Journal of New Materials for Electrochemical Systems 8, 257-267 (2005)
c J. New. Mat. Electrochem. Systems
Bipolar Plates for PEM Fuel Cells - From Materials to Processing
Xiao Zi Yuana∗ , Haijiang Wanga , Jiujun Zhanga , David P. Wilkinsona,b
a
b
National Research Council Canada, Institute for Fuel Cell Innovation
3250 East Mall, Vancouver, BC, Canada V6T 1W5
Department of Chemical and Biological Engineering, University of British Columbia,
Vancouver, BC, Canada V6T 1Z4
( Received January 27, 2005 ; received in revised form February 8, 2006 )
Abstract: This paper is a review of recent work done on the development of the bipolar plates for PEM fuel cell applications. The type of
materials used for the plate’s fabrication and the various properties related to PEM fuel cell applications are outlined, and a comparison
of the properties and performances of various plates in PEM fuel cells are presented. The graphite plate has not been found to be the most
effective material for PEM fuel cell application. Alternatively, carbon composite plates and metal plates are considered to be the future for
PEM fuel cells. Because bipolar plate processing contributes significantly to the whole cost of these plates, processing is also discussed in this paper.
Key words : Bipolar plate, PEM fuel cell, Graphite, Carbon composite, Stainless steel (SS)
1.
INTRODUCTION
The PEM fuel cell stack hardware consists of the Membrane
Electrode Assembly (MEA), the bipolar plate, seal, and end
plate, etc., as shown in Fig. 1 [5]. Among the components,
the bipolar plate is considered to be one of the most costly and
problematic of the fuel cell stack. In addition to meeting cost
constraints, the bipolar plates must possess a host of other properties. The search for suitable, low-cost bipolar plate materials
becomes a key element of PEMFC stack development.
There has been increasing interest in the potential use of PEM
fuel cells for residential applications and electric vehicles. However, there are still issues to work through before fuel cell applications can be commercialized [1]. For example, the cost of automotive fuel cells should be competitive with today’s internal
combustion engines. The present cost of fuel cells is estimated
to be about $200/kW [2], while the ultimate goal for replacing
the internal combustion engine is $25-50/kW [3] or as low as
$20/kW [2]. Ballard Power Systems’ target cost for the fuel cell
stack, like the US DOE’s, is $30/kW by 2010. New material development, stack technology innovation, and system optimization are the drivers to achieve the cost target. It is exciting that
Japanese scientists have shown that the fuel cell stack cost can
be reduced to the accepted level, which is as low as that of the
present internal combustion engine, and one of the key factors
is the mass production process of the bipolar plate [4].
The bipolar plate is a multi-functional component within a PEM
fuel cell stack. Its primary function is to supply reactant gases
to the gas diffusion electrodes (GDE) via flow channels. Fig.
2 depicts different designs of the flow channels [6]. The effectiveness of reactant transport depends partially on the art of
the flow-field design [7], so the alternative name for a bipolar
plate is the flow-field plate. Bipolar plates must provide electrical connections between the individual cells. They have to remove the water produced at the cathode effectively [8]. Bipolar
plates must also be relatively impermeable to gases, sufficiently
strong to withstand stack assembly and easily mass-produced.
For transport applications, low weight and low volume are essential [9]. As bipolar plates operate in constant contact with
∗ To whom correspondence should be addressed: Fax: +1-604-221-3001;
Email: [email protected]
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X. Z. Yuan et al./ J. New Mat. Electrochem. Systems 8, 257-267 (2005)
9. High chemical stability and corrosion
resistance (<16 µAcm−2 )
10. Low thermal resistance
Possible bipolar plate materials should meet the requirements
of all these design constraints, and in the long run, bipolar plate
materials should be inexpensive and readily available for the
purpose of mass production [11].
Figure 1: PEM fuel cell stack hardware. (Modified from reference [5] with permission.)
the acidic water (pH ≈ 5) that is generated under the operating
conditions of the stack, high chemical stability and corrosion resistance are required. Oxides formed during corrosion can not
only migrate and poison the catalyst but also increase the electrical resistivity of the plates, and therefore result in reduced fuel
cell performance. In addition, the bipolar plate material must be
able to resist a temperature of 80◦ C or more, high humidity, and
an electrical potential [10].
Figure 2: The different channel designs: the original GlobeTech
geometry, the serpentine geometry, the spiral geometry and the
discontinuous channels geometry. (From reference [6].)
To summarize, the PEMFC bipolar plate technical design criteria or major constraints are:
1. Low-cost (<$2/plate)
2. Easy for gas flow
3. High electric conductivity (>100 Scm−1 )
4. Low permeability to gases
5. High manufacturability
6. Reasonable strength
7. Low weight
8. Low volume
The majority of PEM fuel cell stack producers utilize bipolar
plates based on graphite. Now more attention has been paid to
composites or metals. For metallic fuel cell bipolar plates, there
are some institutes and companies that have made substantial
efforts in mass production, such as Oak Ridge National Laboratory, Da Lian Institute of Physical Chemistry (China), Denora
of Italy, Intelligent Energy Ltd, GenCell Corporation, and Sarrnoff Corporation. Some of them have begun to take customer
orders to produce metallic bipolar plates [12]. However, further investigations of bipolar plates are needed to speed up the
process of commercialization of fuel cells.
There are some review papers and a great many research reports related to bipolar plate studies. Middelman et al. [13]
have presented a short bipolar plate review for the PEM fuel
cell focused on the composite bipolar plates. Borup et al. [11]
briefly reviewed different types of bipolar plates for PEM fuel
cells. Recently, Mehta et al. [5] provided a review of bipolar
plate design and manufacture. Hermann et al. [14] reviewed the
material aspects for bipolar plates. The purpose of this work is
to present a comprehensive review of the recent work done on
the development of the bipolar plates for PEM fuel cell applications. Both the materials used for the bipolar plates and the processing for bipolar plate fabrication will be discussed. Advantage and disadvantage of different types of materials for PEM
fuel cell bipolar plates will be presented. The future development work for PEM fuel cell bipolar plates will be highlighted
based on comparison and analysis.
