Sustainable water oxidation enabled by a complex-doped cobalt oxide electrode

Domain: chemistry
Published: 2025-12-31
Editorial outcome: major_revision

Abstract
Achieving sustainable water oxidation presents significant challenges, particularly employing cobalt-based electrocatalysts. Despite promising activities, many cobalt-based electrocatalysts undergo in-situ partial restructuring into disordered (oxy)hydroxides, as indicated by the Pourbaix diagram. This restructuring typically degrades structural integrity and electronic conductivity, undermining catalytic stability. Here, we propose a complex doping strategy to stabilize LiCoO2, a cobalt oxide that can be sourced from spent lithium-ion batteries, for sustainable water oxidation. Specifically, by co-doping LiCoO2 with Ni, Fe, and Pd, we mitigate the reconstructed extent of the in-situ generated spinel phase during water oxidation reaction and enhance electrochemical stability. Furthermore, complex doping improves the surface conductivity and facilitates gas removal, boosting mechanical robustness. Consequently, the optimized LiCo0.79Ni0.1Fe0.1Pd0.01O2 achieves a competitive water oxidation stability of over 2000 hours. Additionally, in membrane electrolyzer tests, LiCo0.79Ni0.1Fe0.1Pd0.01O2 outperforms the benchmark RuO2, delivering 2.5 A cm−2 at 1.58 V and maintaining stability for over 1400 hours. By elucidating the role of each dopant in LiCo0.79Ni0.1Fe0.1Pd0.01O2, this work offers critical insights for the rational design of sustainable water splitting electrodes.

Introduction
Coupling water electrolysis technologies with renewable electricity to produce green hydrogen represents a promising approach for mitigating the intermittency issue of renewable energies and reducing CO2 emissions1. The performance of catalysts in a water electrolyzer significantly impacts the electricity consumption (approximately 70%–90% of the total hydrogen production cost) and operation durability (typically 10,000–100,000 h)2,3. Substantial research has been dedicated to developing low-cost electrocatalysts for water electrolysis, focusing on the anodic oxygen evolution reaction (OER) that poses a significant energy barrier4,5. Despite the considerable achievements in understanding the water-splitting process and developing electrocatalysts, significant challenges remain in transferring the high performance demonstrated by many designed electrocatalysts in rotating disk electrode (RDE) setups to practical electrolyzer applications. For instance, NiFeOxHy, widely accepted as one of the most active OER electrocatalysts in a typical RDE setup, demonstrated considerably poor activity in the membrane electrode assembly (MEA) configuration6–8. Moreover, they can leach Fe in “Fe-free” electrolytes, losing activity and posing threats to ion exchange membranes9. Beyond activity, stability is another big concern. Many OER electrocatalysts undergo alterations in their physicochemical properties due to in-situ reconstruction, a dynamic process that affects structural and catalytic stability10. For instance, cobalt oxides (e.g., Ba0.5Sr0.5Co0.8Fe0.2O3-delta, Co3O4) usually undergo partial reconstructions into layered (oxy)hydroxides, resulting in sacrificed crystallinity and loss of compositing elements11,12. Although this reconstruction can initially enhance activity, it may ultimately impair practical activity and long-term stability due to the less conductive nature of the reconstructed surface13–16. However, previous studies on catalyst reconstructions were usually conducted at ambient temperature and under small current densities17, which limits the applicability of their findings to practical electrolyzer conditions.

Layered LiCoO2 (LCO) is of particular interest among the various cobalt oxides due to the facile tunability of its cobalt redox and associated lithium composition18,19. Also, the abundant availability of LCO from spent lithium-ion batteries makes it an appealing material for recycling and repurposing in other energy applications20. Nevertheless, the intrinsic OER activity of LCO is limited due to the substantial potential barrier that must be overcome to trigger its favorable surface reconstructions18. Several strategies have been employed to enhance its activity, including elemental doping18,21, exfoliation22, cation insertion23, and electrochemical tuning19. While these strategies have shown various improvements, achieving an OER electrode that combines both high performance and long-term durability suitable for practical electrolyzer conditions remains a significant challenge.

Here, we report a complex doping method to improve the practical OER performance of LCO and clarify the critical role of each doping element. By manipulating the catalyst reconstruction extent under elevated electrolysis temperatures, we have developed a high-performance LiCo0.79Ni0.1Fe0.1Pd0.01O2 (LCNFPdO) electrocatalyst with good stability (over 2000 h) and activity that is competitive with benchmark catalysts.

Results
OER activity and stability
Atotalof35dopedLCOcompositionswereexperimentallyscreened,withtheselecteddopingelementsandrationaleoutlinedinFig.S1.Aftersolid-statesynthesis,thelayeredstructure(spacegroup:R-3m)ofthesingleLCOphasewasconfirmedwithX-raydiffraction(XRD,Fig.S2).Subsequently,theOERperformancewasevaluatedacrossarangeoftemperatures(22–80°C)in1MKOHusingcyclicvoltammetry(CV)andlinearsweepvoltammetry(LSV),asillustratedinFigs.S3andS4.AsummaryofthetypicalresultsisprovidedinFig.1aandTableS1.Amongthetestedcompositions,Single-element-dopedsamples(e.g.,LCFO,LCNO,LCPdO)showtheworstOERperformance,binary-element-dopedsamples(e.g.,LCNFO,LCNPdO,LCFPdO)showimprovedperformancecomparedwithsingle-element-dopedsamples,andtheternary-element-dopedLCNFPdOexhibitedthemostpronouncedOERactivity,withasmalloverpotentialof0.351Vat25mAcm−2(22°C),0.337Vat80mAcm−2(50°C),and0.256Vat250mAcm−2(80°C).Thistrendsuggestsanon-additive,possiblysynergisticeffectwhencombiningNi,Fe,andPdinenhancingtheOERperformance.LCNFPdOdemonstratedbetterOERperformancetoothersamples,includingcommercialRuO2andCo3O4,andsignificantlyoutperformedundopedLCO(Fig.S5).Notably,after10CVcycles,LCNFPdOdisplayedanincreasedcurrentdensity,indicatingactivationpossiblyduetoformationofreconstructedspecies18(Fig.1b).Incontrast,theundopedLCOshowedminimalchangeincurrentdensitythroughouttheCVprocess(Fig.S6).Furthermore,thebetterOERactivityofLCNFPdOunderdifferenttemperaturesiscorroboratedbyitssmallerTafelslopes(Fig.S7)andlowerpolarizationresistance(Figs.1candS8).Thetemperature-dependentkineticcurrentdensityplotsofLCNFPdOandLCOwerefitted,confirmingaloweractivationenergy(Ea,78.6kJmol−1)forOERcatalysisbyLCNFPdOthanthatofLCO(86.8kJmol−1),asillustratedinFig.1d.ThespecificOERactivityofLCNFPdO,normalizedbytheelectrochemicalactivesurfacearea(ECSA,Fig.S9andTableS2),wasatleasttwoordersofmagnitudehigherthanthatofRuO2withintheoverpotentialrangeof350mV(Fig.S10),indicatingitsconsiderablepotentialforcatalyzingOER.

Furthermore,LCNFPdOdemonstratedanOERstabilityofover2000h.Specifically,atacurrentdensityof10mAcm−2(Fig.1e),itsdegradationrateof0.012mVh−1wassignificantlylowerthanthoseofLCO(0.086mVh−1)andRuO2(1.594mVh−1).Inaddition,itcouldsustaintheOERwithoutsignificantdegradationformorethan2250hunderaharsherconditionof50mAcm−2,furtherconfirmingitsgoodstability(Fig.1f).TheseresultsdemonstratethatLCNFPdOdisplayscompetitivedurabilityamongcobaltoxide-basedandotherOERcatalysts(TableS3),maintainingstableperformanceunderhigh-temperaturealkalineconditions,andfirmlydistinguishingitasabenchmarkmaterialforOER.

Structuralandcompositionalevolution
ToinvestigatethegoodperformanceofLCNFPdO,wefirstanalyzedtheelementaldistributionusingenergydispersiveX-rayspectroscopy(EDS)inconjunctionwithhigh-angleannulardark-fieldscanningtransmissionelectronmicroscopy(HAADF-STEM,Fig.2a)andscanningelectronmicroscopy(SEM,Fig.S11).Bothtechniquesrevealedhomogeneouselementaldistributioninitspristinestate(designatedas“LCNFPdOpristine”)andOER(denotedas“LCNFPdO22°C”and“LCNFPdO50°C”forsamplespost-testedat22°Cand50°C,respectively).TheFe,Co,Ni,andPdconcentrationsofthepristineLCNFPdOandLCOsamples,asdeterminedbyinductivelycoupledplasmamassspectrometry(ICP-MS)inTableS4,agreedwellwiththeexpectedstoichiometricvalues.Incontrasttothewell-maintainedelements,includingCo,Ni,Fe,andPd,LiconcentrationdecreasedafterOER(TableS4),particularlyatelevatedtemperatures,indicatingcycling-inducedLiloss19,24.TheLCNFPdOwithcomplexdopingretainedahigherLiconcentrationthanLCO,indicatingbetterstructuralintegrity.

Wefurtheranalyzedthecrystalstructure.TheXRDrefinementconfirmedthesinglephaseoflayeredLCNFPdOwithanexpandedc-axislatticeparameterinreferencetothatofLCO(Fig.S12andTablesS5andS6),possiblyduetothelargersizeofthedopantPd2+(0.86Å),Ni2+(0.69Å),andFe3+(0.55–0.65Å)thanthatofCo3+(0.55–0.61Å)25.AfterOERat50°C,bothLCNFPdO50°CandLCO50°Cretainedtheirlayeredstructuresinthebulk(Fig.2b),whichwasfurtherconfirmedbytheXRDrefinement(Fig.S13andTablesS7andS8).ThepossiblesurfacereconstructioncausedbyOERatvaryingtemperatureswasinvestigatedusingSTEM.AfterOER,ahierarchicalconfigurationcomprisingalayeredbulkandareconstructedsurfacewasdetectedinbothLCNFPdO(Figs.2e,g,S15andS16)andLCO(Figs.2f,h,S17andS18),incontrasttothelayeredsurfaceforthepristinesamples(LCNFPdO:Figs.2candS14,LCO:Fig.2d).ThecorrespondingfastFouriertransform(FFT)patternsofLCNFPdOafterOERat22°Candat50°C,asshownintheinsetsinFig.2eandg,detectedstrongreflections(highlightedwithgreencircles)ofspinelstructureatthesurface,e.g.,(002),(220),and(111),whichcanbeattributedtotheLiCo2O4-typespinelstructureduetothestrongerintensityof(111)patternthan(200)pattern18.FromtheSTEMresults(Fig.2fandh),thesurfaceofundopedLCOwasalsoreconstructedintoLiCo2O4-spinelstructure,agreeingwellwithpreviousresearch18.ItisworthnotingthatFFTpatternsevendetectedslightreflections(e.g.,(202')and(002))ofCo3O4-typespinelstructureatthesurfaceofOER-cycledLCO,asindicatedbytheyellowcirclesinFig.2fandh,suggestingseriousLiloss.Moreover,thereconstructedsurfacelayerofOER-cycledLCOwasconsistentlythickerthanthatofLCNFPdOafterOER(Figs.S15–S18),confirmingalargerreconstructionextent.TheXRDinFig.2balsosupportedlessstructuralchangeofLCNFPdOthanthatofLCOaftertheOER.Theaboveresultsshowthatmulti-elementdopingmitigatesreconstructionextent.Specifically,thepeakintensityofthe104-diffractionpattern,anindicativeofstructuraldisorderinlayeredLCO,increasedsignificantlyaftertheOER,incontrasttotheminimumchangeofLCNFPdO26–28.ThecatalystmorphologywasadditionallyexaminedbySEM(Fig.S19),exhibitingminimalalterationbeforeandafterthereaction.

Electronicconfigurationandreconstruction
X-rayabsorptionfinestructure(XAFS)wasemployedtogaininsightsintothemetalvalenceandlocalatomicstructures.FromtheX-rayabsorptionnear-edgestructure(XANES)showninFig.3a,theCoK-edgepeakofLCNFPdOpristineexhibitedaslightlypositiveshiftcomparedtothatofLCOpristine,indicatingahigherCovalencethan+3(Fig.3a,b),whichcouldbeattributedtochargecompensationbytheintroductionoflower-valencedopantssuchasNi2+andPd2+,assupportedbytheBaderchargeanalysis(TableS9andSupplementarydata1).Quantitativeelectronparamagneticresonance(EPR)measurementsofLCOandLCNFPdOsuggestthatoxygenvacanciesarenotamajorfactorinfluencingCovalence(Fig.S20).AfterOERat22°C,anotableshifttohighenergyoftheK-edgepeakwasobservedforbothLCOandLCNFPdO,indicatingCovalenceincrease29.SuchoxidationwasevenmorepronouncedafterOERat50°C,suggestingfacilitatedreconstructionbyhightemperature.Notably,thepre-edgepeakaround7719eV,assignedtothe1sto4ptransitionofCoforbothLCNFPdOandLCO,disappearedafterOER,indicatingtheexistenceofhigh-valenceCo30.ItisworthnotingthattheCovalenceincreaseinLCNFPdOwaslessthanthatofLCOafterOERatallstudiedtemperatures,asshowninFig.3b,supportinglessoxidation.Inadditiontobulk-averageXAFS,CoL3-edgesoftX-rayabsorptionspectra(sXAS)inFig.3candCo2px-rayphotoelectronspectroscopy(XPS)inFigs.3dandS21(fittingparametersinTableS10)detectedaconsistenttrend:OERincreasedsurfaceCovalence,andhightemperaturepromotedthisoxidation.BydeconvolutingtheCo2pXPS,wefoundthattheincreaseinsurfaceCovalencewasalsomitigatedforLCNFPdO(former)comparedtothatofLCO(later):increaseof0.06vs.0.13at22°Cand0.15vs.0.24at50°C.

ThesurfaceCoelectronicstructureevolutionwasalsocharacterizedbyRamanspectra,asshowninFig.S22.TheRamanspectraofbothLCNFPdOandLCOpristineshowedtwocharacteristicpeaksat487cm−1and595cm−1,correspondingtotheEgbendingandA1gstretchingmode31.ButtheRamanspectrumofpristineLCNFPdOwasasymmetricwithashoulderpeakinthelowerwavenumberregion,concurringwithitsincreasedsurfaceCovalencestate32.AfterOERat22°Cand50°C,thesurfacelithiumofLCOwasdeintercalatedandcobaltwasoxidized,assupportedbythenegativeshiftofRamanpeakstolower-wavenumberregionsataround460cm−1and564cm−1thatcanbeassignedtotheEgandA1gmodesofCoO233.However,thetwopeaksofLCNFPdOhadasmallershiftdegreeafterOERthanthoseofLCO,whichwasconsistentwithitshigherLicontentandlowerCooxidationstate.Toprovideoperandoinsightintosurfacereconstruction,weperformedin-situRamanspectroscopyat22°C(Fig.S23).Astheappliedpotentialincreasedfrom1.20Vto1.55Vvs.reversiblehydrogenelectrode(RHE),bothLCOandLCNFPdOdevelopedtwonewpeaksaround465cm−1and565cm−1,consistentwithvibrationsofCoO2resultingfromLideintercalation.Thesefindingscloselymatchourex-situRamanobservations.Notably,LCNFPdOexhibitssmallerRamanpeakshiftsthanLCOunderthesamepolarizationpotential,indicatingamitigatedstructuraltransformation.DifferentfromCo,Ni,Fe,andPdonLCNFPdOsurfaceexhibitedminimalvalencechangesafterOER,asshownbytheXPSandsXASresultsinFigs.S24andS25.

TheFourier-transformedextendedX-rayabsorptionfinestructureinFig.3eandFig.S26showedthatLCNFPdO22°C/50°CandLCO22°C/50°ChaveshorterCo-Obondlengthscomparedwiththepristinesamples,supportingahighercobaltvalenceafterOERandagreeingwellwithXANESresultsabove34.ThecurvefittingoftheFourier-transformedEXAFS(Figs.S27andS28andTablesS11andS12)demonstratedincreasedcoordinationnumbersforbothLCNFPdOandLCOafterOER,whichconcurredwiththeirgreaterCovalence-stateincrease.Fromthewavelettransformofk3-weightedCoK-edgeEXAFSinFig.S29,twointensitymaximawereobservedataroundk=5.9Å−1and7.2Å−1,correspondingtotheCo-OandCo-Cocoordination,respectively.ThemoreobviouscolorchangeofCo-OcoordinationinLCOafterOERsupporteditslargerCovalenceincreasethanLCNFPdO.Asthemajorredoxcenter,cobaltoxidationofdifferentextentsduringOERledtodistinctLilossforchargebalance,assupportedbytheonlineelectrochemicalquartzcrystalmicrobalance(EQCM)tests(Fig.3g,h).DuringCVcyclingatboth22°Cand50°C,theweightlossofLCNFPdOwaslessthanthatofLCO,suggestingareduceddelithiationdegree.ICP-MS(TableS4)corroboratedthisforLCNFPdOandLCOafterOER.Accordingly,ToF-SIMS(Fig.3f)detectedalargerLiconcentrationonthetopsurfaceofLCNFPdOthanthatofLCOafterOER.ThelossofLitriggeredthetransformationofthelayeredsurfaceintoaspinelstructure(Fig.2e).DuringtheOERprocess,Li+extractiondrivestheoxidationofCo3+inLCOtowardCo4+.InLi-deficientLi1-xCoO2,thelayeredstructureisunstableandreadilytransformsintoaLiCo2O4-typespinelphase35,36.Consequently,greaterlithiumlosspromotesdeperreconstructionandtheformationofmorespinelspecies.Incontrast,LCNFPdOundergoeslessCooxidation(Fig.3a–d)andlithiumloss,resultinginathinnerreconstructedlayerthanLCOandthusamitigatedreconstructionextent,asillustratedintheschemeinFig.S30.ThismodulatedreconstructionofLCNFPdOcanbereflectedfromitshigher-wavenumberCo–ORamanbandsthanLCO(Fig.S31)andmarkedlyreducedmetalleachingafterprolongedoperationat100mAcm−2(TableS13).TheaboveresultsmatchedpreviousresearchthatahigherinitialCovalenceinLiCoO2increasetheenergybarrierforfurtherCooxidationandledtolessLilossandcobaltoxidationextentduringOER18.

Despitelessreconstructionextent,thereconstructedsurfaceofLCNFPdOmightbemorefavorableforinteractingwiththeOERintermediates:itsoxygenK-edgesXAS(Fig.3i)detectedanobviousriseoftheresonancenear~535eV,whichwasattributedtotheadsorbedhydroxygroupsonitssurface12,37,38,incontrasttothatofLCO.ThepresenceofmoreadsorbedoxygenspeciesonthesurfaceofLCNFPdOthanLCOaftertheOERwasalsoconfirmedbyO1sXPS(Fig.S32)andToF-SIMS(Fig.S33).Shortly,thecomplexdopinginLCNFPdOeffectivelymitigatedthein-situCoredoxandLiloss,maintainingtheelectrochemicalstabilityandgeneratingmorefavorablesurfacespeciesforOERelectrocatalysis.

Transportcapabilities
TheOERoverpotentialprimarilycompriseskinetic,ohmic,andmasstransfercontributions39.Inadditiontointrinsicactivity,theelectronicconductivityandmasstransferpropertiesofthestudiedcatalystswerealsoexamined.Todistinguishintrinsiccatalyticeffectsfromdiffusionlimitations,wedecomposedtheiR-correctedoverpotentialsusinganextendedButler–Volmer/Tafelanalysis40,asshowninFig.S34.TheextractedkineticlossandmasstransportlossdemonstratethatLCNFPdOoutperformsundopedLCOinbothmetrics,confirmingthatitsgoodOERperformancearisesfromabalancedenhancementofcharge-transferdynamicsandbubble-mediatedmasstransfer.ContrarytoNidoping,Fe,Pd,ortheircombinationswithotherdopingelementsdemonstratedtoenhancetheelectricalconductivityofLCO(Fig.4a).Notably,LCNFPdOmanifestedanelectricalconductivityof2.6mScm−1,approximatelyoneorderofmagnitudegreaterthanthatofLCO(0.35mScm−1)andsurpassingconventionalCooxyhydroxidesandNiFeoxides(Fig.S35).ThehighelectricalconductivitywasfavorabletoloweringtheOERoverpotential(Fig.4b).Toevaluatethemass-transport(e.g.,gasremoval)capabilities,wecheckedthecurrentdensitylimit(Fig.4c,d)andquantifiedthenumberofbubbles(Fig.4e,f)athighOERcurrents.Thecurrentdensitylimitisdefinedasthepointatwhichthepotentialdifferencebetweentestswithoutandwithrotation(1600rpm)reaches10mV.Ahighcurrentlimitindicatesarapidmasstransferratebetweenthecatalystsurfaceandelectrolyte,enablingthedeliveryoflargecurrents.Comparedwithpristineandotherelement-dopedLCO,LCNFPdOachievedthehighestcurrentdensitylimit,highlightingitsbettermasstransfercapability.ThedifferentialTafelanalysisfurthercorroboratesthesignificantlyimprovedmasstransportcapabilityofLCNFPdO(Fig.S36).Fastmasstransfercapabilitymightcontributetopracticalwateroxidationperformance(Fig.4d).Furthermore,thenumberofgeneratedbubbleswascountedatvariouscurrentdensitiesusingahomemadetransparentcell(Fig.S37).AcomparisonwithmultiplecontrolsamplesrevealedthatthecombinationofFeandPddopingmarkedlyacceleratedtheremovalofgasbubbles(Fig.4e,f),whichfavoredtoreducetheconcentrationpolarization41.ThebeneficialeffectofPdinfacilitatingtherapiddiffusionofwatermoleculeshasalsobeenreportedinothercatalyticsystems42.Thesurfaceofagas-evolvingcatalysttypicallyexperiencesacompetitiveadsorptionoftheaqueouselectrolyteandthegeneratedbubbles.Thecontactangletest(Fig.S38)furtherdemonstratedalowerangleofLCNFPdO(30.1°)thanthatofLCO(32.3°),implyingbetterhydrophilicityandbubbleremovalcapability.Fromtheabove(Fig.3i),thereconstructedsurfaceofLCNFPdOenabledtheadsorptionofmoreoxygen-containingspeciesduringtheOER,whichmightfurtherimproveitshydrophilicitytoeffectivelyremoveoxygenbubblesandfavorthesubsequentOERprocesses43–45,maintainingthinnerbubblelayersatlargecurrentdensities(Fig.S39).SuchimprovedadsorptionofOERintermediateswasalsosupportedbythedecayanalysisoftheopencircuitpotential(OCP)inFigs.4g,h,S40,andS41,whichreflectedthecoverageofreactionintermediatesonelectrodesurfaces46.Bycomparison,LCNFPdOdemonstratedlowerdecayslopes(absolutevalue)thanLCOatboth22°Cand50°C(Figs.4gandS40),indicatingthesurfaceofLCNFPdOhadgreateraccessibilitytoreactionintermediates47,48.SuchimprovedtransportpropertymightbeattributedtothesynergisticeffectofNiwithotherdopingelements(Fig.4h),andthecomplexdopingofNi,Fe,andPdfavoreditmost,enhancingthekineticsofOER.Alltheabovecontributedtoreducingthemasstransferoverpotentialandfavoringthemaintenanceofmechanicalstability,particularlyduringlarge-currentOERelectrolysis.

Theoreticalcalculations
To understand the modulated performance, we performed density functional theory (DFT) calculations to investigate the OER stability and activity. Based on the preceding TEM analysis, the surface of LCO and LCNFPdO reconstructed into a LiCo2O4-type spinel structure. Moreover, the surface Li loss was close to 50% (Fig. 3f), and the ICP-MS data (Table S4) confirmed negligible dissolution of Co, Ni, Fe, or Pd for LCNFPdO after OER. As a result, a calculation model of Li0.5Co0.79Ni0.1Fe0.1Pd0.01O2 (i.e., Li0.5Co0.79Ni0.1Fe0.1Pd0.01O2) (Fig. 5a) was applied, which was identified as the most energetically stable configuration using the special quasi-random structures algorithm. The half-delithiated spinel phase model was consistent with previous reports18,35,36. We observed that the Ni and Fe dopants tended to be located with each other, while the Pd and Ni dopants remained separated. These chemical arrangement patterns were then used to construct the complex Li0.5Co0.79Ni0.1Fe0.1Pd0.01O2 catalyst surface for subsequent performance investigations (Fig. S42).

