雙金屬浸出誘導(dǎo)催化劑重構(gòu)用于高活性和高穩(wěn)定性電化學(xué)水氧化

摘要：析氧反應(yīng)（OER）催化劑在催化反應(yīng)過程中不可避免地會發(fā)生表面重構(gòu)，這一過程使得設(shè)計(jì)、構(gòu)筑高性能和高穩(wěn)定性的OER電催化劑充滿挑戰(zhàn)。在此，我們采用雙金屬浸出誘導(dǎo)表面重構(gòu)的策略，構(gòu)建了高活性和高穩(wěn)定性的水氧化電催化劑。在該策略中，通過水熱、離子交換和后續(xù)的退火工藝處理，將由α-CoMoO4、K2Co2（MoO4）3、Co3O4和CoFe2O4四種氧化物晶相組成的材料陣列轉(zhuǎn)換為OER預(yù)催化劑。原位電化學(xué)拉曼光譜和非原位X射線衍射（XRD）分析表明，其中的不穩(wěn)定成分K2Co2（MoO4）3的快速溶解引發(fā)了Mo和K的適度浸出，從而在低電壓下加速了表面富集的α-Co（OH）2向CoOOH活性相的轉(zhuǎn)化。此外，CoFe2O4相耦合重構(gòu)產(chǎn)生新相CoO與無定形層CoOOH，從而形成了CoFe2O4@CoO@CoOOH緊密的多相結(jié)構(gòu)，起到了“納米柵欄”的作用，可有效防止催化劑的過度重構(gòu)，從而賦予重構(gòu)后的催化劑優(yōu)異的催化活性和穩(wěn)定性。本工作為設(shè)計(jì)高電流密度下具有優(yōu)異活性和穩(wěn)定性的OER催化劑提供了新的思路。

關(guān)鍵詞：析氧反應(yīng)；表面重構(gòu)；離子浸出；水分解；電催化；催化劑

中圖分類號：O643

Abstract： Surface reconstruction inevitably occurs during pre-catalysis forthe oxygen evolution reaction （OER）; however， obtaining OERelectrocatalysts with high performance and stability remains a challenge. Inthis study， we have developed a bimetallic leaching-induced surfacereconstruction strategy to fabricate efficient electrocatalysts for wateroxidation. Microcolumn arrays consisting of α-CoMoO4， K2Co2（MoO4）3，Co3O4， and CoFe2O4 four-phase oxides were integrated as pre-catalyst bya hydrothermal， ion-exchange， and subsequent annealing process. In situRaman spectroelectrochemical and ex situ X-ray diffraction （XRD） studiesrevealed that the rapid dissolution of the unstable component K2Co2（MoO4）3triggered the adaptive leaching of Mo and K， which accelerated the transformation of the surface-enriched α-Co（OH）2 tothe active phase of CoOOH at low voltage. Furthermore， the stable CoFe2O4 component couples the reconfigured newphase CoO with the amorphous layer CoOOH to form a compact hierarchical structure of CoFe2O4@CoO@CoOOH， whichplays the role of a nanofence and effectively prevents the catalyst from over-reconstruction， thus achieving excellentcatalytic stability. This work provides a novel idea for designing OER catalysts with excellent activity and stability at highcurrent densities.

Key Words： Oxygen evolution reaction; Surface reconstruction; Ion leaching; Water splitting; Electrocatalysis;Catalyst

1 Introduction

The sustainable development of human civilization faces amultitude of challenging global issues， which include but are notlimited to increasing energy demands， pollution， globalwarming， resource depletion， and rising sea levels 1. As thelargest consumer of energy， China is committed to achievingcarbon neutrality by 2060， aligning with the concept of a“community of human destiny” 2. By harnessing advancedmaterials and green technologies like electrocatalysis，sustainable renewable resources （e.g.， water， carbon dioxide，nitrogen and solar energy） can be converted into high valueaddedproducts （e.g.， hydrogen， oxygen， hydrocarbons andammonia）， thus offering a promising and effective solution tothese pressing problems 3. Electrocatalytic water splitting standsas an exceptionally effective approach for producing renewablehydrogen energy， thereby playing a vital role in reducingreliance on fossil fuels and mitigating greenhouse gasemissions 4. However， a significant obstacle in the process is thesluggish kinetics of the oxygen evolution reaction （OER）， whichsignificantly hampers the efficiency of electrochemicalhydrogen production 5. While Ir/Ru-based materials have beenrecognized as promising and efficient OER electrocatalysts， theyface practical limitations due to their low availability in Earth’scrust and high cost 6. Hence， it is imperative to develop costeffectivenon-precious metal-based electrocatalysts withoutstanding catalytic activity and long-term stability to addressthese formidable challenges.

