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GWU researchers introduce new class of molten air batteries; significantly greater energy capacity than Li-air

Licht
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.

Licht2
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.

Resources

  • 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

Comments

Engineer-Poet

Ralph, the Fiat 500 can get by with a 2-banger of just under 1 liter.  With electric power on tap, it should be adequate for almost all but sport driving.  And I do mean ALL driving; I have climbed long hills with a very heavily loaded vehicle towing a trailer, and calculated my power requirement at 95 horsepower to go 65 MPH.

ai_vin

“Our city has a number of district heating systems that put solar and ”

Notice how Kit has to cut my statement off to attack it? And then make personal attacks on me to press his position?

bman

Forget about HEVs,they are just transient technology.(Except for niche applications) When batteries with adequate capability become available the ICE will go the way of the horse and cart(as charming as they are).A pure BEV will be cheaper, simpler,less maintenance,more durable,more elegant,more efficient and just sweeter.Oh did I mention they will be cheaper? Throw in autonomous control and a carbon body/chassis and I"ll buy.EP I see carbon reinforced/ceramic put thru Additive Manufacturing as lighter and cheaper.

Roger Pham

@bman,
Let's hope that your wish will come true in the future. For now, fast-charging of BEV at the speed of filling up with gasoline is out of the question.

For now, you can still enjoy PEV (Plug-in EV) via PHEV's. You can plug it in once or twice daily and avoid using the ICE. Or for long trips, you can fill up the tank several times a day and can drive all day and night long, coast to coast, with unlimited range.

With engine technology that can run 100,000 miles before major engine maintenance, a PHEV can run for 200-300 k miles or even further, before needing maintenance of the ICE unit, which means that you will not need major maintenance of the ICE at all. After 300,000 miles, the car's frame and interior will be so beat-up that you'll want to buy a new car anyway!

ai_vin

The reason you need speed when you fill up with gasoline is that you can't fill up at home, while you're in bed asleep.

You can do that with a BEV. You can also go out shopping/dining/movies/etc. in a BEV and leave it parked & plugged in for however long it needs to recharge. Gas station owners OTOH tend to frown on you leaving your car where it blocks the next paying oil addict.

Engineer-Poet

If I could plug in at all of my regular stops, "range anxiety" (which is actually "gonna have to kick on the gas engine anxiety") would be a thing of the past for me.

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