Bipolar plate materials for PEMFCs: A conductivity and stability study

Commercial stainless steels, Ni-based alloys and physical vapor deposition (PVD) chromium nitride (CrN)-coated stainless steels were evaluated as possible metallic bipolar plate materials in conditions that resemble a typical PEMFC cathode environment with respect to their interfacial contact resistance (ICR) and corrosion resistance. Results show that stainless steels have a high ICR and undergo corrosion. Although Ni-based alloys showed an ICR value comparable as to that of graphite, their behaviour was not satisfactory in a corrosive acidic medium. Only CrN-coated stainless steels demonstrated to have low ICR values and a very good corrosion resistance.     

Corrosion characteristics of SS316L as bipolar plate material in PEMFC cathode environments with different acidities

The corrosion characteristics of SS316L in simulated proton exchange membrane fuel cell (PEMFC) environments with a wide range of H₂SO₄ concentrations have been systematically studied. Electrochemical methods, both potentiodynamic and potentiostatic, are employed to determine the corrosion parameters and the results show that corrosion resistance decreases with increasing H₂SO₄ concentrations. Scanning electron microscope (SEM) is used to examine the surface morphology of the specimens after potentiostatic polarized in simulated PEMFC cathode environments and the results indicate that local corrosion occurs under all the conditions studied and local corrosion is more severe with higher H₂SO₄ concentrations. Auger electron spectroscopy (AES) analysis is used to identify the composition and the depth profile of the passive film formed on the SS316L surface and the results show that the thickness of passive film decreases with increasing H₂SO₄ concentrations. Interfacial contact resistances (ICR) between SS316L polarized and carbon paper are measured and the results show that ICR decreases with increasing H₂SO₄ concentrations. The corrosion mechanisms of SS316L in PEMFC cathode environments are analysed and discussions on choosing simulated PEMFC cathode corrosion environments for accelerated tests are also provided.     

Factors affecting corrosion behavior of SS316L as bipolar plate material in PEMFC cathode environments

An empirical corrosion model for SS316L in simulated proton exchange membrane fuel cell (PEMFC) environments is developed based on systematic experimental data on the effects of various factors, such as acidity, fluoride ion concentration, temperature and polarization potential. Correlation parameters under different conditions are provided in tabulated forms and comparisons of the empirical model with experimental results are shown in graphical forms. The results show that the empirical model agrees very well with the experimental data except at the short initial polarization time and the model is applicable up to a polarization potential of 0.7 V. The results also show that polarization potential is the most sensitive parameter among all the parameters studied.     

Process modification for coating SnO2:F on stainless steels for PEM fuel cell bipolar plates

Our previous procedure for depositing SnO₂:F on stainless steels for interfacial contact resistance (ICR) reduction and as a protective coating for metal bipolar plates was modified by pre-etching and coating characterization. Only ferrite stainless steels acquired a good quality SnO₂:F coating. The modified SnO₂:F coating decreased the ICR over our previous coating results, confirming the beneficial effect of the pre-etching; however, the corrosion resistance we obtained was significantly reduced.     

Improved anticorrosion properties and electrical conductivity of 316L stainless steel as bipolar plate for proton exchange membrane fuel cell by lower temperature chromizing treatment

The lower temperature chromizing treatment is developed to modify 316L stainless steel (SS 316L) for the application of bipolar plate in proton exchange membrane fuel cell (PEMFC). The treatment is performed to produce a coating, containing mainly Cr-carbide and Cr-nitride, on the substrate to improve the anticorrosion properties and electrical conductivity between the bipolar plate and carbon paper. Shot peening is used as the pretreatment to produce an activated surface on stainless steel to reduce chromizing temperature. Anticorrosion properties and interfacial contact resistance (ICR) are investigated in this study. Results show that the chromized SS 316L exhibits better corrosion resistance and lower ICR value than those of bare SS 316L. The chromized SS 316L shows the passive current density about 3E−7 A cm⁻² that is about four orders of magnitude lower than that of bare SS 316L. ICR value of the chromized SS 316L is 13 mΩ cm² that is about one-third of bare SS 316L at 200 N cm⁻² compaction forces. Therefore, this study clearly states the performance advantages of using chromized SS 316L by lower temperature chromizing treatment as bipolar plate for PEMFC.     

