High-Temperature Synthesized PGM-Free Oxygen Reduction Reaction Catalyst

Platinum group metal-free (PGM-free) catalysts for oxygen reduction reaction (ORR) in polymer electrolyte fuel cells have emerged as a promising alternative to PGM-based catalysts thanks to their abundance in the Earth crust and, consequently, low cost. Metal-nitrogen-carbon (M-N-C) catalysts possibly represent the most attractive PGM-free ORR materials. However, despite the significant progress accomplished in M-N-C catalyst development over the past two decades, this class of materials continues to face challenges that need to be overcome to narrow the performance gap to PGM-based catalysts.[1] Our team has made significant efforts to improve the activity and stability of M-N-C catalysts by developing novel synthesis approaches, such as the ‘dual-zone’ synthesis, targeting specifically improvements in catalyst stability.[2, 3] We have also established standardized protocols for the activity and stability evaluation of PGM-free catalysts under relevant operating conditions of the fuel cell.[4] Additionally, our modeling of PGM-free M-N-C ORR electrocatalysts has provided much needed insight into the materials and helped with their optimization. We have utilized machine learning methods coupled with high-throughput synthesis and characterization to optimize the conditions for a variety of synthesis approaches, which has led to substantial increase in experimentally realized activity.[5, 6] Our prior fundamental studies of ORR activity [1, 2, 7, 8] and materials stability [9] at the density functional theory (DFT) level are actively being linked to achieve simultaneously high activity and long-term stability through a Pareto optimization. Coupling machine learning and DFT efforts is presently underway.In this talk, we will summarize our recent progress in the development of PGM-free M-N-C catalysts for ORR, focusing on improvements in catalyst activity and stability from both experimental and theoretical perspectives. We will highlight various synthesis strategies for improving catalyst activity and stability, and key design principles for M-N-C catalysts, learned from DFT calculations and machine learning-driven, high-throughput catalyst synthesis. We will also discuss the challenges and opportunities in the development of PGM-free catalysts for ORR. Overall, PGM-free catalysts represent a promising pathway for the advancement of low-cost and efficient fuel cell technologies. References Martinez, U. et al., J. Electrochem. Soc., 2019. 166, F3136.Chung, H.T. et al., Science, 2017. 357, 479.Zelenay, P. and D. Myers, ElectroCat (Electrocatalysis Consortium); U.S. Department of Energy Hydrogen and Fuel Cell Program Annual Merit Review 2019. https://www.hydrogen.energy.gov/pdfs/review19/fc160_myers_zelenay_2019_o.pdf.Zhang, H. et al., Nat. Catal., 2022. 5 (5): 455.Kort-Kamp, W.J.M. et al. . J. Power Sources, 2023. 559: 232583.Karim, M.R. et al., ACS Appl. Energy Mater.. 2020. 3 (9): 9083.Holby, E.F., Curr. Opin. Electrochem., 2021. 25, 100631.Anderson, A.B. and E. F. Holby, J. Phys. Chem. C, 2019. 123 (30): 18398.Holby, E.F., G. Wang, and P. Zelenay, ACS Catal., 2020. 10 (24): 14527.

The activity of oxygen reduction reaction (ORR) catalysts often determine performance of polymer electrolyte fuel cells (PEFCs) as measured by their power output, open circuit voltage, and fuel conversion efficiency. Currently, Pt-nanoparticle catalysts, either supported on high surface-area carbons or prepared in a form of contiguous thin layer on conductive or non-conductive supports, represent the state of the art in ORR electrocatalysis at the PEFC cathode. However, the high and variable price and scarceness of Pt have limited its widespread implementation in the -temperature fuel cells to date, especially in automotive transportation. Under these circumstances, platinum group metal-free (PGM-free) ORR catalysts have received growing attention in recent years as a possible replacement for Pt-based formulations. The progress achieved since the development of the first nature-inspired electrocatalysts of oxygen reduction in the seminal work by Jasinski in the 1960s (Nature 201, 1212, 1964), which mostly happened through the broad implementation of the high-temperature synthesis approach, makes replacement of Pt in ORR electrocatalysts with earth-abundant elements, such as Fe, Co, N, and C, a realistic possibility. In this this presentation, we will summarize recent progress in research targeting development of high-performance PGM-free catalysts for oxygen reduction reaction (ORR) at Los Alamos National Laboratory. Two approaches will be discussed in a greater detail: (i) the approach involving fine-tuning of the catalyst porosity and surface area using pore-forming compounds and (ii) the method specifically focusing on the development of atomically dispersed transition metal moieties and avoiding the formation of transition metal-rich nanoparticles during the heat treatment of catalyst precursors. We will demonstrate the impact that porosity/surface area optimization, through the use of either pore formers (cyanamide, ZnCl2) precursor templating (metal organic frameworks) or both, can have on the fuel cell performance of PGM-free catalysts. We will also show how modifications to the electrode structure through the change in the ionomer content and ionomer equivalent weight can lead to substantial improvements in the fuel cell performance at both low- and high current densities (aerial power density of more than 0.50 W/cm2 in H2-air testing at 80°C). Finally, we will recapitulate the challenges still facing PGM-free research that, in spite of all the progress achieved in recent years, is yet to produce materials capable of competing with the incumbent Pt-based catalysts in terms of oxygen reduction activity, performance durability, and cost (specifically, when extended to the overall cost of a fuel cell stack). Acknowledgement Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged.

