aC Nanolayered Coatings via Synergy o

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Applications of Polymer, Composite, and Coating Materials

Enhanced Corrosion Resistance and Interfacial Conductivity of TiC/a-C Nanolayered Coatings via Synergy of Substrate Bias Voltage for Bipolar Plates Applications in PEMFCs x

Peiyun Yi, Weixin Zhang, Feifei Bi, Linfa Peng, and Xinmin Lai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00514 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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ACS Applied Materials & Interfaces

Enhanced Corrosion Resistance and Interfacial Conductivity of TiCx/a-C Nanolayered Coatings via Synergy of Substrate Bias Voltage for Bipolar Plates Applications in PEMFCs Peiyun Yi†, Weixin Zhang†, Feifei Bi†, Linfa Peng*, †, Xinmin Lai†, § †

State Key Laboratory of Mechanical System and Vibration, Shanghai Jiao Tong University, Shanghai 200240, P.R. China §

Shanghai Key Laboratory of Digital Manufacture for Thin-walled Structures, Shanghai Jiao Tong University, Shanghai 200240, P.R. China

ABSTRACT: Proton exchange membrane fuel cells (PEMFCs) are one kind of renewable and clean energy conversion device, of which the metallic bipolar plates are one of the key components. However, high interfacial contact resistance and poor corrosion resistance are still great challenges for the commercialization of metallic bipolar plates. In this study, we demonstrated a novel strategy for depositing TiCx/a-C nanolayered coatings by synergy of 60 V and 300 V bias voltage to enhance corrosion resistance and interfacial conductivity. The synergistic effects of bias voltage on the composition, microstructure, surface roughness, electrochemical corrosion behaviors and interfacial conductivity of TiCx/a-C coatings were explored. The results revealed that the columnar structures in the inner layer were suppressed and the surface became rougher with the 300 V a-C layer outside. The composition analysis indicated that the sp2 content increased with an increase of 300 V sputtering time. Due to the synergy strategy of bias voltage, lower corrosion current densities were achieved both in potentiostatic polarization (1.6 V vs. SHE) and potentiodynamic polarization. With the increase of 300 V sputtering time, the interfacial conductivity was improved. The enhanced corrosion resistance and interfacial conductivity of the TiCx/a-C coatings would provide new opportunities for commercial bipolar plates.

KEYWORDS: amorphous carbon coatings, interfacial contact resistance, corrosion protection, bias voltage, fuel cell 1. INTRODUCTION The development of proton exchange membrane fuel cells (PEMFCs), which are deemed to be a promising power source due to their high energy conversion efficiency,

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relatively fast start-up and low operating temperature, has seen significant progress in the past few years.1-3 Thereby, PEMFCs will play an important role in many applications, such as automobiles, unmanned aerial vehicles, stationary power supplies and military applications.4 However, commercial applications of PEMFCs are still restricted by high cost of component materials and manufacture procedure as well as limited durability of fuel cell stacks under aggressive operating conditions.3 As a key component of the fuel cell stack, bipolar plates (BPPs) occupy over 80% of the total weight and 30% of the cost.2 In recent years, stainless steel (SS) has been generally regarded as an appropriate candidate material for metallic BPPs due to its excellent mechanical properties, splendid malleability and lower fabrication cost, as compared with graphite BPPs.5-6 However, operating under the PEMFCs acidic and humid conditions, SS tends to be corroded and passivated, which will poison the membrane electrode assembly and lead to potential loss.7 In order to enhance corrosion resistance and reduce interfacial contact resistance (ICR) between BPP and gas diffusion layer (GDL), a range of methods have been employed to modify the surface properties of BPPs.8-10 Recently, corrosion resistant and conductive coatings deposited on BPPs for PEMFCs have attracted widespread interest. Various types of coatings have been widely investigated and employed to enhance the interfacial conductivity and corrosion resistance of BPPs, such as noble metal coating,8, 11 carbon-based coating,10, 12 metal nitride coating,13 metal carbide coating,9, 14 and metallic oxide coating.15-16 Much work so far has focused on amorphous carbon (a-C) coatings due to their outstanding


