Investigation on the corrosion resistance of carbon black–graphite-poly(vinylidene fluoride)...

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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 2 4 7 4e1 2 4 8 5

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Investigation on the corrosion resistance of carbonblackegraphite-poly(vinylidene fluoride) composite bipolarplates for polymer electrolyte membrane fuel cells

Renato Altobelli Antunes a,*, Mara Cristina Lopes de Oliveira b, Gerhard Ett b

aEngineering, Modeling and Applied Social Sciences Center (CECS), Federal University of ABC (UFABC), 09210-170 Santo Andre, SP, BrazilbElectrocell Ind. Com. Equip. Elet. LTDA, Technology, Entrepreneurship and Innovation Center (CIETEC), 05508-000 Sao Paulo, SP, Brazil

a r t i c l e i n f o

Article history:

Received 4 May 2011

Received in revised form

22 June 2011

Accepted 26 June 2011

Available online 23 July 2011

Keywords:

Composite bipolar plates

PEM fuel cells

Carbon black

Poly(vinylidene fluoride)

Corrosion

* Corresponding author. Tel./fax: þ55 11 4996E-mail address: renato.antunes@ufabc.ed

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.06.131

a b s t r a c t

The goal of the present work was to evaluate the corrosion resistance of carbon black (CB)-

synthetic graphite (SG)-poly(vinylidene fluoride) (PVDF) composites using electrochemical

impedance spectroscopy (EIS) and potentiodynamic polarization curves. The tests were

conducted in 0.5 M H2SO4 þ 2 ppm HF solution at 70 �C to simulate the typical environment

of polymer electrolyte membrane fuel cells. The fracture surface of the specimens was

characterized by scanning electron microscopy. The through-plane electrical conductivity

was also determined. The corrosion resistance decreased as the carbon black content

increased up to 5 wt.%. The highest electrical conductivity was achieved for the compo-

sition CB ¼ 5 wt.%, PVDF ¼ 15 wt.%, SG ¼ 80 wt.%. A detailed discussion of the EIS data is

given. This approach is unprecedented in the current literature. EIS has proven to be

a valuable tool to the design of electrically efficient bipolar plates.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction development. This is especially true if one envisages bipolar

The overall performance of polymer electrolyte membrane

(PEM) fuel cells has continuously improved in the last two

decades. Since then, this technology has become a well-

established alternative for green power generation [1e3]. Such

achievement was only possible due to the development of

materials capable of attaining the technical requirements

demanded for specific components of PEM fuel cells. In this

way, membrane, catalysts, gas diffusion layers, gaskets and

bipolar plates benefited from materials science related issues

[4e7]. Consequently, both electrical performance and durability

of PEM fuel cells are steadily growing while manufacturing

costs decrease. Nevertheless, this technology is far from being

fully optimized and there is still much to be done toward its

8241.u.br (R.A. Antunes).2011, Hydrogen Energy P

plates. These components have been extensively investigated

[8e10]. It is well known that they are vital to the electrical,

thermal and mechanical performance of PEM fuel cells [11]. A

number of different materials have been tested regarding this

application. Pure graphite, polymeregraphite composites and

metals are the state-of-the-art candidates [12,13]. Each of them

has intrinsic limitations and advantages. Pure graphite has

excellent electrical conductivity, but it is brittle. Graph-

iteepolymer composites overcome the brittleness of pure

graphite, but electrical conductivity is sacrificed. Metals, in

turn, are highly conductive andmechanical resistantmaterials.

However, they lack chemical stability and are prone to corro-

sion in the typical acid and humid environment of PEM fuel

cells. The major challenge of the bipolar plate designer is to

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 2 4 7 4e1 2 4 8 5 12475

select the materials andmanufacturing methods that allow for

the optimum balance of properties, according to the technical

requirements established by the United States of America

Department of Energy (DOE). Some of these targets are shown

in Table 1.

Corrosion resistance is a key property of bipolar plates [14].

Problems arising from corrosion processes have been depicted

bymany authors [15e17]. Metallic materials concentrate most

part of research on the corrosion behavior of these compo-

nents. Composite bipolar plates are considered corrosion

resistant [18] and little attention is paid to the electrochemical

behavior of these materials in the literature. Fu et al. [19]

developed a low contact resistance bipolar plate composed

of a flexible graphite sheet and a stainless steel core. They

evaluated the influence of compacting pressure, temperature,

effective area of the coating and the water content of the

flexible graphite on the electrical resistance of the composite

plate. However, corrosion resistance measurements were not

conducted. Kakati and Deka [20] investigated the effect of the

incorporation of carbon black particles on the electrical

performance and mechanical strength of phenolic resin-

natural graphite composite bipolar plates. Despite the

soundness of the characterization methods, information

about the corrosion behavior of the composites is lacking.

Dweiri and Sahari [21] produced conductive poly-

propyleneegraphiteecarbon black composite bipolar plates

but the corrosion resistance was not assessed. Lee et al. [22]

manufactured three-phase composite bipolar plates by mix-

ing graphite, epoxy resin and woven carbon fabric. The elec-

trical performance of the composites was correlated with

molding parameters such as compaction pressure and

temperature. The corrosion properties of the composites were

not determined. Du et al. [23] showed the suitability of using

compressed expanded graphiteeepoxy resin composites as

bipolar plates for PEM fuel cells. This conclusion was given

based mainly on the electrical and mechanical properties of

the composites. Nevertheless, corrosion issues were not

addressed. The same absence of corrosion studies is found on

several investigations for a variety of composite systems

[24e28]. The reports by Yen et al. [29] and Xiao et al. [30] are

exceptions to this trend. Yen et al. [29] assessed the corrosion

resistance of vinyl esteregraphiteeorganoclay composite

bipolar plates. The corrosion current density of compression

molded plates was less than 10�7 A cm�2. However, no details

were given about the experimental set-up. Thus, it is difficult

for the reader to draw any conclusion about the corrosion

mechanisms that govern the electrochemical behavior of the

Table 1 e DOE technical targets for composite bipolarplates.

