Electrochemical study of charging processes and cycling degradation of bare and ion-exchange membrane-coated microporous activated carbon electrodes for capacitive deionization

We study the charging behavior and cycling performance of microporous activated carbon (AC) electrodes, either uncoated or coated with ion exchange membrane (IEM) films, in aerated 0.1 M NaCl solutions. We use transient and quasi-steady state controlled potential methods, namely cyclic voltammetry (CV) and step potential electrochemical spectroscopy (SPECS), across a range of up to ±0.6 V vs. electrode rest potential, to identify and discriminate capacitive and faradaic processes. The raw data analysis –based on specific capacitance and coulombic efficiency, for CV, and on fitting to a model accounting for surface/micropore capacitive charging and faradaic charge transfer, for SPECS– reveals inherent asymmetries in the electrode response to positive and negative polarization, concerning both electrosorption and faradaic charge transfer. The interface capacitance is higher on the negative side of the zero charge potential (PZC), suggesting specific interactions of Na-ions with AC surface oxygen groups. A secondary source of asymmetry, stemming originally from the different kinetics of oxygen reduction and carbon oxidation, is prone to wane with cycling because of the intervening AC modification. Further, it is arguable to assign to AC electrodes a range of (quasi) ideal polarization, for a faradaic activity of pseudocapacitive character persists in the potential range around EPZC, arguably sustained by redox processes of oxygenated groups on AC. We rely on this analysis to study the electrode cycling behavior and underline, on the one hand, the role of carbon oxidation in determining poor cycling performance under positive polarization and, on the other hand, the weak mitigation effect of the IEM coating on the susceptibility of microporous AC to surface oxidation.