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The characteristics and performance of AISI 1045 steel bipolar plates with chromized coatings for proton exchange membrane fuel cells
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The characteristics and performance of AISI 1045 steel bipolar plates with chromized coatings for proton exchange membrane fuel cells Ching-Yuan Bai a,b,*, Tse-Min Wen c, Kung-Hsu Hou d,**, Nen-Wen Pu b, Ming-Der Ger b a
Graduate School of Engineering Technology, Lunghwa University of Science and Technology, Tau-Yuan 333, Taiwan, ROC Department of Chemical and Materials Engineering, Chung Cheng Institute of Technology, National Defense University, Tau-Yuan 335, Taiwan, ROC c School of Defense Science, Chung Cheng Institute of Technology, National Defense University, Tao-Yuan 335, Taiwan, ROC d Department of Power Vehicles and System Engineering, Chung Cheng Institute of Technology, National Defense University, Tao-Yuan 335, Taiwan, ROC b
article info
abstract
Article history:
The purpose of this study is to produce an anti-corrosive and highly conductive coating on
Received 4 November 2010
1045 steel using a rolling pretreatment along with low-temperature pack chromization.
Received in revised form
The results indicated that a uniform and dense chromized coating was successfully formed
15 December 2010
on the steel. The main constituent phases of the coating were carbides and the minor
Accepted 20 December 2010
phases were chromiumeferric nitrides and oxides. The modified steel plates were used as
Available online 23 January 2011
metal bipolar plates (BPPs) for proton exchange membrane fuel cells (PEMFCs) and their performance was compared with that of conventional graphite BPPs. The contact angle of
Keywords:
water on the rolled-chromized steel BPPs was 98.3 . In addition, the power density of the
AISI 1045 steel
single cells assembled with rolled-chromized steel BPPs was 0.51 W cm2, comparable to
Low-temperature pack chromiza-
those with graphite (0.50 W cm2), in the test conditions of this study.
tion
Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Contact angle
reserved.
Power density
1.
Introduction
Proton exchange membrane fuel cells (PEMFCs) are considered a very promising energy source for electric vehicles. However, the high cost of materials limits the commercial application of PEMFCs. As a multifunctional component, bipolar plates (BPPs) of PEMFCs account for about 80% of the total weight and 45% of the stack cost [1]. Hence, development of inexpensive, thin, and light metallic BPPs is necessary to
meet the requirements for commercial applications of PEMFCs. The materials investigated for the construction of BPPs include graphite, metal, and graphite-polymer composites, among which graphite is the most commonly used due to its low surface contact resistance and high corrosion resistance. However, graphite and graphite composites are brittle and permeable to gases, and cannot be machined to form thin plates with gas channels on each side. Metallic BPPs have received considerable attention due to their high electrical
* Corresponding author. Graduate School of Engineering Technology, Lunghwa University of Science and Technology, Tau-Yuan 333, Taiwan, ROC. Tel.: þ886 2 82093211; fax: þ886 2 82094650. ** Corresponding author. Tel.: þ886 3 3800969; fax: þ886 3 3895570. E-mail addresses: [email protected] (C.-Y. Bai), [email protected] (K.-H. Hou). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.12.110
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conductivity, acceptable material cost, relatively high mechanical strength, and low gas permeability [2]. Although metallic BPPs possess many advantages, they suffer corrosion attack in the slightly acidic environment of PEMFCs. The metallic ions dissociated from metal plates will further poison the membrane and result in the reduction of cell performance [3]. Stainless steel is considered one of the promising candidates for BPPs. On account of its self passivating ability, stainless steel can usually prevent further corrosion by forming a passive film on the surface. Although the passive film can decrease the corrosion rate of stainless steel, it will significantly increase the interfacial contact resistance (ICR) between the BPP and the gas diffusion layer [4]. Recent research studies have explored the preservation of surface conductivity and the chemical stability of metallic BPPs with conductive and corrosion-resistant coatings such as gold, titanium, and chromium nitrides [5e12]. For BPPs in general, high surface tension at the liquidesolid interface is required to increase the water contact angle on the surface and avoid flooding and power degradation [13,14]. Additionally, Liao et al. [15] also pointed out that nanocomposite BPPs possessing relatively small contact angle with water may have difficulty in water transfer at the high current density region. In contrast, hydrophobic graphite BPPs with a higher water contact angle could repel water from the cathode easily and thus exhibit higher current density. Therefore, metallic BPPs have to be modified in order to improve their corrosion resistance, conductivity, and contact angle. The manufacturing cost for the PEMFC components must be reduced to make this technology commercially viable. Thus, considering the cost as well as the performance, AISI 1045 steel was employed as the substrate material in this study to carry out a pack chromization process for the application to BPPs. Pack cementation is one of the thermo-reactive deposition methods, which have been widely used in industry for a long time. However, the conventional operation temperature for pack cementation is always above 1000 C, which brings about variations in mechanical properties, phases, and even configuration of substrates. Therefore, reducing the process temperature is an issue worth paying attention to. Besides, it can help saving the production cost. In general, the operation temperature can be decreased by activating the surface of the base metal [16]. In this work, a rolling process is adopted as the activating pretreatment. Afterward, low-temperature pack chromization is conducted on the rolled steels to form a corrosion-resistant coating. This study investigates the structures and compositions of the rolled-chromized coatings for the application to BPPs in PEMFCs. Moreover, the contact angle of water on the coating surface and the performance of the chromized specimens will be discussed.
2.
thickness, was set at 25%. Prior to pack cementation, the substrate were ground with SiC abrasive papers and ultrasonically cleaned in acetone, and then degreased in ethanol. Next, the substrate was loaded into an alumina crucible with the pack powder mixture and placed the crucible in a furnace evacuated to 103 torr to prevent the substrate from oxidation. The pack powder mixture used as chromizing deposition source contained a master metal (Cr), activator (NH4Cl), and inert filler (Al2O3). The powder mixture was mixed up in accordance with a suitable proportion and homogenized using ball mill for 12 h. The specimens were buried in pack powders filled in a cylindrical alumina crucible. The pack was heated from room temperature to 150 C at a rate of 10 C min1 and the chromization was conducted at the temperature of 700 C for 2 h. Two types of coatings were prepared for comparison: (1) by simple chromization without rolling, or (2) by rolling and chromization. They were named 1045-Cr(700-2) and 1045-R-Cr (700-2), respectively. The surface and cross-sectional images of the coatings were observed using scanning electron microscopy (SEM). The superficial composition of the coatings was characterized by Xray photoelectron spectroscopy (XPS). The C 1s, N 1s, O 1s, Cr 2p3/2, and Fe 2p3/2 spectra (peak intensity versus binding energy (B.E.)) at different etching times were also examined in this experiment. The Al Ka line was used as the X-ray source and set at 10 kV and 20 mA. An argon gun was used to etch the surface to study the depth profile of the film. The estimated sputtering ˚ min1. The quantitative rate for XPS analysis was 60 A compositions of surface film were calculated by using the area sensitivity factor from the equipment handbook. Furthermore, the contact angle of the specimens with water was measured by a FACE-CA-D type contact angle measurement system in order to analyze the hydrophobicity of coatings. Fig. 1 shows a rolled-chromized 1045 steel bipolar plate with an active area of 25 cm2. A single serpentine flow field
Experiments
The raw material used in this work was a commercial AISI 1045 steel with 0.467 percent of carbon. Specimens were rolled and then cut into square sheets with the dimensions of 30 mm 30 mm 2 mm. The processing ratio, defined as the reduced thickness after rolling divided by the original
Fig. 1 e The 1045-R-Cr(700-2) BPPs machined with a single serpentine flow field. The active area is 25 cm2.