Property Value

Weight <0.4 kg kW�1

Flexural strength >25 MPa

Flexibility 3e5% Deflection at mid-span

Electrical conductivity >100 S cm�1

Thermal conductivity >10 W (m K)�1

Gas permeability <2.10�6 cm3 cm�2 s�1 at 80 �C and 3 atm

Corrosion resistance <1 mA cm�2

composites. Conversely, Xiao et al. [30] completely described

their corrosion experiments conducted on poly(arylene

disulfide)-graphite nanosheet composite bipolar plates. The

corrosion current densities were lower than 1 A cm�2 for all

the tested compositions. The incorporation of expanded

graphite (EG) into the composite led to an increase of the

corrosion current density. This behavior was due to the

rougher surface of EG-containing composites as observed

through scanning electron microscopy. Chen et al. [31] have

also characterized the corrosion behavior of composite bipolar

plates but under operating conditions of direct methanol fuel

cells. They underlined the lack of information on the corro-

sion behavior of composite bipolar plates in the literature.

Another example is the excellent report by Kakati et al. [32].

These authors investigated the corrosion behavior of

compression molded phenolic resinegraphiteecarbon black-

ecarbon fiber composite bipolar plates in a simulated PEM fuel

cell environment. The lowest corrosion current density was

0.99 mA cm�2. Furthermore, the addition of carbon black and

carbon fiber was deleterious to the corrosion resistance of the

composite. This behavior was a consequence of the higher

chemical instability of the minor conductive fillers in

comparison with that of the natural graphite particles.

Recently, Wang et al. [33] showed that carbon blackswith high

surface area, small particle size and low density are more

prone to corrosion in a PEM fuel cell environment. Other

authors evidenced that carbon black particles used as

supports for Pt catalysts of PEM fuel cells may oxidize to CO2.

This, in turn, leads to the loss of Pt particles [34,35]. Such

amechanismhas a negative influence on the durability of PEM

fuel cells [36] and does also depend on the crystallinity and

hydrophobicity of the carbon particles [37e39]. However,

despite the possible problems arising from the degradation of

carbon-based materials in PEM fuel cells, carbon black and

carbon fibers are often employed as minor fillers in graph-

iteepolymer composite bipolar plates. The main goal of add-

ing these fillers is to enhance both electrical conductivity and

mechanical resistance of bipolar plates [40,41]. The reduced

particle size of carbon black particles allows for the filling of

voids between graphite particles, increasing the number of

conductive paths and the bulk electrical conductivity of the

composite [42]. The effect of carbon fibers incorporation is

strongly dependent on the anisotropy of individual fibers

within the polymer matrix. Random orientation of short

length fibers is deleterious to the electrical behavior of poly-

mer based composites while long fibers oriented preferen-

tially in a direction parallel to the current flow decrease their

resistivity [43].

Thus, carbon degradation in PEM fuel cells is well-

established in the literature [44,45]. Nevertheless, most

reports focus on the corrosion of carbon supports for catalysts

[46,47]. Carbon materials degradation in bipolar plates has

been hardly studied. Composite bipolar plates are of crucial

technological importance to the successful design of PEM fuel

cells. In this regard, it is surprising that the corrosion behavior

of these components has been systematically unexplored in

the literature. The aim of this work was to study the electro-

chemical behavior of compression molded PVDFegraphi-

teecarbon black composite bipolar plates in a simulated PEM

fuel cell environment. Corrosion mechanisms are discussed

based on electrochemical impedance spectroscopy tests and

potentiodynamic polarization curves. SEM analysis of fracture

surfaces was used to assess the microstructure of the

composites and to correlate the observed morphological

aspects with the corresponding electrochemical behavior.

Fig. 1 e Schematic cross-sectional view of the device used

for the electrical conductivity measurement.

2. Experimental

2.1. Materials

A commercial PVDF resin in powder form was supplied by

Arkema. Synthetic graphite (fuel cell grade) particles were

supplied by Asbury Carbons (typical particle size 40 mm) and

carbon black was supplied by Cabot Corporation (Vulcan XC

2.2. Composite preparation

The components used for molding the composite bipolar

plates were in the powder form. Five different compositions

were prepared as shown in Table 2. Initially, the powderswere

mixed in a Turbula T10B powder blender for 30 min. Then, the

mixture was compression molded in a hydraulic press at

180 �C and a pressure of 400 kg cm�2 for 10 min, forming discs

with 30 mm diameter and 3 mm thickness. After cooling, the

compressed disc was removed from the mold and

characterized.

2.3. Through-plane electrical conductivity

The through-plane electrical conductivity of the composites

was evaluated according to the methodology described by Du

and Jana [48]. Fig. 1 depicts the experimental method

employed for the measurements. The specimens were placed

between gas diffusion layers. Two copper plates were

employed to connect a power supply and the measurement

devices. A current of 10 Awas applied by the power supply and

the resulting voltage drop across the specimen was measured

using a digital multimeter. Initially, the compaction pressure

was increased to 10 kg cm�2 and the voltage drop across the

specimenwas left to stabilize. Next, the pressurewas raised to

20 kg cm�2 and the voltage drop across the specimen was left

to stabilize again. Then, the pressure was released back to

10 kg cm�2. After stabilization, this value was taken as the

voltage drop across the specimen and used for determining its

electrical conductivity. Six specimens of each composition

Table 2 e Compositions used in the preparation of thecomposite bipolar plates.

Specimen Mass (%)

PVDF Synthetic graphite Carbon black

S0 15 85 0

S2 15 83 2

S3 15 82 3

S4 15 81 4

S5 15 80 5

were used for the measurements. The electrical conductivity

reported within this work corresponds to the average value of

the six specimens.

2.4. Corrosion resistance

Corrosion resistance was evaluated in a simulated PEM fuel

cell environment by potentiodynamic measurements and

electrochemical impedance spectroscopy (EIS). A conven-

tional three-electrode cell was used, with a platinum wire as

counter electrode and a saturated calomel electrode (SCE) as

the reference. The electrolyte was comprised of 0.5 M

H2SO4 þ 2 ppm HF aqueous solution maintained at 70 �C. Theopen circuit potential (OCP) was measured for 1 h. This

duration insures free potential steady state required for EIS

and potentiodynamic measurements. Impedance data were

collected under OCP over a frequency range from 100 kHz to

10 mHz at 10 points per decade; a sine wave with �10 mV

amplitude was applied. The evolution of the electrochemical

behavior was accompanied during 8 days of immersion. The

experimental data were fitted using equivalent circuits (ECs)

to give a more quantitative analysis of the EIS response.

Potentiodynamic polarization curves were performed after 8

days of immersion at a scan rate of 1.0 mV s�1. All the

measurements were performed with an Autolab PGSTAT

100 potentiostat/galvanostat equipped with an FRA

(Frequency Response Analyser) module.

