Real Understanding of the Nitrogen-Doping Effect ... - ACS Publications

Feb 9, 2016 - Gurwinder Singh , In Young Kim , Kripal S. Lakhi , Stalin Joseph , Prashant Srivastava , Ravi Naidu , Ajayan Vinu. J. Mater. Chem. A 201...
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Real Understanding of the Nitrogen-Doping Effect on the Electrochemical Performance of Carbon Materials by Using CarbonCoated Mesoporous Silica as a Model Material Alberto Castro-Muñiz,* Yasuto Hoshikawa, Takatoshi Kasukabe, Hiroshi Komiyama, and Takashi Kyotani Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan S Supporting Information *

ABSTRACT: The main aim of the present work is to precisely understand the sole effect of nitrogen doping on the electrochemical performance of porous carbon materials. To achieve this objective, the whole surface of mesoporous silica (SBA-15) was coated with a thin layer of carbon (about 0.4 nm) with and without N-doping by using acetonitrile and acetylene chemical vapor deposition, respectively. The resulting N-doped and nondoped carbon-coated silica samples have mesopore structures identical to those in the original SBA15, and they are practically the same in terms of not only the pore size and pore structure but also the particle size distribution and particle morphology, with the exception of N-doping, which makes them unique model materials to extract the sole effect of nitrogen on the performances of electrochemical capacitors and electrocatalytic oxygen reduction. Moreover, the outstanding features of the carbon-coated silica samples allow even a quantitative understanding of the pseudocapacitance induced by nitrogen functionalities on the carbon surface in an acidic aqueous electrolyte.

1. INTRODUCTION Heteroatom doping, in particular with nitrogen, has often been reported as the cause of significant improvement in the performance of porous carbon materials in many demanding application fields such as electrochemical energy storage, electrocatalysis, photocatalysis, and biomolecular sensing.1−4 Even though the effect of N-doping has been extensively studied, its real influence on the performance of the porous carbon materials is very difficult to evaluate with absolute certainty. This is because the performance of such materials in the fields listed above is also affected by many other factors. In order to assess the sole effect of N-doping, the behavior of a Ndoped carbon material must be compared to that of a nondoped one with nearly identical pore size, shape, and network as well as with very similar particle sizes and particle morphologies. Such a pair of porous carbon materials with those features has not been obtained so far. The preparation of porous carbon materials is usually carried out through either the chemical or physical activation of inexpensive carbon precursors such as lignocellulosic residues or coals. The change in the activation conditions allows some control over the porous texture, but this is still not precise enough. As for N-doping, it can be attained either by the carbonization of N-containing carbon precursors or by posttreatments with N-containing reactive chemical species. In the first case, control of the pore structure remains difficult, and in the latter case, the original pore structure is inevitably altered by the post-treatments to a greater or lesser extent. In short, porous carbons with the same pore structure but with and © XXXX American Chemical Society

without N-doping are hardly obtained by using the traditional preparation methods. Ordered mesoporous oxides, in which the pore size as well as the porous structure can be accurately controlled, are commonly used for biosensing and biocatalysis.5,6 However, these materials are electrical insulators, and they are therefore hardly applied to electrochemical devices. In the last few years, new approaches have been used to solve this problem mainly by using templates. In the hard template method, the ordered mesoporous carbons are synthesized by the infiltration of a carbon precursor into the pore network of a template, usually inorganic oxides such as mesoporous silica.7,8 Another option is the soft template method in which the self-assembly of amphiphilic molecules is used as a template in a procedure similar to the synthesis of ordered mesoporous silica.9 The carbon materials thus obtained show porous structures similar to those of the ordered mesoporous oxides. Many researchers have used both methods to prepare mesoporous carbon materials and have found the methods to be very useful. Moreover, in both methods, N-doping is possible in principle when N-containing carbon precursors are used10 or the post-Nintroduction treatments are performed. However, both the hard and soft template methods give not only the desired mesopores but also a large fraction of additional micropores because the occurrence of cracks produced by gases evolved during the heat Received: October 1, 2015 Revised: February 4, 2016

