Phase transformations in the nickel phosphide system induced by transition-metal doping and their electro-catalytic study

Abstract

Nickel phosphide exists in various compositions, and the synthesis of pure-phase nickel phosphides is of immense interest due to their wide scale applications in different electrocatalytic reactions. We report the facile synthesis of nickel phosphides and rare transition metal-induced phase transformations within this system. Phase selective synthesis of pure Ni₂P or Ni₅P₄ was achieved by decomposition of nickel acetate tetrahydrate Ni(AC)₂·4H₂O in optimized mixed solvent systems, i.e., in tri-octylphosphine oxide (TOPO)/tri-n-octylphosphine (TOP) or hexadecylamine (HDA)/TOP, respectively, by hot injection route. The doping of 5% Cu or Mn in either of the nickel phosphide phases yielded a mixture of phases (Ni₂P/Ni₅P₄). However, increasing the Cu or Mn content to 10% resulted in the complete transformation of phase, i.e., from Ni₂P to pure Ni₅P₄ and vice versa. Lattice stress and size of incorporated dopants, as well as the nature of surfactants employed, were discussed as probable causes of these rare phase transformations. Moreover, in order to establish structure–activity relationship, we studied the comparative effect of transition metal dopants in both nickel rich and nickel deficient phases. Therefore, initially formed and transformed phosphides were investigated as electrocatalysts for overall water splitting and supercapacitance. NiP-5 (Ni₂P formed on 10% Cu doping of Ni₅P₄) delivered a current density of 10 mA cm⁻² with the lowest overpotential of 146 mV among all samples for HER while NiP-3 (Ni₅P₄ formed from Ni₂P on 10% Cu doping) similarly required the least overpotential of 276 mV for the OER at the same current density. NiP-2 (pristine Ni₅P₄) had the highest calculated specific capacitance of 1325 F g⁻¹ at 2 A g⁻¹. These phase transformations resulted in better catalytic activity and stability as well as reaction kinetics indicating suitability in practical water splitting technologies.