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Synthesis of nitrogen-doped ordered mesoporous carbon electrocatalyst: Nanoconfinement effect in SBA-15 template
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e6
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Synthesis of nitrogen-doped ordered mesoporous carbon electrocatalyst: Nanoconfinement effect in SBA-15 template Kai Wan a, Ming-yao Liu a, Zhi-peng Yu a, Zhen-xing Liang a,*, Quan-bing Liu a, Jin-hua Piao b, Yu-ying Zheng c a
Key Laboratory on Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, PR China b School of Food Science and Engineering, South China University of Technology, Guangzhou 510641, PR China c School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, PR China
article info
abstract
Article history:
The effect of the pore structure of SBA-15, which is tuned by varying the hydrothermal
Received 30 May 2016
temperature (90e150 C), is investigated on nitrogen-doped ordered mesoporous carbon. It
Received in revised form
is found that the pore structure of SBA-15 yields effects on both the composition and the
30 June 2016
pore structure of carbon, which thereby affecting its electrocatalytic activity for the oxygen
Accepted 19 July 2016
reduction reaction (ORR). XPS reveals that the nitrogen content of the template-free carbon
Available online xxx
is 2.79 at.%, while dramatically increases to 4.48, 5.01 and 4.16 at.% for the three templateaided carbon catalysts with SBA-15 synthesized at 90, 130 and 150 C, respectively. Nitro-
Keywords:
gen ad/desorption isotherms show that the specific surface area of the template-free
Carbon catalyst
carbon is 257 m2 g1, which increases to 724, 731 and 691 m2 g1 for the above three car-
Confinement effect
bons. Such changes correlate well with the electrochemical results. RRDE results show that
Controllable synthesis
the template-aided carbon catalysts yield better activity and selectivity for the ORR than
Oxygen reduction reaction
does the template-free one. And the best performance is achieved for the carbon catalyst
Template pore structure
with SBA-15 synthesized at 130 C, which coincidentally has the highest nitrogen content and surface area. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Pt has been the mostly widely used electrocatalyst for the oxygen reduction reaction (ORR), of which, however, the high cost seriously hinders the application of fuel cells [1e6]. Enormous efforts have been devoted in developing new cheap electrocatalysts with decent activity [2,7e12]. Nitrogen-doped carbon has attracted an increasing attention
as electrocatalysts due to their unique structure and properties [1,2,13e15]. Nitrogen-doped ordered mesoporous carbon (NOMC) features high specific surface area and ordered pore structure, respectively favoring the electrocatalysis and mass transfer in the porous electrode [16e18]. Hard-template method is a general method to synthesize such materials, in which the rigid template plays a key role in framing out the pore structure. Besides, the nanometer-scale pores further yield a spatial-confinement effect on the
* Corresponding author. E-mail address: [email protected] (Z.-x. Liang). http://dx.doi.org/10.1016/j.ijhydene.2016.07.169 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wan K, et al., Synthesis of nitrogen-doped ordered mesoporous carbon electrocatalyst: Nanoconfinement effect in SBA-15 template, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.169
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e6
carbonization of the precursor. Gadiou [19] first reported that the composition and structure of an ordered mesoporous carbon (OMC) were strongly dependent on the mesoporous structure of SBA-15. Zelenay [20] found that such a spatial confinement could prevent the vaporization of small CN species, which thereby yielded an effect on the composition. Wei [21] used layered montmorillonite (MMT) as a 2-D nanoconfined reactor to selectively synthesize the planar nitrogen, viz. pyrrolic- and pyridinic-type, doped carbon catalyst. They argued that the quaternary-type nitrogen was sp3 hybridization and exist in a 3-D configuration in the graphene layer, which can be sterically hindered in the narrow interspace (0.46 nm) due to the spatial confinement effect. Therefore, the external template cannot only configure the morphology/ structure, but also can affect the composition as well. In our previous work, a novel method to synthesize nitrogen-doped ordered mesoporous carbon with high specific surface area has been developed [11,12,17]. To our knowledge, the state-of-the-art technique for synthesizing the carbon catalyst is generally a pyrolysis-based one, in which the surface composition is mostly determined by the final pyrolysis temperature. This may be the reason that all of the carbon catalysts reported in literature yield almost similar electrochemical performance, which seriously impedes the further development of the carbon catalyst. It is, thus, highly desirable to introduce more experimental parameters for a flexible and controllable synthesis. Based on the nanoconfinement effect in template, it is expected that the composition/structure of the carbon catalyst can be tailored with varying the pore feature of the template. For this reason, the effect of the template SBA-15 on the resultant carbon is extensively investigated in the present work. The pore feature and composition of carbon are well examined by various physiochemical methods, which are then correlated with its electrocatalytic activity to the oxygen reduction reaction (ORR).
