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CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells
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Ti/(Ti,Cr)N/CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells Shengli Wang a,c, Ming Hou a,∗, Qing Zhao a,c, Yongyi Jiang a,c, Zhen Wang a,c, Huizhe Li b, Yu Fu b, Zhigang Shao a,∗
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a b
Q2
c
Fuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China Dalian Sunrise Power Co., LTD, Dalian 116025, Liaoning, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 8 June 2016 Revised 19 August 2016 Accepted 23 August 2016 Available online xxx Keywords: Stainless steel bipolar plate Proton exchange membrane fuel cell Arc ion plating Multilayer coating
1. Introduction
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Proton exchange membrane fuel cell (PEMFC) has attracted much attention over the past decades due to its high efficiency and zero emission [1]. It is promising for automotive, stationary and portable applications. Bipolar plate is the most bulky component in a PEMFC stack with respect to volume and weight [2,3]. Graphite is employed to fabricate bipolar plate currently for its excellent electrical conductivity and chemical stability. Graphite bipolar plate, however, is usually thick and heavy to ensure its low hydrogen permeability and high mechanical strength. What is more, the high cost of graphite bipolar plate hinders it from commercial application in portable devices. Metal, especially stainless steel, is considered to be a promising candidate for bipolar plate fabrication due to its good bulk electrical conductivity, excellent mechanical strength, less gas permeability and cost effectiveness [4–6]. Unfortunately, the corrosion resistance of metal in the PEMFC working environments is inadequate. The releasing ions will future contaminate the membrane and poison the catalyst, besides, the interfacial contact resistance (ICR) between metal material and carbon
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Arc ion plating (AIP) is applied to form Ti/(Ti,Cr)N/CrN multilayer coating on the surface of 316L stainless steel (SS316L) as bipolar plates for proton exchange membrane fuel cells (PEMFCs). The characterizations of the coating are analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Interfacial contact resistance (ICR) between the coated sample and carbon paper is 4.9 m cm2 under 150 N/cm2 , which is much lower than that of the SS316L substrate. Potentiodynamic and potentiostatic tests are performed in the simulated PEMFC working conditions to investigate the corrosion behaviors of the coated sample. Superior anticorrosion performance is observed for the coated sample, whose corrosion current density is 0.12 μA/cm2 . Surface morphology results after corrosion tests indicate that the substrate is well protected by the multilayer coating. Performances of the single cell with the multilayer coated SS316L bipolar plate are improved significantly compared with that of the cell with the uncoated SS316L bipolar plate, presenting a great potential for PEMFC application. © 2016 Published by Elsevier B.V. and Science Press.
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a b s t r a c t
∗
Corresponding authors. Fax: +86 411 84379185. E-mail addresses: [email protected] (M. Hou), [email protected] (Z. Shao).
paper is high, which lowers the efficiency of PEMFC. Recent studies have revealed that bare metals could not be used to fabricate bipolar plate directly in PEMFC [7]. While a promising approach is to form a protecting coating with good corrosion resistance and low interfacial contact resistance on the surface of metal substrate. Noble metals are the most suitable candidates [8,9]. Nevertheless the high cost of the materials prevents them from commercial application. Metal nitrides are potential materials for surface modification such as Cr-nitrides owing to their good corrosion resistance, excellent interfacial conductivity and low cost [7]. Hong et al. [10] formed CrN and Cr2 N compounds on the surface of SS316L by inductively coupled plasma, using a mixture of N2 and H2 at temperatures between 530 K and 650 K. Interfacial conductivity was improved significantly. While the corrosion resistance changed little owing to the formation of Cr-depleted regions. Brady et al. [11–15] Paulauskas et al. [16], Toops et al. [17] and Wang et al. [18–20] conducted a series of experiments to form Cr-nitrides coatings on different kinds of substrates including Ni–50Cr, AISI446, 349TM , Ni–Cr based, Fe–Cr based and Fe–Cr–V based alloys by thermal nitridation. ICRs were reduced for the nitrided samples. In addition, some specimens, especially the nitrided Ni–50Cr, the price of which is high, showed excellent corrosion resistance in both simulated anode and cathode working conditions. Similar works were done by Tian [21] and Lee [22,23].
http://dx.doi.org/10.1016/j.jechem.2016.09.004 2095-4956/© 2016 Published by Elsevier B.V. and Science Press.
Please cite this article as: S. Wang et al., Ti/(Ti,Cr)N/CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.09.004
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S. Wang et al. / Journal of Energy Chemistry xxx (2016) xxx–xxx Table 1. Chemical composition of the 316 L stainless steel. Elements
Fe
Cr
Ni
Mo
Mn
Si
P
C
N
S
Content (wt%)
Balance
16.67
10.15
2.12
1.348
0.576
0.0316
0.0212
0.0136
0.0021
Fig. 1. Schematic illustration of the interfacial contact resistance measurement equipment.
Fig. 3. XRD pattern of the multilayer coating.
Fig. 2. Surface morphology and profile SEM image of the multilayer coated SS316L sample.
