What is a hydrogen–oxygen fuel cell and how does it work?

A hydrogen–oxygen fuel cell is a type of electrochemical cell that converts chemical energy from hydrogen and oxygen into electrical energy. It works by having hydrogen gas (H2) enter the anode side of the cell, where it is split into protons and electrons. The electrons travel through an external circuit, providing electrical power, while the protons move through an electrolyte to the cathode. At the cathode, oxygen gas (O2) combines with the electrons and protons to form water (H2O), completing the reaction.

What are the advantages of using hydrogen–oxygen fuel cells?

Hydrogen–oxygen fuel cells offer several advantages, including high efficiency, as they convert chemical energy directly into electrical energy with minimal energy loss. They produce only water as a byproduct, making them environmentally friendly and reducing pollution. Additionally, they operate quietly and can be used in a variety of applications, from powering vehicles to providing backup power for buildings.

What are the main components of a hydrogen–oxygen fuel cell?

The main components of a hydrogen–oxygen fuel cell include the anode, cathode, and electrolyte. The anode is where hydrogen gas is oxidized, releasing electrons and protons. The cathode is where oxygen gas is reduced, combining with electrons and protons to form water. The electrolyte is a membrane that allows protons to pass through while preventing electrons from doing so, forcing them to travel through an external circuit and generate electricity.

How does a hydrogen–oxygen fuel cell differ from a traditional battery?

A hydrogen–oxygen fuel cell differs from a traditional battery in that it requires a continuous supply of hydrogen and oxygen to generate electricity, whereas a battery stores chemical energy and can operate independently until its energy is depleted. Fuel cells can continuously produce electricity as long as fuel is supplied, making them suitable for applications requiring sustained power. In contrast, batteries need to be recharged once their stored energy is used up.

What are the challenges associated with hydrogen–oxygen fuel cells?

Challenges associated with hydrogen–oxygen fuel cells include the storage and transportation of hydrogen, which is highly flammable and requires special handling. Additionally, the production of hydrogen often relies on fossil fuels, which can negate some environmental benefits. The cost of fuel cell technology is also relatively high, although ongoing research aims to reduce these costs and improve the efficiency and durability of fuel cells.

Why is "Treating a fuel cell like electrolysis (electricity in)" a common mistake in Hydrogen–oxygen fuel cells?

Why it happens: Both involve electrodes. How to avoid it: Electrolysis: electricity IN, makes products. Fuel cell: fuel + oxidant IN, makes electricity. Source: 0620/42 — recurring

Why is "Saying $\text{CO}_{2}$ is produced" a common mistake in Hydrogen–oxygen fuel cells?

Why it happens: Generalising from petrol engines. How to avoid it: H_2/O_2 fuel cell produces ONLY water at the cell.

Why is "Saying hydrogen fuel is automatically clean" a common mistake in Hydrogen–oxygen fuel cells?

Why it happens: Looking only at the cell. How to avoid it: Need to consider how the H_2 is produced — currently mostly from fossil fuels.

ElectrochemistryCambridge IGCSE Chemistry (0620)

Hydrogen–oxygen fuel cells

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Hydrogen-Oxygen Fuel Cells — Cambridge IGCSE 0620 Chemistry Extended (2026)

$\text{H}_{2} + \frac{1}{2}\text{O}_{2} \to \text{H}_{2}\text{O}$ — the only product is water. Generates electrical energy directly. Compare with combustion engines and rechargeable batteries.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

5.2 — Describe the construction of a hydrogen-oxygen fuel cell.
5.2 — State the half-equations and overall equation.
5.2 — Discuss advantages and disadvantages compared with petrol engines.

How a hydrogen-oxygen fuel cell works

H₂ in at one electrode, O₂ at the other. They react to make water; electrons flow through the external circuit.

Construction.

Two porous electrodes (often platinum-coated).
Electrolyte between them — typically KOH solution or a proton-exchange membrane (PEM).
H₂ flows in at one electrode, O₂ at the other.

Reactions (alkaline KOH electrolyte).

At the anode (where H₂ enters): $2\text{H}_{2} + 4\text{OH}^{-} \to 4\text{H}_{2}\text{O} + 4\text{e}^{-}.$
At the cathode (where O₂ enters): $\text{O}_{2} + 2\text{H}_{2}\text{O} + 4\text{e}^{-} \to 4\text{OH}^{-}.$
Overall: $2\text{H}_{2} + \text{O}_{2} \to 2\text{H}_{2}\text{O}.$

Where the electrons go. They leave the anode, flow through the external circuit (powering whatever's connected), and return to the cathode. That flow IS the useful electric current.

Energy conversion. Chemical energy of H₂ + O₂ → electrical energy + a bit of heat.

Cambridge tip. When asked "what is the only chemical product of a hydrogen-oxygen fuel cell?" — answer: water.

H₂ at anode, O₂ at cathode.
Net reaction: $2\text{H}_{2} + \text{O}_{2} \to 2\text{H}_{2}\text{O}$ .
Electrons flow through external circuit.
Only product: water.

Advantages and disadvantages

Clean and efficient, but H₂ is hard to handle.

Advantages of hydrogen fuel cells.

Only product is water. No CO₂, no NOₓ, no particulates.
High efficiency. Direct chemical → electrical conversion (no combustion-engine heat losses). Modern PEM cells $\sim 60\%$ efficient.
Quiet. No moving parts at the energy-conversion stage.
Can be refuelled while running. Unlike batteries which must be removed and recharged.
No emissions of greenhouse gases AT THE POINT OF USE.

Disadvantages.

