Defective crystalline molybdenum phosphides as bifunctional catalysts for hydrogen evolution and hydrazine oxidation reactions during water splitting

Abstract

Molybdenum phosphides (MoP) are emerging as an attractive catalyst for water splitting due to their excellent activity and stability. However, most of the MoP synthesized so far are crystalline MoP with relatively fewer exposed active sites and low electrical conductivity. Here, we use a metallo-organic Mo precursor (MoO₂(acac)₂) to create defect-rich crystalline MoP nanoparticles and uniformly anchor them on reduced graphene oxide (denoted as D-MoP/rGO). High-temperature thermal decomposition of the precursor generates gases, which induce a variety of defects in D-MoP/rGO along with a P-rich surface composition. The electrochemically active surface area of D-MoP/rGO is 8 times that of bulk MoP. D-MoP/rGO requires a small overpotential of 122 mV for the hydrogen evolution reaction (HER) to reach a current density of 10 mA cm⁻². Furthermore, density functional theory (DFT) calculation results reveal that surface P sites are the key active sites, which favor the adsorption of H atoms and also act as an H deliverer to promote the HER. Importantly, D-MoP/rGO is also a highly active catalyst for the hydrazine oxidation reaction (HzOR) and requires a low overpotential of 84 mV to reach a current density of 10 mA cm⁻². Thus, efficient HER||HzOR hybrid water-splitting can be realized using D-MoP/rGO, which can deliver a current density of 100 mA cm⁻² at a small cell voltage of 0.74 V and undergo a stable 12 h operation. In summary, a simple method has been demonstrated to generate high-performance metal phosphide electrocatalysts for total water splitting. Furthermore, the material design concept of creating defective crystalline nanoparticles using gas generating metal precursors can serve as a general approach to create various catalytic active defective crystalline materials.