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.     

Development of ultralight and thin bipolar plates using epoxy-carbon fiber prepregs and graphite composites

Using the excellent mechanical strength and in-plane electrical conductivity of epoxy-carbon fiber prepregs, we design and fabricate ultrathin composite bipolar plates (BPs) for fuel cells. For the successful fabrication of prepreg-based BPs, it is essential to increase the through-plane electrical conductivity of pristine prepregs and reduce the electrical contact resistance due to excessive surface resin extruded during compression molding. In addition, the moldability of prepreg layers should be greatly improved for the proper construction of a serpentine flow field on the prepreg surface. To resolve all these technical issues, we suggest a multilayer BP structure in which prepreg layers, pure graphite layers, and graphite-resin composite layers are combined and compression-molded. The multilayered prepreg BP is approximately 0.6 mm thick and exhibits good electrical behavior (in-plane conductivity of 172 S cm^?1 and through-plane conductivity of 38 S cm^?1), with a high-quality serpentine flow channel configuration. The test results clearly demonstrate that the low through-plane electrical conductivity and poor moldability of pristine prepregs are greatly improved by our proposed BP design and fabrication.     

Contact thermal conductivity of a powder bed in selective laser sintering

Estimation of the temperature field in the powder bed in selective laser sintering process is a key issue for understanding the sintering/binding mechanisms and for optimising the technique. Heat transfer may be strongly affected by formation and growth of necks between particles due to sintering when the contact conductivity becomes predominant in the powder bed effective thermal conductivity. The necks often remain small as compared to the particle size. To calculate the effective contact conductivity of such structures a model of independent small thermal contacts is proposed. The conductivity of the considered cubic-symmetry lattices and the random packing of equal spheres depends on the three structural parameters: the relative density, the coordination number, and the contact size. The present model agrees with the known numerical calculations in the range of contact radius to particle radius ratio below 0.3. The strong dependence on the contact size is qualitatively confirmed by experimental data.     

Semi-empirical modeling of electrical conductivity for composite bipolar plate with multiple reinforcements

Carbon composite bipolar plates were developed by compression molding of novolac type phenol formaldehyde resin with natural graphite, carbon black, and carbon fiber. The General Effective Media equation was adapted to model the electrical conductivity of the bipolar plate. The experimental values of the electrical conductivity of the composites with different reinforcements were well predicted by the model. For resin-graphite system (2-component), the most effective in-plane and through-plane electrical conductivities for 70% graphite content were found to be 201.26 and 40.91 S cm⁻¹, respectively. Similarly, for optimum resin-graphite-carbon black system (3-component), these values were found as 269.55 and 82.77 S cm⁻¹, respectively. The most effective in-plane and through-plane electrical conductivities were found to be 285.54 and 91.79 S cm⁻¹, respectively, for the composite with resin-graphite-carbon black-carbon fiber system (4-component). The predicted electrical conductivities for all the three systems were found to be in well agreement with the experimental values.     

High-temperature,polymer–graphite hybrid composites for bipolar plates: Effect of processing conditions on electrical properties

High-temperature thermoplastic–graphite composites are prepared using polyphenylene sulfide (PPS) and polyether sulfone (PES) containing natural graphite powder. All samples are prepared by high pressure compaction and heating to high temperatures. The effect of a third additional conducting component on the electrical resistance of these composites is studied. A low resistance of the order of 0.1 Ω can be obtained even for a graphite concentration of 50% by addition of the third component. The effect of a mixing/blending technique on the anisotropy of conductivity is investigated. Solution blending of PES with graphite leads to lower anisotropy values than powder mixing and compression moulding. The samples when exposed continuously to a working temperature of 100 °C give a small but significant reduction in electrical resistance. X-ray diffraction studies on composites prepared by different techniques indicate that there is restructuring and crystallite re-orientation of the graphite phase in the samples. A large reduction in the crystallite size is observed for samples prepared by solution blending while re-orientation occurs after heat treatment. The changes in electrical properties can be correlated with these structural transformations in the composites.     

Carbon materials in composite bipolar plates for polymer electrolyte membrane fuel cells: A review of the main challenges to improve electrical performance

The technology of polymer electrolyte membrane (PEM) fuel cells is dependent on the performance of bipolar plates. There is a strong relationship between the material used in the manufacturing of the bipolar plate and its final properties. Graphite-polymer composite bipolar plates are well-established commercial products. Several other carbon based fillers are tested. Carbon nanotubes, carbon fibers, carbon black, graphite nanoplatelets and expanded graphite are examples of such materials. Structural characteristics of these particles such as morphology and size have decisive influence on the final properties of bipolar plates. Furthermore, the volumetric fraction of the filler is of prime importance. There is plenty of information on individual aspects of specific composite bipolar plates in the literature. Notwithstanding, the analysis of structure-property relationship of these materials in a comprehensive source is not found. In this paper, relevant topics on the structural aspects of carbon based fillers and how they influence the final electrical performance of composite bipolar plates are discussed. It is intended that this document contribute to the development of new and maximized products to the PEM fuel cell industry.     

