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)