Hydrogen–oxygen fuel cells
A hydrogen–oxygen fuel cell is an electrochemical device that converts the chemical energy of hydrogen and oxygen directly into electrical energy. Unlike a battery, it does not store energy internally; it generates electricity as long as fuel (hydrogen) and oxidant (oxygen) are supplied from external sources.
The cell consists of two electrodes (anode and cathode) separated by an electrolyte. The overall reaction is the combination of hydrogen and oxygen to form water.
The Overall Reaction:
2H_2(g) + O_2(g) \rightarrow 2H_2O(l)
Key Components for Vehicles:
- Fuel: Hydrogen gas (H_2) is stored in high-pressure tanks on the vehicle.
- Oxidant: Oxygen (O_2) is typically drawn from the ambient air surrounding the vehicle, not stored in a tank. This is a crucial distinction from hydrogen storage.
At the anode (negative electrode), hydrogen molecules lose electrons (oxidation):
2H_2 \rightarrow 4H^+ + 4e^-
The electrons flow through an external circuit to do work (powering a motor), creating an electric current. At the cathode (positive electrode), oxygen gains electrons and combines with hydrogen ions to form water.
Product: The only chemical product is water (H_2O). No carbon dioxide or other pollutants are emitted from the cell itself.
Correct Answer:
2H_2 + O_2 \rightarrow 2H_2O
Marking Points (Typical 2 marks):
- Correct formulae for reactants and products (H_2, O_2, H_2O).
- Balanced equation.
Note: Ensure you include state symbols if required by the question (e.g., (g) for gases, (l) for liquid water), though often just the balanced formulae are sufficient unless specified.
Correction: Hydrogen and oxygen are reactants (inputs). The only product is water. Carbon dioxide is NOT produced in the cell itself. If you see an option saying 'CO₂ is produced', it is incorrect for the fuel cell reaction.
Correction: While some heat is generated, the primary purpose and output of a fuel cell is electricity. This distinguishes it from combustion engines which primarily produce heat to drive pistons.
Context: When comparing hydrogen–oxygen fuel cells to gasoline (petrol) engines in vehicles, you must address both environmental/efficiency aspects and practical engineering challenges.
Advantages (Why use fuel cells?):
- Zero Emissions at Point of Use: The only product is water. There is no carbon dioxide (CO_2) evolved from the cell, unlike petrol engines which burn hydrocarbons to produce CO_2 and other pollutants.
- Higher Efficiency: Fuel cells convert chemical energy directly to electrical energy, bypassing the inefficient heat-to-mechanical conversion steps in internal combustion engines. They are generally more efficient.
- Quiet Operation: No explosions or moving pistons inside the cell make them quieter.
Disadvantages (Why aren't they everywhere?):
- Storage Difficulties (Density): Hydrogen has a very low energy density by volume. To store enough hydrogen for a reasonable driving range, it must be compressed to high pressures (requiring heavy, strong tanks) or cooled to cryogenic temperatures. This makes storage bulky and expensive.
- Safety Risks: Hydrogen is highly flammable with a wide flammability range in air. Leaks can pose significant safety hazards compared to liquid petrol.
- Production of Hydrogen (The 'Green' Issue): Most hydrogen today is produced via steam reforming of methane (natural gas), which releases CO_2. Therefore, the overall carbon footprint depends on how the hydrogen is made. It is only truly 'clean' if produced by electrolysis using renewable electricity.
- Infrastructure: There are very few hydrogen refueling stations compared to petrol stations.
- For advantages, examiners accept: 'no carbon dioxide produced', 'water is the only product', or 'more efficient'.
- For disadvantages, examiners accept: 'hydrogen needs to be stored at high pressure', 'hydrogen is hard to store', 'heavy tanks needed', or 'flammable/safety risk'.
Why this phrasing?
Examiners look for specific technical reasons. Saying 'it's bad for the environment' is too vague. You must specify why (e.g., storage issues) or what is missing (e.g., no CO₂). For disadvantages, vague answers like 'it's expensive' often lose marks unless you explain why (e.g., due to high-pressure storage requirements).
• No carbon dioxide is produced.
• Water is the only product (no pollution).
• It is more efficient.
• Hydrogen needs to be stored at high pressure.
• Hydrogen is difficult/expensive to store (low density).
• Heavy tanks are required for storage.
• Hydrogen is flammable/safety risk.
• Lack of refueling infrastructure.