Electrokinetic insights into cation-coupled reaction pathways in electrochemical CO2 reduction to formic acid or formate

The electrochemical reduction of CO2 to formic acid or formate represents one of the most economically promising route for CO2 utilization. While substantial advances in catalyst design and electrolyzer engineering have been achieved in recent years, critical uncertainties remain regarding the reaction pathway and the often-debated role of alkali metal cations. Resolving these discrepancies requires precise kinetic analysis under well-defined conditions. In this work, we systematically investigate the kinetics of CO2 reduction to formic acid or formate across a wide pH range, enabled by two key developments: the identification of BiPO4 as a stable precatalyst under acidic conditions through comprehensive screening, and the implementation of sensitive ion chromatography for accurate product quantification, even at low current density where conventional methods struggle. Our electrokinetic data suggest that the reaction proceeds via sequential electron and proton transfers rather than proton-coupled electron transfer as proposed by many computational simulations. Notably, the rate-determining step transitions from the proton transfer step at low overpotential to the first electron transfer step at high overpotential, with the proton source dependent on electrolyte pH. Furthermore, through K+ reaction order analysis and crown ether chelation experiments, we demonstrate that the alkali cations are not merely spectators but actively participate in the reaction, likely by stabilizing negatively charged intermediates via electrostatic interactions.