Bipolar plate matrix

Since every component used in a cell (GDL, catalyst layer, bipolar plate, matrix layer, etc.) is critical for the operation of the cell stack, these components have to be within a very tight specification and meet defined key product characteristics. This requires the implementation of a robust quahty control system. For example, substrates in PAFCs have to be checked for porosity, IR, thermal conductivity, thickness, density, compressive strength, flex strength, etc. All of these quality checks impart cost to the substrate. Ability to integrate these measurements into... 

The catalysts and electrode materials used in PAFCs are also similar to those in acidic H2/air fuel cells. Carbon-supported Pt is used as the catalyst at both anode and cathode, porous carbon paper serves as the electrode substrate, and graphite carbon forms the bipolar plates. Since a liquid electrolyte is used, an efficient water removal system is extremely important. Otherwise, the liquid electrolyte is easily lost with the removed water. An electrolyte matrix is needed to support the liquid phosphoric acid. In general, a Teflon -bonded silicon carbide is used as the matrix. 

Life-cycle impact Negative electrode Positive electrode Electrolyte matrix Bipolar plate Total Unit... 

The MCFC is a promising power generating source because of its unique characteristics such as high fuel efficiency and ability to use various carbonaceous fuels. Although Ni-10wt% Cr is used in the state-of-the-art MCFC as anode, it needs to be improved in terms of better creep and sintering resistance. In spite of the development in the alternate cathode material research, lithiated nickel oxide has been the choice of cathode material in the kilowatt-level MCFC stacks developed by many companies. Continuous research in the development of stable electrolyte retention matrix, identification of suitable molten carbonate electrolyte composition, and additives to the electrolyte will be a significant milestone. Also, research in the area of current collector/bipolar plate to overcome... 

Thermoplastic carbon composite materials are a favonrable material combination for bipolar plates becanse they can be mannfactnred by the mass production process of injection monlding [102]. Electrical condnctivity of a carbon composite requires a high content of carbon, nsnally a mixtnre of graphite and active coal. The percolation limit of the graphite in the polymer binder has to be exceeded, leading to direct contact between graphite particles. Additionally, a basic condnctivity of the polymer matrix by the smaller active carbon particles is achieved. The injection... 

As matrix polymers for the bipolar plate standard thermoplastic and technical polymers can be considered. For the developments at the Fraunhofer ICT polypropylene (PP) was used as a suitable polymer because of its material properties and also its low material price. With a service temperature of 100 °C PP is in the uncritical temperature range for the operating conditions in a PEMFC. 

The fuel cell components have thicknesses as follows the anode is 0.8-1.5mm thick the cathode, 0.4-1.5 mm, the matrix, 0.5-1 mm. In a fuel cell of the filter-press type, the individual cells are separated by bipolar plates made of nickel-plated stainless steel, contacting the anode with their nickel side, and the cathode with their steel side. All structural parts are made of nickel or nickel-plated steel. In a working fuel cell, the temperature of the outer part of the matrix electrolyte is lower than that of the inner part, so that in the outer part the electrolyte is solidified. This provides for tight sealing around the periphery of the individual fuel cells. 

PEMFGs use a proton-conducting polymer membrane as electrolyte. The membrane is squeezed between two porous electrodes [catalyst layers (CLs)]. The electrodes consist of a network of carbon-supported catalyst for the electron transport (soHd matrix), partly filled with ionomer for the proton transport. This network, together with the reactants, forms a three-phase boundary where the reaction takes place. The unit of anode catalyst layer (ACL), membrane, and cathode catalyst layer (CCL) is called the membrane-electrode assembly (MEA). The MEA is sandwiched between porous, electrically conductive GDLs, typically made of carbon doth or carbon paper. The GDL provides a good lateral delivery of the reactants to the CL and removal of products towards the channel of the flow plates, which form the outer layers of a single cell. Single cells are connected in series to form a fuel-cell stack. The anode flow plate with structured channels is on one side and the cathode flow plate with structured channels is on the other side. This so-called bipolar plate... 

The main components of a PAFC are bipolar plates, gas diffusion layers, catalyst layers, and matrix layer [3]. Typical cell designs include a sandwich of these layers as arranged in Fig. 12.2 between coolers. Multiple cells per cooler designs are generally employed to improve power density and this unit is called a sub stack. A cell stack assembly consists of multiple sub stacks held between two pressure plates tmder compression to minimize reactant leakage and contact resistance losses. 

Polymers are nsed in fnel cells. Those of particular interest are the polymer electrolyte membrane (PEM) and the phosphoric acid fuel cell (PAFC) designs. The latter design contains the liquid phosphoric acid in a Teflon bonded silicon carbide matrix. In March 2005 Ticona reported that it had bnilt the first fnel cell prototype made solely with engineering thermoplastics. They claimed that this approach rednced the cost of the fuel by at least 50% when compared with fuel cells fabricated from other materials. The 17-cell unit contains injection moulded bipolar plates of Vectra liquid crystal polymer and end plates of Fortron polyphenylene sulfide (PPS). These two materials remain dimensionally stable at temperatures up to 200 "C. The Vectra LCP bipolar plates contain 85% powdered carbon and are made in a cycle time of 30 seconds. 

