Inversed Cation Size Effects on Methanol Formations From CO₂ Electroreduction by Immobilized Cobalt Phthalocyanine

The electrocatalytic reduction of CO₂ to methanol offers a compelling pathway for sustainable fuel synthesis, wherein cations in the electric double layer (EDL) exert a substantial influence on catalytic performance. Although cation modulation of CO₂-to-CO conversion has been extensively documented, its influence on downstream reduction pathways toward CH₃OH has received comparatively little attention. Using multiscale simulation, we establish that methanol synthesis over immobilized cobalt phthalocyanine (CoPc) is kinetically governed by the final proton transfer (*CH₂OH + H₂O → * + CH₃OH + OH^-). The EDL environment substantially accelerates this rate-determining step (RDS). Moreover, the activity exhibits a clear dependence on cation radius, following the trend Li⁺ > Na⁺ > K⁺ > Cs⁺, with smaller cations systematically lowering the proton transfer barrier. This trend stems from the enhanced accessibility of smaller cations to the transition state, where Li⁺ achieves tighter coordination than Cs⁺, conferring greater electrostatic stabilization and a correspondingly reduced barrier. Conversely, smaller cations attenuate the hydrogen-bond network surrounding OH^-, potentially impeding OH^- transfer from the catalyst surface to the bulk electrolyte. These multifaceted cation effects underscore the complex interplay between kinetic promotion and mass transfer limitations in electrocatalytic systems.