Revision notes for AQA GCSE Chemistry Fuel cells. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Fuel cells
What you'll learn
- How a fuel cell is different from a normal chemical cell or rechargeable battery.
- How hydrogen and oxygen produce a potential difference in a hydrogen fuel cell.
- How to write the overall reaction, and the Higher Tier half equations.
- How to evaluate fuel cells compared with rechargeable cells and batteries.
The starting point: chemical cells
A chemical cell uses a chemical reaction to produce a potential difference. The potential difference is the “push” that makes electrons move around an external circuit, such as through a lamp or motor.
A battery is two or more cells connected together, although in everyday speech people often say “battery” for a single cell too.
Definition
Key cell words
- An electrode is a conductor where reactions happen.
- An electrolyte is a substance containing mobile ions, so charge can move through the cell.
- A potential difference is the voltage between two points, measured in volts, V.
- An external circuit is the wire-and-component pathway outside the cell where electrons flow.
In an ordinary chemical cell, the reacting chemicals are already inside the cell. Eventually they get used up, so the cell goes flat.
In a rechargeable cell, the chemical reaction can be reversed by connecting it to a charger. This stores energy again, but it takes time and the cell gradually wears out over many charge-discharge cycles.
What makes a fuel cell different?
A fuel cell is supplied continuously with a fuel, such as hydrogen, and oxygen or air from an external source.
The fuel is not simply stored inside the cell and used up. Instead, the fuel and oxygen keep being fed in, and the cell keeps producing a potential difference as long as the supplies continue.
Key Idea
Fuel cells are refuelled, not recharged
A hydrogen fuel cell can keep producing electricity while hydrogen and oxygen are supplied. It does not need recharging in the same way as a rechargeable cell, but it does need a steady fuel supply.
The word electrochemical means a chemical reaction involving electron transfer that produces electrical energy. In a fuel cell, the fuel is oxidised electrochemically.
Definition
Oxidation and reduction
Oxidation means loss of electrons. Reduction means gain of electrons. The memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain.
The hydrogen fuel cell
In a hydrogen fuel cell:
- hydrogen gas, H₂(g), is supplied to the negative electrode
- oxygen gas, O₂(g), or air is supplied to the positive electrode
- hydrogen reacts and releases electrons
- electrons flow through the external circuit, producing an electric current
- water is produced as the overall product
The overall reaction is:
2H₂(g) + O₂(g) → 2H₂O(l)
So the useful headline is: hydrogen and oxygen react to make water, but the energy is transferred electrically rather than by burning hydrogen in a flame.
This diagram shows an alkaline hydrogen fuel cell, including the ion movement and the Higher Tier half equations.
Common Mistake
Not the same as burning hydrogen
Do not describe a fuel cell as “hydrogen burning in oxygen”. In a fuel cell, hydrogen is oxidised electrochemically, so electrons are forced through an external circuit and can do useful work.
What happens at each electrode?
In a hydrogen fuel cell, the negative electrode is where hydrogen is oxidised. Electrons are produced there, so they leave through the external circuit.
The positive electrode is where oxygen is reduced. Electrons arrive there from the circuit and are used in the reaction.
This means the cell can power a device such as a motor:
- hydrogen reacts at the negative electrode and releases electrons
- electrons flow through the external circuit
- the flow of electrons transfers energy to the device
- reactions at the positive electrode use up the electrons
- ions move through the electrolyte to keep the charges balanced
Tip
Safer electrode wording
For GCSE, it is usually safest to say negative electrode and positive electrode. If you use anode/cathode, remember that in a fuel cell the anode is the negative electrode because oxidation happens there.
Higher Tier: half equations in a hydrogen fuel cell
This part is Higher Tier only.
For the alkaline hydrogen fuel cell, the half equations are:
Negative electrode:
2H₂(g) + 4OH⁻(aq) → 4H₂O(l) + 4e⁻
Hydrogen loses electrons, so hydrogen is oxidised.
Positive electrode:
O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)
Oxygen gains electrons, so oxygen is reduced.
Common Mistake
Half equations depend on the electrolyte
Different types of hydrogen fuel cell can use different electrolytes, so their half equations can look different. For AQA GCSE Chemistry Higher Tier, the alkaline OH⁻ version above is the one commonly taught.
