A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based on Abundant Metals

Abstract

Traditional homogeneous water oxidation catalysts are plagued by instability under the reaction conditions. We report that the complex [Co₄(H₂O)₂(PW₉O₃₄)₂]^10–, comprising a Co₄O₄ core stabilized by oxidatively resistant polytungstate ligands, is a hydrolytically and oxidatively stable homogeneous water oxidation catalyst that self-assembles in water from salts of earth-abundant elements (Co, W, and P). With [Ru(bpy)₃]³⁺ (bpy is 2,2′-bipyridine) as the oxidant, we observe catalytic turnover frequencies for O₂ production ≥5 s⁻¹ at pH = 8. The rate’s pH sensitivity reflects the pH dependence of the four-electron O₂-H₂O couple. Extensive spectroscopic, electrochemical, and inhibition studies firmly indicate that [Co₄(H₂O)₂(PW₉O₃₄)₂]^10– is stable under catalytic turnover conditions: Neither hydrated cobalt ions nor cobalt hydroxide/oxide particles form in situ.