Cu)thinfilmswasdemonstratedbyadecreaseinTafelslopefrom90mV/dec,acommonlyobservedvalueforAgcatalysts,to45mV/dec.Surface enrichment of the Ag-rich phase after ORR testing was indicated by x-ray photoelectronspectroscopyandgrazingincidencesynchrotronx-raydiffractionmeasurements.Bycorrelatingdensityfunctionaltheorywithexperimentalmeasurements,wepostulatethattheactivityenhancementoftheCu-richCuAgthin filmsarisesdueto thenon-equilibriummiscibilityofCuatoms in theAg-richphase,whichfavourablytunesthesurfaceelectronicstructureandbindingenergiesofreactionspecies.

Introduction

Electrocatalystsplayanessentialroleinnumerouscleanenergyconversionandstorageprocessesthatwillbeintegralforasustainableenergyeconomy.Theoxygenreductionreaction(ORR)isoneofthemostheavily investigated reactions,[1-3]owing to its importance for fuel cell,metal-airbatteryandchloro-alkali technologies. Under acidic conditions, Pt and its alloys are the only known catalysts capable ofachievingtechnologicallysuitableactivityanddurability,albeitwiththedrawbackofhighcostandscarcity.WhilesignificantprogresshasbeenmaderecentlytowardsdevelopmentofPtgroupmetalfree(PGM-free)catalystsforacidicORR;[4-7]challengeswithstabilitylimittheirpracticalitytoalkalineconditionsthatprovidemorefavourableORRkineticsandlessharshoperatingenvironments.[8,9]Thechallengesof acidic ORR, coupled with recent advances in hydroxide exchange membrane and ionomerdevelopments,[10, 11] have led to renewed interest in the development of system components foralkalineoperatingfuelcelldevices.[12]

TheORRinalkalineconditionsismorekineticallyfavourablethaninacidicelectrolytes,whichhasenabledtheutilizationofvariousPGM-freecatalysts.[13]Transitionmetaloxides,[14-17]heterogeneoustransitionmetal-nitrogen-carbon (Fe-N-C) nanostructures[3, 9, 18-21] or Ag[22-28] based catalysts have provencapabilitiestowardstheORRinalkalineconditions,withthelattertwoshowingthemostpromisefromapracticalstandpoint.[12]Despitetheirlowcostandhighsurfaceareas,M-N-Cnanostructurespossessarelativelylowdensityofactivesites(numberofactivesitesperunitsurfacearea),whichrequirestheuseofhighcathodeloadingstomeetthedesiredhighperformances.Whilethisisnotprohibitivefromacostperspective, thehighelectrode loadingcombinedwith lowdensityofcarbonmaterials results inthickcatalystlayersthatposechallengeswithrespecttomasstransportandionicconductivity.

Agisanattractivecatalystmaterialthatcanachieverelativelyhighspecific(surfaceareabased)activity(Supporting Information, Table S1). Ag can also be nanostructured into high aspect ratiomorphologies,[26-31]providinganincreasedamountofsurfaceareaavailableforcatalysis.WhileAgisconsidered the most techno-economically viable electrocatalyst material for the ORR in alkalineconditions,[12]AgelectrocatalystsstillhaveroughlyoneorderofmagnitudeloweractivitythanPtonaspecificactivitybasis(TableS1).DuetothesignificantlylowercostofAgincomparisonwithPt(ca.0.6$/gAgversus33$/gPt),[32]theactivitydeficiencyofAgcanbeaddressedbyemployinghigherelectrodeloadings. However, this strategy is only effective to a certain extent, as the aforementioned masstransportissuesofthickercatalystlayersrestricthighcurrent/powerdensityoperation.NewstrategiestoimprovetheintrinsicactivityofAg-basedcatalystsfortheORRinalkalineconditionsarethereforeneededforachievinghighelectrodeperformance.

InthecaseofPt,increasedspecificandmassactivitieshavebeenachievedbypreparingsolidsolutionorintermetallic alloys with transition metals such as Co or Ni.[33-39] Unlike Pt, Ag demonstrates verydifferentbehaviouras it isthermodynamically immiscible inthebulkwithmanytransitionmetals,andthus achieving activity improvements viawell-mixed alloy phases is challenging. Phase segregation incatalysts,whetherthermodynamicallyfavourableoradsorbateinduced,[40]hasnotableimplicationsonthekineticsandselectivityofnumerousreactions,suchasCuCoforsynthesisgasconversiontohigheralcohols,[41-43] PtRu catalysts for small alcohol electrooxidation,[44] or CuPd nanostructures forelectrochemical CO2 reduction.[45] In the case of Ag-based catalysts for the ORR, a recent study[46]demonstrateda rapidheating, rapidquenching technique for thesynthesisofphase-segregatedCoAgnanoparticles.AnalmostorderofmagnitudeORRAg-basedspecificactivityenhancementwasfoundfor

theCoAgnanoparticlesincomparisontopureAg,attributedtotheformationofasurfacealloyarisingfromtheiruniquesynthesistechnique.Thisworkprovidedthefirstconcreteevidencethattheformationof metastable Ag-M alloy structures may be indispensable for achieving appreciable ORR activityenhancements.Therealsoexistreportsonphase-segregatedAg-CubimetallicsfortheORR;[23,47-49]however, in only few of these instances were intrinsic activity enhancements compared to pure Agconclusivelydemonstrated.[48,49]Furthermore,only limitingunderstanding regarding theunderlyingmechanismsoftheORRenhancementforCuAgexists,particularlywithrespecttotheroleCuplays inenhancingtheORRactivityofthisbimetallicsystem.

In this work, we adopt electron beam physical vapour deposition (PVD) as a means to prepare andinvestigatephase-segregatedCuAgthinfilmsaselectrocatalystsfortheORR.WeshowthatPVDintheabsenceofexternalsubstratetemperaturecontrolresultsinmetastablebulkalloyingwithinthesethinfilms.Throughelectrochemicaltestingin0.1MKOHelectrolyte,thinfilmswithanoptimalcompositionofCu70Ag30werefoundtoachieveafour-foldenhancementinactivityoverthatofpureAgonageometricbasis (at 0.8 V vs RHE). Clear improvements to the ORR kinetics are indicated by a Tafel slope thatdecreasesfromca.90mV/decforpureAg,toca.45mV/decforCu70Ag30;alongwiththeemergenceofhysteresis between the anodic and cathodic polarization scans.We use a variety of electrochemical,microscopic and spectroscopic probes in combination with density functional theory (DFT) to helpunderstandtheseelectrochemicalreactiontrends.Wehypothesizethattheactivityenhancementarisesfrom the miscibility of Cu atoms within the Ag-rich phase of these catalysts, resulting in electronicmodificationsthroughbothstrainandligandeffectsthatarefavourablefortheORR.

MethodsandMaterials

ExperimentalDetails

Thin filmswereprepared inacustomdesignedPVDsystem(TechnicalEngineeringServices)equippedwith two electronbeams and a thermal source. Cu andAgwere deposited using the electronbeamsources. Each source is equipped with a quartz crystal microbalance that has been independentlycalibratedtoallowcarefuldepositionrateandcompositioncontrolthroughtheentirethicknessofthethin films. Prior to deposition, 5mmdiameter glassy carbon disks (HTWHochtemperatur-WerkstoffeGmbH)polishedtoamirrorfinishwerefirstdegreasedbysonicationinamixtureofacetone,isopropanolandMilliporewaterfor20minutes.Theyweresubsequentlyrinsed,sonicatedinpureMilliporewaterfor20minutes,andaffixedtoa300mmSi(100)waferusingdoublesidedKaptontape.CuAgthinfilmswerepreparedwithathicknessof100nmwithindividualdepositionratestunedtoachieveatotalrateof0.2nm/sandtargetcompositionsrangingfrompureCutopureAg.Bulkthinfilmcompositionswerefoundtoagreewithnominalcompositionswithin1at.%,determinedby inductivelycoupledplasmaoptimalemissionspectrometryonasolutionpreparedbydigestingthethinfilmsinconcentratedHNO3(avoidingtheformationofinsolubleAgClwhenusingaquaregia).

