Gas-diffusion layer properties

S. Escribano, J. P. Blachot, J. Etheve, A. Morin, and R. Mosdale. Characterization of PEMFCs gas diffusion layers properties. Journal of Power Sources 156 (2006) 8-13. 

Lobato J, Canizares P, Rodrigo MA et al (2010) Direct and inverse neural networks modelling applied to study the influence of the gas diffusion layer properties on PBI-based PEM fuel cells. Int J Hydrogen Energy 35 7889-7897... 

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. 

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... 

Diffusion medium properties for the PEFC system were most recently reviewed by Mathias et al. The primary purpose of a diffusion medium or gas diffusion layer (GDL) is to provide lateral current collection from the catalyst layer to the current collecting lands as well as uniform gas distribution to the catalyst layer through diffusion. It must also facilitate the transport of water out of the catalyst layer. The latter function is usually fulfilled by adding a coating of hydrophobic polymer such as poly(tet-rafluoroethylene) (PTFE) to the GDL. The hydrophobic polymer allows the excess water in the cathode catalyst layer to be expelled from the cell by gas flow in the channels, thereby alleviating flooding. It is known that the electric conductivity of GDL is... 

Numerical models for electrochemical process performance assessment or dimensioning generally assume uniform properties or one-dimensional property variations. For example, plug flow with axial dispersion is usually assumed within fdter-press electrolysers [1], whereas a Darcy flow model is commonly used within the gas diffusion layer of PEM electrolysers and fuel cells [2],... 

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. 

At the macroscopic scale, the full competition of reactant diffusion, electron and proton migration, and charge transfer kinetics unfolds. The water balance further complicates this interplay. Moreover, performance is subject to operation conditions and complex boundary conditions at interfaces to membrane and gas diffusion layer. A vast list of structural characteristics steers this interplay, including thickness, composition, pore size distributions, and wetting properties of pores. 

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. 

There are only a few reported studies that examine mechanical degradation and review the effect of compression of gas-diffusion layers on the performance of fuel cells [133-135]. Lee and Merida [133] characterized some GDL properties after 300 h. of compression at constant temperature in an ex situ test and found that the dry gas-phase permeabilities remained roughly constant. 

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. 

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]. 

Yoneda, M., Takimoto, M., and Koshizuka, S. (2007) Effects of mi-crostmcture of gas diffusion layer on two-phase flow transport properties. ECS Trans., 11, 629-635. 

Becker J et al (2009) Determination of material properties of gas diffusion layers experiments and simulations using phase contrast tomographic microscopy. J Electrochem Soc 156 B1175-B1181... 

Rosen T et al (2012) Saturation dependent effective transport properties of PEFC gas diffusion layers. J Electrochem Soc 159 F536-F544... 

Obviously, the design of the components and properties of materials must accommodate the above-listed processes with minimum obstruction and losses. Because in some of the components more than one process takes place, very often with conflicting requirements, the properties and the design must be optimized. For example, the gas diffusion layer must be optimized so that the reactant gas may easily diffuse, yet at the same time that water, which travels in the opposite direction, does not accumulate in the pores. On top of that, the diffusion layer (or current collector layer as it is sometimes called) must be both electrically and thermally conductive. Similar requirements may be established for almost every fuel cell component. Although a fuel cell seems to be a very simple device, numerous processes take place simultaneously. It is therefore important to understand those processes, their mutual interdependence, and their dependence on components design and materials properties [1]. 

The required properties of the gas diffusion layer follow from its functions ... 

Kleemann J, Finsterwalder F, Tillmetz W (2009) Characterisation of mechanical behaviour and coupled electrical properties of polymer electrolyte membrane fuel cell gas diffusion layers. J Power Sources 190 92-102... 

Abstract The polymer electrolyte fuel cell (PEFC) consists of disparate porous media microstructures, e.g. catalyst layer, microporous layer, gas diffusion layer, as the key components for achieving the desired performance attributes. The microstmcture-transport interactions are of paramount importance to the performance and durability of the PEFC. In this chapter, a systematic description of the stochastic micro structure reconstmction techniques along with the numerical methods to estimate effective transport properties and to study the influence of the porous structures on the underlying transport behavior is presented. 

Akey performance limitation in the polymer electrolyte fuel cell (PEFC) originates from the multiple, coupled and competing, transport interactions in the constituent porous components. The suboptimal transport behavior resulting from the underlying complex and multifunctional microstmctures in the catalyst layer (CL), gas diffusion layer (GDL) and microporous layer (MPL) leads to water and thermal management issues and undesirable performance loss. Therefore, it is imperative to understand the profoimd influence of the disparate porous microstmctures on the transport characteristics. In this chapter, we highhght the stochastic microstmcture reconstmction technique and direct transport simulation in the CL, GDL and MPL porous stmctmes in order to estimate the effective transport properties and imderstand the microstmctural impact on the imderlying transport behavior in the PEFC. 

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