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Anna Strijevskaya, Akira Yamaguchi, Shusaku Shoji, [Shigenori Ueda](https://orcid.org/0000-0001-9425-0614), [Ayako Hashimoto](https://orcid.org/0000-0002-1985-7667), Yu Wen, Aufandra Cakra Wardhana, Ji-Eun Lee, Min Liu, [Hideki Abe](https://orcid.org/0000-0002-8392-7586), Masahiro Miyauchi

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[Nanophase-Separated Copper–Zirconia Composites for Bifunctional Electrochemical CO<sub>2</sub> Conversion to Formic Acid](https://mdr.nims.go.jp/datasets/213024ed-c8cd-40e0-a12e-c75e4c4bc7a8)

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Nanophase-Separated Copper–Zirconia Composites for Bifunctional Electrochemical CO2 Conversion to Formic AcidNanophase-Separated Copper−Zirconia Composites for BifunctionalElectrochemical CO2 Conversion to Formic AcidAnna Strijevskaya, Akira Yamaguchi, Shusaku Shoji, Shigenori Ueda, Ayako Hashimoto, Yu Wen,Aufandra Cakra Wardhana, Ji-Eun Lee, Min Liu, Hideki Abe,* and Masahiro Miyauchi*Cite This: ACS Appl. Mater. Interfaces 2023, 15, 23299−23305 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: A copper−zirconia composite having an evenly distributed lamellar texture, Cu#ZrO2, wassynthesized by promoting nanophase separation of the Cu51Zr14 alloy precursor in a mixture of carbonmonoxide (CO) and oxygen (O2). High-resolution electron microscopy revealed that the material consistsof interchangeable Cu and t-ZrO2 phases with an average thickness of 5 nm. Cu#ZrO2 exhibited enhancedselectivity toward the generation of formic acid (HCOOH) by electrochemical reduction of carbondioxide (CO2) in aqueous media at a Faradaic efficiency of 83.5% at −0.9 V versus the reversible hydrogenelectrode. In situ Raman spectroscopy has revealed that a bifunctional interplay between the Zr4+ sites andthe Cu boundary leads to amended reaction selectivity along with a large number of catalytic sites.KEYWORDS: nanophase separation, Cu51Zr14, Cu#ZrO2, electrochemical CO2 reduction, bifunctional catalysis, in situ Raman,formic acid1. INTRODUCTIONDependence of humanity on fossil fuels and the resultedemissions of carbon dioxide (CO2) into the atmospherecreated a loop, worsening the world environment. Much efforthas been made to reduce the carbon footprint and mitigate theCO2 emissions as well as to diminish the role of unrenewablefossil fuels in industry. One of the most intensively exploredroutes is the direct conversion of CO2 into valuable chemicalproducts including formic acid (FA). The outbreak of COVID-19 results in an increased commercial value of FA due to thehigh demands in poultry, textile, and pharmaceuticalindustries.1 Conventional FA production technologies arebased on the hydrolysis of different formates, such as methylformate, which suffers from poor waste management of toxicbyproducts and huge necessity in water supplies.2 It isdesirable to develop highly efficient and selective reactioncatalysts that enable the direct conversion of CO2 to FAwithout passing through the hydrolysis processes.Copper (Cu) is an active catalyst for direct CO2 conversionin terms of its moderate binding ability toward the *COintermediate and its positive energy for *H adsorption.3,4 Cu-based catalysts showed a Faradaic efficiency (FE) of 77.1% fordirect CO2 conversion to FA in aqueous media for hollowfibers and 82.4% for carbon-anchored Cu nanoparticles.5,6However, such pristine Cu catalysts are poorly selective for FAproduction, leading to undesired generation of chemicalspecies, including carbon monoxide (CO) and hydrogen(H2). One of the most promising routes to improve reactionselectivity is the use of metal−oxide interfaces that canpromote only the target reaction. Indeed, zirconium oxide(ZrO2)-supported Cu nanoparticles (ZrO2/Cu) selectivelycatalyze the conversion of CO2 to ethylene (COE).10 ZrO2possesses basic hydroxyl groups (OH) on the surface toefficiently adsorb CO2 and suppress hydrogen evolution.7−9The COE reaction was promoted via a bifunctional mechanismcomprising the molecular transfer of a *O(CH)O* inter-mediate, which was formed on the ZrO2 surface from adsorbedCO2, and spilt over to the Cu