Storage and fuel cells

Question 1

What are the three types of cells used today?

Answer 1

• Non-reachargeable cells.
• Rechargeable cells.
• Fuel cells

Question 2

Describe and explain non-rechargeable cells.

Answer 2

They provide electrical energy until the chemicals have reacted to such an extent that the voltage falls. The cell is then ‘flat’ and discarded.

Question 3

Describe and explain rechargeable cells.

Give an example of rechargeable cells.

Answer 3

The chemicals in the cell react, providing electrical energy. The cell reaction can be reversed during reacharging - the chemicals in the cell are regenerated and the ell can be used again.

Common examples include:

• nickel and cadmium batteries, used for general AA batteries.
• Lithium-ion and lithium polymer batteries, used in laptops.

Question 4

Describe and explain fuel cells.

Answer 4

The cell reaction uses external supplies of a fuel and an oxidant, which are consumed and needs to be provided continuously. The cell will continue to provide electrical energy so long as there is a supply of fuel and oxidant.

Question 5

Describe and explain the hydrogen-oxygen fuel cell.

• Draw a diagram also.

Answer 5

• A fuel cell used the energy from the reaction of a fuel with oxygen to create a voltage.
• The reactants flow in and products flow out while the electrolyte remains in the cell.

The redox equilibria are shown below. The more negative of the two systems is the negative terminal of the cell.

2H₂O(l) + 2e^- ⇔ H₂(g) + 2OH^-(aq) E^ø=-0.83V negative terminal.

1/2O₂(g) + H₂O(l) + 2e^- ⇔ 2OH^-(aq) E^ø= +0.40V

positive terminal

The more negative hydrogen system provides the electrons. This equilibrium is reversed when writing the half-equations at each electrode.

Overall: H₂(g) + 1/2O₂(g) → H₂O(l)

E^ø_cell= 1.23V

Question 6

Explain the advantages of fuel cells.

Answer 6

• The reactants flow in and products flow out while the electrolyte remains in the cell.
• Fuel cells can operate virtually continously so long as the fuel and oxygen continue to flow into the cell. Fuel cells do not have to be reacharged.

Question 7

What fuel is used in fuel cells?

Answer 7

Modern fuel cells are based on hydrogen, or hydrogen-rich fuels such as methanol, CH₃OH.