2.
BIPOLAR PLATE MATERIALS REVIEW
The most commonly used bipolar plate material is graphite.
Graphite plates exhibit excellent resistance to corrosion and low
bulk resistivity. It is well known that the interfacial ohmic losses
between the MEA and the bipolar plate can significantly reduce
the overall power output of a PEM fuel cell. Compared to stainless steel, however, these losses from graphite bipolar plate materials are insignificant, whereas the existence of a passive film
on the surface of stainless steel greatly reduces electrical conductivity [15].
With respect to corrosion resistance, graphite materials are preferable. However, the conductivity of this material is still much
lower than that of metallic materials. The conductivity values
of some bipolar plate materials are compared in Table 1 [16].
Bipolar Plates for PEM Fuel Cells - From Materials to Processing / J. New Mat. Electrochem. Systems 8, 257-267 (2005)
Table 1: Comparison of the conductivity values of some bipolar
plate materials [16]
Materials
Polymers
Graphite
Polymer/graphite
composites
Conductivity
(Scm−1 )
~1
103
10
Materials
(Scm−1 )
Fe alloys
Ti
Gold
Conductivity
5300×103
2400×103
45000×103
On the other hand, the machining process is required to make
gas flow-fields on the surface of bipolar plates. Material and
machining costs of graphite bipolar plates are very high, which
account for as much as 60% of the stack cost [17]. Furthermore,
low mechanical properties (brittleness) and the porous nature of
graphite prevent people from utilizing thin sheet graphite. This
results in an increase in the weight as well as the volume of
these plates [18].
These disadvantages of graphite material have led to substantial development efforts to replace it. Alternatives fall into three
categories: carbon-carbon composite, carbon-polymer composite and metals [19]. It has been estimated that for composites
and metal alloys, the cost of bipolar plates would be only 1529% of the stack cost [17].
2.1
Carbon – carbon composite
As a candidate to meet those requirements for PEM fuel cell
bipolar plates, the carbon composite has been investigated extensively.
SGL 001, a low-cost graphite composite, has been developed
by SGL Technik GmbH. The electrical conductivity, corrosion,
chemical compatibility, gas tightness and mechanical strength
were tested and the tests proved that SGL 001 was a promising
candidate for high performance and cost efficient PEM fuel cell
bipolar plates [3]. A concept stack that used graphite composite bipolar plates was developed by Scholta et al. [20]. They
reported that the design limit of 10kW could be significantly
exceeded.
Besmann et al. [21]-[23] have developed carbon-carbon composite bipolar plates by slurry-molding a chopped-fiber preform
and sealing it with chemically vapor-infiltrated carbon. The
composite exhibits low-cell resistance and has high electronic
conductivity (200-300 Scm−1 ) . The manufacturing process of
the composite can be carried out continuously; therefore mass
production is not a big problem, and processing costs should
decrease significantly. The material has a low density (0.96
gcm−3 ) due to the porosity of the material. Its weight is approximately half that of other potential materials with the same dimensions. Corrosion testing indicated minimal corrosion in fuel
259
cell environments, and cell testing indicated high efficiency, but
with a somewhat steep drop-off in voltage at high current densities. The reason of this drop was likely due to leakage from seals
around the edge of the plate in the cell. Long-term tests were
also carried out to investigate surface resistance. The results
showed that the surface resistance changed less than 0.001Ωcm2
after 2000 hours of operation. Their on-going efforts have focused on optimizing the fabrication process and characterizing
prototypical components.
Two kinds of carbon composite were studied by Cho et al. [17]
using the same compound powder: Composite A was fabricated
into a plate by hot-pressing the compound powder and then machining the flow fields into its surface; Composite B was manufactured directly into a bipolar plate by molding the compound
powder under certain pressure and temperature. Compared with
machined graphite, both of the carbon composites satisfied the
development targets of PEMFC bipolar plates. The electrical
and physical properties were acceptable after 500 hours’ longterm operation.
Graftech Inc., a wholly owned subsidiary of UCAR (one of the
world’s largest manufacturers and providers of high quality natural and synthetic graphite and carbon-based products and services), develops and manufactures flexible graphite (GRAFOIL
[24] for use in the manufacture of flow-field plates for proton
exchange membrane fuel cells. Graftech’s advanced flexible
graphite technologies and engineering capabilities will play a
significant role in supporting the commercialization of
BALLARD fuel cells.
However, for carbon-carbon composites, the lack of mechanical strength is inherent, which limits the size and accordingly
the volumetric power density. So among the three categories:
carbon-carbon composite; carbon-polymer composite; and metals, it is likely that only carbon-polymer composites and metal
systems will meet the long-term cost requirements for fuel cell
technology [19].
2.2
Carbon-polymer composites
A carbon-polymer composite bipolar plate is a promising alternative to graphite, and has the advantages of low-cost, good
corrosion resistance and low weight. In particular, flow-fields
can be molded directly into the composites, and therefore, the
machining process, which is required for graphite and metal,
can be eliminated.
