Fibre orientation effect on polypropylene/milled carbon fiber composites in the presence of carbon nanotubes or graphene as a secondary filler: Application on PEM fuel cell bipolar plate

Conductive polymer composite (CPC) materials are extensively used for the bipolar plate in Polymer Electrolyte Membrane Fuel Cells (PEMFCs). The produced CPC materials through the extrusion process strongly relate to electrical conductivity and mechanical properties. In this study, milled carbon fibre (MCF) reinforced polypropylene (PP) incorporating carbon nanotube (CNT), or graphene nanoplatelets (xGNP) are pre-mixed using the extrusion process to orientate the fibres before undergoing the compression moulding at 13.8 MPa and 200 °C for 15 min. The CNT composites exhibited the higher through-plane conductivity of 14.8 S/cm as compared to xGnP composites with 4.9 S/cm at 70 wt% of MCF. The flexural strength improved after being compressed to 99.6 MPa and 172.5 MPa for 70 wt% of CNT and xGNP, respectively. This reveals that the oriented fibres can boost the CPC performance.     

Effect of carbon fillers on properties of polymer composite bipolar plates of fuel cells

Bipolar plates are major components of fuel cell (FC) stacks and they make up a large portion of the stack volume and cost. In order to reduce their weight and fabrication cost, polymer composite materials with various carbon conducting fillers are tested for use as composite bipolar plates for FCs. The composite materials are prepared by using graphite with a small vol.% of carbon black (CB), multi-walled carbon nanotubes (MWNTs) or carbon fibres (CF) in an epoxy resin. The electrical conductivity and flexural properties of the composites are measured as a function of the carbon conductive filler content. The highest electrical conductivity is observed at a total conducting filler content of 75 vol.%. The addition of a small amount of hybrid conducting filler enhances the electrical conductivity up to certain threshold, viz. 5 vol.% of CB, 2 vol.% of MWNTs, and 7 vol.% of CF. Above these thresholds, the electric conductivity decreases with increasing filler content, due to the lack of sufficient resin to bind the fillers tightly. The hybrid filler system has better properties than the single filler system. The experimental results indicate that there is an optimum composition range with respect to electrical conductivity and mechanical properties.     

Preparation and properties on the graphite/polypropylene composite bipolar plates with a 304 stainless steel by compression molding for PEM fuel cell

Graphite/polymer composites have high corrosion resistance, low contact resistance and low fabrication cost but low cell efficiency and mechanical strength. This study examined the electrical and mechanical properties of graphite/polypropylene composite bipolar plates. Carbon nanotubes (CNTs) were used to improve the electrical properties of the graphite/PP composites. Although the electrical properties increased when excess conducting filler was added to the composite, the mechanical strength decreased significantly. 304 stainless steel (304 SS) plates with different thicknesses were used as the support material of a graphite/PP composite bipolar plate. The 304 SS-supported graphite/PP composite bipolar plate had an optimum CNTs/graphite/PP composite composition of 1.2, 83 and 17 wt.%, respectively. The flexural strength of the 304 SS-supported graphite/PP composites increased from 35 to 58 MPa with increasing 304 SS thickness from 0.5 to 1 mm. The power density of the graphite bipolar plate and 304 SS-supported graphite/PP composite bipolar plate were 968 and 877 mW cm⁻², respectively. The 304 SS complemented the mechanical strength of the graphite/PP composite bipolar plate as well as the cell efficiency.     

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.     

Electrically conductive LCP–carbon composite with low carbon content for bipolar plate application in polymer electrolyte membrane fuel cell

Lightweight polymer–carbon composites with high specific electrical conductivity at a carbon content below 40 vol.% were developed. The electrical and mechanical properties and the hydrogen permeability of carbon fiber and particle reinforced liquid crystalline polymers were examined. Vectra^® A 950, SIGRAFIL^® carbon fibers and Vulcan^® XC 72 R carbon black were employed. The composites are found to have sufficient mechanical properties and a hydrogen permeability low enough to be utilised as bipolar plate material in fuel cell applications. The density of the new composite is 20% lower than the density of commercial bipolar plates made from carbon reinforced polymeric composite materials, due to the lower carbon content. The current density at 0.5 V in an operating fuel cell is only 20% lower compared to commercial materials with more than 80 vol.% carbon content and meets the requirements for bipolar plate application.     

