Superior Inorganic Ion Cofactors of Tetraborate Species Attaining Highly Efficient Heterogeneous Electrocatalysis for Water Oxidation on Cobalt Oxyhydroxide Nanoparticles

A heterogeneous catalyst incorporating an inorganic ion cofactor for electrochemical water oxidation was exploited using a CoO­(OH) nanoparticle layer-deposited electrode. The significant catalytic current for water oxidation was generated in a Na₂B₄O₇ solution at pH 9.4 when applying 0.94 V versus Ag/AgCl in contrast to no catalytic current generation in the K₂SO₄ solution at the same pH. HB₄O₇^– and B₄O₇^2– ions were indicated to act as key cofactors for the induced catalytic activity of the CoO­(OH) layer. The Na₂B₄O₇ concentration dependence of the catalytic current was analyzed based on a Michaelis–Menten-type kinetics to provide an affinity constant of cofactors to the active sites, K_m = 28 ± 3.6 mM, and the maximum catalytic current density, I_max = 2.3 ± 0.13 mA cm^–2. The I_max value of HB₄O₇^– and B₄O₇^2– ions was 1.4 times higher than that (1.3 mA cm^–2) for the previously reported case of CO₃^2– ions. This could be explained by the shorter-range proton transfer from the active site to the proton-accepting cofactor because of the larger size and more flexible conformation of HB₄O₇^– and B₄O₇^2– ions compared with that of CO₃^2– ions.