Steel production, where the hydrogen is a chemical reagent, might be one of the few exceptions where hydrogen is really the best choice.

Not to mention liquid H2 has quite a bit lower energy density than liquid fossil fuels. Many rockets have used kerosene in their first stages for this reason. Not the end of the world for aircraft, but certainly another major disadvantage.

But the energy density per mass is still better than Li-ion batteries.

Li-ion batteries are improving at a faster rate, and you can use your gravitational energy to recharge the battery.

How does that work? You run your propellor in reverse like a wind turbine?

This doesn't sound completely ridiculous but is there even any proof of concept for that? The gravitational potential energy doesn't seem all that high - most airplane fuel is burnt maintaining height, not gaining it - to justify the complexity of recovering some small fraction of it. Still, I once thought the same about regenerative braking in electric cars.

Hydrogen is a storage mechanism; there'd be a way to reclaim your energy either way

It's also prone to explode, and is a potent (indirect) greenhouse gas if it leaks into the atmosphere. Agreed that it will have some niche applications though. As well as the byproduct situation, there are likely situations where batteries wouldn't be feasible due to insufficient capacity or too great of weight. Aerospace maybe?

Hydrogen (and helium) are too light to stay in the atmosphere and are lost once released.

How is hydrogen a greenhouse gas (and what do you mean by "indirect") ?

I think it'd be an "indirect" GG because it's electricity intensive to obtain it, and that likely comes from a carbon emitting source. The same argument is often made about electric vehicles.

In this case it's about how it interacts with OH radicals in the atmosphere, meaning other GHG stay in the atmosphere longer. So it's not a greenhouse gas, but it makes others hang around longer.

> The reaction of hydrogen with OH radicals has a further side-effect of reducing the availability of OH radicals with potential impacts on the build-up of greenhouse gases.

https://assets.publishing.service.gov.uk/government/uploads/...

Well you don't typically get hydrogen from H2O, you get it from CH4. So your byproduct isn't oxygen, it is carbon. Unlike electric vehicles, it doesn't get greener as the grid changes to wind/solar/nuclear/hydro. There's just more hydrogens per hydrocarbon than water. Not to mention dealing with oxidation.

> Namely: It'll diffuse through just about anything on a fairly short time scale if it isn't kept as a liquid at cryogenic temperatures, and it causes almost all metals to become brittle.

Both claims are wildly wrong. Neither actually happens except at high temperatures and only with certain metals.

Could you elaborate about “wildly wrong”?

I am not an expert on this, but random internet searches such as [0] indicate that common materials, like high strength steel, are susceptible to both diffusion and embrittlement at room temperature

[0] https://www.energy.gov/sites/prod/files/2014/03/f10/pipeline...

Those tests seem to be at >200˚C if I’m not mistaken. Hardly room temperature. Regardless, the main trick is to avoid using certain types of high-strength steel, line exposed surfaces with polymers, or using composites where ever possible. The problem is for the most part solved.

So you're admitting the problem exists, which is my entire point.

It's hardly a theoretical-only problem, either:

https://en.wikipedia.org/wiki/Eastern_span_replacement_of_th...

In the same sense all chemicals have handling issues. In practice, it is basically solved.

That's probably due to them using a vulnerable alloy, and was likely exposed to hydrogen while at a high temperatures. This is nothing like a gas tank filled will hydrogen.

What's a high temperature though? -50C?

Well into the hundreds of degrees C.

That's why some bet on ammoniac.