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Synergistic interaction and controllable active sites of nitrogen and sulfur co-doping into mesoporous carbon sphere for high performance oxygen reduction electrocatalysts
Applied Surface Science 440 (2018) 627–636
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Full Length Article
Synergistic interaction and controllable active sites of nitrogen and sulfur co-doping into mesoporous carbon sphere for high performance oxygen reduction electrocatalysts Taeseob Oh, Myeongjin Kim, Dabin Park, Jooheon Kim ⇑ School of Chemical Engineering & Materials Science, Chung-Ang University, 211 Heukseok-dong, Dongjak-gu, Seoul 156-756, Republic of Korea
a r t i c l e
i n f o
Article history: Received 14 November 2017 Revised 29 December 2017 Accepted 22 January 2018
Keywords: Oxygen reduction reaction Metal-free electrocatalyst Mesoporous carbon sphere Nitrogen and sulfur co-doping Synergistic effect
a b s t r a c t Nitrogen and sulfur co-doped mesoporous carbon sphere (NSMCS) was prepared as a metal-free catalyst by an economical and facile pyrolysis process. The mesoporous carbon spheres were derived from sodium carboxymethyl cellulose as the carbon source and the nitrogen and sulfur dopants were derived from urea and p-benzenedithiol, respectively. The doping level and chemical states of nitrogen and sulfur in the prepared NSMCS can be easily adjusted by controlling the pyrolysis temperature. The NSMCS pyrolyzed at 900 °C (NSMCS-900) exhibited higher oxygen reduction reaction activity than the mesoporous carbon sphere doped solely with nitrogen or sulfur, due to the synergistic effect of co-doping. Among all the NSMCS samples, NSMCS-900 exhibited excellent ORR catalytic activity owing to the presence of a highly active site, consisting of pyridinic N, graphitic N, and thiophene S. Remarkably, the NSMCS900 catalyst was comparable with commercial Pt/C, in terms of the onset and the half-wave potentials and showed better durability than Pt/C for ORR in an alkaline electrolyte. The approach demonstrated in this work could be used to prepare promising metal-free electrocatalysts for application in energy conversion and storage. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction The oxygen reduction reaction (ORR) plays an important role in energy conversion and storage in devices such as, fuel cells and metal-air batteries [1,2]. A major concern is the sluggish reaction kinetics of ORR at the cathode, which in turn limits the performance of fuel cells and metal-air batteries. Hence, electrocatalysts are required to decrease the activation energy of the sluggish reaction and boost the performance of ORR. Among the electrocatalyst materials, platinum-based catalysts were widely known as the most active catalysts for ORR, owing to their low onset potential and high current density [3–6]. However, Pt based catalysts have several limiting factors related to large-scale industrial application, including high cost, limited supply, and poor durability. In this regard, numerous studies have been conducted to obtain lowcost and efficient ORR electrocatalysts based on non-precious metal [7–9] or metal-free catalysts [10–12]. Among the metal-free catalysts, carbon-based materials such as graphene [13], carbon nanotube [14], carbon nanodots [15], and ⇑ Corresponding author. E-mail addresses: [email protected] (T. Oh), [email protected] (M. Kim), [email protected] (D. Park), [email protected] (J. Kim). https://doi.org/10.1016/j.apsusc.2018.01.186 0169-4332/Ó 2018 Elsevier B.V. All rights reserved.
nanosheets [16] have been considered alternative electrocatalysts due to their low cost and better stability. Although there has been significant progress in developing carbon-based catalysts, metalfree carbon catalysts still have problems with respect to specific surface area and the presence of pores. Therefore, increasing the specific surface area and adjusting the pore sizes can become important factors in achieving an efficient triple-phase (solidliquid-gas) region to facilitate mass transfer of electrolyte and oxygen [17]. Yao Zheng et al. reported that C3N4 into a mesoporous carbon enhanced the electron transfer of C3N4 [18] and Liang et al. reported that C3N4/C with 150 nm ordered pores showed high ORR activity by facilitating reactant transport through the macroporous structure [19]. Small pore size is often reported to be unfavorable for mass transfer, while macropores can shorten the electrolyte diffusion pathway to the interior surfaces. In addition, mesopores can provide large accessible areas for charge and ion transport. However, these porous carbon-based materials are still poorer than the commercial Pt catalysts in ORR performance. To overcome the low ORR activity, doping with heteroatoms (N, S, B, P) has been researched as a new way to enhance the catalytic activity [20,21]. Doping the carbon materials with heteroatoms could induce changes in the atomic charge density and the spin density. This
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charge redistribution with the heteroatom dopants could facilitate the O2 chemisorption on the carbon material surface and weaken the O-O bonding to facilitate the ORR process [22]. Liangti Qu et al. reported that nitrogen-doped graphene was an effective electrocatalyst [23] and Kuanping Gong et al. reported high electrocatalytic activity for nitrogen-doped carbon nanotube [24]. As reported earlier, nitrogen doping was widely investigated for improving the ORR activity, because of the electronegativity difference between the carbon and nitrogen atoms [22]. Other heteroatoms with lower electronegativity such as B and P could also change the charge density, thereby generating active sites for ORR. On the other hand, Zhi Yang et al. reported sulfur-doped graphene as ORR catalysts [11] and Yan Sun et al. reported sulfurdoped carbon spheres as electrocatalysts [25]. The reason for the ORR activity of sulfur is the two lone pairs of electrons on the sulfur atom can contribute to the O2 chemisorption [26] and its large atomic radius compared to that of the carbon atom, in spite of having similar electronegativity. Furthermore, heteroatom co-doped carbon materials revealed enhanced ORR performance owing to the synergistic effect between the dopants. The fundamentals of heteroatom co-doping in carbon materials can be understood from the theoretical researches on density functional theory (DFT) calculations of spin and charge densities [27]. The heteroatom doping level, the nature of the heteroatom dopants and the chemical states formed between the carbon atoms and the dopants are the key factors affecting the spin and the charge densities [28]. For instance, Shuangyin Wang et al. reported that nitrogen and boron codoped carbon nanotube [29] and graphene [30] had better ORR activity, compared to the carbon materials solely doped with boron and nitrogen. Moreover, nitrogen and sulfur co-doped into carbonaceous materials has been researched for oxygen reduction eletrocatalysts with various morphologies such as ordered mesoporous carbon [31], carbon dots [32], carbon nanofiber networks [33], graphene with carbon nanospheres [34] and mesoporous graphene [28]. This is because, the difference in atomic radius between sulfur and other atoms and the difference in electronegativity between nitrogen and other atoms can change the charge and the spin density and improve ORR performance. However, these studies showed only the synergistic effect of doping nitrogen and sulfur in carbon materials and rarely controlled the heteroatom doping level and the chemical states formed between the carbon atoms and the dopants. Therefore, it is attractive to control active sites of doping level and chemical states in nitrogen and sulfur co-doped carbonaceous materials. Herein, nitrogen and sulfur co-doped three-dimensional mesoporous carbon spheres (NSMCS) were successfully synthesized from sodium carboxymethyl cellulose as the carbon source (see Scheme 1). The prepared NSMCS exhibited high specific surface area and uniform nanostructure morphology. The high specific surface area and the mesopores can provide low resistance pathways for oxygen diffusion in the pores and good charge propagation, leading to improved catalytic activity. Nitrogen and sulfur codoping was achieved by a facile pyrolysis method using urea and p-benzenedithiol, respectively, while controlling the doping ratio of nitrogen and sulfur. We can successfully control active sites of chemical states and doping level in NSMCS and find optimal condi-
