Evaluating the phase-dependent electrocatalytic activity of manganese phosphides for the hydrogen evolution reaction

Abstract

To accomplish the mass production of hydrogen by electrochemical water splitting, it is essential to explore cost-effective, electrochemically active, and durable electrocatalysts for the hydrogen evolution reaction (HER). Transition metal phosphides (TMPs) have been regarded as promising HER electrocatalysts due to their remarkable electrochemical properties including high activity and stability. The intrinsic HER activity of TMPs can be effectively manipulated depending on the nature of TM, the stoichiometric ratio of TM and phosphorus (P) in TM_xP_y from TM-rich to P-rich phases, and the corresponding surface electronic structures. Recently, earth-abundant manganese phosphides (Mn–P) have shown superior HER electrocatalytic activities. However, the systematic understanding of phase-dependent HER activity has been limited by different morphological contributions over Mn–P phases due to different synthesis processes and particle growth behavior. In this study, a series of Mn–P compounds with Mn₂P, MnP, and MnP₄ phases have been introduced as HER electrocatalysts. The series of compounds were synthesized by using high energy mechanical milling to compare their intrinsic HER activities. Among the Mn–P compounds, the MnP/Gr nanocomposite showed the highest HER electrocatalytic activity. In addition, an anion exchange membrane (AEM) electrolysis cell catalyzed by MnP/Gr and NiFeOOH showed a high performance of approximately 0.8 A cm⁻² at 2.0 V_cell.