Engineering nickel phosphides for electrocatalytic hydrogen evolution: a doping perspective

Hydrogen fuel is regarded as the future clean energy alternative to fossil fuels. Water electrolysis has been touted as a key technology component in realizing a future hydrogen economy. The electrocatalytic hydrogen evolution reaction (HER), a fundamental step in water electrolysis, has therefore been the subject of immense investigation. In particular, the search for a low-cost and catalytically active HER electrocatalyst remains at the forefront of challenges from its practical and widespread implementation. Among transition metal-based compounds, nickel phosphides (Ni_xP_y) have emerged as one of the most nascent and low-cost candidates with unique properties and unprecedented HER performances. Motivated by the complexity, tunability and diversity of Ni_xP_y structure, the doping of Ni_xP_y emerges as a pivotal and effective strategy to optimize its catalytic activity at the intrinsic level. Nevertheless, the precise impacts of doping on Ni_xP_y have in-so-far been elusive. As such, this review aims to provide a conspectus of various doping approaches and expound the miscellaneous effects engendered through the doping of Ni_xP_y. Insights to respective doping nature and their corresponding active origins and mechanistic pathways which promote HER activity are distilled. This review serves as a roadmap towards the rational design of Ni_xP_y to catalyze a highly efficient HER.