Experimental Synthesis of nitrogen-doped ordered mesoporous carbon (NOMC) NOMC was synthesized by the nanocasting method. The details were reported in our previous publications [17,18,22]. Briefly, SBA-15 was first synthesized in a hydrothermal method using Pluronic P123 as the structure-directing agent. 4.0 g Pluronic P123 was dissolved in 126 ml deionized (DI) water and 20 ml hydrochloric acid (37 wt.%), and then 9.2 ml TEOS was added and stirred for 20 h at 35 C. The slurry was hydrothermally treated for 12 h. Finally, the product was filtered, dried, and finally calcined at 550 C for 6 h in air to remove the template. Second, the carbon precursor (pyrrole) was impregnated into SBA-15. 0.90 ml newly distilled pyrrole (99%, Xiya Reagent), together with 1.0 g SBA-15, was added into a vacuum container, which was then held in an oven at 133 C for 2 h. After that, the container cooled to the room temperature, and the light-yellow powders were finally obtained. The powders were added into 40 ml 2.0 mol l1 FeCl3 aqueous solution, which was then vigorously stirred for
24 h at room temperature for polymerization. The product was filtrated and thoroughly washed with DI water to remove metal salt. After dried, the black-colored powders were pyrolysed at high temperatures for 3 h in argon (99.999%). Finally, the silica template was removed in 10 M NaOH at 120 C for 24 h, followed by washing with DI water. The samples were referred to as C-PY-Sx. Here, x refers to the hydrothermal temperature, viz. 90, 130, and 150 C. The above synthesis process, except without using SBA-15 template, was repeated to synthesize the template-free carbon material, viz. C-PY.
Physicochemical characterizations Thermogravimetric analyses (TGA, TA Instrument SDT 2960) was performed at 5 C min1 from room temperature to 900 C in air at a flow rate of 20 ml min1. X-ray photoelectron spectroscopy (XPS, Physical Electronics PHI 5600) measurement was carried out with a multi-technique system using an Al monochromatic X-ray at a power of 350 W. Scanning electron microscopy (SEM) was conducted on a Nova NanoSEM 430 scanning electron microscope. Nitrogen ad/desorption isotherms were collected at 77 K using Micromeritics TriStar II 3020 analyzer. The total surface area was analyzed with the well-established Brunauer-Emmer-Teller (BET) method, the microporous (MP) surface area was obtained with the t-plot method, and the pore size distribution was analyzed by the BarretteJoynereHalenda (BJH) method.
Electrochemical characterization The electrochemical behavior of the catalyst was characterized by the cyclic voltammetry (CV) and linear sweeping voltammetry (LSV) using a three electrode cell with an electrochemical work station (Zennium) at room temperature (25 C). A platinum wire and a double junction Ag/AgCl reference electrode (PINE) were used as the counter and reference electrodes, respectively. The working electrode was a rotating ring-disk electrode (Glassy carbon disk: 5.0 mm in diameter, platinum ring: 6.5 mm inner diameter and 7.5 mm outer diameter, RRDE). The thin-film electrode on the disk was prepared as follows. 10 mg of the catalyst was dispersed in 1.0 ml Nafion/ethanol (0.84 wt.% Nafion) by sonication for 120 min. Then, 10 mL of the dispersion was transferred onto the glassy carbon disk by using a pipette, yielding the catalyst loading of 0.50 mg cm2. For comparison, the ORR electrocatalytic activity, of a 40 wt.% Pt/C commercial catalyst (HiSPEC4000, Johnson Matthey) with 20 mg cm2 of Pt loading, was also measured. The electrolyte solution, 0.10 M KOH, was first bubbled with argon for 60 min. Then, the CV tests were conducted at 20 mV s1 in the potential range between 0 and 1.23 V (vs. reversible hydrogen electrode, RHE) for 20 cycles. The linear sweep voltammetry (LSV) curves were obtained by scanning the disk potential from 1.2 down to 0 V at 5 mV s1 in the oxygensaturated electrolyte solution under 1600 rpm, from which the ORR polarization curve was extracted by subtracting the capacitive current. During the data collection, the potential of the ring was set to be 0.5 V (vs. RHE) to determine the yield of hydrogen peroxide.
Please cite this article in press as: Wan K, et al., Synthesis of nitrogen-doped ordered mesoporous carbon electrocatalyst: Nanoconfinement effect in SBA-15 template, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.169
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e6
H2 O2 ð%Þ ¼
(1)
-1
4jid j jid j þ ir =N
C-PY C-PY-S90 C-PY-S120 C-PY-S150
700 600 500
3
n¼
800
Quantity Adsorbed (cm g STP)
The electron-transfer number (n) and hydrogen peroxide yield (H2O2%) in the ORR was calculated from the following equations:
2ir =N 100 jid j þ ir =N
(2)
where id is the disk current, ir the ring current, and N the collection efficiency (¼20.50%).
