Impact of Local Environment on Selectivity of Ag and Cu Gas Diffusion Electrodes with Strong Hydrophobic Substrate during the Electrochemical Reduction of CO2

Electrochemical CO 2 reduction (CO 2 RR) is a promising pathway to convert detrimental CO 2 emissions into sustainable fuels and chemicals, and close the carbon loop 1 . To ensure the industrial applicability of this technology, electrocatalysts need to be highly active, selective towards a desired product, and offer long-term stability. High activity can be achieved with gas diffusion electrodes (GDEs), in which the CO 2 reactant is supplied directly from the gas phase, however reaching high selectivity toward a single product over time remains a challenge. Recently, the control of the local environment around the catalyst has been identified to be a key factor to ensure high selectivity and stability 2 . In this work, we show that the local environment around the catalyst drives the selectivity of the catalyst depending on the pore size and hydrophobicity of the GDE. GDEs with large pores and weak hydrophobicity lead to poor selectivity toward carbon products, while GDEs with small pore and strong hydrophobicity yield a greatly improved Faradaic efficiency (FE) towards CO in the case of Ag (FE > 70%) 2 and towards C 2 H 4 in the case of Cu (FE > 50%) 3 at 200 mA/cm 2 for several hours. The further increase of current density is detrimental to the performance of the GDEs, which points to a limitation of the transport of CO 2 to the electrocatalyst. Additionally, CO reduction experiments show a qualitatively similar behavior but at much lower current densities. These findings are consistent with the presence of an electrolyte layer wetting the electrocatalyst that forces the electrocatalytic reaction to take place at double phase boundaries. Finally, we provide evidence for the correlation between the loss of selectivity towards carbon products over time and the precipitation of carbonates by operando synchrotron X-rays diffraction experiments 4 . In summary, our results highlight the importance of controlling the local environment near the catalyst to enhance the selectivity towards carbon products during CO 2 RR. References 1 A. Senocrate, C. Battaglia, J. Energy Storage 2021 , 36, 102373 2 A. Senocrate, F. Bernasconi, D. Rentsch, K. Kraft, M. Trottmann, A. Wichser, D. Bleiner, C. Battaglia, ACS Appl. Energy Mater. 2022 , 5, 14504 3 F. Bernasconi, A. Senocrate, P. Kraus, C. Battaglia, Energy & Environ. Science Catalysis , 2023 , 1, 1009 4 F. Bernasconi, A. Senocrate, N. Plainpan, M. Mirolo, Q. Wang, C. Battaglia, submitted