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Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells
international journal of hydrogen energy xxx (xxxx) xxx
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Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells Lun Wang a, Youkun Tao b,c,**, Zhen Zhang a, Yajun Wang c, Qi Feng a, Haijiang Wang c,d, Hui Li a,d,* a
Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong Province, PR China b SUSTech Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen, Guangdong Province, PR China c Department of Mechanical and Energy Engineering, Southern University of Science and Technology, Shenzhen, Guangdong Province, PR China d Shenzhen Southerntech Fuel Cell Corp., Ltd., Shenzhen, Guangdong Province, PR China
article info
abstract
Article history:
Superior corrosion resistance and high electrical conductivity are crucial to the metallic
Received 2 November 2018
bipolar plates towards a wider application in proton exchange membrane fuel cells. In this
Received in revised form
work, molybdenum carbide coatings are deposited in different thicknesses onto the sur-
22 December 2018
face of 316 L stainless steel by magnetron sputtering, and their feasibility as bipolar plates
Accepted 25 December 2018
is investigated. The microstructure characterization confirms a homogenous, compact and
Available online xxx
defectless surface for the coatings. The anti-corrosion performance improves with the increase of the coating thickness by careful analysis of the potentiodynamic and poten-
Keywords:
tiostatic data. With the adoption of a thin chromium transition layer and coating of a
Proton exchange membrane fuel cell
~1052 nm thick molybdenum carbide, an excellent corrosion current density of
Bipolar plate
0.23 mA cm2 is achieved, being approximately 3 orders of magnitude lower than that of the
Molybdenum carbide
bare stainless steel. The coated samples also show a low interfacial contact resistance
Stainless steel
down to 6.5 mU cm2 in contrast to 60 mU cm2 for the uncoated ones. Additionally, the
Corrosion current density
hydrophobic property of the coatings’ surface is beneficial for the removal of liquid water
Interfacial contact resistance
during fuel cell operation. The results suggest that the molybdenum carbide coated stainless steel is a promising candidate for the bipolar plates. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong Province, PR China. ** Corresponding author.SUSTech Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen, Guangdong Province, PR China E-mail addresses: [email protected] (Y. Tao), [email protected] (H. Li). https://doi.org/10.1016/j.ijhydene.2018.12.184 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Introduction The proton exchange membrane fuel cell (PEMFC) has great promise as a new power generation technology for the stationary and transportation applications due to its high power density, high efficiency, zero carbon emission and fast response [1,2]. The major components of PEMFCs include the membrane electrode assembly (MEA), the gas diffusion layer (GDL) and the bipolar plate (BP). The bipolar plate is particularly important because it accounts for ~80% of the total stack weight and more than 30% of the stack cost. The BP plays multi-roles in a PEMFC stack, such as providing the mechanical support for the single cells, separating and guiding the reactant gas flows, collecting the electrical current, providing channels for removing the water product and facilitating water management [3]. Therefore, the BPs must have excellent electrical conductivity, high thermal conductivity, low gas permeability and certain mechanical strength [4]. On these properties, the Department of Energy (DOE) of the United States proposed some criteria to be achieved in 2020 towards the commercialization of BPs [5]. The criteria are summarized in Table 1. Conventionally, graphite is used for bipolar plates due to its excellent electrical conductivity, thermal conductivity and chemical inertness, and good power output has been achieved with PEMFC stacks using the graphite BPs [6]. However, graphite is brittle and needs complex technique to be shaped into BPs, resulting in dramatic cost increase hindering the commercialization of PEMFCs [7]. In recent years, more and more studies were focused on the application of metallic materials (especially stainless steels, SS) in bipolar plates. Metallic BPs are very attractive due to the excellent mechanical strength, good electrical conductivity, low gas permeability, high thermal conductivity, ease of shaping into sheets and low cost production at large scales. However, the major drawbacks lie in that metallic BPs are prone to corrosion in the perfluorosulfonic acid and electrochemical environment of PEMFCs [8,9]. The dissolved metal cations can migrate into MEAs and reduce the performance. Moreover, a non-conductive oxide layer may be formed on the surface of BPs due to corrosion, causing significant increase of contact resistance between BPs and GDLs thereby decreasing the stack performance [10,11]. To solve these problems, protective and conductive surface coatings are demanded for metallic bipolar plates. Up to now a variety of protective coating materials, such as graphite diamond-like carbon, conductive polymers, noble
Table 1 e The US DOE Technical Targets in 2020: Bipolar plates [5]. Property Electrical conductivity Flexural strength H2 permeation coefficient Corrosion current density Thermal conductivity Interfacial contact resistance Plate weight Cost
Value 1
>100 S cm >25 MPa <2$106 cm3sec1cm2 at 80 C, 3 atm. <1 mA cm2 >10 W mK1 <10 mU cm2 0.4 kg/KWnet 3 $/KWnet
metal and metal nitrides, have been investigated for improving the corrosion resistance of the metallic BPs. For example, Sisan et al. prepared carbon film for the SS 316 L BPs using Physical Vapor Deposition (PVD) technique, and a film of only 200 nm thickness exhibited sufficient corrosion tolerance for use in PEMFCs [12]. Also, dense carbon films have been obtained through Chemical Vapor Deposition (CVD) in C2H2/ H2 for protection of the SS 304 BPs [13,14]. The carbon films, which consisted of highly crystallized graphite and amorphous carbon, exhibited decent electrical conduction and corrosion resistance comparable to that of graphite BPs. Feng et al. reported SS 316 BPs with a carbon coating of several micrometers’ thickness [15], showing not only a good corrosion resistance but also hydrophobicity that can benefit the water management. Additionally, Cr doped carbon was reported as another effective anti-corrosion coating for SS 316 BPs [16]. It was also found that the addition of Cr prompted the sp3 of carbon to change into sp2, resulting in a decrease of the contact resistance. Although carbon coating is advantageous considering the simple chemical composition and costeffective manufacturing, its bonding to the SS substrate is usually insufficient and may fall off the substrate [12]. Further, carbon can be oxidized during PEMFC stack start-up where there is a high voltage instantaneously [17]. Some conductive polymers have also been investigated as coating materials of metallic BPs. For example, a polyaniline (PANI) coated SS 304 bipolar plate has demonstrated high corrosion resistance [18], although the contact resistance between the BPs and GDL was still too high for PEMFC applications [19]. Noble metals (e.g. Au or Ag) having excellent chemical stability and electrical conductivity could be good candidates for coatings of the metallic BPs [20]. However, the high cost limits their application in PEMFCs. Similar to the noble metals, some low-cost transition metal carbides and nitrides also exhibit good corrosion tolerance and electrical conductivity, therefore attracting a lot of interest to serve as anti-corrosion coatings in PEMFCs. For instance, a ~2 mm thick TiN conductive coating [21] can successfully reduce the corrosion current density of the SS 316 BPs down to 1 mA cm2. Also, densified TiC coatings have been applied onto the SS 304 BPs [22]. During the deposition process, any defects such as macroparticles or pinholes in the coating layer should be avoided since they are harmful to the lifetime of PEMFCs [23]. Barranco et al. employed cathodic arc evaporation technique for the deposition of CrN on Al-5083 [24], and studied the effect of coating thickness on the coating structure as well as the corrosion resistance. It was found that the number of defects can be significantly reduced with increase of the coating thickness [24]. Composite or multi-layered coatings were also developed to achieve defectless surface and improve the corrosion resistance of the BPs. Nam et al. prepared multilayered coatings on the SS 316 L via RF sputtering [25], and a low corrosion current density ~0.7 mA cm2 was obtained with a 0.1 mm thick CrN inner layer and a 0.9 mm thick TiN outer layer. Molybdenum is one important alloying element to stainless steel 316 for improvement of the anti-pitting corrosion resistance to acids. Molybdenum carbide has been well known as a low cost wear-resistive material [26] and an effective catalyst [27,28]. It has a unique interstitial crystal structure
Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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with the smaller carbon atoms filling half of the total octahedral interstices in the lattice of the bigger metal atoms. Molybdenum carbide exhibits simultaneously some extent of metal-like (e.g. electrical conduction) and carbon-like (e.g. chemical inertness in acid) properties, which are demanded by the BPs of PEMFCs [29e32]. To the best of our knowledge, there is little study so far on the molybdenum carbide coatings for metallic BPs of PEMFCs in literature. In this study, we deposited molybdenum carbide on SS 316 L by magnetron sputtering and investigated its feasibility as BPs. Instead of using Mo target together with carbon-source gases or carbon target [33], the magnetron sputtering deposition of molybdenum carbide was conducted directly with a single Mo2C target in this work, which is simple, easy to handle and costeffective. The effect of coating thickness and adoption of an additional transition layer on the corrosion resistance as well as the contact resistance were investigated.
