Effects of thermal oxi-nitridation on the corrosion resistance and electrical conductivity of 446M stainless steel for PEMFC bipolar plates

Stainless steel has attracted interest as a bipolar plate material for polymer electrolyte membrane fuel cells due to its excellent mechanical properties, good corrosion resistance, and low cost. However, the application of thermal nitridation for the improvement of electrical conductivity deteriorates the corrosion resistance under PEMFC operating conditions due to the discontinuous formation of external Cr-nitride. In this study, nitridation with pre-oxidation of 446M stainless steel was performed in order to improve both the corrosion resistance and the electrical conductivity. 446M stainless steels with oxide and nitride on the surface were evaluated to assess their feasibility as a bipolar plate material for PEMFCs. The results were compared with those obtained using as-received and only nitrided 446M stainless steels. The oxide formed by the pre-oxidation protects the surface of 446M stainless steel from corrosion in corrosive environments, especially under cathode conditions, and the Cr-nitride formed by the subsequent nitridation serves as an electro-conductive channel. As a result, the pre-oxidized, nitrided 446M stainless steel exhibits improved corrosion properties and electrical conductivity under PEMFC operating environments.     

Ferritic stainless steels as bipolar plate material for polymer electrolyte membrane fuel cells

Both interfacial contact resistance (ICR) measurements and electrochemical corrosion techniques were applied to ferritic stainless steels in a solution simulating the environment of a bipolar plate in a polymer electrolyte membrane fuel cell (PEMFC). Stainless steel samples of AISI434, AISI436, AISI441, AISI444, and AISI446 were studied, and the results suggest that AISI446 could be considered as a candidate bipolar plate material. In both polymer electrolyte membrane fuel cell anode and cathode environments, AISI446 steel underwent passivation and the passive films were very stable. An increase in the ICR between the steel and the carbon backing material due to the passive film formation was noted. The thickness of the passive film on AISI446 was estimated to be 2.6 nm for the film formed at −0.1 V in the simulated PEMFC anode environment and 3.0 nm for the film formed at 0.6 V in the simulated PEMFC cathode environment. Further improvement in the ICR will require some modification of the passive film, which is dominated by chromium oxide.     

Effect of immersion in NaOH solution on ferritic stainless steel as a bipolar plate for polymer electrolyte membrane fuel cells

The surface of 446M ferritic stainless steel (FSS) is modified by immersing in NaOH solution to understand its effect on the interfacial contact resistance (ICR) and corrosion. Immersion in NaOH solution under optimum condition can lead to decrease the ICR value of 446M FSS with no decrease in the corrosion resistance. Immersion of 446M FSS in NaOH solution increases the ratio of Cr oxy-hydroxide/oxide, which contributes to decrease the ICR value. This means that the bound water present in the form of the OH⁻ group in the passive film acts as a donor-type impurity and provides the active sites for electrical conduction in the oxide. This imparts positive effect on the electrical conduction and leads to decrease in the ICR value even after the long-term immersion in the simulated PEMFC environment.     

Electrodeposition of ruthenium oxide on ferritic stainless steel bipolar plate for polymer electrolyte membrane fuel cells

The effect of RuO₂ electrodeposition on ferritic stainless steel as a bipolar plate is evaluated in terms of the surface morphology, interfacial contact resistance (ICR), potentiostatic polarization, contact angle, and X-ray photoelectron spectroscopy. The surface morphology of deposited RuO₂ is greatly stabilized by addition of HNO₃ in 10 mM RuCl₃·xH₂O solution. The RuO₂-deposition on stainless steel shows a high contact angle indicating the high surface energy and hydrophobic characteristics. The ICR measurement indicates that the deposition of conductive RuO₂ on stainless steel is very effective in decreasing ICR value. Moreover, after potentiostatic polarization, the ICR value shows only 2.4 and 2.2 mΩ cm² at 150 N/cm² under air and H₂ purged environments, respectively. In electrochemical test, even though the current density of RuO₂-deposited stainless steel is slightly higher than that of bare stainless steel, it is acceptable value for the relevant DOE 2015 target for metallic bipolar plates (less than 1 μA/cm²). Because the RuO₂-deposition on stainless steel shows a low ICR value and good corrosion resistance and high contact angle, the RuO₂-deposition is a sufficiently feasible method for the bipolar plate material of PEMFC.     

