GWU researchers introduce new class of molten air batteries; significantly greater energy capacity than Li-air
13 September 2013
|Generalized form of the molten air battery. Licht et al. Click to enlarge.|
Researchers at George Washington University led by Dr. Stuart Licht have introduced the principles of a new class rechargeable molten air batteries that offer amongst the highest intrinsic electric energy storage capabilities.
In a paper just accepted and published online by the RSC journal Energy & Environmental Science, Licht and his colleagues show three examples of the new battery’s electron transfer chemistry. These are the iron, carbon and VB2 molten air batteries with respective 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 the lithium-air battery.
In 2008 a zirconia stabilized VB2 air battery was presented. [Earlier post.] This 11e- (eleven electron) per molecule, room temperature, aqueous electrolyte battery has the highest volumetric energy capacity for a battery, with an intrinsic capacity greater than that of gasoline and an order of magnitude higher than that of conventional lithium ion batteries. The challenge has been to recharge the battery; that is to electrochemically reinsert 11e- into the battery discharge products. Here, this challenge is resolved through the introduction of a new class of molten air batteries.
...Unlike prior rechargeable molten batteries, the [molten air] battery is not burdened by the weight of the active chargeable cathode material. The rechargeable molten air electrode instead uses oxygen directly from the air to yield high battery capacity. This electrode will be shown to be compatible with several high capacity multiple electron redox couples. Three demonstrated new batteries chemistries are the metal (iron), carbon and VB2 molten air batteries with intrinsic volumetric energy capacities greater than that of the well known lithium air battery due to the latter’s single electron transfer and low density limits.—Licht et al.
As an example of the process, during the charging of the iron molten battery, iron oxide is converted to iron metal via a three-electron reduction, and oxygen is released to the air. During discharge, iron metal is converted back to iron oxide. Li2CO3, which melts at 723°C, and lower melting carbonate eutectics are effective electrolytes, the researchers found. Simple steel foil cathodes and nickel foil anodes are effective for either iron oxide or carbon dioxide splitting.
Iron has been widely explored for battery storage due to its availability as a resource and its capability for multiple electron charge transfer. Retention of the intrinsic anodic storage capacity of these batteries has been an ongoing challenge...In 2010 we introduced the molten carbonate electrolytic conversion of iron oxide to iron as a CO2-free synthesis alternative to the conventional greenhouse gas intensive industrial production of iron metal. The unexpected, high solubility of iron oxide in lithiated molten carbonate electrolytes was demonstrated to lead to the facile splitting of iron oxide to iron metal with the concurrent release of oxygen.
Here, we consider this unusual electrolytic splitting as a battery “charging”. We couple this with the known primary discharge of the air cathode as used in the widely studied molten carbonate fuel cell, including those using coal as a fuel, to explore the first example of a molten air rechargeable battery. In lieu of iron we also explore the alternative use of carbon and VB2 as alternative high capacity discharge redox couples for these rechargeable cells.—Licht et al.
|Illustration of the charge/discharge of the iron molten air battery in molten carbonate. The charging or discharging process is indicated by red or blue text & arrows. Licht et al. Click to enlarge.|
For the carbon molten air battery, carbon formation during molten carbonate electrolysis provides the “charging”; molten carbonate cells have been widely probed as robust fuel cells. The combination of these two processes form the basis for the carbon molten air battery.
On the other hand, the researchers note, the foundation of understanding of the electrochemical VB2 molten air system “is small; there is little or no prior information pertaining to an electrochemical path for recharge of the molten vanadate (V2O5) and molten borate (B2O3) discharge products”.
The studies reported in the paper are less advanced that with the other two chemistries due to the scarcity of prior fundamental electrochemical knowledge of the molten system.
Advancing the new class of molten air batteries will require the exploration of an extensive range of parameters such as electrolyte and gas composition; electrode morphology; temperature; and cycle rates, the authors point out.
For example, while these experiments have been conducted in the 700 °C to 800 °C temperature range, the molten carbonate electrolyte has a wide range of electrolyte opportunities, such as through the use of mixed alkali carbonate eutectics which exhibit a minimum melting point below 400 °C. Enhancements of the morphology and modifications of the electrocatalytic nature of the air electrode should improve energy efficiency of the cell. A range of cell configurations with lower polarization (with similar discharge potentials, but supporting significantly higher current density) will be reported in a future study.—Licht et al.
Stuart Licht, Baochen Cui, Jessica Stuart, Baohui Wang and Jason Lau (2013) Molten Air - A new, highest energy class of rechargeable batteries. Energy Environ. Sci. doi: 10.1039/C3EE42654H
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