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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.

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

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

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

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

In commercial electrolysis, the cost of electricity input drives the cost of hydrogen production. Therefore, electrolysis is typically run at high catalyst loading and constant power input over long periods of time. Lowering water-splitting hydrogen production costs to a level comparable to steam methane reformation requires: coupling electrolysis with low-cost power input (wind, solar) to reduce feedstock costs; and dropping catalyst loading to reduce the capital cost at lower capacity.[1,2] While minimal durability loss is seen in commercial electrolyzers, catalyst losses can be masked by high loading (several mg cm‒2). At low loading, however, these losses become more apparent.[3] In this study, electrolyzer durability was evaluated at low iridium-anode loading (0.1‒0.5 mg cm‒2) and with different power inputs (potentials, intermittency). Higher loading tended to delay the onset of durability losses; in contrast, a loading of 0.1 mg cmelec ‒2 resulted in incremental but immediate loss when exposed to high potential. Increasing the upper potential limit appeared to increase iridium dissolution and migration and resulted in higher performance losses. This trend was expected and iridium dissolution is anticipated to be a primary factor in electrolyzer loss at low loading.[4] Introducing cycling (square/triangle wave), however, significantly increased the rate of performance decay and was less expected from half-cell tests and even though less time was spent at elevated potential.[3] Changing the rate of potential increase (saw tooth profiles) confirmed that rapid input increases accelerated loss and may be due to localized potential spikes occurring within the catalyst layer. A variety of system control-based mitigation strategies have been evaluated for lessening durability losses when handling intermittent power sources. Several iridium catalyst types (oxides, surface areas) have been tested in half- and single-cells. Catalyst development efforts often focus on metallic- or hydroxide-based iridium structures, due to higher activity in ex-situ tests. These performance advantages, however, largely disappear in single-cell tests and may be due to surface/near-surface oxidation during conditioning protocols. Metallic/hydroxide durability losses further tend to be larger in both half- and single-cells and may be related to the kinetics of iridium metal/hydroxide/oxide dissolution. In single-cell testing, we have focused on how loading, test parameters, and catalyst type affect proton exchange membrane-based electrolyzer durability. These tests have significant implications on lowering the cost of electrolysis-based hydrogen production and on coupling electrolysis with renewable power inputs. [1] H2 at Scale: Deeply Decarbonizing our Energy System. Presented at Annual Merit Review, U.S. Department of Energy; Washington, DC, June 6−10, 2016. https://www.hydrogen.energy.gov/pdfs/review16/2016_amr_h2_at_scale.pdf. [2] Denholm, P.; O’Connell, M.; Brinkman, G.; Jorgenson, J. Overgeneration from Solar Energy in California: A Field Guide to the Duck Chart; Vol. NREL/TP-6A20-65023; National Renewable Energy Laboratory: Golden, CO, 2015. Available at the following: http://www.nrel.gov/docs/fy16osti/65023.pdf. [3] Alia, S. M.; Rasimick, B.; Ngo, C.; Neyerlin, K. C.; Kocha, S. S.; Pylypenko, S.; Xu, H.; Pivovar, B. S. J. Electrochem. Soc. 2016, 163, F3105−F3112. [4] Cherevko, S.; Geiger, S.; Kasian, O.; Kulyk, N.; Grote, J. P.; Savan, A.; Shrestha, B. R.; Merzlikin, S.; Breitbach, B.; Ludwig, A.; Mayrhofer, K. J. J. Cat. Today 2016, 262, 170.

In pursuit of vehicle electrification, considerable efforts have been devoted to elucidating the degradation mechanisms of proton exchange membrane (PEM) fuel cells(1-5). While the majority of these studies are based around post-mortem analyses of fuel cell materials (e.g. changes in nanoparticle sizes and shape distributions), a full accounting of the losses that contribute to MEA performance degradation requires looking beyond well-researched catalyst nanoparticle degradation processes(2). Indeed, the performance of a PEM fuel cell is affected by many internal and external factors, such as fuel cell design and assembly, material degradation, operational conditions, and impurities or contaminants(6). It is well accepted that state-of-the-art PtCo-alloy cathode electrocatalysts will release Co cations under operation, and the subsequent migration of these cations into the membrane can result in unacceptably large performance losses(1, 7, 8). The effect of Co cations changes with operating conditions giving their mobility in the fuel cell ionomer phase. Despite this general knowledge, a thorough accounting of the Co cation positions and concentrations has been impossible due to an inability to monitor these processes under realistic (humidified) MEA operating conditions. Our approach in this work was to leverage the unique capabilities of beamline 2-ID-D at the Advanced Photon Source to: 1) directly monitor Co2+ diffusion and mobility in an operating H2/air MEA for the first time, and 2) provide real-time data to feed into predictive degradation/loss models. For these proof-of-concept studies, we pre-exchanged standard Nafion® membranes with Co cations and used standard Pt/C for both the anode and cathode catalysts. XRF maps of the Co signal revealed that the Co cation distributions in the MEA membrane were affected by the feed-gases and current densities. These distributions, under certain circumstances, were time-dependent. Future experiments will leverage this technique to probe the effects of different MEA designs and operating conditions on overall current distributions. Acknowledgements We are grateful to Barry Lai and Zhonghou Cai at APS 2-ID-D to setup and assistance during the u-XRF experimental runs. This research used resources of 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. References S. Chen, H. A. Gasteiger, K. Hayakawa, T. Tada and Y. Shao-Horn, J. Electrochem. Soc., 157, A82 (2010). T. A. Greszler, T. E. Moylan and H. A. Gasteiger, Handbook of Fuel Cells - fundamentals, Technology and Applications,Vol. 6 in Advances in Electrocatalysis, Materials, Diagnostics and Durability, W. Vielstich, H. Yokokawa and H. A. Gasteigers W. Vielstich, H. Yokokawa and H. A. Gasteigers, p. 728, John Wiley & Sons, Ltd, (2009). J. Wu, X. Z. Yuan, J. J. Martin, H. Wang, J. Zhang, J. Shen, S. Wu and W. Merida, J. Power Sources, 184, 104 (2008). K. Yu, D. J. Groom, X. Wang, Z. Yang, M. Gummalla, S. C. Ball, D. J. Myers and P. J. Ferreira, Chem. Mater., 26, 5540 (2014). J. A. Gilbert, A. J. Kropf, N. N. Kariuki, S. DeCrane, X. Wang, S. Rasouli, K. Yu, P. J. Ferreira, D. Morgan and D. J. Myers, J. Electrochem. Soc., 162, F1487 (2015). W. Gu, D. R. Baker, Y. Liu and H. A. Gasteiger, Handbook of Fuel Cells - fundamentals, Technology and Applications,Vol. 6 in Advances in Electrocatalysis, Materials, Diagnostics and Durability, W. Vielstich, H. Yokokawa and H. A. Gasteigerss W. Vielstich, H. Yokokawa and H. A. Gasteigerss, p. 631, John Wiley & Sons, Ltd, (2009). Y. Cai, A. Kongkanand, W. Gu and T. E. Moylan, ECS Trans., 69, 1047 (2015). J. H. Dumont, A. M. Baker, S. Maurya, Y. S. Kim, R. Mukundan, D. J. Myers and R. L. Borup, ECS Trans., 80, 861 (2017).

The primary PEMFC catalyst layer architecture used since the early 1990’s 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 [1,2]. The thin catalyst layer consists of microscale carbon particles each supporting nanoscale platinum catalyst particles all loosely embedded in a matrix of ionomer creating a porous structure. The fuel cell catalyst layer is a complex structure that facilitates the electrochemical conversion of hydrogen and oxygen; it provides pathways for reactant transport, provides both electrical and proton conductivity 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 result product water must also be managed both in terms of product removal and optimal hydration of the membrane/ionomer providing the proton conductivity. Various methods are used to produce these thin-layer-electrode films including decal transfer (in both H+ and Na+ forms), direct membrane application, catalyst-coated-membranes and catalyst-coated-gas-diffusion-media. However the resulting structures all must result in providing the 3-phase boundary regions for good performance. 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 and catalyst utilization. A demonstration of the 3-phase boundary is shown in Figure 1 by high resolution TEM [3]. Platinum nanoparticles are observed attached to the carbon support with a thin layer of surrounding ionomer. However, this observed structure is not consistent through-out the electrode; regions where agglomerates of ionomer occur, including Pt nanoparticles separated from the carbon support. One consortia working to better understand and improve the catalyst layer structure is FC-PAD (Fuel Cell-Performance and Durability) consortia, comprised of five U.S. National Labs: Argonne, Berkeley, Los Alamos, Oak Ridge and National Renewable Energy Lab. Recent results from the FC-PAD consortia include elemental mapping of the electrode layers (see Figure 2). This type of elemental mapping suggests an inhomogeneous distribution in the electrode layer. Such an inhomogeneous distribution suggests that catalyst utilization is not fully optimized. Additionally, the appropriate pore structure in the catalyst layer is also necessary for providing transport paths for reactants to reach reaction sites as well as allow water mobility throughout the catalyst layer. Various methods have also been utilized to map the electrode layer pore-structure including digitizing of microscopy data, MIP and BET analysis. Because performance is largely influenced by the ionomer/catalyst interface within the catalyst layers of a MEA, the “ideal” interface should contain 100% ionomer contact with the platinum nanoparticles 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. Partial ionomer contact renders a portion of the catalyst to be electrochemically inactive and can also limit proton conduction. However, increasing ionomer content (to increase contact with the catalyst) leads to a thicker ionomer film, which results in an increased gas and water diffusional barrier. Some novel MEA fabrications various developers are exploring include: NSTF (Nano Structure Thin Film) catalyst and electrodes Catalyst support functionalization to control the catalyst/ionomer interface Stratified catalyst layers Ionomer “nanofiber” inclusion in the catalyst layer Ionomer-coated MWCNTs High temperature membrane electrode assemblies 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 S. Wilson and S. Gottesfeld, Thin- film catalyst layers for polymer electrolyte fuel cell electrodes, J. Appl. Electrochem., 22, 1 (1992) MS Wilson, S Gottesfeld, High Performance Catalyzed Membranes of Ultra‐low Pt Loadings for Polymer Electrolyte Fuel Cells, Journal of the Electrochemical Society, Electrochem. Soc. 1992. Moore, R Borup, KS Reeves, ECS Trans. 3(1), 717-733 (2006) Figure 1