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was machined onto its surface. Single PEM cells were fabricated with the steel BPPs. The membrane electrode assembly (MEA) consisted of Nafion 112 membranes and Torry paper with a catalyst loading of 0.2 mg cm2 on the anode and 0.4 mg cm2 on the cathode. A 200-mm thick silicone rubber film was used as a gasket to prevent gas leakage. The end plates made of glass fiber were machined in order to form the gas ports and installation holes of the cell. In addition, Au electroplated copper plates were used as the current collector. Subsequently, every bolt was evenly loaded with a compaction force of 20 kg to assemble the components mentioned above. A finished single PEM cell is shown in Fig. 2. The IeV and IeP curves for the single cells were measured and plotted in the operation range of 0.9e0.35 V. The long-term performance tests of the cells were conducted at an operation voltage of 0.6 V for 100 h. The as-built single cells were operated at 60 C under ambient pressure. Pure hydrogen and air completely humidified at 70 C were used as the reactant gases at the anode and cathode sides, respectively. The flow rate of the anode and cathode gases (QA and QC) was controlled at 300 cm3 min1. The residue gas in the cell was expelled by a nitrogen purge before the cell operation. Moreover, the metallic ions dissociated from the BPPs in the water by-product and in the MEA were examined by an inductively coupled plasma-mass spectrometer (ICP-MS) and XPS, respectively.
3.
Results and discussion
3.1.
Microstructure and composition of coatings
Fig. 3a and b show the SEM surface images and EDS spectra of 1045-R-Cr(700-2) bipolar plates, respectively. It is clearly found that the surface of rolled-chromized steel exhibits a granular morphology, and the coating contains a significant amount of Cr-carbide and some Cr-nitride and oxide distributing near the surface of the coating. The cross-sectional SEM image of 1045-R-Cr(700-2) is shown in Fig. 4. It reveals that a uniform and continuous chromized layer was deposited on the steel
Fig. 3 e (a) The SEM surface image and (b) EDS analysis of 1045-R-Cr(700-2).
surface. A pearlite structure comprising a-Fe (dark) and Fe3C (white) phases was observed under the coating. In particular, the textural structure of the steel substrate resulting from the rolling pretreatment almost disappeared in the micrograph of 1045-R-Cr(700-2) due to the constituent rearrangement under thermal energy. The coating thickness of 1045-R-Cr (700-2) is approximately 2 mm. The cross-sectional structure of the 1045Cr(700-2) specimen was also observed (not shown). It is found
Fig. 2 e The prototype single cell stack with chromized steel BPPs; (a) front view and (b) lateral view.
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Fig. 4 e The cross-sectional SEM image of 1045-R-Cr(700-2).
that the 1045-Cr(700-2) coating structure is similar to that of 1045-R-Cr(700-2). However, the coating thickness of 1045-Cr (700-2) is only about 1.25 mm. Thus, it is interesting to note that a combination of rolling work and chromization can generate a thicker coating than a simple chromization process. During the rolling process, severe plastic deformation gives rise to the formation of dislocations with an extremely high density and even a large amount of new grain boundaries, which may serve as numerous fast diffusion “channels” for Cr atoms. In addition, grain-boundary diffusion is the dominant mechanism when the operation temperature of pack cementation ranges from 500 C to 700 C. Hence, the pretreatment with rolling work apparently not only accelerates the diffusivity of Cr but also reduces the working temperature. Fig. 5 displays the glancing incident XRD results of 1045-R-Cr(700-2), and it reveals that the primary products in these coatings are carbides, including (Cr, Fe)23C6, (Cr, Fe)7C3, and Cr2C3. But a minor (Cr, Fe)2N1x phase is also detected in these specimens [39], which coincides with the EDS result. Fig. 6 shows the C 1s, Cr 2p3/2, N 1s, O 1s, and Fe 2p3/2 spectra in the XPS analyses for 1045-Cr(700-2). The binding energy of C1s was 285 eV at the etching time of 0 s, which is
1200
+
Relative intensity
1100 1000 900
Cr2N+(Cr,Fe)2N1-x Cr2C3 (Cr,Fe)7C3 (Cr,Fe)23C6 -Fe -FeCr
800 +
700 600 500 400 30
35
40
45
50
55
60
65
2θ Fig. 5 e XRD patterns of 1045-R-Cr(700-2). [39].
70
attributed to the contaminated carbon on the surface. As the etching time increased from 900 s to 8100 s, the binding energy (282.6e282.9 eV) of metal carbides was detected. All binding energies of the Cr 2p3/2, except the one on the top surface detected at the etching time of 0 s, were in the range of 574.8e574.5 eV, which are found to be in forms of chromium carbide and chromium-ferric carbides,. According to the literature [17,18], if the energy resolution of electron spectrometer is not good enough, the contributions of compounds can’t be resolved. The difference of Cr 2p3/2 binding energy in the carbide configuration is only 0.3 eV, while the resolution of the electron spectrometer is also 0.3 eV for the equipment in this work. It is therefore determined that the configuration of chromium carbide and chromium-ferric carbide definitely exists in the coatings. The binding energy of N 1s core-level electrons, ranged from 397.0 eV to 397.3 eV at the etching time of 900e2700 s, corresponds to chromiumeferric nitrides. The result also indicated that the thickness of nitride layer in the 1045-Cr(700-2) coating is approximately 270 nm. The binding energy of O 1s peak varied from 530.5 eV to 531.1 eV. Based on the reference data of binding energy for chromium oxides (530.2e531.0 eV) and chromium hydroxides (530.85 eV), it is speculated that small amounts of chromium oxide and chromium hydroxide exist in the coatings. The signal of Fe 2p3/2 on the surface is too weak to make any judgment. As the etching time increased, the photoelectron peak of Fe 2p3/2 shifted slightly from 708.6 eV to 708.3 eV, indicating that some ferric oxides with different valence states exist in the coatings. The C 1s, Cr 2p3/2, N 1s, O 1s, and Fe 2p3/2 spectra in XPS analyses for the 1045-R-Cr(700-2) specimen are shown in Fig. 7. The signal of C 1s photoelectron at the etching time of 0 s is mainly associated with the surface contamination of carbon, hydrogen, and oxygen. As the etching time increased from 900 s to 8100 s, the C 1s binding energy peak was located at 283.5e283.8 eV, which can be ascribed to carbides existing in the coating. This result is in agreement with the previous literature in which the binding energy of Cr3C2 was reported in the range of 283.2e283.6 eV [19]. The signal of Cr 2p3/2 on the surface is too weak to be detected. As the etching time increased to the interval between 900 s and 7200 s, the peak shifted to 576.1 eV, which could be identified as Cr(CO)6 [20]. The N 1s binding energy peak ranging from 397.2 eV to 397.8 eV at the etching time of 900e3600 s is related to the formation of metal nitride with a depth of about 360 nm [21] in the coatings. This is caused by the nitridation reaction of chromium and iron, in which the nitrogen is decomposed from the activatoreNH4Cl. The signal of Fe 2p3/2 photoelectron at the etching time of 0e900 s is too weak to be detected, indicating that the Fe concentration is very low in the superficial layer of the coatings. After 1800 s etching time, the signal was ascribed to ferric oxide [22], which may be dispersed in the chromium carbide base and cause a decrease in the conductivity of coatings. Furthermore, the binding energy of O 1s core-level electron suggested that the oxygen was mainly bound to carbon, chromium, and hydrogen [23] in the depth range of 0e180 nm beneath the surface of coatings. In summary, the XPS results of 1045-Cr(700-2) and 1045-R-Cr(700-2) indicated that the primary products in the chromized coatings are carbides, and some minor products such as nitrides and oxides are also detected in the coatings, which is consistent with the literature [11].
