C A R B O N 4 7 ( 2 0 0 9 ) 2 4 1 3 – 2 4 1 8

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The electrical conductivity of a composite bipolar platefor fuel cell applications

B.K. Kakatia, V.K. Yamsania, K.S. Dhathathreyanb, D. Sathiyamoorthyc, A. Vermaa,*

aDepartment of Chemical Engineering, IIT Guwahati, Guwahati 781039, Assam, IndiabCentre for Fuel Cell Technology, Medavakkam, Chennai 601302, IndiacPowder Metallurgy Division, BARC, Mumbai 400705, Maharashtra, India

A R T I C L E I N F O

Article history:

Received 2 April 2009

Accepted 23 April 2009

Available online 3 May 2009

0008-6223/$ - see front matter � 2009 Elsevidoi:10.1016/j.carbon.2009.04.034

* Corresponding author: Fax: +91 361 2582291E-mail address: anil.verma@iitg.ernet.in (

A B S T R A C T

A graphite/phenol formaldehyde resin composite bipolar plate was developed for fuel cell

applications. The electrical conductivity of the composite was measured with the help of a

four-probe technique. A basic model was modified to predict the electrical conductivity of

the plate for a wide range of graphite content. The model was highly dependent on the

shape factor and orientation factor of the conductive graphite filler in the composite.

The concept of digital image processing was used to quantitatively determine the shape

and orientation factors of the bipolar plate. The experimental values of the electrical con-

ductivities were well predicted by the model. The most effective in-plane and through-

plane electrical conductivities, at 75% graphite content, were found to be 165 and

103.3 S cm�1, respectively.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The proton exchange membrane fuel cell (PEMFC) is a promis-

ing power source for residential and automotive applications

due to its attractive features such as high power density, rela-

tively low operating temperature, convenient fuel supply,

longer lifetime and modularity [1]. Bipolar plate is a vital com-

ponent of low temperature fuel cell, which contributes to 80%

of the total weight of the PEMFC stack [2,3]. Recent cost anal-

ysis shows that 38% of the total cost of PEMFC stack is incurred

by the bipolar plate followed by the costs of the electrodes,

membrane, and catalyst (platinum) as 32%, 12% and 11%,

respectively [3]. Different type of materials like metal sheet,

polymer coated metal sheet, graphite, flexible graphite, C–C

composite, advanced composites, etc. are under investigation

for the development of low cost and light weight bipolar plates

for PEMFC application. Hermann et al. [4] reviewed and dis-

cussed different types of materials for the bipolar plate. They

have suggested that the metal or coated metal might be a good

er Ltd. All rights reserved

.A. Verma).

choice keeping in mind the reduction in thickness and easy

processibility of flow field design on the bipolar plate. How-

ever, the corrosion of the metal plate and uneven expansion

of coated metal at the fuel cell temperature (80–90 �C) are

the limitations. Therefore, development of composite bipolar

plate using non-metallic materials is the candidate of interest.

Many experimental studies have been carried out by vari-

ous researchers on the electrical conductivity of a composite

with binary composition [5–7]. The most studied combination

of the composite bipolar plate was graphite and phenol form-

aldehyde (PF) resin. The high electrical conductivity and low

specific gravity (2.21 gm cm�3) of natural graphite, and good

mechanical strength of PF resin are the main reason behind

it [5,8]. The properties of the graphite/resin composite bipolar

plate extensively depend on the electrical conductivity of

both graphite and the resin matrix. Typically, PF resin is elec-

trically insulating material with electrical conductivity in the

order of 10�15 S cm�1, whereas the electrical conductivity of

natural graphite is of the order of 106 S cm�1.

.

Fig. 1 – Modelling of real composite showing (a) particles

and clusters within the polymer matrix and (b) field induced

due to the orientation of the particles.

