7.5 Hydrogen versus petrol as a fuel

Two fuels

Petrol is a mixture of hydrocarbons from crude oil. Hydrogen can be used in cars either by burning it in an engine or in a fuel cell.

• hydrogen + oxygen → water: 2H₂ + O₂ → 2H₂O
• petrol + oxygen → carbon dioxide + water (plus some pollutants)

Advantages of hydrogen

• The only product is water: no carbon dioxide (a greenhouse gas), no carbon monoxide or soot at the point of use.
• Hydrogen can be made from water, which is a renewable and plentiful raw material.
• It releases more energy per kilogram than petrol.

Disadvantages of hydrogen

• It is a gas, so it takes up a large volume. It must be stored at high pressure in heavy, strong tanks.
• It is highly flammable and forms explosive mixtures with air.
• Making hydrogen needs energy. It is usually made from fossil fuels (natural gas), which releases carbon dioxide, or by electrolysis of water, which needs electricity.
• There are very few refuelling stations, and hydrogen cars are expensive.

Petrol

• Easy to store and transport as a liquid, with many filling stations.
• But it comes from crude oil, a finite resource, and burning it releases carbon dioxide and pollutants such as carbon monoxide, soot and nitrogen oxides.