Emerging investigator series: a comparison of strong and weak-acid functionalized carbon electrodes in capacitive deionization

Capacitive deionization (CDI) is a promising membraneless technology widely explored for water desalination and selective separations. The active elements in CDI are often inexpensive activated carbon electrodes, which store ions in charged micropore electric double layers. Various surface functionalizations of CDI electrodes have been explored to enhance salt storage capacity, long-term stability, and ion selectivity, including use of strong or weak-acid functional groups in CDI cathodes. However, a direct comparison of CDI performance between weak and strong-acid functionalized cathodes has not been presented. Here we fill this knowledge gap by cycle testing a single CDI cell with either a weak or strong-acid functionalized cathode for varying cycle times. We provide measurements for cell salt storage capacity and charge efficiency, as well as quantify salt capacity degradation rates. Detailed ex situ material characterizations yield insight into material behavior and mechanisms for electrode degradation. This data allows us to conclude that strong-acid functionalized cathodes are more pH stable and stable to charge-discharge cycling, but that current weak-acid functionalization methodologies provide cathodes with higher initial salt and charge storage capacity. Overall, the data presented here provides insights into the proper selection of surface functionalization for CDI cathodes.