Synthesis of highly uniform N-doped porous carbon spheres derived

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Materials and Interfaces

Synthesis of highly uniform N-doped porous carbon spheres derived from their phenolic-resin-based analogues for high performance supercapacitors Xiying Li, Yufeng Song, Lei You, Li Gao, Yong Liu, Wei Chen, and Liqun Mao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04823 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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Synthesis of highly uniform N-doped porous carbon spheres derived from their phenolic-resin-based analogues for high performance supercapacitors Xiying Li,* Yufeng Song, Lei You, Li Gao, Yong Liu, Wei Chen, Liqun Mao Henan Engineering Research Center of Resource & Energy Recovery from Waste, Institute of Functional Polymer Composites, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, PR China

*Corresponding author. *Corresponding author. E-mail: [email protected] Phone: +86-371-22868833. Fax: +86-371-22868833.


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Abstract: A novel route is developed to synthesize highly uniform N-doped porous carbon spheres (NCS) with a tailorable size ranging from 86 to 205 nm depending on their precursors of phenolic-resin-based analogues. Solid-state NMR and FTIR spectra confirm that the as-synthesized polymer spheres are composed of typical phenolic resin. After activation treatment with KOH, the NCS with the diameter of 86 nm possesses high surface area 1462 m2 g-1 coupled with hierarchical (micro and meso) pore structures. Upon these advantages, the prepared carbon sample endows the supercapacitor with high specific capacitance up to 247 F g-1 at 1 A g-1 with excellent rate performance and high cycling stability. Also, the specific energy density is 6.74 Wh kg−1 at a current density of 0.1 A g-1. Therefore, the as-prepared uniform N-doped porous carbon spheres represent a promising candidate for an efficient electrode material. Keywords: N-doped porous carbon sphere; Supercapacitor; Photonic crystal; Phenolic resin

1. Introduction: Nanoporous carbon materials possess unique properties, such as high surface area, tunable particle size porosity, easily doped heteroatoms, electrical conductivity, chemical inertness and biocompatibility, thereby gaining widespread applications involving adsorption, energy storage and conversion, catalysis, drug delivery and photonic crystal.1-11 To date, nanoporous carbonaceous materials have been synthesized by several methods including self-assembly with hard

templates,[12,13]

hydrothermal

carbonization,14,15

modified

Stöber

method,5,16-28

soft-templating strategies by organic-organic self-assembly.6-8,29-34 However, most of these methods, if not all, are not suitable for the preparation of carbon spheres with uniform size distribution. Recently, more and more efforts were devoted to exploring versatile and feasible strategies to prepare porous carbon nanospheres because of their special advantages in better pore accessibility, shorter pathways for fast mass diffusion and reduced viscous effects.3,4, 14,23,25,27,31 Despite many reports on the synthesis of phenolic resin spheres and their carbon analogues in micro and nano scales, very few methods are applicable to prepare highly uniform phenolic resin 1



and carbon nanospheres with the size less than 200 nm, especially for affording three-dimensional periodic structures in a large domain via self-assembly. 6,7,17-20,35 The

well-known Stöber method was proposed for the synthesis of highly uniform silica particles

in 1968, which were obtained from the hydrolysis and condensation of TEOS catalyzed by hydroxide ammonium in the mixture of water and ethanol.36 Based on the similar underlying mechanism existing between the condensation of TEOS and the polymerization of resorcinol-formaldehyde, Stöber method can be extended to prepare phenolic resin spheres.5,16 Using the modified Stöber method, solid silica particles with highly uniform size were synthesized and assembled into periodic arrangement by using lysine as catalyst instead of hydroxide ammonium.37 More recently, Lu group reported the formation of highly uniform carbon nanospheres based on benzoxazine chemistry, which was achieved by the polymerization of resorcinol, formaldehyde and 1,6-diaminohexane (simultaneously acting as catalyst) with the addition of tri-block copolymer Pluronic F127 as the soft template. The size of as-prepared phenolic resin spheres was precisely tailored by controlling reaction temperatures.6 Also, Zhao group developed a low-concentration hydrothermal route for synthesizing highly ordered body-centered-cubic mesoporous carbon nanospheres with uniform sizes (20-140 nm) that allow their application in drug delivery.7-9 For the previous synthesis of uniform carbon spheres below 200 nm, highly diluted solutions were applied to prevent the interpartcile crosslink causing severe aggregation. However, this route has a low yield that severely limits its practical applications. Therefore, it is imperative to establish efficient ways to prepare uniform carbon nanospheres within 200 nm in size to meet the ever-increasing demands in drug delivery, biodiagnostics, packed bed catalysis and particle templates. 2


