Contamination of Low Platinum Catalyst Loading Cathodes for Proton Exchange Membrane Fuel Cells

Challenges and opportunities in modelling of proton exchange membrane fuel cells (PEMFC)

In recent years, polymer electrolyte membrane fuel cells (PEMFCs) have garnered significant attention due to their remarkable attributes including high power density, energy efficiency, and zero-emission characteristics. This surge in interest has primarily been driven by the potential of PEMFC-powered fuel cell electric vehicles (FCEVs) to substantially reduce carbon dioxide emissions in the transportation sector, thereby propelling the advancement of the hydrogen economy on a global scale. However, the widespread commercialization of FCEVs hinges upon addressing key challenges surrounding the durability and resilience of fuel cell systems.One critical issue that has emerged is the phenomenon of hydrogen starvation in the anode, particularly during start-up/shutdown or cold start scenarios, which can lead to rapid and severe degradation of PEMFC performance and durability through cell reversal. This degradation manifests quickly, often within minutes, resulting in abrupt cell failure. Extensive prior research has been devoted to unraveling the mechanisms underlying degradation induced by hydrogen starvation and cell reversal in PEMFCs, with the goal of identifying effective mitigation strategies to minimize damage to membrane electrode assembly (MEA) components.In this study, we introduce an innovative materials approach aimed at mitigating cell damage by employing a thin film Pt anode catalyst supported on Nafion nanowires in a co-axial configuration, referred to as coaxial nanowire electrode (CANE). Leveraging the high roughness of Nafion nanowires, CANE eliminates the need for conventional carbon support and exhibits exceptional tolerance to reversal-induced stress. We present a comprehensive evaluation of CANE as a reversal-tolerant anode, comparing its performance to that of conventional supported catalysts (Pt/C). Furthermore, we explore the potential for further enhancing durability by integrating iridium with the CANE anode. Finally, we will delve into our efforts to evaluate the feasibility of implementing this concept in structures that can be manufactured at scale. This research contributes to the ongoing efforts to enhance the robustness and longevity of PEMFC systems, thereby advancing the prospects of FCEVs as a sustainable and viable solution for future transportation needs.AcknowledgementThis research was supported by the Hydrogen and Fuel Cell Technologies Office (HFTO), Office of Energy Efficiency and Renewable Energy, US Department of Energy (DOE) through the Million Mile Fuel Cell Truck (M2FCT) consortia, technology managers G. Kleen and D. Papageorgopoulos. Authors would also like to acknowledge support for this work from the Laboratory Directed Research and Development (LDRD) program at Los Alamos National Laboratory (LANL) (2020200DR).

