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On-going research is aimed at commercializing low temperature fuel cell technology systems as zero emission alternatives for automotive applications in order to reduce greenhouse gas emissions and air pollution. These systems use polymer electrolyte fuel cells (PEFCs) to generate electricity via an electrochemical process using hydrogen and ambient air (oxygen) to produce water. Advantages of PEFCs include quick start-up time, low operating temperature, low weight, high efficiency, and relatively simple design. An important area of current research is the identification of factors which affect fuel cell performance degradation during operation and ultimately, its durability. A technique that has yielded new insights in the investigation and identification of failure modes in fuel cells is lab-based X-ray computed tomography (XCT), which is most advantageous due to its on-demand availability. The Fuel Cell Research Laboratory (FCReL) at Simon Fraser University currently operates Canada’s only facility for multi-length scale XCT, comprising of two state-of-the-art laboratory-based XCT scanners from Carl Zeiss X-ray Microscopy (Zeiss Xradia 520 Versa and 810 Ultra) with complementary resolution and field of view capabilities. This unique combination offers unprecedented access to investigations at multi length scales; with an ability to probe fuel cell components at the micro as well as the nano scale. Figure 1 illustrates the fuel cell holder and its orientation with respect to the X-ray beam as well as an exploded view of the miniature fuel cell design. An overview of recent fuel cell degradation investigations at FCReL using the XCT technique will be shown. The XCT based workflow facilitates determination and quantification of material structure and properties changes resulting from degradation stresses associated with operational parameters such as temperature, relative humidity, and voltage. The non-destructive nature of lab-based XCT visualization coupled with the ability to scan the same fuel cell multiple times without inducing damage [1] has enabled detailed studies of fuel cell degradation evolution in four dimensions (3D space, 1D time) [2 - 4]. The new knowledge gained from this procedure has led to root cause identification with respect to membrane and catalyst layer crack initiation and propagation [5 - 7], sealing issues [8,9], and subsequent mitigation toward enhanced fuel cell durability. Acknowledgement This research was supported by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, Western Economic Diversification Canada, Canada Research Chairs, and Ballard Power Systems. References: [1] R.T. White, M. Najm, M. Dutta, F.P. Orfino, E. Kjeang, J. Electrochem. Soc. 163 (2016) F1206-F1208 [2] R.T. White, A. Wu, M. Najm, F.P. Orfino, M. Dutta, E. Kjeang, J. Power Sources 350 (2017) 94-102 [3] R. T. White, S. H. Eberhardt, Y. Singh, T. Haddow, M. Dutta, F. P. Orfino, E. Kjeang, Scientific Reports, (2019) 9:1843 [4] R. T. White, D. Ramani, S. H. Eberhardt, M. Najm, F. P. Orfino, M. Dutta, and E. Kjeang, J. Electrochem Soc, 166 (2019) F914-F925 [5] Y. Singh, R. T. White, M. Najm, T. Haddow, V. Pan, F. P. Orfino, M. Dutta, and E. Kjeang, J. Power Sources, 412 (2019) 224 [6] D. Ramani, Y. Singh, R. T. White, M. Wegener, F. P. Orfino, M. Dutta, and E. Kjeang, International Journal of Hydrogen Energy, 45 (2020) 10089-10103 [7] D. Ramani, Y. Singh, R. T. White, T. Haddow, M. Wegener, F. P. Orfino, L. Ghassemzadeh, M. Dutta, and E. Kjeang, Electrochimica Acta 380 (2021) 138194 [8] Y. Chen, Y. Singh, D. Ramani, F. P. Orfino, M. Dutta, and E. Kjeang, J. Power Sources, 520 (2022) 230674 [9] Y. Chen, Y. Singh, D. Ramani, F. P. Orfino, M. Dutta, and E. Kjeang, J. Power Sources, 520 (2022) 230673 [10] J. Stoll, F. P. Orfino, M. Dutta, and E. Kjeang, J. Electrochem Soc, (2021) 168 024516 Figure 1

Perfluorosulfonic acid ionomer membranes are subjected to simultaneous chemical and mechanical degradation under fuel cell operation. Despite the importance of membrane durability, the understanding of its structural degradation and failure modes has been considerably restricted by conventional 2D imaging. In this work, non-invasive micro X-ray computed tomography (XCT) is adopted to visualize the 3D membrane decay at different life stages during combined chemical and mechanical degradation. A detailed survey exhibits damage density of 6 and 10 cracks per mm2 observed at the near-final and final end of life stages respectively. Through-thickness membrane cracks with unbranched I-shaped cracks and Y- and X- shaped cracks with one and two branches respectively are observed. The observed damage development at each life stage is correlated to supplementary diagnostic data including hydrogen leak rate, open circuit voltage, and tensile strength. In particular, large X-shaped cracks formed due to embrittlement from underlying chemical degradation are deemed to have a critical impact on the eventual failure development by facilitating large hydrogen leaks. Overall, the comprehensive 3D perspective enabled by XCT imparts new knowledge pertaining to the degradation process, and could also be extended to other fuel cell failure modes and degradation mechanisms.

Membrane electrode assembly (MEA) edges are sensitive regions that could strongly influence the durability of polymer electrolyte membrane fuel cells. Membrane failure in poorly designed edges can promote gas crossover and often lead to premature cell failures as well as reduced durability; hence, shortened fuel cell lifetimes. Currently, there is limited knowledge addressing the mechanisms of edge failure in the literature and this gap is explored in this work. Two different MEA edge designs for our small scale fixture and research scale MEA (2), where edge effects are more pronounced, were implemented to study their robustness during a combined chemical/mechanical membrane degradation accelerated stress test (AST)[1]. Four-dimensional in-situ visualization[2], enabled by X-ray computed tomography, was performed to understand and thus mitigate the design issues responsible for edge failures. Interfacial interaction between the adhesive-containing polyimide (PI) gasket layer and the catalyst coated membrane (CCM) was identified as a key contributor to premature edge failures, which introduced significant voltage decay and fluctuations due to permanent membrane deformation subjected to the hygrothermally severe AST conditions. CCM slipping was observed at the edges likely due to adhesive melting, which led to excessive CCM cracks. This issue was mitigated in a subsequent design by using a non-adhesive polytetrafluoroethylene (PTFE) layer at the CCM interface along with changes in gasket coverage area, which led to: (i) delayed onset of edge failure; (ii) nearly five times reduction in edge crack size; (iii) elimination of membrane tearing; and (iv) minimal impact of edge failures on cell performance. This mitigation enabled a robust MEA edge wherein the performance-impacting failure was shifted from the edges to the active area regions where operational factors are expected to play a role in eventual degradation of performance. The active area is broadly composed of uncompressed channel regions and compressed land regions. Membrane cracks, observed exclusively in channel regions, were the predominant cause of performance loss by opening paths for internal gas crossover. All membrane cracks were driven by membrane buckling into macro pores in gas diffusion layer (GDL) through surface pores on microporous layer (MPL). Cracks were initiated from the catalyst layer (CL) surface, and gradually penetrated the entire membrane thickness. In-situ diagnostics data showed significant gas crossover through membrane cracks, which induced dramatic open circuit voltage loss. Formation of membrane buckling was highly dependent on preliminary non-uniformities in CL and GDL such as cracks and macro pores that cause uneven stress distribution. In land regions, membrane creep into macro GDL pores was observed instead of buckling, but creep did not lead to membrane crack. However, both creep and buckling were induced by similar mechanisms. The accumulation of ionomer and catalyst at creep spots led to local membrane thinning and loss in effective platinum surface area. Although mechanical stress was the main contributor to membrane failure, it is believed that chemical stress accelerated the degradation process since the number of AST cycles needed to achieve ultimate MEA failure was significantly reduced compared to our previously reported pure mechanical degradation tests[2]. Overall, a robust MEA edge design was the most significant outcome from this work. In addition, new knowledge was gained on membrane degradation under combined chemical/mechanical AST, which could contribute to enhanced fuel cell durability. Keywords: fuel cell; membrane durability; edge design; mechanical degradation; chemical degradation; X-ray computed tomography Acknowledgement Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program. Reference [1] D. Ramani, et al., J. Electrochem. Soc. 165 (2018) F3200. [2] Y. Singh, et al., Journal of Power Sources. 412 (2019) 224–237. Figure 1

