Redox 4: Fuel Cells

The half-equation for the reaction occurring at the (+) electrode in a hydrogen-oxygen fuel which has an alkaline electrolyte would be

• H₂(g) + 2OH^-(aq)→ 2H₂O(l) + 2e^-
• H₂(g) → 2H⁺(g) + 2e^-
• O₂(g) + 4H⁺(aq) + 4e^- → 2H₂O(l)
• O₂(g) + 2H₂O(l) + 4e^- → 4OH^- (aq)

A simplified diagram of the phosphoric acid fuel cell (PAFC), and their half-reactions are shown. The cell operates at 190^oC using an electrolyte of liquid phosphoric acid.
During operation of the PAFC, electrode I is

• positively charged, and the pH near the electrode will increase
• positively charged, and the pH near the electrode will decrease
• negatively charged, and the pH near the electrode will increase
• negatively charged, and the pH near the electrode will decrease

A simplified diagram of the phosphoric acid fuel cell (PAFC), and their half-reactions are shown. The cell operates at 190^oC using an electrolyte of liquid phosphoric acid.
Consider the following statements in regards to the PAFC:
I. The electrodes are likely to be porous and have a catalytic function.
II. More electrical energy will be produced in the PAFC for an equivalent mass of hydrogen, than using the fuel to produce steam for the generation of electricity by a turbine and generator.
III. The water generated in the cell reaction will dilute the electrolyte progressively and lower the efficiency of the cell.
Which of the above statements about PAFC are correct?

• I and II only
• I and III only
• II and III only
• I, II and III

Methane gas is used as a fuel in an acidic fuel cell. The half-equation occurring at the anode will be

• O₂(g) + 4H⁺(aq) + 4e^- →2H₂O(g)
• CH₄(g) + 2H₂O(g) → CO₂(g) + 8H⁺ (aq) + 8e^-
• CH₄(g) + 4H⁺(g)→ CO₂(g) + 4H₂O(l)
• CO₂(g) + 8H⁺(aq) + 8e^- → CH₄(g) + 2H₂O(g)

The energy released in a chemical reaction is directly converted to electrical energy in al

• solar cell
• electrolytic cell
• fossil-fuel power station
• hydrogen/oxygen fuel cell

In a fuel cell based on the oxidation of methane, the equation for the anode half-reaction is:
CH₄(g) + 2H₂O(l) → CO₂(g) + 8H⁺(aq) + 8e^-
The corresponding equation for the half-reaction at the cathode is

• 2H₂O(l) + 4e^- → 4H⁺(aq) + O₂(g)
• 4H⁺(aq) + O₂(g) → 2H₂O(l)
• 2H₂O(l) → 4H⁺(aq) + O₂ (g) + 4e^-
• 4H⁺(aq) + O₂(g) + 4e^- → 2H₂O(l)

A fuel cell currently under development for powering small electronic devices is based on the reaction of methanol and oxygen using an acidic electrolyte. The reductant in the cell reaction and the half-reaction at the anode are

• Methanol; O₂(g) + 4H⁺(aq) + 4e^- → 2H₂O(l)
• Oxygen; O₂(g) + 4H⁺(aq) + 4e^- → 2H₂O(l)
• Methanol; CH₃OH(g) + H₂O(l) → CO₂(g) + 6H⁺(aq) + 6e^-
• Oxygen; CH₃OH(g) + H₂O(l) → CO₂(g) + 6H⁺(aq) + 6e^-

A fuel cell can be constructed that uses the two half-reactions as shown.
Which one of the following would occur at the negative electrode of the cell as it generates electricity?

• production of H⁺
• formation of H₂O
• consumption of CO₂
• reduction of CH₃OH

A fuel cell can be constructed that uses the two half-reactions as shown.
Which one of the following statements about this fuel cell is most likely to be correct?

• An external power supply is used to recharge the cell.
• Gaseous products are recycled into the cell to improve efficiency.
• Chemical energy is not completely converted into electrical energy.
• More H⁺ ions are produced at the anode than are consumed at the cathode.

Which one of the following state is true for both fuel cells and rechargeable cells?

• All reactants are stored within the cell.
• Reaction products are continuously removed from the cell.
• Electrons pass from the reductant to the anode as electricity is produced.
• Electrical energy is converted to chemical energy as the cell is recharged.