Controlling the Morphology of CuO to Construct Rich Cu⁰/Cu⁺ Interfaces for CO₂ Electroreduction to Multi-Carbon Products
Authors
Weiren Chen¹ᐟ², Xixiong Jin¹ᐟ², Min Wang¹ᐟ², Bohan Ai¹ᐟ², Zixuan Wei¹ᐟ², Guobao Jiang³, Hongqi Shi³, Lingxia Zhang¹ᐟ²ᐟ⁴
- State Key Laboratory of High Performance Ceramics, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, P.R. China
- Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, P.R. China
- Wuhan Second Ship Design and Research Institute, Wuhan, P.R. China
- School of Chemistry and Materials Science, Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences, Hangzhou, P.R. China
Correspondence: Xixiong Jin (jinxixiong@mail.sic.ac.cn), Hongqi Shi (flags_s@163.com), Lingxia Zhang (zhlingxia@mail.sic.ac.cn)
Received: 29 September 2025 | Revised: 8 December 2025 | Accepted: 16 December 2025
Published in: Advanced Functional Materials, 2026; 36: e26003
DOI: https://doi.org/10.1002/adfm.202526003
Keywords
CO₂ electroreduction | Cu⁰/Cu⁺ interfaces | CuO | morphology control | multi-carbon products
Abstract
ItremainsasignificantchallengetoelectrocatalyticallyconvertCO 2 intomulti-carbon(C 2+)productswithbothhighselectivity andindustrial-gradecurrentdensity.Herein,wereportapH-directedstrategyforgrowingCuOcatalystswithpreciselycontrolled morphologies.Amongthem,nanorod-likeCuO(R-CuO)achievesanexceptionalperformancewithaFaradaicEfficiencyof84.0% at500mA⋅cm−2towardC products.X-rayphotoelectronspectroscopyrevealsthatduringelectrochemicalCO reductionreaction 2 2 (CO RR),R-CuOpreservestherichestsurfaceCu+ species(73.8%),therebyfurnishingabundantCu0/Cu+ interfacesstemming 2 fromtheunique“fragmentation-agglomeration”reconstructionprocess.XASresultsdisclosealowcoordinationnumber(CN= 8.71)fortheCu─CuscatteringpathandahighCN(0.72)fortheCu─Opathinpost-reactionR-CuO,underscoringitsdefective structureandCu+ retention.InsitucharacterizationsfurtherrevealthattherobustCu0/Cu+ interfacesmarkedlyenhancethe adsorptionofOH−andCOintermediate,acceleratingtherate-determiningC─CcouplingbetweenCOand*COHintermediates anddrivingselectiveC formation.Therefore,wehaveestablishedarelationshipbetweenthemorphologyofCuOandthepivotal 2 Cu0/Cu+interfaces,providingarationalstrategyfordynamicallytailoringinterfaceevolutiontowardselectiveC productionfrom 2 CO RR. 2
1. Introduction
andfabricationofhighlyefficientelectrocatalystsfortheCO RR 2 isacriticalmission. TheelectrochemicalCO reductionreaction(CO RR),whichcan 2 2 directlytransforminertCO moleculesintovalue-addedproducts Cu-basedcatalystshavebeenattheforefrontofCO electroreduc- 2 2 likecarbonmonoxide(CO),formate(HCOO−),andmulti-carbon tiontoC 2+productseversincethepioneeringworkbyHorietal. (C 2+) compounds, has emerged as an effective means for the [1]whichoffermoderateadsorptionenergyfor*COintermediate, utilizationofCO 2 .Itnotonlyhelpsalleviatetheenvironmental promotingtheC─CcouplingprobabilitytoproduceC 2+products, crisisbutalsoenablestheproductionofgreenfuelsandchemicals other than forming CO in the early stage [2, 3]. However, the [1–5]. For the realization of large-scale applications, the design moderate *H binding strength simultaneously intensifies the ©2025Wiley-VCHGmbH AdvancedFunctionalMaterials,2026;36:e26003 1of11 https://doi.org/10.1002/adfm.202526003
competitionofthehydrogenevolutionreaction(HER).Theoreti- andgrowthprocessofCuOnanocrystals(SchemeS1andFigures calcalculationsconductedbyXiaoetal.revealedthatthesurface S1andS2,SupportingInformation)[11,12].NaOHandNH ⋅H O 3 2 Cu+ species markedly lowers both kinetic and thermodynamic rapidly react with Cu2+ to form Cu(OH) and Cu(NH ) 2+, 2 3 4 barriers to CO activation, leading to a synergistic effect with respectively(FigureS3).Incontrast,theintroductionofHMTA 2 adjacentCu0 speciesthatacceleratesC─Ccoupling[4].Hence, intotheCu2+ precursorsolutionleadstoslightturbiditydueto theconstructionandstabilizationofCu0/Cu+ interfacesduring the formation of the Cu(HMTA) 2+ complex. Subsequently, all 4 CO RR has become a pivotal challenge to steer the selectivity threesolutionsweretransferredtoan85◦Cwaterbathforfurther 2 towardC 2+products. thermaldecomposition. Toaddressthisproblem,variousstrategieshavebeendeveloped, When NaOH is used as the precipitant, sufficient Cu(OH) 2 including protective species introduction [5, 6], pulsed voltage nanowires are directly formed and thermally decomposed into application[7,8],orself-sacrificialsiteconstruction[9,10].For CuO nanorods. However, an excess of OH− ions neutralizes instance,Wangetal.stabilizedCuδ+ sitesandsuppressedHER the surface charge of the formed CuO nanorods, resulting in by encapsulating Cu nanoparticles in a hydrophobic SiO layer grain aggregation. Concurrently, a diffusion layer of OH− ions 2 [5]; Xu et al. maintained Cu+ species via pulsed electrolysis, formsonthesurfaceofthenanorods,whichrestrainsanisotropic achieving a C 2+ Faradic Efficiency (FE C2+) of 81.2% [7]; Jiang grain growth and promotes preferential growth along lower- etal.introducedI toformCuI,establishingaredoxcyclethat energy crystal