Environmental Engineering Reference
In-Depth Information
the PEM, resulting in a so-called catalyst-coated membrane (CCM) [Gasteiger et al.,
2005].
It would certainly be desirable to evaluate catalyst performance and understand size
and structural effects directly under the conditions of fuel cell operation. However,
determination of kinetic parameters in a single-cell fuel cell is associated with a
number of limitations. Let us consider some of them.
The first limitation is related to interference of the anode and the cathode. The finite
permeability of the Nafion
w
membrane to fuel and oxygen results in crossover of fuel
from the anode to the cathode, and oxygen crossover in the opposite direction. This
may have a significant influence on electrode kinetics.
The second limitation is concerned with the high time constant t
cell
determined
by the high capacitance of the CLs and by substantial cell resistance. t
cell
for a
thick MEA of large geometric area may exceed several minutes, thus strongly limiting
the application of transient methods and methods based on the concept of impedance.
The third limitation is concerned with the numerous contributions to the cell vol-
tage V
cell
, which, along with the difference in the electrode reversible potentials
DE
eq
, comprises overpotentials at the cathode, h
C
, and the anode, h
A
, as well as the
ohmic drop DE
ohmic
:
V
cell
¼
DE
eq
h
j j
h
j j
DE
ohmic
(15
:
8)
For isolating the overpotential of the working electrode, it is common practice to
admit hydrogen to the counter-electrode (the anode in a PEMFC; the cathode in a
direct methanol fuel cell, DMFC) and create a so-called dynamic reference electrode.
Furthermore, the overpotential comprises losses associated with sluggish electroche-
mical kinetics, h
C,A
, as well as a concentration polarization h
conc
C,A
, related to hindered
mass transport:
h
C,A
¼
h
C,A
þ
h
conc
(15
:
9)
C,A
Ohmic losses DE
ohmic
originate from (i) membrane resistance, (ii) resistance of CLs
and diffusion layers, and (iii) contact resistance between the flow field plates.
Although it is common practice to split current - voltage characteristics of an MEA
into three regions—“kinetic” (low currents), “ohmic” (intermediate currents), and
“mass transport” (high currents) [Winter and Brodd, 2004]—implicit separation of
h
i
, h
con
i
, and DE
ohmic
is not always straightforward, and thus studies of size and
structural effects under conditions of non-negligible mass transport and ohmic contri-
butions may be fraught with significant errors.
In practice, reduction of the size of metal particles supported on porous carbon
materials is usually implemented by decreasing the metal loading, which can be
done either by decreasing the metal percentage at constant carbon surface area or
by keeping the metal percentage constant but increasing the specific surface area of
the support. These changes may affect not only kinetic but also mass transport and
ohmic losses [Kaiser et al., 2007]. Evaluation of the influence of particle size on
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