Electrophoretic deposition of polypyrrole/Vulcan XC-72 corrosion protection coatings on SS-304 bipolar plates by asymmetric alternating current for PEM fuel cells

In this study, corrosion protection composite films composed of polypyrrole (formed from the electropolymerization of pyrrol monomer) and Vulcan XC-72, labeled as PPy-C, were deposited on SS-304 substrates by EPD using an asymmetric AC signal (AAC) in acetone and acetone:methanol solutions. Prior to deposition, Vulcan powders were functionalized in order to create functional groups that develop a negative surface charge. Under the applied electric field, both pyrrole monomer and carbon colloidal particles migrated simultaneously toward the SS anode surface to create the corrosion protection PPy-C coatings. The effect of applied voltages (50, 70, 90 and 120 V) and deposition time (2, 6, 10 and 15 min) on the film characteristics was evaluated. The coatings were analyzed by SEM and the results showed improved characteristics of the films deposited by AAC, in terms of homogeneity and porosity, compared to samples prepared by a continuous DC process. Polarization curves were conducted in order to evaluate the corrosion characteristics of the bipolar plates in 0.1 M H₂SO₄ at room temperature. The experimental results indicated that the PPy-C composites significantly enhanced the corrosion resistance of the SS substrates. For example, the T1 sample increased the corrosion potential by about 541 mV and decreased the corrosion current density by two orders of magnitude, compared to uncoated SS. FTIR studies confirmed the formation of PPy after electropolymerization of the pyrrole monomer under the EPD process. Even though this characterization has been carried out in a half cell, it has been proposed that the PPy-C-coated SS bipolar plates may find application in the acidic atmosphere of PEMFCs.     

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.     

Protective coatings for metal bipolar plates of fuel cells: A review

Proton exchange membrane fuel cell has attracted much attention in recent years due to their advantages of environmental protection and high resource utilization, which is important for improving the global environment. Bipolar plates are the important components of fuel cell, which accounts for most of the weight and high cost. Compared with graphite bipolar plates, metal bipolar plates are easier to machining and have lower cost because of its good mechanical properties. However, in the acidic environment of proton exchange membrane fuel cell operation, metal bipolar plates are prone to corrosion, which leads to lower output efficiency of fuel cell and seriously affected the application. Applying a protective coating to the metal bipolar plates is an effective way to improve its corrosion resistance. This paper mainly introduces the research progress of several anti-corrosion coatings for metal bipolar plates in recent years, and summarizes the challenges and future requirements of metal bipolar plates.     

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%.     

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.     

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.     

Assessment of the durability of low-cost Al bipolar plates for High Temperature PEM fuel cells

The present research aims to assess the durability of a single cell formed by a commercial high-temperature membrane electrode assembly (MEA) and Al bipolar plates (Al-BPPs) coated with Ni-P. To this end, an accelerated degradation test based on voltage load cycles is carried out, and the effects of the degradation on the MEA and its impact on the Al-coated bipolar plates are analyzed. They are compared with the results obtained testing a similar cell formed by the same MEA with graphite plates. MEAs from both cells were analyzed by different characterization techniques before and after the degradation tests (XRD, SEM, TEM). The performance of the Al-based cell is initially better, but it suffers a faster deterioration mainly due to the degradation of the Ni-P surface coating. A green deposit is formed, probably due to the chemical reaction of the phosphoric acid leached from the MEA. These deposits cause an increase in the contact resistance of the plates, local channel blockages and, eventually, a larger degradation. This work proves that PBI membranes still need to be improved to ensure their long-term durability.     

