Study On Synthesis Of Poly(3,4-ethylenedioxythiophene)/Transition Metal Oxides Nanocomposite As An Advanced Supercapacitor Electrode Material

At present conductive polymer/inorganic metal oxides hybrid material has become one of the hot research areas in the field of energy storage as supercapacitor electrode materials. It attributed to the combination of advantages of both conductive polymers and inorganic metal oxides component. On the one hand, conductive polymers exhibits advantages such as light weight and low price, good environmental stability, high conductivity under the doping state, excellent charge storage ability and reversibility, and on the other hand, inorganic metal oxides component shows higher specific faradaic pseudocapacitance. Currently, there are many methods for preparation of organic/inorganic hybrid materials, such as, sol-gel technique, intercalation in-situ polymerization, blending method, in situ generation method, insitu polymerization, electrochemical polymerization, and molecular self-assembly method, etc. All the above methods have their own advantages and disadvantages. Up to now, the conventional method of preparing organic–inorganic hybrids is that inorganic particles were synthesized first and then embedded in the polymer matrix via in-situ chemical oxidative polymerization or electrochemical deposition method. And the resulted product is mostly composed of inorganic components unevenly coated with organic polymers. The possible disadvantage is poor controllability in whole pore structure sizes and their distributions in the as-obtained organic-inorganic hybrid, which cannot meet the requirements as the supercapacitor electrode materials for the construction of the fast ions and electrons transport channels. Therefore it undoubtedly becomes a key to design organic-inorganic hybrids with controllable fine structure and performance in their applications as supercapacitor electrode materials.This study intends to prepare conductive polymer/inorganic metal oxide hybrid supercapacitor electrode materials with superior comprehensive performance. In this work, a novel one-pot method has been developed for the synthesis of the above hybrid consisting of inorganic metal oxides and conductive poly（3,4-ethylenedioxythiophene）-polystyrenesulfonate（PEDOT） via a solvothermal-coprecipitation route coupled with oxidative polymerization of organic monomer 3,4-ethylenedioxythiophene（EDOT） by neutral KMnO₄ solution as an oxidizing agent. In this present work, KMnO₄ both as oxidant and coprecipitation agent, and PSS both as polymer dopant and molecule-directing agent, were accomplished in the present work in order to obtain an ordered 3D network structure of PEDOT: PSS-metal oxide nanocomposites. This method is thus named as “oxidative polymerization-chemical coprecipitation-hydrothermal” route in the thesis. And the specific research contents and corresponding results are as follows:（1） PEDOT:PSS/MnO₂ nanocomposites were synthesized via the hydrothermal method by using KMnO₄ as an oxidant, and characterized by the FT-IR and XRD techniques. The influence of the synthetic conditions such as the molar ratio of oxidant KMnO₄ to organic monomer, hydrothermal temperature and time etc on the performance of the resulted product were investigated. Characterization of SEM results showed that the PEDOT:PSS/MnO₂ composites present nanorod-like structure. Electrochemical tests show that the as-synthesized material has superior rate performance with a capacitance retention of 89% when the current density increases from 1 A·g-1 to 20 A·g-1.（2） This chapter focuses on the discussion of influence of different dopants on the morphologies and the electrochemical properties of the resultant composite. Five kinds of dopant such as sodium p-methylbenzene sulfonate, sodium amino sulfonate, sodium camphorsulfonate, sodium dodecylbenzene sulfonate and sodium sulfosalicylate were chosen, and the effects of different molar ratio of dopant to monomer on the properties of the as-prepared materials were investigated. It was found that the PEDOT/Ni-Mn-Co-O hybrid using sodium camphorsulfonate as the doping agent has the best electrochemical performance with a capacitance retention of 98.6% at the current density of 5 A·g-1 after 1000 cycles. The results show that the basic principles for selecting appropriate dopant are as follows: moderate molecular weight with a certain amount of carbon chain length, and multi-branched or multiple sulfonate groups in the molecule.（3） The PEDOT:PSS/Ni-Mn-Co-O nanocomposite was prepared via the “oxidative polymerization-chemical coprecipitation-hydrothermal” method by using poly（vinylbenzene sulfonate） as dopant. SEM characterization confirmed that the nanosheets and nanowires intertwined 3D network structure was formed with a specific surface area of 64.8 m2·g-1 and an electrical conductivity of 160 S·cm-1. The electrochemical performance of as-obtained material was tested in 6 M KOH electrolyte by using a three-electrode system. Results show that the specific capacitance of the material was 1234.5 F·g-1 at the current density of 1 A·g-1; and the capacity retention was 84.7% after 1000 cycles at the current density of 5 A·g-1.（4） The ratio of organic components to inorganic components was further optimized in order to improve the cycle stability of composite materials in the process of charging and discharging. The two complexation methods were designed to achieve the above target by changing the ratios of organic monomer EDOT to pyrrole（py）, and the ratios of dopant PSS to sodium camphorsulfonate（CSA）, respectively. The two kinds of PEDOT:PPy-PSS/Ni-Mn-Co-O and PEDOT:PSS-CSA/Ni-Mn-Co-O nanostructured composites were prepared by the same method of “oxidative polymerization-chemical coprecipitation-hydrothermal”. The experimental results show that the capacitance retention of the two materials was 85% and 91%, respectively, at the same current density of 5 A·g-1 after 2000 times cycles.The research provides a new idea and guidance for the preparation of organicinorganic hybrids as advanced supercapacitor electrode materials to meet different customers’ requirements.