Atomistic MD Simulations of CLs

In MD simulations, the molecular adsorption concept is used to interpret the Pt-C interactions during the fabrication processes. The Pt complexes are mostly attached to the hydrophilic sites on carbon particles, viz. carbonyl or hydroxyl groups [108]. The adsorption is based on both physical and chemical adsorption. Carbon particle preparations, impregnation, and reduction are three main steps of the catalyst preparation. The point of zero charge (PZC) determines the pH range at which the impregnation step should be carried out. The PZC is an important parameter in catalyst preparation. 

Several studies have focused on extensive MD simulations of Pt nanoparticles adsorbed on carbon in the presence or absence of ionomers [109-113]. Lamas and Balbuena performed classical molecular dynamics simulations on a simple model for the interface between graphite-supported Pt nanoparticles and hydrated Nation [113]. In MD studies of CLs, the equilibrium shape and structure of Pt clusters are usually simulated using the embedded atom method (EAM). Semi-empirical potentials such as the many-body Sutton-Chen potential (SC) [114] are popular choices for the close-packed metal clusters. Such potential models include the effect of the local electron density to account for many-body terms. The SC potential for Pt-Pt and Pt-C interactions provides a reasonable description of the properties of small Pt clusters. The potential energy in the SC potential is expressed by 

Mesoscale Model of Self-Organization in Catalyst Layer Inks 

The remainder of this section describes a CGMD methodology used to unravel self-organization phenomena in the CL and to analyze their impact on physicochemical properties (Malek et al., 2007 Marrink et al., 2007). In particular, the focus will be on structure and distribution of ionomer. Moreover, it will explore the implications of ionomer morphology and porous structure on water distribution (wettability), Pt utihzation, and proton transport properties. Validation of the emerging structural picture by experimental data on adsorption and transport properties will be discussed briefly. 

The methodology for performing CGMD studies of self-organization in catalyst layer mixtures has been introduced by Malek et al., (2007 2011). 

The simulation protocol for the formation of Pt-decorated primary C particles (PPCs) mimics catalyst dispersions obtained by pertinent fabrication techniques. In practice, two methods are used to obtain PPCs (1) impregnation of carbon nanoparticles with Pt precursor or (2) adsorption of Pt oxide or Pt metal colloids onto the carbon surface (Antolini, 2003 Antolini et al., 2002). In the case of impregnation with Pt precursor, diffusion into the pores of each individual support particle can occur. For the second mechanism, colloidal Pt or Pt oxide particles adsorb on the external surface of the support particles as a result of size exclusion, the accessibility of the inner pores is limited and, therefore, Pt particles are mostly formed on the surface of CNPs. 

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