Hydrogen-Oxygen Fuel Cells

4.2 Hydrogen–oxygen fuel cells

Definition and Basic Principle

• A hydrogen–oxygen fuel cell is an electrochemical cell that converts the chemical energy of hydrogen (H₂) and oxygen (O₂) directly into electrical energy.
• Overall reaction:
  • 2H₂(g) + O₂(g) → 2H₂O(l)
• Only chemical product is water, making it a zero-carbon emission energy source (if hydrogen is produced from renewable sources).
• Operates continuously if fuel (H₂) and oxidant (O₂) are supplied.

Structure and Components

• Anode (negative electrode):
  • Made of porous material coated with a platinum catalyst.
  • Hydrogen gas is supplied here.
• Cathode (positive electrode):
  • Made of porous material coated with platinum catalyst.
  • Oxygen gas is supplied here.
• Electrolyte:
  • Can be an acid (proton exchange membrane fuel cell, PEMFC) or an alkaline solution (alkaline fuel cell, AFC).
  • PEM allows H⁺ ions to pass through but not electrons.
• External circuit:
  • Electrons flow from anode to cathode through the circuit, providing electrical power.

Reactions in a Hydrogen–Oxygen Fuel Cell

1. At the anode (oxidation):
   • Hydrogen molecules split into protons (H⁺) and electrons (e⁻).
   • Equation: H₂(g) → 2H⁺(aq) + 2e⁻
   • In an alkaline fuel cell, OH⁻ ions are involved instead:
     • H₂(g) + 2OH⁻(aq) → 2H₂O(l) + 2e⁻
2. At the cathode (reduction):
   • Oxygen molecules react with protons (H⁺) from the electrolyte and electrons from the circuit to form water.
   • Equation (acid electrolyte): O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l)
   • In alkaline fuel cells:
     • O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)
3. Overall cell reaction:
   • Acid electrolyte: 2H₂(g) + O₂(g) → 2H₂O(l)
   • Alkaline electrolyte: Same overall equation.

Advantages over Gasoline/Petrol Engines

• Environmental benefits:
  • Only water is produced — no CO₂ emissions if H₂ is from renewable sources.
  • No production of nitrogen oxides (NOₓ), sulfur oxides (SOₓ), or particulate matter.
• High efficiency:
  • Fuel cells can be more efficient than internal combustion engines (up to ~60% vs ~25–30% for petrol engines).
• Quiet operation:
  • Almost silent running, suitable for urban environments.
• Continuous operation:
  • As long as fuel and oxidant are supplied, the fuel cell keeps generating electricity.
• Modular scalability:
  • Can be designed for small devices or large vehicles.

Disadvantages compared to Gasoline/Petrol Engines

• Hydrogen storage and transport:
  • Hydrogen gas is difficult to store — it has low energy density by volume and needs high-pressure tanks or cryogenic storage.
• Fuel infrastructure:
  • Limited hydrogen refuelling stations globally.
• Production of hydrogen:
  • Most hydrogen today is made from natural gas (methane) via steam reforming, which produces CO₂ unless carbon capture is used.
  • Renewable hydrogen via electrolysis is currently expensive.
• Durability and cost:
  • Platinum catalysts are expensive.
  • Fuel cells have shorter lifespans compared to conventional engines due to catalyst degradation.
• Cold start issues:
  • Performance may be reduced at low temperatures.

Key Comparisons Table