tions for efficient ORR activity. The existence of nitrogen and sulfur in the carbon sphere can modify the charge distribution of NSMCS, which can change the charge and spin density, facilitating oxygen chemisorption on carbon sphere surface and weak oxygen bonding to improve ORR activity Among the NSMCS samples, the NSMCS pyrolyzed at 900 °C exhibited higher ORR activity owing to the presence of a highly active site consisting of pyridinic N, graphitic N, and thiophene S. When used as electrocatalyst at the cathode, the NSMCS-900 exhibited excellent ORR performance and better durability and stability than the commercial Pt/C in alkaline electrolyte. 2. Experimental 2.1. Preparation of materials The mesoporous carbon spheres (MCS) were synthesized through the hard template method. In a typical procedure, 0.32 g cetyl trimethyl ammonium bromide (CTAB) was added to a mixture containing 106 mL of deionized water and 56 mL of ethanol to form solution A. Then, 2 mL ammonium hydroxide and 2 mL tetraethyl orthosilicate (TEOS) were added under vigorous stirring for 4 h, until the solution A was homogeneously dispersed. The resulting mixture became white because of the formation of silica spheres. 1 g sodium carboxymethyl cellulose (average Mw 90,000) was completely dissolved in 50 mL deionized water under vigorous stirring for 1 h to form solution B. Subsequently, solution B was dissolved in solution A under vigorous stirring at room temperature. After carrying out the reaction for 12 h, the white solution was evaporated at 90 °C in a water bath. The products were dried at 80 °C in a vacuum oven for 8 h and carbonized at 800 °C for 4 h at a ramp rate of 5 °C min1 under a N2 atmosphere, in a tubular furnace. Finally, the MCS were obtained after etching with 10 wt% HF solution. The nitrogen and sulfur doped mesoporous carbon spheres (NSMCS) were prepared by a one pot facile method. 200 mg of MCS, 800 mg of urea and 800 mg of p-benzenedithiol were dissolved in deionized water and ultrasonicated for 1 h before stirring for 1 h to facilitate the adsorption of urea and p-benzenedithiol on the MCS. The mixture was slowly evaporated at 90 °C in an oven. The dried samples were annealed at different temperatures (700, 800, 900 and 1000 °C) for 2 h under a N2 atmosphere at a heating rate of 5 °C min1 in a tubular furnace and are denoted as NSMCS700, NSMCS-800, NSMCS-900 and NSMCS-1000, respectively. The NMCS and SMCS were also synthesized using an identical procedure for comparison. 2.2. Characterization methods The morphology and structure of the nanomaterials were characterized by field-emission scanning electron microscopy (FE-SEM, SIGMA, Carl Zeiss) and high-resolution transmission electron microscopy (HR-TEM, JEM-3010). Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) measurements were done to observe the specific surface area, pore volume and average pore size of the samples. Raman spectra were recorded on a Raman
Scheme 1. Schematic illustration of synthesis of NSMCS for oxygen reduction reaction.
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spectrometer (T64000, HORIBA, FR), equipped with an argon laser at 514 nm excitation wavelength. Chemical composition and binding energy of the samples were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo U.K. K-Alpha) with an Al Ka X-ray source (1486.6 eV). 2.3. Electrochemical measurements The electrochemical properties of the NSMCS catalysts were studied using a potentiostat (CHI 600E, CH Instrument, USA) with
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a three-electrode system at room temperature. A platinum wire, Ag/AgCl electrode and glassy carbon electrode (GCE, 5.0 mm diameter) were used as counter, reference and working electrodes, respectively. Before performing electrochemical measurements, the glassy carbon electrode was polished with 0.05 lL and 1 lL of alumina and diamond polish and then rinsed with deionized water. The catalyst ink was prepared by dispersing 2.0 mg of the sample in 1000 lL solvent mixture containing deionized water, Nafion solution (5 wt%) and absolute ethanol (V:V:V = 8:1:1) by ultrasonication at 0.5 h. Then, 10 lL of the catalyst ink was
Fig. 1. (a, b) Low and high-magnification FE-SEM image of NSMCS-900. (c, d) Low and high magnification HR-TEM image of NSMCS-900. (e) Nitrogen adsorption–desorption isotherms of NSMCS-900. (f) Pore size distribution curves of NSMCS-900.
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dropped on the surface of a glassy carbon electrode and dried in oven at 80 °C for 0.5 h. The ORR activity experiments were performed by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements on a rotating disk electrode (RDE). The CV measurement was evaluated at a scan rate of 100 mV/s and a potential range of 0.2 to 1.0 V in 0.1 M KOH solution, saturated with O2 or N2. The LSV measurement was performed on the RDE at a scan rate of 10 mV/s and a potential range of 0.2 to 1.0 V by varying the rotation speed from 400 to 2400 rpm in O2 saturated 0.1 M KOH solution. To investigate the stability and durability of the catalysts, 2000 continuous LSV cycles were performed and chronoamperometric measurement was performed at a constant potential of 0.4 V at 1600 rpm for 20,000 s. All electrochemical experimental results were presented with respect to reversible hydrogen electrode (RHE). The conversion between Ag/AgCl and RHE electrode is performed by using the Nernst equation.
ERHE ¼ EAg=AgCl þ E0Ag=AgCl ðreferenceÞ þ 0:0591 pH
ð1Þ
ERHE is the converted potential vs. RHE, EAg/AgCl is the experimental potential measured with respect to Ag/AgCl reference electrode, and E0Ag/AgCl is the standard potential of Ag/AgCl at 25 °C (0.1976 V). 3. Results and discussion 3.1. Characterization of NSMCS The morphology and nanostructure of NSMCS-900 were characterized using FE-SEM and HR-TEM, as shown in Fig. 1. The FE-SEM images (Fig. 1a) of NSMCS showed that all the carbon particles displayed spherical nanostructure without aggregation. In Fig. 1b, the average diameters of these carbon spheres were about 200–300 nm and the surface of the NSMCS-900 was porous. Furthermore, the HR-TEM images (Fig. 1c and d) of the NSMCS-900 clearly showed that the three dimensionally interconnected mesopores were randomly located to construct the whole carbon sphere. The mesoporous features shown in the TEM images were further confirmed using nitrogen adsorption-desorption measurements. BrunauerEmmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) analyses yielded the specific surface area and the pore size distribution of
NSMCS-900. The sample exhibited type IV isotherm (Fig. 1e) with a hysteresis loop, according to the IUPAC classification [35]. The hysteresis loop can be observed at a relative pressure range 0.4–1.0, indicating the presence of randomly located mesopores [36], in accordance with the TEM observation (Fig. 1d). The formation of the mesopores on carbon spheres can be attributed to the etching of silica by HF solution. The BET specific surface area and total pore volume of the NSMCS-900 were calculated to be 354.5 m2 g1 and 2.315 cm3 g1 respectively. The average pore size (Fig. 1f) of NSMCS-900 was 10.8 nm, corresponding to the adsorption of nitrogen on the mesopores. Such a porous structure of the MCS nanocomposites was beneficial to provide efficient transport pathways for oxygen and increased the electrode/electrolyte contact area, which are important for high performance electrocatalysis [17]. The graphitic structure of NSMCS-900 was probed using Raman spectroscopy. A typical D band and a G band peak appeared at 1350 and 1580 cm1 respectively, as shown in Fig. 2a. The D band was related to the sp3 defect sites and the G band was associated with the sp2 hybridized bonded pairs. The ID/IG ratio is widely used to evaluate the density of defects in graphite materials [37]. All the NSMCS samples showed higher ID/IG ratio than the undoped MCS (0.991), indicating that the sulfur and nitrogen doping induced defects in the carbon lattice. In addition, it is obvious that the ID/IG ratio decreased from NSMCS-700 (1.024) to NSMCS-1000 (1.011), with increasing annealing temperature, indicating that the graphitization degree of NSMCS is improved. On the other hand, the decrease in ID/IG ratio with the increase in annealing temperature may be due to the decreasing total amount of N and S dopants in the carbon sphere, as confirmed by the following XPS results. The XPS spectra of MCS, NSMCS-700, NSMCS-800, NSMCS-900 and NSMCS-1000 in Fig. 2b, showed the C 1s peak (285 eV), O 1s peak (533 eV), N 1s peak (398 eV) and S 2p peak (164 eV) [38], indicating successful doping with N and S by annealing the mesoporous carbon sphere in the presence of urea and p-benzenedithiol. The XPS data revealed no nitrogen and sulfur content in MCS and showed only carbon and oxygen peaks. After annealing MCS in the presence of urea and pbenzenedithiol, the sulfur and nitrogen peaks were clearly observed. The appearance of S 2s, S 2p and N 1s peaks indicated that both nitrogen and sulfur had been incorporated in the graphitic carbon structure during the pyrolysis process. The amount of
Fig. 2. (a) Raman spectra of MCS, NSMCS-700, NSMCS-800, NSMCS-900 and NSMCS-1000. (b) XPS wide scan spectra of MCS, NSMCS-700, NSMCS-800, NSMCS-900 and NSMCS-1000.
