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.