Hyperbranched Polyurethane/Graphene Mesh Composite Electrode For Flexible Self-Healing Supercapacitors

In recent years,flexible supercapacitors have attracted widespread attentions due to the rapid development of portable wearable electronic devices.However,flexible supercapacitors may fracture or be damaged during repeated stretch-release cycles,which can easily lead to device failure or even cause serious safety accidents.Therefore,it is of great necessary to develop flexible supercapacitors with excellent self-healing capability to improve the safety and lifetime of the devices.Organic electrode materials are commonly used as flexible electrode materials of supercapacitor because of their environmental-friendly,low cost,diverse structures and high designability when compared with traditional inorganic electrode materials like transition metal oxides.Carbonyl compounds,which not only have strong designability but also can achieve high energy density,power density and cycling stability,therefore,they have been considered as the most promising electrode materials for flexible supercapacitors.However,the applications of carbonyl compounds are limited by their low conductivity and easily soluble in electrolyte during charging and discharging process,leading to poor cycling stability and rate-performance.Compounding conductive substrates such as graphene with carbonyl polymers can not only provide the necessary electrical pathways for electrochemical reactions in electrodes,but also inhibit the dissolution of carbonyl polymers.Polyurethanes contain abundant reactive carbonyl groups with high electrochemical activity,intermolecular hydrogen bonds with good self-healing performance,meanwhile,the structures can be designed to enhance their self-healing properties by introducing dynamic covalent bonds into their structures.In this paper,graphene oxide（GO）was oxidatively etched into graphene oxide mesh（GOM）,which was then transformed into aminated graphene mesh（NGM）through p-phenylenediamine assisted solvent-thermal reduction.After that,hyperbranched polyurethane with self-healing group was grafted on the surface of NGM,resulted to polyurethane-grafted NGM composites（NGMU）,and used as self-healable electrodes for flexible supercapacitor.The main work of this thesis includes the following aspects:（1）GOM was prepared by oxidative etching method to create pores on GO surface,and functionalized and modified simultaneously by phenylenediamine-assisted hydrothermal reduction to prepare NGM integrating pseudocapacitance and electrical double-layer capacitance（EDLC）,which significantly improved the specific capacitance and rate performance.The specific capacitance at 1 A?g^-1 is up to 330 F?g^-1,which is a significant improvement compared with 114 F?g^-1 of graphene;the NGM assembled into a symmetric supercapacitor achieves a specific capacitance of 200 F?g^-1at 0.5 A?g^-1.The NGM based supercapacitor also achieves a high capacitance retention rate of 82%after 10,000charge/discharge cycles at 6 A?g^-1.（2）Urea/urethane-graphene（GOU）,urea/urethane-graphene mesh（GOMU）and urea/urethane-aminated graphene mesh（NGMU）were prepared by reacting GO,GOM and NGM with diisocyanate and ternary alcohol amine,respectively.The specific capacitances of the GOU,GMU,and NGMU composite electrodes were 255 F?g^-1,450.6 F?g^-1,and 895 F?g^-1,at 5 m V?s^-1,respectively,which were higher than those of the non-urea/urethane grafted samples,owing the introduction of pseudocapacitance contribution from the active carbonyl groups.The specific capacitance of NGMU was as high as 368 F?g^-1 at 6 A?g^-1,and the capacitance retention was 75.5%after 10,000 cycles at a current density of 4 A?g^-1.（3）The hyperbranched structure of polyurethane can enhance the carbonyl utilization rate and its enolization reaction rate,and the introduced disulfide bonds can enhance its self-repairing ability.The specific capacity of the flexible electrode reached 232 and 176 F?g^-1at 1and 10 A?g^-1,respectively,and was assembled into a symmetric supercapacitor with an energy density of 46.1 Wh?kg^-1 at a power density of 1 k W?kg^-1(10 A?g^-1),and the capacitance retention rate was 92%after 10,000 cycles.