Graphite Composite Bipolar Plates Research Articles

High-performance polybenzoxazine based composites PEMFC bipolar plates with a multi-layer structure for surface enrichment of conductive phase

In this study, a multilayer high-performance polybenzoxazine/expanded graphite composite bipolar plate with a ‘graphite-composites-graphite’ structure was prepared by compressing molding. The surface graphite layer and the conductive network were constructed by hot compressing of graphite paper and the polybenzoxazine/expanded graphite composites. The results show that the multilayer synergy makes composite BP reach an in-plane conductivity of 278.85 S cm−1, an area specific resistance of 9.70 mΩ cm2 and a flexural strength of 75.75 MPa. The ‘graphite-composites-graphite’ structure can make the power density of a single cell assembled by the multilayer composite BP increase by 111.02% and 113.80%. In conclusion, the multilayer bipolar plate with a ‘graphite-composites-graphite’ structure is a promising polymer composite bipolar plate for the proton exchange membrane fuel cell (PEMFC).

Carbon Black Doped Graphite Composite Bipolar Plate Compounds for Improving Conductivity

Bipolar plates are a key part of fuel cells. The optimal hot press temperature, pressure, and time were determined in this study. The press condition can directly affect the relative density, thereby improving the conductivity and mechanical property. Several carbon black doped samples were also successfully prepared on the basis of the optimized preparation method. Results show that carbon black optimized the conductivity due to the “conduction bridge”. The maximum conductivity and bending strength reached 345 S/cm and 32.5 MPa, respectively. Key words: fuel cells; bipolar plate; graphite; composite materials.

Life Cycle Assessment and Key Parameter Comparison of Hydrogen Fuel Cell Vehicles Power Systems

Hydrogen fuel cell vehicles (HFCVs) are regarded as potential solutions to the problems of energy security and environmental pollution. To explore the energy consumption and pollutant emissions of fuel cell vehicle power systems, data inventories of an HFCV power system were established, and quantitative evaluation calculations and prediction analysis were carried out for fuel life cycle energy consumption and greenhouse gas emissions of Chinese fuel cell vehicles in 2030 based on the technology roadmap for new energy vehicles by modeling with GaBi software. The effects of different types of bipolar plates, different energy control strategies, and different hydrogen production methods on the environment were studied, with uncertainty analysis as the key parameter. The results showed that fossil energy consumption (ADPf), global warming potential (GWP, CO2 equivalent), and acidification potential (AP, SO2 equivalent) for the HFCV power system in the fuel life cycle were 1.35×105 MJ, 9108 kg, and 15.79 kg, respectively. The energy consumption and greenhouse gas emissions in the production of the power system were higher than those in the use stage, mainly because of the fuel cell stack and hydrogen storage tank. In the manufacturing process of metal bipolar plates, graphite composite bipolar plates, and graphite bipolar plates, graphite composite bipolar plates had the most comprehensive environmental benefits. Optimizing the energy control strategy will reduce hydrogen energy consumption. When the hydrogen energy consumption was reduced by 22.8%, the life cycle energy consumption and greenhouse gas emissions of the power system were reduced by 10.4% and 8.3%, respectively. For life cycle power systems, the use of hydrogen from electrolysis operated with water power reduced the GWP by approximately 39.6% relative to steam methane reforming. In contrast, the application of hydrogen from electrolysis operated with the Chinese electricity grid mix resulted in an increase in GWP of almost 53.7%. Measures to reduce fossil energy consumption and global warming potential in the life cycle of fuel cell vehicle powertrains include optimizing energy control strategies to reduce hydrogen energy consumption, scaling up the hydrogen production industry using water electrolysis for renewable energy power generation, and focusing on key technologies of fuel cell stacks to improve performance.

Study on carbon matrix composite bipolar plates with balance of conductivity and flexural strength

In order to balance the conductivity and flexural strength of graphite composite bipolar plates, the influence of conductive filler on the properties of graphite composite bipolar plate was comprehensively studied by using phenolic resin as binder, natural flake graphite as conductive substrate and functional carbon materials with different structures as auxiliary filler. The results show that the particle size of conductive substrate has an important influence on the conductivity enhancement of auxiliary filler. The influence of conductive particle size on auxiliary filler electrical conductivity improvement was first investigated in this research. The effects of various auxiliary filler concentrations on improving electrical conductivity and flexural strength were then examined. This research has substantial implications for the balance of electrical conductivity and flexural strength of graphite composite bipolar plates.

