Understanding the first activation step of electrochemical CO2 reduction over Ag and Ni-N-C active sites

Understanding the first elementary step of CO2 activation over electrode surfaces is a prerequisite for its following precise valorization process. Even for CO2-to-CO conversion, the simplest 2e reduction reaction, the identity of its rate-limiting step (RLS) and the dependence of its reaction kinetics remain controversial. Herein, we comparatively evaluate the electrocatalytic kinetics of CO2 reduction over Ni-N-C single atom catalyst, polycrystalline and oxide-derived Ag nanocatalysts at well-defined reactant mass transport conditions using a membrane electrode assembly electrolyzer. We find the reaction kinetics for CO evolution is independent of CO2 partial pressure on Ni-N-C but be dependent on polycrystalline Ag, whereas this dependency is weakened on oxide-derived Ag with more under-coordinated surface sites exposed. The first principles calculations reveal the initial electron transfer to CO2 is the RLS for both oxide-derived and polycrystalline-Ag, while the subsequent proton transfer to CO2 is the RLS for Ni-N-C. Our findings indicate that the distinct electrostatic interactions between active sites and the adsorbed CO2 primarily contribute to these differences.