Fabrication And Electrochemical Properties Of Carbon Fiber Based Composite Electrodes

With the continuous increase in the electrical device usage,energy storage systems have attracted considerable attention from several researchers and companies all over the world.In particular,supercapacitors have been regarded as one of the most promising energy storage devices because of their higher power density and greater endurance than that of secondary batteries,while they can maintain nearly the same energy density as that of batteries.Fiber-based supercapacitors are a promising way to integrate charge storage into items such as clothing without compromising on design.Carbon fibers,one of widely used electrode materials,are well known not only for their high conductivity and high mechanical strength but also their low density and inertness under ambient conditions,as well as their ability to be easily woven into fabrics,which make them attractive as one dimensional（1 D）electrode materials for various energy storage devices.Unfortunately,the carbon fiber can hardly meet the requirement of high specific capacitance and energy density.It is envisaged that it is a practical and promising approach to efficiently incorporate pseudocapacitive materials including carbon materials with functional groups,transitional metal oxides,metal nitrides or conducting polymers into/onto weavable/knittable carbon fiber yarns,which combines the good capacitive energy storage properties of pseudocapacitive components with excellent electrical conductivity and mechanical flexibility of carbon fibers.Therefore,oxidization andmicrowave-assisted strategy,electrodeposition and sol-gel method were applied to make carbon fiber composites electrodes,and its structure,morphology and electrochemical property were studied.Then 1 D flexible supercapacitors were fabricated based on it and electrochemical tests were conducted to show the electrochemical properties.The main contents are followed:1.Carbon fibers,with reduced oxidized graphite layers on the surface obtained using oxidization and microwave-assisted strategy,were developed as 1 D electrodes.After oxidization,graphite layers formed on the surface of carbon fibers,and FTIR,EDS and XPS methods were applied to prove abundant oxygen functional groups formed on the graphite layers.The oxygen content of the oxidized carbon fibers（CF-O）was 25.67%,which was much higher than that of original carbon fibers（CF,1.74%）.With the reduction of hydrazine hydrate,the oxygen content of samples（CF-R）decreased while the nitrogen content increased.The Raman spectrum showed that CF-O kept the same structure of CF.Then with microwave processing,the SEM images showed there are pores appeared on the surface of CFs.When microwave processing time reached at 10 min（CF-10）,lots of oxidized graphite layers were exploded and detached from the surface of carbon fibers.The electrochemical test showed that the samples,with a processing time of 6min（CF-6）,can get the best specific capacitance of 218.9 F g^-1 among the samples with microwave processing.The specific capacitance of CF-R can reach 213.3 F g^-1 at the current density of 0.1 A g^-1,which was about 40 times more than that of CF.The electrodes based on CF-R endowed optimal cycling ability without a decline in specific capacitance after 10000 cycles.However,the still comparatively low conductivity,caused by the oxygen functional groups,made it unavailable to be used for the electrode matrix.2.1D flexible solid-state supercapacitors were fabricated by directly electrodepositing ultrathin MnO₂ nanosheets on commercial carbon fiber yarns.The deposition process was well controlled and the composition of MnO₂ in fiber electrodes was optimized to enable fiber SCs to possess high specific capacitance.Conductive carbon fibers concurrently served as current collectors in fiber SCs and as flexible substrates for the deposition of MnO₂.TGA and ICP methods were used to confirm the composition of MnO₂ in fiber electrodes,which was the same with the theoretical results.The SEM and EDS images showed that an electrodeposited layer and Mn element uniformly covered on the surface of carbon fibers,respectively.The layer,consisted of nano MnO₂ layers,proved by the XRD and XPS spectrums and TEM images.Under a three electrode test system,a single MnO₂/CFs fiber electrode exhibited a specific volumetric capacitance of 58.7 F cm^-33 with a specific gravimetric capacitance of 428 F g^-11 based on the MnO₂ mass.Two hybrid carbon fiber electrodes were assembled together in parallel with polyvinyl pyrrolidone/Na₂SO₄ gel,which was used as both an electrolyte and a separator.The assembled 1D flexible device exhibited a high volumetric energy density of 3.8 mWh cm^-3 at a power density of 89mW cm^-3 with a good flexibility（CV curves almost unchanged after 2000 bending times）and a superior long cycle stability（an 85.8%capacitance retention after10000cycles）.However,the conductivity of electrodes still needed to be improved when the conductivity of MnO₂ did not meet the needs of getting good rates property.3.An amide-based non-oxide sol-gel method was used to produce smooth titanium oxide nitride（Ti（O,N））coatings on carbon fibers.Firstly,carbon fibers were annealed under ammonia at 1000°C（CF-NH₃）.Then sol preparation and Ti（O,N）coating were carried out at high temperature under nitrogen using tetrakisdimethylamido titanium（Ti（NMe₂）₄）and n-propylamine（ⁿPrNH₂）mixed hexane solution.The SEM and EDS images showed that the Ti and N element uniformly covered the surface of carbon fibers.The XRD spectrum and Rietveld refinement proved the coating consisted of Ti（O,N）.And with the TGA results,the overall compositions of the fibers can be calculated while the residue was identified by XRD as TiO₂.The XPS spectrum showed the nitrogen content increased after ammonia treatment and proved the coating was made of Ti（O,N）again.The prepared samples with different sol concentrations were made and their electrochemical properties were studied.Both the coating and a nitridation of the fibers resulting from the process increased the capacitance of the fibers measured in 1 M H₂SO₄ aqueous electrolyte,with significant redox capacitance contributions.The CV curves of the carbon fibers with Ti（O,N）coating had redox peaks at-0.56/-0.75 V when tested at the scan rates of 1 mV s^-1.A discharging flat appeared in the charging/discharging curves and proved the existence of pseudocapacitance again at the current density of 0.1 A g^-1.The capacitance of CF-TiN-1 was 38.3 F g^-1 when tested at the current density of 0.1 A g^-1.The specific capacitance plots of single electrode showed the sol concentration had little effect on it.So the electrodes,made using the lowest concentration sol（Ti（NMe₂）₄,0.43 m L;ⁿPrNH₂,0.30 mL;hexane,7.50 m L）,were chosen to fabricate 1D flexible symmetric capacitors using PVA/H₂SO₄ gel.The capacitance of 1D flexible symmetric capacitors,made from the coated fibers and an acid gel electrolyte,retained their capacitance very effectively on bending up to 180°and the capacitance retention can be 100%when tested at the current density of 0.1 A g^-1.they also provided competitive energy(2.2 mWh cm^-3)and power densities(103.1 mW cm^-3).The good conductivity of Ti（O,N）made the supercapacitors a good rate ability and the retaining ratio was 86.4%when the power density increased to 1031 mW cm^-3 with an energy density of 1.9 mWh cm^-3.