Battery electrodes offer higher specific capacity and lower self-discharge than capacitive electrodes, the team notes.

At the current growth rate, humans will consume 90% of available fresh water by 2025, by which time the population living in water-stressed areas is expected to increase to 3.9 billion. Seawater desalination is becoming a feasible source for fresh water, free from local and global variations in rainfall. For this reason, the use of seawater desalination has steadily increased in recent years.

Seawater desalination processes require electric or thermal energy to separate saline seawater into two streams, a fresh water stream containing a low concentration of dissolved salts and a concentrated brine stream. A variety of desalination technologies have been developed over the years. Reverse osmosis requires a large electrical energy input, which accounts for 44% of its cost, and it is based on selective membranes, which are prone to fouling and require frequent replacement. Recent research on high-flux membranes based on carbon nanotubes could potentially reduce the energy consumption, while alternative electrical-based desalination technology such as ion concentration polarization in nanodevices can potentially avoid the fouling.

Thermal energy based multistage flash distillation requires energy intensive heating to temperatures above 90 °C, accounts for 50% of its cost. Multiple effect distillation is gaining popularity due to its higher efficiency and lower top brine temperatures (about 70 °C) than multistage flash technology. Forward osmosis is a promising new process that utilizes lower temperatures (60 °C) but still requires the use of membranes. Solvent extraction using directional solvents like decanoic acid at mild temperatures (30−50 °C) has been demonstrated recently by Bajpayee and co-workers.

Previous electrochemical approaches relying on the electrical double-layer built at the surface of high surface area carbonaceous electrodes are capacitive deionization (CDI) and the newly developed capacitive double layer expansion (CDLE). Despite these recent advances, the 50−80% target for reduction in desalination costs by 2020 set by the National Research Council roadmap will not be achieved by incremental improvements to existing technologies, and therefore, a new approach is needed.

—Pasta et al.

The desalination battery includes a Na_2-xMn₅O₁₀ nanorod positive electrode and Ag/AgCl negative electrode. In their paper, the team reports demonstrating 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:

1. 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.

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

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

4. 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.

—Pasta et al.

Resources

• Mauro Pasta, Colin D. Wessells, Yi Cui, and Fabio La Mantia (2012) A Desalination Battery. Nano Letters doi: 10.1021/nl203889e