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According to the previous pathways for the preparation of uniform carbon spheres, the combined effects of both the reactive activities of monomers (including resorcinol,6 aminophenol,4,17,26 2,4-dihydroxybenzoic acid,13 phloroglucinol38,39 and low-molecular-weight phenolic resol7), and the alkalinity and molecule structure of catalysts (hydroxide ammonium6, amino acid4,13

and

organic amine18,25,39,40) account for the uniformity of carbon spheres from the viewpoint of the devising at molecular level41. Herein, we report a simple and reliable pathway for the synthesis of phenolic-resin-based spheres and the derived carbon counterparts with highly uniform size which can thereby assemble into photonic crystal. Using phenol and formaldehyde as raw monomers, polymer spheres are synthesized via a two-step hydrothermal route, bearing very high uniform size ranging from 113 nm to 270 nm. The synthesis is performed in aqueous phase, which contains bis-tris (2,2-bis(hydroxymethyl)-2,2’,2’’-nitrilotriethanol) acting as the catalyst and CTAB or OTAB (hexa(octa)decyltrimethyl ammonium bromide) acting as stabilizing agent of emulsion droplets. Meanwhile, these phenolic spheres can be converted into N-doped nanoporous carbon spheres and retain the spherical morphology and uniform size distribution after calcination. Furthermore, the synergistic effect of high surface area with hierarchical (micro and meso) pore structures together with small particle size of these activated carbon products favor a good electrochemical performance upon supercapacitor. 2. Experiment section 2.1. Materials These chemicals were purchased from ACROS and used without further purification, including phenol (C6H6O, 99%), formaldehyde (HCHO, 37 wt%), cetyltrimethyl ammonium bromide (CTAB),

octadecyltrimethylammonium

bromide 3


(OTAB),

Bis-tris


(2,2-bis(hydroxymethyl)-2,2’,2’’-nitrilotriethanol). 2.2 Synthesis of uniform polymer/carbon spheres Highly uniform polymer spheres (PS) were synthesized through a two-step hydrothermal route in a one-pot reaction. Typically, an aqueous solution was prepared in a 30 ml glass vial with the mixture of 20 mL distilled water, 0.33 g phenol, 0.1 g Bis-tris and 0.3 g CTAB or OTAB. The solution was firstly placed into thermostatic water bath operating at 80 oC and stirred for an hour at the speed of 600 rpm. Subsequently, formaldehyde with a mass of 0.7 g is introduced follow by the first hydrothermal treatment remaining for 28 h. Next, the reaction mixture was transferred to 50 ml Teflon cup and sealed in a stainless-steel autoclave for the second-step hydrothermal treatment, which lasted for 6 h at 200 oC in a thermostatic oven. The solid product (uniform polymer spheres) was collected by centrifugation at the speed of 13,400 rpm for 10 min and dried at 80 °C for 10 h. Hereafter, the as-prepared polymer spheres using CTAB and OTAB as surfactants were named as PS-CTAB and PS-OTAB, respectively. Subsequently, the dried samples of PS-CTAB and PS-OTAB were converted to their corresponding N-doped carbon spheres (NCS-CTAB and NCS-OTAB) after calcination in the tube furnace at 800 °C under N2 environment. The whole procedure for pyrolysis was controlled at a heating rate of 5 oC/min to 800 °C followed by maintaining at this temperature for 2 h. In order to further enhance the specific surface area of NCS-CTAB/OTAB to satisfy the requirements for supercapacitor, the as-prepared carbon spheres were subjected to activation by mixing KOH with a weight ratio of 1:3 (NCS to KOH). The activation was carried out in tube furnace which was operated at a heating rate of 10 oC/min to 850 oC and retained this temperature for 2 h. Afterward, the activated products (NCSA-CTAB/OTAB) were washed with water for 4


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several times until pH value of the decanted solution became 7.