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Nevertheless, forming a continuous and external Cr-nitrides layer on the surface of the substrates, whose composition is strictly restricted, is arduous by thermal nitridation or plasma nitridation. Physical vapor deposition (PVD) technology is a capable process to obtain the specific Cr-nitrides layer with less confinement. Tian
[24] deposited CrN/Cr coating on the surface of SS316L by PVD technology. The coated sample exhibited improved corrosion resistance and surface conductivity compared with SS316L substrate under both simulated anode and cathode PEMFC operating environments. Lavigne et al. [25] assembled a stack with five single cells using CrN coated SS316L bipolar plates, which showed good performances after 200 h dynamic cycling. Ho et al. [26] investigated multilayer TiN/CrN coating deposited on SS316L substrate by cathodic arc deposition technique. They found that the multilayer coating deposited at rotation speed of 2 rpm showed the best corrosion resistance due to the multilayer structure and higher thickness of the coating. Barranco et al. [27] studied the corrosion behavior of CrN coated aluminum. Though the coated samples, the coating thicknesses of which ranged from 3 μm to 5 μm, showed superior performances than those of the as-received Al substrate in all cases, pitting holes were found in the coatings. According to our previous studies [28–32], Cr-nitrides coating, prepared by arc ion plating technique with a device to reduce the droplets, exhibited good corrosion resistance and interfacial conductivity. In the present research, a layer of Ti, which is well known to have very high resistance to general corrosion and local corrosion [33,34], was deposited on the surface of the SS316L substrate to prevent corrosion from penetrating to the substrate through the flaws in the coating. Then a transition layer of (Ti,Cr)N was deposited before the CrN layer to improve the adhesion between the Ti layer and the CrN layer for the similar values of lattice parameters can improve the compatibility of the adjacent layers [35]. At last, a layer of CrN was deposited. The behaviors of the single cell with the multilayer coated SS316L bipolar plate were also studied. While the bare SS316L substrate was tested as reference. In addition, arc ion plating technique is cost effective to obtain dense films and realize low-temperature deposition.
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2. Experimental
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The austenitic 316L stainless steel was chosen as the base metal, the chemical composition of which is shown in Table 1.
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Please cite this article as: S. Wang et al., Ti/(Ti,Cr)N/CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.09.004
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Fig. 4. ICRs of the multilayer coated SS316L sample and the SS316L substrate.
Fig. 5. Potentiodynamic polarization curves of the multilayer coated SS316L sample and the SS316L substrate in 0.5 M H2 SO4 solution at 70 °C.
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The substrate with a thickness of 0.1 mm was polished with No. 120 0–20 0 0 SiC waterproof abrasive papers, ultrasonically cleaned in ethyl ethanol and distilled water for 30 min, rinsed with distilled water and dried in air to improve the adhesion of the coating. A layer of Ti was deposited for 10 min. Then N2 and Cr were introduced with the Ti target on to deposit the transition layer of (Ti,Cr)N. At last, the Ti target was turned off and the Cr-nitrides layer was prepared for 40 min. High purity metal of chromium (99.9%) on the left of the facility and titanium (99.9%) on the right of the facility were used as the sputtering targets in the deposition processes. Phase structure of the coating was detected with X-ray diffraction (Empyream, Cu target, 40 KV, 40 mA). Surface morphology and the profile of the sample were observed with scanning electron microscopy (JSM-7800F). ICR between the coated sample and gas diffusion layer (Toray® carbon paper) was measured using the method proposed by David [4,5] and developed by Wang [6]. Both the coated sample and the
carbon paper are wafers with the same diameter of 60 mm as big as the copper plates. Two pieces of carbon papers are sandwiched between the sample and two gold plated copper plates as shown in Fig. 1. During the test, an increasing compaction force with a step of 10 N/s was controlled by a WDW Electromechanical Universal Testing Machine. A constant electrical current of 5.0 A, sourced by a PSP-2010 Programmable Power Supply, was provided via the two gold plated copper plates. The coated sample and the unpolished SS316L substrate were tested. Potentiodynamic and potentiostatic tests were conducted to investigate the corrosion behaviors of the coated sample in 0.5 M H2 SO4 solution at 70 °C to simulate the PEMFC working environment. A CHI630D electrochemical workstation controlled by a computer was used in the electrochemical experiments. A conventional three-electrode system was used with a graphite sheet as the counter electrode, the saturated calomel electrode (SCE) as the reference electrode and the sample as the working electrode. The dimensions of the sample were 15 mm × 15 mm × 0.1 mm. The
Please cite this article as: S. Wang et al., Ti/(Ti,Cr)N/CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.09.004
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Fig. 6. Potentiostatic polarization curves of the multilayer coated SS316L sample and the SS316L substrate in 0.5 M H2 SO4 solution at 70 °C. (a) Cathode behavior at 0.6 VSCE , (b) anode behavior at −0.1 VSCE .
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edges were sealed with epoxy resin exposing 10 mm × 10 mm surface to the electrolyte. All electrode potentials were referenced to SCE unless otherwise specified. Prior to electrochemical measurements the specimens were stabilized at open circuit for 30 min in the testing solution. Potentiodynamic polarization was performed in the anodic direction at a sweeping rate of 0.5 mV/s. To investigate the performance of the coating in the simulated PEMFC working conditions, potentiostatic polarizations were conducted for 2 h at the potential of 0.6 VSCE and −0.1 VSCE , respectively, with the current density–time curves recorded. Compound bipolar plate, like Murphy’s [36], was fabricated with thin graphite flow fields and the sample which works as the metal gas barrier to preliminarily evaluate the feasibility of the sample to be used in bipolar plate manufacturing. A membrane electrode assembly (MEA) was prepared applying commercial Pt/C catalyst and Nafion 211 membrane. The active electrode area was 5 cm2 . A single cell was constructed with the prepared MEA and bipolar plate. The inlet gas (H2 and air) temperature, cell temperature, operating pressure and humidification were 65 °C, 65 °C, 0.05 MPa and 100%, respectively. The performances of the single cell were characterized by polarization curve and cell power
density curve. Electrochemical impedance spectroscopy (EIS) was measured at the output current density of 10 0 0 mA/cm2 and the sweeping frequency ranges from 0.1 Hz to 10 kHz with the potential amplitude of 10 mV.
