Nitrogen-Doped Mesostructured Carbon Supported Metallic Cobalt

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Nitrogen-Doped Mesostructured Carbon Supported Metallic Cobalt Nanoparticles for Oxygen Evolution Reaction Alexander Baehr, Gun-hee Moon, and Harun Tüysüz ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b01183 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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ACS Applied Energy Materials

Nitrogen-Doped Mesostructured Carbon Supported Metallic Cobalt Nanoparticles for Oxygen Evolution Reaction Alexander Bähr, Gun-hee Moon and Harun Tüysüz*

Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany KEYWORDS: mesostructured carbon, pore confinement, soft-templating, nitrogen doping, metallic cobalt, oxygen evolution reaction ABSTRACT: A series of metallic cobalt nanoparticles supported on mesostructured nitrogen-doped carbons was successfully synthesized through soft-templating by using poly(ethylene oxide)-b-polystyrene (PEO-b-PS) as a structure directing agent. The formation of metallic cobalt nanoparticles and nitrogen-doping into carbon structures were simultaneously achieved by ammonia treatment. The physicochemical properties of the resulting materials and consequently their performance for the oxygen evolution were systematically altered by varying cobalt loading (5-89 wt.%), pyrolysis atmosphere (argon or ammonia) and temperatures (600-800 °C). Thereby, up to 37 wt.% of the cobalt nanoparticles were confined in the pores of the mesostructured nitrogen-doped carbon materials with a high BET surface area. At temperatures above 700 °C, the cobalt additionally catalyzes the graphitization of the carbon support. The catalyst with a cobalt loading of 37 wt.% pyrolyzed at 700 °C under ammonia atmosphere shows the highest turnover frequency (TOF) of 311 h-1 in the oxygen evolution reaction due to the improved electronic properties of the carbon support from the incorporation of nitrogen atoms combined with a large amount of accessible cobalt sites. This class of materials shows even higher activity in comparison with ordered mesoporous Co3O4.

INTRODUCTION Over the past several decades electrochemical water splitting has attracted huge attention to produce hydrogen from renewable energy sources since hydrogen is a clean, storable and portable chemical energy carrier.1 In order to optimize the faradaic efficiency for the water electrolysis, a significant research effort has focused on the oxygen evolution reaction (OER, 2 H2O → O2 + 4 H+ + 4 e-, 1.23 VNHE) due to its high overpotential which is kinetically less favorable than that of the hydrogen evolution reaction (HER, 2 H+ + 2 e- → H2, 0 VNHE).2 Among diverse electrocatalysts reported, noble-metal oxides, for example IrO2 and RuO2, showed the highest OER activity, however, high costs and low abundancy limited their utilization for practical applications.3–5 In this regard, non-noble metal oxides and especially cobalt-based oxides and composite materials have been considered as a promising alternative in virtue of a good OER activity assisted by higher oxidation states (i.e. Co3+ and Co4+, latter only forming under reaction conditions), earth abundancy, controllable morphology by using diverse synthetic methodologies and a high stability against electro-corrosion.6–13 For example, mesoporous cobalt oxide catalysts, fabricated by hard-templating methods, provide a large surface area as well as a high porosity which is beneficial to expose more catalytically active sites and accelerate the mass transfer

kinetics of not only electrolyte but also oxygen gas evolved, respectively.14–17 However, unsupported cobalt oxide suffers from its low conductivity (1 ∙ 10-3 S m-1)18 and also lower number of the active centers, which limits their OER performance.19 The challenge can be addressed by adopting the strategy to synthesize highly dispersed metallic cobalt catalysts with higher conductivity (1.8 ∙ 107 S m-1)20 on a conductive support with a high surface area and porosity. It has been demonstrated that supported metallic cobalt catalysts on carbon materials provides a promising OER activity due to a synergetic effect induced by a stabilization with neighbored hetero-atoms such as nitrogen, phosphorus or boron21–24 or a protecting carbon shell.25–27 Nevertheless, an in-depth study mainly focusing on the effect of the size of the metallic cobalt nanoparticles and a structural change of the support materials, which are governed by pyrolysis atmosphere and temperature on the electrocatalytic OER has not been done so far. Carbon-based materials are known as a good support in electrocatalysis due to the versatile opportunities to control their morphology, functional groups and electronic structure along with a high electrical conductivity.28,29 When some carbon atoms are replaced by hetero-elements such as nitrogen, oxygen, sulfur or phosphorous (i.e. doping), the physicochemical properties of carbon materials are significantly changed; a better electrical conductivity and addition of active sites

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Synthesis of PEO-b-PS. To polymerize the PEO-Br with styrene, 30 g styrene, 6 g PEO-Br, 0.16 g CuBr and 0.2 g PMDETA were added to a Schlenk flask. The liquids were degassed by three freeze-pump-thaw cycles. The suspension was then stirred and heated to 110 °C for 3 hours under argon atmosphere. The resulting dark green gel was cooled to room temperature and then dispersed in 200 mL THF. The CuBr was removed by filtration with an Al2O3 column until the filtrate turned colorless (from yellow). The THF was removed under vacuum at 40 °C and the PEO-b-PS was precipitated in 400 mL hexane. After centrifugation, the white solid was dried under vacuum and room temperature. Synthesis of Phenolic Resin. The procedure for the synthesis of phenolic resin was adapted from Teng et al.46 In short, phenol was melted at 55 °C and 14.96 mL were transferred into a round bottom flask. At 45 °C, 1.36 mL NaOH solution (10 wt.%) were added and stirred for 10 minutes. Then, 9.68 mL formaldehyde solution was added and the solution was heated to 70 °C for one hour. At room temperature, the solution was neutralized with 1.5 mL 2 M HCl. The residual water was removed under vacuum at 40 °C to yield brownish oil like resin. Synthesis of CoOx Nanoparticles Supported on Mesostructured Carbon. The following materials were synthesized according to Wang et al.47 For the synthesis of CoOx nanoparticles on ordered mesoporous carbon, 8 g phenolic resin and 1.6 g Co(NO3)2•6 H2O (which represents a nominal cobalt loading of 4 wt.%) were dissolved in 100 mL THF and stirred for 30 minutes. Meanwhile, 2 g PEO-b-PS were dissolved in 100 mL THF. The PEO-b-PS solution was added dropwise to the phenolic resin/Co(NO3)2•6 H2O solution and then stirred for 2 hours. Afterwards, the solution was poured into a crystallizing dish and THF was evaporated at room temperature. The resulting polymer film was dried at 50 °C and 100 °C for 24 hours each. After scratching off the composite from the crystallizing dish, it was pyrolyzed in a tube furnace under argon atmosphere (100 mL min-1) with a heating rate of 5 K min-1 to the desired temperature and a dwelling time of 5 hours. Synthesis of Cobalt Nanoparticles Supported on nitrogen-doped Mesoporous Carbon. For the reduction of the CoOx nanoparticles and the introduction of nitrogen into the carbon support, the obtained composite of PEO-b-PS and phenolic resin/Co(NO3)2•6 H2O mentioned above was pyrolyzed under ammonia atmosphere with a flow rate of 100 ml min-1, a heating rate of 5 K min-1 and a dwelling time of 5 hours. Samples of different cobalt loadings were synthesized by using various amounts of cobalt precursor (0.8, 3.2, 6.4 and 12.8 g) as well as a sample without cobalt for comparison. The pyrolysis temperature was varied between 600, 700 and 800 °C. In order to investigate the effect of the in-situ deposition of metallic cobalt, a sample was also prepared by incorporating cobalt through a conventional ex-situ impregnation route (see Supporting Information for synthetic details). Characterization. The scanning transition electron microscopy (STEM) micrographs and elemental mapping