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2.2. Carbon Coating on SBA-15. Before the carbon coating, the surface of SBA-15 was grafted with trimethylsilyl (TMS) groups in exactly the same manner as in our previous study.18,19 Then the resulting silylated SBA-15 (TMS-SBA15) was coated with a thin layer of N-doped carbon by the following CVD method. About 0.5 g of TMS-SBA15 was placed in a horizontal quartz reactor (i.d. 40 mm), and the sample was heated to 800 °C at a heating rate of 5 °C min−1 under N2 flow (400 mL (STP) min−1). Immediately after the reactor reached 800 °C, acetonitrile vapor was introduced into the reactor. The vapor was generated by bubbling N2 through liquid acetonitrile in a saturator at 0 °C. Under these conditions, the flow rate of the acetonitrile vapor is estimated to be 18.4 mL (STP) min−1. The acetonitrile CVD was conducted for different time intervals (60, 90, and 120 min). For comparison, TMS-SBA15 was coated with a nondoped carbon layer by using acetylene gas as a CVD source under the same CVD conditions as reported previously18 (CVD temperature 600 °C, CVD time 130 min). After acetonitrile or acetylene CVD, some of the samples were subjected to further heat treatment at 900 °C for 1 h under N2 flow. 2.3. Characterization of Materials. The amount of carbon loading for each carbon-coated SBA-15 sample was estimated from its weight loss when it was combusted at 800 °C for 1 h in air using a thermobalance (Shimadzu DTG-60H). Water adsorption isotherms at 25 °C were obtained in a volumetric apparatus (BEL Japan BELMAX). The porous texture of the pristine and carbon-coated SBA-15 samples was analyzed by the physical adsorption of N2 at −196 °C (BEL Japan BELMINI). For these adsorption measurements, the samples were degassed under vacuum at 150 °C for 6 h beforehand. The BET surface area, SBET, was calculated by the BET equation from the data in a relative pressure range of ca. 0.01−0.2, and the total pore volume, Vt, was obtained from the amount of adsorbed N2 at a relative pressure of 0.975 using the Gurvitsch rule. The micropore volume, Vmi, was calculated by applying the Dubinin−Radushkevich method to the adsorption isotherms, and the mesopore volume was estimated as the difference between Vt and Vmi. The pore size distribution (PSD) was calculated from the N2 adsorption data using the nonlocal density functional theory on the assumption that every sample has cylindrical pores. The surface chemistry of the coated samples was analyzed by Xray photoelectron spectroscopy (XPS) in a spectrometer (JEOL JPS9200) operating at a pressure of 10−7 Pa with a Mg Kα X-ray source using a power of 300 W. In order to observe the pore structure of the samples, transmission electron microscope (TEM) images were taken in a JEOL JEM-2010 at an acceleration voltage of 200 kV. The carbon structure was analyzed with a Raman spectrometer (NRS-3000FL, JASCO, laser wavelength 532 nm). For the measurement of the electrical conductivity (σ), each sample (ca. 50 mg) was placed between two stainless steel plungers in a homemade polyetherketone cylindrical mold with an area (A) of 48 mm2 and a load was then applied using an AG-X Plus testing machine from Shimadzu. The load and the resulting displacement of the plungers were directly monitored with the testing machine, and the resistance (R) of the material was measured between the two plungers. The conductivity was calculated to be σ = dR−1A−1, where d is the sample length obtained from the displacement of the plungers. 2.4. Performance Tests. The electric double layer performance tests were carried out at 25 °C in two types of aqueous electrolytes, acidic (1 M H2SO4) and neutral (1 M NaCl), using a beaker-type three-electrode cell with a Ag/AgCl saturated KCl reference electrode (BAS Inc. Japan). All of the potentials appearing hereafter will refer to this reference, i.e., vs Ag/AgCl. To make the working electrode, a paste was prepared by mixing the sample with acetylene carbon black (CB, Denka black, Denki Kagaku Kogyo Kabushiki Kaisha) as a conductive additive and poly(tetrafluoroethylene) (PTFE 6-J, Du Pont-Mitsui Fluorochemicals Company, Ltd.) as a binder with the following weight ratio: sample/CB/PTFE = 85:10:5. Then a sheet made from the paste of ca. 7 mg was sandwiched in a metallic mesh (acidic electrolyte, stainless steel SUS 304 100 mesh; neutral electrolyte, titanium 100 mesh; both are from Nilaco Corp.), pressed at ca. 20 MPa for 1 min, and used as a working electrode. The counter electrodes were prepared in a similar manner but using an activated carbon fiber (A20

