Gas diffusion layer materials

In a second prototype, the reaction temperature was reduced to 250 °C, which reduced the carbon monoxide concentration from 1.2 to < 1%. Later, the first fuel processor prototype was linked to a meso-scale high-temperature fuel cell developed at Case Western University by Holladay et al. [117], which was tolerant to carbon monoxide concentrations up to 10%. Hence no CO clean-up was necessary to run the fuel processor. A 23 mW power output was demonstrated according to Holladay et al. [118], This value was lower than expected, which was attributed to several factors. First, the hydrogen supply was lower in the reformate. Second, the presence of carbon monoxide (2%) lowered the cell voltage. Third, the presence of carbon dioxide (25%) generated a magnified dilution effect at the gas diffusion layer material of the fuel cell, which was considerably less porous than conventional materials. 

Fliickiger R et al (2008) Anisotropic, effective diffusivity of porous gas diffusion layer materials for PEFC. Electrochim Acta 54 551-559... 

The influence of different gas diffusion layer materials on the characteristic curve of a PEMFC is presented inFig. 4.3. All cells had the same geometry, were equipped with the same MEA, and were operated at the same operating conditions, but the... 

The typical electrode degradation modes are (1) corrosion of the catalyst metal (both particle growth and dissolution) and (2) corrosion of the carbon materials in electrodes (catalyst support and gas diffusion layer materials). 

Several empirical and semiempirical expressions are available which attempt to describe the behavior of capillary pressnre in terms of a porous media and fluid properties. A generic Leverett function from soil science has been commonly employed to describe the capillary transport behavior of the porons media in multiphase models. Since many porous media share similar characteristic behavior, for PEFC gas diffusion layer material, a Leverett-type function has been nsed to represent this behavior as a first step toward achieving an accurate two-phase transport model. Udell [45] used Leverett s approach [46] to develop a semiempirical relation correlating capillary pressure and saturation data for clean unconsolidated sands of various permeability and porosity by means of defining a capillary pressure function ... 

CNF is an industrially produced derivative of carbon formed by the decomposition and graphitization of rich organic carbon polymers (Fig. 14.3). The most common precursor is polyacrylonitrile (PAN), as it yields high tensile and compressive strength fibers that have high resistance to corrosion, creep and fatigue. For these reasons, the fibers are widely used in the automotive and aerospace industries [1], Carbon fiber is an important ingredient of carbon composite materials, which are used in fuel cell construction, particularly in gas-diffusion layers where the fibers are woven to form a type of carbon cloth. 

A PEFC consists of two electrodes in contact with an electrolyte membrane (Fig. 14.7). The membrane is designed as an electronic insulator material separating the reactants (H2 and 02/air) and allowing only the transport of protons towards the electrodes. The electrodes are constituted of a porous gas diffusion layer (GDL) and a catalyst (usually platinum supported on high surface area carbon) containing active layer. This assembly is sandwiched between two electrically conducting bipolar plates within which gas distribution channels are integrated [96]. 

T. H. Ko, Y. K. Liao, and C. H. Liu. Effects of graphitization of PAN-based carbon fiber cloth on its use as gas diffusion layers in proton exchange membrane fuel cells. New Carbon Materials 22 (2007) 97-101. 

As can be seen in the different boundary conditions, the main effects of having ribs are electronic conductivity and transport of oxygen and water, especially in the liquid phase. In terms of electronic conductivity, the diffusion media are mainly carbon, a material that is fairly conductive. However, for very hydro-phobic or porous gas-diffusion layers that have a small volume fraction of carbon, electronic conductivity can become important. Because the electrons leave the fuel cell through the ribs, hot spots can develop with large gradients in electron flux density next to the channel. " Furthermore, if the conductivity of the gas-diffusion layer becomes too small, a... 

In this chapter, after recalling the working principles and the different kinds of fuel cells, the discussion will be focused on low-temperature fuel cells (AFC, PEMFC, and DAFC), in which several kinds of carbon materials are used (catalyst support, gas-diffusion layer [GDL], bipolar plates [BP], etc.). Then some possible applications in different areas will be presented. Finally the materials used in fuel cells, particularly carbon materials, will be discussed according to the aimed applications. To read more details on the use of carbon in fuel cell technology, see the review paper on The role of carbon in fuel cell technology recently published by Dicks [6],... 

Proton exchange membrane (PEM) fuel cells are the primary choice for transportation systems, but they can also be useful for stationary power production or local hydrogen production. Most of the challenges of PEM fuel cell commercialization center around cost and materials performance in an integrated system. Some specific issues are the cost of catalyst materials, electrolyte performance, i.e., transport rates, and water collection in the gas diffusion layer (GDL). 

