Hydrogel-Polymer Electrolyte for Electrochemical Capacitors with High Volumetric Energy and Life Span

AC/AC (AC=activated carbon) electrochemical capacitors (ECs) were designed with a 1 mol L^-1 lithium sulfate hydrogel- polymer electrolyte (HPE) based on carboxymethyl cellulose sodium salt (CMC). The electrochemical performance of the ECs was compared with that of cells composed of common separators such as a polyolefin membrane (Celgard 3501, polypropylene separator coated by surfactant) and a glass microfiber membrane (Whatman GF/A). The ECs with Celgard 3501 and home-made CMC-HPE demonstrated a higher volumetric energy than that with Whatman GF/A. However, after 120 h of floating at 1.5 V, the capacitance of the EC with Celgard 3501 decreased dramatically by 25 %, whereas with CMC-HPE and Whatman GF/A, the decrease was only 4 and 6 %, respectively. Post-mortem observations of the Celgard 3501 separator after floating suggested that the surfactant layer was removed, which caused a decrease of separator wettability, as confirmed by the slow evolution of the electrolyte contact angle on its surface. Hence, CMC-HPE is a very attractive option to develop aqueous-electrolyte-based ECs with an excellent life span and high volumetric energy density.