NiCu mixed metal oxide catalyst for alkaline hydrogen evolution in anion exchange membrane water electrolysis

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Electrochimica Acta 371 (2021) 137837

ContentslistsavailableatScienceDirect

Electrochimica Acta

journalhomepage:www.elsevier.com/locate/electacta

NiCu mixed metal oxide catalyst for alkaline hydrogen evolution in anion exchange membrane water electrolysis

Alaa Y. Faid

^a^,^∗

, Alejandro Oyarce Barnett

^b^,^c

, Frode Seland

^a

, Svein Sunde

^a

aDepartment of Materials Science and Engineering, Norwegian University of Science and Technology, Trondheim, Norway

bSINTEF Industry, New Energy Solutions Department, Trondheim, Norway

cDepartment of Energy and Process Engineering, Norwegian University of Science and Technology, Norway

a rt i c l e i nf o

Article history:

Received 13 August 2020 Revised 15 January 2021 Accepted 18 January 2021 Available online 21 January 2021 Keywords:

Hydrogen evolution Water electrolysis Nickel based catalyst In situ Raman

Anion exchange membrane

a b s t r a c t

Wereportontheoptimizationofnickel-coppercatalystsforsuperiorperformanceasacathodecatalystin anionexchangemembrane(AEM)waterelectrolysis.ThebifunctionalsystemofNiCumixedmetaloxide (MMO)nanosheetsincludes Nimetallic,NiO, andCuOoxidesproviderapidkinetics forthehydrogen- evolution reaction(HER)of the Volmer step. In-situRaman spectroscopy for NiCuMMO proved that nickelhydroxidewassustainedunderHERconditionsforatleast30,000s,whichmayexplainwhythe exceptionalactivityofNiCuMMOascomparedtoothernickel-coppercatalystsismaintainedovertime.

The activityofthe NiCuMMO forthe HERactivity inalkalineelectrolytes increased as KOH concen- trationraisedfrom0.1 Mto1 M.TheNiCuMMOnanosheetsshowedsuperiorstabilityunderalkaline HERconditionsfor30h.TheuseofNaﬁonionomerintheinkresultedinahigherHERcurrentdensity ascomparedtoinkswithaFumionanionionomer.ThemaximumHERperformancewasachievedata Naﬁonionomertocatalystweightratioof0.5.UsingIrblackastheanode,theNiCuMMOcathodegave anAEMelectrolyzer performance of1.85 A/cm² at2Vin1MKOH at50 °C.The NiCuMMOcatalyst developedheredeliversAEMperformancecomparabletoPEMwaterelectrolysis.

ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/)

1. Introduction

Eﬃcientandcheap renewablehydrogenproductionfromwater electrolysisisacrucialchallengeforasustainablesociety[1,2].An- ionexchangemembrane(AEM)waterelectrolysisaimstocombine the low cost ofalkaline with the advantagesof protonexchange membrane(PEM)electrolyzers [3,4].Nickelisthemostactiveand cheapestnon-noblemetalcatalystreportedforhydrogenevolution reaction (HER)in alkaline electrolysis[5,6].Nickel initially shows ahighHERactivity,buttheactivitydeterioratesrapidlyovertime due tothe formationof metal hydride underHER conditions[7]. AlloyingnickelwithotherelementssuchasMo,P, S,Cuincreases HER activity and stability [8]. Sluggish HER kinetics in alkaline electrolytescauseshighoverpotentialsthusremainsachallengeto developahighlyactiveandstableHERcatalyst[9,10].Themecha- nismoftheHERinalkalinemediaisusuallydiscussedintermsof the Volmer,Heyrovsky,andTafelreactions [11],andafundamen-

∗Corresponding author.

E-mail address: [email protected] (A.Y. Faid).

talunderstandingofthefactorsinﬂuencingtheratesofthesesteps mayprovideimportantcluesforcatalystdesign.

NiCu has been reported as an active HER catalyst in alkaline electrolytes.NiCu catalysts have been synthesizedusing different processes such as freeze casting [12] electrodeposition [13]and powder metallurgy[14]. He etal.obtained a currentof

−10 mA/cm² at 117 mV with NiCu synthesized by galvanostatic depositionandwithan atomicratioofNitoCuequaltoone [15]. ElectrodepositedNiCunanosheetsexhibitedenhancedHERactivity withan onset potential of48 mV vs. RHE [16]. Solmaz etal.re- portedthatNiCushowedhigherHERactivitythanNiandCudue totheroughnesseffectandsynergisticinteractionbetweenNiand Cuatoms[17,18].Oshchepkovetal.foundthat highmassactivity ofNi_0.95Cu_0.05/CisduetotheelectronicinﬂuenceofCuonNi[19]. However, thereisa deﬁciency intheliterature ofresultsdemon- stratingNiCucatalystasacathodeinrealalkalineelectrolyzers.

Mixedmetaloxide(Ni/NiO-transitionmetaloxide(TMO))com- positestructuresexhibitsuperiorHERactivity[20–22]NiOattracts OH_ads whilethemetallicNiattractsH_adsintermediateduringHER, thusloweringthefreeenergyoftheﬁrststepintheHER,viz.the formation of adsorbedhydrogen through the Volmerstep of the reaction. The other TMO oxide, such as Cr₂O₃ or Fe₃O₄ appears

https://doi.org/10.1016/j.electacta.2021.137837

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A.Y. Faid, A.O. Barnett, F. Seland et al. Electrochimica Acta 371 (2021) 137837

to stabilize the composite NiO component underHER conditions [20–23].Therefore,itisbeneficialto designHERcatalystcontain- ing both a catalytic element and elements(or oxides) stabilizing mixed oxidation states forthe catalytic element. Below we suggest NiCu mixedmetal oxide(MMO)to catalyzethe HERso that the catalyst containsboth Ni (which has an affinity forH_ad) and NiO(whichhasanaffinityforOH_ad),thesimultaneouspresenceof thesebeingstabilizedbyCuOunderHERconditions.

Ionomers are frequently employed in electrochemical testing is to promote ink uniformity and coating quality [24]. The pres- enceoftheionomermayalsoincreaseionicconductivityandmin- imizes mass-transport limitations related to the diffusion of the ionicspecies[24].Theliteraturehasreportedthat catalyticlayers containingNaﬁonionomerresultinhigherHERactivitycompared tocatalyticlayers withotheranionexchangeionomers[4,21].The activity difference has been attributedto several factors, such as ionomer head groups, and ionomer backbone chemistry [4,21]. However,tuningionomertocatalystratioisrequiredforoptimum catalystutilization[25].Notonlyitsactivityandstabilitybutalso theinteraction ofNiCumetal mixedoxide catalystwithionomers andeffectsofanaqueouselectrolyte,isthereforealsoimportantif thiscatalystistoplayaroleinupscalingtoAEMwaterelectrolyzer devices,preferablyincludingnotonlyexperimentsinaqueouscells butalsoelectrolyzertesting.

Inthiswork,weinvestigatetheHERactivityofvariousnickel- copper catalysts such as NiCualloy, NiCuoxide, andNiCu mixed metaloxide(MMO)synthesizedbychemicalreduction.Aswewill showbelow,NiCuMMOshowsanexceptionalHERactivityinalka- linemedia.InsituRamanspectroscopyunderHERconditionswas carried out to investigate the state of copper and nickel species present and how these states vary over time for various nickel coppercatalysts andcorrelatethiswiththeiractivityfortheHER.