PEM fuel cells (PEMFCs) show great promise to increase the fuel efficiency for transportation applications. However, cost reduction and improvement in durability is required for mass commercialization. To achieve high platinum dispersions, platinum is typically supported on high surface area carbon. However, elemental carbon is not the thermodynamically favored state under PEMFC operating conditions of acidity and potential. Catalyst support, e.g., carbon, corrosion occurs during fuel cell operation, which can be greatly exacerbated by SD/SU cycles when hydrogen/air fronts are present on the fuel cell anode. Carbon corrosion results in thinning of the catalyst layer and changes in surface hydrophobicity, contributing to degradation of the catalyst and accelerating performance losses. The high cost of the noble metal(s) used as the catalyst makes this a crucial area requiring durability improvement. Porous electrodes lose substantial porosity during PEMFC operation, which does not appear to correlate directly with loss of carbon during corrosion reactions. Compaction of the electrode layer reduces the pore volume within the electrode and can negatively impact performance due to increased mass transport losses. To examine the stability of electrode structures and further understand the combined effects of carbon corrosion on performance, we have tested various types of carbon support materials, as well as platinum and platinum-alloy catalysts under simulated drive-cycles and accelerated stress tests. Post characterization has included porosity measurements by mercury intrusion porosimetry (MIP), transmission electron microscopy (TEM) characterization of electrode cross-sections and 2D image analysis, and focus ion beam-SEM (FIB-SEM). Advanced alloy catalysts maintain their electrode structure for some period of time during ASTs, but eventually display similar pore collapse and loss as that observed for Pt-only catalysts supported on high-surface-area-carbons. The loss of pore volume is shown by impedance measurements to result in large increases in mass transport losses. The initial pore size distribution within the catalyst layer shows a range of sizes with an average pore diameter of 120 nm. This average pore diameter holds for up to 500 AST cycles; however, after 1000 AST cycles the average pore diameter is reduced to 40 nm. The initial pore sizes exhibit a distribution centered around 50-99 nm, but range in size up to ~ 300 nm (See Figure 1a). After the AST, most of the remaining pores are < 50 nm, with a few pores measured above 50 nm (See Figure 1b). During the carbon corrosion AST, PtCo alloy catalyst particles show a loss of Co, changing from 18 at.% Co initially to 14 at.% Co after the AST, further degrading the performance of the electrode. Acknowledgments The authors wish to acknowledge the financial support of the Fuel Cell Technologies Office and Fuel Cell Component R&D Team Lead: Dimitrios Papageorgopoulus and Technology Development Manager: Nancy Garland. The authors also wish to acknowledge General Motors and Ion Power, Inc. for supplying materials used in this study. 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 cm2active area test section with parallel flow channels was operated as a differential cell at 80 °C at constant voltage. Shown in the figure are the imaged water content at 0.4 V, 100% inlet relative humidity for the I/C = 0.9 and I/C = 1.1. Figure Caption: Neutron images of the water content of an operating fuel cell at 0.4 V; a) I/C of 1.1, b) 2I/C of 0.9. In both cases, the cathode is toward the top of the image and the anode toward the bottom. The authors from LANL are supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos. The authors from NIST are supported by the U.S. Department of Commerce, the NIST Radiation and Physics Division, the Director's office of NIST, the NIST Center for Neutron Research, and the Department of Energy interagency agreement No. DE_AI01-01EE50660, program manager: Nancy Garland. Figure 1

Stratified catalyst layers can increase fuel cell performance compared to standard flat catalyst layers1,2 due to improved mass transport because of their irregular thickness and porosity. Stratified catalyst layer structures are expected to have enhanced performance in mass transport region due to improvements in water removal from the catalyst layer. Custom electrodes are fabricated with a custom designed spray coating procedure and catalyst ink recipe. Results from multiple fabrication approaches will be discussed to achieve the electrode structure with the highest performance. One approach used involves a topographical patterning of the catalyst layer, and is based on Ion Power proprietary manufacturing techniques. The second approach uses glass epoxy masks during the spray coating process to create a patterned electrode on the GDL with varying thickness. The mask dimensions mirror the bipolar plate flow-field at the cathode, exposing either the land or channel regions. This way the two extreme cases were investigated and the resulting performance compared. The application of more catalyst material in the channels was found to be more beneficial than when applied in the land regions, likely due to rapid reaction times via better access of the incoming reactant gases through the channels. However, the presence of a thicker catalyst layer under the channel may be an issue for product water removal and oxygen diffusion at high current densities. The best performance was observed when in addition to applying the catalyst preferentially in the channels, a carbon-ionomer filler was used in the land region. The filler is used for both mechanical stabilization of the catalyst layer and to improve ionic and electronic conductivity within the catalyst layer. Two ionomer to carbon (I/C) ratios were tested in the carbon-ionomer filler. The high I/C ratio matches the I/C ratio of the Pt/C ink and is equal to 0.9. This case achieved a significant improvement in the kinetics of the catalyst, but had no effect on mass transport. When the I/C ratio was reduced to 0.6 in the carbon-ionomer filler, there was a visible improvement in mass transport. This is likely due to lower ionomer content which results in less water retention in the catalyst later. In order to better understand the performance mechanisms taking place, the effect of adding hydrophobic agents in the carbon filler is examined. This includes the addition of Polytetrafluoroethylene (PTFE) and Fluorinated ethylene propylene (FEP) to further support water removal at high current densities. Finally, the effect of using thin membranes will be discussed, as by stratifying the catalyst layer, much of the proton conduction occurs in a limited portion of the membrane. Membrane electrode assemblies (MEAs) using 25 micron thick membranes will be compared to significantly thinner membranes of 5 and 10 microns. 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. T. E. Springer, M. S. Wilson, and S. Gottesfeld, J. Electrochem. Soc., 140 (12), 3513–3526 (1993). 2. R. Borup and T. Rockward, US Dep. Energy Annu. Merrit Rev., Project ID: FC052 (2015). https://www.hydrogen.energy.gov/pdfs/review15/fc052_rockward_2015_p.pdf

Alloying Pt with transition metals, such as Co and Ni, enhances the catalytic activity for the oxygen reduction reaction (ORR) at cathodes of polymer electrolyte fuel cells (PEFCs). The alloying process necessarily involves a high-temperature thermal annealing step that grows the size of catalyst nanoparticles above 4-5 nm. Particles of this size are known to be more stable under cyclic potentials as they are less prone to Pt dissolution and coalescence than finer scale 2-3 nm Pt particles. However, transition metals are unstable at operating fuel cell potentials and can leach out of the catalyst in the cathode and into the ionomer, resulting in loss of ORR activity. Three formulations of Pt-Co alloy catalysts supported on high surface area carbon were subjected to accelerated stress tests (ASTs) in H2-N2 environment. The AST protocol consisted of repeated square waves with 0.6 V lower potential limit (LPL), 0.95 V upper potential limit (UPL), and 3-s holds at LPL and UPL. Changes in cell performance were monitored by measuring polarization in H2-air, high frequency resistance (HFR), electrochemical surface area (ECSA), impedance in air and helox (79% He, 21% O2), and mass activity. These measurements were made at beginning of life (BOL) and after 1000 (1k), 20,000 (20k) and 30,000 (30k) cycles. The three catalysts had a different initial Co content (34%, 20%, and 15%) and electrode microstructure. At BOL, transmission electron microscopy (TEM) analysis of the high-Co content (H) catalyst electrode, 0.2 mg/cm2 Pt loading, showed a “spongy” porous (hollow) catalyst nanoparticle morphology with ~6.7 nm median Pt-Co particle size after conditioning. The BOL medium-Co content (M) catalyst electrode, 0.1 mg/cm2 Pt loading, showed a crystalline fully alloyed catalyst nanparticle morphology with ~4.4 nm particle size after conditioning. The BOL low-Co content (L) catalyst electrode, 0.1 mg/cm2 Pt loading, also showed catalysts with a fully alloyed crystalline morphology with ~4.5 nm median particle size. Post-mortem TEM analysis of aged electrode specimens showed different amounts of Co dissolution from the nanoparticles. There is an apparent direct relationship between the initial alloy Co content the amount of Co dissolved after potential cycles. WAXS analysis confirms significant Co loss from the nanoparticles and also indicates that the Pt-Pt d-spacing in Pt-Co alloy approaches the values typical of pure Pt particles. The effect of Co dissolution on the loss in specific activity for ORR is summarized in Fig. 1a. The data suggest that the specific activity depends not only on the initial Co content but also on the catalyst nanoparticle morphology and size. Even with 27-50% Co loss, the specific activity of Pt-Co alloy remains higher than 1000 mA/cm2-Pt, and exceeds the activity (650 mA/cm2-Pt) of pure Pt of similar particle size. The measured specific activities are consistent with a kinetic model and catalyst-specific choice of kinetic parameters. The modeled ORR kinetic losses depend on specific activity, ECSA, catalyst loading, and kinetic parameters (electrode morphology). As shown in Fig. 1b, the H-catalyst electrode shows the smallest kinetic overpotentials (ηc s) due to high Pt loading, but has the fastest increase in ηc s with ageing because of low Co retention in the electrode. The L-catalyst electrode shows higher ηc s due to lower Pt loading but has a smaller increase in ηc s with ageing because of high Co retention. For this catalyst, increase in is mostly due to loss of ECSA with ageing. The M-catalyst electrode shows similar increase in ηc s with ageing as the L-catalyst electrode, but this increase is due to the combined effects of Co dissolution and ECSA loss. The modeled increase in mass transfer overpotentials (ηm) with ageing correlates with Pt loading and ECSA loss and depends on the initial electrode morphology. There is no clear evidence of a relationship between Co dissolution (i.e., effect of Co2+ on O2 permeability in ionomer film) and the resulting increase in ηm. Acknowledgments The authors wish to acknowledge the financial support of the Fuel Cell Technologies Office and Fuel Cell Component R&D Team Lead, Dr. Dimitrios Papageorgopoulus, and Technology Development Manager, Dr. Nancy Garland. The authors also wish to acknowledge General Motors, IRD and Umicore for supplying materials used in this study. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. Figure 1

