Fe-Based O2-Reduction Catalysts Synthesized Using Na2CO3 as a

Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Fe-Based O2‑Reduction Catalysts Synthesized Using Na2CO3 as a Pore-Inducing Agent Kathrin Ebner,† Juan Herranz,*,† Viktoriia A. Saveleva,† Bae-Jung Kim,† Sebastian Henning,† Marleǹ e Demicheli,† Frank Krumeich,‡ Maarten Nachtegaal,§ and Thomas J. Schmidt†,∥ †

Electrochemistry Laboratory, Paul Scherrer Institut, 5232 Villigen, Switzerland Laboratory of Inorganic Chemistry, ETH Zürich, 8093 Zürich, Switzerland § Paul Scherrer Institut, 5232 Villigen, Switzerland ∥ Laboratory of Physical Chemistry, ETH Zürich, 8093 Zürich, Switzerland ‡

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S Supporting Information *

ABSTRACT: The cost reductions required for the large-scale commercialization of polymer electrolyte fuel cells (PEFCs) could be achieved by substituting state-of-the-art PEFC cathode catalysts based on platinum with more abundant and affordable materials. In this context, this work presents a new approach for synthesizing Fe-based oxygen reduction reaction (ORR) catalysts using sodium carbonate (Na2CO3) as an inexpensive but effective pore-inducing agent offering microporosity control. By employing (scanning) transmission electron microscopy, a qualitative relation between the heat-treatment temperature and the formation of larger isolated Fe-based phases in particulate form was identified, mainly unveiling an effect of this variable on the Fe-speciation. Complementary bulk characterization, namely, X-ray absorption spectroscopy, on the other hand confirmed that the majority of the iron in the samples was present in single atomic sites. Electrochemical activity measurements in liquid environment as well as in a fuel cell demonstrate that the resulting materials display ORR-activities among the highest for this class of catalysts and synthesis conditions. KEYWORDS: energy conversion, electrocatalysis, electrochemistry, fuel cells, oxygen reduction

1. INTRODUCTION The global energy demand of the transportation sector is growing steadilyan average annual increase of 1% from 2015 to 2040 has been estimated by the U.S. Energy Information Administration.1 To minimize the environmental impact of this sector, the required reduction in transportation-related emissions enforces the ongoing shift toward electro-mobility solutions. Because of their high power density and fast refueling times,2 polymer electrolyte fuel cells (PEFCs) are excellently suited energy conversion devices for propelling emission-free electric vehicles employing hydrogen as a carbon-free fuel. However, their high cost has prevented deeper market penetration.3 A major contributor to this excessive cost is the large amount of Pt used to catalyze the sluggish oxygen reduction reaction (ORR) taking place at PEFC cathodes (≈ 0.4 mgPt·cm−2).4,5 This motivates the search for low-cost replacements based on abundant elements.6,7 The most promising candidates known up to date are materials of the so-called Me/N/C-family, which require the presence of a transition metal (Me) as well as nitrogen-containing functionalities and a carbon-based support. Encouragingly, certain catalysts of this class have even been shown to meet the initial PEFC performance targets set by the U.S. Department of Energy for transportation applications,8−11 if accompanied by material-related challenges © XXXX American Chemical Society

(i.e., insufficient stability and high-current density performance) that have been extensively reviewed in recent articles.7,12 Beyond these unresolved issues, these Pt-group metal (PGM)-free ORR-catalysts are expected to offer an estimated 2 orders of magnitude decrease in production costs of the catalyst material,6 but those of the highly active kind discussed above are typically synthesized using expensive metal organic frameworks (MOFs) that could jeopardize their cost advantages.7 More precisely, a technologically advisible cathode catalyst cost can be estimated on the basis of the target values for well-established Pt-based materials set by the U.S. Department of Energy for automotive PEFCs;13,14 assuming that 80% of the latter Pt-loading target of ≤0.125 mgPt·cm−2 were to correspond to the cell’s cathode, and considering a Pt-cost of ≈45 $·gPt−1,15 the cathode catalyst costs should be kept ≤4 × 10−3 $·cmcathode−2. Comparatively, if one considers the amount of the MOF (ZIF-8, Zeolitic imidazolate framework) used to synthesize the best-performing PGM-free catalyst presented in ref 8 (accounting to 80 wt. % of the initial precursors’ mass), along with the synthesis’ yield of 13%,8 a customary PGM-free catalyst loading of ≈4 Received: November 23, 2018 Accepted: January 14, 2019 Published: January 14, 2019 A

