Synthesis of hybrid carbon spheres@nitrogen-doped graphene/carbon nanotubes and their oxygen reduction activity performance

Abstract

The hybrid architecture of carbon spheres@nitrogen-doped graphene/carbon nanotubes (CS@N-G/CNT) was synthesized by a hydrothermal and ultrasonic-assisted method. It consisted of one-dimensional CNTs and two-dimensional N-doped graphene (N-G) supporting zero-dimensional carbon spheres (CSs). The electrocatalytic activity of the hybrid CS@N-G/CNT shows a better oxygen reduction reaction (ORR) than CS, CNT, N-G, and CS@N-G in alkaline media. More importantly, it displays both a higher mass activity and better stability than a commercial Pt/C. The high electrocatalytic activity and stable oxygen reduction electrocatalyst could be attributed to the higher surface areas and larger pore volume, as well as the synergistic effects of the different dimensional carbon materials, which was beneficial to the diffusion of the electrolyte and O₂ during the ORR.

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1. Introduction

Due to the growing challenge of energy shortage and the severe environmental pollution caused by fossil fuels, fuel cells have been recognized as one of the most promising and environmentally friendly power generators by converting chemical energy directly into electricity.¹ This energy conversion technology has received intensive research because of its high energy conversion efficiency, virtually no pollution, and potential large-scale applications.² While, efficient catalysts for the oxygen reduction reaction (ORR) at the anode play a key role in fuel cells. Platinum nanoparticles are known to be the most active catalysts. However, its large-scale commercial application is limited by their high cost, sensitivity to methanol or CO and a decrease of catalytic activity in the overall fuel cells. Consequently, it is essential to search for non-noble metal catalysts which exhibit both high catalytic activity and long-term operation stability to reduce or replace Pt-based ORR catalysts in fuel cells.

Recently, various nitrogen-doped carbon materials (carbon nanotubes, graphene, and mesopore carbon) have been used as metal-free electrocatalysts, which could dramatically reduce the cost and increase the efficiency of fuel cells.^3–5 Among them, newly developed nitrogen-doped graphene have attracted intensive attention because the incorporation of N lead to a positive charge on adjacent carbon atoms, which facilitates oxygen adsorption and subsequently weakens the O–O bonds.⁶ These nitrogen-doped catalysts show comparable ORR activities to commercial Pt/C catalyst, and better long-term stability, stronger tolerance to crossover and poison effect.⁷ However, as-prepared N-Gs tend to aggregate in the electrolyte due to van der Waals interaction, making it difficult to express its intrinsic physical and chemical properties.⁸ Therefore, numerous composites of N-G with various nanomaterials have been prepared and used in conductive electrode. By acting as spacers for each other, graphene–CNTs nanohybrids could alleviate graphene stacking and CNTs bundling by forming a 3D architecture. For instance, graphene coated a CNT/S hybrid was fabricated in hierarchical sandwich-type architecture, showing excellent rate performance and cyclic stability in lithium–sulfur batteries.⁹ Dai et al. synthesized graphene/carbon nanotubes (GN/CNT) hybrids with three-dimensional (3D) microstructures, and these hybrid films possessed an interconnected network of carbon structures with well-defined nanopores to be promising for supercapacitor electrodes.¹⁰ Nevertheless, most of them used polymeric binders or surfactants, which would cover the active sites and reduce the electric conductivity of the electrode.¹¹ Thus, facile and cost-effective preparation of clean and interconnected 3D N-G based materials is interesting but still a challenge.

Carbon spheres with nanopores/mesoporous have already exhibited promising applications in many fields such as water purification, fuel cell catalysis, energy storage, and gas sensors.^12–14 Unfortunately, the applications of carbon spheres are limited by their low specific surface areas and low conductivity, which need to be improved substantially. Carbon spheres always form a “point-to-point” conductive mode, where high loading and intimately contacted particles are needed to form effective electron conveying channels.¹⁵ So, it was applied to increase the conductivity of the whole composite electrode that constructing a conductive network by introduction of graphene through the “plane-to-point” mode.¹⁵ Some previous researches on graphene wrapped carbon spheres structures have been reported. Lü et al. reported that graphene-wrapped nitrogen-containing carbon spheres were fabricated by co-pyrolysis of graphene oxide and polyaniline–lignosulfonate composite at 800 °C, whose structures exhibited a high specific capacitance and good cycling stability during repeating cyclic voltammetry measurement.¹⁶ Yang et al. synthesized sulfur-doped carbon spheres via solvothermal method using sucrose as carbon source and benzyl disulfide as sulfur source, which exhibited improved electrocatalytic activities for the ORR and long-term stability.¹⁷ Moreover, the carbon spheres existing between graphene nanosheets by van der Waals interactions, not only prevent graphene from restacking and increases basal spacing, but also provide an efficient triple phase (solid–liquid–gas) region for easy mass transfer of O₂ and electrolyte during the ORR process.^12,17