treatment for the carbonization is not avoidable. It is often the case that the performance of these mesoporous carbons is strongly influenced (mostly in a positive way) by such uncontrollable micropores. A possible approach that makes the ordered mesoporous oxides electrically conductive but retains their ordered porous structure is to coat their surfaces with a very thin layer of carbon. An anodic aluminum oxide (AAO) film with uniform, straight nanochannels can be regarded as one of such mesoporous oxides because its carbon coating is very easy. We indeed used the carbon-coated AAO films as a model material, but their small surface areas (ca. 30 m2 g−1) and large mesopore size (16 nm) gave rise to a very low electrochemical capacitance value, which hampered the precise and quantitative understanding of the doping effect.11 In this context, ordered mesoporous silica is much more suitable than AAO for the present purpose. The coating of ordered mesoporous silica with a very thin layer of carbon has been attained to some extent in some of the hard template studies,12,13 although it was not their intention. Some researchers, including our group, have intentionally coated mesoporous silica with a thin carbon layer by means of the esterification of aromatic compounds over the silica surface.14 Moreover, we have demonstrated that carbon-coated mesoporous silica can be used as a porous electrode.15−17 However, this coating method needs a timeconsuming washing process to remove the unreacted aromatic compounds, which makes the method difficult to scale up. In addition, so far we have not found any N-containing aromatic compounds that could be applied to the esterification reaction with the silica surface. In other words, N-doping is virtually not possible in the above carbon coating method. In the present work, instead of the above wet process, we try to coat mesoporous silica (SBA-15) with a thin layer of Ndoped carbon by means of chemical vapor deposition (CVD) using the method recently developed in our group.18 Our previous study has revealed that, when the silica surface is silylated, the Si radicals formed during the CVD process significantly catalyze the pyrolytic carbon deposition from hydrocarbon molecules having a C−C triple bond such as acetylene, and carbon-coated SBA-15 has been obtained as a result. Here we use acetonitrile instead of acetylene as a CVD source. The main aim of this work is to precisely understand the sole effect of N-doping on the performance as an electrode material in electrochemical applications (capacitors and electrocatalytic oxygen reduction) using the carbon-coated silica samples (SBA15) as model materials. To reach this objective, first we have investigated if the silylation is still effective for the acetonitrile CVD to obtain a coating of a thin layer of N-doped carbon. Then we have compared the physicochemical properties of the N-doped material with those of the nondoped one prepared by acetylene CVD to evaluate their usefulness as model materials.

2. EXPERIMENTAL SECTION 2.1. SBA-15 Synthesis. The mesoporous silica, SBA-15, was prepared as follows. First, 3.48 g of Pluronic P123 was dissolved in 26.00 g of distilled water by stirring the mixture for 1 h at room temperature. Then, 103.74 g of 2 N HCl and 10.18 g of tetraethoxysilane (TEOS) were added, and the mixture was stirred for 5 min. The solution was further stirred at 35 °C for 25 h to allow the hydrolysis of the TEOS. Afterward, the material was aged at 80 °C for 24 h. The mixture was filtered, thoroughly washed with both distilled water and ethanol, and vacuum dried at 80 °C for 8 h. Finally, the surfactant was removed by calcination at 550 °C for 6 h. B

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Langmuir from Osaka Gas Co. Ltd.) as the active material, and the total mass of the electrode was ca. 10 mg. After a cellulose separator (TF-4850, Tokyo Sangyo Yoshi Co. Ltd.) was sandwiched between the working and counter electrodes, they were soaked in the electrolyte that was degassed in vacuum for 1 h and were kept in the electrolyte for 3 days. Before the measurements, N2 was bubbled in the electrolyte for 30 min. Cyclic voltammetry (CV) with a potential window of −0.1−0.8 V, galvanostatic charge/discharge cycling, and impedance spectroscopy with a frequency range of 10−2−10−5 Hz were carried out with a Biologic VMP3 potentiostat. The electrochemical oxygen reduction reaction tests were performed at 25 °C in a phosphate buffer solution (pH 6.86) as an electrolyte using a Ag/AgCl (KCl saturated) reference electrode (BAS Inc. Japan) and a platinum coil as a counter set in a homemade threeelectrode cell, which has two different gas lines to saturate the electrolyte with N2 or O2. The working electrode was prepared by depositing ink on a rotary electrode of glassy carbon (5 mm diameter). The ink was obtained from 10 mg of each carbon-coated sample, 350 μL of ethanol, and 95 μL of Nafion (5% dispersion solution DE521 CS type, Wako Pure Chemical Industries, Ltd.), and the resulting mixture was ultrasonicated for 30 min. Then a 10 μL drop of the ink was put on the glassy carbon electrode and dried for 30 min at 60 °C. Note that before use the glassy carbon electrode was polished in a suspension of alumina, washed with ultrapure water (Millipore), and ultrasonicated until a clean mirrorlike surface was obtained (the surface was checked with an optical microscope). After the electrolyte (25 mL in volume) was degassed by bubbling N2 gas for 30 min, the gas was allowed to flow on top of the electrolyte and cyclic voltammetry was carried out with a potential window of −0.8−0.4 V. Then the gas was switched to O2 gas, and the cyclic voltammetry was again performed. In addition, the samples were subjected to linear sweep voltammetry at different rotational rates (400, 800, 1200, 1600, 2000, 2400, and 3200 rpm). These electrochemical characterizations were made with a Hokuto Denko HZ-5000 potentiostat and an HR502 dynamic electrode controller.

