Structural Engineering of 3D Carbon Materials from Transition Metal

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Structural Engineering of 3D Carbon Materials from Transition Metal Ion-Exchanged Y Zeolite Templates Gun-hee Moon, Alexander Bähr, and Harun Tüysüz Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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Chemistry of Materials

Structural Engineering of 3D Carbon Materials from Transition Metal Ion-Exchanged Y Zeolite Templates Gun-hee Moon, Alexander Bähr, and Harun Tüysüz*

Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, Mülheim an der Ruhr, D-45470, Germany E-mail: [email protected]

Submitted to Chemistry of Materials (Revised)

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ABSTRACT A series of three-dimensional ordered microporous carbon materials (3D CMs) were prepared through nanocasting route by using transition metal ion-exchanged Y zeolite (M-Y) as template and ethylene gas a carbon source. The different d-π coordination and the formation of metal nanoparticles during thermal treatment altered textural parameters of the final carbon products. After a detailed structural analysis and characterization, the most promising cobaltcarbon sample was further treated with NH3 for nitrogen doping and evaluated for oxygen reduction reaction (ORR). This new class of material indicated good electrochemical stability and similar activity in comparison with commercial Pt/C (20 wt.%) electrocatalyst. The protocol developed here allows in-situ incorporation of diverse transition metals as well as doping of various hetero-elements into three-dimensional carbon framework and has a big potential for different catalytic applications.


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INTRODUCTION Nanostructured carbon materials including 0-dimensional (0D) fullerenes, 1D carbon nanotubes (CNT), 2D graphene, and 3D nano-architectures have received great attention. The unique

physicochemical properties of this materials arise from the delocalization of π

electrons, their electronic structure, surface energy, functionality, and distortion of carbon lattices.1 Graphitic carbon materials have been widely utilized for energy and environmental applications because of their high conductivities that can facilitate the transfer of electrons supplied from the conduction band of semiconductors or the cathodic bias to desired electron acceptor.2-6 Although a great deal of effort has been performed to precisely tailor the structure, the fabrication of 3D carbon materials with well-ordered channels preserving a high conductivity still remains a big challenge.7-8 Very recently, Ryoo reported the preparation of microporous 3D carbon materials (3D CMs) with high conductivity from lanthanumexchanged zeolite templates, demonstrating the role of lanthanum ions as a catalyst for the carbonization of ethylene to graphitic carbons inside the zeolite frameworks.9 Carbon-based composites, such as those containing (i) transition metal species10-14, (ii) hetero-element-doping (doping or co-doping for N, B, S, P, O, etc. elements),15-17 and (iii) both transition metal species and hetero-element-doped structured carbon materials18, 19 show unusual catalytic nature from synergistic coupling effects and the formation of more active sites. The high specific surface area of carbon materials makes them good supports for the dispersion of nanoparticles, and the high conductivity enabled by the delocalized π electrons can facilitate charge transfer to enhance various redox reactions. Furthermore, the doping of hetero-atoms changes the energy bandgap, the spin density, and the charge density of surrounding carbon atoms,20 providing many advantages for energy conversion and storage applications such as fuel cells, batteries, photocatalysis, (photo)electrochemical cells, etc. For example, the overpotential for the oxygen reduction reaction (ORR) can be effectively reduced via not only the parallel adsorption of diatomic O2, induced by a charge redistribution 3 ACS Paragon Plus Environment


arising from N-doping, but also the formation of Co-O-C and Co-N-C bonds between Co3O4 and N-doped graphene in the Co3O4/N-doped graphene hybrid systems.21 In this work, we fabricated microporous 3D CMs using Y zeolites exchanged with transition metal ions (Co2+, Mn2+, Cu2+, and Ni2+) as a hard template. The synthetic procedure can be summarized as follows: (i) the replacement of Na+ in Y zeolite templates with transition metal ions (M-Y, M: Co2+, Mn2+, Cu2+, and Ni2+), (ii) the carbonization and graphitization (M-C) of the ion-exchanged templates, (iii) the removal of the zeolite templates using NaOH, and (iv) the removal of transition metal species and trace amorphous silicates using HCl (M-C-A; noted as 3D CMs) (Scheme 1). We found that the catalytic conversion of ethylene to polymeric carbon bonds was enabled by the transition metal ions (not the zerovalent transition metals), while the ethylene polymerization and graphitization proceeded inside zeolite frameworks through a d-π coordination, stabilizing the ethylene and pyrocondensation intermediates. The morphology of the NaY zeolite was well-reproduced for 3D CMs obtained from M-Y templates (M: La3+, Co2+, Mn2+, and Cu2+), while only CNTs were formed in the Ni-Y template on account of the preference for 1D growth of carbon moieties when metallic Ni nanoparticles are formed. Although further studies on the fabrication of 3D CMs by La3+ and Ca2+ exchanged zeolites have been reported by Ryoo’s group since 2016,22,

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the transition metal-exchanged zeolites can be widely utilized for

various catalytic reactions and are cost-effective. Furthermore, the synthetic concept can be readily expanded for the structural engineering of 3D CMs as starting materials with high flexibility for the target applications. Herein, we focused on in-situ incorporation of transition metal species in 3D CMs, using cobalt-exchanged materials as an example of using the synthetic concept to obtain materials as catalysts for electrochemical ORR. Mild acid and NH3 treatments adopted for the selective removal of cobalt species and to enable N-doping, respectively, are shown to have a large effect on the ORR activities of the materials, resulting in a comparable performance with the commercial Pt/C (20 wt.%) electrocatalyst. 4 ACS Paragon Plus Environment