2.3 Tuning the size of polymer/carbon spheres In our strategy for the synthesis of highly uniform PS and NCS, the size is controlled by changing the amounts of precursors of phenol and formaldehyde at a constant mole ratio of 1:2.46. For example, by changing the amounts of phenol at 0.28, 0.33 and 0.38 g, the as-prepared PS assume the uniform size of 153, 200, and 270 nm, respectively. Meanwhile, the size of the corresponding carbon spheres was tuned from 116 to 152 and 205 nm. 2.4 Characterization of polymer and carbon spheres TEM images were captured using JEOL 2010F instrument operated at 200 keV. The TEM samples were prepared by dropping the suspensions of the PS and CS on a 200 mesh copper grid coated by carbon layer coating. SEM images were recorded on JEOL 7610F scanning electron microcopy. Similarly, the samples were prepared by dropping the suspension on a small piece of silicon wafer. Dynamic light scattering (DLS) measurements were monitored at 25 °C on a Malvern Zetasizer NanoZS Instrument equipped with ALV-5000/EPP Multiple Tau Digital Correlator and a JDS Uniphase 1145P 22 mW He-Ne laser (632.8 nm in wavelength). The MAS 13C

NMR spectra were recorded on a broker MSL-400WB spectrometer using CP-TOSS program

with 7.5 mm of MAS probe, 4 kHz of spinning rate, repetition time of 3 s, contact of 1 s. X-ray photoelectron spectroscopy (XPS) spectra were obtained on an ESCALab MKII X-ray photoelectron spectrometer with Mg Kα (1253.6 eV) excitation source. The Raman spectra were recorded on a thermo scientific DXR Smart Raman spectrometer with laser excitation at 532 nm. Nitrogen adsorption isotherms were measured at 77 K on Quantachrome QuadraWin volumetric 5



adsorption analyzers using nitrogen of 99.998% purity. Prior to adsorption measurements, each sample was degassed under a vacuum for 6 h at 300 °C. The specific surface area of the samples was calculated using the Brunauer-Emmett-Teller (BET) method in adsorption isotherm branch in the relative pressure ranging from 0.10 to 0.30. Incremental pore size distributions were obtained from the nitrogen adsorption isotherms by the NL-DFT (Nonlocal Density Functional Theory) method provided by Quantachrome. The total pore volumes (Vt) were estimated on the total adsorption at a relative pressure of 0.99. XRD patterns were recorded on D8 Advance diffractometer (Bruker, Germany) diffractometer with Cu Ka (λ=0.154 nm) radiation operating at 40 kV and 40 mA. The thermogravimetric (TG) measurements were performed on a TA Instrument TGA Q500 thermogravimetric analyzer using a high-resolution mode. FTIR (Fourier Transform Infrared Spectroscopy) spectra were collected using a Bruker Vector 22 FTIR spectrometer in the frequency range of 4000-500 cm-1. 2.5 Supercapacitor electrode fabrication and electrochemical measurements Electrochemical measurements were carried out on Modulab XM MTS Test System using a three-electrode system. The working electrodes were prepared by mixing 80 wt% active material, 10 wt% carbon black and 10 wt% polytetrafluoroethylene (PTFE) binder in water. The slurry of the mixture was painted on a piece of the nickel foam (1 cm2) and pressed under a pressure of 15 MPa followed by drying at 100 oC. The mass loading of the active materials is about 6.0 mg cm-2. Electrochemical performance was performed in 6 M KOH aqueous electrolyte with three-electrode configuration. The performance upon the carbon samples was evaluated by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and life cycle test. CV was recorded from 0 to 1.0 V at various sweep rates, and discharge–charge curves were implemented from -1.0 6


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to 0 V working at different current densities. Csp was calculated from the discharge curve through the following equation: Csp 

2 I  t

(1)

m  V

where I is the discharge current (A), t is the discharge time (s), V is the voltage change during the discharge stage, and m is the total mass of the active material in working electrodes (g). The 6M KOH aqueous solution was used as electrolyte in the two electrodes symmetric cell. The test electrodes were prepared by loading the slurry consisting of 80 wt % of active material, 10 wt % of carbon black and 10 wt % of PTFE binder. After pressing onto a nickel foam with 1.77 cm2 surface area, the active materials were loaded and the areal loading mass on each nickel foam was calculated as 8.5 mg cm−2. The electrodes were subsequently exposed to vacuum drying at 100℃ for 4 h. Then, the two electrodes were separated by a piece of porous polypropylene (PP) membrane soaked with electrolyte, thereby preparing a sandwich structure wrapped with parafilm. Cyclic voltammetry and galvanostatic charge-discharge tests were carried out on an Arbin BT2000 battery testing system with a potential range from 0 to 1.0 V at various sweep rates, and discharge–charge curves were measured at different current densities. The energy density and power density of the electrochemical capacitor were calculated through the following equations: Cm  E

1 8

P

4 I  t

(2)

m  V

 Cm  ( V )

2

E

(3) (4)

t

where Cm (F g-1) is the measured device capacitance, I (A) refers to the discharge current, V

