Conventional Catalyst Layers

At the macroscopic scale, catalyst utilization is severely limited by the statistical eonstraints imposed by the 3-phase random eomposite structure. Only catalyst particles that are simultaneously accessible to electrons, protons, and oxygen could be electrochemically active. These requirements are included in flic factors /(Xp,e,X,)/Xp.e and g(5j in Equation 8.2. It was suggested in [3, 6, 47, 50] 

At the mesoscopic scale, the effectiveness faetor Tj, of agglomerates is a strong function of agglomerate radius and cathodie transfer coefficient, as discussed in Section 8.5.2. At values 0.9 and 50 nm, Ej, could be markedly smaller 

A calculation of the overall effectiveness of catalyst utilization in conventional CCLs has been recently performed in [56], including all of the aforementioned detrimental factors. This estimate suggests that less than 10% of the catalyst is 

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Cyclic voltammetric studies indicated that the activity of the Pt-Ru films increased with operating temperature just as in conventional catalyst layers produced from unsupported catalyst inks. Membrane electrode assemblies were fabricated from Pt-Ru films of the most active compositions, and a power density of 800 mW/mg was realized for anodes that were deposited with about 0.1 mg/cm of Pt-Ru (see Figure 1). Applying the catalyst layers by sputter deposition on the electrode was found to yield better performance than applying them on the membrane. This was attributed to the enhanced electrical connectivity achieved when the catalyst layer is applied on the electrode. However, this is only true for very thin films. When thicker composite films are produced, such as those planned later in this project, good electrical connectivity may be achieved even with membrane deposition. 

Due to the thickness range, L 10 — 20pm, the operation of conventional catalyst layers depends decisively on the availability of sufficient gas porosity for the transport of reactants. The pertinent theory of gas diffusion electrodes dates back to the 1950s with major contributions by A.N. Frumkin and O.S. Ksenzhek [79-81]. Specifically, catalyst utilization and specific effective surface area in composite electrodes were always the focus of attention (see, e.g. [82,83] and the articles quoted therein). 

A concentration of the platinum loading close to the electrolyte interface via Pt-covered polymer nanofibers has been proposed by 3 M [31]. The concentration of the platinum catalyst coated onto nanofibers within approximately 300 nm distances from the electrolyte membrane surface, however, is leading to specific operation characteristics. While high power densities can be achieved under comparatively dry operating conditions even at elevated temperatures, the thin catalyst layers show a tendency for flooding under wet conditions at low temperatures. A summary of the behavior of this type of catalyst layers under specific operating conditions is given in [56]. The low temperature behavior has been improved by addition of a conventional catalyst layer on to this thin film catalyst layer [57]. 

Du CY, Yang T, Shi PF, Yin GP, Cheng XQ (2006) Performance analysis of the ordered and the conventional catalyst layers in proton exchange membrane fuel cells. Electrochim Acta 51(23) 4934-4941... 

As an example of how to use the insights conveyed in this chapter we provide an explicit comparison of overall effectiveness of Pt utilization (by atom number or catalyst weight) for conventional 3-phase composite and ultrathin CCLs, in Table 8.2. For conventional catalyst layers, the main detrimental factors arise at the nanoparticle scale and at the macroscopic scale due to triple-phase boundary requirements. For the nanostructured ultrathin CCLs it is assumed that a sputter-deposited continuous Pt layer is needed to provide electronic conductivity. It was suggested in [153] on the basis of cyclic voltammetry measurements that the irregular surface morphology of such catalysts corresponds to grain sizes of 10 nmwith... 

Another approach for increasing the amount of water in the anode catalyst layer is to boost the water content in the catalyst layer components. A conventional catalyst layer, for instance, may contain up to 30% by weight amount of fully hydrated perfluorosulfonic ionomer. Thus, an increase in water content can be accomplished by increasing the amount of the water-containing ionomer used in the catalyst layer or by employing a different ionomer with higher water content. Alternatively, more hygroscopic materials may be incorporated into the anode catalyst layer to retain more water therein. 

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. 

Although the majority of authors who have investigated CNTs as supports for Pt and PtRu particles claim higher activity or performance compared to conventional catalysts, it is not clear why these enhancement arise. It seems unlikely that the CNTs provide any electronic enhancement to Pt(Ru) reactivity, so it is likely that CNTs provide benefits for catalyst layer structure. Part of this may be related to surface area because CNTs can have relatively high surface areas and are often compared to XC72 supported catalysts that have only a moderate surface area ( 250 m g ). Given the current high expense of these materials ( 10 kgr ), further benefits of their use need to be identified before fhey can be practically considered as candidates for fuel cell catalyst supports. 

Catalyst layer ink can be deposited on gas diffusion layers to form a CCGDL, as discussed in the previous section. Alternatively, the catalyst ink can be applied directly onto the proton exchange membrane to form a catalyst-coated membrane (CCM). The most obvious advantage of the CCM is better contact between the CL and the membrane, which can improve the ionic connection and produce a nonporous substrate, resulting in less isolated catalysts. The CCM can be classified simply as a conventional CCM or as a nanostructured thin-film CCM. 

Although the sputter deposition technique can provide a cheap and directly controlled deposition method, the performance of PEM fuel cells with sputtered CLs is still inferior to that of conventional ink-based fuel cells. In addition, other issues arise related to the physical properties of sputtered catalyst layers, such as low lateral electrical conductivity of the thin metallic films [96,108]. Furthermore, the smaller particle size of sputter-deposited Ft can hinder water transport because of the high resistance to water transport in a thick, dense, sputtered Ft layer [108]. Currently, the sputter deposition method is not considered an economically viable alternative for large-scale electrode fabrication [82] and further research is underway to improve methods. 

Saha et al. [109] have proposed an improved ion deposition methodology based on a dual ion-beam assisted deposition (dual IBAD) method. Dual IBAD combines physical vapor deposition (PVD) with ion-beam bombardment. The unique feature of dual IBAD is that the ion bombardment can impart substantial energy to the coating and coating/substrate interface, which could be employed to control film properties such as uniformity, density, and morphology. Using the dual lABD method, an ultralow, pure Ft-based catalyst layer (0.04-0.12 mg Ft/cm ) can be prepared on the surface of a GDL substrate, with film thicknesses in the range of 250-750 A. The main drawback is that the fuel cell performance of such a CL is much lower than that of conventional ink-based catalyst layers. Further improvement... 

Zhang, H., Wang, X., Zhang, J., and Zhang, J. Conventional catalyst ink, catalyst layer, and MEA preparation. In PEM fuel cell electrocatalysts and catalyst layers Fundamentals and applications, ed. J. Zhang. London Springer, 2008. 

Due to their high electrical and thermal conductivity, materials made out of metal have been considered for fuel cells, especially for components such as current collectors, flow field bipolar plates, and diffusion layers. Only a very small amount of work has been presented on the use of metal materials as diffusion layers in PEM and DLFCs because most of the research has been focused on using metal plates as bipolar plates [24] and current collectors. The diffusion layers have to be thin and porous and have high thermal and electrical conductivity. They also have to be strong enough to be able to support the catalyst layers and the membrane. In addition, the fibers of these metal materials cannot puncture the thin proton electrolyte membrane. Thus, any possible metal materials to be considered for use as DLs must have an advantage over other conventional materials. 

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. 

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. 

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