Herein we report a comparative study for a series of cheap, robust, and efficient water oxidation electrocatalysts made up of the heptanuclear disk clusters ([M-7(OH)(6)L-6](ClO4)(2)center dot solvent (M = Co, Ni or mixed Co-Ni) and L = 2-ethoxy-6-(N-methyl-iminomethyl) phenolate) for renewable energy production. Experiments performed on the three complexes immobilized on conductive substrates reveal the catalytic efficiency in the order Co-Ni > Ni > Co based on threshold and overpotentials as well as electro-impedance parameters. Remarkably, the Co-Ni mixed metal cluster shows low onset overpotential of only 270 mV and a small Tafel slop of 53.4 mV/dec. Moreover, the Faradaic efficiency exceeds 90% for the Co-Ni cluster. Their characteristics are major improvements on those reported for transition metal complexes. These complexes could be proposed as promising electrocatalysts for water oxidation reaction. (C) 2017 Elsevier Ltd. All rights reserved.