The ionomer membrane facilitates critical functions in a polymer electrolyte membrane fuel cell. Dynamic operational duty cycles can gradually introduce various damage features within the membrane, which could lead to overall performance failure of the fuel cell. Membrane durability is affected by chemical, mechanical, and thermal stressors that often act synergistically to degrade the ionomer membrane during fuel cell operation [1]. However, to understand the effect of each individual stressor, a useful approach is to isolate and study these degradation mechanisms separately. For instance, pure mechanical membrane degradation is typically affected in situ by actively cycling the membrane hydration levels under electrochemically inert conditions. These conditions generate cyclic mechanical stresses within the constrained membrane, leading to fatigue-induced fracture and electrode delamination under prolonged exposures [2]. When the cracks penetrate completely through the membrane thickness and spread across a substantial planar area, internal gas crossover is accelerated through convective transport, and ultimate fuel cell failure could result [3]. Furthermore, the mechanical membrane damage is strongly influenced by the microstructure of the adjoining membrane electrode assembly (MEA) components and cell design. Failure analysis investigations, using both 2D and 3D imaging, have consistently found significant proportion of membrane cracks to be co-located with the electrode cracks, which is typically attributed to the local stress concentration effects that could favour membrane crack initiation at these sites [2,4–6]. Recently, X-ray computed tomography (XCT) based 4D in situ visualization approach adopting a custom X-ray transparent fuel cell assembly [7] provided further insights into such interaction effects by demonstrating the temporal development of new membrane cracks underneath pre-existing electrode features [8]. Given the dominant role of electrode cracks in membrane fracture, the present work was undertaken to explore low crack density electrodes as a mitigation strategy against mechanical membrane degradation. This was achieved by subjecting two separate MEAs with different initial cathode catalyst layer (CCL) crack densities, i.e., high and low crack designs, to accelerated stress test (AST) involving humidity cycling for generating pure mechanical degradation. Periodic XCT-based in situ visualization was performed to elucidate differences in the magnitude and evolutionary processes of the membrane/MEA degradation between the two examined cases. Failure mechanisms responsible for mechanical membrane fracture were investigated in relation to the role of neighbouring components ― both electrodes and gas diffusion layers (GDLs). Membrane damage was found to be significantly curtailed through minimization of ab initio crack density in the CCL. While through-thickness membrane cracks developed within a similar timeframe in both high and low cathode crack density MEAs, the coverage area of fatigue-induced membrane fracture in the latter case was nearly six times lower after 2000 AST cycles, thereby establishing the adopted treatment of electrode morphology as an effective mitigation strategy in slowing the mechanical membrane degradation rate. Hydration-dehydration cycles, however, still altered this initial morphology by introducing electrode cracks at early stages of the AST. These electrode cracks, as an intermediate step, exclusively governed the subsequent initiation and propagation of membrane cracks. Two distinct membrane failure mechanisms were identified, each characterized by the presence and absence, respectively, of the MEA’s permanent buckling deformation into void spaces of the GDL (Figure 1). The buckling phenomenon was found to be strongly influenced by the non-uniform GDL microstructure and had a significant contribution towards the eventual frequency and coverage of membrane fracture. This finding suggests that critical component interactions during membrane failure extend beyond the immediately adjacent electrode layers. Additional comparative in situ visualizations of the fuel cell assembly held at varying hydration states showed that the size and geometry of the membrane fracture features that had developed in the low cathode crack MEA varied substantially between dry and wet environments. Acknowledgements Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program. References [1] Rodgers, et al., Chem. Rev. 112 (2012) 6075–6103. [2] Sadeghi Alavijeh, et al., J. Electrochem. Soc. 162 (2015) F1461–F1469. [3] Lai, et al., J. Fuel Cell Sci. Technol. 6 (2009) 21002. [4] Singh, et al., J. Electrochem. Soc. 164 (2017) F1331–F1341. [5] Singh, et al., J. Power Sources. 345 (2017) 1–11. [6] Huang, et al., J. Polym. Sci. Part B 44 (2006) 2346–2357. [7] White, et al., J. Power Sources. 350 (2017) 94–102. [8] Singh, et al., J. Power Sources. 412 (2019) 224–237. Figure 1