surface across the Cu−ZrO2interface to generate C2+ products via an *OCCO*intermediate.10 The recent study by Li et al. showed thatinterfacial ZrO2 favored the stabilization and retention of Cu+species in the CuO@ZrO2 catalyst, which resulted in increased*CO coverage and promoted coupling.11Recently, nanophase separation of alloys has garneredconsiderable attention as an accessible and scalable path todiscover a high density of catalytically active metal−oxideinterfaces with a narrow size distribution.12 Nickel−yttriumoxide (Ni#Y2O3) and rhodium−cerium oxide (Rh#CeO2)were materialized from Ni2Y and Rh2Ce precursor alloys,respectively, to demonstrate the enhanced catalytic perform-ances for the dry reforming of methane.13,14Here, we report the use of nanophase separation of acopper−zirconium (Cu51Zr14) precursor alloy to obtain ananocomposite catalyst comprising a metal−metal oxideinterface of Cu and ZrO2 (Cu#ZrO2). The Cu#ZrO2 catalystReceived: February 28, 2023Accepted: April 21, 2023Published: May 4, 2023Research Articlewww.acsami.org© 2023 The Authors. Published byAmerican Chemical Society23299https://doi.org/10.1021/acsami.3c02874ACS Appl. Mater. Interfaces 2023, 15, 23299−23305Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on July 9, 2024 at 07:28:55 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Anna+Strijevskaya"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Akira+Yamaguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Shusaku+Shoji"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Shigenori+Ueda"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Ayako+Hashimoto"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yu+Wen"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Aufandra+Cakra+Wardhana"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Aufandra+Cakra+Wardhana"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Ji-Eun+Lee"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Min+Liu"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hideki+Abe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Masahiro+Miyauchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acsami.3c02874&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=abs1&ref=pdfhttps://pubs.acs.org/toc/aamick/15/19?ref=pdfhttps://pubs.acs.org/toc/aamick/15/19?ref=pdfhttps://pubs.acs.org/toc/aamick/15/19?ref=pdfhttps://pubs.acs.org/toc/aamick/15/19?ref=pdfwww.acsami.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acsami.3c02874?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://www.acsami.org?ref=pdfhttps://www.acsami.org?ref=pdfhttps://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://acsopenscience.org/open-access/licensing-options/exhibited enhanced selectivity for the electrochemical CO2reduction reaction toward FA production in aqueous mediathrough the bifunctional interplay of Cu and ZrO2 across theirinterface. Spectroscopic analyses have shown that the CO2admolecules on the ZrO2 surface spill over to the Cu surfaceacross the interface and react with the OH species to generateFA via sequential reaction steps involving *CO2− and*OC(H)O*.2. MATERIALS AND METHODS2.1. Synthesis of Cu51Zr14 Alloy Precursor. An ingot of theCu51Zr14 precursor alloy was obtained by arc-melting of Cu and Zrmetals in an argon (Ar) atmosphere. For this process, Cu foil(99.99%, Nilaco Corporation) and Zr chunks (99.99%, NilacoCorporation) were weighted in a molar ratio of 51:14 placed on awater-cooled copper hearth and subjected to a plasma arc torch. TheCu51Zr14 precursor alloy ingot was powdered with an agate mortar inair and sieved to adjust the size of particles to between 40 and 50 μm.The Cu51Zr14 powder was placed on a ceramic boat and heated at 400°C for 12 h in a stream of mixture gases of carbon monoxide (CO)-and oxygen (O2) at a molar ratio of 2:1 and a total flow rate of 100mL min−1. The blackish-gray Cu51Zr14 powder was converted into adark-purple nanocomposite of metal Cu and zirconium oxide (ZrO2),i.e., Cu#ZrO2.2.2. Characterization. The synthesized Cu#ZrO2 material andthe Cu51Zr14 precursor alloy were characterized by powder X-raydiffraction (pXRD) over a 2θ range of 20−80° with a RigakuSmartLab