(723c) Transport Phenomena In Polymer Electrolyte Membranes Using Molecular Dynamics

Authors: 
Chung, P. S., Carnegie Mellon University
Biegler, L. T., Carnegie Mellon University
Jhon, M. S., Carnegie Mellon University


Recently, fuel cells have gained considerable attention as an alternative energy source exhibiting attractive features of higher efficiency (more than twice the internal combustion engines), near zero greenhouse emissions, as well as simple and modular design. Amongst the various fuel cells, polymer electrolyte membranes PEM fuel cells (PEMFCs) are especially promising due to their higher efficiencies, low temperature of operation leading to short start-up times, and various applications including automobiles and portable electronics. Here PEM, which plays a critical role in determining fuel cell cost and performance, has been examined via molecular dynamics (MD) simulations.

Based on our previous studies [1, 2], Nafion® molecules were modeled via a coarse-grained, bead-spring model, which neglects the detailed atomic structural information while keeping the essence of the molecular structure. Lennard Jones and finitely extensible nonlinear elastic models were used to describe potential energies among the beads in the system. In addition, to examine the water uptake in PEM, long-range Coulombic interactions between water molecules and the beads were explicitly incorporated in our model. With the total potential energy constructed, MD simulations were performed using the Langevin equation.

Using these simulations, we examined the static and dynamic properties of PEM to obtain a relationship between the PEM nano/micro-structure, its water uptake, and the macroscopic properties such as the conductivity and water diffusion. In order to examine the PEM swelling mechanism and its effect, we calculated PEM density and the radius of gyration of the molecules for different amounts of water uptake. We also investigated the transport processes of protons and water molecules under various water uptake conditions. Specifically, we calculated the self-diffusion coefficient of protons to examine the diffusion process, and the proton conductivity as a function of water uptake. In addition, different types of PEMs were examined by modifying the molecular structure.

The findings from this study, which relate the PEM diffusion coefficient to its structure and composition, can serve to model the non-homogeneity in PEM diffusion processes. This information can be employed to build nano/microscopic models for the PEM water management, as well as to construct an integrated modeling and optimization framework for PEMFCs [3].

References:

1. Q. Guo, H. G. Chen, B. C. Smith, and M. S. Jhon, J. Appl. Phys., 97, 10P301 (2005).

2. Q. Guo, P. S. Chung, H. G. Chen, and M. S. Jhon, J. Appl. Phys., 99, 08N105 (2006).

3. P. Jain, L.T. Biegler, and M.S. Jhon, ?Parametric Study and Estimation in CFD-based PEM Fuel Cell Models,? AIChE J., to appear (2008).