Quantitative Information about Electrosorption of Ionic Liquids in Carbon Nanopores from Electrochemical Dilatometry and Quartz Crystal Microbalance Measurements

Abstract

Electrochemical energy storage using nanoporous carbons and ionic liquids enables large cell voltages and is a promising way to increase the energy density of electrical double-layer capacitors. The structure of the double layer in solvent-free electrolytes is fundamentally different from other systems with organic or aqueous solvents. In our study, we investigate the physical behavior of nanoporous carbon electrodes in contact with ionic liquids with a multilength scale approach by combining electrochemical quartz-crystal microbalance and electrochemical dilatometry. Synergistic combination of both in situ methods allows one to correlate system properties on particle and electrode level. We find that the charging mechanism at low charge is characterized by the exchange of more smaller ions by fewer larger ions. At higher charges, the system is changing to preferred counterion adsorption, which is resulting in a strong increase in the electrode volume. The maximum linear strain for a bulk electrode is 2% in our study, which is quite high for a supercapacitor system.