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Journal of New Materials for Electrochemical Systems 11, 21-29 (2008)
© J. New Mat. Electrochem. Systems
Development of Highly Filled Graphite Compounds as Bipolar Plate Materials for Low
and High Temperature PEM Fuel Cells
*T. Derieth 1 , G. Bandlamudi 1 , P. Beckhaus 1 , C. Kreuz 1 , F. Mahlendorf 1, 2 and A. Heinzel 1, 2
1 ZBT gGmbH, Germany, 2 University Duisburg-Essen, Germany
Received: October 27, 2006, Accepted: July 23, 2007
Abstract: This paper focuses on the interactions between carbon filling materials and polypropylene as binder in compound based low
temperature bipolar plates. The influences of size and morphology of graphite on the compounding and moulding processes are disclosed
here. Without losses in electrical conductivity the throughput for the compounding process has been improved by a factor of twelve, only
depending on the morphology of graphite. Moreover, the way of bipolar plate production by compression- or injection moulding leads to
significant differences in their electrical conductivity. The process of compression moulding enables a higher processable filling load but
due to an obtained alignment of graphite flakes within an injection moulded plate, this material tends to offer an adequate conductivity
with fewer quantities of carbon filling particles.
Additional to this work first approaches in the development of plate materials suitable for high temperature PEM operation (up to
200 °C) are presented. First plates based on compression moulded compounds have been analysed and operated in a high temperature
PEM stack recently. In this paper the latest results of ZBT’s material development project regarding high temperature compound based
bipolar plate technologies are presented.
Keywords: compound, composite, bipolar plate , electrically, conductive, fuel cell
1. INTRODUCTION
*To whom correspondence should be addressed: Email: t.derieth@zbt-duisburg.de
The polymer electrolyte membrane fuel cell (PEMFC) is a very
promising energy converter and there is a significant and growing
interest from both, industry and government, to establish this technology
and bringing it into the market. Attractive features like
high power density, relatively low operating temperature, rapid
load change, continuous and convenient fuel supply and a low
noise output makes the PEMFC ideally suitable for residential,
mobile and portable applications like, e.g. auxiliary power units
(APU’s). Inside a PEMFC, besides the membrane-electrodeassemblies
(MEA) and the gas-diffusion-layer (GDL), the bipolar
plates constitute the most important element of a PEMFC stack.
Ideally they separate the reactant gases and distribute them on each
side over the whole active area of the MEA. It is required that the
bipolar plates are gastight and electrically and thermally conductive.
Another characteristic feature required of bipolar plates is
their chemical resistance to the operation conditions inside a fuel
cell stack. In essence, the bipolar plate has to endure a steady electrical
potential, a humid environment of pure water, a low pHvalue
(down to pH 4) [1] and temperatures of about 80°C in case
of low temperature PEMFC and up to 180°C for high temperature
PEMFCs. Even beyond the obvious challenges of the chemistry, it
is basically and significant that the PEMFCs and the bipolar plates
must be price competitive to other power units to establish the fuel
cell into the market. So, for wide spread commercialisation of
PEMFC, cost reduction of the bipolar plates is necessary and at
best to realise by low cost basis materials and mass production.
In the majority of cases bipolar plates based on metal, pure
graphite or carbon filled compound material.
First, metallic plates usually have high bulk electrical and thermal
conductivities, good mechanical properties and insignificant
gas permeability. But for metal based bipolar plates the choice of
suitable metals is due to the aggressive fuel cell environment limited
to expensive corrosion resistant metals. A lot of investigations
have been done to develop metallic plates of stainless steel, titanium
or other noble metals. Stainless steel is being considered
most promising for commercialisation [2,3] but stainless steel
usually shows low surface conductivities. However, due to the
required chemical resistance to the operation conditions inside a
fuel cell stack and the low corrosion resistance of non noble met-
21

22 T. Derieth et al. / J. New Mat. Electrochem. Systems
als, their surfaces must be protected by a layer which has been
found to be very difficult to accomplish [4,5].
Second, pure graphite based bipolar plates offer the advantages
of an excellent chemical resistance, good thermal and electrical
conductivity combined with a lower density than metal plates.
