Catalyst coated membranes (CCMs) in polymer electrolyte fuel cells are subjected to mechanical stresses in the form of fatigue and creep that deteriorate the durability and lifetime of the cells. The present article aims to determine the effect of in-situ hygrothermal fatigue on the microstructure and mechanical properties of the CCM. The fatigue process is systematically explored by the application of two custom-developed accelerated mechanical stress test (AMST) experiments with periodic extraction of partially degraded CCMs. Cross sectional and top surface scanning electron microscope (SEM) images of the end-of-test CCMs reveal the formation of mechanically induced cracks and delamination due to cyclic tensile and compressive fatigue stress. Tensile and expansion tests are conducted at different stages of degradation to evaluate the evolution in the mechanical and hygrothermal properties of the CCM. The tensile test results indicate gradual reductions in final strain, ultimate tensile strength, and fracture toughness with increasing number of fatigue cycles. The decay in tensile properties is attributed to the microstructural damage and micro-cracks formed during the AMST. Moreover, it is shown that the hygrothermal expansion of the CCM is more sensitive to conditioning than mechanical degradation. Polymer electrolyte fuel cells (PEFCs) are a prime candidate to replace gasoline and diesel internal combustion engines for transportation applications due to their environmental benefits combined with rapid start-up, high efficiency, and high power density at relatively low operating temperature.1 The commercialization of PEFCs is dependent on the development of membrane electrode assemblies (MEA) capable of meeting the automotive industry durability targets.2 However, the current PEFC technology is facing insufficient longevity, mainly because of the deterioration of the proton exchange membrane (PEM) component.1 Hence, an essential step to accomplish the commercialization requirements for PEFCs is to enhance the membrane durability and lifetime. Among various types of membranes utilized in PEFCs, perfluorosulfonic acid (PFSA) ionomer membranes (e.g., Nafion from DuPont) are the most widely used materials due to the superior chemical stability attributed to the chemically inert C-F bonds of the polytetrafluoroethylene (PTFE) base structure. 3Chemical and mechanical degradation mechanisms are recognized as the principal root causes for lifetime limiting failures of PFSA ionomer membranes in fuel cells. Understanding of the degradation mechanisms, their interactions, and the corresponding failure modes could provide valuable insight toward decelerating the rate of the membrane degradation and thereby extend the lifetime.2 Chemical degradation is caused by the attack of radical species in the form of hydroxyl (•OH) and hydroperoxyl (•OOH) radicals generated through decomposition of hydrogen peroxide (H 2 O 2 ) by metal contaminants.2,4,5 Hydroxyl radicals also form as a by-product of the electrochemical reaction bet...

Interlayer exchange coupling in transition metal multilayers has been intensively studied for more than three decades and is incorporated into almost all spintronic devices. With the current spacer layers, only collinear magnetic alignment can be reliably achieved; however, controlling the coupling angle has the potential to markedly expand the use of interlayer exchange coupling. Here, we show that the coupling angle between the magnetic moments of two ferromagnetic layers can be precisely controlled by inserting a specially designed magnetic metallic spacer layer between them. The coupling angle is controlled solely by the composition of the spacer layer. Moreover, the biquadratic coupling strength, responsible for noncollinear alignment, is larger than that of current materials. These properties allow for the fabrication and study of not yet realized magnetic structures that have the potential to improve existing spintronic devices.

Spintronic devices would greatly benefit from a noncollinear alignment between magnetizations of adjacent ferromagnetic layers for maximum performance and reliability. We demonstrate that such an alignment can be created and controlled by coupling two ferromagnetic layers across a magnetic spacer layer consisting of a nonmagnetic material, Ru, alloyed with a ferromagnetic element, Co. Changing the composition and thickness of the spacer layer enables the control of the relative angle between the magnetizations of the ferromagnetic layers between [Formula: see text] and [Formula: see text]. The onset of noncollinear alignment between the ferromagnetic layers coincides with the advent of magnetic order in the spacer layer. This study maps the concentration and thickness ranges of RuCo spacer layers that give rise to noncollinearity between ferromagnetic Co layers. The experimental results are successfully reproduced by simulating our structures with an atomistic model. This model assumes that Co atoms in the RuCo spacer layer have magnetic moments and that neighboring Co atoms are ferromagnetically coupled, while Co atoms separated by one or more Ru atoms are antiferromagnetically coupled.