diffractometer equipped with a D/teX Ultra detector. Theelemental composition and crystallographic structure of the sampleswere identified with scanning transmission electron microscopy(STEM, JEM-ARM200F, JEOL) imaging equipped with an energy-dispersive X-ray spectrometer (JED-2300, JEOL) operating at anacceleration voltage of 200 kV. A field-emission scanning electronmicroscope [JSM-7500F (JEOL)] was used to observe the externalmorphology of the materials. X-ray photoelectron spectroscopy (XPS)was performed after 5 min and 5 h of reaction using an ULVAC-PHI1600 instrument with a monochromatic Al Kα source. The bindingenergies were corrected using a carbon (C 1s) signal at 284.80 eV.Hard X-ray photoemission spectroscopy (HAXPES) measurementswere performed at BL15XU of SPring-8 (Super Photon Ring 8 GeV,Hyo̅go Prefecture, Japan). The excitation photon energy and totalenergy resolution were set to 5.95 keV and 240 meV, respectively.The measurements were done at room temperature, and the pressureof the analysis chamber of HAXPES was 1.1 × 10−7 Pa.2.3. Catalyst Preparation. Carbon paper with an averagegeometrical area of 2 cm2 was sonicated for 20 min consequently inethanol and ultrapure water (Millipore Q), after which it was dried inair at 80 °C for 1 h. The catalyst ink was prepared by sonication of 5mg of Cu#ZrO2 in a mixture of ethanol (440 μL) and ultrapure water(1785 μL) with an addition of 5 μL of 5% Nafion (Sigma-Aldrich).The prepared ink was then drop-casted onto carbon paper and driedin air at 80 °C for 1 h to ensure evaporation of ethanol. For thepurpose of comparison of catalytic activities, we performed the sameevaluation protocol on commercial Cu powder (Nilaco Corporation,average particle size: 30 μm), Cu2O powder (Nilaco Corporation),and a mixture of Cu- and yttria-stabilized zirconia (YSZ, 5.2% Y-doped, Sigma-Aldrich) in a molar ratio 51 to 14. YSZ was chosen asthe control due to similarity of phases between ZrO2 in Cu#ZrO2 andYSZ. The catalyst loading was always 5 mg for any of the materials.2.4. Electrochemical CO2 Reduction. Catalytic activity andselectivity of Cu#ZrO2 were evaluated in a custom-made H-type cellthat was filled with electrolyte solution of 0.1 M KHCO3 (CO2saturated, pH 6.8). A silver−silver chloride (Ag/AgCl) electrode anda platinum wire were used as the reference electrode (RE) andcounter electrode (CE), respectively. The reaction selectivity wasevaluated based on the Faradaic efficiency (FE) that was calculated asfollowsFigure 1. pXRD patterns of the Cu51Zr14 precursor alloy (A) and Cu#ZrO2 (B). HAXPES profiles of the Cu51Zr14 precursor alloy (black) andCu#ZrO2 (red) in the Zr 3d region (C) and the Cu 2p region (D).ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c02874ACS Appl. Mater. Interfaces 2023, 15, 23299−2330523300https://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig1&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c02874?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-as= × ×F n N QFE / (1)where F, n, N, and Q are the Faradaic constant, number of electronsinvolved in the reaction, amount of the product in moles, and the totalcharge that flows between the working and counter electrodes,respectively. Gas product identification was performed using a gaschromatograph equipped with a dielectric-barrier discharge ionizationdetector (Shimadzu Tracera 2010). A proton nuclear magneticresonance spectrometer (Bruker UltraShield Plus, 400 MHz) wasused for the qualification of liquid products.2.5. In Situ Spectroscopic Analyses by FT-IR and RamanSpectra. Two sets of measurements were conducted for obtainingdiffuse reflectance infrared Fourier transform (DRIFT) spectra. Foreach of the experiments, a fresh catalyst sample was used, and eithernitrogen (N2) or CO2 was bubbled through deuterium water (D2O)for 20−30 min prior to measurements.In situ Raman spectroscopy was performed using a custom-madesteady-flow setup (Figure S1). CO2-saturated 0.1 M KHCO3 solutionis made to flow through a closed chamber with a transparent uppercover from the IN to OUT direction. A working electrode, a RE, anda CE were connected to a flow reactor, and the bias potential wasapplied