DEVELOPMENT OF GRAPHENE MATERIAL STRUCTURE TO IMPROVE THE PERFORMANCE OF ELECTRODES FOR ENERGY STORAGE DEVICES

• Main Author: Bityasmawan A, Oktaviardi
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/85462
• Institution: Institut Teknologi Bandung

Energy storage devices such as supercapacitors and lithium ion batteries have received a lot of attention from scientists in recent years to support the use of renewable energy. Electrodes are an important component in the performance of energy storage devices because they play a direct role to store and transfer charge. Among various types of materials, graphene has been widely studied for application in energy storage device electrodes because it has fascinating properties including high electrical conductivity, large surface area, and good electrochemical stability. The electrochemical exfoliation method is a promising method for top-down graphene fabrication because it has several advantages such as easy to control the properties of graphene, simple preparation, environmentally friendly, and produces relatively high electrical conductivity. However, graphene produced using this method easily experiences aggregation during drying process due to the low content of oxygen functional groups. Aggregation can also be caused by the use of binders when fabricating electrodes. It can result to diminishing the specific capacitance and rate capability especially on supercapacitor electrodes. In addition, even though the oxidation that occurs during electrochemical exfoliation is moderate, the presence of oxygen-containing functional groups can still cause defects, thereby reducing the electronic conductivity of graphene. Therefore, it is necessary to optimize the synthesis process to obtain a low oxygen functional group content, especially for applications that require high electron transfer, including for energy storage device electrodes. In this study, we have modified the structure, lateral size, functionalization, and deposition techniques of exfoliated graphene (EG) synthesized through electrochemical exfoliation to enhance the performance of energy storage devices such as supercapacitors and lithium-ion batteries. Post-ultrasonication treatment after electrochemical exfoliation aims to minimize graphene layer aggregation. The results indicate that this treatment can increase the degree of exfoliation and reduce the lateral size of the resulting EG, thus improving the performance of the supercapacitor electrodes. The best degree of exfoliation was achieved by ultrasonication at a power of 480 W (EG 480), as evidenced by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Raman spectroscopy characterizations. This optimal degree of exfoliation resulted in the highest specific capacitance among the samples, as confirmed by Cyclic Voltammetry (CV) and Galvanostatic Charge Discharge (GCD) characterizations. Furthermore, we have developed EG-based supercapacitor electrodes without the use of binders through an electrophoretic deposition process. This method can produce homogeneous graphene deposits, exhibit low charge transfer resistance, and prevent graphene layer aggregation, resulting in specific capacitance values of up to 145.95 F g-1 at a current density of 0.5 A g-1 . This value exceeds the specific capacitance obtained from electrodes fabricated using Polyvinylidene Fluoride (PVDF) binder, which is 97.16 F g-1 at 0.5 A g-1 . Subsequently, the symmetric supercapacitor device, consisting of electrodes fabricated through electrophoretic deposition at a voltage of 5V, achieved the highest rate capability up to 82.31% (at 10 A g-1 compared to 0.5 A g-1 ) and excellent cycle stability (95% after 10,000 cycles at 5 A g-1 ). The maximum energy density and power density achieved were 8.28 Wh (kg)-1 and 10,337.38 W (kg)-1 , respectively. This study reveals that the combination of electrochemical exfoliation and electrophoretic deposition can produce high-performance graphene-based supercapacitors without the need for binders, using a simple and environmentally friendly method. In addition, in this study we also utilized the resulting EG material as a wrapping material for cathode of lithium ion battery. For this application, EG should have a high electronic conductivity to ensure the charge transfer on surface of the cathode material occurs rapidly and maximize redox reactions to achieve a high specific capacity. The EG material utilized for this application was synthesized using variation of electrolyte concentration (NH4)2SO4 to control the oxidation degree of EG. The oxidation degree in graphene affects the electronic conductivity of EG which further impacts the rate capability of the NCA cathode. The electrolyte concentration that produces optimum electronic conductivity is 0.3M with a value of 125.40 ± 5.48 S cm-1 . When mixing this with NCA cathode, the NCA/EG 0.3M composite material produces the highest rate capability which can maintain a specific capacity of 67.72% at a current density of 5C compared to 0.1C. These results provide insight that a high electronic conductivity of graphene is needed to achieve the best rate capability. In addition, the cycle stability test by performing repeated charging-discharging revealed that the NCA/EG 0.3M sample shows better stability than bare NCA with a capacity retention value of 77.80% after 100 cycles.