N Codoped Carbon Nanocages with Single-Atom Feature as

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

Article

Fe/N Codoped Carbon Nanocages with Single-Atom Feature as Efficient Oxygen Reduction Reaction Electrocatalyst Nan Jia, Qiaozhen Xu, Fengqi Zhao, Hong-Xu Gao, Jiaxin Song, Pei Chen, Zhongwei An, Xinbing Chen, and Yu Chen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00970 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 20, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 14 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

ACS Applied Energy Materials

Fe/N Codoped Carbon Nanocages with Single-Atom Feature as Efficient Oxygen Reduction Reaction Electrocatalyst Nan Jia,† Qiaozhen Xu,† Fengqi Zhao,‡ Hong-Xu Gao,‡ Jiaxin Song,† Pei Chen, *,† Zhongwei An,† Xinbing Chen, *,† and Yu Chen† †

Key Laboratory of Applied Surface and Colloid Chemistry (MOE); Shaanxi Key Laboratory for Advanced Energy Devices; Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, PR China. ‡ National Key Laboratory of Science and Technology on Combustion and Explosion, Xi’an Modern Chemistry Research Institute, 168 East Zhangba Road, Xi’an 710065, PR China. ABSTRACT: The electrochemical kinetics of oxygen reduction reaction (ORR) determines the energy conversion efficiency of fuel cells and metal-air batteries. Thus, developing efficient and low cost electrocatalysts for the ORR has attracted mounting attention. Herein, a facile synthetic route is presented to achieve atomic dispersion of Fe-Nx species in N doped porous carbon nanocages, resulting in a single-atom ORR electrocatalyst. The obtained Fe/N codoped carbon nanocages exhibit a comparable half-wave potential (0.82 V) and higher limiting current density compared to the commercial Pt/C electrocatalyst. In addition, by using it as air electrode electrocatalyst, the assembled Zn-air battery device shows a specific capacity of 705 mAh g−1 at 5 mA cm−2 and a negligible voltage loss after continuous operation for 67 h. The superior performance of Fe/N codoped carbon nanocages is attributed to the porous structure and the synergetic effect of atomically dispersed Fe-Nx species and N doping. KEYWORDS:

nanocages,

single-atom,

oxygen

reduction

reaction,

activity,

stability

triazine-based frameworks,31 and the pyrolysis of biological enzymes containing Fe-N center.32 However, most of such electrocatalysts are fabricated by the direct pyrolysis of the precursors containing carbon and iron, which makes it difficult to control the dispersion state of Fe atoms. Therefore, a facile and effective approach to achieve single-atom Fe-N-C electrocatalysts is desirable but remains a great challenge. Considering the large-scale commercial application, the synthesis of single-atom Fe-N-C electrocatalysts using low cost and mass-producible raw material as N and C sources is very attractive. Among various N and C sources, aminophenol-formaldehyde resin is extremely popular because of economy, facile synthesis, controllable morphology, and porous structure. In addition, the N and O atoms in aminophenol-formaldehyde resin are able to capture Fe3+ ion through coordination.33 After treating with alkaline solution, a small amount of coordinated Fe3+ are left in the resin.34 As known, because of the thermodynamical instability, Fe atom is liable to migration and agglomeration into nanoparticles at high temperature,23 indicating that the higher iron content in the precursor, the easier to form inactive Fe or FeOx nanoparticles for the ORR. Therefore, the lower iron precursor content may be the more benefit to the existence of single atomic Fe. Inspired by these pioneer works, we introduced trace amount of Fe3+ into the shell of the aminophenol-formaldehyde resin/SiO2 composite nanoparticles through Fe3+ coordination with N and O atom in alkali media, and then carbonized, etched SiO2 with HF, Fe/N codoped carbon nanocages (Fe-NCCs) with atomic dispersion of Fe‐Nx species were successfully synthesized. The Fe-NCCs exhibited a comparable half-wave potential (E1/2=0.82

1. INTRODUCTION With the reduction of fossil fuel and the emphasis on green environmental protection, it is desirable to explore sustainable and non-polluting power sources, such as fuel cells and metal-air batteries.1-6 The cathodic reaction, oxygen reduction reaction (ORR), mainly controls the energy conversion efficiency of fuel cells and metal-air batteries7-11. At present, platinum (Pt) is usually used as electrocatalyst for the ORR due to its excellent activity. However, the high cost, scarcity, low stability, and poor methanol tolerance limit its widely industrial applications.12-14 Thus, developing lowcost, efficient, and stable ORR electrocatalysts becomes an inevitable tendency.15-18 In recent years, iron-nitrogen-doped carbon nanomaterials (FeN-C) have been emerging as one of the most promising nonprecious metal electrocatalysts because of their high activity for the ORR.19-24 In such Fe-N-C materials, Fe-Nx coordinated species and pyridinic or graphitic N within the carbon framework are generally considered as active centers, especially, Fe-Nx sites are regarded as the most important ones to be responsible for high ORR activity.20, 25-28 It is expected that single atomic dispersion of Fe-Nx sites may remarkably enhance catalytic performance due to the maximum metal utilization, and thus the design and synthesis of carbon-based nanomaterials with Fe-Nx active sites at a single atomic scale have received great attentions. Some single-atom FeN-C electrocatalysts have been prepared using various strategies, such as zeolitic imidazolate frameworks precursor pyrolysis,23, 29 the direct ball milling of iron phthalocyanine and graphene nanosheets,30 embedding Fe-N4 species in porous porphyrinic 1

ACS Paragon Plus Environment

ACS Applied Energy Materials 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

V vs. RHE) with the commercial Pt/C electrocatalyst (E1/2=0.84 V vs. RHE), good methanol tolerance, and excellent stability for the ORR in 0.1 M KOH. Moreover, as air cathode electrocatalyst of Zn-air battery, it illustrated 1.23 V discharge voltage at 5 mA cm−2, specific capacity of 705 mAh g−1, and good stability. These results clearly demonstrated that Fe-NCCs were a potential Ptalternative electrocatalyst for the ORR.

