10.02.5 Fuel Cells
Syllabus
Fuel cells are supplied by an external source of fuel (eg Hydrogen) and Oxygen or air.
The fuel is oxidised electrochemically within the fuel cell to produce a potential difference.
The overall reaction in a Hydrogen fuel cell involves the oxidation of Hydrogen producing water.
Hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries.
Students should be able to:
evaluate the use of Hydrogen fuel cells in comparison with rechargeable cells and batteries
(HT only) write the half equations for the electrode reactions in the Hydrogen fuel cell.
What does this mean?
What is a fuel cell?
An 'Alkaline battery' will eventually run out because the reaction isn't reversible
A conventional cell has chemicals that can run out as the reaction goes on.
In a rechargeable cell the reaction can be reversed providing we plug it in to a source of energy from time to time.
A car battery shouldn't run out because the reaction is reversible and is recharged as the vehicle is driven
A fuel cell simply continually re-fuels the cell so that it carries on generating a voltage without ever running out of chemicals (providing we don't run out of fuel).
We could just burn Hydrogen in Oxygen and release heat energy to generate electricity with.
But that's potentially explosive and a much less efficient way of generating electricity from Hydrogen than Oxygen.
In the fuel cell the Hydrogen and Oxygen never meet - they are separated by the electrolyte.
Almost all the energy released becomes electrical energy.
How does the Hydrogen and Oxygen react if they never meet?
In the simplest fuel cell the Hydrogen gas breaks up to become Hydrogen ions.
The HT paper will expect you to be able to write the half-equation
Half-eqn 1: H2(g) --> 2H+(aq) + 2 e-
The electrons move away along a wire (they are the electric current); the Hydrogen ions pass through the electrolyte to the other side of the cell. where they can react with the Oxygen.
The simplest half-equation for this is
Half-eqn 2: 4H+(aq) + O2(g) + 4 e- --> 2 H2O(g)
So, only water vapour is produced.
You may notice that the number of electrons in the two half-equations doesn't match.
In reality all the electrons made & used must match so we multiple Half-eqn 1 by 2
2 x Half-eqn 1: 2H2(g) --> 4H+(aq) + 4 e-
The overall equation is what we get when we add the two half-equations together and cancelling what appears in both.
2H2(g) --> 4H+(aq) + 4 e-
+
4H+(aq) + O2(g) + 4 e- --> 2 H2O(g)
Overall : 2H2(g) + O2(g) --> 2 H2O(g)
Advantages and Disadvantages of Fuels Cells
Advantages
Disadvantages
Only water is emitted as waste - although CO2 is produced by burning fossil fuels to make the Hydrogen so it isn't Carbon-neutral.
Highly efficient - little energy wasted as heat
Can operate indefinitely if fuel is available (conventional cells run-out, rechargeable cells become less and less efficient)
Currently quite expensive
Hydrogen is hard to store and potentially dangerous
Low voltage
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