Journal of Power Sources 185 (2008) 226–232
Contents lists available at ScienceDirect
Journal of Power Sources
journal homepage: www.elsevier.com/locate/jpowsour
Optimization of the polypyrrole-coating parameters for
proton exchange membrane fuel cell bipolar plates using
the Taguchi method
Yan Wang, Derek O. Northwood ∗
Department of Mechanical, Automotive, and Materials Engineering, University of Windsor, 401 Sunset Avenue, Windsor, Ontario, Canada N9B 3P4
a r t i c l e
i n f o
Article history:
Received 9 June 2008
Received in revised form 11 July 2008
Accepted 15 July 2008
Available online 25 July 2008
Keywords:
PEMFCs
Bipolar plates
Polypyrrole
Taguchi
Corrosion
a b s t r a c t
In order to overcome the high price, weight and volume of non-porous graphite bipolar plates, metallic
bipolar plates are being investigated as a substitute material. However, metallic materials can corrode
under proton exchange membrane fuel cell (PEMFC) working conditions, leading to a degradation in the
performance of the membrane. Previous work had shown that a polypyrrole coating on SS316L can significantly increase the corrosion resistance of the base material. In this study, a Taguchi design of experiment
method was used to optimize the process parameters for the polypyrrole coating so as to produce the
maximum corrosion resistance. Potentiodynamic and potentiostatic tests were used to determine the
corrosion resistance of the polypyrrole-coated SS316L. Scanning electron microscopy (SEM) with energy
dispersive X-ray (EDX) was used to characterize the coating thickness and coating appearance. Inductively
coupled plasma optical emission spectroscopy (ICP-OES) was used to determine the metal ion concentration in the solution after corrosion. The interfacial contact resistance of SS316L with carbon paper was
measured both before and after coating with polypyrrole.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
With escalating oil prices (about US$140/barrel) and increasing environmental concerns, increasing attention is being paid to
fuel cell technology. Proton exchange membrane fuel cells (PEMFCs) are receiving wide attention because of their low operation
temperature and fast start-up.
Bipolar plates are one of the most important components in
PEMFCs and are designed to accomplish many functions, including:
separate the individual cells in the stack; facilitate water management within the cell; carry current away from the cell; distribute
the fuel and oxidant in the stack; and facilitate heat management
[1]. However, the non-porous graphite bipolar plates account for
about 80% of the total weight, 45% of stack cost and almost the
whole volume [2]. Therefore, they are one of the major barriers to
the more widespread application of PEMFCs.
Currently, two major types of materials, metallic and composite materials, are being researched to substitute for non-porous
graphite for the fabrication of bipolar plates for PEMFCs. Both
∗ Corresponding author. Tel.: +1 519 253 3000x4785; fax: +1 519 973 7007.
E-mail addresses: wang167@uwindsor.ca (Y. Wang), dnorthwo@uwindsor.ca
(D.O. Northwood).
0378-7753/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jpowsour.2008.07.036
materials have their advantages and disadvantages as bipolar plate
materials. Metallic materials have good mechanical stability, electrical conductivity and thermal conductivity and can be recyclable.
However, in a PEMFC environment, metals are prone to corrosion
and the resulting metal ions can readily migrate to, and poison,
the membrane [3,4]. The dissolved metal ions can lower the ionic
conductivity of the membrane and, thus, the performance of the
PEMFC. Furthermore, any corrosion layer will lower the electrical
conductivity of the bipolar plates, and increase the potential loss
because of a higher electrical resistance. Hence, in order to be suitable materials of bipolar plates, metals should have both a very high
corrosion resistance and high electrical conductivity [5].
Conductive polymers are a new type of material, which have a
high redox potential and properties of both metals and plastics. The
electrochemical polymerization of a conductive polymer has been
used to coat metallic materials in order to fabricate bipolar plate
materials [6–8]. In our earlier research [8], we have coated polypyrrole on SS316L surface and it showed that the corrosion resistance
was increased about 10 times compared to the bare metal. However, this previous study [8], examined a very limited number of
process parameters and there was a need to determine whether
the corrosion resistance could be further improved.
