Direct atomic-level insight into the active sites of a high-performance PGM-free ORR catalyst

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Science  04 Aug 2017:
Vol. 357, Issue 6350, pp. 479-484
DOI: 10.1126/science.aan2255

Replacing platinum in air-fed fuel cells

Replacing expensive and scarce platinum catalysts in polymer electrolyte membrane fuel cells for the oxygen reduction reaction (ORR) with ones based on non-noble metals would speed up the adoption of hydrogen fuel vehicles. Most of the candidate replacement catalysts that have shown high performance do so only when running on pure oxygen. Chung et al. developed an iron-nitrogen-carbon catalyst from two nitrogen precursors that forms a high-porosity structure and exhibits high ORR performance when running on air. The proposed catalytically active site is FeN4.

Science, this issue p. 479


Platinum group metal–free (PGM-free) metal-nitrogen-carbon catalysts have emerged as a promising alternative to their costly platinum (Pt)–based counterparts in polymer electrolyte fuel cells (PEFCs) but still face some major challenges, including (i) the identification of the most relevant catalytic site for the oxygen reduction reaction (ORR) and (ii) demonstration of competitive PEFC performance under automotive-application conditions in the hydrogen (H2)–air fuel cell. Herein, we demonstrate H2-air performance gains achieved with an iron-nitrogen-carbon catalyst synthesized with two nitrogen precursors that developed hierarchical porosity. Current densities recorded in the kinetic region of cathode operation, at fuel cell voltages greater than ~0.75 V, were the same as those obtained with a Pt cathode at a loading of 0.1 milligram of Pt per centimeter squared. The proposed catalytic active site, carbon-embedded nitrogen-coordinated iron (FeN4), was directly visualized with aberration-corrected scanning transmission electron microscopy, and the contributions of these active sites associated with specific lattice-level carbon structures were explored computationally.

The widespread integration of polymer electrolyte fuel cells (PEFCs) into vehicles will require substantial reductions in overall stack cost (1). The main stack cost contributor (~46%) is the expensive Pt-based electrocatalyst (2). The oxygen reduction reaction (ORR) at the cathode is inherently slower by six orders of magnitude than the hydrogen oxidation reaction at the anode (3) and contributes more to this cost. Thus, the implementation of high-performance and durable platinum group metal–free (PGM-free) ORR catalysts could greatly enable large-scale commercialization of fuel cell–powered vehicles.

Toward this goal, heat-treated metal-nitrogen-carbon (M-N-C) catalysts have been developed where the metal component is usually inexpensive and earth-abundant Fe or Co (46). A carbon-hosted FeN4 structure is the most commonly proposed active site, arrived at on the basis of both theory and experimental data from x-ray absorption, x-ray photoelectron spectroscopy, and Mössbauer spectroscopies (711), but this active site structure has not been directly observed and confirmed in the activated catalysts. Fuel cell performance has been improved through better control of the pore structure of PGM-free catalysts (12, 13) to improve mass transport properties, as well as accessibility of the active site(s). Serov et al. (13) incorporated commercial silica powder as a template in the high-temperature synthesis of M-N-C catalysts, followed by removal of the porosity-inducing template species with hydrofluoric acid. Others have used metal-organic frameworks (MOFs) as precursors for PGM-free catalysts, starting with a Co-imidazolate framework (14). Among several MOF approaches, only Zn-MOF–derived PGM-free catalysts demonstrated high fuel cell performance (11, 12, 15), mainly attributed to the catalyst porosity resulting from Zn evaporation during the heat treatment (12).

Despite improvements in fuel cell performance with these approaches, most M-N-C PEFC cathode studies have been performed under H2-O2 conditions. The use of O2 likely masks the true effects of concentration polarization, commonly observed in H2-air cells (stacks) for automotive applications. Thus, demonstration of high PGM-free catalyst performance under practical H2-air conditions is required to validate such catalysts for practical systems.

