Mechanical behavior of fuel cell membranes under humidity cycles and effect of swelling anisotropy on the fatigue stresses

Abstract

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.

Introduction

Proton exchange membrane fuel cells (PEMFCs) are viable candidates for powering vehicles in a proposed future hydrogen economy. However, in order to replace the internal combustion engine, the durability and reliability of the fuel cell systems must be improved. For automotive applications the current target is 5000 h (150,000 miles equivalent) operational life over a full range of vehicle operating temperatures (−40 to 40 °C) [1].

During the operation of the cell, the proton exchange membrane (PEM) provides an ionic conductive path for protons from the anode to the cathode, while acting as an electronic insulator and a gas barrier to prevent mixing of oxygen and hydrogen. Any type of discontinuity in the membrane reduces the performance (i.e. output power, total voltage) and lifetime of the cell [2], [3], [4], [5], [6]. Although the electro-chemical and thermo-mechanical interactions among cell components (electrocatalysts, membranes, gas diffusion layers, and bipolar plates) affect the durability, it has been found that the membrane itself is a major source of failure, including mechanical damage and chemical degradation [2], [3], [5], [6], [7], [8]. Thus, the membrane must be durable enough to withstand mechanical stresses and chemical attacks so that the fuel cell can sustain its function under the severe internal operating conditions.

Several forms of mechanical damage in the membrane electrode assembly (MEA) are commonly observed, including through-the-thickness tears or pinholes in the membrane, or delamination between the polymer membrane and the electrodes [4], [5], [6], [7], [8], [9], [10]. A number of studies related to the influence of the membranes’ mechanical properties on the durability and performance of the cell are available in literature [2], [3], [9], [11], [12], [13]. It is commonly believed that the mechanical stresses, due to the hydration–dehydration cycles in the membrane precipitate these damage mechanisms [9], [14], [15], [16]. Kolde et al. [13] investigated the material properties and fuel cell performance of several commercial fuel cell membranes of various thicknesses. The results indicated that membranes exhibiting good dimensional stability in the in-plane directions during hydration–dehydration cycles are desirable for improving the cell performance and product reliability. A key parameter is therefore the swelling coefficient of the membrane [14], [16], [17]. Previous work simulating the mechanical response of a PEM during fuel cell operation [14] showed (through numerical means) that compressive stresses develop upon hygro-thermal loading. In some cases, these stresses can exceed the yield strength, causing permanent deformation. This, in turn, results in tensile residual stresses after dehydration. These in-plane tensile residual stresses are believed to be a significant contributor to the mechanical failures observed in the membranes, since they may cause the propagation of the through-the-thickness cracks in the membrane.

Even though crack initiation mechanisms in polymers are not completely understood, cyclic loading results in fatigue crack growth, which is one of the mechanisms governing the lifetime of these materials. The fatigue crack growth rate is governed by the stress amplitude and stress level [18], [19]. Therefore, investigating the stresses in the membrane during hygro-thermal cycling, which is the objective of this paper, is an important part of understanding the failure mechanisms in the membrane.

A common method to experimentally simulate the fuel cell operation is with accelerated cycling through a range of inlet relative humidities (RH) at elevated temperatures (80–95 °C) [3], [7], [13], [15], [20], [21]. Accelerated open-circuit voltage decay tests were conducted by Protsailo [20] using Nafion^® membranes to identify the long-term effects of high temperature and low RH operation on the performance of the perfluorosulfonic acid (PFSA) membranes. They found that membrane lifetime increases with increased inlet humidity and decreases with an increase in operating temperature. In the case of reduced relative humidity, the membrane failure is accelerated by a degradation of the mechanical properties of the PFSA [20]. In order to demonstrate purely mechanical failure modes, several investigators conducted RH cyclic tests in the absence of electro-chemical reactions [7], [15]. Mathias et al. [15] conducted an RH cycling test for an MEA made from a cast, 25 μm Nafion^®1 membrane. The MEA was held at a constant 80 °C and cycled between maximum and a minimum RH values in the absence of chemical degradation [15]. Recently, different test procedures were introduced to study the purely mechanical, and combined mechanical and chemical durability of fuel cell membranes for the development of new materials [7], [21]. Crum and Liu [7] conducted accelerated and non-accelerated tests (representing realistic automotive duty cycles) for various GORE-SELECT^®2 membranes (produced by W.L. Gore & Associates Inc.). These tests were run at a constant temperature (80 °C) and varying inlet and outlet RH at the anode and cathode. In order to separate the effects of chemical degradation and mechanical fatigue, they performed the tests in an inert environment (nitrogen) to suppress chemical activity. Similar trends in the durability of membranes were observed for the both accelerated and non-accelerated tests [7]. Thus, accelerated RH cycling tests may be an effective way of simulating the automotive fuel cell duty cycles in order to observe the failure mechanisms and to monitor the durability and the changes in the material properties of the membrane [7], [21].

In this work, we study the mechanical response of the membranes through numerical simulations. We employ a mechanics based model, that includes temperature and humidity dependent material properties, to determine the evolution of the stresses in the membrane due to the hydration–dehydration cycles at a fixed temperature (85 °C) corresponding to the maximum operating temperature of the fuel cell. The influence of the cell assembly design in terms of the clamping method is investigated (Fig. 1). In addition, since the swelling of the membrane induces the compressive stresses and subsequent residual tension, swelling anisotropy of the membrane will be investigated.

In the following, we review the theory for isotropic elasto-plasticity, followed by the definition of the geometry of the unit cell with boundary and loading conditions used in the numerical model. The material properties of the membrane used in this model are based on data obtained from our tensile tests of the membrane reported previously [17]. The details of the hygro-thermal loading cycles and the simulation of swelling strains during these cycles are studied. In Section 3, we introduce a series of assumed anisotropic swelling characteristics and evaluate their effects on the stresses.