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MATERIALS AND METHODS Materials Synthesis. Lanthanum and transition metal ions (La3+, LaCl3•xH2O (99.999 %, STREM); Co2+, Co(NO3)2•6H2O (ACS reagent, Aldrich); Mn2+, MnCl2•4H2O (> 99.0 %, Aldrich); Cu2+, Cu(NO3)2•3H2O (99-104 %, Aldrich); Ni2+, Ni(NO3)2•6H2O (> 97.0 %, Aldrich)) were added to aqueous suspensions containing 1 g of NaY zeolites (Molecular sieve, NaY, Aldrich). The initial concentration of cations was fixed at 0.2 M. The pH was adjusted to around 3.5 using HCl for La3+ and Mn2+, and HNO3 for the other cations. The solution was vigorously stirred at room temperature overnight. After M-Y (M: La3+, Co2+, Mn2+, Cu2+, and Ni2+) samples were collected by centrifugation, they were washed with distilled water several times and then dried at 50 oC overnight. Carbonization and graphitization were then accomplished in three steps: (1) Heating up to 600 oC under N2 flow, (2) Carbonization at 600 o

C for 1 h by flowing ethylene, water vapor, and N2 gases (water vapor was fed through by

bubbling N2 carrier gas through a water bath), and (3) Graphitization at 850 oC for 2 h under N2 flow. The samples prepared after carbonization and graphitization of M-Y were labelled as M-C (M: La3+, Co2+, Mn2+, Cu2+, and Ni2+). The zeolite templates were removed using NaOH (2 M) at 80 oC for 8 h and the process was repeated after centrifugation. Then, the samples were washed using distilled water several times. After base treatment, the transition metal species and trace amorphous silicate was removed using HCl (0.1 M) overnight, followed by washing with distilled water several times and drying at 60 oC in the oven. The samples of MC after base and acid treatment were noted as M-C-A (M: La3+, Co2+, Mn2+, Cu2+, and Ni2+). Graphite oxide (GO) was synthesized using the modified Hummer method,24 and the oxygen-containing functional groups were eliminated by heat treatment at 1,000 oC under Ar atmosphere. CNTs were purchased from Hanwha chemical (Multi-walled carbon nanotube, CH-150), and iron impurities were removed using concentrated nitric acid. After base and acid treatment, nitrogen-doping was accomplished for Co-C-A using NH3 treatment at 850 oC for 1 h. To confirm the concentration effect of HCl on the ORR 5 ACS Paragon Plus Environment


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activities, the acid treatment was modified using HCl with different concentrations (0, 0.5, 1, and 2 M) for 4 h before NH3 treatment. The samples of Co-C-A after NH3 treatment were labelled as N-Co-C-A (0 to 2 M) depending on the concentration of HCl. The NH3 treatment was carried out at 1,000 oC for rGO and CNT to effectively introduce nitrogen into the carbon lattices (so called, N-rGO and N-CNT). All samples were washed by distilled water several times to remove the physisorbed NH3. Electrochemical Activity Measurement. Electrochemical ORR performance was measured (SP-150 potentiostat, Bio-logic Science Instrument) using a three-electrode system (working electrode: a rotating disk electrode (AFMSRCE, PINE Research Instrumentation), reference electrode: a hydrogen electrode (HydroFlex, Gaskatel), and counter electrode: Pt wire) in 0.1 M KOH as an electrolyte. Oxygen gas was purged for 30 min to saturate the electrolyte before the measurement, and the temperature of electrolyte cell was fixed at 25 oC. The working electrode was prepared by drop-casting the solution (9.81 µL of suspension containing 0.98 mL of ethanol, 20 µL of Nafion (Nafion® 117 solution, 5% in a mixture of lower aliphatic alcohols and water, Aldrich), and 2 mg of the samples) on glassy carbon electrodes (5 mm in diameter, 0.196 cm2 surface area). Each time, the electrode was polished using an Al2O3 suspension (5 and 0.25 µm, Allied High Tech Products, Inc.), and was dried using Ar gas after sonication in distilled water for 1 min. Cyclic voltammetry (CV) was performed for 50 cycles from 0 to 1.0 VRHE with a scan rate of 100 mV/s to stabilize the electrode, and then linear sweep voltammetry (LSV) was continuously conducted from 0 to 1.1 V with a different rotating disk from 2,400 to 1,000 rpm. The number of electrons transferred to dissolved oxygen was calculated by the equations (1 and 2) as below.

= + ( )/

(1)

= . ( ) ()


(2)

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where jD = disk current density (mA/cm2), jk = kinetic current density (mA/cm2), ω = electrode rotating speed (rad/s), n = number of electrons, F = Faraday constant (96,485 C/mol), A = surface area of electrode (0.196 cm2), CO2 = concentration of dissolved O2 (1.2 x 10-3 mol/L), ν = kinetic viscosity (0.01 cm2/s), and DO2 = diffusion coefficient of dissolved O2 in 0.1 M KOH (1.9 x 10-5 cm-5/s), respectively. Before the durability test, the CV curves from 0 to 1 V were performed for 50 cycles with a scan rate of 100 mV/s, and then chronoamperometry was measured at constant bias of 0.5 V at 800 rpm under O2-saturated condition.