2.5. Scanning electron microscopy (SEM) analyses

The morphology of the fracture surfaces of the composites

was observed using a scanning electronmicroscope Leica/LEO

440i. The specimens were fractured after immersion in liquid

nitrogen.

3. Results and discussion

3.1. Through-plane electrical conductivity

According to Derieth et al. [49], as the electrons need to pass

through the bipolar plate, the through-plane conductivity of

a bipolar plate is more important than the in-plane conduc-

tivity. Thus, only the through-plane conductivity was

measured in this work. The effect of carbon black addition on

the through-plane electrical conductivity of the composite

bipolar plates is shown in Fig. 2. Despite the commercial

availability of PVDF-based composite bipolar plates [50] there

are few investigations on the electrical properties of these

materials in the literature [51e53]. A data sheet of the

commercial product supplied by Eisenhuth [50] reports

a through-plane electrical conductivity of 20 S cm�1. Cun-

ningham and Baird [51] achieved a value of 32 S cm�1 for the

same property. Only the in-plane conductivity can be found in

references [52,53]. As shown in Fig. 2 the through-plane

conductivity of the specimens developed in this work

reached 59 S cm�1 for a carbon black content of 5.0 wt.% that is

significantly higher than those found in the literature for the

same polymer matrix. The electrical conductivity depends on

a complex interaction between conductive fillers, polymer

matrix and processingmethods [13]. Hence, it is not surprising

that different authors report discrepant values for the same

property. Notwithstanding, the good electrical conductivity

found in this work has to be emphasized. It indicates that the

selection of the materials used for the manufacturing of the

composite bipolar plates was accurate as well as the prepa-

ration method.

There was a sharp increase of the electrical conductivity

above a CB concentration of 3.0 wt.%. The beneficial effect of

incorporating carbon black on graphiteepolymer composite

bipolar plates is reported by several authors [54e56]. The total

filler loading required to reach high conductivity can be

significantly reduced when carbon black is added to the

composite formulation. The small carbon black particles

make conducting tunnels between the larger graphite parti-

cles, increasing the overall conductivity of the composite. This

behavior depends on the concentration of the CB particles.

The conductivity decreases above a critical content of CB

particles as their wetting by the resin becomes poor, impairing

the compaction of the composite plate. This phenomenonwas

Fig. 2 e Through-plane electrical conductivity of the

composite bipolar plates as a function of the carbon black

content.

confirmed in the present work. We prepared specimens with

a CB concentration of 10 wt.% and determined the corre-

sponding through-plane electrical conductivity according to

themethod described in Section 2.3. The value decreased from

59 S cm�1 for the 5 wt.%-containing CB particle composites to

34 S cm�1. This value is even lower than the conductivity of

the CB-free composite.

3.2. Corrosion resistance

3.2.1. Electrochemical impedance spectroscopy (EIS)Bode (phase angle) and Nyquist plots of the S0 specimen after

different periods of immersion in 0.5 M H2SO4 þ 2 ppm HF

aqueous solution at 70 �C are shown in Fig. 3. Electrochemical

AC techniques such as EIS have a low amplitude perturbation

signal that is an attractive feature to study the corrosion

processes of electrical conductors in aqueous electrolytes [57].

EIS is well established as a powerful tool for investigating the

mechanisms of electrochemical reactions [58]. Bode diagrams

(Fig. 3a) indicate the presence of two time constants inde-

pendently of the immersion time. The first one is character-

ized by a shoulder with a well-defined peak at nearly �70� in

the frequency range between 100 Hz and 10 Hz. This time

constant will be denoted as HF (high frequency) throughout

Fig. 3 e EIS spectra of the S0 specimen after different

periods of immersion in 0.5 M H2SO4 D 2 ppm HF aqueous

solution at 70 �C: (a) Bode and (b) Nyquist plots.

Fig. 4 e EIS spectra of the CB-free and CB-added specimens

after different 1 day of immersion in 0.5 M H2SO4 D 2 ppm

HF aqueous solution at 70 �C: (a) Bode and (b) Nyquist plots.

the text. The second one arises at lower frequencies and will

be denoted as LF (low frequency) throughout the text. It is seen

that the HF time constant did not vary significantly with the

elapsed time with respect to both peak height (phase angle)

and position (frequency). Variations in the HF time constant

have been attributed to surface processes of conductive elec-

trodes such as the thickening of passive films in metallic

alloys [59], the deposition of degradation by-products [60],

alterations of the dielectric properties of conductive coatings

[61,62] and the barrier properties of protective coatings [63].

For conductive-insulator composites the HF time constant

may be considered to arise from the impedance response of

the polymer matrix [60]. In this regard, if no degradation

products are formed during the immersion of the composite,

it is expected that the HF would be unchanged as the polymer

matrix is roughly inert in the electrolyte. This trend is

confirmed for the graphiteePVDF composite as shown in

Fig. 3a. Conversely, the LF time constant is less stable as

indicated by the phase angle decrease observed for longer

immersion times. This time constant would be caused by the

response of graphite to charge transfer reactions undergone

by the composite in the acidic electrolyte. The decrease of the

phase angles with the elapsed time at low frequencies indi-

cates that the impedance response becomes less capacitive

[64]. It is likely that the electrolyte penetrates through the

pores of the composite, thus accentuating the charge transfer

reactions. As shown in Fig. 3b, Nyquist plots evidence that the

impedance of the graphiteePVDF composite specimens

sharply decreased with time in the low frequency domain.

This is associated with a decrease of the corrosion resistance

[65]. Despite the chemical inertness of the PVDF matrix, the

results suggest a marked interaction between the conductive

graphite phase and the electrolyte. It is noteworthy that such

interaction was only monitored during 8 days of immersion

which can be considered as a very short operating period for

a bipolar plate. Actual components are designed to operate

duringmuch longer times [66]. Therefore, it is obvious that the

corrosion behavior of graphiteepolymer composite bipolar

plates should not be neglected during the development of

specific compositions.