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performance of electric conductivity and corrosion resistance as well as abundant deposits of graphite material worldwide. Fukutsuka et al. 10 used plasma-assisted CVD technique to coat carbon films on SUS304 and revealed that carbon-coated SUS304 had higher electric conductivity compared with uncoated SUS304. And the dynamic polarization results showed that the corrosion current density was less than 1μA cm-2 at 0.6 V (vs. SCE). However, columnar structures usually generate during the deposition of a-C coatings17 and the corrosion media will penetrate across the columnar structures to corrode the substrate, which will weaken the corrosion resistance of substrate.18-19 In addition, the oxidation of a-C films, leading to the decrease of interfacial conductivity, is still a big challenge in the real application of PEMFCs. Thus, multilayer coatings are introduced to suppress columnar structures. Zhu and coworkers reported that the Ti/Cr nanoscale multilayer coatings, deposited on the depleted uranium substrate, restrained the columnar structures and blocked the penetrating defects.19 Yi et al. deposited multilayered Cr-C/a-C: Cr coatings and the corrosion current density at 0.6 V of potentiodynamic polarization was 0.276 μA cm2 20

.

Wan et al. inserted the Al2O3 interlayer to CrN coatings and proved that the

corrosion resistance was improved.18 In a word, it is generally accepted that multilayer coatings are conducive to improve corrosion resistance and the improvement is increased with more interfaces in the multilayer films. However, the multilayer coatings mentioned above are deposited with different elements or transition metal compounds layer by layer, which may bring out the galvanic corrosion behaviors.



Therefore, the intent of this study is to propose a novel strategy for depositing multilayer coatings by synergy of different negative substrate bias voltage during the magnetron sputtering process. Here, we deposit a-C films with the bias voltage synergy strategy and try to illuminate the synergistic effects on composition, microstructure, electrochemical corrosion behavior and interfacial conductivity of the TiCx/a-C multilayer coatings. The feasibility of the substrate bias voltage synergy strategy is confirmed by the enhanced corrosion resistance and interfacial conductivity of the TiCx/a-C coatings. This study will open up a new avenue to advanced multifunctional materials for anti-corrosive applications. 2. EXPERIMENTAL DETAILS 2.1. Sample Preparation. Commercial 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) and monocrystalline silicon (100) wafers were used as the substrates. Prior to experiments, the SS316L sheets were ultrasonically degreased in acetone, deionized water and absolute ethyl alcohol for 15, 10 and 15 minutes, respectively. After that, all of the specimens were dried out by plasma blower. The TiCx/a-C coatings were deposited on the substrates with non-reactive sputtering technique in the closed field unbalanced magnetron sputter ion plating system (UPD850/4, Teer Coating, Ltd.). The coating system was equipped with two titanium targets (99.99%) and two graphite targets (99.99%). And the substrates were fixed on the specimen carousel in front of the targets at a distance of 150 mm. After that, the vacuum chamber was firstly pumped to get a


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base pressure below 3×10-5 Torr. During deposition, high purity argon (99.99%) was introduced to work as sputtering gas. In order to get further clean substrates surface, the substrates were firstly plasma-sputtered for 30 minutes with a negative substrate bias voltage of 700 V. And then the titanium seed layer was deposited to serve as a protective layer and the TiCx transition layer was deposited immediately. Afterwards, the a-C inner layer (nominated as 60 V a-C layer) was deposited with a negative substrate bias voltage of 60 V. Subsequently, the interface layer was deposited with the substrate bias voltage increased linearly from 60 V to 300 V in one minute. At last, the a-C outer layer (nominated as 300 V a-C layer) was deposited with a negative substrate bias voltage of 300 V. As shown in Table 1, we deposited a-C layers with different sputtering time of bias voltage. For convenience, we designated the samples as ‘60 V (60)’, ‘60 V/300 V (40/19)’, ‘60 V/300 V (20/39)’, ‘300V (60)’, respectively. The visualized schematic illustrations of the coating structures are shown in Scheme 1. Table 1. Sputtering Time Parameter of TiCx/a-C Coatings. Sample NO.

