The best source of hydrogen comes from the ground. You pump this stuff out - it's long molecules consisting of a carbon backbone covered in hydrogen. You run it through something called a "reformer" to strip the hydrogen off. The reformer could be put on a car, but it'd be more efficient to do it on an industrial scale. Actually wait, if you just use a combustion process you can get energy from both the hydrogen and…
Excess wind and solar power can be used to generate hydrogen from water.
Additionally, if there was capacity why not use it to charge lithium batteries?