Corrosion resistant quaternary Al–Cr–Mo–N coating on type 316L stainless steel bipolar plates for proton exchange membrane fuel cells

Insufficient corrosion resistance, electrical conductivity and wettability of bipolar plates are some of the important issues affecting the performance of hydrogen fuel cells. To address these issues, an amorphous Al–Cr–Mo–N coating is deposited on type 316L stainless steel using direct current (DC) magnetron sputtering. The electrochemical corrosion behaviour is investigated under simulated fuel cell anode (H₂-purging) and cathode (air-purging) environment consisting of 0.5 M H₂SO₄ + 2 ppm NaF at 70 ± 2 °C. The corrosion current density is reduced to 0.02 μA cm⁻² comparable to the commercially used Ta/TaN coatings. The polarization resistance increases by two orders of magnitude and the interfacial contact resistance (ICR) reduces significantly due to the application of the coating. Further, the coating shows better water management due to high hydrophobicity than the bare stainless steel.     

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.     

Electrochemical behavior of surface treated metal bipolar plates used in passive direct methanol fuel cell

Corrosion resistance performance of SS316L treated by passivation solution was investigated in a simulated environment of the passive direct methanol fuel cell (DMFC). Electrochemical impedance spectroscopic (EIS) test showed that polarization resistance of untreated and treated SS316L were 1191 Ω cm² and 9335 Ω cm², respectively. The above result agreed with the Tafel slope analysis of potentiodynamic polarization curves. Comparing the untreated and treated SS316L in the simulated environment of DMFC anode working conditions, it was observed that the corrosion current density of treated SS316L as estimated by 4000 s potentiostatic test reduced from 38.7 μA cm⁻² to 0.297 μA cm⁻², meanwhile, the current densities of untreated and treated SS316L in cathode working conditions were 3.87 μA cm⁻² and 0.223 μA cm⁻², respectively. It indicated that the treated SS316L should be suitable in both anode and cathode environment of passive DMFCs. The treated SS316L bipolar plates have been assembled in a passive single fuel cell. A peak power density of 1.18 mW cm⁻² was achieved with 1 M methanol at ambient temperature.     

Tantalum nitride coated AISI 316L as bipolar plate for polymer electrolyte membrane fuel cell

Tantalum nitride (TaN) thin films are deposited on AISI 316L stainless steel by inductively coupled, plasma-assisted, reactive magnetron sputtering at various N₂ flow rates. TaN film behavior is investigated in simulated polymer electrolyte membrane fuel cell (PEMFC) conditions by using electrochemical measurement techniques for application as bipolar plates. The results of a potentio-dynamic polarization test under PEMFC cathodic and anodic conditions indicate that the corrosion current density of the TaN_x films is of the order of 10⁻⁷ A cm⁻² (at 0.6 V) and 10⁻⁸ A cm⁻² (at −0.1 V), respectively; these results are considerably better than the individual results for metallic Ta films and AISI 316L stainless steel. The TaN_x films exhibit superior stability in a potentio-static polarization test performed under PEMFC cathodic and anodic conditions. The interfacial contact resistance of the films is measured in the range of 50-150 N cm⁻², and the lowest value is 11 mΩ cm² at a compaction pressure of 150 N cm⁻².     

Formable chromium nitride coatings for proton exchange membrane fuel cell stainless steel bipolar plates

Among the different coatings developed for proton exchange membrane fuel cell steel bipolar plate, nitride-based coatings present several advantages compared to gold or polymeric coating: high chemical stability, low interfacial contact resistance and reasonable cost. In this work, 50 nm thick chromium nitride coatings are deposited by reactive magnetron sputtering on 316L stainless steel foil. They are optimized to fulfill the Department of Energy targets in terms of interfacial contact resistance (ICR) and corrosion resistance, with values of 8.4 mΩ cm⁻² (at 100 N cm⁻²) and 0.10 μA cm⁻² (in 0.6 M H₂SO₄ solution at 0.48 V_vs. SCE potential) respectively. Moreover, they retain their excellent properties after high deformation (biaxial deformation of 20% in x-axis and 5% in y-axis), giving the possibility to achieve, in line, the stamping of a bipolar plate from a coated foil. The etching of the substrate, prior to the coating deposition, appears to be determinant to obtain low and stable corrosion current and ICR. The removing of interfacial oxyde leads to better coating adhesion and improves the corrosion resistance and electrical conductivity. The enhancement of the properties (low ICR and high corrosion resistance) is durable, with no signicant change of the ICR value up to 200 days after deposition.     

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

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

Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells

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.     

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.     

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.     