Increasing the lifetime and reliability of proton exchange membrane fuel cells (PEMFCs) is one of the main challenges facing the fuel cell industry. Under automotive operating conditions, the membrane in PEMFCs is subjected to chemical and mechanical degradation, which gradually leads to loss in performance and subsequent failure. The US Department of Energy (DOE) has developed standardized protocols for in-situ chemical and mechanical accelerated stress tests (ASTs) [1]. The decay in membrane properties under pure chemical, pure mechanical, and combined chemical – mechanical AST [2-4] were recently evaluated. In the area of mechanical membrane degradation, material fatigue is expected to dominate, and an ex-situ fatigue based AST technique was recently developed by our group [5]. The objective of the present work is to compare the action of the ex-situ fatigue AST to that of the more established in-situmechanical AST protocol. For this purpose, fatigue lifetime, failure modes, and decay in mechanical properties were investigated and compared for both AST techniques. In-situ mechanical AST was conducted on a five-cell research-scale fuel cell stack by applying wet–dry cycles from 0% to 90% relative humidity under a modified DOE mechanical AST protocol. CCM samples were periodically extracted from the stack after certain numbers of AST cycles (every 4,000 cycles) and replaced by fresh cells. In order to investigate the formation of mechanical damage in the membrane, leak tests followed by a systematic microstructural study using scanning electron microscopy (SEM) were applied on the extracted samples. Post-mortem analysis on the degraded samples using an infrared camera showed traces of mechanical defects facilitating gas crossover through the membrane and leading to failure, as depicted in Figure 1. The decay in mechanical properties was evaluated through conducting tensile and expansion experiments on the degraded samples at different AST cycles, in accordance with our recently established CCM characterization procedures [6]. The ex-situ fatigue AST experiments, on the other hand, were applied on fresh CCM specimens utilizing fatigue stresses via a dynamic mechanical analyzer (DMA) equipped with an environmental chamber. Systematic fatigue experiments in our group, proved the capability of cyclic loadings in order to benchmark the mechanical durability of the materials. It was observed that membrane fatigue life is a strong function of temperature and relative humidity [5]. Employing the outcomes of the baseline fatigue data, approximate CCM fatigue lifetimes were extrapolated and predicted at the desired test conditions. Depending on the total fatigue lifetime, fatigue experiments were interrupted at different fractions of the CCM lifetime in conjunction with the corresponding in-situ extractions. In a similar manner, tensile and expansion experiments were performed on ex-situ fatigue extracted specimens to evaluate the decay in mechanical properties under cyclic loading. The results of the two methods are comprehensively compared and utilized to shed light on the overall fundamental understanding of the pure mechanical membrane degradation mechanism and the associated CCM fatigue lifetime. The capabilities of the ex-situfatigue experiment as a rapid, inexpensive mechanical AST are discussed. Figure 1. Formation of leak observed during the in-situmechanical AST. The bright region captured by an infrared camera indicates the main leak location near the inlet. Acknowledgements: This research was supported by Ballard Power Systems and the Natural Sciences and Engineering Research Council of Canada through an Automotive Partnership Canada grant. References: [1] http://www1.eere.energy.gov/hydrogenandfuelcells/mypp [2] Y.P. Patil, et al., J Membrane Sci. 356, 7, 2010. [3] J. Kang and J. Kim, Int J Hydrogen Energ, 35, 13125, 2010. [4] C. Lim, et al., J Power Sources, 257, 102, 2014. [5] R. Khorasany et al., J Power Sources, (under review). [6] M.A. Goulet, et al., J Power Sources,234, 38-47, 2013.

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