Multiscale Modeling and Numerical Simulation of Materials Degradation Mechanisms in PEM Fuel Cells: Theory and Practice

Abstract

Because of the materials structural complexity and multiphysics character of modern PEMFCs, their durability prediction and optimization remain nowadays challenging. An analysis of PEMFC materials aging through consistent cell level models capturing the impact of the materials chemical and structural properties on the performance degradation, is required to elucidate the efficiency limitations and their location, the degradation and failure mechanisms [1-3].This tutorial comprehensively discusses both theoretical and practical aspects of multiscale modeling and numerical simulation of PEMFC materials aging for their durability prediction. First, fundamental concepts of physical electrochemistry, elementary kinetics and transport processes in relation to catalyst, catalyst support, diffusion media and ionomer aging are revisited. Then, new emerging computational techniques for the numerical simulation of PEMFC degradation are reviewed and remaining challenges are discussed. In particular, the new Prof. Franco's in-house simulation package "MS LIBER-T" is presented [4-6]. This software is a multiscale and multiphysics code which introduces significant progress on algorithimic aspects and computational capabilities compared to previously developed packages (MEMEPhys) [7]. The model captures the impact of the chemistry and structure of the materials through numerical databases providing kinetic activation energies (calculated from Density Functional Theory) and transport parameters as function of the composite electrode microstructures (calculated from Monte Carlo and Coarse Grain Molecular Dynamics simulations) for different degradation scenarios (e.g. electrode porosity change with the amounts of degradated ionomer and corroded carbon) [8-9]. Practical implementation of mathematical descriptions of materials aging mechanisms within MS LIBER-T is demonstrated in the tutorial through a test case of relevance for automotive applications of PEMFCs: the impact of simple current square signals onto the cell performance decay and competitive cathode catalyst dissolution, carbon corrosion, ionomer degradation within the electrodes and in the membrane and MPL/GDL carbon corrosion. The impact of parameters such as the cell temperature, the current amplitude and the cycle period are studied, and synergies but also cancellation effects between materials aging mechanisms are discussed in comparison with available experimental data. Furthermore, the impact on the MEA aging kinetics of the GDLs hydrophobicity and of water droplets formation within the channels is also explored and GDL hydrophobicity structurations mitigating materials degradation are proposed. Finally, the tutorial discusses pedagogical methods to teach and disseminate these modeling techniques, with illustrative feedback examples from MSc. students within the Erasmus Mundus Master on Materials for Energy Storage and Conversion (MESC) [10-11].