Phosphorus-Induced One-Step Synthesis of NiCo₂S₄ Electrode Material for Efficient Hydrazine-Assisted Hydrogen...

Rational control of the reaction parameters is highly important for synthesizing active electrocatalysts. NiCo₂S₄ is an excellent spinel-based electrocatalyst that is usually prepared through a two-step synthesis. Herein, a one-step hydrothermal route is reported to synthesize P-incorporated NiCo₂S₄. We discovered that the inclusion of P caused formation of the NiCo₂S₄ phase in a single step. Computational studies were performed to comprehend the mechanism of phase formation and to examine the energetics of lattice formation. Upon incorporation of the optimum amount of P, the stability of the NiCo₂S₄ lattice was found to increase steadily. In addition, the Bader charges on both the metal atoms Co and Ni in NiCo₂S₄ and P-incorporated NiCo₂S₄ were compared. The results show that replacing S with the optimal amount of P leads to a reduction in charge on both metal atoms, which can contribute to a more stable lattice formation. Further, the electrochemical performance of the as-synthesized materials was evaluated. Among the as-synthesized nickel cobalt sulfides, P-incorporated NiCo₂S₄ exhibits excellent activity toward hydrazine oxidation with an onset potential of 0.15 V vs RHE without the assistance of electrochemically active substrates like Ni or Co foam. In addition to the facile synthesis method, P-incorporated NiCo₂S₄ requires a very low cell voltage of 0.24 V to attain a current density of 10 mA cm^-2 for hydrazine-assisted hydrogen production in a two-electrode cell. The free energy profile of the stepwise HzOR has been investigated in detail. The computational results suggested that HzOR on P-incorporated NiCo₂S₄ was more feasible than HzOR on NiCo₂S₄, and these findings corroborate the experimental evidence.