Localised corrosion processes of austenitic stainless steel bipolar plates for polymer electrolyte membrane fuel cells

This research addresses the problem of localised corrosion of stainless steel PEMFC bipolar plates. The susceptibility to pitting and crevice corrosion of austenitic AISI 304 stainless steel has been investigated both by post-mortem microscopic analysis of the end-plates of a laboratory single-cell and by studies of electrochemically corroded stainless steels, in the presence of specially-designed crevice-formers simulating the operating conditions of a PEMFC. This work is based on optical and scanning-electron microscopies as well as potentiostatic and potentiodynamic measurements. The crevice-formers we considered were: Teflon, graphite and AISI 304. The samples, coupled to the crevice-formers have been tested in aqueous solutions containing Cl⁻, SO₄²⁻ and F⁻. From the E-log i plot, the values of corrosion, pitting, crevice and protection potential have been obtained and perfect and imperfect passivity conditions have been identified.     

Surface and corrosion properties of modified passive layer on 304 stainless steel as bipolar plates for PEMFCs

In this study, the relation between surface chemistry and corrosion properties of modified 304 stainless steels (304SS) was investigated. 304SS samples were submitted to plasma nitriding performed at two different temperatures: 420 °C (low) and 520 °C (high). Then, a thermo reactive deposition (TRD) was used in a mixture of ferro niobium, alumina and ammonium chloride. Finally, a pickling treatment was performed to access high corrosion resistant surfaces. Surface treated samples, both before and after pickling, were investigated by using Scanning Electron Microscopy (SEM) and Potentiodynamic (PD) techniques. X-ray Photoelectron Spectroscopy (XPS) was performed for the analysis of the surface layers of the samples after pickling.The surface layers were mostly comprised of iron and chromium oxides and hydroxides. XPS results proved the presence of a surface layer mostly constituted by iron oxides and oxyhydroxides for both the treated samples that resulted more homogenous for the sample nitrided at low temperature. Chromium was detected by XPS in the predominant form of oxide on the surface of the sample nitrided at low temperature.     

Corrosion behavior of tantalum coatings on AISI 316L stainless steel substrate for bipolar plates of PEM fuel cells

Corrosion resistance of tantalum coatings 30 μm thick deposited by chemical vapor deposition on SS316L coupons has been evaluated by electrochemical impedance spectroscopy (EIS). To this end, anodic and cathodic operating conditions of proton exchange membrane fuel cells (PEMFC) have been simulated in a three-electrode heated corrosion cell. Interfacial contact resistance (ICR), contact angle and durability tests have been performed in long-term tests (>100 h) polarizing the electrode to 1.193 V vs. Ag/AgCl. Results obtained by different experimental techniques show a dense coating structure with a high polarization resistance, mainly formed by surface crystals of α-Ta (bcc), Ta₂O₅ and carbon. An atomic ratio (in %) of oxide to metallic species (Ta_ox/Ta_met) of 4.8 was verified from XPS spectra, which is slightly increased to 6.23 after the anodizing treatment. The modified surface composition yielded a coating capacity higher than the amorphous oxide, favoring the in-plane electrical conduction. After the treatment, no noticeable changes were observed neither in surface morphology nor in contact angle (>90°). ICR values in the range of 22.3–32.6 mΩ cm² were obtained for a clamping pressure of 140 N cm⁻². No morphological changes or loss of coating adherence were observed during the long-term tests.     

Influence of fluoride ions on corrosion performance of 316L stainless steel as bipolar plate material in simulated PEMFC anode environments

Corrosion performance of 316L stainless steel as a bipolar plate material in proton exchange membrane fuel cell (PEMFC) is studied under different simulated PEMFC anode conditions. Solutions of 1 × 10⁻⁵ M H₂SO₄ with a wide range of different F⁻ concentrations at 70 °C bubbled with hydrogen gas are used to simulate the PEMFC anode environments. Electrochemical methods, both potentiodynamic and potentiostatic, are employed to study the corrosion behavior. Scanning electron microscope (SEM) and atomic force microscope (AFM) are used to examine the surface morphology of the specimen after it is potentiostatic polarized in simulated PEMFC anode environments. X-ray photoelectron spectroscopy (XPS) analysis is used to identify the compositions and the depth profile of the passive film formed on the 316L stainless steel surface after it is polarized in simulated PEMFC anode environments. Mott–Schottky measurements are used to characterize the semiconductor passive films. The results of potentiostatic analyses show that corrosion currents increase with F⁻ concentrations. SEM examinations show that no localized corrosion occurs on the surface of 316L stainless steel and AFM measurement results indicate that the surface topography of 316L stainless steel becomes slightly rougher after polarized in solutions with higher concentration of F⁻. From the results of XPS analysis and Mott–Schottky measurements, it is determined that the passive film formed on 316L stainless steel is a single layer n-type semiconductor.     