Experimental Sample preparation Commercial SS 316 L plates were used as the substrate in this study. The SS 316 L plates were cut into small pieces in the dimension of 15 mm 15 mm x 2 mm by wire electrical discharge machining. To remove the oxide layer and clean the surface, the SS 316 L samples were polished in turns with No. 600, 800, 1000, 1200 and 1500 SiC waterproof abrasive papers. The samples were then rinsed with acetone and distilled water, and finally dried with nitrogen purge gas. A magnetron sputtering equipment (500CK-500ZF dual chamber coating machine, Beijing KYKY Technology Co., Ltd.) equipped with a transition chamber and a vacuum chamber was used for deposition. The samples were put into the transition chamber and the vacuum chamber pressure was evacuated to a base pressure below 103 Pa using a rotary pump and a diffusion pump. Prior to the deposition, the SS 316 L was sputtered by Ar ions to remove any passive layers on the surface. High purity argon (99.99%) was flown into the chamber at a rate of 50 sccm. Then the molybdenum carbide coatings were deposited with a Mo2C target and the power was set as 300 W. By varying the deposition time (0.5 h, 1 h and 1.5 h), the coatings were deposited in three different thicknesses, i.e. 375 nm, 741 nm and 1052 nm as measured with a step profiler. Accordingly, these three samples were labelled as MC375, MC741 and MC1052 (Table 2). A blank sample (MC0), which was a bare SS 316 L plate without coating, was also included in Table 2. In another case, a transition layer was adopted in-between the coating and the substrate for further improvement of the
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corrosion resistance: a ~100 nm thick Cr layer was coated on the 316 L SS using a Cr target, followed by the deposition of molybdenum carbide for another 1.5 h. This sample will be mentioned as Cr-MC1052 in the later section.
Coating characterization To assess the corrosion resistance, the potentiodynamic polarization and potentiostatic polarization tests were conducted on the samples using an electrochemical workstation (Solartron Analytical 1400 Cell Test System). The electrochemical test was implemented in the solution of 0.5 M H2SO4 þ 2 ppm HF at 70 C, which was thoroughly bubbled with H2 and air for simulation of the aggressive anode and cathode environment in PEMFCs, respectively. A three-electrode system was employed in the electrochemical test, where the coating sample acted as the working electrode, the saturated calomel electrode (SCE) was used as the reference electrode and a platinum mesh was used as the counter electrode. All the working electrodes had an active area of 1 cm2. The threeelectrode cells were stabilized at open circuit potential for half hour before polarization curve scanning. Then the potentiodynamic polarization was conducted at a scanning rate of 1 mV s1 in the potential range of 0.5 V vs. SCE. ~0.9 V vs. SCE. By analysis of the polarization curves the corrosion current density can be obtained, which is commonly used for evaluating the anti-corrosion property. In order to investigate the stability of the coatings, the potentiostatic polarization was conducted at potential of 100 mV and 600 mV vs. SCE for 1e2 h with H2 and air bubbling the solution of 0.5 M H2SO4 þ 2 ppm HF at 70 C for simulation of the PEMFC operation environment. In PEMFC stacks, bipolar plates are placed in direct contact with GDLs. The interfacial contact resistance (ICR) between the BPjGDL needs to be investigated, considering its contribution to the performance loss of the PEMFCs. Similar to the method reported in literature [30,34,35], a sandwich configuration was used for measuring the ICR between the samples and the GDL. It consists of two pieces of carbon paper (AvCarb Material Solutions) and one bipolar plate sample (flat plate without channels) which was placed in-between. Two pieces of Au-coated copper plates were used as the clamping plates. A sketch for the configuration of the ICR measurement is shown in Fig. 1. A constant current (I) of 5 A was applied to the BP sample through the two gold-coated copper plates and the voltage drop (U) was recorded with increasing the compaction force ranging from 0 to 300 Ncm2. The ICR can be obtained as followings [15]: firstly, measuring the total resistance R1 (as in Fig. 1a) of the whole CujGDLjsamplejGDLjCu sandwich structure; secondly, measuring the resistance R2 (as in Fig. 1b) of the two gold-coated Cu plates with one GDL in between (CujGDLjCu). All measurements were repeated for three times.
Table 2 e The thickness of molybdenum carbide coatings with different deposition times.
According to Ohm's law: R ¼ (U/I) x S
Samples
The first measurement: R1 ¼ 2 RGDLjCu þ 2 RGDLjBP þ RBP þ 2 (2) RGDL
Deposition time (min) Thickness (nm)
MC0 MC375 MC741 MC1052
CrMC1052
0
30
60
90
10 þ 90
0
375
741
1052
100 þ 1052
The second measurement: R2 ¼ 2 RGDLjCu þ RGDL
(1)
(3)
Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Fig. 1 e Schematic of the contact resistance measurement: a) total resistance R1 (CujGDLjsamplejGDLjCu) and b) R2 (CujGDLjCu).
where S is the exposed surface area of the BP sample, RGDLjCu is the ICR between the gold-coated copper plate and the GDL, RGDLjBP is the ICR between the GDL and the BP, RBP is the bulk resistance of BP and the RGDL is the bulk resistance of the GDL. Therefore, the ICR between the BP sample and the GDL can be calculated as: RGDLjBP ¼ (R1 - R2 - RGDL - RBP) / 2
(4)
previously used for SS316L flat plates. The single cell has an active electrode area of 5 cm2. The single cell was operated at temperature of 65 C, with inlet gas pressure of 3 atm. and back pressure at 1 atm. Hydrogen and oxygen gases were fed to the anode and cathode, respectively. The humidity of the inlet gas flow was RH ¼ 80% for both the cathode and the anode, and the stoichiometric ratio is 2.0/2.5 for H2/air, respectively. After activation and stabilization for 30 min, the i-V curves were measured for the single fuel cell and the power density can be obtained.
Because the bulk of the GDL and BP are excellent conductors, the values of RGDL and RBP are very small and can be ignored. Then, the RGDLjBP can be rewritten as:
Results and discussion
RGDLjBP ¼ (R1 - R2) / 2
Surface morphology and composition
(5)
The surface morphology and cross-section of the molybdenum carbide coating were observed by scanning electron microscopy (FEI Quanta-200) at an accelerating voltage of 5e15 kV. The chemical elements distribution was analyzed by the energy dispersive X-ray spectroscopy (EDS). In order to determine the chemical composition and bonding structure of the coatings, X-ray photoelectron spectroscopy (XPS, ThermoFisher Scientific) characterization was conducted with an Al Ka X-ray source. The spot size of the XPS is 400 nm and the chamber pressure is 1.6 107 mbar. The hydrophobic property is important for water management in PEMFCs, therefore the contact angle of the coated SS 316 L surface with liquid water was measured using a VCA Optima XE instrument. A drop of water was dripped on the sample surface at room temperature, and the contact angle was calculated with the help of UTHSHCSA image tool software. The measurement was repeated for six times and then the average value was calculated.