In recent years， transition non-precious metal （e.g.， Ni， Co，Cu， Fe and Mo） compounds including metal oxides/hydroxides/hydroxy oxides 7， phosphides 8， sulphur compounds 9， etc. havebeen well developed and show high catalytic performances.Most of these compounds act as pre-catalysts， undergoingsurface reconstruction to form metal hydroxides/oxides duringoperation， which are considered the actual active phase inelectrocatalysis 10. These reconstructed catalysts show improvedOER catalytic performance compared to pre-catalysts and pureoxides/hydroxides. In situ surface reconstruction facilitateshigher specific surface areas and allows tuning of local atomicand electronic structures at active sites， optimizing theadsorption of reaction intermediates and accelerating the OERprocess 11. However， the reported surface reconstructions usuallyoccur at high anodic oxidation potentials in alkaline conditions，leading to the collapse of the pristine catalysts and lowutilization 12，13. What’s worse， most of the reported pre-catalystsare reconstituted to a depth of only a few nanometers， limitingthe content and reactivity of the “true” catalysts 14. Duringelectrochemical reconstruction， ion leaching behavior in the precatalystsaccelerates electrolyte penetration and enhancesfavorable OH– adsorption on the catalyst surface， facilitating thephase transition to the actual active phase 15. Therefore，electrochemically unstable components can be suitablyintroduced into the pre-catalysts for effective reconstruction. Onthe contrary， in case of severe dissolution of components，excessive catalyst reconstruction will destabilize the catalysts 16.Therefore， the challenge lies in appropriately triggering ionleaching to promote surface reconstruction while controlling thedepth of surface reconstruction to prevent damage to the maincatalyst structure and ultimately obtain OER catalysts with highcatalytic performance and stability.

Thermodynamically unstable high valence metal atoms （Mo）and low valence， highly soluble atoms （K） are prone to adaptiveleaching， forming metal cation vacancies during electrochemical water oxidation 7，17. Synthesizing precursors containing K， Mo，F(xiàn)e， and Co in a one-step process is challenging due to significantdifferences in the atomic sizes and valence states of Mo， K， andthe 3d metal elements （Fe and Co）. While by leveraging thedisparity in stabilization constants between metal ions andcyanide （CN?）， a Prussian blue-like cubic framework canfacilitate ion exchange， enabling rapid transformation. Inspiredby the above ideas， we report here a pre-catalyst by anchoringK2Co2（MoO4）3， Co3O4 and CoFe2O4 nanoparticles onto an α-CoMoO4 microcolumn array （denoted as KFCM-100）. Theintegrated electrode demonstrated excellent OER catalyticperformance in 1.0 mol?L?1 KOH. It exhibited lowoverpotentials for the OER at different current densities， withvalues of only 192 mV （at 10 mA?cm?2）， 233 mV （at 100mA?cm?2）， and 259 mV （at 500 mA?cm?2）. Furthermore， thecatalyst showed outstanding long-term stability， even at largecurrent densities of 300 mA?cm?2. In situ Ramanspectroelectrochemical and ex situ X-ray diffraction （XRD）studies revealed that during the electrochemical process， theunstable K2Co2（MoO4）3 underwent dissolution， and Mo and Kadaptively leach to form metal cation vacancies， opening thestructure and maximizing the contact area with the electrolyte.Moreover， K2Co2（MoO4）3 served as a “raw material warehouse”for α-Co（OH）2， accelerating the surface enrichment of α-Co（OH）2 and facilitating its rapid phase transition to CoOOH atlow operating voltages. Simultaneously， Co3O4 can be in situconverted to CoO during the OER process. The structurecharacterization results after the OER stability test show that theultra-wide amorphous reconstruction layer of CoOOH and thesub-internal CoFe2O4@CoO create a tight hierarchical structureof CoFe2O4@CoO@CoOOH， which acts as a “nanofence”preventing the severe reconstruction of the internal α-CoMoO4and resulting in exceptional catalytic stability.

2 Experimental section

The Supporting Information includes a comprehensivedescription of the chemical materials， physical characterizations，and electrochemical process employed in the synthesizedcatalyst.

2.1 Synthesis of CoMoO4?0.75H2O （CM）

A piece of nickel foam （NF） with a size of 2 cm × 4 cm wasinitially cleaned by immersing it in acetone， followed byanhydrous ethanol， and then deionized water for 15 min each，sequentially， to remove surface impurities. The cleaned NF wasthen vacuum-dried for later use. Meanwhile， a solution wasprepared by dissolving 700 mg of Co（NO3）2?6H2O and 741 mgof H24Mo7N6O24?4H2O in 60 mL of deionized water withcontinuous stirring for 15 min， resulting in a purple-red solution.The pretreated NF and the mixed solution were then combinedin a Teflon-lined stainless steel autoclave （100 mL） and heatedat 150 °C for 6 h. The obtained purplish-red CoMoO4?0.75H2Osample （referred to as CM） was subsequently washed withdeionized water and anhydrous ethanol before vacuum dryingfor further use.

2.2 Synthesis of CoMoO4?0.75H2O-Prussian blue-likeCoFe （CM-PBA-x， x = 20， 60， 100 or 140）composites

A solution was first prepared by dissolving 300 mg ofpotassium ferricyanide （K3[Fe（CN）6]） in 30 mL of deionizedwater under stirring at room temperature for 30 min. Theresulting solution was then combined with the CM sample in a50 mL Teflon-lined stainless steel autoclave and heated in anoven at 100 °C for 8 h. Subsequently， the autoclave was allowedto cool naturally to room temperature， resulting in the formationof a dark purple-black CM-PBA-100 sample. This sample wasthoroughly washed with deionized water and anhydrous ethanol，followed by drying in a vacuum oven at 60 °C.

