Journal of New Materials for Materials for Electrochemical Systems 4, 233-238 (2001)
c J. New. Mat. Electrochem. Systems
Advancements in Fuel Cell Stack Technology at International Fuel Cells
D.J. Wheeler∗, J.S.Yi, R. Fredley, D. Yang, T. Patterson Jr., and L. VanDine
INTERNATIONAL FUEL CELLS, LLC
195 Governor’s Highway, South Windsor, CT 06074
( Received June 11, 2001 ; receved in revised form October 8, 2001 )
Abstract: The International Fuel Cells’ cell stack concept has specialized design characteristics to address the difficulties in removing liquid water
from a PEM cell. Product water is removed by two mechanisms: 1) Transport of liquid water through the porous bipolar plates into the coolant.
2) Evaporation into the reactant gas streams until the gas streams are fully saturated.
Key words : PEM, bipolar plates, passive water management, thermal management, Humidification, air utilization, fuel utilization.
1. INTRODUCTION
by preventing dryout of these membranes, a common mode of
failure. A third function afforded by the water filled pores in
the bipolar plate is thermal management. The development of
localized hot spots, i.e. a localized temperature increase, is typically the result of high reactant concentrations producing a localized increase in current density. The flooded pores of the
bipolar plate supply water for evaporation, which in turn results
in local cooling of the cell. Alternatively, regions within the cell
that become cooler, sometimes caused by a localized decrease
in one or both of the reactants, can lead to precipitation from the
reactant gases at these local cold spots. The liquid water that accumulates due to this precipitation is rapidly removed into the
pores of the porous bipolar plates and transported to the coolant
stream.
The basic components of the International Fuel Cell’s PEM cell
at first glance appear similar to other PEM cell designs with
membrane, catalyst, substrates, and bipolar plates. Closer inspection identifies a very different construction and composition for the bipolar plate, the IFC bipolar plate is porous and
serves multiple functions for the PEM fuel cell. The pores of
the plates are filled with liquid water that communicates directly
with the coolant stream, and these filled pores engender product water removal with transport of the product water from the
cathode through the pores into the coolant stream. The product
water contained in the coolant stream is then carried to an accumulator/reservoir. The water management characteristics are
schematically shown in Figure 1a. For comparison, the product water removal for a PEM cell having solid plates is shown
in Figure 1b where the product water removal is constrained to
the flow field. Table 1 identifies the product water flow regimes
associated with porous plates and solid plates.
The automatic passive water maintenance within the cell greatly
simplifies the control system for a PEM cell stack while participating in maintaining the water balance within a cell. Separation
of the reactants in adjacent cells is the fourth contribution of the
water filled pores within the cell stack where the separation is
accomplished using a water barrier. The porous plate concept
facilitates simplified design of reactant flow fields which leads
to operation of the cell stack at ambient pressure.
The flooded pores serve a second purpose of supplying water to
the incoming reactant gases and humidifying those gases prior
to and at entry to the cell. This is a critical function to preserve the perfluorinated sulfonic acid membranes in a PEM cell
∗ email
: [email protected]
233
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D.J. Wheeler et al./ J. New Mat. Electrochem. Systems 4, 233-238 (2001)
Table 1: Product Water Removal
Regime
Description
Liquid transfers
into plates & reactant gases saturated
Solid Plate Product Water Removal
Plug
Alternate plugs
of liquid and gas
Stratified
Smooth liquid flow
along bottom,
gas above
Wavy
Stratified flow
with waves
Slug
Periodic separation
of wave and froth
Annular
Liquid film around
channel, gas flow
in core
Gas
Velocity
Probable Cell
Performance Impact
Stable, High
Performance
<3 ft/sec
Unstable, Low
Performance
Stable, low
performance
Porous Plates
Spray
Almost all liquid
as fine entrained
droplets
2 to 10
ft/sec
15 ft/sec
>20 ft/sec
>200
ft/sec
Unstable, low
performance
Unstable
performance
Stable, average to
good performance,
function of film
thickness
Stable, Good
Performance
to be pumped through the porous plate into the coolant water
flow field.
