[Co9(H2O)6(OH)3(HPO4)2(PW9O34)3]16−

Water oxidation catalysis is generally considered one of the major hurdles in the development of an efficient water splitting process. Most available catalysts are either too expensive, too unstable, or too slow for real technological applications. In this chapter we illustrate the major features of an interesting candidate, the nonanuclear polyoxometalate [Co9(H2O)6(OH)3(HPO4)2(PW9O34)3]16 − (Co9). This POM, as others of its kind, has demonstrated to be a robust and fast catalyst promoting oxygen evolution in homogeneous and heterogeneous conditions. In addition to its excellent performance, and prospects, we are including many experimental data that confirm its genuine catalytic activity. This is a key issue since the harsh conditions of oxygen evolution are prone to provoke a major evolution in the catalyst or precatalyst. Being all inorganic and obtained from earth-abundant metals, Co9 matches most of the requirements to be successfully implemented in an overall water splitting process.