Bipolar plate design and fabrication using graphite reinforced composite laminate for proton exchange membrane fuel cells

A bipolar plate is designed to have high electric conductivity, low corrosion and good mechanical strength characteristics. The two most common materials adopted for bipolar plates are carbon and metal. The carbon bipolar plate has good electric conductivity and corrosion resistance but brittle. The metal bipolar plate has good mechanical strength, acceptable electrical conductivity but worse corrosion resistance. The main objective of this paper is to design and fabricate graphite composite laminate based PEMFC bipolar plate. A thermoset type phenolic resin is adopted as the matrix with a plain weave type woven graphite fiber cloth adopted as the composite laminate reinforcement. In the fabrication process, thermoset phenol-formaldehyde resin is first printed onto the plain-weave woven carbon fiber cloth and the waiting until air-dry as prepregs. Several layers of prepregs were then stacked into a mold and heated. The resin contained in the prepregs melted and cured into a composite laminate. The carbonization process is further conducted to increase the electric conductivity. The flow channels are carved and the bipolar plate is completely fabricated. The developed bipolar plates are assembled into a single cell PEMFC and tested. The composite bipolar plate performance with or without carbonization are also studied. The back side bipolar plate electric conductivity would also significantly affect the cell performance. Therefore, increasing the back side conductivity could increase the cell performance.     

A graphite-coated carbon fiber epoxy composite bipolar plate for polymer electrolyte membrane fuel cell

A PEMFC (polymer electrolyte membrane fuel cell or proton exchange membrane fuel cell) stack is composed of GDLs (gas diffusion layers), MEAs (membrane electrode assemblies), and bipolar plates. One of the important functions of bipolar plates is to collect and conduct the current from cell to cell, which requires low electrical bulk and interfacial resistances. For a carbon fiber epoxy composite bipolar plate, the interfacial resistance is usually much larger than the bulk resistance due to the resin-rich layer on the composite surface.In this study, a thin graphite layer is coated on the carbon/epoxy composite bipolar plate to decrease the interfacial contact resistance between the bipolar plate and the GDL. The total electrical resistance in the through-thickness direction of the bipolar plate is measured with respect to the thickness of the graphite coating layer, and the ratio of the bulk resistance to the interfacial contact resistance is estimated using the measured data. From the experiment, it is found that the graphite coating on the carbon/epoxy composite bipolar plate has 10% and 4% of the total electrical and interfacial contact resistances of the conventional carbon/epoxy composite bipolar plate, respectively, when the graphite coating thickness is 50 μm.     

Preparation and properties of carbon nanotube-reinforced vinyl ester/nanocomposite bipolar plates for polymer electrolyte membrane fuel cells

Novel multiwalled carbon nanotubes (MWNTs) were prepared using poly(oxypropylene)-backboned diamines of molecular weights M_w 400 and 2000 to disperse acid-treated MWNTs, improving the performance of composite bipolar plates in polymer electrolyte membrane fuel cells. A lightweight polymer composite bipolar plate that contained vinyl ester resin, graphite powder and MWNTs was fabricated using a bulk molding compound (BMC) process. Results demonstrate that the qualitative dispersion of MWNTs crucially determined the resultant bulk electrical conductivity, the mechanical properties and the physical properties of bipolar plates. The flexural strength of the composite bipolar plate with 1 phr of MWNTs was approximately 48% higher than that of the original composite bipolar plate. The coefficient of thermal expansion of the composite bipolar plate was reduced from 37.00 to 20.40 μm m⁻¹ °C⁻¹ by adding 1 phr of MWNTs, suggesting that the composite bipolar plate has excellent thermal stability. The porosity of the composite bipolar plate was also evaluated. Additionally, the bulk electrical conductivity of the composite bipolar plate with different MWNTs types and contents exceeds 100 S cm⁻¹. The results of the polarization curves confirm that the addition of MWNTs leads to a significant improvement on the single cell performance.     

Development of composite-metal hybrid bipolar plates for PEM fuel cells

In this study, in order to increase the electrical conductivity, a carbon composite-metal hybrid bipolar plate has been developed using pre-forming method followed by a plasma surface treatment. A pre-formed metal foil between the carbon fiber/polymer composite plates promotes the metal foil to follow the shape of the channels of the bipolar plates without tearing and permits a continuous flow of electrons. The pre-formed metal foil also reduces the residual stress between the composite and metal foils, which helps prevent delamination between the composite and metal foils. The composite surface has been treated with plasma to increase the contact area between the carbon fiber and the gas diffusion layer (GDL). The composite-metal hybrid bipolar plates have only 1.4% of the total electrical resistance of that of the conventional composite bipolar plates. Unit cell test results have proved that the developed composite-metal hybrid bipolar plates with reduced total electrical resistance increase the cell performance.     