1

2

3

4

60 V a-C layer (min)

60

40

20

0

0

1

1

0

0

19

39

60

60 V→300 V interface (min) 300 V a-C layer (min)

Scheme 1. Schematic Illustrations of the Cross-sectional Structures in the TiCx/a-C Nanolayered Coatings: (a) 60V (60), (b) 60V/300V (40/19), (c) 60V/300V (20/39), (d) 300V (60).



2.2. Coating Characterization. The cross-sectional and surface morphologies of silicon wafers were observed with scanning electron microscope (SEM, MAIA3, TESCAN). In addition, the surface morphologies were also revealed by atomic force microscope (AFM, Dimension Icon, Bruker, USA). Raman spectroscopy (Senterra R200-L, Bruker Optics, Germany) and X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD, SHIMADZU/Kratos, Japan) were employed to explore the composition and the atom binding state of the TiC x/a-C coatings. With the help of an electrochemical workstation (Corr-Test 310, China), electrochemical corrosion experiments were carried out to evaluate the corrosion resistance of the TiCx/a-C multilayered coatings in the simulated PEMFCs cathode environment (pH=3 H2SO4 solution, 0.1 ppm HF, bubbled with air, 80 °C), which is more corrosive than anode environment.21 Traditionally, the three-electrode system was adopted for the corrosion tests, including a platinum net worked as the counter electrode, an Ag/AgCl electrode acted as the reference electrode and a coated sheet served as the working electrode. The potentiodynamic polarization tests were carried out at a scan rate of 1 mV/s (from -0.6 V to +1.0 V, vs. Ag/AgCl) and the potentiostatic polarization experiments were conducted at a high potential of 1.6 V (vs. SHE) for one hour. The 1.6 VSHE potentiostatic polarization tests are significant because the high potential


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corrosion is a practical phenomenon and technological problem when the fuel cell vehicle operates in the complex and fickle conditions, e.g. start-up and shut-down process.22 Moreover, the high potential electrochemical polarization is an accelerated method to evaluate the corrosion resistance of coatings.23-24 Prior to all of the electrochemical corrosion tests, the three-electrode system was kept running for one hour to stabilize at an open circuit potential. All of the corrosion tests were measured at least three times. The corrosion potential (Ecorr) and corrosion current density (icorr), calculated by the conventional Tafel extrapolation, were introduced to evaluate the protective efficiency. In addition, the water contact angle was measured by the Contact Angle System (OCA20, Dataphysics Co., Germany). The ICR values between the coated sheets and the GDL (commercial TGP-H-060 carbon papers) were evaluated by conventional method and the experimental procedures were minutely described in our previous work.12 A digital micro-ohm meter (ZY9858, China) was employed to detect the ICR under a compaction pressure of 1.4 MPa, regarded as the conventional compaction pressure in fuel cell stack. As shown in Scheme 2, the ICR between GDL and TiCx/a-C coatings can be calculated as eq 1

ICR 

 R2  R1   A 2

contact

(1)

In this equation, R1 and R2 are measured in Step 1 and Step 2, respectively. And Acontact is the contact area between the GDL and the coated SS316L sheets. Scheme 2. Schematic Illustrations of ICR measurement: (a) Step 1, (b) Step 2.



3. RESULTS AND DISCUSSION 3.1. Synergistic Effect on the Morphology of TiCx/a-C Coatings. Here, the surface topographies of TiCx/a-C nanolayered films are explored by AFM and the results are shown in Figure 1. As shown in Figure 1a-b, the surface morphology of the TiCx/a-C coatings is smooth. In contrast, as depicted in Figure 1c-d, it is conspicuous that the surface morphologies of coatings have many conical protuberances, about 90-130 nm, and thus the surfaces seem to be rougher.


