Na₂SO₄‐Polyacrylamide Electrolytes and Enabled Solid‐State Electrochemical Capacitors

Abstract

Abstract A highly ionic‐conductive and high‐performance neutral pH polymer electrolyte comprises of Na 2 SO 4 and polyacrylamide (PAM) was developed for solid electrochemical double‐layer capacitors (EDLCs). Na 2 SO 4 was compared with Li 2 SO 4 baseline in liquid electrolyte and exhibited higher ionic conductivity and identical stability window. Na 2 SO 4 −PAM electrolytes were optimized by varying their salt‐to‐polymer ratio. While all compositions show excellent stability for over 30‐day tracking period, an ionic conductivity of ca. 30 mS cm −1 was attained at 10,000 : 1 molar ratio of Na 2 SO 4 :PAM, among the highest reported for neutral pH polymer electrolytes. Raman spectroscopy verified that the electrolyte maintained well‐hydrated ions throughout the 30 days monitoring period. Solid EDLCs were constructed using activated carbon electrodes and Na 2 SO 4 −PAM electrolyte. They exhibited a wide 1.9 V operating window at 50 mV s −1 CV scan rate, with comparable capacitance to its liquid baseline. The solid EDLCs outperformed many symmetric devices in terms of energy and power densities. They also displayed good rate capability (up to 500 mV s −1 ) and excellent cycle life (>8,000 cycles).