Composite-based flexible supercapacitors for wearable applications

Abstract

Flexible and wearable electronics are becoming increasingly popular nowadays. Flexible energy storage devices like flexible supercapacitors can serve as the power source for next generation wearables to offer flexibility and safety that current batteries lack. The thesis aims to develop safe, mechanically durable, and flexible supercapacitors using hydrogel polymer electrolytes and carbon-based flexible electrodes. The first part of the thesis studies the processing and performance improvement of the hydrogel polymer electrolytes. We found that both ionic conductivity and interfacial capacitance of such electrolytes can be modulated by exposing them to different humidity. Orders of magnitude increase in ionic conductivity was observed when samples were treated at high humidity. We applied high-humidity treatment to flexible supercapacitor prototypes and achieved significant increases in both energy density and power density. The second part of the thesis presents the design and testing of a highly durable supercapacitor. We modeled the flexible supercapacitors as laminate composites, in which the polymer electrolytes are matrix and electrode materials are fillers. We adjusted structures and compositions of fillers to simultaneously optimize the flexibility, strength, and energy storage capacity of the device. The prototype supercapacitor showed an aerial capacitance of over 40mF/cm2 with thickness and flexibility comparable to wearable fabrics. Moreover, the device is foldable, machine-washable, and durable under repeated bending and pressing.