http://www.newsweek.com/hydrogen-power-seawater-solar-1373678

Often referred to as the "Holy Grail" of energy storage, the economical and efficient conversion of water into hydrogen and oxygen with electricity (electrolysis) has been a goal of scientists for decades.

The amount of energy stored per kilogram of hydrogen is roughly two and a half times larger than natural gas and, most importantly, the only byproduct of burning hydrogen for energy is water. In the future, hydrogen could power everything from cars to buildings—but producing hydrogen from water on a global scale could put a strain on purified water sources that are needed for drinking, as well as many other industrial processes.