Characterization and performance of UNS S63019 (21-4N) as bipolar plate material in a simulated polymer electrolyte membrane fuel cell environment

The austenitic stainless steel UNS S63019 was evaluated regarding its potential as bipolar plate material in a polymer electrolyte membrane fuel cell (PEMFC) environment. Segregated grains of niobium carbide (NbC_x) were identified in polished cross-sections of the alloy, offering a possible pathway for enhanced electrical conductivity through the passive surface oxide. Additionally, the alloy was tested for corrosion resistance in a simulated PEMFC environment. It was considered that perhaps the elevated nitrogen concentration in the alloy would provide some benefit for corrosion resistance.Results for interfacial contact resistance (ICR) testing of the air-formed surface film on UNS S63019 showed decreased electrical conductivity as compared to UNS S30400. Niobium carbide particles did not improve film conductivity due to a non-conductive niobium oxide layer that formed on the surface. Corrosion resistance of the alloy was also poor as compared with UNS S30400, demonstrating that elevated nitrogen concentration in the alloy was not adequate in itself to enhance corrosion resistance. Poor corrosion resistance was attributed primarily to high carbon content in the alloy which combined with a significant amount of chromium to form carbides.     

Controlling the compactness and sp2 clusters to reduce interfacial damage of amorphous carbon/316L bipolar plates in PEMFCs

In proton exchange-membrane fuel cells, the compactness and composition of amorphous carbon (a-C) films directly affect the interface-induced degradation of a-C-coated metallic bipolar plates, but their roles and synergistic effects are ambiguous. In this study, by changing the working pressure, the compactness and composition of a-C are adjusted, and the relationship among the plasma state, microstructure, and performance of coated 316L stainless steel are studied. Results show that, at 2 mTorr, a high ionization degree of the plasma causes high compactness (highest density 2.52 g/cm³) and large sp² cluster size in a-C films, resulting in their best performance. At 11 mTorr, the low ionization degree causes their loose structure and large sp² cluster size. The possible galvanic coupling effect can result in serious interface damage and the highest Fe ions concentration of 13.05 ppm after a potentiostatic test. Thus, high compactness and large sp² cluster sizes should be obtained simultaneously to improve their performance.     

Preparation of Cr-based multilayer coating on stainless steel as bipolar plate for PEMFCs by magnetron sputtering

In the present study, a multilayer composing of Cr₃Ni₂/Cr₂N/CrN is sputtered onto stainless steel. The potential of using the coated stainless steel as the bipolar plate for polymer electrolyte membrane fuel cell (PEMFC) is evaluated. The coated stainless steel exhibits improved corrosion resistance and higher electrical conductivity. The coated surface also demonstrates a hydrophobic characteristic. By using single cell test, the multilayer-coated SS304 plate exhibits an improved performance in terms of I-V properties.     

Effects of low-temperature nitridation on the electrical conductivity and corrosion resistance of 446M stainless steel as bipolar plates for proton exchange membrane fuel cell

Low-temperature nitridation was used to form a protective and conductive layer on stainless steel. The surface characterization reveals that a continuous and protective Cr-nitride/oxide layer (CrN and Cr₂O₃) forms on the 446M stainless steel surface after low-temperature nitridation. The electrical conductivity of the sample is investigated in terms of the interfacial contact resistance. This value for nitrided 446M at low temperature is 6 mΩ cm², which is much lower than that of the bare 446M stainless steel (about 77 mΩ cm²) at a compaction force of 140 N/cm². The corrosion resistance of low-temperature nitrided 446M stainless steel is examined in potentiodynamic and potentiostatic tests under simulated polymer electrolyte membrane fuel cell (PEMFC) conditions with pH 3 H₂SO₄ at 80 °C. In a simulated anode condition, the current density is −1 × 10⁻⁶ A/cm². In a simulated cathode condition, the current density is 1 × 10⁻⁷ A/cm². Low-temperature nitrided 446M stainless steel shows superior electrical conductivity and corrosion resistance than bare 446M stainless steel.     