Single cell tests In order to evaluate the performance in PEMFCs, the molybdenum carbide coated SS316L bipolar plates were fabricated and assembled with a MEA (i.e. membrane electrode assembly) for single cell test. The bipolar plates have a single serpentine type of flow channel design. After channel preparation, the plates were coated with ~100 nm thick Cr transition layer followed with coating of ~1 mm thick molybdenum carbide layer. The coating process was exactly the same with
SEM images in Fig. 2 show the surface morphology of the SS 316 L blank sample without coating and the one coated with molybdenum carbide. According to Fig. 2a, there were some micro-scratches on the surface of the pre-treated SS 316 L without coating before the electrochemical tests. After coating, the SS 316 L exhibited a compact, flat and homogeneous surface (Fig. 2b) without showing any scratches. This indicates that the coating can effectively cover the SS316L surface and eliminate the major defects like micro-scratches. The cross-section of the coated SS 316 L is shown in Fig. 2c. From the SEM picture, a dense coating layer was formed on the substrate without showing voids or delamination inbetween. A thin transition layer in-between the coating layer and the substrate can be observed under SEM for the cross-section of the sample Cr-MC1052, the composition of which was confirmed as chromium according to the EDS line scanning (Fig. 2d). Also, the EDS results revealed that the coating layer consisted of molybdenum and carbon. Fig. 2e shows the surface morphology of bare SS316L (MC0) after the electrochemical tests. There exists a number of micro-holes and pitting on the surface due to corrosion. In contrast, no pitting can be observed for the coated sample Cr-MC1052 (Fig. 2f) after test, indicating that the molybdenum carbide coatings can prevent the SS316L from the aggressive acid environment. The EDS elemental mapping analysis in Fig. 3 shows that the chemical elements of coating layer were mainly C and Mo, which distributed uniformly on the surface. The XRD characteristic peaks (Fig. 3c) of the SS316L substrate are indicated
Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Fig. 2 e a) SEM image of the bare surface of SS 316 L (MC0), b) SEM surface image of the coated sample Cr-MC1052, c) SEM cross-section view of Cr-MC1052, and d) EDS elemental analysis along the line in c), e) SEM image of bare SS316L (MC0) after test, and f) SEM image of the coated sample (Cr-MC1052) after test. by *. The peaks marked with * indicate mixed phases of molybdenum carbides with different stoichiometric composition most likely including Mo2C [PDF#31-0871, 35-0787], MoC [PDF#08-0384, 06-0546] and Mo3C2 [PDF#42-0890]. The X-ray photoelectron spectroscopy (XPS) measurement (Fig. 4) was conducted for the analysis of the surface composition and valence state of the coatings. As shown in Fig. 4a, the peak fitting of Mo 3 d profile suggested that there were four states (2þ, 3þ, 4 þ and 6þ) for Mo in the surface coating layer. The peaks located at 235.7 eV and 232.6 eV can be attributed to the Mo6þ and Mo4þ, respectively, which may result from the surface oxidation of coating materials during the electrochemical testing or when exposed to air [36,37]. The peaks located at 228.2 eV and 231.3 eV can be ascribed to the characteristic doublets of the Mo2þ state of Mo2C, which are in good agreement with the results reported in literature [38,39]. The deconvoluted C 1s XPS spectrum was shown in Fig. 4b. The peak at 284.8 eV was the characteristic of C]C [40], and the peaks at 286.3 eV and 282.9 eV were associated with the carbon species of C]O [41] and MoeC [41,42], respectively. The binding energy peak at 289.0 eV may be ascribed to OeC]O [43].
Contact angle In order to assess the hydrophobicity, which is an important property of BPs for water management in PEMFC stacks, the contact angle between molybdenum carbide coated SS 316 L plates and liquid water was measured. As shown in Fig. 5, the contact angle with water for the blank sample MC0 and the coated sample MC1052 was measured as 72.6 and 90.1 , respectively. This means the coated SS 316 L can display a more hydrophobic behavior than the bare SS 316 L. Also, compared with the Mo2N-coated SS 304 bipolar plates in literature [44] showing a contact angle of 68 , the molybdenum carbide coated SS 316 L BPs in this work exhibited much better hydrophobicity and is expected to benefit the water management in PEMFC stacks.
Adhesion test The adhesion test was carried out with a Nano Scratch Tester. The scratch length was 150 mm and the maximum scratch load was 50 mN. The results are shown in Fig. 6. It is observed in Fig. 6a that the sample MC1052 has a deeper displacement into
Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Fig. 3 e a) The element mapping by SEM/EDS for the molybdenum carbide coated SS 316 L,b) The green and violet colors indicate the element distribution of Mo and C, respectively, c) XRD characterization of the coated sample MC1052. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4 e XPS spectra of a) the Mo 3 d and b) the C 1s for molybdenum carbide coatings.
Fig. 5 e Contact angle measurement of a) the blank sample MC0 and b) the coated flat-plate sample Cr-MC1052. surface with the scratch going than Cr-MC1052. Fig. 6b shows a larger load on the sample of Cr-MC1052 than on MC1052, indicating more strength will be needed to destroy the coating
for Cr-MC1052. Therefore, it is concluded that the Cr transition layer can effectively enhance the bonding strength between the substrate and the coating layers.
Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Electrochemical test To simulate the anode environment The potentiodynamic polarization curves of the blank and the coated samples were measured in the solution of H2-bubbled 0.5 M H2SO4 þ 2 ppm HF at 70 C, which was conventionally used as simulation of the PEMFCs environment. The results are shown in Fig. 7. The bare SS 316 L plate shows a corrosion curve very typical for stainless steels, which includes a cathodic polarization region, an anode polarization region and a passive region [44,45]. The corrosion potential is associated with the thermodynamic stability of the coating, and is often used for representing the corrosion resistance. All the coated SS 316 L samples shows a higher corrosion potential in comparison to the bare sample. From a thermodynamics point of view, this increased corrosion potential indicates an improved chemical inertness and a better stability in the corrosion environment. Also, the corrosion current density can be calculated based on the extension of Tafel's slope on the polarization curves [46], and the values were listed in Table 3. The tests were performed on three specimens for each condition, and the corrosion current density has a deviation within range of ±1.02 107 A cm2. The bare SS 316 L plate shows a passive current density of 5.9 104 A cm2, which is too high to be used in PEMFCs. The corrosion current densities of the coated samples (approx. 1 106 A cm2 or even lower) were significantly smaller than that of the bare sample, indicating that the molybdenum carbide coating can effectively prevent the SS 316 L BPs from corrosion. Moreover, it can be observed that the higher thickness of the coating resulted in better corrosion resistance of the SS 316 L (Table 3). With increase of the coating thickness from 375 nm to 741 nm and 1052 nm, the corresponding corrosion current density decreased from 1.1 106 A cm2 to 0.49 106 A cm2 and finally down to 0.31 106 A cm2. The results indicate that a ~741 nm or ~1052 nm thick molybdenum carbide layer is sufficient for protecting the SS 316 L bipolar plates from corrosion in PEMFCs, exactly meeting the DOE's target on corrosion current density of below 1 106 A cm2. To further improve the anti-corrosion property and also enhance the adhesion between the coating and the substrate, a transition layer was adopted in the sample Cr-MC1052 by
Fig. 7 e Potentiodynamic polarization measurement in the H2-bubbled solution of 0.5 M H2SO4 þ 2 ppm HF for the SS 316 L with different molybdenum carbide coatings.
depositing a Cr thin-layer on the surface of SS 316 L prior to coating with molybdenum carbide. The potentiodynamic polarization was conducted under the same condition as before. The polarization curves and the derived corrosion current density are shown in Fig. 7 and Table 3 to compare with the samples having no transition layer. Employing the same molybdenum carbide coatings (1052 nm), the sample with the addition of a Cr transition layer exhibited a lower corrosion current density (2.3 107 A cm2) than the one without transition layer (3.1 107 A cm2), indicating an improved corrosion resistance for the SS 316 L bipolar plate. To investigate the durability of the coated samples in PEMFC environment, the potentiostatic polarization curves were measured at 0.1 V vs. SCE in the H2 bubbled solution of 0.5 M H2SO4 þ 2 ppm HF at 70 C. Fig. 8 shows a similar tendency in current density curves for the coated SS 316 L samples. The current density decreased in a relatively fast rate for the initial short period, and then gradually slowed down reaching a lower platform (<1 106 A cm2). The results confirmed that all coated samples were stable in the simulated anode environment with a sufficiently low corrosion rate meeting the criterion for PEMFC applications. Due to the protective coatings, the SS 316 L samples shows a stabilized
Fig. 6 e The adhesion test with Nano Scratch Tester for the molybdenum carbide coated SS316L plate with and without Cr transition layer: a) the displacement into surface and b) the load profile with change of the scratch distance. Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Table 3 e The anti-corrosion properties of SS 316 L BPs with different coating thickness and with the addition of a Cr transition layer tested in simulated anode condition. Samples
Coating thickness (nm) Corrosion Potential (mV vs SCE) Corrosion current density (A$cm2)
Molybdenum carbide coated SS 316 L MC0
MC375
MC741
MC1052
Cr-MC1052
0 283 5.9 104
375 221 1.1 106
741 187 4.9 107
1052 102 3.1 107
1052 45 2.3 107
current density at 1e2 orders of magnitude lower in comparison to the uncoated one (MC0). A lower current density indicated a higher stability and a longer life-time of the BPs. Apparently, increasing the coating thickness from 375 nm to 741 nm and 1052 nm effectively improved the corrosion resistance. The sample Cr-MC1052, which had a Cr transition layer deposited prior to coating of molybdenum carbide, exhibited the best anti-corrosion property of all (Fig. 8). The enhanced performance could be ascribed to the elimination of any possible tiny defect (such as nano-pinholes or voids) due to the transition layer as well as improvement of coatings’ adhesion to the substrate. The results are in consistent with those shown in Table 3.
