Synthesis Of Doped-or Ternary Metal Sulfides/Reduced Graphene Oxides Composites And Their Electrochemical Performance

Increasing energy demands and over-consumption of fossil fuels have brought about global environmental problems and air pollution.Therefore,it is vital to develop cost-effective and environment-friendly energy sources.Hydrogen with the ideal calorific value and high efficiency is considered the most promising clean energy.Lithium-ion batteries with high energy density,low self-discharge and long life is identified as the most efficient energy storage system.Both are potential alternatives to traditional energy sources.Looking for efficient,electrochemically stable and cheap hydrogen evolution catalysts and preparing novel negative electrode materials with excellent electrochemical performance remains challenging.Because of their high theoretical capacity,low cost and abundant reserves,metal sulfide nanomaterials（such as MoS₂,SnS₂,Bi₂S₃,CoS₂ and NiS₂）have been studied extensively.The low electronic conductivity and the electrode pulverization caused by volume change during lithium ions insertion/extraction process would lead to their poor cyclic stability and rate capability.In addition,MoS₂ also has outstanding catalytic hydrogen evolution activity,but the defects of easy aggregation and low conductivity greatly limit its catalytic activity.Owing to large specific surface area,excellent electrical conductivity and intrinsically mechanical property,graphene is believed as one of the most promising carbon matrices.The graphene can not only provide an elastic space to buffer the volume change of the electrode materials during the repetitive charge/discharge process,but also facilitate the faster electron transport during electrode reaction process.In addition,it has also been reported that metal element（such as Co or Ni）doping,hybridization with other metal sulfides and preparation of ternary sulfides also contribute to the improvement of conductivity of metal sulfides.In this paper,the electrochemical performance of metal sulfide/graphene composite is further improved by cobalt doping,complexing with another metal sulfide and fabrication of ternary metal sulfides.The characterization and electrochemical tests of the composites were carried out.The formation mechanism,the mechanism of lithium storage/electrocatalytic reaction and the reasons for improvement of electrochemical properties were discussed.The results demonstrate that the prepared materials have excellent lithium storage or hydrogen evolution properties,the main research conclusions as follows:（1）Co-doped CoxMo1-xS₂/reduced graphene oxides（CoxMo1-xS₂/RGO,x = 0.1,0.2,0.3 or 0.5）composites:First,graphene oxide sheets were prepared from graphite power by the modified Hummers method.Then the Co-doped CoxMO1-xS₂/RGO composites were prepared through a hydrothermal route between graphene oxide,CoCl2·6H2O,Na2MoO4·2H2O and L-cysteine.The effects of Co-doping on the microstructure and the electrochemical performance of CoxMo1-xS₂/RGO for lithium ion storage have been investigated.The results show that resonable Co doping can change the structure and morphology of the composites.Especially when x = 0.2,a large number of curly MoS₂ sheets were dispersed on the surface of RGO.The Co0.2Mo0.8S₂/RGO composite exhibit a high reversible capacity of 1236 mAh/g at the current density of 100 mA/g and enhanced high-rate capability of 895 mAh/g at the current density of 1000 mA/g.After 100 cycles,the capacity still remains 1223 mAh/g.In addition,the Co0.2Mo0.8S₂/RGO exhibits much higher Coulombic efficiency of 89.2%at the first cycle,in comparison with that（67.2%）of MoS₂/RGO.（2）Bi₂S₃-SnS₂/reduced graphene oxides（Bi₂S₃-SnS₂/RGO）composites:The Bi₂S₃-SnS₂/RGO composite was synthesized via a hydrothermal reaction between SnCl4·5H2O,Bi（NO3）3·5H2O and L-cysteine in the presence of graphene oxide sheets.The results showed that granular Bi₂S₃ was encapsulated or dispersed on the surface of reduced graphene oxides nanosheets or SnS₂ nanosheets.After 100 cycles at the current density of 100 mA/g,Bi₂S₃-SnS₂/RGO composites exhibited high reversible capacity of 1276 mAh/g.At the current density of 1000 mA/g,its reversible specific capacity increased to 877 mAh/g.（3）NiCo₂S₄/reduced graphene oxides（NiCo₂S₄/RGO）composites:It was synthesized via a hydrothermal reaction between Ni（Ac）2·4H2O,Co（Ac）2·4H2O and L-cysteine in the presence of graphene oxide sheets.NiCo₂S₄ nanoparticles uniformly dispersed on the surface of reduced graphene oxides.NiCo₂S₄/RGO composite delivers a reversible capacity as high as 1106 mAh/g in the initial 150 cycles at the current density of 100 mA/g and enhanced high-rate capability of 713 mAh/g at the current density of 1000 mA/g.（4）Co_0.1Mo_0.9S₂/reduced graphene oxides(Co_0.1Mo_0.9S₂/RGO)composites:Co_0.1Mo_0.9S₂/RGO composite was synthesized via a hydrothermal reaction between Ni（Ac）2·4H2O,Co（Ac）2·4H2O and L-cysteine in the presence of graphene oxide sheets.It exhibits a lot of short Co-doped MoS₂ sheets are well anchored on the surface of the reduced graphene oxides and exposes more active sites for hydrogen evolution reaction（HER）.Electrochemical tests also showed that the Co_0.1Mo_0.9S₂/RGO composite shows a remarkable catalytic activity toward HER with a low Tafel slope of 44.25 mV/dec,a lowest overpotential of 0.142 V vs.RHE and excellent durability（minimal degradation after 1000 cycles）.