H2Tech - Q4 2021 - 20

SPECIAL FOCUS FUTURE OF HYDROGEN ENERGY
be carried out at different polarizations
of the fuel cell to investigate phenomena
at different loads. The combination of
modeling impedance spectroscopy and
parameter estimation with experimental
data can then provide accurate descriptions
of transport and reaction properties
in fuel cells during operation at different
loads. Over time, the models and
experiments may reveal the source of
deterioration of a cell. This implies that
the proper actions in design and material
selection can be taken to increase
performance and slow deterioration.
FIG. 4 shows a so-called Nyquist plot
FIG. 5. A unit cell model may consider a part of the membrane-electrode assembly,
as well as the metallic plates that serve as the current collectors and feeders.
of the results from a high-fidelity model
of a fuel cell unit cell. This is a small
experimental cell where the conditions
can be accurately controlled. The model
shows the effect of the reaction kinetics
at the active sites of the cathode. As
the catalyst deteriorates, the cathodic
semicircle grows (so the impedance
grows). However, there is no change at
very high frequencies since the kinetics
are unable to react to very fast perturbation.
The ohmic losses in the cells are
constant. In this way, it is possible to
separate other losses, too, such as ohmic
or transport losses.
Modeling and simulations offer an
effective, almost unique way of studying
the processes. As mentioned above, it is
difficult to measure the phenomena that
occur in the active layer during operation.
Instead, these phenomena can be
modeled in detail, and their impact at
the macroscopic level can also be modeled
in so-called multiscale models.2
ExFIG.
6. Left: Stack with crossflow configuration, i.e., the O2
and H2
channels run in a 90° angle
relative to each other. Right: One cell in the stack. FIGS. 1, 2, 5 and 6 illustrate the transition
from the microscale to the stack scale.
periments can be designed to verify the
implications of the microdesign.
One example is the connection of
physics-based models for impedance
spectroscopy with measurements, as
shown in FIG. 4. This allows for scientists
and engineers to separate processes in
different time scales, such as diffusion
(slow) and current conduction (fast).
Studying what limits the response to
perturbations at different time scales
may reveal which process limits the performance
at the microscale.3,4,5
Once the processes are understood,
FIG. 7. Relative humidity in a section of a serpentine channel PEMFC, in contrast to the straight
channel configuration in FIG. 6. The section is small enough to include all of the relevant
transport and reaction processes in the fuel cell in a high-fidelity model. Simulation in COMSOL
Multiphysics.
20 Q4 2021 | H2-Tech.com
more direct methods may be used. An
example is the innovation of using ordered
porous structures in the active
layer to lower tortuosity. Ordered structures
may increase the transport of reactants,
improve access to the catalyst
surface, and yield a uniform current
density distribution in the active layer.1,6
The results may be improvements in
performance without requiring a higher
platinum load or causing the accumulation
of water or harmful byproducts that

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H2Tech - Q4 2021

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