Rationally Dispersed Molybdenum Phosphide on Carbon Nanotubes for the Hydrogen Evolution Reaction

Molybdenum phosphide (MoP) is viewed as a potential electrocatalyst for the electrochemical hydrogen evolution reaction (HER). However, crystallization of MoP occurs at rather high temperature (>600 °C). At this temperature, coalescence and agglomeration, which affect the performance severely, become inevitable. Herein, an oxalate-guided nonhydrolytic method is demonstrated for the preparation of MoP with smaller particle size and better dispersion qualities onto the surface of carbon nanotubes (CNTs). Molybdenum is coordinated with the oxalate group using oxalic acid, which modifies the self-assembling of molybdenum at the molecular level and renders discrete nucleation and growth of MoP on CNTs. Phosphoric acid (crystalline) was used as a source of phosphorus. The method is simple with the potential to scale-up. A probable mechanism for the growth of MoP on CNTs is proposed. The as-derived MoP/CNT electrode exhibits excellent performance, outperforming most of the MoP-based electrocatalysts, for hydrogen evolution in both acidic and basic media. In addition, the electrode possesses excellent stability. The higher performance of the electrode is rationalized in terms of small particle size with uniform dispersion, high specific and electrochemically active surface area, electrical conductivity, interfacial charge transfer kinetics, and turnover frequency. Estimation of Tafel slope is consistent with electrochemical desorption of hydrogen gas following the Volmer-Heyrovsky mechanism as the rate-determining step.