CO2 electrolysis converts the greenhouse gas CO2 into valuable fuels and chemicals, such as carbon monoxide, ethylene, ethanol, etc. Currently, Cu is the only known monometallic catalyst capable of producing multicarbon products from electrochemical CO2 reduction reaction (eCO2RR), while the poor selectivity limits its further use. It has been found that introducing Ag atoms into the Cu lattice can modulate product preference. However, the synergistic effects between Cu and Ag, and thus, the catalytic performance, are strongly influenced by catalyst morphology, electrolyzer configuration, reaction conditions, etc. Operando measurements can provide explicit information on the catalyst dynamic variation during the reaction, but their operation and analysis are challenging. Herein, we prepared CuAg multiphase alloy catalysts by magnetron sputtering, which allowed for investigating the intrinsic interaction between Cu and Ag. eCO2RR performance exhibited an improved selectivity toward carbonyls at the expense of hydrogen and hydrocarbons. The partially alloyed Cu and Ag phases were confirmed by operando X-ray diffraction. By means of combining operando X-ray measurements and density functional theory (DFT) calculations, the preferred carbonyl production is attributed to the reduced electron density and compressive strain of Cu due to Ag incorporation, which leads to a deeper d-band center and therefore weakened intermediate adsorption and oxophilicity. This work provides evidence of the intrinsic structural and electronic interaction between Cu and Ag during eCO2RR. The obtained information will facilitate the design of bi/multi-phase metallic or alloy electrocatalysts.

Copper is known to be versatile in producing various products from CO2 reduction (eCO2RR), and the product preference is dependent on reaction environments. Literature have reported alkaline electrolytes favor acetate production, and proposed hypotheses on the reaction pathway accordingly. Our work shows acetate can additionally come from the non-faradaic chemical oxidation of acetaldehyde in alkaline environments, which occurs quite rapidly in comparison to typical measurement times. This adds uncertainty into both acetaldehyde and acetate production. With an electrochemistry-mass spectrometry combined (EC-MS) system, we present real-time detection of acetaldehyde as a function of applied potential on single crystal Cu electrodes during electrochemical CO reduction (eCORR). In 0.1M KOH, the (100) and (211) facets had an acetaldehyde production onset of -0.35 V vs. RHE, whereas (111) and (110) exhibited no detectable acetaldehyde production up to -0.6 V vs. RHE. Moreover, the quantified acetaldehyde-to-ethylene production ratio provides insightful information on the acetaldehyde-to-ethylene bifurcation point in eCO2RR, and thus help understand the reaction pathways.

Electrochemical CO2 reduction (ECO2R) converts greenhouse gas CO2 into valuable fuels and chemicals, and thus helps with closing the anthropogenic carbon cycle. Currently, Cu is the only known material being capable of producing a variety of hydrocarbons and alcohols, while the poor selectivity limits its further use. Ethanol and ethylene have been proved to go through similar pathways but their bifurcation is yet to be fully understood. It has been found that introducing Ag atoms into Cu lattice could shift the product distribution toward ethanol compared to ethylene. However, previous studies have proposed contradictory speculations: DFT calculations predict the introduced Ag atoms prefer to dope on the undercoordinated sites on Cu surface [1], while experiments have proved that C-C coupling occurs at these sites and is promoted when they are occupied by Ag [1]–[3]. Literature also interpreted various mechanisms of the interaction between Cu and, such as the constrained effect [1], [4], “spillover” [5], and different C-C coupling pathways between *CO and *CHx (x=1,2) at the boundaries [6]. The oxidation state [7] and faceting [1] as well as the composition of CuAg catalysts over time have been observed during the reaction course, but explicitly real-time information remains scarce. To provide more mechanistic information on the above controversies, we prepared both bimetallic (with miscible Cu and Ag phases s) and surface alloy (with separated Cu and Ag phases) CuAg thin films by physical vapor deposition (PVD) and galvanic exchange, respectively. Ex situ X-ray Photoelectron Spectroscopy (XPS) and O perando X-ray Absorption Spectroscopy (XAS), including X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) on surface alloy CuAg are performed under ECOR/ECO2R conditions to investigate the oxidation state and coordination numbers of Cu and Ag. By this means, electron transfer and the interface miscibility between Cu and Ag are identified. Combined with DFT calculations, we speculated possible doping sites of Ag atoms in the Cu lattice and potential adsorption sites of the produced intermediates for ethanol/ethylene formation. Besides, variations of the oxidation state, electric and geometric local structure, as well as the transformation in crystallinity of the CuAg catalysts over time are monitored by correlating XAS with operando Grazing Incidence X-ray Diffraction (GIXRD). The produced CO intermediates are substantiated to be the reason for Cu-enrichment occurring on the CuAg electrodes as speculated. References: [1] D. Higgins et al., “Guiding Electrochemical Carbon Dioxide Reduction toward Carbonyls Using Copper Silver Thin Films with Interphase Miscibility,” ACS Energy Lett., vol. 3, no. 12, pp. 2947–2955, 2018, doi: 10.1021/acsenergylett.8b01736. [2] L. Wang et al., “Selective reduction of CO to acetaldehyde with CuAg electrocatalysts,” Proc. Natl. Acad. Sci. U. S. A., vol. 117, no. 23, pp. 12572–12575, 2020, doi: 10.1073/pnas.1821683117. [3] C. Hahn et al., “Engineering Cu surfaces for the electrocatalytic conversion of CO2: Controlling selectivity toward oxygenates and hydrocarbons,” Proc. Natl. Acad. Sci. U. S. A., vol. 114, no. 23, pp. 5918–5923, 2017, doi: 10.1073/pnas.1618935114. [4] E. L. Clark, C. Hahn, T. F. Jaramillo, and A. T. Bell, “Electrochemical CO2 Reduction over Compressively Strained CuAg Surface Alloys with Enhanced Multi-Carbon Oxygenate Selectivity,” J. Am. Chem. Soc., vol. 139, no. 44, pp. 15848–15857, 2017, doi: 10.1021/jacs.7b08607. [5] S. Lee, G. Park, and J. Lee, “Importance of Ag-Cu Biphasic Boundaries for Selective Electrochemical Reduction of CO2 to Ethanol,” ACS Catal., vol. 7, no. 12, pp. 8594–8604, 2017, doi: 10.1021/acscatal.7b02822. [6] Y. C. Li et al., “Binding Site Diversity Promotes CO2 Electroreduction to Ethanol,” J. Am. Chem. Soc., vol. 141, no. 21, pp. 8584–8591, 2019, doi: 10.1021/jacs.9b02945. [7] S. B. Scott et al., “Absence of Oxidized Phases in Cu under CO Reduction Conditions,” ACS Energy Lett., vol. 4, no. 3, pp. 803–804, 2019, doi: 10.1021/acsenergylett.9b00172.