Graphite Bipolar Plates Research Articles

Method for exposing carbon fibers on composite bipolar plates

The carbon/epoxy composite bipolar plate is an ideal substitute for the brittle graphite bipolar plate for proton exchange membrane fuel cells (PEMFCs) and vanadium redox flow batteries (VRFBs) because of its high mechanical properties and easy manufacturing. However, due to the resin-rich area that forms on its surface, the carbon/epoxy composite bipolar plate requires an expanded graphite coating to decrease the areal specific resistance (ASR). The expanded graphite coating not only increases the manufacturing costs but also has very low mechanical properties.In this work, an innovative manufacturing method that exposes carbon fibers on the surface of the carbon/epoxy composite bipolar plate was developed. Soft release films were inserted between the mold and the composite to prevent the formation of a resin-rich area and to expose carbon fibers on the surface of the bipolar plate. The developed method considerably decreased the ASR of the carbon composite bipolar plate without an expanded graphite coating, which satisfied the target established by the Department of Energy (DOE) of United States. The effects of the soft layer on the mechanical and electrical properties of the carbon composite bipolar plate were investigated.

Development of 500 W PEM fuel cell stack for portable power generators

Polymer Electrolyte Membrane Fuel Cell (PEMFC) portable power generators are gaining importance in emergency applications. In this study, an air-cooled PEMFC stack was designed and fabricated for net 500 W power output. Gas Diffusion Electrodes (GDE's) were manufactured by ultrasonic spray coating technique. Stack design was based on electrochemical data obtained at 0.60 V was 0.5 A/cm2 from performance tests of a single cell having the same membrane electrode assemblies (MEA) that had an active area of 100 cm2. Graphite bipolar plates were designed and machined by serpentines type flow. The stack comprising of 24 cells was assembled with external fixing plates. The stack temperature was effectively regulated by the cooling fan based on on-off control system. A maximum power of 647 W was obtained from the stack. The PEMFC stack was stable during start-up and shutdown cycling testing for 7 days at 65 °C in H2/air at a constant cell voltage.

Thermal conductivity of a graphite bipolar plate (BPP) and its thermal contact resistance with fuel cell gas diffusion layers: Effect of compression, PTFE, micro porous layer (MPL), BPP out-of-flatness and cyclic load

This paper reports on measurements of thermal conductivity of a graphite bipolar plate (BPP) as a function of temperature and its thermal contact resistance (TCR) with treated and untreated gas diffusion layers (GDLs). The thermal conductivity of the BPP decreases with temperature and its thermal contact resistance with GDLs, which has been overlooked in the literature, is found to be dominant over a relatively wide range of compression. The effects of PTFE loading, micro porous layer (MPL), compression, and BPP out-of-flatness are also investigated experimentally. It is found that high PTFE loadings, MPL and even small BPP out-of-flatness increase the BPP-GDL thermal contact resistance dramatically. The paper also presents the effect of cyclic load on the total resistance of a GDL-BPP assembly, which sheds light on the behavior of these materials under operating conditions in polymer electrolyte membrane fuel cells.

Active screen plasma nitriding of 316 stainless steel for the application of bipolar plates in proton exchange membrane fuel cells

Proton exchange membrane fuel cell (PEMFC) has attracted considerable interest because of its superb performance, and many researches are focused on the development of high-performance, long-life bipolar plates. Stainless steel bipolar plates offer many advantages over the conventional graphite bipolar plates, such as low material and fabrication cost, excellent mechanical behaviour and ease of mass production. However, the insufficient corrosion resistance and relatively high interfacial contact resistance (ICR) become the major obstacles to the widespread use of stainless steel bipolar plates. In this work, active screen plasma nitriding (ASPN), a novel plasma nitriding technique, was used to modify the surface of 316 austenitic stainless steel. A variety of analytical techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), glow discharge optical emission spectrometer (GDOES), were employed to characterize the nitrided samples. The results reveal that a nitrogen supersaturated S-phase layer has been successfully produced on the surface of all nitrided 316 stainless steel samples. The interfacial contact resistance (ICR) value can be decreased dramatically after ASPN treatment and the corrosion resistance can also been improved. In addition, better corrosion resistance can be achieved by active screen plasma nitriding with a stainless steel screen than with a carbon steel screen. This technique could be used to improve the performance and lifespan of bipolar plates for fuel cells.

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.