2414 C A R B O N 4 7 ( 2 0 0 9 ) 2 4 1 3 – 2 4 1 8

Barton et al. [9] developed a model based on general effec-

tive media equation to predict the electrical conductivity of

composite bipolar plate. To validate the model they have

developed a composite bipolar plate using varying amount

of synthetic graphite (elliptical disc in nature) in liquid crystal

polymer. They have concluded that the results were not

encouraging. However, the same model worked well, when

they replaced synthetic graphite with carbon black (spherical

in nature). Lux [10] has reviewed the models proposed to ex-

plain the electrical conductivity of the binary mixtures made

of conductive and insulating materials. He has described the

importance of filler distribution, filler/matrix interaction, pro-

cessing techniques and orientation of the filler in the insulat-

ing resin matrix. However, the filler/matrix interaction, filler

distribution in the resin, and filler orientation were discussed

qualitatively.

Mamunya et al. [11] have proposed a model for Nickel/re-

sin and Cu/resin composite systems considering the shape

and spatial distribution of the filler particles near the percola-

tion threshold for an undisclosed application. The proposed

model was not applicable for the composite bipolar plate be-

cause the conductive graphite particle content in the resin

matrix was far more than the percolation threshold [12,13].

Ondracek and coworkers have shown a model for electrical

conductivity of iron and iron carbide composite [14]. The elec-

trical conductivity of the iron and iron carbide was 105 and

227 S cm�1, respectively. They have considered the shape fac-

tor and orientation factor of the filler (iron) in the matrix (iron

carbide) and found a good agreement between the model and

experimental values of the electrical conductivities. However,

the model was not applicable for the systems, where the two

components of the mixture had wide difference in their elec-

trical conductivities. It is worth mentioning that the electrical

conductivities of the PF resin and graphite in the composite

bipolar plate are far apart in the order of 1020–1022 S cm�1.

Moreover, the Ondracek paper described only a qualitative

method to find out the shape and orientation factors of the fil-

ler in the composite. In this paper, modified Ondracek model

is presented to predict the electrical conductivity of fuel cell’s

composite bipolar plate for a wide range of compositions.

Concept of digital image processing (DIP) was utilized to

quantitatively find out the orientation and shape factors of

the electrical filler (graphite) particles in the composite bipo-

lar plate. To validate the model, composite bipolar plates were

developed by compression molding technique using different

composition of natural graphite and PF resin. Electrical con-

ductivity of the developed bipolar plates was investigated

using four-probe technique as per the ASTM-C611 standard.

2. Theoretical background

2.1. Model for electrical conductivity of the composite

Ondracek model (Eq. (1)) was used as a basis to predict the

electrical conductivity of the composite,

1� Cf ¼rp

r

� �m rf � r

rf � rp

rþ n � rp

rp � n � rf

� �r

ð1Þ

where, r is the electrical conductivity of the composite, rp the

conductivity of the polymer matrix, rf the conductivity of the

conductive phase (filler) and Cf is the volume fraction of the

filler. The constants m, n and r are the functions of shape fac-

tor (Ff) and orientation factor ðcos af Þ as defined by Eqs. (2)–(4),

m ¼Ff ð1� 2Ff Þ

1� ð1� Ff Þ cos2 af � 2Ff ð1� cos2 af Þð2Þ

n ¼1� ð1� Ff Þ cos2 af � 2Ff ð1� cos2 af Þ

2Ff ð1� cos2 af Þ þ ð1� Ff Þ cos2 afð3Þ

r ¼ mþ ð1� Ff Þ2Ff

2Ff ð1� cos2 af Þ þ 1� Ff cos2 afð4Þ

Ondracek’s model fails to predict the electrical conductivity of

the composite system when the difference between the elec-

trical conductivities of filler and insulating matrix is extre-

mely high [10]. This is due to the negative value of the term

ðrp � n � rf Þ in Eq. (1). Thus the model is modified to fit for elec-

trical conductivity of composite bipolar plate especially for

fuel cell application. The proposed modified model is given

by Eq. (5),

1� Cf ¼rp

r

� �m0 rf � r

rf � rp

rþ n � rp

rp � n � rf

��������r

ð5Þ

where m0 ¼ dm and d is the dimensionless parameter, which

can be found out empirically. The other terms have their

usual meaning.

