Cation-induced changes in the inner- and outer-sphere mechanisms of electrocatalytic CO₂ reduction

The underlying mechanism of cation effects on CO₂RR remains debated. Herein, we study cation effects by simulating both outer-sphere electron transfer (OS-ET) and inner-sphere electron transfer (IS-ET) pathways during CO₂RR via constrained density functional theory molecular dynamics (cDFT-MD) and slow-growth DFT-MD (SG-DFT-MD), respectively. Our results show without any cations, only OS-ET is feasible with a barrier of 1.21 eV. In the presence of K⁺ (Li⁺), OS-ET shows a very high barrier of 2.93 eV (4.15 eV) thus being prohibited. However, cations promote CO₂ activation through IS-ET with the barrier of only 0.61 eV (K⁺) and 0.91 eV (Li⁺), generating the key intermediate (adsorbed CO₂^δ-). Without cations, CO₂-to-CO₂^δ-(ads) conversion cannot proceed. Our findings reveal cation effects arise from short-range Coulomb interactions with reaction intermediates. These results disclose that cations modulate the inner- and outer-sphere pathways of CO₂RR, offering substantial insights on the cation specificity in the initial CO₂RR steps.