State-of-the-art polymer electrolyte membrane fuel cells (PEMFC) cathode catalysts are platinum or platinum alloy nanoparticles loaded on a high surface area carbon support. Although these catalysts have been successfully implemented in passenger fuel-cell electric vehicles (FCEVs) (e.g., the Toyota Mirai), these catalysts are facing many performance and durability challenges that limit their applications in medium-duty and heavy-duty transportation. Among these challenges is the corrosion of the carbon support, which represents a source of efficiency loss (i.e., maximum voltages must be “clipped” to prevent catalyst degradation) and limitation of PEMFC lifetime. Corrosion of the carbon support affects catalyst utilization and the transport properties of the cathode as it can result in loss of electrical connection to the platinum alloy nanoparticles and destruction of electrode porosity.1-4 This talk introduces a new class of PEM fuel cell cathode catalyst supports called electron-conductive three-dimensional inorganic nanocrystals. The highly-conductive nanocrystals are fabricated by a chemical vapor deposition (CVD) assisted route using a home-built vertical plug-flow fused-quartz reactor. The developed synthesis method can produce gram scale amounts and operates in batch mode. Platinum nanoparticles have been introduced to the surface of newly- developed supports using different synthetic routes in order to demonstrate the excellent support durability after being subjected to different synthetic treatments. The ORR activity, catalyst performance durability, and catalyst support durability have been evaluated by means of the thin-film rotating disk electrode (TF-RDE) technique. The platinum content in the prepared catalysts has been measured using inductively coupled plasma mass spectrometry (ICP-MS) and X-ray fluorescence (XRF). The morphology, crystal structure, and spatial distribution of the metal nanoparticles have been characterized by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM), X-ray powder diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDS), respectively. The electronic structure of the conductive 3-D inorganic nanocrystals has been identified by means of electron energy-loss spectroscopy (EELS), X-ray diffraction (XRD), and thermal gravimetric analysis (TGA) to be graphene-like with sp2 hybridized bonds.AcknowledgementsThis work was supported by the United States Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office. Argonne is a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC.References F. Cetinbas, R. K. Ahluwalia, N. N. Kariuki, and D.J. Myers, J. Electrochem. Soc., 165(13), F1051 (2018).Kongkanand and M. F. Mathias, J. Phys. Chem. Lett., 7, 1127 (2016).Subbaraman, D. Strmcnik, A. P. Paulikas, V. R. Stamenkovic, and N. M. Markovic, Chemphyschem, 11, 2825 (2010).Cullen, David A., K. C. Neyerlin, Rajesh K. Ahluwalia, Rangachary Mukundan, Karren L. More, Rodney L. Borup, Adam Z. Weber, Deborah J. Myers, and Ahmet Kusoglu. "New roads and challenges for fuel cells in heavy-duty transportation." Nature Energy (2021): 1-13.

The polymer electrolyte membrane (PEM) fuel cell is a promising alternative power source for electric vehicles, forklifts, and stationary backup power due to its high power density, low operating temperature, and clean emissions. The gas diffusion layer (GDL) facilitates the transport of heat, reactant gases, water vapor, and liquid water in the PEM fuel cell. The GDL is a highly porous material composed of dense arrays of carbon fibers, resins, and polytetrafluoroethylene (PTFE). Additionally, it is typically coated with a micro porous layer (MPL), which results in the reduction of contact resistance at the catalyst layer interface and the overall improvement in water management. Effective water management must be achieved within the GDL over the entire operating lifetime in order to improve the commercial success of PEM fuel cells. Investigations into the effects of ageing on water management within the GDLs can be useful tools to guide this success. Water is required to maintain polymer electrolyte membrane hydration and ionic conductivity; however, excess water tends to accumulate in the catalyst layer, GDL, and flow channels. This accumulation limits mass transport and leads to performance losses. Therefore, effective water management in the GDLs is essential for improving cell performance, durability, and stability. Most GDL research has been focused on the impact of GDL materials and microstructure design on overall performance, rather than durability. Decreased GDL hydrophobicity has been found after only a few hundred hours of operation (1, 2). Therefore, a clear understanding of the influence of GDL degradation on the liquid water behavior is essential. As a tool to visualize this behaviour, synchrotron X-ray radiography is a powerful technique for visualizing liquid water transport behavior within an operating PEM fuel cell. High intensity X-rays generated at the BMIT-BM beamline of the Canadian Light Source were used for identifying liquid water inside the GDLs and capturing the multiphase flow behavior in an operating PEM fuel cell. Figure 1 is an example synchrotron X-ray radiograph of the fuel cell in operando, illustrating the spatial distribution of liquid water. The observed pixel brightness was correlated to the presence of liquid water, and the saturation profile across the GDL thickness was obtained. In this study, as-received fresh GDLs were immersed in a 35wt % solution of H2O2 at 90o C for 12 hours in order to perform an accelerated artificial ageing process. As-received fresh and artificially aged GDLs were then used to assemble specialized fuel cells for synchrotron imaging. Both the electrochemical performance and liquid water transport behavior were observed for a range of current densities. Additionally, the effect of changes in relative humidity on performance was compared to investigate the effect of artificial ageing. Notable differences in cell performance and liquid water transport behavior were observed at steady state operating conditions. The cell with aged GDLs showed lower cell voltage and higher quantities of liquid water than those of as-received fresh GDLs. The impacts of aging on the liquid water distributions within the GDL and on cell performance will be presented in this work. The objective of current work is to highlight the potential degrading effect on liquid water management in long term fuel cell operation.