Shape and orientation factors are assumed to be charac-

teristic of an ellipsoid, which allow the calculation of conduc-

tivity of the mixtures. The orientation factor is represented by

cos a, where a is the angle between the major axis of the ellip-

soid (generated by the graphite particle or clusters) and the

electric field as shown in the Fig. 1. The orientation factor of

individual particle will be different from particle to particle

C A R B O N 4 7 ( 2 0 0 9 ) 2 4 1 3 – 2 4 1 8 2415

in the real composite. Therefore, the average orientation fac-

tor of the graphite particles in the composite was considered

as the weighted average of all the orientation factors as given

by Eq. (6),

Average orientation factorðcos aÞ ¼P

ni cos aiPni

ð6Þ

where, ni is the number of particles having the same orienta-

tion factor.

The shape factor is defined as the ratio of minor to major

axis of the ellipsoid. The weighted average of the shape factor

of individual particle or cluster was used.

3. Experimental

3.1. Materials

Composite bipolar plates were fabricated using industrial

grades of resole type PF resin and natural graphite. Natural

graphite powder with purity of 95% and average size of

47 lm (300 mesh size) was received from Nickunj Eximp Entp.

Pvt., Ltd., Gujarat, India. PF resin (resole) was procured from

Tipco Industries Ltd., Gujarat, India.

3.2. Development of composite bipolar plate

An appropriate amount of resin volume fraction was diluted

with acetone and mixed thoroughly with natural graphite

powder using a mechanical stirrer. The mixture was then al-

lowed to dry at 70 �C. The completely dried mixture of resin

and graphite was again ground to powder form for better mix-

ing. The obtained powder was hot pressed, under 100 kg cm�2

pressure, in a mold using compression molding machine at

the curing temperature of 96 �C to obtain a preform. The pre-

form was then post cured at a temperature of 220 �C for 1 h to

obtain the composite bipolar plate. The size of the bipolar

plate was 7.0 · 7.0 · 0.3 cm3 and a minimum of four samples

were prepared and characterized for each composition.

3.3. Determination of shape and orientation factors ofgraphite in the composite

The determination of shape and orientation factors in the

developed composite bipolar plate carries real challenge.

Therefore, to cope up with the challenge, we have utilized

the concept of DIP technique. DIP uses scanning electron

microscope (LEO; model 1430vp) images to investigate the

morphology of the composite bipolar plate. SEM micrographs

were digitized and converted to binary image using MATLAB�

software [15]. The graphite particles having particle size less

than 0.2 lm were filtered out from the binary image using

Gaussian filter of the Matlab� software. All the pixels on the

boundary of the graphite particles were stored as a matrix

in the form of an array B[b1, b2, b3, . . ... bn]. Each element of

the array was a matrix containing the coordinates of the

boundary points. Then all the possible distances between

any pair of coordinates were calculated. So, a set containingnC2 number of elements were generated, which represented

the distance between any two boundary points. The maxi-

mum and minimum distances were represented as major

and minor axes of the ellipsoid, respectively, for either a

graphite particle or cluster as shown in Fig. 1.

3.4. Porosity of the composite

The total porosity of the composite bipolar plate sample was

measured using BET-surface area analyzer (make: CoulterTM,

model: SA3100).

