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Metal alloy catalysts (e.g., Pt-Co) are widely used in fuel cells for improving the oxygen reduction reaction kinetics. Despite the promise, the leaching of the alloying element contaminates the ionomer/membrane, leading to poor durability. However, the underlying mechanisms by which cation contamination affects fuel cell performance remain poorly understood. Here, we provide a comprehensive understanding of cation contamination effects through the controlled doping of electrodes. We couple electrochemical testing results with membrane conductivity/water uptake measurements and impedance modeling to pinpoint where and how the losses in performance occur. We identify that (1) ∼44% of Co2+ exchange of the ionomer can be tolerated in the electrode, (2) loss in performance is predominantly induced by O2 and proton transport losses, and (3) Co2+ preferentially resides in the electrode under wet operating conditions. Our results provide a first-of-its-kind mechanistic explanation for cation effects and inform strategies for mitigating these undesired effects when using alloy catalysts.

Platinum group metal-free (PGM-free) catalysts are a low-cost alternative to expensive PGM catalysts for polymer electrolyte fuel cells. However, due to the low volumetric activity of PGM-free catalysts, the catalyst layer thickness of the PGM-free catalyst electrode is an order of magnitude higher than PGM based electrodes. The thick PGM-free electrodes suffer from increased transport resistance and poor water management, which ultimately limits the fuel cell performance. This manuscript presents the study of water management in the PGM-free electrodes to understand the transport limitations and improve fuel cell performance. In-operando neutron imaging is performed to estimate the water content in different components across the fuel cell thickness. Water saturation in thick PGM electrodes, with similar catalyst layer thickness to PGM-free electrodes, is lower than in the PGM-free electrodes irrespective of the operating conditions, due to high water retention by PGM-free catalysts. Improvements in fuel cell performance are accomplished by enhancing water removal from the flooded PGM-free electrode in three ways: (i) enhanced water removal with a novel microporous layer with hydrophilic pathways incorporated through hydrophilic additives, (ii) water removal through anode via novel GDL in the anode, and (iii) lower water saturation in PGM-free electrode structures with increased catalyst porosity.

Metal alloy catalysts, such as Pt-Co, reduce the activation energy of oxygen reduction reaction, leading to improved proton exchange membrane fuel cell (PEMFC) performance. However, leaching of non-noble elements contaminates the ionomer and membrane, which has a negative impact on the durability of PEMFCs [1,2]. For the commercial success of metal alloy catalysts, understanding the mechanisms of how cation contamination affects PEMFC performance is crucial. Here, we investigate the effect of cobalt cation contamination effects through intentional doping of decal electrodes. Electrochemical testing results are coupled with membrane conductivity and water uptake measurements, as well as impedance modeling to identify the mechanisms of performance loss. Our results provide a comprehensive understanding of how cation contamination affects performance, which can inform mitigation strategies and new materials development that can enable the use of metal alloy catalysts in PEMFCs. Acknowledgement This work 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. Financial support for this work from the Laboratory Directed Research and Development (LDRD) program at Los Alamos National Laboratory (LANL) is gratefully acknowledged (Projects 2020200DR and 20210915PRD2). ChungHyuk Lee acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC). References Cai et al., ECS Trans., 69, 1047 (2015) P. Braaten et al., J. Electrochem. Soc., 166, F1337 (2019) Figure 1

Nafion is one of the standard proton exchange membranes that serves as the electrolyte membrane in fuel cells. However, it has been found to chemically degrade over time due to the generation of harmful peroxide and hydroxyl radicals. Attack by such radicals on the Nafion side chains causes polymer chain unzipping, and the membrane loses its mechanical integrity over time [1]. Cerium is proven to be an effective radical scavenger due to rapid transition between its +3 and +4 oxidation states and regenerates swiftly in the fuel cell environment. However, over time, cerium migrates due to the electrochemical potential, high water flux, and ionic diffusion [2] [3]. To resolve this issue, we functionalized a fraction of the side chains of Nafion with crown ether to complex with cerium and thereby prevent its migration. Crown ether has shown the ability to enhance cerium stability in recent studies [4] [5]. Cerium complex formation with crown ether was studied using UV visible spectroscopy, and enhanced retention was verified by testing drop in cerium concentration in the acidic medium, and migration of cerium under an external potential gradient. Membrane durability testing was conducted to verify that the stabilized membrane indeed shows higher lifetime in fuel cell operation. [1] F. D. Coms, “The Chemistry of Fuel Cell Membrane Chemical Degradation,” ECS Transactions, 16(2): 235–255 (2008). [2] S. M. Stewart, D. Spernjak, R. Borup, A. Datye, and F. Garzon, “Cerium Migration through Hydrogen Fuel Cells during Accelerated Stress Testing,” ECS Electrochemistry Letters, 3(4): F19–F22 (2014). [3] Baker, Andrew M., et al. ''Cerium ion mobility and diffusivity rates in perfluorosulfonic acid membranes measured via hydrogen pump operation.'' Journal of The Electrochemical Society 164.12 (2017): F1272. [4] Park, Junghwa, Yongman Park, and Dukjoon Kim. ''Chemical stability enhancement of crown ether grafted sulfonated poly (arylene ether ketone) fuel cell membrane by cerium ion fixation.'' Journal of Polymer Science Part A: Polymer Chemistry 57.2 (2019): 101-109. [5] Tinh, Vo Dinh Cong, and Dukjoon Kim. ''Enhancement of oxidative stability of PEM fuel cell by introduction of HO• radical scavenger in Nafion ionomer.'' Journal of Membrane Science (2020): 118517.

Governments and industries across the world have initiated implementation of clean hydrogen (H2) to achieve zero emissions, yet significant challenges remain for large scale adoption. With the decreasing renewable electricity cost PEM water electrolyzers (PEMWEs) show significant potential for at scale deployment due to their high faradaic efficiency and high operating current density. To reduce the per kilogram price of hydrogen and increase the overall energy efficiency, PEMWEs need to be operated at high differential pressures (30 to 50 bar) to eliminate the need for compression during storage and transportation1. To achieve high faradaic efficiency, thinner membranes are required to reduce voltage losses within the cell. However, thinner membranes exhibit a higher gas permeation rate which leads to increased crossover of hydrogen from the cathode through the membrane to the anode. Such increased crossover of H2 not only reduces the efficiency but can also result in flammable gas mixtures (lower flammability limit for H2 in O2 is 4 %) as Iridium/Iridium oxide (anode catalyst) is inefficient in oxidizing H2 1. The addition of a gas recombination catalyst (GRC) to the membrane is an useful strategy to oxidize permeating H2 and minimize the H2:O2 ratio at the anode. In this study, a high pressure (up to 30 bar) electrolyzer setup is coupled with online gas chromatography-mass spectrometry to understand the H2 gas permeation in membranes as a function of H2 partial pressure and operating current density (simulated by varying the flow rates) as shown in Figure 1 . Effect of the GRC, its loading, and its location on the H2 permeation will be elucidated. Differences induced by the addition of anode and cathode catalyst layers on H2 permeation rates will also be presented. Figure 1. Ex-situ H2 permeation rate at different cathode operating pressure at 80 °C. The anode was maintained at 1 bar of O2 partial pressure Acknowledgement This research is supported by the U.S. Department of Energy (DOE) through the Hydrogen and Fuel Cell Technologies Office, program manager Dave Peterson Reference Bernt, M., Schröter, J., Möckl, M. & Gasteiger, H. A. Analysis of Gas Permeation Phenomena in a PEM Water Electrolyzer Operated at High Pressure and High Current Density. J. Electrochem. Soc. 167, 124502 (2020). Figure 1

This talk will discuss several characterization techniques that have traditionally been used to characterize the performance and durability of fuel cells and discuss their implications to electrolysis systems. With the announcement of the DOE’s hydrogen shot to get the price of clean hydrogen to $1/kg in a decade, there has been an increased emphasis on improving the durability of Polymer electrolyte membrane electrolysis systems. This talk will discuss various in-operando, in-situ and ex-situ characterization techniques used to study and track the durability of both electrolysis and fuel cell systems. The presentation will include work from the DOE’s Million Mile Fuel Cell Truck (M2FCT) and Hydrogen from Next generation of Electrolyzers of Water (H2NEW) consortiums. In specific, the talk will discuss various electrochemical characterization techniques including cyclic voltammograms, Impedance Spectroscopy and limiting current measurements that can be used to highlight changes in the polarization behavior of fuels cells and electrolyzers. We will also discuss the usefulness of Fluoride emission rate of the effluent water and CO2 concentration in the exhaust in tracking membrane and carbon degradation respectively. Finally, we will present results from various in-situ/ex-situ techniques including Xray analysis and microscopy to track changes in catalyst composition and morphology over time and correlate those changes to device performance.

Polymer electrolyte membrane fuel cells (PEMFCs) are a viable zero-emissions option for the electrification of the heavy duty transportation sector. However, PEMFCs still suffer from degradation of materials over the fuel cell lifetime. Cation contaminants can be generated from corrosion of bipolar plates and balance of plant components, water contaminants, and environmental sources (e.g., Fe3+, Ca2+, Na+), making them present in the fuel or oxidant stream during operation(1). Cations have been shown to be detrimental to the performance of the PEMFC by reducing water uptake, ionic conductivity, and O2 transport, resulting in performance loss and degradation. Metal cations such as Fe3+ can also lead to chemical degradation of the membrane ionomer (2-4). It is critical to understand the mechanism and rate of cation transport from the bipolar plate channel to the membrane to develop mitigation strategy to suppress the cation transport. In this work, we present the study of the cation (Fe3+) transport mechanism through the gas diffusion layer (GDL) by introducing a cation solution in the cathode channel. Transport rates across the GDL are studied using an ex-situ GDL holder where Fe solution is introduced in the GDL substrate side with water transported through to the microporous layer side (MPL) and is collected and analyzed for Fe concentration, as shown in Figure 1a. Effect of the Fe concentration on transport rates is also studied using computational modeling. Understanding of the transport mechanism is then leveraged to identify mitigation solutions and suppress cation transport from the flow field to the electrode using a GDL with dual MPL architecture as shown in Figure 1b. Optimization of the dual MPL architecture for both durability and performance is also presented. Acknowledgements This research is supported by the 2021-2022 ECS Toyota Young Investigator fellowship and U.S. Department of Energy (DOE) Hydrogen and Fuel Cell Technologies Office, through the Million Mile Fuel Cell Truck Consortium (M2FCT). Authors acknowledge the Laboratory Directed Research and Development (LDRD) program at Los Alamos National Laboratory (LANL). References D. D. Papadias, R. K. Ahluwalia, J. K. Thomson, H. M. Meyer, M. P. Brady, H. L. Wang, J. A. Turner, R. Mukundan and R. Borup, Journal of Power Sources, 273, 1237 (2015). R. K. Ahluwalia, D. D. Papadias, N. N. Kariuki, J. K. Peng, X. P. Wang, Y. F. Tsai, D. G. Graczyk and D. J. Myers, Journal of the Electrochemical Society, 165, F3024 (2018). J. P. Braaten, X. M. Xu, Y. Cai, A. Kongkanand and S. Litster, Journal of the Electrochemical Society, 166, F1337 (2019). A. Kneer, J. Jankovic, D. Susac, A. Putz, N. Wagner, M. Sabharwal and M. Secanell, Journal of The Electrochemical Society, 165, F3241 (2018). Figure 1

Extrusion-based 3D printing processes have the lowest cost for equipment and materials used in additive manufacturing (AM). Recently, the AM technology has been extended beyond the typical polymer-based parts to include the capability of printing metal parts via Bound Metal Deposition (BMD). This technological advancement has, in turn, increased the potential application range of printed components particularly for Polymer Electrolyte Fuel Cells (PEFC). Although AM offers some design and cost advantages over traditional manufacturing, the finished part, post processing, must yield acceptable bi-polar plate properties outlined by the Hydrogen and Fuel Cell Technologies Office (HFTO). The post-processing techniques employed are de-binding, sintering, and surface treatment which are performed in a single step process. Here we focus on corrosion resistance, electrical conductivity, area specific resistance and porosity as the key parameters to qualify the candidate bi-polar plate material. The objective of this study is to investigate the effects of thermal processing parameters on the commercial metal composite filament via various characterization methods such as linear sweep voltammetry, area specific resistance, and x-ray computed tomography.