planes, such as CuO (002). Eventually, the CuO 2 enabled a FE C2+ over 70% in acidic electrolyte [10]. Despite nanorods stack in a 2D manner, leading to the formation of a theirefficacy,theseapproachesoftenrelyonexternalmodifiers nanosheet-like structure (denoted as S-CuO). This morphology orcomplexoperationconditions,potentiallycompromisingthe is corroborated by scanning electron microscope (SEM) and intrinsicpropertyofthecatalyst. transmission electron microscope (TEM) images (Figure 1a,d; FigureS4)[13]. Herein, to overcome this limitation, we propose an intrinsic- design strategy that engineers the architecture of the catalysts In the case of NH ⋅H O, the ammonia ligands in Cu(NH ) 2+ 3 2 3 4 todrivethespontaneous,self-sustainingformationofabundant are gradually replaced by OH− ions released from NH ⋅H O 3 2 Cu0/Cu+ interfaces during CO RR. Three distinct CuO cata- at elevated temperatures. The initial deficiency of OH− ions 2 lystswithcontrolledmorphologies(nanosheet,nanoflower,and promotestheaggregationofgrainsintolargercrystalnuclei.As nanorod)weresynthesizedbyadjustingthepHoftheprecipitant thereactionproceeds,therisingconcentrationofOH−facilitates solution.Remarkably,nanorod-likeCuO(R-CuO)withpreferen- the outward growth of nanosheets similar to S-CuO from the tiallyexposedCuO(111)crystalplane,achievinganoutstanding crystal nuclei, ultimately yielding a nanoflower-like structure FE of84.0%towardethylene,ethanol,andaceticacidproducts (denotedasF-CuO)(Figure1b,e;FigureS5). C2 at an industrial-grade current density of 500 mA⋅cm−2. High- resolution transmission electron microscopy (HRTEM) images When HMTA is employed, the release of OH− ions pro- reveal that R-CuO undergoes a distinctive “fragmentation- ceeds much more slowly. Initially, HMTA undergoes gradual agglomeration” reconstruction process. This structural evolu- thermal decomposition upon heating, releasing CHCO and tion during CO RR exposes large number of active sites, with NH [14]. The NH then dissolves in water to generate OH− 2 3 3 the largest electrochemical active surface area (ECSA) (271.4) ions. The newly formed Cu(OH) quickly decomposes into 2 among these three CuO catalysts. Furthermore, in situ Raman CuO grains under elevated temperature and limited OH− sup- spectroscopy confirms that R-CuO has been reconstructed to ply. These CuO grains act as substrates for the continued form Cu0/Cu+ interfaces, which remain highly stable during deposition of Cu(OH) , eventually resulting in a nanorod- 2 CO RR,therebyenhancingtheC─Ccoupling.X-rayabsorption like morphology (denoted as R-CuO) (Figure 1c,f; Figure S6 2 spectrum (XAS) of R-CuO after CO RR further reveals a low and Scheme S4). This synthesis method focuses on the estab- 2 coordinationnumber(8.71)fortheCu─Cuscatteringpath,which lishment of a clear relationship between the release kinet- promotes the adsorption of the CO intermediate. Moreover, ics of OH− ions of precipitants and the final nanostructure in situ attenuated total reflection Fourier transform infrared morphology. (ATR-FTIR)spectroscopyconfirmsthattheabundantCu0/Cu+ interfacesonR-CuOconsiderablyacceleratetherate-determining Throughout the synthesis, PEG-6000 (polyethylene glycol) was step,thecouplingbetweenCOand*COHintermediates,which introducedasasurfactant.ThePEGmonomer(─CH ─CH ─O─) 2 2 accounts for the high selectivity toward C products. These forms chain structures that adsorb onto crystal nuclei and 2 findings offer valuable insights for morphological engineering effectivelyseparateCuOnanoparticles[15,16].SEMimagesreveal of electrocatalysts and the regulation of triple-phase interface that R-CuO synthesized without PEG-6000 exhibits significant evolutionduringCO RR. aggregation(FigureS7),underscoringtheessentialroleofPEG- 2 6000inobtainingsmall-sizedcatalysts. 2 MaterialsandMethods HR-TEM analysis shows that the nanosheets of S-CuO exhibit latticefringeswithaspacingof0.258nm,correspondingtothe
2. Results and Discussion
2.1 Material Synthesis and Characterizations
CuO (002) crystal plane (Figure S8). In contrast, the epitaxial nanosheetsinF-CuOarecharacterizedbythecoexistenceofboth Three precipitant solutions with progressively decreasing CuO (002) and (111) crystal planes (Figure S9). Meanwhile, the pH (controlled using NaOH, NH ⋅H O, and hexam- epitaxialnanorodsinR-CuOhaveadiameterofapproximately 3 2 ethylenetetramine) were employed to regulate the nucleation 8 nm and display uniform lattice fringes with an interplanar 2of11 AdvancedFunctionalMaterials,2026 16163028, 2026, 32, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202526003 by Zhejiang University Of Technology, Wiley Online Library on [12/05/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
