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.     

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.     

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-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.     

Effect of Ni content on the electrical and corrosion properties of CrNiN coating in simulated proton exchange membrane fuel cell

In this paper, CrNiN coatings with various Ni content are deposited on 304ss bipolar plates by closed field unbalanced magnetron sputter ion plating from CrNi alloy targets. Simulative working environment of proton exchange membrane fuel cell (PEMFC) is applied to test the electrical and corrosion properties of uncoated 304ss and CrNiN-coated samples. The influence of Ni content on microstructure, phase structure, contact angle with water and electrochemical performance is investigated. Results show that all the coated samples significantly enhanced the corrosion resistance of the 304ss, and the CrN-coated 304ss sample without Ni has the best corrosion resistance of 153.8 and ?141.9 mV in the simulated anodic and cathodic environments, respectively. Electrochemical impedance spectroscopy (EIS) studies reveal that the resistance of CrN coating is higher than that of other coated samples and 304ss in the cathodic environment. Furthermore, Interfacial contact resistance (ICR) studies revealed that CrN coating has a superior ICR of 11 mΩ cm² at a compaction force of 160 N cm^?2. In addition, the contact angle of the CrNiN coatings with water is approximately 114°, which is beneficial for water management in PEMFC. Analysis result indicates that the enhanced performance of the coated 304ss bipolar plates is related to the high film density determined by closed field unbalanced magnetron sputter ion plating, and the synergistic function of the CrNiN layered structure.     

Performance of a proton exchange membrane fuel cell stack using conductive amorphous carbon-coated 304 stainless steel bipolar plates

In this study, 304 stainless steel (SS) bipolar plates are fabricated by flexible forming process and an amorphous carbon (a-C) film is coated by closed field unbalanced magnetron sputter ion plating (CFUBMSIP). The interfacial contact resistance (ICR), in-plane conductivity and surface energy of the a-C coated 304SS samples are investigated. The initial performance of the single cell with a-C coated bipolar plates is 923.9 mW cm⁻² at a cell voltage of 0.6 V, and the peak power density is 1150.6 mW cm⁻² at a current density of 2573.2 mA cm⁻². Performance comparison experiments between a-C coated and bare 304SS bipolar plates show that the single cell performance is greatly improved by the a-C coating. Lifetime test of the single cell over 200 h and contamination analysis of the tested membrane electrode assemble (MEA) indicate that the a-C coating has excellent chemical stability. A 100 W-class proton exchange membrane fuel cell (PEMFC) short stack with a-C coated bipolar plates is assembled and shows exciting initial performance. The stack also exhibits uniform voltage distribution, good short-term lifetime performance, and high volumetric power density and specific power. Therefore, a-C coated 304SS bipolar plates may be practically applied for commercialization of PEMFC technology.     

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.     

Corrosion resistance of CrN film deposited by high-power impulse magnetron sputtering on SS304 in a simulated environment for proton exchange membrane fuel cells

Insufficient corrosion resistance and conductivity are two major problems hindering the wide application of stainless steel (SS) bipolar plates in proton exchange membrane fuel cells. This study explores the use of CrN monolayer and multilayer films to improve the performance of SS304 bipolar plates, which are realized by high-power impulse magnetron sputtering with single pulse width or alternating pulse width. The effect of pulse width on the film structure and composition was characterized by various characterization techniques. Furthermore, corrosion and interfacial contact resistance (ICR) test results show that monolayer CrN film using a single long pulse of 40 μs has the lowest corrosion current density of 0.08 μA/cm² (0.6 V vs. Ag/AgCl) and ICR of 3.15 mΩ cm² in all coated samples. As the pulse width increases, vacancy-like defects in film decrease and the density increases, which is attribute to high bombardment flux and high average power of long pulse during deposition.     

