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 Na2B4O7solution at pH 9.4 when applying 0.94 V versus Ag/AgCl in contrast to no catalytic current generation in the K2SO4solution at the same pH. HB4O7–and B4O72–ions were indicated to act as key cofactors for the induced catalytic activity of the CoO(OH) layer. The Na2B4O7concentration 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, Km= 28 ± 3.6 mM, and the maximum catalytic current density, Imax= 2.3 ± 0.13 mA cm–2. The Imaxvalue of HB4O7–and B4O72–ions was 1.4 times higher than that (1.3 mA cm–2) for the previously reported case of CO32–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 B4O72–ions compared with that of CO32–ions.