Figure 5b shows the calculated dissolution free energies (ΔGdiss) of LiCo2O4 and Li0.5Co0.79Ni0.1Fe0.1Pd0.01O2 (Supplementary data 1) at varying potentials in alkaline media. A higher ΔG value indicates greater catalyst instability under electrochemical working conditions. For LiCo2O4, as the applied potential shifts from 1.23 V to 2.00 V, the calculated ΔG decreases from 0.45 to 0.19 eV/atom. These large ΔG values suggest that LiCo2O4 is unstable under working conditions and tends to decompose into CoOOH(s) and LiOH(aq). This result is in line with our experimental observations. In contrast, doping with Ni, Fe, and Pd considerably reduces the ΔG to 0.21 eV/atom at 1.23 V and 0 eV/atom at 2.00 V. Notably, when the applied potential exceeds 1.55 V, the ΔG becomes 0.03 eV/atom. This value falls well within the stable region where kinetics, driven by solid-solid phase transformation49, play a key role. Specifically, the formation Co3O4(s) and NiPdO3(s) remarkably lowers the ΔG, consequently enhancing the catalyst stability of LCNFPdO under operating conditions. This result aligns well with our experimental observations, as no obvious leaching was detected for LCNFPdO via ICP-MS (Table S4), unlike the Fe-containing hydroxides9. Close examination of the decomposition products reveal that this enhanced catalyst stability primarily arises from the Pd dopant, which facilitates the formation of NiPdO3(s) and Li0.5Co0.79Ni0.1Fe0.1Pd0.01O2(s).

Determining the surface adsorption state under electrochemical working conditions is a crucial first step in studying surface activity. Therefore, we calculated the surface Pourbaix diagram for LCNFPdO, as shown in Fig. 5c. Under typical alkaline OER conditions (e.g., pH = 14, potential = 1.6 V), the catalyst surface is covered by O* at the Ni-Co bridge site. At potentials higher than 1.8 V, O* adsorption becomes accessible at the Co, Ni, and Fe top sites. Coupling two adsorbed O* species at these top sites directly forms O2 gas during DFT structure optimization, a result attributed to the weak binding of O* on the transition metal oxide50. This potential-independent O2 formation indicates that an oxide pathway mechanism is unlikely for LCNFPdO. Furthermore, a surface with oxygen vacancies is only stable at potentials below −0.61 V vs. SHE, which also rules out the lattice oxygen mechanism for OER. In light of these findings, the adsorbate evolution reaction mechanism was applied to investigate the OER activity in this work.

To elucidate the role of each dopant, we calculated the OER activity for LCNFPdO and six other systems for comparison (Table S14 and Supplementary data 1). With the exception of the LCNPdO system, all catalysts reside on the right side of the volcano, as shown in Fig. 5d. This indicates that the formation of O* is the potential-determining step (PDS) due to weak O* binding. For single-element doping, the calculated overpotentials (ηOER) for LCFO (0.90 V) and LCPdO (0.76 V) are higher than that of pristine LCO (0.68 V), suggesting poorer OER activity. In contrast, Ni doping reduces the overpotential to 0.61 V, indicating enhanced activity. Among the binary-doped systems, LCFPdO shows a slight improvement over LCO with a calculated η of 0.67 V. The LCNFO system exhibits significantly enhanced activity with an overpotential of 0.54 V, which can be attributed to electronic optimization between Ni and Fe. In contrast, the LCNPdO system has a higher overpotential of 0.77 V and notably undergoes a PDS shift from O* to OOH* formation, highlighting Pd’s role in electronic restructuring. For the ternary-doped system, LCNFPdO achieves the lowest overpotential of 0.46 V, suggesting that Pd further optimizes the NiFe electronic framework. Taken together, these results indicate that Ni is a pivotal active site that synergizes with Co, and its activity is further amplified by Fe through electronic cooperation. To establish fundamental mechanistic insights, we conducted integrated Bader charge and density of states (DOS) analyses. Bader charge analysis (Table S9) reveals that Ni/Fe co-doping facilitates a substantial charge transfer enhancement of about 0.3e on Co sites. Concurrently, a 0.1e enhancement in charge transfer at Co sites was observed for LCPdO relative to LCO. This demonstrates Pd’s significant contribution to electronic structure modulation. Complementarily, total DOS analysis (Fig. S43 and Supplementary data 1) demonstrates Fe-doping reduces the band gap by 0.4 eV, while LCNFPdO manifests metallic conductivity through further electronic restructuring. The enhanced conductivity is an important indicator of catalysts. Collectively, these synergistic electronic modifications constitute the fundamental mechanism driving enhanced catalytic activity.

Based on our experimental results, we speculate that the mass transport of reactants (H2O) and products (O2) may also influence OER performance. Therefore, the adsorption energies for H2O and O2 were calculated as shown in Fig. 5e. The calculated O2 adsorption energies for LCNFPdO and LCO were 1.10 eV and 0.35 eV, respectively. This result suggests that LCNFPdO possesses weakened O2 adsorption compared to LCO. Such weakened O2 adsorption facilitates the desorption of O2 from the electrode surface. The calculated H2O adsorption energies for LCNFPdO and LCO were −1.07 eV and −0.33 eV, respectively. This result suggests that LCNFPdO has a stronger tendency for H2O adsorption compared with LCO, which is beneficial for enhancing OER efficiency. Comprehensive comparison in Fig. S44 demonstrates Fe doping inhibits H2O/O2 adsorption, whereas Ni and Pd synergistically enhances surface binding. This adsorption dichotomy stems from Co-site electronic modulation: augmented charge transfer elevates valence states and local electronegativity, intensifying adsorbate interactions.

AEMWE performance
To verify the practical application, the catalysts studied were fabricated into the MEA for the AEMWE tests (Fig. 6a). At the same voltage, LCNFPdO achieved a current density much larger than those of RuO2, IrO2, α-Co(OH)2, Co3O4, and β-Co(OH)2 (Fig. 6b). This performance was also accompanied by a lower polarization impedance (Fig. 6c). With iR correction, LCNFPdO delivered 2.5 A cm−2 at only 1.58 V (Fig. 6b), suggesting great application potential. The three-electrode electrolyzer tests confirmed a lower mass transport resistance of LCNFPdO than LCO, as shown in Fig. S45. In addition, the AEMWE cell with LCNFPdO as the anode sustained the current density of 500 mA cm−2 without any degradation for over 1420 h (Fig. 6d). Such a performance is comparable to the highest values reported for cobalt-oxide–based electrocatalysts (Table S15).

Discussion
This work proposed a complex-element doping strategy to yield a sustainable cobalt oxide electrode for highly active and stable OER. The multi-element doping mitigated the reconstructed extent of the in-situ formed spinel surface during the OER, forming a stable surface with significantly enhanced activity at industrial temperatures. The obtained LCNFPdO outperformed RuO2 in OER activity and demonstrated stability of over 2000 h without serious degradation, setting a benchmark for cobalt-based OER electrocatalysts. In addition, the mechanism underlying the complex doping-improved performance was also clarified: (a) The introduction of lower-valence Ni and Pd dopants increased the cobalt valence in pristine LCNFPdO, mitigated its in-situ cobalt oxidation during the OER, thus reducing surface reconstruction extent and improving OER stability. Furthermore, the reconstructed LiCo2O4-type spinel phase of low thickness favored a higher intrinsic OER activity by facilitating the adsorption of oxygen intermediates on the catalyst surface, contributing to higher OER activity. (c) The doping of Fe and Pd also enhanced the electronic conductivity and gas transport properties, resulting in fast reaction kinetics and rapid removal of bubbles. Shortly, these results shed light on the directional designing of active OER catalysts by multiple elements doping and represent a significant advancement in manipulating in situ generated active species to prepare high-performance OER catalysts at industrial temperatures.

Methods
Sample preparation
The layered catalysts were prepared via a solid-state method based on a previous literature51. A stoichiometric amount of specific raw reactants, including Li2CO3 (99.9%, Aladdin), Co3O4 (99.9%, Macklin), Fe2O3 (≥99%, Aladdin), NiO (99%, Aladdin), Pd(NO3)2 (0.216 M, Kojima Chemicals Co., Ltd.), Na2CO3 (≥99.5%, Macklin), K2CO3 (99.5%, Macklin), (MgCO3)4·Mg(OH)2·5H2O (98%, Macklin), MnO2 (99%, Aladdin), RuO2 (Ru 75.2% min, Alfa Aesar), Ce(NO3)3 (≥98%, Wako), Cr2O3 (99%, Aladdin), La2O3 (99.9%, Aladdin), TiO2 (P25, Macklin), MoO2 (99%, Aladdin), ZnO (99%, Dieckmann), Cu(OH)2 (≥96%, Macklin), CoCl2·6H2O (99.99%, Aladdin), CoSO4·7H2O (99%, Aladdin), and LiF (99%, Aladdin), were mixed well and then sintered for 12 h in ambient air with a heating ramping rate of 10 °C min−1. The thermal treatment temperature for catalyst preparation has been optimized, and 700–850 °C is suitable for LCO (Fig. S46), among which 800 °C is chosen as the sintering temperature in this research considering proper kinetics and high-purity phase.

α-Co(OH)2 was synthesized by CoCl2·6H2O (1 mmol, 99.99%, Aladdin) and NaCl (99.5%, Aladdin, 2.5×g), both of which were dissolved in ultrapure water (150 mL, 18.2 MΩ cm) with stirring (solution A). NH3·H2O (0.133 mL, 25–28%, Aladdin) was mixed with ultrapure water (50 mL) and dropped into solution A under stirring by a peristaltic pump (10 mL h−1). The precipitation was washed by ultrapure water and finally dried.

β-Co(OH)2 was prepared by dropping CoSO4·7H2O (0.1 M, 10 mL, 99%, Aladdin) into a stirring NaOH aqueous solution (0.2 M, 10 mL, 96%, Aladdin). After stirring for one hour, the precipitation was rinsed three times by centrifugation with ultrapure water and finally dried.

The XRD of synthesized α-Co(OH)2 and β-Co(OH)2 are presented in Fig. S47.

RuO2 (Alfa Aesar), IrO2 (Aladdin), Co3O4 (99.9%, Macklin), and NiFe2O4 (≥99.5%, Aladdin) were used without further treatment.

Characterizations
XRD measurements were conducted on an X-ray diffractometer (PANalytical X’Pert3 Powder, Cu Kα, λ=0.15406 nm). The corresponding Rietveld refinements were performed with the General Structure Analysis System (GSAS-II) program. The XAFS and sXAS were collected at the TLS 01C1 and TLS 20A1 beamlines of the National Synchrotron Radiation Research Center, Taiwan. The XANES and EXAFS data were processed and analyzed using the Demeter software package Athena52, and the energy at the maximum value of the first derivative of its XANES was defined as the E0 for energy correction. The EXAFS results of the Co K edge were fitted by Artemis52. Wavelet transform was used to analyze the EXAFS data by a self-developed program53. XPS results were obtained on a Thermo Fisher Nexsa spectrometer using an Al Kα source. For ex-situ XRD and XAS characterizations, catalyst powders were loaded onto carbon paper substrates prior to the OER measurements. The preparation of post-OER sample involved 10 CV cycles (5 mV s−1, 1.02–1.62 V vs. RHE, 1 M KOH, 22 °C or 50 °C) during which the current stabilized. Subsequently, a 30-min chronoamperometry hold at 1.62 V vs. RHE was applied during which no notable current decay was observed. The samples were then rinsed with deionized water and dried under vacuum at 25 °C for 12 h before characterization.

HAADF-STEM images were taken by JEOL-2100F equipped with an energy dispersive X-ray spectroscopy detector. A field emission SEM (Zeiss SIGMA 500) mounted with an energy-dispersive X-ray detector was used for morphology and elemental analysis. Raman spectra were collected in a Raman spectrometer (Renishaw inVia Qontor) with the laser at 100 mW (0.05% intensity) and 532 nm. In-situ Raman spectroscopy was performed using a GSL (Gaoss Union) electrochemical cell coupled to the instrument. Measurements employed a 532 nm excitation source with a three-electrode configuration: carbon cloth (Toray Industries) working electrode, Pt wire counter electrode (Gaoss Union), and Ag/AgCl reference electrode (Gaoss Union, 1 M KCl solution) in 1 M KOH electrolyte. The system was stabilized at each potential for 15 min prior to spectral acquisition, which commenced precisely at the 10-min interval. An Iontof 5 spectrometer was used for the ToF-SIMS measurements. The analysis was done with a pulsed 30 keV Bi+ primary ion source and the depth profiling by sputtering with a 1–2 keV Cs+ beam. The pressure was 10−7 Pa. IonSpec software was used for data acquisition and post-processing analysis. Negative ion profiles were recorded due to their sensitivity to fragments originating from oxide matrixes. ICP-MS (NexION 350D Perkin-Elmer) with a mass resolution of 0.3–3.0 a.m.u. was conducted to measure element concentration. The mass variation was characterized by EQCM (10 M Gamry Instruments Inc.) with a working electrode of a gold quartz crystal (5 MHz). EPR spectra were recorded with a Bruker EMXplus X-band spectrometer (≈9.84 GHz) at 25 °C. The spectra were averaged over 5 scans with 1000 points per sweep. Each 2 mg sample was measured under identical settings. The g‑factor was calculated using g=hν/βH. Oxygen vacancy concentrations were quantified by comparing EPR peak areas.

Electrochemical assessment
The OER activity was evaluated in 1 M KOH (≥95%, Macklin) with a three-electrode set-up measured by an electrochemical workstation (Gamry 1010E). The electrolyte was freshly prepared before each experiment using ultrapure (18.2 MΩ·cm) water (150 mL) as the solvent. It was then stored sealed in a chemical-resistant glass reagent bottle and used on the same day. Glassy carbon (Gaoss Union, 3 mm, <50 °C) and carbon paper (Toray Industries, 0.5 cm × 0.5 cm, to avoid catalyst peeling off at the reaction temperature ≥50 °C) were utilized as working electrode, and graphite rod (Gaoss Union), and Ag/AgCl (Gaoss Union, 1 M KCl filling solution) served as counter and reference electrode, respectively. The catalyst ink was prepared by dispersing the mixture of 6 mg of active material with 2 mg of Vulcan XC-72 carbon into a solution made of 0.95 mL ethanol (AR, Uni-Chem) and 0.05 mL of 5 wt% Nafion under ultrasonic treatment for 30 min, and 1 mg cm−2 active material was covered on working electrode by drop casting. The measured potentials were calibrated to the RHE scale using the following equation: ERHE = EAg/AgCl + E0Ag/AgCl + 2.303·RT·pH/F, where E is the measured potential with respect to the Ag/AgCl reference electrode and E0 is the standard reference electrode potential and obtained by Eq. (1)54:

E0Ag=AgCl(T) = 0.23659 − 4.8564×10−4(T−273.15) − 3.4205×10−6(T−273.15)2 − 5.869×10−9(T−273.15)3 (1)

T is the operation temperature, R is the ideal gas constant (8.31 J K−1 mol−1), F is the Faraday constant (96485 C mol−1). The pH value of 1 M KOH electrolyte was measured as 13.8±0.1, 13.2±0.1, and 12.5±0.1 was obtained in 25, 50, and 80 °C, respectively. CV and LSV were conducted at a scan rate of 5 mV s−1 in the tests. The ECSA was measured within a non-Faradaic potential window of 1.15–1.20 V vs. RHE at scan rates of 1, 2, 4, 8, and 10 mV s-1, as shown in Fig. S9a, c, e, and g. For each scan rate, the current difference at a selected potential (1.175 V vs. RHE) was calculated as (Ia-Ic)/2, where Ia and Ic are the anodic and cathodic currents at the selected potential, respectively. The double-layer capacitance (Cdl) was obtained by plotting (Ia-Ic)/2 versus scan rate (mV s−1) and fitting the data to a linear regression (Fig. S9b, d, f, and h). The slope of this line yielded Cdl in units of capacitance (F). The ECSA was then calculated using ECSA = Cdl/(Cs·m), where m is the loading mass of the catalysts on the electrode and Cs is the reference capacitance per unit area (40 μF cm−2 55). Notably, the catalyst ink used for ECSA testing was prepared without the addition of XC-72 carbon. The test temperature was controlled by a thermostatic water bath (Sunne Inc., Shanghai, China). EIS was obtained using a Gamry 1010E electrochemical workstation over a frequency range from 1 MHz to 0.1 Hz at 1.65 V vs. RHE with an AC amplitude of 10 mV.

MEA preparation and single-cell tests
The MEAs were fabricated using a hand spray-coating method for both the anode and cathode catalyst layers. The anode catalyst ink was prepared by dispersing the catalyst powder in a mixed solvent of deionized water and isopropanol (mass ratio=1:4:5). Subsequently, 5 wt% Nafion solution was added, resulting in a Nafion content of 7 wt% in the final ink. The ink was ultrasonicated for 1 h to ensure homogeneous dispersion and then uniformly sprayed onto a nickel foam substrate (Kunshan Shengshijingxin New Material Co., Ltd, China, 1 cm2). The anode catalyst loading was controlled at 10.0±0.5 mg cm−2 (1 mg cm−2 for RuO2 and IrO2), determined by weight difference before and after coating. For the cathode, a 5 wt% Pt/C catalyst (Alfa Aesar) was used. The ink formulation consisted of catalyst, deionized water, and isopropanol in a weight ratio of 1:8:9. After adding 5 wt% Nafion solution (final Nafion content 10 wt%), the ink was ultrasonicated for 1 h and then sprayed onto a carbon paper substrate (TGP-H-120T, Toray). The Pt loading was 0.5 mg cm−2.

A commercial Sustainion® membrane (X37-50, Grade T, Dioxide Materials) was employed as the anion exchange membrane for all single-cell tests. Prior to use, the membrane was pre-treated by soaking in 1 M KOH for 12 h to ensure full ion exchange. The MEA was assembled with an active geometric area of 1.0 cm2 (as shown in Fig. 6a). The single cell hardware was composed of nickel-coated stainless-steel end plates to ensure chemical stability in alkaline media.

All electrochemical measurements were conducted in 1 M KOH electrolyte, supplied to both anode and cathode chambers. Polarization curves were recorded at 80 °C by a Gamry 5000E electrochemical workstation with 30 s hods at each current density ranging from 0.10 A cm−2 to 2.50 A cm−2, followed by a reverse scan. Voltage and current density data were recorded during the downward scan. The durability of the MEA was evaluated by operating the cell at a constant current density of 500 mA cm−2 at 60 °C with a programmable DC power supply (Tommense TM-K1560SPL).

High-frequency resistance (HFR) was measured via EIS using a Gamry 5000E electrochemical workstation over a frequency range from 1 MHz to 1 Hz at a cell potential of 1.8 V with an AC amplitude of 10 mV. The HFR was obtained from the intercept of the Nyquist plot with the real axis. Three independent measurements were performed, and the mean and standard deviation were subsequently calculated from the replicate data. The voltage data are presented without iR compensation unless specifically indicated.

Three-electrode AEMWE preparation
The anode and cathode used in three-electrode AEMWE are the same with the two-electrode AEMWE introduced above, and the only difference is the membrane. Versogen A40 membrane (thickness 40 μm) was used. Prior to use, the membrane was equilibrated under ambient laboratory conditions for 1 h before removing the protective films. This step allows the bicarbonate form of the membrane to convert to the hydroxide form, which is required to achieve optimal ionic conductivity. To ensure full conversion to the hydroxide form, the membrane was immersed in 0.5 M KOH aqueous solution at 22 °C for 1 h, after which the solution was replaced with freshly prepared 0.5 M KOH and the membrane was immersed for an additional 1 h. Following the two-step soaking process, the membrane was rinsed thoroughly with deionized water until the rinse water reached approximately neutral pH (~7). During handling, exposure of the membrane to ambient air was minimized to prevent CO2 uptake, which would convert the membrane back to the bicarbonate form. Three-electrodes system was established based on our homemade cell (Fig. S45a). The membrane was extended to an extra reference electrode chamber (Fig. S45b) and filled with 0.01 M KOH to ensure good ionic conduction with Ag/AgCl reference electrode. The testing procedure is the same with two-electrode single cell.

Anode applied potential breakdown analysis: the sources of potential loss at the anode was identified using a three-electrode AEMWE setup (Fig. S45a, b) to study the transport properties of LCNFPdO and LCO. The polarization curves of individual anode and cathode versus Ag/AgCl (1 M KCl solution) reference electrode and the calculated full cell voltage are shown in Fig. S45c, d.

The calculated full cell voltage is obtained from the individual electrode potentials according to Eq. (2):

Ecalculatedfullcell(j) = Eanodevs:Ag=AgCl(j) − Ecathodevs:Ag=AgCl(j) (2)

where Eanodevs:Ag=AgCl and Ecathodevs:Ag=AgCl are the potentials versus Ag/AgCl reference electrode obtained during tests.

The total anode potential breakdown model can be written as Eq. (2) 56:

Eanode(j) = Erev,OER + ΔEkinetic(j) + ΔEohmic(j) + ΔEmasstransport(j) (3)

where Eanode (vs. RHE) is the anode potential applied, Erev,OER is the reversible potential for OER, and ΔEkinetic, ΔEohmic, and ΔEmasstransport are the kinetic, ohmic, and mass transport potential losses, respectively. The Eanode is obtained using Eq. (3):

Eanode(j) = EAg=AgCl(j) + E0Ag=AgCl + 2.303× RT/F × pH (4)

And the Erev,OER = 1.1812 V (vs: RHE) is calculated using Eq. (5) 57:

E0rev,OER = 0.4011 − 1.6816×10−3×(T−298.15) + 2.303× RT/F × pH (5)

where T is the operation temperature (353.15 K), and the pH of 1 M KOH at 80 °C (353.15 K) is 12.5±0.1. E0Ag=AgCl = 0.1732 V (vs: SHE) is the standard potential versus standard hydrogen electrode (SHE) and is calculated according to Eq. (1).

The kinetic potential loss can be obtained using a Tafel (Eq. (6)) from the iR-corrected polarization curve at low current density region,

ΔEkinetic = b×log(j/j0) − Erev,OER (6)

where j0 is the exchange current density, j is the current density, and b is the Tafel slope and can be determined by fitting the low-current region (dominated by charge-transfer processes) of the polarization curve. The ohmic potential loss can be obtained using Ohm’s law (Eq. (7)),

ΔEohmic = HFR×j (7)

Thus, the mass transport potential loss can be obtained by Eq. (8):

ΔEmasstransport(j) = Eanode(j) − Erev,OER − ΔEkinetic(j) − ΔEohmic(j) (8)

Computational methods
All spin-polarized DFT calculations were conducted using the Vienna ab initio simulation package within the projected-augmented wave method58,59. The Perdew-Burke-Ernzerhof functional was used to describe the exchange-correlation interaction60. The DFT+U method was utilized to correct the self-interaction error with Hubbard U parameters of 3.32, 5.30, and 6.20 eV for Co, Fe, and Ni, respectively61,62. The convergence criteria of the electronic total energy and structure relaxation were set to 10−5 eV and 0.05 eV Å-1, respectively. A plane wave energy cutoff of 450 eV was applied.