Abstract Proton exchange membrane fuel cell (PEMFC) electrodes have a complex structure of carbon supported platinum (Pt) nanoparticles intermixed with proton conducting ionomer. This structures creates a porous network facilitating transport of electrons, protons, and reactants. These catalyst layers have been shown to exist with a non-homogenous distribution of ionomer, with aggregates and agglomerates of carbon and ionomer. To better understand the catalyst layer structure, we have applied atomic force microscopy (AFM) to investigate fuel cell catalyst layer properties at high resolution. Various ionomer-carbon (I/C) ratios were analyzed in order to identify correlations between carbon agglomerate size, pore distribution, and structure. Surface and cross-sectional images of topography, adhesion, and roughness were collected. Scanning electron microscopy (SEM) was performed to allow for additional depictions of the samples. Several other techniques are planned to further explore structures. Electrochemical capabilities of the AFM will be utilized to inspect the effects of current on the aforementioned properties and provide better control in measurements. Additionally, samples will be placed in a water-filled holder to study effects of swelling through in situ AFM. Transmission electron microscopy (TEM) will also be performed to complement these measurements. A Bruker AFM was used in PeakForce tapping mode with a ScanAsyst Air tip to obtain the images in Figure 1. Figure 1a shows the height map of a sample with an I/C ratio of 1. Figure 1b is the adhesion map of the same sample, showing clear spatial adhesion contrast. In this map, lighter colors correspond to the more adhesive ionomer. Darker areas signify hard materials like carbon. Figure 1c depicts the adhesion map overlaid on the height map, indicating how the adhesion and topography relate. The arrows in Figure 1c illustrate where the ionomer thickness can be measured between carbon aggregates. Figure 2a summarizes the average ionomer thickness calculated for a range of I/C ratios from 0.25 to 1.25. There is a positive correlation between I/C ratio and average ionomer thickness, suggesting that increased availability of ionomer leads to thicker surface coatings and more separation between the carbon aggregates. Figure 2b shows the relationship between I/C ratio and aggregate diameter. These values were measured by processing the images in ImageJ and reducing aggregates to approximated spheres. Although no clear relationship is apparent now, further investigation with differing I/C ratios may reveal a trend. Acknowledgements : This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Technical Development Manager: Greg Kleen and Fuel Cells Program Manager: Dimitrios Papageoropoulos). This work was also performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Figure 1

Cerium enhances the durability of polymer electrolyte membrane (PEM) fuel cells by scavenging reactive radical species which are generated during operation. However, during cell fabrication, conditioning, discharge, and during fuel cell operation, Ce dissolves and is transported within the membrane-electrode-assembly (MEA) due to gradients in ionic potential (migration),1–3 ionic concentration (diffusivity),1,4 and ionomer hydration (convection).5,6 Ce migration is detrimental because (i) its accumulation in the CL ionomer can diminish the electrode’s proton conductivity, which results in sub-optimal performance, especially at high current density7,8; and (ii) its depletion may leave an ionomer region more susceptible to radical attack. Therefore, it is necessary to quantify these transport mechanisms under a range of operating conditions in order to understand their effects on cell performance/durability and determine the necessity of further Ce stabilization. To understand the effects of potential gradients and relative humidity (RH) on Ce migration, ex situ experiments were performed using Nafion™ NR-211 (Ion Power, Inc.) which was doped in-house using Ce(III) nitrate to ~5% ion-exchanged Ce. PEM specimens were operated in 4-electrode H2 pump mode in a standard conductivity cell (Scribner Associates) under a range of temperatures and RHs. The evolution of Ce profiles as a function of charge transfer was quantified using X-ray fluorescence, which were fit using our previously-developed transient, 1-D transport model in order to decouple the competing effects of migration and diffusion.1 The model was updated to capture convection due to hydration gradients, as well as variations in λ (defined as nH2O/nSO3 -) and conductivity as a function of Ce exchange fraction. As shown in Figure 1a, diffusion experiments performed in the absence of potential gradients showed excellent agreement with simultaneous 2-parameter migration and diffusivity fitting of a single migration experiment, validating the efficiency and effectiveness of this technique. Figure 1 shows modeled values for (a) diffusivity and (b) migration over a range of temperatures and λs. Convection experiments were also conducted using a differential RH cell, adapted from the GM/Giner design.9 Since no potential gradient is present here, the convection coefficients were fit by employing the isothermal diffusivity-λ relations determined from the H2 pump experiments. Results of all three transport coefficients will be presented over a range of relevant operating conditions and 2-D cell model results demonstrating the impact on cell performance will be discussed. The findings of these studies may also be analogously applied to Co and Fe cations, which dissolve from Pt-Co electrocatalysts and system balance of plant components, respectively, and are expected show transport characteristics in PFSA. Acknowledgement: This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Fuel Cells Program Manager: Dimitrios Papageorgopoulos and Technical Development Manager: Greg Kleen). References A. M. Baker, S. K. Babu, R. Mukundan, S. G. Advani, A. K. Prasad, D. Spernjak, and R. L. Borup, J. Electrochem. Soc., 164, 1272–1278 (2017). A. M. Baker, D. Torraco, E. J. Judge, D. Spernjak, R. Mukundan, R. L. Borup, S. G. Advani, and A. K. Prasad, ECS Trans., 69, 1009–1015 (2015). Y. Cai, J. M. Ziegelbauer, A. M. Baker, W. Gu, R. S. Kukreja, A. Kongkanand, M. F. Mathias, R. Mukundan, and R. L. Borup, J. Electrochem. Soc., 165, F3132–F3138 (2018). F. D. Coms and A. B. McQuarters, ECS Trans., 86, 395–405 (2018). A. M. Baker, R. Mukundan, D. Spernjak, E. J. Judge, S. G. Advani, A. K. Prasad, and R. L. Borup, J. Electrochem. Soc., 163, F1023–F1031 (2016). Y.-H. Lai, K. M. Rahmoeller, J. H. Hurst, R. S. Kukreja, M. Atwan, A. J. Maslyn, and C. S. Gittleman, J. Electrochem. Soc., 165, F3217–F3229 (2018). D. Banham, S. Y. Ye, T. Cheng, S. Knights, S. M. Stewart, M. Wilson, and F. Garzon, J. Electrochem. Soc., 161, F1075–F1080 (2014). E. L. Redmond, S. M. Wriston, and J. L. Szarka III, ECS Trans., 80, 633–641 (2017). S. Kumaraguru, "Durable High Power Membrane Electrode Assembly with Low Pt Loading," Fuel Cell R&D Annual Merit Review Proceedings, (2018). Figure 1

Abstract Fuel cells have potential for small power applications that require long periods, as much as several decades, of uninterrupted power. Small microwatt fuel cell stacks of varying membrane thickness were designed to allow for passive water management. The passive water management system was incorporated into the stack design by use of exposed Nafion membrane for lateral water transport. 1 An experimental and theoretical study of water management across the operating temperature range was conduction, including operando neutron imaging and the development of a multiphysics two-dimensional model. Stack testing was done between -55 °C to 85 °C using a baseline output current of 10 µA and periodic pulses of 4.5 mA lasting 100 ms, every 100 s, or an average current of 16 µA. The stack spatial water concentration was monitored during various operating conditions through in situ neutron imaging. Figure 1 shows the main points of interest on the stack. Operating temperatures ranged from -55 °C to 85 °C to analyze water formation and subsequent water removal. Freezing and below-freezing temperatures were controlled by use of a liquid nitrogen cooled series of insulated boxes to prevent external water condensation during imaging. These measurements suggest successful water transport laterally, enabling operation from the sub-freezing environment to temperatures near the boiling point of water. Figure 2a depicts a constant operation for two hours at 16 µA and -20 ºC and the subsequent drying process in Figure 2b. Water production due to gas crossover in the stacks was quantified at below-freezing temperatures. At temperatures at and below -20 °C, the water production due to gas crossover was greater than the amount of water removal via the passive water management strategy with the current turned off, rather than a drying of the cell as expected. With the gases turned off as well, drying occurs. This indicates that the cell design must allow for water (ice) build-up during operation at these sub-freezing temperatures for successful operation. To avoid large amounts of hydrogen loss due to membrane crossover, we employ multiple layers of membrane to make a thick, mechanically reinforced and chemically stabilized membrane. A two-dimensional multiphysics model for evaluation of water management was also developed to further describe and explore the passive water management system under different conditions. The fundamental model in its present form shows a similar shape but indicates faster water build-up than experimental data. This discrepancy is currently being resolved to allow for a more accurate representation. Acknowledgments: This work was supported by the U.S. Department of Energy. References: Chintam, M. S. Wilson, T. Rockward, S. Stariha, A. M. Baker, E. L. Brosha, D. S. Hussey, J. M. LaManna, D. L. Jacobson, J. Rau, R. L. Borup, ECS Transactions 86 (13), 233-244. Figure 1

