R.1.3.5 - Fuel cells

Deduce half-equations for the electrode reactions in a fuel cell.

(alkaline electrolyte)

Oxidation at hydrogen anode:

• H2 (g) + 2OH- (aq) arrow 2H2O (l) + 2e-
  Reduction at oxygen cathode:

• O2 (g) + 2H2O (l) + 4e- arrow 4OH- (aq)

Problems

• Hydrogen has to be extracted from other sources.

• Hydrocarbons, including fossil fuels, can be processed to break down into hydrogen and carbon dioxide.

• Alternative is to electrolyze water, but this needs electricity.

• For the whole process to be environmentally clean, the hydrogen should be generated using renewable sources.

Methanol fuel cell

• Methanol is a stable liquid, has a high energy density, and is easy to transport.

• In the direct methanol fuel cell (DMFC) the fuel is oxidised under acidic conditions on a catalyst surface to form carbon dioxide.

• The H⁺ ions formed are transported across a proton exchange membrane from the anode to the cathode where they react with oxygen to produce water, and electrons are transported through an external circuit from the anode to the cathode.

• Water is consumed at the anode and is produced at the cathode.

Methanol fuel cell half equations:

A methanol fuel cell operates through a series of half-reactions involving the oxidation of methanol and the reduction of oxygen. The half-equations typically include:

1. Anode half-reaction:

   • Methanol oxidation: CH₃OH + H₂O → CO₂ + 6H⁺ + 6e⁻

2. Cathode half-reaction:

   • Oxygen reduction: O₂ + 4e⁻ + 4H⁺→ 2H₂O

These half-equations show how methanol is oxidized to carbon dioxide, while oxygen is reduced to water, producing electricity in the process.

Overall = CH₃OH + 1.5O₂ → CO₂ + 2H₂O.