Though, the machining of pure graphite plates is a complicate and
time intensive step (milling the flow channels into plates) these
plates are not mass producible and hence high priced. As well
pure graphite plates are brittle and porous, they have to be coated
to be impermeable to the fuel and oxygen [6]. The facts mentioned
above make these plates, due to their cost and time intensive
manufacturing only suitable for very special applications but exclude
standard mass production to fabricate PEMFC components
as a low cost product.
Third, to fulfil the technical requirements for bipolar plates
combined with the possibility of mass production, a lot of investigations
have been done in developing carbon based compound
materials [7,8]. These compound materials are made of commercially
available polymers as binder, graphite or other carbon content
[9,10] and possibly further additives. Compound based bipolar
plates offer a lot of advantages. Due to the polymer matrix the
compound at least partly retains the process ability of the polymer.
Hence conventional processing methods like extruding, compression
moulding or injection moulding can be used to manufacture
bipolar plates mass producible and low priced. To obtain compound
material with good conductivity, a high content of carbon
fillers must be dispersed within a minor polymer matrix. The process
can take place in a discontinuous way using a kneader or continuously
using an extruder whereas the most homogenous material
can be obtained by using a twin screw extruder [11]. Such
produced compound is suitable for further thermal forming processes,
like compression moulding or injection moulding to bipolar
plates. The carbon filling materials and the thermoplastic binder
have significant effects on the compounding and moulding processes
as well as on the resulting conductivities of the compound.
The main objective of this work has been to determine the influences
of size and morphology of different kinds of graphite regarding
the entire production chain to bipolar plates
(compounding, compression- and injection moulding) and their
effects on the electrical conductivity. In addition, the role of the
chosen method to produce plates with their effects is disclosed
here. This part focuses on developing new compound materials for
low temperature bipolar plates and only polypropylene functions
as matrix.
Moreover investigations are initialised and ongoing in developing
thermoplastic bonded, high temperature bipolar plates
(dimensionally stable > 180°C). This paragraph is intended to give
a small overview over the first investigations regarding high temperature
bipolar plates which have been performed at ZBT in the
recent months.
2. EXPERIMENTAL
2.1. Preparation of the compounds
Initially, the investigation started with a screening of different
types of graphite without any further additives. Polypropylene
functions as binder. Each compound was produced under similar
process conditions (number of revolutions, temperature profile,
screw configuration and nozzle). In general, compounds offer a
filling load in the range of 75 – 85 % to achieve an adequate conductivity.
Within the scope of this investigation all compounds
have been produced with a similar filling load of 78 % (for assurance
of processability and comparability).
The extruder (Thermo Electron, Germany) is equipped with two
gravimetrical metering units (K-Tron, Switzerland) for the thermoplastics
and the filling materials. The two gravimetric metering
units were controlled by software (K-Tron) to proof the weight
ratio and under these conditions it is possible to reach an accuracy
of the compound of up to 0.2 %. During compounding the different
parameters torque, pressure and melt temperature were recorded
by software PolyLab (Thermo Electron) and subsequently
evaluated.
The production of compound material took place in a twin
screw extruder in which the thermoplastic is continuously fed into
the extruder where it is preheated, compacted, and then plasticised.
According to the desired weight ratio, an adequate amount
of the conductive filling materials is added downstream to the
plasticised polymer resulting in conductive particles getting homogeneously
dispersed with the thermoplastic. The produced compound
serves as basis material for the subsequently following
compression moulding or injection moulding process.
2.2. Injection moulding to bipolar plates
At ZBT, injection moulding is the standard production method
to manufacture bipolar plates in a one-step process directly with
the required structures (Kraus Maffei, Germany, F max =3000 KN,
p max =350 Mpa).
The injection moulding technique for the processing of carbon
filled compounds is a non trivial task. The high content of filler
results in high viscosity and high flow resistance of the mass.
Additionally, injection pressure, injection velocity and mass temperature
must all be increased as compared to injection moulding
of pure thermoplastics [3]. Within the scope of this investigation,
exceptionally injection moulded plates (low temperature materials)
were produced without any structures to realise comparable
measurement of conductivity between compression moulded and
injection moulded plates.