increased the activity and stability for various catalytic reactions.30–37 For practical applications, the control of size and dispersion of transition metal particles and tailoring of the morphology of carbon supports should be attained in one step. In general, the size of nanoparticles can be controlled by the addition of stabilizers, and some studies reported that well-ordered porous carbon materials with a pore size from a few to several nanometers can stabilize the size of nanoparticles by the confinement in the pore as well.38–40 This class of ordered porous materials show outstanding activity for the hydrogenation,41 42 hydrodeoxygenation, oxygen reduction, 40,43 and overall water splitting.44 Herein, we describe a facile soft-templating route with the advantage that the formation of metallic cobalt particles confined in the pore of a conductive nitrogen-doped carbon support is simply achieved by a one-step process, which leads to a high OER activity. The support material further helps to stabilize the nanoparticles, consequently prevents sintering and maintaining dispersion for a large amount of catalytic active sites. The influence of various parameters including cobalt loading (5-89 wt.%) and pyrolysis atmosphere (argon and ammonia) and temperature (600-800°C) upon the structural change of the composite materials and their effect on the OER activity were investigated systematically. Notably, the electrocatalytic performance is markedly enhanced for the sample prepared under ammonia treatment at 700 °C, where the highest turnover frequency (TOF) of 311 h-1 is recorded. This value is even higher than those of mesoporous Co3O4 synthesized by a hard templating method.

EXPERIMENTAL SECTION Chemicals. Phenol, formaldehyde (37 wt.% in H2O), tetrahydrofuran (THF), pyridine, monomethyl poly(ethylene oxide) (Mn: 5000 g mol-1) (denoted as PEO), α-bromoisobutyryl bromide, Cu(I)Br, N,N,N’,N’’,N’’-pentamethyldiethylenetriamine (PMDETA), Al2O3, Co(NO3)2•6 H2O and styrene were purchased from Sigma Aldrich and used without further purification except styrene, which was extracted with 1 M NaOH and H2O each three times to remove the stabilizer. Synthesis of PEO-Br. The synthesis of PEO-Br and PEO-b-PS is following the procedure according to previously published work from Deng et al.45 In detail, to functionalize the terminal OH-group of PEO, 20 g PEO were dissolved under stirring in 60 mL THF and 40 mL pyridine at 35 °C. The solution was cooled to 0 °C and 3 g α-bromoisobutyryl bromide were added dropwise over 30 minutes. The precipitating white gel was stirred with a glass rod during the whole time. After the complete addition of the bromide, the suspension was stirred overnight at 30 °C. The resulting suspension was filtrated and the filtrate was precipitated with 200 mL cold methyl tert-butyl ether (MTBE). The resulting PEO-Br was washed 3 times with cold MTBE and then dried under vacuum over several days. 2


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ACS Applied Energy Materials minute in water and finally drying in argon flow. For the ink solutions, 750 µL H2O, 250 µL 2-propanol, 4.8 mg catalyst and 25 µL Nafion (Nafion 117 solution, 5% in a mixture of lower aliphatic alcohols and water, Aldrich) were mixed and further dispersed for one hour by ultrasonication. Afterwards, 5.25 µL of ink solution (represents a loading of 0.125 mg cm-2) were drop casted onto the cleaned glassy carbon electrode and dried at 50 °C. For the activity measurements, the electrode was rotated at 2000 rpm and treated with 6 linear sweep voltammograms (LSV) in the range from 0.7 to 1.7 V vs. RHE with a scan rate of 10 mV s-1 and 50 cyclic voltammograms (CV) in the range from 0.7 to 1.6 V vs. RHE with a scan rate of 50 mV s-1. After the 50 CVs, another 3 LSVs were recorded with the same scan range and scan rate as the previous ones where the third LSV was chosen to represent the activity data. The IR drop was compensated at 85% for all measurements and the reproducibility of the results was checked with at least 3 electrodes per sample. For the stability test, chronopotentiometric measurement was conducted at a constant current density of 10.2 mA cm-2 in 1M KOH electrolyte with 2000 rpm without any LSV and CV scans.

