Fuel Cells (SL IB Chemistry)

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 hydrogen-oxygen fuel cell combines both elements to release energy and water

• 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 reaction product, so fuel cells present obvious environmental advantages over other types of cells
• The reaction is the same as hydrogen combusting in oxygen, but since the reaction takes place at room temperature without combustion, all the bond energy is converted into electrical energy instead of heat and light
• There are no harmful oxides of nitrogen produced, which are usually formed in high-temperature combustion reactions where air is present
• 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 a highly flammable gas and the production and storage of hydrogen carries safety hazards
• Very thick walled cylinders and pipes are needed to store hydrogen which has economic impacts
• The production of hydrogen is a by-product of the crude oil industry, which means it relies on a non-renewable, finite resource
• Until a cheap way is found to make hydrogen, its widespread use in fuel cells will be limited
• Hydrogen has high energy density, that is, the amount of energy contained in 1g of the fuel is high compared to other fuels, but because it is a gas, its energy density per unit volume is low which means larger containers are needed compared to liquid fuels

Methanol fuel cell

• The methanol fuel cell works in a similar fashion 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 uses methanol as the course of hydrogen ions and produces carbon dioxide and water while releasing energy 

• 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

• Methanol is easier to store and transport than hydrogen
• The fuel cell does not require high pressure or temperature
• The membrane has a longer lifespan as it is operating in an aqueous environment
• Methanol has a greater energy density (energy per unit volume) than hydrogen
• Methanol can be produced from renewable resources through fermentation
  • This means that methanol is cleaner than hydrogen because its production has less impact on the environment in terms of greenhouse gases.

Disadvantages of methanol fuel cells over hydrogen fuel cells

• Methanol is very toxic and highly flammable
• Methanol is most commonly made from non-renewable fossil fuels
• The fuel cell produces a lower voltage / lower power per unit mass of the cell
• The fuel cell has a low efficiency because methanol can pass through the available membrane materials
• The reaction at the anode requires a more highly efficient catalyst that contains expensive precious metals, usually ruthenium and palladium
• The fuel cell produces the greenhouse gas, carbon dioxide