CIE A Level Chemistry (9701) 2019-2021

Revision Notes

5.3.10 Batteries & Fuel Cells

Batteries

Rechargeable batteries

  • The electrochemical cells discussed so far represent non-rechargeable batteries
    • Electrons flow from an anode to a cathode
    • Once there are no electrons left at the anode, the battery can no longer be used
  • In rechargeable batteries, the electron flow can be reversed from cathode to anode in order to ‘recharge’ the battery
    • For example, mobile phones makes use of a lithium-ion battery
    • When in use, electrons flow from the anode of the battery to the cathode
    • Connecting the phone to a power source enables electrons to flow back to the anode therefore charging the phone battery
  • There are several types of electrochemical cells, with several advantages, that can be used as rechargeable batteries

Hydrogen fuel cell

  • This fuel cell uses the half-equations of a reaction between hydrogen and oxygen to generate a current
    • The only byproduct is water
    • As a result, the hydrogen fuel cell is considered largely environmentally friendly
  • For this fuel cell to work, hydrogen and oxygen gas are present at the two electrodes of the cell
  • An electrolyte solution separates the two electrodes
  • At the anode:
    • Hydrogen gas is pumped in
    • Each hydrogen atom oxidises and loses an electron – producing protons (H+)

H2 → 2e + 2H+

    • These electrons flow through an external circuit to the cathode, while producing a small current
  • At the cathode:
    • Oxygen gas is pumped in
    • Each oxygen atom gains electrons from the external circuit and becomes reduced
    • Oxide (O2-) ions react with protons to form water

O2 + 4H+ + 4e→ 2H2O

  • The overall reaction is:

2H2 + O2 → 2H2O

  • Advantages of the hydrogen fuel cell
    • The only byproduct of the redox reactions is water – environmentally friendly
    • Uses oxygen from the air
  • However, the use of hydrogen is somewhat problematic
    • The sources of hydrogen aren’t always environmentally friendly
    • Storage of hydrogen gas is dangerous as well due to its explosive nature

Nickel-metal hydride rechargeable batteries

  • Nickel-metal hydride (NiMH) rechargeable batteries are widely used in digital cameras
  • One of the most advantageous aspects is that high currents can be supplied using a NiMH battery
  • NiMH batteries are not suitable for re-charging until the battery has fully lost its charge

Lithium-ion rechargeable batteries

  • Lithium-ion batteries are widely used in mobile phones, laptops and other portable charging devices
  • The electrons travel through an external circuit when the phone is being used
  • When the battery is low on charge, connecting to a power source allows the electrons to flow back to the anode
  • There are 2 electrode separated by an electrolyte
    • A lithium cobalt oxide electrode
    • A graphite electrode
    • Lithium salt electrolyte
  • The half-equation at the cathode (negative electrode):

Li ⇄ Li+ + e

  • The half-equation at the anode (positive electrode):

Li+ + CoO2 + e ⇄ Li+[CoO2]

  • Unlike the NIMH battery, Li-ion batteries do not need to be fully uncharged to re-charge
    • For example, a smartphone does not need to be on 0% battery to be put on to charge

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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