Carbon Composite Bipolar Plate Research Articles

Fuel starvation in automotive PEMFC stacks: hydrogen stoichiometry and electric cell-to-cell interaction

Fuel gross starvation (FGS) in a polymer electrolyte membrane fuel cell is an error state, during which the supplied amount of fuel is insufficient to sustain the requested electrical current. A novel experimental technique was developed to intentionally provoke well-controlled fuel starvation situations of one single cell in a multi-cell fuel cell stack. This modification was implemented in a 20 cell stack of automotive-sized cell geometry and carbon composite bipolar plates (BP). The intentional fuel starvation situation was analyzed using a printed circuit board to measure the current density distribution (CDD) in addition to a multipoint cell voltage monitoring (CVM) to measure local cell voltages. The provoked detrimental subsidiary reactions of the anode were found to take place spatially separated from the normal hydrogen oxidation reaction. It was therefore possible to determine and intentionally vary the hydrogen stoichiometry of the fuel starved cell. This error state caused intense distortions of the starved cells CDD and local cell voltages. The maximum difference obtained between outlet and inlet voltage of the modified cell was 1.4 V. Compared to the average current density, a more than 4-times higher maximum local current density was measured in the affected cell. Adjacent cells were also affected via electric cell-to-cell interaction. Characteristic patterns therefore became visible in the cell voltage distribution, measured by the inlet and outlet CVM. The use of carbon composite BP is favoring the occurrence of these patterns due to their relatively high electric sheet resistance. Using the new hardware setup, we could investigate the relation between the hydrogen stoichiometry of the affected cell during FGS and the observed irregular redistribution of current density and local cell voltages.

Conductive nanoparticle-embedded carbon composite bipolar plates for vanadium redox flow batteries

Electrochemical reduction and oxidation reactions of vanadium ions can enable efficient power management by a secondary battery, such as a vanadium redox flow battery (VRFB). To increase the energy efficiency of a VRFB system, the electrical resistance of the cell component, such as a bipolar plate, should be reduced. In this study, particles of polyaniline (PANI), a conductive polymer, were embedded in a carbon/epoxy composite by spraying on the prepreg surface to increase the electrical conductivity of the bipolar plate. The effect of the PANI particles on the electrical and mechanical properties was investigated. Furthermore, the optimum PANI content of 3 wt% achieved an area-specific resistance of 16.7 mΩ·cm2, 55.2 % lower than that of the neat specimen. The tensile strength was improved to 586 MPa, a 13.8 % increase compared with the neat specimen. Finally, the cell performance of the fabricated bipolar plate was verified via permeability and single-cell charge/discharge tests.

Characterization of Commercial Polymer–Carbon Composite Bipolar Plates Used in PEM Fuel Cells

Bipolar plates represent a crucial component of the PEM fuel cell stack. Polymer-carbon composites are recognized as state-of-the-art materials for bipolar plate manufacturing, but their use involves a compromise between electrical and heat conductivity, mechanical strength and costs. Thus, all key parameters must be considered when selecting a suitable plate satisfying the demands of the desired application. However, data relevant to commercial materials for such selection are scarce in the open literature. To address this issue, 13 commercially available polymer-carbon composites are characterised in terms of the following parameters: through-plane conductivity, hydrogen permeability, mechanical strength, water uptake, density, water contact angle and chemical stability. None of the materials tested reached the DOE target for electrical conductivity, while five of the materials met the target for flexural strength. The overall best-performing material showed a conductivity value of 50.4 S·cm-1 and flexural strength of 40.1 MPa. The data collected provide important supporting information in selecting the materials most suitable for the desired application. In addition, the key parameters determined for each bipolar plate supply important input parameters for the mathematical modelling of fuel cells.

