Catalyst layer, optimal model

The reactions are still most often carried out in batch and semi-batch reactors, which implies that time-dependent, dynamic models are required to obtain a realistic description of the process. Diffusion and reaction in porous catalyst layers play a central role. The ultimate goal of the modehng based on the principles of chemical reaction engineering is the intensification of the process by maximizing the yields and selectivities of the desired products and optimizing the conditions for mass transfer. 

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. 

Wang, G., Mukherjee, P P, and Wang, C. Y. Optimization of polymer electrolyte fuel cell cathode catalyst layers via direct numerical simulation modeling. Electrochimica Acta 2007 52 6367-6377. 

The previous discussion asserts that design, fabrication, and implementation of stable and inexpensive materials for membranes and catalyst layers are the most important technological challenges for PEFC developers. A profound insight based on theory and modeling of the pertinent materials will advise us how fuel cell components with optimal specifications can be made and how they can be integrated into operating cells. 

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. 

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 ... 

The macrohomogeneous model was exploited in optimization studies of the catalyst layer composition. The theory of composifion-dependent performance reproduces experimental findings very well. - The value of the mass fraction of ionomer that gives the highest voltage efficiency for a CCL with uniform composition depends on the current density range. At intermediate current densities, 0.5 A cm < jo < 1.2 A cm , the best performance is obtained with 35 wt%. The effect of fhe Nation weight fraction on performance predicted by the model is consistent with the experimental trends observed by Passalacqua et al. ... 

Overall, the interface models are basically 0-D. They assume that all of the relevant variables in the catalyst layers are uniform in their values across the layer. This has some justification in that the catalyst layers are very thin, and it is adequate if other effects that are modeled are more significant however, the catalyst layers should be modeled in more detail to ensure that all the relevant interactions are accounted for and to permit optimization of such parameters as catalyst loading. 

Fuel cell performance is affected by MEA composition, including catalyst loading, PTFE content in the gas diffusion layer, and Nafion content in the catalyst layer and membrane, each of which affects the performance in different ways, yielding distinct characteristics in the electrochemical impedance spectra. Even different fabrication methods may influence a cell s performance and electrochemical impedance spectra. With the help of the model described above, impedance spectra can provide us with a useful tool to probe structure-performance relationships and thereby optimize MEA structure and fabrication methods. 

An appropriate model of CCL operation should account for the interdiffusion of oxygen, nitrogen, and water vapor, migration of protons and electrons, and kinetics of the electrochemical reaction. The pertinent theory was developed in Refs. 8, 9, 15. A model similar to that was more recently studied analytically [17, 107, 108], and we will dwell on these transparent closed form results. We will see that having such solutions at hand helps in revealing the reserves for optimization of the structure and function of the catalyst layers. 

Dr. Qianpu Wang received his Diploma in Metallurgy Engineering from the Central South University, China in 1986 and his Ph.D. in Multiphase Flow from the Norwegian University of Science and Technology, Norway in 2001. He then joined the National Research Council of Canada Institute for Fuel Cell Innovation (NRC-IFCI) as a post-doctoral fellow until 2004. Thereafter he has been a research officer at NRC-IFCI. His research interests include fundamental understanding of mass transport limitations, catalyst utilization in catalyst layers, and fuel cell/stack modeling and optimization. 

Q. Wang, D. Song, T. Navessin, S. Holdcroft, Z. Liu, A mathematical model and optimization of the cathode catalyst layer structure in PEM fuel cells , Electrochim. Acta 50 (2004) 725. 

The two-step strategy in the physical modeling of catalyst layer operation is depicted in Figure 3.5. The first step relates structure to the physical properties of the layer, considered as an effective medium. The second step relates these effective properties to electrochemical performance. Relations between structure and performance are complicated by the formation of liquid water, affecting effective properties and performance. Solutions for such a model provide relations between structure, properties, and performance. These relations allow predictions of architectures of materials and operating conditions that optimize catalyst layer and fuel cell operation to be made. 

The most promising approach to the optimization of catalyst layers would be a concerted experimental-theoretical strategy. Since theory and modeling inevitably have to invoke simplifying assumptions, offering a pure theoretically driven optimization would be irresponsible. Ex situ diagnostics is needed to characterize structural details and explore their relations to effective properties. The availability of such experimental data defines the level of detail of structure-property relationships that a theory should be permitted to employ. 

This chapter is devoted entirely to performance models of conventional catalyst layers (type I electrodes), which rely on reactant supply by gas diffusion. It introduces the general modeling framework and employs it to discuss the basic principles of catalyst layer operation. Structure-based models of CCL rationalize distinct regimes of performance, which are discernible in polarization curves. If provided with basic input data on structure and properties, catalyst layer models reproduce PEFC polarization curves. Consistency between model predictions and experimental data will be evaluated. Beyond polarization curves, performance models provide detailed maps or shapes of reaction rate distributions. In this way, the model-based analysis allows vital conclusions about an optimal design of catalyst layers with maximal catalyst utilization and minimal transport losses to be drawn. 

Kulikovsky, A. A. 2012f. A model for optimal catalyst layer in a fuel ceU. 

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