Catalyst/hydrated membrane interface

The next question is which of the above-discussed scenarios is most Kkely to be found at the catalyst/hydrated membrane interface. It is generally accepted that the proton conductivity of the membrane depends on the characteristics of ionic clusters formed surrounding the polymer hydrophilic sites, both within the bulk polymeric structure and at the interface with the catalyst [79]. The ionic clusters located at the membrane/catalyst interface are the ones that close the circuit of this electrochemical system. That is, these ionic clusters act as bridges through which protons and other hydrophilic reactants and products may pass from the membrane to the catalyst surface and vice versa during fuel cell operation. To get some insights into the possible formation of ionic clusters, we have analyzed the conformation of a hydrated model nation membrane over Pt nanoparticles deposited on a carbon substrate via classical MD simulations [80] at various degrees of hydration. 

Figure 17.1. Carbon particles surrounded by a pol5imer electrolyte membrane. Inset the catalyst/hydrated pol5mer interface in the nanoscale regime.

Figure 4. Final snapshots at >, = 12.8 (a) hydrated Nafion (b) membrane/vapor interface (c) mem-brane/vapor/catalyst support interface (d) mem-brane/vapor/catalyst interface. In the bulk hydrated membrane we can find the nano segregation of the hydrophilic and hydrophobic domains. Wetting of the catalyst surface is observed while there is none on the catalyst support. Gray, CFx groups orange, sulfur red, oxygen atom of H2O or SO3 green, oxygen atom of HsO" white, hydrogen.

The problem we would like to address is schematically shown in Figure 17.1 A catalytic interface (inset) is surrounded by a proton-carrier hydrated membrane the reactants, intermediates, and products diffuse in and out to reach or leave the catalyst surface, and the substrate is the electronic conductor material which may also influence the reactive system given the nanoscale of the actual... 

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. 

These models were developed for single PEM fuel cell taking into account interfacial kinetics at the Pt/ionomer interface, gas-transport and ionic-conductivity limitations in the catalyst layer, and gas-transport limitations in the cathode backing. These models have been improved for dynamic simulation of fuel cell stack with additional feature on anode fuel flow and cathode flow, and membrane hydration air supply models (Pukrushpan, 2003). 

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