RESULTS AND DISCUSSION 3D CMs were successfully fabricated by the step-by-step procedure starting with the ionexchange (M-Y; M: Co2+, Mn2+, Cu2+, and Ni2+) of the Na zeolite, following by carbonization and graphitization (M-C), and finally removal of zeolite templates and transition metal species by NaOH and HCl (M-C-A), respectively. As a control experiment, La-Y template was also used for the synthesis of 3D CMs. In principle, transition metal ions can be localized at different crystallographic sites including the (i) hexagonal prism (Site I), (ii) sodalite cage (Site I’), (iii) supercage (Site II), and (iv) β cage (Site II’) during the ion exchange process. The localization of cations maximizes their interaction with oxygen atoms present in aluminosilicate, together with minimizing the electrostatic repulsion among neighboring cations.25 As shown in Figure S1a, the color of NaY was changed from white to pink, blue, and green after exchange with Co2+, Cu2+, and Ni2+, respectively. The exchange with La3+ and Mn2+ did not result in any color change (the color of La3+ and Mn2+ precursor was white and light pink, respectively.). In ion-exchanged Y zeolites, the cations are mainly placed in the supercage at ambient condition, but they can migrate to the hexagonal or sodalite cage during dehydration process.26-28 Due to heating up to 600 oC under inert atmosphere, some of



the transition metal ions could migrate inside the zeolite, thereby providing uniformly dispersed catalytic active sites for the carbonization of ethylene. The carbonization was carried out under continuous flowing ethylene and water vapor at 600 oC for 1 h, with a mechanistic pathway proposed as follows: (i) La3+ ions can stabilize ethylene as a result of d-π interaction, (ii) carbide was formed by the reaction between La3+ and ethylene, (iii) active carbonaceous moieties were generated by the reaction of carbide with water vapor, and (iv) carbon frameworks were formed inside zeolite by migration and further polymerization of active carbonaceous species.9 Although the color of all M-Y samples was changed to black after the CVD process, that of NaY was grey, indicating that the carbonization of ethylene was not enabled in NaY due to an absence of d-orbitals in Na+ (Figure S1b). After carbonization, all samples were calcined at 850 oC under flowing nitrogen to increase graphitic domains. X-ray diffraction (XRD) patterns of M-C (M: La3+, Co2+, Mn2+, Cu2+, and Ni2+) demonstrate that the peak intensity of the zeolite templates was reduced relative to that of Y-C, and zero-valent metallic species were formed in the M-C samples (M: Co2+, Cu2+, and Ni2+) but not for Mn-C (Figure S2). From a thermodynamic point of view, the reduction potential of Co2+ (Eo = - 0.28 V), Cu2+ (Eo = + 0.34 V), and Ni2+ (Eo = - 0.25 V) to the zero-valent state is more positive than that of Mn2+ (Eo = - 1.18 V)29; hence the formation of reduced cobalt, copper and nickel was observed in the XRD. To verify the carbonization pathway and the formation of transition metal species, CoC was selected as a standard material for detailed characterization. While the carbonization conditions were kept the same, the graphitization temperature was modified from 600 to 900 o

C. As shown in Figure S3, the peak intensity of Y zeolite was decreased, whereas that of

metallic Co became visible with increasing temperature. The trend is more obvious in Figure S4, where the relative peak intensity change at 23.7o (assigned to Y zeolite) and at 44.2o (originated from metallic Co) was plotted with respect to the graphitization temperature. At 600 oC, the change of peak intensity for both Y zeolite and metallic Co was negligible, which 8 ACS Paragon Plus Environment

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implies that the carbonization did not destroy the template and cobalt ions were still welllocalized inside zeolites. This also indicates that the d-π coordination between ethylene and cobalt ions is similar as that which occurs with lanthanum ions. With increasing temperature from 700 to 900 oC, the peak intensity at 23.7o and 44.2o was linearly decreased and increased, respectively. The result suggests that the formation of metallic Co nanoparticles could be attributed to the reduction of the un-stabilized metal ions, which stem from the breakage of the zeolite templates. To better understand the influence of the carbonization process on the formation of metallic Co, the same experiment was carried out without CVD process of ethylene (Figure S5). The color of Co-Y was changed from pink to purple (not shown), while the original peaks of Y zeolite were preserved and metallic Co peaks were not observed in the XRD pattern. Therefore, it makes sense to suppose that the zeolite template was broken by the shrinkage of the carbon structures during heat treatment because of the volumetric decrease of sp3 carbon networks. Hence, cobalt ions not stabilized by the zeolite structure could diffuse out, be reduced, and then agglomerate as Co nanoparticles. Figure S6 shows the XRD pattern and transmission electron microscope (TEM) image with energy-disperse X-ray (EDX) spectroscopies for Co-C after the removal of the zeolite template by hot NaOH (2 M). In the XRD patterns, while the typical peaks of Y zeolite completely disappeared, the diffraction patterns derived from zeolite-like ordered carbon structure (2θ = 6.6o), cobalt hydroxide (red line), and metallic cobalt (blue line) were clearly measured (Figure S6a). The small particles with a different contrast and the sheet-like morphologies were discovered in the TEM images (Figure S6b). Based on the spot EDX analysis, it was verified that the metallic cobalt contained oxygen due to oxide layers on the surface (Figure S6c), and the atomic ratio of cobalt to oxygen for the sheet-like structure was consistent with that of Co(OH)2 (Figure S6d). With regard to the different interactions between cobalt ions and oxygen atoms, the metallic cobalt species were possibly generated by the reduction of cobalt ions localized in the 9 ACS Paragon Plus Environment


