How a hydrogen fuel cell makes electricity
Hydrogen–oxygen fuel cell reaction · Redox · Electrochemical
A fuel cell car runs on hydrogen and its only exhaust is water clean enough that Apollo astronauts drank it.
Reactants approach
Drag to rotate
ΔH = -572 kJ · exothermic, releases heat
ΔH per 2 mol H2; ΔG = -474 kJ is the maximum electrical work.
Meet the molecules
- Reactantgas
H2
hydrogen
- Reactantgas
O2
oxygen
- Productliquid
H2O
water
What you notice
Hydrogen fuel cell cars such as the Toyota Mirai, and many city buses, carry hydrogen compressed to about 700 times atmospheric pressure. Instead of burning it, a stack of fuel cells combines the hydrogen with oxygen from the air and turns much of the energy directly into electricity, which drives an electric motor. The only thing leaving the tailpipe is water, sometimes visible as drips or a puff of vapour. Fuel cells also power spacecraft: on the Apollo missions they supplied both electricity and the crew's drinking water.
Safety: Hydrogen burns with a nearly invisible flame and ignites across a wide range of concentrations in air, so its tanks and pipes are built to strict standards.
Step by step
- 1
Hydrogen splits at the anode
A platinum catalyst splits each H2 molecule into two H+ ions and two electrons: H2 → 2H+ + 2e–.
- 2
Electrons take the long way
A polymer membrane lets H+ ions through but blocks electrons, so the electrons flow through an external circuit and power the motor.
- 3
Oxygen is reduced at the cathode
At the other electrode, O2 combines with the arriving electrons and H+ ions: O2 + 4H+ + 4e– → 2H2O.
- 4
Energy accounting
Of the 572 kJ released per 2 mol of H2, at most 474 kJ (ΔG) can become electrical work, giving a maximum of 1.23 V per cell. The rest leaves as heat.