Bio-based Nitrogen and Oxygen Co-doped Carbon Materials for

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Bio-based Nitrogen and Oxygen Codoped Carbon Materials for Supercapacitor Yuan Liu, Lijun Cao, Jun Luo, Yunyan Peng, Qing Ji, Jinyue Dai, Jin Zhu, and Xiaoqing Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05947 • Publication Date (Web): 20 Dec 2018 Downloaded from http://pubs.acs.org on December 24, 2018

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ACS Sustainable Chemistry & Engineering

Bio-based Nitrogen and Oxygen Co-doped Carbon Materials for High-performance Supercapacitor

Yuan Liu, †, ‡, §, ⊥ Lijun Cao, †, ‡ Jun Luo, § Yunyan Peng, †, ‡ Qing Ji, †, ‡ Jinyue Dai, †, ⊥Jin

Zhu, †, ⊥ and Xiaoqing Liu*, †, ⊥

†Ningbo

Institute of Materials Technology and Engineering, Chinese Academy of

Sciences, No. 1219 Zhongguan West Road, Zhenhai District, Ningbo 315201, China ‡University

of Chinese Academy of Sciences, No.19 (A) Yuquan Road, Shijingshan

District, Beijing 100049, China §Engineering

Research Center for Materials Protection of Wear and Corrosion of

Guizhou Province, Guiyang University, No.103 Jianlong Road, Nanming District, Guiyang 550005, China ⊥ Key

Laboratory of Bio-based Polymeric Materials Technology and Application of

Zhejiang Province, No. 1219 Zhongguan West Road, Zhenhai District, Ningbo 315201, China

* Correspondence to: Xiaoqing Liu (E-mail: [email protected])

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Nitrogen and oxygen co-doped carbon materials (NOPC-x and NOPCbis-CN-x) have been prepared from the bio-based polybenzoxazines by a soft-template method. Subsequently, their textural properties and surface chemistry are characterized by nitrogen isothermal adsorption, raman spectroscopy, X-ray diffraction, transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Results show that the higher content of cyano group in benzoxazine monomers endows the carbon materials with higher specific surface area, higher graphitization degree, better pore size distribution and more electrochemically active N-3, O-2 as well as O-3. The electrochemical performances of the obtained carbons are evaluated by means of cyclic voltammetry (CV), galvanostatic charge/discharge tests (GCD), and electrochemical impedance spectroscopy (EIS). The result show that the NOPC-bis-CN-x systems possess much better electrochemical properties than those of NOPC-x. Especially for NOPC-bis-CN-3, it displays the highest specific capacitance of 167.3 F g-1 at 1 A g-1 and a good retention capability of more than 80 % at a current density of 10 A g-1, as well as higher pseudo-capacitance and good cycling stability. The excellent electrochemical performance together with the renewable features and the facile preparation route of NOPC-bis-CN-3 make it a promising bio-based candidate for supercapacitor electrode.

KEYWORDS ： Bio-based benzoxazines, Nitrogen and oxygen co-doped, Porous carbon, Supercapacitor

INTRODUCTION


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Supercapacitor, a relatively new environmental friendly and fast storage/release energy device, has captured more and more attention.1-4 According to the difference of the energy storage/release mechanism, supercapacitors are divided into two categories: electrochemical double layer capacitors (EDLCs) and pseudo-capacitors.5 While fast reversible Faradic reactions endow the pseudo-capacitors with Faradaic pseudocapacitance, the energy in EDLCs arises from the accumulation of charges at the interface between the electrode and electrolyte.

6, 7

Generally speaking, the porous

carbon has been considered as a promising candidate for EDLCs fabrication due to its unique features, including outstanding chemical stability, high specific surface area, acceptable price, optimal conductivity and so on.

8-10

Unfortunately, the EDLCs can

output limited energy and power because of its limited charge storage. As pointed by Wang and coworkers, the pseudo-capacitance of metal oxide is almost 10 - 100 times higher than double layer capacitance. 11 Although new preparation techniques, such as preparation of the carbon areogel or selective etching can enhance the capacitance of the EDLCs, the capacitance is still in the range of 110 ~ 130 F g -1. 12, 13 Meanwhile, the oxides cannot be used alone, because of their inherent insulation characteristics.14 Naturally, combination of the double layer capacitance and Faradaic pseudocapacitance has stimulated the research interest of many scientists, for example, Li et al. reported MnO2·xH2O/carbon aerogel composite electrodes whose specific capacitance was as high as 226.3 F g