Measurement of local electrode potentials in operating fuel cells can provide important information about spatial variations in performance and durability. These measurements are useful for improved understanding of localized effects associated with contaminants, startup/shutdown, electrode irregularities, membrane cracks/holes, and fuel starvation. Accurate measurement of local electrode potentials requires incorporation of multiple reference electrodes into the fuel cell hardware. By connecting external reference electrodes to the outside of the MEA using ionomer salt bridges, local potential measurements can be obtained without disturbing cell operation [1]. Contaminants such as CO can cause strong localized electrode poisoning in operating fuel cells. Incorporation of multiple reference electrodes has enabled measurement of the distribution of anode losses in a fuel cell exposed to CO, as depicted in Fig. 1. Similar techniques can be used to measure and deconvolute losses associated with the anode and cathode. However, robust ionic connectivity between the MEA and the external reference electrodes is required to enable quantitative potential measurements. Excessive impedance in this ion-conducting pathway can result in slow response times, reference potential drift, and non-quantitative measurements. This work describes improved techniques for integrating external reference electrodes, allowing more accurate quantification of local electrode potentials. Fig. 1. Change in cell voltage and local anode potential (vs SHE) during exposure to CO gas (~400 s) and removal of CO (~1,800 s). References E. Brightman, G. Hinds, Journal of Power Sources 267 (2014) 160-170. Acknowledgements This research is sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and the U.K. National Measurement System. Figure 1

Cerium ions are additives which enhance the durability of polymer electrolyte fuel cells (PEFCs) by scavenging reactive radical species which are generated during operation.1 These radicals attack vulnerable carboxyl, sulfonic acid, and ether groups in the ion conducting polymer, or ionomer, phases in the polymer electrolyte membrane (PEM) and catalyst layers (CLs) within the membrane electrode assembly (MEA) of the cell.2 During cell fabrication, conditioning, and discharge, Ce rapidly migrates between the PEM and CLs, which reduces efficiency of the Pt catalyst and may negate the addition of the Ce additive.3,4 Therefore, in order to stabilize Ce and localize it to areas of highest radical generation, it is critical to understand the relative influences of different migration mechanisms. In this work, we identified the fundamental Ce migration mechanisms during PEM fuel cell operation. Using a novel elemental analysis technique based on X-ray fluorescence (XRF), we characterized Ce migration due to potential and concentration gradients, water flux, and degradation of Ce-exchanged sulfonic acid groups within the PEM. During accelerated stress tests (ASTs), which are designed to reproduce and enhance stressors the cell will experience in the field, we have observed that < 1% of Ce exits the cell during open circuit voltage (OCV) between 30% and 100% RH. It does, however, irreversibly migrate from the PEM into the CLs at a rate correlated to AST aggressiveness. As a result, Ce accumulation in the CL reduces performance, while Ce migration out of the PEM lowers the PEM’s ability to resist the harmful effects of reactive radicals. In order to determine the relevant Ce transport coefficients during cell operation, Ce migration was induced in PEMs using H2 pump experiments under a range of applied potentials at 50% and 100% RH. Ce profiles were intermittently measured using XRF and fit with a 1-D Nernst-Planck equation. Using this model, diffusion and ion mobility coefficients were extracted, which arise due to concentration and potential gradients during the H2 pump experiments, respectively. The results show the rapid and significant effects of such gradients on Ce movement. Using this knowledge, we propose system control and additional material engineering strategies to mitigate Ce migration due to potential and concentration gradients, in order to reduce performance losses and improve cell durability. We have also determined that the degradation of Ce-exchanged ionomer side chains during aggressive ASTs results in significant Ce migration from the PEM into the CLs. In order to counter undesirable migration due to ionomer degradation, we propose the use of more efficient radical scavengers or improved localization of scavengers near areas of highest radical generation. Zirconia-doped ceria (CZO) has previously been shown to enhance peroxide reactivity while minimizing byproduct radical generation.5 We have demonstrated that adding CZO to the cathode CL, where significant peroxide and radical generation occurs6,7, results in identical initial cell polarization performance compared to CZO-free cells. During PEM chemical stability ASTs, however, CZO reduces gas crossover (Figure 1a) and fluoride emission (Figure 1b) rates, which implies that it is a more effective radical scavenger. Less ionomer degradation results in stable performance of the CZO-enhanced MEA after 500 hours of ASTs. Furthermore, after >1400 hours of ASTs, the Ce concentration in the PEM was maintained at its initial value due to the protective capabilities of CZO. Acknowledgements This research is supported by the U.S. DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells Program manager: Dimitrios Papageorgopoulos. Prof. Ajay Prasad and Prof. Suresh Advani also acknowledge support from the University of Delaware’s Fuel Cell Bus Program. This program is funded by the Federal Transit Administration at the Center for Fuel Cell Research at the University of Delaware. References Coms, F. D.; Liu, H.; Owejan, J. E. ECS Trans. 16, 1735–1747 (2008). Rodgers, M. P.; Bonville, L. J.; Kunz, H. R.; Slattery, D. K.; Fenton, J. M. Chem. Rev. 112, 6075–6103 (2012). Stewart, S. M.; Spernjak, D.; Borup, R.; Datye, A.; Garzon, F. ECS Electrochem. Lett. 3, F19–F22 (2014). Baker, A. M.; Mukundan, R.; Spernjak, D.; Judge, E. J.; Advani, S. G.; Prasad, A. K.; Borup, R. L. J. Electrochem. Soc. 163, F1023–F1031 (2016). Stewart, S. M.; Borup, R. L.; Wilson, M. S.; Datye, A.; Garzon, F. H. ECS Trans. 64, 403–411 (2014). Sethuraman, V. A; Weidner, J. W.; Haug, A. T.; Motupally, S.; Protsailo, L. V. J. Electrochem. Soc. 155, B50-B57 (2008). Gubler, L.; Dockheer, S. M.; Koppenol, W. H. J. Electrochem. Soc. 158, B755-B769 (2011). Figure 1

Current state-of-the-art fuel cell cathodes rely on high loading of platinum (Pt) or Pt-alloy on high surface area carbon supports mixed with Nafion. The cost of the high Pt loading is still a hindrance to the commercialization of polymer electrolyte fuel cells (PEFCs) 1. The electrode structure consists of a random aggregation of the functional components (catalyst, ionomer, and pore space) 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 a higher loading and active area of Pt to maintain an acceptable level of performance. The ionomer binder adds an additional transport resistance and becomes significant at lower Pt loadings2. Decreasing this transport resistance would remove a significant barrier to low-cost, ultra-high power density fuel cells. Rational design of the electrode structure in PEFCs could improve performance and reduce cost. However, in current state-of-the-art fuel cells, each unit volume of electrode contains a random mix of components that must simultaneously provide catalytic activity, water transport, O2transport, electronic conductivity, and protonic conductivity. Since each unit must provide all functions, units cannot be optimized for individual functions. By separating the different electrode functions into discrete electrode elements, each element can be optimized for specific functions as shown in Figure 1a. Arranging these optimized discrete elements in a controlled, low-tortuosity configuration enables transport limitations to be reduced or eliminated. In previous work, Middlemen et al. showed under optimal conditions oriented catalyst layer structure can be fabricated but didn’t present any electrochemical evaluation 3. Komini Babu et al. fabricated vertically oriented Nafion nanofibers and deposited Pt via vapor deposition 4. The Nafion nanofiber electrode showed increase in performance compared to Pt sputtered on Nafion 115 electrode but lower in performance to Pt/C based electrode. In this work, we propose an alternate electrode structure to enable good transport behavior and high cell performance through controlled deposition of electrode components in an optimized, organized structural configuration. Figure 1b shows the SEM image of Nafion nanofibers for proton transport, fabricated by solution casting on a porous template. Providing effective proton transport through these low-tortuosity percolating highways allows the catalyst domain to have a lower ionomer/catalyst ratio, reducing transport resistance. 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. F. T. Wagner, B. Lakshmanan, and M. F. Mathias, J. Phys. Chem. Lett., 1, 2204–2219 (2010). 2. N. Nonoyama, S. Okazaki, A. Z. Weber, Y. Ikogi, and T. Yoshida, J. Electrochem. Soc., 158, B416 (2011). 3. E. Middelman, Fuel Cells Bull., 2002, 9–12 (2002). 4. S. K. Babu, R. W. Atkinson, A. B. Papandrew, and S. Litster, ChemElectroChem, 2, 1752–1759 (2015). 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, and discharge, Ce migrates through-plane between the PEM and catalyst layers (CLs) due to concentration and potential gradients.1,2 In addition, we have observed in-plane Ce migration due to water gradients and also identified degradation of Ce-exchanged PEM side chains as another possible mechanism for Ce migration.3 Ce migration is detrimental because (1) its accumulation in the CL ionomer can diminish the electrode’s proton conductivity, which generates performance losses4; and (2) its depletion may leave an ionomer region more susceptible to radical attack. Therefore, it is critical to understand the relative influence of different migration mechanisms under a range of operating conditions in order to stabilize Ce in the PEM and localize it to areas of highest radical generation. To understand the effects of potential gradients and relative humidity (RH) on Ce migration, ex situ experiments were performed using uncatalyzed Nafion® XL PEMs (DuPont) which contain ~6 μg/cm2 ion-exchanged Ce. PEM specimens were operated in H2 pump mode in a standard conductivity cell (BekkTech) at 80°C with 50% and 100% RH H2. The evolution of Ce profiles was quantified using X-ray fluorescence (XRF). By comparing the resulting profiles at 4 C of charge transfer for the different potential and RH conditions (Figure 1a), we observe decreased Ce transference at low RH. Under these conditions, decreased PEM water content causes a disproportionate reduction in Ce conductivity relative to proton conductivity. In these experiments, Ce ion mobility induced by a potential gradient leads to a concentration gradient, which, in turn, induces Ce diffusion in the opposite direction. Therefore, the profiles shown in Figure 1a arise from a combination of ion mobility and back-diffusion due to the resulting concentration gradient. In order to decouple these effects, a transient, 1-D model was developed based on Nernst-Einstein ion mobility and Fickian diffusion, in order to solve for the ion mobility and diffusion coefficients. Experimental and model results for the 2 V, 100% RH case are shown in Figure 1b. At 100% RH, both diffusion and ion mobility coefficients were determined to be an order of magnitude higher than at 50% RH. In addition to migration within the PEM, Ce is stabilized in the cathode CL, likely in the CL ionomer and/or carbon CL supports.3 Ce accumulation in the cathode catalyst layer was measured to degrade MEA performance. Different mechanisms have been proposed for the performance loss, including increased proton resistance within the CL, and a reduction of oxygen reduction reaction kinetics. However, the effect of each mechanism on performance loss is difficult to quantify. The relative influence of Ce poisoning on the cathode CL performance will also be discussed. Quantifying the different Ce migration mechanisms has provided a better understanding of the effects of Ce migration in the PEM and CL and the associated losses in operating performance. These results demonstrate that stable Ce compounds, which can be localized to areas of highest reactive radical generation, need to be developed to enhance PEFC durability without compromising performance. Acknowledgements This research is supported by the U.S. DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells Program manager: Dimitrios Papageorgopoulos. Prof. Ajay Prasad and Prof. Suresh Advani also acknowledge support from the University of Delaware’s Fuel Cell Bus Program. This program is funded by the Federal Transit Administration at the Center for Fuel Cell Research at the University of Delaware. References Stewart, S. M.; Spernjak, D.; Borup, R.; Datye, A.; Garzon, F. ECS Electrochem. Lett. 3, F19–F22 (2014). Baker, A. M.; Mukundan, R.; Spernjak, D.; Advani, S. G.; Prasad, A. K.; Borup, R. L. ECS Trans. 75, 707–714 (2016) Baker, A. M.; Mukundan, R.; Spernjak, D.; Judge, E. J.; Advani, S. G.; Prasad, A. K.; Borup, R. L. J. Electrochem. Soc. 163, F1023–F1031 (2016). Banham, D.; Ye, S. Y.; Cheng, T.; Knights, S.; Stewart, S. M.; Wilson, M.; Garzon, F. J. Electrochem. Soc. 161, F1075–F1080 (2014). Figure 1

