“Sweat-chargeable” on-skin supercapacitors for practical wearable energy applications

Supercapacitors (SCs) have received worldwide attention as energy devices for wearable smart systems. Biocompatibility and self-charging property of SCs require serious consideration, but the related studies are just in their infancy. Here, a “sweat-chargeable” on-skin SC using biocompatible NaCl electrolyte and freestanding polyaniline/carbon nanotube (PANI/CNT) electrodes is presented. Although the capacitance of the biocompatible SC is about half of that using conventional skin-irritated H2SO4 electrolyte, the potential window extends from 0.8 V to 1.4 V. Thus, a higher energy density can be obtained. The self-charging property is achieved by simply adding a piece of Zn foil on one electrode with silver paste, which provides energy source and maintains a super compact two-electrode configuration of SC. The sweat absorbed from human body could trigger the redox reaction between the added Zn foil and oxygen in air, resulting in the self-charging of the device to 0.652 V in 6 min. To demonstrate its wearability in real applications, a “sweat-chargeable” band is prepared that can spontaneously power a timer or a pedometer when the user is exercising. This work has made a further step in the development of biocompatible and practical wearable energy devices.