5.11 - Fuel Cells

The hydrogen-oxygen fuel cell

• A fuel cell produces electricity directly from a chemical reaction between a fuel and oxygen.
• The fuel and oxygen are supplied continuously from outside the cell, so it produces electricity for as long as it's supplied with fuel.
• Inside the cell, the fuel is oxidised, setting up a potential difference (voltage) that drives an electric current through a circuit.

The hydrogen-oxygen reaction

In a hydrogen-oxygen fuel cell, hydrogen reacts with oxygen to produce water:

hydrogen + oxygen → water

2H₂ + O₂ → 2H₂O

• Unlike burning hydrogen, which releases energy as heat, a fuel cell converts the chemical energy directly into electricity.
• Water is the only chemical product - No carbon dioxide or other polluting exhaust gases are released.

Evaluating hydrogen-oxygen fuel cells

Advantages

• Only produces water and heat - No carbon dioxide or polluting exhaust gases are released during use.
• High efficiency - Electricity is produced directly from the reaction with almost no moving parts, so little energy is lost to heat or friction.
• Quick to refuel and doesn't lose capacity - Refilling a hydrogen tank takes minutes compared to hours for recharging a battery, and fuel cells don't lose their ability to hold charge over time.

Disadvantages

• Hydrogen takes up a lot of storage space - Hydrogen is a gas at room temperature, so it must be stored at high pressures or low temperatures to fit enough in a vehicle.
• Hydrogen is flammable and explosive when mixed with air, making storage and transport hazardous.
• Hydrogen production often creates pollution - It's typically obtained from hydrocarbons (fossil fuels) or electrolysis powered by fossil fuels.
• High cost - Fuel cells and hydrogen refuelling infrastructure are expensive to build.

Half equations in a hydrogen fuel cell

• The two gases enter separate compartments and react at the electrodes, which are separated by an electrolyte (often an acid).
• The electrodes are made of porous carbon coated with a catalyst.
• Unlike electrolysis, the anode is negative (where oxidation happens) and the cathode is positive (where reduction happens).

At the anode (oxidation)

Hydrogen loses electrons to form hydrogen ions:

2H₂ → 4H⁺ + 4e^-

• The electrons travel through the external circuit to the cathode, creating the electric current.
• The hydrogen ions move through the electrolyte to the cathode.

At the cathode (reduction)

Oxygen gains electrons and reacts with hydrogen ions to produce water:

O₂ + 4H⁺ + 4e^- → 2H₂O

Overall reaction

Combining the two half equations gives the overall redox reaction:

2H₂ + O₂ → 2H₂O