Gas-diffusion layer membrane

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. 

In this section, operating conditions and modes that contribute to voltage decay or limit performance will be discussed. In section Materials Degradation and the Relation to Performance Loss and Shortening the PEMFC Lifetime, the durability issues related to the different components of the fuel ceU, that is, catalyst, the gas diffusion layers, membranes, bipolar plates, and seals will be presented in more detail. 

A combination of synergistic improvements in the catalyst, support, gas diffusion layers, membrane, and essentially the entire porous electrode structure in conjunction with bipolar plates/flow fields is expected to improve the mass-transport of reactant gases, protons, and water management. Thus, an increase in the peak current density (A/cm ) and peak power density (W/cm ) will result this in turn will lower the stack volume, the amount of catalyst, and membrane material used and raise the kW/L, kW/kg, and lower the /kW stack metrics. It should be noted that the rated or peak power for automotive stacks is based in part on maintaining an electrical efficiency of >50% this dictates that the cell voltage has to be maintained above 0.60 V. At this time, volumetric power densities of practical stacks in fuel ceU vehicles have been reported to be as high as 2 kW/L... 

Figure 6.10 shows a typical temperature distribution in the electrode-gas diffusion layers, membrane, and gas channels at a given cross section. 

In Part I, degradation phenomena of stack components, catalysts, membranes, gas-diffusion layers, membrane-electrode assemblies, bipolar plates, and sealings are discussed on the basis of their materials chemistry. Accelerating methods and recent progress in durability improvement are also reviewed by prominent authors in the field. 

The main components of a PEM fuel cell are the flow channels, gas diffusion layers, catalyst layers, and the electrolyte membrane. The respective electrodes are attached on opposing sides of the electrolyte membrane. Both electrodes are covered with diffusion layers, and the flow channels/current collectors. The flow channels collect current from the electrodes while providing the fuel or oxidant with access to the electrodes. The gas diffusion layer allows gases to diffuse to the electro-catalysts and provides electrical contact throughout the catalyst layers. Within the anode catalyst layer, the fuel (typically H2) is oxidized to produce electrons and protons. The electrons travel through an external circuit to produce electricity, while the protons pass through the proton conducting electrolyte membrane. Within the cathode catalyst layer, the electrons and protons recombine with the oxidant (usually 02) to produce water. 

An analysis of the individual PEM components offers evidence of almost unbroken R D see Fig. 13.10 (Jochem et al., 2007). The overall importance of the membrane is striking. Furthermore, the numbers of annual applications for bipolar plates (BPP) and the gas-diffusion layer (GDL) decrease after 2002, while the increase in membrane applications flattens out. This correlates with the equally lower number of fuel cell patents in the field of mobile applications. 

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]. 

Figure 2.1 shows a schematic structure of the fuel cell membrane electrode assembly (MEA), including both anode and cathode sides. Each side includes a catalyst layer and a gas diffusion layer. Between the two sides is a proton exchange membrane (PEM) conducting protons from the anode to the cathode. 

There are two main types of thin-film catalyst layers catalyst-coated gas diffusion electrode (CCGDL), in which the CL is directly coated on a gas diffusion layer or microporous layer, and catalyst-coated membrane, in which the CL is directly coated on the proton exchange membrane. In the following sections, these catalyst layers will be further classified according to their composition and structure. 

Antoine et al. [28] inveshgated the gradient across the CL and found that the Pt utilization was dependent on the CL porosity. In a nonporous CL, catalyst utilization was increased through the preferential locahon of Pt close to the gas diffusion layer in a porous CL, catalyst utilization efficiency was increased through the preferential location of Pt close to the polymer electrolyte membrane. In PEM fuel cells, fhe CL has a porous structure, and better performance is expected if higher Pf loading is used af preferential locahons close to the membrane/catalyst layer interface. 

Catalyst layer ink can be deposited on gas diffusion layers to form a CCGDL, as discussed in the previous section. Alternatively, the catalyst ink can be applied directly onto the proton exchange membrane to form a catalyst-coated membrane (CCM). The most obvious advantage of the CCM is better contact between the CL and the membrane, which can improve the ionic connection and produce a nonporous substrate, resulting in less isolated catalysts. The CCM can be classified simply as a conventional CCM or as a nanostructured thin-film CCM. 

Similar to screen printing, the spray coating method [95] is widely used for catalyst fabrication, especially in labs. The major difference between the two is that the viscosity of the ink for spray coating is much lower than that for screen printing. The application apparatus can be a manual spray gun or an auto-spraying system with programmed X-Y axes, movable robotic arm, an ink reservoir and supply loop, ink atomization, and a spray nozzle with adjustable flux and pressure. The catalyst ink can be coated on the gas diffusion layer or cast directly on the membrane. To prevent distortion and swelling of the membrane, either it is converted into Na+ form or a vacuum table is used to fix the membrane. The catalyst layer is dried in situ or put into an oven to remove the solvent. 

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. 

V. Gurau, M. J. Bluemle, E. S. De Castro, et al. Characterization of transport properties in gas diffusion layers for proton exchange membrane fuel cells. 2. Absolute permeability. Journal of Power Sources 165 (2007) 793-802. 