supercage in that the interaction of Co2+ with the anionic zeolite templates is relatively weak. On the other hand, the cobalt ions captured at hexagonal or sodalite cages with a small window could be stabilized by the strong interaction even at a high temperature. Thus, cobalt hydroxide can be formed by the reaction of Co2+ with hydroxide during NaOH treatment.30 Trace amounts of cobalt were also measured in the carbon materials (Figure S6e), however, aluminum elements were not detected in the spot EDX analysis. After the removal of zeolite templates in M-C (M: La3+, Co2+, Mn2+, Cu2+, and Ni2+),the 3D CMs were prepared by overnight acid treatment using 0.1 M HCl to remove transition metal species and remaining trace amorphous silicate. Scanning electron microscope (SEM) images (Figure 1) demonstrated that the duplication of the NaY template was successfully achieved for M-C-A (M: La3+, Co2+, Mn2+, and Cu2+), except for Ni-C-A, which consisted of 3D CMs and thread-like fibers. The morphology of the templated carbon is clearer in TEM images with low (Figure S7) and high (Figure S8) magnifications. The particle sizes of the templated carbon were confirmed in the range of a few hundred nm to a few µm, and individual metallic nanoparticles were not observed in any of the samples. It should be noted that the result was not limited to a local region but was observed throughout in SEM images with low magnification (Figure S9). Moreover, the surface of the 3D CMs was smooth, and their morphologies and sizes were consistent with those of NaY templates. Meanwhile, the production of CNTs in Ni-C-A was due to the superior catalytic behavior of Ni nanoparticles to promote the 1D growth of carbon species under CVD processes.31 In the XRD data, no impurity peaks, originating from the zeolite templates or transition metal species, were detected in any of the samples (Figure 2a). The reflection at the low angle region (2θ = 6.6o), derived from the ordered zeolite-like structure, was visible for all samples except Ni-C-A (Inset in Figure 2a). The two broad peaks centered at 22o (002) and 43o (100) were caused by the amorphous graphitic carbon.32 These reflections were prominent in Ni-C-A only because of the formation of CNTs, which will be discussed later. 10 ACS Paragon Plus Environment

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Although the formation of the carbides as an intermediate was proposed during carbonization, no transition metal carbide peaks were detected in the XRD data (Figure 2a and Figure S2S6a). Two scenarios could be possible: one is that the intensity of atomic scale carbides was too weak to be detected, and the other is that a radical polymerization, initiated by the transition metal ions localized in the zeolite templates, might be involved in the formation of the sp2 carbon networks. In the SAXS profile (Figure 2b), the shift of the Bragg peaks to lower or higher q was not observed amongst the 3D CMs. Meanwhile, the peak intensity was negligible in Ni-C-A. Although the diffraction intensities were not strong (typical nature of microporous carbon materials), the still visible peaks possessing a higher q (4.80) of 3D CMs than those of NaY (not shown, q = 4.46) demonstrated that the zeolite-like ordered structure was well-maintained but the pore size of 3D CMs was a little increased compared with that of NaY.33 As shown in Figure 2c, the Raman spectra of all samples revealed quite similar trends with two broad peaks at 1,588 cm−1 (G-band) and 1,332 cm−1 (D-band). These can be defined as sp2-hybridized carbon atoms with the E2g vibrational mode in-plane, and bond disorders and defects such as dangling bonds, functional groups, five- and seven- membered carbon rings, etc. with the A1g breathing mode, respectively.34 The small peak around 1,270 cm-1 in Ni-C-A originated from the CNTs.35 Not only the broad peak at 1,332 cm−1 and the narrow peak at 1,588 cm−1, but also the lower peak intensity of IG to ID in 3D CMs predicts the presence of curved structures, functional groups introduced during base and acid treatment, and defect sites on nano-graphene layers.36-38 The Raman result was not unique in our system, but the broad and high intensity D-band was also measured in a previous report.9 The functional groups of 3D CMs were monitored using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) (Figure 2d). The peaks at 1,587 and 906 cm-1 were assigned as C=C and =CH2 vibrations, respectively.39, 40 The peaks at 1,054 and 1,222 cm-1 originated from C-O stretching and phenolic C-O vibration, respectively.41 The oxygen11 ACS Paragon Plus Environment


containing functional groups could be introduced during base and acid treatment of the M-C samples. In Ni-C-A, the peak intensities, marked with dots, were relatively weak owing to the interference by the background signal of CNTs. The ATR-FTIR analysis suggests that the carbonization of ethylene, catalyzed by different transition metal ions, yielded similar functional groups, containing a small content of C-O groups. Figure 3a shows the scanning transmission electron microscopy (STEM) image, and more specific STEM images for different magnifications, dark field (DF) and bright field (BF) mode for Co-C-A; the top side view for a slice of Co-C-A with a secondary electron (SE) and transmission electron (TE) mode are presented in Supporting Information (Figure S10 - S12). In Figure S10, the STEM images with SE mode clearly show that the smooth surface possessed a high porosity, and no individual impurity particles were identified on the external surface. The porous nature of 3D CMs was clearer in the STEM images with BF mode (Figure S11), revealing that micropores were uniformly formed in the entire 3D CMs without any bulk coke moieties blocking the pores. To investigate the porosity inside of the 3D CMs, the STEM measurement was focused on the slice of Co-C-A (Figure S12). Although the carbon structure was broken during cutting the samples, thereby providing nonuniform and large pores, it was observed that the inside of the 3D CMs was largely porous. High resolution TEM (HRTEM) image resolved the relatively well-ordered microporous structure of the 3D CMs (Figure 3b). However, the measurement was limited since the carbon structure rapidly decomposed under high electron beam irradiation. Figure 3c presents the N2 sorption isotherm of Co-C-A at 77 K. The isotherm curves showed not only the sharp increase at low relative pressure (P/P0) but the near plateau to P/P0 of 1.0 for N2 sorption, which indicates a majority of pores are micropores. The isotherm curve shape of NaY resembled that of all the M-C-A samples except for Ni-C-A (Figure S13). The non-local density functional theory (NLDFT) pore size distribution showed that the maximum peak is around 1.1 nm for the 3D CMs, which is higher than NaY (0.85 nm). The result is 12 ACS Paragon Plus Environment

