January 30, 2012

Desalination Battery

Nanoletter - Water desalination is an important approach to provide fresh water around the world, although its high energy consumption, and thus high cost, call for new, efficient technology. Here, we demonstrate the novel concept of a “desalination battery”, which operates by performing cycles in reverse on our previously reported mixing entropy battery. Rather than generating electricity from salinity differences, as in mixing entropy batteries, desalination batteries use an electrical energy input to extract sodium and chloride ions from seawater and to generate fresh water. The desalination battery is comprised by a Na2-xMn5O10 nanorod positive electrode and Ag/AgCl negative electrode. Here, we demonstrate an energy consumption of 0.29 Wh l–1 for the removal of 25% salt using this novel desalination battery, which is promising when compared to reverse osmosis ( 0.2 Wh l–1), the most efficient technique presently available.

A four-step charge/discharge process allows the electrodes to separate seawater into fresh water and brine streams:

Fully charged electrodes, which do not contain mobile sodium or chloride ions when charged, are immersed in seawater. A constant current is then applied in order to remove the ions from the solution.

The fresh water solution in the cell is extracted and then replaced with additional seawater.

The electrodes are then recharged in this solution, releasing ions and creating brine

The brine solution is replaced with new seawater, and the desalination battery is ready for the next cycle.

Fresh water is produced during the initial discharge of the electrodes (Steps 1−2), while recharging the electrodes results in the production of a brine stream (Steps 3−4).

The desalination battery has simple construction, uses readily available materials, has a promising energy efficiency, operates at room temperature with fewer corrosion problems than existing desalination technology, and it could potentially be Na+ and Cl− selective, which would end the need for resalination. Its primary limitation of low total ion extraction arises from the low specific charge capacity of the NMO sodium ion electrode (35 mAh g−1 vs an average value of 160 mAh g−1 for Li- intercalating cathodes). This low charge capacity limits the volume of water that can be desalinated within one cycle, and therefore the overall efficiency of desalination. However, the battery research community has recently shown increasing interest in aqueous sodium ion batteries for grid scale power storage applications.

In the near future, higher capacity sodium ion electrodes, as well as improved chloride electrodes will make the desalination battery a feasible method for seawater desalination...The work reported here demonstrates the concept of a desalination battery. Further developments will result in a versatile technology for the desalination of seawater, either independently or through integration with other desalination methods.
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