This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Research Article pubs.acs.org/acscatalysis
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
Downloaded via 5.189.202.40 on July 12, 2018 at 13:15:14 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: While the field of carbon-based metal-free electrocatalysts for oxygen reduction reaction (ORR) has experienced great progress in recent years, the fundamental issue of the origin of ORR activity is 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 explored little. 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 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 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
■
ORR activity.9 Generally, a large number of intrinsic structural and edge defects exist in the doped sp2 carbon nanomaterials for accommodation of the dopants. Therefore, the ORR activities of doped carbon materials should originate at least partially from the intrinsic defects, which has been explored little 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 have 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 a high onset potential of ∼0.11 V versus the normal hydrogen electrode (NHE) in a 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.
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 and sulfur6 demonstrate excellent ORR activity with high stability and CO and methanol tolerance and, thus, attract considerable interest.7 Despite the great progress that has been made on this topic, the origin of the ORR activity is 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 shown in this study, which is even better than that of the B-doped carbon nanotubes4 and comparable to that of the N-doped carbon nanostructures.3 Recent experimental progress also suggests the contribution of carbon defects to © 2015 American Chemical Society
■
EXPERIMENTAL SECTION Sample Preparation. Carbon nanocages were synthesized by the in situ MgO template method recently developed by our Received: August 19, 2015 Revised: September 30, 2015 Published: October 5, 2015 6707
DOI: 10.1021/acscatal.5b01835 ACS Catal. 2015, 5, 6707−6712
Research Article
ACS Catalysis
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 the D peak to the G peak. (d) Schematic structural characters of the carbon nanocages. I, II, and III in panels a and d represent three typical defective locations, i.e., the corner, the broken fringe, and the hole, respectively.
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 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 a 1:1 hydrochloric acid solution and repeatedly flushed with deionized water to remove the MgO template. The resulting pure carbon nanocages are termed CNC700, CNC800, and CNC900, corresponding to growth temperatures of 700, 800, and 900 °C, respectively. 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 a Thermo Fisher Scientific Surfer Gas Adsorption Porosimeter at 77 K after the sample had been 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 the N2 isotherm, the micropore size distribution was calculated by using the Horvath−Kawazoe (HK) method (