Polymer electrolyte membrane fuel cells (PEMFCs) are promising alternative to internal combustion engines, owing to their capability to produce on-demand power with zero local carbon emissions. However, PEMFCs suffer from limitations related to performance, particularly for heavy-duty vehicle applications [1]. A major contributor to the performance loss is the cathode electrode; uncontrolled conventional electrode structures lead to non-ideal transport pathways of O2 and H+ [2], subsequently leading to performance loss. Here, we report an alternative electrode structure termed groovy electrodes, fabricated via patterned Si templates. Si templates with desired patterns are fabricated through photolithography and deep-reactive ion etching techniques (Fig. 1a), and an electrode layer is coated onto the template and subsequently transferred to the membrane or the gas diffusion layer. The resulting electrode features ordered grooves with consistent depth and width (Fig. 1b-c). We will present how this alternative structure enables improved H+ transport (enabled by higher ionomer content) with grooves facilitating effective O2 transport to reaction sites. We will also demonstrate the benefits of a groovy electrode for the durability of PEMFCs. Reference Cullen et al., Nat. Energy, 6, 462 (2021). Ramaswamy et al., J. Electrochem. Soc., 167, 064515 (2020). Figure 1

Micro- to milli-watt fuel cell stacks and systems are designed and tested for applications that require continuous operation for multiple decades and at very wide ambient temperature ranges. Development presumes generic load profiles consisting of small baseline currents with frequent, periodic excursions to higher currents, both delivered at the 3.3 V standard integrated circuit voltage level. Stack design and testing was done between -55˚C to 80˚C using a baseline output current of 10 uA and periodic pulses of 4.5 mA lasting 100 ms, every 100 s. To simplify testing, the stacks are paired with 3.3V output miniature dc-dc converters to supply the respective 33 uW and 15 mW power requirements, with the stack then initially providing about 40 uW and 17 mW reflecting the converter efficiencies at the two sets of output currents/input voltages. Since maximum cell power tends to occur at >0.5 V/cell, 8-cell stacks were chosen so that, if called upon, the maximum power point still exceeds the ~4 V minimum buck converter input. Concerns about air availability and purity impel the use of H2 & O2 reactants; accommodating the fittings and other design constraints lead to a 13x19mm footprint (active area = 0.2 cm2), as shown in Figure 1. Since the stack reactant supplies are “dead-ended”, provision needs to be made to remove product water. In contrast to typical (higher power) fuel cell applications, the primary system challenges with the microwatt system are reactant cross-over and leakage current. As an example of the severity of the issue, a 2020 technical target1 for membrane leakage current is an equivalent DC resistance of 1000 Wcm2. A similar value in this case would yield a cumulative leakage current of about 1.4 mA, causing an otherwise 30 y reactant supply for this system to last less than a year. Moreover, cross-over losses are generally hand-in-hand with leakage. The simple straightforward solution is to use thick membranes, done here primarily by stacking multiple layers of DuPont XL, which provides the benefits of radical scavengers and PTFE reinforcement scrims to enhance long-term chemical and dimensional stability. Other loss mitigation configurations and strategies are being pursued in tandem. The need to provide 17 mW pulses not only limits the maximum membrane thicknesses (and stack resistances), but subjects the individual cells to voltage cycles between about 0.6V and OCV, similar to a catalyst durability AST (accelerated stress test). These issues can be alleviated by pairing the stack with a supercapacitor to minimize the voltage swings during the 17 mW pulses, thus allowing the stack to supply a fairly constant 65 uW (accounting for converter efficiency). Consequently, life and environmental testing are underway to demonstrate the utility of the configuration, but the preferred goal is to achieve the desired results without introducing additional components with potential lifetime and temperature range issues. Long-term room temperature life-testing is underway on stacks with and without supercaps. As of March 2018, a stack subjected to the full 17 mW pulses is approaching 5000 h of operation and has shown little if any drop in performance beyond an initial modest decline. A second stack paired with supercaps has been in operation nearly as long and likewise shows minimal sustained losses, even though it is being subjected to 10x the standard pulse rate. Environmental chamber testing has demonstrated prolonged operation at temperatures ranging from -40˚ to 80˚C. Stack resistance decreases between room and high (80˚C) temperatures, overtly because the proton transport kinetics are enhanced in the already extremely dry membranes. While such dry operation was believed warranted to avoid product water freezing in the catalyst layer causing transport issues, the ability of the stacks to provide 17 mW pulses is challenged by the high stack resistance. Consequently, we have begun investigating the degree to which we can allow water content to increase while maintaining reactant access. Additional future efforts include environmental testing down to -55˚C and further stack design evolution to improve performance and functionality. Figure 1. An 8-cell stack with nitrided titanium bipolar and endplates. Reference 1 DOE Technical Targets for Polymer Electrolyte Membrane Fuel Cell Components. https://www.energy.gov/eere/fuelcells/doe-technical-targets-polymer-electrolyte-membrane-fuel-cell-components Figure 1

Neutron imaging is a completely non-destructive probe of the water content in operating fuel cells. However, the spatial and temporal resolution of the method limits the application range. We report on our continued imaging detector resolution improvements, where we have achieved a spatial resolution of 2 µm. We applied this new detector capability to measure the water content of two different membrane electrode assemblies where the ionomer to carbon mass ratio (I/C) was the only variable. A 1 cm2 active area test section with parallel flow channels was operated as a differential cell at 80 °C at constant voltage.

Doped ceria compounds demonstrate excellent radical scavenging abilities and are promising additives to improve the chemical durability of polymer electrolyte membrane (PEM) fuel cells. In this work, Ce0.85Zr0.15O2 (CZO) nanoparticles were incorporated into the cathode catalyst layers (CLs) of PEM fuel cells (based on Nafion XL membranes containing 6.0 μg cm−2 ion-exchanged Ce) at loadings of 10 and 55 μg cm−2. When compared to a CZO-free baseline, CZO-containing membrane electrode assemblies (MEAs) demonstrated extended lifetimes during PEM chemical stability accelerated stress tests (ASTs), exhibiting reduced electrochemical gas crossover, open circuit voltage decay, and fluoride emission rates. The MEA with high CZO loading (55 μg cm−2) demonstrated performance losses, which are attributed to Ce poisoning of the PEM and CL ionomer regions, which is supported by X-ray fluorescence (XRF) analysis. In the MEA with the low CZO loading (10 μg cm−2), both the beginning of life (BOL) performance and the performance after 500 hours of ASTs were nearly identical to the BOL performance of the CZO-free baseline MEA. XRF analysis of the MEA with low CZO loading reveals that the BOL PEM Ce concentrations are preserved after 1408 hours of ASTs and that Ce contents in the cathode CL are not significant enough to reduce performance. Therefore, employing a highly effective radical scavenger such as CZO, at a loading of 10 μg cm−2 in the cathode CL, dramatically mitigates degradation effects, which improves MEA chemical durability and minimizes performance losses.

Ceria nanoparticles (NPs) have unique catalytic properties which make them suited to scavenge degrading radical species and their precursor peroxides during PEM fuel cell operation. However, in the acidic environment of the fuel cell, ceria dissolves and the resulting cations migrate within the MEA, causing performance and durability losses. In this work, ex situ testing was used to evaluate the peroxide decomposition, selectivity towards radical generation, and solubility of Gd, Pr, and Zr-doped ceria NPs over a range of crystallite sizes and dopant levels. These doped materials exhibit better peroxide scavenging activity and dissolution resistance than undoped ceria. In these materials, activity is largely governed by increased surface area due to high internal porosity at smaller crystallite sizes compared to undoped ceria. Of the compounds tested, ceria NPs doped with 15 at% Zr (10 nm) and 5 at% Pr (17 nm) exhibited greater dissolution resistance than undoped ceria. Stabilization of the former doped NPs is attributed to crystallite agglomeration, while the increased stability of the latter is proposed to be due to its internally-porous, mesoscale structure suggested by its sorption isotherm. Both materials are more dissolution-resistant and active peroxide decomposers compared to undoped ceria but exhibit increased byproduct radical generation.

Polymer electrolyte membrane fuel cells provide an alternative option to fossil fuel-based energy conversion devices. However, the corrosion of fuel cell components, specifically the bipolar plates, introduces contaminants (e.g., Fe, Ni) into the membrane electrode assembly (MEA). These contaminants accelerate the ionomer degradation by acting as a Fenton’s reagent, decreasing the fuel cell’s durability. This study presents the mechanism and the diffusion media properties affecting the transport of cation contaminants into the MEA. Cation contaminant transport was studied after altering the gas diffusion layers (GDLs) wettability, emulating the GDL properties after prolonged operation, by ex situ hydrogen peroxide treatment or in situ electrochemical potential cycling. A GDL with crack-free microporous layer (MPL) showed a lower cation transport rate to the catalyst layer than MPL with cracks after both ex situ and in situ treatment. A novel GDL was developed from modification of the conventional GDL via the addition of a hydrophobic layer to the GDL substrate, which suppressed the contaminant cation transport significantly. This novel GDL also showed improved fuel cell performance.

Cerium is a radical scavenger which improves polymer electrolyte membrane (PEM) fuel cell durability. During operation, however, cerium rapidly migrates in the PEM and into the catalyst layers (CLs). In this work, membrane electrode assemblies (MEAs) were subjected to accelerated stress tests (ASTs) under different humidity conditions. Cerium migration was characterized in the MEAs after ASTs using X-ray fluorescence. During fully humidified operation, water flux from cell inlet to outlet generated in-plane cerium gradients. Conversely, cerium profiles were flat during low humidity operation, where in-plane water flux was negligible, however, migration from the PEM into the CLs was enhanced. Humidity cycling resulted in both in-plane cerium gradients due to water flux during the hydration component of the cycle, and significant migration into the CLs. Fluoride and cerium emissions into effluent cell waters were measured during ASTs and correlated, which signifies that ionomer degradation products serve as possible counter-ions for cerium emissions. Fluoride emission rates were also correlated to final PEM cerium contents, which indicates that PEM degradation and cerium migration are coupled. Lastly, it is proposed that cerium migrates from the PEM due to humidification conditions and degradation, and is subsequently stabilized in the CLs by carbon catalyst supports.

