Polyoxometalate Water Oxidation Catalytic Systems

Catalytic water oxidation is indispensable to solar fuel production. We have developed a family of soluble water oxidation catalysts (WOCs) based on polyoxometalates (POMs) that address many issues of concern or interest to the solar fuels research community: POM WOCs are soluble so their properties can be extensively probed experimentally and computationally, they are carbon-free and thus oxidatively resistant, thermally robust and hydrolytically stable over wide pH ranges dictated by the POM metal(s) and structure. Many POM WOCs have now been reported but truly in-depth studies of their reactivity and stability have been needed for clarity. This review addresses these issues: it elaborates pitfalls in the study of these systems, including common misinterpretations of experimental data, it discusses POM hydrolytic chemistry, and describes the many techniques used to date to probe POM WOC reactivity and stability, including some new ones that unequivocally distinguish between water oxidation by POMs versus hydrated metal cations and metal oxides that can be in equilibrium with or form from the POM.