-1,

while the capacitance of carbon aerogel

electrode alone was only 112 F g -1. 15 Wei et al. pasted MnFe2O4/carbon material onto nickel foam, and the pure contribution of MnFe2O4 on specific capacitance of asprepared electrode was 135 F g -1.16 However, compared with the carbons, these composites often involves in tedious and boring preparation processes, which hinders their large-scale preparation. Recently, many researchers disclose that direct



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introduction of heteroatoms (N, O, P) into carbon skeleton can also generate extra pseudo-capacitance because of their electron-donor characteristics.17,

18

Miao et al.

prepared N, S co-doped hierarchical porous carbon rods via in-suit carbonization of a protic salt; with the high doping content of the N and S, the obtained carbon rods featured of high gravimetric capacitance of 282 F g -1 @ 1 A g -1. 19 Furthermore, these heteroatoms can also enhance the conductivity and promote the wettability to electrolyte of the porous carbons.20-24 Thus, porous carbons containing heteroatoms are potential candidates for combination of the double layer capacitance and pseudocapacitance at the same time, moreover, they can be prepared by an easy and scalable route. Nowadays, the massive use of fossil resources has not only posed a heavy burden on environment but also threatened human health seriously. It is a matter of great significance to explore the sustainable and environmentally favorable materials to replace the petroleum-based ones, even partially.

25-27

The conversion of renewable

resources into biofuels, chemicals and useful materials has been regarded as a great strategy for sustainable development. Accordingly, the electrode materials prepared from renewable feedstock, such as natural cotton,

28

cellulose,

29

banana peel

30

etc.,

have been reported. Ostensibly, the direct usage of natural products shows the advantages of simplicity and convenience. However, this method is facing the dilemma of weak structural controllability due to the poor stability and inhomogeneity of natural products, and accordingly it is difficult to predict or control the structures of obtained porous carbons. Unlike the natural products, the bio-based polymers are characterized by definite molecular structures and great sustainability, which overcomes the drawbacks of the natural products and retains the advantages of the natural products at the same time.


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In this work, two kinds of bio-based benzoxazines with definite chemical structures are synthesized from renewable vanillin, and the heteroatoms contained porous carbons are obtained by directly carbonization and activation of the polymers of these bio-based benzoxazines. As shown in Scheme 1, two different benzoxazines (Boz-Va and Bozbis-VaCN), containing cyano groups or not, are synthesized. Expectably, the introduction of cyano groups will add extra nitrogen atoms into the carbon skeletons after carbonization, which will influence the pore structures and electronic conductivity of the resulted porous carbons. We hope that through structure design and control, the relationship between the chemical structure of benzoxazines and the properties of resulted carbon materials could be figured out. In addition, supercapacitor has been regarded as an environmental friendly power source, if its electrode materials could be derived from bio-based feedstock, the “green + green” strategy will be achieved, which is a remarkable step towards sustainable development. This is the main objective of this work.

Scheme 1. Synthesis and chemical structures of vanillin-based benzoxazines (Boz-Va and Bozbis-VaCN)

EXPERIMENTAL SECTION



Preparation of porous carbons from Boz-Va and Boz-bis-VaCN. The mixtures of benzoxazine monomers and F127 with different weight ratios were dissolved in DMSO and charged in 3 different Teflon pans. Then the DMSO was vapored at 120 °C for 12h and the residues were cured at 160 °C for 2 h, 180 °C for 2 h, 200 °C for 2 h, 220 °C for 2 h and 250 °C for 2 h (same protocol was applied for their curing behaviors investigation). Subsequently, cured products were pre-carbonized at 600 °C for 3 h under Ar flow at a heating rate of 1 °C min -1. The resulted carbons were mixed with KOH [KOH/carbon, 2: 1 (w/w)] and activated at 800 °C for 1 h under argon atmosphere. After cooling to room temperature, the carbons were washed with deionized water until the value of pH reached 7, followed by drying at 110 °C. The similar procedure was applied for different systems. NOPC and NOPC-bis-CN represented the carbon materials derived from Boz-Va and Boz-bis-VaCN respectively. When different content of F127 was used, the samples were denoted as NOPC-x and NOPC-bis-CN-x, where x are 1, 2 and 3 for all the obtained materials. For example, NOPC-1, NOPC-2, and NOPC-3 respectively represent three types of carbon materials, in which the weight ratio of Boz-Va to F127 were 2:1, 1:1, and 1:2; NOPC-bis-CN-x was named with similar regulation.