Increasing the utilization of Pt and Pt alloy catalysts in polymer electrolyte fuel cell cathodes is critical to improving the high power density operation, particularly at low Pt loadings. State of the art electrodes are fabricated in an ink deposition process that leads to uncontrolled electrode architecture with random aggregates of functional domains (catalyst, ionomer, and pore volume) (1). The randomness in the domains induces high tortuosity transport pathways for ions and fluids, which cause severe transport resistance during high current density operation. Thin ionomer films cause additional transport resistance and poisoning of the Pt catalyst, which becomes more significant at low Pt loadings. Reducing the amount of ionomer in the catalyst domain without affecting the ionic transport resistance is key to improving the utilization of the Pt and reducing the transport resistance at low Pt loading. Rational design of the electrode structure with controlled low tortuous ionic transport pathways could improve performance. The introduction of the ionomer pathways could also enable reduction of the ionomer volume in the catalyst domain, reducing the transport resistance. Middelmen et al. proposed electrode structures consisting of aligned components in a low tortuosity configuration to improve performance (2). In this work, we present an alternative electrode structure based on a vertically aligned array of Nafion pillars in the cathode catalyst layer, as shown in Figure 1a. Figure 1b shows the SEM image of the Nafion pillars. Pt supported on carbon catalyst was deposited on the Nafion pillars to fabricate a meso-structured electrode. Nafion pillars provide high conductive and low tortuous pathways for protons, reducing the effective transport distance, and enabling reduction of the ionomer binder in the catalyst domain. Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos. References 1. S. Litster and G. McLean, Journal of Power Sources, 130, 61 (2004). 2. E. Middelman, Improved PEM fuel cell electrodes by controlled self-assembly, in, p. 9 (2002). Figure 1

Cerium ions enhance the chemical stability and lifetime of polymer electrolyte membrane (PEM) fuel cell components by rapidly and reversibly scavenging degrading radical species which are generated during operation [1]. However, during cell fabrication and discharge, these ions readily migrate between the membrane and catalyst layers (CLs) of the membrane electrode assembly (MEA) [2]. Complete washout of cerium from the MEA has also been observed [3]. It is necessary to understand the mechanisms and magnitude of cerium migration during fuel cell operation, since cerium ions are ineffective outside of the catalyzed area of the MEA. These results can be applied to improve cerium stability in the active area of the MEA and localize it to areas of high radical generation, which can further extend the lifetime of PEM fuel cells. Table 1: AST conditions and flow fields Test Time (h) RH (%) Flow field I 500 30 50 cm2 single serpentine II 2,000 100 25 cm2 tri-serpentine III 2,000 100 25 cm2 single serpentine Membrane chemical stability accelerated stress tests (ASTs) [4] were performed on cerium-containing MEAs at 90°C in single-cell hardware (Fuel Cell Technologies), compressed with 8 x bolts at 50 in-lb of torque, using the conditions and hardware shown in Table 1. Nafion XL (DuPont) membranes were used, which contain a nominal cerium loading of 6 μg Ce/cm2. Carbon-supported platinum electrodes (TKK, 48% Pt, 0.1-0.2 mg Pt/cm2loading) and Sigracet 25BC GDLs (SGL) were also used. X-ray fluorescence (XRF) was performed on MEA components before and after the ASTs in order to measure in-plane cerium content in the membrane and CLs. After 500 hours of OCV operation at 30% RH, cerium moved uniformly from the membrane of MEA I into the CLs (not shown). Here, migration is attributed cell component hot pressing and interactions with the CLs [2]. Membrane cerium was reduced to 3.7± 0.68 μg Ce/cm2, while anode and cathode CL concentrations were increased to 2.3 ± 0.06 and 3.4 ± 0.22 μg Ce/cm2, respectively. At 100% RH, cerium also remained in the active area, however, membrane concentration increased from inlet to outlet (Figure 1a). This gradient may arise due to the increased presence and flow of liquid water during 100% RH operation. Only trace amounts of cerium remained in the CLs, except near the outlet, which suggests that under humidified conditions, the effects of CL interactions on cerium migration are reduced. After 2,000 hours of operation at 100% RH, flow field compression was observed to have implications on cerium migration out of the active area. The cerium profile of MEA III (Figure 1b) shows that it migrated from areas of high compression in the active area (shown in red) into low compression regions outside of the active area. In contrast, compression was higher around the active area of MEA II (Figure 1a), which prevents cerium from leaching from it. However, concentration was non-uniform, as discussed above. These preliminary results indicate that cerium migration and leaching out of the active area are affected by membrane water content and cell clamping pressure. It is believed that other factors such as electrical potential and temperature influence migration, as well. The authors wish to acknowledge the financial support of the Fuel Cell Technologies Program and the Technology Development Manager Nancy Garland. References [1] F. D. Coms, H. Liu, J. E. Owejan, ECS Trans., 16, 1735-1747 (2008). [2] S. M. Stewart, D. Spernjak, R. L Borup, A. Datye, F. Garzon, ECS Lett., 3, F19-F22 (2014). [3] M. V. Lauritzen, S. Knights, T. Cheng, D. W. Banham, E. Kjeang, A. Sadeghi Alavijeh, Fuel Cells 2014 Science & Technology, April 2014, Amsterdam, The Netherlands [4] U.S. DOE, Cell Component Accelerated Stress Test Protocols for PEM Fuel Cells, 2010. Figure 1

Investigation of membrane/electrode assembly (MEA) microstructure has become an essential step to optimize the MEA manufacturing processes or to study the degradation of the different MEA components. Transmission electron microscopy (TEM) is a tool of choice as it provides direct imaging of the different components. TEM is then widely used for analyzing catalyst nanoparticle structures and distribution, as well as their chemical composition. The carbon support can also be imaged; for example, the degree of graphitization can be investigated. However, the ionomer network inside the electrode is more difficult to be imaged due to the fact that the ionomer mainly forms an ultrathin layer surrounding the carbon support. Moreover, these two components, having similar density, present no difference in contrast. The presence of the ultrathin ionomer layer can only be revealed in some favorable zones on high resolution TEM images1. The entire ionomer network inside the catalyst layer has never been imaged, even though this microstructural parameter plays a crucial role in ionic conduction through the catalyst layer. In this work, we show how the advanced TEM techniques of electron tomography and X-ray energy dispersion spectroscopy (EDS) elemental mapping provide new possibilities for imaging this ionomer network. Electron tomography in HAADF-STEM (high annular dark field – scanning transmission electron microscopy) mode was successfully used to image the 3D morphology of the ultrathin ionomer layer surrounding the carbon particles2. For this experiment, the ionomer contrast was enhanced by selectively staining the ionic domains with Cs+ions. In order to avoid high contrast of the Pt nanoparticles (compared to the ionomer contrast), a model active layer consisting of ionomer and carbon black without Pt nanoparticles was made using usual electrode manufacturing process. The influence of the ionomer/carbon black ratio introduced in the ink was studied by preparing two samples with two different ionomer/carbon black ratios equal to 0.5 and 0.2 w/w. Figure 1 shows the 3D-rendered volume extracted from the reconstructed tomogram for the two samples. The 3D repartition of the ultrathin ionomer layer (in blue) surrounding the carbon black (in grey) is clearly revealed. The quantitative tomogram data analyses showed that doubling the amount of ionomer in the catalyst layer does not change the mean thickness of the ionomer layer, measured around 7 nm, but leads to a twofold increase in its degree of carbon particle coverage. These electron tomography analyzes are of great interest for finding the optimum manufacturing process that will lead to the maximum carbon coverage without increasing the ionomer layer thickness. Indeed, the optimized ionomer network structure have to ensure both ionic contact with a maximum of Pt nanoparticles and the connectivity of the ionic conduction paths to the membrane without inhibiting the gas diffusivity. On the other hand, recent developments of high performance EDS detectors offer new possibilities to acquire chemical elemental maps in a very short time (few seconds or minutes). Fluorine being one of the main elements of the ionomer, ionomer distribution within the catalyst layer can be visualized by acquiring fluorine EDS elemental maps in a MEA ultramicrotomed TEM sample (Figure 2). One limitation of this technique is that a high EDS signal requires a high electron dose, especially when high spatial resolution is required. Unfortunately, the ionomer is highly sensitive to electron beam radiation damage and a continuous F loss happens during the TEM observation. Measurement of the F loss under different acquisition conditions has shown that F loss can be minimized by controlling the electron dose and by cooling the specimen during analysis3. Using these conditions, it is now possible to obtain quantitative data on the active layer ionomer content that in some electrodes can reveal distribution heterogeneity. But more importantly, these analyses allowing a quantitative comparison of the ionomer content in different electrodes offer the possibility to study the active layer ionomer evolution after MEA ageing tests. 1K. L. More, R. Borup, K. S. Reeves, ECS Trans 3, 717-733 (2006). 2 M. Lopez-Haro, L. Guetaz, T. Printemps, A. Morin, S. Escribano, P.H. Jouneau, P. Bayle-Guillemaud, F. Chandezon, G. Gebel, Nat Commun, 5, 5229 (2014). 3 D. A. Cullen, R. Koestner, R. S. Kukreja, Z. Y. Liu, S. Minko, O. Trotsenko, A. Tokarev, L. Guetaz, H. M. Meyer, C. M. Parish, K. L. More, J. Electrochem. Soc., 161 (10) F1111-F1117 (2014). Figure 1

Perfluorosulfonic-acid (PFSA) membranes are frequently subjected to high humidity and temperature cycles during fuel-cell operation. It is of great interest to understand how the properties of the membrane change with ageing conditions and time. In this study, we investigate how the properties of as-received and pretreated Nafion membranes change after exposure to hygrothermal ageing, including the chemical structure, mechanical properties, water uptake, ionic conductivity, and morphology. Our findings demonstrate that anhydrides form during ageing via a condensation reaction, which results in chemical crosslinks that impair the membrane functionalities by reducing water uptake and conductivity and increasing the storage modulus and α relaxation temperature. In addition, a membrane aged in a 75% relative humidity environment exhibits more dramatic changes compared to that aged in 100% conditions. It is also shown that the impact of ageing can be recovered through a post-treatment by boiling the membrane in strong acid.

