Size-selective electrocatalytic

Electrocatalytic effects eventually exerted by zeolite-attached species on selected processes also become significantly dependent on the size of electrolyte cations. 

The purpose of this chapter is to selectively summarize recent advances in the molecular modeling of anode and cathode electrocatalytic reactions employing different computational approaches, ranging from first-principles quantum-chemical calculations (based on density functional theory, DFT), ab initio and classical molecular dynamics simulations to kinetic Monte Carlo simulations. Each of these techniques is associated with a proper system size and timescale that can be adequately treated and will therefore focus on different aspects of the reactive system under consideration. 

Since the rate of all electrocatalytic reactions is strictly related to the active surface area, besides the surface chemistry, the morphology of the electrocatalyst needs to be tailored. Morphology is not only related to the metal-phase area but also to the presence of micro- and macro pores in the electrocatalyst support that could facilitate or hinder the mass transport properties. All these characteristics determine the cell performance even if the relative influence of each parameter is still not known in detail. It is thus necessary to select appropriate procedures for the optimization of these characteristics, i.e. composition, structure, particle size, porosity, etc. Generally a combination of physico-chemical and electrochemical analyses carried out on different electrocatalysts indicates the system that best suits the scope of application in a DMFC. 

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. 

This effect is even more evident for the larger molecule, 2-propanol, which yielded an active Pt atom ratio of 0.1, even for 400 nm X 3 p,m/Pt. These results clearly indicate an effect of molecular size for the honeycomh/Pt electrodes for the catalytic oxidation of alcohols. The electrocatalytic activities of the Pt-modified nanohoneycomh films were found to he dependent on the structural parameters of the honeycomb pores and the molecular sizes of the alcohols, indicating that the selectivity of the electrodes can be controlled by variation of the pore dimensions. 

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