Production of the great majority of carbon-polymer composites involves the hot molding of carbon or graphite filler in a
thermosetting (epoxy resin, phenolic resins, furan resin, vinyl
ester) or a thermoplastic (poly-vinylidene fluoride, polypropylene, polyethylene) matrix. Typical carbon contents range between 50 and 80 % by weight. The carbon content used in
preparing composite bipolar plates is extremely important be-
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cause it ultimately determines the electrical conductivity and
mechanical properties. High carbon content will enhance the
electrical conductivity of the composite bipolar plate, but will
make the bulk-molding compound (BMC) process more difficult. There is a trade-off between mechanical strength and electrical conductivity [25]. Carbon-polymer composites should be
developed with at least 10 Scm−1 (48 Scm−1 was reported in
Chang et al.’s carbon-plastic composites [26]), well above that
of the membrane, though still below the conductivity values of
5.3x106 Scm−1 for iron-based alloys and 2.4x106 Scm−1 for
Titanium, also seen in Table 1.
Although polyethylene and polypropylene have been used as
matrices, most of the thermoplastic composite materials used
in fuel cell bipolar plates are composed of graphite powder in
poly-vinylidene fluoride (PVDF), with or without short carbon
fibers for reinforcement [27]. For example, Del Rio et al. [28]
reported the use of PVDF incorporating with various amounts
of carbon powder. However, PVDF is relatively expensive, and
any thermoplastic composite must be cooled before being removed from the mold. Thermosetting resins such as phenolics,
epoxies, and polyesters generally offer shorter process cycle
times than thermoplastic resins because they can be removed
from the mold while still hot. ZBT (Zentrum für Brennstoffzellen Technik GmbH) [10] has developed a number of composites for bipolar plate materials, which contain commercially
available thermoplastic and graphite. Additional additives were
also added to increase the conductivity of the compound material. Thermoplastic materials were used as the polymer matrix due to their advantages, such as high chemical resistance,
good mechanical properties, impermeability, and low cost. As a
result, low-cost carbon-polymer compounds with specific bulk
conductivities between 5 and 150 Scm−1 have been achieved.
The composites generally show high corrosion resistance in the
PEM fuel cell environment.
Vinyl esters are widely used in the chemical process industry
for their excellent corrosion resistance. Busick et al. [27] combined a thermosetting vinyl ester matrix, which is commercially
available, with graphite powder to produce relatively inexpensive composites. Traditional or non-traditional fiber reinforcements and other additives were added to improve the properties of the composites. These composite materials exhibit several advantages compared to the existing bipolar plate materials. For example, graphite/vinyl ester composites are less brittle
than machined graphite and show less corrosion problems than
stainless steel. The commercial availability of the raw materials, the light-weight, and low cost are also advantageous. The
total materials and manufacturing cost was estimated to be below $10/kg. Kuan et al. [18] also discussed a composite bipolar
plate consisting of vinyl ester and graphite powder that was prepared by a BMC process. The composite bipolar plates with
optimum composition, including graphite content and graphite
size, were very similar to that of graphite bipolar plates.
Recently, Wu et al. [29] proposed making carbon or carbon nanotube (CNT)-filled PET/PVDF blends in a 3D structure. Fig. 3
presents a schematic of the carbon-filled polymer blends consisting of binary polymer blends. Carbon particles have been
shown to be preferentially distributed in one polymer phase,
and both polymer phases are continuous in 3D space. The CNTfilled PET phase provides an electrical short circuit for the composite, and the PVDF phase provides the strength and elongation for the composite. The triple continuous, carbon-filled
polymer blends possess two characteristics, which satisfy current performance objectives. One is the improved processability
due to the low carbon content. Another is the minimal degradation in tensile properties due to the presence of a continuous
neat polymer phase. Thus, the triple continuous, carbon-filled
polymer blends are promising for the manufacture of low-cost
conductive polymers with superior conductivity and strength for
bipolar plate applications of PEM fuel cells. They speculated
that further improvements in electrical and mechanical properties were possible due to the segregation of CNT in the PET
phase of the blends.
Figure 3: Schematic of the microstructure of the carbon-filled
polymer blend composite. (Modified from reference [29] with
permission.)
In addition, a heterogeneous composite has been developed [30].
This composite consists of two regions, unlike the traditional
carbon composites. Flexible and loose carbon fiber bunches
form the “rib” of the plate, and plain plastic forms the rest of
the plate. The advantage of the heterogeneous composite design lies in its low contact resistance, lower stack weight and
volume, full electrode utilization, and low cost. Full electrode
utilization was accomplished by applying a small compression
force on the ribs, thus the fuel flowing into the catalyst layer
under the rib-contact-area was not constricted as it would be on
hard, flat surfaces. This design will make significant contributions to the development of bipolar plates due to the possibility
of mass production and the availability of cheaper materials.
However, research on its long-term durability is still needed.
Bipolar Plates for PEM Fuel Cells - From Materials to Processing / J. New Mat. Electrochem. Systems 8, 257-267 (2005)
To sum up, carbon-polymer composites have been extensively
studied by many researchers because they offer various advantages as stated at the beginning of the section. With the exception of its low conductivity, this material still has its problems.
The degradation of the mechanical property of this composite
material due to the nature of polymer’s inherent properties can
cause strength reduction. Resin additives often contain heavy
atoms such as calcium, magnesium, or zinc. These ions may
get in touch with the proton exchange membrane and result in
membrane contamination. Further investigations are needed.
2.3
Metal
Metal is considered to be a good material for a bipolar plate due
to its good electrical conductivity, excellent mechanical properties, and low cost. Typical metals such as aluminum and stainless steel can easily meet the volume requirements. When used
as regular bipolar plates, they can be machined to form a sheet
approximately 0.05 inches thick, and when used as coolant bipolar plates, 0.1 inches thick [31]. With regard to Open Circuit
Potential (OCP) , the metallic bipolar plate may perform better
than the graphite bipolar plate, which is operated at low operating voltage, has a low power density and a shorter cell lifetime
[12].