DOI: 10.1021/acsaem.8b02036 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials mgcatalyst·cmcathode−2,8,9,16 and ZIF-8’s retail price of ≈14 $·g−1 (on a 100 g basis),17 the MOF alone would account for a cathode-specific cost of ≈3 × 10−1 $·cmcathode−2. While this estimate strongly depends on the MOF’s price, which will likely diminish due to the expected increase in the demand for this product, sufficient upscaling of MOFs’ production remains a challenge,18 and thus, it is hard to fathom that mere economics of scale will completely close this ≈2 order of magnitude gap with the benchmark cost value calculated above. On top of this, these highly active materials8−11 are customarily synthesized using ammonia in the heat-treatment steps involved in their preparationa requirement of significant environmental impact that is often overlooked in the pursuit of high activities. Motivated by these drawbacks, this work introduces a new PGM-free catalyst synthesis based on the use of environmentally benign gases during the synthesis’ heat-treatment steps and implementing comparably inexpensive precursors.17 Specifically, instead of a MOF, Na2CO3 (1) is used as a thermally decomposable pore-inducing agent that is not only of remarkably lower cost (i.e., > 250-fold cheaper than ZIF8)17 but also novel to the field to the best of our knowledge.19,20 Inspired by the proposed structure of the active sites in Fe/N/C catalysts, whereby an Fe2+-ion is 4-fold coordinated by N-moieties embedded in a carbonaceous matrix (cf. 4 in Scheme 1, note that the exact structure remains controversial),21−23 this approach further relies on the use of polyacrylonitrile (PAN) (3) as the C- and N- source, as well as Fe2+-phenanthroline (2) as the iron-precursor. As depicted in Scheme 1, these three compounds first undergo a mixing and ball milling step, followed by a first heat treatment in N2 atmosphere for 30 min at temperatures between 550 and 950 °C. Next, the resulting material is acid washed, water-rinsed and dried, and submitted to a second heat-treatment step for 1 h at 950 °C in an atmosphere of Ar with 5% H2. Further synthetic details can be found in the Experimental Section below. By investigating the influence of the synthesis parameters on the ORR-activity and physiochemical properties (i.e., surface area, surface N-content, and composition) of the resulting materials, the synthesis conditions were optimized to yield materials with satisfactory activities in liquid electrolyte (≈ 1 A·gcatalyst−1 at 0.8 V vs the reversible hydrogen electrode). Further, the possibly heterogeneous composition (i.e., Fespeciation)16,24−29 of these Fe/N/C-type materials caused by the high-temperature heat treatments typically implemented in their syntheses was addressed by characterizing selected catalysts employing (scanning) transmission electron microscopy ((S)TEM) as well as X-ray absorption spectroscopy (XAS). Finally, the catalysts’ ORR-activity in the PEFC operating environment was shown to be among the highest for PGM-free catalysts prepared under comparable synthesis conditions (i.e., MOF- and NH3-free).

Scheme 1. Schematic Representation of the Novel PGMFree Catalyst Synthesis Introduced in This Studya

a

Sodium carbonate (1), an Fe2+-phenanthroline complex (2) and PAN (3) are i) mixed, dried and ball milled, ii) heat treated in N2 at a temperature T1 for 30 min, iii) acid washed overnight, and iv) submitted to a second heat treatment at 950 °C (in 5% H2 in Ar) to yield ORR-active catalysts, for which one proposed active site structure is depicted in (4).21

basis) and phenanthroline (Sigma-Aldrich, > 99%) in a molar ratio of 1:5) needed for an initial Fe-content of 0.1 wt. % Fe on the basis of all precursors’ masses was added. After it was further stirred (≈ 1 h), the solvent was evaporated while drying the mixture overnight at 110 °C. The resulting dry powder was ground in an agate mortar, and 2.5 g of it was placed in a 45 mL zirconia crucible along with 5 mm diameter zirconia balls in a catalyst-to-ball weight ratio of 1:4.45. After sealing the crucible, the powder underwent eight ball milling cycles (planetary ball mill, Fritsch, Pulverisette) at 300 rpm, with each cycle lasting 10 min and a 5 min break in-between cycles. Next, a weighed amount (≈ 1 g) of the ground precursors mixture was placed in an alumina crucible and submitted to the first heat treatment in N2 (100 mL·min−1, Messer AG, 5.0) at a set temperature T1 for 30 min (furnace: Gero RETTH-KS 400/6). For this, the temperature was first increased at 300 K·h−1 to 150 °C, held for 3 h, and then raised again with the same ramp up to T1. After the temperature was held for 30 min at T1, the heating was stopped, and the furnace and sample were left to cool down through natural convection. This heat-treated powder was then acid washed overnight in 0.1 M HClO4 (room temperature, Merck Suprapur, 70%), recovered through vacuum-filtration using a 0.45 μm filter (Merck Duropore, membrane filters), washed with ultrapure water (18.2 MΩ·cm, ELGA Purelab Ultra) until the washing solution had a neutral pH, and dried at 80 °C. Finally, the dry, acid-washed powder was submitted to the second heat-treatment step in an atmosphere of Ar with 5% of H2 (100 mL·min−1, Messer Inoxline H5) at 950 °C for 60 min (using the

2. EXPERIMENTAL SECTION 2.1. Preparation of Materials. For the catalyst synthesis, polyacrylonitrile (PAN, Sigma-Aldrich, average molecular weight: 150 000 g·mol−1) and sodium carbonate (Sigma-Aldrich, anhydrous, 99.999% trace metal basis) were stirred separately overnight in dimethylformamide (DMF, Sigma-Aldrich, anhydrous 99.8%) in the desired weight ratios while keeping the temperature at ≈80 °C. After the resulting suspensions were combined and stirred hot for another hour, the amount of Fe2+-phenanthroline complex (pre-prepared in DMF by mixing Fe2+-acetate (Sigma-Aldrich, > 99.99% trace metal B


