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Researchers present lower temperature version of ultra-high capacity molten air battery

27 May 2014

Last year, researchers at George Washington University led by Dr. Stuart Licht introduced the principles of a new class rechargeable molten air batteries that offer amongst the highest intrinsic electric energy storage capabilities. (Earlier post.) The iron, carbon and VB2 molten air batteries they proposed offered intrinsic volumetric energy capacities of 10,000 (for Fe to Fe(III)); 19,000 (C to CO32-) and 27,000 Wh liter-1 (VB2 to B2O3 + V2O5), compared to 6,200 Wh liter-1 for a lithium-air battery.

Now, in a new paper in the RSC’s Journal of Materials Chemistry A, Baochen Cui and Licht report on a lower-temperature iron molten air battery that they suggest would be more compatible with electric vehicle applications.

Licht
The iron molten air battery; illustration of the charge/discharge in molten carbonate. The charging or discharging process is indicated by red or blue text & arrows. Cui and Licht, SI. Click to enlarge.

A rechargeable molten air battery (MAB) uses an air cathode, a molten electrolyte and a high capacity multi-electron anode. Discharging MABs couple the cathodic reduction of O2 (from the air) with anodic multi-electron/molecule oxidation to yield the high intrinsic storage capacities. As examples, the VB2 MAB offers 11-electron oxidation; the carbon, 4-electron; and the iron, 3-electron.

In earlier work, the team demonstrated the Molten Air Battery chemistries at temperatures of 730 °C to 800 °C.

Unlike the challenges to study of the Carbon or VB2 Molten Air Batteries by constraining their intrinsic capacity, the capacity of an Iron Molten Air Battery can be controlled by limiting the iron added to the cell. As one example of the recently introduced molten air battery class, we probe here the rechargeable nature of the Iron Molten Air Battery. Of the three examples of molten air batteries provided to date, the Iron Molten Air example provides the easiest route to purposely restrict the battery capacity by limiting the iron reactant (by allowing free flow entry of air, but by constraining the concentration of dissolved iron salt in the electrolyte). We will probe sustainable current densities and discharge efficacy, and then demonstrate a pathway to lower temperature rechargeable Iron Molten Air batteries.

—Cui and Licht, Supplementary Information

To achieve the lower temperature, the team moved from a lithium carbonate (Li2CO3) electrolyte which melts at 723 ˚C to the alkali carbonate eutectic (having the lowest melting point possible) Li0.87Na0.63K0.50CO3 which melts at around 393 ˚C.

The solubility of iron in the eutectic electrolyte is high, and at 750 °C approaches half the solubility of the high solubility in the pure lithium carbonate electrolyte. The eutectic has the advantage of a greater molten temperature range—extending several hundred degrees lower than the pure lithium system). Compared to pure lithium carbonate, the alternative molten media has the disadvantage of lower conductivity, but the advantage of even greater availability, and the wider operating temperature domain.

In the paper, Bao and Licht compared iron MABs with the Li0.87Na0.63K0.50CO3 electrolyte at 600 ˚C or less with a battery with the 730 °C Li2CO3 electrolyte.

High voltage efficiency and cycling is observed at 600 ˚C, but polarization is excessive at 395 ˚C. In contrast to the low temperature advantage the eutectic electrolyte has two challenges. Li2Co3 is more conductive than electrolytes containing Na2CO3 or K2CO3, and Li2O is more stabilizing than Na2O or K2O in carbonates or chlorides. We hope to explore if a new BaCO3 additive can offset the disadvantages.

—Cui and Licht

Resources

  • Baochen Cui and Stuart Licht (2014) “A Low Temperature Iron Molten Air Battery,” J. Mater. Chem. A, Accepted Manuscript doi: 10.1039/C4TA01290A

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Comments

I would think that that a molten air battery can be first and very soon be used in naval aviation and space application where batteries that can store a lot of power are needed. The bigger the application the lesser the heat control issue is. How about a space vehicle with molten air batteries that supply energy to microwave propulsion lift off to orbit. These batteries are good for this use even if they are not rechargeable yet and can reach energy densities greater than gasoline. Moreover there is no need to store an oxidizer. Microwave propulsion is available at every kitchen and is who knows how much more efficient than rocket propulsion. All this savings will translate to a launch price lowered by orders of magnitude.

Seer;

You've made many mistakes in your post but I'll just highlight one: "Moreover there is no need to store an oxidizer."

Molten AIR battery.

Molten air battery may hold the key toward electric flight for small airplanes. This will make general aviation much safer and much more cost-effective than ever before. In combination with a type of autopilot that can take off, guide, and land the airplane without pilot intervention will make general aviation much more accessible to the general public who do not have the high skills required to safely pilot a small airplane. Satellite weather display and prediction will prevent getting into bad weather.

When all elses fail, even when the autopilot malfunction, there is available on-board parachute to take the whole plane safely to the ground.

Thus, an average person can book a flight with an auto-piloted electric plane at a small airport near home at any time, on demand, and travel to any other small airports within perhaps a 1000-mile radius without having to go thru the hassle of an airline flight, in cluding pre-booking weeks in advance in order to avoid the cut-throat prices!

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