400 300 200 100 0
Results and discussion
0.0
Fig. 1 shows the nitrogen ad/desorption isotherms of SBA-15 synthesized at 90, 130, 150 C respectively. It can be seen that all isotherms are basically same in shape and sorption (type IV), indicating the mesoporous nature of SBA-15. The obtained information for the pore parameters are listed in Table S1. The hydrothermal temperature is found to yield a significant effect on the pore structure [23,24]. With increasing the temperature from 90 to 150 C, the specific surface area decreases from 733 to 438 m2 g1, and the pore diameter increases from 4.6 to 10.1 nm [25]. In addition, the morphology of the particles is also subject of great changes. The synthesized SBA-15 particles are found to be interconnected, and the boundaries become clearer at higher hydrothermal temperatures. Such a change is expected to yield effects on the resultant carbon, as shown below. In summary, the pore structure of SBA-15 can be controlled by varying the hydrothermal temperature, which then affects the resultant carbon. The nitrogen ad/desorption isotherms of the nitrogendoped carbon, are depicted in Fig. 2. It can be distinguished that the shape of the isotherms greatly varies for the carbon materials and the isotherms of the templates-aided carbon are basically of the type IV. The key pore parameters are also calculated and listed in Table 1. Firstly, it is seen that the template exerts a dramatic effect on the pore features of the
800
S90 S130 S150
600
3
-1
Quantity Adsorbed (cm g STP)
700
500 400 300
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Fig. 2 e Nitrogen sorption isotherms of the synthesized carbon materials.
carbon materials. Without using the template, the specific surface area of C-PY carbon is close to 257 m2 g1, and the specific pore volume is 0.16 cm3 g1. Detailed analysis reveals that the pores are predominantly micropores, of which the specific surface area is 160 m2 g1. In comparison, by using the SBA-15 template, the specific surface area dramatically increases to 724, 731, 691 m2 g1 for C-PY-S90, C-PY-S130, C-PYS150, respectively. Also, the specific volume is found to be increased and the pore diameter is controlled in the range of 4.0e7.4 nm. The formation of micropore can be attributed to the thermodecomposition of the precursor polypyrrole, and the mesoporous framework should result from the replica of the SBA-15 template [17]. This result indicates that the distribution of the pore size, which is important for the mass transfer in the electrode, can be well controlled by using different nanostructured templates. The morphology of carbon was characterized via SEM, as shown in Fig. 3. It is seen that the morphology of carbon basically follows that of the template (see Fig. S1), which evolves from the interconnected short bars to necklace-like connected spherical particles. In comparison, the templatefree carbon C-PY shows featureless morphology, however with serious agglomeration (see Fig. 3a). Besides, it is noted, that the presence of SBA-15 yields a noticeable effect on the thermodecomposition of polypyrrole (see Fig. S2), which can be attributed to both the chemical interaction and the spatial nanoconfinement effect in the SBA-15 template. Such an effect is further supported by the analysis of the elemental composition.
200
Table 1 e Pore feature of the synthesized carbon materials.
100 0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Fig. 1 e Nitrogen sorption isotherms of SBA-15 synthesized at 90, 130, and 150 C.
Sample C-PY C-PY-S90 C-PY-S130 C-PY-S150
ABET/m2 g1 AMP/m2 g1 DBJH/nm V/cm3 g1 257 724 731 691
160 160 94 196
4.3 4.0 5.2 7.4
0.16 0.62 0.94 1.12
Please cite this article in press as: Wan K, et al., Synthesis of nitrogen-doped ordered mesoporous carbon electrocatalyst: Nanoconfinement effect in SBA-15 template, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.169
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Fig. 3 e SEM images of a) C-PY, b) C-PY-S90, c) C-PY-S130, and d) C-PY-S150.
XPS analysis was used to gain the elemental composition on the carbon surface. Survey spectra are depicted in Fig. S3 and the elemental compositions are listed in Table 2. It is found that the nitrogen content of the template-free carbon CPY is 2.7 at.%, which is much lower than those in templateaided ones. The result reveals the existence of a special interaction between the template and polypyrrole (vide supra), which favors the nitrogen doping into carbon. It is further seen that the nitrogen content is dependent on the pore structure of SBA-15, and the hydrothermal temperature of 130 C yields the highest nitrogen content (5.01 at.%). As mentioned above, such an effect may result from the chemical interaction between polypyrrole and silica [26], and the spatial confinement in the mesopores of SBA-15 as well [19]. And such an interaction offers more flexible strategies to control the composition of carbon in the pyrolysis-based synthesis protocols. As acknowledged, an effective doping of nitrogen is required to generate the ORR active sites in the carbon-based catalyst. As such, the SBA-15 template not only configures the mesoporous structure with high specific surface area, but also promotes the nitrogen doping with high intrinsic activity, both of which are expected to improve the electrochemical performance of carbon.
