Electrochemical conversion of carbon dioxide (CO₂) gas is a simple and cost-benefit method to produce economically valuable materials such as carbon monoxide (CO), multi-carbon (C₂₊) oxygenate, and hydrocarbons. In particular, ethanol (EtOH) production is attractive to be applied for conventional transport fuels. However, there has been a lack of techniques to yield predominant EtOH from CO₂. Copper (Cu), as the well-known and exclusive C₂₊ catalyst, thermodynamically prefers the production of (C₂H₄) compared to EtOH. Previous studies suggested that increased concentration of carbon monoxide (CO) as the intermediate of CO₂ ameliorated the EtOH selectivity.^1,2 Because the solubility of CO is very little in an aqueous electrolyte solution, surface diffusion of CO has been considered to enhance the local CO concentration. It suggests that catalyst interface is crucial to increasing EtOH yield, while little knowledge was developed.

We prepared porous Ag inverse opal (AgIO) frameworks with a uniform pore size of ~ 600 nm. The CO₂ gas was electrochemically reduced to CO with > 90% selectivity at -1.05 V vs. RHE. In addition, the CO settled in the pore, providing an opportunity for sequential electrochemical/chemical reactions. We deposited the ultra-thin Cu layer (2~10 nm) upon the Ag surface through the pulse electrodeposition technique (PED). This catalyst, indicated as PEDCu/AgIOs, performed ~33 % Faradaic efficiency (FE) of EtOH at -1.05 V vs. RHE. Moreover, PEDCu/AgIOs showed a high oxygenates ratio relative to hydrocarbons (FE_oxygenate/FE_C2H4) of 2.28, while H₂ evolution was significantly suppressed. These results suggested that as-prepared thin Cu film was partially segregated, and the Ag surface was exposed, forming the mixed Ag and Cu interfaces in the given electrochemical condition. This hypothesis was confirmed by negligible EtOH from the Ag-free CuIOs structure. More importantly, the resulting EtOH selectivity outperformed Cu nanoparticles (~7 nm diameter) dispersed on AgIOs (7.45 % FE) and aggregated Cu nanostructures on AgIOs (17.65 % FE) prepared by the constant current mode of electrodeposition. We, therefore, anticipated that CO spillover was promoted by the remaining and ultra-thin Cu layer. Further experiments were conducted by controlling of the thickness, pore size, and interpore size of AgIO frameworks to understand their roles. PEDCu/AgIOs with larger AgIOs thickness and smaller pore size showed higher EtOH selectivity, while the interpore size did not significantly affect the product selectivity. In the presentation, I will discuss key parameters to determine EtOH selectivity and the presumable pathway of EtOH production in details.

References

1. Ting, L. R. L.; Piqué, O.; Lim, S. Y.; Tanhaei, M.; Calle-Vallejo, F.; Yeo, B. S., Enhancing CO₂ Electroreduction to Ethanol on Copper–Silver Composites by Opening an Alternative Catalytic Pathway. ACS Catal. 2020, 10 (7), 4059-4069.
2. Gurudayal; Perone, D.; Malani, S.; Lum, Y.; Haussener, S.; Ager, J. W., Sequential Cascade Electrocatalytic Conversion of Carbon Dioxide to C-C Coupled Products. ACS Appl. Energy Mater. 2019, 2 (6), 4551-4559.