Increasing demand for clean energy have triggered researches on alternative energy sources and devices to reduce use of fossil fuel. Hydrogen has been considered as one of the most promising energy source for future due to its high energy density and no air pollutant emission. Splitting water into hydrogen and oxygen is an environmentally friendly method for producing hydrogen gas. This technology can store excess electric energy in the form of chemical bonds of hydrogen, which can resolve an issue about surplus electric power of present renewable energy systems caused by irregular energy source such as airflow and sunlight.

Water electrolysis reaction is divided into two half reactions; hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). High overpotential of both HER and OER is the most significant problems to hamper reaction rate and overall efficiency of water electrolysis, especially OER has much higher overpotential than HER. Therefore, recently major efforts have been devoted to exploring active catalysts for the OER in water electrolysis cell.

Among many kinds of candidate materials for OER catalyst, cobalt (Co) and various Co based materials, including nanostructured Co₃O₄, CoSe₂, Co based perovskites, CoP, CoB and Co/N-doped carbon, have drawn much attention for use in the alkaline water electrolysis system. These Co based catalysts have low OER ovepotential in alkaline media comparable with precious metal based catalysts, such as IrO₂ and RuO₂. However, previous studies has focused mainly on the exploring desirable composites for high OER activity without careful mechanistic study. OER mechanism on Co based catalysts and descriptors for designing more efficient catalysts have been unclear yet.

Herein, we report novel hybrid type catalysts, which composed of Co and molybdenum carbide (Mo₂C), as efficient OER catalysts for alkaline water electrolysis, and evaluate the OER mechanism by investigating the effects of surface acidity of the catalysts on the OER activity in alkaline media. Mo₂C has very similar electronic structure with platinum (Pt) group metal. So, it can be promising candidate as an efficient electrocatalyst for water electrolysis system. We synthesized Co-Mo₂C hybrids using facile solution based process. Synthesized Co-Mo₂C hybrids exhibit enhanced activity and durability compared with Co and ruthenium dioxide (RuO₂) catalysts in alkaline media (0.1 and 1 M KOH). This result is ascribed to increase in surface acidity by formation of Co-Mo bimetallic surface on the Co-Mo₂C hybrids. Increase in surface acidity leads to increase in hydroxide ion (OH^-) adsorption on the catalyst surface, which can promote the OER kinetics in alkaline media.

Fig. 1. XRD patterns of Co-Mo₂C hybrids.

Fig. 2. OER activities of Co-Mo₂C hybrids, Co, RuO₂ and Mo₂C in 0.1 M KOH solution.