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> Storage tanks for compressed hydrogen enjoy the square-cube law.

Not really. Wall thickness is roughly proportional to diameter, and surface area to the square, so you don't gain anything in terms of storage mass ratio by building bigger tanks.

> But the round-trip efficiency of the tank is virtually 100%

This is oversimplifying quite a bit. Compressing hydrogen, the lightest gas, is very energy intensive per unit of mass, and this energy is not fully recoverable upon decompression (due to general pump efficiency and thermal losses in the intercooler).

Also hydrogen loves to leak through tank walls, and can make metals brittle (this was covered in the article)

27 tons of iron oxide have a volume of 5m^3 and can be stored in pretty much a hole in the ground.

2.7 tons of hydrogen have a volume of almost exactly 30000 m^3, requiring storing it under high pressure in specialized containers. Hydrogen is famous for being hard to store without losses.

For long-term storage storage and losses are a problem.

> But the round-trip efficiency of the tank is virtually 100%. The efficiency of the iron-based storage is only 50%

Maybe I'm missing something, but why? As you mentioned it takes 29kj to restore 3 moles of H2 out of (3 moles of H20 + 1 mole of Fe2O3). Where does 50% comes from?

i.e. the paper[0] states that first "discharging" produced 7.09kg of H2 out of 8.71 theoretically possible

the efficiency is super low, but again, according to the paper, "most of the energy input was due to thermal losses at the reactor surface (83.9%)", which also benefits from square/cube law.

[0] https://pubs.rsc.org/en/content/articlelanding/2024/se/d3se0...

Iron oxide is completely inert. You can store it in piles, under the elements. Iron powder less so, you need to keep it dry, but again you can just pile it up. The only tricky part is moving dense dry solids around at the huge scales required.

Edit: wait I forgot that direct reduced iron powder exothermically reacts with oxygen and water in the air, that's how single-use instant hand warmers work. So yeah you gotta isolate the iron powder a bit more than stick it under a tarp.

I've been playing too much Factorio lately so of course my mind goes towards rail systems (could repurpose coal plants) in combination with pneumatics.

single-use instant hand warmers are catalyzed by salt and a little water. iron powder is indeed hazardous by itself but not like that

I'm picturing in my head a freight train car parked at a small desolate compound, standardized iron reactant cartridges dripping tar-like preservation liquid robotically unloaded, and local FCEVs and BEVs gathering to charge there. That might make an interesting Sci-Fi cutscene.

There are alternatives to iron that have higher efficiency and lower prices. For instance https://hydrogenious.net/ does exactly that but with benzene like structures. The advantage of this is that you can reuse existing infrastructure for transport and you have higher transport efficiency: while the square cube law exist, the same thing holds for the forces on the chamber walls which have to increase in thickness. Hydrogen tanks are also very expensive as they have to be manufactured to tight tolerances (and they need to be replaced rate often due to hydrogen creep weakening chamber walls)

> I don't see this making any sense for large scale energy storage. Storage tanks for compressed hydrogen enjoy the square-cube law.

This system doesn't store hydrogen. It stores elemental iron (produced from iron oxide, i.e., iron ore, and hydrogen from solar power splitting water into hydrogen and oxygen), and uses steam to get the hydrogen out (and convert the iron to iron oxide) only when the hydrogen is needed.

<< I can't see the niche that this idea can apply to.

Tbh, I am not sure either. I think the main benefit of this is FeO is inert under temps/conditions humans consider normal so maybe it long term storage is not that far fetched. I like the idea. I am just unsure about its practical applications.

Hydrogen is difficult to store and transport, you really want natural gas or jet fuel