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consistent with the SAXS data, showing the positive shift of q in 3D CMs. The surface area and pore volume for all samples are summarized in Table S1. The surface area and pore volume varied depending on the particular metal ion used where the highest surface area was observed for the cobalt and manganese samples. It could be possible that the transition metal ions localized in Y zeolites stimulated the activation of 3D CMs, similar to the chemical activation of carbon materials by alkaline earth metals such as AlCl3, ZnCl2, and MgCl2.42 The activation mechanism is still debatable,43 and the question regarding how the well-ordered microporous structure could be maintained during the activation process cannot be easily answered in this stage. The small surface area and pore volume of Ni-C-A can be attributed to the formation of CNTs. Figure 3d shows the C 1s core level peaks as measured by X-ray photoelectron spectroscopy (XPS). Four different species were fitted at 284.5, 286.6, 288.4, and 290.6 eV corresponding to C-C, C-O-C, C=O, and O-C=O, respectively.44 As mentioned in the description of the ATR-FTIR spectra, the oxygen containing functional groups could be introduced by base and acid treatment. Despite the successful fabrication of the 3D CMs, their applications for energy conversion devices are quite limited since most pores are comprised of small sized pores. The micropores significantly hinder assess of electrolyte ions or reactant molecules even if the pore structure is well-oriented. This can give rise to a high resistance via diffusion rate limitation and steric interaction.45 Moreover, gas bubbles evolved inside 3D CMs reduce the contact area between the electrolyte and catalytic active sites, and eventually collapse the structure of the electrode materials.46, 47 Therefore, the structural modification of 3D CMs is required to overcome these mentioned drawbacks for electrochemical energy conversion devices. Because of the unique structural properties of the 3D CMs prepared here, they should be more suited for the electrochemical ORR application instead of water-splitting, since the generation of gas bubbles is not involved during ORR. Consequently, to enhance the electrocatalytic activity of 3D CMs, we introduced active sites including transition metal 13 ACS Paragon Plus Environment


species and nitrogen elements, together with increasing the pore size. Since the superior electrocatalytic activity of cobalt species for ORR has already been reported,48 and a high surface area and porosity was demonstrated from our synthetic method in Co-C-A, this material (obtained after base treatment of Co-C) was chosen to be systematically modified and evaluated for ORR. To vary the amount of cobalt species within the carbon structure, we simply modified the synthetic procedure by selectively leaching some cobalt by HCl with different concentration (0 to 2 M) for 4 h. Following the acid treatment, N-doping was introduced by a post NH3 treatment at 850 oC for 1 h. These samples were labelled as N-Co-C-A (0 to 2 M HCl), where N and A indicate the nitrogen doping and acid treatment, respectively. The acid treatment using different concentrations resulted morphology change of the N-doped 3D CMs (Figure S14). Without acid treatment, individual particles with a broad size distribution and sheet-like agglomeration appeared inside the 3D CMs. On the other hand, HCl with highest concentration (2 M) was the most effective for removing the cobalt species, however, this result in collapsing of 3D CMs structure. The morphologies of the 3D CMs were relatively well-maintained when the concentration of HCl was 0.5 and 1 M. The magnetic properties of each sample were confirmed using a niobium magnet (Figure S15). Without acid treatment, the magnetic behavior was obvious regardless of the NH3 treatment due to the existence of metallic cobalt species. However, after the acid treatment, all samples except N-Co-C-A (0.5 M) did not show any magnetic properties, which indicates that the cobalt species were not present in the metallic form anymore. The correlation between the HCl concentration and the cobalt content will be discussed later. To elucidate the effect of HCl on the morphology and the element composition of 3D CMs, the N-Co-C-A (1 M) sample was selected for detailed characterization. The selected area EDX mapping marked in the STEM image (Figure 4a) confirmed the presence of carbon (Figure 4b), nitrogen (Figure 4c), and cobalt (Figure 4d) atoms, which were homogeneously 14 ACS Paragon Plus Environment

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dispersed in the entire N-doped 3D CMs. Similar results were also collected in HRTEM-EDX spot analysis, indicating that cobalt and nitrogen were not occupied in localized domains, but rather were uniformly spread out over the sample (Figure S16). In Figure 4e, the Co 2p XPS band shows the presence of cobalt elements, mainly the oxidation state Co2+ as seen from the satellite peak around 786.9 eV.49-51 XPS analysis did not show the existence of any metallic cobalt, which is also confirmed by the non-magnetic behavior of the sample (Figure S15f). The deconvolution of the N 1s spectrum in Figure 4f provided three peaks at 398.1, 399.8, and 401.4 eV, representing the formation of pyridinic, pyrrolic, and graphitic nitrogen, respectively.52 According to the literature, the NH3 treatment temperature significantly affects the N-doping state, where pyridinic N is a primary state at 550 oC but graphitic N is started to be generated with increasing temperature.53 Since the NH3 treatment was carried out at 850 oC, the graphitic N was predominant in N-Co-C-A (1 M). The survey scan spectrum shows the elemental change before and after NH3 treatment, exhibiting the prominent peaks of carbon, oxygen, and silicon elements in both samples, but cobalt and nitrogen elements were only identified after NH3 treatment (Figure S17). The result implies that the cobalt species were located inside 3D CMs before NH3 treatment, but some of them could migrate to the external surface during NH3 treatment. Again, a small amount of silicon always remains when NaOH is utilized as an etching reagent to remove silica and aluminosilicate templates. The ATR-FTIR spectroscopy results showed the change in functional groups in Ndoped 3D CMs depending on the concentration of HCl (Figure S18). A broad peak at 1,148 and 1,082 cm-1 specified the C=N stretching and C-N vibration, respectively.54,

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The

prominent peaks at 1,585 and 960 cm-1 were responsible for C=N (and/or C=C) and Si-O-Co (and/or Si-O-Si) vibration, respectively.54, 56 Without acid treatment, the peak at 960 cm-1 was the only distinct feature (instead of the vibration peaks related with C-N bonds), but other Ndoped samples had almost identical functional groups.



