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 (O^2-) 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