To simulate the cathode environment The potentiodynamic polarization curves of the blank and the coated samples were measured in the solution of air-bubbled 0.5 M H2SO4 þ 2 ppm HF at 70 C. The results are shown in Fig. 9. Similar to the anode environment, the coated sample exhibits significantly better anti-corrosion performance than the bare SS316L in the simulated cathodic condition. The corrosion current density was measured to be 5.3 106 A cm2 for the bare SS316L. While for the coated sample (Cr-MC1052), an excellent corrosion current density of 9.1 108 A cm2 was achieved, almost two orders lower than the uncoated one. This corrosion current density is also smaller than measured for the anode condition (2.3 107 A cm2), indicating the bipolar plate of anode side is more prone to corrosion than cathode side. The results of the simulated cathodic corrosion tests are also presented in Table 4. Fig. 10 shows the results of potentiostatic polarization
Fig. 8 e Potentiostatic polarization measurement in the H2bubbled solution of 0.5 M H2SO4 þ 2 ppm HF for the SS 316 L with different molybdenum carbide coatings.
measurements in the air-bubbled solution at 0.6 V vs SCE. The sample Cr-MC1052 displays good stability of anti-corrosion during the test and finally levels off at 1.1 107 A cm2.
Interfacial contact resistance The results of the interfacial contact resistance (ICR) measurement were shown in Fig. 11. All samples including the bare and the coated SS 316 L show ICR of a similar tendency with increasing compaction force. The ICR decreased dramatically in the low force range (0e50 N cm2), then decreased slowly and finally stabilized at a lower platform with further increase of the compaction force. The explanation is given as below: at low compaction forces, the effective contact area is actually insufficient between the SS 316 L BPs and the carbon paper considering they are both rigid, resulting in a poor interfacial conduction or a high ICR value. With the increase of the compaction force and deformation of the carbon paper, the effective contacting points increase, providing more conduction paths and resulting reduced ICR. The bare SS 316 L shows a large ICR of 60 mU cm2 at a compaction force of 210 N cm2. In contrast, all the molybdenum carbides coated SS 316 L show significantly smaller ICR values down to ~10 mU cm2 or below. For MC1052 which had a ~1052 nm thick molybdenum carbide coating, the ICR was measured to be 7.9 mU cm2 at 210 N cm2. The ICR was further decreased to 6.5 mU cm2 when a Cr transition layer was adopted. The improved conduction could be attributed to the enhanced bonding strength between the substrate and the coating with help of the transition layer. Thin transition layers
Fig. 9 e Potentiodynamic polarization measurement in the air-bubbled solution of 0.5 M H2SO4 þ 2 ppm HF for the sample MC0 (bare SS316L) and Cr-MC1052.
Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Table 4 e The anti-corrosion properties of SS 316 L BPs with bare 316 L SS and Cr-MC1052 tested in simulated cathode condition. Samples
Coating thickness (nm) Corrosion Potential (mV) Corrosion current density (A$cm2)
Molybdenum carbide coated SS 316 L MC0
Cr-MC1052
0 271 5.3 106
1052 112 9.1 108
Fig. 12 e Interfacial contact resistances between GDL and the coated SS 316 L bipolar plates as a function of compaction force after potentiostatic polarization tests.
Fig. 10 e Potentiostatic polarization measurement in the air-bubbled solution of 0.5 M H2SO4 þ 2 ppm HF for the bare sample MC0 (bare SS316L) and coated sample CrMC1052.
Fig. 13 e Performance of the single fuel cell assembled with the coated SS316L bipolar plates (Cr-MC1052).
Fig. 11 e Interfacial contact resistance between GDL and the coated SS 316 L BPs as a function of compaction force.
are often used before film deposition and could serve as the so-called ‘‘seed layer’’ benefiting the crystallization of the coating layer [47]. The transition layers usually have an interdiffusion effect with substrate and coating material, enhancing the interfacial connection and conduction between them [47]. It is expected to facilitate the electrical conduction and the in-plane current distribution of the coating layer, which would in turn contribute to a smaller resistance of the coarse GDL texture contacting rigid bipolar plates. However, a
more clear and detailed explanation needs further investigations. After the potentiostatic polarization test, the contact resistance was measured again for examination of any property change in surface contacting conduction. The result is shown in Fig. 12. The ICR of the bare 316 L SS increases to 80 mU cm2 after the corrosion test, while the coated sample Cr-MC1052 still keeps a low contact resistance at 6.8 mU cm2, demonstrating good anti-corrosion property and stability. Considering the DOE's target of ICR <10 mU cm2, the molybdenum carbide coated SS 316 L in this work can satisfy the demand for a low ICR for commercializable metallic BPs.
Single cell test Finally, a single fuel cell has been assembled and tested for assessment of the as developed SS316L bipolar plates working in real PEMFCs. The bipolar plates have a single serpentine type of flow channel, and were coated with the same process as that for flat-plate samples of Cr-MC1052. The i-V performances and power density curve of the fuel cell are shown in
Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Fig. 13. The fuel cell operating at 65 C with humidity of RH ¼ 80% for both electrodes demonstrated good performances. A cell voltage of 0.46 V was measured at the current density of 2.0 Acm2, where the maximum output power density reaches 0.92 Wcm2.
Conclusions Molybdenum carbide coatings were applied onto SS 316 L as protective coatings and successfully tested as bipolar plates of PEMFCs. The molybdenum carbide was deposited in different thicknesses by DC magnetron sputtering, and a smooth and dense surface was obtained without showing any major defect. The anti-corrosion property was investigated by potentiodynamic and potentiostatic polarization tests in the simulated PEMFC environments. The corrosion current of the SS 316 L was found to decrease with increasing the coating thickness of molybdenum carbide. Moreover, the thicker coating contributed to a lower interfacial contact resistance with GDL. The performance could be further improved with the adoption of a thin Cr transition layer on the SS 316 L prior to coating molybdenum carbide. A very low corrosion current density of 2.3 107 A cm2 was achieved in H2-bubbled solution of 0.5 M H2SO4 þ 2 ppm HF at 70 C and 9.1 108 A cm2 in Air-bubbled solution of 0.5 M H2SO4 þ 2 ppm HF at 70 C The coated SS316L BPs demonstrated good stability within a preliminary durability test. Also, a low interfacial contact resistance of 6.5 mU cm2 was obtained and after test the interfacial contact resistance still keeps a low interfacial contact resistance. Furthermore, the coated surfaces exhibited hydrophobic properties, which will benefit the water management in PEMFCs stacks. The overall evaluation suggested the molybdenum carbide coated SS 316 L can be a promising candidate for the metallic bipolar plates of PEMFCs.
Acknowledgements Thanks for Huhang Ma from the Materials Characterization and Preparation Center, Huqiang Yi and Xiaoming Zhang from the Department of Material Science and engineering, Southern University of Science and Technology, for guidance of the sputtering experiment and help on single cell testing. Thanks for the financial support by the Shenzhen Peacock Plan (KQTD2016022620054656), the National Key Research and Development Program of China (No. 2017YFB0102701), the Development and Reform Commission of Shenzhen Municipality (2017) (No.1106 and No.1181), the Shenzhen Key Laboratory Project (ZDSYS201603311013489) and the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2016ZT06N500).
