Fuel Cells (AQA A Level Chemistry): Revision Note

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

• 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, which 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 (acidic fuel cells also exist)

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

• The half equations are

$2{\mathrm{H}}_2\left(\mathrm{g}\right)+4{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$  E^ꝋ = -0.83 V

${\mathrm{O}}_2\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+4{\mathrm{e}}^{-}\to 4{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)$   E^ꝋ = +0.40 V 

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

$2{\mathrm{H}}_2\left(\mathrm{g}\right)+\cancel{4{\mathrm{OH}}^{-}}\left(\mathrm{aq}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)+\cancel{4{\mathrm{e}}^{-}}\to 2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+\cancel{4{\mathrm{e}}^{-}}+\cancel{4{\mathrm{OH}}^{-}}\left(\mathrm{aq}\right)$

$2{\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$ E^ꝋ = +1.23 V

• Although the half-equations differ depending on the electrolyte, the overall reaction is always  2H₂ (g) + O₂ (g)  →   2H₂O (l), so the EMF is the same

Benefits

• 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, a much larger proportion of the chemical energy is converted into electrical energy compared to combustion in an engine, where significant energy is lost as heat

• 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

Risks and problems

• Hydrogen is a highly flammable gas, and the production and storage of hydrogen carry 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

•  While steam reforming of natural gas is currently the main industrial source, hydrogen can also be produced by electrolysis of water using renewable electricity

• 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

Related topics

• Standard Electrode Potentials

• Commercial Cells

• Lithium Cells

• Oxidation & Reduction