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j.energy.2018.08.099

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Energy 162 (2018) 933e943
Contents lists available at ScienceDirect
Energy
journal homepage: www.elsevier.com/locate/energy
Strategy of alternating bias voltage on corrosion resistance and
interfacial conductivity enhancement of TiCx/a-C coatings on metallic
bipolar plates in PEMFCs
Weixin Zhang a, Peiyun Yi a, *, Linfa Peng a, Xinmin Lai b
a
b
State Key Laboratory of Mechanical System and Vibration, Shanghai Jiao Tong University, Shanghai 200240, PR China
Shanghai Key Laboratory of Digital Manufacture for Thin-walled Structures, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 March 2018
Received in revised form
3 August 2018
Accepted 12 August 2018
Available online 13 August 2018
Proton exchange membrane fuel cells (PEMFCs) are deemed to be a promising renewable energy for
variety of applications. However, metallic bipolar plates, one of key components in PEMFCs, still suffer
from severe corrosion and degradation of interfacial conductivity under the humid and acid operating
condition. Herein we proposed a novel strategy to enhance interfacial conductivity and corrosion
resistance of TiCx/a-C coatings for metallic bipolar plates by alternating substrate bias voltage during the
deposition process. The effects of the alternating substrate bias voltage strategy on the composition and
morphology of the multilayered TiCx/a-C coatings had been explored. Both the corrosion resistance and
the interfacial conductivity of the multilayered TiCx/a-C coatings were improved with more alternating
periods of bias voltage. The effects of the enhanced performance had been discussed, and it was found
that the alternating bias voltage strategy will restrain the columnar structures in the a-C layers and
promote the generation of sp2-rich clusters on the surface. This versatile strategy based on moderately
alternating cycles of substrate bias voltage exhibits great potential in many applications.
© 2018 Published by Elsevier Ltd.
Keywords:
Proton exchange membrane fuel cell
Bipolar plate
Amorphous carbon
Bias voltage
Corrosion resistance
Interfacial contact resistance
1. Introduction
As one kind of promising energy generator with low operating
temperature, low emission and high efficiency [1,2], proton exchange membrane fuel cells (PEMFCs) have attracted substantial
academic interest on account of their potential applications in
many fields, which include but are not limited to automobiles
[3e5], drones [6], military equipment [7], and stationary power
stations [8]. However, the commercialization of PEMFCs still suffers
from fairly high cost and limited durability of stack under aggressive operating conditions [9].
Typically, PEMFCs stack is composed of membrane electrode
assemblies, bipolar plates (BPPs), and seals. Metallic BPPs, one of
key multifunctional components in PEMFCs, occupy over 80% of
the weight and 45% of the cost in PEMFCs stack [4,10,11]. The
corrosion resistance and interfacial conductivity performance of
BPPs are crucial for the PEMFCs stack performance [1]. In the past
decade, stainless steel has been regarded as the alternative
* Corresponding author.
E-mail address: yipeiyun@sjtu.edu.cn (P. Yi).
https://doi.org/10.1016/j.energy.2018.08.099
0360-5442/© 2018 Published by Elsevier Ltd.
material for BPPs [3,4,11,12]. Hence, BPPs are required to possess
excellent corrosion resistance and interfacial conductivity under
humid and acid operating condition. Up to now, tremendous efforts have been made for amorphous carbon (a-C) films coated on
BPPs due to their excellent corrosion resistance and admirable
interfacial conductivity with gas diffusion layer [13e15]. Yi et al.
[16] used closed field unbalanced magnetron sputter ion plating
method to deposit a-C films on stainless steel 304 BPPs. The
interfacial contact resistance (ICR) value was 5.4 mU cm2 at
1.5 MPa. However, the columnar structures and pinholes, usually
generated in the a-C coatings during the deposition process, easily
lead to the so-called pitting corrosion [17], which is harmful to the
electrochemical stability of BPPs and comprehensive performance
of PEMFCs stack.