The electrochemical activityofNiCu MMOwasfurtheroptimized intermsofthetype ofionomerbinder,KOHconcentration inthe aqueouselectrolyte,andtheionomertocarbonratio.AnAEMwa- ter electrolyzer basedonmembraneelectrode assembly (MEA)of NiCuMMOatthecathodeandIrblackattheanodewasfabricated, tested, andcompared toPt/IrMEA.The NiCuMMO/IrMEAshows comparableperformancetoPt/IrMEAwhichindicatesthatitcould replacescarceandexpensivePtcatalyst.

2. Experimental 2.1. Catalystsynthesis

NiCu alloy and NiCu oxide were synthesized by mixing 10 mmol of nickel nitrate hexahydrate Ni(NO₃)₂.6H₂O (97.0%, Sigma Aldrich) and 10 mmol Copper(II) sulfate pentahydrate (98.0%, SigmaAldrich) in500mlwater(18.2M^cm,³^ppb^TOC, Milli-Q ultrapure water). The precursor mixture was stirred for 15minat750rpm.200mlof0.15MNaBH₄(98%,SigmaAldrich) was added dropwise while bubbles were observed. The solution mixture was stirred for another 1 hour to ensure the complete chemical reduction of precursors. The resulting precipitate was centrifuged 5timesat8000rpmfor6minandcleanedwithwa- ter and ethanolthree times. The produced precipitate wasdried in avacuum oven at80°C overnight.The driedpowder wasan- nealed in an airatmosphere to obtainNiCu oxide or5%H₂/Arto obtainNiCualloy.Theannealingwasdone at500°Cfor6hwith arampingrateof10°C/min.

In ordertomakeNiCuMMO,100 mlof1M Na₂CO₃ (≥99.5%, Sigma-Aldrich) were added to nickel-copper precursors solution until thesolutionbecamemilky andpH reached10.Themixture wasthen stirredfor another15 min,followed bythe addition of NaBH₄ dropwise.Theproducedcatalystwassubjectedtothesame procedureforcleaninganddryingasabove.Thedriedpowderwas

annealedin5%H₂/Aratmosphereat500°Cfor6hwitharamping rateof10°C/min.

For catalyst supported on carbon, Ketjen black EC-600JD (Ak- zoNobel)wasdispersedintheprecursors’ solutionmixturetoget (60 wt% catalyst supported on carbon) and stirred for another 1hourbeforeaddingNaBH₄ andcompletethechemicalreduction step.

2.2. Structuralandelectrochemicalcharacterization

Scanning electron microscopy (SEM, Carl Zeiss supra 55) and energydispersiveX-ray(EDX)spectroscopyintheSEMdevicewere usedtostudythemorphologyandelementalcompositionofcata- lysts.ThecatalystmorphologywasfurtherstudiedusingHitachiS- 5500viascanningtransmissionelectronmicroscopy(STEM)mode.

BrukerD8A25DaVinciX-raydevice(Cu-K

α

^radiation^with^a^wave-

lengthof1.5425 ˚A)wasusedtoexaminethecrystallinecharacter- isticsofcatalysts.X-raydiffraction(XRD)patternswere takenbe- tween15[2

θ

^]^and⁷⁵^[2

θ

^]ûsingâ^step^sizeôf^0.3^[2

θ

^].^WITec^al-

pha300R Confocal Raman device with a532 nm laser wasused to collect the Raman vibrational characterstics of catalyst powders.X-rayphotoelectronspectroscopy(XPS)wasdoneviaanAxis UltraDLD instrument (Kratos Analytical) equippedwith Al X-ray monochromaticsource.

Electrochemical investigation of the catalysts was carried out ina three-electrode cellusing arotating diskelectrode (Pine Re- search,)with an (Ivium-n-Stat) potentiostat. Carbon paper(Toray 090, Fuel cell store) was used as the counter-electrode while Hg/HgOelectrode(PineResearch)wasservedasthereferenceelec- trode.Theworkingelectrodewascatalystdepositedonglassycar- bon(GC)electrodes(5mmdiameter,PineResearch).TheGCelec- trodewaspolishedusingaluminasuspension (5and0.05

μ

^m,^Al-

liedHigh-TechProducts,Inc.)onpolishingpads.TheGCelectrode was then washed, sonicated in 1 M KOH for 5 min, and ﬁnally rinsed with water. The catalyst ink was prepared by dispersing 10mgcatalystpowderin1.0mLofasolution[500

μ

^L^water,⁵⁰⁰

μ

^Lisopropanol]. TheionomerusedwaseitherNaﬁon (5wt%,Alfa Aesar)oranion exchange ionomerFumionFAA-3(10 wt% fumat- ech)with an ionomerto catalyst weight ratio of0.2. The Naﬁon ionomer to catalyst weight ratio in the ink was then optimized from a selection of weight ratios equal to 0.1, 0.3, 0.5, 0.7, and 0.9. The ink wasthen sonicatedfor30 minin an icebath.Cata- lystloadingontheGCsurfacewaskept250μg/cm².

Thecatalystinkwasspin-coatedona GCelectrodeturned up- side down and rotated to assure a homogenous catalyst distri- bution. A waterdrop wasdeposited on the electrode before im- mersed inthe electrolyte toprevent air bubblesfrom formingat theelectrode surface.Alltheelectrochemical measurementswere conductedinN₂-saturated1MKOHelectrolyteatroomtempera- ture(20 ±2).Theelectrolytewaspurgedfor30minwithN₂ gas before usingandduringthe experimenttoremove anydissolved gassesduring electrochemicalmeasurements. The electrolyte was preparedbyusingKOH(SigmaAldrich,85%),andwater(18.2M cm,Milli-Q^R Integralultrapurewater).Theelectrolytewaspuriﬁed accordingtotheprocedurereportedbyTrotochaudetal.[26].

Theworkingelectrodeunderwentelectrochemicalactivationby cycling between −0.8 to −1.5 V vs Hg/HgO at a scan rate of 100 mV/sfor50 cycles.The linearsweep voltammetry(LSV) polarization curves were recorded in a potential range of −0.8 to

−1.5Vvs.Hg/HgOat1mV/ssweeprateundercontinuousstirring at 1600 rpm to avoid the accumulation of gas bubbles over the GC electrode. The electrochemical impedance spectroscopy (EIS) measurements were collectedat speciﬁc overpotentials(−100 to

−250mV)inafrequencyrangeof0.1−10⁵ Hzwithanamplitude of10mValternativecurrent(AC)perturbation.Inthiswork,ohmic resistance (IR) drop was compensated at 85% of high-frequency

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Fig. 1. a) Schematics of the concept of in-situ electrochemical Raman spectroscopy reproduced with permission from American Chemical Society (ACS) [27] , b), c) actual images, and d) cell design of the in-situ Raman electrochemical cell used in this work [28] .

resistance,which wasmeasured by theEIStechnique.The poten- tialwascompensatedbythefollowingequation:

Ecompensated=Emeasured−iR (1)

where Ecompensated and E_measured are compensated and measured potentials,respectively.

