In Situ X-ray Photoelectron Spectroscopy Study of Lithium Interaction

Article pubs.acs.org/JPCC

In Situ X‑ray Photoelectron Spectroscopy Study of Lithium Interaction with Graphene and Nitrogen-Doped Graphene Films Produced by Chemical Vapor Deposition L. G. Bulusheva,*,†,‡ M. A. Kanygin,†,‡ V. E. Arkhipov,† K. M. Popov,† Yu. V. Fedoseeva,†,‡ D. A. Smirnov,§ and A. V. Okotrub†,‡ †

Nikolaev Institute of Inorganic Chemistry SB RAS, 3 Acad. Lavrentiev Avenue, 630090 Novosibirsk, Russia Novosibirsk State University, 2 Pirogova Street, 630090 Novosibirsk, Russia § Institute of Solid State Physics, Dresden University of Technology, 01062 Dresden, Germany ‡

S Supporting Information *

ABSTRACT: It is commonly accepted that the presence of nitrogen atoms in a graphene lattice improves many properties of carbon materials and particularly enhances their electrochemical capacity in Li ion batteries. Here, we present model experiments for revealing the difference in interaction of lithium with N-doped and N-free graphene samples by monitoring the changes in their electronic states after the deposition of Li vapors. Graphene and N-doped graphene films have been grown by chemical vapor deposition on copper substrates using methane and acetonitrile as precursors. The electronic structure of the films transferred onto SiO2/Si substrates was examined by X-ray photoelectron spectroscopy (XPS) before and after deposition of lithium from a Li evaporation source under vacuum conditions. A comparison between two graphene samples using in situ XPS measurements has detected a higher accumulation of lithium on the N-doped graphene, which implies its high prospects in energy storage applications. Analysis of the XPS core-level binding energy shifts showed that charge density donated by lithium is localized near the nitrogen defects, especially around the nitrogen atoms directly substituting for carbon atoms. testing of graphene films grown on copper substrates in Li ion coils showed almost twice increase in the reversible Li ion capacitance for the N-doped graphene.10 Density functional theory (DFT) calculations explain this by an increase of charge transfer from lithium to N-doped graphene sheet.11 Moreover, nondefective graphene lattice provides only one stable position for lithium, namely, at a hexagon center, resulting in maximal lithium content limited by the formula LiC6. The nitrogen defects produce more additional active sites for lithium adsorption. Pyridinic and pyrrolic nitrogen atoms located at vacancies and lattice edges in hexagonal and pentagonal rings, respectively, are considered as most attractive for lithium.12 Comparative ex situ X-ray photoelectron spectroscopy (XPS) measurements of N-doped thermally exfoliated graphene after electrochemical lithiation and delithiation have revealed the largest variation in the intensity of the component corresponding to pyrollic nitrogen.13 This result does not fully correlate with DFT calculations, which have identified the strongest lithium interaction with pyridinic nitrogen.6,11,14 The discrepancy between experiment and theory could be related to a modification of graphene material during the preparation steps,

1. INTRODUCTION Currently, rechargeable Li ion batteries are the most widespread energy storage systems for various portable devices and vehicles.1 Rapidly growing human demands require power sources with higher specific energy. That is why development of new electrode materials that can enable better battery performance becomes very important. Typically, graphitic carbons possessing high electrical conductivity and good mechanical stability during cycling are employed as Li ion battery anodes.2 Lithium intercalation capacity for graphite is, however, limited to 372 mAh/g, which is not sufficient for further increase of battery performance. Doping by foreign elements can potentially improve the electrochemical performance of graphite. Nitrogen is considered as an optimal element for insertion in graphene lattice. This atom has a size close to that of carbon, which facilitates the replacement of carbon by nitrogen. In addition to this benefit, there is a reasonable amount of nitrogen-containing molecules, and hence, there are many possibilities to influence the structure and composition of N-doped graphitic materials by choosing an appropriate precursor and synthesis procedure.3−5 Examination of N-doped carbon nanotubes,6 porous carbons,7,8 and few-layered graphenes9 in electrodes of Li ion batteries demonstrates an enhancement of the specific capacities as compared to their N-free carbon counterparts. A © XXXX American Chemical Society

Received: December 17, 2016 Revised: February 14, 2017 Published: February 14, 2017 A