금속분리판의 Electro Polishing 및 CrN 코팅을 통한 PEMFC 성능 향상을 위한 연구

As an important component of a fuel cell, the bipolar plate comprises a large proportion in the fuel cell's volume, weight and price. The bipolar plate is the most widely used; however, graphite bipolar plate is large in volume, brittle and therefore easily broken during assembling. In addition, due to its poor machinability, production costs a lot, unless mass production. Compared with the graphite bipolar plate, the metal bipolar plate has good machinability, high electric conductivity and strong mechanical strength; however, it corrodes easily and has a high contact resistance, so in order to prevent corrosion and reduce the contact resistance, the basic metal needs to be processed by use of electro polishing and coating. The water which is produced by electrochemical reactions in the fuel cell must be discharged smoothly. In this study, in order to prevent corrosion the processes of electro polishing and CrN coating were used. According to the presence or absence of these processes, the contact angles can be measured and different metal bipolar plates can be made, these plates can be used for comparing and analyzing the performance of the fuel cell.

Electro and Surface Properties of Graphene-Modified Stainless Steel for PEMFC Bipolar Plates

Chemical converted graphene (CCG) were coated on 316L stainless steel as a bipolar plate which is a component of proton exchange membrane fuel cell (PEMFC) by electro spray coating (ESC). Scanning electron microscope (SEM) and X-ray diffraction (XRD) were used to examine the thickness and surface properties of coating layer. Electrochemical potentiodynamic test was conducted in acidic atmosphere (0.1N H2SO4+2ppm F-) at 80°C using Versastat 4 and analysis program for corrosion resistance measurement. After packing bipolar plates for PEMFC stack, the electrical performances of graphite, bare SS316L and graphene coated SS316L bipolar plates were evaluated by PEMFC evaluating device. The chemical converted graphene was founded on the surface of coated SS316L, and the thickness was 12μm. Graphene coated bipolar plate showed high corrosion resistance of 1.32×10-7A/cm2beside bare SS316L bipolar plate. In electrical performance evaluation, the graphene coated bipolar plate was shown 0.978V on Voc and 0.5A/m2on the reduction potential (0.6V). Although the electrical performance of the graphene coated bipolar plate is lower than graphite bipolar plate, the thickness and weight is lower than graphite bipolar plate. These advantages can enable the PEMFC system more efficiently and economically.

Thermal Contact Resistance Between Gas Diffusion Layer and Graphite Bipolar Plate: Modeling and Experiments

An analytic, mechanistic robust model is developed to predict the thermal contact resistance (TCR) between fibrous porous media such as GDLs and flat surfaces. The model, which accounts for the salient and realistic geometrical parameters, mechanical deformation, and thermal spreading/constriction resistances, is successfully validated with new experimental data of the TCR between GDLs and graphite bipolar plates. Several parametric studies are performed to reveal the effect of fiber specifications such waviness and also GDL properties on the TCR. For instance, it is found that, interestingly enough, fiber length does not have any effect on TCR at constant porosity. From the parametric studies, the critical values of key parameters effective on TCR are also identified, which can be very useful in GDL manufacturing and fuel cell design in viewpoint of heat management. The presented model can be readily plugged into fuel cell models for simulations and modeling purposes. Overall, the model is developed in a general form to be also applicable, with only minor modifications, to other fibrous media such as fibrous catalyst layers, metal foams, and heat exchangers.

Fabrication of titanium bipolar plates by rubber forming and performance of single cell using TiN-coated titanium bipolar plates

In this study, a rubber forming method is used to fabricate titanium bipolar plates for proton exchange membrane fuel cells. A titanium blank with a thickness of 0.1 mm is compressed using a stamping mold equipped with a 200-ton hydraulic press to fabricate bipolar plates. A forming experiment is carried out by changing different parameters such as the punch velocity, punch pressure, rubber thickness, rubber hardness, and draft angle of the channel. The optimum forming conditions are found to be a rubber thickness of 10 mm, rubber hardness equivalent to that of Shore A 20, punch velocity of 30 mm s−1, punch pressure of 55 MPa, and punch draft angle of 30°. The fabricated titanium bipolar plate is coated with a TiN layer. A single cell with a TiN-coated titanium bipolar plate is examined and compared to those with uncoated titanium and graphite bipolar plates. The initial performances (in terms of current densities) of the single cells with the uncoated titanium, TiN-coated titanium, and graphite bipolar plates are 396, 799, and 1160 mA cm−2, respectively, at a cell voltage of 0.6 V.