Hydrogen storage. H₂ is the smallest molecule, leaks easily. Must be stored under high pressure or as a liquid at very low temperature.
Hydrogen production. Most H₂ today is made from natural gas (steam reforming) — releases CO₂. Only "green hydrogen" (from electrolysis of water using renewable electricity) is truly carbon-free.
Cost. Platinum electrodes are expensive. Infrastructure (refuelling stations) is limited.
Energy density. Storing enough H₂ for long-range vehicles is challenging.

Comparison with combustion engines.

Property	Fuel cell	Combustion engine
Efficiency	$\sim 60\%$	$\sim 25\%$
CO₂ emissions	None directly	Major emitter
Other emissions	None	NOₓ, particulates
Refuelling time	Minutes	Minutes
Range	$300-400\,\text{km}$ typical	$500+\,\text{km}$
Cost	High (currently)	Lower (mass-produced)

Comparison with batteries.

Batteries store chemical energy internally; must be recharged.
Fuel cells use externally-supplied fuel; can run as long as fuel is available.
Both produce electricity directly; fuel cells offer faster refuelling.

Pros: only water; high efficiency; quiet; no emissions.
Cons: H₂ storage / production / cost.
Beats combustion engines on efficiency and emissions.
Different to batteries (external fuel vs internal store).

How it’s examined

Hydrogen fuel cells appear most years on Paper 4 (4-6 marks): half-equations, overall equation, advantages over petrol engines. Examiner reports flag students saying 'no pollution' — they should specify 'no pollution AT POINT OF USE' and acknowledge that production of H₂ may not be clean.

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (5.2); 0620/22 May/Jun 2024 — Q9 (fuel cell); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-06.

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Step-by-step worked examples — Hydrogen–oxygen fuel cells

Step-by-step solutions to past-paper-style questions on hydrogen–oxygen fuel cells, written exactly the way a tutor would explain them at the board.

1Overall reaction in a fuel cell
Extended• Adapted from 0620/42 May/Jun 2024 Q12• overall
Question
Write the overall equation for a hydrogen-oxygen fuel cell.
Step-by-step solution
1. Step 1
  Hydrogen burns with oxygen to form water — the SAME overall reaction.
  $2\text{H}_{2}(\text{g}) + \text{O}_{2}(\text{g}) \to 2\text{H}_{2}\text{O}(\text{l})$
Answer
$2\text{H}_{2} + \text{O}_{2} \to 2\text{H}_{2}\text{O}$
2Half-equations in a fuel cell
Extended• half-equations
Question
Write the half-equations at each electrode (alkaline cell).
Step-by-step solution
1. Step 1
  Anode (negative — fuel side).
  $2\text{H}_{2} + 4\text{OH}^{-} \to 4\text{H}_{2}\text{O} + 4e^{-}$
2. Step 2
  Cathode (positive — oxygen side).
  $\text{O}_{2} + 2\text{H}_{2}\text{O} + 4e^{-} \to 4\text{OH}^{-}$
Answer
Anode: $2\text{H}_{2} + 4\text{OH}^{-} \to 4\text{H}_{2}\text{O} + 4e^{-}$ . Cathode: $\text{O}_{2} + 2\text{H}_{2}\text{O} + 4e^{-} \to 4\text{OH}^{-}$ .
3Compare with a petrol engine
Extended• compare
Question
State two advantages and one disadvantage of a hydrogen-oxygen fuel cell compared with a petrol engine.
Step-by-step solution
1. Step 1
  Advantages: water is the only product (no $\text{CO}_{2}$ or pollutants); higher efficiency than combustion.
2. Step 2
  Disadvantage: hydrogen is hard to store (low density, must be compressed or liquefied); production of $\text{H}_{2}$ may itself use fossil fuels.
Answer
Only water emitted, higher efficiency. − Hydrogen is hard to store; production may not be clean.

Key Definitions and Keywords — Hydrogen–oxygen fuel cells

Definitions to memorise and the exact keywords mark schemes credit for hydrogen–oxygen fuel cells answers — sharpened from recent examiner reports for the 2026 0620 sitting.

Fuel cell
Examiner keyword
An electrochemical cell that converts the energy of a fuel directly into electricity, using continuous supplies of fuel and oxidant.
Electrochemical reaction
Examiner keyword
A chemical reaction in which electron transfer between species generates (or is driven by) electrical energy.

Common Mistakes and Misconceptions — Hydrogen–oxygen fuel cells

The traps other students keep falling into on hydrogen–oxygen fuel cells questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Treating a fuel cell like electrolysis (electricity in)
0620/42 — recurring
Why it happens
Both involve electrodes.
How to avoid it
Electrolysis: electricity IN, makes products. Fuel cell: fuel + oxidant IN, makes electricity.
✕Saying $\text{CO}_{2}$ is produced
Why it happens
Generalising from petrol engines.
How to avoid it
$\text{H}_{2}/\text{O}_{2}$ fuel cell produces ONLY water at the cell.
✕Saying hydrogen fuel is automatically clean
Why it happens
Looking only at the cell.
How to avoid it
Need to consider how the $\text{H}_{2}$ is produced — currently mostly from fossil fuels.

Practice questions

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Past paper style quiz

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Hydrogen–oxygen fuel cells — frequently asked questions

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Hydrogen–oxygen fuel cells

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Overall reaction in a fuel cell

2Half-equations in a fuel cell

3Compare with a petrol engine

Fuel cell

Electrochemical reaction

✕Treating a fuel cell like electrolysis (electricity in)

✕Saying CO2\text{CO}_{2}CO2​ is produced

✕Saying hydrogen fuel is automatically clean

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Hydrogen–oxygen fuel cells — frequently asked questions

✕Saying $\text{CO}_{2}$ is produced