I find this animation particularly useful to help students visualise the oxidative and reductive reactions happening at the anode and cathode respectively. A Proton Exchange Membrane(PEM) cell uses a specially treated membrane/electrolyte that divides the two electrodes and blocks electrons from passing directly through it, forcing them to instead flow through the external circuit set up between the electrodes, producing electricity. 
 
 As you can see, the hydrogen gas molecules enter into each cell from the anode side and lose electrons due to the effect of platinum catalyst (H2 is oxidised --> electrode is anode). This creates a continuous flow of electrons to the other electrode: 
 
 Anode:  2H2(g) + 4OH–(aq) → 4H2O(l) + 4e– 
 
 Simultaneously, oxygen from the air is fed in from a separate inlet.  The positively-charged hydrogen ions that pass through the special membrane receive the incoming negatively-charged electrons and combine with the oxygen molecules to form water molecules. 
 
 At the cathode, oxygen molecules receive the incoming electrons and are reduced:
 Cathode: O2(g) + 2H2O(l) + 4e– → 4OH–(aq) 
 
 So, two electrode plates + membrane make one unit of cell. In a typical fuel cell battery, there can be a series of cells clamped in contact to generate the required power.
 The dawn of this simply build of cells has been touted as the new future of clean energy as ONLY WATER is produced from the process. 
 
  2H2(g) + O2(g) → 2H2O(l) 
 
 However, there are concerns about the use of hydrogen as a fuel source. 
 
 Based on what you've learnt of hydrogen, what do you think are some concerns?