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nitrogen and sulfur was 4.90% and 1.84% (N/C and S/C atomic ratio), respectively, in NSMCS-700 and 1.63% and 0.58% in NSMCS-1000. In addition, intensity of C 1s peaks are increased with increase in annealing temperature [39]. The decrease in nitrogen and sulfur contents and increase in carbon content with increasing annealing temperature are in accordance with the Raman spectra. To further understand the bonding configurations of N and S in NSMCS, we analyzed the catalysts using high resolution XPS. The
N 1s spectra in the high resolution XPS of NSMCS, annealed from 700 °C to 1000 °C are given in Fig. 3. The XPS spectra showed four types of N species, including pyridinic N, pyrrolic N, graphitic N and oxidized N, at binding energies of around 398.5, 400.7, 401.9 and 404.6 eV, respectively [40,41]. As the annealing temperature was increased from 700 °C to 1000 °C, the total nitrogen content decreased from 4.90% (NSMCS-700) to 1.63% (NSMCS-1000), as shown in Table 1. The pyridinic N and pyrrolic N contents were 52.4% and 10.1% in NSMCS-700, whereas contents of pyridinic N
Fig. 3. High-resolution N 1s XPS spectra of (a) NSMCS-700, (b) NSMCS-800, (c) NSMCS-900 and (d) NSMCS-1000.
Table 1 Nitrogen and sulfur atomic percentage of various chemical states in NSMCS samples. Samples
N/C (%)
Pyridinic N (%)
Pyrrolic N (%)
Graphitic N (%)
Oxidized N (%)
S/C (%)
Thiophene S (%)
Oxidized S (%)
NSMCS-700 NSMCS-800 NSMCS-900 NSMCS-1000
4.90 4.07 3.11 1.63
52.4 50.7 45.7 31.3
10.1 8.1 5.0 4.2
28.0 33.4 42.4 58.5
9.5 7.8 6.9 6.0
1.84 1.27 1.07 0.58
69.7 73.4 80.5 82.1
30.3 26.6 19.5 17.9
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and pyrrolic N were decreased for 31.3% and 4.2% in NSMCS-1000. The decrease in pyridinic N and pyrrolic N contents with increasing annealing temperature was due to the lower stability of the samples at higher temperature. Moreover, the content of oxidized N decreased with increasing temperature. However, the content of graphitic N increased from 28.0% in NSMCS-700 to 58.5% in NSMCS-1000. This increase of graphitic N at high temperature, which indicates that graphitic N is more stable than pyridinic N and pyrrolic N. The results showed that the nitrogen doping level can be easily controlled by adjusting the annealing temperature. Similarly, the S 2p peaks in the high resolution XPS of NSMCS annealed from 700 °C to 1000 °C are given in Fig. 4. There were two types of S configuration, thiophene S and oxidized S. The thiophene S gave two peaks at 164.1 and 165.2 eV [12], which are in agreement with the reported S 2p3/2 and S 2p1/2, owing to the ACASACA bonds and conjugated AC@SA bonds, respectively. The broad and weak peak centered at 168.5 eV was attributed to the oxidized S groups (-C-SOx-C-) [42]. The total sulfur content
decreased from 1.84% (NSMCS-700) to 0.58% (NSMCS-1000) with increasing annealing temperature (Table 1). Noticeably, the contents of thiophene S and oxidized S differed with annealing temperature. The content of thiophene S increased from 69.7% in NSMCS-700 to 82.1% in NSMCS-1000, whereas the content of oxidized S decreased from 30.3% in NSMCS-700 to 17.9% in NSMCS1000. It can be seen that the oxidized S groups can be transformed to thiophene S at higher annealing temperatures and the sulfur doping level can be controlled by the annealing temperature. 3.2. Electrochemical performance Electrochemical measurements of the catalysts toward oxygen reduction reaction were performed using the rotating disk electrode method. Cyclic voltammetry experiments for the MCS, NMCS, SMCS and NSMCS-900 were first performed in an O2 saturated 0.1 M KOH solution at a scan rate of 100 mV s1, as shown in Fig. 5a. For the pristine MCS, a small cathodic peak was observed at
Fig. 4. High-resolution S 2p XPS spectra of (a) NSMCS-700, (b) NSMCS-800, (c) NSMCS-900 and (d) NSMCS-1000.
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0.660 V, while heteroatom doped MCS showed a well-defined cathodic peak resulting from O2 reduction. The peak potentials of heteroatom doped catalyst were 0.711 V, 0.704 V and 0.771 V for NMCS, SMCS and NSMCS-900, respectively. Compared to pristine MCS and solely doped MCS, dual doped MCS exhibited a higher peak current density and more positive peak potential, indicating good catalytic performance toward oxygen reduction reaction. Linear sweep voltammetry (LSV) measurements were also performed
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in O2 saturated 0.1 M KOH at a scan rate of 10 mV s1 and a rotation rate of 1600 rpm to confirm the ORR performance. Fig. 5b and c show the LSV curves of MCS, NMCS, SMCS, NSMCS-900 and Pt/C and compare the onset and half wave potentials. The onset and half wave potentials of the MCS electrode were 0.779 V and 0.697 V. In accordance with the CV data, the ORR activity of undoped MCS showed poor activity, whereas NSMCS-900 showed good performance for the ORR. The NSMCS-900 displayed high
Fig. 5. (a) CV curves of the MCS, SMCS, NMCS and NSMCS-900 in the O2 saturated 0.1 M KOH electrolyte at a scan rate of 100 mV s1. (b) LSV curves of the MCS, SMCS, NMCS and NSMCS-900 in the O saturated 0.1 M KOH at rotation rate of 1600 rpm and scan rate of 10 mV s1. (c) Onset and half-wave potentials of MCS, SMCS, NMCS and NSMCS900.
Fig. 6. (a) CV curves of the MCS, NSMCS-700, 800, 900, 1000 and Pt/C in the O2 saturated 0.1 M KOH electrolyte at scan rate of 100 mV s1. (b) LSV curves of the MCS, NSMCS700, 800, 900, 1000 and Pt/C in the O2 saturated 0.1 M KOH electrolyte at rotation rate of 1600 rpm and scan rate of 10 mV s1. (c) Onset and half-wave potentials of NSMCS700, 800, 900, 1000 and Pt/C. (d) LSV curves of NSMCS-900 at various rotation rates from 400 to 2400 rpm. (e) Koutecky-Levich plots for NSMCS-900 obtained from LSV curves at different electrode potentials from 0.15 to 0.55 V. (f) The electron transfer numbers at different electrode potentials from 0.15 to 0.55 V.
