Spectroelectrochemical investigation of electrocatalytic water oxidation by a mononuclear ruthenium complex in a homogeneous solution

• Electrocatalytic water oxidation was studied by Potential-step chronocoulospectrometry.
• [Ru(EtOtpy)(bpy)OH2]2+ can work for electrocatalytic water oxidation under weakly acidic conditions.
• The rate-determining step for the electrocatalysis is oxidation of RuIVO to RuVO species.

Electrochemical water oxidation by [Ru(EtOtpy)(bpy)OH2]2+ (1) (EtOtpy = 4′-ethoxy-2,2′:6#6°6#′-terpyridine, bpy = 2,2′-bipydidine) was investigated in a homogeneous solution under weakly acidic conditions (pH 5.3). The cyclic voltammogram of a 1 aqueous solution showed that successive proton-coupled electron transfer reactions of RuIIOH2 / RuIIIOH and RuIIIOH/RuIVO redox pairs and high anodic current above 1.1 V vs SCE. Electrocatalytic water oxidation was corroborated by the bulk electrolysis at 1.5 V; the significant amount of O2 was evolved compared with the blank during the electrolysis. Potential-step chronocoulospectrometry (PSCCS) from 0.0 V to 1.52 V vs SCE was conducted to observe the change of 1 in solution during the electrocatalysis. The in situ UV visible spectral change showed oxidation of 1 (RuIIOH2) to RuIIIOH and to further oxidation of RuIIIOH to RuIVO, and that the RuIVO species mainly exists in a steady state after 200 s in the electrocatalysis. The in situ UV–vis spectral change in a reverse potential step from 1.52 V to 0.22 V vs SCE exhibited that RuIIOH2 completely recovers by two electron re-reduction process from the steady state in the electrocatalysis. The observation of RuIVO in a steady state suggests that a rate determining step in the catalytic cycle is oxidation of RuIVO to RuVO rather than the OO bonding formation by nucleophilic attack of water to RuVO in the electrocatalysis in a homogeneous solution.