Proton exchange membrane fuel cell vehicles using hydrogen as fuel has become one of the key development directions in the field of new energy vehicles. However, hydrogen is flammable, it is easy to trigger serious safety accidents once hydrogen leak occurs. Therefore, the application of hydrogen leakage diagnosis methods is essential for the safe operation of proton exchange membrane fuel cell system. This paper reviews the existing methods of diagnosing hydrogen leakage for proton exchange membrane fuel cell system. The principles, research status and application scopes of different methods are elaborated, and these methods are analyzed based on three aspects of on-line diagnosis, real-time performance and calculation complexity. Furthermore, suggestions are provided for the selection of on-line hydrogen leakage diagnosis methods for fuel cell vehicles. Based on this review, readers can easily understand the current status of hydrogen leakage diagnosis methods for proton exchange membrane fuel cell system, and it can provide references for the application selection of the hydrogen leakage diagnosis methods for fuel cell vehicles.

To date, the world has been making a massive shift away from fossil fuels towards cleaner energy sources. For the past decade, polymer electrolyte membrane fuel cells (PEMFCs) powered by hydrogen have attracted much attention as a promising candidate for eco-friendly vehicles, i.e. fuel cell electric vehicles (FCEVs), owing to their high power density, high efficiency and zero emission features. Since the world’s first mass production of Tucson ix35 FCEV by Hyundai in 2013, global automotive original equipment manufacturers (OEMs) have focused on commercialising FCEVs. In 2018, Hyundai also unveiled the second generation of the mass-produced FCEV (i.e. Nexo) with improved performances and durability compared with its predecessor. Since then, the global market for PEMFCs for a variety of FCEV applications has been growing very rapidly in terms of both passenger vehicles and medium- and heavy-duty vehicles such as buses and trucks, which require much higher durability than passenger vehicles, i.e. 5000 h for passenger vehicles vs. 25,000 h for heavy-duty vehicles. In addition, PEMFCs are also in demand for other applications including fuel cell electric trains, trams, forklifts, power generators and vessels. We herein present recent advances in how hydrogen and PEMFCs will power the future in a wide range of applications and address key challenges to be resolved in the future.

This study presents a complete three-dimensional, two-phase transport model for proton exchange membrane fuel cells based on the two-fluid method, which couples the mass, momentum, species, and electrical potential equations. The different liquid water transport mechanisms in the flow channels, gas diffusion layers, catalyst layers, and membrane are modeled using two different liquid water transport equations. In the flow channels, gas diffusion layers, and catalyst layers, the generalized Richards equation is used to describe the liquid water transport including the effect of the pressure gradient, capillary diffusion, evaporation and condensation, and electro-osmotic, while in the membrane, the liquid water transport equation only takes into account the effect of back diffusion and electro-osmotic. Springer’s model is utilized on the catalyst layer-membrane interface to maintain continuum of the liquid water distribution. The model is used to investigate the effect of flow channel aspect ratio on the performance of fuel cells with single and triple serpentine flow fields. The predictions show that for both flow fields, the cell performance improves with decreasing aspect ratio. The aspect ratio has less effect on the cell performance for the triple serpentine flow field than for the single serpentine flow field due to the weaker under-rib convection.

