Environmental Engineering Reference
In-Depth Information
the relationship for voltage losses associated with mass transport polarization is
obtained:
V
tr
¼
RT
i
L
i
L
i
2F
ln
ð
3
:
35
Þ
which takes into account for the fast voltage drop associated with mass transport
losses when the current density approaches its limiting value. This rapid voltage
decrease is not represented in Fig.
3.5
as it would occur at current values dan-
gerous for the stack. In these conditions the sudden lack of uniformity of indi-
vidual cell voltages, accompanied by the straight down potential diminution, could
cause the reversal of the electrochemical reaction in some cells, which would
operate as electrolysis cells, with direct mixing of hydrogen and oxygen, strong
rise of local temperature, and possible damages to the MEA.
All polarization phenomena described above can occur at both anode and
cathode of a PEM fuel cell, but due to the higher reaction rate of the hydrogen
anodic oxidation with respect to the oxygen cathodic reduction, the anodic
polarization can be considered much smaller.
If the contribution of ohmic losses (Eq.
3.32
) is substituted in Eq.
3.31
, the
following general relationship between cell potential and current density can be
obtained:
i
þ
i
in
E
¼
E
V
act
;
in
V
ohm
;
in
¼
E
RT
i
þ
i
in
i
0
aF
ln
ð
Þ
R
in
ð
3
:
36
Þ
Equation
3.36
is valid for current densities lower than the limiting value i
L
, then
it does not consider the mass transport resistance, but with the proper values of the
parameters a, i
in
, and i
0
it can be used to fit the polarization curve of Fig.
3.5
,
taking into account for the cell active area and number of cells.
3.3.2 Effect of Operative Parameters on the Polarization Curve
The operative parameters which can be regulated to optimize the stack perfor-
mance are MEA humidification, reactant pressure, stack temperature, and stoi-
chiometric ratio. While the role of membrane humidification, already partially
discussed in
Sect. 3.2
, is closely considered in
Sect. 4.5
and in case studies
(
Chaps. 6
and
7
), the influence of the other parameters is examined here with
reference to the stack of Fig.
3.5
. These effects have already been described from a
thermodynamic point of view (see
Sect. 3.1
), while kinetic implications are con-
sidered in this section for their importance in determining the stack efficiency.
As an example, Fig.
3.6
shows the effect of the relative air pressure on the stack
voltage for two different current values, 12 and 25 A [
45
].
During the above experiment the pressure difference between anode and
cathode
is
maintained
under
40 kPa,
according
to
instructions
of
the
stack
