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

Herein, the synthesis of three nickel(II) dithiophosphonate complexes of the type [Ni{S₂ P(OR)(4-C₆ H₄ OMe)}₂ ] [R=H (1), C₃ H₇ (2)] and [Ni{S₂ P(OR)(4-C₆ H₄ OEt}₂ ] [R=(C₆ H₅ )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, Ni₂ P, Ni₅ P₄ , and rhombohedral NiS, respectively. Similarly, the decomposition of complex 1 in TOPO/TOP and HDA/TOP yielded hexagonal Ni₂ P and Ni₅ P₄ , respectively, and that of complex 3 in similar solvents led to hexagonal Ni₅ P₄ , 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 Ni₂ P (hex). Similarly, NiS (rhom) and Ni₂ 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. Ni₅ P₄ (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 Ni₅ P₄ (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.