In a fuel cell driven vehicle, the energy is obtained from the combustion of

Concept: A fuel cell is an electrochemical cell that converts the chemical energy of a fuel directly into electrical energy. The fuel is oxidised at the anode and oxygen is reduced at the cathode, and the electrons released travel through the external circuit as current. Because the energy is released by an electrode reaction rather than by burning in a cylinder, the working substance is chosen for how cleanly and readily it can be oxidised at an electrode.

Application: In a fuel-cell driven vehicle the gas supplied to the stack is hydrogen, and the electrode reactions are:

1. Anode: 2H2 → 4H+ + 4e− — hydrogen molecules split into protons and electrons, and the electrons are pushed into the external circuit where they drive the traction motor.

2. Cathode: O2 + 4H+ + 4e− → 2H2O — oxygen drawn from the air takes up those electrons together with the protons that crossed the electrolyte.

3. Overall: 2H2 + O2 → 2H2O — the energy of this hydrogen–oxygen reaction is delivered as electrical energy, with water as the only product.

Contrast: The other candidates are all carbon-based fuels used in ordinary internal-combustion vehicles:

• Methane, CH4 — the chief constituent of natural gas; its oxidation releases carbon dioxide along with water.

• LPG, a liquefied mixture of propane C3H8 and butane C4H10 — its oxidation likewise releases carbon dioxide.

• CNG — natural gas compressed to about 200–250 bar; chemically it is the same methane, so it too gives carbon dioxide.

• A vehicle fuel cell can draw on any of these only indirectly, after a reformer first converts them into hydrogen.

Cross-check: The only tailpipe emission of a fuel-cell driven vehicle is water vapour. That is consistent only with a carbon-free fuel, which confirms hydrogen as the energy source.