Proton exchange membrane fuel cells (PEMFCs) represent a promising technology for clean energy conversion. The key chemical transformation within a fuel cell is conducted by catalyst nanoparticles tethered to a carbon support. Although the carbon support plays a critical role in maximizing the performance and durability of the catalyst nanoparticles and mass transport, predictably tuning the chemical environment and pore architecture of the carbon support remains a challenge. Metal–organic frameworks are a class of crystalline porous coordination polymers known for their structural and chemical tunability. Recently, metal–organic frameworks (MOFs), almost exclusively zeolitic imidazolate framework-8 (ZIF-8), have been used as precursors to generate carbon supports where the MOF architecture serves as a template to generate the carbon skeleton. As a result, these materials have an improved homogeneous porous landscape and, encouragingly, superior PEMFCs performance and durability. Herein, we leverage the vast library of MOFs and report design principles for generating precisely tuned carbon support architectures through systematic changes to the structure and chemical composition of MOFs. Advanced fuel cell testing of PEMFCs featuring these novel carbon architectures offers fine level insight into the effects of pore size, connectivity and chemistry on the performance of PEMFCs.Acknowledgement: This research was supported by the Hydrogen and Fuel Cell Technologies Office (HFTO), Office of Energy Efficiency and Renewable Energy, US Department of Energy (DOE) through the Million Mile Fuel Cell Truck (M2FCT) consortium, technology managers G. Kleen and D. Papageorgopoulos.

Fuel cells represents one of the most promising ways to obtain sustainable energy because they provide power and heat cleanly and efficiently, using diverse domestic fuels like hydrogen produced from renewable resources, biomass-based fuels, and natural gas. Fuel cells open the way to integrated “open energy systems” and they have the potential to reduce greenhouse gas emissions in many applications. For instance, applications have been demonstrated in combined heat and power (CHP) systems, light-duty highway vehicles, fuel cell electric vehicles (FCEVs) and Auxiliary power units (APUs) [1].Transportation is one of the wider areas of application for fuel cells. While Internal Combustion Engines (ICEs) generate power without any catalysts, needing precious metals only for catalytic converters in order to eliminate noxious gases, the catalysts found in electrodes are key to electric power generation by Polymer Electrolyte Membrane Fuel Cells (PEMFCs). Moreover, efforts in the catalysis of the PEMFC reactions (1) and (2) are the key to enhance sustainable power generation. The cathode is the most studied electrode as the major source of losses in efficiency and power density due to the five-orders-of-magnitude slower kinetics of the oxygen reduction reaction (ORR) on Pt, Reaction 2 [2].2H2 →4H+ Anode Reaction (1)O2+4H++ 4e-→2H2O Cathode Reaction (2)Pt is expensive and of limited availability. This greatly impacts the cost of PEMFCs, and consequently limits their potential for mass commercialization. In order to reduce the Pt loading in cathodes without decreasing their performance there are three options: (1) enhancing Pt mass activity for ORR via alloying or core-shell nanostructuring, (2) improving mass-transport properties of Pt-based cathodes, and (3) developing well-performing non-precious metal catalysts (NPMCs) for ORR [2, 3].The aim of this study is to estimate the voltage drop due to transport processes inside a non-precious metal loaded cathode catalyst layer, namely, electron, proton and oxygen transport. There are many problems with obtaining accurate fits to polarization data, both in the kinetic region of the polarization curve as Figure 1 shows, and in mass-transport-controlled regions.In an NPM catalyst layer, we need to identify and classify the main parameters to describe catalyst layers such as catalyst loading and thickness, diffusion coefficients, active catalyst surface area, volume fraction of polymer in the agglomerate. For this we will use the Jaouen model [4] as a starting point and then validate the result with our catalyst characterization and structure measurements. It is also critical to use adequately complex experimental data sets to be analyzed and to include as many independently and experimentally determined parameters as are available to reduce the degrees of freedom for fitting. Only then will a modeling study serve to reveal the physical basis of the important performance differences resulting from the use of given catalyst layer structures, on one hand, and different non-precious reaction pathways on the other. Figure 1.Kinetic model vs. experimental results for NPMC. Acknowledgement We gratefully acknowledge the support of the NSF EPSCoR program and Colciencias for support of this work.

