Influence of aqueous electrolyte concentration on parasitic reactions in high-voltage electrochemical capacitors

Abstract

Increasing operating voltage is a straightforward approach to increase the specific energy of aqueous electrolyte based electrical double-layer capacitors (EDLCs). A broader operating voltage window, however, comes at the expense of accelerated cell ageing processes. Two complementary in situ gas analysis techniques, i.e., internal cell pressure measurement and Online Electrochemical Mass Spectrometry (OEMS), were applied to study the influence of electrolyte salt concentration on the electrochemical degradation of the electrode and decomposition of electrolyte during cycling of symmetrical carbon based EDLCs. Higher concentrations of Li2SO4 aqueous electrolyte salt increase the coulombic efficiency of the cell and reduce the amount of volatile side reaction products (e.g., CO, CO2, and H2) arising from carbon corrosion and water electrolysis. The lower amount of degradation products when increasing the salt to solvent ratio is believed to result from a favorable stabilization of the increasingly coordinated electrolyte solvent-salt bonds, which in turn increases the energy barrier for H2O decomposition. Higher salt concentrations not only increase the electrochemical reversibility at high cell voltages but also increase cell charging capacities as a result of increased electrolyte conductivity, which all-in-all facilitates implementation of future high energy aqueous EDLCs.