from the open-circuit potential (OCP) to −1.0 V vs thereverse hydrogen electrode (RHE). CO2 gas was continuouslybubbled through the electrolyte solution of 0.1 M KHCO3 from 20min before the experiments onward to keep the setup always saturatedwith CO2. The gaseous products of reaction are removed from thechamber with flow of liquid.3. RESULTS AND DISCUSSIONThe pXRD pattern for the alloy precursor (Figure 1A)indicates that the main phase of the precursor was Cu51Zr14(P6/m, a = 1.12444 nm, c = 0.82815 nm, α = β = 90°, γ =120°)15 containing Cu5Zr (F4̅3m, a = 0.68700 nm, α = β = γ =90).16 The pXRD pattern for Cu#ZrO2, shown in Figure 1B,indicates that the prepared Cu#ZrO2 mainly consisted ofmetallic Cu (face-centered cubic, Fm3̅m; JCPDS no. 85-1326)and tetragonal ZrO2 (t-ZrO2, P42/nmc) with an inclusion ofCu2O (Pn3̅m)17 and monoclinic ZrO2 (m-ZrO2, P21/c).18Note that most of the ZrO2 phase in Cu#ZrO2 crystallized in atetragonal form (t-ZrO2), which is thermodynamically lessfavorable than the monoclinic form (m-ZrO2). Figure 1C,Dshows the HAXPES profiles of Cu51Zr14 and Cu#ZrO2. TheHAXPES profile of Cu#ZrO2 in the Zr-3d core region isconsistent with the reported data of ZrO2 (Figure 1C).19 TheZr0 3d5/2 emission peak positioned at 179.5 eV on theHAXPES profile of Cu51Zr14 was not visible on that ofCu#ZrO2, indicating that the metallic Zr0 in Cu51Zr14 was fullyoxidized to ZrO2 in Cu#ZrO2.20,21 Moreover, HAXPES hasconfirmed that the Cu phase in Cu#ZrO2 retained the metallicstate of Cu0, the same as in the Cu51Zr14 precursor alloy (2p3/2:932.6 eV; 2p1/2: 952.5 eV) (Figure 1D).For the investigation of the nanostructure of Cu#ZrO2, weperformed cross-sectional STEM (Figure 2). Elementalmapping images were first acquired with energy-dispersivespectrometry (EDS) for each of the constituent elements suchas Cu, Zr, and O. All the STEM−EDS images show a lamellartexture with an average thickness of 5 nm. The Zr- and Ospecies show the same special distributions as part of the ZrO2phase in Cu#ZrO2, where the Cu species are exclusivelydistributed to the ZrO2 phase (Figure 2D).An annular-dark field STEM (ADF-STEM) image ofCu#ZrO2 near the surface boundary shows that the lamellarstructure consists of dark- and bright-contrasted phases (Figure2E). High-resolution ADF-STEM on the bright-contrastedphase demonstrates a long-range atomic ordering (Figure 2F).A fast Fourier transform (FFT) pattern calculated for the areain the green square box in Figure 2F is assigned to the ⟨010⟩zone axis of the tetragonal zirconia (t-ZrO2) (Figure 2F inset),which indicates that the bright-contrasted phase consists of t-ZrO2. The neighboring, dark-contrasted Cu phase shows anatomic fringe of the Cu(110) plane with a spacing ofapproximately 0.25 nm.Figure S2 in the Supporting Information shows thatCu#ZrO2 exhibited larger current densities under CO2bubbling than those under an Ar atmosphere (Figure S2A).The changes in current densities in a potential range moreanodic than −0.3 V were attributed to a charging caused by theadsorption of CO2 molecules on the catalyst. Results of cyclicvoltammetry shown in Figure S2B indicate the absence of self-oxidation of the catalyst during the electrolysis.Figure 3A shows the FE of Cu#ZrO2 for CO2 reduction atdifferent bias potentials. As indicated by the error bars, the FEvalues were scattered, but the total FEs were close to 100%under sufficiently negative potentials over −0.4 V vs the RHE.At potentials of more positive than −0.6 V, the major share ofFE was occupied by hydrogen evolution. In contrast, atpotentials more negative than −0.6 V, FA became one of themost predominant products: the corresponding FEs were 72%(partial current density: 2.4 mA/cm2) and 83.5% (partialFigure 2. STEM−EDS images of the Cu#ZrO2 catalyst with a scalebar of 100 nm. Elemental mappings acquired at Zr L (A), O K (B),Cu K (C), and overlapped elemental mapping (D). ADF-STEMimage showing a lamellar structure in Cu#ZrO2 (E). High-resolutionADF-STEM image at the Cu−ZrO2 boundary and an FFT patternfrom the green square area (the inset) (F).ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c02874ACS Appl. Mater. Interfaces 2023, 15, 23299−2330523301https://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig2&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c02874?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ascurrent density: 16 mA/cm2) at −0.6 and −0.9 V, respectively.Hydrogen evolution was only 17% at −0.9 V, which isattributed to the lower hydrogen production selectivity of ZrO2as reported by Soloveichik9 and Li.11 The other detectedproducts were trace amounts of CH4 and C2H6, constitutingless than 1% of FE each as shown in the SupportingInformation (Table S1). Based on the result from DFTcalculations by Xiao et al.,22 we can attribute these to enhanced*CO adsorption and dimerization on the Cu+/Cu0 pair. Wealso measured the partial current densities for the FAproduction (jform) over Cu#ZrO2 and control catalystsincluding Cu powder and a powder mixture of Cu- andyttria-stabilized ZrO2 (Cu + YSZ) (Supporting Information,Figure S3A) since the present ZrO2 structure in Cu#ZrO2 issimilar to that of YSZ. Among those catalysts, Cu#ZrO2exhibited the largest jform at all given potentials. Moreover,Tafel analysis (Figure S3B) showed a slope of 155.9 mV/decade for synthesized Cu#ZrO2, which is smaller than thoseof Cu + ZrO2 and Cu, indicating faster reaction kinetics forCO2 reduction of our Cu#ZrO2 catalyst. We also confirmedthe absence of reaction products over Cu#ZrO2 under theargon (Ar) atmosphere, indicating that the products in theCO2 atmosphere were purely from CO2 reduction.23−25Overall, these results, while being comparable to recent Cu-and Bi-based catalysts, provide valuable insights at interface-related reactions, as shown in the Supporting Information(Table S2).26,27Figure 3B−E shows FEs for FA, H2, and CO over thedifferent catalysts at the time after 3 h of electrolysis at −0.9 VFigure 3. Product distribution of electrochemical CO2 reduction overCu#ZrO2, quantified at the time after 3 h of electrolysis at −0.9 V vsRHE (A).Performance of electrochemical CO2 reduction over amixture of Cu + YSZ (B), Cu#ZrO2 (C), commercial Cu (D), andCu2O (E).Figure 4. DRIFTS spectra recorded on YSZ (A) and Cu#ZrO2 (B) under D2O−CO2 adsorption. In situ Raman spectra recorded duringelectrochemical CO2 reduction at 0.0 V (purple), −0.2 V (blue), −0.4 V (red), and −0.6 V vs RHE (green) over Cu#ZrO2 in CO2-saturated 0.1 MKHCO3 (C,D).ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c02874ACS Appl. Mater. Interfaces 2023, 15, 23299−2330523302https://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?fig=fig4&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c02874?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asvs RHE. The FE for FA generation over the Cu#ZrO2 and Cu+ YSZ catalysts reached 80 and 40%, respectively, whereasneither Cu nor Cu2O catalysts promoted FA generation. Theresults showed that the simple mixing of Cu and YSZ is notsufficient to achieve a high FE for FA generation. For thepurpose of catalyst characterization after exposure to reactionconditions, FE-SEM and STEM−EDS images were taken after3 h and 30 min of the electrocatalysis (Figures S4 and S5A−E). Figure S5 indicates the retention of the ZrO2 phase nearthe surface of the catalyst grain. Existence of ZrO2 even aftercathodic bias application was further confirmed by XPS(Figure S6) and X-ray fluorescence analyses (Table S3). Theseresults indicate that the catalyst surface is re-constructed at theinitial bias application to a Cu-rich nanoporous structure thatcomprises a stable electrocatalysis center after several hours tostably produce FA. The results of 10 h of the stability test areshown in our Supporting Information (Figure S2C), and wecould see the increase of current densities in the initial 4 h ofelectrocatalytic reaction, which is attributed to the ZrO2reduction. After 4 h, the current densities became stable. FE-SEM images, taken before and after the CO2 reductionreaction, are shown in the Supporting Information (Figure S4).In order to investigate the role of Cu- and ZrO2 sites in themolecular adsorption on initial stages of the FA generationreaction, we performed a set of DRIFTS over Cu#ZrO2 andYSZ in the atmosphere of deuterium oxide (D2O) vapor andCO2 (Figure 4A,B). The D2O vapor atmosphere is useful sinceit provides a high signal-to-noise ratio in the region of CO2-related species.28,29 The results indicate that the broadDRIFTS band around 650 cm−1 is attributed to thephysisorbed linear CO2,30 which is recognized on both thecontrol YSZ and Cu#ZrO2 under a CO2 atmosphere (Figure4A,B). A broad band only observed for Cu#ZrO2 at around1200 cm−1 (Figure 4B) is likely attributed to the CO2molecules chemisorbed to the Cu surface.31Further, we performed in situ Raman spectroscopy onCu#ZrO2 using a home-made flowing system filled with aCO2-saturated aqueous electrolyte of 0.1 M KHCO3 (see theSupporting Information, Figure S1). Figure 4C,D shows theRaman spectra in a potential range from the OCP of 0.0 to−0.6 V vs RHE. A prominent band is recognized at 2886 cm−1over the Cu#ZrO2 surface in the CO2 atmosphere at −0.2 V vsRHE (Figure 4D). This band is assigned to the C−Hstretching of *O(CH)O*, which strongly correlates with thegeneration of FA.32,33 When the potential was more negativethan −0.4 V, the peak intensity of *O(CH)O* at 2886 cm−1was decreased, and no other intermediate species were seen inthis region. These results suggest that the intermediate specieswere rapidly converted into FA and desorbed from the catalystsurface under application of high cathodic bias.34,35 Theasymmetric stretching *CO2− of adsorbed CO2 is observed at1532 cm−1, while 1610 cm−1 is assigned to the O−C−Ovibration of the adsorbed formate (Figure 4C). Summarizingthis spectroscopic data, we propose a possible molecularscenario for the Cu#ZrO2 catalyst. The CO2 molecules thatwere physisorbed on the Zr4+ site to result in a reflectanceband in DRIFTS 650 cm−1 (Figure 4B) were furthertransferred to the Cu0 site across the Cu−ZrO2 interfaceperimeter for further chemical adsorption (1200 cm−1 inDRIFTS, 1532 cm−1 in Raman). The protonation of adsorbedCO2 proceeds through sequential steps involving the formationof *O(CH)O* species (2886 cm−1 in Raman) and of theformate species that is identified as a band at 1610 cm−1 inRaman.4. CONCLUSIONSIn conclusion, nanophase-separated Cu#ZrO2 was successfullyobtained by internal oxidation of the Cu51Zr14 precursor alloy.Microscopic characterizations have demonstrated that theCu#ZrO2 material consists of nanometer-thick lamellae of Cumetal and tetragonal ZrO2, leading to a stable and widespreadCu−ZrO2 interface. The Cu#ZrO2 material exhibits enhancedselectivity toward the electrocatalytic CO2-to-FA conversiondue to the uniformly distributed catalytic sites as well as theabundant metal−oxide interface, which play an important rolein amended reaction selectivity. DRIFTS and in situ Ramanspectroscopy have shown that CO2 is adsorbed on the Zr4+ siteand further protonated into *O(CH)O* over the neighboringCu site to generate FA via a bifunctional interplay between Cu-and ZrO2 in Cu#ZrO2.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsami.3c02874.Additional experimental data, including experimentalsetup for in situ Raman measurements (PDF)■ AUTHOR INFORMATIONCorresponding AuthorsHideki Abe − National Institute for Materials Science,Tsukuba, Ibaraki 305-0044, Japan; Graduate School ofScience and Technology, Saitama University, Saitama 338-8570, Japan; orcid.org/0000-0002-8392-7586;Email: Abe.Hideki@nims.go.jpMasahiro Miyauchi − Department of Materials Science andEngineering, School of Materials and Chemical Technology,Tokyo Institute of Technology, Meguro, Tokyo 152-8552,Japan; orcid.org/0000-0001-8889-2645;Email: mmiyauchi@ceram.titech.ac.jpAuthorsAnna Strijevskaya − Department of Materials Science andEngineering, School of Materials and Chemical Technology,Tokyo Institute of Technology, Meguro, Tokyo 152-8552,Japan; Uzbek-Japan Innovation Center of Youth, Tashkent100095, UzbekistanAkira Yamaguchi − Department of Materials Science andEngineering, School of Materials and Chemical Technology,Tokyo Institute of Technology, Meguro, Tokyo 152-8552,Japan; orcid.org/0000-0002-3550-4239Shusaku Shoji − Department of Materials Science &Engineering, Cornell University, Ithaca, New York 14853-1501, United StatesShigenori Ueda − National Institute for Materials Science,Tsukuba, Ibaraki 305-0044, Japan; orcid.org/0000-0001-9425-0614Ayako Hashimoto − National Institute for Materials Science,Tsukuba, Ibaraki 305-0044, Japan; Graduate School of Pureand Applied Sciences, University of Tsukuba, Tsukuba,Ibaraki 305-8571, JapanYu Wen − National Institute for Materials Science, Tsukuba,Ibaraki 305-0044, Japan; Graduate School of Pure andACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c02874ACS Appl. Mater. Interfaces 2023, 15, 23299−2330523303https://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c02874?