So far, multilayer coatings have been widely adopted to suppress
columnar structures and improve corrosion resistance. Tüken and
coworkers [18] used the cyclic voltammetry technique to obtain the
multilayer coatings by synthesis of thin polyphenol film on the
polypyrrole layer. Compared with single layer coating, the multilayer coatings showed barrier effect and provided a much effective
protection of mild steel for much longer periods in the corrosion
tests. Barshilia et al. [19] used the reactive direct current (DC)
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W. Zhang et al. / Energy 162 (2018) 933e943
magnetron sputtering method to deposit TiN/NbN multilayer
coatings. The corrosion behaviors of the multilayer coating with a
thickness of ~1.5 mm were measured and the results revealed that
the corrosion resistance was increased with number of interfaces in
the coatings. Zhang et al. [20] adopted arc ion plating technique to
deposit the sandwich-like Cr/CrN/Cr multilayers coatings on
stainless steel. Compared with the bare substrate, the corrosion
current density of the multilayer coatings decreased by one order of
magnitude. After potentiostatic polarization under simulated
cathode environment, the ICR values increased from 35 mU cm2 to
80 mU cm2. Even so, multilayer coatings fabricated by different
materials may bring out the so-called local galvanic corrosion due
to the perforated pinholes in the coatings [17,21]. Hence, a new
deposition strategy is a necessity for anti-corrosive and conductive
coatings without columnar structures in consideration of the
operating condition of PEMFCs.
Here, we propose a novel and effective strategy to deposit
compact TiCx/a-C multilayered coatings. To be specific, the
multilayered TiCx/a-C coatings are deposited layer by layer with
alternate substrate bias voltage during DC magnetron sputtering
process. In each alternating cycle, the higher substrate bias
voltage may bring out higher flux of energetic particles in the
plasma and enhance the intensity of the bombardment and thus
suppress the generation of columnar microstructures and pinholes. The superiority of alternating substrate bias voltage strategy is confirmed by the enhanced corrosion resistance and
interfacial conductivity of TiCx/a-C coatings consisting of alternating 150 V a-C layer and 600 V a-C layer. The optimized TiCx/aC coated BPPs are promising for the commercialization application of PEMFCs. In addition, the proposed strategy in this study
opens a new door to anti-corrosive applications and advanced
energy materials.
2. Experimental
2.1. Sample preparation
Commercial stainless steel 316L (SS316L) sheets (Cr:16.21 wt.%,
Ni:9.50 wt.%,
Mo:3.22 wt.%,
Co:0.42 wt.%,
Mn:1.47 wt.%,
Si:0.34 wt.%, Cu:0.21 wt.%, V:0.04 wt.%, Fe: balance) with a diameter
of 60 mm and single crystalline (110) silicon wafers were prepared
as substrates. Prior to sputtering, substrates were ultrasonically
cleaned in ethyl alcohol, acetone and deionized water and then a
plasma blower was used to dry the substrates. After that, substrates
were fixed on the sample carousel in front of the targets with a
distance of 130 mm in the self-designed balanced DC magnetron
sputtering system. The coating system was equipped with two high
purity graphite targets (99.99%) and two high purity titanium targets (99.99%). And the chamber was firstly vacuumed to get a
background pressure below 5.0E-3 Pa. During the sputtering process, argon (99.9%) was introduced as the sputtering gas. As illustrated schematically in Fig. 1a, before the deposition, the plasma
cleaning was carried out to get a cleaner and more active surface.
Then the protective titanium seed layer was deposited, as shown in
Fig. 1b. After that, the TiCx transition layer was deposited immediately by decreasing the sputtering current of titanium targets and
graphite targets at the same time. Afterwards, the alternating
substrate bias voltage strategy was introduced to deposit the
multilayered a-C coatings, as depicted in Fig. 1def. The a-C layers
were deposited layer by layer by alternating the negative substrate
bias voltage of 150 V and 600 V by n periods (n ¼ 0, 5,10,15), and the
total deposition time of a-C multilayered coatings was 2 h. The
detailed parameters of the multilayer TiCx/a-C coating and the a-C
layers were given in Tables 1 and 2.