FIGURE 1 SEMimagesof(a)S-CuO,(b)F-CuO,and(c)R-CuO.TEMimagesof(d)S-CuO,(e)F-CuO,and(f)R-CuO.(g–i)HR-TEMimagesof R-CuO(inset:SAEDpattern).(j)Enlargedlocalimagewithintheyellowboxandits(k)fastFouriertransform(FFT)pattern.(l)XRDpatterns,(m)N 2 adsorption–desorptionisotherms(inset:curvesofporesizedistribution)and(n)AugerCuLMMspectraofallthreeCuOcatalysts. distance of 0.218 nm, assigned to the CuO (111) crystal plane X-raydiffraction(XRD)patternsconfirmthatallthreesamples (Figure 1g–j; Figure S10). These results demonstrate that the consistexclusivelyofthemonoclinicCuOphase(PDF#45-0937), concentrationofOH− ionsduringsynthesisplaysacriticalrole with diffraction peaks at 35.5◦, 38.7◦, and 46.2◦assigned to the in determining the morphology and dominant crystal facets of (002), (111), and (-112) crystal planes, respectively (Figure 1l). theresultingCuOnanocrystals.AsshowninFigureS11,R-CuO Notably,thepreferentiallyexposedcrystalplaneshiftsfrom(002) displays the highest packing density and the smallest particle to(111)asthepHoftheprecipitantsolutiondecreases,consistent diameter with an average diameter of approximately 400 nm. withtheHR-TEMimages. AdvancedFunctionalMaterials,2026 3of11 16163028, 2026, 32, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202526003 by Zhejiang University Of Technology, Wiley Online Library on [12/05/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
Furthermore, the exposure of the CuO (111) crystal plane, concentration,prospectivelyservesasahighlyeffectivecatalyst which possesses higher surface energy compared to the (002) for promoting CO adsorption, diffusion, and activation during 2 crystal plane, leads to increased surface defects and the poor- CO RR. 2 est crystallinity in R-CuO. The CuO structure of all three catalysts is further supported by Raman spectroscopy, which shows characteristic peaks at 276, 321, and 610 cm−1. These
2.2 Catalytic Performance on CO₂RR
2 correspondtotheA (Cu─Osymmetricstretching),B (Cu─O g g1 bending), and B (Cu─O asymmetric stretching) vibrational The CO RR performance of all three CuO catalysts (S-CuO, g2 2 modes,respectively(FigureS12)[16,17].Additionally,FTIRspec- F-CuO, and R-CuO) was evaluated using a flow-cell setup. troscopyrevealsabroadbandaround3340cm−1,attributedtothe Linear sweep voltammetry (LSV) curves show that all cata- O─H stretching vibration of surface-adsorbed water molecules lysts exhibit an onset potential of −0.25 V vs. RHE in a CO 2 (FigureS13)[18]. atmosphere (Figure 2a). Among them, R-CuO achieves the highest current density under identical conditions, reaching The N adsorption–desorption isotherms reveal that R-CuO 530 mA⋅cm−2 at 1.75 V vs. RHE, indicating superior intrinsic 2 possessesthelargestBrunauer-Emmett-Tellersurfacearea(S ) conductivitythatpromoteselectrontransferandenhancesCO BET 2 of 49.88 m2⋅g−1, which is 3.0 and 4.5 times those of S-CuO activation during CO RR. By contrast, under an Ar-fed envi- 2 (16.84m2⋅g−1)andF-CuO(11.19m2⋅g−1),respectively(Figure1m). ronment, the onset potentials shift negatively to −0.5 V vs. This largely enhanced S likely stems from the abundance RHE and the current density decreases significantly, suggest- BET of small CuO crystal nuclei formed during the synthesis of ing a stronger tendency for CO RR over the competing HER 2 R-CuO. The pore size distribution further indicates that R- (FigureS18). CuO exhibits a microporous structure with pore sizes around 0.5 nm, presumably originating from the interstices between Under constant current test, R-CuO demonstrates exceptional the nanorods, whereas S-CuO and F-CuO lack such porosity. CO RRperformance.TheFaradaicEfficiency(FE)forC prod- 2 2 Additionally, CO -temperature programmed desorption (CO - uctsincreasessteadilyfrom59.9±1.3%to84.0±2.0%asthetotal 2 2 TPD)confirmsthatR-CuOexhibitsthehighestCO adsorption currentdensityrisesfrom100to500mA⋅cm−2(Figure2b;Figure 2 capacity among the three catalysts (Figure S14). The com- S19andTableS1).TheFEsforindividualC products,including 2 bination of the highest S and the presence of abundant C H , CH CH OH, and CH COOH, reach 63.3 ± 2.1%, 17.4 ± BET 2 4 3 2 3 microporespromotesefficientadsorptionandproductdiffusion 0.5%,and3.3±0.6%,respectively(Figure2c).At600mA⋅cm−2, duringCO RR,therebycontributingtoenhancedoverallcatalytic R-CuO maintains a high C partial current density of 489.6 ± 2 2 performance. 11.0 mA⋅cm−2, with only a slight decline in FE(C ) to 81.6 ± 2 1.8%(Figure2d;FiguresS20andS21).ThehighestFEforC H 2 4 ThecompositionofthethreeCuOcatalystswascomprehensively (65.5 ± 2.9%) is achieved at 400 mA⋅cm−2, corresponding to a characterized by energy dispersive spectra (EDS) and X-ray partialcurrentdensityof261.9±11.6mA⋅cm−2.Incomparison, photoelectronspectroscopy(XPS).EDSmappingandXPSsurvey S-CuO and F-CuO show less prominent CO RR performance 2 spectra confirm that Cu and O are the primary elemental across the tested current density range. Although their FE(C ) 2 components in all three catalysts (Figures S15 and S16). In the risesfrom100to300mA⋅cm−2,furtherincreaseincurrentdensity high-resolution Cu 2p spectra, the spin-orbit doublet observed leads to the selectivity degradation of C . The highest FE(C ) 2 2 at binding energies of 934.0 and 953.7 eV corresponds to Cu2+ for S-CuO and F-CuO are 69.8 ± 1.2% at 400 mA⋅cm−2 and species in the Cu 2p and 2p