2.3. Compression moulding to samples and to bipolar
plates
For investigations in high temperature materials the extruded or
kneaded compounds were transferred to a mould (Ø= 20 mm) and
then compression moulded in a 100 t hydraulic press (Weber Pressen,
Germany). During compression moulding the higher tendency
of the polymer component to flow ultimately leads to accumulation
of the polymer in a thin insulating film at the surface of the
plate. For the following measurement of electrical conductivity of
these samples, these thin films of approximately 100 mm were
removed by grinding both sides carefully.
High temperature bipolar plates (140 × 70 × 4 mm) were compression
moulded in a 285 t hydraulic press (Gegner, Austria).
Such produced plates serve for the following machining steps
(milling the structures into the plate)
2.4. Measurement of electrical conductivity
Four pole measurement of through plane electrical conductivity
took place in an apparatus which offers the possibility to deter-

Development of Highly Filled Graphite Compounds as Bipolar Plate Materials for Low and High Temperature PEM Fuel Cells
/ J. New Mat. Electrochem. Systems
23
Stamps
F press
Gold layer
Gas diffusion layer
Gold pins
Sample
RΩ
RV
Rtot
RC
Gold pins
F press
Figure 1. Four pole measurement apparatus with separate resistances
(principle sketch). R tot = total resistance of measurement,
R Ω = bulk/material resistance, R C = contact/surface resistance,
R V = volume resistance of the sample, R V = 2 × R C + R W
Figure 2. SEM-image of flake like graphite - scale bar 2 µm
mine the conductivity of bipolar plates with and without structure ∗ .
For measurement of electrical conductivity the samples were
placed between two gas diffusion layers (W.L. Gore & Associates
GmbH, Fuel Cell Technologies, Carbel ® -Cl-Gas Diffusion Media)
which are fixed on gold plated stainless steel stamps integrated in a
hydraulic press. The measurement was performed under pressure to
simulate conditions more similar to a fuel cell. For four sets of
pressure of 0.5, 1.5, 2.5 and 4 Mpa a current of 500, 1000, 1500
and 2000 mA (Wenking HP 96-20) was applied and the resulting
voltage recorded. Principle of the measurement apparatus is shown
in Figure 1.
For unification and comparability, the results of the resistance
were divided by the thickness and multiplied by the area, to yield
the resistivity independent of its geometry.
2.5. Analysis of structure
Scanning electron microscopy (field emission-SEM) (Leo 1530
Gemini) was conducted to analyse the internal structure of compression
moulded and injection moulded bipolar plates. For SEM
measurements, the plates were broken in liquid nitrogen to prevent
thermal deformation. The plate samples were placed in the sample
holder, vacuum was applied and SEM images were taken of the
broken surface (cross sectional direction).
3. RESULTS AND DISCUSSION
* Exceptionally, injection moulded plates were produced without any structures to
realise comparable measurement of conductivity between compression moulded and
injection moulded plates
** 20, 10 and 5 mm, d50-value: size distribution measured by laser diffraction -50% of
particles are smaller and equal than the specified value
Figure 3. SEM-image of spherical graphite – scale bar 10 µm
3.1. Influence of size and morphology of graphite
particles
Initially the investigations started with a set of graphite materials
of exactly equal morphology (flake like, high aspect ratio, Figure 2)
but different sizes ∗∗ . Another kind of graphite, spherical in morphology
(low aspect ratio, Figure 3) has been compared to these
three types of graphite with high aspect ratio. All pure graphite
materials were compounded under identical process conditions,
with an equal filling load (gravimetric) and polypropylene as
binder. In Table 1 the investigated kinds of graphite are listed.
With growing size of particles, the rate of production has been
observed to be higher, as presented in Figure 4. The higher surface
area of many smaller particles results in a higher need of binder to
get dispersed. This in turn has an effect on the output rate and the
maximum filling load. This might be one reason, but the bulk density
also accompanies with the particles size and morphology of the
Table 1. Tested kinds of graphite –influence of size and morphology
of different kinds of graphite on the compounding process and
the resulting conductivities for compression moulded and injection
moulded bipolar plates.
