papers Phase transformation in nickel-rich phosphides (Ni2P2 P and Ni12P5) via transition metal (Cu, Mn) substitution enabling high supercapacitance and water splitting

The electrochemical properties of the Ni-P system can be optimized by modulating the surface-active sites via phase engineering. The preparation of selected phases of nickel phosphides applicable in electrochemical reactions is restricted by its multiplicity of phases and synthetic hurdles though they are highly desired. Herein, we report the facile preparation of nickel phosphides via a hot-injection method and the rare transformation of Ni2P to Ni12P5 induced by transition metal doping. The decomposition of nickel acetylacetonate, Ni(acac)2, in trioctylphosphine oxide (TOPO)/tri-n-octylphosphine (TOP) formed phase-pure Ni2P while a mixture of phases (Ni2P/Ni12P5) was obtained from the hexadecylamine (HDA)/TOP system. Transition metal-induced phase transformations were observed under similar reaction conditions. Doping Ni2P in TOPO/TOP with 5 and 10 % Mn and Cu resulted in Ni2P, Ni2P/Ni12P5, Ni2P and Ni12P5, respectively. On the other hand, a mixture of phases; Ni2P/Ni8P3, Ni2P/Ni8P3, Ni2P/Ni12P5 and Ni12P5 were obtained by doping Ni2P/Ni8P3 formed in HDA/TOP with 5 and 10 % Mn and Cu, respectively. Supercapacitance and water-splitting performance of pristine and transformed phosphide electrodes were investigated to examine the effect of phase engineering on catalyst performance. The highest specific capacitance of 748 F/g at 2 A/g was achieved by the NiP-2 (5 % Mn-doped Ni2P) electrode. NiP-3 (5 % Cu-doped Ni2P) and NiP-4 (10% Cu-doped Ni2P), prepared in TOPO/TOP, showed the best performance for HER and OER with overpotentials of 233 and 271 mV to attain 10 mA/cm2 current density, respectively. The transformation of Ni2P to Ni12P5 not only results in better catalytic performance but also improved reaction kinetics, durability and stability of formed electrodes.