Catalyst layers uniformity

Fig. 7 Schematic of proton transport in and through Nafion membranes. Protons are generated by hydrogen oxidation on a catalyst surface. The protons move laterally in the catalyst layer (uniform potential field) until they find an opening into the hydrophilic domains of the Nafion. The protons are transported across the membrane by electric field-assisted motion. At low applied potentials, the transport across the membrane is limiting, but at high applied potentials, the proton motion is limited by diffusion in the catalyst layer to the openings of the hydrophilic domains...

Another approach has been developed to fabricate electrodes with loading as low as 0.1 mg Pt/cm (32). The electrode structure was improved by increasing the contact area between the electrolyte and the platinum clusters. The advantages of this approach are that a thinner catalyst layer of 2 to 3 microns and a uniform mix of catalyst and ionomer are produced. For example, a cell with a Pt loading of 0.17 to 0.13 mg/cm has been fabricated. The cell generated 3 A/cm at > 0.4V on pressurized O2 and 0.65 V at 1 A/cm on pressurized air (32,... 

To overcome these disadvantages, a thin-film CL technique was invented, which remains the most commonly used method in PEM fuel cells. Thin-film catalyst layers were initially used in the early 1990s by Los Alamos National Laboratory [6], Ballard, and Johnson-Matthey [7,8]. A thin-film catalyst layer is prepared from catalyst ink, consisting of uniformly distributed ionomer and catalyst. In these thin-film catalyst layers, the binding material is not PTFE but rather hydrophilic Nafion ionomer, which also provides proton conductive paths for the electrochemical reactions. It has been found that the presence of hydrophobic PTFE in thin catalyst layers was not beneficial to fuel cell performance [9]. 

Saha et al. [109] have proposed an improved ion deposition methodology based on a dual ion-beam assisted deposition (dual IBAD) method. Dual IBAD combines physical vapor deposition (PVD) with ion-beam bombardment. The unique feature of dual IBAD is that the ion bombardment can impart substantial energy to the coating and coating/substrate interface, which could be employed to control film properties such as uniformity, density, and morphology. Using the dual lABD method, an ultralow, pure Ft-based catalyst layer (0.04-0.12 mg Ft/cm ) can be prepared on the surface of a GDL substrate, with film thicknesses in the range of 250-750 A. The main drawback is that the fuel cell performance of such a CL is much lower than that of conventional ink-based catalyst layers. Further improvement... 

The microstructure of a catalyst layer is mainly determined by its composition and the fabrication method. Many attempts have been made to optimize pore size, pore distribution, and pore structure for better mass transport. Liu and Wang [141] found that a CL structure with a higher porosity near the GDL was beneficial for O2 transport and water removal. A CL with a stepwise porosity distribution, a higher porosity near the GDL, and a lower porosity near the membrane could perform better than one with a uniform porosity distribution. This pore structure led to better O2 distribution in the GL and extended the reaction zone toward the GDL side. The position of macropores also played an important role in proton conduction and oxygen transport within the CL, due to favorable proton and oxygen concentration conduction profiles. 

The macrohomogeneous model was exploited in optimization studies of the catalyst layer composition. The theory of composifion-dependent performance reproduces experimental findings very well. - The value of the mass fraction of ionomer that gives the highest voltage efficiency for a CCL with uniform composition depends on the current density range. At intermediate current densities, 0.5 A cm < jo < 1.2 A cm , the best performance is obtained with 35 wt%. The effect of fhe Nation weight fraction on performance predicted by the model is consistent with the experimental trends observed by Passalacqua et al. ... 

The model was also used to explore novel design ideas. It was predicted and subsequently confirmed in experiment that functionally graded layers result in improved performance compared to standard CCLs with uniform composifion.243 244 design, the catalyst layer is fabricated as a sublayer... 

Overall, the interface models are basically 0-D. They assume that all of the relevant variables in the catalyst layers are uniform in their values across the layer. This has some justification in that the catalyst layers are very thin, and it is adequate if other effects that are modeled are more significant however, the catalyst layers should be modeled in more detail to ensure that all the relevant interactions are accounted for and to permit optimization of such parameters as catalyst loading. 