The effect of pH and halides on the corrosion process of stainless steel bipolar plates for proton exchange membrane fuel cells

Stainless steel is attractive as material for bipolar plates in proton exchange membrane fuel cells, due to its high electrical conductivity, high mechanical strength and relatively low material and processing cost. Potentiostatic and potentiodynamic tests were performed in H₂SO₄ solutions on AISI 316L stainless steel bipolar plates with etched flow fields. The effect of pH and presence of small amounts of fluoride and chloride on the corrosion rate and interfacial contact resistance of the stainless steel bipolar plate were investigated. The tests performed in electrolytes with various pH values revealed that the oxide layer was thinner and more prone to corrosion at pH values significantly lower than the pH one expects the bipolar plate to experience in an operating proton exchange membrane fuel cells. The use of solutions with very low pH in such measurements is thus probably not the best way of accelerating the corrosion rate of stainless steel bipolar plates. By use of strongly acidic solutions the composition and thickness of the oxide layer on the stainless steel is probably altered in a way that might never have happened in an operating proton exchange membrane fuel cell. Additions of fluoride and chloride in the amounts expected in an operating fuel cell (2 ppm F⁻ and 10 ppm Cl⁻) did not cause significant changes for neither the polarization- nor the contact resistance measurements. However, by increasing the amount of Cl⁻ to 100 ppm, pitting was initiated on the stainless steel surface.     

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.     

Investigation on the corrosion resistance of carbon black-graphite-poly(vinylidene fluoride) composite bipolar plates for polymer electrolyte membrane fuel cells

The goal of the present work was to evaluate the corrosion resistance of carbon black (CB)-synthetic graphite (SG)-poly(vinylidene fluoride) (PVDF) composites using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves. The tests were conducted in 0.5 M H₂SO₄ + 2 ppm HF solution at 70 °C to simulate the typical environment of polymer electrolyte membrane fuel cells. The fracture surface of the specimens was characterized by scanning electron microscopy. The through-plane electrical conductivity was also determined. The corrosion resistance decreased as the carbon black content increased up to 5 wt.%. The highest electrical conductivity was achieved for the composition CB = 5 wt.%, PVDF = 15 wt.%, SG = 80 wt.%. A detailed discussion of the EIS data is given. This approach is unprecedented in the current literature. EIS has proven to be a valuable tool to the design of electrically efficient bipolar plates.     

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.     

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.     

Evaluation of coated metallic bipolar plates for polymer electrolyte membrane fuel cells

Metallic bipolar plates for polymer electrolyte membrane (PEM) fuel cells typically require coatings for corrosion protection. Other requirements for the corrosion protective coatings include low electrical contact resistance, good mechanical robustness, low material and fabrication cost. The authors have evaluated a number of protective coatings deposited on stainless steel substrates by electroplating and physical vapor deposition (PVD) methods. The coatings are screened with an electrochemical polarization test for corrosion resistance; then the contact resistance test was performed on selected coatings. The coating investigated include Gold with various thicknesses (2 nm, 10 nm, and 1 μm), Titanium, Zirconium, Zirconium Nitride (ZrN), Zirconium Niobium (ZrNb), and Zirconium Nitride with a Gold top layer (ZrNAu). The substrates include three types of stainless steel: 304, 310, and 316. The results show that Zr-coated samples satisfy the DOE target for corrosion resistance at both anode and cathode sides in typical PEM fuel cell environments in the short-term, but they do not meet the DOE contact resistance goal. Very thin gold coating (2 nm) can significantly decrease the electrical contact resistance, however a relatively thick gold coating (>10 nm) with our deposition method is necessary for adequate corrosion resistance, particularly for the cathode side of the bipolar plate.     

Polyaniline and polypyrrole coatings on aluminum for PEM fuel cell bipolar plates

The conducting polymers polypyrrole and polyaniline were deposited on 6061 aluminum using cyclic voltammetry and painting, respectively. These samples are intended for proton exchange membrane fuel cell applications where surface contact resistance as well as bulk corrosion resistance are requirements for the bipolar plates that separate the cells. Corrosion current and voltage were measured on the samples as well as contact resistance between coated samples as a function of contact pressure. The polypyrrole samples showed neither improved corrosion resistance nor acceptable contact resistance. The painted polyaniline samples, however, showed about an order of magnitude reduction in corrosion current with only a minor increase in contact resistance. It is believed that in the more acidic environment of a fuel cell, the polyaniline will become even more conductive and that further reduction in contact resistance should be possible.     

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.     

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.     

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.     