2.2. Coating characterization
In order to explore the effects of alternating substrate bias
voltage strategy on the morphology, atomic force microscope (AFM,
Dimension Icon, Bruker, USA) and ultra-high resolution scanning
electron microscope (SEM, MAIA3, TESCAN, Czech Republic) were
used to evaluate the surface and cross-sectional morphology of
TiCx/a-C multilayered coatings. X-ray photoelectron spectroscopy
(XPS, AXIS Ultra DLD, SHIMADZU/Kratos, Japan) was introduced to
detect the composition as well as the atom binding state of TiCx/a-C
coatings before and after corrosion.
In order to estimate the corrosion resistance of TiCx/a-C coatings, electrochemical experiments were carried out with the help of
a commercial electrochemical workstation (Corr-Test 310, China)
under the simulated PEMFCs cathode operating condition (pH ¼ 3
H2SO4 solution containing 0.1 ppm HF with air bubbles at 80 C),
including the potentiodynamic polarization (scans from 0.6 V
to þ1.0 V at 0.1 mV/s, vs. Ag/AgCl), potentiostatic polarization at
0.6 V (vs. Ag/AgCl, abbreviated as 0.6 V polarization below) for 24 h
and the high potential 1.6 V potentiostatic polarization (vs. SHE,
abbreviated as 1.6 V polarization below) for 1 h. It should be
mentioned that the high potential 1.6 V polarization tests are significant because the severe high potential electrochemical corrosion is inevitable during the vehicle start-stop condition [22,23].
Electrochemical impendence spectroscopy (EIS) was also employed
to evaluate the corrosion resistance of bare substrate and coated
samples. Concretely, the EIS experiments were carried out at a
potential of 0.6 V (vs. Ag/AgCl) with an amplitude of 10 mV over a
wide range of frequency from 0.01 Hz to 100 kHz in the simulated
PEMFCs cathode environment. Prior to all the electrochemical
corrosion tests, the electrochemical workstation was required
running for 1 h to stabilize at open circuit potential. Additionally,
the electrochemical tests were carried out at least three times.
ICR is another important parameter to evaluate the interfacial
electrical conductivity of BPPs. A commercial micro-ohm meter
(ZY9858, China) was used to measure the ICR value between the
coated samples and gas diffusion layer (commercial TGP-H-060
carbon papers) under a compaction force of 1.4 MPa, which was
considered as the typical assembly pressure in PEMFC stack. And
the detailed testing procedures were reported in our previous work
[24].
3. Results and discussion
3.1. Effects of alternating bias voltage on the morphology
Fig. 2 shows the three-dimensional morphological AFM images
and corresponding root-mean-square (RMS) roughness of TiCx/a-C
coatings with different alternating bias voltage periods. The RMS
roughness is calculated by the NanoScope Analysis 1.40 software.
As shown in Fig. 2, the topography of the TiCx/a-C multilayered
coatings is composed of many hilly pinnacles. As the increase of
alternating bias voltage periods, the number of hillocks decreases.
Without alternating bias voltage (n ¼ 0), the RMS roughness is the
highest, i.e. ~1.28 nm. As the alternating periods further increase to
5 and 10, the RMS roughness is decreased slightly to ~0.89 and
~0.87 nm, respectively. And the sample with the highest alternating
bias voltage periods has the lowest RMS roughness of ~0.71 nm.