In the present work, template-free hybrid CS@N-G/CNT was designed and constructed by two steps: firstly, CSs were wrapped by N-doped graphene (CS@N-G) under hydrothermal. It followed “plane-to-point” conducting mode with exceptional electron transport properties, which was attribute to the supporting role of N-Gs for loading carbon spheres. Moreover, carbon spheres intercalated between N-Gs could also be effectively rendering an intimate contact of N-Gs. Secondly, CNTs were introduced in the CS@N-G composite by self-assembly using ultrasonic method. The strong coupling between N-Gs and CNTs in CS@N-G/CNT hybrid facilitated efficient electronic conduction, which benefited the reaction kinetics of ORR. The hybrid CS@N-G/CNT structure combined the advantages of CSs and N-Gs, including the porous CSs structure and the high surface of N-doped graphene, and highly conductivity of CNT. Thus, the as-synthesized hybrid CS@N-G/CNT exhibited high electrocatalytic activity for ORR with long-term stability in alkaline media. Furthermore, the present study provides a facile way to design and develop low-cost and excellent non-precious metal catalyst for ORR in alkaline solution.

2. Experiments

2.1 Materials

Nafion solution (5 wt% in ethanol) and Pt/C (20 wt% Pt on Vulcan XC-72) were provided from Alfa Aesar Co. Acid-treated CNTs (diameter: 20–30 nm) were obtained from Chengdu Organic Chemical Co, Ltd., Chinese Academy of Sciences. Graphene oxide sheets (GO) were purchased from Nanjing XF Nano Material Tech Co.,Ltd., Nochromix glass-cleaning reagent was provided from Sigma-Aldrich, Inc., and other reagents were of analytical grades and used without further purification.

2.2 Synthesis of carbon spheres

Carbon spheres (CSs) were prepared according to the literature.¹⁸ Typically, 60 mL of 0.5 M glucose solution was transferred into a stainless steel autoclave of 100 mL, sealed and heated at 180 °C for 6 h. After cooling down to room temperature, the obtained blank products were washed several times with ethanol and distilled water by centrifugation, and dried in vacuum at 60 °C.

2.3 Preparation of carbon spheres@nitrogen-doped graphene composites

Typically, 30 mL of 2.0 mg mL⁻¹ graphene oxide (GO) solution was adjusted the pH to 10 by 30% ammonia. Then 2 mL of hydrazine hydrate was added as previous report.¹⁹ 0.06 g of carbon spheres (CSs) was added in the above solution, and then was ultrasonicated for 0.5 h. After that, the suspension was transferred into a Teflon-lined autoclave and heated at 200 °C for 3 h. After cooling down to room temperature, the obtained composite was collected by centrifugation and washed with deionized water and ethanol for three times, and dried in vacuum at 60 °C, and denoted as (CS@N-G). For comparison, the N-Gs (without CSs) were prepared.

2.4 Fabrication of hybrid carbon spheres@nitrogen-doped graphene/carbon nanotubes

The prepared CS@N-G composite was dispersed in DMF with a concentration of 4 mg mL⁻¹. To prepare carbon nanotubes supported CS@NG, 4 mg mL⁻¹ of the carbon nanotubes were dispersed in DMF. Then, both two suspensions were ultrasonicated for 1 h. The obtained hybrid carbon spheres@nitrogen-doped graphene/carbon nanotube (denoted as CS@N-G/CNT) dispersion was purified by filtration and washing with DMF at least three times. The synthesis route of hybrid CS@N-G/CNT was shown in Fig. 1.

Fig. 1 The synthesis process of hybrid CS@N-G/CNT.

