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.     

Investigation of bio-inspired flow channel designs for bipolar plates in proton exchange membrane fuel cells

Proton exchange membrane (PEM) fuel cell performance is directly related to the flow channel design on bipolar plates. Power gains can be found by varying the type, size, or arrangement of channels. The objective of this paper is to present two new flow channel patterns: a leaf design and a lung design. These bio-inspired designs combine the advantages of the existing serpentine and interdigitated patterns with inspiration from patterns found in nature. Both numerical simulation and experimental testing have been conducted to investigate the effects of two new flow channel patterns on fuel cell performance. From the numerical simulation, it was found that there is a lower pressure drop from the inlet to outlet in the leaf or lung design than the existing serpentine or interdigitated flow patterns. The flow diffusion to the gas diffusion layer was found be to more uniform for the new flow channel patterns. A 25 cm² fuel cell was assembled and tested for four different flow channels: leaf, lung, serpentine and interdigitated. The polarization curve has been obtained under different operating conditions. It was found that the fuel cell with either leaf or lung design performs better than the convectional flow channel design under the same operating conditions. Both the leaf and lung design show improvements over previous designs by up to 30% in peak power density.     

Evaluation of silver-coated stainless steel bipolar plates for fuel cell applications

In this study, computer-aided design and manufacturing (CAD/CAM) technology were applied to develop and produce stainless steel bipolar plates for DMFC (direct methanol fuel cell). Effect of surface modification on the cell performance of DMFC was investigated. Surface modifications of the stainless steel bipolar plates were made by the electroless plating method. A DMFC consisting of silver coated stainless steel as anode and uncoated stainless steel as cathode was assembled and evaluated. The methanol crossover rate (R_c) of the proton exchange membrane (PEM) was decreased by about 52.8%, the efficiency (E_f) of DMFC increased about 7.1% and amounts of methanol electro-oxidation at the cathode side (M_co) were decreased by about 28.6%, as compared to uncoated anode polar plates. These measurements were determined by the transient current and mathematical analysis.     

Corrosion resistance of chromized 316L stainless steel for PEMFC bipolar plates

Corrosion resistance of the chromized 316L stainless steel was studied in a proton exchange membrane fuel cell (PEMFC) operating condition. Cr-rich surface layer was formed by pack cementation technique and electrochemical properties of the chromized surface were examined by potentiodynamic and potentiostatic tests. Results showed that the Cr-rich layers underneath the free surface passivated the surface and protect the surface from corrosion in 0.5 M H₂SO₄ solution at 80 °C. However, the Cr-rich layers showed columnar grains with voids when the stainless steel was pack cemented for an extended period of time, resulting in drastic degradation of corrosion resistance. The optimum condition for the best corrosion resistance in the PEMFC operating condition was obtained without sacrificing the interfacial contact resistance.     

Development of titanium bipolar plates fabricated by additive manufacturing for PEM fuel cells in electric vehicles

Bipolar plates (BPs) are one of the main parts of proton exchange membrane (PEM) fuel cell stacks, which constitute a significant percentage of a PEM fuel cell system in terms of cost, weight, and structural strength. Although frequently used graphite BPs have low density, high conductivity, and high corrosion resistance, machining the desired flow channels on these plates is challenging. On the other hand, BPs made of various materials rather than graphite can be also fabricated by additive manufacturing methods. These methods can be considered as a reasonable alternative to conventional machining for the fabrication of graphite BPs in PEM fuel cells regarding material cost, fabrication of flow channels, and some post-processes in which the large-scale manufacturing of graphite BPs is more complex. This study offers a comparison of formed stainless-steel, additive manufactured titanium and machined composite graphite plates having the same flow-field geometry as a bipolar plate. In addition, titanium BPs are coated with gold and their performances are compared. Among the cells tested, the highest peak power of 639 mWcm^?2 is measured from the cell with 450 nm gold coated titanium BP, whereas those of the cell with conventional graphite and stainless-steel BP are only around 322 mWcm^?2 and 173 mWcm^?2, respectively. Moreover, a new titanium bipolar plate design providing high specific power density is also presented.     

