Cells & Batteries Triple Only
An exothermic reaction normally releases its energy as heat. A cell is a clever way of getting that energy out as electricity instead: it contains chemicals that react to produce a voltage, which can push a current around a circuit. The simplest cell is just two different metals (the electrodes) dipped into an electrolyte — a liquid or solution containing ions that are free to move — and connected by a wire.
A simple cell: two different metals in an electrolyte. The bigger the difference in reactivity between the metals, the bigger the voltage.
The voltage of a simple cell depends on the type of electrodes (the two metals) and the electrolyte. The key rule for the metals:
- The bigger the difference in reactivity between the two metals, the bigger the voltage.
- Two of the same metal give a difference of zero, so the voltage is 0 V.
This means you can use cell voltages to put metals in order of reactivity — the pairs that are furthest apart in the reactivity series give the largest readings.
From cells to batteries
A single cell only gives a small voltage. A battery is simply two or more cells connected together in series, which adds their voltages up to give a greater total. Four 1.5 V cells in series, for example, make a 6.0 V battery.
Rechargeable or not?
- In a non-rechargeable cell or battery (such as an alkaline battery), the reaction stops when one of the reactants is used up. It cannot be recharged because the reaction is not reversible.
- A rechargeable cell or battery (such as a lithium-ion battery) can be recharged because passing an external current through it reverses the reactions, turning the products back into reactants.
This part of the course is deliberately light-touch. You need that a cell is two different metals (electrodes) in an electrolyte whose chemicals react to produce electricity, that a bigger difference in reactivity gives a bigger voltage, and that a cell goes flat once a reactant is used up — unless an external current can reverse the reaction (rechargeable). You don’t need the detail of what happens at each electrode: in AQA’s words, students “do not need to know details of cells and batteries other than those specified.” The Higher-Tier note below gives an optional first look, but the full picture — oxidation and reduction at the electrodes, electrode potentials and so on — is electrochemistry you’ll meet at A‑level.
In a simple cell the reaction is redox. The more reactive metal loses electrons (it is oxidised) and is the negative electrode; the less reactive metal gains them (it is reduced). For a zinc–copper cell:
Zn → Zn2+ + 2e− (oxidation — the negative electrode)
Cu2+ + 2e− → Cu (reduction — the positive electrode)
🧪 Exam-style questions
Which combination would produce a voltage greater than zero?
A single cell produces 1.5 V. How many of these cells must be connected in series to make a 12 V battery?
Show answer
- Cells in series add their voltages, so divide the target voltage by one cell’s voltage: 12 ÷ 1.5. 1 mark
- = 8 cells. 1 mark
An alkaline battery is non-rechargeable. Why does it eventually stop working?
Hydrogen Fuel Cells Triple Only
A fuel cell is different from an ordinary cell: instead of running down when its chemicals are used up, it is continuously supplied with fuel from outside. In a hydrogen fuel cell, hydrogen and oxygen (from the air) are fed in, the hydrogen is oxidised electrochemically, and this produces a voltage (a potential difference). The only product is water.
hydrogen + oxygen → water 2H2 + O2 → 2H2O
The diagram below runs as an interactive. Choose an acidic or an alkaline electrolyte and watch the cell work: each side has an entry tube at the top and an exit tube below — hydrogen and oxygen are fed in, the gas that doesn’t react leaves again, and the product water leaves with it.
Choose an electrolyte to start the fuel cell — then watch the ions cross the electrolyte, electrons drive the lamp, and water leave at the bottom.
In the animation the lamp is only there to show that the circuit is complete and that a current is flowing. The fuel cell itself is just a power supply — like a battery — so it could drive any electrical component instead: a motor to turn a car’s wheels, a phone to charge, a heater, and so on. The lamp simply makes the current easy to see.
Back in electrolysis we said the anode was positive — so why is it the negative terminal here? The trap is assuming that “anode” means “the positive electrode”. It doesn’t. Anode just means the electrode where oxidation happens (and cathode the one where reduction happens). Whether that electrode turns out positive or negative depends on whether electricity is being used or generated.
In electrolysis, a power supply forces electrons onto the cathode. So:
- the cathode is negative — it receives electrons from the power supply;
- the anode is positive — electrons are pulled away from it by the power supply.
At the anode, negative ions lose those electrons — for example, in a chloride:
2Cl− → Cl2 + 2e−
Losing electrons is oxidation, so oxidation happens at the anode.
In a cell, nothing forces the electrons: the cell generates electricity, because the chemical reaction itself pushes electrons around the circuit. Hydrogen is oxidised at the anode:
H2 → 2H+ + 2e−
Those electrons are produced at the anode, so the anode becomes the negative terminal. They then flow through the wire to the positive cathode — the very current that lights the lamp above.
