Efficient Electrocatalytic Water Oxidation by a Dinuclear Ruthenium(II) Complex with Vicinal Aquo and Hydroxo Groups Adsorbed on a TiO₂ Electrode

Abstract

Dinuclear ruthenium­(II) complexes, proximal,proximal-[Ru2(Hcptpy)2L­(μ-Cl)]3+ (Ru 2 (μ-Cl), Hcptpy = 4′-(4-carboxyphenyl)-2,2′;6′,2″-terpyridine and L = 5-phenyl-2,8-di­(2-pyridyl)-1,9,10-anthyridine) and proximal,proximal-[Ru2(cptpy)2L­(OH)­(OH2)]+ (Ru 2 (OH)­(OH 2 )), were synthesized with the aid of quantitative photoisomerization of a mononuclear ruthenium­(II) complex, distal-[Ru­(Hcptpy)­L­(OH2)]2+ (d-RuOH 2 ). Ru 2 (μ-Cl) and Ru 2 (OH)­(OH 2 ) were chemically adsorbed on a nanoporous TiO2 electrode via 4-carboxyphenyl linker moieties on the complexes, and the stability of these complexes adsorbed on the electrode was considerably improved by addition of 0.1 M KPF6 in a phosphate buffer solution due to the low solubility of PF6 salts of these complexes in water. Ru 2 (OH)­(OH 2 ) worked efficiently for electrocatalytic water oxidation on the electrode with an overpotential (ηO2 ) of 530 mV at a pH of 7.0 and a high catalytic current of 5.1 mA cm–2 at 1.6 V versus saturated calomel electrode (SCE) compared with Ru 2 (μ-Cl). This suggests that the dinuclear structure with vicinal OH2 and OH– ligands on each Ru center in Ru 2 (OH)­(OH 2 ) is important for efficient water oxidation catalysis. In bulk electrolysis at 1.16 V versus SCE using the Ru 2 (OH)­(OH 2 )/TiO2 electrode, O2 was evolved with 87% of Faraday efficiency. After the electrocatalysis, 75% of Ru 2 (OH)­(OH 2 ) remained on the electrode in Ru2 II(OH)­(OH2) (or Ru2 II(OH)2) and Ru2 III(OH)2 states, and 21% was eluted to the electrolyte solution in the higher oxidation states of Ru2 III(OH)2 and/or Ru2 IV(O)­(OH), even with the suppression effect on the complex elusion by addition of KPF6 maintained. The mechanistic investigation revealed the important catalytic aspect involving the active Ru2 IV(O)­(OH) state, which could be responsible for the O–O bond formation by intramolecular coupling of their oxos on the electrode surface.

Highly Efficient and Durable Electrocatalysis by a Molecular Catalyst with Long Alkoxyl Chains Immobilized on a Carbon Electrode for Water Oxidation

“…The E p values of the first redox process decreased with an increase in pH with a slope of −0.024 V pH –1 , indicating 1-proton-coupled 2-electron redox process from the Ru II Ru II (OH)­(H 2 O) state for 1 to Ru III Ru III (OH) 2 . The E p value (0.58 V at pH 7.0) for 1 is lower than those (0.77 V at pH 7.0) for proximal , proximal -[Ru 2 L­(cptpy) 2 (OH)­(OH 2 )] + on the nanoporous TiO 2 electrode mainly due to the electron-donating ability of the C 8 O groups introduced on tpy ligands. The second redox process provided the slope of −0.056 V pH –1 , indicating 1-proton-coupled 1-electron redox process from Ru III Ru III (OH) 2 to Ru III Ru IV (OH)­(O).…”

“…The onset potential (0.88 V) for the formation of the Ru IV Ru IV (O) 2 species (transformed to the Ru III Ru III (μ-OO) active species) affords the low overpotential (η onset ) value of 0.26 V, which is compared advantageously with those (η onset = 0.13–0.74 V) of molecular WOC-based anodes reported previously for water oxidation (Table S5). ,− The η onset value is lower by 0.16 V than that (η onset = 0.42 V) of proximal , proximal -[Ru 2 L­(cptpy) 2 (OH)­(OH 2 )] + on the nanoporous TiO 2 electrode, being ascribable to the electron-donating ability of the C 8 O groups introduced on the tpy ligands. The introduced C 8 O groups act as a dual role of the linker of 1 on the CP surface and decrease the η onset value.…”

“…The former reaction is considered to be a bottleneck in water splitting, which suffers from sluggish kinetics and high η due to its multielectron process with O–O bond formation from two molecules of water. − Therefore, much effort has been made to develop highly active and durable water oxidation catalysts (WOCs) of transition-metal complexes − and oxides. − Although various types of WOCs have been studied so far, molecular WOCs based on transition metals offer the advantages of fine tuning of their catalytic activities by rational ligand design and extensive investigations on the water oxidation mechanism by a combination of experimental and theoretical studies. For the water oxidation mechanism by molecular catalysts, O–O bond formation involving two molecules of water is a key step as it is most commonly the rate-determining step in the catalytic cycle. − Two main classes of mechanisms have been proposed for the O–O bond formation in water oxidation: radical coupling interaction of two metal-oxyl radicals (I2M) − and water nucleophilic attack (WNA) on a high-valent metal-oxo center. ,− …”

“…Moreover, for water oxidation on the electrode surface, the issue on the intermolecular distance between the adjacent metal-oxo centers on WOCs remains in the intermolecular I2M mechanism compared with the intramolecular mechanism. However, there are only a few successful illustrations of electrocatalytic water oxidation via the intramolecular I2M mechanism for efficient molecular WOC-based anodes. ,, …”

“…However, there are only a few successful illustrations of electrocatalytic water oxidation via the intramolecular I2M mechanism for efficient molecular WOC-based anodes. 19,28,33 We previously reported that a dinuclear Ru complex, proximal,proximal-[Ru 2 L(tpy) 2 (OH)(OH 2 )] 3+ (tpy = 2,2′; 6′,2″-terpyridine) chelated by a backbone L ligand (L = 5phenyl-2,8-di(2-pyridyl)-1,9,10-anthyridine) is active for electrocatalytic water oxidation in a homogeneous aqueous solution 25 and that the intramolecular O−O bond is formed between oxos derived from the vicinal aquo and hydroxo ligands. 36 From the view of the design of the molecule-based anode, we applied the dinuclear Ru derivative, proximal,proximal-[Ru 2 L(cptpy) 2 (OH)(OH 2 )] + (Hcptpy = 4′-(4-carboxyphenyl)-2,2′; 6′,2″-terpyridine) to a nanoporous TiO 2 electrode surface via the 4-carboxyphenyl linker moieties introduced on tpy auxiliary ligands.…”