Page 281 - Biofuels Refining and Performance

260 Chapter Nine

where ∆G is the change in Gibbs free energy for the reaction, n is the
number of moles of electrons involved in the reaction per mole of H , and
2
F (Faraday’s constant) 96,487 C (coulombs joules/volt). At a constant
pressure of 1 atm, the change in Gibbs free energy in the fuel cell process
(per mole of H ) is calculated from the reaction temperature (T ) and from
2
changes in the reaction enthalpy (H ) and entropy (S ).
G H T S
285,800 J (298 K)( 163.2 J/K)
237,200 J

For the hydrogen–air fuel cell at 1 atm pressure and 25 C (298 K), the
cell voltage is

G
E
nF
237,200 J
52a 2 b 5 1.23 V
2 3 96,487 J/V

As temperature rises from room temperature to the PEM fuel cell
operating temperature (80 C or 353 K), the change in values of H and S
is very small, but T changes by 55 C. Thus the absolute value of G
decreases. Assuming negligible change in the values of H and S,

G 285,800 J/mol (353 K)(163.2 J/mol
K)
228,200 J/mol
Therefore,

228,200 J
E 52a 2 b 5 1.18 V
2 3 96,487 J/V

Thus, for standard pressure of 1 atm, the maximum cell voltage
decreases from 1.23 V at 25 C to 1.18 V at 80 C. An additional correc-
tion is needed for using air instead of pure oxygen, and also for using
humidified air and hydrogen instead of dry gases. This further reduces
the maximum voltage from the hydrogen–air fuel cell to 1.16 V at 80 C
and 1 atm pressure. With an increase in load current, the actual cell
potential is decreased from its no-load potential because of irreversible
losses, which are often called polarization or overvoltage (h). These origi-
nate primarily from three sources:
Activation polarization (h )
act
Ohmic polarization (h ohm )
Concentration polarization (h conc )