The decreased RMS roughness of TiCx/a-C coatings is attributed
to the effect caused by the alternating bias voltage strategy. In this
study, the TiCx/a-C coatings are deposited in a balanced magnetron
sputtering system at room temperature. It is well accepted that the
higher bias voltage will bring out the higher energy state of particles in plasma and undoubtedly, the more intense bombardment
comes into being from energetic particles [25e27]. Hence, in an
W. Zhang et al. / Energy 162 (2018) 933e943
935
Fig. 1. Schematic illustrations of depositing process of TiCx/a-C coatings with alternating substrate bias voltage strategy: (a) plasma cleaning, (b) depositing titanium seed layer, (c)
depositing TiCx transition layer, (d) depositing 150 V a-C layer, (e) depositing 600 V a-C layer, (f) after alternating substrate bias voltage by n periods (n ¼ 0,5,10,15).
Table 1
Detailed deposition parameters of multilayer TiCx/a-C coatings.
Bias Voltage (V)
Sputtering Current (A)
Depositing Time (sec)
Plasma cleaning
Titanium seed layer
TiCx transition layer
700
150
150
1800
240
240
a-C layers
150/600
0.1
9
Ti: 9 / 0
C: 0 / 10
10
7200
Table 2
Detailed deposition parameters of a-C layers with different alternating periods.
No.
Alternate periods
Depositing time of 150 V a-C layer (sec)
Depositing time of 600 V a-C layer (sec)
#
#
#
#
n¼0
n¼5
n ¼ 10
n ¼ 15
7200
1152
576
384
0
288
144
96
1
2
3
4
Fig. 2. AFM micrographs in dimensions of 5 5 mm2 showing the surface morphology and RMS roughness of the TiCx/a-C multilayered coatings via different alternating periods of
bias voltage: (a) n ¼ 0, (b) n ¼ 5, (c) n ¼ 10, (d) n ¼ 15.
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W. Zhang et al. / Energy 162 (2018) 933e943
alternating bias voltage period, the deposition process with high
substrate bias voltage of 600 V appears to be more forceful and
invasive due to the incident energetic ions from plasma. Consequently, the energy of incident ions dissipates locally and causes
the local surface melting process, which will flatten the surface
locally [28]. Besides, in every alternating period, the enhanced intensity of bombardment caused by the bias voltage of 600 V will
minimize the RMS roughness by transferring the adatoms from
crest to valley [29]. Additionally, although the total deposition time
is unchanged, the effect of modulation is more and more obvious
and the surface morphology becomes smoother as the increase of
alternating periods.
Fig. 3 shows the surface and cross-sectional SEM micrographs of
the TiCx/a-C coatings deposited with different alternating bias
voltage periods. As shown in Fig. 3a, the sample without alternating
bias voltage has bigger grain size on the surface. When the alternating periods rise, the surface morphology of TiCx/a-C coatings
changes obviously from blocky to finely granular, as depicted in
Fig. 3bed. In other words, the topographies of TiCx/a-C coatings
become smoother and flatter due to the coordinating effect of
alternating bias voltage, which are consistent with the AFM micrographs in Fig. 2. Therefore, the change of the SEM surface
topography is associated with the effect caused by the alternating
150 V a-C layer and 600 V a-C layer, as illustrated above.
The cross-sectional morphology of TiCx/a-C coatings is shown in
Fig. 3a-d. The white region on the bottom is the titanium seed layer
and TiCx transition layer. Above the white region, it is referred to
the a-C multilayered layers deposited by alternating bias voltage.
Without alternating bias voltage, the columnar microstructures
could be clearly seen in Fig. 3a’. When the alternating periods go up
to 5, the cross-section of TiCx/a-C multilayered coating becomes
more compact and has fewer columnar microstructures. With
further increased alternating periods of bias voltage, the TiCx/a-C
coatings are almost column-free. The above-mentioned phenomenon is attributed to the intermittent bombardment generated
from higher flux of the impinging ions in the plasma with high
substrate bias voltage of 600 V in every alternating cycle. Besides,
the TiCx/a-C coatings are inclined to deposit layer by layer so that it
hardly shows any distinct penetrating channels through the a-C
layer.
As for the thickness of TiCx/a-C coatings, it is apparent that the
thickness decreases monotonously by increasing alternate periods
of bias voltage. More precisely, the total thickness decreases from
~214 nm to ~176 nm, ~157 nm and ~156 nm, respectively. This result
reflects the more prominent compaction effects are caused by more
alternating periods of bias voltage.