Formation of a protective TiN layer by liquid phase plasma electrolytic nitridation on Ti–6Al–4V bipolar plates for PEMFC

The current work mainly investigates the corrosion resistance and conductivity of TiN layer on Ti–6Al–4V with liquid phase plasma electrolytic nitridation for PEMFC bipolar plate (BP). The X-ray diffraction (XRD) results show that TiN phase presents in the nitriding samples. The surface morphology analysis indicates that higher CH₄NO₂ concentration tends to form more compact structure. The potentiodynamic polarization test indicates that sample N-600 prepared by 600 g/L CH₄NO₂ concentration possesses the lowest corrosion current density of 0.57 μA cm⁻² in the simulated PEMFC cathode potential (+0.6 V_SCE), which completely satisfies the DOE 2020 technical targets. Meanwhile, sample N-600 also exhibits the highest corrosion resistance and stability in the potentiostatic polarization, electrochemical impedance spectroscopy (EIS) and high potential (+1.6 V_SCE) polarization test. Interfacial contact resistance (ICR) results show that sample N-600 possesses the lowest ICR value of 6 ± 0.4 mΩ cm², which fully meets the DOE 2020 requirements.     

Cr–N–C multilayer film on 316L stainless steel as bipolar plates for proton exchange membrane fuel cells using closed field unbalanced magnetron sputter ion plating

To combine the advantages of chromium nitride (CrN) and amorphous carbon (a-C) film, this study proposes a novel Cr–N–C multilayer film on 316L stainless steel (SS316L) as bipolar plates for proton exchange membrane fuel cells (PEMFCs) using closed field unbalanced magnetron sputter ion plating (CFUBMSIP) method. The characterizations of Cr–N–C film are analyzed by X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD), and scanning electron microscopy (SEM). Scratch tests indicate that the adhesion strength between the film and SS316L substrate has been greatly improved which is beneficial to prevent the multilayer film from spalling. Interfacial contact resistance (ICR) between coated SS316L sheets and simulated gas diffusion layer (GDL) decreases to 2.64 mΩ cm² at 1.4 MPa. Potentiodynamic results reveal that the anodic corrosion potential of coated samples is more positive than the operation potential and the cathodic passivation current density is only 0.61 μA cm⁻² at 0.6 V. Potentiostatic test, contamination analysis and surface morphology results reveal that the substrate is well protected by the Cr–N–C film. This research demonstrates that the novel Cr–N–C film exhibits excellent ex-situ performance including strong adhesion strength, high corrosion resistance and low ICR.     

Investigation of acidity on corrosion behavior and surface properties of SS304 in simulated PEMFC cathode environments

This study presents the influence of acidity on the corrosion performance and surface properties of AISI 304 stainless steel (SS304) in the simulated cathode condition of proton exchange membrane fuel cells (PEMFC) with various concentrations of H₂SO₄. The electrochemical tests indicate that the corrosion resistance of SS304 samples decreases gradually with the solution acidity ascending, but the stable current densities (0.043–0.547 μA cm^?2) in the simulated solutions after polarization (0.6 V, 5 h) are all lower than that of the relevant DOE 2025 target (i_corr < 1 μA cm^?2). Obvious pitting corrosion occur in the solutions with H₂SO₄ concentration higher than 10^?3 M. The surface wettability and interfacial contact resistance (ICR) of the potentiostatically polarized SS304 show an upward trend with the solution acidity increasing, and whether the SS304 samples are polarized or not, their ICR (0.274–1.232 Ω cm²) is far higher than the latest DOE 2025 technical target (<0.01 Ω cm²). The results reveal that surface modification is indispensable for SS304 as bipolar plates, and more attention should be paid to possessing high and stable pitting resistance, hydrophobicity, and interfacial conductivity in an acid environment.     

Long-term polarization accelerated degradation of nano-thin C/Ti coated SS316L bipolar plates used in polymer electrolyte membrane fuel cells

Dynamic potentials of polymer electrolyte membrane fuel cells (PEMFCs) lead to the corrosion of metal bipolar plates and significantly increase their interfacial contact resistance (ICR). Herein, two nano-thin C/Ti coatings with different thicknesses are prepared on SS316L (C/Ti/SS) and examined under three types of polarization potential modes. The C₁₀₀Ti₆₀ coating has improved corrosion resistance than C₂₀Ti₁₄₀ coating. The C₁₀₀Ti₆₀ coating exhibits a low ICR of 2.67 mΩ cm² and a small corrosion rate of 1.47 μA/cm², highlighting the high electrical conductivity and corrosion resistance. Moreover, C₁₀₀Ti₆₀/SS achieves a slight degradation of ICR after polarization at 0.67 V and cyclic polarization between 0.43 and 0.73 V. However, the high potential of 1.43 V induces severe delamination of C layer, resulting in a remarkable increase of ICR. Analysis suggests that the Ti layer improves the oxidation resistance and the C layer could provide effective protection when the potential is below 1.13 V.     

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.     

Influence of temperature on corrosion behavior,wettability, and surface conductivity of 304 stainless steel in simulated cathode environment of proton exchange membrane fuel cells

The effects of temperature on corrosion behavior, wettability, and surface conductivity of 304 stainless steel (SS304) in simulated cathode environment of proton exchange membrane fuel cells (PEMFC) are investigated systematically using electrochemical tests and surface analyses. The results indicate that although the corrosion resistance of SS304 is decreased with the rising of solution temperature, the current density of SS304 at the working potential in the simulated PEMFC cathode environment can still meet the 2025 U.S. Department of Energy (DOE) technical target (i_corr < 1 μA cm^?2). Meanwhile, the surface wettability and ICR of SS304 samples after potentiostatic polarization show a continuous increase with the rise of the simulated solution temperature. The surface conductivity of SS304 both before and after polarization cannot reach the 2025 DOE technical target (<0.01 Ω cm²) and needs to be improved by surface modification.     

