Sulfonate group improves the solubility and electrocatalytic performance of Ru-based bda- and pda-type water oxidation catalysts under neutral conditions

Four ruthenium water oxidation catalysts that bear carboxylate and sulfonate groups in the active site have been synthesized and analyzed for their catalytic activity. The developed catalysts are modified from highly active species that traditionally have two carboxylate groups in the active site and are used to probe the effects of sulfonate substitution as well as the effects of other structural changes of the catalyst. The sulfonate-containing catalysts show higher electrochemical activity in pH 7 phosphate buffer with 3-5 times larger catalytic current, improved durability with sacrificial oxidant, and increased solubility compared to their dicarboxylate counterparts. Density functional theory calculations suggest that the sulfonate group provides more favorable geometry for water nucleophilic attack, of which is both the most energetically favorable mechanism calculated and experimentally predicted mechanism under electrochemical conditions. Further experimental studies have been performed to show that under certain conditions catalysts can perform well electrochemically under pH conditions as low as 1.6 and that various structural components can greatly change solubility and catalytic operation.