Low Interfacial Contact Resistance Research Articles

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 (Cr 2N, CrN, TiN, V 2N, 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.

Pre-oxidized and nitrided stainless steel alloy foil for proton exchange membrane fuel cell bipolar plates. Part 2: Single-cell fuel cell evaluation of stamped plates

Thermal (gas) nitridation of stainless steel alloys can yield low interfacial contact resistance (ICR), electrically conductive and corrosion-resistant nitride containing surface layers (Cr 2N, CrN, TiN, V 2N, VN, etc.) of interest for fuel cells, batteries, and sensors. This paper presents results of proton exchange membrane (PEM) single-cell fuel cell studies of stamped and pre-oxidized/nitrided developmental Fe–20Cr–4V weight percent (wt.%) and commercial type 2205 stainless steel alloy foils. The single-cell fuel cell behavior of the stamped and pre-oxidized/nitrided material was compared to as-stamped (no surface treatment) 904L, 2205, and Fe–20Cr–4V stainless steel alloy foils and machined graphite of similar flow field design. The best fuel cell behavior among the alloys was exhibited by the pre-oxidized/nitrided Fe–20Cr–4V, which exhibited ∼5–20% better peak power output than untreated Fe–20Cr–4V, 2205, and 904L metal stampings. Durability was assessed for pre-oxidized/nitrided Fe–20Cr–4V, 904L metal, and graphite plates by 1000+ h of cyclic single-cell fuel cell testing. All three materials showed good durability with no significant degradation in cell power output. Post-test analysis indicated no metal ion contamination of the membrane electrode assemblies (MEAs) occurred with the pre-oxidized and nitrided Fe–20Cr–4V or graphite plates, and only a minor amount of contamination with the 904L plates.

Laser surface modification of AISI 410 stainless steel with brass for enhanced thermal properties

Brass coating was applied to AISI410 steel using high power laser in a laser engineered net shaping (LENS™) system. The influence of laser treatment on interfacial microstructure and thermal performance was evaluated as a function of coating thickness. Laser deposition resulted in a diffused and metallurgically sound interface between metallurgically incompatible brass coating and AISI410 steel substrate. The thermal conductivity of AISI410 steel increased from 27 W/mK to a maximum of 37 W/mK depending on the coating thickness, almost 50% gain. The absence of sharp interface between the coating and the substrate, as a result of laser processing, resulted in a low interfacial thermal contact resistance. Thermal performance tests showed that the brass coating can enhance the heat transfer rate of stainless steel substrate. These results show that novel and efficient feature based coatings can be exploited using laser-based advanced manufacturing technologies for various industrial applications.

Plasma Nitrided Type 349 Stainless Steel for Polymer Electrolyte Membrane Fuel Cell Bipolar Plate—Part II: Nitrided in Ammonia Plasma

Austenitic 349 stainless steel was nitrided in an NH3 plasma. A low interfacial contact resistance was obtained with the nitrided steel. Glancing angle X-ray diffraction suggests that the nitrided layer is very thin and possibly amorphous. X-ray photoelectron spectroscopy (XPS) studies show that the nitrided layer is composed of mixed oxides and nitrides of Fe3+ and Cr3+. Contaminations of V and Sn were also observed, though their influence on the as-nitrided surface conductivity is not clear. The nitrided samples were investigated in a simulated polymer electrolyte membrane fuel cell (PEMFC) environment, and showed excellent corrosion resistance. The XPS depth profile indicated that the passive film, which formed on the plasma-nitrided steel in the PEMFC anode environment, is composed of mixed oxides and nitrides, in which chromium oxide/nitride dominates the surface chemistry. No V or Sn was detected on the surface after the polarization tests. For the PEMFC bipolar plate application, nitridation in NH3 plasma is a promising surface treatment approach, though more research is needed to investigate the influence of the plasma density and substrate bias on the surface conductivity and performance of the nitrided steel in PEMFC environments.

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 cm 2 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.

Electrochemical Properties and Contact Resistance of Titanium in PEFC Bipolar Plate Environment

In order to assess the applicability of titanium as a separator material for polymer electrolyte fuel cells (PEFCs), the electrochemical surface characteristics of commercially available pure titanium were examined in a simulated PEFC environment. The results of the electrochemical tests showed that the anode and cathode operating potentials of PEFC, -0.36V and 0.64V (vs.Ag/AgCl(3.33M KCl)), lay in the cathodic and anodic polarization regions for Ti, respectively. Under the PEFC cathode condition, a barrier-type film with high charge transfer resistance and high interfacial contact resistance grew on the specimen. Under the PEFC anode condition, an air-formed oxide film dissolved and an oxidation film with low charge transfer resistance and low interfacial contact resistance grew.