Additionally， a series of control experiments were conducted，including material preparation at room temperature （20 °C） andhydrothermal treatments at temperatures of 60 and 140 °C.These control samples were designated as CM-PBA-20， CMPBA-60， and CM-PBA-140， respectively.

2.3 Synthesis of K2Co2（MoO4）3@CoFe2O4@Co3O4@α-CoMoO4-400 （KFCM-x， x = 20， 60， 100 or 140）

The prepared CM-PBA-x samples （x = 20， 60， 100， or 140）were annealed in air at 400 °C for 2 h with a heating rate of5 °C?min?1， resulting in the formation of KFCM-x （x = 20， 60，100， or 140）.

2.4 Synthesis of K2Co2（MoO4）3@CoFe2O4@Co3O4@α-CoMoO4-300 （KFCM-x-300， x = 20， 60， 100 or140）

The prepared CM-PBA-x samples （x = 20， 60， 100， or 140）were annealed in air at 300 °C for 2 h with a heating rate of5 °C?min?1， resulting in the formation of KFCM-x-300 （x = 20，60， 100， or 140）.

2.5 Synthesis of K2Co2（MoO4）3@CoFe2O4@Co3O4@α-CoMoO4-500 （KFCM-x-500， x = 20， 60， 100 or140）

The prepared CM-PBA-x samples （x = 20， 60， 100， or 140）were annealed in air at 500 °C for 2 h with a heating rate of5 °C?min?1， resulting in the formation of KFCM-x-500 （x = 20，60， 100， or 140）.

2.6 Synthesis of α-CoMoO4/β-CoMoO4 （CM-y，y = 300， 400 or 500）

The prepared CM sample were annealed in air at 300，400 or500 °C for 2 h with a heating rate of 5 °C?min?1， resulting in theformation of CM-y （y = 300， 400 or 500）.

2.7 Preparation of RuO2 electrocatalyst

For the preparation of control samples， a commercial RuO2supported on NF electrode was fabricated following thefollowing steps. 5.0 mg of RuO2 was first dispersed in a mixedsolvent containing 2.0 mL of 20.0 μL Nafion （5%， wt）， 1.0 mLof （CH3）2CHOH， and 980.0 μL of deionized water. The mixturewas subjected to ultrasonication for 30 min to create ahomogeneous ink. Subsequently， the homogeneous ink wasloaded onto the NF electrode with a loading of 2.0 mg?cm–2.

2.8 Preparation of catalyst-modified GC electrodes

The catalyst powders （KFCM-100， CM-PBA-100， CM andcommercial RuO2） were initially removed from the nickel foamsubstrate by scraping or sonication and subsequently dried.Different catalyst-modified GC-RDE （Glass Carbon-RotatingDisk Electrode） were served as the working electrode. Thefabrication of the modified electrode was carried out as follows：An appropriate quantity of catalyst was dispersed in 1 mL of amixed solution （Vabsolute alcohol ： Vdeionized water ： VNafion = 1 ： 1 ： 0.03）and subjected to ultrasonication for over 30 min until a uniformink was formed. Subsequently， an appropriate amount of the inkwas dispersed onto the surface of the polished electrode andallowed to dry naturally. The loading amount of the catalystmodifiedelectrode was about 0.28 mg?m?2.

