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Effect of Heteroatoms in Ordered Microporous Carbons on Their Electrochemical Capacitance Hiroyuki Itoi, Hirotomo Nishihara, and Takashi Kyotani Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02667 • Publication Date (Web): 28 Oct 2016 Downloaded from http://pubs.acs.org on November 2, 2016

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Effect of Heteroatoms in Ordered Microporous Carbons on Their Electrochemical Capacitance Hiroyuki Itoi,*,† Hirotomo Nishihara,*,ǂ,§ and Takashi Kyotani ǂ †

Department of Applied Chemistry, Aichi Institute of Technology, Yachigusa 1247, Yakusa-

cho, Toyota, 470-0392, Japan. ǂ

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-

8577, Japan. §

PRESTO, the Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi 332-

0012, Japan. KEYWORDS Electrochemical capacitor, Zeolite-templated carbon, Boron-doping, Nitrogen-doping

ABSTRACT

Micropores play a more important role in enhancing the electrochemical capacitance than mesopores and macropores; therefore, the effect of heteroatom doping into micropores on the electrochemical behavior is interesting. However, heteroatom doping into porous carbon materials would potentially change their pore structures and pore sizes, which also affect their electrochemical capacitive behaviors. To gain insight into the intrinsic effects of heteroatoms on

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the electrochemical capacitive behaviors, zeolite-templated carbon (ZTC) may be the most suitable candidate. ZTC is an ordered microporous carbon with a uniform micropore size of 1.2 nm, a high surface area, and a large micropore volume. In this work, a series of ZTCs containing oxygen, nitrogen, or boron as heteroatoms, with an ordered pore structure and the same pore size, are prepared. By examining their electrochemical capacitive behaviors in an organic electrolyte, the effect of heteroatom doping can be isolated and discussed without considering the effects of pore structure and pore size. Acid anhydride groups are found to generate pseudocapacitance in two potential ranges, −1.0 to −0.3 V (vs. Ag/AgClO4) and −0.2 to 0.4 V. B is introduced into the ZTC framework solely as –B(OH)2, which is found to be an electrochemically inert species. N is introduced as pyridine (3.0%), pyridone/pyrrole (23.8%), quaternary (66.6%), and oxidized N (6.6%), and these species exhibit noticeable pseudocapacitance in the microporous carbon.

1. Introduction

Electrochemical capacitors (ECs) are rechargeable energy storage devices with higher power density and longer cycling life than secondary batteries.1−3 Although the energy density of ECs is limited to ca. 1/10 to 1/100 of that of secondary batteries, ECs are considered to be a key energy storage device for future electrical appliances and electric vehicles because of the fascinating advantages mentioned above and an expectation of further improvement in their energy density. ECs store charge on the basis of two major mechanisms: the formation of an electric doublelayer and fast redox reactions, which occur mainly on the surfaces of the active materials, i.e., pseudocapacitance. To enhance the former, a high specific surface area and appropriate pore size are in principle required.1,4 At the same time, the conductivity of the active materials should be sufficiently high, and the pore structure (i.e., the size, connectivity, and alignment) needs to be


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optimized to achieve high power density.5 Thus, many nanostructured carbons have been examined as electrode materials. However, enhancement of the electric double-layer capacitance cannot directly yield a higher energy density. Hence, it is necessary to develop pseudocapacitance.6 Typical pseudocapacitive materials are metal oxides7,8 and conducting polymers.9,10 Most of these materials have the great advantage of high electrochemical capacitance, but they also have the drawbacks of limited rate capability and poor cycling life. Pseudocapacitance can also be achieved via heteroatom doping by introducing functional groups into porous carbon materials or replacing carbon atoms in a graphene sheet with heteroatoms. The introduction of functional groups containing oxygen,11−14 nitrogen,15−22 or boron,23−26 or substitution with N or B24,27 has been extensively studied and found to enhance the conductivity and/or wettability of the carbon framework, which contributes to improved performance. Although there are many published studies on heteroatom doping for better performance in ECs, the intrinsic effect of heteroatom doping on the electrochemical capacitance is in dispute because the doping would potentially change the porous properties of the carbon matrices at the same time. In addition, heteroatoms are generally introduced as diverse chemical species, and it is difficult to observe the effect of a specific functional group.28,29 Another problem with conventional pseudocapacitance research is that most materials have been examined using aqueous electrolytes, which have a limited potential range (~1 V) compared to organic electrolytes (~3 V), resulting in poor energy density.5 Therefore, investigations into the effect of heteroatom doping alone would ideally use heteroatom-doped carbons with the same pore structure. In light of this background, we have demonstrated the precise effect of N or B doping into mesopores on the electrochemical capacitance in both aqueous and organic electrolytes by using straight mesopores with a uniform pore size of ca. 16 nm in a carbon-coated anodic