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Figure 1.

AFM surface morphologies and roughness of the TiCx/a-C coatings: (a)

60V (60), (b) 60V/300V (40/19), (c) 60V/300V (20/39), (d) 300V (60). Besides, the root-mean-square roughness (Rq) of the TiCx/a-C coatings is calculated by the NanoScope Analysis software (Version 1.40). The Rq of Sample 1 is 2.6 nm and the Rq decreases slightly to 1.7 nm as the 300 V sputtering time increases to 19 minutes. While further increasing the 300 V sputtering time, the surface roughness leaps to 16.1 nm and 18.4 nm for Sample 3 and Sample 4, respectively. The synergistic effect of bias voltage on the surface morphology is analyzed as follows. It’s well accepted that the higher substrate bias voltage can intensify the bombardment of particles in the plasma, accelerate the speed of the impinging ions and enhance the flux of energetic ions, including C+ and Ar+ in our case, during the travel from plasma to the substrate.25 Three effects are believed to influence the surface morphology as shown in Figure S1 of the Supporting Information, including the enhanced bombardment effect, the surface relaxation effect and the re-sputtering effect. Specifically, the enhanced intensity of bombardment caused by the bias voltage of 300 V will pin the 60 V a-C layer and promote the adatoms to transfer from the crest to the valley and thus smooth the surface.17,

26

Moreover, continuous impingement of

energetic particles will raise the substrate temperature and cause local melting. So the surface relaxation process occurs and thus flattens the surface locally.27 Additionally, the substrate temperature will also influence the growth mode of carbon atoms or ions. Generally, the further increased substrate temperature (>150 °C) will promote the incident particles to migrate across the surface and sp2-rich clusters are formed.28 Hence,



due to the re-sputtering effect, the weak sp2-rich clusters will be etched away while the stiff parts remain well or be etched at lower rate, thus the hillock-like agglomerates distribute on the surface, and the surface morphology seems to be rougher.29 Therefore, the surface morphology of the TiCx/a-C coatings are intricate competition results of the above-mentioned effects. As the sputtering time of 300 V aC layer increases to 19 minutes, the surface morphology is smoothed due to the bombardment effect and the surface relaxation effect. However, as the further increased depositing time of 300 V a-C layer, the re-sputtering effect is dominative and the persistently incident energetic ions will etch the weak sp2-rich structures on the top layer, as shown in Figure 1c-d. Figure 2 shows the cross-sectional and surface SEM micrographs of TiCx/a-C multilayer coatings. As shown in Figure 2a and d, the undesired columnar structures are noticeable in the a-C layer. However, with the synergy strategy of substrate bias voltage, the a-C layer becomes columnar-free, as presented in Figure 2b-c. In addition, the surface morphologies of the SEM micrographs, as depicted in Figure 2a’-d’, are consistent with the AFM results. Similarly, there are synergistic effects of 60 V and 300 V on the thickness of a-C layers. To be specific, the thickness of a-C layer increases, ranging from 242 nm to 279 nm, as the 300 V a-C layer depositing time increases to 19 minutes. In fact, as we have discussed above, the re-sputtering effect is not obvious if the 300 V depositing time is no longer than 20 minutes. Thus, the increased thickness of a-C layer is mainly


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depended on the growth rate at a bias voltage of 300 V. In fact, with higher substrate bias voltage, the higher plasma density will be formed and the higher plasma flux of energetic ions will travel from the targets to the substrate, thus the deposition rate increases.30 As a result, the samples with the 300 V a-C outer layer are thicker than the sample ‘60V (60)’. Besides, another noticeable phenomenon is that the thickness decreases as the deposition time of 300 V a-C layer goes up from 19 minutes to 60 minutes. Based on the re-sputtering effect mentioned above, this phenomenon may be attributed to the carbon atoms etching effect during the 300 V substrate bias voltage magnetron sputtering process, and the etching process is becoming more and more severe as the increase deposition time of 300 V a-C layer.