Characterization. Nuclear magnetic resonance spectra were recorded on a Bruker AVANCE III 400MHz in Dimethyl sulfoxide-d6 (DMSO-d6) using tetramethylsilane as the internal standard. Fourier transform infrared spectra (FTIR) were obtained from a Nicolet iS30 using KBr disks with 32 scans in the range of 4000 ~ 400 cm -1 at the resolution of 4 cm

-1.

The curing behavior of each benzoxazine monomer was

determined by a Mettler-Toledo TGA/DSC Analyzer (Mettler-Toledo, Switzerland). About 5 mg of benzoxazine monomer was heated from 25 to 300 °C at a heating rate


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of 10 °C min

-1

under a N2 atmosphere with the flow rate of 50 mL min

-1.

Thermogravimetric analysis (TGA) was performed on a Mettler-Toledo TGA/DSC Analyzer (Mettler-Toledo, Switzerland). About 5 mg of cured benzoxazine without F127 was heated from 50 to 800 °C at a heating of 10 °C min -1 under a N2 atmosphere. N2 adsorption isotherms were measured on a Micromeritics ASAP2020M analyzer (Micromeritics Instrument Corporation, USA) at -196 °C. Before measurements, all the samples were degassed at 110 °C for 30 min and then at 300 °C for 400 min. The surface area was determined by the Brunauer-Emmett-Teller (BET) method based on the adsorption data at 100 mmHg. The pore size distributions (PSDs) were calculated from the density functional theory (DFT) model assuming the split-shaped pores. Raman spectra were recorded on a Renishaw inVia Reflex raman spectroscopy (Renishaw, UK) with a laser beam of 532 nm as the excitation source. X-ray photoelectron spectra (XPS) were obtained on a Kratos Axis Ultra DLD (Kratos, Japan) using the Mg Kα as the excitation source. Before XPS analysis, the samples were grounded into powders and all the reported binding energies of spectra were calibrated by C 1s binding energy set at 284.8 eV. Elemental analysis was performed on a CHNO elemental analyzer (Vario EL Cube, Elementar, Germany). X-ray diffractometer (XRD, D8 ADVANCE, BRUKER) at a scanning rate of 5° min-1 in the 2θ range of 5°-80° to evaluate the microstructures of the obtained carbons. Transmission electron microscopy (TEM) images were recorded using a JEM-2100 TEM (JEOL, Japan) operated at an accelerating voltage of 200 kV. Before imaging, the samples were dispersed ultrasonically in absolute ethanol for 30 minutes, and then 3-4 drops of the mixtures were placed on carbon-supported copper grids with a pipette and dried with an infrared lamp for 5 min. Electrochemical Measurements. The electrochemical performances were recorded on



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an Autolab PGSTAT 302N (Metrohm AG Switzerland) using a three-electrode system at room temperature. For the preparation of working electrode, the obtained carbon materials, conductive carbon black and polyvinylidene fluoride (PVDF) were mixed together and subjected to ultrasonication with the weight ratio of 8:1:1 to form a homogeneous slurry. Then the mixture was pressed on a nickel foam with the pressure ranged from 10 to 12 MPa after the slurry dried at 80 ° C for 3 h. The loading mass of the active material on the electrode was around 4.0 mg for each electrode. The platinum wire and Hg/HgO electrodes were applied as the counter electrode and the reference electrode respectively. Cyclic voltammetry (CV) curves and galvanostatic chargedischarge (GCD) were performed in 6 M KOH solution. The gravimetric specific capacitance Cg was determined by the equation 1: 18 I∆t

(1)

Cg = (m∆V)

where I, Δt, ΔV, and m refer to the applied current, discharge time, voltage change and mass of the active carbon materials, respectively. In addition, the potential window of the cycling was confined in the range of -0.9 to 0 V. The carbon with the highest value of gravimetric specific capacitance was chosen to prepare a cell supercapacitor whose test electrode and counter electrode were with nearly the same weight, and the two electrodes were separated by a piece of glass microfiber filter. The CV and GCD performances were also recorded in 6 M KOH solution, and the electrochemical impedance measurements were recorded in a frequency ranged from 100 kHz to 10 mHz with an alternating-current amplitude of 5 mV. The specific capacitance Cg of the electrode in was calculated by equation 2:31 4I∆t

(2)

Cg = (m∆V)

where I, Δt, ΔV, and m refer to the applied current, discharge time, voltage change, total mass of active carbon materials in two electrodes. In addition, the potential window of


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the cycling was confined in the range of -0.9 to 0 V. Energy densities and power densities of the supercapacitor device were calculated using Eqs. (3) and (4).32 1

E = 2 ∙ C(∆V)2 1

P=2∙

(3)

I ∙ ∆V

(4)

m

where I, C, ΔV, and m represent the applied current, specific capacitance, voltage change, total mass of the active carbon materials in the electrode. In addition, the potential window of the cycling was confined in the range of -0.9 to 0 V, and the specific capacitance can be calculated by equation 2.

