Mediator Enhanced Water Oxidation Using Rb4[RuII(bpy)3]5[{RuIII 4O4(OH)2(H2O)4}(γ-SiW10O36)2] Film Modified Electrodes

The water insoluble complex Rb4[RuII(bpy)3]5[{RuIII 4O4(OH)2(H2O)4}(γ-SiW10O36)2], ([RuIIbpy]5[RuIII 4POM]), was synthesized from Rb8K2[{RuIV 4O4(OH)2(H2O)4}(γ-SiW10O36)2] and used for electrocatalytic water oxidation under both thin- and thick-film electrode conditions. Results demonstrate that the [RuIIbpy]5[RuIII 4POM] modified electrode enables efficient water oxidation to be achieved at neutral pH using thin-film conditions, with [Ru(bpy)3]3+([RuIIIbpy]) acting as the electron transfer mediator and [RuV 4POM] as the species releasing O2. The rotating ring disc electrode (RRDE) method was used to quantitatively determine the turnover frequency (TOF) of the catalyst, and a value of 0.35 s–1 was obtained at a low overpotential of 0.49 V (1.10 V vs Ag/AgCl) at pH 7.0. The postulated mechanism for the mediator enhanced catalytic water process in a pH 7 buffer containing 0.1 M LiClO4 as an additional electrolyte includes the following reactions (ion transfer for maintaining charge neutrality is omitted for simplicity): [RuIIbpy]5[RuIII 4POM] → [RuIIIbpy]5[RuV 4POM] + 13 e– and [RuIIIbpy]5[RuV 4POM] + 2H2O → [RuIIIbpy]5[RuIV 4POM] + O2 + 4H+. The voltammetry of related water insoluble [RuIIbpy]2[S2M18O62] (M = W and Mo) and [FeIIPhen] x [RuIII 4POM] materials has also been studied, and the lack of electrocatalytic water oxidation in these cases supports the hypothesis that [RuIIIbpy] is the electron transfer mediator and [RuV 4POM] is the species responsible for oxygen evolution.