Hydrogen Fuel Cells (Cambridge O Level Chemistry)

• A fuel is a substance which releases energy when burned
• Hydrogen is used as a fuel in rocket engines and in fuel cells to power some cars
• A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode
  • H₂ → 2H⁺ + 2e^– 
  • O₂ + 4e^– → 2O^2– 
• The hydrogen-oxygen fuel cell produces electricity by combining both elements, releasing energy and water
• The overall equation for the reaction within a hydrogen fuel cell is:

hydrogen   +   oxygen   →   water 

• The diagram below shows the setup of a hydrogen fuel cell 
• The air entering provides the oxygen
• The fuel entering is hydrogen
• The only chemical product made is water 

Diagram showing the movement of hydrogen, oxygen and electrons in a hydrogen-oxygen fuel cell

• Hydrogen-oxygen fuel cells are becoming more common in the automotive industry to replace petrol or diesel engines

Advantages

• They do not produce any pollution: the only product is water whereas petrol engines produce carbon dioxide, and oxides of nitrogen
• They release more energy per kilogram than either petrol or diesel
• No power is lost in transmission as there are no moving parts, unlike an internal combustion engine
• Quieter so less noise pollution compared to a petrol engine

Disadvantages

• Materials used in producing fuel cells are expensive
• Hydrogen is more difficult and expensive to store compared to petrol as it is very flammable and easily explodes when under pressure 
• Fuel cells are affected by low temperatures, becoming less efficient
• There are only a small number of hydrogen filling stations across the country
• Hydrogen is often obtained by methods that involve the combustion of fossil fuels, therefore releasing carbon dioxide and other pollutants into the atmosphere