Iron-Doped Ni₅P₄ Ultrathin Nanoporous Nanosheets for Water Splitting and On-Demand Hydrogen Release via NaBH₄ Hydrolysis

The search for efficient overall water-splitting electrocatalysts plays an important role for low-cost hydrogen production. Metal phosphides are promising for integrating hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) properties in one bifunctional electrode, but several challenges still exist and need to be solved. We report a free-standing carbon cloth electrode consisting of iron (Fe)-doped Ni₅P₄ nanosheet arrays via a hydrothermal reaction and subsequent phosphorization, which effectively drives both HER and OER in the base. The as-prepared Fe-doped Ni₅P₄ with ultralow Fe conent (2.5 atom %) delivers a current density of 10 mA cm^-2 at 131 mV for HER and needs 257 mV to reach a current density of 100 mA cm^-2 for OER. Overall, water splitting requires only 1.50 V to reach a current density of 10 mA cm^-2. The introduction of trace Fe in Ni₅P₄ is effective to modulate the adsorption energy of the key intermediates for both HER and OER, which is promising for the rational design of transition-metal phosphides based on bifunctional water-splitting catalysts with high performance. In particular, Fe-doped Ni₅P₄ could effectively catalyze the hydrolysis of NaBH₄ with a hydrogen generation rate of 0.175 L g^-1 min^-1 and an activation energy of 53.41 kJ mol^-1, comparable to most transition-metal-based electrocatalysts.