Detailed Fuel Cell Description

The MCFC membrane electrode assembly (MEA) comprises three layers a porous lithiated NiO cathode structure and a porous Ni/NiCr alloy anode structure, sandwiching an electrolyte matrix (see detail below). To a first approximation, the porous, p-type semiconductor, nickel oxide cathode structure is compatible with the air oxidant, and a good enough electrical conductor. The nickel anode structure, coated with a granular proprietary reform reaction catalyst, is compatible with natural gas fuel and reforming steam, and is an excellent electrical conductor. As usual, the oxygen is the actual cathode and the fuel the anode. Hence the phrase porous electrode structure . 

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A brief description of various electrolyte cells of interest follows. A detailed description of these fuel cells may be found in References (1) and (2). 

With the increased computational power of today s computers, more detailed simulations are possible. Thus, complex equations such as the Navier—Stokes equation can be solved in multiple dimensions, yielding accurate descriptions of such phenomena as heat and mass transfer and fluid and two-phase flow throughout the fuel cell. The type of models that do this analysis are based on a finite-element framework and are termed CFD models. CFD models are widely available through commercial packages, some of which include an electrochemistry module. As mentioned above, almost all of the CFD models are based on the Bernardi and Verbrugge model. That is to say that the incorporated electrochemical effects stem from their equations, such as their kinetic source terms in the catalyst layers and the use of Schlogl s equation for water transport in the membrane. 

A schematic of a typical fuel-cell catalyst layer is shown in Figure 9, where the electrochemical reactions occur at the two-phase interface between the electrocatalyst (in the electronically conducting phase) and the electrolyte (i.e., membrane). Although a three-phase interface between gas, electrolyte, and electrocatalyst has been proposed as the reaction site, it is now not believed to be as plausible as the two-phase interface, with the gas species dissolved in the electrolyte. This idea is backed up by various experimental evidence, such as microscopy, and a detailed description is beyond the scope of this review. Experimental evidence also supports the picture in Figure 9 of an agglomerate-type structure where the electrocatalyst is supported on a carbon clump and is covered by a thin layer of membrane. Sometimes a layer of liquid water is assumed to exist on top of the membrane layer, and this is discussed in section 4.4.6. Figure 9 is an idealized picture, and... 

This chapter presents the design and application of a two-stage combinatorial and high-throughput screening electrochemical workflow for the development of new fuel cell electrocatalysts. First, a brief description of combinatorial methodologies in electrocatalysis is presented. Then, the primary and secondary electrochemical workflows are described in detail. Finally, a case study on ternary methanol oxidation catalysts for DMFC anodes illustrates the application of the workflow to fuel cell research. 

High temperature membranes, that can operate at temperatures above 100 °C, are desirable to promote heat rejection, speed up electrode reaction rates, and to improve tolerance to impurities. This is an active area of materials research. Unfortunately, space constraints preclude a detailed description of fuel cell technologies and the underlying issues. Instead, the reader is referred to excellent reviews and books that exist on this topic.45 47... 

The initial emphasis on evaluation and modeling of losses in the membrane electrolyte was required because this unique component of the PEFC is quite different from the electrolytes employed in other, low-temperature, fuel cell systems. One very important element which determines the performance of the PEFC is the water-content dependence of the protonic conductivity in the ionomeric membrane. The water profile established across and along [106]) the membrane at steady state is thus an important performance-determining element. The water profile in the membrane is determined, in turn, by the eombined effects of several flux elements presented schematically in Fig. 27. Under some conditions (typically, Pcath > Pan), an additional flux component due to hydraulic permeability has to be considered (see Eq. (16)). A mathematical description of water transport in the membrane requires knowledge of the detailed dependencies on water content of (1) the electroosmotic drag coefficient (water transport coupled to proton transport) and (2) the water diffusion coefficient. Experimental evaluation of these parameters is described in detail in Section 5.3.2. 

A more detailed description of different types of batteries and other electric energy storage systems for electric vehicles can be found in Sect. 5.3, while a description of the main characteristics and properties of fuel cells for automotive application is given here, starting from some basic concepts of electrochemistry and thermodynamic, and focusing the attention on the operative parameters to be regulated to obtain the best performance in the specific application. 

X-ray diffraction analysis is used routinely by every catalyst manufacturer to determine the phase composition of the catalysts produced and the size of coherently scattering domains, and hardly needs a detailed description. An aspect that we would like to emphasize concerns the influence of the enviromnent on the oxidation state of carbon-supported metal nanoparticles. Quite often, authors try to correlate electrochemical performance with the phase composition of as-prepared samples. It has, however, been demonstrated convincingly in a number of publications by both x-ray diffraction [155] and x-ray absorption spectroscopy [156] that as-prepared fuel cell catalysts and samples stored under ambient conditions are often in the form of metal oxides but are reduced under the conditions of PEMFC or DMFC operation. The most dramatic changes are observed for samples with high metal dispersions, while larger particles are affected only marginally [17]. One should keep in mind, however, that the extent of the particle oxidation depends critically on the preparation procedure. 

A detailed description of all different kinds of fuel cells, their techniques and applications can be found in [30]. 

