Enhancing charge storage capacity of cellulose-sweat-based electrolyte flexible supercapacitors with electrochemically exfoliated free-standing carbon yarn electrodes

The Internet of Things (IoT) provides an interface between different electronic devices such as flexible electronics, and e-textiles to capture and receive real-time data and help humans to devise systems that will adequately respond to these environmental stimuli. The main limitations of these devices to work 24/7 are the lack of continuous power supply and easy integration into textiles to perform their functions. The other issues are poor adhesion of active materials with substrates and peeling-off of active material from the electrode substrates and consequently, degradation of electrochemical performance. A potential and evolving strategy is fabricating a current collector-less and integrable carbon yarn-based energy storage device. Herein, we are presenting a facile and novel technique to exfoliate carbon yarn fibers to enhance their electrochemical performance by 3 orders of magnitude. Activated carbon yarn wires acting as current collector-less electrodes along with cellulose acetate-based composite separators offer a large surface area to simulated sweat electrolyte ions and show a gravimetric capacitance of 11.28 Fg⁻¹ at the scan rate of 5 mVs⁻¹. Activated carbon yarn-based symmetric supercapacitor device in a simulated sweat solution electrolyte offers excellent cyclic and bending stability with over 95 % capacitance retention in both tests.

Theoretical insight

Supercapacitors (SCs) comprise many active and passive elements. The most passive and vital elements are current collectors, separators, binders, electrolytes, and packaging. Two key elements, current collector and binders can be eliminated by developing current collector-free or free-standing electrodes. Carbonaceous materials such as graphene [1], [2], carbon nanotubes (CNT) [3], porous carbon [2], and carbon onions[4], [5] are common alternatives of active materials for SCs electrodes owing to their low cost, chemical stability, large surface area, and high electrical conductivity. These active materials show exceptional attributes such as long cyclic life, and high-rate capability owing to their intrinsic operation mechanism e.g., surface charge storage due to large surface area. However, they also suffer from low specific capacitance ascribed to low surface area exposed to electrolyte ions and low charge storage due to poor wettability. The most efficient technique to address this problem is to incorporate doped heteroatoms or surface functional groups such as surface oxygen groups present on the surface of carbon yarn. The inclusion of these doped heteroatoms and functional groups boosts the intrinsic properties, such as electrical conductivity, and wettability. The increased electro-active surface area offers more active sites for electrolyte ions, resulting in more charge storage and higher pseudocapacitance [6], [7], [8].

Carbon yarn comprised of long-chain carbon filaments of 2.5–5 µm in radius, excellent conductivity, high chemical and mechanical stability, and light weightiness make it a potential candidate as an active material or a free-standing electrode for SCs. However, due to its low specific capacitance, limited surface area, and low porosity, carbon yarn failed to be directly exploited as a free-standing or current collector-less electrode for flexible supercapacitor applications.