Figure 1: a) Porous Plate Schematic Representation of Water
Management. b) Solid Plate Schematic Representation of Water
Management.
2.
2.1
RESULTS AND DISCUSSION
Water Removal in the Water Transport Plates
Key to the removal of the product water through the porous plate
is the controlled porosity and wettability of the porous plate,
typically a carbon plate. These parameters control the water permeability of the porous plates. In Figure 2, the permeability is
given as a function of the porosity. The wettability is assured by
treating the plates with metal oxides, typically tin oxide[1]. The
through-plane and in-plane permeability of the porous plates is
given in Figure 3 as a function of the density of the plates. In
Figure 3, we identify the minimum required cathode and anode
permeability of the plates to maintain the flow fields free of liquid water. The movement of the water through the porous plate,
i.e., from the cathode to the coolant channels as shown in Figure 1a, is assisted by maintaining a small pressure differential
between the reactant and the coolant [2-4]. On the cathode, the
oxidant reactant gas is pressurized to a pressure that exceeds the
coolant loop pressure. The resultant ∆P will cause liquid water
appearing on the cathode side of membrane electrode assembly
At the anode, the fuel flow field pressure will be maintained at
a level which allows coolant water migration from the coolant
loop through the fine pore plates toward the membrane, but
which prevents flooding of the anode surface of the membrane
with coolant water. The water management afforded by the
porous plates facilitates removal of the product water but also
humidification of the membrane.
2.2
Humidification
Dry reactants or partially humidified reactants become fully saturated within the PEM cell. For solid plate systems and for
porous plate systems the product water readily humidifies the
oxidant. The evaporation of water into the fuel stream for the
solid plate systems must come from the membrane with the possibility of decreasing the water content of the membrane, and
increasing the probability of membrane dryout and cell failure.
On the other hand, the porous plate supplies water at the fuel
inlet and provides an alternative humidification source.
Out-of-cell testing using a Visi-rig that allows direct observation of the rate of humidification and penetration distance of the
reactants into the cell to reach saturation demonstrates the humidification concept associated with the porous plates. In the
Visi-rig only the porous plates supply water for humidification
to the inlet reactants, and there is no membrane to supply water
for humidification. Figure 4 shows a pattern identifying a penetration region on the air side observed in a Visi-rig experiment.
Advancements in Fuel Cell Stack Technology . / J. New Mat. Electrochem. Systems 4, 233-238 (2001)
Figure 2: Water Permeability of Porous Plates vs. Open Porosity.
235
Figure 4: Visi-rig photograph showing depletion of water at the
upper left corner of cell.
Figure 3: Through-Plane and In-Plane Permeability with Minimum Anode with Cathode Porous Plate Permeability Identified.
Figure 5: Penetration depth to achieve 100% relative humidity:
Evaluation of humidification strategy.
At the upper air inlet section of the porous plate some drying is
observed across the air inlet with the addition of dry air. The
extent of the drying and the pattern of the drying are specific
to this particular Visi-rig experiment. None-the-less, the benefits of using a Visi-rig to study reactant gas humidification are
evident.
50% recycle is used, the distance required to reach 100% relative humidity actually increases due to the increased reactant
velocity. The impact of these two prehumidification approaches
is less than the effect of doubling the contact area. Prehumidification or air recycle are not as effective as increasing the contact
area of the porous plate at the inlet.
