Preparation of the Catalyst Layer

In accordance with the component distribution in a catalyst layer, two categories of catalyst layer have been employed during the development of PEM fuel cells. The first is the uniform catalyst layer, in which all components distribute uniformly this kind of CL is widely used in the fuel cell industry. The second is the non-uniform catalyst layer, in which one or more components have gradients across the whole catalyst layer, from the GDL side to the membrane side or from fuel cell inlet to outlet. The following sections will discuss these two categories in detail. 

PTFE-bonded hydrophobic electrodes are the modified versions of gas diffusion electrodes developed for PAFCs. In preparation, the catalyst particles are mixed with PTFE emulsion to form a catalyst ink, which is then cast onto the GDL. In order to provide ionic transport to the catalyst sites, the PTFE-bonded catalyst layers are generally impregnated with an ionomer, commonly Nation, by brushing or spraying. A typical preparation process is detailed as follows  

Coat a PTFE-bonded hydrophobic ink onto the GDL and then dry to remove the solvent  

Bake the electrode subsequently at 240-340 °C for 40 minutes to remove surfactants in the PTFE emulsion and hydrophobilize the electrodes  

Spray a certain amount of Nation solution onto the surface of the catalyst layer to form the ionic paths  

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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. 

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... 

An important consideration for the electronics of semiconductor/metal supported catalysts is that the work function of metals as a rule is smaller than that of semiconductors. As a consequence, before contact the Fermi level in the metal is higher than that in the semiconductor. After contact electrons pass from the metal to the semiconductor, and the semiconductor s bands are bent downward in a thin boundary layer, the space charge region. In this region the conduction band approaches the Fermi level this situation tends to favor acceptor reactions and slow down donor reactions. This concept can be tested by two methods. One is the variation of the thickness of a catalyst layer. Since the bands are bent only within a boundary layer of perhaps 10-5 to 10 6 cm in width, a variation of the catalyst layer thickness or particle size should result in variations of the activation energy and the rate of the catalyzed reaction. A second test consists in a variation of the work function of the metallic support, which is easily possible by preparing homogeneous alloys with additive metals that are either electron-rich or electron-poor relative to the main support metal. 

Figure 24 shows a cross section of a Nafion membrane catalyzed by direct application of catalyst inks to its two major surfaces, as observed by SEM [52], The thin slice of MEA required for SEM imaging was generated by microtome from the MEA encapsulated in epoxy. This figure actually describes an MEA prepared for a DMFC, with PtRu black and Pt black catalyst layers of relatively high loading, resulting in catalyst layers 10 and 14 pm thick (Fig. 24). The SEM image well depicts two generic characteristics of CCMs prepared by direct, ink-based application of the catalysts to the ionomeric membrane the interface between the catalyst layer and the membrane is sharp on the SEM scale and the thickness of the catalyst layer measured from the...

To avoid high-pressure drop and clogging problems in randomly packed micro-structured reactors, multichannel reactors with catalytically active walls were proposed. The main problem is how to deposit a uniform catalyst layer in the microchannels. The thickness and porosity of the catalyst layer should also be enough to guarantee an adequate surface area. It is also possible to use methods of in situ growth of an oxide layer (e.g., by anodic oxidation of a metal substrate [169]) to form a washcoat of sufficient thickness to deposit an active component (metal particles). Suzuki et al. [170] have used this method to prepare Pt supported on nanoporous alumina obtained by anodic oxidation and integrate it into a microcatalytic combustor. Zeolite-coated microchannel reactors could be also prepared and they demonstrate higher productivity per mass of catalyst than conventional packed beds [171]. Also, a MSR where the microchannels are coated by a carbon layer, could be prepared [172]. 

Pt-doped carbon aerogels have been used successfully in the preparation of cathode catalyst layers for oxygen reduction reaction (ORR) in PEMFC systems [83-86]. Thus, different Pt-doped carbon aerogels with a Pt content of around 20 wt% were prepared by impregnation [83]. Results obtained with these Pt catalysts were compared with others supported on carbon blacks Vulcan XC-72 and BP2000, which are commonly used as electrocatalysts. The accessibility of the electrolyte to Pt surface atoms was lower than expected for high-surface-area... 

The right choice of a carbon support greatly affects cell performance and durability. The purpose of this chapter is to analyze how structure and properties of carbon materials influence the performance of supported noble metal catalysts in the CLs of the PEMFCs. The review chapter is organized as follows. In Section 12.2 we give an overview of carbon materials utilized for the preparation of the catalytic layers of PEMFC. We describe traditional as well as novel carbon materials, in particular carbon nanotubes and nanofibers and mesoporous carbons. In Section 12.3 we analyze properties of carbon materials essential for fuel cell performance and how these are related to the structural and substructural characteristics of carbon materials. Sections 12.4 and 12.5 are devoted to the preparation and characterization of carbon-supported electrocatalysts and CLs. In Section 12.6 we analyze how carbon supports may influence fuel cell performance. Section 12.7 is devoted to the corrosion and stability of carbon materials and carbon-supported catalysts. In Section 12.8 we provide conclusions and an outlook. Due to obvious space constraints, it was not possible to give a comprehensive treatment of all published data, so rather, we present a selective review and provide references as to where an interested reader may find more detailed information. 

