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

The selectivity between CO and HCOOH in the 2e – CO 2 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 BiN 4 /G without energy barrier, simultaneously boosting CO 2 RR 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.