Fuel cells compared with petrol engines
Evaluate the point-of-use benefits alongside hydrogen supply, storage and practical costs.
Extended only (Supplement) · Chemistry 0620, 2026–2028.
Before you begin
- State the fuel-cell reactants and product.
- Understand that producing a fuel can require energy and other resources.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which product contains carbon in the comparison of petrol combustion and the hydrogen–oxygen fuel-cell reaction?
What you will learn
- Describe advantages and disadvantages of hydrogen–oxygen fuel cells relative to petrol engines.
- Distinguish operation emissions from the impact of making and supplying hydrogen.
Compare the same part of each system
A hydrogen–oxygen fuel cell produces water at the point of use; it does not produce carbon dioxide in that cell reaction. Petrol contains carbon, and its combustion releases carbon dioxide. A fuel-cell vehicle can convert fuel energy efficiently into electrical power for its motor, while a petrol engine loses substantial energy as heat. Use supplied efficiency data for any numerical comparison rather than assuming one fixed value for all designs.
Point-of-use product is only one comparison. Hydrogen manufacture and delivery can cause emissions, depending on the feedstock and energy source. Electrolysis supplied by low-emission electricity differs from using electricity generated from fossil fuels. Hydrogen made from fossil feedstocks can also involve carbon dioxide emissions. A claim that a fuel-cell reaction produces only water does not establish that its whole supply chain has zero emissions.
Weigh storage and practical trade-offs
Hydrogen has low energy content per volume compared with liquid petrol, making compact storage difficult. Pressurised storage needs suitable tanks and controls; alternative storage routes also have technical demands. Fuel-cell equipment and hydrogen production, distribution and refuelling provision can add cost and complexity. Compare these needs with the fuel supply and equipment available for the stated petrol-engine case, rather than asserting one current price or availability everywhere.
A balanced answer links each point to its consequence: water-only operation can reduce local exhaust pollution, but producing the fuel may still have a climate impact; an efficient electrical output can reduce the fuel energy needed for a task, but storage and equipment requirements can make deployment harder. Use the question's evidence to recommend an option for its specific context, not a universal winner.
Pause and explain
Which is the most evidence-based conclusion about a hydrogen fuel-cell vehicle?
Worked example
A hypothetical comparison gives fuel-cell useful electrical efficiency 50% and petrol-engine useful mechanical efficiency 25%, both for the same 100 MJ fuel-energy input. Compare useful output, then explain why these numbers do not settle total environmental impact.
Show the worked solution
- Fuel-cell useful output is 100×0.50=50 MJ; petrol-engine useful output is 100×0.25=25 MJ under the stated simplified data.
- The fuel cell supplies twice the stated useful output for the same input, but this comparison does not include hydrogen production, delivery or storage.
- Water is the fuel-cell reaction product; whole-system emissions still depend on the fuel supply and energy sources.
Answer 50 MJ versus 25 MJ in the stated example; whole-system impacts require fuel-supply evidence.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Give one operation advantage and one storage disadvantage of hydrogen–oxygen fuel cells compared with petrol engines, explaining each consequence.
Give me a hint
Connect the chemical product with local exhaust and the gas's volume with storage equipment.
Compare my reasoning
- The cell reaction produces water rather than CO₂, reducing carbon-containing emissions at that point of use.
- Hydrogen's low energy content per volume makes compact storage difficult.
- Suitable storage systems therefore add technical demands compared with storing a liquid petrol fuel.
Water-only cell reaction benefits local operation; hydrogen storage adds technical demands.
Look for these in your work
- Each comparison explains a consequence.
- Operation and storage are kept distinct.
Two identical fuel cells use hydrogen from different sources: one from electrolysis powered by low-emission electricity, the other from a fossil-feedstock process with stated CO₂ emissions. Compare their cell products and what can be concluded about the fuel-supply emissions.
Give me a hint
An identical cell reaction does not make different upstream processes identical.
Compare my reasoning
- Both hydrogen–oxygen cells produce water in the cell reaction.
- The stated fossil-feedstock process has CO₂ emissions that must enter the whole-system comparison.
- The low-emission electricity case can reduce production emissions, but a full comparison still needs the actual production and delivery data.
Same water product in both cells; different fuel-production emissions must be evaluated separately.
Look for these in your work
- The chemical reaction is not changed by the source label.
- The conclusion stays within the evidence supplied.
Using the hypothetical 50% and 25% efficiencies, find the fuel energy needed for 40 MJ useful output from each system. Give one reason this calculation alone cannot decide which vehicle is preferable.
Give me a hint
Rearrange useful output=efficiency×input, then add a missing comparison factor.
Compare my reasoning
- Fuel-cell input is 40/0.50=80 MJ.
- Petrol-engine input is 40/0.25=160 MJ.
- Hydrogen production emissions, storage needs, equipment costs or available refuelling provision are additional factors not quantified by the efficiency calculation.
80 MJ fuel-cell input versus 160 MJ petrol input; additional supply, storage or practical evidence is needed.
Look for these in your work
- Both calculations meet the same useful-output target.
- One relevant unmeasured trade-off is identified.
Common mistakes
- Equating water-only operation with zero emissions throughout hydrogen production.
- Claiming an efficiency or price is universal without supplied evidence.
- Listing a disadvantage without explaining its consequence for the stated use.
What can you explain now?
Does water-only fuel-cell operation prove that producing its hydrogen caused no carbon dioxide emissions?
Compare with the explanation
No
Fuel-cell operation and fuel production are different stages; production emissions depend on the source and process.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Write a short balanced comparison with two advantages and two disadvantages. Separate cell operation from fuel supply and avoid a universal conclusion unsupported by the question's data.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.