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Fig. 6 e XPS C (1s), Cr (2p3/2), N (1s), Fe (2p3/2), and O (1s) spectra of 1045-Cr(700-2).
3.2.
Contact angle test
Water management is an important factor that affects the cell performance in PEMFC systems. While a PEM fuel cell is operated, the by-product, water, produced by the cathode reaction will accumulate at the flow field. If water could not be removed in time, it will block the channel through which the reactant gases access to the electrode, and result in reduced performance of PEM fuel cells [24]. Generally, a hydrophobic material exhibits a contact angle more than 90 between liquidevapor and liquidesolid surfaces as a water droplet is placed onto its surface [25]. Hence, bipolar plates have to possess excellent hydrophobicity with a water contact angle higher than 90 to avoid flooding and power degradation. The contact angle images of bare, 1045-Cr(700-2), and 1045-R-Cr(700-2) steel samples are shown in Fig. 8. It is observed that the contact angles of water on 1045-R-Cr(700-2) and 1045-Cr(700-2) specimens are 98.3 and 94.0 , respectively, both of which are bigger than that of bare material (84.6 ). Referring to previous studies [26,27], the
superficial composition and surface roughness of coatings are the main factors that affect the water-solid contact angles. The values of surface roughness for bare 1045, 1045-Cr(700-2), and 1045-R-Cr(700-2) measured by a profilometer are 0.14 mm, 0.12 mm, and 0.13 mm, respectively, which are very small and close. Hence, it is speculated that the dominant factor which affects the contact angle isn’t surface roughness but rather superficial composition of the BPPs in this work. The Fe2O3 compound covering the surface of bare 1045 steel would possess better affinity with water than the chromium carbide on the surface of the chromized specimens because oxygen is more electronegative than carbon. For this reason, the hydrophobic property of bare 1045 steel is the worst among the tested specimens. In contrast, the coatings containing primarily chromium carbides show better hydrophobicity. The quantitative compositions at the coating surface can be calculated from the XPS spectra by using the area sensitivity factor in the equipment handbook. As can be seen in Table 1, the chromium concentration in the 1045-R-Cr(700-2) and 1045-Cr(700-2) coatings are
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Fig. 7 e XPS C (1s), Cr (2p3/2), N (1s), Fe (2p3/2), and O (1s) spectra of 1045-R-Cr(700-2).
61.9 at.% and 57.21 at.%, respectively. In addition, the oxygen concentration in 1045-Cr(700-2) (1.41 at.%) is higher than that of 1045-R-Cr(700-2) (0.44 at.%). It indicates that the coatings of 1045-R-Cr(700-2) contain a larger amount of chromium carbide and a smaller amount of chromium oxide than the 1045-Cr(7002) specimens. Therefore, 1045-R-Cr(700-2) has a bigger contact angle with water and a better hydrophobicity than 1045-Cr(7002). The results of contact angle tests reveal that low-temperature pack chromization in combination with a rolling pretreatment can significantly enhance the hydrophobic property of the 1045 steel BPPs and improve the performance of single cells.
3.3.
Single cell performance
The initial IeV and IeP curves of single cells assembled with rolled-chromized steel, simple chromized steel, 1045 steel, and graphite BPPs are compared in Fig. 9. The results show that the open circuit voltage (OCV) of the cells fabricated with
1045-R-Cr(700-2), 1045-Cr(700-2), and graphite is 0.948 V, 0.945 V, and 0.947 V, respectively. The voltage and performance for the single cell with 1045-R-Cr(700-2) BPPs are almost equivalent to those with graphite BPPs at low and middle current densities. At a cell voltage of 0.6 V, the current density of the single cells assembled with 1045-R-Cr(700-2) and graphite BPPs are 671.6 and 679.2 mA cm2, respectively, which are very close. However, the cell with 1045-Cr(700-2) BPPs showed inferior performance at low and middle current densities. The maximum power densities for the single cells assembled with 1045-R-Cr(700-2) or 1045-Cr(700-2) BPPs are 0.51 and 0.47 W cm2, respectively. The performance of the single cell with 1045-R-Cr(700-2) BPPs is very close to that with graphite BPPs (0.50 W cm2) in the test conditions of this work. The properties of BPPs that affect the performance of fuel cells are the corrosion resistance, interfacial contact resistance, and hydrophobicity, which are generally related to the superficial composition of coatings. The average elemental
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PREN ¼ %Cr þ 3:3ð% MoÞ þ 16ð% NÞ The superficial layer of 1045-R-Cr(700-2) possessed higher nitrogen concentration (1.02 at.%) than that of 1045-Cr(700-2) (0.44 at.%). Therefore the 1045-R-Cr(700-2) coatings with larger amounts of nitride are expected to show better corrosion resistance in the severe PEMFC environments. Additionally, it
has been reported in the literature that nitrides produced by many methods on different substrate exhibit good electrical conductivity, which is important for BPPs [10,36,37]. Based on the constituent analyses of the chromized specimens, it is determined that the coatings produced with a rolling pretreatment along with low-temperature pack chromization possess better corrosion resistance, conductivity [38], and hydrophobicity than the simple chromized steels. These results conform to the performance of single cells exactly. In our previously published work, three kinds of Fe-based alloys, SS 316, SS 420, and SS 430 steel, treated with rolling and low-temperature pack chromization have been evaluated for the application of BPPs in PEMFCs [39]. In that study, the single cell with 420-R-Cr(700-2) BPPs showed the highest peak power density of 0.46 W cm2, which is lower than that of the cell with 1045-R-Cr(700-2) BPPs here. Therefore, the single cell
1045-R-Cr(700-2)
0.60 0.55 0.50 0.45
2
concentrations of the coatings on the 1045-Cr(700-2) and 1045R-Cr(700-2) specimens in the depth range of 0e600 nm beneath the surface are listed in Table 1. The chromium concentration of 1045-R-Cr(700-2) is obviously higher than that of 1045-Cr(700-2). The higher the Cr concentration in the coating, the better the corrosion resistance [28,29]. Moreover, the concentration of Fe in the coatings would also affect the corrosion resistance of the chromized steel BPPs. The Fe in the coatings would be selectively dissolved [30] due to the higher mobility of iron cations as compared to chromium cations in the passive film [31,32]. The Fe concentrations in the 1045-RCr(700-2) and 1045-Cr(700-2) coatings are 2.41 at% and 7.13 at %, respectively, so more Fe cations would be dissociated from the coating of a simple chromized specimen than from that of a rolled-chromized specimen. These ions will further poison the MEA. According to the literature [33], addition of nitrogen to a molybdenum-free stainless steel is beneficial for the development of passivity when the stainless steel is immersed in a sulfuric acid solution. Moreover, the addition of nitrogen will also remarkably increase the resistance to pitting and crevice corrosion, as is indicated clearly in the pitting resistance equivalent number (PREN) defined by the following equation [34,35].