Metallic bipolar plates are currently being studied by many fuel
cell researchers. The materials related to possible metal bipolar plates include metals/alloys, such as SS 316L, High Si iron,
Cr-Ni steel, Nickel silver alloy, Titanium, Ni steel, Monel alloy, and Aluminum alloy; intermetallic compounds, such as Iron
Aluminides, Titanium Aluminides and Nickel Aluminides; and
composites, such as Cu alloy with C fibers, and Al composites
[32]. Normally, a metallic bipolar plate needs to be coated in order to prevent corrosion. Davies et al. [8] reported that it would
be feasible to use uncoated metallic bipolar plates by optimizing the chemical composition of the alloy without the obvious
voltage drop seen in graphite bipolar plates.
Different kinds of metals and alloys have been selected for use
in bipolar plates. Ta, Hf, Nb, Zr, and Ti exhibited satisfactory
corrosion resistance over a large voltage range; however, these
materials are much too costly to be considered for commercial
applications [33]. The cost requirement drove the researchers
back to the cheaper Fe-based materials. Hornung and Kappelt
[34] investigated the Fe-based metallic bipolar plates. Their
goal was to investigate cheaper Fe-based metals to replace the
more expensive gold-plated, nickel-based metal. The Fe-based
alloys contained Cr, Mo, N and Ni. They found that the corrosion resistance of the Fe-based metallic bipolar plate was about
the same as that of the gold-plated nickel-based metallic bipolar
plates.
Nevertheless, as for Fe-based metallic bipolar plates, stainless
steel is attractive and has been studied extensively. Stainless
steel offers many advantages over conventional graphite ma-
261
terials, such as high strength, ease of manufacture, and relative low cost. Because they can be shaped into thin sheets,
the power/volume ratio will be improved significantly. Brandon et al. estimated that by using SS bipolar plates, a 7kW
stack might cost less than $20/kW [35]. Wang et al. [36] studied the characteristics of stainless steel with different Cr contents. The higher the Cr content in the stainless steel, the better its corrosion-resistance, because Cr can form a passive film
on the stainless steel surface. Kim et al. [37] determined the
effects of alloying elements (Cr, Mo) on the passivation and
the transpassive transition behavior of various commercial SS.
The Fe and Ni atoms were not found to be as stable as Cr
atoms under the PEM fuel cell operating environment because
Fe and Ni atoms might dissolve due to the electrochemical effect [1]. Wang et al. [38] studied ferritic stainless steel samples
of AISI434, AISI436, AISI441, AISI444, and AISI446. The
results suggest that AISI446 SS is superior to the others.
As a matter of fact, most of the work on stainless steel bipolar
plate material has focused on the austenitic stainless steel, and
frequently, SS 316/L has been chosen for metallic bipolar plates.
Comparisons of properties between graphite and SS316 in fuel
cell conditions are shown in Table 2 [32]. The performances
in comparison to Graphite and SS316L both in the coated and
bare form are shown in Fig. 4 [39]. Coated SS 316L is a good
replacement for graphite, as seen in Fig. 4, but the cell performance of uncoated SS 316L is not as satisfactory as that of
graphite. This attributes to the corrosion of the metal plates. As
we know, the fuel cell works in an acid environment because
of the protons produced in the anode and moisture from the inlet gases and in the MEA. Also, there are potential differences
between the metallic plates and the MEA due to the discharging process of the cell reaction. Therefore, both chemical and
electrochemical corrosion may occur. The released multivalent
cations from the corrosion reaction can go into the polymeric
membrane by diffusion or migration to make exchanges with
protons in the membrane. These cations increase the membrane
resistance and therefore lower the power density of the stack.
Furthermore, oxidation of the bipolar plate at the surface may
also lead to an increase in the contact resistance. Normally,
corrosion rates are very high for Al and Ni, but are lower for
some SS due to passivation. On one hand, the nature of stainless
steel passivation brings significant ohmic losses, which limit the
overall power output of a PEM fuel cell. On the other hand, the
protective passive film (Cr2 O3 ) that is formed on stainless steel
in the normal manufacturing process is not uniform, and can
produce surface defects. These surface defects known as contaminants, scratches, and microcracks will accelerate localized
corrosion [40]. In short, corrosion is a big problem for stainless
steel when used as bipolar plates.
To overcome the problem of corrosion, the usual practice is to
coat the stainless steel bipolar plate for protection. The anode side of the stainless steel flow plate has been found to be
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X. Z. Yuan et al./ J. New Mat. Electrochem. Systems 8, 257-267 (2005)
Table 2: Comparison of the main properties between graphite and SS 316 [32]
Properties
Density
Thickness
Units
Graphite
SS 316
gcm−3
2.25
8.02
mm
2.50-4.00
0.16
Electrical
resistivity
Scm−1
1000
5×106
Corrosion
current
mAcm−2
<0.01
<0.1
Permeability
for hydrogen
cm3 cm−2 s−1
10−2 ~10−6
<10−12
Modulus
of elasticity
MPa
4800
193000
reduce the costs of the coatings, materials that have the potential for low-cost coatings need to be applied. Additionally, the
coating process must also be a low-cost process.
Tantalum is a tough, corrosion resistant metal. It would be a
very good bipolar plate material for PEM fuel cells if it were
not for its high weight and price. But relatively thin tantalum
coatings can be applied to the surface of other inexpensive and
lighter metals. Gladczuk et al. [31] successfully deposited high
quality tantalum coatings with a thickness of a few micrometers
using DC magnetron sputtering on aluminum and steel. The
coating provides a passive protection layer on the surfaces of
these substrates.
Figure 4: Performance of stainless steel bipolar plates. (Modified from reference [39] with permission.)
the main source of contamination, which is enhanced greatly
by the direct contact of the stainless steel and the membrane.
Therefore it is necessary to make a coating with an appropriate pretreatment between stainless steel and the membrane [41].
