Synthesis of Ni-Co Oxides/Sulfides for Electrochemical Capacitors
Authors: Xiao Peng , Li Yan Hong , Zhang Yunhuai 
Published: February 2020

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 S2- 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 1.8 V 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.

As we all know, dimensionality and rational design of electrode architectures plays a crucial role in determining materials' fundamental properties and the electrochemical performance of supercapacitor. For a proof-of-concept, Ni-Co layered double hydroxides (LDH), NiCo2O4 and NiCo2S4 nanosheets supported on carbon fiber paper (CFP) substrate are prepared in our next work. When tested as the pseudo-capacitor positive electrode, the self-support nanosheets on CFP demonstrate good performance and rate capability as well as excellent cycling life, which contribute to the unique 2D nanosheets structure supported on 3D conductive CFP substrate with open permeable channels, facilitating electrolyte penetration and ensuring more efficient ion diffusion and faster electron transport. The asymmetric supercapacitor based on pseudocapacitance of both electrodes is further first realized by using NiCo2S4 nanosheets and FeOOH nanorods as positive and negative materials, respectively. And it exhibits high energy density and power density as well as outstanding cycling life.

Figure 1