Nanostructured 2D Transition Metal Dichalcogenides (TMDs) As Electrodes For Enhanced Supercapacitor Performance

To satisfy the increasing global demand of energy,it is urgent to develop new energy sources and energy storage devices.In this respect,supercapacitors are potential energy storage devices due to their properties of high-power density,long cyclic stability,and rapid charge/discharge ability.The performance of the supercapacitors mainly relies on the electrode materials with high electrical conductivity,easy charge(ion/electron)transportation,adequate porosity,and high surface area.To date,2D materials such as transition metal dichalcogenides(TMDs),reduced graphene oxide(RGO),transition metal oxides(TMOs),and their composites have been investigated as commendable electrode materials for high performance supercapacitors due to their outstanding merits including high specific surface area,additional electrochemical active sites,high conductivity,and single-layer morphology.This work mainly focuses on the synthesis of highly defective 1T-molybdenum disulfide(1T-MoS2),its composites with graphene and tungsten trioxide(WO3),and the functionalization of exfoliated 1T-MoS2 nanosheets.The hydrothermal process documented here is simple,ecofriendly,low cost,and less time consumption This fabrication route enables to synthesize MoS2 nanosheets with controllable defect density and phases(1T and 2H).These highly defective 1T-MoS2 hybrid and functionalized nanostructures are investigated as electrode materials for high performance supercapacitors.The first chapter of this thesis is a general introduction of two dimensional dichalcogenides(2D TMDs),fabrication techniques,and their applications.The second chapter provides the details of the materials synthesis,structural characterization and electrochemical techniques for the study of supercapacitor applications.The rest chapters(3,4,and 5)cover the following abstract1.Highly defective MoS2 nanosheets of 1T phase are synthesized on a three-dimensional reduced graphene oxide(3D RGO)network through a one-pot hydrothermal method.By switching the reaction time,the resulting MoS2's phases(1T or 2H)and defect-density are successfully controlled.Reducing the reaction time from 20 to 6 h increases the defect-density and induces a phase transition from hybrid 2H/1T to 1T phase.High defect-density coupled with high 1T-phase portion in MoS2 nanosheets/RGO heterostructures enhances the electrochemical performance for supercapacitor application by inducing additional active sites,which provide fast charge transfer rate and a large number of ion diffusion channels.Among all samples,the defect rich 1T-phase MoS2 nanosheets/RGO heterostructure synthesized in 6 h(M/RGO-6)gives an outstanding specific capacitance of 442.0 F g"1 at a current density of 1 A g"1.The M/RGO-6 also exhibits excellent cycling stability with capacitance retention of 84.5%over 3,000 cycles at 5 A g"1.These make M/RGO-6 attractive to the electrodes of high-performance supercapacitors.2.A new ternary composite of 1T-molybdenum disulfide,hexagonal tungsten trioxide,and reduced graphene oxide(M-W-rGO)is synthesized by using a one-pot hydrothermal process.The synergetic effect of 1T-MoS2 and hexa-WO3 nanoflowers improves the electrochemical performance for supercapacitors by inducing additional active sites and hexagonal tunnels,respectively,which leads to high storage capacity and easy transfer of electrolyte ions.The ternary M-W-rGO composite has a high specific capacitance of 836 F g-1 at 1 A g-1,which nearly doubles that of binary composites of M-rGO and W-rGO.Also,the ternary M-W-rGO shows high capacitance retention of 86.35%after 3,000 cycles at high current density of 5 A g"1.This study provides a new ternary composite as electrode material for high performance supercapacitors.3.Covalently functionalized 1T-MoS2(FC-1T-MoS2)nanosheets are synthesized through a chemical reaction between electrochemically exfoliated 1T-MoS2 nanosheets and iodobenzene.The NH2Ph ring functionality and 1T-MoS2 phase improves the electrochemical performance for supercapacitors by enhancing the electron density and electronic properties,which leads to fast charge transportation and high charge storage capacity.The FC-1T-MoS2 has a high specific capacitance of 501 F g-1 at current density of 1 A g-1.The performance is 2.6 times of magnitude higher than that of bare 1T-MoS2 nanosheets.This study exposes the effect of functionalization of MoS2 nanosheets for supercapacitor applications.This work could have implication for the controllable engineering of defect density and phases on MoS2,impacts on one-pot synthesis of ternary composite of MoS2 with graphene and WO3,and the functionalization of exfoliated MoS2 nanosheets.It could also open a new avenue for the development of electrode materials for high performance supercapacitors.