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Available online at www.sciencedirect.com
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Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells Lun Wang a, Youkun Tao b,c,**, Zhen Zhang a, Yajun Wang c, Qi Feng a, Haijiang Wang c,d, Hui Li a,d,* a
Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong Province, PR China b SUSTech Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen, Guangdong Province, PR China c Department of Mechanical and Energy Engineering, Southern University of Science and Technology, Shenzhen, Guangdong Province, PR China d Shenzhen Southerntech Fuel Cell Corp., Ltd., Shenzhen, Guangdong Province, PR China
article info
abstract
Article history:
Superior corrosion resistance and high electrical conductivity are crucial to the metallic
Received 2 November 2018
bipolar plates towards a wider application in proton exchange membrane fuel cells. In this
Received in revised form
work, molybdenum carbide coatings are deposited in different thicknesses onto the sur-
22 December 2018
face of 316 L stainless steel by magnetron sputtering, and their feasibility as bipolar plates
Accepted 25 December 2018
is investigated. The microstructure characterization confirms a homogenous, compact and
Available online xxx
defectless surface for the coatings. The anti-corrosion performance improves with the increase of the coating thickness by careful analysis of the potentiodynamic and poten-
Keywords:
tiostatic data. With the adoption of a thin chromium transition layer and coating of a
Proton exchange membrane fuel cell
~1052 nm thick molybdenum carbide, an excellent corrosion current density of
Bipolar plate
0.23 mA cm2 is achieved, being approximately 3 orders of magnitude lower than that of the
Molybdenum carbide
bare stainless steel. The coated samples also show a low interfacial contact resistance
Stainless steel
down to 6.5 mU cm2 in contrast to 60 mU cm2 for the uncoated ones. Additionally, the
Corrosion current density
hydrophobic property of the coatings’ surface is beneficial for the removal of liquid water
Interfacial contact resistance
during fuel cell operation. The results suggest that the molybdenum carbide coated stainless steel is a promising candidate for the bipolar plates. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen, Guangdong Province, PR China. ** Corresponding author.SUSTech Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen, Guangdong Province, PR China E-mail addresses: [email protected] (Y. Tao), [email protected] (H. Li). https://doi.org/10.1016/j.ijhydene.2018.12.184 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Introduction The proton exchange membrane fuel cell (PEMFC) has great promise as a new power generation technology for the stationary and transportation applications due to its high power density, high efficiency, zero carbon emission and fast response [1,2]. The major components of PEMFCs include the membrane electrode assembly (MEA), the gas diffusion layer (GDL) and the bipolar plate (BP). The bipolar plate is particularly important because it accounts for ~80% of the total stack weight and more than 30% of the stack cost. The BP plays multi-roles in a PEMFC stack, such as providing the mechanical support for the single cells, separating and guiding the reactant gas flows, collecting the electrical current, providing channels for removing the water product and facilitating water management [3]. Therefore, the BPs must have excellent electrical conductivity, high thermal conductivity, low gas permeability and certain mechanical strength [4]. On these properties, the Department of Energy (DOE) of the United States proposed some criteria to be achieved in 2020 towards the commercialization of BPs [5]. The criteria are summarized in Table 1. Conventionally, graphite is used for bipolar plates due to its excellent electrical conductivity, thermal conductivity and chemical inertness, and good power output has been achieved with PEMFC stacks using the graphite BPs [6]. However, graphite is brittle and needs complex technique to be shaped into BPs, resulting in dramatic cost increase hindering the commercialization of PEMFCs [7]. In recent years, more and more studies were focused on the application of metallic materials (especially stainless steels, SS) in bipolar plates. Metallic BPs are very attractive due to the excellent mechanical strength, good electrical conductivity, low gas permeability, high thermal conductivity, ease of shaping into sheets and low cost production at large scales. However, the major drawbacks lie in that metallic BPs are prone to corrosion in the perfluorosulfonic acid and electrochemical environment of PEMFCs [8,9]. The dissolved metal cations can migrate into MEAs and reduce the performance. Moreover, a non-conductive oxide layer may be formed on the surface of BPs due to corrosion, causing significant increase of contact resistance between BPs and GDLs thereby decreasing the stack performance [10,11]. To solve these problems, protective and conductive surface coatings are demanded for metallic bipolar plates. Up to now a variety of protective coating materials, such as graphite diamond-like carbon, conductive polymers, noble
Table 1 e The US DOE Technical Targets in 2020: Bipolar plates [5]. Property Electrical conductivity Flexural strength H2 permeation coefficient Corrosion current density Thermal conductivity Interfacial contact resistance Plate weight Cost
Value 1
>100 S cm >25 MPa <2$106 cm3sec1cm2 at 80 C, 3 atm. <1 mA cm2 >10 W mK1 <10 mU cm2 0.4 kg/KWnet 3 $/KWnet
metal and metal nitrides, have been investigated for improving the corrosion resistance of the metallic BPs. For example, Sisan et al. prepared carbon film for the SS 316 L BPs using Physical Vapor Deposition (PVD) technique, and a film of only 200 nm thickness exhibited sufficient corrosion tolerance for use in PEMFCs [12]. Also, dense carbon films have been obtained through Chemical Vapor Deposition (CVD) in C2H2/ H2 for protection of the SS 304 BPs [13,14]. The carbon films, which consisted of highly crystallized graphite and amorphous carbon, exhibited decent electrical conduction and corrosion resistance comparable to that of graphite BPs. Feng et al. reported SS 316 BPs with a carbon coating of several micrometers’ thickness [15], showing not only a good corrosion resistance but also hydrophobicity that can benefit the water management. Additionally, Cr doped carbon was reported as another effective anti-corrosion coating for SS 316 BPs [16]. It was also found that the addition of Cr prompted the sp3 of carbon to change into sp2, resulting in a decrease of the contact resistance. Although carbon coating is advantageous considering the simple chemical composition and costeffective manufacturing, its bonding to the SS substrate is usually insufficient and may fall off the substrate [12]. Further, carbon can be oxidized during PEMFC stack start-up where there is a high voltage instantaneously [17]. Some conductive polymers have also been investigated as coating materials of metallic BPs. For example, a polyaniline (PANI) coated SS 304 bipolar plate has demonstrated high corrosion resistance [18], although the contact resistance between the BPs and GDL was still too high for PEMFC applications [19]. Noble metals (e.g. Au or Ag) having excellent chemical stability and electrical conductivity could be good candidates for coatings of the metallic BPs [20]. However, the high cost limits their application in PEMFCs. Similar to the noble metals, some low-cost transition metal carbides and nitrides also exhibit good corrosion tolerance and electrical conductivity, therefore attracting a lot of interest to serve as anti-corrosion coatings in PEMFCs. For instance, a ~2 mm thick TiN conductive coating [21] can successfully reduce the corrosion current density of the SS 316 BPs down to 1 mA cm2. Also, densified TiC coatings have been applied onto the SS 304 BPs [22]. During the deposition process, any defects such as macroparticles or pinholes in the coating layer should be avoided since they are harmful to the lifetime of PEMFCs [23]. Barranco et al. employed cathodic arc evaporation technique for the deposition of CrN on Al-5083 [24], and studied the effect of coating thickness on the coating structure as well as the corrosion resistance. It was found that the number of defects can be significantly reduced with increase of the coating thickness [24]. Composite or multi-layered coatings were also developed to achieve defectless surface and improve the corrosion resistance of the BPs. Nam et al. prepared multilayered coatings on the SS 316 L via RF sputtering [25], and a low corrosion current density ~0.7 mA cm2 was obtained with a 0.1 mm thick CrN inner layer and a 0.9 mm thick TiN outer layer. Molybdenum is one important alloying element to stainless steel 316 for improvement of the anti-pitting corrosion resistance to acids. Molybdenum carbide has been well known as a low cost wear-resistive material [26] and an effective catalyst [27,28]. It has a unique interstitial crystal structure
Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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with the smaller carbon atoms filling half of the total octahedral interstices in the lattice of the bigger metal atoms. Molybdenum carbide exhibits simultaneously some extent of metal-like (e.g. electrical conduction) and carbon-like (e.g. chemical inertness in acid) properties, which are demanded by the BPs of PEMFCs [29e32]. To the best of our knowledge, there is little study so far on the molybdenum carbide coatings for metallic BPs of PEMFCs in literature. In this study, we deposited molybdenum carbide on SS 316 L by magnetron sputtering and investigated its feasibility as BPs. Instead of using Mo target together with carbon-source gases or carbon target [33], the magnetron sputtering deposition of molybdenum carbide was conducted directly with a single Mo2C target in this work, which is simple, easy to handle and costeffective. The effect of coating thickness and adoption of an additional transition layer on the corrosion resistance as well as the contact resistance were investigated.