TheHg/HgOpotentialswereconvertedtoRHEbymeasuringthe voltage atzerocurrentofthe HERcurve ina hydrogen-saturated electrolyteonPtelectrodes.TheHg/HgOreferenceelectrodepoten- tial wasconverted toRHEin 1MKOHusing thefollowingequa- tion:

EvsRHE=EvsHg/HgO+0.9 (2)

Allthereportedcurrentdensitieswerenormalizedtothegeomet- ricareaoftheelectrode.

The electrochemical active surfacearea (ECSA) was measured by theelectrochemical double-layercapacitance method.Thenca- pacitancefrom0.9to1VvsRHEatscanratesof50,100,150,200, 250 mV/s. The CV used for electrochemical double-layer capacitance(C_dl)calculation wasacquiredina potentialwindow where noFaradaicprocessoccurred.ToderivetheC_dl,thefollowingequa- tionwasused:

C_dl=Ic

ν

⁽³⁾

where C_dl is the double-layer capacitance (mF/cm²) of the elec- troactive materials, Ic is thecharging current(mA/cm²), and

ν

^is

thescanrate(mV/s).

Chronoamperometrywasmeasuredataﬁxedpotential(−0.4V vs. RHE) for30 h. The stabilityof the catalyst material wasalso evaluated using an acceleratedstress test (AST). AST wascarried outbycyclingtheelectrodebetween−0.8to−1.3Vatascanrate of 100 mA/cm² for5000 cycles.The Hg/HgOreference electrode wascalibrated versus a reversible hydrogen electrode (RHE)in 1 and0.1M KOH.The electrochemicaldatashownareaveragedata from3inksfromeverypowderforeachcatalyst.

2.3. InsituRamanmeasurements

InsituRamanmeasurementswerecarriedoutwithalab-made Teﬂon cell. The catalyst deposited on GC (pine research), a car- bonpaper(fuel cellstore),andHg/HgO(Pine Research)wasused asa working, counter, andreferenceelectrode, respectively asin Fig. 1. In situ Raman spectra were collected using a WITec al- pha300RConfocalRamanmicroscope[532nmlaserwithapower of5.0mW]coupled withZeiss ECEpiplan10xobjectiveandG1:

600g/mmBLZ=500nmgrating.TheGCsurfacewaspolishedwith

μ

^m-sized^alumina^powders,^then^sonicatedⁱⁿ¹^M^KOH^for⁵^min

andthenrinsedwithwateranddriedinair.Theexperimentswas carriedoutusingpuriﬁedN₂-saturated1MKOHelectrolytes.The laser is emitted on the working electrode through a transparent quartzglasswindowthatreducescontaminationandinterference.

Alltheexperimentswereconductedatroomtemperature(20±2

°C).Allthedatapointswereprocessedusingoriginsoftware.

In situRaman-chronoamperometry studywasdone at−0.4 V vs. RHE for30,000 s forNiCu catalysts.The Raman spectra were

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Fig. 2. Schematic of the membrane electrode assembly for AEM water electrolyzer.

collected at the applied potential in 1 M KOH every 10 sweeps (10 s/sweep)from 100 to2000 cm⁻¹. The spectrum shiftofsili- conwaferRamanpeakat520.7cm⁻¹wasusedforcalibration.

2.4. Membraneelectrodeassembly(MEA)fabricationand pretreatment

Catalyst inks were fabricated by mixingcatalyst powder with water: isopropanol (1:1), and ionomer (Fumion FAA-3-SOLUT-10 (Fuel Cell Store)). The solution wassonicated for 30 min to en- sureﬁne andwell-dispersedink. Cathodecatalysts loadings were 1 mg/cm² for Pt/C (60 wt% metal on support, Alfa Aesar) and 5mg/cm² for60wt%NiCuMMO/Ketjenblack.AnIrblackbench- markcatalystwithaloadingof3mg/cm²(AlfaAesar)wasusedat theanodeforallMEAs.Catalystlayersweresprayedat60°Cusing a Coltechairbrush (0.35mm nozzle) onToray 090 carbon paper (25cm²,FuelCellStore)forthecathode,andTifelt(Bekaert Inc.) coatedwithAuforanodeascatalystcoatedsubstrates(CCSs).The area ofcarbonpaperequals theareaofTi feltandrepresentsthe electrodesurfacearea(25cm²).TheTifeltwaspretreatedbyetch- ing in HCl(37wt%, SigmaAldrich) for2 minto remove thenon conductive surfaceoxide andthen sonicated for5 min in water andethanolbeforebeingsputter-coatedwithAuusinganEdwards sputtercoatertoreduce interfacialcontactresistance(ICR)within thecell.Thecoatingwascarriedoutatavapordepositionpressure of0.15atmat20mAfor2minoneachside.Theionomercontent amounted to 25and 7wt% ofthe total solidsinink for cathode and anode, respectively. The membrane, Fumapem-3-PE-30, was sandwiched between cathode andanode gas diffusion electrodes asinFig.2.TheMEAswereconditionedandexchangedtotheOH formin1MKOHovernight.TheAEMwaterelectrolyzersetupcon- sisted ofa 5 L Teﬂon tank with heaters and a peristaltic pump.

TestswereconductedatT=50°C.TheconcentrationsofKOHem- ployed were1 and0.1M KOH(ACSreagent, ≥85%,pellets, Sigma Aldrich).Theﬂowratesofthepumpswere250ml/min.

2.5. Polarizationcurveandelectrochemicalimpedancemeasurements

A high-current potentiostat (HCP-803, Biologic) was used to controlcellvoltageandmeasureimpedanceinthesingle-cellmea- surements. The polarization curve was recordedgalvanostatically,

rampingthecurrentfrom0to2A/cm²atarateof80mA/cm²per minute. Electrochemical impedance spectroscopy (EIS) was em- ployedtodeterminethecellresistancesandperformedatdifferent currentdensities,suchas0.2A/cm²,intheACfrequencyrangeof 100kHz–1Hz.TheNiCuMMOcatalyticlayerswerepostanalyzed bySEMandEDX.

3. Resultsanddiscussions 3.1. Structuralcharacterization

SEM and STEM images of the nickel-copper catalyst synthe- sizedby chemicalreduction withtheaddition ofNa₂CO₃ andan- nealedin5%H₂/Ar(NiCuMMO) areshowninFig.3.The Figs.3a and 3b show that NiCu MMO catalysts have dense areas of ag- glomeratednanosheetmorphology.TheSTEMimageinFig.3cdis- plays that NiCu MMOnanosheets are loaded on the carbonsup- port (Ketjen black EC-600JD) with a dark thick region of NiCu nanosheets.Fig.3dconﬁrmedthelooselystackednanosheetsmor- phologyofNiCuMMOcatalysts.Similarcatalyst morphologypro- duced by chemical reduction by sodium borohydride has given various names from nanocotton [29], nanosponges [30–32, and nanosheets[33–39] andinthiswork,we willreferto thesecata- lystsasnanosheets.Duringthechemicalreductionprocess,sodium borohydridereactsquicklywithtransitionmetalcationstoprecip- itatemetalborideMxByspecies[39–42].InthecaseofNiCuMMO, Na₂CO₃ wasaddedduringthesynthesis processtoprecipitateox- ide species [21]. We investigated another nickel-copper catalyst withouttheadditionofNa₂CO₃andannealedtheresultedpowder intheair(NiCuoxide) and5%H₂/Ar(NiCualloy)andthey exhib- itedalsoanagglomeratednanosheetsmorphologysimilartoNiCu MMO as seen in (Fig. S1). Energy dispersive x-ray spectroscopy (EDX)of NiCuMMO is shownin Fig. 4a. The EDXspectrum displayspeakscorresponding toNi,Cu,O,andCwithNi:Cu weight percentageas52.3:47.7,which isingoodagreement withprecur- sors percentage. The EDX spectrum displayspeakscorresponding toNi,Cu,O,andC.Impuritiesorremainingelementsfromthesyn- thesisprocessappeartobeabsent.

