Effect of electrolyte cations on the selectivity of electrochemical <scp>CO</scp> ₂ reduction in a flow cell system

Abstract Electrochemical carbon dioxide reduction reaction (eCO 2 RR) offers a promising pathway for the sustainable production of hydrocarbons, with potential for industrial‐scale implementation. While electrolyte cations are known to influence product selectivity, their effects in flow cell systems still remain elusive. In this study, we systematically investigate the role of alkali metal cations under alkaline conditions on copper‐based eCO 2 RR in a flow cell configuration, with a particular focus on their interfacial interactions between the cations and the catalyst surface, and their impact on product distribution and electrochemical performance. Our results show that electrolytes containing lithium ions exhibit negligible eCO 2 RR activity, attributed to the strong interaction between Li + and CO 2 , which hinders the reaction. Sodium‐containing electrolytes favor the formation of C1 product (CH 4 ), while potassium and cesium ions promote the generation of C2 products (C 2 H 4 and C 2 H 5 OH). The variation in selectivity is found to be closely related to the hydration radius of the cations, which influences the structure of the electric double layer and modulates the interfacial electric field strength. These findings offer valuable insights into the rational design of electrolyte compositions for steering product selectivity in future eCO 2 RR applications.