Flexible, aqueous-electrolyte supercapacitors based on water-processable dioxythiophene polymer/carbon nanotube textile electrodes

Abstract

Water-processed, aqueous electrolyte supercapacitors are demonstrated incorporating an alternating copolymer of a functionalized 3,4-propylenedioxythiophene unit with a 2,2′-bis(3,4-ethylenedioxythiophene) unit (PE₂) into carbon nanotube textiles. The side chains functionalizing PE₂ allow the solubility to be tuned from the organic soluble precursor polymer, to a water-soluble polyelectrolyte, and finally to a solvent resistant form by a dilute acid wash (SR-PE₂). Thin SR-PE₂ films (60 μg cm⁻²) are electroactive over a wide potential window with a mass capacitance of 54 F g⁻¹ at a discharge rate of 1.6 s (38 A g⁻¹). These films exhibit a 65% capacitance retention at an 80 millisecond discharge time (560 A g⁻¹). Flexible, free-standing electrodes were fabricated by depositing SR-PE₂ onto a nonwoven carbon nanotube textile (CNT-T) and were assembled into symmetrical, flexible supercapacitors where the addition of polymer resulted in a boost in volumetric capacitance by 400 percent compared to the bare CNT-T electrode. Independent of whether the SR-PE₂ films are cast from an organic solvent or water, device electrodes exhibit 11.4 F cm⁻³ (45 mF cm⁻², 15.9 F g⁻¹) which leads to an energy density of 0.10 mW h cm⁻³ at 7.5 mW cm⁻³ considering the total device volume. The CNT-T imparts a high degree of flexibility allowing the supercapacitors to retain 88% of their charge storage capacity when bent to a 0.8 mm radius as well as complete capacitance retention after 2000 bending cycles to a 3.5 mm radius. The devices assembled in air with a benign KCl electrolyte maintained 83% capacitance retention after 10 000 charge/discharge cycles highlighting the stability of these materials and utility for this processing approach for high throughput energy storage devices.