Synthesis of Ni-Co Oxides/Sulfides for Electrochemical Capacitors
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.