Energy changes: fuel cells and hydrogen as a fuel

What you'll learn

This revision guide covers fuel cells and hydrogen as a fuel, focusing on the chemistry behind these clean energy technologies. You'll understand how fuel cells generate electricity through chemical reactions, write half equations for the processes occurring at each electrode, and evaluate hydrogen as an alternative fuel. This topic links directly to sustainability and the global transition away from fossil fuels.

Key terms and definitions

Fuel cell — an electrochemical cell that converts chemical energy from a fuel and oxygen into electrical energy continuously, as long as fuel is supplied.

Hydrogen fuel cell — a type of fuel cell that uses hydrogen gas as the fuel and oxygen (usually from air) as the oxidant to produce electrical energy, with water as the only product.

Electrode — a conductor through which electricity enters or leaves an electrochemical cell; in fuel cells, the anode (negative electrode) and cathode (positive electrode) are where oxidation and reduction occur.

Oxidation — the loss of electrons by a substance, or the gain of oxygen; at the fuel cell anode, hydrogen is oxidised.

Reduction — the gain of electrons by a substance, or the loss of oxygen; at the fuel cell cathode, oxygen is reduced.

Half equation — an equation showing either the oxidation or reduction reaction that occurs at one electrode, including electrons.

Electrolyte — a substance containing free ions that conducts electricity; in fuel cells, it allows ions to move between electrodes while preventing electron flow through it.

Zero-emission vehicle — a vehicle that produces no harmful exhaust emissions at the point of use, such as cars powered by hydrogen fuel cells.

Core concepts

What is a fuel cell?

A fuel cell is an electrochemical device that continuously converts the chemical energy stored in a fuel directly into electrical energy. Unlike batteries, which store a fixed amount of chemical energy and eventually run down, fuel cells operate continuously as long as fuel and oxygen are supplied.

The key features of fuel cells include:

• They produce electricity through chemical reactions, not combustion
• They require a constant supply of fuel (typically hydrogen) and oxygen
• They contain two electrodes (anode and cathode) separated by an electrolyte
• They produce water and heat as by-products when using hydrogen
• They are more efficient than combustion engines as they convert chemical energy directly to electrical energy

Fuel cells are used in various applications including:

• Powering vehicles (cars, buses, ships)
• Generating electricity for buildings and remote locations
• Providing backup power systems
• Space exploration (NASA has used them since the 1960s)

How hydrogen fuel cells work

In a hydrogen fuel cell, hydrogen gas (H₂) is supplied to the anode and oxygen gas (O₂) is supplied to the cathode. The overall reaction produces water and releases electrical energy.

At the anode (negative electrode):

Hydrogen molecules are oxidised, losing electrons to form hydrogen ions (protons):

2H₂ → 4H⁺ + 4e⁻

This is an oxidation reaction because electrons are lost.

At the cathode (positive electrode):

Oxygen molecules gain electrons (from the external circuit) and combine with hydrogen ions to form water:

O₂ + 4H⁺ + 4e⁻ → 2H₂O

This is a reduction reaction because electrons are gained.

The electrolyte:

The electrolyte allows hydrogen ions (H⁺) to move from the anode to the cathode but does not allow electrons to pass through. This forces electrons to flow through an external circuit, creating an electric current that can power devices.

Overall equation:

2H₂ + O₂ → 2H₂O

This is the same equation as burning hydrogen in oxygen, but in a fuel cell the energy is released as electricity rather than heat and light.

Writing half equations for fuel cells

For AQA GCSE Chemistry, you need to write and interpret half equations for reactions at each electrode in a hydrogen fuel cell.

Key rules for half equations:

• Show the electrons (e⁻) on the correct side
• Balance atoms first, then charges
• At the anode, oxidation occurs — electrons appear on the right (as products)
• At the cathode, reduction occurs — electrons appear on the left (as reactants)
• Remember: "An Ox" and "Red Cat" (Anode Oxidation, Reduction Cathode)

Anode half equation:

H₂ → 2H⁺ + 2e⁻

Or, to match with cathode equations using 4 electrons:

2H₂ → 4H⁺ + 4e⁻

Cathode half equation:

O₂ + 4H⁺ + 4e⁻ → 2H₂O

When writing half equations, ensure:

• Atoms are balanced on both sides
• Charges are balanced on both sides
• The number of electrons in oxidation equals the number in reduction
• State symbols may be required (g) for gases, (l) for liquids, (aq) for aqueous ions

