Hydrogen-Oxygen Fuel Cells Exam Questions

• Hydrogen gas is supplied to one electrode (the anode/fuel electrode) and oxygen gas to the other electrode (the cathode) [1]
• At the anode, hydrogen is oxidised / loses electrons [1]
• Electrons flow through an external circuit from the anode to the cathode, producing an electric current. At the cathode, oxygen gains electrons and reacts with hydrogen ions to form water [1]

Examiner note: A fuel cell converts chemical energy directly into electrical energy. Unlike a battery, it is continuously supplied with fuel and does not run down as long as hydrogen and oxygen are supplied.

(a) 2H2 + O2 → 2H2O [1]

(b) Water [1]

Examiner note: The reaction is the same as burning hydrogen in oxygen, but in a fuel cell the energy is released as electricity rather than heat. The product is pure water, which is why fuel cells are considered “clean.”

Advantages (any two):

• The only product at the point of use is water — no carbon dioxide, sulfur dioxide, or nitrogen oxides are produced [1]
• Hydrogen is a renewable energy source if produced by electrolysis using renewable electricity [1]
• Fuel cells are more efficient than combustion engines [1]

Disadvantages (any two):

• Hydrogen is difficult and expensive to store (requires high pressure or low temperature) [1]
• Hydrogen is highly flammable / explosive, posing safety risks [1]
• The infrastructure for hydrogen refuelling stations is not widely available [1]
• Producing hydrogen by electrolysis requires electricity, which may come from fossil fuels [1]

Examiner note: Do not say “hydrogen fuel cells produce no pollution.” Producing the hydrogen itself may cause pollution if fossil fuels are used for the electrolysis. Say “no pollution at the point of use.”

Fuel cell advantage: Fuel cells can be refuelled quickly (like filling a tank) whereas batteries take hours to recharge [1] Fuel cell disadvantage: Hydrogen storage infrastructure is limited / hydrogen is expensive to produce and transport [1]

Battery advantage: Batteries use electricity directly from the grid, which is widely available / easier to recharge at home [1] Battery disadvantage: Batteries have a limited range / contain toxic materials that cause disposal problems / degrade over time [1]

Examiner note: Both are “cleaner” than petrol engines at the point of use. The question tests your ability to compare two alternative technologies, not to compare them with fossil fuels.

• In a fuel cell, a chemical reaction produces electricity (chemical energy → electrical energy) [1]
• In an electrolytic cell, electricity is used to drive a chemical reaction (electrical energy → chemical energy) — the fuel cell operates in the reverse direction [1]

Examiner note: Fuel cells and electrolytic cells are the reverse of each other. A fuel cell is similar to a battery — both produce electricity from chemical reactions. Electrolysis uses electricity to cause a reaction.

(a) Steam reforming of methane produces carbon dioxide as a by-product / uses a fossil fuel [1]

(b) The electricity must come from a renewable source such as wind, solar, or hydroelectric power [1]

(c) 2H2O → 2H2 + O2 [1]

Examiner note: This is a key evaluation point. Hydrogen is only as clean as the method used to produce it. If the electricity for electrolysis comes from a coal-fired power station, the overall process still generates CO2. “Green hydrogen” requires renewable electricity.