Example
Combining the half equations
Show that the two half equations give the overall reaction.
-
Add the negative electrode equation and the positive electrode equation together:
2H₂(g) + 4OH⁻(aq) + O₂(g) + 2H₂O(l) + 4e⁻ → 4H₂O(l) + 4e⁻ + 4OH⁻(aq) -
Cancel the electrons, because 4e⁻ appears on both sides. Electrons are transferred within the cell, not used up overall.
-
Cancel the hydroxide ions, because 4OH⁻(aq) appears on both sides. They are regenerated in the cell.
-
Cancel 2H₂O(l) from both sides, leaving:
2H₂(g) + O₂(g) → 2H₂O(l)
Why fuel cells are considered useful
Hydrogen fuel cells are a possible alternative to rechargeable cells and batteries, especially in transport or places where quick refuelling is useful.
Their biggest attraction is that the product at the point of use is water. If the hydrogen is made using renewable electricity, the overall process can have low carbon dioxide emissions.
However, that does not automatically make every hydrogen fuel cell system “pollution-free”. Hydrogen has to be manufactured, transported and stored.
Common Mistake
Saying zero emissions too broadly
It is better to say water is the only product at the point of use. Producing the hydrogen may still release carbon dioxide if fossil fuels are used.
Fuel cells compared with rechargeable cells and batteries
| Feature | Hydrogen fuel cell | Rechargeable cell or battery |
|---|---|---|
| Energy supply | Needs hydrogen and oxygen or air supplied from outside | Stores chemicals inside the cell |
| Refuelling/recharging | Can be refuelled quickly if hydrogen is available | Must be recharged using electricity |
| Product at point of use | Water | No exhaust gases, but electricity source matters |
| Practical issues | Hydrogen is difficult to store and highly flammable | Batteries can be heavy and take time to recharge |
| Cost and infrastructure | Fuel cells and hydrogen tanks can be expensive | Charging networks and battery technology are already common |
| Lifetime and waste | Catalysts and components can be costly | Batteries degrade and need recycling or disposal |
A balanced evaluation should include both environmental and practical points.
Example
Choosing between fuel cells and rechargeable batteries
A bus company is choosing between hydrogen fuel cell buses and battery-electric buses. The buses need long range and quick turnaround at a central depot.
-
Compare the energy supply. Hydrogen fuel cells may suit long routes because the buses can be refuelled, rather than waiting for a long battery recharge.
-
Consider the environmental impact. If the depot makes hydrogen using renewable electricity, the fuel cell buses have water as the only point-of-use product and low carbon dioxide emissions overall.
-
Consider practical disadvantages. Hydrogen needs high-pressure storage, safety systems and refuelling equipment, so the starting cost may be high.
-
Make a judgement. Hydrogen fuel cells could be a good choice for this depot if quick refuelling and long range are more important than the extra infrastructure cost.
The core idea to remember
A hydrogen fuel cell turns the energy from the reaction between hydrogen and oxygen into electrical energy.
It does this by separating the oxidation and reduction reactions so that electrons have to travel through an external circuit.
Key Idea
Fuel cell summary
Hydrogen is oxidised at the negative electrode, oxygen is reduced at the positive electrode, electrons flow through the external circuit, and the overall product is water.
Exam technique
In the exam
-
When describing a fuel cell, include the words external supply, hydrogen, oxygen or air, oxidised electrochemically, and potential difference.
-
For evaluation questions, give both sides: water is the only product at point of use, but hydrogen production, storage, cost and infrastructure matter.
-
For Higher Tier half equations, check that electrons cancel and that the overall equation becomes 2H₂(g) + O₂(g) → 2H₂O(l).
Self review
Check yourself
- Why does a fuel cell not need recharging in the same way as a rechargeable cell?
- What is the overall reaction in a hydrogen fuel cell?
- What are two advantages and two disadvantages of hydrogen fuel cells compared with rechargeable batteries?
Chemical cells and fuel cells (chemistry only)
Guide 2 of 2
You've reached the end
Test yourself on this topic, or move on to the next guide.
How was this guide?