The performance and durability of proton exchange membrane fuel cells (PEMFCs) is of great interest for vehicular power applications. In traditional PEMFCs, Pt catalysts are used in both the anode and the cathode. However, efforts to improve PEMFCs often exposes the membrane electrode assembly of PEMFCs with other metallic cations. For example, reducing the cost of PEMFCs and in particular of Pt-based catalysts can be accomplished with Pt-Ni or Pt-Co alloying catalysts [1-3]. Additionally, Cerium (Ce) is incorporated in membranes in order to improve the chemical durability of the membrane [4]. While these materials effectively improve performance and durability, they have shown to leach out contaminants such as Ce3+, Co2+, and Ni2+ions into the ionomer [5-7]. Such leaching can reduce the efficacy of the additives, decrease ionomer conductivity and reduce cell performance. In this work, we have studied the impact of metallic cationic contaminants (Ce, Co and Ni) in the catalyst layer ionomer on the oxygen reduction reaction (ORR) of a Pt microelectrode in various conditions. A well-established microelectrode mini-cell set-up was used for the electrochemical measurements of the ORR activity at the Pt-ionomer interface with and without the presence of cationic contaminants in concentrations observed during fuel cell operation. Nafion was doped with cation contaminant and subsequently deposited on Pt micro-electrodes. X-ray fluorescence (XRF) was used to verify that the contaminant concentrations in the thin films was equivalent to experimentally determined concentrations in the CL ionomer [4]. Results obtained using electrochemical and other characterization techniques will be presented to further understand the role of the contaminants in the performance of Pt electrodes for the ORR. These techniques include laser profilometry to measure the thin film thickness, conductivity cells and impedance measurements to understand the impact of contaminants on the ionomer resistance. Results indicate that in presence of Ni, we observe a decrease in ionomer conductivity (Nafion® 211), from 102 mS cm-1 in proton form to 35 mS cm-1 with 5.2 mgNi cm-3 at 80°C, 100% RH. This performance correlates with a decrease in ORR performance, most notably lower limiting currents 1.1x10-1 mA cm2 in proton form vs. 6.3x10-2 mA cm-2 with 7 mgNi cm-3and lower ORR on-set potentials at 25°C, 100% RH. References 1. Kang, Y., et al., Multimetallic core/interlayer/shell nanostructures as advanced electrocatalysts. Nano letters, 2014. 14(11): p. 6361-6367. 2. Cui, C., et al., Compositional segregation in shaped Pt alloy nanoparticles and their structural behaviour during electrocatalysis. Nat Mater, 2013. 12(8): p. 765-771. 3. Chi, M., et al., Surface faceting and elemental diffusion behaviour at atomic scale for alloy nanoparticles during in situ annealing. Nature Communications, 2015. 6: p. 8925. 4. Coms, F.D., The chemistry of fuel cell membrane chemical degradation. ECS Transactions, 2008. 16(2): p. 235-255. 5. Baker, A.M., et al., Cerium migration during PEM fuel cell accelerated stress testing. Journal of The Electrochemical Society, 2016. 163(9): p. F1023-F1031. 6. Antolini, E., J.R.C. Salgado, and E.R. Gonzalez, The stability of Pt–M (M = first row transition metal) alloy catalysts and its effect on the activity in low temperature fuel cells: A literature review and tests on a Pt–Co catalyst. Journal of Power Sources, 2006. 160(2): p. 957-968. 7. Mani, P., R. Srivastava, and P. Strasser, Dealloyed binary PtM3 (M = Cu, Co, Ni) and ternary PtNi3M (M = Cu, Co, Fe, Cr) electrocatalysts for the oxygen reduction reaction: Performance in polymer electrolyte membrane fuel cells. Journal of Power Sources, 2011. 196(2): p. 666-673. LA-UR-17-23298

FC-PAD (Fuel Cell – Performance and Durability) consortium coordinates U.S. national laboratory activities related to PEM fuel cell performance and durability. The cost and durability of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are two major barriers to the commercialization of these systems for transportation power applications. Transportation conditions include operation in the presence of fuel and air impurities, start/stop, freeze/thaw, and humidity and load cycling that result in mechanical and chemical stresses. MEA durability decreases with decreasing catalyst loading, making cost reduction even more difficult. While there has been significant progress in improving PEM Fuel Cell durability with lower cost materials, further improvements are needed to meet the commercialization targets. Degradation in the cathode catalyst layer is the primary cause of performance loss, with both recoverable and irreversible losses occurring. One recoverable loss for PEMFCs includes catalyst poisoning by membrane degradation products [1] which requires removal from the catalyst surface and then from the electrode layer for the performance recovery[2]. Irreversible degradation modes include catalyst dissolution and ripening, loss of alloying agents from Pt-X catalysts, plus the effects of the various forms of carbon used in PEMFC components which include changing hydrophobicity, carbon corrosion and loss of porosity of electrode layers and Gas Diffusion Layers (GDLs). These increasing losses are primarily in the cathode catalyst layer and are attributed to both increasing transport and kinetic losses. The FC-PAD consortium is examining these degradation mechanisms to help develop improved materials and operating strategies. Corrosion of the carbon electrocatalyst support has been measured during drive-cycle operating conditions and increases with increase potential cycling from 0.4 to 0.9 V. Carbon corrosion is one of the major contributors to degradation which leads to changes in the catalyst layer structure and reduces its activity. Reduction in catalyst layer thickness is observed during operation, exacerbated during drive cycles. This reduction can be due to the loss of carbon through carbon corrosion or due to compaction; both effects likely lead to a loss of void volume. Membrane additives which increase membrane life-times, have been measured to migrate into the catalyst layer and appear to be associated with the carbon in the catalyst layers. Low potentials (0.2V) appear to be required to remove membrane fragment adsorbates which decrease catalyst activity. Pt alloy catalysts lose most of their alloying agents during operation; the alloying agents migrated throughout the ionomer. Durability implications of using Pt-X alloy catalysts will be discussed. Results related to the mentioned degradation mechanisms will be presented including characterization from TEM, SEM, XRF, XRD and electrochemical testing. Acknowledgments This work was funded through the DOE FC-PAD Consortium with thanks to DOE EERE FCTO, Fuel Cell Team Leader: Dimitrios Papageoropoulos References: [1] Yu Seung Kim, Melinda Einsla, James E. McGrath, and Bryan S. Pivovar, The Membrane–Electrode Interface in PEFCs: II. Impact on Fuel Cell Durability Journal of The Electrochemical Society, 157 11 B1602-B1607 2010. [2] Jingxin Zhang, Brian A. Litteer, Frank D. Coms, and Rohit Makharia, Recoverable Performance Loss Due to Membrane Chemical Degradation in PEM Fuel Cells, Journal of The Electrochemical Society, 159 (7) F287-F293 (2012)

The FC-PAD (Fuel Cell – Performance and Durability) consortium coordinates national laboratory activities related to fuel cell performance and durability, provides technical expertise, and integrate activities with industrial developers. The national laboratory core teams have the responsibility to carry out foundational research and capabilities development, and provide support for the individual projects’ research efforts. This consortium incorporates National-Laboratory investigators related to durability, transport, and performance, and combines them into one highly coordinated effort. The consortium formalized already existing and effective collaborations amongst the National Laboratories that established leadership in PEMFC performance and durability research and development. The consortium coordinates work under in different Thrust Areas including component thrust areas and cross-cutting thrust areas. The six different thrust areas are: Component Thrust Areas: Electrocatalysts and Supports Electrode Layers Ionomers, Gas Diffusion Layers, Bipolar Plates, Interfaces Cross-cutting Thrust Areas: Modeling and Validation Operando Evaluation: Benchmarking, ASTs, and Contaminants Component Characterization and Diagnostics The structure of FC-PAD utilizes multiple cross-cutting thrust areas from theoretical modeling to characterization including benchmarking new materials that are provided to FC-PAD. The FC-PAD structure brings together world-class scientists into one integrated consortium, yet it provides a flexible structure to strategically use the widely varying expertise. The FC-PAD consortium is examining degradation mechanisms to help develop improved materials and operating strategies. Corrosion of the carbon electrocatalyst support has been measured during drive-cycle operating conditions and increases with increase potential cycling from 0.4 to 0.9 V. Carbon corrosion is one of the major contributors to degradation which leads to changes in the catalyst layer structure and reduces its activity. Reduction in catalyst layer thickness is observed during operation, exacerbated during drive cycles. This reduction can be due to the loss of carbon through carbon corrosion or due to compaction; both effects likely lead to a loss of void volume. Membrane additives which increase membrane life-times, have been measured to migrate into the catalyst layer and appear to be associated with the carbon in the catalyst layers. Low potentials (0.2V) appear to be required to remove membrane fragment adsorbates which decrease catalyst activity. Pt alloy catalysts lose most of their alloying agents during operation; the alloying agents migrated throughout the ionomer. Durability implications of using Pt-X alloy catalysts will be discussed. Results related to the mentioned degradation mechanisms will be presented including characterization from TEM, SEM, XRF, XRD and electrochemical testing. Consortium members include Argonne National Laboratory, Lawrence Berkeley National Laboratory, Los Alamos National Laboratory, the National Renewable Energy Laboratory and Oak Ridge National Laboratory. Acknowledgments This work was funded through the DOE FC-PAD Consortium with thanks to DOE EERE FCTO, Fuel Cell Team Leader: Dimitrios Papageoropoulos

