Recent advances in ruthenium complex-based light-driven water oxidation catalysts

•Ruthenium complex-based light-driven water oxidation catalysts are reviewed.•Photosensitizer–catalyst covalent assemblies are highlighted.•Photosensitizer–catalyst modified electrodes are highlighted.•Future directions and efforts are suggested. The light driven splitting of water is one of the most attractive approaches for direct conversion of solar energy into chemical energy in the future. Ruthenium complexes as the water oxidation catalysts (WOCs) and light sensitizers have attracted increasing attention, and have made a great progress. This mini-review highlights recent progress on ruthenium complex-based photochemical and photoelectrochemical water oxidation catalysts. The recent representative examples of these ruthenium complexes that are in homogeneous solution or immobilized on solid electrodes, are surveyed. In particular, special attention has been paid on the supramolecular dyads with photosensitizer and WOC being covalently hold together, and grafted onto the solid electrode.