were recorded with a Hitachi HD-2700 electron microscope at 200 kV and TEM micrographs were recorded with a H-7100 electron microscope (Hitachi) at 100 kV. Elemental analysis was conducted by “Mikroanalytisches Laboratorium Kolbe” (Oberhausen, Germany) by using inductively coupled plasma optical emission spectroscopy. Powder X-ray diffraction (XRD) patterns were recorded by a Stoe theta/theta diffractometer in Bragg-Brentano geometry using Cu Kα radiation (1.5406 Å). Small angle X-ray scattering (SAXS) patterns were recorded with an Anton Parr SAXSess by using Cu Kα radiation. The detector was exposed for 0.1 s for 30 frames over a width of 3 mm. N2 physisorption at 77 K was performed with a 3Flex device from Micromeritics. Prior to the measurements, the samples were activated under vacuum at 350 °C for 10 hours. The micro- and mesopore volume was determined by applying the NLDFT model for slit pores on a carbon surface at 77 K and the surface area was calculated with the Brunauer-Emmett-Teller (BET) method applied for the relative pressure range from 0.055 to 0.11. X-ray photoelectron spectroscopy (XPS) measurements were performed with a Kratos HSi spectrometer with a hemispherical analyzer. The monochromatized Al Kα X-ray source (E=1486.6 eV) was operated at 15 kV and 15 mA. For the narrow scans, an analyzer pass energy of 40 eV was applied. The hybrid mode was used as lens mode. The base pressure during the experiment in the analysis chamber was 4 ∙ 10-7 Pa. To account charging effects, all spectra have been referred to C 1s at 284.5 eV. Raman spectra were collected before and after chronoamperometry using a Raman Laser (LASER-785 Series, Ocean Optics). The ink solution (containing 4.8 mg of the sample, 750 µL water, 250 µL 2-propanol and 50 µL Nafion solution) was coated onto the gold substrate (gold foil with a thickness of 0.127 mm, 99.99 % trace metals basis, Aldrich) to be utilized as a working electrode. The Raman analysis was carried out right after applying the voltage of 1.55 VNHE in 1 M KOH for 1 h and then washing by deionized water. The SEM micrographs were recorded with a Hitachi S-3500N electron microscope. Electrocatalytic Activity Measurement. The electrochemical OER activity measurements of the synthesized materials were conducted in a three-electrode set-up controlled by a potentiostat (SP150 potentiostat, Biologic Science Instrument) using a rotating disk electrode (AFMSRCS, PINE Research Instrumentation) as working electrode with a diameter of 5 mm (geometric area 0.196 cm2), a hydrogen electrode (HydroFlex, Gaskatel) as reference electrode and a platinum wire as counter electrode. 1 M KOH solution was used as electrolyte. Prior to the measurement, the electrolyte was purged with argon for at least 30 minutes to remove oxygen and during the measurement the cell was kept under argon atmosphere. The temperature of the cell was kept at 25 °C. The working electrodes were prepared by polishing the glassy carbon electrode over cloths with Al2O3 suspensions (1 µm and 0.05 µm, Allied High Tech Products Inc.), then ultrasonication for one

RESULTS AND DISCUSSION Herein, by combining the pyrolysis and nitrogen doping steps we design a series of nitrogen-doped mesostructured carbon supported metallic cobalt nanoparticles with ratios of 5, 9, 37, 65 and 89 wt.%. The first step of the synthesis procedure yields a composite consisting of the di-block copolymer and cobalt-loaded phenolic resin. During the pyrolysis under ammonia atmosphere, the polymer template is decomposed while the phenolic resin is carbonized and doped with nitrogen. At the same time, the cobalt ions are reduced to metallic nanoparticles and mostly confined in the pores of the carbon. To determine the actual cobalt and nitrogen content of the samples, inductively coupled plasma optical emission spectroscopy (ICP-OES) was conducted. The actual cobalt loading of 5, 9, 37, 65 and 89 wt.% differs a lot from the nominal loadings of 2, 4, 8, 16 and 32 wt.% by factors of roughly 2-4. The difference between actual and nominal loading increases with higher cobalt loadings. This can be explained by catalytic carbon decomposition over cobalt during the pyrolysis step under ammonia atmosphere. Moreover, it is reported that carbon/cobalt composite catalysts are active in the ammonia decomposition.48 Thus, if ammonia gets decomposed during the pyrolysis (2 NH3 → 3 H2 + N2), the carbon/cobalt catalyst is exposed to an atmosphere of nitrogen and hydrogen, which hydrogenates the carbon support to form volatile organic compounds. Furthermore, the cobalt/carbon ratio is also varied during the pyrolysis due to carbon degradation that occurs from the reaction of carbon with oxygen functionalities from the phenolic resin to form gaseous CO and CO2. The amount of doped nitrogen in the carbon support for the 37 wt.% cobalt sample decreases from 1.7 3



to 0.8 and 0.5 wt.% by increasing the pyrolysis temperature from 600 to 700 and 800 °C, respectively. A decrease in nitrogen content is expected, since at high temperatures the cleavage of unstable C-N bonds can form thermodynamically more stable C-C bonds under the release of volatile nitrogen containing compounds.49 The loading amount of cobalt significantly influences its particle size and the final morphology of the carbon structure. These properties of the materials were analyzed by using transmission electron microscopy (TEM). As seen from TEM micrographs of the sample series that was pyrolyzed at 700 °C (Figure 1a-f, low magnification TEM images are given in Figure S1), for low loading amounts of 5 and 9 wt.%, most of the cobalt particles have a size of roughly 10 nm and are confined within pores of the carbon support similar to the sample with 37 wt.% cobalt (Figure 1a-d). At higher cobalt loadings (65 and 89 wt.%), the cobalt particles are not exclusively confined in the pores, thus particles are sintered together due to particle migration and agglomeration, which causes formation of larger particles (Figure 1e-f). Next to the particle size, the morphology and structure of the carbon support is affected dramatically by the loading amount of cobalt, as well. Up to a loading of 37 wt.%, the support mainly maintains its ordered mesoporous structure (Figure 1a-d), but for higher loadings is no ordered structure observable (Figure 1e-f). After such kind of high loading, the applied synthesis method reaches its limits to form ordered mesoporous carbon frameworks. The long range ordering of this loading series was further investigated by small angle X-ray scattering (SAXS). The SAXS patterns (Figure S2) show that only the control sample without cobalt nanoparticles has a long range order of mesoporous structure. Applying the Bragg equation on the reflex at the scattering vector of 0.49 nm-1, a unit cell parameter of 12.8 nm was determined for the repeating mesoporous structure. This unit cell size is similar to comparable ordered mesoporous carbon materials.42 Further, the result is in good agreement with the TEM micrographs, where the pore size is around 10 nm. Although the other samples with a cobalt loading up to 37 wt.% show domains with ordered mesopores structures in the TEM micrographs, their long range order is not sufficient enough to show defined reflexes in the SAXS pattern. For further analysis of effects of the pyrolysis temperature on properties of the prepared materials, the sample with a cobalt loading of 37 wt.% was chosen since this material had the best ratio of cobalt loading to confined nanoparticles. As can be seen from the TEM micrographs in Figure 1d, g-h, the change in pyrolysis temperature was also a crucial parameter to alter the structure of the composites. For the sample that is pyrolyzed at 600 °C under ammonia atmosphere, the composite material has an ordered mesoporous carbon structure supported with cobalt nanoparticles of around 10 nm. Increasing the pyrolysis temperature to 700 and 800 °C, the morphology of the carbon support and consequently the dispersion of the cobalt particles was altered. As it is well-known, cobalt is a good catalyst for 4

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carbon nanotube (CNT) formation,50 so increasing the pyrolysis temperature destroys ordered structure as a result of the formation of CNT. Thus, this causes sintering and formation of larger cobalt nanoparticles (Figure 1g).

