Non-Platinum Group Metal Oxygen Reduction Catalysts for a Hydrogen Fuel Cell Cathode: A Mini-Review

The oxygen reduction reaction (ORR) activity of the Fe/N/Carbon catalysts derived from the pyrolysis of zeolitic-imidazolate-framework-8 (ZIF-8) has been still lower than that of commercial Pt-based catalysts utilized in the proton exchange membrane fuel cells (PEMFCs) due to low density of accessible active sites. In this study, an efficient carbon-supporting strategy is developed to enhance the ORR efficiency of the ZIF-derived Fe/N/Carbon catalysts by increasing the accessible active site density. The enhancement lies in (i) improving the accessibility of active sites via converting dodecahedral particles to graphene-like layered materials and (ii) enhancing the density of FeNx active sites via suppressing the formation of nanoparticles as well as providing extra spaces to host active sites. The optimized and efficient Fe/N/Carbon catalyst shows a half-wave potential (E1/2) of 0.834 V versus reversible hydrogen electrode in acidic media and produces a peak power density of 0.66 W cm-2 in an air-fed PEMFC at 2 bar backpressure, outperforming most previously reported Pt-free ORR catalysts. Finally, the general applicability of the carbon-supporting strategy is confirmed using five different commercial carbon blacks. This work provides an effective route to derive Fe/N/Carbon catalysts exhibiting a higher power density in PEMFCs.

The development of platinum group metal-free (PGM)-free catalysts with comparable performance and durability to low- PGM catalysts would hasten the cost competitiveness of fuel cells with alternative technologies. Therefore, addressing electrocatalyst development represents the most pressing technical barrier in fuel cell deployment and in meeting the ultimate U.S. Department of Energy (DOE) automotive fuel cell system cost target of $30/kW. ElectroCat (The Electrocatalysis Consortium), like other DOE Energy Materials Network (EMN) consortia, aligns research and development (R&D) activities at national laboratories and, through DOE-funded projects, facilitates access of academic and industry partners to the world-class capabilities and expertise at the national labs. These capabilities, such as (1) catalyst synthesis (including in particular high-throughput methods), (2) advanced characterization expertise, and (3) quantum chemical modeling address the salient knowledge gaps in PGM-free catalyst development: ascertaining the structure-function relationship, increasing active site density, improving electrode structure to maximize active site utilization and mitigate mass transport limitations, and improving durability. In the near-term, the consortium will work to develop PGM-free catalysts and electrodes to meet the 2025 DOE current density technical target of 0.044 A/cm2 at 0.9 VIR-free, analogous to the goal for low-PGM catalysts. In 2018, ElectroCat R&D efforts increased initial activity by 68% compared to the 2016 baseline of 16 mA/cm2. The presentation will highlight current R&D advancements, core national lab experimental and computational capabilities, as well as high-throughput and machine-learning approaches to expedite the development of PGM-free catalysts and electrodes.

