Hydrogen as a Fuel and Fuel Cells
Learn why hydrogen is a potential fuel, how a hydrogen fuel cell generates electricity directly, and how production affects its environmental impact.
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Hydrogen can supply chemical energy to a fuel cell, which converts that energy into electrical energy. Its point-of-use product is water, but evaluating hydrogen also requires considering how it is produced and stored.
1. Definition
A. Alternative Fuels
Alternative fuels are fuels used in place of conventional fossil fuels. They may be chosen to reduce pollution or greenhouse-gas emissions, but the label alone does not guarantee that benefit.
B. Hydrogen as a Fuel
A hydrogen–oxygen fuel cell produces water without direct carbon dioxide emissions at the point of use. Its overall chemical change, shown with water collected as a liquid, is: 2H₂(g) + O₂(g) → 2H₂O(l)
2. Key Ideas
- Burning fossil fuels produces CO₂ (a greenhouse gas). Link: Fossil Fuels.
- Burning hydrogen produces water, not CO₂.
- Hydrogen must be made (it is an energy carrier, not a primary energy source).
- In a hydrogen fuel cell, hydrogen reacts with oxygen to generate electrical energy directly.
- The product of the overall reaction is water.
Hydrogen produces no CO₂ when used, but if hydrogen is made from fossil fuels, CO₂ can still be produced during manufacture.
3. Detailed Explanations
- A hydrogen fuel cell generates electrical energy directly as hydrogen reacts with oxygen; water is the chemical product.
- Electrolysis of water requires electrical energy to produce hydrogen and oxygen.
- Hydrogen can be obtained from water or hydrocarbons; the production route affects the overall environmental impact.
A. Why Look for Alternative Fuels?
Combustion of fossil fuels releases:
- CO₂ (greenhouse gas → global warming),
- CO (toxic, from incomplete combustion),
- SO₂ and nitrogen oxides (acid rain / irritation).
B. Using Hydrogen as a Fuel (Combustion)
Hydrogen can also be burned to release heat. With pure oxygen, the chemical product is water; in the hot flame, it is water vapour: 2H₂(g) + O₂(g) → 2H₂O(g)
Burning hydrogen in air can also produce nitrogen oxides at high temperatures because air contains nitrogen and oxygen. Do not turn “no carbon dioxide from hydrogen itself” into “every hydrogen combustion system has no air pollution”. A fuel cell generates electricity electrochemically rather than by burning the hydrogen.
Hydrogen is highly flammable and mixtures with air can ignite explosively. Keep it away from ignition sources and test only small samples using the instructed gas-test procedure.
C. How Hydrogen Is Obtained
Hydrogen must be manufactured. It may be produced from fossil-fuel feedstocks, which can release carbon dioxide, or by the electrolysis of water. The environmental impact of electrolysis depends on the electricity source.
Hydrogen is a clean fuel at the point of use, but the overall environmental benefit depends on how hydrogen is produced.
D. Hydrogen–Oxygen Fuel Cell
A hydrogen fuel cell generates electrical energy directly from the reaction between hydrogen and oxygen. You do not need to learn its construction, electrode reactions or operating mechanism.
The overall chemical change is: 2H₂(g) + O₂(g) → 2H₂O(l)
Follow the three core ideas in order:
- hydrogen is obtained from water or hydrocarbons;
- hydrogen reacts with oxygen in the fuel cell;
- electrical energy is generated directly, and water forms. Heat is also released; electrical energy is not a chemical product to put on the right of the chemical equation.
E. Advantages and Limitations (O-Level)
| Point | Fuel cell |
|---|---|
| Pollution at point of use | water only (no CO₂) |
| Energy transfer | chemical energy → electrical energy |
| Limitations | hydrogen storage/transport and flammability; hydrogen production may release CO₂ |
4. Common Mistakes
- Writing “hydrogen is clean” without stating at the point of use.
- Saying hydrogen is a primary energy source. It must first be manufactured from water or hydrocarbons.
- Saying the fuel cell burns hydrogen. The syllabus wording is that hydrogen reacts with oxygen to generate electricity directly.
- Describing only the water product and ignoring emissions from hydrogen production.
5. Exam Tips
Link the ideas in one sentence: “Hydrogen obtained from water or hydrocarbons reacts with oxygen in a fuel cell to generate electrical energy directly, forming water.”
- If asked to evaluate hydrogen, distinguish point-of-use emissions from emissions during production.
- Construction details, electrode signs and half-equations are not required.