The membrane electrolyte assembly (MEA) is the key component of a proton exchange membrane fuel cell (PEMFC). The MEA usually consists of gas diffusion layers as outer layers to the inner catalyst coated membrane (CCM). There are various methods to prepare CCMs and decal transfer is currently a common method which was first introduced by Wilson and Gottesfeld [1] and further developed by other researchers [2]–[4]. The initial step of this method is that the catalyst ink is coated onto an intermediate substrate material creating a catalyst coated film (CCF). This step is followed by a transfer of the active layer onto the membrane by hot pressing the membrane and CCF between heated press platens maintained at a specific high temperature for an optimized time and pressure to ensure bonding.Extensive studies on the procedure of decal transfer, substrate material and its preparation, catalyst solvents, etc. have increased the transfer yield of decal transfer to more than 95% [5]. Additionally, in the last decade, industrial development has reduced its cost; however, the total cost for fuel cell manufacturing is still relatively high. One reason for this high production cost is the contamination sensitivity of the CCMs by external particles that can potentially be detrimental to fuel cell operation [6]. This requirement necessitates the use of cleanrooms and quality control equipment to prevent contaminants entering the manufactured components, thus increasing the overall costs. The purpose of the present research is to improve the understanding of the interactions between foreign contaminants and the manufacturing process in the context of MEA production quality. The specific objectives are to i) understand the impact of external particles on the catalyst layer decal transfer process and ii) improve the robustness of the decal transfer method in the presence of external particles.This research investigates the impact of incidental external particles that may be found on the membrane surface or the CCF prior to hot pressing. 60µm Silica microspheres (Si-M) were selected as a representative of solid particles and CCMs with purposely introduced Si-Ms were fabricated and several decal transfer methods and support materials were tested and imaged using the X-ray computed tomography (XCT) technique to analyze the impact on the fuel cell integrity and operation at the presence of the Si-M. While regular decal transfer protocols result in excessive membrane thinning in the presence of these particles, it was observed that by changing the rate of applied pressure and using alternative support materials when transferring the cathode catalyst onto a half-coated membrane, it was possible to reduce membrane thinning under the particles by more than 20% while maintaining transfer quality and cell performance (Fig. 1). In addition, it was observed that although anisotropic mechanical properties of the membrane can adversely affect the CCM topography after the decal transfer, a tuned protocol is able to prevent unwanted deformations and potential stress concentrations. Overall, it is envisioned that the outcomes of this work may enable relaxed quality control measures and manufacturing site cleanroom standards by reducing the potential effects of external particles on MEA production quality. Acknowledgements 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, Ballard Power Systems, and W.L. Gore & Associates. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program. Keywords: fuel cell; membrane durability; X-ray computed tomography; decal transfer; manufacturing References Wilson, M. S. & Gottesfeld, S. Thin-film catalyst layers for polymer electrolyte fuel cell electrodes. J. Appl. Electrochem. 22, 1–7 (1992).Shahgaldi, S., Alaefour, I. & Li, X. Impact of manufacturing processes on proton exchange membrane fuel cell performance. Appl. Energy 225, 1022–1032 (2018).Cho, H. J. et al. Development of a novel decal transfer process for fabrication of high-performance and reliable membrane electrode assemblies for PEMFCs. Int. J. Hydrogen Energy 36, 12465–12473 (2011).Thanasilp, S. & Hunsom, M. Effect of MEA fabrication techniques on the cell performance of Pt-Pd/C electrocatalyst for oxygen reduction in PEM fuel cell. Fuel 89, 3847–3852 (2010).Liang, X., Pan, G., Xu, L. & Wang, J. A modified decal method for preparing the membrane electrode assembly of proton exchange membrane fuel cells. Fuel 139, 393–400 (2015).James, B. D., Moton, J. M. & Colella, W. G. Mass Production Cost Estimation of Direct H2 PEM Fuel Cell Systems for Transportation Applications: 2018 Update. ASME 2014 12th Int. Conf. Fuel Cell Sci. Eng. Technol. collocated with ASME 2014 8th Int. Conf. Energy Sustain. V001T07A002–V001T07A002 (2018). Figure 1