Figure 1. Carbon content as a function of the CVD time (a) and the amount of CVD source introduced into the reactor (b).

acetylene CVD. However, when the carbon amount is plotted against the total amount of carbon source (acetonitrile or acetylene) fed into the reactor (Figure 1b), it is found that the acetonitrile CVD needs a larger amount than the acetylene CVD, even though the temperature of the former CVD is higher. Thus, the carbon deposition is much less effective in the case of using acetonitrile as the CVD source. Regardless of the difference in chemical reactivity between the two CVD sources, the silylation is very useful in enhancing the carbon deposition in both cases. In our previous work,18 we have proposed a possible mechanism for the carbon coating from the acetylene CVD on the mesoporous silica grafted with TMS groups. Briefly, the silicon radicals produced by the thermal decomposition of TMS groups catalyze the trimerization of acetylene to form benzene through free radical reactions of the triple bonds in acetylene molecules, which induces the carbonization on the silica surface. Since the decomposition of TMS groups occurs in the range of 500−700 °C,18 silicon radicals must already be formed in the present acetonitrile CVD at 800 °C. However, it is reported that the polymerization or trimerization of the CN functional groups in acetonitrile molecules is possible only through ionic reaction pathways when the molecules form a complex with Lewis acids such as metal chlorides.23 It is thus unlikely that nitrile trimerization occurs by the catalysis of Si radicals during acetonitrile CVD. From many studies on the pyrolysis of acetonitrile, it can be concluded that the major products are hydrogen cyanide and methane.24−26 Once they are formed in the reactor under the present CVD conditions, most of them are readily swept away by the carrier gas because both of them are rather stable even at 800 °C. This can explain why there is almost no carbon deposition on the silica surface in the pristine SBA-15. Together with the two major products, smaller quantities of other gases such as ethylene and acrylonitrile are formed, and these gases can be easily converted to acetylene.25,26 Besides, the pyrolysis of acrylonitrile may produce a small amount of cyanoacetylene.25 These acetylene species should undergo trimerization if silicon radicals are present on the silica surface, and then the resulting trimers formed on the silica surface may function as nuclei for further pyrolytic carbon growth. 3.2. Physicochemical Properties of Carbon-Coated SBA-15. Porous textural parameters of the pristine and carboncoated SBA-15 samples are summarized in Table 1, where the specific surface area is decreased by more than half upon carbon coating by acetonitrile CVD. This is due to not only the pore narrowing by the carbon deposition on the pore walls but also the shrinkage of SBA-15 framework during the heat treatment, as we will demonstrate later. Concerning the carbon-coated samples, the values of the specific surface area, the pore volumes, and the pore size are gradually decreased with increasing acetonitrile CVD time from 60 to 120 min, suggesting that the carbon deposition on the silylated SBA-15

3. RESULTS AND DISCUSSION 3.1. Effect of Trimethylsilylation on the Carbon Deposition from Acetonitrile CVD. First, the pristine (nonsilylated) SBA-15 was subjected to the acetonitrile CVD at 800 °C, but almost no carbon deposition was observed on the unmodified silica surface. Considering that this CVD temperature (800 °C) is high enough for the pyrolytic carbon deposition from acetonitrile for both anodic aluminum oxide and zeolite Y,11,20 we can judge that the silica surface has much lower catalytic activity toward carbon deposition. This may be the main reason that other researchers carried out acetonitrile CVDs at temperatures higher than 800 °C in their hard template studies using mesoporous silica.21,22 However, the CVD temperature should be as low as possible for the purpose of controlling the amount of carbon deposited on the mesoporous silica surface and the homogeneity of the resulting carbon layer, as we have already pointed out in the case of acetylene CVD.18 Although the acetonitrile CVD at 800 °C does not produce any carbon deposition on the surface of SBA-15, once the silica surface was modified with the TMS groups its activity toward carbon deposition has been greatly improved as shown in Figure 1a, where the amount of carbon loaded onto the silylated silica surface is plotted as a function of the CVD period together with the previous data from the acetylene CVD at 600 °C over the silylated SBA-15. Upon increasing the CVD time from 60 to 120 min, the carbon loading shows an almost linear increase from 17 to 26 wt %. In comparison with the case of the acetylene CVD, apparently the acetonitrile CVD needs a shorter time to gain a given amount of carbon than the C

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Table 1. Amount of Carbon and Porous Textural Parameters Derived from N2 Adsorption Isotherms at −196 °C on SBA-15 and Carbon-Coated SBA-15 Samples treatments for SBA-15 samples pristine carbon coated with acetonitrile CVD

carbon coated and heat treated heat treateda carbon coated with acetylene CVD a

CVD time (min) − 60 90 120 90 − 130

HTT at 900 °C C (wt %) − × × × ○ ○ ○

− 17 21 26 22 − 22

SBET (m2 g−1)

Vt (cm3 g−1)

Vmi (cm3 g−1)

Vmes (cm3 g−1)

dDFT (nm)

743 352 316 214 300 502 303

0.77 0.32 0.27 0.18 0.26 0.52 0.30

0.32 0.15 0.12 0.09 0.12 0.20 0.12

0.45 0.17 0.15 0.09 0.14 0.32 0.18

6.3 5.1 4.9 4.7 4.9 5.7 5.1

Pristine SBA-15 was heat treated at 800 °C for 90 min and then at 900 °C for 1 h.

Figure 2. TEM micrographs of CN-SBA15 (a) and C-SBA15 (b).