Anticorrosion properties of Ta-coated 316L stainless steel as bipolar plate material in proton exchange membrane fuel cells

In order to reduce the cost, volume and weight of the bipolar plates used in the proton exchange membrane fuel cells (PEMFC), more attention is being paid to metallic materials, among which 316L stainless steel (SS316L) is quite attractive. In this study, metallic Ta is deposited on SS316L using physical vapor deposition (PVD) to enhance the corrosion resistance of the bipolar plates. Simulative working environment of PEMFC is applied for testing the corrosion property of uncoated and Ta-coated SS316L. X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical methods (potentiodynamic and potentiostatic polarization) are also used for analyzing characteristics of uncoated and Ta-coated SS316L. Results show that, Ta-coated SS316L has significantly better anticorrosion property than that of uncoated SS316L, with corrosion current densities of uncoated SS316L being 44.61 μA cm⁻² versus 9.25 μA cm⁻² for Ta-coated SS316L, a decrease of about 5 times. Moreover, corrosion current densities of Ta-coated SS316L in both simulative anode (purged with H₂) and cathode (purged with air) conditions are smaller than those of uncoated SS316L.     

Evaluation of anticorrosion and contact resistance behavior of poly(orthophenylenediamine)- coated 316L SS bipolar plate for proton exchange membrane fuel cell

The present work was focused on the corrosion properties and contact resistance behavior of poly(orthophenlyenediamine) (PoPD) coating on 316L SS bipolar plates. To reduce the corrosion rate and increase the interfacial conductivity of 316L SS bipolar plates, PoPD coating was deposited using an electropolymerization technique by the various monomer concentration of orthophenlyenediamine (oPD) on its surface. The presence of 1, 2, 4, 5- tetra substituted benzene nuclei of phenazine units in the polymer coating was confirmed by infrared spectroscopy. X-ray photoelectron spectroscopy analysis has confirmed the (%) of chemical composition in PoPD coating. The results of scanning electron microscopy analysis revealed that the uniform and compact coating with complete cover on 316L SS. The corrosion properties were investigated in 0.5 M H₂SO₄ and 2 ppm HF solution at 80 °C. The polarization test results showed that the PoPD coating reduced the corrosion current density both in the PEMFC anode and cathode environments. The charge transfer resistance values were in the order of 316L SS ? 0.02 M PoPD ? 0.06 M PoPD ? 0.04 M PoPD. A very low interfacial contact resistance and good adhesion strength was observed for 0.04 M PoPD coating. The higher contact angle of 0.04 M PoPD coating explained the hydrophobic property and more benefit of water management in the PEMFC environment. The results of the analysis of total metal ion releases clearly explained that the low level of metal ions released for 0.04 M PoPD coating. The overall studies revealed the PoPD coating with optimized 0.04 M oPD concentration showed best performance and provided more anodic protection to 316L SS bipolar plates.     

Electrochemical characterization of Fe‐26Mn‐3A1‐7Cr alloy bipolar plate for PEMFC

Corrosion behaviors of Fe‐26Mn‐3A1‐7Cr alloy are investigated in the solution of 0.05 M H₂SO₄ + 2 ppm F^‐, and interfacial contact resistances (ICRs) are measured before and after potentiostatic polarization at operation potentials. The results show that passive current densities of Fe‐26Mn‐3A1‐7Cr alloy are about 10^‐5 A cm^‐2. Stable passive film forms on the surface of Fe‐26Mn‐3A1‐7Cr alloy in the cathodic side, and no pit is observed, while serious dissolution occurs in the anodic one. The ICR of Fe‐26Mn‐3A1‐7Cr alloy in anode side is about three times higher than that in cathode one. Copyright © 2012 John Wiley & Sons, Ltd.     

