Page 71 - Introduction to Transfer Phenomena in PEM Fuel Cells

60 Introduction to Transfer Phenomena in PEM Fuel Cells
The electrical potential, that is, the difference between two electrode
potentials, is written as:
E theo = E theo _ cathode − E theo _ anode [2.24]
According to the Nernst law, we can write:
– for the anode:
( )
Δ G anode = Δ G 0 anode − RTln P H 2 [2.25]

– for the cathode:
( )
Δ G = Δ G 0 − RT ln P [2.26]
cathode
cathode
2 O 2
The energy that is actually recoverable is that of the standard free
0
enthalpy (Gibbs free energy standard), which is denoted as (ΔG ):
– for the anode:

Δ
Δ G 0 anode = Δ H 0 anode − T S 0 anode [2.27]
– for the cathode:
Δ
Δ G 0 cathode = Δ H 0 cathode − T S 0 cathode [2.28]

The variation of enthalpy and entropy is a function of the temperature,
T 0 = 298.15 K:

T
0 
0
0
Δ H(T) = Δ H(T ) + Δ C .dT [2.29]
p
T 0
and for the entropy:

T 1
0 
0
0
Δ S(T) = Δ S(T ) + Δ C.dT [2.30]
p
T 0 T