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.     

Molybdenum nitride modified AISI 304 stainless steel bipolar plate for proton exchange membrane fuel cell

A molybdenum nitride diffusion coating has been prepared on the surface of AISI 304 stainless steel (304 SS) by plasma surface diffusion alloying method as bipolar plate for proton exchange membrane fuel cell (PEMFC). X-ray diffraction data shows that the molybdenum nitride is face-centered-cubic Mo₂N phase. The results of scanning electron microscopy in combination with energy-dispersive X-ray analysis spectrometer indicate that the as-prepared molybdenum nitride diffusion coating consists of a ∼3.5 μm surface layer (molybdenum nitride) and a ∼0.5 μm subsurface layer (Mo and N solid solution). In addition, the average contact angle with water for modified 304 SS is 91°, demonstrating the better hydrophobic property of the surface modified 304 SS as compared to the untreated ones with average contact angle of 68°. Potentiodynamic and potentiostatic testing in simulated PEFMC operating conditions (0.05 M H₂SO₄ + 2 ppm F⁻ solution at 70 °C purged with either hydrogen or air) as well as interfacial contact resistance (ICR) measurement imply that the molybdenum nitride modified 304 SS exhibits improved corrosion resistance and promising ICR.     

Chromium nitride films on stainless steel as bipolar plate for proton exchange membrane fuel cell

A series of chromium nitride films are prepared on stainless steel substrates by pulsed bias arc ion plating (PBAIP) at different N₂ flow rate as bipolar plates for proton exchange membrane fuel cell (PEMFC). The film chemical composition and phase structure are characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffractometry (XRD). The characterization results indicate that the nitrogen content of deposited films varies from 0.28 to 0.50, and the phase structure changes from mixtures of Cr + Cr₂N, pure Cr₂N through Cr₂N + CrN, to pure CrN. The interfacial contact resistance between samples and carbon paper is measured by Wang's method, and a minimum value of 5.8 mΩ cm² is obtained under 1.2 MPa compaction force. The anticorrosion property is examined by potentiodynamic test in the simulated corrosive circumstance of the PEMFC under 25 °C, and the lowest corrosive current density of 5.9 × 10⁻⁷ A cm⁻² is obtained at 0.6 V (vs. SCE). Stainless steel substrates coated by the film with lowest contact resistance are chosen as the bipolar plates to assemble cells. An average voltage value of 0.62 V is achieved at 500 mA cm⁻², which is close to that of the cell with Ag-plated bipolar plates.     

Rapid coating preparation strategy for chromium nitride coated titanium bipolar plates of proton exchange membrane fuel cells

It is critical to develop a coating with sufficient comprehensive performances and efficient preparation strategy for the commercial application of metallic bipolar plate in proton exchange membrane full cell (PEMFC). In this work, chromium nitride coatings prepared by a rapid multi-arc ion plating (MIP) process with various nano thicknesses are investigated in the simulated PEMFC cathodic environments. Both the corrosion resistance and conductivity of the coatings increase with the growth of the coating thickness, which can be attributed to the increasing equivalent diameter, density, and area fraction of the droplets formed on the coating surfaces. The chromium nitride coating with a thickness of approximately 1.0 μm has the lowest I_0.6 V (0.594 μA cm^?2) and interfacial contact resistance (ICR, 6.54 mΩ cm²@1.4 MPa after corrosion test), achieving the 2025 technical targets proposed by the US Department of Energy for bipolar plates. This work shows that rapid preparation by MIP within 12 min is a potential strategy for chromium nitride coated titanium bipolar plates of PEMFCs at industrial scale.     

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 Cr₂N) 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 V₂O₃–Cr₂O₃, 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.     

A review of modified metal bipolar plates for proton exchange membrane fuel cells

In recent years, the proton exchange membrane fuel cell (PEMFC) has been widely studied due to its high energy efficiency and non-polluting products. As a key component of PEMFC, bipolar plates (BPPs) play an important role in isolating reaction gas, distributing flow field, collecting electrons and conducting heat. Metal BPPs have excellent manufacturing performance, low cost, and mechanical strength. Therefore, it is considered to be a powerful substitute for traditional graphite BPP. The surface modification of metal BPP is essential for its application in PEMFC. In this review, the latest developments in popular coatings were reviewed from the perspective of corrosion resistance, conductivity and contact angle of metal BPPs in PEMFC environments. The strengths and weaknesses of different surface modification methods were analyzed. Meanwhile,the development trend of future commercialization was also considered.     

