Environmental Engineering Reference
In-Depth Information
As water in a PEM fuel cell is initially produced in the vapor phase, the membrane
hydration from this phase is more relevant with respect to water uptake from liquid
phase. In particular, when water is collected by vapor phase, two distinct mech-
anisms can be individuated: at lower vapor concentration ion solvation occurs
inside the membrane, while in conditions of pre-saturated vapor phase polymer
swelling is observed with accommodation of a larger amount of water. At this
regard an experimental polynomial equation has been proposed to correlate water
content of membrane with water partial pressure [
10
]:
2
þ
36
3
p
p
sat
p
p
sat
p
p
sat
k
¼
0
:
043
þ
17
:
18
39
:
85
ð
3
:
20
Þ
where k is the number of water molecules per sulfonic group present in the
co-polymer, p is water partial pressure, and p
sat
is the saturation pressure.
The maximum amount of water that can be taken by the polymeric membrane
depends also on the state of water, in particular a Nafion membrane in liquid water
can take approximately 50% of water more than from vapor phase. However, an
excess of liquid water in contact with the membrane of a PEM fuel cell can
determine flooding phenomena which strongly decrease its proton conductivity. On
the other hand, an increase of temperature up to 90C favors a strong improvement
in proton conductivity with respect to room temperature, while low humidity
conditions have been found to cause fast degradation of Nafion-like membranes
[
11
]. As a consequence, the necessity of an accurate humidification of the poly-
meric membrane is one of the key aspects affecting both performance and reli-
ability of PEM fuel cells (see
Sect. 4.5
). If the currently used polymeric membranes
exhibit their interesting properties (high conductivity, chemical stability, and
mechanical flexibility) in highly hydrated conditions, then the maintenance of high
humidity at temperatures higher than 90C would require heavy pressurization, that
could imply energy costs not compatible with high efficiency expected from these
systems. For this reason many studies are currently being carried out about the
possibility to realize proton exchange membranes characterized by more water-
independent proton conductivity, or by higher water retention capability, then able
to operate in PEM fuel cells at temperature higher than 100C (in the range
120-150C) and low pressures. Such membranes could also provide an improved
CO tolerance, being well known from thermodynamic analyses that adsorption
capacity of platinum toward carbon monoxide decreases at higher temperatures. In
particular it has been found that while CO tolerance is 10-20 ppm at 80C, it
becomes 1000 ppm at 130C and rises up to 30,000 ppm at 200C[
12
,
13
]. Recent
results confirm that a typical reformate gas with CO percentage comprised in the
range 2-5% can be fed to PEM fuel cells working at temperature above 180C
directly from the fuel processor [
14
]. In this way the need for CO cleanup by
catalytic selective oxidation of the fuel reforming stream could be eliminated,
significantly improving the overall system efficiency. In addition, as the rate of heat
removal is proportional to the temperature difference between the system and the
environment, the increase in PEM fuel cell working temperature above 120C
