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 (NixPy) 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 NixPy structure, the doping of NixPy emerges as a pivotal and effective strategy to optimize its catalytic activity at the intrinsic level. Nevertheless, the precise impacts of doping on NixPy 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 NixPy. 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 NixPy to catalyze a highly efficient HER.