Single Atomic Iron Catalysts for Oxygen Reduction in Acidic Media: Particle Size Control and Thermal Activation.

The high cost platinum (Pt) catalyst used for the electrocatalysis of oxygen reduction reaction (ORR) is one of the bottlenecks for the widespread deployment of polymer membrane electrolyte fuel cells (PEMFCs). Fe-N-C catalysts, produced by the pyrolysis of iron, nitrogen and carbon precursors together, have been found as one of the promising alternatives to replace Pt with abundant elements and low cost.1-3 The Fe-N coordination structure bonded with carbon, generated during high-temperature pyrolysis, has been identified as the active site for the high ORR activity of Fe-N-C catalysts. However, it remains a grand challenge to prepare Fe-N-C catalysts with abundant Fe-N active sites to achieve high ORR activity, since undesired inactive species (e.g., Fe/Fe3C) are easily obtained during high-temperature pyrolysis due to the improper design and the poor chemistry control in the integration of all precursors, lowering the active sites density of catalysts. We have developed an approach to synthesize Fe-N-C catalysts with exclusively atomic Fe-N active sites instead of inactive Fe agglomeration by using well-defined Fe-containing metal-organic frameworks (MOFs) precursors.4 The morphology of MOF precursors can be directly transferred to Fe-N-C catalysts with the retained and homogenous morphology after pyrolysis. Such well-defined precursors and resulting homogenous catalysts allow us to precisely control and tune the composition and morphology of final Fe-N-C catalysts to understand how the property change of catalysts impact their ORR activity of catalysts. In this presentation, we will discuss the effect of particle size of catalysts on their ORR activity and the critical role of pyrolysis temperature on the formation of Fe-N active sites. The Fe-N-C catalysts with size from 20 nm to 1000 nm are prepared by adjusting the size of MOF crystals in the precursor synthesis. Similar to Pt nanoparticles, the unique size control of the Fe-N-C catalysts enables us to increase the accessible number of Fe-N active sites for ORR. The 50 nm catalyst shows the best ORR activity with a half-wave potential of 0.85 V vs. RHE, only leaving 30 mV gap with Pt/C (60 µgPt/cm2) in 0.5 M H2SO4 along with the excellent stability. When the particle size of catalyst is reduced to 20 nm, significant agglomeration of particles are found in the catalyst, resulting in ORR activity decrease of the catalyst. Using our homogenous model catalysts, the formation of active sites during pyrolysis is investigated by correlated the measured-ORR activity with the bonds change of precursors at various pyrolysis temperature. 800 oC is found to be the critical temperature to form the Fe-N active sites with notable ORR activity, which is related to the generation of new Fe species likely bonded with pyridinic N in the carbon structure. These high performance Fe-N-C catalysts exhibit a promising potential to replace Pt for ORR in future PEMFCs. (1) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Science 2011, 332, 443. (2) Zhang, H.; Osgood, H.; Xie, X.; Shao, Y.; Wu, G., Nano Energy, 2017, 31, 331-350. (3) Wu, G.; Santandreu, A.; Kellogg, W.; Gupta, S.; Ogoke, O.; Zhang, H.; Wang, H.-L.; Dai, L., Nano Energy, 2016, 29, 83–110. (4) Zhang, H.; Hwang, S.; Wang, M.; Feng, Z.; Karakalos, S.; Luo, L.; Qiao, Z.; Xie, X.; Wang, C.; Su, D.; Shao, Y.; Wu, G. Journal of the American Chemical Society, 2017, 139, 14143–14149.