2.5 Preparation of electrodes

A glassy carbon (GC) disk electrode with a 5.0 mm diameter (disk: 0.071 cm²), was used as the working electrode. Before any measurements, the electrode surface was polished by an aqueous suspension of Al₂O₃ (0.05 μm) and rinsed with distilled water. The GC electrode was then placed in an ultrasonic cleaning bath rinsed again with acetone, ethanol and distilled water, and then dried in an air-flow environment at room temperature. The catalyst powder (4.0 mg) was dispersed ultrasonically in 1.0 mL ethanol to form a homogeneous black suspension with 4 mg mL⁻¹, and a 10 μL aliquot of this suspension was cast on the pretreated GCE surface. The catalyst loading was controlled by 0.57 mg cm⁻². After being dried in air, a 10 μL diluted Nafion solution (5 wt% in ethanol) was put on the top of the product suspension, and then dried in air to prevent sample detachment from the working electrode during electrochemical testing.

2.6 Characterization

The X-ray diffraction (XRD) patterns of the assemblies were detected by a Rigaku (Tokyo, Japan) D/max-2400 X-ray diffractometer with Cu Kα radiation. Scanning electron microscopy (SEM) images of the products were obtained using a JEOL JSM-6700 field emission scanning electron microscope. Fourier transform infrared spectroscopy (FT-IR) spectra of the assemblies were measured by a Germany EQUINOX 55 spectrometer (KBr pellets) in the 4000–400 cm⁻¹ range. Raman spectra were characterized using Invia Laser-Raman spectrometer (Renishaw, United Kingdom) with confocal microscope system. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos AXILS ULTRA instrument equipped with a monochromated Al K X-ray source. The binding energies were referenced to the C 1s line at 284.6 eV from adventitious carbon. The Brunauer–Emmett–Teller (BET) surface area was measured at 77 K through N₂ adsorption desorption isotherms by a Micromeritics ASAP 2020 analyzer.

Linear sweep voltammeter (LSV) and cyclic voltammogram (CV) measurements were performed with a CorrTest electrochemical work station (CS310 Wuhan CorrTest Instrument Co. Ltd.) in a conventional three-electrode cell using the coated GC (3 mm diameter) as the working electrode, platinum foil as the auxiliary electrode, and Ag/AgCl as reference electrode. 0.1 M KOH solutions were used as electrolyte in the experiments. All solutions were purged with pure oxygen (or nitrogen) prior to each electrochemical experiment for ∼30 min. During the test, the trachea was put on the liquid surface. After this test, the solutions were de-aerated by bubbling oxygen (or nitrogen) again for ∼10 min, and then do the next sample. LSV measurements were done in O₂ saturated solution, with the scan rate of 20 mV s⁻¹. CV measurements were done in N₂/O₂ saturated solution, with the scan rate of 50 mV s⁻¹. Before CV measurements, the electrode was repeatedly potentiodynamic swept from −0.2 to +1.0 V in an oxygen-protected 0.1 M KOH for 20 min until a steady voltammogram curve was obtained.

The stabilities of the CS@N-G/CNT and commercial Pt/C catalysts were carried out using chronoamperometric technique in presence of oxygen saturated 0.1 M KOH at constant potential of −0.5 V vs. Ag/AgCl and with electrode rotation speed of 1600 rpm. Before the testing, the glassware was cleaned via Nochromix, subsequently rinsed with boiling double distilled water for 6 times,²⁰ and then dried at room temperature. All the experiments were performed at room temperature of 25 ± 1 °C and ambient pressure.

3. Results and discussion

3.1 Morphology and structural analysis

As a facile and inexpensive method, hydrothermal has been widely applied to preparation of various nanomaterials. In the present work, the reduction and nitrogen doping of graphene, and prepared the CS@N-G composite are achieved simultaneously by the hydrothermal approach. Fig. 2a–f show the representative SEM of micrographs of CSs, GO, N-Gs, CS@N-G and as-synthesized hybrid CS@N-G/CNT. As seen from Fig. 2a, the CSs show good spherical shape with smooth surfaces and a diameter of 1–2 μm. According to the LaMer model and Sun's work,^18,21 the carbon spheres were obtained from glucose under hydrothermal conditions at 180 °C, containing polymerization and carbonization steps.²¹ Moreover, the CSs exhibit nanopores structures, which are formed by small amount of the interstitial spaces produced by carbon clusters.²² Fig. 2b shows the typical SEM image of GO with a curled morphology consisting of thin wrinkled paper-like structures.²³ After hydrothermal reduction, the N-G exhibits a typical aggregated and crumpled structure (Fig. 2c). From Fig. 2d, it displays morphology of the as-synthesized CS@N-G composite. It can be seen that carbon spheres are wrapped with N-G sheets. These carbon spheres contact the N-Gs closely, which maybe benefits for the effective charge transfer for the ORR. Fig. 2e shows the SEM images of as-synthesized CS@N-G/CNT hybrid. It can be seen that the surface of graphene-wrapped carbon spheres appears crinkled and rough, which can provide more spaces for the electrolyte to transport more freely.¹⁶ The magnified SEM image of CS@N-G/CNT marked with the red circle region is shown in Fig. 2f. It can be seen that lots of CNTs maintain the cross-link structure and firmly attach to the N-G sheets, indicating the strong interaction between them, which helps improving the electric conductivity of the composite material. It is noticeable that these carbon spheres become smaller (0.5–1 μm) after second hydrothermal (Fig. 2f), which is accordant with the previous result.²⁴

