Intramolecular Hydroxyl Nucleophilic Attack Pathway by a Polymeric Water Oxidation Catalyst with Single Cobalt Sites

Exploring efficient water oxidation catalysts (WOCs) is the primary challenge in converting renewable energy into fuels. Here we report a molecularly well-defined heterogeneous WOC with Aza-fused-π-conjugated-microporous-polymer (Aza-CMP) coordinated single cobalt sites (Aza-CMP-Co). The single cobalt sites in Aza-CMP-Co exhibited superior activity under alkaline and near-neutral conditions. Moreover, the molecular nature of the isolated catalytic sites makes Aza-CMP-Co a reliable model for studying the heterogeneous water oxidation mechanism. By combining the experimental and theoretical results, a pH-dependent nucleophilic attack pathway for O-O bond formation was proposed. Under alkaline conditions, the intramolecular hydroxyl nucleophilic attack (IHNA) process that the adjacent -OH group nucleophilically attacks Co⁴⁺=O was identified as the rate-determining step. The process leads to lower activation energy and boosted kinetics than that of the intermolecular water nucleophilic attack pathway. This study provides significant insights into the crucial function of electrolyte pH in water oxidation catalysis and enhancement of the water oxidation activity by regulating the IHNA pathway.