Significant progress has been made in platinum group metal (PGM)-free oxygen reduction reaction (ORR) catalysts derived from Fe. However, Fenton chemistry due to the presence of Fe and peroxide in electrodes causes serious degradation of ionomer and membrane, limiting the use of these Fe-based catalysts in proton exchange membrane fuel cells (PEMFCs). Therefore, PGM-free catalysts that are also free of Fe are urgently needed to enable durable and inexpensive PEMFCs. Here, we report a new type of highly dispersed nitrogen-coordinated single-atom Co site catalyst. This catalyst is derived from Co-doped metal-organic frameworks through a one-step controlled thermal activation. A combination of aberration-corrected electron microscopy couple with electron energy loss spectra and X-ray absorption spectroscopy clearly verify the co-location of Co and N at the atomic level in the form of CoN4. Such a catalyst with properly controlled Co doping content and thermal activation achieved comparable activity to state of the art Fe-based catalysts, showing a respectful half-wave potential of 0.80 V vs. RHE in acids, only 60 mV lower than Pt catalysts (60 mgPt/cm2) in challenging acidic media. Exceptional stability was observed in both potential cycling and constant potential (e.g., 0.7 V) tests. The high ORR performance is attributed to the presence of well-dispersed single CoN4 active sites embedded in the porous carbon matrix without formation of inactive Co aggregates. Fuel cell tests further confirmed that the intrinsic high ORR activity and stability translate to a high-performance cathode in PEMFCs. This atomic single Co site catalyst is a promising step toward replacement of Fe in PGM-free catalysts for advanced fuel cell technologies.

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.

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

The overall objective of this work was development proton exchange membrane (PEM) fuel cell catalysts for the oxygen reduction reaction (ORR) having ultra-low Pt loading with low cost, high performance and durability. The approach was based on development of a catalyst bridging the current platinum group metal (PGM) catalyst technology and the non-platinum cathode catalyst developed at USC. To balance the cost and the limited supply of platinum, the ultra-low platinum alloy project at (USC) developed highly stable and kinetically-activated carbon composite supports (A-CCS) as well as highly-active and stable hybrid cathode catalyst (HCC), Pt*/A-CCS (Pt* = Compressive Pt-lattice catalyst) that retain their activity after accelerated testing to simulate startup/shutdown and drive cycle potential cycling thus accomplishing the DOE 2017 targets. The HCC technology is based on a two-step patented process. 1-10 In order to develop a hybrid catalysts, in the first step, the following major constraints in the PEM fuel cells interfaces were addressed : (a) chemical and electrochemical stability of the support at high potentials, low pH and high temperature and (b) the support onset potential and kinetic activity for oxygen reduction reaction to be similar to that of a platinum catalyst. To accomplish these requirements, the A-CCS was synthesized with optimized: (i) BET surface area, porosity, pore-size and distribution and active sties for ORR (ii) hydrophilic/hydrophobic ratio, (iii) structural properties (amorphous/crystalline ratio). In the second step, compressive Pt-lattice catalyst (Pt*) was synthesized through a USC-developed annealing procedure that controls the particle size during annealing and forms monolayers of Pt* by diffusing Co atoms present in the support into Pt which is deposited on A-CCS support. In this step, Pt/Pt*-support interaction was optimized through inclusion of active surface functional groups, and through optimization of transition metal content in the alloy necessary for the formation of compressive Pt-lattice catalyst. The Pt*/A-CCS shows high mass activity (0.41 A/mg Pt), excellent support stability, and enhanced catalyst durability under accelerated stress test (AST) conditions. Initial mass activity, mass activity and ECSA loss after 30K cycles (0.6 -1.0 V- catalyst durability), potential loss after 30 K cycles (0.6-1.0-catalyst durability), potential loss after 5 K cycles (1.0-1.5V-support stability) and mass activity and ECSA loss after 5 k cycles (1.0-1.5V-support stability) will be compared in the presentation with 2020 DOE Technical targets for PEM catalyst and supports. References Ákos Kriston, Andreas Pfrang, Branko N. Popov, and L. Boon-Brett, “Development of a Full Layer Pore-Scale Model for the Simulation of Electro-Active Material Used in Power Sources,” Electrochem. Soc., 161 (2015) E3235-E3247.Kim, T. Xie, W.S. Jung, F. Gadala-Maria, P. Ganesan, B.N. Popov, Development of Catalytically Active and Highly Stable Catalyst Supports for Polymer Electrolyte Membrane Fuel Cells, J. Power Sources, 273, (2015) 761-774.Wonsuk Jung, Tianyuan Xie, Taekeun Kim, Prabhu Ganesan, Branko N. Popov, Highly Active and Durable Co-Doped Pt/CCC Cathode Catalyst for Polymer Electrolyte Membrane Fuel Cells, Acta, 167, (2015) 1-12.Takeun Kim , Branko N. Popov, Development of highly-active and stable Pt/C Catalyst for Polymer Electrolyte Membrane Fuel Cells Under Simulated Start up/Shut Down Cycling, International Journal of Hydrogen Energy, 41, (2016) 1328-1336.N. Popov, Jong-Won Lee, Sehkyu Park, Chapter Electrocatalyst for Low Temperature Fuel Cells,” John Willey and Sons, VCH, RE 9783527341320 (2016).Taekeun Kim, Tianyuan Xie, Won Suk Jung , Branko N. Popov, Development of Ultra-low Highly Active and Durable Hybrid Compressive Platinum Lattice Cathode Catalyst for Polymer Electrolyte Membrane Fuel Cells, International Journal of Hydrogen Energy, 42, (2017), 12507-12520.Won Suk Jung, Branko N. Popov, Improved Durability of Pt Catalyst Supported on N-doped Mesoporous Graphitized Carbon for Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells, Carbon, 122 (2017) 746-755.Won Suk Jung and Branko N. Popov, New Method to Synthesize Highly Active and Durable Chemically Ordered fct-PtCo Cathode Catalyst for PEMFC’s, ACS Appl. Mater. Interfaces, (2017), 9, 23879-23686.Won Suk Jung and Branko N. Popov, Hybrid cathode catalyst with Synergistic effect Between Carbon Composite Catalyst and Pt for Ultra –low Pt Loading in PEMFC’s, Catalysis Today, 295, (2017) 65-74.\\Won Suk Jung and Branko N. Popov, Effect of Pretreatment on Durability of fct-Structured Pt-Based Alloy Catalyst for Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells, ACS Sustainable Chem. and Eng, (09)(2017) DOI: 10.1021.

