Tuning composition of CuCo 2 S 4 -NiCo 2 S 4 solid solutions via solvent-less pyrolysis of molecular precursors for efficient supercapacitance and water splitting.

Mixed metal sulfides are increasingly being investigated because of their prospective applications for electrochemical energy storage and conversion. Their high electronic conductivity and high density of redox sites result in significant improvement of their electrochemical properties. Herein, the composition-dependent supercapacitive and water splitting performance of a series of Ni (1- x ) Cu x Co 2 S 4 (0.2 ≤ x ≤ 0.8) solid solutions prepared via solvent-less pyrolysis of a mixture of respective metal ethyl xanthate precursors is reported. The use of xanthate precursors resulted in the formation of surface clean nanomaterials at low-temperature. Their structural, compositional, and morphological features were examined by p-XRD, SEM, and EDX analyses. Both supercapacitive and electrocatalytic (HER, OER) properties of the synthesized materials significantly vary with composition (Ni/Cu molar content). However, the optimal composition depends on the application. The highest specific capacitance of 770 F g -1 at a current density of 1 A g -1 was achieved for Ni 0.6 Cu 0.4 Co 2 S 4 (NCCS-2). This electrode exhibits capacitance retention ( C R ) of 67% at 30 A g -1 , which is higher than that observed for pristine NiCo 2 S 4 (838 F g -1 at 1 A g -1 , 47% C R at 30 A g -1 ). On the contrary, Ni 0.4 Cu 0.6 Co 2 S 4 (NCCS-3) exhibits the lowest overpotential of 124 mV to deliver a current density of 10 mA cm -2 . Finally, the best OER activity with an overpotential of 268 mV at 10 mA cm -2 was displayed by Ni 0.8 Cu 0.2 Co 2 S 4 (NCCS-1). The prepared electrodes exhibit high stability, as well as durability.