Successive cation and anion exchange strategy towards hierarchical Ni–Co sulfide arrays for high-performance hybrid supercapacitors

Abstract

The specific energy of hybrid supercapacitors (HSCs) is mainly limited by the low specific capacity and poor rate performance of the battery electrode. Herein, a series of Ni–Co sulfides with hierarchical architectures on Ni foam were constructed by successive cation and anion exchange reactions, and were investigated as battery electrodes for HSCs. This special hierarchical structure allows the active materials to be effectively exposed and participate in the redox reaction, and it also enables the rapid transport of electrons and electrolyte ions. In addition, the synergistic effect between Ni and Co constitutions can effectively improve the electrochemical activity and reversible specific capacity of HSCs. Benefiting from these advantages in combination, Ni–Co sulfide with an optimum cation reaction time exhibits a high specific capacity of 929 C g⁻¹ at 2 A g⁻¹, which is better than Co sulfides and other Ni–Co sulfide samples. The Ni–Co sulfide electrode has been used as the positive electrode for HSCs, which demonstrates a specific energy of 51.9 W h kg⁻¹ at a specific power of 838.7 W kg⁻¹ and a value of 14.1 W h kg⁻¹ that can be retained at 11.6 kW kg⁻¹. Our work developed a robust method for transforming monometal sulfides to their bimetal counterparts while maintaining the hierarchical structures, which is very promising for multifunctional material designing and performance optimization.