The flexible supercapacitor, with high power density, rapid charge-discharge rate, and long cycle life, is one of the most promising energy storage devices, and has potential applications in portable electronic devices, such as roll-up displays, electronic papers, wearable devices, mobile phones, and computers. On account of their outstanding mechanical properties, high yield, low cost, renewability, and environmental friendliness, natural fibers such as flax and cotton have been regarded as promising substrates or precursors for flexible supercapacitor electrodes.
Free-standing and flexible activated flax fabrics (AFFs) with hierarchical meso/microporous structure have been prepared through a novel one-step synthetic strategy of rapid carbonization/activation of flax fabrics in CO2. The fast heating and the high partial pressure of CO2 inhibit the decomposition of flax during heating up and thus keep more char materials for gasification at high temperature. It is found that such a process could retain a considerable amount of oxygen groups and creates relatively large pores, bringing a one-step process to carbonize and activate flax fabrics at the same time, and offering the freedom of tuning the mesopore volume/total pore volume (Vmeso/Vtotal) ratio. Notably, the Vmeso/Vtotal ratio is significantly increased from 27.6 % to 67.0 %. As a result, the flexible electrodes show excellent electrochemical performance in aqueous electrolyte, including a large specific capacitance of 205 F g-1 or 140 F cm-3 (at current density of 0.1 A g-1), a good rate capability (139 F g-1 or 95 F cm-3 at 35 A g-1) and an excellent cycling stability (~96.6 % retention after 3000 cycles). The excellent rate performance can be attributed to the improved ion transport (due to large pore size) and the good wettability (due to the oxygen group) of electrolyte.
In another work aiming at increasing the specific capacitance of natural fiber-based flexible supercapacitor electrode, MnO2/carbonized cotton textile with high mass-loading and tunable morphology has been developed. After a simple carbonization process, the cotton cloth with high surface area (585 m2 g-1) served as 3D binder-free and flexible scaffolds to anchor MnO2 nanostructures. The morphology of MnO2 nanostructures was tuned and optimized into curled sheet-like, which provided large surface area and could also release large local stress. Electrochemical measurements showed that the curled sheet-like MnO2 had a specific capacitance of 465 F g-1 at 0.1 A g-1, and exhibited an excellent cyclic stability with a specific capacitance retention ratio of 95% after 5000 cycles (at 10 A g-1). Due to the flexible nature of cotton textile, the hybrid electrodes could be bent freely, and the capacitance and cyclability almost remained unchanged even at a bending angle of 150 degrees.