# Can a battery powered by iron and air really provide a feasible power source for automobiles?

I'm referring to the Shuttle Battery recently announced (read more on it here) that suggests that:

The energy density of the device is up to 7,700 Wh/L. It is more than 5 times higher than state-of-the-art (SOA) Li-ion (550Wh/L), and comparable to fossil fuel (10,000 Wh/L).

It goes on to say that:

It is rechargeable because it is based on a simple reversible chemical reaction, 3Fe + 4H2O = 4H2 + Fe3O4 and 2H2 + O2 = 2H2O. The proof of concept cell demonstrated zero capacity loss after 200 cycles of discharge and charge.

Now the question is: Is this even remotely possible? Or is there a red flag here somewhere that I can't seem to pick up?

• Yes, technically $\frac{7700}{550} = 14$ is greater than 5, but I wouldn't normally describe 14 times as "more than 5 times". Nov 21 '13 at 22:49
• @user1915639 I didn't notice that! :) Nov 22 '13 at 2:47

This is certainly possible. In fact, there are a large number of metal-air batteries being developed all over the world.

It is, however, strange that such high energy density is achieved, because iron ion itself would not offer very high energy density. My guess is that likely they did it by having an extremely high matter density. So this battery may be small in size for its capacity, but at the same time it may also be much heavier than a conventional battery.

That could be why they brand it as a shuttle power supply(well, energy density isn't really relevant for a bus anyways). A very heavy battery on a hand held device is never good. Cost is the more important factor there, but it is hard to tell at the moment. 0 loss over 200 cycles seems like a fairy tale, but I will give them the benefit of the doubt at the moment.

This battery will also likely to have a considerable loss of charge when not in use. Again, this might not be a huge problem for buses, where you keep driving non-stop during days then kept it in the charging station overnight.

Anyways, don't expect this to be on your iPhone 10 or your laptop. Even if it works as presented, it might not be that ideal for portable electronic devices, where people worry much more about weight and stand-by time than if it occasionally explodes, sadly.

• Makes sense - i was thinking about its feasibility for electric vehicles like Tesla cars, for example, where the battery pack is quite often the heaviest component - and mileage isn't great. Thank you for the answer Nov 18 '13 at 7:36

Current Li-ion cells have $\ce{LiCoO4}$ or $\ce{LiFePO4}$ cathodes where the $\ce{Co}$ or $\ce{Fe}$ are only oxidised or reduced by 1 electron per atom, so if they have managed to make a cell with iron giving up $\frac{8}{3}$ electrons per atom they will get a higher capacity. I find it hard to believe they have got this reaction to happen reversibly and at high current density (i.e. high power/weight ratio) at room temperature, though - if you put iron in water at room temperature it doesn't react very quickly, and hydrogen-oxygen fuel cells also can't get high power densities at moderate temperatures.

• Good point. I noticed they used the term hermetical sealing, which I would guess means none of the components escape into the air. Nov 22 '13 at 2:32