Arc ion plated Cr/CrN/Cr multilayers on 316L stainless steel as bipolar plates for polymer electrolyte membrane fuel cells

Arc ion plating (AIP) is applied to coat sandwich-like Cr/CrN/Cr multilayers on stainless steel 316L (SS316L) as bipolar plates for polymer electrolyte membrane fuel cell (PEMFC). Phase structure, hardness, adhesion property, interfacial contact resistance (ICR) between bipolar plates and carbon papers, and electrochemical corrosion property in the simulated PEMFC conditions are investigated. Cr phase with crystal plane of (1 1 0), (2 1 1), (3 2 2), and CrN phase with (3 2 1) are observed in the multilayer. The coating is found smooth, continuous and dense in cross-sectional observation by SEM, and the sandwiched structure of the coating is also confirmed by EDX results. Scratch tests show that the multilayer exhibits strong adhesion strength with steel substrate, which is beneficial to prevent layers from peeling off mechanically. After the coating treatment, the performance of the bipolar plate is greatly improved. Knoop hardness of the bipolar plates increases from 324 HK to 692 HK. The ICR decreases by one order of magnitude; furthermore, the corrosion resistance was also enhanced. Our analysis indicates that the improvement is attributed to high adhesion force of the smooth and dense coating and the synergistic function of Cr/CrN/Cr multilayer structure.     

Chromium nitride/Cr coated 316L stainless steel as bipolar plate for proton exchange membrane fuel cell

Chromium nitride/Cr coating has been deposited on surface of 316L stainless steel to improve conductivity and corrosion resistance by physical vapor deposition (PVD) technology. Electrochemical behaviors of the chromium nitride/Cr coated 316L stainless steel are investigated in 0.05 M H₂SO₄ + 2 ppm F⁻ simulating proton exchange membrane fuel cell (PEMFC) environments, and interfacial contact resistance (ICR) are measured before and after potentiostatic polarization at anodic and cathodic operation potentials for PEMFC. The chromium nitride/Cr coated 316L stainless steel exhibits improved corrosion resistance and better stability of passive film either in the simulated anodic or cathodic environment. In comparison to 316L stainless steel with air-formed oxide film, the ICR between the chromium nitride/Cr coated 316L stainless steel and carbon paper is about 30 mΩ cm² that is about one-third of bare 316L stainless steel at the compaction force of 150 N cm⁻². Even stable passive films are formed in the simulated PEMFC environments after potentiostatic polarization, the ICR of the chromium nitride/Cr coated 316L stainless steel increases slightly in the range of measured compaction force. The excellent performance of the chromium nitride/Cr coated 316L stainless steel is attributed to inherent characters. The chromium nitride/Cr coated 316L stainless steel is a promising material using as bipolar plate for PEMFC.     

Evaluation of the durability of ZrC as support material for Pt electrocatalysts in PEMFCs: Experimental and computational studies

Zirconium carbide (ZrC) is evaluated as support material for corrosion resistance to replace carbon in Pt electrocatalyst for the first time. The commercial ZrC (1 m²g^-1) is activated using solid sodium carbonate producing high surface area activated ZrC (a-ZrC, 134 m²g^-1). Both ZrC and a-ZrC exhibit good corrosion resistance properties during carbon corrosion tests. As supports in Pt/ZrC and Pt/a-ZrC electrocatalysts, the ECSA losses are below 40% after standard accelerated stress test protocols satisfying the standard set targets of DoE. The Pt/ZrC does not show significant shift in the onset potential in the linear sweep voltammetry after start-up and shut-down protocol tests. The shift in E_1/2 potentials for Pt/ZrC and Pt/a-ZrC catalysts, respectively, are 13 mV and 24 mV, which are less compared to 30 mV for Pt/C. The computational analysis shows strong-metal-support-interaction between ZrC and Pt which is responsible for catalyst durability because of higher dissolution potential.     

Effects of niobium and titanium addition and surface treatment on electrical conductivity of 316 stainless steel as bipolar plates for proton-exchange membrane fuel cells

Niobium and titanium are added to 316 stainless steel, and then heat treatment and surface treatment are performed on the 316 stainless steel and the Nb- and Ti-added alloys. All samples exhibit enhanced electrical conductivity after surface treatment but have low electrical conductivity before surface treatment due to the existence of non-conductive passive films on the alloy surfaces. In particular, the Nb- and Ti-added alloys experience a remarkable enhancement of electrical conductivity and cell performance compared with the original 316 stainless steel. Surface characterization reveals the presence of small carbide particles on the alloy surface after treatment, whereas the untreated alloys have a flat surface structure. Cr₂₃C₆ forms on the 316 stainless steel, and NbC and TiC forms on the Nb- and Ti-added alloys, respectively. The enhanced electrical conductivity after surface treatment is attributed to the formation of these carbide particles, which possibly act as electro-conductive channels through the passive film. Furthermore, NbC and TiC are considered to be more effective carbides than Cr₂₃C₆ as electro-conductive channels for stainless steel.