Fuel Cells (DP IB Chemistry): Revision Note

Fuel Cells

What are fuel cells?

• A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode

• These cells are becoming more common in the automotive industry to replace petrol or diesel engines

How do fuel cells work?

• As the fuel enters the cell it becomes oxidised which sets up a potential difference or voltage within the cell

• Different electrolytes and fuels can be used to set up different types of fuel cells

• An important cell is the hydrogen-oxygen fuel cell 

The hydrogen-oxygen fuel cell

• The fuel cell consists of

  • a reaction chamber with separate inlets for hydrogen and oxygen gas

  • an outlet for the product - water

  • an electrolyte of aqueous sodium hydroxide

  • a semi-permeable membrane that separates the hydrogen and oxygen gases

• The half equations are:

2H₂ (g) + 4OH^– (aq)  →  4H₂O (l) +  4e^–                     E^θ = –0.83 V 

O₂ (g) +  2H₂O  +  4e^– →  4OH^– (aq)                      E^θ = +0.40 V 

• The overall reaction is found by combining the two half equations and cancelling the common terms:

2H₂ (g) + 4OH^– (aq) + O₂ (g) +  2H₂O   +  4e^– →   4H₂O (l) +  4e^– + 4OH^– (aq)

 2H₂ (g) + O₂ (g)  →   2H₂O (l)           E^θ = +1.23 V

Advantages of hydrogen fuel cells

• Water is the only product so is environmentally friendly with no harmful emissions

• The reaction occurs at room temperature and avoids combustion, so energy is efficiently converted into electricity rather than heat

• No nitrogen oxides (NOₓ) are produced, unlike in high-temperature combustion

• Fuel cells have been used on spacecraft, where the product can be used as drinking water for astronauts

Disadvantages of hydrogen fuel cells

• Hydrogen is highly flammable, so poses significant safety hazards

• Requires heavy, thick-walled storage tanks, increasing cost and complexity

• Hydrogen is mostly sourced from fossil fuels, making it non-renewable

• Until cheap and sustainable hydrogen production is developed, usage will remain limited

• Although hydrogen has high energy density by mass, its low energy density by volume means larger containers are needed compared to liquid fuels

Methanol fuel cell

• The methanol fuel cell works in a similar way to the hydrogen fuel cell

• The main difference is that the fuel or source of hydrogen ions, H⁺, is methanol rather than hydrogen 

The methanol fuel cell

• The methanol fuel cell consists of:

  • a reaction chamber with separate inlets for methanol and oxygen 

  • outlets for the carbon dioxide and water products

  • an electrolyte, typically a proton exchange membrane

  • a semi-permeable membrane that separates the hydrogen and oxygen gases

• This use of methanol as the source of hydrogen ions means that the half-equations and overall equation for the cell will be different

• The half equations are:

CH₃OH (aq) + H₂O (l)  →  CO₂ (g) + 6H⁺ (aq) +  6e^–  

O₂ (g) + 4H⁺ (aq) + 4e^– →  2H₂O (l)  

• The overall reaction is found by:

  • Multiplying the O₂ equation by 1.5

1.5O₂ (g) + 6H⁺ (aq) + 6e^– →  3H₂O (l)

• Combining the two half equations

CH₃OH (aq) + H₂O (l) + 1.5O₂ (g) + 6H⁺ (aq) + 6e^– → CO₂ (g) + 6H⁺ (aq) +  6e^– + 3H₂O (l) 

• Cancelling the common terms

 CH₃OH (aq) + 1.5O₂ (g) → CO₂ (g) +2H₂O (l) 

Advantages of methanol fuel cells over hydrogen fuel cells

• Easier to store and transport than hydrogen

• Operates at low pressure and temperature

• Longer membrane lifespan due to aqueous conditions

• Higher energy density (more energy per unit volume)

• Can be made from renewable resources (e.g. fermentation of biomass)

  • Lower greenhouse gas emissions than hydrogen produced from fossil fuels

Disadvantages of methanol fuel cells over hydrogen fuel cells

• Toxic and highly flammable

• Commonly made from non-renewable fossil fuels

• Lower voltage and power output per unit mass

• Low efficiency: methanol can cross the membrane and reduce performance

• Requires expensive catalysts (e.g. ruthenium, palladium)

• Produces carbon dioxide, a greenhouse gas