Development of multifunctional carbon composite bipolar plate for vanadium redox flow batteries

The carbon/epoxy composite bipolar plate is a promising substitute for the conventional graphite bipolar plate for the vanadium redox flow battery due to its high mechanical property and productivity. The carbon/epoxy composite bipolar plate, a multifunctional structure, requires expanded graphite coating or additional surface treatments to decrease the interfacial contact resistance. However, expanded graphite coating has low durability under vanadium redox flow battery operating condition and surface treatments are costly to implement. In this work, excess resin-absorbing method is developed with polyester fabric to uniformly remove the resin-rich layer and expose carbon fibers on the surface of the carbon/epoxy composite bipolar plate. This method not only decreases the interfacial contact resistance by exposing carbon fibers but also forms a unique ditch pattern which is desirable to fix carbon felt electrode in place. Durability against acidic environment, mechanical property, and gas permeability of the composite bipolar plate manufactured by excess resin-absorbing method is investigated.

Cathode/anode integrated composite bipolar plate for high-temperature PEMFC

Carbon fiber reinforced polymer composites are ideal substitutes for the brittle graphite bipolar plate in lightweight proton exchange membrane fuel cells (PEMFCs) because of their high specific strength and stiffness. Among the various types of carbon composite bipolar plates, non-woven carbon felt based composites have recently attracted attention because they require much lower curing pressures than unidirectional carbon fiber or carbon fabric composites.In this work, a carbon felt composite bipolar plate is developed for a high-temperature PEMFC (HT-PEMFC) using randomly oriented non-woven carbon felt and a cyanate ester-modified epoxy. The cathode and anode are integrated into a single bipolar plate to reduce the thickness and weight of the HT-PEMFC stack. The developed composite bipolar plate exhibits superior electrical properties because of the bare carbon fibers exposed on the surface by the soft layer method. In addition, the bipolar plates are dimensionally stable under high-temperature and high-pressure conditions. Finally, a cell test is performed on a 3-cell HT-PEMFC stack to investigate the cell performance.

Carbon composite bipolar plate for high-temperature proton exchange membrane fuel cells (HT-PEMFCs)

Abstract A carbon/epoxy composite bipolar plate is an ideal substitute for the brittle graphite bipolar plate for lightweight proton exchange membrane fuel cells (PEMFCs) because of its high specific strength and stiffness. However, conventional carbon/epoxy composite bipolar plates are not applicable for high-temperature PEMFCs (HT-PEMFCs) because these systems are operated at higher temperatures than the glass transition temperatures of conventional epoxies. Therefore, in this study, a cyanate ester-modified epoxy is adopted for the development of a carbon composite bipolar plate for HT-PEMFCs. The composite bipolar plate with exposed surface carbon fibers is produced without any surface treatments or coatings to increase the productivity and is integrated with a silicone gasket to reduce the assembly cost. The developed carbon composite bipolar plate exhibits not only superior electrical properties but also high thermo-mechanical properties. In addition, a unit cell test is performed, and the results are compared with those of the conventional graphite bipolar plate.

Optimum design of the carbon composite bipolar plate (BP) for the open cathode of an air breathing PEMFC

The proton exchange membrane fuel cells (PEMFCs) are a promising source of portable power due to its high energy density, low emissions and wide power range. However, PEMFCs have some limitations for portable applications because of the supplementary power consumption and the mass increase by balance of plants, such as the fuel pump, air compressor and humidifier.In this study, a light mass PEMFC system was developed using carbon composite bipolar plates (BPs) and an air breathing stack design with open cathode channel configurations to reduce the mass of the stack. However, under the high compaction pressure, the failure occurred in the carbon composite BPs for the open cathode.To enhance the durability of the BP against the compaction pressure, the finite element analysis, the compressive test and the fracture toughness test were performed. The maximum compaction pressure on the BP was calculated by applying the maximum strain failure criterion to the composite BP. Then, the optimum thickness and stacking sequence of the open cathode bipolar plate were determined by considering the mass and the maximum compaction pressure.

