C16: Chemical Cells and Fuel Cells
How cells and fuel cells produce electricity
Chemical Cells And Fuel Cells
📋 Key Definitions
Chemical cell: A device that converts chemical energy into electrical energy. It contains two different metals (electrodes) in an electrolyte. A voltage is produced because one metal is more reactive than the other.
Fuel cell: A cell that produces electrical energy from a fuel (e.g. hydrogen) and an oxidant (e.g. oxygen), as long as they are supplied. Unlike chemical cells, fuel cells do not run down or need recharging.
Voltage: The potential difference produced by a cell. The greater the difference in reactivity between the two metals, the greater the voltage produced.
Hydrogen fuel cell:
Overall: 2H₂ + O₂ → 2H₂O
At the anode: 2H₂ → 4H⁺ + 4e⁻
At the cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O
🔋 Chemical Cells
A simple chemical cell consists of two different metals connected by a wire, with both metals dipped into an electrolyte (a liquid that conducts electricity). The more reactive metal loses electrons more readily, creating a flow of electrons through the wire — this is an electric current.
How a chemical cell works: The more reactive metal becomes the negative electrode because it more readily gives up electrons. These electrons flow through the external circuit to the less reactive metal. The further apart the two metals are in the reactivity series, the greater the voltage.
| Metal Pair | Reactivity Difference | Voltage Produced |
|---|---|---|
| Lithium and Copper | Large (far apart) | High |
| Zinc and Copper | Moderate | Medium |
| Iron and Zinc | Small (close together) | Low |
Worked Example 1: Predicting voltage
Which cell produces the larger voltage: Mg/Cu or Zn/Cu?
Magnesium is further from copper in the reactivity series than zinc is.
The greater the difference in reactivity, the greater the voltage.
Mg/Cu produces the larger voltage.
Worked Example 2: Why cells stop working
Explain why a simple chemical cell eventually stops producing a voltage.
Over time, the chemicals in the cell are used up. The more reactive metal is gradually consumed as it dissolves into the electrolyte. Once one of the reactants is depleted, the cell can no longer produce a voltage and must be replaced or recharged.
⚡ Types of Chemical Cell
| Type | Features | Examples |
|---|---|---|
| Primary (non-rechargeable) | Chemicals are used up; cannot be recharged | Zinc-carbon, alkaline batteries |
| Secondary (rechargeable) | Can be recharged by reversing the reaction | Lithium-ion, lead-acid car batteries |
🚀 Hydrogen Fuel Cells
A hydrogen fuel cell produces electricity by reacting hydrogen with oxygen. The only product is water, making it a very clean energy source.
How a hydrogen fuel cell works: Hydrogen enters at the anode and is oxidised to H⁺ ions and electrons. The electrons flow through the external circuit (producing electricity) to the cathode. Oxygen enters at the cathode and reacts with H⁺ ions and electrons to form water.
Worked Example 3: Half-equations for hydrogen fuel cell
Write the half-equations for a hydrogen fuel cell.
At the anode (+): 2H₂ → 4H⁺ + 4e⁻ (hydrogen is oxidised)
At the cathode (−): O₂ + 4H⁺ + 4e⁻ → 2H₂O (oxygen is reduced)
Overall: 2H₂ + O₂ → 2H₂O
Worked Example 4: Comparing fuel cells and chemical cells
Compare a hydrogen fuel cell with a rechargeable chemical cell.
Both convert chemical energy to electrical energy. However, a fuel cell will continue to produce electricity as long as hydrogen and oxygen are supplied — it does not run down. A rechargeable cell stores a fixed amount of chemicals and will run down, but can be recharged. A fuel cell does not need recharging but needs a constant fuel supply.
⚖️ Advantages and Disadvantages of Hydrogen Fuel Cells
| Advantages | Disadvantages |
|---|---|
| Only product is water — no pollution or greenhouse gases | Hydrogen is flammable — risk of explosions |
| No carbon dioxide produced — helps tackle climate change | Hydrogen is often produced from fossil fuels or by electrolysis of water (which uses electricity) |
| Continuous supply of fuel — does not run down | Hydrogen is difficult to store and transport (low density gas) |
| More efficient than combustion engines | Limited refuelling infrastructure for hydrogen |
| Quiet operation with no moving parts | Expensive to produce and maintain |
❓ Practice Questions
Q1: Foundation Explain why a chemical cell needs two different metals and an electrolyte.
Q2: Foundation Which would produce the larger voltage — a cell using iron and copper, or a cell using magnesium and copper? Explain your answer.
Q3: Higher Write the half-equations and overall equation for a hydrogen fuel cell.
