Soft-template assisted hydrothermal synthesis of size-tunable, N-doped porous carbon spheres for supercapacitor electrodes

Results in Physics 12 (2019) 1984–1990

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Soft-template assisted hydrothermal synthesis of size-tunable, N-doped porous carbon spheres for supercapacitor electrodes

T

Zhongguan Lianga, Luomeng Zhanga, Hao Liua, Jianping Zengb, Jianfei Zhoua, Hongjian Lia, ⁎ Hui Xiaa, a b

School of Physics and Electronics, Central South University, Changsha 410083, China School of Physics and Electronics, Hunan University, Changsha 410082, China

ARTICLE INFO

ABSTRACT

Keywords: Soft-template Nitrogen-doped Porous carbon spheres Supercapacitor Energy storage and conversion

Porous carbon spheres ranging in size from nanometer to micrometer have been attracting worldwide attention owing to their excellent performance and wide application. In this paper, nitrogen-doped porous carbon spheres (NPCSs) are successfully prepared by a facile hydrothermal strategy under the assistance of triblock copolymer F108 as soft template. The effect of the ethanol/water volume ratios, concentration of F108 and ammonia, and the alkyl chain of alcohol on the morphologies and particle size of the resulted NPCSs are investigated systematically. Results show that the particle size can be tuned from 160 to 2000 nm by adjusting the synthesis parameters. The obtained NPCSs exhibit a large specific surface area (1481 m2 g−1) and suitable nitrogen-doped concentration (2.4 at%). As electrode material for supercapacitors, the NPCSs present a high specific capacitance of 365 F g−1 at 0.5 A g−1 and outstanding cycling stability with 93.9% retention after 10 000 cycles.

Introduction Green, environmental-friendly and sustainable energy storage devices have been received considerable attention with increasing of the global climate change and traditional energy depletion [1]. Supercapacitors (SCs) are the promising energy storing device due to its advantages of high power density, short charging time and excellent cycle stability [2–5]. The electrochemical performance of SCs was highly dependent on the physical and chemical properties, surfacefunctionalization, and the pore structure of electrode materials [6–8]. Although great efforts have been done for high performance electrode materials in recent decades, carbon materials still dominate the SCs applications owing to their stable physicochemical properties, good electrical conductivity and long life span [9–15]. Nitrogen-doped porous carbon spheres (NPCSs) with the unique structural features, enhanced surface properties and advanced porosities have shown that it’s a promising candidate electrode material for SCs [16–20]. Currently, many synthetic methods have been reported for preparing carbon spheres (CSs), including hard-templating [21], softtemplating [22], self-assembly [23], hydrothermal [24] and the Stöber method [25] et al. Among them, the hydrothermal method has been

⁎

widely used for the synthesis of CSs at relatively low temperature (160–200 °C) due to its low cost and high efficiency. A series of functionalities CSs could be obtained by this method which the different precursors were used [18,26,27]. However, its wider applications are still limited because of difficulties in controlling the porosity and size of the CSs [28]. Therefore, using hydrothermal method to synthesize of CSs with well-defined graded porous (containing micropores and mesoporous simultaneously) and tunable size is still a significant challenge, and the further research in this direction is required. Herein, we demonstrate a facile soft-templating assisted hydrothermal method for the preparation of size-tunable, N-doped porous CSs. This synthesis method is a feasible strategy for preparation CSs and is applicable to extensive industrial production at low cost. The synthesized CSs with uniform and controllable particle size, which can be tuned from 160 to 2000 nm by adjusting the ethanol/water volume ratios, concentration of F108 and ammonia, or the alkyl chain of alcohol. Benefit from the synergetic contribution of the high specific surface area and the pseudocapacitance provided by the N- and Odoped, the resultant NPCSs as electrode materials for supercapacitors exhibit an outstanding electrochemical performance.

