Environmental Engineering Reference
In-Depth Information
methods able to dynamically control the rotational speed of the wheel according to
the process variables.
4.5 Water/Humidification Management System
Polymeric proton exchange membrane needs to be maintained properly humidified
to guarantee a satisfactory ion conductivity during stack operation (see
Sect. 3.2
).
In fact it exists a strong relationship between proton conductivity and water
content of Nafion material used as membrane in PEMFC [
24
,
25
]. Unfortunately
the water produced at cathode side and the air moisture could be not sufficient to
maintain properly wet the membranes in all working conditions, because of
complex phenomena involving water within MEA [
26
] (Fig.
4.7
).
The Fig.
4.7
refers to water fluxes occurring inside a MEA fed by pure
hydrogen in dead-end configuration (see
Sect. 4.2
) and by not saturated air stream.
The following four events are evidenced:
1. The water is produced on cathode electrode surface (O
2
? 2H
+
? 2e
-
? H
2
O).
2. A back diffusion occurs from cathode side to anode through the electrolyte
because of concentration gradient.
3. H
2
O
molecules
are
dragged
by
protons
during
stack
power
requirement
(electro-osmotic drag).
Purge
cathode
stream
water
removal
cathode
stream
water
removal
electro-osmotic drag
electro-osmotic drag
H
2
O
production
H
2
O
production
back diffusion
back diffusion
H
2
INLET
AIR
INLET
Fig. 4.7
Water behavior inside MEA of PEMFC
