Numerical Simulation of Cerium Ion Migration and Distribution in a Polymer Electrolyte Membrane

Abstract

Ionomer membrane degradation in polymer electrolyte fuel cell (PEFC) becomes one of the most urgent and critical problems that decrease the longevity of a PEFC, thereby limiting its development with good efficiency and affordability 1. Adding ceria into the PEFC system has proven to be a promising and sustainable way to mitigate this chemical membrane degradation, as the cerium ions can quench the radicals before they attack the ionomer membrane 2. Understanding the distribution and movement of cerium ions within a fuel cell membrane is essential for improving the design of a PEFC system, in terms of its operating environment, material properties, boundary conditions, etc., whereas this has rarely been studied. Due to the experimental limitations of measuring cerium ions transport in an operating fuel cell, numerical simulation becomes a practical approach to quantitatively analyse cerium ions migration under certain conditions. In this study, we aim to provide: (1) a reliable and practical simulation approach to investigate 2D distribution of the cerium ions under specific conditions; and (2) a flexible strategy to analyse the migration of cerium ions attributed to a variety of factors. These techniques would support future research into the development of a promising fuel cell not only with longevity but also at a sustainable cost. From the simulation, we quantified many factors that have impacts on the migration of cerium ions. The diffusion coefficient of cerium ion with respect to the local water content, as well as how the ions are drugged by the local water diffusion, was obtained at first by performing molecular dynamics simulations. Then, by combining these results into a computational fluid dynamics simulation using finite volume method, transport of the cerium ions under certain conditions was resolved (see Figure 1). Along the direction of the path from anode to cathode side, the ion transport in the membrane is affected more substantially by the electric field rather than the water convection and its self-diffusion. Meanwhile in the membrane cross-section plane, it is mainly affected by the water diffusion and its self-diffusion. As the primary finding, the electric field has the greatest influence on the transport of cerium ions, while local distribution of the water content plays an important role when the electric potential difference is small. Credibility of these preliminary simulation results was confirmed by comparing against experimental observations at the similar fuel cell operating environments. Detailed settings of the material properties and results on the reactions of cerium ions to various fuel cell environments will be discussed in the presentation. Acknowledgement This study is support by New Energy and Industrial Technology Development Organisation (NEDO) and the integrated supercomputer system at Institute Fluid Science, Tohoku University. Reference [1] Wang, Y.; Chen, K. S.; Mishler, J.; Cho, S. C.; Adroher, X. C. A Review of Polymer Electrolyte Membrane Fuel Cells: Technology, Applications, and Needs on Fundamental Research. Appl. Energy 2011, 88 (4), 981–1007. [2] Wong, K. H.; Kjeang, E. In-Situ Modeling of Chemical Membrane Degradation and Mitigation in Ceria-Supported Fuel Cells. J. Electrochem. Soc. 2017, 164 (12), F1179–F1186. Figure 1