Ionomer membrane degradation during fuel cell operation is conjointly produced by three different modes, namely, thermal, mechanical, and chemical. Amongst these, chemical degradation is generally considered a major contributor to lifetime limitations.[1] Chemical degradation of the ionomer membrane is caused by the formation of radical species, such as hydroxyl (OH•) and hydroperoxyl (HOO•) groups, which causes polymer chain scission and unzipping leading to membrane thinning and eventual rupture and shorting events. Furthermore, the chemical degradation deteriorates the mechanical properties of the membrane which dramatically accelerates its overall degradation rate. Scanning electron microscopy (SEM) based studies have been conventionally used to conduct two-dimensional characterization of degradation-induced structural features in fuel cell membranes; however, SEM imaging is inherently destructive and thus prohibits any tracking of structural changes over time. Laboratory-based X-ray computed tomography (XCT) is as an alternative imaging technique, which has not only enabled three-dimensional (3D) membrane failure analysis revealing novel insights on membrane failure [2,3], but has also facilitated studies on damage growth characterization, [4] and more recently on evolutionary aspects of mechanical membrane degradation through identical-location imaging. [5] In the present work, an XCT-based 4D in situ imaging method, featuring three dimensions in space and one dimension in degradation time, is applied to investigate the evolution of pure chemical membrane degradation through the use of a custom designed fixture housing a fully operational small-scale fuel cell. [6] Membrane electrode assemblies (MEAs) with three distinct membrane types, namely, i) non-reinforced, ii) mechanically reinforced, and iii) chemically and mechanically mitigated membranes, are separately examined and compared. The MEAs were subjected to open circuit voltage (OCV) hold under high temperature (75°C) and low humidity (30% RH) with 0.3 slpm H2 and 0.5 slpm air supplied to anode and cathode, respectively. Various in situ electrochemical diagnostics were periodically performed to monitor MEA and membrane health. XCT-based 3D datasets were obtained at various stages of degradation to facilitate identical location tracking and analysis of membrane morphology as a function of degradation time and enable the characterization of damage evolution in the three membrane types. Membrane thinning and other damage features within all three membranes are comprehensively examined from various perspectives by studying planar and cross-sectional views extracted from the 3D XCT datasets. Preliminary observations show that electrode-shorting under land regions was the key failure mode for membranes without chemical mitigation, i.e., non-reinforced and mechanically reinforced membranes (Fig. 1). In contrast, the chemically mitigated membrane did not fail up to 850 h of operation, highlighting the scavenging effect against the radicals. No membrane pinhole or crack development was observed in any of the three membranes, which is consistent with previous 3D post mortem results under pure chemical degradation.[3] The present 4D in situ imaging approach further revealed locally amplified membrane thinning preceding the eventual formation of shorts due to absence of membrane material and associated loss of electrode separation. The observed phenomenon further suggests that mechanical stresses, even in the absence of wet/dry cycling, may influence the local shorting events. Overall, the new findings from this work demonstrate the distinct advantage of XCT technology towards improving the fundamental understanding of membrane degradation by capturing critical failure modes and mechanisms at their different developmental stages. Acknowledgements This research was supported by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program. W. L. Gore & Associates, Inc. is acknowledged for additional support. References [1] W. Chen et al. J. Power Sources 51 (2006) 2391–2399. [2] Y. Singh et al. J. Power Sources 345 (2017) 1–11 [3] Y. Singh et al. J. Electrochem. Soc. 164 (2017) F1331-41 [4] D Ramani et al. J. Electrochem. Soc. 165 (2018) F3200-08 [5] Y. Singh et al. J. Power Sources 412 (2019) 224–37 [6] R.T. White et al. J. Power Sources 350 (2017) 94-102 Figure 1: Cross-sectional identical location MEA images obtained by XCT at beginning of life (BOL) and end of test (EOT) for separate MEAs with: a) non-reinforced; b) mechanically reinforced; and c) chemically and mechanically mitigated membranes subjected to pure chemical degradation. The encircled areas indicate the shorting sites at EOT. Figure 1

The hydrogen fuel cell is a promising technology that supports the development of sustainable energy systems and zero emission vehicles. One of the key technical challenges for the use of fuel cells in the transportation sector is the high durability requirements 1–3. One of the key components that control the overall life time of a hydrogen fuel cell is the ionomer membrane that conducts the protons and allows the separation between the anode and the cathode. During fuel cell operation, the membrane is subjected to two categories of degradation: mechanical and chemical. These degradations lead to reduction in the performance, crossover of reactants between anode and cathode and ultimately total failure of the fuel cell. The mechanical degradation occurs when the membrane swells and shrinks under the variation of the local hydration level. This leads to fatigue of the ionomer structure and ultimately irreversible damage. However, under pure mechanical degradation the damage takes a very long time to occur 4,5. Sadeghi et al. 5 observed failure of the membrane after 20,000 of accelerated mechanical stress testing. This translates into a longer lifetime in comparison to what is observed in field operation 6. The chemical degradation on the other hand is caused by the presence of harmful chemicals such as OH radicals that attack the side chains and the main chains of the ionomer 7,8. Such attacks weaken the structural integrity of the membrane and make it prone to severe mechanical damage. Hence understanding the effect of combining both categories of membrane degradation is the key to accurate prediction of the time to failure of the fuel cell. In this work we propose a novel model that represents accurately the structural properties of the membrane and couples the chemical and the mechanical degradations to estimate when the ultimate failure is initiated. The model is based on a network of agglomerated fibrils corresponding to the basic building block of the membrane structure 9–11. The mechanical and chemical properties are defined for each fibril and probability functions are used to evaluate the likelihood of a fibril to break under certain operating conditions. The description of the fundamentals behind the approach will be presented. Two set of simulations will be presented and discussed. The first one corresponding to standard testing scenarios that were used to validate the model. The second set of results will highlight the impact of coupling both degradation mechanisms on the estimation of the failure initiation time. The main strengths of the model and the future development will be discussed as well. T. Sinigaglia, F. Lewiski, M. E. Santos Martins, and J. C. Mairesse Siluk, Int. J. Hydrogen Energy, 42, 24597–24611 (2017). T. Jahnke et al., J. Power Sources, 304, 207–233 (2016). P. Ahmadi and E. Kjeang, Int. J. Energy Res., 714–727 (2016). X. Huang et al., J. Polym. Sci. Part B Polym. Phys., 44, 2346–2357 (2006). A. Sadeghi Alavijeh et al., J. Electrochem. Soc., 162, F1461–F1469 (2015). N. Macauley et al., J. Power Sources, 336, 240–250 (2016). K. H. Wong and E. Kjeang, J. Electrochem. Soc., 161, F823–F832 (2014). K. H. Wong and E. Kjeang, ChemSusChem, 8, 1072–1082 (2015). P.-É. A. Melchy and M. H. Eikerling, J. Phys. Condens. Matter, 27, 325103–6 (2015). J. A. Elliott et al., Soft Matter, 7, 6820 (2011). L. Rubatat, G. Gebel, and O. Diat, Macromolecules, 37, 7772–7783 (2004).

