Unraveling the rate-limiting step of two-electron transfer electrochemical reduction of carbon dioxide

Electrochemical reduction of CO₂ (CO₂ER) has received significant attention due to its potential to sustainably produce valuable fuels and chemicals. However, the reaction mechanism is still not well understood. One vital debate is whether the rate-limiting step (RLS) is dominated by the availability of protons, the conversion of water molecules, or the adsorption of CO₂. This paper describes insights into the RLS by investigating pH dependency and kinetic isotope effect with respect to the rate expression of CO₂ER. Focusing on electrocatalysts geared towards two-electron transfer reactions, we find the generation rates of CO and formate to be invariant with either pH or deuteration of the electrolyte over Au, Ag, Sn, and In. We elucidate the RLS of two-electron transfer CO₂ER to be the adsorption of CO₂ onto the surface of electrocatalysts. We expect this finding to provide guidance for improving CO₂ER activity through the enhancement of the CO₂ adsorption processes by strategies such as surface modification of catalysts as well as careful control of pressure and interfacial electric field within reactors.