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aluminum oxide film.24 The mesoporous materials were free from ultramicropores, micropores, and macropores; hence, the effect of heteroatom doping into mesopores was isolated and discussed successfully. Taking a similar approach, we recently demonstrated that the use of Ndoped and undoped carbon-coated mesoporous silica samples provides a quantitative understanding of the pseudocapacitance induced by N species on the carbon surface in an acidic aqueous electrolyte.30

In this work, we demonstrate O, N, and B doping into micropores and focus on an organic electrolyte to discuss the effect of heteroatoms in micropores. The behavior of ions and solvent molecules in micropores differs from that in mesopores in terms of the motion, arrangement, solvation, and interaction with the pore surface.31−33 Moreover, micropores are much more effective for enhancement of the capacitance than mesopores and macropores;34 therefore, the effect of introducing heteroatoms into micropores is of interest. Zeolite-templated carbon (ZTC) was chosen as the model microporous carbon used in this study, as it is the best candidate for examining the effect of heteroatom doping. ZTC is an ordered microporous carbon that is prepared as a negative replica of the zeolite template.35 It is characterized by structural regularity, an orderly array of interconnected micropores, and a uniform micropore size (1.2 nm). Its framework consists of crosslinked single-layer nanographenes without stacking,24 and ZTC is thus free of any additional types of pores such as ultramicropores and mesopores. In addition, its framework allows selective introduction of functional groups.36,37 By taking advantage of the ordered microstructure and molecular-level structural controllability, we focus on the effect of acid anhydride and –B(OH)2 groups for O and B species, respectively. Moreover, ZTC doped with a variety of N species is also prepared, and the electrochemical behavior of N-doped ZTC is investigated.


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2. Experimental 2.1. Sample preparation ZTC was prepared by the method described elsewhere.34 Briefly, zeolite Y (Na form, SiO2/Al2O3 = 5.6, obtained from Tosoh Co., Ltd.) was impregnated with furfuryl alcohol (FA), and the mixture was heated at 150 °C under N2 flow for 12 h to polymerize the FA. The resulting composite of polyfurfuryl alcohol (PFA) and zeolite was heated to 700 °C under N2 flow to carbonize the PFA, and additional carbon was introduced into the zeolite at this temperature by chemical vapor deposition (CVD) with propylene (7.0 vol % in N2) for 2 h. After subsequent heat treatment under N2 flow at 900 °C for 3 h, the zeolite was removed by HF under stirring for 5 h. Finally, ZTC was obtained after drying at 150 °C under vacuum for 6 h. The ZTC thus obtained contains a large amount of oxygen (7.8 wt %), which is derived from several types of oxygen functional groups.38 Heat treatment at 400 °C for 1 h under N2 flow partially removes only acid anhydride groups.38 This temperature was also selected to ensure thermal stability of the ZTC framework, which tends to shrink above 400 °C. The resulting sample is denoted as ZTC-h. To introduce –B(OH)2 groups into ZTC, we have developed the new method shown in Figure 1. ZTC is first impregnated with dimethylamine borane (DMAB) (5.0 wt % in odichlorobenzene) under vacuum at room temperature. After the sample is separated by filtration, it is heat-treated at 120 °C for 6 h and then at 400 °C for 2 h under N2 flow. When it is first heated to 120 °C, the DMAB embedded in the ZTC decomposes (its decomposition temperature is ca. 75 °C) to release reactive BH3, which reacts with the edge sites of ZTC through the hydroboration reaction (Figure 1a to b),39 followed by hydrogen elimination during the subsequent heat treatment at 400 °C (Figure 1b to c). The resulting B–H bonds (Figure 1c) are easily hydrolyzed by H2O upon air exposure (Figure 1c to d).40,41 Thus, ZTC can be selectively