The physical chemistry of electrolytic solutions is a special area of physical chemistry with a large number of reference literatures. Classical descriptions are given in the books of Hamed and Owen or Robinson and Stokes. A more recent advanced treatment is found in the book of Barthel, Krienke, and Kunz. The special problems of ionic-conducting polymers and ionic solid electrolytes are described in various other reviews. Grajd described polymer electrolytes. A classical treatment of ionic solid electrolytes is the book by Rickert or the Kudo and Fueki compilation. Because these materials are used in batteries and fuel cells, there is much hterature for this research field including such detailed reviews in the book by Julien and Nazri. Another source for details and data compilations is the CRC Handbook of Solid State Electrochemistry f... 

So far, we have focused on the formal description of current generation in the catalyst layer and discussed major effects of structure and composition on exchange current density and catalyst utilization. In the remainder of this chapter, we will explore in detail, how electrocatalytic activity interferes with other processes at the catalyst surface (e.g. surface diffusion) and transport in the bulk phases. The key measure of catalyst layer performance is the current density that could be extracted from a cell for a given cell potential. This links the spatially varying concentrations and reaction rates with the global performance, rated in terms of power density and fuel cell efficiency. 

As conclusion to this section, we have stressed the influence of the heterogeneous surface structure on catalyst activity. Two complementary model approaches have been presented that could be used to establish the effective value of 7 . Kinetic MC simulations provide the most versatile tool for exploring structure-reactivity relations. It allows incorporating detailed surface structures and finite mobilities of adsorbates. In the limit of high surface mobilities of adsorbates, an analytical MF model could replace the stochastic description. The MF model still accounts for the heterogeneous surface structure. It is, thus, more realistic for real catalyst systems in fuel cells than homogeneous surface models. 

Schematic illustration of the membrane electrode assembly (MEA) of a PEM fuel cell (top) and details which have been subjected to modeling and simulation work described in the following chapters. Atomic level simulations have been performed for water and proton transport within the hydrophilic domaine of hydrated ionomers and for the electrochemical processes taking place at the electrocatalysts surfaces. The latter include the introduction of polarizable solvents and electrostatic potential variations. Mesoscale modeling is aiming at a better description and understanding of the development of ionomer microstructures.

In 2002 I have invented a transient, multiscale and multiphysics single fuel cell model, called MEMEPhys. This model, that 1 have continuously developed since then, accounts for the coupling between self-consistent physical-based mechanistic descriptions of the PEM and the CL phenomena (e.g. reactants, water and charge transport and detailed electrochemistry) and different materials aging mechanisms [59, 205-215], The model is designed for simulating hydrogen-feed PEMFC, PEM Water Electrolyzers and Li Ion batteries, but could be easily extended to simulate DAFCs. 

Detailed desoiptions of fuel cell technology are extensively presented in several recent monographs by Appleby and Foulkes [1993], Kordesch and Simader [1996], Vielstich et al. [2003], Kurzweil [2003], Blumen and Mugerwa [1993] a brief description of the main types will be given in Sections 4.S.4.2 to 4.S.4.4. 

As became obvious in the previous section, a detailed description of a complete fuel cell is computationally very demanding. The stack models thatt are discussed in this section are on a higher abstraction level. They serve mainly as one component of a complete fuel-cell system. The discussion of system simulation is beyond the scope of this chapter, and at this point only the characteristics of the stack models are mentioned. 

Life cycle assessment of SOFC technology is still uncommon due to the relatively early stage in technical development. However, several studies have been performed since the end of the 1990s. Since there is a lack of standard commercial equipment that could serve as a basis and reference point for analysis, LCA studies mostly refer to hypothetical concepts and/or extrapolate from laboratory and early market prototypes to commercial units. While the first studies had only little access to operation data at aU (for the fuel cell system itself but also for production processes), the main effort was set in the assessment of inventory data using assumptions, simplifications, and correlations [79, 80]. The main outcomes of these studies were the identification of weak points and the setting of benchmarks for further development. With more information about fuel cells available today and a simultaneous advancement in LCA methodology, the studies became more reliable and detailed, regarding system description [81] as well as the assessment of environmental impacts coimected with inputs and outputs [82]. Especially the extensive data of these two studies found their way to commercial databases for LCA [83] and thereby became available to LCA practitioners. In 2005, the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU)... 

Direct Alcohol Fuel Cells (DAFC). DAFC (or, more commonly, direct methanol fuel cells or DMFC) use alcohol without reforming. Mostly, this refers to a PEFC-type fuel cell in which methanol or another alcohol is used directly, mainly for portable applications. A more detailed description of the DMFC or DAFC is provided in Chapter 3 ... 

This chapter section summarizes the most widely used electrochemical techniques for HT-PEM MEA characterization. As it has been already mentioned in Sect. 17.1, the focus of the chapter is not a description on fundamentals of each one of the electrochemical techniques used. Besides, several references can be found in literature describing in detail the fundamentals of the electrochemical techniques [24—26]. Although a description of each one of the techniques is given, the main focus of this section is to show what kind of detailed information the reader can subtract from each electrochemical technique for a correct diagnosis of the HT-PEMFC behavior. Thus, the combination of the electrochemical characterization techniques can help to identify the different mechanisms and processes that take place in the fuel cell and lead to its degradation. 

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