Perfluorosulfonic acid (PFSA) ionomer membranes in fuel cells are susceptible to operational mechanical stresses resulting in fatigue and/or creep failures that compromises their durability and lifetime. The fatigue based mechanical degradation is typically a result of repeated wet/dry humidity cycles that cause micro crack initiation and /propagation within the membrane [1]. Fatigue induced membrane failure in the form of cracks/tears/pinholes leads to a gradual increase in gas crossover and ultimately to fuel cell failure. Membranes with less conductive mechanical reinforcements have been developed to alleviate mechanical degradation yielding demonstrated improvements in lifetime and durability. Nevertheless, development of membrane damage remains a critical failure mode and the fundamental understanding of membrane mechanical degradation is a subject of ongoing research. Scanning electron microscopy (SEM) based studies have been used to characterize the degradation-induced structural changes in fuel cell membranes; however, SEM imaging is inherently destructive and inhibits any tracking of structural changes at a particular location over time. Hence, membrane degradation evolution studies are limited to ex situ analysis at various stages of degradation and with different samples. Recently, laboratory-based X-ray computed tomography (XCT) was introduced as an alternative imaging technique, which has enabled three-dimensional (3D) failure analysis of fuel cell membranes revealing novel insights on membrane failure [2,3]. In the present work, the XCT-based 3D failure analysis approach is extended to an in situ investigation of pure mechanical membrane degradation by utilizing a custom designed fixture. This X-ray transparent fixture houses a gas diffusion electrode (GDE) based MEA with a reinforced membrane, which is subjected to wet/dry cycling of N2 gas flowing through both anode and cathode sides, thereby producing a pure mechanical fatigue type degradation within the membrane. XCT-based 3D identical location tracking of membrane morphology as a function of degradation time facilitates a novel four-dimensional (4D) in situ workflow [4], which enables the characterization of the damage growth or evolution. Preliminary results show that no through- thickness membrane cracks developed until 3000 wet/dry cycles. However, minor crazes initiated on the cathode side membrane surface (Fig. 1) between 2000 and 2500 cycles. Crack initiation and growth within the reinforced membrane are comprehensively examined from various perspectives by simultaneously studying the two-dimensional (2D) planar and cross-sectional views. A clear interaction of membrane cracks with defect features, such as delamination and catalyst layer cracks is observed. Furthermore, the centrally located reinforcement layer is found to restrict the through-thickness growth of membrane cracks at several locations in the early stages of damage initiation. Overall, the size and density of membrane crack formation at a given number of cycles is found to be considerably reduced with the use of a reinforced membrane when compared to a non-reinforced membrane. A detailed study to understand the variation in degradation mechanisms between reinforced and non-reinforced membranes is carried out. Overall, the work summarized here is a unique study on the evolution of reinforced membrane degradation with a 4D perspective. The new findings from this work demonstrate the distinct advantage of XCT technology in gaining an improved fundamental understanding of membrane degradation by capturing critical failure modes and mechanisms at their different developmental stages. Acknowledgement This research was funded by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada (APC) grant. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program. The authors thank Kevin Dahl and Alex Boswell for technical support. References [1] R.M.H. Khorasany, et al. J. Power Sources 274 (2015) 1208-1216 [2] Y. Singh et al. J. Power Sources 345 (2017) 1–11 [3] Y. Singh et al. J. Electrochem. Soc. 164 (2017) F1331-41 [4] R.T. White et al. J. Power Sources 350 (2017) 94-102 Figure 1

Membrane failure is an important factor for the overall durability of polymer electrolyte fuel cells. Lifetime limiting failure occurs when the membrane loses its ability to reliably separate the reactant gases such that potentially combustive conditions may arise due to mixing of hydrogen and oxygen. While diffusive gas crossover is regularly present at benign rates, critical leak rates require significant convective fluxes that can only occur in the presence of large holes and cracks that span the full thickness of the membrane. Membrane degradation and damage development during fuel cell operation takes place through complex interplay between chemical degradation due to radical attack of the ionomer and mechanical degradation due to hygrothermal variations under mechanically constrained conditions. The combined action of chemical and mechanical degradation, which is difficult to avoid during dynamic fuel cell operation, is known to drastically accelerate the accumulation of membrane damage and be the primary cause of ultimate failure [1]. However, the process through which failure occurs is only partially understood. Membrane failure analysis has historically been performed by 2D imaging techniques such as optical and electron microscopy, which is limited to surface views of the electrodes and cross-sectional snapshots of the internal MEA structure. These techniques are also destructive in nature, demand tedious sample preparation with risk of artifacts, operate under vacuum, and/or are suitable only for electrically conductive samples. Our group recently proposed the use of X-ray computed tomography (XCT) to overcome these limitations and open up a novel non-destructive 3D characterization method for imaging of membranes inside an MEA [2]. This approach leverages recent advances in laboratory-based XCT technology to non-destructively acquire 3D images of membrane damage features and to develop a unique 3D failure analysis framework for fuel cell membranes [3]. This methodology is systematically applied to fuel cells subjected to pure chemical, pure mechanical, and combined chemical and mechanical membrane degradation in order to reveal the different types of membrane failures that may occur during fuel cell operation and assign their presence to specific degradation mechanisms. Novel discoveries made using this technique include distinct identification of I, Y, and X branched cracks (Figure 1), formation of exclusive membrane cracks, interaction of membrane and catalyst layers cracks and delamination sites, and electrode shorts due to excessive chemical membrane degradation. However, regular 3D visualization of membrane failures cannot reveal the root cause of each damage feature. Therefore, our recent efforts have focused on further exploiting the non-destructive nature of XCT scans to perform 4D membrane visualization by means of 3D scans at different points in time [4]. This allows us to track the propagation of membrane damage during the degradation process and also enables back-tracking of failure modes to determine the actual root cause by inspecting same-location scans performed at earlier times. Using this methodology, we have determined under which circumstances catalyst layer cracks may propagate into membrane cracks and vice versa. Overall, 3D visualization of membrane degradation and failure by XCT is shown to be a highly promising method to help understand complex failure modes and degradation mechanisms in fuel cells. Acknowledgements: Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program. References: 1. Lim, C., Ghassemzadeh, L., Van Hove, F., Lauritzen, M., Kolodziej, J., Wang, G.G., Holdcroft, S. and Kjeang, E., J. Power Sources 257 (2014) 102-110 2. White, R.T., Najm, M., Dutta, M., Orfino, F.P. and Kjeang, E., J. Electrochem. Soc. 163 (2016) F1206-F1208 3. Singh, Y., Orfino, F.P., Dutta, M. and Kjeang, E., J. Power Sources 345 (2017) 1-11 4. White, R.T., Wu, A., Najm, M., Orfino, F.P., Dutta, M. and Kjeang, E., J. Power Sources 350 (2017) 94-102 Figure 1. Representative reconstructed false color tomogram of an MEA, showing planar, cross-sectional, and 3D virtual views of an X-shaped membrane crack and adjacent catalyst layer cracks (M = membrane; ACL = anode catalyst layer; CCL = cathode catalyst layer). Figure 1

