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Desalination with a battery

04 August 2017 - Startups

NanoBismithName2017.PNGA battery can be used as a desalination plant. The technique is called capacitive deionization: apply a current over two electrodes and use them to temporarily to store the sodium ions and chloride ions and create deionised water. At discharge release the ions again to produce concentrated salt waste water. The big competitor in desalination reverse osmosis is too expensive in terms of energy input. Main problem to solve for effective use of a battery with actual sea water: be able to store even more salt ions into an electrode pair.

In the latest JACS Nam & Choi present a new experimental set-up (DOI) to accomplish just this. In their work one electrode consisted of NaTi2(PO4)3, a NASICON type material capable of storing lots of sodium ions as Na2Ti2(PO4)3. The other electrode was elemental bismuth (prepared by cathodic deposition from a Bismuth chloride solution), capable of storing chloride ions as bismuth oxychloride (BiOCl) with additional water reduction. The bismuth electrode was in the form of a nanocrystalline bismuth foam as a high surface area was critical. The foam was courtesy of hydrogen gas bubbles resulting from water reduction during the formation process.

In an electrochemical set-up nearly 50% of the bismuth was found to be active and available for conversion to BiOCl, this equates to a maximum loading capacity of 80 mg of chlorine in 1 gram of bismuth. For comparison: with 1 gram of carbon electrode this number of only 16 gram. In a cycle of 200 capture and releases around 60% of capacity survived (the electrodes tend to pulverise and erode).

The Na/Bi set-up was also tested. The discharge step should be spontaneous but was not (sluggish solid-state chemistry), so both charging and discharging require an energy input. Fortunately, The discharge step could be improved upon by letting it take place in an acidic medium, in this way protons could facilitate the uptake of the oxygen.