Catalyst and gas diffusion layers

I use the term "electrode" for the electron conductors. A different terminology is also in use, where the name "electrode" is reserved for the catalyst and gas diffusion layer assembly. 

Fundamental properties of the materials such as polymer electrolyte membranes, catalysts and gas diffusion layers making up the so called Membrane Electrode Assembly (MEA) as well as requirements to bipolar plates and sealing concepts necessary for stack integration are discussed. 

Fig. 1 Schematic of a typical micro-fuel cell stracture with a sohd electrolyte membrane, catalysts, and gas diffusion layers on both sides. The structure is sandwiched by the two current collector bipolar plates acting also as the flow fields (After Morse 2007)...

Figure 3.1 Schematic of the oxygen concentration profile across the cathode catalyst and gas-diffusion layers. In this chapter, the oxygen concentration in the catalyst layer is assumed to be constant c = cj.

Das PK, Li X, Liu Z (2010) Effective transport coefficients in PEM fuel cell catalyst and gas diffusion layers beyond Bruggeman approximation. Appl Energy 87 2785-2796... 

Note that in the fuel cell Uterature, the term electrode may have two meanings either a catalyst layer only, or a combination of catalyst and gas diffusion layers. The actual meaning must be inferred from... 

For the detailed information, pore structure porosimetry techniques are used. These methods enable measurement of pore diameter, pore shape, pore volume, and pore distribution in the electrode catalyst and gas diffusion layers. However, for PEMFC, these layers have hydrophobic and hydrophilic pores and there is no suitable technique available for characterization of such complex pore structures. Combination of multiple porosimetry techniques are employed to characterize layers with both hydrophobic and hydrophilic pores. The pore structure characterization techniques include capillary flow porosimetry, water intrusion porosimetry, and mercury intrusion porosimetry (Jena and Gupta, 2002). In water... 

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. 

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... 

Before discussing the details of the numerical experiments performed in this study, the primary mechanisms governing the two-phase transport in the PEFC catalyst layer and gas diffusion layer are discussed, which essentially build the foundations behind the specific assumptions and justifications pertaining to the subsequent two-phase simulations. 

The catalyst layer and gas diffusion layer play a crucial role in the overall PEFC performance due to the transport limitation in the presence of liquid water and flooding phenomena. The... 

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],... 

The reliability/durability of these fuel cells is another major barrier hindering commercialization. Developing durable catalysts, membranes, gas diffusion layers, and bipolar plates are currently the major areas of concentration in the search for technical breakthroughs. 

Figure 1. From the macroscale to the nanoscale a membrane electrode assembly has a polymer electrolyte membrane sandwiched between two catalyst layers and gas diffusion layers. The catalyst layer is composed of carbon particles impregnated with catalyst nanoparticles. Effective utilization of the catalyst particles depends on their local environment.

Various types of Nafion membranes and gas diffusion layers were compared as well. The electrode deposition technique was optimized to achieve performance comparable to the state-of-the-art for ink deposition approach. Several deposition techniques were investigated for printing the electrode layers, Figure 6 compares the performance achieved with two MEAs, printed by method A and method B with otherwise identical composition. Improved Method B yields performance of 1.9 g Pt/ kW at 0.8 V and the above stated test conditions. Further optimization of the electrode structure is in progress for catalysts with various loadings on high-surface-area carbon supports. 

The proper construction of a stable, well-dispersed, three-dimensional catalyst layer is one of the most critical determinants of performance for a PEM fuel cell. The membrane isolates the reactants from one another and provides an ionic current path from one electrode to another, and the flow fields and gas-diffusion layers distribute the reactants to the catalyst layer, but all of the relevant electrochemical reactions are carried out in the catalyst layers themselves. It is the proper construction of the so-called three-phase interface that allows the reactants and products to be brought into intimate contact and makes possible the operation of the fuel cell. Indeed, it is the tailoring of this layer by Raistrick et al. [1] in 1991 that demonstrated the practical feasibility of lowering precious metal loadings by a faetor of 40 over previous designs and helped to usher in the past deeade of inereased activity and investment in fuel cell development. 

The electrolyte membrane is in contact with catalyst layers consisting of platinum nanoparticles typically supported on a porous carbon black. The thickness of the catalyst layer is in the range from 5 to 20 pm which is contacted by a gas diffusion layer (GDL) of thickness 100-250 pm. The functional layers electrolyte membrane, catalytic layer and gas diffusion layer are making up the active part of the MEA. A detailed discussion of electro catalysts and catalyst layers is given in [9]. 

This section is devoted to a brief description of the main comptments of DAFC as an introduction to the most exhaustive analysis in Chaps. 2, 3,4, and 5 for electrocatalysts for methanol, ethanol, and higher alcohols, in Chap. 6 for proton exchange and alkaline membranes, and Chap. 7 for carbonous materials used as catalysts support, gas diffusion layers and bipolar plates. 

The analysis of carbon materials used as catalyst support, gas diffusion layer, and current collector and bipolar plates is performed in Chap. 7. A number of carbon materials including carbon blacks, nanotubes, nanofibers, and structured porous carbon materials are analyzed and compared as catalyst support in direct methanol fuel cells. Commercial and non-commercial gas diffusion layers are described along with the role of the mesoporous layer on the fuel cell performance. Finally, synthetic graphite and carbon composites used as current collector and bipolar plates are discussed, focusing on their mechanical and electrical properties and production costs. 

Electrode porous catalyst layer and gas diffusion layer Degradation effect on oxygen diffusion polarizations Aoki et al., 2010... 

Owing to the complex architecture of the fuel cell and especially the presence of a flow field and gas diffusion layer, only a few spectroscopic operando techniques are available. The most powerful ones include the use of X-rays, which to a certain extent may penetrate through the above-mentioned carbon materials. One of these techniques, XAS, is very useful in this respect since it provides information about catalyst structure, electronic properties [107-110], and, in certain cases, surface species on these catalyst nanoparticles [30]. Several spectroscopic test cells have been proposed [109, 110], one of which was demonstrated recently to allow for in situ investigation of fuel cell catalysts without any compromise regarding cell design [111]. Another useful X-ray technique is XRD, which has recently been applied successfully to monitor oscillations in particle growth on Pt/C catalysts [112]. 

The influence of CO poisoning at the anode of an HT-PEFC was investigated by Bergmann et ul. [28]. The dynamic, nonisothermal model takes the catalyst layer as a two-dimensional plane between the membrane and gas diffusion layer into account. The effects of CO and hydrogen adsorption with respect to temperature and time are discussed in detail. The CO poisoning is analyzed with polarization curves for different CO concentrations and dynamic CO pulses. The analysis of fuel-cell performance under the influence of CO shows a nonlinear behavior. The presence of water at the anode is explicitly considered to take part in the electrooxidation of CO. The investigation of the current response to a CO pulse of 1.31% at the anode inlet showed a reversible recovery time of 20 min. 

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... 

Yan et al. (2006) investigated the effect of cold-start temperatures (-5 °C, -10 °C and -15 °C) on fuel cell performance. It was reported that the fuel cell was able to start at -5 °C if it was pre-purged with dry nitrogen and air, although at -10 °C the PEFC could start by using the highest air stoichiometry (4) and 100 mA cm current load. At -15 °C the fuel cell could not start. SEM studies of MEAs subjected to temperatures below 0 °C revealed severe damage on the membrane and gas diffusion layer as well as delamination of the catalyst layer from the membrane (Yan et al., 2006). 

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). 

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