Preparation And Application Of Carbon Cloth Electrodes Loading Modified Polyaniline And Mixed Transition Metal Oxide

Supercapacitors are considered to be potential for energy storage and conversion devices in future generations due to its ability of achieving high output power density, maintaining stability at high charge- discharge current and after a long-term cycle. Based on the different charge-storage mechanism, supercapacitors can be divided into two categories, the electric double layer supercapacitors utilizing carbon materials with high specific surface area and the pseudo-capacitors using redox-active transition metal oxides or conductive polymers. Electric double layer supercapacitors only make use of close interface electric double layer while pseudo-capacitors additionally utilize redox reactions to store charge, so pseudo-capacitance performance can reach tens of times over electric double layer capacitors under same conditions. Supercapacitor utilizing transition metal oxides and conducting polymers are expected to bridge the gap between conventional capacitors and batteries to form units of practical energy storage. To supercapacitors, the electrode and electrolyte are their key parts, these two factors determine the nature and form of supercapacitors in all aspects. In recent years, the research direction gradually steers from only preparing and characterizing electrode materials to assembling devices and optimizing their performance. Most reports combined electrode materials and collectors through the use of additives or by the pressure, this process resulted in adverse effects on the performance and stability of the devices. With the gradual development of portable and wearable devices, the demand of quasi-solid flexible energy storage devices increases rapidly. The strategy of directly loading or growing electrode materials on the planar, flexible and porous substrate will be able to effectively meet the application requirements.Here, based on carbon cloth(CC) with excellent flexibility and high mechanical strength, electrodes loading with conductive polymer composite and spinel transition metal oxides mixture were designed, respectively. Then quasi-solid polymer electrolyte was used to assemble electrodes into supercapacitors. Such design and implementation made the device can be miniaturized and ultra-thin. In terms of conducting polymer, for the poor water dispersibility of polyaniline, we focused on improving its water dispersibility and keeping capacitance performance of polyaniline, so nitrogen-doped graphene doped polyaniline-polyacrylic acid composite was prepared, and the effects on the electrochemical properties of the electrode system were characterized and evaluated; On the other hand, basing on the high conductivity and multi active sites of the spinel transition metal oxides, it was planned to diversify active sites and increase the specific surface area by preparing cobalt nickel-cobalt iron nanorods porous structure and electrochemical properties of the electrodes were characterized and evaluated. The main contents are summarized as follows:(1) By dropping aniline into acidic aqueous solution of polyacrylic acid and ammonium persulfate at a low temperature of 0～5℃ under vigorous stirring, in situ oxidative polymerization achieved polyaniline-polyacrylic acid composite. Thereafter, the purified product dispersion was added with pure polyaniline nanoparticles and dispersed by ultrasonic, the polyaniline weight content of the product after drying was 20%, which reached the upper limit to form a complete film without defects. At this point, compared with pure polyaniline, capacitance performance of the composite material decreased, further adding of nitrogen-doped graphene nanosheets to improve the capacitance performance. When the nitrogen-doped graphene nanosheets weight content of the product after drying was about 1%, the capacitance performance of the composite was optimized. Analysis results by scanning electron microscopy, XRD and other characterization methods showed that polyacrylic acid improved its water dispersibility and made the composite have film-forming properties by adsorbing and covering polyaniline nanoparticles partially; nitrogen-doped graphene nanosheets brought out sheet-bridged mode and contributed its own conductive advantages to connect points in the composite network, improving the conductivity of the composite system. Films on Pt plate made from the optimal composite (20 wt.% PANI,1.3 wt.% NG) have a high capacitance of 399 F/g (1.20 F/cm2 or 240 F/cm3) at the scan rate of 10 mV/s. A supercapacitor made from carbon cloth (CC) electrodes impregnated with the composite material has a more than ten times larger capacitance (68 F/g at 1 A/g) than previously reported capacitors based on polyaniline-acrylic acid mixture containing 60% by weight fraction of polyaniline polymer (capacitance value 5.05F/ g) and retains these properties under large bending angles. Also, the high energy and power densities (5.8 Wh/kg at 1.1 kW/kg) of the capacitor are accompanied by long-term electrochemical stability (83.2% retention after 2000 cycles) and a superior rate capability (the capacitance at 10 A/g is still 81% of that at 1 A/g).(2) Cobalt nickle and cobalt iron precursor was deposited on the surface of carbon cloth by hydrothermal method. After being calcined in air, the carbon cloth electrode loading with cobalt nickel-cobalt iron mixture was obtained. The graphene impregnated carbon cloth electrode was prepared. The as-prepared CC/NiCo2O4-FeCo2O4 and CC/graphene electrodes were with a loading density of 2mg/cm2 and 1.5mg/cm2, respectively. These two components were assembled into bipolar supercapacitor with quasi-solid polymer electrolyte. First, two electrodes were assembled with 3 M potassium hydroxide solution as the electrolyte, and a piece of filter as the separator; then other electrodes were assembled with polyvinyl alcohol-potassium hydroxide as the electrolyte sol and its film as the separator. The specific capacitance values of CC/NiCo2O4-FeCo2O4 electrodes were as high as 490 F/g in 3 M KOH solution at a current density of 2 A/g. The specific capacitance values of CC/NiCo2O4-FeC0204 electrodes were enhanced due to the three-dimensional structure of CC substrate, which provided conductive network, efficient ion diffusion path, and high surface area for NiCo2O4-FeCo2O4 nanoparticles and electrolyte. The as-assembled asymmetric supercapacitor showed a voltage window of 1.6 V and offered a high energy density of 0.16 mWh/cm2 at a power density of 5.74 mW/cm2 in 3 M KOH solution. Moreover, quasi-solid asymmetric supercapacitors were also assembled which reached one quarter of the capacitance perfprmance of those with KOH solution.