Figure 3. XPS spectra corresponding to N 1s with their deconvoluted components (dashed lines) (a) and first-order Raman spectra (b) of the carbon-coated SBA-15 after the heat treatment at 900 °C.

°C for 1 h under N2 flow. Despite further heat treatment, the surface area, the pore volumes, and the pore sizes do not change very much (Table 1). In addition, we let pristine SBA15 undergo the same heat treatment history as the above sample but without the introduction of acetonitrile vapor. It is clear from the data of this heat-treated SBA-15 in Table 1 that only the heat treatment reduces the porosity of the pristine SBA-15 because of the thermal shrinkage of the SBA-15 framework. The difference in pore size between the heattreated SBA-15 and the carbon-coated one can be taken as a measurement of the thickness of the carbon layer on average. The carbon layer is thereby estimated to be 0.4 nm, which corresponds to a thickness of almost one carbon layer. Furthermore, this value agrees with the one estimated from the amount of carbon loading (22 wt %) as explained above. For comparison, the SBA-15 sample coated with a nondoped carbon layer was prepared by the acetylene CVD over the

took place in a controlled manner. In other words, the samples retain the mesoporous structure of the original SBA-15 even with as large a carbon loading as 26 wt %. This is evident from the shape of the N2 adsorption−desorption isotherms and the pore distribution curves for these samples (Figure S1 in Supporting Information). The amount of carbon that is needed for the complete coating of the whole SBA-15 surface can be calculated theoretically from the pore structure of SBA-15 using the data obtained from the N2 adsorption isotherms. The calculated carbon loading amounts for the complete coating with one and two carbon layers are 20.0 and 30.2 wt %, respectively. The acetonitrile CVD for 90 min therefore suffices to obtain the coating mass that would cover all pore walls of the SBA-15. With the aim of increasing the electrical conductivity of the carbon layer, immediately after acetonitrile CVD for 90 min, coated SBA-15 was subjected to a further heat treatment at 900 D

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Langmuir silylated SBA-15 at 600 °C for 130 min (Table 1). Note that this coated sample was also heat-treated at 900 °C. Table 1 shows that the two carbon-coated SBA-15 samples which underwent the 900 °C treatment have the same carbon content (22 wt %) and have very similar porous textural parameters. Besides, the two materials have exactly the same particle size distribution and particle morphology (SEM images in Figure S2 in Supporting Information) because both of them were obtained from the same lot of SBA-15 powder. Henceforth, these samples coated with the acetonitrile and acetylene CVDs will be referred to as CN-SBA15 and C-SBA15, respectively, and CN-SBA15 will always be compared with C-SBA15. This comparison makes it possible to understand the sole effect of N-doping because the structure of the two samples is the same in terms of not only the pore structure but also the particle size and the particle morphology. The TEM images of CN-SBA15 and C-SBA15 show an ordered mesoporous structure inherited from the original SBA15 (Figure 2). Even though both samples contain a large amount of carbon (22 wt %), the ordered mesopores do not look clogged with the carbon deposit and no carbon heaps are observed on the external surfaces, supporting the idea that a uniform carbon coating on the silica surface has also been achieved in the present acetonitrile CVD. The introduction of N atoms into the carbon layers of CNSBA15 was confirmed by XPS. The resulting spectrum of CNSBA15 in the N 1s region is shown in Figure 3a, where the spectrum is deconvoluted into three components centered at 404.3, 400.9, and 398.5 eV, which can be attributed to oxidized N, quaternary N, and pyridinic-type N, respectively.27,28 The major nitrogen functionalities are quaternary and pyridinic N species, and their ratio is 1:0.44. In addition to the N 1s spectrum, the XPS measurement for C 1s was performed and the atomic ratio of N/C is determined to be 0.054. The XPS spectrum of C-SBA15, as expected, does not have any peaks in the N 1s region (Figure 3a). The Raman spectra of both samples (Figure 3b) exhibit the typical features for disordered carbons, i.e., two wide bands centered at around 1330 cm−1 (D band) and 1580 cm−1 (G band). The ID/IG ratios are 1.19 and 1.01 for CN-SBA15 and C-SBA15, respectively, and the former sample has a somewhat broader G band than the latter one, indicating that the former has a little lower degree of crystallinity than the latter. The lower crystallinity can be explained by the fact that N-doping usually induces defects in the carbon lattice.29 The water adsorption isotherms (Figure S3 in Supporting Information) show that a breakthrough pressure at which the water adsorption rapidly increases can be distinguished between the two samples. In the case of C-SBA15, the relative H2O pressure of this point is 0.87, whereas for CN-SBA15 this value is shifted down to 0.73. In addition, the hysteresis between the adsorption and desorption branches of the isotherms is wider for CN-SBA15. All of these findings clearly indicate that the presence of nitrogen enhances the water adsorption and CNSBA15 is more hydrophilic than C-SBA15. Silica is an insulator, but carbon is a good electrical conductor. We therefore expect that the carbon-coated silicas are endowed with some electrical conductivity, even though they are powder. In Figure 4, the electrical conductivity of the powder samples with no additives is plotted as a function of the loading pressure. Both carbon-coated samples exhibit similar behavior: a rapid increase in the conductivity with the applied load and then leveling off at ca. 20 MPa. C-SBA15 and CN-

Figure 4. Electrical conductivity as a function of loading pressure for the carbon-coated samples.

