[adrian_b]

The efficiency of a complete cycle of storing then retrieving energy into hydrogen is quite low and there are thermodynamic reasons (due to the phase changes between liquid and gas) that limit the achievable efficiency.

Hydrogen might be a possible choice when high energy per mass or power per mass is desired, but it is a very bad choice for the purpose of this new iron-air battery, i.e. stationary storage with very high energy capacity and very low cost.

Iron-air might indeed be the best choice for medium-time energy storage, with low cost and good full-cycle efficiency.

For very long energy storage times, e.g. years, synthetic hydrocarbons would be preferable to hydrogen, due to much easier storage and handling.

[Hypx_]

All metal-air batteries have similar thermodynamic properties. If you can make a iron-air battery with good efficiency, than you can make a hydrogen-air battery with good efficiency too.

Since water is significantly more available than just about any other material, hydrogen-air cells should be the ideal battery for anything that isn't volume limited. Which is frankly a lot of cases. Unless there's some specific need for an iron-air battery where hydrogen-air can't be used, it's hard to conceive of a situation where we wouldn't use hydrogen-air.

Synthetic hydrocarbons are basically extensions of hydrogen electrochemistry. You are just adding carbon to the hydrogen made with the electrolysis step of a hydrogen-air battery. It's even possible to make a hydrocarbon-air cell such as direct-alcohol fuel cells or solid oxide fuel cells.

[adrian_b]

You are right about metal-air batteries, but IIRC hydrogen-air ideal efficiency is still significantly lower than the ideal efficiency for carbon-air (using fuel cells with solid carbon) or metal-air batteries.

You are also right about synthetic hydrocarbons. The extra hydrocarbon synthesis step lowers the total efficiency, compared with using hydrogen.

Nevertheless, the lower efficiency is more than compensated by the simpler methods used for storage and handling, which require much less expensive materials and a much lower volume and total mass.

The benefits of using hydrocarbons for long-term energy storage have been amply demonstrated by the living beings that have been using this method for billions of years, many of which can easily achieve an autonomy of months without eating, while doing activities that would make a present-time robot using batteries inoperational after a few hours at most.

[Hypx_]

As I understand, the byproduct of a carbon-air cell is carbon dioxide. This doesn't mean such a cell can't make sense, but it has some fundamental downsides that are hard to mitigate. It will requirement a very specific form of carbon capture where the result is hard carbon for such cells to make sense.

That might be partially true. Especially in the case of hydrogen-to-ammonia where the process of conversion is much more straightforward. However, storing hydrogen in salt caverns for years at a time is already proven. So further conversion steps might not prove any real value except where volume of storage is important.