Morphology of graphite Size [µm] Bulk (Apparent) Density [g/l]
flake like- high aspect ratio 5,10,20 d50 * 100,115,155
spherical- low aspect ratio 20 d50 * 513

1
24 T. Derieth et al. / J. New Mat. Electrochem. Systems
14000
1400
output rate [g/h]
12000
10000
8000
6000
spherical graphite 20 µm
flake like graphite 20 µm
flake like graphite 10 µm
flake like graphite 5 µm
(bar⋅s)
1350
1300
1250
1200
1150
1100
spherical graphite 20 µm
flake like graphite 20 µm
flake like graphite 10 µm
flake like graphite 5 µm
4000
1050
2000
1000
950
0
900
Figure 4. Influence of size and morphology of different kinds of
graphite on the compounding process. Only depending on the morphology
of the compounded graphite the output rate is enhanced
by the factor of at least five
Figure 5. Integral of pressure over the time- a suitable indicator for
the viscosity- more challenging materials are characterised by a
higher integral value
Table 2. Obtained output rate during compounding depending on
morphology and size
Graphite
flake like 5 µm
Output rate [g/h]
1000
flake like 10 µm 2000
flake like 20 µm 2500
spherical 20 µm 12000
Table 3. Pressure integral over the time period during injection for
the investigated compounds
Graphite Pressure integral [bar s]
flake like 5 µm 1296
flake like 10 µm 1203
flake like 20 µm 1141
spherical 20 µm 1116
Bipolar plate with
flowfield, gas inlet
and further
structures
Overfeed (thin film)
Figure 6. Bipolar plate with overfeed- the overfeed film prompts to
increase the filling load of the compound
graphite materials. To specify, a higher density is beneficial for the
compounding process and with increasing particle size the density
also increases and as a consequence the rate of production improves.
On the other hand a significant increase of the output rate has
been observed by compounding the spherical graphite (Figure 4,
Table 2). The density of this type graphite is significantly higher
than that of flake like ones, predominantly due to their morphology
and this improves the output rate immensely.
In consequence, the spherical graphite (20 µm) enables a maximum
output rate enhanced by a factor of 5-12 than of graphite,
flake like in morphology.
To realise injection moulded bipolar plates the highly filled compound
material requires high injection pressure over the time period
during injection. The pressure integral over the time ( ∫ p dt) is
a suitable indicator for the viscosity and more challenging materials
are characterised by a higher integral value.
As expected, it has been observed that these materials which
were difficult in compounding were also difficult in injection
moulding. Hence, for the compounds filled with small sized graphite
a higher integral ( ∫ p dt) has been obtained. In Figure 5 the
results are presented.
It is noted that Roßberg and Trapp [3] reported that small particle
size of graphite is helpful in reducing the viscosity (increase of
flow ability) of the compound-melt. The lower binder absorption of
small particles allows to disperse more graphite particles within the
binder. According to this, it is to annotate here that for comparison
of compounds with same filling content (gravimetric) the total
surface area of many small sized particles is higher than of larger
particles, which in turn results in a higher need of binder. The
higher total need of binder reduces the flow ability and thus gives
rise to complexities in their production chain as shown in Figure 4
and Figure 5.

Development of Highly Filled Graphite Compounds as Bipolar Plate Materials for Low and High Temperature PEM Fuel Cells
/ J. New Mat. Electrochem. Systems
25
120
25
bulk-conductivity (S/cm)
100
80
60
40
flake like
graphite, 5 µm
spherical
graphite, 20 µm
+ 3.6 %
flake like
graphite, 10 µm
flake like
graphite, 20 µm
volume-conductivity (S/cm)
20
15
10
flake like
graphite, 5 µm
spherical
graphite, 20 µm,
+ 3.6 %
flake like
graphite, 10 µm
flake like
graphite, 20 µm
20
spherical
graphite, 20 µm
5
spherical
graphite, 20 µm
0
0 1 2 3 4
pressure (Mpa)
0
0 1 2 3 4
pressure [Mpa]
Figure 7. Bulk conductivities of the investigated materials
(measurement was applied on unstructured bipolar plate samples
20 × 20 mm, filling load 78%) - smaller particles result in better
values of conductivity but spherical morphology enables a higher
filling load which in turn results in relatively good values of conductivity
Within this investigation, during the process of injection moulding,
it has been observed that a thin overfeed film has been found to
be formed (Figure 6) around the plates when the material used was
spherical in morphology and not flake like (same parameters for
injection moulding). This phenomenon is an indicator for good
flow ability (low viscosity) of the molten compound material and
normally allows to increase the filling load with the result of avoiding
the thin overfeed film and primary to realise bipolar plates of
better conductivity.