Reduced contact resistance of PEM fuel cell's bipolar plates via surface texturing

Polymer electrolyte fuel cell performance strongly depends on properties of the stack bipolar plates. Stainless steel, being an attractive material for bipolar plates, raises major concern as having a high contact resistance. It is assumed that most of this contact resistance is governed by electrical properties of the developed oxide surface film. Accurate consideration of existing data and measurements of mechanically treated stainless steel/carbon interface reveals a substantial influence of surface topography on the contact resistance. Contact resistance may change tenfold, depending on substrate surface treatment and roughness. A model describing carbon/stainless steel interface is introduced, explaining the observed behavior.     

Corrosion of metal bipolar plates for PEM fuel cells: A review

PEM fuel cells are of prime interest in transportation applications due to their relatively high efficiency and low pollutant emissions. Bipolar plates are the key components of these devices as they account for significant fractions of their weight and cost. Metallic materials have advantages over graphite-based ones because of their higher mechanical strength and better electrical conductivity. However, corrosion resistance is a major concern that remains to be solved as metals may develop oxide layers that increase electrical resistivity, thus lowering the fuel cell efficiency. This paper aims to present the main results found in recent literature about the corrosion performance of metallic bipolar plates.     

Localised corrosion processes of austenitic stainless steel bipolar plates for polymer electrolyte membrane fuel cells

This research addresses the problem of localised corrosion of stainless steel PEMFC bipolar plates. The susceptibility to pitting and crevice corrosion of austenitic AISI 304 stainless steel has been investigated both by post-mortem microscopic analysis of the end-plates of a laboratory single-cell and by studies of electrochemically corroded stainless steels, in the presence of specially-designed crevice-formers simulating the operating conditions of a PEMFC. This work is based on optical and scanning-electron microscopies as well as potentiostatic and potentiodynamic measurements. The crevice-formers we considered were: Teflon, graphite and AISI 304. The samples, coupled to the crevice-formers have been tested in aqueous solutions containing Cl⁻, SO₄²⁻ and F⁻. From the E-log i plot, the values of corrosion, pitting, crevice and protection potential have been obtained and perfect and imperfect passivity conditions have been identified.     

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.     

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.     

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

Thermal (gas) nitridation of stainless steel alloys can yield low interfacial contact resistance (ICR), electrically conductive and corrosion-resistant nitride containing surface layers (Cr₂N, CrN, TiN, V₂N, VN, etc.) of interest for fuel cells, batteries, and sensors. This paper presents results of proton exchange membrane (PEM) single-cell fuel cell studies of stamped and pre-oxidized/nitrided developmental Fe-20Cr-4V weight percent (wt.%) and commercial type 2205 stainless steel alloy foils. The single-cell fuel cell behavior of the stamped and pre-oxidized/nitrided material was compared to as-stamped (no surface treatment) 904L, 2205, and Fe-20Cr-4V stainless steel alloy foils and machined graphite of similar flow field design. The best fuel cell behavior among the alloys was exhibited by the pre-oxidized/nitrided Fe-20Cr-4V, which exhibited ∼5-20% better peak power output than untreated Fe-20Cr-4V, 2205, and 904L metal stampings. Durability was assessed for pre-oxidized/nitrided Fe-20Cr-4V, 904L metal, and graphite plates by 1000+ h of cyclic single-cell fuel cell testing. All three materials showed good durability with no significant degradation in cell power output. Post-test analysis indicated no metal ion contamination of the membrane electrode assemblies (MEAs) occurred with the pre-oxidized and nitrided Fe-20Cr-4V or graphite plates, and only a minor amount of contamination with the 904L plates.     