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 effective utilization of clean energy and finding alternatives to fossil resources are highly important to ensure the sustainability of human society and are always among the major goals of both chemistry and material science research. Advanced electrochemical devices, such as fuel cells, water electrolysers and metal-air batteries, represent the most promising strategies for clean-energy utilization. In an electrochemical device, the redox reactions are spatially separated by a membrane, allowing direct extraction/transfer of electrons at an electrode-electrolyte interface, which leads to higher intrinsic energy conversion efficiencies, milder process conditions, easy product separation and excellent design features for coupling to renewable energy infrastructure. The performance of such electrochemical processes is fundamentally determined by the physicochemical properties of the electrochemical interfaces, encompassing both the electrocatalyst and the structure of the adjacent electrochemical double layer. Specifically, electrocatalysts play key roles in electrochemical reactions and often limit the performance of entire systems due to their insufficient activity, low durability or high cost. Ideally, the rate, efficiency, and selectivity of the above electrochemical reactions can be substantially improved by developing high-performance electrocatalyst. One of the central tasks for chemists and material scientists is to design and fabricate the high-efficient efficiency but low-cost electrocatalysts systems. The current promising electrochemical reactions mainly focus on the realization of the reversible conversion between chemical and electricity energy, e.g., the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and hydrogen evolution reaction (HER). Coupling of the above electrochemical reactions provide a solid foundation for various essential electrochemical devices, such as direct hydrogen fuel cells (HOR + ORR); electrolysers (OER + HER); rechargeable zinc (Zn)-air battery (ORR + OER). Therefore, this thesis aims to design and synthesize high-performance electrocatalysts for HER, ORR and OER based on earth-abundant materials with proper hierarchical 2D or 3D nanostructures. Combined with the advanced characterization techniques and density functional theory (DFT) calculations, the relationship between the electrochemical activity and active sites of these earth-abundant electrocatalysts were detailedly explored and confirmed. Furthermore, to emphasize the hierarchical 2D or 3D nanostructures, the actual performance of these electrocatalysts was all evaluated in practical devices including Zn-air battery and proton exchange membrane fuel cell (PEMFC), specifically as follows: (1) The vast majority of the reported HER electrocatalysts performs poorly under alkaline conditions due to the sluggish water dissociation kinetics. In the first work, a hybridization catalyst construction concept is presented to dramatically enhance the alkaline HER activities of catalysts based on 2D transition metal dichalcogenides (TMDs) (MoS2 and WS2). A series of ultrathin 2D-hybrids are synthesized via facile controllable growth of 3d metal (Ni, Co, Fe, Mn) hydroxides on the monolayer 2D-TMD nanosheets. The resultant Ni(OH)2 and Co(OH)2 hybridized ultrathin MoS2 and WS2 nanosheet catalysts exhibit significantly enhanced alkaline HER activity and stability compared to their bare counterparts. The combined theoretical and experimental studies confirm that the formation of the heterostructured boundaries by suitable hybridization of the TMD and 3d metal hydroxides is responsible for the improved alkaline HER activities because of the enhanced water dissociation step and lowers the corresponding kinetic energy barrier by the hybridized 3d metal hydroxides. (2) Nitrogen-coordinated iron atoms on carbon matrix (Fe-N-C) materials are the most active Pt-group-metal-free ORR catalysts but still suffering their low stability and relatively lower activity compared to platinum-based materials. In the second work, Fe and Ni dual sites atomically dispersed in hierarchically ordered macroporous carbon support (Fe-Ni/N-HOMC) was designed and successfully prepared. Isolated atomic Fe- N4 and Ni-N4 active sites were confirmed via various characterizations. The ORR activity and stability of Fe-Ni/N-HOMC in both acid and alkaline electrolyte were much higher than commercial Pt/C and the mono-Fe doping counterpart, which was among the state-of-the-art ORR electrocatalysts. In addition, this 3D ordered interconnected macroporous structure with abundant mesopores and micropores could greatly increase the accessible ORR active site and also enhance the mass transport during the ORR process. When employed as cathodes for PEMFC, we found the excellent ORR activity of Fe-Ni/N-HOMC was completely translated to the cathode in the fuel cell. (3) High-performance bifunctional electrocatalysts with ORR and OER activity is the key to developing efficient rechargeable Zn-air batteries. In the third work, a high-performance bifunctional electrocatalysts for both OER and ORR were synthesized via further hybridizing as-prepared Fe-Ni/N-HOMC with NiFe layer double hydroxides (LDHs). Layered double hydroxides (LDHs) have been reported to be promising OER electrocatalysts with ultrahigh OER performances. The as-synthesized new composites exhibited almost the same ORR activity as Fe-Ni/N-HOMC, revealing that hybridization of NiFe-LDHs would not deteriorate the initial ORR activity. Moreover, the remarkable enhancement of OER activity was observed after the hybridization, which was attributed to the strong coupling of uniformly dispersed small NiFe-LDH nanoparticles with the carbon substrate. The prototype Zn-air battery was assembled using these new composites, which displayed the ultralow voltage gap and long-term stability. (4) Compared with Fe-N-C or Co-N-C based ORR electrocatalysts, the Cu-nitrogen-carbon composites were attracted little attention. However, the natural multicopper oxidases (MCOs) enzymes, such as laccase, can serve as efficient ORR catalyst with almost no overpotential. Inspired by their tris-copper centers in MCO, one novel Cu-nitrogen-carbon composite (Cu SAs/N-CS) with atomic Cu coordination sites were synthesized via the pyrolysis of the Cu-involved metal-organic-framework. The copper contents in Cu SAs/N-CS reaches as high as 3.17 wt.%, and the average distances of adjacent copper sites was around only 3.1 Å. Due to the synergetic effect of abundant single atomic copper active sites with closer distance and ultrathin carbon nanosheet structure, Cu SAs/N-CS exhibited superior ORR activity exceeding commercial Pt/C catalyst, methanol tolerance, and long-term stability in both alkaline and neutral electrolyte. In summary, four kinds of new composites were successfully designed and prepared as high-performance electrocatalysts for HER, ORR and OER. Multi-dimensional heterostructures, atomic metal coordination sites and 3D hierarchically porous structure were designed and observed, which contributed greatly to improve activities of these composites. This thesis suggests several new viewpoints in the design of electrocatalysts based on earth-abundant materials: (i) offering new strategies for the preparation of novel 2D and 3D heterostructures as electrocatalysts; (ii) expanding methods for the synthesis of atomic metal coordination sites and evaluating their activities for ORR; (iii) evaluating the practical performances of achieved electrocatalysts in proton exchange membrane fuel cell and Zn-air battery; (iv) attempting to explain reaction mechanisms of some electrocatalysts by DFT calculation.