The satisfied surface coating should significantly improve the
corrosion-resistance and the electrical conductivity at an acceptable cost. Note that the pre-treatment of the surface can influence corrosion-resistance. These coatings must be defect-free
because any defects, such as pinholes, will increase the corrosion rate. Classifications of viable coating materials that include
carbon-based and metallic are listed below [11]:
Carbon-based:
Graphite
Conductive polymer
Diamond, diamond like carbon (DLC)
Organic self-assembled monolayers
Metallic:
Noble metals
Metal nitrides
Metal carbides
Sub/super Stoich.
Oxides
Also seen in Fig. 4, the gold-coated SS 316L exhibits the best
performance among graphite plates, with and without a coating.
However, noble metals, such as gold, should not be chosen as
the coating material due to the high material price. In order to
Transition metal nitrides have been widely used as functional
coatings due to their unique properties of excellent hardness,
electrical conductivity, and chemical stability. Most conventional coating methods are expensive for PEM fuel cell applications and leave pinholes, which severely accelerate the localized corrosion rate. Brady et al. [42][43] designed a protective
metal nitride surface layer using thermal (gas) nitridation. The
thermally grown nitride layer had the potential of resisting corrosion without significant contamination of a Nafion membrane
or degradation of electrical conductivity. They reported that pinhole defects were not a big problem for thermal nitridation because thermodynamic and kinetic factors favor the surface reactions at elevated temperatures. In the meantime, their results
showed that nitrided Ni-Cr base alloys were potential material
for the use in a bipolar plate. Their future work is focusing on
alloys with a lower content of Cr and the use of less expensive
Ni(Fe)- or Fe-base substrates. The corrosion behavior of TiN
coated 316 SS was also investigated by Li et al. [44]. TiN coatings were found to have brilliant corrosion resistance and passivity in simulated cathode environment. They concluded that
TiN coatings were full of promise for the use of 316 SS bipolar
plates due to the excellent corrosion resistance and metal-like
conductivity. But the electrochemical stability of TiN coatings
was not provided.
As for the carbon-based coatings, a film made by pyrolyzing a
high carbon content polymer sprayed on the surface of SS 316L
was reported by Cunningham et al. [25]. The layers on SS
316L were dense, conductive and adhesive. The fuel cell performance increased when the carbon-based coating protected
Bipolar Plates for PEM Fuel Cells - From Materials to Processing / J. New Mat. Electrochem. Systems 8, 257-267 (2005)
plates were used. However, the long-term stability of the coatings hasn’t been assessed, and the cost of the coating material
used in their work is still high. Lee et al. [12] studied the physical vapor deposition (PVD) coating of YZU001 diamond-like
film on the 5052 aluminum alloy, and found that PVD coated
5052 aluminum performed better than that of graphite material
at low voltage and shorter cell life. Compared to a SS 316L
plate with natural passive film, the YZU001 coated aluminum
plate may have higher corrosion rate, but exhibited better cell
performance because of less contact resistance. Nevertheless,
its fate should be decided by the long-term durability.
For commercial applications, fuel cells must be able to operate
for longer periods of time. Gottesfeld [45][46] concluded that
the fuel cell using stainless steel as bipolar plates can work up to
1,000h, but surface treatments were needed in order to extend
its long-term stability. As we know, the necessary operation
time for fuel cells is several thousand hours. Many efforts have
been made to study the long-term stability of the SS316 alloy in
the PEM fuel cell. Some researchers have reported a significant
drop in performance due to contamination of the Nafion membrane by Fe ions during the first 100 hours of operation. In Kumar’s experiments [32], no drop in performance was observed
even after 1000 hours of continuous operation. They suggested
that it could be attributed to a better flow-field design, which
helped to remove the metal ions efficiently from the cell. In
Wind et al.’s work [1], the most promising bipolar plates were
coated stainless steel 316L. The lifetime curves of single cells
using coated stainless steel 316L coated with different coating
materials by various methods such as electroplating, evaporation, sputtering and chemical vapor deposition are shown in Fig.
5. The performance of most of the tested cells with coated stainless steel 316L bipolar plates is similar to those with graphite
bipolar plates. The endurance testing indicates that these layers
are stable for at least 1000h.
Another interesting development of the SS 316 bipolar plate is
Kumar and colleagues’ work [47]-[50]. They used SS 316 as the
bipolar plate, plus a porous material, such as Ni-Cr metal foam,
SS 316 metal foam, or the carbon cloth, in the gas flow-field
of the bipolar plates instead of the conventional channel type of
design. The performance of a fuel cell with Ni–Cr metal foam
was shown to be the highest, and decreased in the order SS316 metal foam, conventional multi-parallel flow-field channel
design, and then carbon cloth. This trend was explained based
on the effective permeability of the gas flow-field in the bipolar plates. The use of porous metal foam offers a more uniform
distribution of local current density compared with that of traditional flow field design, hence, the performance of the fuel
cell will be enhanced. The performance was predicted to increase further if the size, shape and distribution of the pores in
the metal foam were properly tailored.
In brief, metallic materials, especially stainless steel can easily
meet the requirements of bipolar plates for fuel cells. The big
problem is the metal corrosion, which will cause poisoning of
membrane or the catalyst. Using an appropriate pre-treatment
and a coating will prevent direct contact between stainless steel
and the membrane. As stated above, many researchers have
studied the different coatings using different methods for metallic bipolar plates, and some of the results are promising. Furthermore, proper flow field design may help to avoid the buildup of the metal ions and therefore minimize or eliminate the
performance loss of fuel cells.
3.