Applicability of extra low interstitials ferritic stainless steels for bipolar plates of proton exchange membrane fuel cells

Ferritic stainless steels can be attractive bipolar plate materials of proton exchange membrane fuel cells (PEMFC), provided that the stainless steels show sufficient corrosion resistance, for instance, by eliminating interstitial elements such as carbon and nitrogen. In the present study, thus, ferritic stainless steels (19Cr2Mo and 22Cr2Mo) with extra low interstitials (ELI) are evaluated to determine the required level of chromium content to apply them for PEMFC bipolar plates. In a simulated PEMFC environment (0.05 M SO₄²⁻ (pH 3.3) + 2 ppm F⁻ solution at 353 K), the 22Cr2Mo stainless steel showed lower current density during the polarization in comparison with the 19Cr2Mo one. The polarization behavior of the 22Cr2Mo stainless steel resembles that of the type 316 one (17Cr12Ni2Mo). Similar values of interfacial contact resistance (ICR) are observed for both ferritic stainless steels. The 22Cr2Mo stainless steel bipolar plate is found to be stable throughout the cell operation, while the 19Cr2Mo stainless steel corroded within 1000 h. After the cell operation, the 22Cr2Mo stainless steel retains the chromium enriched passive film, while the chromium enriched surface film is not found for the 19Cr2Mo one, showing iron oxide/hydroxide based film. X-ray fluorescence (XRF) analysis of the membrane electrode assemblies (MEAs) after the cell operation indicates that the 22Cr2Mo stainless steel was less contaminated with iron species. The above results suggest that the 22Cr2Mo stainless steel can be applicable to bipolar plates for PEMFC, especially 22 mass% of chromium content in ferritic stainless steel with ELI system is, at least, demanded to ensure stable cell performance.     

Pre-oxidized and nitrided stainless steel alloy foil for proton exchange membrane fuel cell bipolar plates: Part 1. Corrosion, interfacial contact resistance, and surface structure

Thermal (gas) nitridation of stainless steel alloys can yield low interfacial contact resistance (ICR), electrically conductive and corrosion-resistant nitride containing surface layers (Cr₂N, CrN, TiN, V₂N, VN, etc.) of interest for fuel cells, batteries, and sensors. This paper presents results of scale-up studies to determine the feasibility of extending the nitridation approach to thin 0.1 mm stainless steel alloy foils for proton exchange membrane fuel cell (PEMFC) bipolar plates. Developmental Fe-20Cr-4V alloy and type 2205 stainless steel foils were treated by pre-oxidation and nitridation to form low-ICR, corrosion-resistant surfaces. As-treated Fe-20Cr-4V foil exhibited target (low) ICR values, whereas 2205 foil suffered from run-to-run variation in ICR values, ranging up to 2× the target value. Pre-oxidized and nitrided surface structure examination revealed surface-through-layer-thickness V-nitride particles for the treated Fe-20Cr-4V, but near continuous chromia for treated 2205 stainless steel, which was linked to the variation in ICR values. Promising corrosion resistance was observed under simulated aggressive PEMFC anode- and cathode-side bipolar plate conditions for both materials, although ICR values were observed to increase. The implications of these findings for stamped bipolar plate foils are discussed.     

Growth of Cr-Nitrides on commercial Ni–Cr and Fe–Cr base alloys to protect PEMFC bipolar plates

Nitridation of Cr-bearing alloys can yield low interfacial contact resistance (ICR), electrically conductive and corrosion-resistant CrN or Cr₂N base surfaces of interest for a range of electrochemical devices, including fuel cells, batteries, and sensors. This paper presents results of exploratory studies of the nitridation of commercially available, high Cr (30–35 wt%) Ni–Cr alloys and a ferritic high Cr (29 wt%) stainless steel for proton exchange membrane fuel cell (PEMFC) bipolar plates. A high degree of corrosion resistance in sulfuric acid solutions designed to simulate bipolar plate conditions and low ICR values were achieved. Oxygen impurities in the nitriding environment were observed to play a significant role in the nitrided surface structures that formed, with detrimental effects for the Ni–Cr base alloys, but beneficial effects for the stainless steel alloy. Positive results from single-cell fuel cell testing are also presented.     

Surface coating and texturing on stainless-steel plates to decrease the contact resistance by using screen printing

Stainless steel is an attractive material for use in bipolar plates of polymer-electrolyte-membrane fuel cells, except for its high interfacial contact resistance (ICR). Inexpensive surface treatment is required to decrease the ICR. A carbonaceous conductive composite was coated on stainless-steel plate surfaces by using a screen-printing technique. A grid-like texture of the same material as the coating was also printed on the coated plate surface. The cross section showed that conductive carbon particles were well dispersed in the coating layer, which favors through-plane electrical conductivity. The coated and textured plates exhibited a much lower ICR than that of bare stainless steel. The ICR of textured plates was lower than that of coated plates under lower compaction pressures. A single cell with coated and textured bipolar plates exhibited higher power densities than that of bare stainless-steel bipolar plates.     