3.2. Effects of alternating bias voltage on the composition
The binding energy state of carbon atoms in TiCx/a-C coatings is
analyzed by XPS spectra, as shown in Fig. 4. The narrow scanning C
1s spectra are pretreated by the Shirley background function. After
that, as depicted in Fig. 4aeb, the spectra are deconstructed into
three peaks, i.e. C]C (284.6 eV), CeC (285.7 eV), CeO (287.6 eV), by
a Lorentzian-Gaussian function [30]. And the relative content of sp2
and sp3 before and after 1.6 V polarization tests are calculated and
shown in Fig. 4ced, respectively. As indicated in Fig. 4c, the sp2
relative content of the as deposited samples is increased very
slightly, while the sp3 relative content decreases as the increase of
alternating periods. After the severe corrosion, the sp2 relative
content rises slightly by increasing alternate periods of bias voltage.
The increased sp2 content is attributed to alternate high substrate
bias voltage of 600 V [31,32]. In other words, due to the alternating
bias voltage strategy, there are more sp2-rich clusters on the surface
of the TiCx/a-C coatings. This phenomenon is closely related to the
interfacial conductivity of the TiCx/a-C coatings, which will be
discussed in the following part.
In order to evaluate the oxidation of carbon, the oxygen atomic
contents are measured and shown in Fig. 5. The oxygen atomic
content is also related with the interfacial conductivity of TiCx/a-C
coatings, which will be further discussed in the following part. It is
worth mentioning that before the corrosion tests, the surface oxides detected by XPS mainly come from the oxidation reactions
during sputtering process and atmosphere [33]. Before corrosion
tests, it is clearly that the oxygen atomic concentration decreases
slightly with the increase of alternating periods, meaning a more
stable state against the oxidation. But after the severe electrochemical corrosion, the carbon corrosion should be taken into account. Under the PEMFCs simulated cathode environment, the
carbon corrosion is inevitable, and the reactions are shown below
[34].
C þ 2H2 O/CO2 þ 4H þ þ 4e ; E0 ¼ 0:207Vðvs:SHEÞ
(1)
C þ H2 O/CO þ 2H þ þ 2e ; E0 ¼ 0:518Vðvs:SHEÞ
(2)
After the 1.6 V polarization tests, the oxygen content of all the
samples increases apparently. Similarly, the surface oxygen atomic
content of TiCx/a-C coatings drops back with the increase of
alternating periods, except that the sample with 15 cycles of
alternating bias voltage increases slightly, which indicates the
improvement is restricted because the 600 V a-C layer in each
alternating period is becoming thinner. The result suggests that
with the alternating bias voltage strategy, the a-C coatings on the
surface have better performance against the carbon oxidation
process under the simulated PEMFCs operating condition.
3.3. Effects of alternating bias voltage on the electrochemical
corrosion behaviors
In this study, potentiodynamic polarization, 0.6 V polarization
and 1.6 V polarization are employed to evaluate the electrochemical
corrosion behaviors of TiCx/a-C multilayered coatings in the
simulated PEMFCs operating conditions from different aspects.
Compared with the bare substrate, the TiCx/a-C coatings have
higher corrosion potential and lower corrosion current density in a
wide range of potential from 0.6 V to 1.0 V, as can be seen in
Fig. 6a. Additionally, the corrosion current density at a potential of
0.6 V of all the TiCx/a-C coatings is much lower than 1E-6 A/cm2, i.e.
the US DOE 2020 technical target [35]. Besides, the corrosion current density decreases slightly with more alternating periods,
ranging from 4.56E-7 A/cm2 to 2.97E-7 A/cm2.
The conventional Tafel extrapolation method is also introduced
to calculate the protective efficiency, as presented in eq (3).
Pi ¼
icorr
1 0
icorr
!
