Polyvinyl alcohol-acid redox active gel electrolytes for electrical double-layer capacitor devices

Since the mechanism of charge storage in electrical double-layer capacitors (EDLCs) relies on diffusion of ions into the pores of the electrodes, in general, a much lower capacitance is expected for gel-based electrolytes than liquid electrolytes. However, in this work, we have found that the specific capacitance in gel-based electrolytes made of polyvinyl alcohol (PVA) and an acid (H2SO4 or H3PO4) is even higher than the specific capacitances of similar devices with liquid acid-based electrolytes. We have discovered that the reason is due to the gel being a redox active material with the capability of charge storage in the volume of the electrolyte. In this work, solid-state and flexible devices with both H2SO4-PVA and H3PO4-PVA electrolytes were fabricated and characterized. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods were applied to estimate the capacitance associated to the gel electrolytes. Also, a relatively high cycling stability of 97.5% for H2SO4-PVA and 95% for H3PO4-PVA was obtained after 1000 charging-discharging cycles. A mechanism of charge storage is proposed to explain the redox active behavior of the gel electrolyte. The presented results are promising for employment of PVA gel electrolytes in some low-cost applications.