Improved corrosion resistance and interfacial contact resistance of 316L stainless-steel for proton exchange membrane fuel cell bipolar plates by chromizing surface treatment

The electrochemical performance and electrical contact resistance of chromized 316 stainless-steel (SS) are investigated under simulated operating condition in a proton-exchange membrane fuel cell (PEMFC). The corrosion resistance of the chromized stainless steel is assessed by potentiodynamic and potentiostatic tests and the interfacial contact resistance (ICR) is examined by measuring the electrical contact resistance as a function of the compaction force. The results show that the chromizing surface treatment improves the corrosion resistance of the stainless steel due to the high-chromium concentration in the diffuse coating layer. On the other hand, the excess Chromium content on the surface increases the contact resistance of the steel plate to a level that is excessively high for commercial applications. This study examines the root cause of the high-contact resistance after chromizing and reports the optimum process to improve the corrosion resistance without sacrificing the ICR by obtaining a chrome carbide on the outer layer.     

Effect of plastic deformation on the corrosion resistance of ferritic stainless steel as a bipolar plate for polymer electrolyte membrane fuel cells

Stainless steel is one of the best candidate materials for bipolar plate of polymer electrolyte membrane fuel cell (PEMFC) and there have been several manufacturing techniques for stainless steel bipolar plate. The deformation from manufacturing process for bipolar plate can induce the corrosion problem of bipolar plate. The deformed and the stamped stainless steels were examined by evaluating the corrosion resistance to understand the effect of the deformation on the stainless steel as a bipolar plate. The deformation of the stainless steel can significantly affect the corrosion resistance and the deformation from the shaping process for bipolar plate can induce the local anodic sites on the bipolar plate. Therefore, from the corrosion point of view, the shaping process for the bipolar plate is an important factor and the corrosion possibility by shaping process should be considered when selecting the optimum shaping method.     

Carbon-coated stainless steel as PEFC bipolar plate material

Stainless steel is quite attractive as bipolar plate material for polymer electrolyte fuel cells (PEFCs). Passive film on stainless steel protects the bulk of it from corrosion. However, passive film is composed of mixed metal oxides and causes a decrease in the interfacial contact resistance (ICR) between the bipolar plate and gas diffusion layer. Low ICR and high corrosion resistance are both required. In order to impart low ICR to stainless steel (SUS304), carbon-coating was prepared by using plasma-assisted chemical vapor deposition. Carbon-coated SUS304 was characterized by Raman spectroscopy and atomic force microscopy. Anodic polarization behavior under PEFC operating conditions (H₂SO₄ solution bubbled with H₂ (anode)/O₂ (cathode) containing 2 ppm HF at 80 °C) was examined. Based on the results of the ICR evaluated before and after anodic polarization, the potential for using carbon-coated SUS304 as bipolar plate material for PEFC was discussed.     

Electrochemical nitridation of a stainless steel for PEMFC bipolar plates

AISI446 steel has been electrochemically nitrided in 0.1 M HNO₃ + 0.5 M KNO₃ solution at room temperature. XPS analysis revealed surface NH₃ and a deeper nitride layer. The surface layer of the stainless steel modified by electrochemical nitridation was thus composed of a nitrogen-incorporated oxide film. The nitrided steel showed very low interfacial contact resistance (ca. 18 mΩ cm² at 140 N/cm²) and excellent corrosion resistance in simulated PEMFC environments. Electrochemical nitridation provides an economic way to modify the stainless steel’s surface, and is very promising for application to fuel cell bipolar plates.     

Active screen plasma nitriding of 316 stainless steel for the application of bipolar plates in proton exchange membrane fuel cells

Proton exchange membrane fuel cell (PEMFC) has attracted considerable interest because of its superb performance, and many researches are focused on the development of high-performance, long-life bipolar plates. Stainless steel bipolar plates offer many advantages over the conventional graphite bipolar plates, such as low material and fabrication cost, excellent mechanical behaviour and ease of mass production. However, the insufficient corrosion resistance and relatively high interfacial contact resistance (ICR) become the major obstacles to the widespread use of stainless steel bipolar plates. In this work, active screen plasma nitriding (ASPN), a novel plasma nitriding technique, was used to modify the surface of 316 austenitic stainless steel. A variety of analytical techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), glow discharge optical emission spectrometer (GDOES), were employed to characterize the nitrided samples. The results reveal that a nitrogen supersaturated S-phase layer has been successfully produced on the surface of all nitrided 316 stainless steel samples. The interfacial contact resistance (ICR) value can be decreased dramatically after ASPN treatment and the corrosion resistance can also been improved. In addition, better corrosion resistance can be achieved by active screen plasma nitriding with a stainless steel screen than with a carbon steel screen. This technique could be used to improve the performance and lifespan of bipolar plates for fuel cells.     

