BINDER-FREE TiO2-BASED FLEXIBLE SUPERCAPACITORS FOR LOW-POWER WEARABLE ELECTRONICS

Abstract

The increasing popularity of flexible/wearable electronic systems requires flexible, compact, light-weight and compatible energy storage devices to work as power sources for their realization. Therefore, a flexible SC is a compatible and promising candidate for storage of energy in flexible/wearable electronics applications as compared to batteries or micro-batteries due to their incompatibility, bulkiness and limited life cycles. However, one of the major drawbacks of supercapacitors is low energy density. Hence, researchers are doing continuous efforts to increase the energy densities of supercapacitors by developing new materials, techniques and design configurations. Moreover, in the conventional SCs, active material sites are not fully utilized, since electrodes are stacked in a sandwich configuration which results in a larger ionic diffusion length. This problem can be overcome using an in-plane interdigitated micro-supercapacitor (MSC) that can reduce the ionic diffusion length, and enhances the ion exchange procedure. Moreover, an MSC device can be integrated on-chip for the realization of a suitable self-powered electronic system. Further, during the fabrication of an SC device, small amount of binder is generally mixed in the active material which simply adds an extra weight to the electrode material, increases the contact resistance between active material particles, restricts the transportation of charge towards the current collector and further restricts the flow of electrolyte inside the active material by blocking the ion diffusion path. As a result, the electrochemical performance of a supercapacitor system can be degraded. In this regard, the binder-free electrodes of TiO2 nanofibers as an active material have been fabricated via electrophoretic deposition technique and electrochemically tested in a three-electrode system. The fabricated electrodes exhibit high electrochemical performance, such as higher specific capacitance (408 Fg-1 @ 1 Ag-1), minimum value of ESR (0.54 Ω), and higher diffusion coefficient (2.81× 10-10 cm2 s-1), as compared to various reported electrodes consisting binders. After that, an in-plane interdigitated MSC device has been fabricated utilizing binder-free deposition of TiO2 nanofibers over Mo-coated flexible PET substrate for application in low power flexible/wearable electronic area. The optimization of the inter-space between adjacent comb-like finger electrodes has also been performed. The fabricated device exhibits good electrochemical performance, such as high areal capacitance (9.4 mF/cm2 @10mV/s), maximum areal energy density of 0.64 μWh/cm2, areal power density of 307.2 μW/cm2, and 80% capacitance retention after 10,000 cycles. i Furthermore, to get higher energy and power output for practical applications in these devices, the insulating nature of the substrate (PET) in a flexible SC may affect their electrical and electrochemical performance because many on-chip devices need to be integrated or connected in series for a high energy output. The values of energy and power densities for a flexible SC device can be improved by direct incorporation of the active materials over a low density, highly flexible and conductive textile substrate and the utilization of these electronic textiles as a smart solution for the next-generation wearable electronics industry. Hence, by considering above-mentioned benefits, a flexible and binder-free yarn-based supercapacitor (YSC) is fabricated utilizing carbon yarn as substrate and TiO2 nanofibers with small amount of MWCNT as active materials through facile electrophoretic deposition technique for application in flexible/wearable energy storage area. The flexible YSC device is fabricated utilizing the gel electrolyte with a small amount of redox additive to enhance the energy density of the device. The fabricated device shows excellent electrochemical characteristics (36.8 mF/cm at 0.1 mA/cm, 90% capacitance retention after 10,000 cycles) and mechanical flexibility with a high energy output (95% capacitance retention after 2000 bending cycles, and maximum energy density of 11.5 μWh/cm and power density of 368 μW/cm). Moreover, three-similar YSCs are knitted into a wearable fabric to glow a red LED for more than 5 min even during the bending condition at different bending angles, to prove the potential of the device in flexible/wearable electronics. Further, to enhance the energy density of the flexible SC device, the direct deposition of active materials over a high surface area, conductive carbon cloth substrate is an effective strategy to develop a high-performance device and to improve practical applicability. In this regard, carbon cloth (CC) textile is coated by TiO2 nanofibers through electrophoretic deposition technique for application in flexible/wearable supercapacitors. Then a fully flexible symmetric SC device is fabricated utilizing the gel electrolyte with a redox additive to enhance the energy density of the device. The fabricated device is electrochemically tested under various conditions, showing the high electrochemical performance (468 mF/cm2 at 1 mA/cm2 with 97% capacitance retention after 30,000 cycles) with the maximum energy output (maximum areal energy density of 0.14 mWh/cm2 and areal power density of 3.6 mW/cm2) for a TiO2-based flexible SC device. Further, 19 red LEDs are glown using two series connected devices upto 4 min to prove the capability of the device to store energy. Hence, the proposed research work shows the candidature of advanced, flexible, and binder-free supercapacitor devices towards the achievement of a high specific energy goal for integration with other electronic circuitry in future multifunctional flexible/wearable electronics systems.