(V) represents the potential change within the discharge time t (s),

m (g) is the total

mass of the active material in the cell. E (J g-1) corresponding to the energy density and P (W 7



g-1) is the power density. Electrochemical impedance spectroscopy (EIS) was recorded with a voltage amplitude of 5 mV in a frequency range from 0.01 Hz to 100 kHz. 3. Results and discussion 3.1 Synthesis and characterization of polymer and carbon nanospheres Based on the emulsion polymerization process,3,42 the plausible

scheme is provided to elucidate

the preparation of polymer spheres as shown in Figure 1. The first stage of hydrothermal reaction was carried out at80 oC. After the addition of formaldehyde, the reaction between phenol and formaldehyde gives rise to the mixtures of addition and condensation compounds which can react further to form crosslinking network. The primary reactions encompass (1) the generation of hydroxymethyl derivatives of phenol, (2) the condensation of hydroxymethyl derivatives to further produce methylene and methylene ether bridged compounds and (3) the disproportionation of methyl ether bridges to allow methylene bridges accompanying the elimination of formaldehyde as a by-product.43,44 Thereafter, these derived compounds form little emulsion droplets which are diffused into the hydrophobic core of CTAB micelles driven by the minimum of surface energy. 6,9 Meanwhile, the existing micelles of CTAB might undergo disassembling and reconstruction by immigrating to the surface of emulsion droplets ， thereby acting as stabilizing agents. This process is very slow as evidenced by the reacting solution being transparent for 16 h reaction (see Figure S1). Meanwhile, only CTAB micelles are detected with hydrodynamic diameter of about 2.2 nm, as demonstrated in Figure 2 (a). After staying at 80 °C for 28 h for incubation, the polymer spheres with a low crosslinking degree exhibit spherical morphology with the size ranging from 100 to 130 nm as shown in Figure 2(b), which is in good agreement with the DLS results in Figure 2 (a). In our experiments, the slow formation of emulsion droplets is influenced 8


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by two crucial factors. First, the monomer of phenol is less active than resorcinol, aminophenol and phloroglucinol with high reactivity of electrophilic substitution, arising from the enhanced electron density at the ortho and para positions donated by activating groups. Second, the solution containing Bis-tris with high buffer capability offers a neutral environment with the pH=6.96 before addition of formaldehyde, thereby retarding the electrophilic substitution to form hydroxylmethyl derivatives of phenol. Additionally, even after the synthesis of polymer spheres, the pH value of the suspension slowly reduced to 6.34, suggesting the whole reaction process was exposed to a nearly neutral condition. In the second phase of hydrothermal treatment at 200 oC, the condensation products of both phenolic derivatives and their oligomers transfer to the surface of the preceding polymer spheres and further react by crosslinking, simultaneously accompanying the reconstruction of isolated CTAB molecules by adsorption onto the surface of the growing emulsion droplets. Finally, the monodisperse PS-CTAB are obtained with the diameter about 200 nm as shown in Figure 3 (a)-(c). Obviously, the PS-CTAB with uniform size can assemble into periodic structure within a large domain after solvent evaporation. The second stage is governed by fast condensation with the rapid consumption of precursors that inhibits the formation of new polymer spheres and eventually results in the monodisperse PS-CTAB via the monomer addition.45,46 Moreover, the sample of PS-CTAB is easily converted to the corresponding N-doped carbon spheres (NCS-CTAB) with the original periodic structure remaining the same after pyrolysis as shown in Figure 3 (d)-(f), which benefits from the high thermal stability of phenolic resin.6 The centrifuged sample of PS-CTAB exhibits bright green color and the corresponding NCS-CTAB sample is bright and shiny as demonstrated in Figure S2, which further confirms the periodic arrangement 9



of PS-/NCS-CTAB into photonic crystals which is similar to the results of Xu group.17-20 The pyrolysis treatment causes a linear shrinkage of PS-CTAB around 24% and yields the NCS-CTAB with the size of 152 nm, while their polymer analogue PS-CTAB is 200 nm. In fact, the linear shrinkage results from the further crosslinking and is accompanied with the loss of water and formaldehyde during carbonization process. As shown in Figure S3, the TG curve of PS-CTAB displays a high carbon yield upon carbonization (about 50%) originating from their phenolic-resin-based analogues.6 In order to identify the chemical composition, the PS-CTAB is characterized by solid state 13C

NMR and FTIR. These results based on

13C

NMR spectra in Figure 4 (a) and FTIR

spectroscopy in Figure 4 (b) demonstrates that the sample of PS-CTAB is composed of the typical phenolic resin.32,47,48 The signal at 150 ppm is assigned to carbon directly attached to OHgroup in phenolic ring. The signal at 128 ppm corresponds to aromatic carbons and the signal at 28 ppm is assigned to methylene linkages between phenolic rings. In addition, the broad signals ranging from 10 ppm to18 ppm are ascribed to methyl groups arising from the decomposition of dimethylene ether linkages at high-temperatures (>160o).48 The other broad signals in the range of 45-75 ppm result from the methylene carbons of the ether groups, which partly overlap with the signals of Bis-tris. Furthermore, four weak shoulder peaks of