RESULTS AND DISCUSSION Synthesis and Structure Identification of Boz-Va and Boz-bis-VaCN The synthesis routes of Boz-Va and Boz-bis-VaCN are illustrated in Scheme 1. Due to the presence of phenolic hydroxyl, the vanillin-based monofunctional benzoxazine (Boz-Va) is easy to be prepared. After two cyano groups were introduced into vanillin via the Knoevenagel reaction, the other benzoxazine monomer containing four cyano groups was prepared (Boz-bis-VaCN) by similar synthesis route. Before curing reaction and property investigation, their chemical structures were identified by 1H NMR and 13C

NMR (Figure S1). The peaks presented at 4.75 (H2) and 5.65 ppm (H3) with an

equal integral ratio are designated to the protons in oxazine ring (Ph-CH2-N and OCH2-N). It is easy to note that the signal at 9.85 ppm standing for H3 in Boz-Va (CH=O) has shifted to 8.35 ppm in Boz-bis-VaCN (CH=C(CN)2). As for the 13C-NMR spectra, the characteristic peaks showing at 53.57 (Ph-CH2-N) and 79.65 ppm (O-CH2-N) confirm the formation of oxazine ring. Besides, Boz-Va displays the unique peak at



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192.65 ppm (marked as C3), which stands for the carbon atom in aldehyde group. After the incorporation of cyano groups, the signal at 192.65 ppm disappears and three new peaks marked as C4, C5 and C6 are clearly observed at 78.95, 115.25 and 116.25 ppm in Boz-bis-VaCN, the characteristic signals for carbon atoms in CH=C(CN)2 groups. Based on above NMR spectra, the chemical structures of Boz-Va and Boz-bis-VaCN can be confirmed. As for the bio-based polymers, the bio-based content is also a significant factor to evaluate their comprehensive performance. According to the USDA (United States Department of Agriculture) standard, 33 the amount of bio-based carbon in the product as a percentage of the weight of the total organic carbon in the product can be taken as the bio-based content. Based on this standard, the bio-based contents of Boz-Va and Boz-bis-VaCN are calculated to be 66.2 % and 50.0 %, respectively, which are all higher than 50 % and could definitely be regarded as the “Bio-based materials”.

Curing Behaviors of Boz-Va and Boz-bis-VaCN The reason for Boz-bis-VaCN exhibits a broader curing peak than that of Boz-Va (Figure 1a) might be that, besides the ring opening reaction of oxazine ring in Boz-bisVaCN, cyano groups probably formed the triazine rings at a higher temperature, which partially coincided with the curing of benzoxazines. In previous literatures,

8, 9, 34

the

formation of triazine rings associated with the ring opening reaction of oxazine ring were also reported. In addition, the peak curing temperature of Boz-bis-VaCN is shown at 212.6 °C, lower than 223.8 °C for Boz-Va, which probably due to the catalytic effect of cyano groups. The typical thermal degradation behaviors of P(Boz-Va) and P(Boz-bis-VaCN) is showed in Figure 1b. P(Boz-Va) demonstrated a char yield at 800 °C of 60.3 %, while


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P(Boz-bis-VaCN) shows the higher char yield of 64.5 % at the same temperature. High carbon forming capacity is one of the most outstanding advantages that makes polybenzoxazines good candidates for the preparation of carbon materials. 35 In Figure 1c, the formation of triazine rings in P(Boz-bis-VaCN) has been further confirmed by FT-IR spectra. Before curing reaction, besides the characteristic absorption bands of oxazine rings (1072, 1246 and 1257 cm -1), the sharp signal indicating the presence of cyano group is displayed at 2225 cm

-1.