DFT method: The Brillouin zone was sampled using the Gamma scheme with a 1×2×1 k-point grid for the surface models with 5 atomic layers of LiCo2O4 63. A vacuum thickness of 16 Å was added along the z direction to avoid periodic interactions, and the dipole correction was applied as well. The (111) surface was selected for exploring the catalytic activity, as it is the most stable surface of the spinel structures64. The doped structure was generated by the special quasi-random structures algorithm implemented in the ATAT package65 and the most energetically favorable one was then selected for the activity and stability studies66. The OER activity was evaluated by the minimum overpotential required for the four elementary reactions given by:

*+H2O(l) → OH*+H++e− (9)

OH* → O*+H++e− (10)

O*+H2O(l) → OOH*+H++e− (11)

OOH* → O2+H++e− (12)

where * represents a surface site. The reaction free energy of these elementary steps was calculated as: ΔG=ΔE+ZPE−TΔS=ΔE+ΔGcorr, where ΔE is the adsorption energy, ZPE represents zero-point energy, and ΔS is the entropy change at 300 K. Free energy corrections were explicitly calculated per 67:

ΔGcorr = ZPE + ∫Cp dT−TS (13)

with vibrational frequencies computed for all intermediates at 300 K and 1 atm (Table S16). The catalyst stability was assessed by the Strongly Constrained and Appropriately Normed (SCAN) functional and calculated bulk Pourbaix diagram68,69. In SCAN calculations, the plane-wave energy cut-off was 520 eV. The electronic energy and structure relaxation were converged to 10−5 eV and 0.02 eV Å−1, respectively. The k-point grid density was 100 per reciprocal volume.

The Hubbard U parameters for Ni, Fe, and Co were empirically calibrated by fitting to experimental redox potentials of their corresponding metal oxides (Table S17)70. This approach ensures the computed U values accurately reproduce redox energetics critical to surface catalytic processes.

Surface model construction: To construct a reasonable surface model, three principles governed surface model construction: (1) We constructed the NiFePd-doped LCO bulk structure by optimizing 128 quasi-random structures and selecting the most stable one. It is observed that the Ni and Fe dopants tend to be located with each other, while the Pt and Ni dopants sit away from each other. These chemical insights were applied to construct the complex LCFNPdO catalyst surface; (2) Based on previous studies, the (111) facet is the most stable surface in spinel LCO and is therefore used as a surface model; (3) The presence of Pd in the sublayer is more stable than that on the top layer (Formation energy difference: −0.10 eV). Meanwhile, the more active Ni and Fe are often exposed on the topmost layer71,72. The final surface structure model was finally determined by considering all these factors.

All relevant adsorption sites on LCNFPdO were evaluated, including top sites of Co, Fe, and Ni atoms and Ni-Co bridge sites (Table S18). Structural relaxation revealed: (1) Intermediates at Ni/Co top sites consistently migrated to Ni-Co bridge sites, producing identical structures and energies; (2) Fe sites exhibited poor catalytic activity despite electronic modulation effects. The Ni-Co bridge site was consequently identified as the optimal active center.

