In the last five years and currently, research on solar fuels has been intense and no sub-area in this field has been more active than the development of water oxidation catalysts (WOCs). In this timeframe, a new class of molecular water oxidation catalysts based on polyoxometalates have been reported that combine the advantages of homogeneous and heterogeneous catalysts. This review addresses central issues in green energy generation, the challenges in water oxidation catalyst development, and the possible uses of polyoxometalates in green energy science.

Floriated ZnFe₂O₄ with porous nanorod structures were successfully synthesized via mild hydrothermal and calcination processes by using cetyltrimethylammonium bromide (CTABr) as a template-directing reagent. The resulting ZnFe₂O₄ was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS) and nitrogen adsorption measurement. It was found that the floriated ZnFe₂O₄ nanostructures were composed of porous nanorods with an average length of 122 nm and diameter of 29 nm. The obtained ZnFe₂O₄ with a bandgap of ∼1.94 eV was firstly used as a visible-light-driven photocatalyst for hydrogen production, and exhibits remarkable photostability in an aqueous suspension by using CH₃OH as a sacrificial reagent. Moreover, the possible photo-reaction mechanism for the hydrogen production from CH₃OH aqueous solution was proposed for better understanding the photocatalytic behavior of ZnFe₂O₄ without Pt-loading.