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.     

Electrically conductive polymer composite coating on aluminum for PEM fuel cells bipolar plate

Aluminum has many advantages for commercial bipolar plate of PEM fuel cell such as light weight, low cost and easy manufacturing. However, it has a low corrosion resistance under a PEM fuel cell operation condition that is a special issue of all metal bipolar plates. In this study, polypropylene composite coated with aluminum bipolar plates were fabricated to improve the corrosion resistance. However, contact resistance of polymer composite coated aluminum bipolar plate is highly increased due to high contact resistance between aluminum substrate and composite layer. Two different types of inter layers were added to improve the contact resistance. Carbon paper attached and carbon black added samples were fabricated between aluminum substrate and composite. Polyamide-imide/carbon black composite adhesive was used for carbon paper attached on the aluminum plate. The contact resistance of carbon paper attached sample was lower than that of carbon black added sample. And, corrosion resistance was tested by potentiodynamic and potentiostatic methods. The composite coated aluminum attached to carbon paper exhibited properties suitable for PEM fuel cells.     

Study on dimensional and corrosion properties of thixoformed A356 and AA7075 aluminum bipolar plates for proton exchange membrane fuel cells

Metallic bipolar plates for polymer exchange membrane (PEM) fuel cells are currently manufactured by stamping of thin sheets. However, there are dimensional and shape errors of microchannels because of forming limitation such as spring back of thin sheets after stamping. On the other hand, stamping process is limited to commercially available sheet alloys, which restricts the development of a high corrosion resistant substrate aluminum alloy. Here, thixoforming (a commercial semisolid route) that is applicable to a wide range of aluminum alloys is proposed for net-shape micromanufacturing of aluminum bipolar plates with high dimensional stability. High corrosion resistance cast A356 (Cu-free) and wrought AA7075 (∼2% Cu) aluminum billets are used for this study. Initial billets are heated at different semisolid temperatures. Subsequently, the semisolid slurries are injected into the die cavity. A356 and AA7075 aluminum bipolar plates are successfully fabricated by thixoforming with very small deviation of 0.7% and 1.5% from the nominal value of 0.300 mm in the microchannel depth, respectively. A multilayer coating of TiN/CrN is deposited on the surface of thixoformed bipolar plates through a commercially available magnetron sputtering technique. Electrochemical corrosion tests show that coated-thixoformed A356 (Cu-free) bipolar plates have significantly lower corrosion current densities than coated-thixoformed AA7075 (∼2% Cu) bipolar plates. This seems to be due to the deleterious effect of Cu alloying element on the corrosion resistance of aluminum alloys that clearly confirms the importance of substrate material development for corrosive PEM fuel cell environment. It is suggested that specific high corrosion resistance aluminum alloy for PEM fuel cell application can be simply designed and then thixoforming can be efficiently and cost effectively employed to fabricate net-shape aluminum bipolar plates.     

Performance of PEMFC stack using expanded graphite bipolar plates

The bipolar plates are in weight and volume the major part of PEM fuel cell stack, and also a significant effect to the stack cost. To develop the low-cost and low-weight bipolar plate for PEM fuel cell, we have developed a kind of cheap expanded graphite plate material and a production process for fuel cell bipolar plates. The plates have a high electric conductivity and low density, and can be stamped directly forming fuel cell bipolar plates. Then, 1 and 10 kW stacks using expanded graphite bipolar plates are successfully assembled. The contact resistance of the bipolar plate is investigated and the electrochemical performances of the fuel cell stacks are tested. Good fuel cell performance is obtained and the voltage distribution among every single cell in the stacks is very uniform.     

Review on current research of materials,fabrication and application for bipolar plate in proton exchange membrane fuel cell

Bipolar plate (BP) in proton exchange membrane (PEM) fuel cells provides conducting paths for electrons between cells, distributes and blocks the reactant gases, removes waste heat, and provides stack structural integrity. It is a key component to ensure the aforementioned functions while maintaining a low cost of fuel cell stack. This paper presents a comprehensive review about the BP materials (metallic, non-porous graphite and composite materials) and the corresponding fabrication methods, flow field layouts, and PEM fuel cells applications. Among the materials, the metallic BP has attracted high attention in automotive application due to its superior mechanical and physical properties, competitive cost compared with non-porous graphite and composite materials, but the fabrication technology and corrosion resistance are the major concerns for metallic bipolar plates. In recent studies, the protective coatings reported such as the conductive polymer, metal nitride/carbide and noble coatings have become the hot topics. They have been widely applied in different kinds of metallic bipolar plates, and the metal nitride coatings exhibit relatively low corrosion current and moderate interfacial contact resistance in comparison to other coatings. In future, developing excellent corrosion resistance and electrical conductivity coatings or novel metallic materials for bipolar plates will greatly enhance PEM fuel cells application in transportation field.     

