Research Article www.acsami.org
Promoting the Electrochemical Performances by Chemical Depositing of Gold Nanoparticles Inside Pores of 3D Nitrogen-Doped Carbon Nanocages Ling Jiang,† Li Mi,† Kan Wang,† Yafeng Wu,† Ying Li,† Anran Liu,† Yuanjian Zhang,† Zheng Hu,‡ and Songqin Liu*,† †
Key Laboratory of Environmental Medicine Engineering, Ministry of Education, Jiangsu Engineering Laboratory of Smart Carbon-Rich Materials and Device, School of Chemistry and Chemical Engineering, Southeast University, Nanjing 210096, P. R. China ‡ Jiangsu Provincial Lab for Nanotechnology and Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210046, P. R. China S Supporting Information *
ABSTRACT: Carbon Nanomaterials are excellent electrode materials due to their extraordinary conductivity, prolific structures, and morphologies. Herein, a novel nanocarbonbased material (Au@NCNC) was synthesized by embedding gold nanoparticles (AuNPs) inside the pores of threedimensional hierarchical nitrogen-doped carbon nanocages (NCNC) through an in situ chemical deposition method. The resultant Au@NCNC was employed as an electrochemical catalyst for the oxygen reduction reaction (ORR) and as an electrode material for supercapacitors. The conductivity and hydrophilicity of Au@NCNC were much more improved than those of pristine NCNC. Meanwhile, the bubble adhesive force on the Au@NCNC film was much lower underwater than that of NCNC, which provided easy accessibility to the active sites of reactants, such as hydrated O2. Therefore, the deposition of AuNPs inside pores of NCNC facilitated the transfer of electrons and diffusion of ions, promoting the electrocatalytic performance of Au@NCNC. As a result, Au@NCNC exhibited high performance toward ORR, which manifested in high numbers of electron transfer (3.7−3.9), high kinetic current density, enhanced electrocatalytic stability, and remarkable methanol durability. Moreover, Au@NCNC displayed high specific capacitance, good rate capability, and cycling stability with ∼97% of its initial capacitance retained at the high current density of 10 A g−1 after 5000 cycles. This could be attributed to the synergetic effect of ultrafine gold nanoparticles, the hierarchical porous structure, and the hydrophilic surface of NCNC as well. This work offers an excellent alternative for Pt-based catalysts in fuel cells, ORR, and supercapacitive electrode materials by enhancing the conductivity and surface hydrophilicity of electrocatalysts. KEYWORDS: gold nanoparticles, 3D nitrogen-doped carbon nanocages, hydrophilic, electrocatalytic oxygen reduction reaction, capacitances performances of the carbon-based materials.16,17 For example, the ORR of N-CNTs, vertically aligned on a quartz substrate, showed a four-electron pathway, low overpotential, and good long-term operation stability.18 On the other hand, the carbon nanomaterials with porous structures have attracted enormous interest as well due to their multiscale pore structure and large surface area, which provide efficient access for mass transport and a number of electrocatalytic active sites for ORR.19−21 Tachibana and co-workers synthesized a nitrogen-doped carbon material with a highly porous structure. A high pyrolysis temperature produced the most favorable microstructure, which manifested into the resultant, nitrogen-doped porous
1. INTRODUCTION Carbon-based nanomaterials, including fullerene, carbon nanotubes (CNTs), and graphene, have attracted tremendous concerns and been widely applied in the fields of energy conversion and storage devices due to their outstanding conductivity, thermal stability, and robust mechanical properties.1−3 Over the past few years, great efforts have been dedicated to improving the catalytic performances of carbonbased nanostructure materials toward the oxygen reduction reaction (ORR) or supercapacitors.4−7 Among these, heteroatom doping is considered as an effective method to improve the catalytic performances of carbon-based nanomaterials, such as nitrogen-doped CNTs and graphene,8−11 boron-doped CNTs and graphene,12,13 and sulfur-doped graphene.14,15 The heteroatom doping can modulate the surface chemistry and affect the electron distribution and thus promote the catalytic © 2017 American Chemical Society
Received: July 6, 2017 Accepted: August 29, 2017 Published: August 29, 2017 31968
DOI: 10.1021/acsami.7b09830 ACS Appl. Mater. Interfaces 2017, 9, 31968−31976
Research Article
ACS Applied Materials & Interfaces
Figure 1. (A, B) SEM images and corresponding elemental mappings of NCNC and Au@NCNC and (C, D) nitrogen adsorption and desorption isotherms of NCNC and Au@NCNC, inset: the corresponding pore size distributions.