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functionalized by –B(OH)2 groups. The resulting sample is denoted as ZTC-B. Nitrogen-doped ZTC (N-ZTC) was also prepared by the method reported elsewhere.42 Briefly, acetonitrile CVD (4.2 vol % in N2) was performed on the PFA/zeolite composite mentioned above at 850 °C for 2 h, followed by heat treatment at 900 °C for 1 h. Then, zeolite was removed using the same procedure as that for ZTC.

Figure 1. Possible reaction scheme of boron doping into ZTC. (a) Hydroboration of the edge sites of ZTC with borane released from DMAB. (b) Hydrogen elimination at 400 °C. (c) Hydrolysis of two B–H bonds by two H2O molecules upon air exposure. (d) Final structure of doped boron (BCO2).

2.2. Characterizations The elemental composition of C, H, and N was determined by conventional elemental analysis. Inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo Fisher Scientific IRIS Advantage DUO) was performed to determine the elemental concentration of boron in BZTC. The oxygen content was calculated as the remainder. X-ray diffraction (XRD) patterns were collected using a Shimadzu XRD-6100 analyzer with Cu Kα radiation (λ = 1.5418 Å). Nitrogen adsorption/desorption isotherms (at −196 °C) were measured using a BELSORP-MAX (Microtrac BEL Co.) apparatus. The specific surface area (SBET) was calculated on the basis of


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Brunauer–Emmett–Teller (BET) method using an adsorption isotherm at p/p0 = 0.01–0.05 because all the samples are microporous.43 The total pore volume (Vtotal) was calculated using the amount adsorbed at p/p0 = 0.96. The micropore volume (Vmicro) was determined by the Dubinin– Radushkevich method. X-ray photoelectron spectroscopy (XPS) was performed on a KRATOS ESCA-3400 instrument using Mg Kα radiation (1253.6 eV). The binding energy was calibrated by the position of the C 1s peak (284.6 eV). 2.3. Electrochemical measurements For the electrochemical analysis, a three-electrode cell was used. As an organic electrolyte, 1 M tetraethylammonium tetrafluoroborate in propylene carbonate (TEABF4/PC) was used. The active material was mixed with carbon black (Denka black, Denki Kagaku Kogyo Kabushiki Kaisha) and poly(tetrafluoroethylene) (PTFE 6-J, Du Pont-Mitsui Fluorochemicals Company, Ltd.) at a weight ratio of 18:1:1 to form an electrode sheet. About 5 mg of the sheet was sandwiched by Pt mesh (80 mesh) and pressed at 30 MPa to form a working electrode. A counter electrode was prepared using an activated carbon fiber, A20 (Unitika Ltd., Japan), by the same process as that for the working electrodes. Electrochemical measurements were made on a multichannel potentiostat/galvanostat instrument (VMP3, Bio-Logic, France) at 25 °C. To avoid electrochemical reaction of ZTC in high and low potential ranges,44 all the electrochemical measurements were conducted in a potential range of −1.5 to 0.5 V (vs. Ag/AgClO4). Cyclic voltammetry (CV) was performed at a scan rate of 1 mV s−1 for four cycles. Galvanostatic charge/discharge cycling (GC) was recorded at current densities ranging from 50 to 2000 mA g−1. The gravimetric capacitance was calculated from the cation desorption process in the discharge curve, i.e., −1.5 V to the open-circuit potential (OCP).45 Impedance spectroscopy was performed at −0.4 V over a frequency range from 100 kHz to 10 mHz with a peak amplitude of 10 mV.