Commercially viable polymer electrolyte membrane fuel cell (PEMFC) systems for transportation and stationary power applications require optimal combination of performance and durability at a competitive cost. Optimized water management across a fuel cell is key to improving performance and maintaining high conductivity of the ionomer in PEM and catalyst layers (CLs), while preventing excessive flooding in CLs, gas diffusion layers (GDLs), channels, and manifolds. Durability is also greatly affected by water transport as certain degradation mechanisms are accelerated depending on the humidity levels. As well, catalyst layers that suffered carbon corrosion due to long operation and/or high number of starts/stops are more susceptible to flooding. Imaging in situ and ex situ enables crucial insight into the microstructure of the fuel cell components and materials, associated water transport, and their effect on cell performance, dynamic response, and degradation mechanisms. Several case studies are presented to highlight the use of optical imaging, neutron imaging, and micro X-ray computed tomography (microXCT) to investigate fuel cell components across a range of length scales. Laboratory-scale microXCT instruments are used ex situ to resolve the three-dimensional structure of the GDL and electrodes, before and after various accelerated stress tests. Optical imaging in operating fuel cells offers a cost-effective combination of high temporal resolution (tens of frames per second) and high spatial resolution (several µm). The imaging requires special cell design with visual access to the imaged area (GDL surface and channels [1] or catalyst layer surface [2]). We employed optical visualization in a variety of studies, such as to visualize liquid water dynamics in the cathode and anode flow fields, or at the interface between the CL and the GDL. Besides water transport studies, optical imaging can quantify in-plane gas distribution in the flow fields during startups and shutdowns [3]. Neutron imaging is a powerful tool to visualize and quantify water transport in operating fuel cells. It has a high sensitivity to small amounts of water inside a cell, while having high transmission through the common fuel cell hardware. At a lower spatial resolution of 250 µm, neutron imaging is used to measure in-plane water distribution with high temporal resolution and large field of view of up to 20 cm by 20 cm. We studied the performance with novel NSTF (nano structured thin film) electrodes and GDL materials [4], and also visualized water and ice formation when cell was subjected to repeated starts at sub-freezing temperatures [5]. Since neutron imaging provides water content integrated along the neutron beam, we combined neutron imaging with the optical visualization to study water transport in different flow field geometries [6] as well as in an operating electrolyzer [7]. Such approach with simultaneous imaging provides additional information about the water location, and in certain situations allows distinguishing between channel and GDL water, or between anode and cathode channels and manifolds. High-resolution neutron imaging is able to measure water distribution across the cell thickness at spatial resolution as high as 10 µm. Image processing procedure is developed to measure the water uptake and observe Schroeder’s paradox in situ in PEMs [8,9]. Further, properties of the microporous layer (MPL) of the GDL were manipulated to prevent excessive flooding in cathode catalyst layers. Anode flooding is evidenced by optical visualization, simultaneous imaging, and high-resolution neutron imaging. Increased water transport across the membrane, from cathode to anode, can be detrimental as liquid water on the anode side may cause localized fuel starvation and cause irrecoverable degradation, similar to accelerated carbon corrosion during unassisted startups and shutdowns. However, for certain novel cathodes, which are prone to flooding, e.g. nano-structured thin film (NSTF) electrodes and non-precious group metal (non-PGM) catalysts, water removal through the anode has proven to be a viable option to improve the performance by reducing cathode water content. The authors acknowledge support from US Department of Energy EERE FCTO, Los Alamos National Laboratory LDRD (Laboratory Directed Research and Development) program, Federal Transit Administration, US Department of Commerce, and NIST Center for Neutron Research. References: D. Spernjak et al., J. Power Sources 170 pp334 (2007) F.-Y. Zhang et al., J. Electrochem. Soc. 154 (11) B1152 (2007) Y. Ishigami et al., J. Power Sources 269 pp556 (2014) A. J. Steinbach et al., ECS Trans. 33 (1) 1179 (2010) N.Macauley et al, J. Electrochem. Soc. submitted (2016) D. Spernjak et al., J. Power Sources 195 pp3553 (2010) O.F. Selamet et al., Int. J. Hydrogen Energy 38 pp5823 (2013) D. Hussey et al., J. Appl. Phys. 112 (10), 104906 (2012) D. Spernjak et al., ECS Trans. 33 1451 (2010)

In order to simultaneously meet the automotive PEMFC targets of cost, performance and durability, Pt-alloys electrocatalysts are being intensively investigated.1,2,3 Pt-alloys show promise of higher ORR mass activities that will permit the use of lower Pt loadings (~0.1 mgPt/cm2) in the cathode catalyst layers. In this work we have studied several Pt-Co/C catalysts that exhibit mass activities that exceed the DOE target of 440 mA/mgPt at 0.90V, 80oC, 100% RH for ORR activity. The performance at rated power density of 1W/cm2 at 0.60V is more difficult to achieve at low loadings due to unanticipated transport resistances. In addition to carrying out ORR activity studies both in TF-RDE set-ups4,5 as well as in MEAs of subscale cells, we have also applied complementary diagnostics of wet and dry H2-Air curves, microscopy of catalyst powders and catalyst layer films, EIS and dilute oxygen limiting current studies6 on these materials. Rigorous base-lining was carried out to obtain the ORR activity of a commercial Pt/C catalyst in order to benchmark the Pt-Co/C catalysts under study using identical protocols.7,8 Although we easily achieve the mass activity targets at 0.90V, we only approach the rated power targets indicating that further understanding of the transport resistances at high current densities and low Pt loadings is necessary to identify and mitigate the losses. Acknowledgements: This work was funded through the DOE FC-PAD Consortium. References Mathias MF, Makharia R, Gasteiger HA, Conley JJ, Fuller TJ, Gittleman CJ, Kocha SS, Miller DP, Mittelsteadt CK, Xie T, Van SG. Two fuel cell cars in every garage. Electrochem. Soc. Interface. 2005;14(3):24-35. Gasteiger HA, Kocha SS, Sompalli B, Wagner FT. Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. Applied Catalysis B: Environmental. 2005 Mar 10;56(1):9-35. Kongkanand A, Mathias MF. The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells. The journal of physical chemistry letters. 2016 Mar 11;7:1127-37. Kocha SS, Zack JW, Alia SM, Neyerlin KC, Pivovar BS. Influence of ink composition on the electrochemical properties of Pt/C electrocatalysts. ECS Transactions. 2013 Mar 15;50(2):1475-85. Shinozaki K, Zack JW, Pylypenko S, Pivovar BS, Kocha SS. Oxygen Reduction Reaction Measurements on Platinum Electrocatalysts Utilizing Rotating Disk Electrode Technique II. Baker DR, Caulk DA, Neyerlin KC, Murphy MW. Measurement of oxygen transport resistance in PEM fuel cells by limiting current methods. Journal of The Electrochemical Society. 2009 Sep 1;156(9):B991-1003. Kocha SS. Principles of MEA Preparation in Handbook of Fuel Cells: Fundamentals, Technology, and Applications; Vol. 3, edited by W. Vielstich, A. Lamm, and HA Gasteiger. Ch.;43:538. Neyerlin KC, Gu W, Jorne J, Clark A, Gasteiger HA. Cathode catalyst utilization for the ORR in a PEMFC analytical model and experimental validation. Journal of The Electrochemical Society. 2007 Feb 1;154(2):B279-87.

The development of stratified catalyst layers promises an increase in catalyst layer performance compared to conventional flat catalyst layers.1,2 Irregular catalyst layer thickness and porosity can lead to enhanced gas and water transport in and out of the catalyst layer, respectively. The stratified structure is expected to have the same performance in the kinetic region where the performance is controlled by the overall Pt loading. However at high current densities, the thinner sections of the stratified catalyst layers should allow for better mass transport properties. Electrode fabrication is done in house with a custom designed spray coating procedure and catalyst ink recipe. Various approaches are being explored to achieve the desired electrode structure. One approach is to densify the catalyst layer in localized regions, and is based on Ion Power proprietary manufacturing techniques. This can involve the use of glass epoxy masks during the spray coating process to directly create a patterned electrode on the membrane that has thicker and thinner regions. Figure 1 illustrates the effect of stratification on the fuel cell performance of a MEA. The second approach is to pattern thicker and thinner sections of an electrode using masks. During spray coating, one mask exposes the lands and the other mask exposes the channels. This way the two extreme cases can be investigated and the resulting performance compared. The concept of having more catalyst material in the channels may be beneficial due to rapid reaction times during influx of reactant gases through the channels. However, the presence of a thicker catalyst layer under the channel may later become an issue for product water removal and oxygen diffusion. On the other hand, spray coating more of the electrode in the land areas may support better product water removal and oxygen diffusion by leaving a thinner catalyst layer under the channels. The best performance is expected from a combination of ordered thin and thick regions in the catalyst layer. Results from various MEAs tested in a single cell on a fully automated test station to evaluate catalyst performance will be presented. The evaluation of new catalyst ink recipes with reduced ionomer to carbon ratios is included in this study to further increase mass transport by reducing ionomer swelling in the catalyst layer at high relative humidity. 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. T. E. Springer, M. S. Wilson, and S. Gottesfeld, J. Electrochem. Soc., 140 (12), 3513–3526 (1993). 2. R. Borup and T. Rockward, US Dep. Energy Annu. Merrit Rev., Project ID: FC052(2015). https://www.hydrogen.energy.gov/pdfs/review15/fc052_rockward_2015_p.pdf Figure 1. Performance of textured and baseline MEA at 275kPa in H2/Air @ 80oC and 100%RH. Figure 1

