Modeling of Self-Organization in PEMs

Basic requirements on feasible systems and approaches for computational modeling of fuel cell materials are (i) the computational approach must be consistent with fundamental physical principles, that is, it must obey the laws of thermodynamics, statistical mechanics, electrodynamics, classical mechanics, and quantum mechanics (ii) the structural model must provide a sufficiently detailed representation of the real system it must include the appropriate set of species and represent the composition of interest, specified in terms of mass or volume fractions of components (iii) asymptotic limits, corresponding to uniform and pure phases of system components, as well as basic thermodynamic and kinetic properties must be reproduced, for example, density, viscosity, dielectric properties, self-diffusion coefficients, and correlation functions (iv) the simulation must be able to treat systems of sufficient size and simulation time in order to provide meaningful results for properties of interest and (v) the main results of a simulation must be consistent with experimental findings on structure and transport properties. 

Requirement (i) must be enforced in a rigorous fashion. The only possible relief is that in some cases the laws of classical mechanics suffice to establish a consistent approach, rendering computationally intensive quantum mechanical approaches obsolete. Other requirements on the above list offer room for adaptation to peculiar simulation objectives, priorities in terms of physical properties of interest and structural system specifications. 

Studies of proton transport in PEMs or at interfaces, as well as studies of processes at the electrified interface, usually demand quantum mechanical simulations to incorporate electronic structure effects and hydrogen bond dynamics. Studies of structure formation and transport properties in heterogeneous media demand computationally efficient algorithms that enable simulations of sufficient length ( 20 nm) and time 

At least three major scales must be distinguished in simulations of heterogeneous media (i) the atomistic scale, required to account for electronic structure effects in catalytic systems or for molecular and hydrogen bond fluctuations that govern the transport of protons and water (ii) the scale of the electrochemical double layer, ranging from several A to a few nm at this level, simulations should account for potential and ion distributions in the metal-electrolyte interfacial region and (iii) the scale of about 10 nm to 1 pm, to describe transport and reaction in heterogeneous media as a function of composition and porous structure. 

Computational approaches that have been employed to understand the structure and transport properties of water and protons in swollen Nation membranes include ab initio (Eikerling et al., 2003 Roudgar et al., 2006, 2008 Vartak et al., 2013), classical all-atom (Cui et al., 2007 Devanathan et al., 2007a,b Goddard et al., 2006 Spohr et al., 2002 Vishnyakov and Neimark, 2000, 2001), and coarse-grained descriptions (Galperin and Khokhlov, 2006 Wescott et al., 2006) of the system. 

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