Experimental Sample preparation Commercial SS 316 L plates were used as the substrate in this study. The SS 316 L plates were cut into small pieces in the dimension of 15 mm 15 mm x 2 mm by wire electrical discharge machining. To remove the oxide layer and clean the surface, the SS 316 L samples were polished in turns with No. 600, 800, 1000, 1200 and 1500 SiC waterproof abrasive papers. The samples were then rinsed with acetone and distilled water, and finally dried with nitrogen purge gas. A magnetron sputtering equipment (500CK-500ZF dual chamber coating machine, Beijing KYKY Technology Co., Ltd.) equipped with a transition chamber and a vacuum chamber was used for deposition. The samples were put into the transition chamber and the vacuum chamber pressure was evacuated to a base pressure below 103 Pa using a rotary pump and a diffusion pump. Prior to the deposition, the SS 316 L was sputtered by Ar ions to remove any passive layers on the surface. High purity argon (99.99%) was flown into the chamber at a rate of 50 sccm. Then the molybdenum carbide coatings were deposited with a Mo2C target and the power was set as 300 W. By varying the deposition time (0.5 h, 1 h and 1.5 h), the coatings were deposited in three different thicknesses, i.e. 375 nm, 741 nm and 1052 nm as measured with a step profiler. Accordingly, these three samples were labelled as MC375, MC741 and MC1052 (Table 2). A blank sample (MC0), which was a bare SS 316 L plate without coating, was also included in Table 2. In another case, a transition layer was adopted in-between the coating and the substrate for further improvement of the
3
corrosion resistance: a ~100 nm thick Cr layer was coated on the 316 L SS using a Cr target, followed by the deposition of molybdenum carbide for another 1.5 h. This sample will be mentioned as Cr-MC1052 in the later section.
Coating characterization To assess the corrosion resistance, the potentiodynamic polarization and potentiostatic polarization tests were conducted on the samples using an electrochemical workstation (Solartron Analytical 1400 Cell Test System). The electrochemical test was implemented in the solution of 0.5 M H2SO4 þ 2 ppm HF at 70 C, which was thoroughly bubbled with H2 and air for simulation of the aggressive anode and cathode environment in PEMFCs, respectively. A three-electrode system was employed in the electrochemical test, where the coating sample acted as the working electrode, the saturated calomel electrode (SCE) was used as the reference electrode and a platinum mesh was used as the counter electrode. All the working electrodes had an active area of 1 cm2. The threeelectrode cells were stabilized at open circuit potential for half hour before polarization curve scanning. Then the potentiodynamic polarization was conducted at a scanning rate of 1 mV s1 in the potential range of 0.5 V vs. SCE. ~0.9 V vs. SCE. By analysis of the polarization curves the corrosion current density can be obtained, which is commonly used for evaluating the anti-corrosion property. In order to investigate the stability of the coatings, the potentiostatic polarization was conducted at potential of 100 mV and 600 mV vs. SCE for 1e2 h with H2 and air bubbling the solution of 0.5 M H2SO4 þ 2 ppm HF at 70 C for simulation of the PEMFC operation environment. In PEMFC stacks, bipolar plates are placed in direct contact with GDLs. The interfacial contact resistance (ICR) between the BPjGDL needs to be investigated, considering its contribution to the performance loss of the PEMFCs. Similar to the method reported in literature [30,34,35], a sandwich configuration was used for measuring the ICR between the samples and the GDL. It consists of two pieces of carbon paper (AvCarb Material Solutions) and one bipolar plate sample (flat plate without channels) which was placed in-between. Two pieces of Au-coated copper plates were used as the clamping plates. A sketch for the configuration of the ICR measurement is shown in Fig. 1. A constant current (I) of 5 A was applied to the BP sample through the two gold-coated copper plates and the voltage drop (U) was recorded with increasing the compaction force ranging from 0 to 300 Ncm2. The ICR can be obtained as followings [15]: firstly, measuring the total resistance R1 (as in Fig. 1a) of the whole CujGDLjsamplejGDLjCu sandwich structure; secondly, measuring the resistance R2 (as in Fig. 1b) of the two gold-coated Cu plates with one GDL in between (CujGDLjCu). All measurements were repeated for three times.
Table 2 e The thickness of molybdenum carbide coatings with different deposition times.
According to Ohm's law: R ¼ (U/I) x S
Samples
The first measurement: R1 ¼ 2 RGDLjCu þ 2 RGDLjBP þ RBP þ 2 (2) RGDL
Deposition time (min) Thickness (nm)
MC0 MC375 MC741 MC1052
CrMC1052
0
30
60
90
10 þ 90
0
375
741
1052
100 þ 1052
The second measurement: R2 ¼ 2 RGDLjCu þ RGDL
(1)
(3)
Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Fig. 1 e Schematic of the contact resistance measurement: a) total resistance R1 (CujGDLjsamplejGDLjCu) and b) R2 (CujGDLjCu).
where S is the exposed surface area of the BP sample, RGDLjCu is the ICR between the gold-coated copper plate and the GDL, RGDLjBP is the ICR between the GDL and the BP, RBP is the bulk resistance of BP and the RGDL is the bulk resistance of the GDL. Therefore, the ICR between the BP sample and the GDL can be calculated as: RGDLjBP ¼ (R1 - R2 - RGDL - RBP) / 2
(4)
previously used for SS316L flat plates. The single cell has an active electrode area of 5 cm2. The single cell was operated at temperature of 65 C, with inlet gas pressure of 3 atm. and back pressure at 1 atm. Hydrogen and oxygen gases were fed to the anode and cathode, respectively. The humidity of the inlet gas flow was RH ¼ 80% for both the cathode and the anode, and the stoichiometric ratio is 2.0/2.5 for H2/air, respectively. After activation and stabilization for 30 min, the i-V curves were measured for the single fuel cell and the power density can be obtained.
Because the bulk of the GDL and BP are excellent conductors, the values of RGDL and RBP are very small and can be ignored. Then, the RGDLjBP can be rewritten as:
Results and discussion
RGDLjBP ¼ (R1 - R2) / 2
Surface morphology and composition
(5)
The surface morphology and cross-section of the molybdenum carbide coating were observed by scanning electron microscopy (FEI Quanta-200) at an accelerating voltage of 5e15 kV. The chemical elements distribution was analyzed by the energy dispersive X-ray spectroscopy (EDS). In order to determine the chemical composition and bonding structure of the coatings, X-ray photoelectron spectroscopy (XPS, ThermoFisher Scientific) characterization was conducted with an Al Ka X-ray source. The spot size of the XPS is 400 nm and the chamber pressure is 1.6 107 mbar. The hydrophobic property is important for water management in PEMFCs, therefore the contact angle of the coated SS 316 L surface with liquid water was measured using a VCA Optima XE instrument. A drop of water was dripped on the sample surface at room temperature, and the contact angle was calculated with the help of UTHSHCSA image tool software. The measurement was repeated for six times and then the average value was calculated.
