Hydrogen–oxygen fuel cells and useful electricity
Distinguish a cell that supplies electricity from electrolysis that needs an electrical input.
Core and Extended · Chemistry 0620, 2026–2028.
Before you begin
- Read a balanced equation involving H₂, O₂ and H₂O.
- Recognise that energy can be transferred between chemical and electrical forms.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which balanced equation represents hydrogen reacting with oxygen to form water?
What you will learn
- State the reactants, useful output and only chemical product of a hydrogen–oxygen fuel cell.
- Distinguish fuel-cell operation from the overall water-electrolysis process.
Fuel and oxygen supply a reaction
A hydrogen–oxygen fuel cell uses a reaction between hydrogen and oxygen to produce electricity. Water is its only chemical product in the overall reaction 2H₂+O₂→2H₂O. Heat is also transferred, but heat is an energy transfer, not an additional chemical substance. Fuel and oxygen must be supplied for continued operation.
The overall reaction is similar to hydrogen combustion, but a fuel cell provides an electrical output through electrode reactions rather than simply burning a hydrogen flame to drive an engine. Do not say that electricity is a new molecule, or that water is the fuel used up by the hydrogen–oxygen cell. Hydrogen and oxygen are its input reactants.
Electrolysis and the fuel cell have opposite energy roles
Water electrolysis uses an electrical input to obtain hydrogen and oxygen. A hydrogen–oxygen fuel cell uses hydrogen and oxygen to obtain an electrical output and water. This distinction connects the chemical equation with the energy direction; reversing the overall reaction does not make an energy-free cycle.
Producing, storing and delivering hydrogen requires resources and energy. The Core statement about water as the cell's only chemical product describes operation of that cell, not the environmental impact of the entire fuel supply. The Extended comparison lesson considers those wider steps without changing the Core reaction identification.
Pause and explain
Which statement describes the fuel cell rather than water electrolysis?
Worked example
Complete an input–output diagram for a hydrogen–oxygen fuel cell and compare it with water electrolysis.
Show the worked solution
- Fuel-cell chemical inputs are H₂ and O₂; useful output is electricity and chemical product is H₂O.
- Its balanced overall reaction is 2H₂+O₂→2H₂O.
- Water electrolysis instead needs an electrical input to split water into hydrogen and oxygen.
Answer Fuel cell: H₂ + O₂ → water with electrical output. Electrolysis: water → H₂ + O₂ with electrical input.
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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.
List the chemical inputs, chemical product and useful energy output of a hydrogen–oxygen fuel cell.
Give me a hint
Keep material substances separate from energy transfers.
Compare my reasoning
- Chemical inputs are hydrogen and oxygen.
- The chemical product is water.
- Useful energy output is electricity; heat can also be transferred to the surroundings.
Inputs H₂ and O₂; product water; useful electrical output.
Look for these in your work
- Reactants and product are named correctly.
- Energy is distinguished from chemical material.
Explain why a fuel cell's production of water does not mean water is its fuel.
Give me a hint
Fuel names the input that supplies chemical energy, not the product formed.
Compare my reasoning
- Hydrogen is supplied as a reactant together with oxygen.
- Their reaction forms water and gives an electrical output.
- Water is on the product side, so its appearance does not identify it as the fuel.
Hydrogen is the supplied fuel; water is the reaction product.
Look for these in your work
- The equation's input and output sides are used.
- The term fuel is connected with the chemical input.
A claim says water electrolysis followed by a fuel cell supplies free energy forever because the water returns. Explain the missing energy accounting.
Give me a hint
A recycled material does not mean every energy transfer is recovered.
Compare my reasoning
- Electrolysis requires an electrical input to make hydrogen and oxygen.
- The fuel cell returns water while supplying electrical output from those reactants.
- Energy losses and other required inputs mean returning the water does not create unlimited or free energy.
The cycle returns material but requires electrolysis energy and has losses; it cannot supply free energy forever.
Look for these in your work
- Material conservation is distinguished from energy recovery.
- The electrical input is included.
Common mistakes
- Calling heat a second chemical product.
- Claiming a fuel cell supplies unlimited electricity without fuel.
- Treating hydrogen production and fuel-cell operation as one energy-free cycle.
What can you explain now?
What is the only chemical product of the overall hydrogen–oxygen fuel-cell reaction?
Compare with the explanation
Water
Hydrogen reacts with oxygen to form water while the cell supplies electricity; electrical output and heat are not chemical products.
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Make it stick
Draw separate fuel-cell and electrolysis diagrams with arrows for substances and energy. Name the fuel cell's only chemical product and explain why it still needs a fuel supply.
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.