Graphene Quantum Dots as an Electrode Material for Hybrid Battery-Supercapacitors

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A battery-supercapacitor hybrid (BSH) is a promising type of energy storage device that combines the advantages of both batteries and supercapacitors, allowing them to benefit from high energy density, high power density, long life cycle, fast charging times and high-temperature capabilities1. However, fabricating reliable BSH exhibiting high performance remains very challenging and little studied. This project aims to address this research gap by exploiting the unique properties of graphene quantum dots (GQDs), i.e. to synthesise and embed them in the fabrication of BSH electrodes controllably in a low-cost process. To realise the potential high for performance BSHs, the search for methods to improve their properties further, making them competitive against advanced batteries and conventional supercapacitors, is ongoing. One option is to integrate graphene quantum dots (GQDs, nanometre-sized semiconductor particles with tuneable electronic properties) onto both electrodes of the device. GQDs retain the characteristics of graphene (fast-moving electrons, high thermal conductivity, quantum tunnelling, etc.), but exhibit the quantum confinement effect, resulting in an opening of the material's bandgap, allowing them to behave as a semiconductor, in addition to size and edge effects. Furthermore, GQDs display a strong adsorption capacity for a wide range of electrolyte ions, including lithium, sodium and other ions commonly used in conventional energy storage devices. The incorporation of GQDs into a supercapacitor-type electrode increases the energy density and capacitance potential2, while the advantages to including GQDs on a battery-type electrode include high reversible specific capacity, improved rate capability, and longer life cycle3. BSHs without the addition of GQDs have the potential to maintain a life cycle equal to 4000% versus that of a standard lithium-ion battery1,4. The inclusion of GQDs will result in a further increase of the life cycle since they can regulate the internal structure of the electrodes, preventing damage and therefore reducing the need for their frequent replacement, extensive mineral mining, CO2 emissions during production and environmental issues surrounding the disposal of spent lithium-ion batteries. Despite the high potential of GQDs, to the best of my knowledge, so far, there are no reports on reliable BSHs where GQDs are integrated into both electrodes. Moreover, the role and effect of GQD structure (zigzag and armchair edges, and functionalisation degree) on the properties of the GQDs, and the resulting electrodes and device are not defined yet. Therefore, this project aims to fill this gap and by developing reliable GQD-based electrodes for BSH applications. We aim to: (i) Establish the relationship between the structure and properties of the GQDs and electrodes produced using a systematic characterisation of the GQDs and electrode behaviours; (ii) use molecular dynamic simulations to optimise the experimental design and to predict the response of the device produced; (iii) produce a prototype demonstrator of a hybrid power device based on the most promising simulation and experimental results.