Highly conductive epoxy/graphite composites for bipolar plates in proton exchange membrane fuel cells

Carbon-filled epoxy composites are developed for potential application as bipolar plates in proton exchange membrane (PEM) fuel cells. These composites are prepared by solution intercalation mixing, followed by compression molding and curing. Electrical conductivity, thermal and mechanical properties, and hygrothermal characteristics are determined as function of carbon-filler content. Expanded graphite and carbon black are used as synergistic combination to obtain desired in-plane and through-plane conductivities. These composites show high glass transition temperatures (T_g ∼ 180 °C), high thermal degradation temperatures (T₂ ∼ 415 °C), in-plane conductivity of 200–500 S cm⁻¹ with 50 wt% carbon fillers, in addition to offering high values of flexural modulus, flexural strength, and impact strength, respectively 2 × 10⁴ MPa, 72 MPa, and 173 J m⁻¹. The presence of carbon fillers helps reduce water uptake from 4 to 5 wt% for unfilled epoxy resins to 1–2 wt%. In addition, morphology, electrical, mechanical, and thermal properties remain unchanged on exposure to boiling water and acid reflux. This data indicate that the composites developed in this work meet many attributes of bipolar plates for use in PEM fuel cells.     

A single-type aluminum/composite hybrid bipolar plate with surface modification for high efficiency PEMFC

An aluminum/composite hybrid bipolar plate with a bypass aluminum channel has been developed. The mechanical abrading technique and electromagnetic-carbon technique are employed for surface modification of aluminum and composite, respectively. The optimum processes of surface modification techniques for aluminum and composite are investigated to promote intimate contact between aluminum and carbon fibers of composite for low electrical resistance. After the surfaces of the aluminum and carbon fiber prepregs are treated with the surface modification techniques, they are co-cured with the single-type of bipolar plate to lower the electrical contact resistance. In this study, it has been found that the hybrid bipolar plate has only 3% of the electrical resistance of the conventional composite bipolar plates.     

Study on the mesocarbon microbeads/polyphenylene sulfide composite bipolar plates applied for proton exchange membrane fuel cells

Thermoplastic/graphite composite bipolar plates based on polyphenylene sulfide (PPS) and mesocarbon microbeads (MCMB) were prepared by compression molding at a pressure of 40 MPa and 400 °C. Electrical conductivity, bulk density, flexural strength, water and ethanol absorption were determined as function of PPS content. The influences of molding time, actived carbon and carbon fiber on the properties of the composite bipolar plates were investigated, the cross section of the composite plates were analyzed by scanning electron microscope (SEM). We found that the optimized PPS content is 20 wt% and the required molding time is 30 min. In particular, the composite plates containing 20 wt% PPS demonstrated in-plane conductivity as high as 133.7 S cm⁻¹, through-plane conductivity 21.37 S cm⁻¹, in addition to showing the value of density, flexural strength, water and ethanol absorption as 1.98 g cm⁻³, 38.82 MPa, 0.0409 and 0.352 g cm⁻³. The addition of actived carbon degraded all the performance of the bipolar plate, while addition of carbon fiber improved almost all the performance of bipolar plate except bulk density and through-plane conductivity. The performances of fuel cell with this composite bipolar plate were tested, no distinct variation occurred after the composite plates operating in fuel cell. These data indicates the chemical and mechanical stability of the composite plates and their potential application in fuel cell.     

Characteristics of composite bipolar plates for polymer electrolyte membrane fuel cells

As alternative bipolar plate materials for polymer electrolyte membrane fuel cell (PEMFC), two types of carbon composite were developed and characterized. Electrical and physical properties of the currently used graphite and newly developed carbon composites were evaluated in terms of bulk and contact resistance, flexural strength, density, gas tightness, water absorption, and depth deviation of the flow channel. The test results showed that the carbon composites were very promising candidates for PEMFC bipolar plate material. In single cell tests, the carbon composite bipolar plates exhibited good initial and long-term performance compared with the graphite bipolar plates.     

