Graphene Hybrid Fibers For Flexible Supercapacitors

To meet the rapid advances of flexible,portable,and even wearable electronics,such as rollup display and electronic skin,considerable efforts have been dedicated to develop matchable energy storage and conversion systems as power sources,such as flexible supercapacitors?SCs?,lithium-ion batteries,solar cells,etc.Among them,supercapacitors are of significant interest as energy storage devices due to their high power density,fast charge-discharge rates,long cycling life,and relatively simple configuration.In our daily lives,fiber is one of the most common flexible materials.If the SCs are made in a wire or fiber format,it can satisfy the development of flexible and wearable electronics.Graphene fiber formed by individual graphene nanosheets is one of the promising candidates as flexible electrodes,because they are light,highly conductive,bendable,even can be woven into wearable cloths.However,the energy density of the neat graphene fiber-based supercapacitor is low,which largely limits their practical applications.This dissertation focuses on the preparation and structure control of continous graphene hybrid fibers by a simple wet-spinning method,and explored their application as flexible supercapacitor.The main results are as follows:1.Preparation of continuous conductive carbon black?CB?/rGO hybrid fibers and their application as flexible solid-state supercapacitors.CB/rGO hybrid fibers were fabricated through a simple,scalable wet-spinning method.When the CB content is increased,the mechanical strength decrease,while the electrical conductivity increase at first,and then decreases.By introducing CB as spacers,the restacking of rGO sheets was effectively hindered.The hybrid fibers possess very high surface area and a hierarchically porous nanostructure.A flexible solid-state SCs assembled from the hybrid fiber with 40 % CB exhibited high capacitance of 97.5 F cm-3,excellent cycling stability,superior energy density of 2.8 mWh cm-3 and ultrahigh power density up to 1200 mW cm-3.Meanwhile,it maintained excellent physical and capacitive stability under bending deformation,indicating a good flexibility for flexible and wearable applications.2.Preparation of continuous CNT/rGO hybrid fibers and their application as flexible solid-state supercapacitors.Porous CNT/rGO hybrid fibers were fabricated through a simple,scalable wetspinning method.With the increasing CNT content,the mechanical strength decrease and the conductivity increase.The hybrid fibers exhibit a well-defined interconnected porous network structure composed of hierarchical pores.The porous fiber electrode exhibits an exceptionally high rate capability and power handling performance and can be charged and discharged at a CV scan rate up to 50 V s-1,about two orders of magnitude higher than other graphene fiber-based SCs.More importantly,the relaxation time constant ?0 of our devices is extremely short?about 74 ms?.An all solid-state flexible SC was constructed using the hybrid fiber with 10 % CNT as flexible electrode.The supercapacitor exhibits a high volumetric capacitance?8.2 F cm-3,normalized by the total volume of two fiber electrodes?,excellent cycling stability?97.6 % capacitance retention over 10000 cycles?,high energy density?1.45 mWh cm-3?and outstanding power density?7.6 W cm-3?.Meanwhile the device is flexible and robust enough to tolerate the long-term and repeated bending.3.Preparation of continuous activated carbon?AC?/rGO hybrid fibers and their application as flexible solid-state supercapacitors.A bottom-up method was developed to fabricate AC/rGO fiber employing graphene oxide?GO?as both dispersant and binder.After chemical reduction,the fiber has high electrical conductivity?185 S m-1?,high specific surface area?1476.5 m2 g-1?and good mechanical flexibility.An all solid-state flexible SC was constructed using the prepared fiber as electrode,which is free of binder,conducting additive and additional current collector.The fibershaped SC shows high capacitance?27.6 F cm-3 or 43.8 F g-1,normalized to the two electrode volume?,superior cyclability?90.4 % retention after 10000 cycles?and good bendability?96.8 % retention after bending 1000 times?.4.Preparation of continuous MnO₂ nanowire /rGO hybrid fibers and their application as flexible solid-state supercapacitors.Hierarchically structured MnO2 nanowire/graphene hybrid fibers were fabricated through a simple,scalable wet-spinning method.The hybrid fibers form mesoporous structure with large specific surface area of 139.9 m2 g-1.The mass loading of MnO2 can be as high as 40 wt %.The conductivity and mechanical strength of the fibers decreases with the increasing content of MnO2.Due to the synergistic effect between MnO2 nanowires and graphene,the main pseudocapacitance of MnO2 and the electric double-layer capacitance of graphene are improved simultaneously.In view of the practical demonstration,a highly flexible solid-state supercapacitor is fabricated by twisting of two MnO2/graphene fibers coated by polyvinyl alcohol/H3PO4 electrolyte.The supercapacitor exhibits a high volumetric capacitance?66.1 F cm-3,normalized by the total volume of two fiber electrodes?,excellent cycling stability?96 % capacitance retention over 10000 cycles?,high energy and power density?5.8 mWh cm-3 and 0.51 W cm-3,respectively?.5.Flexible all-solid-state asymmetric supercapacitor based on transition metal oxide nanorods/reduced graphene oxide hybrid fibers with high energy density.Transition metal oxide nanorods/ reduced graphene oxide?rGO?hybrid fibers were prepared by a facile,scalable wetspinning method.Due to the synergetic effects between transition metal oxide nanorods and rGO,the electrochemical performance of the hybrid fibers were greatly improved.An all-solid-state asymmetric supercapacitor was constructed by using MnO2 nanorods/rGO hybrid fiber as positive electrode,MoO₃ nanorods/rGO hybrid fiber as negative electrode and H3PO4/poly?vinyl alcohol??PVA?as electrolyte.Based on the different working potential window between MnO2 and MoO₃,the optimized ASC can be cycled reversibly at a high voltage of 1.6 V and deliver a superior volumetric energy density of 18.2 mWh cm-3 at a power density of 76.4 mW cm-3.Besides,the ASC exhibits remarkable cycling stability and excellent flexibility and mechanical stability.