The cost and durability of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are the two major barriers to the commercialization of these systems for stationary and transportation power applications.1 Given the stringent performance requirements that PEMFCs have to meet for automotive applications, Pt-alloy catalysts are being considered as cathode catalysts. While PtCox alloy catalysts exhibit better mass activity than Pt/C catalysts and can meet the beginning of life performance requirements, their durability is not proven. Several studies have indicated that the base metal leaches out of these catalysts over time and any benefits due to alloying is lost over the lifetime of the Membrane Electrode Assembly (MEA).2 The U.S. DOE Fuel Cell Tech Team has recommended various ASTs for PEMFC components.3,4 Los Alamos National Laboratory (LANL) has previously reported on the durability of several Pt/C catalysts during both simulated drive cycle testing and accelerated stress tests (ASTs).5 A catalyst AST that involved applying a square wave with 3 sec at 0.6V and 3 sec at 0.95V was determined to be an appropriate AST for evaluating the durability of Pt electrocatalysts.6In this study the performance of PtCo/C cathode catalyst based MEAs subjected to this AST were evaluated. The durability of PtCo/C catalysts was found to be comparable or better than that of the Pt/C based catalysts. Moreover the activity of the PtCo/C catalyst based MEA was also enhanced with respect to the Pt/C MEA. Figure 1 illustrates the evolution of the electrochemical surface area (ECSA) over 30,000 cycles of the square wave AST. The PtCo/C based MEA exhibited little change in ECSA over the course of the test. The ECSA initially increased due to insufficient conditioning and then decreasing due to catalyst particle size increase. In contrast, the Pt/C catalyst showed significant increase in ECSA over the course of the test. Finally the starting ECSA of the PtCo/C catalyst based MEA was significantly lower than that of the Pt/C MEA, while the end of test (EOT) ECSA was almost identical. These results are consistent with the small (≈ 2nm) starting particle size of the Pt and relatively large (≈ 5nm) starting particle size of the PtCo catalyst. The performance of the two MEAs before and after the durability AST is presented in Figure 2. It should be noted that the Pt/C based MEA had a loading of 0.15mg.Pt/cm2 whereas the PtCo/C MEA had an initial loading of 0. 21mg.Pt/cm2. The beginning of test (BOT) performance of the PtCo/C based MEA was significantly better in the kinetic region due to both the alloying effect and higher Pt loading. However, the BOT performance of the Pt/C MEA was better in the mass transport region due to extra mass transport limitations observed in alloy catalysts. The End of test (EOT) performance of the PtCo/C based MEA is only slightly higher than that of the EOT performance of the Pt/C based MEA, consistent with the fact that most of the Co has leached out of the PtCo catalyst. In this presentation the durability implications of using PtCo alloy catalysts will be discussed in detail with results presented from TEM, SEM, XRF, XRD and electrochemical testing. References 1. R. Borup, et al., Chemical Reviews, V. 107, No. 10, 3904-3951 (2007). 2. S.C. Ball, S.L. Hudson, B.R.C. Theobald and D. Thompsett, “PtCo, a Durable Catalyst for Automotive Proton Electrolyte Membrane Fuel Cells?” ECS Trans., 11 (1), 1267 (2007). 3. DOE Cell Component AST and polarization curve Protocols for PEM Fuel Cells (Electrocatalysts, Supports, Membranes and MEAs), Revised December 16, 2010. 4. N. L. Garland, T.G. Benjamin, J. P. Kopasz, ECS Trans., V. 11 No. 1, 923 (2007). 5. R. Mukundan, G. James, J. Davey, D. Langlois, D. Torraco, W. Yoon, A. Z. Weber, and R. Borup, “Accelerated Testing Validation” ECS Trans., 41 (1), 613 (2011) 6. R. Borup et al. DOE Hydrogen and Fuel Cell Program, FY15 Annual Progress Report. https://www.hydrogen.energy.gov/pdfs/progress15/v_e_1_borup_2015.pdf Acknowledgements The authors wish to acknowledge the financial support of the Fuel Cell Technologies Program and Technology Development Managers: Dimitrios Papageorgopoulos and Nancy Garland. The authors also wish to acknowledge SGL Carbon for the GDLs used in this study. Figure 1

The use of a three-electrode cell, in which the potential of a working electrode can be accurately measured vs. a reference electrode (RE), is a standard practice in liquid-phase electrochemical experiments. However, the geometric constraints of a working PEMFC make use of a RE challenging. Incorporation of a RE into an operating PEMFC typically requires experimental compromises that prevent precise and accurate measurements from being obtained, and do not allow accurate spatial resolution of potential variation along the electrodes. As a result, PEMFC experiments are often performed using the PEMFC anode, which remains within a few mV of 0 V vs RHE during typical operation, as a pseudo RE. Recently, the National Physical Laboratory (NPL) developed an effective technique for accurate in situ monitoring of local electrode potentials in a PEMFC. By inserting REs from the back side of the MEA, the NPL configuration eliminates the edge effects, lack of anode vs. cathode specificity, Ohmic losses, and current density distribution disruptions that have compromised previous fuel cell RE experiments. We report here on the use of this RE technique to monitor local electrode potentials in an operating PEMFC during exposure to CO gas (Figure 1). CO is a common impurity in H2 gas and is known to poison Pt-based anodes. Levels of CO present in H2 fuel are expected to be in the ppb range for cells operating on pure H2 and the ppm range or higher for cells operating on reformate. Use of an RE can reveal the distribution of CO losses, and can also enable losses on the anode to be distinguished from those on the cathode. The results of this work demonstrate a new diagnostic capability that enables improved understanding of local conditions during PEMFC operation during transient or steady state operation. Figure 1. Cell voltage (orange) and anode potential (blue) of a MEA operating at 1 A/cm2 with anode exposed to 20 ppm CO. CO was removed from the anode feed at t = 47 min. Gore MEA, with anode/cathode loading of 0.4/0.1 mg/cm2, 80°C, 100% RH, 100 kPaabs. Figure 1

The cost and durability of the electro-catalysts used in the proton exchange membrane fuel cells (PEMFCs) are two critical factors affecting the early market penetration for both transportation and stationary applications. Although progress has been made in the rational designs of highly active ORR electro-catalysts with excellent performance characterized by rotating disc electrode (RDE), [1, 2] there still remains a huge challenge to implement these RDE findings into an actual membrane electrode assembly (MEA) for PEMFC systems. Because the ORR performance is largely influenced by the ionomer/catalyst interface within the catalyst layers of a MEA, the “ideal” interface should contain 100% ionomer coverage for maximizing 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. Additionally, the appropriate pore structure in the catalyst layer is also necessary for providing transport paths for both reactants (O2 and H2) to reach reaction sites as well as allow water mobility throughout the catalyst layer. In some of the more conventional MEA fabrication processes, the ionomer coverage of the catalyst particles can be partial or non-uniform. This is partly due to the lack of control of ionomer deposition onto the catalyst surface. As a consequence, some of the catalyst becomes electrochemically inactive. Furthermore, insufficient ionomer coverage suppresses the amount of H+ transport, thus impacting the oxygen reduction reaction (ORR). On the other hand, the aggregation of ionomer with increasing ionomer content (to increase catalyst coverage) in a catalyst layer leads to a thicker ionomer film, which results in an increased gas and water diffusional barrier. We will present a novel approach to construct an innovative ionomer/catalyst interface with high ionomer coverage and a thin ionomer layer. This is done by an electrostatic charge attraction of a negatively charged “-SO3 -” on the surface of ionomer particles and a positively charged “-NH3 +” on the surface of catalyst particles (catalyst surface charge is realized by chemically grafting different functional groups via diazonium reaction). In our approach, the improved ionomer/catalyst interface is formed during the ink preparation. Using a unique method of combined ultra-small angle x-ray scattering (USAXS) and cryo-TEM, we observed a significant increase on carbon aggregate size after the NH3 +- functionalized carbon black (CB) particles were mixed with Nafion ionomer, whereas only a negligible size change was observed when SO3 - functionalized CB was used. These results suggest that the coulombic attraction facilitates the mixing of catalyst and ionomer. The ionomer/catalyst interface was characterized by Scanning Transmission Electron Microscopy (STEM) combined with Energy-dispersive X-ray Spectroscopy (EDS) mapping. Figure 1(b) shows the EDS mapping of F distribution for both NH3 +-CB and SO3 --CB. We will report on the performance of MEAs using this approach. Reference [1] C. Chen, Y. Kang, Z. Huo, Z. Zhu, W. Huang, H.L. Xin, J.D. Snyder, D. Li, J.A. Herron, M. Mavrikakis, M. Chi, K.L. More, Y. Li, N.M. Markovic, G.A. Somorjai, P. Yang, V.R. Stamenkovic, Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces, Science, 343 (2014) 1339-1343. [2] X. Huang, Z. Zhao, L. Cao, Y. Chen, E. Zhu, Z. Lin, M. Li, A. Yan, A. Zettl, Y.M. Wang, X. Duan, T. Mueller, Y. Huang, High-performance transition metal–doped Pt3Ni octahedra for oxygen reduction reaction, Science, 348 (2015) 1230-1234. Figure 1

A major problem with performance for transportation applications is associated with degradation. The performance degradation losses can be separated into two categories: irreversible and recoverable losses. Significantly more work has examined the irreversible degradation losses compared with the recoverable losses because the recoverable losses often can be simply recovered by performing polarization curves[1]. Delineating the component contributions of recoverable performance losses and mitigation of these losses appears to be a growing concern for fuel cell developers. While these degradation mechanisms are recoverable, recovery processes are not necessarily trivial to accomplish in operando in a vehicle. MEA performance degradation is sensitive to operating conditions and both irreversible and reversible materials changes. Some known recoverable losses for PEMFCs include the loss of cathode activity due to surface oxide (hydroxide) formation at high potentials,[2] catalyst poisoning by membrane degradation products [3] and recoverable transport losses due to water transport [1,4]. Platinum oxidation at the cathode results in decreased ORR (Oxygen Reduction Reaction) and with Pt often considered to have two different Tafel slopes; one for a metallic Pt surface, and one for an oxidized Pt surface. In other cases, indications are that membrane degradation products, such as (bi)sulfate are readily adsorbed onto the catalyst and are a large source of recoverable degradation. These require removal from the catalyst surface and then from the electrode layer for the performance recovery.[5] To examine the relative contributions to recoverable degradation, we have conducted in situ and ex situ experiments to separate the effects of Pt oxidation, (bi)sulphate adsorption, water management plus other operational effects including spatial degradation mapping, potential cycling and different levels of RH. One method to identify the effect of membrane degradation products on the performance decay is to compare the effect of non-chemically-stabilized membranes versus the effect of chemically-stabilized membranes. This is shown in Figure 1, where the OCV is shown during two-24-hr periods (with recovery in-between the 24-hr periods) for MEAs with (a) a non-chemically stabilized membrane and (b) a chemically stabilized membrane. Past results have shown significantly more degradation products of fluoride and sulphate anions without the chemical stabilization. Both MEAs show decreasing OCVs, however the decay is significantly more for MEA using the non-chemically stabilized membrane. Segmented cell measurements, made in a 10x10 co-flow cell, do not indicate a substantial spatial difference in the rate of decay during these types of tests. Various methods of recovering the losses were examined to separate the Pt oxidation from the other effects, e.g. decreasing the potential to 0.6 V to reduce the Pt oxide films compared with much lower recovery voltages to desorb adsorbed anions. Recovery protocols in fuel cell mode (H2/air) are also compared without current generation (H2/N2) at potentials of 0.6 V, 0.4 V, 0.3V and 0.2 V including with and without liquid water injection. Liquid water injection was found to be detrimental to the recovery of the fuel cell performance and a voltage of 0.3 V required to good recovery with little difference below that potential. 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] Cleghorn, S. J. C.; Mayfield, D. K.; Moore, D. A.; Moore, J. C.; Rusch, G.; Sherman, T. W.; Sisofo, N. T.; Beuscher, U., A polymer electrolyte fuel cell life test: 3 years of continuous operation, Journal of Power Sources (2006), 158 (1), 446-454. [2] Yu Seung Kim and Piotr Zelenay in Polymer Electrolyte Fuel Cell Durability, eds. Felix N. Büchi, Minoru Inaba, Thomas J. Schmidt, Springer New York (2009) pp 223-240 [3] Yu Seung Kim, Melinda Einsla, James E. McGrath, and Bryan S. Pivovar, The Membrane–Electrode Interface in PEFCs: II. Impact on Fuel Cell Durability Journal of The Electrochemical Society, 157 11 B1602-B1607 2010. [4] Qing Li, Dusan Spernjak, Piotr Zelenay, Yu Seung Kim, Micro-crack formation in direct methanol fuel cell electrodes, Journal of Power Sources 271 (2014) 561e569 [5] Jingxin Zhang, Brian A. Litteer, Frank D. Coms, and Rohit Makharia, Recoverable Performance Loss Due to Membrane Chemical Degradation in PEM Fuel Cells, Journal of The Electrochemical Society, 159 (7) F287-F293 (2012) Figure 1