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.

Nano-structured platinum group metal-free catalysts and their integration in fuel cell electrode architectures

The energy industry is set to be revolutionized by polymer electrolyte membrane fuel cells (PEMFCs). Commercial PEMFC stationary units for industrial / home use are already on sale in Japan, and PEMFC vehicles will be commercialized in various countries in 2015. Platinum-decorated carbon black electrocatalyst powder is a crucial aspect of PEMFC technology. However, Pt is expensive, and limits PEMFC durability due to aggregation, dissolution, ripening, and carbon corrosion. Pt-free catalysts are therefore desirable for next-generation fuel cells. Non-precious, Pt-free catalysts for the electrochemical oxygen reduction reaction (ORR) in acid media have been the subject of intense research for the past few decades. One of the most popular materials in this field are pyrolysed mixtures of Fe/C/N-containing precursors, subjected to various heating, acid washing, and milling procedures. The mechanism for the ORR in these catalysts is not well understood, and is still debated. There are two main camps; those who see Fe as part of the catalytic center; and those who see Fe as a generator of active sites, whilst nitrogen would play the active role in the ORR. The history of this field is well-summarized in a recent review.1 Resolution of this debate is hampered by the complicated chemical structure in these materials; the possible combinations of Fe, N, and C atoms that could be ORR active are many. Therefore our approach is to simplify the system by removing Fe from the equation, and clarifying the fundamental catalytic activity of nitrogen-doped carbons. Initially we synthesized carbon nitride, and pyrolysed carbon nitrides supported on carbon black and carbon nanotubes. These materials had some inherent ORR activity, but Fe contamination was an issue,2,3,4 as with most materials approaches to this problem. We therefore developed synthesis of a truly metal-free nitrogen-doped graphene foam (GFN), which showed high inherent 4-electron oxygen reduction in acid medium.5,6 Here we synthesize GFN by combustion of nitrogen-containing sodium alkoxide, followed by washing, 1000˚C heat treatment in N2 and H2, and graphitization at 1400˚C. This is a 3D carbon with micron-scale pores encapsulated by thin defective graphene walls with a thickness of around 2 nm, with a surface area of > 700 m2/g and a nitrogen content of around 0.5 at.%. (Fig. 1). The material was confirmed to be Fe-free by ICP analysis. Linear sweep voltammograms (LSVs) of GFN (Fig. 2) show relatively high activity for an entrirely metal free catalyst (maxinum current density, -4 mA/cm2, mass activity (at 0.6 V), 2.79 A/gcat). The onset potential (at -10 µA/cm2) is around 0.85 V, similar to many Fe-containing catalysts. The electron transfer cooeficient (calculated from the ring electrode) is 3.6, incdicating majority 4-electron transfer. We conclude that Fe-free active sites may contribute significantly to the 4-electron ORR in Fe/C/N-based catalysts (although this does not mean that Fe-centers have no activity).Figure 1. (a) Schematic showing some PEMFC degradation mechanisms; (b) nitrogen-doped graphene schematic; (c) SEM image of GFN; (d) XPS N1s spectrum; linear sweep voltammograms for GFNin acid, at various rotation speed; (e) disc electrode, (f) ring electrode. The International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) is supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan. 1. J-. P. Dodelet, Electrocatalysis in Fuel Cells, Springer (2013):271 2. Lyth, S. M., et al. J. Phys. Chem. C113.47 (2009): 20148 3. Lyth, S. M., et al. J. Electrochem. Soc. 158.2 (2011): B194 4. Lyth, S. M., et al. J. Nanosci. & Nanotech. 12.6 (2012): 4887 5. Lyth, S. M., et al. e-J. Surf. Sci. & Nanotech. 10 (2012): 296. Liu, J, et al. J. Electrochem. Soc. 161.4 (2014):F544-F550