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c02874/suppl_file/am3c02874_si_001.pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hideki+Abe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-8392-7586mailto:Abe.Hideki@nims.go.jphttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Masahiro+Miyauchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0001-8889-2645mailto:mmiyauchi@ceram.titech.ac.jphttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Anna+Strijevskaya"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Akira+Yamaguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-3550-4239https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Shusaku+Shoji"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Shigenori+Ueda"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0001-9425-0614https://orcid.org/0000-0001-9425-0614https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Ayako+Hashimoto"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yu+Wen"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c02874?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asApplied Sciences, University of Tsukuba, Tsukuba, Ibaraki305-8571, JapanAufandra Cakra Wardhana − Department of MaterialsScience and Engineering, School of Materials and ChemicalTechnology, Tokyo Institute of Technology, Meguro, Tokyo152-8552, JapanJi-Eun Lee − Biofunctional Catalyst Research Team, RIKENCenter for Sustainable Resource Science, Wako, Saitama 351-0198, JapanMin Liu − Hunan Joint International Research Center forCarbon Dioxide Resource Utilization, School of Physical andElectronics, Central South University, Changsha 410083,Public Republic of China; orcid.org/0000-0002-9007-4817Complete contact information is available at:https://pubs.acs.org/10.1021/acsami.3c02874Author ContributionsA.S. dealt with the experimental investigation, data analysis,and first draft writing; A.Y. took care of the supervision andelectrochemical analysis; S.S. took charge of the conceptualiza-tion; A.H. and Y.W. performed the electron microscopeanalysis; A.C.W. did the XPS measurements and analysis; J.-E.L. took charge of the in situ Raman spectroscopy analysis;S.U. performed the HAXPES measurement and analysis; M.L.was in charge of the supervision; H.A. took charge of theestablishment of the synthetic method of the nanophase-separated structure, supervision, and writing the manuscript;M.M. conducted the supervision, funding, and writing the finalmanuscript and was the project leader.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work was funded by JST SICORP (no. JPMJSC18H7)and the Japan International Cooperation Agency (JICA)-Knowledge Co-creation Program and was also supported bythe JST CREST program (grant no. JPMJCR15P1) toward“Innovative Catalysts”. We also thank the Human JointInternational Research Center for Carbon Dioxide ResourceUtilization, School of Physical and Electronics, Central SouthUniversity, Changsha, Republic of China. The TEM character-ization was carried out using the NIMS TEM Station facilityand supported by the Global Research Center for Environmentand Energy. HAXPES measurements were performed withapproval from the NIMS Synchrotron X-ray Station (proposalnos. 2, 019A4600, 2019B4600, 2020A4600, 2020A4605). Theauthors thank Dr. Kadirova Z. Ch. for the program initiation.■ REFERENCES(1) Rieser, K.-P. BASF Increases Prices for Formic Acid and forPropionic Acid; The BASF News Release, 2020; Vol. 20, p 364.(2) Hietala, J.; Vuori, A.; Johnsson, P.; Pollari, I.; Reutemann, W.;Kieczka, H. Formic Acid. Ullmann’s Encycl. Ind. 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