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ACS Applied Energy Materials same heating and cooling programs described above). The resulting PGM-free catalyst was stored under vacuum. 2.2. Electrochemical Measurements. The method of choice for screening the PGM-free catalysts’ electrochemical activity toward the oxygen reduction reaction was rotating disk electrode (RDE) voltammetry.30 A catalyst ink was prepared by dispersing a weighed amount of PGM-free catalyst in a mixture of ultrapure water (18.2 MΩ·cm, ELGA Purelab Ultra) and isopropanol (Sigma-Aldrich, HPLC gradient grade, 99.9%) in a volume ratio of 7:3, and Nafion (5%, Sigma-Aldrich) was added to reach a Nafion-to-catalyst-mass ratio of 0.3. After sonication, the volume of ink required for a catalyst loading of 500 μgcatalyst·cm−2 was dropcasted onto a mirror-polished, 5 mm diameter glassy carbon disk (HTW - Hochtemperatur-Werkstoffe GmbH) embedded at the center of a polytetrafluoroethylene (PTFE) RDE (Pine Research). This disk was then employed as the working electrode, while a reversible hydrogen electrode (RHE) and a piece of gold mesh shielded in a separated glass compartment (connected through a #3 porous glass frit) were used as the reference and the counter electrodes, respectively. 0.1 M perchloric acid (Kanto Chemicals, 60%, Cica Reagent Ultrapure) saturated with either N2 (Messer AG, 5.0) or O2 (Messer AG, 5.5) at room temperature served as the electrolyte. All tests were performed with a Bio-Logic VSP-300 potentiostat. Following immersion of the catalyst-coated RDE in the O2saturated electrolyte, the surface was electrochemically conditioned by recording 25 potential cycles between 0.05 and 1.0 V vs RHE at 50 mV·s−1 in stagnant electrolyte. Next, three cyclic voltammograms were recorded at a scan rate of 5 mV·s−1 and an electrode rotation rate of 1600 rpm in the same potential window. The electrolyte was then deaerated by bubbling N2 for 20 min, after which the capacitive current (icap) was determined by recording another three cyclic voltammograms between 0.05 and 1.0 V vs RHE at 5 mV·s−1. Finally, the resistance between working and reference electrode required for correcting the recorded curves for ohmic losses (cf. below) was determined by potentiostatic electrochemical impedance spectroscopy, applying a 10 mV perturbation between 200 kHz and 0.1 Hz while holding the potential at 0.45 V vs RHE. For calculating the electrochemical activity, the ORR-specific faradaic current density (iF) was first determined via eq 1: iF = i − icap

electrochemical impedance spectroscopy, see above) of 0.8 V versus RHE. A selected catalyst was further tested in a polymer electrolyte fuel cell (PEFC). To fabricate the membrane electrode assembly (MEA), the catalyst was first spray-coated on a Nafion XL 100 (DuPont) membrane to yield a loading of 0.7 mgcatalyst·cm−2, whereby the catalyst ink was prepared by mixing and sonicating (30 min) 15 mg of the catalyst, 656 μL of a mixture of ultrapure water (18.2 MΩ·cm, ELGA Purelab Ultra) and isopropanol (Sigma-Aldrich, HPLC gradient grade, 99.9%) in a ratio of 7:3, and 345 μL of Nafion (5%, Sigma-Aldrich, for a Nafion-to-catalyst-ratio of 1.0). The MEAs were hotpressed for 5 min at 120 °C with a pressure of 1 bar using a commercially available gas diffusion electrode (0.4 mgPt·cm−2 HISPEC 9100 Pt/HAS on Sigracet GDL 25 BC, Johnson Matthey) as the anode and a gas diffusion layer (Sigracet GDL 25 BC) at the cathode side. The MEAs were tested in a differential fuel cell32 (active area 1 cm2) at a temperature of 80 °C and 100% relative humidity. Cyclic voltammograms (CVs) in a potential range from 0.05 to 1.0 V were obtained with a H2 flow rate of 50 mL min−1 at the anode; the N2 flow at the cathode was stopped for the measurement. Subsequently the pressure was set to 1.5 barabs and a polarization curve (recorded galvanostatically, 3 min per point) was measured with H2 and O2 flow rates of 300 and 750 mL min−1 at anode and cathode, respectively. Because of the small size of the device, the gas flow velocities remain in the same order of magnitude, even though the stoichiometric ratios are higher than in a technical cell (≥30/≥ 30).33 For testing the durability, the cell potential was held at 0.5 V for one hour and another polarization curve was measured just afterward. All tests were performed with a Bio-Logic VSP-300 potentiostat equipped with a 10 A/5 V current booster. The resulting electrochemical mass activity was calculated from the kinetic current density at an IRcorrected cell potential of 0.8 V (resistance determined by electrochemical impedance spectroscopy applying the same parameters as in the RDE measurements). 2.3. Characterization of Materials. The final Fe-content in the catalysts was determined by inductively coupled plasma optical emission spectrometry (ICP-MS) provided by BACHEMA AG. Samples were prepared by suspending a weighted amount of catalyst (≈ 3 mg) in 5 mL of aqua regia (HCl: 37%, VWR, for trace analysis; HNO3: 65%, VWR, for analysis) overnight, evaporating to dryness and adding 10 mL of 1 M H2SO4(96%, VWR, for trace analysis). The suspensions were centrifuged and filtered to remove any insoluble residue, and their liquid fractions were analyzed by ICP-MS. N2-physisorption measurements were performed at 77 K in a Quantachrome Autosorb 1 instrument after outgassing a weighed amount (≈ 20 mg) of the catalyst in a 9 mm large bulb cell overnight at 200 °C. The total surface area of all catalysts was calculated by applying the multipoint Brunauer−Emmet−Teller (BET) equation in the relative pressure range 0.03 ≤ p/p0 ≤ 0.09. By employing a nonlocal density functional theory method assuming slit-like pores in carbon materials (provided in the Quantachrome Autosorb Software), information about the pore size distribution was obtained. Specifically, the surface area corresponding to pore sizes ≤2 nm in width was assigned to micropores. For the STEM investigation, each material was dispersed in ethanol and a few drops of the suspension were deposited onto a perforated carbon foil supported on a copper grid (Agar Scientific, holey carbon film - copper, 400 mesh). STEM measurements were performed on an aberration-corrected, dedicated STEM microscope, a HD-2700CS (Hitachi), which was operated at an acceleration potential of 200 kV (cold field emitter). A probe corrector (CEOS) that was incorporated in the microscope column between the condenser lens and the probeforming objective lens provides a resolution below 0.1 nm. Several detectors could be selected allowing for bright field (BF) and (highangle) annular dark field ((HA)ADF) imaging as well as morphology studies (secondary electron detector). The images were recorded with frame times between 20 and 40 s. Analytical investigations were done with an energy-dispersive X-ray spectrometer (EDXS, EDAX)

