(35a) A Study of Channel Dimension Effect On Performance of Interdigitated Flow Field PEM Fuel Cells

            Novel water management
and reactant distribution strategies are important to the development of next
generation polymer electrolyte membrane fuel cell systems (PEMFCs).
Over-hydration (flooding) or under-hydration (membrane drying) of channels and critical
membrane electrode assembly components can have detrimental effects on cell
performance and lifetimes. Improving these strategies in PEMFCs leads to higher
power density and reduced stack size for vehicle applications, which in turn
reduces weight and improves the price competitiveness of fuel cell systems.

            Parallel and
interdigitated flow fields, Figure 1, are two common types of PEMFC designs
that have benefits and drawbacks depending upon operating conditions and
flow-field geometry. Parallel flow fields suffer from longer diffusion lengths
which inhibit delivery of reactants and removal of byproduct water.
Interdigitated flow fields induce convective transport, known as cross flow,
through the porous GDL between adjacent channels and therefore are superior at
water removal beneath land areas, which can lead to higher cell performance.
Unfortunately, forcing flow through the GDL results in higher pumping losses as
the inlet pressure for interdigitated flow fields can be up to an order of
magnitude greater than that for a parallel flow field. Additionally, the
pressure gradient between inlet and outlet channels in an interdigitated flow
field may not be evenly distributed along the channel length. This may cause
cross flow maldistribution in the same way Z-manifolds
may unevenly distribute gasses across the system. Such maldistribution
of reactant gasses along the active area of a cell may lead to areas of low
cross flow, and areas of excessive cross flow. This, in turn, can cause areas
of low oxygen concentration, water build up, which can lead to higher pressure
losses and uneven membrane hydration all of which reduce overall cell
performance. This research seeks to examine the affect varying channel dimension
sizes have on the performance of a fuel cell under interdigitated

conditions.
The features of interest are channel length, width, and depth. The width of
lands for the bipolar plates mirrors the width of channels on that plate. Changing
feature size affects the distribution of liquid water, as well as the
distribution of gaseous reactant in the cell. This work will help develop a
better understanding of cross flow for interdigitated flow fields.

            For
the length study portion of this body of work, a PEMFC was designed that could
be configured to run under varying interdigitated channel length configurations
through the use of separate manifold exit ports. The lengths studied were 5 cm, 15 cm and 25 cm each with a width of 2
cm. The flow field is comprised of 10 channels with width/depth/land dimensions
all of 1 mm. The cell was controlled using stoichiometry
to account for the change in active area at the different channel lengths.
Tests were performed at stoichiometric conditions of
1.5 on the anode and 2.0 on the cathode (normal), and 2.0 on the anode and 4.0
on the cathode (high). Humidity, temperature and cell compression were held
constant for all trials. Pressure was measured at the inlet of the cell while
the outlets were exhausted to atmosphere. The cathode was run in
interdigitated configuration, while the anode was run in parallel to
isolate the effects of the cathode. Polarization and inlet pressure curves were
produced for each length case at the two different stoichiometries.
A total of three trials for each case were conducted in random order so as to
verify repeatability. Figure 2 displays the average polarization, inlet
pressure and net system power data, taking into account pumping losses, for
each different length. Reduced cell performance with increasing channel length
is a trend that was observed.

            This experimental work
was complemented by a modeling study of the reactant flow through the GDL at
these various lengths and flow rates using the multiphysics
package COMSOL. Using the dimensions from the experimental cell, interdigitated
flow fields at varying lengths and conditions were simulated in three
dimensions. The model focuses on the cathode flow field and GDL layer and does
not take into account the electrochemical reaction. As this study is interested
in how channel length affects pressure and cross flow distribution, modeling
the entire electrochemical process is not necessary since the electrochemical reaction
has little effect on bulk cross flow; additionally, the experimental portion of
this work characterizes cell performance. Reaction by-product water is a factor;

however, no models can currently
account for this completely. Figure 3 displays some of the key results from the
modeling work. The pressure distribution in inlet and outlet channels was shown
to be more variable in the 25 cm cell than in the 5 cm cell. As a result, the
cross flow between adjacent channels in the long cell is maldistributed
along its length compared with the short cell, which shows a very consistent
cross flow rate. This effect may help explain why the shorter channel length
configuration produced the best performance. The center
region of the 25 cm cell, where the cross flow rate is lower, may be subject to
reduced oxygen concentration and increased water buildup. Regions toward
the entrance and exit, while receiving higher cross flow rates, may be
oversupplied and contributing to increased pressure drop due to higher
velocities. 

The even cross flow rate in the
short cell appears to be more advantageous when these results are compared to
the experimental data and suggests that interdigitated designs are sensitive to
maldistribution along the channel length. The results
of this work were compared to a similar earlier study on parallel flow field channel
length; the overall trends are summarized in Figure 4. These findings suggest
that for large aspect ratio fuel cells (e.g. long channel lengths and small
channel widths) if interdigitated designs are attempted, attention must be paid
to the distribution of reactant gas cross flow. Compared to parallel flow
fields, which follow a reverse trend due to increased slug removal with longer
channel length, interdigitated design flow patterns should be oriented so
channel length is minimized. Neutron radiography is being conducted to study
the water removal characteristics of interdigitated

cells of varying length. Through-plane
images of in-situ cell operation should allow for comparison of inlet and
outlet water content as well as comparison of GDL water distribution.

            The affect of width
and depth of channels are currently being examined. Channel dimensions of interest
are (width x depth, in millimeters) 1x1, 0.5x0.5, 0.25x0.25, 0.5x1, and 0.25x1.
The effect of these features is being explored through the use of
interchangeable bipolar plates. The bipolar plates are 20 cm long and 1.5 cm
wide, for a total active area of 30 cm². The bipolar plates are machined
out of aluminum and have a nonreactive gold coating on the active areas to
prevent corrosion and eliminate unaccounted for reactions on the surface of the
plate. The use of gold further reduces the interfacial contact resistance with the
GDL, decreasing the voltage loss due to ohmic
resistance.

            These bipolar plates
can fit into a cell superstructure that has the ability to change from parallel
configuration to interdigitated configuration through the use of valves and a
novel manifold ? channel connection design. The valves close the inflow of gas
to every other channel, and close the outflow of gas to the remaining channels.
This can be done without taking apart the cell, allowing for direct in situ
comparison of parallel and interdigitated flow fields.

            The work of this study
has already been started, with preliminary testing of a nickel plated version
of the 1 x 1 mm plates already completed. These results can serve as a baseline
for comparison of the gold plated set of bipolar plates. This preliminary
testing has further given a better understanding of how the system behaves
during operation and improvements which may be made in the new plates.

            Further research will be required in
the area of modeling fuel cells with electrochemical reactions to

determine
the effect of these channel dimensions on cell performance. Neutron radiography
will need to further be performed on the plates with varying channel widths and
depths to isolate the water removal characteristics of these designs. Coupling
this data with performance trends should give researchers and cell industry a
better understanding of design consequences.