Oxygen Reduction Reaction Activity Of Catalysts Research Articles

Practice of Ecological Aesthetics in Green Production of Bimetallic Carbide Catalyst for Oxygen Reduction Reaction: Integrating Technological Development with Ecological Protection

This work summarizes the disciplinary connotation of ecological aesthetics, discusses the social and philosophical background of the origination of ecological aesthetics, and applies ecological aesthetics to the research on the production processes of catalytic materials. It is found that compared with conventional chemical processes, catalytic materials synthesized using green chemical processes that conform to ecological aesthetics have advantages in raw material cost, energy consumption, environmental protection, operational complexity, and product performance. Based on this, it is proposed that, as green chemical processes develop to a certain extent, they will unify anthropocentrism and ecocentrism, and meet both human needs and ecological protection requirements. The mentioned green chemical processes adopt biomass lotus leaf stems as a carbon source to produce non-noble metal bimetallic carbide (C19Cr7Mo24)-based catalysts for oxygen reduction reaction (ORR). Its initial half-wave potential (E1/2) for catalyzing ORR in an alkaline medium is 0.903 V, the E1/2 retention rate after 50,000 cycles is 98.9%, and its peak power density in H2/O2 fuel cell reaches 1.47 W cm−2, making it one of the most active non-noble metal catalysts for ORR reported so far; its stability is unparalleled.

Engineering ORR Electrocatalysts from Co8Pt4 Carbonyl Clusters via ZIF‐8 Templating

AbstractTo reduce the costs of proton exchange membrane fuel cells, the amount of Pt necessary to drive efficient oxygen reduction reaction (ORR) should be minimized. Particle nanostructuring, (nano‐)alloying, and metal‐doping can yield higher activities per Pt mass through tailoring catalysts owning a high number of active sites and precise electronic properties. In this work, the atom‐precise [NBnMe3]2[Co8Pt4C2(CO)24] (Co8Pt4) cluster is encapsulated and activated in a zeolitic imidazolate framework (ZIF)‐8, which unlocks the access to defined, bare Pt−Co nanoclusters, Co8±xPt4±yNC@ZIF‐8, for the fabrication of highly active ORR catalysts. Upon controlled C‐interfacing and ZIF‐8‐digestion, Co‐doped Pt NPs (Pt27Co1) with a homogenous and narrow size distribution of (1.1±0.4) nm are produced on Vulcan® carbon. Restructuring of the Pt27Co1/C catalyst throughout the ORR measurement was monitored via high‐angle annular dark field‐scanning transmission electron microscopy and X‐ray photoelectron spectroscopy. The measured ORR mass activity of (0.42±0.07) A mgPt−1 and the specific activity of (0.67±0.06) mA cmECSA−2 compare favourably with the catalyst obtained by direct C‐interfacing the pristine Co8Pt4 cluster and with state‐of‐the‐art Pt/C reference catalysts. Our results demonstrate the potential of ZIF‐8‐mediated Pt−Co NP synthesis toward devising ORR catalysts with high Pt‐mass activity.

Manipulation of Electronic States of Pt Sites via d-Band Center Tuning for Enhanced Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells.

Expediting the torpid kinetics of the oxygen reduction reaction (ORR) at the cathode with minimal amounts of Pt under acidic conditions plays a significant role in the development of proton exchange membrane fuel cells (PEMFCs). Herein, a novel Pt-N-C system consisting of Pt single atoms and nanoparticles anchored onto the defective carbon nanofibers is proposed as a highly active ORR catalyst (denoted as Pt-N-C). Detailed characterizations together with theoretical simulations illustrate that the strong coupling effect between different Pt sites can enrich the electron density of Pt sites, modify the d-band electronic environments, and optimize the oxygen intermediate adsorption energies, ultimately leading to significantly enhanced ORR performance. Specifically, the as-designed Pt-N-C demonstrates exceptional ORR properties with a high half-wave potential of 0.84 V. Moreover, the mass activity of Pt-N-C reaches 193.8 mA gPt-1 at 0.9 V versus RHE, which is 8-fold greater than that of Pt/C, highlighting the enormously improved electrochemical properties. More impressively, when integrated into a membrane electrode assembly as cathode in an air-fed PEMFC, Pt-N-C achieved a higher maximum power density (655.1 mW cm-2) as compared to Pt/C-based batteries (376.25 mW cm-2), hinting at the practical application of Pt-N-C in PEMFCs.