Evaluation of Silver-coated Magnesium Bipolar Plate for Lightweight PEM Fuel Cell Stack

A magnesium bipolar plate whose surface was protected by a thinly deposited silver layer was investigated as an alternative to the existing graphite bipolar plate of polymer electrolyte membrane(PEM) fuel cells. A thin silver layer was deposited on a magnesium alloy substrate by physical vapor deposition at 180℃. A number of tests were conducted on the fabricated magnesium-based bipolar plates to determine their suitability for use in PEM fuel cell stacks. A test on corrosion resistance under the same pH conditions as those in a PEM operation demonstrated that the silver layer protected the magnesium alloy substrate, while an unprotected substrate suffered from severe corrosion. The contact resistance of the fabricated bipolar plate was less than 20 mΩ-cm2, which was superior to conventional bipolar plates. A single-cell was constructed using the fabricated bipolar plates and its power output was measured. Owing to the enhanced conductivity caused by low contact resistance, a slight increase was observed in the current density and output voltage. Based on the low density of the magnesium substrate and magnesium’s easy machinability, a weight reduction of 30- 40% is possible for the stack while producing the same power output.

Fabrication of Aluminum Bipolar Plates by Semi‐solid Forging Process and Performance Test of TiN Coated Aluminum Bipolar Plates

AbstractAluminum bipolar plates that can replace graphite bipolar plates for PEM fuel cells are made by applying a semi‐solid forging process. A semi‐solid slurry is made using electromagnetic stirring (EMS), and the resulting slurry is injected into a forging die attached to a 200 ton hydraulic press. The slurry is then compressed with a punch, flowed into a die cavity, and solidified into the bipolar plate form. A356 (cast Al alloy), A6061 (wrought Al alloy), and A1100 (pure Al) are used to make the plates. Titanium nitride (TiN) coating is deposited on the aluminum bipolar plates. An atomic force microscope (AFM) is used to measure the surface roughness of the plates. TiN coated A356 and A1100 plates have a surface roughness of Ra < 1.2 μm. The plate thickness is 1.2 mm. The active area of the channel is 70 mm × 70 mm, with a depth and width of 0.3 and 1.0 mm, respectively. The three TiN‐coated aluminum plates are combined with a unit cell for a performance test. Our results show that a current density value of 473 mA cm–2 (about 41% of the current density value of commercial graphite plates) can be obtained.

Performance Analysis of a Direct Methanol Fuel Cell Stack With Bipolar Plate Incorporated With Innovative Flow-Field Combination

Abstract Increasing interest in utilizing direct methanol fuel cells for portable applications has prompted the need for understanding of their operating characteristics. Approximately 80% of a direct methanol fuel cell stack's volume and weight arise from the bipolar plates. The bipolar plates have grooved anode and cathode flow fields, and have a critical influence on the cell stack performance and stability. However, there is little published data regarding design expansion from single cell to stack, and literature regarding the fuel/oxidant distribution in each cell is especially scant. Hence, this topic is the subject of the present study, which reports the design of a complete direct methanol fuel cell consisting of five single cells including a graphite bipolar plate, as well as an innovative anode and cathode flow channel design. By observing variations in operating parameters, such as applied load and the flow of methanol solution and air, the impact of each parameter on the output performance and stability of the stack was investigated.

Contact resistance between bipolar plate and gas diffusion layer in high temperature polymer electrolyte fuel cells

One of the major contributors to the ohmic loss in fuel cells originates at the interface between adjacent cell components. The compressive pressure used to achieve contact in cells should be carefully estimated to ensure that resistive losses arising from contact behavior remain minimal. In present work, a generic model is developed, capable of estimating contact resistance as a function contact pressure at the interface of graphite bipolar plate and carbon fiber based gas diffusion layer at different temperatures. A good agreement is observed between the results obtained from the model and experiments. Compressive pressure in the ranges of 3–4 MPa is found optimum for achieving low contact resistance. The contact resistance obtained for carbon paper and BPP while using recommended pressure lies between ∼9 and 4 mΩ cm2 considering the operating regime of HT-PEMFC (120–180 °C). Operating under similar conditions, the contact resistance values for carbon cloth and BPP is ∼13 to 7 mΩ cm2.