To achieve adequate conductivities, the composite bipolar plates generally consist of a polymer, which functions as a binding matrix, and a high content of conductive filler materials. Composite plates offer an economic route for producing bipolar plates. For example, the composite material can be produced in an extruder and subsequently injection molded or compression molded to bipolar plates. 

First, a few studies on metal-filled composite bipolar plates are briefly described. At Los Alamos National Laboratory (LANL) composite bipolar plates filled with porous graphite and stainless steel and bonded with polycarbonate (Hermann, 2005) has been developed. Kuo (2006) investigated in composite bipolar plates based on austenitic chromium-nickel-steel (SS316L) incorporated in a matrix of PA 6. Their results showed that these bipolar plates are chemically stable. Furthermore, Bin et al. (2006) reported a metal-filled bipolar plate using polyvinylidene fluorid (PVDF) as the matrix and titanium silicon carbide (TijSiCj) as the conductive filler and obtained an electrical conductivity of 29 S cm" with 80 wt% filling content. 

Especially for composite bipolar plates the long-term stability of the plastic binder materials is a very appropriate classifier for the reliability of a bipolar plate. The quantification of the period of induction of the used polymer—as explained later—is essential as after this period a rapid aging of the polymer takes place resulting in a sudden decomposition of the polymeric matrix in the composite bipolar plates. 

Additionally, it has to be pointed out that for composite bipolar plates a high content of filler materials is necessary that leads to more brittle properties. Therefore, such losses in plastic properties lead to mechanical stress to the plate structure owing to swelling of the material. Furthermore, the intermediate formation of peroxides during exceptional fuel cell operation may also lead to decomposition of the polymeric matrix of bipolar plates. This leads in consequence to accelerated embrittlement of the composite material that finally would affect the stability and gas tightness of the bipolar plate. 

It can be seen that the same polypropylene matrix leaches a higher content of impurities into methanol when compared with water. This indicates that for use in a direct methanol fuel cell another type of polymer might be more appropriate, even though such polypropylene plates successfully operate in PEM fuel cells. Due to the chemically corrosion stable nature and the low additive content of PPS, it can be supposed that such plates are an appropriate choice for use in direct methanol fuel cells. At the time of writing, the authors apply a simultaneous ex situ test in methanol with such PPS-bonded bipolar plates. 

In case of the polymeric materials several factors play a role, but generally it can be concluded that polymers that contain hydrolyzable groups, such as esters or amides, are susceptible to corrosion and decomposition in fuel cell environment. They should not be used as composite matrix material. The following properties are conducive to increase the long-term stability and should be taken into account for a targeted selection of a suitable polymeric matrix for composite bipolar plates. 

The PAFC stack consists of a repeating arrangement of a ribbed bipolar plate, the anode, electrolyte matrix, and cathode. In a similar manner to that described for the PEM cell, the ribbed bipolar plate serves to separate the individual cells and electrically connect them in series, whilst providing the gas supply to the anode and the cathode, respectively, as shown in Figures 1.9 and 1.10. Several designs for the bipolar plate and ancillary stack components are being used by fuel ceU developers, and these aspects are described in detail elsewhere (Appleby and Foulkes, 1993). A typical PAFC stack may contain 50 or more cells connected in series to obtain the practical voltage level required. 

Bipolar plate Anode Matrix Cathode Oxidator gas... 

The substrate fiber matrix acts to bridge the many structural and functional gaps between the catalyst layers and bipolar plates, as well as ensuring the entire membrane electrode assembly (MEA) active area is utilized. MPLs act as a further transitional... 

A basic PAFC stack configuration is given in Fig. 13. The anode-acid holder matrix-cathode sandwich is held between two bipolar plates and this array is repeated to the end. At each end there is an one-side grooved plate—one with anode groove and the other end with cathode groove. Each of these plates have through holes, when assembled act as reactant gas flow headers (inlet and outlet). The inlet header of one gas has an opening in one side of each bipolar plate, and similarly the outlet header of the same gas collects excess unreacted reactants/products from the same side of the bipolar plates. The inlet and outlet headers of the other gas similarly feed and collect gas from the other side of the bipolar plates. 

The latest generation of PAFC units employ mixture of various technologies. Ribbed electrode design as well as Toray developed thin diffusion layer with separate acid reservoirs are employed depending upon the design, capacity and the availability of the fuel cell materials. For fully hydrophobised electrode substrate, one has to provide acid links from the reservoirs located in bipolar plates to the acid holder matrix. This is possible, by making part of the substrate hydrophilic, and is achieved by filling carbon or SiC particles inside the substrate to allow acid transport to the matrix by capillary action (DeCasperis et al., 1981, Schroll and Hartford, 1974). 

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