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3. Results and discussion
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3.1. Characterization of the multilayer coating
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Fig. 2 is the surface morphology and profile SEM image of the sample. From Fig. 2(a), we can see that the coating is flat, continuous and dense without microcrack or vacancy in the deposited multilayer even in the higher magnification image (insert in Fig. 2(a)). The microparticles scattering on the coating surface, which are detrimental to the coating [37], are reduced significantly compared with the conventional arc ion plating technology [28]. Fig. 2(b) shows the profile of the sample, the coating also shows continuous and dense microstructure, which is accordant with Fig. 2(a). The multilayer coating well adheres to the substrate and the thickness of the coating is uniform (about 190 nm). Fig. 3 presents the XRD pattern of the coating. Phases of
Please cite this article as: S. Wang et al., Ti/(Ti,Cr)N/CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.09.004
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Fig. 7. SEM images of the multilayer coated SS316L sample (a) and the SS316L substrate (b) after potentiostatic polarization tests in simulated cathode working conditions.
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CrN(1 1 1), (2 0 0), (3 1 1) and Cr2 N(0 0 2), (1 1 1), (1 1 2) are observed, indicating that the outermost Cr-nitrides layer consists of CrN and Cr2 N phases. The presence of Ti and TiN diffraction peaks confirms that the designed multilayer coating is formed.
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3.2. Interfacial contact resistance (ICR)
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Interfacial conductivity, which affects the power output of fuel cells [38], is an important property of metal bipolar plate for PEMFC. ICR between the sample and Toray® carbon paper, therefore, is used to assess the interfacial conductivity of the sample. As shown in Fig. 4, the values of contact resistance decrease with the increasing compact force attributing to the enlargement of the effective contact area between the sample and carbon paper [39]. The coated sample exhibits excellent interfacial conductivity over the whole compact force range. The ICR values of the coated sample, ranging from 8.0 m/cm2 to 4.1 m/cm2 under 80–190 N/cm2 , are about two orders of magnitude lower than those of the SS316L substrate, ranging from 838.9 m/cm2 to 312.1 m/cm2 under 80– 190 N/cm2 . ICR of the coated sample is 4.9 m/cm2 at 150 N/cm2 reaching DOE 2020 target of < 20 m/cm2 [40], while that of the SS316L substrate is too high to meet the requirement attributing to the high ICR between the passive film on the SS316L substrate and the carbon paper. The excellent interfacial conductivity of the multilayer coated sample may be attributed to the following reasons: (i) the inner Ti layer and the formed nitrides layers prevent the formation of metal oxide semiconductors which yield a high value of interfacial contact resistance on the surface of metal substrate [41,42]; (ii) the electrical conductivity of the outmost Cr-nitrides layer is eminent [24,30]. Besides, the transitionlayer of Cr-nitrides and Ti-nitride may also be beneficial to electron conductivity between the inner Ti layer and the outmost Cr-nitrides layer. Therefore improved output power density of the single cell with the multilayer coated SS316L bipolar plate is anticipated.
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3.3. Corrosion resistance
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Fig. 5 shows the potentiodynamic polarization test results of the coated sample and SS316L substrate in 0.5 M H2 SO4 at 70 °C. Corrosion current density (Icorr ) of the SS316L substrate is 177.83 μA/cm2 determined by Tafel extrapolation method. Nevertheless Icorr of the coated sample is 0.12 μA/cm2 , which is significantly reduced. Corrosion potential (Ecorr ) of the coated sample and the SS316L substrate is 0.075 VSCE and −0.3 VSCE , respectively. An obvious active anodic dissolution region of the SS316L substrate is observed. While the coated sample can be self-passivated in the natural state, indicating that a self-passivating protective coating is formed on the surface of SS316L substrate. The modified sample with the multilayer coating, compared with the SS316L substrate, presents lower corrosion current density in the entire anodic polarization region. Potentiodynamic polarization tests show that the anticorrosion of the coated sample is superior to the SS316L substrate under the testing conditions. In order to further study the corrosion behaviors of the coated sample in actual working conditions of PEMFC, potentiostatic polarization tests were conducted at 0.6 VSCE and −0.1 VSCE to simulate the cathode and anode working conditions, respectively. In the simulated cathode working conditions (Fig. 6a), the corrosion current density of the SS316L substrate stabilizes at about 2.6 μA/cm2 . For the coated sample, corrosion current density is reduced and stabilizes at about 0.67 μA/cm2 . As for the simulated anode working conditions (Fig. 6b), lower corrosion current densities are observed indicating that corrosion issue is more serious in the cathode side compared with that in the anode side, agreeing well with the result reported by Feng et al. [43]. For the SS316L substrate, the corrosion current density is 0.54 μA/cm2 . While the coated sample presents a negative current density, which means that the specimen is cathodic protected [44]. Potentiostatic polarization tests also show that the coated sample excels the SS316L substrate in anticorrosion behavior, which is consistent with the potentiodynamic polarization tests. The improved anticorrosion behaviors of the coated sample are attributed to good anticorrosion property of the coating materials [28,45]. Besides, the continuity and densification of the coating as well as the multilayer structure of the coating prevents corrosion from penetrating into the SS316L substrate. The nethermost Ti barrier layer, which has high resistance to local corrosion, can protect the SS316L substrate if there is flaw in the nitride layers. It should be noted that the test conditions are more severe than the actual PEMFC working conditions, namely accelerated conditions according to Feng et al. [46]. Fig. 7 presents the surface morphology of the coated sample and the SS316L substrate after potentiostatic polarization at 0.6 VSCE . The dense multilayer coating of the coated sample protects the substrate from corrosion and no obvious corrosion pit or deposit is found on the surface of the coating, as shown in Fig. 7(a). Local corrosion evidences, however, are observed on the surface of SS316L substrate, as shown in Fig. 7(b). It can be concluded that corrosion of the SS316L substrate is severe than that of the coated sample.