Aluminate cement/graphite conductive composite bipolar plate for proton exchange membrane fuel cells

Aluminate cement/graphite conductive composite bipolar plate for proton exchange membrane fuel cells (PEMFC) was prepared by mold pressing at room temperature. The effect of size of graphite particles on the conductivity and the flexural strength of composite bipolar plate were discussed. Resistance to acid corrosion, thermal property and pore size distribution of this composite bipolar plate were also investigated in this paper. The experiment results show that the conductivity and the flexural strength of this composite bipolar plate can be improved by choosing uniform size graphite as conductive fillers. The corrosion current is about 10^−4.5 A cm⁻² from polarization curves of this composite bipolar plate, which shows that this composite bipolar plate is acid corrosion-resistant. Al and Ca ions may leach from this composite bipolar plate after 1 M H₂SO₄ acid corrosion. But Al and Ca ions leaching from this composite bipolar plate are only a little percentage of the total Al and Ca ions content in the composite bipolar plate after acid corrosion at 30 °C. This composite bipolar plate is also thermally stable from room temperature to 400 °C. The large amount of pore in this composite bipolar plate is gel capillary pores because of the hydration and solidification of aluminate cement, which make it possess humidifying function during the PEMFC operating.     

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.     

Performance of the gold-plated titanium bipolar plates for the light weight PEM fuel cells

In this present work, we are attempting to develop a light weight and corrosion resistant bipolar plate for the proton exchange membrane fuel cell. A titanium bipolar plate substrate has been chosen as the base metal due to its low cost, ease of manufacture into stampable bipolar plates, and its light weight. Our approach to obtain a smaller and lighter weight single fuel cell is to coat titanium with a corrosion resistant coating. Gold (Au) was investigated. The cell performance of the gold-plated bipolar plates is close to and even better than the PEM fuel cells with graphite and pure titanium bipolar plates. Gold-plated titanium bipolar plates can be employed to produce fuel cells with light weight, low coating cost and low contact resistance, ideal for portable applications.     

Effect of different graphite materials on the electrical conductivity and flexural strength of bipolar plates fabricated using selective laser sintering

Selective Laser Sintering provides a way to fabricate graphite composite bipolar plates for use in fuel cells. This significantly reduces time and cost at the research and development stage of bipolar plates, as compared with the conventional fabrication methods such as compression molding and injection molding. Different graphite materials, including natural graphite, synthetic graphite, carbon black, and carbon fiber, were investigated using the selective laser sintering process to fabricate bipolar plates. The effect of each material on the electrical conductivity and flexural strength of the bipolar plates was studied experimentally. With a proper combination of these materials, bipolar plates with electrical conductivity ranging from 120 to 380 S/cm and flexural strength ranging from 30 to 50 MPa have been obtained, which satisfy the requirements set by the Department of Energy and also are comparable with those developed by compression molding and injection molding. A modified percolation model was proposed to predict the electrical conductivity of the fabricated bipolar plates with different compositions. The analytical results calculated from the proposed model agree well with the experimental results. Finally, a single PEM (Proton Exchange Membrane) fuel cell unit was assembled using the fabricated bipolar plates, and its in-situ performance was studied.     

Expanded graphite-based electrically conductive composites as bipolar plate for PEM fuel cell

In this study, expanded graphite-based composite bipolar plates are developed from expanded graphite (EG), which is synthesized by chemical intercalation of natural graphite and rapid expansion at high temperature. The expanded graphite synthesized in this study has an expansion ratio between 75–100 cc/gm. The composite bipolar plate with varying weight percentage of EG gives different bulk density, electrical conductivity, mechanical properties and air tightness. The critical weight percentage of filler content is 50 to achieve the desired electrical conductivity and mechanical properties of bipolar plate as per U.S. DOE targets. The composite bipolar plate with 50 wt% of EG gives bulk density of 1.50 g/cm³, electrical conductivity >120 S/cm, bending strength 54 MPa, modulus 6 GPa and shore hardness 50. I–V characteristic of a cell assembly with EG-based composite plates are similar with the performance of a cell with commercial composite plates. These lightweight bipolar plates reduced the volume and weight of ultimate fuel cell stack and helped in improving the fuel cell performance.     

