Fuel coating catalyst

The earliest models of fuel-cell catalyst layers are microscopic, single-pore models, because these models are amenable to analytic solutions. The original models were done for phosphoric-acid fuel cells. In these systems, the catalyst layer contains Teflon-coated pores for gas diffusion, with the rest of the electrode being flooded with the liquid electrolyte. The single-pore models, like all microscopic models, require a somewhat detailed microstructure of the layers. Hence, effective values for such parameters as diffusivity and conductivity are not used, since they involve averaging over the microstructure. 

Note that, despite the typically high operating temperatures of fuel cells, radiative heat transfer was neglected. Lee and Aris (16) have discussed such effects in parallel-channel monoliths. The importance of radiative transport depends on the emissivity of the surface for the low (about 0.1) emissivity of Pt-coated catalyst-electrodes, their analysis suggests that radiative effects can be neglected. 

Note finally that, as mentioned in the Introduction, the corrosion of the substrate may also damage irreversibly a microstructured device under the severe conditions of fuel processing reactions. For example, under water vapor pressure, many detrimental effects can occur, such as surface migration of Ni in stainless-steel alloys, surface oxidation of metals (Fe to Fe203), surface enrichment with trace elements able to alloy/react with the coated catalyst (Sn, Pb, Cl ions) and poison it or surface substrate restructuring. 

Atanasoski RT, Atanasoska LL, Cullen DA, Haugen GM, More KL, Vemstrom GD (2012) Fuel cells catalyst for start-up and shutdown conditions electrochemical, XPS, and STEM evaluatitm of sputter-deposited Ru, Ir, and Ti on Pt-coated nanostmctured thin film supports. Electrocatalysis. Electrocatal 3 284—297... 

An early proposal to apply coated catalyst systems for reforming applications, which is not yet heading for mobile fuel cell systems, was made by Ramshaw more than 20 years ago [466]. 

On one hand, the term characterization is related to pure and basis material questions by investigating physical and chemical parameters like viscosity of inks [1] needed to coat catalysts on membranes or gas diffusion layers (GDLs). Or it can be related to electrochemical data of fuel cells (FC) components like the determination of the ion conductivity of membranes [2-5]. It is obvious to the reader that a really large variety of different characterization methods exist in order to determine several physical, chemical, and electrochemical parameters of fuel cell components. 

Fig. 19. The EPR spectra obtained with aqueous POBN solution at the cathode side of the in situ fuel cell. Catalyst-coated membrane based on Nafion membrane (a) and on a fluorine-free membrane (b, = 15.10 G, au = 2.68 G). (From Ref. 55 with permission.)...

George, P. P. et al. 2008. Selective coating of anatase and rutile TiOj on carbon via ultrasound irradiation Mitigating fuel ceU catalyst degradation. Journal of Fuel CeU Science and Technology 5 041012(1-9). 

Needless to say MEA is the core compartment of DMFC with its electrochemical reaction function. Figure 13.3 shows the typical microstructure of MEA, danon-strating the interface of catalyst and membrane. Its function is to deliver materials, such as catalyst and membrane, and physical functions for fuel delivery and recovery. Mobile application MEAs minor functions, such as fuel delivery and recovery, have become more important. MEA can be defined as three compartments of membrane, a catalyst layer and diffusion electrode with a microporous layer. The catalyst layer consists of catalyst and interface materials with membrane. This layer has to be designed for effective utilization of the catalyst in order to minimize the use of precious metals while maintaining the produced proton path to the membrane. For this reason, this layer has to be electron- and ion-conductive with low fuel flow resistance. The membrane is located at the center of the MEA, with the catalyst layer coated (catalyst-coated membrane, CCM) in some cases. Its ion conduction would be made a lot easier by reducing the impedance at the interface with the catalyst. 

Catalytic Unit. The catalytic unit consists of an activated coating layer spread uniformly on a monolithic substrate. The catalyst predominantly used in the United States and Canada is known as the three-way conversion (TWC) catalyst, because it destroys aU three types of regulated poUutants HC, CO, and NO. Between 1975 and the early 1980s, an oxidation catalyst was used. Its use declined with the development of the TWC catalyst. The TWC catalytic efficiency is shown in Figure 5. At temperatures of >300° C a TWC destroys HC, CO, and NO effectively when the air/fuel mixture is close to... 

Lead compounds were not found on the surrounding activated coating layer, rather only associated with the precious metal. The Pt sites are less poisoned by lead than are Pd or Rh sites because the Pt sites are protected by the sulfur in the fuel. Fuel sulfur is converted to SO2 in the combustion process, and Pt easily oxidizes SO2 to SO on the catalyst site. The SO reacts with the lead compounds to form PbSO, which then moves off the catalyst site so that lead sulfate is not a severe catalyst poison. Neither Pd nor Rh is as active for the SO2 to SO reaction, and therefore do not enjoy the same protection as Pt. 

It was quite recently reported that La can be electrodeposited from chloroaluminate ionic liquids [25]. Whereas only AlLa alloys can be obtained from the pure liquid, the addition of excess LiCl and small quantities of thionyl chloride (SOCI2) to a LaCl3-sat-urated melt allows the deposition of elemental La, but the electrodissolution seems to be somewhat Idnetically hindered. This result could perhaps be interesting for coating purposes, as elemental La can normally only be deposited in high-temperature molten salts, which require much more difficult experimental or technical conditions. Furthermore, La and Ce electrodeposition would be important, as their oxides have interesting catalytic activity as, for instance, oxidation catalysts. A controlled deposition of thin metal layers followed by selective oxidation could perhaps produce cat-alytically active thin layers interesting for fuel cells or waste gas treatment. 

Microelectronic circuits for communications. Controlled permeability films for drug delivery systems. Protein-specific sensors for the monitoring of biochemical processes. Catalysts for the production of fuels and chemicals. Optical coatings for window glass. Electrodes for batteries and fuel cells. Corrosion-resistant coatings for the protection of metals and ceramics. Surface active agents, or surfactants, for use in tertiary oil recovery and the production of polymers, paper, textiles, agricultural chemicals, and cement. 

Catalysts were expensive, however, so the petroleum industry did not solve the problem of cheap, lead-free, knock-free gasoline until the 1970s, after General Motors adopted the catalytic converter. Lead compounds inactivate the catalysts, and sophisticated catalytic cracking techniques had to be developed to replace the fuel additive. Ironically, an even more difficult job was finding a substitute for the protective coating that tetraethyl lead formed on exhaust valve seats not even newly developed, extremely hard materials prevent wear and tear on them as well as tetraethyl lead did. 

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