Catalyst layer self-organization

At the mesoscopic scale, interactions between molecular components in membranes and catalyst layers control the self-organization into nanophase-segregated media, structural correlations, and adhesion properties of phase domains. Such complex processes can be studied by various theoretical tools and simulation techniques (e.g., by coarse-grained molecular dynamics simulations). Complex morphologies of the emerging media can be related to effective physicochemical properties that characterize transport and reaction at the macroscopic scale, using concepts from the theory of random heterogeneous media and percolation theory. 

For typical catalyst layers impregnated with ionomer, sizes of hydrated ionomer domains that form during self-organization are of the order of 10 nm. The random distribution and tortuosity of ionomer domains and pores in catalyst layers require more complex approaches to account properly for bulk water transport and interfacial vaporization exchange. A useful approach for studying vaporization exchange in catalyst layers could be to exploit the analogy to electrical random resistor networks of... 

This section provides a comprehensive overview of recent efforts in physical theory, molecular modeling, and performance modeling of CLs in PEFCs. Our major focus will be on state-of-the-art CLs that contain Pt nanoparticle electrocatalysts, a porous carbonaceous substrate, and an embedded network of interconnected ionomer domains as the main constituents. The section starts with a general discussion of structure and processes in catalyst layers and how they transpire in the evaluation of performance. Thereafter, aspects related to self-organization phenomena in catalyst layer inks during fabrication will be discussed. These phenomena determine the effective properties for transport and electrocatalytic activity. Finally, physical models of catalyst layer operation will be reviewed that relate structure, processes, and operating conditions to performance. 

Mesoscale Simulations of Self-Organization in Catalyst Layers... 

Surface reconstruction is driven by stabilization of the adsorbate after adsorption of carbon atoms on more reactive surface atoms. Ciobica et al. (74) demonstrated that an overlayer of Cads leads to the Co(lll) to Co(lOO) reconstruction on fee cobalt (the stable phase of small cobalt particles). Because of the change in metal atom density in the surface layer, the reconstruction may be associated with faceting and hence creation of step-edge sites, which are highly active in the Fischer-Tropsch reaction (5). Hence, surface reconstruction and formation of a stable carbide overlayer may actually be the processes occurring during the initial activation of the catalyst. This phenomenon has been described by Schulz (101) as self-organization. 

Atomistic and coarse-grained MD simulations of self-organization in catalyst layers suggest that the resulting structures are inherently unstable. Applicable solvents with different dielectric constants correspond to different stable conformations in terms of agglomerate sizes, sizes of ionomer domains, pore space... 

Malek K, Eikerling M, Wang Q, Navessin T, Liu Z. Self-organization in catalyst layers of polymer electrolyte fuel cells. J Phys Chem C 2007 111 13627-34. 

A catalyst ink is prepared by mixing the catalyst powder that consists of primary Pt/C particles in dispersion media that are mixtures of water, alcohols, or other organic compounds (Xie et al., 2008). lonomer is added until a desired ionomer-to-carbon mass ratio is reached. For a review of catalyst layer fabrication approaches, see Chapter 19 in Zhang (2008). The added ionomer self-assembles into a separate interconnected phase in the pore space, primarily in secondary pores. The final CL structure depends on materials used, ink composition, dispersion medium, fabrication conditions, and the protocol of MEA fabrication and drying. 

In the section Structure Eormation in Catalyst Layers and Effective Properties aspects related to the self-organization phenomena in CL inks will be discussed. These phenomena determine the effective properties for transport and electrocatalytic activity. Thereafter, catalyst layer performance models that involve parameters related to structure, processes, and operating conditions will be presented. 

These parameters and conditions determine complex interactions between Pt nanoparticles, carbon support, ionomer molecules, and solvent, which control the catalyst layer formation process. Self-organization of ionomer and carbon/Pt in the colloidal ink leads to the formation of phase-segregated and agglomerated morphologies. The choice of a dispersion medium determines whether ionomer exists in solubilized, colloidal, or precipitated form. This influences the microstructure and pore size distribution of the CL (Uchida et al., 1996). It is believed that mixing of ionomer with dispersed Pt/C catalysts in the ink suspension, prior to deposition to form a CL, enhances the interfacial area of Pt with water in pores and with Nation ionomer. 

Coarse-grained molecular dynamics (CGMD) simulations have become a viable tool to unravel self-organization phenomena in complex materials and to analyze then-impact on physicochemical properties (Malek et al., 2007 Marrink et al., 2007 Peter and Kremer, 2009). Various MD simulations to study microstructure formation in catalyst layers will be discussed. The impact of structures obtained on pore surface wettability, water distribution, proton density distribution, and Pt effectiveness will be evaluated. 

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

CGMD simulations have become a viable tool in studying self-organization processes in catalyst layers of PEFCs. Stmctural parameters of interest for such studies involve composition and size distributions of Pt/C agglomerates, pore space morphology, surface wettability, as well as the structure and distribution of ionomer. The latter aspect has important implications for electrochemically active area, proton transport properties, and net electrocatalytic activity of the CL. 

Simulations of physical properties of realistic Pt/support nanoparticle systems can provide interaction parameters that are used by molecular-level simulations of self-organization in CL inks. Coarse-grained MD studies presented in the section Mesoscale Model of Self-Organization in Catalyst Layer Inks provide vital insights on structure formation. Information on agglomerate formation, pore space morphology, ionomer structure and distribution, and wettability of pores serves as input for parameterizations of structure-dependent physical properties, discussed in the section Effective Catalyst Layer Properties From Percolation Theory. CGMD studies can be applied to study the impact of modifications in chemical properties of materials and ink composition on physical properties and stability of CLs. 

This chapter focuses on steady-state phenomena during electrode operation. The structure that is formed during self-organization in catalyst layer inks and during MEA fabrication is considered to be constant. Unless stated otherwise, one-dimensional modeling approaches will be explored. This means that species transport occurs in through-plane direction only, perpendicular to the electrode plane. Moreover, in most part of this chapter, conditions are assumed to be isothermal. 

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