Structure-Property Relationship in Ionomer Membranes

Perfluorosulfonic acid (PFSA) ionomer membranes are ion-conducting polymers with high water sorption capacity and thermo-mechanical stability. The morphology of PFSA changes during sorption affecting the mechanical and physical properties of the membrane. In this work, we investigate the structure-property relationship in swollen PFSA membranes using three proposed nanostructural descriptions and determine Young\u27s modulus of the membrane at a wide range of temperatures (−20–85°C) and water volume fractions (0–0.5) for these configurations. Comparing the mechanics-based models with experimental data we propose that ion-rich water domains in PFSA membrane are spherical at low water content, spherical with connecting channels at intermediate water content and cylindrical at high water contents. Furthermore, our findings indicate that the scaling behavior for Young\u27s modulus of PFSA ionomers is similar to that of aerogels. This suggests, from a mechanics perspective, that aerogels and ionomers may have a similar interconnected porous nanostructure where some solid regions do not contribute to the mechanical load-bearing capacity

Aspects of Fatigue Failure Mechanisms in Polymer Fuel Cell Membranes

The swelling-driven fatigue behavior of polymer fuel cell membranes during relative humidity (RH) cycling is investigated. In particular, swelling-induced membrane stresses are obtained from a numerical model simulating fuel cell RH cycle tests, and compared to the lifetimes obtained experimentally from tests conducted in the absence of electrochemical effects. A strong correlation between the lifetimes of the membranes in the actual tests and model results is obtained. In general, higher RH (or swelling) amplitude results in larger stress amplitudes and shorter lifetime, that is, fewer cycles to failure. Tensile stresses are needed for forming local cavities in the membrane, which may eventually lead to craze formation. Cavitation is less likely to occur in compressed membrane at high humidities. The stress–lifetime plots for polymer fuel cell membranes exhibit similar features to those observed for other polymers. The crazing criterion for polymers suggests that craze initiation during RH cycling is more likely to occur in the low compression regions, such as under the channels, which is in agreement with experimental observations. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1506–1517, 201

Mechanical Behavior of Fuel Cell Membranes under Humidity Cycles and Effect of Swelling Anisotropy on the Fatigue Stresses

The mechanical response of proton exchange membranes in a fuel cell assembly is investigated under humidity cycles at a constant temperature (85°C). The behavior of the membrane under hydration–dehydration cycles is simulated by imposing a humidity gradient from the cathode to the anode. Linear elastic, plastic constitutive behavior with isotropic hardening and temperature and humidity dependent material properties are utilized in the simulations for the membrane. The evolution of the stresses and plastic deformation during the humidity cycles are determined using finite element analysis for two clamping methods and various levels of swelling anisotropy. The membrane response strongly depends on the swelling anisotropy where the stress amplitude decreases with increasing anisotropy. These results suggest that it may be possible to optimize a membrane with respect to swelling anisotropy to achieve better fatigue resistance, potentially enhancing the durability of fuel cell membranes

Long discontinuous fiber composite structure: Forming and structural mechanics

Cost effective composite structure has motivated the investigation of several new approaches to develop composite structure from innovative material forms. Among the promising new approaches is the conversion of planar sheet to components of complex curvature through sheet forming or stretch forming. In both cases, the potential for material stretch in the fiber direction appears to offer a clear advantage in formability over continuous fiber systems. In the present study, the authors have established a framework which allows the simulation of the anisotropic mechanisms of deformation of long discontinuous fiber laminates wherein the matrix phase is a viscous fluid. The initial study focuses upon the establishment of micromechanics models for prediction of the effective anisotropic viscosities of the oriented fiber assembly in a viscous matrix. Next, the developed constitutive relation is employed through an analogy with incompressible elasticity to exercise the finite element technique for determination of local fiber orientation and laminate thickness after forming. Results are presented for the stretch bending of a curved beam from an arbitrary composite laminate and the bulging of a clamped sheet. Structural analyses are conducted to determine the effect of microstructure on the performance of curved beams manufactured from long discontinuous fiber composites. For the purposes of this study, several curved beams with ideal and non-ideal microstructures are compared for response under pure bending. Material parameters are determined from a separate microstructural analysis

Mechanical Behavior of Fuel Cell Membranes under Humidity Cycles and Effect of Swelling Anisotropy on the Fatigue Stresses

The mechanical response of proton exchange membranes in a fuel cell assembly is investigated under humidity cycles at a constant temperature (85°C). The behavior of the membrane under hydration–dehydration cycles is simulated by imposing a humidity gradient from the cathode to the anode. Linear elastic, plastic constitutive behavior with isotropic hardening and temperature and humidity dependent material properties are utilized in the simulations for the membrane. The evolution of the stresses and plastic deformation during the humidity cycles are determined using finite element analysis for two clamping methods and various levels of swelling anisotropy. The membrane response strongly depends on the swelling anisotropy where the stress amplitude decreases with increasing anisotropy. These results suggest that it may be possible to optimize a membrane with respect to swelling anisotropy to achieve better fatigue resistance, potentially enhancing the durability of fuel cell membranes

Numerical Investigation of Mechanical Durability in Polymer Electrolyte Membrane Fuel Cells