Platinum is widely used as the catalyst in polymer electrolyte fuel cells (PEFCs) due to its high catalytic activity and stability. However, the high cost of this precious metal is one of main barriers to widespread commercialization of PEFCs in automotive and stationary applications. Many research efforts have focused on reducing this cost barrier by either decreasing the platinum loading by developing more active Pt-based catalysts, such as nanostructured alloys, or by developing platinum group metal-free (PGM-free) catalysts based on earth-abundant materials, such as iron, cobalt, carbon, and nitrogen [1-4]. The overall approach of this project is to accelerate the development of PGM-free catalysts by developing high-throughput materials synthesis, characterization, and performance evaluation methodologies. A multi-channel flow double electrode (m-CFDE) cell, based on previous work by M. Watanabe’s group [5], has recently been designed and tested for the simultaneous screening of the catalytic activity of multiple samples in an aqueous environment [6]. An automated robotic catalyst ink dispensing system has been developed for precise deposition of nano-liter ink droplets across the m-CFDE glassy carbon electrodes (1 mm × 3 mm). The nano-liter injector with exchangeable micro-tips deposits the electrode ink onto the tiny surface glassy carbon at a precise injection speed. The removable electrode plugs of the m-CFDE cell are fixed on a numerically-controlled XY table to uniformly distribute the PGM-free catalyst across the glassy-carbon electrode. This automated deposition stage provides better reproducibility and uniform distribution over the conventional hand deposition using micropipettes as is typically utilized for ink deposition for rotating disk electrode measurements. In the performance evaluation aspect, a commercial combinatorial membrane-electrode assembly (MEA) test system has been further developed and utilized to simultaneously evaluate the fuel cell performance of 25 electrodes with different catalyst and ionomer-to-carbon ratios. The automated electrode deposition system in combination with a custom-designed heated vacuum table mounted on the XY table has also been used to make a catalyst-coated membrane (CCM) with 25 segmented electrodes. The results of application of this high-throughput methodology to a promising class of PGM-free catalysts will be discussed. Acknowledgements This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the auspices of the Electrocatalysis Consortium (ElectroCat). Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, under contract DE-AC-02-06CH11357.

To reduce greenhouse gases (GHG) emission, we need to move away from fossil-based energy sources. Electrochemical energy conversion systems (EECSs), like fuel cells, are broadly considered to be alternative systems to the currently dominant internal combustion energy-generation systems (ICESs). Among several types of fuel cells, polymer electrolyte fuel cells (PEFCs) are the most suitable one for vehicle applications. Nanoparticle platinum (Pt) catalysts are now used as state-of-the-art catalyst for the PEFCs. However high price, scarcity, and monopolized global distribution of Pt place significant limitations on these energy conversion systems. Particularly oxygen reduction reaction (ORR) at the cathode is inherently slower by six orders of magnitude than hydrogen oxidation reaction at the anode, thus requiring higher Pt loading. Platinum group metal free (PGM-free) ORR catalyst development thus has been a continuous research theme for several decades by many research groups. Metal-nitrogen-carbon (M-N-C) type catalysts have demonstrated the highest activity and durability among several types of PGM-free catalysts. Recently there has been a significant improvement in ORR activity, however further improvement in activity is still needed to compete with Pt catalyst. Furthermore good durability of these catalysts has not been demonstrated. Understanding of active site that has not been clarified yet is a core for solving these issues. Recently we directly observed FeN4 moiety in (CM+PANI)-Fe-C catalysts [1]. If FeN4 is an active site for ORR, increasing the number of this moiety, i.e., making atomically dispersed (AD) Fe, will be a pathway for improving the ORR activity. Thus to achieve high density of FeN4, we