When hydrogen reacts with oxygen by burning, the chemical energy is given out as heat. A fuel cell converts chemical energy directly into electrical energy – there is no heat. The overall reaction is: 2H 2 + O 2 → 2H 2O with electrons being exchanged from the cathode to the anode.
In a flame of pure hydrogen gas, burning in air, the hydrogen (H2) reacts with oxygen (O2) to form water (H2O) and releases energy. If carried out in atmospheric air instead of pure oxygen, as is usually the case, hydrogen combustion may yield small amounts of nitrogen oxides, along with the water vapor.