The relationship between the mechanical behavior and water transport in the membrane electrode assembly (MEA) is numerically investigated. Swelling plays a key role in the mechanical response of the MEA during fuel cell operation because swelling can be directly linked to the development of stresses. Thus, in the model introduced here, the stresses and the water distribution are coupled. Two membranes are studied: unreinforced perfluorosulfonic acid (PFSA) and an experimental reinforced composite membrane. The results suggest that open-circuit voltage operations lead to a uniform distribution of stresses and plastic deformation, whereas under current-load operation, the stresses and the plastic deformation are generally lower and localized at the cathode side of the MEA. For the experimental reinforced membrane investigated, the in-plane swelling and, consequently, the stresses and plastic deformation are lower than in an unreinforced PFSA membrane. This reduction is a favorable outcome for improving durability. The model also suggests that the mechanical constraints due to the clamping of the cell may limit the swelling of the membrane and consequently change the water distribution

Stresses in Proton Exchange Membranes Due to Hygro-Thermal Loading

Durability of the proton exchange membrane (PEM) is a major technical barrier to the commercial viability of polymer electrolyte membrane fuel cells (PEMFC) for stationary and transportation applications. In order to reach Department of Energy objectives for automotive PEMFCs, an operating design lifetime of at least 5000 h over a broad temperature range is required. Reaching these lifetimes is an extremely difficult technical challenge. Though good progress has been made in recent years, there are still issues that need to be addressed to assure successful, economically viable, long-term operation of PEM fuel cells. Fuel cell lifetime is currently limited by gradual degradation of both the chemical and hygro-thermomechanical properties of the membranes.Eventually the system fails due to a critical reduction of the voltage or mechanical damage. However, the hygro-thermomechanical loading of the membranes and how this effects the lifetime of thefuel cell is not understood. The long-term objective of the research is to establish a fundamental understanding of the mechanical processes in degradation and how they influence the lifetime of PEMFCs based on perfluorosulfuric acid membrane. In this paper, we discuss the finite element models developed to investigate the in situ stresses in polymer membranes

Time-Dependent Mechanical Behavior of Proton Exchange Membrane Fuel Cell Electrodes

The electrodes used for Proton Exchange Membrane Fuel Cells (PEMFCs) are typically painted or sprayed onto the membrane during manufacturing, making it difficult to directly characterize their mechanical behavior as a stand-alone material. An experimental-numerical hybrid technique is devised to extract the electrode properties from the experimentally measured properties of Nafion® 211 membrane1 and a membrane electrode assembly (MEA) based on Nafion® 211 membrane at various temperatures, humidities, and strain rates. Within the linear regime, the rule-of-mixtures assuming an iso-strain condition is used to calculate the rate-dependent Young\u27s modulus of the electrodes. Beyond the linear regime, reverse analysis is conducted using finite element models of the MEA to determine the non-linear behavior of the electrodes. The mechanical damage mechanisms that occur in the MEA during tensile loading are also investigated through interrupted tension tests and then incorporated into the finite element models for determining the electrode behavior. The results suggest that the electrodes have similar behavior to Nafion® 211 membrane as functions of strain rate, temperature and humidity, but with lower Young\u27s modulus and proportional limit

Time-Dependent Mechanical Behavior of Proton Exchange Membrane Fuel Cell Electrodes

The electrodes used for Proton Exchange Membrane Fuel Cells (PEMFCs) are typically painted or sprayed onto the membrane during manufacturing, making it difficult to directly characterize their mechanical behavior as a stand-alone material. An experimental-numerical hybrid technique is devised to extract the electrode properties from the experimentally measured properties of Nafion® 211 membrane1 and a membrane electrode assembly (MEA) based on Nafion® 211 membrane at various temperatures, humidities, and strain rates. Within the linear regime, the rule-of-mixtures assuming an iso-strain condition is used to calculate the rate-dependent Young\u27s modulus of the electrodes. Beyond the linear regime, reverse analysis is conducted using finite element models of the MEA to determine the non-linear behavior of the electrodes. The mechanical damage mechanisms that occur in the MEA during tensile loading are also investigated through interrupted tension tests and then incorporated into the finite element models for determining the electrode behavior. The results suggest that the electrodes have similar behavior to Nafion® 211 membrane as functions of strain rate, temperature and humidity, but with lower Young\u27s modulus and proportional limit

Effect of Time-Dependent Material Properties on the Mechanical Behavior of PFSA Membranes Subjected to Humidity Cycling

A viscoelastic-plastic constitutive model is developed to characterize the time-dependent mechanical response of perfluorosulphonic acid (PFSA) membranes. This model is then used in finite element simulations of a representative fuel cell unit, (consisting of electrodes, gas diffusion layer and bipolar plates) subjected to standardized relative humidity (RH) cycling test conditions. The effects of hold times at constant RH, the feed rate of humidified air and sorption rate of water into the membrane on the stress response are investigated. While the longer hold times at high and low humidity lead to considerable redistribution of the stresses, the lower feed and sorption rates were found to reduce the overall stress levels in the membrane. The redistribution and reduction in stress magnitudes along with inelastic deformation during hydration eventually lead to development of residual tensile stresses after dehydration. Simulations indicate that these tensile stresses can be on the order of 9–10 MPa which may lead to mechanical degradation of the membrane. The simulation results show that time-dependent properties can have a significant effect on the in-plane stress response of the membrane