The beneficial effect of carbon black on the electrical

conductivity of the reference graphiteePVDF composite has

been outlined in the previous section. It is also important to

investigate this effect on the electrochemical behavior of the

composites. Thus, EIS plots of CB-added specimens were

obtained and the results were compared with the reference

graphiteePVDF composite. Fig. 4 presents the EIS plots of CB-

free and CB-added specimens after 1 day of immersion in

0.5 M H2SO4 þ 2 ppm HF aqueous solution at 70 �C. The plots

obtained after 5 and 8 days of immersion are shown in Figs. 5

and 6, respectively. It is clear that the electrochemical

behavior of the reference composite was significantly altered

by the carbon black particles. Bode plots of all CB-added

composites are characterized by two time constants as well

as for the reference material. However, the HF time constant

appears at progressively lower phase angles (peak heights) as

the CB content increases. Moreover, the peak corresponding

to the HF time constant if shifted to lower frequencies with

increasing CB contents. This trend is observed for 1, 5 and 8

days of immersion. The phase angle drop can be due to the

development of higher porosity because of the incorporation

of carbon black particles within the reference composite. This

behavior is typical of porous electrodes as described by the De

Levie theory [67]. The peak shifting to lower frequencies is

related to an increase of the HF time constant of the system.

The time constant is the product of a capacitor and a resistor

in parallel. Thus, this deviation can be caused by either

capacitance or resistance changes. Both parameters are

influenced by the area exposed to the electrolyte [68]. In this

context, as porosity is expected to increase with CB additions,

new surfaces are exposed to the electrolyte. This can lead to

an unbalanced change of capacitance and resistance, thus

shifting the time constant to lower frequencies. Furthermore,

as shown in Fig. 2, the electrical conductivity of the composite

increases with the incorporation of the conductive CB parti-

cles reaching a maximum at 5 wt.%. Then, the electrical

resistance decreases accordingly and the capacitance

increases, shifting the time constant to lower frequencies. The

formation of new conducting paths with CB addition has

a profound effect on the electrochemical response of the

composites. This is also observed at the low frequency domain

of the Bode plots, where the phase angles diminish with the

incorporation of the conductive CB particles. The high

after different 5 days of immersion in 0.5 M H2SO4 D 2 ppm

after different 8 days of immersion in 0.5 M H2SO4 D 2 ppm

conductivity of the CB particles is also evidenced in the

Nyquist plots. As denoted in Figs. 4b, 5b and 6b the impedance

of the CB-added composites was sharply depressed as the CB

content increased. Similar effects have been reported by

several authors [69e71].

The results of the EIS measurements suggest a direct

relationship between the formation of conductive paths

within the composite structure and its electrochemical

behavior. In order to give amore quantitative interpretation of

the impedance response, the experimental data were fitted

using equivalent electrical circuits (EECs) through a non-linear

least square (NLLS) fitting procedure. The capacitive behavior

was simulated using constant phase elements (CPEs) instead

of pure capacitors accounting for the inhomogeneity of the

material surfaces [72]. The parameters obtained from the

modeling of the EIS data are summarized in Table 3. Q and n

are the magnitude and the exponent of the constant phase

element (CPE), respectively. The CPE impedance (ZCPE) is

described by equation (1) [73]. Q is identified with the capaci-

tance of the CPE [74], ju is the complex variable for sinusoidal

perturbations and n varies between �1 and 1. The value of n is

associated with the non-uniform distribution of current due

to roughness and surface defects [75].

ZCPE ¼ �QðjuÞn��1

The EIS response was dependent on the composition of the

specimen and on the immersion time. In this sense, three

different EECs were employed to simulate the experimental

data. These EECs are shown in Fig. 7. The physical meaning of

each parameter is given as follows for model 1:R1 is the elec-

trolyte resistance; Q1 and R2 model the compositeeelectrolyte

interface where R2 represents the pore resistance to the pene-

tration of the electrolyte and Q1 is the associated capacitance;

Q2 and R3 model the LF time constant of the EIS plots and are

related to the charge transfer reactions taking place within the

pores of the composite due to the penetration of the electrolyte.

This circuit was proposed by Alias and Brown [60] tomodel the

EIS response of carbonfiber-epoxy and carbonfiberevinyl ester

composites. Themodel is valid for the S0 specimenafter 1 and 5

days of immersion. It is seen from Table 3 that R2 is nearly

independent of the immersion time. Conversely, R3 diminishes

from 1 to 5 days. This is consistent with the decrease of the

phase angles and of the impedance values at low frequencies

observed in the Bode and Nyquist plots shown in Fig. 3.

After 8 days of immersion, the circuit shown in Fig. 7b

(model 2) provided the best fitting to the experimental results.

Table 3 e Evolution of the parameters of the equivalent electrical circuits with time.

Time(days)

Specimen Parameters

R1(U cm2)

Q1(10�4 F cm�2 sa�1)

n1 R2(kU cm2)

Q2(10�4 F cm�2 sa�1)

n2 R3(kU cm2)

Q3(10�4 F cm�2 sa�1)

1 S0 2.19 2.60 0.85 1.46 1.40 0.71 175.7 e e

S2 2.64 5.78 0.82 0.58 3.27 0.62 e e

S3 2.19 5.85 0.81 0.54 3.45 0.74 e e e

S4 2.31 5.95 0.82 1.26 4.27 0.66 e e e

S5 2.48 23.9 0.72 0.37 11.09 0.33 e e e

5 S0 1.97 4.44 0.83 1.53 4.08 0.78 148.5 e e

S2 2.04 9.46 0.78 2.38 5.13 0.63 e e e

S3 2.39 10.6 0.76 0.53 10.2 0.52 e e e

S4 2.17 8.75 0.78 0.89 17.7 0.46 e e e

S5 2.20 33.8 0.70 0.75 114 0.72 e e e

8 S0 1.78 4.54 0.84 1.15 17.4 0.66 e e e

S2 1.88 9.92 0.77 1.33 10.2 0.48 e e e

S3 2.39 8.18 0.59 1.76 � 10�4 3.18 0.86 1.80 6.89 0.60

S4 1.84 25.7 0.72 1.11 65.2 0.68 e e e

S5 3.85 36.0 0.70 0.64 148 0.76

Barsan et al. [76] used this model to simulate the EIS response

of a graphiteecellulose acetate conductive composite.