3 Results and discussion

3.1 Synthesis and characterization ofelectrocatalysts

Fig. 1 illustrates the schematic process of fabricating the selfsupportingKFCM-100 pre-catalyst on nickel foam （NF）. Firstly，CoMoO4?0.75H2O precursor （denoted as CM） was in situ grownon nickel foam （NF） by a hydrothermal process. With this step，smooth microcolumns with diameter of 1–2 μm and length of 10μm was obtained， as depicted in the field-emission scanningelectron microscopy （FE-SEM） images （Fig. S1）. The CMmatrix microcolumns with high aspect ratios not only provide alarger specific surface area and abundant active centers forelectrochemical reactions， but also reduce bubble attachmentwith promote bubble release and electrolyte penetration andtransport. In the XRD pattern shown in Fig. S2， all diffractionpeaks consistent with the standard spectrum of the monocliniccrystal system CoMoO4?0.75H2O （PDF#97-015-3169）. The insitu ion exchange reaction was then carried out to form Prussianblue-like CoFe PBA （denoted as CM-PBA-100） on the surfaceof CoMoO4?0.75H2O. In contrast to the CM precursor， denseCoFe-PBA nanoparticles are embedded on CM nanoarrays，forming a rough surface （Fig. S3）. It is worth noting that cavitiesappeared on the top surface of CM-PBA microcolumns withhigher surface energies， which become more pronounced atreaction temperature exceeding 60 °C. These outcomes can beascribed to the greater surface dissolution of CoMoO4?0.75H2Oand its outward diffusion rate being greater than inward diffusionrates of K+ and [Fe（CN）6]3?， leading to the formation of cavitieson the top surface of the microcolumn 18. The XRD pattern inFig. S4 confirms that CM-PBA consists of CoMoO4?0.75H2Oand K2CoFe（CN）6. The Fourier-transform infrared （FT-IR）spectra （Fig. S5） show two absorption bands in the 2050–2200cm?1 range， which are attributed to the characteristic stretchingvibrations of ―CN 19. In addition， two peaks at 2096 and 2140cm?1 are clearly observed in all Raman spectra （Fig. S6）， whichcorrespond to the vibrations of the ―CN group 20， consistentwell with the XRD results. Finally， the CM-PBA-100 wasconverted into a KFCM-100 electrode consisting of multipleoxide nanoparticles tightly attached to the α-CoMoO4 backboneby calcination at 400 °C for 2 h under air atmosphere （denotedas KFCM-100）. The -CN group connecting Co and Fe atoms inCoFe-PBA is thermally decomposed with releasing CO2 andNOx gases， which may create pores as they escape 21. Nitrogenadsorption-desorption isotherms were employed to determinethe surface area and total pore volume of the samples. Brunauer-Emmett-Teller （BET） surface area analyses revealed that thespecific surface areas of CM， CM-PBA-100， KFCM-100-300，KFCM-100， and KFCM-100-500 are 2.930， 12.650， 13.879，10.542， and 8.960 m2?g?1， respectively （Fig. S7 and Table S1）.The decrease in specific surface area of KFCM-100 compared toCM-PBA-100 and KFCM-100-300 may be attributed tonanoparticle aggregation during high-temperature heattreatment 22. In addition， the increase in the number of poresbelow 5 nm and the widening of pore size distribution rangeimply the formation of pores inside the post-heat treatedsamples.

The morphology and structure of various types of catalystsafter heat treatment were analyzed by FE-SEM. Upon furtherheat treatment， the structure of the microcolumns remainsunchanged， but a substantial number of particles appeared on therough surface of the microcolumns. Such nanostructure showed an increased surface area with exposure of more active sites （Fig.2a and Figs. S8–11）. XRD was used to monitor the compositionand structural evolution of the products. The XRD patterns ofmaterials obtained at different heat treatment temperatures arepresented in Fig. 2b. The diffraction peaks of the obtainedmaterials can be well indexed to α-CoMoO4 （PDF#04-009-3678）， K2Co2（MoO4）3 （PDF#00-035-0070）， spinel Co3O4（PDF#04-001-8014）， and inverse spinel CoFe2O4 （PDF#97-018-4063）. It should be noted that heat treatment of CM-PBA-20 andCM-PBA-60 at 300 °C in an air atmosphere did not completelyremove the crystallization water in CoMoO4?0.75H2O （Fig.S12）. In addition， the Co3O4 phase was not detected in theannealed products of CM-PBA-20 and CM-PBA-60 even whenthe carbonization temperature was increased to 400 and 500 °C，which may be attributed to the low content of the Co3O4 phase 23.Under high-temperature hydrothermal conditions， the unstableC―N bond breaks， causing an increase in the pH of the aqueoussolution. Simultaneously， the rapid decomposition ofCoMoO4?0.75H2O generates an excess of Co2+， whichrecombines to form a small amount of Co（OH）2 in anatmosphere of water and CO2， and ultimately oxidizing duringthe high-temperature heat treatment to form spinelCo3O4 24. This hypothesis is supported by the presence of newcharacteristic Raman peaks attributed to β-Co（OH）2 and α-Co（OH）2， as shown in the Raman spectrum in Fig. S6 25. CMwas also subjected to heat treatment at various temperatures（Figs. S12d and S13）. When the heat treatment temperatureexceeded 400 °C， CoMoO4?0.75H2O could be converted to bothα-CoMoO4 and β-CoMoO4 （PDF#00-021-0868）， potentially dueto the introduction of foreign elements to change the phasetransition mechanism of CoMoO4?0.75H2O 18.

Raman spectroscopy was used to characterize the structure ofthe synthesized samples， as shown in Fig. 2c. The Raman peaksin the low-frequency region of 250–370 cm?1 are attributed tothe vibrations of MoO4 tetrahedra， while the peaks in the midfrequencyregion of 650–980 cm?1 are associated with theCo―O―Mo stretching mode 8. The presence of an energy bandat 700 cm?1 implies the existence of α-CoMoO4， representing thevibration of the Mo―O―Mo bridge with octahedralcoordination of molybdenum atoms 26. The Raman characteristicpeaks of the heat-treated samples show blue-shift as comparedto the CM precursor containing crystallization water， whichmay be due to the increase in vibrational frequency as a resultof lattice shrinkage and shortening of the bond lengths （Fig.S14） 27. However， no Raman signals of CoFe2O4 and Co3O4were detected， which may be related to the low contents ofCoFe2O4 and Co3O4 28.

