A fuel cell converts chemical energy from the fuel (usually H2) to electrical energy, which is used to power engines. With reference to the diagram below, H2 is fed into the left compartment where it is oxidised to H+ at the anode, which is porous and impregnated with a Pt catalyst. The protons then migrate across the electrolyte (H3PO4) that is contained in a polymer exchange membrane that only allows the passage of H+.
At the cathode, O2 in the air that is fed into the right compartment reacts with H+ and is reduced to form water. The cathode is again porous and impregnated with a Ni catalyst. The overall redox reaction is:
2H2 (g) + O2 (g) → 2H2O (l)