Electroreduction of carbon dioxide (CO2) to valuable chemicals and fuels is a promising approach for utilizing renewable electricity and mitigating CO2 emissions. This process, known as CO2 reduction reaction (CO2RR), converts electrical energy into stored chemical energy by reorganizing the molecular bonds in CO2 and water to produce various carbon-containing products. While many metal catalysts produce a mix of single-carbon (C1) products, only copper (Cu)-based catalysts can effectively transform CO2 into multi-carbon (C2+) products through the coupling of adsorbed *CO intermediates. Ethanol (EtOH), a significant chemical feedstock and liquid fuel, is particularly attractive due to its wide applications, high energy density, and potential for large-scale energy storage and transport. However, achieving high current density and Faradaic efficiency (FE) for EtOH production using Cu-based catalysts remains a significant challenge. EtOH and ethylene (C2H4), both 12-electron reduced products, share initial intermediates such as HCCOH. The more saturated structure of EtOH makes its intermediates harder to stabilize on pure Cu surfaces compared to C2H4, leading to lower EtOH yields. Therefore, improving the selectivity and efficiency of CO2RR towards EtOH is crucial for advancing this technology as a renewable chemical feedstock. Previous research has focused on optimizing Cu-based catalysts through various strategies including morphology and facet control, vacancy engineering, dopant modification, and defect control. Modification of Cu with other CO2-active metals to form bimetallic catalysts has shown promise. This study explores the use of a silver (Ag)-modified copper oxide catalyst to enhance ethanol production, addressing the challenges associated with low selectivity and efficiency.

Numerous studies have explored strategies to enhance the selectivity and efficiency of CO2RR towards multi-carbon products, particularly ethanol. These strategies include controlling the morphology and exposed facets of the copper catalyst, introducing vacancies to modify the electronic structure, incorporating dopants or modifiers to alter the binding energies of reaction intermediates, and engineering defects to create active sites. Several studies have highlighted the potential of bimetallic catalysts, combining copper with other metals such as gold or silver, to improve catalytic performance. The modification of Cu with other CO2-active metals is attractive for boosting EtOH production by creating unique bimetallic sites. For example, Jaramillo et al. demonstrated that Cu-Au bimetallic catalysts can promote asymmetric C-C coupling, stabilizing reaction intermediates and boosting EtOH production at high current densities. This work builds upon these existing studies by exploring the use of silver as a modifier for Cu-based catalysts to enhance EtOH production. The literature suggests that the interaction between copper and silver could lead to changes in the electronic structure and surface properties of the copper, which may influence the adsorption and reactivity of CO intermediates and ultimately improve selectivity toward ethanol.

The researchers prepared pristine Ag-modified Cu2O nanocubes (Cu2O/Ag NCs) using a one-pot seed-mediated method. Cu2O NCs were synthesized by reducing Cu(OH)2 with ascorbic acid (AA), followed by the addition of AgNO3. TEM and other characterization techniques such as XPS, SEM-EDS, and XRD confirmed the formation of the heterostructure with Ag nanoparticles dispersed on the Cu2O surface. The activation and in-situ characterization of the Cu2O/Ag2.3% NCs were conducted under CO2RR conditions using a flow cell with a gas diffusion electrode (GDE). This revealed the actual state of the catalyst during CO2RR and its evolution. Several characterization techniques including HAADF-STEM, SEM, EDS mapping, ex-situ XRD, and operando X-ray absorption spectroscopy (XAS) were employed to investigate changes in the catalysts structure, phase, and coordination environment during activation. CO2RR performance of activated catalysts was evaluated via electrolyzing at specified currents. Linear sweep voltammetry, FE calculations for liquid and gaseous products using NMR and GC, and analysis of partial current densities were performed. The ratio of FE for ethanol to FE for ethylene was examined to understand the selectivity of the catalyst. The electrochemical active surface area (ECSA) was calculated via different methods to ensure a fair comparison with reported data. A series of Cu2O/Ag NCs with varying Ag content were prepared and assessed for CO2RR performance. The stability of the optimized catalyst was evaluated through long-term chronopotentiometry testing. Furthermore, the catalyst performance was examined in a commercially relevant membrane electrode assembly (MEA) to evaluate its practicality. The mechanism for boosted EtOH production was investigated using in-situ attenuated total reflectance infrared absorption spectroscopy (ATR-IRAS) to identify reaction intermediates. CO reduction reaction (CORR) on dCu2O/Ag was studied to understand whether the enhanced C-C coupling and EtOH generation follow the classic CO-tandem mechanism. CO-TPD and CO2-TPD were used to analyze the binding strength of CO and CO2 on the catalyst surface.