(1)

where i is the current density measured in the polarization curve at 1600 rpm recorded in the O2-saturated solution (see above). Two means of quantification were then employed to assess the catalytic ORR-activity: (i) To facilitate comparison of catalysts displaying a large variety of ORR-activity values (i.e., including those featuring ORRonsets 15 A·gcatalyt−1 (corresponding to >20 mA cm−2 in Figure 5a). Such performance losses are often related to the mass transport properties of the catalyst upon processing into a catalyst layer (CL),8 which are determined by its agglomerate morphology and pore size distribution and can be studied using focused ion beam scanning electron microscopy (FIB-SEM).49 The CL cross-

Figure 4. EXAFS spectra for the samples synthesized at T1 = 650 (blue), 700 (green) or 750 °C (gray) (a) and the corresponding FT EXAFS spectra (b). In the inset, the Fourier-transformed EXAFS of Fe-foil (black dashed line) and the 650 °C sample (blue solid line) are shown for comparison (c). Note that the shaded area in (a) represents the k-range used for the Fourier transformation.

neighbor at ≈3.06 Å. This hints at the presence of iron in Fe3C or metallic nanoparticles. The XANES spectra (Figure S7), however, did not present any of the features characteristic for Fe-based particles commonly detected in Fe/N/C-type materials (i.e., Fe and Fe3C, see the spectra in Figure S7 for comparison), but did closely match those reported for Fe/N/ C-catalysts containing a large majority45 of their iron in the form of FeN4-like sites.22 In summary, these XAS results indicate that the Fe in these three catalysts is preponderantly present in the form of molecularly dispersed, FeN4-like sitesa conclusion that may appear to disagree with the above (S)TEM observations, whereby Fe-particles could be spotted in all catalysts. While at the current time we cannot pinpoint the exact reason for the disagreement among techniques, it may arise from an agglomeration of FeN4-sites resulting in a strong Z-contrast in STEM but that would not alter the XAS spectra. Alternatively, this apparent inconsistency could be due to the

Table 1. Structural Parameters Obtained by Best Fitting the FT EXAFS Spectra of the Samples Synthesized at 650−750 °Ca T1/°C

scatterer

N

R/Å

σ2/Å2

ΔE0/eV

R-factor

650 700 750

O1.1 O1.1 O1.1

4.8 ± 1.4 5.2 ± 0.5 5.6 ± 1.1

1.98 ± 0.03 1.96 ± 0.01 1.99 ± 0.02

0.009 ± 0.005 0.010 ± 0.002 0.008 ± 0.004

−2.6 ± 3.6 −5.6 ± 1.2 −1.5 ± 2.4

0.030 0.002 0.016

N refers to the number of nearest neighbors, R refers to the 1st shell Fe−O/N bond distance, σ2 is the Debye−Waller factor, ΔE0 is the energy shift, and the R-factor is a statistic parameter for which values below 0.02 are regarded as indicative of a good-quality EXAFS fit.46 Note that the fits were performed in the R-range of 1−2.2 Å. a

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morphology and CL preparation, as to ensure optimum performance of these novel materials in the fuel cell. Beyond this initial performance, the great majority of PGMfree catalysts reportedly suffer from a fast deactivation in the first hours of PEFC operation.12 This instability also affects our catalyst, for which the ORR-activity drops to ≈20% of its initial value after one hour of potentiostatic operation at 0.5 V (see the polarization curve marked as “final” in Figure 5a). While an understanding of the origin of this effect remains beyond the purpose of this manuscript, several mechanisms have been discussed in the literature. The Dodelet group has ascribed this effect to the flooding of the pores in the catalyst layer,50 which leads to poor O2-diffusion to the ORR-active sites and an apparent loss of utilization. This hypothesis goes hand in hand with the CL cross-section image in Figure 5b, which reveals the low amount of the open porosity (>50 nm in width) crucial for efficient mass transport along the CL.51 Most recently, though, this phenomenon has further been related by the same group to the specific demetalation of the FeN4-sites hosted in the micropores of such PGM-free catalysts.52 Other authors, however, proposed H2O2- or radical-attack53 and anion adsorption following N-protonation48 as likely causes for this performance drop.12,54

4. CONCLUSIONS In summary, we have presented a new, inexpensive and NH3free approach for synthesizing Fe-based ORR-catalysts using Na2CO3 as a porosity-inducing agent that permits excellent microporosity control. Tuning the temperature of the first heat-treatment step revealed the effect of this parameter on the N- and (micro)pore-contents in the final catalysts, as well as on their Fe-speciation and corresponding ORR-activity. Specifically, whereas carbon-encapsulated Fe-based particles or agglomerates were present in the sample prepared at 750 °C, these seemed to be preponderantly removed in a material prepared at 650 °C. Regardless of these qualitative differences, XAS results indicated that these catalysts featured a similar Fespeciation, whereby the majority of the contained iron is present in the form of FeN4-like ORR-active sites. Finally, the initial PEFC performance of the most active catalyst prepared in this manner is among the best reported for PGM-free catalysts synthesized without NH3 or MOFs, highlighting the

