Membrane Electrode Assembly Model Research Articles

Computational Modelling of Cation Contamination in PEFCs

Polymer electrolyte membrane (PEM) is a key component of polymer electrolyte membrane fuel cell (PEFC). It is vulnerable to cationic impurities that are present in the air and fuel stream or that may have originated from the cell, stack or balance of plant components. Almost all foreign cations have higher affinity to sulfonic acid group than proton and the presence of foreign cation in the membrane or catalyst layers results in reduced ionic conductivity, increased electro-osmotic drag, and reduced water content (1).In this study, a steady, one dimensional cation (Na+) contamination model of membrane electrode assembly (MEA) of PEFC (shown in figure 1) is developed by solving the coupled transport phenomena related to the foreign cation, water, and oxygen. Concentrated solution theory is utilized to solve the cation transport equation based on Serincan et al.’s model (2) and dilute solution theory for water, hydrogen, and oxygen transport. A spherical agglomerate model is considered in the cathode catalyst layer (CCL) to describe effects of foreign cations in the catalyst layer. To separate the effects of cations on different transport phenomena, first no contamination case is solved and then contamination effects are added one by one to the cation transport, water transport, and oxygen transport and finally to the Nernst equation.Cation (Na+) contamination in the air stream found to cause several detrimental effects on the performance of PEFC. The main effects are: (a) cation replaces proton and occupies sulfonic acid sites in ionomer. It can occupy up to 96% sites in the cathode catalyst layer (CCL); (b) It causes serious water depletion in the ionomer due to decrease in water diffusivity; (c) It decreases the oxygen diffusivity causing a decrease in dissolved oxygen concentration in CCL; and (d) It decreases the open circuit voltage (OCV) in CCL due to reduction in proton concentration in the ionomer phase. These contamination effects on each parameter decrease the protonic current density and combined effects result in a reduction of current density from 0.72 A/cm2 to 0.25 A/cm2at a cell voltage of 0.7 V (shown in figure 2). Acknowledgements The authors are grateful for the financial support of the Department of Energy (DOE)-EERE, DE-EE00000467 (University of Hawaii, prime contractor, Jean St-Pierre, PI).

Characterization of the PEM Fuel Cell Catalyst Layer Microstructure by Nonlinear Least-Squares Parameter Estimation

Models of polymer electrolyte membrane fuel cells (PEMFC) have become increasingly complex, using many parameters to define the behavior of species and the performance of the cell. A framework is presented here to couple an agglomerate electrode based, multi-dimensional, multi-physics mathematical model of membrane electrode assembly (MEA) model with an optimization-based nonlinear least-squares parameter estimation algorithm. The framework is used to estimate the micro-structural parameters of the catalyst layer such as agglomerate size. The results show that a set of data over a range of operating conditions can be accurately described by using a unique set of structural parameters that match experimental visualization of the catalyst layer. Extension of this methodology can be used to systematically estimate any model parameters in order to reduce uncertainty in model predictions.

Intelligent structure design of membrane cathode assembly for direct methanol fuel cell

The performance and the structural model of membrane electrode assembly (MEA) have been developed and experimentally verified with fundamental calculations of the direct methanol fuel cell (DMFC). The model provides information concerning the influence of the operating and structural parameters. The composition and performance optimization of MEA structure in DMFC has been investigated by including both electrochemical reaction and mass transport process. In the experimentation, the effect of Nafion content and loading method in the catalyst layer of cathode for DMFC was investigated. For the spray method electrode (SME), the cell performance and cathode performance using a dynamic hydrogen electrode (DHE) as a reference electrode was improved in comparison with those of the PME electrode by decreasing cathode potential. From ac impedance measurements of the cathode, the adsorption resistance of the SME electrode was decreased compared with that of the PME electrode. The higher cell performance was mostly dependent on the adsorption resistance. In the modelling, the cathode overpotential was decreased with increasing ionomer content, due to increasing ionic conductivity for proton transfer and the larger reaction site. The resistance to oxygen transport was increased at the same time, and became dominant at higher ionomer loadings, leading to an increase in the voltage loss. The ratio of ionomer to void space in the cathode affected the cathode polarization, which had the lowest resistance of oxygen diffusion at the ratio of 0.1–0.2. Copyright © 2005 John Wiley & Sons, Ltd.