In this study, the Taguchi method for design of experiment
was used for the optimization of the polypyrrole-coating param-
227
Y. Wang, D.O. Northwood / Journal of Power Sources 185 (2008) 226–232
Table 1
Experimental layout using a L9 orthogonal array
Experiment number
1
2
3
4
5
6
7
8
9
Column number
A (applied current)
B (time)
C (concentration of polypyrrole)
D (concentration of H2 SO4 )
1
1
1
2
2
2
3
3
3
1
2
3
1
2
3
1
2
3
1
2
3
2
3
1
3
1
2
1
2
3
3
1
2
2
3
1
eters for the PEM fuel cell bipolar plates. A classical optimization
method would design experiments that identify all possible combinations for a given set of variables. This approach is called the
full factorial design and it takes into account a large number of
experiments, which can be costly and time consuming. Taguchi [9]
proposed a design of experiment method, which minimizes the
number of experiments to a practical level for optimization processes. Taguchi’s parameter design has proved to be an effective
approach producing high quality products at a relatively low cost.
Some initial results from this Taguchi DOE have been reported in
Ref. [10].
the galvanostatic method was used to coat polypyrrole on SS316L.
In the galvanostatic coating, different currents, coating times, concentrations of polypyrrole and sulphuric acid were utilized to coat
the polypyrrole. The electrochemical instrumentation used was a
Solartron 1285 potentiostat. A typical three-electrode system was
used. SS316L, Pt and saturated calomel electrodes are the working
electrode, counter electrode and reference electrode, respectively.
The coating temperature was ambient temperature.
2. Experimental details
Scanning electron microscopy (SEM) and energy dispersive
X-ray analysis (EDX) analysis were used to observe the surface
morphologies and analyze the chemical compositions. If only the
surface morphology was to be examined, a JEOL JSM-5800LV SEM
was used. If both surface morphology and chemical analysis were
required, an environmental scanning electron microscope (ESEM)
facility equipped with EDX was used [FEI Quanta 200F with a solid
state back scattered detector (BSD)].
2.1. Taguchi design of experiment
The Taguchi method uses a special of orthogonal arrays to study
all the designed factors with a minimum of experiments. Orthogonality means that factors can be evaluated independently of one
another; the effect of one factor does not interfere with the estimation of the influence of another factor [9]. Four factors (applied
current (A), time (B), the concentration of polypyrrole (C), the concentration of H2 SO4 (D)) with three levels were selected as shown
in Table 1. The factors and levels were used to design a experimental layout using a L9 (34 ) array. The experiments were repeated 3
times in order to ensure reliability. For the applied current, levels
of 0.0002 A (1), 0.0005 A (2), 0.001 A (3) were used. For coating time,
levels of 10 min (1), 30 min (2), 60 min (3) were used. For the concentration of polypyrrole, levels of 0.05 mol L−1 (1), 0.1 mol L−1 (2),
0.2 mol L−1 (3) were used. For the concentration of H2 SO4 , levels of
0.05 mol L−1 (1), 0.1 mol L−1 (2), 0.2 mol L−1 (3) were used.
2.2. Polypyrrole coating
SS316L was chosen as the base material primarily because of its
good corrosion resistance and relatively cheap price. The chemical composition of the SS316L is shown in Table 2. In this study,
2.4. Electrochemistry
Both potentiodynamic and potentiostatic tests were used to
analyze the corrosion characteristics of the uncoated and coated
samples. In the potentiodynamic tests, the initial potential was
−0.1 V vs. open circuit potential (OCP), and the final potential was
1.2 V vs. saturated calomel electrode (SCE) and the scan rate was
1 mV s−1 . In the potentiostatic tests, at the anode, the applied potential was −0.1 V vs. SCE purged with H2 and at the cathode, the
applied potential was 0.6 V vs. SCE purged with O2 [11]. The setting time was 10 h with 1 point s−1 . Both the potentiodynamic and
potentiostatic tests were conducted at 70 ◦ C because PEM fuel cells
are generally operated at temperatures between 50 ◦ C and 100 ◦ C
[12].
