Hierarchical Graphene Supercapacitors

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Abstract

Electrochemical supercapacitors are promising devices for energy storage applications. However, their uptake is currently limited by their relatively low energy density. The recent discovery of graphene has strengthened supercapacitor research, due to graphene̢۪s high surface area, conductivity, strength, and flexibility. However, the synthesis of large quantities of defect-free graphene and its subsequent incorporation into supercapacitors has proved difficult due to aggregation and restacking of the graphene. Hence, in order to retain the high surface area of graphene, it needs to be incorporated into hierarchical structures. Given these issues, this thesis aimed to produce high quality graphene flakes via electrochemical exfoliation. These flakes were then processed into hierarchical structures (foams and fibres) for supercapacitor devices. The graphene was exfoliated using a reductive process, with two different cells designs explored. The influence of the microstructure of the initial graphite on the exfoliation process was also studied. The hierarchical foams were produced by depositing the graphene onto nickel foam. It was found that the degree of exfoliation has a marginal effect on the capacitance of the device. This electrochemically exfoliated graphite was also wet-spun with polyacrylonitrile (PAN) and carbonised to produce carbon fibre-graphene composites. It was found that the carbonised materials had a higher capacitance than the precursor material (33 F g-1 and 47 F g-1 respectively). As a comparison, wet-spun graphene oxide fibres were synthesised with polyvinyl alcohol and were subsequently carbonised and reduced. These fibres gave comparable capacitance results to the carbonised polyacrylonitrile fibres (47 F g-1 and 40 F g-1 respectively).

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