Fig. 2 High magnification SEM images of (a) CS, (b) GO, (c) N-G, (d) CS@N-G, and (e) CS@N-G/CNT, (f) high-magnification image taken from the area marked with a red circle in (e).

XRD patterns of the as-synthesized products were shown in Fig. 3. The broad peak around 2θ = 21° indicates an amorphous phase, corresponding to the carbonaceous composition of the CSs.²⁵ The pattern of the GO exhibits an intense, sharp peak centered at 2θ = 10.9° and 44.1°, contributing to graphite-like (002) and (100) of graphene oxide. The diffraction peak of 10.9° disappeared, and another broad peak at 2θ = 28° can be observed for CS@N-G composite, indicating the efficient reduction of graphene oxide during the hydrothermal.²⁶ After induction of CNT in the CS@N-G/CNT hybrid, no obvious reflection peak from other impurities is observed in the XRD, indicating the formation of the hybrid, which is in accordance with the result of FTIR.

Fig. 3 XRD patterns of (a) CS, (b) GO, (c) CS@N-G and (d) CS@N-G/CNT.

FTIR analysis was further performed to analyze the surface properties of products. As shown in Fig. 4, for CSs, the peaks at 1704 cm⁻¹ could be assigned to the C=O vibration, 1617 cm⁻¹ to C=C stretching vibration, 1380 cm⁻¹ to the symmetric stretching vibration of deprotonated carboxylic group COO⁻, respectively.²⁷ Other range of 3450–3500 cm⁻¹ can be characterized as O–H stretching vibration, 1021 cm⁻¹ and 1302 cm⁻¹ to C–OH stretching and O–H bending vibrations in C–OH, respectively, indicating the existence of large numbers of residual hydroxyl groups on the CSs. Partially of the residue of OH or CHO groups are covalently bonded to the carbon frameworks of GO, making the CS@N-G more stable in aqueous systems.¹⁸ Almost all the characteristic peaks of GO disappeared for CS@N-G after the hydrothermal, including the C=O stretching at 1720 cm⁻¹, HO–C=O stretching at 1630 cm⁻¹, and C–O stretching at 1030 cm⁻¹. It only can be seen that skeletal vibration absorption peak of graphene is at about 1570 cm⁻¹. This result suggests that GO was reduced to graphene during the hydrothermal.²⁸ Moreover, for hybrid CS@N-G/CNT, the peak at 1570 cm⁻¹ shifts to position at 1555 cm⁻¹ owing to the C=C stretching vibration band in CNTs, which testify that the CNTs has been successfully introduction into the hybrid.²⁹

Fig. 4 IR spectra of (a) CS, (b) GO, (c) CS@N-G and (d) CS@N-G/CNT.

Raman spectra were employed to characterize the properties of carbon materials and in particular disorder and defect structures. As shown in Fig. 5, all the Raman spectra display a broad D band peak at 1349 cm⁻¹ and G band peak at 1585 cm⁻¹, corresponding to the defects in the graphitic layers and the graphitic layers, respectively. Moreover, the intensity ratio of D band to G band (I_D/I_G) reflects the density of defects in graphene-based materials. The ratio of CS is 0.58, indicating an amorphous carbon structure.³⁰ The value of CS@N-G (0.98) is higher than that of GO (0.84), indicating that a partial reduction of the GO. Interestingly, for CS@N-G/CNT, the ratio of I_D/I_G further increases to 1.03, suggesting that more defects have been doped into the CS@N-G/CNT after introduction of CNT. At the same time, the D band and G band slightly shifted to 1333 cm⁻¹ and 1560 cm⁻¹, respectively. This red shift can be ascribed to the strong coupling between CSs and N-G or CNTs, similar phenomena have been reported in other work.³¹

Fig. 5 Raman spectra of (a) CS, (b) GO, (c) CS@N-G and (d) CS@N-G/CNT.