states, respectively (Figure 64.2 ± 5.9% at 300 mA⋅cm−2, respectively. Overall, the CO RR 3/2 1/2 2 S16b) [19]. Furthermore, the Cu LMM Auger electron spectra performancefollowstheorder:R-CuO>S-CuO>F-CuO.This (AES) show kinetic energy peaks centered at 917.6 eV for all performancedifferenceisprimarilyattributedtovariationsinH 2 three catalysts, consistence with the presence of Cu2+ species selectivity. While S-CuO and F-CuO exhibit FE(H ) exceeding 2 (Figure 1n) [20]. The XPS O 1s spectra can be deconvoluted 20% across all current densities, R-CuO effectively suppresses into two contributions: adsorbed oxygen (O ) at 531.5 eV and FE(H )tobelow10%between200to600mA⋅cm−2(FigureS22). ads 2 lattice oxygen (Cu─O bond) at 529.5 eV (Figure S16c) [19]. As ContactanglemeasurementsfurtherrevealthatR-CuOpossesses thepHoftheprecipitantsolutiondecreasesduringsynthesis,the the highest hydrophobicity among the three catalysts (Figure arearatiooftheO peakgraduallyincreases,andthebinding S23). The minimized H production enables R-CuO to more ads 2 energy of the Cu─O peak shifts toward lower energy. These efficientlyconvertCO into*COintermediatesandsubsequently 2 changesindicateahigherconcentrationofoxygenvacancieson intohigh-valueC products. 2 the surface of R-CuO compared to S-CuO and F-CuO, which may be attributed to the dominant exposure of the CuO (111) In addition to FE and partial current density, the half-cell crystalplaneinR-CuO.Thisconclusionisfurthercorroborated conversion efficiency (HPCE) serves as another crucial metric by the electron paramagnetic resonance (EPR) spectrum, in forassessingCO RRperformance.Amongthethreecatalysts,R- 2 which R-CuO exhibits the most intense oxygen vacancy signal CuOachievesthehighestHPCEof20.7±1.3%at300mA⋅cm−2, amongthethreecatalysts(FigureS17)[21].Ahighconcentration which can be attributed to its outstanding C H selectivity 2 4 of oxygen vacancies enhances CO adsorption on the catalyst (Figure2e;FigureS24).Incomparison,theHPCEvaluesforS- 2 surface and provides additional defect sites to facilitate the CuOandF-CuOare17.8±0.6%and18.6±1.4%at300mA⋅cm−2, reaction process [21]. Taking into account the comprehensive respectively.HPCEisinfluencednotonlybyproductselectivity structural and compositional properties characterized above, but also by the reaction potential of the catalyst. Analysis of we conclude that R-CuO, with its high specific surface area, thepotential-time(p-t)curvesallowscalculationoftheaverage abundant microporous structure, and elevated oxygen vacancy reaction potential for each material (Figures S25 and S26). The 4of11 AdvancedFunctionalMaterials,2026 16163028, 2026, 32, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202526003 by Zhejiang University Of Technology, Wiley Online Library on [12/05/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
FIGURE 2 (a)LSVcurvesofallthreeCuOcatalystsunderaCO -fedenvironmentbeforeCO RR.(b)FEtowardC productsofallthreeCuO 2 2 2 catalysts.(c)ProductdistributiononR-CuOatdifferentappliedcurrentdensities.(d)PartialcurrentdensitytowardC productsand(e)HPCEofall 2 threeCuOcatalystsatdifferentappliedcurrentdensities.(f)Thelong-termstabilityofR-CuOat400mA⋅cm−2.(g)ThecomparisonofR-CuOwiththose state-of-the-artCu-basedelectrocatalystsforCO RR. 2 resultsindicatethatR-CuOoperatesatasignificantlyloweraver-
2.3 Structural, Compositional, and Morphological Evolution
age reaction potential compared to S-CuO and F-CuO, further MorphologicalEvolutionofCuOCatalysts corroborating its excellent electrical conductivity and intrinsic activityinCO RR.Therefore,thecombinationofhighC product GiventheinevitablereconstructionoftheCuOintothemetallic 2 2 selectivity and low reaction potential enables R-CuO to deliver Cu phase during electrochemical processes, it is crucial to thehighestHPCEamongthecatalystsstudied. compare the electrochemical properties of the catalysts before and after CO RR. Post-reaction LSV curves show an increase 2 R-CuO was subjected to a continuous 30 h electrolysis test in current density for all three catalysts in both CO and Ar 2 at 400 mA⋅cm−2 (Figure 2f; Figure S27) and maintained an atmospheres. Notably, R-CuO maintains the highest current FE(C )above68%for26h,althoughselectivitytowardH ,CH , density,reaching600mA⋅cm−2 at1.75Vvs.RHEunderaCO - 2 2 4 2 and CH COOH gradually increased over time. After 26 h, we fedenvironment(FigureS28),consistentwithitslowestaverage 3 observed performance degradation, commonly associated with reactionpotential. waterfloodingissuesonthecarbonpapersubstrateduringlong- term testing. We further compared R-CuO with state-of-the-art Electrochemical impedance spectroscopy (EIS) reveals that R- Cu-based catalysts reported in the literature for CO RR. As CuOexhibitsthesmallestcharge-transferresistance(R )of164.3 2 ct summarized in Figure 2g and Table S2, R-CuO demonstrates Ω,asindicatedbythesmallestradiusintheNyquistplotamong notableadvantagesinC production,highlightingitscompetitive all three catalysts (Figure S29 and Table S3). This trend aligns 2 performanceamongcurrentefficientcatalysts. withthevariationsinspecificsurfacearea(S ),implyingthat BET AdvancedFunctionalMaterials,2026 5of11 16163028, 2026, 32, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202526003 by Zhejiang University Of Technology, Wiley Online Library on [12/05/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