To better understand the nitrogen and cobalt content in N-doped 3D CMs, ICP-AES and SEM-EDX analysis were conducted. It should be mentioned that the absolute value of the composition in N-doped 3D CMs cannot be determined by SEM-EDX (a high uncertainty of nitrogen and oxygen) and ICP-AES (a high uncertainty for light elements); however, the relative comparison can be used for a rough estimate. As seen in Figure S19 and Figure S20, the cobalt and nitrogen content were decreased and increased with increasing the concentration of HCl, respectively. It is very notable that the cobalt species remaining in the 3D CMs prohibit the N-doping into the carbon lattice. This is due to the fact that NH3 can be decomposed into N2 and H2 over cobalt-based catalysts at a high temperature; thus it is not reactive anymore.57, 58 Accordingly, nitrogen was not detected in N-Co-C-A (0 M) owing to a large amount of cobalt remaining in the sample. In the N2 sorption isotherm (Figure 4g), the sharp increase near P/P0 of 0 was effectively reduced, but there were inflection points observed below P/P0 of 0.5, which indicates that the pores consisted of micro- and mesopores in N-Co-C-A (1 M). As summarized in Table S1 and Figure S21, the surface area and pore volume reached a maximum in N-Co-C-A (0.5 M) and decreased with increasing HCl concentration. After detailed structural analyses, the electrocatalytic performance of the materials were investigated for ORR, which is a key and challenging reaction in fuel cell applications. All samples were loaded onto glassy carbon electrodes (GCEs), and cyclic voltammetry (CV) was performed for 50 cycles to stabilize the electrode in 0.1 M KOH electrolyte under O2saturated conditions. The ORR catalytic activities of N-Co-C-A (0 to 2 M HCl) were first investigated by LSV with different rotating speeds from 1,000 to 2,400 rpm (Figure 5a and Figure S22). The current density regularly increased for all samples as the rotating rate was accelerated, whereas not only the curve shape but also the onset potential and the limiting current density were significantly changed depending on the concentration of HCl used in the acid treatment. In particular, without acid treatment, the current density was the lowest 16 ACS Paragon Plus Environment

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compared with other the samples, and the wide current plateau over a negative potential region was absent. For N-Co-C-A (0.5 M), the noise fluctuation behavior appeared in the current plateau region, which was different from the other samples. This could be from the predominantly micropore surface area and volume, which hindered the access of the electrolyte and dissolved oxygen to the active sites. In principle, the introduction of hetero-elements into sp2 carbon lattices induces charge redistribution and the O-O bond can be weakened by parallel diatomic O2 adsorption, which can lower the overpotential for ORR. The pyridinic N present in graphene can provide one p electron to the neighboring carbon atoms and their electron-donating nature increases the bonding of O2 with N and/or the adjacent C atoms. Although the mechanism is still controversial, it has been accepted that pyridinic N and graphitic N are responsible for the change of onset potential and limiting current density, respectively.20 Cobalt-based catalysts on carbon materials have been also considered as active sites for ORR, where N-doped carbon supports synergistically facilitate the kinetics.59 Based on the characterization results, it was confirmed that the nitrogen and cobalt content increased and deceased with the concentration of HCl used in the treatment, respectively. Therefore, when the LSV curves of N-Co-C-A with different cobalt amounts were compared (Figure S23a), the tendency did not match with the physisorption data. The higher efficiency of N-Co-C-A (1 M) than that of N-Co-C-A (0.5 M) even despite its lower surface area and pore volume can be explained by the optimized ratio of cobalt to nitrogen and the larger pore size as well. An absence of nitrogen in N-Co-CA (0 M) and low cobalt content in N-Co-C-A (2 M) did give rise to the low efficiency, in addition to its low surface area and pore volume as well. The Koutecky-Levich plots for each sample, obtained at 0.3 V from the LSV curves with different rotating speeds, increased in proportion (J-1 vs w-1/2) (Figure S23b). The electron-transfer number (Figure S23c) and the kinetic limiting current (Jk) (Figure S23d), calculated based on the linearly fitted Koutecky-