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3.4. Single cell performances
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Fig. 8 presents single cell performances with graphite, the multilayer coated SS316L and SS316L substrate as bipolar plate, respectively. OCVs are almost the same in all the cases. Cell behaviors with the multilayer coated SS316L bipolar plate are slightly lower than those of the cell with graphite bipolar plate but much higher than those of the cell with uncoated SS316L bipolar plate over the entire current density range. For the cell with the multilayer coated SS316L bipolar plate and graphite bipolar plate, the cell voltage is 654.8 mV and 675.0 mV, respectively, when
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Please cite this article as: S. Wang et al., Ti/(Ti,Cr)N/CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.09.004
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Fig. 8. Polarization curves for single cell with multilayer coated SS316L bipolar plate, uncoated SS316L bipolar plate and graphite bipolar plate, respectively.
Fig. 9. Electrochemical impedance spectra for single cells with multilayer coated SS316L bipolar plate, uncoated SS316L bipolar plate and graphite bipolar plate, respectively.
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the current density is 10 0 0 mA/cm2 . The maximum cell power density of the cell with the multilayer coated SS316L bipolar plate is 811.65 mW/cm2 at the current density of 1500 mA/cm2 . And the maximum cell power density of the cell with the graphite bipolar plate is 860.48 mW/cm2 at the current density of 1600 mA/cm2 . There is no significant difference in performance of the two cells discussed above. But the behaviors of the cell with the uncoated SS316L bipolar plate are inferior. The diversity of performances between cells with the multilayer coated SS316L bipolar plate and the uncoated SS316L bipolar plate is related to the total resistances in the cells. Better cell performances can be anticipated with lower total resistance. Therefore, the relative high output power density of the cell with the multilayer coated SS316L bipolar plate is attributed to its low internal resistance. EIS of the single cell with graphite, the multilayer coated SS316L and SS316L substrate as bipolar plate was investigated at the current density of 10 0 0 mA/cm2 , respectively, as shown in Fig. 9. Intersection point of the semicircle and X-axis in high frequency region reflects ohmic resistances, which is attributed to the membrane resistance and/or interfacial resistance
predominantly. Comparing the curves, it can be observed that the ohmic resistance of the cell with the multilayer coated SS316L bipolar plate is 0.111 cm2 , which is slightly higher than that of the cell with graphite bipolar plate(0.088 cm2 ) and much lower, however, than that of the cell with the uncoated SS316L bipolar plate (0.593 cm2 ). Membrane resistance is identical among the three cases. The differences of ohmic resistances between the cell with graphite bipolar plate and the other two cells reflect the increase of interfacial contact resistances. The interfacial contact resistances of the cell with the uncoated SS316L bipolar plate is much higher than that of the cell with the multilayer coated SS316L bipolar plate, which does not increase significantly. The results of EIS are in accordance with ICR tests, which can explain the differences of the single cell performances adequately. Obviously, the multilayer coated SS316L bipolar plate is promising for practical application owing to its excellent interfacial conductivity and anticorrosion properties. Further studies, however, are necessary to clarify the stability of the multilayer coating in realistic PEM fuel cell stack environments for a long term.
Please cite this article as: S. Wang et al., Ti/(Ti,Cr)N/CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.09.004
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4. Conclusions
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Smooth and dense Ti/(Ti,Cr)N/CrN multilayer coating was prepared on the surface of SS316L substrate by arc ion plating as bipolar plate for PEMFCs. Interfacial contact resistance of the coated sample is much lower than that of the SS316L substrate. In potentiodynamic and potentiostatic tests, the coated sample presents superior anticorrosion properties than the SS316L substrate. The corrosion current density of the coated sample is more than two orders of magnitude lower than that of the SS316L substrate. Performances of the single cell with the multilayer coated SS316L bipolar plate are improved significantly compared with that of the cell with the uncoated SS316L bipolar plate. Furthermore, the multilayer coating is cost effective and the arc ion plating processes are environment-friendly. The multilayer coated SS316L is a potential candidate as bipolar plate material for PEMFCs. Long-term stability and stack performance of the multilayer coated bipolar plate, however, are necessary to be investigated in the following work.
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Acknowledgments
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This work was financially supported by the National Basic Research Program of China (973 Program) (no. 2012CB215500), the National Key Technology Research and Development Program of China (no. 2015BAG06B00), Major Program of the National Natural Science Foundation of China (no. 61433013) and National Natural Science Foundation of China (no. 21206012).