ZIF-8-derived nanocarbon composite-based highly active platinum group metal-free bimetallic electrocatalysts for oxygen reduction reaction in proton exchange membrane fuel cells

ConspectusProton-exchange membrane fuel cells (PEMFCs) are efficient and clean hydrogen energy technologies for transportation and stationary applications. Highly active and durable low-cost cathode catalysts for the oxygen-reduction reaction (ORR) under challenging acidic environments are desperately needed to address the cost and durability issues of PEMFCs. The most promising platinum group metal (PGM)-free catalysts for the ORR in acidic media are atomically dispersed and nitrogen-coordinated metal site catalysts denoted as M–N–C, M = Fe, Co, or Mn. Due to significant efforts in the past few decades, these catalysts have demonstrated much-improved ORR activity and promising initial fuel cell performance approaching traditional Pt/C catalysts. However, the insufficient long-term stability (up to 5000 h) under PEMFC operation represents a primary technical barrier to making current PGM-free catalysts less viable yet in PEMFCs. In this Account, we highlight recent advances in synthesizing efficient PGM-free catalysts for the ORR in PEMFCs, emphasizing effective strategies to improve mass and intrinsic activity and the possible degradation mechanisms. In particular, a chemical doping method based on the zeolitic imidazolate framework (ZIF)-8 represents the key to developing efficient M–N–C catalysts containing atomically dispersed and nitrogen-coordinated single metal active sites (i.e., MN4). The newly acquired understanding of the formation mechanism of MN4 active sites during the thermal activation and its correlation to catalytic properties guide the rational catalyst design rather than relying on current trial-and-error approaches. Considerable efforts have further been invested in increasing the active site density and enhancing intrinsic activity by regulating carbon-phase structures and the local coordination environment. These highly active catalysts usually suffer from significant activity loss during the ORR. Therefore, breaking the activity–stability trade-off is the key to simultaneously achieving activity and stability in one catalyst, which is discussed on the basis of our recent successes in regulating local carbon structures surrounding active single metal sites. Significant research efforts toward understanding the degradation mechanisms and improving the lifetime of PGM-free catalysts are still crucial for viable applications in the future. Novel electrode designing strategies are needed to translate the PGM-free catalysts’ ORR activity to solid-state electrolyte-based membrane electrode assemblies (MEAs) with robust three-phase (i.e., gas–liquid–solid) interfaces for efficient charge and mass transports for performance improvement. On the basis of our effort at the University at Buffalo supported by ElectroCat Consortium associated with U.S. DOE’s Hydrogen and Fuel Cell Technologies Office, we provide a perspective on PGM-free cathode catalysts concerning remaining bottlenecks and future opportunities, aiming to inspire the community in both mechanistic understanding and technological development.