3.5. Electrical conductivity of the composite

Electrical conductivity of the sample was measured as per the

ASTM C611 method using conventional four probe method at

a constant current supply ranging from 100 to 500 mA. The

electrical conductivity was measured at least 5-times for each

sample at 25 �C. The experimental error was within ±5%. The

schematic of the electrical conductivity measurement set-up

has been reported elsewhere [16]. Keithly electrometer (mod-

el-6514) was used as the constant current source. The electri-

cal conductivity of the composite bipolar plate was

determined using Eq. (7),

Conductivity ¼ i� dV � l� b

S � cm�1 ð7Þ

where l (cm) and b (cm) are the width and thickness of the

sample, respectively. The constant current supplied through

the sample is represented by i (A) and V (V) is the voltage drop

between two points separated by a distance d (cm). The same

set-up was used to measure the through-plane electrical con-

ductivity by changing the orientation of the bipolar plate such

that the thickness of the plate being the shortest path for the

electricity through the set-up.

4. Results and discussion

4.1. Digital image processing

Fig. 2 shows representative SEM and digitally processed

images of 75% graphite and 25% PF composite bipolar plate.

Fig. 2a, a portion of the SEM image, shows that the flaky

graphite particles are well distributed and oriented in the

plane of the composite. Fig. 2b shows the binary image of

the SEM for further study. In the Fig. 2b, resin and graphite

particles are represented by white and black colour, respec-

tively. The point like particles of diameter equal 0.2 lm or less

were filtered out from Fig. 2b and are shown in Fig. 2c. The dif-

ference between original and filtered binary image is dis-

tinctly visible in the inset of the respective picture.

Fig. 2c was used for determining the shape and orientation

factors of the graphite particles in the composite. It is to be

noted that the shape factor obtained will not be only for a par-

ticle but will also represent the continuous cluster of the

graphite particles as discussed earlier. High electrical conduc-

tivity is an essential characteristic of the composite bipolar

plate, which can be achieved by the continuous path of the

electrical conductor in the direction of external current col-

lector of the fuel cell. Moreover, for any graphite particle,

the shape factor may vary depending upon its inclination

with the plane. Thus, low shape factor is a desired property

compared to high shape factor of the same size particles

Fig. 2 – (a) SEM micrograph of the composite bipolar plate, (b) binary image of the same micrograph and (c) filtered binary

image where the point like particles in the binary image is removed; magnified view of a particular portion is shown in the

inset to show the effect of filtering.

2416 C A R B O N 4 7 ( 2 0 0 9 ) 2 4 1 3 – 2 4 1 8

due to the projection of the graphite particle. Therefore, low

shape factor will serve the purpose as the graphite particles

are highly conductive in the plane. Fig. 3 shows the effect of

graphite volume fraction on the average shape factor. The

40 50 60 70 80 900.2

0.3

0.4

0.5

Ave

rage

sha

pe f

acto

r

Graphite volume fraction / %

Fig. 3 – Effect of graphite content on average shape factor of

the filler.

average shape factor decreases slowly with the increase in

the graphite content. This decrease was due to the replace-

ment of the excessive resin by graphite particles. At 75%

graphite content the average shape factor sharply decreased

to 0.41 from its previous value of 0.49. When the graphite con-

tent was further increased the shape factor again increased

because of the creation of pores that was resulted due to

insufficient quantity of the resin in the matrix. Thus, 75% is

the optimum graphite content at which the graphite particles

do not have any excessive resin and the graphite particles are

well connected with each other and well bonded with the

resin.

The above explanation is verified (Fig. 4) with the help of

porosity analysis of the composite. Moreover, the other

mechanical properties of the composite also reduced (not

shown) drastically on further increase in the graphite content

(above 75%) in the composite bipolar plate.

The orientation of the graphite particles in the composite

is an important parameter as graphite has anisotropic electri-

cal properties. It has high electrical conductivity along the ba-

sal plane in comparison to the perpendicular direction to the

basal plane. The average orientation factor versus volume

fraction of the composite is shown in Fig. 5a. From the

Fig. 5a, it can be seen that the average orientation factor is

40 50 60 70 80 90

1

2

3

4

5

6P

oros

ity

/%

Graphite volume fraction / %

Fig. 4 – Effect of graphite content on porosity of the

composite.

