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

A new mononuclear Ru aquo complex [Ru(C 8 Otpy)(H 2 dcbpy)(OH 2 )] 2+ with 4,4′-dicarboxy-2,2′-bipyridine (H 2 dcbpy) and 4′-octyloxy-2,2′:6′,2′′-terpyridine (C 8 Otpy) 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(C 8 Otpy)(dcbpy)(OH 2 )] ( RuOH 2 ) is deprotonated to [Ru(C 8 Otpy)(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 II OH/Ru III OH. 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 IV O) exist in a steady state of the electrocatalysis in the initial stage. This suggests that the electrochemical oxidation from Ru IV O to Ru V O 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 IV O could also compete with the electrochemical oxidation of Ru IV O to Ru V O was suggested by the electrocatalytic water oxidation at a low applied potential of 1.4 V prior to the formation potential of Ru V O. 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. A new Ru complex was synthesized to investigate electrocatalytic water oxidation on a mesoporous ITO electrode using multi-potential-step chronocoulospectrometry.