> I do wonder about the efficiency

This will highly depend on the insulation and the duration of storage.

Likely not useful for your personal car that stands a week in sunlight but maybe for s. th. Like public transportation

I don’t think anyone believes hydrogen to be relevant for ground transportation anymore.

But for industry, grid energy storage (perhaps longer term, paired with existing gas power plants, to deal with dunkelflaute) and perhaps some roles in sea and air transportation… there are plenty of areas where efficient hydrogen storage would be useful.