Synthesis of high-concentration B and N co-doped porous carbon polyhedra and their supercapacitive properties

Abstract

Taking phenylboronic acid as a boron source and ZIF-11 as a carbon template, boron and nitrogen co-doped porous carbon polyhedra (BN-PCPs) were prepared through a simple procedure for the first time. The as-prepared BN-PCPs were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and typical electrochemical methods. The results show that the developed BN-PCPs possess 10.68 atom% B and 8.1 atom% N with uniform distribution, and excellent electrochemical capacitive properties with high specific capacitance of 262 F g⁻¹ at 20 mV s⁻¹ in 1.0 M H₂SO₄ aqueous solution and excellent long-term charge–discharge stability (no obvious degradation during 40 000 charge–discharge cycles at 20 A g⁻¹).

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Introduction

Carbon-based materials, especially porous carbon materials, have exhibited superior performance as adsorbents,¹ catalyst supports² and electrode materials for energy devices such as supercapacitors,^3–5 batteries,⁶ and fuel cells.⁷ Metal–organic frameworks (MOFs), as a new subclass of crystalline porous materials with high specific surface area, large pore volume, exceptional chemical and thermal stability, have received great interest in being employed as the templates and precursors to prepare porous carbon materials for various applications.^8–10 Recently, as a classic example of MOFs with nitrogenous organic ligands, exceptionally high thermal stability and chemical robustness, zeolitic imidazolate frameworks (ZIFs), have received wide attention.¹¹ Up to now, two classes of materials derived from ZIFs have been widely studied: (i) N-doped porous carbons synthesized by direct carbonization,^1,12–14 which is widely applied in gas storage, sensing, and supercapacitors, etc. (ii) Transition-metal oxides or hydroxides derived from transition metal-containing ZIFs,^15–17 which is explored in lithium batteries, catalysts and supercapacitors, etc.

On the other hand, it is well-known that introducing heteroatoms (such as N, B, P, etc.) into the porous carbon materials is one of the most effective ways to improve the electronic properties of the carbon-based materials. Many works have demonstrated that N-doped porous carbon-based materials have better electronic properties than the related materials without N-doping.^18,19 This arouses the interest of researchers to prepare the B-doped materials. Boron can enter the carbon lattice by substituting carbon at the trigonal sites²⁰ and acts as electron acceptor due to its three valence electrons, leading to shift the Fermi level to the conducting band and therefore modifying the electronic structure of doped carbon.^21–23 Moreover, more defects and oxygen functional groups can be introduced during the B-doping process,^24,25 which is beneficial to the improvement of the electrochemical properties of carbon-based materials. Boron has been investigated for decades as a substitution in porous carbon,²⁶ graphene^27,28 and carbon nanotubes²⁹ to improve the properties of the related supercapacitor, oxygen reduction, lithium storage and sensing etc. However, not similar as N-doping, B-doping usually has a quite low doping efficiency. Cheng et al. reported B-doped mesoporous carbons with 0.6 atom% B using boric acid as the dopant.²⁶ Taking BCl₃ gas as the B source, Wu et al. also prepared B-doped grapheme with 0.88 atom% B.²⁸ The synthesis of high-concentration B-doped carbon materials still is a challenge. On the other hand, to the best of our knowledge, B-doped porous carbons derived from ZIFs-based materials have not been reported previously.

In this work, taking phenylboronic acid as boron source and ZIF-11 as the carbon template, we successfully prepared high-concentration B and N co-doped porous carbon polyhedra (BN-PCPs) (Scheme 1). The morphology, structure and the chemical states of B and N of the prepared BN-PCPs were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. On the other hand, the electrochemical capacitive properties of the developed BN-PCPs were investigated by typical electrochemical methods.

Scheme 1 Schematic illustration of the fabrication procedure for BN-PCPs and N-PCPs.

Experimental

Reagents

Zinc acetate dihydrate, benzimidazole, toluene, ammonium hydroxide (25 wt%), anhydrous methanol, anhydrous ethanol and phenylboronic acid were purchased from Sinopharm Chemical Reagent Co. Ltd, China. All chemicals are of analytical grade and used without any further purification. All aqueous solutions were prepared with ultrapure water obtained from a Millipore system (>18 MΩ cm).