Previously, our group has developed high-activity Fe-N-C catalysts derived from polymers and simple organic compounds, such as polyaniline (PANI) (1618) or cyanamide (CM) (19, 20). Here, PANI and CM precursors were deliberately combined to synthesize a catalyst that exhibits both hierarchical pore structures and remarkable activity, reflected by the current densities recorded in the kinetic region of the air cathode operation, at fuel cell voltages greater than ~0.75 V. These current densities are the same as those obtained with a Pt cathode at a loading of 0.1 mgPt cm−2. Atomic-level images of FeN4 active sites were obtained with low-voltage (60 kV), aberration-corrected scanning transmission electron microscopy (AC-STEM). Electron energy-loss spectroscopy (EELS) with single-atom resolution confirmed such FeN4 structures embedded within carbon basal planes. However, considerably higher concentrations of dispersed Fe single atoms were observed along the surfaces of graphitic domains or step-edges in multilayer graphene. On the basis of the prevalence of such edge-hosted structures observed in the catalyst, these specific configurations were explored by using theoretical approaches, which pointed to a relatively high ORR activity for multilayer graphene (basal-plane) edge-hosted FeN4 structures. Notably, these structures can be operational in the catalysis of a variety of other electrochemical reactions, some of which have already been reported to be very promising (21).

In the synthesis of the dual nitrogen–precursor catalyst, aniline and CM were first dissolved in 1.5 M HCl solution, followed by the addition of iron (III) chloride as the iron precursor and ammonium persulfate as oxidant for the oxidative PANI polymerization. The solution was stirred at room temperature for ~4 hours to allow full polymerization of aniline and then heat treated (22). After completion of the (CM+PANI)-Fe-C catalyst synthesis, a surface area of ~1500 m2 g−1 was achieved, as determined by the Brunauer-Emmett-Teller (BET) method. The volumes of mesopores (pore diameter 2 to 50 nm) and micropores (pore diameter <2 nm), as measured with nitrogen adsorption measurements, were ~0.25 and ~0.61 cm3 g−1, respectively (Fig. 1A). In their extensive PGM-free catalyst rotating disk electrode (RDE) studies involving five different laboratories and four different syntheses, Jaouen et al. (23) found that micropore surface area was the primary factor governing the ORR activity of PGM-free catalysts. The micropore surface area of the (CM+PANI)-Fe-C was ~1600 m2 g−1, which would predict high ORR activity on the basis of Jaouen et al.’s findings. PANI-Fe-C, i.e., the catalyst obtained without using CM as a pore former, was much more aggregated (Fig. 1B) than the combined (CM+PANI)-Fe-C catalyst (Fig. 1C), which contained fewer pores with diameter >50 nm and a BET surface area of only ~1000 m2 g−1. These morphological features depict what we refer to as a “hierarchical pore structure.”

Fig. 1 Hierarchical pore structure.

(A) Micropore and mesopore size distributions. The micropore size distribution had peaks associated with three pore widths: 0.8, 1.1, and 1.5 nm. dV/dlog (D) is the differential pore volume distribution, where V is pore volume and D is pore diameter. (B and C) SEM images of PANI-Fe-C and (CM+PANI)-Fe-C catalysts, respectively, demonstrating the effect of CM in macropore formation.

We attribute the resulting hierarchical pore structure of (CM+PANI)-Fe-C to the relatively low CM-decomposition temperature compared to that of PANI. The decomposition of CM apparently formed gases responsible for pore formation in the PANI-rich phase at higher heat-treatment temperatures. This hypothesis is supported by the results of thermogravimetric analysis (TGA) in fig. S1, pointing to a drastic loss (~50% by weight) of the (CM+PANI)-Fe-C precursor between 100° and 300°C. The TGA of PANI-Fe-C precursors (fig. S1) showed a more gradual weight loss (no more than 20%) up to 300°C.

The vital role of macropores in providing greater accessibility to the catalytically active sites and establishing a more open framework for improving the ionomer distribution within catalyst layers was revealed by high-angle annular dark-field (HAADF)–STEM imaging of a microtomed cross section of a (CM+PANI)-Fe-C catalyst electrode (fig. S2, A and B). Figure S2B shows a fluorine energy-dispersive x-ray (EDX) spectroscopy elemental map acquired from the same area shown in fig. S2A. Fluorine EDX mapping is an established method to investigate ionomer distributions within a catalyst layer (24). Ionomer impregnated the macroporous catalyst regions (“C” in fig. S2, A and B) but not the dense catalyst regions (“D” in fig. S2, A and B) of the electrode. A stark difference in the ionomer distribution between porous and dense catalyst regions is highlighted in fig. S2, C and D.

