5.2.3Energy Changes
Electrode Reactions in Hydrogen Fuel Cells
5 min read 5 parts Lesson 8 of 8 Unit quiz
Section 1 of 5Structure of a Hydrogen Fuel Cell
A hydrogen fuel cell converts the chemical energy of hydrogen and oxygen directly into electrical energy.
Anode
- Anode The negative electrode, where hydrogen is supplied and loses electrons to form hydrogen ions.
Cathode
- Cathode The positive electrode, where oxygen gains electrons and combines with hydrogen ions to form water.
Electrolyte
- Electrolyte A membrane or solution that lets hydrogen ions pass through but does not allow electrons to pass,
- so electrons must travel round the external circuit.
External circuit
- External circuit The wire through which electrons flow from the anode to the cathode, producing the electric current.
Products
- Products Water is the only chemical product when the cell runs on pure hydrogen and oxygen, although some heat is also released.
Section 2 of 5Electrode Half-Equations
Each electrode reaction can be written as a half-equation, and adding the two gives the overall reaction.
1
At the anode each hydrogen molecule splits into two hydrogen ions and releases two electrons, which travel into the external circuit.
2
The hydrogen ions pass through the electrolyte towards the cathode, while the electrons go round the external circuit.
3
At the cathode oxygen molecules gain electrons and react with hydrogen ions, forming water molecules.
4
The electrons used at the cathode are replenished by those arriving from the anode, so the current continues while fuel is supplied.
5
Adding the two half-equations and cancelling the electrons and hydrogen ions gives the overall equation 2H₂(g) + O₂(g) → 2H₂O(l).
| Electrode | Half-equation | Type of change |
|---|---|---|
| Anode (negative) | 2H₂(g) → 4H⁺(aq) + 4e⁻ | Oxidation: electrons are lost |
| Cathode (positive) | O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l) | Reduction: electrons are gained |
| Overall | 2H₂(g) + O₂(g) → 2H₂O(l) | Electrons cancel out |
Section 3 of 5Advantages and Limits
Fuel cells have clear benefits, but their practical use depends on hydrogen supply and cost.
Advantages and Limits
Efficiency
- Efficiency Fuel cells are not limited by the heat-engine constraints of combustion, so they can convert more of the fuel energy into electricity.
Emissions
- Emissions Running on pure hydrogen produces no carbon dioxide at the point of use, although hydrogen production may release it.
Fuel supply
- Fuel supply Hydrogen must be stored and transported safely, which is a practical difficulty for widespread use.
Cost
- Cost Catalysts such as platinum make fuel cells expensive, which limits their use in some vehicles.
Section 4 of 5Worked Example
Question
- A hydrogen fuel cell uses 4.0 g of hydrogen gas completely.
- Calculate the amount in moles of electrons released and the mass of water produced.
- Relative formula mass of H₂ = 2, so moles of H₂ = 4.0 ÷ 2 = 2.0 mol.
- Each H₂ molecule releases 2 electrons (H₂ → 2H⁺ + 2e⁻), so electrons released = 2.0 × 2 = 4.0 mol.
- From 2H₂ + O₂ → 2H₂O, moles of water produced = moles of hydrogen = 2.0 mol.
- Relative formula mass of H₂O = 18, so mass of water = 2.0 × 18 = 36 g.
Answer 4.0 mol of electrons are released and 36 g of water is produced.
Section 5 of 5Exam Checklist & Tips
5.2.3 – Key Points for the Exam
- Anode The negative electrode of a fuel cell, where oxidation takes place.
- Cathode The positive electrode of a fuel cell, where reduction takes place.
- Electrolyte The medium that conducts ions between the electrodes but not electrons.
- Half-equation An equation showing only the electrons, ions or molecules involved in one electrode reaction.
- Oxidation A reaction in which a species loses electrons.
- Reduction A reaction in which a species gains electrons.
Common Mistakes
- Labelling the anode as positive; in a hydrogen fuel cell the anode is the negative electrode.
- Forgetting that electrons flow through the external circuit rather than through the electrolyte.
- Writing water as the product of the anode reaction, when water is formed at the cathode.
- Leaving the charge unbalanced in a half-equation; check the charges as well as the atoms.
Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.