100%
(3)
In the above equation, Pi represents the protective efficiency of
the multilayered coatings, icorr and i0corr are the corrosion current
density of coated samples and bare substrate, respectively. The
Tafel extrapolation result and protective efficiency are depicted in
Fig. 6b. Owing to the alternating bias voltage strategy, the value of
icorr decreases obviously and the value of Ecorr rises by ~0.3 V. The Pi
is also increased slightly with the increase of alternating periods. In
other words, the corrosion resistance of multilayered TiCx/a-C
coatings is enhanced in a wide range of potential from 0.6 V
to þ1.0 V due to the alternating bias voltage strategy.
The 1.6 V polarization tests are carried out to evaluate the
corrosion resistance of coatings in an accelerated way and appraise
W. Zhang et al. / Energy 162 (2018) 933e943
937
Fig. 3. SEM surface and cross-sectional images of the TiCx/a-C multilayered coatings deposited with different alternating periods of substrate bias voltage: (a, a’) n ¼ 0, (b, b’) n ¼ 5,
(c, c’) n ¼ 10, (d, d’) n ¼ 15.
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W. Zhang et al. / Energy 162 (2018) 933e943
Fig. 4. Deconvolution of XPS spectra of TiCx/a-C multilayered coatings: (a) as deposited, (b) after potentiostatic polarization (1.6 V vs. SHE, 1 h), (c, d) relative content of sp2 and sp3
before and after potentiostatic polarization (1.6 V vs. SHE, 1 h).
the high potential electrochemical corrosion behaviors of TiCx/a-C
coatings in the PEMFCs operating condition especially in the
startup or shutdown process [22]. As revealed in Fig. 6c, the
corrosion current density of the sample without alternating bias
voltage goes up all the way and the corrosion current density is
~5.37E-5 A/cm2 at last. On account of the alternating bias voltage
strategy, the corrosion current density of other three samples stabilizes in a lower way, i.e. ~2.09E-5 A/cm2, ~1.15E-5 A/cm2 and
~8.09E-6 A/cm2, respectively. Additionally, the corrosion current
density decreases with more alternating periods, meaning better
corrosion resistance against severe high potential electrochemical
corrosion.
In addition, the 0.6 V polarization experiments are carried out
and the polarization curves are displayed in Fig. 6d. All of the coated
samples show stabilized corrosion current densities throughout the
corrosion time and the corrosion current densities are lower than
the DOE 2020 target of 5E-8 A/cm2 [35]. Similarly, the corrosion
current densities are also decreased with the increase of alternating
periods, i.e. ~1.10E-8 A/cm2, ~5.79E-9 A/cm2, ~3.39E-9 A/cm2, and
~2.21E-9 A/cm2, respectively.
In addition, EIS is also introduced to estimate the corrosion
resistance of bare SS316L and coated samples, as depicted in Fig. 7a.
One time constant model, expressed in Fig. 7b, is generally adopted
to describe corrosion behaviors of the bare substrate [36]. While
the two time constant model, as shown in Fig. 7c, are widely
accepted to evaluate the corrosion performance of coated samples
[37]. In the equivalent electrical circuits, Rs, Rct, and Rpore represent
the resistance of solution, the charge transfer resistance and the
electric resistance of the ionic current through the pores in the
coatings, respectively. CPEcoat and CPEdl indicate the double-layer
capacitance of coatings and double-layer capacitance of the interface between substrate and solution.
As shown in Fig. 7a, the responses of bare substrate and TiCx/a-C
coated samples are essentially specific semi-circles with different
radii. Apparently, the impedance cycle of coated samples is larger
by increasing alternate periods of bias voltage. The fitted parameters of the equivalent electric circuits are given in Table 3. It is
observed that the Rpore increases with more alternating periods of
bias voltage, indicating the decrease of porosity in TiCx/a-C coatings, which is consistent with the results in Fig. 6b. Besides, the
W. Zhang et al. / Energy 162 (2018) 933e943
939
Fig. 5. Surface oxygen atomic content of TiCx/a-C coatings before and after potentiostatic polarization (1.6 V vs. SHE) for 1 h.