synthesized fiber-type zeolitic imidazolate framework (ZIF-F) as a precursor for (AD)-Fe-N-C catalysts (Fig. 1 (a)), in which FeN4 structure already exist. Heat-treatment converts this ZIF-F into fibrous N-doped carbons (Fig. 1(b). Fe atoms are dispersed atomically without aggregation in the fibrous N-doped carbons (Fig. 1(c)). Importantly, electron energy loss spectroscopy (EELS) demonstrates that N is cordinated to the Fe atoms (Fig. 1(d)). Thus we could construct FeNx moieties within the (AD)-Fe-N-C catalysts. In this presentation, we will present the activity and durability of this (AD)-Fe-N-C catalyst in rotating disk electrode (RDE) and fuel cells in conjunction with diverse analysis tools. This will give some insights for the nature of activity/durability for M-N-C type ORR catalysts. Hoon T, Chung, David A. Cullen, Drew Higgins, Brian T. Sneed, Edward F. Holby, Karren L. More, Piotr Zelenay, “Direct atomic-level insignt into the active sites of a high-performance PGM-free ORR catalyst”, Science, 357, 479 (2017). Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Electrocatalysis Consortium (ElectroCat). Figure 1

Hydrogen-fueled polymer electrolyte fuel cell (PEFCs) systems are currently utilized as high efficiency, low emission power sources for a variety of applications, most notably for electric vehicles such as the Toyota Mirai. The high cost of platinum, widely used as the electrocatalyst in PEFCs, is one of main barriers to the widespread and economically-viable implementation of fuel cell vehicles. Many research efforts have successfully developed platinum group metal-free (PGM-free) electrocatalysts for the oxygen reduction reaction (ORR), including those in the U.S. Department of Energy, Fuel Cell Technologies Office’s Electrocatalysis Consortium (ElectroCat)1-3. High-throughput catalyst synthesis, activity evaluation, and performance testing are core activities within ElectroCat aimed at accelerating the discovery and implementation of ORR electrocatalysts. High-throughput catalyst activity screening is accomplished using a newly-developed multi-channel flow double electrode, allowing activity measurements of four catalysts simultaneously deposited on glassy carbon electrodes using an automated robotic catalyst ink dispensing system with a nano-liter injector4. The purpose of this project is to accelerate the development of PGM-free catalysts using high-throughput performance evaluation in a 25-electrode array cell (NuVant) which has a common counter electrode and five independent flow channels. This system allows simultaneous performance testing and characterization of 25 types of catalyst in a membrane-electrode assembly (MEA) with five gas supply manifolds. Electrode fabrication is one of the most challenging steps in the catalyst-coated membrane (CCM) fabrication process for the array cell. A custom-designed heated vacuum table mounted on the computer-controlled XY table is used to make the 25-electrode CCM to avoid shrinkage and swelling of the catalyst layer and the membrane during the fabrication. The nano-pipette automated ink deposition system is being used for precise and even catalyst ink deposition across the small circular array electrodes. Scanning electron microscopy of the electrode is being utilized to confirm uniform and robust microstructures after the direct fabrication method. The 25-electrode set-up can accelerate PGM-free catalyst evaluation and optimization of electrode composition with the additional advantage of minimizing the amount of material required since the area of each electrode in the array is less than 1 cm2. Acknowledgements This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the auspices of the Electrocatalysis Consortium (ElectroCat). Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, under contract DE-AC-02-06CH11357. References U. Martinez, S. K. Babu, E. F. Holby, K. Artyushkova, L. Lin, H. T. Chung, G. M. Purdy, and P. Zelenay, 232nd ECS Meeting, National Harbor, MD, October 1-5, 2017.S.T. Thompson, A.R. Wilson, P. Zelenay, D.J. Myers, K.L. More, K.C. Neyerlin, and D. Papageorgopoulos, Solid State Ionics, 319 (2018) 68-76.H.T. Chung, D.A. Cullen, D. Higgins, B.T. Sneed, E.F. Holby, K.L. More, P. Zelenay, Science 357 (2017) 479–484.J. Park, N. Kariuki, D. J. Myers, H. Chung, U. Martinez, P. Zelenay, 233nd ECS Meeting, Seattle, WA, May 13-17, 2018.