FIGURE 3 (a) ECSA of all three CuO catalysts before and after CO RR. (b) XRD patterns and (c) Auger Cu LMM spectra of 2 all three CuO catalysts with etching after CO RR. Peak fitting of Cu species in Cu LMM spectra of (d) S-CuO, (f) F-CuO, and 2 (h)R-CuO.HR-TEMimagesof(g)S-CuO,(h)F-CuO,and(i)R-CuOafterCO RR. 2 differencesinmorphologyandthenumberofexposedactivesites ToquantitativelyanalyzethesurfaceCu0 andCu+ speciesafter significantlyinfluencecharge-transferbehavior.Toquantifythe CO RR,XPScharacterizationwasperformed,withAr+sputtering 2 active site density, ECSA was estimated based on the double- (etchingdepth∼2nm)toremovesurfacecontaminants.Before layercapacitance(C )(FiguresS30andS31)[22].AfterCO RR, etching,allthreecatalystsexhibitedsurfacesdominatedbyCu+ dl 2 theECSAofR-CuOreaches271.4,whichis6.8timesthatofthe species (916.7 eV), withsimilar Cu0/Cu+ ratios (78–80%) across pristinecatalyst(Figure3a).Incomparison,theECSAofS-CuO thesamples(FiguresS34andS35).AfterAr+ etching,theCu2p andF-CuOafterreactionisonly4.8and2.3timesthatoftheir spectra display spin-orbit doublets at 932.6 and 952.5 eV, corre- pristinecatalysts,respectively.ThesubstantiallylargerECSAof spondingtoCu0/Cu+2p and2p species,respectively(Figure 3/2 1/2 R-CuOallowsexposureofmoreactivesites,therebyenhancing S36). Further analysis using Cu LMM spectra reveals distinct CO adsorptionandconversiontoC products. differencesamongthecatalysts.Thepeakat915.8eV,indicative 2 2 of Cu0 species, becomes evident, reflecting the true chemical To elucidate the active sites of catalysts during CO RR, we stateduringCO RR(Figure3c).Peakdeconvolutionshowsthat 2 2 systematically investigated their structural and compositional R-CuO retains the highest Cu+ content (73.8%), significantly evolution.ExsituXRDanalysisofR-CuOindicatesaprogressive exceedingthatofS-CuO(61.4%)andF-CuO(56.5%).Thisresult phase transformation from CuO to Cu O, followed by reduc- demonstrates the superior stability of Cu+ species in R-CuO, 2 tion to metallic Cu (Figure S32). Similarly, S-CuO and F-CuO enablingthemaintenanceofabundantCu0/Cu+interfacesunder also undergo complete conversion to metallic Cu after CO RR reactionconditions(FigureS37).TounderstandtheoriginofCu+ 2 (Figure3b).Ramanspectroscopyfurtherconfirmsthepresence species under negative applied potential, EPR spectroscopy is of surface Cu O species, with characteristic vibrational peaks performedinthe1mKOHelectrolyteusedduringCO RR.The 2 2 observed at 140 cm−1 (T mode), 513 cm−1 (T mode), and 612 results confirm the presence of ⋅OH radicals in the electrolyte, 1u 2g cm−1(T mode)(FigureS33)[23].Aspreviouslynoted,Cu0/Cu+ which originates from rapid oxygen exchange between CO 2− 1u 3 interfaces act as active sites for promoting the electroreduction and H O during CO RR, consistent with the previous report 2 2 fromCO toC products. (FigureS38)[24].R-CuOappearstofavorthelocalenrichmentof 2 2 6of11 AdvancedFunctionalMaterials,2026 16163028, 2026, 32, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202526003 by Zhejiang University Of Technology, Wiley Online Library on [12/05/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
⋅OHradicalsandthedynamicstabilizationofmoreCu+ species 30min,astrongbandappearsat520cm−1,which,consideringthe becauseofitsuniquereconstructedmorphology. presence of surface-adsorbed OH−, is assigned to CuO /(OH) , x y representingamixedoxide/hydroxidesurfacelayer,accompanied However,thespatialdistributionofCu0andCu+speciesrequires byastretchingvibrationvs(Cu─OH )at491cm−1[33].Theinten- ad further investigation. SEM and TEM analyses show that S- sityoftheCuO /(OH) bandreflectsahighsurfaceconcentration x y CuO and F-CuO largely retain their original morphological ofOH−ions,whichcanbeascribedtotheabundanceofCu+sites structure and particle size after CO RR (Figures S39–S42 and dynamicallygeneratedonR-CuOduringCO RR.Additionally,a 2 2 SchemeS2).Incontrast,R-CuOundergoesstructuralreorganiza- bandobservedat594cm−1isassignedtoadsorbedoxygenonthe tion,transformingfromnanorodsintocauliflower-likeparticles Cu(111)crystalplane(Cu─O ),akeyintermediateintheforma- ads accompanied by partial fragmentation (Figures S43 and S44 tionofsurfaceCu O.ThisindicatesthatsurfaceCu0speciesare 2 and Scheme S3). We propose that these fragments originate oxidizedtoCu+speciesby⋅OHradicalspresentintheelectrolyte from the primary CuO crystal nuclei that constitute the initial [30].TheretentionofCu+enhancestheadsorptionofOH−ions, nanorods.Underharshelectrochemicalconditions,thenanorods reinforcing the localized alkaline microenvironment. Although fracture due to insufficient structural stability, followed by re- S-CuO and F-CuO undergo a similar structural transformation