Hydrogen-oxygen fuel cell

Follow the hydrogen

Press play and watch the two routes. The ions go through the membrane. The electrons take the long way round.

Predict, then check

Think about the reaction you just balanced.

A spacecraft is powered by hydrogen fuel cells. What comes out of the cell’s outlet, and can the crew use it?

What you need to know

• A fuel cell makes a voltage that can push a current, when it is supplied with a fuel and with oxygen (or air).
• Unlike an ordinary cell or battery, it does not run down or need recharging, as long as the supplies keep coming.
• Have a goYour friend is sure a fuel cell will go flat after a few hours, just like their phone battery. What would you tell them?

  It doesn’t go flat. It keeps working while fuel and oxygen keep being supplied.

  A battery runs down, but a fuel cell is kept going by a constant supply of fuel and oxygen, not by recharging.

• In a hydrogen fuel cell, hydrogen is the fuel and water is the only product: 2H2 + O2 → 2H2O.
• Hydrogen flows in at the anode and oxygen flows in at the cathode, with the electrolyte and its membrane between them.
• At the anode, hydrogen is oxidised: it loses electrons, H2 → 2H+ + 2e-.
• The hydrogen ions move through the membrane towards the cathode. The electrons go the long way, round the external circuit.
• Have a goA classmate says the electrons shoot straight through the membrane to the cathode. Where do they actually go, and what goes through the membrane?

  The electrons go round the external circuit. The hydrogen ions go through the membrane.

  The electrons are taken by the anode and pass round the external circuit. Only the hydrogen ions move through the membrane.

• At the cathode, oxygen is reduced: O2 + 4H+ + 4e- → 2H2O. The water leaves through the outlet.
• Have a goOne H2 gives out 2e- at the anode. The cathode half equation takes in 4e- for each O2. How many H2 does it take to supply one O2?

  2 hydrogen molecules (2 × 2e- = 4e-).

  The cathode takes 4e- for each O2, and each H2 supplies only 2e-, so you need twice as many hydrogen molecules as oxygen molecules.

• To combine the half equations, equalise the electrons, add them, and cancel the 4H+ and 4e- on both sides.
• The cell is seen as unpolluting because water is its only product, so no carbon dioxide or other pollutants are made in it.
• The hydrogen comes from the electrolysis of water, which takes energy itself, or from fossil fuels.

The big picture

A fuel cell makes a voltage as long as it is supplied with a fuel and oxygen. In a hydrogen fuel cell the reaction 2H2 + O2 → 2H2O is split into two half equations: hydrogen is oxidised at the anode and oxygen is reduced at the cathode. The hydrogen ions cross the membrane, the electrons go round the external circuit, and water is the only product.

⚠ Watch out

Two traps. Don’t call a fuel cell “pollution-free” with no caveat: it makes only water, but making the hydrogen takes energy or fossil fuels. And don’t pick the anode by a charge sign: oxidation happens at the anode, reduction at the cathode.

Memory hook

AN OX, RED CAT: oxidation at the anode, reduction at the cathode. And remember the journey: hydrogen in, oxygen in, water out, with the electrons taking the long way round.

Check yourself

Cover the page and sketch the cell. Label both inlets, the outlet, the ions’ route and the electrons’ route, then add each half equation.