To significantly reduce the cost of proton exchange membrane (PEM) fuel cells, current Pt must be replaced by platinum-metal-group (PGM)-free catalysts for the oxygen reduction reaction (ORR) in acid [1-5]. We presented a new class of high-performance atomic iron carbon catalysts through controlled chemical doping of iron ions into zinc-zeolitic imidazolate framework (ZIF), a type of metal-organic framework (MOF). The novel synthetic chemistry enables accurate size control of Fe-doped ZIF catalyst particles with a wide range from 20 to 1000 nm without changing chemical properties, which provides a great opportunity to increase the density of active sites that is determined by the particle size. We elucidated the active site formation mechanism by correlating the chemical and structural changes with thermal activation process for the conversion from Fe-N4 complex containing hydrocarbon networks in ZIF to highly active FeNx sites embedded into carbon. A temperature of 800oC was identified as the critical point to start forming pyridinic nitrogen doping at the edge of the graphitized carbon planes. Further increasing heating temperature to 1100oC leads to increase of graphitic nitrogen, generating possible synergistic effect with FeNx sites to promote ORR activity. The best performing catalyst, which has well-defined particle size around 50 nm and abundance of atomic FeNx sites embedded into carbon structures, achieve a new performance milestone for the ORR in challenging acidic media including a half-wave potential of 0.85 V vs RHE. This has been approaching to Pt and holds great promise for future PEM fuel cells.

Low cost and high performance cathode catalysts for oxygen reduction reaction (ORR) in acidic Nafion® based polymer electrolyte fuels still remain a grand challenge in the field.1-5 Here, we present an atomic iron-dispersed carbon catalyst with homogeneous microstructure. The iron atoms were found atomically embedded into partially graphitized carbon. Due to the significantly improved uniformity and increased density of active sites, the ORR activity of this new catalyst reached to a new milestone, showing a half-wave potential of 0.84 V vs RHE in 0.5 M H2SO4 by rotating disk electrode along with fuel cell performance (0. 044 A/cm2 at 0.87 ViR-free in a H2-O2 cell) and stability at a practical operation voltage of 0.7 V. The high-performance platinum group metal (PGM)-free catalyst is derived from a well-defined iron doped metal-organic framework (e.g., zeolitic imidazolate framework, ZIF-8) precursor with ordered crystal structure through a single carbonization step in N2 atmosphere. Obtaining the optimal doping content of Fe into the ZIF-8 during the precursor synthesis played a key role in achieving the atomically dispersed iron morphology associated with the improvement of activity and stability. Unlike previous studied Fe-based catalysts, the particle size and shape of this catalyst were well-controlled and highly homogeneous and can be directly transferred from the morphology of Fe-doped ZIF precursors. Notably, higher Fe doping contents yielded larger crystal sizes in the precursors. Furthermore, optimal iron doping content is crucial for activity and stability enhancement, which correlated to iron distribution, carbon structures, surface areas/porosity, and nitrogen doping. Doping high Fe content is desirable to provide more active sites. However, atomic iron tends to agglomerate and form clusters when Fe content is above 5 at%. Higher Fe content yields highly graphitized carbon, but mitigates the total pore volumes, which inhibits active formation in micropores and mass transfer through meso/macro pores. Clearly, doped Fe content affects the critical nitrogen doping including the total content and doping position. In principle, high Fe content in catalyst should coordinate more nitrogen. Oppositely, instead of coordinating with nitrogen, Fe just tends to form inactive metallic iron and iron carbides. Therefore, one future focus is to develop effective solution synthesis to chemically doped Fe into MOFs capable of forming atomic iron distribution in carbon, rather than aggregates. This work provides evidence that superior ORR activity arises from atomically and homogenously dispersed FeNx active sites located in the carbon fringes. The elucidation of structure-property correlations revealed that Fe content in the precursor is the foremost variable in controlling the synthetic chemistry of these catalysts, and affects various properties including particle size, porosity, graphitization, and nitrogen-doping. Most importantly, this work strongly supports the theoretical prediction that iron-based catalysts are able to achieve comparable catalytic activity of Pt in the highly challenging environment of acid media.

