Size-dependent catalytic activity of cobalt phosphides for hydrogen evolution reaction

Abstract Transition metal phosphides are a class of promising electrocatalysts for hydrogen evolution reaction (HER) to replace noble metals. In this work, we for the first time synthesize carbon supported CoP nanoparticles with the average particle sizes from 3.3 to 9.2 nm, via a solvothermal process followed by low-temperature topological phosphorization, and the size-dependent HER activity of the CoP is investigated by virtue of TEM, XRD, XPS and the electrochemical techniques. It is discovered that the 9.2nm-CoP particles possess high intrinsic HER catalytic activity as compared to the 3.3nm-CoP, although the smaller one displays a high mass activity due to the large surface area. Detailed studies manifest that the small CoP particles suffer from serious oxidation once exposing to air. In contrast, most cobalt remains in the quasi-metallic state in the relatively large CoP particles, which is beneficial for the desorption of Hads, the rate determining step of the HER process over CoP surface. In addition, the low charge transfer resistance across the liquid/solid interfaces also contributes to the excellent HER activity of the relatively large CoP particles.