Magnetic configuration: To address magnetic complexity in transition metal systems, we implemented a rigorous configuration selection protocol. The pristine slab model was first optimized to determine its lowest-energy magnetic moment configuration; Identical NUPDOWN values were subsequently applied to corresponding adsorbate systems; All configurations must satisfy four criteria: absence of antiferromagnetic ordering, adherence to the ΔGOOH−ΔGOH scaling relationship (2.8–3.3 V), total magnetization within 4 μB tolerance, and the selection of the lowest-energy state for multiple valid configurations; Systems failing these criteria were recalculated with NUPDOWN adjustments (±1 μB increments) and modified initial moments. Total magnetization data for all configurations are documented in Table S19.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a multi-element doping strategy (Ni, Fe, Pd) to enhance the stability of LiCoO2 (LCO) for OER, mitigating surface reconstruction. While the experimental work is thorough and the reported improvements in OER performance are noteworthy, the manuscript lacks the mechanistic depth and conceptual clarity expected for publication in Nature Communications. The conclusions rely largely on macroscopic observations without sufficient atomic-level insight. A more rigorous theoretical framework is needed to support the proposed structure–function relationships. Key concerns are outlined below.\n1. Ambiguity in doping mechanism: (1) The authors state that Ni and Pd doping increases the Co valence state while simultaneously suppressing its oxidation. These statements appear contradictory, as a higher Co valence is typically associated with increased oxidation behavior. This inconsistency should be addressed. Additionally, the explanation that the increase in Co valence arises solely from the introduction of low-valence Ni and Pd is overly simplistic. The Co valence state is influenced by several factors, such as the electronegativity of the dopants, orbital hybridization and the possible generation of oxygen vacancies due to valence mismatch. A more thorough investigation of these underlying mechanisms is necessary to support this conclusion, rather than relying on the dopants’ valence states alone.\n(2) A clear mechanistic rationale of mitigating reconstruction should be provided. The authors report that LCNFPdO undergoes reconstruction into a spinel-type LiCo2O4 phase during OER, but do not clarify how multi-element doping alters the reconstruction pathway to favor this specific structure. If reconstruction is indeed observed, evidenced by the formation of LiCo2O4, then the claim of “mitigating reconstruction” is not accurate. It would be more appropriate to state that doping induces the formation of a stable spinel reconstruction layer. The authors are encouraged to elaborate on how this reconstructed structure contributes to enhanced stability. Additionally, it remains unclear what the final reconstruction product of undoped LCO is? Does it also form LiCo2O4?\n(3) The individual roles of Ni, Fe, and Pd are not clearly distinguished. Possible synergistic or antagonistic effects between dopants are not discussed. A systematic investigation, such as single-element doping experiments or theoretical simulations, would clarify the contributions of each element.\n2. Limited insight into mass transport mechanism: The authors claimed that the doping of Fe and Pd can enhance the electronic conductivity and gas transport properties, which fast reaction kinetics and rapid removal of bubbles. However, the conclusion is based solely on macroscopic observations (bubble statistics and limiting current density), the underlying mechanism remains speculative. Does Pd affect the wettability of the catalyst surface or reduce bubble adhesion? or could Fe doping alter local charge distributions to facilitate ion transport?\n3. Inconsistencies in data interpretation: (1) The introduction notes that Fe is prone to dissolution under OER conditions (Line 69), yet Table S3 shows no Fe loss after extended high-temperature and high-current operation for LCNFPdO catalyst. This discrepancy should be clarified.\n(2) An activation process is reported during the initial CV cycles (line 111), indicated by increased current density. However, no explanation is offered. Could this be due to the formation of a reconstructed layer, and if so, how does that align with the claim of suppressed reconstruction?\n(3) The discussion (line 249) uses O-K edge sXAS and O 1s XPS to support more adsorbed oxygen species on LCNFPdO, due to reconstructed surface. Yet previous findings indicate that LCO undergoes more extensive surface reconstruction. Why is OH adsorption not more prominent in LCO? This inconsistency requires further clarification.\n4. Insufficient experimental details: (1) The method for post-reaction sample collection for XRD and XAS is not described. Please include details such as how powders were collected and prepared.\n(2) Key parameters for AEMWE testing are missing, including membrane electrode assembly method, catalyst loading, and membrane type.\n5. Inappropriate language and formatting errors: (1) Avoid superlative terms like “the longest reported lifetime” or “none of these strategies”, unless supported by comprehensive comparative data.\n(3) Figure 3e is discussed twice with nearly identical description (Lines 161 and 245).\n(4) Line 191 links the peak intensity of the 104-diffraction pattern to structural disorder. This requires a citation or clearer justification.\n(5) Line 252 contains a double comma and should be corrected; Line 333 incorrectly places a period before the citation \" (Figure 39)\".", "reviewer_2": "The urgent need to address instability in cobalt-based oxide electrodes represents a critical bottleneck in advancing sustainable water electrolysis technologies. In this context, the manuscript NCOMMS-25-35487-T by Wang et al. makes a compelling contribution. The authors report a sophisticated doping strategy to stabilize LiCoO₂—a cobalt oxide widely deployed in lithium-ion batteries—as an anode for alkaline water electrolysis.\n1. Please provide a clear and quantitative definition of “reconstruction extent”and specify how it is measured in this experiments.\n2. Please expand the discussion on how lithium loss quantitatively correlates with the formation of the spinel phase during OER\n3. Please describe the electrolyzer testing methods in more detail, including MEA preparation, cell assembly, operational parameters, and evaluation criteria\n4. Please consider including post-test characterizations or comparative data to verify electrode structural stability after long-term operation. Also, briefly discuss possible degradation modes and compare durability with commercial catalysts .", "reviewer_3": "The authors have screened a series of element-doped LiCoO₂ materials and identified LiCo₀.₇₉Ni₀.₁Fe₀.₁Pd₀.₀₁O₂ (LCNFPdO) as the most active catalyst for alkaline water electrolysis. While the ex-situ characterization of LCNFPdO is comprehensive, the manuscript lacks a robust discussion—and corresponding experimental evidence—linking the reconstructed surface structure to the observed OER performance. Furthermore, the reported cell voltage of 1.58 V at 2.5 A cm⁻² (iR-corrected; ~2.1 V without compensation) appears impressive, but a fair comparison to state-of-the-art catalysts should be made using uncorrected voltages. My detailed comments follow:\n1. The authors acknowledge Li leaching during OER, yet no direct structural comparison is made between LCO and LCNFPdO after reconstruction. In Fig. 1b, the CV curves for LCO and LCNFPdO cross in opposite ways before and after cycling—suggesting fundamentally different surface evolution. The manuscript should include a more in-depth characterization/discussion on the structure and the structural difference after reconstruction and how these differences relate to the observed OER activity. Moreover, what chemical understanding was obtained after screening/characterizing/testing a series of dopants was not discussed.\n2. In Figs. S21–S22, the authors claim that LCNFPdO is less oxidized post-OER than LCO. However, the Co L₃-edge sXAS peak shifts in Fig. S21 are subtle and not convincingly resolved. Likewise, the Co 2p XPS fits in Fig. S22 appear overly broad for the Co²⁺ and Co⁴⁺ components—e.g., the Co²⁺ peak for LCO in Fig. S22c seems significantly wider than that for LCNFPdO. I recommend re-fitting/re-analyzing these results after carefully reviewing the peaking fitting constraints.\n3. Almost all characterizations and oxygen adsorption were collected ex-situ. Given that bias removal and air exposure can alter the surface, the authors should either perform in-situ or—at minimum—provide evidence demonstrating that these ex-situ spectra reliably reflect the status of surface under OER conditions.\n4. The manuscript attributes higher currents in LCNFPdO partly to enhanced mass transport. However, LSV data were recorded in the mixed-kinetic/diffusion regime, so mass-transport effects are conflated with intrinsic kinetics. A deconvolution of kinetic and diffusion contributions should be made to support the claim. Additionally, hydrophobicity differences at the level reported are too small and not statistically meaningful to draw firm conclusions (contact angles 30.1° vs. 32.3°).\n5. Reporting 1.58 V at 2.5 A cm⁻² only after iR correction may mislead readers regarding practical performance. The authors should also compare the uncorrected performance for LCNFPdO with the state-of-the-art catalysts (Table S10). The author may refer to this comprehensive performance summary: Supplementary Table 2 in Nature Catalysis 7, 944–952 (2024). This will allow a fair, “apples-to-apples” comparison under realistic operating conditions.", "reviewer_4": "In the manuscript by Wang et al., titled \"Sustainable Water Oxidation Enabled by a Complex-doped Cobalt Oxide Electrode\", the authors present a very interesting combined experimental and theoretical investigation of a LiCoO2-based electrocatalyst doped with Ni, Fe, and Pd for the oxygen evolution reaction. The proposed system exhibits high catalytic activity, delivering 2.5 A cm-2 at 1.58 V, while maintaining excellent long-term stability. The authors demonstrate that complex doping suppresses cobalt redox during OER, effectively mitigating surface reconstruction, enhancing electrochemical stability, improving surface conductivity, and facilitating gas removal—thereby boosting mechanical robustness. I find these findings highly significant, as they directly address the persistent challenge of catalyst degradation under industrial OER conditions, and in my view, merit publication in Nature Communications. The work is well-written and scientifically sound. However, several critical points should be addressed to enhance the overall quality of the manuscript before it can be recommended for publication.\n1. The authors constructed the NiFePd-doped LiCoO2 bulk structure by optimizing 128 quasi-random configurations and selecting the most stable one. However, it’s not clear how the surface shown in Fig. S39 was built. Different surface cuts can expose different dopants and lead to variations in stability, so some clarification is needed.\n2. OER intermediates (*OH, *O, *OOH) were optimized on both LiCoO2 and NiFePd-doped LiCoO2 surfaces. However, the doped surface contains a large number of non-equivalent adsorption sites with potentially different activities. It’s not clear how the optimization was performed in this case—were all relevant sites considered, or just one specific configuration? Some clarification is needed. Moreover, the POSCAR files of the optimized key structures should be provided in the Supplementary Information to ensure reproducibility of the results.\n3. The manuscript assumes a conventional single-site OER mechanism as shown in Supplementary Note 2. However, given the doped and reconstructed nature of the surface, with multiple metal centers in close proximity, could dual-site or cooperative mechanisms play a role? It would be useful if the authors could comment on whether such possibilities were considered and how the dual-site mechanism can affect the theoretical overpotential.\n4. The choice of Hubbard U parameters for Ni, Fe, and Co should be justified. Have the authors performed any self-consistent calculations to determine the U values? The U = 6.2 eV used for Ni appears somewhat large in my opinion.\n5. It is really disappointing how the authors calculated the changes in free energy, especially ΔG_corr. They simply used approximate values for zero-point energy and entropy corrections suggested in early works by Nørskov for metal surfaces. This methodology is outdated, especially for a study submitted to a high-impact journal like Nature Communications. Such corrections can vary significantly for complex metal oxides and should be explicitly calculated for each intermediate. Moreover, the manuscript does not explain how solvent effects were treated, which is critical, as solvation can substantially impact the computed free energy values.\n6. The system contains transition metal atoms with unpaired d-electrons (Fe, Co, and Ni), and in my experience, it can be very challenging to identify the correct spin ordering in such complex systems. The authors should clearly state the magnetic configuration used and explain how it was chosen to ensure the lowest-energy structure.\nOverall, this work addresses an important topic and may be suitable for Nature Communications, but in its current form, it requires substantial revision before it can be considered for publication."}, "author_response": "Response to Reviewers\nManuscript ID: NCOMMS-25-35487-T\nArticle title: Sustainable Water Oxidation Enabled by a Complex-doped Cobalt Oxide\nElectrode\nWe would first like to express our appreciation to the editor for giving our manuscript a\nchance to be reviewed and to the reviewers for their constructive comments, which helped\nstrengthen our scientific findings. Below, we provide point-to-point responses to each\ncomment, which are written in blue font. Our responses are in black font, with manuscript\nadditions highlighted in yellow.\nReviewer #1 (Remarks to the Author):\nThis manuscript presents a multi-element doping strategy (Ni, Fe, Pd) to enhance the\nstability of LiCoO (LCO) for OER, mitigating surface reconstruction. While the\n2\nexperimental work is thorough and the reported improvements in OER performance are\nnoteworthy, the manuscript lacks the mechanistic depth and conceptual clarity expected\nfor publication in Nature Communications. The conclusions rely largely on macroscopic\nobservations without sufficient atomic-level insight. A more rigorous theoretical\nframework is needed to support the proposed structure–function relationships. Key\nconcerns are outlined below.\nResponse:\nWe thank the reviewer for the constructive comments. In this response, we strengthened\nthe atomic‐scale evidence and added more theoretical analysis to support our conclusions.\nIn our initial submission, we provided atomic-level characterizations and calculations to\nunderstand the modulated reconstruction and improved OER performance. First, our\nmultiple atomic‐level characterizations demonstrated how Ni/Fe/Pd co‐doping modulated\nthe surface reconstruction of LiCoO (LCO) during the OER. For example, time‐of‐flight\n2\nsecondary ion mass spectrometry (TOF‐SIMS, Fig. 3e) and online electrochemical quartz\ncrystal microbalance (EQCM, Fig. 3f-g) measurements quantitatively show that the\nNi/Fe/Pd co-doped LCO (LCNFPdO) undergoes significantly less Li loss under OER than\npristine LCO, accompanied by less cobalt valence state increase during the OER, which\nwas confirmed by synchrotron X-ray absorption fine structure (XAFS) in Fig. 3a-b and soft\nX-ray absorption spectroscopy (sXAS) in Fig. S21. This reduced Li loss and cobalt valence\nincrease led to the formation of a thinner spinel-type reconstruction surface of LCNFPdO\nthan that of undoped LCO, as detected by the scanning transmission electron microscopy\n(STEM, Fig. 2e-h and Fig. S15-S18) and selected-area electron diffraction (SAED, Fig. 2e-\nh). Moreover, our density functional theory (DFT) calculations establish a rigorous\ntheoretical framework for these observations. The DFT results show that Ni/Fe/Pd dopants\nthermodynamically stabilize the spinel phase compared to the undoped LCO (Fig. 5b).\nComputed reaction energetics on this spinel surface also reveal a lower overpotential for\nthe OER rate‐determining step on LCNFPdO (Fig. 5d), consistent with the improved\ncatalytic activity we observe. Finally, this controlled reconstruction yields a mechanically\nrobust and conductive catalyst interface. The crystalline spinel layer in LCNFPdO remains\nhighly conductive (as confirmed by the conductivity data in Fig. 4a–b). DFT calculations of\ngas adsorption (Fig. 5e) further indicate that the doped surface promotes efficient oxygen\nbubble release, improving wettability and preventing bubble blockage. These atomic-scale\nand mesoscale analyses explain why LCNFPdO maintains high activity and stable\nperformance under prolonged, high-current OER operation, whereas undoped LCO would\ndevelop a resistive, inactive surface layer.\nIn the revised manuscript, we added more characterizations and calculations. First, we\nadded in situ Raman spectroscopy to provide direct insight into the reconstruction pathway.\nDuring the OER, new Raman peaks emerge at ~465 cm-1 and ~565 cm-1 (Figure R13, Fig.\nS24 of the revised SI), corresponding to the characteristic vibration modes of the spinel\nphase. The appearance of these specific bands in situ confirms that LCNFPdO follows a\nreconstruction route to a spinel phase. In other words, instead of forming an amorphous or\ndisordered oxide, the material transforms into a well‐ordered spinel structure. This\noperando evidence directly links our macroscopic observations to a specific structural\nchange at the atomic level. We also conducted additional DFT calculations to deepen the\nmechanistic understanding of the OER performance and the individual role of each doping\nelement. For example, facilitating O evolution can usually be observed through the final\n2\nstep (4.92- ΔG )1. At the atomic scale, Ni doping brings bridge sites between Ni and Co\nOOH\nas active sites. On this basis, the doping of Fe and Pd redistributes the electronic structure\nof catalysts, which weakens the bonds of OOH* and facilitates O evolution (Table R1).\ndirectly link the observed performance enhancements to specific atomic-scale changes\ninduced by the dopants.\nTable R1 The calculated Gibbs free energy steps for different catalysts. (Table S14 in the\nrevised SI)\nCatalysts ΔG[OH] ΔG[O]-ΔG[OH] ΔG[OOH]-ΔG[O] 4.92-ΔG[OOH] ΔG[OOH]-ΔG[OH] overpotential\nLCNFPdO 1.18 1.69 1.10 0.96 2.79 0.46\nLCNFO 1.48 1.77 1.32 0.36 3.09 0.54\nLCNPdO 1.40 1.02 2.00 0.50 3.02 0.77\nLCFPdO 0.84 1.90 1.31 0.87 3.21 0.67\nLCNO 0.44 1.84 1.00 1.64 2.84 0.61\nLCFO 1.05 2.13 1.20 0.54 3.33 0.90\nLCPdO 1.04 1.99 0.81 1.08 2.80 0.76\nLCO 0.88 1.91 0.87 1.26 2.78 0.68\nIn summary, we have provided extensive atomic-level evidence (TOF-SIMS, EQCM,\nSTEM/SAED, in situ Raman, and XAS) and a robust theoretical framework (dissolution\nfree energies, surface Pourbaix diagram, and Gibbs free energy) to clarify the structure–\nfunction relationships in our doped LCO system. Together, these results form a cohesive\nmechanism: multi-element doping directs a favorable, crystalline spinel reconstruction\npathway that enhances catalytic kinetics and preserves material integrity. We hope this\ndetailed mechanistic picture addresses the reviewer’s concerns about mechanistic depth\nand conceptual clarity and directly underpins the structure–stability relationships we report.\n1. Ambiguity in doping mechanism: (1) The authors state that Ni and Pd doping\nincreases the Co valence state while simultaneously suppressing its oxidation. These\nstatements appear contradictory, as a higher Co valence is typically associated with\nincreased oxidation behavior. This inconsistency should be addressed. Additionally, the\nexplanation that the increase in Co valence arises solely from the introduction of low-\nvalence Ni and Pd is overly simplistic. The Co valence state is influenced by several\nfactors, such as the electronegativity of the dopants, orbital hybridization and the\npossible generation of oxygen vacancies due to valence mismatch. A more thorough\ninvestigation of these underlying mechanisms is necessary to support this conclusion,\nrather than relying on the dopants’ valence states alone.\nResponse：\nWe appreciate the reviewer’s insightful comment. In LCNFPdO, the initial Co valence is\nhigher than that of undoped LCO due to multi-element doping. This higher cobalt valence\nmakes it require a larger voltage to further oxidize cobalt2,3, as the cobalt valence increase\nin layered LiCoO is charge balanced by Li loss that is more difficult (requiring larger\n2\nvoltage) when cobalt valence is at a higher state. As a result, the cobalt valence increase\nof LCNFPdO is less during the OER with less Li loss (Table S4). The above interpretation\nis supported by multiple characterizations. XAFS and X-ray Photoelectron Spectroscopy\n(XPS) analyses (Fig. 3a–b, d and Fig. S22) show that LCNFPdO exhibits a higher initial\nCo valence and a smaller extent of Co oxidation during OER compared with LCO.\nAs redox-active materials, cobalt valence changes during the OER in both LCO and\nLCNFPdO are highly related to lithium composition: the charge change caused by lithium\nloss is solely balanced by the cobalt oxidation (we have ruled out the possible redox\nchange of Ni, Fe, and Pd after the OER, see Fig. S25-26). Our previous work (Nature\nCatalysis 4, 212–222 (2021)) has demonstrated that a higher initial cobalt valence is\nassociated with a larger energy barrier to induce lithium loss during the OER, leading to\nless cobalt oxidation extent after the OER2. The findings of our present work are consistent\nwith those of the published one: LCNFPdO with a higher-valence state cobalt has less\nlithium loss (Fig. 3e-g and Table S5) and further lower cobalt oxidation extent during the\nOER. We have newly included a schematic diagram (Figure R1, Fig. S31, and analysis\nadded in lines 259-265 in the revised manuscript) to describe this.\nFigure R1. Schematic diagram of the in-situ reconstruction process of LCO and LCNFPdO.\n(Fig. S31 in the revised SI)\nRegarding the doping-induced valence change, we have performed additional DFT\ncalculations to support the increase of Co valence state through Ni/Fe/Pd co-doping.\nIn\nLCNFPdO, the Bader charges of Co, Ni, and Fe are -1.40e, -1.19e, and -1.58e, respectively.\nThe Bader charge of Co in LCO is -1.11e (Table R2). Hence, the valence state of Co\nincreases, the electronegativity increases, and the adsorption of oxygen species enhances.\nTable R2 Calculated Bader charges for selected atomic sites (Co, Ni, Fe) in various\ncatalysts. Site 1, site 2, and site 3 are the location of Co site (Ni-Co bridge), the location\nof Ni site (Ni-Co bridge), and the location of Fe site in LCNFPdO, respectively. (Table S9\nin the revised SI)\nCatalyst Site Charge Catalyst Site Charge Catalyst Site Charge\nLCNO Co -1.26 LCFO Co -1.29 LCPdO Co -1.20\nNi -1.16 Co -1.34 Co -1.31\nCo -1.40 Fe -1.61 Co -1.39\nLCNFO Co -1.40 LCNPdO Co -1.28 LCFPdO Co -1.22\nNi -1.20 Ni -1.21 Co -1.29\nFe -1.60 Co -1.39 Fe -1.60\nLCO Co -1.11 LCNFPdO Co -1.40\nCo -1.33 Ni -1.19\nCo -1.38 Fe -1.58\nTo examine the possible role of oxygen vacancies, we combined theoretical and\nexperimental approaches. We performed electron paramagnetic resonance (EPR)\nmeasurements for both LCO and LCNFPdO, as shown in Figure R2. The similar peaks\nand measured signal intensities (2.08 × 1012 spins/mg for LCO vs. 2.00 × 1012 spins/mg for\nLCNFPdO) indicate that Ni/Fe/Pd co-doping does not significantly alter the oxygen\nvacancy concentration compared with LCO. This strongly suggests that oxygen vacancies\nare not a major factor in the observed shift in initial Co valence state. The analysis was\nadded to lines 204-206 in the revised manuscript.\nFigure R2 Quantitative EPR analysis of oxygen vacancies for pristine LCO and\nLCNFPdO at 25 °C. (Fig. S20 in the revised SI)\nThe surface Pourbaix diagram (Figure R3) shows that at pH = 14, oxygen vacancies can\nonly exist when the potential is below -0.61 V vs. SHE. However, the dashed line\nrepresenting the OER onset potential is significantly higher than the vacancy formation\nrange, confirming that no stable vacancies exist during OER operation. Therefore,\noxygen vacancy is not considered in our work.\nOverall, our updated explanation incorporates not only valence considerations but also\norbital hybridization and oxygen vacancies. We believe these additions resolve the\napparent contradiction and clarify the dual role of doping in modifying both the initial\nelectronic state and structure evolution during OER.\nFigure R3 Surface Pourbaix diagram of LCNFPdO. (Fig. 5c in the revised manuscript)\n(2) A clear mechanistic rationale of mitigating reconstruction should be provided. The\nauthors report that LCNFPdO undergoes reconstruction into a spinel-type LiCo2O4\nphase during OER, but do not clarify how multi-element doping alters the reconstruction\npathway to favor this specific structure. If reconstruction is indeed observed, evidenced\nby the formation of LiCo2O4, then the claim of “mitigating reconstruction” is not accurate.\nIt would be more appropriate to state that doping induces the formation of a stable spinel\nreconstruction layer. The authors are encouraged to elaborate on how this reconstructed\nstructure contributes to enhanced stability. Additionally, it remains unclear what the final\nreconstruction product of undoped LCO is? Does it also form LiCo2O4?\nResponse：\nWe thank the reviewer for this constructive comment. We agree with the reviewer that\nsurface reconstruction of LCNFPdO was observed, but compared with LCO, the\nreconstruction extent characterized by Li loss (Fig. 3e-g and Table S4), cobalt oxidation\nextent (Fig. 3a, b, and d), and reconstruction layer thickness (Fig. S15-18), was less.\nThat is why we used the term “mitigate” (to cause to become less harsh) instead of “suppress”\nto describe this less reconstruction phenomenon. In the revised manuscript, we have\nrevised our terminology to \"mitigate the reconstruction extent\" to more precisely describe\nthis, as shown in lines 191-192 in the revised manuscript. We also agree with the reviewer\nthat complex doping contributed to the formation of a more stable spinel reconstruction\nlayer, and we have highlighted the importance of doping-modulated reconstruction in\nachieving this. Complex doping modulated the in-situ cobalt redox and the associated\nlithium loss during the OER, as evidenced by Co K-edge XANES (Fig. 3a-b), TOF-SIMS\n(Fig. 3e), EQCM (Fig. 3f-g), and ICP-MS (Table S4). This led to the formation of a stable\nspinel phase, in contrast to the disordered spinel surface for undoped LCO.\nRegarding the stability of the reconstructed spinel phase, we applied DFT calculations in\nour previous submission to understand it. Please find this part from Fig. 5b, and we copied\nthe corresponding descriptions here for your reference:\n“Fig. 5b shows the calculated dissolution free energies (ΔG ) of LiCo O and\ndiss 2 4\nLi Co Ni Fe Pd O at varying potentials in alkaline media. A higher ΔG value\n0.5 0.79 0.1 0.1 0.01 2 diss\nindicates greater catalyst instability under electrochemical working conditions. For LiCo O ,\n2 4\nas the applied potential shifts from 1.23 V to 2.00 V, the calculated ΔG decreases from\ndiss\n0.45 to 0.19 eV/atom. These large ΔG values suggest that LiCo O is unstable under\ndiss 2 4\nworking conditions and tends to decompose into CoOOH(s) and LiOH(aq). This result is in\nline with our experimental observations. In contrast, doping with Ni, Fe, and Pd\nconsiderably reduces the ΔG to 0.21 eV/atom at 1.23 V and 0 eV/atom at 2.00 V. Notably,\ndiss\nwhen the applied potential exceeds 1.55 V, the ΔG becomes 0.03 eV/atom. This value\ndiss\nfalls well within the stable region where kinetics, driven by solid-solid phase transformation,\nplay a key role. Specifically, the formation Co O (s) and NiPdO (s) remarkably lowers the\n3 4 3\nΔG , consequently enhancing the catalyst stability of LCNFPdO under operating\ndiss\nconditions. This result aligns well with our experimental observations, as no obvious\nleaching was detected for LCNFPdO via ICP-MS (Table S5), unlike the Fe-containing\nhydroxides. Close examination of the decomposition products reveals that this enhanced\ncatalyst stability primarily arises from the Pd dopant, which facilitates the formation of\nNiPdO (s) and Li Co Ni Fe Pd O (s).”