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