2.5. Metal ion concentration in solution after potentiostatic tests
Table 2
Chemical composition of SS316L (wt%)
Metal
C
Mn
P
S
Si
Cr
Ni
Mo
Cu
N
Fe
2.3. SEM
SS316L
0.021
1.82
0.029
0.01
0.58
16.32
10.54
2.12
0.47
0.03
Balance
Inductively coupled plasma optical emission spectrometry (ICPOES) (IRIS #701776, Thermo Jarrell Ash Corporation) was used to
investigate the metal ion concentrations in solution after corrosion.
Liquid samples were introduced into the instrument via a Meinhard
concentric glass nebulizer (TK-30-K2, JE Meinhard Associates Inc.,
California, USA) combined with a cyclonic spray chamber. The areosol was then introduced into a radial orientation argon plasma
resulting in characteristic emission lines that are simultaneously
resolved using argon purged echelle optics and a themostatted
charge injection device detector. The total solution volume for ICPOES is 7 mL.
228
Y. Wang, D.O. Northwood / Journal of Power Sources 185 (2008) 226–232
Fig. 1. SEM micrographs of SS316L coated with polypyrrole: (a) experiment 1, (b) experiment 2, (c) experiment 3, (d) experiment 4, (e) experiment 5, (f) experiment 6, (g)
experiment 7, (h) experiment 8, (i) experiment 9 (for process parameters, see Table 1).
Y. Wang, D.O. Northwood / Journal of Power Sources 185 (2008) 226–232
229
Table 3
The thickness of polypyrrole coating with different coating conditions
Coating condition
Coating thickness (m)
1
2
3
4
5
6
7
8
9
7
21
46
18
43
50
25
43
54
±
±
±
±
±
±
±
±
±
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
2.6. Contact resistance tests
The interfacial contact resistance measurements were made
using a standard testing method [13,14]. All interfacial contact resistance measurements were carried out at room temperature. In our
setup, two pieces of Toray conductive carbon paper (Fuel Cell Store)
were sandwiched between the metal sample and the copper plates.
A GW Instel GOM-802 milliohm meter was used to measure the
electrical resistance. The compaction force was gradually increased
using a Tinus Olsen test machine and the contact resistance was
measured.
3. Results and discussion
3.1. SEM of polypyrrole-coated materials
Fig. 1a–i are SEM micrographs of polypyrrole-coated SS316L produced at different process conditions. Two types of polypyrrole
structure can be seen. In Fig. 1a, b, d and g, the polypyrrole film
is quite thin and the polishing marks from the 240 grit polishing
of the SS316L are still visible. In Fig. 1c, e, f, h and i, we can see
polypyrrole particles, which are between 1 m and 3 m in size.
Comparing the particle sizes in the different coatings, it was found
that the polypyrrole particle size increased with both increasing
coating current and coating time.
From cross-sectional micrographs, we determined the thickness of the polypyrrole coating for different coating conditions,
and these are given in Table 3. We can see that the polypyrrole coatings have different thickness with different coating
conditions. The thickness of polypyrrole coatings for coating
conditions 1, 2, 4, 7 are relatively thin (less than 30 m)
compared to coating conditions 3, 5, 6, 8, 9 (more than
30 m). Therefore, in general, the coating thickness increased
with increased coating time, coating current and polypyrrole
concentration.
Our earlier studies [15] have shown that the nucleation and
growth behavior of polypyrrole on SS316L could be divided
into three stages. The first stage is an incubation period. The
second stage is a combination of instantaneous nucleation
and two-dimensional growth and instantaneous nucleation and
three-dimensional growth. The third stage is a combination of
instantaneous nucleation and three-dimensional growth and progressive nucleation and three-dimensional growth.
3.2. Corrosion resistance of polypyrrole coatings
A linear polarization method was used to obtain the polarization
resistance of SS316L at 70 ◦ C:
Rp =
ˇa ˇc
2.3icorr (ˇa + ˇc )
(1)
Fig. 2. Potentiostatic curve for polypyrrole-coated SS316L in the simulated anode
and cathode environments (condition 5).
where ˇa , ˇc , icorr , and Rp are the Tafel slopes of the anodic and
cathodic reactions, the corrosion current density and polarization
resistance, respectively [16].