\n3 0.5 0.79 0.1 0.1 0.01 2\nIn addition to the electrochemical stability of the reconstruction layer discussed above, we\nalso considered mechanical robustness. Our experiments demonstrated that the\nreconstructed surface of LCNFPdO has better gas removal capability than LCO, especially\nat large currents (Fig. 4e-h). This may be related to the weakened O adsorption capability\n2\nof reconstructed LCNFPdO than that of reconstructed LCO, as supported by the DFT\ncalculation in Fig. 5e. We also copied the corresponding descriptions from our previous\nsubmission for your reference here:\n“The adsorption energies for H O and O were calculated as shown in Fig. 5e. The\n2 2\ncalculated O adsorption energies for LCNFPdO and LCO were 1.10 eV and 0.35 eV,\n2\nrespectively. This result suggests that LCNFPdO possesses weakened O adsorption\n2\ncompared to LCO. Such weakened O adsorption facilitates the desorption of O from the\n2 2\nelectrode surface. The calculated H O adsorption energies for LCNFPdO and LCO were\n2\n−1.07 eV and −0.33 eV, respectively. This result suggests that LCNFPdO has a stronger\ntendency for H O adsorption compared with LCO, which is beneficial for enhancing OER\n2\nefficiency.”\nWe have also checked the reconstruction product of undoped LCO in our previous\nsubmission. From the STEM results (Fig. 2e-g, f-h), the surface of undoped LCO was\nreconstructed into LiCo O -spinel structure, agreeing well with our previous publication in\n2 4\nNature Catalysis2. Depending on the above results, we established the calculation model\nof spinel LiCo O -type structure for DFT calculations in Fig. 5a.\n2 4\nIn the revised manuscript, we added more descriptions for the formation of this phase in\nlines 258-266: “During the OER process, Li+ extraction drives the oxidation of Co3+ in LCO\ntoward Co4+. In Li-deficient Li CoO , the layered structure is unstable and readily\n1-x 2\ntransforms into a LiCo O -type spinel phase35,36. Consequently, greater lithium loss\n2 4\npromotes deeper reconstruction and the formation of more spinel species. In contrast,\nLCNFPdO undergoes less Co oxidation and lithium loss, resulting in a thinner\nreconstructed layer than LCO and thus a mitigated reconstruction extent, as illustrated in\nthe scheme in Fig. S31.”\n(3) The individual roles of Ni, Fe, and Pd are not clearly distinguished. Possible\nsynergistic or antagonistic effects between dopants are not discussed. A systematic\ninvestigation, such as single-element doping experiments or theoretical simulations,\nwould clarify the contributions of each element.\nResponse:\nWe thank the reviewer for this important comment. To address the individual and combined\neffects of Ni, Fe, and Pd doping, we carefully analyzed both experimental and\ncomputational results.\nFirst, we would like to highlight that Fig. 1a offers an experimental basis for understanding\nboth the individual and combined effects of dopants on OER performance across\ntemperatures. Generally, Single-element-doped samples (e.g., LCFO, LCNO, LCPdO)\nshow the worst OER performance, binary-element-doped samples (e.g., LCNFO, LCNPdO,\nLCFPdO) show improved performance compared with single-element-doped samples, and\nthe ternary-element-doped LCNFPdO exhibits significantly lower overpotentials compared\nwith undoped LCO, especially at elevated temperatures. This trend suggests a non-\nadditive, possibly synergistic effect when combining Ni, Fe, and Pd and supports the\nhypothesis that co-doping introduces cooperative interaction between dopants, enhancing\nthe OER performance, as we have demonstrated in the original manuscript (Fig. 1, Fig. 4,\nand Fig. 5).\nTo further get the thermodynamic insights, we added more DFT calculations. As shown in\nFigure R4, all samples except for LCNPdO are on the right side of the volcano plot,\nindicating that their potential determining step (PDS) is the formation of O*. Consistent with\nthe above experimental results, the ternary-element-doped LCO is exclusively superior to\nother doped counterparts in reducing the theoretical OER overpotential, supporting a\nsynergistic activity enhancement.\nWe further analyze the role of each element. For single-element-doped LCO, only Ni\ndoping shows a beneficial effect in reducing the theoretical overpotential. Within the two-\nelement doping systems, the combination of Ni/Pd and Fe/Pd doping led to only\ncomparable overpotential to that of undoped LCO. Particularly, the PDS of LCNPdO\nchanges from the formation of O* to OOH*, implying that Pd doping significantly affects the\nadsorption behavior of reaction intermediates, possibly by regulating the catalyst’s\nelectronic structure. In contrast, Ni/Fe doping further reduced the overpotential compared\nwith Ni and Fe single doping, implying their synergistic effect. When three elements of Ni,\nFe, and Pd are co-doped simultaneously, Pd further optimizes the electronic structure of\nLCO compared with the NiFe-doped one, resulting in the lowest theoretical overpotential.\nOverall, Ni doping is favorable to reduce the overpotential, and it works synergistically with\nFe and Pd to optimize the electronic configuration of LCO and achieve the best OER\nactivity. It should be noted that Pd doping is also favorable to improve the stability of LCO\nin Fig. 5b.\nFigure R4 The volcano map of Ni/Fe/Pd-doped LCO systems. (Fig. 5d in the revised\nmanuscript)\nWe have supplemented the relevant statements in the revised manuscript:\n“To elucidate the role of each dopant, we calculated the OER activity for LCNFPdO and six\nother systems for comparison (Table S13). With the exception of the LCNPdO system, all\ncatalysts reside on the right side of the volcano, as shown in Fig. 5d. This indicates that\nthe formation of O* is the potential-determining step (PDS) due to weak O* binding. For\nsingle-element doping, the calculated overpotentials (η ) for LCFO (0.90 V) and LCPdO\nOER\n(0.76 V) are higher than that of pristine LCO (0.68 V), suggesting poorer OER activity. In\ncontrast, Ni doping reduces the overpotential to 0.61 V, indicating enhanced activity.\nAmong the binary-doped systems, LCFPdO shows a slight improvement over LCO with a\ncalculated η of 0.67 V. The LCNFO system exhibits significantly enhanced activity with\nOER\nan overpotential of 0.54 V, which can be attributed to electronic optimization between Ni\nand Fe. In contrast, the LCNPdO system has a higher overpotential of 0.77 V and notably\nundergoes a PDS shift from O* to OOH* formation, highlighting Pd's role in electronic\nrestructuring. For the ternary-doped system, LCNFPdO achieves the lowest overpotential\nof 0.46 V, suggesting that Pd further optimizes the NiFe electronic framework. Taken\ntogether, these results indicate that Ni is a pivotal active site that synergizes with Co, and\nits activity is further amplified by Fe through electronic cooperation.”\n2. Limited insight into mass transport mechanism: The authors claimed that the doping\nof Fe and Pd can enhance the electronic conductivity and gas transport properties, which\nfast reaction kinetics and rapid removal of bubbles. However, the conclusion is based\nsolely on macroscopic observations (bubble statistics and limiting current density), the\nunderlying mechanism remains speculative. Does Pd affect the wettability of the catalyst\nsurface or reduce bubble adhesion? or could Fe doping alter local charge distributions\nto facilitate ion transport?\nResponse：\nWe appreciate the reviewer’s comment on this point. In our previous submission, we\nprovided some mechanistic insights into the improved transport properties in addition to\nthe experimental observations, as we conducted DFT calculations to compare the\nadsorption capabilities of H O and O on the reconstructed surface of LCNFPdO and LCO.\n2 2\nPlease find the results in Fig. 5e and the corresponding descriptions copied below:\n“Based on our experimental results, we speculate that the mass transport of reactants (H O)\n2\nand products (O ) may also influence OER performance. Therefore, the adsorption\n2\nenergies for H O and O were calculated as shown in Fig. 5e. The calculated O adsorption\n2 2 2\nenergies for LCNFPdO and LCO were 1.10 eV and 0.35 eV, respectively. This result\nsuggests that LCNFPdO possesses weakened O adsorption compared to LCO. Such\n2\nweakened O adsorption facilitates the desorption of O from the electrode surface. The\n2 2\ncalculated H O adsorption energies for LCNFPdO and LCO were −1.07 eV and −0.33 eV,\n2\nrespectively. This result suggests that LCNFPdO has a stronger tendency for H O\n2\nadsorption compared with LCO, which is beneficial for enhancing OER efficiency.”\nTo further strengthen this, we have carried out DFT calculations of H O and O adsorption\n2 2\nenergies for various LCO-doped compositions, as shown in Figure R5. The results indicate\nthat Pd doping significantly strengthens water adsorption (e.g., –1.0 to –1.4 eV for\nLCNFPdO and LCFPdO vs. –0.33 eV for pristine LCO), consistent with enhanced surface\nwettability. In addition, Fe doping significantly weakened the O adsorption (1.93 eV for\n2\nbecomes much less favorable (+1.20 eV), suggesting a strong synergistic effect that\npromotes gas desorption. Taken together, these results demonstrate that Pd, particularly\nin combination with Ni and Fe, enhances wettability and reduces bubble adhesion, in good\nagreement with our experimental observations.\nFigure R5 The calculated adsorption energies of H O (a) and O (b) gas on the catalysts. (Fig.\n2 2\nS45 in the revised SI)\nRegarding the electronic conductivity, we calculated the DOS and band gap. As shown in\nFigure R6, Fe doping changed the electronic distributions and reduced the band gap from\n2.3 eV in LCO to 1.9 eV, favoring the enhancement of electronic conductivity. Further\ndoping of Ni and Pd causes the density of states of the material to cross the Fermi level,\nindicating partially conductive properties and agreeing well with the experimentally\nobserved excellent electrical conductivity. These results indicate that Fe doping effectively\naccelerates charge transfer by altering the electronic structure. We have added the above\nresults to the revised manuscript at lines 416-419.\nFigure R6 The density of states of different catalysts. (Fig. S44 in the revised SI)\n3. Inconsistencies in data interpretation: (1) The introduction notes that Fe is prone to\ndissolution under OER conditions (Line 69), yet Table S3 shows no Fe loss after\nextended high-temperature and high-current operation for LCNFPdO catalyst. This\ndiscrepancy should be clarified.\nResponse:\nThank the reviewer for the comment. In the introduction, we mentioned that “NiFeO H can\nx y\nleach Fe in 'Fe-free' electrolytes, losing activity”, as reported in Nature Energy, 2020, 5,\n222-2304. However, it does not mean that all the Fe-containing electrocatalysts will suffer\nfrom significant Fe leaching during the OER. Leaching or not should be influenced by the\nphysicochemical properties of electrocatalysts (e.g., structure, composition) and water\nelectrolysis conditions.\nIn this study, Fe is a doped element in layered LCO that has a different structure from\nhydroxides. Moreover, the reconstructed surface is spinel-type oxide instead of Fe-\ncontaining hydroxides. As a result, the reported Fe leaching in NiFeO H may not apply to\nx y\nour material. Our DFT calculation in Fig. 5b supported the stability of the reconstructed\nLCNFPdO surface. As a result, ICP-MS results detected no significant Fe leaching after\nthe OER (Table S4 of the revised SI). We have added some clarifications in the revised\nmanuscript: (See lines 375-377 of the revised manuscript): “This result aligns well with our\nexperimental observations, as no obvious leaching was detected for LCNFPdO via ICP-\nMS (Table S5), unlike the Fe-containing hydroxides8.”\n(2) An activation process is reported during the initial CV cycles (line 111), indicated by\nincreased current density. However, no explanation is offered. Could this be due to the\nformation of a reconstructed layer, and if so, how does that align with the claim of\nsuppressed reconstruction?\nResponse:\nThank the reviewer for these questions. Yes, the activation process is due to the formation\nof a reconstructed layer with higher activity. Compared with the original layered structure,\nthe reconstructed spinel-type structure is more favorable for OER. A similar phenomenon\nthat spinel LCO is more active than layered LCO is also reported in our previous work Nat.\nCata. 2021, 4, 212-2222 and other publications5–8. The relevant information was updated\nin line 115 of the revised manuscript. In our previous submission, we never claimed that\nwe suppressed the reconstruction; instead, we claimed that we mitigated reconstruction\nby a complex doping method. As suggested by the reviewer, we have revised it to be\n“mitigated reconstruction extent” in the revised manuscript.\n(3) The discussion (line 249) uses O-K edge sXAS and O 1s XPS to support more\nadsorbed oxygen species on LCNFPdO, due to reconstructed surface. Yet previous\nfindings indicate that LCO undergoes more extensive surface reconstruction. Why is OH\nadsorption not more prominent in LCO? This inconsistency requires further clarification.\nResponse:\nThank the reviewer for the constructive comment. We want to clarify that a more extensive\nreconstruction extent does not necessarily lead to enhanced adsorption of more OH\nspecies. In fact, as redox-active materials, the charge balance for cobalt oxidation in LCO\nduring the in-situ reconstruction was achieved by Li deintercalation, instead of by the\nadsorption of OH. Consequently, even though LCO experienced more cobalt oxidation\nduring the OER, the OH adsorption, which should be more influenced by the nature of the\nreconstructed surface layer, was not necessarily more favored than that of LCNFPdO. As\nshown in Fig. 5e, DFT calculations suggest that OH adsorption on the reconstructed\nLCNFPdO surface is energetically more favorable than on the reconstructed LCO surface.\nThis supports our O-K edge sXAS and O 1s XPS findings of a higher proportion of\nadsorbed oxygen species on LCNFPdO after the OER. We have clarified this in the revised\nmanuscript at lines 437-439.\n4. Insufficient experimental details: (1) The method for post-reaction sample collection\nfor XRD and XAS is not described. Please include details such as how powders were\ncollected and prepared.\nResponse:\nWe appreciate the reviewer’s suggestion. For ex-situ XRD and XAS characterizations,\ncatalyst powders were loaded onto carbon paper substrates for OER measurements.\nFollowing OER testing, samples were rinsed thoroughly with deionized water, dried under\nvacuum at 25 °C, and subsequently collected for characterization. These experimental\ndetails have been incorporated in lines 520-524 into the revised manuscript.\n(2) Key parameters for AEMWE testing are missing, including membrane electrode\nassembly method, catalyst loading, and membrane type.\nResponse:\nWe appreciate the reviewer’s suggestion. In the revised manuscript, we added a dedicated\nsubsection covering MEA fabrication and single cell testing protocols. Please find the\ndescriptions in lines 564-592 of the revised manuscript, which we have also copied below:\n“The MEAs were fabricated using a hand spray-coating method for both the anode and\ncathode catalyst layers. The anode catalyst ink was prepared by dispersing the catalyst\npowder in a mixed solvent of deionized water and isopropanol (mass ratio = 1:4:5).\nSubsequently, 5 wt% Nafion solution was added, resulting in a Nafion content of 7 wt% in\nthe final ink. The ink was ultrasonicated for 1 hour to ensure homogeneous dispersion and\nthen uniformly sprayed onto a nickel foam substrate (Kunshan Shengshijingxin New\nMaterial Co., Ltd, China). The anode catalyst loading was controlled at 10.0 ± 0.5 mg cm-2,\ndetermined by weight difference before and after coating. For the cathode, a 5 wt% Pt/C\ncatalyst (Alfa Aesar) was used. The ink formulation consisted of catalyst, deionized water,\nand isopropanol in a weight ratio of 1:8:9. After adding 5 wt% Nafion solution (final Nafion\ncontent 10 wt%), the ink was ultrasonicated for 1 hour and then sprayed onto a carbon\npaper substrate (TGP-H-120T, Toray). The Pt loading was 0.5 mg cm-2.\nA commercial Sustainion® membrane (X37-50, Grade T, Dioxide Materials) was employed\nas the anion exchange membrane for all single-cell tests. Prior to use, the membrane was\npre-treated by soaking in 1 M KOH for 12 hours to ensure full ion exchange. The MEA was\nassembled with an active geometric area of 1.0 cm2 (as shown in Fig. 6a). The single cell\nhardware was composed of nickel-coated stainless-steel endplates to ensure chemical\nstability in alkaline media.\nAll electrochemical measurements were conducted in 1 M KOH electrolyte, supplied to\nboth anode and cathode chambers. Polarization curves were recorded at 80 °C by a Gamry\n5000E electrochemical workstation with a scan rate of 5 mV s-1. The durability of the MEA\nwas evaluated by operating the cell at a constant current density of 500 mA cm-2 at 60 °C.\nHigh-frequency resistance (HFR) was measured via EIS using a Gamry 5000E\nelectrochemical workstation over a frequency range from 1 MHz to 1 Hz at a cell potential\nof 1.8 V. The HFR was obtained from the intercept of the Nyquist plot with the real axis.”\nThese details will improve the reproducibility and transparency of our AEMWE evaluation.\n5. Inappropriate language and formatting errors: (1) Avoid superlative terms like “the\nlongest reported lifetime” or “none of these strategies”, unless supported by\ncomprehensive comparative data.\nResponse:\nWe thank the reviewer for catching the excessive use of superlatives. To address this,\nwe\nrevised the sentence to avoid these claims in line 44 and lines 142-145 in the revised\nmanuscript. We have also provided a more direct comparison with stability performances\nin recent literature to contextualize our results scientifically and fairly. (Table R4 and Table\nR6), In addition, the sentence “However, none of these strategies have yielded a high-\nperformance and durable OER electrode for practical electrolyzer applications” is refined\nto “While these strategies have shown various improvements, achieving an OER electrode\nthat combines both high performance and long-term durability suitable for practical\nelectrolyzer conditions remains a significant challenge.” (lines 86-89 in the revised\nmanuscript). This wording accurately acknowledges the progress in the field while clearly\nstating the existing gap, without overstating our position.\n(3) Figure 3e is discussed twice with nearly identical description (Lines 161 and 245).\nResponse:\nThank you for pointing this out. We have removed the repeated discussion at Line 161,\nensuring that Fig. 3e is now described only once—at the most appropriate location (line 256\nin the revised manuscript). This eliminates the redundancy and improves clarity of the\nmanuscript.\n(4) Line 191 links the peak intensity of the 104-diffraction pattern to structural disorder.\nThis requires a citation or clearer justification.\nResponse:\nWe appreciate the reviewer’s attention to detail. We have supplemented the manuscript\nwith clear literature references explaining this correlation. Recent studies in top-tier\njournals have firmly established that: the decrease of I /I intensity ratio of the XRD\n(003) (104)\nof LCO, in some cases falling below 1.2, strongly indicates structural disorder9–11.\nTherefore, our original interpretation that a higher relative intensity of the (104) diffraction\npeak signifies increased structural disorder in LCO is well supported. We have revised the\nsentence on line 195 in the revised manuscript to include these citations.\n(5) Line 252 contains a double comma and should be corrected; Line 333 incorrectly\nplaces a period before the citation \"(Figure 39)\".\nResponse:\nThank you for identifying these editorial oversights. We have corrected the punctuation at\nline 252 (removing the double comma) and adjusted the citation placement at line 333 to\nproperly place the period after the reference. We also checked the full article to prevent\nsimilar errors.\nReviewer #2 (Remarks to the Author):\nThe urgent need to address instability in cobalt-based oxide electrodes represents a\ncritical bottleneck in advancing sustainable water electrolysis technologies. In this\ncontext, the manuscript NCOMMS-25-35487-T by Wang et al. makes a compelling\ncontribution. The authors report a sophisticated doping strategy to stabilize LiCoO₂—a\ncobalt oxide widely deployed in lithium-ion batteries—as an anode for alkaline water\nelectrolysis.\n1. Please provide a clear and quantitative definition of “reconstruction extent” and specify\nhow it is measured in this experiments.\nResponse:\nThank you for acknowledging our work and proposing those insightful comments. We\nproposed the concept of “reconstruction extent” in our previously published paper (Chem,\n9, 7: 1645-1657, 2023)12. Here, we defined the reconstruction extent as the degree of the\nreconstruction level, and we can evaluate it by comparing the thickness of the\nreconstructed layer during the OER. Since the formation of this reconstructed layer\ndepends on in-situ cobalt oxidation and lithium loss during the OER: larger cobalt oxidation\ncontributed to more lithium loss and the formation of a thicker reconstruction layer. As a\nresult, the reconstruction extent can also be reflected or quantitatively evaluated by\ncomparing the cobalt valence increase and lithium loss. These parameters are easier to\nquantify and more statistically significant than the thickness of the reconstructed surface.\nWe have added the above points at lines 262-264 in the revised manuscript, which is also\ncopied below for your reference: “LCNFPdO undergoes less Co oxidation and lithium loss,\nresulting in a thinner reconstructed layer than LCO and thus a mitigated reconstruction\nextent.”\n2. Please expand the discussion on how lithium loss quantitatively correlates with the\nformation of the spinel phase during OER\nResponse:\nThank you for your comment. In layered LiCoO , lithium extraction is directly correlated to\n2\ncobalt oxidation. As Li+ ions are removed during OER, Co3+ of LiCoO is oxidized toward\n1-X 2\nare subject to phase change into LiCo O -type spinel structure5–8. Thus, more lithium loss\n2 4\nleads to a higher reconstruction extent and the formation of more spinel species. In our\nexperiments, ICP-MS (Table S4), TOF-SIMS (Fig. 3e), and online EQCM (Fig. 3f-g) clearly\ndemonstrate that undoped LCO undergoes substantially more Li extraction than doped\nLCNFPdO during OER. XANES (Fig. 3a) and XPS (Fig. 3d and Fig. S22) data confirm that\nthe cobalt valence increase was larger for LCO compared to LCNFPdO. And XRD (Fig. 2b)\nand STEM imaging (Fig. S15-S18) further support that LCO exhibits a thicker\nreconstructed surface layer versus LCNFPdO, indicating more extensive phase\ntransformation in the undoped sample.\nWe hope this expanded explanation strengthens the link between Li depletion, Co\noxidation, and spinel formation in our manuscript, effectively addressing the reviewer’s\nrequest for clearer quantitative discussion. We have added the above points in the revised\nmanuscript in lines 258-262: “During the OER process, Li+ extraction drives the oxidation\nof Co3+ in LCO toward Co4+. In Li-deficient Li CoO , the layered structure is unstable and\n1-x 2\nreadily transforms into a LiCo O -type spinel phase35,36. Consequently, greater lithium loss\n2 4\npromotes deeper reconstruction and the formation of more spinel species.”\n3. Please describe the electrolyzer testing methods in more detail, including MEA\npreparation, cell assembly, operational parameters, and evaluation criteria.\nResponse:\nThank you for your valuable comment. In response to your suggestion, we have revised\nthe Methods section and added an “MEA preparation and single cell tests” section to\nprovide a more detailed and structured description of the electrolyzer testing process in the\nrevised manuscript (see lines 564–592). And we copied the descriptions here for your\nreference:\n“The MEAs were fabricated using a hand spray-coating method for both the anode and\ncathode catalyst layers. The anode catalyst ink was prepared by dispersing the catalyst\npowder in a mixed solvent of deionized water and isopropanol (mass ratio = 1:4:5).\nSubsequently, 5 wt% Nafion solution was added, resulting in a Nafion content of 7 wt% in\nthe final ink. The ink was ultrasonicated for 1 hour to ensure homogeneous dispersion and\nthen uniformly sprayed onto a nickel foam substrate (Kunshan Shengshijingxin New\nMaterial Co., Ltd, China). The anode catalyst loading was controlled at 10.0 ± 0.5 mg cm-2,\ndetermined by weight difference before and after coating. For the cathode, a 5 wt% Pt/C\ncatalyst (Alfa Aesar) was used. The ink formulation consisted of catalyst, deionized water,\nand isopropanol in a weight ratio of 1:8:9. After adding 5 wt% Nafion solution (final Nafion\ncontent 10 wt%), the ink was ultrasonicated for 1 hour and then sprayed onto a carbon\npaper substrate (TGP-H-120T, Toray). The Pt loading was 0.5 mg cm-2.\nA commercial Sustainion® membrane (X37-50, Grade T, Dioxide Materials) was employed\nas the anion exchange membrane for all single-cell tests. Prior to use, the membrane was\npre-treated by soaking in 1 M KOH for 12 hours to ensure full ion exchange. The MEA was\nassembled with an active geometric area of 1.0 cm2 (as shown in Fig. 6a). The single cell\nhardware was composed of nickel-coated stainless-steel endplates to ensure chemical\nstability in alkaline media.\nAll electrochemical measurements were conducted in 1 M KOH electrolyte, supplied to\nboth anode and cathode chambers. Polarization curves were recorded at 80 °C by a Gamry\n5000E electrochemical workstation with a scan rate of 5 mV s-1. The durability of the MEA\nwas evaluated by operating the cell at a constant current density of 500 mA cm-2 at 60 °C.\nHigh-frequency resistance (HFR) was measured via EIS using a Gamry 5000E\nelectrochemical workstation over a frequency range from 1 MHz to 1 Hz at a cell potential\nof 1.8 V. The HFR was obtained from the intercept of the Nyquist plot with the real axis.”\nThese revisions ensure that all key parameters and procedures related to AEMWE testing\nare presented with sufficient clarity and reproducibility.\n4. Please consider including post-test characterizations or comparative data to verify\nelectrode structural stability after long-term operation. Also, briefly discuss possible\ndegradation modes and compare durability with commercial catalysts.\nResponse:\nWe appreciate this valuable suggestion. To evaluate the structural stability of the electrodes\nafter long-term OER operation, we conducted post-test Raman characterizations on both\nLCO and LCNFPdO after 100 hours of continuous operation at 100 mA cm-2. As shown in\nFigure R7, the characteristic Co–O related Raman peaks of LCO appear at lower\nwavenumbers compared to those of LCNFPdO, suggesting that Co in LCO undergoes\nmore extensive oxidation during prolonged OER. These findings support the enhanced\ndurability of LCNFPdO and its stabilized reconstruction surface.\nLCNFPdO 588\nLCO\n462.3\n)\nu. 569.7\na.\n(\ny\nsit 458.6\nn\ne\nnt\nI\n200 400 600 800\nRaman shift (cm-1)\nFigure R7 Raman spectra of LCO and LCNFPdO after OER at 100 mA cm-2 for 100h. (Fig.\nS32 in the revised SI)\nIn addition, we conducted ICP-MS analysis of the electrode after both a simple soaking\ntest (1 M KOH, 100 h, without potential applied) and the long-term stability test (100 mA\ncm-2, 100 h). The results are summarized in Table R3. After stability testing, more Li (61.303\nmol%) and Co (0.155 mol%) of LCO were dissolved compared to those of LCNFPdO: Li\n(24.935 mol%) and Co (0.060 mol%), together with minimum leaching of Ni, Fe, and Pd.\nThese findings demonstrate that multi-element doping effectively suppresses Li loss and\nCo dissolution and improves stability under harsh OER conditions.