Performance of catalyst layers

A. A. Kulikovsky. Performance of catalyst layers of polymer electrolyte fuel cells Exact solutions. Electrochem. Comm., 4(4) 318-323, 2002. 

In this chapter, we report on several analytical models for the performance of catalyst layers and fuel cells. The simple models discussed below are iUustraled with examples demonstrating these models in use. 

Figure 8.2. Illustration of structures and phenomena at multiple scales that control the performance of catalyst layers in PEFCs. Left Pt nanoparticle on carbonaceous substrate ( 3 nm) middle phase-segregation and agglomerate formation at mesoscale ( 50 nm) right the catalyst layer as a complex composite medium at the macoscopic scale ( 10 pm).

Kulikovsk AAy. Performance of catalyst layers of pol3uner eleetrotyte fuel eeUs exact solutions. Electrochem Comm 2002 4(4) 318-23. 

With both approaches, it is key to establish the current regions where samples are under kinetic control to allow the correct comparison. Many reported comparisons of catalysfs in MEA sfrucfures point to differences in performance, which are attributed to intrinsic catalyst differences when it is clear that differences are due to mass fransport effects because of catalyst layer structure. To help overcome these difficulties, it is recommended that, for catalyst evaluation, pure reactants be used (e.g., O2 instead of air) and at relatively high stoichiometries. Use of current-voltage curves should be corrected for elecfrolyte or membrane resistances and Tafel analysis used to identify fhe kinefically confrolled current regions. 

The fabrication of catalyst layers for PEM fuel cells involves maintaining a delicate balance between gas and water transport, and electron and proton conduction. The process of CL fabrication should be guided by both fuel cell performance and cost reduction. 

Wang, Mukherjee, and Wang [124] investigated the effects of catalyst layer electrolyte and void phase fractions on fuel cell performance using a random microstructure. The model predicted volume fractions of 0.4 and 0.26 for void and electrolyte phases, respectively, as the optimal CL compositions. 

In this chapter, we will mainly address the vital topics in theoretical membrane research. Specifically, we will consider aqueous-based proton conductors. Our discussion of efforts in catalyst layer modeling will be relatively brief. Several detailed accounts of the state of the art in catalyst layer research have appeared recently. We will only recapitulate the major guidelines of catalyst layer design and performance optimization and discuss in some detail the role of the ionomer as a proton-supplying network in catalyst layers with a conventional design. 

This section provides a comprehensive overview of recent efforts in physical theory, molecular modeling, and performance modeling of CLs in PEFCs. Our major focus will be on state-of-the-art CLs that contain Pt nanoparticle electrocatalysts, a porous carbonaceous substrate, and an embedded network of interconnected ionomer domains as the main constituents. The section starts with a general discussion of structure and processes in catalyst layers and how they transpire in the evaluation of performance. Thereafter, aspects related to self-organization phenomena in catalyst layer inks during fabrication will be discussed. These phenomena determine the effective properties for transport and electrocatalytic activity. Finally, physical models of catalyst layer operation will be reviewed that relate structure, processes, and operating conditions to performance. 

At macroscopic level, the overall relations between structure and performance are strongly affected by the formation of liquid water. Solution of such a model that accounts for these effects provides full relations among structure, properties, and performance, which in turn allow predicting architectures of materials and operating conditions that optimize fuel cell operation. For stationary operation at the macroscopic device level, one can establish material balance equations on the basis of fundamental conservation laws. The general ingredients of a so-called "macrohomogeneous model" of catalyst layer operation include ... 

In Song et al. s same work [5], the effect that Nafion content in the catalyst layer had upon electrode performance was also investigated, following their work on the optimization of PTFE content in the gas diffusion layer. The optimization of Nafion content was done by comparing the performance of electrodes with different Nafion content in the catalyst layer while keeping other parameters of the electrode at their optimal values. Figures 6.8 and 6.9 show the polarization curves and impedance spectra of fuel cells with electrodes made of catalyst layers containing various amounts ofNafion . 

In Section 3, the slow rate of the ORR at the Pt/ionomer interface was described as a central performance limitation in PEFCs. The most effective solution to this limitation is to employ dispersed platinum catalysts and to maximize catalyst utilization by an effective design of the cathode catalyst layer and by the effective mode of incorporation of the catalyst layer between the polymeric membrane electrolyte and the gas distributor/current collector. The combination of catalyst layer and polymeric membrane has been referred to as the membrane/electrode (M E) assembly. However, in several recent modes of preparation of the catalyst layer in PEFCs, the catalyst layer is deposited onto the carbon cloth, or paper, in much the same way as in phosphoric acid fuel cell electrodes, and this catalyzed carbon paper is hot-pressed, in turn, to the polymeric membrane. Thus, two modes of application of the catalyst layer - to the polymeric membrane or to a carbon support - can be distinguished and the specific mode of preparation of the catalyst layer could further vary within these two general application approaches, as summarized in Table 4. 

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. 

The reaction rates Qa and Qc are reduced by the lack of methanol/oxygen in the parts of the catalyst layers shaded by current collectors. It is possible to remove the precious catalyst from the shaded regions practically without degradation of performance [18], However, it is much better to change the design of current collectors. To prevent partial shading of catalyst layer by current collectors, it is... 

Study the dependence of performance on catalyst layer structure. 

Overall, this structure-based approach has proven its assets in electrode diagnostics and in providing guidelines for the optimized structural design of catalyst layers [11-15]. Vital performance aspects that can be rationalized are catalyst utilization, distributions of reactants, reaction rates, and electrode potential, as well as effects on the overall fuel cell power density and water balance. 

The exchange current density is the key property of catalyst layers. It determines the value of the overpotential needed to attain the targeted fuel cell current density. This property, thus, links fundamental electrode theory with practical aspects of fuel cell performance. The following parameterization distinguishes explicitly the effects of different structural characteristics,... 

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. 

The structural approach relates local distributions of reactant concentrations, electrode potential, and reaction rates in the catalyst layer to the global performance of the layer. Characteristic length scales, which represent the interplay between mass transport and kinetic processes, could be defined. They help distinguishing active and inactive zones in the catalyst layer. 

Overall, the theory of composition-dependent performance reproduces experimental findings [13,22,23,94,95] very well. In spite of the widely recognized importance of catalyst layer optimization, structural data are, however, still scarce in the open literature on fuel cells. 

The requirement to characterize the detailed structure of catalyst layers in MEAs has become more and more clamorous to improve MEA perform-... 

Also promising are the layer-by-layer technique which allows the intercalation of polyelectrolytes to prepare membranes with enhanced properties for DAFC, like that described by Hammond and coworkers [167,495] not only because it is easy to perform, but also offer a lot of alternatives to modify commercial membranes or integration with the codeposition of catalyst layers on the membrane to form integrated ME As. 

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