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ORR onset and half wave potentials of 0.885 V and 0.812 V, which were more positive than those of NMCS (0.833 V and 0.765 V) and SMCS (0.808 V and 0.693 V). The enhanced ORR performance was due to nitrogen and sulfur doping. Doping of nitrogen in the carbon transformed the sp3 hybridized carbon structure to a sp2 hybridized carbon structure [43]. The sp2 hybridized structure disturbed the electroneutrality of the graphitic materials owing to the large difference in electronegativity between carbon (2.55) and nitrogen (3.04) and created favorable sites for the surface adsorption of O2 [22]. This charge separation could effectively weaken O2 bonding and facilitate a four-electron reduction process. Nevertheless, since the electronegativity of carbon (2.55) and sulfur (2.58) is very similar, sulfur doping may not be favorable for enhancing ORR performance. However, the lone pairs of sulfur can contribute to favorable interaction with O2. In addition, sulfur can induce defects and strain in the carbon structure, creating a charge difference, facilitating the chemisorption of O2, due to its large atomic radius (110 pm), compared to carbon (70 pm) and nitrogen (65 pm) [44,45]. Thus, sulfur can easily donate electron and facilitate the four-electron reduction process. Thus, NSMCS showed good ORR performance due to the synergistic effect of nitrogen and sulfur. To obtain the appropriate nitrogen content and sulfur content for a better ORR performance, the influence of different annealing temperatures on the catalytic properties of the NSMCS was further explored. Fig. 6a and b show the CV and LSV curves for NSMCS annealed from 700 to 1000 °C. All the NSMCS samples showed a well-defined cathodic peak resulting from ORR and the peak current and potential of NSMCS was improved in the order, NSMCS-1000 < NSMCS-700 < NSMCS-800 < NSMCS-900. In Fig. 6b, LSV curve of NSMCS-700 through 1000 and Pt/C are shown along with the onset and half wave potentials for comparison. In Fig. 6c, the NSMCS-700 showed 0.821 V and 0.739 V for the onset and half wave potentials, which shifted to positive values up to NSMCS-900. However, NSMCS-1000 had the most negative onset and half wave potential of 0.822 V and 0.726 V. Surprisingly, in comparison, the onset and half wave potentials of the Pt/C catalyst were 0.931 V and 0.840 V, similar to those of NSMCS-900. Moreover, the LSV curve of NSMCS-900 showed a high limiting current density (5.07 mA cm2) at 0.1 V. This value was as similar as the limiting current density of Pt/C (5.17 mA cm2) and much higher than that of NSMCS-
700 (3.56 mA cm2), NSMCS-800 (4.40 mA cm2) and NSMCS1000 (3.15 mA cm2) at 0.1 V. These results indicated that the dual doped NSMCS-900 is very similar in activity to commercial Pt/C. Further investigation of the ORR process of all the NSMCS catalysts and Pt/C was carried out using LSV curve in O2 saturated 0.1 M KOH solution under various rotation speeds ranging from 400 to 2400 rpm at a scan rate of 10 mV s1. Fig. 6d shows that the catalytic current density of NSMCS-900 increased with increasing rotation speed, indicating a diffusion controlled reaction. The Koutecky-Levich (K-L) plot of NSMCS-900 was obtained at different potentials in Fig. 6e. All the catalysts showed good linearity at potentials between 0.15 V and 0.55 V and the number of electrons transferred (n) and kinetic current density (JK) were derived from the Koutecky-Levich (K-L) equation [46,47]:
1 1 1 1 1 þ ¼ þ ¼ J J L J K Bw1=2 J K
ð2Þ
1=6 B ¼ 0:201nFC O2 D2=3 O2 m
ð3Þ
In Eq. (2), J is the measured current density, JL is the diffusion limiting current density, JK is the kinetic limiting current density and w is the electrode rotation speed in rpm. B is related to the diffusion current density and is given by Eq. (3). F is Faraday constant (96,485 C mol1), C O2 is the bulk concentration of O2 (1.2 106 mol cm3), DO2 is the diffusion coefficient of O2 in 0.1 M KOH solution (1.9 105 cm2 s1) and m is the kinetic viscosity of the electrolyte (0.01 cm2 s1). Generally, the calculated electron transfer number increased as the potential became more negative and the annealing temperature increased from 700 to 900 °C. However, a higher calcination temperature (1000 °C) may damage the mesoporous carbon sphere and nullify nitrogen and sulfur contents, resulting in decreased catalytic activity. In Fig. 6f, the value of electron transfer number for NSMCS-900 was between 3.5 and 3.8 at the potential range between 0.15 V and 0.55 V. This result showed that NSMCS-900 exhibited the highest n value among the NSMCS catalysts, indicating a four electron ORR process. Combined with the XPS measurements results, it can be observed that the high nitrogen and sulfur content may not improve the ORR performance. According to the density functional
Fig. 7. (a) Changes in the LSV curves of the NSMCS-900 after 2000 potential cycles. (b) Durability evaluation of NSMCS-900 and Pt/C electrodes for 20,000 s at 0.57 V with rotation rate of 1600 rpm.
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theory calculation for dual doping of nitrogen and sulfur, high doping levels cannot determine the number of active sites that can participate in ORR [48]. In addition, previous studies showed that pyridinic N and graphitic N were usually regarded as active sites and pyrrolic N and oxidized N were regarded as inactive sites for ORR [49,50]. Thiophene S, existed only at the defect and edge sites of the carbon structure with high catalytic activity as pyridinic N [51]. However, the oxidized S groups were regarded as inactive sites [42]. In Table 1, the nitrogen ratio of active sites/inactive sites in NSMCS increased in the order, NSMCS-700 (80.4/19.6) < NSMCS800 (84.1/15.9) < NSMCS-900 (88.1/11.9) < NSMCS-1000 (89.8/10.2). Similarly, the sulfur ratio of active sites/inactive sites in NSMCS increased in the order, NSMCS-700 (69.7/30.3) < NSMCS-800 (73.4/26.6) < NSMCS-900 (80.5/19.5) < NSMCS-1000 (82.1/17.9). This result indicated that the ratio of active sites increased with increasing annealing temperature. However, the nitrogen and sulfur content of as-synthesized NSMCS-1000 catalysts was only 1.63% and 0.58%. The overall doping effect was limited owing to the presence of only a few active doping sites. Therefore, the NSMCS-900 catalyst was superior to the other NSMCS catalysts, indicating the most appropriate doping and synergic effect of nitrogen and sulfur in the catalytic reaction. The long-term stability and durability experiments of NSMCS900 were also carried out as shown in Fig. 7. After performing 2000 continuous cycles, only the current density and the half wave potential of NSMCS-900 decreased slightly and there was no significant change in the onset potential in the LSV curve (Fig. 7a). As shown in Fig. 7b, the changes in current density over time, of NSMCS-900 and Pt/C were evaluated by chronoamperometric measurements at 1600 rpm in a continuous O2 saturated 0.1 M KOH electrolyte at a constant potential of 0.57 V for 20,000 s. Remarkably, Pt/C catalyst revealed a significant decrease (71%) in the initial current after 20,000 s measurement, indicating poor durability. In contrast, the NSMCS-900 showed only a slight loss of current (86%) after continuous 20,000 s ORR and remained stable throughout the experiment. These results indicated that NSMCS-900 had better ORR stability and durability than Pt/C. 4. Conclusion In summary, we have successfully demonstrated facile pyrolysis of urea and p-benzenedithiol as precursors for nitrogen and sulfur co-doped mesoporous carbon spheres while controlling the doping ratio of nitrogen and sulfur. The NSMCS-900 obtained by annealing the precursors at 900 °C exhibited excellent ORR activity in alkaline electrolyte, including high kinetic current density of 5.07 mA cm2 at 0.1 V and long-term durability, compared to commercial Pt/C. In addition, when NSMCS-900 was applied as metal-free catalyst for ORR, the reaction kinetics study confirmed a fourelectron transfer pathway. The ORR performance of NSMCS-900 originated from the high surface area, pore volume, synergistic effect of nitrogen and sulfur doping and optimal chemical states formed between the dopants and the carbon atoms. The approach demonstrated in this work would yield promising metal-free electrocatalysts for application in energy conversion and storage. Acknowledgement This research was supported by the Chung-Ang University Graduate Research Scholarship in 2017 and the NRF Grant funded by the Ministry of Education (2017R1D1A1B03029212). References [1] B.C. Steele, A. Heinzel, Materials for fuel-cell technologies, Nature 414 (2001) 345–352.