SBA15 reach conductivity values of 323 and 397 S m−1, respectively, and the introduction of N atoms seems to be effective at increasing the electrical conductivity to some extent. For the sake of comparison, the commercial acetylene carbon black used as a conductive additive in the paste electrodes (Experimental Section) was also tested by the same method, and the resulting conductivity value at the maximum loading pressure (ca. 100 MPa) was found to be 1125 S m−1, which is just about 3 times higher than those of the carbon-coated composites. The observed good conductivity of the carboncoated samples implies that the thin carbon layer covers not only the pore walls but also the external surface of every particle, which allows good interparticle contact. 3.3. Electrochemical Performance. In the previous section, we have demonstrated that the carbon-coated SBA15 materials are electrically conductive and that both nondoped and doped materials show almost identically ordered porous structures replicated from the original SBA-15. These features make it possible to apply C-SBA15 and CN-SBA15 as electrodes in electrochemical devices and to use them as model materials for understanding the sole effect of N-doping on the electrochemical performance. 3.3.1. Capacitive Behavior in Neutral Aqueous Electrolyte. Figure 5a shows the cyclic voltammograms of the paste electrodes obtained in 1 M NaCl electrolyte for the pristine and carbon-coated samples. For better comparison purposes, the specific capacitance was expressed in terms of the specific surface by dividing the gravimetric capacitance by the BET surface area. As expected, the pristine SBA-15 does not show any capacitance, but both carbon-coated samples give a clear CV pattern, demonstrating that the samples exhibit capacitance and work very well as porous electrodes. Moreover, it should be noted that the two CV patterns are almost the same. This is because the two samples have almost the same pore structure, the same particle size distribution, and the same particle morphology, although they differ from each other in surface chemistry. This finding indicates that the introduction of N atoms does not play any role in the neutral aqueous electrolyte. 3.3.2. Capacitive Behavior in Acidic Aqueous Electrolyte. When the electrolyte is changed to the acidic aqueous one (1 M H2SO4), the presence of N atoms becomes a very important factor affecting the capacitive behavior. The cyclic voltammograms of both samples in 1 M H2SO4 are shown in Figure 5b, where the N-doped carbon-coated sample (CN-SBA15) presents a better performance with a wider CV pattern that exhibits broader anodic and cathodic peaks than the nondoped one (C-SBA15). Since the two materials are very similar in every aspect (pore structure, particle size distribution, and E

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Figure 5. Cyclic voltammograms (sweep rate 0.5 mV s−1) of the pristine and carbon-coated samples measured in 1 M NaCl (a) and 1 M H2SO4 (b) and the contribution of the pseudocapacitance aroused from N-functionalities calculated as the difference between CV voltammograms of doped and nondoped carbon-coated SBA-15 in an acidic electrolyte (c).

Figure 6. Specific capacitance as a function of the current density for the carbon-coated samples (a) and Nyquist plots of the carbon-coated SBA-15 samples at 0.3 V vs Ag/AgCl (b) obtained in 1 M H2SO4.

particle morphology) with the exception of N-doping, we can attribute the difference in the two CV patterns solely to the presence of N atoms in the carbon layers. One might be inclined to credit the different behavior of the N-doped carboncoated sample to the slightly better wettability and conductivity of this sample. However, if this were the case, then the performance in the neutral electrolyte would have also been different. Hence, the enhancement in capacitance can be explained only by the well-known pseudocapacitance induced by nitrogen functionalities in acidic aqueous electrolytes, which has been well documented by many researchers.30−33 3.3.3. Origin of Pseudocapacitance. Taking a closer look at the two patterns in 1 M H2SO4 (Figure 5b), we can find that each pattern has a couple of more defined redox peaks in the range of 0.2−0.4 V. These peaks originate most probably from the quinone/hydroquinone redox couple, which has often been observed in this region,34 and the peak heights look almost the same between C-SBA15 and CN-SBA15. Then, in order to extract the contribution of the pseudocapacitance induced by the nitrogen functionalities, we have subtracted the former CV pattern from the latter one. Figure 5c illustrates the resulting difference CV pattern, which is almost rectangular without any clear peaks. The absence of peaks suggests that the effects of the quinone/hydroquinone redox couple in both samples have canceled each other out. More importantly, the difference CV pattern reveals the pseudocapacitance due to only the nitrogen functionalities in CN-SBA15. However, before analyzing the difference CV pattern, we should check the possibility of the electrochemical oxidation of the nitrogen functionalities during

