All-carbon solid-state yarn supercapacitors from activated carbon and carbon fibers for smart textiles

Smart textiles are intelligent devices that can sense and respond to environmental stimuli. They require integrated energy storage to power their functions. An emerging approach is to build integratable fiber-/yarn-based energy storage devices. Here, we demonstrate all-carbon solid-state yarn supercapacitors using commercially available activated carbon and carbon fiber yarns for smart textiles. Conductive carbon fibers concurrently act as current collectors in yarn supercapacitors and as substrates for depositing large surface area activated carbon particles. Two hybrid carbon yarn electrodes were twisted together in polyvinyl alcohol/H₃PO₄ polymer gel, which is used as both an electrolyte and a separator. A 10 cm long yarn supercapacitor, with the optimum composition of 2.2 mg cm^-1 activated carbon and 1 mg cm^-1 carbon fiber, shows a specific length capacitance of 45.2 mF cm^-1 at 2 mV s^-1, an energy density of 6.5 μW h cm^-1, and a power density of 27.5 μW cm^-1. Since the yarn supercapacitor has low equivalent series resistance at 4.9 Ω cm^-1, longer yarn supercapacitors up to 50 cm in length were demonstrated, yielding a high total capacitance of up to 1164 mF. The all-carbon solid-state yarn supercapacitors also exhibit excellent mechanical flexibility with minor capacitance decreases upon bending or being crumpled. Utilizing three long yarn supercapacitors, a wearable wristband was knitted; this wristband is capable of lighting up an LED indicator, demonstrating strong potential for smart textile applications.