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3. Results and discussion 3.1. Characterization of ZTCs The elemental compositions of the samples are summarized in Table 1,which shows that ZTC originally contains a large amount of O (7.8 wt %). Our previous study using temperatureprogrammed desorption and Fourier transform infrared (FT-IR) analyses has confirmed that the O in ZTC is derived from oxygen functional groups of acid anhydride (15%), ether (66%), hydroxyl (15%), and carbonyl (4%).38 Moreover, we have found that these functional groups decompose at ~400, ~600, ~780, and ~1000 °C, respectively.38 Thus, the present heat treatment at 400 °C for 1 h should remove most of the acid anhydride. As a result of the heat treatment, the oxygen content was reduced to 6.6 wt % in ZTC-h. Hence, the effect of acid anhydride can be discussed by comparing the electrochemical properties of ZTC and ZTC-h. B-ZTC has 0.8 wt % of B. According to the results reported by Wang et al.,46 the capacitance is noticeably affected by only a small amount of B doping, i.e., 0.16 wt %, as determined by ICP-OES, even in the presence of a large amount of O (9.2 atom %). Therefore, the present doping amount is sufficiently large that we can discuss the effect of B doping. The present B-ZTC also contains a large amount of O, which is ascribed to –B(OH)2 and also to some extra oxygen functional groups. However, as shown below, the extra oxygen is electrochemically inert, and its effect is negligible. N-ZTC contains 5.8 wt % of N, which is sufficient to reveal the effect of N doping.24 At the same time, N-ZTC contains more oxygen (9.1 wt %) than ZTC does (7.8 wt %). The effect of oxygen is discussed later.


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Table 1. Elemental compositions and textural properties of the samples.

Sample ZTC ZTC-h B-ZTC N-ZTC

C 90.3 91.6 86.7 83.9

Elemental compositiona (wt %) H N O (diff.) 1.9 0 7.8 1.8 0 6.6 1.9 0.2 10.4 1.2 5.8 9.1

B – – 0.8 –

SBET (m2 g−1) 3690 3590 3280 2780

Vtotal (cm3 g−1) 1.68 1.65 1.50 1.31

Vmicro (cm3 g−1) 1.56 1.50 1.32 1.21

a

Elemental composition determined by conventional elemental analysis (C, H, N) and ICPOES (B). Oxygen content was calculated as the remainder.

Figure 2 shows the XRD patterns of the four types of ZTC prepared in this work. The XRD pattern of ZTC is characterized by the presence of a sharp peak at 2θ = 6.4° due to the ordered microporous framework (pore size is ca. 1.2 nm) derived from the ordered structure of zeolite Y.35 In addition, there is almost no carbon (002) peak at 2θ = 26° because the framework of ZTC comprises interconnected nanographenes free from stacking of graphene sheets.38 The XRD patterns of the other three ZTCs also show these features. Further evidence for the absence of stacked carbon outside the ZTC particles is available in the Supporting Information (Figures S1 and S2). Thus, the four types of ZTC have basically the same ordered micropore structure. Figure 3a shows the nitrogen adsorption/desorption isotherms of the ZTCs. All the ZTCs display type I isotherms,47 indicating that they are microporous carbons. The uptake near p/p0 = 1 corresponds to capillary condensation of nitrogen in interparticle spaces between the ZTC fine particles (particle size is ca. 200 nm). The specific surface areas and pore volumes are summarized in Table 1. All the ZTCs have large micropore volumes as well as large specific surface areas. The pore size distributions (PSDs) calculated by density functional theory (DFT) are shown in Figure 3b. All the ZTCs display similar narrow PSDs around 1.2 nm, reflecting their ordered microporous structure. Their mesopore volumes, which can be calculated by


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subtracting the micropore volume from the total pore volume (Vtotal − Vmicro), are negligible. Moreover, we also confirmed the absence of ultramicropores in all of the ZTCs from their αs plots (Figure S3). Therefore they have the same pore size. Although N-ZTC exhibits a low surface area and a small pore volume in comparison with the other ZTCs, that is not an intrinsic problem in this study (for details, see the Supporting Information). Thus, Figures 2 and 3 prove that ZTC, ZTC-h, B-ZTC, and N-ZTC have similarly ordered microporous structure and that they can therefore be used as model microporous materials to investigate the effects of O, B, and N doping.

Figure 2. XRD patterns of ZTCs.


