Tailoring supercapacitance and water splitting performance of nickel sulfides (NiS and Ni ₃ S ₂ ): a comparative study of colloidal and solventless synthesis

To overcome the potential issue of active site blockage by surfactants in colloidal synthesis, alternative synthetic approaches must be explored. In this study, we investigated both solvent-free and colloidal thermolysis routes to synthesize nickel sulfides (NiS and Ni₃S₂) using sulfur-based Ni complexes, [Ni(S₂CO(C₂H₅))₂] (Ni-Xan) and [Ni(S₂CN(C₂H₅)₂)₂] (Ni-DTC) as precursors. The solvent-free decomposition of these complexes produced ligand-free NiS and Ni₃S₂ in the absence or presence of triphenylphosphine (TPP), respectively. In contrast, colloidal thermolysis in oleylamine (OLA) led to phase-selective nickel sulfide formation (NiS and Ni₃S₂), with TPP facilitating desulfurization. The electrochemical performance of the synthesized materials was evaluated in water splitting and supercapacitance applications. Among the tested materials, NiS synthesized from Ni-Xan in OLA exhibited the highest specific capacitance (809.2 F g^-1 at 1 A g^-1) and energy density (34.9 Wh kg^-1), while NiS derived from Ni-DTC in OLA achieved the highest power density (281.7 Wh kg^-1). Additionally, the Ni₃S₂ electrode obtained <i>via</i> the colloidal route demonstrated superior HER performance, requiring only 197 mV (Tafel slope: 159 mV dec^-1) to reach a current density of 10 mA cm^-2. These findings underscore that simply eliminating surfactants and adopting a solvent-free method is not inherently sufficient to achieve high electrochemical performance. This study provides insights into the limitations of solvent-free synthesis and outlines potential prerequisites that may guide future optimization for improved electrochemical performance.