Figure 2.

Cross-sectional and surface SEM micrographs of (a, a’) 60V (60), (b, b’)

60V/300V (40/19), (c, c’) 60V/300V (20/39), (d, d’) 300V (60). 3.2. Synergistic Effect on the Composition of TiCx/a-C Coatings. The XPS spectra are employed to evaluate the binding energy of carbon atoms of a-C layer in TiCx/a-C coatings. As shown in Figure 3a-b, the narrow scanning XPS


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spectra are deconstructed into three peaks, i.e. C=C (284.5 eV), C-C (285.2 eV), C-O (287.2 eV), by a Lorentzian-Gaussian method.31 The calculated sp2 and sp3 fraction before and after 1.6 VSHE potentiostatic polarization tests are indicated in Figure 3c-d, respectively. With increasing the sputtering time of 300 V a-C outer layer, the sp2 fraction increases monotonously while the sp3 content decreases monotonously before and after corrosion.

Figure 3.

Deconvolution of XPS narrow-scan spectra of TiCx/a-C coatings at C=C,

C-C, C-O: (a) and (b) before and after 1.6 VSHE potentiostatic polarization for one hour, (c) and (d) sp2 content and sp3 content before and after 1.6 VSHE potentiostatic polarization for one hour.



Raman spectroscopy is quite an effective method to explore the bonding structure of a-C films due to its non-destructive and nonintrusive peculiarity.32-33 In this work, Raman spectra is also adopted to detect the composition of the TiCx/a-C coatings, as depicted in Figure 4a-b, which have been deconvoluted into two Gaussian peaks centered at ~1350 cm-1 and ~1580 cm-1, respectively.34 The ID/IG ratios are calculated by the area ratio of D band and G band and the results are presented in Figure 4c. As the increased sputtering time of 300 V a-C layer in the TiCx/a-C coatings, the ID/IG ratio increases monotonously, which is consistent with the tendency mentioned in reference.35 It is also consistent with the results of XPS spectra by considering the relation between ID/IG ratio and sp2 content.36 In other words, the sp2 content goes up while the sp3 content decreases with the increased sputtering time of 300 V a-C layer, which is concordant with the results revealed in reference.37 The reasonable reasons for the XPS and Raman results may be specifically attributed to the so-called subplantation model and thermal spike model.28, 38 To be specific, compared with the deposition process of 300 V, the 60 V deposition is more approximate to the so-called internal growth, i.e. the successive incorporation of carbon atoms is inclined to generate the increased internal stress and thus leads to the generation of the more diamond-like phase. However, at a substrate bias voltage of 300 V, the substrate will suffer enhanced intensity of ions bombardment by the energetic particles in the plasma and furthermore, the continuous bombardment will lead to the increased substrate temperature. Consequently, the mobility of carbon atoms increases and the film growth process is inclined to the so-called surface growth process, i.e. the


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sp2-rich clusters come into being on the top layer. And as the deposition time of 300 V a-C layer increases, the more and more sp2-rich clusters will aggregate on the surface, which will enhance the interfacial conductivity.

Figure 4.

Deconvolution of Raman spectra: (a) as deposited, (b) after 1.6 VSHE

potentiostatic polarization, (c) ID/IG before and after 1.6 VSHE potentiostatic polarization for one hour. 3.3. Synergistic Effect on the Electrochemical Corrosion Behavior. Operating under the humid and acid environment, corrosion resistance is one of the most significant parameters affecting the performance of PEMFCs, especially when the



bipolar plates have hydrophilic surfaces in consideration of water management.39 As shown in Figure S2 and Figure S3 of the Supporting Information, the TiCx/a-C coatings are hydrophilic. Therefore, excellent corrosion resistance is required to protect the SS316L substrates. Here, the potentiodynamic polarization curves of TiCx/a-C multilayered coatings are plotted in Figure 5a. The corrosion current density at 0.6 VAg/AgCl of Sample 1 is 0.44 μA/cm2, much lower than the DOE 2020 technical target.5 And Sample 4 possesses the highest corrosion current density at 0.6 VAg/AgCl, i.e. 0.98 μA/cm2. However, lower corrosion current densities of 0.25 μA/cm2 and 0.32 μA/cm2 are achieved due to the synergy strategy of bias voltage, as shown in Figure 5b.

