Catalyst layer design

The properties and composition of the CL in PEM fuel cells play a key role in determining the electrochemical reaction rate and power output of the system. Other factors, such as the preparation and treatment methods, can also affect catalyst layer performance. Therefore, optimization of the catalyst layer with respect to all these factors is a major goal in fuel cell development. For example, an optimal catalyst layer design is required to improve catalyst... 

Pt is, however, an expensive and limited resource. For a 60 kW fuel cell vehicle, the cost of Pt would be over 2,400 at current cost and loading of Pt. Even worse, replacing combustion engines in all existing vehicles by fuel cell drive systems at no penalty in power would exceed the known reserves of Pt. Catalyst layer design, therefore, strives to reduce the Pt loading markedly at no penalty in the fuel cell voltage. 

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. 

Equations (6.59)-(6.61) represent a highly simplified scheme for evaluating various catalyst layer designs. Refinements of this crude framework for evaluating catalyst layer performance should address all transport limitations, account for water accumulation, and include two- and three-dimensional effects. 

In spite of the widely recognized importance of an advanced catalyst layer design, detailed structural data for catalyst layers are still scarce in the open literature on fuel cells [116, 117]. In one of the rare experimental studies, Uchida et al. showed the effect of the variation of the PFSI (and PTFE) content on catalyst layer performance [101]. An attempt to rationalize the experimentally observed composition dependence theoretically was first undertaken in Ref. 17. The prerequisites for an adequate theoretical study... 

Implications of Kinetic Limitations on Catalyst Layer Design... 

In summary it seems that the ideal catalyst layer design in a microstructured devices is achieved by solving quantitatively the opposite trends between (i) porosity, which means effective diffusion and high specific activity, and (ii) denseness, which means high thermal conductivity and layer stability. The resulting maximum should lead to an optimal reactor design in terms of heat transfer and productivity determined by intrinsic kinetics. 

Equations 8.8 to 8.10 provide a simple means for evaluating various catalyst layer designs. This can be applied for CL but it becomes inaccurate for... 

The objective of catalyst layer design is twofold from a materials scientist s perspective, the objective is to maximize the electrochemically active surface area (ECSA) per unit volume of the catalytic medium Secsa, by (i) catalyst dispersion in nanoparticle form or as an atomistically thin film and (ii) optimization of access to the catalyst surface for electroactive species consumed in surface reactions. From a fuel cell developers point of view, the objective is to optimize pivotal performance metrics like voltage efficiency, energy density, and power density (or specific power) under given cost constraints and lifetime requirements. These performance objectives are achievable by integration of a highly active and sufficiently stable catalyst into a structurally well-designed layer. 

This chapter provides a systematic account of the pertinent challenges and approaches in catalyst layer design. The hierarchy of structural effects and physical phenomena discussed includes materials design for high surface area and accessibility, statistical utilization of Pt evaluated on a per-atom basis, transport properties of charged species and neutral reactants in composite media with nano- to meso-porosity, local reaction conditions at internal interfaces in partially electrolyte-filled porous media, and global performance evaluated in terms of response functions for electrochemical performance and water handling. 

The reaction penetration depths. Id or la, are highly insightful parameters to evaluate catalyst layer designs in view of transport limitations, uniformity of reaction rate distributions, and the corresponding effectiveness factor of Pt utilization, as discussed in the sections Catalyst Layer Designs in Chapter 1 and Nonuniform Reaction Rate Distributions Effectiveness Factor in Chapter 3. Albeit, these parameters are not measurable. The differential resistances, Rd or Ra, can be determined experimentally either as the slope of the polarization curve or from electrochemical impedance spectra (Nyquist plots) as the low-frequency intercept of the CCL semicircle with the real axis. The expressions in Equation 4.33 thus relate the reaction penetration depths to parameters that can be measured. 

For a Pt/C-based catalyst layer in PEM fuel cells, according to the three-phase boundary theory, Pt catalysts that are not in the three-phase reaction zone are useless in the PEM fuel cell reaction as they are not accessible for reactants, electrons, or protons these Pt catalysts are thus inactive. To compare different catalyst layer designs, the Pt utilization (Mpt(%)) can be calculated according to the following equation ... 

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