With the rapid development of portable electronic devices, electrified transportation, and renewable energy conversion, there has been an increasing and urgent demand for advanced energy storage devices [1], [2], [3]. Electrochemical double-layer capacitors (EDLCs), also known as supercapacitors, owing to their high power density (> 10 kW Kg−1), rapid charging/discharging rate, ultralong cycle lifetime and wide operating temperatures, have attracted tremendous attention. However, compared with widely used batteries, commercial supercapacitors suffer from a low energy density [4], [5], [6], [7]. Especially, the energy density at a high power density is too low to meet the requirement of practical usage [8].

A high cell operating voltage provides both high energy density (E = 1/2 CV2) and power density (P = V2/4Rs) [7,9]. Thus, the current trend in the development of supercapacitor involves the switch from an aqueous electrolyte with low operating voltage of 1 V due to the thermodynamic decomposition of water, to an organic electrolyte which allows a much higher voltage window of about 2.5 V [9], [10], [11], [12], [13]. However, the much bigger size of organic ions arises new challenge to the pore size engineering of electrode materials: pore sizes above 0.4 nm can be active in electrochemical double layer charging in aqueous solution [14], while the optimal pore size in organic electrolytes is as large as 0.8 nm [15]. Moreover, if the carbon is exclusively microporous, the equivalent resistance and diffusion resistance will be rather high due to ions trapping effects [16]. Thus, the existence of a certain amount of mesopores (i.e., pores between 2 and 50 nm) is necessary to meet the requirement of fast ion diffusion [9,15,17,18].

Carbon-based materials such as porous carbon materials, carbon onions, carbon nanotubes, and graphene are the most widely used electrode materials because of the often-cited desirable physical and chemical properties [16,[19], [20], [21]. Among them, activated carbon cloth (ACC) is a highly conductive textile with excellent flexibility, mechanical strength, and ultrahigh surface area compared with non-activated carbon cloth [22], [23], [24], [25], [26]. It holds great promise as an electrode material for flexible or wearable supercapacitors and has exhibited excellent capacitive properties in aqueous electrolyte [20,22,[27], [28], [29]. However, the diameter of each solid carbon fiber unit of the ACC is quite large (~5 µm) [24,30]. The organic ions have to overcome a long ion diffusion length, which will result in high diffusion resistance for organic ions [31]. From the perspective of device physics, the key to realizing the potentials of ACC in organic electrolyte supercapacitors is to ensure effective ion diffusion in the radial direction.

In the present work, we show that such an objective could be achieved by a simple one-step etching & doping (E&D) treatment of commercial cotton fabrics in NH3 atmospheres. The E&D process in the NH3 atmospheres resulted in a tubular fiber-based textile structure, high specific surface area (up to 2116 m2 g−1), controllable hierarchical pore size distribution, as well as in-situ N-doping in the resulted hierarchical porous tubular carbon microfibers clothes (HPACCs). Serving as binder-free flexible electrodes in TEABF4/AN electrolyte, the HPACCs delivered a high specific capacitance (up to 215.9 F g−1 at 1 A g−1), extremely high rate capability (89% from 1 A g−1 to 200 A g−1), small IR (0.23 V at 100 A g−1), outstanding cycling stability (98% capacitance retention over 20 000 cycles), as well as high flexible and recoverable abilities.

0 0