| The gradual increase in demand for portable devices and the development of micro electronics are driving research on supercapacitors.And these electronic devices need not only energy or power density and cycle life,but also flexibility and comfort.Therefore,the preparation of light weight,highly transparent and bendable flexible devices is the primary goal of current research and development.Graphene quantum dots,as a new type of nanomaterial,have functional groups with high surface abundance,which are important for improving solvent dispersion,simplifying various reactions and electrolyte permeation,and also increasing the electrochemical activity of carbon electrodes.Given the advantageous properties of graphene quantum dots,their use as an electrode material for flexible micro-supercapacitors not only helps flexible devices maintain their electrical conductivity and electrode robustness,but also enables mechanical stability of the devices.In this paper,graphene quantum dots were selected as the electrode material for flexible devices and polyethylene terephthalate with high flexibility and oxidation resistance was used as the flexible substrate.The flexible micro-supercapacitors with interdigital electrode width to gap ratios of 6μm/2μm,30μm/10μm,60μm/20μm,and 90μm/30μm were prepared using an modified liquid-air interface self-assembly method,and lithography process.X-ray photoelectron spectroscopy,raman spectroscopy,transmission electron microscopy and atomic force microscopy were subsequently employed to characterize the graphene quantum dots electrode film.The results showed that the electrode film was continuous and complete over a large area,in which the contents of carbon,nitrogen and oxygen elements were 41.85%,26.79%and 31.36%,respectively,and the graphene quantum dots were spheres of regular shape and uniform size as well as the average particle size w about 1.51 nm,and contained a large number of defects and abundant functional group structures.The cyclic voltammetry,galvanostatic charge discharge,cyclic stability and electrochemical impedance spectroscopy tests were also performed on flexible devices bent at 0°,45°and 75°,respectively,using an electrochemical workstation.The flexible devices achieved a surface energy density in the order of 8.82 n Wh cm-2and a surface power density of 693.41μW cm-2.The loss of various electrochemical performances of the flexible devices after bending was small and there was no detachment and cracking between the electrode film,collector and flexible substrate during the bending process,which indicated that the mechanical performances of the flexible substrate and the electrode material and the interface between them were well integrated.Series and series-parallel flexible micro-supercapacitors were prepared using the same experimental procedure as the inserted finger configuration,and the devices were tested for cyclic voltammetry,galvanostatic charge discharge,cyclic stability and electrochemical impedance spectroscopy under bending at 0°,45°and 75°for the flexible devices,respectively.The surface energy density of series-connected device reacheed 1.16 n Wh cm-2and the surface power density reacheed 110.56μW cm-2;the surface energy density of series-parallel-connected devices reacheed 0.75 n Wh cm-2and the surface power density reacheed 166.73μW cm-2.The voltage of the two composite configurations was 2 V.The results showed that series and series-parallel devices not only provided high voltages and currents,but also maintained good cycling stability and capacitive characteristics under bending.In summary,flexible micro-supercapacitors were not only flexible under unpredictable mechanical deformations such as bending,but also maintained electrochemical stability and had good potential for future applications in many fields. |
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