Transmission electron microscopy （TEM） images （Fig. 2d）provided further insights into the structure， revealing thepresence of numerous voids on the surface of the KFCM-100microcolumns. Fig. 2e and the magnified HR-TEM image of thecorresponding region reveal the presence of multiple sets of lattice fringes （f1， f2， g1， and g2， Figs. 2f，g） for KFCM-100. Byutilizing fast Fourier transform diffractograms （FFT）， inversefast Fourier transform images （IFFT） and lattice distance profilesof the corresponding regions， the well-defined lattice spacings of0.326， 0.297， 0.289， and 0.252 nm were obtained， which areassigned to the （2 12） and （0 1 6） planes of K2Co2（MoO4）3， （11 2） plane of Co3O4 and （3 1 1） plane of CoFe2O4， respectively.The bright annulus observed in the selected area electrondiffraction （SAED， Fig. 2h） pattern is associated withK2Co2（MoO4）3， Co3O4， and CoFe2O4， indicating the mixed andpolycrystalline texture of KFCM-100. The high-angle annulardark-field scanning transmission electron microscopy （HAADFSTEM）images further confirm the uniform distribution of theelements of K， Fe， Co， Mo， and O （Fig. 2i）. Energy dispersiveX-ray spectroscopy （EDS） analysis revealed that KFCM-100consists of the following elements with atomic percentages： K（7.10%， atomic fraction）， Fe （3.05%）， Co （16.77%）， Mo（28.05%）， and O （40.17%） （Fig. S15）.

X-ray photoelectron spectroscopy （XPS） tests were carriedout to further elucidate the changes in chemical composition andchemical valence of the synthesized samples. XPS confirmed thepresence of K， Fe， Co， Mo and O elements in the KFCM-100catalyst （Fig. 3a）， in agreement with the EDS results. Note thatthe signal of Ni comes from the debris of Ni accidentally obtained by stripping the sample powder on the nickel foam. Inthe K 2p region of KFCM-100 （Fig. 3b）， the peaks at 293.78 and296.48 eV are attributed to K 2p3/2 and P 2p1/2 of K2Co2（MoO4）3，respectively， which are in agreement with the results of the XRDanalysis. The corresponding high-resolution Fe 2p spectra （Fig.3c） were deconvoluted into three spin-orbit doublet peaks， wherethe peaks at 709.50/722.34 eV and 711.60/724.70 eV wereassigned to species bonded to Fe2+ and Fe3+， respectively. Thedoublet peak at 717.78/729.00 eV was attributed to thevibrational satellite peak 20，29. The peak appearing near 713.18eV is attributed to the linear superposition of Auger peaks of Coand Ni 29，30. Notably， the binding energy of Fe in KFCM-100positively shifts by 0.3 eV compared to CM-PBA-100， which isconsistent with the results of the formation of the CoFe2O4phase. Similarly， the Co 2p high-resolution XPS spectrum ofKFCM-100 in Fig. 3d reveals the co-existence of Co2+ and Co3+.Specifically， the peaks of Co2+ are located at 782.38 and 797.53eV， and the peaks of Co3+ are located at 780.23 and 796.13 eV.The presence of Co3+ confirms the formation of Co3O4 inKFCM-100. In the Mo 3d high-resolution XPS spectrum （Fig.3e）， the valence state of the Mo element in KFCM-100 wasdetermined to be +6. In addition， the binding energies of Co 2pand Mo 3d in CM-PBA-100 negatively shift with respect to theCM precursor， which is indeed related to the ion-exchangeinteraction between CoMoO4?0.75H2O and K3[Fe（CN）6]. The O1s spectra obtained from the KFCM-100 sample show twocharacteristic peaks （Fig. 3f）， corresponding to lattice oxygen（530.18 eV） and surface-adsorbed oxygen （532.28 eV），respectively 31. The presence of OH? was detected in the O 1sspectrum of the CM-PBA-100 sample， further confirming that asmall amount of Co（OH）2 was formed on the surface of CMPBA-100.

3.2 Evaluation of the Electrochemical OERPerformance

The OER catalytic performances of the different catalystswere investigated using a standard three-electrode system in 1.0mol?L?1 KOH. Prior to electrochemical measurements， theHg/HgO reference electrode was calibrated versus the reversiblehydrogen electrode （RHE） （Fig. S16）. For the OER tests， alllinear scanning voltammetry （LSV） curves were recorded bynegative potential sweep at a scan rate of 1 mV?s?1 to minimizethe capacitive current background to avoid possibleoverestimation of performance. It is worth noting that all LSVcurves were obtained after repeated sweep linear voltammetry（CV） cycles to obtain stable curves， thus ensuring completephase transition and surface reconstruction. As depicted in Fig.4a， LSV polarization curves of the catalysts indicate that KFCM-100 exhibits significantly higher OER catalytic activitycompared to NF， commercially available RuO2， precursor CM，CM-400 and CM-PBA-100. The KFCM-100 requires only 192，233， and 259 mV of overpotential to achieve current densities of10， 100， and 500 mA?cm?2， respectively （Fig. 4b）. Themacroscopic internal surface area of nickel foam may be manytimes larger than the geometrical internal surface area， and theGC electrode was chosen as a substrate due to its good electricalconductivity， very low surface area， and low background current，which can reflect the intrinsic activity of the loaded catalysts 6.Therefore， the OER electrocatalysis was also carried out byusing catalyst-modified GC electrode. As shown in Fig. S17， theoverpotential of KFCM-100@GC for obtaining a current densityof 10 mA?cm–2 is 270 mV， which is still better than that of CMPBA-100@GC （298 mV）， CM@GC （307 mV）， and commercial RuO2@GC （308 mV）. Obviously， self-supported electrodeshave the advantages of high specific surface area， tight bindingto the catalyst， and no need of polymer binders （Nafion）compared to conventional powder electrocatalysts. It is worthnoting that the OER performances of the samples with heattreatedat different temperatures follow the same result： the bestOER performance was obtained for the sample that heat-treatedat 400 °C （Fig. S18）. This can be attributed to the fact that highoxygen content during high-temperature annealing repairsoxygen vacancies or reduces surface defects， which areresponsible for increasing catalytic activity 32，33.