Q4: Foundation Give two advantages and two disadvantages of hydrogen fuel cells compared to petrol engines.
Q5: Higher Explain why a hydrogen fuel cell does not need recharging but a rechargeable battery does.
✅ Answers
- Two different metals are needed so one is more reactive than the other, creating a voltage (electrons flow from the more reactive metal). The electrolyte allows ions to move between the electrodes, completing the circuit.
- Magnesium and copper produces the larger voltage. Magnesium is further from copper in the reactivity series than iron is. The greater the difference in reactivity, the greater the voltage.
- Anode: 2H₂ → 4H⁺ + 4e⁻. Cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O. Overall: 2H₂ + O₂ → 2H₂O
- Advantages: only water produced (no pollution); no CO₂ emissions. Disadvantages: hydrogen is flammable; hydrogen is difficult to store and transport; hydrogen may be produced from non-renewable sources.
- A fuel cell does not need recharging because hydrogen and oxygen are continuously supplied from external sources — the cell produces electricity as long as fuel is provided. A rechargeable battery has a fixed amount of chemicals inside; when these are used up, the battery must be recharged to reverse the chemical reactions.
🎯 Exam Tips
- For voltage questions: the further apart two metals are in the reactivity series, the greater the voltage
- Learn the hydrogen fuel cell half-equations — they are commonly tested
- In the fuel cell: oxidation happens at the anode (hydrogen loses electrons), reduction at the cathode (oxygen gains electrons)
- When discussing advantages/disadvantages of fuel cells, give balanced arguments
- Remember that although hydrogen fuel cells are clean, the hydrogen itself may not be — if produced from fossil fuels, there are still environmental issues
- Do not confuse chemical cells (which run down) with fuel cells (which keep going as long as fuel is supplied)
🔢 Maths Skills
Mathematical Skills
Comparing voltages: The voltage produced by a chemical cell depends on the difference in reactivity between the two metals. The further apart two metals are in the reactivity series, the greater the voltage.
Example: If a Zn/Cu cell produces 1.1 V and a Mg/Cu cell produces a higher voltage, this is consistent with Mg being further above Cu in the reactivity series than Zn is.
⚠️ Common Misconceptions
Watch Out!
Fuel cells burn hydrogen like a combustion reaction. Wrong: fuel cells burn hydrogen Correct: fuel cells use an electrochemical process — hydrogen is oxidised at the anode and oxygen is reduced at the cathode; there is no combustion and no flame
Rechargeable batteries last forever. Wrong: rechargeable batteries last forever Correct: rechargeable batteries degrade over time — each charge-discharge cycle slightly reduces their capacity until they eventually need replacing
✍️ 6-Mark Question
Extended Answer
6 marks: Evaluate hydrogen fuel cells versus rechargeable batteries for use in vehicles.
Hydrogen fuel cells produce only water as a product, so they produce no CO₂ or polluting emissions at the point of use. They do not run down as long as hydrogen fuel is supplied, so refuelling is quick. However, hydrogen is difficult to store and transport (it is a low-density gas), there is limited refuelling infrastructure, and producing hydrogen often requires fossil fuels or electricity (which may come from non-renewable sources). Rechargeable batteries are widely available with extensive charging infrastructure. They are cheaper to produce and can be recharged from the mains. However, they have a limited range per charge, take a long time to recharge, and the batteries degrade over time. Battery production also involves mining lithium and other metals, which has environmental impacts. Overall, both have advantages and disadvantages; the best choice depends on the application, available infrastructure and energy sources.
Mark scheme: 1 mark for fuel cell advantage; 1 mark for fuel cell disadvantage; 1 mark for battery advantage; 1 mark for battery disadvantage; 1 mark for second advantage/disadvantage for either; 1 mark for balanced evaluation/conclusion.
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
The table shows data for three vehicle power sources:
| Power source | Range (km) | Refuel time | CO₂ at point of use | Cost per km |
|---|---|---|---|---|
| Petrol engine | 600 | 5 min | High | High |
| H₂ fuel cell | 500 | 5 min | Zero | Medium |
| Li-ion battery | 300 | 60 min | Zero | Low |
Question: A taxi company needs vehicles with long range and quick refuelling. Which power source is most suitable? Justify your answer using the data.
Answer: The hydrogen fuel cell is most suitable. It offers a long range (500 km) and quick refuelling (5 min), which matches the taxi company's requirements. Unlike the petrol engine, it produces zero CO₂ at the point of use. The Li-ion battery has zero CO₂ but its range is only 300 km and it takes 60 minutes to recharge, which would cause significant downtime for a taxi that needs to be on the road.
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