Preparation of graphene derivatives for supercapacitor application

The research on the carbon nanomaterials has begun since the world war one. Various types of carbon nanomaterials such as fullerenes, activated carbon, carbon nanotubes are subjected to intense research. The main advantage of nanomaterials over the conventional counterparts is their tiny size which provides higher surface area to volume ratio, which is very useful in wide applications. The depletion of fossil fuels as the main source for energy has prompted intense search for alternative energy storage such as batteries and supercapacitors. The main advantage of supercapacitors is their ability to rapidly store and release energy and is very useful for portable devices and shows promising potential to replace fossil fuels in automotive industries. However, the current generation of supercapacitors which is made of graphite has poor energy storage capacity due to poor ion adsorption/desorption between the electrolyte and active sites of the carbon electrode. In addition, it also known that graphite has poor fast ion transport which is essential for high current density application such as heavy industries and automotive applications. However, the discovery of monolayer carbon lattice of graphene which has superlative properties encourages an intense research to exploit its properties, especially for supercapacitor application. In this work, a suitable method was developed to prepare various nanographene derivatives and their nanohybrids. The work also covers the determination of their physical, chemical and electrochemical properties for supercapacitor application. The preparation of various sizes of nanographene derivatives such as graphene oxide, nanographene oxide and graphene oxide quantum dots (GOQD) by a combination of chemical oxidation and step by step centrifugation speeds were carried out. Locally produced, herringbone graphite nanofibers (HGNF) was used as the starting material. Particle size distribution (PSD) study shows micron-, nano- and quantum dots graphene oxide (GO) were obtained. Importantly, the formation mechanism was identified due to pitted and peeled out of HGNF into smaller carbon fragments. In addition, the formation of various functional groups was recorded on the FTIR and XPS spectra. Interestingly, the photoluminescence (PL) results indicate the GO with 100 nm lateral size starts to exhibit the fluorescence property. The huge differences in the physical and chemical properties between GOQD and GQD were observed in this work. The morphological analysis indicates that the GOQD was agglomerated while GQD was well dispersed. Similarly, the FTIR and XPS studies show huge percentage difference of C-C/C-O between GOQD and GQD, which is 40% and 60%, respectively. It is believed that this percentage difference has affected the emitted colour of the quantum structure from blue at 425 nm to green at 450 nm. This is due to the shifted of the emission wavelength from lower to higher wavelength as shown by the PL studies. After the study on the individual component of these graphene derivatives was accomplished, then the preparation of two types of nanohybrids, namely nanographene/graphene quantum dots and graphene oxide/graphene quantum dots were carried out. The electrochemical studies were carried out to determine the supercapacitor performances of the nanographene derivatives. It was found that the specific capacitance values were increased for smaller sizes of the graphene sheets. This is due to better ion adsorption/desorption between electrolyte and smaller sizes of nanographene derivatives as the electrode. This is in agreement with the higher integrated area of cyclic voltammograms and longer discharging time in charge/discharge profiles for smaller sizes of nanographene derivative. More importantly, it was found that the availability of abundant edges of GOQD is essential for fast ion transport which crucial for high current density supercapacitor application. The impedance spectroscopy also indicates lower resistances as the sizes were decreased. Moreover, the electrochemical studies also indicate better supercapacitor performances were obtained when some functional groups were removed from the quantum dots particles. In addition, a computational modeling study using density functional theory (DFT) was carried out for the graphene oxide quantum dots and the graphene quantum dots to predict its charge density distribution mapping. Generally, it was found that superior supercapacitor performances were obtained for the both types of nanohybrids. The double layer capacitor (DLC) characteristic was also observed for the in situ formation of nanographene/GQD nanohybrid. It is believed that the coexistent of both GQD and nanographene provided abundant active sites for ions adsorption/desorption interactions between the electrolyte and the nanohybrid. Interestingly, this nanohybrid could retain more than 300 F/g of specific capacitance values, even at high current densities. The formation of random-stacked GQD and individual GQD played a crucial role to retain the impressive capacitor performance. In addition to good retention, the nanohybrid has shown a remarkable improvement for specific energy and specific power. Another type of nanohybrid also was prepared using the wet chemical method by mixing GO and GOQD. A hybrid characteristic was observed in the cyclic voltammetry analysis, which is due the presence of GOQD and GO. Generally, the nanohybrid exhibits superior capacitor performance compared to the individual components. This is due to better ions adsorption/desorption interactions as more ions were allowed to diffuse. The charge/discharge profiles indicated the superiority of the capacitor performances of the nanohybrid at different current densities. In addition, it was also found an improvement in the specific energy and specific power of the resulting nanohybrid.

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