During automotive operation of polymer electrolyte fuel cells, dynamic duty cycles can gradually introduce various damage features within the ionomer membrane, viz. cracks, tears, pinholes, thinning, delamination etc., collectively leading to ultimate operational failure of the fuel cell. There are two main degradation mechanisms that drive damage development within the membrane: (i) chemical degradation due to radical attack; and (ii) mechanical degradation due to hygrothermal variations under mechanically constrained conditions. Although both of these mechanisms are simultaneously active during fuel cell operation, analyzing them in an uncoupled manner could aid in assessing the role of individual damage features in overall membrane failure which could potentially result in the development of effective failure mitigation strategies. Failure analysis studies of fuel cell membranes typically employ scanning electron microscopy (SEM) which is two-dimensional (2D) in nature and is inherently destructive, thereby inhibiting any possibility to track degradation-induced structural changes over time. Consequently, membrane degradation evolution studies are limited to ex situ analysis of different samples at various stages of degradation. In our recent work, laboratory-based X-ray computed tomography (XCT) was utilized to perform three-dimensional (3D) ex situ failure analysis of fuel cell membranes and the 3D nature of this imaging technique revealed novel insights into membrane failure that had thus far eluded the traditional 2D investigations [1]. Additionally, XCT imaging is also non-destructive and non-invasive [2], and the present work leverages these attributes with the objective to extend the 3D failure analysis approach to an in situ investigation of pure mechanical membrane degradation , following a unique 4D approach wherein the fourth dimension represents time or degradation state. This is achieved by utilizing a custom designed X-ray transparent fuel cell fixture [3] with which 3D visualization of identical membrane locations can be performed periodically, thereby offering a novel approach to track the structural/morphological evolution of the membrane in its true sense as a function of degradation. The custom designed X-ray transparent fixture was made with a 9 mm (length) x 4 mm (width) active area and consisted of two co-flow parallel straight channels each having 1 mm width and separated by a 250 μm wide central land region with additional land regions at the two peripheral sides. After assembling a single fuel cell within it, pure mechanical degradation in form of in situ hygrothermal fatigue was generated within the membrane by subjecting the assembled fuel cell held at 80°C to successive cycles of 2 min wet and 2 min dry states with nitrogen gas on both anode and cathode sides to eliminate chemical degradation. A laboratory-based XCT system, ZEISS Xradia 520 Versa® , was used to obtain 3D tomographic images at two different length scales: (i) low resolution (2.1 μm voxel size) large field of view (FOV) scans for inspecting the overall membrane damage; and (ii) high resolution (1.1 μm voxel size) zoomed scans of selected regions of interest for a detailed structural investigation. Tomographic data of identical locations were acquired periodically at every 500 wet/dry cycles to track membrane damage development over time. No cracks had appeared within the membrane up to 1500 wet/dry cycles, whereas a significant number of through-thickness membrane cracks had developed at 2000 cycles. This result suggests that fatigue-driven mechanical degradation progresses non-linearly over time via distinct crack initiation/propagation events. The majority of the membrane crack development occurred under the channel regions which is consistent with higher tensile stresses predicted in these regions by simulation studies [4]. A strong correlation was observed between the presence of beginning-of-life (BOL) MEA defects, mainly cathode catalyst layer cracks and membrane—catalyst layer delamination, and eventual formation of membrane cracks at those locations (cf. Figure 1). In many cases, the shape of newly developed membrane cracks resembled that of the BOL catalyst layer cracks suggesting that localized stress concentration effects may influence both crack initiation and propagation within the membrane. Overall, the novel approach for 4D same-location tracking of membrane degradation reported in this work shows significant potential for improved fundamental understanding of the membrane crack development process during mechanical degradation in fuel cells. Acknowledgements Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant. References [1] Y. Singh et al., J. Power Sources. 345 (2017). [2] R.T. White et al., J. Electrochem. Soc. 163 (2016). [3] R.T. White et al., J. Power Sources 350 (2017). [4] R.M.H. Khorasany et al., J. Power Sources 252 (2014). Figure 1

Membrane stability is an important consideration for the overall durability and lifetime of polymer electrolyte fuel cells. During operating conditions, the ionomer membrane is subjected to chemical degradation due to the formation of radicals and their subsequent attack on the chemical bonds of the polymer [1,2]. The membrane is also subjected to mechanical degradation due to the hygrothermal cycles during operation, which causes the membrane to expand and contract repeatedly leading to the development of cyclic stresses [3,4]. Recent studies show that the mechanical and chemical degradation processes are connected and, once combined, cause accelerated degradation and reduced integrity of the membrane [3–6]. On one hand, the chemical degradation causes a change in the ionomer membrane mechanical properties, transforming it from a ductile material to a brittle material [3]. This transformation makes the membrane less resistant to crack formation and propagation and increases the mechanical damage that can be caused by typical hydrothermal cycles. On the other hand, it was observed that applying compression to the membrane increases the rate of chemical decomposition [7]. Understanding the interaction between the chemical and mechanical degradation processes is therefore an important step in developing more durable membranes and improving the performance and lifetime of the fuel cell. We propose a statistical model to establish a correlation between the chemical degradation at the level of the polymer backbone chains and the initiation of microcracks due to mechanical stress. The representation of the morphology of the ionomer is based on the fibrillary structure described in [6,8]. In this representation a bundle of backbone chains is considered as a basic building block of the structure. We generate a network of bundles representative of a microcrack initiation site. A crack initiation in the membrane structure is the tipping point that leads to an accelerated degradation in terms of hydrogen leaks and ultimate failure of the fuel cell under regular operating conditions. Once a crack is initiated, it increases the amount of reactant crossover leading to an increasing chemical decomposition [4]. The crack also constitutes a stress concentration site where the cyclic mechanical load participates further in its propagation. The combination of these processes accelerates the overall rate degradation leading to a complete failure of the ionomer. Clearly, the crack initiation process is a critical root cause for the accelerated cycle of degradation of the ionomer. Our model allows for the estimation of the time of the crack initiation under relevant mechanical stresses and follows a realistic chemical degradation pattern. The statistical analysis provides a comprehensive understanding of the interaction between the local amounts and distribution of chemical decomposition sites and the mechanical stressors causing the initiation of the physical damage in the ionomer structure. [1] K.H. Wong, E. Kjeang, J. Electrochem. Soc. 161 (2014) F823. [2] K.H. Wong, E. Kjeang, ChemSusChem 8 (2015) 1072. [3] A. Sadeghi Alavijeh, M.A. Goulet, R.M.H. Khorasany, J. Ghataurah, C. Lim, M. Lauritzen, E. Kjeang, G.G. Wang, R.K.N.D. Rajapakse, Fuel Cells 15 (2015) 204. [4] A. Kusoglu, A.Z. Weber, J. Phys. Chem. Lett. 6 (2015) 4547. [5] R.M.H. Khorasany, A. Sadeghi Alavijeh, E. Kjeang, G.G. Wang, R.K.N.D. Rajapakse, J. Power Sources 274 (2015) 1208. [6] P.-É.A. Melchy, M.H. Eikerling, J. Phys. Condens. Matter 27 (2015) 325103. [7] A. Kusoglu, M. Calabrese, A.Z. Weber, ECS Electrochem. Lett. 3 (2014) F33. [8] L. Rubatat, G. Gebel, O. Diat, Macromolecules 37 (2004) 7772.