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Figure 3. (a) Nitrogen adsorption/desorption isotherms of ZTCs measured at −196 °C and (b) the corresponding PSDs calculated by DFT method.

XPS has been more reliable and useful for determining the type of B- and N-containing functional groups in carbon materials than other spectroscopy techniques such as FT-IR and Raman spectroscopy. Additionally, conventional organic elemental analysis and ICP-OES provide the precise bulk amounts of nitrogen and boron, respectively. Hence, B-ZTC and N-ZTC are analyzed by XPS and bulk elemental analyses in this work. Figure 4a shows the B 1s XPS spectrum of B-ZTC. The chemical form of boron in B-ZTC is clearly identified as BCO2 (192.1 eV).48 In previous works on the effect of B doping on the capacitance,23,24,26 boron was doped as a variety of species including BC3, BC2O, BCO2, and BO3. Therefore, it was unclear which one is effective for capacitance enhancement. Hence, B-ZTC provides information about the effect of doping with the BCO2 species. Because the final process in B-ZTC preparation was heat treatment at 400 °C, acid anhydride must be removed from B-ZTC, as in the preparation of ZTCh. Figure 4b shows the N 1s XPS spectrum of N-ZTC. The N 1s spectrum is divided into four components, pyridine (398.0 eV), pyridone/pyrrole (400.9 eV), quaternary (401.3 eV), and


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oxidized N (403.7 eV).49 The ratios of these components are 3.0%, 23.8%, 66.6%, and 6.6%, respectively, so the main component in N-ZTC is quaternary N. Many research groups have reported the electrochemical capacitor performance of various types of N-doped porous carbons.15,50−53 However, there are few publications on N-doped ordered microporous carbons, i.e., N-doped ZTCs. Ania et al. reported the electrochemical capacitance of N-doped ZTC with a surface area of 1680 m2 g−1 measured in 1 M H2SO4.52 On the other hand, Portet et al. reported the electrochemical capacitance of N-doped ZTC with a surface area of ~1990 m2 g−1 measured in Et4NBF4/acetonitrile.53 Although these works also used zeolite Y as a template, the surface areas of these N-doped ZTCs are lower than that of N-ZTC in this work (2780 m2 g−1) because of the low structural regularity and the presence of stacked carbon outside of the ZTC particles.52,53 The low structural regularity causes imperfect replication of the zeolite template owing to partial collapse and/or aggregation of the carbon framework, resulting in the formation of additional types of pores such as ultramicropores and mesopores. Compared to these previously reported materials, the structure of the present N-ZTC is much closer to that of ZTC. Consequently, the present N-ZTC can provide more precise data regarding the effect of N doping in micropores on the capacitance.


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Figure 4. X-ray photoelectron spectra: (a) B 1s spectrum of B-ZTC and (b) N 1s spectrum of NZTC. Thick and thin lines correspond to measured and deconvoluted spectra, respectively. Dotted line in (b) corresponds to the sum of all the deconvoluted spectra.

3.2. Effect of heteroatoms on the electrochemical capacitance Figure 5 shows the CV patterns (fourth scans) of the ZTCs measured in 1 M TEABF4/PC. In this potential range, ZTC is electrochemically stable,44 and there are almost no differences among the CV patterns at different cycles for all the samples. We have previously reported that ZTC is electrochemically oxidized in an organic electrolyte above 0.5 V, and electrochemically active quinone and furan groups are introduced as a result.44 These groups yield broad peaks around −0.6 and 0.4 V, respectively. In this work, ZTC is not electrochemically oxidized, so there are no such additionally introduced groups to affect the results. The CV pattern of ZTC is characterized by a current drop around −0.3 to 0.3 V, near the OCP (−0.2 V). We previously ascribed it to semiconductive segments in the ZTC framework consisting of single and curved nanographene.54 It is well known that graphene nanoribbons