Single cell tests In order to evaluate the performance in PEMFCs, the molybdenum carbide coated SS316L bipolar plates were fabricated and assembled with a MEA (i.e. membrane electrode assembly) for single cell test. The bipolar plates have a single serpentine type of flow channel design. After channel preparation, the plates were coated with ~100 nm thick Cr transition layer followed with coating of ~1 mm thick molybdenum carbide layer. The coating process was exactly the same with
SEM images in Fig. 2 show the surface morphology of the SS 316 L blank sample without coating and the one coated with molybdenum carbide. According to Fig. 2a, there were some micro-scratches on the surface of the pre-treated SS 316 L without coating before the electrochemical tests. After coating, the SS 316 L exhibited a compact, flat and homogeneous surface (Fig. 2b) without showing any scratches. This indicates that the coating can effectively cover the SS316L surface and eliminate the major defects like micro-scratches. The cross-section of the coated SS 316 L is shown in Fig. 2c. From the SEM picture, a dense coating layer was formed on the substrate without showing voids or delamination inbetween. A thin transition layer in-between the coating layer and the substrate can be observed under SEM for the cross-section of the sample Cr-MC1052, the composition of which was confirmed as chromium according to the EDS line scanning (Fig. 2d). Also, the EDS results revealed that the coating layer consisted of molybdenum and carbon. Fig. 2e shows the surface morphology of bare SS316L (MC0) after the electrochemical tests. There exists a number of micro-holes and pitting on the surface due to corrosion. In contrast, no pitting can be observed for the coated sample Cr-MC1052 (Fig. 2f) after test, indicating that the molybdenum carbide coatings can prevent the SS316L from the aggressive acid environment. The EDS elemental mapping analysis in Fig. 3 shows that the chemical elements of coating layer were mainly C and Mo, which distributed uniformly on the surface. The XRD characteristic peaks (Fig. 3c) of the SS316L substrate are indicated
Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Fig. 2 e a) SEM image of the bare surface of SS 316 L (MC0), b) SEM surface image of the coated sample Cr-MC1052, c) SEM cross-section view of Cr-MC1052, and d) EDS elemental analysis along the line in c), e) SEM image of bare SS316L (MC0) after test, and f) SEM image of the coated sample (Cr-MC1052) after test. by *. The peaks marked with * indicate mixed phases of molybdenum carbides with different stoichiometric composition most likely including Mo2C [PDF#31-0871, 35-0787], MoC [PDF#08-0384, 06-0546] and Mo3C2 [PDF#42-0890]. The X-ray photoelectron spectroscopy (XPS) measurement (Fig. 4) was conducted for the analysis of the surface composition and valence state of the coatings. As shown in Fig. 4a, the peak fitting of Mo 3 d profile suggested that there were four states (2þ, 3þ, 4 þ and 6þ) for Mo in the surface coating layer. The peaks located at 235.7 eV and 232.6 eV can be attributed to the Mo6þ and Mo4þ, respectively, which may result from the surface oxidation of coating materials during the electrochemical testing or when exposed to air [36,37]. The peaks located at 228.2 eV and 231.3 eV can be ascribed to the characteristic doublets of the Mo2þ state of Mo2C, which are in good agreement with the results reported in literature [38,39]. The deconvoluted C 1s XPS spectrum was shown in Fig. 4b. The peak at 284.8 eV was the characteristic of C]C [40], and the peaks at 286.3 eV and 282.9 eV were associated with the carbon species of C]O [41] and MoeC [41,42], respectively. The binding energy peak at 289.0 eV may be ascribed to OeC]O [43].
Contact angle In order to assess the hydrophobicity, which is an important property of BPs for water management in PEMFC stacks, the contact angle between molybdenum carbide coated SS 316 L plates and liquid water was measured. As shown in Fig. 5, the contact angle with water for the blank sample MC0 and the coated sample MC1052 was measured as 72.6 and 90.1 , respectively. This means the coated SS 316 L can display a more hydrophobic behavior than the bare SS 316 L. Also, compared with the Mo2N-coated SS 304 bipolar plates in literature [44] showing a contact angle of 68 , the molybdenum carbide coated SS 316 L BPs in this work exhibited much better hydrophobicity and is expected to benefit the water management in PEMFC stacks.
Adhesion test The adhesion test was carried out with a Nano Scratch Tester. The scratch length was 150 mm and the maximum scratch load was 50 mN. The results are shown in Fig. 6. It is observed in Fig. 6a that the sample MC1052 has a deeper displacement into
Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Fig. 3 e a) The element mapping by SEM/EDS for the molybdenum carbide coated SS 316 L,b) The green and violet colors indicate the element distribution of Mo and C, respectively, c) XRD characterization of the coated sample MC1052. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4 e XPS spectra of a) the Mo 3 d and b) the C 1s for molybdenum carbide coatings.
Fig. 5 e Contact angle measurement of a) the blank sample MC0 and b) the coated flat-plate sample Cr-MC1052. surface with the scratch going than Cr-MC1052. Fig. 6b shows a larger load on the sample of Cr-MC1052 than on MC1052, indicating more strength will be needed to destroy the coating
for Cr-MC1052. Therefore, it is concluded that the Cr transition layer can effectively enhance the bonding strength between the substrate and the coating layers.
Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Electrochemical test To simulate the anode environment The potentiodynamic polarization curves of the blank and the coated samples were measured in the solution of H2-bubbled 0.5 M H2SO4 þ 2 ppm HF at 70 C, which was conventionally used as simulation of the PEMFCs environment. The results are shown in Fig. 7. The bare SS 316 L plate shows a corrosion curve very typical for stainless steels, which includes a cathodic polarization region, an anode polarization region and a passive region [44,45]. The corrosion potential is associated with the thermodynamic stability of the coating, and is often used for representing the corrosion resistance. All the coated SS 316 L samples shows a higher corrosion potential in comparison to the bare sample. From a thermodynamics point of view, this increased corrosion potential indicates an improved chemical inertness and a better stability in the corrosion environment. Also, the corrosion current density can be calculated based on the extension of Tafel's slope on the polarization curves [46], and the values were listed in Table 3. The tests were performed on three specimens for each condition, and the corrosion current density has a deviation within range of ±1.02 107 A cm2. The bare SS 316 L plate shows a passive current density of 5.9 104 A cm2, which is too high to be used in PEMFCs. The corrosion current densities of the coated samples (approx. 1 106 A cm2 or even lower) were significantly smaller than that of the bare sample, indicating that the molybdenum carbide coating can effectively prevent the SS 316 L BPs from corrosion. Moreover, it can be observed that the higher thickness of the coating resulted in better corrosion resistance of the SS 316 L (Table 3). With increase of the coating thickness from 375 nm to 741 nm and 1052 nm, the corresponding corrosion current density decreased from 1.1 106 A cm2 to 0.49 106 A cm2 and finally down to 0.31 106 A cm2. The results indicate that a ~741 nm or ~1052 nm thick molybdenum carbide layer is sufficient for protecting the SS 316 L bipolar plates from corrosion in PEMFCs, exactly meeting the DOE's target on corrosion current density of below 1 106 A cm2. To further improve the anti-corrosion property and also enhance the adhesion between the coating and the substrate, a transition layer was adopted in the sample Cr-MC1052 by
Fig. 7 e Potentiodynamic polarization measurement in the H2-bubbled solution of 0.5 M H2SO4 þ 2 ppm HF for the SS 316 L with different molybdenum carbide coatings.
depositing a Cr thin-layer on the surface of SS 316 L prior to coating with molybdenum carbide. The potentiodynamic polarization was conducted under the same condition as before. The polarization curves and the derived corrosion current density are shown in Fig. 7 and Table 3 to compare with the samples having no transition layer. Employing the same molybdenum carbide coatings (1052 nm), the sample with the addition of a Cr transition layer exhibited a lower corrosion current density (2.3 107 A cm2) than the one without transition layer (3.1 107 A cm2), indicating an improved corrosion resistance for the SS 316 L bipolar plate. To investigate the durability of the coated samples in PEMFC environment, the potentiostatic polarization curves were measured at 0.1 V vs. SCE in the H2 bubbled solution of 0.5 M H2SO4 þ 2 ppm HF at 70 C. Fig. 8 shows a similar tendency in current density curves for the coated SS 316 L samples. The current density decreased in a relatively fast rate for the initial short period, and then gradually slowed down reaching a lower platform (<1 106 A cm2). The results confirmed that all coated samples were stable in the simulated anode environment with a sufficiently low corrosion rate meeting the criterion for PEMFC applications. Due to the protective coatings, the SS 316 L samples shows a stabilized
Fig. 6 e The adhesion test with Nano Scratch Tester for the molybdenum carbide coated SS316L plate with and without Cr transition layer: a) the displacement into surface and b) the load profile with change of the scratch distance. Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Table 3 e The anti-corrosion properties of SS 316 L BPs with different coating thickness and with the addition of a Cr transition layer tested in simulated anode condition. Samples
Coating thickness (nm) Corrosion Potential (mV vs SCE) Corrosion current density (A$cm2)
Molybdenum carbide coated SS 316 L MC0
MC375
MC741
MC1052
Cr-MC1052
0 283 5.9 104
375 221 1.1 106
741 187 4.9 107
1052 102 3.1 107
1052 45 2.3 107
current density at 1e2 orders of magnitude lower in comparison to the uncoated one (MC0). A lower current density indicated a higher stability and a longer life-time of the BPs. Apparently, increasing the coating thickness from 375 nm to 741 nm and 1052 nm effectively improved the corrosion resistance. The sample Cr-MC1052, which had a Cr transition layer deposited prior to coating of molybdenum carbide, exhibited the best anti-corrosion property of all (Fig. 8). The enhanced performance could be ascribed to the elimination of any possible tiny defect (such as nano-pinholes or voids) due to the transition layer as well as improvement of coatings’ adhesion to the substrate. The results are in consistent with those shown in Table 3.
