Monitoring the ion population at a carbon electrode/aqueous electrolyte interface at various pHs using electrochemical dilatometry

Abstract

Changes in the electrode thickness in electrochemical capacitors depend strongly on the electrolyte pH. Hence, different ions (or ions with various solvation shells) participate in electrical double-layer formation. In this study, operando electrochemical dilatometry experiments incorporating various electrochemical techniques, including cyclic voltammetry, large amplitude sinusoidal voltammetry, and step potential electrochemical spectroscopy, were performed on ECs operating in 1 mol L−1 Li2SO4 with different pHs (6, 9, and 12) and 1 mol L-1 LiOH (for comparative purposes). A microporous carbon (Kuraray YP-80F) was used as the electrode material. Comparable trends for the cell behaviour were observed for electrolytes with pHs of 6 and 9; however, the cell containing the pH 12 solution exhibited similar behaviour to that containing the LiOH solution, suggesting that during positive polarization, the primary role was played by OH−, because the same salt concentration was used in all cases. This result raises an essential concern regarding the actual ion population at the electrode/electrolyte interface, which affects the water decomposition potentials and triggers various parasitic reactions impacting long-term electrochemical capacitor performance.