Improving Long-Term Anode Stability in Capacitive Deionization Using Asymmetric Electrode Mass Ratios

Abstract

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.

Enhancing the Total Salt Adsorption Capacity and Monovalent Ion Selectivity in Capacitive Deionization Using a Dual-Layer Membrane Coating Electrode

“…ACF exhibited a significant increment of the oxygen content from 13.4 to 29.3%, suggesting the serious oxidation caused by Faradaic reactions. Electrode oxidation would cause the positive shift of anode potential and increment of electrode resistance, thereby leading to the decay in the SAC. , The mild change of the oxygen content for the D-MCE before and after cycling indicated its slight oxidation, which well explained its stable performance. The deposited dual-layer membrane acted as a barrier to inhibit the transport of H 2 O 2 and isolate ACF to contact with feed solution, thereby preventing ACF oxidation.…”

Capacitive Deionization for the Extraction and Recovery of Butyrate

“…While it is beyond the scope of this study, there are several operational and material-based approaches that may be applied to mitigate carbon oxidation. For example, modifying the cell configuration, , charging at lower potentials that are far from the limits of the electrochemical window ,, or for less time, minimizing the oxygen content by purging the solution with an inert gas, choosing materials with large surface area and pore diameter, and applying a surface coating with a protective layer have all been reported. Further evaluation of these strategies is needed in future studies to improve anode stability.…”

Faradaic Rectification in Electrochemical Deionization and Its Influence on Cyclic Stability

“…27 It is worth noting that the precise control of electrode potential usually plays a crucial role in electrochemical deionization for selective-ion recovery 8 and asymmetrical electrochemical deionization construction. 28 (2) Until now, the anion-exchange membrane (AEM) is regarded as playing the key role in stabilizing MCDI as it can avoid the growing cation expulsion after oxidation of the carbon anode, 29 while the contribution of the cation-exchange membrane (CEM) to MCDI stability has not yet been reported. Nevertheless, the exploration of CEM's contribution to MCDI stability would lead to improved system designs for practical electrochemical deionization.…”