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Fuel cellsEdexcel International A Level Chemistry: Revision notes

Section 1

What is a fuel cell?

A fuel cell uses the energy released when a fuel reacts with oxygen to generate a voltage directly. Fuel and oxygen are supplied continuously to the electrodes, the electrodes and electrolyte are not used up, and so the cell provides current for as long as reactants are supplied. This differs from a battery, where the reactants are sealed in and eventually run out.

The fuel is oxidised at the negative electrode and oxygen is reduced at the positive electrode. Electrons flow through the external circuit and ions move through the electrolyte.

Key termsfuel cell

Section 2

Hydrogen–oxygen fuel cell: acidic electrolyte

With an acidic electrolyte (such as a proton-exchange membrane):

  • Negative electrode: H₂ → 2H⁺ + 2e⁻ (oxidation)
  • Positive electrode: O₂ + 4H⁺ + 4e⁻ → 2H₂O (reduction)
  • Overall: 2H₂ + O₂ → 2H₂O

The H⁺ ions travel through the electrolyte to the positive electrode. E°cell = (+1.23) − (0.00) = +1.23 V. The only product is water.

To balance the electrons, the hydrogen half-equation is doubled; the E° value is not.

Key termsproton-exchange membrane

Section 3

Hydrogen–oxygen fuel cell: alkaline electrolyte

With an alkaline electrolyte such as aqueous potassium hydroxide:

  • Negative electrode: H₂ + 2OH⁻ → 2H₂O + 2e⁻ (oxidation)
  • Positive electrode: O₂ + 2H₂O + 4e⁻ → 4OH⁻ (reduction)
  • Overall: 2H₂ + O₂ → 2H₂O (the same)

OH⁻ ions move through the electrolyte from the positive to the negative electrode. No OH⁻ is used up overall, because it is consumed at one electrode and made at the other.

Common mistake

In an alkaline cell there are no free H⁺ ions to use. Write OH⁻ and H₂O in the half-equations, not H⁺.

Section 4

Methanol and other hydrogen-rich fuels

Hydrogen is difficult to store (a gas, flammable), so hydrogen-rich fuels such as methanol (and ethanol or methane) can be used instead. In an acidic methanol fuel cell:

  • Anode: CH₃OH + H₂O → CO₂ + 6H⁺ + 6e⁻
  • Cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O
  • Overall: 2CH₃OH + 3O₂ → 2CO₂ + 4H₂O

To balance the electrons, multiply the anode by 2 and the cathode by 3 (12 electrons each).

Calculation: 1.00 g of CH₃OH is 0.03125 mol, which releases 6 × 0.03125 = 0.1875 mol of electrons, so charge Q = 0.1875 × 96 500 = 1.81 × 10⁴ C.

Key termshydrogen-rich fuel

Section 5

Advantages and disadvantages

Advantages:

  • Continuous power as long as fuel is supplied, with no recharging
  • Hydrogen cells give only water at the point of use
  • High efficiency compared with burning the fuel in an engine

Disadvantages:

  • Hydrogen is difficult to store (flammable, needs pressure) and is mostly made from methane, releasing CO₂
  • Methanol cells release CO₂ and are less efficient
  • Expensive platinum catalysts
  • Lack of refuelling infrastructure

A good evaluation reaches a justified conclusion.

Must Know

  • Fuel cell: fuel + O₂ react to give a voltage, with continuous supply of reactants
  • Acid H₂/O₂: H₂ → 2H⁺ + 2e⁻ and O₂ + 4H⁺ + 4e⁻ → 2H₂O
  • Alkaline H₂/O₂: H₂ + 2OH⁻ → 2H₂O + 2e⁻ and O₂ + 2H₂O + 4e⁻ → 4OH⁻
  • E°cell = +1.23 V; only water is formed in the hydrogen cell
  • Methanol: CH₃OH + H₂O → CO₂ + 6H⁺ + 6e⁻

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Fuel cells

  1. A hydrogen–oxygen fuel cell with an acidic electrolyte is used to power a small vehicle. Standard electrode potentials: 2H⁺(aq) + 2e⁻ ⇌ H₂(g) 0.00 V; O₂(g) + 4H⁺(aq) + 4e⁻ ⇌ 2H₂O(l) +1.23 V.
    Explain why the electrolyte, which allows H⁺ ions to pass from one electrode to the other, is needed in this fuel cell.2 marks
  2. A hydrogen–oxygen fuel cell uses aqueous potassium hydroxide as the electrolyte.
    Write the half-equation for the reaction at the negative electrode of this fuel cell and use oxidation numbers to show that it is an oxidation.2 marks
  3. A methanol fuel cell uses an acidic electrolyte. The half-equations are: anode CH₃OH(l) + H₂O(l) → CO₂(g) + 6H⁺(aq) + 6e⁻; cathode O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l). Use F = 96 500 C mol⁻¹ and M(CH₃OH) = 32.0 g mol⁻¹.
    Combine the half-equations to deduce the overall equation for the reaction in the methanol fuel cell.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).