Table 2 e Elemental composition (at. %) the carbon materials measured by XPS. Sample C-PY C-PY-S90 C-PY-S130 C-PY-S150
C
N
O
Fe
N:C
94.34 91.34 90.46 91.70
2.79 4.48 5.01 4.16
2.74 3.56 3.56 3.58
0.13 0.32 0.28 0.18
0.030 0.049 0.055 0.045
Fig. 4 presents the ORR polarization curves and the corresponding yield of hydrogen peroxide in an O2-saturated 0.10 M KOH solution. First, the template-aided carbon catalysts show superior ORR electrocatalytic activity to that of the template-free one. For example, the half wave potential, E1/2, is much higher for the template-aided carbon (>0.860 V) than the template-free one (0.758 V). The superior performance should be attributed to their larger specific surface area and higher nitrogen content. Second, a considerable difference in the electrocatalytic activity of the three template-aided carbon catalysts can easily be distinguished, which follows the order of C-PY-S90 (E1/2: 0.862 V), C-PY-S150 (E1/2: 0.868 V) < C-PY-S130 (E1/2: 0.886 V). All the carbon catalysts exhibit a high selectivity with the yield of H2O2 below 20%, among which the best-performed catalyst C-PY-S130 shows the lowest H2O2 yield (<10%) and its electron transfer number is above 3.8. Such a trend in the electrocatalytic activity and selectivity correlates well with the change in the nitrogen content and the specific surface area, as discussed above. Finally, it is noted that the template-aided carbon catalyst even exhibit a higher electrocatalytic activity than does the commercial Pt/C catalyst (E1/2: 0.844 V), which is of great interests to be used in practical fuel cells. In our previous work, the active sites for the ORR were suggested to be the nitrogen-activated carbon atoms rather than the FeN2(or 2þ2)Cy species, on which the ORR proceeds by a surface-confined redox-mediation mechanism in both acid and alkaline media [27]. Here, Tof-SIMS was further used to detect the molecular fragments on the surface of the carbon catalysts. It is seen that only a very tiny fraction of FeNxCþ y fragments is observed in the spectra (see Fig. S4), suggesting most iron is not coordinated with nitrogen atoms. To confirm
Please cite this article in press as: Wan K, et al., Synthesis of nitrogen-doped ordered mesoporous carbon electrocatalyst: Nanoconfinement effect in SBA-15 template, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.169
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5
Fig. 4 e RRDE results of the carbon materials in O2-saturated 0.10 M KOH solution: a) ORR polarization curves, b) electron transfer number and H2O2 yield.
the nature of the active sites, the peak fitting of C1s is performed and the fitting results are shown in Fig. S5. The fraction of the CeN groups are extracted and then plotted versus the kinetics current density (at 0.90 V) in Fig. 5. The change in the electrocatalytic activity basically follows the content of the nitrogen-activated carbon atoms. And this result strongly confirms our previous claim on the chemical nature of the active sites for the ORR [17,27]. In summary, it is found that the template-aided synthesis protocol can offer more flexible control over the composition, structure, and consequently the electrocatalysis of the nitrogen-doped carbon catalysts.
which significantly enhanced the nitrogen doping into carbon. Then, the pore structure of the template was tuned by varying the hydrothermal temperature. With increasing this temperature from 90 to 150 C, it was found that both the nitrogen content and specific surface area of the resultant carbon followed a volcano relationship. Such a trend correlated well with the change in the electrocatalytic activity/ selectivity of the oxygen reduction reaction. As such, the pore structure of the template can be introduced as one effective parameter that controls the composition of the carbon catalyst. The above findings provide insights into the importance of the template pore structure and offer flexible strategies to control the properties of carbon in the pyrolysisbased synthesis protocols.
Conclusions The effect of the SBA-15 template on the resultant nitrogendoped ordered mesoporous carbon was extensively investigated in the present work. It was found that the nanoscale pores in the template yielded a spatial confinement effect on the carbonization of the precursor during the pyrolysis,
2.5
Current density Fraction of C-N
20
15
2.0
1.5 10 1.0
Fraction of C-N / %
-2 Current density @ 0.90 V / mA cm
3.0
Acknowledgements The work described in this paper was jointly supported by the National Natural Science Foundation of China (Nos. 21476087, 21576101), Innovation Project of Guangdong Department of Education (No. 2014KTSCX016), Science & Technology Research Project of Guangdong Province (Nos. 2013B010405005, 2014A010105041), and Fundamental Research Funds for the Central Universities.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.07.169.
5 0.5
references
0.0
0
C-PY
C-PY-S90
C-PY-S130
C-PY-S150
Fig. 5 e Plot of the nitrogen-activated carbon fraction vs. the current density at 0.90 V of the carbon catalysts.