Densely accessible single atom Fe sites dispersed on porous carbon as highly stable and active ORR catalyst for PEMFC

Single-atom Fe–N–C catalysts have received ever-growing interest in proton exchange membrane fuel cells (PEMFCs) application for their utmost atomic utilization in oxygen reduction reaction (ORR). However, developing atomically dispersed Fe–N–C catalysts with both satisfactory activity and excellent stability presents great challenges. Herein, a template-activator combined strategy is developed to construct ORR catalysts with densely accessible single-atom Fe sites dispersed on hierarchically porous carbon networks. The optimized catalyst exhibits excellent oxygen reduction activity with a half-wave potential of 0.84 V and surprising stability with 93.6% retention of the initial current density after 24 hours of chronoamperometric test in acidic media. Notably, when assembled in a H2–O2 fuel cell, it shows remarkable long-term durability with only 16% loss in current density during the first 10 hours.

Bimetallic niobium-iron oxynitride as a highly active catalyst towards the oxygen reduction reaction in acidic media

Developing cost-effective acidic oxygen reduction reaction (ORR) catalysts with high performance is of great significance for proton exchange membrane fuel cells (PEMFCs) but very challenging. Transition-metal oxynitrides have high tolerance to harsh acidic media due to their excellent corrosion resistance, but they suffer from low acidic ORR activity. Here we report the discovery of a carbon-supported bimetallic niobium-iron oxynitride as a highly active and robust ORR catalyst in acidic media. This catalyst shows much higher ORR activity than its monometallic niobium oxynitride counterpart and exhibits a record high ORR activity among transition-metal oxynitrides, with an optimal ORR half-wave potential of 0.75 V vs. RHE, approaching those of atomically dispersed metal-N-C materials. It is revealed that the optimal catalyst has two types of Fe species with low oxidation state and two additional oxygen adsorption sites with high reactivity in comparison to its monometallic niobium oxynitride counterpart, therefore resulting in its remarkable ORR activity. Our work provides a new direction to explore efficient acidic ORR catalysts with low costs.

Self‐Assembled Perovskite Nanocomposite With Beneficial Lattice Tensile Strain as High Active and Durable Cathode for Low Temperature Solid Oxide Fuel Cell

AbstractThe sluggish kinetics of oxygen reduction reaction (ORR) at low temperatures and the fast degradation of the cathode are the main obstacles to the commercialization of solid oxide fuel cells (SOFCs). However, it is still very challenging to achieve both high catalytic activity and favorable stability for single‐phase materials. Herein, a highly active and durable nanocomposite cathode (Ba0.5Sr0.5)0.75Pr0.25Co0.575Fe0.3W0.125O3‐δ (BSPCFW) for low‐temperature SOFC (≤650 °C) is presented, which self‐assembles into two cubic perovskites: the simple perovskite Pr0.38Ba0.25Sr0.37Co0.62Fe0.38O3‐δ (PBSCF‐c) and the B‐site cations ordered double perovskite Ba1.30Sr0.70Co1.0Fe0.25W0.75O6‐δ (BSCFW‐c). The former PBSCF‐c serves as the highly conductive and active catalyst for ORR, while the latter BSCFW‐c with a large lattice parameter introduces a key beneficial lattice tensile strain into the PBSCF‐c phase through a coherent interface, which significantly promotes the ORR activity at low temperatures with the area specific resistance of 0.034 Ω cm2 at 650 °C, and the long‐term stability of 2 years storage and 1280 h operation in symmetrical cell. The introduction of the beneficial lattice tensile strain in self‐assembled composite catalysts is an effective way to synergistically enhance the electrochemical activity and durability of the electrode materials for electrochemical devices.

