Batteries Fuel Cells and Hydrogen
Electrochemical Cells (Batteries)
An electrochemical cell (commonly called a battery) produces a voltage (potential difference) from chemical reactions. They convert chemical energy into electrical energy.
Simple Cells
A simple cell is made by connecting two different metals in an electrolyte (a solution that conducts electricity, such as an acid or salt solution).
How it works:
1. The more reactive metal loses electrons more readily (oxidation)
2. These electrons flow through the external wire to the less reactive metal
3. This flow of electrons is an electric current
4. The voltage (potential difference) depends on the difference in reactivity between the two metals
Key Points
- The greater the difference in reactivity between the two metals, the higher the voltage
- If the same metal is used for both electrodes, the voltage is zero
- The more reactive metal is the negative electrode (it loses electrons)
- The less reactive metal is the positive electrode
| Metal pair | Approximate voltage |
|---|---|
| Zinc and copper | ~1.1 V |
| Magnesium and copper | ~2.7 V |
| Magnesium and zinc | ~1.6 V |
| Iron and copper | ~0.8 V |
Non-Rechargeable Batteries
In non-rechargeable (primary) batteries, the chemical reactions that produce the current are irreversible. When the reactants are used up, the battery goes flat and must be disposed of. Example: alkaline batteries (zinc–manganese dioxide).
Rechargeable Batteries
In rechargeable (secondary) batteries, the reactions are reversible. Passing a current in the opposite direction reverses the chemical reaction, restoring the reactants. Example: lithium-ion batteries (in phones, laptops, electric vehicles), lead-acid batteries (in cars).
Advantages of rechargeable batteries:
- Can be reused many times
- Less waste going to landfill
- Lower long-term cost
Disadvantages:
- Higher initial cost
- Eventually degrade (capacity decreases)
- Contain toxic metals that need careful disposal/recycling
Fuel Cells
A fuel cell is an electrochemical cell that produces a voltage by reacting a fuel (usually hydrogen) with oxygen. Unlike a battery, it does not store chemical energy internally — it is supplied continuously with fuel and oxygen.
Hydrogen Fuel Cell
The overall reaction in a hydrogen fuel cell is:
2H₂(g) + O₂(g) → 2H₂O(l)
The only product is water — no polluting gases are produced.
At the anode (negative): hydrogen is oxidised
2H₂ → 4H⁺ + 4e⁻
At the cathode (positive): oxygen is reduced
O₂ + 4H⁺ + 4e⁻ → 2H₂O
The electrons flow from anode to cathode through the external circuit, providing electrical energy.
Advantages of Hydrogen Fuel Cells
- No pollution at point of use — the only product is water
- No carbon dioxide — does not contribute to climate change during use
- High efficiency — more energy is converted to electricity than in combustion engines
- No noise pollution compared to internal combustion engines
Disadvantages of Hydrogen Fuel Cells
- Hydrogen production: most hydrogen is currently made from fossil fuels (steam reforming of methane), which does produce CO₂. Electrolysis of water is cleaner but energy-intensive
- Storage: hydrogen is a gas with very low density, requiring high-pressure tanks or cryogenic (very cold) storage
- Infrastructure: hydrogen refuelling stations are rare
- Flammability: hydrogen is highly flammable and explosive — safety concerns
- Platinum catalyst: fuel cells use expensive platinum as a catalyst
- Cost: fuel cell technology is currently expensive
Hydrogen as a Fuel
Hydrogen can also be burned directly as a fuel:
2H₂(g) + O₂(g) → 2H₂O(g)
This is a clean combustion reaction producing only water vapour. However, the same storage and production challenges apply.
Comparing Energy Sources
| Feature | Fossil fuel car | Hydrogen fuel cell car | Electric (battery) car |
|---|---|---|---|
| Emissions | CO₂, CO, NOx, particulates | Water only | None at point of use |
| Energy source | Petrol/diesel | Hydrogen | Grid electricity |
| Range | Good | Moderate | Moderate |
| Refuelling | Quick, widely available | Quick but limited stations | Slow charging |
| Overall CO₂ | High | Depends on H₂ source | Depends on grid source |
Exam Tips
- Know the difference between a battery (stores chemicals inside) and a fuel cell (supplied continuously)
- In a simple cell, the voltage depends on the reactivity difference between the metals
- Hydrogen fuel cells produce only water — but you must also discuss how the hydrogen was made for full marks
- Six-mark questions often ask you to evaluate hydrogen fuel cells vs fossil fuels — give advantages AND disadvantages with specific detail