N₂ adsorption/desorption analysis was used to assess the specific surface area and the pore structure. As shown in Fig. 6, the isotherm of the CS@N-G/CNT is of typical type-IV with a desorption hysteresis at a pressure range of 0.7–1.0 P/P₀. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, the result indicates the existence of mesopores in the hybrid.³² Moreover, when the relative pressure is close to 1, the amount of the adsorbed N₂ rapidly increases, indicating that macropores exist in the CS@N-G/CNT, which can be confirmed by the narrow pore size distribution curve centered at 86.0 nm, corresponding Barrett–Joyner–Halenda (BJH) pore-size distribution plot inset of Fig. 6. Both the CSs and CS@N-G exhibit type-I isotherms, which are characteristic feature of a microporous materials, with a peak pore diameter of ca. 1.8 nm for CSs and ca. 1.9 nm for CS@N-G. For comparison, Table 1 summarizes the BET data of the CS and CS@NG composite obtained in this work. The BET surface area of the hybrid CS@N-G/CNT (132.28 m² g⁻¹) is higher than that of CSs (13.56 m² g⁻¹) and CS@N-G (93.42 m² g⁻¹), which is favorable for the accessibility of the electrolyte. Furthermore, the CS@N-G/CNT hybrid exhibits much larger pore volume (0.48 cm³ g⁻¹) than the CSs (0.031 cm³ g⁻¹) and CS@N-G (0.098 cm³ g⁻¹). The high specific surface area, large pore volume, and porous structure can provide more active sites to adsorb reactant molecules and facilitate accessibility of reactant.

Fig. 6 N₂ adsorption–desorption isotherms with the corresponding BJH pore-size distribution plot (the inset) for the synthesized: CS, CS@N-G and CS@N-G/CNT.

Table 1 BET specific surface area, pore volume and pore size of various samples

Sample	SBET (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore size (nm)
CS	13.56	0.031	1.8
CS@N-G	93.42	0.098	1.9
CS@N-G/CNT	132.28	0.48	2.5, 3.0, 86.0

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The chemical bonding structures of CS@N-G/CNT have been analyzed by X-ray photoelectron spectroscopy (XPS) in the Fig. 7. The XPS spectra clearly show the existence of C, O and N elements on the surface of the composite. And from the high-resolution XPS spectra of N 1s, it can be seen that the N 1s of the composite can be deconvoluted into the pyridinic (∼398.1 eV), pyrrolic (∼399.7 eV), and graphitic (∼401.1 eV) nitrogen, indicating that N atoms are in the three different bonding characters in the composite. The pyridinic and pyrrolic nitrogen are assigned to the one or two P electron to the π conjugated system,³³ graphitic nitrogen is attributed to the replacing the C by N atoms in the graphene.¹⁹

Fig. 7 (a) XPS spectra of the CS@N-G/CNT. (b) High-resolution N 1s spectra of CS@N-G/CNT.