Figure 1: TEM micrographs of samples pyrolyzed at 700 °C under ammonia atmosphere with cobalt loadings of 0 wt.% (a), 5 wt.% (b), 9 wt.% (c), 37 wt.% (d), 65 wt.% (e) and 89 wt.% (f) as well as for the sample with 37 wt.% cobalt pyrolyzed at 600 (g) and 800 °C (h).

The crystal structure of cobalt and crystallinity of carbon support were further investigated by using X-ray diffraction (XRD). The powder XRD patterns of the samples pyrolyzed at different temperatures under ammonia atmosphere (Figure 2) show that the carbon changes from a rather amorphous structure to a graphitic structure (pdf no. 75-1621) when increasing the pyrolysis temperature from 600 to 800 °C, as evidenced by the reflex at 26°.51 The sample pyrolyzed under argon atmosphere shows the formation of graphitic carbon already at 700 °C. For the samples pyrolyzed under ammonia atmosphere, the cobalt is mainly in a metallic phase (pdf no. 15-0806) due to the strongly reducing atmosphere. The sample treated at 600 °C shows only a small reflex of CoO (pdf no. 09-0402) at 42° indicating that conditions at 600 °C under ammonia are not reducing enough to fully transform the cobalt species to metallic cobalt nanoparticles. Further, the sample prepared under argon atmosphere shows a mixture of mainly metallic cobalt and CoO. At high temperature,


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carbon can act as reducing agent under argon atmosphere as well, which can reduce the cobalt oxide to metallic cobalt nanoparticles. The change of the structural properties of the sample at pyrolysis temperatures of 600, 700 and 800 °C under ammonia was investigated by N2 physisorption (Figure S3). With increasing pyrolysis temperature, the surface area decreases from 561 to 407 and 378 m2 g-1 (Table 1). The micropore volume also decreases from 0.19 to 0.13 and 0.09 cm3 g-1 at increasing temperatures. However, the mesopore volume increases with increasing pyrolysis temperature from 0.19 to 0.25 and 0.48 cm3 g-1. These trends show that with increasing temperatures the micropores consequently grow into mesopores. The formation of CNTs, as they were already observed in the TEM micrographs, can explain this behavior, since CNTs mainly possess mesoporous morphologies with only small amounts of micropores. It is noteworthy that the surface area of the samples pyrolyzed at 700 °C is decreased from 890 m2 g-1 to 5 m2 g-1 when the loading is increased from 0 wt.% to 89 wt.%. This is expected since the majority of the composite material’s surface area originates from the carbon support.

metallic cobalt as bulk phase of the cobalt particles, the oxidation at the surface of the cobalt particles is probably caused by a well reported surface passivation of metallic cobalt when it is exposed to air.52 A loss of intensity for the N 1s signal (Figure S4b) with increasing temperature indicates a decreasing nitrogen concentration in the surface layers of the material. That is in good agreement with the observations of the ICP-OES results. For the sample pyrolyzed at 600 °C under ammonia, the nitrogen species can be differentiated between equal amounts of pyridinic (398.4 eV) and pyrrolic (400.4 eV) nitrogen species,53 whereas the intensity of the sample pyrolyzed at 700 °C is too low for a proper determination of the nitrogen species. For the sample treated at 800 °C, no N 1s signal was detectable due to very low nitrogen content at the surface. Finally, to analyze the cobalt particle distribution as well as the location of nitrogen in the composite material, a STEM analysis combined with elemental mapping was conducted on the selected sample of 37 wt.% cobalt pyrolyzed at 700 °C under ammonia atmosphere. As seen in Figure 3, cobalt particles confined in the pores of the carbon support and nitrogen homogeneously distributed among the carbon support. The detailed characterization confirmed that nitrogen doped mesostructured carbon supported metallic cobalt nanoparticles could be prepared by combining pyrolysis and nitrogen doping steps. The cobalt particles are confined up to a relative high loading of 37 wt.%. At temperatures of 700 °C and above, the cobalt catalyzes the formation of CNTs from the carbon framework leading to a more graphitic carbon support and an overall lower surface area of the material. The impacts of the variation of the structural and textural parameters of the prepared composite materials have been further evaluated for electrochemical OER.

Metallic Co (PDF: 15-0806) Graphite (PDF: 75-1621) CoO (PDF: 09-0402)

700°C, Ar

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


800°C, NH3 700°C, NH3 600°C, NH3

10

20

30

40

50

60

70

80

2/° Figure 2. XRD patterns of the samples with 37 wt.% cobalt at different pyrolysis temperatures (600, 700 and 800 °C) and atmospheres (ammonia and argon).

Further, the samples prepared at different pyrolysis temperatures were investigated by X-ray photoelectron spectroscopy (XPS) to explore oxidation state of the cobalt nanoparticles and the amount and nature of nitrogen species. The Co 2p regions of the samples with 37 wt.% cobalt (Figure S4a) show CoII ions with binding energies of 780.9 eV (Co 2p3/2) and 796.5 eV (Co 2p1/2) with their respective satellites at 786.7 and 803.3 eV for pyrolysis temperatures at 600 and 700 °C, respectively. The Co 2p spectrum of the sample pyrolyzed at 800 °C shows CoII ions as well, although the spectrum is slightly shifted to higher binding energies. This shift can be explained by the different interaction of the cobalt particle with more graphitized and conductive carbon support. A characteristic peak for metallic cobalt at 778.1 eV is not observable. Since the XRD patterns show

Figure 3. STEM micrograph (a) and EDX mapping of carbon (b), cobalt (c) and nitrogen (d) of the sample with 37 wt.% cobalt pyrolyzed at 700 °C under ammonia.

5



The effect of the pyrolysis atmosphere and temperature on the OER activity of the materials is shown in Figure 4a-b. The sample with 37 wt.% cobalt pyrolyzed under ammonia atmosphere shows a much lower overpotential compared to the sample that was treated under argon atmosphere (Figure 4a). In detail, compared to the samples pyrolyzed under argon atmosphere, the overpotential at 10 mA cm-2 for the ammonia treated sample was reduced from 431 mV to 373 mV and the current density at 1.7 V increased from 25 mA cm-2 to 104 mA cm-2 (Table 1). Since the structural parameters of the samples pyrolyzed in ammonia and argon are quite similar regarding cobalt particle size and its distribution

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(Figure S5), long range structural order (Figure S6) and surface area (Figure S7, Table 1), this difference in activity should be attributed to the incorporation of nitrogen into the carbon structure. Nitrogen doping enhances the OHadsorption on positively charged carbon atoms next to electron withdrawing nitrogen atoms and increase the electrocatalyst efficiency.54 Another effect of the nitrogen doping can be the improvement of conductivity of the carbon material due to the introduction of electrons close to the Fermi level.55,56 Additionally, the nitrogen in the carbon support can act as a binding site for the cobalt nanoparticles and thus enhancing their stability compared to the non-doped cobalt/carbon material.57

Figure 4. LSV curves of samples with 37 wt.% cobalt pyrolyzed at 700°C under ammonia and argon atmosphere (a), of samples with 37 wt.% cobalt pyrolyzed under ammonia atmosphere from 600 to 800 °C (inset shows the nitrogen content of the measured samples determined by ICP-OES) (b), of samples pyrolyzed at 700 °C under ammonia atmosphere with the respective loading (c) and the resulting mass activities (at 1.7 V vs. RHE) and TOF (at an overpotential of 400 mV) (d).