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

Hydrogen proton exchange membrane fuel cells are a key clean energy technology suited to address hard-to-abate carbon emission sources in medium- and heavy-duty transportation sectors such as trucks, rail, ships, and aviation. Researchers continue to develop novel, functionalized/modified, high-surface-area, and mesoporous carbon supported catalysts to enhance electrochemical performance and durability. However, these novel carbon supports, as well as platinum group metal-free catalysts, often require unique ink formulation methods due to differences in initial catalyst aggregate size, surface functional groups, and surface wettability. These factors affect the homogeneity of the catalyst ink, film coatings, and catalyst layer structure. As a result, determining sufficient ink mixing protocols requires trial-and-error verification. This work examines how different ink mixing strategies, such as probe sonication and low-to-high speed ball milling, affect the structure of the catalyst layer and the cell performance for an emerging functionalized high-surface-area carbon-platinum catalyst (COR-Cat-C4™ by pH Matter). A high-throughput ink drop ‘witness sample’ coating method was applied to image the catalyst structure through the mixing process and to determine the necessary duration for homogeneity. Using this approach and other characterizations, our study further evaluates the relationship between mixing intensity and the resulting aggregate size distribution. In turn, we attempt to correlate the resulting size distribution with electrode coating quality, morphology, transport properties, and fuel cell performance. The inks were characterized systematically and specific metrics have been identified for optimal coating both in terms of quality and functionality. Overall, we observe that smaller aggregates reduce free ionomer agglomerates, improve electrode film uniformity, and decrease the catalyst protonic resistance, leading to enhanced cell performance.This work was supported by the DOE Office of Energy Efficiency and Renewable Energy (EERE) under award number DE-EE0011348-001.

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.

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.

Today, the development of active and stable catalysts still represents a challenge to be overcome in the research field of low-temperature fuel cells.[...]

Platinum group metal-free (PGM-free) catalysts for oxygen reduction are being pursued as a solution for scarcity and cost of catalysts utilizing platinum. Transition metal-nitrogen-carbon (M-N-C) materials are currently the most popular of PGM-free catalysts finding use in low temperature fuel cells. Iron-containing Fe-N-C are being synthesized lately in many variations ranging from atomically dispersed to metallic nano-particles containing types. Despite intense effort, much of the structure of these materials remains unknown and the interpretation of the activity and selectivity data rely on substantial hypothesis-based structural assumptions. 57Mössbauer spectroscopy is gaining significant importance in the experimental characterization of pyrolyzed Fe-N-C catalysts [1]. Isomer shifts and quadrupole splittings (QS), two main features of the 57Mössbauer spectra of Fe-bearing materials, vary systematically as a function of 57Fe oxidation, spin states, and coordination and can be used to accurately determine the local chemical environment of Fe ions in the Fe-N-C materials. All Fe-N-C catalysts have shown at least two clearlydistinct doublets in their Mössbauer spectra, often labeledas D1 and D2. The D1 doublet has an isomer shift value ranging from 0.33 to 0.4 mm/s and quadrupole splitting ranging from 0.7 to 1.2 mm/s, which is usually empirically attributedto Fe-N4defects with Fe2+in its low spin state. The assignment of the D2 doublet with isomer shifts ranging from 0.36 to 0.52 mm/s and with quadrupole splitting ranging from 2.4 to 2.7 mm/s is more ambiguous.It is attributedto Fe-N4 defects similar to iron phthalocyanine with Fe2+in its intermediate spin state or to a Fe-N2+2 defect in which Fe binds to four pyridinic nitrogen atoms from two adjacent nitrogen-doped graphene layers. However, the assignment of these doublets has been basedon experimental reports on carbon-supported FeN4macrocycles, which differs greatly from the case in which the Fe-Nxmoieties are integratedintothe graphene matrix. Therefore, there is a need for combining density functional theory (DFT) calculations of isomer shifts and quadrupole splittings on more realistic models, and compare them to those experimentally extracted with different fitting approaches from the Mössbauer spectraof real Fe-N-C materials. The DFT theory has been applied previously to calculate 57Fe isomeric shifts and quadrupolesplitting for different Fe-containing molecules, but has not yet been appliedtothe Fe-Nxdefects incorporated in the graphene matrix. In this work, both cluster and periodic approacheswere used to calculate thequadrupolesplitting in Fe-N4defects that are identified as the active sites in the pyrolyzed Fe-N-C materials. Because of the well-documented dependence of the DFT computed QS on the exchange-correlation functionals and basis sets [2],the methodology has been first tested for benchmark systems, shown in the Figure. Extensive computations of QS in Fe-N4models with Fe in various coordinationsand with different oxidation/spin states have also been carried out. Furthermore, the DFT calculated values were compared with experimental data to interpret quadrupole doublets ubiquitous in the Mössbauer spectra of pyrolyzed Fe-N-C catalysts. This paper will bring to discussion both computational approaches and correlations of 57Mössbauerspectroscopy “fingerprints” with observed and hypothesized structural motifs of Fe-containing active sites in PGM-free electrocatalysts for oxygen reduction reaction. Zitolo A., Goellner V., Armel V., Sougrati M.-T., Mineva T., Stievano L., Fonda E., Jaouen F., Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials, Nature Materials, 14 , 937 (2015). Pápai M. and Vankó G., On Predicting Mössbauer Parameters of Iron-Containing Molecules with Density-Functional Theory, J. Chemical Theory and Computation, 9, 5004 (2013). Figure 1

