Capacitive deionization (CDI) is an emerging desalination technology that may provide a low-energy alternative to pressurized-membrane processes. In CDI, a voltage or current is applied across a pair of electrodes [typically porous activated carbon (AC)] and ions (e.g., Na+, Cl-) from the feedwater are removed via electrosorption. In most CDI designs, flow is directed parallel to the electrodes (flow-by). Alternatively, flow can be directed through or perpendicular to the electrode (flow-through). Flow-through configurations are unique relative to flow-by because they are asymmetrical (i.e., the cathode or anode can be the upstream electrode). As such, electrode orientation may yield differences in salt removal and charge efficiency due to the sequential nature that ions and/or faradaic reactions occur in flow-through. Accordingly, we examined the performance [measured as salt adsorption capacity (SAC) and charge efficiency] in flow-through CDI as a function of electrode orientation. Our results show that positioning the cathode upstream increases the SAC and charge efficiency by up to 28.9% (6.5 ± 0.2 mg-NaCl/g-AC) and 21.8% (26.4 ± 0.7%), respectively, compared to the anode upstream orientation. Dramatic differences in effluent pH were observed as well. The pH dropped sharply to 3.7±0.1 when the cathode was located upstream, but positioning the anode upstream increased the pH to 9.7±0.1. This observation suggested that faradaic reactions were occurring, and that the reaction type and/or extent was orientation dependent. Effluent hydrogen peroxide (H2O2) measurements confirmed this hypothesis. Much larger concentrations (69±0.2 µM) were recorded when the anode was upstream compared to the cathode upstream configuration (26±0.9 µM). This discrepancy could not be explained by H2O2 consumption at the anode because it lacked sufficient oxidative potentials (relative to theoretical predictions) to drive H2O2 oxidation. We hypothesize that the variation of H2O2 production is due to orientation-driven differences in dissolved oxygen (DO) and proton (H+) concentrations at the cathode-electrolyte interface. The role of DO and pH on possible faradaic reactions in the two orientations are being explored. In summary, electrode orientation in flow-through CDI affects the extent and rates of faradaic reactions, which in turn has an appreciable impact on CDI performance.<br/>

Effect of electrode orientation on salt removal in flow-through capacitive deionization

2019 Spring National Meeting

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