Why the Gibbs free energy of water -237.14kJ/mol is chosen to calculate the Nernst voltage in PEMFuelCel​lSystemExa​mple, but not the one of water vapor -228.60 kJ/mol?

In the code of FuelCell.ssc, the activity of water vapor (a_H2O_C, RH%) is utilized to calculate the Nernst voltage, but not the activity of liquid water(1).

so, is the Nernst voltage based on the product of liquid water, or water vapor?

[Yifeng Tang]

Not sure ...

Using the liquid water value of -237.14kJ/mol gives me a standard potential of 1.2289V

G_H2O  =  -237.14e3 ; % J/mol
F      = 96485.33212; % C/mol
E_cell = G_H2O/(-2*F) % V

E_cell = 1.2289

This seems to agree well with references I found. For example https://users.encs.concordia.ca/~pillay/fuel-cells.html, Table 4.2.

Using -228.60 kJ/mol for G gives me 1.1846V instead.

G_H2O  =  -228.60e3 ; % J/mol
F      = 96485.33212; % C/mol
E_cell = G_H2O/(-2*F) % V

E_cell = 1.1846