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

Catalysis of the oxygen reduction reaction (ORR) is a key challenge for high power density fuel cells and metal-air batteries. Today, the polymer electrolyte membrane fuel cell (PEMFC) is the most advanced fuel cell type for transportation applications, where high power density is required. The advanced technology readiness of the PEMFC leans on the existence of stable and extremely thin polymer membranes and of highly active platinum-based catalysts for oxygen reduction and hydrogen oxidation. The acidic nature of PEM is however disadvantageous to the electrocatalysis of the sluggish ORR. While advances in platinum-based catalysts are still ongoing,1 economic and resource-based constraints are incentives to start designing today the next generation of ORR catalysts. The latter should ideally be free of rare elements and, in particular, free of Platinum-Group Metals (PGMs) since those metals are all mined from the same ore. Important advances in non-PGM catalysts have been witnessed since 2009,2-4 which is promising toward the long-haul goal of more affordable PEMFC stacks and decreased reliance on limited natural resources. The three aspects of i) synthesis, ii) electrochemical characterisation and iii) spectroscopic characterisation of Fe- and Co-N-C catalysts synthesized using Metal-Organic Frameworks (MOFs) will be discussed. The design of novel sacrificial MOFs and approaches for preparing the catalyst precursors that are subsequently pyrolyzed will be presented. The versatility of MOF materials with respect to the ligand chemistry and topological structure is immense, resulting in thousands of different possible materials. Such materials just started being investigated as sacrificial precursors for the synthesis of Fe(Co)-N-C materials.5-6-7 Results obtained from the electrochemical investigation of the key characteristics of those ORR catalysts, namely initial ORR activity, power performance and durability, will be discussed. In particular, the degradation mechanisms related to transient high-voltage and H2O2 generation during steady-state operation are important.8 Both these degradation types correspond to various oxidative attacks of the carbon-based catalysts. The degradation induced by minute amounts of H2O2 can lead to a decreased ORR activity and/or decreased mass-transport properties of the cathode, depending on the nature of the transition metal element. Ex situ chemical treatment of catalysts with various amounts of H2O2 is a useful approach for simulating the steady-state operando degradation in fuel cell. In this respect, cobalt will be shown to be advantageous compared to iron for long term durability of metal-N-C catalysts. Third, a better identification of the nature and structure of the metal-centred active sites with spectroscopic methods (Mössbauer spectroscopy and X-ray absorption spectroscopy) will be presented. Comparison of computed spectra to experimentally measured spectra is important for identifying the active site structure and for supporting the DFT calculation approaches on model sites with some experimental evidence for the existence of metal-Nx-Cy moieties embedded in or at the edge of graphene sheets.