A 2D through-the-Membrane Transient Model for Polymer Electrolyte Membrane Fuel Cells

Abstract

Effective and efficient water and thermal management remain an integral part of the research and development efforts aimed at improving performance and durability of polymer electrolyte membrane (PEM) fuel cells. Understanding the transport phenomena occurring in the cell is key to developing successful water and thermal management strategies. To this end, recent experimental observations have shed light on some aspects of the transport in the cell, such as the phase change induced flow [1], and have significantly contributed to the current understanding in this area. Furthermore, mathematical modeling has been an indispensable tool in this endeavor and has provided insight into the physical phenomena in the cell. However, most of the models developed to date for this purpose have focused on steady state performance [2, 3]. While successful transient modeling efforts have been reported in the literature [4], a complete understanding of the dynamic behavior of the cell requires further research. Motivated by the automotive industry’s requirements to better understand this dynamic behavior, in this talk we present our latest modeling efforts emphasizing the transient response of the cell. In particular, we present a fully coupled two-dimensional finite element model with two-phase flow and non-isothermal effects. The model resolves transport across various length scales, including the diffusion media and the catalyst layers, where a recently developed homogenous model is employed to account for the local oxygen transport resistance [5]. Moreover, anisotropy in material properties is considered to model in-plane variations. Oxygen reduction reaction (ORR) is described using the modified double-trap model that captures doubling of the Tafel slope [6]. ORR dependency on relative humidity is also taken into account [7, 8]. Values reported in the literature for time constants of membrane water sorption and desorption are discussed. Based on this discussion, a factor scaling the time constant of the original diffusion equation used for membrane water transport is introduced and its importance is discussed through transient modeling results. In general, the model can be used to investigate the transient response of the fuel cell under various operating conditions. A steady state version of the model can help in making design decisions to improve cell performance. Finally, the model can serve as a benchmark to parameterize and validate more computationally efficient models that can be used for real-time applications. Acknowledgement: Financial support for this work was provided by Ford Motor Company through the Ford University Alliance Program.