Power density(Wcm )
Fig. 8 e The contact angle images of the (a) 1045, (b) 1045-Cr (700-2), and (c) 1045-R-Cr (700-2) specimens with water.
Fig. 9 e IeP and IeV curves of the single cells assembled with rolled-chromized steel, simple chromized steel, 1045 steel, and graphite BPPs; cell temperature [ 60 C; QA [ QC [ 300 cm3 minL1.
0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0
Table 1 e Average elemental concentrations of the coatings in the depth range of 0e600 nm beneath surface. Specimens names C(at%) Cr(at%) Fe(at%) N(at%) O(at%) 1045-R-Cr(700-2) 1045-Cr(700-2)
34.23 34.48
61.9 57.21
2.41 7.13
1.02 0.44
0.44 1.41
1000
2000
3000
4000
5000
6000
Time(min)
Fig. 10 e Long-term performance of the single cell assembled with 1045-R-Cr(700-2) BPPs measured at a cell voltage of 0.5 V; cell temperature [ 60 C; QA [ QC [ 300 cm3 minL1.
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Table 2 e The concentrations of metallic ions in the water produced from 1045-R-Cr (700-2) BPPs after the 100-h cell test. Elements
Concentration (ppm)
Cr Fe Ni
0.08717 0.06634 0.07548
with rolled-chromized 1045 steel exhibits much improved performance and is more suitable for the application to PEMFCs. In order to evaluate the long-term performance, the cell with 1045-R-Cr(700-2) BPPs was tested for 100 h. The current density at a cell voltage of 0.6 V was plotted as a function of operation time in Fig. 10. During the entire operation period for 100 h, the power density of the cell with 1045-R-Cr(700-2) BPPs stayed in the range of 0.54e0.50 W cm2, indicating that the cell was operating without distinct performance degradation.
a
Cr
1400
Cr2p3/2 Counts / s
1350 1300 1250
4.
1200 1150 594 592 590 588 586 584 582 580 578 576 574 572
Binding Energy (eV)
b
Fe
Fe2p3/2
3000 2800
Counts / s
2600 2400 2200 2000 1800 1600 1400 730
720
710
Binding Energy (eV)
c
Specifically, a fluctuating phenomenon appeared on the testing curve. This was due to the temporary accumulation and quick removal of the water on the flow field, which was produced by the cathode reaction. Moreover, typical PEM fuel cell atmospheres are reported to be weakly acidic (pH 3.0e5.0) due to the presence of SO2 4 , Cl , F , etc., ions, resulting in a great source of metallic ions, such as Cr, Fe, Ni, which are dissolved from BPPs and will contaminate the proton exchange membranes. In order to estimate the contamination effects of metallic ions dissociated from BPPs on proton exchange membranes during the operation time, the concentrations of metallic ions in the water, which was produced after the operation of the single cell for 100 h, were measured by ICP-MS, and the results are shown in Table 2. The concentrations of Cr, Fe, and Ni ions are 0.08717, 0.06634, and 0.07548 ppm, respectively. Apparently, the concentrations of metallic ions are very small. In addition, the surface composition of MEA is also examined by XPS analysis. The binding energy profiles of Cr 2P3/2, Fe 2P3/2, and Ni 2P3/2 are shown in Fig. 11. Apparently the concentrations of Cr (Fig. 11a), Fe (Fig. 11b), and Ni (Fig. 11c) on the surface of MEA are too low to be detected. Although the contamination of metallic ions is very little in the test results, current efforts towards longer operation test of the single cells are still in progress in our laboratory. Relevant performance and contamination effects of metallic ions will be investigated and presented in the future.
Ni
4400
Ni2p3/2
Conclusions
AISI 1045 steel was used as the substrate for surface modification by rolling pretreatment in combination with lowtemperature pack chromization, and its properties and performance for the application to PEMFCs were evaluated. The XPS results indicate that the composition of 1045-R-Cr (700-2) is composed of chromium carbides, chromium nitrides, and chromium oxides. The atomic concentrations of Cr and O in the 1045-R-Cr(700-2) coating are 61.9% and 0.44%, respectively. The contact angle of water on 1045-R-Cr(700-2) is the highest among all specimens due to the higher chromium carbide and lower oxide contents in the superficial layer of the coatings. The single cell with rolled-chromized BPPs exhibited a peak power density of 0.51 W cm2 at the initial stage of cell operation, which is very close to that of the cell with graphite BPPs (0.50 W cm2). Based on the excellent performance presented above, the rolled-chromized AISI 1045 steel could be considered a good candidate for BPPs in PEMFC applications.
4200
Counts / s
4000
references
3800 3600 3400 3200 3000 2800 890
880
870
860
850
Binding Energy (eV)
Fig. 11 e The XPS binding energy profiles of (a) Cr, (b) Fe, and (c) Ni on the surface of MEA after the single cell operation for 100 h.