Thin chromium nitride PVD coatings on stainless steel for conductive component as bipolar plates of PEM fuel cells: Ex-situ and in-situ performances evaluation

In this paper, two types of chromium PVD coatings (100 nm) have been elaborated on 316L stainless steel (SS) by adjusting the nitrogen flow rate. The first coating is a mixture of Cr₂N and Cr, the second one is a single phase CrN. It is shown that the performances of the material are strongly dependant of the nature of the passive film formed on the chromium nitride layers due to the galvanic coupling between the coating and the substrate. The CrN coated SS shows very good corrosion resistance in simulated PEMFC media. The surface conductivity of the SS is also greatly improved and the CrN coated SS shows an interfacial contact resistance of 10 mΩ cm² at 140 N cm⁻². Five single cells of stainless steel bipolar plates coated with the CrN film were assembled for performance test. This 5 cell stack does not show any mean voltage degradation over 200 h dynamic cycling. Moreover, the performances of the CrN coated SS bipolar plates are very close to the Au-coated SS bipolar plates.     

Catalyst layer-free carbon-coated steel—An easy route to bipolar plates of polymer electrolyte membrane fuel cells: Characterization on structure and electrochemistry

Stainless steel coated with carbon by CVD process has been evaluated as a low-cost and small-volume substitute for graphite bipolar plate in polymer electrolyte membrane fuel cell (PEMFC). Carbon film was grown at 690–930 °C under gas-mixture of C₂H₂–H₂. Scanning electron microscopy and X-ray diffractometry were used to characterize surface morphology and crystal structure of resultant carbon films, which were found to depend much on reaction temperature. Interfacial contact resistance (ICR), hydrophobicity and chemical stability of obtained specimens were measured to compare with commercial highly oriented pyrolytic graphite (HOPG). All carbon films investigated in this study show improved ICR and hydrophobicity of SUS304 substrate to the level of HOPG. Amorphous carbon layer with continuous film structure prepared at 810 °C shows the best protection of SUS304 substrate against the attack of H_(aq)⁺ (anodic side) and the best resistance of the coated carbon from gasification (cathodic side) in the simulated PEMFC environment.     

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 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.     

Carbon film-coated 304 stainless steel as PEMFC bipolar plate

Carbon film-coated stainless steel (CFCSS) has been evaluated as a low-cost and small-volume substitute for graphite bipolar plate in polymer electrolyte membrane fuel cell (PEMFC). In the present work, AISI 304 stainless steel (304SS) plate was coated with nickel layer to catalyze carbon deposits at 680°C under C₂H₂/H₂ mixed gas atmosphere. Surface morphologies of carbon deposits exhibited strong dependence on the concentration of carbonaceous gas and a continuous carbon film with compact structure was obtained at 680 °C under C₂H₂/H₂ mixed gas ratio of 0.45. Systematic analyses indicated that the carbon film was composed of a highly ordered graphite layer and a surface layer with disarranged graphite structure. Both corrosion endurance tests and PEMFC operations showed that the carbon film revealed excellent chemical stability similar to high-purity graphite plate, which successfully protected 304SS substrate against the corrosive environment in PEMFC. We therefore predict CFCSS plates may practically replace commercial graphite plates in the application of PEMFC.     

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.     

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.     

Effect of manufacturing conditions on the corrosion resistance behavior of metallic bipolar plates in proton exchange membrane fuel cells

Metallic bipolar plates are one of the promising alternatives to the graphite bipolar plates in proton exchange membrane fuel cell (PEMFC) systems. In this study, stainless steel (SS304, SS316L, and SS430), nickel (Ni 270), and titanium (Grade 2 Ti) plates with an initial thickness of 51 μm were experimented as bipolar plate substrate materials in corrosion resistance tests. In addition to unformed blanks, SS316L plates were formed with stamping and hydroforming processes to obtain bipolar plates under different process conditions (stamping force, hydroforming pressure, stamping speed, hydroforming pressure rate). These bipolar plates, then, were subjected to corrosion tests, and the results were presented and discussed in detail. Potentiodynamic polarizations were performed to observe corrosion resistance of metallic bipolar plates by simulating the anodic and cathodic environments in the PEMFC. In order to determine the statistical significance of the corrosion resistance differences between different manufacturing conditions, analysis of variance (ANOVA) technique was used on the corrosion current density (I_corr, μA cm⁻²) values obtained from experiments. ANOVA for the unformed substrate materials indicated that SS430 and Ni have less corrosion resistance than the other substrate materials tested. There was a significant difference between blank (unformed) and stamped SS316L plates only in the anodic environment. Although there was no noteworthy difference between unformed and hydroformed specimens for SS316L material, neither of these materials meet the Department of Energy‘s (DOE) target corrosion rate of ≤1 μA cm⁻² by 2015 without coating. Finally, stamping parameters (i.e. speed and force levels) and hydroforming parameters (i.e. the pressure and pressure rate) significantly affected the corrosion behavior of bipolar plates.     