Enhanced Electrochemical Oxygen Reduction By Modulating Electronic Structures of Platinum Nanoparticles By Nitrogen-Introduced Carbon Support

With universal attention to sustainable development, the eco-friendly generation of energy has been emerging as one of the most important projects. Fuel cells, electrochemical energy conversion devices using hydrogen as the energy source, are one of the key solutions to this urgent problem. To get an effective energy conversion efficiency, understanding the reaction of fuel cells that occurred in each part is essential. Oxygen reduction reaction (ORR), which occurred in the cathode, is the main reason for reducing the efficiency because of its sluggish reaction kinetics. Therefore, it is necessary to develop highly active ORR catalysts to get efficient fuel cells. Among the various elements, platinum (Pt) has been known as the best active metal for its optimized binding properties with oxygen so plenty of research related to Pt-based materials has been reported so far. Most of them focused on electrocatalytic active material such as Pt-alloy nanocrystals or facet-controlled nanostructures where the electrochemical reaction occurred, while the surface state of the support has been largely neglected. Generally, carbon support for nanocrystals has been regarded as inert for reaction pathways on metal nanocrystals. In this study, we demonstrate that surface N-functionalization of carbon support can modulate electronic structures of supported Pt nanocrystals. Electronic structure-modified Pt nanocrystals show less affinity toward oxygen adsorption species, which means attenuated surface coverage and thereby much higher availability of Pt active sites during electrocatalysis. Consequently, reaction kinetics and mass activity are remarkably improved as much as Pt-M (M=Transition metals) alloy nanocrystals. This study will suggest the importance of surface engineering in carbon support as a factor in enhancing the electrocatalytic performance of nano-particulate electrocatalysts.

Design of nanosheet/nanotube composites of Fe, N-doped carbon for enhanced oxygen reduction in zinc-air batteries

Low-density active sites, low-mass transfer rate, and poor stability of carbon-based catalytic materials from the pyrolysis of conventional metal-nitrogen complexes can hamper the oxygen reduction reaction (ORR). Herein, a composite structure of nitrogen-doped carbon nanosheets grafted with carbon nanotubes are synthesized by Zn-ZIF-L breaking and co-functionalized by coupling Fe2O3 nanoparticles with active Fe-Nx species to form an active ORR catalyst (D-FeNC/MOF). Under the synergetic effect of high-density active sites and multi-size pore distribution, the diffusion resistance of oxygen in the pores can be decreased to facilitate the transfer of reactants and products, and the improvement of ORR efficiency. Comparing to the commercial Pt/C catalyst, the d-FeNC/MOF has exhibited a more positive half-wave potential of 0.875 V (vs. RHE) and a higher catalytic stability. Besides, the zinc-air battery assembled with d-FeNC/MOF as a cathode catalyst can also offer a maximum power density of 170 mW cm−2 and a specific energy density of 842 Wh kg−1. This work can provide a new way for designing non-Pt catalysts for various electrochemical energy devices.

Co-Doped Ni3V2O8 Nanofibers Achieving d-d Orbital Coupling for Electrocatalytic Oxygen Reduction.

Efficient and robust non-platinum-group metal electrocatalysts for O2 reduction are a prerequisite for practical high-performance fuel cells and metal-air batteries. Herein, we reported an integrated principle of gradient electrospinning and controllable pyrolysis to fabricate various Co-doped Ni3V2O8 nanofibers with high oxygen reduction reaction (ORR) activity. The representative Co1.3Ni1.7V2O8 nanofibers showed outstanding ORR performance in an alkaline solution with a half-wave potential (E1/2) of 0.874 V vs RHE, along with high long-term stability. Furthermore, the introduction of Co could effectively restrain the growth of nanoparticles and change the electronic structure of Ni3V2O8. Control experiments and theoretical calculations demonstrated that upon Co-doping, the hybridization between the 3d orbital for both Co and Ni guaranteed the stable adsorption interaction with O2 over Ni and Co metal centers. Meanwhile, the weakened binding ability of Ni3V2O8 to OH* reduced the ORR free energy. Overall, the synergistic effect of Co and Ni metal cations essentially reflected the origin of ORR activity on the Co-doped Ni3V2O8 nanofibers. This work offers new insights and practical guidance for designing highly active ORR catalysts for electrochemical clean energy conversion and storage.