Figure 5. Polarization curves obtained in a PEFC test (at 1.5 barabs, 80 °C, 100% relative humidity and H2- and O2-stoichiometries ≥30) of a PGM-free catalyst heat treated at T1 = 700 °C (with ≈0.7 mgcatalyst·cm−2), before and after one hour of potentiostatic operation at 0.5 V (a). Corresponding IR-corrected Tafel plots, whereby points along the dashed line (with a slope of 70 mV·dec−1) correspond to kinetically controlled currents (b). Cross-section image of a catalyst layer (T1 = 750 °C) acquired with FIB-SEM (c).

section image obtained using this technique and displayed in Figure 5c reveals low open porosity (i.e., low content of pores >50 nm in width), which causes poor O2- and H2O-transport. Thus, forthcoming work will focus on optimizing the catalyst

Table 2. Comparison of the Initial ORR-Activities of the Most Active Catalyst in This Study to the Best-Performing Materials Presented in the Literature Synthesized without Using NH3 in the Heat-Treatment Steps catalyst ref 31e ref 31f ref 55g ref 45h ref 16i ref 56j ref 57k ref 58l this work (with T1 = 700 °C)

initial Fecontent /wt. %

final Fecontent /wt. %

catalyst-mass specific ORRactivity at 0.8 V vs RHE, in RDE/A g−1

catalyst-mass specific ORR-activity at 0.8 V, in PEFC/A g−1

Fe-mass specific mass ORRactivity at 0.8 V vs RHE, in RDE/A g−1

Fe-mass specific mass ORR-activity at 0.8 V, in PEFC/A g−1

26a 26a 1 1 -

2.4 3.4 0.9b 4.1 1.4d 0.4

1.2 3.15 0.7 ∼5c ∼4c ∼1.5c 23.0 1.0

6.4 ∼50c ∼14c 39.4 ∼55c ∼14

50 556 ∼36 1643 250

188 5556 956 3929 3500

0.1

a

Calculated from given precursor masses. bEstimated from given N- and Fe-contents (EELS) assuming the catalyst to consist of only Fe, N and C. Visually derived from Tafel plot/polarization curve. dOn the basis of XPS-derived values. eReferred to as (Fe, Fe)1 + N2/H2. fReferred to as (Fe, Fe)2 + N2/H2. gReferred to as Fe-ODAN-1%. hReferred to as Fe1.0d. iReferred to as (CM+PANI)−Fe-C. jData shown for the best-performing material heat-treated at 1050 °C. kReferred to as PFETTPP-700. lReferred to as Fe/N/C-SCN. c

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for Automotive Applications: 2016 Update; Strategic Analysis Inc.: Arlington, 2017. (5) Rabis, A.; Rodriguez, P.; Schmidt, T. J. Electrocatalysis for Polymer Electrolyte Fuel Cells: Recent Achievements and Future Challenges. ACS Catal. 2012, 2, 864−890. (6) Jaouen, F. Heat-Treated Transition Metal-NxCy Electrocatalysts for the O2 Reduction Reaction in Acid PEM Fuel Cells. Non-Noble Met. Fuel Cell Catal. 2014, 29−118. (7) Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594−3657. (8) Proietti, E.; Jaouen, F.; Lefevre, M.; Larouche, N.; Tian, J.; Herranz, J.; Dodelet, J.-P. Iron-Based Cathode Catalyst with Enhanced Power Density in Polymer Electrolyte Membrane Fuel Cells. Nat. Commun. 2011, 2, 416−425. (9) Shui, J.; Chen, C.; Grabstanowicz, L.; Zhao, D.; Liu, D.-J. Highly Efficient Nonprecious Metal Catalyst Prepared with Metal−organic Framework in a Continuous Carbon Nanofibrous Network. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 10629−10634. (10) Strickland, K.; Miner, E.; Jia, Q.; Tylus, U.; Ramaswamy, N.; Liang, W.; Sougrati, M.; Jaouen, F.; Mukerjee, S. Highly Active Oxygen Reduction Non-Platinum Group Metal Electrocatalyst without Direct Metal−nitrogen Coordination. Nat. Commun. 2015, 6, 7343−7351. (11) Serov, A.; Artyushkova, K.; Niangar, E.; Wang, C.; Dale, N.; Jaouen, F.; Sougrati, M. T.; Jia, Q.; Mukerjee, S.; Atanassov, P. NanoStructured Non-Platinum Catalysts for Automotive Fuel Cell Application. Nano Energy 2015, 16, 293−300. (12) Banham, D.; Ye, S.; Pei, K.; Ozaki, J.; Kishimoto, T.; Imashiro, Y. A Review of the Stability and Durability of Non-Precious Metal Catalysts for the Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cells. J. Power Sources 2015, 285, 334−348. (13) U.S. Department of Energy. DOE Technical Targets for Polymer Electrolyte Membrane Fuel Cell Components. See the following: https://www.energy.gov/eere/fuelcells/doe-technicaltargets-polymer-electrolyte-membrane-fuel-cell-components (accessed Nov 6, 2018). (14) U.S. DRIVE-Fuel Cell Technical Team. Fuel Cell Technical Team Roadmap Hydrogen Storage Technologies Roadmap. 2017; pp 1−34. (15) Johnson Matthey - Platinum Price Chart. See the following: http://www.platinum.matthey.com/prices/price-charts# (accessed Nov 6, 2018). (16) Chung, H. T.; Cullen, D. A.; Higgins, D.; Sneed, B. T.; Holby, E. F.; More, K. L.; Zelenay, P. Direct Atomic-Level Insight into the Active Sites of a High-Performance PGM-Free ORR Catalyst. Science 2017, 357, 479−484. (17) Sigma Aldrich US. See the following: https://www. sigmaaldrich.com/united-states.html (accessed Nov 6, 2018). (18) Rubio-Martinez, M.; Hadley, T. D.; Batten, M. P.; ConstantiCarey, K.; Barton, T.; Marley, D.; Mönch, A.; Lim, K. S.; Hill, M. R. Scalability of Continuous Flow Production of Metal-Organic Frameworks. ChemSusChem 2016, 9, 938−941. (19) Patel, M. U. M.; Demir-Cakan, R.; Morcrette, M.; Tarascon, J. M.; Gaberscek, M.; Dominko, R. Li-S Battery Analyzed by UV/Vis in Operando Mode. ChemSusChem 2013, 6, 1177−1181. (20) Lai, C.; Gao, X. P.; Zhang, B.; Yan, T. Y.; Zhou, Z. Synthesis and Electrochemical Performance of Sulfur/Highly Porous Carbon Composites. J. Phys. Chem. C 2009, 113, 4712−4716. (21) Kramm, U. I.; Herranz, J.; Larouche, N.; Arruda, T. M.; Lefèvre, M.; Jaouen, F.; Bogdanoff, P.; Fiechter, S.; Abs-Wurmbach, I.; Mukerjee, S.; Dodelet, J.-P. Structure of the Catalytic Sites in Fe/ N/C-Catalysts for O2-Reduction in PEM Fuel Cells. Phys. Chem. Chem. Phys. 2012, 14, 11673−11688. (22) Zitolo, A.; Goellner, V.; Armel, V.; Sougrati, M.-T.; Mineva, T.; Stievano, L.; Fonda, E.; Jaouen, F. Identification of Catalytic Sites for Oxygen Reduction in Iron- and Nitrogen-Doped Graphene Materials. Nat. Mater. 2015, 14, 937−942.