\nTable R3 The leached element amount from the electrode determined by ICP-MS after soaking\nin 1M KOH for 100h and after stability test at 100 mA cm-2 for 100h (The values in the table\nrepresent the mean of three independent measurements along with their standard errors.).\n(Table S13 in the revised SI)\nSample Li (mol%) Co (mol%) Ni (mol%) Fe (mol%) Pd (mol%)\nLCNFPdO 3.139±0.06 0.008+0.00 0.005±0.00 0.152±0.00 0.005±0.00\nafter soaking 2 01 01 4 01\nLCNFPdO 24.935±0.2 0.060±0.00 0.009±0.00 1.577±0.00 0.014±0.00\nafter stability 30 1 02 9 01\nLCO after 61.303±1.0 0.155±0.00 / / /\nstability 73 1\nTaken together, the Raman and ICP results confirm that Ni/Fe/Pd co-doping substantially\nenhances the durability of LCO by stabilizing its surface and mitigating cation leaching.\nCompared with undoped LCO, the doped system shows markedly improved resistance to\ndegradation, approaching the stability levels expected for advanced OER catalysts. The\nabove discussion has been added to lines 265-270 in the revised manuscript.\nA comprehensive durability comparison of our catalyst with benchmark electrocatalysts\nunder basic conditions is shown in Table R4, which summarizes the OER stability metrics,\nincluding representative commercial catalysts such as RuO and NiFeO . Notably,\n2 x\nLCNFPdO demonstrates exceptional stability, maintaining continuous performance for\nover 2,000 hours, significantly exceeding the operational longevity of other catalysts\nevaluated.\nTable R4 Summary of OER stabilities of the state-of-the-art catalysts. (Table S3 in the\nrevised SI)\nj Duration Degradation\nCatalyst Electrolyte Ref.\n(mA cm-2) (h) description\n10 2000 0.012 mV h−1\nLCNFPdO 1 M KOH\nNo significant This\n50 2250\ndegradation work\nRuO2 1 M KOH 10 100 1.594 mV h−1\nNo significant\nCo3O4@CoO 0.5 M KOH 8 1000 13\ndegradation\nNo significant\nNiFeOx 1 M KOH 10 100 14\ndegradation\nNiFeP 1 M KOH ~30 50 J decreased by 12% 15\nPotential increased by\nFe2P-WO2.92 1 M KOH 10 100 16\n8.4%\nPotential increased by\nNiTe/NiS 1 M KOH 50 50 17\n6%\nNiCo2S4 1 M KOH 10 110 J decreased by 5% 18\nCeOx/NiFeOx 1 M KOH 20 96 0.141 mV h−1 19\nNiFe-Graphene 1 M KOH 10 136 0.051 mV h−1 20\nNo significant\nRuO2/CoOx 1 M KOH 10 200 21\ndegradation\nFe-V doped\nNiOOH\nReviewer #3 (Remarks to the Author):\nThe authors have screened a series of element-doped LiCoO₂ materials and identified\nLiCo₀.₇₉Ni₀.₁Fe₀.₁Pd₀.₀₁O₂ (LCNFPdO) as the most active catalyst for alkaline water\nelectrolysis. While the ex-situ characterization of LCNFPdO is comprehensive, the\nmanuscript lacks a robust discussion—and corresponding experimental evidence—\nlinking the reconstructed surface structure to the observed OER performance.\nFurthermore, the reported cell voltage of 1.58 V at 2.5 A cm⁻² (iR-corrected; ~2.1 V\nwithout compensation) appears impressive, but a fair comparison to state-of-the-art\ncatalysts should be made using uncorrected voltages. My detailed comments follow:\nResponse:\nThank the reviewer for acknowledging our work and proposing those comments. In our\noriginal submission, we applied systematic experimental tests and DFT calculations to\nelucidate the mechanism underlying the modulated reconstruction-improved OER\nperformance. Specifically, the OER activity improvement comes from 1) the formation of\nmore favorable reconstructed active species, i.e., spinel-type Li Co Ni Fe Pd O\n0.5 0.79 0.1 0.1 0.01 2\nthat lowered the energy barrier of binding OER intermediates compared with LCO (Fig.\n5(d)); 2) Complex doping improved electrical conductivity (Fig. 4a and b). The OER stability\nimprovement can be attributed to 1) complex doping lowering the aqueous decomposition\nfree energy of the reconstructed LCNFPdO (Fig. 5b) under the OER potentials, favoring\nelectrochemical stability; 2) the reconstructed LCNFPdO surface possessed a weakened\nO adsorption compared to reconstructed LCO (Fig. 5e), favoring gas removal, especially\n2\nunder large OER current densities (Fig. 4c-h), which contributed to the mechanical\nrobustness.\nFollowing the suggestions, we added more points in our revised manuscript, including: 1.\nDirect linkage between reconstructed surface structure and OER performance, supported\nby additional discussion and in-situ Raman analysis. 2. Benchmarking uncorrected cell\nvoltage of LCNFPdO (2.03 V at 2.0 A cm-2) against recent top-tier AEMWE catalysts,\nensuring a transparent comparison with current literature. We believe these additions will\nsignificantly strengthen the manuscript by clarifying the mechanistic link between surface\nreconstruction and catalyst performance, and by positioning LCNFPdO among leading\nAEMWE catalysts under practical conditions.\n1. The authors acknowledge Li leaching during OER, yet no direct structural comparison\nis made between LCO and LCNFPdO after reconstruction. In Fig. 1b, the CV curves for\nLCO and LCNFPdO cross in opposite ways before and after cycling—suggesting\nfundamentally different surface evolution. The manuscript should include a more in-\ndepth characterization/discussion on the structure and the structural difference after\nreconstruction and how these differences relate to the observed OER activity.\nResponse:\nThank the reviewer for the comment. In our previous submission, we thoroughly\ncharacterized the structures of LCO and LCNFPdO after OER-induced lithium leaching\nand surface reconstruction, and please find the corresponding contents in the section\n“Structural and compositional evolution” in the manuscript. Briefly, from the XRD and STEM\nin Fig. 2 and Fig. S15-S18, we directly visualize the formation of spinel phase layers on\nboth LCO and LCNFPdO after OER. The reconstructed layer is noticeably thicker in LCO\nthan in LCNFPdO. These characterizations and discussions have been given in the\nmanuscript:\n“We further analyzed the crystal structure. The XRD refinement confirmed the single phase\nof layered LCNFPdO with an expanded c-axis lattice parameter in reference to that of LCO\n(Fig. S12, Table S5-S6), possibly due to the larger size of the dopant Pd2+ (0.86 Å), Ni2+\n(0.69 Å), and Fe3+ (0.55-0.65 Å) than that of Co3+ (0.55-0.61 Å)24. After OER at 50 °C, both\nLCNFPdO 50 °C and LCO 50 °C retained their layered structures in the bulk (Fig. 2b),\nwhich was further confirmed by the XRD refinement (Fig. S13, Table S7-S8). The possible\nsurface reconstruction caused by OER at varying temperatures was investigated using\nSTEM. After OER, a hierarchical configuration comprising a layered bulk and a\nreconstructed surface was detected in both LCNFPdO (Fig. 2e, g, Fig. S15-S16) and LCO\n(Fig. 2f, h, Fig. S17-S18), in contrast to the layered surface for the pristine samples\n(LCNFPdO: Fig. 2c and Fig. S14, LCO: Fig. 2d). The corresponding fast Fourier transform\n(FFT) patterns of LCNFPdO after OER at 22 ºC and at 50 °C, as shown in the insets in Fig.\n2e and g, detected strong reflections (highlighted with green circles) of spinel structure at\nthe surface, e.g., (002), (2̅20) and (11̅̅̅1̅), which can be attributed to the LiCo O -type spinel\n2 4\nstructure due to the stronger intensity of (111) pattern than (200) pattern17. From the STEM\nresults (Fig. 2f and h), the surface of undoped LCO was also reconstructed into LiCo O -\n2 4\nspinel structure, agreeing well with previous research17. It is worth noting that FFT patterns\neven detected slight reflections (e.g., (202̅) and (002)) of Co O -type spinel structure at the\n3 4\nsurface of OER-cycled LCO, as indicated by the yellow circles in Fig. 2f and h, suggesting\nserious Li loss. Moreover, the reconstructed surface layer of OER-cycled LCO was\nconsistently thicker than that of LCNFPdO after OER (Fig. S15-S18), confirming a larger\nreconstruction extent. The XRD in Fig. 2b also supported less structural change of\nLCNFPdO than that of LCO after the OER. The above results show that multi-element\ndoping mitigates reconstruction extent. Specifically, the peak intensity of the 104-diffraction\npattern, an indicative of structural disorder in layered LCO, increased significantly after the\nOER, in contrast to the minimum change of LCNFPdO25-27. The catalyst morphology was\nadditionally examined by SEM (Fig. S19), exhibiting minimal alteration before and after the\nreaction.”\nIn the revised manuscript, we also added sentence to explain the correlation between Li\nleaching and the spinel surface formation in lines 258-265: “During the OER process, Li+\nextraction drives the oxidation of Co3+ in LCO toward Co4+. In Li-deficient Li CoO ,\nthe\n1-x 2\nlayered structure is unstable and readily transforms into a LiCo O -type spinel phase35,36.\n2 4\nConsequently, greater lithium loss promotes deeper reconstruction and the formation of\nmore spinel species. In contrast, LCNFPdO undergoes less Co oxidation and lithium loss,\nresulting in a thinner reconstructed layer than LCO and thus a mitigated reconstruction\nextent.”\nIn the CV curves in Fig. 1b, such “curve crossing”—where the cathodic scan shows higher\ncurrent than the anodic scan—is a recognized signal of in situ catalyst formation or\nactivation. As highlighted in Chemical Society Reviews (2019, 48, 2927–2945), an initial\ninduction phase often manifests as enhanced current on the return scan due to increasing\nconcentrations of reactive species, such as the formation of active oxide or spinel phases\nduring the forward scan23. Applying this to LCNFPdO, the appearance of curve crossing\nindicates a dynamic enhancement of surface reactivity: during the anodic scan, a more\nactive, spinel phase cobalt surface species Li Co Ni Fe Pd O forms in situ. As\n0.5 0.79 0.1 0.1 0.01 2\nthe scan reverses, this newly generated active layer contributes to higher reduction\ncurrents. In contrast, the in-situ formed spinel LiCo O phase for undoped LCO lacks such\n2 4\nactivity enhancement and thus does not exhibit similar curve-crossing behavior. In fact, the\nobserved CV crossing underscores that LCNFPdO undergoes more beneficial surface\nrestructuring during OER, forming a more redox-active catalyst layer that becomes\nelectrochemically dominant, thereby supporting our structural and spectroscopic evidence\nfor a reconstructive, activity-enhancing mechanism.\nDuring revision, we added in-situ Raman spectra (Figure R13, Fig. S24 in the revised SI),\nobtained under anodic polarization. Both LCO and LCNFPdO show the appearance of\npeaks at ~465 cm-1 (E ) and ~565cm-1 (A ), characteristic of spinel phase formed during\ng 1g\nOER. Importantly, the shift is consistently smaller for LCNFPdO, indicating a less extensive\nreconstruction under identical conditions. The thinner spinel surface in LCNFPdO\ncorrelates with lower overpotentials, reduced charge-transfer resistance, and enhanced\nstructural integrity under high-current OER, supporting both its improved activity and\ndurability compared to LCO. By consolidating pre- and post-OER structural data along with\nin-situ evidence, we have now provided a clear, direct structural comparison that\ndemonstrates how Li leaching correlates with differential reconstruction as well as the\ndifferent OER performance in LCO versus LCNFPdO.\nFor the structure-performance relationship, in our previous submission, we applied\nsystematic experimental tests and DFT calculations to elucidate the mechanism underlying\nthe modulated reconstruction-improved OER performance. Specifically, the OER activity\nimprovement comes from 1) the formation of more favorable reconstructed active species,\ni.e., spinel-type Li Co Ni Fe Pd O that lowered the energy barrier of binding OER\n0.5 0.79 0.1 0.1 0.01 2\nintermediates compared with LiCo O (Fig. 5(d)); 2) Complex doping improved the electrical\n2 4\nconductivity (Fig. 4a-b). The OER stability improvement can be attributed to 1) complex\ndoping lowering the aqueous decomposition free energy of the reconstructed LCNFPdO\n(Fig. 5b) under OER potentials, favoring electrochemical stability; 2) the reconstructed\nLCNFPdO surface possessed a weakened O adsorption compared to reconstructed LCO\n2\n(Fig. 5e), favoring gas removal, especially under large OER current densities (Fig. 4c-h),\nwhich contributed to the mechanical robustness.\nThese discussions have been given in the revised manuscript as follows:\n“To provide operando insight into surface reconstruction, we performed in-situ Raman\nspectroscopy at 22 °C (Fig. S24). As the applied potential increased from 1.20V to 1.55V\nvs. RHE, both LCO and LCNFPdO developed two new peaks around 465 cm-1 and 565\ncm-1, consistent with vibrations of CoO resulting from Li deintercalation. These findings\n2\nclosely match our ex-situ Raman observations. Notably, LCNFPdO exhibits smaller Raman\npeak shifts than LCO under the same polarization potential, indicating a mitigated structural\ntransformation.”\n“Despite less reconstruction extent, the reconstructed surface of LCNFPdO might be more\nfavorable for interacting with the OER intermediates: its oxygen K-edge sXAS (Fig. 3h)\ndetected an obvious rise of the resonance near ~535 eV, which was attributed to the\nadsorbed hydroxy groups on its surface11,35,36, in contrast to that of LCO. The presence of\nmore adsorbed oxygen species on the surface of LCNFPdO than LCO after the OER was\nalso confirmed by O 1s XPS (Fig. S33) and ToF-SIMS (Fig. S34). Shortly, the complex\ndoping in LCNFPdO effectively stabilized the reconstructed Li Co Ni Fe Pd O -\n0.5 0.79 0.1 0.1 0.01 2\nspinel phase, maintaining the electrochemical stability and generating more favorable\nsurface species for OER electrocatalysis.”\nMoreover, what chemical understanding was obtained after\nscreening/characterizing/testing a series of dopants was not discussed.\nResponse:\nWe appreciate the reviewer’s constructive suggestion. In our previous submission, we\nused systematic experiments to highlight the beneficial role of complex doping in improving\nthe OER performance. Fig. 1a offers an experimental basis for understanding both the\nindividual and combined effects of dopants on OER performance across temperatures.\nGenerally, Single-element-doped samples (e.g., LCFO, LCNO, LCPdO) show the worst\nOER performance, binary-element-doped samples (e.g., LCNFO, LCNPdO, LCFPdO)\nshow improved performance compared with single-element-doped samples, and the\nternary-element-doped LCNFPdO exhibits the best OER performance, especially at\nelevated temperatures. This trend suggests a non-additive, possibly synergistic effect\nwhen combining Ni, Fe, and Pd and supports the hypothesis that co-doping introduces\ncooperative interaction between dopants, as we have demonstrated in the original\nmanuscript (Fig. 1, Fig. 4, and Fig. 5).\nTo further get the thermodynamic insights, we added more DFT calculations. As shown in\nFigure R8, all samples except for LCNPdO are on the right side of the volcano plot,\nindicating that their potential determining step (PDS) is the formation of O*. Consistent with\nthe above experimental results, the ternary-element-doped LCO is exclusively superior to\nother doped counterparts in reducing the theoretical OER overpotential, supporting a\nsynergistic activity enhancement.\nFigure R8 The volcano map of NiFePd-doped LCO systems. (Fig. 5d in the revised\nmanuscript)\nWe further analyze the role of each element. For single-element-doped LCO, only Ni\ndoping shows a beneficial effect in reducing the theoretical overpotential. Within the two-\nelement doping systems, the combination of Ni/Pd and Fe/Pd doping led to only\ncomparable overpotential to that of undoped LCO. Particularly, the PDS of LCNPdO\nchanges from the formation of O* to OOH*, implying that Pd doping significantly affects the\nadsorption behavior of reaction intermediates, possibly by regulating the catalyst’s\nelectronic structure. In contrast, Ni/Fe doping further reduced the overpotential compared\nwith Ni and Fe single doping, implying their synergistic effect. When three elements of Ni,\nFe, and Pd are co-doped simultaneously, Pd further optimizes the electronic structure of\nLCO compared with the NiFe-doped one, resulting in the lowest theoretical overpotential.\nOverall, Ni doping is favorable to reduce the overpotential, and it works synergistically with\nFe and Pd to optimize the electronic configuration of LCO and achieve the best OER\nactivity. It should be noted that Pd doping is also favorable to improve the stability of LCO\nin Fig. 5b.\nWe have supplemented the relevant statements in the revised manuscript:\n“To elucidate the role of each dopant, we calculated the OER activity for LCNFPdO and six\nother systems for comparison (Table S14). With the exception of the LCNPdO system, all\ncatalysts reside on the right side of the volcano, as shown in Fig. 5d. This indicates that\nthe formation of O* is the potential-determining step (PDS) due to weak O* binding. For\nsingle-element doping, the calculated overpotentials (η ) for LCFO (0.90 V) and LCPdO\nOER\n(0.76 V) are higher than that of pristine LCO (0.68 V), suggesting poorer OER activity. In\ncontrast, Ni doping reduces the overpotential to 0.61 V, indicating enhanced activity.\nAmong the binary-doped systems, LCFPdO shows a slight improvement over LCO with a\ncalculated η of 0.67 V. The LCNFO system exhibits significantly enhanced activity with\nOER\nan overpotential of 0.54 V, which can be attributed to electronic optimization between Ni\nand Fe. In contrast, the LCNPdO system has a higher overpotential of 0.77 V and notably\nundergoes a PDS shift from O* to OOH* formation, highlighting Pd's role in electronic\nrestructuring. For the ternary-doped system, LCNFPdO achieves the lowest overpotential\nof 0.46 V, suggesting that Pd further optimizes the NiFe electronic framework. Taken\ntogether, these results indicate that Ni is a pivotal active site that synergizes with Co, and\nits activity is further amplified by Fe through electronic cooperation. To establish\nfundamental mechanistic insights, we conducted integrated Bader charge and density of\nstates (DOS) analyses. Bader charge analysis (Table S9) reveals that Ni/Fe co-doping\nfacilitates a substantial charge transfer enhancement of about 0.3e on Co sites.\nConcurrently, a 0.1e enhancement in charge transfer at Co sites for LCPdO relative to\nLCO. This demonstrates Pd's significant contribution to electronic structure modulation.\nComplementarily, total DOS analysis (Fig. S44) demonstrates Fe-doping reduces the\nband gap by 0.4 eV, while LCNFPdO manifests metallic conductivity through further\nelectronic restructuring. The enhanced conductivity is an important indicator of catalysts.\nCollectively, these synergistic electronic modifications constitute the fundamental\nmechanism driving enhanced catalytic activity.”\nThese findings support a synergistic mechanism among the three dopants, where their\ncomplementary roles enhance surface energetics, leading to superior OER performance.\nBy screening, we can also get some insights into the individual role of those doping\nelements in influencing the physicochemical properties: for example, Pd and Fe doping are\nfavorable to improve the mass transport and favor the gas removal. Fe also contributes to\nconductivity enhancements (Fig. 4a-b).\nWe added more theoretical calculations to support experimental observations. After\nscreening the adsorption energies of H O (a) and O on different catalysts by DFT, as\n2 2\nshown in Figure R9, we find that Pd doping significantly strengthens water adsorption (e.g.,\n–1.0 to –1.4 eV for LCNFPdO and LCFPdO vs. –0.33 eV for pristine LCO), consistent with\nenhanced surface wettability. In addition, Fe doping significantly weakened the O\n2\nadsorption (1.93 eV for LCFO) compared with LCO (0.45 eV). When Pd and Fe/Ni are co-\ndoped, O adsorption becomes much less favorable (+1.20 eV) compared with LCO (0.45\n2\neV), suggesting a strong synergistic effect that promotes gas desorption. Taken together,\nthese results demonstrate that Pd, particularly in combination with Ni and Fe, enhances\nwettability and reduces bubble adhesion, in good agreement with our experimental\nobservations.\nFigure R9 The calculated adsorption energies of H O (a) and O (b) gas on the catalysts. (Fig.\n2 2\nS45 in the revised SI)\nRegarding the electronic conductivity, we calculated the DOS and band gap. As shown in\nFigure R10, Fe doping changed the electronic distributions and reduced the band gap from\n2.3 eV in LCO to 1.9 eV, favoring the enhancement of electronic conductivity. Further\ndoping of Ni and Pd causes the density of states of the material to cross the Fermi level,\nindicating partially conductive properties and agreeing well with the experimentally\nobserved excellent electrical conductivity. These results indicate that Fe doping effectively\naccelerates charge transfer by altering the electronic structure. We have added the above\nresults to the revised manuscript.\nFigure R10 The density of states of different catalysts. (Fig. S44 in the revised SI)\n2. In Figs. S21–S22, the authors claim that LCNFPdO is less oxidized post-OER than\nLCO. However, the Co L₃-edge sXAS peak shifts in Fig. S21 are subtle and not\nconvincingly resolved.\nResponse:\nWe appreciate the reviewer's valuable comment. Following it, we meticulously reexamined\nthe samples and calibrated the energy using the L peak of NiO as reference sample during\n3\nthe sXAS test and reprocessed the data accordingly. After calibration, the trend remains\nsimilar to our previously reported one. In the pristine state, Co in LCNFPdO has a higher\nvalence state than in undoped LCO. After OER, the Co valence increase in LCNFPdO is\nless than that in LCO under all tested conditions. Those results agreed well with our\nXANES and XPS results in Fig. 3a-b and Fig. S22. This confirms that multi-element doping\neffectively moderates Co oxidation during OER. This updated figure is provided below and\nhas been incorporated into the revised Supporting Information.\nFigure R11 sXAS of Co L3-edge of LCO and LCNFPdO. (Fig. S22 in the revised SI)\nLikewise, the Co 2p XPS fits in Fig. S22 appear overly broad for the Co²⁺ and Co⁴⁺\ncomponents—e.g., the Co²⁺ peak for LCO in Fig. S22c seems significantly wider than\nthat for LCNFPdO. I recommend re-fitting/re-analyzing these results after carefully\nreviewing the peaking fitting constraints.\nResponse:\nThank you for your constructive suggestion. In response, we have carefully re-analyzed\nthe Co 2p XPS spectra presented in Fig. S22, with particular attention to the fitting\nconstraints for Co2+ and Co4+ components24,25. Specifically, we carefully adjusted the full-\nwidth at half maximum (FWHM) limits and ensured consistent fitting protocols across all\nsamples. The updated fitting results (Figure R12), along with a detailed summary of the\nfitting parameters (Table R5), are now included in the revised SI (Fig.S22 and Table S10).\nFigure R12 Co 2p XPS of LCO and LCNFPdO at pristine state (a), after OER at 22 ºC\n3/2\n(b), and at 50 °C (c). The peaks were fitted with Co2+, Co3+, and Co4+. (Fig. S22 in the\nrevised SI)\nTable R5 XPS fitting parameters of Figure R12 (Table S10 in the revised SI)\nPeak Average Co\nSample Peak binding FWHM (eV) Area (%) valence\nenergy (eV) state\nCo3+ 780.06 1.10 48.86\nLCO\nCo2+ 780.83 1.67 31.64 +2.88\nPristine\nCo4+ 781.89 2.00 19.50\nCo3+ 780.18 1.17 34.73\nLCNFPdO\nCo2+ 781.08 1.13 35.95 +2.93\nPristine\nCo4+ 781.95 1.97 29.32\nCo3+ 779.95 1.96 63.95\nLCO 22 ºC Co2+ 781.54 1.10 17.47 +3.01\nCo4+ 782.71 2.00 18.58\nCo3+ 780.05 1.61 67.04\nLCNFPdO\nCo2+ 781.47 1.28 16.90 +2.99\n22 ºC\nCo4+ 782.64 2.00 16.06\nCo3+ 780.06 1.80 60.26\nLCO 50 ºC Co2+ 781.21 1.32 13.72 +3.12\nCo4+ 782.25 2.00 26.02\nLCNFPdO Co3+ 779.82 1.58 50.38 +3.08\n50 ºC Co2+ 780.88 1.55 20.93\nCo4+ 781.99 2.00 28.69\n3. Almost all characterizations and oxygen adsorption were collected ex-situ. Given that\nbias removal and air exposure can alter the surface, the authors should either perform\nin-situ or—at minimum—provide evidence demonstrating that these ex-situ spectra\nreliably reflect the status of surface under OER conditions.\nResponse:\nWe sincerely appreciate the reviewer’s constructive comment. To directly address the\nconcern regarding ex-situ characterizations, we first highlight that our study includes online\nEQCM measurements conducted during OER operation, which monitor lithium\ndeintercalation in real time (Fig. 3f-g) since there is no obvious leaching of other metals\nfrom ICP-MS. As redox-active materials, cobalt valence in LCO is dependent on Li\ncomposition. By monitoring the mass change caused by Li loss, we could predict the trend\nof cobalt valence change. These results agree with our postmortem characterizations,\ndemonstrating consistent Co oxidation and surface transformation metrics.\nMoreover, in this revision, we include in-situ Raman spectroscopy data that clearly show\nthe emergence of two characteristic peaks at ~ 465 cm-1 (E mode) and ~565 cm-1 (A\ng 1g\nmode), corresponding to the CoO phase generated due to the delithiation of LiCoO during\n2 2\nanodic polarization (Figure R13). Both LCO and LCNFPdO display these features under\napplied polarization potentials, consistent with our ex-situ Raman results in Fig. S23 of the\nrevised SI. Importantly, LCNFPdO shows smaller Raman peak shifts compared to LCO\nunder the same applied potential, confirming that its reconstruction is more limited and\ncorroborating STEM (Fig. S15-18), XAFS (Fig. 3a-b), and TOF-SIMS (Fig. 3e) findings in\nour manuscript.\nFinally, the above findings are consistent with our previous operando investigations\npublished in Nature Catalysis, 4, 212–222, 2021. With operando XAFS, it was found that\nthe higher cobalt valence in LCO made it more resistant to further oxidation during OER—\nan observation that aligns well with our EQCM results and confirms the consistency of our\nex-situ analyses.\nTaken together, online EQCM during OER, the newly added in-situ Raman evidence, and\nsupporting operando XAFS data all converge to affirm that the ex-situ spectroscopy\nmeasurements used in our study provide a reliable depiction of the catalytic surface under\noperating conditions.\nFigure R13 In situ Raman spectra of (a) LCNFPdO and (b) LCO during OER in 1M KOH\nelectrolyte. The potentials are versus RHE (Fig. S24 in the revised SI)\nThe analyses are added in the revised manuscript: “To provide operando insight into\nsurface reconstruction, we performed in-situ Raman spectroscopy at 22 °C (Fig. S24). As\nthe applied potential increased from 1.20V to 1.55V vs. RHE, both LCO and LCNFPdO\ndeveloped two new peaks around 465 cm-1 and 565 cm-1, consistent with vibrations of\nCoO resulting from Li deintercalation. These findings closely match our ex-situ Raman\n2\nobservations. Notably, LCNFPdO exhibits smaller Raman peak shifts than LCO under the\nsame polarization potential, indicating a mitigated structural transformation.”\n4. The manuscript attributes higher currents in LCNFPdO partly to enhanced mass\ntransport. However, LSV data were recorded in the mixed-kinetic/diffusion regime, so\nmass-transport effects are conflated with intrinsic kinetics. A deconvolution of kinetic and\ndiffusion contributions should be made to support the claim.\nResponse:\nThank you for your suggestions. In our original manuscript, mass transport characteristics\nwere analyzed by evaluating the current density limits under high-current-density\nconditions(Fig. 4c, d), wherein the contribution of intrinsic kinetics should be minimal or\nnegligible26: “To evaluate the mass-transport (e.g., gas removal) capabilities, we checked\nthe current density limit (Fig. 4c, d) and quantified the number of bubbles (Fig. 4e, f) at high\nOER currents. The current density limit is defined as the point at which the potential\ndifference between tests without and with rotation (1600 rpm) reaches 10 mV. A high\ncurrent limit indicates a rapid mass transfer rate between the catalyst surface and\nelectrolyte, enabling the delivery of large currents. Compared with pristine and other\nelement-doped LCO, LCNFPdO achieved the highest current density limit, highlighting its\nsuperior mass transfer capability. The differential Tafel analysis further corroborates the\nsignificantly improved mass transport capability of LCNFPdO (Fig. S37).”