A new phosphonium-based ionic liquid to synthesize nickel metaphosphate for hydrogen evolution reaction

The electrochemical hydrogen evolution reaction (HER) is a promising strategy for production of hydrogen; however, it is still restricted by appropriate efficient and low-cost electrocatalysts. Herein, for the first time, the n-octylammonium hypophosphite, a kind of protic ionic liquid (IL), was used as a new phosphorus source for the manufacture of nickel metaphosphate (Ni₂P₄O₁₂) electrocatalysts. In contrast to traditional multi-step fabrication processes, the n-octylammonium hypophosphite acted as both reactant and solvent to synthesize Ni₂P₄O₁₂ by a one-step calcination approach. The obtained Ni₂P₄O₁₂ as an alkaline HER catalyst required a low overpotential of 116 mV at −10 mA cm⁻² and a small Tafel slope of 97 mV dec⁻¹, comparable to the majority of reported Ni-based materials and other phosphate catalysts. Furthermore, this catalyst exhibited robust stability with no distinct attenuation of current density after a long-term durability test in 1 M KOH. Therefore, this task-specific IL strategy with a simple reaction system, reducing the occurrence of side reactions, provided a new perspective on design of high-efficiency metaphosphate electrocatalysts. (paper)