An investigation into nickel implanted 316L stainless steel as a bipolar plate for PEM fuel cell

A nickel-rich layer about 100 μm in thickness with improved conductivity was formed on the surface of austenitic stainless steel 316L (SS316L) by ion implantation. The effect of ion implantation on the corrosion behavior of SS316L was investigated in 0.5 M H₂SO₄ with 2 ppm HF solution at 80 °C by potentiodynamic test. In order to investigate the chemical stability of the ion implanted SS316L, the potentiostatic test was conducted in an accelerated cathode environment and the solutions after the potentiostatic test were analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES). The results of potentiodynamic test show that the corrosion potential of SS316L is shifted toward the positive direction from −0.3 V versus SCE to −0.05 V versus SCE in anode environment and the passivation current density at 0.6 V is reduced from 11.26 to 7.00 μA cm⁻² in the cathode environment with an ion implantation dose of 3 × 10¹⁷ ions cm⁻². The potentiostatic test results indicate that the nickel implanted SS316L has higher chemical stability in the accelerated cathode environment than the bare SS316L, due to the increased amount of metallic Ni in the passive layer. The ICP results are in agreement with the electrochemical test results that the bare SS316L has the highest dissolution rate in both cathode and anode environments and the Ni implantation markedly reduces the dissolution rate. A significant improvement of interfacial contact resistance (ICR) is achieved for the SS316L implanted with nickel as compared to the bare SS316L, which is attributed to the reduction in passive layer thickness caused by the nickel implantation. The ICR values for implanted specimens increase with increasing dose.     

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.     

Properties of C-doped CrTiN films on the 316L stainless steel bipolar plate for PEMFC

C doped CrTiN films were deposited on 316L stainless steel by magnetron sputtering technology to investigate corrosion resistance and electrical conductivity. The sputtering current of the C target alter to obtain various C contents. The carbon target currents are 0 A, 3 A and 6 A, respectively. The result of SEM confirms that deposited films have a dense and uniform microstructure. CrTiN coating consist of Cr, CrN and TiN phases. With the increase of C carbon target currents, Cr crystal structure vanishes, and the amorphous carbon and carbides appear. The result of the potentiodynamic polarization test in the simulate PEMFC environment reveals that C doped CrTiN coating can improve samples’ corrosion resistance. At 1.1 V (vs. SHE) potentiostatic tests, the C-6A has the lowest current density, 6.09 × 10⁻⁷ A/cm². Interfacial contact resistance decreases with the addition of C atoms. The C-6A coated sample has the lowest interfacial contact resistance values, 5.5  mΩ cm².     

Carbon-polymer composite coatings for PEM fuel cell bipolar plates

A carbon-polymer composite coating on stainless steel 316L substrates was investigated for the use as bipolar plate material for polymer electrolyte membrane fuel cells. The coating consisted of 45 vol% graphite, 5 vol% carbon black and 50 vol% epoxy binder. The coating was applied by a spraying technique followed by hot-pressing while the binder cured. An interfacial contact resistance of 9.8 mΩ cm² at a compaction pressure of 125 N cm⁻² was measured. Ex-situ electrochemical tests showed that the carbon-polymer composite coated plates had smaller increases in the interfacial contact resistance after polarization than bare stainless steel plates at potentials of 0.0191 and 0.6191 V_SHE. At 1.0 V_SHE, the resistance increased similarly for both the coated plate and the bare stainless steel plate, and reached unacceptable values. The porosity of the coating was estimated with scanning electron microscope imaging of the cross-section of the coating to be about 50%.     

Novel hybrid coating of TiN and carbon with improved corrosion resistance for bipolar plates of PEM water electrolysis

Surface modification of Ti based bipolar plates (BPs) could be an effective solution to prevent the formation of low-conductive oxide film, promote the corrosion resistance and electronic conductivity as well. In this work, a novel hybrid coating composed of nano-sized TiN and carbonaceous phases (TiN–C) was successfully established on a Ti substrate via a combined electrophoretic deposition and thermal treatment process. Various physiochemical characterizations revealed that the coating was uniform, and the formation of the nano-sized TiN and carbonaceous composites reduced the interfacial contact resistance significantly. The potentiodynamic and potentiostatic polarization tests indicated that the hybrid coating TiN–C prepared at 400 °C had a corrosion current density of only 1.41 μA cm⁻², which was an order of magnitude lower than that (36.02 μA cm⁻²) of the bare Ti. Furthermore, the as-prepared coating showed excellent durability in both the simulated PEMWE environment and the PEMWE cell under polarization at 1.7 V for more than 300 h.