Performance of surface chromizing layer on 316L stainless steel for proton exchange membrane fuel cell bipolar plates

Stainless steels as proton exchange membrane fuel cell bipolar plates have received extensive attention in recent years. The pack chromizing layer was fabricated on 316L stainless steel to improve the corrosion resistance and electrical conductivity. The corrosion properties were investigated in 0.5 M H₂SO₄ + 2 ppm HF solution at 70 °C purged with hydrogen gas and air. Higher electrochemical impedance and more stable passive film were obtained by chromizing the 316L stainless steel. Potentiodynamic polarization results showed the corrosion current densities were reduced to 0.264  μA cm⁻² and 0.222  μA cm⁻² in two simulated operating environments. In addition, the interfacial contact resistance was decreased to 1.4 mΩ⋅cm² under the compaction force of 140 N⋅cm⁻² and maintained at low values after potentiostatic polarization for 4 h. The excellent corrosion and conductive performances could be attributed to the chromium carbides and high alloying element content in chromizing layer.     

Performance of a high Cr and Ni austenitic stainless steel bipolar plates in proton exchange membrane fuel cell working environments

Electrochemical behavior of a high Cr and Ni austenitic stainless steel (HCN) is investigated and 316L SS in a simulated proton exchange membrane fuel cell environments is also investigated, and interfacial contact resistance (ICR) is measured before and after potentiostatic polarization. Both stainless steels underwent passivation in both anode and cathode environments for proton exchange membrane fuel cell. Passive current density of HCN is lower than that of 316L SS. An increase in ICR between carbon paper and HCN results from passive film formed during the potentiostatic polarization.     

High nitrogen stainless steel as bipolar plates for proton exchange membrane fuel cells

To investigate the applicability of high nitrogen (HN) austenitic stainless steel as bipolar plates for proton exchange membrane fuel cells (PEFCs), the polarization tests were carried out in synthetic solutions (0.05 M SO₄²⁻ (pHs 2.3, 4.3 and 5.5) +2 ppm F⁻) at 353 K. Interfacial contact resistance between the stainless steel and gas diffusion layer was measured before and after polarization. A single cell employing the HN stainless steel as bipolar plates was operated for 1000 h at 0.5 A cm⁻² (12.5 A). The single cell exhibited voltage drop of 17 mV during the operation. Corrosion products were scarcely detected for the HN stainless steel bipolar plate, as confirmed by scanning electron microscopy. After the polarization tests and single cell operation, XPS analyses were carried out to examine the resulting surface states. In the synthetic solutions to pH 4.3, the passive films mainly consisted of oxides enriched with Cr. When the solution pH was 5.5, on the other hand, the films were mainly composed of Fe-oxides. After the single cell operation for 1000 h, it was found that the passive films of the rib surface for the gas inlet part was mainly composed by Fe-oxides. On the other hand, the passive films for the ribs from center to gas outlet part were mainly made up of Cr-oxides. By combining the simulated and real operation environments, it is believed that the corrosion resistive Cr-oxides passive layer of the HN stainless steel obtained by the presence of nitrogen incorporated into the stainless steel could contribute to the maintenance of the higher cell voltage during the extensive cell operation.     

Design and manufacturing of stainless steel bipolar plates for proton exchange membrane fuel cells

Stainless steel bipolar plates (BPPs) are regarded as promising alternatives to traditional graphite BPPs in proton exchange membrane fuel cells (PEMFCs). This technology has experienced more than 20 years development and has been partially applied in industrial production. This review surveys recent progress of entire development process for stainless steel BPPs in terms of flow field design, microforming process, joining process and coating process. Besides, assembly process considering dimensional error, shape error and assembly error are comprehensively summarized as well. Finally, technical challenges and future trends are presented for the application of stainless steel BPPs for PEMFCs.     

Thermal nitridation of chromium electroplated AISI316L stainless steel for polymer electrolyte membrane fuel cell bipolar plate

The electrical and corrosion properties of surface-nitrided AISI316L stainless steel are evaluated to assess the potential use of this material as a bipolar plate for a polymer electrolyte membrane fuel cell. Chromium is electroplated on the surface of the AISI316L stainless steel before nitridation. The nitriding condition is selected so as to form Cr₂N nitride only and the result is compared with that of a CrN + Cr₂N nitride coating. The stainless steels with the Cr₂N nitride protective coating layer exhibit better interfacial contact resistance and corrosion resistance than the as-rolled or (CrN + Cr₂N)-coated AISI316L stainless steels.     

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.     