Figure 5.

Potentiodynamic polarization of SS316L and TiCx/a-C coatings in

simulated PEMFCs cathode environment: (a) potentiodynamic polarization curves


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as a function of potential, (b) corrosion current density of SS316L substrate and TiCx/a-C coatings at 0.6 V (vs. Ag/AgCl). In addition, the Ecorr and icorr are also introduced to further investigate the anticorrosion ability of coatings and the results are presented in Table 2 and Figure 6. The protective efficiency (Pi) is calculated according to the eq 2 below18

Pi  (1 

icorr )  100% 0 icorr

(2)

where, Pi is the protective efficiency of TiCx/a-C coatings, icorr and i0corr refer to the corrosion current density of TiCx/a-C coated samples and bare SS316L substrate, respectively. Apparently, the coated samples have lower icorr and higher Ecorr as compared with the bare substrate. In addition, the Ecorr increases monotonously as the increased sputtering time of 300 V a-C layer, while the icorr has a sharp drop from SS316L to Sample 1 and then keeps almost unchanged from Sample 1 to Sample 3 and increases slightly for Sample 4. Therefore, due to the synergy strategy of bias voltage, Sample 2 and Sample 3 have higher protective efficiency. Table 2. Electrochemical Corrosion Parameters of SS316L Substrate and TiCx/a-C Coatings in Simulated PEMFCs Cathode Environment. Sample

icorr (μA/cm2)

Ecorr (mV)

SS316L

4.49

-234.42

60V (60)

0.060

92.351

60V/300V (40/19)

0.051

165.778

60V/300V (20/39)

0.085

181.44



300V (60)

Figure 6.

0.180

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190.298

Corrosion potential and corrosion current density of bare SS316L

substrate and TiCx/a-C coated samples. Insert: protective efficiency of TiCx/a-C coatings. For the purpose of obtaining a deeper insight into the corrosion resistance of the TiCx/a-C multilayer coatings, the potentiostatic polarization tests (1.6 V vs. SHE) have been carried out and the results are exhibited in Figure 7. Without the synergy strategy of bias voltage, the Sample 1 and Sample 4 possess corrosion current densities of ~60 μA/cm2 and ~168 μA/cm2, respectively. Significantly, due to the synergy strategy of bias voltage, the polarization curves of Sample 2 and Sample3 are nearly the same and lower corrosion current density of ~20 μA/cm2 is achieved, indicating similar corrosion resistance under the 1.6 VSHE electrochemical corrosion condition. Therefore,


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compared with the a-C single layer, the corrosion resistance is enhanced due to the a-C multilayer coatings deposited by the synergy strategy of bias voltage.

Figure 7.

Potentiostatic polarization (1.6 V vs. SHE) curves of TiCx/a-C coatings as

a function of corrosion time. Based on the results mentioned above, it is confirmed that the corrosion resistance of TiCx/a-C coatings is enhanced due to the bias voltage synergy strategy. This significant improvement could be contributed to the following factors. On the one hand, the interface layer generates between 300 V outer layer and 60 V inner layer as the bias voltage changes from 60 V to 300 V. As presented in Scheme 3, the effect of interface on the enhancement of corrosion resistance could be ascribed to the suppression of columnar microstructures and micro-pinholes, i.e. the following deposited layer will block those columnar structures or pinholes.19, 40 On the other hand, the substrate bias voltage of 300 V will significantly enhance the kinetic energy of ions in the plasma,



thus strengthen the intensity of concurrent ion bombardment. Therefore, the interface layer and the 60 V inner a-C layer will become more compact17, which are supported by the cross-sectional SEM micrographs in Figure 2. Scheme 3. Schematic Illustrations of the Different Corrosion Behaviors between (a) a-C single layer and (b) a-C multilayer in PEMFCs Cathode Environment.