Research progress in carbon-based composite molded bipolar plates

Bipolar plates are significant components in proton exchange membrane fuel cells (PEMFCs), thus playing a decisive role in the weight, volume, efficiency, durability, and cost of battery stacks. At present, the preparation technologies of metal and graphite plate stacks have become relatively mature, and have been widely used in the fields of commercial and passenger vehicles. However, production of composite material bipolar plates has not been marketed on a large scale in China due to the incomplete localization of raw material formulation, absence of mass production lines and high production cost. Therefore, it is of great significance to find low-cost raw materials, screen raw material compositions and ratios, optimize processing conditions including molding temperature, pressure and time, and shorten the processing cycle for the industrialization of composite bipolar plates. In this review, the characteristics of metal bipolar plates, graphite bipolar plates and composite material bipolar plates were compared. Moreover, the molding process and its advantages in composite bipolar plate production were introduced. Subsequently, recent progress in research of carbon-based composite molded bipolar plates was summarized. This includes resin/graphite composite bipolar plates using thermosetting resins such as phenol formaldehyde resin, epoxy resin and vinyl ester resin as the binder, and carbon black, carbon fiber and carbon nanotube reinforced composite bipolar plates. The effects of the types and ratios of raw materials and molding process conditions on the performance of bipolar plates were emphasized. Finally, the industrialization status of composite bipolar plates was discussed, and the problems faced by major composite bipolar plate manufacturers at home and abroad were pointed out. Prospects in the development of composite bipolar plates were also explored. Additionally, unified performance test standards for composite bipolar plates were recommended to make the performance of products developed by different enterprises comparable.

Influence of oxyfluorinated graphite on fluorinated ethylene–propylene composites as bipolar plates

Numerous studies have reported that good adhesion and fluorination of carbon materials in a fluoropolymer matrix enhance their electrical and mechanical properties. However, a composite reinforced with oxyfluorinated graphite has not been reported for improving mechanical properties. This paper discusses the fabrication of conductive fluorinated ethylene–propylene (FEP)/oxyfluorinated graphite (f-graphite) composite bipolar plates (BPs) via compression molding. To investigate the effects of fluorinating graphite, graphite with a large particle size of 500 μm was mixed with FEP powder with a small particle size of 8 μm through ball milling. The FEP/graphite composites exhibited high anisotropic electrical conductivity with the in-plane conductivity much higher than the through-plane conductivity because of the planar orientation of the graphite sheets. Therefore, the mechanical properties of the composites such as flexural strength tended to deteriorate with increasing graphite content. In particular, the FEP/f-graphite composites exhibited excellent flexural strength of 12 MPa, much higher than that of FEP/graphite composites at 9 MPa with a graphite content of 80 wt%. The interfacial interaction between FEP and f-graphite led to improved physical compatibilization, which contributed to enhance the mechanical properties of these composites. Our results are a step toward developing BPs for use in high-temperature fuel cells and heat-sink components. Graphic abstract

Comparison of Carbon Based Bipolar Plate Materials for Polymer Electrolyte Membrane Fuel Cells

