The physical properties and limitations of hydrogen are well-known. Many people repeat them in other posts in this thread.
Here's a very factual explanation of why hydrogen internal combustion engines won't work: https://www.youtube.com/watch?v=vJjKwSF9gT8
(In summary, you can't compress hydrogen enough to fit in a reasonably-sized tank. The energy that it takes to compress hydrogen will be expensive.)
Maybe it's the optimum (I'm not convinced). But to narrow down on one approach that doesn't even quite work seems premature.
If you wanted to just throw money at the problem, I'd just throw it at nuclear - reliable, carbon free, well tested and doesn't need much storage. But hey I guess that's the wrong answer to some.
My background, FWIW, is also not energy, so I'm just happy to see the research net cast wide and well funded throughout.
And most global warming activists aren't climate scientists.
Meanwhile the military industrial complex, who DGAF about the environment but crave lavish taxpayer subsidies for a nuclear economy, is shouting down solar, wind and storage - largely because it competes for subsidies and its low cost renders it a no brainer alternative.
They are louder, richer, more persuasive (cynically leveraging environmental messaging) and considerably more powerful than people who care more for the environment than military supply chains.
As with global warming itself, appeal to moderation doesn't work - the best solution doesnt lie half way between these two extremes.
> which solutions are really helpful in reducing CO2.
There, you have already decided that lowering the amount of CO2 should be our concern, but should it?
I say: let's collect facts and count numbers and compare which solutions are really helpful in enhancing the quality of our life.