Research on the effect of inlet humidifier area of the porous
plate to the incoming reactants shows that doubling this humidifier area relative to the baseline decreases the penetration distance to achieve full humidification by 50% The relationship of
evaporative contact area for humidifying the incoming reactants
appears to be dominant factor for humidification. In Figure 5,
different humidification strategies are evaluated and the results
are shown. The penetration distance from the reactant inlet
to achieve 100% relative humidity improves from the baseline
when prehumidification (50% relative humidity) is used. When
2.3
Thermal Control
Non-uniform temperature distributions are observed in all fuel
cells and are primarily the result of the differences in the concentration of reactants within the cell. For example, the high
concentration of oxygen at the oxidant inlet can result in a localized increase in current density that increases the local temperature. Alternatively, cool regions, relative to the hot zone or even
the average cell temperature, can exist where depletion of the
reactants occurs. Research on different flow field patterns and
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D.J. Wheeler et al./ J. New Mat. Electrochem. Systems 4, 233-238 (2001)
A hot zone to cold zone temperature change from 80o C to 60o C
at atmospheric pressure would result in a condensation flux of
about 0.51 grams/cm2 -hr at typical operating conditions. This
condensation will result in liquid water formation in the flow
field and for solid plates can lead to a flow regime as described
in Table 1, .e.g., plug flow or stratified flow. On the other hand,
the porous plate, under the control of the pressure differential
between the reactant flow field and the coolant stream, transfers
the condensed liquid to the coolant stream and maintains the
reactant flow field free of any obstructions.
2.4
Figure 6: Calculated Thermal Map of an IFC Single Cell showing different temperature zones.
designs is well documented and includes: 1) counter flow, i.e.
the reactants in the cell flow in opposite directions, 2) cross flow
where the reactants flow orthogonal to each other, and 3) the
simple case of parallel flow. Complex flow geometry is reported
for pressurized systems, and distribution of reactants for these
pressurized cell designs can also lead to temperature variations
within the cell. International Fuel Cells has developed thermal
profile models that correspond well with experimental data for
the temperature variations observed in a cell. Figure 6 identifies the temperature zones calculated using the thermal model
for cross flow of the reactants. Extreme temperature zones are
specifically labeled in the figure.
For a PEM fuel cell, a localized increase in temperature will
increase the quantity of water vapor in the reactants at that location since the water content follows the steam table. The water for this localized evaporation is supplied by the water filled
porous plate for the IFC design. A solid plate design requires
the localized water to be supplied by evaporation of the product
water or from the membrane.
Conversely, water vapor in the reactant stream condenses on
colder walls of the flow field as it moves from the hot zone to a
cold zone. The molar condensation flux is:
ṅcond = ṅHZ − ṅCZ
Psat
Psat
= ṅHZtot ?
− ṅCZtot ?
Ptot HZ
Ptot CZ
(1)
where ṅcond = molar condensation flux, ṅHZ = hot zone condensation flux, ṅCZ = cold zone condensation flux, ṅHZtot =
hot zone total flux, ṅCZtot = cold zone total flux, Psat = saturation pressure, and Ptot = total pressure.
Utilization
Optimization of the fuel cell requires high utilization for both
the fuel and oxidant. Achieving high utilization decreases the
overall parasitic power consumed in operating the power plant.
For example, the parasitic power consumption by the blower
used in an atmospheric pressure fuel cell power plant is given
by equation 2
P arasiticP ower =
1
U 1+f
(2)
U 1+f is the utilization to the 1 + f power where f = 1 for
laminar flow and when f = 2 for turbulent flow.
The reduction of the parasitic power consumption with the increase in utilization benefits the power plant by reducing the size
and power requirements for the air blower or turbo-compressor
in the case of pressurized systems.
2.5 Influence of Utilization on Water Balance
For a fuel cell power plant to remain in water balance, i.e., not
be a net consumer of water, the water in the reactant gas stream
must be recovered. Recovery of the water is typically done using a condenser that is a net consumer of energy. A high water
content in the exhaust stream contributes to the parasitic energy
consumption and devices to collect the water add weight and
volume to the fuel cell system. Minimization of the water evaporated into the exit reactants is an objective that will result in a
simplified fuel cell power plant. One method to minimize the
water removal by the spent reactant gas stream is through increased utilization of the reactants.