Developing durable and highly active low-Pt electrocatalysts for oxygen reduction reaction (ORR) in acidic media is necessary for highly effective polymer electrolyte membrane fuel cells (PEMFCs). Intermetallic structure is thermodynamically more stable than random alloy metal compounds. In oxygen reduction reaction (ORR), Pt3Ni1 with (111) facet is to be the most effective electrocatalyst for ORR. Therefore, to develop a highly efficienct electrocatalyst for oxygen reduction reaction (ORR) in effective polymer electrolyte membrane fuel cell (PEMFC), an ordered PtNi/C intermetallic alloy on a carbon black support has been designed. To enhance ORR performance and reduce Pt usage, a Pt-transition metal alloy catalyst has been developed. Alloying Pt with a second metal on a carbon support material can improve activity due to the change in lattice parameter and the electronic effect. According to the d-band theory, the downshift effect of the d-band induced by the upshift of the fermi level is predicted to improve ORR activity by weakening the binding energy. In case of alloying Pt and Ni, it can be predicted that the difference in electronegativity between Pt and Ni will induce a fermi level shift, resulting in a d-band center shift effect, which can enhance ORR activity. Compared to random alloy structured electrocatalysts, intermetallic structures of ordered PtNi were more thermodynamically stable. Moreover, intermetallic structure stabilizes the alloy, improving the durability of metal components against chemical corrosion in acidic media, such as HClO4 or H2SO4. Therefore, to improve catalyst’s durability, heat treatment was used to align the structure, making it structurally stable and reducing Pt-Pt distance. Heat treatment screening was carried out between 500℃ and 800℃ using a thermal shock method to minimize particle size growth. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and rotating disc electrode (RDE) were used to investigate the compositional difference and crystallinity of the intermetallic structure, electrochemical ORR activity and durability between disordered PtNi/C (d-PtNi/C) and ordered PtNi/C (o-PtNi/C). The electronic effect of the three Nicore@Ptshell models, namely ordered Nicore@Ptshell (o-PtNi), disordered Nicore@Ptshell (d-PtNi), and pure Pt147 (pure Pt), was estimated through density functional theory (DFT). The d-band theory and Bader charge analysis were used to estimate ORR activity and charge transfer, respectively. Additionally, the durability in acidic media of each model was estimated through dissolution potential calculations (Udiss). This work has supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT in Republic of Korea (MSIT) (NRF-2022R1A2C2093090). This work was supported by the Technology Innovation Program (20019175) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). This research was supported by the Ministry of Trade, Industry, and Energy (MOTIE), Korea, under “Digital manufacturing platform" (No. P0022331) supervised by the Korea Institute for Advancement of Technology (KIAT).