Preparation and properties of thin epoxy/compressed expanded graphite composite bipolar plates for proton exchange membrane fuel cells

Although the composite bipolar plates prepared by the method of the vacuum resin impregnation in compressed expanded graphite (CEG) sheets have been applied in the KW-class stacks, there have been few investigations of the preparation and properties of them so far. In this research, the influences of the microstructure on the physical properties of the thin epoxy/CEG composites (the thickness is 1 mm) are investigated for the first time and the optimum preparation conditions are obtained. Results demonstrated that the mechanical property and the impermeability of the composites increases evidently with the resin content changing from 4% to 30%, while the electrical properties keep nearly constant. It can be attributed to the continuous expanded graphite (EG) conductive network of the raw CEG sheet. The epoxy (30 wt.%)/CEG composite is shown to be the optimum composite, displaying in-plane conductivity of 119.8 S cm⁻¹, through-plane resistance of 17.13 mΩ cm², density of 1.95 g cm⁻³, gas permeability of 1.94 × 10⁻⁶ cm³ cm⁻² s⁻¹ and flexural strength of 45.8 MPa. The alcohol scrubbing is the optimum method of surface post-processing. The performance of a single cell with the optimum composite bipolar plates is tested and demonstrated to be outstanding. Above all, the composite prepared by resin vacuum impregnation in the CEG sheet is a promising candidate for bipolar plate materials in PEMFCs.     

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.     

Hygrothermal effects on properties of highly conductive epoxy/graphite composites for applications as bipolar plates

The hygrothermal effects on mechanical, thermal, and electrical properties of highly conductive graphite-based epoxy composites were investigated. The highly conductive graphite-based epoxy composites were found to be suitable for applications as bipolar plates in proton exchange membrane (PEM) fuel cells. The hygrothermal aging experiments were designed to simulate the service conditions in PEM fuel cells. Specifically, the composite specimens were immersed in boiling water, aqueous sulphuric acid solution, and aqueous solution of hydrogen peroxide. The water uptake, changes in surface appearance and dimensions, glass transition behavior and thermal stability, and electrical and mechanical properties were evaluated. The water uptake at short time increased linearly with the square root of time as in linear Fickian diffusion. The presence of graphite significantly reduced both the rate and extent of water uptake. No discernible changes in specimen dimensions, surface appearance, and morphology of the composites were observed. The electrical conductivity and mechanical properties remained almost unchanged. The wet specimens showed slight reduction of glass transition temperature (T_g) due to plasticization of epoxy networks by absorbed water, while the re-dried specimens showed small increase of T_g. The composites maintained high electrical conductivity of about 300–500 S cm⁻¹ and good mechanical properties and showed thermal stability up to 350 °C.     

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.     

石墨烯/碳纳米管/环氧树脂复合材料的导热性能的实验研究

在微电子领域中,随着元器件的体积微小化,要求导热材料具备体积小、高导热的特点。高分子导热复合材料能很好的解决器件在不同的工作环境中仍能保持正常的散热问题。以环氧树脂(EP)为基体,石墨烯粉末(GP)和多壁碳纳米管(MWCNTs)为导热填料,采用溶剂和超声分散法,制备出石墨烯/碳纳米管/环氧树脂复合材料。实验采用瞬态电热技术测量其导热系数,结果显示,石墨烯与碳纳米管协同作为导热填料时,复合材料导热性优于单独添加导热填料(GP或MWCNTs),且随着GP所占比例的增大复合材料的导热系数越大。当GP和MWCNTs比例分别为0.7%和0.3%时,复合材料导热系数为0.940 W/(m·K),相比于纯EP导热系数提高了286.83%。     

Electromagnetic-carbon surface treatment of composite bipolar plate for high-efficiency polymer electrolyte membrane fuel cells

Polymer electrolyte membrane (PEM) or proton-exchange membrane fuel cell systems are environmentally friendly power sources for many applications. Bipolar plates are essential components of a PEM fuel cell. Recently, composite bipolar plates have received considerable interest due to their superior performance. The most important properties of bipolar plates are electrical resistance and contact resistance, which are largely dependent on the surface morphology of the bipolar plate, because low electrical resistance improves the efficiency of PEM fuel cells. In this study, a selective surface preparation technology is developed using an electromagnetic field and carbon black (electromagnetic-carbon surface treatment). The carbon black is heated by an electromagnetic field on the surface of the bipolar plate with a high rate of temperature rise. The non-electrically conducting surface resin is removed, without damaging the carbon fibre to give a low electrical resistance. It is found that the surface-treated composite bipolar plate has a lower electrical resistance than those of conventional composite bipolar plates, and that the electromagnetic-carbon surface treatment can be applied for production of the composite bipolar plates in a fast and efficient process.