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[5] Wind J, Spah R, Kaiser W, Bohm G. Metallic bipolar plates for PEM fuel cells. J Power Sources 2002;105:256e60. [6] Hermann A, Chaudhuri T, Spagnol P. Bipolar plates for PEM fuel cells: a review. Int J Hydrogen Energy 2005;30:1297e302. [7] Tawfik H, Hung Y, Mahajan D. Metal bipolar plates for PEM fuel cellea review. J Power Sources 2007;163:755e67. [8] Wang H, Brady MP, More KL, Meyer III HM, Turner JA. Thermally nitrided stainless steels for polymer electrolyte membrane fuel cell bipolar plates, part 2: beneficial modification of passive layer on AISI446. J Power Sources 2004;138:79e85. [9] Jung HY, Huang SY, Ganesan P, Popov BN. Performance of gold-coated titanium bipolar plates in unitized regenerative fuel cell operation. J Power Sources 2009;2:972e5. [10] Liu J, Chen F, Chen YG, Zhang DM. Plasma nitrided titanium as a bipolar plate for proton exchange membrane fuel cell. J Power Sources 2009;187:500e4. [11] Bai CY, Ger MD, Wu MS. Corrosion behaviors and contact resistances of the low-carbon steel bipolar plate with a chromized coating containing carbides and nitrides. Int J Hydrogen Energy 2009;34:6778e89. [12] Fu Y, Lin G, Hou M, Wu B, Li H, Hao L, et al. Optimized Cr-nitride film on 316L stainless steel as proton exchange membrane fuel cell bipolar plate. Int J Hydrogen Energy 2009;34:453e8. [13] Pozio A, Silva RF, Francesco MD, Giorgi L. Nafion degradation in PEMFCs from end plate iron contamination. Electrochim Acta 2003;48:1543e9. [14] Cho EA, Jeon US, Ha HY, Hong SA, Oh IH. Characteristics of composite bipolar plates for polymer electrolyte membrane fuel cells. J Power Sources 2004;125:178e82. [15] Liao SH, Yen CY, Weng CC, Lin YF, Ma CC. Preparation and properties of carbon nanotube/polypropylene nanocomposite bipolar plates for polymer electrolyte membrane fuel cells. J Power Sources 2008;185:1225e32. [16] Wang ZB, Lu J, Lu K. Chromizing behaviors of a low carbon steel processed by means of surface mechanical attrition treatment. Acta Mater 2005;53:2081e9. [17] Tabet N, Allam I, Yin RC. X-ray photoelectron spectroscopy investigation of the carburization of 310 stainless steel. Appl Surf Sci 2003;220:259e72. [18] Detroye M, Reniers F, Buess-Herman C, Vereecken J. AESeXPS study of chromium carbides and chromium iron carbides. Appl Surf Sci 1999;144e145:78e82. [19] Schmidt C, Oetzmann H, Hess P, Nowak R. XPS characterization of chromium films deposited from Cr(CO)6 at 248 nm. Appl Surf Sci 1989;43:11e6. [20] Barber M, Connor JA, Guest MF. On core level shifts and the charge distribution in substituted metal carbonyls. J Electron Spectros Relat Phenomena 1972;1:110e2. [21] Kovac CA, Clabes JG, Goldberg MJ. Metal-polymer chemistry. 2. Chromium-polyimide interface reactions and related organometallic chemistry. J Vac Sci Technol A 1988;6:991e6. [22] Briggs D, Seah MP. Practical surface analysis. 2nd ed., vol. 1. John Willey & Sons; 1993.
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The characteristics and performance of AISI 1045 steel bipolar plates with chromized coatings for proton exchange membrane fuel cells Ching-Yuan Bai a,b,*, Tse-Min Wen c, Kung-Hsu Hou d,**, Nen-Wen Pu b, Ming-Der Ger b a
Graduate School of Engineering Technology, Lunghwa University of Science and Technology, Tau-Yuan 333, Taiwan, ROC Department of Chemical and Materials Engineering, Chung Cheng Institute of Technology, National Defense University, Tau-Yuan 335, Taiwan, ROC c School of Defense Science, Chung Cheng Institute of Technology, National Defense University, Tao-Yuan 335, Taiwan, ROC d Department of Power Vehicles and System Engineering, Chung Cheng Institute of Technology, National Defense University, Tao-Yuan 335, Taiwan, ROC b
article info
abstract
Article history:
The purpose of this study is to produce an anti-corrosive and highly conductive coating on
Received 4 November 2010
1045 steel using a rolling pretreatment along with low-temperature pack chromization.
Received in revised form
The results indicated that a uniform and dense chromized coating was successfully formed
15 December 2010
on the steel. The main constituent phases of the coating were carbides and the minor
Accepted 20 December 2010
phases were chromiumeferric nitrides and oxides. The modified steel plates were used as
Available online 23 January 2011
metal bipolar plates (BPPs) for proton exchange membrane fuel cells (PEMFCs) and their performance was compared with that of conventional graphite BPPs. The contact angle of
Keywords:
water on the rolled-chromized steel BPPs was 98.3 . In addition, the power density of the
AISI 1045 steel
single cells assembled with rolled-chromized steel BPPs was 0.51 W cm2, comparable to
Low-temperature pack chromiza-
those with graphite (0.50 W cm2), in the test conditions of this study.
tion
Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Contact angle
reserved.
Power density
1.
Introduction
Proton exchange membrane fuel cells (PEMFCs) are considered a very promising energy source for electric vehicles. However, the high cost of materials limits the commercial application of PEMFCs. As a multifunctional component, bipolar plates (BPPs) of PEMFCs account for about 80% of the total weight and 45% of the stack cost [1]. Hence, development of inexpensive, thin, and light metallic BPPs is necessary to
meet the requirements for commercial applications of PEMFCs. The materials investigated for the construction of BPPs include graphite, metal, and graphite-polymer composites, among which graphite is the most commonly used due to its low surface contact resistance and high corrosion resistance. However, graphite and graphite composites are brittle and permeable to gases, and cannot be machined to form thin plates with gas channels on each side. Metallic BPPs have received considerable attention due to their high electrical
* Corresponding author. Graduate School of Engineering Technology, Lunghwa University of Science and Technology, Tau-Yuan 333, Taiwan, ROC. Tel.: þ886 2 82093211; fax: þ886 2 82094650. ** Corresponding author. Tel.: þ886 3 3800969; fax: þ886 3 3895570. E-mail addresses: [email protected] (C.-Y. Bai), [email protected] (K.-H. Hou). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.12.110
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conductivity, acceptable material cost, relatively high mechanical strength, and low gas permeability [2]. Although metallic BPPs possess many advantages, they suffer corrosion attack in the slightly acidic environment of PEMFCs. The metallic ions dissociated from metal plates will further poison the membrane and result in the reduction of cell performance [3]. Stainless steel is considered one of the promising candidates for BPPs. On account of its self passivating ability, stainless steel can usually prevent further corrosion by forming a passive film on the surface. Although the passive film can decrease the corrosion rate of stainless steel, it will significantly increase the interfacial contact resistance (ICR) between the BPP and the gas diffusion layer [4]. Recent research studies have explored the preservation of surface conductivity and the chemical stability of metallic BPPs with conductive and corrosion-resistant coatings such as gold, titanium, and chromium nitrides [5e12]. For BPPs in general, high surface tension at the liquidesolid interface is required to increase the water contact angle on the surface and avoid flooding and power degradation [13,14]. Additionally, Liao et al. [15] also pointed out that nanocomposite BPPs possessing relatively small contact angle with water may have difficulty in water transfer at the high current density region. In contrast, hydrophobic graphite BPPs with a higher water contact angle could repel water from the cathode easily and thus exhibit higher current density. Therefore, metallic BPPs have to be modified in order to improve their corrosion resistance, conductivity, and contact angle. The manufacturing cost for the PEMFC components must be reduced to make this technology commercially viable. Thus, considering the cost as well as the performance, AISI 1045 steel was employed as the substrate material in this study to carry out a pack chromization process for the application to BPPs. Pack cementation is one of the thermo-reactive deposition methods, which have been widely used in industry for a long time. However, the conventional operation temperature for pack cementation is always above 1000 C, which brings about variations in mechanical properties, phases, and even configuration of substrates. Therefore, reducing the process temperature is an issue worth paying attention to. Besides, it can help saving the production cost. In general, the operation temperature can be decreased by activating the surface of the base metal [16]. In this work, a rolling process is adopted as the activating pretreatment. Afterward, low-temperature pack chromization is conducted on the rolled steels to form a corrosion-resistant coating. This study investigates the structures and compositions of the rolled-chromized coatings for the application to BPPs in PEMFCs. Moreover, the contact angle of water on the coating surface and the performance of the chromized specimens will be discussed.
