State the energy change conversion involved in a fuel cell.

(i)     Identify the two half-equations that take place at the positive electrode (cathode) and negative electrode (anode) in a hydrogen-oxygen fuel cell with an alkaline electrolyte.

Positive electrode (cathode) half-equation:

Negative electrode (anode) half-equation:

(ii)     State the overall reaction, identifying the states of all species involved.

(iii)   One commercial version of the hydrogen-oxygen fuel cell (with alkaline electrolyte) operates at a temperature of 353 K. The electrodes of the fuel cell are made of graphite but both are covered with a thin layer of platinum. State the function of the platinum.

(iv)     Outline the function of the thin polymer membrane used in the corresponding hydrogen-oxygen fuel cell with an acidic electrolyte.

(v) Other than cost, state one disadvantage of a fuel cell.

chemical (energy) to electrical (energy);

(i)     Positive electrode (cathode) half-equation:

\({{\text{O}}_2} + {\text{2}}{{\text{H}}_2}{\text{O}} + {\text{4}}{{\text{e}}^ - } \to {\text{4O}}{{\text{H}}^ - }/\frac{1}{2}{{\text{O}}_2} + {{\text{H}}_2}{\text{O}} + {\text{2}}{{\text{e}}^ - } \to {\text{2O}}{{\text{H}}^ - }\);

Negative electrode (anode) half-equation:

\({\text{2}}{{\text{H}}_2} + {\text{4O}}{{\text{H}}^ - } \to {\text{4}}{{\text{H}}_2}{\text{O}} + {\text{4}}{{\text{e}}^ - }/{{\text{H}}_2} + {\text{2O}}{{\text{H}}^ - } \to {\text{2}}{{\text{H}}_2}{\text{O}} + {\text{2}}{{\text{e}}^ - }/\frac{1}{2}{{\text{H}}_2} + {\text{O}}{{\text{H}}^ - } \to {{\text{H}}_2}{\text{O}} + {{\text{e}}^ - }\);

Award [1 max] if correct half-equations are given but incorrect electrodes.

Allow e instead of e^–.

Penalise use of reversible arrow once only in 9 (b)(i) and 11 (a).

(ii)     \({\text{2}}{{\text{H}}_2}{\text{(g)}} + {{\text{O}}_2}{\text{(g)}} \to {\text{2}}{{\text{H}}_2}{\text{O(l)}}/\frac{1}{2}{{\text{O}}_2}{\text{(g)}} + {{\text{H}}_2}{\text{(g)}} \to {{\text{H}}_2}{\text{O(l)}}\);

State symbols required.

Allow H₂O(g).

(iii)     catalyst/electrocatalyst / speeds up reaction but not consumed in reaction itself / provides surface for (initial) decomposition of molecules into atoms;

(iv)     allows flow of ions/H⁺/protons (from anode/negative electrode to cathode/positive electrode) / prevents reactants mixing/moving from one compartment to another / salt bridge / prevents flow of electrons through membrane / OWTTE;

(v)     storage/transport difficulties of gases / potentially explosive/hydrogen is flammable / needs constant supply of fuel / can contain heavy metal(s) / often operated at high temperature / low power to mass ratio / susceptible to poisoning due to impurities in fuel / OWTTE;

Allow a named gas (hydrogen or oxygen) for storage/transport difficulties.

Allow problems related to corrosion.

Accept answers based on ethanol and methanol fuel cells (but needs to be stated) such as difficult to use in cold weather/less clean product (CO₂) formed.

(a) was well answered. In (b), many candidates missed the fact that the fuel cell was with an alkaline electrolyte, even though alkaline was marked clearly in bold on the examination paper. Parts (iii), (iv) and (v) were very well answered and many candidates scored all three marks.

(a) was well answered. In (b), many candidates missed the fact that the fuel cell was with an alkaline electrolyte, even though alkaline was marked clearly in bold on the examination paper. Parts (iii), (iv) and (v) were very well answered and many candidates scored all three marks.