The influence of the pyrolysis temperature under ammonia atmosphere is displayed in Figure 4b. The sample with 37 wt.% cobalt treated at 600 °C is much less active for the OER than the samples pyrolyzed at 700 and 800 °C which show almost equal activities regarding overpotential (373 and 376 mV at 10 mA cm-2) and current density (104 and 105 mA cm-2). As observed in the XRD patterns, at high pyrolysis temperatures of 700 and 800 °C the carbon becomes more graphitic leading to a higher

conductivity of the support. Although for the sample pyrolyzed at 600 °C the surface area is larger and the cobalt particles are smaller in comparison with samples with higher pyrolysis temperatures, it seems that the overall surface area, size of cobalt nanoparticles and support morphology do not play a significant role for the OER activity. Instead, the conductivity of the carbon material appears to be much more dominant for the OER performance. Therefore, a combination of graphitic

6


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ACS Applied Energy Materials of the conventionally impregnated sample for OER was 7 times lower than that of the one step synthesized one (i.e., 13.5 and 104 mA cm-2, respectively, Figure S10)). The cobalt content loaded on the supported catalyst plays a critical role in catalysis in fact that loading amount alters the structure but also number of the active centers. The influence of the loading amount of cobalt was further investigated for OER. As seen in Figure 4c, only a moderate activity of the materials could be observed for low cobalt loading of 5 and 9 wt.%, where current densities of 4 and 15 mA cm-2 at 1.7 V have been recorded. (see Table 1 for overpotential and current density values of all samples).

carbon and nitrogen doping results in an improved OER activity, whereas only nitrogen doping at 600 °C into amorphous carbon leads to higher overpotentials of 465 mV at 10 mA cm-2 and lower current density of 11 mA cm-2 at 1.7 V. As a control experiment, ex-situ incorporated cobalt nanoparticles on N-doped carbon were synthesized by conventional impregnation method. The SEM and TEM images reveal that the size of cobalt particles was much larger than that of the samples obtained by the one-step approach and the bulky cobalt particles were predominantly deposited on the outside of the carbon support instead of their confinement inside the mesopores (Figure S8-9). The current density at 1.7 V

Table 1: BET Surface areas, overpotentials and current densities of the samples treated under different pyrolysis atmosphere and temperature and with varying cobalt loading. Synthesis Conditions

BET surface area / m2 g-1

Overpotential @ 10 mA cm-2 / mV

Current Density @ 1.7 V/ mA cm-2

37 wt.% Co

NH3

407

373

104

700 °C

Ar

473

431

25

600 °C

561

465

11

700 °C

407

373

104

800 °C

378

376

105

0 wt.% Co

890

-

1

5 wt.% Co

506

-

4

700 °C

9 wt.% Co

532

452

15

NH3

37 wt.% Co

407

373

104

65 wt.% Co

163

369

137

89 wt.% Co

5

388

44

37 wt.% Co NH3

However, the current densities were enhanced with increasing the loading amount (up to 65 wt.% cobalt) to 137 mA cm-2. Further increasing of cobalt loading to 89 wt.% reduces the activity of the sample dramatically and results in a current density of 44 mA cm-2 at 1.7 V. Those values are comparable to similar nitrogen-doped carbon supported metallic cobalt catalyst where a cobalt loading of 16.8 wt.% results in an overpotential of 410 mV.58 To evaluate the activity of the materials normalized to the cobalt amount, the mass activity at 1.7 V and the turnover frequency (TOF) at an overpotential of 400 mV were calculated from those LSV curves under the assumption that every cobalt atom is electrochemically active and 100 % Faraday efficiency is achieved (see Supporting Information, Eq. 3 and Eq. 4 for calculation).16,59 It should be kept in mind that the TOF calculation does not give absolute values since not every

cobalt atom is electrochemically accessible and the faradaic efficiency is a little bit reduced due to cobalt oxidation as well (Figure S11). The normalized evaluation of the materials shows that the mass activities first increase steadily with increasing cobalt loading from 0.6 A mgCo-1 at 5 wt.% cobalt to 2.1 A mgCo-1 at 37 wt.% cobalt, but then markedly decreases with higher cobalt loading to 0.4 A mgCo-1 at 89 wt.% cobalt (Figure 4d). The TOF follows the same trend, where the values increase from 52 h-1 at 5 wt.% cobalt to 311 h-1 at 37 wt.% and then dropping to 67 h-1 at 89 wt.%. The result supports that a cobalt loading of 37 wt% is optimal to form accessible cobalt sites with the described synthesis procedure. At lower loadings, where the cobalt particles mainly show the same fine size distribution, the carbon framework might not be as conductive as at higher loadings due to low degree of graphitization of 7


ACS Applied Energy Materials support. This is also confirmed by XRD patterns of the loading series (Figure S12), where the broad reflex of amorphous carbon decreases and the reflex of graphitic domains at 26° increases with increasing cobalt loading. At very high cobalt loadings, most of the cobalt particles are sintered as proven by the TEM micrographs, thus the active sites are located in the bulk of the large particles and are not accessible by the electrolyte for OER. The activities of the supported catalysts is further compared with an ordered mesoporous Co3O4 synthesized by a hard templating method using KIT-6 silica as template (Figure S13).17 The supported sample with 37 wt.% cobalt shows a roughly four-times higher TOF than the templated pure Co3O4 (TOF of 71 h-1), showing that supporting cobalt species on a nitrogen-doped carbon leads to synergistic effects due to the improved conductivity and charge transfer compared to a pure metal oxide catalyst. 1.75

Potential @ 10.2 mA cm-2 / V vs RHE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.70

1.65

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CONCLUSION This work presents a novel approach for the synthesis of nitrogen-doped mesostructured carbon supported metallic cobalt nanoparticles where pyrolysis and nitrogen doping steps are combined in one step. A series of composite materials was prepared through soft-templating by varying cobalt loading, pyrolysis temperature and atmosphere. Up to a relatively high loading of 37 wt.%, the cobalt particles were confined in the pores of the mesoporous carbon framework. The pyrolysis temperatures above 700 °C led to a partial destruction of the mesostructured carbon framework by forming CNTs. The parameters for the sample synthesized with 37 wt.% cobalt pyrolyzed at 700 °C under ammonia atmosphere are found to be optimal for high active OER catalysts, since they provide an optimal ratio of accessible cobalt sites combined with a well conducting nitrogen-doped carbon support. This class of materials shows even higher activity in comparison with ordered mesoporous Co3O4. This study serves as a showcase for a synthesis concept that combines the incorporation of nitrogen into the carbonaceous support and the reduction of metal precursors in a single step for the design of effective OER catalysts.