agglomeration of the fragments into porous, cauliflower-like sequence (CuO → Cu O → CuO /(OH) ) during CO RR, the 2 x y 2 particles. This unique “fragmentation-agglomeration” mecha- intensities of the characteristic peaks related to CuO /(OH) , x y nism allows R-CuO to expose a greater number of active sites, vs(Cu─OH ),ρ(Cu─CO),andv(Cu─CO)aremarkedlyweaker ad which aligns with two key experimental observations. First, a thanthoseforR-CuO.ThisdemonstratesthatR-CuOmaintains significantlylargerincreaseinECSAofR-CuOafterthereaction more stable and abundant Cu0/Cu+ interfaces under operating comparedtoS-CuOandF-CuO.Second,thehigherconcentration conditions. of Cu+ species and more abundant Cu0/Cu+ interfaces in R- CuO, as confirmed by XPS, are critical for enhancing catalytic Adsorbed species were detected in the spectral range of 750 to performance. HR-TEM images reveal that all three catalysts 2500 cm−1. During CO RR, CO molecules introduced at the 2 2 after CO RR contain lattice fringes corresponding to Cu (111) triple-phaseinterfacereactwithtwoOH− ionstoform*CO 2−, 2 3 (0.208nm)andCu O(111)(0.256nm)crystalplanes,alongwith as indicated by the band at 1066 cm−1 (CO + OH− → HCO −, 2 2 3 composite structures indicative of Cu0/Cu+ interfaces. Both S- HCO − + OH− → CO 2− + H O) [32]. Thus, the intensity of 3 3 2 CuOandF-CuOarepredominantlycomposedof2Dnanosheets. the *CO 2− band serves as an indirect indicator of the local 3 In this structure, 2D nanosheets dominate, within which Cu alkalinity near the catalyst surface. Starting from 20 min, R- (111) and Cu O (111) crystal planes are randomly oriented. CuO exhibits a strong *CO 2− signal, while those of S-CuO 2 3 This disordered arrangement results in limited and inefficient and F-CuO remain considerably weaker. In the spectrum of F- Cu0/Cu+ interfaces (Figure 3g,h), which may further impede CuO, an additional band at 1036 cm−1 attributed to *HCO − 3 CO adsorptionatactivesites[25].Incontrast,thereconstructed becomes distinguishable from *CO 2− as the reaction proceeds 2 3 R-CuOexhibitsaroughenedmorphologycomposedoffinerparti- to 40 min. A broad feature around 1560 cm−1 is assigned to cles.Thistransformationpromotespreferentialsurfaceoxidation, the asymmetric stretching vibration vs(*CO −), associated with 2 leading to an outer-inner structure with Cu+ species enriched the formation of HCOO− during CO RR [34, 35]. Two broad 2 on the surface and Cu◦ concentrated in the core (Figure 3i). bandsbetween1900∼2000cm−1and2000–2100cm−1correspond ThisorganizedconfigurationfacilitatesefficientCO adsorption to the C≡O stretching vibrations. The former is attributed to 2 at the well-defined Cu0/Cu+ interfaces during CO RR, thereby a bridge-adsorption configuration (*CO ), and the latter to 2 Bridge enhancingtheselectivityforC products. anatop-adsorptionconfiguration(CO )[36].UnlikeCO , 2 Atop Atop *CO isconsideredunfavorableforC─CcouplinginCO RR Bridge 2 [37]. On R-CuO, *CO is the dominant configuration and Atop
2.4 CO₂RR Mechanism on CuO Catalysts
can be further deconvoluted into *CO (∼2042 cm−1) and 2 Terrace CO (∼2077cm−1)[34,38].TheCO bandintensifiesand Step Step To validate the proposed hypothesis, time-dependent in situ blueshiftsto2091cm−1 overtime,indicatinganincreasingpro- Raman spectroscopy was employed to monitor the structural portionoflow-coordinationCusitesonR-CuOduringCO RR. 2 evolutionofthecatalystsandtheformationofintermediateson In contrast, CO intermediates on F-CuO primarily adopt the theirsurface(FiguresS45–S47).Atopencircuitpotential(OCP), bridge-adsorptionmode,withmarkedlyweakerCO signals. Atop the characteristic bands at 298 and 610 cm−1 correspond to the This suggests stabilized CO bridge-adsorption that hinders CuO phase in R-CuO. After applying a negative potential for subsequent C─C coupling. For direct comparison of spectral 10min,theCuObandsweakensignificantly,whilethoseofCu O evolutionamongthethreecatalysts,color-mappedRamanspec- 2 at 520 and 620 cm−1 are intensified [26]. Simultaneously, new tra are provided (Figure 4a,b; Figure S48). All CO RR-related 2 bandsemergednear290and360cm−1,attributedtotheρ(Cu– intermediatepeakintensitiesaresignificantlystrongeronR-CuO CO)rotationmodeandv(Cu–CO)stretchingmode,respectively thanonS-CuOorF-CuO,particularlythecharacteristicfeature [27–29]. The intensity of these bands initially increases and ofCuO /(OH) ,whichisascribedtotheabundanceofCu0/Cu+ x y thendecreasesasCO RRproceeds,indicatingaccumulationand interfaces. 2 subsequentconsumptionofCOintermediateforC─Ccoupling. By 25 min, the Cu O characteristic band disappears, and a To elucidate the coordination environment of R-CuO after 2 bending vibration band v (Cu─OH ) emerges at 700 cm−1 [25, CO RR, XAS was conducted. The X-ray absorption normalized b ad 2 30], suggesting that OH− ions are adsorbed onto Cu0 sites at near-edge structure (XANES) spectra show that the absorption the triple-phase interface, thereby creating a localized alkaline edgeofallthreepost-reactioncatalystsliesbetweenthoseofCu microenvironment conducive to C─C coupling [31, 32]. After foil(Cu0)andCu O(Cu+),confirmingthecoexistenceofmetallic 2 AdvancedFunctionalMaterials,2026 7of11 16163028, 2026, 32, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202526003 by Zhejiang University Of Technology, Wiley Online Library on [12/05/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
