Ionomer phase

Microstructures of CLs vary depending on applicable solvenf, particle sizes of primary carbon powders, ionomer cluster size, temperafure, wetting properties of carbon materials, and composition of the CL ink. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules, which control the catalyst layer formation process. The choice of a dispersion medium determines whefher fhe ionomer is to be found in solubilized, colloidal, or precipitated forms. This influences fhe microsfrucfure and fhe pore size disfribution of the CL. i It is vital to understand the conditions under which the ionomer is able to penetrate into primary pores inside agglomerates. Another challenge is to characterize the structure of the ionomer phase in the secondary void spaces between agglomerates and obtain the effective proton conductivity of the layer. 

Table 2. Oxygen concentrations and oxygen diffusion coefficients in aqueous acid, recast ionomer, and bulk ionomer phases [1].

The performance of perfluorooctane (CgF g) Coated electrodes which were prepared by the method described above were evaluated in half-cell experiments for their oxygen reduction activity. Although perfluorooctane has a, relatively low boiling point (- 100 0) and evaporative losses may occur, as well as losses due to the supply stream of oxygen carrying material away, it is believed that some of the perfluorocarbon is persistent due to adsorption phenomena with the carbon substrate, electrocatalyst, and/or ionomer phase. As shown in Fig. 1.23, it is evident that there is some effect upon the oxygen reduction characteristics when perfluorooctane is introduced as an electrode coating and evaluated under room temperature conditions. When the activity of treated and untreated Nafion -coated electrodes are compared at room... 

Neutralizing the acid groups in the dispersion leads to formation of an ionomer membrane in the latex film [63]. As shown in Figure 14.24, this ionomer phase has a profound effect on slowing down the interdifiusion and on broadening the distribution of diffusion rates. In spite of the complexity of the system, there is a quite simple explanation for these effects. The shell polymer has a higher Tg than that of the core. The Tg of the shell phase depends upon composition the... 

Figure 13.5 Connolly surfaces, taken from MD simulations by Jang et al. [73], showing the regions of hydrophilic and hydrophobic interface between the water and ionomer phases in polymeric models for Nafion with different monomeric sequences (a) random copolymer and (b) diblock copolymer.

The catalyst layer must also maintain open porosity in order to homogeneously supply reactants to the reaction sites. Therefore, additives in the catalyst layer can be used to maintain open gas transport pores by creating hydrophobic zones as well as additives preventing the radical attack to the ionomer phase. Stability of the ionomer and the other additives in the electrolyte layer are important for the overall endurance of the MEA. The ratio of catalyst mass to ionomer and additive mass requires a careful multi parameter optimization and is depending on the overall amount of platinum present in the catalyst layer and the ratio of noble metal to support material in the catalyst powder [54, 55]. 

In addition, water is transported through both void space and ionomer phase consequently, the effective diffusion coefficient can be defined as [30]... 

To describe the time-dependent behavior, a transient term should be added to the governing equations, as shown in Eq. (31.1), to account for the storage rates of mass, momentum, species, energy, and charges. In the catalyst layer, both the ionomer phase and void space can hold water, hence an effective factor, in Eq. (31.1) can be introduced to simplify the model expression [16] ... 

In the catalyst layer, the presence of liquid water also complicates the mass transport processes. On the one hand, the liquid may fill part of the pore space, affecting the diffusional transport of the reactants in the pores. On the other hand, the relative humidity also strongly affects the ionic conductivity of the ionomer phase. As the humidity and/or presence of liquid water strongly varies with temperature and current density, the transport properties for gas and proton transport change significantly during operation. 

The catalyst layer (CL) is also a porous structure. However, the pore size is much smaller (between 10 and few hundreds of nanometers) as compared to tens of micrometers in the GDL. Further, because the electrochemical conversion occurs here, charge is transported by both protons and electrons and the electrochemically interlinked charge transport becomes more complicated than the one in the GDL. Figure 4 shows a small schematic cutout of the CL showing the tortuous arrangement of the electronically (carbon) and ion-conducting (ionomer) phases. Reactants are predominantly transported in the pore space (void). 

In the catalyst layer educts and water are transported in the void and the ionomer phases. Again, because of the porous and tortuous... 

The proton resistance of the catalyst layer can be reduced by adding more ionomer phase or low EW ionomer to the system, but this is outweighed by reduced oxygen transport. As gas phase diffusion is several orders of magnitude faster than diffusion through liquid water, it is essential to create a system that does not completely fill with liquid water, that is, contains fairly hydrophobic materials and not too small pore sizes. The state-of-the-art carbon blacks do not seem to meet this criterion. CNT or other more graphitic structures seem better suited. Also alternative supports should be selected with their potential for improved mass transport. Oxides may be less suitable in that respect but this certainly requires... 

Electrochemical studies on Pt thin films, Pt wires, and Pt nanoparticles on carbon have shown that at potentials <0.85 V, the dissolution process has a Nemstian potential dependence and that the equilibrium electrochemical potential of the ft dissolution decreases with particle size [116]. Accordingly, higher equilibrium Pt concentrations were measured for Pt/C than predicted for bulk Pt [111] at 80°C. According to mechanism 1, Pt " dissolved into the ionomer phase can redeposit on other (larger) particles in the cathode, due to the higher equilibrium potential of dissolution on these particles [117]. These particles need to be on the carbon... 

The loss of the alloyed non-noble metal does not always lead to a decrease of activity. As already mentioned, in some cases, an increase of activity was found, ascribed to as surface roughening or electrOTiic effects [61]. On the other hand, in other cases, the activity decreases because the beneficial effect on the electronic structure of Pt is lost. In any case, metal ions leaching from the catalyst will have a negative effect on the membrane and ionomer phase in the electrode. 

Independent computational strategies are generally needed in order to simulate the structure-related properties of CLs at different length- and time-scales. For instance, the stmcture of the ionomer-phase in CLs cannot be straightforwardly inferred from that in membrane simulations, as there are distinct forces and correlations between Nafion, water, and carbon/Pt particles in CLs. 

On the cathode side of a PEFC under normal operating conditions, one is concerned about excessive amounts of liquid water and flooding of gaseous supply charmels due to a net electroosmotic flux through the PEM and the production of water. Under these conditions, it is reasonable to assume full hydration of the ionomer phase in the CCL. Proton conductivity of the layer is, thus, eonsidered constant. Due to the assumption of capillary equilibrium in pores, pore-filling is... 

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