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Full Length Article
Synergistic interaction and controllable active sites of nitrogen and sulfur co-doping into mesoporous carbon sphere for high performance oxygen reduction electrocatalysts Taeseob Oh, Myeongjin Kim, Dabin Park, Jooheon Kim ⇑ School of Chemical Engineering & Materials Science, Chung-Ang University, 211 Heukseok-dong, Dongjak-gu, Seoul 156-756, Republic of Korea
a r t i c l e
i n f o
Article history: Received 14 November 2017 Revised 29 December 2017 Accepted 22 January 2018
Keywords: Oxygen reduction reaction Metal-free electrocatalyst Mesoporous carbon sphere Nitrogen and sulfur co-doping Synergistic effect
a b s t r a c t Nitrogen and sulfur co-doped mesoporous carbon sphere (NSMCS) was prepared as a metal-free catalyst by an economical and facile pyrolysis process. The mesoporous carbon spheres were derived from sodium carboxymethyl cellulose as the carbon source and the nitrogen and sulfur dopants were derived from urea and p-benzenedithiol, respectively. The doping level and chemical states of nitrogen and sulfur in the prepared NSMCS can be easily adjusted by controlling the pyrolysis temperature. The NSMCS pyrolyzed at 900 °C (NSMCS-900) exhibited higher oxygen reduction reaction activity than the mesoporous carbon sphere doped solely with nitrogen or sulfur, due to the synergistic effect of co-doping. Among all the NSMCS samples, NSMCS-900 exhibited excellent ORR catalytic activity owing to the presence of a highly active site, consisting of pyridinic N, graphitic N, and thiophene S. Remarkably, the NSMCS900 catalyst was comparable with commercial Pt/C, in terms of the onset and the half-wave potentials and showed better durability than Pt/C for ORR in an alkaline electrolyte. The approach demonstrated in this work could be used to prepare promising metal-free electrocatalysts for application in energy conversion and storage. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction The oxygen reduction reaction (ORR) plays an important role in energy conversion and storage in devices such as, fuel cells and metal-air batteries [1,2]. A major concern is the sluggish reaction kinetics of ORR at the cathode, which in turn limits the performance of fuel cells and metal-air batteries. Hence, electrocatalysts are required to decrease the activation energy of the sluggish reaction and boost the performance of ORR. Among the electrocatalyst materials, platinum-based catalysts were widely known as the most active catalysts for ORR, owing to their low onset potential and high current density [3–6]. However, Pt based catalysts have several limiting factors related to large-scale industrial application, including high cost, limited supply, and poor durability. In this regard, numerous studies have been conducted to obtain lowcost and efficient ORR electrocatalysts based on non-precious metal [7–9] or metal-free catalysts [10–12]. Among the metal-free catalysts, carbon-based materials such as graphene [13], carbon nanotube [14], carbon nanodots [15], and ⇑ Corresponding author. E-mail addresses: [email protected] (T. Oh), [email protected] (M. Kim), [email protected] (D. Park), [email protected] (J. Kim). https://doi.org/10.1016/j.apsusc.2018.01.186 0169-4332/Ó 2018 Elsevier B.V. All rights reserved.
nanosheets [16] have been considered alternative electrocatalysts due to their low cost and better stability. Although there has been significant progress in developing carbon-based catalysts, metalfree carbon catalysts still have problems with respect to specific surface area and the presence of pores. Therefore, increasing the specific surface area and adjusting the pore sizes can become important factors in achieving an efficient triple-phase (solidliquid-gas) region to facilitate mass transfer of electrolyte and oxygen [17]. Yao Zheng et al. reported that C3N4 into a mesoporous carbon enhanced the electron transfer of C3N4 [18] and Liang et al. reported that C3N4/C with 150 nm ordered pores showed high ORR activity by facilitating reactant transport through the macroporous structure [19]. Small pore size is often reported to be unfavorable for mass transfer, while macropores can shorten the electrolyte diffusion pathway to the interior surfaces. In addition, mesopores can provide large accessible areas for charge and ion transport. However, these porous carbon-based materials are still poorer than the commercial Pt catalysts in ORR performance. To overcome the low ORR activity, doping with heteroatoms (N, S, B, P) has been researched as a new way to enhance the catalytic activity [20,21]. Doping the carbon materials with heteroatoms could induce changes in the atomic charge density and the spin density. This
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charge redistribution with the heteroatom dopants could facilitate the O2 chemisorption on the carbon material surface and weaken the O-O bonding to facilitate the ORR process [22]. Liangti Qu et al. reported that nitrogen-doped graphene was an effective electrocatalyst [23] and Kuanping Gong et al. reported high electrocatalytic activity for nitrogen-doped carbon nanotube [24]. As reported earlier, nitrogen doping was widely investigated for improving the ORR activity, because of the electronegativity difference between the carbon and nitrogen atoms [22]. Other heteroatoms with lower electronegativity such as B and P could also change the charge density, thereby generating active sites for ORR. On the other hand, Zhi Yang et al. reported sulfur-doped graphene as ORR catalysts [11] and Yan Sun et al. reported sulfurdoped carbon spheres as electrocatalysts [25]. The reason for the ORR activity of sulfur is the two lone pairs of electrons on the sulfur atom can contribute to the O2 chemisorption [26] and its large atomic radius compared to that of the carbon atom, in spite of having similar electronegativity. Furthermore, heteroatom co-doped carbon materials revealed enhanced ORR performance owing to the synergistic effect between the dopants. The fundamentals of heteroatom co-doping in carbon materials can be understood from the theoretical researches on density functional theory (DFT) calculations of spin and charge densities [27]. The heteroatom doping level, the nature of the heteroatom dopants and the chemical states formed between the carbon atoms and the dopants are the key factors affecting the spin and the charge densities [28]. For instance, Shuangyin Wang et al. reported that nitrogen and boron codoped carbon nanotube [29] and graphene [30] had better ORR activity, compared to the carbon materials solely doped with boron and nitrogen. Moreover, nitrogen and sulfur co-doped into carbonaceous materials has been researched for oxygen reduction eletrocatalysts with various morphologies such as ordered mesoporous carbon [31], carbon dots [32], carbon nanofiber networks [33], graphene with carbon nanospheres [34] and mesoporous graphene [28]. This is because, the difference in atomic radius between sulfur and other atoms and the difference in electronegativity between nitrogen and other atoms can change the charge and the spin density and improve ORR performance. However, these studies showed only the synergistic effect of doping nitrogen and sulfur in carbon materials and rarely controlled the heteroatom doping level and the chemical states formed between the carbon atoms and the dopants. Therefore, it is attractive to control active sites of doping level and chemical states in nitrogen and sulfur co-doped carbonaceous materials. Herein, nitrogen and sulfur co-doped three-dimensional mesoporous carbon spheres (NSMCS) were successfully synthesized from sodium carboxymethyl cellulose as the carbon source (see Scheme 1). The prepared NSMCS exhibited high specific surface area and uniform nanostructure morphology. The high specific surface area and the mesopores can provide low resistance pathways for oxygen diffusion in the pores and good charge propagation, leading to improved catalytic activity. Nitrogen and sulfur codoping was achieved by a facile pyrolysis method using urea and p-benzenedithiol, respectively, while controlling the doping ratio of nitrogen and sulfur. We can successfully control active sites of chemical states and doping level in NSMCS and find optimal condi-