Corrosion resistance characteristics of stamped and hydroformed proton exchange membrane fuel cell metallic bipolar plates

Metallic bipolar plates have several advantages over bipolar plates made from graphite and composites due to their high conductivity, low material and production costs. Moreover, thin bipolar plates are possible with metallic alloys, and hence low fuel cell stack volume and mass are. Among existing fabrication methods for metallic bipolar plates, stamping and hydroforming are seen as prominent approaches for mass production scales. In this study, the effects of important process parameters of these manufacturing processes on the corrosion resistance of metallic bipolar plates made of SS304 were investigated. Specifically, the effects of punch speed, pressure rate, stamping force and hydroforming pressure were studied as they were considered to inevitably affect the bipolar plate micro-channel dimensions, surface topography, and hence the corrosion resistance. Corrosion resistance under real fuel cell conditions was examined using both potentiodynamic and potentiostatic experiments. The majority of the results exhibited a reduction in the corrosion resistance for both stamped and hydroformed plates when compared with non-deformed blank plates of SS304. In addition, it was observed that there exist an optimal process window for punch speed in stamping and the pressure rate in hydroforming to achieve improved corrosion resistance at a faster production rate.     

Ni–Cr Co-implanted 316L stainless steel as bipolar plate in polymer electrolyte membrane fuel cells

A Ni–Cr enriched layer about 60 nm thick with improved conductivity is formed on the surface of austenitic stainless steel 316L (SS316L) by ion implantation. The electrochemistry results reveal that a proper Ni–Cr implant fluence can greatly improve the corrosion resistance of SS316L in the simulated PEMFC environment. The samples after the potentiostatic test are also analyzed by XPS and the ICR values are measured. The XPS results indicate that the composition of the passive film change from a mixture of Fe oxides and Cr oxide to a Cr oxide dominated passive film after the potentiostatic test. Hence, the ICR increases after polarization due to depletion of iron in the passive film. Nickel is enriched in the passive film formed in the simulated PEMFC cathode environment after ion implantation thereby providing better conductivity than that formed in the anode one.     

Evaluation of Ni-Cr-P coatings electrodeposited on low carbon steel bipolar plates for polymer electrolyte membrane fuel cell

Polymer electrolyte membrane fuel cell (PEMFC) stacks suffer from the high cost and low volumetric energy of non-porous graphite bipolar plates. To resolve this problem, a bilayer coating consisting of Ni and Ni–Cr–P is deposited on AISI 1020 low-carbon steel using pulse electrodeposition. Ni/Ni–Cr–P-coated AISI 1020 is evaluated as a bipolar plate material for PEMFCs. Ni/Ni–Cr–P-coated substrates exhibit better corrosion resistance in both cathodic (air-purging) and anodic (H₂-purging) environment than the bare AISI 1020 substrate and lower interfacial contact resistance (ICR) than bare AISI 1020 and stainless steel. Further, it is expected to show better water management as the Ni/Ni–Cr–P coating is more hydrophobic than the bare substrate. Preliminary studies show that Ni/Ni–Cr–P-coated AISI 1020 plate can be a suitable candidate for replacing graphite as the bipolar plate of PEMFCs.     

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.