Ex situ evaluation of nanometer range gold coating on stainless steel substrate for automotive polymer electrolyte membrane fuel cell bipolar plate

The bipolar plate in polymer electrolyte membrane (PEM) fuel cell helps to feed reactant gases to the membrane electrode assembly (MEA) and collect current from the MEA. To facilitate these functions, the bipolar plate material should exhibit excellent electrical conductivity and corrosion resistance under fuel cell operating conditions, and simultaneously be of low-cost to meet commercialization enabling targets for automotive fuel cells. In the present work, we focus on the benchmarking of 10 nm gold coated SS316L (a.k.a. Au Nanoclad^®) bipolar plate material through ex situ tests, which is provided by Daido Steel (Japan). The use of nanometer range Au coatings help to retain the noble properties of gold while significantly reducing the cost of the bipolar plate. The area specific resistance of the flat sample is 0.9 mΩ cm² while that for the formed bipolar plate is 6.3 mΩ cm² at compaction force of 60 N cm⁻². The corrosion current density was less than 1 μA cm⁻² at 0.8 V/NHE with air sparge simulating cathodic conditions. Additionally, gold coated SS316L showed anodic passivation of SS316L, thereby exhibiting robustness towards coating defects including surface scratches that may originate during the manufacturing of the bipolar plate. These series of ex situ tests indicate that 10 nm gold coated SS316L has good potential to be considered for commercial bipolar plates in automotive fuel cell stack.     

Corrosion behavior of polyphenylene sulfide–carbon black–graphite composites for bipolar plates of polymer electrolyte membrane fuel cells

The aim of this work was to study the corrosion behavior of polyphenylene sulfide (PPS) – carbon black – graphite composites regarding their application as bipolar plates of polymer electrolyte membrane (PEM) fuel cells. Electrochemical impedance spectroscopy (EIS), potentiostatic and potentiodynamic polarization tests were used to characterize the electrochemical response of the composites in a simulated PEM fuel cell environment. Cross-sectional views of fractured specimens were observed by scanning electron microscopy (SEM). The results showed that the corrosion behavior depends on the carbon black content incorporated into the composite formulation. There was a trend of decreasing the corrosion resistance for higher carbon black contents. This behavior could be explained based on the porosity and electrical conductivity of the composites.     

Efficient composite bipolar plate reinforced with carbon fiber and graphene for proton exchange membrane fuel cell

The composite bipolar plates are developed using natural graphite, carbon black, and carbon fiber, along with 1% graphene with phenol formaldehyde (resole) resin. The graphene is developed by thermo-chemical exfoliation of natural graphite and characterized by XRD, Raman, FESEM, and AFM analyses. The synthesized graphene is monolayer graphene with a minimum thickness of 1 Å. The bipolar plates are developed using compression molding technique and thoroughly characterized considering stringent benchmarks (US-DOE and Plug Power Inc.) for PEMFC viz., electrical conductivity, flexural strength, deflection at mid-point, and corrosion current density. The composite bipolar plate showed excellent corrosion resistance to the rigorous fuel cell environment. All the required properties are achieved by the developed composite bipolar plate for PEMFC application. The fuel cell is fabricated with the developed bipolar plate and the performance of the fuel cell is studied. The incorporation of graphene has improved the fuel cell performance significantly.     

Fabrication of high strength and a low weight composite bipolar plate for fuel cell applications

The bipolar plate is one of the most important components in a PEM fuel cell. A polymer composite bipolar plate possessing high strength (81 MPa) and high stiffness (20 GPa) has been developed by making use of carbon fiber network in a specific form as the filler component. Such high strength is very much desired, especially when the fuel cells are used for mobile applications, since it is the bipolar plate that provides mechanical support to all the other cell components. The addition of carbon black and the effect of particle size of the natural graphite flakes used as other reinforcements also play a crucial role in controlling the physical and electrical properties of the composite plates. The plate when used in the unit fuel cell assembly showed I–V performance comparable to that of the commercially available bipolar plates.     

