Humidification-Thermal Cycling-Based Mechanical Degradation Analysis of Low-Temperature PEMFC Using a Full-Scale Transient Physics-Based Model

Abstract

Low-Temperature Polymer electrolyte membrane fuel cells (LT-PEMFCs) have garnered huge attention as green energy technology due to low environmental impact and high efficiency. However, the long-term durability of these systems remains a critical concern, particularly regarding mechanical degradation of the proton exchange membrane. It is one of the most critical PEMFC components, facilitating ionic conduction and restricting the reactant gas cross-over. An examination of aged membranes through ex-situ analysis has uncovered various degradation features, including the emergence of crazes, cracks, pinholes, membrane thinning, and elongation1. Partly these defects appear due to cyclic stress arising from the repetitive expansion/compression of the membrane under water absorption/desorption2,3. Consequently, these defects reduce the open circuit voltage and increase the ohmic loss in the cell4. The crossover of reactants beyond a certain limit can instigate the formation of localized hotspots, further accelerating membrane degradation and eventually catastrophic failure of the cell. Multiple investigations have highlighted the influence of clamping force and humidity-temperature cycling on LT-PEMFC performance5,6. Nevertheless, most of these studies have primarily focused on structural mechanics, overlooking other critical aspects such as electrochemical reactions, fluid flow, heat transfer, and species transport. This type of decoupled approach does not provide real-time stress evolution inside the membrane with the cell operation. Moreover, existing models are often either two-dimensional or simplified single-channel 3D representations, with simplifications like isothermal operation, simplified reaction kinetics and more5,7,8. These models lack spatiotemporal resolution of the membrane deformation or stress distribution under diverse operating conditions.In this study, we present a comprehensive analysis of mechanical degradation in the LT-PEMFC membranes through the implementation of a transient physics-based model. The research focuses on understanding the intricate interplay between mechanical stress, environmental factors, and the resulting degradation mechanisms within the membrane. A transient model is developed using COMSOL Multiphysics to simulate the mechanical behaviour of the membrane under various operational conditions and stress scenarios encountered during the fuel cell's lifespan. It is a full-scale Multiphysics model that comprises charge transfer, species transport, porous media flow, electrochemical reactions, heat generation, ionomer water transport, liquid water transport, and solid mechanics. The model accuracy is confirmed with the lab experiment on 32 cm2 cell with multi-serpentine channels at 80% RH & 75 °C. This model incorporates factors such as humidity, temperature variations, and clamping pressure. By analyzing stress accumulation and its effects on the Nafion membrane, we aim to identify critical locations that lead to mechanical degradation and permanent deformation. Furthermore, the transient response of the cell parameters like liquid water dynamics, stress accumulation and membrane water content dynamics are studied under the urban dynamometer driving schedule (UDDS), which has not been reported earlier. Preliminary results of the liquid water dynamics in Figure 1 (a) suggest that the liquid water accumulates during the higher current density operations between 1500 and 1800 seconds. Also, the liquid water saturation is unable to reach its initial state of 0.12 during a sudden change in the power demand that leads to the cumulative accumulation of liquid water inside diffusion media. Though It improves the water content and ionic conductivity of the membrane, it increases the chance of cell flooding and stress accumulation due to membrane swelling. Figure 1 (b) shows the cell deformation under the clamping pressure of 1 and 2 MPa, reducing the species transport inside the diffusion media. The outcomes of this study aim to provide deeper insights into the stress accumulation and permanent deformation mechanisms affecting membrane degradation. Understanding these degradation pathways is crucial for the development of improved materials and design strategies that enhance the durability and long-term performance of PEMFCs, ultimately contributing to the advancement and widespread adoption of clean energy technologies.