On the Challenge of Obtaining an Accurate Solvation Energy Estimate in Simulations of Electrocatalysis

The effect of solvation on the free energy of reaction intermediates adsorbed on electrocatalyst surfaces can significantly change the thermochemical overpotential, but accurate calculations of this are challenging. Here, we present computational estimates of the solvation energy for reaction intermediates in oxygen reduction reaction (ORR) on a B-doped graphene (BG) model system where the overpotential is found to reduce by up to 0.6 V due to solvation. BG is experimentally reported to be an active ORR catalyst but recent computational estimates using state-of-the-art hybrid density functionals in the absence of solvation effects have indicated low activity. To test whether the inclusion of explicit solvation can bring the calculated activity estimates closer to the experimental reports, up to 4 layers of water molecules are included in the simulations reported here. The calculations are based on classical molecular dynamics and local minimization of energy using atomic forces evaluated from electron density functional theory. Data sets are obtained from regular and coarse-grained dynamics, as well as local minimization of structures resampled from dynamics simulations. The results differ greatly depending on the method used and the solvation energy estimates are deemed untrustworthy. It is concluded that a significantly larger number of water molecules is required to obtain converged results for the solvation energy. As the present system includes up to 139 atoms, it already strains the limits of computational feasibility, so this points to the need for a hybrid simulation approach where efficient simulations of much larger number of solvent molecules is carried out using a lower level of theory while retaining the higher level of theory for the reacting molecules as well as their near neighbors and the catalyst. The results reported here provide a word of caution to the computational catalysis community: activity predictions can be inaccurate if too few solvent molecules are included in the calculations.

Zinc-Mediated Template Synthesis of Hierarchical Porous N-Doped Carbon Electrocatalysts for Efficient Oxygen Reduction.

The development of highly active and low-cost catalysts for use in oxygen reduction reaction (ORR) is crucial to many advanced and eco-friendly energy techniques. N-doped carbons are promising ORR catalysts. However, their performance is still limited. In this work, a zinc-mediated template synthesis strategy for the development of a highly active ORR catalyst with hierarchical porous structures was presented. The optimal catalyst exhibited high ORR performance in a 0.1 M KOH solution, with a half-wave potential of 0.89 V vs. RHE. Additionally, the catalyst exhibited excellent methanol tolerance and stability. After a 20,000 s continuous operation, no obvious performance decay was observed. When used as the air-electrode catalyst in a zinc-air battery (ZAB), it delivered an outstanding discharging performance, with peak power density and specific capacity as high as 196.3 mW cm-2 and 811.5 mAh gZn-1, respectively. Its high performance and stability endow it with potential in practical and commercial applications as a highly active ORR catalyst. Additionally, it is believed that the presented strategy can be applied to the rational design and fabrication of highly active and stable ORR catalysts for use in eco-friendly and future-oriented energy techniques.

Post-Synthesis Heat Treatment of Doped PtNi-Alloy Fuel-Cell Catalyst Nanoparticles Studied by In-Situ Electron Microscopy

