Catalyst-coated diffusion

In the preparation of MEA, there are two options. One is to coat the CL onto the DM such as carbon paper, or carbon cloth, the other is to coat the CL onto the PEM, as shown in Figure 3.17. The CL coated on the diffusion medium is called Catalyst-coated Diffusion Medium (CDM), and the CL coated on the membrane is called Catalyst Coated Membrane (CCM). Using a hot-press process, by sandwiching PEM between two CDMs, or CCM between two DMs, an MEA can be fabricated for fuel cell testing. 

The catalytic (supported or unsupported) interface in the vast majority of direct liquid fuel cell studies is realized in practice either as a catalyst coated membrane (CCM) or catalyst coated diffusion layer (CCDL). Both configurations in essence are part of the electrode design category, which is referred to as a gas diffusion electrode, characterized by a macroporous gas diffusion and distribution zone (thickness 100-300 pm) and a mainly mesoporous, thin reaction layer (thickness 5-50 pm). The various layers are typically hot pressed, forming the gas diffusion electrode-membrane assembly. Extensive experimental and mathematical modeling research has been performed on the gas diffusion electrode-membrane assembly, especially with respect to the H2-O2 fuel cell. It has been established fliat the catalyst utilization efficiency (defined as the electrochemically available surface area vs. total catalyst surface area measured by BET) in a typieal gas diffusion electrode is only between 10-50%, hence, flie fuel utilization eflfieieney can be low in such electrodes. Furthermore, the low fuel utilization efficiency contributes to an increased crossover rate through the membrane, which deteriorates the cathode performance. 

The technology just described, of applying catalysts to the diffusion layers and then hot-pressing all components, results in a catalyst-coated diffusion layer (CCDL). Another technology, which results in a catalyst-coated membrane (CCM), involves all components of the catalytic layer being evaporated as a Ihin layer directly onto the membrane. 

In practice, the catal5Tic layers are prepared by brushing or spraying catalyst ink (a suspension of the catalyst particles in water and/or an organic solvent with addition of ionomer) either onto diffusion media (carbon paper or carbon cloth, also referred to as substrates), resulting in so-called catalyst-coated substrates (CCS), or directly onto... 

There are two main types of thin-film catalyst layers catalyst-coated gas diffusion electrode (CCGDL), in which the CL is directly coated on a gas diffusion layer or microporous layer, and catalyst-coated membrane, in which the CL is directly coated on the proton exchange membrane. In the following sections, these catalyst layers will be further classified according to their composition and structure. 

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. 

Firstly, reactions of hydrocarbons will be discussed. Vehicle exhaust contains a complex mixture of hydrocarbons (ICubo et al., 1993). These will have different reactivity and their different molecular masses lead to different propensities for mass transport, the larger molecules diffusing more slowly to the catalyst coating. While the full (vast) range of hydrocarbons in the exhaust could not practically be included in a model, it is desirable to include a small number of representative hydrocarbons to emulate the range of activity and transport properties of the full mixture. 

With a monolith reactor, diffusion from the channels to the catalyst coated on the channel walls is the sole means by which reactants are able to reach the catalyst (Section III). It seems reasonable that a similar diffusion process occurs in a coated filter. 

In fast reactions, mass transfer or intraparticle diffusion becomes controlling. Thinner catalyst coatings, Taylor flow, etc. can be applied to optimize these... 

Conventional welding may well be applied for prototypes and small series production especially for bonding of reactor/device periphery such as inlet diffusers and fluidic connections. In the case of chemical reactors, overheating needs to be avoided if precoating techniques are applied to avoid damage to the catalyst coating. 

Heatric claims to have developed a robust and renewable technology to apply catalyst coats to the passages within a PCR [163]. This coating has to be executed after the diffusion bonding but the coating procedure is not described. Such a development would certainly be a milestone in micro reaction technology as the standard wash-coat methods are usually applied to free accessible surfaces and not to closed channels. 

As shown in Figure 1.6, the optimized cathode and anode structures in PEMFCs include carbon paper or carbon cloth coated with a carbon-PTFE (polytetrafluoroethylene) sub-layer (or diffusion layer) and a catalyst layer containing carbon-supported catalyst and Nafion ionomer. The two electrodes are hot pressed with the Nafion membrane in between to form a membrane electrode assembly (MEA), which is the core of the PEMFC. Other methods, such as catalyst coated membranes, have also been used in the preparation of MEAs. 

