Ionomers catalyst interaction with

The reaction reduces the proton concentration and thus the proton conductivity of the ionomer in both the PEM and the catalyst layers. Metal ions will do similar harm, and the impact of highly charged cations such as Fe + is most harmful. It not only replaces H+ but may also restructure the hydrophilic domains of the ionomer (e.g., crosslinking the side chains by ionically interacting with the -SO3 groups). Obviously, the impact caused by cations is cumulative. 

In addition to the effect of support on the catalyst dispersion, the interaction with the ionomer (Nafion ) is crucial for the electrochemical performance of the catalyst layer in the fuel cell. Catalyst nanoparticles that are isolated from the ionomer network are electrochemically inactive. Furthermore, the distribution of the ionomer will affect the ohmic resistance and the mass transport of the reactants and/or products in the catalyst layer. Hence, the interface between the catalyst/support/ionomer will influence the overall polarization behavior of the anode. 

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. 

NH3 in H2 primarily interacts with the polymer electrolyte membrane and ionomer in the catalyst layer (Uribe et al, 2002). The influence of NH3 on the membrane performance will be considered in more detail in Section 8.3.3 dedicated to membrane contamination. The effect of NHj on HOR is not completely understood. Different reports show that NHj mayor may not strongly adsorb on the Pt catalyst and disrupt HOR (Uribe et al., 2002 Zhang et al, 2009). This disagreement is likely due to different compositions of anode catalyst layers in Uribe et al. (2002) and Zhang et al. (2009). 

Coarse-grained molecular d5mamics simulations in the presence of solvent provide insights into the effect of dispersion medium on microstructural properties of the catalyst layer. To explore the interaction of Nation and solvent in the catalyst ink mixture, simulations were performed in the presence of carbon/Pt particles, water, implicit polar solvent (with different dielectric constant e), and ionomer. Malek et al. developed the computational approach based on CGMD simulations in two steps. In the first step, groups of atoms of the distinct components were replaced by spherical beads with predefined subnanoscopic length scale. In the second step, parameters of renormalized interaction energies between the distinct beads were specified. 

The present chapter has presented a comprehensive review of electrode kinetic and catalytic aspects associated with methanol, ethanol, and formic acid oxidation. The prevalent point of view in selecting and organizing the vast amount of information in this area was that of practical applicability in order to advance the technology of direct fuel cells. Emphasis was placed on the catalytic system , starting with catalyst preparation methods and focusing on the interaction of catalyst/support/ionomer/chemical species. The development of catalytic systems was followed, from fundamental electrochemical and surface science studies to fuel cell experiments (whenever experimental data was available). Advances in both fundamental electrocatalysis and electrochemical engineering hold promise for the development of high-performance and cost-effective direct liquid fuel cells. 

To improve the structure-dynamics relationships of CLs, the effects of applicable solvents, particle sizes of primary carbon powders, wetting properties of carbon materials, and composition of the catalyst layer ink should be explored. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules and, therefore, control the catalyst layer formation process. Mixing the ionomer with dispersed Pt/C catalysts in the ink suspension prior to deposition will increase the interfacial area between ionomer and Pt/C nanoparticles. The choice of a dispersion medium determines whether ionomer is to be found in the solubilized, colloidal, or precipitated forms. 

St-Pierre (2009) developed a zero-dimensional model that considers competitive adsorption for a contaminant with O2 or H2 at the cathode or anode side, respectively. This model assumes that contaminant transport through the gas flow channels, GDLs and ionomer in the catalyst layers is much faster compared to surface kinetics. The rate determining step is considered to be due to contaminant reaction or desorption of reaction product from the platinum surface. Other model assumptions include the absence of lateral interaction between adsorbates, first-order reaction kinetics, constant pressure, and constant temperature at the cathode/anode sides. Using a set of parameters, St-Pierre (2009) successfully used his model in order to describe experimental transient data obtained in the presence of SOj, NOj, and HjS in the cathode airstreams. 

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