1.60

ASSOCIATED CONTENT

1.55

The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.”

1.50

1.45 0

50

100

150

200

250

Time / min

Figure 5. Chronopotentiometric measurement of the sample with 37 wt.% cobalt pyrolyzed at 700 °C under ammonia atmosphere at a constant current density of 10.2 mA cm-2.

At the last, the sample with the highest TOF value in the OER was tested for stability using a chronopotentiometry at a fixed current density of 10.2 mA cm-2 over the time (Figure 5). The resulting profile of the potential curve indicates an activation of the electrocatalyst in the beginning of the experiment. This can be ascribed to the in-situ oxidation of the cobalt nanoparticles under bias condition that is able to induce (i) the formation of amorphous cobalt species with higher oxidation states (i.e., Co(OH)2, CoOOH, CoO2, etc.) and (ii) the geometric rearrangement of cobalt nanoparticles, which were confirmed by post-structural analyses through XPS, Raman, and TEM (Figure S14 and S15).60 After this initial activation period, the potential required to keep the 10.2 mA cm-2 stays constant at 1.6 mV for additional 3 hours. The small fluctuations in the potential curve are ascribed to the formation of oxygen bubbles on the electrode surface, which get detached from the electrode when they reach a critical size.

It contains SAXS patterns, N2 physisorption isotherms, XPS spectra, TEM micrographs, calculation of mass activity and TOF, XRD patterns, Raman spectra and electrochemical data of ordered mesoporous Co3O4.

AUTHOR INFORMATION Corresponding Author *E-Mail: [email protected]

Funding Sources This work was supported by the Carbon2Chem project funded by the Bundesministerium für Bildung und Forschung (BMBF) of the German government and the MAXNET Energy research consortium of the Max Planck Society.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Dr. C. Weidenthaler for the analysis of XPS data, Mr. N. Pfänder and Mr. B. Spliethoff for their support with STEM and TEM measurements and Mrs. S Palm for her help with the SEM measurements.

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REFERENCES (1) Lewis, N. S.; Nocera, D. G. Powering the planet: Chemical challenges in solar energy utilization. P. Natl. Acad. Sci. USA 2006, 15729–15735. (2) Dau, H.; Limberg, C.; Reier, T.; Risch, M.; Roggan, S.; Strasser, P. The Mechanism of Water Oxidation: From Electrolysis via Homogeneous to Biological Catalysis. ChemCatChem 2010, 2, 724–761. (3) Fabbri, E.; Habereder, A.; Waltar, K.; Kötz, R.; Schmidt, T. J. Developments and perspectives of oxide-based catalysts for the oxygen evolution reaction. Catal. Sci. Technol. 2014, 4, 3800– 3821. (4) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3, 399–404. (5) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar water splitting cells. Chem. Rev. 2010, 110, 6446–6473. (6) McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices. J. Am. Chem. Soc. 2015, 137, 4347–4357. (7) Subbaraman, R.; Tripkovic, D.; Chang, K.-C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in activity for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts. Nat. Mater. 2012, 11, 550–557. (8) Smith, R. D. L.; Prévot, M. S.; Fagan, R. D.; Trudel, S.; Berlinguette, C. P. Water oxidation catalysis: Electrocatalytic response to metal stoichiometry in amorphous metal oxide films containing iron, cobalt, and nickel. J. Am. Chem. Soc. 2013, 135, 11580–11586. (9) Hocking, R. K.; Brimblecombe, R.; Chang, L.-Y.; Singh, A.; Cheah, M. H.; Glover, C.; Casey, W. H.; Spiccia, L. Wateroxidation catalysis by manganese in a geochemical-like cycle. Nat. Chem. 2011, 3, 461–466. (10) Esswein, A. J.; McMurdo, M. J.; Ross, P. N.; Bell, A. T.; Tilley, T. D. Size-Dependent Activity of Co3O4 Nanoparticle Anodes for Alkaline Water Electrolysis. J. Phys. Chem. C 2009, 113, 15068– 15072. (11) Deng, X.; Tüysüz, H. Cobalt-Oxide-Based Materials as Water Oxidation Catalyst: Recent Progress and Challenges. ACS Catal. 2014, 4, 3701–3714. (12) Moon, G.-h.; Yu, M.; Chan, C. K.; Tüysüz, H. Highly Active Cobalt-Based Electrocatalysts with Facile Incorporation of Dopants for the Oxygen Evolution Reaction. Angew. Chem. Int. Ed. 2019, 58, 3491–3495. (13) Fang, G.; Wang, Q.; Zhou, J.; Lei, Y.; Chen, Z.; Wang, Z.; Pan, A.; Liang, S. Metal Organic Framework-Templated Synthesis of Bimetallic Selenides with Rich Phase Boundaries for Sodium-Ion Storage and Oxygen Evolution Reaction. ACS nano 2019, 13, 5635–5645. (14) Deng, X.; Chan, C. K.; Tüysüz, H. Spent Tea Leaf Templating of Cobalt-Based Mixed Oxide Nanocrystals for Water Oxidation. ACS Appl. Mater. Interfaces 2016, 8, 32488– 32495. (15) Yu, M.; Chan, C. K.; Tüysüz, H. Coffee-Waste Templating of Metal Ion-Substituted Cobalt Oxides for the Oxygen Evolution Reaction. ChemSusChem 2018, 11, 605–611. (16) Tüysüz, H.; Hwang, Y. J.; Khan, S. B.; Asiri, A. M.; Yang, P. Mesoporous Co3O4 as an electrocatalyst for water oxidation. Nano Res. 2013, 6, 47–54.