Corrosion protection of type 304 stainless steel bipolar plates of proton-exchange membrane fuel cells by doped polyaniline coating

Polyaniline coating doped with dodecylbenzesulfonate anions is electrodeposited galvanostatically on type 304 stainless steel used as bipolar plates of proton-exchange membrane fuel cell from a basic solution of 0.3 M aniline monomer solution containing sodium dodecylbenzesulfonate as a supporting electrolyte. Electrochemical measurements in 1 M H₂SO₄ and in 0.3 M HCl show that the polyaniline coating increases the free corrosion potential of the steel by more than 300 mV and 450 mV, respectively, with a corrosion rate more than two orders of magnitude lower than that of the uncoated steel. Long-term exposure studies show that the coating is highly stable and inhibits the corrosion of the steel effectively.     

An investigation into the effects of a nano-thick gold interlayer on polypyrrole coatings on 316L stainless steel for the bipolar plates of PEM fuel cells

Polypyrrole is one of the most important conductive polymers because it is easily oxidized, water soluble and commercially available. Also, polypyrrole coatings have potential applications in batteries, fuel cells, electrochemical sensors, anti-corrosion coatings and drug delivery systems. In this study, a very thin gold layer was first coated on SS316L, and then a polypyrrole coating was laid on top. The nucleation and growth mechanisms of polypyrrole on the gold-coated SS316L were studied by electrochemical nucleation and growth techniques. SEM was used to characterize the polypyrrole coating morphology. Potentiodynamic tests were performed to determine the corrosion parameters of the polypyrrole coatings. Potentiostatic tests of the coated SS316L were conducted in simulated anode and cathode environments of a PEM fuel cell. The simulated anode environment was at a potential of about −0.1 V versus SCE purged with H₂ and the simulated cathode environment was at a potential of about 0.6 V versus SCE purged with O₂. After coating with Au and polypyrrole, the polarization resistance of SS316L is increased about six times, and the corrosion current density is decreased about seven times, compared to the base SS316L. Also, our calculations show that the metal ion concentration in solution for the polypyrrole/Au/SS316L had met the target of 10 ppm after 5000 h fuel cell operation.     

Niobium diffusion modified austenitic stainless steel bipolar plate for direct methanol fuel cell

An austenitic stainless steel with a niobium diffusion protective layer is evaluated for bipolar plate of direct methanol fuel cell (DMFC). Corrosion resistance of niobium diffusion modified AISI 304 stainless steel (niobized 304 SS) is investigated in simulated DMFC cathodic environment (0.5 M H₂SO₄ + 2 ppm HF + 0.1 M methanol solution at 50 °C) and anodic environment (0.5 M H₂SO₄ + 2 ppm HF + (1 M, 10 M and 20 M) methanol solution at 50 °C), respectively. Potentiodynamic, potentiostatic as well as electrochemical impedance spectroscopy tests show that, comparing with a bare 304 SS, the corrosion current density of niobized 304 SS is reduced greatly while the polarization resistance is raised in the simulated DMFC cathodic environment. Corrosion tests in the simulated anodic environment are applied to examine the effect of methanol on the corrosion behaviour of niobized 304 SS. It is interesting to find that the niobized 304 SS shows better corrosion resistance in the higher methanol concentration solutions.     

Investigation of formability of metallic bipolar plates via stamping for light-weight PEM fuel cells

Bipolar plates (BPs) are one of the main members which constitute a significant percentage of a fuel cell system in terms of cost, weight and structural strength. Although frequently used graphite BPs have low density, high conductivity and corrosion resistance, machining the desired flow channels on the graphite plates is an important issue. On the other hand, metallic BPs can be considered a reasonable alternative material to graphite in the view of the material cost, fabrication of flow channels and some post-processes in which the large-scale manufacturing of graphite BPs is more complex compared to cutting and stamping processes for metal ones. This study offers a comparison of the formability of four different metals with four flow channel depths as bipolar plates formed by stamping. 304 Stainless Steel (SS 304), pure Titanium - Grade2 (CP–Ti) and Aliminium (Al 6016 and Al 3104) are chosen as the BP materials. A serpentine type flow channel with two different channel widths are formed on to 0.1 mm thick sheets. The channel width is chosen as 1.2 mm and 1.8 mm for the channel depths of 0.36 mm–0.55 mm, and 0.54 mm–0.82 mm, respectively. The stamping processes of the BPs materials are simulated via commercially available eta/Dynaform v5.9.4. software and formability characteristics are obtained for sixteen various cases. As a result, it is determined that SS 304 is the more appropriate material in the view of the formability for such a complex form.     