3.4. Synergistic Effect on the ICR. ICR is another significant parameter to appraise the coating performance under the PEMFCs operating condition. The interfacial conductivity deteriorates inevitably under the PEMFCs real operating condition. As shown in Figure 8, with increased sputtering time of 300 V a-C layer, the ICR values of the as deposited samples decrease from 4.55 mΩ cm2 (Sample 1) to 1.85 mΩ cm2 (Sample 3) and increase slightly to 1.92 mΩ cm2 (Sample 4). After 1.6 VSHE potentiostatic polarization, the ICR values of all the coated specimens increase remarkably and especially, the ICR value of Sample 1 leaps to 23.35


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mΩ cm2, about five times of its as deposited state. After corrosion, the ICR values decrease monotonously with the increased sputtering time of the 300 V a-C layer, ranging from 23.35 mΩ cm2 to 3.97 mΩ cm2. These results reveal that the 300 V a-C layer has a positive effect on the improvement of interfacial electrical conductivity and the improvement goes up with the increased 300 V sputtering time.

Figure 8.

ICR of TiCx/a-C coatings before and after 1.6 VSHE potentiostatic

polarization for one hour. In general, ICR is commonly influenced by the coating surface state and the composition of thin films. As will be described below, the ICR of TiCx/a-C films may be closely related to three factors, i.e. the sp2 content, degree of a-C corrosion and the coating surface roughness. Firstly, it is widely accepted that carbon will be oxidized under the PEMFCs cathode conditions. According to reference,41 the following reactions will proceed if the applied potential meets the conditions: C  2H 2O  CO2  4H   4e , E0  0.207 V (vs. SHE )


(3)


C  H 2O  CO  2H   2e , E0  0.518 V (vs. SHE )

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(4)

Hence, it is clear that the oxidation of carbon is ineluctable under PEMFCs operating conditions, especially when the fuel cell stacks start or shut down. Therefore, the high potential corrosion should be taken into consideration since the oxidation of carbon would cause the degradation of electrical conductivity.42 As presented in Figure 9, the increased content of oxygen atoms reduces monotonously as the increased sputtering time of 300 V a-C layer, indicating the weaker degree of a-C corrosion. It must be pointed out that although the degree of a-C corrosion in Sample 4 is the lowest, its substrate protective efficiency is worst due to the columnar structures in the single a-C layer.

Figure 9.

Oxygen atomic percent of TiCx/a-C coatings before and after 1.6 VSHE

potentiostatic polarization for one hour. Secondly, the sp2 content of coatings will also affect the interfacial conductivity. Obviously, the sp2 content decreases a lot after 1.6 VSHE electrochemical corrosion, and


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it implies the increase of ICR after corrosion to some extent, as shown in Figure 3c-d. Meanwhile, another remarkable trend is that the sp2 content has a significant increase as the 300 V sputtering time grows, which is coincident with the results of ICR shown in Figure 8. Furthermore, as shown in Figure 4c, the ratio of ID/IG also has the concordant tendency, i.e. the sp2 content increases according to the increased ID/IG ratio, and thus the interfacial conductivity tends to improve with more sp2-rich clusters on the surface. Finally, ICR values are also influenced by the coating surface topography although this effect is secondary. Generally, the moderate increase of surface roughness would contribute to the decrease of ICR because the actual contact area increases with the rougher surface.42 Therefore, as shown in Figure 1, the increased surface roughness will also promote the improvement of the interfacial conductivity to some extent. 4. CONCLUSIONS In summary, we have demonstrated a novel synergy strategy of substrate bias voltage to suppress the columnar structures in the a-C coatings for metallic bipolar plates in PEMFCs. The composition analysis revealed that the sp2 content increased as the increased sputtering time of 300 V a-C layer. The cross-sectional morphology micrographs confirmed that the a-C layers became column-free due to the synergy strategy of bias voltage. Besides, the corrosion resistance was also enhanced due to the synergistic effect of bias voltage no matter in potentiodynamic polarization tests or 1.6 VSHE potentiostatic polarization experiments. Furthermore, the interfacial electric conductivity also had significant improvements due to higher content of sp2 clusters in