The XRD patternof NiCu MMOin Fig. 4b showspeaks at2

θ

valuesof32.5°,35.6°,37.2°,38.9°,43.2°,44.56°,48.9°,51.93°,and 62.8°. The diffraction peaksat 2

θ

^valuesôf ^44.5^° ând^51.93^° âre

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Fig. 3. a) and b. SEM images, c) STEM image NiCu MMO supported on Ketjen black, and d) high magniﬁcation STEM image of NiCu MMO nanosheets.

associatedwithNi(111)andNi(200)crystalplanesofnickelface- centered cubic (FCC) structure with (JCPDS card No. #04–0850) [43].Thepeaksat2

θ

^values^at^37.2^°,^43.2^°,^and^62.8^°^correspond

to(111),(200),and(220)diffractionplanesofNiO(JCPDScardno.

#47–1049)[44].Thediffractionpeaksat2

θ

^values^of^32.5^°,^35.6^°,

38.9°,48.9°valuescorrespondtoCuOcrystalstructure(JCPDScard no.#80–0076)[45].

NiCu alloyshows peaksat 2

θ

^values ^of^44.5^° ^and^51.93^°^that

correspond to pureNi (JCPDS No.04–0850) [43] whilethe peaks at44°and51.2°correspondtopureCu(JCPDSNo.04–0836)[46]. While NiCu oxide shows peaks at 2

θ

^values ^of ^37.2^°, ^43.2^°, ^and

62.8° correspond to NiO (JCPDS card no. #47–1049) and peaks at 2

θ

^values ^of ^32.5^°, ^35.6^°,^38.9^°,^48.9^° ^of^CuO ^crystal^structure

(JCPDS card No. #80–0076) [44,45]. The NiCu MMO vibrational modes were characterizedby Raman spectroscopyin Fig.4c.The Raman spectrumin Fig.4cshowsRamanpeaksat 490,606,810, 1020, and1100 cm⁻¹ respectively. The Raman peak at490 cm⁻¹ corresponds to Cu(OH)₂ while the Raman peak 606 cm⁻¹ corresponds to theBg Ramanmode of CuO[47–50.The Raman peaks at810,1020,and1100cm⁻¹ correspondto two-phonon(2P)NiO vibrationalmodes[51–55.

XPS analysis provides sensitive information aboutthe surface chemical composition of NiCu MMO catalyst. NiCu MMO survey spectrum is shownin Fig.4d. The survey spectrumindicates the presence of Ni,Cu, B, O, andC peaks.Ni 2phigh-resolution XPS spectrum isshownin Fig.5a and5b.TheNi 2p XPSspectrum is divided into two main peaks (Ni-2p_1/2 and Ni-2p_3/2) due to the spin-orbiteffectandtwooxidationstatesfornickel(Ni⁰ andNi²⁺) canbedeconvoluted.TheXPSpeaksat853.8eVand871.4eVcan beassignedtoNi2p_3/2andNi2p_1/2ofNi⁰[20,56].TheXPSpeaks

locatedat 855.4eVwitha satellite at860.9 eVcorrespond to Ni 2p_3/2 of Ni²⁺. The peak at872.5 eV witha satellite at879.4 eV canbeattributedtoNi2p_1/2ofNi²⁺[20,56]Cu-2phigh-resolution spectrumis showninFig. 5cand5d.The XPSpeaks at932.6eV and952.4eVcorrespondtoCu2p_3/2andCu2p_1/2ofCu⁰ [57].The XPSpeakat933.7eVcorrespondstoCuO[58].Thepeaksat934.8 and954.4eVareassociatedwithCu(OH)₂[58].Cu(OH)₂appearsto formduetoCuOreactionwithchemisorbedwateronthecatalyst surface.

Thehigh-resolution Ni2pXPSspectrum inNiCualloyexhibits peaks at 852.4 and 869.5 eV which correspond to Ni 2p_3/2 and Ni2p_1/2peaksofmetallicNi^o)Fig.S2a([59].TheCu2pspectrum showstwo peaksat932.5and952.3eVwhichareassignedtoCu 2p_3/2andCu2p_1/2ofmetallicCu⁰ (Fig.S2b)[16].

Thehigh-resolutionXPSspectrumofNi2pinNiCuoxideshows that the Ni 2p_3/2 main peak andits satellite at 854and862 eV, andtheNi2p_1/2mainpeakanditssatelliteat872and879eV,re- spectivelyconﬁrmingthepresenceofNi⁺²state(Fig.S2c)[60].The high-resolutionXPSspectrumoftheCu2pspectrumofNiCuoxide showspeaks at933.7, 943.1, 954.3, 962.9eV. The peaksat 933.7 and 954.3 eV correspond to the Cu 2p_3/2 and Cu 2p_1/2, respectively.Also, thereare two satellite peakscentered atabout943.1 and962.9eV,demonstratingthepresenceofCu⁺² state(Fig.S2d) [61].

Basedonthestructuralcharacterization.NiCumixedmetalox- ide (MMO) nanosheets have Ni, NiO, CuO phases and hydroxide speciessuchasCu(OH)₂ whichcanbe beneﬁcialforHERinalka- lineelectrolytes[62]aswewillseefromtheelectrochemicalmea- surements.NiCualloycontainspure NiandpureCuphaseswhile NiCuoxidecontainsNiOandCuOphases.

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Fig. 4. a) EDX spectrum of NiCu MMO catalyst, b) XRD spectrums of NiCu alloy, NiCu MMO, and NiCu oxide catalysts, c) Raman spectrum and, d) XPS survey spectrum of NiCu MMO catalyst.

3.2. Electrochemicalcharacterization

Fig. 6a showslinear sweepvoltammetry (LSV) curvesof NiCu alloy,NiCuMMO,andNiCuoxidein1and0.1MKOH.Allcatalyst loadings were equal to 250 μg/cm². NiCu MMO has the highest HER activityin1 MKOH byachieving−10mA/cm² at −200mV comparedtothe−250and−300mVforNiCualloyandNiCuox- ide,respectively,toobtainthesamecurrentdensity.Asseenfrom Fig. 6a, thecurrentdensity normalizedto geometric surfacearea forNiCuMMOat−0.35VvsRHEin1MKOHisﬁvetimeshigher than0.1MKOH.However,theactivitytrendforthenickel-copper catalysts isthe same in 0.1M KOH. Fig. 6bshows a comparison betweentheNiCuMMOHERactivityanddatafromtheliterature.