Page 2 of 14

glassy carbon electrode surface, and dried at room temperature. The catalyst loading is 0.10 mg cm-2. The cyclic voltammetric (CV) and linear sweep voltammetry (LSV) tests were measured in N2- or O2-saturated 0.1 M KOH solution. The electron transfer number is determined by the KouteckyLevich equation: 1/j=1/jk+1/Bω0.5 (1) Here, jk is the kinetic current density, and B is expressed by the following expression: B = 0.2nF(DO2)2/3ν-1/6CO2 (2) where n represents the number of electrons transferred per oxygen molecule; F is the Faraday constant (F = 96 485 C mol-1); DO2 is the diffusion coefficient of O2 in 0.1 M KOH (1.9×10-5 cm2 s-1); ν is the kinematic viscosity of the electrolyte solution (0.01 cm2 s-1); CO2 is the concentration of dissolved O2 (1.2×10-6 mol cm-3). The constant 0.2 is adopted when the rotation speed is expressed in rpm36. The electrochemically active surface area (ECSA) of samples was estimated using a cyclic voltammetry method (detailed information is in the supporting information)

2. EXPERIMENTAL SECTION 2.1. Reagents and chemicals 3-aminophenol (C6H7NO), ethanol (C2H5OH, 99 wt%), formaldehyde (HCHO, 37 wt%), iron trichloride (FeCl3), cetyltrimethyl ammonium bromide (CTAB), hydrofluoric acid (HF), and ammonia (NH3·H2O, 25 wt%) were purchased from Sinopharm Chemical Reagent Co. Ltd. Potassium hydroxide (KOH) and tetraethyl orthosilicate (TEOS) were supplied by Alfa Aesar and Aladdin Reagent Co. Ltd., respectively. Commercial 20 wt.% Pt/C elelctrocatalyst was supplied by E-TEK, Inc. 2.2. Preparation of Fe-NCCs

2.5. Rotating ring-disk electrode (RRDE) measurement

Typically, 0.6 mL of NH3·H2O, 0.15 g of CTAB, and 0.15 g of C6H7NO were added in 21 mL of C2H5OH solution. Then, 0.75 mL of TEOS was added in the mixture. After stirring for 0.5 h, the 0.21 mL of HCHO solution was added. A atrovirens mixture was obtained after stirring for 24 hours at room temperature. 0.714 mL of 0.2M FeCl3 solution (the molar ratio of nC6H7NO/FeCl3=1:0.014) was added in the mixture. Then, the atrovirens mixture was hydrothermally treated for 24 hours at 100 °C. The solid product was recovered, dried and carbonized at 800 °C for 2 hours under a N2 flow with a heating rate of 1°C min-1. Finally, Fe-NCCs were obtained after etching the silica by 50% HF solution for 10 hours. For comparison, Fe-free N-doped carbon nanocages (NCCs) 35 and N-free Fe-doped carbon nanocages (Fe-CCs) were also synthesized by similar processes.

For the RRDE measurements, an electrode (PINE, E7R9) consisted of a GC disk (0.247 cm2) and a Pt ring (collection efficiency 37%) was used. The electrocatalyst inks and electrodes were prepared by the same method as mentioned above. Based on the RRDE result, the electron transfer number and the HO2- yield (%) are determined by the followed equations n = 4Id/(Id + Ir/N) (3) H2O2 (%) = 100 × (4 − n)/2 (4) where Id is the disk current density, Ir is the ring current density, and N = 0.37 is the current collection efficiency. 2.6. Zn-Air battery measurements All electrochemical The performance of Zn-air batteries was evaluated in home-built electrochemical devices. For the air cathode, as-prepared electrocatalyst ink was coated onto the PTFEtreated carbon fiber paper (1 × 1 cm2) with loading of 1.0 mg cm-2 for all electrocatalysts. A polished zinc foil and 6 M KOH were used as the anode and electrolyte, respectively. Polarization curves and galvanostatic discharge tests were carried out on a CHI760E electrochemical workstation and LAND testing system, respectively.

2.3. Physical characterization The surface area and pore volume of the samples were measured on a physical adsorption instrument (ASAP 2400). The crystalline structure, morphology and surface composition of the sample were physically characterized by X-ray diffraction (XRD, D/maxrC), transmission electron microscopy (TEM, JEM-2100F) equipped with Energy dispersive spectrometer (EDS), scan electron microscopy (SEM, SU-8020), spherical aberration corrected transmission electron microscope (ACTEM, FEI Titan G2 80-200 ChemiSTEM), Raman Spectrometer (In Via Reflex), and X-ray photoelectron spectroscopy (XPS, ESCALAB 250). The surface area and pore volume of the samples were measured on a physical adsorption instrument (ASAP 2400).

2.7. Instruments The crystalline structure, morphology and surface composition of the sample were physically characterized by X-ray diffraction (XRD, D/max-rC), transmission electron microscopy (TEM, JEM-2100F) equipped with Energy dispersive spectrometer (EDS), scan electron microscopy (SEM, SU-8020), spherical aberration corrected transmission electron microscope (ACTEM, FEI Titan G2 80-200 ChemiSTEM), Raman Spectrometer (In Via Reflex), and X-ray photoelectron spectroscopy (XPS, ESCALAB 250). The surface area and pore volume of the samples were measured on a physical adsorption instrument (ASAP 2400).

2.4. Electrochemical measurement Electrochemical experiments were performed on computercontrolled CHI 760E electrochemical workstation with glassy carbon rotating disk electrode (RDE) (0.196 cm2, PINE, E3). at room temperature, using a standard three-electrode system. A saturated calomel electrode acted as reference electrode, a carbon rod served as counter electrode, and a glassy carbon disk electrode coated with the electrocatalyst was used as working electrode. All potentials in this work were reported with respect to reversible hydrogen electrode (RHE). The electrocatalyst-modified working electrode was prepared as follows. The elelctrocatalyst ink was obtained by mixing 10 mg of electrocatalyst and 5 mL of water/Nafion solution, and sonicating the mixture for 60 min. Then, 10 µL of the resulting suspension was drop-cast onto the polished

3. RESULTS AND DISCUSSION 3.1 Characterization of Fe-NCCs The NCCs were synthesized according to our previous work35. The assemble of SiO2 and 3-aminophenol/formaldehyde resin nanoparticles driven by CTAB surfactant is responsible for the formation of NCCs.35 Fe-NCCs were prepared with the same way as NCCs but there is an extra procedure of adding small quantity 2


Page 3 of 14 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


of FeCl3 into the mixture. The morphology and size of Fe-NCCs were investigated by SEM and TEM. The SEM image clearly shows that Fe-NCCs consist of nanospheres with a diameter of ca. 130 nm (Figure 1A), and TEM image further demonstrates that the nanospheres have a porous internal structure with pore size of about 6 nm and a relatively compact shell with thickness of ca. 10 nm (Figure 1B). The morphology and size of Fe-NCCs are well in agreement with that of NCCs (Figure S1). This result indicates that FeCl3 has little influence on the morphology of nanocages. EDS data show that the content of N and Fe elements are 4.21 and 0.26 at.%, respectively (Figure S2). EDS mappings further prove the presence of C, N, and Fe elements, and the signals of Fe, N and C are completely superimposed on each other, at least on the nanoscale (Figure 1C), suggesting that Fe may be bonded with N or C. In order to deeply understand the Fe element distribution at an atomic scale, the sub-Ångström-resolution HAADF-STEM technique was used to characterize Fe-NCCs. The high resolution transmission electron microscopy (HRTEM) image clearly reveals some isolated bright dots (Figure 1D), which can be assigned to Fe species due to the different Z-contrast between Fe and C elements. The size of these bright dots is only ca. 0.1 nm, revealing Fe single atom rather than Fe cluster or small nanocrystal.27. The exclusive formation of atomically dispersed Fe sites in electrocatalysts is expected to show high catalytic activity for the ORR due to the lowest size limit and full atom utility.37-38

XPS. Probably, this is partly because of its low content and partly because of its relatively deep location in the carbon nanocages, which exceed the detection limit and detection depth of XPS. This result pushed us to investigate the location distribution of Fe element in the carbon nanocage to clarify its existent state as far as possible. A practical way is to track the evolution of Fecontaining species with preparation steps. Therefore, XRD and EDS line scanning analysis were carried out to characterize the intermediate products, namely, the hydrothermal treated product, and the carbonized product before and after HF etching, respectively. For these samples, no iron-based compounds were detected by XRD (Figure S3), while EDS line scanning data give us some information. For the hydrothermal treated product, relative to C and N elements, Fe mainly distributes in the outer shell of nanocages (Figure 2C), indicating that Fe-containing species mainly assembled on the shell of SiO2/phenolic resin composite nanospheres that are pre-generated in the agitation process at room temperature (Figure S4). After carbonizing the hydrothermal treated products, the signals of Fe and N are roughly coincident and run throughout the nanosphere (Figure 2D), implying that Fe atoms diffused in inside of the nanospheres and coordinated simultaneously with nitrogen atoms during carbonization.40 Although the subsequent HF etching process results in a dramatic reduction of the signal intensity of iron (Figure 2E) due to the formation of soluble [FeF6]3- complex ions, its distributions are still complete overlap that of nitrogen, indicating the presence of Fe-Nx species. Based on these analysis, the evolution of iron distribution with the experimental process is illustrate in Scheme 1. Given all the results of EDS mapping, HAADF-STEM, XRD and XPS, it is safe to conclude that Fe single atoms exist in the form of Fe-Nx.

Figure 1. Typical (A) SEM image, (B) TEM image, (C) HAADFSTEM image and corresponding element maps of the Fe-NCC, and (D) HAADF-STEM image of Fe-NCCs and size distribution histogram of bright dots. The XRD was used to further elucidate the existing form of iron (Figure 2A). Compared with NCCs, no obvious characteristic diffraction peaks of Fe or FeOx are detected, excluding the presence of any large Fe-containing crystalline nanoparticles. In addition, there is no obvious change in the intensity and halfwidth of peak at 26° belonging to the graphitic structure, indicating a little influence of Fe atoms on the graphitization degree due to its low content. The chemical state of single-atom Fe as well as its local environment is key for catalytic performance. Therefore, the XPS was subsequently employed to determine the possible chemical state of Fe. The high-resolution N 1s scan shows five peaks, besides the peaks of pyridinic-N (398.2 eV), pyrrolic-N (399.9 eV), oxidized-N (405.1 eV), and graphitic-N (401.0 eV) species,39 an additional peak at 399.1 eV is observed (Figure 2B), which originates from the direct bonding of Fe and N atoms.27 Unfortunately, no Fe element was detected by

Figure 2. (A) XRD patterns of Fe-NCCs. (B) N1s XPS spectra of Fe-NCCs. (C-E) HAADF-STEM image and corresponding EDS lines scanning of hydrothermal treated product, and the carbonized product before and after HF etching. (F) N2 adsorption-desorption isotherms of Fe-NCCs. Insert in Figure 3



1(C): the pore-size distribution of Fe-NCCs. (G) Raman spectra of Fe-NCCs.

Page 4 of 14

fact, compared with the reported single-atom Fe/N-doped porous carbon23, the catalytic activity of Fe-NCCs in this work is slightly inferior, which probably arises from the extremely low Fe content (0.26 at.%) and low abundance at the outermost layer of carbon nanocages, based on the above mentioned EDS results (Figure 2C-E)). Further studies on fine control of Fe doping amount are currently in progress.

Scheme 1. Schematic representation of the synthetic procedure for Fe-NCCs.

Figure 3. (A) CV curves of Fe-NCCs in N2/O2-saturated 0.1 M KOH solution at scan rate of 50 mV s-1. (B) ORR polarization curves of Fe-NCCs, NCCs, Fe-CCs and Pt/C electrocatalysts in O2-saturated 0.1 M KOH solution at scan rate of 10 mV s-1 and a rotation rate of 1,600 rpm. The ORR reaction kinetics at Fe-NCCs was assessed by LSV. Normally, the limiting diffusion current density increases with rotation rate. The transferred electron number is calculated to be ca. 3.88 by Koutecky-Levich equation at the potential ranging from 0.3 to 0.7 V (Figure 4A), suggesting there is a four-electron pathway for the ORR. The results of the RRDE show a negligible ring current (Figure 4B), and the calculated percentage of H2O2 is 3.9-16.7 % in the range of 0.3-0.7 V. The average value of the electron transfer number is 3.74, which is in agreement with the calculation result of the Koutecky−Levich plots. Apart from the activity, the stability of ORR electrocatalysts is also a critical parameter for the practical application. Herein, the multiple recycle tests were performed to evaluate the stability of Fe-NCCs and Pt/C electrocatalyst.42 As shown in Figure 4C, a very little change in E1/2 value (∆E1/2=3 mV) is observed for Fe-NCCs even after 5000 cycles, whereas a big loss of E1/2 value (∆E1/2=35 mV) is observed for the Pt/C electrocatalyst under the same condition, revealing an excellent long-term stability of Fe-NCCs for the ORR in an alkaline media. Notably, after 5000 cycles, the E1/2 value (0.817 V) of Fe-NCCs is more positive than that of commercial Pt/C electrocatalyst (0.805 V), even though after 10000 cycles (Figure S8), the E1/2 value (0.81 V) of Fe-NCCs is still more positive than that of commercial Pt/C electrocatalyst (0.799 V), showing its promising practical application as Pt alternative electrocatalyst.

The structural features of Fe-NCCs were further characterized by N2 adsorption-desorption isotherm and Raman spectra. The N2 gas adsorption isotherm of Fe-NCCs has a steep uptake below P/P0 = 0.02 and a hysteresis loop above P/P0 = 0.4 (Figure 2F), suggesting the co-presence of micropores and mesopores. The pore size distribution (insert in Figure 2F), derived from adsorption branch and calculated using the quenched solid state density functional theory method, shows that most of the pores fall into the size range of 1 to 10 nm and the microporous and mesoporous size are calculated to be 1.5 and 4.8 nm, respectively, which is consistent with the observed from TEM image. The specific surface area is estimated to be ca. 872 m2 g-1. The porous structure and high specific surface area are beneficial to the fast mass transfer during the ORR.41 The Raman spectrum shows the typical D and G bands, and the ratio of the intensity of D to G band ( ID/IG) is 0.86 (Figure 2G), indicating the co-presence of amorphous and graphitized carbon in Fe-NCCs.

3.2 The ORR performance of Fe-NCCs To evaluate the electrocatalytic performance of Fe-NCCs, we first performed the CV measurements in 0.1 M KOH electrolyte (Figure 3A). An obvious cathodic peak at 0.78 V is observed from the case of O2-saturated electrolyte, indicating that Fe-NCCs have obviously electrocatalytic activity for the ORR in alkaline media. The ORR activity of Fe-NCCs was further studied with the LSV technique. For comparison, the ORR activity of the NCCs, FeCCs, and commercial Pt/C (20 wt%) electrocatalyst were also investigated under the same experimental conditions (Figure 3B). Although Fe-NCCs, Fe-CCs, and NCCs exhibit the similar characteristics in morphology (Figure S5), phase structure (Figure S6) and surface area (Figure S7), the onset and half-wave potential of Fe-NCCs for the ORR (Eon= 0.93 V, E1/2=0.82 V) is much higher than that of the NCCs (Eon= 0.82 V, E1/2=0.78 V) and Fe-CCs (Eon= 0.79 V, E1/2=0.71 V). The results demonstrate that the formation of Fe-Nx species effectively enhance the electrocatalytic activity. Meanwhile, E1/2 value (0.82 V) of FeNCCs for the ORR is also more positive than the most recently reported of various carbon-based nanomaterials (Table S1). Particularly, the Eon and E1/2 of Fe-NCCs are very close to that of commercial Pt/C electrocatalyst (Eon= 0.98 V, E1/2=0.84 V). In

To better understanding the role of Fe in enhancing the ORR performance, a series of Fe-NCCs with different Fe content were synthesized by changing the molar ratio of nC6H7NO to FeCl3 in the precursor solution. TEM images, EDS data and N2 adsorptiondesorption isotherm (Figure S9) show that, their images have no obviously change, and the Fe content increases with the dosage of FeCl3, but the relative content is low, and the highest one is only 0.39%. Meanwhile, the BET specific surface area (SSA) slowly decreases with the increase of Fe content. The relation curve of the E1/2 value versus BET SSA and Fe content exhibits that, both these two factors have nonlinear influence on the E1/2 value. Similarly, there was little correlation between the ECSA and E1/2 (Figure S10, S11 and Table S2). Further study on this issue will go on incessantly. As known that SCN- ion can poison the Fe-Nx active sites lead to catalytic deactivation for the ORR because SCN- ions have strong 4


affinity to Fe.43-44 To clarify the active species of Fe-NCCs, an electrochemical poisoning test was carried out with KSCN. As shown in Figure 4D, the E1/2 negatively shifts by 36 mV after introduction of 0.1 M KSCN, in line with the results reported in the literature.44 Thus, maintaining the Fe as isolated atoms and incorporating nitrogen are essential to deliver the high performance, even the Fe content in Fe-NCCs is only 0.26 at.%. In addition, it is noted that the E1/2 is still up to 0.78 V after the Fe-Nx species is positioned, which is identical to that of NCCs, suggesting some active sites originating from nitrogen doping. Although there is still controversy on the type of active nitrogen for the ORR, many researches have suggested that pyridinic N, or both graphitic N and pyridinic N moieties facilitate the ORR,45-46 in which graphitic N determines the limiting current density, while pyridinic N improves the ORR onset potential.47 For the samples of NCCs and Fe-NCCs (Figure S12), they have the low content of nitrogen (3.05 and 3.87 at.%, respectively), but the relative content of the sum of graphitic N and pyridinic N are up to 38.5 and 41 at.%, respectively, which should be responsible for their activity for the ORR. Hence, it can conclude that both Fe-Nx species and doped nitrogen atoms have contribution to the superior catalytic performance of Fe-NCCs.

V) and longer discharge time (67 h) compared with that (1.25 V, 57 h) of Pt/C electrocatalyst at a current density of 5 mA cm-2. For Fe-NCCs catalyzed air cathodes, the calculated specific capacities (Figure 5C) of the batteries are 705 mAh g−1 at 5 mA cm−2, which is comparable to that catalyzed by 20% Pt/C (722 mAh g−1). Compared with those control catalysts (Table S3), the high discharging voltages and specific capacities of Fe-NCCs are indicative of their excellent electrocatalytic performance for Zn– air batteries. After refilling the Zn foil and electrolyte without changing the air cathode, we can “recharge” the battery, and a long-term galvanostatic discharge test for Fe-NCCs was carried out at 5 mA cm-2. As observed, no obvious voltage drop occurred during the test process (Figure 5D), indicating excellent stability of Fe-NCCs electrocatalyst. The superior catalytic performance attributes to the high activity of catalysis species and the fast electron/ion and gas diffusion in hierarchical porous structures. Considering the much lower cost of Fe-NCCs than commercial Pt/C electrocatalyst, it has an obvious advantage in practical application.

1.2

70

0.9

50

60 40

0.6

30 20

0.3 0.0 0

240

10 0

1.0 0.5 0.0 0

-2

Fe-NCCs 5mA cm Fe-NCCs 10mA cm-2 Fe-NCCs 25mA cm-2 Pt/C 5mA cm-2

1.5 1.0

Fe-NCCs 5mA cm-2 Fe-NCCs 10mA cm-2 Fe-NCCs 25mA cm-2 Pt/C 5mA cm-2

1.5

1.6

C

2.0

Voltage (V)

40 80 120 160 200 Current density (mA cm-2)

B

2.0

Voltage (V)

Voltage (V)

90 80

Fe-NCCs Pt/C

A

1.5

Power density (mW cm-2)

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


10

20

30 40 50 Time (h)

60

70

D

1.2

Voltage (V)

Page 5 of 14

0.8

Recharge Recharge

0.4

0.5 0.0

0.0 0

200 400 600 Specific capacity (mAh g-1)

0

20

40 Time (h)

60

80

Figure 5. (A) Polarization and power-density curves, and (B) galvanostatic discharge curves at various current densities (5, 10, and 25 mA cm-2) of the Zn-air batteries using Fe-NCCs and Pt/C as ORR electrocatalysts; (C) specific capacities, and (D) longterm galvanostatic discharge test at a current density of 5 mA cm-2 for the Zn-air batteries using Fe-NCCs as the ORR electrocatalyst. The battery is recharged by refilling Zn foil and the electrolyte. A 6 M KOH solution was used as the electrolyte.

Figure 4. (A) LSV curves of Fe-NCCs in 0.1 mol L–1 KOH solution at different rotation rates. Insert: The Koutechy-Levich plots for Fe-NCCs. (B) RRDE measurements for Fe-NCCs electrode in O2-saturated 0.1M KOH. Insert: Percentage of peroxide in the total oxygen reduction products, and the number of electron transfer. (C) ORR polarization curves of the Fe-NCCs and Pt/C electrocatalysts in O2-saturated 0.1 M KOH solution before and after 5000 cycles. (D) ORR polarization curves of FeNCCs electrocatalysts in O2-saturated 0.1 M KOH + 0.01M KSCN solution.

It is known that the chemical composition stability is one of the key factors for the actual application, therefore, EDS and XPS analysis were carried out for the Fe-NCCs after undergoing three discharge cycles over 60 hours. EDS and XPS results (Figure S13, S14) show that the oxygen content obviously increased from 9.06 at% (Figure S2) to 14.11%, accompanied with a remarkable reduction of the carbon content. Meanwhile, the relative content of graphitic N, pyridinic N and Fe-Nx species slightly decrease. Thus, it can be inferred that some framework carbon atoms are oxidized during the long-term discharge, while the nitrogen and Fe atoms maintain a certain degree of stability, probably due to their low neat content in the catalyst. Further improving the catalytic activity and stability of the Fe-NCCs should be emphasized in the future work.

3.3 Zn-air battery tests To investigate the practical application of Fe-NCCs, the discharge behaviours of a homemade Zn-air battery was investigated by using Fe-NCCs as air cathode electrocatalyst. Figure 5A presents the polarization and power-density curves of Zn-air batteries. The open-circuit voltage of ca. 1.36 V and the peak power density of 66 mW cm-2 at 0.55 V are competed with that of Pt/C electrocatalyst (1.47 V, 70 mW cm-2 at 112 mA cm-2). Furthermore, Galvanostatic discharge curves in Figure 5B reveal that Fe-NCCs based battery possesses a comparable voltage (1.21

4. CONCLUSIONS In this work, Fe-NCCs with isolated single-atom iron were pre5



pared by a simple interfacial assembly synthetic route and subsequent carbonization. Due to the synergetic effect of the hierarchical porous structure, Fe-Nx species and nitrogen doped in carbon framework, Fe-NCCs exhibited excellent catalytic performance for the ORR in the alkaline media, as reflected in comparable E1/2, higher diffuse current, better long-term stability than that of the commercial Pt/C electrocatalyst. Moreover, Zn-air batteries assembled with Fe-NCCs as cathode electrocatalyst showed comparable discharge behaviours to that of Pt/C electrocatalyst. These results demonstrate that Fe-NCCs as electrocatalyst for the ORR is a potential alternative of noble metal materials in metal-air batteries.

Page 6 of 14

(1) Chu, S.; Majumdar, A. Opportunities and Challenges for a Sustainable Energy Future. Nature 2012, 488, 294-303. (2) Zhou, M.; Wang, H.-L.; Guo, S. Towards High-Efficiency Nanoelectrocatalysts for Oxygen Reduction Through Engineering Advanced Carbon Nanomaterials. Chem. Soc. Rev. 2016, 45, 1273-1307. (3) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253-278. (4) Mao, S.; Wen, Z.; Huang, T.; Hou, Y.; Chen, J. Highperformance Bi-Functional Electrocatalysts of 3D Crumpled Graphene-Cobalt Oxide Nanohybrids for Oxygen Reduction and Evolution Reactions. Energ. Environ. Sci. 2014, 7, 609-616. (5) Shi, Q.; Wang, Y.; Wang, Z.; Lei, Y.; Wang, B.; Wu, N.; Han, C.; Xie, S.; Gou, Y. Three-Dimensional (3D) Interconnected Networks Fabricated Via In-Situ Growth of N-Doped Graphene/Carbon Nanotubes on Co-Containing Carbon Nanofibers for Enhanced Oxygen Reduction. Nano Res. 2015, 9, 317-328. (6) Su, C.-Y.; Cheng, H.; Li, W.; Liu, Z.-Q.; Li, N.; Hou, Z.; Bai, F.-Q.; Zhang, H.-X.; Ma, T.-Y. Atomic Modulation of FeCoNitrogen-Carbon Bifunctional Oxygen Electrodes for Rechargeable and Flexible All-Solid-State Zinc-Air Battery. Adv. Energy Mater. 2017, 7, 1602420. (7) Zhang, R.; He, S.; Lu, Y.; Chen, W. Fe, Co, NFunctionalized Carbon Nanotubes in Situ Grown on 3D Porous NDoped Carbon Foams as a Noble Metal-Free Catalyst for Oxygen Reduction. J. Mater. Chem. A 2015, 3, 3559-3567. (8) Liu, X.; Amiinu, I. S.; Liu, S.; Pu, Z.; Li, W.; Ye, B.; Tan, D.; Mu, S. H2O2-Assisted Synthesis of Porous N-Doped Graphene/Molybdenum Nitride Composites with Boosted Oxygen Reduction Reaction. Adv. Mater. Interfaces 2017, 4, 1601227. (9) He, D.; Jiang, Y.; Lv, H.; Pan, M.; Mu, S. Nitrogen-Doped Reduced Graphene Oxide Supports for Noble Metal Catalysts with Greatly Enhanced Activity and Stability. Appl. Catal. BEnviron. 2013, 132-133, 379-388. (10) Wang, M.; Yang, Y.; Liu, X.; Pu, Z.; Kou, Z.; Zhu, P.; Mu, S. The Role of Iron Nitrides in the Fe-N-C Catalysis System Towards the Oxygen Reduction Reaction. Nanoscale 2017, 9, 7641-7649. (11) Kou, Z.; Guo, B.; He, D.; Zhang, J.; Mu, S. Transforming Two-Dimensional Boron Carbide into Boron and Chlorine DualDoped Carbon Nanotubes by Chlorination for Efficient Oxygen Reduction. ACS Energy Lett. 2018, 3, 184-190. (12) Wang, T.; Wang, C.; Jin, Y.; Sviripa, A.; Liang, J.; Han, J.; Huang, Y.; Li, Q.; Wu, G. Amorphous Co-Fe-P Nanospheres for Efficient Water Oxidation. J. Mater. Chem. A 2017, 5, 2537825384. (13) Su, C.-Y.; Cheng, H.; Li, W.; Liu, Z.-Q.; Li, N.; Hou, Z.; Bai, F.-Q.; Zhang, H.-X.; Ma, T.-Y. Atomic Modulation of FeCo– Nitrogen-Carbon Bifunctional Oxygen Electrodes for Rechargeable and Flexible All-Solid-State Zinc–Air Battery. Adv. Energy Mater. 2017, 7, 1602420. (14) Cheng, H.; Li, M. L.; Su, C. Y.; Li, N.; Liu, Z. Q. Cu‐Co Bimetallic Oxide Quantum Dot Decorated Nitrogen-Doped Carbon Nanotubes: A High-Efficiency Bifunctional Oxygen Electrode for Zn-Air Batteries. Adv. Funct. Mater. 2017, 27, 1701833. (15) Xu, H.; Yan, B.; Li, S.; Wang, J.; Wang, C.; Guo, J.; Du, Y. Facile Construction of N-Doped Graphene Supported Hollow PtAg Nanodendrites as Highly Efficient Electrocatalysts toward Formic Acid Oxidation Reaction. ACS Sustain. Chem. Eng. 2018, 6, 609-617. (16) Zhang, J.; Zhang, D.; Zhang, R.; Zhang, N.; Cui, C.; Zhang, J.; Jiang, B.; Yuan, B.; Wang, T.; Xie, H.; Li, Q. Facile Synthesis of Mesoporous and Thin-Walled Ni-Co Sulfide

ASSOCIATED CONTENT Supporting Information. characterization details are available in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Additional information for SEM and TEM images of the NCCs; EDS pattern of the Fe-NCCs; XRD patterns of hydrothermal treated product, carbonized product and Fe-NCCs; TEM image of SiO2/phenolic resin composite nanospheres pre-generated in the agitation process; typical TEM image of Fe-NCCs, NCCs, and FeCCs; XRD patterns of Fe-NCCs, NCCs, and Fe-CCs; N2 adsorption–desorption isotherms of Fe-NCCs, NCCs, and Fe-CCs; Table of the half wave potential of recently reported noble metalfree carbon nanostructures for the ORR in an alkaline solution; ORR polarization curves of the Fe-NCCs and Pt/C electrocatalysts in O2-saturated 0.1 M KOH solution before and after 10000 cycles; TEM images, EDS data, ORR LSV curves, N2 adsorption-desorption isotherms, and the relation of E1/2 with BET surface and Fe content, Cyclic voltammogram in the non-faradic potential region at varying scan rates, the corresponding plot of average current density vs. scan rate, of a series of Fe-NCCs samples with different Fe content; Table of electrochemical active surface areas analysis for the samples of Fe-NCCs with different Fe content; The contents of nitrogen species of the samples of NCCs and Fe-NCCs; Table of the performance of discharge Znair batteries with the recently reported catalysts; EDS pattern and XPS results of the Fe-NCCs after undergoing three discharge cycles in Zn-air battery over 60 hours.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (P. Chen); [email protected] (X. Chen)

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by National Science Foundation Committee of China (51773112 and 21473111), Program for Key Science & Technology Innovation Team of Shaanxi Province (2015KCT-13), the Fundamental Research Funds for the Central Universities (GK201801001, GK201703030, 2018CBLZ005, and GK201602002), and National Training Program of Innovation and Entrepreneurship for Undergraduates (201710718018).

REFERENCES 6


Page 7 of 14 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


Nanotubes as Efficient Electrocatalysts for Oxygen Evolution Reaction. ACS Appl. Energy Mater. 2018, 1, 495-502. (17) Bin, D.; Yang, B.; Ren, F.; Zhang, K.; Yang, P.; Du, Y. Facile Synthesis of PdNi Nanowire Networks Supported on Reduced Graphene Oxide with Enhanced Catalytic Performance for Formic Acid Oxidation. J. Mater. Chem. A 2015, 3, 1400114006. (18) Liu, M.; Zhang, R.; Chen, W. Graphene-Supported Nanoelectrocatalysts for Fuel Cells: Synthesis, Properties, and Applications. Chem. Rev. 2014, 114, 5117-5160. (19) Xia, B. Y.; Yan, Y.; Li, N.; Wu, H. B.; Lou, X. W.; Wang, X. A Metal-Organic Framework-Derived Bifunctional Oxygen Electrocatalyst. Nat. Energy 2016, 1, 150006. (20) Sa, Y. J.; Seo, D.-J.; Woo, J.; Lim, J. T.; Cheon, J. Y.; Yang, S. Y.; Lee, J. M.; Kang, D.; Shin, T. J.; Shin, H. S.; Jeong, H. Y.; Kim, C. S.; Kim, M. G.; Kim, T.-Y.; Joo, S. H. A General Approach to Preferential Formation of Active Fe-N-x Sites in FeN/C Electrocatalysts for Efficient Oxygen Reduction Reaction. J. Am. Chem. Soc. 2016, 138, 15046-15056. (21) Zhang, G.; Jin, X.; Li, H.; Wang, L.; Hu, C.; Sun, X. NDoped Crumpled Graphene: Bottom-Up Synthesis and Its Superior Oxygen Reduction Performance. Sci. China Mater. 2016, 59, 337-347. (22) Yin, P.; Yao, T.; Wu, Y.; Zheng, L.; Lin, Y.; Liu, W.; Ju, H.; Zhu, J.; Hong, X.; Deng, Z.; Zhou, G.; Wei, S.; Li, Y. Single Cobalt Atoms with Precise N-Coordination as Superior Oxygen Reduction Reaction Catalysts. Angew. Chem. Int. Edit. 2016, 55, 10800-10805. (23) Chen, Y.; Ji, S.; Wang, Y.; Dong, J.; Chen, W.; Li, Z.; Shen, R.; Zheng, L.; Zhuang, Z.; Wang, D.; Li, Y. Isolated Single Iron Atoms Anchored on N-Doped Porous Carbon as an Efficient Electrocatalyst for the Oxygen Reduction Reaction. Angew. Chem. Int. Edit. 2017, 56, 6937-6941. (24) Fu, G.; Cui, Z.; Chen, Y.; Li, Y.; Tang, Y.; Goodenough, J. B. Ni3Fe-N Doped Carbon Sheets as a Bifunctional Electrocatalyst for Air Cathodes. Adv. Energy Mater. 2017, 7, 1601172. (25) Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594-3657. (26) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. HighPerformance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt. Science 2011, 332, 443-447. (27) Zhu, C.; Fu, S.; Song, J.; Shi, Q.; Su, D.; Engelhard, M. H.; Li, X.; Xiao, D.; Li, D.; Estevez, L.; Du, D.; Lin, Y. SelfAssembled Fe-N-Doped Carbon Nanotube Aerogels with SingleAtom Catalyst Feature as High-Efficiency Oxygen Reduction Electrocatalysts. Small 2017, 13, 1603407. (28) Zitolo, A.; Goellner, V.; Armel, V.; Sougrati, M.-T.; Mineva, T.; Stievano, L.; Fonda, E.; Jaouen, F. Identification of Catalytic Sites For Oxygen Reduction in Iron- and NitrogenDoped Graphene Materials. Nat. Mater. 2015, 14, 937-942. (29) Zhang, H.; Hwang, S.; Wang, M.; Feng, Z.; Karakalos, S.; Luo, L.; Qiao, Z.; Xie, X.; Wang, C.; Su, D.; Shao, Y.; Wu, G. Single Atomic Iron Catalysts for Oxygen Reduction in Acidic Media: Particle Size Control and Thermal Activation. J. Am. Chem. Soc. 2017, 139, 14143-14149. (30) Chen, X.; Yu, L.; Wang, S.; Deng, D.; Bao, X. Highly Active and Stable Single Iron Site Confined in Graphene Nanosheets for Oxygen Reduction Reaction. Nano Energy 2017, 32, 353-358. (31) Yi, J.-D.; Xu, R.; Wu, Q.; Zhang, T.; Zang, K.-T.; Luo, J.; Liang, Y.-L.; Huang, Y.-B.; Cao, R. Atomically Dispersed Iron– Nitrogen Active Sites within Porphyrinic Triazine-Based

Frameworks for Oxygen Reduction Reaction in Both Alkaline and Acidic Media. ACS Energy Lett. 2018, 3, 883-889. (32) Jiang, W.-J.; Hu, W.-L.; Zhang, Q.-H.; Zhao, T.-T.; Luo, H.; Zhang, X.; Gu, L.; Hu, J.-S.; Wan, L.-J. From Biological Enzyme to Single Atomic Fe-N-C Electrocatalyst for Efficient Oxygen Reduction. Chem. Commun. 2018, 54, 1307-1310. (33) Yano, J.; Kawakami, H.; Yamasaki, S.; Kanno, Y. Cation Capturing Ability and the Potential Response of a Poly (oaminophenol) Film Electrode to Dissolved Ferric Ions. J. Electrochem. Soc. 2001, 148, E61-E65. (34) Tucceri, R. Redox mediation and permeation processes at deactivated poly(o-aminophenol) films. A Study Applying Rotating Disc Electrode Voltammetry and Electrochemical Impedance Spectroscopy. J. Electroanal. Chem. 2009, 633, 198206. (35) Jia, N.; Weng, Q.; Shi, Y.; Shi, X.; Chen, X.; Chen, P.; An, Z.; Chen, Y. N-Doped Carbon Nanocages: Bifunctional Electrocatalysts for the Oxygen Reduction and Evolution Reactions. Nano Res. 2018, 11, 1905-1916. (36) Zhao, Y.; Lai, Q.; Wang, Y.; Zhu, J.; Liang, Y. Interconnected Hierarchically Porous Fe, N-Codoped Carbon Nanofibers as Efficient Oxygen Reduction Catalysts for Zn-Air Batteries. ACS Appl. Mater. Inter. 2017, 9, 16178-16186. (37) Fei, H.; Dong, J.; Arellano-Jimenez, M. J.; Ye, G.; Kim, N. D.; Samuel, E. L. G.; Peng, Z.; Zhu, Z.; Qin, F.; Bao, J.; Yacaman, M. J.; Ajayan, P. M.; Chen, D.; Tour, J. M. Atomic Cobalt on Nitrogen-Doped Graphene for Hydrogen Generation. Nat. Commun. 2015, 6, 16178-16186. (38) Tang, C.; Zhang, Q. Nanocarbon for Oxygen Reduction Electrocatalysis: Dopants, Edges, and Defects. Adv. Mater. 2017, 29, 1604103. (39) Mo, Z.; Zheng, R.; Peng, H.; Liang, H.; Liao, S. NitrogenDoped Graphene Prepared by a Transfer Doping Approach for the Oxygen Reduction Reaction Application. J. Power Sources 2014, 245, 801-807. (40) Xu, Z.; Wang, L.; Wang, W.; Li, N.; Chen, C.; Li, C.; Yang, C.; Fu, H.; Kuang, L. Migration Behavior, Oxidation State of Iron and Graphitization of Carbon Nanofibers for Enhanced Electrochemical Performance of Composite Anodes. Electrochim. Acta 2016, 222, 385-392. (41) Lin, L.; Qing, Z.; Xu, A.-W. Noble Metal-Free Fe-N/C Catalyst for Highly Efficient Oxygen Reduction Reaction under Both Alkaline and Acidic Conditions. J. Am. Chem. Soc. 2014, 136, 11027-11033. (42) Zhao, Y.; Watanabe, K.; Hashimoto, K. Self-Supporting Oxygen Reduction Electrocatalysts Made from a Nitrogen-Rich Network Polymer. J. Am. Chem. Soc. 2012, 134, 19528-19531. (43) Zeng, S.; Lyu, F.; Nie, H.; Zhan, Y.; Bian, H.; Tian, Y.; Li, Z.; Wang, A.; Lu, J.; Li, Y. Y. Facile Fabrication of N/S-Doped Carbon Nanotubes with Fe3O4 Nanocrystals Enchased for Lasting Synergy as Efficient Oxygen Reduction Catalysts. J. Mater. Chem. A 2017, 5, 13189-13195. (44) Zhang, W.; Xu, X.; Zhang, C.; Yu, Z.; Zhou, Y.; Tang, Y.; Wu, P.; Guo, S. 3D Space-Confined Pyrolysis of Double-Network Aerogels Containing In-Fe Cyanogel and Polyaniline: A New Approach to Hierarchically Porous Carbon with Exclusive Fe-N-x Active Sites for Oxygen Reduction Catalysis. Small Methods 2017, 1, 1700167. (45) Guo, D.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J. Active Sites of Nitrogen-Doped Carbon Materials for Oxygen Reduction Reaction Clarified using Model Catalysts. Science, 2016, 351, 361-365. (46) Xing, T.; Zheng, Y.; Li, L. H.; Cowie, B. C.; Gunzelmann, D.; Qiao, S. Z.; Huang S. M.; Chen,Y. Observation of Active 7



Sites for Oxygen Reduction Reaction on Nitrogen-Doped Multilayer Graphene. ACS Nano. 2014, 8, 6856-6862. (47) Lai, L. F.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.; Tang, C. H.; Gong, H.; Shen, Z. X.; Lin J. Y.; Ruoff, R. S. Exploration of the Active Center Structure of Nitrogen-Doped

Page 8 of 14

Graphene-Based Catalysts for Oxygen Reduction Reaction. Energy Environ. Sci. 2012, 5, 7936-7942.

8


Page 9 of 14 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


Table of Contents artwork

ACS Paragon8Plus Environment


Figure 1


Page 10 of 14

Page 11 of 14 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


Figure 2



Figure 3

Figure 4


Page 12 of 14

Page 13 of 14 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


Figure 5

Scheme 1



Graphic abstract


Page 14 of 14

N Codoped Carbon Nanocages with Single-Atom Feature as

Recommend Documents