Fuel Cells
Electrode Potentials Worksheet
1. An acidic hydrogen fuel cell is an alternative to batteries. The reactants for the reaction are stored hydrogen and oxygen from the air.
a) At the anode, oxidation of one of the reactants takes place. Write the equation for this reaction.
H2
→
2H+ + 2e–
b) At the cathode, the other reactant is reduced by the product from the anode reaction. Write the equation for this reaction.
O2 + 4H+ + 4e–
→
2H2O
c) The Eo value for the reduction reaction at the cathode is +1.23V. State the EMF of the fuel cell.
The standard hydrogen electrode (anode reaction reversed) is 0.00V.
+1.23V – 0.00V = +1.23V
d) State why the hydrogen fuel cell does not need to be recharged.
The reactants (hydrogen and oxygen) are constantly supplied to the cell from an external tank and the air, so the reaction continues as long as fuel is provided.
2. An alkaline hydrogen cell is an alternative to the acidic hydrogen fuel cell.
a) State the Eo value of the alkaline hydrogen fuel cell.
+1.23V (The overall reaction is the same as the acidic cell).
b) At the anode, hydrogen is oxidised in the presence of hydroxide ions. Write the half equation for this reaction.
H2 + 2OH–
→
2H2O + 2e–
c) Explain why fuel cell vehicles are considered to be a more environmentally friendly alternative to internal combustion engine vehicles.
Hydrogen can be produced by the hydrolysis of water. This process requires only water and electricity. If the electricity is produced from renewable sources such as wind or solar power, the entire cycle releases no carbon dioxide.
3. Methanol fuel cells are used in situations where hydrogen is not easily accessible.
a) The overall reaction for a methanol fuel cell is the same as the combustion of methanol. Write the equation for this reaction.
CH3OH + 1.5O2
→
CO2 + 2H2O
b) At the cathode, oxygen is reduced to form water. Write the equation for the reaction at the cathode.
O2 + 4H+ + 4e–
→
2H2O
c) At the anode, methanol reacts with water to form carbon dioxide and hydrogen ions. Write the equation for the reaction at the anode.
CH3OH + H2O
→
CO2 + 6H+ + 6e–
d) Sketch a graph of the EMF of a fuel cell against time.
The EMF remains constant over time because the concentration of reactants within the cell remains constant as they are continuously supplied.