According to the authors, the composite can be regarded as an

assembly of small capacitors due to the conducting graphite

particles randomly dispersed within the insulating polymer

matrix. R1 is the electrolyte resistance; CPE1 represents an

interfacial charge separation between the insulator and the

conducting particles in parallel with a polarization resistance

(R2) that is associated with the corrosion resistance of the

composite. CPE2 represents intrinsic capacitive properties of

the composite electrode. This model was not suitable to

simulate the experimental data obtained for 1 and 5 days of

immersion, giving rise to more significant errors associated

with each parameter. Thus, the electrochemical behavior of

the S0 composite evolved with time, being fitted by model 1

(Fig. 7a) up to 5 days and by model 2 (Fig. 7b) after 8 days. It is

hypothesized that this evolution is a consequence of the

development of porosity with time, thus exposing more

conductive graphite particles to the electrolyte, increasing the

capacitance and drastically diminishing the resistance to

charge transfer reactions. Indeed, the Nyquist plots shown in

Fig. 3b clearly indicate that the impedance dropped sharply

from 1 to 8 days of immersion. Furthermore, as seen in

Fig. 7 e EECs employed to simulate EIS experimental data

Table 3, R2 for 8 days of immersion (associated with the

corrosion resistance of the composite) is significantly lower

than the R3 values (associated with the corrosion resistance of

the composite) obtained for 1 and 5 days of immersion. The

increase of Q2 with time also points to the development of

a higher surface area exposed to the electrolyte which is likely

to be associated with the penetration of corrosive species

through the pores of the composite with time.

The addition of CB particles altered the EIS response of the

composites. The model proposed by Barsan et al. [76] (Fig. 7b)

yielded the best fitting to the experimental data for all the CB-

added specimens. Conversely, while the circuit shown in

Fig. 7a produced the best fitting for the S0 specimen after 1 and

5 days of immersion, it produced a poor fitting quality for the

CB-added composites. It is speculated that this behavior is

strongly determined by the increase of the interfacial charge

separation between the PVDF matrix and the conducting

particles, now represented by both graphite and carbon black.

The volume fraction of conducting particles is increased with

the addition of the CB particles aswell as the intrinsic porosity

of the composite. The concomitant action of these factors

yields two results. Firstly, a reduction of the corrosion resis-

tance (also related to an increase of electrical conductivity)

of the composites: (a) Model 1; (b) Model 2; (c) Model 3.

Table 4 e Electrochemical parameters obtained from thepolarization curves of Fig. 8.

Specimen Ecorr(mV) icorr (mA cm�2)

S0 62 1.10

S2 35 2.86

S3 20 2.34

S4 �68 9.46

S5 �221 26.6

denoted by the low values of R2 (see Table 3). Secondly, an

increase of the area exposed to the electrolyte denoted by the

relatively high values of Q1 in comparison with the S0 spec-

imen. This model applied well to S2, S4 and S5 specimens

throughout thewhole test. It is important to notice that the Q1

and Q2 values increasedwith the incorporation of CB particles

and with time. This suggests that the area exposed to the

electrolyte increased. In the same way, R2 decreased with CB

addition due to the low electrical resistivity of the composites.

The S3 specimen has to be discussed separately. While the

circuit shown in Fig. 7b provided a good fitting to the data

obtained after 1 and 5 days immersion, it failed at simulating

the data for 8 days of immersion. In this case, a new time

constant was introduced in the circuit as shown in Fig. 7c. It is

speculated that this time constant represents the EIS response

of a low-resistance oxidation layer developed on the surface of

the S3 specimenwith time. This time constant refers to the EIS

response at high frequencies and is modeled by the Q1 and R2

parameters of the circuit shown in Fig. 7c. CPE2 corresponds to

the interfacial charge separation between the insulator and

the conducting particles in parallel with a polarization resis-

tance (R3) that is associated with the corrosion resistance of

the composite. CPE3 represents intrinsic capacitive properties

of the composite electrode. The reason for the development of

this new time constant can be due to the loosening of

conductive particles with the elapsed time. EIS proved to be

very sensitive to the electrochemical behavior of the

composites. The results obtained from this technique can be

successfully related to the electrical behavior of the compos-

ites based on both experimental and modeling data.

3.2.2. Potentiodynamic polarizationPotentiodynamic polarization curves of the CB-free and CB-

added specimens after 8 days of immersion in 0.5 M

H2SO4 þ 2 ppm HF aqueous solution at 70 �C are shown in

Fig. 8. The values of the electrochemical parameters obtained

from these curves are shown in Table 4. The corrosion current

Fig. 8 e Potentiodynamic polarization curves of the CB-free

and CB-added specimens after different 8 days of

immersion in 0.5 M H2SO4 D 2 ppm HF aqueous solution at

70 �C.

densities (icorr) were obtained from Tafel plots considering

both the anodic and cathodic branches of the polarization

curves. It is noteworthy that the corrosion potential (Ecorr)

diminishes for increasing CB contents while icorr has an

opposite trend. Therefore, it is clear that the corrosion resis-

tance of the specimens decreases as the composite becomes

more conductive with the addition of the carbon black parti-

cles. This behavior is consistent with the results obtained

from the EIS modeling. If, on one hand, the increase of elec-

trical conductivity is a highly desirable effect of CB incorpo-

ration, on the other hand the corrosion resistance decreases

accordingly. Balancing these effects is a main challenge to the

successful design of composite bipolar plates. The icorr values

shown in Table 4 are higher than the values reported by Kakati

et al. [32] and Xiao et al. [30] for other composite bipolar plates.

However, it is noteworthy that the values reported in this

work were obtained after 8 days of immersion, while the

results from references [30] and [32] are related to very short

periods. The immersion time is critical to the corrosion rate of

the composites. This work gives a first step into the investi-

gation of the long-term corrosion of composite materials for

bipolar plates.

3.3. SEM analyses

SEM images of the fracture surfaces of the composites are

shown in Fig. 9. The aim of observing the cross-sectional

views of the specimens was to assess their internal struc-

ture. The electrochemical behavior of the composites would

be strongly determined by the inner distribution of the PVDF

matrix and the conductive fillers as discussed in the previous

sections. In this regard, the presence of internal voids plays

a major role and the incorporation of carbon black particles

would strongly influence the formation of such imperfections.

The reference PVDFegraphite composite (Fig. 9a) presents

a typical layered microstructure. The fracture surface clearly

shows the presence of pores. The evolution of the electro-

chemical behavior of this specimen was related to such

intrinsic porosity as indicated by the EIS modeling results.

According to Maheshwari et al. [77] carbon black particles

require high amounts of resin for a proper bulk compaction

due to their small size (high surface area). Consequently, the

porosity of graphiteepolymer composites is likely to increase

with the addition of carbon black. This effect was speculated

as one of the reasons for the EIS response of the CB-added

specimens as discussed in Section 3.2.1. As observed in

Fig. 9, several pores were found on the fracture surface of the

CB-added composites. However, it was not possible to identify

if the amount of pores of these specimens was higher than

Fig. 9 e SEM images of the fracture surface of the composites: (a) S0; (b) S2; (c) S3; (d) S4; (e) S5.

that of the CB-free composite. Yet, EIS data point toward this

direction. The most important structural effect of incorpo-

rating the CB particles into the reference PVDFegraphite

composite appears to be the formation of new conducting

paths. The presence of new conductive particles is clearly

seen for the S4 and S5 specimens as shown in Fig. 9d and e.

