Optimization factor Catalyst layer, optimal

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... 

In order to make catalyst layers with high platinum utilization and better performance, we need to determine how various factors affect Pt utilization. Although this objective has been receiving more attention, we have not achieved a fundamental understanding of the relationships of composition, structure, effective properties, and fuel cell performance—a fact that may limit the optimal design and fabrication of CLs. 

The bed effectiveness is an important factor for an optimal design of a PPR for a given application. A lower bed effectiveness means that more catalyst and a larger reactor is needed for the same process duty. Another important factor is pressure drop. As discussed above, a PPR with thin catalyst layers will have a high degree of catalyst utilization, so the amounts of catalyst and reactor space required are low. However, with thinner catalyst layers the construction cost of the PPR will be higher. 

Consider, for example, a catalyst layer with thickness / = 10 pm and I = 1 A cm-2 (corresponding to D tv 10-4 cm2s 1 at T = 300 KPo2 = 1 atm), which obeys the phase diagram in Fig. 14 (at defined composition). The optimum current density range of operation for this electrode would be 0.5-1.0 A cm-2. If, however, for example, for reasons of maximum efficiency, the target current density of fuel cell operation is 0.25 A cm-2 then the electrode thickness should be increased to at least l = 30 pm. This would mean that (since composition is kept fixed) the material costs of the catalyst layer would increase as well by a factor of 3. It is thus obvious that a phase diagram of the catalyst layer is a powerful tool for a rational choice of the optimal thickness of the layer. 

Advanced design Control of the catalyst layer thickness and composition provide the means to optimize catalyst utilization and performance issues. Taking together all factors listed above, only 10-20% of the catalyst is effectively... 

Finally, the efficiency factor of the catalyst should be dose to unity, implying that there should be no diffusion limitation in the catalyst layer. Here porosity generally enhances the diffusion, which brings the optimization process back to the previous features. 

At this point, it is important to realize that the ultimate optimization target of electrode design is not Pt utilization, which is a static statistical property of a catalyst layer, but more importantly the effectiveness factor, which includes as well the effects of non-uniform reaction rate distributions due to mass transport phenomena at finite current densities in the operating fuel cell. In simple ID... 

Although the catalyst activity is the most important factor in improving cell performance of HT-PEMFC, the catalyst layer structure can also be optimized to increase the concentration of O2 in phosphoric acid near the catalyst sites. To establish a diffusion path for O2 in gas phase, two possible approaches can be taken. One is the dispersion of the hydrophobic binder such as PTFE within the catalyst layer. For the MEAs that contain the high content of phosphoric acid, it would be best to use the PTFE binder in the catalyst layer. The other way is to control the pore size within the catalyst layer so that the pores that are filled with phosphoric acid and pores that provide path for O2 diffusion in the gas phase can be separated according to the pore size. The piimaiy pores between catalyst particles are known to be filled with phosphoric acid, while the larger secondary pores between the agglomerates of catalyst particles provide O2 diffusion path [47]. If the O2 diffusion path within the catalyst layer can be established and maintained without the use of the hydrophobic binder which increases the ohmic resistance in MEAs [45], the cell performance of HT-PEMFC can be improved. 

Exploration of the pillar-clay sheet reactivity and connectivity also indicate the important role of the specific clay type. 27 1 and 29si-MASNMR experiments have shown distinctive differences between pillaring mechanisms in trioctahedral hectorite and dioctahedral montmorillonite. Whereas Plee et al. (22) concluded that chemical crosslinking may occur between the pillar and tetrahedral layer in a beidellite montmorillonite, Pinnavaia et al. (23) showed that it did not occur in a hectorite. These are the first observations of a complex process that may depend upon several structural and chemical factors, such as substitution of Al in the tetrahedral layer, or the need for vacancies in the octahedral layer to allow rotation of structural units or migration of reactant species to facilitate crosslinking. Ongoing research should further elucidate refinements on these mechanisms, and direct the technology towards more optimized catalysts - presumably those which form chemical bonds between the pillar and clay layer. 

Because the surface electronic processes play a fundamental role in catalytic activities, heterogeneous catalytic activity is determined primarily by the surface morphology and composition of the nanoparticle catalyst. The structure and composition of a few atomic layers below the surface play a secondary role, while the bulk of the catalyst remains a spectator of the catalytic activity. At the same time, cost considerations necessitate the optimization of dispersion and homogeneity of the catalytic sites, particularly when expensive noble metals are involved. Consequently, research towards the improvement of existing catalysts and the design of new ones focuses on two aspects tailoring of the surface structure, and minimizing the mass of the catalytically inert material. Therefore, catalyst fabrication techniques that allow control over those factors are desirable. 

When the substrate is first transported in a boundary layer surrounding the particle, before diffusing within the catalyst support where reaction occurs, external resistance needs to be considered (Calabro et al, 2008 Truskey et al., 2004). An example is the case of a packed bed bioreactor, where fluid-dynamics play a significant role in the optimization of system performances. In such a case the kinetic contribution has to be expressed in terms of overall effectiveness factor ri y. To estimate it, the mass balance. Equation [1.29], has to be solved by imposing the continuity of mass flux at the wall. For a flat-sheet support it corresponds to ... 

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