X-rayphotoelectronspectroscopywascollectedusingaPHIVersaprobeequippedwithAl-Kαradiation(1486eV).Arsputterdepthprofilingwasaccomplishedinsputtertimeincrementsof30swithanArbeamenergyof1kV,currentof0.5uAandrastersizeof2mmx2mm.CompositionalquantificationofXPSspectrawasconductedusingMultipak.Scanningelectronmicroscopy(SEM)imagingwasconductedonan FEI XL30 Sirion operating in secondary electron detection mode. Atomic force microscopy (AFM)

images were collected using a Park XE-70. X-ray diffraction (XRD) measurements were taken on aPANalytical Materials Research Diffractometer equipped with a Cu-Kα (l = 0.154 nm) x-ray source.SymmetricscanswerecollectedonCuAgthinfilmsgrownon3cmx4cmquartzsheetstoincreasetheareaofmaterialunderilluminationandeliminatebackgroundinterferencesfromglassycarbonsubstrates.Grazing-incidencex-raydiffraction(GI-XRD)measurementswereobtainedatbeamline7-2oftheStanfordSynchrotronRadiationLightsource(SSRL)usingaphotonenergyof14keV.Forthesemeasurements,CuAgthinfilmsweregrownondegeneratelyn+dopedSi(100)waferswitha2nmTistickinglayer.Thisallowedfor comparison between as-deposited and post-electrochemistry films, while avoiding theaforementioned interference from the glassy carbon substrates. GI-XRD patterns were refractioncorrectedusingamodifiedversionofthetechniquereportedbyToneyandBrennan.[50]

Electrochemical characterizationwas conducted by rotating disc electrode (RDE) testing using a PineResearchInstrumentsrotatorsetupandaBiologicVMP-300potentiostat.Athree-electrodecellwasused,employingagraphiterodcounterelectrodeandaAg/AgClreferenceelectrode.TheCuAgthinfilmcoatedglassycarbondiscswereaffixedinaTeflonRDEtip(PineResearchInstrumentation)andusedasworkingelectrodes.Theelectrolytewas0.1MKOHpreparedfromsemiconductorgradepellets(SigmaAldrich)andMilliporewater(R=18MW.cm).Allpotentialsinthismanuscriptareversusthereversiblehydrogenelectrode(RHE)basedoncalibrationoftheAg/AgClelectrodebeforeandaftereachmeasurementversusacleanPtwireinH2saturatedelectrolyte.Priortoelectrochemicalmeasurements,theworkingelectrodewasinsertedintoAr-saturatedelectrolyteunderpotentialcontrolat0.2VvsRHE,andthenconditionedwith20cyclicvoltammetry(CV)cyclesat50mV/sfrom0to1.0VvsRHE.TheelectrolytewasthenpurgedwithO2foratleast15minutespriortoORRactivityevaluation,whichwasconductedbyCVat20mV/sfrom0 to1.0VvsRHE.Allpolarizationmeasurementswerecorrected foruncompensatedelectrolyteresistancealongwithbackgroundcapacitivecontributions.[51]

ComputationalDetails

SimulationswerecarriedoutusingtheAtomicSimulationEnvironment(ASE).[52]Theelectronicstructurecalculations were performed using the Quantum ESPRESSO program package.[53] The electronicwavefunctions were expanded in series of plane waves with a cut-off energy of 550 eV and acorresponding electron density cut-off of 5500 eV. Core electrons were approximated with ultrasoftpseudopotentials.[54]TheRPBEexchange-correlationfunctionalwasused,[55]asithasbeenshowntoaccuratelydescribechemisorptionpropertiesontransitionmetals.TheoptimizedlatticeconstantsforAgandCucalculatedusingRPBEare4.21Åand3.71Å,respectively.Foradsorptionstudies,supercellsoflateralsize(2x2)andfour-layerswereusedandavacuumregionofatleast18Åwasusedtodecoupletheperiodicimagesinthedirectionperpendiculartothesupercell.ThefirstBrillouinzonesweresampledwith (4x4x1) Monkhorst-Pack61 k-points. The adsorbates and top two layers of the slab atoms wereallowedtorelaxuntilthemaximumforceoneachatomwasbelow0.05eV/Aforeachcalculation.

Catalyticactivitiesofdifferentsurfacestructuresweredeterminedbythebindingenergiesofthereactionintermediates (i.e., OOH*, O* and OH*).[56] To estimate the adsorption energies of differentintermediatesatzeropotentialandpH=0,weusethecomputationalhydrogenelectrode(CHE)approachintroducedpreviously.[56] In thismodel, theelectrodepotential (U,versus theRHE), is consideredbyshiftingtheelectronenergyby-eUwhenanelectronistransferred.Theoverpotentialisdefinedasthemaximumpositivefreeenergydifferencebetweeneachtwosuccessivereactionsteps.

ResultsandDiscussion

Tounderstandthecrystalstructureoftheas-preparedCuAgthinfilmsofvaryingnominalcompositions,XRDpatternscollectedfromsymmetricscansareshowninFigure1a,alongwithpureAgandCuthinfilmsforcomparison.Eachpolycrystallinefilmshowsastrong(111)peakconsistentwithapreferentialoutofplane<111>orientation,withtheexceptionofCu90Ag10thatdemonstratesnocleartexturing.The<111>orientationforthemajorityofthethinfilmslikelyarisesduetothetendencyofface-centeredcubicmetalsunder no external influence (i.e., epitaxy) to grow with the minimal surface energy {111} facetexposed.[57, 58] Clear phase segregation of the Cu50Ag50 and Cu70Ag30 thin films is indicated by thepresenceoftwodistinct(111)diffractionpeaks,attributedtoaAg-richandaCu-richphase.Forallsamples,ofparticularinterestistheclearshiftintheAg-rich(111)peakstohigherdiffractionanglesatincreasednominalCucompositions.ThisisdepictedmoreclearlyinFigure1b,andsuggestsadecreaseinthelatticeconstantoccurringintheAg-richphase.AcomplementaryshiftintheCu-richphase(111)peaktolowerdiffractionanglesindicatesalatticeconstantincreaseinthisphaseatincreasednominalAgcompositions.Atomic compositions of each phase were estimated using Vergard’s law[59] and are shown in theSupportingInformation(TableS2).ThisanalysisimpliesthatCu90Ag10istheonlybimetallicwithasinglephase,sinceonlyinthissamplethenominalcompositionmatchesthephasecompositionpredictedbythe (111)diffractionpeak (10.1at%Ag).WhileaCu-richphasepeak isnotobserved forCu20Ag80, theAg(111)peakpositioncorrespondsto13.1at.%Cu,wellbelowthebulkcomposition.Thisimpliesthereisasecond,albeitminorCu-richphase,withanon-discernablediffractionpeakduetothelowerscatteringfactorofCuversusAg.Overall,XRDpatternsprovideevidencethatelectronbeamPVDisamenabletopreparing metastable solid solution phases in thermodynamically immiscible CuAg. This has beendemonstratedpreviouslybyPVDtechniquesusingnearroomtemperaturesubstrates[60,61]duetothe“vapourquenching”natureoftheprocessthatrenderscondensedatomswithinsufficientkineticmobilitytosegregateintoequilibrium(i.e.,pure)phases.[62]ThistechniquehaspreviouslybeenappliedbyourgrouptoprepareNiAgthinfilmswithmetastablemiscibilityforelectrochemicalhydrogenevolutionandoxidationreactions.[63]

Figure1:(a)SymmetricXRDpatternsforCu,AgandCuAgthinfilmswithvaryingcompositions,and(b)magnified(111)diffractionpeakswithAg(38.1°)andCu(43.3°)locationsshownbydottedlines.