Perfluorosulfonic acid (PFSA) ionomer membranes are subjected to simultaneous chemical and mechanical degradation under fuel cell operation. Open circuit voltage (OCV) operation is typically employed as an in-situ accelerated stress test (AST) to expedite the chemical degradation which plays a key role in membrane thinning. The in-situ mechanical degradation is typically produced using wet/dry humidity cycles leading to micro crack initiation/propagation in the membrane [1]. Characterization of morphological changes in the membrane electrode assembly (MEA) is helpful in understanding the degradation mechanisms at play during fuel cell operation. 2D visualization techniques, such as optical and electron microscopy [2], have typically been used to determine the structural features of the MEA. However, these techniques are destructive and 2D in nature, and demand elaborate sample preparation. The X-ray computed tomography (XCT) technique overcomes these limitations by integrating several 2D images acquired at diverse incident angles into a virtual 3D image. Furthermore, modern XCT systems achieve sub µm resolution which is adequate for imaging membrane cracks and other damage features. In this work, the XCT technique is employed to investigate the structural evolution of membrane degradation over time in the presence of combined chemical and mechanical stressors. Additionally, the observed trends are correlated with various material and diagnostic properties to develop a better understanding of the degradation patterns. Partially degraded MEAs extracted after different numbers of operating cycles when subjected to cyclic open circuit voltage (COCV) AST protocol are visualized using the XCT technique and the evolution of membrane degradation is studied by mapping the 3D structural/morphological changes over time. Crack distribution and morphology are examined from various perspectives by studying the 2D planar and cross-sectional views of the 3D reconstructed images. No through-thickness membrane crack is detected up to 60% of the MEA lifetime which suggests that sizeable crack development occurs mainly during the 60-85% period of the MEA lifetime. A detailed survey exhibits eight cracks at end of life (EOL) and five cracks at 85% degradation across a survey area of 0.44 mm2. All identified membrane cracks are observed to have a single distinct fragment of X or I-shape on the surface of the membrane. The X-cracks in their initial stage of development observed after 85% degradation are likely to grow along their width, while I-cracks tend to be slender, as shown in Figure 1. In addition, about 31% of cracks after 85% degradation and 50% of cracks at EOL are exclusive membrane cracks without any connectivity with the catalyst layers. The membrane crack width is almost uniform along its length after 85% degradation but has greater variation along the length at EOL resulting in an increased maximum crack width to crack length ratio likely caused by in-plane stresses [3]. The rapid decrease in the fracture strain of the membrane during the AST suggests increasing brittleness, which represents the most pronounced change in the mechanical properties [4]. This illustrates that the membrane encounters reduced mechanical strength and leads to more branching of cracks with increasing AST cycles. The combined effect of mechanical and chemical degradation is likely to have created favorable conditions for crack initiation and propagation inside the mechanically weakened membrane. The work summarized here is a unique attempt to study the evolution of membrane degradation with a 3D perspective. These new findings demonstrate that adoption of XCT technology can provide a distinct advantage in understanding the pattern of membrane degradation, thereby enabling the capture of critical failure modes that may be invoked at different stages of fuel cell operation. Acknowledgement This research was funded by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada (APC) grant. References [1]R.M.H. Khorasany, A.S Alavijeh, E. Kjeang, G.G. Wang, R.K.N.D. Rajapakse. J. Power Sources 274 (2015) 1208-1216 [2]K.S Alcantara, D.C. Martınez-Casillas, K.B. Zheng, Q. Zhu, M. Abdellah, D. Haase, T. Pullerits, O. Solorza-Feria, S.E. Canton. Int. J. Hydrogen Energy, 39 (10) 5358–5370. [3]Y. Singh, O. Luo, F.P. Orfino, M. Dutta, E. Kjeang, 3D Analysis of PEM Fuel Cell Membrane Cracks using X-Ray Computed Tomography, in: 228th Meet. Electrochem. Soc., Phoenix, 2015. [4] C. Lim, L. Ghassemzadeh, F. Van Hove, M. Lauritzen, J. Kolodziej, G.G. Wang, S. Holdcroft, E. Kjeang. J. Power Sources. 257 (2014) 102. Figure 1

Recent studies have shown that a suitable approach for analyzing the mechanical durability of fuel cell stacks is to study the mechanical behavior of catalyst coated membranes (CCMs) under hygrothermal loading conditions [1–4]. Previous studies have also indicated that under cyclic mechanical and hygrothermal loadings, cracks are initiated and propagated inside the membrane [5,6] eventually leading to the cell/stack failure. The crack propagation rate of pure membranes is found to be a strong function of applied stress, temperature, and humidity [7]. Given the significant difference in mechanical properties of pure membrane and CCM [1], their crack propagation characteristics can also be expected to differ. In an operating fuel cell, this implies that a crack could potentially propagate differently when it has penetrated through the entire CCM compared to when it is confined within the membrane. To investigate this, a series of experiments were conducted to characterize the rate of crack propagation in the CCM. Rectangular specimens with a width of 10 mm and artificially created double edge cracks were used. The initial crack length on each side was approximately 0.7 mm. Under pre specified levels of environmental conditions (temperature and relative humidity), the specimens were subjected to cyclic mechanical loading [3–6] which led to crack propagation as shown in Fig. 1. The rate of crack propagation as a function of loading and environmental conditions was measured and analyzed. It is found that at elevated levels of relative humidity and temperature, the sensitivity of propagation rate to the applied mechanical loading increases. In a parallel study, a fracture mechanics model based on Paris Law theory and capable of simulating the ex situ crack propagation in the CCM during typical fuel cell operating conditions is developed. The model incorporates the characteristic time, temperature, and humidity dependent elastic-viscoplastic mechanical behaviour of CCMs [1] through a sub model developed using the finite element method (FEM) in COMSOL Multiphysics® . The stress-strain relationship of CCM simulated by the FEM sub model is validated at all combinations of 23 ºC and 70ºC temperature, 50% and 90% relative humidity; and 0.0001 s-1 and 0.001 s-1 strain rates. Fundamental fracture mechanics parameters, viz. J-integral, stress intensity factor (K), and configuration correction factor (ccf) are obtained iteratively for incremental changes in the crack length. These parameters together with the experimental crack propagation data enable the construction of Paris Curves at various temperature and humidity conditions. Information from the Paris Curves is used to predict the time taken by a CCM crack to increase from initial crack length ai to final crack length af under typical fuel cell conditions. The CCM crack propagation data collected and simulation capability developed during this work are considered to be important contributions towards developing a holistic understanding of mechanical fatigue and fracture phenomenon which are active during fuel cell operation and which ultimately lead to its failure. Acknowledgements: This research is supported by Ballard Power Systems and the Natural Sciences and Engineering Research Council of Canada through an Automotive Partnership Canada (APC) grant. References [1] M.A. Goulet, R.M.H. Khorasany, C. De Torres, M. Lauritzen, E. Kjeang, G.G. Wang, et al., J.of Power Sources. 234 (2013) 38–47 [2] R.M.H. Khorasany, M.-A. Goulet, A. Sadeghi Alavijeh, E. Kjeang, G.G. Wang, R.K.N.D. Rajapakse, J. Power Sources. 252 (2014) 176–188. [3] A. Sadeghi Alavijeh, M.-A. Goulet, R. Khorsany, J. Ghataurah, C. Lim, M. Lauritzen, et al., Fuel Cells. (2015) 204–213. [4] A. Sadeghi Alavijeh, R.M.H. Khorasany, A. Habisch, G.G. Wang, E. Kjeang, J. Power Sources. 285 (2015) 16–28. [5] R.M.H. Khorasany, A. Sadeghi Alavijeh, E. Kjeang, G.G. Wang, R.K.N.D. Rajapakse, J. Power Sources. 274 (2015) 1208–1216. [6] R.M.H. Khorasany, A. Sadhegi, E. Kjeang, G.G. Wang, R.K.N.D. Rajapakse, J. Power Sources. 279 (2015) 55–63. [7] Y. Singh, R.M.H. Khorasany, A. Alavijeh, E. Kjeang, G. Wang, R.K.N.D. Rajapakse, Fracture Properties of Fuel Cell Membranes, in: 226th Meet. Electrochem. Soc., Cancun, 2014. Figure 1