To simulate the cathode environment The potentiodynamic polarization curves of the blank and the coated samples were measured in the solution of air-bubbled 0.5 M H2SO4 þ 2 ppm HF at 70 C. The results are shown in Fig. 9. Similar to the anode environment, the coated sample exhibits significantly better anti-corrosion performance than the bare SS316L in the simulated cathodic condition. The corrosion current density was measured to be 5.3 106 A cm2 for the bare SS316L. While for the coated sample (Cr-MC1052), an excellent corrosion current density of 9.1 108 A cm2 was achieved, almost two orders lower than the uncoated one. This corrosion current density is also smaller than measured for the anode condition (2.3 107 A cm2), indicating the bipolar plate of anode side is more prone to corrosion than cathode side. The results of the simulated cathodic corrosion tests are also presented in Table 4. Fig. 10 shows the results of potentiostatic polarization
Fig. 8 e Potentiostatic polarization measurement in the H2bubbled solution of 0.5 M H2SO4 þ 2 ppm HF for the SS 316 L with different molybdenum carbide coatings.
measurements in the air-bubbled solution at 0.6 V vs SCE. The sample Cr-MC1052 displays good stability of anti-corrosion during the test and finally levels off at 1.1 107 A cm2.
Interfacial contact resistance The results of the interfacial contact resistance (ICR) measurement were shown in Fig. 11. All samples including the bare and the coated SS 316 L show ICR of a similar tendency with increasing compaction force. The ICR decreased dramatically in the low force range (0e50 N cm2), then decreased slowly and finally stabilized at a lower platform with further increase of the compaction force. The explanation is given as below: at low compaction forces, the effective contact area is actually insufficient between the SS 316 L BPs and the carbon paper considering they are both rigid, resulting in a poor interfacial conduction or a high ICR value. With the increase of the compaction force and deformation of the carbon paper, the effective contacting points increase, providing more conduction paths and resulting reduced ICR. The bare SS 316 L shows a large ICR of 60 mU cm2 at a compaction force of 210 N cm2. In contrast, all the molybdenum carbides coated SS 316 L show significantly smaller ICR values down to ~10 mU cm2 or below. For MC1052 which had a ~1052 nm thick molybdenum carbide coating, the ICR was measured to be 7.9 mU cm2 at 210 N cm2. The ICR was further decreased to 6.5 mU cm2 when a Cr transition layer was adopted. The improved conduction could be attributed to the enhanced bonding strength between the substrate and the coating with help of the transition layer. Thin transition layers
Fig. 9 e Potentiodynamic polarization measurement in the air-bubbled solution of 0.5 M H2SO4 þ 2 ppm HF for the sample MC0 (bare SS316L) and Cr-MC1052.
Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Table 4 e The anti-corrosion properties of SS 316 L BPs with bare 316 L SS and Cr-MC1052 tested in simulated cathode condition. Samples
Coating thickness (nm) Corrosion Potential (mV) Corrosion current density (A$cm2)
Molybdenum carbide coated SS 316 L MC0
Cr-MC1052
0 271 5.3 106
1052 112 9.1 108
Fig. 12 e Interfacial contact resistances between GDL and the coated SS 316 L bipolar plates as a function of compaction force after potentiostatic polarization tests.
Fig. 10 e Potentiostatic polarization measurement in the air-bubbled solution of 0.5 M H2SO4 þ 2 ppm HF for the bare sample MC0 (bare SS316L) and coated sample CrMC1052.
Fig. 13 e Performance of the single fuel cell assembled with the coated SS316L bipolar plates (Cr-MC1052).
Fig. 11 e Interfacial contact resistance between GDL and the coated SS 316 L BPs as a function of compaction force.
are often used before film deposition and could serve as the so-called ‘‘seed layer’’ benefiting the crystallization of the coating layer [47]. The transition layers usually have an interdiffusion effect with substrate and coating material, enhancing the interfacial connection and conduction between them [47]. It is expected to facilitate the electrical conduction and the in-plane current distribution of the coating layer, which would in turn contribute to a smaller resistance of the coarse GDL texture contacting rigid bipolar plates. However, a
more clear and detailed explanation needs further investigations. After the potentiostatic polarization test, the contact resistance was measured again for examination of any property change in surface contacting conduction. The result is shown in Fig. 12. The ICR of the bare 316 L SS increases to 80 mU cm2 after the corrosion test, while the coated sample Cr-MC1052 still keeps a low contact resistance at 6.8 mU cm2, demonstrating good anti-corrosion property and stability. Considering the DOE's target of ICR <10 mU cm2, the molybdenum carbide coated SS 316 L in this work can satisfy the demand for a low ICR for commercializable metallic BPs.
Single cell test Finally, a single fuel cell has been assembled and tested for assessment of the as developed SS316L bipolar plates working in real PEMFCs. The bipolar plates have a single serpentine type of flow channel, and were coated with the same process as that for flat-plate samples of Cr-MC1052. The i-V performances and power density curve of the fuel cell are shown in
Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184
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Fig. 13. The fuel cell operating at 65 C with humidity of RH ¼ 80% for both electrodes demonstrated good performances. A cell voltage of 0.46 V was measured at the current density of 2.0 Acm2, where the maximum output power density reaches 0.92 Wcm2.
Conclusions Molybdenum carbide coatings were applied onto SS 316 L as protective coatings and successfully tested as bipolar plates of PEMFCs. The molybdenum carbide was deposited in different thicknesses by DC magnetron sputtering, and a smooth and dense surface was obtained without showing any major defect. The anti-corrosion property was investigated by potentiodynamic and potentiostatic polarization tests in the simulated PEMFC environments. The corrosion current of the SS 316 L was found to decrease with increasing the coating thickness of molybdenum carbide. Moreover, the thicker coating contributed to a lower interfacial contact resistance with GDL. The performance could be further improved with the adoption of a thin Cr transition layer on the SS 316 L prior to coating molybdenum carbide. A very low corrosion current density of 2.3 107 A cm2 was achieved in H2-bubbled solution of 0.5 M H2SO4 þ 2 ppm HF at 70 C and 9.1 108 A cm2 in Air-bubbled solution of 0.5 M H2SO4 þ 2 ppm HF at 70 C The coated SS316L BPs demonstrated good stability within a preliminary durability test. Also, a low interfacial contact resistance of 6.5 mU cm2 was obtained and after test the interfacial contact resistance still keeps a low interfacial contact resistance. Furthermore, the coated surfaces exhibited hydrophobic properties, which will benefit the water management in PEMFCs stacks. The overall evaluation suggested the molybdenum carbide coated SS 316 L can be a promising candidate for the metallic bipolar plates of PEMFCs.
Acknowledgements Thanks for Huhang Ma from the Materials Characterization and Preparation Center, Huqiang Yi and Xiaoming Zhang from the Department of Material Science and engineering, Southern University of Science and Technology, for guidance of the sputtering experiment and help on single cell testing. Thanks for the financial support by the Shenzhen Peacock Plan (KQTD2016022620054656), the National Key Research and Development Program of China (No. 2017YFB0102701), the Development and Reform Commission of Shenzhen Municipality (2017) (No.1106 and No.1181), the Shenzhen Key Laboratory Project (ZDSYS201603311013489) and the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2016ZT06N500).
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Please cite this article as: Wang L et al., Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.184