Hydrogen is the fuel
When hydrogen reacts with oxygen, a lot of electrical energy is produced (this is an exothermic reaction).
The only product formed is water.
hydrogen + oxygen → water
2H2 + O2 → 2H2O
The fuel cell is constantly supplied with the fuel, hydrogen, and oxygen, which react to produce water, generating electrical engery, which can be used to power things like cars.
The fuel cell generates electricity
The electrolyte in a hydrogen fuel cell is potassium hydroxide (KOH).
1. The cathode is a platinum catalyst. Oxygen is supplied to the cathode, and reacts with water. The oxygen gains electrons to make hydroxide ions (this is a reduction reaction because it gains electrons).
2. The anode is also a planinum catalyst; hydrogen is supplied to the anode and it reacts with hydroxide ions (made in step 1) to make water. This is an oxidation reaction (electrons are lost from hydrogen).
3. The electrons leave the anode and travel through the external circuit which uses the electricity supplied by the fuel cell. (Then they go to the cathode - see step 1).
1. If the hydrogen comes from a renewable resource, generating energy this way is renewable. (For example, water is the product, so by decomposing the product you have a renewable source of hydrogen).
2. As the only product is water, it does not produce pollution which contributes to global warming, so is cleaner than fossil fuels
3. On spacecraft, fuel cells are often used to generate electricity, and can also produce water for the astronaughts to drink
4. Hydrogen fuel cells are lightweight and compact, with no moving parts.
1. To produce a large current or voltage, you need lots of fuel cells wired together, which is expensive.
2. Hydrogen is explosive and hard to store.
3. If the hydrogen doesn't come from a renewable resource (for example, if it comes from methane) then the electricity produced is not renewable.
4. Fuel cells often contain poisonous catalysts which are hard to dispose of (in this way they cause pollution).
5. The production of hydrogen and oxygen requires electricity which will probably come from the burning of fossil fuels, and therefore causes pollution.