Synthesis Of Functionalized Bioinspired Porous Carbon And Its Application In Energy Storage

Lithium-ion batteries(LIBs)are considered as promising energy storage devices for the portable devices and electric vehicles owing to their high energy density,high operating voltage,long run time,low self-discharge features and no memory effect.On one side,commercial graphitic carbon does not meet the increasing demand of high energy densities for LIBs as anode because of their low specific capacity and less lithium insertion.On the other hand,electrochemically active materials with high specific capacities such as metal oxides,transition metal dichalcogenides,and silicon nanoparticles suffer large volume changes during charge-discharge process,causing pulverization and capacity fading of the electrodes.Therefore,new substitute electrodes are developed as porous carbon with hierarchical structure.In this regard,biomass-derived porous carbon materials have gained great attention due to its fantastic features such as high specific surface areas,unique morphology,thermal and chemical stability,low cost and especially their hierarchical porous structure.The permeation of electrolyte is made better using the hierarchical porous structure.The micropores make a great contribution to increase the surface area and the mesopores provide the diffusion channels for fast transportation of electrolyte.The macropores are sufficient to act as an ion buffering reservoirs for the minimization of diffusion path in the interior surface.Porous carbon can be prepared using chemical such as glucose and also through some complicated methods such as template,non-template,and polymer carbonization processes.As compared to these approaches,porous carbon derived from biomass is given preference over the others due to its low cost and perfect hierarchical structures inherited from the biomass.Herein,a biomass sugarcane bagasse has been used as a precursor to derive a series of hierarchical porous carbon materials with allium-giganteum like microstructures.Then,the obtained hierarchical porous carbon has been used as the main matrix to make a composite with other materials such as graphene,iron oxide hydroxide,and transition metal dichalcogenides.We have prepared different composites of porous carbon,which has high specific capacity and can maintain for long cycles with high energy density as an anode.The detail is given in the following paragraphs.(1)Firstly,we designed a composite based on functionalization of porous carbon and reduced graphene oxide to use as an anode for long cycling performance in LIBs.Porous carbon has been synthesized from sugarcane bagasse with a hierarchical structure and a high specific surface area of 1620 m²g^-1.Then,the resultant porous carbon is combined with reduced graphene oxide by using a hydrothermal method to produce a novel hierarchical modified porous carbon/graphene(MPC/rGO)composite material.Prior to the formation of composite material,the porous carbon derived from bagasse was treated with nitric acid for surface modification to get a strong chemical attachment with reduced graphene oxide(rGO).Graphene can store more lithium ions on both sides of the sheets with edges.As an anode for lithium-ion battery,the MPC/rGO composite has delivered a high reversible discharging capacity of 617.3 mAh g^-1 after 600 cycles at a current density of 200 mA g^-1.In contrast,the physical mixture of MPC and rGO prepared by using mortar-pestle has exhibited only 272.5mAh g^-1 at the same test condition.The chemically bonded rGO does not only improve the electrical conductivity of MPC/rGO but also prevent it from structural collapsing upon a long term cycling.The remarkable electrochemical performance shown by the MPC/rGO composite is attributed to the chemical bonds formed between the reduced graphene oxide sheets and the hierarchical porous carbon.(2)In order to improve the specific capacity of porous carbon,then we designed a composite consisting of hydrothermally modified porous carbon(HMPC)and?-FeOOH via a simple and effective hydrolysis method.Porous carbon was fabricated from bagasse by hydrothermal treatment in H₂SO₄ solution and then carbonized.The hydrothermal treatment was carried out to make the interconnected structure of porous carbon.Subsequently,functionalized with concentrated nitric acid enabled the in-situ formation of akaganeite nanorods using a simple hydrolysis method,resulting in the composite HMPC/?-FeOOH.When tested as an anode for lithium-ion batteries,the obtained HMPC/?-FeOOH composite showed a high discharge capacity of 898.8 mAh g^-1 at 200 mA g^-1 after 350 cycles and achieved a specific capacity of446.1 m Ah g^-1 at a current rate of 1 A g^-1 after 1000 cycles together with a remarkable coulombic efficiency of 99.9%.As a comparison,the HMPC exhibited a discharging capacity(465.7 mAh g^-1)which is nearly half of the capacity of the composite material at the same current density.This excellent electrochemical performance of the HMPC/?-FeOOH composite can be attributed to the synergistic effect of the interconnected porous structure of porous carbon and the electrochemically active tunnel type?-FeOOH nanorods.(3)To further improve the properties,we designed another composite that combines graphene and metal sulfide with the bio-derived porous carbon.The human cultivated sugarcane bagasse wastes are converted to hydrothermally modified porous carbon(HMPC)with high specific surface area.Meanwhile,a three dimensional(3D)aerogel was constructed based on in-situ growth of transition metal dichalcogenides(1T-MoS₂)on hydrothermally modified porous carbon(HMPC)with wrapping of reduced graphene oxide(rGO)through hydrothermal reaction.The resultant composites HMPC/MoS₂/rGO(1:2:0.5),and HMPC/MoS₂/rGO(1:1:0.5)formed a 3D hierarchical structured aerogel.Prepared by this simple and effective method have been evaluated as an anode for LIBs and as a supercapacitor,respectively.This HMPC/MoS₂/rGO(1:2:0.5)composite(ternary component)delivered a high specific capacity of 952 mAh g^-1 at 200 mA g^-1 after 200 cycles and outstanding high-rate capability as an anode for LIBs,while another composite sample HMPC/Mo S₂/rGO(1:1:0.5)showed the best performance as a supercapacitor by delivering a specific capacitance of 385 Fg^-1 at 1 Ag^-1.Such an excellent and stable performance of the composite in energy storage can be attributed to the synergistic effect of MoS₂ nanosheets,graphene sheets,and porous carbon.The 1T-Mo S₂layered structured nanosheets are tightly lying on the surface of HMPC,making their strong contact with each other and thus reducing the diffusion path for both electrons and lithium ions.While the electrical conductivity of the composite system is enhanced by the graphene sheets.According to the work done in this thesis,natural biomass wastes are used effectively for the benefit of mankind.Moreover,we have achieved excellent results as expected from such a friendly material,which can be used in the future for energy storage application on industrial scale.