Corresponding author. E-mail address: [email protected] (H. Xia).

https://doi.org/10.1016/j.rinp.2019.01.074 Received 26 October 2018; Received in revised form 25 January 2019; Accepted 25 January 2019 Available online 30 January 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

Results in Physics 12 (2019) 1984–1990

Z. Liang, et al.

Experimental

Characterization

Materials synthesis

Scanning electron microscopy (SEM) images were recorded on a Nova NanoSEM230 instrument. Transmission electron microscopy (TEM) was observation with a Tecnai G2 F20 S-TWIX instrument. X-ray photo-electron spectroscopy (XPS) analysis was conducted on an ESCALAB 250Xi instrument with Al Kα radiation. Nitrogen adsorptiondesorption isotherms were measured at 77 K on an ASAP 2020 instrument.

In a typical synthesis of the NPCSs, 0–6 mL ammonia aqueous solution (25 wt%) was mixed with 80 mL mixture solvent containing ethanol and deionized water (H2O) (the volume ratio of ethanol/H2O were 0.6:1, 0.51:1, 0.43:1 and 0.36:1). Then, 0–0.75 g Triblock copolymer Pluronic F108 (Mw = 14,600, PEO132-PPO50-PEO132) were added in above solution and stirring at room temperature for 10 min. Subsequently, 1.2 g phenol and 4.5 mL formaldehyde (37 wt%) were added and continue stirring for 20 min. After that, the resulting solution was followed by hydrothermal reaction at 170 °C for 6 h in a 100 mL Teflon-lined autoclave. The obtained pale yellow phenol–formaldehyde (PF) resin polymer spheres were rinsed by H2O and ethanol for several times, and air-dried at 80 °C more than 12 h. After collection, the products were annealed at 600 °C for 3 h under N2 flow, and followed by KOH activated in mass ratio of 1:2 at 700 °C for 1 h to obtain the NPCSs.

Electrochemical measurement All the electrochemical performance measurements of cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopes (EIS) were performed on a CHI660E electrochemical workstation with a classical three-electrode system in 6 M KOH electrolyte solution using platinum foil and Hg/HgO as the counter electrode and reference electrode, respectively. The working electrodes were fabricated by the mixing of the NPCSs, Polyte

Fig. 1. (a–d) SEM and (e–h) TEM images of the NPCSs prepared at F108 concentration of 0.392 mmol L−1, ammonia concentration of 0.46 mol L−1, and different ethanol/H2O volume ratios of: (a), (e) 0.6:1; (b), (f) 0.51:1; (c), (g) 0.43:1 and (d), (h) 0.36:1. (i–l) The particle size distribution of NPCSs corresponding to the SEM images (a–d).

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Results in Physics 12 (2019) 1984–1990

Z. Liang, et al.

Fig. 2. SEM images of the NPCSs prepared at ethanol/H2O volume ratios of 0.51:1, ammonia concentration of 0.46 mol L−1, and different F108 concentration of: (a) 0 mmol L−1, (b) 0.196 mmol L−1, (c) 0.392 mmol L−1, and (d) 0.587 mmol L−1.

trafluoroethylene (60 wt%) and acetylene black with a mass proportion of 8:1:1, and the mass of the active materials in each piece working electrode was about 3 mg cm−2. The gravimetric specific capacitance was calculated by the following equation:

