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 Na 2 B 4 O 7 solution at pH 9.4 when applying 0.94 V versus Ag/AgCl in contrast to no catalytic current generation in the K 2 SO 4 solution at the same pH. HB 4 O 7 – and B 4 O 7 2– ions were indicated to act as key cofactors for the induced catalytic activity of the CoO(OH) layer. The Na 2 B 4 O 7 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 HB 4 O 7 – and B 4 O 7 2– ions was 1.4 times higher than that (1.3 mA cm –2 ) for the previously reported case of CO 3 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 HB 4 O 7 – and B 4 O 7 2– ions compared with that of CO 3 2– ions.