PROCESSING OF BIPOLAR PLATES
It is known that bipolar plate processing contributes significantly
to the total cost of bipolar plates. The commercialization of
PEM fuel cells strongly depends on the development of mass
production process. Pros and cons of different types of bipolar
plate processing will be discussed in the following sections.
3.1
Figure 5: Lifetime curves of single cells with bipolar plates
made of stainless steel 316L coated with different coating materials. (Modified from reference [1] with permission.)
263
Carbon-carbon composites
Very little has been reported about the processing of the machined graphite bipolar plates with the exception of Woodman
et al. [51], who introduced the fact that PEMFC graphite bipolar plates could be either machined or molded with a flow-field
arrangement for gas flow to the electrodes. For the carbon composites, Meissner [52] presented compression molding, which
molds mixtures containing graphite with additives or binders in
an atmosphere without oxygen. Besmann [23] described slurrymolding by adding carbon fibers into graphite mixtures and using chemical vapor infiltration to strengthen the material. This
method helps to increase the surface density and smoothness,
and therefore enhance the impermeability and electrical contact.
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3.2
X. Z. Yuan et al./ J. New Mat. Electrochem. Systems 8, 257-267 (2005)
Coated metals
The bulk resistance of stainless steel is insignificant with respect
to the surface resistance caused by the passive film from corrosion. Thus, coating is necessary for stainless steel. Processing
the coated metallic bipolar plates consists of the fabrication of
the base plate, the pretreatment of the base plate, and the coating processes. In plate formation, Woodman et al. [51] put
forward the methods of machining or stamping to form the base
plate. Mehta et al. [5] identified another five processing techniques: Cold Closed Die Forging, Die Casting, Investment Casting, Powder Metal Forging, and Electroforming. They found
that for the larger faced plate more approaches could be utilized,
but for the smaller faced plate some techniques were not applicable, such as investment casting, powder metal forging and
die-casting. Thus far, no further discussion has been reported
about their processing techniques in plate formation. Another
unique method for plate formation was introduced by Allen [5].
In this method, flow channels and manifolds are stretched and
formed into finite subsections by producing some depth for the
flow ribs using a special tool. This method is less typical in
plate formation.
Before the surface coating treatment, a pre-treatment process
is necessary. A standard cleaning process contains grinding,
degreasing, ultrasonic cleaning, and de-ionization. The coating methods that are related to the metallic plate include Physical Vapor Deposition (electron beam evaporation, sputtering
and glow discharge decomposition), Chemical Vapor Deposition and Liquid Phase Chemical Techniques (Electroplating,
Electrolessplating, Electrolytic Anodization), and painting [5].
Some of these methods have been reported. For example, Gladczuk et al. [31] introduced DC magnetron sputtering to obtain
tantalum coatings on steel and aluminum. However, the cost
estimation in most of the related studies was not discussed.
3.3
Carbon-polymer composites
For carbon-polymer composite bipolar plates, flow-fields can be
molded directly into these composites. Processing of composite plates may contain several steps, but it is flexible for incorporating the PEM components. Several methods of processing
have been exploited for the fabrication of the composite bipolar
plate. The main processing approaches are compression molding, injection molding, two-component injection molding, and
preform molding [13].
To begin with a powder compound, the process of the compression molding is to feed the powder compound into a heated
mold, then the compound will flow and fill the mold cavity. For
a thermo-set binder, several minutes are needed for the crosslinking reaction of the binder. Whereas for a thermoplastic
binder, a longer time is normally required because the mold
must be cooled down until the temperature drops below the
melting point of the binder [13]. This method was also reported
by Busick et al. [9]. They mixed various additives including
a catalyst, inhibitor, and thickener into thermosetting vinyl ester resin. Graphite power was then added to the mixture, and
finally, the plate was formed by compression molding at about
1000-2000 psi.
Injection molding is another approach for the mass production
of the composite bipolar plates. It is a difficult process due to
the conductivity of the composite. Injection molding has its
advantages and disadvantages. The advantages are automated
production, short cycle time and accurate size, and the disadvantages include poor conductivity and excessive mold wear
[13]. Wu et al [29] used injection molding and produced CNTfilled PET/PVDF blends. This method has also been reported
by ZBT. Their work focused on injection molding as a standard
mass production technique to manufacture bipolar plates in a
one-step process. Using injection molding, the cost for a representative plate of 100 cm2 in size with a thickness of 2.5mm
and an electrical bulk conductivity of 50 Scm−1 has been estimated. When the number of produced plates increased, the sum
of the material costs and the processing cost dropped significantly [10].
The two-component molding process has been exploited by NedStack [13], and it is mainly for the use of fuel cell plates. In the
active area of the plates, the relatively high conductive compound was used, whereas in the border area of the plates, the
non-conductive compound was employed. The two-component
injection molding process was also reported to be more favorable for small size plates than for large ones.
For larger bipolar plates, a preform molding process was also
developed by NedStack [13]. The conductive composite material was heated outside the mold. When the temperature reached
above the melting point of the binder, it was inserted into a cold
mold and then molded to shape. This method is reported to be
a break-through in mass production and cost reduction because
the molding cycle is much faster than other plate molding processes.
As stated above, NedStack has contributed a great deal to the
development of the composite processing. The new processing
methods were introduced, and the costs were analyzed as well.
High power density of 2kW/kg and 2kW/l has been achieved.
From the cost’s standpoint, the conductive composite materials
and the processing methods of mass production developed by
NedStack are acceptable.
4.
