Two ways to power a vehicle

1. A petrol car burns petrol, a mixture of hydrocarbons, inside its engine, and the hot gases produced push the pistons that turn the wheels.
2. A fuel-cell car uses a hydrogen-oxygen fuel cell to generate electricity, which drives an electric motor.
3. Both vehicles carry their fuel in a tank and both take oxygen from the air.
4. Comparing them means looking at what each one releases, how efficiently each uses its fuel, and how easily each fuel can be produced, stored and supplied.

Advantages of fuel cells

1. Water is the only product: a fuel cell releases only water, so no carbon dioxide is produced while the vehicle is running.
   1. Burning petrol releases carbon dioxide, which is a greenhouse gas linked to climate change.
2. No other harmful exhaust gases: a fuel cell does not produce carbon monoxide, unburnt hydrocarbons, soot or oxides of nitrogen.
   1. A petrol engine produces toxic carbon monoxide and soot through incomplete combustion when the oxygen supply is limited.
   2. A petrol engine also produces oxides of nitrogen when nitrogen and oxygen from the air react at the high temperature inside the engine.
   3. These pollutants cause breathing problems, and oxides of nitrogen also contribute to acid rain.
3. Higher energy efficiency: a fuel cell transfers a larger proportion of the energy in its fuel to useful energy, because it releases energy directly as electricity rather than as heat in a flame.
   1. A fuel cell typically transfers about $40\%$ to $60\%$ of the energy in its fuel usefully, compared with about $20\%$ to $30\%$ for a petrol engine.
4. A fuel that need not run out: hydrogen can be made from water by electrolysis, so it does not depend on crude oil, which is a finite resource.

Disadvantages of fuel cells

1. Making hydrogen needs energy: hydrogen is not found as a free gas in nature, so it has to be manufactured.
   1. Most hydrogen today is made from methane in natural gas, a process that releases carbon dioxide.
   2. Hydrogen made by electrolysis needs a large amount of electricity, which is often generated by burning fossil fuels in power stations.
   3. A fuel-cell vehicle is only low in carbon dioxide overall if its hydrogen is made using renewable energy, such as wind or solar power.
2. Storage is difficult: hydrogen is a gas with a very low density, so a useful amount takes up far more space than petrol holding the same energy.
   1. It must be squeezed into strong, heavy tanks at very high pressure or cooled to a liquid at about $-253{}^{\circ }C$.
3. Flammable and explosive: hydrogen forms explosive mixtures with air, so any leak from a tank, pipe or refuelling pump is a serious hazard.
4. Few refuelling stations: there are very few hydrogen filling stations compared with petrol stations, which makes long journeys difficult.
5. High cost: fuel cells use expensive catalysts such as platinum, so fuel-cell vehicles cost more to buy than petrol vehicles.
6. Advantages of petrol: petrol is a liquid that is easy to store and transport, and a network of filling stations already exists.