Hydrophobic octadecylamine-functionalized graphene/TiO2 hybrid coating for corrosion protection of copper bipolar plates in simulated proton exchange membrane fuel cell environment

In the present work, G-TiO₂ and G-ODA-TiO₂ hybrids were prepared by concurrent surface functionalization and reducing of graphene oxide (GO) using octadecylamine (ODA). The G-TiO₂ and G-ODA-TiO₂ powders were deposited on the copper surface by electrophoretic deposition (EPD) technique. The wettability of coatings revealed the preferable hydrophobic characteristic of G-ODA-TiO₂ compared to G-TiO₂ and bare copper with water contact angles of 130°, 101°, and 87°, respectively. The anti-corrosion performance of specimens in a 0.5 M H₂SO₄ solution was appraised by the potentiodynamic polarization (Tafel analysis), which clearly showed that G-TiO₂ and G-ODA-TiO₂ coatings can act as a great barrier for copper in the corrosive H₂SO₄ solution. The corrosion inhibition of G-TiO₂ and G-ODA-TiO₂ was about 2 and 15 times higher than bare copper. Moreover, the hydrophobic G-ODA-TiO₂ coating on copper reached a much lower interfacial contact resistance (ICR) than the other samples.     

Effects of low-temperature nitridation on the electrical conductivity and corrosion resistance of 446M stainless steel as bipolar plates for proton exchange membrane fuel cell

Low-temperature nitridation was used to form a protective and conductive layer on stainless steel. The surface characterization reveals that a continuous and protective Cr-nitride/oxide layer (CrN and Cr₂O₃) forms on the 446M stainless steel surface after low-temperature nitridation. The electrical conductivity of the sample is investigated in terms of the interfacial contact resistance. This value for nitrided 446M at low temperature is 6 mΩ cm², which is much lower than that of the bare 446M stainless steel (about 77 mΩ cm²) at a compaction force of 140 N/cm². The corrosion resistance of low-temperature nitrided 446M stainless steel is examined in potentiodynamic and potentiostatic tests under simulated polymer electrolyte membrane fuel cell (PEMFC) conditions with pH 3 H₂SO₄ at 80 °C. In a simulated anode condition, the current density is −1 × 10⁻⁶ A/cm². In a simulated cathode condition, the current density is 1 × 10⁻⁷ A/cm². Low-temperature nitrided 446M stainless steel shows superior electrical conductivity and corrosion resistance than bare 446M stainless steel.     

Chromium-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.     

Conducting polymer-coated stainless steel bipolar plates for proton exchange membrane fuel cells (PEMFC)

Stainless steel satisfies many of the requirements for proton exchange membrane (PEM) fuel cell bipolar plates except its corrosion under fuel cell operating conditions. Metal oxide formation leads to contact resistance, and metal dissolution can cause contamination of the membrane electrode assembly (MEA). These problems can be solved by coating stainless steel plates with corrosion resistant and conductive layers. In this study, 304 stainless steel was coated electrochemically with the conducting polymers polyaniline (PANI) and polypyrrole (PPY). Cyclic voltammetry was used for the polymerization and deposition of these polymers. The polymer-coated stainless steel plates were tested for corrosion and contact resistance under PEM fuel cell conditions, which showed improved corrosion resistance with acceptable contact resistance.     

Corrosion behavior of tantalum coatings on AISI 316L stainless steel substrate for bipolar plates of PEM fuel cells

Corrosion resistance of tantalum coatings 30 μm thick deposited by chemical vapor deposition on SS316L coupons has been evaluated by electrochemical impedance spectroscopy (EIS). To this end, anodic and cathodic operating conditions of proton exchange membrane fuel cells (PEMFC) have been simulated in a three-electrode heated corrosion cell. Interfacial contact resistance (ICR), contact angle and durability tests have been performed in long-term tests (>100 h) polarizing the electrode to 1.193 V vs. Ag/AgCl. Results obtained by different experimental techniques show a dense coating structure with a high polarization resistance, mainly formed by surface crystals of α-Ta (bcc), Ta₂O₅ and carbon. An atomic ratio (in %) of oxide to metallic species (Ta_ox/Ta_met) of 4.8 was verified from XPS spectra, which is slightly increased to 6.23 after the anodizing treatment. The modified surface composition yielded a coating capacity higher than the amorphous oxide, favoring the in-plane electrical conduction. After the treatment, no noticeable changes were observed neither in surface morphology nor in contact angle (>90°). ICR values in the range of 22.3–32.6 mΩ cm² were obtained for a clamping pressure of 140 N cm⁻². No morphological changes or loss of coating adherence were observed during the long-term tests.     

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.     

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.     

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.