Octahedral-shaped PtNi-alloy nanoparticles are highly active oxygen reduction reaction catalysts for the cathode in proton exchange membrane fuel cells. However, one major drawback in their application is their limited long-term morphological and compositional stability. Here, we present a detailed in situ electron microscopy characterization of thermal annealing on octahedral-shaped PtNi catalysts as well as on doped octahedral PtNi(Mo) and PtNi(MoRh) catalysts. The evolution of their morphology and composition was quantified during both ex situ and in situ experiments using energy dispersive X-ray spectroscopy in a scanning transmission electron microscope under a hydrogen atmosphere and in vacuum. Morphological changes upon heating, i.e., a gradual loss of the octahedral shape and a continuous rounding of the particles, were observed, as well as evidence for increased alloying. Furthermore, the evolution of the shape of the PtNi(Mo) nanoparticles was quantified using in situ experiments under hydrogen atmosphere in a transmission electron microscope. The shape change of the particles was quantified using segmentation maps created by a neural network. It has been demonstrated that morphological changes crucially depend on the composition and surface doping: doping with Mo or Mo/Rh significantly stabilizes the structure, allowing for persistence of a truncated octahedral shape during heat treatments.

Enhancement Mechanism of Pt/Pd-Based Catalysts for Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is one of the key catalytic reactions for hydrogen fuel cells, biofuel cells and metal-air cells. However, due to the complex four-electron catalytic process, the kinetics of the oxygen reduction reaction are sluggish. Platinum group metal (PGM) catalysts represented by platinum and palladium are considered to be the most active ORR catalysts. However, the price and reserves of Pt/Pd are major concerns and issues for their commercial application. Improving the catalytic performance of PGM catalysts can effectively reduce their loading and material cost in a catalytic system, and they will be more economical and practical. In this review, we introduce the kinetics and mechanisms of Pt/Pd-based catalysts for the ORR, summarize the main factors affecting the catalytic performance of PGMs, and discuss the recent progress of Pt/Pd-based catalysts. In addition, the remaining challenges and future prospects in the design and improvement of Pt/Pd-based catalysts of the ORR are also discussed.

Pt17 nanocluster electrocatalysts: preparation and origin of high oxygen reduction reaction activity.

We recently found that [Pt17(CO)12(PPh3)8]z (Pt = platinum; CO = carbon monoxide; PPh3 = triphenylphosphine; z = 1+ or 2+) is a Pt nanocluster (Pt NC) that can be synthesized with atomic precision in air. The present study demonstrates that it is possible to prepare a Pt17-supported carbon black (CB) catalyst (Pt17/CB) with 2.1 times higher oxygen reduction reaction (ORR) activity than commercial Pt nanoparticles/CB by the adsorption of [Pt17(CO)12(PPh3)8]z onto CB and subsequent calcination of the catalyst. Density functional theory calculation strongly suggests that the high ORR activity of Pt17/CB originates from the surface Pt atoms that have an electronic structure appropriate for the progress of ORR. These results are expected to provide design guidelines for the fabrication of highly active ORR catalysts using Pt NCs with a diameter of about 1 nm and thereby enabling the use of reduced amounts of Pt in polymer electrolyte fuel cells.

Operando X-Ray Absorption Spectroscopic Study on Electrochemical Oxygen Reduction Mechanism of Novel Platinum-Based Nanostructured Catalysts

