ELECTROCHEMICAL ANALYSIS ON THE CHARGE TRANSPORT PROPERTIES OF HETEROGENEOUS SUPERCAPACITOR ELECTRODE MATERIALS

Abstract

The design and exploration of heterogeneous materials for energy storage system are investigated here. The charge transport property of the electrode materials is tuned through different architectures and chemical compositions. This dissertation describes the motivation, design, and fabrication of heterogeneous materials grown on cellulose fibers or as free standing ordered nanoarrays with AAO technology. Their advanced electrochemical properties were enabled by the more feasible charge transfer processes enhanced by structural or compositional regulations. In the first approach, cellulose fibers with porous structure and electrolyte absorption properties are explored as prospective substrate for the deposition of energy material. These substrates are lighter, more flexible and better performing than the traditional substrates (e.g. gold, stainless steel), and are able to also function as an interior

electrolyte reservoir. We demonstrated the value of this internal electrolyte reservoir by comparing a series of hierarchical hybrid supercapacitor electrodes based on home-made cellulose paper or polyester textile integrated with carbon nanotubes (CNTs) by simple solution dip, and electrodeposited with MnO₂. Besides substrate, direct modification on the active materials itself was also conducted to advance the charge transfer process. In this approach, heterogeneous materials were synthesized either through single step or two step fabrications processes assisted with AAO technology, revealing well-ordered 1D nanoarrays. Solely MnO₂ nanowires were compared with a series of RuO₂-MnO₂ composite nanowires with different loading amounts manipulated through the co-electrochemical deposition. The RuO₂ has direct beneficial influences on the charge transfer process as evidenced by the impedance behavior, and shows capability to enhance both the power and energy densities of MnO₂ materials. Additionally, a general route to grow metal oxides into/onto polymer nanowires matrixes through the redox-exchange reaction between high oxidation state metal ions and different polymers were explored here. These heterogeneous materials embed the metal oxide materials (MnO₂, RuO₂) into/onto polymer arrays (PEDOT, PPY) where good electrical conductivity and ion diffusion path are well maintained, realizing the maximum utilization of synergistic materials. These heterogeneous supercapacitor electrode materials engineered in aspects of substrate, structure, and compositions show promising capability in directing the future energy material toward high performance and easy packing to meet the requirements dictated by different fields.