Co-catalytic Effects of CoS₂ on the Activity of the MoS₂ Catalyst for Electrochemical Hydrogen Evolution

MoS₂ is a promising material to replace the Pt catalyst in the electrochemical hydrogen evolution reaction (HER). It is well known that the activity of the MoS₂ catalyst in the HER is significantly promoted by doping cobalt atoms. Recently, the Co-Mo-S phase, in which cobalt atoms decorate the edge positions of the MoS₂ slabs, has been identified as a co-catalytic phase in the Co-doped MoS₂ (Co-MoS_x) with low Co content. Here, we report the effect of the incorporation of cobalt atoms in the chemical state of the Co-MoS_x catalyst, which gives rise to the co-catalytic effect. Co-MoS_x catalysts with various Co contents were prepared on carbon fiber paper by a simple hydrothermal process. On the Co-MoS_x catalyst with high Co content (Co/Mo ≈ 2.3), a dramatically higher catalytic activity was observed compared to that for the catalyst with low Co content (Co/Mo ≈ 0.36). Furthermore, the co-catalytic phase in the Co-MoS_x catalyst with the high Co content was found not to be the Co-Mo-S phase but was identified as CoS₂ by Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and transmission electron microscopy. It is believed that CoS₂ is an alternative choice to co-catalyze HER on MoS₂-based catalysts.