For large-scale commercialization of polymer electrolyte membrane fuel cells (PEMFCs), a significant reduction or, ultimately, replacement of platinum as cathode catalyst for the oxygen reduction reaction (ORR) is of great interest. As a first step, the similarity of palladium and platinum makes Pd an often-discussed alternative material for the electrocatalytic reduction of oxygen. Moreover, apart from Pt, Pd is located on the very top of the well-known volcano plot for the ORR activity of pure metals published by Nørskov et al. (1). However, materials for the oxygen reduction in acidic media have to address both high catalytic activity and long-term stability. Especially voltage cycling stability is an important factor for technical applications. Here, a high tolerance towards dynamic fuel cell operating conditions is required. Although Pd has been known to be less stable against dissolution compared to Pt, e.g., shown in a recent study by Cherevko et al.(2), a comparative study about the effect of Pd particle size is yet outstanding. In this work, a systematic study of both ORR activity and stability against degradation during voltage cycling is conducted on Pd nanoparticles supported on carbon black (Pd/C), on unsupported Pd-black, and on conventional Pt/C as comparison, using commercial catalysts with various particle sizes. First, the activity for the ORR is evaluated in a rotating (ring) disk electrode experiment, indicating a 5-7 fold lower ORR mass activity at 0.85 V vs. the reversible hydrogen electrode (VRHE) of morphologically similar Pd/C compared to Pt/C. However, the ORR mass activity is a function of the initial electrochemically active surface area (ECSA) of the studied Pd catalysts, with a rather strong dependency for small ECSAs and little dependence at higher ones. Accelerated voltage cycling between 0.5 and 1.0 VRHE is conducted in order to correlate catalyst degradation properties with respect to initial ECSA. To monitor the degradation process, the electrochemically active surface area of the Pd catalysts is evaluated frequently during the accelerated voltage cycling. Additional information on the ECSA degradation is gathered by comparing particle size distributions, determined by transmission electron microscopy (TEM), of pristine catalyst material to that of cycled electrodes. Figure 1 shows cyclic voltammograms taken at various times during accelerated voltage cycling of a 40 wt.% Pd/C catalyst. A clear trend towards degradation in both Hupd (<0.4 VRHE) and (hydr-)oxide (>0.55 VRHE) region can be observed. Adversely, currents in the electrochemical double layer region (0.4 VRHE ≤ E ≤ 0.55 VRHE) stay unaffected, indicating a loss of active Pd surface without significant corrosion of the carbon support over the measured timescale. Both the ORR activity and the voltage cycling stability depend on the initial ECSA in opposing directions. In this work, we thus address the question, whether Pd catalysts with initially large particle size may offer substantially better corrosion stability without significant loss in ORR mass activity.

Two-Dimensional Metal–Organic Frameworks with Unique Oriented Layers for Oxygen Reduction Reaction: Tailoring the Activity through Exposed Crystal Facets

Platinum group metal (PGM) catalysts are the major electrocatalysts for oxygen reduction reaction (ORR) in the polymer electrolyte membrane fuel cells (PEMFCs). However, the cost of PGM catalysts is very high. Particular, the cost becomes unaffordable if the PEMFC is in massive application. In order to remove this cost obstacle of fuel cell commercialization, PGM-free catalysts have been considered as the replacement of PGM catalysts for ORR because of the low cost and the reasonable performance. Fe-C-N complex is the one of the most active centers in PGM-Free catalyst groups. This type of catalyst shows very promising activity in rotation disk electrode (RDE) testing. The half wave potential could reach 0.91 V versus standard hydrogen electrode (SHE). However, in a membrane electrode assembly (MEA), the performance of PGM-Free catalysts is not good enough to replace the PGM catalysts. Since the PGM-free catalysts are so different from the PGM catalysts in terms of catalytic activity, stability, surface conditions, particle size etc, the fabrication of PGM-Free catalyst MEA cannot simply copy the method of making PGM MEA. In addition, the thicknesses of catalyst layers of PFM-free are significantly thicker than that of PGM, for example, 10 times. We proposed a novel method of fabricating PGM-Free catalyst MEA, so that the intrinsic catalyst activity from RDE can be translated into MEA performance. The method is based on the catalyst coated membrane (CCM) method using optimized ionomer to carbon (I/C) ratio and solvent mixture of catalyst ink. Using this method, the PGM-free catalyst MEA achieved the current density 44.9 mA cm-2 at 0.9 ViR-free in H2/O2 and 150 mA cm-2 at 0.8 V in H2/air, which surpassed the performance targets of US Department of Energy (DOE) for PGM-Free catalyst MEA. The property (solvent composition, dispersion of catalyst and ionomer in an ink), structure (pore structure) and the MEA performance have been characterized using ultra-small angle x-ray scattering (USAXS), cyro-TEM, mercury intrusion porosimetry (MIP), SEM/EDAX, RDE and MEA testing. A property-structure-performance relationship has been established.