PEM fuel cells (PEMFCs) show great promise to increase the fuel efficiency for transportation applications; however, for this application, they must show performance and durability with the requirements for transportation. For transportation applications, the fuel cell will be subjected to frequent power cycling. For example, the DOE/Fuel Cell Tech Team (FCTT) protocol for durability includes load cycling from 0.02 A/cm2 to 1.2 A/cm2 every 0.5 min. The cathode catalyst and catalyst layer have been shown as susceptible to degradation causing loss of performance due to both loss of kinetics for the oxygen reduction reaction and loss of mass transport. Catalyst support-carbon corrosion can result in thinning of the catalyst layer contributing to degradation in performance. To examine the effect of power cycling in situ on carbon corrosion and electrode degradation, we directly measured the catalyst support degradation by measuring CO2 in the cathode outlet by NDIR (Non-Dispersive Infra-Red) while operating a single-cell fuel cell. CO2 present in air was removed by a lime bed prior to introduction to the fuel cell. We operated with a modified DOE/FCTT durability protocol using controlled voltage, and varied the potential limits to explore the effects of the upper potential limit, lower potential limit, the potential step size and time at potential. The upper potential limit was varied from 0.95 to 0.55V; the lower potential limit from 0.40V to 0.80V, with times ranging from 0.5 min to 5 min. The corrosion of three different types of carbon were explored, high surface area (E), vulcan (V), and graphitized (EA). The catalyst support carbon corrosion occurs under normal fuel cell operating conditions and is exacerbated by the voltage cycling inherent in these steps in potential. A series of carbon corrosion spikes during potential cycling is shown in Figure 1 for E-type carbon, varying the upper potential from 0.95 V to 0.60V while keeping the lower potential constant at 0.40V. Sharp spikes in the carbon corrosion rate are observed during a step increase in cell potential with the magnitude of the spikes decreasing as the high cell potential is reduced from 0.95 V to 0.6 V. The carbon corrosion rate at high cell potential (0.95V) decreases with time at potential, indicating formation of passivating carbon surface oxides. Carbon corrosion was measured during the drive cycle measurements for all three types of carbon, with the relative carbon corrosion rates of E>V>EA. The series of step potential carbon corrosion spikes where the potential was varied for the lower potential from 0.40 V to 0.60V while keeping the upper potential constant at 0.95V shows similar results in terms of the carbon corrosion. The magnitude of the spikes decrease as the lower cell potential is raised. These results indicate that the size of the step in potential has a more significant impact on the carbon corrosion rate than does the absolute value of the potential for normal cathode operating potentials. The peak in CO2 evolution occurs when the cell potential increases from high power operation to low power near open circuit. This correlates with when CO2 evolution is observed during cyclic voltammograms, which occurs during the positive sweep at ~ 0.55 to 0.60 V. The evolution of this CO2 peak suggests that oxygen is adsorbed onto the carbon and/or CO is formed on the Pt surface. During long-term operation, a reduction in catalyst layer thickness is observed during drive cycle operation, which can be due to the loss of carbon through carbon corrosion or possibly due to compaction; both effects likely lead to a loss of void volume. This reduction in thickness includes a sharp decrease in catalyst layer thickness within the first 100 hours of operation (30%), eventually reaching ~50% of its thickness after 1000 hours. Most of this reduction in electrode thickness does not appear to be directly due to carbon corrosion as there is little evidence for carbon corrosion from microscopic analysis, especially during the early stages of operation, where the thickness reduction is substantially more than what should be due to carbon corrosion. These results show that carbon corrosion occurs during normal potential operation, and is exacerbated by potential variations during operation. To minimize the carbon corrosion, the size of the steps in potential should be minimized. For a more stable catalyst, another requirement is for the Pt particles to be stabilized on graphitized carbon supports. Acknowledgments Funding for this work is from DOE EERE FCTO, Technology Development Manager Nancy Garland Figure 1

A potentiostatic polarization method is used to evaluate the corrosion behavior of SS316L in simulated anode and cathode environments of polymer electrolyte fuel cells. A passive barrier oxide film is observed to form and reach steady state within ∼10 h of polarization, after which time the total ion release rates are low and nearly constant at ∼0.4 μg cm−2 h−1 for all potentials investigated. The equilibrium film thickness, however, is a function of the applied potential. The main ionic species dissolved in the liquid are predominately Fe followed by Ni, that account for >90% of the steady-state corrosion current. The dissolution rate of Cr is low but increases systematically at potentials higher than 0.8 V. The experimental ion release rates can be correlated with a point defect model using a single set of parameters over a broad range of potentials (0.2–1 V) on the cathode side. The interfacial contact resistance measured after 48 h of polarization is observed to increase with increase in applied potential and can be empirically correlated with applied load and oxide film thickness. The oxide film is substantially thicker at 1.5 V possibly because of alteration in film composition to Fe-rich as indicated by XPS data.

Electrospun PFSA-Pt/C nanofiber mats have demonstrated promise as electrodes for PEM fuel cells that are highly scalable and show improved performance and durability compared to traditional sprayed electrodes.1,2 As a result, it is of great interest to understand the underlying mechanisms responsible for these gains, so that these electrode structures may be optimized in order to further increase performance and durability. Neutron radiography was used to measure water formation through the cross-sections of sprayed gas diffusion electrodes (denoted GDE) and electrospun PFSA-Pt nanofiber electrodes (denoted NF) during operation. Both GDE and NF-containing MEAs were assembled in custom hardware specifically designed for use within the neutron beam and operated in a differential cell configuration at 80°C, with variable RH and current density. Initial polarization results (Figure 1a) demonstrated a lower OCV for the NF electrodes, which were attributed to shorting at the through-holes within the active area, specific to the custom-built imaging hardware. Regardless, the performance of NF electrodes is improved at high (2 A/cm2) current densities. Through-thickness water profiles at 0.2 V (Figure 1b) show that the water concentration is around 2x lower within the MEA and GDLs in the NF-containing MEA compared to the baseline GDE. The lower water contents suggests improved performance in the mass transport region, commensurate with the observed polarization curves. Hardware issues are currently being addressed, and further test results exploring effects of RH, current density, and NF composition, will be discussed. Acknowledgements This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Fuel Cells Program Manager: Dimitrios Papageorgopoulos and Technical Development Manager: Greg Kleen). References P. N. Pintauro, “Fuel Cell Membrane-Electrode-Assemblies with Ultra-Low Pt Nanofiber Electrodes,” Fuel Cell R&D Annual Merit Review Proceedings, 2018. Brodt, R. Wycisk, N. Dale, and P. Pintauro, “Power Output and Durability of Nanofiber Fuel Cell Cathodes with PVDF and Nafion/PVDF Binders”, J. Electrochem. Soc., 163 , F401-F410 (2016). Figure 1

The Fuel Cell Performance and Durability (FC-PAD) consortium was formed to advance performance and durability of polymer-electrolyte fuel cells (PEFCs) at a pre-competitive level in order to further enable their commercialization. This consortium coordinates national laboratory activities related to fuel-cell performance and durability, provides technical expertise, and harmonizes activities with industrial developers. The consortium serves as a resource that amplifies DOE EERE’s impact by leveraging the core capabilities of several labs in conducting low technology-readiness-level research. Consortium members include Argonne National Laboratory, Lawrence Berkeley National Laboratory, Los Alamos National Laboratory, the National Renewable Energy Laboratory, and Oak Ridge National Laboratory. This consortium incorporates national laboratory investigators with proven experience (developed in prior projects) related to durability, transport, and performance, and combines them into one highly coordinated effort. The consortium is divided into six thrusts: three thrust areas related to components ((1) Electrocatalysts and Supports, (2) Electrode Layers, (3) Ionomers, Gas Diffusion Layers, Bipolar Plates, Interfaces), and three cross-cutting thrust areas ((4) Modeling and Validation, (5) Operando Evaluation: Benchmarking, ASTs, and Contaminants and (6) Component Characterization and Diagnostics.) In addition, four external-lab FC-PAD projects are led by 3M Company, General Motors, United Technologies Research Center, and Vanderbilt University. The core national lab team supports those four projects utilizing national lab capabilities. The major challenge addressed by this consortium is to develop the knowledge base, design rules, and understanding of underlying phenomena in order to mitigate durability concerns and optimize performance through understanding the science of integration, while simultaneously reducing cost. Current research focuses on achieving high performance and durability in low Pt-loaded PEFCs. FC-PAD has conducted significant analysis of MEA electrode structures, analyzed the structures to understand the performance and durability losses, comprehensively defined the current commercial material baseline materials, and used this information to developed new electrode architectures that are designed to minimize existing electrode layer losses. In this talk, in addition to discussion of the consortium itself, recent results will be highlighted including analysis of electrode layers from inks to structure to conditioning to performance. Acknowledgments We want to acknowledge all of the FC-PAD researchers, particularly the Research Thrust Coordinators: Deborah Myers (ANL), K.C. Neyerlin (NREL), Ahmet Kusoglu (LBNL), Rajesh Ahluwalia (ANL), Rangachary Mukundan (LANL), Karren More (ORNL). This work is funded through the DOE FC-PAD Consortium with thanks to DOE EERE FCTO.

The Consortium for Fuel Cell Performance and Durability (FC-PAD) was formed by the U.S. Department of Energy Fuel Cell Technologies office (FCTO) in 2015 to improve the understanding of performance and durability issues in polymer electrolyte membrane fuel cells (PEMFCs) and to help meet the DOE 2020 targets. This talk will summarize some of the durability work performed within the consortium on commercial dealloyed-PtCo/C catalyst-based membrane electrode assemblies (MEAs). Commercial MEAs using Nafion®HP membranes, ElystPt300670 (30wt.%PtCo/high surface area C) cathode catalyst, and ElystPt200380 (20wt.%Pt/Graphitized carbon) anode catalyst were obtained from Umicore. The MEAs were assembled in a 5cm2differential cell reported by D. R. Baker et al.1and conditioned extensively before performing beginning of life (BOL) characterization. The conditioning protocol included a recovery step where the cell was operated at high RH and low temperatures and voltages to flush out impurities and maximize the performance.2,3The samples were then subjected to the DOE-recommended square wave catalyst accelerated stress test (AST) which is a 3 sec hold at 0.6V and 3 sec hold at 0.95V at 80oC and 100%RH.4Characterizations were performed after 15,000 and 30,000 cycles of the AST with the recovery protocol applied to the MEA before characterization. Electrochemical characterization included mass activity, electrochemical surface area, polarization curves, Electrochemical Impedance Spectroscopy (EIS), and oxygen transport measurements. The BOL and end-of-test catalysts were also characterized by electron microscopy, energy dispersive X-ray spectroscopy, X-ray absorption spectroscopy, and small-angle X-ray scattering to determine particle size and Pt to Co content. The oxygen reduction reaction mass activity, electrochemical surface area, and performance in the kinetic region of the catalysts degrade with AST cycling. This loss can be attributed to both Co leaching from the catalyst and catalyst particle size increase. The effect of this degradation was most pronounced in the high current region for the MEA with low Pt loading. For example, in Figure 1, the voltage loss at 1A/cm2after 30,000 AST cycles is ~50mV for the 0.15mgPt/cm2MEA and ~200mV for the 0.05mgPt/cm2MEA. This is primarily due to increases in the pressure-independent oxygen transport resistance. The non-Fickian oxygen transport resistance of low-loaded (0.05 mgPt/cm2) MEAs is significantly larger than those of the high-loaded MEAs even at BOL and worsens significantly with ageing of the catalyst. EIS experiments in HelOx (21% O2, balance He) also revealed that there was an increase in the pressure-dependent gas transport during the AST cycling. However, little or no change was observed in the ionomer sheet resistance over the course of the durability testing, independent of the catalyst loading. These results will be compared with recent results reported for pure Pt catalyst-based MEAs subjected to similar tests.5In this talk, the analysis of the cathode catalyst composition and morphology before and after the testing will be presented in detail and correlated to the observed performance losses. Acknowledgements This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Fuel Cells Program Manager: Dimitrios Papageorgopoulos and Technical Development Manager: Greg Kleen). References D. R. Baker, D. A. Caulk, K. C. Neyerlin, and M. W. Murphy, J. Electrochem. Soc.,V156(9), B991-B1003 (2009). https://doi.org/10.1149/1.3152226. Zhang, B. A. Litteer, F. D. Coms, and R. Makharia,. J. Electrochem. Soc., V159(7),F287-F293 (2012). https://doi.org/10.1149/2.063207jes. Zhang et al, U.S Patent Application Publication US 2011/0195324 A1, August 11 (2001). United States Department of Energy Fuel Cells Technologies Office Multi-Year Research, Development, and Demonstration Plan. https://www.energy.gov/sites/prod/files/2017/05/f34/fcto_myrdd_fuel_cells.pdf. G. S. Harzer, J. N. Schwammiein, A. M. Damjanovic, S. Ghosh, and H. A. Gasteiger, J. Electrochem. Soc.,V165(6), F3118-F3131 (2018). https://doi.org/10.1149/2.0161806jes Figure 1

