In Site Investigations On Energy Storage Behavior Of Conductive Polymer By Electrochemical Analysis Techniques

ABSTRACT:Apart from the properties of easy synthesis, low price, environmental friendliness, electronically conducting polymers possess electrochemical redox reversibility and high conductivity. Therefore, they have a good prospect of application in electrochemical storage field. Among many polymers, polyaniline (PANI) and polypyrrole (PPy) have attracted considerable attention due to its high conductivity, good redox reversibility and environmental stability. In present, most investigations on them focused on the performance of a conducting polymer film, including chemical and electrochemical doping process and mechanism of the polymer film. On the other hand, as for the energy storage applications, PANI and PPy as electrode materials are mostly used in the form of solid polymer film. In this case, due to the use of substrate materials, adhesives and other non electroactive material, the specific capacity of the battery was low.In this paper, the electrochemical quartz crystal microbalance (EQCM) technique was used to study the electrochemical doping behavior of PANI films in HC1, ZnCl2,Pb(CH3SO3)2+CH3SO3H solutions, and the electrochemical doping behavior of PPy film in the solution of MnSO4+H2SO4. On the basis of investigations on polymer films, we further studied the electrochemical redox and charge transfer behavior of conductive polymer suspension electrodes. Findings showed that in the static state, PANI or PPy suspension electrode possessed the same redox characteristic as the conductive polymer film; whereas, under the flow condition, the electrochemical behaviors of conductive polymer suspension electrode were dependent on the particle concentration and particle size, and conductive polymer particles can be quickly renewed by the flow of suspension, showing good charge transfer characteristic. The charge/discharge and charge storage performance of conductive polymer suspension electrodes were investigated in various flow battery systems. Experiments showed that the conductive polymer suspension electode possessed higher specific capacity than that of film electrode. The details of the paper are as follows:(1) PANI and PPy films were electrochemically grown onto the PQC Au electrode with various thicknesses by cyclic voltammetry in a solution of HClO4and aniline. Investigations on the doping and dedoping behavior of the PANI film were carried out by EQCM technique in the solution of HCl, ZnCl2or Pb (CH3SO3)2+CH3SO3H. Research findings showed that water molecules are involved in the doping process of the first redox process of PANI film in HCl solution. In second redox process, deprotonation and protonation took place mainly, accompanied with the dedoping and doping of the counter anion from/into the PANI materials. The doping of PANI in solution of ZnCl2is accompanied with pseudo protonation of zinc ions. In the Pb(CH3SO3)2+CH3SO3H solution,the doping and dedoping behavior of PANI are similar to that in HCl. Research shows that the redox process of PPy films in the solution of MnSO4+H2SO4corresponded to the doping and dedoping of SO42-anions. At the same time the charge storage capacity of conducting polymers in different media were quantitatively measured by EQCM technique. Findings showed that as for charge transfer and storage of energy storage materials, EQCM was an effective quantitative analysis method.(2) PANI and PPy powders were respectively synthesized by chemical and electrochemical method. And then conductive polymer particles with different particle size were prepared in shear way. The chemical structure and micro morphology of as-prepared polymer materials were characterized by using scanning electron microscopy (SEM), infrared spectroscopy (IR), particle size analysis technology and other modern analysis methods. The electrochemical behaviors of PANI, PPy suspension electrodes were studied with the cyclic voltammetry and potentiostatic technique. The peak currents of cyclic voltammetry of PANI or PPy suspension electrodes in static conditions were almost independent of the particle concentration, which was similar to that of conducting polymer film. In contrast, in the flow state, it was difficult for the conductive polymer particles to adsorb on the electrode surface, and the current depended only on the number of polymer particles colliding with the electrodes. Therefore, the potentiostatic currents of the conductive polymer suspension electrodes enhanced with the content increase of polymer particles. On this basis, we proposed the charge transfer mechanism of conductive polymer particles, which could be used to explain the experimental phenomena.(3) By the cyclic voltammetry and constant potential test of the PANI suspension electrodes in ZnCl2solution, we proved that the PANI suspension electode possessed good redox reversibility, charge storage and charge transfer characteristics. Therefore, PANI suspension electrode was used as positive electrode to fabricate a Zn-PANI flow battery with zinc as negative electrode. The coulombic efficiency came to97%at the current density of20mA·cm-2, while the coulombic efficiency of10mA·cm-2arrived at118%.. Transition from solid film to a flow-through mode for the cathode material enables the battery to operate at higher current density. The discharge current density of30mA·cm-2was achieved, and, if appropriate configuration can be constructed, to achieve higher power. Based on redox potential and content of the PANI materials studied here, the PANI cathode has maximum theoretical energy density of66.5Wh·L-1, this value is higher than state-of-the-art all-vanadium RFB (～40Wh·L-1).(4) In the flow mode, the PANI suspension electrode, which were dispersed in Pb(CH3SO3)2+CH3SO3H solution, exhibited good charge transfer property and excellent redox performance due to continuous renewal of PANI microparticles by the flow of suspension. In experiment PANI suspension electrode, which replaced lead electrode, was used to design PANI suspension-dioxide lead redox flow battery. The charge discharge behaviors of PANI suspension and dioxide lead single electrode were studied respectively. Rearch findings showed that there was electrode polarization to some extent for two electrodes, so the voltage efficiency of battery lowered. PANI particles as negative electrode materials avoid problems to the lead electrode in soluble lead flow battery and improve the cycle performance of battery. PANI suspension-two lead oxide redox flow battery showed good cycle performance. The discharge capacity of first cycle reached116.3mAh·g-1, and then the capacity decreased slightly with the cycle number. After60cycles, the discharge capacity remains97.2%of the original value, and the average discharge capacity loss is only0.05%per cycle. The coulombic efficiency shows no significant change with the number of cycles over the experiment.(5) PPy microparticles with various sizes were prepared with PPy power in high speed shear way, and then PPy suspension electrodes were obtained by dispersing PPy particles in MnSO4+H2SO4solution. On the basis of studies on redox property and charge/discharge properties of PPy suspension and dioxide manganese single electrode, the PPy suspension-dioxide manganese redox flow battery was designed. The mean charge voltage of flow battery is1.3V, the average discharge voltage of0.95V, and the open circuit voltage of the battery is about1.25V. The discharge capacity density reached132.3mAh·g-1in the present battery system (calculated by the weight of PPy), exhibiting a significant improvement on the discharge capacity density in comparison with the conventional PPy film batteries. Considering the435g L-1upper limit content of the flowable suspension, the highest energy density the highest energy density can reach54.7Wh·L-1(39.1Wh·kg-1).The paper systematically studied the electrochemical behavior of PANI, PPy particles suspension for the first time. And the charge capacities of PANI and PPy were determined by using EQCM technique. We creatively developed the conductive polymer suspension redox flow battery. Conductive polymer suspension electrode can be used as flowable positive and negative electrode respectively in different redox flow battery system, and showed excellent charge storage and charge transfer properties.