2022 journal article
Improving Long-Term Anode Stability in Capacitive Deionization Using Asymmetric Electrode Mass Ratios
ACS ES&T ENGINEERING, 2(1), 129–139.
Activated carbon (AC) electrodes are used for desalination in capacitive deionization (CDI) because they provide a large, porous surface area at a low cost. In short-term tests, AC electrodes can achieve relatively high salt adsorption capacities. In long-term tests, desalination performance degrades significantly. The poor performance has been attributed to the corrosion of carbon in the anode electrode. Here, we show a simple strategy to improve anode stability by changing the mass ratio of AC electrodes (asymmetric configuration) in flow-through CDI (FT-CDI). Increasing the anode mass by double and triple decreased the anode half-cell potential by up to 21 and 64%, respectively, relative to cells with single anodes. Less positive anode potentials were associated with smaller shifts in the anode potential of zero charge and lower acidity in the effluent. Additional evidence for improved anode stability in the asymmetric configurations was the significantly lower charge transfer resistance and oxygen to carbon ratio content of the anodes at the end of the tests. After 48 h of operation, the salt adsorption capacity decreased by 68% in symmetric cells, but, in asymmetric cells, it was more stable and decreased by only 47% (double anode) and 17% (triple anode). These trends were consistent when the anode was located upstream or downstream of the cathode. We attribute the improved anode stability to reduced oxidation in the asymmetric cells, which was driven by lower oxidative half-cell potentials. This, in turn, maintained a stable desalination performance over long-term operation. Our results demonstrate that increasing the anode mass is an effective strategy to extend anode stability and improve long-term salt removal in FT-CDI.