A Salen-based Dinuclear Cobalt(II) Polymer with Direct and Indirect Synergy for Electrocatalytic Hydrogen Evolution

Optimizing the spatial arrangement and geometric configuration of dinuclear metal sites within catalysts to leverage the dinuclear metal synergistic catalysis (DMSC) effect is a promising strategy for enhancing catalytic performance. In this work, we report a Salen-based dinuclear cobalt covalent organic polymer (Co₂-COP) that exhibits both direct and indirect DMSC synergistic effects, significantly improving catalytic efficiency for the electrocatalytic alkaline hydrogen evolution reaction (HER). Notably, one of the Co atoms in this structural unit features an OH⁻ anion. The OH⁻ anion facilitates both H₂O adsorption through p-p orbital overlapping interaction and the subsequent OH* intermediate removal by pre-attracting cations. As a result, Co₂-COP exhibits superior HER activity that surpasses its single-atom counterpart by a factor of 36. Control experiments and theoretical calculations revealed that the enhanced catalytic efficiency of Co₂-COP is attributed to both the direct DMSC effect between two Co^II ions, and the indirect DMSC involving the OH⁻ anion and alkali cations. This synergistic interaction significantly facilitates water activation and accelerates the removal of the OH* intermediate, all of which are intricately linked to the unique dinuclear structure of the material.