DOI: 10.1021/acs.jpcc.6b12687 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C such as dispersion in a solution, vacuum filtration, thermal shock, etc., as well as effects of electrolyte, binder, and, especially, a N-containing solvent, taken for electrode mass preparation. Here we present XPS investigations of N-free and N-doped graphene films, both obtained by the chemical vapor deposition (CVD) method on copper foils and then transferred onto SiO2/Si substrates, before and after deposition of lithium. The deposition was carried out in high-vacuum conditions using a Li evaporation source, and samples were not exposed to air between the measurements. Such model experiments can give more clear information on the chemistry of carbon, nitrogen, and lithium, since the graphitic material is not covered by any overlayers, which can be formed due to interactions with electrolytes used in electrochemical cells.15,16

sample surface were collected. The near-edge X-ray absorption fine structure (NEXAFS) C K-edge spectra were acquired in the total-electron yield (TEY) mode. The angle between the incident radiation and the sample surface was 55°. The spectra were normalized to the primary photon current from a goldcovered grid. The XPS energies were calibrated to positions of the Au 4f7/2 component. The XPS N 1s lines were approximated using a Gaussian/Lorentzian function after subtraction of Shirley’s spectral background. The energy positions of XPS lines were determined with an accuracy of ∼0.05 eV.

3. RESULTS AND DISCUSSION SEM images detected the covering of Cu foils by graphitic layers after decomposition of CH4/H2 at 1050 °C and CH3CN/H2 at 900 °C (Figure 1a,b). Successful transfers of

2. METHODS Graphene and N-doped graphene films were grown on Cu substrates by low-pressure CVD of methane and acetonitrile at 1050 and 900 °C, respectively. Before the synthesis, an electrolytic Cu foil (35 μm in thickness) was polished using a mixture of H3PO4, HNO3, and CH3COOH taken in a volume ratio of 63:20:17 for 4 min; washed by distilled water; and finally cleaned with 2-propanol. A Cu substrate of 1 × 1 cm2 was loaded in a tubular quartz reactor, which was evacuated to a base pressure of ∼5 Pa and then flushed with H2 gas by maintaining a pressure of 10 000 Pa. The reactor was heated to a required temperature and the Cu foil was held at this temperature for 20 min to remove an oxidized surface layer. A mixture of CH4 (200 Pa) and H2 (2000 Pa) or CH3CN (400 Pa) and H2 (2000 Pa) was used for the synthesis of graphene or N-doped graphene. After 10 min of synthesis, the reactor was cooled to room temperature with a rate of ∼10 °C/s. The resulting CVD samples on Cu foils were coated with a thin layer of 7% poly(methyl methacrylate) (PMMA) diluted in anisole, and then the solvent was removed by heating samples at 100 °C for 1 h. The Cu substrates were etched using a water solution of ferric chloride (30 vol %), the PMMA-coated graphene films were transferred onto SiO2/Si substrates, and then the PMMA coating was removed by an annealing in H2 atmosphere (∼5 Pa) at 200 °C for 30 min and then at 400 °C for 30 min. The films on Cu and SiO2/Si substrates were surveyed using, respectively, scanning electron microscopy (SEM) on a Hitachi S3400 N microscope and optical microscopy on an Olympus BX51 microscope. SiO2/Si substrates with graphene and N-doped graphene were affixed to a molybdenum holder and all further manipulations and measurements were carried out using equipment of the Russian−German beamline (RGBL) at the Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung (BESSY II). To remove contaminations from graphene surface, the samples were annealed in a preparation chamber evacuated to ∼5 × 10−7 Pa at 600 °C for 30 min. After cooling to room temperature, the samples were exposed to Li vapor from a well-outgassed alkali metal dispenser (SAES Getters) for 5 min at a current of ∼8 A. The holder with samples was transferred into a measuring chamber with a vacuum at least 6 × 10−8 Pa. The XPS C 1s spectra were recorded at excitation photon energies of 330, 400, and 830 eV, with an experimental resolution better than 100 meV. The XPS N 1s spectra and Li 1s spectra were measured at 500 and 400 eV, respectively. The photoelectrons emitted normal to the

Figure 1. SEM images of (a) N-doped graphene and (b) graphene on copper foils and optical images of (c) N-doped graphene and (d) graphene transferred onto SiO2/Si substrates. Solid, dotted, and dashed curves show graphene domains with gradually increased thickness.