In consequence, one further compound based on the spherical
graphite, has been produced with an increased filling content
(3.6 % increased filling load than before). Injection moulded bipolar
plates were produced in the same approach as mentioned before
and subsequently measurement of electrical conductivity was applied.
3.2. Measurement of electrical conductivity
While the electrons need to pass through the bipolar plate, the
through-plane conductivity is obviously more important than the
in-plane conductivity for a bipolar plate. Hence, only the through
plane conductivity has been measured and the results are presented
in Figure 7 and Figure 8.
Figure 8. Volume conductivities of the investigated materials- R V =
2 × R C + R W (measurement was applied on unstructured bipolar
plate samples 20 × 20 mm)
These compounds were not being optimized regarding their conductivity;
the main focus on this investigation was to ensure comparability
with regard to the electrical conductivity. There was no
additional carbon content inside these compounds. In consideration
of this, the electrical conductivity of the compound for the graphite
with a particle size of 5 µm is a respectable value and it has been
found, that with decreasing size of particles the electrical conductivity
increases immensely while the process also becomes more
difficult.
By comparing the spherical compound (20 µm) and the flake like
compound (20 µm), it is obvious that the conductivity of the spherical
compound is much lower in case of the same filling content.
However, the spherical graphite enables a higher processable filling
load and this in turn increases the values for the electrical conductivity
while keeping the processability improved by factor of at
least five up to twelve.
Based on these results and within the focus of a mass production
technology, it is obvious that there is need for some more improvement.
As a consequence, one screening, with different spherical
types of graphite and mixtures of these ones with carbon blacks has
been initiated.
Why carbon blacks (CB)? The nanometer sized carbon blacks are
forming out conductive bridges through the insulating polymer
matrix between the micro sized graphite flakes. But the influence
of carbon blacks is significant in both, on one hand the positive
Table 4. Comparison of all investigated materials- as a result, a compound has been developed, combining good values of conductivity and
easy manufacturing during the entire production chain
Graphite
Volume-conductivity by 2,5 Mpa Bulk-conductivity by 2,5 Mpa Output rate during compounding [kg/h]
flake like 20 µm 7.4 34.0 2.5
flake like 10 µm 13.6 58.7 2
flake like 5 µm 20.6 96.9 1
spherical 5.1 22.6 12
spherical + 3,6 % 14.7 68.4 10
optimised spherical compound incl. carbon black 22.9 88.5 12

26 T. Derieth et al. / J. New Mat. Electrochem. Systems
Voltage U [V]
24,0
20,0
16,0
12,0
8,0
4,0
0,0
0 100 200 300 400 500 600 700 800
Current Density j [mA/cm²]
Figure 9. LT fuel cell stack with 24 cells and investigated injection
moulded bipolar plates
Figure 10. SEM-image of a compression moulded plate - graphite
particles oriented perpendicular to the current flow in a fuel cell
effect for the conductivity and on the other hand a negative effect
on the process (high surfaces, abrasive behaviour). In a field of
eventually appropriate materials, combing graphite, carbon blacks
and the polymer, one composition exposed to be the best compromise
between processability and conductivity. As a result of this
screening, one compound has been achieved with a conductance
better than the compound, based on the flake like graphite (5 µm).
Nevertheless the processability for the compounding process retains
improved by a factor of 12.
