EDIT: My search-engine abilities apparently failed me. I'm seeing 0.44V as the voltage... well... whatever Iron's voltage is... it is pretty small. That's my point.
There's a reason we didn't make Iron batteries. Iron definitely stores electrons, but the voltage dropoff leaves a lot to be desired...
Iron stores 2 electrons at very low voltage. That fundamentally limits its energy density.
Li-ion stores ... some number... of electrons at 3.7V. Looking at the periodic table, I think 1 electron? In any case, the big 3.7V drop really changes its energy / power characteristics.
You'll need additional cells to increase either voltage or current (doesn't matter which: electric-engineers can convert either voltage or current into power). But without fundamentally good voltage or current (ie: stored electron) stats, you're just not going to have much energy storage.
And if each of those cells are heavier because you're using say... a heavier element (ex: Iron) instead of lighter elements (ex: Hydrogen or Li-ion), then its that much worse.
But the status-quo for chemical energy storage (that is burned / re-released) is maybe Hydrogen?
If you don't care about weight / density issues, then a giant steel-tank containing many/many tons of Hydrogen Syn-gas might be a better storage option than iron?
Hard to say really. I'm willing to see people experiment with the technology. Iron is one of the most abundant elements after-all, so any use of Iron will almost certainly have raw cost-benefits.
But will it be cheaper to use the iron as energy storage? Or is it cheaper to turn the iron into steel, and then pump Hydrogen into the Steel like a balloon and pressurize-store Hydrogen at 10,000PSI or whatever?
That's usable but not great and the installations tend to be costly and somewhat fragile due to all kinds of attributes that hydrogen atoms have which they impart on the materials that that installation is made out of. For instance, hydrogen tends to make metals brittle so regular ductwork isn't going to work. Hydrogen installations also have the bad habit of going 'foom' due to the extremely low activation energy.
So there is some pressure to try to find a storage system that is both cheap, doesn't use exotic materials, is chemically stable and reasonably energy dense and has a round-trip efficiency as high as can be achieved.
This is a complex problem to put it mildly and there is a whole raft of technologies that are being examined all of which have different trade offs making them more or less applicable to certain applications.
The 'Iron cycle' is one of those, it isn't a winner on all or even a majority of dimensions but it has some interesting properties, notably: the cycle is uses only commonly available materials, it doesn't require long term storage of the hydrogen (which can be produced and used in the same cycle step), the materials involved are stable in both forms (pure iron vs iron oxyde), it is reasonably (not super) energy efficient and it has an energy density which for stationary applications is not problematic.
The biggest questions at this point in time are: does it scale and does it do so cost effectively. If the answer to both of those are 'yes', which this experiment in scaling up the cycle to 5MW should answer then it will at least be a viable contender, much more so than many other schemes that I've seen come by in the last couple of years.
Most of the bulk hydrogen storage plans involve underground storage at a pressure that would not cause trouble. Where more density is needed, e.g. aircraft fuel, liquid form is favored, again at low pressure.
And for liquid storage you have to count on some H2 boiling off and for underground storage you will need to be extremely careful about possible ignition sources.
You need to worry about material degradation mainly at high pressure where the material has to be very strong, and where the hydrogen is driven into the crystal structure, weakening it. At low pressure, ordinary aluminum is resistant to hydrogen degradation, and doesn't need to be strong.
Whether liquid hydrogen is kept refrigerated below its boiling point, or kept well insulated but allowed to boil off, will depend on detailed engineering analysis. As the energy needed to produce it gets cheaper, the extra complication of keeping it too cold to boil gets less attractive. Maybe to refrigerate the tank, it needs to be above-ground so servicing is possible; but without, it can be wholly underground and thereby better insulated.