Cg =

I t m V

concentration can be used to control the diameters of the NPCSs. By increasing the F108 concentration from 0.196 to 0.392 and 0.587 mmol L−1, the particle diameters decrease from 1800 to 1300 and 750 nm, respectively, as show in Fig. 2b–d. However, the NPCSs show non-uniform size and encountered agglomeration at the condition of without F108 (Fig. 2a). It is indicated that F108 can act as a surfactant to effectively inhibit the agglomeration of PF resin, and also play the role of structural-directing agent. The NPCSs size can also be adjusted by the ammonia concentration in this synthesis method. Fig. 3a–c present the SEM images of NPCSs synthesized at different ammonia concentration. The particle diameter decreased from 1700 to 1300 and 180 nm with increasing the ammonia concentration from 0.16 to 0.46 and 0.89 mol L−1. Notably, there is no product obtained in the absence of ammonia. Thus, ammonia is a catalyst which can promote the whole hydrothermal reaction process. In addition, further studies reveal that the alcohol (such as methanol, ethanol, isopropanol and n-butyl alcohol) with different alkyl chains also have effect on the size of the NPCSs, as show in Fig. 4. When methanol is used, which has one alkyl chain, the obtained NPCSs diameter is around 160 nm. Change the number of alkyl chain to two (ethanol), three (isopropanol) and four (n-butyl alcohol), the particle size increases to 1300, 1600 and 2000 nm. The more the number of alkyl chain the higher molecular weight of alcohol are, makes the higher viscosity of reaction solution and the stronger surface tension, and thus form larger size emulsion droplets [25], and results in the NPCSs with a larger diameter. The results shown above suggest that the NPCSs prepared using this method have tunable size from 160 to 2000 nm by adjusting the ethanol/H2O volume ratios, concentration of F108 and ammonia, or the alkyl chain of alcohol. Scheme 1 illustrates the possible synthesis

(1)

where I (A), Δt (s), ΔV (V), and m (g) is the applied current, discharge time, potential window, and the active material mass of the electrodes, respectively. Results and discussion In this work, the NPCSs are prepared using resol (phenol-formaldehyde) as a carbon precursor and a triblock copolymer Pluronic F108 as a soft template via a facile hydrothermal strategy. Fig. 1a–d shown the SEM images of NPCSs synthesized at different ethanol/H2O volume ratios from 0.6:1 to 0.36:1. It is indicates that the NPCSs have spherical particles with a uniform size. TEM images present in Fig. 1e–h are further confirms the NPCSs have perfect spherical morphology, smooth surface and highly dispersity. When the ethanol/H2O volume ratio is 0.6:1, the TEM image (Fig. 1e) demonstrates that those NPCSs have a mean size of 1800 nm with good dispersity. With decreasing the ethanol/H2O volume ratio to 0.51:1, 0.43:1 and 0.36:1, the diameters of NPCSs are reduced to 1300, 600 and 190 nm, respectively. Fig. 1i–l present the corresponding particle size distribution. With increasing the quotient of water, the surface tension decreases [29], lead to form smaller size emulsion droplets, and final to decrease the diameter of NPCSs. Not only the ethanol/H2O volume ratio but also the F108

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Z. Liang, et al.

Fig. 3. SEM images of the NPCSs prepared at ethanol/H2O volume ratios of 0.51:1, F108 concentration of 0.392 mmol L−1, and different ammonia concentration of: (a) 0.16 mol L−1, (b) 0.46 mol L−1, and (c) 0.89 mol L−1.

Fig. 4. SEM images of the NPCSs prepared at F108 concentration of 0.392 mmol L−1, ammonia concentration of 0.46 mol L−1, and alcohol/H2O volume ratios of 0.51:1 by using alcohols with different alkyl chains: (a) methanol, (b) ethanol, (c) isopropanol, and (d) n-butyl alcohol.

H2O volume ratios of 0.51:1, F108 concentration of 0.392 mmol L−1 and ammonia concentration of 0.46 mol L−1 was selected as a sample further used to characterization analysis. As shown in Fig. 5a, the Raman spectrum of the NPCSs exhibits two typical peaks at 1352 cm−1 (D band) and 1581 cm−1 (G band), which are corresponds to the crystal defects and the hexagonal graphitic of carbon materials, respectively. The intensity ratio (ID/IG) of carbon materials reflects the graphitization degree. The ID/IG value is about 0.87, present the NPCSs have an amorphous structures and high graphitization degree. The elementary compositions of the as-prepared NPCSs were determined by XPS. Fig. 5b shows three obvious peaks at binding energy of 285.7, 399.7 and 534.0 eV, which corroborates the existence of C, N and O element. The corresponding relative proportions of C, N and O are 91.4 at%, 2.4 at% and 6.2 at%, respectively. Usually, N- and O-doped have been considered an effective way to optimize the carbon materials, not only enhance the pseudocapacitance but also improve the surface wettability