Modern technological developments and many fields of pure and applied research depend on the quantitative information about the spatial element distribution in thin solid layers and thin-film systems. For example, without the use of thin films the experimental studies on the physics of semiconductor are very difficult. Similarly the diffusion processes in solids, sandwich-like thin films structures in microelectronics, anti-reflecting or selectively transparent optical films, catalysts, coatings, composites - all rely on material properties on an atomic scale. The development of these new materials as well as the understanding of the basic physical and chemical properties that determine their specific characters are not possible without the knowledge of their compositional structure, in particular in the interface regions. 

Advances in chemical reaction engineering and catalytic materials have allowed catalytic combustion for thermal energy generation to be commercialized in consumer and industrial applications. The development of catalysts coated on one side of a metal substrate, coupled with the use of diffusion barriers, has allowed controlling the combustion temperature to suit diverse applications. Catalytic materials have been developed to remain active for thousands of hours under conditions deemed too severe just a few years ago. We may expect that the need for clean distributed power increases the demand for gas turbines fitted with catalytic combustors and promotes the development of catalytic burners to be used in fuel processors for fuel cell power systems. 

The PEM cell design chosen for tlie current work employs a significantly different geometry than the Westinghouse cell. The PEM electrolyzer consists of a membrane electrode assembly (MEA) inserted between two flow fields. Behind each flow field is a back plate, copper current collector and stainless steel end plates. The MEA consists of a Nafion proton-exchange-membrane with catalyst-coated gas diffusion electrodes bonded on either side. 

Membrane electrode assemblies (MEAs) are typically five-layer structures, as shown in Figure 10.1. The membrane is located in the center of the assembly and is sandwiched by two catalyst layers. The membrane thickness can be from 25 to 50 pm and, as mentioned in Chapter 10, made of perfluorosulfonic acid (Figure 11.3). The catalyst-coated membranes are platinum on a carbon matrix that is approximately 0.4 mg of platinum per square centimeter the catalyst layer can be as thick as 25 pm [12], The carbon/graphite gas diffusion layers are around 300 pm. Opportunities exist for chemists to improve the design of the gas diffusion layer (GDF) as well as the membrane materials. The gas diffusion layer s ability to control its hydrophobic and hydrophilic characteristics is controlled by chemically treating the material. Typically, these GDFs are made by paper processing techniques [12],... 

Consider a straight tube of radius R with circular cross section and expensive metal catalyst coated in the inner wall. Reactant A is converted to products via first-order irreversible chemical reaction on the catalytic surface at r = R. Hence, diffusion of reactant A in the radial direction, toward the catalytic surface, is balanced by the rate of consumption of A due to heterogeneous chemical reaction. The boundary condition at the mathematically well-defined catalytic surface (i.e., r = R) is... 

Interdiflusion between the components of catalyst coatings and substrates can also lead to catalyst deactivation. If Nb, Ta, Ti, V or Zr diffuses through palladium or other noble metal protective layers and reacts on the outer surface to form stable oxides, carbides or nitrides, the catalytic dissociation of molecular hydrogen can be poisoned. Interdiffusion, linked to loss of hydrogen flux, has been reported by Edlund and McCarthy [47] and Pagheri et al. [56]. Membrane surfaces can also become depleted of palladium if palladium diffuses into the substrates. Apparent complete loss of palladium has been observed by Rothenberger et al. for 40 nm thick Pd films on Ta foils after 48 h use at 1173 K (900 °C) [41]. 

Fig. 14.1 Functional layers in a fuel cell. Functional layers can be integrated to a component such as catalyst coated membrane (CCM), catalyst coated substrate (CCS)/gas diffusion electrode (GDE) or bipolar plate (BPP)...

Individual layers can be integrated to components such as the electrolyte membrane and the two catalyst layers to a Catalyst Coated Membrane (CCM). Another option is the integration of the gas diffusion layer and the catalyst layer to a Gas Diffusion Electrode (GDE). The gas functions of distribution, gas separation and coolant distribution are commonly integrated into the BiPolar Plate (BPP). 

Fig. 14.3 MEA-configurations (a) catalyst coated membrane (CCM, 3-layer MEA), (b) CCM with gasketing frame (5-layer MEA), (c) 5-layer MEA with gas diffusion layers attached (7-layer MEA)...

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