Hydrogen as a Fuel

Burning dihydrogen releases an unusually large amount of energy for its mass -- more energy per kilogram than any conventional fossil fuel -- and its combustion produces only water, with no carbon dioxide, soot or sulfur compounds:

This combination of very high energy density and a completely clean combustion product is why hydrogen is so often discussed as the fuel of the future, both burnt directly (as in some rocket engines, where liquid hydrogen and liquid oxygen are burnt together) and used in internal combustion engines adapted to run on hydrogen gas.

Hydrogen fuel cells offer a second, generally more efficient way to extract useful energy from hydrogen: instead of burning the gas, a fuel cell combines hydrogen and oxygen electrochemically at two separate electrodes (hydrogen is oxidized at the anode, oxygen is reduced at the cathode), producing an electric current directly, with water again the only chemical by-product. Because a fuel cell is not a heat engine, it is not limited by the same thermodynamic efficiency ceiling that limits an internal combustion engine, so it can convert a substantially higher fraction of the chemical energy released into usable electrical work; fuel cells of this kind already power some buses, cars and spacecraft. …