Electrochemical separation technologies such as capacitive deionization (CDI) have evolved to be an energy-efficient desalination possible solution for treating brackish and low salinity waters.¹ The achievable ion removal capacity in using capacitive carbon electrodes ranges between 0.1-0.2 mmol/g of carbon electrode.² Further, the capacity retention of carbon electrodes is poor due to electrode ageing. In this work, we investigate the use of sodium titanium phosphate or NaTi₂(PO₄)₃ , a Na-ion superconductor (NASICON) material as a sodium-ion selective cathode for electrochemically-driven aqueous separations. Additionally, we also discuss the importance of self-discharge or diffusive loss of sorbed ions during open circuit conditions specifically in comparing CDI system performance with that of insertion type electrodes such as NaTi₂(PO₄)₃.

NaTi₂(PO₄)₃, has until now only been evaluated in concentrated aqueous electrolytes and specifically for aqueous Na-ion batteries. It is however important in the context of water treatment and desalination, that we consider its performance in dilute electrolytes. NaTi₂(PO₄)₃  shows reversible sodium-ion removal behavior over a range of sodium ion concentrations. Round trip coulombic efficiency values for these systems typically range between 40-70%, depending on the operating pH and dissolved oxygen levels in water. The causes of this efficiency loss are competing charge consumption by the hydrogen evolution reaction (HER) during the ion removal step and the chemical de-sodiation of the cathode by dissolved oxygen during ion retrieval step. ^{3, 4}These efficiencies are improved through the use of an ion exchange resin and controlling the composition of the water near the electrode.

In addition, retained-ion loss characteristics of electrodes used for water treatment are important from the standpoint of being able to intermittently operate the electrochemical separation process. Our studies show that in even presence of dissolved oxygen, a common constituent in natural and industrial waters, the self-discharge properties of the titanium phosphate electrode are superior to porous carbon electrodes used in CDI systems. In the light of our findings, present a discussion on the utility of NaTi₂(PO₄)₃as a insertion based electrode for sodium removal from aqueous streams.

References

1. M. A. Anderson, A. L. Cudero and J. Palma, Electrochimica Acta, 55, 3845 (2010).

2. S. Porada, R. Zhao, A. van der Wal, V. Presser and P. M. Biesheuvel, Progress in Materials Science, 58, 1388 (2013).

3. W. Wu, S. Shabhag, J. Chang, A. Rutt and J. F. Whitacre, Journal of The Electrochemical Society, 162, A803 (2015).

4. A. I. Mohamed, NaTi2(PO4)3 as an Aqueous Anode: Degradation Mechanisms and Mitigation Techniques, in Material Science and Engineering, p. 150, Carnegie Mellon University (2017).