Fig. 6. Corrosion behaviors of TiCx/a-C multilayered coatings: (a) potentiodynamic polarization, (b) corrosion potential and corrosion current density of SS316L and TiCx/a-C
multilayered coatings. Insert: calculated protective efficiency of TiCx/a-C multilayered coatings. (c) 1.6 V potentiostatic polarization (vs. SHE) for 1 h, (d) 0.6 V potentiostatic polarization (vs. Ag/AgCl) for 24 h.
940
W. Zhang et al. / Energy 162 (2018) 933e943
Fig. 7. EIS results of bare SS316L and TiCx/a-C coated samples: (a) Nyquist plots, (b) equivalent circuit model of bare substrate, (c) equivalent circuit model of TiCx/a-C coated
samples.
Table 3
EIS fitting parameters of SS316L and TiCx/a-C coated samples according to the equivalent circuit models.
Sample
Rs (U cm2)
Rpore (U cm2)
Rct (U cm2)
CPEdl (F cm2)
CPEcoat (F cm2)
SS316L
n¼0
n¼5
n ¼ 10
n ¼ 15
619.8
662.6
665.1
673.7
664.5
97888
98298
118910
145010
68971
656240
757630
854260
1069000
2.39E-05
8.70E-06
8.04E-06
7.62E-06
5.99E-06
1.33E-05
1.23E-05
1.17E-05
1.05E-05
coated samples show relatively higher Rct values than the bare
substrate and the Rct values rise distinctly as the increase of alternating periods, especially the sample with the highest alternating
periods is two orders of magnitude higher than that of bare SS316L.
Therefore, the alternating bias voltage strategy is successful to
enhance the corrosion resistance of TiCx/a-C coatings against the
corrosive mediums at the steel-electrolyte interface.
Confirmed from the potentiodynamic polarization tests, EIS
results, 0.6 V and 1.6 V polarization experiments, the alternating
bias voltage strategy is successful to deposit more compact coatings, which will enhance the corrosion resistance in the simulated
PEMFCs cathode environment. The successfully enhanced corrosion
resistance of TiCx/a-C multilayered coatings is attributed to the
following reasons. On the one hand, due to the higher bias voltage
of 600 V in each alternating period, the kinetic energy of energetic
particles in the plasma is enhanced and the flux of the impinging
ions improves, thus the bombardment from the energetic is
enhanced in every alternate period, as compared with the one-fold
bias voltage strategy. In other words, the intermittent energetic
bombardment tends to restrain the porosity in the a-C layers and
make the multilayered TiCx/a-C coatings more compact, which can
be inferred and supported from the AFM micrographs in Fig. 2 and
the SEM cross-sectional images in Fig. 3. On the other hand, with
more alternating bias voltage periods, there are more interfaces
and thinner thickness of the interlayers in the a-C multilayered
coatings, as shown in Fig. 8. It is said that the diameter of the
columnar structures in the a-C coatings grows with the increased
coating thickness [38]. Hence, the reduced thickness of each
interlayer can restrain the generation of columnar structures. In
addition, as shown in Fig. 8, the increasing interfaces are conducive
to block the possible pinholes or columnar structures, and the
number of penetrating corrosion channels are significantly
decreased [19,39]. Therefore, the TiCx/a-C coated samples deposited by alternating bias voltage have more excellent corrosion
resistance.
3.4. Effects of alternating bias voltage on the interfacial
conductivity
ICR is another important performance of BPPs. As depicted in
Fig. 9, the initial ICR values of different coatings decrease slightly as
the alternating period goes up, ranging from ~4.27 mU cm2 to
W. Zhang et al. / Energy 162 (2018) 933e943
941
Fig. 8. Schematic illustrations of different corrosion behaviors between: (a) single layer (n ¼ 0) and (b) multilayered TiCx/a-C coatings deposited by alternating bias voltage strategy.