무인기용 경량 PEM 연료전지 스택용 마그네슘 분리판의 성능평가

표면에 얇은 은(Ag)층이 증착된 마그네슘 분리판을 PEM 연료전지의 그라파이트 분리판의 대체 재질로 검토하였다. <TEX>$180^{\circ}C$</TEX>의 온도 환경에서 마그네슘 모재 표면에 <TEX>$3{\mu}m$</TEX>의 얇은 은층을 물리적 증착방법(PVD)을 이용하여 증착하였다. 제작된 마그네슘 분리판을 대상으로 PEM 연료전지 스택 적용 가능성을 확인하기 위하여 다수의 실험을 수행하였다. PEM 연료전지의 동작환경과 동일한 pH에서의 부식실험을 통하여 보호막이 형성된 마그네슘 분리판은 부식으로부터 모재를 적절히 보호하였지만 보호막이 형성되지 않은 경우 심각한 부식이 발생됨을 확인하였다. 제작된 마그네슘 분리판의 접촉저항은 <TEX>$20m{\Omega}-cm^2$</TEX>이하로 기존의 분리판 대비 우수한 성능을 보였다. 이러한 낮은 접촉저항으로 인하여 전기전도도가 개선되어 연료전지의 성능이 향상됨을 확인하였다. 마그네슘 모재의 낮은 밀도와 기계가공의 용이성 때문에 동일한 연료전지 스택의 출력을 기준으로 약 30~40 %의 중량절감이 가능한 것으로 판단되었다. A magnesium bipolar plate whose surface was protected by thinly deposited silver layer was investigated as an alternative to existing graphite bipolar plate of PEM fuel cells. Thin silver layer of <TEX>$3{\mu}m$</TEX> was deposited on a magnesium alloy substrate by physical vapor deposition (PVD) method in an environment of <TEX>$180^{\circ}C$</TEX>. A number of tests were conducted on the fabricated magnesium based bipolar plates to determine their suitability for use in PEM fuel cell stacks. The test on corrosion resistance in the same pH condition as in a PEM operation demonstrated the layer protected the magnesium alloy substrate, while unprotected substrate suffered from severe corrosion. The contact resistance of the fabricated bipolar plate was less than <TEX>$20m{\Omega}-cm^2$</TEX> which was superior to the conventional bipolar plates. A single cell was constructed using the fabricated bipolar plates and power output was measured. Due to the enhanced conductivity caused by low contact resistance, slight increase was observed in current density and output voltage. With low density of the magnesium substrate and ease on machining, the weight reduction of the stack of 30~40 % is possible to produce the same power output.

Corrosion characteristics and interfacial contact resistances of TiN and CrN coatings deposited by PVD on 316L stainless steel for polymer electrolyte membrane fuel cell bipolar plates

In a polymer membrane fuel cell stack, the bipolar plate is a key element because it accounts for over 50% of total costs of the stack. In order to lower the cost of bipolar plates, 316L stainless steels coated with nitrides such as TiN and CrN by physical vapor deposition were investigated as alternative materials for the replacement of traditional brittle graphite bipolar-plates. For this purpose, interfacial contact resistances were measured and electrochemical corrosion tests were conducted. The results showed that although both TiN and CrN coatings decreased the interfacial contact resistances to less than <TEX>$10m{\Omega}{\cdot}cm^2$</TEX>, they did not significantly improve the corrosion resistance in simulated polymer electrolyte membrane fuel cell environments. A CrN coating on 316L stainless steel showed better corrosion resistance than a TiN coating did, indicating the possibility of using modified CrN coated metallic bipolar plates to replace graphite bipolar plates.

Corrosion Resistance Measurements of Amorphous Ni40Ti40Nb20 Bipolar Plate Material for Polymer Electrolyte Membrane Fuel Cells

Metallic bipolar plates have the advantages of better manufacturability, higher strength over graphite bipolar plates. The higher strength and toughness of the metallic materials permits the reduction of the width of the bipolar plate so, the volume and mass of the fuel cell can also be reduced. In this paper we are investigating the use of Ni-based amorphous material as a bipolar plate for polymer electrolyte membrane fuel cell (PEMFC). The major requirements of the metallic bipolar plate material are low weight, high corrosion and low contact resistance. The corrosion property of the present alloy has been investigated under conditions that simulate the fuel cell environment. Hydrogen gas and air were bubbled into a 1 N H2SO4solution at 70 °C, throughout the experiment to simulate the respective anodic and cathodic PEMFC environment. The Ni-base amorphous alloys displayed higher corrosion resistance than stainless steel.

