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 , military equipment , and stationary power stations . However, the commercialization of PEMFCs still suffers from fairly high cost and limited durability of stack under aggressive operating conditions . 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 . In the past decade, stainless steel has been regarded as the alternative * Corresponding author. E-mail address: firstname.lastname@example.org (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.  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 , 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  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.  used the reactive direct current (DC) 934 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.  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 . 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. 936 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 . 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 . 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 . 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 . 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 . 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 . 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. 938 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 . 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 . 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 . While the two time constant model, as shown in Fig. 7c, are widely accepted to evaluate the corrosion performance of coated samples . 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 . 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 . 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 . Hence, the oxygen atomic content is also closely bound up with the interfacial conductivity and the performance of the PEMFC stack. As declared in 942 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. 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