Cobalt Molybdenum Phosphide As a Non-Noble-Metal Catalyst for the Hydrogen Evolution Reaction

Abstract

To replace Pt electrocatalysts for the HER in acidic electrolytes, several approaches have been exploited on transition metals through modifying the d–states at the Fermi level of the metal center by the alloying nonmetal elements such has sulfur, nitrogen and carbon.1 To further reduce the overpotential and enhance the stability, ternary compounds such as nickel molybdenum nitride2 have been shown enhanced performance in the HER. Very recently, metal phosphides, known as efficient hydrotreating catalysts, have also been demonstrated as HER catalysts, such as MoP,3 CoP,4 and Ni2P.5 Unlike bulk molybdenum dichalcogenides, which show poor activity, hexagonal molybdenum phosphide (MoP) exhibits good HER activity in bulk form, however, its performance is still limited by the intrinsic strong hydrogen binding energy. In this study, attempts to synergize the electronic properties of cobalt phosphide and molybdenum phosphide by synthesizing ternary cobalt molybdenum diphosphide lead to the formation of an efficient electrocatalyst for the HER. X-ray diffraction data and high-resolution transmission electron microscopy show that this cobalt molybdenum phosphide crystallizes in space group P63/mmc and a formula of CoMoP2 (Figure 1). Incorporation of the sequentially stacked layers of CoP6 and MoP6 prisms on a single CoMoP2 structure brings about a significant enhancement in the HER kinetics and fast charge transfer. Characterization using X-ray absorption, electrochemical impedance analysis and electrochemical activity and Tafel behavior will be discussed.