Carbon agglomeration

Figure 8.5 SEM micrographs of calcium carbonate agglomerates a, vaterite and calcite h, calcite). After Zauner and Jones, 2000c)...

Figure 19 TEM micrograph of catalyst-Nafion interface showing metal particles supported on carbon agglomerate and Nafion ionomer micelles.

It should be added that the method presented here also permits us to obtain adsorbents with an increased specific surface area as compared to the initial carbon-silica adsorbent. This process can be easily accounted for if one takes into consideration the fact that the vapours of the pyrolysed substance transport by the carrier gas, with its adequately high flow rate over the modified carbon-silica adsorbent, may not diffuse into the narrow pores on time but will undergo decomposition either in wide and more easily accessible pores or on active, catalytically acting, corners and walls of the carbon agglomerates formed in the process of pyrolysis of dichloromethane. (Diffusion is a rather slow process, slower than adsorption.) Thus the size of the specific surface area of silica gels modified by dichloromethane may be increased or slightly changed (e.g. Adsorbent X, see Table 7). 

Experimental. Samples for electrophoretic mobility (EM) measurements were prepared as previously described for the titration experiments except that violent mixing with destruction of carbon agglomerates was desirable here and was promoted with a 5-10 minute treatment of the capped serum bottles in an ultrasonic bath. The indicator was, of course, deleted in the EM sample preparation procedure. Following mixing, a few drops of the mixture was transferred to a 25 ml flask of benzene and given another ultrasonic treatment. This procedure produced a fairly stable colloid suspension of the carbon black which was used to fill the glass-Teflon commercially available EM cell manufactured by Zeta-Meter, Inc. 

Figure 12.1 Schematic representation of a membrane-electrode assembly of a PEMFC. Platinnm particles and carbon agglomerates are colored in black and dark gray, respectively.

Figure 7.8 False-color image of an ultrathin section of a fuel cell electrode the inset shows a typical carbon agglomerate. (Adapted from our other book chapter [128]. Copyright Woodhead Publishing. Reproduced with permission.)...

The use of anhydrous carbonate gives higher active agglomerates. This is because the (hydro-phobic) HLAS does not wet the hydrated carbonate as well as the anhydrous carbonate. This applies to zeolite as well. Typically, an LAS carbonate agglomerate could reduce activity from 20 to -18% due to hydrated powders. 

For the commercialization of PEFCs, in order to reduce the Pt loading without serious loss of cell performance and durability, it has been proposed that maximizing the Efpt is necessary. In the relationship between the Pt catalyst powder and ionomer in a conventional CL, much of the Pt existing in mesopores of the carbon agglomerates cannot connect with the ionomer. Moreover, the primary particles of the carbon support contain many nanopores. The Pt in such nanopores also does not connect with the ionomer [16, 20]. Therefore, we must improve the Efpi by improving the catalyst loading method, the support materials, and development of new ionomers. In addition, we should try to improve the ORR activity of the catalyst material itself. Of course, adequate durability of the MEA under practical conditions must also be secured. 

This coarse-grained molecular dynamics model helped consolidate the main features of microstructure formation in CLs of PEFCs. These showed that the final microstructure depends on carbon particle choices and ionomer-carbon interactions. While ionomer sidechains are buried inside hydrophilic domains with a weak contact to carbon domains, the ionomer backbones are attached to the surface of carbon agglomerates. The evolving structural characteristics of the catalyst layers (CL) are particularly important for further analysis of transport of protons, electrons, reactant molecules (O2) and water as well as the distribution of electrocatalytic activity at Pt/water interfaces. In principle, such meso-scale simulation studies allow relating of these properties to the selection of solvent, carbon (particle sizes and wettability), catalyst loading, and level of membrane hydration in the catalyst layer. There is still a lack of explicit experimental data with which these results could be compared. Versatile experimental techniques have to be employed to study particle-particle interactions, structural characteristics of phases and interfaces, and phase correlations of carbon, ionomer, and water in pores. 

The ink is mixed vigorously until the carbon agglomerates are dispersed. Therefore, propeller stirrer or dissolver disks can be used. 

Final microstructures obtained reveal a high sensitivity of carbon agglomeration and ionomer structure formation to the wetting properties of carbon particles and the strength of ionomer-carbon interactions. While ionomer sidechains are confined in hydrophilic domains, with a weak contact to carbon domains, ionomer backbones are preferentially attached to the surface of carbon agglomerates for the given hydrophobic type of C. As expected, the correlation between hydrophilic species... 

Carbon agglomerate Polymer Cathode catalyst Gas diffusion Pt particle ° membrane layer layer... 

Figure 1.3 illustrates the cathode catalyst layer. In the cathode catalyst layers, protons transfer in the polymer (ionomer), a material similar to the membrane, which covers the surface of carbon agglomerates. Electrons transfer in the carbon agglomerates in a chain structure. Oxygen transfers... 

Using the models and the calculated results, a simplified equation, termed an evaluation equation in this section, was derived to show the relationships among major parameters in the cathode catalyst structure (Tabe et al., 2011). From the analysis of the evaluation equation, optimal structural parameters were identified. The results indicate tiiat the dominant parameters of the CL structure are the polymer electrolyte thickness covering carbon agglomerates and the CL thickness. 

Inside an agglomerate (Afs) the phase s reconstruction sequence in the computing domain is (i) carbon agglomerate generation, (ii) platinum on carbon placement-generation, (iii) the rest is ionomer. 

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