All the components of hydrogen, with the possible exception of low cost electrolyzers, don't need to be prototyped. It's existing technology. It's not like (say) molten salt reactors, in which fundamental development remains to be done.
It's existing technology, but it's a novel application of that technology. We haven't used hydrogen electrolysis as a form of grid storage before. And we certainly haven't used it for grid storage at the Terawatt hour scale. And that the scale we'll need to make wind and solar viable. 1 TWh isn't even 30 minutes of global electricity consumption.
We both know the answer: there aren't any.
Back in the 1950s people thought nuclear power would be cheaper than fossil fuels. They thought it'd be effectively free. The energy density of uranium is so much better, so clearly generating electricity with it would be much cheaper. But actually deploying a technology at scale reveals more and more challenges.
Your proposal for hydrogen storage is in the same phase that nuclear power was in during the 1950s. A solution that exists on paper, but one that hasn't actually encountered and overcome the challenges of implementing it at scale. Same with thermal batteries, synthetic methane, and so on. These are proposals that haven't passed the test of actual implementation at scale.
Hydrogen is being stored in a few places. That the storage isn't larger isn't because of any technical obstacles, it's because there's no reason to store it now. In particular, when we can burn natural gas without CO2 charges, using the hydrogen for energy storage is pointless.
This doesn't mean hydrogen CAN'T be stored, it just means the market conditions for widespread adoption of an off-the-self technology aren't there yet.
And it certainly doesn't answer the question of whether or not this represents a viable grid-storage solution, since we haven't built it at remotely close to the scale required.
It's not "if it isn't already being done, it can't be done"
It's "if it isn't already being done, it is extremely reckless to assume that it can be done cheaply at a massive scale".
Screw it, let's just use fusion. Nobody has actually built a fusion plant? Well, who cares if it hasn't already been done, that's a "foolish argument" in your own words. /s
Hydrogen could also be stored in depleted gas fields and in deep saline aquifers. The storage capacity available is more than adequate.
We haven't done this to provide 100 MWh of storage. How on earth can we be confident it'll be easy to provide 1 TWh of storage, or 10 TWh?
People mostly talk about lithium ion storage because that's what's actually available, besides geographically limited options like hydroelectricity. Until there's a company that's building dozens of gigawatt hours of hydrogen storage it's a moot point. It's a technology that exists the laboratory, not one that's commercially available.
We could easily have Petawatt-hour scale hydrogen energy storage.
Back in the 1950s people thought that nuclear power would be effectively free. But actually building it at scale exposed challenges of implementation that weren't foreseen. The cost of a system on paper and the cost after overcoming the challenges of actually building it are two very different things. For hydrogen storage, you only have the former.
Nuclear's problem are fundamentally political in nature. If we really cared about green energy, nuclear power could easily be built out at scale.
The industries that manipulate tens of millions of tons of hydrogen each year would be astounded to hear this statement of yours. What are those facilities made of, unobtainium?
Unfortunately, those alloys that experience embrittlement includes the ones used in steam turbines: https://www.sciencedirect.com/science/article/abs/pii/S09215...
They aren't "off the shelf" technology yet.