This new developed material serves as feedstock for the subsequently
following step of injection moulding and such produced
plates are operating in large quantities in PEM-Fuel-Cell-Stacks at
ZBT. It should be noted here, that it is possible to produce bipolar
plates with a better electrical conductivity which as well means a
better cell performance. For high performance application like
automotive a better material might be required whereas for the
most applications (i.e. auxiliary power units (APU) or combined
heat and power units (CHP)) this compound material is well appropriate
and even low priced producible. Figure 9 shows an I-V-curve
of an operating 24 cell low temperature PEM-stack including bipolar
plates based on this compound.
3.3. Influence of production method on the internal
structure of bipolar plates
Another important issue is to analyse the different effects of the
filling components and/or the content in a bipolar plate, depending
on the different production methods compression- or injection
moulding.
Due to the required degree of lower viscosity for the injection
moulding process, the compression moulding allows a higher processable
filling load of graphite than injection moulding. This results
in a higher bulk conductivity of the basis compound for compression
moulded plates. But plates which have been produced by injection
moulding using a less filled compound show a similar total
resistivity or better conductivities when compared to compression
Core Surface layer – approx. 200 µm
Alignment
perpendicular to
current flow in a
fuel cell
Low resistivity
within the plate due
to the alignment of
the graphite flakes
plane to the current
flow in a fuel cell
Figure 11. SEM-image of an injection moulded plate - graphite particles being oriented within the core of the plate plane to the current flow
in a fuel cell

Development of Highly Filled Graphite Compounds as Bipolar Plate Materials for Low and High Temperature PEM Fuel Cells
/ J. New Mat. Electrochem. Systems
27
moulded plates with equal filling load.
To specify, graphite is an anisotropic material with regard to the
direction of its current flow. Due to its chemical structure, graphite
along its crystalline layered structure is electrically conductive. On
the other hand, perpendicular to its plane, graphite is an electrical
insulator. Due to its morphology, flake like kinds of graphite (high
aspect ratio) offer a significantly higher anisotropic behaviour
which is of high importance as explained in following paragraphs.
The process of compression moulding results in graphite particles
oriented in a plane perpendicular to the direction of compaction
force during moulding. Which as well as means perpendicular
to the current flow in a fuel cell (Figure 10). This possibility exists
in all polymer/graphite systems as long as the aspect ratios of the
graphite particles are not unity. This in turn influences the “in plane
to through plane ratio” of conductivity non beneficial and results in
higher in plane conductivities but worse through plane conductivities.
Contrary to this, the injection moulding process results in different
structures to be formed within an injection moulded bipolar
plate. SEM-pictures of broken injection moulded plates (crosssectional)
have shown two separate areas with different orientations
of the graphite flakes within the polymer matrix (Figure 11).
First, in the middle of the bipolar plate (core), an alignment of
the graphite flakes convenient to the direction of the current flow in
a fuel cell (anisotropy effect) has been found to be formed. Second,
a distribution of the graphite flakes perpendicular to the current
flow in a fuel cell has been observed at the surface of the bipolar
plate (approx. 200 – 400 µm) similar to compression moulded
plates. The focal point is that the alignment within the core of the
plates - in plane to the current flow in a fuel cell - increase the
through plane conductivity of injection moulded plates. This in
consequence, offers the possibility to realise less filled bipolar
plates with electrical conductivities, similar to compression
moulded bipolar plates containing a higher filling load. In addition,
this alignment is also the explanation why flake like graphite based
injection moulded plates, offer higher values of conductivity (when
compared for same filling load).
3.4. Carbon based, thermoplastic bonded high temperature
bipolar plates
The high temperature polymer electrolyte membrane fuel cells
(HT PEMFCs) offer a great promise both in pure hydrogen (H 2 )
based and reformed hydrogen (RH) based power supply systems
owing to many advantages when compared to their low temperature
counterparts (LT PEMFCs). The HT PEMFCs are operated in
the 130 ºC - 200 ºC temperature range, enabling the heat produced
in the stack to be put to profitable use and offer high CO tolerance.
As the energy supply in the form of H 2 infrastructure is not yet
realised in most parts of the world, onsite H 2 production from liquid
and gaseous fuels (with high energy densities) is more realistic
and in such a scenario, HT PEMFCs are more appropriate and offer
many fold advantages when compared to the LT PEMFCs.