13C

spectrum of PS-CTAB are

discernible and their positions match well with those of Bis-tris as shown in Figure 4 (a), suggesting the involvement of Bis-tris in the polymer products. The presence of Bis-tris may be from the formation of complex hydrogen bonding between hydroxyl groups of Bis-tris and PS-CTAB as shown in Figure 4 (a). The FTIR spectrum of PS-CTAB is shown in Figure 4 (b). The peak at 3451 cm-1 10


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corresponds to the O-H stretching band. The peaks at 2928 and 2853 cm-1 respectively correspond to the -CH2- in-phase and out-of-phase stretching vibration. The C=C aromatic ring stretching band is presented at 1603 cm-1. In the phenol-formaldehyde resin spectrum, additional characteristic signals of methylene bridge C-H bend at 1473 cm–1 and methylene-ether bridge C-O-C bend at 1100 cm–1 are present. The peak at 1370 cm-1 is ascribed to the phenol O-H in-plane stretching band. The bands at 1266 and 1014 cm–1 represent asymmetric stretch of phenolic C-C-OH and stretching vibration of aliphatic C-OH bonded with Bis-tris, respectively.49,50 The peak at 1144 cm-1 is due to C-O stretching band. Furthermore, the peak at 1224 cm-1 is assigned to the stretching vibration of C-N (ternary amine) in Bis-tris, which also confirms the presence of Bis-tris in the sample of PS-CTAB. The involvement of Bis-tris can endow the calcined carbon samples with nitrogen species. Furthermore, the role of surfactants in acting as the stabilizing agents is investigated by FT-IR and TG analysis. For FT-IR characterization of the PS-CTAB, two controlled samples were exposed to water washing five times (PS-CTAB-W5) and treatment in 5% HCl ethanol solution three times (PS-CTAB-HCl3), respectively, in comparison with direct centrifuged sample (PS-CTAB-W0). From the Figure S4, the characteristic peak of CTAB at 720 cm-1, assigned to the characteristic peak of -(CH2)n- (here n>4), completely disappears for the samples of PS-CTAB-W5 and PS-CTAB-HCl3. However, the FT-IR spectra of the untreated sample display a weak peak at 720 cm-1 thus confirming little amount of CTAB molecules in PS-CTAB. These results shed light on the presence of CTAB molecules on the surface instead of involvement into of PS-CTAB inside. Moreover, from the Figure S4 (b), the samples of PS-CTAB-W0 and PS-CTAB-W5 exhibit similar thermal decomposition behavior with the exception of slightly earlier decomposition of 11



PS-CTAB-W0 at the first stage (about 220~320 oC) relative to the sample of PS-CTAB-W5, which may be associated with the presence of CTAB molecules in PS-CTAB-W0. As a consequence, both the results upon FT-IR and TG measurements support the hypothesis that CTAB/OTAB molecules act as stabilizing agents for the preparation of uniform penolic-based spheres. Considering the crucial effect of surfactants on controlling the morphology and size of polymer particles, we further examine the influence of OTAB on tuning the size of polymer spheres. As shown in Figure 5, the uniform size of PS-OTAB is considerably reduced to 113 nm in comparison with PS-CTAB at 200 nm. Similarly, the sample of PS-OTAB have periodic arrangements in a large domain via centrifugation or evaporation-induced self-assembly. According to statistical results of DLS as shown in Figure S5, the hydrodynamic diameters of PS-CTAB and PS-OTAB correspond to 216 nm and 124 nm, respectively, which are somewhat larger than their size from the SEM and TEM images (located at 200 nm and 113 nm) due to the formation of the hydrated layer.51 The reason for the reduction in size for PS-OTAB can be explained in the light of the diffusion rate of the monomers, which are retarded by the longer alkyl chains of OTAB molecules around the polymer spheres. Similarly, NCS-OTAB faithfully inherited the periodic structure from PS-OTAB after pyrolysis. The resulting NCS-OTAB with the size of 86 nm underwent the same linear shrinkage about 24% to NCS-CTAB. Additionally, TEM images clearly demonstrate micropores on the surface of NCS-OTAB in Figure 5 (f). If we further reduce the amount of phenol to 0.28 and 0.24 g, the size of the resulting PS becomes 91 and 67 nm, respectively, while remaining the highly uniform as shown in Figure S6. Meanwhile, the size of the derived NCS correspondingly decreases to 69 and 51 nm, which is close to the smallest 12


