Page 68 - Introduction to Transfer Phenomena in PEM Fuel Cells

Charge Transfer Phenomena 57
2.2.4. Empty voltage
The theoretical potential of a fuel cell is the potential difference between
the cathode and the anode, this voltage is denoted as (E). The superscript
character (0) is used to designate the battery voltage under standard
temperature and pressure conditions (25°C, 1 atm) [BLU 07]. It is said to be
empty because it is calculable without the presence of an electric charge that
consumes the current delivered by the fuel cell.
At equilibrium, the Faraday equation connecting the free energy to the
electrode potential is written as:

0
⋅
⋅
Δ G = − n F E 0 [2.9]
–1
where (F) is the Faraday constant and its value is 96 485 C.mol and the
number of electrons exchanged is always equal to 2, hence:

0
E = −Δ G 0 [2.10]
2F
In the case where the water formed is in liquid form, the theoretical
voltage of an H 2/O 2 fuel cell is:

−Δ G 0 − 237.34
0
E = = = 1.23V [2.11]
×
2F 2 96485
In the case where the water formed is in vapor form, the theoretical
voltage of a H 2/O 2 fuel cell is:

−Δ G 0 − 228.6
0
E = = = 1.185V [2.12]
×
2F 2 96485
The difference is due to the change in the Gibbs free energy during water
vaporization.