Microporous layer carbon blacks

In this chapter, diffusion layers will be considered as fhe porous media that help the transport of fhe reactant fluids and producfs from one surface to another. In addition, the MPL will be defined as fhe additional layer or layers (made ouf of carbon black and water-repellenf particles) located between the CL and the DL. It is important to note that although "microporous layer" and "diffusion layer" are the common names for these components, as well as the ones used in this chapter, a number of different names can be found in the literature. 

X. Wang, H. Zhang, J. Zhang, et al. A bifunctional microporous layer with composite carbon black for PEM fuel cells. Journal of Power Sources 162 (2006) 474 79. 

J. Yu, M. N. Islam, T. Matsuura, et al. Improving the performance of a PEMFG with Ketjenblack EG-600JD carbon black as the material of the microporous layer. Electrochemical and Solid State Letters 8 (2005) A320-A323. 

The black rust is covered by a very compact microporous layer, made up of green rust and calcium carbonates. This film influences the high-frequency loop of the impedance diagrams. 

CB, and in particular Vulcan XC-72, is the standard support material in fuel cell research. Therefore, it is hardly surprising that most recent publications do no longer focus on ways to describe, analyze, and optimize the support, but rather report on strategies to, for example, increase the dispersion of the catalytically active metal nanoparticles on the carbon surface by various treatments [43-47]. Apart from that, also the investigation of CB composites, such as CB blended with CNT [48] and CB/reduced graphene oxide [49], came into the focus of recent fuel cell research, as well as its application in the microporous layer (MPL) of the GDLs, which are not subject of this chapter [50, 51]. In addition, the comparison of low-surface-area Vulcan XC-72 with high-surface-area black pearls or Ketjen-black with respect to their electrochemical properties, capacitive behavior [52], and durability [53] has been a frequent subject of recent publications. 

The presence of a so-called microporous layer at the interface with the catalyst layer has a positive effect on the contact resistance. For carbon black-based catalyst layers, it is usually assumed that electron transport losses are negligible. This may not be the case when less well-conducting oxide or carbide supports are considered. 

The GDL is usually made of a carbon-based porous substrate, such as carbon paper or carbon cloth, with a thickness of about 0.2 to 0.5 mm and a dual-layer structure. A schematic of the GDL between the flow field and the catalyst layer is presented in Figure 1.13. The first layer of the GDL, in contact with the flow field and the inlet gas in the flow channels, is a macro-porous carbon substrate, serving as a current collector, a physical support for the catalyst layer, and an elastic component of the MEA. The elastic component is necessary for the fuel cell to handle the compression needed to establish an intimate contact. The second layer of the GDL, in contact with the catalyst layer, is a thiimer microporous layer consisting of carbon black powder and some hydrophobic agent, which provides proper surface pore size and hydrophobicity to avoid flooding and to enhance intimate electronic contact at its interface with the catalyst layer [33]. 

To enhance intimate eleetronic contact with the catalyst layer and furflier improve both gas and water transport, a microporous layer (MPL) composed of carbon black powder and a hydrophobic agent such as PTFE is applied to the GDL substrate. The resulting pores in the MPL are primarily between 0.1-0.5 pm in diameter, much smaller than the pore size of the carbon paper (10-50 pm), so the MPL also prevents the eatalyst ink from penetrating the GDL substrate, which decreases catalyst utilization. Thus, in that configuration, the GDL is divided into two layers a macro-porous carbon substrate and a microporous composite layer. Figure 22.6 shows the sehematie of a double-layer GDL. 

The electrochemically active electrode materials in Li-ion batteries are a lithium metal oxide for the positive electrode and lithiated carbon for the negative electrode. These materials are adhered to a metal foil current collector with a binder, typically polyvinylidene fluoride (PVDF) or the copolymer polyvinylidene fluoride-hexafluroropropylene (PVDF-HFP), and a conductive diluent, typically a high-surface-area carbon black or graphite. The positive and negative electrodes are electrically isolated by a microporous polyethylene or polypropylene separator film in products that employ a liquid electrolyte, a layer of gel-polymer electrolyte in gel-polymer batteries, or a layer of solid electrolyte in solid-state batteries. 

Figure 10.22 shows a schematic of the fabrication procedure for this MEA [82]. Typically, a microporous layer, compounded by 1 mg cm Ke en Black 300 carbon (Azko Nobel, UK) and Teflon (40wt.%) as binder, is first deposited on Toray Graphite Paper (TGPH-090, 20% wet-proofed) by air brushing, using isopropanol as the solvent to make this layer hydrophobic for gas diffusion. 

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