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Please cite this article in press as: Wan K, et al., Synthesis of nitrogen-doped ordered mesoporous carbon electrocatalyst: Nanoconfinement effect in SBA-15 template, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.169
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Synthesis of nitrogen-doped ordered mesoporous carbon electrocatalyst: Nanoconfinement effect in SBA-15 template Kai Wan a, Ming-yao Liu a, Zhi-peng Yu a, Zhen-xing Liang a,*, Quan-bing Liu a, Jin-hua Piao b, Yu-ying Zheng c a
Key Laboratory on Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, PR China b School of Food Science and Engineering, South China University of Technology, Guangzhou 510641, PR China c School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, PR China
article info
abstract
Article history:
The effect of the pore structure of SBA-15, which is tuned by varying the hydrothermal
Received 30 May 2016
temperature (90e150 C), is investigated on nitrogen-doped ordered mesoporous carbon. It
Received in revised form
is found that the pore structure of SBA-15 yields effects on both the composition and the
30 June 2016
pore structure of carbon, which thereby affecting its electrocatalytic activity for the oxygen
Accepted 19 July 2016
reduction reaction (ORR). XPS reveals that the nitrogen content of the template-free carbon
Available online xxx
is 2.79 at.%, while dramatically increases to 4.48, 5.01 and 4.16 at.% for the three templateaided carbon catalysts with SBA-15 synthesized at 90, 130 and 150 C, respectively. Nitro-
Keywords:
gen ad/desorption isotherms show that the specific surface area of the template-free
Carbon catalyst
carbon is 257 m2 g1, which increases to 724, 731 and 691 m2 g1 for the above three car-
Confinement effect
bons. Such changes correlate well with the electrochemical results. RRDE results show that
Controllable synthesis
the template-aided carbon catalysts yield better activity and selectivity for the ORR than
Oxygen reduction reaction
does the template-free one. And the best performance is achieved for the carbon catalyst
Template pore structure
with SBA-15 synthesized at 130 C, which coincidentally has the highest nitrogen content and surface area. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Pt has been the mostly widely used electrocatalyst for the oxygen reduction reaction (ORR), of which, however, the high cost seriously hinders the application of fuel cells [1e6]. Enormous efforts have been devoted in developing new cheap electrocatalysts with decent activity [2,7e12]. Nitrogen-doped carbon has attracted an increasing attention
as electrocatalysts due to their unique structure and properties [1,2,13e15]. Nitrogen-doped ordered mesoporous carbon (NOMC) features high specific surface area and ordered pore structure, respectively favoring the electrocatalysis and mass transfer in the porous electrode [16e18]. Hard-template method is a general method to synthesize such materials, in which the rigid template plays a key role in framing out the pore structure. Besides, the nanometer-scale pores further yield a spatial-confinement effect on the
* Corresponding author. E-mail address: [email protected] (Z.-x. Liang). http://dx.doi.org/10.1016/j.ijhydene.2016.07.169 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Wan K, et al., Synthesis of nitrogen-doped ordered mesoporous carbon electrocatalyst: Nanoconfinement effect in SBA-15 template, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.169
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carbonization of the precursor. Gadiou [19] first reported that the composition and structure of an ordered mesoporous carbon (OMC) were strongly dependent on the mesoporous structure of SBA-15. Zelenay [20] found that such a spatial confinement could prevent the vaporization of small CN species, which thereby yielded an effect on the composition. Wei [21] used layered montmorillonite (MMT) as a 2-D nanoconfined reactor to selectively synthesize the planar nitrogen, viz. pyrrolic- and pyridinic-type, doped carbon catalyst. They argued that the quaternary-type nitrogen was sp3 hybridization and exist in a 3-D configuration in the graphene layer, which can be sterically hindered in the narrow interspace (0.46 nm) due to the spatial confinement effect. Therefore, the external template cannot only configure the morphology/ structure, but also can affect the composition as well. In our previous work, a novel method to synthesize nitrogen-doped ordered mesoporous carbon with high specific surface area has been developed [11,12,17]. To our knowledge, the state-of-the-art technique for synthesizing the carbon catalyst is generally a pyrolysis-based one, in which the surface composition is mostly determined by the final pyrolysis temperature. This may be the reason that all of the carbon catalysts reported in literature yield almost similar electrochemical performance, which seriously impedes the further development of the carbon catalyst. It is, thus, highly desirable to introduce more experimental parameters for a flexible and controllable synthesis. Based on the nanoconfinement effect in template, it is expected that the composition/structure of the carbon catalyst can be tailored with varying the pore feature of the template. For this reason, the effect of the template SBA-15 on the resultant carbon is extensively investigated in the present work. The pore feature and composition of carbon are well examined by various physiochemical methods, which are then correlated with its electrocatalytic activity to the oxygen reduction reaction (ORR).
