16. CHEMICAL CELLS AND FUEL CELLS CGSE EDEXCEL CHEMISTRY

16. CHEMICAL CELLS AND FUEL CELLS CGSE EDEXCEL CHEMISTRY

Specification 



Comparing cells





Exothermic reactions transfer energy to their surroundings. This is usually transferred by heating, but the energy can be transferred in other ways. Chemical cells use chemical reactions to transfer energy by electricity.[Energy stored as chemical energy in the cells are changed to electrical energy and transferred outside the cell]

Chemical cells

Chemical cells include the familiar batteries used in torches and mobile phones. There are different designs of chemical cells, with different reactions depending on the type of cell.

Chemical cells produce a voltage until one of the reactants is used up. When this happens, we say the battery 'goes flat'.

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Fuel cells

Fuel cells work in a different way to chemical cells. 
Fuel cells produce a voltage continuously, as long as they are supplied with:
  • a fuel
  • oxygen (from the air)

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Hydrogen-oxygen fuel cells

    In a hydrogen-oxygen fuel cell, hydrogen and oxygen are used to produce a voltage. Water is the only product. The overall reaction in a hydrogen-oxygen fuel cell is:
    Hydrogen + oxygen → water
    2H2(g) + O2(g) → 2H2O(l)
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    Evaluating fuel cells

    Fuel cells have different strengths and weaknesses, depending on the intended use. For example, fuel cells are used in spacecraft and vehicles.

    Fuel cells in spacecraft

    Hydrogen-oxygen fuel cells are used in spacecraft. Their strengths include:
    • they have no moving parts to maintain
    • they are small for the amount of electricity they produce
    • the water they produce is useful as drinking water

    Hydrogen-oxygen fuel cells must be supplied with hydrogen fuel and oxygen. This could be a problem once a spacecraft leaves the Earth. However, spacecraft in orbit (such as the International Space Station) have solar cells. These convert light into electricity, so the hydrogen and oxygen can be replaced by the electrolysis of water. Solar cells only work when they are in the light, so the fuel cells allow electricity to be produced even when the spacecraft is in the dark.

    Fuel cells in vehicles

    Some cars and buses contain hydrogen-oxygen fuel cells. These power an electric motor, which turns the wheels. Other vehicles use engines fuelled by petrol or diesel. The table shows some strengths and weaknesses of each type of vehicle.



    Hydrogen, diesel and petrol are all highly flammable fuels, but hydrogen is more difficult to store because it is a gas and the other fuels are liquids. As there are far fewer hydrogen filling stations, it may be more difficult to travel long distances in a fuel cell vehicle. However, fuel cell vehicles have fewer moving parts than vehicles fuelled by petrol or diesel, so they may be more reliable or easier to maintain.
    Glossary

    1. battery
      A chemical supply of electrical energy. For example, common battery voltages include 1.5 V and 9 V.
    2. cell
      A store of internal energy that can be transferred as an electric current in a circuit.
    3. electricity
      The flow of charge through a conductor.
    4. electrolysis
      The decomposition (breakdown) of a compound using an electric current.
    5. energy
      The capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow.
    6. exothermic
      Reaction in which energy is given out to the surroundings. The surroundings then have more energy than they started with so the temperature increases.
    7. flammable
      Able to ignite and burn.
    8. fuel
      Material that is used to produce heat, like coal, oil or gas.
    9. fuel cell
      Device that produces a voltage continuously when supplied with a fuel and oxygen.
    10. reactant
      A substance that reacts together with another substance to form products during a chemical reaction. 
    11. solar cell
      A device that converts light energy into electrical energy.
    12. voltage
      The potential difference across a cell, electrical supply or electrical component. It is measured in volts (V).
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