Cation Intercalation Materials and Cell Design for Low-Energy Electrochemical Desalination

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Due to the growing population and human-made climate change spurred by the use of fossil fuels, the urgent need for energy-efficient desalination has motivated research into electrochemical desalination as a technology to remove salt ions from water. Prussian Blue analogues (PBAs) are a promising class of redox-active intercalation compounds with high capacity and cycle life used in sodium-ion batteries. While their functionality was previously shown to enable desalination in symmetric cation intercalation desalination (CID) cell architectures [S. Porada, A. Shrivastava, P. Bukowska, P. M. Biesheuvel, and K. C. Smith, Electrochim. Acta 255, 369 (2017)], they have low electronic conductivity which limits their charge and discharge rates while causing high energy consumption due to ohmic losses [A. Shrivastava and K. C. Smith, J. Electrochem. Soc. 165, A1777 (2018)]. A common technique to increase electronic conductivity in battery electrodes is the inclusion of additives such as carbon black, however increasing conductive particle content reduces active particle loading and overall salt absorption capacity. To overcome these limitations, the effects of volume fraction of either C45 or Ketjen Black conductive additive on electrode transport properties were studied, and we demonstrated that orders of magnitude improvement in electronic conductivity could be achieved using the smaller Ketjen Black particles [E. R. Reale, A. Shrivastava, and K. C. Smith, Water Res. 165, 114995 (2019)]. Through this discovery, the low electronic conductivity of PBA electrodes was overcome, and new higher-conductivity electrodes were successfully fabricated and used in a flow-through cation intercalation desalination cell built with a custom-designed 3D printed flow field. The new CID cell functioned using a PBA anode and cathode separated by an anion exchange membrane to transport salt into one half of the cell while desalinating the other, and displayed energy consumption an order of magnitude lower than that of other electrochemical desalination technologies while removing similar quantities of salt from an influent feed. For a salt concentration of 100 mM, a salinity comparable to brackish water found in areas such as estuaries and aquifers, approximately 25% of influent salt was removed from one feed and transferred into the other. Further improvements can be made through the use of intercalation materials with higher volumetric storage capacity and increased mobility for sodium, which were both limiting factors when using PBAs which lowered utilization and prevented higher salt removal. Early tests with such electrodes have shown their electronic conductivity to be several times higher than PBA electrodes, further reducing energy consumption. Another modification to the cell is the use of an in-house designed recirculation system to increase salt removal over multiple passes rather than single-pass flow. The goal of our research is to develop CID technology into a form of energy-efficient desalination capable of surpassing reverse osmosis in salt removed for a given energy input.