the graphene and N-doped graphene onto SiO2/Si substrates using a PMMA coating were confirmed by optical microscopy (Figure 1c,d). Graphene monolayer absorbs ∼2.3% of white light,17 which makes possible its visualization after the deposition on a SiO2/Si substrate with appropriate thickness of surface oxide.18 The change of the contrast in the optical image of graphene/SiO2/Si sample corresponds to the difference in the thickness of graphene regions.19 As compared to N-doped graphene (Figure 1c), the optical contrast of N-free graphene (Figure 1d), obtained under the same observation conditions, is markedly darker, indicating the larger film thickness. In the image of N-doped graphene there are three regions with different contrast (highlighted by curves in Figure 1c), thus suggesting that this film consists of mainly two and three layers at least. Small particles in the sample images could be attributed to the decomposition products of PMMA. In an inert atmosphere, PMMA depolymerizes at about 360 °C,20 i.e., before the temperature of the final stage of the heating of sample in H2 environment (400 °C) used us for removal of polymer coating from the surface. However, the thermal degradation kinetics of PMMA is dependent on its structure, particularly cross- or linear-linking,21 which may influence the carbonaceous residue on the graphene surface. B


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The Journal of Physical Chemistry C

lattice causes the n- or p-type doping23,24 that should raise or lower the Fermi level relative to that for pure graphene. Actually, an increase of the binding energy of C 1s electrons has been detected for the samples enriched with graphitic N,25,26 and an opposite shift was observed for the graphene containing mainly pyridinic N.27 The shift has been shown to increase with nitrogen concentration.26 The line broadening of the N-doped graphene C 1s spectrum can be assigned to a rise of graphene conductivity with the doping and occurrence of carbon atoms in various chemical states. The C−N bonds in graphene materials are often associated with the C 1s spectral components located at ∼285.8 and ∼287.6 eV.28 The former component involves the bonding of carbon with graphitic N, while the latter component can be attributed to carbon interactions with pyridinic N.29 In our case, the C 1s spectrum of N-doped graphene exhibits an enhanced intensity around 286 eV (Figure 2a), which may be contributed by carbon bound to graphitic N. However, considering that the sample does not contain a very high amount of nitrogen, a marked broadening of the C 1s line at the high-energy side is mainly associated with an increased density of states near the Fermi level with nitrogen doping.30,31 NEXAFS spectra measured at the C K-edge of N-doped and N-free graphene films also found differences in their electronic structures. Both spectra have two main resonances at 285.4 and 291.7 eV assigned to 1s → π* and 1s → σ* transitions, respectively (Figure 2b). Incorporation of N atoms in graphene results in a lowering of the π*-resonance intensity, due to the moving of a part of the carbon π-electron density toward nitrogen32−34 and smearing of the σ*-resonance with an addition of imperfections in the lattice.35 The peaks at 287.1 and 288.6 eV are attributed to the transitions with participations of covalently functionalized carbon atoms.36 The XPS measurements detected a strong change in the electronic state of carbon after deposition of Li vapors onto a film surface (Figure 2c). As compared to the initial samples, the C 1s spectra of the lithiated ones shift toward higher binding energies and become wider. The deposited lithium atoms donate electronic density to graphene sheets, and the filling of previously empty states causes a raising of the Fermi level. Similar changes in the electronic structure of epitaxial graphene have been previously observed upon deposition of potassium on the surface of monolayers37 and bilayers.38 An extra shift of the position of the C 1s line for N-doped graphene relative to that for N-free graphene indicates a larger charging of the former film owing to apparently more adsorbed lithium. Actually, the compositions of lithiated graphene and Ngraphene samples were respectively LiC14 and LiC10, as determined from the XPS data obtained at 400 eV. For this excitation energy, the inelastic mean free paths of photoelectrons from the C 1s and Li 1s level are ∼0.47 and ∼1.64 nm, respectively.39 Assuming that lithium penetrates in the depth of the films and taking into account the interlayer distance of 0.37 nm in lithium-intercalated graphite,40 the probing sample depth is two and five surface layers by C 1s and Li 1s photoelectrons, respectively. Thus, the compositions of samples derived from the XPS data underestimate the Li to C ratios. The displacement of the C 1s line after the lithium deposition is ∼0.42 eV for graphene film and ∼0.73 eV for N-doped graphene film (Figure 2a,c), while this value has been determined for LiC6, prepared from highly oriented pyrolytic graphite (HOPG), to be equal to 0.75 eV.41 The fact that the