Both the electrical conductivity and the corrosion resistance

of the composites were markedly affected by these perco-

lating sites.

4. Conclusions

The direct relationship between the corrosion resistance of

PVDFegraphite composite bipolar plates and the addition of

minor conductive fillers was unequivocally shown in this

work. The electrical conductivity was increased with the

addition of carbon black particles up to 5 wt.%. Higher load-

ings yielded a decrease of the electrical performance. The

incorporation of carbon black significantly influenced the

corrosion resistance of the composite. EIS results indicated

that porosity is a key factor to the evolution of the electro-

chemical behavior of the composites. The EIS response was

successfully modeled with equivalent electrical circuits,

showing that the capacitance increased and the resistance

decreased with the incorporation of carbon black. The

formation of new conducting paths with the addition of the

small carbon black particles explains this variation. Cross-

sectional SEM images of the fractured composites evidenced

the presence of pores and additional percolating sites impar-

ted by the carbon black particles. Furthermore, EIS results

proved to correlate well with electrical conductivity

measurements. In spite of the several investigations on the

electrical behavior of composite bipolar plates for PEM fuel

cells, the corrosion resistance is often disregarded. EIS was

found to be a powerful analytical tool to assess the corrosion

properties of composite bipolar plates and to understand the

mechanisms that govern the electrochemical behavior of

these components. This approach is not found in the current

literature. The combination of EIS and potentiodynamic

polarization curves exposes the importance of evaluating the

corrosion behavior of composite bipolar plates for long

immersion times. Moreover, it was clearly demonstrated that

electrical conductivity and corrosion resistance are inversely

proportional. The successful design of composite bipolar

plates depends on the compromise between these properties.

Acknowledgments

The authors are grateful to CNPq (The Brazilian Research

Council) for the financial support to this work (Project 558134/

2008-4). Mara C. L. de Oliveira is thankful for the post-doctoral

grant (Proc. 150396/2009-0).

r e f e r e n c e s

[1] Gencoglu MT, Ural Z. Design of a PEM fuel cell system forresidential application. Int J Hydrogen Energy 2009;34:5242e8.

[2] Hwang JJ, Chang WR, Weng FB, Su A, Chen CK. Developmentof a small vehicular PEM fuel cell system. Int J HydrogenEnergy 2008;33:3801e7.

[3] Sisworahardjo NS, Yalcinoz T, El-Sharkh MY, Alam MS.Neural network model of 100 W portable PEM fuel cell andexperimental verification. Int J Hydrogen Energy 2010;35:9104e9.

[4] Wang Y, Chen KS, Mishler J, Cho SC, Adroher XC. A review ofpolymer electrolyte membrane fuel cells: technology,applications and needs on fundamental research. ApplEnergy 2011;88:981e1007.

[5] Zhang S, Yuan X, Wang H, Merida W, Zhu H, Shen J, et al. Areview of accelerated stress tests of MEA durability in PEMfuel cells. Int J Hydrogen Energy 2009;34:388e404.

[6] Tan J, Chao YJ, Yang M, Lee W-K, Zee JWV. Chemical andmechanical stability of a silicone gasket material exposed toPEM fuel cell environment. Int J Hydrogen Energy 2011;36:1846e52.

[7] Fu Y, Lin G, Hou M, Wu B, Li H, Hao L, et al. Optimized Cr-nitride film on 316L stainless steel as proton exchangemembrane fuel cell bipolar plate. Int J Hydrogen Energy 2009;34:453e8.

[8] Dhakate SR, Sharma S, Chauhan N, Seth RK, Mathur RB. CNTsnanostructuring effect on the properties of graphite compositebipolar plate. Int J Hydrogen Energy 2010;35:4195e200.

[9] Zhang D, Duan L, Guo L, Tuan W-H. Corrosion behavior ofTiN-coated stainless steel as bipolar plate for protonexchange membrane fuel cell. Int J Hydrogen Energy 2010;35:3721e6.

[10] Hui C, Hong-bo L, Li Y, Jian-xin L, Li Y. Study on thepreparation and properties of novolac epoxy/graphitecomposite bipolar plate for PEMFC. Int J Hydrogen Energy2010;35:3105e9.

[11] Hermann A, Chaudhuri T, Spagnol P. Bipolar plates for PEMfuel cells: a review. Int J Hydrogen Energy 2005;30:1297e302.

[12] Brandon NP, Skinner S, Steele BCH. Recent advances inmaterials for fuel cells. Annu Rev Mater Res 2003;33:183e213.

[13] Antunes RA, De Oliveira MCL, Ett G, Ett V. Carbon materialsin composite bipolar plates for polymer electrolytemembrane fuel cells: a review of the main challenges to

improve electrical performance. J Power Sources 2011;196:2945e61.

[14] Antunes RA, Oliveira MCL, Ett G, Ett V. Corrosion of metalbipolar plates for PEM fuel cells: a review. Int J HydrogenEnergy 2010;35:3632e47.

[15] Cho KH, Lee WG, Lee SB, Jang H. Corrosion resistance ofchromized 316L stainless steel for PEMFC bipolar plates. JPower Sources 2008;178:671e6.

[16] Feng K, Shen Y, Liu D, Chu PK, Cai X. NieCreCo-implanted316L stainless steel as bipolar plate in polymer electrolytemembrane fuel cells. Int J Hydrogen Energy 2010;35:690e700.

[17] Han D-H, Hong W-H, Choi HS, Lee JJ. Inductively coupledplasma nitriding of chromium electroplated AISI 316Lstainless steel for PEMFC bipolar plate. Int J Hydrogen Energy2009;34:2387e95.

[18] Heinzel A, Mahlendorf F, Niemzig O, Kreuz C. Injectionmolded low cost bipolar plates for PEM fuel cells. J PowerSources 2004;131:35e40.

[19] Fu Y, Hou M, Liang D, Yan X, Fu Y, Shao Z, et al. The electricalresistance of flexible graphite as flowfield plate in protonexchange membrane fuel cells. Carbon 2008;46:19e23.

