Improving capacitance and cyclability in microbial cellulose based ultracapacitors
Young, Nathaniel James
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Microbial Cellulose (MC) is a highly porous macromolecule with intrinsic properties that make it a useful substrate for conductive materials within ultracapacitors. MC has the potential to increase capacitance by serving as a high surface area substrate for conductive polymers and carbonaceous materials. Electrode surface area is a critical parameter in ultracapacitors because capacitance depends on the available active sites that are accessible to counter ions. Commercial ultracapacitors increase electrode surface area by adding microsize carbonaceous materials. Most commercial devices also require adhesive compounds to bind the conductive material to the substrate. Adhesive compounds increase sheet resistance and hinder overall capacitance. MC membranes possess highlyordered surface hydroxyl groups that readily bind to different types conductive materials and reduce the need for additive adhesive compounds. This thesis investigates three unique methods for converting a MC membrane into a working ultracapacitor electrode. In the first method, polypyrrole and carbon nanotubes (CNTs) are added to a medium of Acetobacter that incorporates the material into a homogeneous crystalline matrix of beta1,4 glucan chains. The resulting MC is a fully integrated membrane with a homogeneous embedded layer of conductive material. SEM imaging shows the conductive material is incorporated primarily at the core of the membrane. As a result, this electrode suffered from high sheet resistance and did not generate any significant capacitance. In the second method, a conductive ink consisting of CNTs, carboxymethyl cellulose (CMC), polypyrrole, and DI water was used to coat the surface of a dried cellulose membrane. After 12 hours, the ink dries and leaves a shiny black conductive layer on the membrane’s surface. CMC’s role in the ink is to increase viscosity and help bind the conductive material to the membrane surface. CMC is also a dielectric material that acts as an insulator to the polypyrrole and CNTs, and ultimately impedes electrical energy storage. In the final method, a MC membrane was soaked in aqueous and non aqueous pyrrole solutions, and polymerized with FeCl3 and Fe2(SO4)3. Single and double membrane device configurations were also investigated. Surface polymerization of pyrrole monomers proved to be the best method for converting microbial cellulose into a working electrode with good capacitance and cyclability.