The NiCu MMOshows one ofthe best massactive HER catalytic activitiesreportedinTableS1,TableS2,andFig.6b.

The LSVcurvesinFig.6ashowthat theHERactivityincreases with increasing KOH electrolyte concentration, which in agree- ment withliterature [63,64].Lasia etal.found that theratecon- stants of Volmer and Heyrovsky reactions depend on the bulk OH⁻concentrations[65].AnappropriateriseoftheKOHelectrolyte concentration increases hydroxide ion activity [64–67]. Recently Wang etal.showedthat thehigh HERactivityat highKOHcon- centrationisduetoH₃O⁺intermediatesgeneratedonnanocatalyst surface[68].

Electrocatalyticactive surfacearea (ECSA) measurements were carried out to evaluate the intrinsic catalytic activity of nickel- copper catalysts.The ECSAwasestimatedby measuringthe elec-

trochemical double-layercapacitance (C_d_l) from cyclicvoltammo- gramsatvarious scanratesoveranon-faradaic(totally polarized) potential range, as in Fig. S3 in the Supplementary Information.

The NiCu MMO catalysts exhibit the largest double-layer capacitance C_dl of 9.16 (mF/cm²) compared to those ofthe NiCualloy (6.58 mF/cm²) and the NiCu oxide (3.80 mF/cm²), showing that a larger ECSA ofNiCu MMO allows more exposed active sites to promote HER performance. The speciﬁc surface area of the NiCu catalystswasalsoinvestigatedwithBrunauer–Emmett–Teller(BET) measurement(FigureS3,ESI†).Thespeciﬁcsurfaceareahasasimi- lartrendasECSA.NiCuMMOpossessesasurfaceareaof156m²/g which isfar higher thanthat ofNiCu alloy(112 m²/g) andNiCu oxide(92m²/g).Whennormalizedtoelectrochemicalsurfacearea (Fig.S3,ESI†)thedifferencesincatalystactivitybecomeless,espe- ciallyinthelower potentialrange.However,NiCuMMOstill pos- sessesthehighestintrinsicactivity.

ThelinearregionsofTafelplotsinFig.6careﬁttedtotheTafel equation, yielding Tafel slopes of 120, 130, and 195 mV/dec for NiCu MMO, NiCu alloy, and NiCu oxide respectively. The kinetic parametersforthenickel-coppercatalysts (jo andb) presentedin Table1werederivedfromtheTafelequation:

η

⁼^a⁺^b^log^j ⁽⁴⁾

Where

η

^(V)^is^the^appliedoverpotential,j(mA/cm²)isthecurrent density,b(V/dec)istheTafelslope,anda(V)istheintercept.

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Fig. 5. High-resolution XPS spectrum of a) Ni 2P 3/2, b) Ni 2p 1/2,c) Cu 2p 3/2,d) Cu 2p 1/2of NiCu MMO catalyst.

Table 1

Overpotential at −10 mA/cm ², Tafel slope b, charge transfer resistance ( α), and exchange current density j 0for NiCu catalysts in 1 M KOH.

Catalyst Overpotential (V) at −10 mA/cm ² Tafel Slope mV/decade charge transfer coeﬃcient( α) J o(μA/cm ²)

NiCu alloy −0.250 130 ±2 0.454 9.70

NiCu oxide −0.300 195 ±1 0.302 4.62

NiCu MMO −0.200 120 ±2 0.492 9.98

Theexchangeofcurrentdensityjocanbeobtainedbyextrapo- latingtheTafelplotstothex-axisorassuming

η

^is^zero.

a=

(

²^.³^RT

)

^/

( α

^F

)

^log^j^o

b=

(

².3RT

)

/

( α

^F

)

⁽⁵⁾

whereRisthegasconstant(8.314kJmol⁻¹K⁻¹),Tisthetemper- atureinK,

α

^is^thecharge-transfercoeﬃcient,andFistheFaraday constant(96,485Cmol⁻¹).

Table 1 summarizes the kinetic parameters for nickel-copper catalysts. NiCu MMO shows the lowest Tafel slope and highest chargetransfercoeﬃcientandexchangecurrentdensityoverNiCu alloyandNiCuoxidewhichconﬁrmsthesuperioractivityofNiCu MMO.

InviewoftheTafelslopebeingcloseto120mV/dec,itislikely thatthechargetransfercoeﬃcientrepresentsthesymmetryfactor oftheVolmerstepinthiscase.TheTafelslopesreﬂectanintensive property of the HER catalysts fromwhich some indication about

thereactionmechanismoftheHERandtherate-determiningstep (rds)canbe obtained.TheVolmerreactioninvolvestheelectrore- ductionofwatermoleculeswithhydrogenadsorptionasinEq.(6), whilethe Heyrovsky’sreaction involveselectrochemical hydrogen desorptioneq (7).The Tafelreactioninvolveschemicaldesorption Eq.(8)[65].

M+H2O+e⁻↔MH_ads+OH⁻ Volmer (6) MHads+H2O+e⁻↔H2+M+OH⁻ Heyrovsky (7)

MH_ads+MH_ads↔H₂+M Tafel (8) A detailed analysis showsthat rds for the HER at NiCu MMO is the Volmer reaction, then a Tafel slope in the order of 120 mV wouldresult.Whetherthenextstepinthereactionsequenceisthe HeyrovskyorTafelstep[65]cannotbedeterminedbythisanalysis, however.

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Fig. 6. a) HER LSV curves NiCu alloy, NiCu MMO, and NiCu oxide in 0.1 and 1 M KOH using Naﬁon ionomer (ionomer to catalyst weight ratio = 0.2), b) Comparison of mass activity and overpotential to achieve to −10 mA/cm ²for hydrogen evolution catalysts, data reproduced from Kibsgaard et al . [69] and Table S2 (supplementary information) c) Corresponding Tafel slopes of NiCu alloy, NiCu MMO, and NiCu oxide, d) Impedance complex plane plots of NiCu alloy, NiCu MMO, and NiCu oxide at −250 mV.

Fig. 6d shows impedance complex plane plots of NiCu alloy, NiCu oxide, and NiCu MMO in 0.1 and 1 M KOH at an applied potential of −250mV vs. RHEafter subtracting ohmic resistance.

Inthecomplexplaneplots,onlyonesemicircleisobserved,which can be attributedtoa charge transferprocess relatedto theHER [70–72].Thecharge transferresistance(Rct)isrepresentedbythe diameterofthe semicircle.Theradius ofthesemicircledecreases athigherKOHconcentration,signifyingalowerchargetransferre- sistance(Rct)andahigherrateofhydrogenevolution.NiCuMMO exhibit R_ct value of6.96 ât ân âpplied ^potential ôf −250mV comparedtoNiCualloy(10.38⁾ ândôxide^(13.81⁾^which^fur- therconfirm thesuperioractivity,fasterreactionkineticandhigh electrontransferefficiencyofNiCuMMO[72].