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carbon spheres reported so far.3,42 Similarly, the size of PS and CS can also be precisely controlled by changing the amounts of phenol and formaldehyde meanwhile keeping their mole ratio at 1:2.46 (phenol to formaldehyde) in the case of the adaption of CTAB as stabilizing agent. If the amount of phenol is increased to 0.38 g, the size of as-synthesized PS concomitantly becomes around 270 nm and the size of the derived NCS is about 205 nm as shown in Figure 6 (a) and (b). In the other case, if the amount of phenol is changed as 0.28 g, the size of the resulting PS is reduced to 152 nm and the size of the corresponding NCS is tuned as 116 nm as shown in Figure 6 (c) and (d). Likewise, these monodisperse PS, which are obtained via changing the amounts of precursors, can form photonic crystals that display blue, green and brown color via simple centrifugation treatment corresponding to the amount of phenol at 0.28, 0.33 and 0.38 g, respectively, as shown in Figure 6 (e). The XRD patterns of the NCS-CTAB/OTAB are shown in Figure 7 (a). The powder XRD patterns of NCS-CTAB/OTAB exhibit the presence of two broad peaks at about 2θ = 24 o and 44 o representing the (002) and (100) planes, respectively, which indicates the formation of graphitic carbon (sp2-bonded carbon). It is apparent that the peak intensity of NCS-OTAB at 2θ = 24

o

is

higher than that of NCS-CTAB, which may result from the smaller size of NCS-OTAB with larger external surface area that is susceptible to graphitization. Also, Raman spectra are collected for NCS-CTAB and NCS-OTAB in Figure 7 (b). Two peaks at ~1325 cm-1 and ~1575 cm-1 are observed for both materials and correspond to the D and G bands, respectively. The intensity ratio of the D band to the G band (IG/ID) is employed to evaluate the graphitization degree of carbon. The IG/ID values are calculated as 1.16 for NCS-CTAB and 0.90 for NCS-OTAB, which suggests 13



that these carbon samples are partly graphitized, consistent with the XRD results. To further identify the nitrogen functionalities and contents, XPS is used to characterize the carbon samples. The XPS survey spectra of NCS-CTAB/OTAB in Figure 7 (c) and (e) reveal the presence of C, O and N elements. The contents of C, O and N are also summarized in Table 1. The nitrogen contents are calculated from XPS spectra of the samples of NCS-CTAB/OTAB, which are almost equal and account for 2.31 and 2.32 at.%, respectively. As known, N doping can change the electron distribution of the neighboring C atoms and enhance electronic conductivity thus favoring supercapacitor performance.52,53 The N 1s spectra of the two carbon samples are illustrated in Figure 7 (d) and (f) respectively. The N 1s spectra can be deconvolved into three fitted peaks, which are pyridinic (N-6), quaternary (N-Q) and pyrrolic nitrogen (N-5), which are positioned at 398.5, 401and 399.8 eV, respectively. The contents of various types of nitrogen are summarized in Table 1. Due to the creation of more microporous pores, activated carbon material can possess high specific area that favors the accumulation of more charges on the electrode/electrolyte interface and considerably improves supercapacitor performance. In our experiments, the activated carbon samples are obtained by pyrolysis of the mixture of NCS-CTAB/OTAB and KOH. Compared to the carbon spheres of NCS-OTAB, the activated products almost remain their original spherical morphology regardless of coarsened surface as shown in Figure 8. Also, TEM image with high magnification demonstrates that some micropores are interconnected to form mesopores especially in the vicinity of external edge, which is possibly due to some carbon loss during activation process. Nitrogen adsorption isotherms are obtained for the carbon samples before and after activation as 14