[20] Kakati KB, Deka D. Differences in physic-mechanicalbehaviors of resol(e) and novolac type phenolic resin basedcomposite bipolar plate for proton exchange membrane(PEM) fuel cell. Electrochim Acta 2007;52:7330e6.

[21] Dweiri R, Sahari J. Electrical properties of carbon-basedpolypropylene composites for bipolar plates in polymerelectrolyte membrane fuel cell (PEMFC). J Power Sources2007;171:424e32.

[22] Lee HS, Kim HJ, Kim SG, Ahn SH. Evaluation of graphitecomposite bipolar plate for PEM (proton exchangemembrane) fuel cell: electrical, mechanical and moldingproperties. J Mater Proc Technol 2007;187e188:425e8.

[23] Du C, Ming P, Hou M, Fu J, Shen Q, Liang D, et al. Preparationand properties of thin epoxy/compressed expanded graphitecomposite bipolar plates for proton exchange membranefuel cells. J Power Sources 2010;195:794e800.

[24] Liao S-H, Hung C-H, Ma C-CM, Yen C-Y, Lin Y-F, Weng C-C.Preparationandpropertiesof carbonnanotube-reinforcedvinylester/nanocomposite bipolar plates for polymer electrolytemembrane fuel cells. J Power Sources 2008;176:175e82.

[25] Bin Z, Bingchu M, Chunhui S, Runzhang Y. Study on theelectrical and mechanical properties of polyvinylidenefluoride/titanium silicon carbide composite bipolar plates. JPower Sources 2006;161:997e1001.

[26] Dhakate SR, Mathur RB, Sharma S, Borah M, Dhami TL.Influence of expanded graphite particle size on theproperties of composite bipolar plates for fuel cellapplication. Energy Fuels 2009;23:934e41.

[27] Liao S-H, Hsiao M-C, Yen C-Y, Ma C-CM, Lee S-J, Su A, et al.Novel functionalized carbon nanotubes as cross-linksreinforced vinyl ester/nanocomposite bipolar plates forpolymer electrolyte membrane fuel cells. J Power Sources2010;195:7808e17.

[28] Du C, Ming P, Hou M, Fu J, Fu Y, Luo X, et al. The preparationtechnique optimization of epoxy/compressed expandedgraphite composite bipolar plates for proton exchangemembrane fuel cells. J Power Sources 2010;195:5312e9.

[29] Yen C-Y, Liao S-H, Lin Y-F, Hung C-H, Lin Y-Y, Ma CCM.Preparationandpropertiesofhighperformancenanocompositebipolar plate for fuel cell. J Power Sources 2006;162:309e15.

[30] Xiao M, Lu Y, Wang SJ, Zhao YF, Meng YZ. Poly(arylenedisulfide)/graphite nanosheets composites as bipolar platesfor polymer electrolyte membrane fuel cells. J Power Sources2006;160:165e74.

[31] Chen W, Liu Y, Xin Q. Evaluation of a compression moldedcomposite bipolar plate for direct methanol fuel cell. Int JHydrogen Energy 2010;35:3783e8.

[32] Kakati BK, Sathiyamoorthy D, Verma A. Electrochemical andmechanical behavior of carbon composite bipolar plate forfuel cell. Int J Hydrogen Energy 2010;35:4185e94.

[33] Wang M-X, Liu Q, Sun H-F, Ogbeifun N, Xu F, Stach EA, et al.Investigation of carbon corrosion in polymer electrolyte fuelcells using steam etching. Mater Chem Phys 2010;123:761e6.

[34] Wang J, Yin G, Shao Y, Zhang S, Wang Z, Gao Y. Effect ofcarbon black support corrosion on the durability of Pt/Ccatalyst. J Power Sources 2007;171:331e9.

[35] Hu J, Sui PC, Kumar S, Djilali N. Modelling and simulationsof carbon corrosion during operation of a polymerelectrolyte membrane fuel cell. Electrochim Acta 2009;54:5583e92.

[36] Oh H-S, Kim K, Ko Y-J, Kim H. Effect of chemical oxidation ofCNFs on the electrochemical carbon corrosion in polymerelectrolyte membrane fuel cells. Int J Hydrogen Energy 2010;35:701e8.

[37] Oh H-S, Lim KH, Roh B, Hwang I, Kim H. Corrosion resistanceand sintering effect of carbon supports in polymerelectrolyte membrane fuel cells. Electrochim Acta 2009;54:6515e21.

[38] Lim KH, Oh H-S, Jang S-E, Ko Y-J, Kim H-J, Kim H. Effect ofoperating conditions on carbon corrosion in polymerelectrolyte membrane fuel cells. J Power Sources 2009;193:575e9.

[39] Hung C-C, Lim P-Y, Chen J-R, Shih HC. Corrosion of carbonsupport for PEM fuel cells by electrochemical quartz crystalmicrobalance. J Power Sources 2011;196:140e6.

[40] Lee JH, Jang YK, Hong CE, Kim NH, Li P, Lee HK. Effect ofcarbon fillers on properties of polymer composite bipolarplates of fuel cells. J Power Sources 2009;193:523e9.

[41] Hwang IU, Yu HN, Kim SS, Lee DG, Suh JD, Lee SH, et al.Bipolar plate made of carbon fiber epoxy composite forpolymer electrolyte membrane fuel cells. J Power Sources2008;184:90e4.

[42] Mathur RB, Dhakate SR, Gupta DK, Dhami TL, Aggarwal RK.Effect of different carbon fillers on the properties of graphitecomposite bipolar plate. J Mat Proc Technol 2008;203:184e92.

[43] Kim JW, Kim NH, Kuilla T, Kim TJ, Rhee KY, Lee JH. Synergyeffects of hybrid carbon system on properties of compositebipolar plates for fuel cells. J Power Sources 2010;195:5474e80.

[44] Cai M, Ruthkosky MS, Merzougui B, Swathirajan S,Balogh MP, Oh SH. Investigation of thermal andelectrochemical degradation of fuel cell catalysts. J PowerSources 2006;160:977e86.

[45] Arico AS, Stassi A, Modica E, Ornelas R, Gatto I,Passalacqua E, et al. Performance and degradation of hightemperature polymer electrolyte fuel cell catalysts. J PowerSources 2008;178:525e36.

[46] Ko Y-J, Oh H-S, Kim H. Effect of heat-treatment temperatureon carbon corrosion in polymer electrolyte membrane fuelcells. J Power Sources 2010;195:2623e7.