\nBesides, we have now applied a quantitative deconvolution method based on the extended\nButler–Volmer equation to extract the kinetic overpotential (η ) and mass transport\nk\noverpotential (η ) components from our iR-corrected LSV data in Fig. S427. The kinetic\nmt\noverpotential (η ) was obtained using the high-overpotential approximation, where the\nk\nButler–Volmer equation simplifies to the Tafel law (equation (R1)):\n𝜂 =𝐴+𝐵∙log(𝑗) (R1)\n𝑘\nThe parameters A and B were determined by fitting the low-current region (dominated by\ncharge-transfer processes) in Fig. S4.\nSince our LSV curves are iR-corrected, the total overpotential (η) includes only kinetic and\nmass-transport components. The mass transfer overpotential was obtained by subtracting\nthe kinetic overpotential from the overall overpotential according to equation (R2).\n𝜂 =𝜂 −𝜂 (R2)\n𝑚𝑡 𝑡𝑜𝑡𝑎𝑙 𝑘\nwhere 𝜂 is obtained directly from the polarization data.\n𝑡𝑜𝑡𝑎𝑙\nUsing this protocol, the deconvolution (see Figure R14) shows that LCNFPdO exhibits\nsignificantly lower kinetic overpotential and reduced mass-transport overpotential\ncompared to LCO across the operating range. These improvements confirm that our multi-\ndoping strategy effectively enhances both intrinsic OER kinetics and mass-transport\nperformance.\nThis part of discussion was added to the revised manuscript from lines 295-301 in the\nrevised manuscript and Fig. S35 in the revised SI.\nFigure R14 (a) Deconvolution of the overpotentials of LCO and LCNFPdO into kinetic\npotential and mass transport overpotential. (b) The mass transport overpotential of LCO\nand LCNFPdO. (Fig. S35 in the revised manuscript)\nAdditionally, hydrophobicity differences at the level reported are too small and not\nstatistically meaningful to draw firm conclusions (contact angles 30.1° vs. 32.3°).\nResponse:\nThank you for the constructive comment. In our study, the contact angle was measured\nprior to electrochemical activation, serving only as a hydrophobicity reference for the\nprecursor. The surface of the reconstructed phase Li Co Ni Fe Pd O is more\n0.5 0.79 0.1 0.1 0.01 2\nhydrophilic than LiCo O according to the theoretical calculations in Fig. 5d, but it is difficult\n2 4\nto experimentally measure the contact angle after reconstruction because of the formation\nof partially reconstruction surface after the OER. In addition, recent literature confirms that\neven slight variations in contact angle can meaningfully influence mass transport and gas\nevolution behaviors28. Moreover, the better hydrophilicity induced by Ni/Fe/Pd doping is\nalso supported by additional evidence: the improved limiting current density, reduced\nbubble residence times, and thinner bubble layers (Figs. 4e–f, Fig. S40). These trends,\ntaken together, convincingly demonstrate that the observed hydrophilicity change is indeed\nmeaningful and contributes to superior mass-transport performance.\n5. Reporting 1.58 V at 2.5 A cm⁻² only after iR correction may mislead readers regarding\npractical performance. The authors should also compare the uncorrected performance\nfor LCNFPdO with the state-of-the-art catalysts (Table S10). The author may refer to this\ncomprehensive performance summary: Supplementary Table 2 in Nature Catalysis 7,\n944–952 (2024). This will allow a fair, “apples-to-apples” comparison under realistic\noperating conditions.\nResponse:\nThank the reviewer for the comment. We agree with the reviewer that uncorrected cell\nvoltage data is favorable for practical benchmarking. However, the uncorrected voltage is\nsignificantly influenced by the cell assembly and membrane choice, which may vary a lot\namong different research groups and make it difficult to evaluate the performance of\nstudied electrocatalysts only. That is why we provided both corrected and uncorrected\nvalues in our previous submission. Following the reviewer’s suggestion, we have now\nadded a comparison table including uncorrected voltages (without iR compensation) of\nLCNFPdO versus leading AEMWE catalysts reported in recent high-impact publications\n(Table R6). This additional data foregrounds practical performance and contextualizes our\nresults within the highest standards of AEMWE catalysts. We have updated the\ncorresponding Table S16 in the revised SI. We also cited the paper Nature Catalysis 7,\n944–952 (2024) in our updated manuscript.\nTable R6 Comparing the performance of AEM water electrolyzer with recent reported\nresults. (Table S16 in the revised SI)\nPerformanc\nStability\ne Refe\nAnode Cathode Membrane T(°C) J J renc\n(A cm- E (V) (A cm- Hours (h) e\n2) 2)\nSustainion 0.5 1.638\nThis\nLCNFPdO Pt/C X37- 50 80 1 1.779 0.5 1420\nwork\nGrade T 2 2.03\nPt /CoHPO Pt /CoHPO Alkymer 80 1 1.8 0.4 100 29\n1 1\nLi Mn Ru\n2 0.85 0.15\nPt/C QAPPT 60 1.2 1.9 / / 30\nO\nFe Ni Co Pt/C / 60 0.72 1.8 1 550 32\n10 45 45\nNiFe Foam NiMo Foam PPTQ-OH 80 1.94 2 0.5 80 33\nFumasep\nNiFe LDH@Ni\nRu/Zn-N-C FAA-3-PK- 25 0.3 2 0.25 60 34\nfoam\n130\n1 ‰ Pt- Sustainion\nNiFe LDH NiCoP@MXe X37-50 grade 25 1 1.95 0.5 500 35\nne RT\nCo-1T-MoS - Fumasep\nbpe-350 FAA-3–50\nFe Ni OOH- 0.2 160\nx y\nPt/C PAP-TP-85 80 0.54 1.8 37\n20F 0.5 60\nSustainion\nBi/BiCeO H Pt/C 50 1 1.97 1 100 38\n1.8\nX37-50\nIrO Pt/C FAA-3 90 1 1.8 0.5 1 39\nIrO Pt/C 60 0.75 1.9 / / 40\nNiFe NiMo m-PBI 60 1 2.04 0.5 160 41\nNiMn O Pt/KB 40% FAA3 80 0.53 2 ~0.3 1000 42\n2 4\nIrO PtRu/C HTMA-DAPP 60 1.74 2 / / 43\n2\n1.0 1750\nCAPist-L1 Ni Mo/MoO PAP-TP-85 80 7.3 2.0 44\n4 2\n2.0 1150\n4.0 600\nReviewer #4 (Remarks to the Author):\nIn the manuscript by Wang et al., titled \"Sustainable Water Oxidation Enabled by a\nComplex-doped Cobalt Oxide Electrode\", the authors present a very interesting\ncombined experimental and theoretical investigation of a LiCoO2-based electrocatalyst\ndoped with Ni, Fe, and Pd for the oxygen evolution reaction. The proposed system\nexhibits high catalytic activity, delivering 2.5 A cm-2 at 1.58 V, while maintaining excellent\nlong-term stability. The authors demonstrate that complex doping suppresses cobalt\nredox during OER, effectively mitigating surface reconstruction, enhancing\nelectrochemical stability, improving surface conductivity, and facilitating gas removal—\nthereby boosting mechanical robustness. I find these findings highly significant, as they\ndirectly address the persistent challenge of catalyst degradation under industrial OER\nconditions, and in my view, merit publication in Nature Communications. The work is\nwell-written and scientifically sound. However, several critical points should be\naddressed to enhance the overall quality of the manuscript before it can be\nrecommended for publication.\nResponse: We sincerely thank the reviewer for the positive evaluation of our work and for\nrecognizing the significance of our findings. At the same time, we highly appreciate the\nreviewer’s critical comments, which are constructive and valuable for improving the clarity\nand overall quality of the manuscript. We have carefully revised the manuscript accordingly,\nand our detailed responses to each point are provided below.\nComment 1. The authors constructed the NiFePd-doped LiCoO bulk structure by\n2\noptimizing 128 quasi-random configurations and selecting the most stable one.\nHowever, it’s not clear how the surface shown in Fig. S39 was built. Different surface\ncuts can expose different dopants and lead to variations in stability, so some clarification\nis needed.\nResponse: Thank you for your comments. The NiFePd-doped LCO bulk structure was\ndetermined by optimizing 128 quasi-random structures and selecting the most stable one.\nOur calculations reveal that Ni and Fe dopants exhibit clustering tendencies, while Pd\nand Ni dopants preferentially occupy separated sites. To determine the energetically\nfavorable surface structure for the complex LCFNPdO catalyst, we applied these chemical\ninsights to surface structure ordering. Next, consistent with established literature45, we\nadopted the (111) facet of spinel LCO as our surface model due to its superior stability.\nFinally, we explored the energetics of the different terminations. We found Pd prefers\nsublayer positions over top-layer exposure (formation energy difference: -0.10 eV).\nMeanwhile, Ni and Fe elements are often exposed on the topmost layers in the studied\nmodels46,47. These considerations collectively identify the finalized structure model. We\nhave added the above details in Supplementary Note 2 of the revised SI, which is also\ncopied below for your reference:\n“To construct a reasonable surface model, three principles governed surface model\nconstruction: (1) We constructed the NiFePd-doped LCO bulk structure by optimizing 128\nquasi-random structures and selecting the most stable one. It is observed that the Ni and\nFe dopants tend to be located with each other, while the Pt and Ni dopants sit away from\neach other. These chemical insights were applied to construct the complex LCFNPdO\ncatalyst surface; (2) Based on previous studies, the (111) facet is the most stable surface\nin spinel LCO and is therefore used as a surface model; (3) The presence of Pd in the\nsublayer is more stable than that on the top layer (Formation energy difference: -0.10 eV).\nMeanwhile, the more active Ni and Fe are often exposed on the topmost layer. The final\nsurface structure model was finally determined by considering all these factors.”\nComment 2. OER intermediates (*OH, *O, *OOH) were optimized on both LiCoO2 and\nNiFePd-doped LiCoO2 surfaces. However, the doped surface contains a large number\nof non-equivalent adsorption sites with potentially different activities. It’s not clear how\nthe optimization was performed in this case—were all relevant sites considered, or just\none specific configuration? Some clarification is needed. Moreover, the POSCAR files\nof the optimized key structures should be provided in the Supplementary Information to\nensure reproducibility of the results.\nResponse: Thank you for your suggestions. We have systematically evaluated all relevant\nadsorption sites on the NiFePd-doped LCO surface, including top sites of Co, Fe, and Ni\natoms and bridge sites between Ni and Co (as referenced in Table R7). During structural\nrelaxation, intermediates initially placed at top sites of Ni or Co consistently migrated to\nbridge sites between Ni and Co, resulting in identical final configurations and Gibbs free\nenergies. For Fe sites, they play a more important role in optimizing the electronic structure\nof surrounding atoms, and their poor activity is insufficient to serve as active sites. Finally,\nwe chose the bridge site as the most reasonable active site. The key structures are\nprovided in Supplementary Note 2 of the revised SI, and we have added the clarification\nas follows:\n“All relevant adsorption sites on NiFePd-doped LCO were evaluated, including top sites\nof Co, Fe, and Ni atoms and Ni-Co bridge sites (Table S18). Structural relaxation revealed:\n(1) Intermediates at Ni/Co top sites consistently migrated to Ni-Co bridge sites, producing\nidentical structures and energies; (2) Fe sites exhibited poor catalytic activity despite\nelectronic modulation effects. The Ni-Co bridge site was consequently identified as the\noptimal active center.”\nTable R7 The calculated adsorbed Gibbs free energy of all relevant sites. (Table S18 in the\nrevised SI)\nSite ΔG[OH] ΔG[O] ΔG[OOH] overpotential\nNi 1.17 2.85 3.96 0.46\nCo 1.17 2.85 3.96 0.46\nFe 1.13 3.65 4.33 1.30\nBridge 1.17 2.85 3.96 0.46\nComment 3. The manuscript assumes a conventional single-site OER mechanism as\nshown in Supplementary Note 2. However, given the doped and reconstructed nature of\nthe surface, with multiple metal centers in close proximity, could dual-site or cooperative\nmechanisms play a role? It would be useful if the authors could comment on whether\nsuch possibilities were considered and how the dual-site mechanism can affect the\ntheoretical overpotential.\nResponse: We appreciate the reviewer's insightful suggestion. It is widely acknowledged\nthat the surface state under electrochemical working conditions plays a key role in\ndetermining the reaction mechanisms. Before diving into the dual-site mechanism, we\nfirst examined the catalyst surface state under typical OER working conditions in alkaline\nenvironments through the surface Pourbaix diagram analysis (Figure R15). The dashed\nline represents the OER onset potential, which falls within the clean surface regime (our\nslab model). Meanwhile, the potential from O* bridge adsorption (O* model) to O* bridge\nan O* (Fe) adsorption is 1.83 V vs. RHE, which is higher than the typical OER reaction\npotential. Therefore, the selection of surface adsorption species in our model is\nreasonable. Specifically, at potentials below 0.60 V vs. SHE, the surface remains clean.\nAs the potential increases to 0.60-1.00 V vs. SHE, oxygen adsorption occurs exclusively\nat bridge sites (O* model). For further adsorption, Co, Ni, and Fe top sites are potential\ncandidates. However, additional oxygen adsorption at Co and Ni top sites directly\ngenerates O due to coordination saturation. For Fe sites, the potential required for\n2\nfurther oxygen adsorption at pH=14 reaches 1.00 V vs. SHE, indicating an energetically\nunfavorable process. Consequently, the saturated coordination environment around\nactive sites makes dual-site catalysis unlikely.\nFigure R15 Surface Pourbaix diagram of NiFePd-doped LCO. (Fig. 5c in the revised\nmanuscript)\nComment 4. The choice of Hubbard U parameters for Ni, Fe, and Co should be justified.\nHave the authors performed any self-consistent calculations to determine the U values?\nThe U = 6.2 eV used for Ni appears somewhat large in my opinion.\nResponse: We appreciate the reviewer's comment. In this work, we determined the\nHubbard U fitting for Ni, Fe, and Co by fitting to experimental redox reaction energies of\ntheir corresponding metal oxides (Table R8)48. This empirical calibration ensures that the\nchosen U values faithfully reproduce redox processes relevant to surface catalysis. For\nexample, for Ni, U = 6.2 eV accurately captures the Ni2+ /Ni3+ redox transition. The full set\nof U values is summarized in Table R8 and reproduced in the table below.\nTable R8 The list of Calibrated U values used in GGA+U calculations. (Table S17 in the revised\nSI)\nElement System Fitting Reaction Redox Couple Calibrated U\n(eV)\nCo Oxides 6CoO+O →2Co O Co2+→Co2.67+ 3.32\n2 3 4\nFe Oxides 6FeO+O →2Fe O Fe2+→Fe2.67+ 5.3\n2 3 4\n4Fe O +O →6Fe O Fe2.67+→Fe3+\n3 4 2 2 3\nNi Oxides Li O+2NiO+1/2O →2LiNiO Ni2+→Ni3+ 6.2\n2 2 2\nComment 5. It is really disappointing how the authors calculated the changes in free\nenergy, especially ΔG_corr. They simply used approximate values for zero-point energy\nand entropy corrections suggested in early works by Nørskov for metal surfaces. This\nmethodology is outdated, especially for a study submitted to a high-impact journal like\nNature Communications. Such corrections can vary significantly for complex metal\noxides and should be explicitly calculated for each intermediate. Moreover, the\nmanuscript does not explain how solvent effects were treated, which is critical, as\nsolvation can substantially impact the computed free energy values.\nResponse: We appreciate the reviewer's comments. We have computed free-energy\ncorrections for each catalyst, and the results are summarized in Table R9. The applied\ncorrections vary only slightly (≈0.03 eV) across different catalyst surfaces; therefore,\nthese updates do not change our previous conclusions. We have updated all results to\nincorporate these corrections and have added the corresponding values as Table S16 in\nthe revised SI. The relevant additions are copied as follows:\n“Free energy corrections were explicitly calculated per:\nΔG = ZPE +∫C dT −TS\ncorr p\nwith vibrational frequencies computed for all intermediates at 300 K and 1 atm (Table\nS16).”\nWe considered the effect of solvation by adding a water layer above the surface of NiFePd-\nLCO49. The solvent effects for OH*, O*, and OOH* were 0.45, 0.36, and 0.27, respectively.\nIn Figure R16, it is worth mentioning that, except for NiPd, the potential energy\ndetermining step of the catalyst in this system is the transition from OH* to O* (The effect\nof correction is less than 0.1 eV). Therefore, the influence of solvents did not change the\nprevious conclusion.\nTable R9 Calculated free energy corrections for reaction intermediates and molecules at 300\nK and 1 atm on various catalysts. (Table S16 in the revised SI)\nCatalysts Adsorbates Molecules Total\nCorrection Correction Correction\n(eV) (eV) (eV)\nOH* O* OOH* H O H OH* O* OOH*\n2 2\nLCNFPdO 0.34 0.05 0.40 0 -0.04 0.32 0.01 0.34\nLCO 0.37 0.06 0.39 0 -0.04 0.35 0.02 0.33\nLCFO 0.36 0.04 0.40 0 -0.04 0.34 0 0.34\nLCNO 0.35 0.05 0.38 0 -0.04 0.33 0.01 0.32\nLCPdO 0.36 0.05 0.41 0 -0.04 0.34 0.01 0.35\nLCNFO 0.35 0.05 0.39 0 -0.04 0.33 0.01 0.33\nLCFPdO 0.34 0.05 0.39 0 -0.04 0.32 0.01 0.33\nLCNPdO 0.34 0.06 0.40 0 -0.04 0.32 0.02 0.34\nFigure R16 The volcano of NiFePd-doped LCO systems. (Fig. 5d in the revised manuscript)\nComment 6. The system contains transition metal atoms with unpaired d-electrons (Fe,\nCo, and Ni), and in my experience, it can be very challenging to identify the correct spin\nordering in such complex systems. The authors should clearly state the magnetic\nconfiguration used and explain how it was chosen to ensure the lowest-energy structure.\nResponse: We completely agree with the reviewer that our studied systems are complex.\nIn this work, we applied the ferromagnetic state for adsorption energies calculations due\nto the prohibitive computational costs in exploring the large antiferromagnetic states.\nTo keep calculations consistent, we implemented the following procedure: First, we\ndeliberately optimized the surface structure to determine the lowest-energy magnetic\nconfiguration. We then constrained the total magnetic moment of the corresponding\nadsorption systems to this determined value. All calculations were required to satisfy four\ncriteria: (1) no antiferromagnetic ordering appears; (2) the calculated ∆G - ∆G\nOOH OH\nadheres to the scaling relationship (approximately 2.8–3.3); (3) the total magnetization\nof the entire model remains within a range of about 4 μB; and (4) When multiple magnetic\nconfigurations satisfy conditions (1)-(3), we select the configuration with the lowest\nenergy. When calculations failed to meet these criteria, we recalculated with adjusted\nNUPDOWN values (±1 μB increments) and modified initial moments. The total\nmagnetization values for all systems are listed in Table R10. We have supplemented the\nrelevant statements in the Method section:\n“To address magnetic complexity in transition metal systems, we implemented a rigorous\nconfiguration selection protocol: The pristine slab model was first optimized to determine\nits lowest-energy magnetic moment configuration; Identical NUPDOWN values were\nsubsequently applied to corresponding adsorbate systems; All configurations must\nsatisfy four criteria: absence of antiferromagnetic ordering, adherence to the ∆G -\nOOH\n∆G scaling relationship (2.8-3.3 V, considering the influence of solvents), total\nOH\nmagnetization within 4 μB tolerance, and the selection of the lowest-energy state for\nmultiple valid configurations; Systems failing these criteria were recalculated with\nNUPDOWN adjustments (±1 μB increments) and modified initial moments. Total\nmagnetization data for all configurations are documented in Table S19.”\nTable R10 Total magnetization for different catalysts. 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C 121,\n11455–11463 (2017).\nResponse to Reviewers\nManuscript ID: NCOMMS-25-35487-A\nArticle title: Sustainable Water Oxidation Enabled by a Complex-doped Cobalt Oxide\nElectrode\nWe would first like to express our appreciation to the editor for giving our manuscript\nanother chance to be reviewed and to the reviewers for their recognition and constructive\ncomments, which helped strengthen our scientific findings. Below, we provide point-to-\npoint responses to each comment, which are written in blue font. Our responses are in\nblack font, with manuscript additions highlighted in yellow.\nREVIEWER COMMENTS\nReviewer #1:\nThe authors have adequately addressed my key concerns. I recommend acceptance.\nResponse:\nWe greatly appreciate the reviewer’s recognition.\nReviewer #2 (Remarks to the Author):\nThe authors have revised the manuscript carefully. I have no more comment.\nResponse:\nWe greatly appreciate the reviewer’s recognition.\nReviewer #4 (Remarks to the Author):\nThe authors have addressed the reviewers’ comments and considerably improved the\nmanuscript. It is suitable for publication in its present form.\nResponse:\nWe greatly appreciate the reviewer’s recognition.\nReviewer #3 (Remarks to the Author):\nThe referee appreciates the authors’ careful discussion of the roles of the constituent\nelements in this ternary-doped catalyst and the breadth of the post-test characterization.\nSome additional comments are provided below, focusing on improving the clarity and\nstrengthening the mechanistic narrative:\n1. Please provide a clearer description in Fig. 2 (and Methods) of the exact\nelectrochemical protocols preceding collection of the “22 °C” and “50 °C” post-test\nsamples. In particular: Please specify which tests were performed immediately before\nsampling (e.g., CV activation, chronopotentiometry at what current density for how long,\netc.). Please specify whether the samples were collected after CV only or after the\nstability test.\nResponse: Thank the reviewer for acknowledging our revisions and proposing new\ncomments. In the revised Fig. 2 and Methods section, we have added detailed sample\npreparation procedures. Briefly, the preparation of post-OER sample involved 10 CV cycles\n(5 mV s-1, 1.02–1.62 V vs. RHE, 1 M KOH, 22 °C or 50 °C) during which the current\nstabilized. Subsequently, a 30-minute chronoamperometry hold at 1.62 V vs. RHE was\napplied during which no notable current decay was observed. The samples were then\nrinsed with deionized water and dried under vacuum at 25 °C for 12 hours before\ncharacterization.\n2. The referee appreciates that the authors conducted comprehensive characterization\nof the reconstructed catalyst, but it is recommended to re-organize the characterization\nresults and corresponding discussion to create a clearer narrative flow. For example, in\nthe discussion of the microscopic results, the authors claim that LCNFPdO undergoes\nless structural change than LCO. This mitigation of structure/oxidation-state changes is\nfurther supported by XPS, XAS, and Raman experiments. However, the CV data in Fig.\n1b shows a significant change in the CV curves for LCNFPdO, indicating that the\ncatalyst’s reactivity has changed dramatically. The authors briefly note in line 271 that,\ndespite less reconstruction, the reconstructed surface of LCNFPdO might be more\nfavorable for OER because there are more adsorbed hydroxide groups on its surface.\nThen, beginning at line 354, the authors discuss activity enhancement for the\nreconstructed LCNFPdO surface using theoretical calculations. However, it is not clear\nhow the theoretically predicted energetically stable configuration of\nLi₀.₅Co₀.₇₉Ni₀.₁Fe₀.₁Pd₀.₀₁O₂ connects with the experimentally obtained reconstructed\nsurface structure; for example, no composition analysis of the surface layer is provided.\nResponse:\nThank the reviewer’s insightful comment. The formation of the LiCo O -type spinel phase\n2 4\nis indeed a well-established thermodynamic phenomenon in layered Li CoO at a\n1-x 2\ndelithiation level of x ≤ 0.5. This is supported by the seminal work of Ceder and co-workers,\nwho outlined the phase transition pathway from layered LiCoO to spinel LiCo O and\n2 2 4\nfinally to Co O (Phys. Rev. B 58, 2975, 1998 (687 citations); Chem. Mater. 2007, 19, 543–\n3 4\n552 (324 citations)) during Li loss1,2. Compositions with x ≤ 0.5 tend to form LiCo O -type\n2 4\nspinel phase, while compositions with x > 0.5 tend to transform into Co O -type phase.\n3 4\nTherefore, LiCo O represents a thermodynamically stable configuration within the partially\n2 4\ndelithiated regime, consistent with our DFT calculations. Our and others’ previous work\nalso documented that under the OER potential window, the surface of LCO would change\ninto LiCo O -type spinel phase structure3–5. Our experimental data further robustly confirm\n2 4\nthis model in our samples:\n1) TEM analysis (Fig. 2e-h) directly identified a LiCo O -type instead of Co O -type\n2 4 3 4\nspinel structure on the reconstructed surface of both LCO and LCNFPdO.\n2) ToF-SIMS depth profiling (Fig. 3f) provides semi-quantitative support, revealing a\nsurface Li intensity for LCO that is approximately 50% of the internal bulk,\ncorresponding to a delithiation level of x ~ 0.5. While LCNFPdO exhibits a slightly\nhigher surface/bulk Li ratio, indicating less Li loss (x < 0.5) and a thinner\nreconstructed layer. The substantial Li deficiency was still sufficient to drive its\ntransformation into a LiCo O -type spinel surface, as directly observed by TEM.\n2 4\n3) ICP-MS data (Table S4) confirm negligible dissolution of Co, Ni, Fe, or Pd for\nLCNFPdO after OER, suggesting those elements maintained their original ratio.\nCollectively, these results validate that our Li Co Ni Fe PdO\n40 63 8 8 160\n(Li Co Ni Fe Pd O ) DFT model is a thermodynamically sound and experimentally\n0.5 0.79 0.1 0.1 0.01 2\njustified representation of the reconstructed surface observed in our study.\nWe have revised the main text as the following:\n“To understand the modulated performance, we performed density functional theory (DFT)\ncalculations to investigate the OER stability and activity. Based on the preceding TEM\nanalysis, the surface of LCO and LCNFPdO reconstructed into a LiCo O -type spinel\n2 4\nstructure. Moreover, the surface Li loss was close to 50% (Fig. 3f) and the ICP-MS data\n(Table S4) confirmed negligible dissolution of Co, Ni, Fe, or Pd for LCNFPdO after OER.\nAs a result, a calculation model of Li Co Ni Fe PdO (i.e., Li Co Ni Fe Pd O )\n40 63 8 8 160 0.5 0.79 0.1 0.1 0.01 2\n(Fig. 5a) was applied, which was identified as the most energetically stable configuration\nusing the special quasi-random structures algorithm. The half-delithiated spinel phase\nmodel was consistent with previous reports18,35,36.”\n3. To make a clear narrative flow, it is recommended that the authors re-examine the\ncharacterization results and identify the most convincing and reliable structural\ndifferences on which to base their hypothesis. For example, a large portion of the\ndiscussion focuses on the Co oxidation states, leading to the conclusion that lower Co\noxidation states in LCNFPdO benefit OER. However, the key supporting XPS results\n(Fig. S22) do not clearly support this argument. While the referee appreciates the\nclarification of the XPS fitting parameters, the fitting results do not clearly show a\nsignificantly lower Co oxidation state, particularly for the 22 °C case. The 50 °C case\nmay show a larger difference in the Co³⁺ peak, but the signal-to-noise ratio is too low in\nthe LCNFPdO sample, leading to large fitting errors.\nResponse:\nWe appreciate the reviewer’s comments. We carefully examined the evolution of Co\noxidation state because it is strongly correlated with Li content, and together they\ndetermine the reconstruction extent, as cobalt oxidation is charge balanced by Li loss, and\nLi deficiency in layered LCO/LCNFPdO inevitably drives surface reconstruction into a\nspinel-type structure. Beyond XPS, we employed multiple complementary techniques,\nincluding Co K-edge XAFS (Fig. 3a–b) and Co L -edge sXAS (Fig. 3c) to examine the\n3\ncobalt valence change in our previous submission, all showing a consistent trend that\nLCNFPdO undergoes less oxidation than LCO during OER.\nIn addition, we agree with the reviewer that our XPS fitting demonstrated similar absolute\nCo oxidation state at 22 °C, but this does not affect our conclusion, which is based on\nthe relative change in valence before and after OER. As shown in Figure R1, the Co\nvalence increase from XPS in LCNFPdO at both 22 °C and 50 °C is obviously smaller than\nthat in LCO. This consistent trend across multiple characterization methods strongly\nsupports our conclusion that multielement co-doping effectively moderates the cobalt\noxidation and reconstruction extent of LCO. We have revised the manuscript to improve\nthe above clarity.\nFigure R1 The Co valence change of LCO and LCNFPdO after OER at 22 °C and 50 °C\naccording to the XPS fitting results in Fig. S22 and Table S10. The error bar represents XPS\nmeasurement uncertainty.