2.
thickness, was set at 25%. Prior to pack cementation, the substrate were ground with SiC abrasive papers and ultrasonically cleaned in acetone, and then degreased in ethanol. Next, the substrate was loaded into an alumina crucible with the pack powder mixture and placed the crucible in a furnace evacuated to 103 torr to prevent the substrate from oxidation. The pack powder mixture used as chromizing deposition source contained a master metal (Cr), activator (NH4Cl), and inert filler (Al2O3). The powder mixture was mixed up in accordance with a suitable proportion and homogenized using ball mill for 12 h. The specimens were buried in pack powders filled in a cylindrical alumina crucible. The pack was heated from room temperature to 150 C at a rate of 10 C min1 and the chromization was conducted at the temperature of 700 C for 2 h. Two types of coatings were prepared for comparison: (1) by simple chromization without rolling, or (2) by rolling and chromization. They were named 1045-Cr(700-2) and 1045-R-Cr (700-2), respectively. The surface and cross-sectional images of the coatings were observed using scanning electron microscopy (SEM). The superficial composition of the coatings was characterized by Xray photoelectron spectroscopy (XPS). The C 1s, N 1s, O 1s, Cr 2p3/2, and Fe 2p3/2 spectra (peak intensity versus binding energy (B.E.)) at different etching times were also examined in this experiment. The Al Ka line was used as the X-ray source and set at 10 kV and 20 mA. An argon gun was used to etch the surface to study the depth profile of the film. The estimated sputtering ˚ min1. The quantitative rate for XPS analysis was 60 A compositions of surface film were calculated by using the area sensitivity factor from the equipment handbook. Furthermore, the contact angle of the specimens with water was measured by a FACE-CA-D type contact angle measurement system in order to analyze the hydrophobicity of coatings. Fig. 1 shows a rolled-chromized 1045 steel bipolar plate with an active area of 25 cm2. A single serpentine flow field
Experiments
The raw material used in this work was a commercial AISI 1045 steel with 0.467 percent of carbon. Specimens were rolled and then cut into square sheets with the dimensions of 30 mm 30 mm 2 mm. The processing ratio, defined as the reduced thickness after rolling divided by the original
Fig. 1 e The 1045-R-Cr(700-2) BPPs machined with a single serpentine flow field. The active area is 25 cm2.
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was machined onto its surface. Single PEM cells were fabricated with the steel BPPs. The membrane electrode assembly (MEA) consisted of Nafion 112 membranes and Torry paper with a catalyst loading of 0.2 mg cm2 on the anode and 0.4 mg cm2 on the cathode. A 200-mm thick silicone rubber film was used as a gasket to prevent gas leakage. The end plates made of glass fiber were machined in order to form the gas ports and installation holes of the cell. In addition, Au electroplated copper plates were used as the current collector. Subsequently, every bolt was evenly loaded with a compaction force of 20 kg to assemble the components mentioned above. A finished single PEM cell is shown in Fig. 2. The IeV and IeP curves for the single cells were measured and plotted in the operation range of 0.9e0.35 V. The long-term performance tests of the cells were conducted at an operation voltage of 0.6 V for 100 h. The as-built single cells were operated at 60 C under ambient pressure. Pure hydrogen and air completely humidified at 70 C were used as the reactant gases at the anode and cathode sides, respectively. The flow rate of the anode and cathode gases (QA and QC) was controlled at 300 cm3 min1. The residue gas in the cell was expelled by a nitrogen purge before the cell operation. Moreover, the metallic ions dissociated from the BPPs in the water by-product and in the MEA were examined by an inductively coupled plasma-mass spectrometer (ICP-MS) and XPS, respectively.
3.
Results and discussion
3.1.
Microstructure and composition of coatings
Fig. 3a and b show the SEM surface images and EDS spectra of 1045-R-Cr(700-2) bipolar plates, respectively. It is clearly found that the surface of rolled-chromized steel exhibits a granular morphology, and the coating contains a significant amount of Cr-carbide and some Cr-nitride and oxide distributing near the surface of the coating. The cross-sectional SEM image of 1045-R-Cr(700-2) is shown in Fig. 4. It reveals that a uniform and continuous chromized layer was deposited on the steel
Fig. 3 e (a) The SEM surface image and (b) EDS analysis of 1045-R-Cr(700-2).
surface. A pearlite structure comprising a-Fe (dark) and Fe3C (white) phases was observed under the coating. In particular, the textural structure of the steel substrate resulting from the rolling pretreatment almost disappeared in the micrograph of 1045-R-Cr(700-2) due to the constituent rearrangement under thermal energy. The coating thickness of 1045-R-Cr (700-2) is approximately 2 mm. The cross-sectional structure of the 1045Cr(700-2) specimen was also observed (not shown). It is found
Fig. 2 e The prototype single cell stack with chromized steel BPPs; (a) front view and (b) lateral view.
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Fig. 4 e The cross-sectional SEM image of 1045-R-Cr(700-2).
that the 1045-Cr(700-2) coating structure is similar to that of 1045-R-Cr(700-2). However, the coating thickness of 1045-Cr (700-2) is only about 1.25 mm. Thus, it is interesting to note that a combination of rolling work and chromization can generate a thicker coating than a simple chromization process. During the rolling process, severe plastic deformation gives rise to the formation of dislocations with an extremely high density and even a large amount of new grain boundaries, which may serve as numerous fast diffusion “channels” for Cr atoms. In addition, grain-boundary diffusion is the dominant mechanism when the operation temperature of pack cementation ranges from 500 C to 700 C. Hence, the pretreatment with rolling work apparently not only accelerates the diffusivity of Cr but also reduces the working temperature. Fig. 5 displays the glancing incident XRD results of 1045-R-Cr(700-2), and it reveals that the primary products in these coatings are carbides, including (Cr, Fe)23C6, (Cr, Fe)7C3, and Cr2C3. But a minor (Cr, Fe)2N1x phase is also detected in these specimens [39], which coincides with the EDS result. Fig. 6 shows the C 1s, Cr 2p3/2, N 1s, O 1s, and Fe 2p3/2 spectra in the XPS analyses for 1045-Cr(700-2). The binding energy of C1s was 285 eV at the etching time of 0 s, which is
1200
+
Relative intensity
1100 1000 900
Cr2N+(Cr,Fe)2N1-x Cr2C3 (Cr,Fe)7C3 (Cr,Fe)23C6 -Fe -FeCr
800 +
700 600 500 400 30
35
40
45
50
55
60
65
2θ Fig. 5 e XRD patterns of 1045-R-Cr(700-2). [39].