Proton exchange membrane (PEM) fuel cells have gained increasing interest from academia and industry, due to its remarkable advantages including high efficiency, high energy density, high power density, and fast refueling, also because of the urgent demand for clean and renewable energy. One of the biggest challenges for PEM fuel cell technology is the high cost, attributed to the use of precious platinum group metals (PGM), e.g., Pt, particularly at cathodes where sluggish oxygen reduction reaction takes place. Two primary ways have been paved to address this cost challenge: one named low-loading PGM-based catalysts and another one is non-precious metal-based or PGM-free catalysts. Particularly for the PGM-free catalysts, tremendous efforts have been made to improve the performance and durability—milestones have been achieved in the corresponding PEM fuel cells. Even though the current status is still far from meeting the expectations. More efforts are thus required to further research and develop the desired PGM-free catalysts for cathodes in PEM fuel cells. Herein, this paper discusses the most recent progress of PGM-free catalysts and their applications in the practical membrane electrolyte assembly and PEM fuel cells. The most promising directions for future research and development are pointed out in terms of enhancing the intrinsic activity, reducing the degradation, as well as the study at the level of fuel cell stacks.

In proton exchange membrane fuel cells (PEMFCs), appropriate water management is critical to achieve high power density operation with increased robustness. Proton exchange membranes (PEMs) require sufficient hydration to fulfill its function as proton conductor, while flooding at the cathode side can hamper the transport of reactant, resulting in deterioration of cell operation. Liquid water transport and accumulation should be affected by materials impregnated in membrane electrode assemblies (MEAs) and this causes variation of cell performance. This motivated us to investigate effects of impregnated materials in MEAs on liquid water accumulation behaviors and resultant cell performance. For this purpose, soft X-ray radiography has been proof it is a powerful incitement for observing liquid water in PEMFCs because of the high spatial and temporal resolution. [1-5]In this study, we focused our attention on micro porous layers (MPLs) with vapor growth carbon fiber (VGCF, Unitetek international co. LTD, Taiwan) impregnated in MEA, because VGCF-modified MPLs have been shown for better cell performance especially in high current density operation, suggesting an improvement on liquid water transport [6]. We first examined polarization curves of two types of MEAs with or without VGCF in MPL. In Fig.1, we observed better cell performance with a MEA with VGCF-modified MPL than those with a conventional MEA without VGCF. In the experiment, we fabricated VGCF MPL coated on gas diffusion substrate (SIGRACET® 24BA). We also used commercial gas diffusion layer (SIGRACET® 24BC) for comparison. In both MEAs, we fabricated catalyst coated membranes (CCMs) by decal transfer method. Nafion® 212 had been selected as the membrane and the same batch of catalyst layer with 0.27mgPt cm-2 loading had been used. The hydrogen and air were fed at 200mL min-1 and 500mL min-1 for a 5cm-2 (23 mm x 23 mm) active area normal cell, respectively. The channel width and depth were 1.0 and 1.0 mm. The rib-to-channel ratio was 1 for normal cell. For a X-ray cell (0.03cm-2, 1 mm x 3 mm), hydrogen and air were both fed at 15mL min-1. The channel width and depth were 0.5 and 0.5 mm. The rib-to-channel ratio was 1 for X-ray cell. Cell operating temperature was set at 80°C and relative humidity of the reactants was kept at 92%. In situ visualization of liquid water distribution in both MEAs by soft X-ray radiography clearly shows their difference as shown in Fig.2. Form the original image Fig.2 (a) and (b), we can found VGCF sample (Fig.2(b)) was difficult to recognize the MPL. The one of the reason is the MPL infiltrate into gas diffusion substrate and another reason is the VGCF MPL maybe has high porosity thus it didn’t absorb much soft X-ray cause to similar intensity of GDL and VGCF-MPL. In both cases (Fig.2(c) and (d)), liquid water was observed under the rib, however, for VGCF-modified MPL at 0.7A cm-2 (Fig.2(d)), less spatially-dispersed liquid water was observed under the rib, while liquid water closed to the catalyst layer was identified in the conventional MEA (Fig.2(c)). At high current density operation (1.2A cm-2), liquid water in the MEA with VGCF was still accumulated closed to the rib. These observations by soft X-ray radiography indicate that VGCF-MPL plays a role to guide liquid water removal in MEAs, which resulted in better cell performance, although physicochemical properties of VGCF-MPL should be unveiled in the future.This work has been supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