great potential of this synthesis route for the preparation of inexpensive ORR-catalysis. Future work will focus on further boosting this ORR-activity by increasing the amount of iron in the initial precursors’ mixture while improving the CL’s mass transport properties and studying in detail the reasons for the catalysts’ instability.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b02036.

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Details on the XPS results and fits, STEM and EDX results, EXAFS fits, XANES spectra of catalysts and references as well as electrochemical characterization (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Juan Herranz: 0000-0002-5805-6192 Bae-Jung Kim: 0000-0002-6052-0150 Frank Krumeich: 0000-0001-5625-1536 Thomas J. Schmidt: 0000-0002-1636-367X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Swiss National Science Foundation through the Ambizione Energy grant PZENP2_173632 is greatly acknowledged by the authors K.E., V.A.S., and J.H. T.J.S. thanks Innosuisse and the Swiss Competence Center for Energy Research Heat & Electricity Storage. Furthermore, the authors thank the SuperXAS beamline at the Swiss Light Source as well as ScopeM at ETHZ for the use of their facilities.

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ABBREVIATIONS Me, transition metal; ORR, oxygen reduction reaction; PEFC, polymer electrolyte fuel cell; PGM, Pt-group metal; PAN, polyacrylonitrile; SI, Supporting Information; TEM, transmission electron microscopy; CL, catalyst layer; FIB-SEM, focused ion beam scanning electron microscopy; XPS, X-ray photoelectron spectroscopy; RDE, rotating disc electrode; MOF, metal organic framework; RHE, reversible hydrogen electrode; CV, cyclic voltammogram; SLS, Swiss light source; FT, Fourier transformed; EXAFS, extended X-ray absorption fine structure; XANES, X-ray absorption near edge structure

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REFERENCES

(1) U.S. Energy Information Administration (EIA). International Energy Outlook 2017 Overview - Report Number: DOE/EIA0484(2017); EIA: Washington, DC, 2017. (2) Wagner, F. T.; Lakshmanan, B.; Mathias, M. F. Electrochemistry and the Future of the Automobile. J. Phys. Chem. Lett. 2010, 1, 2204− 2219. (3) Gröger, O.; Gasteiger, H. A.; Suchsland, J.-P. Review Electromobility: Batteries or Fuel Cells? J. Electrochem. Soc. 2015, 162, A2605−A2622. (4) James, B. D.; Huya-Kouadio, J. M.; Houchins, C.; DeSantis, D. A. Mass Production Cost Estimation of Direct H2 PEM Fuel Cell Systems I


