Multi-potential-step chronocoulospectrometry for electrocatalytic water oxidation by a mononuclear ruthenium aquo complex immobilized on a mesoporous ITO electrode

Abstract

A new mononuclear Ru aquo complex [Ru(C₈Otpy)(H₂dcbpy)(OH₂)]²⁺ with 4,4′-dicarboxy-2,2′-bipyridine (H₂dcbpy) and 4′-octyloxy-2,2′:6′,2′′-terpyridine (C₈Otpy) ligands was synthesized to investigate electrocatalytic water oxidation by the complex immobilized on a mesoporous indium-doped tin oxide (meso-ITO) electrode using a multi-potential-step chronocoulospectrometric (MPSCCS) technique. UV-visible absorption spectroscopic data indicated that [Ru(C₈Otpy)(dcbpy)(OH₂)] (RuOH₂) is deprotonated to [Ru(C₈Otpy)(dcbpy)(OH)]⁻ (RuOH) on the meso-ITO surface even at pH 5.9 of the electrolyte solution. The cyclic voltammogram (CV) of the RuOH/meso-ITO electrode showed a pH-independent redox response at E_1/2 = 0.80 V vs. Ag/AgCl in the pH range of 5–12, being assigned to a non-proton-coupled 1e⁻ redox process of Ru^IIOH/Ru^IIIOH. The MPSCCS measurement of the RuOH/meso-ITO electrode between 0.2 and 1.5 V vs. Ag/AgCl showed that Ru^IV species (tentatively Ru^IVO) exist in a steady state of the electrocatalysis in the initial stage. This suggests that the electrochemical oxidation from Ru^IVO to Ru^VO could compete with the water nucleophilic attack for O–O bond formation involved in the rate-determining step under the employed conditions. The possibility that the water nucleophilic attack on Ru^IVO could also compete with the electrochemical oxidation of Ru^IVO to Ru^VO was suggested by the electrocatalytic water oxidation at a low applied potential of 1.4 V prior to the formation potential of Ru^VO. The MPSCCS measurement at 1.4 V for 1 h showed that RuOH is gradually transformed into an alternative catalyst (most likely RuO_x nanoparticles) on the electrode. The MPSCCS technique is promising to reveal the redox reactions and catalytic aspects of molecular catalysts immobilized on an electrode for water oxidation.