Rotating ring-disk electrode (RRDE) testing in 0.5 M H2SO4 revealed a half-wave potential of 0.80 V versus the reversible hydrogen electrode (RHE), with a well-defined mass transport–limited current density (Fig. 2A). Ring-current values from RRDE measurements verified that the H2O2 yield remained <2.5% at all electrode potentials. This low H2O2 yield corresponds to an average electron-transfer number per O2 molecule (ne) of >3.95. The ORR activity and selectivity by means of RRDE rank among the highest for PGM-free catalysts reported (11, 13, 17, 25, 26). The catalyst performance loss after 30,000 RDE potential cycles was ~5% (fig. S3).

Fig. 2 Electrochemical and fuel cell performances.

(A) ORR performance of (CM+PANI)-Fe-C catalyst. Steady-state RDE polarization plots were obtained by using a 20-mV potential step and 25-s potential hold time at every step. Electrolyte 0.5 M H2SO4, temperature 25 ± 1°C, rotation rate 900 rpm. (B and C) H2-air fuel cell polarization plots. Cathode: ~4.0 mg cm−2 of (CM+PANI)-Fe-C; air 200 ml min−1 (2.5 stoichiometry at 1.0 A cm−2) and 760 ml min−1 (9.5 stoichiometry at 1.0 A cm−2); 100% relative humidity (RH); and 1.0 bar partial pressure. Anode: 2.0 mgPt cm−2 Pt/C; H2 200 ml min−1; 100% RH; and 1.0 bar partial pressure. Membrane Nafion 211, cell 80°C, electrode area 5 cm2. (D) H2-O2 fuel cell polarization plots. Cathode: ~4.0 mg cm−2 of (CM+PANI)-Fe-C; O2 200 ml min−1 (40 ml min–1 cm–2); 100% RH; 0.3, 1.0, and 2.0 bar partial pressures. Anode: 2.0 mgPt cm−2 Pt/C; H2 200 ml min−1; 100% RH; 1.0 bar partial pressure. Membrane Nafion 211, cell 80°C; 5 cm2 electrode area.

The catalyst’s hierarchical pore structure plays a crucial role in exposing a large fraction of graphite (002) basal-plane edges of the carbon phases comprising the catalyst to dioxygen. These surface-terminated basal planes are dominant in the carbon structures and are most notably associated with small, randomly oriented graphitic domains that form the predominant carbon phase, with Fe present at the exposed plane edges on the surfaces. The high ORR activity measured through RRDE testing cannot be directly correlated to high fuel cell performance because of the fundamental differences in operating environment, temperature, and electrode structures between RDE and fuel cells. Thus, evaluation of PGM-free catalysts under realistic fuel cell operation, i.e., H2-air conditions, is critical to assess performance from a practical standpoint, especially because the conventionally used H2-O2 test conditions do not capture the important effects of mass transport in the cathode.

Performance of (CM+PANI)-Fe-C cathode catalyst layers (CCLs) in the membrane electrode assembly (MEA) was investigated under realistic conditions for the cathode operated on air (Fig. 2B). The maximum power density reached with an electrode containing 35 weight % (wt %) Nafion ionomer was 0.39 W cm−2 at 1.0 bar partial pressure of H2 and air (the sum of the partial pressures of O2 and N2). Electrodes with increased ionomer content (50 and 60 wt % Nafion) were also fabricated and exhibited enhanced current densities at fuel cell voltages >0.7 V resulting from more effective filling and lining of macropores by the ionomer that improved catalyst utilization (27). However, the performance improvement at low current densities came at the expense of hindered water removal, manifesting itself as a performance decrease in the mass transport region (below 0.6 V). The current density of ~75 mA cm−2 at a reference voltage of 0.8 V [~90 mA cm−2, iR corrected (i, current; R, resistance), fig. S4A] was 1.5 times the highest current density reported to date, despite the use of much lower air pressure relative to previous studies (11). We speculate that the large number of macropores formed by the inclusion of CM during catalyst synthesis, coupled with an improved ionomer dispersion that resulted from the presence of these macropores in the CCL (fig. S2), contributed considerably to this superior H2-air performance.