(17) Deng, X.; Schmidt, W. N.; Tüysüz, H. Impacts of Geometry, Symmetry, and Morphology of Nanocast Co3O4 on Its Catalytic Activity for Water Oxidation. Chem. Mater. 2014, 26, 6127–6134. (18) Shaju, K. M.; Guerlou-Demourgues, L.; Godillot, G.; Weill, W.; Delmas, C. Strategies for Synthesizing Conductive Spinel Cobalt Oxide Nanoparticles for Energy Storage Applications. J. Electrochem. Soc. 2012, 159, A1934-A1940. (19) Burke, M. S.; Enman, L. J.; Batchellor, A. S.; Zou, S.; Boettcher, S. W. Oxygen Evolution Reaction Electrocatalysis on Transition Metal Oxides and (Oxy)hydroxides: Activity Trends and Design Principles. Chem. Mater. 2015, 27, 7549–7558. (20) Rumble, J. R.; Lide, D. R.; Bruno, T. J. CRC handbook of chemistry and physics: A ready-reference book of chemical and physical data, 99th edition; CRC Press: Boca Raton, 2018. (21) Chen, P.; Xu, K.; Fang, Z.; Tong, Y.; Wu, J.; Lu, X.; Peng, X.; Ding, H.; Wu, C.; Xie, Y. Metallic Co4N Porous Nanowire Arrays Activated by Surface Oxidation as Electrocatalysts for the Oxygen Evolution Reaction. Angew. Chem. 2015, 127, 14923– 14927. (22) Jiang, N.; You, B.; Sheng, M.; Sun, Y. Electrodeposited Cobalt-Phosphorous-Derived Films as Competent Bifunctional Catalysts for Overall Water Splitting. Angew. Chem. 2015, 127, 6349–6352. (23) Chen, P.; Xu, K.; Tong, Y.; Li, X.; Tao, S.; Fang, Z.; Chu, W.; Wu, X.; Wu, C. Cobalt nitrides as a class of metallic electrocatalysts for the oxygen evolution reaction. Inorg. Chem. Front. 2016, 3, 236–242. (24) Chen, Z.; Wang, Q.; Zhang, X.; Lei, Y.; Hu, W.; Luo, Y.; Wang, Y. N-doped defective carbon with trace Co for efficient rechargeable liquid electrolyte-/all-solid-state Zn-air batteries. Sci. Bull. 2018, 63, 548–555. (25) Xiao, Q.; Zhang, Y.; Guo, X.; Jing, L.; Yang, Z.; Xue, Y.; Yan, Y.-M.; Sun, K. A high-performance electrocatalyst for oxygen evolution reactions based on electrochemical post-treatment of ultrathin carbon layer coated cobalt nanoparticles. Chem. Commun. 2014, 50, 13019–13022. (26) Su, J.; Xia, G.; Li, R.; Yang, Y.; Chen, J.; Shi, R.; Jiang, P.; Chen, Q. Co 3 ZnC/Co nano heterojunctions encapsulated in Ndoped graphene layers derived from PBAs as highly efficient bifunctional OER and ORR electrocatalysts. J. Mater. Chem. A 2016, 4, 9204–9212. (27) Shi, Y.; Wang, Y.; Yu, Y.; Niu, Z.; Zhang, B. N-doped graphene wrapped hexagonal metallic cobalt hierarchical nanosheet as a highly efficient water oxidation electrocatalyst. J. Mater. Chem. A 2017, 5, 8897–8902. (28) Moon, G.-h.; Shin, Y.; Choi, D.; Arey, B. W.; Exarhos, G. J.; Wang, C.; Choi, W.; Liu, J. Catalytic templating approaches for three-dimensional hollow carbon/graphene oxide nanoarchitectures. Nanoscale 2013, 5, 6291–6296. (29) Yu, Z.; Tetard, L.; Zhai, L.; Thomas, J. Supercapacitor electrode materials: Nanostructures from 0 to 3 dimensions. Energy Environ. Sci. 2015, 8, 702–730. (30) Dai, L.; Xue, Y.; Qu, L.; Choi, H.-J.; Baek, J.-B. Metal-free catalysts for oxygen reduction reaction. Chem. Rev. 2015, 115, 4823–4892. (31) Deng, D.; Novoselov, K. S.; Fu, Q.; Zheng, N.; Tian, Z.; Bao, X. Catalysis with two-dimensional materials and their heterostructures. Nat. Nanotechnol. 2016, 11, 218–230. (32) Liu, X.; Dai, L. Carbon-based metal-free catalysts. Nat. Rev. Mater. 2016, 1, 760. (33) Wu, M.; Wang, K.; Yi, M.; Tong, Y.; Wang, Y.; Song, S. A Facile Activation Strategy for an MOF-Derived Metal-Free Oxygen Reduction Reaction Catalyst: Direct Access to Optimized Pore Structure and Nitrogen Species. ACS Catal. 2017, 7, 6082–6088. (34) Xu, Y.; Kraft, M.; Xu, R. Metal-free carbonaceous