CNTs nanostructuring effect on the properties of graphite composite bipolar plate

Intent of present investigation is to improve the properties of graphite–polymer composite bipolar plate by nanostructuring. This involves the incorporation of different vol.% of multiwall carbon nanotubes (MWNTs) in graphite–polymer composite bipolar plate. It has been found that by inclusion of 1 vol.% of MWNTs in graphite composite plate, the electrical and thermal conductivity of nanocomposite increased by 100%. The thermal conductivity of nanocomposite plate increases from 1 W/m K to 13 W/m K in through-plane and in-plane from 25 W/m K to 50 W/m K at 1 vol.% of MWNTs. This significant enhancement is due to the orientation of MWNTs in all the directions of composite, positive synergistic effect of MWNTs and heat transfer along the axis directions. However, bending strength of nanocomposite increases by 25% and maximum augmentation is in case of 1 vol.% of MWNTs. The improvement in conductivity of nanocomposite plate is due to an increase in the electron transfer ability within the composite plate which influences the I–V performance of ultimate fuel cell. These observations confirm that the optimal content of MWNTs is 1 vol.%, in graphite–polymer composite.     

Development of bipolar plates for fuel cells from graphite filled wet-lay material and a thermoplastic laminate skin layer

In this paper, a method with the potential to rapidly produce thermoplastic polymer composite bipolar plates with improved formability and through-plane conductivity is described. In our earlier work, it was reported that composite bipolar plates made with graphite filled wet-lay materials exhibited excellent mechanical properties and in-plane electrical conductivity. However, the through-plane conductivity and formability of the materials needed improvement. In this work, laminate polymer composite plates consisting of a wet-lay based core and a fluoropolymer/graphite skin layer are manufactured in an effort to improve formability and through-plane conductivity. These plates are characterized by their through-plane and in-plane conductivity, half-cell resistance, and mechanical properties at ambient and elevated temperatures. The laminate plates with PPS based wet-lay core exhibited bulk conductivities of above 300 S cm⁻¹, tensile strength of up to 34 MPa, and flexural strength of up to 54 MPa. Compared to the bipolar plates consisting of wet-lay material only, the bipolar plates with laminate structure exhibited an increase in through-plane conductivity of 25–35%, as well as a decrease in half-cell resistance by a factor of up to 5. The laminate bipolar plates can be manufactured in several ways with two of them being discussed in detail in the paper.     

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.     

Development of bipolar plates for fuel cells from graphite filled wet-lay material and a compatible thermoplastic laminate skin layer

In this paper a method with the potential to lead to the rapid production of thermoplastic polymer composite bipolar plates with improved mechanical properties, formability, and half-cell resistance is described. In our previous work it was reported that laminate structure composite bipolar plates made with a polyphenylene sulfide (PPS) based wet-lay material as the core and a polyvinylidene fluoride (PVDF)/graphite mixture as the laminate exhibited improved formability, through-plane conductivity, and half-cell resistance over that of wet-lay based bipolar plates. However, the mechanical strength of the laminate plates needed improvement. In this work laminate polymer composite plates consisting of a PPS/graphite-based laminate mixture and a PPS based wet-lay core are manufactured in an effort to improve mechanical strength. Additionally, our existing channel design has been altered to reduce the channel depth from 0.8 to 0.5 mm in an effort to improve the half-cell resistance by reducing the total plate thickness. The plates are characterized by their half-cell resistance and mechanical properties at ambient and elevated temperatures. The PPS based laminate plates exhibited half-cell resistances as low as 0.018 Ω cm², tensile strength of up to 37 MPa, and flexural strength of up to 60 MPa at ambient temperature. The laminate bipolar plates can be manufactured in several ways with two of them being discussed in detail in the paper.     

Graphite-epoxy composites for fuel-cell bipolar plates: Wet vs dry mixing and role of the design of experiment in the optimization of molding parameters

Bipolar plates (BPs) are key components of Proton Exchange Membrane Fuel Cells mainly employed in hydrogen-powered electric vehicles. Here, a reliable and detailed experimental method to prepare graphite-epoxy composites suitable for manufacturing BPs is reported. Dry and wet mixing procedures were compared and a simple composition was optimized, with regard to electrical conductivity. The adoption of wet mixing of the components and the choice of the conductive filler were the main factors that contributed to the achievement of good electrical and mechanical properties. The addition of a small percentage of carbon black as a secondary filler was also advantageous. The effects of molding parameters (pressure, temperature and time) on a fixed-composition graphite-epoxy composite were modeled using a Design Of Experiments approach, which provided valuable information for future improvements. Conductivity values well above the US DOE requirements were obtained.     