SEMimagingofthethinfilmelectrodes(Figure2a-c,FigureS1)showedsurfaceflatnesswitha lackoftopologicalfeatures.Cu70Ag30thinfilms,primarilyfocusedoninthemainmanuscriptduetoitsoptimalORRactivity(videinfra),werefoundtoconsistofirregularlyshaped,quasisphericalcrystallites(Figure2a).SEMimagingofotherCuAgcompositionsshowsverysimilarfeaturesandinplanegrainsizes(FigureS1). Pure Cu thin films (Figure 2b) show grain anisotropy, consisting ofwhat appear to be conjoinedadjacentcrystallites.PureAg(Figure2c)onaveragealsoshowslargercrystallitesthanCuAg,withahighdegreeofvariabilityintheaveragesize.ThesetrendsareinagreementwithXRDpatternsthatshowpeakbroadeningfortheCuAgthinfilms,indicatingthatthesmalleraveragecrystallitesizeobservedinplanebySEMholdstruefortheoutofplanedirectionprobedbyXRD.ThelackoftopologicalfeaturesofthethinfilmswascorroboratedbyAFMimagingoftheCu70Ag30thinfilm(Figure2d).Aroughnessfactorof1.002andrmsvalueof0.85nmindicatedthethinfilmsareveryflat.ThisdemonstratesthesuitabilityofPVDforpreparingflatsurfacesthatcanbeusedforevaluatinggeometricelectrochemicalactivity.[63-66]

Figure2:SEMimagesof(a)Cu70Ag30,(b)Cuand(c)Ag,alongwith(d)AFMimagingofCu70Ag30thinfilmsonglassycarbonelectrodes.

CVsobtainedinAr-sparged0.1MKOHareshowninFigure3aforpureCu,pureAgandCu70Ag30thinfilms.AdditionalCVsofvariousCuAgcompositionsarealsoshownintheSupportingInformation(FiguresS2).The CV profile of Cu (Figure 3a) shows several typical redox features consistentwith those reported

previously.[67-71]Theoxidativepeakcenteredat0.54VvsRHEisprecededbyasmallanodicfeatureat0.47VvsRHE.ThisbehaviourarisesfromthetwostepformationofCu2O,involvingsubmonolayerislandoxide growth at the lower potential, followed by complete surface oxide coverage at the higherpotential.[69]Belowthispotentialregion,smalloxidationwavesareobservedintherangeof0.34to0.43VvsRHE.TheseoxidationfeaturesaretypicalofCu[67]andhavebeenshowntobefacetdependent.[68]Insituscanning tunnelingmicroscopyonsinglecrystalCuelectrodeswaspreviouslyusedtoassociatetheseanodiccurrentswiththeadsorptionofhydroxidespeciesthatserveasprecursorsforsubsequentCu2Ogrowth.[69,72]Thelargeoxidativecurrentobservedaboveca.0.76VvsRHEisduetotheformationofasocalledduplexsurfacefilm,consistingofaninnerCu2OlayerandouterCuOandCu(OH)2layer.[69,70] In thecathodicsweepdirection, thisduplexsurface film isnoteffectively reduceduntil thebroadcathodicwavecenteredatca.0.28VvsRHE.[67,71]

WithmuchsmallercurrentmagnitudesthanthoseobservedforCu,theAgthinfilmelectrodesshowCVprofiles(Figure2a,magnifiedCVprovidedinFigureS2d)withtypicalfeaturesthathavebeeninvestigatedpreviously on single crystal[73-75] or polycrystalline[76] electrodes. Particularly, redox features areobservedinthe0.30to0.53VvsRHErangeandrelatedtotheadsorption/desorptionoflooselyboundhydroxidespecies.[73,74,77]Athigherpotentials,oxidativemaximaoccuratca.0.80andabove0.95VvsRHEintheanodicdirection,withcorrespondingreductivefeaturesobservedinthecathodicdirection.These are attributed to more strongly bound hydroxide or perhaps underpotential deposited oxidespecies,[73, 75, 77] which serve as precursors for Ag2O formation at potentials higher than thoseinvestigated in this study (i.e., > 1.1 V vs RHE). The Cu70Ag30 thin film shows CV features (Figure 2a)expectedforamixtureofCuandAg,albeitwithtwonotableexceptions.Thefirst isthattheoxidativefeatureseenonpureCuandassociatedwiththeformationoftheduplexCu2O/CuO/Cu(OH)2structureissignificantly hindered, suggesting that Ag potentially plays a role in partially inhibiting this oxidativeprocess.Secondistheclearemergenceofareductionpeakwithashouldercenteredatca.0.7VvsRHE.ThispotentiallyindicatesthatAgcanfacilitatethereductionofCu-oxide/hydroxidespeciesatpotentialsmuchhigherthanobservedforpureCu,ahypothesisthatwillbediscussedinfurtherdetailbelow.

ORRpolarizationcurvesforCu,AgandCu70Ag30thinfilmsinO2-saturated0.1MKOHareshowninFigure3b.PureCuprovidesverypoorORRactivity.CorrelatingitsORRpolarizationcurveswithCVprofilesinAr-spargedelectrolyte(Figure3a),itisapparentthatCuonlycatalyzestheORRinitsmetallicstate.Thisgivesrisetothehysteresisobservedinthecathodicandanodicscans,andisconsistentwithresultsreportedpreviouslyintheliterature.[78]Interestingly,alocalizedmaximuminORRcurrentdensityisobservedatca. 0.48 V vs RHE in the anodic scan direction. This occurs in the potential regionwhere Cu adsorbshydroxide species,[69, 72] which potentially could be underlying the current density enhancementobservedpriortotheonsetofCu2Oformation.AsimilarroleofhydroxidespecieswaspreviouslyobservedonAu(100)singlecrystals,withthepresenceofadsorbedOHadsfavouringashiftfromthe2electronto4electronORR.[79]TheORRactivityofpureAgshowsnohysteresisbetweentheanodicandcathodicscans,withacurrentdensity-potentialprofilesimilartothosereportedpreviouslyforAgsinglecrystalelectrodesinthesameelectrolyte.[80]AlsoshowninFigure3b,isthatupontheadditionofCutoAg,anover80mVimprovement inhalf-wavepotential isobservedforCu70Ag30.Additionally,apotentialsweepdirectionhysteresisemerges,withaca.45mVincreaseinhalf-wavepotentialfortheanodicversusthecathodicsweep direction. This observed hysteresis is common for Pt-based catalysts and associated with anadditional overpotential required to remove active site blocking OHads species in the cathodic scandirection;[2,51]however,thisbehaviourhasnotbeenobservedpreviouslyforAg-basedORRcatalysts

andclearlyarisesduetothepresenceofCu.ThisphenomenonisconsistentoverallcompositionsofCuAgevaluated,withFigure3cillustratingthattheORRactivitydecreasedintheorderofCu70Ag30>Cu90Ag10>Cu50Ag50>Cu20Ag80.

SEM (Figure S1) and AFM (Figure S3) imaging on electrochemically tested samples showed minimalchangestothethinfilmsurfacetopography,withonlyamarginalincreaseintheroughnessfactorfrom1.002 (RMS=0.85nm) to1.004 (RMS=0.94nm) forCu70Ag30 asa resultofORR testing. This lackofdiscernable changes to the two-dimensional nature of the thin film surface after electrochemicalmeasurementsindicatedthattheactivityenhancementobservedwasnotduetosignificantsurfaceareaenhancement. This is supported by Tafel plot analysis, with kinetically corrected current densities(electrodegeometricbasis)forallCuAgcompositionsshowninFigure3dalongsidepureAg.DashedlinesrepresentingTafelslopesof90mV/decand45mV/decareincludedforvisualcomparison.TheAgthinfilmshowsaTafelslopethatcloselyresembles90mV/dec,avalueinlinewithpreviousreportsonAg-basedcatalysts.[80-82]CuAgthinfilmelectrodeswithnominalCucompositionsof50at%orgreatershowa striking reduction in Tafel slope to ca. 45 mV/dec. This corroborates the fact that the activityenhancements are notmerely a surface area effect, and indicates rate determining step, adsorbate,and/oractivesitestructure inducedchangesthat improvetheORRkineticsonthesesurfaces.[83]TheORRenhancementofCuAgisdepictedinFigure3e,showingkineticallycorrectedORRcurrentdensitiesforthevariouselectrodesmeasuredat0.8VvsRHE.Cu70Ag30displaysakineticcurrentdensityof1.62mA/cm2

geo,a4-foldenhancementoverthatofpureAg(0.38mA/cm2geo).OwingtothedecreasedTafel

slope of Cu70Ag30 versus that of Ag, an order of magnitude activity improvement is achieved at anelectrodepotentialof0.77VvsRHE,andamorethan30-foldactivity improvementat0.70Vvs.RHE.ComparisonoftheORRactivityofCu70Ag30withthatofAgsinglecrystals,Fe-N-Ccatalystsandstate-of-the-artPt/CisprovidedinTableS1.