3.2 Electrocatalytic activity of catalysts for oxygen reduction

To evaluate the electrocatalytic ORR performance of catalysts, linear sweep voltammetry (LSV) measurements were performed on a rotating ring-disk electrode (RRDE). Based on LSV, the ORR onset potential could be determined from the slope of the fitted linear line.³⁴ A series of samples including N-G, CS, CNT, CS@N-G, mixed CS/N-G/CNT and CS@NG/CNT (with the same mass loading) on GCE were studied. The onset potential and current density at −1.0 V (vs. Ag/AgCl) of these catalysts can be obtained from Fig. 8, which are summarized in Table 2. It can be seen that CS@N-G/CNT/GCE manifests more positive onset potential (−0.105 V vs. Ag/AgCl) than those of CS@N-G/GCE (−0.130 V vs. Ag/AgCl) and mixed N-G/CS/CNT/GCE (−0.117 V vs. Ag/AgCl). Moreover, the current density of the catalysts follows the order: CS@N-G/CNT > mixed CS/N-G/CNT > CNT > CS@N-G > N-G > CS. More positive onset potential and larger current density indicate higher catalytic activity. Above results indicate that physically mixed CS/N-G/CNT catalysts and CS@N-G/CNT show higher ORR catalytic activities than CS, N-G and CNT alone in terms of the onset potential, which might be caused since the additional CNTs in the composites. These results demonstrate that CNTs presented in the composite could functioned as tunnels for the rapid electron transfer,³⁵ furthermore, as spacer between N-G sheets and CSs network structure for supplying more active sites to adsorb sufficient oxygen from solution on its surface, which facilitates to enhance the ORR performance. It is notable that the catalytic activity of physically mixed CS/N-G/CNT is inferior to that of the CS@N-G/CNT, indicating that the high catalytic activity of CS@N-G/CNT is due to the strong coupling between CSs and N-Gs. These results suggest that CS@N-G/CNT catalyst shows the highest electrocatalytic activity towards ORR in the present work. Although the onset potential for CS@N-G/CNT (−0.105 V) is less positive than that of Pt/C (−0.052 V vs. Ag/AgCl), the CS@N-G/CNT electrode exhibits a higher current density with respect to the Pt/C (Fig. 11a), which infers an increased number of active sites for ORR. In addition, the performance of CS@N-G/CNT electrocatalyst is comparable to other non-noble metal catalysts (Table 3).^36–40 The high electrocatalytic performance of as-synthesized CS@N-G/CNT is attributed to unique structure of CS@N-G/CNT and the synergetic effects of the nanopores of CSs, appropriate content of N (2.58 at%) in the composite (Fig. 7) and large specific surface area of grapheme and CNT.

Fig. 8 Linear sweep voltammograms of N-G/GCE, CS/GCE, CNT/GCE, CS@N-G/GCE, mixed CS/N-G/CNT/GCE and CS@N-G/CNT/GCE electrodes in O₂-saturated 0.1 M KOH at scan rate of 20 mV s⁻¹.

Table 2 Comparative electrochemical properties and corresponding experimental data for various ORR electrocatalysts in oxygen-saturated 0.1 M KOH solution

Sample	Onset potential (V vs. Ag/AgCl)	J^a (mA cm^−2)
a J, current density at the potential of −1.0 V vs. Ag/AgCl, obtained by subtracting the background currents from those performed in the oxygen-purged electrolyte.
N-G	−0.162	1.81
CS	−0.199	1.13
CNT	−0.175	3.29
CS@N-G	−0.130	2.61
Mixed CS/N-G/CNT	−0.117	4.94
CS@N-G/CNT	−0.105	5.92
Pt/C	−0.052	4.96

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Table 3 Comparison the performance of various electrocatalysts for ORR

Catalysts	Onset potential (V vs. Ag/AgCl)	Electron transfer number (n)	Ref.
3D-N–RGO/MnO	−0.15 to −0.35	3.03	36
PPy/Co	−0.1	3	37
N–CNT	−0.139	3.2	38
α-Fe2O3/CNTs	−0.15	3.05–3.45	39
Co–N–GN	−0.098	3.44–3.72	40
CS@N-G/CNT	−0.105	3.5	This work

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Cyclic voltammetry measurements were performed CS@N-G/CNT in O₂ (solid line)/N₂ (dashed line)-saturated 0.1 M KOH electrolyte within the potential range of 0.2 to −1.0 V (vs. Ag/AgCl) (Fig. 9). As shown in Fig. 9a, featureless voltammetric currents can be observed for CS@N-G/CNT/GCE in N₂-saturated 0.1 M KOH solution. In contrast, a well-defined cathodic peak centered at −0.23 V (vs. Ag/AgCl) appears in O₂-saturated 0.1 M KOH solution. Compared to bare GCE (Fig. 9b), the CS@N-G/CNT catalyst has more positive onset potential and higher current densities, revealing high catalytic activity of the CS@N-G/CNT hybrid for O₂ reduction.

Fig. 9 CVs for ORR at (a) the GC electrodes, (b) GC coated by CS@N-G/CNT in nitrogen protected (dash curves) or oxygen-saturated (solid curves) in a 0.1 M KOH at the scan rate of 50 mV s⁻¹.