By comparison, also shown in Fig. 2B is the performance of a 10 wt % Pt/C cathode (0.1 mgPt cm−2) prepared by a decal transfer method. Under identical test conditions, nearly the same current densities as those measured for (CM+PANI)-Fe-C catalyst were obtained in the kinetic region (>0.75 V). Compared with a state-of-the-art 50 wt % Pt/C MEA with a cathode Pt loading of 0.1 mgPt cm−2 (28), the Pt/C MEA data shown in Fig. 2B demonstrated the same performance in the kinetic region, albeit with lower current densities in the mass transport region, likely because of the much greater thickness of the 10 wt % Pt/C cathode, effectively illustrating the importance of catalyst-layer thickness on mass transport properties. The lower performance of the (CM+PANI)-Fe-C catalyst in the mass transport region was due to the thickness of the (CM+PANI)-Fe-C layer, which was approximately four times as thick as the 10 wt % Pt/C layer (on the basis of a total carbon loading of ~4 mg cm−2 for PGM-free versus 0.9 mg cm−2 for Pt/C). These results are corroborated by an improvement in fuel cell performance of the (CM+PANI)-Fe-C cathode with 35 wt % Nafion when the air flow was increased from 200 to 760 standard cubic centimeters per minute (Fig. 2C). Higher current densities were observed across the entire range of investigated voltages, including an increase in maximum power density from 0.39 to 0.42 W cm−2. All these results further validate that (CM+PANI)-Fe-C–based cathodes suffer from mass transport limitations, which must be addressed by further improvements to the intrinsic ORR activity of PGM-free catalysts to match the performance of Pt-based electrodes.

The performance of this (CM+PANI)-Fe-C MEA can be compared favorably to that of other MEAs reported in peer-reviewed literature for PGM-free catalysts (11, 29, 30) under realistic PEFC operating conditions. Among reported results, the highest power density of ~0.4 W cm−2 (11) was obtained at a pair of 2.5 bar that would be reduced by more than 30% if measured at a pair of 1.0 bar, the condition used throughout our testing, which was based on Serov et al.’s power density dependence on pressure measurements (29).

Experiments under H2-O2 conditions were also performed to minimize mass transport effects and achieve better insight into the true activity of the (CM+PANI)-Fe-C catalyst in the fuel cell (Fig. 2D, iR-corrected polarization plots shown in fig. S4B). Three different partial pressures of O2—0.3, 1.0, and 2.0 bar—were applied. At all of these pressures, the performance losses caused by mass transport were barely noticeable down to voltages as low as 0.2 V. This performance reflected much-improved mass transport within the catalyst layer versus H2-air conditions. The maximum power density values of ~0.87 and 0.94 W cm−2 reached at Embedded Image of 1.0 and 2.0 bar, respectively, were the highest ever achieved with PGM-free ORR catalysts operating on oxygen (31).

To better understand the source of high ORR activity, the atomic-level structure and chemistry of the (CM+PANI)-Fe-C catalyst were studied in detail by AC-STEM and EELS. Figure 3A shows a bright-field (BF)–STEM image of the overall morphology of the principal structures present in the (CM+PANI)-Fe-C catalyst. The catalyst consisted of primary fibrous carbon particles and secondary few-layer graphene sheets. Carbon tubes can be produced from CM (20, 32) and graphene-like structures produced from PANI (16, 17) in the presence of Fe, although no nanotubes were observed in the (CM+PANI)-Fe-C. These two carbon phases each contained similar amounts of N [3.5 atomic % (at %)] and Fe (0.2 at %), as determined by EELS. The nitrogen and iron contents within the near-surface regions of the catalyst measured by x-ray photoelectron spectroscopy (XPS) were 5.0 and 0.3 at %, respectively, which were slightly higher values than those obtained by highly localized STEM-EELS measurements. Analysis of the XPS N1s peak indicated the presence of pyridinic–, pyrrolic–, and graphitic–nitrogen bonded species (fig. S5). The dense fibrous carbon particles were composed of randomly oriented, intertwined, turbostratic graphitic domains on the order of a few nanometers in size, as shown in Fig. 3B and fig. S6A.

Fig. 3 STEM images and EEL spectra of the (CM+PANI)-Fe-C catalyst.

(A) BF-STEM image of a typical (CM+PANI)-Fe-C catalyst showing primary fibrous carbons and secondary graphene sheets. (B) Atomic-resolution HAADF-STEM image of Fe atoms distributed across the surface of fibrous carbon phase showing randomly oriented, intertwined graphitic domains. (C) HAADF-STEM image of individual Fe atoms (labeled 1, 2, and 3) in a few-layer graphene sheet. (D) EEL spectra of the N k-edge (Nk) and Fe L-edge (FeL) acquired from single atoms (1 and 2) and few-layer graphene (3), demonstrating the presence of N around the Fe atoms.

