A Molecular Silane-Derivatized Ru(II) Catalyst for Photoelectrochemical Water Oxidation

Photoanodes in dye-sensitized photoelectrosynthesis cells integrate molecular chromophore/catalyst assemblies on mesoporous n-type metal oxide electrodes for light-driven water oxidation. One limitation for sustainable photoanodes is the stability of chromophore/catalyst assembly on electrode surfaces for long periods. Progress has been made in stabilizing chromophores based on atomic layer deposition, polymer dip coating, C-C cross-coupling by electropolymerization, and silane surface binding, but little progress has been made on catalyst stabilization. We report here the silane-derivatized catalyst, Ru(bda)(L)₂ (bda = 2,2′-bipyridine-6,6′-dicarboxylate, L = 4-(6-(triethoxysilyl)hexyl)pyridine), catalyst 1, which is stabilized on metal oxide electrode surfaces over an extended pH range. A surface stabilization study shows that it maintains its reactivity on the electrode surface toward electrochemical oxidation over a wide range of conditions. Its electrochemical stability on electrode surfaces has been systematically evaluated, and its role as a catalyst for water oxidation has been explored. On surfaces of mesoporous nanostructured core/shell SnO₂/TiO₂, with a TiO₂ stabilized inner layer of the Ru(II) polypyridyl chromophore, [Ru(4,4′-(PO₃H₂)₂bpy)(bpy)₂]²⁺ (RuP²⁺ bpy = 2,2′-bipyridine), highly efficient photoelectrochemical water oxidation catalysis occurs to produce O₂ with a maximum efficiency of ∼1.25 mA/cm². Long-term loss of catalytic activity occurs with time owing to catalyst loss from the electrode surface by axial ligand dissociation in the high oxidation states of the catalyst.