Polymer electrolyte membrane (PEM) fuel cells can be separated into two types, the high-temperature (HT) PEM and low-temperature (LT) PEM fuel cell. As main advantages operating at higher temperatures improved tolerances to impurities like carbon monoxide, easier heat rejection and simpler water management can be mentioned. In comparison to LT-PEM fuel cells where sulfuric acid is applied polybenzimidazole (PBI) membranes doped with phosphoric acid (PA) were used in HT-PEM fuel cells. Nevertheless, degradation and long term stability are still major challenges. Here, it is necessary to understand the degradation mechanisms regarding membrane and catalyst degradation, carbon corrosion as well as loss of phosphoric acid and degradation of bipolar plates (BPP). Titration investigations of carbon based bipolar plates after operation in PEM fuel cells showed that phosphoric acid can be found within the composite matrix [1, 2] which can lead to corrosion of the graphite material. In this work, a variety of graphite-polymer composite bipolar plate materials for PEM fuel cells were tested for their stability and compared with different methods. The electrochemical stability was investigated by means of corrosion measurements within a three-electrode cell setup. The bipolar plate material was applied as working electrode and a platinum wire with an activated felt was served as counter electrode. Potentials are shown with respect to the standard hydrogen electrode (SHE). Two different methods of aging were used: chemical storage and potentiostatic treatment. For determination of stability and corrosion rates OCP, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were evaluated. Additionally, the bipolar plate materials were compared by electrical conductivity measurements (in-plane and through-plane), contact angle measurements as well as structural changes of the surface area. Here different imaging methods like scanning electron microscopy (SEM), micro-computed tomography (µ-CT), atomic force microscopy (AFM) and confocal microscopy were applied. First results of two graphite composite bipolar plate materials with different types and amount of polymer binder show different electrochemical behavior after chemical aging for 3 and 7 days in 0.5 M H2SO4. The bipolar plate material with higher polymer content (BPP-01) reveals lower double layer capacitance and a lower amount of oxygen functional groups in the potential range between 0.5-0.7 V vs. SHE (Figure 1) in comparison to the bipolar plate with higher polymer content (BPP-02). The reason for this behavior could be the lower amount of electrochemical active graphite at the BPP/electrolyte interface from BPP-01 in comparison to BPP-02. In addition, conductivity measurements show that BPP01 has a higher sheet resistance (53.6 +/- 5.6 mOhm) compared to BPP-02 (46.2 +- 4.5 mOhm) in good agreement to the content of polymer and graphite in the composite materials. The various methods can be used to identify the difference of graphite compound materials and correlate it to degradation mechanism. Further results will be given. [1] C. Hartnig and T. J. Schmidt, Electrochim. Acta, 56 (2011), 4237-4242.[2] N. Pilinski, M. Rastedt and P. Wagner, ECS Trans., 69 (2015), 323-335. Figure 1

Comparison of Carbon Based Bipolar Plate Materials for Polymer Electrolyte Membrane Fuel Cells

Two types of graphite composite based materials, BPP-01 with higher amount of polymer binder in comparison to BPP-02, are investigated and reported for their suitability as bipolar plates (BPPs) in polymer electrolyte membrane fuel cells (PEMFC). Through-plane electrical conductivity of pristine BPP-01 and BPP-02 are 10.31 and 11.38 S cm-1, respectively. The electrochemical behaviour of aged BPPs is examined by cyclic voltammetry (CV) in sulfuric acid. Chemically aged BPP-01 exhibits lower double layer capacitance than BPP-02. In order to observe cathodic and anodic corrosion behaviour of BPPs, potentiostatic polarisation experiments are also performed. BPP-02 undergoes higher corrosion rate than BPP-01 during cathodic polarisation. BPP-02 surface change can be observed even with bare eyes. Conversely, physical changes do not occur after anodic polarisation for both BPPs which were thoroughly characterised by profilometer, optical microscope and micro-computed tomography (μ-CT). The higher polymer content based graphite composite bipolar plate (BPP-01) endures the potential applied in PEMFC applications.

Fabrication of micro-flow channels on graphite composite bipolar plates using microplaning

A new method of manufacturing micro-flow channels on graphite composite bipolar plate (GCBPP) microplaning using specially designed multi-tooth tool is proposed. In this method, several or even dozens of parallel micro-flow channels ranging from 100 µm to 500 µm in width can be produced simultaneously. But, edge chippings easily occur on the rib surface of GCBPP during microplaning due to brittleness of graphite composites. Experimental results show that edge chippings result in the increase of contact resistance between bipolar plate and carbon paper at low compaction force. While the edge chippings scarcely exert influence on the contact resistance at high compaction force. Contrary to conventional view, the edge chippings can significantly improve performance of microfuel cell and big edge chippings outperform small edge chippings. In addition, the influence of technical parameters on edge chippings was investigated in order to obtain big, but not oversized edge chippings.

Plasma-Coating of Ti and Ta on Graphite Composite As Corrosion-Improved Bipolar Plate in PEM Electrolysis