The gas diffusion layer (GDL) is a critical component of proton exchange membrane fuel cells (PEMFC). It serves as the primary mediator in electrical and thermal conductivity as well as gas and water transport within the cell. Currently, the production of GDLs is dependent on the use of high-cost materials and manufacturing methods. The most common base material for GDLs is polyacrylonitrile (PAN). PAN, in addition to its high base cost ($15 - 20/kg), requires high graphitization temperatures (> 1700°C) in order attain sufficient conductivity and post-carbonization treatment with Teflon to increase the hydrophobicity for improved water transport. The high strength of PAN based GDLs has been required to prevent fiber intrusion into the channel of the flow field. Some state-of-the-art flow fields no longer utilize the land/channel design, and as such may not require the use of high strength materials like PAN in order to be functional. This presents the opportunity to create novel GDLs with equivalent or superior performance at lower cost. We are investigating methods to significantly reduce the cost of GDL production while maintaining durability and performance characteristics necessary to PEM fuel cells. First, we are developing the use of inexpensive, natural fiber materials, such as cotton, jute, and bamboo. Second, we are developing methods to reduce the carbonization temperatures and eliminate manufacturing steps, such as the need for a microporous layer. Figure. SEM images of commercial (A) (without microporous layer), cotton (B), and jute (C) based GDLs. The cotton and jute GDLs had been carbonized at 1100°C in N2 atmosphere. Figure 1

We have investigated the mechanism of increase in oxygen transport resistance (Rm) due to aging of electrodes with low-loaded de-alloyed PtCo cathode catalysts supported on high surface area carbon (d-PtCo/C). Commercially available membrane electrode assemblies with 0.05, 0.1 and 0.15 mg/cm2 Pt loadings in d-PtCo/C cathode catalysts were assembled into cells and subjected to accelerated stress tests (ASTs) that consisted of 0.6-0.95 V square wave potentials with 3-s hold at upper and lower potential limits. Catalyst degradation was characterized by measuring mass activity for the oxygen reduction reaction, electrochemically active surface area (ECSA) and polarization performance in H2/air at beginning of test (BOT) and after 15k and 30k (EOT) potential cycles in H2/N2. Concurrently, limiting current densities (iL) were measured for different pressures (1.3, 1.8, 2.3, 2.8 and 3.2 atm) and oxygen concentrations (0.01, 0.02 and 0.04 O2 mole fraction in dry air) at 80oC and 90% relative humidity (RH). We observed that the O2 transport resistance inferred from the measured limiting current densities depends not only on pressure (P), catalyst loading and aging, but also on the current density itself. The observed increase in Rm with iL is almost linear and independent of pressure, suggesting the following new representation for Rm: Rm = {Rg P/Pr + Rd P/Pr} + RKn + {Rf + Ri i/ir}, Rf = RO2/SPt In this representation, the various terms denote the resistance for O2 transport across the boundary layer in the gas channel (Rg), across the gas diffusion layer (Rd), across the microporous layer and secondary pores in cathode electrode (RKn), and across the ionomer film on Pt particles supported on carbon (Rf). Note that Rf is inversely proportional to roughness (SPt, cmPt 2/cm2), and Ri is the portion of Rm that depends linearly on the normalized current density (ir = 1 A/cm2). As in earlier formulations, Rg and Rd are pressure dependent (Pr = 1 atm) as they are related to molecular diffusivity of O2 in gas channel (GC) and in gas diffusion layer (DM), whereas RKn and Rf are pressure-independent as they are related to Knudsen diffusion in 2 permeability across the ionomer film. Using the new representation for the three cells with different loadings, we determined Ri from the current density dependence of Rm at BOT as 0.306 ± 0.1 s/cm; Ri was found to be nearly independent of pressure. We estimated Rg + Rd from the pressure dependence of Rm at BOT as 0.409 ± 0.032 s/cm. Knowing Rg, Rd, and Ri, we calculated RKn + Rf from Rm, and plotted it as a function of 1/ SPt to determine RKn from the intercept and Rf from the slope. Using all the available data at BOT and after 15k and 30k potential cycles, the estimated values are 0.07 s/cm for RKn and 9.15 s/cm for RO2. This value of RO2 compares favorably with the reported literature data for fresh electrodes. To explain the current density dependence of Rm, we conducted O2 transport simulations on an electrode microstructure determined from nano-computed X-ray tomography at an APS (Advanced Photo Source) beamline. The microstructure was digitized at a resolution of 2.5 nm to reconstruct the connectivity and heterogeneous size and spatial distributions of secondary pores, primary pores, carbon, catalyst particles, and ionomer phases. Assuming that the secondary pores are dry at 90% RH, the primary pores were progressively filled with liquid water to match the measured increase in electrode resistance at higher limiting current densities. The water-filling algorithm follows the concept of capillary condensation whereby the smallest primary pores are allowed to preferentially saturate first. In these simulations, changing the primary pore saturation from 0 (dry) to 1 (flooded) results in the limiting current density increasing from 0 to 1 A/cm2 and the electrode resistance from 0.2 to 0.65 s/cm. Figure 1 summarizes the effects of Pt loading and catalyst aging on O2 transport resistance at one set of reference conditions: 1.5 atm operating pressure, 4% O2 mole fraction, 80oC and 90% RH. At BOT, reducing Pt loading to 0.05 from 0.15 mg/cm2 causes a 31% increase in Rm and a 23% drop in iL. After 30k cycles, Rm increases by 12% for the 0.15 mg/cm2 loading and by 43% for the 0.05 mg/cm2 loading, and is 66% higher in the lower loaded cell even though iL is 40% smaller. More telling is the increase in the electrode resistance (Rf + Ri i/ir) after 30 k cycles: 35% for 0.15 mg/cm2 loading and 115% for 0.05 mg/cm2 loading. Figure 1

State-of-the-art polymer electrolyte membrane (PEM) fuel cells often use PtM (M= Co or Ni) as cathode catalysts and Ce-based additives in the membrane to enhance their performance and durability. During operation, however, the transition metals dissolve and Co2+ and Ce3+ cations transport from their initial positions, which decreases their efficacy and can cause fouling of ionomer regions, resulting in performance losses and local membrane failure.1 Despite general knowledge of cation transport through diffusion, migration, and convection, a thorough accounting of the cation positions and concentrations has not been available owing to an inability to monitor these processes under realistic MEA operating conditions and cell geometries. Our approach has been to leverage the unique capabilities of synchrotron microprobe X-ray fluorescence (µXRF), which enable point data collection on the millisecond-scale with spot sizes of Here, we report the development of techniques to directly observe transient, µm-scale migration of cations, in operando. An initial cell design to isolate 1-D through-thickness cation transport was tested using traditional µXRF at the Advanced Photon Source (APS). These experiments resulted in unexpected depletion of cations from the analysis surface of the cell, more preferentially from cathode side. Results agreed qualitatively with 3-D cell modeling, which suggested that steep potential gradients form between the air/cell interface and the interior of the cell, which drives cation migration deep into the cell.2 In order to map ions beyond the front cell edge, follow-on experiments utilized the confocal µXRF capabilities at the Cornell High Energy Synchrotron Source (CHESS). This setup relies on Si collimating optics to define a fluorescent voxel which can be controlled to generate 3-D elemental maps, enabling the measurement of cation transport in the interior of the cell. Steady state depth scans taken at 50 mA/cm2 show the depletion of ions from the surface (Figure 1a, inset), more preferentially from the cathode side, in agreement with the APS experiments. As shown in Figure 1, these depth profile scans were used to determine an optimized depth of ~180 µm, where depth profiles converged and were located far enough away to avoid edge effects, but near enough to the surface to get sufficient signal (~90% attenuation measured at a depth of 200 µm). Transient scans at this optimized depth were performed at 80°C and 50% RH with a current density of 100 mA/cm2. These measurements, shown in Figure 1b, reveal that Ce3+ cations initially situated in the PEM migrate from there into cathode catalyst layer (cCL), forming a gradient in the PEM after 15 minutes. Upon removal of load, Ce3+ rapidly diffuses and equilibrates between ionomer regions of the CL and PEM. These results demonstrate that cation transport in the MEA has a rapid response to the operating condition and generates concentration gradients under load, which validates the model findings and improves the understanding of the root cause of the performance loss. For the first time in fuel cell study, 3-D cation transport inside the MEA was monitored during cell operation. The findings and techniques developed in this study have significant implications for future designs of cell geometry and operating conditions. These results demonstrate that cation transport in the MEA has a rapid response to the operating condition and generates concentration gradients under load, which validates the model findings and improves the understanding of the root cause of the performance loss. For the first time in fuel cell study, 3-D cation transport inside the MEA was monitored during cell operation. The findings and techniques developed in this study have significant implications for future designs of cell geometry and operating conditions. Acknowledgements This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Fuel Cells Program Manager: Dimitrios Papageorgopoulos and Technical Development Manager: Greg Kleen). This work is based upon research conducted at the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357 and the Cornell High Energy Synchrotron Source (CHESS) which is supported by the National Science Foundation under award DMR-1332208. References A. M. Baker, R. Mukundan, D. Spernjak, E. J. Judge, S. G. Advani, A. K. Prasad, and R. L. Borup, J. Electrochem. Soc., 163, F1023–F1031 (2016). Y. Cai, J. M. Ziegelbauer, A. M. Baker, W. Gu, R. S. Kukreja, A. Kongkanand, M. F. Mathias, R. Mukundan, and R. L. Borup, J. Electrochem. Soc., 165, F3132–F3138 (2018). Figure 1