To investigate the factorsunderlyingORRactivity improvements, the surfaceatomic compositionandstructure of the CuAg thin films were evaluated. XPS was conducted before and immediately afterelectrochemical ORR testing (Figure S4). The as-prepared thin films exhibit surface compositionsconsistentwithnominal bulk values (black squares in Figure3f). FollowingORR testing, significantAgsurfaceenrichmentoftheCuAgthinfilmwasobserved(redcirclesinFigure3f),limitedonlytothetopseveralnanometers(FigureS5).ThislikelyresultsfromsurfacesegregationofAgduetoitslowersurfaceenergy in comparison to Cu, or due tomild dissolution of surface Cu species during electrochemicalcycling.[84,85]These results indicate that it isaAg-enrichedsurface that facilitates theORRcatalysismeasuredbyRDE.BesidestheAgsurfaceenrichment,highresolutionXPSspectraalsoshowedahigherproportionofCuOtoCuo/Cu2Opresentinthethinfilmsfollowingelectrochemicaltesting(FigureS4).

Figure3:(a)CVcurvesinAr-saturatedand(b)ORRpolarizationcurvesinO2-saturated0.1MKOHforCu,AgandCu70Ag30.(c)ComparativeORRpolarizationcurvesforvariousCuAgcompositions.(d)KineticallycorrectedTafelplotsand(e)geometricbasedactivityvaluesat0.8VvsRHEforallelectrodes.(f)XPS

determinedsurfacecompositionsofCuAgelectrodesbeforeandafterelectrochemicaltesting.

The crystal structure of as-prepared and ORR tested thin films was investigated further by GI-XRDmeasurementsatSSRLbeamline7-2.X-rayincidenceangleswerevariedfromnearthecriticalangleto1°tostudythethinfilmstructureasafunctionofdepth.Figure4aand4bshowthediffractionpatternsforCu70Ag30 before and afterORR, respectively.Measurements are shown for the shallowest (0.19°) anddeepest(1.0°)measuredgrazingincidenceangles,correspondingtoaca.6nmpenetrationdepthandabulkmeasurement,respectively.WhilethebulkappearsunchangedthroughORRtesting,theincreasein

therelativeintensityofthe(111)peakoftheAg-richphaserelativetothatoftheCu-richphasepost-ORRindicatesasurfaceenrichmentofAg,consistentwithresultsfromXPS.Interestingly,thesurfacelatticeparametersarenotappreciablydifferentbetweenas-depositedandpost-ORRsamples.ThisindicatesthattheAg-richsurfacephaseisstillcomprisedofasignificantamountofCu(22at.%),anditisthisAg-richphasethatcouldbeunderlyingthemeasuredORRactivityenhancements.Theemergenceofaveryminorpeakata2-thetaangleof21°isalsoobservedfollowingelectrochemistry,attributedtoCu-oxide.

Figure4–GI-XRDmeasurementsonas-preparedandafterORRelectrochemicaltestingCu70Ag30thinfilmswith(a)x-rayincidentangleof0.19°correspondingtoanestimated6nmpenetrationdepthand

(b)x-rayincidentangleof1°correspondingtoabulkmeasurement.

TheORRactivityenhancementachievedthroughsubstitutionofAgforrelativelyinactive(andinexpensive)Cuatomsisofinterestfrombothafundamentalandtechnologicalperspective.Tounderstandthenatureofactive sitespotentiallyunderlying thenotableORRactivityenhancement,density functional theory(DFT) calculationswere performed. Theheterogeneous nature of CuAg electrodes, alongwith awidevarietyofsurfaceconfigurationspossibleunderelectrochemicalconditionsledustoinvestigateseveraldifferentstructureswithexamplesshown inFigure5a.XRDmeasurementsshowingmetastableCu/Agmiscibilityand latticespacingshifts suggestsan interplaybetweenatomicensemble, ligandandstraineffectsonORRactivity(foradetaileddiscussionofthesephenomenathereaderisreferredtoreferences[86,87]).Assuminganassociative reactionmechanismwithOOH*,O*andOH*as intermediates,[56]activitywasevaluatedbasedonthecalculatedlimitingpotential(UL),definedasthehighestpotentialatwhich all reaction steps become exergonic. This theoretical basis has played a key role towardsunderstanding the nature of active site structures, and has been used to guide catalyst design andoptimization efforts[88-90] with a close correlation being made between the thermodynamics andkineticsforthefour-electronORR.[91]

ToprobetheroleofstraineffectsonORRactivity,Agsurfacestructuresrangingfrom-5%compressiveto+5%tensilestrainwerestudied(Figure5a).Ag(111)hasanoxygenbindingenergyontheweaksideof

optimalfortheORR,[56]leadingtoacalculatedlimitingpotentialof0.4VvsRHE.Itiswellknownthatcompressivelatticestrainweakensthesurfacebindingenergyofadsorbedintermediatesduetoloweringof the d-band position, while tensile strain has the opposite effect.[92, 93] Figure 5b and S6correspondinglyshowthattheORRactivityofAgdecreasesundercompressivestrain,andincreasesundertensilestrain.AsXRDindicatedthatthesmallersizeofCuatomsleadstolatticecompressionoftheAg-richphase, this analysis suggests thatpurely straineffects arenot responsible for thehighactivityofCu70Ag30.However,giventheCu/AgmiscibilityshownbyXRD,weinvestigatedthecaseoftrappedsurfaceCuatomsintheAg-richphase(Cua-Ag,Figure5a).Toadditionallyaccountforstraineffects,weconsideredCua-Agwith varying degrees of host Ag lattice strain as a simple representative case (Figure S7). ThecalculatedlimitingpotentialofunstrainedCua-Agis0.64VvsRHE,indicatinganincreaseinactivityversuspureAg(111).Withacompressivestrainof2%,Cua-Ag(-2%)showsanevenfurtherincreaseincalculatedlimiting potential to 0.72 V vs RHE (Figure 5b), followed by a decrease in activity with increasingcompressivestrain (FigureS7).Tensilestrainontheotherhand leads toasignificantdecrease inORRactivitycomparedtoCua-AgandisunlikelyinthesethinfilmsbasedonXRDanalysis.Usingthed-bandmodel,[90, 94, 95] these reactivity trends are attributed to a dramatic difference in the d-projecteddensityofstates(PDOS)ofaCuatominCua-AgversusCu(111)(FigureS8).Thisdifferenceresults inanegative shiftof thed-bandedgeofCua-Ag relative to theFermi level, culminating ina loweroxygenbindingenergyforCua(TableS3)andaccompaniedincreaseinORRactivity.Thisshiftismostprominentfor Cua-Ag(-2%), which is thereby calculated to provide the highest activity among all Cua-Agconfigurationsinvestigated.Furthermore,thisistheexpectedstructureinthesethinfilmcatalystsbasedon XRD analysis.While the Cua atoms in Cua-Ag are predicted to provide the enhancedORR activity,electronicstructurechangestoAginCua-Ag,andtoAginAga-Cuwerealsoprobed(detailsprovidedintheSupportingInformation).TheseconfigurationswerefoundtohavesimilarelectronicstructurestoAginAg(111), indicating that they neither hinder activity nor were underlying the significant ORR activityenhancementsobservedexperimentallyincomparisontopureAg.

Another possibility for activity enhancement is the presence of Cu-oxide species under ORR relevantpotentials(Figure2a),whichledustoinvestigatetheinfluenceofCuOandCu2Oonactivity.WhileourcalculationssuggestedthatCu-oxidespeciesarenotdirectlyresponsiblefortheobservedORRactivityincrease(FigureS9),XPSandGI-XRDcharacterizationofpost-ORRCuAgelectrodesledustoinvestigatethepossibilityofanAgoverlayerondifferentCu-oxidesubstrates(Figure5a).Severalofthesestructuresresulted inmajor reconstructiondue to latticemismatching,however reasonably stableAgoverlayerscouldbeformedonCuO(100)andCu2O(111),exhibiting limitingpotentialsof0.60and0.40VvsRHE,respectively.With additional details provided in the Supporting Information (Figure S9), DFT analysissuggeststhattheORRactivityenhancementcouldpotentiallyarisefromtheformationofAgoverlayersonCu-oxideunderreactionconditions.