As discussed in the preceding paragraph, HT PEMFCs are a
promising technology, but the various materials which are required
in a working HT PEMFC are challenging to realize.
This paragraph is intended to give a small overview over the first
investigations regarding high temperature bipolar plates which
have been performed at ZBT. More detailed information’s will be
published in a following paper.
Based on the experience in successfully produced low temperature
compounds, the development of high temperature compounds
currently focuses on the selection of suitable thermoplastics and the
optimised filling mixture as mentioned before, serves as basis to
achieve respectable electrical conductivities.
As mentioned before, the focus of ZBT is to process bipolar
plates with a mass production technique to produce low priced
articles (in this case injection moulding). Realising high temperature
plates by injection moulding is due to the typical flow behaviour
of high temperature polymers, combined with high filling
loads much more challenging than for low temperature compounds.
Hence, for first investigations, ZBT has chosen the easiest way to
produce high temperature plates, which means unstructured and by
compression moulding while the required structures have been
milled into the plates afterwards.
The choice of appropriate HT-thermoplastics for carbon filled
HT-bipolar plates is mainly determined by the following requirements.
• Dimensionally stable at elevated temperatures (> 160°C)
• High melt flow rate
• Chemical stability in the presence of fuel, oxidant, product
water and mineral acids
• Creep strength even at high temperature and good ageing
resistance
• Gas tightness / low coefficient of permeability
High temperature materials are divided into two main categories—semi-crystalline
and amorphous—based on their difference in
molecular structure.
Amorphous high temperature resins have a randomly ordered
molecular structure which does not have a sharp melt point but
instead softens gradually as the temperature rises. These materials
change viscosity when heated, but seldom are as easy flowing as
semi-crystalline materials.
Semi-crystalline materials have a highly ordered molecular structure
with sharp melt points. They do not gradually soften with a
Table 5. Tested thermoplastic for high temperature compound materials
Thermoplastic Company t m /t g [°C] Structure
Polyphenylen oxide, PPO (different types) GE-Plastics 280-310 100 % amorphous
Polyethersulphone, PES BASF 225 100 % amorphous
Polysulphone, PSU BASF 180 100 % amorphous
Polyether imide, PEI GE-Plastics 370-410 100 % amorphous
Polyamide, PA 12 Degussa HPP 220 30 -35 % semi crystalline
Polyphenylen sulphide, PPS Ticona 280 28 – 35 % semi crystalline

28 T. Derieth et al. / J. New Mat. Electrochem. Systems
1,00
0,90
0,80
Cell at 170°C with Thermoset based HT-Bipolar
Plates
Cell at 170°C with ZBT-HT-Bipolar Plates
0,70
Cell Voltage (V)
0,60
0,50
0,40
0,30
0,20
0,10
0,00
0 5 10 15 20 25
Load Current (A)
Figure 12: Single HT-cell operating successfully at temperatures
up to 200°C at ZBT- bipolar plates produced by ZBT
Figure 13. I-V characteristic of the tested single cell compared
with bipolar plates based on phenol resin
1,0
0,9
0,8
temperature increase but, rather, remain hard until a given quantity
of heat is absorbed and then rapidly change into a low viscosity
liquid. They have excellent chemical resistance. Table 5 shows the
choice of thermoplastics which have been investigated.
First tests took place in a small lab scale kneader. During the
process of compounding it has been observed, that depending on
the grade of crystallinity, the thermoplastics exhibit significant
differences in their quality to disperse the graphite particles.
The characteristic of amorphous polymers of slowly softening
over a large temperature range was reflected in their behaviour of
dispersing the filling particles. Bringing the filling particles into the
polymer matrix during kneading turned out as very difficult to accomplish.
The particles must be mixed in, step by step very carefully.
Otherwise it occurred that a small amount of molten compound
turned around the rotors of the kneader and shifts the residue
of filling content inside the kneader without dispersing any further
particles. Nevertheless it was possible to produce some highly
filled compounds (> 80 %) based on PPO and PEI which were
pressed to unstructured plates.