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Please cite this article as: S. Wang et al., Ti/(Ti,Cr)N/CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.09.004
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Ti/(Ti,Cr)N/CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells Shengli Wang a,c, Ming Hou a,∗, Qing Zhao a,c, Yongyi Jiang a,c, Zhen Wang a,c, Huizhe Li b, Yu Fu b, Zhigang Shao a,∗
Q1
a b
Q2
c
Fuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China Dalian Sunrise Power Co., LTD, Dalian 116025, Liaoning, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 8 June 2016 Revised 19 August 2016 Accepted 23 August 2016 Available online xxx Keywords: Stainless steel bipolar plate Proton exchange membrane fuel cell Arc ion plating Multilayer coating
1. Introduction
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Proton exchange membrane fuel cell (PEMFC) has attracted much attention over the past decades due to its high efficiency and zero emission [1]. It is promising for automotive, stationary and portable applications. Bipolar plate is the most bulky component in a PEMFC stack with respect to volume and weight [2,3]. Graphite is employed to fabricate bipolar plate currently for its excellent electrical conductivity and chemical stability. Graphite bipolar plate, however, is usually thick and heavy to ensure its low hydrogen permeability and high mechanical strength. What is more, the high cost of graphite bipolar plate hinders it from commercial application in portable devices. Metal, especially stainless steel, is considered to be a promising candidate for bipolar plate fabrication due to its good bulk electrical conductivity, excellent mechanical strength, less gas permeability and cost effectiveness [4–6]. Unfortunately, the corrosion resistance of metal in the PEMFC working environments is inadequate. The releasing ions will future contaminate the membrane and poison the catalyst, besides, the interfacial contact resistance (ICR) between metal material and carbon
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Arc ion plating (AIP) is applied to form Ti/(Ti,Cr)N/CrN multilayer coating on the surface of 316L stainless steel (SS316L) as bipolar plates for proton exchange membrane fuel cells (PEMFCs). The characterizations of the coating are analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Interfacial contact resistance (ICR) between the coated sample and carbon paper is 4.9 m cm2 under 150 N/cm2 , which is much lower than that of the SS316L substrate. Potentiodynamic and potentiostatic tests are performed in the simulated PEMFC working conditions to investigate the corrosion behaviors of the coated sample. Superior anticorrosion performance is observed for the coated sample, whose corrosion current density is 0.12 μA/cm2 . Surface morphology results after corrosion tests indicate that the substrate is well protected by the multilayer coating. Performances of the single cell with the multilayer coated SS316L bipolar plate are improved significantly compared with that of the cell with the uncoated SS316L bipolar plate, presenting a great potential for PEMFC application. © 2016 Published by Elsevier B.V. and Science Press.
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a b s t r a c t
∗
Corresponding authors. Fax: +86 411 84379185. E-mail addresses: [email protected] (M. Hou), [email protected] (Z. Shao).
paper is high, which lowers the efficiency of PEMFC. Recent studies have revealed that bare metals could not be used to fabricate bipolar plate directly in PEMFC [7]. While a promising approach is to form a protecting coating with good corrosion resistance and low interfacial contact resistance on the surface of metal substrate. Noble metals are the most suitable candidates [8,9]. Nevertheless the high cost of the materials prevents them from commercial application. Metal nitrides are potential materials for surface modification such as Cr-nitrides owing to their good corrosion resistance, excellent interfacial conductivity and low cost [7]. Hong et al. [10] formed CrN and Cr2 N compounds on the surface of SS316L by inductively coupled plasma, using a mixture of N2 and H2 at temperatures between 530 K and 650 K. Interfacial conductivity was improved significantly. While the corrosion resistance changed little owing to the formation of Cr-depleted regions. Brady et al. [11–15] Paulauskas et al. [16], Toops et al. [17] and Wang et al. [18–20] conducted a series of experiments to form Cr-nitrides coatings on different kinds of substrates including Ni–50Cr, AISI446, 349TM , Ni–Cr based, Fe–Cr based and Fe–Cr–V based alloys by thermal nitridation. ICRs were reduced for the nitrided samples. In addition, some specimens, especially the nitrided Ni–50Cr, the price of which is high, showed excellent corrosion resistance in both simulated anode and cathode working conditions. Similar works were done by Tian [21] and Lee [22,23].
http://dx.doi.org/10.1016/j.jechem.2016.09.004 2095-4956/© 2016 Published by Elsevier B.V. and Science Press.
Please cite this article as: S. Wang et al., Ti/(Ti,Cr)N/CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.09.004
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S. Wang et al. / Journal of Energy Chemistry xxx (2016) xxx–xxx Table 1. Chemical composition of the 316 L stainless steel. Elements
Fe
Cr
Ni
Mo
Mn
Si
P
C
N
S
Content (wt%)
Balance
16.67
10.15
2.12
1.348
0.576
0.0316
0.0212
0.0136
0.0021
Fig. 1. Schematic illustration of the interfacial contact resistance measurement equipment.
Fig. 3. XRD pattern of the multilayer coating.
Fig. 2. Surface morphology and profile SEM image of the multilayer coated SS316L sample.