TheequivalentcircuitfortheNiCualloy,NiCuMMO,andNiCu oxide in Fig. 6d is characterized by a single time constant, and we modeled the impedanceby a series resistance (Rs, relatedto ohmic solutionresistance),inserieswithoneparallel circuitcon- sisting of a charge transfer resistance (Rct) anda constant phase element (CPE)relatedtothedouble-layercapacitance.Thisequiv- alentcircuithaspreviouslybeenusedinliteraturetodescribeHER on polycrystalline Ni and Ni-based materials [73]. (The constant phase element (CPE)wasused insteadof capacitance duetofre- quencydispersionandtheappearanceofdepressedsemicirclesin the impedance plane plots). The charge transfer resistance (Rct) represents thekineticsofthe HERattheelectrode/electrolyte in- terface. The absence of Warburg impedance indicates that mass transport israpid enough so that the reaction is kinetically con-

trolled[70,71,74].Theimpedancecomplexplaneplotsfordifferent appliedoverpotentialsareshowninFig.S4aintheESI†,andthese show thattheRctdecreases withincreasingpotential asitwould if the current-voltage relationship is described by Eq. (4) above.

(The lower Rct value at higher potential reﬂects the exponential dependenceof thecurrenton theoverpotential andthusthe ac- celeratedelectron transfer andhigherrates ofthe HER athigher overpotential[70,71,74].Ascanbeshownbya simpledifferentia- tionofEqn.4above,theTafelslopemaybeobtainedfromplotsof potential vs. log (R_ct)⁻¹. From our plots, we obtain 120 mV/dec, see Supplementary Information, Figure S4b, which is the same as that obtained from the LSV curves. The same Tafel slope be- ing obtainedwith impedancespectroscopy thus validates theiR- correctedlinear-sweepvoltammograms.

Fig. 7a shows the current vs. time recorded in chronoamperometric measurements performed by applyinga constant potential of −400 mV for 30 h on NiCu alloy, NiCu MMO, and NiCu oxide.For all samplesbut NiCuMMO in0.1 MKOH, thecurrent density decreases (i.e. the activity decreases) rapidly during the ﬁrst few minutes. For NiCuMMO in 0.1 M KOH andNiCu oxide in1MKOH,thecurrentdensitythenlevelsoff and remainscon- stantatapproximately−11mA/cm²and−50mA/cm²,respectively.

NiCu oxide shows stable performance at −50 mA/cm² for 30 h.

ForNiCu alloyin 1 M KOH,the currentdensity slowlyincreases (activity increases) with time from approximately −90 mA/cm² to −100 mA/cm² after 30 h. For the NiCu MMO sample in 1 M KOH,the current densityreaches a minimum activity after

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Fig. 7. a) Chronoamperometry measurement of NiCu alloy, NiCu MMO, and NiCu oxide using Naﬁon ionomer in KOH electrolytes, b) In situ Raman spectra of NiCu MMO under chronoamperometric HER conditions ( −0.4 V vs RHE) at different time intervals in 1 M KOH, In situ Raman spectra of c) NiCu oxide and d) NiCu alloy under HER conditions ( −0.4 V vs RHE) at different time intervals, e) HER LSV curves of NiCu alloy, NiCu MMO, and NiCu oxide using Naﬁon and Fumion ionomers in 1 M KOH, f) Current density at −0.25 V at different ionomer to catalyst weight ratios (I/C) of NiCu MMO.

approximately 30 min at which the current density is approximately−180mA/cm²,andthenslowlyincreasestoa littlebelow

−200 mA/cm² at 30 h. The chronoamperometric measurements conﬁrmthehigherHERactivityofNiCuMMOin1MKOHthanin 0.1MKOHandoverNiCualloyandNiCuoxideinthesameelec- trolyte. The current densities observed fromchronoamperometry areingoodagreementwiththoseobservedintheLSVs.

Fig. 7b showsin-situ Raman spectra of NiCu MMO under an applied potentialof −0.4VvsRHEatdifferenttime intervals.All the spectrain Fig.7b display peaksat292,530,1060, 1350,and

1585cm⁻¹.Thepeak at292cm⁻¹ canbeassignedto copperhy- droxideCu(OH)₂ species[47]whilethe peakat530cm⁻¹ can be assignedtonickelhydroxideNi(OH)₂ [75].Thepeakat1060cm⁻¹ can be assignedto carbonates[76] while thepeaks at1346,and 1585 cm⁻¹ correspond Dband, andG band peaks of carbon respectively [77,78]. The spectra show clear peaks of Ni(OH)₂ and Cu(OH)₂atthebeginningofHER.However,theCu(OH)₂peaksde- creasedmoresigniﬁcantlythantheNi(OH)₂peaks.Inotherwords, whereas both Cu(OH)₂ andNi(OH)₂ both exist during the entire periodof30,000s,bothCuandNihydroxidesgetslowlyreduced,

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butCumoresothanNi.Thereductionoftheseelementsisconsis- tent withthePourbaixdiagramsofCu andNiwhich predictthat bothNi(OH)₂andCu(OH)₂wouldbereducedtometallicnickeland copperatthispotential[79].

The NiCuoxide alsodisplayedpeakscorresponding toNi(OH)₂ and Cu(OH)₂, but these peaks disappeared completely after 15,000 s, as shown in Fig. 7c. However, the HER activity of the NiCu oxideismuch lowerthan thatof NiCuMMO.Thisconﬁrms theimportance ofthepresenceofmetallicspeciesonthecatalyst surface,asfoundbyDanilovicetal.[80],forsuperiorHERactivity.

Finally, the Raman spectrum for the NiCu alloy catalyst also showspeaksrelatedtoNi(OH)₂ andCu(OH)₂surfacespecieswhen thecatalyst isimmersedinKOH,whichconﬁrmedthehypothesis that Ni metal will convertto oxide species once incontact with KOH[80,81].ThehydroxidespeciesontheNiCualloygetsreduced ratherrapidly(<5000s)atthesurface,asshowninFig.7d,compared to NiCu MMO andNiCu oxide and did not lead to excep- tionalactivitycomparedtoNiCuMMO.

NiCu MMO thus showed the best HER activity in the alkaline electrolytes with a Tafel slope of 120 mV/dec. The bifunc- tional system ofNiCu MMO catalyst includes Ni metal, NiO, and CuOoxides,andprovidearapidVolmerstepandthus rapidover- all HER reactionkinetics. Theimproved HERkinetics ofthe NiCu MMOcanbeattributedtothepresenceofbothNiandNiOwhere NiO sites to facilitatewater dissociationand bindOH_ad while Ni metallic bindsH_ads andCuOstabilizesNiO underHER conditions.

Similarly, Bates et al. found that the synergistic HER enhance- ment of Ni/NiOisdue toNiO content andCr₂O₃ appears to stabilize NiO under HER conditions [82]. The in situRaman results show that the presence of both metal and oxide phases is es- sential to sustain a high HER activity, the performance of NiCu MMO relativeto that ofNiCualloyor NiCuoxide.We relate this totheinsituRaman datashowingthatcopperhydroxidegetsre- ducedandnickelhydroxideistosomeextentpreservedunderHER conditions.

We attribute the rapid decayin electrocatalytic activity in all samples to an initial and rapid adjustment of the surface state of all catalysts, whereas the long-term behavior is more complex. For the NiCu oxide, there is no further change in the surface state after 15000s (Fig. 7c), and the electrocatalytic activity remains the same asthat immediately after the initial transient.