Effect of the addition of milled carbon fiber as a secondary filler on the electrical conductivity of graphite/epoxy composites for electrical conductive material

In this work, carbon composite bipolar plates consisting of synthetic graphite and milled carbon fibers as a conductive filler and epoxy as a polymer matrix developed using compression molding is described. The highest electrical conductivity obtained from the described material is 69.8 S/cm for the in-plane conductivity and 50.34 S/cm for the through-plane conductivity for the composite containing 2 wt.% carbon fiber (CF) with 80 wt.% filler loading. This value is 30% greater than the electrical conductivity of a typical graphite/epoxy composite with 80 wt.% filler loading, which is 53 S/cm for the in-plane conductivity and 40 S/cm for the through-plane conductivity. The flexural strength is increased to 36.28 MPa compared to a single filler system, which is approximately 25.22 MPa. This study also found that the General Effective Media (GEM) model was able to predict the in-plane and through-plane electrical conductivities for single filler and multiple filler composites.

Novel fabrication process for carbon fiber composite bipolar plates using sol gel and the double percolation effect for PEMFC

Bipolar plates are a key component of polymer electrolyte membrane fuel cells (PEMFCs) in terms of cost and weight. Recently, bipolar plates based on carbon fiber-reinforced composites have been the main focus of development activities by researchers because they provide high strength and stiffness as well as excellent corrosion resistance. Although carbon composite bipolar plates have these advantages, they also have low electrical properties that compromise the efficiency of PEMFCs.In this study, a bipolar plate for PEMFCs was developed with a carbon fiber/polypropylene (PP)/polyethylene (PE) composite filled with multiwall carbon nanotubes (MWCNTs) by compression molding. Carbon fiber/PP/PE composite sheets filled with MWCNTs were made by using the sol impregnation method to enhance the degree of impregnation and dispersion of MWCNTs. In addition, the double percolation effect was introduced to improve the electrical conductivity. The electrical resistance, flexural strength, and gas permeability were investigated via the four-point probe method, three-point bending test, and unsteady-state permeability measurement method, respectively. The results were also compared with the technical target from the Department of Energy (DOE).

Durability of graphite coated carbon composite bipolar plates for vanadium redox flow batteries

Composite bipolar plates for vanadium redox flow batteries (VRFBs) and fuel cells are coated with expanded graphite to decrease the interfacial contact resistance of the carbon composite. The bipolar plates of vanadium redox flow batteries are exposed to electro-chemical corrosion, which might degrade the graphite coating layer. From two types of graphite foil, i.e., pyrolytic graphite and expanded flake-type graphite, the former has higher durability because its graphene sheets are crystallized in a planar direction with a highly oriented structure, which has a stronger van der Waals bonding.In this study, the characteristics of the pyrolytic graphite and expanded flake-type graphite are investigated with respect to the porosity of graphite. The durability of the graphite coating of the carbon/epoxy composite bipolar plate during the electrochemical reaction in vanadium electrolytes, based on highly concentrated sulfuric acid, is investigated. The areal specific resistances (ASRs) of the bipolar plates coated with pyrolytic graphite are measured, from which the ASR is expressed in terms of modulus of the graphite based on the Hertzian contact model. A single cell test is performed to evaluate the performance of the developed bipolar plate.

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.

Surface crack closing method for the carbon composite bipolar plates of a redox flow battery

The vanadium redox flow battery (VRFB) is the most promising energy storage system (ESS) due to its safety, durability and scalability. The electrode and the bipolar plate are the most important components that affect the efficiency of the VRFB. Based on previous research, an expanded graphite-coating method for the composite bipolar plate of the VRFB was employed to reduce interfacial contact resistance between the electrode and the bipolar plate. However, damage to the coating layer of the composite bipolar plate during actual operating condition can reduce durability of the composite bipolar plate.In this work, a surface crack closing method for the damaged composite bipolar plates of a redox flow battery was developed. To optimize the surface crack closing process, compressive tests of the expanded graphite were performed. The effect of the surface crack closing process was estimated by measuring the area-specific resistance (ASR) before and after an electro-chemical operation test. Based on the results of the analysis, the optimal surface crack closing process for the composite bipolar plate was suggested.

