It is very important to exploit robust electrocatalysts for the water splitting in an alkaline medium. Hence, a series of Mo-doped Co9S8 nanorod array on Ni foam (Mo-Co9S8/NF) was successfully synthesized through hydrotherma and sulfuration processes for the first time and used as an efficient and stable difunctional electrocatalyst for the overall water splitting. Such Mo-Co9S8-3//Mo-Co9S8-2 electrodes couple display superior water splitting performance with the requirement of a cell voltage of 1.50 V to drive a catalytic current density of 10 mA cm(-2), which is lower than that of RuO2//Pt/C (1.52 V). The activity of the catalyst is greatly enhanced by the molybdenum ion doping and the instability of the sulfide is resolved. The experiment result shows that the relationship between the current density and pH is different in neutral and alkaline media, which is most be likely assigned to the change of O-O formation by transforming the reactants from water molecule to the hydroxy ion. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.