Recently, development of alternative energy for future has been under investigation to prevent environmental pollution caused by the huge use of fossil fuels. Among them, hydrogen is known as a clean energy source which does not emit pollutants. Effective hydrogen production using water electrolysis is one of the promising methods due to the simple procedure and high purity of product. However, its high cost limits the practical application of water electrolysis owing to the highly loaded platinum which is known as the best active catalyst for hydrogen evolution reaction (HER) To solve this problem, transition metals based compounds have been investigated as catalysts for HER.

In this study, the cobalt phosphide was fabricated on the carbon paper by using electrodeposition method. The morphology and composition of cobalt phosphide was controlled by bath composition and deposition potential/time. The morphology and composition of prepared CoP_x catalyst was characterized by field emission scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. Furthermore, transmission electron microscopy was utilized to examine the crystal structure and surface composition of CoP_x catalyst at the surface. All electrochemical measurements to explore the HER activity were conducted by using conventional three electrodes cell system with N₂-purged 0.5 M H₂SO₄ electrolyte. The HER catalytic activity and electrochemical surface area of CoP_x was measured by using cyclic voltammetry. Especially, the electrochemical surface area was estimated by double layer capacitance measurement varying the scan rates. As a result, it was revealed that the intrinsic HER activity of prepared CoP_x catalyst has significant relationship with their morphology and composition. Furthermore, the developed catalyst was adopted as hydrogen-evolving electrode for proton exchange membrane water electrolyzer (PEMWE).