Flexible Molecular Precursors for Selective Decomposition to Nickel Sulfide or Nickel Phosphide for Water Splitting and Supercapacitance

Abstract

Abstract Herein, the synthesis of three nickel(II) dithiophosphonate complexes of the type [Ni{S 2 P(OR)(4‐C 6 H 4 OMe)} 2 ] [R=H ( 1 ), C 3 H 7 ( 2 )] and [Ni{S 2 P(OR)(4‐C 6 H 4 OEt} 2 ] [R=(C 6 H 5 ) 2 CH ( 3 )] is described; their structures were confirmed by single‐crystal X‐ray studies. These complexes were subjected to surfactant/solvent reactions at 300 °C for one hour as flexible molecular precursors to prepare either nickel sulfide or nickel phosphide particles. The decomposition of complex 2 in tri‐octylphosphine oxide/1‐octadecene (TOPO/ODE), TOPO/tri‐ n ‐octylphosphine (TOP), hexadecylamine (HDA)/TOP, and HDA/ODE yielded hexagonal NiS, Ni 2 P, Ni 5 P 4 , and rhombohedral NiS, respectively. Similarly, the decomposition of complex 1 in TOPO/TOP and HDA/TOP yielded hexagonal Ni 2 P and Ni 5 P 4 , respectively, and that of complex 3 in similar solvents led to hexagonal Ni 5 P 4 , with TOP as the likely phosphorus provider. Hexagonal NiS was prepared from the solvent‐less decomposition of complexes 1 and 2 at 400 °C. NiS (rhom) had the best specific supercapacitance of 2304 F g −1 at a scan rate of 2 mV s −1 followed by 1672 F g −1 of Ni 2 P (hex). Similarly, NiS (rhom) and Ni 2 P (hex) showed the highest power and energy densities of 7.4 kW kg −1 and 54.16 W kg −1 as well as 6.3 kW kg −1 and 44.7 W kg −1 , respectively. Ni 5 P 4 (hex) had the lowest recorded overpotential of 350 mV at a current density of 50 mA cm −2 among the samples tested for the oxygen evolution reaction (OER). NiS (hex) and Ni 5 P 4 (hex) had the lowest overpotentials of 231 and 235 mV to achieve a current density of 50 mA cm −2 , respectively, in hydrogen evolution reaction (HER) examinations.