Page 155 - Introduction to Transfer Phenomena in PEM Fuel Cells

144 Introduction to Transfer Phenomena in PEM Fuel Cells
The sorption phenomena thus reflect the equilibrium between a liquid
phase (adsorbed) and a gaseous phase (desorbed) at an interface. Transfers
are therefore subject to a phase change at this interface.
Several authors have focused on measuring the enthalpy of water sorption
in a Nafion® polymer membrane. Watari et al. [WAT 03] reviewed the
values used in the literature and reported values varying between 42 and
−1
52 kJ.mol according to the water content of the membrane or its
pretreatment. However, there was no clear correlation between measured
values and experimental conditions. More recently, Burnett et al. [BUR 06]
took measurements using Nafion®112 and obtained values between 43 and
−1
60 kJ.mol .
It is not currently possible to know the exact sorption enthalpy as a
function of the operating conditions. However, the measured values are close
to the latent heat of water vaporization L vap (Atkins and De Paula [ATK 02]
−1
give L vap = 41.6 kJ.mol to 353 K). In what follows, the sorption enthalpy of
water in the membrane will be considered equal to the latent heat of
vaporization of the water L vap [ROS 99].

According to Ramousse et al. [RAM 09], the algebraic character of the
calculated water flows makes it possible to treat sorption and desorption in
–2
the same way. The heat flow densities are expressed in [W.m ]:

Q sorp =Δ H sorp ⋅ N a H O [4.37]
a
HO
2
2
and:

Q sorp =−Δ H sorp ⋅ N c H O [4.38]
c
HO
2
2
In order for the production of water in liquid form at the cathode to be
generated, we write:
j
N c HO = N a HO + 2F [4.39]
2
2