Preparation of BN-PCPs and N-PCPs

ZIF-11 was synthesized according to the literature.³⁰ Briefly, benzimidazole (2.4 g) was dissolved in anhydrous methanol (60.2 mL) and then 53.1 mL toluene and 15 mL ammonium hydroxide (25 wt%) were added into the solution under continuously stirring at room temperature. After that, zinc acetate dihydrate (1.1 g) was added and the mixture was vigorously stirred for 3 h to complete the crystallization process at room temperature. Finally, the product was collected from the milky colloidal dispersion by centrifugation and washed with adequate anhydrous ethanol, then dried in vacuum at 60 °C overnight.

The as-synthesized ZIF-11 was thermal annealed in the vacuum furnace at 800 °C for 2 h with a heating rate of 5 °C min⁻¹ in N₂ atmosphere. The obtained product was labeled as N-PCPs-800. To prepare BN-PCPs, N-PCPs-800 (500 mg) were well mixed with the phenylboronic acid suspension (1.74 g phenylboronic acid was dispersed in 50 mL anhydrous ethanol by severe ultrasonication for 1 h). Then, the mixture was dried in vacuum at 70 °C to remove anhydrous ethanol and carbonized at 1000 °C for 2 h with a heating rate of 5 °C min⁻¹ under a flow of nitrogen gas. The boron and nitrogen co-doped porous carbon polyhedra were obtained and labeled as BN-PCPs. For the preparation of N-PCPs, the obtained N-PCPs-800 were further heated at 1000 °C for 2 h.

Characterization

The morphology and structure of the prepared materials were characterized by scanning electron microscopy (SEM, Hitachi S-4800, Japan), transmission electron microscopy (JEM-3010F, JEOL, Japan) and Raman spectroscopy (Labram-010, JY, France). Powder X-ray diffraction (PXRD) was performed on an X-ray diffractometer (XRD, D/MAX-RA, Japan). The chemical states of B and N in BN-PCPs were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific K-Alpha 1063, UK) using Al Kα radiation. Nitrogen adsorption–desorption isotherms and Brunauer–Emmett–Teller (BET) surface area of the material were investigated by Quantachrome Autosorb-1C/TCD Automatic Chemisorption & Physisorption Analyzer.

Electrochemical measurements

Both the cyclic voltammetry (CV) and galvanostatic charge–discharge measurements were performed on a CHI 1140A electrochemical workstation (Chenhua Instrument Company of Shanghai, China) at room temperature. A conventional three-electrode configuration was utilized with a modified GC electrode (diameter, 5 mm) as the working electrode, a platinum wire as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. For the preparation of the working electrode, 10 μL BN-PCPs suspension (2 mg mL⁻¹ in deionized water) was transferred onto the surface of the GC electrode. Then, the electrode was dried at room temperature and finally coated with 5 μL Nafion solution (0.5 wt% in ethanol). For comparison, the N-PCPs modified working electrode was also prepared according to the same procedure.

Electrochemical impedance spectroscopy (EIS) was carried out on CHI 660D electrochemical workstation (Chenhua Instrument Company of Shanghai, China) with an AC signal amplitude of 5 mV in the frequency range of 10 mHz to 100 kHz. 1.0 M H₂SO₄ aqueous solution was used as the electrolyte throughout all electrochemical measurements.

The specific capacitance was calculated from the CV curves by eqn (1), in which C is the specific gravimetric capacitance (F g⁻¹), I stands for the current (A), V is the potential (V), s means the scan rate (V s⁻¹), ΔV is the potential window (V) and m means the mass of the active electrode material.

For the galvanostatic charge–discharge curves, the discharge specific capacitance of the electrode could be calculated by using the eqn (2),

where I, Δt, ΔV and m are the discharge current (A), discharge time (s), potential window (V) and the mass of the electro-active material (g), respectively.

Power density (P) and energy density (E) were also estimated using the following equations:

Results and discussions

Structural characterization

The SEM and TEM images of ZIF-11, N-PCPs and BN-PCPs were shown in Fig. 1. The original ZIF-11 (Fig. 1A) shows the typical rhombic dodecahedron morphology, which is consistent with that observed in the reported literature.³¹ From Fig. 1B, it can be observed that the N-PCPs prepared by direct carbonization of the ZIF-11 crystals at 1000 °C have average size of 3 μm and retain their original shapes and size. However, with the B-doping into the N-PCPs, the size of the BN-PCPs decreases to about 1 μm. From the inset image in Fig. 1C, it can be easily observed that lots of flocculent substances exist on the surface of BN-PCPs. Furthermore, from the high resolution transmission electron microscopy (HRTEM) images and SAED patterns of BN-PCPs (Fig. 1D) and N-PCPs (Fig. 1E), it can be obtained that the out layers of BN-PCPs and N-PCPs are amorphous carbon species, which is also confirmed by the XRD results shown in Fig. 2. Also, the element mapping images of BN-PCPs (Fig. 1F–I) show that the doped B and N elements are uniformly distributed in BN-PCPs. These imply that B has been doped into N-PCPs and B, N-co-doped carbon polyhedra have been prepared successfully.