The corrosion potential increased and the corrosion current density decreased after coating polypyrrole on SS316L: see Table 4. In
our earlier studies [17,18], the polarization resistance and corrosion
current density of uncoated SS316L were found to be 328 cm2
and 40 A cm−2 at 70 ◦ C, respectively, for the same test conditions.
From the linear polarization data, the polarization resistance for
the thin (less than 30 m) polypyrrole coatings (Fig. 1a, b, d and
g) was around 1500 cm2 , which is about 5 times higher than for
the uncoated samples. Also, the polarization resistance of relatively
thick (more than 30 m) polypyrrole coating (Fig. 1c, e, f and h) is
about 3500 cm2 , which increased the polarization resistance of
SS316L by more than 10 times. The polarization resistance and corrosion current density for uncoated SS316L are about 4200 cm2
and 5 A cm−2 at 20 ◦ C, respectively [17,18]. Therefore, the polarization resistance and corrosion current density for a relatively thick
polypyrrole-coated material at high temperature (70 ◦ C) are of the
same order as those of the uncoated SS316L at ambient temperature
(20 ◦ C).
3.3. Potentiostatic tests with polypyrrole-coating chosen on basis
of linear polarization tests
In actual PEMFC working conditions, the anode is at a potential
of about −0.1 V vs. SCE and the cathode is at a potential of about
0.6 V vs. SCE. Under these PEMFC conditions, any corrosion that
takes place is not the same as the corrosion at OCP. In order to study
the corrosion behavior of metallic bipolar plates in actual PEMFC
working conditions, potentiostatic tests were conducted at −0.1 V
vs. SCE purged with H2 to simulate the anode working conditions
and at 0.6 V vs. SCE purged with O2 to simulate the cathode working
conditions.
Polypyrrole-coated SS316L (condition 5) was chosen for the
potentiostatic tests because it had one of the higher corrosion resistances and the coating time required was relatively short (30 min).
The total test time for the potentiostatic tests was 10 h for the simulated anode and cathode conditions. However, the software used
could only store 16,800 points (about 4.67 h) at 1 point s−1 speed.
Therefore, Fig. 2 shows only part of the total potentiostatic test
curve.
Fig. 2 shows that the corrosion current density is negative in
the simulated anode side. However, it is positive in the simulated
cathode side. This is because OCP of polypyrrole-coated SS316L is
around 0.2 V vs. SCE. The simulated anode and cathode are −0.1 V vs.
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Y. Wang, D.O. Northwood / Journal of Power Sources 185 (2008) 226–232
Table 4
Polarization resistance of polypyrrole-coated SS316L at 70 ◦ C
Results
No.
1
2
3
4
5
6
7
8
9
ˇa (V)
Test 1
Test 2
Test 3
0.128
0.163
0.185
0.179
0.163
0.189
0.204
0.141
0.113
0.173
0.259
0.192
0.067
0.192
0.123
0.247
0.265
0.467
0.322
0.137
0.178
0.159
0.208
0.191
0.152
0.163
0.185
ˇc (V)
Test 1
Test 2
Test 3
0.107
0.126
0.118
0.050
0.058
0.049
0.025
0.051
0.031
0.051
0.050
0.054
0.027
0.025
0.027
0.021
0.027
0.021
0.056
0.063
0.054
0.018
0.017
0.018
0.023
0.026
0.029
Ecorr (V)
Test 1
Test 2
Test 3
0.129
0.128
0.130
0.137
0.129
0.127
0.145
0.147
0.142
0.142
0.140
0.141
0.146
0.145
0.147
0.154
0.160
0.168
0.158
0.155
0.156
0.171
0.179
0.188
0.233
0.215
0.222
2.08
2.53
2.67
2.72
2.50
2.13
2.66
2.24
2.29
2.38
2.85
2.69
icorr (A cm−2 )
Test 1
Test 2
Test 3
17.91
16.93
20.64
Rp ( cm2 )
Test 1
Test 2
Test 3
13.90
12.06
12.83
1415
1825
1518
1222
1542
1339
2.78
2.99
2.41
3483
5446
4389
10.85
12.53
11.35
1578
1454
1614
4023
3801
3605
SCE and 0.6 V vs. SCE, respectively. Therefore, the potential of the
simulated anode is anodic to the potential of polypyrrole-coated
SS316L and the potential of the simulated cathode is cathodic to
the potential of polypyrrole-coated SS316L. This negative current
density can provide cathodic protection for the metallic bipolar
plate material. The negative current arises because of the following
reactions [8]:
2H+ + 2e− → H2
+
metalelectrode
2H2 O → 4H + O2 + 4e
−
(2)
platinumelectrode
(3)
3.4. SEM and EDX tests of polypyrrole-coated SS316L after
potentiostatic tests
Fig. 3 presents SEM micrographs and associated EDX spectra of
the coatings after potentiostatic testing. The polypyrrole coatings
were still intact and covered the whole surface after 10-h potentiostatic tests in both the simulated anode and cathode environments.