DISCUSSION
This review regarding the different types of bipolar plate material and their fabrication processes for use in PEM fuel cells
combines the efforts of many researchers. Substantial work has
been done on the development of attractive materials for the
Bipolar Plates for PEM Fuel Cells - From Materials to Processing / J. New Mat. Electrochem. Systems 8, 257-267 (2005)
265
Table 3: Comparison of the advantages and disadvantages of different bipolar plate materials
Material
Graphite
Advantages
Excellent corrosion resistance
Low bulk resistivity
Low contact resistance
Carbon-carbon
composites
Low density
High corrosion resistance
Low contact resistance
Low cost
Good corrosion resistance
Low weight
No machining process
Commercial availability of
the raw materials
Good electrical conductivity
High thermal conductivity
Low cost
Excellent mechanical properties
Ease of fabrication
Small volume
Carbon-polymer
composites
Metal
bipolar plates, and so far huge progress has been achieved. As
presented in Table 3, each type of bipolar plate material has its
advantages and disadvantages, but none of the materials meet
all the design criteria.
Graphite is an excellent material for fuel cell bipolar plate, but
both the material cost and the processing cost are very high for
large-scale production. Although graphite plates are not considered to be the most effective material for PEM fuel cell application, they have been preferred for space applications due
to the superior corrosion resistance without coating when compared to metal plates. Table 3, also shows that the advantages
of the graphite polymer composites and the metallic material
far exceed the disadvantages, and since the carbon-carbon composite is not expected to obtain the satisfied price targets from
the viewpoint of processing, the most promising alternatives for
fuel cell commercialization are the graphite polymer composites and the metallic material with coatings. Generally speaking,
carbon polymer composites and sheet metal are potentially lowcost materials and are especially suitable for mass production
because flow-fields can be molded directly into carbon polymer
composites and thin sheet metal can be stamped to plates in an
established mass production process.
However, the cost of each type of bipolar plate was not seriously compared in this work because current approaches and
results for practical applications lack exact cost estimations of
the materials and their processing. The literature also lacks data
about long-term stability of the testing materials. Most of the re-
Disadvantages
Poor mechanical properties (brittleness)
Porosity
High weight and volume
High processing cost
Low mechanical strength
Low bulk electrical conductivity
High price
Low mechanical strength
Low electrical conductivity
Severe corrosion (membrane
poisoning and formation of
insulating surface oxide)
searchers have focused on the short-term properties of the bipolar plate candidates. There is still a great deal of work needed to
obtain satisfactory bipolar plate materials. For carbon polymer
composites, resin optimization appears to be the best approach
to improve composite strength, and additional additives are the
solution to increase the conductivity. It is clear that non-coated
stainless steel has the problem of a surface-insulating layer. The
solution to the metallic bipolar plates is to coat the material for
protection using physical, chemical or electrochemical methods, and the coating should significantly improve the corrosionresistance and the electrical conductivity at an acceptable cost.
Attention should be paid to the fact that an optimal bipolar plate
should not only meet the property requirements of a bipolar
plate at a low material cost, but also meet the demand of an
easy fabrication process that is also low in cost. The art of the
flow-field design is another important feature of bipolar plates
for the performance of the fuel cells. A variety of flow channel
designs have been developed in different applications. These
different flow field designs have pros and cons for different materials and applications. Optimal flow field design of the bipolar
plates will help to achieve the cost price target and the fuel cell
performance. Finally, the fate of the materials will be determined by all these factors including material, processing, and
flow field design.
266
5.
X. Z. Yuan et al./ J. New Mat. Electrochem. Systems 8, 257-267 (2005)
ACKNOWLEDGMENTS
The authors gratefully acknowledge the Canadian National Fuel
Cell program for supporting this project.
REFERENCES
[1] J. Wind, R. Späh, W. Kaiser, G. Böhm, J. Power Sources,
105, 256 (2002).
[2] I. Bar-On, R. Kirchain, R. Roth, J. Power Sources, 109, 71
(2002).
[3] J. Scholta, B. Rohland, V. Trapp, U. Focken, J. Power
Sources, 84, 231 (1999).
[4] H. Tsuchiya, O. Kobayashi, Int. J. Hydrogen Energy, 29,
985 (2004).
[5] V. Mehta, J. S. Cooper, J. Power Sources, 114, 32 (2003).
[6] T. Mennola in “Cathode Flow Field Geometry in
a PEMFC”, Laboratory of Advanced Energy Systems, Helsinki University of Technology, Finland,
http://www.tkk.fi/Units/AES/papers/papers/mennola01a.pdf
[19] D. R. Hodgson, B. May, P. L. Adcock, D. P. Davies, J.
Power Sources, 96, 233 (2002).
[20] J. Scholta, N. Berg, P. Wilde, L. Jörissen, J. Garche, J.
Power Sources, 127, 206 (2004).
[21] T. M. Besmann, J. W. Klett, and T. D. Burchell, Mat. Res.
Soc. Symp. Proc. 496, 243 (1998).
[22] T. M. Besmann, J. W. Klett, J. J. Henry Jr., E. Lara-Curzio,
J. Electrochem. Soc., 147, 4083 (2000).
[23] T. M. Besmann, J. J. Henry, Jr., E. Lara-Curzio, J. W. Klett,
D. Haack, K. Butcher, Mat. Res. Soc. Symp. Proc. 756,
415 (2003).
[24] J. J. Hwang, H. S. Hwang, J. Power Sources, 104, 24
(2002).
[25] N. Cunningham, D. Guay, J. P. Dodelet, Y. Meng, A. R.
Hlil, A. S. Hay, J. Electrochem. Soc., 149, A905 (2002).
[26] H. Chang, P. Koschany, C. Lim, J. Kim, J. New Mat. Electrochem. Systems, 3, 55 (2000).
[27] D. N. Busick, M. S. Wilson, Electrochem. Soc. Proc., 9827, 435 (1999).
[7] J. S. Cooper, J. Power Sources 129, 152 (2004).
[8] D. P. Davies, P. L. Adcock, M. Turpin, S. J. Rowen, J.
Power Sources, 86, 237 (2000).
