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.     

Corrosion resistance and interfacial contact resistance of TiN coated 316L bipolar plates for proton exchange membrane fuel cell

TiN coating is successfully deposited on 316L by multi-arc ion plating. Corrosion behavior of TiN coated 316L is studied in 0.05 M H₂SO₄ + 2 ppm F⁻ simulating proton exchange membrane fuel cell (PEMFC) environments using electrochemical method, and interfacial contact resistance (ICR) is measured before and after potentiostatic polarization at operation potential for PEMFC. The TiN coated 316L exhibits promising ICR and improved corrosion resistance in simulated aggressive PEMFC environments. Only general overall corrosion is observed after potentiostatic polarization. Stable passive film has formed on the surface of the TiN coated 316L after potentiostatic polarization at the operation potential and results in a slight increase in the ICR. These results indicate that the TiN coated 316L is a candidate bipolar plate material for PEMFC stacks.     

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.     

Inductively coupled plasma nitriding of chromium electroplated AISI 316L stainless steel for PEMFC bipolar plate

Chromium electroplated AISI 316L stainless steel was nitrided using inductively coupled plasma (ICP) for application in the bipolar plate of a polymer electrolyte membrane fuel cell (PEMFC). A continuous and thin chromium nitride layer was formed at the surface of the samples after ICP nitriding for 2 h at 400 °C. The interfacial contact resistance (ICR) and corrosion resistance in simulated PEMFC operating conditions were higher than the required values, while they varied with the applied dc bias voltage during the nitriding process. The ICR value decreased with an increase in bias voltage. Potentiodynamic polarization measurements showed that all of the nitrided samples had excellent corrosion resistance with a current density of ∼10⁻⁷ A cm⁻² at the cathode. It was also found that the oxygen content at the surface was not increased after the corrosion test. X-ray diffractometry (XRD), field emission scanning electron microscopy (FE-SEM), and Auger electron spectroscopy (AES) were used to analyze the effect of plasma nitriding.     

High-density plasma nitriding of AISI 316L for bipolar plate in proton exchange membrane fuel cell

Austenitic stainless steel (AISI 316L) is nitrided by inductively coupled plasma using a gas mixture of N₂ and H₂ at temperatures between 530 K and 650 K, and the corrosion resistance as well as the interfacial contact resistance (ICR) are measured in a simulated proton exchange membrane fuel cell (PEMFC) environment.After plasma nitriding, a nitrogen-expanded austenite layer, the so-called S-phase is formed in all nitrided samples. The ICR value of the nitrided samples decreases to approximately 10 mΩcm² after plasma nitriding. The sample nitrided at 590 K shows the best corrosion property, while the corrosion resistance of the sample nitrided at higher temperatures decreases because of the formation of Cr-depleted regions in the nitrided sample. By using high-density plasma, the process temperature can be reduced to such a low temperature that Cr depletion is not significant, but a dense S-phase is formed.     

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.     

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.     

Contact resistance characteristics of coated metallic bipolar plates for PEM fuel cells – investigations on the effect of manufacturing

The main purpose of this study is to understand the interfacial contact resistance (ICR) characteristics of coated metallic bipolar plates (BPP) manufactured through stamping and hydroforming. To this goal, 51 μm thick SS316L stainless steel sheet blanks were formed into BPPs using two forming techniques (stamping and hydroforming); then these formed plates were coated with three different PVD coatings (CrN, TiN, ZrN) at three different coating thicknesses (0.1, 0.5 and 1 μm). Contact resistance of the formed and coated BPP samples were measured before and after they were exposed to the proton exchange membrane fuel cells (PEMFC) operating conditions (i.e., corrosive environment). ICR tests indicated that CrN coating increased the contact resistance of the samples, unexpectedly. TiN samples showed the best performance in terms of low ICR; however, their ICR dramatically increased after short-term exposure to corrosion. ZrN coating, as well, improved conductivity of the SS316L BPP samples and demonstrated similar ICR performance before and after exposure to corrosion.     

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.     

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.     

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.     

Nanocomposite-carbon coated at low-temperature: A new coating material for metallic bipolar plates of polymer electrolyte membrane fuel cells

A nanocomposite-carbon layer is coated onto the surface of 316L stainless steel (SS316) using a beam of accelerated C₆₀ ions at low temperature. The coating is composed of textured graphite nanocrystals ranging in size from 1 to 2 nm, with the graphene plane normal to the coating plane; the nanocrystals are separated by amorphous carbon. This orientation of the graphene layer provides low film resistivity in the direction of the substrate normal. Corrosion resistance tests performed in aggressive anodic and cathodic environments of a polymer electrolyte membrane fuel cell (PEMFC) show that the nanocomposite-carbon coated SS316L exhibits better anticorrosion properties than does bare SS316L. The interfacial contact resistance (ICR) of the nanocomposite-carbon coated SS316L is 12 mΩ cm², which is similar to that of graphite at a compaction force of 150 N cm⁻² and lower than a target of ∼20 mΩ cm². A low value of ICR is maintained even after corrosion tests in aggressive anodic and cathodic environments. The fabricated nanocomposite-carbon coated SS316L exhibits excellent corrosion resistance and low interfacial contact resistance under simulated PEMFC bipolar plate conditions.     