In the EDX spectra, the elemental peaks for C, N, O and S are from
the polypyrrole, whereas the Fe, Cr, Ni and Si peaks arise from the
SS316L base material since the polypyrrole coating is quite thin.
3.5. ICP-OES tests for coating subjected to potentiostatic testing
In a ‘real’ PEM fuel cell, metal ions generated from corrosion
can migrate to the membrane and levels as low as 5–10 ppm can
degrade the membrane performance [19,20]. Therefore, metal ion
concentration in solution is a very important parameter for the
metallic bipolar plate performance. Metal ion concentrations were
3094
4261
4102
11.88
14.72
10.05
1746
1275
1792
2643
3050
3123
3649
3421
4052
determined to find the ion concentration left in the solution after
the 10 h potentiostatic tests in the simulated anode and cathode
environments.
Table 5 summarizes the metal ion concentrations in the 10 h
potentiostatic tests in the simulated anode and cathode conditions for a PEM fuel cell. Comparing the data in Table 5, we find
that metal ion concentration at the cathode is much higher than
that at the anode for all the elements analysed for both uncoated
and polypyrrole-coated samples. Also, comparing the results of
uncoated and polypyrrole-coated SS316L, we find that the metal
ion concentration in solution for polypyrrole-coated SS316L is only
about half that for the uncoated samples. For example, the Fe ion
concentrations for uncoated SS316L are 771 g L−1 or 1246 g L−1
in the simulated anode and cathode environments, respectively.
The Fe ion concentrations for polypyrrole-coated SS316L are
434 g L−1 or 826 g L−1 in the simulated anode and cathode environments, respectively. Therefore, based on our research results,
metal corrosion is more severe in the cathode environment, which
is consistent with the potentiostatic test results. The metal ion concentrations measured in the simulated anode conditions seems at
odds with the potentiostatic tests because the negative current
should provide cathodic protection for SS316L. The reason why
we still get a relatively high metal ion concentration at the anode
is that the cathodic protection is only partial and it cannot provide full protection for the SS316L. The applied potential is not
negative enough to provide full protection for SS316L. Therefore,
SS316L can be corroded in both the anode and cathode environments, and corrosion in the cathode environment is the hot
spot.
Table 5
Metal ion concentrations after potentiostatic tests
Environment
Dissolved metal concentration (g L−1 )
Fe
Base solution
Uncoated SS316L at anode side
Uncoated SS316L at cathode side
Coated SS316L at anode side
Coated SS316L at cathode side
Note: IDL is the identification limit.
21
771
1246
314
656
Total concentration of metal ions (g L−1 )
Cr
Ni
Mn
<IDL
129
230
59
60
<IDL
81
180
50
91
1
20
36
11
19
22
1001
1692
434
826
Y. Wang, D.O. Northwood / Journal of Power Sources 185 (2008) 226–232
Fig. 3. SEM and EDX after 10-h potentiostatic tests in the simulated anode and cathode environments of PEM fuel cells (a) anode (b) cathode.
231
232
Y. Wang, D.O. Northwood / Journal of Power Sources 185 (2008) 226–232
4. Conclusions
Fig. 4. Interfacial contact resistances for SS316L and polypyrrole-coated SS316L.
