The Proximal Protonation Source in Cu-NH_x-C Single Atom Catalysts Selectively Boosts CO₂ to Methane Electroreduction

Regulating the coordination environment of active sites has proved powerful for tapping into their catalytic activity and selectivity in homogeneous catalysis, yet the heterogeneous nature of copper single-atom catalysts (SACs) makes it challenging. This work reports a bottom-up approach to construct a SAC (rGO@Cu-N(H_x)-C) by inlaying preformed amine coordinated Cu²⁺ units into reduced graphene oxide (rGO), permitting molecular level revelation on how the proximal N-site functional groups (N-H or N-CH₃) impact on the carbon dioxide reduction reaction (CO₂RR). It is demonstrated that the N-H moiety of rGO@Cu-NH_x-C can serve as an in situ protonation agent to accelerate the CO₂-to-methane reduction kinetics, delivering a methane current density (163 mA/cm²) 2.42-times that with the -CH₃ substituted counterpart rGO@Cu-N-C. Operando spectroscopic studies and theoretical calculations elucidate that the high methane faradaic efficiency (77.1 %) achieved here is enabled by opening up the energetically favorable formyl pathway (*OCHO pathway) against the traditional *CO pathway that normally leads to various CO₂RR products other than methane. Our strategy sets the stage to precisely modulate single-atom catalysts for efficient and selective electrochemical CO₂ reduction.