\nIn the revised text:\n“Consequently, greater lithium loss promotes deeper reconstruction and the formation of\nmore spinel species. In contrast, LCNFPdO undergoes less Co oxidation (Fig. 3a–d) and\nlithium loss, resulting in a thinner reconstructed layer than LCO and thus a mitigated\nreconstruction extent, as illustrated in the scheme in Fig. S30.”\n4. It is suggested to study the deconvolution of the losses and study the transport\nproperties in AEMWE condition. The mass transport loss should not be explained by the\nlower OER overpotential as it should be reflected by kinetic loss. Also, it is recommended\nto carefully check the benchmark catalysts performance (as ~2.3V@ 0.5A/cm2 for Ir\ncatalyst might cause concerns on MEA fabrication) shown in Fig.6b and compared to\ntypical commercial benchmark performance.\nResponse:\nWe appreciate the reviewer’s insightful suggestions. In our previous submission we never\nclaim that the lowered OER overpotential was attributed to reduced mass transport loss.\nTo prevent confusion, we have revised the statement into “Fast mass transfer capability\nmight contribute to practical water oxidation performance.”\nAs suggested by the reviewer, we identified the sources of potential loss at the anode using\na three-electrode AEMWE setup (Figure R2a-b) to study the transport properties of\nLCNFPdO and LCO. The polarization curves of individual anode and cathode versus\nAg/AgCl reference electrode and the calculated full cell voltage are shown in Figure R2c-\nd.\nFigure R2 a, digital photograph of the homemade three-electrode AEMWE system with an\nAg/AgCl reference electrode. b, the close-up image of the custom-designed chamber that\nallows contact between the reference electrode and the electrolyte. c-d, polarization curves of\nindividual anode and cathode potential versus Ag/AgCl reference electrode, and the calculated\nfull cell voltage of LCNFPdO (c) and LCO (d). e-f, Applied-potential breakdown for the anode of\nAEMWE with LCNFPdO (e) and LCO (f) loaded on nickel foam as the OER electrode. 8 cm2\nactive area, 80 °C, 1M KOH as electrolyte, Pt/C loaded on carbon paper as the HER electrode,\nand Versogen A40 membrane as the anion exchange membrane.\nThe calculated full cell voltage is obtained from the individual electrode potentials\naccording to equation R1:\n𝐸 =𝐸 −𝐸 (𝑅1)\n𝑐𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑 𝑓𝑢𝑙𝑙 𝑐𝑒𝑙𝑙 𝑎𝑛𝑜𝑑𝑒 𝑣𝑠.𝐴𝑔/𝐴𝑔𝐶𝑙 𝑐𝑎𝑡ℎ𝑜𝑑𝑒 𝑣𝑠.𝐴𝑔/𝐴𝑔𝐶𝑙\nThe total anode potential breakdown model can be written as R26:\n𝐸 (𝑗)=𝐸 +∆𝐸 (𝑗)+∆𝐸 (𝑗)+∆𝐸 (𝑗) (𝑅2)\n𝑎𝑛𝑜𝑑𝑒 𝑟𝑒𝑣,𝑂𝐸𝑅 𝑘𝑖𝑛𝑒𝑡𝑖𝑐 𝑜ℎ𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡\nwhere 𝐸 is the anode potential applied, 𝐸 is the reversible potential for OER,\n𝑎𝑛𝑜𝑑𝑒 𝑟𝑒𝑣,𝑂𝐸𝑅\nand ∆𝐸 , ∆𝐸 , and ∆𝐸 are the kinetic, ohmic, and mass transport\n𝑘𝑖𝑛𝑒𝑡𝑖𝑐 𝑜ℎ𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡\npotential losses, respectively. The 𝐸 is calculated using equation R37:\n𝑟𝑒𝑣,𝑂𝐸𝑅\n𝑅𝑇\n𝐸0 =0.4011−1.6816×10−3×(𝑇−298.15)+(2.303× )×𝑝𝐻 (𝑅3)\n𝑟𝑒𝑣,𝑂𝐸𝑅 𝐹\nwhere T is the operation temperature (378.15 K), R is the ideal gas constant (8.31 J K-1\nmol-1), F is the Faraday constant (96485 C mol-1), and the pH of 1 M KOH at 80 °C (378.15\nK) is 12.458. The kinetic potential loss can be obtained using a Tafel equation (R4) from\nthe iR-corrected polarization curve at low current density region,\n𝑗\n∆𝐸 =𝑏×log −𝐸 (𝑅4)\n𝑘𝑖𝑛𝑒𝑡𝑖𝑐 𝑗 𝑟𝑒𝑣,𝑂𝐸𝑅\n0\nwhere j is the exchange current density, j is the current density, and b is the Tafel slope\n0\nand can be determined by fitting the low-current region (dominated by charge-transfer\nprocesses) of the polarization curve. The ohmic potential loss can be obtained using Ohm’s\nlaw (R5),\n∆𝐸 =𝐻𝐹𝑅×𝑗 (𝑅5)\n𝑜ℎ𝑚𝑖𝑐\nwhere HFR is the high-frequency resistance. Thus, the mass transport potential loss can\nbe obtained by equation R6:\n∆𝐸 (𝑗)=𝐸 (𝑗)−𝐸 −∆𝐸 (𝑗)−∆𝐸 (𝑗) (𝑅6)\n𝑚𝑎𝑠𝑠 𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡 𝑎𝑛𝑜𝑑𝑒 𝑟𝑒𝑣,𝑂𝐸𝑅 𝑘𝑖𝑛𝑒𝑡𝑖𝑐 𝑜ℎ𝑚𝑖𝑐\nThe anode potential breakdown from the three-electrode AEMWE tests (cathode\noverpotential analysis not shown here) revealed that LCNFPdO anode show lower mass\ntransport loss compared to LCO anode, especially at high current densities, as shown\nin Figure R2e-f, suggesting the better mass transport capabilities of LCNFPdO. We note\nthat while the performance trend of LCNFPdO superior to LCO is consistent, the absolute\nperformance values presented here differ from those reported in the main text, which is\nowing to differences in the electrolyzer setup and membrane use.\nRegarding the benchmark Ir catalyst performance (~2.3 V @ 0.5 A cm-2), this data was\nreproduced from the literature (Energy Environ. Sci., 2016, 9, 184–192)9 rather than\nmeasured by us, as indicated in the figure caption in our previous submission\n(screenshot provided below in Figure R3 and highlighted in yellow). To prevent confusion,\nwe have experimentally tested the benchmark IrO OER catalyst in AEMWE under the\n2\nsame condition as those for other studied catalysts, and its performance (2.03 V @ 1 A cm-\n2) agrees well with literature values summarized in Table S15, as shown in Figure R4\n(Extracted from Fig. 6b in the revised manuscript). By comparison, it is clear that our\ndeveloped LCNFPdO is still superior to the IrO in the AEMWE performance, showing great\n2\npotential.\nFigure R3 Fig. 6 and its figure legend in the original manuscript.\nFigure R4 AEMWE polarization curves of IrO and LCNFPdO as anode catalysts at 80 ºC\n2\nwith a working electrode area of 1 cm2 without iR-correction, cathode: Pt/C. (Extracted\nfrom Fig. 6b in the revised manuscript)\nFinally, we want to emphasize that AEMWE performance is heavily depending on\nexperimental factors such as catalyst loading, membrane type, and compression\npressure during MEA fabrication10. These factors may explain why the reported Ir\nbenchmark performance appeared comparatively low.\nWe have revised the manuscript to include the above points. Given that this is our final\nopportunity for revision, we hope we have addressed your comments in full. Thank you\nonce again for helping us improve this work.\nReference:\n1. Van der Ven, A. First-principles investigation of phase stability in Li𝑥CoO2. Phys. Rev. B 58,\n2975–2987 (1998).\n2. L. Wang, T. Maxisch & Ceder, G. A First-Principles Approach to Studying the Thermal\nStability of Oxide Cathode Materials. Chem. Mater. 19, 543–552 (2006).\n3. Wang, J. et al. Redirecting dynamic surface restructuring of a layered transition metal\noxide catalyst for superior water oxidation. Nat. Catal. 4, 212–222 (2021).\n4. Sun, Y. et al. Navigating surface reconstruction of spinel oxides for electrochemical water\noxidation. Nat. Commun. 14, 2467 (2023).\n5. Meng, Y. S. & Dompablo, M. E. A. First principles computational materials design for\nenergy storage materials in lithium ion batteries. Energy Environ. Sci. 2, 589–609 (2009).\n6. Gerhardt, M. R. et al. Method—Practices and Pitfalls in Voltage Breakdown Analysis of\nElectrochemical Energy-Conversion Systems. J. Electrochem. Soc. 168, 074503 (2021).\n7. Bratsch, S. G. Standard Electrode Potentials and Temperature Coefficients in Water at\n298.15 K. J. Phys. Chem. Ref. Data 18, 1–21 (1989).\n8. Hausmann, J. N., Traynor, B., Myers, R. J., Driess, M. & Menezes, P. W. The pH of Aqueous\nNaOH/KOH Solutions: A Critical and Non-trivial Parameter for Electrocatalysis. ACS Energy\nLett. 6, 3567–3571 (2021).\n9. Gardner, G. et al. Structural basis for differing electrocatalytic water oxidation by the cubic,\nlayered and spinel forms of lithium cobalt oxides. Energy Environ. Sci. 9, 184–192 (2016).\n10. K. Volk, E., E. Kreider, M., Kwon, S. & M. Alia, S. Recent progress in understanding the\ncatalyst layer in anion exchange membrane electrolyzers – durability, utilization, and\nintegration. EES Catal. 2, 109–137 (2024)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have adequately addressed my key concerns.I recommend acceptance.", "reviewer_2": "The authors have revised the manuscript carefully. I have no more comment.", "reviewer_3": "The referee appreciates the authors’ careful discussion of the roles of the constituent elements in this ternary-doped catalyst and the breadth of the post-test characterization. Some additional comments are provided below, focusing on improving the clarity and strengthening the mechanistic narrative:\n1. Please provide a clearer description in Fig. 2 (and Methods) of the exact electrochemical protocols preceding collection of the “22 °C” and “50 °C” post-test samples. In particular: Please specify which tests were performed immediately before sampling (e.g., CV activation, chronopotentiometry at what current density for how long, etc.). Please specify whether the samples were collected after CV only or after the stability test.\n2. The referee appreciates that the authors conducted comprehensive characterization of the reconstructed catalyst, but it is recommended to re-organize the characterization results and corresponding discussion to create a clearer narrative flow. For example, in the discussion of the microscopic results, the authors claim that LCNFPdO undergoes less structural change than LCO. This mitigation of structure/oxidation-state changes is further supported by XPS, XAS, and Raman experiments. However, the CV data in Fig. 1b shows a significant change in the CV curves for LCNFPdO, indicating that the catalyst’s reactivity has changed dramatically. The authors briefly note in line 271 that, despite less reconstruction, the reconstructed surface of LCNFPdO might be more favorable for OER because there are more adsorbed hydroxide groups on its surface. Then, beginning at line 354, the authors discuss activity enhancement for the reconstructed LCNFPdO surface using theoretical calculations. However, it is not clear how the theoretically predicted energetically stable configuration of Li₀.₅Co₀.₇₉Ni₀.₁Fe₀.₁Pd₀.₀₁O₂ connects with the experimentally obtained reconstructed surface structure; for example, no composition analysis of the surface layer is provided.\n3. To make a clear narrative flow, it is recommended that the authors re-examine the characterization results and identify the most convincing and reliable structural differences on which to base their hypothesis. For example, a large portion of the discussion focuses on the Co oxidation states, leading to the conclusion that lower Co oxidation states in LCNFPdO benefit OER. However, the key supporting XPS results (Fig. S22) do not clearly support this argument. While the referee appreciates the clarification of the XPS fitting parameters, the fitting results do not clearly show a significantly lower Co oxidation state, particularly for the 22 °C case. The 50 °C case may show a larger difference in the Co³⁺ peak, but the signal-to-noise ratio is too low in the LCNFPdO sample, leading to large fitting errors.\n4. It is suggested to study the deconvolution of the losses and study the transport properties in AEMWE condition. The mass transport loss should not be explained by the lower OER overpotential as it should be reflected by kinetic loss. Also, it is recommended to carefully check the benchmark catalysts performance (as ~2.3V@ 0.5A/cm2 for Ir catalyst might cause concerns on MEA fabrication) shown in Fig.6b and compared to typical commercial benchmark performance.", "reviewer_4": "The authors have addressed the reviewers’ comments and considerably improved the manuscript. It is suitable for publication in its present form."}, "author_response": "Response to Reviewers\nManuscript ID: NCOMMS-25-35487-A\nArticle title: Sustainable Water Oxidation Enabled by a Complex-doped Cobalt Oxide\nElectrode\nWe would first like to express our appreciation to the editor for giving our manuscript\nanother chance to be reviewed and to the reviewers for their recognition and constructive\ncomments, which helped strengthen our scientific findings. Below, we provide point-to-\npoint responses to each comment, which are written in blue font. Our responses are in\nblack font, with manuscript additions highlighted in yellow.\nREVIEWER COMMENTS\nReviewer #1:\nThe authors have adequately addressed my key concerns. I recommend acceptance.\nResponse:\nWe greatly appreciate the reviewer’s recognition.\nReviewer #2 (Remarks to the Author):\nThe authors have revised the manuscript carefully. I have no more comment.\nResponse:\nWe greatly appreciate the reviewer’s recognition.\nReviewer #4 (Remarks to the Author):\nThe authors have addressed the reviewers’ comments and considerably improved the\nmanuscript. It is suitable for publication in its present form.\nResponse:\nWe greatly appreciate the reviewer’s recognition.\nReviewer #3 (Remarks to the Author):\nThe referee appreciates the authors’ careful discussion of the roles of the constituent\nelements in this ternary-doped catalyst and the breadth of the post-test characterization.\nSome additional comments are provided below, focusing on improving the clarity and\nstrengthening the mechanistic narrative:\n1. Please provide a clearer description in Fig. 2 (and Methods) of the exact\nelectrochemical protocols preceding collection of the “22 °C” and “50 °C” post-test\nsamples. In particular: Please specify which tests were performed immediately before\nsampling (e.g., CV activation, chronopotentiometry at what current density for how long,\netc.). Please specify whether the samples were collected after CV only or after the\nstability test.\nResponse: Thank the reviewer for acknowledging our revisions and proposing new\ncomments. In the revised Fig. 2 and Methods section, we have added detailed sample\npreparation procedures. Briefly, the preparation of post-OER sample involved 10 CV cycles\n(5 mV s-1, 1.02–1.62 V vs. RHE, 1 M KOH, 22 °C or 50 °C) during which the current\nstabilized. Subsequently, a 30-minute chronoamperometry hold at 1.62 V vs. RHE was\napplied during which no notable current decay was observed. The samples were then\nrinsed with deionized water and dried under vacuum at 25 °C for 12 hours before\ncharacterization.\n2. The referee appreciates that the authors conducted comprehensive characterization\nof the reconstructed catalyst, but it is recommended to re-organize the characterization\nresults and corresponding discussion to create a clearer narrative flow. For example, in\nthe discussion of the microscopic results, the authors claim that LCNFPdO undergoes\nless structural change than LCO. This mitigation of structure/oxidation-state changes is\nfurther supported by XPS, XAS, and Raman experiments. However, the CV data in Fig.\n1b shows a significant change in the CV curves for LCNFPdO, indicating that the\ncatalyst’s reactivity has changed dramatically. The authors briefly note in line 271 that,\ndespite less reconstruction, the reconstructed surface of LCNFPdO might be more\nfavorable for OER because there are more adsorbed hydroxide groups on its surface.\nThen, beginning at line 354, the authors discuss activity enhancement for the\nreconstructed LCNFPdO surface using theoretical calculations. However, it is not clear\nhow the theoretically predicted energetically stable configuration of\nLi₀.₅Co₀.₇₉Ni₀.₁Fe₀.₁Pd₀.₀₁O₂ connects with the experimentally obtained reconstructed\nsurface structure; for example, no composition analysis of the surface layer is provided.\nResponse:\nThank the reviewer’s insightful comment. The formation of the LiCo O -type spinel phase\n2 4\nis indeed a well-established thermodynamic phenomenon in layered Li CoO at a\n1-x 2\ndelithiation level of x ≤ 0.5. This is supported by the seminal work of Ceder and co-workers,\nwho outlined the phase transition pathway from layered LiCoO to spinel LiCo O and\n2 2 4\nfinally to Co O (Phys. Rev. B 58, 2975, 1998 (687 citations); Chem. Mater. 2007, 19, 543–\n3 4\n552 (324 citations)) during Li loss1,2. Compositions with x ≤ 0.5 tend to form LiCo O -type\n2 4\nspinel phase, while compositions with x > 0.5 tend to transform into Co O -type phase.\n3 4\nTherefore, LiCo O represents a thermodynamically stable configuration within the partially\n2 4\ndelithiated regime, consistent with our DFT calculations. Our and others’ previous work\nalso documented that under the OER potential window, the surface of LCO would change\ninto LiCo O -type spinel phase structure3–5. Our experimental data further robustly confirm\n2 4\nthis model in our samples:\n1) TEM analysis (Fig. 2e-h) directly identified a LiCo O -type instead of Co O -type\n2 4 3 4\nspinel structure on the reconstructed surface of both LCO and LCNFPdO.\n2) ToF-SIMS depth profiling (Fig. 3f) provides semi-quantitative support, revealing a\nsurface Li intensity for LCO that is approximately 50% of the internal bulk,\ncorresponding to a delithiation level of x ~ 0.5. While LCNFPdO exhibits a slightly\nhigher surface/bulk Li ratio, indicating less Li loss (x < 0.5) and a thinner\nreconstructed layer. The substantial Li deficiency was still sufficient to drive its\ntransformation into a LiCo O -type spinel surface, as directly observed by TEM.\n2 4\n3) ICP-MS data (Table S4) confirm negligible dissolution of Co, Ni, Fe, or Pd for\nLCNFPdO after OER, suggesting those elements maintained their original ratio.\nCollectively, these results validate that our Li Co Ni Fe PdO\n40 63 8 8 160\n(Li Co Ni Fe Pd O ) DFT model is a thermodynamically sound and experimentally\n0.5 0.79 0.1 0.1 0.01 2\njustified representation of the reconstructed surface observed in our study.\nWe have revised the main text as the following:\n“To understand the modulated performance, we performed density functional theory (DFT)\ncalculations to investigate the OER stability and activity. Based on the preceding TEM\nanalysis, the surface of LCO and LCNFPdO reconstructed into a LiCo O -type spinel\n2 4\nstructure. Moreover, the surface Li loss was close to 50% (Fig. 3f) and the ICP-MS data\n(Table S4) confirmed negligible dissolution of Co, Ni, Fe, or Pd for LCNFPdO after OER.\nAs a result, a calculation model of Li Co Ni Fe PdO (i.e., Li Co Ni Fe Pd O )\n40 63 8 8 160 0.5 0.79 0.1 0.1 0.01 2\n(Fig. 5a) was applied, which was identified as the most energetically stable configuration\nusing the special quasi-random structures algorithm. The half-delithiated spinel phase\nmodel was consistent with previous reports18,35,36.”\n3. To make a clear narrative flow, it is recommended that the authors re-examine the\ncharacterization results and identify the most convincing and reliable structural\ndifferences on which to base their hypothesis. For example, a large portion of the\ndiscussion focuses on the Co oxidation states, leading to the conclusion that lower Co\noxidation states in LCNFPdO benefit OER. However, the key supporting XPS results\n(Fig. S22) do not clearly support this argument. While the referee appreciates the\nclarification of the XPS fitting parameters, the fitting results do not clearly show a\nsignificantly lower Co oxidation state, particularly for the 22 °C case. The 50 °C case\nmay show a larger difference in the Co³⁺ peak, but the signal-to-noise ratio is too low in\nthe LCNFPdO sample, leading to large fitting errors.\nResponse:\nWe appreciate the reviewer’s comments. We carefully examined the evolution of Co\noxidation state because it is strongly correlated with Li content, and together they\ndetermine the reconstruction extent, as cobalt oxidation is charge balanced by Li loss, and\nLi deficiency in layered LCO/LCNFPdO inevitably drives surface reconstruction into a\nspinel-type structure. Beyond XPS, we employed multiple complementary techniques,\nincluding Co K-edge XAFS (Fig. 3a–b) and Co L -edge sXAS (Fig. 3c) to examine the\n3\ncobalt valence change in our previous submission, all showing a consistent trend that\nLCNFPdO undergoes less oxidation than LCO during OER.\nIn addition, we agree with the reviewer that our XPS fitting demonstrated similar absolute\nCo oxidation state at 22 °C, but this does not affect our conclusion, which is based on\nthe relative change in valence before and after OER. As shown in Figure R1, the Co\nvalence increase from XPS in LCNFPdO at both 22 °C and 50 °C is obviously smaller than\nthat in LCO. This consistent trend across multiple characterization methods strongly\nsupports our conclusion that multielement co-doping effectively moderates the cobalt\noxidation and reconstruction extent of LCO. We have revised the manuscript to improve\nthe above clarity.\nFigure R1 The Co valence change of LCO and LCNFPdO after OER at 22 °C and 50 °C\naccording to the XPS fitting results in Fig. S22 and Table S10. The error bar represents XPS\nmeasurement uncertainty.\nIn the revised text:\n“Consequently, greater lithium loss promotes deeper reconstruction and the formation of\nmore spinel species. In contrast, LCNFPdO undergoes less Co oxidation (Fig. 3a–d) and\nlithium loss, resulting in a thinner reconstructed layer than LCO and thus a mitigated\nreconstruction extent, as illustrated in the scheme in Fig. S30.”\n4. It is suggested to study the deconvolution of the losses and study the transport\nproperties in AEMWE condition. The mass transport loss should not be explained by the\nlower OER overpotential as it should be reflected by kinetic loss. Also, it is recommended\nto carefully check the benchmark catalysts performance (as ~2.3V@ 0.5A/cm2 for Ir\ncatalyst might cause concerns on MEA fabrication) shown in Fig.6b and compared to\ntypical commercial benchmark performance.\nResponse:\nWe appreciate the reviewer’s insightful suggestions. In our previous submission we never\nclaim that the lowered OER overpotential was attributed to reduced mass transport loss.\nTo prevent confusion, we have revised the statement into “Fast mass transfer capability\nmight contribute to practical water oxidation performance.”\nAs suggested by the reviewer, we identified the sources of potential loss at the anode using\na three-electrode AEMWE setup (Figure R2a-b) to study the transport properties of\nLCNFPdO and LCO. The polarization curves of individual anode and cathode versus\nAg/AgCl reference electrode and the calculated full cell voltage are shown in Figure R2c-\nd.\nFigure R2 a, digital photograph of the homemade three-electrode AEMWE system with an\nAg/AgCl reference electrode. b, the close-up image of the custom-designed chamber that\nallows contact between the reference electrode and the electrolyte. c-d, polarization curves of\nindividual anode and cathode potential versus Ag/AgCl reference electrode, and the calculated\nfull cell voltage of LCNFPdO (c) and LCO (d). e-f, Applied-potential breakdown for the anode of\nAEMWE with LCNFPdO (e) and LCO (f) loaded on nickel foam as the OER electrode. 8 cm2\nactive area, 80 °C, 1M KOH as electrolyte, Pt/C loaded on carbon paper as the HER electrode,\nand Versogen A40 membrane as the anion exchange membrane.\nThe calculated full cell voltage is obtained from the individual electrode potentials\naccording to equation R1:\n𝐸 =𝐸 −𝐸 (𝑅1)\n𝑐𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑 𝑓𝑢𝑙𝑙 𝑐𝑒𝑙𝑙 𝑎𝑛𝑜𝑑𝑒 𝑣𝑠.𝐴𝑔/𝐴𝑔𝐶𝑙 𝑐𝑎𝑡ℎ𝑜𝑑𝑒 𝑣𝑠.𝐴𝑔/𝐴𝑔𝐶𝑙\nThe total anode potential breakdown model can be written as R26:\n𝐸 (𝑗)=𝐸 +∆𝐸 (𝑗)+∆𝐸 (𝑗)+∆𝐸 (𝑗) (𝑅2)\n𝑎𝑛𝑜𝑑𝑒 𝑟𝑒𝑣,𝑂𝐸𝑅 𝑘𝑖𝑛𝑒𝑡𝑖𝑐 𝑜ℎ𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡\nwhere 𝐸 is the anode potential applied, 𝐸 is the reversible potential for OER,\n𝑎𝑛𝑜𝑑𝑒 𝑟𝑒𝑣,𝑂𝐸𝑅\nand ∆𝐸 , ∆𝐸 , and ∆𝐸 are the kinetic, ohmic, and mass transport\n𝑘𝑖𝑛𝑒𝑡𝑖𝑐 𝑜ℎ𝑚𝑖𝑐 𝑚𝑎𝑠𝑠 𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡\npotential losses, respectively. The 𝐸 is calculated using equation R37:\n𝑟𝑒𝑣,𝑂𝐸𝑅\n𝑅𝑇\n𝐸0 =0.4011−1.6816×10−3×(𝑇−298.15)+(2.303× )×𝑝𝐻 (𝑅3)\n𝑟𝑒𝑣,𝑂𝐸𝑅 𝐹\nwhere T is the operation temperature (378.15 K), R is the ideal gas constant (8.31 J K-1\nmol-1), F is the Faraday constant (96485 C mol-1), and the pH of 1 M KOH at 80 °C (378.15\nK) is 12.458. The kinetic potential loss can be obtained using a Tafel equation (R4) from\nthe iR-corrected polarization curve at low current density region,\n𝑗\n∆𝐸 =𝑏×log −𝐸 (𝑅4)\n𝑘𝑖𝑛𝑒𝑡𝑖𝑐 𝑗 𝑟𝑒𝑣,𝑂𝐸𝑅\n0\nwhere j is the exchange current density, j is the current density, and b is the Tafel slope\n0\nand can be determined by fitting the low-current region (dominated by charge-transfer\nprocesses) of the polarization curve. The ohmic potential loss can be obtained using Ohm’s\nlaw (R5),\n∆𝐸 =𝐻𝐹𝑅×𝑗 (𝑅5)\n𝑜ℎ𝑚𝑖𝑐\nwhere HFR is the high-frequency resistance. Thus, the mass transport potential loss can\nbe obtained by equation R6:\n∆𝐸 (𝑗)=𝐸 (𝑗)−𝐸 −∆𝐸 (𝑗)−∆𝐸 (𝑗) (𝑅6)\n𝑚𝑎𝑠𝑠 𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡 𝑎𝑛𝑜𝑑𝑒 𝑟𝑒𝑣,𝑂𝐸𝑅 𝑘𝑖𝑛𝑒𝑡𝑖𝑐 𝑜ℎ𝑚𝑖𝑐\nThe anode potential breakdown from the three-electrode AEMWE tests (cathode\noverpotential analysis not shown here) revealed that LCNFPdO anode show lower mass\ntransport loss compared to LCO anode, especially at high current densities, as shown\nin Figure R2e-f, suggesting the better mass transport capabilities of LCNFPdO. We note\nthat while the performance trend of LCNFPdO superior to LCO is consistent, the absolute\nperformance values presented here differ from those reported in the main text, which is\nowing to differences in the electrolyzer setup and membrane use.\nRegarding the benchmark Ir catalyst performance (~2.3 V @ 0.5 A cm-2), this data was\nreproduced from the literature (Energy Environ. Sci., 2016, 9, 184–192)9 rather than\nmeasured by us, as indicated in the figure caption in our previous submission\n(screenshot provided below in Figure R3 and highlighted in yellow). To prevent confusion,\nwe have experimentally tested the benchmark IrO OER catalyst in AEMWE under the\n2\nsame condition as those for other studied catalysts, and its performance (2.03 V @ 1 A cm-\n2) agrees well with literature values summarized in Table S15, as shown in Figure R4\n(Extracted from Fig. 6b in the revised manuscript). By comparison, it is clear that our\ndeveloped LCNFPdO is still superior to the IrO in the AEMWE performance, showing great\n2\npotential.\nFigure R3 Fig. 6 and its figure legend in the original manuscript.\nFigure R4 AEMWE polarization curves of IrO and LCNFPdO as anode catalysts at 80 ºC\n2\nwith a working electrode area of 1 cm2 without iR-correction, cathode: Pt/C. (Extracted\nfrom Fig. 6b in the revised manuscript)\nFinally, we want to emphasize that AEMWE performance is heavily depending on\nexperimental factors such as catalyst loading, membrane type, and compression\npressure during MEA fabrication10. These factors may explain why the reported Ir\nbenchmark performance appeared comparatively low.\nWe have revised the manuscript to include the above points. Given that this is our final\nopportunity for revision, we hope we have addressed your comments in full. Thank you\nonce again for helping us improve this work.\nReference:\n1. Van der Ven, A. First-principles investigation of phase stability in Li𝑥CoO2. Phys. Rev. B 58,\n2975–2987 (1998).\n2. L. Wang, T. Maxisch & Ceder, G. A First-Principles Approach to Studying the Thermal\nStability of Oxide Cathode Materials. Chem. Mater. 19, 543–552 (2006).\n3. Wang, J. et al. Redirecting dynamic surface restructuring of a layered transition metal\noxide catalyst for superior water oxidation. Nat. Catal. 4, 212–222 (2021).\n4. Sun, Y. et al. Navigating surface reconstruction of spinel oxides for electrochemical water\noxidation. Nat. Commun. 14, 2467 (2023).\n5. Meng, Y. S. & Dompablo, M. E. A. First principles computational materials design for\nenergy storage materials in lithium ion batteries. Energy Environ. Sci. 2, 589–609 (2009).\n6. Gerhardt, M. R. et al. Method—Practices and Pitfalls in Voltage Breakdown Analysis of\nElectrochemical Energy-Conversion Systems. J. Electrochem. Soc. 168, 074503 (2021).\n7. Bratsch, S. G. Standard Electrode Potentials and Temperature Coefficients in Water at\n298.15 K. J. Phys. Chem. Ref. Data 18, 1–21 (1989).\n8. Hausmann, J. N., Traynor, B., Myers, R. J., Driess, M. & Menezes, P. W. The pH of Aqueous\nNaOH/KOH Solutions: A Critical and Non-trivial Parameter for Electrocatalysis. ACS Energy\nLett. 6, 3567–3571 (2021).\n9. Gardner, G. et al. Structural basis for differing electrocatalytic water oxidation by the cubic,\nlayered and spinel forms of lithium cobalt oxides. Energy Environ. Sci. 9, 184–192 (2016).\n10. K. Volk, E., E. Kreider, M., Kwon, S. & M. Alia, S. Recent progress in understanding the\ncatalyst layer in anion exchange membrane electrolyzers – durability, utilization, and\nintegration. EES Catal. 2, 109–137 (2024)."}]

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