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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 E1/2 = 0.80 V vs. Ag/AgCl in the pH range of 5–12, being assigned to a non-proton-coupled 1e− redox process of RuIIOH/RuIIIOH. The MPSCCS measurement of the RuOH/meso-ITO electrode between 0.2 and 1.5 V vs. Ag/AgCl showed that RuIV species (tentatively RuIVO) exist in a steady state of the electrocatalysis in the initial stage. This suggests that the electrochemical oxidation from RuIVO to RuVO 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 RuIVO could also compete with the electrochemical oxidation of RuIVO to RuVO was suggested by the electrocatalytic water oxidation at a low applied potential of 1.4 V prior to the formation potential of RuVO. The MPSCCS measurement at 1.4 V for 1 h showed that RuOH is gradually transformed into an alternative catalyst (most likely RuOx 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. [ABSTRACT FROM AUTHOR]

Catalytic water oxidation represents a bottleneck for developing artificial photosynthetic systems that store solar energy as renewable fuels. A variety of molecular water oxidation catalysts (WOCs) have been reported over the last two decades. In view of their applications in artificial photosynthesis devices, it is essential to immobilize molecular catalysts onto the surfaces of conducting/semiconducting supports for fabricating efficient and stable water oxidation anodes/photoanodes. Molecular WOC‐based anodes are essential for developing photovoltaic artificial photosynthesis devices and, moreover, the performance of molecular WOC on anodes will provide important insight into designing extended molecular WOC‐based photoanodes for photoelectrochemical (PEC) water oxidation. This Review concerns recent progress in the development of molecular WOC‐based anodes over the last two decades and looks at the prospects for using such anodes in artificial photosynthesis. [ABSTRACT FROM AUTHOR]

• A mononuclear ruthenium aquo complex was stably anchored on a TiO 2 surface. • The monolayer of the complex was formed on the TiO 2 surface. • The charge transport occurs via electron hopping between the complexes on TiO 2. • A high and steady catalytic current for O 2 evolution was provided. A mononuclear Ru aquo complex, [Ru(C 8 Otpy)(H 2 dcbpy)(OH 2)]2+ (C 8 Otpy = 4′-octyloxy-2,2′:6′,2′′-terpyridine, H 2 dcbpy = 4,4′-dicarboxy-2,2′-bipyridine) was stably adsorbed on a nanoporous TiO 2 surface to afford a TiO 2 electrode anchoring the complex. The adsorption isothermal of the complex on the TiO 2 surface was analyzed by the Langmuir adsorption model to provide a maximum coverage of Г max = 4.4 × 10−8 mol cm−2 and an adsorption equilibrium constant of K ads = 1.1 × 104 M−1, suggesting that the monolayer of the complex is formed on the TiO 2 surface. The cyclic voltammetry measurement of the complex on the TiO 2 surface suggested the diffusion-controlled charge transport via electron hopping between the complexes through the TiO 2 layer. Pourbaix diagram showed that the complex undergoes a non-H+-coupled 1e- redox reaction of a RuIIOH/RuIIIOH pair of the complex on the TiO 2 electrode due to pH buffering ability of the TiO 2 surface in a range of pH 5−11. Bulk electrolysis at 1.7 V vs Ag/AgCl using the complex-anchoring TiO 2 electrode provided a high and steady catalytic current density of 0.39 mA cm−2 with 83 % Faradaic efficiency for O 2 evolution during 1 h electrolysis. However, we presume that the complex is transformed to RuO x nanoparticles on the TiO 2 electrode during the electrocatalysis, on the basis of our earlier report on the complex / mesoporous ITO electrode system (Dalton Trans. , 2020 , 49, 1416−1423.). The RuOH /TiO 2 electrode is the efficient anode for water oxidation under the acidic conditions, irrespective of the molecular catalyst of RuOH and the alternative catalysts formed via its oxidative transformation. [ABSTRACT FROM AUTHOR]