Lifetime and reliability of flexible aqueous supercapacitors: constant voltage floating and bending experiments

We have developed thin and flexible supercapacitors that can be combined with energy harvesting components. The substrate material is a laminate foil made of polyethylene terephthalate and aluminum. Activated carbon is used as the electrode, graphite ink as the current collector and aqueous NaCl or Li₂SO₄ as electrolyte. Constant voltage floating experiments were done to estimate the practical voltage window of aqueous supercapacitors with respect to lifetime. We measured the capacitance, equivalent series resistance, efficiency and leakage current over time at constant voltage between 0.9 and 1.6 V. The results indicate that 1.5-1.6 V is feasible for a short time, i.e. about 1-100 hours depending on the materials. Lower voltages are substantially more stable: after 2000 hours at 1.2 V the capacitance decreased by about 10 % and at 0.9 V by only ca. 3 %. Potassium iodide was tested as an additive to the aqueous electrolyte, which clearly increased (up to 3-fold) capacitance but simultaneously shortened the lifetime. The potential for scalable manufacturing of supercapacitor components by printing was demonstrated in an upscaling trial. The supercapacitors were bent to less than 1 cm minimum radius without deterioration of their electrical properties.