Study on the electrical and mechanical properties of phenol formaldehyde resin/graphite composite for bipolar plate

With phenol formaldehyde resin (PF) powder and graphite powder as raw materials, a kind of conductive composite for bipolar plate is obtained by hot-pressure molding. The effects of PF resin content, molding temperature and time on conductivity and bending strength of the composite were investigated in this paper; and the optimum PF resin content, molding temperature and time were obtained. The results show that: the conductivity decreases and bending strength increases with the increasing of PF resin content; the conductivity varies wave-like and bending strength increases firstly and then decreases with the increasing of molding temperature; the effects of molding time on properties of the composite are similar to that of molding temperature; and the best conductivity and bending strength of the composite are 142 s cm⁻¹ and 61.6 MPa, respectively, when its PF resin content is 15% molded at 240 °C for 60 min.     

The effect of compression molding parameters on the electrical and physical properties of polymer composite bipolar plates

The performance of polymer electrolyte membrane fuel cell (PEMFC) greatly depends on the properties of the components. Bipolar plate is one of the key components of PEMFC and the properties of the plates on fuel cell performance are critical. Electrical conductivity and surface roughness of the bipolar plate appear very important properties to minimize ohmic resistance and optimize water management, respectively. The composite bipolar plates having conductive fillers and thermosetting resin binder are produced by compression molding. Response Surface Methodology (RSM) has been applied to optimize the production conditions of bipolar plate. Electrical conductivity, physical appearance and roughness are chosen as response parameters and molding temperature, pressure and time are chosen as independent parameters. The main challenges of this study are observation of the individual and combined effects of these parameters on bipolar plate properties by RSM. The optimization of the production conditions of polymer composite plate is carried out by aiming maximum electrical conductivity and minimum time. The maximum electrical conductivity of 107.4 Scm^?1 is obtained at temperature of 187 °C, pressure of 119 bar and time of 5 min. It is found that the polymer composite plates produced by the compression molding process at minimum time satisfied the electrical conductivity target of Department of Energy (DoE).     

Effects of thermal oxi-nitridation on the corrosion resistance and electrical conductivity of 446M stainless steel for PEMFC bipolar plates

Stainless steel has attracted interest as a bipolar plate material for polymer electrolyte membrane fuel cells due to its excellent mechanical properties, good corrosion resistance, and low cost. However, the application of thermal nitridation for the improvement of electrical conductivity deteriorates the corrosion resistance under PEMFC operating conditions due to the discontinuous formation of external Cr-nitride. In this study, nitridation with pre-oxidation of 446M stainless steel was performed in order to improve both the corrosion resistance and the electrical conductivity. 446M stainless steels with oxide and nitride on the surface were evaluated to assess their feasibility as a bipolar plate material for PEMFCs. The results were compared with those obtained using as-received and only nitrided 446M stainless steels. The oxide formed by the pre-oxidation protects the surface of 446M stainless steel from corrosion in corrosive environments, especially under cathode conditions, and the Cr-nitride formed by the subsequent nitridation serves as an electro-conductive channel. As a result, the pre-oxidized, nitrided 446M stainless steel exhibits improved corrosion properties and electrical conductivity under PEMFC operating environments.     

Effect of polyaniline on the electrical conductivity and activation energy of electrospun nylon films

Nano films of nylon 6 and polyaniline (0, 1, 2, 3, 4, 5, and 6 wt. %) were prepared by the electrospinning technique. The addition of polyaniline increased the electrical conductivity of nylon 6. Pure nylon had an electrical conductivity of 3.8 × 10^?3 S/cm, while the conductivity of nylon 6 with 1% polyaniline was 10.2 × 10^?3 S/cm. In addition, the electrospinning process increased the electrical conductivity of bulk nylon 6 from 10^?14 S/c to 10.2 × 10^?3 S/cm. The viscosity and surface tension of nylon 6 decreased with increasing polyaniline content. The morphology of the prepared films was observed with SEM, and the average diameter of the fibre diameters, which was measured statically from the SEM images, was found to be 74 nm for a nylon film with 4 wt.% polyaniline, and 180 nm for pure nylon. The nanofibre films showed an enhanced electrical conductivity with increasing polyaniline concentration, from 2.627 × 10^?10 S/cm for a pure nylon film to 3.44 × 10^?7 S/cm for a nylon film with 6 wt.% polyaniline. The activation energy decreased with increasing polyaniline concentration. The activation energy was 0.135, 0.0899, 0.0864, 0.0811, 0.078, 0.075 and 0.07299 eV for pure nylon, and nylon with 1, 2, 3, 4, 5, and 6 wt.% polyaniline, respectively. The activation energy of the prepared nylon films decreased in comparison with the activation energy of a pure nylon 6 film, while the electrical conductivity increased as the amount of polyaniline was increased.     