mechanism. The first step, the emulsion droplets formed through hydrogen bonding interaction between the F108 micelles, alcohol, H2O, ammonia and PF precursors. The second step is the hydrothermal reaction process, the PF resin spheres are formed through cross-linking polymerize of the emulsions by ammonia molecules catalysis. The third step, the PF resin spheres via high temperature carbonization remove the soft template F108 and followed by KOH activation to obtain the NPCSs. The SEM results shown the NPCSs have perfect spherical morphology and smooth surface, and without collapse after high temperature carbonization. It is indicated that the frameworks of triblock copolymer F108/PF resin composites with a high degree of crosslinking. These synthesized NPCSs may have some potential applications such as catalysis, adsorption, electrode materials for supercapacitors and lithium-ion batteries. In order to understand the structure property of the as-prepared material, the NPCSs material prepared at ethanol/ 1987

Results in Physics 12 (2019) 1984–1990

Z. Liang, et al.

Scheme 1. The preparation process of NPCSs using the hydrothermal method.

Fig. 5. (a) Raman spectra, (b) XPS survey spectra, (c) nitrogen adsorption/desorption isotherms and (d) pore size distribution curves of the NPCSs material prepared at ethanol/H2O volume ratios of 0.51:1, F108 concentration of 0.392 mmol L−1 and ammonia concentration of 0.46 mol L−1.

The NPCSs have a high specific surface area of 1481 m2 g−1 owing to the high micropore contents. Furthermore, the large pore volume of 0.897 cm3 g−1 can facilitates the ionic transportation and exchange, and then benefits to the electrochemical performances. So, it is reasonable to infer that the NPCSs will have excellent electrochemical performances as electrode materials for supercapacitors. In order to investigate the structural and electrochemical performances advantages of the NPCSs, the CV, GCD and EIS were carried out to evaluate the as-prepared material as supercapacitor electrodes. Fig. 6a presents the CV curves of NPCSs electrode at scan rates of 10, 20, 50 and 100 mV s−1. The quasi-rectangular shapes are observed at all scan rates, suggest that the NPCSs electrodes have an ideal electrical double-layer capacitor behavior. In addition, the reversible humps demonstrated in the potential window of −0.9 V to −0.3 V due to the redox reaction provided by N- and O-doped. According to the GCD

of the materials [30]. The specific surface areas and pore structures of NPCSs were investigated by the N2 adsorption/desorpotion isotherm and porosity characteristic. Fig. 5c presents the isotherm of the NPCSs, it is belong to the pseudo-type I isotherm. A steep increase at low relative pressures of P/P0 < 0.01 and a high N2 adsorption horizontal plateau at relative pressures of 0.1 < P/P0 < 0.8 are observed, which suggesting that the NPCSs existence of plenty of micropores and has high specific surface area. In addition, a little hysteresis loop with a H3 character at relative pressures of 0.8 < P/P0 < 1, indicating the presence of few mesopores. The pore size distribution curve (Fig. 5d) verifies that the coexistence of developed micropore and few mesopore in NPCSs. The micropores may result from the high temperature pyrolysis of F108 and PF polymer, and the chemical activity of KOH. While the mesopores could be correspond to the voids from the stacking of NPCSs particles. 1988

Results in Physics 12 (2019) 1984–1990

Z. Liang, et al.

Fig. 6. The electrochemical characteristics of the NPCSs material prepared at ethanol/H2O volume ratios of 0.51:1, F108 concentration of 0.392 mmol L−1 and ammonia concentration of 0.46 mol L−1. (a) CV curves of the NPCSs electrode at different scan rates from 10 to 100 mV s−1, (b) GCD curves of the NPCSs electrode at different current densities from 0.5 to 20 A g−1, (c) Specific capacitance of the NMCSs electrode as a function of current densities, and (d) Nyquist plot of the NPCSs electrode and the inset gives the magnify plots at high frequency range, (e) Bode plot of the NPCSs electrode, (f) Cycling performance of the NPCSs electrode at current density of 20 A g−1 for 10 000 cycles.

