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Batteries and Fuel Cells

Learn and apply Batteries and Fuel Cells in the published Chemistry course sequence.

  • GCE A-Level H2 Chemistry 9476-2027
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Batteries and Fuel Cells: Orientation

This lesson applies electrochemistry to real devices: how batteries differ from fuel cells, how to write the key half-equations, and how to use anode/cathode definitions consistently.

Before the harder applications, review Writing Redox Equations from Half-Equations and keep the Electrochemistry hub as your route map.

Definitions (Must Know)

A. Primary and secondary cells

  • Primary cell: non-rechargeable (cell reaction not easily reversed).
  • Secondary cell: rechargeable (cell reaction can be reversed using an external power supply).

B. Fuel cell

A fuel cell generates electricity continuously from a fuel and oxidant supplied from outside the cell.

Detailed Explanations

A. The hydrogen–oxygen fuel cell (alkaline version, common syllabus)

Half-equations (alkaline electrolyte):

Anode (oxidation): 2H₂ + 4OH⁻ → 4H₂O + 4e⁻

Cathode (reduction): O₂ + 2H₂O + 4e⁻ → 4OH⁻

Overall: 2H₂ + O₂ → 2H₂O

What to write:

  • hydrogen is oxidised at the anode (electrons released)
  • oxygen is reduced at the cathode (electrons gained)
  • electrons flow through the external circuit to do useful work

Because the oxidation and reduction happen at different electrodes, therefore electrons must flow through the external circuit from anode to cathode, producing an electric current.

B. Comparing fuel cells with combustion (exam-style points)

Advantages:

  • high efficiency (more energy converted directly to electrical work)
  • water is the main product (no CO₂ if pure hydrogen used)

Limitations:

  • hydrogen storage/transport challenges
  • catalysts can be expensive
  • fuel purity requirements (catalyst poisoning)

C. Primary vs secondary (what exam questions often want)

  • Primary: convenient, low maintenance, but waste and limited by reactant amount.
  • Secondary: rechargeable, but may be heavier/less energy-dense and degrade over time.

D. Workflow: writing the overall equation from half-equations

  1. Identify oxidation (anode) and reduction (cathode).
  2. Make electron numbers match (multiply if needed).
  3. Add half-equations and cancel electrons and any common ions.

Mini example (fuel cell, alkaline): Add the given half-equations and cancel 4e⁻ and 4OH⁻ to get 2H₂ + O₂ → 2H₂O.

Alkaline hydrogen–oxygen fuel cellHydrogen enters the negative anode and oxygen enters the positive cathode. Electrons released at the anode flow through an external load to the cathode. Hydroxide ions move through the electrolyte from cathode to anode. Water leaves the anode side. The overall reaction is two hydrogen molecules plus oxygen forming water.Alkaline hydrogen–oxygen fuel cellexternal loadelectrical energy outelectrons: anode → cathodeANODE (−)CATHODE (+)hydrogen isoxidisedoxygen isreducedelectrolyteions carry chargeH₂(g) inO₂(g) inH₂O(l) outOH⁻ ion movementcathode → anode through electrolyteOverall: 2H₂(g) + O₂(g) → 2H₂O(l)chemical energy → electrical energy
Use the particle and charge paths together: hydrogen is oxidised at the anode, electrons power the external circuit, oxygen is reduced at the cathode and hydroxide ions carry charge through the alkaline electrolyte.

Worked Examples

Modelled example 1

Primary versus Secondary Cells

Core

Problem

State one difference between a primary and secondary cell and link it to reaction reversibility.
Study the worked solution

  1. Classify primary cells

    Method

    Call a primary cell non-rechargeable.

    Reason

    Its discharge chemistry is not readily reversed by applying an external current.

    Working

    Primary: used until reactants are depleted.

  2. Classify secondary cells

    Method

    Call a secondary cell rechargeable.

    Reason

    An external current can reverse the cell reaction sufficiently to regenerate reactants.

    Working

    Secondary: discharge reaction can be reversed during charging.

Common misconception 2

Gas Roles in an Alkaline Fuel Cell

Find and correct the mistake

Learner claim

A learner says oxygen is consumed at the anode because the anode is the “positive oxygen electrode”. Correct the anode/cathode assignment using oxidation and reduction.

Use processes rather than memorised signs

Anode gas
Cathode gas

View solution step by step

  1. Assign the anode

    Method

    Consume hydrogen where oxidation occurs.

    Reason

    The anode is defined by oxidation in both galvanic and electrolytic cells.

    Working

    Anode: H₂ oxidised.

  2. Assign the cathode

    Method

    Consume oxygen where reduction occurs.

    Reason

    The cathode is defined by reduction.

    Working

    Cathode: O₂ reduced.

Challenge 3

Overall Hydrogen–Oxygen Fuel-Cell Equation

Minimal support

Process-integration transfer

Using the gas roles from the alkaline fuel cell, write the overall equation after cancelling electrons and any alkaline medium species.

Identify net reactants and product

Net product
H₂ : O₂ : H₂O

Hints

Hint 1: net chemistry
Hydrogen is oxidised and oxygen reduced; hydroxide and water may appear in half-equations but cancel in the net equation.
Hint 2: balance
Balance oxygen first, then hydrogen.
View solution step by step

  1. Identify net species

    Method

    Keep hydrogen and oxygen as reactants and water as product.

    Reason

    Electrons and medium species cancel when the electrode processes are added.

    Working

    H₂ + O₂ → H₂O skeleton.

  2. Balance the equation

    Method

    Use coefficients 2, 1 and 2.

    Reason

    This conserves four H atoms and two O atoms.

    Working

    2H₂ + O₂ → 2H₂O.

Mind Stretchers

Mind stretcher 1Extension

Why might a fuel cell be more efficient than burning the same fuel in an engine?

Show Hint

Compare the energy-conversion pathways and the main forms in which energy leaves each system.

Show Answer

Mark scheme:

  • A fuel cell converts chemical energy directly into electrical work, reducing energy losses as heat.
  • Heat engines are limited by thermodynamic efficiency and lose significant energy as waste heat.

Mind stretcher 2: Evaluating a transport-cell claimExtension

Question. A manufacturer says a new fuel-cell stack is better for a vehicle because it is smaller, lighter and produces a higher voltage than the previous stack. Explain why these are genuine possible advantages but are not, by themselves, enough to prove that the whole vehicle system is better.

Show Hint

Distinguish cell-stack properties from the mass, storage, supply and lifecycle requirements of the complete system.

Show Answer

Smaller size and lower mass can leave more space or payload, while a higher voltage can reduce the number of cells needed for a target output. However, a fuel cell needs a continuous external fuel and oxidant supply. Storage tanks, fuel production and transport, catalyst cost, durability and total system efficiency must also be compared. The cell-stack data support specific advantages but not the wider conclusion on their own.