In The Science Behind Hydrogen Hybrid Cars – The Electrolysis of Water we discussed the basic composition of a hydrogen fuel cell and how it works, now let’s look at the actual scientific equations of the process. (This gets a little advanced so pull out your old chem. books!)
At the negatively charged cathode (electrode plate) in the water of your hydrogen cell car, a reduction reaction happens. The electrons (e−) from the cathode cross over to hydrogen cations (positively charged ion that’s attracted to a negative electrode in an electrolyte) that then form the hydrogen gas, (half the reaction balanced with acid):
Cathode (reduction): 2H+(aq) + 2e− → H2(g)
On the positively charged anode (electrode plate) in the water of your hydrogen cell car, an oxidation reaction happens. This produces oxygen gas and gives electrons to the cathode to complete the circuit:
Anode (oxidation): 2H2O(l)→O2(g) (+) 4H+(aq) (+) 4e−
You can balance these half reactions in your hydrogen cell car with a base like those listed below. You don’t have to balance all of these half reactions with a base or acid although many like the improvement of the reduction or oxidation of the water. For half reactions, they both must balance with either base or acid:
Cathode (reduction): 2H2O(l) (+) 2e− → H2(g) (+) 2OH−(aq)
Anode (oxidation): 4OH−(aq) → O2(g) (+) 2H2O(l) + 4e−
You can combine either of these half reactions to get the same total breakdown of water into hydrogen and oxygen:
Overall reaction: 2H2O(l) → 2H2(g) (+) O2(g)
As stated before you’ll get twice the number of hydrogen molecules, as you will oxygen molecules. If everything is equal like the pressure and temperature for both gasses, the hydrogen produced will have twice the volume of the oxygen. Therefore, the number of electrons that push through the water will be double the number of hydrogen molecules generated and four times as many as the number of oxygen molecules
If we look at the thermodynamics of the electrolysis in you hydrogen cell car, we see that normal temperatures and pressures don’t produce the best results. Based on the “Nernst Equation” the normal potential for the fuel cell is:
-1.23 V at 25°C at pH 7 (H+ = 1.0×10-7 M) or -1.23 V at 25°C at pH 0 (H+ = 1.0 M)
The negative voltage shows that the energy for the electrolysis of water is more than zero. This is shown using the G=-nFE equation from the kinetics of chemicals, where “n” equals the moles of electrons and “F” equals the Faraday constant. This reaction won’t happen without the added energy, usually from an outside power source.
Please stay with us for The Science Behind Hydrogen Hybrid Cars – Electrolytes and Electrolysis when we’ll talk more about how the electrolytes affect the process in your hydrogen cell car.