Solid-state electronic conductivity of ruthenium nanoparticles passivated by metal–carbon covalent bonds

Stable ruthenium nanoparticles were prepared by passivation of the metal cores (diameter 2.7–3.2 nm by transmission electron microscopy) with ruthenium–carbon covalent bonds. Electrochemical study showed that the electronic conductivity of the particle films exhibited metal-like temperature dependence, and it decayed exponentially with the length of the alkyl spacer of the aliphatic protecting ligands, with an electronic coupling coefficient ( β) of 0.48 Å −1. This was ascribed to the strong Ru–C bonding interaction and low interfacial contact resistance where the spilling of core electrons into the organic protecting shell led to enhanced interparticle charge transfer.

Durability and characterization studies of polymer electrolyte membrane fuel cell’s coated aluminum bipolar plates and membrane electrode assembly

Coated aluminum bipolar plates demonstrate better mechanical strength, ease of manufacturability, and lower interfacial contact resistance (ICR) than graphite composite plates in polymer electrolyte membrane (PEM) fuel cell applications. In this study, coated aluminum and graphite composite bipolar plates were installed in separate single PEM fuel cells and tested under normal operating conditions and cyclic loading. After 1000 h of operation, samples of both the bipolar plates and the membrane electrode assembly (MEA) were collected from both the cathode and the anode sides of the cell and characterized to examine the stability and integrity of the plate coating and evaluate possible changes of the ionic conductivity of the membrane due any electrochemical reaction with the coating material. Scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis were performed on the land and valley surfaces of the reactant flow fields at both the anode and the cathode sides of the bipolar plates. The measurements were superimposed on the reference to identify possible zones of anomalies for the purpose of conducting focused studies in these locations. The X-ray diffraction (XRD) analysis of samples scraped from the anode and cathode electrodes of the MEA showed the tendency for catalyst growth that could result in power degradation. Samples of the by-product water produced during the single fuel cell operation were also collected and tested for the existence of chromium, nickel, carbon, iron, sulfur and aluminum using mass spectroscopy techniques. The EDX measurements indicated the possibility of dissociation and dissolution of nickel chrome that was used as the binder for the carbide-based corrosion-resistant coating with the aluminum substrate.

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.

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.

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.

Protective nitride formation on stainless steel alloys for proton exchange membrane fuel cell bipolar plates

Gas nitridation has shown excellent promise to form dense, electrically conductive and corrosion-resistant Cr-nitride surface layers on Ni–Cr base alloys for use as proton exchange membrane fuel cell (PEMFC) bipolar plates. Due to the high cost of nickel, Fe-base bipolar plate alloys are needed to meet the cost targets for many PEMFC applications. Unfortunately, nitridation of Fe-base stainless steel alloys typically leads to internal Cr-nitride precipitation rather than the desired protective surface nitride layer formation, due to the high permeability of nitrogen in these alloys. This paper reports the finding that it is possible to form a continuous, protective Cr-nitride (CrN and Cr 2N) surface layer through nitridation of Fe-base stainless steel alloys. The key to form a protective Cr-nitride surface layer was found to be the initial formation of oxide during nitridation, which prevented the internal nitridation typically observed for these alloys, and resulted in external Cr-nitride layer formation. The addition of V to the alloy, which resulted in the initial formation of V 2O 3–Cr 2O 3, was found to enhance this effect, by making the initially formed oxide more amenable to subsequent nitridation. The Cr-nitride surface layer formed on model V-modified Fe–27Cr alloys exhibited excellent corrosion resistance and low interfacial contact resistance under simulated PEMFC bipolar plate conditions.

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 (H 2SO 4 solution bubbled with H 2 (anode)/O 2 (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.

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 Cr 2N 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.

Contact resistance prediction and structure optimization of bipolar plates

The objective of this work is to investigate the effect of clamping force on the interfacial contact resistance and the porosity of the gas diffusion layer (GDL) in a proton exchange membrane fuel cell (PEMFC). An optimal rib shape for the bipolar plate is developed to analyze the electrical contact resistance. We found that the electrical contact resistance is determined by both the clamping force and the contact pressure distribution. A minimum contact resistance can be obtained in the case of a constant contact pressure distribution. The porosity of the GDLs underneath the rib of the bipolar plate decreases with increasing the clamping force, and the void volume is changed with the deformation of the GDLs. It is found that there exists an optimal rib width of the bipolar plates to obtain a reasonable combination of low interfacial contact resistance and good porosity for the GDL.