ThecurrenttransientisthusfullyconsistentwiththeRamanspec- tra forNiCuoxideinFig.7c.SincetheRaman spectraoftheNiCu alloy (Fig. 7d) indicate a surface at which hydroxides are completely absent after 5000 s, however, the slow increase in catalytic activity with time in Fig. 7a for this catalyst may be related toa slowchange inthecomposition or surfacearea,i.e.to catalyst instability.For NiCu MMO in 1 M KOH,the initial tran- sientisfollowedbyaslowerincreaseincatalyticactivity.Acorre- spondinglyslowchangeinthesurfacestate,c.f.theRamanspectra in Fig. 7b, appear to persist throughout the chronoamperometry experiment.

We relate thisdifference tothe synthesis. Forthe NiCu alloy, only athinlayerofthe hydroxideswill formasthe NiCualloyis exposed to theKOHsolution.Thislayeris rapidlyreducedasthe catalystissubjectedtoanegativepotential.However,sincethereis noindicationofanymetalphaseintheNiCuoxideinthediffrac- tograms,wemayassumethat duringexposuretonegativepoten- tials thesecatalysts will bereduced continuously untilthe entire catalyst is converted tometal. Forthe NiCuMMO, thisseems to have combined behavior (mixed metal oxide (Ni-NiO-CuO) catalyst),sincetheoxidationduetotheannealingisnotcomplete,c.f.

thediffractogramsinFig.4bwhichdisplaysasubstantialpeakcor- responding to Ni(111).This catalyst heterogeneityof metallic and oxide phaseswill causemixedbehavior ofacontinuousbutslow change in the surface state throughout the experiments, which

mayberelatedto aslowdiffusion-limitedprocessinthe sample.

Thesurfaceisthereforealsoslowlyreorganizedandwillconsistof amixofphasesandaslowlychangingactivity.

NiCu MMO also showed good stability during an accelerated stress testconsistingof5000potential cyclesfrombetween−0.8 to−1.3Vatascan rateof100mV/s. TheLSVforNiCuMMObe- foreand afterthe procedure showedonly a 20mV difference in thepotential required toachieve - 100 mA/cm² asshown inFig.

S5ESI†.

Fig.7eshowsTheHERactivityusingNafionandanionexchange ionomer(Fumionionomer)ofNiCualloy,NiCuMMO,andNiCuox- ide.TheactivityfortheHERofnickel-coppercatalystsdecreasedif Fumionionomerreplaced Nafioninthe catalystink, andresulted in a potential shift of 30 mV at −100 mA/cm² as compared to the Nafion ionomer. Catalyst inks with Nafion resulted in higher HERactivitycompared tocatalystinks withFumion ionomer.We assign the difference in activity between catalysts in inks with Nafion and those with Fumion ionomers to the nature of the ionomerbackbone andits chemistry (ammonium-, imidazolium-, phosphonium-basedcompoundsinanionexchangeionomerssuch asFumion,orsulphonicacidgroups(SO₃⁻)inNafion)[83,84].The SO₃⁻moietyinNafioninteractsonlyweaklywiththecatalystsur- face, andthe effectof SO₃⁻ adsorption on electrocatalyst performance is expected to be negligible, particularly in the HER re- gionwherethenegativechargeonthecatalystsurfacewouldrepel sulfonatespecies[21].The quaternaryammonium (QA)functional groupusedforOH⁻transportinanionexchangeionomer(AEI),on the other hand,appears to poison NiCu MMOcatalyst andblock activecatalyst sites.Fumionionomershowshighertotal polarization resistance than Nafion as shownin the impedance complex plane plotofNiCuMMOusingNafionandFumionionomers (Fig.

S6.a).Thesmallsemicircleatthelow-frequencyregionforFumion ionomer (Fig. S6.a) has been suggested to correspond to quater- naryammoniumadsorption [85].Theresultsshowthat theanion ionomernotonlyservesasabinderbutalsoaffectstheelectrocat- alyst’sHERactivity[4].

WeconsequentlyinvestigatedtheimpactoftheNafionionomer content to find the composition at which the HER performance peaks forNiCu MMO. The results are shownin Fig. 7f. The HER activitythusincreaseswithincreasingNafion ionomertothe cat- alystweightratio(I/C),andamaximumappearsataweightratio of I:C of0.5. The NiCuMMO atI/C = 0.5achieves −10 mA/cm² at170 mV, whichindicates better catalyst utilization, lower total polarization resistance,andoptimum HER performance asshown inFig.S6bandS6c.Thelowperformancewithalowionomercon- tentisattributedtothepoordispersionoftheink.Athighionomer content,theHERactivityissmallduetoincreasedaggregationof Nafion and the associated blocking of mass transport andactive sites [25]. The moderate I/C ratio indicates that Nafion improves thecatalystdispersionanddistributionandreducedtransferresis- tance.The optimizedionomercontent provides an efficientpath- wayforOH⁻ (intheaqueouselectrolyte)andelectronsandforms astablereactioninterface[86].

3.3. Anionexchangemembrane(AEM)electrolysis

To test the activity of NiCu MMO in an actual AEM electrol- ysisenvironment, NiCu MMO andIr blackMEAs were fabricated and mounted in an AEM water electrolysis cell as explained in the Supplementary Information and Fig. S7 ESI†. Two types of MEAs willbe mentioned in Results Pt/Ccell andNiCuMMO cell for NiCu MMO-Ir and Pt/C-Ir cellsrespectively. Fig. 8 shows the impedancecomplex-plane plot at 0.2A/cm² forNiCu MMOcells (Fig.8a)andPt/Ccells(Fig.8b)in0.1and1MKOH.Theimpedance complex-plane plots appear to consist of two partly overlapping

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Fig. 8. a) Impedance complex plane plot of NiCu MMO/Ir cell, b) Impedance complex plane plot of Pt/Ir cell, c) Raw polarization curves and d) HFR-corrected Polarization curves of NiCu MMO/Ir and Pt/Ir cell in 1 and 0.1 M KOH.

and depressed semicircles. The ohmic resistance of the cell was determinedfromthehigh-frequencyresistance(HFR),i.e.,fromthe interceptwiththereal(Re)axes[87].

InFig.8,weshow theequivalentcircuitthat isusedtoﬁtthe impedance datataken at0.2A/cm² in bothNiCu MMO andPt/C cells. We assign the low-frequency arcto mass transport [87,88]

and the high-frequency arcto electrode kinetics contributions to thecellvoltagefromtheNiCuMMOandPt/Ccathodes.Thefitted electrical circuitiscomprisedofa seriescombinationoftwo par- allel circuits eachconsisting ofaresistance andaconstant phase element(CPE),inserieswitharesistor,R.TheR correspondsto the ohmic resistance of the cell (catalyst layer, currentcollectors andmembrane).TheRctdescribesthechargetransferresistanceof the cathode andanode. CPE₁ isthe constant phase element that represents the electrode roughness. The circuithas an additional RCcombination, constantphaseelement,andtheresistance(CPE₂ andR₁),whichissuggestedtodescribethemasstransportrelated to bubbleformationatthe electrode-electrolyteinterface [88].All parametersextractedfromthefittingoftheimpedancedatatoare presented inTable S3.For1 MKOH,NiCu MMOcell hasan HFR of0.195^.cm² ^while^Pt/C^basedÂEMWE^cell^hasân^HFRôf^0.115 ^.cm²^.^NiCu^MMO^cell^displaysân^HFRôf^0.295^.cm²^while^Pt/C achieves0.225^.cm²ⁱⁿ^0.1^M^KOH.^NiCu^MMO⁽⁵^mg/cm²⁾^higher loadings resulted in thickercatalyst layers and higherHFR compared toPt/C. The HFR increasesasKOHconcentration decreases to 0.1MKOH.ThisHFR increasewithdecreasingKOHconcentra-

tionmayindicateinsuﬃcientionicconductivityofthemembrane [26].