You will not be expected to use the terms “anode” and “cathode” for electrochemical cells at GCSE — but if the labels ever seem to flip, this is why.
The reaction is redox: hydrogen is oxidised and oxygen is reduced. With an acidic electrolyte:
Anode (oxidation): 2H2 → 4H+ + 4e−
Cathode (reduction): O2 + 4H+ + 4e− → 2H2O
Add them together and the H+ and e− cancel, leaving the overall 2H2 + O2 → 2H2O.
Some hydrogen fuel cells use an alkaline electrolyte (containing OH− ions) instead of an acidic one. The overall reaction is the same, but the half equations are written using OH−:
Anode (oxidation): H2 + 2OH− → 2H2O + 2e−
Cathode (reduction): O2 + 2H2O + 4e− → 4OH−
It is called an alkaline fuel cell simply because its electrolyte contains OH− ions.
Oxygen is the thing that does the oxidising (the oxidising agent), so in a fuel cell oxygen is itself reduced — never say oxygen is oxidised. And when comparing hydrogen with a fossil fuel, the marks come from the application point: using hydrogen instead of petrol or methane would reduce the rate of climate change, because the only product is water (no carbon dioxide).
Fuel cells vs rechargeable batteries
A favourite exam question asks you to evaluate hydrogen fuel cells against rechargeable batteries — for example to power a car. There are good points on both sides:
| Hydrogen fuel cells — for | Hydrogen fuel cells — against |
|---|---|
| Refuel in minutes (no long recharge); keeps making electricity for as long as hydrogen is supplied, rather than running down; can give a greater range; lighter than a large battery for long journeys; only product is water, so no pollutants at the point of use; no toxic battery chemicals to dispose of | Hydrogen is hard to store and is highly flammable/explosive; few hydrogen filling stations; making hydrogen needs a lot of energy and often uses fossil fuels; expensive to produce |
🧪 Exam-style questions
Balance the overall equation for the reaction in a hydrogen fuel cell.Type a balancing number in each box (leave it as 1 if none is needed), then press Check. Any correct set of numbers is accepted.
Show answer
2H2 + O2 → 2H2O
- Two water molecules need 4 H and 2 O, so you need 2H2 and 1O2. The numbers are 2 : 1 : 2. 1 mark
Which is an advantage of a hydrogen fuel cell over a rechargeable battery for powering a car?
The table shows data about two ways of powering electric cars.
| Hydrogen fuel cell | Rechargeable lithium-ion battery | |
|---|---|---|
| Time taken to refuel or recharge in minutes | 5 | 30 |
| Distance travelled before refuelling or recharging in miles | Up to 415 | Up to 240 |
| Distance travelled per unit of energy in km | 22 | 66 |
| Cost of refuelling or recharging in £ | 50 | 3 |
| Minimum cost of car in £ | 60 000 | 18 000 |
Evaluate the use of hydrogen fuel cells compared with rechargeable lithium-ion batteries to power electric cars. Use the table and your own knowledge. This is a levels-of-response question — use points from both the data and your own knowledge, weigh both sides, and finish with a justified conclusion. Plan, then compare with the model answer.
Show a model answer
How it is marked (levels of response):
- Level 3 (5–6): a judgement, strongly linked and logically supported by a sufficient range of correct reasons.
- Level 2 (3–4): some logically linked reasons; perhaps a simple judgement.
- Level 1 (1–2): relevant points, but not logically linked.
From the table — for the fuel cell: refuelling takes only 5 minutes compared with 30 minutes to recharge, and the range is much greater (up to 415 miles vs up to 240 miles), so it suits long journeys.
From the table — for the battery: it uses energy far more efficiently (66 km per unit of energy vs 22 km); recharging costs £3 against £50 for refuelling; and the car itself is far cheaper (£18 000 vs £60 000).
From your own knowledge — for the fuel cell: the only product is water, so no pollutants at the point of use; hydrogen can be renewable if made by electrolysis using renewable energy; lithium-ion batteries have a finite life (eventually they cannot be recharged), can catch fire, and may release toxic chemicals on disposal.
From your own knowledge — for the battery: charging points are far more widely available than hydrogen filling stations; hydrogen is difficult to store (high pressure or very low temperature) and is highly flammable/explosive; hydrogen is often made from fossil fuels, so it is not necessarily renewable; the catalyst in a fuel cell eventually becomes poisoned, so fuel cells also have a finite life.
To reach Level 3 you must end with a justified conclusion that links back to your reasons — for example, “the battery is better for most drivers because the car costs a third as much and each charge is far cheaper, but the fuel cell would suit long-distance driving because of its 415-mile range and 5-minute refuelling, provided the hydrogen is made using renewable energy.” Either judgement can earn full marks — what matters is that the data and knowledge points support it.