Ni–P and Ni–Co–P coated aluminum alloy 5251 substrates as metallic bipolar plates for PEM fuel cell applications

This study is aimed to replace graphite bipolar plates in PEM fuel cells with surface modified aluminum alloy. To improve the surface characteristics of aluminum alloy 5251 (AA5251) substrate, Ni–P and Ni–Co–P coatings were deposited using electroless and electroplating deposition techniques [power supply and chronoamperometry]. Surface morphology and chemical composition of prepared coatings have been investigated using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques. The corrosion behaviour of Ni–P and Ni–Co–P coated AA5251 was studied in (0.5 M H₂SO₄ + 2 ppm HF) solution by potentiodynamic polarization technique. Lower corrosion current densities and more positive corrosion potentials were gained after coating AA5251 with Ni–P and Ni–Co–P deposits. Much better corrosion resistance was shown by coatings containing cobalt. Potentiostatic tests were carried out at +160 mV (MMS) in air-saturated solution to simulate cathode environment in PEM fuel cells. The current density of Ni–Co–P (1:1)/AA5251 was stabilized at a value lowered by 4 times relative to that at bare AA5251 substrate. Interfacial contact resistance values between coated substrates and carbon paper were measured. Ni–P and Ni–Co–P coatings prepared by electroless method showed ICR values, twice that at ones prepared by electroplating power supply technique.     

Evaluation of a compression molded composite bipolar plate for direct methanol fuel cell

A polymer–graphite composite bipolar plate of direct methanol fuel cell (DMFC) was fabricated by a compression molding method. The electrical conductivity and electrochemical behavior of the composite material under DMFC operating conditions were evaluated. The results show that the composite bipolar plate has a good electrical conductivity. Moreover, the through-plane conductivity of the composite material is higher than the in-plane one, which is ascribed to the anisotropic property of the composite bipolar plate resulted from the compression molding process. Corrosion tests show that the stable current density is below 10 μA cm⁻² under both anode and cathode conditions of DMFC. The discharge test of the DMFC single cell also presents a satisfactory result.     

Investigations on novel low-cost graphite composite bipolar plates

PEM fuel cells are viewed as one of the most environmentally friendly propulsion systems for automotive travel in the future. The PEM fuel cell is still too expensive for wide-spread commercialization. To achieve this cost target and at the same time meeting several technical requirements for mass production, a novel type of low-cost bipolar plates has been developed by SGL Technik GmbH. In this paper, test results of novel SGL bipolar plate materials concerning electrical conductivity (including material resistivity and contact resistances), corrosion, chemical compatibility, gas tightness and mechanical strength are presented. Based on the measurements of resistivity and cell performance, the investigated material appears to be a good choice for stable high performance PEMFC bipolar plates.     

Study on the preparation and properties of novolac epoxy/graphite composite bipolar plate for PEMFC

In this study, the graphite/polymer composite bipolar plate was manufactured by a bulk-molding compound process. Low-cost novolac epoxy was chosen to compound with natural graphite and black carbon. The electrical properties and mechanical properties of composite bipolar plate were studied. The aging behavior was characterized according to the changes in property before and after the immersion test. The results show that the composite bipolar plates have good corrosion resistibility in the simulated solution of 0.005 mol L⁻¹ H₂SO₄ + 2 × 10⁻⁶ mol L⁻¹ HF. TGA result shows that the novolac epoxy/NG composite has excellent thermal stability. The optimum processing conditions for preparing composite bipolar plate are: resin content about 15 wt.%; molding pressure 200 MPa; curing temperature 180 °C; graphite particle size −200 mesh. Under the optimum conditions, the composite bipolar plates have been produced, the electrical conductivity can attain to 120 S/cm and flexural strength is higher than 38 MPa.     

Self-assembled graphene film to enable highly conductive and corrosion resistant aluminum bipolar plates in fuel cells

We report in this paper a novel method to form protective graphene film on aluminum substrate, which is particularly applicable to bipolar plates in proton exchange membrane (PEM) fuel cells. By simply immersing an aluminum sheet in an aqueous solution of graphene oxide (GO), a layer of cross-linked GO gel forms on the aluminum sheet, taking advantage of dissociated aluminum ions as a cross-linker. Then the cross-linked GO is converted to graphene at 400 °C in hydrogen atmosphere. The chemistry of the self-assembled GO layer and its conversion to graphene film is revealed by FTIR and XPS. Under simulated fuel cell environment the graphene coated aluminum sheet shows a corrosion current density of <1 × 10^?6 A/cm², which is around four orders of magnitude lower than a bare aluminum sheet. Meanwhile, the graphene film on aluminum results in a much lower and more stable interfacial contact resistance (ICR) of <5 mΩ cm². These enable the graphene coated aluminum sheet to meet the U.S. DOE targets of 2020 for bipolar plates in terms of both the corrosion and electrical resistance. Thus the proposed method is very promising for protecting aluminum bipolar plates in PEM fuel cells.     

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.