Esnek Tüm Katı Hal Süper Kapasitörler için Grafen Oksit (GO) ve Kitosan Bazlı Biyouyumlu Kompozit 3D Mimarilerin Hazırlanması

Abstract

Today, an increasing number of elderly people or patients with chronic diseases need new generation flexible, implantable, and biocompatible devices to monitor their health conditions moment by moment. Although many people currently use implantable medical devices (IMDs), they require power from an external source or an internal battery. Flexible and biocompatible implantable electronic devices provide an effective strategy to monitor health conditions within the biological body and provide information flow when necessary. In this way, monitoring of patients' current health status, continuity of routine monitoring processes and emergency interventions when necessary can be carried out effectively. It is critical that the power systems of the electronic devices used for this purpose are flexible and high-performance, as well as biocompatible and implantable. Unfortunately, traditional energy sources can cause biocompatibility issues such as immunological rejection, inflammatory reactions and secondary biological toxicity due to corrosion and leakage of battery packs. Additionally, existing energy storage devices require surgical replacement every 6-10 years, posing additional risk to the user. Therefore, alternative energy storage devices that do not have such disadvantages are needed. Supercapacitors are proposed as promising candidates for powering implantable electronic devices, thanks to their high power density, long cycle life, and ability to charge-discharge quickly within seconds.

For this purpose, the aim of the current PhD thesis is to design a solid-state supercapacitor with high electrochemical performance, based on graphene oxide/chitosan (GO/CS) as electrode materials and poly(vinyl alcohol) (PVA)/potassium chloride (KCl) as gel polymer electrolyte (GPE). In this thesis, PVA a hydrophilic and biocompatible polymer, is utilized as the matrix and KCl is used as the ion generator. This thesis focuses on optimizing the key properties of the gel electrolytes, including ionic conductivity, film formation, mechanical-thermal properties, and biocompatibility. Since the ionic conductivity of an electrolyte depends on the charge carrier concentration and mobility, in the first part of this thesis, the effect of KCl amount and polymer molecular weight (Mw) on ionic conductivity of PVA/KCl/H₂O-based gel polymer electrolytes were systematically investigated for the first time. The result of characterization tests confirms the combination of PVA and KCl. Finally, by considering the electrochemical impedance spectroscopy (EIS) test and mechanical-thermal properties results, GPE with Mw=195000 g mol-1 and PVA/KCl (w/w:1/2) was found to have the greatest ionic conductivity (3.48 ±0.25 mS cm-1). Even after 5000 cycles, it retained 88% of the initial specific capacitance. Our findings reveal that GPEs exhibit exceptional mechanical strength as well as unique ionic conductivity characterized by minimal interfacial resistance. Moreover, the gel electrolyte exhibited exceptional biocompatibility, evident in a cell viability test where 72.3% of cells remained unaffected after 72 h of exposure to PVA/KCl gel.

For the first time, symmetric solid-state supercapacitor based on GO/CS composite fiber was developed by wet-spinning method for different weight ratios of GO/CS. In this way, at an optimum concentration of chitosan, the supercapacitor containing RGO/CS (w/w: 90/10) showed the highest specific capacitance of 523.06± 53.57 F g-1 at scan rate of 5 mV s-1. Chitosan effectively prevents re-stacking of GO nanosheets and provides high specific surface area and high conductivity, thus leading to excellent energy storage performance. Additionally, film-based GO/CS electrodes were prepared using the solution mixing method for various GO/CS weight ratios. The supercapacitor containing RGO/CS (w/w: 90/10) showed the highest specific capacitance of 872.75± 68.35 F g-1 at scan rate of 5 mV s-1. The highest specific capacitance of the film-based supercapacitor can be attributed to variations in the material's surface area, porosity, and accessibility of active sites in the film morphology. Film generally provides a larger surface area, more active sites for electrochemical reactions, and better accessibility of electrolyte ions to the electrode surface compared to fibers, leading to higher specific capacitance. Additionally, the film structure may contribute to improved electrochemical performance by providing improved electron and ion transport within the electrode, better porous structure, and potentially flexibility. Therefore, the results of characterization and microscopic tests of GO/CS film confirm the combination of GO and chitosan. Moreover, the film composite electrode retained 87.31% of its initial capacitance even after 10,000 cycles. The symmetric solid-state supercapacitor exhibits excellent energy and power densities of 234.97 W h kg-1 and 1499.98 kW kg-1, respectively. Beyond its electrochemical benefits, the integration of CS improved biocompatibility and mechanical strength, making it a suitable candidate for implantation. In evaluating the cell viability of the RGO/CS composite, the results showed a viability rate of 76.4% after 72 hours. The recorded cell viability of 76.4% indicates that a significant proportion of cells maintained their viability and well-being after exposure to the RGO/CS electrode.

Following the optimization studies conducted on the structures of electrolyte and electrode materials, in the third stage of the thesis, a flexible solid-state supercapacitor based on RGO/CS (w/w: 90/10) and PVA/KCl gel electrolyte (Mw=195000 g mol-1, PVA/KCl (w/w:1/2)) was fabricated using PET as a flexible current collector substrate, instead of stainless-steel substrate. The specific capacitance value was found as 191.07±19.24 F g-1 at 5 mVs-1 (186.64±21.69 F g-1 at 1 A g-1), and this value was lower than that of the stainless-steel substrate. This may be due to the stable platform for uniform adhesion of the electrode material and good electricity conductor of stainless steel which is beneficial for efficient charge transfer and overall excellent durability and electrochemical performance.

Furthermore, a flexible solid-state supercapacitor based on RGO/CS (w/w: 90/10) and PVA/KCl gel electrolyte (Mw=195000 g mol-1, PVA/KCl (w/w:1/2)) was fabricated using laser 3D-printing technology on polyimide (PI) film. The interdigital RGO/CS electrodes displayed a hair comb formation and showed an outstanding adhesive properties and durability. In this stage of the thesis, the adaptability of the obtained supercapacitors to 3D printing technology was demonstrated.

These findings offer a promising route towards the fabrication of stable and high-performance biocompatible electrode material and gel polymer electrolytes that can be used in solid-state supercapacitors, thus laying the foundation for their integration into flexible, safer, and wearable bioelectronics. Developed with environmentally friendly materials, the electrode paves the way for the production of solid-state supercapacitors with advanced electrochemical and biocompatibility properties, making them suitable for integration into sustainable next-generation biomedical devices.