Platinum and platinum-alloy oxygen reduction reaction (ORR) catalysts have been implemented in the prototype, and by now also in commercial fuel cell power systems for automotive applications, successfully meeting the demanding conditions of relatively low temperature (80°C) and high acidity operating conditions of the polymer electrolyte fuel cell (PEFC). However, due to the high price of Pt, the catalyst cost is projected to account for as much as a half of the total PEFC stack cost.1 In this context, the potential replacement of Pt-based catalysts, especially in the fuel cell cathode, with non-precious metal catalysts (NPMCs) is expected to significantly reduce PEFC stack cost. Achieving such a cost benefit is, however, contingent upon reaching sufficient activity and durability with NPMC for the ORR, a requirement that the electrocatalysis research community has been striving to meet. In this presentation, we expand upon the idea of combining nitrogen precursors during NPMC synthesis to improve the performance and performance durability of non-precious metal ORR catalysts. Previously, we introduced a new non-precious metal ORR catalyst derived from two nitrogen precursors with different decomposition temperatures: easily decomposing cyanamide (CM) and more thermally stable polyaniline (PANI).2 In this approach, CM acts as an efficient pore-forming agent that allows for the formation of a highly porous structure, which in particular benefits the high-current response of the resulting electrocatalyst. Compared to the alternative approach utilizing metal-organic frameworks (MOFs),3,4 the use of cyanamide promises a very significant reduction in the NPMC cost, without sacrificing ORR activity. In an effort to further improve the PEFC performance of the two nitrogen-precursor NPMCs, we have explored approaches involving the use of ammonia treatment, the addition of various carbon supports, and inclusion of multiple transition metals. In this presentation, we will summarize the results of these studies, which have involved the extensive use of characterization techniques, such as X-ray photoelectron spectroscopy, scanning electron microscopy, scanning transmission electron microscopy, X-ray diffraction, surface area/pore distribution measurements, and nano-scale X-ray computed tomography. In the conclusion, we will also define critical factors for enhancing the performance of NPMCs in the PEFC cathode and outline directions for future research. Acknowledgement Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged. Microscopy research supported through a user project at ORNL’s Center for Nanomaterials Sciences, which is an Office of Science User Facility. References Spendelow et al., DOE Fuel Cell Technologies Office, Fuel Cell System Cost-2013, (2013).Hoon T. Chung et al., “Combined nitrogen precursor approach to develop highly active non-precious metal oxygen reduction reaction catalyst.” in preparation.S. Ma et al., “Cobalt Imidazolate Framework as Precursor for Oxygen Reduction Reaction Electrocatalysts,” Chem. Eur. J. 17, 2063 (2011).Proietti et al., “Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells,” Nat. Commun. 2, 416 (2011)

The development of durable and highly active platinum group metal-free (PGM-free) electrocatalysts is key to overcoming the greatest barrier in the commercialization of fuel cell electric vehicles: the high cost and limited availability of PGM catalysts utilized in polymer electrolyte fuel cells. While the majority of research on PGM-free catalysts has focused on developing active and selective materials for the sluggish oxygen reduction reaction (ORR), minimal work has been devoted to the understanding of the physical and chemical structure of active PGM-free catalysts and its relationship with the long-term stability of these novel materials. This understanding is imperative for the further development of next-generation PGM-free catalysts with improved activity and durability that could ultimately replace PGM-based ORR catalysts in fuel cells. PGM-free catalysts with high volumetric activity, four-electron selectivity (i.e., low hydrogen peroxide yield), and long-term stability in highly acidic and strongly oxidizing environment of the fuel cell cathode are desired as replacements for PGM-based ORR catalysts. For this presentation, state-of-the-art PGM-free ORR catalysts, developed by our group [1, 2], which are obtained via high-temperature treatment of dual nitrogen precursors, cyanamide (CM) and polyaniline (PANI), different transition metal precursors (Fe, Co, Mn), and with or without carbon support are used as model PGM-free catalyst systems. Depending on the transition metal used, catalysts with different physical and electrochemical characteristic properties are obtained. Specifically, Fe-based catalysts are generally known for their high activity and low peroxide yield generation. However, Co- and Mn-based catalysts generate highly graphitic structures which could improve catalyst long-term stability. In this work, structure-activity-durability relationships are developed based on comprehensive physical, chemical, and electrochemical characterization of highly active (CM+PANI)-Me-C catalysts (Me = Fe, Co, Mn), correlating properties such as carbon structure, elemental speciation, and CO2 generation. Acknowledgement Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged. [1] Chung, H.T., Holby, E.F., Purdy, G.M., Babu, S.K., Litster, S., Cullen, D.A. More, K.L., Zelenay, P. (2015). “Combining Nitrogen Precursors in Synthesis of Non-Precious Metal ORR Catalysts with Improved Fuel Cell Performance.” ECS Meeting s, MA2015-02 (37), 1278. [2] Martinez, U., Holby, E.F., Dumont, J.H., Chung, H.T., Zelenay, P. (2016) “Non-PGM ORR Catalysts Based on Transition Metals Alternative to Iron.” ECS Meeting s, MA2016-01, 1719.