Because of the extremely high density of graphitic domains that terminate on the surfaces of the fibrous carbon particles, the surfaces of these particles were dominated by exposed edges and steps of graphite (002) basal planes. We consider the exposure of such basal-plane edges in the (CM+PANI)-Fe-C catalyst to be the most important factor contributing to its high ORR activity. Raman spectra for (CM+PANI)-Fe-C (fig. S7) confirmed an abundance of the (002) basal-plane edges as verified by a high D band–to–G band intensity ratio (ID/IG) (1.07). This value was used as a metric to evaluate structural disorder within the catalyst. A broad (002) peak in the x-ray diffraction pattern (fig. S8) also indicated a very small graphitic domain size coupled with turbostratic alignment of the basal planes within the domains.

Atomic-resolution STEM images of the fibrous phase (Fig. 3B and fig. S6B) showed single atoms dispersed across the carbon surface (dots exhibiting bright contrast), which were confirmed to be primarily Fe by EELS (fig. S6C). On the basis of the weak signal acquired in EEL spectra (due to data acquisition from a very limited area of ~1 to 2 Å2 combined with the instability of the individual Fe under the electron beam), it was not possible to determine the valence state of the individual Fe from EEL fine-structure analysis. However, according to Li et al.’s work (11), it is likely that atomic Fe is present both in Fe2+ and Fe3+ states. Besides being present as highly dispersed single atoms, excess Fe also existed in larger, isolated particulate form as either iron or iron sulfide (fig. S9), distributed randomly within the catalyst. These particulates were typically embedded or encased within few-layer graphene or graphitic shells. These structural data were consistent with results observed previously for heat-treated M-N-C catalysts (16, 20, 26) and are not expected to contribute to the catalytic activity of the catalyst. Individual Fe atoms were also observed to be embedded in the few-layer graphene sheet phase (Fig. 3C and fig. S10). The Moiré pattern originating from overlapping and rotated layers of the graphene honeycomb lattice is visible in these images.

The stacked, few-layers of graphene provided a stabilizing effect for these Fe atoms and allowed for more detailed spectroscopic analysis. A similar graphene-stabilizing effect was previously observed for MoS2 (33). EEL spectra obtained directly around the Fe (Fig. 3D) showed that N was associated with the Fe, which was otherwise absent in the surrounding graphene-only regions (Fig. 3D). High-resolution EEL spectrum imaging (fig. S11) confirmed this N-Fe association. Quantification of the Fe-to-N ratio from the EELS data acquired for several of these sites yielded an average composition of FeN4, consistent with previously proposed active sites for this type of PGM-free catalyst (7, 911). However, previous active-site determinations were based on bulk material analyses with Mössbauer, XPS, and x-ray absorption spectroscopy, which average the obtained signal, versus direct microscopic evidence for the formation of individual FeN4 complexes embedded in few-layer graphene, away from any exposed edges. Recently Fei et al. (34) observed atomic cobalt on nitrogen-doped graphene; however, the Co-N bond proposed was based on results from an indirect bulk technique, i.e., wavelet transform of extended x-ray absorption fine structure, as opposed to our direct observations of single complexes with STEM-EELS reported here.

It must be emphasized that the FeN4 complexes observed away from exposed basal-plane edges (bulk hosted) represent a relatively small fraction of the total Fe and N observed by STEM imaging of the catalyst; most of the highly dispersed Fe atoms were predominantly positioned at exposed basal-plane edges and steps (edge hosted) in both of the carbon phases in (CM+PANI)-Fe-C catalyst, fibrous carbon particles (primary phase), and few-layer graphene sheets (secondary phase). The tendency of Fe to occupy edge and step sites was further demonstrated for the (CM+PANI)-Fe-C catalyst in small regions of single-layer graphene identified within the mostly few-layer graphene sheets (fig. S12). However, the specific N coordination of these edge-positioned Fe atoms could not be easily verified with STEM-EELS because of the instability of these atoms under even the low-energy electron beam used (60 keV). Although many candidate active sites could account for the observed Fe distribution, edge-hosted FeN4 structures would simultaneously satisfy the indirect evidence of FeN4 structures identified by Mössbauer, XPS, and x-ray absorption spectroscopies, as well as the predominance of Fe observed at the basal-plane edges for both the fibrous and graphene-sheet phases present.