Proton exchange membrane fuel cell (PEMFC) electrodes are commonly composed of Pt nanoparticles supported on conductive carbon interspersed with ionomer, creating a porous network facilitating transport of electrons, protons, both reactant and product gasses, and liquid water. For the electrochemical reactions to occur at the metallic Pt active sites in the cathode, the electrons, protons, and oxygen all need to be present, which presumably requires the confluence of carbon, ionomer, and Pt all accessible at a pore. Following reaction at this 3-phase boundary, the pore network needs to remove product water from the electrode. Though the structure of PEMFC electrode layers has been studied for many years, the details are still unclear. The distribution of pore sizes, thickness and distribution of ionomer, and structure of carbon agglomerates are not well understood. This lack of understanding hinders rational design of catalyst layers that utilize reduced Pt loading while maintaining the performance and durability necessary for large-scale commercialization. Part of the difficulty in obtaining accurate morphological information on these complex structures is that it is unclear what techniques provide accurate analysis of the pore structures within these materials. Common pore size analysis techniques include nitrogen adsorption (BET), mercury intrusion (MIP), x-ray tomography (XCT), transmission electron microscopy (TEM), and focused ion beam tomography (FIB-SEM). When multiple techniques are applied, even to the same sample, the results for pore size distribution and connectivity can vary widely. It is unclear which, if any, of these techniques yield results representative of the true pore structure within the catalyst layer. Further, it is not known if the lack of agreement results from sample preparation methods and analysis conditions used for each technique, or fundamental limitations of the techniques when applied to PEMFC electrode materials. In this work, we examine results of pore analysis from multiple techniques. In addition to standard measurement techniques including BET, TEM, MIP, and FIB-SEM tomography, we employ techniques for measurement of pore size by analysis of the solid/liquid phase transition temperature using cryoporometry (Figure 1). We will discuss the results of these multiple techniques, address their utility and limitations, and attempt to present a pore size distribution in better agreement with reality that can be used as input for modeling efforts. Acknowledgement: This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Technical Development Manager: Greg Kleen and Fuel Cells Program Manager: Dimitrios Papageoropoulos). Figure 1. Comparison of pore size distributions from nitrogen isotherm analysis using BJH1 (top) and cryoporometry (bottom). Top figure is PSD for TEC10E50E (CB), TEC10EA30E (GCB), and acetylene black supported Pt with specific surface areas of 779 and 219 m2 g-1 (AB800 and AB250 respectively). Bottom figure is PSD for TEC10E50E. 1) Young-Chul Park, Haruki Tokiwa, Katsuyoshi Kakinuma, Masahiro Watanabe, Makoto Uchida, Effects of carbon supports on Pt distribution, ionomer coverage and cathode performance for polymer electrolyte fuel cells, Journal of Power Sources, 315 (2016) pp.179-191 Figure 1

Catalyst support materials are an essential component of proton exchange membrane (PEM) fuel cells to decrease catalyst Pt loading. To be classified as a good catalyst support, a material must have high surface area, high electrical conductivity, be durable in harsh environments, and be economically feasible for large scale commercial production. In light of this, carbon is one such material that has widely been used in PEM fuel cells as a catalyst support. Various carbon materials of differing morphologies have been utilized as supports where high surface area is beneficial to greater Pt accessibility and higher performance, while graphitization and lower surface areas are beneficial to durability especially at high potentials. Here, we present a durability analysis of platinum cobalt catalyst supported on five structurally different carbon supports which are as follows: high surface area carbon TEC36E32 (29% Pt / 2.3% Co / 526.6 m2/g-Cat), high surface area graphitized carbon TEC36EA32 (29.3% Pt / 3.3% Co / 108.5 m2/g-Cat), Vulcan carbon TEC36V32 (29.1% Pt / 3.6% Co / 160 m2/g-Cat), graphitized Vulcan carbon TEC36VA32 (29.3% Pt / 3.4% Co / 70 m2/g-Cat), and acetylene black TEC36F32 (29.3% Pt / 3.4% Co / 523.6 m2/g-Cat). Each of the aforementioned carbon supported dealloyed-PtCo catalysts were spray coated onto one side of a DuPont® XL membrane at a loading of approximately 0.1 mgcat/cm2. The other side of the membrane, which served as the anode, was kept the same for all samples and spray coated in the same manner at the same loading with platinum supported on Vulcan carbon TEC10V20E (20% Pt). The membrane electrode assemblies (MEAs) were then incorporated into 5 cm2 differential cells with SGL Sigracet® Gas diffusion layers (GDLs). Subsequent to conditioning, catalyst performance was evaluated via the following characterizations: local oxygen transport resistance measurement, fuel cell polarization curves, sheet resistance measurement, impedance measurement in air and HelOx, mass activity measurement, and cyclic voltammetery measurements. DOE recommended square wave catalyst accelerated stress testing (AST) was utilized to degrade the catalysts with a recovery protocol and characterization performed after 15,000 and 30,000 cycles of the AST.1 Figure 1 illustrates the performance of TEC36V32 and TEC36E32 cathode catalyst MEAs where power densities of 0.643 W/cm2 and 0.716 W/cm2 were obtained respectively at 80oC, 100%RH and 150kPa pressure. In addition to this characterization, evolution of the resistances that arise from oxygen transport during the catalyst durability testing, both pressure dependent and pressure independent, will be presented for the various carbon supports. Further characterization of the catalysts after the testing will also be presented to provide insight on the influence of carbon support materials on catalyst performance with cell degradation. Acknowledgement: This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Fuel Cells Program Manager: Dimitrios Papageorgopoulos and Technical Development Manager: Greg Kleen). References: 1United States Department of Energy Fuel Cells Technologies Office Multi-Year Research, Development, and Demonstration Plan. https://www.energy.gov/sites/prod/files/2017/05/f34/fcto_myrdd_fuel_cells.pdf. Figure 1

PEMFCs are subject to many mechanisms that abruptly decrease power output or slowly degrade long-term performance. Los Alamos National Laboratory researchers pioneered many experimental investigations and forensic characterization studies to explore these effects. These include catalyst degradation, membrane performance loss and failure, gas diffusion layer degradation, and fuel and air poisoning mechanisms. This retrospective highlights a 25+ year effort at understanding and mitigating these effects. In the early 1990s fuel cell performance loss was observed in fuel cells which by today’s standards, had relatively high catalyst loading and utilized thick Nafion 117 membranes [1]. Carbon monoxide was already well known to poison the hydrogen oxidation reaction. The processes of Pt coarsening, Pt-M, dealloying, carbon corrosion, membrane free radical attack, foreign ion-proton exchange, ammonia and sulfurous gas poisoning, were investigated in subsequent years. Many characterization techniques, in addition to voltammetry, were employed to study these processes including: XRD, SEM, EDS, TEM, XPS, XRF, X-ray microscopy & tomography, neutron imaging and ion and gas chromatography [2]. Modeling methods from atomistic calculations to macroscopic effective medium transport models were developed to further provide insights towards mechanisms and mitigation strategies. The efforts continue today as ultralow loading platinum group metal catalysts and alloys, PGM free electrocatalysts, supports and novel membranes are being evaluated for next generation fuel cells. References Wilson, M. S.; Garzon, F. H.; Sickafus, K. E.; Gottesfeld, S., Surface area loss of supported platinum in polymer electrolyte fuel cells. Journal of The Electrochemical Society 1993, 140(10), 2872-2877. Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, D., et.al, Scientific aspects of polymer electrolyte fuel cell durability and degradation. Chemical reviews 2007, 107(10), 3904-3951.

Although fuel cells are being deployed in cars in limited commercialization, they still fall short of the DOE targets for this technology, which are required for widespread consumer acceptance. The FC-PAD (Fuel Cell Performance and Durability) consortium was formed to advance the performance and durability of polymer electrolyte membrane fuel cells (PEMFCs) at a pre-competitive level to further enable their commercialization. The primary component that FC-PAD works to understand and improve is the catalyst layer. The primary catalyst layer architecture in use is a porous dispersed Pt-supported carbon catalyst with recast solid ionomer component forming a continuous (percolating) network through the electrode for proton transport; the carbon facilitating the electrical conduction; the active electrochemical reactions conducted on the metallic Pt sites. These thin-layer-electrode films were pioneered at Los Alamos National Labs in the late 1980’s under the direction of Dr. Shimshon Gottesfeld [1,2] As ubiquitous as the thin layer electrode structure has been used, it is still an active field of research to properly understand the structure and improve the reactant transport, water removal and catalyst utilization. The fuel cell catalyst layer is a complex structure that facilitates the electrochemical conversion of hydrogen and oxygen; it provides pathways for reactant transport, and provides both electrical and proton conducting pathways. This has been traditionally referred to as the 3-phase boundary of an electrolyte, an electrode, and a gaseous fuel. In addition to the gas-reactant transport to the active site, the product water must also be managed both in terms of product removal and optimal hydration of the membrane/ionomer providing the proton conductivity. Recent FC-PAD results have examined the structure of the electrode layer. Because performance is largely influenced by the ionomer/catalyst interface within the catalyst layers of a MEA, the “ideal” interface should contain platinum particles with 100% available proton and electrical pathways to maximize catalyst utilization. In addition, the thickness of ionomer film over the catalyst nanoparticles should be optimal to facilitate gas diffusion and water balance without sacrificing its protonic conductivity. LANL’s recent FC-PAD examinations have included using neutron reflectometry to examine the interaction of ionomer (with and without cations such as cerium present as a radical scavenger) with platinum and carbon simulating the electrode structure. Other fundamental properties of the electrode layer being examined includes, the structure and size of catalyst agglomerates, the porosity and relative hydrophobicity of electrode layer pores, and relative distribution of ionomer. A combination of AFM (atomic force microscopy) BET, MIP (mercury intrusion porosimetry), IGC (inverse gas chromatograph) have been employed to help better define these catalyst layer fundamental properties. These types of measurements suggest an inhomogeneous distribution in the electrode layer. Such an inhomogeneous distribution indicates that catalyst utilization is not fully optimized. This talk will discuss many of the factors affecting catalyst layer performance and provide strategies for catalyst layer optimization. Acknowledgments This work was funded through the DOE FC-PAD Consortium with thanks to DOE EERE FCTO, Fuel Cell Team Leader: Dimitrios Papageoropoulos and Technical Development Manager: Greg Kleen. References Wilson, M. S.; Gottesfeld, S., Thin- film catalyst layers for polymer electrolyte fuel cell electrodes, Journal of Applied Electrochemistry (1992), 22(1), 1-7. Wilson, M. S.; Gottesfeld, S, High Performance Catalyzed Membranes of Ultra‐low Pt Loadings for Polymer Electrolyte Fuel Cells, Journal of the Electrochemical Society, Electrochem. Soc. 1992

An important step in achieving fundamental understanding of fuel cell failure mechanisms and development of technology to mitigate these failures is accomplished by analysis of directed lifetime and failure test results. Several lifetime, accelerated stress, and drive cycle test protocols have been developed and carried out. The two major ASTs that have been developed to evaluate membrane degradation are 1) Open circuit voltage tests, which are designed to accelerate chemical degradation, and 2) Relative humidity cycling tests, which are designed to accelerate mechanical degradation. The results from these tests have been compared to field tests. The ultimate goal is to use the laboratory tests to predict data in the field. An overall predictive decay model is being developed through a combination of specific modeling and tests.

Neutron imaging was been widely used to study the water distribution in proton exchange membrane fuel cell flow fields and gas diffusion layer. However, due to the limitation of spatial resolution, there has been little focus on the water transport process in the membrane and catalyst layer. Here we report on measurements made on thick membranes under saturation gradients which show no “jump condition” and on thick cathode catalyst layers to understand the water transport issues in a non-precious metal catalyst. Finally, we speculate on the possibility of obtaining neutron images with ~1 µm spatial resolution.