changes manifested Au@NCNC possessing much higher electrocatalytic performances than those of pristine NCNC.
carbon material with fast electron transfer and mass transport as well as abundant active sites toward ORR.22 Similarly, N-doped carbon spheres with multiscale pore structures were fabricated and used as metal-free electrocatalysts. The resulting material displayed high kinetic-limiting current density and good runstability toward ORR in alkaline media.23 Gold nanoparticles (AuNPs), especially those of sizes less than 2 nm, are reported to possess much higher electrocatalytic activity for ORR.24,25 Tang et al. reported a new method to graft metal clusters, such as Au, Pt, and Pd clusters, on graphene sheets. These noble-metal cluster/graphene hybrids displayed good ORR performances, such as low onset potential as compared to that of Pt/C, good methanol tolerance, and stability.26 Moreover, the surface hydrophilicity of electrocatalysts has great influence on the properties of ORR or supercapacitors. Kaskel and colleagues demonstrated that the ultrahydrophilic nonprecious carbon electrocatalyst exhibited enhanced catalysis efficiency because a high surface hydrophilicity was able to provide a high dispersion of metal-related active sites, which increased the accessibility of reactants to the active centers and thus promoted the electrocatalytic performances by increasing the effective surface area and mass utilization efficiency of catalysts.27 In this work, a new electrocatalyst (Au@NCNC) was synthesized by embedding gold nanoparticles (AuNPs) inside the pores of three-dimensional (3D) nitrogen-doped carbon nanocages (NCNC). The NCNC was prepared by an in situ MgO template method. The as-prepared NCNC possessed a large surface area, good hydrophilicity, and high electrochemical performance toward ORR and supercapacitors. By depositing AuNPs inside the pores of NCNC, the conductivity and hydrophilicity of Au@NCNC were much improved compared to those of NCNC. In addition, a very low bubble adhesive force was observed on Au@NCNC. All of these
2. EXPERIMENTAL SECTION 2.1. Chemicals. Sodium borohydride (NaBH4), chloroauric acid (HAuCl4·4H2O), and Nafion (0.05 wt % in ethanol) were received from Sigma-Aldrich (Shanghai, China). The nitrogen-doped carbon nanocage (NCNC) was prepared at 850 °C according to the MgO template method with pyridine as the precursor.28,29 All other chemicals were of analytical grade and used without further purification. Ultrapure water (18.2 MΩ cm) used throughout the experiment was acquired from a Smart2 water purification system. Gold nanoparticles deposited inside pores of NCNC were prepared as follows: 1.0 mg of negatively charged NCNC (Figure S1) was ultrasonically dispersed in 10 mL of water. Then, 200 μL of 25 mM HAuCl4 aqueous solution was added into the suspension (0.45 mM) and stirred intensely for 24 h. After that, 1.0 mL of freshly prepared NaBH4 solution (0.5 mg mL−1) was quickly added and magnetically stirred for another 30 min. The resulting mixture was washed with ultrapure water, centrifuged, and freeze-dried to obtain the final product, which was denoted as Au@NCNC. 2.2. Instruments. Transmission electron microscope (JEM-2010, JEOL, Japan) was used to collect transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images. Scanning electron microscopy (SEM) images was obtained on an Ultra Plus field emission scanning electron microscope (Zeiss, Germany). X-ray photoelectron spectroscopy (XPS) was performed on a PHI5000 VersaProbe (Ulvac-Phi, Japan) system with an Mg Kα excitation. N2 adsorption/desorption isotherms were determined by a Thermo Fisher Scientific Surfer gas adsorption porosimeter, which were used to calculate the specific surface areas and pore size distributions based on Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda, at 77 K. X-ray diffractometry was conducted to obtain X-ray diffraction (XRD) using graphitemonochromated Cu Kα radiation (λ = 1.54056 Å). The Zeta Plus Potential Analyzer (Brookhaven Instruments Corporation) was used to determine the ζ potential of different materials. The dynamic 31969