Figure5-(a)VariousconfigurationsoftheCuAgsystemexaminedbyDFTfortheireffectonORRactivity.(b)ORRactivitymapshowingcalculatedlimitingpotentialsforthevariousstructures

investigatedbyDFT.

While there are likely several active site configurations at play, DFT investigations suggest twoconfigurationsthatcouldperhapsberesponsibleforthesignificantactivityenhancementobservedforthe CuAg thin films versus pure Ag. Particularly: (i) Ag overlayers on Cu-oxide species, identified bymodellingAg/CuO(100); and (ii) Cu atoms in aAg-rich phase (Cua-Ag),with the activity enhancementmaximizedunder-2%compressivestrainofthehostAglattice.Thesefindingsareincontrasttopreviouswork[48]thatusedDFTtospeculatethataAgoverlayeronaAg3Cu(111)slabwasresponsibleforORRactivity enhancements. In our work, correlating the two configurations identified by computationalinvestigationswithexperimentalresultsleadsustobelievethatitistheCua-Agconfigurationunderlyingthe observed ORR activity enhancement. This is based on XRD measurements clearly indicating thepresence of an Ag-rich phase with Cu atom substituents. The highest activity Cu70Ag30 thin film alsopossessesacompressivestrainof ca. -2.4%versuspureAg,veryclose to themostactiveCua-Ag(-2%)structuredeterminedcomputationally.GI-XRDfurtherindicatedthattheproportionofthiscompressivelystrainedCua-AgstructureatthesurfaceofCu70Ag30increasesasaresultofORRtesting.WealsobelievethatCua-AgasthesourceofimprovedactivitycouldgiverisetotheCuAgactivityhysteresisobservedinFigure3band3c.Notably,thebaselineCVprofileofCu70Ag30(Figure3a)demonstratesredoxfeaturesintheregionofenhancedORRactivity(i.e.,0.70to0.85VvsRHE)thatarenotobservedonpureAgorCuthinfilms.WetherebysuggestthattheseredoxfeaturesarisefromtheinfluencethatAghasonCuatomsintermsofweakenedoxygenbindingenergy.ByfacilitatingthereductionofCu-oxidespeciesatpotentialshigherthanpossibleonpureCu,itispossiblethatthesereducedCuatomsinCua-Agcangiverisetotheactivity enhancement in this potential region. This could explain the additional overpotential in the

cathodicsweepdirectionforCuAg,asitisrequiredtoreduceaportionoftheseCu-oxidespeciesbeforeenhancedORRactivityisobserved.ValidationofthishypothesisstillrequireselucidationoftheatomicandelectronicstructureoftheseCuAgthinfilmsurfacesunderoperatingconditions,andwillbefeaturedinafuturepublication.Regardless,ORRactivityimprovementsareconclusivelydemonstratedforCuAgthinfilms.CorrelatingDFTandstructuralcharacterization,theseactivityenhancementsarepostulatedtoariseduetothemiscibilityofCuatomswithintheAg-richphase.

Conclusions

CuAg thin filmswerepreparedbye-beamPVDand tested forORRactivity inalkalineelectrolyte.Theeffect of composition on activity was investigated, with Cu70Ag30 providing a 4-fold ORR activityimprovementelectrodegeometricareabasis)versuspureAgat0.8VvsRHE.ImprovedORRkineticsoftheCu-richthinfilms(<50at%Cu)wasdemonstratedbyaTafelslopeof45mV/dec,whichismuchlowerthanthatofAg(90mV/dec),allowingforevengreateractivityenhancementathigheroverpotentials.XRDoftheaspreparedthinfilmsdemonstratedsignificantinter-metalmiscibilitytoformaCu-richandanAg-rich phase, despite the fact that thesemetals are completely thermodynamically immiscible attemperaturesbelowca.200°C.XPSandsynchrotronGI-XRDcharacterizationofthethinfilmsafterORRmeasurementsindicatedasurfaceenrichmentoftheAg-richphase.ExperimentalmeasurementswerecorrelatedwithDFTtounderstandobservedORRactivityimprovements.TheseresultssuggestthattheactivityenhancementarisesduetothepresenceofCuatomswithintheAg-richphaseofthecatalyst,whichtunesboththegeometricandelectronicstructure,allowingfor improvedbindingenergieswithreactingspecies.

ASSOCIATEDCONTENT

Supporting Information Available: Electrochemical activity comparison with literature reported data,resultssummaryofVergard’sLawanalysisofXRDpatterns,supplementaryexperimentalcharacterization(SEM,CV,AFM,XPSdepthprofiling)andadditionalcomputationaldetailsandanalysis.Rawdatafromallmaintextfiguresarealsoprovided,inlinewithDepartmentofEnergyrequirements.

Acknowledgements

Thisworkwas supported by theU.S.Department of Energy,Office of Science,Office of Basic EnergyScience,viaGrantDE-SC0008685totheSUNCATCenterof InterfaceScienceandCatalysis.Partofthisworkwasperformedat theStanfordNanoSharedFacilities (SNSF),supportedbytheNationalScienceFoundation under award ECCS-1542152. Use of Stanford Synchrotron Radiation Lightsource, SLACNationalAcceleratorLaboratoryissupportedbytheUSDepartmentofEnergy,OfficeofScience,OfficeofBasicEnergyScience,underContractDE-AC02-76SF00515.D.H.gratefullyacknowledgessupportfromtheBantingPostdoctoralFellowshipsprogram,administeredbytheGovernmentofCanada.B.M.G.gratefullyacknowledgessupport fromtheUSDepartmentofDefensethroughtheNationalDefenseScienceandEngineeringGraduateFellowship.M.E.-E.gratefullyacknowledgesfundingfromtheDanishCouncil forIndependentResearchundertheSapereAude–ResearchTalentProgram.M.G.-M.acknowledgesTrinityCollegeDublin for financial support.Qizhan Tam andMakotoMitani are gratefully acknowledged for

assistancewithAFMimagingandprocessing.AlanLandersisgratefullyacknowledgedforhisdevelopmentofrefractioncorrectionmethodsforthegrazingincidencex-raydiffraction.

