Nanocellulose and Polypyrrole Composites for Electrical Energy Storage
Abstract: To meet the predicted increase in demand for energy storage in tomorrow's society, the development of inexpensive, flexible, lightweight and sustainable energy-storage materials is essential. In this respect, devices based on electroactive organic molecules, such as conducting polymers, are highly interesting. The aim of this thesis was to evaluate the use of nanocellulose as a matrix material in composites of cellulose and the electroactive polymer polypyrrole (PPy), and the use of these composites in all-polymer paper-based energy-storage devices. Pyrrole was polymerized using FeCl3 onto cellulose nanofibers in the form of a hydrogel. The resulting PPy-coated fibers were washed with water and dried into a high surface area, conductive paper material. Variations in the drying technique provided a way of controlling the porosity and the surface area of wood-based cellulose nanofibers, as the properties of the cellulose were found to have a large influence on the composite structure. Different nanocellulose fibers, of algal and wood origin, were evaluated as the reinforcing phase in the conductive composites. These materials had conductivities of 1–6 S/cm and specific surface areas of up to 246 m2/g at PPy weight fractions around 67%. Symmetrical supercapacitor devices with algae-based nanocellulose-PPy electrodes and an aqueous electrolyte showed specific charge capacities of around 15 mAh/g and specific capacitances of around 35 F/g, normalized with respect to the dry electrode weight. Potentiostatic charging of the devices was suggested as a way to make use of the rapid oxidation and reduction processes in these materials, thus minimizing the charging time and the effect of the IR drop in the device, and ensuring charging to the right potential. Repeated charging and discharging of the devices revealed a 10–20% loss in capacity over 10 000 cycles. Upon up-scaling of the devices, it was found that an improved cell design giving a lower cell resistance was needed in order to maintain high charge and discharge rates. The main advantages of the presented concept of nanocellulose-PPy-based electrical energy storage include the eco-friendly raw materials, an up-scalable and potentially cost-effective production process, safe operation, and the controllable porosity and moldability offered by the nanocellulose fiber matrix. Integrating energy storage devices into paper could lead to un- precedented opportunities for new types of consumer electronics. Future research efforts should be directed at increasing the energy density and improving the stability of this type of device as well as advancing the fundamental understanding of the current limitations of these properties.
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