A copper tube about 90 cm. long and 7.6 cm. in diameter, packed with copper turnings which are plated with silver, is used as the reaction vessel. The reactor is closed at the ends by disks of copper fastened on by silver solder. An inlet tube of -in. copper tubing is silver-soldered to the vessel at one end and an outlet tube is attached in the same manner at the other end. In the preparation of the catalyst about 4.5 kg. of annealed thin copper ribbon (in the form of chore balls of the type normally used for cleaning dishes) t is plated with 50 to 100 g. of silver by displacement from a solution containing Ag(CN)2 ion. The plated ribbon is washed, dried, and packed firmly into the reactor to which one end-plate is attached. J The second end-plate is then attached and the reactor is insulated with a layer of asbestos paper and wound with Nichrome ribbon to permit electrical heating. Before use, the reactor is heated to 200° and treated with fluorine until the gas appears to be absorbed no longer. About 40 g. of fluorine is required. 

Figure 1.8 shows a scheme of the microstructxu e of the catalyst layer, where the presence of TPRs allows the simultaneous flows of and electrons toward the PEM and GDL, respectively. Note that the thickness of the proton conducting membrane covering the carbon-supported catalyst nanoparticles is of the order of nanometers, depending of the concentration of proton conducting binder (usually Nafion) in the ink used to prepare the MEA. 

There are a lot of articles and patents that describe the preparation of several types of GDEs. These electrodes can be used as anodes or cathodes, depending on the electrocatalyst present in them. We will report below general procedmes for their preparation, mainly focused to the manufacture of hydrogen diffusion electrodes [5]. The main steps in the fabrication of GDEs are the preparation of the gas diffusion medium and the preparation and apphcation of the catalyst layer. 

The main preparation methods for H2 technical electrodes for low temperature fuel cells have been examined. It has been demonstrated that the electrochemical behavior of the electrodes depends on their fabrication, thus affecting the fuel cell operation. The preparation of the catalyst of the active layer also influences its physical properties and electrochemical performance. Different electrochemical approaches to study HOR on model, as a first approximation, and technical electrodes, are exhaustively analyzed and their kinetic parameters are discussed to evaluate their performance and system modelling. The existence of a gap between the knowledge obtained from studies on model electrodes and technical electrodes is emphasized. To optimize the performance of practical fuel cell electrodes, the preparation of high surface area catalysts with the same characteristics as those shown at the atomic level then seems necessary. In this sense, mechanistic studies are fimdamental to... 

Alumina sol may be prepared from aluminium alkoxide or pseudo-boehmite [A10(0H)xH20]. Addition of additives such as urea provides the porosity of the catalyst layer through thermal composition during calcination [136] ... 

On the same topic of DMFC performance with supported vs. unsupported catalysts Smotkin and co-workers concluded that at 363 Kthe supported PtRu (1 1) catalyst with a toad of 0.46 mg cm performed as welt as an unsupported PtRu (1 1) with over four times higher load, i.e., 2 mg cm [266]. It is likely that these differences between various studies are related not only to the intrinsic activity of the respective anode catalys layers but also to the manufacturing procedures such as catalyst layer preparation and application techniques, MEA hot pressing conditions (temperature, pressure and time), presence or absence of other binders (such as PTFE) and fuel cell compression. All these MEA manufacturing variables can affect, in a poorly understood manner at present, the structure, morphology and composition of the catalyst layer in the operating fuel cell. Therefore, in fuel cell experiments it is difficult to isolate the truly physico-chemical effect of the support on the catalytic activity. 

Colloid Method In order to improve gas transport through the ionomer-bonded hydrophilic catalyst layer, some modified hydrophilic electrodes have also been developed. One technique is called the colloid method, which changes Nafion into a colloid state. The colloid ionomer (such as Nafion) can adsorb catalyst particles to form larger catalyst/Nafron agglomerates. It is believed that the colloid method benefits the construction of the CL microstructure and enhances gas transportation [41-45]. Uchida et al. [41, 42] first proved that butyl acetate with a e of 5.01 was the best solvent to form PFSI colloids for the preparation of a catalyst layer. The detailed steps are as follows ... 

Sputtering-deposition Electrode. Sputtering is widely used in the preparation of thin catalyst layers. The resultant CLs have demonstrated high performance even at an ultra-low level of Pt loading [24, 54, 55]. In general, the sputtering process is carried out in an evacuated chamber. The typical sputtering steps are as follows ... 

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