To evaluate the OER kinetics of the catalysts， the Tafel slopeswere calculated using the Tafel equation （η = a + b log（j）， whereη is the overpotential， j is the current density， and b is the Tafelslope）. As shown in Fig. 4c， KFCM-100 exhibits the smallestTafel slope of 41.51 mV?dec?1， which is significantly smallerthan those of CM （83.54 mV?dec?1）， CM-400 （58.34 mV?dec?1），CM-PBA-100 （55.29 mV?dec?1）， NF （154.71 mV?dec?1）， andRuO2/NF （97.16 mV?dec?1）. By comparing the Tafel slopes ofKFCM-20 and KFCM-60， it can be observed that theintroduction of Co3O4 can effectively enhance the OER kineticprocess （Fig. S19）. Impressively， the KFCM-100 demonstratescompetitive alkaline OER performance among the reported nonpreciousmetal OER catalysts （Table S2）.

The competition between methanol oxidation reaction （MOR）and OER is evident， and researchers have employed methanol asa molecular probe to measure and compare the yield andadsorption strength of OH* 34. The current difference betweenOER and MOR may in some cases imply a moderate strength ofOH* adsorption， as both reactions involve the adsorption ofhydroxide ions （OH*） on the electrode surface. A small currentdifference suggests that they have similar current densities in thepotential range for both reactions， which implies a relativelymoderate adsorption strength of OH* in both reactions sinceOH* adsorption can influence both. If the OH* adsorption is toostrong， it may limit the precipitation of oxygen and the oxidationof methanol， resulting in a larger current difference. Accordingto Sabatier’s principle， the moderate OH* adsorption isfavorable for the OER performance 35，36. The overpotential ofKFCM-100 exhibits a tendency to decrease and then increase，and the current difference between OER and MOR is thesmallest， indicating a moderate OH* adsorption strength （Fig.S20）.

The double-layer capacitance （Cdl） was estimated using CVmethod in the non-Faraday region to compare theelectrochemically active surface area （ECSA） of the catalysts.The results indicated that KFCM-100 possesses the largest Cdlvalue of 2.81 mF?cm?2， surpassing those of CM （0.80 mF?cm?2），CM-400 （0.85 mF?cm?2）， and CM-PBA-100 （1.00 mF?cm?2），RuO2 （0.69 mF?cm?2） and pristine NF （0.38 mF?cm?2）. Thisresult indicates that KFCM-100 possesses the largestelectrochemically active surface area and the largest number ofpotential active sites （Figs. S21–S23）.

Electrochemical impedance spectroscopy （EIS）measurements were conducted to gain insights into theelectrode/electrolyte interfacial dynamics during OER in thepresence of different catalysts. Figs. 4d and S24–S26 depict theNyquist plots of the catalysts at 1.48 V vs. RHE. These plots canbe fitted by a series connection of a solution resistor （Rs） withtwo R-CPE units. Each R-CPE unit comprises a charge-transferresistor （Rp or Rct） and a constant-phase element （CPE1 or CPE2）connected in parallel （equivalent circuits can be found in theinsets of Figs. 4d and S24–S26; Table S3 for fitted parameters）.The high-frequency arc （Rct） and the low-frequency arc （Rp）represent two resistances observed at the electrochemicalinterface. They are related to interfacial charge transfer andadsorption of reaction intermediates during OER. Obviously， thelatter is a much slower process than the former. The Rp resistanceis attributed to the electron transfer through the catalyst to thesubstrate electrode， while the larger charge transfer resistance Rctcorresponds to the electrochemical oxidation of water byelectron transfer at the liquid-solid interface. Thus， the KFCM-100 possesses the smallest charge transfer resistance of about1.361 Ω with excellent charge transfer kinetics， which isconsistent with its corresponding low overpotential and smallTafel slope.

Long-term durability is an indispensable indicator forassessing catalyst performance. In this regard， the multi-stepchronopotentiometric curve of KFCM-100 electrode wasmeasured （Fig. 4e）， where the current density changes from 100to 500 mA?cm?2 and then returned to 100 mA?cm?2 withnegligible voltage change at each stage. Furthermore， the OERelectrocatalytic stability of KFCM-100 was assessed usingchrono-potentiometric （CP） curve tests. CP tests were performedin 1.0 mol?L?1 KOH solution for 50 h at a current density of 300mA?cm?2. As shown in Fig. 4f， the voltage of the KFCM-100sample showed almost no decay during the CP test. Moreover，the LSV polarization curves of KFCM-100 before and aftertesting almost overlap （Fig. 4f inset）， indicating that KFCM-100has excellent OER performance and long-term stability.

