Question

What are fuel cells? Draw a labelled diagram of the hydrogen–oxygen fuel cell and give the reactions that occur in the cell.

Hint:

A fuel cell converts a fuel's combustion energy straight into electricity; for H₂–O₂ in KOH the net reaction is 2H₂ + O₂ → 2H₂O. Recall the two half-cell reactions.

Answer 1

Step 1: Definition of a fuel cell. A fuel cell is a galvanic (voltaic) cell that converts the chemical energy released by the combustion of a fuel (such as H₂, CH₄, CH₃OH) directly and continuously into electrical energy. Unlike an ordinary cell, the reactants (fuel and oxygen) are supplied from outside without a break, so the cell keeps working as long as the fuel is fed.

Step 2: Construction of the H₂–O₂ fuel cell. It has two porous carbon (graphite) electrodes containing a finely divided catalyst (Pt or Pd). The electrolyte between them is a hot concentrated solution of KOH (or NaOH). Hydrogen gas is bubbled at the anode and oxygen gas at the cathode.

Step 3: Labelled diagram (in words).
H₂ in → [porous C anode (–)] | conc. KOH electrolyte | [porous C cathode (+)] ← O₂ in
The two electrodes are joined through an external wire carrying a voltmeter/load; electrons flow from anode to cathode in the wire, water leaves the cell.

Step 4: Electrode reactions.
At the anode (oxidation):
\[ 2H_2(g) + 4OH^-(aq) \rightarrow 4H_2O(l) + 4e^- \]
At the cathode (reduction):
\[ O_2(g) + 2H_2O(l) + 4e^- \rightarrow 4OH^-(aq) \]
Overall (net) cell reaction:
\[ 2H_2(g) + O_2(g) \rightarrow 2H_2O(l) \]

Step 5: Note. The only product is water, so the cell is pollution-free, and its efficiency (about 70%) is much higher than a thermal power plant. Such cells were used in the Apollo space programme, where the water produced was used for drinking.