in short:
yes it is possible to store excess energy by doing electroloysis on water into H2 and O2 molecules, storing it and using it to provide power later.
The problem is this:
to do this efficiently, you require additives to the water to help aid electrolysis.
Also to convert it back to electricity (if that is your goal) - you either need a gas turine or a fuel cell = $$$$
to burn it in a car, it would have to be collected, then compressed using a compressor. Regulating this reaction might be dificult, as would require a vaccume etc to ensure no "air" got into the process.
a while ago i was thinking about using solar pannels on a house to generate "H2 and O2 @ home. however i very quickly realised it would be dificult to build something like this out of hobby parts, as you can't let other gasses get in, and the H2 and O2 needs to be compressed into bottles / storage.
I ahve no doubt that a system will eventually become commonplace.
ie
"reactor"
has an inlet for a water source - ie a garden hose connector
two outlets for H2 and O2 separated.
I actualy think it could be designed really nicely, to almost look like an alien powersource using creative blue lighting etc.
it stil suffers that you need lots of pwoer, and additives to teh water though.
maybe we shuld all work towards a working prototype.