Preparation And Electrochemical Performances Of Biomass-derived Carbon Materials And Their Composites

The increasing demands of energy,the environmental pollution,increasing fossil fuel consumption and greenhouse gas emissions have become urgent global objectives for the sustainable development of economy and society.Developing high efficiency electrochemical energy storage devices is considered as an attractive route to keep the balance between energy and environment.Among various energy storage systems,supercapacitors are regarded as one of promising devices due to their fast charge/discharge rate,long cycling stability,high power density and environmental friendliness.However,the energy density of supercapacitors is still relatively low,which is directly related to the specific capacitance of electrode materials and the operating voltage of the assembled systems,restricting their further applications.Rational design of material structure has been turned out to improve the utilization of electroactive materials,leading to high specific capacitance.Meanwhile,the fabrication of asymmetric systems can enlarge the operating voltage of supercapacitors.Therefore,in this dissertation,we mainly enhance the energy density of supercapacitors through the rational design of the structure of electrode materials and the fabrication of asymmetric systems.Porous carbon materials derived from various biomasses have aroused intensive interests from scientific community due to their low cost,abundant resource,eco-friendliness and easy fabrication.Firstly,three-dimensional honeycomb-like hierarchical structured carbon(HSC)has been fabricated by a one-step carbonization/activation of abundant and low cost bacterial cellulose for ultrahigh-energy-density supercapacitors.Benefitting from its interconnected honeycomb-like hierarchical and open structure with high specific surface area,the prepared HSC exhibited superhigh specific capacitance of 422 F·g-1 at 2 mV·s-1 with remarkable rate performance(73.7%at 500 mV·s-1)in 6 mol·L-1 KOH aqueous electrolyte.Meanwhile,the symmetric supercapacitor could deliver a high energy density of 37.3 Wh·kg-1 in 1 mol·L-1 Na2SO4 aqueous electrolyte.Secondly,Ni-Co-Al double hydroxide(NiCoAl)has been synthesized through solvthermal approach.The highest specific capacitance is 1480 F·g-1 at 2 mV·s-1.Then NiCoAl nanosheets have been grown on the surfaces of graphene nanosheets.The resultant composite exhibited high specific capacitance of 2304 F·g-1 at 2 mV·s-1.To evaluate practical application,an asymmetric supercapacitor(ASC)fabricated with graphene/NiCoAl double hydroxide as the positive electrode and HSC as the negative electrode achieves conspicuously high energy density of 110.3 Wh·kg-1 and could still retain 21.2 Wh·kg-1 even at a high power density of 23.8 kW·kg-1,which is highly comparable with or even higher than those of the previously reported asymmetric supercapacitors in aqueous electrolytes.Furthermore,our asymmetric supercapacitor exhibits excellent cycling stability along with 83%capacitance retention after 7000 cycles.Thirdly,C-BC/MoS2 was controllably synthesized by a hydrothermal approach,the prepared C-BC/MoS2 hybrid showed a typical crinkly structure with ultrathin MoS2 nanosheets.The obtained the C-BC/MoS2 composite displayed maximum specific capacitance of 320.7 F·g-1 at 5 mV·s-1 in 1 mol·L-1 Na2SO4 aqueous electrolyte.Additionally,C-BC/MnO2 composition were synthesized by an in-situ redox method.The specific capacitance of the flower-like C-BC/MoS2 reached up to 267.5 F·g-1 at 2 mV·s-1,Meanwhile,the specific capacitance increased by about 106.4%of initial capacitance after 8500 cycles at scan rate of 200 mV·s-1,Show the good cycle stability.Finally,an ASC was fabricated with C-BC/MnO2 and C-BC/MoS2 as positive and negative electrodes,respectively.The fabricated ASC delivered a high energy density 69.2 Wh·kg-1 at a power density of 249 W kg-1 due to a wide operation window and its ultrahigh specific capacity.The specific capacitance retained 91%of the initial capacitance after 11000 cycles at scan rate of 200 mV·s-1,thus the C-BC/MoS2 hybrid is a promising electrode candidate for next-generation energy-storage devices.