9



electrocatalysts and photocatalysts for water splitting. Chem. Soc. Rev. 2016, 45, 3039–3052. (35) Yu, D.; Xue, Y.; Dai, L. Vertically Aligned Carbon Nanotube Arrays Co-doped with Phosphorus and Nitrogen as Efficient Metal-Free Electrocatalysts for Oxygen Reduction. J. Phys. Chem. Lett. 2012, 3, 2863–2870. (36) Ju, M. J.; Jeon, I.-Y.; Lim, K.; Kim, J. C.; Choi, H.-J.; Choi, I. T.; Eom, Y. K.; Kwon, Y. J.; Ko, J.; Lee, J.-J.; Baek, J.-B.; Kim, H. K. Edge-carboxylated graphene nanoplatelets as oxygen-rich metalfree cathodes for organic dye-sensitized solar cells. Energy Environ. Sci. 2014, 7, 1044–1052. (37) Wang, Q.; Lei, Y.; Wang, D.; Li, Y. Defect engineering in earth-abundant electrocatalysts for CO2 and N2 reduction. Energy Environ. Sci. 2019, 12, 1730–1750. (38) Ma, T.-Y.; Liu, L.; Yuan, Z.-Y. Direct synthesis of ordered mesoporous carbons. Chem. Soc. Rev. 2013, 42, 3977–4003. (39) Meng, Y.; Gu, D.; Zhang, F.; Shi, Y.; Yang, H.; Li, Z.; Yu, C.; Tu, B.; Zhao, D. Ordered mesoporous polymers and homologous carbon frameworks: Amphiphilic surfactant templating and direct transformation. Angew. Chem. Int. Ed. 2005, 44, 7053– 7059. (40) Moon, G.-h.; Bähr, A.; Tüysüz, H. Structural Engineering of 3D Carbon Materials from Transition Metal Ion-Exchanged Y Zeolite Templates. Chem. Mater. 2018, 30, 3779–3788. (41) Wang, G.-H.; Deng, X.; Gu, D.; Chen, K.; Tüysüz, H.; Spliethoff, B.; Bongard, H.-J.; Weidenthaler, C.; Schmidt, W.; Schüth, F. Co3O4 Nanoparticles Supported on Mesoporous Carbon for Selective Transfer Hydrogenation of α,β-Unsaturated Aldehydes. Angew. Chem. Int. Ed. 2016, 55, 11101–11105. (42) Wang, G.-H.; Cao, Z.; Gu, D.; Pfänder, N.; Swertz, A.-C.; Spliethoff, B.; Bongard, H.-J.; Weidenthaler, C.; Schmidt, W.; Rinaldi, R.; Schüth, F. Nitrogen-Doped Ordered Mesoporous Carbon Supported Bimetallic PtCo Nanoparticles for Upgrading of Biophenolics. Angew. Chem. Int. Ed. 2016, 55, 8850–8855. (43) Liu, G.; Li, X.; Ganesan, P.; Popov, B. N. Development of non-precious metal oxygen-reduction catalysts for PEM fuel cells based on N-doped ordered porous carbon. Appl. Catal. B 2009, 93, 156–165. (44) Xu, M.; Huang, L.; Fang, Y.; Han, L.; Yu, Y.; Dong, S. The unified ordered mesoporous carbons supported Co-based electrocatalysts for full water splitting. Electrochim. Acta 2018, 261, 412–420. (45) Deng, Y.; Yu, T.; Wan, Y.; Shi, Y.; Meng, Y.; Gu, D.; Zhang, L.; Huang, Y.; Liu, C.; Wu, X.; Zhao, D. Ordered mesoporous silicas and carbons with large accessible pores templated from amphiphilic diblock copolymer poly(ethylene oxide)-bpolystyrene. J. Am. Chem. Soc. 2007, 129, 1690–1697. (46) Teng, W.; Wu, Z.; Fan, J.; Chen, H.; Feng, D.; Lv, Y.; Wang, J.; Asiri, A. M.; Zhao, D. Ordered mesoporous carbons and their corresponding column for highly efficient removal of microcystin-LR. Energy Environ. Sci. 2013, 6, 2765. (47) Wang, Z.; Zhu, Y.; Luo, W.; Ren, Y.; Cheng, X.; Xu, P.; Li, X.; Deng, Y.; Zhao, D. Controlled Synthesis of Ordered Mesoporous

Page 10 of 11

Carbon-Cobalt Oxide Nanocomposites with Large Mesopores and Graphitic Walls. Chem. Mater. 2016, 28, 7773–7780. (48) Torrente-Murciano, L.; Hill, A. K.; Bell, T. E. Ammonia decomposition over cobalt/carbon catalysts—Effect of carbon support and electron donating promoter on activity. Catal. Today 2017, 286, 131–140. (49) Liu, J.; Webster, S.; Carroll, D. L. Temperature and flow rate of NH3 effects on nitrogen content and doping environments of carbon nanotubes grown by injection CVD method. J. Phys. Chem. B 2005, 109, 15769–15774. (50) Guo, T.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Catalytic growth of single-walled nanotubes by laser vaporization. Chem. Phys. Lett. 1995, 243, 49–54. (51) Wei, Z.; Wang, J.; Mao, S.; Su, D.; Jin, H.; Wang, Y.; Xu, F.; Li, H.; Wang, Y. In Situ-Generated Co0-Co3O4/N-Doped Carbon Nanotubes Hybrids as Efficient and Chemoselective Catalysts for Hydrogenation of Nitroarenes. ACS Catal. 2015, 5, 4783–4789. (52) Brundle, C. R.; Chuang, T. J.; Rice, D. W. X-Ray Photoemission Study of the Interaction of Oxygen and Air with Clean Cobalt Surfaces. Surf. Sci. 1976, 60, 286–300. (53) Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Evolution of Nitrogen Functionalities in Carbonaceous Materials during Pyrolysis. Carbon 1995, 33, 1641–1653. (54) Zhao, Y.; Nakamura, R.; Kamiya, K.; Nakanishi, S.; Hashimoto, K. Nitrogen-doped carbon nanomaterials as nonmetal electrocatalysts for water oxidation. Nat. Commun. 2013, 4, 2390. (55) Czerw, R.; Terrones, M.; Charlier, J.-C.; Blase, X.; Foley, B.; Kamalakaran, R.; Grobert, N.; Terrones, H.; Tekleab, D.; Ajayan, P. M.; Blau, W.; Rühle, M.; Carroll, D. L. Identification of Electron Donor States in N-Doped Carbon Nanotubes. Nano Lett. 2001, 1, 457–460. (56) Cordero-Lanzac, T.; Rosas, J. M.; García-Mateos, F. J.; Ternero-Hidalgo, J. J.; Palomo, J.; Rodríguez-Mirasol, J.; Cordero, T. Role of different nitrogen functionalities on the electrochemical performance of activated carbons. Carbon 2018, 126, 65–76. (57) Nie, R.; Miao, M.; Du, W.; Shi, J.; Liu, Y.; Hou, Z. Selective hydrogenation of C C bond over N-doped reduced graphene oxides supported Pd catalyst. Appl. Catal. B 2016, 180, 607–613. (58) Liu, S.; Wang, Z.; Zhou, S.; Yu, F.; Yu, M.; Chiang, C.-Y.; Zhou, W.; Zhao, J.; Qiu, J. Metal-Organic-Framework-Derived Hybrid Carbon Nanocages as a Bifunctional Electrocatalyst for Oxygen Reduction and Evolution. Adv. Mater. 2017, 29, 1700874– 1700883. (59) Lin, Y.; Wu, K.-H.; Lu, Q.; Gu, Q.; Zhang, L.; Zhang, B.; Su, D.; Plodinec, M.; Schlögl, R.; Heumann, S. Electrocatalytic Water Oxidation at Quinone-on-Carbon: A Model System Study. J. Am. Chem. Soc. 2018, 140, 14717–14724. (60) Yeo, B. S.; Bell, A. T. Enhanced activity of gold-supported cobalt oxide for the electrochemical evolution of oxygen. J. Am. Chem. Soc. 2011, 133, 5587–5593.

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Nitrogen-Doped Mesostructured Carbon Supported Metallic Cobalt

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