Product water removal through evaporation to the reactant gases
is one of the more straightforward methods to eliminate product
water from the fuel cell. The total amount of water removed
through the evaporation of water into the reactant stream is, of
course, a function of the utilization of the reactant. The high utilization, in turn, allows for an increase in the cell exit temperature. The relationship between the cell exit temperature (System
Exhaust Dew Point (o C)) for a fuel cell using hydrogen as the
fuel is given by the following:
Advancements in Fuel Cell Stack Technology . / J. New Mat. Electrochem. Systems 4, 233-238 (2001)
237
Figure 7: Relationship between system air utilization and Exhaust Dew Point for a Hydrogen fuel cell at atmospheric pressure and 3 Bar.
Figure 9: Performance as a function of oxygen utilization for a
hydrogen air fuel cell at different current densities.
2.6
Performance Optimization of Cell Stack
Figure 8: Effect of Optimization of the Porous Plate Flow Fields
on Utilization at 0.9 A/cm2 .
UAir =
)
2.381 PP(T
T
)
1 − 0.5 PP(T
T
(3)
where P (T ) is the temperature dependent vapor pressure and
PT is the total pressure. The relationship is graphically shown
in Figure 7.
Design of the reactant flow fields of the bipolar plates to maximize utilization of reactants has resulted in decreased water removal in these streams. Figures 8 & 9 show the results with
such improved flow field designs where air utilization were increased from 60% to 80% over a range of current densities.
The data in Figures 7, 8, & 9 demonstrate that increased operating temperatures can be achieved through higher utilization
while maintaining water balance. The contribution of the increased temperature to the fuel cell system is a decrease in the
radiator size.
Figure 10: Comparison of Performance; Optimized Porous
Bipolar plate to previous standard design.
Optimization of the cell stack based on the improved flow field
designs, product water removal through porous parts, and optimization of the utilization has resulted in a 175% increase in
the power density for a cell operating at 0.6 V. This is shown in
Figure 10.
The performance improvements associated with the single cell
optimization of the porous bipolar plate water management and
the concomitant improvement in the flow field design directly
translate from the single cell to a 20-cell stack and a 276 cell
75 kW cell stack as shown in Figure 11. Differences in performance for the single cell and the two cell stacks are within
experimental error. At a current density of 1.2 A/cm2 the cell
voltage is 0.6 V for this hydrogen air cell while operating at
atmospheric pressure. Analysis of the performance curves in
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D.J. Wheeler et al./ J. New Mat. Electrochem. Systems 4, 233-238 (2001)
[4] C. Reiser, U.S. Patent No. 5,833,903
Figure 11: Performance of Single Cell, 20 Cell Stack, and
75 kW stack using Optimized Porous Plates
Figures 10 & 11 indicate some curvature at current densities
greater than 1 A/cm2 indicating some remaining mass transport
polarization in the cell.
3.
CONCLUSION
The improvements of the porous bipolar plates permits water
removal at atmospheric pressure and eliminates liquid water in
the flow fields. The water filled porous bipolar plates supply water to humidify the incoming reactants and this, combined with
elimination of the liquid water in the flow fields, permits thermal management of the cell. Water formation in the cell due to
temperature variations across the planform of the cell are eliminated. Increasing the air utilization leads to higher operating
temperatures and improved single cell and multiple cell stack
performance.
Table 2: Price of High Velocity Water Management
Porous Plates
Evaporation
Evaporation &
Entrainment
Air
Velocity
ft/sec
Pressure Drop
psia
8
28
>30
0.4
1.5
5.0
inches of
water
12
40
140
REFERENCES
[1] J. Bett et al., U.S. Patent No. 5,840,414
[2] A. Meyer et al., U.S. Patent No. 5,503,944
[3] C. Reiser, U.S. Patent No. 5,700,595