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.