maybe the approach is wrong. for us humans to fully utilize the benefits of hydrogen, we should remove the wheel and make automobiles hover or fly first before we open the idea of running hydrogen machines
if you say hydrogen has an abundance in the atmosphere, then stop thinking inside the box and make cars as light as air
like this one:
http://news.bbc.co.uk/1/hi/world/americas/4643575.stm
so instead of trying to extract the hydrogen from the H20 which will require more power than an V6 gasoline engine, why not let the cars fly/hover first, and let the flying cars sniff hydrogen in the hydrogen-abundant atmosphere