The impedance data were converted to Tafel impedance. The TafelslopecanbeestimatedfromtheTafelimpedance,forakinet- icallylimitedprocess, asthediameteroftheimpedance arc[89]. TheTafelimpedanceshowninFig.S8ESI†istheimpedancemul- tipliedwith thesteady-state currentdensityat whichit wasob- tained.WethusestimatetheTafelslopein1MKOHtobe40mV forPtand65mVforNiCuMMOat0.2A/cm².TheTafelslopefrom the impedancedata isin therange of50 millivolts, whereas the slopesfromthepolarizationcurvearetwicethisvalue(seeFig.S9 ESI) suggestedthat thepolarization curves are dominatedby the ohmicresistance.Fig.8cand8dshowthe potentiostaticpolariza- tioncurvesofbothHFR-correctedanduncorrectedvoltagesforthe AEMWE atdifferentKOHconcentrations forNiCu MMOandPt/C cells.

Fig.8cand8dshowtheAEMelectrolyzerperformanceofNiCu MMOandPt/Ccathodecatalystsin1and0.1MKOHat50°Cusing Irblackasan anode. In1M KOH,withNiCu MMOacell perfor- manceof1.85A/cm² at2Vachieved,whichmaybecomparedto Pt /Ir cell that delivers 2 A/cm² at 2 V in 1 M KOH while both cellsachieved1A/cm² at2Vin0.1MKOH.TheincreaseinKOH electrolyte concentration leadsto a higher AEMelectrolyzer performance.

Fig.8dshowedthatNiCuMMOcellexhibitshigherperformance than Pt/C catalyst when HFR-corrected. NiCu MMO (5 mg/cm²)

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showshigherresistancethanPt/C(1mg/cm²)in1and0.1MKOH (Fig.8aand8b).Sincethecellhardware,components,electrolyte, temperatureisthesameandtheonlydifferenceisthecathodecat- alyst, the originof high resistance isthe higher loading andthe presenceofoxidespeciesinNiCuMMO(Ni-NiO-CuO).Thisleadsto ahigherresistanceintheNiCuMMOcatalyticlayeritselfascom- paredtothePt/C,withitslowerloadingandmetallicconductivity.

The results suggest that the differencesin the activityof the samples (Fig. 8c) are not merely due to their different intrinsic activities, butalso partly due to low electronic resistance in the catalytic layer.Thiscontribution totheresistancewill beparticu- larlysignificantforpoorlyconductingoxidessuchasthoseofNiCu MMO. The high-frequency resistance(HFR) corrected polarization curvesinFig.8dconfirmthattheelectronicresistanceofthecath- odecatalystlayersignificantlyaffectscellperformance.Similarre- sultscanbefoundintheliterature.Yuetal.[90]showedthatfor catalysts with widely different conductivity the ranking depends on whetheriR compensation isapplied or not.Xu etal. [91] re- ferredthedifferencesinAEMelectrolyzerperformancepartiallyto differences in the OER catalyst phases electrical conductivity. Fi- nally, D. Chung et al. [92] showed that poorly conductive MoS₂ HER activityisaffectedby theohmic lossesandrecommend that electricalconductivityshouldbeconsideredwhendesigningactive catalystsforwaterelectrolysis.

The NiCu MMO/Ir MEA activity shows a good reproducibility forthreedifferentMEAs in1MKOHat50°CasinFig.S10ESI†. The post-mortemanalysisofNiCuMMOcatalytic layersshowsno visible cracks which prove the stability ofcatalytic layers during AEMwaterelectrolysisasindicatedinFig.S11.EnergydispersiveX- ray(EDX)mappingofNiCuMMOcatalyticlayersandcross-section showsthepresenceofnickel,copper,carbon,andathinpotassium layeraftertheelectrolysisexperimentFig.S12,andS13ESI†.

The excellent performance of 1.85 A/cm² at 2 V in 1 M KOH obtained forthe NiCu MMO hydrogencatalyst outperforms most of those summarized in (Fig. S14 and Table S4 ESI†) allows for an active andcheap catalystforAEMwaterelectrolysisoperation on acommercialscale[93,94]andcomparabletothestate ofthe artperformanceofPEMelectrolysisassummarizedbyAyersetal.

[95]. Conclusions

NiCu mixed metal oxide (MMO) nanosheets synthesized by chemical reduction showed an exceptional activity for the HER comparedtoNiCualloyandNiCuoxidecatalysts,withhigherper- formance in1MKOHthan 0.1MKOH.TheimprovedHERkinet- icsoftheNiCuMMObifunctionalsystemcanbeattributedtothe presence ofboth Ni and NiO where NiO sites to facilitate water dissociationandbindOH_ad whileNi metallicbinds H_ads andCuO stabilizes NiO under HER conditions.In situ Raman spectroscopy attheNiCuMMOcatalystsshowedthatasubstantialfractionofin situformednickelhydroxideremainedafter30,000s atHERcon- ditions, which mayexplain why theNiCu MMOis able tomain- tain its veryhigh activityascompared to that ofNiCu alloyand NiCu oxide over longer periods of time. Despite that anion exchange ionomers would be expected to be suitable ionomers in an AEMenvironment,theapplicationofanionexchangeionomers incatalyticlayersresultedinalowerHERactivityascomparedto catalytic layers withNaﬁon as the ionomer.Using Ir black asan anodecatalyst,cellswithNiCuMMOnanosheetsascathodecata- lyst achievedAEM electrolyzerperformance of1.85A/cm² at2V in1MKOHat50°C.

DeclarationofCompetingInterest

“Therearenoconﬂictstodeclare.”

Creditauthorshipcontributionstatement

AlaaY. Faid: Conceptualization, Methodology, Investiga- tion, Writing - original draft, Writing - review & editing.

AlejandroOyarce Barnett: Funding acquisition, Supervision, Writing - review & editing. FrodeSeland: Supervision, Writing - review & editing. Svein Sunde: Funding acquisition, Supervision, Writing-review&editing.

Acknowledgments

This work was performed within HAPEEL project “Hydrogen ProductionbyAlkalinePolymerElectrolyteElectrolysis” ﬁnancially supported by the Research Council of Norway-ENERGIX program contractnumber268019andtheINTPARTproject261620.TheRe- search CouncilofNorwayis acknowledgedforthesupport tothe Norwegian Micro- and Nano-Fabrication Facility, NorFab, project number245963/F50.

Supplementarymaterials

Supplementary material associated with this article can be found,intheonlineversion,atdoi:10.1016/j.electacta.2021.137837. References

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