Graphene synthesized from the exfoliation of graphite has a great potential in polymer electrolyte membrane fuel cell (PEMFC) applications.[1] The highly graphitic structure composed of the conjugated carbons endows graphene with extremely high electric conductivity and chemical/electrochemical stability. Improved stability is required for PEMFC catalyst support materials as the stability plays a critical role in determining the overall durability of PEMFC systems. While graphene is an attractive material, there are three major barriers when using graphene-supported Pt-based catalysts in PEMFCs: 1) the lack of bonding sites for catalyst landing on the basal plane of graphene causes the migration/aggregations of Pt nanoparticles when subjected to harsh accelerated stress tests (ASTs), 2) the highly hydrophobic surface of graphene is difficult to wet when mixed with the Nafion ionomer particles, leading to a poor catalyst/ionomer interface, 3) 2D graphene sheets tends to restack back to graphite structure through π-π interactions, which can severely block the O2 diffusion and retard the catalytic reactions, in particular when PEMFCs are operated at high current density (>1.5 A/cm2),and 4) the exfoliation of natural graphite using wet chemistry method usually yield graphene that contains some defects and oxygenated functional groups, which can de-stabilizes its conjugated electronic structure which negatively affects electronic conductivity and stability. Furthermore, when subjected to harsh potential cycling (e.g. 1.0-1.5 V; potentials which can be observed during start-up/shut-down or local fuel starvation), these defect sites are vulnerable to carbon corrosion. To overcome these challenges, we use a novel approach to make the 2D graphene sheets into 3D composite materials with many channels and pores to facilitate the facile mass transport by developing the highly stable hierarchical polybenzimidazole (PBI) -grafted graphene hybrids supported Pt catalysts for PEMFCs. PBI- functionalization is found to homogenize the chemical environment of graphene surface, resulting in a uniform dispersion of Pt nanoparticles that exposes more active sites for electro-catalytic reactions. In order to construct appropriate pore structures in the catalyst layers, spacers were introduced using graphitized carbon materials or metal oxides. The surface charge was imposed on the surfaces of these spacer-particles as opposed to PBI-graphene, so that the columbic attraction forces drive them to anchor onto graphene sheets. As a result, the secondary pore volume (characterized by mercury porosimetry) significantly increases, leading to the improved mass transport of reactants (H2 and O2) and water, and consequently an improvement in the PEMFCs performance at high current density (i.e. 2.0 A/cm2). Additionally, we prepared graphene nanoplatelet with smaller dimensions as the catalyst supports, which leads to extra voltage gains at the high current density. Covalent grafting polymers onto graphene sheets containing semi-flexible backbones on graphene defects can help to re-build the conjugated carbons and de-localize the electron, as well seal the dangling bond resulting from graphene synthesis. The PEMFC durability studies carried out by our research group has shown significant improvement on support stability when they were cycled between 1.0 V to 1.5 V for 5000 cycles compared to traditional carbon support materials,[RB1]during which the mass activity is only decreased by 47% for Pt/PBI-graphene and 25% for Pt/PBI-nanographene. The reported results are very close to DOE 2020 targets on catalyst support stability [2]. In addition, these materials show improved Pt stability. Strengthened interaction between functional groups and Pt particles, as characterized by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) greatly improves the catalyst stability. Our developed PBI-graphene supported Pt demonstrated a 27% decrease in mass activity and 15% loss of ECSA after 10,000 cycles between 0.6 V and 1.0 V. This exceeds the DOE 2020 targets for automotive PEM fuel cell electro-catalysts.[2] The unique advantages of the hierarchical PBI-functionalized graphene/nanographene hybrids demonstrates the potential for these materials to be the next generation of catalyst supports for PEMFCs. [1]. Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183 [2]. U.S. DRIVE FuelCell Tech Team Cell Component Accelerated Stress Test Protocols for PEM Fuel Cells (2013)

Polymer electrolyte membrane (PEM) fuel cell performance and materials degradation, particularly associated with the cathode catalyst layer (CCL), can be directly attributed to the structure and chemistry of individual material components, as well as their uniformity/homogeneity within a CCL. The individual material constituents used to form the CCL within the membrane electrode assemblies (MEAs), which include the electrocatalyst, catalyst support, and ionomer films, and especially the critical interfaces that are formed between these various constituents, are critically importance in controlling fuel cell perfomance. Understanding the specific microstructural characteristics of the individual materials within the CCL, and how the materials interact to “form” the CCL, is important for identifying materials optimization parameters that can significantly enhance performance and durability. Materials in several states/conditions, e.g., prior to incorporation in the CCL (as-synthesized), after MEA preparation (CCL), and after fuel cell testing, are beig evaluated and quantified using a combination of advanced electron microscopy methods, which are used to interrogate the materials constituents and interfacial structures and chemistries from the μm- to the Å-level. The as-processed (prior to and following incorporation into a CCL) microstructural evidence is directly correlated with observations of materials-specific degradation mechanisms that contribute to fuel cell performance loss, and are then used to identify potential processing variables (materials-based mitigation strategies) to improve the microstructure and compositional homogeneity within the electrode structure, and enhance MEA durability and stability during fuel cell operation. Research efforts at Oak Ridge National Laboratory are focused on the high-resolution microstructural and microchemical characterization of MEAs fabricated using different electocatalysts (typically Pt-based) and catalyst loadings, carbon-based support materials, and ionomer solutions, as well as the same MEAs subjected to accelerated stress tests (ASTs) designed to degrade specific MEA components. While a significant microscopy effort has been aimed towards understanding catalyst degradation (e.g., coarsening, de-alloying), recently, high-resolution analytical microscopy methods have been used to directly image/map the distribution and chemistry of the ionomer films/layers within CCLs, results of which are being combined with high-resolution imaging and 3-D tomography data of powder materials and MEAs, to provide unprecedented insight into the MEA architecture and interfaces (ionomer/support, ionomer/catalyst, catalyst/support, ionomer/pore). This presentation will focus on understanding materials distributions within CCLs as a function of materials used and ink/MEA processing variables, e.g., initial ionomer and/or ink chemistry, electrocatalyst (type, content, and dispersion), and the type of carbon support used. Additionally, the stability of the ionomer films, electrocatalysts, and support structures in CCLs following ASTs designed specifically for either catalyst degradation or carbon corrosion, will be evaluated. ________________________________________________________________________ Research sponsored by (1) the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and (2) Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), which is a U.S. Department of Energy, Office of Science User Facility.

The durability of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) is one of the key barriers to their commercialization in automotive applications.1 The development of accelerated stress tests (ASTs) is a critical step in the evaluation and improvement of the durability of PEMFCs. The availability of validated component ASTs can lead to the accelerated development of materials with improved durability. However the ASTs need to faithfully reproduce the degradation mechanisms of interest and not introduce new degradation modes while attempting to accelerate the degradation rates. Furthermore, the acceleration factors with respect to the intended application need to be determined accurately to provide manufacturers with a valuable tool that enables informed decision making regarding cost, performance and durability. For example the DOE – Fuel Cell Technologies program has set lifetime durability targets of 5500 hours for automotive fuel cells and the U.S. Drive Fuel Cell Technical Team (FCTT) has developed AST protocols for the various components.2,3 In this paper we will compare the degradation observed during the ASTs to the degradation observed during simulated drive cycle operation and report on the acceleration factors observed for various materials. We will also propose new ASTs that have greater acceleration factors than the reported FCTT ASTs while retaining the degradation mechanisms of interest. The FCTT has recommended potential cycling from 0.6 V to 1.0 V at 50 mV/sec as the electrocatalyst AST. This potential cycling is performed at 80 °C in 100% RH with H2 (anode) and N2 (cathode) and results in Pt particle growth and loss in electrochemical surface area (ECSA). This loss in ECSA and associated performance loss correlates well to the degradation observed in the field and during simulated drive cycle operation. Figure 1 compares the degradation observed during this AST (Old AST) to that observed during the wet portion of the FCTT: Drive cycle (current cycling from 0.02 to 1.2 A/cm2 at 80 °C and saturated conditions). It is seen that this AST has a 5X acceleration factor for 3 different MEAs that were evaluated. In order to further accelerate this AST, we modified the triangle wave to a square wave and lowered the upper potential to 0.95 V. This new AST consisted of a square wave with upper and lower potentials of 0.95 V and 0.6 V with 3 seconds duration, and was based on literature reports.4 This New AST (Fig. 1) demonstrated a 100X acceleration factor compared to the drive cycle which is a 20 times improvement over the old AST. Moreover, this new AST retained the Pt growth mechanism while minimizing carbon corrosion. The FCTT has 2 recommended ASTs for membranes with one focusing on chemical degradation (OCV hold at 90 °C and 30% RH) and another focusing on mechanical degradation (RH cycling in Air). Our previous results have indicated that both these ASTs are not representative of membrane failure in the field.5 In this paper we will present a new AST that combines the mechanical and chemical degradation and is representative of field failure. This AST consists of cycling the RH between saturated and dry conditions in a H2/Air atmosphere at 90 °C. The duration of the wet and dry cycles are 30s and 45s respectively. The fluoride release rate observed during this AST using DuPont XL® membranes, illustrated in Figure 2, is comparable to the chemical AST and higher than in the mechanical AST. References 1. R. Borup, et al., Chemical Reviews, V. 107, No. 10, 3904-3951 (2007). 2. DOE Cell Component AST and polarization curve Protocols for PEM Fuel Cells (Electrocatalysts, Supports, Membranes and MEAs), Revised December 16, 2010. 3. N. L. Garland, T.G. Benjamin, J. P. Kopasz, ECS Trans., V. 11 No. 1, 923 (2007). 4. A. Ohma, K. Whinohara, A. Liyama, T. Yoshida, and A. Daimaru, ECS Trans., V. 41 No. 1 , 775 (2011). 5. R. Mukundan et al., ECS Trans., V. 50 No. 2, 1003 (2013). Acknowledgements The authors wish to acknowledge the financial support of the Fuel Cell Technologies Program and the Technology Development Manager: Nancy Garland. The authors also wish to acknowledge Ion Power, Inc. for supplying the MEAs and SGL Carbon for the GDLs used in this study. Figure 1