Fig. 9. ICR of TiCx/a-C coatings before corrosion, after 0.6 V (vs. Ag/AgCl) potentiostatic polarization for 24 h and 1.6 V (vs. SHE) potentiostatic polarization for 1 h.
~3.58 mU cm2. After the 0.6 V polarization for 24 h, the ICR value of
the a-C single layer coatings rises to ~10.34 mU cm2. However,
owing to the alternating bias voltage strategy, the ICR values of the
multilayered coatings are lower than 10 mU cm2, lower than the
DOE 2020 technical target [35]. In addition, after the severe 1.6 V
polarization, the ICR value of the single layer coating is
14.90 mU cm2. However, by increasing alternate periods of bias
voltage, the ICR values are decreased monotonously, i.e.
11.13 mU cm2, 8.92 mU cm2 and 8.30 mU cm2, respectively. In a
word, the alternating bias voltage strategy is not only successful to
enhance the corrosion resistance but also the interfacial electrical
conductivity.
The enhanced interfacial conductivity can be contributed to the
following factors. On the one hand, the alternating bias voltage
strategy is conducive to improve the sp2 content in the a-C coatings,
as revealed in Fig. 4. As is well accepted, the sp2 content is related to
the electrical conductivity in the a-C films. According to Fig. 4c, the
initial sp2 content goes up slightly with the increase of alternating
periods, and it conforms to the initial ICR results in Fig. 9. And as
shown in Fig. 4d, the sp2 content after corrosion also increases
monotonously by increasing alternate periods of bias voltage,
which is consistent with the ICR results after 1.6 V polarizations
tests.
On the other hand, it is sure that the carbon corrosion is inevitable in the PEMFCs environment, especially when the PEMFC stack
suffers from the start or shut down processes [9]. Hence, the oxygen
atomic content is also closely bound up with the interfacial conductivity and the performance of the PEMFC stack. As declared in
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W. Zhang et al. / Energy 162 (2018) 933e943
Fig. 5, the oxygen atomic content is fortunately decreased as the
increase of alternate periods whether before corrosion or after 1.6 V
polarization experiments. That is to say, the carbon oxidation of
TiCx/a-C coatings is remarkably restrained due to the alternating
bias voltage strategy, which will improve the interfacial conductivity to some extent.
4. Conclusion
The TiCx/a-C coatings are appropriate for the bipolar plate applications against the humid and acidic environment in PEMFCs. In
this study, a novel deposition strategy is proposed to enhance the
corrosion resistance and interfacial conductivity of multilayered
TiCx/a-C coatings, i.e. depositing the TiCx/a-C coatings by alternating the substrate bias voltage during DC magnetron sputtering
process. Confirmed by various corrosion tests, it was proved that
the multilayered TiCx/a-C coatings deposited by alternating bias
voltage strategy had better corrosion resistance and the improvement was more remarkable as the alternating periods of bias
voltage increased. In addition, the interfacial contact resistance
before and after corrosion was also measured and the results
revealed that the alternate bias voltage strategy also contributed to
the interfacial conductivity significantly, and the improvement
went up with the increasing alternate periods of bias voltage. Besides, the effects of the improvements were also discussed and
supported by AFM micrographs, SEM images and XPS results. The
sample deposited with 15 alternating periods has the lowest
corrosion current density of 2.97E-7 A/cm2 at a potential of 0.6 V
and the lowest ICR of 3.58 mU cm2, which show great potential for
the commercial application of bipolar plates. Our works can be
further improved by optimizing the magnetic field of the sputtering
system, and using pulsed direct current sputtering power sources.
In addition, our approach in this study can readily be extended to
more multilayered coatings for use in anti-corrosive applications
and advanced energy materials.
Notes
The authors declare no competing financial interest.
Acknowledgments
This study was supported by National Key R&D Program of
China (No 2017YFB0102900) and National Natural Science Foundation of China (Grant No. U1737214).
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