Replacing rare and expensive noble-metal catalysts with inexpensive and earth-abundant ones is of great importance to split water either electrochemically or photoelectrochemically. In this study, two amorphous cobalt oxide catalysts (Co−W film and Co−Mo film) with high activity for electrocatalytic water oxidation were prepared by fast, simple electrodeposition from aqueous solutions of Na₂WO₄ and Na₂MoO₄ containing Co²⁺. In solutions of Na₂WO₄ and Na₂MoO₄, sustained anodic current densities up to 1.45 and 0.95 mA cm⁻² were obtained for Co−W film at 1.87 V versus a reversible hydrogen electrode (RHE) and Co−Mo film on fluorine-doped tin oxide (FTO) substrates at 1.85 V versus RHE. For the Co−W film, a much higher current density of 4.5 mA cm⁻² was acquired by using a stainless-steel mesh as the electrode substrate. Significantly, in long-term electrolysis for 13 h, the Co−W film exhibited improved stability in cobalt-free buffer solution in comparison with the previously reported Co−Pi film.

The activity of eleven separated iron complexes and nine in situ-generated iron complexes towards catalytic water oxidation have been examined in aqueous solutions with Ce(NH₄)₂(NO₃)₆ as the oxidant. Two iron complexes bearing tridentate and tetradentate macrocyclic ligands were found to be novel water oxidation catalysts. The one with tetradentate ligand exhibited a promising activity with a turnover number of 65 for oxygen evolution.

Chromophore-catalyst molecular assemblies towards visible light-driven water oxidation were synthesized by covalent integration of a light-harvesting complex [Ru(bpy)₃]²⁺ (bpy=2,2′-bipyridine) and a Co₄O₄ cubane water oxidation catalyst. The two components were assembled either in linear or macrocyclic configurations. In the presence of the sacrificial reagent, the Ru–Co metallocycle exhibits remarkable photocatalytic activity for oxygen evolution, which is one order of magnitude higher than that of a multicomponent system and exceeds that of a linear assembly by a factor of five, offering access to highly active photocatalyst through molecular design.