Advantages of hydrogen fuel cells

Hydrogen fuel cells offer several environmental and practical benefits:

Environmental advantages:

• The only product when using pure hydrogen and oxygen is water — no carbon dioxide, sulfur dioxide, or nitrogen oxides are produced
• Zero emissions at the point of use make them ideal for reducing urban air pollution
• No particulates or harmful pollutants released
• Reduced contribution to climate change compared to fossil fuels
• Quieter operation than combustion engines

Practical advantages:

• More efficient than combustion engines (typically 40-60% efficiency versus 25-30% for petrol engines)
• Can be refuelled relatively quickly (compared to recharging batteries)
• Lightweight fuel compared to batteries for the same energy output
• Reliable power source for remote locations
• Can operate in extreme temperatures

Energy security:

• Hydrogen can be produced from various sources including water, reducing dependence on oil imports
• Can be generated using renewable electricity (solar, wind, tidal)
• Provides energy storage solution for excess renewable electricity

Challenges and disadvantages of hydrogen as a fuel

Despite the advantages, several challenges limit widespread adoption of hydrogen fuel cells:

Production challenges:

• Most hydrogen is currently produced from natural gas (a fossil fuel) through steam reforming, which produces carbon dioxide
• Producing hydrogen by electrolysis of water requires large amounts of electricity
• If the electricity comes from fossil fuels, the overall environmental benefit is reduced
• "Green hydrogen" (produced using renewable electricity) is currently expensive

Storage and distribution:

• Hydrogen is a very low-density gas, requiring compression to high pressures (up to 700 times atmospheric pressure) for storage
• Compressed hydrogen storage tanks are heavy and expensive
• Alternatively, hydrogen can be stored as a liquid, but this requires cooling to -253°C
• Building a hydrogen refuelling infrastructure is costly
• Hydrogen can leak easily due to its small molecular size
• Safety concerns as hydrogen is highly flammable

Cost issues:

• Fuel cells contain expensive catalysts, often platinum
• Hydrogen vehicles are currently more expensive than conventional cars
• Limited refuelling infrastructure exists
• Production costs remain high compared to fossil fuels

Efficiency considerations:

• The overall process (producing hydrogen, compressing/storing it, then converting it back to electricity) can be less efficient than using electricity directly in battery vehicles
• Energy losses occur at each conversion step

Comparing hydrogen fuel cells with other energy sources

Hydrogen fuel cells versus petrol/diesel engines:

Advantages of hydrogen:

• No harmful emissions at point of use
• Higher efficiency
• Renewable if produced from water using renewable electricity

Advantages of petrol/diesel:

• Established infrastructure
• Higher energy density per volume
• Lower vehicle cost
• Easier and safer to store

Hydrogen fuel cells versus batteries:

Advantages of hydrogen:

• Faster refuelling (minutes versus hours)
• Longer range for equivalent weight
• Better for heavy vehicles (lorries, buses)

Advantages of batteries:

• Higher overall efficiency (electricity to vehicle motion)
• Simpler technology
• Existing electrical grid infrastructure
• No need for hydrogen production and compression

Hydrogen fuel cells versus combustion of hydrogen:

Both use hydrogen as fuel, but fuel cells:

• Are more efficient (direct conversion to electricity)
• Operate at lower temperatures
• Produce no nitrogen oxides (formed at high combustion temperatures)
• Are quieter with no moving parts in the cell itself

Worked examples

Example 1: Half equations

Question: A hydrogen fuel cell uses hydrogen and oxygen to produce electricity and water.

(a) Write the half equation for the reaction at the negative electrode (anode). [2 marks]

(b) Write the half equation for the reaction at the positive electrode (cathode). [2 marks]

(c) Identify which half equation represents oxidation and explain your answer. [2 marks]

Answer:

(a) 2H₂ → 4H⁺ + 4e⁻ [1 mark for correct species, 1 mark for balancing]

Or H₂ → 2H⁺ + 2e⁻ is also acceptable

(b) O₂ + 4H⁺ + 4e⁻ → 2H₂O [1 mark for correct species, 1 mark for balancing]

(c) The anode/negative electrode half equation represents oxidation [1 mark] because hydrogen loses electrons / electrons are on the right-hand side of the equation / the oxidation state of hydrogen increases [1 mark].

Example 2: Evaluating hydrogen as a fuel

Question: Hydrogen fuel cells are being developed to power vehicles.

