Capacitive Performance Of Flexible N-doped Graphene/Cotton Fiber Composite Electrode

Energy storage devices has arisen due to their potential applications in wearable and portable electronic products,typically roll-up displays,electronic paper,and wearable systems for personal multimedia.Despite these advances,the real bottleneck hindering flexible electronics from becoming ubiquitous in practical products is in constructing flexible and deformable energy storage devices.Current electronic devices are still too heavy,thick and bulky to match flexibility requirements.In order to realize the needed mechanical properties,energy storage devices that are light,thin and flexible must be developed.In this work,cotton fiber,which is low cost and renewable,was chosen as the precursor of electrode material for flexible supercapacitor.And single cotton or composite flexible electrodes by combining with graphene oxide and nitrogen doping were designed and constructed.The main contents and results are as follows:(1)The synthesis of cotton fiber carbide via direct carbonization method.The effects of carbonation parameters on the morphology,mechanical properties and electrochemical properties of cotton fiber as capacitor electrode materials were investigated.The results show that the increase of carbonization temperature will lead to the decrease of tensile ultimate strength,that is,mechanical properties decrease.The process of ammonia treatment can improve the extensibility of the fiber.Moreover,the specific capacitance of the fibers treated by ammonia is higher than that of the argon treated ones.In addition,the conductivity and rate performance of the fibers carbonized at 900?is worse than that of 800?carbonized ones.(2)The synthesis of nitrogen doped fiber/graphene layered composite material via vacuum filtration method.The influence of the mixed mass ratio of graphene and cotton fiber on the morphology and electrochemical properties of the composites was investigated.The results showed that the mixing ratio of 8:2(fiber/graphene)composite with high specific capacitance(up to 138 F/g at the current density of 1 A/g),better rate performance(when the current density is 20 F/g.The capacitance retention rate is 32%),and the optimal impedance characteristics.The results also showed that the capacitance performance of the cotton fiber/graphene composites with a mixture ratio of 8:2 is the best as the ammoniation time is 3 h.(3)The synthesis of 3D nitrogen doped cotton fiber/graphene composite of was prepared by freeze-drying method.The effects of graphene oxide concentration and ammoniation time on the capacitance properties of composite materials were studied,and the flexibility and capacitance properties of symmetrical capacitance devices were characterized.The results of a series of electrochemical analysis showed that the composite of graphene oxide(GO-5)concentration presented the highest specific capacitance value in different density current(the specific capacitance reached 291 F/g,and still able to maintain 41%of the capacity at the current density of 20 F/g).Moreover,the cotton fiber/graphene composite,which processed in ammoniation with 3 h(NGC-3)has the highest proportion of pyridine nitrogen and pyrrole nitrogen,as well as the lowest proportion of quaternary nitrogen and nitrogen oxygen groups.In addition,NGC-3 has the highest specific pore volume and the specific surface area of 631 m~2/g.After 10000 cycles of charge and discharge test with variable current density,the specific capacitance calculated by 10000th cycles increased by 11%compared with the initial value of NGC-3.More importantly,the shape of the CV curve is all quasi rectangular and there is no significant change as the symmetrical device NGC-3//NGC-3 is bent from the initial state to 90 degree or 180 degree,and finally,recovering to the initial state.The energy density of the device NGC-3//NGC-3 of the flexible device reaches 5 Wh/kg under the current density of 0.2 A/g.