Contrary to the amorphous resins, it was undemanding to realise
highly filled compounds, based on both investigated semicrystalline
polymers (PA, PPS). Due to the typical behaviour of this
category of polymer, the melt is of low viscosity and flows readily
around the filling particles which results in a good quality of dispersion
and leads to a better dispersed more homogenous material
and plate in the end.
First tests of gas tightness have shown that these plates which are
based on the amorphous thermoplastics are more brittle than plates
made of the semi crystalline thermoplastics and they do not prevent
gas diffusion/convection through the material. Hence, the semi
crystalline materials PA and PPS are preferable as matrix for high
temperature materials and finally, due to the higher chemical stability
and the higher heat deflection temperature, PPS has been selected.
Larger quantities were produced at the extrusion line and
plates have been pressed in the same approach as mentioned before.
Within the pressed unstructured plates the required structures
Voltage U [V]
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
0 100 200 300 400 500 600 700 800
Current density j [mA/cm 2 ]
HT PEM 170 °C LT PEM full humidified 55 °C LT PEM wet diff. Layer 50 °C LT PEM dried out 55 °C
Figure 14. Comparison of a HT single fuel cell with a LT fuel cell
under various conditions, 50 cm² active area, HT Fuel Cell without
humidification
like flowfield and cooling channels were milled in and by now first
bipolar plates based on the investigated compound material have
been integrated into a high temperature PEM-stack. This fuel cell
stack has been operated successfully at ZBT with temperatures of
up to 200°C and has shown good performance (Figure 11 and Figure
12).
Figure 13 shows a comparison between a ZBT high temperature
stack and a ZBT low temperature stack under various conditions.
The high temperature stack has been operated with compression
moulded bipolar plates however the low temperature stack has been
run with injection moulded plates. As mentioned before, the process
of compression moulding enables a higher processable filling
load than injection moulding and hence the load of the high temperature
plates has been three percent higher.
4. CONCLUSION
The experiments have shown that these compounds have good
electrical conductivity and are mass producible, to be effectively
used in bipolar plates. The morphology and bulk density of the
tested graphite material used in these compounds, greatly influence

Development of Highly Filled Graphite Compounds as Bipolar Plate Materials for Low and High Temperature PEM Fuel Cells
/ J. New Mat. Electrochem. Systems
29
the production rate of the compounds, which in turn influences the
production rate of the bipolar plates. Graphite, whose morphology
and bulk density result in better electrical conductivity might give
rise to complexities in their entire production chain. Furthermore
the particle size of the graphite material also has a role to play here:
compounds with smaller graphite particles tend to offer better conductivities
when compared to larger graphite particles. A central
result of the investigations is that graphite materials which allow
higher ease of manufacturing also allow a higher filling load, thus
improving the conductivity. Analysis of the internal material structure
of injection moulded plates shows - contrary to compression
moulded plates - an alignment of the graphite flakes convenient to
the direction of the current flow in a fuel cell which in turn increases
the electrical conductivity.
The development of high temperature compounds focuses on the
selection of suitable thermoplastics. Investigations have disclosed
that depending on the grade of crystallinity, the thermoplastics
exhibit significant differences in their behaviour concerning dispersion
during compounding and compression moulding. By now
bipolar plates based on the investigated compound material have
been produced by compression moulding and have been integrated
into a high temperature PEM-stack. This fuel cell stack has been
operated successfully at temperatures up to 200 °C and has shown
good performance.
[8] Kazuya N., Hitoshi I., Hideyuki N., Effect of particle size of
graphite on electrical conductivity of graphite/polymer composites,
Composites Interfaces, Vol. 6, No. 5, 483 (1999).
[9] Stewart Jr CR. US Patent 4670300, 1987.
[10]Buswick D, Wilson M., US Patent 6248467, 2001.
[11]R. Kaiser, H. G. Fritz, Highly Conductive Polymer Compounds
for Fuel Cell Applications, Presented at PPS- 17, The Polymer
Processing Society, 17 th Annual Meeting, Montreal, Canada,
May 21-24 2001.
5. ACKNOWLEDGEMENTS
Financial support by the European Union and the state of North
Rhine Westphalia for the starting of the ZBT GmbH is gratefully
acknowledged.
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