Evaluate the use of hydrogen fuel cells in cars compared to petrol engines. [6 marks]

Answer:

A good answer should include advantages and disadvantages with a conclusion:

Advantages:

• Hydrogen fuel cells produce only water as a product, whereas petrol engines produce carbon dioxide (greenhouse gas) and other pollutants [1 mark]
• Fuel cells are more efficient at converting chemical energy to electrical energy than petrol engines are at producing kinetic energy [1 mark]
• Hydrogen can be produced from renewable sources, making it sustainable, unlike petrol which is a finite fossil fuel [1 mark]

Disadvantages:

• Currently, most hydrogen is produced from natural gas which produces carbon dioxide, reducing environmental benefits [1 mark]
• Hydrogen requires storage at very high pressure or very low temperature, making it more difficult and expensive to store than petrol [1 mark]
• There are few hydrogen refuelling stations compared to petrol stations, limiting where vehicles can refuel [1 mark]

Maximum 6 marks available for relevant points including advantages, disadvantages, and ideally a balanced evaluation.

Example 3: Overall equation

Question: In a hydrogen fuel cell, hydrogen reacts with oxygen to produce water.

(a) Write the balanced symbol equation for this reaction. [2 marks]

(b) Explain why this reaction is described as exothermic. [2 marks]

Answer:

(a) 2H₂ + O₂ → 2H₂O [1 mark for correct species, 1 mark for balancing]

(b) The reaction is exothermic because it releases energy [1 mark] to the surroundings in the form of heat and electricity [1 mark].

Or: bonds formed in water molecules release more energy than is required to break bonds in hydrogen and oxygen molecules.

Common mistakes and how to avoid them

• Mixing up oxidation and reduction at electrodes — Remember "An Ox Red Cat": Anode = Oxidation (electrons lost, appear as products), Cathode = Reduction (electrons gained, appear as reactants). Hydrogen is oxidised at the anode, oxygen is reduced at the cathode.

• Forgetting to balance half equations — Always check that both atoms AND charges balance on each side. For example, O₂ + 4H⁺ + 4e⁻ → 2H₂O has 2 oxygen atoms and 0 charge on both sides.

• Confusing fuel cells with batteries — Fuel cells require continuous supply of reactants and produce electricity as long as fuel flows; batteries contain a fixed amount of reactants and need recharging. Make this distinction clear in exam answers.

• Stating hydrogen is a renewable fuel — Hydrogen is not a fuel source but an energy carrier. It must be produced from other sources. Whether it's sustainable depends on the production method (renewable electricity for electrolysis versus steam reforming of natural gas).

• Oversimplifying advantages or disadvantages — In evaluation questions, provide specific details. Instead of "hydrogen is cleaner," write "hydrogen fuel cells produce only water, unlike petrol engines which produce carbon dioxide and nitrogen oxides."

• Ignoring the production method when discussing environmental impact — Always consider the full lifecycle. Hydrogen produced from fossil fuels still contributes to carbon emissions, even though the fuel cell itself produces no carbon dioxide.

Exam technique for "Energy changes: fuel cells and hydrogen as a fuel"

• Command words matter — "Write the half equation" (2 marks) requires a balanced equation with electrons; "Describe" means give characteristics without explanation; "Explain" requires reasons using because/therefore; "Evaluate" needs advantages AND disadvantages with possibly a conclusion (typically 4-6 marks).

• Half equation questions — Show electrons clearly with correct positioning (products for oxidation, reactants for reduction). Balance atoms first, then ensure charges balance. State symbols may earn an extra mark if requested. Write e⁻ not just e.

• Six-mark evaluation questions — Use a structured approach with separate paragraphs for advantages and disadvantages. Aim for 3 developed points on each side. Include specific chemical facts (not vague statements). Link points to the question context. Quality of written communication is assessed.

• Show understanding of context — When discussing hydrogen vehicles, mention infrastructure challenges; when discussing environmental benefits, acknowledge the production method matters; when comparing with batteries, consider specific applications like buses versus cars.

Quick revision summary

Fuel cells convert chemical energy directly into electrical energy. In hydrogen fuel cells, hydrogen is oxidised at the anode (2H₂ → 4H⁺ + 4e⁻) and oxygen is reduced at the cathode (O₂ + 4H⁺ + 4e⁻ → 2H₂O), producing only water overall. Advantages include zero emissions at point of use and high efficiency. Disadvantages include production methods often using fossil fuels, expensive storage requiring high pressure, limited infrastructure, and high costs. Hydrogen is an energy carrier, not a source. Evaluation questions require balanced discussion of benefits and limitations.