Polymer electrolyte fuel cells (PEFCs) have been developed for electric device applications such as fuel cell vehicles (FCV). PEFC cathode catalyst requires an excess Pt due to the degradation of oxygen reduction reaction (ORR) activity. Many research efforts aim to design Pt-based nanomaterials with controllable composition and structure to increase their activity while reducing the amount of Pt, like core-shell structure. The core-shell structure is obtained by Adzic’s method (Cu-UPD)1, where the underpotential deposition (UPD) of Cu mediator on Pt and the displacement of Cu mediator by Pt are conducted. This method requires the highly precise potential control for the UPD of Cu by means of potentiostats. In contrast, our previous research demonstrated the spontaneous two-step method (Galvanic cell) to synthesize spherical Pd@Pt core-shell catalyst without a special control of electrochemical equipment and reducing/stabilizing agents2. In this study, this Galvanic cell method was applied for Pd@Pt core-shell nanowire catalysts. Compared with the conventional Cu-UPD method, this method can synthesize the catalysts on a larger scale and form more uniform monolayer Pt shells, resulting in a higher ORR activity.Ultrathin Pd nanowires were prepared using surfactants as templates3. Briefly, palladium nitrate, ODA, and DTAB were dissolved in toluene under strong magnetic stirring. The mixture was placed in an inert atmosphere and sonicated for 20 min. NaBH4 was added to the above solution as a reducing agent. One hour later, the organic phase was recovered using distilled water and chloroform as extractants. After centrifugation, Pd nanowires can be obtained. After mixing the catalyst with carbon, the Pd/C powder is dispersed in the N2-saturated 0.01M CuSO4 and 0.5 M H2SO4 solution, with continual stirring. Then, a Cu wire is immersed in the above mixture2. Near 5 hours later, Cu wire is extracted, and successively N2-saturated 10 mM K2PtCl4 solution is added to the suspension to obtain Pd-nanowire@Pt/C. Electrochemical tests were performed in 0.1 M HClO4, and the catalyst ORR activity was evaluated at 0.9 V. XAS measurements of Pt L-edge were also performed under the same conditions.In contrast to the previously reported method (Cu-UPD)4, our proposed method does not require electrochemical equipment, or reducers/stabilizers, and uniform Pt shells in Pd-nanowire@Pt/C can be obtained. As shown in Figure 1, Core-shell catalysts have a lower white line intensity than Pt/C, meaning that they are more difficult to oxidize. The ligand effect causes surface Pt compression, and after calculating, the first-shell Pt-Pt bond length in the Pd-NW@Pt Gal. catalyst is shorter than that of the nanoparticle, which means a stable structure and less oxygen species generation. So, the catalysts synthesized using the Galvanic Cells method can provide higher ORR activity (specific activities: 0.8 mA/cm2 and mass activities: 1.3 A/mgPt) compared to the Cu-UPD method. Acknowledgement This work was supported by the project (JPNP20003) and a NEDO FC-Platform project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References; [1] Zhang, J.; Mo, Y.; Vukmirovic M. B.; Klie R; Sasaki K.; Adzic R R.; J. Phys. Chem. B 2004 108 (30), 10955-10964.[2] Wang, X.; Orikasa, Y.; Inaba, M.; Uchimoto, Y.; et al. ACS Catal., 2020, 10, 430– 434.[3] Teng, X.; Han, W.; Ku, W.; Hucker, M.; et al. Angew. Chem., Int. Ed. 2008, 47, 2055– 2058.[4] Koenigsmann, C., Adzic, R. R.; et al. J. Am. Chem. Soc., 2011 , 133, 9783– 9795. Figure 1

(Invited) Enhancing Oxygen Reduction Reaction Activities of Pt Catalysts By Atomic Layer Deposited Metal Oxides

The oxygen reduction reaction (ORR) posts a significant kinetic limit to hydrogen fuel cells, especially for polymer electrolyte membrane fuel cells (PEMFCs) operating at low temperatures. Highly active and durable electrocatalysts are required for reducing platinum group metal use and ensuring reliable device performance. The use of atomic layer deposition (ALD) and ALD transition metal oxides as dopants or templates are applied in controlled synthesis of highly active and durable Pt ORR catalysts. With density functional theory, these catalysts can also be designed with reliable theoretical guidance. The feasibility of their use in membrane electrode assemblies will be demonstrated.

Synergistic Hybrid Electrocatalysts of Platinum Alloy and Single-Atom Platinum for an Efficient and Durable Oxygen Reduction Reaction.

Pt single-atom materials possess an ideal atom economy but suffer from limited intrinsic activity and side reaction of producing H2O2 in catalyzing the oxygen reduction reaction (ORR); platinum alloys have higher intrinsic activity but weak stability. Here, we demonstrate that anchoring platinum alloys on single-atom Pt-decorated carbon (Pt-SAC) surmounts their inherent deficiencies, thereby enabling a complete four-electron ORR pathway catalysis with high efficiency and durability. Pt3Co@Pt-SAC demonstrates an exceptional mass and specific activities 1 order of magnitude higher than those of commercial Pt/C. They are durable throughout 50000 cycles, showing only a 10 mV decay in half-wave potential. An in situ Raman analysis and theoretical calculations reveal that Pt3Co core nanocrystals modulate electron structures of the adjacent Pt single atoms to facilitate the intermediate absorption for fast kinetics. The superior durability is attributed to the shielding effect of the Pt-SAC coating, which significantly mitigates the dissolution of Pt3Co cores. The hybridizing strategy might promote the development of highly active and durable ORR catalysts.

