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^- 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.