Surface engineered CoP/Co₃O₄ heterojunction for high-performance bi-functional water splitting electro-catalysis

In the electrochemical water splitting process, integrating hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in the same electrolyte with the same catalyst is highly beneficial for increasing the energy efficiency and reducing the fabrication cost. However, most OER catalysts are unstable in the acidic solution, while HER shows poor kinetics in the alkaline solution, which hinders the scale-up application of electro-catalytic water splitting. In this work, a CoP/Co₃O₄ heterostructure is firstly fabricated and then O and P defects are introduced via surface engineering (s-CoP/Co₃O₄). The as-prepared material was employed as the catalyst towards electrochemical water splitting in an alkaline environment. In alkaline HER, a current density of -10 mA cm^-2 can be achieved at an overpotential of 106 mV vs. RHE. In the OER process, the overpotential of s-CoP/Co₃O₄ electrode is only 211 mV vs. RHE at 10 mA cm^-2 in 1 M KOH, and the corresponding Tafel slope is only 58.4 mV dec^-1 so that the s-CoP/Co₃O₄ electrode could be used as the bifunctional catalyst for alkaline water splitting. This work provides a simple and low-cost approach to fabricate a Co-based heterojunction electrode with unsaturated metal sites to improve the electro-catalytic activities towards water splitting.