Two-step hydrothermal synthesis of NiCo2S4/Co9S8 nanorods on nickel foam for high energy density asymmetric supercapacitors

Abstract It is still a huge challenge to obtain a high-energy-density asymmetric supercapacitors and develop an active electrode material with excellent electrochemical characteristics. Although NiCo 2 S 4 has been considered as one of the promising positive electrode materials for asymmetric supercapacitors, the electrochemical performance of the NiCo 2 S 4 -based positive electrodes is still relatively low and cannot meet the demand in the devices. Herein, NiCo 2 S 4 /Co 9 S 8 nanorods with a large capacitance are synthesized via a simple two-step hydrothermal treatment. A high-performance asymmetric supercapacitor operating at 1.6 V is successfully assembled using the NiCo 2 S 4 /Co 9 S 8 nanorods as positive electrode and activated carbon as negative electrode in 3 M KOH aqueous electrolyte, which demonstrates a fairly high energy density of 49.6 Wh kg −1 at a power density of 123 W kg −1 , an excellent capacitance of 0.91 F cm −2 (139.42 F g −1 ) at current density of 1 mA cm −2 as well as a remarkable cycling stability due to the high physical strength, the large specific surface area, and the good conductivity for NiCo 2 S 4 /Co 9 S 8 nanorods and the brilliant synergistic effect for NiCo 2 S 4 and Co 9 S 8 electrode materials. The as-prepared NiCo 2 S 4 /Co 9 S 8 nanorods open up a new platform as positive electrode material for high-energy-density asymmetric supercapacitors in energy-storage.