Unusual doping induced phase transitions in NiS via solventless synthesis enabling superior bifunctional electrocatalytic activity

Transition metal sulfides have been investigated as promising bifunctional materials for catalytic energy generation and energy storage applications. Although various strategies such as tuning the size, phase or defects and composition engineering have led to catalytic enhancement, there still remains the requirement for better performance for practical applications. In this study, we have used a potentially scalable solventless route for phase selective synthesis of α-NiS or β-NiS. Both phases were doped with different transition metals (Cu, Co and Fe) for enhanced catalytic performance. Interestingly, besides commonly observed thermal assisted phase transition, dopant (Co, Cu, and Fe) induced α- to β-phase transition or vice versa was also observed which has rarely been reported for NiS. The effect of dopants on the crystal structure and electrocatalytic activity has been investigated. The best supercapacitive behavior was observed for Co-doped α-NiS which showed a specific capacitance of 1586 F g−1 at a current density of 0.5 A g−1 and a high rate capability. On the other hand, Fe-doped α-NiS displayed the best electrocatalytic activity for both the OER (266 mV at 10 mA cm−2) and the HER (146 mV at 10 mA cm−2), with Tafel slopes of 79 and 113 mV dec−1 respectively. The Fe-doped α-NiS catalyst was also used as both the anode and cathode in an electrolyzer, in which an overpotential of about 410 mV at 10 mA cm−2 was observed. The prepared electrodes demonstrate outstanding stability and flexibility.