[9] D. Busick, M. Wilson, Mat. Res. Soc. Symp. Proc. 575,
247 (2000).
[10] A. Heinzel, F. Mahlendorf, O. Niemzig, C. Kreuz, J.
Power Sources, 131, 35 (2004).
[11] R. L. Borup, N. E. Vanderborgh, Mat. Res. Soc. Symp.
Proc. 393, 151 (1995).
[12] S. J. Lee, C. H. Huang, Y. P. Chen, J. Mater. Process. Technol., 140, 688 (2003).
[13] E. Middelman, W. Kout, B. Vogelaar, J. Lenssen, E. de
Waal, J. Power Sources, 118, 44 (2003).
[14] A. Hermann, T. Chaudhuri, P. Spagnol, Int. J. Hydrogen
Energy, 30, 1297 (2005).
[28] C. Del-Rio, M. C. Ojeda, J. L. Acosta, M. J. Escuder,
E. Hontanon, L. Daza, J. Appl. Polymer Sci., 83, 2817
(2002).
[29] M. Wu, L. L. Shaw, J. Power Sources, 136, 37 (2004).
[30] M. S. Lee, L. J. Chen, Z. R. He, S. H. Yang, ASME J. Fuel
Cell Sci. Technol., 2, 14 (2005).
[31] L. Gladczuk, C. Joshi, A. Patel, J. Guiheen, Z. Iqbal, M.
Sosnowski, Mat. Res. Soc. Symp. Proc., 756, 423 (2003).
[32] A. Kumar, R. G. Reddy in “Fundamentals of Advanced
Materials for Energy Conversion Proceedings”, Eds. D.
Chandra and R. G. Bautista, Seatle, USA, Feb. 17-21,
2002, p. 41.
[33] L. Ma, S. Warthesen, D. A. Shores, J. New Mat. Electrochem. Systems, 3, 221 (2000).
[34] R. Hornung, G. Kppelt, J. Power Sources, 72, 20 (1998).
[15] D. P. Davies, P. L. Adcock, M. Turpin, S. J. Rowen, J.
Appl. Electrochem., 30, 101 (2000).
[35] N. P. Brandon, S Skinner, B. C. H. Steele, Annu. Rev.
Mater. Res., 33, 183 (2003).
[16] B. C. H. Steele, A. Heinzel, Nature, 414, 345 (2001).
[36] H. Wang, M. A. Sweikart, J. A. Turner, J. Power Sources,
115, 243 (2003).
[17] E. A. Cho, U. S. Jeon, H. Y. Ha, S. A. Hong, I. H. Oh, J.
Power Sources, 125, 178 (2004).
[18] H. C. Kuan, C. Chi, M. Ma, K. H. Chen, S. M. Chen, J.
Power Sources, 134, 7 (2004).
[37] J. S. Kim, W. H. A. Peelen, K. Hemmes, R. C. Makkus,
Corrosion Science, 44, 635 (2002).
[38] H. Wang, J. A. Turner, J. Power Sources, 128,193 (2004).
Bipolar Plates for PEM Fuel Cells - From Materials to Processing / J. New Mat. Electrochem. Systems 8, 257-267 (2005)
[39] P. L. Hentall, J. B. Lakeman, G. O. Mepsted, P. L. Adcock,
and J. M. Moore, J. Power Sources, 80, 235 (1999).
[40] S. J. Lee, C. H. Huang, J. J. Lai, Y. P. Chen, J. Power
Sources, 131,162 (2004).
[41] R. C. Makkus, A. H. H. Janssen, F. A. de Bruijn, R.
K.A.M. Mallant, J. Power Sources 86, 274 (2000)
[42] M. P. Brady, K. Weisbrod, C. Zawodzinski, I. Paulauskas,
R. A. Buchanan, L. R. Walker, Electrochem. Solid State
Lett., 5, A245 (2002).
[43] M. P. Brady, K. Weisbrod, I. Paulauskas, R. A. Buchanan,
K. L. More, H. Wang, M. Wilson, F. Garzon, L. R. Walker,
Scripta Materialia, 50, 1017 (2004).
[44] M. Li, S. Luo, C. Zeng, J. Shen, H. Lin, C. Cao, Corrosion
Science, 46, 1369 (2004).
[45] J. Scholta, B. Rohland, J. Garche in “Proceedings of
the Second International Symposium on New Materials
for Fuel Cell and Modern Battery Systems II”, Eds. O.
Savadogo, P.R. Roberge, Montreal, Canada, July 6-10,
1997, p. 330.
[46] S. J. C. Cleghorn, X. Ren, T. E. Springer, M. S. Wilson, C.
Zawodzinski, T. A. Zawodzinski and S. Gottesfeld, Int. J.
Hydrogen Energy, 22, 1137 (1997).
[47] A. Kumar, R. G. Reddy, J. Power Sources, 129, 62 (2004).
[48] A. Kumar, R. G. Reddy, J. New Mat. Electrochem. Systems, 6, 231 (2003).
[49] A. Kumar, R. G. Reddy in “Advanced Materials for Energy Conversion II Symposium”, Eds. D. Chandra, R.
G. Bautista, L. Schlapbach, Charlotte, USA, Mar. 14-18,
2004, p. 317.
[50] A. Kumar, R. G. Reddy, J. Power Sources, 114, 54 (2003).
[51] A.S. Woodman, E.B. Anderson, K.D. Jayne, M.C. Kimble, in “Proceedings AESF SUR/FIN ‘99”, Eds. D. Foulke,
R. Leeds and F. Lowenheim, Cincinnati, USA, June 21-24,
1999, p. 21.
[52] R. Meissner, M. Irgang, K. Eger, P. Weidlich, H. Dreyer,
US patent 5,736,076,7 April, 1998.
267