Experimental Synthesis of nitrogen-doped ordered mesoporous carbon (NOMC) NOMC was synthesized by the nanocasting method. The details were reported in our previous publications [17,18,22]. Briefly, SBA-15 was first synthesized in a hydrothermal method using Pluronic P123 as the structure-directing agent. 4.0 g Pluronic P123 was dissolved in 126 ml deionized (DI) water and 20 ml hydrochloric acid (37 wt.%), and then 9.2 ml TEOS was added and stirred for 20 h at 35 C. The slurry was hydrothermally treated for 12 h. Finally, the product was filtered, dried, and finally calcined at 550 C for 6 h in air to remove the template. Second, the carbon precursor (pyrrole) was impregnated into SBA-15. 0.90 ml newly distilled pyrrole (99%, Xiya Reagent), together with 1.0 g SBA-15, was added into a vacuum container, which was then held in an oven at 133 C for 2 h. After that, the container cooled to the room temperature, and the light-yellow powders were finally obtained. The powders were added into 40 ml 2.0 mol l1 FeCl3 aqueous solution, which was then vigorously stirred for
24 h at room temperature for polymerization. The product was filtrated and thoroughly washed with DI water to remove metal salt. After dried, the black-colored powders were pyrolysed at high temperatures for 3 h in argon (99.999%). Finally, the silica template was removed in 10 M NaOH at 120 C for 24 h, followed by washing with DI water. The samples were referred to as C-PY-Sx. Here, x refers to the hydrothermal temperature, viz. 90, 130, and 150 C. The above synthesis process, except without using SBA-15 template, was repeated to synthesize the template-free carbon material, viz. C-PY.
Physicochemical characterizations Thermogravimetric analyses (TGA, TA Instrument SDT 2960) was performed at 5 C min1 from room temperature to 900 C in air at a flow rate of 20 ml min1. X-ray photoelectron spectroscopy (XPS, Physical Electronics PHI 5600) measurement was carried out with a multi-technique system using an Al monochromatic X-ray at a power of 350 W. Scanning electron microscopy (SEM) was conducted on a Nova NanoSEM 430 scanning electron microscope. Nitrogen ad/desorption isotherms were collected at 77 K using Micromeritics TriStar II 3020 analyzer. The total surface area was analyzed with the well-established Brunauer-Emmer-Teller (BET) method, the microporous (MP) surface area was obtained with the t-plot method, and the pore size distribution was analyzed by the BarretteJoynereHalenda (BJH) method.
Electrochemical characterization The electrochemical behavior of the catalyst was characterized by the cyclic voltammetry (CV) and linear sweeping voltammetry (LSV) using a three electrode cell with an electrochemical work station (Zennium) at room temperature (25 C). A platinum wire and a double junction Ag/AgCl reference electrode (PINE) were used as the counter and reference electrodes, respectively. The working electrode was a rotating ring-disk electrode (Glassy carbon disk: 5.0 mm in diameter, platinum ring: 6.5 mm inner diameter and 7.5 mm outer diameter, RRDE). The thin-film electrode on the disk was prepared as follows. 10 mg of the catalyst was dispersed in 1.0 ml Nafion/ethanol (0.84 wt.% Nafion) by sonication for 120 min. Then, 10 mL of the dispersion was transferred onto the glassy carbon disk by using a pipette, yielding the catalyst loading of 0.50 mg cm2. For comparison, the ORR electrocatalytic activity, of a 40 wt.% Pt/C commercial catalyst (HiSPEC4000, Johnson Matthey) with 20 mg cm2 of Pt loading, was also measured. The electrolyte solution, 0.10 M KOH, was first bubbled with argon for 60 min. Then, the CV tests were conducted at 20 mV s1 in the potential range between 0 and 1.23 V (vs. reversible hydrogen electrode, RHE) for 20 cycles. The linear sweep voltammetry (LSV) curves were obtained by scanning the disk potential from 1.2 down to 0 V at 5 mV s1 in the oxygensaturated electrolyte solution under 1600 rpm, from which the ORR polarization curve was extracted by subtracting the capacitive current. During the data collection, the potential of the ring was set to be 0.5 V (vs. RHE) to determine the yield of hydrogen peroxide.
Please cite this article in press as: Wan K, et al., Synthesis of nitrogen-doped ordered mesoporous carbon electrocatalyst: Nanoconfinement effect in SBA-15 template, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.169
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H2 O2 ð%Þ ¼
(1)
-1
4jid j jid j þ ir =N
C-PY C-PY-S90 C-PY-S120 C-PY-S150
700 600 500
3
n¼
800
Quantity Adsorbed (cm g STP)
The electron-transfer number (n) and hydrogen peroxide yield (H2O2%) in the ORR was calculated from the following equations:
2ir =N 100 jid j þ ir =N
(2)
where id is the disk current, ir the ring current, and N the collection efficiency (¼20.50%).