93.9% retention over 10 000 cycles at a current density of 20 A g−1 (Fig. 6f). Those all demonstrate that the as-prepared NPCSs are an ideal electrode material for high performance supercapacitors.

profiles (Fig. 6b), the symmetrical triangular shapes show a reversible charge/discharge process and without obvious IR drop. The specific capacitances at different current densities are recorded in Fig. 6c. A high specific capacitances of 365 F g−1 can be obtained at 0.5 A g−1 and maintains 242 F g−1 at 20 A g−1, it is reflecting a good rate performance for NPCSs electrodes. The electrochemical performance comparisons of the NPCSs to other CSs materials which have reported in the literatures are summarized in Table 1. As a result, the specific capacitance of the NPCSs has prominent advantages over most CSs, which is attributed to the synergetic contribution of the high specific surface area and the pseudocapacitance provided by the N- and Odoped. Fig. 6d presents the Nyquist plot, a small equivalent series resistance of 0.78 Ω indicates a good electrical conductivity of the prepared NPCSs materials. Furthermore, an almost vertical line is observed in the low frequency region, and the Bode plot (Fig. 6e) shows the phase angle (−84.5°) is close to −90° which approximate an ideal capacitor. In addition, the NPCSs electrode shows good cycling stability with

Conclusions In summary, the NPCSs have been successfully synthesized through a facile soft-templating assisted hydrothermal method. The triblock copolymer F108 was used as a soft template, and ammonia as both of the catalyst and nitrogen source. Results show that the prepared NPCSs have uniform size and good dispersity, and the particle size can be tuned from 160 to 2000 nm by adjusting the ethanol/H2O volume ratios, concentration of F108 and ammonia, or the alkyl chain of alcohol. The NPCSs have a high specific surface area of 1481 m2 g−1 and suitable nitrogen-doped concentration of 2.4 at%. As a result, the NPCSs as electrode materials for supercapacitors exhibited a high specific capacitance (365 F g−1 at current density of 0.5 A g−1) and outstanding 1989

Results in Physics 12 (2019) 1984–1990

Z. Liang, et al.

Table 1 Comparison of electrochemical performances of recently reported porous carbon spheres. Carbon materials

Specific capacitance

N-OMCSa ACNSb MCNSc PCNSd N-UCNse MCMsf MMCSsg O-MCSh NPCSs

288 243 224 132 269 289 314 226 365

a b c d e f g h

(0.1 A g−1) (0.2 A g−1) (0.2 A g−1) (0.2 A g−1) (1.0 A g−1) (1.0 A g−1) (0.5 A g−1) (1.0 A g−1) (0.5 A g−1)

[11]

Cycling performance

Electrolyte

Ref.

[12]

100% (20 000) 96.1% (10 000) 93% (10 000) 97.5% (10 000) 90.3% (10 000) 90.3% (10 000) 96% (500) 75.8% (10 000) 93.9% (10 000)

6M 6M 6M 6M 6M 6M 6M 6M 6M

[19] [31] [12] [32] [18] [33] [34]

[13]

This work

[16]

KOH KOH KOH KOH KOH KOH KOH KOH KOH

[14] [15]

[17]

N-doped ordered mesoporous CSs. Activated carbon nanospheres. Monodisperse carbon nanospheres. Porous carbon nanospheres. N-containing ultramicroporous carbon nanospheres. Mesoporous carbon microspheres. Micro- and mesoporous CSs. Order-mesoporous CSs.

[18] [19] [20]

[21]

cycling stability (93.9% capacitance retention after 10 000 cycles). In addition, these NPCSs are also potentially for application in catalysis, adsorption, water purification and energy storage and conversion.

[22] [23]

Acknowledgements

[24]

This work is supported by the Graduate Independent Exploration and Innovation Project of Central South University (No. 2018zzts008).

[25]

References

[26]

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