Studying the transition from pseudocapacitive to battery-like desalination for ion selectivity (SELECT)

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The energy transition can only succeed through its analog of the water transition: green energy meets blue water. New technologies for energy-efficient water treatment must be developed for a sustainable future. Electrochemical methods such as capacitive deionization (CDI), made possible by ion electrosorption using electrically charged carbon electrodes, are of particular interest. However, due to the concentration-dependent permselectivity, CDI is limited to very low ion concentrations (brackish water) and depends on the specific surface area of the electrode material. It is also possible to desalinate water by loading Faradaic materials, such as those that enable intercalation (Faradaic deionization: FDI). A typical FDI material is metal oxides that are capable of cation intercalation. FDI also works with high salt concentrations (seawater) and, due to the higher charge storage capacity, also enables higher desalination capacities. However, the slower ion diffusion in metal oxides limits the desalination rates of FDI systems. CDI and FDI have in common that the aqueous medium to be desalinated flows past two electrodes.In addition to pseudo-capacitive desalination, the technology of desalination batteries represents one of two FDI mechanisms. In desalination batteries, ions are immobilized at specific intercalation potentials. In the case of pseudocapacitors, incorporating ions is possible continuously at every applied potential. Battery-like processes, which are more subject to diffusion limitations, can potentially allow high selectivity for an ionic species. In contrast, the faster process of pseudocapacity can be less selective. As a function of the crystal structure, the transition between these two processes has not yet been addressed in detail. Neither does how the ion selectivity changes during material aging. Therefore, it is of great interest to know how desalination capacity, rate, energy efficiency, and ion selectivity change. This is because ion-selective and high-performance desalination technologies are of great importance for the recovery of raw materials, the removal of pollutants and the generation of clean water for the drinking water supply, and the production of hydrogen via electrolysis.Our project will specifically apply metal oxides (vanadium oxide and molybdenum oxide) to carbon materials with an outer surface (carbon nano onions and carbon nanotubes). To do this, we use the atomic layer deposition (ALD) method that can be controlled on the nanoscale. In addition to comprehensive material characterization, we will record the electrochemistry in half and full cells and conduct desalination experiments with low and high ion concentrations. The electrode aging, the ion immobilization mechanism, and the resulting performance parameters of desalination and ion selectivity are addressed in detail.