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shown in Figure 9, and the deduced textural parameters are further summarized in Table 2. These isotherms display a pronounced adsorption volume at low pressure, which indicates type Ӏ isotherm characteristic of microporous materials with pore diameter less than 2 nm. According to multi-point BET model, the specific surface areas of NCS-CTAB and NCS-OTAB are 441 and 497 m2 g-1, respectively. After activation, the specific surface areas of activated NCSA-CTAB and NCSA-OTAB are significantly increased to 1325 and 1462 m2 g-1, respectively. However, it should be noted that the isotherm of the NCSA-OTAB displays a definite hysteresis loop, which suggests that the mesopores available possibly evolved from the penetration between micropores during activation. In fact, during pyrolysis process, external section of NCS, directly in contact with KOH, is preferentially etched away in comparison with core section partly due to reductive gas (for example CO) diffusion via various pathways with different length. In this regard, mesopores more easily come out for NCS-OTAB due to its larger external area and smaller size relative to NCS-CTAB and allows a hysteresis loop for NCS-OTAB. In the case of NCSA-OTAB sample, the external surface area, mainly arising from mesopores, accounts for 149 m2 g-1 calculated by t-plot method, and the size of mesopores is centered at 3.86 nm (Figure 9 (b) from NL-DFT equilibrium model. As known, the mesopores can provide a larger pool for the access to liquid electrolyte and promotes ion transport, thereby favoring capacitance enhancement. Also, the micro pore size distributions of the activated samples become more concentrated, which are centered at 0.68 and 1.06 nm for NCSA-CTAB and NCSA-OTAB, respectively, compared with 1.61 and 1.54 nm for their counterparts before activation. Similar to NCSA-OTAB, the mesopores are available for the sample of NCSA-CTAB and the size is centered at 3.38 nm deduced from NL-DFT equilibrium model, as demonstrated in Figure 9 (b). Moreover, the external surface area 15



is about 107 m2 g-1 calculated by t-plot method, XPS is employed to further analyze the nitrogen functionalities and contents upon the NCSA-CTAB and NCSA-OTAB, and the results are given in Figure S7. According to Figure S7 and Table 1, XPS reveals that the nitrogen contents of NCSA-CTAB and NCSA-OTAB respectively drop to 0.61 and 0.69 at.%, which nonetheless approach the lowest detection threshold of XPS. Different from original carbon samples before activation, the activated counterparts contain single nitrogen specie N-5 corresponding to binding energy at 399.8 eV, as shown in Figure S7 (b) and (d). It is therefore believed that the activation by KOH at high temperature not only influences the nitrogen content but also changes the nitrogen functionalities containing in carbon materials. 3.2 Electrochemical supercapacitive performance To investigate the electrochemical performance of the activated carbon samples as electrode material for supercapacitor, cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) measurements are carried out in a three-electrode system. In the case of NCSA-OTAB as electrode material, CV, GCD measurements and cycling stability are summarized in Figure 10. According to Figure 10 (a), quasi rectangular shaped CV curves is remained even at the high sweep rate up to 200 mV s-1, suggesting that NCSA-OTAB electrode material possesses excellent capacitive performance and ideal electric double-layer capacitor (EDLC) behavior. Figure 10 (b) shows representative GCD curves of NCSA-OTAB electrode material within the potential range of -1 to 0 V at various current densities from 1 to 30 A g-1. The charge-discharge curves exhibit nearly linear and symmetric features, which represent electrochemical double-layer capacitor (EDLC) with good capacitive property and electrochemical reversibility. Also, there is no obvious 16


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electrode-potential drop emerging on GCD curves, indicating low ion transport resistance and short ion diffusion route of the electrodes.27,28 When current density remains constant at 1 A g-1, the specific capacitance of electrode material based on NCSA-OTAB is up to 247 F g-1, which is much higher than that of the NCSA-CTAB 176 F g-1 as shown in Figure S8. The higher specific capacitance of NCSA-OTAB sample is ascribed to its small uniform size, high surface area and hierarchical (mciro and meso) pore structure. As a matter of fact, the combined effects of smaller particle size and hierarchical (mciro and meso) pore structure of the NCSA-OTAB sample can promote ion transport rate thus favoring the high specific capacitance.2,54,55 In addition, the content of oxygen element of NCSA-OTAB is about 8.96 at.% which is much larger than that of NCSA-CTAB being 5.30 at.% as shown in Table 1. Generally, the hetero-atoms involvement in carbon framework can efficiently improve the supercapacitor performance by boosting the surface polarity and wettability of carbon electrode towards electrolyte.56,57 The rate capability is obtained as displayed in Figure 10 (c) by examining the specific capacitance at different current densities. The capacitance retains 149 F g-1 when the current density is increased to 30 A g-1, corresponding to 64% of capacitance retention at 1 A g-1. Apart from the high capacitance and good rate capability, the electrode material of NCSA-OTAB also exhibits a good cycling stability, retaining 97% capacitance retention even after 5,000 cycles at 1 A g-1 (Figure 10 (d)). Therefore, the carbon spheres here may be a promising electrode material candidate for energy storage. Electrochemical impedance spectroscopy (EIS) is a convincing measurement that offers information addressing the internal resistance of the electrode material and resistance between the electrode and electrolyte. Figure 11 shows the Nyquist plots of the NCSA-CTAB/OTAB in a 17