tions for efficient ORR activity. The existence of nitrogen and sulfur in the carbon sphere can modify the charge distribution of NSMCS, which can change the charge and spin density, facilitating oxygen chemisorption on carbon sphere surface and weak oxygen bonding to improve ORR activity Among the NSMCS samples, the NSMCS pyrolyzed at 900 °C exhibited higher ORR activity owing to the presence of a highly active site consisting of pyridinic N, graphitic N, and thiophene S. When used as electrocatalyst at the cathode, the NSMCS-900 exhibited excellent ORR performance and better durability and stability than the commercial Pt/C in alkaline electrolyte. 2. Experimental 2.1. Preparation of materials The mesoporous carbon spheres (MCS) were synthesized through the hard template method. In a typical procedure, 0.32 g cetyl trimethyl ammonium bromide (CTAB) was added to a mixture containing 106 mL of deionized water and 56 mL of ethanol to form solution A. Then, 2 mL ammonium hydroxide and 2 mL tetraethyl orthosilicate (TEOS) were added under vigorous stirring for 4 h, until the solution A was homogeneously dispersed. The resulting mixture became white because of the formation of silica spheres. 1 g sodium carboxymethyl cellulose (average Mw 90,000) was completely dissolved in 50 mL deionized water under vigorous stirring for 1 h to form solution B. Subsequently, solution B was dissolved in solution A under vigorous stirring at room temperature. After carrying out the reaction for 12 h, the white solution was evaporated at 90 °C in a water bath. The products were dried at 80 °C in a vacuum oven for 8 h and carbonized at 800 °C for 4 h at a ramp rate of 5 °C min1 under a N2 atmosphere, in a tubular furnace. Finally, the MCS were obtained after etching with 10 wt% HF solution. The nitrogen and sulfur doped mesoporous carbon spheres (NSMCS) were prepared by a one pot facile method. 200 mg of MCS, 800 mg of urea and 800 mg of p-benzenedithiol were dissolved in deionized water and ultrasonicated for 1 h before stirring for 1 h to facilitate the adsorption of urea and p-benzenedithiol on the MCS. The mixture was slowly evaporated at 90 °C in an oven. The dried samples were annealed at different temperatures (700, 800, 900 and 1000 °C) for 2 h under a N2 atmosphere at a heating rate of 5 °C min1 in a tubular furnace and are denoted as NSMCS700, NSMCS-800, NSMCS-900 and NSMCS-1000, respectively. The NMCS and SMCS were also synthesized using an identical procedure for comparison. 2.2. Characterization methods The morphology and structure of the nanomaterials were characterized by field-emission scanning electron microscopy (FE-SEM, SIGMA, Carl Zeiss) and high-resolution transmission electron microscopy (HR-TEM, JEM-3010). Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) measurements were done to observe the specific surface area, pore volume and average pore size of the samples. Raman spectra were recorded on a Raman
Scheme 1. Schematic illustration of synthesis of NSMCS for oxygen reduction reaction.
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spectrometer (T64000, HORIBA, FR), equipped with an argon laser at 514 nm excitation wavelength. Chemical composition and binding energy of the samples were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo U.K. K-Alpha) with an Al Ka X-ray source (1486.6 eV). 2.3. Electrochemical measurements The electrochemical properties of the NSMCS catalysts were studied using a potentiostat (CHI 600E, CH Instrument, USA) with
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a three-electrode system at room temperature. A platinum wire, Ag/AgCl electrode and glassy carbon electrode (GCE, 5.0 mm diameter) were used as counter, reference and working electrodes, respectively. Before performing electrochemical measurements, the glassy carbon electrode was polished with 0.05 lL and 1 lL of alumina and diamond polish and then rinsed with deionized water. The catalyst ink was prepared by dispersing 2.0 mg of the sample in 1000 lL solvent mixture containing deionized water, Nafion solution (5 wt%) and absolute ethanol (V:V:V = 8:1:1) by ultrasonication at 0.5 h. Then, 10 lL of the catalyst ink was
Fig. 1. (a, b) Low and high-magnification FE-SEM image of NSMCS-900. (c, d) Low and high magnification HR-TEM image of NSMCS-900. (e) Nitrogen adsorption–desorption isotherms of NSMCS-900. (f) Pore size distribution curves of NSMCS-900.
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dropped on the surface of a glassy carbon electrode and dried in oven at 80 °C for 0.5 h. The ORR activity experiments were performed by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements on a rotating disk electrode (RDE). The CV measurement was evaluated at a scan rate of 100 mV/s and a potential range of 0.2 to 1.0 V in 0.1 M KOH solution, saturated with O2 or N2. The LSV measurement was performed on the RDE at a scan rate of 10 mV/s and a potential range of 0.2 to 1.0 V by varying the rotation speed from 400 to 2400 rpm in O2 saturated 0.1 M KOH solution. To investigate the stability and durability of the catalysts, 2000 continuous LSV cycles were performed and chronoamperometric measurement was performed at a constant potential of 0.4 V at 1600 rpm for 20,000 s. All electrochemical experimental results were presented with respect to reversible hydrogen electrode (RHE). The conversion between Ag/AgCl and RHE electrode is performed by using the Nernst equation.
ERHE ¼ EAg=AgCl þ E0Ag=AgCl ðreferenceÞ þ 0:0591 pH
ð1Þ
ERHE is the converted potential vs. RHE, EAg/AgCl is the experimental potential measured with respect to Ag/AgCl reference electrode, and E0Ag/AgCl is the standard potential of Ag/AgCl at 25 °C (0.1976 V). 3. Results and discussion 3.1. Characterization of NSMCS The morphology and nanostructure of NSMCS-900 were characterized using FE-SEM and HR-TEM, as shown in Fig. 1. The FE-SEM images (Fig. 1a) of NSMCS showed that all the carbon particles displayed spherical nanostructure without aggregation. In Fig. 1b, the average diameters of these carbon spheres were about 200–300 nm and the surface of the NSMCS-900 was porous. Furthermore, the HR-TEM images (Fig. 1c and d) of the NSMCS-900 clearly showed that the three dimensionally interconnected mesopores were randomly located to construct the whole carbon sphere. The mesoporous features shown in the TEM images were further confirmed using nitrogen adsorption-desorption measurements. BrunauerEmmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) analyses yielded the specific surface area and the pore size distribution of
NSMCS-900. The sample exhibited type IV isotherm (Fig. 1e) with a hysteresis loop, according to the IUPAC classification [35]. The hysteresis loop can be observed at a relative pressure range 0.4–1.0, indicating the presence of randomly located mesopores [36], in accordance with the TEM observation (Fig. 1d). The formation of the mesopores on carbon spheres can be attributed to the etching of silica by HF solution. The BET specific surface area and total pore volume of the NSMCS-900 were calculated to be 354.5 m2 g1 and 2.315 cm3 g1 respectively. The average pore size (Fig. 1f) of NSMCS-900 was 10.8 nm, corresponding to the adsorption of nitrogen on the mesopores. Such a porous structure of the MCS nanocomposites was beneficial to provide efficient transport pathways for oxygen and increased the electrode/electrolyte contact area, which are important for high performance electrocatalysis [17]. The graphitic structure of NSMCS-900 was probed using Raman spectroscopy. A typical D band and a G band peak appeared at 1350 and 1580 cm1 respectively, as shown in Fig. 2a. The D band was related to the sp3 defect sites and the G band was associated with the sp2 hybridized bonded pairs. The ID/IG ratio is widely used to evaluate the density of defects in graphite materials [37]. All the NSMCS samples showed higher ID/IG ratio than the undoped MCS (0.991), indicating that the sulfur and nitrogen doping induced defects in the carbon lattice. In addition, it is obvious that the ID/IG ratio decreased from NSMCS-700 (1.024) to NSMCS-1000 (1.011), with increasing annealing temperature, indicating that the graphitization degree of NSMCS is improved. On the other hand, the decrease in ID/IG ratio with the increase in annealing temperature may be due to the decreasing total amount of N and S dopants in the carbon sphere, as confirmed by the following XPS results. The XPS spectra of MCS, NSMCS-700, NSMCS-800, NSMCS-900 and NSMCS-1000 in Fig. 2b, showed the C 1s peak (285 eV), O 1s peak (533 eV), N 1s peak (398 eV) and S 2p peak (164 eV) [38], indicating successful doping with N and S by annealing the mesoporous carbon sphere in the presence of urea and p-benzenedithiol. The XPS data revealed no nitrogen and sulfur content in MCS and showed only carbon and oxygen peaks. After annealing MCS in the presence of urea and pbenzenedithiol, the sulfur and nitrogen peaks were clearly observed. The appearance of S 2s, S 2p and N 1s peaks indicated that both nitrogen and sulfur had been incorporated in the graphitic carbon structure during the pyrolysis process. The amount of
Fig. 2. (a) Raman spectra of MCS, NSMCS-700, NSMCS-800, NSMCS-900 and NSMCS-1000. (b) XPS wide scan spectra of MCS, NSMCS-700, NSMCS-800, NSMCS-900 and NSMCS-1000.
