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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.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • describe hydrogen, derived from water or hydrocarbons, as a potential fuel, reacting with oxygen to generate electricity directly in a hydrogen fuel cell (details of the construction and operation of a fuel cell are not required).

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 fossil fuels to reduce pollution and/or reduce greenhouse gas emissions.

B. Hydrogen as a Fuel

Hydrogen is considered a “clean fuel” at the point of use because its combustion produces only water: 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.
Clean fuel trap

Hydrogen produces no CO₂ when used, but if hydrogen is made from fossil fuels, CO₂ can still be produced during manufacture.

3. Detailed Explanations

Quick Recall
  • Hydrogen combustion gives water only: 2H₂ + O₂ → 2H₂O (no CO₂ at the point of use).
  • A hydrogen fuel cell generates electrical energy directly as hydrogen reacts with oxygen.
  • Hydrogen can be made from water or from 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 burns in oxygen to form water: 2H₂(g) + O₂(g) → 2H₂O(l)

Laboratory Warning

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.

Keyword you can write safely

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:

  1. hydrogen is obtained from water or hydrocarbons;
  2. hydrogen reacts with oxygen in the fuel cell;
  3. electrical energy is generated directly, and water forms.

E. Advantages and Limitations (O-Level)

PointFuel cell
Pollution at point of usewater only (no CO₂)
Energy transferchemical energy → electrical energy
Limitationshydrogen 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

Fuel cell marks

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

Core

Problem

Write the overall equation, including state symbols, for hydrogen reacting with oxygen in a hydrogen–oxygen fuel cell.
Study the worked solution

  1. 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).

  2. Balance hydrogen

    Method

    Place 2 before hydrogen and water.

    Reason

    This gives four hydrogen atoms on each side without changing the oxygen balance.

    Working

    2H₂(g) + O₂(g) → 2H₂O(l).

  3. 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

About 5 min

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

Likely lower-emission route
Best reason

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

  1. 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.

  2. 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

Find and correct the mistake

Learner claim

“A hydrogen fuel cell makes only water, so using hydrogen always solves global warming.” Improve the claim so that it is scientifically accurate.

Try this before viewing the solution

Point-of-use product
Missing system boundary

View solution step by step

  1. 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: H₂ + O₂ → H₂O; no direct CO₂.

  2. Restore the lifecycle boundary

    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

    Safe conclusion: clean at point of use; overall benefit depends on how hydrogen is produced and supplied.

Examiner practice 4

Describe hydrogen as a fuel

4 marks

Examination question

Describe hydrogen as a potential fuel and explain what happens in a hydrogen fuel cell. [4 marks]

Try this before viewing the solution

View solution step by step

  1. State possible sources

    1 mark

    Method

    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.

  2. Describe the fuel-cell change

    2 marks

    Method

    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

    Hydrogen + oxygen → electrical energy.

  3. Name the product

    1 mark

    Method

    State that water forms.

    Reason

    The overall chemical equation is 2H₂ + O₂ → 2H₂O.

    Working

    Product: water.

Challenge 5

Transfer the Pollution Reasoning

Minimal support

Problem

In a poorly ventilated room, a carbon-containing fuel burns with a smoky yellow flame. Name one dangerous product, explain why it forms, and state its hazard.

Try this before viewing the solution

Oxygen condition
Dangerous product

Hints

Hint 1: combustion type
A smoky yellow flame indicates incomplete combustion.
Hint 2: hazard word
Name the carbon-containing gas and use the word “toxic”.
View solution step by step

  1. Use the observations

    Method

    Infer incomplete combustion because oxygen is limited.

    Reason

    There is insufficient oxygen to oxidise all carbon fully to carbon dioxide.

    Working

    Poor ventilation + smoky flame → incomplete combustion.

  2. Name product and hazard

    Method

    Identify carbon monoxide, CO, and state that it is toxic.

    Reason

    Carbon monoxide can form during incomplete combustion and is dangerous when inhaled, especially in an enclosed room.

    Working

    Dangerous product: toxic CO.

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.

8. Quiz

Quiz Time!

Test yourself on hydrogen as a fuel, fuel cell basics, and the “clean fuel” exam trap.

Go to Quiz Page