[NeedMoreTea]

That is exactly how Orkney plans to enable the next ferries to be hydrogen powered, for the huge energy density compared to batteries. They already generate more from renewables than they can use for electric, heat and vehicles. Surplus is already splitting water. More here: http://www.surfnturf.org.uk/page/transport

Germany is trialling it for energy storage, has a number of trial sites running I believe, and already have some hydrogen powered trains.

Unless you need the energy density of hydrogen the simpler solutions are probably better.