Polymer electrolyte fuel cells (PEFCs) offer the prospects to revolutionize the transportation by powering not only automotive but also and heavy-duty vehicles. In PEFCs, ionomers are used to function as a proton-exchange membrane (PEM) facilitating ion transport between the catalyst layers (CLs). In addition, ionomer also function as an electrolyte binder mixed with the catalyst supports and particles to facilitate mass transport of reactants to catalytic sites. When confined to nanometer thicknesses in a heterogeneous CL structure, ionomer properties exhibit a range of properties form bulk (membrane) to thin films, and influenced by ionic and interfacial interactions.Diverse functionalities, operating environment and performance metrics in fuel cells require optimized ionomer multi-functionalities for both PEM and CLs. While reducing the thickness of PEM is commonly considered an effective strategy for improving device performance by minimizing transport losses, this was considered purely from the perspective of geometric effects and cost constraints. However, how thickness affects intrinsic properties of an ionomer from membrane to electrode structures remains to be an open question. On one hand, efforts towards improving performance and durability of PEFCs require chemically and mechanically supported PEMs, which introduce additional interactions with the secondary support materials. An improvement in transport functionality in these ionomer, however, compromises stability and separator functionality, thereby setting a constraint to their optimization. On the other hand, improving CL ionomers require an understanding of their structure in the presence of confinement and electro-catalyst interactions. Between these duality, a new paradigm for fuel-cell ionomers, where one needs to understand and tune ionomer interactions with secondary particles across a wide spectrum of thicknesses, from bulk to nm.This talk will present an overview of ionomers from this perspective with a focus on structure-property relationship of perfluorosulfonic acid (PFSA) ionomer, in both CL and PEM regime, which enables identification of the structural transitions relevant to fuel-cell functionality. Building upon a systematic investigation of membranes in micrometer thickness range (100 to a few µm), combined with nanometer-thick supported films in sub-micron range, a comprehensive material property map is generated for PFSAs, from mechanical properties, to water and ion transport. Our data links thickness-dependent changes in bulk membrane functionality to confinement-induced changes in catalyst-ionomer structure. Given the distinct functionality expected of ionomers in these regimes, our results are used to first expand the design space of ionomer using previously overlooked thickness as a design lever, and then to identify key transition regimes to elucidate the governing phenomena and demonstrate the combinatory roles of thickness and interactions in ionomer performance. The results will be evaluated to provide a new paradigm for ionomers in hybrid structures, from composites membrane to electrodes, and to expand the ionomer parameter space enabling optimization strategies for performance and durability in fuel cells. Acknowledgment AK acknowledges the ECS-Toyota Fellowship and the funding from the Fuel-Cell Performance and Durability (FC-PAD) consortium through the Fuel Cell Technologies Office, Energy Efficiency, and Renewable Energy Office of the U.S. Department of Energy (DOE), (contract no. DE-AC02-05CH11231)

Several emerging and growing applications like biosensors, gas separation membranes, protective coatings, photonics, nanocomposites and microelectronics depend on polymers of thicknesses under 100nm[1]. Ion conducting polymers (ionomers) of thickness <100nm have induced significant advances in the performance of polymer-electrolyte fuel cells (PEFCs) by serving as binder in the catalyst layers (CLs), aiding proton conduction and extending the reaction zone of the CL porous electrode.[2] Thin and ultra-thin polymer films provide ease of processing, application specific tunable properties and reduction in material cost. However, due to deviation of nano-confined polymers from bulk behavior, they pose a challenge of predictability. This is evident in PEFCs where thin-film ionomers are large contributors to mass-transport losses at low catalyst loadings required for commercialization. In-situ investigations over the past 10 years have provided a significant amount of insight into these losses in CLs.[3] As a result, it would be misleading to employ the same principles of understanding used for bulk polymers for nano-confined polymers without modification. In our study, we aim to understand nano-confined ionomers in two ways. First, looking at impact of interfaces by using ionomers supported on different substrates and tuning substrate-ionomer interaction via reducing and oxidizing environment. Second, by examining the impact of cooperative motion and chain dynamics using thermal expansion study of nano-confined ionomers. Combining these two studies we can gain new understanding of effect of confinement on performance. Understanding ionomer-catalyst surface interface and interaction can elucidate not only how ionomers impact catalyst performance but also how catalyst surface impacts ionomer performance. In our study we use water uptake and modulus to probe the effect of platinum surface on thin-film ionomers under oxidizing(Air) and reducing(H2) conditions. Several in-situ and ex-situ methods have demonstrated that oxide formation on the active platinum catalyst surface can reduce the catalytic activity, further hampering activity in the cathode catalyst layer[4]. Despite this, our studies show continued growth of oxides on platinum surface results in higher density of water maintained in the ionomer. This indicates that nano-confined ionomers have greater water stability in the cathode than on the anode catalyst layer, enhancing proton transport in the cathode catalyst layer. Using transition temperature as a proxy for material behavior, we also looked at relaxation behavior of nano-confined ionomers under slow heating conditions. For glass-forming polymers like perfluorinated sulfonic-acid (PFSA) ionomers (such as Nafion and 3M-ionomer) cooperative motion and dynamics of chains have been used to explain changes in modulus, thermal expansion, relaxation behavior, and gas transport. Thermal transition temperatures are often used as a broad physical description for these kinetically and thermodynamically-driven changes in polymer behavior[1]. In this study, we used heated cell ellipsometry to explore the presence of transition temperatures and their dependence on thickness and substrate in thin-film ionomers. We are able to see both α and β transition temperatures using ion exchanged nano-confined ionomers supported on silicon substrates. Our results demonstrate increased rate of expansion for thin films as compared to bulk and dramatic change of transition temperatures around 50-70nm indicating impact of ionomer-surface interaction and influence of electrostatic interaction on chain relaxation and gas transport. Acknowledgement We would like to thank Douglas Kushner for providing assistance and guidance in this study. This work made use of facilities at the Joint Center for Artificial Photosynthesis at Lawrence Berkeley National Laboratory. This work was funded in part by the University of California Chancellor's Graduate Fellowship, National Science Foundation Graduate Fellowship and the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U. S. Department of Energy under contract number DE-AC02-05CH11231.