(Keynote) Identification of Active Species and Mechanisms in Non-Precious Metal Oxygen Reduction Catalysts By Reductive Treatments and Magnetic Measurements

This talk addresses the Oxygen Reduction Reaction (ORR) utilizing Fe- and Co-based non precious metal (NPM) catalysts. There has long been uncertainty as to the active site for ORR in these materials due to the heterogeneity of the as-prepared catalyst. We report on two innovations directed at understanding the active sites in these materials. In the first, we interrogate the ORR activity in acid of a Fe porphyrin on different supports. While the activity is high when the Fe porphyrin is adsorbed on XC72 (a graphitic carbon), this activity is much lower when the porphyrin is adsorbed on either MoS2 or g-C3N4. Numerous physical characterization techniques show the electronic structure around the Fe center is the same for all three supports. Only the Fe porphyrin supported on XC72 exhibits a pH dependence in its ORR activity. This observation, coupled with the increased hydrophilicity of XC72 relative to the other supports, suggests that the support-electrolyte interaction controls the ORR activity. Modification of MoS2 to increase its hydrophilicity results in a more active ORR catalyst. In the second area, we use magnetometry and other techniques to examine the effect of two different reductive gas treatments on pyrolyzed ORR catalysts. We find that reductive gas treatments yield more superparamagnetic/ferromagnetic Fe species, with specific preparations yielding materials that consist almost entirely of reduced Fe (particularly Fe4N). Interestingly, catalysts exhibiting large percentages of superparamagnetic/ferromagnetic materials exhibit high activities for ORR catalysis as high as those reported in the literature.

Optimization of Synthesis Variables Towards Improved Activity and Stability of Fe-N-C PGM-Free Catalysts

Materials in the Fe-N-C family are the most promising platinum group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs).1-3 Although significant progress has been made in recent years in improving both the ORR activity and durability of the Fe-N-C catalysts, further improvements are needed, especially in long-term performance durability in hydrogen-air PEFCs, to enable their use in applications such as propulsion power for light-duty vehicles.3 The most active ORR catalysts in the Fe-N-C family were synthesized by heat treating iron salts or other iron-containing compounds with zinc-based zeolitic imidazolate frameworks (ZIFs) and/or phenanthroline (as carbon and nitrogen sources), or by heat treating iron-substituted ZIFs. For this family of PGM-free materials, it has been shown that many synthesis variables, such as the metal and carbon-nitrogen macrocycle content, the heat treatment temperature, atmosphere, and temperature profile all affect the activity and durability of the resulting catalysts.4-7 Optimization of these variables and testing the resulting catalyst properties is not a trivial task, and only a limited portion of the composite composition and temperature space has been explored for this family of catalysts.To accelerate optimization of the synthesis variables to obtain improved ORR activity and stability for the Fe-N-C catalysts, high-throughput synthesis and characterization methods were developed and utilized. An automation platform, a multi-port ball-mill, and parallel fixed bed reactors in Argonne’s High-throughput Research Laboratory were used to rapidly synthesize the PGM-free catalysts with systematically-varied synthesis conditions. A multi-channel flow double electrode (m-CFDE) cell and other cells were designed and constructed for the simultaneous testing the ORR activity and stability of the multiple catalysts synthesized. The ORR activity and stability of the catalysts were correlated with their Fe speciation, as determined using Fe K-edge X-ray absorption spectroscopy (XAFS), electrochemically-determined surface areas, and other variables, which is beneficial for the further improved catalyst activity and stability.References B. Pivovar, Nature Catalysis, 2 (2019) 562.S. Thompson and D. Papageorgopoulos, Nature Catalysis, 2 (2019) 558.L. Osmieri, J. Park, D.A. Cullen, P. Zelenay, D.J. Myers, and K.C. Neyerlin, Curr. Opin. Electrochem., 25 (2021) 100627.X. Wang, H. Zhang, H. Lin, S. Gupta, C. Wang, Z. Tao, H. Fu, T. Wang, J. Zheng, G. Wu, and X. Li, Nano Energy, 25 (2016) 110.H. Zhang, S. Hwang, M. Wang, Z. Feng, S. Karakalos, L. Luo, Z. Qiao, X. Xie, C. Wang, D. Su, Y. Shao, and G. Wu, J. Am. Chem. Soc., 139 (2017) 14143-14149.E. Proietti, F. Jaouen, M. Lefevre, N. Larouche, J. Tian, J. Herranz, and J.-P. Dodelet, Nature Comm. 2 (2011) 1.A. Zitolo, V. Goellner, V. Armel, M.-T. Sougrati, T. Mineva, L. Stievano, E. Fonda, and F. Jaouen, Nature Materials, 14 (2015) 937. This work was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) under the auspices of the Electrocatalysis Consortium (ElectroCat). This work utilized the resources of the Advanced Photon Source, a U.S. DOE Office of Science user facility operated by Argonne National Laboratory for DOE Office and was authored by Argonne, a U.S. Department of Energy (DOE) Office of Science laboratory operated for DOE by UChicago Argonne, LLC under contract no. DE-AC02-06CH11357.

