Chemical membrane degradation through the Fenton's reaction is one of the main lifetime-limiting factors for polymer-electrolyte fuel cells. In this work, a comprehensive, transient membrane degradation model is developed to capture and elucidate the complex in situ degradation mechanism. A redox cycle of iron ions is discovered within the membrane electrolyte assembly, which sustains the Fe(II) concentration and results in the most severe chemical degradation at open circuit voltage. The cycle strength is critically reduced at lower cell voltages, which leads to an exponential decrease in Fe(II) concentration and associated membrane degradation rate. When the cell voltage is held below 0.7 V, a tenfold reduction in cumulative fluoride release is achieved, which suggests that intermediate cell voltage operation would efficiently mitigate chemical membrane degradation and extend the fuel cell lifetime.

Chemical membrane degradation is a major limiting factor for polymer electrolyte fuel cell (PEFC) durability and lifetime. While the main degradation mechanisms are established in the literature, the in-situ trends of their action are often only known qualitatively. This motivates the development of a comprehensive in-situ chemical membrane degradation model addressed in this work. The numerical algorithms developed are strategically designed to be compatible with state-of-the-art computational membrane electrolyte assembly (MEA) performance models; here, we emphasize the integration of the developed degradation model into a 1-D MEA transport-reaction model to determine the linkages between the MEA macroscopic phenomena, in-situ operating conditions, and the temporal membrane degradation process. Concentrations of hydrogen peroxide, radical, and degraded ionomer species are modeled to interrogate the evolution of ionomer molecular structure with respect to the chemical membrane degradation. The proposed degradation mechanism includes the initiation and propagation of side chain degradation culminating in main chain scission and fragmentation, and demonstrates a good agreement with the most recent experimental findings. The integrated MEA model is further applied to simulate the macroscopic effects of chemical membrane degradation characterized in a recent accelerated stress test. Comparisons between the numerical and experimental results are discussed.

A transient, isothermal, two-dimensional model coupling cell performance and chemical membrane degradation in a polymer electrolyte membrane fuel cell (PEMFC) is developed. The model is based on the conservation of and thermodynamic equilibrium between charged and neutral species, including radicals. The model is validated against experimental polarization behavior and chemical degradation under an open circuit voltage (OCV) hold test at 368.15 K. The four-step chemical degradation of a PFSAbased membrane is assumed to start by an attack by hydroxyl radical at the terminal ether bond in the side chain. The source of the attacking hydroxyl radical is an indirect hydrogen peroxide formation and the subsequent decomposition at Fenton's reagent in the membrane. Simulation of degradation rate (defined as the loss of cell voltage with time at a fixed cell operating condition and at a point of time with a known degradation history) under an OCV-hold test agree qualitatively with the degradation rates reported in the literature.

Ceria-supported membrane electrode assemblies (MEAs) have recently been proposed to address chemical membrane degradation in polymer electrolyte fuel cells. Although ceria is known to effectively protect the membrane at open circuit voltage (OCV) conditions, its effectiveness has not been demonstrated for cell voltages below OCV and associated conditions relevant for field operation. In the present work, a comprehensive, transient in situ chemical degradation model for ceria stabilized MEAs is developed and applied to investigate the mitigation effectiveness of ceria additive. At high cell voltages, abundant Ce 3+ ions are available in the membrane to quench hydroxyl radicals which is the primary mitigation mechanism observed at OCV conditions. However, the mitigation is suppressed at low cell voltages, where electromigration drives Ce 3+ ions into the cathode catalyst layer (CL). Without an adequate amount of Ce 3+ in the membrane, the hydroxyl radical scavenging is significantly reduced, leading to a ten-fold reduction in mitigation effectiveness at cell voltages below 0.7 V. The simulated results also suggest that significant ceria precipitation may occur in the cathode CL due to the increased local Ce 3+ concentration at low to medium cell voltages. Ceria-supported MEAs may therefore experience higher rates of chemical membrane degradation at low cell voltages than at OCV. Hydrogen powered polymer electrolyte fuel cells (PEFCs) generally use perfluorosulfonic acid (PFSA) ionomer membranes to separate the two electrodes in the membrane electrode assembly (MEA). Their high proton conductivity at low temperatures, relatively low reactant permeation, and superior electrical insulation lead to high fuel cell performance. However, the ionomer membrane can be degraded in the fuel cell environment which reduces its stability and limits its lifetime.1,2 Chemical degradation initiates the overall degradation processes 3,4 and further damages the membrane when combined with mechanical stress, strain, and fatigue induced by hygrothermal fluctuations in the MEA. [5][6][7][8][9][10][11][12] The resulting physical damage in the form of cracks and holes eventually causes hydrogen leaks across the membrane which is considered one of the main lifetime limiting failure modes in fuel cells. 4,[13][14][15][16] The primary chemical degradation in PEFCs is caused by reactions of reactive radicals with the PFSA ionomer membrane.17-21 For instance, hydrogen peroxide (H 2 O 2 ) can be generated via the twoelectron oxygen reduction reaction (ORR) resulting in hydroxyl radical (·OH) formation due to decomposition of hydrogen peroxide in the presence of metal contaminants. 22,23 The hydroxyl radical formed is highly reactive and can attack the ionomer membrane in terms of side chain cleavage and unzipping 18,20,21,24,25 which is responsible for the deteriorated physicochemical properties observed in degraded membranes. Radical scavenging is therefore proposed to mitigate the chemical damage by quenching the radicals before they attack t...

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