Synthesis And Electrochemical Properties Of Copper/Cobalt/Nickel Sulfides

Supercapacitors,also known as electrochemical capacitors,have attracted much research attention because of their high power density,fast charging process,long cycle life,and environmental friendliness.These advantages give it practical application in many fields,such as portable devices,electric vehicles,etc.Among all the potential electrode materials,transition metal sulfides have several unique properties,e.g.high theoretical specific capacity,abundant reserves,low toxicity.However,challenges still remain in low electrical conductivity,poor cycle performance,and lack of Faradaic electrochemical reaction active sites.Here,we reported a comprehensive work about the improvement of the electrochemical performance for electrode materials:NiS,NiCo₂S₄,and CuCo₂S₄.Two approaches were investigated to enhance their electrochemical performance:optimizing the morphology and chemical doping to prepare unitary,binary and ternary metal sulfides.1.CuCo₂S₄ materials were synthesized by a two-step method including hydrothermal and subsequent annealing.Crystalline CuCo₂S₄ were synthesized by annealing amorphous precursors at evaluated temperatures.A high temperature causes high-degree of crystallinity.The crystalline materials keep the same morphology and structure as the amorphous.The electrochemical performance of crystalline CuCo₂S₄was higher than that of amorphous CuCo₂S₄.The results in this work establish that the degree of crystallinity of CuCo₂S₄ materials will influence the electrochemical performances when being used as electrode materials for supercapacitor.The optimal material has the highest specific capacitance of 424 F g^-1 at 1 A g^-1 and good stability of 95%capacitance retaining after 20000 cycles.2.NiS and NiCo₂S₄ electrode materials were synthesized by a simple two-step hydrothermal method.The phase,structure and morphology were characterized,and the electrochemical properties were measured.Hollow NiS microspheres contained two phases of?-NiS and?-NiS.The irreversible transformation of?-NiS led to poor comprehensive electrochemical performance.Nevertheless,NiCo₂S₄ has self-assembled a three-dimensional mesoporous structure formed by many ultra-thin nanosheets.The electrochemical performance was much higher than that of NiS,indicating that ultra-thin nanosheets can improve the specific surface area and porosity volume.The higher conductivity and the synergistic effect of binary metal sulfide contributed to the improvement of electrochemical performance.3.Single-phase CuCo_xNi_2-xS₄ materials with different Ni/Co ratios have been successfully synthesized by a facile two-step hydrothermal method.Proper introduction of Ni can enhance the electrochemical performance by helping to form hierarchical structure,increasing the specific surface area and porosity,as well as providing more electrochemical active sites and offering more diffusion channels for electrochemical reactions without changing the structure and phase.The specific capacitance increased with the increase of Ni content,while too much Ni content would deteriorate the cycle stability of the electrode material.The optimized CuCo_1.25Ni_0.75S₄ electrode exhibits the best comprehensive electrochemical performance(647 F g^-1 at 1 A g^-1)and impressive cycling stability?⁹8%specific capacitance retention after 10,000 charge-discharge cycles?.This study shows a promising potential to fabricate high-performance ternary metal sulfides of CuCo_xNi_2-xS₄ for supercapacitor application.