Previous theoretical studies (8, 35) suggest that edge-hosted FexNy structures had lower formation energies—i.e., an increased relative stability—than bulk-hosted structures. Thus, the equilibrium concentration of the edge-hosted structures is expected to be substantially higher than bulk-hosted structures. Because most of the Fe observed by AC-STEM imaging is present at the basal-plane edges and steps, and theoretical studies suggest higher concentration of edge-hosted FeNx structures, we used quantum chemical modeling to study the relative ORR pathways of edge-hosted FeN4 sites versus bulk-hosted FeN4 sites to gain further insight into the possible predominant active sites in the (CM+PANI)-Fe-C catalyst.

In addition to relative equilibrium concentrations, previous quantum chemistry studies of active site structures of M-N-C catalysts suggest that edge-adjacent Fe2N5 configurations have high ORR activity (36). Using the same methodology (22), we calculated the reaction pathways (fig. S13) and thermodynamic limiting potential, Ul, that serves as descriptor for relative ORR activity for the graphene bulk-hosted and nanoribbon zigzag edge-hosted structures. Similar to the previously reported edge-hosted Fe2N5 structure (36), both bulk-hosted (Fig. 4, A, C, and E) and zigzag edge-hosted (Fig. 4, B, D, and F) FeN4 structures spontaneously evolved an OH ligand at relevant potentials. With the OH ligand attached, the bulk-hosted FeN4 does not exothermically bind O2, and a bound OOH spontaneously dissociates to a bound O and free OH. These results suggest that such an OH-modified site may not act as a single reaction site on an associative ORR pathway. However, the zigzag edge-hosted FeN4 with OH ligand does bind O2 and, on an associative pathway, has a limiting potential of 0.80 V. This Ul is equal to the highest reported value (35, 36), in which the initial chemical adsorption of O2 was considered. Thus, quantum chemistry calculations show that the FeN4 structures follow different ORR reaction pathways, depending on whether they are hosted in the bulk or at the edge of the graphene, and that the edge-hosted sites, when spontaneously ligated by OH in the fuel cell environment, lead to highly ORR-active structures.

Fig. 4 Model structures used in theoretical studies with spontaneously formed OH ligand.

Views from above (A and B), from side (C and D), and from tilted perspective (E and F). (A), (C), and (E) show bulk-hosted FeN4 and (B), (D), and (F) show zigzag edge-hosted FeN4 structures with OH ligands. C, gray; Fe, bronze; H, white; N, blue; O, red.

Though FeN4 sites at the exposed basal-plane edges could not be directly discerned with AC-STEM, the considerably higher concentration of Fe associated with edges and steps observed by AC-STEM imaging combined with the quantum chemical calculations supported higher populations as well as higher ORR activities with the edge-hosted FeN4 compared with the bulk-hosted FeN4. Thus, we can plausibly suggest that edge-hosted FeN4 sites are likely the major contributors to the overall high activity observed in both the RDE and MEA testing of the (CM+PANI)-Fe-C catalyst.

Supplementary Materials

Materials and Methods

Figs. S1 to S13

References (3747)

DFT Data File S1

References and Notes

  1. See supplementary materials.
  2. H2-O2 fuel cell performance in this work was measured at a relatively low O2 flow rate of 40 ml min–1 cm–2. The maximum power density reported to date is ~0.9 W cm–2. It was obtained at Embedded Image of 1.0 bar and a high O2 flow rate of 260 ml min–1 cm–2 (12) and, independently, at a high Embedded Imageof 2.0 bar and high O2 flow rate of 80 ml min–1 cm–2 (15).
  3. Acknowledgments: This work was supported by the Office of Energy Efficiency and Renewable Energy of the U.S. Department of Energy (DOE) through the Fuel Cell Technologies Office. Microscopy was performed as part of a user project supported by Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Computational resources were provided by the Institutional Computing program of Los Alamos National Laboratory. We thank A. Dattelbaum and J. Spendelow (Los Alamos National Laboratory), R. Adzic (Brookhaven National Laboratory), G. Wu (University at Buffalo), and P. Atanassov (University of New Mexico) for worthwhile discussions. All results are presented in the main paper and supplementary materials. The binary nitrogen precursors PGM-free catalyst synthesis method has been patented by H.T.C. and P.Z. as U.S. patent number US20160351915 A9, Non-precious metal catalysts.
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