Fuel cells are one of the most the most promising energy conversion devices for both residential and transportation applications due to their high electrical efficiencies, low operating temperatures, and zero tailpipe emissions.1-2 Significant advancements have been made in the development of ORR electrocatalysts to enable attainment of the DOE catalyst activity target of 440 mA/mgPt at 0.9V and 150 kPa. Specifically, utilizing nanostructured carbon materials with internal porosity resulted in increased platinum dispersion, higher electrochemically available surface areas, and higher mass acitivites.3 Additionally, transition metal Pt alloys, specifically, Ni and Co, have shown to increase the activity per Pt site, enabling improvements in mass acitivity.4 However, when these different sets of Pt or PtM/Carbon materials are incorporated into dispersed PEMFC electrodes the time, potential and environmental parameters, also known as “break-in” or “conditioning”, required to achieve optimum performance can be vastly different. While several publications have presented baseline electrocatalyst performance values,5-7 and/or demonstrated different methods for fabricating electrodes and standardizing MEA performance,8-9 the impact of MEA conditioning on performance has rarely been discussed in detail. The absence of a detailed discussion of lab-scale MEA fabrication methods in conjunction with conditioning can lead to the reporting of erroneous experimental observations related to purported improvements in electrocatalytic activity or device design. Previous studies by Neyerlin et. al.10 have demonstrated the importance of MEA conditioning on observed mass activity and high current density performance alike. In this study, we expand upon our previous findings, examining the influence of conditioning on 4 different commercially available catalysts: (i) 50wt.% Pt/Vulcan (TKK), (ii) 50 wt% Pt/HSC (TKK), (iii) 30 wt.% PtCo (Umicore) and (iv) 30 wt.% Pt/HSC (Umicore) at three different loadings (0.05, 0.1 and 0.15 mgPt cm-2). The objective was to determine the influence of the break-in processes on oxygen reduction reaction (ORR) mass and specific activity (MA, SA), electrochemical surface area (ECA) and H2-Air polarization curves to formulate a fair and comparative assessment of state-of-the-art electrocatalysts. Ex-situ techniques such as, SAXS, XAFS, nano-CT, and microscopy will be utilized along with operando effluent collection and performance data to glean the fundamental impact of the conditioning process. Preliminary results have shown that the conditioning procedures might need to be re-considered based on the particular catalyst and catalyst layer loading in order to obtain peak performances. The results from this study will not only provide possible pathways towards improving the performance of low loaded high activity catalysts that is needed to meet DOE targets for fuel cell commercialization, but also demonstrate the importance of implementing systematic protocols. References M. K Debe, Nature, 2012, 486, 43−51. Handbook of Fuel Cells: Advances in Electrocatalyst, Materials, Diagnostics, and Durability; Vielstich, W., Yokokawa, H., Gasteiger, H.A., Eds.; John Wiley & Sons: Hoboken, NJ, 2009. S. H. Joo, S. J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature, 2001, 412(6843), 169-172. D. Wang, H. L. Xin, R. Hovden, H. Wang, Y. Yu, A. Muller, F. J. DiSalvo, H. D. Abruña, Nature materials, 2013, 12(1), 81-87. H. A. Gasteiger, S. S. Kocha, B. Sompalli, and F. T. Wagner, Appl. Catal. B: Env., 2005, 56, 9. H. A. Gasteiger, J. E. Panels, and S. G. Yan, J. Power Sources, 2004, 127, 162. Y. Garsany, O. A. Baturina, K. E. Swider-Lyons, S. S. Kocha, Anal. Chem., 2010, 82 (15), 6321-6328. M. B. Sassin, Y. Garsany, B. D. Gould, K. E. Swider-Lyons, Anal. Chem., 2017, 89 (1), 511–518

Fuel cells have the potential to provide very low uninterrupted power for long periods (several decades) of time. The specifications for these applications can vary tremendously from transportation fuel cells that are a large driving force behind fuel cell development. For these applications, the fuel and oxidant supply are often required to be stored for the entire lifetime of the application; these systems have higher inherent safety and overall higher system energy density than batteries for these prolonged operational times. Many of the technical challenges are application dependent but can include: Fuel/oxidant loss (due to membrane/plates/seals permeability) Membrane degradation and thinning (due to high cell voltages) Passive system water management Operation in uncontrolled environments (including operation a large temperature range and unpredictable/uncontrollable ambient atmospheres) Control of power transients (microwatt to watt power levels) The primary degradation modes expected for these applications involve the electrode layer, loss of catalytic activity, membrane thinning, changing hydrophobicity of carbon materials (which impacts water management) and corrosion of metallic bipolar plates (including increasing contact resistance). Electrode degradation involving performance loss of the cathode catalyst layer includes catalyst particle agglomeration, catalyst support corrosion, losses of catalyst layer porosity and loss of catalyst layer proton conductivity due to ionomer degradation. Depending on the systems load requirements, potential cycles can be many millions of cycles over decades long continuous operation. To minimize catalyst degradation, we expect that a Pt-black electrode will be superior compare to the Pt supported on carbon electrodes that are the preferred for transportation applications. To examine the durability of these catalysts we are conducting Accelerated Stress Testing (ASTs); Figure 1 shows an on-going test of a Pt-black catalyst over 1.3 million cycles. The square wave cycling induces Pt dissolution and re-precipitation (Ostwald ripening) of the platinum thus decreases the catalyst surface area. From Figure 1, catalyst surface area loss appears to have reached 30-35% loss, but has not substantially increased after about 750,000 cycles. To avoid large amounts of hydrogen loss due to membrane cross-over, we are exploring multiple layers of membrane to make a thick, mechanically reinforced and chemically stabilized membrane. A primary method to examine membrane degradation is to examine radical attack of the membrane at OCV conditions. Membrane thinning was measured at approximately 1.7 - 4.2 micron/year over a 2.4 year test with H2/air, at 25 C, which over a 20 year operational life extrapolates to34 - 83 micron. This suggests that a membrane of > 200 micron thick will be required for a 20 year operational life at low power levels. Figure 1

While high oxygen reduction reaction (ORR) activities for polymer electrolyte fuel cells (PEFCs) have been demonstrated for high surface area carbon-supported Pt alloy nanoparticle catalysts in aqueous cell rotating disk electrode tests and when operating on oxygen in membrane-electrode assemblies (MEAs), the performance improvement expected from the high intrinsic ORR activities is not realized at moderate to high current densities. 1 One possible reason may arise from the complex requirements for full utilization of the electrocatalytic sites and for adequate reactant transport in the cathode catalyst layer. Knowledge about the structural features of the catalyst layer, understanding the distribution in this porous layer and understanding the proton mobility in ionomer between structural elements of the catalyst could help to design more efficient electrodes for PEFCs. Small angle X-ray scattering (SAXS), which probes objects with dimensions of 1 to 100 nm, is emerging as a powerful tool for structural investigations of PEFC electrodes to estimate the size, shape, and structure of the catalyst and ionomer formulations as well as of fuel cell electrodes. 2 Ultra-small angle X-ray scattering (USAXS) is the scattering at ultra-low angles by structures up to several micrometers in size. USAXS combined with SAXS can probe the length scales relevant to the carbon structures in PEFC electrodes (10 nm to 6000 nm). 3, 4 This presentation will highlight X-ray scattering microstructural characterization of state-of-the-art alloy cathode catalysts including catalyst powders, catalyst-ionomer-solvent dispersions, optimized electrodes and MEAs at different stages of testing and using different testing protocols. The microstructural results will be compared with the physico-chemical properties of the electrode components, the electrode fabrication processes, as well as the fuel cell performance and in-cell diagnaotics to establish a relationship between electrode microstructure and electrode performance. Acknowledgments This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the support of the Fuel Cell Performance and Durability Consortium (FC-PAD). The Advanced Photon Source was supported by the U.S. Department of Energy, Office of Basic Energy Sciences. Argonne is a U.S. Department of Energy Office of Science Laboratory operated under Contract No. DE-AC-02-06CH11357 by UChicago Argonne, LLC. References Kongkanand, A.; Mathias and M. F., Phys. Chem. Lett. 2016, 7, 1127–1137. Khaneft, M.; Holderer, O.; Ivanova, O.; Luke, W.; Kentzinger, E; Appavou, M. S.; Zorn R.; Lehnert, W. Fuel Cells 2016, 16 (4), 406–413. Yang, F.; Xin, L.; Uzunoglu, A.; Qiu, Y.; Stanciu, L.; Ilavsky, J.; Li, W.; Xie, J., ACS Appl. Mater. Interfaces 2017, 9, 6530−6538. Xin, L.; Yang, F.; Xie, J.; Yang, Z.; Kariuki N. N.; Myers, D. J.; Peng, J-K; Wang, X.; Ahluwalia, R. K.; Yu, K.; Ferreira, P. J.; Bonastre, A. M.; Fongalland, D.; Sharman, J. Electrochem. Soc. 2017 164(6): F674-F684.

State-of-the-art fuel cell catalyst layers consist of a random mixture of functional components (catalyst, ionomer, and pore) that are formed in an uncontrolled ink deposition process. The random nature of these structures makes it difficult to optimize the functional domains and hence causes severe mass transport limitations during high-power operation, resulting in a loss in performance and requiring that the fuel cell be oversized to maintain an acceptable level of performance. The ionomer binder adds an additional transport resistance and becomes significant at lower Pt loadings [1]. Decreasing this transport resistance would remove the main barrier to low-cost, ultra-high power density fuel cells. Rational design of the electrode structure in PEFCs could improve performance and reduce cost. By separating the different electrode functions into discrete electrode elements, each element can be optimized for specific functions. Arranging these optimized discrete elements in a controlled, low-tortuosity array configuration enables transport limitations to be reduced or eliminated. We have demonstrated this approach through fabrication of freestanding arrays of vertically-oriented ionomer channels with different aspect ratios, and incorporation of these channels into fuel cell electrodes where they serve as non-tortuous proton-transport highways (Figure 1). Providing effective proton transport through these low-tortuosity percolating highways allows the catalyst domain to have a lower ionomer/catalyst ratio, reducing transport resistance. Proof of concept was demonstrated by achieving a 15% increase in performance with channels with an aspect ratio of 8. Further work is underway to increase channel aspect ratio and improve integration of channels with surrounding catalyst layer, leading to further transport improvements and increases in performance. Fig. 1. Left: freestanding array of ionomer channels. Right: catalyst layer incorporating ionomer channels. References N. Nonoyama, S. Okazaki, A. Z. Weber, Y. Ikogi, and T. Yoshida, J. Electrochem. Soc., 158, B416 (2011). Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium. Figure 1