(148a) Transport and Reaction Processes in Non-Homogeneous Porous Media for Fuel Cell Applications

The key technological issues in polymer electrolyte fuel cell (PEFC) technology stem from the performance inefficiencies within its membrane electrode assembly (MEA). A typical MEA comprises of the gas diffusion layer (GDL), the catalyst layer (CL), and the polymer electrolyte membrane, which are heterogeneous porous material sub-components, and through which heat & mass transport processes occur with or without electrochemical reaction. For overcoming the performance limitations within MEA, a detailed understanding of these transport processes is required. Traditional non-homogeneous media theories [1-3] estimate the effective molecular transport coefficients (without fluid flow information) through averaged phase hold-up and/or tortuosity parameters, therefore, do not account for the microscopic structural information within the media.

We recently adopted an agglomerate CL model, which captures the essential transport processes and accounts for species, proton, and electron transports within the GDL/CL sub-components as well as reaction/transport processes within the CL, and incorporated it into our in house state-of-the-art interior point optimization algorithm, IPOPT. [4] We examined the effect of various agglomerate shapes (sphere, cylinder, and plate) on simulations and optimizations via introducing agglomerate effectiveness factor and effective transport properties. We also investigated the agglomerate size effect via scaling analysis and obtained up to three-fold enhancement in the overall current density as the agglomerate size decreased by ten times.

We further incorporated mesoscopic structural information for estimating transport properties in non-homogeneous media via lattice Boltzmann methods (LBMs). The advantages of clear physical pictures, geometric flexibility, an inherently transient nature, and fully parallel algorithms, make LBM to be an attractive tool for multi-phase, multi-scale simulation for PEFC devices.[5-7] LBM has been employed previously for simulating individual transport processes (momentum, energy, or mass) however, simulating combination of these phenomena, present in a PEFC MEA requires a clever modification in the LBM scheme via equilibrium distribution function or relaxation time. We first developed the standard LBM with Bhatnagar-Gross-Krook approximation for porous media, and verified with the Maxwell-Eucken formulation, and accounted for local structural information via correlation functions. Prior to the real application to a PEFC MEA, we extend our LBM scheme to systematically estimate the effective transport properties in non-homogeneous media especially including fluid flow.

Our methodology will be used for addressing PEFC sub-component level issues through meso-/micro-scopic modeling, and is planned for integration with entire PEFC system with optimization.

References:

1. J. C. Maxwell, A Treatise on Electricity and Magnetism (Clarendon, Oxford, 1881), Vol. 1, p. 435.

2. D.A.G. Brugemann, Ann. Phys.(5) 24, 637 (1935).

3. Jianfeng Wang, James K. Carson, Mike F. North, Donald J. Cleland, International Journal of Heat and Mass Transfer 49 (2006) 3075?3083

4. P. Jain, L.T. Biegler, and M.S. Jhon, Electrochem Solid-State Lett, 11 (10), B193 (2008).

5. W.T. Kim, S.S. Ghai, Y. Zhou, I. Staroselsky, H.D. Chen, and M.S. Jhon, IEEE Trans. Magn., 41, 3016 (2005).

6. W.T. Kim, M.S. Jhon, Y. Zhou, I. Staroselsky, and H.D. Chen, J. Appl. Phys., 97, 10P304 (2005).

7. Y.S. Xu, Y. Liu, X.Z. Xu, and G.X. Huang, J. Electrochem. Soc. , 153, A607 (2006).