Phase transformation in nickel-rich phosphides (Ni2P and Ni12P5) via transition metal (Cu, Mn) substitution enabling high supercapacitance and water splitting
Gwaza Eric Ayom, Malik Dilshad Khan, Felipe M. de Souza, Wang Lin, Ram K. Gupta, Neerish Revaprasadu
Abstract
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 Ni 2 P to Ni 12 P 5 induced by transition metal doping. The decomposition of nickel acetylacetonate , Ni(acac) 2 , in tri-octylphosphine oxide (TOPO)/tri- n -octylphosphine (TOP) formed phase-pure Ni 2 P while a mixture of phases (Ni 2 P/Ni 12 P 5 ) was obtained from the hexadecylamine (HDA)/TOP system. Transition metal-induced phase transformations were observed under similar reaction conditions. Doping Ni 2 P in TOPO/TOP with 5 and 10 % Mn and Cu resulted in Ni 2 P, Ni 2 P/Ni 12 P 5 , Ni 2 P and Ni 12 P 5 , respectively. On the other hand, a mixture of phases; Ni 2 P/Ni 8 P 3 , Ni 2 P/Ni 8 P 3 , Ni 2 P/Ni 12 P 5 and Ni 12 P 5 were obtained by doping Ni 2 P/Ni 8 P 3 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 Ni 2 P) electrode. NiP-3 (5 % Cu-doped Ni 2 P) and NiP-4 (10 % Cu-doped Ni 2 P), prepared in TOPO/TOP, showed the best performance for HER and OER with overpotentials of 233 and 271 mV to attain 10 mA/cm 2 current density , respectively. The transformation of Ni 2 P to Ni 12 P 5 not only results in better catalytic performance but also improved reaction kinetics, durability and stability of formed electrodes.