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could have a band gap when their width is less than a few nanometers.55 ZTC has a very narrow nanographene framework, and its narrowness could make the framework semiconductive. The effect of the semiconductive framework on the CV patterns is typically seen in single-walled carbon nanotubes (SWCNTs), which are generally a mixture of metallic and semiconductive nanotubes with diverse chiral vectors.56,57 SWCNTs show so-called “butterfly” patterns in which the current increases and decreases linearly along with the potential on both sides of the OCP owing to electrochemical doping.57 The CV pattern of ZTC shows broad peaks at −1.0 to −0.3 V and −0.2 to 0.4 V and differs slightly from such typical butterfly patterns. The first major difference between ZTC and SWCNTs is their framework structures, which consist of crosslinked nanographenes and cylindrical graphenes, respectively. The second major difference has to do with the surface functional groups. ZTC has many oxygen functional groups, whereas SWCNTs rarely have them. By comparing the CV patterns of ZTC and ZTC-h, it is possible to see the effect of acid anhydride directly. In ZTC-h, the broad peaks are weakened, and its CV pattern becomes closer to the typical butterfly shape seen for SWCNTs. Hence, the broad peaks in ZTC are ascribed to the pseudocapacitance of acid anhydride. As described above, quinone and furan groups also produce broad peaks around −0.6 and 0.4 V, respectively. Therefore, it is necessary to consider the possibility that these electrochemically active groups are present in the pristine ZTC. However, we can rationally rule out this possibility because quinone and furan groups are not decomposed below 400 °C.44 The disappearance of the pseudocapacitance of the pristine ZTC in ZTC-h is thus concluded to be due to the absence of acid anhydrides. This also means that pristine ZTC rarely contains electrochemically active quinone and furan groups. Reversible redox reactions of 3,4,9,10-perylene-tetracarboxylic acid-dianhydride molecules have been reported in an aprotic organic electrolyte solution,58 whereas redox reactions of acid


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anhydrides in carbon materials have rarely been reported. Figure 5 indicates that the pseudocapacitance derived from acid anhydride is present in two different potential regions. Such a two-step reaction is known to occur in acid anhydride molecules.58 The capacitances of the samples are listed in Table 2. Because of the pseudocapacitance of acid anhydride, ZTC shows higher gravimetric (Cg) and normalized (Cs) capacitances than ZTC-h.

Figure 5. Cyclic voltammograms (fourth scans) of ZTCs measured at a scan rate of 1 mV s−1 in 1 M TEABF4/PC at 25 °C. Table 2. Gravimetric and normalized capacitances of the samples

Sample ZTC ZTC-h B-ZTC N-ZTC a

Cg a − (F g 1) 175 151 146 180

Cs b − (µF cm 2) 4.8 4.2 4.4 6.5

Gravimetric capacitance measured at 50 mA g−1.

b

Normalized capacitance calculated as Cs = Cg/SBET.


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B-ZTC shows a butterfly-shaped CV pattern without any apparent peaks. Although B-ZTC contains a relatively large amount of oxygen (Table 1), the oxygen functional groups are thus found to be electrochemically inactive in the potential range in Figure 5. In addition, its capacitance is very close to that of ZTC-h, indicating that B-ZTC, i.e., the –B(OH)2 group (the most probable form of the BCO2 species), does not exhibit pseudocapacitance. Several reports have described capacitance enhancement by B doping.23,24,26 However, the origin of the pseudocapacitance has been unclear because the B-doped carbons generally contain a variety of B species such as BC3, BC2O, BCO2, and BO3. The results obtained in this work make it possible to rule out the contribution of BCO2 groups to the enhanced capacitance. Additionally, BO3 is usually derived from inorganic impurities such as H3BO3 and is not effective for capacitance enhancement. Hence, the present work suggests that BC3 or BC2O species should be responsible for the reported capacitance enhancement in an organic electrolyte.23,24 As for N-ZTC, its voltammogram does not show a significant decrease in the current around the OCP, and its capacitance is remarkably increased (especially Cs). Thus, N doping is found to be very effective for enhancing the capacitance of microporous carbon in the organic electrolyte. Because N-ZTC contains a slightly larger amount of oxygen (Table 1) than ZTC does, the effect of the oxygen should also be taken into account. As demonstrated above, acid anhydrides yield broad peaks at −1.0 to −0.3 V and −0.2 V to 0.4 V. Other redox-active species are quinone and furan groups, which produce broad peaks around −0.6 and 0.4 V, respectively.44 The CV pattern of N-ZTC shows current enhancement around −0.3 to 0.2 V, entirely unlike those of any oxygen functional groups. These results suggest the presence of pseudocapacitance derived from the fast redox reactions of N species in an organic electrolyte.24 Additionally, there is another possible reason, based on the increase in the mobile charge carriers, for this


