Multi-metallic Layered Catalysts for Stable Electrochemical CO 2 Reduction to Formate and Formic Acid.

Electrochemical CO 2 reduction (ECR) to value-added products such as formate/formic acid is a promising approach for CO 2 mitigation. Practical ECR requires long-term stability at industrially relevant reduction rates, which is challenging due to the rapid degradation of most catalysts at high current densities. Herein, we report the development of a bismuth (Bi) gas diffusion electrode on a polytetrafluoroethylene-based electrically conductive silver (Ag) substrate (Ag@Bi), which exhibits high Faradaic efficiency (FE) for formate of over 90 % in 1 M KOH and 1 M KHCO 3 electrolytes. The catalyst also shows high selectivity of formic acid above 85 % in 1 M NaCl catholyte, which has a bulk pH of 2-3 during ECR, at current densities up to 300 mA cm -2 . In 1 M KHCO 3 condition, Ag@Bi maintains formate FE above 90 % for at least 500 hours at the current density of 100 mA cm -2 . We found that the Ag@Bi catalyst degrades over time due to the leaching of Bi in the NaCl catholyte. To overcome this challenge, we deposited a layer of Ag nanoparticles on the surface of Ag@Bi to form a multi-layer Ag@Bi/Ag catalyst. This designed catalyst exhibits 300 hours of stability with FE for formic acid ≥70 % at 100 mA cm -2 . Our work establishes a new strategy for achieving the operational longevity of ECR under wide pH conditions, which is critical for practical applications.