Chemical cells and fuel cells
Cells and batteries
A turns chemical energy into electrical energy. It contains chemicals that react to produce a — for example, two dipped in an .
The voltage depends on the metals and the electrolyte. The bigger the difference in reactivity between the two metals, the bigger the voltage. Magnesium and copper give a bigger voltage than zinc and copper, because magnesium is further above copper in the reactivity series.
A is two or more cells connected in series, which gives a bigger voltage.
Key terms
Rechargeable and non-rechargeable
In non-rechargeable cells, such as alkaline batteries, the chemical reactions stop when one of the reactants has been used up. The cell then has to be recycled.
Rechargeable cells, such as those in phones, can be used again: passing an external electric current through them reverses the chemical reactions.
Hydrogen fuel cells
A is supplied continuously with a fuel and oxygen (or air). Inside, the fuel is — it isn't burned — to produce a voltage.
In a hydrogen fuel cell, the only product is water: 2H₂ + O₂ → 2H₂O
| Hydrogen fuel cells | Rechargeable batteries |
|---|---|
| Only waste product is water | Toxic metal compounds to dispose of |
| Keep working while fuel is supplied | Run down and need recharging |
| Hydrogen is a flammable gas, hard to store | Easy to store and use |
| Hydrogen is often made using fossil fuels or electricity | Charged using electricity, which may come from fossil fuels |
Key terms
Fuel cell half equations (Higher tier only)
In a hydrogen fuel cell with an acid electrolyte:
- At the , hydrogen is — it loses electrons:
2H₂ → 4H⁺ + 4e⁻
- At the positive electrode (cathode), oxygen is — it gains electrons:
O₂ + 4H⁺ + 4e⁻ → 2H₂O
Add them together and the H⁺ ions and electrons cancel, leaving the overall reaction: 2H₂ + O₂ → 2H₂O. The electrons flow through the external circuit from the anode to the cathode.