Tuning the oxidation state of SnOx and mass transport to enhance catholyte-free CO2-to-formate electrolysis

Abstract

Electrochemical CO₂ conversion to formate is a promising potential pathway to facilitate carbon neutrality with industrial feasibility. However, designing an active catalyst and optimizing the CO₂ electrolyzer to enable energy-efficient CO₂ conversion is a continuing challenge. Herein, we demonstrate that the initial surface oxidation state of tin oxide (SnO_x) catalysts is a key and enduring factor in determining the CO₂-to-formate conversion efficiency. Comparing the selectivity and energy efficiency of formate generation on thermally-evaporated and annealed SnO_x catalysts reveals that catalysts that are initially SnO-rich at the surface show improved overall efficiency relative to catalysts that are initially SnO₂-rich. Moreover, we show that controlling the flow rate of CO₂ strongly affects overall CO₂-to-formate conversion activity in partially-concentrated CO₂ streams in a catholyte-free electrolyzer, which emphasizes the importance of mass transport of CO₂ to design an efficient CO₂ electrolyzer. These findings provide insights into the critical importance of the chemical state in non-stoichiometric transition metal oxide catalysts like SnO_x catalysts and CO₂ mass transport for CO₂-to-formate conversion, offering fundamental guidelines and an efficient carbon-negative CO₂ conversion.