Chemical cells
- Cells contain chemicals that react to produce electricity.
- A simple cell can be made by connecting two different metals in contact with an electrolyte.
- The voltage depends on the type of electrodes and the electrolyte: the bigger the difference in reactivity between the two metals, the bigger the voltage.
- Batteries are two or more cells connected in series to give a bigger voltage.
Rechargeable and non-rechargeable
- In non-rechargeable cells and batteries, such as alkaline batteries, the chemical reactions stop when one of the reactants has been used up.
- Rechargeable cells and batteries can be recharged, because the chemical reactions are reversed when an external electric current is supplied.
Fuel cells
- Fuel cells are supplied with a fuel (such as hydrogen) and oxygen or air from outside. The fuel is oxidised electrochemically to produce a potential difference.
- In a hydrogen fuel cell, hydrogen is oxidised to produce water: 2H₂ + O₂ → 2H₂O. Water is the only product.
- (Higher) At the negative electrode: 2H₂ → 4H⁺ + 4e⁻. At the positive electrode: O₂ + 4H⁺ + 4e⁻ → 2H₂O.
Fuel cells compared with batteries
- Advantages of hydrogen fuel cells: they only produce water, they don't need recharging (they work while fuel is supplied), and they have fewer parts to wear out.
- Disadvantages: hydrogen is a gas that is hard to store and is flammable; it is often made using fossil fuels; there are few places to refuel.
- Batteries are easy to store and use, but contain toxic chemicals, eventually wear out, and recharging uses electricity that may come from fossil fuels.
Key terms
- Cell
- A device containing chemicals that react to produce electricity.
- Battery
- Two or more cells connected in series.
- Electrolyte
- A solution or liquid containing ions that allows charge to flow in a cell.
- Rechargeable cell
- A cell whose reactions can be reversed by supplying an external current.
- Fuel cell
- A cell that uses a fuel and oxygen from outside to produce electricity.
- Hydrogen fuel cell
- A fuel cell in which hydrogen reacts with oxygen to produce water and electricity.
- Potential difference
- The voltage produced by a cell.