With the emergence of various wearable devices, supercapacitors have gained immense attention because of their fast response rates. However, most supercapacitors use hazardous electrolyte materials, such as H2SO4, KOH, and acetonitrile. Leakage of these types of electrolytes during use would be very harmful to human skin. Therefore, a supercapacitor that does not employ hazardous materials is an attractive option for use in the energy-storage components of wearable devices. Herein, we successfully demonstrate a Na-ion supercapacitor (NISC) with a polypyrrole/carbon-coated heat-treated carbon felt electrode and an aqueous 0.4 M NaCl electrolyte, which is not harmful. Furthermore, our NISC with polypyrrole/carbon-coated heat-treated carbon felt exhibits a high specific capacitance (31.09 Fg(-1)) and a fast response rate (chargeable at 0.5-s intervals). The proposed NISC with no harmful materials in the electrolyte has an excellent response rate. It will establish useful guidelines for the energy-storage components in wearable devices Copyright (c) 2017 John Wiley & Sons, Ltd.