To remove the surface contaminations, the samples on SiO2/ Si substrates have been annealed in a high-vacuum chamber of the spectrometer at 600 °C for 30 min. The core-level lines of carbon, oxygen, and silicon were detected as dominant in the XPS survey spectra measured at an excitation photon energy of 830 eV [see Figure S1, Supporting Information (SI)]. High intensities of the Si 2s and 2p lines confirm that the synthesized films are fairly thin. The film thickness was evaluated using the attenuation law for light photons in matter and taking into account a ratio of intensities of the C 1s line to the Si 2p line and atomic cross-section factors. The obtained values of 2.55 ± 0.02 nm for graphene film and 1.62 ± 0.02 nm for N-doped graphene film correspond to about seven layers in the former sample and about four layers in the latter sample, which well correlates with the optical microscopy data. The concentration of nitrogen in N-doped graphene was determined from the XPS spectrum to be ∼1.8 atom %. A comparison of the XPS C 1s spectra of the two samples revealed a shift ∼0.15 eV toward the higher binding energy and broadening of the line for N-doped graphene (Figure 2a). Both

Figure 2. XPS C 1s spectra (a) and NEXAFS C K-edge spectra (b) of annealed graphene and N-doped graphene films on SiO2/Si substrates. XPS C 1s (c) and Li 1s (d) spectra of graphene and N-doped graphene films after the deposition of lithium. The C 1s intensities were normalized to the maximal value, and the Li 1s spectra are presented in absolute intensities.

of these facts are due to changes in the electronic state of the graphitic network with nitrogen incorporation. The effect of possible PMMA residues on spectral shapes is excluded, due to the absence in both spectra of the peaks at about 289 eV corresponding to the CO moiety.22 The high-energy shift of the sp2−C 1s maximum for N-doped graphene as compared to that for N-free graphene could be attributed to a raising of the Fermi level. The shift of the Fermi level depends on the chemical state of the incorporated nitrogen. The direct substitution of carbon by nitrogen produces a 3-fold coordinated atom, called graphitic N, and 2-fold coordinated nitrogen located at the edges of vacancies or graphene domains is referred to as pyridinic N. The DFT calculations have shown that the presence of graphitic N or pyridinic N in graphene C


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The Journal of Physical Chemistry C spectrum of N-doped graphene with one lithium atom per 10 carbon atoms showed almost the same high-energy shift as that observed for the LiC6 compound could be assigned to the stronger interaction of lithium with N-graphene. Actually, the binding energy of Li 1s electrons is ∼57 eV for lithium deposited on N-doped graphene (Figure 2d), and this energy coincides with the reference value for lithiated HOPG with LiC6 composition.41 The position of the XPS Li 1s line for the lithiated graphene film is ∼56.7 eV. The reduced binding energy is attributed to a smaller concentration of the accumulated lithium,42 and this correlates with a smaller absolute intensity of the Li 1s line of lithiated graphene as compared to that of N-doped graphene (Figure 2d). Since the spectra were recorded at the same parameters, the ratio of the integral intensities of 1.7 corresponds to the ratio of lithium amounts in the samples. As mentioned above, at an excitation energy of 400 eV the Li 1s photoelectrons probe the top five layers of lithium-intercalated graphene film, and the larger amount of lithium in N-doped graphene evaluated from XPS data can be related to the smaller thickness of this film. An increase of the excitation photon energy allows probing the sample more deeply. The XPS C 1s spectra of lithiated graphene obtained at 400 and 830 eV remained at the same energy (see Figure S2, SI) as the spectrum recorded at 330 eV (Figure 2c). The shape of the spectrum of lithiated graphene is independent of the excitation energy. These facts indicate a uniform distribution of Li throughout the depth of the film. Note that after the deposition of lithium, the samples have not been additionally heated; hence, Li atoms are able to penetrate between graphene layers, even at room temperature. The C 1s line for lithiated N-doped graphene measured at 400 eV was peaked at almost the same energy as that at 330 eV, while it was shifted to ∼285.1 eV with an increase of the excitation photon energy to 830 eV (see Figure S2, SI). However, the centers of gravity of the spectra have almost the same energy position. These changes in the spectral shape evidence that most of the deposited Li atoms were accumulated on the top surface and between two upper layers of N-doped graphene film. Such behavior is related to the smaller thickness of this film and the stronger interaction of N-doped graphene with lithium allowing the limit of one Li atom per one hexagon to be exceeded. The XPS Si 2p spectrum of graphene/SiO2/Si sample detected a substantial decrease of the intensity after the deposition of lithium (see Figure S3, SI) due to the increase of the total thickness of the film with the intercalation of lithium. The tiny spectral changes observed for the N-doped graphene/ SiO2/Si sample are due to probing the entire depth of the intercalated sample. At the used excitation energy of 830 eV, the mean free path of Si 2p photoelectrons is 1.94 nm,39 which exceeds the overall sample thickness of 1.62 ± 0.02 nm. A shoulder at ∼102.4 eV in the spectrum of the lithiated N-doped graphene corresponds to LixSiOy species,43 thus indicating that lithium atoms have reached the substrate. The change in the electronic state of nitrogen upon interaction of N-doped graphene with lithium was monitored by XPS N 1s spectra. The N 1s spectrum of initial samples exhibited two peaks located at ∼400.8 and ∼398.7 eV (Figure 3), which were attributed to graphitic N and pyridinic N atoms,23,26,27,44 respectively. A ratio of the integral intensities of these peaks shows a slight domination of the graphitic N in the synthesized sample. From the literature, the binding energies for graphitic N and pyridinic N configurations are within the ranges of 400.0−401.7 and 398.0−398.6 eV, respectively,45 and