400 300 200 100 0
Results and discussion
0.0
Fig. 1 shows the nitrogen ad/desorption isotherms of SBA-15 synthesized at 90, 130, 150 C respectively. It can be seen that all isotherms are basically same in shape and sorption (type IV), indicating the mesoporous nature of SBA-15. The obtained information for the pore parameters are listed in Table S1. The hydrothermal temperature is found to yield a significant effect on the pore structure [23,24]. With increasing the temperature from 90 to 150 C, the specific surface area decreases from 733 to 438 m2 g1, and the pore diameter increases from 4.6 to 10.1 nm [25]. In addition, the morphology of the particles is also subject of great changes. The synthesized SBA-15 particles are found to be interconnected, and the boundaries become clearer at higher hydrothermal temperatures. Such a change is expected to yield effects on the resultant carbon, as shown below. In summary, the pore structure of SBA-15 can be controlled by varying the hydrothermal temperature, which then affects the resultant carbon. The nitrogen ad/desorption isotherms of the nitrogendoped carbon, are depicted in Fig. 2. It can be distinguished that the shape of the isotherms greatly varies for the carbon materials and the isotherms of the templates-aided carbon are basically of the type IV. The key pore parameters are also calculated and listed in Table 1. Firstly, it is seen that the template exerts a dramatic effect on the pore features of the
800
S90 S130 S150
600
3
-1
Quantity Adsorbed (cm g STP)
700
500 400 300
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Fig. 2 e Nitrogen sorption isotherms of the synthesized carbon materials.
carbon materials. Without using the template, the specific surface area of C-PY carbon is close to 257 m2 g1, and the specific pore volume is 0.16 cm3 g1. Detailed analysis reveals that the pores are predominantly micropores, of which the specific surface area is 160 m2 g1. In comparison, by using the SBA-15 template, the specific surface area dramatically increases to 724, 731, 691 m2 g1 for C-PY-S90, C-PY-S130, C-PYS150, respectively. Also, the specific volume is found to be increased and the pore diameter is controlled in the range of 4.0e7.4 nm. The formation of micropore can be attributed to the thermodecomposition of the precursor polypyrrole, and the mesoporous framework should result from the replica of the SBA-15 template [17]. This result indicates that the distribution of the pore size, which is important for the mass transfer in the electrode, can be well controlled by using different nanostructured templates. The morphology of carbon was characterized via SEM, as shown in Fig. 3. It is seen that the morphology of carbon basically follows that of the template (see Fig. S1), which evolves from the interconnected short bars to necklace-like connected spherical particles. In comparison, the templatefree carbon C-PY shows featureless morphology, however with serious agglomeration (see Fig. 3a). Besides, it is noted, that the presence of SBA-15 yields a noticeable effect on the thermodecomposition of polypyrrole (see Fig. S2), which can be attributed to both the chemical interaction and the spatial nanoconfinement effect in the SBA-15 template. Such an effect is further supported by the analysis of the elemental composition.
200
Table 1 e Pore feature of the synthesized carbon materials.
100 0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Fig. 1 e Nitrogen sorption isotherms of SBA-15 synthesized at 90, 130, and 150 C.
Sample C-PY C-PY-S90 C-PY-S130 C-PY-S150
ABET/m2 g1 AMP/m2 g1 DBJH/nm V/cm3 g1 257 724 731 691
160 160 94 196
4.3 4.0 5.2 7.4
0.16 0.62 0.94 1.12
Please cite this article in press as: Wan K, et al., Synthesis of nitrogen-doped ordered mesoporous carbon electrocatalyst: Nanoconfinement effect in SBA-15 template, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.169
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Fig. 3 e SEM images of a) C-PY, b) C-PY-S90, c) C-PY-S130, and d) C-PY-S150.
XPS analysis was used to gain the elemental composition on the carbon surface. Survey spectra are depicted in Fig. S3 and the elemental compositions are listed in Table 2. It is found that the nitrogen content of the template-free carbon CPY is 2.7 at.%, which is much lower than those in templateaided ones. The result reveals the existence of a special interaction between the template and polypyrrole (vide supra), which favors the nitrogen doping into carbon. It is further seen that the nitrogen content is dependent on the pore structure of SBA-15, and the hydrothermal temperature of 130 C yields the highest nitrogen content (5.01 at.%). As mentioned above, such an effect may result from the chemical interaction between polypyrrole and silica [26], and the spatial confinement in the mesopores of SBA-15 as well [19]. And such an interaction offers more flexible strategies to control the composition of carbon in the pyrolysis-based synthesis protocols. As acknowledged, an effective doping of nitrogen is required to generate the ORR active sites in the carbon-based catalyst. As such, the SBA-15 template not only configures the mesoporous structure with high specific surface area, but also promotes the nitrogen doping with high intrinsic activity, both of which are expected to improve the electrochemical performance of carbon.
