Engineering an Ni₉S₈@Fe-NiP-O₂ core-shell catalyst: high activity and stability for oxygen/hydrogen evolution in...

To address the issues of low activity, inadequate stability, and susceptibility to deactivation in ammonia-containing alkaline water electrolysis catalysts, Ni₉S₈@Fe-NiP-O₂ bifunctional electrocatalysts with a fractional core-shell structure were effectively synthesised using a phosphating oxidation coupling strategy. The Ni₉S₈@N₃Fe-PBA precursor underwent structural transformation to form porous cubic-like particles, exhibiting significant electronic interaction between the Ni₉S₈ core and the Fe-NiP-O₂ shell, which effectively modulated the surface electronic structure. Electrochemical assessments indicate that in a 1 M KOH solution, the oxygen evolution reaction (OER) overpotential of Ni₉S₈@Fe-NiP-O₂ is at 235.6 mV at 20 mA cm^-2, while the hydrogen evolution reaction (HER) overpotential is as low as 125.4 mV. The Tafel slope and charge transfer resistance are markedly superior to those of Ni(OH)₂, Ni₉S₈, and other comparison samples, exhibiting the highest electrochemical double layer capacitance (C _dl) and electrochemical active surface area (ECSA). In a 25% ammonia aqueous solution, the catalyst exhibits exceptional catalytic activity for both OER and HER, with superior overpotential, kinetics, and mass transfer characteristics. The comprehensive water electrolysis assessment indicates that Ni₉S₈@Fe-NiP-O₂ demonstrates great efficiency and stability in water electrolysis across alkaline systems, ammonia aqueous solutions, and membrane electrode assemblies (MEAs). The current density in the MEA system can attain 950 mA cm^-2 at 60 °C. The results show that the core-shell synergistic effect, multi-stage porous structure and multi-component electronic regulation jointly endow the catalyst with excellent bifunctional catalytic activity and ammonia resistance.