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capacitance enhancement.24,59 Derradji et al. reported that N doping decreases the carbon electrical resistance, and quaternary N in particular is effective up to a N content of 12 atom %.60 Zhao et al. characterized individual N atoms using scanning tunneling microscopy and various spectroscopies and reported that doping of quaternary N into a graphene monolayer increases the mobile charge carriers.61 Wiggins-Camacho et al. reported enhanced capacitance in N-doped multi-walled CNTs due to an increase in the density of mobile charge carriers and observed the correlation between the amount of pyridinic N and the density of mobile charge carriers.62 To evaluate the contribution of the pseudocapacitance, the Nyquist plots and GC of ZTCs were examined, as shown in Figure 6a and b, respectively. Note that the Nyquist plot of ZTC is independent of the potential over the potential range in Figure 5, and all the Nyquist plots shown in Figure 6a were collected at −0.4 V (for details, see the Supporting Information). The size of the semicircle in Figure 6a, expressed as Re, corresponds to the sum of the internal cell resistance and the charge transfer resistance.34,36 In this study, all of the ZTCs were prepared using the same template, i.e., zeolite Y, and they therefore have the same particle size.54 Accordingly, the electrode densities of the ZTCs are all the same, so the effects of the interparticle resistance and the contact resistance between the electrode sheet and the current corrector can be ignored. Moreover, all of the ZTCs have the same ordered microporous structure, and the ion diffusion resistance inside the micropores can also be considered to be the same. Hence, the difference in Re can be ascribed to the differences in the resistance of the ZTC framework and the charge transfer resistance. In Figure 6a, N-ZTC shows a larger resistance than ZTC-h, although N doping, especially with quaternary N, increases the framework conductivity.24 Thus, the large resistance can be ascribed to the effect of pseudocapacitance. We have reported an increase in the Re value of ZTC after electrochemical functionalization by quinone groups,36 which supports


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our explanation of the Nyquist plots observed in this study. Accordingly, the capacitance enhancement by N doping can be ascribed mainly to the occurrence of pseudocapacitance, and this result is the same as that in mesopores.24 Because N-ZTC does not show defined peaks in Figure 5, there should be several complex redox reactions derived from different N species doped in N-ZTC (Figure 4b), as also observed in mesopores.24 The Re value of ZTC is as high as that of N-ZTC because the pseudocapacitance of acid anhydride is present. The Re value of BZTC is close to that of ZTC-h, and this is consistent with the absence of pseudocapacitance in Figure 5. On the other hand, in Figure 6b, all the samples show similar good rate performance regardless of the presence of pseudocapacitance. We previously reported that quinonefunctionalized ZTC shows a very large pseudocapacitance in 1 M H2SO4 electrolyte, with a good rate performance that is almost at the same level as that of mesoporous activated carbon.36 Although the reason for such an excellent rate performance involving pseudocapacitance is still unclear, this might be a specific feature of micropores.

Figure 6. (a) Nyquist plots of ZTCs measured over a frequency range from 100 kHz to 10 mHz with a peak amplitude of 10 mV. (b) Dependence of capacitance on the current density. Both analyses were conducted in 1 M TEABF4/PC at 25 °C.