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Nevertheless, forming a continuous and external Cr-nitrides layer on the surface of the substrates, whose composition is strictly restricted, is arduous by thermal nitridation or plasma nitridation. Physical vapor deposition (PVD) technology is a capable process to obtain the specific Cr-nitrides layer with less confinement. Tian
[24] deposited CrN/Cr coating on the surface of SS316L by PVD technology. The coated sample exhibited improved corrosion resistance and surface conductivity compared with SS316L substrate under both simulated anode and cathode PEMFC operating environments. Lavigne et al. [25] assembled a stack with five single cells using CrN coated SS316L bipolar plates, which showed good performances after 200 h dynamic cycling. Ho et al. [26] investigated multilayer TiN/CrN coating deposited on SS316L substrate by cathodic arc deposition technique. They found that the multilayer coating deposited at rotation speed of 2 rpm showed the best corrosion resistance due to the multilayer structure and higher thickness of the coating. Barranco et al. [27] studied the corrosion behavior of CrN coated aluminum. Though the coated samples, the coating thicknesses of which ranged from 3 μm to 5 μm, showed superior performances than those of the as-received Al substrate in all cases, pitting holes were found in the coatings. According to our previous studies [28–32], Cr-nitrides coating, prepared by arc ion plating technique with a device to reduce the droplets, exhibited good corrosion resistance and interfacial conductivity. In the present research, a layer of Ti, which is well known to have very high resistance to general corrosion and local corrosion [33,34], was deposited on the surface of the SS316L substrate to prevent corrosion from penetrating to the substrate through the flaws in the coating. Then a transition layer of (Ti,Cr)N was deposited before the CrN layer to improve the adhesion between the Ti layer and the CrN layer for the similar values of lattice parameters can improve the compatibility of the adjacent layers [35]. At last, a layer of CrN was deposited. The behaviors of the single cell with the multilayer coated SS316L bipolar plate were also studied. While the bare SS316L substrate was tested as reference. In addition, arc ion plating technique is cost effective to obtain dense films and realize low-temperature deposition.
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2. Experimental
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The austenitic 316L stainless steel was chosen as the base metal, the chemical composition of which is shown in Table 1.
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Please cite this article as: S. Wang et al., Ti/(Ti,Cr)N/CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.09.004
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Fig. 4. ICRs of the multilayer coated SS316L sample and the SS316L substrate.
Fig. 5. Potentiodynamic polarization curves of the multilayer coated SS316L sample and the SS316L substrate in 0.5 M H2 SO4 solution at 70 °C.
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The substrate with a thickness of 0.1 mm was polished with No. 120 0–20 0 0 SiC waterproof abrasive papers, ultrasonically cleaned in ethyl ethanol and distilled water for 30 min, rinsed with distilled water and dried in air to improve the adhesion of the coating. A layer of Ti was deposited for 10 min. Then N2 and Cr were introduced with the Ti target on to deposit the transition layer of (Ti,Cr)N. At last, the Ti target was turned off and the Cr-nitrides layer was prepared for 40 min. High purity metal of chromium (99.9%) on the left of the facility and titanium (99.9%) on the right of the facility were used as the sputtering targets in the deposition processes. Phase structure of the coating was detected with X-ray diffraction (Empyream, Cu target, 40 KV, 40 mA). Surface morphology and the profile of the sample were observed with scanning electron microscopy (JSM-7800F). ICR between the coated sample and gas diffusion layer (Toray® carbon paper) was measured using the method proposed by David [4,5] and developed by Wang [6]. Both the coated sample and the
carbon paper are wafers with the same diameter of 60 mm as big as the copper plates. Two pieces of carbon papers are sandwiched between the sample and two gold plated copper plates as shown in Fig. 1. During the test, an increasing compaction force with a step of 10 N/s was controlled by a WDW Electromechanical Universal Testing Machine. A constant electrical current of 5.0 A, sourced by a PSP-2010 Programmable Power Supply, was provided via the two gold plated copper plates. The coated sample and the unpolished SS316L substrate were tested. Potentiodynamic and potentiostatic tests were conducted to investigate the corrosion behaviors of the coated sample in 0.5 M H2 SO4 solution at 70 °C to simulate the PEMFC working environment. A CHI630D electrochemical workstation controlled by a computer was used in the electrochemical experiments. A conventional three-electrode system was used with a graphite sheet as the counter electrode, the saturated calomel electrode (SCE) as the reference electrode and the sample as the working electrode. The dimensions of the sample were 15 mm × 15 mm × 0.1 mm. The
Please cite this article as: S. Wang et al., Ti/(Ti,Cr)N/CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.09.004
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Fig. 6. Potentiostatic polarization curves of the multilayer coated SS316L sample and the SS316L substrate in 0.5 M H2 SO4 solution at 70 °C. (a) Cathode behavior at 0.6 VSCE , (b) anode behavior at −0.1 VSCE .
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edges were sealed with epoxy resin exposing 10 mm × 10 mm surface to the electrolyte. All electrode potentials were referenced to SCE unless otherwise specified. Prior to electrochemical measurements the specimens were stabilized at open circuit for 30 min in the testing solution. Potentiodynamic polarization was performed in the anodic direction at a sweeping rate of 0.5 mV/s. To investigate the performance of the coating in the simulated PEMFC working conditions, potentiostatic polarizations were conducted for 2 h at the potential of 0.6 VSCE and −0.1 VSCE , respectively, with the current density–time curves recorded. Compound bipolar plate, like Murphy’s [36], was fabricated with thin graphite flow fields and the sample which works as the metal gas barrier to preliminarily evaluate the feasibility of the sample to be used in bipolar plate manufacturing. A membrane electrode assembly (MEA) was prepared applying commercial Pt/C catalyst and Nafion 211 membrane. The active electrode area was 5 cm2 . A single cell was constructed with the prepared MEA and bipolar plate. The inlet gas (H2 and air) temperature, cell temperature, operating pressure and humidification were 65 °C, 65 °C, 0.05 MPa and 100%, respectively. The performances of the single cell were characterized by polarization curve and cell power
density curve. Electrochemical impedance spectroscopy (EIS) was measured at the output current density of 10 0 0 mA/cm2 and the sweeping frequency ranges from 0.1 Hz to 10 kHz with the potential amplitude of 10 mV.