Table 2 e Elemental composition (at. %) the carbon materials measured by XPS. Sample C-PY C-PY-S90 C-PY-S130 C-PY-S150
C
N
O
Fe
N:C
94.34 91.34 90.46 91.70
2.79 4.48 5.01 4.16
2.74 3.56 3.56 3.58
0.13 0.32 0.28 0.18
0.030 0.049 0.055 0.045
Fig. 4 presents the ORR polarization curves and the corresponding yield of hydrogen peroxide in an O2-saturated 0.10 M KOH solution. First, the template-aided carbon catalysts show superior ORR electrocatalytic activity to that of the template-free one. For example, the half wave potential, E1/2, is much higher for the template-aided carbon (>0.860 V) than the template-free one (0.758 V). The superior performance should be attributed to their larger specific surface area and higher nitrogen content. Second, a considerable difference in the electrocatalytic activity of the three template-aided carbon catalysts can easily be distinguished, which follows the order of C-PY-S90 (E1/2: 0.862 V), C-PY-S150 (E1/2: 0.868 V) < C-PY-S130 (E1/2: 0.886 V). All the carbon catalysts exhibit a high selectivity with the yield of H2O2 below 20%, among which the best-performed catalyst C-PY-S130 shows the lowest H2O2 yield (<10%) and its electron transfer number is above 3.8. Such a trend in the electrocatalytic activity and selectivity correlates well with the change in the nitrogen content and the specific surface area, as discussed above. Finally, it is noted that the template-aided carbon catalyst even exhibit a higher electrocatalytic activity than does the commercial Pt/C catalyst (E1/2: 0.844 V), which is of great interests to be used in practical fuel cells. In our previous work, the active sites for the ORR were suggested to be the nitrogen-activated carbon atoms rather than the FeN2(or 2þ2)Cy species, on which the ORR proceeds by a surface-confined redox-mediation mechanism in both acid and alkaline media [27]. Here, Tof-SIMS was further used to detect the molecular fragments on the surface of the carbon catalysts. It is seen that only a very tiny fraction of FeNxCþ y fragments is observed in the spectra (see Fig. S4), suggesting most iron is not coordinated with nitrogen atoms. To confirm
Please cite this article in press as: Wan K, et al., Synthesis of nitrogen-doped ordered mesoporous carbon electrocatalyst: Nanoconfinement effect in SBA-15 template, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.169
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Fig. 4 e RRDE results of the carbon materials in O2-saturated 0.10 M KOH solution: a) ORR polarization curves, b) electron transfer number and H2O2 yield.
the nature of the active sites, the peak fitting of C1s is performed and the fitting results are shown in Fig. S5. The fraction of the CeN groups are extracted and then plotted versus the kinetics current density (at 0.90 V) in Fig. 5. The change in the electrocatalytic activity basically follows the content of the nitrogen-activated carbon atoms. And this result strongly confirms our previous claim on the chemical nature of the active sites for the ORR [17,27]. In summary, it is found that the template-aided synthesis protocol can offer more flexible control over the composition, structure, and consequently the electrocatalysis of the nitrogen-doped carbon catalysts.
which significantly enhanced the nitrogen doping into carbon. Then, the pore structure of the template was tuned by varying the hydrothermal temperature. With increasing this temperature from 90 to 150 C, it was found that both the nitrogen content and specific surface area of the resultant carbon followed a volcano relationship. Such a trend correlated well with the change in the electrocatalytic activity/ selectivity of the oxygen reduction reaction. As such, the pore structure of the template can be introduced as one effective parameter that controls the composition of the carbon catalyst. The above findings provide insights into the importance of the template pore structure and offer flexible strategies to control the properties of carbon in the pyrolysisbased synthesis protocols.
Conclusions The effect of the SBA-15 template on the resultant nitrogendoped ordered mesoporous carbon was extensively investigated in the present work. It was found that the nanoscale pores in the template yielded a spatial confinement effect on the carbonization of the precursor during the pyrolysis,
2.5
Current density Fraction of C-N
20
15
2.0
1.5 10 1.0
Fraction of C-N / %
-2 Current density @ 0.90 V / mA cm
3.0
Acknowledgements The work described in this paper was jointly supported by the National Natural Science Foundation of China (Nos. 21476087, 21576101), Innovation Project of Guangdong Department of Education (No. 2014KTSCX016), Science & Technology Research Project of Guangdong Province (Nos. 2013B010405005, 2014A010105041), and Fundamental Research Funds for the Central Universities.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.07.169.
5 0.5
references
0.0
0
C-PY
C-PY-S90
C-PY-S130
C-PY-S150
Fig. 5 e Plot of the nitrogen-activated carbon fraction vs. the current density at 0.90 V of the carbon catalysts.
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Please cite this article in press as: Wan K, et al., Synthesis of nitrogen-doped ordered mesoporous carbon electrocatalyst: Nanoconfinement effect in SBA-15 template, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.169