Flexible supercapacitors based on cotton textile electrodes

This work focuses on the design and development of flexible solid-state supercapacitors based on cotton textile electrodes with activated carbon (AC) and manganese oxide (MnO2) as negative and positive electrodes, respectively and a polymer/electrolyte membrane. The cotton textile was chosen as support because its low cost, flexibility and easily to be conformed. Because it is an insulator needs special treatments to become an electrically conductive material. To be accomplish this, a first carbon conductive layer was deposited on both sides of pristine cotton substrate starting from a slurry of acetylene black and carbon nanofibers and succesively a second active layer of both AC and MnO2 was further deposited on two different substrates to form the negative and positive composite electrodes, respectively. The negative electrode was composed by 80 wt. % AC; 10 wt % of carbon nanofibers (CNF); 10 wt % of poly(vinylidene fluoride) (PVDF) binder and, the positive one by 70 wt% of MnO2, 10 wt % of carbon black (CB), 10 wt % of CNF, 10 wt % of PVDF. A flexible supercapacitor is ther eafter realized using two cotton electrodes coupled with an Aquivion® PFSA (Solvay) electrolyte membrane. The supercapacitor was then electrochemically characterized to determinate the specific capacitance, voltage range stability and cycling stability. For better investigate the durability of the developed supercapacitor, a new procedure combining galvanostatic charge/discharge cycles and floating conditions at 1.6 V has been designed and applied experimentaly. The SC as a whole was subjected at different electrochemical characterizations (CV, G-C/D, EIS), which were carried out to check the cell performances at starting conditions and at different cycle intervals until the test was deliberately interrupted. As a result, the capacitance of the SC, calculated from the G-CD profile at 0.2 Ag-1, increased from the initial value of 134 Fg-1 to 164 Fg-1 after 7800 charge/discharge cycles and 108h under maximum voltage of 1.6V. After which, the capacitance decreased to reach a enough stable value of 144 Fg-1 after 20 K cycles and 280h at 1.6 V (see Figure 1). The study also showed that initial CVs were perfectly rectangular, while the subsequent ones showed distortions and peaks that appeared during the progression of stability test. A remarkable 7% increase in specific capacity of the SC was achieved after 20 K cycles and 280 hours of floating at 1.6V, despite the very harsh conditions of long-term durability assessment.