Storage cells and fuel cellsEdexcel A-Level Chemistry: Revision notes

A storage cell (rechargeable cell) keeps its reactants inside the cell. While it discharges, a spontaneous redox reaction produces a voltage, with oxidation at the negative electrode and reduction at the positive electrode. When it is recharged, an external voltage larger than the cell emf is applied in the opposite direction, which reverses the reaction.

This works only if the products stay at the electrodes, so the reaction is reversible. Electrode potentials are used to find the emf of the cell and to decide which half-cell is positive.

Lead–acid cell (E°: PbO₂/PbSO₄ = +1.69 V; PbSO₄/Pb = −0.36 V):

$E^\ominus_{cell} = +1.69 - (-0.36) = +2.05$ V, and six cells in series give a 12.3 V car battery.

Negative electrode: $\mathrm{Pb + SO_4^{2-} \rightarrow PbSO_4 + 2e^-}$. Positive electrode: $\mathrm{PbO_2 + 4H^+ + SO_4^{2-} + 2e^- \rightarrow PbSO_4 + 2H_2O}$.

Lithium-based cells are widely used in phones, laptops and electric vehicles. Lithium has a very negative electrode potential ($E^\ominus$(Li⁺/Li) = −3.04 V) and a low relative atomic mass, so a lithium cell gives a high emf and stores a lot of energy per unit mass. On discharge, lithium ions move through the electrolyte from the negative electrode to the positive electrode; on recharge they move back, so the reaction is reversible.

The general approach is the same for any cell: the half-cell with the more negative $E^\ominus$ is the negative electrode, $E^\ominus_{cell} = E^\ominus_{positive} - E^\ominus_{negative}$, and the half-equations are reversed when the cell is recharged.

A fuel cell converts the energy released when a fuel reacts with oxygen directly into electrical energy. Fuel and oxygen are supplied continuously from outside, so the cell gives a steady voltage for as long as they are supplied and does not need recharging. The electrodes are usually coated with a catalyst such as platinum, and the two gases are kept apart by an electrolyte.

Hydrogen is the most common fuel, but methanol and other hydrogen-rich fuels can also be used. Oxidation of the fuel occurs at the negative electrode and reduction of oxygen at the positive electrode; electrons flow through the external circuit.

Acidic electrolyte:

• Negative electrode: $\mathrm{H_2 \rightarrow 2H^+ + 2e^-}$
• Positive electrode: $\mathrm{O_2 + 4H^+ + 4e^- \rightarrow 2H_2O}$

Alkaline electrolyte:

• Negative electrode: $\mathrm{H_2 + 2OH^- \rightarrow 2H_2O + 2e^-}$
• Positive electrode: $\mathrm{O_2 + 2H_2O + 4e^- \rightarrow 4OH^-}$

Both give the same overall reaction, $\mathrm{2H_2 + O_2 \rightarrow 2H_2O}$, and $E^\ominus_{cell} = +0.40 - (-0.83) = +1.23$ V in the alkaline cell.

A methanol fuel cell with an acidic electrolyte uses $\mathrm{CH_3OH + H_2O \rightarrow CO_2 + 6H^+ + 6e^-}$ at the negative electrode, giving $\mathrm{2CH_3OH + 3O_2 \rightarrow 2CO_2 + 4H_2O}$ overall.

• Supply: a fuel cell is fed fuel continuously and is refuelled; a storage cell has fixed reactants and is recharged.
• Speed: refuelling takes minutes; recharging takes hours.
• Products: a hydrogen fuel cell emits only water at the point of use; methanol cells emit CO₂.
• Storage and safety: hydrogen is flammable and must be stored under pressure or as a liquid.
• Sustainability: most hydrogen is made from fossil fuels, so the benefit depends on its source. Hydrogen from electrolysis using renewable electricity is low-carbon.
• Efficiency: energy is lost making hydrogen, so a battery can be more efficient overall.