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nitrogen and sulfur was 4.90% and 1.84% (N/C and S/C atomic ratio), respectively, in NSMCS-700 and 1.63% and 0.58% in NSMCS-1000. In addition, intensity of C 1s peaks are increased with increase in annealing temperature [39]. The decrease in nitrogen and sulfur contents and increase in carbon content with increasing annealing temperature are in accordance with the Raman spectra. To further understand the bonding configurations of N and S in NSMCS, we analyzed the catalysts using high resolution XPS. The
N 1s spectra in the high resolution XPS of NSMCS, annealed from 700 °C to 1000 °C are given in Fig. 3. The XPS spectra showed four types of N species, including pyridinic N, pyrrolic N, graphitic N and oxidized N, at binding energies of around 398.5, 400.7, 401.9 and 404.6 eV, respectively [40,41]. As the annealing temperature was increased from 700 °C to 1000 °C, the total nitrogen content decreased from 4.90% (NSMCS-700) to 1.63% (NSMCS-1000), as shown in Table 1. The pyridinic N and pyrrolic N contents were 52.4% and 10.1% in NSMCS-700, whereas contents of pyridinic N
Fig. 3. High-resolution N 1s XPS spectra of (a) NSMCS-700, (b) NSMCS-800, (c) NSMCS-900 and (d) NSMCS-1000.
Table 1 Nitrogen and sulfur atomic percentage of various chemical states in NSMCS samples. Samples
N/C (%)
Pyridinic N (%)
Pyrrolic N (%)
Graphitic N (%)
Oxidized N (%)
S/C (%)
Thiophene S (%)
Oxidized S (%)
NSMCS-700 NSMCS-800 NSMCS-900 NSMCS-1000
4.90 4.07 3.11 1.63
52.4 50.7 45.7 31.3
10.1 8.1 5.0 4.2
28.0 33.4 42.4 58.5
9.5 7.8 6.9 6.0
1.84 1.27 1.07 0.58
69.7 73.4 80.5 82.1
30.3 26.6 19.5 17.9
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and pyrrolic N were decreased for 31.3% and 4.2% in NSMCS-1000. The decrease in pyridinic N and pyrrolic N contents with increasing annealing temperature was due to the lower stability of the samples at higher temperature. Moreover, the content of oxidized N decreased with increasing temperature. However, the content of graphitic N increased from 28.0% in NSMCS-700 to 58.5% in NSMCS-1000. This increase of graphitic N at high temperature, which indicates that graphitic N is more stable than pyridinic N and pyrrolic N. The results showed that the nitrogen doping level can be easily controlled by adjusting the annealing temperature. Similarly, the S 2p peaks in the high resolution XPS of NSMCS annealed from 700 °C to 1000 °C are given in Fig. 4. There were two types of S configuration, thiophene S and oxidized S. The thiophene S gave two peaks at 164.1 and 165.2 eV [12], which are in agreement with the reported S 2p3/2 and S 2p1/2, owing to the ACASACA bonds and conjugated AC@SA bonds, respectively. The broad and weak peak centered at 168.5 eV was attributed to the oxidized S groups (-C-SOx-C-) [42]. The total sulfur content
decreased from 1.84% (NSMCS-700) to 0.58% (NSMCS-1000) with increasing annealing temperature (Table 1). Noticeably, the contents of thiophene S and oxidized S differed with annealing temperature. The content of thiophene S increased from 69.7% in NSMCS-700 to 82.1% in NSMCS-1000, whereas the content of oxidized S decreased from 30.3% in NSMCS-700 to 17.9% in NSMCS1000. It can be seen that the oxidized S groups can be transformed to thiophene S at higher annealing temperatures and the sulfur doping level can be controlled by the annealing temperature. 3.2. Electrochemical performance Electrochemical measurements of the catalysts toward oxygen reduction reaction were performed using the rotating disk electrode method. Cyclic voltammetry experiments for the MCS, NMCS, SMCS and NSMCS-900 were first performed in an O2 saturated 0.1 M KOH solution at a scan rate of 100 mV s1, as shown in Fig. 5a. For the pristine MCS, a small cathodic peak was observed at
Fig. 4. High-resolution S 2p XPS spectra of (a) NSMCS-700, (b) NSMCS-800, (c) NSMCS-900 and (d) NSMCS-1000.
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0.660 V, while heteroatom doped MCS showed a well-defined cathodic peak resulting from O2 reduction. The peak potentials of heteroatom doped catalyst were 0.711 V, 0.704 V and 0.771 V for NMCS, SMCS and NSMCS-900, respectively. Compared to pristine MCS and solely doped MCS, dual doped MCS exhibited a higher peak current density and more positive peak potential, indicating good catalytic performance toward oxygen reduction reaction. Linear sweep voltammetry (LSV) measurements were also performed
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in O2 saturated 0.1 M KOH at a scan rate of 10 mV s1 and a rotation rate of 1600 rpm to confirm the ORR performance. Fig. 5b and c show the LSV curves of MCS, NMCS, SMCS, NSMCS-900 and Pt/C and compare the onset and half wave potentials. The onset and half wave potentials of the MCS electrode were 0.779 V and 0.697 V. In accordance with the CV data, the ORR activity of undoped MCS showed poor activity, whereas NSMCS-900 showed good performance for the ORR. The NSMCS-900 displayed high
Fig. 5. (a) CV curves of the MCS, SMCS, NMCS and NSMCS-900 in the O2 saturated 0.1 M KOH electrolyte at a scan rate of 100 mV s1. (b) LSV curves of the MCS, SMCS, NMCS and NSMCS-900 in the O saturated 0.1 M KOH at rotation rate of 1600 rpm and scan rate of 10 mV s1. (c) Onset and half-wave potentials of MCS, SMCS, NMCS and NSMCS900.
Fig. 6. (a) CV curves of the MCS, NSMCS-700, 800, 900, 1000 and Pt/C in the O2 saturated 0.1 M KOH electrolyte at scan rate of 100 mV s1. (b) LSV curves of the MCS, NSMCS700, 800, 900, 1000 and Pt/C in the O2 saturated 0.1 M KOH electrolyte at rotation rate of 1600 rpm and scan rate of 10 mV s1. (c) Onset and half-wave potentials of NSMCS700, 800, 900, 1000 and Pt/C. (d) LSV curves of NSMCS-900 at various rotation rates from 400 to 2400 rpm. (e) Koutecky-Levich plots for NSMCS-900 obtained from LSV curves at different electrode potentials from 0.15 to 0.55 V. (f) The electron transfer numbers at different electrode potentials from 0.15 to 0.55 V.