A. M. Kannan, A. Menghal, and 1. V. Barsukov. Gas diffusion layer using a new type of graphitized nanocarbon PUREBLAGK(R) for proton exchange membrane fuel cells. Electrochemistry Communications 8 (2006) 887-891. 

A. M. Kannan, L. Cindrella, and L. Munukutla. Functionally graded nanopo-rous gas diffusion layer for proton exchange membrane fuel cells under low relative humidity conditions. Electrochimica Acta 53 (2008) 2416-2422. 

G. Inoue, Y. Matsukuma, and M. Minemoto. Evaluation of the thickness of membrane and gas diffusion layer with simplified two-dimensional reaction and flow analysis of polymer electrolyte fuel cell. Journal of Power Sources 154... 

T. Koido, T. Furusawa, and K. Moriyama. An approach to modeling two-phase transport in the gas diffusion layer of a proton exchange membrane fuel cell. Journal of Power Sources 175 (2008) 127-136. 

J. Benziger, J. Nehlsen, D. Blackwell, T. Brennan, and J. Itescu. Water flow in the gas diffusion layer of PEM fuel cells. Journal of Membrane Science 261 (2005) 98-106. 

Transport properties of hydrated PFSA membranes strongly depend on nanophase-segregated morphology, water content, and state of water. In an operational fuel cell, these characteristics are indirectly determined by the humidity level of the reactant streams and Faradaic current densities generated in electrodes, as well as the transport properhes of catalyst layers, gas diffusion layers, and flow... 

A graphite (or metal) plate serves as the plenum for the gas supply and for heat removal. A catalyzed carbon layer is applied to the membrane surface. A thicker gas diffusion layer or porous carbon paper... 

The fuel-cell sandwich describes the 1-D cross section of the fuel cell (see Figure 1) and is shown in Figure 5. For the single dimension, flow is taken to be normal to the various layers. Flow in the other directions is discussed in section 5. The fuel-cell sandwich contains the gas channels or flow fields, diffusion media, catalyst layers, and membrane. Additional layers are sometimes incorporated into the sandwich, such as separating the diffusion media into microporous and gas-diffusion layers. Fuel cells operate in the following manner. 

Figure 4. Membrane wrinkling and shearing under die gas diffusion layer in die assembled cell/stack.

Water content affects many processes within a fuel cell and must be properly managed. Proton conductivity within the polymer electrolyte typically decreases dramatically with decreasing water content (especially for perfhiorinated membranes such as Nation ), while excessive liquid water in the catalyst layers (CLs) and gas diffusion layers (GDLs) results in flooding, which inhibits reactant access to the catalyst sites. Water management is complicated by several types of water transport, such as production of water from the cathode reaction, evaporation, and condensation at each electrode, osmotic drag of water molecules from anode to cathode by... 

Proper water management in proton exchange membrane fuel cells (PEMFCs) is critical to PEMFC performance and durability. PEMFC performance is impaired if the membrane has insufficient water for proton conduction or if the open pore space of the gas diffusion layer (GDL) and catalyst layer (CL) or the gas flow channels becomes saturated with liquid water, there is a reduction in reactant flow to the active catalyst sites. PEMFC durability is reduced if water is left in the CL during freeze/thaw cycling which can result in CL or GDL separation from the membrane,1 and excess water in contact with the membrane can result in accelerated membrane thinning.2... 

A typical PEFC, shown schematically in Fig. 1, consists of the anode and cathode compartments, separated by a proton conducting polymeric membrane. The anode and cathode sides each comprises of gas channel, gas diffusion layer (GDL) and catalyst layer (CL). Despite tremendous recent progress in enhancing the overall cell performance, a pivotal performance/durability limitation in PEFCs centers on liquid water transport and resulting flooding in the constituent components.1,2 Liquid water blocks the porous pathways in the CL and GDL thus causing hindered oxygen transport to the... 

Polymer electrolyte fuel cell (PEFC) is considered as one of the most promising power sources for futurist s hydrogen economy. As shown in Fig. 1, operation of a Nation-based PEFC is dictated by transport processes and electrochemical reactions at cat-alyst/polymer electrolyte interfaces and transport processes in the polymer electrolyte membrane (PEM), in the catalyst layers consisting of precious metal (Pt or Ru) catalysts on porous carbon support and polymer electrolyte clusters, in gas diffusion layers (GDLs), and in flow channels. Specifically, oxidants, fuel, and reaction products flow in channels of millimeter scale and diffuse in GDL with a structure of micrometer scale. Nation, a sulfonic acid tetrafluorethy-lene copolymer and the most commonly used polymer electrolyte, consists of nanoscale hydrophobic domains and proton conducting hydrophilic domains with a scale of 2-5 nm. The diffusivities of the reactants (02, H2, and methanol) and reaction products (water and C02) in Nation and proton conductivity of Nation strongly depend on the nanostructures and their responses to the presence of water. Polymer electrolyte clusters in the catalyst layers also play a critical... 

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