Figure 3. Comparison of XPS N 1s spectra of N-doped graphene film before (top) and after (bottom) Li vapor deposition.

such variations can be explained by different local surroundings of the probed atoms. The energy position of the graphitic N component was shown experimentally to increase from ∼400.9 to ∼401.4 eV with a decrease of the pyridinic N fraction in graphene from 80 to 30%.23 The DFT calculations of Ncontaining graphene models detected the variations of the N 1s level energy depending on the distance between nitrogen inclusions and their location in the basal plane or at the edge.46 The lithiation of N-doped graphene causes a shift of both components to the higher binding energies, and the shift value is markedly larger for the N-pyridinic component (Figure 3). As a result, the separation of the components decreases as compared to that in the spectrum of the initial sample (Table 1). At the first approximation, we can consider the interaction between lithium and N-doped graphene as charge transfer from Li atoms to graphene. To check how such an interaction affects the energies of N 1s levels, we analyzed the Kohn−Sham eigenvalues calculated for ground states of the N-doped graphene model and that charged negatively. The core level energies in the ground state of a system cannot be directly compared with XPS data obtained for an excited state. The quantitative agreement with the experiment is provided by the Slater’s transition state approach accounting for the hole on the core level.47 However, these values are also difficult to assign to a certain configuration of nitrogen, because the binding energies depend on the local bonding configuration of the probing atom in the experimental sample. Therefore, the results of calculations are considered relative to the N 1s binding energy of a chosen standard, for example, −NH246 or graphitic N,47 and the values of the N 1s chemical shift, obtained within different approaches, are very close to each other.47 On the basis of the XPS N 1s spectrum, we constructed a model with graphitic N and two pyridinic atoms located at a monatomic vacancy and the edge of a graphene fragment (Figure S4, SI). The N 1s eigenvalues for pyridinic atoms differed by ∼1 eV, confirming the strong dependence of the N 1s binding energy on the atom location in graphene. The separation between eigenvalues of graphitic N and edged pyridinic N is closer to the experimental value (Table 1), and therefore, these species are analyzed further. As the result of the model charging by −2e, the distance between N 1s levels of these atoms reduces relative to that for the zero-charged model, in fully agreement with the experimental behavior (Table 1). From the calculations, negative charging of N-doped graphene D


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The Journal of Physical Chemistry C

Table 1. Separation between XPS N 1s Components for Initial and Lithiated N-Doped Graphenes in Comparison with N 1s Level Distances Calculated for the Corresponding Nitrogen Atoms in Zero and Negatively Charged Models (Figure S4, SI) and Shift of XPS N 1s Components (N 1s levels) Derived from the Spectra (calculations) after Li Deposition (model charging by −2e) separation between N 1s (eV) experiment calculation

shift of N 1s (eV)

initial N-graphene

after Li/charging

graphitic N

pyridinic N

2.1 ± 0.1 (δN) 3.35 (δN)