References

(1)Debe,M.K.,Electrocatalystapproachesandchallengesforautomotivefuelcells.Nature,2012,486,43.(2)Gasteiger,H.A.,Kocha,S.S.,Sompalli,B.,Wagner,F.T.,ActivitybenchmarksandrequirementsforPt,Pt-alloy,andnon-PtoxygenreductioncatalystsforPEMFCs.AppliedCatalysisB:Environmental,2005,56,9.(3)Higgins,D., Zamani,P., Yu,A.,Chen, Z., Theapplicationof grapheneand its composites inoxygenreductionelectrocatalysis:aperspectiveandreviewofrecentprogress.Energy&EnvironmentalScience,2016,9,357.(4) Jaouen,F.,Proietti,E., Lefevre,M.,Chenitz,R.,Dodelet, J.-P.,Wu,G.,Chung,H.T., Johnston,C.M.,Zelenay,P.,Recentadvances innon-preciousmetalcatalysis foroxygen-reductionreaction inpolymerelectrolytefuelcells.Energy&EnvironmentalScience,2011,4,114.(5)Banham,D.,Ye,S.,Pei,K.,Ozaki,J.-i.,Kishimoto,T.,Imashiro,Y.,Areviewofthestabilityanddurabilityofnon-preciousmetalcatalystsfortheoxygenreductionreactioninprotonexchangemembranefuelcells.JournalofPowerSources,2015,285,334.(6)Chen,Z.,Higgins,D.,Yu,A.,Zhang,L.,Zhang,J.,Areviewonnon-preciousmetalelectrocatalystsforPEMfuelcells.Energy&EnvironmentalScience,2011,4,3167.(7)Chung,H.T.,Cullen,D.A.,Higgins,D.,Sneed,B.T.,Holby,E.F.,More,K.L.,Zelenay,P.,Directatomic-levelinsightintotheactivesitesofahigh-performancePGM-freeORRcatalyst.Science,2017,357,479.(8)Ge,X.,Sumboja,A.,Wuu,D.,An,T.,Li,B.,Goh,F.W.T.,Hor,T.S.A.,Zong,Y.,Liu,Z.,OxygenReductioninAlkalineMedia:FromMechanismstoRecentAdvancesofCatalysts.ACSCatalysis,2015,5,4643.(9)Li,Q.,Cao,R.,Cho,J.,Wu,G.,NanocarbonElectrocatalystsforOxygenReductioninAlkalineMediaforAdvancedEnergyConversionandStorage.AdvancedEnergyMaterials,2014,4,1301415.(10) Wang, Y.-J., Qiao, J., Baker, R., Zhang, J., Alkaline polymer electrolyte membranes for fuel cellapplications.ChemicalSocietyReviews,2013,42,5768.(11) Varcoe, J.R., Atanassov, P., Dekel, D.R., Herring, A.M., Hickner, M.A., Kohl, P.A., Kucernak, A.R.,Mustain,W.E.,Nijmeijer,K.,Scott,K.,Xu,T.,Zhuang,L.,Anion-exchangemembranesinelectrochemicalenergysystems.Energy&EnvironmentalScience,2014,7,3135.(12)Setzler,B.P.,Zhuang,Z.,Wittkopf,J.A.,Yan,Y.,Activitytargetsfornanostructuredplatinum-group-metal-freecatalystsinhydroxideexchangemembranefuelcells.NatNano,2016,11,1020.(13)Ramaswamy,N.,Mukerjee,S.,FundamentalMechanisticUnderstandingofElectrocatalysisofOxygenReductiononPtandNon-PtSurfaces:AcidversusAlkalineMedia.AdvancesinPhysicalChemistry,2012,2012,17.(14)Gorlin,Y.,Jaramillo,T.F.,ABifunctionalNonpreciousMetalCatalystforOxygenReductionandWaterOxidation.JournaloftheAmericanChemicalSociety,2010,132,13612.(15)Gorlin, Y., Chung, C.-J.,Nordlund,D., Clemens,B.M., Jaramillo, T.F.,Mn3O4SupportedonGlassyCarbon:AnActiveNon-PreciousMetalCatalystfortheOxygenReductionReaction.ACSCatalysis,2012,2,2687.(16)Suntivich, J.,Gasteiger,H.A.,Yabuuchi,N.,Nakanishi,H.,Goodenough, J.B.,Shao-Horn,Y.,Designprinciplesforoxygen-reductionactivityonperovskiteoxidecatalystsforfuelcellsandmetal–airbatteries.NatChem,2011,3,546.(17)Liang,Y.,Li,Y.,Wang,H.,Zhou,J.,Wang,J.,Regier,T.,Dai,H.,Co3O4nanocrystalsongrapheneasasynergisticcatalystforoxygenreductionreaction.NatMater,2011,10,780.(18) Chung, H.T., Won, J.H., Zelenay, P., Active and stable carbon nanotube/nanoparticle compositeelectrocatalystforoxygenreduction.Naturecommunications,2013,4,1922.

(19)Chen,Z.,Higgins,D.,Tao,H.,Hsu,R.S.,Chen,Z.,Highlyactivenitrogen-dopedcarbonnanotubesforoxygenreductionreactioninfuelcellapplications.TheJournalofPhysicalChemistryC,2009,113,21008.(20)Qiu,K.,Chai,G.,Jiang,C.,Ling,M.,Tang,J.,Guo,Z.,HighlyEfficientOxygenReductionCatalystsbyRationalSynthesisofNanoconfinedMaghemiteinaNitrogen-DopedGrapheneFramework.ACSCatalysis,2016,6,3558.(21) Xia, B.Y., Yan, Y., Li, N., Wu, H.B., Lou, X.W., Wang, X., A metal–organic framework-derivedbifunctionaloxygenelectrocatalyst.NatureEnergy,2016,1,15006.(22)Gu,S.,Sheng,W.,Cai,R.,Alia,S.M.,Song,S.,Jensen,K.O.,Yan,Y.,AnefficientAg–ionomerinterfaceforhydroxideexchangemembranefuelcells.ChemicalCommunications,2013,49,131.(23) Qaseem, A., Chen, F.,Wu, X., Johnston, R.L., Pt-free silver nanoalloy electrocatalysts for oxygenreductionreactioninalkalinemedia.CatalysisScience&Technology,2016,6,3317.(24)Xin,L.,Zhang,Z.,Wang,Z.,Qi, J.,Li,W.,CarbonsupportedAgnanoparticlesashighperformancecathodecatalystforH2/O2anionexchangemembranefuelcell.FrontiersinChemistry,2013,1.(25) Singh, P., Buttry, D.A., Comparison of Oxygen Reduction Reaction at Silver Nanoparticles andPolycrystallineSilverElectrodesinAlkalineSolution.TheJournalofPhysicalChemistryC,2012,116,10656.(26)Ohyama,J.,Okata,Y.,Watabe,N.,Katagiri,M.,Nakamura,A.,Arikawa,H.,Shimizu,K.-i.,Takeguchi,T.,Ueda,W.,Satsuma,A.,Oxygenreductionreactionoversilverparticleswithvariousmorphologiesandsurfacechemicalstates.JournalofPowerSources,2014,245,998.(27)Alia,S.M.,Duong,K.,Liu,T.,Jensen,K.,Yan,Y.,SupportlessSilverNanowiresasOxygenReductionReactionCatalystsforHydroxide-ExchangeMembraneFuelCells.ChemSusChem,2012,5,1619.(28)Garlyyev,B.,Liang,Y.,Butt,F.K.,Bandarenka,A.S.,EngineeringofHighlyActiveSilverNanoparticlesforOxygenElectroreductionviaSimultaneousControloverTheirShapeandSize.AdvancedSustainableSystems,2017,1,1700117.(29)Wang,N.,Cao,X.,Chen,Q.,Lin,G.,AgNanobelts:Synthesis,MorphologicalEvolution,andTheirUseasElectrocatalystsforOxygenReduction.Chemistry–AEuropeanJournal,2012,18,6049.(30)Lee,C.-L.,Chiou,H.-P.,Syu,C.-M.,Wu,C.-C.,Silvertriangularnanoplatesaselectrocatalystforoxygenreductionreaction.ElectrochemistryCommunications,2010,12,1609.(31)Zhou,Y.,Lu,Q.,Zhuang,Z.,Hutchings,G.S.,Kattel,S.,Yan,Y.,Chen,J.G.,Xiao,J.Q.,Jiao,F.,OxygenReductionatVeryLowOverpotentialonNanoporousAgCatalysts.AdvancedEnergyMaterials,2015,5,1500149.(32)HistoricalCharts&Data,2017.(33)Chen,C.,Kang,Y.,Huo,Z.,Zhu,Z.,Huang,W.,Xin,H.L.,Snyder,J.D.,Li,D.,Herron,J.A.,Mavrikakis,M.,Chi,M.,More,K.L.,Li,Y.,Markovic,N.M.,Somorjai,G.A.,Yang,P.,Stamenkovic,V.R.,HighlyCrystallineMultimetallicNanoframeswithThree-DimensionalElectrocatalyticSurfaces.Science,2014,343,1339.(34)Huang,X.,Zhao,Z.,Cao,L.,Chen,Y.,Zhu,E.,Lin,Z.,Li,M.,Yan,A.,Zettl,A.,Wang,Y.M.,Duan,X.,Mueller,T.,Huang,Y.,High-performancetransitionmetal–dopedPt₃Nioctahedraforoxygenreductionreaction.Science,2015,348,1230.(35)Stamenkovic,V.,Mun,B.S.,Mayrhofer,K.J.J.,Ross,P.N.,Markovic,N.M.,Rossmeisl,J.,Greeley,J.,Nørskov, J.K., Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the SurfaceElectronicStructure.AngewandteChemie,2006,118,2963.(36)Stamenkovic,V.R.,Fowler,B.,Mun,B.S.,Wang,G.,Ross,P.N.,Lucas,C.A.,Marković,N.M.,ImprovedOxygenReductionActivityonPt₃Ni(111)viaIncreasedSurfaceSiteAvailability.Science,2007,315,493.(37)Wang,D.,Xin,H.L.,Hovden,R.,Wang,H.,Yu,Y.,Muller,D.A.,DiSalvo,F.J.,Abruña,H.D.,Structurallyordered intermetallic platinum–cobalt core–shell nanoparticleswithenhancedactivity and stability asoxygenreductionelectrocatalysts.NatMater,2013,12,81.