3.3 Exploration after stability testing

The morphology and structure of KFCM-100 were furtherinvestigated after 50 h of OER stability testing at 300 mA?cm?2anode current density to probe the origin of its excellent OERactivity and stability. Here， the SEM， XRD， TEM and XPSanalyses were performed. Following the extended OER stabilitytest， the one-dimensional nanorod structure of KFCM-100 wasmaintained， indicating its excellent stability， even though thecatalyst’s surface underwent some reconstruction and oxidation（Fig. S27）. TEM and HR-TEM images after OER （Fig. S28） alsoshow the well-preserved microrod morphology of KFCM-100，with the formation of an ultra-wide amorphous reconstitutedactive layer of 50–80 nm on the surface. The ultra-wideamorphous reconstituted active layer provides an “armor” insidethe catalyst to protect it from excessive corrosion 37，38. Thecontinuous and diffuse rings observed in the constituent electron diffraction patterns indicate the formation of stable materialunder alkaline and oxidizing conditions （Fig. S28c）. Thecorresponding HAADF-STEM images show low content signalsof elements K and Mo due to their leaching duringelectrooxidation （Fig. S28d–i）. Furthermore， the EDS elementalanalysis as well as semi-quantitative analysis based on XPSresults （Fig. S29 and Table S4） also indicate the leaching of Kand Mo from KFCM-100 during the OER. The high-resolutionXPS of K， Fe， and Mo further confirms the good electronicstructure stability of KFCM-100 （Fig. S30）. The increased Co3+content in the samples after long time stability test is related tothe formation of CoOOH during the OER process （Fig. S30c） 39.In addition， the O 1s spectra showed a greatly enhancedhydroxide peak， supporting the formation of M-OOH （Fig.S30e） 40. The XRD of the post-OER samples showed thegeneration of a new phase of CoO， whereas the diffraction peaksignals originally belonging to K2Co2（MoO4）3 disappeared as aresult of the severe leaching during the electrochemical process（Fig. S31）. Under alkaline conditions， CoO is formed in KFCM-100 through a structural transformation of Co3+ from Co3O4 andserves as one of the primary active sites in the OER reaction， asconfirmed by relevant studies 41. Surprisingly， the CoFe2O4 andα-CoMoO4 compositions in KFCM-100 still exist even after the50 h OER test at a high current density. Consequently， in situelectrochemical-Raman was further performed to investigate thereason for the no decrease in OER activity and the increase instability after the formation of hydroxyl oxide layer.

A specially designed device， coupled with an in situelectrochemical-Raman system， was employed to monitor thereal-time evolution of KFCM-100 during the OER in a liquidelectrolyte （Figs. S32 and 5）. At open circuit voltage （OCP），Raman signals were observed to undergo a blueshift anddecrease in intensity due to the reduced laser power andspontaneous etching of the samples in the electrolyte 42，43. Asshown in Fig. 5a，d， the CM exhibits a typical CoMoO4?0.75H2ORaman characteristic peak， which remains almost unchanged asthe potential increases from OPC to 1.43 V， except for a gradualdecrease in its intensity. In the case of CM-PBA-100， at voltagesbelow 1.43 V， new peaks appeared at 477， 502， and 595 cm?1，attributed to the formation of β-Co（OH）2 44，45， which isassociated with the transformation of CoFe PBA in alkalineconditions （Fig. 5b，e）. When the applied potential increased fromOCP to 1.23 V for KFCM-100， a significant new peak at around895 cm?1 emerged， attributed to the dissolution of unstableK2Co2（MoO4）3 （Fig. 5c，f） 46，47. It is noteworthy that the leachingof K2Co2（MoO4）3 was accompanied by the formation of two newbands at 523 and 633 cm?1， corresponding to the Co-OHvibrations in α-Co（OH）2 25. Obviously， alkaline electrolyte willtrigger the rapid leaching of K2Co2（MoO4）3 from the catalyst，leading to the rapid disintegration of the Co-MoO4 ligand. When active Co2+ species diffuse outward from the interior of thecatalyst and are exposed to the alkaline electrolyte， they rapidlybind to OH– ions and eventually enriched α-Co（OH）2 is formedon the outer surface of KFCM-100. At the bias voltage of 1.33V， the characteristic peak of α-Co（OH）2 disappears and twocharacteristic peaks of CoOOH at 463 and 535 cm?1 appear，which indicates that Co2+ （α-Co（OH）2） begin to beelectrooxidized to Co3+ （CoOOH） 48. In contrast， thecharacteristic peaks of CoOOH are only observed at 1.43 V forCM and CM-PBA-100， suggesting that the K2Co2（MoO4）3leaching effectively promotes the in situ oxidation of KFCM-100 to CoOOH at low working potentials. Surprisingly， theRaman peak at 895 cm?1 did not disappear immediately with theappearance of CoOOH， but persisted until the bias voltagereached 1.53 V.