Recent advances in the development of polymer electrolyte fuel cells have proven that application of a thin micro-porous layer (MPL) on top of the macro-porous fibrous gas diffusion layer (GDL) can significantly enhance the fuel cell performance. Therefore, specific information on the structure of this layer could assist the understanding of the underlying mechanisms. Porosity and pore size distribution (PSD) are the main quantities that reveal important information about the structure. Mercury intrusion porosimetry (MIP) technique has been used previously to characterize the pore phase of the MPL [1]. Focused ion beam integrated with scanning electron microscopy (FIB-SEM) technique is another method of determining the MPL structure, in terms of both solid and pore phases [1,2]. However, these two approaches are generally destructive in nature, and may therefore skew the results by altering the MPL structure either during imaging or during post-processing. In addition, neither method can distinguish between carbon and PTFE inside the MPL [3]. The recent emergence of lab scale micro- and nano-scale X-ray computed tomography facilities provides a new opportunity for obtaining the 3D structure of porous materials by means of a non-destructive approach that can provide high quality images. This approach was used to study the cathode catalyst layer by Nano-scale X-ray computed tomography (NXCT) [4]. The reliability of the approach was assessed by comparing the obtained images by NXCT with transmission electron microscopy (TEM) images [4]. To our knowledge, there is only one published investigation with the focus on MPL imaging using NXCT [5]. In [5], an MPL was used as the sample for comparison of NXCT and FIB-SEM imaging capabilities, and some properties were calculated based on the obtained structures. By increasing the application of NXCT in the field of fuel cell materials, there will be a need for standardized imaging techniques. Hence, the objective of the present work is to develop suitable NXCT imaging, reconstruction, and post-processing protocols for MPLs. For this purpose, commercially available Sigracet SGL 24BC is used as the sample GDL material in this analysis. Two different sample preparation techniques are compared. Furthermore, various algorithms for automatic thresholding of the images are discussed and the most appropriate one for MPL segmentation is recommended. Fig. 1 shows the obtained raw versus segmented image on a cross section of the MPL, for which the porosity is obtained to be 50-52%. Shown in Fig. 2 is also the pore size distribution for the same MPL.MPL thermo-physical properties are also calculated based on the reconstructed images and compared to literature data. Moreover, for the first time, we explore the viability of determining the nanoscale PTFE distribution inside the MPL using the low energy X-ray beam of the Zeiss Xradia 810 Ultra facility. This instrument is considered state-of-the-art for laboratory NXCT and generates a monochromatic X-ray beam with 5.4 keV of energy. The low energy of the X-ray beam provides a good image contrast even for chemical compounds with low effective atomic numbers. Aknowledgment Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant. References [1]A. Nanjundappa, A.S. Alavijeh, M. El Hannach, D. Harvey, E. Kjeang, Electrochimica Acta 110 (2013) 349. [2]H. Ostadi, P. Rama, Y. Liu, R. Chen, X.X. Zhang, K. Jiang, Journal of Membrane Science 351 (2010) 69. [3]M. El Hannach, R. Singh, N. Djilali, E. Kjeang, Journal of Power Sources 282 (2015) 58. [4]W.K. Epting, J. Gelb, S. Litster, Advanced Functional Materials 22 (2012) 555. [5]E. a. Wargo, T. Kotaka, Y. Tabuchi, E.C. Kumbur, Journal of Power Sources 241 (2013) 608. Figure 1

