Electrode Materials and Energy Consumption for Desalination by Capacitive Deionization
Date
2020-09-04
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Abstract
Capacitive deionization (CDI) is a brackish water desalination technology which uses capacitive charge storage within porous carbon electrodes to adsorb salt from water flowing through the cell. Selecting an appropriate electrode material is a critical step to optimize a CDI system. In this work the performance of two materials, a reduced graphene oxide foam – magnetite composite and a nanoporous carbon scaffold (NCS) were compared to that of activated carbon. Faradaic losses were lowest for the activated carbon electrodes but still consumed over a third of the applied charge. While quantifiable desalination was only achieved with the activated carbon electrodes, NCS displayed the highest volume specific, ideal (i.e. assuming 100% charge efficiency) average salt adsorption rate (ASAR). For adsorption half-cycles <3 minutes, NCS also demonstrated the highest mass specific ideal ASAR, due to the thin NCS electrodes and their large pore widths. A stack of activated carbon electrodes was constructed to investigate the dependence of CDI specific energy consumption on cell voltage. The minimum specific energy consumption achieved was 115 Wh/mol using an average cell voltage of 0.8 V. The Faradaic loss fraction was between 43% - 46% for all tested voltages. Shunt current significantly reduced the charge efficiency of the 10-cell CDI stack. A resistive-capacitive circuit model quantified this effect for stack voltages between 4 V – 10 V. The shunt current losses were greater for higher stack voltages, reducing the charge efficiency by approximately 18% to 30% for stack voltages between 4 V and 10 V, respectively. If dissolved oxygen is present, carbon anodes can be gradually oxidized, increasing their potential of zero charge and reducing their salt adsorption capacity and charge efficiency. The i-mD model was used to estimate the magnitude of these effects, suggesting that after 80 cycles the electrode PZC had shifted by 0.22, 0.14, 0.11, or 0.10 V for average cell voltages of 0.4, 0.6, 0.8 or 1.0 V, respectively. These results show that while the energy consumption of CDI can be reduced by optimizing the cell voltage, greater improvements can be achieved by minimizing shunt current and selecting electrode materials to minimize Faradaic losses.
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Capacitive Deionization
Citation
Vandersleen, J. K. (2020). Electrode Materials and Energy Consumption for Desalination by Capacitive Deionization (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.