Through-the-Membrane Transient Phenomena in PEM Fuel Cells: A Modeling Study

Abstract

This paper presents a 2D, fully coupled and comprehensive transient model that accounts for micro-structural features of various cell layers. The model benefits from state of the art sub-models for reaction kinetics and incorporates the polymer relaxation dynamics. Furthermore, a mixed wettability model is utilized to simulate the transient two phase conditions in the porous layers. The model is validated with transient experimental data under various conditions. A comprehensive simulation study is presented to investigate the impact of operating temperature and relative humidity on the transient response. The effects of cathode Pt loading and operation mode, i.e., current control versus voltage control, are also studied. The cell response is found to be dominated by water transport through its thickness. Additionally, it is found that reducing the Pt loading can influence the performance by changing the water balance in the cell, which has rarely been highlighted in the literature. In particular, at low temperature more water is transported toward the anode when the cathode Pt loading is reduced, since the resistance to water back diffusion is lowered with reduced thickness of the cathode catalyst layer. This trend is reversed at a higher temperature due to increased volumetric heat generation with reduced thickness. The model can help in understanding various transport phenomena and is expected to be useful for inspecting spatio-temporal temperature, potential, and species distributions across the cell's thickness and optimizing the cell design and choice of materials.

A Mathematical Model toward Real-Time Monitoring of Automotive PEM Fuel Cells

“…+18λ is the water volume fraction in ionomer [40] ( a = RH, and s is the local liquid saturation [9] exp(2 + 0.2λ)…”

“…where λ * is the dynamic quasi-equilibrium water content for the ionomer used to model the effects of stress relaxation [9,27] (see Table 1). In addition to S ad , both electrochemical and chemical water productions contribute to source term for ionomer water content.…”

“…In particular, earlier models were mostly lumped parameter with no distribution [4], or 1D, capturing the distributions in either the through-the-membrane [7] or along-the-channel [8] directions. More recently, most of the models have evolved to 2D computational domains capturing land-channel variations [9][10][11] or along the channel distributions [12][13][14].…”

“…Pseudo 3D [15,16] and 3D [17,18] models have also become popular. Additionally, the transient phenomena have been incorporated in many recent models [9,11,12,14,19]. In most cases, the evolution to higher dimensions and incorporation of transient behavior are accompanied by a higher complexity of the modeled physics.…”

“…In most cases, the evolution to higher dimensions and incorporation of transient behavior are accompanied by a higher complexity of the modeled physics. More specifically, accounting for multi-step oxygen reduction reaction (ORR) kinetics [20] and intricate two-phase flow and non-isothermal phenomena [9][10][11]18] has become common in recent literature. This ever-increasing complexity of the models has shed some light into the transport phenomena in the porous layers of the cell that were previously unidentified [21].…”