An amorphous dual action electrocatalyst based on cobalt sulfide for the hydrogen and oxygen evolution reactions

The fabrication of electrocatalysts that are active for more than one of the water splitting reactions has gained significant momentum. Here we demonstrate such a material produced via an electrochemical process that is based on amorphous cobalt sulfide films doped with oxygen which are active for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) under alkaline conditions. The optimum electrochemical protocol was found to be a repetitive potential cycling approach rather than a constant potential to create an amorphous CoSx film containing oxygen. Samples with a Co : S ratio of 1.56 : 1 were found to be active for the HER in 0.5 M H2SO4, phosphate buffer and 0.1 M NaOH. Significantly this activity is comparable to highly crystalline nanomaterials of cobalt sulfide. Density functional theory calculations indicated that a reduced S–Co coordination number, as encountered in amorphous materials, leads to an optimum binding energy for hydrogen adsorption on the material which facilitates good electron transfer kinetics. In addition, this material was also active for the OER in alkaline conditions with evidence of conversion to cobalt oxide which gave a low overpotential of 370 mV for an applied current density of 10 mA cm−2 with a Tafel slope of 67 mV dec−1. This simple approach shows promise for the fabrication of a dual action electrocatalyst for electrochemical water splitting under alkaline conditions