Development of carbon/PEEK composite bipolar plates with nano-conductive particles for High-Temperature PEM fuel cells (HT-PEMFCs)

Proton Exchange Membrane Fuel Cell (PEMFC) systems are environmentally friendly power sources that have many potential applications. Although bipolar plates made of carbon fiber composites have high strength and stiffness and also resist corrosion in an acidic environment, they exhibit only marginal electrical conductivity, which compromises the efficiency of PEM fuel cells.In this work, to increase both the manufacturing productivity and the electrical conductivity of a High-Temperature PEM fuel cell (HT-PEMFC) bipolar plate, PEEK (Poly Ether–ether Ketone), a high temperature thermoplastic polymer was employed as the matrix of the carbon composite bipolar plate. To increase the electrical conductivity and strength, a continuous carbon fiber composite and conductive nano-particles were used. The electrical resistance and the mechanical properties were measured at both low and high temperatures. Environmental durability tests were performed to verify the durability of the developed bipolar plates.

Development of carbon composite bipolar plate (BP) for vanadium redox flow battery (VRFB)

A vanadium redox flow battery (VRFB) is one of the most promising energy storage systems (ESSs) due to its safety, durability and scalability. However, high cost of its components has been obstacle for commercialization of VRFB. Especially, bipolar plates (BPs) which are the main components of VRFB, are fabricated using graphite, which increases not only its manufacturing cost, but also decreases the reliability of VFRB, especially for the large area BP due to the brittleness of graphite although the graphite has high electrical conductivity and chemical stability.In this work, a carbon composite BP for the VRFB has been developed optimally considering its electrical as well as chemical stabilities against strong acids. Charge/discharge tests of the VRFB using the developed carbon composite BPs were performed to observe its energy and voltage efficiencies, from which the durability of the composite BP was estimated.

Conductive particles embedded carbon composite bipolar plates for proton exchange membrane fuel cells

Proton exchange membrane (PEM) fuel cell systems are environmentally friendly power sources that have many potential applications. Although carbon fiber composite bipolar plates have high strength and stiffness with good corrosion resistance in an acid environment, they exhibit only a marginal electrical conductivity, which compromises the efficiency of PEM fuel cells.In this work, electrically conductive particles (graphite powder and carbon black) are sprayed onto carbon–epoxy composite prepregs of a bipolar plate to decrease the electrical resistivity of the bipolar plate. The electrical resistance and mechanical properties are measured using conventional test methods. In addition, a unit cell performance assessment is conducted with the developed carbon composite bipolar plates and compared with that of the conventional bipolar plate.

Surface modifications of gasketless carbon composite bipolar plates for gas tightness of PEM fuel cells

A carbon fiber composite gasketless bipolar plate for proton exchange membrane (PEM) fuel cells was developed using the groove of a trapezoidal shape, which develops large contact stresses due to a wedge mechanism. The surface of the composite groove was modified to increase gas tightness without using rubber gaskets. Graphite, polyimide (PI) and polyethylene terephthalate (PET) film were co-cure bonded on the surface of the bipolar plate during the compression molding process to change its surface hardness, which could affect the gas tightness. Additionally, plasma surface treatment and mechanical abrasion were used to modify the surface morphologies. Corrosion and gas tightness tests on the surface modified bipolar plates were performed to verify the reliability of the gasketless sealing method.

Effects of binder type and heat treatment temperature on physical properties of a carbon composite bipolar plate for PEMFCs

Abstract This study investigated a developed process for producing a composite bipolar plate having excellent conductivity by using coal tar pitch and phenol resin as binders. We used a pressing method to prepare a compact of graphite powder mixed with binders. Resistivity of the impregnated compact was observed as heat treatment temperature was increased. It was observed that pore sizes of the GCTP samples increased as the heat treatment temperature increased. There was not a great difference between the flexural strengths of GCTP-IM and CPR-IM as the heat treatment temperature was increased. The resistivity of GPR700-IM, heat treated at 700°C using phenolic resin as a binder, was 4829 µΩ·cm which was best value in this study. In addition, it is expected that with the appropriate selection of carbon powder and further optimization of process we can produce a composite bipolar plate which has excellent properties. Key words: polymer electrolyte membrane fuel cell, bipolar plate, graphite, resistivity, flexural strength