Fig. 1 SEM and TEM images of ZIF-11 (A), N-PCPs (B) and BN-PCPs (C). HRTEM images of BN-PCPs (D) and N-PCPs (E). Inset plots in (D) and (E) is the corresponding SAED patterns of BN-PCPs and N-PCPs. Element mapping images of boron (F), nitrogen (G), carbon (H) and oxygen (I) in BN-PCPs.

Fig. 2 XRD patterns of ZIF-11 (A), N-PCPs (B) and BN-PCPs (C).

Fig. 2 shows the X-ray diffraction (XRD) patterns of ZIF-11, N-PCPs and BN-PCPs. From Fig. 2A, the structure of the original ZIF-11 prepared in this work matches well with that reported previously.³¹ For N-PCPs and BN-PCPs, two broad and weak diffraction peaks at 2θ values of around 25° and 44° can be observed from the wide-angle XRD patterns (Fig. 2B and C), which correspond to (002) and (101) diffractions of turbostratic carbon, respectively.²¹ The weak (002) peaks indicate that the obtained N-PCPs and BN-PCPs have a low graphitic crystallinity.³² Furthermore, no diffraction peaks of ZIF-11 are found in Fig. 2B and C, meaning the complete decomposition and carbonization of ZIF-11 during the carbonization process. Because of the high temperature at 1000 °C during the carbonization process which leads to the reduction of ZnO and the evaporation of the Zn metal (boiling point, 904 °C),³³ no peaks related to Zn or ZnO are observed in Fig. 2B and C.

Raman spectra (Fig. 3) show the D-bands and G-bands of the BN-PCPs and N-PCPs, providing some additional information of the carbon lattice. Both the two samples display two bands at ca. 1330 cm⁻¹ (D-band) and ca. 1590 cm⁻¹ (G-band), which are related to the defects and hexagonal graphene plane, respectively. It is noted that the I_D/I_G of BN-PCPs (1.23) is larger than that of N-PCPs (1.17), indicating that more defects and imperfections are introduced into the porous carbon lattice by B-doping.

Fig. 3 Raman spectra of N-PCPs (A) and BN-PCPs (B).

In order to further explore the porous textures of BN-PCPs and N-PCPs, the nitrogen adsorption–desorption isotherms were measured, and the corresponding adsorption–desorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distribution curves are shown in Fig. 4. For both BN-PCPs and N-PCPs, type IV isotherms with an unclosed hysteresis loop can be observed in Fig. 4A, implying that disorder micropores exist in the materials.^34,35 This is further supported by the results of micropore-surface area showed in Table S1 (see ESI section†). On the other hand, the BET surface area, total pore volume and average pore size of the N-PCPs are measured to be about 612 m² g⁻¹, 0.37 cm³ g⁻¹ and 2.4 nm, respectively (Table S1, see ESI section†). However, with high-concentration B-doping, the specific surface area decreases to 57 m² g⁻¹, and the total pore volume and average pore size are 0.10 cm³ g⁻¹ and 7.2 nm, respectively. Furthermore, it is noted that the S_micro value (provided by micropores) of N-PCPs is about 537 m² g⁻¹ and much larger than that of BN-PCPs (5 m² g⁻¹). This implies that lots of micropores in N-PCPs disappear during B-doping process.

Fig. 4 (A) Nitrogen adsorption–desorption isotherms and (B) pore size distribution of BN-PCPs and N-PCPs.

The prepared N-PCPs and BN-PCPs were further characterized by XPS and the corresponding results are shown in Fig. 5. It is noted that BN-PCPs shows the C, B and N signals, implying the successfully doping of B and N into the porous carbons as expected. The XPS survey gives the high contents of B (10.68 atom%) and N (8.10 atom%) in BN-PCPs and 5.05 atom% N in N-PCPs (Table S1, see ESI section†). The B-doping in the BN-PCPs not only introduces the B heteroatoms but also increases the N content in the porous carbons by the formation of B–N bond. Interestingly, the concentration (10.68 atom%) of the doped B in BN-PCPs is the highest among all the B-doped porous carbons prepared by various methods.³⁶

Fig. 5 The survey scan of XPS on N-PCPs (A) and BN-PCPs (B), and high resolution of C 1s XPS peaks (C), B 1s XPS peaks (D), N 1s XPS peaks (E) of BN-PCPs and N 1s XPS peaks of N-PCPs (F).

