Applications of Electrode Potentials

Full Notes

Storage Cells

Storage cells, also known as rechargeable batteries, are electrochemical cells that store energy for use when needed.

They involve two half-cells, and their operation is based on redox reactions.

• In discharging, the cell acts like a galvanic (voltaic) cell, releasing electrical energy.
• In charging, an external current is applied to reverse the redox reaction.

Matt’s Exam Tip - The half equations that occur during charging are simple the reverse direction of the half equations that occur when discharging.

Example: Lithium-Ion Battery

Common batteries (such as in mobile phones) contain lithium and are called ‘lithium ion cells’.

The electrode reactions occurring can be simplified to:

Positive electrode (cathode) Li⁺ + CoO₂ + e⁻ → Li[CoO₂]⁻

Negative electrode (anode) Li → Li⁺ + e⁻

When a lithium ion battery is recharged, an external input of electricity is used to force the above reactions to happen in reverse – reforming the original reactants.

These batteries are lightweight, have a high energy density, and are rechargeable over many cycles.

Fuel Cells

Fuel cells produce electricity by reacting a fuel with oxygen, without combustion.

The most common type is the hydrogen-oxygen fuel cell.

• They operate continuously as long as fuel and oxygen are supplied.
• They are highly efficient and produce only water as a waste product.

Fuel cells are often used in spacecraft and submarines (quiet, low-emission power) and increasingly in cars and buses (environmentally friendly transport).

Hydrogen-Oxygen Fuel Cells

There are two main types of hydrogen-oxygen fuel cell: Acidic and alkaline. Both use hydrogen as the fuel and oxygen as the oxidant.

Acidic electrolyte based fuel cell

Reaction summary

• Anode (oxidation): H₂ → 2H⁺ + 2e⁻
• Cathode (reduction): O₂ + 4H⁺ + 4e⁻ → 2H₂O
• Overall reaction: 2H₂ + O₂ → 2H₂O

The fuel is hydrogen and this gets oxidised to form H⁺(aq) ions, at the anode.

The electrons released from the hydrogen oxidation get transferred to oxygen gas molecules (from air) and the oxygen is reduced to O²⁻(aq) ions at the cathode.

The ‘flow’ of electrons from the electrode where the H⁺(aq) ions are oxidised to the electrode where the oxygen molecules are reduced creates an electrical current.

H⁺(aq) ions are able to move through a permeable membrane to then combine with oxide ions that are formed at the cathode. This creates water molecules that are the only chemical product of the fuel cell.

A permeable membrane is important as it is allows H⁺(aq) ions through, whilst forcing the electrons lost by H₂ molecules to pass through a wire to get the anode – this forcing of electrons through a wire is what creates an electrical current.

Alkaline electrolyte based fuel cell

Reaction summary

• Anode (oxidation): H₂ + 2OH⁻ → 2H₂O + 2e⁻
• Cathode (reduction): O₂ + 2H₂O + 4e⁻ → 4OH⁻
• Overall reaction: 2H₂ + O₂ → 2H₂O

The fuel is also hydrogen and this again gets oxidised to form H⁺(aq) ions at the anode.

In an alkaline hydrogen fuel cell, the electrolyte contains hydroxide ions. Rather than H⁺ ions moving from the anode to combine with oxide ions, hydroxide ions combine directly with the H⁺ ions and form water molecules.

The electrons released from the hydrogen oxidation still get transferred to oxygen gas molecules (from air) and the oxygen is reduced at the cathode in the presence of water, forming hydroxide ions.

The ‘flow’ of electrons from the electrode where the H⁺(aq) ions are oxidised to the electrode where the oxygen molecules are reduced creates an electrical current.

Despite using different electrolytes, both cell types have the same overall redox equation and generate clean energy efficiently.

Summary

• Storage cells use redox reactions to store and release electrical energy.
• Fuel cells continuously convert hydrogen and oxygen into water and electrical energy.
• Hydrogen fuel cells can run in acidic or alkaline media with different half-equations.
• Fuel cells are efficient and have water as the only product.