Surface conductivity and stability of metallic bipolar plate materials for polymer electrolyte fuel cells

Different types of commercial stainless steels, Ni-based alloys and nitride-coated steels were evaluated as metallic bipolar plate in terms of interfacial contact resistance (ICR) and corrosion resistance in conditions typical of PEFC anode and cathode environments. Results show that stainless steel have a high ICR and undergo corrosion in both anode and cathode. Moreover, although Ni-based alloys showed an ICR comparable with graphite, their behaviour was not satisfactory in corrosive acidic medium. Only nitride-coated stainless steel demonstrated to have low ICR and very good corrosion resistance.

Thermally nitrided stainless steels for polymer electrolyte membrane fuel cell bipolar plates

Thermal nitridation of a model Ni–50Cr alloy at 1100 °C for 2 h in pure nitrogen resulted in the formation of a continuous, protective CrN/Cr2N surface layer with a low interfacial contact resistance. Application of similar nitridation parameters to an austenitic stainless steel, 349™, however, resulted in a discontinuous mixture of discrete CrN, Cr2N and (Cr,Fe)2N1−x (x = 0–0.5) phase surface particles overlying an exposed γ austenite-based matrix, rather than a continuous nitride surface layer. The interfacial contact resistance of the 349™ was reduced significantly by the nitridation treatment. However, in the simulated PEMFC environments (1 M H2SO4 + 2 ppm F− solutions at 70 °C sparged with either hydrogen or air), very high corrosion currents were observed under both anodic and cathodic conditions. This poor behavior was linked to the lack of continuity of the Cr-rich nitride surface formed on 349™. Issues regarding achieving continuous, protective Cr-nitride surface layers on stainless steel alloys are discussed.

Thermally nitrided stainless steels for polymer electrolyte membrane fuel cell bipolar plates

Thermal nitridation of AISI446 mod-1 superferritic stainless steel for 24 h at 1100 ◦ C resulted in an adherent, inward growing surface layer based on (Cr, Fe)2N1−x (x = 0–0.5). The layer was not continuous, and although it resulted in low interfacial contact resistance (ICR) and good corrosion resistance under simulated polymer electrolyte membrane fuel cell (PEMFC) cathodic conditions; poor corrosion resistance was observed under simulated anodic conditions. Nitridation for 2 h at 1100 ◦ C resulted in little nitrogen uptake and a tinted surface. Analysis by SEM, XPS, and AES suggested a complex heterogeneous modification of the native passive oxide film by nitrogen rather than the desired microns-thick exclusive Cr-rich nitride layer. Surprisingly, this modification resulted in both good corrosion resistance under simulated cathodic and anodic conditions and low ICR, well over an order of magnitude lower than the untreated alloy. Further, little increase in ICR was observed under passivating polarization conditions. The potential of this phenomenon for PEMFC bipolar plates is discussed. © 2004 Elsevier B.V. All rights reserved.

Sinter bonding of molybdenum-iron-boron cermet to steel substrate

Superior corrosion resistance and high electrical conductivity are crucial to the metallic bipolar plates towards a wider application in proton exchange membrane fuel cells. In this work, molybdenum carbide coatings are deposited in different thicknesses onto the surface of 316 L stainless steel by magnetron sputtering, and their feasibility as bipolar plates is investigated. The microstructure characterization confirms a homogenous, compact and defectless surface for the coatings. The anti-corrosion performance improves with the increase of the coating thickness by careful analysis of the potentiodynamic and potentiostatic data. With the adoption of a thin chromium transition layer and coating of a ∼1052 nm thick molybdenum carbide, an excellent corrosion current density of 0.23 μA cm−2 is achieved, being approximately 3 orders of magnitude lower than that of the bare stainless steel. The coated samples also show a low interfacial contact resistance down to 6.5 mΩ cm2 in contrast to 60 mΩ cm2 for the uncoated ones. Additionally, the hydrophobic property of the coatings’ surface is beneficial for the removal of liquid water during fuel cell operation. The results suggest that the molybdenum carbide coated stainless steel is a promising candidate for the bipolar plates.