Levich plots (eqn. 1 and 2), indicate that the 4 electrons transfer was not favorable and the dynamic overpotential was relatively high for N-Co-C-A (0 M). The ORR performance was confirmed before and after NH3 treatment, which was further compared with a commercial Pt/C (20 wt.%), 2D CMs (reduced graphene oxide, rGO and N-rGO), and 1D CMs (carbon nanotubes, CNT and N-CNT). As shown in Figure 5b, the N-doping effect was obvious in LSV curves, where the broad reduction peak around 0.43 V for rGO and the inflection point for Co-C-A suggest the two-electron ORR process from O2 to OOH-. However, the wide current plateau without unique reduction peaks in N-rGO and NCo-C-A (1 M) reveals the diffusion-controlled ORR process with a tendency of a fourelectrons transfer to oxygen.60 Compared with N-rGO, the better electrocatalytic activity of N-Co-C-A (1 M) can be attributed to its high surface area and pore volume, the existence of cobalt-based catalysts, and well-defined pore distribution stemmed from 3D structure, facilitating the electrolyte and O2 diffusion inside pore, thereby providing the optimized contact between the active sites and electrolyte. As shown in Figure S24, the sheet-like morphologies of both rGO and N-rGO were not well-ordered, and the materials had small surface area and pore volume (Figure S25a, S25b, and Table S1). Notably, the onset potential and the limiting current density for ORR of N-Co-C-A (1 M) were comparable to those of a commercial Pt/C (20 wt.%) catalyst (Figure 5b). A quasi-rectangular CV curve lacking obvious redox peaks in a range of 0.8 to 1.0 V was obtained in both Pt/C and N-Co-CA (1 M) under Ar atmosphere (Figure S26). The larger area CV curve in N-Co-C-A (1 M) was attributed to the supercapacitance effect, typically observed in porous CMs. However, the maximum peaks originating from the reduction of oxygen were clearly measured at 0.81 and 0.83 V in Pt/C and N-Co-C-A (1 M), respectively, under O2-saturated conditions, where the current density of the latter was higher than that of the former. To obtain the electron-transfer number and Jk for all samples, the LSV curves were collected at different rotating rates (Figure S27). 18 ACS Paragon Plus Environment

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The Koutecky-Levich plots for each sample are presented in Figure 5c, and the results were well-matched with linearly fitted curves. The electron-transfer number was derived from the Koutecky-Levich equation, which was displayed with a potential applied in Figure 5d. As noted in the literature, the electron-transfer number is recorded as intermediate values from 2.3 to 2.8 in most N-doped carbon samples;61 that for N-rGO was estimated around 2.6, which was within the reported range. However, the electron-transfer number of N-Co-C-A (1 M) was calculated to be around 3.7, which is comparable to 3.9 obtained in Pt/C (Figure 5d). Meanwhile, the value was measured at about 1.6 and 1.9 for rGO and Co-C-A, respectively. Fundamentally, the products can be changed by the number of electrons transferred to O2 and applied bias. For example, superoxide anion radicals (O2 + e- → O2-; Eo = - 0.33 V), hydrogen peroxide (O2 + 2 H+ + 2 e- → H2O2; Eo = 0.68 V), and water (O2 + 4 H+ + 4 e- → 2 H2O; Eo = 1.23 V) can be generated by one-, two-, and four-electrons transfer, respectively.62 When twoelectron transfer is favorable, hydroxyl radials are produced with further one electron transfer to H2O2 (H2O2 + e- → •OH + OH-). The hydroxyl radical, a strong oxidant, and H2O2 itself could corrode the membrane and electrode materials in fuel cell.61, 63 Therefore, the selective production of water as a result of the reduction of oxygen is highly desired for practical applications. Here, the electron-transfer number of N-Co-C-A (1 M) was recorded close to 4. As shown in Figure 5e, the Jk value of N-Co-C-A (1 M) was higher than that of all samples including Pt/C. Without NH3 treatment, the high Jk value was also measured in Co-C-A, which could be ascribed to the cobalt-based catalysts incorporated in the 3D CMs. According to previous studies, excellent ORR performance is characterized by (i) a high disk and kinetic limiting current density, (ii) an onset potential with a more negative value, (iii) and an electron-transfer number close to 4, all of which were achieved in N-Co-C-A (1 M). The ORR performance was also investigated for 1D CNTs before and after N-doping (Figure S28). The onset potential was more negative and the current-limiting current was lower relative to those of N-Co-C-A (1 M). Although it was reported that vertically well19 ACS Paragon Plus Environment


aligned N-doped CNTs had high performance for ORR,64 the aggregated CNTs with a low surface area and porosity (Figure S25c, S25d, and Table S1) resulted in low ORR activity. Moreover, the electron-transfer number in the range of 2 to 3 and low Jk values were observed regardless of NH3 treatment. The durability of N-Co-C-A (1 M) was evaluated using chronoamperometry under a constant potential at 0.5 V for 17 h, which revealed that the newly developed catalyst has higher stability than Pt/C (Figure 5f).

CONCLUSIONS A series of 3D microporous carbon materials with high surface area and pore volume were prepared by using transition metal ion-exchanged zeolites as a hard template. The advantage of the synthetic method is the simple in-situ incorporation of transition metal species within 3D carbon materials during the synthesis procedure where the amount of the metallic species can be controlled by a post selective leaching. Among the diverse possible electrochemical applications of the carbon-based materials, the main emphasis was given ORR. A high disk and kinetic limiting current density, an onset potential comparable to a commercial Pt/C (20 wt.%), and an electron-transfer number close to 4 were achieved with N-doped cobalt-carbon material. Furthermore, this new class of material indicates very good durability and superior activity in comparison with 1D CNTs and 2D graphene. This methodology can be extended to diverse catalytic applications due to the simple and flexible synthetic procedure.

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.XXX. Characterization conditions, analysis results of as-prepared samples (XRD, TEM, STEM, SEM, EDX, BET isotherm & DFT pore size distribution, XPS, ATR-FTIR, and ICP-AES),


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and the different electrochemical ORR results induced by the different concentration of acid and the dimension of carbon materials

ACKNOWLEDGEMENTS This work was supported by the MAXNET Energy research consortium of the Max Planck Society and the Carbon2Chem project funded by the Bundesministerium für Bildung und Forschung (BMBF) of the German government. We thank Mr. B. Spliethoff, Mr. H.-J. Bongard, and Ms. S. Palm for assistance with electron microscopy measurements and EDX analysis, Dr. C. Weidenthaler for XPS data analysis, and Prof. C. K. Chan (Arizona State University) for assistance with revising the manuscript.

