Stability improvement of carbon-based electrodes in aqueous electrocapacitive devices

Abstract

Activated carbon (AC) electrodes are irreplaceable in many electrochemical cells due to their unique combination of specific surface area and relative abundance. However, in many devices the aqueous conditions at the surface of carbon will lead to irreversible parasitic faradaic reactions, resulting in the formation of oxygenated functional groups and a reduction in electrode service life. In this study, we employ capacitive deionization (CDI) as an ideal system to investigate the cycling stability of different AC electrodes with modified surface chemistry, so as to reveal the coupled reaction-degradation phenomena that are often limiting deployment of aqueous electrocapacitive devices. It is found that the incorporation of AC cathodes with rich oxygen-containing surface groups significantly alleviates an inversion effect during long-term operation of a CDI system. At the same time, the salt adsorption capacity (SAC), the charge efficiency and the SAC retention rate of cells with modified commercially available AC improve significantly as compared to those with unmodified commercially available AC. To pinpoint the responsible faradaic processes, multiple parameters of the effluent are monitored to quantify the generation rate of OH⁻ species, which leads to a beneficial acidic-electrolyte environment. Consequently, the carbon-oxidation reactions are suppressed due to the increased redox potential in an acid environment and the potential of the zero charge (PZC) of the aged anode remains nearly unchanged after long-term cycling. Importantly, the carbon treatment methods demonstrated herein are industrially scalable, and combined with the unraveled mechanism, suggest many opportunities to stimulate the design of the specialized AC materials with extreme stability for enabling a new generation of aqueous electrocapacitive technologies.