1.8 ± 0.1 (δLi−N) 2.79 (δLi−N)

+0.2 ± 0.05 +4.26 (ΔNgr)

+0.5 ± 0.05 +3.70 (ΔNpyr)

hence these atoms may act as the centers for lithium clusterization. This conclusion is in line with theoretical results on the enhanced electrochemical double-layer capacitance in the graphene with substitutional nitrogen (graphitic N).48

causes raising of the N 1s level energy, and the shift is larger for the graphitic N. The shift of the core level depends on the electrostatic potential at the atom, and calculations of electron density surfaces for N-doped graphene model with zero charge and −2e charge show that its value changes more strongly for the graphitic N atom than for the pyridinic N atoms (see Figure S5, SI). The upshift of N 1s levels seems to conflict with the XPS data, where the binding energies of N 1s electrons in the lithiated N-doped graphene grow (Table 1). However, this conflict is resolved positively when one takes into account that these binding energies are measured relative to the Fermi level. The changes in electronic structure of N-doped graphene after the lithiation can be schematically illustrated as follows (Figure 4). Donation of electrons from lithium to N-doped

4. CONCLUSIONS We performed model experiments to revealing a difference in graphene and N-doped graphene toward interaction with lithium. Thin films initially grown on Cu foil from methane or acetonitrile have been transferred onto SiO2/Si substrates using a polymer-assisted procedure. The success of the transfer was identified by optical microscopy images showing large film sizes. Contrast darkness indicated the thicker N-free graphene film, while the N-doped graphene mostly consisted of three and four layers. The deposition of Li was carried out simultaneously on the surface of both films in the vacuum chamber of an X-ray spectrometer. A comparison of the XPS data revealed that the N-doped graphene interacts substantially more strongly with lithium than the pure graphene, which can explain the higher efficiency of different N-doped graphitic carbons as electrode materials in Li ion batteries. Moreover, we suppose that especially graphitic N atoms incorporated into graphene lattice are helping to accumulate lithium, since they modify the electron density distribution on the surrounding carbon atoms.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 4. N 1s and C 1s electron binding energies in N-doped graphene and lithiated N-doped graphene relative to the vacuum level.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b12687. XPS survey spectra of graphene and N-doped graphene films; XPS C 1s spectra of lithiated graphene and lithiated N-doped graphene measured at excitation photon energies of 400 and 830 eV, XPS Si 2p spectra measured at 830 eV before and after the lithium deposition of the samples, and N-doped graphene model and electron density surfaces for the model with zero and −2e charge (PDF)

graphene results in a raising of the Fermi level. This modifies the electrostatic potential at the graphene surface, the value of which depends on the local surroundings of an atom. A larger modification at graphitic N atoms, where electrons are involved in a joint delocalized π-system of graphene, causes a greater upshift of the N 1s level (ΔNgr) as compared to that of pyridinic N (ΔNpyr). The different level shifts are supported by a decrease of the separation of the corresponding N 1s components determined by the comparison of the XPS spectra of initial Ndoped graphene (δN) and that after the deposition of lithium (δLi−N). The displacement of the Fermi level after the sample lithiation is larger than the upshift of the core levels of nitrogen and carbon, and owing to this, the N 1s and C 1s spectral components move to the higher binding energies with respect to their positions in the initial sample. From the comparison of the shifts of different core-level binding energies, we conclude that the charging of N-doped graphene has a smaller effect on the electronic state of carbon (ΔC shift) and a greater influence on the electronic state of nitrogen, especially graphitic N atoms. This can be related to the denser lithium population around the nitrogen inclusions as compared to that in nondoped graphene lattice. Graphitic N atoms have more modified carbon sites in the vicinity, compared to the pyridinic N located at edges, and

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

L. G. Bulusheva: 0000-0003-0039-2422 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. E. A. Maksimovskiy for the SEM images and Dr. L. V. Yashina and Dr. D. M. Itkis for fruitful discussion of the results. We are also grateful to the bilateral Program “Russian− German Laboratory at BESSY II” for the assistance with XPS E


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The Journal of Physical Chemistry C and NEXAFS measurements. The work was financially supported by Ministry of Education and Science of the Russian Federation (RFMEFI61614x0007) and Bundesministerium für Bildung und Forschung (project no. 05K2014) in the framework of the joint Russian−German research project “SYnchrotron and NEutron STudies for Energy Storage (SYNESTESia)”.

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