Regulating the structure and morphology of nickel sulfides for electrochemical energy storage: The role of solvent pH

Nickel sulfides with different crystal forms show apparent difference in microstructure: NiS owns layered structure, while the NiS2 displays bulk structure. This leads to great difference in their performances. Hence, exploring a simple and versatile route for targeted synthesis of specific phase is vital for expanding the application of nickel sulfide. Herein, we designated a pH-dependent hydrothermal strategy to tune the crystal phase and structure of nickel sulfide. DFT was used to simulate the environment around nickel ions under acid and alkaline solvents, and the results show acidic conditions make S atom more competitive to approach Ni atom than alkaline conditions. The experimental results demonstrate that reducing the pH value of solvent favors sulfur-rich phase and increasing the pH value benefits sulfur-deficient nickel sulfides. Especially in strong acidic (1 M HCl) and alkaline (2 M KOH) conditions, bulk NiS2 with microscale and layered8-NiS with nanoscale can be obtained, respectively. The electrochemical results indicate the highest specific capacity of 8-NiS electrode (242.2 mAh g(-1) at 1 A g(-1)), two times higher than the NiS2 electrode (104.7 mAh g(-1) at 1 A g(-1)). The assembled TU-2MKOH//rGO hybrid supercapacitor (HSC) delivers a high energy density of 61.1 Wh kg(-1) at power density of 399.2 W kg(-1). This work gives some insights into phase regulation mechanism of nickel sulfide and this mechanism holds great potential in phase and composition control of other transition metal sulfides.