Performance And Mechanism Of Biomass Materials Modified Graphene-Based Flexible Electrodes

Graphene-based supercapacitors have a promising future in flexible electronic devices due to the advantages of long cycle life,high power density,charge discharge rate and excellent mechanical flexibility etc.However,graphene is easy to cause serious accumulation and flocculation in the process of assembling electrode.The key to improve its electrochemical performance is to adjust the three-dimensional structure of graphene-based electrode materials.In this paper,abundant and renewable biomass carbon materials and organic compounds are selected to be composited with graphene,respectively.The microstructure and performance of the composite electrodes have been adjusted and optimized,and then applied to flexible supercapacitors.Carbon material is the most traditional electrode material of supercapacitor.Biomass carbon materials have the advantages of rich resources,renewable,low price and easy processing.Firstly,the feasibility and effect of biomass carbon materials prepared from waste fiberboards on the graphene-based electrode performance were explored.It is found that the doping of porous activated carbon particles can effectively increase the specific surface area of the composite electrode.Graphene oxides act as conductive bridge,ensuring the high strength and flexibility of the electrode.Under the condition of no adhesive,the flexible and self-supporting graphene/activated carbon film electrode was prepared by constant temperature water bath and vacuum filtration methods.The specific capacitance of the composite film electrode is 5.7 times of that of the pure graphene electrode.After 1000 bending and folding cycles,the capacitance retention rate is 90%.In order to further improve the energy density,the urchin type Mn O₂nanoparticles were grown on the surface of the composite film,the specific capacitance of the ternary composite film is 17.2 times higher than that of the pure graphene electrode,but the mechanical flexibility is decreased.The waste fiberboards were carbonized and further prepared into nitrogen and oxygen co-doped carbon dots by acid etching method,and then used to modify graphene.Thus,the doping of nitrogen and oxygen elements is realized,the active sites and electron supplying ability of graphene-based composites are increased.The Ni Co₂S₄ nano layers are grown on the surface by anode constant current deposition method.It is found that the specific capacitance of the composite film electrode is 1348 F g^-1,the assembled flexible symmetrical supercapacitor has a high energy density of 85.1 Wh kg^-1.After 1000cycles of bending and folding,the capacity retention rate is 96.8%.Compared with activated carbon and metal oxides,the functionalized carbon dots and ternary sulfides have more significant effect on the improved performance of graphene-based electrode.Based on the advantages of natural organic compounds,they are non-toxic,environment-friendly,rich in resources and easy to extract.From the perspective of chemical modification,the effects of natural organic compounds with different structures on the on the graphene-based electrode performance were investigated.It is found that magnesium phytate and graphene sheets can form ionic chelations and hydrogen bonds.The hydrogel electrode prepared by hydrothermal method has a very stable structure and long cycle life,and its charge transfer resistance is only 0.3?.The capacitance of 10000 charge/discharge cycles remains almost unchanged.When using tannic acid molecules to modify graphene,a large number of benzene rings can form?-?conjugation with graphene sheets,and the phenolic hydroxyl groups on the benzene rings can be converted to quinone groups,resulting in reversible redox reaction,and the porous hydrogels with sponge like structure are formed.The specific capacitance(533 F g^-1)and energy density(37 Wh kg^-1)are improved.The porous gel skeleton formed by pectin-Mg²⁺/Ca²⁺ions is used as template to prepare graphene composite hydrogel electrode by hydrothermal method.And its specific capacitance(839.2 F g^-1)is further improved,particularly the coulomb efficiency is up to 191.8%.In addition,the?-cyclodextrin is used as modifer,grafting ethylenediamine on the?-cyclodextrin molecules firstly to modify graphene,the composite film electrode was prepared by constant temperature water bath and vacuum filtration methods and has excellent mechanical strength and flexibility?98.2%of capacitance retention for 1000bending cycles?while the specific capacitance is also increased.Then,?-cyclodextrin molecules are used as carbon source to form inclusion complex with Fe,Co ions to prepare graphene composite film electrode.The assembled flexible asymmetric supercapacitor has a high working voltage of 1.8 V,and the energy density is up to 5.99 m Wh cm^-3.Compared with the activated carbon materials,the electrochemical performance and flexibility of the graphene-based film electrode decorated by?-cyclodextrin have been greatly improved.