Aerogel-Derived Fe-N-C Catalysts for Oxygen Electro-Reduction. Linking Their Pore Structure and PEMFC Performance

Iron-based catalysts are intensely investigated for application at the cathode of proton exchange membrane (PEM) and, more recently, of anion exchange membrane fuel cells (AEMFC). Besides optimising their kinetic activity toward the oxygen reduction reaction (ORR), the design and control of their pore structure plays a key role in achieving the highest possible performance in fuel cell. Fe-N-C catalysts derived from the metal-organic framework ZIF-8 are highly active but also mainly microporous, which can lead to mass-transport issues. The silica templating approach typically leads to slightly less active Fe-N-C catalysts, but featuring a combination of micropores and mesopores, which can facilitate mass-transport. Aerogel methods for the synthesis of Fe-N-C catalysts with a hierarchical pore size distribution has been hitherto under-investigated. In contrast to the silica templating method, it is more environmental friendly since no dissolution of a template is needed (no HF). However, highly active Fe-N-C catalysts prepared by aerogel method have not yet been reported in PEM system (even if promising results were recently obtained in rotating disk electrode [1-2]), and it is therefore also unclear if the hierarchical pore size distribution derived from this method is beneficial for improving the mass-transport properties of Fe-N-C cathodes.This presentation will first report the synthesis method and the key parameters investigated, such as the ratio of melamine to resorcinol, the nature and amount of the iron precursor, the addition of an additionnal N-rich ligand, on their ORR activity. The materials were characterized for their texture (N2 sorption, Hg porosimetry), Fe coordination (X-ray absorption and Mössbauer spectroscopy) and surface chemistry (XPS) and morphology. The most promising materials were investigated in PEMFC, for their initial power performance and durability. The results show that, by playing with these synthesis parameters, the aerogel method can yield highly active ORR catalysts (Figure 1), and can give a control on their hierarchical porosity, especially in tuning the mesoporous volume and the average pore width of mesopores. Most catalysts display an atomic dispersion of Fe, explaining their high ORR activity. The presentation will particularly discuss the link between the pore size distribution in this set of materials (micro, meso and macropores), and the mass-transport properties in PEMFC, under pure O2 or air at the cathode. Their durability will also be discussed, from accelerated stress tests performed in rotating disk electrode and/or PEMFC.Acknowledgments:This study was financially supported by the French National Research Agency under the project ANIMA (ANR-19-CE05-0039).