Along with the curing reaction, a new

absorption band at around 1620 cm -1 emerges, indicating the reduction of cyano groups and the formation of triazine rings at a higher temperature. The crosslinking networks of polybenzoxazines combining with the extra rings (triazine rings) would ensure the high char yield after carbonization and activation. In addition, introduction of heteroatoms into the carbon skeletons would improve the electronic conductivity and surface wettability of carbon materials.

17-24

Compared with that of P(Boz-Va), more

nitrogen atoms in the P(Boz-bis-VaCN) would affect their electronic properties, which has been discussed in the following sections.



Figure 1. Thermal properties of benzoxazine monomers and cured resins: (a) DSC curves for Boz-Va and Boz-bis-VaCN; (b) Thermal degradation behaviors of P(Boz-Va) and P(Boz-bisVaCN); (c) FT-IR results supporting the formation of triazine; (d) Schematic illustration for the formation of triazine ring

Morphology Analysis of NOPC-x and NOPC-bis-CN-x In order to obtain the carbon materials with hierarchical pore structure, the usually used surfactant F127 was mixed with Boz-Va and Boz-bis-VaCN at different weight ratio, and then the pre-carbonization was conducted at 600 °C. The graphitization degree of prepared carbon materials (NOPC-x and NOPC-bis-CN-x) was investigated by XRD and Raman spectra. The XRD patterns of NOPC-x and NOPC-bis-CN-x are presented in Figure 2a. It is easy to identify two broad diffraction peaks showing at 2θ = 25° and 2θ = 44°. These two peaks are corresponding to the diffractions of graphitic carbon,


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representing the 002 and 100 planes, respectively. The peak at 2θ = 25° is caused by the irregular and amorphous carbon, while the signal at 2θ = 44° is correlated to the graphitic carbons. 36 The diffraction peaks at 2θ = 44° for NOPC-bis-CN-x are relatively stronger than those for NOPC-x, implying that the cyano groups are beneficial to shaping more regular carbons. Moreover, the overall similar XRD patterns for NOPCx and NOPC-bis-CN-x reveals that F127 has no evident effect on the graphitization of polybenzoxazines. Figure 2b shows the Raman spectra of NOPC-x and NOPC-bis-CN-x. Two broad bands can be distinguished at 1345 cm-1 (D band) and 1590 cm-1 (G band) for all of the samples. As previously reported, the D band corresponds to the disarrayed and imperfect structures arising from turbostratic carbon layers, and G band correlates to the vibrations of sp2-bonded carbon atoms. 37, 38 In addition, the integral ratio of D and G band (ID/IG) can be taken as an index to determine the graphitic degree of carbon materials. In Figure 2b, the values of ID/IG for NOPC-x and NOPC-bis-CN-x are 1.68, 1.67, 1.68, 1.49, 1.51 and 1.44, respectively. The values of ID/IG for NOPC-bis-CN-x obtained are generally smaller than those of NOPC-x (Boz-Va), and the reason is ascribed to the formation of triazine rings from cyano groups at a high temperature, which improves the regularity of the carbon materials. In Kuo’s work,37 they synthesized two benzoxazine monomers (BZPh and BZCN) from phenol and 4cyanophenol. After carbonization, the carbons derived from BZPh showed the ID/IG of 1.07, higher than that of BZCN-based carbon (0.98). The influence of cyano group on the graphitization degree of carbon materials was similar to our result. Moreover, it is easy to note that the ID/IG values unchanged with the change of the contents of F127, indicating the introduction of F127 into PBZs has no influence on the graphitization degree of carbon materials, which is in good agreement with the results of XRD tests.



Figure 2. (a) XRD patterns, (b) Raman spectra, (c) N2 adsorption-desorption isotherms, and (d) DFT pore distributions of NOPC-x and NOPC-bis-CN-x.

The N2 adsorption-desorption isotherms and the pore size distributions (PSD) of NOPC-x and NOPC-bis-CN-x are displayed in Figure 2c and 2d. Except for NOPC-2, neither NOPC-1 nor NOPC-3 gives a recognizable hysteresis loop. The PSD curves also corroborate that limited number of mesopores (2-50 nm) and hardly any macropore (larger than 50 nm) are observed in NOPC-1 and NOPC-3 (Figure 2d). As for NOPCbis-CN-x, they all indisputably exhibit type IV isotherms, especially for NOPC-bis-CN2 and NOPC-bis-CN-3, their N2 sorption isotherms are very similar, displaying conspicuous hysteresis loops and relatively high values of N2 uptake at low relative pressure (P/P0

Bio-based Nitrogen and Oxygen Co-doped Carbon Materials for

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