The Effect of Addition of Multi Wall Carbon Nano Tubes (MWCNT) Reinforcement to the Characteristics of Carbon Composite Bipolar Plate

Polymer electrolyte membrane fuel cell (PEFMC) or also known as proton exchange membrane fuel cell is a chemical conversion device that converts hydrogen and oxygen into electrical energy, heat, and water. One of the most important components of PEMFC is bipolar plate, in which it allows for electrons to flow from the anode to the cathode. The objective of this study was to analyze the effect of the addition of multi walled carbon nanotubes (MWCNT) to increase the mechanical and electrical properties of bipolar plate carbon polymer composite. We utilized graphite waste product from electric arc smelting as reinforcementand carbon black made from coconut husk by pyrolysis process as a filler. Bipolar plates were made by compression moulding method at a pressure of 55 MPa and a temperature of 100 o C for 4 hours. Characterization in this study includes density testing, porosity testing, flexural testing, electrical conductivity testing, and observation of the flexural fracture morphology using FESEM. Based on this study, it showed that the addition of 5 %wt MWCNT yielded optimal properties of the bipolar plate (the density was 1.51 g/cm3, the value of porosity was 1.94 %, the flexural strength was 63.31 MPa, and the electrical conductivity was 2.30 S/cm). In conclusion, adding MWCNT as reinforcement in PEMC bipolar plates could reduce the density and the porosity. Thus, it could improve the electrical conductivity and flexural strength of the bipolar plate carbon polymer composites.

The Effects of Carbon Black Loading on the Characteristics of Carbon Composite Bipolar Plate by Utilizing Graphite Waste Products

Current energy resources derived from fossil fuels thinning, and the issue global warning make the relevant parties that concern about the environment has been trying to find alternative renewable energy. Among the renewable energy options, the fuel cell is one of the many alternatives studied by the researchers in the world. One type of fuel cell that is currently being investigated is the proton exchange membrane fuel cell cel. The utilization of graphite and carbon black waste product is expected to result in light-weight and cost-effective bipolar plates.by using recycle materials. In this paper, we used graphite EAF as reinforcement together with carbon black that comes from the coconut husk pyrolysis process and epoxy resin as a binder. We examined the effects of carbon black loading at 5 %wt and 10 %wt, which influenced by differential pressure applied on compression molding process on density, porosity, flexural strength and electrical conductivity of the resulting polymer carbon composite bipolar plate. Pressure was applied from 30 MPa - 60 MPa in increments of 5 MPa while maintaining constant temperature operation at 70oC for 4 hours. Maximum value of bipolar plate was achieved by 5 %wt CB at application 55 MPa, density of 1.69 g/cm3, the flexural strength was measured to be 48 MPa with the porosity of 0.7%, and electrical conductivity of 1.03 S/cm.We demonstrated that waste product such as graphite EAF and carbon black are a good candidate for manufacturing of polymer carbon composite bipolar plates.

Development of Carbon Composite Bipolar Plates for Vanadium Redox Flow Batteries

Carbon composite bipolar plates with various carbon black contents were prepared by a compression molding method. The electrical conductivity and electrochemical stability of the bipolar plates have been evaluated. It is found that the electrical conductivity increases with increasing carbon black contents up to 15 wt %. When the carbon black contents are greater than 15 wt %, the electrical conductivity decreases because of a poor compatibility between epoxy resin and carbon black, and a weakening of compaction in the carbon composite bipolar plate. Based on the results, it could be concluded that there are optimum carbon black contents when preparing the carbon composite bipolar plate. Corrosion tests show that the carbon composite bipolar plate with 15 wt % carbon black exhibits better electrochemical stability than a graphite bipolar plate under a highly acidic condition. When the optimized carbon composite bipolar plate is applied to vanadium redox flow cells, the performance of flow cells with the carbon composite bipolar plate is comparable to that of flow cells with the graphite bipolar plate.