Corrosion of catalyst support-carbon during unprotected start-up and shut-down of H2-air polymer electrolyte fuel cells (PEFC) is a serious concern. Many studies have investigated the underlying mechanisms of current reversals that occur as H2 sweeps the air in the anode compartment during start-up and, conversely, as air sweeps the H2 in anode during shut-down. Carbon corrodes during these transients as high interfacial potentials exceeding 1.5 V develop in the air/air portion of the cell. Protective measures have been proposed to limit these high interfacial potentials and to limit the exposure time to high potentials. Material-based mitigation strategies have also been proposed that include suppressing the oxygen reduction reaction (ORR) on anode and promoting the oxygen evolution reaction (OER) on cathode. Catalyst support-carbon can also corrode below the open-circuit potentials typical of normal operation of automotive fuel cells. Figure 1a is a typical trace of the measured carbon corrosion rate during a simulated drive cycle in which the potential was varied between 0.4 V and 0.95 V. The hold times were 0.5 min at the low potential and 5 min at the high potential. The data are for an Ion Power supplied MEA with 0.15 mg/cm2 Pt loading in cathode, 0.23 Pt/C ratio, and high surface area Ketzenblack (E-type) carbon support. The data are for H2-air at fixed flow rates, 80oC cell temperature, and 100% relative humidity (RH). The corrosion trace in Fig. 1a has a few particularly interesting features. The larger spike in corrosion rate occurs as the cell potential is raised from 0.4 V to 0.95 V. However, the corrosion rate declines rapidly and approaches zero as the cell is held at the high potential for a long time. The smaller spike in corrosion rate occurs as the cell potential is reduced from 0.95 V to 0.4 V. The corrosion rate also declines as the cell is held at the low potential but does not approach zero. Similar features in carbon corrosion rates were observed in other MEAs with Vulcan XC-72 (V-type) and graphitized Ketzenblack (EA-type) carbons as catalyst supports. We have formulated a simple carbon corrosion model to quantitatively capture the essential features of the data. As indicated in Fig. 1a, the surface carbon sites are represented as vacant (C#), active carbon oxides (C#OH), and passive carbon oxides (C#Ox). The active carbon oxides are formed at low potentials. The spike in carbon corrosion after a step increase in potential is associated with the direct oxidation of C#OH to CO2 through its reaction with H2O. Holding the cell at a high potential leads to the formation of C#Ox and displacement of C#OH. The corrosion rate goes down as C#Ox passivates the carbon surface. Also, at potentials higher than 0.4 V, Pt sites begin to convert to PtOH, and at still higher potentials, PtOH converts to PtO. In the model shown in Fig. 1a, a step decrease in potential results in the sites occupied by passive C#Ox being vacated. The vacated sites are available to be occupied by active C#OH which reacts with the OH-like species on adjacent catalyst surface to produce a spike in carbon corrosion. With time, PtOH gradually reduces to Pt, and the carbon corrosion rate decreases to a lower, non-zero value. As partial validation, Fig. 1b shows the modeled steady-state corrosion rate and compares it with the asymptotic rates measured in the experiments when the cell with E-type carbon is held at high potentials for 5 min. Both the model and the data exhibit peak steady-state corrosion rates for E-type carbon at about 0.6 V. As seen in Fig. 1a, the transient corrosion rates can be much higher under potential cycling. Acknowledgments: This work is supported by DOE-EERE-FCTO with Dr. Nancy Garland as the Technology Development Manager. Figure 1

The cost and durability of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are the two major barriers to the commercialization of these systems for stationary and transportation power applications.1 The cost and durability of fuel cells are inter-related as more often than not lower cost components are less durable than their higher cost counterparts. Given the long expected life times of these fuel cell systems in the field (> 5500 hours for transportation and > 60,000 hours for stationary applications), there is a pressing need to develop Accelerated Stress Tests (ASTs).2 The ASTs should be representative of degradation in the field and are expected to provide estimates of fuel cell lifetime using short experiments. The U.S. DOE Fuel Cell Tech Team has recommended various ASTs for PEMFC components.3,4 While these ASTs are good at evaluating the relative durability of the various materials used in PEMFCs, there is little information available on acceleration factors of these ASTs for various applications. In this paper we will present various diagnostic techniques that are utilized in conjunction with these ASTs to better evaluate the durability of fuel cell components and their effect on fuel cell performance. Diagnostic techniques used to evaluate electrocatalyst durability include, performance curves, electrochemical surface area, mass activity, electrode capacitance, AC impedance, evolved CO2 from the cathode, catalyst particle size, and electrode thickness. Figure 1 illustrates the performance of PEMFCs subjected to two different support ASTs; a 1.2V hold and a 1 to 1.5 V potential cycle @ 500 mV/sec. The observed degradation rate is 150 – 200 times faster during the potential cycling compared to the potential hold. This is consistent with the 100 times faster increase in Pt particle size observed during the potential cycling experiment (Figure 2). In addition to the Pt particle size increase and associated kinetic losses, there are clear mass transport losses associated with these ASTs. This is illustrated in Figure 3 where the lower frequency mass transport resistance dramatically increases during the AST especially for carbons that corrode faster. Diagnostic techniques used to evaluate membrane durability include performance curves, hydrogen cross over, high frequency impedance, shorting resistance, fluoride emission, membrane thickness, membrane morphology and membrane mechanical strength. The importance of these diagnostic techniques in the development of a new membrane AST will be discussed. References 1. R. Borup, et al., Chemical Reviews, V. 107, No. 10, 3904-3951 (2007). 2. S. Zhang, X. Yuan, H. Wang, W. Merida, H. Zhu, J. Shen, S. Wu and J. Zhang, Int. J. Hydrogen Energy, V 34, 388-404 (2009). 3. DOE Cell Component AST and polarization curve Protocols for PEM Fuel Cells (Electrocatalysts, Supports, Membranes and MEAs), Revised December 16, 2010. 4. N. L. Garland, T.G. Benjamin, J. P. Kopasz, ECS Trans., V. 11 No. 1, 923 (2007). Acknowledgements The authors wish to acknowledge the financial support of the Fuel Cell Technologies Program and the Technology Development Manager: Nancy Garland. The authors also wish to acknowledge Ion Power, Inc. for supplying the MEAs and SGL Carbon for the GDLs used in this study. Figure 1

Fuel cells have the potential to replace the internal combustion engine in vehicles and provide power in stationary because they are energy-efficient, clean, and fuel-flexible. The durability of MEAs (Membrane Electrode Assemblies) of PEM (Polymer Electrolyte Membrane) fuel cells is a major barrier to the commercialization of these systems for transportation applications. Power transients, shut-down/start-up, temperature cycling, RH (relative humidity) cycling, are all operating conditions imposed by the rigid requirements for transportation applications. These operational transients have the effect of inducing a number of chemical and structural degradation mechanisms on electrocatalysts and MEAs, changing their performance. Past improvements have largely been made possible because of the fundamental understanding of the underlying degradation mechanisms. By investigating component and cell degradation modes; defining the fundamental degradation mechanisms of components and component interactions new materials can be designed to improve durability. One of the major degradation mechanisms involves the catalyst and catalyst layer, including corrosion of the catalyst support leading to structural changes in the catalyst layer. Cathode catalyst layer thinning is regularly observed with increased losses associated with both catalyst activity and to mass transport limitations which are coupled to loss of porosity in the catalyst layer. Thinning of the catalyst layer can be due to loss of carbon through carbon corrosion or due to compaction and loss of void volume. Carbon corrosion is induced by air/hydrogen (or hydrogen/air) fronts during shut-down/start-up which cause high potentials leading to carbon support corrosion. The degree of the effect is dependent on many factors including the types of carbon utilized as the catalyst support, water content, and the anode catalyst layer. An example of cathode catalyst layer is changing porosity during carbon corrosion accelerated stress testing, where the initial catalyst layer porosity is approximately 40%, decreasing to 33% over 10 hours, and is only about 7% after 40 hours, as measured by TEM. A reduction of the median pore diameter of the carbon particles in the electrode structure is also measured. Electrode structure degradation has been shown to occur in microscopically localized regions, with support corrosion leading to detachment and agglomeration of catalyst particles, while weakening of the carbon structure allows collapse of electrode pores and can severely limits gas transport. Post-analysis, after drive cycle operation, shows localized bands of carbon corrosion that are correlated increased Pt particle sizes and closer Pt-Pt interparticle spacings. Thinning of the catalyst layer can be due to loss of carbon through carbon corrosion or due to compaction and loss of void volume. Membrane degradation has been noted to affect both the membrane and the electrode structure. After testing, post-characterization detects electrode regions (channel area) where less Nafion (less fluorine signal) compared to the rest of the electrode surface. Other regions show bands where no electrode is bonded to the membrane surface. Characterization also shows regions (under the lands) where a part of the membrane remained attached to the catalyst layer. In this region, there was a delamination of the membrane from the PTFE reinforcement. Delamination of the membrane from the PTFE reinforcement has also been noted with a large amount of particles (analyzed as Si-O by EDS) present along the two reinforcement interfaces. At the cathode side, the membrane (between the reinforcement and the cathode electrode layer) has virtually been removed. The material design and fabrication methods of producing the catalyst layer can greatly affect the catalyst layer durability. Utilization of different electrode materials can stabilize the catalyst layer void volume, thus preventing a portion of the performance loss.