In Proton Exchange Membrane (PEM) Electrolysis bipolar plates have to be more corrosion resistant than bipolar plates in PEM Fuel Cells. Several materials have been evaluated as cost-efficient material in PEM Electrolyzers. Since the electrochemical conditions in PEM Electrolyzers are quite harsh, cost-efficient materials like stainless steel or graphite composites are not stable. Coating the material with corrosion resistant material by magnetron sputtering is one possibility to enhance the stability. Gago et al.[1] have been coating stainless steel for example with Ti by vacuum plasma spraying. However, Ti is only useable if the oxidation potential is not too high, otherwise it will form an insulating TiO2 layer. Tantal has good corrosion protective characteristics and is not forming an insulating layer.[2] We developed a plasma process to deposit protective coatings on graphite composite bipolar plates. Therefore we used unbalanced magnetron sputtering of Ti and Ta. A new magnetron sputtering process was applied to keep the temperature of the graphite composites constant. Furthermore a combined plasma-process was used to enhance the stability of the deposited layer by activating the graphite composite. The Ti and Ta coatings were tested in a heated (T = 353 K) electrochemical cell amperometric at 2 V vs. RHE and by cyclic voltammetry in oxygen saturated 0.5 M H2SO4. We figured out the layer quality with the 4-Point Probe before and after each experiment. Furthermore we evaluated the film characteristics by microscope (SEM, light microscope), XRD and XPS. We could demonstrate that the coating is protecting the graphite composite as bipolar plate material in harsh electrochemical environment which was choosen close to real conditions in PEM Electrolyzers. Thereby it should be possible to lower the cost of bipolar plates in PEM electrolyzers substantial. [1] A. Gago, A. Ansar, N. Wagner, J. Arnold, K.A. Friedrich, Titanium coatings thermal spraying bipolar plates PEM electrolyzer, in: 64th Annu. Meet. Int. Soc. Electrochem., 2013. [2] O.E. Kongstein, N. Guillet, A. Ødegård, Rapport technique DEHT-DR-10/051, 2010.

Preparation, Characterization, and Properties of Resole Type Phenolic Resin/Exfoliated Graphite Composite Bipolar Plates for PEM Fuel Cell

Preparation, Characterization, and Properties of Resole Type Phenolic Resin/Exfoliated Graphite Composite Bipolar Plates for PEM Fuel Cell

Fabrication of Graphite Composite Bipolar Plate for Miniature PEMFCs by Micro Planing with Multi-Cutter

This study explores the feasibility of using a novel process, micro planing with multi-cutter, to fabricate graphite composite bipolar plates of proton exchange membrane fuel cells (PEMFCs). Through the use of multi-cutter, this study succeeds in machining micro flow channels with dimensions of 0.2 mm×0.23 mm×0.4 mm (channel width × rib width × depth size) on graphite composite bipolar plate (21 mm × 80 mm × 1.5 mm), in a reaction area of 10 mm × 60 mm. The graphite composite plates fabricated by multi-cutter are tested for cell performance under varying back-pressures. Results show that the cell performance increase with the rising back-pressures and can be promoted apparently by decreasing the size of flow channels.

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.

Gas-Phase Hybrid Sulfur Electrolyzer Stack

Membrane-electrode assemblies (MEAs) were developed for the gas-phase Hybrid Sulfur electrolyzer, and the addition of a low-permeability anode prelayer (LPAP) significantly reduced the observed SO2 crossover rate under nominal 25-cm² electrolyzer operating conditions vs. untreated controls utilizing the same base membrane. An 80-cm² bipolar electrolyzer stack having common fluid manifolds was designed and fabricated. Preliminary stack testing indicates similar performance and efficiency to the small, single-cell electrolyzer, but that some corrosion was observed in the anolyte paths of the graphite composite bipolar plates. Follow-up experiments are discussed.

Effect of Moisture and Temperature on Mechanical Properties of Graphite Composite Bipolar Plate for Proton Exchange Membrane Fuel Cell (PEMFC)

The fuel cell is considered to be a promising alternative energy source for the near future. A bipolar plate, which is the major component of the fuel cell, is always exposed to the moisture during operation, because one of the important functions of the bipolar plate is to facilitate water management within the cell. In order to use graphite composite materials as bipolar plate material for PEMFC, the effect of the moisture environment on the mechanical properties of bipolar plate is studied. Two types of specimens, one made of particulate graphite 90%-epoxy 10% composite 'with' woven carbon fabric, and the other 'without' carbon fabric, were fabricated. Both types of specimens were kept in three different environmental conditions: (1) dry at room temperature, (2) immersed in 85° C water for 100 h, and (3) immersed in 85° C water for 300 h. A series of experiments were performed with the samples of the two types: water absorption rate, strength and modulus from the bending and tensile tests were measured. The results showed that a steep increase of water absorption rate occurred at the beginning of the test followed by a slow increase afterwards. Total water absorption rate was lower for the specimens with fabric since the inserted carbon fabric has lower porosity and lower water absorption than the specimens without fabric made of graphite/epoxy composite. Bending strength and modulus decreased for both types of specimens. However, addition of carbon fabrics to the graphite-particle/epoxy composite increased the tensile strength significantly.