the present CV measurements. For this purpose, we lowered the upper limit potential from 0.8 to 0.6 V and made the CV measurement with a narrow potential window from −0.1 to 0.6 V. Note that 0.6 V is close to the open circuit potential (0.5 V). The resulting CV patterns (Figure S4 in Supporting Information) are similar to the ones obtained by using 0.8 V as an upper limit potential. In addition, with XPS, we analyzed the nitrogen functionalities in CN-SBA15 after the whole electrochemical runs in the range of −0.1 to 0.8 V and confirm that there is no change in the N 1s XPS peak before and after the electrochemical measurements (Figure S5 in Supporting Information). Thus, we can rule out the possibility of the electrochemical oxidation of the nitrogen functionalities in CNSBA15. Pyridinic N species which are one of the nitrogen functionalities in CN-SBA15 are believed to provide pseudocapacitance through a redox reaction involving protons, as in the case of quinone groups, but the difference CV pattern (Figure 5c) is rectangular, suggesting that the redox reaction should equally take place at every potential over the whole range used in this study. The possible reason for the occurrence of the faradic processes in such a wide range is that there are pyridinic N species with a wide variety of redox potentials, as we discussed previously.11 The presence of quaternary N species in the carbon framework is known to enhance the capacitive performance through the change in the carbon electronic structure,4,35,36 but this is not the case for CN-SBA15 because this sample exhibits almost the same performance as CSBA15 in the neutral aqueous medium. F

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Figure 7. Cyclic voltammograms (sweep rate, 50 mV s−1; dashed lines, N2-saturated; solid lines, O2-saturated) (a) and linear sweep voltammetry (2400 rpm sweep rate, 10 mV s−1 from 0.4 to −0.8 V) (b) for oxygen reduction of the carbon-coated samples in O2-saturated PBS at pH 6.86.

catalyze the oxygen reduction reaction (ORR), but the price and scarcity of this material make the search for alternative materials indispensable. For example, carbon materials doped with nitrogen have been thoroughly studied as electrocatalysts in acidic and alkaline electrolytes.2,37,38 Initially, we tested the ORR performance of CN-SBA15 in 0.1 M KOH, and the results were very promising. Nevertheless, in spite of the fact that the activity of the samples was retained, the electrolyte slightly dissolved the silica due to its high alkalinity. Alternative fuel cells in which biological systems are used as electrocatalysts, so-called biofuel cells, work under mild conditions, usually in neutral electrolytes at moderate temperatures. In this regard, the search for materials that may cause oxygen reduction in the absence of either biological or metallic catalysts in neutral media is a technical challenge that has been addressed recently.39−42 The electrocatalytic activity of the carbon-coated samples toward oxygen reduction was thus tested in a neutral medium using phosphate buffer solution (PBS) at pH 6.86 as an electrolyte. Figure 7a shows cyclic voltammograms in the N2saturated electrolyte. The shapes of the voltammograms are slightly distorted because of the relatively low conductivity of the carbon-coated samples. However, it is worth pointing out that, in this case, the electrodes do not include any additive to increase the conductivity but still give a good response. As the electrical conductivity tests showed, the nitrogen-doped sample is slightly more conductive, so the current intensity is higher. The electrolyte was then saturated with oxygen (by bubbling it for 30 min). The resulting cyclic voltammograms (Figure 7a) clearly show an increase in the reduction current for both samples. A cathodic shoulder can be observed in the voltammogram of CN-SBA15 between 0 and −0.2 V. Such a feature is not observed as clearly in the C-SBA15 voltammogram. In any case, both carbon-coated samples show electrocatalytic activity toward oxygen reduction regardless of the nitrogen doping. The polarization curves of both materials in O2-saturated electrolyte (Figure 7b) show two-step behavior similar to that of other mesoporous carbon materials under similar ORR conditions.41 The onset potential at which ORR takes place (calculated by the tangent lines method) for C-SBA15 was ca. −0.23 V, which is shifted to 0.01 V when the carbon layer is doped with nitrogen. These values are similar to those previously reported for carbon nanofibers41 and higher than for N-doped graphene.39,43 The number of electrons transferred during ORR (n) was calculated from the Koutecky−Levich plots (Figures S6 and S7 and the calculation procedure in Supporting Information).