40 50 60 70 80 90

50

100

150

200

250

Experimental

Modelled

Ele

ctri

cal C

ondu

ctiv

ity

/ S.c

m-1

Graphite volume fractions / %

Fig. 6 – Effect of the graphite content on electrical

conductivity (in-plane) of the bipolar plate.

C A R B O N 4 7 ( 2 0 0 9 ) 2 4 1 3 – 2 4 1 8 2417

maximum for 75% graphite content in the composite. At 75%

filler volume fraction the average orientation factor of the fill-

ers is 0.4755, which means that the average angle of inclina-

tion is ±61.60�. This shows that the graphite particles are

homogeneously distributed in the composite at that particu-

lar orientation factor [12]. A representative histogram is

shown in the Fig. 5b for 75% graphite content in the compos-

ite. The histogram shows the pattern of the angle of inclina-

tion with the number of particles in the composite.

4.2. Electrical conductivity of the composite

Fig. 6 shows the electrical conductivity, where the experimen-

tal data are shown by symbols and the model predictions are

shown by lines for different graphite content in the composite

bipolar plate. From the Fig. 6 it can be seen that the conduc-

tivity of the composite increases with the increase in graphite

content and follows inverse ‘‘S’’ pattern. In the lower region of

the pattern (40–55%) the electrical conductivity increases with

a slightly higher rate due to the decrease in insulating resin in

the smearing region of the graphite particles. In the middle

40 50 60 70 80 90

0.38

0.40

0.42

0.44

0.46

0.48

0.50

Ave

rage

ori

enta

tion

fac

tor

(cos

α)

Graphite volume fraction / %

(a)

Fig. 5 – (a) Effect of filler content on the average orientation fact

with different orientation factors (75% graphite content).

region (55–75%) of the pattern, the rate decreases. It may be

because the available resin was just sufficient to fill the inter-

stices of the graphite particles. However, in the top section

(75–85%) of the pattern the electrical conductivity rate further

increases with slightly higher rate due to the compacted

graphite particles, where the resin content was not enough

to provide any insulating barrier.

The experimental data are well predicted by the model as

shown by the line in the figure. The parameter d for PF resin

and graphite system was found to be 1.7346. The higher value

of the electrical conductivity of the composite is desirable.

However, the selection of the graphite composition is guided

by the shape and orientation factors as discussed earlier. As

per the recent benchmark given by Department of Energy,

USA the recommended value of electrical conductivity for

bipolar plate is >100 S cm�1 [17–19]. Thus, the composite at

75% graphite is a suitable bipolar plate for the fuel cell appli-

cation as it shows the electrical conductivity of 165 S cm�1.

Through plane electrical conductivity of the composite bipo-

lar plates was also measured and for 75% graphite content

it was found to be 103.3 S cm�1.

or of the composite, and (b) histogram of graphite particles

2418 C A R B O N 4 7 ( 2 0 0 9 ) 2 4 1 3 – 2 4 1 8

5. Conclusions

Composite bipolar plate has been developed using graphite

and PF resin for the fuel cell application. The composite bipo-

lar plates were developed using compression molding tech-

nique and the electrical conductivity of the composite was

measured using four-probe technique. A model has been de-

rived for the electrical conductivity of composite bipolar plate

with binary mixture of PF resin and natural graphite. The

model showed good correlation with the experimental results

for wide range of graphite volume fraction in the composite.

The electrical conductivity of the composite was dependent

on the individual electrical conductivity of the graphite and

resin. Moreover, the size, shape and orientation of the con-

ducting filler within the resin matrix affected the electrical

conductivity of the composite. The shape and orientation fac-

tors were calculated with the help of DIP of SEM micrograph

and were successfully incorporated in the model. The electri-

cal conductivity of the composite bipolar plate was found to

be 165 (in-plane) and 103.3 S cm�1 (through-plane) at 75%

graphite content.

Acknowledgement

The authors gratefully acknowledge the financial support of

the BRNS, Department of Atomic Energy, Government of In-

dia, for the above project (No. 2007/36/19-BRNS/1000).

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