(38)Higgins,D.C.,Wang,R.,Hoque,M.A.,Zamani,P.,Abureden,S.,Chen,Z.,Morphologyandcompositioncontrolledplatinum–cobalt alloynanowirespreparedby electrospinning as oxygen reduction catalyst.NanoEnergy,2014,10,135.(39)Bu,L.,Guo,S.,Zhang,X.,Shen,X.,Su,D.,Lu,G.,Zhu,X.,Yao,J.,Guo,J.,Huang,X.,Surfaceengineeringofhierarchicalplatinum-cobaltnanowiresforefficientelectrocatalysis.2016,7,11850.(40)Zafeiratos,S.,Piccinin,S.,Teschner,D.,Alloysincatalysis:phaseseparationandsurfacesegregationphenomenainresponsetothereactiveenvironment.CatalysisScience&Technology,2012,2,1787.(41)Prieto,G.,Beijer,S.,Smith,M.L.,He,M.,Au,Y.,Wang,Z.,Bruce,D.A.,deJong,K.P.,Spivey,J.J.,deJongh,P.E.,DesignandSynthesisofCopper–CobaltCatalystsfortheSelectiveConversionofSynthesisGastoEthanolandHigherAlcohols.AngewandteChemieInternationalEdition,2014,53,6397.(42)Courty,P.,Durand,D., Freund,E., Sugier,A.,C1-C6alcohols fromsynthesis gason copper-cobaltcatalysts.JournalofMolecularCatalysis,1982,17,241.(43)Mendes,L.V.P.,Snider,J.L.,Fleischman,S.D.,Kibsgaard,J.,McEnaney,J.M.,Aranda,D.A.G.,Jaramillo,T.F.,PolyolSynthesisofCobalt–CopperAlloyCatalystsforHigherAlcoholSynthesisfromSyngas.CatalysisLetters,2017,147,2352.(44)Huang,S.-y.,Chang,S.-m.,Lin,C.-l.,Chen,C.-h.,Yeh,C.-t.,PromotionoftheElectrochemicalActivityofaBimetallicPlatinum−RutheniumCatalystbyOxidation-InducedSegregation.TheJournalofPhysicalChemistryB,2006,110,23300.(45) Ma, S., Sadakiyo, M., Heima, M., Luo, R., Haasch, R.T., Gold, J.I., Yamauchi, M., Kenis, P.J.A.,Electroreduction of Carbon Dioxide to Hydrocarbons Using Bimetallic Cu–Pd Catalysts with DifferentMixingPatterns.JournaloftheAmericanChemicalSociety,2017,139,47.(46)Holewinski, A., Idrobo, J.-C., Linic, S.,High-performanceAg–Coalloy catalysts for electrochemicaloxygenreduction.NatChem,2014,6,828.(47)Qaseem,A.,Chen,F.,Zhang,N.,inAdvancedCatalyticMaterials-PhotocatalysisandOtherCurrentTrends,eds.L.E.NorenaandJ.-A.Wang,InTech,Rijeka,2016,p.Ch.15.(48)Zhang,N.,Chen,F.,Wu,X.,Wang,Q.,Qaseem,A.,Xia,Z.,Theactivityoriginofcore-shellandalloyAgCubimetallicnanoparticlesfortheoxygenreductionreaction.JournalofMaterialsChemistryA,2017,5,7043.(49)Wu,X.,Chen,F.,Zhang,N.,Qaseem,A.,Johnston,R.L.,EngineeringBimetallicAg–CuNanoalloysforHighly Efficient Oxygen Reduction Catalysts: A Guideline for Designing Ag-Based ElectrocatalystswithActivityComparabletoPt/C-20%.Small,2017,13,1603876.(50)Toney,M.F.,Brennan,S.,Observationof theeffectof refractiononx raysdiffracted inagrazing-incidenceasymmetricBragggeometry.PhysicalReviewB,1989,39,7963.(51) Shinozaki, K., Zack, J.W., Richards, R.M., Pivovar, B.S., Kocha, S.S., Oxygen Reduction ReactionMeasurements on Platinum Electrocatalysts Utilizing Rotating Disk Electrode Technique: I. Impact ofImpurities,MeasurementProtocolsandAppliedCorrections.JournalofTheElectrochemicalSociety,2015,162,F1144.(52)Bahn,S.R.,Jacobsen,K.W.,Anobject-orientedscriptinginterfacetoalegacyelectronicstructurecode.ComputinginScience&Engineering,2002,4,56.(53)Paolo,G.,Stefano,B.,Nicola,B.,Matteo,C.,Roberto,C.,Carlo,C.,Davide,C.,Guido,L.C.,Matteo,C.,Ismaila,D.,AndreaDal,C.,Stefanode,G.,Stefano,F.,Guido,F.,Ralph,G.,Uwe,G.,Christos,G.,Anton,K.,Michele,L.,Layla,M.-S.,Nicola,M.,Francesco,M.,Riccardo,M.,Stefano,P.,Alfredo,P.,Lorenzo,P.,Carlo,S., Sandro, S., Gabriele, S., Ari, P.S., Alexander, S., Paolo, U., Renata,M.W., QUANTUM ESPRESSO: amodular and open-source software project for quantum simulations of materials. Journal of Physics:CondensedMatter,2009,21,395502.(54)AlwaleedAhmed,A.,AndreaDal,C.,Ultrasoftpseudopotentialsandprojectoraugmented-wavedatasets:applicationtodiatomicmolecules.JournalofPhysics:CondensedMatter,2011,23,425501.

(55)Hammer,B.,Hansen,L.B.,Nørskov,J.K., Improvedadsorptionenergeticswithindensity-functionaltheoryusingrevisedPerdew-Burke-Ernzerhoffunctionals.PhysicalReviewB,1999,59,7413.(56)Nørskov,J.K.,Rossmeisl,J.,Logadottir,A.,Lindqvist,L.,Kitchin,J.R.,Bligaard,T.,Jónsson,H.,OriginoftheOverpotentialforOxygenReductionataFuel-CellCathode.TheJournalofPhysicalChemistryB,2004,108,17886.(57) Thornton, J.A., Influence of apparatus geometry and deposition conditions on the structure andtopographyofthicksputteredcoatings.JournalofVacuumScienceandTechnology,1974,11,666.(58)Evans,D.,Wilman,H.,Crystalgrowthandorientationindepositscondensedfromthevapour.ActaCrystallographica,1952,5,731.(59)Denton,A.R.,Ashcroft,N.W.,Vegard'slaw.PhysicalReviewA,1991,43,3161.(60)Saunders,N.,Miodownik,A.,Phase formation inco-depositedmetallicalloy thin films. Journalofmaterialsscience,1987,22,629.(61)Barber,Z.H.,Thegrowthofsputter-depositedsilver-copperfilms.Vacuum,1990,41,1102.(62)Mader,S.,Metastablealloyfilms.JournalofVacuumScienceandTechnology,1965,2,35.(63)Tang,M.H.,Hahn,C.,Klobuchar,A.J.,Ng,J.W.D.,Wellendorff,J.,Bligaard,T.,Jaramillo,T.F.,Nickel-silver alloy electrocatalysts for hydrogen evolution and oxidation in an alkaline electrolyte. PhysicalChemistryChemicalPhysics,2014,16,19250.(64)Hahn,C.,Abram,D.N.,Hansen,H.A.,Hatsukade,T.,Jackson,A.,Johnson,N.C.,Hellstern,T.R.,Kuhl,K.P.,Cave,E.R.,Feaster,J.T.,Jaramillo,T.F.,SynthesisofthinfilmAuPdalloysandtheirinvestigationforelectrocatalyticCO2reduction.JournalofMaterialsChemistryA,2015,3,20185.(65)Snyder,J.,Danilovic,N.,Paulikas,A.P.,Tripkovic,D.,Strmcnik,D.,Markovic,N.M.,Stamenkovic,V.R.,ThinFilmApproachtoSingleCrystallineElectrochemistry.TheJournalofPhysicalChemistryC,2013,117,23790.(66)Hahn,C.,Hatsukade,T.,Kim,Y.-G.,Vailionis,A.,Baricuatro,J.H.,Higgins,D.C.,Nitopi,S.A.,Soriaga,M.P., Jaramillo, T.F., Engineering Cu surfaces for the electrocatalytic conversion of CO2: Controllingselectivitytowardoxygenatesandhydrocarbons.ProceedingsoftheNationalAcademyofSciences,2017,114,5918.(67)Droog,J.M.M.,Alderliesten,C.A.,Alderliesten,P.T.,Bootsma,G.A.,Initialstagesofanodicoxidationof polycrystalline copper electrodes in alkaline solution. Journal of Electroanalytical Chemistry andInterfacialElectrochemistry,1980,111,61.(68)Droog,J.M.M.,Schlenter,B.,Oxygenelectrosorptiononcoppersinglecrystalelectrodesinsodiumhydroxidesolution.JournalofElectroanalyticalChemistryandInterfacialElectrochemistry,1980,112,387.(69)Kunze,J.,Maurice,V.,Klein,L.H.,Strehblow,H.-H.,Marcus,P.,InSituScanningTunnelingMicroscopyStudyoftheAnodicOxidationofCu(111)in0.1MNaOH.TheJournalofPhysicalChemistryB,2001,105,4263.(70)Kunze,J.,Maurice,V.,Klein,L.H.,Strehblow,H.-H.,Marcus,P.,InsituSTMstudyoftheduplexpassivefilmsformedonCu(111)andCu(001)in0.1MNaOH.CorrosionScience,2004,46,245.(71)Reyter,D.,Odziemkowski,M.,Bélanger,D.,Roué,L.,ElectrochemicallyActivatedCopperElectrodes:SurfaceCharacterization,ElectrochemicalBehavior,andProperties for theElectroreductionofNitrate.JournalofTheElectrochemicalSociety,2007,154,K36.(72)Maurice,V.,Strehblow,H.-H.,Marcus,P., InsituSTMstudyoftheinitialstagesofoxidationofCu(111)inaqueoussolution.SurfaceScience,2000,458,185.(73)Savinova,E.R.,Scheybal,A.,Danckwerts,M.,Wild,U.,Pettinger,B.,Doblhofer,K.,Schlogl,R.,Ertl,G.,StructureanddynamicsoftheinterfacebetweenaAgsinglecrystalelectrodeandanaqueouselectrolyte.FaradayDiscussions,2002,121,181.(74)Jovic,B.M.,Jovic,V.D.,Stafford,G.R.,CyclicvoltammetryonAg(111)andAg(100)faces insodiumhydroxidesolutions.ElectrochemistryCommunications,1999,1,247.