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Thus, the effect of heteroatom doping into ZTC in the organic electrolyte is summarized as follows. 1) Acid anhydride in microporous carbon generates pseudocapacitance at −1.0 to −0.3 V and −0.2 to 0.4 V (vs. Ag/AgClO4). 2) The –B(OH)2 group does not produce pseudocapacitance. Thus, the BCO2 species, which is usually detected by XPS, is not effective for enhancing the capacitance. BC3 or BC2O is most likely the species responsible for the enhanced capacitance in the organic electrolyte. 3) N doping enhances the capacitance of microporous carbons mainly by providing pseudocapacitance. Its enhancement mechanism is similar to that in mesopores. Generally, highly porous carbon materials have higher surface areas than the theoretical surface area of single graphene (2630 m2 g−1) owing to the contribution of edge planes. Such highly porous carbons contain more or less semiconductive nanosized graphene, and this could be the reason for the decrease in Cs, such as that seen in ZTC-h. It has indeed been reported that a linear relationship between the capacitance and the surface area levels off when the surface area exceeds approximately 1200 m2 g−1.63 According to the results obtained in this work, the intrinsically low normalized capacitance in nanographene-based high-surface-area carbon materials can be remarkably improved by doping with redox-active species, especially N species.

4. Conclusions A series of microporous carbons doped by heteroatoms, with an ordered pore structure and the same pore size, were prepared using ZTC as a model platform. By exploiting the consistent ordered pore structure and unimodal pore size of 1.2 nm, the effects of acid anhydride, –B(OH)2, and N functionalities were investigated in an organic electrolyte. Acid anhydride in micropores


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generates pseudocapacitance at −1.0 to −0.3 V and −0.2 to 0.4 V (vs. Ag/AgClO4). On the other hand, the –B(OH)2 group does not contribute to the electrochemical capacitance, suggesting that BC2O and BC3 are effective species for enhancement of the electrochemical capacitance in an organic electrolyte. N doping most remarkably enhanced the capacitance of microporous carbons, mainly by contributing pseudocapacitance. This study provides insight into the development of electrochemical capacitors by heteroatom doping, especially for materials built on a nanosized graphene framework with a high surface area.

ASSOCIATED CONTENT Supporting Information. TEM images, high resolution αs-plots, and Nyquist plots collected at different potentials. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] *E-mail address: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by JSPS KAKENHI Grant Number 15K21478 and partially supported by the Ministry of Education, Science, Sports and Culture; a Grant-in-Aid for Scientific


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Research (B), 26286020. This research was supported also by Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials. REFERENCES (1) Conway, B. E., Electrochemical Supercapacitors, Scientific Fundamentals and Technological Applications; Kluwer Academic/Plenum Press: New York, 1999. (2) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; Van Schalkwijk, W. Nanostructured materials for advanced energy conversion and storage devices. Nat. Mater. 2005, 4, 366-377. (3) Kotz, R.; Carlen, M. Principles and applications of electrochemical capacitors. Electrochim. Acta 2000, 45, 2483-2498. (4) Chmiola, J.; Yushin, G.; Gogotsi, Y.; Portet, C.; Simon, P.; Taberna, P. L. Anomalous increase in carbon capacitance at pore sizes less than 1 nanometer. Science 2006, 313, 1760-1763. (5) Nishihara, H.; Kyotani, T. Templated Nanocarbons for Energy Storage. Adv. Mater. 2012, 24, 4473-4498. (6) Conway, B. E.; Birss, V.; Wojtowicz, J. The role and utilization of pseudocapacitance for energy storage by supercapacitors. J. Power Sources 1997, 66, 1-14. (7) Lang, X.; Hirata, A.; Fujita, T.; Chen, M. Nanoporous metal/oxide hybrid electrodes for electrochemical supercapacitors. Nat. Nanotechnol. 2011, 6, 232-236. (8) Bao, L.; Zang, J.; Li, X. Flexible Zn2SnO4/MnO2 Core/Shell Nanocable−Carbon Microfiber Hybrid Composites for High-Performance Supercapacitor Electrodes. Nano Lett.2011, 11, 12151220.


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Table of Contents Graphic and Synopsis

Zeolite-templated carbon (ZTC) is an ordered microporous carbon with a uniform micropore size of 1.2 nm, a high surface area, and a large micropore volume. In this work, a series of ZTCs containing oxygen, nitrogen, or boron as heteroatoms, with the same pore structure and pore size, are prepared. By examining their electrochemical capacitive behaviors in an organic electrolyte, the effect of heteroatom doping can be isolated and discussed without considering the effects of pore structure and pore size.


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Effect of Heteroatoms in Ordered Microporous ... - ACS Publications

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