Synthesis Of Cobalt Sulfides And Their Nanocomposites For Applications In Energy Conversion And Storage

Advanced energy conversion and storage（ECS） devices（including fuel cells, solar cells, Li-ion batteries and supercapacitors） are expected to play a major role in the development of sustainable technologies that alleviate the energy and environmental challenges we are currently facing. The successful utilization of ECS devices depends critically on the materials. Recent progress has demonstrated that nanostructured transition metal sulfides are very promising candidates for efficient ECS systems based on their unique physical and chemical properties, such as conductivity, and thermal stability and cyclability. Among them, cobalt sulfides are charactized by their various possible stoichionmtries, such as Co1-xS, CoS, CoS2, Co3S4, Co₉S₈, and crystal structures, resulting in various properties for a wide range of applications.In this paper, Co₉S₈ hollow spheres with a dimater of about1 μm are synthesized by a template- and surfactant-free solution method in a mixture of ethylene glycol（ EG） and N,N-dimethyl formamide（ DMF）. The structure and morphology of the as-prepared Co₉S₈ hollow spheres are investigated by X-ray diffraction（ XRD）, scanning electron microscopy（ SEM） combined with energy dispersive X-ray spectroscopy（EDS） and transmission electron microscopy（ TEM）. The formation mechanism of Co₉S₈ hollow spheres are proposed based on the influence of volumetric ratio of EG and DMF, the spesies and ratio of cobalt salt and sulfur source and the reaction time on the morphology. The as-prepared Co₉S₈ hollow spheres deliver high discharge and charge capacities（1050 mAh/g and 591 mAh/g） in the first cycle at a constant current density of 0.1C, and still ratain a reversible capacity of 650 mAh/g after 75 cycles, with a reversible capacity of 600 mAh/g at different current densities from 0.1 C-2 C, proving its excellent cycling durability and high-rate capacity.On the basis of Co₉S₈ hollow spheres, Co₉S₈/graphene（Co₉S₈/rGO） nanocompsites is successfully fabricated by a simple reflux method, which exibit a remarkably improved Li-storage performance, excellent cycling stability as well as high-rate capability compared with pristine Co₉S₈ hollow spheres. Its discharge and charge capacities in the first cycle are 1690 mAh/g and 1210 mAh/g（at 0.1 C）, respectively. It obtains a reversible capacity of 1110 mAh/g at 0.1 C after 75 cycles, with a reversible capacity of 760 mAh/g at different current densities from 0.1 C-2 C.In addition, a cation exchanging method is introduced to prepare Co3S4/CuS hollow spheres（¹ μm）, with nanosheets inserting on the surface of the compsites. CoNi2S4/NiS hollow spheres with a diameter of about 300 nm are prepared by a reflux method. What’t more, The ratio of cobalt and nickel or copper may geatly influences the morphology and structure of the corresponding composites.Finally, photocatalytic activities of Co₉S₈, Co₉S₈/rGO and Co3S4/CuS are studied. After illumination of 90 minutes, the decoloring efficiency of Co3S4/CuS, Co₉S₈/rGO and Co₉S₈ reach 99. 8%, 97.3% and 85.6% respectively. Both Co3S4/CuS and Co₉S₈/rGO have higher photocatalytic activities than Co₉S₈.