Improving the activity and stability of Fe/N/C catalyst in oxygen reduction reaction (ORR) is a huge challenge in the commercial application of polymer electrolyte membrane fuel cells (PEMFCs). In the past decade, there have been significant break‐throughs in the performance of transition metal catalysts, but little progress has been made in their stability. Herein, a zinc‐based zeolite imidazole framework (ZIF‐8) and tungsten carbide engaged strategy was reported to prepare Fe/N/C catalyst. Particularly, physical vapor deposition (PVD) was used to trap tungsten carbide nanoparticles with particle size of less than 3 nm limited into the FeNC catalytic micropores to synthesis composite catalyst (WC@FeNC). Compared with original Fe/N/C cata‐lysts, confined WC nanoparticles in Fe/N/C porous has improved the ORR activity (2.7 mA mg−1 vs. 2.2 mA mg−1 at 0.85 V vs. RHE) as well as stability (decay 18.7 mV vs. 21.6 mV after 10 h charged) in 0.1 M H2SO4. This work puts forward some unique insights for improving the stability of transition metal oxygen reduction catalysts.

The development of low cost and high performance cathode for the oxygen reduction reaction (ORR) remains a grand challenge for transportation applications of polymer electrolyte membrane fuel cells (PEMFCs). Fe and Co-based platinum group metal (PGM)-free catalysts have been previously studied, however facing poor long-term durability and may potentially accelerate membrane degradation caused by oxygen radicals1. To overcome the above weakness, Li et al.2 has developed Mn based PGM-free catalyst which can mitigate membrane degradation, meanwhile showing a competitive catalyst activity compared to Fe based catalyst. The activity of the Mn-N-C catalysts measured using rotating disk electrodes (RDEs) in acidic electrolytes approaches state-of-the-art Fe-N-C catalysts3. More importantly, the Mn-N-C catalysts have demonstrated enhanced stability using potential cycling (0.6-1.0 V) in O2-saturated acidic electrolytes. In addition to the Mn-based PGM-free catalyst development, the electrode design is also critical for the MEA performance. The oxygen reduction reaction (ORR) may occur at different interface in the catalyst layers for PGM and PGM-free catalysts. For PGM catalysts, the ORR takes place on the Pt/ionomer interface, while Pt is on the surface of the catalyst support. However, since the Mn active sites are embedded inside the metal organic framework (MOF), it is more difficult to establish the catalyst/ionomer interface PGM-free catalyst. Hereby, the ionomer with lower equivalent weight (EW=830) has been proposed to replace the conventional Nafion ionomer (EW=1100). The shorter chain of the low EW ionomer can penetrate into the micro-pores in the catalyst layer easier, which may promote their interaction with the catalyst active sites. In addition to the low EW ionomer, the ionomer to carbon (I/C) ratio also has a great impact on the proton, gas and water transports that can further affect the MEA performance. The particle size of the catalyst also affects the MEA performance. In the RDE studies, the Mn catalyst with a smaller average particle size (50 nm) shows a better ORR activity than the one with a larger average particle size (80nm). However, in the MEA studies, catalysts with larger average particle size seems to demonstrate better performance, likely due to optimal pore structures of electrodes. This work will provide a systematical understanding and guidance of the MEA design not only for Mn-based catalyst but also for other PGM-free catalysts. Acknowledgement: The project is financially supported by the Department of Energy’s Fuel Cell Technology Office under the Grant DE-EE0008075. Reference: Wang, X. X., Prabhakaran, V., He, Y., Shao, Y., Wu, G., Adv. Mater. 2019, 1805126.Li, J.; Chen, M.; Cullen, D. A.; Hwang, S.; Wang, M.; Li, B.; Liu, K.; Karakalos, S.; Lucero, M.; Zhang, H.; Lei, C.; Xu, H.; Sterbinsky, G. E.; Feng, Z.; Su, D.; More, K. L.; Wang, G.; Wang, Z.; Wu, G., Nature Catalysis 2018, 1 (12), 935-945.Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P., Science 2011, 332 (6028), 443-447.