Graphene composites for energy storage applications
Foo, Ce Yao
Date of Issue2014
School of Materials Science and Engineering
Flexible energy storage devices are one of the many emerging technologies which have been highly sought after. This is particularly due to the anticipated release of flexible electronics by manufacturing giants such as Samsung and Apple, as well as wearable electronic products such as light-emitting shirts. All these products require power sources which are not only able to generate enough power, but must also display a certain degree of flexibility and mechanical robustness. In our work, highly flexible and free-standing graphene-based composite electrodes have been fabricated. These composite electrodes do not require current collectors as it is by itself an electrode which comprises entirely of the active materials for energy storage. This effectively removes the need for additives such as binders, which unnecessarily adds to the resistance and weight of the electrode. Furthermore, by encompassing materials which exhibit both pseudocapacitive and electrical double-layer capacitive (EDLC) behvarior, we are able to fully harness the potential of the composite electrode and improve the overall charge storage capability. Graphene-based derivatives such as reduced graphene oxide (rGO) and electrochemically exfoliated graphene (EEG) which exhibits high conductivity were prepared and used as a free-standing electrode. Incorporation of materials such as YzOs, Mn02 and polypyrrole were included to introduce the pseudocapacitive aspect of charge generation. The abundant oxygen functional groups in GO acts as anchoring sites for the growth of metal oxides.[ll Growth of these materials on graphene which exhibits high surface area is especially beneficial as these accessible areas allow easier penetration of electrolyte ions, enhancing electrochemical interactions. The addition of graphene also alleviates the conductivity issues of metal oxides, making it possible to achieve higher mass loadings, better cycling stability and improved reversibility. Addition of a carbon-based support was also shown to further improve the mechanical properties. OH-rich nanocellulose (NC) fibers possess great chemical affinity with rGO which have abundant oxygen functional groups, this holds the graphene sheets together and improve their interaction, giving rise to improved mechanical properties such as Youngs' modulus and tensile strength. To show the scalability of our methodology, we emphasize the importance of fabricating high mass electrodes which are more realistic to the requirements of practical applications. High capacitive performance will never be achievable during scalability if the active material mass is very low. We also demonstrate the ability of our free-standing electrodes to power up commercially available products such as LED decorative lights by fabricating it into a device, signifying the feasibility of our work to be used in commercial applications. The device also demonstrated excellent capacitive behavior in the bent state as well, showing the excellent mechanical stability of our electrodes.