The next set of models treats the catalyst layers using the complete simple porous-electrode modeling approach described above. Thus, the catalyst layers have a finite thickness, and all of the variables are determined as per Table 1 with a length scale of the catalyst layer. While some of these models assume that the gas-phase reactant concentration is uniform in the catalyst layers,most allow for diffusion to occur in the gas phase. 

As mentioned, the reaction distribution is the main effect on the catalyst-layer scale. Because of the facile kinetics (i.e., low charge-transfer resistance) compared to the ionic resistance of proton movement for the HOR, the reaction distribution in the anode is a relatively sharp front next to the membrane. This can be seen in analyzing Figure 10, and it means that the catalyst layer should be relatively thin in order to utilize the most catalyst and increase the efficiency of the electrode. It also means that treating the anode catalyst layer as an interface is valid. On the other hand, the charge-transfer resistance for the ORR is relatively high, and thus, the reaction distribution is basically uniform across the cathode. This means... 

If external mass-transfer limitations can be neglected, then the surface concentration in eq 58 (via eq 13) can be set equal to the bulk concentration, which is assumed uniform throughout the catalyst layer in the simple agglomerate models. Otherwise, the surface concentration is unknown and must be... 

Figure 13. Plot of cathode potential as a function of current density for a macrohomogeneous embedded model where the proton conductivity is assumed to be uniform (0.044 S/m), curve a, or varies with water production (changing humidity) across the catalyst layer, curve b. (Reproduced with permission from ref 98. Copyright 2002 The Electrochemical Society, Inc.)...

Diffusion medium properties for the PEFC system were most recently reviewed by Mathias et al. The primary purpose of a diffusion medium or gas diffusion layer (GDL) is to provide lateral current collection from the catalyst layer to the current collecting lands as well as uniform gas distribution to the catalyst layer through diffusion. It must also facilitate the transport of water out of the catalyst layer. The latter function is usually fulfilled by adding a coating of hydrophobic polymer such as poly(tet-rafluoroethylene) (PTFE) to the GDL. The hydrophobic polymer allows the excess water in the cathode catalyst layer to be expelled from the cell by gas flow in the channels, thereby alleviating flooding. It is known that the electric conductivity of GDL is... 

The substrate was usual 1.1 mm glass with sputtered aluminium layer. Ni was used as a catalyst. For optimization of cathode surface topography, three types of samples were prepared, with different catalyst distribution on the substrate. Catalyst layer in the first sample was uniform sputtered Ni layer. In the second and the third samples, the catalyst sputtering was produced through screens with 1 mm h 50 pm holes correspondingly. The distance between holes compared with the twice island diameter. The total area of each cathode sample was about 0.5 cm2. Cathodes are shown on Fig. 1. 

Equivalent circuits for the catalyst layer are similar to those for porous electrodes, where charge-transfer resistance, capacitance, and Warburg resistance should be considered. The catalyst layer can be conceived of as a whole uniform unit or as a non-uniform circuit. In the case of a uniform unit, the equivalent circuits are similar to the modified ones discussed in Section 4.2.2 2, and the equations in that section apply. In many cases, such as in the presence of adsorbents, the surface is covered by the adsorbed species. For example, in direct methanol fuel cells and in H2/air fuel cells, CO adsorption should be considered. One example is illustrated in Ciureanu s work [7], as shown in Figure 4.31. 

It is known that the catalyst layer is far from uniform, especially in the case of a gradient catalyst layer. Thus, profiling properties, such as conductivity, in the catalyst layer are important. Both an electronic conductor (carbon) and an ionic conductor (Nafion ) exist in the catalyst layer, which can be considered a conductive polymer. The conductive polymer electric circuit model has been applied to the catalyst layer, and an ionic conductivity profile was obtained [8], as shown in Figure 4.33. 

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