The durability of proton exchange membranes (PEMs) is a critical factor influencing the reliability and longevity of PEM fuel cells (PEMFCs). In the quest for durable fuel cell systems, expanded polytetrafluoroethylene (ePTFE) reinforced ionomer membranes stand as a promising avenue for designing membrane electrode assembly (MEA) compared to plain perfluorinated sulfonic acid (PFSA) membranes. The incorporation of ePTFE reinforcement introduces a complex microstructure that influences the membrane's ability to withstand chemical attack, mechanical stress, and oxidative damage during operation1,2. Despite several recent studies demonstrating enhanced durability in accelerated stress testing (AST)3–5, the intricate relationship between the membrane's microstructure, ePTFE reinforcement, and chemo-mechanical degradation in fuel cell operation has not been explored. In the present work, the durability of MEAs composed with ePTFE-reinforced ionomer membranes of 15 µm nominal thickness in PEMFCs was investigated using a combined chemical and mechanical accelerated stress test (CM-AST). The chemo-mechanical degradation of the ePTFE-reinforced membranes was characterized using X-ray computed tomography (XCT). The cell exhibited two degradation stages under CM-AST. The initial phase of 25 AST cycles was marked by a notable decline in cell performance. Subsequently, in the second phase, the cell maintained consistent operation until the conclusion of testing at the 65th cycle. The cell degradation rate for the first stage was 62 µV h-1 and the overall degradation rate was 58 µV h-1. The hydrogen crossover current density slowly increased from 3.3 mA cm−2 to 5.1 mA cm-2 at 80 °C under atmospheric pressure. A comprehensive analysis of cells using XCT showed significant decay of the catalyst layers and membrane thinning. 3D visualization by XCT scans of the entire active surface area confirmed the absence of pinholes or cracks in the membrane that explained the observed low hydrogen crossover. Chemical degradation was observed in the ionomer-rich layers, while no signs of ePTFE reinforcement degradation were found. Therefore, the reinforcement layer plays a key role in mitigating the chemical and mechanical degradation by limiting the gas crossover rather than altering the degradation process. The results concluded that by optimizing the microstructure of ePTFE, reinforced membranes can pave the way for more reliable and durable PEMFC systems.Keywords: Composite ionomer membranes, ePTFE reinforcement, PEMFC, chemo-mechanical degradation, accelerated stress testing, X-ray computed tomography References (1) Liu, W.; Suzuki, T.; Mao, H.; Schmiedel, T. Development of Thin, Reinforced PEMFC Membranes through Understanding of Structure-Property-Performance Relationships. ECS Trans. 2013, 50 (2), 51–64. https://doi.org/10.1149/05002.0051ecst.(2) Shi, S.; Weber, A. Z.; Kusoglu, A. Structure/Property Relationship of Nafion XL Composite Membranes. Journal of Membrane Science 2016, 516, 123–134. https://doi.org/10.1016/j.memsci.2016.06.004.(3) Ramani, D.; Singh, Y.; White, R. T.; Wegener, M.; Orfino, F. P.; Dutta, M.; Kjeang, E. 4D in Situ Visualization of Mechanical Degradation Evolution in Reinforced Fuel Cell Membranes. International Journal of Hydrogen Energy 2020, 45 (16), 10089–10103. https://doi.org/10.1016/j.ijhydene.2020.02.013.(4) Sadeghi Alavijeh, A.; Bhattacharya, S.; Thomas, O.; Chuy, C.; Kjeang, E. A Rapid Mechanical Durability Test for Reinforced Fuel Cell Membranes. Journal of Power Sources Advances 2020, 2, 100010. https://doi.org/10.1016/j.powera.2020.100010.(5) Tang, H.; Pan, M.; Wang, F.; Shen, P. K.; Jiang, S. P. Highly Durable Proton Exchange Membranes for Low Temperature Fuel Cells. J. Phys. Chem. B 2007, 111 (30), 8684–8690. https://doi.org/10.1021/jp073136t.