Materials commonly used for the gas diffusion layers are carbon paper or woven carbon mats (examples of which are shown in Fig. 3.41). They combine the cormectivity allowing electron transport with a pore structure suitable for hydrogen or oxygen gas access to the catalyst layer. In cell manufacture, the catalysts may be deposited either on the gas diffusion layer or on the membrane. 

A number of different methods exist for the production of catalyst layers [97-102]. They use variations in composition (contents of carbon, Pt, PFSI, PTFE), particle sizes and pds of highly porous carbon, material properties (e.g., the equivalent weight of the PFSI) as well as production techniques (sintering, hot pressing, application of the catalyst layer to the membrane or to the gas-diffusion layer, GDL) in order to improve the performance. The major goal of electrode development is the reduction of Pt and PFSI contents, which account for substantial contributions to the overall costs of a PEFC system. Remarkable progress in this direction has been achieved during the last decade [99, 100], At least on a laboratory scale, the reduction of the Pt content from 4.0 to 0.1 mg cm-2 has been successfully demonstrated. 

Dalla Betta et al. first proposed an inert porous layer, or diffusion barrier, to prevent temperature runaway, and loosely interpreted the effect in terms of a reduction in the rate of combustion. A more rigorous interpretation of the effect of an inert porous layer on catalyst temperature was provided by McCarty et al, who also described the desired properties for diffusion layer materials, including a high thermal conductivity and low specific combustion activity. These authors stated that the high washcoat temperatures found in catalytic combustion of natural gas were due to the high diffusivity of methane in air, which causes the diffusion rate to the catalyst surface to match the rate of heat dissipation by conduction to the gas phase. The diffusion barrier decreases the rate of diffusion of methane to the catalyst surface, thus reducing the catalyst temperature. Modeling work by Hayes et al. confirmed those concepts. ... 

The gas diffusion layers, one next to the anode and the other next to the cathode, are usually made of a porous carbon paper or carbon cloth, typically 100 pm to 300 pm thick. Fig. 14 shows a porous GDL made of carbon paper, which is partially covered by catalyst layer. The porous nature of the backing layer ensures effective diffusion of feed and product components to and from the electrode on the MEA. The correct balance of hydrophobicity in the backing material, obtained by PTFE treatment, allows the appropriate amount of water vapor to reach the MEA, keeping the membrane humidified while allowing the liquid water produced at the cathode to leave the cell. The permeability of oxygen in the GDL affects the limiting current density of ORR, and thus the performance of PEMFC.[ l... 

Membrane electrode assemblies (MEAs) are typically five-layer structures, as shown in Figure 10.1. The membrane is located in the center of the assembly and is sandwiched by two catalyst layers. The membrane thickness can be from 25 to 50 pm and, as mentioned in Chapter 10, made of perfluorosulfonic acid (Figure 11.3). The catalyst-coated membranes are platinum on a carbon matrix that is approximately 0.4 mg of platinum per square centimeter the catalyst layer can be as thick as 25 pm [12], The carbon/graphite gas diffusion layers are around 300 pm. Opportunities exist for chemists to improve the design of the gas diffusion layer (GDF) as well as the membrane materials. The gas diffusion layer s ability to control its hydrophobic and hydrophilic characteristics is controlled by chemically treating the material. Typically, these GDFs are made by paper processing techniques [12],... 

The cost of non-active materials (gas diffusion layer, membrane, and bipolar plates) dominate stack cost at very low platinum loadings, while ohmic losses limit the benefit of increasing platinum loading beyond some point. 

The experimental dependence between the current density and width of air channels of bipolar plate has been obtained [3, 4]. Based on these data optimal width of channels 0.4-0.7 mm and current transfer prominent elements 0.2-0.7 mm was prescribed (exact values are determined by technological and material aspects of bipolar plate production, and also by gas diffusion layer parameters (thickness, porosity, mechanical characteristics, electric resistance). 

An experiment to determine contact resistance between the gas diffusion layer (GDL) and the separators was designed using the following materials SUS304, carbon A, carbon B, and alumina ceramics. The results are shown in Figure 6-3. 

With respect to fuel-cell technology itself, the small portable units use commercially available membrane electrode assemblies (MEA) and gas diffusion layers (GDL). As the operating temperature of small fuel-cell stacks usually lies below 50 °C, the requirements with respect to material stability of MEA and GDL, but also of sealing gaskets and bipolar plates are comparable lower than for other applications. For example, it is well known that metallic bipolar plates show significantly lower corrosion below 50 °C than at typical operation temperature of 80 °C [6,7], so that a sufficient lifetime for portable applications can be achieved with stainless steel. 

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