Article

ACS Applied Energy Materials (23) Fei, H.; Dong, J.; Feng, Y.; Allen, C. S.; Wan, C.; Volosskiy, B.; Li, M.; Zhao, Z.; Wang, Y.; Sun, H.; An, P.; Chen, W.; Guo, Z.; Lee, C.; Chen, D.; Shakir, I.; Liu, M.; Hu, T.; Li, Y.; Kirkland, A. I.; Duan, X.; Huang, Y. General Synthesis and Definitive Structural Identification of MN4C4 single-Atom Catalysts with Tunable Electrocatalytic Activities. Nat. Catal. 2018, 1, 63−72. (24) Kramm, U. I.; Herrmann-Geppert, I.; Fiechter, S.; Zehl, G.; Zizak, I.; Dorbandt, I.; Schmeißer, D.; Bogdanoff, P. Effect of IronCarbide Formation on the Number of Active Sites in Fe−N−C Catalysts for the Oxygen Reduction Reaction in Acidic Media. J. Mater. Chem. A 2014, 2, 2663−2670. (25) Varnell, J. A.; Tse, E. C. M.; Schulz, C. E.; Fister, T. T.; Haasch, R. T.; Timoshenko, J.; Frenkel, A. I.; Gewirth, A. A. Identification of Carbon-Encapsulated Iron Nanoparticles as Active Species in NonPrecious Metal Oxygen Reduction Catalysts. Nat. Commun. 2016, 7, 12582−12591. (26) Choi, C. H.; Baldizzone, C.; Polymeros, G.; Pizzutilo, E.; Kasian, O.; Schuppert, A. K.; Ranjbar Sahraie, N.; Sougrati, M. T.; Mayrhofer, K. J. J.; Jaouen, F. Minimizing Operando Demetallation of Fe-N-C Electrocatalysts in Acidic Medium. ACS Catal. 2016, 6, 3136−3146. (27) Wu, G.; Johnston, C. M.; MacK, N. H.; Artyushkova, K.; Ferrandon, M.; Nelson, M.; Lezama-Pacheco, J. S.; Conradson, S. D.; More, K. L.; Myers, D. J.; Zelenay, P. Synthesis-Structure-Performance Correlation for Polyaniline-Me-C Non-Precious Metal Cathode Catalysts for Oxygen Reduction in Fuel Cells. J. Mater. Chem. 2011, 21, 11392−11405. (28) Jaouen, F.; Proietti, E.; Lefèvre, M.; Chenitz, R.; Dodelet, J.-P.; Wu, G.; Chung, H. T.; Johnston, C. M.; Zelenay, P. Recent Advances in Non-Precious Metal Catalysis for Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells. Energy Environ. Sci. 2011, 4, 114−130. (29) Chung, H. T.; Won, J. H.; Zelenay, P. Active and Stable Carbon Nanotube/Nanoparticle Composite Electrocatalyst for Oxygen Reduction. Nat. Commun. 2013, 4, 1922−1925. (30) Schmidt, T. J.; Gasteiger, H. A.; Stab, G. D.; Kolb, D. M.; Behm, R. J. Characterization of High-Surface-Area Electrocatalysts Using a Rotating Disk Electrode Configuration. J. Electrochem. Soc. 1998, 145, 2354−2358. (31) Kramm, U. I.; Herrmann-Geppert, I.; Behrends, J.; Lips, K.; Fiechter, S.; Bogdanoff, P. On an Easy Way to Prepare MetalNitrogen Doped Carbon with Exclusive Presence of MeN4-Type Sites Active for the ORR. J. Am. Chem. Soc. 2016, 138, 635−640. (32) Oberholzer, P.; Boillat, P. Local Characterization of PEFCs by Differential Cells: Systematic Variations of Current and Asymmetric Relative Humidity. J. Electrochem. Soc. 2013, 161, F139−F152. (33) Villers, D.; Jacques-Bédard, X.; Dodelet, J.-P. Fe-Based Catalysts for Oxygen Reduction in PEM Fuel Cells. J. Electrochem. Soc. 2004, 151, A1507−A1515. (34) Krumeich, F.; Müller, E.; Wepf, R. A.; Nesper, R. Characterization of Catalysts in an Aberration-Corrected Scanning Transmission Electron Microscope. J. Phys. Chem. C 2011, 115, 1080− 1083. (35) Jaouen, F.; Herranz, J.; Lefèvre, M.; Dodelet, J.-P.; Kramm, U. I.; Herrmann, I.; Bogdanoff, P.; Maruyama, J.; Nagaoka, T.; Garsuch, A.; Dahn, J. R.; Olson, T.; Pylypenko, S.; Atanassov, P.; Ustinov, E. A. Cross-Laboratory Experimental Study of Non-Noble-Metal Electrocatalysts for the Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2009, 1, 1623−1639. (36) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-Ray Absorption Spectroscopy Using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537−541. (37) ICSD database. See the following: https://icsd.fiz-karlsruhe.de/ search/basic.xhtml (accessed Nov 21, 2018). (38) Jaouen, F.; Lefèvre, M.; Dodelet, J. P.; Cai, M. Heat-Treated Fe/N/C Catalysts for O2 Electroreduction: Are Active Sites Hosted in Micropores? J. Phys. Chem. B 2006, 110, 5553−5558. (39) Jaouen, F.; Dodelet, J. P. Average Turn-over Frequency of O2 electro-Reduction for Fe/N/C and Co/N/C Catalysts in PEFCs. Electrochim. Acta 2007, 52, 5975−5984.

