Cation-Directed Mechanistic Bifurcation in Electrochemical CO2 Reduction: Rate-Determining Step Switching and Opposite Temperature Dependencies on Ni Single-Atom Catalysts

Abstract While electrolyte cations are widely recognized as a key regulator of electrochemical CO2 reduction (CO2RR) activity, whether different cation classes merely modulate reaction rates or fundamentally redirect the underlying rate-determining step (RDS), and consequently the system's response to environmental variables, remains mechanistically unresolved. Herein, we elucidate the distinct CO2RR kinetic dependencies on a Ni-nitrogen-doped carbon single-atom (Ni–NC) catalyst using a systematic series of alkali-metal, organic quaternary ammonium, and ammonium cations. Using electrochemical analyses and in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy, we reveal a cation-induced switch in the RDS, supported by differences in intermediate distributions and protonation on Ni–NC. These divergent mechanistic pathways dictate opposite temperature dependencies of CO2RR activity: jCO increases with temperature under K+ conditions but decreases under NH4+. Alkali metal cations provide strong stabilization of the key intermediate, *CO2–, which renders the subsequent proton transfer involved in the RDS. In contrast, weak stabilization with quaternary ammonium cations causes the CO2 adsorption step to be involved in the RDS. Ammonium (NH4+), acting as a proton source that enhances protonation, yields higher performance than alkali metal cations while shifting the RDS back toward CO2 adsorption. This study provides critical mechanistic insights into cation-dependent RDS shifts and their subsequent impact on temperature dependency.