In addition， we utilized ex situ XRD technique to study thephase evolution of KFCM-100 during the OER. To activate theKFCM-100， CV scanning was conducted in a typical threeelectrodesystem in the potential range from 0.91 to 1.71 Vversus the reversible hydrogen electrode （RHE）， with a scan rateof 100 mV?s?1 without iR compensation. As shown in Fig. S33，the KFCM-100 exhibits a dynamic self-optimization processthroughout the CV cycles with surface reconstruction andgeneration of new active species. K2Co2（MoO4）3 was notdetected in the XRD spectra of KFCM-100 after differentnumbers of CV cycles， suggesting its rapid leaching during theOER process （Fig. S34）. In addition， XRD spectra of KFCM-100 were collected after immersion in a fresh 1.0 mol?L?1 KOHelectrolyte solution for 24 h （Fig. S35）. The disappearance of theK2Co2（MoO4）3 phase and the newly generated Co（OH）2 phasesuggest that the unstable K2Co2（MoO4）3 in KFCM-100spontaneously leached out， leading to the enrichment ofCo（OH）2 on the catalyst surface and accelerating surfacereconstruction. Such result is consistent with those of the in situelectrochemical-Raman analysis.

Based on the above analysis， a schematic diagram on theformation of ion leaching-induced surface reconstruction ofKFCM-100 was proposed （Fig. S36）. It can be proposed that thethermodynamically unstable high-valent Mo atoms and lowvalentsoluble K atoms open up the structure of the catalystthrough the vacancies formed by adaptive leaching during theOER process， which facilitates the penetration of the electrolyteand OH– adsorption. Simultaneously， K2Co2（MoO4）3 can beused as the main “raw material warehouse” for α-Co（OH）2， i.e.，the internal Co-MoO4 coordination is rapidly broken， and theexposed Co2+ quickly combines with OH– in the solution to formsurface-enriched α-Co（OH）2， which is conducive to the rapidphase transition to active CoOOH. Accompanied by the leachingof K2Co2（MoO4）3， the amorphous CoOOH broadenscontinuously， which provides a corrosion-resistant “armor”inside the catalyst， which may be the reason for the retention ofCoFe2O4 and α-CoMoO4 in KFCM-100 after long-term stabilitytest. Meanwhile， the stabilized nanoscale CoFe2O4 component and the new phase CoO are distributed on the near surface of theα-CoMoO4 microcolumns， forming a compact hierarchicalstructure of α-CoMoO4@（CoFe2O4@CoO）@CoOOH. Thetightly connected （CoFe2O4@CoO）@CoOOH acts as a“nanofence” to prevent the internal α-CoMoO4 from leachingout excessively during the reconfiguration process， thusimproving the catalyst stability. This work provides ideas for thedevelopment of OER catalysts with excellent stability at largecurrent densities.

4 Conclusions

In summary， we developed an electrocatalyst by integratingK2Co2（MoO4）3， Co3O4 and CoFe2O4 components on α-CoMoO4microcolumns， which result in a great enhancement of the OERperformance. Precise control of the catalyst’s morphology andcomposition was achieved by adjusting the hydrothermal andannealing temperatures. The KFCM-100 electrocatalystdemonstrated outstanding OER activity， achieving a high currentdensity of 500 mA?cm–2 with an overpotential of 259 mV， as wellas impressive long-term stability， maintaining its performancefor 50 h at 300 mA?cm?2. In situ electrochemical Ramanspectroscopic studies revealed the adaptive leaching ofpotassium （K） and molybdenum （Mo） from KFCM-100 duringthe OER process， which promoted surface reconstruction. Thenative CoFe2O4 component coupled with the reconstituted CoOand the indeterminate layer CoOOH prevented the severereconstruction of the KFCM-100 catalyst. This work opens anew avenue for in situ integration of multiple actives on a 3Dplatform with open conductive networks for the design andfabrication of integrated cost-effective and high-performanceelectrocatalysts for energy conversion.

Conflicts of Interest： There are no conflicts of interest to declare.

Author Contributions： Methodology， Investigation，F(xiàn)ormal analysis， Data curation， Resources， Writing-originaldraft： Wentao Xu; Investigation， Validation， Formal analysis：Xuyan Mo; Investigation， Validation： Yang Zhou， Yanhua Wu，Dan Li， Zuxian Weng， Tangqi Lan and Kunling Mo;Investigation： Xinlin Jiang and Huan Wen; Supervision，F(xiàn)unding acquisition： Youjun Fan; Supervision， Writing-reviewand editing， Funding acquisition： Fuqin Zheng and Wei Chen.

Supporting Information： available free of charge via theinternet at http：//www.whxb.pku.edu.cn.

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廣西自然科學(xué)基金（2019GXNSFGA245003， 2021GXNSFBA220058）， 國家自然科學(xué)基金（22002026， 22272036）， 廣西科技基地與人才課題（桂科AD23026272）和廣西師范大學(xué)科研基金（2022TD）資助項(xiàng)目