Inspired from the Mn-based water oxidation complex in photosystem II that generates oxygen from water with an exceptionally high efficiency 15, 16, 17, numerous OER catalysts have been synthesized and evaluated using relatively cheaper 3 d transition metals (Mn, Fe, Co and Ni) 8, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36. In this regard, the development of cost-effective, abundant-element-based and efficient OER catalysts is highly demanded. Noble-metal catalysts, such as Pt, IrO x and RuO x, exhibit outstanding OER catalytic activity however, the high costs of these catalysts prohibit their practical use 10, 11, 12, 13, 14. This inefficiency is primarily due to the rigid O–O double bond formation and the sluggish proton-coupled electron-transfer reactions 7, 8, 9. Although it has been studied for more than a half century, the inefficiency of the oxygen evolution reaction (OER) has not yet been resolved and is still regarded as a bottleneck in the integration of an overall water splitting system 1, 2, 3, 4, 5, 6, 7, 8. Splitting water into hydrogen and oxygen molecules via solar energy has been considered as one of the most environment-friendly ways to effectively use renewable energy resources from harvest to redistribution 1, 2, 3, 4, 5, 6. Our findings emphasize the importance of local cobalt coordination in the catalysis and suggest the possible effect of polyanions on the water oxidation chemistry. First-principles calculations suggest that the surface reorganization by the pyrophosphate ligand induces a highly distorted tetrahedral geometry, where water molecules can favourably bind, resulting in a low overpotential ( ∼0.42 eV). Although they exhibit various catalytic activities and stabilities during water oxidation, Na 2CoP 2O 7 with distorted cobalt tetrahedral geometry shows high activity comparable to that of amorphous cobalt phosphate under neutral conditions, along with high structural stability. Here we select four cobalt-based phosphate catalysts with various cobalt- and phosphate-group coordination as a platform to better understand the catalytic activity of cobalt-based materials. Although extensive studies have focused on the metal-oxide catalysts, the effect of metal coordination on the catalytic ability remains still elusive. The development of efficient and stable water oxidation catalysts is necessary for the realization of practically viable water-splitting systems.

Cobalt oxidation number