(75)Horswell, S.L.,Pinheiro,A.L.N.,Savinova,E.R.,Danckwerts,M.,Pettinger,B.,Zei,M.-S.,Ertl,G.,AComparative Study ofHydroxideAdsorption on the (111), (110), and (100) Faces of Silverwith CyclicVoltammetry,ExSituElectronDiffraction,andInSituSecondHarmonicGeneration.Langmuir,2004,20,10970.(76) Orozco, G., Pérez, M.C., Rincón, A., Gutiérrez, C., Adsorption and Electrooxidation of CarbonMonoxideonSilver.Langmuir,1998,14,6297.(77) Savinova, E.R., Kraft, P., Pettinger, B., Doblhofer, K., In situ Raman spectroscopy studies of theinterface between silver(111) electrodes and alkaline NaF electrolytes. Journal of ElectroanalyticalChemistry,1997,430,47.(78)Vazquez,M.V.,deSanchez,S.R.,Calvo,E.J.,Schiffrin,D.J.,Theelectrochemicalreductionofoxygenonpolycrystallinecopperinboraxbuffer.JournalofElectroanalyticalChemistry,1994,374,189.(79)Staszak-Jirkovský,J.,Subbaraman,R.,Strmcnik,D.,Harrison,K.L.,Diesendruck,C.E.,Assary,R.,Frank,O.,Kobr,L.,Wiberg,G.K.H.,Genorio,B.,Connell,J.G.,Lopes,P.P.,Stamenkovic,V.R.,Curtiss,L.,Moore,J.S., Zavadil, K.R.,Markovic,N.M.,WaterasaPromoterandCatalyst forDioxygenElectrochemistry inAqueousandOrganicMedia.ACSCatalysis,2015,5,6600.(80) Blizanac, B.B., Ross, P.N., Marković, N.M., Oxygen Reduction on Silver Low-Index Single-CrystalSurfacesinAlkalineSolution: RotatingRingDiskAg(hkl)Studies.TheJournalofPhysicalChemistryB,2006,110,4735.(81) Blizanac, B.B., Ross, P.N., Markovic, N.M., Oxygen electroreduction on Ag(111): The pH effect.ElectrochimicaActa,2007,52,2264.(82)Wiberg,G.K.H.,Mayrhofer,K.J.J.,Arenz,M.,InvestigationoftheOxygenReductionActivityonSilver–ARotatingDiscElectrodeStudy.FuelCells,2010,10,575.(83)Shinagawa,T.,Garcia-Esparza,A.T.,Takanabe,K.,InsightonTafelslopesfromamicrokineticanalysisofaqueouselectrocatalysisforenergyconversion.ScientificReports,2015,5,13801.(84)Ambrose,J.,Barradas,R.G.,Shoesmith,D.W.,Investigationsofcopperinaqueousalkalinesolutionsbycyclicvoltammetry.JournalofElectroanalyticalChemistryandInterfacialElectrochemistry,1973,47,47.(85) Ambrose, J., Barradas, R.G., Shoesmith, D.W., Rotating copper disk electrode studies of themechanismofthedissolution-passivationsteponcopperinalkalinesolutions.JournalofElectroanalyticalChemistryandInterfacialElectrochemistry,1973,47,65.(86)Stephens,I.E.L.,Bondarenko,A.S.,Gronbjerg,U.,Rossmeisl,J.,Chorkendorff,I.,Understandingtheelectrocatalysisofoxygenreductiononplatinumanditsalloys.Energy&EnvironmentalScience,2012,5,6744.(87)Vej-Hansen,U.G.,Escudero-Escribano,M.,Velázquez-Palenzuela,A.,Malacrida,P.,Rossmeisl,J.,L.Stephens, I.E., Chorkendorff, I., Schiøtz, J., New Platinum Alloy Catalysts for Oxygen ElectroreductionBasedonAlkalineEarthMetals.Electrocatalysis,2017,8,594.(88)Seh,Z.W.,Kibsgaard,J.,Dickens,C.F.,Chorkendorff,I.,Nørskov,J.K.,Jaramillo,T.F.,Combiningtheoryandexperimentinelectrocatalysis:Insightsintomaterialsdesign.Science,2017,355.(89)Norskov,J.K.,Bligaard,T.,Rossmeisl,J.,Christensen,C.H.,Towardsthecomputationaldesignofsolidcatalysts.NatChem,2009,1,37.(90)Mavrikakis,M.,Hammer,B.,Nørskov,J.K.,EffectofStrainontheReactivityofMetalSurfaces.PhysicalReviewLetters,1998,81,2819.(91)Hansen,H.A.,Viswanathan,V.,Nørskov,J.K.,UnifyingKineticandThermodynamicAnalysisof2e–and4e–ReductionofOxygenonMetalSurfaces.TheJournalofPhysicalChemistryC,2014,118,6706.(92)Stamenkovic,V.,Mun,B.S.,Mayrhofer,K.J.J.,Ross,P.N.,Markovic,N.M.,Rossmeisl,J.,Greeley,J.,Nørskov, J.K., Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the SurfaceElectronicStructure.AngewandteChemieInternationalEdition,2006,45,2897.

(93)Stephens,I.E.L.,Bondarenko,A.S.,Perez-Alonso,F.J.,Calle-Vallejo,F.,Bech,L.,Johansson,T.P.,Jepsen,A.K.,Frydendal,R.,Knudsen,B.P.,Rossmeisl,J.,Chorkendorff,I.,TuningtheActivityofPt(111)forOxygenElectroreductionbySubsurfaceAlloying.JournaloftheAmericanChemicalSociety,2011,133,5485.(94)Hammer,B.,Norskov,J.K.,Whygoldisthenoblestofallthemetals.Nature,1995,376,238.(95) Kitchin, J.R., Nørskov, J.K., Barteau, M.A., Chen, J.G., Role of Strain and Ligand Effects in theModificationof theElectronicandChemicalPropertiesofBimetallic Surfaces.PhysicalReviewLetters,2004,93,156801.

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