Preparation And Electrochemical Properties Of Nickel Sulfide/carbon Composites

With the continuous development of digitalization,intelligence and transportation electrification,there is an urgent demand to establish various clean,stable and high-efficiency electrochemical energy storing systems.As an advanced energy conversion and storage device,supercapacitors have been highly favored by researchers,while the development and optimization of their electrode materials are the key to enhance the energy storage performance.Recently,nickel sulfide has become a potential electrode material due to its excellent capacitive properties,abounding electrochemical reaction sites,tunable composition and low electronegativity.Unfortunately,nickel sulfides often suffer from the low electrical conductivity,agglomeration during preparation and volume expansion during electrochemical processes,resulting in undesirable electrochemical properties.To solve the above problems,carbon materials with diverse structures,high stability and good electrical conductivity are synergistically coupled with nickel sulfide by optimizing the preparation method,regulating the microscopic morphology and designing the hierarchical structure.Moreover,the energy storage mechanism and constitutive relationship of the electrode materials are precisely analysed by means of physical characterization,structural morphology analysis and electrochemical testing.The major work of this paper is as follows.1.To address the problem that nickel sulfide tends to agglomerate during the synthesis process to the extent that the active area is reduced,porous hollow carbon spheres/Ni S composites（PHCSs/Ni S）with three dimensional double shell structure by in situ hydrothermal method are successfully prepared.The hollow carbon spheres derived from waste yeast cell walls are used as a substrate to effectively suppress the aggregation of Ni S nanoparticles and ensure more active sites,thereby enhancing the energy storage performance of the composites.In addition,the specific capacity of PHCSs/Ni S composites are improved by adjusting the content of nickel ions during the reaction.The optimized PHCSs/Ni S-3 sample exhibits an excellent specific capacitance of 1436.9 F g^-1at 1.0 A g^-1 and a high capacitance retention of 81.3%after 5000 cycles.The hybrid supercapacitor assembled with PHCSs/Ni S-3 as the positive electrode and commercial activated carbon as the negative electrode displays a high energy density of 24.4 Wh kg^-1.This work cleverly realizes the crossover between environmental and energy fields and provides a feasible strategy to synthesize economical and efficient electrode materials.2.Porous hollow carbon spheres as the substrate can effectively improve the aggregation problem of Ni S,but the cycle stability of the composites is still poor.After investigation,it is found that the cycle life of the composites can be further improved by the modulation of micromorphology and stable combination of carbon material and Ni S.Herein,a series of Ni S/carbon hexahedrons build by self assembling nanoplates or nanorods via nitrilotriacetic acid（NTA）assisted hydrothermal strategy are synthesized.This results demonstrate that the complexation of NTA not only enables the chemical bonding of carbon materials and Ni S,but also modulates the micromorphology of the unit structures constituting the hexahedron.Benefiting from synergetic effect of hierarchical assembly mode and stable composite structure,the optimized Ni S/NTA-2exhibits superb electrochemical performance,including excellent specific capacitance of1530.4 F g^-1 at 1 A g^-1 and impressive cycling durability with 85.6%for up to 5000cycles.Besides,the constructed Ni S/NTA-2//AC hybrid supercapacitor device possesses a spectacular energy density of 35.1 Wh kg^-1.Based on this strategy,high performance energy storage devices can be further developed through uti lizing the modulation effect of metal complexes to design morphology controlled electrode materials.3.The advantage of the high specific capacitance of Ni S was not fully exploited in the composite material in the previous work.Herein,a novel and scalable“inner-outer synergistic”strategy is developed for superior Ni S_NF/CF@Ni S_NP composites via combining the two forms of Ni S with carbon fibers（CFs）.Uniformly sized Ni S nanoparticles and mutually independent Ni S nanoflakes are introduced into the internal and external regions of the carbon fibers,which not only alleviates the aggregation of nanoparticles,but also realizes the dual introduction of Faraday capacitance.Profiting from the felicitous designed architecture,the Ni S_NF/CF@Ni S_NP-3 electrode delivers a reversible specific capacitance of 1691.1 F g^-1 at 1 A g^-1.Furthermore,the hybrid supercapacitor of Ni S_NF/CF@Ni S_NP-3//AC demonstrates a maximum energy density of31.2 Wh kg^-1 and power density of 4004.3 W kg^-1 along with impressive cycling durability of 87.8%retention for up to 5000 cycles.This demonstrated“inner-outer simultaneous exploitation”engineering provides a new insights into the development of various types of fiber based energy storage materials.