Synthesis and Ex-Situ characterizations of diamond-like carbon coatings for metallic bipolar plates in PEM fuel cells

Metallic bipolar plates can significantly increase the power density of polymer electrolyte membrane fuel cells; however, they require corrosion-protection coatings with desirable physical and chemical properties. In this study, diamond-like carbon (DLC) film coatings are investigated as such coatings potentially for metallic-based bipolar plates, with the focus on the relation between the processes and properties of the coatings under different coating deposition conditions of Plasma Enhanced Chemical Vapor Depsoition (PECVD) method. Various characterization techniques are applied to study the adhesion, structure, morphology, wettability, corrosion, and electrical resistivity of the film coatings. XPS, EDAX, and SEM analyses are used to identify the ratio of sp³ (diamond-like) and sp² (graphite-like) bonds in the coatings, surface elements, and surface morphology, respectively. Potentiodynamic polarization test is utilized to investigate the corrosion behaviors of substrates with and without DLC coatings. Further, the electrical resistivity of the DLC films is measured by the four-point probe method. The results indicate that higher deposition power along with the absence of argon gas results in more sp³ than sp² bonds in the coating, and the electrical resistivity is increased accordingly. The coating films deposited from methane (CH₄) exhibit superior adhesion to the stainless steel (SS316) substrates over those generated from acetylene (C₂H₂) gas. Coating films deposited on the metallic substrates change the surface wettability appreciably. Further, polarization tests show that coatings generated with a low power of 250 W and higher argon gas percentage of 30% provide better anti-corrosion protection for metallic-based bipolar plates.     

Durability and characterization studies of polymer electrolyte membrane fuel cell’s coated aluminum bipolar plates and membrane electrode assembly

Coated aluminum bipolar plates demonstrate better mechanical strength, ease of manufacturability, and lower interfacial contact resistance (ICR) than graphite composite plates in polymer electrolyte membrane (PEM) fuel cell applications. In this study, coated aluminum and graphite composite bipolar plates were installed in separate single PEM fuel cells and tested under normal operating conditions and cyclic loading. After 1000 h of operation, samples of both the bipolar plates and the membrane electrode assembly (MEA) were collected from both the cathode and the anode sides of the cell and characterized to examine the stability and integrity of the plate coating and evaluate possible changes of the ionic conductivity of the membrane due any electrochemical reaction with the coating material. Scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis were performed on the land and valley surfaces of the reactant flow fields at both the anode and the cathode sides of the bipolar plates. The measurements were superimposed on the reference to identify possible zones of anomalies for the purpose of conducting focused studies in these locations. The X-ray diffraction (XRD) analysis of samples scraped from the anode and cathode electrodes of the MEA showed the tendency for catalyst growth that could result in power degradation. Samples of the by-product water produced during the single fuel cell operation were also collected and tested for the existence of chromium, nickel, carbon, iron, sulfur and aluminum using mass spectroscopy techniques. The EDX measurements indicated the possibility of dissociation and dissolution of nickel chrome that was used as the binder for the carbide-based corrosion-resistant coating with the aluminum substrate.     

Micro-roll forming of stainless steel bipolar plates for fuel cells

Stainless steel bipolar plates for use in proton exchange membrane (PEM) fuel cells have been identified as a lighter and cheaper alternative to graphite plates. Current manufacturing of metal bipolar plates by hydroforming or micro-stamping leads to excessive stretching of the material and therefore limits the channel depths that can be formed. Low channel depths for the bipolar plates will result in low overall fuel cell efficiency. In comparison, the bending-dominated deformation mode present in roll forming provides the potential to form metal bipolar plates with less thinning and to greater channel depths. In this work, the roll forming process is employed for the first time to form thin stainless steel sheets to micro-scale channel sections of the kind required for bipolar plates. This paper describes the process and machine design as well as the establishment of the forming methodology. Experimental trials are performed and the final part quality is evaluated in terms of material thinning, longitudinal bow and cross-sectional shape. The process was numerically analysed to understand the causes of the forming problems and shape defects observed in the experimental trials. The results of this work show that roll forming of micro-scale corrugated bipolar sheets is feasible. Furthermore, the findings provide a summary of both the practical difficulties and the possible advantages of using micro-roll forming to manufacture improved thin metal micro-corrugations for bipolar plates.