The chemical states of C, N and B in BN-PCPs were further analyzed with fine-scanned XPS spectra. As shown in Fig. 5C, the XPS C 1s peak can be decomposed into five peaks at about 284.2, 284.6, 285.2, 286.1 and 288 eV, attributed to C–B, C=C, C–N or C–C, C=O and O–C=O bonds, respectively. B 1s XPS is also investigated (Fig. 5D). It is noted that the B 1s spectra are mainly deconvoluted into three peaks according to B–C, B–N or BCO₂ or BC₂O and BO bonds. Furthermore, for N 1s XPS peak, as shown in Fig. 5E, in addition to the pyridinic N (N1), pyrrolic N (N2) and graphitic N (N3), a novel C–N–B bond is found at about 399.1 eV. Compared to the N 1s of N-PCPs (Fig. 5F), because of the B-doping, the pyridine oxide or the oxidized nitrogen (N4) is not observed in BN-PCPs. It was reported that the N1 and N2 configurations induced the pseudocapacitance enhancement, and the N3 improved the conductivity of carbon materials,^37,38 while pyridine oxide or oxidized nitrogen (N4) had no obvious contribution to the capacitive properties of the material.³⁹ The high-concentration B content, the increased N content and the disappear of N4 in BN-PCPs imply that BN-PCPs may have excellent capacitive properties.

Electrochemical capacitive properties

Electrochemical capacitive properties of BN-PCPs and N-PCPs were investigated by cyclic voltammetry in 1.0 M H₂SO₄ aqueous solution and the results are shown in Fig. 6. From Fig. 6A, the cyclic voltammogram of BN-PCPs at 20 mV s⁻¹ shows a quasi-rectangular shape with a pair of board peaks, which indicates that the capacitance of BN-PCPs is a combination of carbon electric double layer capacitance and pseudocapacitance. It is noted that BN-PCPs possess stronger redox peaks than N-PCPs and consequently display the larger specific capacitance of 262 F g⁻¹ at 20 mV s⁻¹, which is more than 3 times higher than that of N-PCPs (84 F g⁻¹). What is more, the specific capacitance of BN-PCPs at 20 mV s⁻¹ (262 F g⁻¹) is also higher than that of some carbon materials measured in aqueous solution using a 3-electrode configuration (Table S2, see ESI section†). The high capacitance of BN-PCPs may result in the factors as follows: (1) the introduction of pseudocapacitance. With the high-concentration B-doping and the increased N content in BN-PCPs, the obvious occurrence of pseudocapacitance can be easily observed. As it has been reported that the presence of B-, N- and O-containing functional groups in carbon materials may result in introducing pseudocapacitance on the surface,^26,40,41 the BN-PCPs with a high capacitance are really due to the B-, N- and O-functional groups, as characterized by XPS. On the other hand, nitrogen, boron and oxygen incorporated into the carbon matrix can also enhance the wettability between the electrode materials and the electrolyte, resulting in the pseudocapacitance effect. (2) The occurrence of space-charge-layer capacitance in carbon. The charge density of space charge layer and the density of states (DOS) at the Fermi level mainly affect the double layer capacitance, which is tailorable by the heteroatoms doping in carbon lattice.^42,43 Boron atom with three valence electrons, one electron less than carbon atom, can introduce a hole charge carrier once replacing a carbon atom in the graphene lattice, which will enhance the charge density and DOS, and hence improve the double layer capacitance.²¹ (3) The decrease of equivalent series resistance (ESR) due to the B-doping and the increased N content. As shown in Fig. 6B, the charge-transfer resistance of BN-PCPs is only about 1 Ω, and much smaller than that of N-PCPs (about 200 Ω). On the other hand, as shown in Fig. 6C, BN-PCPs show high retention of 72% while N-PCPs is only 52% as the sweep rate increases from 20 mV s⁻¹ to 200 mV s⁻¹, indicating the excellent power characteristics of BN-PCPs due to their mesoporous structure, which is in agreement with the pore size distribution analysis (Fig. 4B).