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ORR onset and half wave potentials of 0.885 V and 0.812 V, which were more positive than those of NMCS (0.833 V and 0.765 V) and SMCS (0.808 V and 0.693 V). The enhanced ORR performance was due to nitrogen and sulfur doping. Doping of nitrogen in the carbon transformed the sp3 hybridized carbon structure to a sp2 hybridized carbon structure [43]. The sp2 hybridized structure disturbed the electroneutrality of the graphitic materials owing to the large difference in electronegativity between carbon (2.55) and nitrogen (3.04) and created favorable sites for the surface adsorption of O2 [22]. This charge separation could effectively weaken O2 bonding and facilitate a four-electron reduction process. Nevertheless, since the electronegativity of carbon (2.55) and sulfur (2.58) is very similar, sulfur doping may not be favorable for enhancing ORR performance. However, the lone pairs of sulfur can contribute to favorable interaction with O2. In addition, sulfur can induce defects and strain in the carbon structure, creating a charge difference, facilitating the chemisorption of O2, due to its large atomic radius (110 pm), compared to carbon (70 pm) and nitrogen (65 pm) [44,45]. Thus, sulfur can easily donate electron and facilitate the four-electron reduction process. Thus, NSMCS showed good ORR performance due to the synergistic effect of nitrogen and sulfur. To obtain the appropriate nitrogen content and sulfur content for a better ORR performance, the influence of different annealing temperatures on the catalytic properties of the NSMCS was further explored. Fig. 6a and b show the CV and LSV curves for NSMCS annealed from 700 to 1000 °C. All the NSMCS samples showed a well-defined cathodic peak resulting from ORR and the peak current and potential of NSMCS was improved in the order, NSMCS-1000 < NSMCS-700 < NSMCS-800 < NSMCS-900. In Fig. 6b, LSV curve of NSMCS-700 through 1000 and Pt/C are shown along with the onset and half wave potentials for comparison. In Fig. 6c, the NSMCS-700 showed 0.821 V and 0.739 V for the onset and half wave potentials, which shifted to positive values up to NSMCS-900. However, NSMCS-1000 had the most negative onset and half wave potential of 0.822 V and 0.726 V. Surprisingly, in comparison, the onset and half wave potentials of the Pt/C catalyst were 0.931 V and 0.840 V, similar to those of NSMCS-900. Moreover, the LSV curve of NSMCS-900 showed a high limiting current density (5.07 mA cm2) at 0.1 V. This value was as similar as the limiting current density of Pt/C (5.17 mA cm2) and much higher than that of NSMCS-
700 (3.56 mA cm2), NSMCS-800 (4.40 mA cm2) and NSMCS1000 (3.15 mA cm2) at 0.1 V. These results indicated that the dual doped NSMCS-900 is very similar in activity to commercial Pt/C. Further investigation of the ORR process of all the NSMCS catalysts and Pt/C was carried out using LSV curve in O2 saturated 0.1 M KOH solution under various rotation speeds ranging from 400 to 2400 rpm at a scan rate of 10 mV s1. Fig. 6d shows that the catalytic current density of NSMCS-900 increased with increasing rotation speed, indicating a diffusion controlled reaction. The Koutecky-Levich (K-L) plot of NSMCS-900 was obtained at different potentials in Fig. 6e. All the catalysts showed good linearity at potentials between 0.15 V and 0.55 V and the number of electrons transferred (n) and kinetic current density (JK) were derived from the Koutecky-Levich (K-L) equation [46,47]:
1 1 1 1 1 þ ¼ þ ¼ J J L J K Bw1=2 J K
ð2Þ
1=6 B ¼ 0:201nFC O2 D2=3 O2 m
ð3Þ
In Eq. (2), J is the measured current density, JL is the diffusion limiting current density, JK is the kinetic limiting current density and w is the electrode rotation speed in rpm. B is related to the diffusion current density and is given by Eq. (3). F is Faraday constant (96,485 C mol1), C O2 is the bulk concentration of O2 (1.2 106 mol cm3), DO2 is the diffusion coefficient of O2 in 0.1 M KOH solution (1.9 105 cm2 s1) and m is the kinetic viscosity of the electrolyte (0.01 cm2 s1). Generally, the calculated electron transfer number increased as the potential became more negative and the annealing temperature increased from 700 to 900 °C. However, a higher calcination temperature (1000 °C) may damage the mesoporous carbon sphere and nullify nitrogen and sulfur contents, resulting in decreased catalytic activity. In Fig. 6f, the value of electron transfer number for NSMCS-900 was between 3.5 and 3.8 at the potential range between 0.15 V and 0.55 V. This result showed that NSMCS-900 exhibited the highest n value among the NSMCS catalysts, indicating a four electron ORR process. Combined with the XPS measurements results, it can be observed that the high nitrogen and sulfur content may not improve the ORR performance. According to the density functional
Fig. 7. (a) Changes in the LSV curves of the NSMCS-900 after 2000 potential cycles. (b) Durability evaluation of NSMCS-900 and Pt/C electrodes for 20,000 s at 0.57 V with rotation rate of 1600 rpm.
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theory calculation for dual doping of nitrogen and sulfur, high doping levels cannot determine the number of active sites that can participate in ORR [48]. In addition, previous studies showed that pyridinic N and graphitic N were usually regarded as active sites and pyrrolic N and oxidized N were regarded as inactive sites for ORR [49,50]. Thiophene S, existed only at the defect and edge sites of the carbon structure with high catalytic activity as pyridinic N [51]. However, the oxidized S groups were regarded as inactive sites [42]. In Table 1, the nitrogen ratio of active sites/inactive sites in NSMCS increased in the order, NSMCS-700 (80.4/19.6) < NSMCS800 (84.1/15.9) < NSMCS-900 (88.1/11.9) < NSMCS-1000 (89.8/10.2). Similarly, the sulfur ratio of active sites/inactive sites in NSMCS increased in the order, NSMCS-700 (69.7/30.3) < NSMCS-800 (73.4/26.6) < NSMCS-900 (80.5/19.5) < NSMCS-1000 (82.1/17.9). This result indicated that the ratio of active sites increased with increasing annealing temperature. However, the nitrogen and sulfur content of as-synthesized NSMCS-1000 catalysts was only 1.63% and 0.58%. The overall doping effect was limited owing to the presence of only a few active doping sites. Therefore, the NSMCS-900 catalyst was superior to the other NSMCS catalysts, indicating the most appropriate doping and synergic effect of nitrogen and sulfur in the catalytic reaction. The long-term stability and durability experiments of NSMCS900 were also carried out as shown in Fig. 7. After performing 2000 continuous cycles, only the current density and the half wave potential of NSMCS-900 decreased slightly and there was no significant change in the onset potential in the LSV curve (Fig. 7a). As shown in Fig. 7b, the changes in current density over time, of NSMCS-900 and Pt/C were evaluated by chronoamperometric measurements at 1600 rpm in a continuous O2 saturated 0.1 M KOH electrolyte at a constant potential of 0.57 V for 20,000 s. Remarkably, Pt/C catalyst revealed a significant decrease (71%) in the initial current after 20,000 s measurement, indicating poor durability. In contrast, the NSMCS-900 showed only a slight loss of current (86%) after continuous 20,000 s ORR and remained stable throughout the experiment. These results indicated that NSMCS-900 had better ORR stability and durability than Pt/C. 4. Conclusion In summary, we have successfully demonstrated facile pyrolysis of urea and p-benzenedithiol as precursors for nitrogen and sulfur co-doped mesoporous carbon spheres while controlling the doping ratio of nitrogen and sulfur. The NSMCS-900 obtained by annealing the precursors at 900 °C exhibited excellent ORR activity in alkaline electrolyte, including high kinetic current density of 5.07 mA cm2 at 0.1 V and long-term durability, compared to commercial Pt/C. In addition, when NSMCS-900 was applied as metal-free catalyst for ORR, the reaction kinetics study confirmed a fourelectron transfer pathway. The ORR performance of NSMCS-900 originated from the high surface area, pore volume, synergistic effect of nitrogen and sulfur doping and optimal chemical states formed between the dopants and the carbon atoms. The approach demonstrated in this work would yield promising metal-free electrocatalysts for application in energy conversion and storage. Acknowledgement This research was supported by the Chung-Ang University Graduate Research Scholarship in 2017 and the NRF Grant funded by the Ministry of Education (2017R1D1A1B03029212). References [1] B.C. Steele, A. Heinzel, Materials for fuel-cell technologies, Nature 414 (2001) 345–352.
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