text: why can't we use water as fuel (h2o)? hydrogen and oxygen? both very flammable and abundant.?   there is one major problem. electrolyzing the two elements (separating the h2 from o) requires more energy to separate it than it releases.  there is no cost effective way to break the bonds, currently.  the only plus about h2 for fuel cell cares is that it lasts longer than batteries (h2 does not die out), 1 l of compressed h2 takes you as far as 1 l of petrol, and it is clean burning.  however, the only energy abundant enough to separate the bonds comes from coal, oil, and natural gas.  and since the 1st law of thermodynamics is correct we will be using more energy to make h2 to power our cars than we will at our homes.