Covalently Modified Carbon Cloth To Fabricate Composite Flexible Electrode For High Performance Flexible Supercapacitor

With the development of science and technology,flexible electronic devices such as foldable mobile phones,flexible electronic sensors and electronic skins gradually appear in peoples’vision.However,one of the factors that restricts practical application of flexible electronic devices is attributed to flexible energy storage devices that couldn`t meet the actual demand.Supercapacitors possessed low energy density,have been used together with batteries in electronic products.Therefore,it is not only necessary for flexible supercapacitors（FSSC）to solve the problem of low energy density,but also need to ensure the stability of electrochemical performance under various conditions of mechanical motion.In order to meet the practical application requirements,flexible electrode,as the key component of FSSC,has to build stable structure to resist strong forces and improve the energy density at the same time.To solve problem mentioned above,this thesis proposes to prepare an"integrated"flexible electrode by covalently bonding the electrode active material with the flexible fluid collector（carbon cloth）through chemical reaction,which could ensure stable structure of flexible electrode to achieve the purpose of improving the stability of electrochemical performance and capability density at the same time.In order to prepare electrode by methods of covalent modification successfully,we select amino phthalocyanine,an organic molecule with both electrochemical activity and functional groups,and inorganic materials such as graphene,molybdenum disulfide and carbon-coated Mn₂O₃ as active materials of electrode.Chemical modifications were carried out on inorganic electrical active materials and carbon cloth to graft functional groups on their surfaces,providing conditions for formation of covalent bonds.Based on this research strategy,the following experiments are designed and studied:（1）Flexible electrode was prepared by amidation of carbon cloth and phthalocyanine（GCC-PcNH₂）.Carboxyl group as the active site was grafted on the surface of carbon cloth of plasma treatment（GCC）with acidification of maleic anhydride,then amino phthalocyanine was reacted with carboxyl group to in situ grow on carbon fiber.GCC-PcNH₂synthesized by covalent reaction has low internal resistance（0.782Ω）.At the scanning rate of 1 mA/cm²,specific capacitance of GCC-PcNH₂ is up to 2.425 F/cm² and the energy density is 0.165 mWh/cm².After 10000 cycles,the capacitance retention rate of GCC-PcNH₂ is90.46%,showing good cycle performance.The specific capacitance of the symmetric supercapacitor made of GCC-PcNH₂ is 1.610 F/cm²,and the capacitance remains 94.80%after 1000 times of bending at 180°.The result shows that covalent bonding is beneficial to the electrochemical stability of flexible electrodes.（2）Flexible electrode was prepared by coupling carbon cloth and rGO（GCC-PANI-rGO）.Considering the limited active sites astricted improvement of electrochemical performance of electrode,rGO is easy to be chemically modified to covalently introduced into a flexible electrode for improving the energy density.Firstly,aniline group was grafted on the surface of carbon cloth（GCC）and rGO by diazotization and SUZUKI reaction respectively.Finally,rGO and GCC were covalently combined by polymerization reaction of aniline to form flexible electrode（GCC-PANI-rGO）.With formation of conjugated system,GCC-PANI-rGO has low R_ct（0.428Ω）.At 1 mA/cm² current density,specific capacitance of GCC-PANI-rGO reaches 11.825 F/cm² and energy density is 1.642 mWh/cm².At 40 mA/cm² current density,the specific capacitance（C_A）retention rate of GCC-PANI-rGO reaches 97.73%after 15,000charge-discharge times.C_Aof FSSC prepared by GCC-PANI-rGO remains 108.78%after 10,000 bending cycles.The result shows that"integrated"flexible electrode with conjugated system constructed by the method of covalent binding of graphene and carbon cloth,achieves the purpose of improving the energy density and electrochemical performance stability at the same time.（3）Flexible electrode was prepared by coupling carbon cloth and MoS₂（GCC-PANI-MoS₂）.Molybdenum disulfide has a high theoretical capacitance,and its special layered structure enables it to be covalently combined with carbon cloth after chemical modification,which further increases the energy density of the electrode.Firstly,molybdenum disulfide is converted to 1T phase by n-butyl lithium intercalation to improve its conductivity.Then,aniline groups are grafted on the surface of GCC and MoS₂by diazotization and SUZUKI reaction respectively.Finally,MoS₂ and GCC are covalently combined by polymerization reaction of aniline to prepared flexible electrode GCC-PANI-MoS₂.Due to phase transition of MoS₂ and integrated structure of electrode,the electrode has a small charge transfer resistance（0.496Ω）.Specific capacitance of GCC-PANI-MoS₂ is 12.034 F/cm²,and the energy density is 1.651 mWh/cm² at the5 mA/cm² current density.At current density of 60 mA/cm²,the GCC-PANI-MoS₂ maintains 106.47%initial specific capacitance after 15,000 cycles.The capacitance of flexible supercapacitor prepared by GCC-PANI-MoS₂ also retains 103.83%and 94.37%after 10,000 cycles and 10,000 180°bending cycles respectively.The result shows that MoS₂ is introduced into flexible electrode covalently,increasing energy density of electrode on the premise of ensuring the stability of electrode.（4）Flexible electrode was prepared by coupling carbon cloth and Mn₂O₃@C（GCC-PANI-Mn₂O₃@C）.In order to solve the problem that metal oxides with high energy density are limited to chemical modification,carbon coated metal oxides can be prepared by using MOF as precursor,which provides the possibility for covalently introducing metal oxides into flexible electrodes.Mn-MOF was synthesized,calcined at high temperature to obtain Mn₂O₃@C.Then,aniline groups are grafted on the surface of carbon and Mn₂O₃@C by diazotization and SUZUKI reaction.Finally,Mn₂O₃@C and GCC are covalently combined by aniline polymerization reaction for preparing GCC-PANI-Mn₂O₃@C flexible electrode.The whole skeleton structure of electrode has conjugate system,so charge transfer resistance（2.27Ω）of GCC-PANI-Mn₂O₃@C effectively reduced,specific capacitance of GCC-PANI-Mn₂O₃@C up to 13.650F/cm²,and the energy density is 1.896 mWh/cm² at 5 mA/cm² current density.GCC-PANI-Mn₂O₃@C retains 85.76%initial specific capacitance after 10,000 cycles,the FSSC prepared by GCC-PANI-Mn₂O₃@C maintains 84.72%and 110.36%specific capacitance after 10,000 cycles and 10,000180°bending cycles respectively.The results show that the synthesized Mn₂O₃@C is covalently introduced into the flexible electrode with conjugated system,which not only stabilizes the electrode performance but also improves the energy density of the electrode,providing a new idea for the preparation of more flexible metal oxide electrodes.