A systematic methodology to determine what is limiting the performance of a fuel cell will be presented. The basis of this methodology is to utilize in-situ cell data combined with simple analytical techniques. The key analytical techniques are based on theoretically-predicted limiting cases for the different types of overpotential that can limit the performance of a polymer-electrolyte fuel cell (PEFC). These limiting cases are compared to PEFC performance data utilizing simple graphical techniques in order to provide semi-quantitative insight into the mechanisms primarily responsible for the cell polarization observed operating under the operating conditions of interest. This diagnostic methodology is ideally suited to investigate changes in the performance of a fuel cell. For example, the use of this methodology to investigate durability losses in PEFCs has been described [1]. However, the same techniques can be used to determine why the performance of one type of cell is different than a cell of a different configuration, or why the performance of a given cell varies with different operating conditions. Examples of each of these types of cell-performance investigations will be used to illustrate the methodology. The diagnostic method recommended here is a systematic, step-by-step method. The first step is to determine what major type(s) of overpotential are responsible for the changes in performance, namely: kinetic, ohmic, or mass transport. Polarization-change curves, such as those depicted in Figure 1, are a simple and useful tool to assist in apportioning performance changes to the different types of overpotential. The limiting cases depicted in Figure 1 were constructed with a relatively simple model of cell polarization [1]. These simple limiting cases, as well as some actual polarization-change curves examples, will be presented. Once the major types of overpotential have been identified, additional cell diagnostics can be selected to establish the sources (i.e., cell components and/or locations) of these changes. In-cell diagnostics for each of the major types of polarization are also recommended. For example, to determine whether mass-transport resistance is increasing within or external to the catalyst layer (or catalyst agglomerate) one can utilize an oxygen-gain analysis, which is based on a relatively simple model of the cathode potential and the two limiting cases predicted by this model [2]. To determine whether mass-transport losses are primarily due to oxygen transport and/or proton transport (i.e., ohmic losses in the catalyst layer), one can utilize an oxygen-dependence analysis, which is also based on limiting cases predicted by a simple model of the cathode [3]. The examples used to illustrate these diagnostic techniques will be on state-of-the-art PEFCs, so that the actual performance limitations being investigated should also be of interest to PEFC research community, such as mass-transport losses in PEFCs with ultra-low catalyst loadings. Acknowledgements Thanks to the organizers of this Symposia for the invitation to present. The author would also like to thank his many fuel-cell collaborators at United Technologies Corporation (both past and present). Funding from U.S. Department of Energy, EERE’s Fuel Cell Technologies Office under contract numbers DE-AC02-05CH11231 and DE-AC02-06CH11357 has enabled much of the recent work at UTRC on PEFCs that will be used as examples here, and is also gratefully acknowledged.