6. Worked Examples
Modelled example 1
Write the Fuel-Cell Overall Equation
Problem
Study the worked solution
Choose reactants and product
Method
Use hydrogen and oxygen as reactants and water as the product.Reason
The fuel cell combines the supplied gases while converting chemical energy into electrical energy.Working
H₂(g) + O₂(g) → H₂O(l).Balance oxygen, then hydrogen
Method
Place 2 before water to give two oxygen atoms, then 2 before hydrogen to give four hydrogen atoms.Reason
One oxygen molecule has two oxygen atoms. Two water molecules contain those two oxygen atoms and four hydrogen atoms, which require two hydrogen molecules.Working
2H₂(g) + O₂(g) → 2H₂O(l).Audit the equation
Method
Check both atom counts and distinguish the overall reaction from electrolysis.Reason
Electrolysis of water runs in the reverse chemical direction and consumes electrical energy.Working
Fuel cell: 2H₂(g) + O₂(g) → 2H₂O(l).
Guided practice 2
Compare two hydrogen sources
Problem
Route A obtains hydrogen from a hydrocarbon. Route B obtains hydrogen from water using wind-generated electricity. Which route is more likely to have lower carbon dioxide emissions during production?
Try this before viewing the solution
Hints
Hint 1: use the full pathway
Both routes give hydrogen, so compare the feedstock and energy source used to make it.
View solution step by step
Compare production
Method
Select Route B as more likely to have lower production emissions.
Reason
Wind-generated electricity does not require fossil-fuel combustion, while obtaining hydrogen from a hydrocarbon can release carbon dioxide.
Working
Lower-emission route: water + wind electricity.
Keep the conclusion bounded
Method
State that the answer depends on the actual production energy and process.
Reason
“Hydrogen” alone does not determine its total greenhouse-gas impact.
Working
Compare the full production pathway, not only the fuel-cell product.
Common misconception 3
Check the “clean fuel” claim
Learner claim
Try this before viewing the solution
View solution step by step
Credit the valid part
Method
State that water is formed and no carbon dioxide is emitted by the fuel cell itself at point of use.Reason
Hydrogen contains no carbon.Working
Point of use: 2H₂ + O₂ → 2H₂O; no direct CO₂.Include production and supply
Method
State that hydrogen must be manufactured and that fossil-fuel feedstocks or fossil-generated electricity can release carbon dioxide.Reason
The overall greenhouse-gas impact depends on production energy, feedstock, storage and transport rather than the cell exhaust alone.Working
Supported conclusion: clean at point of use; overall benefit depends on how hydrogen is produced and supplied.
Examiner practice 4
Describe hydrogen as a fuel
Exam-style practice
Try this before viewing the solution
View solution step by step
State possible sources
1 markMethod
State that hydrogen can be obtained from water or hydrocarbons.Reason
Hydrogen must be manufactured before it can be used as a fuel.Working
Sources: water or hydrocarbons.Describe the fuel-cell change
2 marksMethod
State that hydrogen reacts with oxygen and generates electrical energy directly.Reason
These are the required reactants and energy output; construction details are not needed.Working
Reactants: hydrogen and oxygen. Useful energy output: electrical energy.Name the product
1 markMethod
State that water forms.Reason
The overall chemical equation is 2H₂ + O₂ → 2H₂O.Working
Product: water.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark the source, reactants, direct electricity generation and product.
The carbon-fuel comparison is now in Fossil Fuels, where incomplete combustion and its products are explained.
7. Mind Stretchers
Mind stretcher 1: Energy Source vs Energy CarrierExtension
Explain why hydrogen is not a “primary energy source”.
Show Answer
Hydrogen must be manufactured (e.g., from methane or by electrolysis of water). Energy has to be supplied to obtain it, so hydrogen is an energy carrier, not a primary energy source.
Mind stretcher 2: Compare Two Hydrogen SourcesExtension
Source 1: hydrogen made from methane. Source 2: hydrogen made by electrolysis using electricity from wind. Which is more likely to reduce greenhouse gas emissions? Explain.
Show Answer
Hydrogen made using wind electricity is more likely to reduce greenhouse gas emissions because no fossil fuel is burned to produce the electricity. Making hydrogen from methane can release CO₂ during production.
Try independently: Hydrogen for a fuel cell is made by electrolysis using electricity from a fossil-fuel power station. A learner says, “The cell produces water, so no carbon dioxide is associated with this electricity supply.” Identify what is correct, what is missing, and one change that could reduce the production emissions.
Show answer and reasoning
The hydrogen–oxygen fuel cell itself forms water without emitting carbon dioxide. The claim omits the electricity used to make the hydrogen: fossil-fuel combustion at the power station can emit carbon dioxide. Using a lower-emission electricity source for electrolysis can reduce those production emissions. The point-of-use product and the full supply-chain emissions are different questions.
Practise and check
Practise explaining the fuel-cell reaction, direct electricity generation and the production boundary.
Open the Redox Chemistry topic checkSyllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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