70
attributed to the contaminated carbon on the surface. As the etching time increased from 900 s to 8100 s, the binding energy (282.6e282.9 eV) of metal carbides was detected. All binding energies of the Cr 2p3/2, except the one on the top surface detected at the etching time of 0 s, were in the range of 574.8e574.5 eV, which are found to be in forms of chromium carbide and chromium-ferric carbides,. According to the literature [17,18], if the energy resolution of electron spectrometer is not good enough, the contributions of compounds can’t be resolved. The difference of Cr 2p3/2 binding energy in the carbide configuration is only 0.3 eV, while the resolution of the electron spectrometer is also 0.3 eV for the equipment in this work. It is therefore determined that the configuration of chromium carbide and chromium-ferric carbide definitely exists in the coatings. The binding energy of N 1s core-level electrons, ranged from 397.0 eV to 397.3 eV at the etching time of 900e2700 s, corresponds to chromiumeferric nitrides. The result also indicated that the thickness of nitride layer in the 1045-Cr(700-2) coating is approximately 270 nm. The binding energy of O 1s peak varied from 530.5 eV to 531.1 eV. Based on the reference data of binding energy for chromium oxides (530.2e531.0 eV) and chromium hydroxides (530.85 eV), it is speculated that small amounts of chromium oxide and chromium hydroxide exist in the coatings. The signal of Fe 2p3/2 on the surface is too weak to make any judgment. As the etching time increased, the photoelectron peak of Fe 2p3/2 shifted slightly from 708.6 eV to 708.3 eV, indicating that some ferric oxides with different valence states exist in the coatings. The C 1s, Cr 2p3/2, N 1s, O 1s, and Fe 2p3/2 spectra in XPS analyses for the 1045-R-Cr(700-2) specimen are shown in Fig. 7. The signal of C 1s photoelectron at the etching time of 0 s is mainly associated with the surface contamination of carbon, hydrogen, and oxygen. As the etching time increased from 900 s to 8100 s, the C 1s binding energy peak was located at 283.5e283.8 eV, which can be ascribed to carbides existing in the coating. This result is in agreement with the previous literature in which the binding energy of Cr3C2 was reported in the range of 283.2e283.6 eV [19]. The signal of Cr 2p3/2 on the surface is too weak to be detected. As the etching time increased to the interval between 900 s and 7200 s, the peak shifted to 576.1 eV, which could be identified as Cr(CO)6 [20]. The N 1s binding energy peak ranging from 397.2 eV to 397.8 eV at the etching time of 900e3600 s is related to the formation of metal nitride with a depth of about 360 nm [21] in the coatings. This is caused by the nitridation reaction of chromium and iron, in which the nitrogen is decomposed from the activatoreNH4Cl. The signal of Fe 2p3/2 photoelectron at the etching time of 0e900 s is too weak to be detected, indicating that the Fe concentration is very low in the superficial layer of the coatings. After 1800 s etching time, the signal was ascribed to ferric oxide [22], which may be dispersed in the chromium carbide base and cause a decrease in the conductivity of coatings. Furthermore, the binding energy of O 1s core-level electron suggested that the oxygen was mainly bound to carbon, chromium, and hydrogen [23] in the depth range of 0e180 nm beneath the surface of coatings. In summary, the XPS results of 1045-Cr(700-2) and 1045-R-Cr(700-2) indicated that the primary products in the chromized coatings are carbides, and some minor products such as nitrides and oxides are also detected in the coatings, which is consistent with the literature [11].
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Fig. 6 e XPS C (1s), Cr (2p3/2), N (1s), Fe (2p3/2), and O (1s) spectra of 1045-Cr(700-2).
3.2.
Contact angle test
Water management is an important factor that affects the cell performance in PEMFC systems. While a PEM fuel cell is operated, the by-product, water, produced by the cathode reaction will accumulate at the flow field. If water could not be removed in time, it will block the channel through which the reactant gases access to the electrode, and result in reduced performance of PEM fuel cells [24]. Generally, a hydrophobic material exhibits a contact angle more than 90 between liquidevapor and liquidesolid surfaces as a water droplet is placed onto its surface [25]. Hence, bipolar plates have to possess excellent hydrophobicity with a water contact angle higher than 90 to avoid flooding and power degradation. The contact angle images of bare, 1045-Cr(700-2), and 1045-R-Cr(700-2) steel samples are shown in Fig. 8. It is observed that the contact angles of water on 1045-R-Cr(700-2) and 1045-Cr(700-2) specimens are 98.3 and 94.0 , respectively, both of which are bigger than that of bare material (84.6 ). Referring to previous studies [26,27], the
superficial composition and surface roughness of coatings are the main factors that affect the water-solid contact angles. The values of surface roughness for bare 1045, 1045-Cr(700-2), and 1045-R-Cr(700-2) measured by a profilometer are 0.14 mm, 0.12 mm, and 0.13 mm, respectively, which are very small and close. Hence, it is speculated that the dominant factor which affects the contact angle isn’t surface roughness but rather superficial composition of the BPPs in this work. The Fe2O3 compound covering the surface of bare 1045 steel would possess better affinity with water than the chromium carbide on the surface of the chromized specimens because oxygen is more electronegative than carbon. For this reason, the hydrophobic property of bare 1045 steel is the worst among the tested specimens. In contrast, the coatings containing primarily chromium carbides show better hydrophobicity. The quantitative compositions at the coating surface can be calculated from the XPS spectra by using the area sensitivity factor in the equipment handbook. As can be seen in Table 1, the chromium concentration in the 1045-R-Cr(700-2) and 1045-Cr(700-2) coatings are
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Fig. 7 e XPS C (1s), Cr (2p3/2), N (1s), Fe (2p3/2), and O (1s) spectra of 1045-R-Cr(700-2).
61.9 at.% and 57.21 at.%, respectively. In addition, the oxygen concentration in 1045-Cr(700-2) (1.41 at.%) is higher than that of 1045-R-Cr(700-2) (0.44 at.%). It indicates that the coatings of 1045-R-Cr(700-2) contain a larger amount of chromium carbide and a smaller amount of chromium oxide than the 1045-Cr(7002) specimens. Therefore, 1045-R-Cr(700-2) has a bigger contact angle with water and a better hydrophobicity than 1045-Cr(7002). The results of contact angle tests reveal that low-temperature pack chromization in combination with a rolling pretreatment can significantly enhance the hydrophobic property of the 1045 steel BPPs and improve the performance of single cells.
3.3.