Nitride films as protective layers for metallic bipolar plates of polymer electrolyte membrane fuel cell stacks

The idea of using nitride films coated by sputtering Nb and Cr, using N₂ as a reaction gas, as protective layers for metallic bipolar plates (BP) of polymer electrolyte membrane fuel cell (PEMFC) stacks, was explored by experiments. Specimens were fabricated from austenite 304 stainless steels, which are frequently used as bipolar plate materials for fuel cells due to their good corrosion resistance and high strength at elevated temperature. The results of XRD analysis and Gaussian function analysis of the coated films suggest that NbN or NbN/NbCrN films were induced depending on the process parameters. The NbN/NbCrN multiphase films were induced at high Cr target powers and low gas ratios among the process parameters selected in this investigation. The result of ICR measurement of the films suggests that the effect of the Cr target power, i.e. the effect of the Cr amount in the film, on the ICR of the films is not significant, while the effect of the gas ratio on the ICR of the films is noticeable. For the films deposited at different gas ratios, the ICR of the film generally decreases as the gas ratio increases. In general, all the 304 SS specimens coated by the NbN single phase or NbN/NbCrN multiphase films at the given process parameters showed significantly improved corrosion resistance in comparison with the bare 304 SS. Among NbN single phase and NbN/NbCrN multiphase films, the performance of NbN/NbCrN multiphase films was more stable. Depending on the process parameters, the polarization curves of the specimens coated with NbN films showed rapid increase of the current density due to the pitting. Therefore, as corrosion resistance coating for metallic BP of PEMFC stacks, NbN/NbCrN multiphase film may be preferred to NbN single phase films.     

Development of bipolar plates with different flow channel configurations for fuel cells

Bipolar plates include separate gas flow channels for anode and cathode electrodes of a fuel cell. These gases flow channels supply reactant gasses as well as remove products from the cathode side of the fuel cell. Fluid flow, heat and mass transport processes in these channels have significant effect on fuel cell performance, particularly to the mass transport losses. The design of the bipolar plates should minimize plate thickness for low volume and mass. Additionally, contact faces should provide a high degree of surface uniformity for low thermal and electrical contact resistances. Finally, the flow fields should provide for efficient heat and mass transport processes with reduced pressure drops. In this study, bipolar plates with different serpentine flow channel configurations are analyzed using computational fluid dynamics modeling. Flow characteristics including variation of pressure in the flow channel across the bipolar plate are presented. Pressure drop characteristics for different flow channel designs are compared. Results show that with increased number of parallel channels and smaller sizes, a more effective contact surface area along with decreased pressured drop can be achieved. Correlations of such entrance region coefficients will be useful for the PEM fuel cell simulation model to evaluate the affects of the bipolar plate design on mass transfer loss and hence on the total current and power density of the fuel cell.     

Conductive amorphous carbon-coated 316L stainless steel as bipolar plates in polymer electrolyte membrane fuel cells

Amorphous carbon (a-C) film about 3 μm in thickness is coated on 316L stainless steel by close field unbalanced magnetron sputter ion plating (CFUBMSIP). The AFM and Raman results reveal that the a-C coating is dense and compact with a small size of graphitic crystallite and large number of disordered band. Interfacial contact resistance (ICR) results show that the surface conductivity of the bare SS316L is significantly increased by the a-C coating, with values of 8.3–5.2 mΩ cm² under 120–210 N/cm². The corrosion potential (E_corr) shifts from about −0.3 V vs SCE to about 0.2 V vs SCE in both the simulated anode and cathode environments. The passivation current density is reduced from 11.26 to 3.56 μA/cm² with the aid of the a-C coating in the simulated cathode environment. The a-C coated SS316L is cathodically protected in the simulated anode environment thereby exhibiting a stable and lower current density compared to the uncoated one in the simulated anode environment as demonstrated by the potentiostatic results.     

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.     

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.