Characterizing and mitigating the degradation of oxidized cathodes during capacitive deionization cycling

Capacitive deionization (CDI) is a fast-emerging technology typically applied to brackish water desalination and ion-selective separations. In a typical cell, feedwater is desalinated via ion electrosorption into micropore electric double layers (EDLs) of charging porous carbon electrodes. Several studies have previously demonstrated that oxidizing the cathode via a nitric acid pretreatment enhances the cell's salt adorption capacity (SAC). It was recently reported that oxidized cathodes can degrade rapidly during cell cycling, yet the mechanisms and mitigation strategies remain unknown. Here, we experimentally characterize the performance and degradation of nitric acid-oxidized commercial carbon cloth cathodes. For a full cycle time (FCT) of 100 min and 1 V applied, we observed a 42.5% reduction of SAC by the 100th cycle, and measured a reduction in cathode micropore chemical charge concentration at pH = 7 from −1.5 M to −0.25 M after cycling. We further found that cell charging time and electrode mass are major determinants of degradation rate, for example, reducing FCT to 30 min and 10 min allows for SAC decay of only ∼14% and <2%, respectively, over 100 cycles. The insights provided here allow us to posit degradation mechanisms, and develop long-lasting, high performance CDI cells with oxidized cathodes.