Single cell performance
The initial IeV and IeP curves of single cells assembled with rolled-chromized steel, simple chromized steel, 1045 steel, and graphite BPPs are compared in Fig. 9. The results show that the open circuit voltage (OCV) of the cells fabricated with
1045-R-Cr(700-2), 1045-Cr(700-2), and graphite is 0.948 V, 0.945 V, and 0.947 V, respectively. The voltage and performance for the single cell with 1045-R-Cr(700-2) BPPs are almost equivalent to those with graphite BPPs at low and middle current densities. At a cell voltage of 0.6 V, the current density of the single cells assembled with 1045-R-Cr(700-2) and graphite BPPs are 671.6 and 679.2 mA cm2, respectively, which are very close. However, the cell with 1045-Cr(700-2) BPPs showed inferior performance at low and middle current densities. The maximum power densities for the single cells assembled with 1045-R-Cr(700-2) or 1045-Cr(700-2) BPPs are 0.51 and 0.47 W cm2, respectively. The performance of the single cell with 1045-R-Cr(700-2) BPPs is very close to that with graphite BPPs (0.50 W cm2) in the test conditions of this work. The properties of BPPs that affect the performance of fuel cells are the corrosion resistance, interfacial contact resistance, and hydrophobicity, which are generally related to the superficial composition of coatings. The average elemental
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PREN ¼ %Cr þ 3:3ð% MoÞ þ 16ð% NÞ The superficial layer of 1045-R-Cr(700-2) possessed higher nitrogen concentration (1.02 at.%) than that of 1045-Cr(700-2) (0.44 at.%). Therefore the 1045-R-Cr(700-2) coatings with larger amounts of nitride are expected to show better corrosion resistance in the severe PEMFC environments. Additionally, it
has been reported in the literature that nitrides produced by many methods on different substrate exhibit good electrical conductivity, which is important for BPPs [10,36,37]. Based on the constituent analyses of the chromized specimens, it is determined that the coatings produced with a rolling pretreatment along with low-temperature pack chromization possess better corrosion resistance, conductivity [38], and hydrophobicity than the simple chromized steels. These results conform to the performance of single cells exactly. In our previously published work, three kinds of Fe-based alloys, SS 316, SS 420, and SS 430 steel, treated with rolling and low-temperature pack chromization have been evaluated for the application of BPPs in PEMFCs [39]. In that study, the single cell with 420-R-Cr(700-2) BPPs showed the highest peak power density of 0.46 W cm2, which is lower than that of the cell with 1045-R-Cr(700-2) BPPs here. Therefore, the single cell
1045-R-Cr(700-2)
0.60 0.55 0.50 0.45
2
concentrations of the coatings on the 1045-Cr(700-2) and 1045R-Cr(700-2) specimens in the depth range of 0e600 nm beneath the surface are listed in Table 1. The chromium concentration of 1045-R-Cr(700-2) is obviously higher than that of 1045-Cr(700-2). The higher the Cr concentration in the coating, the better the corrosion resistance [28,29]. Moreover, the concentration of Fe in the coatings would also affect the corrosion resistance of the chromized steel BPPs. The Fe in the coatings would be selectively dissolved [30] due to the higher mobility of iron cations as compared to chromium cations in the passive film [31,32]. The Fe concentrations in the 1045-RCr(700-2) and 1045-Cr(700-2) coatings are 2.41 at% and 7.13 at %, respectively, so more Fe cations would be dissociated from the coating of a simple chromized specimen than from that of a rolled-chromized specimen. These ions will further poison the MEA. According to the literature [33], addition of nitrogen to a molybdenum-free stainless steel is beneficial for the development of passivity when the stainless steel is immersed in a sulfuric acid solution. Moreover, the addition of nitrogen will also remarkably increase the resistance to pitting and crevice corrosion, as is indicated clearly in the pitting resistance equivalent number (PREN) defined by the following equation [34,35].
Power density(Wcm )
Fig. 8 e The contact angle images of the (a) 1045, (b) 1045-Cr (700-2), and (c) 1045-R-Cr (700-2) specimens with water.
Fig. 9 e IeP and IeV curves of the single cells assembled with rolled-chromized steel, simple chromized steel, 1045 steel, and graphite BPPs; cell temperature [ 60 C; QA [ QC [ 300 cm3 minL1.
0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0
Table 1 e Average elemental concentrations of the coatings in the depth range of 0e600 nm beneath surface. Specimens names C(at%) Cr(at%) Fe(at%) N(at%) O(at%) 1045-R-Cr(700-2) 1045-Cr(700-2)
34.23 34.48
61.9 57.21
2.41 7.13
1.02 0.44
0.44 1.41
1000
2000
3000
4000
5000
6000
Time(min)
Fig. 10 e Long-term performance of the single cell assembled with 1045-R-Cr(700-2) BPPs measured at a cell voltage of 0.5 V; cell temperature [ 60 C; QA [ QC [ 300 cm3 minL1.
3982
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 3 9 7 5 e3 9 8 3
Table 2 e The concentrations of metallic ions in the water produced from 1045-R-Cr (700-2) BPPs after the 100-h cell test. Elements
Concentration (ppm)
Cr Fe Ni
0.08717 0.06634 0.07548
with rolled-chromized 1045 steel exhibits much improved performance and is more suitable for the application to PEMFCs. In order to evaluate the long-term performance, the cell with 1045-R-Cr(700-2) BPPs was tested for 100 h. The current density at a cell voltage of 0.6 V was plotted as a function of operation time in Fig. 10. During the entire operation period for 100 h, the power density of the cell with 1045-R-Cr(700-2) BPPs stayed in the range of 0.54e0.50 W cm2, indicating that the cell was operating without distinct performance degradation.
a
Cr
1400
Cr2p3/2 Counts / s
1350 1300 1250
4.
1200 1150 594 592 590 588 586 584 582 580 578 576 574 572
Binding Energy (eV)
b
Fe
Fe2p3/2
3000 2800
Counts / s
2600 2400 2200 2000 1800 1600 1400 730
720
710
Binding Energy (eV)
c
Specifically, a fluctuating phenomenon appeared on the testing curve. This was due to the temporary accumulation and quick removal of the water on the flow field, which was produced by the cathode reaction. Moreover, typical PEM fuel cell atmospheres are reported to be weakly acidic (pH 3.0e5.0) due to the presence of SO2 4 , Cl , F , etc., ions, resulting in a great source of metallic ions, such as Cr, Fe, Ni, which are dissolved from BPPs and will contaminate the proton exchange membranes. In order to estimate the contamination effects of metallic ions dissociated from BPPs on proton exchange membranes during the operation time, the concentrations of metallic ions in the water, which was produced after the operation of the single cell for 100 h, were measured by ICP-MS, and the results are shown in Table 2. The concentrations of Cr, Fe, and Ni ions are 0.08717, 0.06634, and 0.07548 ppm, respectively. Apparently, the concentrations of metallic ions are very small. In addition, the surface composition of MEA is also examined by XPS analysis. The binding energy profiles of Cr 2P3/2, Fe 2P3/2, and Ni 2P3/2 are shown in Fig. 11. Apparently the concentrations of Cr (Fig. 11a), Fe (Fig. 11b), and Ni (Fig. 11c) on the surface of MEA are too low to be detected. Although the contamination of metallic ions is very little in the test results, current efforts towards longer operation test of the single cells are still in progress in our laboratory. Relevant performance and contamination effects of metallic ions will be investigated and presented in the future.
Ni
4400
Ni2p3/2
Conclusions
AISI 1045 steel was used as the substrate for surface modification by rolling pretreatment in combination with lowtemperature pack chromization, and its properties and performance for the application to PEMFCs were evaluated. The XPS results indicate that the composition of 1045-R-Cr (700-2) is composed of chromium carbides, chromium nitrides, and chromium oxides. The atomic concentrations of Cr and O in the 1045-R-Cr(700-2) coating are 61.9% and 0.44%, respectively. The contact angle of water on 1045-R-Cr(700-2) is the highest among all specimens due to the higher chromium carbide and lower oxide contents in the superficial layer of the coatings. The single cell with rolled-chromized BPPs exhibited a peak power density of 0.51 W cm2 at the initial stage of cell operation, which is very close to that of the cell with graphite BPPs (0.50 W cm2). Based on the excellent performance presented above, the rolled-chromized AISI 1045 steel could be considered a good candidate for BPPs in PEMFC applications.
4200
Counts / s
4000
references
3800 3600 3400 3200 3000 2800 890
880
870
860
850
Binding Energy (eV)
Fig. 11 e The XPS binding energy profiles of (a) Cr, (b) Fe, and (c) Ni on the surface of MEA after the single cell operation for 100 h.
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