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M

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Scheme 1. Schematic diagram for the fabrication of three-dimensional carbon materials (3D CMs) by using Y zeolite as a hard template and ethylene as carbon source.



Figure 1. SEM images of (a) NaY, (b) La-C-A, (c) Co-C-A, (d) Mn-C-A, (e) Cu-C-A, and (f) Ni-C-A. The samples were obtained after base and acid treatment of M-C.


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Normalized intensity (a.u.)

(a)

Nomerized 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


1500

2000

2500

3000

3500

4000

Wavenumber (cm-1)

Figure 2. (a) XRD, (b) SAXS, (c) Raman, and (d) ATR-FTIR spectra of M-C-A (M: La3+, Co2+, Mn2+, Cu2+, and Ni2+). The peaks at 44.1o (110) and 64.1o (200) in (a) were ascribed by the iron sample holder. Inset in (a): Magnification of XRD patterns in the low angle region.



(b)

(a)

600 nm

20 nm

(c)

(d)

600

1.5

1.0

3

400

200

0.5

0.0

0 0.0

C-C C-O-C C=O O-C=O

Intensity (a.u.)

800

dV/dW (cm /g nm)

3

Quantity adsorbed (cm /g STP)

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

Page 28 of 31

1

2

3

4

5

6

Pore width (nm)

0.2

0.4

0.6

0.8

1.0

295

290

285

280

Binding energy (eV)

Relative pressure (P/P0)

Figure 3. (a) STEM image, (b) HRTEM image, (c) N2 adsorption/desorption isotherm, black (square) and red (circle) line are adsorption and desorption isotherm, respectively, the inset shows DFT pore size distribution), (d) high resolution C1s XPS spectrum for Co-C-A.


(c)

200 nm

200 nm

(f)

Co 2p1/2

820

810

800

S 790

780

770

Binding energy (eV)

760

404

402

3

Graphitic N Pyrrolic N Pyridinic N

Intensity (a.u.)

Co 2p3/2

S

200 nm

Quantity adsorbed (cm /g STP)

(e)

(d)

400

398

396

Binding energy (eV)

200 nm

700 600

(g)

500 400

0.6

300

3

(b)

(a)

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


dV/dW (cm /g nm)

Page 29 of 31

200 100 0 0.0

0.4

0.2

0.0

1

2

3

4

5

6

Pore width (nm)

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

Figure 4. (a) STEM image, EDX mapping for (b) carbon (orange), (c) nitrogen (green), and (d) cobalt (purple) elements corresponding to panel (a) marked by red box, high resolution XPS spectra deconvoluted for (e) Co2p and (f) N1s, and (g) N2 adsorption/desorption isotherm; Black (square) and red (circle) line are adsorption and desorption isotherm, respectively, and the inset shows DFT pore size distribution for N-Co-C-A (1 M). In (e), the S indicates the typical satellite peaks of cobalt.



-2 -3

-5 -6 0.0

0.4

0.6

0.8

3

J

2

1 0.07

0.08

0.09

-1/2

(e)

1600 rpm

-1 -2 Pt/C rGO Co-C-A N-rGO N-Co-C-A

-3 -4 -5 0.0

0.10

ω / rpm

0.6

0.8

1.2

3 2 Pt/C rGO Co-C-A N-rGO N-Co-C-A

1 0 0.1

0.2

0.3

0.4

0.5

Potential versus RHE (V) 100

Pt/C N-Co-C-A

95

15

1.0

4

(f)

n = 3.68

0.4

Potential versus RHE (V)

-1/2

20

0.2

(d)

Pt/C rGO Co-C-A N-rGO N-Co-C-A

0.06

0

1.0

5

2 -1

/ mA cm

0.2

Potential versus RHE (V)

4

-1

2

-4

Electron transfer numbers

(c)

-1

(b)

1000 rpm 1200 rpm 1400 rpm 1600 rpm 1800 rpm 2000 rpm 2200 rpm 2400 rpm

Current density (mA/cm )

0

2

Current density (mA/cm )

(a)

90

J/J (%)

-2

n = 2.58

10 n = 3.92

85

0

Jk / mA cm

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

Page 30 of 31

n = 1.86

80

5

75

n = 1.64

70

0

Pt/C

rGO

Co-C-A N-rGO N-Co-C-A

0

20000

40000

60000

Time (s)

Figure 5. (a) LSV curves of N-Co-C-A (1 M) with a different rotating speed. (b) LSV curves of different samples at 1,600 rpm with a scan rate of 10 mV/s, sample labeling; rGO (reduced graphene oxide), Co-C-A (cobalt-carbon sample after template removal and 1M HCl treatment), N-rGO (nitrogen doped reduced graphene oxide), N-Co-C-A (nitrogen doped cobalt-carbon sample after template removal and 1 M HCl treatment). (c) Koutecky-Levich plots for different samples collected at 0.3 V. (d) Number of electron transferred to dissolved oxygen depending on applied bias. (e) Kinetic limiting current (Jk) for different materials; electron transfer numbers at 0.3 V are noted. (f) Durability test of Pt/C and N-Co-C-A (1 M) under applied bias at 0.5 V at 800 rpm for 17 h. 30 ACS Paragon Plus Environment

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Table of Content (TOC)


Structural Engineering of 3D Carbon Materials from Transition Metal

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