Synthesis of Ni-Co Oxides/Sulfides Nanowire Arrays for Ultrahigh Electrochemical Capacitors

Electrochemical capacitors (supercapacitors) have attracted intense attention due to their high power density, fast charge-discharge process, long cycle life and used in many practical applications. Ni-Co composites, also known as ternary metal composites, have excited great interest in recent years because of their high-performance in supercapacitors. Owing to the coupling of two metal species, these materials could render the composites with rich redox reactions and improved electronic conductivity, which are beneficial to electrochemical applications. In particular, the tunable compositions in the Ni-Co oxides provide vast opportunities to manipulate the crystal structure and its physical/chemical properties. During the past years, different shapes and structures of Ni-Co oxides/sulfides were investigated and most of the researches were focused on NiCo₂X₄ (X=O, S). Very few works investigated Ni-Co composites with different Ni-Co molar ratio and its fabrication of free standing electrode structure.

Here we report the interesting formation of Ni-Co oxides and Ni-Co sulfide nanowire arrys (NWAs) with Ni/Co molar ratio at 1: 1 and their derived free standing structure on different substrates for ECs. Ni-Co oxides NWAs were synthesized on ordered TiO₂ nanotubes and Ni foam respectively by a facile hydrothermal method. Ni-Co sulfides NWAs were fabricated through S^2- ion exchange using synthesized Ni-Co oxides nanowires as precursor. The electrochemistry testes showed that this self-supported electrode is able to deliver ultrahigh specific capacitance. To raise the cell voltage and thereby boost the energy density more effectively, we use the Ni-Co sulfides NWAs for the battery-like Faradic electrode and activated carbon for the capacitive electrode to compose an asymmetric cell, and this has extended the cell voltage to 1.8V in an aqueous electrolyte, resulting in high energy density, high power density and good cycling stability all together. To the best of our knowledge, such a prototype device has not been fabricated and explored before.