DOI: 10.1021/acsami.7b09830 ACS Appl. Mater. Interfaces 2017, 9, 31968−31976
Research Article
ACS Applied Materials & Interfaces
Figure 2. TEM image of Au@NCNC (A), inset: partially enlarged TEM image; TEM images of an Au nanoparticle embedded inside the pore of Au@NCNC with slopes from −20 to +20° around the α axis and in the range of −10 to +10° around the β axis (B); HRTEM image of Au nanoparticles (C); and XRD spectra of Au@NCNC (D). contact angle of water was monitored by OCA 30 (DataPhysics Instruments GmbH). The bubble underwater adhesion measurement was carried out on a high-sensitivity micro-electromechanical balance system (Dataphysics DCAT11, Germany). The cyclic voltammetry (CV) was performed on a CHI 832B electrochemical workstation (Shanghai Chenhua Instrument Corporation, China). The linear sweep voltammetry (LSV) measurements were performed on HP-1A model rotating ring-disk electrodes (RDEs; Jiangsu Jiangfen Electroanalytical Instrument Co., Ltd.). The electrochemical impedance spectroscopy (EIS) study of the Au@NCNC- and NCNC-modified electrodes was carried out within a frequency range from 100 kHz to 0.01 Hz under open circuit potential. 2.3. Electrochemical Measurements. All of the electrochemical measurements were performed by using a platinum wire as the counter electrode and an Ag/AgCl (saturated KCl) as the reference electrode. For ORR experiments, 8 μL of the as-prepared samples (1.5 mg mL−1) was dropped onto the cleaned surface of RDE (3 mm diameter) and dried at room temperature. Then 2 μL of Nafion (0.05 wt %) was coated onto the surface of RDE and dried thoroughly in the air. For comparison, 4 μL of commercially available Pt/C (2.0 mg mL−1, 20% Pt containing; Johnson Matthey) was prepared. The galvanostatic charge−discharge was measured by using a three-electrode system with the Au@NCNC- and NCNC-modified electrode (0.1 mg cm−2) as the working electrode in 6 M KOH solution. Prior to electrochemical measurements, the electrolyte solution was purged with either O2 or N2 for 30 min. The rotating disk electrodemeasured currents were corrected by subtracting the background currents measured in the N2-saturated solution. Owing to the loading amount of the catalyst, which had an effect on the ORR catalytic activities, an optimized loading amount of Au@NCNC catalyst (170 μg cm−2) with maximum current density (6.5 mA cm−2) was adopted in this study (Figure S5). The loading amount of 20% Pt/C was chosen as the most common value (112 μg cm−2).
the in situ reduction of gold nanoparticles. The corresponding elemental mappings demonstrated that Au@NCNC contained four elements, C, N, O and Au, and the AuNPs were well dispersed (Figure 1B), whereas NCNC was composed of only three elements, C, N, and O (Figure 1A). Consistent results were obtained in the XPS measurements (Figure S3). The wide scan XPS spectrum of Au@NCNC clearly evaluated the presence of Au 4f, C 1s, N 1s, and O 1s peaks, whereas NCNC only contained C 1s, N 1s, and O 1s (Figure S3A). In addition, the peaks centered at 398.3 (N1), 399.7 (N2), 400.8 (N3), and 402.1 eV (N4) could be assigned to the pyridinic, pyrrolic, quaternary nitrogen, and pyridine-N-oxide groups (Figure S3B).30,31 And the Au 4f signal shown in Figure S3C could be fitted with a couple of doublet peaks at 84.2 and 87.8 eV for Au@NCNC, which, corresponding to the 4f7/2 and 4f5/2 levels of metallic gold atoms,32 confirmed the presence of gold nanoparticles in Au@NCNC. The atomic content profile indicated that the amount of Au 4f increased as the etch depth deepened and then reached a platform at ∼37%, confirming the successful deposition of ultrafine AuNPs into the pores of NCNC (Figure S3D). Nitrogen absorption/desorption isotherms showed that both NCNC and Au@NCNC exhibited a IV-type curve containing large hysteresis loops, respectively, illustrating the coexistence of macropores (>50 nm), mesopores (2−50 nm), and micropores (
