Electrode Potentials & Electrochemical Cells (A Level only) (AQA A-Level Chemistry): Revision Notes

5.4.6 Fuel Cells

What are Fuel Cells?

Fuel cells are electrochemical cells that generate an electric current through redox reactions using a continuous supply of fuel (often hydrogen) and an oxidant (usually oxygen). Unlike traditional batteries, fuel cells do not need recharging as long as fuel and oxidant are available, making them suitable for various applications, including hydrogen-powered vehicles.

The Alkaline Hydrogen-Oxygen Fuel Cell

In an alkaline hydrogen-oxygen fuel cell, hydrogen and oxygen gases are supplied as reactants. The cell operates at a high efficiency and produces only water as a waste product.

The electrode reactions in this type of cell are as follows:

Anode Reaction (Oxidation of Hydrogen):

Hydrogen gas is oxidised at the anode, releasing electrons and forming hydroxide ions in the alkaline environment.

$$\text{H}_2 + 2\text{OH}^- \rightarrow 2\text{H}_2\text{O} + 2e^-$$

Cathode Reaction (Reduction of Oxygen):

Oxygen gas is reduced at the cathode, reacting with water and electrons to form hydroxide ions.

$$\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-$$

Overall Cell Reaction:

Combining the anode and cathode reactions, we get the net cell reaction:

$$2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$$

This reaction produces water as the sole byproduct, making hydrogen fuel cells an environmentally friendly alternative to fossil-fuel-powered engines.

Advantages and Disadvantages of Fuel Cells

Advantages

• Higher Efficiency: Fuel cells are more efficient than internal combustion engines, converting a greater percentage of their available energy into kinetic energy rather than heat.
• Environmentally Friendly: The only waste product from the cell itself is water, with no toxic emissions or $\text{CO}_2$ generated during operation.
• Continuous Operation: Fuel cells don't need to be recharged; they will keep producing electricity as long as they have a fuel and oxidant supply.

Disadvantages

• Hydrogen Production: Hydrogen fuel is commonly produced by electrolysis, a process that often relies on fossil fuels, reducing the overall environmental benefit and making the process less carbon-neutral.
• Storage and Safety: Hydrogen is highly flammable and requires careful handling, especially during storage and transport.
• Limited Infrastructure: Hydrogen refuelling stations are rare, making it challenging for hydrogen fuel cell vehicles to refuel conveniently.