Platinum alloys catalyze oxygen reduction in conventional polymer-electrolyte fuel cells [1]. The platinum is deposited on carbon, and the nature of the carbon support affects cell polarization. Two commonly used carbon blacks are Vulcan and Ketjenblack. Vulcan (V) has a relatively low surface area of (228 m2/g [2]) and ~95% of the Pt resides on the external surface [3]. Ketjenblack (KB) has a higher surface area, 891 m2/g [2], and ~60% of the Pt resides in micropores in the carbon [3]. Pt/KB displays higher catalytic activity, while Pt/V exhibits better performance at high current densities. Pt inside micropores is not in direct contact with ionomer and, consequently, is not poisoned by adsorbed anions. However, Pt inside KB micropores is relatively inaccessible to oxygen and protons, which detrimentally affects performance at high current densities [4].The area-specific impedance of catalyst layers is frequently measured by operating cells with nitrogen at the working electrode, to create a blocking electrode, and hydrogen at the counter electrode [5, 6, 7, 8]. The area-specific impedance of the catalyst layer can be determined from the difference between the maximum value of the real part of the impedance at low frequency and the intercept of the impedance with the real axis at high frequency. The area-specific resistance of a catalyst layer determined in this way is R = l⸱(3κ)-1, where l is the thickness of the catalyst layer and k is the effective conductivity of ionomer.Recently, Harzer et al. deposited platinum on Ketjenblack by different techniques [4]. They fabricated two electrodes with platinum preferentially on the external surface of carbon black, and one electrode with platinum preferentially inside micropores like commercial Pt/KB. They measured higher resistance to oxygen transport, higher values of R, and larger polarization on oxygen at high current densities on catalyst layers having platinum predominantly in micropores. They postulated that higher R was caused by a poorer distribution of ionomer. A different hypothesis is considered in this work: that ohmic losses in micropores contribute significantly to R. Firstly, a formula is developed to describe the impedance of a blocking electrode with substantial resistance inside micropores. Secondly, the model is used to interpret the experiments of Harzer et al. and estimate the conductivity of the pore solution that would fit their results. Acknowledgements: This work was funded under the Fuel Cell Performance and Durability Consortium (FC-PAD), by the Fuel Cell Technologies Office (FCTO), Office of Energy Efficiency and Renewable Energy (EERE), of the U.S. Department of Energy under contract number DE-EE0007652. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Transition metal-nitrogen-carbon (M-N-C) materials have been continuously reported to be one of most promising platinum group metal-free (PGM-free) catalysts for oxygen reduction reaction (ORR)/cathode in low-temperature polymer electrolyte fuel cells (PEFC) and other emerging areas (such as carbon dioxide reduction reaction and nitrogen reduction reaction). They are usually obtained through pyrolysis of low molecular weight (organic) or polymetric precursors. The pyrolysis consists of heat treatment in inert (or reductive) atmosphere. Through the decades of practice, this pyrolysis process has been optimized intuitively and empirically with little-to-no knowledge of the explicit sequence of transformations. Morphological, structural, interfacial and chemical changes during pyrolysis enable the emergence of the new material with atomically dispersed transition metal ions. These ions are incorporated in the graphene-like carbonaceous matrix, doped with nitrogen and forming various Fe-Nxcatalytically active sites. Often, a secondary pyrolysis process is being carries out, after the completion of the initial one, resulting in better defined, and in many cases, more active and stable catalysts (a.k.a. “re-pyrolysis”).Our team have investigated both stages of the pyrolytic synthesis via in situXPS, TEM, EDS, XRD and nano- and micro-XCT methods in two separate studies dedicated to (i) pyrolysis of charge-transfer organic salt (nicarbazin) when mixed with silica template (a.k.a. “sacrificial support method” of M-N-C catalyst synthesis) and (ii) re-pyrolysis (secondary, “purification” pyrolysis) of the M-N-C material as a path to obtain atomically dispersed PGM-free catalyst.During the first pyrolysis we have observed for the first time the explicit melting of the precursor (at exact thermal signature), wetting of the hard template and followed major transformation in morphology: pore structure evolution, initial graphitization, metal and metal carbide phase formation, secondary graphitization templated on the transition metal nanoparticles and secondary micro-pores formation. All these observations have been interpreted through cross-referencing tomography, crystallography, microscopy and spectroscopy in situdata streams.We established that the re-pyrolysis of M-N-C materials could result in the amorphization of carbon, the rearrangement of multitudinous N-containing moieties, and increased content and a more uniform distribution of Fe-Nxsites, yet without major morphology changes in carbonaceous framework. In summary, the re-pyrolysis has multiple advantages and is recommended for all M-N-C electrocatalysts, no matter what precursors and parameters are set for their synthesis.