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3. Results and discussion
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3.1. Characterization of the multilayer coating
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Fig. 2 is the surface morphology and profile SEM image of the sample. From Fig. 2(a), we can see that the coating is flat, continuous and dense without microcrack or vacancy in the deposited multilayer even in the higher magnification image (insert in Fig. 2(a)). The microparticles scattering on the coating surface, which are detrimental to the coating [37], are reduced significantly compared with the conventional arc ion plating technology [28]. Fig. 2(b) shows the profile of the sample, the coating also shows continuous and dense microstructure, which is accordant with Fig. 2(a). The multilayer coating well adheres to the substrate and the thickness of the coating is uniform (about 190 nm). Fig. 3 presents the XRD pattern of the coating. Phases of
Please cite this article as: S. Wang et al., Ti/(Ti,Cr)N/CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.09.004
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Fig. 7. SEM images of the multilayer coated SS316L sample (a) and the SS316L substrate (b) after potentiostatic polarization tests in simulated cathode working conditions.
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CrN(1 1 1), (2 0 0), (3 1 1) and Cr2 N(0 0 2), (1 1 1), (1 1 2) are observed, indicating that the outermost Cr-nitrides layer consists of CrN and Cr2 N phases. The presence of Ti and TiN diffraction peaks confirms that the designed multilayer coating is formed.
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3.2. Interfacial contact resistance (ICR)
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Interfacial conductivity, which affects the power output of fuel cells [38], is an important property of metal bipolar plate for PEMFC. ICR between the sample and Toray® carbon paper, therefore, is used to assess the interfacial conductivity of the sample. As shown in Fig. 4, the values of contact resistance decrease with the increasing compact force attributing to the enlargement of the effective contact area between the sample and carbon paper [39]. The coated sample exhibits excellent interfacial conductivity over the whole compact force range. The ICR values of the coated sample, ranging from 8.0 m/cm2 to 4.1 m/cm2 under 80–190 N/cm2 , are about two orders of magnitude lower than those of the SS316L substrate, ranging from 838.9 m/cm2 to 312.1 m/cm2 under 80– 190 N/cm2 . ICR of the coated sample is 4.9 m/cm2 at 150 N/cm2 reaching DOE 2020 target of < 20 m/cm2 [40], while that of the SS316L substrate is too high to meet the requirement attributing to the high ICR between the passive film on the SS316L substrate and the carbon paper. The excellent interfacial conductivity of the multilayer coated sample may be attributed to the following reasons: (i) the inner Ti layer and the formed nitrides layers prevent the formation of metal oxide semiconductors which yield a high value of interfacial contact resistance on the surface of metal substrate [41,42]; (ii) the electrical conductivity of the outmost Cr-nitrides layer is eminent [24,30]. Besides, the transitionlayer of Cr-nitrides and Ti-nitride may also be beneficial to electron conductivity between the inner Ti layer and the outmost Cr-nitrides layer. Therefore improved output power density of the single cell with the multilayer coated SS316L bipolar plate is anticipated.
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3.3. Corrosion resistance
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Fig. 5 shows the potentiodynamic polarization test results of the coated sample and SS316L substrate in 0.5 M H2 SO4 at 70 °C. Corrosion current density (Icorr ) of the SS316L substrate is 177.83 μA/cm2 determined by Tafel extrapolation method. Nevertheless Icorr of the coated sample is 0.12 μA/cm2 , which is significantly reduced. Corrosion potential (Ecorr ) of the coated sample and the SS316L substrate is 0.075 VSCE and −0.3 VSCE , respectively. An obvious active anodic dissolution region of the SS316L substrate is observed. While the coated sample can be self-passivated in the natural state, indicating that a self-passivating protective coating is formed on the surface of SS316L substrate. The modified sample with the multilayer coating, compared with the SS316L substrate, presents lower corrosion current density in the entire anodic polarization region. Potentiodynamic polarization tests show that the anticorrosion of the coated sample is superior to the SS316L substrate under the testing conditions. In order to further study the corrosion behaviors of the coated sample in actual working conditions of PEMFC, potentiostatic polarization tests were conducted at 0.6 VSCE and −0.1 VSCE to simulate the cathode and anode working conditions, respectively. In the simulated cathode working conditions (Fig. 6a), the corrosion current density of the SS316L substrate stabilizes at about 2.6 μA/cm2 . For the coated sample, corrosion current density is reduced and stabilizes at about 0.67 μA/cm2 . As for the simulated anode working conditions (Fig. 6b), lower corrosion current densities are observed indicating that corrosion issue is more serious in the cathode side compared with that in the anode side, agreeing well with the result reported by Feng et al. [43]. For the SS316L substrate, the corrosion current density is 0.54 μA/cm2 . While the coated sample presents a negative current density, which means that the specimen is cathodic protected [44]. Potentiostatic polarization tests also show that the coated sample excels the SS316L substrate in anticorrosion behavior, which is consistent with the potentiodynamic polarization tests. The improved anticorrosion behaviors of the coated sample are attributed to good anticorrosion property of the coating materials [28,45]. Besides, the continuity and densification of the coating as well as the multilayer structure of the coating prevents corrosion from penetrating into the SS316L substrate. The nethermost Ti barrier layer, which has high resistance to local corrosion, can protect the SS316L substrate if there is flaw in the nitride layers. It should be noted that the test conditions are more severe than the actual PEMFC working conditions, namely accelerated conditions according to Feng et al. [46]. Fig. 7 presents the surface morphology of the coated sample and the SS316L substrate after potentiostatic polarization at 0.6 VSCE . The dense multilayer coating of the coated sample protects the substrate from corrosion and no obvious corrosion pit or deposit is found on the surface of the coating, as shown in Fig. 7(a). Local corrosion evidences, however, are observed on the surface of SS316L substrate, as shown in Fig. 7(b). It can be concluded that corrosion of the SS316L substrate is severe than that of the coated sample.
