In this work, we demonstrate the feasibility of using insertion materials as electrodes in electrochemical de-ionization systems. Insertion compounds are inorganic polyionic crystalline materials that upon exposure a specific redox potential will undergo a change in transition state of the host transition metal species. In doing so, the compound accepts within its host structure, ions that meet the size limitations imposed by its crystal structure and the charge required to maintain electroneutrality. As such, these materials are more selective with regard to the specific ions that are sorbed when compared with porous carbon .electrodes. Additionally, these materials show nearly an order of magnitude higher ion removal capacity than traditional electric double layer carbon materials (less than 0.2 mmol/g) as well as high round trip coulombic efficiencies.3-5
We probe sodium-ion selective materials specifically a NASICON type phase change material, NaTi2(PO4)3 and a layered insertion material Na4Mn9O18 with respect to their sodium ion removal capabilities over a range of inserting ion concentrations as well as over a range of concentrations of non-inserting cations. We demonstrate that in dilute solutions of inserting ions, we encounter higher concentration overpotentials, bulk transport limitations and parasitic charge losses that reduce the coulombic efficiency of the insertion electrodes. Our results also show that at any given concentration of electrolyte, there is a critical current at we balance interfacial ion transport to the migration of ions from bulk to the electrode surface. At this current, we have are fully able to utilize the electrode capacity while minimizing parasitic charge loss. A similar reduction in coulombic efficiency and ion removal capacity is observed when the concentration of non-inserting cations is increased. This can be attributed to due to competing transport of the non-inserting cations to the electrode surface and reduction in the limiting current in presence of non-inserting ions.
Additionally, a penalty in round trip coulombic efficiency is encountered when non-inserting ions are present. As such, the use of insertion compounds could be limited to use in water streams where the inserting ion is the dominant species. To overcome this, we propose alternatives such as improving advective transport and proper selection of operating current densities to overcome some of the above-mentioned challenges and achieve improved separation of the inserting ions, in this case sodium ions from dilute and multi-ionic water streams.
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5. S. Porada, P. Bukowska, A. Shrivastava, P. Biesheuvel and K. C. Smith, arXiv preprint arXiv:1612.08293 (2016).