FIGURE 4 Colormappingimagesoftime-dependentinsituRamanspectraof(a)R-CuOand(b)F-CuO;(c)TheCuK-edgederivativeXANES spectraand(d)k3-weightFourier-transformedEXAFSspectraofallthreeCuOcatalystsafterCO RR(Cufoil,Cu Ostd.,andCuOstdasreferences).(e) 2 2 CoordinationnumberofthefirstshellCu─CuscatteringofallthreeCuOcatalystsafterCO RR.(f)CVcurvesand(g)adsorbedOH−ionsdetectedon 2 Cu(111)crystalplaneofallthreeCuOcatalystsafterCO RR.(h)LSVcurvesforCOstrippingexperimentsforallthreeCuOcatalysts. 2 Cu0 andoxidizedCu+ species(FiguresS49–S51).DerivativeCu Time-dependent in situ Raman spectroscopy confirms that R- K-edge XANES analysis further indicates that the dominant CuOsustainsahighlyalkalinelocalmicroenvironmentandhigh electronic structure is metallic Cu0 (Figure 5c). The k3-weight *COcoverageduringCO RR,bothconducivetoC─Ccoupling. 2 Fourier-transformed extended X-ray absorption fine structure AdditionalOH−adsorptionandCOstrippingexperimentresults (FT-EXAFS) spectra reveal that the coordination number (CN) furthersupportthesefindings(Figure4f,h;FiguresS53andS54). of the Cu─Cu scattering path in R-CuO after CO RR is only BeforeCO RR,nodistinctOH− adsorptionpeakisobservedon 2 2 8.71, significantly lower than those of S-CuO (9.68) and F- any catalyst. After reaction, a small peak emerges near 0.40 V CuO(10.59)(Figure4d,e;TableS4).Theselow-coordinationCu vs.RHE,correspondingtoOH−adsorptionontheCu(111)plane. sites introduce more defects, enhancing the adsorption of *CO Notably,theOH− adsorptionpeakforR-CuOappearsat0.36V intermediate.Meanwhile,R-CuOexhibitsthehighestCNforthe vs.RHE,whichis0.05and0.06VnegativethanthoseonS-CuO Cu–Opath,indicatingagreaterabundanceofCu+species,which and F-CuO, respectively, indicating a stronger affinity for OH− helps strengthen the local alkaline microenvironment (Figure ions and an enhanced local alkaline environment at the triple- S52). These XAS results are consistent with the above XPS and phase interface. The CO stripping experiment also shows that insituRamanspectra. the CO oxidation peak on R-CuO shifts positively compared to 8of11 AdvancedFunctionalMaterials,2026 16163028, 2026, 32, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202526003 by Zhejiang University Of Technology, Wiley Online Library on [12/05/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
FIGURE 5 Thepotential-dependentinsituATR-FTIRspectraof(a)R-CuOand(b)F-CuO.(c)IllustrationofthemechanismforCO -to-C 2 2 productsonR-CuO. thoseontheothercatalysts,confirmingitssuperiorCObinding other characterization peaks, such as COCO intermediates at strength[39,40]. 1562cm−1 andCHOintermediatesat1725cm−1,canbefound. Thus, the proposed reaction pathway of CO RR on R-CuO is: 2 Potential-dependent in situ Attenuated total reflection Fourier CO → *CO − → *COOH → *CO → *COH → *COH + *CO 2 2 transform infrared spectroscopy (ATR-FTIR) was used to fur- →COCOH→C products(Figure5c).Theseresultshighlight 2 ther investigate the CO RR mechanism on the three catalysts the critical role of abundant Cu0/Cu+ interfaces on R-CuO in 2 (Figure5a,b;FigureS55).ForR-CuO,abandat1396cm−1emerges enhancing CO adsorption, activation, and C─C coupling. In 2 asthepotentialdecreases,assignedtotheCOOHintermediate contrast, S-CuO and F-CuO show attenuated or absent signals generated from the protonation of CO −, a key step in CO for these key intermediates, likely due to their fewer Cu0/Cu+ 2 2 reduction to CO intermediates [25, 34, 41]. Bands at 1870 and interfaces,rationalizingtheirinferiorC productactivity. 2 2030cm−1areattributedtoCO andCO intermediates, Bridge Atop respectively, with the latter being significantly more intense, consistentwithinsituRamanresults[42].Additionalbandsat
3. Conclusion
1340and1430cm−1correspondtothe─COand─COHmoietiesin theCOCOHintermediate,suggestingthatC productsformvia In conclusion, we have developed a pH-controlled strategy to 2 couplingbetweenCOand*COH,whichistherate-determining fabricateCuOcatalystswithdistinctmorphologies.Thenanorod- step for generating C products [43]. This pathway is further structuredCuO(R-CuO)demonstratesoutstandingCO RRper- 2 2 supported by a band at 1199 cm−1, associated with the COH formance, achieving a high FE(C ) of 84.0% at 500 mA⋅cm−2 2 intermediatederivedfromCOhydrogenation[44].However,no and a remarkable C partial current density of 489.6 mA⋅cm−2 2 AdvancedFunctionalMaterials,2026 9of11 16163028, 2026, 32, Downloaded from https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.202526003 by Zhejiang University Of Technology, Wiley Online Library on [12/05/2026]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