three-electrode system at open circuit voltage. Obviously, all the Nyquist plots can be divided into three parts, including a depressed semicircle at the high-frequency, a Warburg curve of about 45° slope at middle frequency and a linear part at low frequency.58,59 At high-frequency, the intercept (ESR) on the real axis indicates the contact resistance between the electrode and the current collector, solution resistance and intrinsic resistance of the electrode. Through the equivalent circuit model (the inset), the ESR for NCSA-CTAB and NCSA-OTAB is 0.321 and 0.314 Ω, respectively. Diameter of the semicircle displays the charge transfer resistance (Rct) between the electrode and electrolyte. The value of Rct for NCSA-CTAB and NCSA-OTAB is 0.186 and 0.110 Ω, respectively. It is apparent that the ESR and Rct of NCSA-OTAB are lower than those of NCSA-CTAB, indicating better electron, ion transport properties and excellent electrode materials as supercapacitor materials. Furthermore, two-electrode symmetric cell is installed to estimate the electrochemical performance of activated carbon products. As shown in Figure12 (a), all the CV curves exhibit rectangular-shaped even at a high scan rate of 200 mV s-1, indicating an excellent capacitive performance. According to GCD curves in Figure12 (b) and capacitance retention curves in Figure 12 (c), the gravimetric specific capacitance of NCSA-OTAB is calculated to be 195 F g-1 at 0.1 A g-1, and remains 147 F g-1 at a high current density of 30 A g-1, affording a rate capability of 75.4%. Under the same conditions, the NCSA-CTAB also displays high capacitance retention of 81% (Figure S9). In order to evaluate the energy and power performances of the NCSA-OTAB based symmetric supercapacitors, we further calculate the power density and energy density using the data of Figure 12 via equation 2-4. The NCSA-OTAB based supercapacitor exhibits the specific energy 18


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density of 6.74 Wh kg−1 at a current density of 0.1 A g-1, and the energy density remains 5 Wh kg−1 together even with a high power density of 8411 W kg−1 at a current density of 30 A g-1. According to the results of NCSA-CTAB in Figure S9, as expected, its energy density and power density are slightly lower than those of NCSA-OTAB. These results demonstrate that the NCSA-OTAB based device has a remarkable electrochemical performance and is a promising electrode material candidate. 4. Conclusion In summary, a novel two-step hydrothermal route, which facilitates the incubation of phenolic-resin-based emulsion droplets at low temperature acting as the seeds for the subsequent growth of polymer spheres by monomer addition at high temperature, has been developed to synthesize highly uniform polymer spheres for the precursor of N-doped carbon analogues. Depending on the adjustable size of polymer spheres, the uniform size of N-doped carbon spheres are thereby controlled in the range from 86 to 207 nm. Benefiting from the hierarchical (mciro and meso) pore structure, high specific surface area over 1400 m2/g and narrow size distribution of the activated carbon sample of NCSA-OTAB, the electrode material for supercapacitor affords high specific capacitance, good rate capability and cycle stability, thereby favoring a potential candidate for energy storage. Also, the carbon spheres with highly uniform size, favoring their easy removal after burning in oxygen environment, can be a template to prepare other porous materials Supporting Information Supplementary

data

include

digital

photographs,

SEM

electrochemical performance and TG curve and FT-IR spectra. 19


images

DLS

measurements,


Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21104016), Henan Provincial Department of Education (No. 17A430012) and scientific and technological project of Henan province (Nos. 182102210240, 182300410205). REFERENCE (1)

Hao, G.P.; Li, W.C.; Qian, D.; Wang, G.H.; Zhang, W.P.; Zhang, T.; Wang, A.Q.; Schüth, F.; Bongard, H.J.; Lu, A.H. Structurally designed synthesis of mechanically stable poly(benzoxazine-co-resol)-based porous carbon monoliths and their application as high-performance CO2 capture sorbents. J. Am. Chem. Soc. 2011, 133, 11378-11388.

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Wickramaratne, N.P.; Xu, J.; Wang, M.; Zhu, L.; Dai, L.M.; Jaroniec, M. Nitrogen enriched porous carbon spheres: attractive materials for supercapacitor electrodes and CO2 adsorption. Chem. Mater. 2014, 26, 2820-2828.

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Tiwari, V.K.; Chen, Z.; Gao, F.; Gu, Z.Y.; Sun, X.L.; Ye, Z.B.; Synthesis of ultra-small carbon nanospheres (

Synthesis of highly uniform N-doped porous carbon spheres derived

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