Cation-Modulated Selectivity Switching in 2e– CO₂ Reduction over Heterogeneous Single-Atom Catalysts Via the Protonation of COOH Intermediate

The selectivity between CO and HCOOH in the 2e- CO2 reduction reaction on heterogeneous single-atom catalysts (SACs) remains mechanistically contested. We propose a novel selectivity-determining step using ab initio molecular dynamics with slow-growth sampling method, taking p-block bismuth SACs as an example. The complete free energy calculation results demonstrate the second proton-coupled electron transfer step (COOH protonation) dictates both the rate-determining kinetics and ultimate product selectivity, contrary to the prevailing paradigm focusing on initial *COOH vs *OCHO formation. The alkali metal cation identity (AM+ = Li+, Na+, K+, Cs+) critically modulates this step; Li+ promotes C atom protonation of *COOH, leading to HCOOH. Na+/K+/Cs+ favor O atom protonation to yield CO with activity increasing following cation radius (Li+ < Na+ < K+ < Cs+). Specifically, K+/Cs+ enables the proton preadsorption in one of four N sites of the BiN4/G without energy barrier, simultaneously boosting CO2RR kinetics and suppressing HER, which matches well with experimental measurements by high CO Faradaic efficiency (92%) at the -0.5 V vs reversible hydrogen electrode. This work puts forward an innovative selectivity mechanism toward the strategic design of a CO or HCOOH product on the basis of distinct AM+ cation identity, which can also apply to other electrochemical interface systems, such as the metal-water system.