at600mA⋅cm−2.ItisrevealedthatR-CuOundergoesaunique Academy of Science USA 114 (2017): 6685–6688, https://doi.org/10.1073/ “fragmentation-agglomeration”dynamicreconstructionprocess pnas.1702405114. during CO RR, which results in abundant Cu0/Cu+ interfaces 5.M. Wang, Z. Wang, Z. Huang, M. Fang, Y. Zhu, and L. Jiang, 2 and a 6.8-fold increase in ECSA, enhancing CO 2 adsorption “HydrophobicSiO 2 Armor:StabilizingCuδ+toEnhanceCO 2 Electrore- andactivation.XASresultsdisclosealowcoordinationnumber ductionTowardC 2+ProductsinStrongAcidicEnvironments,”ACSNano (CN = 8.71) for the Cu─Cu scattering path and a high CN 18(2024):15303–15311,https://doi.org/10.1021/acsnano.4c04780. (0.72)fortheCu─Opathinpost-reactionR-CuO,underscoring 6.H.Li,T.Liu,P.Wei,etal.,“High-RateCO 2 ElectroreductiontoC 2+ its defective structure and Cu+ retention. The stable Cu0/Cu+ ProductsOveraCopper-CopperIodideCatalyst,”AngewandteChemie, International Edition 133 (2021): 14450–14454, https://doi.org/10.1002/ interfacesinR-CuOduringCO RRservetwocriticalfunctions: 2 ange.202102657. (1) the high concentration of Cu+ sites strengthens the local alkaline microenvironment at the triple-phase interface, and 7.L.Xu,X.Ma,L.Wu,etal.,“InSituPeriodicRegenerationofCatalyst (2) the defective Cu0 sites facilitate optimal adsorption of *CO During CO 2 Electroreduction to C 2+ Products,” Angewandte Chemie, InternationalEdition 61 (2022): 202210375, https://doi.org/10.1002/anie. intermediate. This synergistic effect of Cu0/Cu+ significantly 202210375. promotesthecouplingkineticsbetweenthe─COmoietyandthe ─COHmoiety,whichistherate-determiningstepinC product 8.C. A. Obasanjo, G. Gao, J. Crane, V. Golovanova, F. P. García De 2 Arquer, and C.-T. Dinh, “High-Rate and Selective Conversion of CO 2 formation. This work provides a clear structure-performance FromAqueousSolutionstoHydrocarbons,”NatureCommunications14 relationshipthatguidestherationaldesignofCu-basedcatalysts (2023):3176,https://doi.org/10.1038/s41467-023-38963-y. through morphological engineering and fundamental insights 9.B.Zhao,F.Chen,C.Cheng,L.Li,C.Liu,andB.Zhang,“C60-Stabilized intothedynamicevolutionofthetriple-phaseinterfacesduring Cu+ Sites Boost Electrocatalytic Reduction of CO 2 to C 2+ Products,” CO 2 RRtofacilitatehigh-valueC 2 production Advanced Energy Materials 13 (2023): 2204346, https://doi.org/10.1002/ aenm.202204346. 10.Y. Jiang, H. Li, C. Chen, Y. Zheng, and S.-Z. Qiao, “Dynamic Cu0 /Cu+InterfacePromotesAcidicCO Electroreduction,”ACSCatalysis14 2 (2024):8310–8316,https://doi.org/10.1021/acscatal.4c01516. Acknowledgements 11.D.P.Singh,A.K.Ojha,andO.N.Srivastava,“SynthesisofDifferent TheauthorsacknowledgefinancialsupportfromtheNationalNatural Cu(OH) 2 andCuO(Nanowires,Rectangles,Seed-,Belt-,andSheetlike) ScienceFoundationofChina(22208363,22405287,51872317),Scienceand Nanostructures by Simple Wet Chemical Route,” Journal of Physical TechnologyCommissionofShanghai(Nos.24ZR1475800,24DZ2201600). ChemistryC113(2009):3409–3418,https://doi.org/10.1021/jp804832g. TheauthorsthankthestaffmembersatBL01BandBL14W1beamlines 12.X.Xu,H.Yang,andY.Liu,“Self-AssembledStructuresofCuOPrimary oftheNationalFacilityforProteinScienceinShanghai(NFPS),Shanghai Crystals Synthesized From Cu(CH COO) –NaOH Aqueous Systems,” 3 2 AdvancedResearchInstitute,ChineseAcademyofSciences,forproviding CrystEngComm14(2012):5289–5298,https://doi.org/10.1039/c2ce25420d. technical support and assistance in data collection and analysis. We appreciate Dr. Rongmin Dun from Core Facilities, Tongji University 13.J. Liu, X. Huang, Y. Li, K. M. Sulieman, X. He, and F. Sun, “Self- Schoolofmedicineforhertechnicalsupport. Assembled CuO Monocrystalline Nanoarchitectures With Controlled Dimensionality and Morphology,” Crystal Growth & Design 6 (2006): 1690–1696,https://doi.org/10.1021/cg060198k. Funding 14.M.Vaseem,A.Umar,S.H.Kim,andY.-B.Hahn,“Low-Temperature National Natural Science Foundation of China (22208363, 22405287, Synthesis of Flower-Shaped CuO Nanostructures by Solution Process: 51872317), Science and Technology Commission of Shanghai (Nos. Formation Mechanism and Structural Properties,” Journal of Physical 24ZR1475800,24DZ2201600). ChemistryC112(2008):5729–5735,https://doi.org/10.1021/jp710358j. 15.R. Ranjbar-Karimi, A. Bazmandegan-Shamili, A. Aslani, and K.
Conflicts of Interest
Kaviani, “Sonochemical Synthesis, Characterization and Thermal and Theauthorsdeclarenoconflictsofinterest. OpticalAnalysisofCuONanoparticles,”PhysicaB:CondensedMatter405 (2010):3096–3100,https://doi.org/10.1016/j.physb.2010.04.021.
Data Availability Statement
16.W.Wang,Z.Liu,Y.Liu,etal.,“ASimpleWet-ChemicalSynthesisand CharacterizationofCuONanorods,”AppliedPhysicsA76(2003),417–420. Thedatathatsupportthefindingsofthisstudyareavailablefromthe correspondingauthoruponreasonablerequest. 17.J.F.Xu,W.Ji,Z.X.Shen,etal.,“RamanSpectraofCuONanocrystals,” JournalofRamanSpecroscopy30(1999):413–415,https://doi.org/10.1002/ (SICI)1097-4555(199905)30:5%3c413::AID-JRS387%3e3.0.CO;2-N. References 18.V.Indovina,M.Occhiuzzi,D.Pietrogiacomi,andS.Tuti,“TheSurface
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