Tunable cation vacancy engineering of FeP toward efficient hydrogen evolution reaction

Transition metal phosphides (TMPs) have emerged as promising and cost-effective electrocatalysts for the hydrogen evolution reaction (HER). However, their practical applications are hindered by insufficient catalytic performance. Herein, we develop a tunable cation vacancy engineering strategy to enhance the HER activity of FeP, one of the most inexpensive TMPs, via facile phosphidation and acid etching of zinc iron hexacyanoferrate (ZnFeHCF) with controllable Zn sacrificial agents. The non-exclusive incorporation of Zn²⁺ into the ZnFeHCF precursor ensures a uniform distribution of vacancies, while the adjustable Zn²⁺ content enables precise control of vacancy concentrations over a broad range (0.3-33.6%). Systematic investigations reveal that an optimal level of cation vacancies elevates the d-band center and strengthens hydrogen adsorption, thereby boosting the HER activity of FeP. Conversely, excessive cation vacancies induce severe lattice distortion and surface oxidation, resulting in deteriorated catalytic performance. At the optimized cation vacancy concentration of 16.2%, FeP exhibits the best HER activity, showing a small overpotential of 119 mV at 10 mA cm^-2, a low Tafel slope of 59.2 mV dec^-1, and excellent stability. This study demonstrates not only an efficient catalyst for the HER but also an effective methodology for cation vacancy engineering of TMPs toward diverse applications.