Page 111 - Introduction to Transfer Phenomena in PEM Fuel Cells

100 Introduction to Transfer Phenomena in PEM Fuel Cells
where:
– T is the critical temperature of (i) or (j);
C
– p c is the critical pressure of (i) or (j);
– M is the molecular mass of (i) or (j);
– a is 0.0002745 for diatomic gases (H 2, O 2 and N 2);
– a is 0.000364 for liquid H 2O;

– b is 1.832 for diatomic gases (H 2, O 2 and N 2);
– b is 2.334 for H 2O vapor.

3.4.3.3. Sorption isotherm
The water, liquid in the membrane and in as a vapor in the diffusers,
undergoes a phase change at the membrane/diffuser interface (at the
electrodes); this phenomenon is known as sorption.

Indeed, the water vapor molecules will bind to the structure of the
membrane, where they will condense. This phenomenon is called adsorption
(or simply sorption); and the reverse phenomenon is called desorption [SER
07]. The sorption mechanism is responsible for a release of heat. The porous
structure of the polymer site allows the adsorption of existing water into the
gas diffusion layers in gaseous form [GOT 08, ZAW 93]. Assuming there is
thermodynamic equilibrium between the water vapor in the support layers and
the liquid water in the polymer, the sorption curve of Hinatsu et al. [HIN 94]
enables the water content of (λ a) and (λ c) to be calculated, according to the
activity of the water vapor at 30 and 80°C [NGU 10]:

2
×
+
×
×
λ 30 C = 0.043 17.81 a − 39.85 a + 36 a 3 [3.38]
°
2
λ 80 C = 0.3 10.8 a − 16 a + 14.1 a 3 [3.39]
×
×
×
+
°
The activity of the water (in the vapor state) in the diffusion layers is
calculated from the water concentration and the saturation pressure at the
relevant cell temperature at the membrane/diffuser interface [NGU 10]:
RT
a = c HO ⋅ P sat () [3.40]
T
2