Crack and pinhole formation in polymeric membranes of fuel cells is understood to be caused by combined chemical, mechanical, and/or thermal mechanisms that are active during operation under typical automotive duty cycles [1] . This type of damage leads to gas leakage through the membrane thereby compromising its functionality and overall durability of the fuel cell system. Due to the innermost location of membrane within the fuel cell membrane electrode assembly, it has been impossible to explore the full features and morphology of membrane cracks with conventional 2D imaging techniques such as optical and electron microscopy [2]. Moreover, these techniques are destructive in nature, demand tedious sample preparation with risk of artifacts, operate under vacuum, and/or are suitable only for electrically conductive samples. The X-ray computed tomography (XCT) technique overcomes these traditional limitations and opens up a novel non-destructive 3D characterization method for imaging interior features of an object. Synchrotron facilities have been typically used for 3D imaging but their access is limited and costly. This work leverages the XCT technology using laboratory-based systems developed by ZEISS Xradia® to image membrane cracks in their natural state within the membrane electrode assembly (MEA) and gain a comprehensive insight into their features and characteristics. An end-of-life (EOL) MEA sample, subjected to cyclic open circuit voltage (COCV) accelerated stress test (AST) protocol [3], is analyzed upon failure using the XCT technique and compared with a similar beginning-of-life (BOL) sample. Based on their reach in the through-plane direction, cracks penetrating the entire membrane thickness are categorized into: (i) Exclusive cracks that remain confined within the membrane; and (ii) Non-exclusive cracks that extend into and penetrate through the anode and/or cathode catalyst layers. The very presence of Exclusive cracks (cf.Fig. 1), without any adjoining cracks in the catalyst layers, indicates the likelihood of crack initiation within the membrane. An examination of 9 membrane cracks spread over a 0.88 mm2area reveals that more than 50% of the cracks fall into the Exclusive category. This percentage is found to be significantly higher than the 13% overall interaction of catalyst layer cracks with the membrane cracks. In the in-plane direction, some membrane cracks are found to propagate as a single entity forming a curved I-shape while the others branch out once forming a Y-shape. Distribution of the two shapes is approx. equal among the analyzed cracks. The smaller Y-cracks ( The total in-plane crack length in the membrane is found to have a linear relationship with the maximum crack width. The crack width remains almost uniform along its length and tapers sharply at the ends indicating that the crack propagation in the membrane is mechanical in nature caused by the cyclic in-plane stresses. The effect of these stresses seems to have been exacerbated by the observed non-uniform reduction in membrane thickness resulting in stress concentration locations within the membrane. During this analysis, a strong probability for crack development is observed at such locations with membrane thickness found to be approx. 30% lower at the crack sites than its average EOL bulk value. The work reported here is unprecedented from the perspective that, for the first time, a truly 3D view of a membrane crack is achieved; thus, making this comprehensive analysis possible. These results, establish the XCT as a breakthrough approach for reliable failure analysis of fuel cell membranes. Acknowledgements This research is financially supported by Ballard Power Systems and Automotive Partnership Canada (APC). The authors thank Chan Lim, Lida Ghassemzadeh and Erin Rogers for providing samples and technical support. References [1] S. Kundu, M.W. Fowler, L.C. Simon, S. Grot, J. Power Sources. 157 (2006) 650–6. [2] F.H. Garzon, S.H. Lau, J.R. Davey, R.L. Borup, ECS Trans. 11 (2007) 1139–49. [3] C. Lim, L. Ghassemzadeh, F. Van Hove, M. Lauritzen, J. Kolodziej, G.G. Wang, et al., J. Power Sources. 257 (2014) 102. Figure 1

The membrane electrode assembly (MEA) in polymer electrolyte membrane (PEM) fuel cells undergoes hygrothermal cyclic loading during normal operating conditions. These cyclic loadings create in plane and through plane cyclic stresses being distributed in the membrane. To understand the mechanical fatigue behavior, samples in dogbone shapes (Figure 1) are prepared and placed under the application of cyclic mechanical loadings in controlled ambient conditions [1]. Both pure PFSA membrane and catalyst coated membrane samples are used in the experiments. The tests are conducted ex-situ and the environmental conditions selected in this study cover the temperature and relative humidity levels from those of room conditions to fuel cell conditions. The experimental results are used to benchmark the effect of environmental and loading conditions on the fatigue lifetime of the membrane. Our previous studies [2,3] show that the catalyst layers play a significant role in the mechanical behavior of catalyst coated membranes. Hence, in this study the effect of catalyst layers on the mechanical durability and final elongation of the membrane before the mechanical failure is also investigated. A strain based fatigue model is then used to develop a finite element numerical scheme for simulating the membrane fatigue lifetime under hygrothermal mechanical loadings. Using the experimental results, the accuracy of the finite element simulations results are verified (Figure 1). The effects of temperature and relative humidity swings on the mechanical longevity of the pure membrane are explored. It is seen that hydration swings have a more profound effect on the fatigue lifetime of the membrane than the temperature swings. Then, using the ex-situ results for the catalyst coated membrane, the impact of the catalyst layers on the membrane/CCM fatigue lifetime is investigated. The finite element model is then used to study the in-situ mechanical stability of pure membranes and catalyst coated membranes under hygrothermal cyclic conditions. Acknowledgements: This research was supported by Ballard Power Systems and the Natural Sciences and Engineering Research Council of Canada through an Automotive Partnership Canada (APC) grant. References [1] R.M.H. Khorasany, A.S. Alavijeh, E. Kjeang, G.G. Wang, R.K.N.D. Rajapakse, J. Power Sources (2014) under review. [2] M.A. Goulet, R.M.H. Khorasany, C. De Torres, M. Lauritzen, E. Kjeang, G.G. Wang, N. Rajapakse, J. Power Sources 234 (2013) 38-47. [3] R.M.H. Khorasany, M.-A. Goulet, A.S. Alavijeh, E. Kjeang, G.G. Wang, R.K.N.D. Rajapakse, J. Power Sources 252 (2014) 176-188. Figure 1: (a) Dogbone sample used in simulations (with dimensions given in mm), membrane fatigue lifetime at room conditions (23oC, 50% RH) under cyclic mechanical loadings with a maximum force of (b) 1.16 N and (c) 0.95

Understanding the crack propagation behavior of common perfluorosulfonic acid (PFSA) ionomer membranes under hygrothermal and mechanical cyclic loading is of vital significance in designing more durable fuel cell stacks (1). Hence, we have developed a test procedure to shed light on the crack propagation characteristics of membranes as a function of loading and environmental conditions. Rectangular specimens with a width of 10 mm and cracks with an average length of 0.7 mm on both sides are used. The specimens are placed under cyclic mechanical loading in a controlled level of environmental conditions (temperature and humidity). The crack propagation rate in the membrane is measured as a function of temperature, humidity and the amplitude of mechanical loading (Figure 1). It is found that a small linear decrease in the magnitude of the force results in an exponential decrement in the crack propagation rate. Furthermore, for a given state of mechanical load, it is found that increasing the relative humidity and/or temperature can significantly increase the crack propagation rate. In a parallel study, a fracture mechanics-based model capable of simulating the in-situ crack initiation and propagation in the membrane during fuel cell operation is also developed. The elastic-viscoplastic nature of PFSA membranes is well-established in (2). A finite element method (FEM) based constitutive model that can simulate this behavior is used to relate the stress and strain at different environmental conditions (3). Numerical simulations reveal the presence of cyclic mechanical stresses within the membrane due to dynamic hygrothermal conditions present in a running fuel cell that can cause the initiation and propagation of cracks through mechanical membrane degradation. The dynamic stress-field thus obtained is coupled with the fracture mechanics model. In-situ test cases for various temperature, humidity and strain rate conditions are run to fully understand their effect on crack propagation rates and ultimate failure of the membranes. Acknowledgements: This research is supported by Ballard Power Systems and the Natural Sciences and Engineering Research Council of Canada through an Automotive Partnership Canada (APC) grant. References 1. R. Borup, J. Meyers, B. Pivovar, Y. S. Kim, R. Mukundan et al, Chem. Rev. 107 3925 (2007). 2. M.A. Goulet, R.M.H. Khorasany, C. De Torres, M. Lauritzen, E. Kjeang, G.G. Wang, N. Rajapakse, J. Power Sources 234 38 (2013). 3. R.M.H. Khorasany, M.A. Goulet, A.S. Alavijeh, E. Kjeang, G.G. Wang, R.K.N.D. Rajapakse, J. Power Sources 252 176 (2014).