The preparation technique optimization of epoxy/compressed expanded graphite composite bipolar plates for proton exchange membrane fuel cells

Vacuum resin impregnation method has been used to prepare polymer/compressed expanded graphite (CEG) composite bipolar plates for proton exchange membrane fuel cells (PEMFCs). In this research, three different preparation techniques of the epoxy/CEG composite bipolar plate (Compression–Impregnation method, Impregnation–Compression method and Compression–Impregnation–Compression method) are optimized by the physical properties of the composite bipolar plates. The optimum conditions and the advantages/disadvantages of the different techniques are discussed respectively. Although having different characteristics, bipolar plates obtained by these three techniques can all meet the demands of PEMFC bipolar plates as long as the optimum conditions are selected. The Compression–Impregnation–Compression method is shown to be the optimum method because of the outstanding properties of the bipolar plates. Besides, the cell assembled with these optimum composite bipolar plates shows excellent stability after 200 h durability testing. Therefore the composite prepared by vacuum resin impregnation method is a promising candidate for bipolar plate materials in PEMFCs.

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

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

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 cm 2, density of 1.95 g cm −3, gas permeability of 1.94 × 10 −6 cm 3 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.

Theoretical Analysis of Conductivity for Composite Bipolar Plate

Conductive composite bipolar plate is made of composite materials filled with high content conductive filler. The conductivity of this bipolar plate is influenced by many factors. It is difficult to use a formula to accurately calculate its conductivity, and current models have certain defaults. In this paper, in view of the contact resistance, a model is estab- lished and the conductivity formula has been educed. The conductivity formula reveals some factors that influence the conductivity, especially the particle size and pressure. Several ways to improve the conductivity of the composite material are discussed. Conductive composite bipolar plate is the mainstream development direction of bipolar plate. The composite mate- rials consisting of conductive graphite filler and resins adhe- sives can be produced into bipolar plates with flow field through compression moulding or injection moulding tech- nology, which is high production efficiency and easy to be mass produced, and it is a significant decline in costs. Poly- mer composite bipolar plate has the same performance of corrosion resistance as graphite bipolar. Graphite polymer composite bipolar plates are made of composite materials filled with high content conductive filler. The high conductivity of bipolar plate demands for high content of graphite. But the mechanical performance will decrease if the content of graphite is much too high. So, the key problem of how to meet the requirement of conduc- tivity of a composite bipolar plate with the addition of a small amount of conductive fillers is needed to be resolved. Theoretical analysis is necessary on the basis of the conduc- tive mechanisms of the composite materials. There are many kinds of theories about the electric conductivity mechanism which is the result of the combination of those theories in general view. However, most of the theories are suit for composite materials filled with low content conductive filler. For those filled with high content, we argues that the mutual contact of the filler particles is the main contribution to the conductivity of composite materials, while the effect of the tunnel current on the conductivity is so little that it can be negligible.

Durability and characterization studies of polymer electrolyte membrane fuel cell’s coated aluminum bipolar plates and membrane electrode assembly

Coated aluminum bipolar plates demonstrate better mechanical strength, ease of manufacturability, and lower interfacial contact resistance (ICR) than graphite composite plates in polymer electrolyte membrane (PEM) fuel cell applications. In this study, coated aluminum and graphite composite bipolar plates were installed in separate single PEM fuel cells and tested under normal operating conditions and cyclic loading. After 1000 h of operation, samples of both the bipolar plates and the membrane electrode assembly (MEA) were collected from both the cathode and the anode sides of the cell and characterized to examine the stability and integrity of the plate coating and evaluate possible changes of the ionic conductivity of the membrane due any electrochemical reaction with the coating material. Scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis were performed on the land and valley surfaces of the reactant flow fields at both the anode and the cathode sides of the bipolar plates. The measurements were superimposed on the reference to identify possible zones of anomalies for the purpose of conducting focused studies in these locations. The X-ray diffraction (XRD) analysis of samples scraped from the anode and cathode electrodes of the MEA showed the tendency for catalyst growth that could result in power degradation. Samples of the by-product water produced during the single fuel cell operation were also collected and tested for the existence of chromium, nickel, carbon, iron, sulfur and aluminum using mass spectroscopy techniques. The EDX measurements indicated the possibility of dissociation and dissolution of nickel chrome that was used as the binder for the carbide-based corrosion-resistant coating with the aluminum substrate.