To further investigate the ORR on CS@N-G/CNT/GCE electrode, rotating-disk electrode (RDE) measurements were carried out in O₂-saturated 0.1 M KOH electrolyte at a scan rate of 20 mV s⁻¹. The polarization curves at different rotation rates for the ORR on CS@N-G/CNT/GCE are shown in Fig. 10a. It can be seen that the current density of CS@N-G/CNT/GCE increases with the increasing rotation rates because more oxygen delivered to the catalyst at higher rotation rates. The number of electrons involved in the reduction of O₂ was calculated by the Koutecky–Levich (K–L) equation:⁴¹

1/J_app = 1/J_k + 1/J_lim (1)

J_lim = 0.62nFD^2/3v^−1/6C₀ω^1/2 (2)

where J_app is the measured or apparent current density, J_k is the kinetic current density, J_lim is the limiting current density, n is the number of electrons transferred per oxygen molecule, F is Faraday constant (96 500 C mol⁻¹), D is the O₂ diffusion coefficient in 0.1 M KOH (1.9 × 10⁻⁵ cm² s⁻¹), C₀ is the concentration of oxygen (1.2 × 10⁻⁶ mol cm⁻³), v is the viscosity of the electrolyte (1.1 × 10⁻² cm² s⁻¹), and ω is the rotation rate of the radian. According to the K–L equation, the K–L plots of I⁻¹ vs. ω^−1/2 at −0.5 V potentials were drawn and shown in Fig. 10b. It shows that the electron exchange number on the CS@N-G/CNT is 3.5, lower than that for the Pt/C electrode (Fig. S1†), suggesting a combination of two-electron and four-electron pathways, and the four-electron pathway is the dominant one, which supports LSV results of its higher ORR activity.

Fig. 10 (a) RDE polarization curves at different rotation rates for ORR on GC with predeposited CS@N-G/CNT/GCE in a 0.1 M KOH solution saturated with oxygen at the scan rate of 20 mV s⁻¹. (b) Plot of I⁻¹ vs. ω^−1/2 for oxygen reduction on GC with precoated CS@N-G/CNT/GCE at potentials −0.65, −0.75 and −0.85 V.

3.3 Study of stability

Stability of an electrocatalyst was also an important factor for practical application. Comparison of ORR catalytic activity and the preliminary stability tests of ORR activity for CS@N-G/CNT and Pt/C were also evaluated using a chronoamperometric method at −0.5 V in 0.1 M KOH solution saturated with O₂ at a rotation rate of 1600 rpm for 9000 s (Fig. 11b). It can be seen the current loss for the CS@N-G/CNT hybrid is about 5%, which is much lower than those of N-doped shell/core structured carbide-derived carbon/SiC particles (loss of 7% after the 2.5 h),⁴² and FePc–Gr composite (loss of 16.5% after 10 000 s),⁴³ while the corresponding current loss for Pt/C catalyst is as high as 39% under the same conditions. As previous reports, it is influenced by many factors, such as, the duration of the test and the surface area of carbon related to the measured currents.^44,45 From the above, the CS@N-G/CNT hybrid exhibits superior durability to Pt/C catalyst. The stability of CS@N-G/CNT could be probably attributed to the strong coupling between the N-G and CNT compared to the physically supported Pt nanoparticles on carbon support. Thus, this result indicates that CS@N-G/CNT hybrid is a good alternative to Pt as cathode catalysts in alkaline solution.

Fig. 11 (a) Reduction of oxygen on Pt/C and CS@N-G/CNT electrodes in O₂-saturated 0.1 M KOH at scan rate of 20 mV s⁻¹. (b) Current–time (i–t) chronoamperometric responses for the ORR of CS@N-G/CNT and commercial Pt/C (20 wt% Pt on carbon) in O₂-saturated 0.1 M KOH solution at −0.5 V and at a rotation rate of 1600 rpm.

4. Conclusions

Hybrid CS@N-G/CNT was successfully synthesized by a hydrothermal and ultrasonic-assisted route. The hybrid consisted of one-dimensional CNTs and two-dimensional N-Gs supporting zero-dimensional CSs. It exhibits high electrocatalytic activity and superior durability to Pt/C, which is mainly attributed to the higher surface areas and larger pore volume (allowing rapid mass diffusion and fast electron transfer), and the synergistic effects of nitrogen doping of graphene, porous CSs and CNTs (enhancing electron conductivity of CS@N-G/CNT). The present study provides a facile way to design and develop low-cost and highly active of non-precious metal catalyst for ORR in alkaline solution.

Acknowledgements

This work has been supported by the National Natural Science Foundation of China under Grant No. 51221001, the “111” Project under Grant No. B08040.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00819d

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