(628h) Atomistic/Molecular Scale Modeling of Polymer Electrolyte Membrane Fuel Cells

            The polymer electrolyte membrane fuel cell (PEMFC) is a mult-physics, mult-scale system with phenomena that occur on various time/length scales.  The system is composed of anode and cathode gas channels (GC) through which fuel enters, gas diffusion layers (GDL) that create uniform gas distribution, catalyst layers where the electrochemical reactions take place, and the polymer electrolyte membrane (PEM) at the center of the device. Within each of the sub-components critical issues of multi-scale physical models must be addressed to make PEMFC viable for widespread commercialization.  Due to the lack of novel methodology to integrate phenomena on each sub-system, demand on the new paradigm of holistic multi-scale modeling becomes critical to resolve the issues. Here, we provide our approach to the PEMFC from atomic/molecular/meso scales within a PEM system.

            The PEM is the heart of the PEMFC system, through which protons are conducted from the anode side to the cathode side.  The complex structure of PEM composed of hydrated perfluorosulfonic acid (PFSA) ionomer enables proton diffusion while it controls fuel transfer through a membrane.  Microstructure of PEM has been investigated via classical molecular dynamics (MD), which exhibits hydrophilic phase agglomeration by sulfonate anion groups [1,2].  At the molecular scale, we examine ion conduction in the hydrated PFSA environment via molecular dynamics simulations.  The effect of factors such as temperature and water content are evaluated in terms of ion diffusion coefficient and membrane structure. 

           To understand molecular simulation fundamentals and the complex microstructure in the PEM, accurate intra- and intermolecular force field parameters for the PFSA, water, and proton species are required.  The atomistic scale study includes ab initio methods to determine accurate force field parameters for the PFSA ionomer’s intramolecular degrees of freedom and its interaction with water and ions. The approach of Seminaro [3] is used for the intramolecular parameterization, and intermolecular potential energies are fit to functional forms which accurately model van der Waals with partial charges accounting for electrostatics.  The force field is evaluated by comparing interaction energies and molecular geometry obtained using ab-initio calculations.  As the bottom most level of our multi-scale modeling approach [4], this force field analysis will be inputted into MD simulations via a systematic coarse graining procedure. The coarse graining procedure will allow for the establishment of a multi-scale integration framework suitable for our optimization model of the PEMFC [5], which ultimately addresses objectives such as lowering cost as well as raising power density and sustainability. 

1. A.Z. Weber and J. Newman, “Modeling Transport in Polymer-electrolyte Fuel Cells,“ Chem. Rev., 104, 4679-4726 (2004).

2. R. Jinnouch and K. Okazakiz, “Molecular Dynamics Study of Transport Phenomena in Perfluorosulfonate Ionomer Membranes for Polymer Electrolyte Fuel Cells,” J. Elec. Soc., 150, E66-E73 (2003).

3. J. Seminario, "Calculation of Intramolecular Force Fields from Second-Derivative Tensors" Int. J. Quantum Chem., 30, 1271-1277 (1996).

4. Kim, D; Chung P.S.; Jain, P.; Vemuri, S.H.; Jhon, M.S., “Multiscale Modeling of Head Disk Interface,” IEEE Trans. Magn., 46 (published on June 2010).

5. Jain, P.; Biegler, L.T.; Jhon M.S., “Parametric Study and Estimation in CFD-based PEM Fuel Cell Models,” AIChE Journal, 54, 2089-2100 (2008).