DeliElectro

So you have some photovoltaic cells to generate electricity, and you have a bottle to store hydrogen.  You can now make the hydrogen by electrolysis, and so save the solar energy for use after dark, or in a different location.

But you need water to electrolyse.  In most places that isn't a problem.  But in deserts, or if you need to make a closed fully-independent system, what would be useful would be an automatic way of extracting the water you need from the atmosphere.

As anyone who has ever inadvertently left a few crystals of sodium hydroxide on a lab bench knows, deliquescence is an intriguing phenomenon.  Many soluble substances (like NaOH) have such a strong affinity for water that they will drag it out of the air and dissolve themselves in it, forming - in the case of the crystals - little droplets on the bench that it is unwise accidentally to lean on.

It ought to be possible to make a deliquescent solution that is also electrolytically appropriate for the production of hydrogen.  Along with the PV cells, this would take sunlight and make a stream of hydrogen from it continuously, extracting water from the atmosphere to replenish itself.  Having no moving parts, it should work forever, with no need for topping up and no need for maintenance.

Mass-produced generators would become possible that could simply be left in sunny places to produce hydrogen for ever...