Structural Design And Performance Of Flexible Wearable Supercapacitors

With the booming development of smart wearable electronics,research and development of lightweight and flexible energy storage devices is vigorously expanding.Flexible supercapacitors as promising power sources attract a lot of attention because of their high power density,quick charge/discharge ability,cycle life,and excellent mechanical flexibility.However,the low energy density of flexible supercapacitors impedes their practical applications.The flexible electrodes are their core components,which determine the electrochemical performance of the flexible supercapacitors.Therefore,it is a promising way to improve the energy density of flexible supercapacitors by developing electrode materials with high electron/ion transport kinetics.In addition,it can further improve the safety and stability of flexible supercapacitors by constructing rational device structures.Hence,this thesis focuses on the preparation and performance improvement of electrode material for planar aqueous supercapacitors,flexible wire-shaped solid-state supercapacitors,and biodegradable micro-supercapacitors to obtain high-performance flexible wearable supercapacitors.The main contents are as follows:（1）Obtaining CuO nanoblades by electrochemical reconstitution and analyzing growth mechanisms:Taking advantage of the metastable nature of Cu（OH）₂,we grown CuO nanoblades on Cu foam by electrochemical reconstitution at room temperature.The growth mechanism is analyzed by regulating the cycle number of galvanostatic charge/discharge,which involves the dissolution of Cu（OH）₂ nanowires,and guides the deposition of the as-dissolved Cu（OH）₄^2-species,and eventually leads to the phase transformation of CuO nanoblades.The CuO nanoblades exhibit high charge storage(～779 m C cm^-2 at 1 m A cm^-2),excellent rate capability and long-term cycling performances.Matching with activated carbon electrode results in hybrid supercapacitors,which displays a wide voltage window（1.7 V）in an aqueous electrolyte,and high energy density(0.17 m Wh cm^-2).The electrochemical restructuring approach is cost-effective,environmentally green and universal,and can be extended to synthesize other metastable hydroxides to in-situ grow corresponding oxides.（2）Synthesizing CuO nanowire with metal Ag nanoparticles electrodes and assembling high-performance planar water-based supercapacitors:Ag nanoparticles were uniformly deposited on CuO nanowire arrays grown on Cu foam by RF sputtering.Metallic Ag nanoparticles on the surface of CuO NWAs and Ag atoms diffused into the CuO lattice could improve the electrical conductivity of electrodes by forming a good ohmic contact,providing positive and stable pathways for the rapid electron/ion transport.Our binder-free Ag/CuO NWAs demonstrate a high stored charge of 1481.3m C cm^-2,which is 1.5 times higher than pristine CuO NWAs.Ag/CuO NWAs electrode displays stored charge retention of 103%after 10000 cycles.The hybrid supercapacitor with Ag/CuO NWAs as the positive electrode and activated carbon as the negative electrode,delivers a high energy density of 0.265 m Wh cm^-2,and a power density of48 m W cm^-2 as well a long cycle life in an aqueous electrolyte,demonstrating their great potential for practical applications.（3）Constructing CuO/Co（OH）₂/rGO hierarchical porous nanostructure and exploring flexible applications of wire-shaped supercapacitors:CuO/Co（OH）₂/rGO hierarchical porous nanoarchitectures were successfully prepared on Cu wires using in-situ wet-chemical approach.Co（OH）₂ nanosheets are utilized to increase charge storage capacity and enlarge electroactive surface area,while supplying abundant faradic active sites.rGO deposition on the nanostructures provides an omnidirectional conductive pathway to optimize electron/ion transfer kinetics,and alleviate the exfoliation and dissolution of the active materials into electrolyte.As a highly flexible wire electrode,binder-free CuO/Co（OH）₂/rGO hierarchical porous nanostructures demonstrate significantly high charge storage(22.3 m C cm^-1)and long-term cycling property（139%retained after 10,000 cycles）.Flexible all-solid-state hybrid WSSC device matched with CuO/Co（OH）₂/rGO and AC electrode exhibits wide potential window（1.45 V）,high energy density(0.133 m Wh cm^-3)and long lifetime stability.Moreover,flexible WSSC device demonstrates excellent mechanical stability with the charge retention of 109%at 180°bending angle,demonstrating their suitability for wearable electronics.（4）Selecting green electrode materials and designing flexible biodegradable micro-supercapacitors:The interdigital micro-supercapacitors with biocompatible agarose/sweat gel as the electrolyte and acid-etched MXene as the active material were successfully prepared by simple laser patterning.Acid etching is utilized to produce nanopores on MXene nanosheets and increase the layer spacing of the active material,and optimize the electron/ion transport kinetics.Benefits from structural optimization of MXene active materials,our micro-supercapacitor exhibits excellent areal capacitance(15.6 m F cm^-2),long-term cycling stability and mechanical flexibility.Moreover,the flexible device can be directly attached to human skin with the capacitance retention of 96%at 90°bending angle.The device can be completely degraded in simulated human body fluids（phosphate buffered solution）,providing research ideas for realizing the next generation of environmentally friendly wearable energy storage systems.