Ni-based amorphous alloy-coating for bipolar plate of PEM fuel cell by electrochemical plating

In this study, the Ni-Cr-P amorphous alloy-coated bipolar plates were produced by electro-plating on the Cu base plates with a flow field. The power generation tests of a single fuel cell with those Ni-Cr-P bipolar plates were conducted at 353 K. It was found that the single fuel cell with those Ni-Cr-P bipolar plates showed excellent I-V performance as well as that with the carbon graphite bipolar plates. It was also found that the single cell with those Ni-Cr-P bipolar plates showed better I-V performance than that with the Ni-P amorphous alloy-coated bipolar plates. Furthermore, the long-time operation test was conducted for 440 h with those Ni-Cr-P bipolar plates at the constant current density of 200 mA·cm−2. As a result, it was found that the cell voltage gradually decreased at the beginning of the measurement before 300 h and then the voltage was kept constant after 300 h.

Effect of small-sized conductive filler on the properties of an epoxy composite for a bipolar plate in a PEMFC

Abstract This paper focused on using a conductive polymer composite (CPC) as a potential replacement for the conventional graphite bipolar plate used in polymer electrolyte membrane fuel cells (PEMFC). Based on the requirements established by the US Department of Energy (DOE), the in-plane electrical conductivity and flexural strength are required to be greater than 100 S/cm and 25 MPa, respectively. The high filler loading is needed to satisfy the high in-plane electrical conductivity. However, the high filler loading reduces the flexural strength and manufacturability of the composite. In this study, the composites were prepared by compounding using an internal mixer followed by compression moulding. The combination of 10 vol% carbon black (CB) as the second filler with synthetic graphite/epoxy (SG/EP) resulted in the following composite properties: 150 S/cm (in-plane conductivity), 55 S/cm (through-plane conductivity), and 38.8 MPa (flexural strength). Used as the second filler, the CB, which had a small-sized diameter, formed conductive networks that filled the voids between the SG and polymer matrix. The in-plane electrical conductivity and flexural strength of the CB/SG/EP composites at the optimum composition exceeded the requirement for bipolar plate applications.

도금된 폴리카보네이트 분리판을 이용한 공기 호흡형 고분자 전해질막 연료전지에 관한 연구

본 연구에서는 도금된 폴리카보네이트를 사용하여 공기호흡형 고분자 전해질막 연료전지의 분리판을 제작하였다. 도금층은 구리 40μm, 니켈 10μm, 금 0.3μm 로 구성되었으며, 구리는 전기 전도층, 니켈은 구리와 금의 결합, 금은 도금층의 부식을 방지하기 위해 사용되었다. 본 분리판을 사용하여 성능을 평가한 결과 120mW/cm<SUP>2</SUP> 의 전력 밀도를 보였으며 이는 동일한 조건에서 그라파이트 분리판을 사용했을 때의 전력밀도와 거의 차이가 나지 않았다. 또한 평판형 12 층 스택의 공기호흡형 연료전지를 구성한 결과 각 전지당 132.7mW/cm<SUP>2</SUP> 의 성능을 보였으며 이를 12 시간 운전해본 결과 안정적인 성능을 보여 공기호흡형 고분자 전해질 연료전지의 분리판으로 적합함을 확인하였다.

Production of Ni-Based Glassy Alloy-Coated Bipolar Plate with Hydrophilic Surface for PEMFC and Its Evaluation by Electrochemical Impedance Spectroscopy

In this study, Ni65Cr15P16B4 glassy alloy-coated bipolar plates with a hydrophilic surface were produced and the power generation properties of a single fuel cell with such plates were investigated, SUS316L and graphite bipolar plates also being produced for comparison. The fuel cell with the glassy alloy-coated bipolar plates showed good I­V performance after repeated I­V scans for membrane activation as did the cell with the graphite bipolar plates. After 30 I­V scans for activation, a long-time power generation test for 100 h was conducted at a current density of 0.2A·cm12. As a result, it was found that the fuel cell with the glassy alloy-coated bipolar plates showed stable power generation behaviour, maintaining the cell voltage at around 0.6V. Impedance measurements were also conducted for the cells and it was found that it was possible to understand the difference of I­V behaviour between the cells with three different bipolar plate materials by calculating the impedance parameters. [doi:10.2320/matertrans.MF201309]