Effect of pH value on corrosion resistance and surface conductivity of plasma‐nitrided 304L bipolar plate for PEMFC

Low temperature plasma nitriding is developed to meet the requirements for corrosion resistance and interfacial contact resistance (ICR) of stainless steel 304L as the bipolar plate for PEMFC. A dense and supersaturated‐nitrogen nitrided layer has formed on the surface of the stainless steel 304L. Electrochemical behavior for the untreated and plasma‐nitrided 304L was measured in H₂SO₄ (pH=1–5)+2 ppm F^? simulating PEMFC environment, and the ICR was evaluated before and after corrosion tests. The experimental results have shown that the ICR for the plasma nitrided 304L is lower than the requirement of U.S. DOE (<10 mΩ cm² to 2010). Corrosion resistance and the ICR at the compaction force of 150–200 N cm^?2 increase with increasing pH value for the untreated and plasma‐nitrided 304L. The passive current densities for the untreated and plasma‐nitrided 304L are all lower than 16 µA cm^?2. The ICR between passive film and carbon paper are increased markedly because of passive film formed on the surface of both studied 304L. However, the passive current density and the ICR are lower for the plasma nitrided 304L than those for the untreated one at the given pH value, which results from the different composition of the stable passive film formed on the surface. The low temperature plasma nitriding provides a promising method for 304L using as bipolar plate for PEMFC. Further research is needed to evaluate the long‐term stability of passive film and the performance of single fuel cell. Copyright © 2010 John Wiley & Sons, Ltd.     

Deposition of gold–titanium and gold–nickel coatings on electropolished 316L stainless steel bipolar plates for proton exchange membrane fuel cells

The effects of electropolishing and coating deposition on electrical resistance and chemical stability were studied for the stainless steel bipolar plates in proton exchange membrane fuel cell (PEMFC). A series of 316L stainless steel plates, selected as the substrate for a proton exchange membrane fuel cell (PEMFC) bipolar plate, were electropolished with a solution of H₂SO₄ and H₃PO₄ at temperatures ranging from 70 to 110 °C. The surface regions of the two electropolished stainless steel plates were coated with gold and either a titanium or nickel layer using electron beam evaporation. The electropolished stainless steel plates coated in 2-μm thick gold with a 0.1-μm titanium or nickel interlayer showed remarkably smooth and uniform surface morphologies in AFM and FE-SEM images compared to the surfaces of the plates that were coated after mechanical polishing only. The electrical resistance and water contact angle of the deposited stainless steel bipolar plates are strongly dependent on the surface modification treatments (i.e., mechanical polishing versus electropolishing). ICP-MS and XPS results indicate that after electropolishing, the coating layers show excellent chemical stability after exposure to an H₂SO₄ solution of pH 3. Finally, it was concluded that before coating deposition, the surface modification using electropolishing was very suitable for enhancing the electrical property and chemical stability of the stainless steel bipolar plate.     

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.     

Corrosion study of SS430/Nb as bipolar plate materials for PEMFCs

Niobium-coated 430 stainless steel (SS430/Nb) specimens were evaluated as possible bipolar plate materials in conditions that resemble a typical PEMFC environment with respect to their interfacial contact resistance (ICR) and corrosion resistance. Results show that SS430/Nb demonstrated to have low ICR values and very good corrosion resistance in comparison with commercial steel and Ni-based alloys. In addition, the ICR values are also comparable with those of graphite.     

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.     

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.     

Anodic behavior of high nitrogen-bearing steels in PEMFC environments

High nitrogen-bearing stainless steels, AISI Type 201 and AL219, were investigated in simulated polymer electrolyte membrane fuel cell (PEMFC) environments to assess the use of these materials in fuel cell bipolar plate applications. Both steels exhibit better corrosion behavior than 316L steel in the same environments. Type 201 steel shows similar but lower interfacial contact resistance (ICR) than 316L, while AL219 steel shows higher ICR than 316L.

X-ray photoelectron spectroscopy (XPS) analysis shows that the air-formed films on Type 201 and AL219 are composed of iron oxides, chromium oxide, and manganese oxide. Iron oxides dominate the composition of the air-formed film, specially the outer layer. Chromium oxide dominates passive films. Surface film thicknesses were estimated. The results suggest that high nitrogen-bearing stainless steels are promising materials for PEMFC bipolar plates.