the 300 V a-C layer. Meanwhile, lower degree of carbon corrosion and coarser surface in the nanolayered TiCx/a-C coatings also contributed to the enhanced interfacial conductivity. From a view of practical application, we suggest that the sample ‘60V/300V (20/39)’ has the best comprehensive performance with ICR of 1.85 mΩ cm2 and corrosion current density of 0.32 μA/cm2. Therefore, the synergy strategy of bias voltage is successful to enhance the corrosion resistance and interfacial conductivity of TiCx/a-C coatings for metallic bipolar plates in PEMFCs. Future research alternating the a-C layers with optimized bias voltage and cycles is likely to further improve the performances and broaden its applications for advanced energy materials and anticorrosive coatings. ASSOCIATED CONTENT Supporting Information Synergistic effect of bias voltage on the surface morphology; Water contact angle pictures of TiCx/a-C coatings; Water contact angle of TiCx /a-C coatings. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.


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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). REFERENCES (1)

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Schematic Illustrations of the Cross-sectional Structures in the TiCx/a-C Nanolayered Coatings: (a) 60V (60), (b) 60V/300V (40/19), (c) 60V/300V (20/39), (d) 300V (60). 11x1mm (600 x 600 DPI)



Schematic Illustrations of ICR measurement: (a) Step 1, (b) Step 2. 26x23mm (600 x 600 DPI)


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AFM surface morphologies and roughness of the TiCx/a-C coatings: (a) 60V (60), (b) 60V/300V (40/19), (c) 60V/300V (20/39), (d) 300V (60). 23x13mm (600 x 600 DPI)



Cross-sectional and surface SEM micrographs of (a, a') 60V (60), (b, b') 60V/300V (40/19), (c, c') 60V/300V (20/39), (d, d') 300V (60). 44x77mm (600 x 600 DPI)


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Deconvolution of XPS narrow-scan spectra of TiCx/a-C coatings at C=C, C-C, C-O: (a) and (b) before and after 1.6 VSHE potentiostatic polarization for one hour, (c) and (d) sp2 content and sp3 content before and after 1.6 VSHE potentiostatic polarization for one hour. 78x79mm (300 x 300 DPI)



Deconvolution of Raman spectra: (a) as deposited, (b) after 1.6 VSHE potentiostatic polarization, (c) ID/IG before and after 1.6 VSHE potentiostatic polarization for one hour. 125x131mm (300 x 300 DPI)


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Potentiodynamic polarization of SS316L and TiCx/a-C coatings in simulated PEMFCs cathode environment: (a) potentiodynamic polarization curves as a function of potential, (b) corrosion current density of SS316L substrate and TiCx/a-C coatings at 0.6 V (vs. Ag/AgCl). 93x129mm (600 x 600 DPI)



Corrosion potential and corrosion current density of bare SS316L substrate and TiCx/a-C coated samples. Insert: protective efficiency of TiCx/a-C coatings. 350x249mm (300 x 300 DPI)


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Potentiostatic polarization (1.6 V vs. SHE) curves of TiCx/a-C coatings as a function of corrosion time. 272x208mm (300 x 300 DPI)



299x208mm (300 x 300 DPI)


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299x208mm (300 x 300 DPI)



45x31mm (600 x 600 DPI)


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aC Nanolayered Coatings via Synergy o

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