Effect of plasma nitriding on behavior of austenitic stainless steel 304L bipolar plate in proton exchange membrane fuel cell

A dense and supersaturated nitrogen layer with higher conductivity is obtained on the surface of austenitic stainless steel 304L by the low temperature plasma nitriding. The effect of plasma nitriding on the corrosion behavior and interfacial contact resistance (ICR) for the austenitic stainless steel 304L was investigated in 0.05 M H₂SO₄ + 2 ppm F⁻ simulating proton exchange membrane fuel cell (PEMFC) environment using electrochemical and electric resistance measurements. The experiment results show that the stable passive film is formed after the potentiostatic polarization at the specified anodic or cathodic potentials under PEMFC operation condition, and the plasma nitriding improves slightly the corrosion resistance and decreases markedly the ICR of 304L. The ICR of the plasma nitrided 304L increases after the potentiostatic polarizations for 4 h, and lower than 100 mΩ cm² at the compaction force of 150 N cm⁻².     

Chromium-containing carbon film on stainless steel as bipolar plates for proton exchange membrane fuel cells

A series of chromium-containing carbon films are deposited on 316L stainless steel (SS316L) as bipolar plates for proton exchange membrane fuel cells (PEMFCs) by pulsed bias arc ion plating (PBAIP). The film characterizations are evaluated by X-ray photoelectron spectroscopy (XPS) and X-ray diffractometry (XRD). Interfacial contact resistance (ICR) between the coated SS316L samples and carbon paper is measured. Potentiodynamic and potentiostatic tests in the simulated corrosive circumstance of PEMFC are conducted to evaluate the corrosion resistance of the coated SS316L samples. The results indicate the films are primarily composed of pure carbon atoms with amorphous structure, including sp³ and sp² carbon atoms. The contents of sp³ and sp² carbon atom are remarkably influenced by the doping chromium. ICR and corrosion resistance of the coated SS316L sample are greatly improved owing to the surface film. The lowest ICR between the coated SS316L sample and carbon paper is only 2.8 mΩ cm² at the compaction force of 120 N cm⁻². The ICR has a close relationship with the contents of sp³ and sp² carbon atom, and the lowest ICR is obtained for the Cr_0.23C_0.77 film with the lowest sp³ carbon atom content and highest sp² carbon atom content. The SS316L sample with Cr_0.23C_0.77 film also exhibits the best corrosion resistance. Finally, the variations of ICR and surface morphology of the coated sample before and after corrosion testing are discussed.     

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.     

Improvement of corrosion resistance and electrical conductivity of 304 stainless steel using close field unbalanced magnetron sputtered carbon film

Carbon film has been deposited on 304 stainless steel (SS304) using close field unbalanced magnetron sputter ion plating (CFUBMSIP) to improve the corrosion resistance and electrical conductivity of SS304 acting as bipolar plates for proton exchange membrane fuel cells (PEMFCs). The corrosion resistance, interfacial contact resistance (ICR), surface morphology and contact angle with water of the bare and carbon-coated SS304 are investigated. The carbon-coated SS304 shows good corrosion resistance in the simulated cathode and anode PEMFC environment. The ICR between the carbon-coated SS304 and the carbon paper is 8.28-2.59 mΩ cm² under compaction forces between 75 and 360 N cm⁻². The contact angle of the carbon-coated SS304 with water is 88.6°, which is beneficial to water management in the fuel cell stack. These results indicate that the carbon-coated SS304 exhibits high corrosion resistance, low ICR and hydrophobicity and is a promising candidate for bipolar plates.     

Development of a galvanostatic analysis technique as an in-situ diagnostic tool for PEMFC single cells and stacks

A new galvanostatic analysis technique was developed for PEMFC single cells and stacks, while conventional potentiodynamic techniques, such as cyclic voltammetry for an electrochemical active surface area (EAS) and linear sweep voltammetry for a crossover current (iH₂)$\left(i_{{\text{H}}_2}\right)$, cannot be directly utilized for stacks. Using a developed relationship for double-layer charging region, the iH₂$i_{{\text{H}}_2}$ and C_dl (double-layer capacitance) of a PEMFC single cell could be determined from the galvanostatic data under an atmosphere of nitrogen (cathodes) and hydrogen (anodes). Then, simply from the elapsed time in hydrogen adsorption/desorption region, EAS or roughness factors could be analyzed for a PEMFC single cell. For a 5-cell PEMFC stack, it was experimentally confirmed that the same analysis technique can be applied to analyze performance distribution in PEMFC stacks. As the characteristics of catalyst layers (EAS and C_dl) and polymer electrolyte membranes (iH₂)$\left(i_{{\text{H}}_2}\right)$ of individual cells can be analyzed without stack disassembly, the developed galvanostatic technique is expected to be utilized for the degradation study and performance monitoring of practical PEMFC stacks.     

Durability and degradation of CrMoN coated SS316L in simulated PEMFCs environment: High potential polarization and electrochemical impedance spectroscopy (EIS)

The high potential on the cathode side originated from the start-up/shut-down processes is the one that cannot be ignored, which will accelerate degradation of the bipolar plates and increase the interfacial contact resistance (ICR) eventually degrade the performance of PEMFCs. Therefore, the coating with corrosion resistance and conductivity is in urgent need of development. CrMoN coating is deposited on SS316L by closed field unbalanced magnetron sputter ion plating (CFUMSIP) with an aim to improve corrosion resistance and conductivity of SS316L under PEMFCs at shut-up/shut-down stages. In terms of high potential polarization test, the corrosion current density and ICR values are found to increase as the applied potential increases. The electrochemical degradation of CrMoN coated SS316L is investigated by electrochemical impedance spectroscopy (EIS). After 40 days of immersion, the charge transfer resistance (R_ct) of the CrMoN-4A coated SS316L is approximately 14 times greater than that of the uncoated SS316L and the ICR values are 11.2 mΩ cm², indicating that the CrMoN-4A coating still has high corrosion resistance and well conductivity after long time immersion.