The number of electrons involved in the redox reactions due to the N-containing surface groups (ne) is roughly calculated from the difference CV pattern in Figure 5c as 4.4 × 1017 e−/ mg of carbon. On the other hand, the XPS data gives the number of pyridinic N atoms (7.1 × 1017 atoms/mg of carbon). Considering that the redox reactions could take place only on the carbon surface exposed to the electrolyte and the average number of carbon layers on the silica surface of SBA-15 is estimated to be 1.2, the number of pyridinic N atoms in the outer layer (npyr) would be overall 5.9 × 1017 atoms/mg of carbon. (See the full details of the calculations in the Supporting Information.) The values of ne and npyr are very similar, which strongly supports the idea that pyridinic N atoms are responsible for the observed pseudocapacitance that is induced by one-electron redox processes through pyridinic N atoms. 3.3.4. Rate Performance. Figure 6a shows the change in the specific capacitances, calculated from the galvanostatic charge/ discharge cycling, with the current density in the acidic electrolyte. The capacitance at 5 mA g−1 takes values of 0.171 and 0.190 F m−2 for the nondoped and doped coated samples, respectively, which are similar to the values shown by conventional activated carbons used as an active material (0.06−0.16 F m−216). The capacitance is always higher in the case of the nitrogen-doped material, while for the nondoped material the capacitance rapidly decreases when the current intensity increases. The capacitance retention values (calculated as the ratio between the capacitances at 5 and 1000 mA g−1) are 78 and 56% for CN-SBA15 and C-SBA15, respectively. Nyquist plots measured in the acidic electrolyte are shown in Figure 6b, where the equivalent series resistances of the two electrodes obtained at the highest frequency (100 kHz) are very similar (∼0.13 Ω), indicating that the small difference in the intrinsic electrical conductivity (Figure 4) does not influence the series resistance of the present capacitor cell. However, there is a large difference in the semicircular feature of the Nyquist plots at high frequencies between C-SBA15 and CN-SBA15. Since the two samples have almost the same pore structure including small voids among particles, the different radii of the semicircles in Figure 6b can be ascribed solely to the difference in the pseudotransfer resistance (surface charge transfer). The smaller radius in CN-SBA15 compared to that in C-SBA15 suggests that the charge resistance caused by the pseudocapacitance induced by the nitrogen functionalities is lower than that by the quinone/hydroquinone redox couple. This may be the reason for the better rate performance of CN-SBA15 (Figure 6a). 3.3.5. Electrocatalytic Oxygen Reduction in Neutral Media. Platinum is used in the electrodes of the fuel cells to G

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Langmuir Thus, n takes a value of 2.0 for C-SBA15 at −0.6 V, meaning that the oxygen reduction follows a two-electron pathway. The number of electrons is increased to 2.9 for CN-SBA15, suggesting that the process is a combination of two-electron and four-electron pathways. As well as in the case of an electrochemical capacitor, we can conclude that the observed better performance of CN-SBA15 toward oxygen reduction is exclusively due to the presence of nitrogen on the carbon layer, which provides not only a higher onset potential but also a greater number of transferred electrons.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.C.-M. acknowledges the receipt of a Clarin postdoctoral contract from the Principado de Asturias government (Spain). We would like to thank Dr. T. Iijima of Nippon Steel & Sumitomo Metal for his helpful suggestion for our study. This work is partially supported by JSPS KAKENHI grant numbers 20651029 and 26600020.

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4. CONCLUSIONS The silylation of the silica surface was proven to be effective in increasing the catalytic activity toward pyrolytic carbon deposition from acetonitrile, and the catalysis makes it possible to perform acetonitrile CVD at as low a temperature as 800 °C in a controlled manner. As a result, not only the outer surface but also the pore walls of mesoporous silica SBA-15 particles were successfully coated with a very thin layer of N-doped carbon (about 0.4 nm) without losing the characteristic mesopore structure derived from SBA-15. It is demonstrated that N-doped carbon-coated silica becomes very useful as a model material for studying the role of nitrogen in electrochemical performance, when compared to the nondoped one prepared from the same SBA-15 using acetylene CVD. This is because both materials are practically the same in terms of the pore structure, particle size, and morphology with the exception of N-doping. Thus, any difference in the performance can undoubtedly be ascribed to the presence of nitrogen. In the performance of electrochemical capacitors, the single contribution of N-functionalities was able to be extracted. The results suggest that the pseudocapacitance in the acidic electrolyte (1 M H2SO4) comes from one-electron redox reactions of protons with pyridinic N surface functional groups having a wide variety of redox potentials. To the best of our knowledge, this is the first study to analyze quantitatively such a direct connection, all thanks to the outstanding features of the carbon-coated mesoporous silica samples as model materials. It was also shown that these model materials reveal the sole effect of nitrogen on the response of carbon materials toward the electrocatalytic reduction of oxygen in a neutral medium (phosphate buffer pH 6.86). The onset potential at which the oxygen reduction takes place is shifted to higher values when nitrogen is present in the carbon layer.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03667. Nitrogen adsorption isotherms at −196 °C and pore size distributions of carbon-coated SBA-15 with different CVD times, SEM micrographs, cyclic voltammograms in 1 M H2SO4, water vapor adsorption isotherms, linear sweep voltammograms, and Koutecky−Levich plots in O2-saturated PBS solution at pH 6.86 for carbon-coated samples (PDF)

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DOI: 10.1021/acs.langmuir.5b03667 Langmuir XXXX, XXX, XXX−XXX