Non-platinum group metal (non-PGM) catalysts with high volumetric activity and low hydrogen peroxide yields are potential replacements for platinum-based catalysts for the oxygen reduction reaction (ORR). State-of-the-art non-PGM ORR catalyst synthesized from dual nitrogen precursors (cyanamide and polyaniline) have been presented before by our group [1]. These complex heterogeneous catalyst systems are formed from high-temperature synthesis of nitrogen, carbon, and transition metal precursors. Although Fe-N-C catalysts have demonstrated high ORR activity and low peroxide yields, Fenton (Fe2+/H2O2) and Fenton-like (Fe3+/H2O2) reactions generating hydroxyl radicals are a great concern for the durability of polymer electrolyte fuel cells due to membrane degradation. In an effort to mitigate these risks, development of iron-free non-PGM catalysts with similar ORR activity to the Fe-N-C system but enhanced durability, alternative transition metal precursors are being pursued. In this presentation, we will present further development of the dual nitrogen precursor synthesis approach replacing Fe with precursors of alternative transition metals: Co, Mn, and Ni. A systematic approach to compare structural and electrochemical characterization of all Me-N-C systems (Me = Fe, Co, Mn, Ni) will be developed for better understanding and further optimization of alternative-metal non-PGM ORR catalysts. Furthermore, atomic-scale structures of simplified Me-Nx active site(s), obtained from density functional theory (DFT), aiding in the synthesis of non-PGM catalysts with high concentration of active site (those with simultaneously high ORR activity and practical stability), will be presented. Relative formation energies serving as descriptors of active-site structure stability will be calculated via DFT with uniform atomic reference states. Previously, O-binding had been utilized as an activity descriptor for ORR catalysts [2], but for non-PGM catalysts, OH-binding is often the potential determining step in the ORR reaction pathway. Thus, this study will present the ORR activity of simplified Me-Nx structures on zig-zag edge of graphene nanoribbons, providing a comparison of both *OH-coordinated and uncoordinated structures and predicting the ORR activity trends from such alternative metal sites to those of iron. Acknowledgement Financial support for this research by DOE-EERE through Fuel Cell Technologies Office is gratefully acknowledged. [1] Chung, H.T., Holby, E.F., Purdy, G.M., Babu, S.K., Litster, S., Cullen, D.A. More, K.L., Zelenay, P. (2015). “Combining Nitrogen Precursors in Synthesis of Non-Precious Metal ORR Catalysts with Improved Fuel Cell Performance.” ECS Meeting s, MA2015-02 (37), 1278. [2] Norskov, J.K., Rossmeisl, J., Logadottir, A., Lindqvist, L., Kitchin, J.R., Bligaard, T, Jónsson, H. (2004). “Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode.” The Journal of Physical Chemistry B, 108 (46), 17886-17892.

Commercial low-temperature polymer electrolyte membrane fuel cells (PEMFCs) rely on expensive platinum-based catalysts to drive the oxygen reduction reaction at the cathode. In order to overcome cost barriers limiting market penetration in both transportation and stationary applications, significant efforts are being made to develop suitable platinum group metal-free (PGM-free) catalysts. The latest classes of PGM-free catalysts show very promising mass activity, but the poor durability of these carbon-based materials is still a significant challenge that must to be addressed. In this effort, advanced analytical electron microscopy methods were employed to study the degradation pathways of PGM-free catalysts which were submitted to accelerated stress tests. Catalysts provided by four different suppliers were tested concurrently in a specially designed segmented fuel cell. Polarization curves were recorded both before and after a durability stress test which involved 10k square wave cycles between 0.95 – 0.60 V in air. Analytical electron microscopy performed across a wide length scale (sub-nanometer to micron) will be presented which links the observed performance losses with changes to the atomically dispersed active sites, carbon support, and ionomer distribution. The advantages and disadvantages of different catalyst precursors (metal organic frameworks, cyanamide/polyaniline, etc.) will also be discussed from a durability standpoint. This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the Electrocatalysis Consortium (ElectroCat). Microscopy was performed as part of a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility

The composition and processing of the catalyst-ionomer ink used to fabricate electrochemical energy conversion device electrodes can impact the final electrode microstructure and performance. Parameters influencing the microstructure and performance are, for example, solvent ratio, ink processing method and time, ionomer chemistry, and carbon morphology/porosity. Polymer electrolyte fuel cell (PEFC) cathodes are typically prepared by first dispersing a catalyst powder, comprised of platinum or platinum alloy nanoparticles supported on carbon blacks, with ionomer in solvents, followed by extensive mixing using a variety of methods. The electrode properties are controlled by the interactions of the ionomer with the catalyst particles and the support in the catalyst-ionomer ink, by the effect of ink solvent composition on those interactions, and by the ink mixing and coating procedures. This presentation will describe the in-situ/ex-situ ultra-small angle X-ray scattering (USAXS) combined with SAXS studies of the evolution of the cathode catalyst layer to determine the effect of ink processing variables and ink composition on the ink properties, especially on the carbon agglomerate break-up for efficient coating of catalyst layers while optimizing fuel cell performance and durability. The effect of ink formulation is examined for Pt-supported Vulcan carbon (Pt/Vu) and Pt and PtCo-supported on high surface area carbon (Pt and PtCo/HSC) with different ionomers. The results provide the fundamental understanding enabling improved membrane-electrode assembly (MEA) performance and durability. Moreover, the results of the evolution of the catalyst layer during the ink drying process as a function of solvent removal rate and solvent identity will be discussed. Acknowledgments This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office under the M2FCT Consortium. This work was authored in Argonne National Laboratory, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357. This research used the resources of the Advanced Photon Source (APS), 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.