GCSE OCR Gateway Organic Chemistry C6.2q Fuel Cells

C6.2q To be able to evaluate the advantages and disadvantages of the chemistry of the hydrogen/oxygen and other fuel cells

A simple Fuel Cell

A simple fuel cell capitalises on the reaction between hydrogen and oxygen to produce water. 

2H₂   +    O₂   ➞   2H₂O

The technological construction of the cell is such that the energy of the reaction is released mainly as electricity not heat as in the combustion of hydrogen in oxygen.

As the reactants are used up more can be added so that a fuel cell can give a continuous supply of electricity unlike an electrochemical cell where the cell potential fades as the reactants are used up.

The technology of the cell can be seen in the diagram below:

There are two porous carbon graphite electrodes coated with a Platinum catalyst.  The electrodes are immersed in corrosive sodium hydroxide solution.    

The porous electrodes allow water, sodium ions and hydroxide ions to migrate around the cell and hydrogen and oxygen molecules to combine at the surface of the metal catalyst: platinum being especially porous to hydrogen. 

Two reactions occur in the cell

1) at the negative electrode:  2H₂   +  4OH^—  ➞    4H₂O    +    4e^—

this is an oxidation half reaction

2) at the positive electrode:  O2   +   2H₂O  +     4e^—    ➞      4OH^— 

this is a reduction half reaction

The advantages of such a fuel cell are that they do not need recharging,  they do not pollute the environment because they only produce water, but they are very expensive to produce. 

This type of fuel cell was famously used in the spacecraft of America’s Apollo moon programme.

The doomed Apollo 13 

developed a fault (“Houston, we have a problem!”) in its oxygen tanks and hence could not maintain the fuel to its fuel cells in the command module. 

Other fuel cells

Alcohol can be used instead of hydrogen as the fuel source in a fuel cell.  

When ethanol replaces hydrogen as the fuel source these are the half-reactions in the cell:

Negative

Terminal:  H₂O +  C₂H₅OH  ➞   CH₃COOH  +    4H⁺    +    4e^—

Positive

Terminal:  [O₂   +   2H⁺   +     2e^—    ➞  2H₂O] ×2

The overall reaction in this fuel cell is:  C₂H₅OH   +   O₂  ➞    CH₃COOH  + H₂O  

One further advantage of fuel cells is that they operate at about 70% efficiency.  Their efficiency is remarkable given that most power plants operate at about 40% and the efficiency of petrol and diesel engines is not much better than that. 

And you can purchase your own model alcohol fuel cell too: