Significant Contribution of Intrinsic Carbon Defects to Oxygen

Subscriber access provided by NEW YORK MED COLL

Article

Significant contribution of intrinsic carbon defects to oxygen reduction activity Yufei Jiang, Lijun Yang, Tao Sun, Jin Zhao, Zhiyang Lyu, Ou Zhuo, Xizhang Wang, Qiang Wu, Jing Ma, and Zheng Hu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01835 • Publication Date (Web): 05 Oct 2015 Downloaded from http://pubs.acs.org on October 6, 2015

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 free 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 accessible to all readers and 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.

ACS Catalysis 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 18

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 Catalysis

Significant contribution of intrinsic carbon defects to oxygen reduction activity Yufei Jiang , Lijun Yang,* Tao Sun, Jin Zhao, Zhiyang Lyu , Ou Zhuo, Xizhang Wang, Qiang Wu, Jing Ma, and Zheng Hu* Key Laboratory of Mesoscopic Chemistry of MOE and Collaborative Innovation Center of Chemistry for Life Sciences, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China. * Correspondence: [email protected]; [email protected]

ABSTRACT: While the carbon-based metal-free electrocatalysts for oxygen reduction reaction (ORR) have experienced great progress in recent years, the fundamental issue on the origin of ORR activity is yet far from being clarified. To date, the ORR activities of these electrocatalysts are usually attributed to different dopants, while the contribution of intrinsic carbon defects has been little touched. Herein, we report the high ORR activity of the defective carbon nanocages, which is better than that of the B-doped carbon nanotubes and comparable to that of the N-doped carbon nanostructures. Density functional theory (DFT) calculations indicate that pentagon and zigzag edge defects are responsible for the high ORR activity. The mutually corroborated experimental and theoretical results reveal the significant contribution of the intrinsic carbon

1 ACS Paragon Plus Environment

ACS Catalysis

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 2 of 18

defects to ORR activity, which is crucial for understanding the ORR origin and exploring the advanced carbon-based metal-free electrocatalysts.

KEYWORDS: carbon nanocages, oxygen reduction reaction, intrinsic carbon defects, density functional calculations, electrocatalysis

INTRODUCTION Fuel cells can directly convert the chemical energy of fuels into electricity with high efficiency, high power density and low emission.1 The main challenge for the large-scale application of fuel cells today is exploring cheap and stable electrocatalysts for cathodic oxygen reduction reaction (ORR).2 In recent years, the metal-free sp2 carbon nanomaterials doped by heteroatoms such as nitrogen,3 boron,4 phosphorus,5 sulfur6 demonstrate excellent ORR activity with high stability and CO and methanol tolerance, thus attract considerable interests.7 Despite the great progresses on this topic, the origin of the ORR activity is still far from being clarified, which is actually the fundamental issue in this field. Our recent studies demonstrate that the ORR activity of doped carbon materials originates from activating carbon π electrons by breaking the integrity of π conjugation.4,8 Specifically, for electron-rich N-doping, the carbon π electrons are activated by conjugating with the lone-pair electrons from N dopants, thus the C atoms neighboring N become active for ORR. For electron-deficient B-doping, the carbon π electrons are activated by conjugating with the vacant 2pz orbital of B, thus the B sites become active for ORR.4,7c,8 With this understanding, intuitively, the intrinsic defects in sp2 carbon could also break the integrity of π conjugation and promote ORR activity. Indeed, we observed that the defective carbon nanocages (CNCs) possess the high ORR activity as presented in this study, which is even better than that of


Page 3 of 18

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 Catalysis

the B-doped carbon nanotubes4 and comparable to that of the N-doped carbon nanostructures.3 Recent experimental progresses also suggest the contribution of carbon defects to ORR activity.9 Generally, a large number of intrinsic structural and edge defects exist in the doped sp2 carbon nanomaterials for accommodating the dopants. Therefore, the ORR activities of doped carbon materials should originate at least partially from the intrinsic defects, which is little touched to date. In this context, understanding the contribution of typical carbon defects is crucial for clarifying the origin of ORR activity and exploring the advanced carbon-based metal-free electrocatalysts. Herein, we use the pure carbon nanocages, which own abundant holes, edges and positive topological disclinations but without any dopants, to address the influence of intrinsic carbon defects on ORR activity. The nanocages prepared at 700 °C present a quite good ORR performance with high onset potential of ~0.11 V vs. NHE in 0.1 mol/L KOH solution. Density functional theory (DFT) calculations indicate that the pentagon and zigzag edge defects are responsible for the high ORR activity. The results demonstrate the significant contribution of intrinsic carbon defects to ORR activity for the carbon-based metal-free electrocatalysts. EXPERIMENTAL SECTION Sample Preparation. Carbon nanocages were synthesized by the in situ MgO template method recently developed in our group with benzene as the precursor.3f,10 In a typical procedure, basic magnesium carbonate (2 g, analytical grade from XILONG CHEMICAL Co., Ltd) was spread in a horizontal quartz tube of 30 mm in diameter, which was then put into a tubular furnace. When the furnace was ramped to the setting temperature at a rate of 10 °C min−1 in argon, benzene (1.5 mL) was introduced into the quartz tube within 30 min by a syringe pump. The reaction system was then cooled to ambient temperature in argon. The as-prepared sample was treated with 1:1


ACS Catalysis

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 4 of 18

hydrochloric acid solution and repeatedly flushed with deionized water to remove the MgO template. The resulting pure carbon nanocages are referred as CNC700, CNC800, or CNC900, corresponding to the growth temperature of 700, 800, or 900 °C. Characterization. The products were characterized by transmission electron microscopy (TEM, JEM2010 operating at 200 kV), X-ray photoelectron spectroscopy (XPS, VG ESCALAB MKII), and Raman spectroscopy (LabRAM Aramis, laser excitation at 532 nm). N2 adsorption and desorption isotherms were measured on Thermo Fisher Scientific Surfer Gas Adsorption Porosimeter at 77 K after the sample was degassed at 300 °C for 6 h. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method based on the adsorption data. From the adsorption branch of N2 isotherm, the micropore size distribution was calculated by using Horvath-Kawazoe (HK) method (pores < 2 nm, p/p0 0~0.15), while the mesopore size distribution obtained by Barrett-Joyner-Halenda (BJH) method (pores 2~50 nm, p/p0 0.15~0.99). Electrochemical Measurements. Electrochemical measurements including cyclic voltammetry (CV), rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) voltammetries were carried out at 25 °C on a MSR electrode rotator (Pine Instrument Co.) coupled with a CHI 760C workstation with the counter electrode of Pt wire and the reference electrode of Ag/AgCl(3M KCl). All of the measured potentials were converted to normal hydrogen electrode (NHE) by adding a value of 0.209 V (the thermodynamic potential for the hydrogen electrode reactions vs. Ag/AgCl). Briefly, the catalyst ink was prepared by adding 2 mg powder of carbon nanocages to a mixture of 0.80 mL of water, 0.20 mL of ethanol and 50 μL of Nafion (Dupont®, 5 wt%) with 1 h ultrasonic treatment. 10 μL of fresh catalyst ink was dropped onto a glassy carbon electrode (GC, 5 mm diameter, Pine Instrument Co.), and dried at room temperature for 12 h. CV, RDE and RRDE curves were recorded in O2-saturated 0.1 mol L-1 KOH at a scan rate of 10 mV s-1. The RRDE


Page 5 of 18

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 Catalysis

curves were collected using a GC disk electrode surrounded by a Pt ring (6.5 mm inside diameter). The electron transfer number (n) and the corresponding peroxide species (HO2-) were calculated by n=4Idisk/(Idisk+Iring/N) and HO2- (%)=(200Iring/N)/(Idisk+Iring/N), where Idisk and Iring were the disk electrode current and ring electrode current, respectively. Pt ring electrode was polarized at 0.5 V (vs. Ag/AgCl), and N is the collection efficiency at the ring electrode (N=0.26). Theoretical details. The pentagon defect was modeled with graphene cluster, which showed a positive topological disclination after geometry optimization. The hole defect was modeled using a ~0.7 nm circular pore centered at a graphene sheet in a 6×8 rectangular supercell. The zigzag and armchair edge defects were modeled with graphene nanoribbons in rectangular supercells. The widths of the nanoribbon models were 5 benzene rings, which were determined from a convergence test for OOH* adsorption on zigzag-edged nanoribbons with different widths (Figure S4). All the vacuum thicknesses were set to be ~10 Å to minimize the interactions between adjacent images. All edged carbon atoms were saturated by hydrogen to eliminate dangling bonds. The possible geometrical structure of a carbon nanocage with pentagon, hole, and edge defects is sketched in Figure S5. All calculations were performed with Vienna Ab initio Simulation Package (VASP 5.2) at the spin-polarized DFT level.11 The Perdew, Burke, and Ernzerhof (PBE) functional was employed to calculate the exchange and correlation energies, with a σ value of 0.05 eV for Gaussian smearing.12 The supercells for the pentagon, hole, zigzag and armchair edge defects were sampled with 1×1×1, 4×4×1, 4×1×1, 1×4×1 Monkhorst-Pack k-point grids, respectively. Free energy diagram calculation. The calculation of ORR free energy diagrams was performed according to the method proposed by Nørskov et al.13 The free energy was calculated by G = E +


ACS Catalysis

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 6 of 18

ZPE – TS, where E is the total energy, ZPE is the zero point energy, T is the temperature in Kelvin and S is the entropy. The vibrational frequencies were calculated in the harmonic normal-mode approximation to determine ZPE and the entropy contributions (Table S3). The free energies at an applied potential (U) were corrected by G(U) = G – neU, where e is the elementary charge and n is the number of transferred electrons. The free energy of (H+ + e- ) at standard conditions of pH = 0 and U = 0 is taken as 1/2H2. The free energy of O2 was obtained from the reaction O2 + 2H2 →2H2O, with a known free energy decrease of 4.92 eV. The free energy of H2O(l) was derived from GH2O(l) = GH2O(g) + RT•ln(p/p0) where R is the ideal gas constant, T = 298.15 K , p = 0.035 bar , and p0 = 1 bar. The free energy of OH- was derived as GOH- = GH2O(l) – GH+, where GH+ = 1/2GH2 – pH•kTln10. The solution effect was considered by adding four water molecules around the absorbates. RESULTS AND DISCUSSION Figure 1 shows the typical high-resolution TEM (HRTEM) image and related characterizations on the carbon nanocages. The CNC700 presents a cuboidal hollow structure with the size of 10~20 nm and shell thickness of 4~7 graphitic layers (Figure 1a). Micropores (~0.6 nm) and mesopores (5~50 nm) co-exist in the samples as reflected by the pore size distribution, which come from the holes penetrating through the shells and the cavities inside/between the nanocages, respectively (Figure 1b).10b The unique morphology endows the carbon nanocages with three defective locations, i.e. the positive topological disclinations at the corner, the surficial broken fringes, and the holes through the shell (Figure 1a,d). With increasing the growth temperature, the average size and wall thickness of the nanocages increase accordingly, leading to the decreasing specific surface areas of 1713 (CNC700), 1009 (CNC800), and 614 m2 g-1 (CNC900). The corresponding


Page 7 of 18

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 Catalysis

ID/IG ratios in Raman spectra decrease from 2.29 via 1.99 to 1.69, indicating the increasing crystallinity, with the highest concentration of defects for the CNC700 (Figure 1c).14 XPS survey spectra show the strong C signal with trace O below 3 at%, without any other impurities (Supporting information SI 1).

Figure 1. Characterizations and schematic structural characters of the carbon nanocages. (a) HRTEM image of CNC700. (b) Pore size distributions. (c) Raman spectra. ID/IG is the area ratio of D peak to G peak. (d) Schematic structural characters of the carbon nanocages. I, II, and III in (a,d) represents three typical defective locations, i.e. the corner, the broken fringe and the hole, respectively.


ACS Catalysis

ORR performances of the carbon nanocages are presented in Figure 2. The CV sweeps show an apparent ORR peak in the O2-saturated solution for all the samples, with the peak position shifting negatively from CNC700 to CNC800 to CNC900 (Figure 2a). The corresponding onset potentials are ca. 0.11 V (vs. NHE), 0.07 V and 0.05 V, as measured by RDE (Figure 2b). The electron transfer number (n) and the corresponding peroxide species (HO2-) are calculated to be ~2.90±0.10 and ~55% in the range of -0.60~0 V, indicating a mixed two-electron and four-electron process. The best ORR performance of CNC700 with the highest defect concentration indicates the contribution of intrinsic carbon defects to ORR (Supporting information SI 2). Actually, such a performance is also the best for pure carbon materials to date, even superior to some doped carbon materials (Supporting information SI 3), indicating the significant contribution of carbon

a

-2

Current Density / mA cm

-2

defects. 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0

b

Current Density / 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 8 of 18

0 -1 -2

CNC900 CNC800 CNC700

-0.8 -0.6 -0.4 -0.2 0.0 Potential / V vs. NHE

0.2

CNC900 CNC800 CNC700

-3 -4 -5

-0.8 -0.6 -0.4 -0.2 0.0 Potential / V vs. NHE

0.2

Figure 2. ORR performances of the CNC700, CNC800 and CNC900 in O2-saturated 0.1 mol L−1 KOH. (a) CV curves. (b) RDE curves. CV and RDE are tested at a scan rate of 10 mV s-1. The rotating speed for RDE is 2500 rpm.


Page 9 of 18

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 Catalysis

As schematically shown in Figure 1d, the defects in carbon nanocages are classified into three types, i.e. (I) the pentagon defects at corners dominating the positive topological disclinations,15 (II) the edge defects at broken fringes on the shells, (III) the hole defects from the micropores. To understand the mechanism of the ORR activities of defects, density functional theory (DFT) method was employed to address the influence of different defective configurations on ORR process. Figure 3 displays the defect models used for theoretical calculations. The pentagon defect was modeled by the graphene cluster with a pentagon ring in the center, which shows a positive topological disclination after geometry optimization (Figure 3a). The hole defect was represented by the ~0.7 nm hexagonal hole in the middle of graphene lattice (Figure 3b). The edge defects were modeled by the zigzag and armchair graphene nanoribbons (Figure 3c,d). All the edged carbon atoms were saturated with hydrogen atoms to eliminate the dangling bonds. The pristine graphene model was also used for comparison (Supporting information SI 4).


ACS Catalysis

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 10 of 18

Figure 3. The defect models used for theoretical calculations. (a) Pentagon (highlight). (b) Hole. (c) Zigzag edge. (d) Armchair edge. The grey and white balls represent C and H atoms, respectively. The free energy diagrams on the ORR process, including the successive steps of O2 hydrogenation to OOH*, O-O bond scission of OOH* to O*, O* protonation to OH*, and OH* removal to form OH-, and the onset potential ranges for different defects are calculated, as plotted in Figure 4.13 For the pristine sp2 carbon, armchair edge and hole defects, the free energy increases for OOH* formation are prohibitively high, hence the ORR process is difficult to occur. As a contrast, for the pentagon and zigzag edge defects, the free energy diagrams are downhill step-bystep for the ORR process (Figure 4a, Supporting information SI 5). Thus, the pentagon and zigzag edge defects are the candidates of ORR active sites from the thermodynamic viewpoint. This is supported by the theoretically predicted onset potential (Utheory onset ) ranges for different defects (Figure 4b).16 The rather negative Utheory ranges for pristine sp2 carbon, armchair edge and hole defects onset indicate their inertness to ORR. For pentagon and zigzag edge defects, their Utheory onset ranges are more positive than the experimentally observed onset potential (Uexp onset) of 0.11 V for CNC700, which indicates their high ORR activities (Supporting information SI 6). As known, practically ORR always occurs at a certain electrode potential, thus we have further evaluated the performances of these two defects with the free energy diagrams at 0.11 V by referring to the Uexp onset of CNC700. For the zigzag edge defect, the free energy diagram still keeps downhill successively for the ORR process. For the pentagon defect, while the formation of OOH* becomes slightly uphill by 0.025 eV, it is still in the reasonable range for a reaction at room temperature (Supporting information SI 7).17 All these theoretical results indicate that the pentagon and zigzag edge defects are the ORR active sites, thus the main contributors for the high ORR activity of the carbon nanocages.


Page 11 of 18

Free Energy / eV

a

pristine U=0 V armchair hole

3 2

1 pentagon zigzag 0 *

*

O2(g) OOH O OH Reaction Step

0.5

*

OH

-

U=0.11 V

0.0 -0.5 -1.0

m ch

le ar

pe nt ag on zi gz ag

-2.0

ai r pr is tin e

-1.5 ho

theory

Uonset / V vs. NHE

b

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 Catalysis

Figure 4. DFT calculations for ORR activities of different defects. (a) Free energy diagrams.

(b)

ranges (bars) and Uexp Utheory onset onset of CNC700 (dash line). The calculations were performed by taking account of the solution effect at 298.15 K, and ‘g’ and ‘*’ indicate the gaseous and chemisorbed state. The underlying reason for the different ORR activities of these defects is attributed to their different electronic structures, which cause the distinct reactivities to the species involved in the ORR process. For example, for zigzag edge, a portion of the active unpaired π electron locates at each edge carbon atom; while for armchair edge, the unpaired π electrons of two neighboring carbon atoms get paired up to form stable covalent bonds.9a,18 Taking the O2 hydrogenation to OOH* (the first step of ORR) as an example, the zigzag edge with unpaired π electrons facilitates electron transfer to O2 and the OOH* is easily formed with a free energy change of -0.222 eV. As a contrast, the armchair edge being lack of unpaired π electrons has to rearrange the neighboring


ACS Catalysis

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 12 of 18

bonds to transfer electron to O2 for OOH* formation, giving a prohibitively high free energy increase of 1.089 eV (Table S4 in Supporting information SI 5). The different electronic structures of zigzag and armchair edges also lead to the different reactivities to other ORR intermediates, and finally the totally different ORR performances. To further identify the individual contribution of pentagon defect and zigzag edge defect, we performed a contrast experiment by annealing. The CNC700 was annealed at 900 °C in Ar for 3 hours, and the obtained sample was denoted as CNC700/900. Such heat treatment doesn’t change the morphology and size of the CNC700 while partially repairs the edge defects, thus the CNC700/900 has the close amount of pentagon defects to the CNC700 while the less edge defects than the CNC700. Consequently, the Uexp onset of CNC700/900 is negatively shifted in reference to that of CNC700, which indicates the ORR active character of the zigzag edge defects. For CNC700/900 and CNC900, both the samples experienced the high temperature treatment at 900 °C. The biggest different between CNC700/900 and CNC900 is that the former has much smaller cage size than the latter, thus owns much more pentagon defects. It is observed the Uexp onset of CNC700/900 is more positive than that of CNC900, which implies the ORR active character of the pentagon defects (Supporting information SI 8). The preceding theoretical and experimental results confirmed that either zigzag edge defect or pentagon defect is responsible for the high ORR activity of carbon nanocages. As known, the ORR activity for carbon-based metal-free electrocatalysts is usually attributed to the contribution of different dopants,3,4,7 while the contribution of defects has long been neglected. Based on the progress of this study, the reason for the appearance of conflicting viewpoints on some key issues in literatures could be understood. For example, for the intensively studied N-doped carbon-based metal-free electrocatalysts, theoretical studies claim that graphitic N is more important than


Page 13 of 18

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 Catalysis

pyridinic N for ORR since the former has the lower work function and the smaller O2 dissociation barrier.19 On the contrary, experimental studies propose that pyridinic N is more important than graphitic N since the catalyst with predominant pyridinic N has higher ORR activity than that with predominant graphitic N.20 Now, we realize that such discrepancy results from the neglect to the vital contribution of intrinsic carbon defects. Usually, the N-doped carbon nanostructures with predominant pyridinic N are obtained at much lower temperature than that with predominant graphitic N, thus with much more intrinsic carbon defects.20,21 The additional contribution of these defects makes the better ORR performance for the former than the latter as experimentally observed. Such kinds of situations suggest the reconsideration on the origin of ORR activity for the doped carbon-based metal-free electrocatalysts due to the unavoidable coexistence of a large number of defects. In other words, combining unique defects with suitable dopants should be a promising strategy to explore the advanced carbon-based metal-free electrocatalysts. CONCLUSION In summary, we observed the high ORR activity of the pure carbon nanocages, which is the best for pure carbon materials to date and even superior to some doped carbon materials. Theoretical calculations revealed the two typical carbon defects, i.e. pentagon and zigzag edge defects, are the contributors to the high activity, as supported by the annealing contrast experiments. The mutually corroborated experimental and theoretical results demonstrate the significant contribution of the intrinsic carbon defects to ORR activity, which takes a crucial step for clarifying the ORR origin, for understanding the controversial viewpoints, and for exploring the advanced carbon-based metal-free ORR electrocatalysts. ASSOCIATED CONTENT


ACS Catalysis

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 14 of 18

Supporting Information Supporting Information Available: The characterization on the carbon nanocages, additional electrochemical results, the performance comparison with the literatures, and additional theoretical results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was jointly supported by NSFC (51232003, 21203092, 21473089, 21373108, 21173115), “973” program (2013CB932902), and Suzhou science and technology plan projects (ZXG2013025).

REFERENCES (1) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345-352. (2) (a) Gasteiger, H. A.; Marković, N. M. Science 2009, 324, 48-49. (b) Debe, M. K. Nature 2012, 486, 43-51. (3) (a) Gong, K. P.; Du, F.; Xia, Z. H.; Durstock, M.; Dai, L. M. Science 2009, 323, 760-764. (b) Wei,W.; Liang, H.; Parvez, K.; Zhuang, X.; Feng, X.; Müllen, K. Angew. Chem. Int. Ed. 2014, 53, 1570-1574. (c) Liang, J.; Zheng, Y.; Chen, J.; Liu, J.; Hulicova-Jurcakova, D.; Jaroniec, M.; Qiao, S. Z. Angew. Chem. Int. Ed. 2012, 51, 3892-3896. (d) Chen, P.; Xiao, T. Y.; Qian,


Page 15 of 18

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 Catalysis

Y. H.; Li, S. S.; Yu, S. H. Adv. Mater. 2013, 25, 3192-3196. (e) Qu, L. T.; Liu, Y.; Baek, J. B.; Dai, L. M. ACS Nano 2010, 4, 1321-1326. (f) Chen, S.; Bi, J.; Zhao, Y.; Yang, L.; Zhang, C.; Ma, Y.; Wu, Q.; Wang, X.; Hu, Z. Adv. Mater. 2012, 24, 5593-5597. (4) Yang, L.; Jiang, S.; Zhao, Y.; Zhu, L.; Chen, S.; Wang, X.; Wu, Q.; Ma, J.; Ma, Y.; Hu, Z. Angew. Chem. Int. Ed. 2011, 50, 7132-7135. (5) (a) Liu, Z. W.; Peng, F.; Wang, H. J.; Yu, H.; Zheng, W. X.; Yang, J. Angew. Chem. Int. Ed. 2011, 50, 3257-3261. (b) Yang, D. S.; Bhattacharjya, D.; Inamdar, S.; Park, J.; Yu, J. S. J. Am. Chem. Soc. 2012, 134, 16127-16130. (6) Yang, Z.; Yao, Z.; Li, G.; Fang, G.; Nie, H.; Liu, Z.; Zhou, X.; Chen, X. a.; Huang, S. ACS Nano 2011, 6, 205-211. (7) (a) Dai, L.; Xue, Y.; Qu, L.; Choi, H. J.; Baek, J. B. Chem. Rev. 2015, 115, 4823-4892. (b) Zheng, Y.; Jiao, Y.; Jaroniec, M.; Jin, Y.; Qiao, S. Z. Small 2012, 8, 3550-3566. (c) Paraknowitsch, J. P.; Thomas, A. Energy Environ. Sci. 2013, 6, 2839-2855. (d) Wang, D. W.; Su, D. Energy Environ. Sci. 2014, 7, 576-591. (e) Yang, L. J.; Zhao, Y.; Chen, S.; Wu, Q.; Wang, X. Z.; Hu, Z. Chin. J. Catal. 2013, 34, 1986-1991. (8) Zhao, Y.; Yang, L.; Chen, S.; Wang, X.; Ma, Y.; Wu, Q.; Jiang, Y.; Qian, W.; Hu, Z. J. Am. Chem. Soc. 2013, 135, 1201-1204. (9) (a) Shen, A.; Zou, Y.; Wang, Q.; Dryfe, R. A. W.; Huang, X.; Dou, S.; Dai, L.; Wang, S. Angew. Chem. Int. Ed. 2014, 53, 10804-10808. (b) Jin, H.; Huang, H.; He, Y.; Feng, X.; Wang, S.; Dai, L.; Wang, J. J. Am. Chem. Soc. 2015, 137, 7588-7591. (10) (a) Xie, K.; Qin, X.; Wang, X.; Wang, Y.; Tao, H.; Wu, Q.; Yang, L.; Hu, Z. Adv. Mater. 2012, 24, 347-352. (b) Lyu, Z.; Xu, D.; Yang, L.; Che, R.; Feng, R.; Zhao, J.; Li, Y.; Wu, Q.; Wang, X.; Hu, Z. Nano Energy 2015, 12, 657-665.


ACS Catalysis

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 16 of 18

(11) (a) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 48, 13115-13118. (b) Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169-11186. (c) Kresse, G.; Hafner, J. Phys. Rev. B 1994, 49, 1425114269. (d) Kresse, G.; Furthmüller, J. Comp. Mater. Sci. 1996, 6, 15-50. (12) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865-3868. (13) Norskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. J. Phys. Chem. B 2004, 108, 17886-17892. (14) Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R. Nano Lett. 2010, 10, 751-758. (15) Ovid'ko, I. A. Rev. Adv. Mater. Sci. 2012, 30, 201-224. (16) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. J. Am. Chem. Soc. 2014, 136, 4394-4403. (17) Scudder, P. H. Electron Flow in Organic Chemistry: A Decision-Based Guide to Organic Mechanisms, 2nd ed.; John Wiley & Sons, Inc.: Hoboken, New Jersey, 2013; p 47. (18) (a) Jiang, D. e.; Sumpter, B. G.; Dai, S. J. Chem. Phys. 2007, 126, 134701. (b) Muge, A.; Yves, J. C. Jpn. J. Appl. Phys. 2011, 50, 070101. (19) (a) Ni, S.; Li, Z. Y.; Yang, J. L. Nanoscale 2012, 4, 1184-1189. (b) Schiros, T.; Nordlund, D.; Pálová, L.; Prezzi, D.; Zhao, L.; Kim, K. S.; Wurstbauer, U.; Gutiérrez, C.; Delongchamp, D.; Jaye, C.; Fischer, D.; Ogasawara, H.; Pettersson, L. G. M.; Reichman, D. R.; Kim, P.; Hybertsen, M. S.; Pasupathy, A. N. Nano Lett. 2012, 12, 4025-4031. (c) Cheon, J. Y.; Kim, J. H.; Kim, J. H.; Goddeti, K. C.; Park, J. Y.; Joo, S. H. J. Am. Chem. Soc. 2014, 136, 88758878. (20) (a) Lai, L.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.; Tang, C.; Gong, H.; Shen, Z.; Lin, J.; Ruoff, R. S. Energy Environ. Sci. 2012, 5, 7936-7942. (b) Wu, J.; Ma, L.; Yadav, R. M.; Yang,


Page 17 of 18

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 Catalysis

Y.; Zhang, X.; Vajtai, R.; Lou, J.; Ajayan, P. M. ACS Appl. Mater. Interfaces 2015, 7, 1476314769. (c) Rao, C. V.; Cabrera, C. R.; Ishikawa, Y. J. Phys. Chem. Lett. 2010, 1, 2622-2627. (21) (a) Chen, H.; Yang, Y.; Hu, Z.; Huo, K. F.; Ma, Y. W.; Chen, Y.; Wang, X. S.; Lu, Y. N. J. Phys. Chem. B 2006, 110, 16422-16427. (b) Sharifi, T.; Hu, G.; Jia, X.; Wågberg, T. ACS Nano 2012, 6, 8904-8912.


ACS Catalysis

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 18 of 18

Table of Contents Significant contribution of intrinsic carbon defects to oxygen reduction activity Yufei Jiang , Lijun Yang, Tao Sun, Jin Zhao, Zhiyang Lyu , Ou Zhuo, Xizhang Wang, Qiang Wu, Jing Ma, and Zheng Hu


Significant Contribution of Intrinsic Carbon Defects to Oxygen

Recommend Documents