Fig. 6 Cyclic voltammograms of BN-PCPs and N-PCPs at 20 mV s⁻¹ (A), electrochemical impedance spectra of BN-PCPs (dot) and N-PCPs (star), and the relationship between the specific capacitance and the scan rate for BN-PCPs and N-PCPs (C) in 1.0 M H₂SO₄ aqueous solution.

Galvanostatic charge–discharge measurements were also performed to further investigate the capacitive behavior of both BN-PCPs and N-PCPs at different current densities and the results are showed in Fig. 7. From Fig. 7A, quasi-triangular curves for BN-PCPs can be observed at all the test currents while the profiles of N-PCPs (Fig. 7B) is a bit twisted, which should be due to the low charge-transfer resistance of BN-PCPs. Significantly, the specific capacitance of BN-PCPs is 360 F g⁻¹ at 2 A g⁻¹, 3 times higher than that of N-PCPs (102 F g⁻¹), which agrees well with the results obtained by cyclic voltammetry. Moreover, the specific capacitance of BN-PCPs can still remain 200 F g⁻¹ even at a high loading current density of 50 A g⁻¹, implies that BN-PCPs possesses good power properties. Furthermore, the Ragone plots of BN-PCPs and N-PCPs are showed in Fig.7C, which illustrate the corresponding energy/specific power densities. With the increasing power density from 900 (1 A g⁻¹) to 16 000 W kg⁻¹ (20 A g⁻¹) for N-PCPs and from 900 (1 A g⁻¹) to 40 000 W kg⁻¹ (50 A g⁻¹) for BN-PCPs, the energy densities drop slower for BN-PCPs. Significantly, the energy density of BN-PCPs calculated at 1 A g⁻¹ was 41.42 W h kg⁻¹, 3 times higher than that of N-PCPs (13.60 W h kg⁻¹), which is attributed to a high pseudocapacitance due to the high-concentration doping of B and N in the carbon matrix. These results indicate that the BN-PCPs exhibit good power characteristic of supercapacitors.

Fig. 7 Galvanostatic charge–discharge profiles of BN-PCPs (A) and N-PCPs (B) at different loading current densities in 1.0 M H₂SO₄ aqueous solution. (1) 1, (2) 2, (3) 5, (4) 10, (5) 20 and (6) 50 A g⁻¹. (C) Ragone plots of BN-PCPs and N-PCPs in 1.0 M H₂SO₄ aqueous solution.

The long-term charge–discharge stability of BN-PCPs was also investigated by galvanostatic charge–discharge method at a high current density of 20 A g⁻¹. As shown in Fig. 8, the capacitance of BN-PCPs retains without any obvious degradation during 40 000 cycles. This reveals that BN-PCPs have excellent long-term charge–discharge stability.

Fig. 8 The charge–discharge stability of BN-PCPs at a current density of 20 A g⁻¹ in 1.0 M H₂SO₄ aqueous solution.

Conclusions

A novel high-concentration B and N co-doped porous carbon polyhedra (BN-PCPs) were prepared through a simple procedure using phenylboronic acid as the dopant and ZIF-11 as the carbon templates. Because of the introduction of pseudocapacitance, the occurrence of space-charge-layer capacitance in carbon and the decrease of equivalent series resistance (ESR), the specific capacitance of the developed BN-PCPs (262 F g⁻¹) is 3 times higher than that of the porous carbon polyhedra without B doping (N-PCPs, 84 F g⁻¹). Also, BN-PCPs posses excellent long-term charge–discharge stability (no obvious degradation during 40 000 charge–discharge cycles at 20 A g⁻¹). These imply that the developed BN-PCPs are the good candidate as the electrode materials of supercapacitors. Also, this work provides a new way to prepare B and N co-doped carbon-based materials with high B and N concentrations.

Acknowledgements

This work was financially supported by NSFC (21275041, J1210040, J1103312), Hunan Provincial Natural Science Foundation of China (12JJ2010), the Specialized Research Fund for the Doctoral Program of Higher Education (20110161110009), and PCSIRT (IRT1238).

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Footnote

† Electronic supplementary information (ESI) available: Table S1 element distributions and porous textures of BN-PCPs and N-PCPs obtained from XPS analysis and nitrogen adsorption–desorption isotherms. Table S2 the values of the specific capacitances of different porous carbons reported in the literatures using three-electrode systems. See DOI: 10.1039/c5ra15249f

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