Fuel Cells

Fuel Cell Basics

A fuel cell is a device that converts chemical energy directly into electrical energy through an electrochemical reaction. Unlike batteries, which store energy internally and eventually run out, fuel cells produce electricity continuously as long as fuel is supplied.

Fuel cells work by combining a fuel (usually hydrogen) with oxygen from the air. The chemical reaction releases energy, which is harnessed as electricity. This process is clean and efficient because it does not involve combustion.

The key difference between fuel cells and batteries is that fuel cells require a constant supply of fuel and oxygen to keep generating electricity, while batteries store energy internally and need recharging or replacement once depleted.

Hydrogen Fuel Cells

Hydrogen fuel cells use hydrogen gas as the fuel and oxygen gas from the air as the oxidant. The overall reaction produces only water and electricity, making it an environmentally friendly energy source.

Overall reaction:

$$2H_2 + O_2 \rightarrow 2H_2O$$

Inside the fuel cell, the reaction is split into two half-reactions occurring at separate electrodes:

Anode Reaction (Oxidation)

At the anode, hydrogen molecules lose electrons (are oxidised) to form hydrogen ions:

$$2H_2 \rightarrow 4H^+ + 4e^-$$

Cathode Reaction (Reduction)

At the cathode, oxygen molecules gain electrons (are reduced) and react with hydrogen ions to form water:

$$O_2 + 4H^+ + 4e^- \rightarrow 2H_2O$$

Electrons flow through an external circuit from the anode to the cathode, creating an electric current that can power devices.

For instance, in a hydrogen fuel cell car, hydrogen gas is stored in tanks and fed to the anode, oxygen comes from the air at the cathode, and the electricity generated powers the electric motor.

Example: Calculate the number of electrons transferred when 1 mole of hydrogen gas reacts in a hydrogen fuel cell.

Each molecule of hydrogen ($H_2$) loses 2 electrons (since $2H \rightarrow 2H^+ + 2e^-$). For 1 mole of $H_2$, the number of electrons is:

$$\text{Electrons} = 2 \times (6.02 \times 10^{23}) = 1.204 \times 10^{24} \text{ electrons}$$

Advantages of Fuel Cells

• High efficiency: Fuel cells convert chemical energy directly into electrical energy, often more efficiently than combustion engines or traditional power stations.
• Low pollution: The only by-product of hydrogen fuel cells is water vapour, so they produce no harmful emissions like carbon dioxide or nitrogen oxides.
• Continuous energy supply: As long as hydrogen and oxygen are supplied, fuel cells can generate electricity continuously without the need for recharging.

Example: A hydrogen fuel cell produces 5 A of current for 2 hours. Calculate the total charge passed through the circuit.

Using the formula $Q = I \times t$:

$$Q = 5 \text{ A} \times (2 \times 3600 \text{ s}) = 5 \times 7200 = 36000 \text{ C}$$

Disadvantages of Fuel Cells

• Hydrogen storage and safety: Hydrogen is a very flammable gas and must be stored carefully at high pressure or low temperature, which can be challenging and expensive.
• Cost and availability of materials: Fuel cells often require expensive catalysts such as platinum, increasing their cost.
• Limited infrastructure: There are few refuelling stations for hydrogen fuel, limiting widespread use, especially in transport.