It is generally considered that PEM fuel cells should have operating lifetimes over 5000 h for transportation applications [21]. If the
corrosion rate remains the same for the 5000 h lifetime of the fuel
cell, Fe, Cr, Ni, Mn ion concentrations will reach 375 ppm, 65 ppm,
40 ppm, 9 ppm at the anode, and 613 ppm, 115 ppm, 90 ppm, 18 ppm
at the cathode after 5000 h for uncoated SS316L. Fe, Cr, Ni, Mn metal
ion concentrations will reach 147 ppm, 29 ppm, 25 ppm, 5 ppm at
the anode, and 318 ppm, 30 ppm, 46 ppm, 9 ppm at the cathode
after 5000 h for polypyrrole-coated SS316L. Let us suppose that 5%
percent of metal ions remain in solution, and then the total metal
concentrations are 25 ppm and 42 ppm at the anode and cathode
for uncoated SS316L, respectively, and 10 ppm at the anode and
20 ppm at the cathode, respectively, after 5000 h for polypyrrolecoated SS316L. Although such levels of metal ion concentration are
still too high for satisfactory PEM fuel cell performance for the
uncoated SS316L, they are approaching satisfactory levels for the
polypyrrole-coated SS316L.
We have earlier shown that the metal ion concentration after
potentiostatic testing can be significantly decreased through the
use of an Au-interlayer between the SS316L base material and the
polypyrrole coating [22].
3.6. Contact resistance tests
Fig. 4 shows the variation of contact resistance of SS316L and
polypyrrole-coated SS316L with compaction pressure. The polypyrrole coating (condition 5) was chosen for the contact resistance
tests in order to be consistent with the corrosion tests. Also, we
have found that the contact resistance does not change significantly when we change the coating thickness. With increasing
compaction pressure from 20 N cm−2 to 220 N cm−2 , the contact
resistance decreased rapidly at low compaction pressures and
then decreased gradually, probably due to a decrease in interfacial resistance [23]. Comparing the two curves, we can see that the
contact resistance of polypyrrole-coated SS316L is lower than that
of SS316L from 20 N cm−2 to 130 N cm−2 , and then contact resistance of polypyrrole-coated SS316L become slightly higher than
that of SS316L from 130 N cm−2 to 220 N cm−2 . The basic finding is
that the contact resistance of SS316L is only changed slightly after
coating with polypyrrole. This is probably because of the good electrical conductivity of the polypyrrole coating. We can know that
the polypyrrole coating has good electrical conductivity from the
SEM pictures. When we did the SEM test for the polypyrrole-coated
SS316L, we do not need coat a thin layer gold on the sample surface
to increase its electrical conductivity.
A Taguchi DOE method was used to optimize the polypyrrolecoating parameters for SS316L for metallic bipolar plate application.
The potentiodynamic and SEM test results have shown that the
relatively thick polypyrrole coatings can provide good corrosion
resistance, with corrosion rates being decreased about 10 times
relative to uncoated samples. In the simulated anode conditions
of a PEM fuel cell, the corrosion current for polypyrrole-coated
SS316L is negative, which can provide partial protection for the
metal. In the simulated cathode conditions of a PEM fuel cell, the
corrosion current is positive. Therefore, the corrosion was more
severe at the cathode. From the ICP-OES test results, we found that
the total metal ion concentrations were 25 ppm and 42 ppm at the
anode and cathode, respectively, for uncoated SS316L, and 10 ppm
and 20 ppm at the anode and cathode, respectively, for polypyrrolecoated SS316L. This is approaching the target of 10 ppm (total metal
ion concentration) for the 5000 h lifetime of a PEM fuel cell. Also
the contact resistance of SS316L with carbon paper does not change
significantly after coating with polypyrrole.
Acknowledgements
The research was financially supported by the Natural Sciences
and Engineering Research Council of Canada (NSERC) through a Discovery Grant awarded to Prof. D.O. Northwood. Yan Wang would
like to acknowledge financial support through an Ontario Graduate
Scholarship.
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