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3.4. Single cell performances
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Fig. 8 presents single cell performances with graphite, the multilayer coated SS316L and SS316L substrate as bipolar plate, respectively. OCVs are almost the same in all the cases. Cell behaviors with the multilayer coated SS316L bipolar plate are slightly lower than those of the cell with graphite bipolar plate but much higher than those of the cell with uncoated SS316L bipolar plate over the entire current density range. For the cell with the multilayer coated SS316L bipolar plate and graphite bipolar plate, the cell voltage is 654.8 mV and 675.0 mV, respectively, when
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Fig. 8. Polarization curves for single cell with multilayer coated SS316L bipolar plate, uncoated SS316L bipolar plate and graphite bipolar plate, respectively.
Fig. 9. Electrochemical impedance spectra for single cells with multilayer coated SS316L bipolar plate, uncoated SS316L bipolar plate and graphite bipolar plate, respectively.
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the current density is 10 0 0 mA/cm2 . The maximum cell power density of the cell with the multilayer coated SS316L bipolar plate is 811.65 mW/cm2 at the current density of 1500 mA/cm2 . And the maximum cell power density of the cell with the graphite bipolar plate is 860.48 mW/cm2 at the current density of 1600 mA/cm2 . There is no significant difference in performance of the two cells discussed above. But the behaviors of the cell with the uncoated SS316L bipolar plate are inferior. The diversity of performances between cells with the multilayer coated SS316L bipolar plate and the uncoated SS316L bipolar plate is related to the total resistances in the cells. Better cell performances can be anticipated with lower total resistance. Therefore, the relative high output power density of the cell with the multilayer coated SS316L bipolar plate is attributed to its low internal resistance. EIS of the single cell with graphite, the multilayer coated SS316L and SS316L substrate as bipolar plate was investigated at the current density of 10 0 0 mA/cm2 , respectively, as shown in Fig. 9. Intersection point of the semicircle and X-axis in high frequency region reflects ohmic resistances, which is attributed to the membrane resistance and/or interfacial resistance
predominantly. Comparing the curves, it can be observed that the ohmic resistance of the cell with the multilayer coated SS316L bipolar plate is 0.111 cm2 , which is slightly higher than that of the cell with graphite bipolar plate(0.088 cm2 ) and much lower, however, than that of the cell with the uncoated SS316L bipolar plate (0.593 cm2 ). Membrane resistance is identical among the three cases. The differences of ohmic resistances between the cell with graphite bipolar plate and the other two cells reflect the increase of interfacial contact resistances. The interfacial contact resistances of the cell with the uncoated SS316L bipolar plate is much higher than that of the cell with the multilayer coated SS316L bipolar plate, which does not increase significantly. The results of EIS are in accordance with ICR tests, which can explain the differences of the single cell performances adequately. Obviously, the multilayer coated SS316L bipolar plate is promising for practical application owing to its excellent interfacial conductivity and anticorrosion properties. Further studies, however, are necessary to clarify the stability of the multilayer coating in realistic PEM fuel cell stack environments for a long term.
Please cite this article as: S. Wang et al., Ti/(Ti,Cr)N/CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.09.004
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4. Conclusions
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Smooth and dense Ti/(Ti,Cr)N/CrN multilayer coating was prepared on the surface of SS316L substrate by arc ion plating as bipolar plate for PEMFCs. Interfacial contact resistance of the coated sample is much lower than that of the SS316L substrate. In potentiodynamic and potentiostatic tests, the coated sample presents superior anticorrosion properties than the SS316L substrate. The corrosion current density of the coated sample is more than two orders of magnitude lower than that of the SS316L substrate. Performances of the single cell with the multilayer coated SS316L bipolar plate are improved significantly compared with that of the cell with the uncoated SS316L bipolar plate. Furthermore, the multilayer coating is cost effective and the arc ion plating processes are environment-friendly. The multilayer coated SS316L is a potential candidate as bipolar plate material for PEMFCs. Long-term stability and stack performance of the multilayer coated bipolar plate, however, are necessary to be investigated in the following work.
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This work was financially supported by the National Basic Research Program of China (973 Program) (no. 2012CB215500), the National Key Technology Research and Development Program of China (no. 2015BAG06B00), Major Program of the National Natural Science Foundation of China (no. 61433013) and National Natural Science Foundation of China (no. 21206012).
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Please cite this article as: S. Wang et al., Ti/(Ti,Cr)N/CrN multilayer coated 316L stainless steel by arc ion plating as bipolar plates for proton exchange membrane fuel cells, Journal of Energy Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2016.09.004
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