This work concerns the study and the modeling of hybrid Proton Exchange Membrane (PEM) Fuel Cell electric vehicle. In fact, the paper deals with the model description of the powertrain which includes two energy sources: a PEM Fuel Cell as a primary source and a supercapacitor as a secondary source. The architecture is two degrees of freedom permitting a stability of the DC bus voltage. The hybridation of primary source with an energy storage system can improve vehicle dynamic response during transients and hydrogen consumption. The proposed energy management algorithm allows us to have a minimum hydrogen consumption. This algorithm is based on supercapacitor state of charge (SOC) control and acceleration/deceleration phases making possible braking energy recovery. The proposed model is simulated and tested using Matlab/Simulink software allowing rapid transitions between sources. The obtained results with the New European Driving Cycle (NEDC) cycle demonstrate a 22% gain in hydrogen consumption.

Proton-Exchange Membrane (PEM) Fuel Cells have the potential to contribute to the future of emission-free, efficient power systems for a wide range of applications. However, operational lifetime of the fuel cells is limited due to failure of cell components, with the membrane being one of the key ones. In PEFCs, chemical decomposition of the membrane due to the attacks of radicals formed during operation along with formation of pinholes, cracks, and delamination influenced by mechanical stressors leads to increased crossover of reactant gases and reduced power loss. Since the chemical and mechanical stressors are largely influenced by the temperature and humidity, it is difficult to determine what mechanisms dominate the membrane lifetime in operation. To understand underlying mechanisms of failure under chemical, mechanical, and combined chemical-mechanical degradation modes, and assess membrane lifetime in shorter time periods, accelerated stress testing (AST) methods are commonly employed. Swelling (or hydration) of the membrane is a key property in fuel cells since it controls both conductivity and stability of the membrane, which could be related to the cell performance and durability, respectively. Swelling-induced operational stresses in the membrane trigger failure mechanisms in the form of cracks or delamination, the impact of which could be further accelerated in the presence of chemical degradation.In this talk, the mechanical aspects of the durability of the polymer fuel cell membranes will be discussed at multiple length-scales. First, membrane's mechanical response during cell operation will be discussed along with the effects of swelling and mechanical properties. We will elaborate on how membrane deformation could be related to the mechanical failure modes ranging from fatigue behavior to the growth of pinholes and cracks, depending on the operating conditions and stress-states. Strategies that could potentially improve membrane durability by tuning its properties will also be discussed. The results will be demonstrated using experimental data as well as modeling methodologies.We will then show how the swelling and properties of membranes change with degradation by focusing on perfluorosulfonic-acid (PFSA) ionomers, the most widely studied membrane in PEM fuel cells. Changes in structure-property relationship of PFSA membranes due to degradation are highlighted based on the investigations on the morphology, mechanical properties and water uptake of degraded membranes. By studying the correlations between measured properties and structural features one can better understand the mechanical aspects of degradation. We will also elaborate how chemical and mechanical stressors could influence each other and control the overall membrane failure. Lastly, mechanistic approaches will be introduced to identify key membrane properties for mechanical durability, to simulate membrane deformation and failure, and to explore models for lifetime prediction. Acknowledgements The authors would like to thank the Los Alamos Fuel Cell Team for their help preparing the degraded membrane samples and for helpful discussions. This work was funded by the Assistant Secretary for Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U.S. Department of Energy under contract number DE-AC02-05CH11231.