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Commercial Application (A2 Only)

Chemistry: AQA A Level, topic 2.4.3

Batteries

Electrochemical cells are used as batteries.

Batteries

Batteries

  • Batteries come in two forms:
  • Rechargeable.
  • Non-rechargeable.
  • Rechargeable batteries can have their reaction reversed by applying an electrical current.
  • This needs more sophisticated cells and is more expensive. But this process can be cheaper overall as you only need to buy one.
Lithium cobalt oxide batteries

Lithium cobalt oxide batteries

  • LiCoO2 batteries were the first lithium ion batteries invented.
  • They have one electrode made of LiCoO2 and one made of graphite.
  • The two half equations are:
  • Li+ + e- ⇌ Li(s), Eθ = −3.04V
  • Li+ + CoO2 + e- ⇌ LiCoO2, Eθ = 0.56V
The reactions

The reactions

  • The Li+/Li(s) cell has a more negative Eθ so is reversed.
  • The overall cell potential is: Eθcell = 0.56 − (−3.04) = 3.6V.
  • To reverse the reaction, you apply an external current of at least 3.6V to push the reactions in the opposite directions.

Fuel Cells

Fuel cells can be used to make electricity. They store the chemicals outside of the cell and put them in when we want electricity.

The hydrogen-oxygen fuel cell

The hydrogen-oxygen fuel cell

  • The most important fuel cell is the alkaline hydrogen-oxygen fuel cell.
  • Hydrogen and oxygen are the fuel.
  • The electrodes are platinum electrodes.
Diagram

Diagram

Anion exchange membranes

Anion exchange membranes

  • The anion exchange membranes do NOT allow the gases through.
  • These membranes do allow hydroxide ions (OH-) and water to pass through.
At the electrodes

At the electrodes

  • The hydrogen gas is delivered to the negative electrode where it reacts with the hydroxide ions. The half equation for this reaction is:
  • 2H2(g) + 4OH-(aq) → 4H2O(l) + 4e-
  • The oxygen gas is delivered to the positive electrode where it reacts with water and electrons. The half equation for this reaction is:
  • O2(g) + 2H2O(l) + 4e- → 4OH-(aq)
Overall reaction

Overall reaction

  • The overall reaction for the hydrogen-oxygen fuel cell is:
  • 2H2(g) + O2(g) → 2H2O(g)

Advantages and Disadvantages of Fuel Cells

Fuel cells can be better electricity-generating devices than the internal combustion engines that are found in cars.

Advantages of fuel cells

Advantages of fuel cells

  • Fuel cells have a high efficiency.
  • This means that they get more energy out of the same amount of fuel than less efficient devices.
  • Their only by-product is water (no CO2(g) is produced).
  • They don’t need to be recharged - they will work as long as the fuel is supplied.
Disadvantages of fuel cells

Disadvantages of fuel cells

  • Hydrogen is flammable and so must be stored very carefully.
  • Energy is needed to produce the reactants of hydrogen and oxygen.
  • This energy usually comes from fossil fuels, so there are some CO2(g) emissions.

Test yourself on Commercial Application (A2 Only)

  • What's the downside to rechargeable batteries?

  • Which of these accurately describes what a fuel cell does?

  • What is the overall reaction for the alkaline hydrogen-oxygen fuel cell?

  • Fuel cells do NOT produce CO2. What do they produce instead?

  • What is the main risk when handling hydrogen?

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