Catalyst layer principles

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. 

In principle, the LFR is a fixed-bed reactor with a very low aspect ratio, i,e the ratio of bed height to bed diameter. Typically, the thickness of the catalyst layers is in the range of 15-75 mm. Hence, the reactor can be considered as a pancake reactor, in which the pancake has been folded for convenient accommodation in the reactor space. Because of the shallowness of the bed and its very large cross section, the pressure drop is much lower than in the case of a fixed bed of more conventional dimensions. 

Constructed on the same principles, a theory of the complex impedance reveals the interplay between the structure of the layer and its performance. The results demonstrate how the impedance spectra could help to determine the catalyst layer parameters. 

In Refs. [18,19], the macrohomogeneous theory was extended to include concepts of percolation theory. The resulting structure-based model correlates the performance of the CCL with the volumetric amounts of Pt, C, ionomer, and pores. A detailed review of macroscopic catalyst layer theory can be found in Ref. [17]. A further extension of this theory in Ref. [25] explores the key role of the CCL for the fuel cell water balance. This function is closely linked to the pore size distribution. Major principles of these models will be reproduced here. The details can be found in the literature cited. 

Figure 1.3 shows the operation principle of a PEM Fuel Cell. Humidified air enters the cathode channel, and a hydrogen gas enters the anode channel. The hydrogen diffuses through the anode diffusion layer towards the catalyst layer, where each hydrogen molecule splits up into two hydrogen protons and two electrons on catalyst smface according to [14] ... 

The principle of operation is shown in Fig. 2. Chlorine gas is produced at the anode (especially optimized dimensionally stable anode) with an anolyte feed concentration of 14 wt % HCl. Anode and cathode are separated by a cation exchange membrane (perfluorosulfonic acid polymer, PFSA, e.g., Nafion of DuPont). The ODC is based on a conductive carbon cloth which operates simultaneously as a gas diffusion layer because a suitable material is incorporated. The oxygen reduction reaction (5) takes place in three-phase boundaries of a thin, porous catalyst layer on the surface. 

The structure of this book aims to emphasize the similarity of fuel cells of different types. Nowadays, when the number of publications on FC science and technology is growing at an alarming rate, the fuel cell community tends to separate into sub-domains interested in only one type of cell. In an attempt to counteract this unfortunate trend the book is structured according to the hierarchical principle (from catalyst layers to stacks), rather than according to the cell type. SOFC, PEFC and DMFC communities can learn a lot from each other. 

Chapter 1 contains an overview of the basic principles of FC operation and of the processes in a FC. Models of catalyst layers for all types of cells are collected in Chapter 2. This chapter is followed by Chapter 3 on the through-plane modelling of PEFC, DMFC and SOFC. Then we proceed to quasi-2D cell modelling, which takes into account feed molecule exhaustion along the channel (Chapter 4). The final Chapter 5 is devoted to modelling of DMFC and SOFC stacks (the ideas in this Chapter can be directly applied to model PEFC stacks as well). 

This coarse-grained molecular dynamics model helped consolidate the main features of microstructure formation in CLs of PEFCs. These showed that the final microstructure depends on carbon particle choices and ionomer-carbon interactions. While ionomer sidechains are buried inside hydrophilic domains with a weak contact to carbon domains, the ionomer backbones are attached to the surface of carbon agglomerates. The evolving structural characteristics of the catalyst layers (CL) are particularly important for further analysis of transport of protons, electrons, reactant molecules (O2) and water as well as the distribution of electrocatalytic activity at Pt/water interfaces. In principle, such meso-scale simulation studies allow relating of these properties to the selection of solvent, carbon (particle sizes and wettability), catalyst loading, and level of membrane hydration in the catalyst layer. There is still a lack of explicit experimental data with which these results could be compared. Versatile experimental techniques have to be employed to study particle-particle interactions, structural characteristics of phases and interfaces, and phase correlations of carbon, ionomer, and water in pores. 

We begin with the discussion of cell thermodynamics and electrochemistry basics (Chapter 1). This chapter may serve as an introduction to the field and we hope it would be useful for the general reader interested in the problem. Chapter 2 is devoted to basic principles of structure and operation of the polymer electrolyte membrane. Chapter 3 discusses micro- and mesoscale phenomena in catalyst layers. Chapter 4 presents recent results in performance modeling of catalyst layers, and in Chapter 5 the reader will find several applications of the modeling approaches developed in the preceding chapters. 

FIG U RE 3.17 The self-consistency problem in Pt electrocatalysis. The metal phase potential determines oxidation state and charging properties at the catalyst surface. These properties in turn determine the local reaction conditions at the Hehnholtz or reaction plane. At this point, structural design and transport properties of the catalyst layer come into play (as illustrated for conventional and ultrathin catalyst layers). Newly developed methods in the emerging field of first-principles electrochemistry attempt to find self-consistent solutions for this conpled problem. 

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. 

From the modeling perspective, all catalyst layers perform the same job with the aid of neutral molecules, they convert a flux of ions or protons into a flux of electrons, or vice versa. Based on these principles, the CL model can be formulated in rather general terms, which would be suitable for catalyst layers of different types. Specific features or requirements of the CL of interest can be taken into account by model parameters, or by adding new terms, if necessary. 

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