Electropolymerized Polyaniline And Its Application In Flexible Supercapacitors

With the characteristics of high theoretical specific capacity,good electrical conductivity,excellent environmental stability,low cost,and easy synthesis,polyaniline（PANI）,as one of conductive polymers,has shown unique advantages for application in the field of energy storage.Moreover,PANI can be processed into small,thin,light,and flexible electrodes due to its features of light weight,adjustable performance and great processability.Therefore,PANI has been extensively studied in the field of flexible energy storage devices,especially flexible supercapacitors.There are two main ways to prepare PANI:chemical oxidation and electrochemical polymerization.However,the chemical oxidation polymerization has a low controllability and incopmplete reaction.The PANI synthesized by chemical oxidation contains residual oxidants,dopants and some oligomers and therefore affect its electrochemical property.By contrast,the electrochemical polymerization method is simple and has a higher controllability.The polymerization process does not requrie additional oxidant.The obtained PANI has a higher purity,more continuous membrane structure,and a better conductivity.Therefore,the electrochemical polymerization method is suitable for mass preparation of PANI electrodes.However,the flexible PANI electrodes by electrochemical method still have some problems.Restricted by the rigid conjugated structure of PANI,the flexibility of PANI electrodes is insufficient,resulting in low cycle stability and short cycle life,which are difficult to meet the actual needs of flexible supercapacitors.Therefore,this thesis focuses on the preparation of PANI electrode by electrochemical method and its electrochemical performance to carry out the following main studies.（1）PANI electrodes with different polymerization time were prepared on graphite paper by galvanostatic polymerization.The growth process of PANI was studied by combining the polymerization curves and the microstructure of PANI at different time.It is found that within a certain time of polymerization,PANI grows uniformly on graphite paper in a compactly stacked form.After a certain period of time,the coral-like fluffy PANI continued to grow with the dense layer as the substrate.In the third stage,PANI continued to grow epitaxially along each branch of coral-like PANI with the diameter of each branch increasing.Then the influence of its morphology and structure on its electrochemical performance was analyzed by electrochemical tests.The results show that the specific capacitance increases with the polymerization time,but the cycle stability is getting worse.（2）In order to study the effect of current collectors with different specific surfaces on the surface capacitance and diffusion capacitance of PANI electrodes,carbon felts and graphite plates are used as the current collectors to polymerize PANI,respectively.The surface capacitance and diffusion control capacitance were calculated according to Dunn’s semi-empirical method,and the effects of PANI with different specific surfaces on the surface capacitance and diffusion control capacitance were analyzed.The carbon felt PANI has a larger specific surface area than graphite plate PANI because of the carbon felt with a larger specific surface area than graphite plate.In addition,related research confirms that there is a high proportion of nanoscale PANI on the carbon felt PANI electrode.These make the carbon felt PANI has a higher surface capacitance and total capacitance at each scan rate under test conditions.At higher scan rate,the total capacitance of the two electrodes are almost all surface capacitance.Therefore,it also shows that the PANI electrode prepared by electropolymerization on the current collector with larger specific surface area has better rate performance.（3）To fabricate high-performance flexible PANI electrode with long cycle life,PVAhydrogel was introduced as the template for the growth of PANI.PVA not only improved the flexiblity of PANI,but also can increase the specific surface area of PANI.In order to study the effect of presence of PVA hydrogel template and the order of PVA hydrogel on electrochemical performance of PANI,this part prepared graphite/PANI（P-PANI）,disordered PANI@PVA hydrogel（DPPF）,and vertical ordered PANI@PVA hydrogel（VPPH）electrode,respectively.By comparision of the pseudocapacitance test results of the three electrodes,the results can be obtained.Compared with the P-PANI,the surface capacity and diffusion control capacity of VPPH electrode are both increasing.Whereas,the DPPF electrode has a highiet diffusion control capacity and the lowest surface capacity.This differences indicate that the order of PVA can increase the specific surface area of PANI effectively,thus obtaining the maximum surface capacity.Besides,the vertical order structure of PVA is also conducive to the diffusion of electrolyte ions,resulting in the promotion of diffusion control capacity.For DPPF electrode,the PANI grew within disordered PVA templates also retained the densed and disordered structure,leading to the specific surface area of PANI decrease significantly.And these make most of PANI cannot contact with the electrolyte directly,but can only participate in the diffusion control redox reaction.The cycling test results show the VPPH（15000 cycles,80%capacitance retention）has a much longer cycle life than DPPF（4300 cycles,80%capacitance retention）and P-PANI（3000 cycles,80%capacitance retention）.The best cycling stability of VPPH electrode benefits from the close contact between PANI and vertical ordered PVA hydrogel which offer a support and cushion for PANI matrix during repeated charge-discharge process.（4）Furthermore,asymmetric flexible supercapacitors（AFSCs）had been fabricated with the vertical PANI@PVA hydrogel anodes and activated carbon cathodes.The AFSCs showed excellent flexibility,high area capacity of 244 m C cm^-2 at 1 m V s^-1(with a specific capacitance of 244 m F cm^-2 for a voltage of 1.0 V),and superior cyclic stability with a capacitance retention of 80%after 7,000 cycles.These results strongly prove that using the polyaniline electropolymerized within a template of vertically ordered PVA as electrodes of flexible supercapacitors has a great potential in actual application.