(40) Yamada, H.; Nakamura, H.; Nakahara, F.; Moriguchi, I.; Kudo, T. Electrochemical Study of High Electrochemical Double Layer Capacitance of Ordered Porous Carbons with Both Meso/Macropores and Micropores. J. Phys. Chem. C 2007, 111, 227−233. (41) Médard, C.; Lefèvre, M.; Dodelet, J. P.; Jaouen, F.; Lindbergh, G. Oxygen Reduction by Fe-Based Catalysts in PEM Fuel Cell Conditions: Activity and Selectivity of the Catalysts Obtained with Two Fe Precursors and Various Carbon Supports. Electrochim. Acta 2006, 51, 3202−3213. (42) Kim, J. H.; Sa, Y. J.; Jeong, H. Y.; Joo, S. H. Roles of Fe−N x and Fe−Fe 3 C@C Species in Fe−N/C Electrocatalysts for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2017, 9, 9567−9575. (43) Hu, Y.; Jensen, J. O.; Zhang, W.; Cleemann, L. N.; Xing, W.; Bjerrum, N. J.; Li, Q. Hollow Spheres of Iron Carbide Nanoparticles Encased in Graphite Layers as Oxygen Reduction Catalysts. Angew. Chem., Int. Ed. 2014, 53, 3675−3679. (44) Deng, D.; Yu, L.; Chen, X.; Wang, G.; Jin, L.; Pan, X.; Deng, J.; Sun, G.; Bao, X. Iron Encapsulated within Pod-like Carbon Nanotubes for Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2013, 52, 371−375. (45) Sougrati, M. T.; Goellner, V.; Schuppert, A. K.; Stievano, L.; Jaouen, F. Probing Active Sites in Iron-Based Catalysts for Oxygen Electro-Reduction: A Temperature-Dependent 57Fe Mössbauer Spectroscopy Study. Catal. Today 2016, 262, 110−120. (46) Newville, M. FEFFIT - Using FEFF to model XAFS data. See the following: http://cars9.uchicago.edu/ifeffit/Documentation (accessed Jan 10, 2019). (47) Jaouen, F.; Goellner, V.; Lefèvre, M.; Herranz, J.; Proietti, E.; Dodelet, J. P. Oxygen Reduction Activities Compared in RotatingDisk Electrode and Proton Exchange Membrane Fuel Cells for Highly Active Fe-N-C Catalysts. Electrochim. Acta 2013, 87, 619−628. (48) Herranz, J.; Jaouen, F.; Lefèvre, M.; Kramm, U. I.; Proietti, E.; Dodelet, J.-P.; Bogdanoff, P.; Fiechter, S.; Abs-Wurmbach, I.; Bertrand, P.; Arruda, T. M.; Mukerjee, S. Unveiling N-Protonation and Anion-Binding Effects Onf Fe/N/C-Catalyst for O2 Reduction in PEM Fuel Cells. J. Phys. Chem. C 2011, 115, 16087−16097. (49) Henning, S.; Shimizu, R.; Herranz, J.; Kühn, L.; Eychmüller, A.; Uchida, M.; Kakinuma, K.; Schmidt, T. J. Unsupported Pt3Ni Aerogels as Corrosion Resistant PEFC Anode Catalysts under Gross Fuel Starvation Conditions. J. Electrochem. Soc. 2018, 165, F3001− F3006. (50) Zhang, G.; Chenitz, R.; Lefèvre, M.; Sun, S.; Dodelet, J. P. Is Iron Involved in the Lack of Stability of Fe/N/C Electrocatalysts Used to Reduce Oxygen at the Cathode of PEM Fuel Cells? Nano Energy 2016, 29, 111−125. (51) Henning, S.; Ishikawa, H.; Kühn, L.; Herranz, J.; Müller, E.; Eychmüller, A.; Schmidt, T. J. Unsupported Pt-Ni Aerogels with Enhanced High Current Performance and Durability in Fuel Cell Cathodes. Angew. Chem., Int. Ed. 2017, 56, 10707−10710. (52) Chenitz, R.; Kramm, U. I.; Lefevre, M.; Glibin, V.; Zhang, G.; Sun, S.; Dodelet, J.-P. A Specific Demetalation of Fe-N4 Catalytic Sites in the Micropores of NC_Ar+NH3 Is at the Origin of the Initial Activity Loss of This Highly Active Fe/N/C Catalyst Used for the Reduction of Oxygen in PEM Fuel Cell. Energy Environ. Sci. 2018, 11, 365−382. (53) Goellner, V.; Armel, V.; Zitolo, A.; Fonda, E.; Jaouen, F. Degradation by Hydrogen Peroxide of Metal-Nitrogen-Carbon Catalysts for Oxygen Reduction. J. Electrochem. Soc. 2015, 162, H403−H414. (54) Martinez, U.; Komini Babu, S.; Holby, E. F.; Zelenay, P. Durability Challenges and Perspective in the Development of PGMFree Electrocatalysts for the Oxygen Reduction Reaction. Curr. Opin. Electrochem. 2018, 9, 224−232. (55) Malko, D.; Lopes, T.; Symianakis, E.; Kucernak, A. R. The Intriguing Poison Tolerance of Non-Precious Metal Oxygen Reduction Reaction (ORR) Catalysts. J. Mater. Chem. A 2016, 4, 142−152. J


Article

ACS Applied Energy Materials (56) Chung, H. T.; Johnston, C. M.; Zelenay, P. Synthesis and Evaluation of Heat-Treated. Cyanamide-Derived Non-Precious Catalysts of Oxygen Reduction. ECS Trans. 2009, 25, 485−492. (57) Yuan, S.; Shui, J. L.; Grabstanowicz, L.; Chen, C.; Commet, S.; Reprogle, B.; Xu, T.; Yu, L.; Liu, D. J. A Highly Active and SupportFree Oxygen Reduction Catalyst Prepared from Ultrahigh-SurfaceArea Porous Polyporphyrin. Angew. Chem., Int. Ed. 2013, 52, 8349− 8353. (58) Wang, Y.-C.; Lai, Y.-J.; Song, L.; Zhou, Z.-Y.; Liu, J.-G.; Wang, Q.; Yang, X.-D.; Chen, C.; Shi, W.; Zheng, Y.-P.; Rauf, M.; Sun, S.-G. S-Doping of an Fe/N/C ORR Catalyst for Polymer Electrolyte Membrane Fuel Cells with High Power Density. Angew. Chem., Int. Ed. 2015, 54, 9907−9910.

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