The study successfully synthesized a novel silver-modified copper-oxide catalyst (dCu2O/Ag2.3%). This catalyst demonstrated significantly enhanced performance in electrocatalytic CO2 reduction to ethanol. Specifically, it achieved a Faradaic efficiency (FE) of 40.8% for ethanol production, along with an energy efficiency (EE) of 22.3% at a high partial current density of 326.4 mA cm−2 at −0.87 V vs reversible hydrogen electrode (RHE). This surpasses most previously reported Cu-based catalysts. In situ characterization revealed that the Ag modification leads to changes in the coordination environment and electronic structure of the Cu surface, optimizing the CO binding strength. The catalyst displayed a mixed atop and bridge CO adsorption configuration, which triggers an asymmetric C-C coupling mechanism responsible for the boosted EtOH selectivity. In-situ ATR-IRAS studies confirmed the presence of key EtOH intermediates (*OC2H5) which are more stable on the dCu2O/Ag2.3% catalyst compared to other catalysts. The results also showed that the ratio of FEethanol/FEethylene was significantly higher for dCu2O/Ag2.3% compared to unmodified Cu2O and Cu2O/Au catalysts, indicating increased ethanol selectivity. The long-term stability tests showed no significant decay in activity over 6 hours of continuous operation in the flow cell, with only a 6% decrease in ethanol selectivity. The enhanced performance was also verified in a commercially relevant membrane electrode assembly (MEA), maintaining high current density and ethanol selectivity. The improved performance is attributed to the intrinsic properties of the Ag-modified oxide-derived Cu sites rather than changes in ECSA or catalyst mass loading.

The findings of this study address the crucial need for efficient and selective CO2RR catalysts for ethanol production. The high FE and EE for ethanol, coupled with the high current density achieved by the dCu2O/Ag2.3% catalyst, demonstrate its significant potential for practical applications. The mechanistic insights, obtained from in-situ ATR-IRAS studies, provide a clear understanding of how Ag modification influences the CO adsorption and subsequent C-C coupling pathways, leading to enhanced ethanol selectivity. This contrasts with previously reported CO-tandem catalysis mechanisms. The superior performance observed in the MEA further confirms the practical applicability of the catalyst. The high stability of the catalyst ensures the economic feasibility of the CO2RR process. These findings contribute significantly to the development of advanced CO2 reduction technologies and pave the way for more efficient and sustainable chemical synthesis methods. The understanding of the asymmetric C-C coupling mechanism is a significant advancement in the field, offering a new strategy for designing catalysts with tailored selectivity.

This work successfully demonstrated that modifying copper oxide catalysts with silver can significantly boost CO2-to-ethanol conversion efficiency. The optimized dCu2O/Ag2.3% catalyst achieved high Faradaic and energy efficiencies for ethanol production at commercially relevant current densities. In-situ studies revealed an asymmetric C-C coupling mechanism as the key to the enhanced selectivity. Future research could focus on exploring other bimetallic combinations, optimizing the catalyst synthesis methods, and investigating the scalability of the process for industrial applications.

While the study demonstrates excellent performance in both flow cell and MEA setups, further long-term stability tests beyond 12 hours are needed to confirm the long-term durability of the catalyst under continuous operation. The study primarily focused on ethanol selectivity; a more comprehensive investigation of other products formed during the reaction would provide a more complete picture of the catalyst's overall performance. The effect of different electrolytes on catalyst performance warrants further exploration. While the mechanism is proposed based on several experimental results, more sophisticated theoretical calculations and simulations would further strengthen the mechanistic understanding.