Effects of resin content and preparing conditions on the properties of polyphenylene sulfide resin/graphite composite for bipolar plate

In the paper, a kind of polyphenylene sulfide (PPS) resin/graphite (G) composite for bipolar plate was prepared by using the PPS resin as adhesive and simple hot pressing. The influences of the resin content, the molding temperature and holding time on the conductivity and the bending strength of the PPS/G composite bipolar plate were investigated firstly and then the optimum content and the preparing conditions of the composite were obtained. The experimental results show that the electrical conductivity decreases and the bending strength reveals a serrated variation with increase in PPS resin content; when the holding time is certain, the conductivity decreases and the bending strength increases with the molding temperature increasing. The experimental results further show that the effect of the holding time on the properties of the composite is different at different molding temperatures. The PPS/G composite with 20% PPS resin content has electrical conductivity of 118.9 S cm⁻¹ and bending strength of 52.4 MPa when it molded at 380 °C for 30 min, and has electrical conductivity of 105 S cm⁻¹, bending strength of 55.7 MPa when it molded at 390 °C for 30 min. The properties of the composites can meet the requirements of United States Department of Energy (DOE).     

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

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

Fabrication of stainless steel bipolar plates for fuel cells using dynamic loads for the stamping process and performance evaluation of a single cell

In this study, a dynamic load (square wave) is applied to bipolar plates in order to reduce forming defects from the stamping process. Four round (R) sizes of die (R 0.05, 0.1, 0.2, 0.3 mm) are applied to edges that ran from the channel to the rib of the stamping die. Fuel cell performance tests are carried out to analyse the depth and shape of bipolar plate channels formed according to the load conditions, and the effect of the die size on the fuel cell performance is evaluated. The depth of the bipolar plate channel increase with the round size of die regardless of the load type. The shape of the channel formed with a die of R 0.05 mm is trapezoidal, while that formed with a die of R 0.3 mm is triangular. Triangular channels have a higher current density than trapezoidal channels. A higher current density can be obtained with a square load than with a conventional straight load because the former produces a deeper and more uniform bipolar plate channel. The current density of a bipolar plate with a triangular channel formed by a square load with a die of R 0.3 mm is 531 mA cm⁻². After TiN coating, the current density is 784 mA cm⁻², which is about 58% of that of a graphite bipolar plate.     

Effect of synthesis process on the microstructure and electrical conductivity of nickel/yttria-stabilized zirconia powders prepared by urea hydrolysis

In this study, NiO/YSZ composite powders were synthesized using hydrolysis on two solutions, one contains YSZ particles and Ni²⁺ ion, and the other contains NiO particles, Zr⁴⁺, and Y³⁺ ions, with the aid of urea. The microstructure of the powders and sintered bulks was further characterized using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results indicated that various synthesis processes yielded NiO/YSZ powders with different morphologies. The NiO precursors would deposit onto the surface of YSZ particles, and NiO-deposited YSZ composite powders were obtained. Alternatively, it was not observed that YSZ precursors deposited onto the surface of NiO particles, thus, a uniform powder mixture of fine NiO and fine YSZ particles was produced. After sintering and subsequent reduction, these powders would lead to the variations of Ni distribution in the YSZ matrix and conductivity of cermets. Owing to the core–shell structure of the powders and the higher size ratio of YSZ and NiO particles, the conductivity of cermet with NiO-deposited YSZ powders containing 23 wt% NiO is comparable to those with a NiO/YSZ powder mixture containing 50 wt% NiO.     

Effects of niobium and titanium addition and surface treatment on electrical conductivity of 316 stainless steel as bipolar plates for proton-exchange membrane fuel cells

Niobium and titanium are added to 316 stainless steel, and then heat treatment and surface treatment are performed on the 316 stainless steel and the Nb- and Ti-added alloys. All samples exhibit enhanced electrical conductivity after surface treatment but have low electrical conductivity before surface treatment due to the existence of non-conductive passive films on the alloy surfaces. In particular, the Nb- and Ti-added alloys experience a remarkable enhancement of electrical conductivity and cell performance compared with the original 316 stainless steel. Surface characterization reveals the presence of small carbide particles on the alloy surface after treatment, whereas the untreated alloys have a flat surface structure. Cr₂₃C₆ forms on the 316 stainless steel, and NbC and TiC forms on the Nb- and Ti-added alloys, respectively. The enhanced electrical conductivity after surface treatment is attributed to the formation of these carbide particles, which possibly act as electro-conductive channels through the passive film. Furthermore, NbC and TiC are considered to be more effective carbides than Cr₂₃C₆ as electro-conductive channels for stainless steel.