Cobalt polyoxometalate-derived CoWO4 oxygen-evolving catalysts for efficient electrochemical and photoelectrochemical water oxidation

•POMs were converted to CoWO4 nanoparticles by annealing.•POM-derived a-CoWO4 exhibited excellent catalytic activity for water oxidation.•Its catalytic activity was closely related to the formation of oxygen vacancies.•It was readily integrated on a desired surface by adsorption and annealing of POMs.•This enabled simple preparation of efficient (photo)anodes for water oxidation.