[47] Li L, Xing Y. Electrochemical durability of carbon nanotubesat 80 �C. J Power Sources 2008;178:75e9.

[48] Du L, Jana SC. Highly conductive epoxy/graphite compositesfor bipolar plates in proton exchange membrane fuel cells. JPower Sources 2007;172:734e41.

[49] Derieth T, Bandlamudi G, Beckhaus P, Kreuz C, Mahlendorf F,Heinzel A. Development of highly filled graphite compoundsas bipolar plate materials for low and high temperature PEMfuel cells. J New Mat Electrochem Syst 2008;11:21e9.

[50] http://www.eisenhuth.de/pdf/SIGRACET_Datenblaetter.pdf(accessed 27.4.11).

[51] Cunningham BD, Baird DG. Development of bipolar plates forfuel cells from graphite filled wet-lay material anda compatible thermoplastic laminate skin layer. J PowerSources 2007;168:418e25.

[52] Del Rıo C, Ojeda MC, Acosta JL, Escudero MJ, Hontanon E,Daza I. New polymer bipolar plates for polymer electrolytemembrane fuel cells: synthesis and characterization. J ApplPolym Sci 2002;83:2817e22.

[53] Chunhui S, Mu P, Runzhang Y. The effect of particle sizegradation of conductive fillers on the conductivity and theflexural strength of composite bipolar plate. Int J HydrogenEnergy 2008;33:1035e9.

[54] Mighri F, Huneault MA, Champagne MF. Electricallyconductive thermoplastic blends for injection andcompression molding of bipolar plates in the fuel cellapplication. Polym Eng Sci 2004;44:1755e65.

[55] Dhakate SR, Sharma S, Borah M, Mathur RB, Dhami TL.Development and characterization of expanded graphite-based nanocomposite as bipolar plate for polymer electrolytemembrane fuel cells (PEMFCs). Energy Fuels 2008;22:3329e34.

[56] Dhakate SR, Mathur RB, Kakati BK, Dhami TL. Properties ofgraphite-composite bipolar plate prepared by compressionmolding technique for PEM fuel cell. Int J Hydrogen Energy2007;32:4537e43.

[57] Juttner K. Electrochemical impedance spectroscopy (EIS) ofcorrosion processes on inhomogeneous surfaces.Electrochim Acta 1990;35:1501e8.

[58] Ates M. Review study of electrochemical impedancespectroscopy and equivalent electrical circuits of conductingpolymers on carbon surfaces. Prog Org Coat 2011;71:1e10.

[59] De Oliveira MCL, Costa I, Antunes RA. Investigation on thecorrosion resistance of PIM 316L stainless steel in PEM fuelcell simulated environment. Mater Sci Forum 2010;660e661:209e14.

[60] Alias MN, Brown R. Corrosion behavior of carbon fibercomposites in the marine environment. Corr Sci 1993;35:395e402.

[61] Lin CH, Duh JG. Electrochemical impedance spectroscopy(EIS) study on corrosion performance of CrAlSiN coatedsteels in 3.5 wt.% NaCl solution. Surf Coat Technol 2009;204:784e7.

[62] Antunes RA, Rodas ACD, Lima NB, Higa OZ, Costa I. Study ofthe corrosion resistance and in vitro biocompatibility of PVDTiCN-coated AISI 316L austenitic stainless steel fororthopedic applications. Surf Coat Technol 2010;205:2074e81.

[63] Palomino LM, Suegama PH, Aoki IV, Montemor MF, DeMelo HG. Electrochemical study of modified cerium-silanobi-layer on Al alloy 2024-T3. Corr Sci 2009;51:1238e50.

[64] Liu C, Bi Q, Leyland A, Matthews A. An electrochemicalimpedance spectroscopy study of the corrosion behaviour ofPVD coated steels in 0.5 N NaCl aqueous solution: Part II. EISinterpretation of corrosion behaviour. Corr Sci 2003;45:1257e73.

[65] Liu C, Bi Q, Ziegele H, Leyland A, Matthews A. Structure andcorrosion properties of PVD Cr-N coatings. J Vac Sci Technol2002;20:772e80.

[66] Wu J, Yuan XZ, Martin JJ, Wang H, Zhang J, Shen J, et al. Areview of PEM fuel cell durability: degradation mechanismsand mitigation strategies. J Power Sources 2008;184:104e19.

[67] De Levie R. On porous electrodes in electrolyte solutions I:capacitance results. Electrochim Acta 1963;8:751e80.

[68] Zeng A, Liu E, Annergren IF, Tan SN, Zhang S, Hing P, et al.EIS capacitance diagnosis of nanoporosity effect on thecorrosion protection of DLC films. Diamond Relat Mater 2002;11:160e8.

[69] Ramanujam BTS, Mahale RY, Radhakrishnan S.Polyethersulfone-expanded graphite nanocomposites:charge transport and impedance characteristics. Comp SciTechnol 2010;70:2111e6.

[70] Thommerel E, Valmalette JC, Musso J, Villain S, Gavarri JR,Spada D. Relations between microstructure, electrical

percolation and corrosion in metal-insulator composites.Mater Sci Eng A 2002;328:67e79.

[71] Wang Y-J, Pan Y, Zhang X-W, Tan K. Impedance spectra ofcarbon black filled high-density polyethylene composites. JAppl Polym Sci 2005;98:1344e50.

[72] Liu C, Bi Q, Leyland A, Matthews A. An electrochemicalimpedance spectroscopy study of the corrosion behaviour ofPVD coated steels in 0.5 N NaCl aqueous solution: Part I.Establishment of equivalent circuits for EIS data modelling.Corr Sci 2003;45:1243e56.

[73] Boukamp BA. A package for impedance/admittance dataanalysis. Solid State Ionics 1986;18-19:136e40.

[74] Abouzari MRS, Berkemeier F, Schmitz G, Wilmer D. On thephysical interpretation of constant phase elements. SolidState Ionics 2009;180:922e7.

[75] Assis SL, Wolynec S, Costa I. Corrosion characterization oftitanium alloys by electrochemical techniques. ElectrochimActa 2006;51:1815e9.

[76] Barsan MM, Pinto EM, Florescu M, Brett CMA. Developmentand characterization of a new conducting carbon compositeelectrode. Anal Chim Acta 2009;635:71e8.

[77] Maheshwari PH, Mathur RB, Dhami TL. Fabrication of highstrength and a low weight composite bipolar plate for fuelcell applications. J Power Sources 2007;173:394e403.

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