Nitrogen-Doped Mesoporous Carbons for Supercapacitor Electrodes

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Nitrogen-doped mesoporous carbons for supercapacitor electrode with high specific volumetric capacitance Xiaoqing Yang, Hong Ma, and Guoqing Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00489 • Publication Date (Web): 06 Apr 2017 Downloaded from http://pubs.acs.org on April 7, 2017

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Nitrogen-doped mesoporous carbons for supercapacitor electrode with high specific volumetric capacitance Xiaoqing Yang*, Hong Ma, Guoqing Zhang School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, PR China

Abstract: For pursuing the miniaturization of supercapacitors in practical use, it is critical to construct an efficient but limited porosity of nanocarbon-based electrode for simultaneously obtaining a high utilization of energy storage places and high coating density. However, current studies dominantly focus on the enhancement of specific mass capacitance (Cm) by increasing the pore volume and surface area, leading to a low coating density and thereby resulting in a low specific volumetric capacitance (CV). We report herein the fabrication of a nitrogen-doped mesoporous carbon (NNCM), whose tunable pore volume coupled with the fixed mesopore size offers us the possibility to control the coating density, thus optimizing the CV and Cm for different application purposes. As a result, NNCM with the highest pore volume and surface area of 2.11 cm3 g-1 and 663 m2 g-1 demonstrates the highest Cm (190 F g-1) but lowest CV (124 F cm-3) because the overhigh porosity reduces the coating density greatly. NNCM with moderate pore volume and surface area of 1.22 cm3 g-1 and 489 * Corresponding authors Tel：+86-020-39322570 (X. Yang) E-mail addresses: [email protected] (X. Yang)

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m2 g-1 shows the highest CV of 200 F cm-3, although it presents a low Cm of 147 F g-1. These results may raise concerns about constructing a suitable porosity to realize a target oriented use, particularly those targeting miniaturized devices.

Keywords: Nitrogen-doped; Porous carbon material; Mesopore; Electrochemical performance; Supercapacitor.


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1. Introduction Nanoporous carbons (NC) with well-defined nanostructure have sparked enormous interest in supercapacitor application due to their relatively low cost, high conductivity and stable physical-chemistry properties

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15

.In

order to increase the specific mass capacitance (Cm) and the corresponding energy density (in W h kg-1), numerous studies have been focusing on constructing a developed porosity of NC, including high pore volume and large surface area 16, 17, 18, 19, 20, 21

. For instance, Zhong et. al

13

13, 14, 15,

prepared a kind of hierarchical porous

carbon with an ultrahigh surface area of 3000 m2 g-1 for supercapacitor application, it showed an extremely high energy density of 118 W h kg-1 at a power density of 100 W kg-1. Nitrogen-doped graphene nanosheets were fabricated by Wen et al.

20

, their

ultrahigh pore volume of 3.42 cm3 g-1 provided rich sites for adsorbing ions and accelerating the electrolyte transfer capability, thus obtaining a high Cm of 246 F g-1 and excellent rate capability (227 F g-1 at 10 A g-1). However, in the pursuit of miniaturization in practical application nowadays, it should be pointed out that these approaches based on Cm will be misleading. A much more suitable benchmark for evaluating the applicability of the NC-based electrode is the specific volumetric capacitance (CV)

21

. Apparently, a high CV requires a

harmonious environment of an acceptable Cm coupled with a high coating density of the NC on the current corrector. This means that the pore volume and corresponding surface area should be limited to a certain extent. Unfortunately, it is contradictory that a low surface area usually leads to the lack of charge accumulation places, thus


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giving rise to a low Cm, while a low pore volume will reduce the mass diffusion/transfer capability of the electrolyte, decreasing the high-rate performance of the materials

22, 23, 24

. For example, in the investigation of Fang et. al

23

, carbon

aerogel (CA) with a relatively low surface area of 592 m2 g-1 presented a low Cm of 50 F g-1. After activation by KOH, the surface area of the activated CA (ACA) was increased to 2371 m2 g-1, giving rise to a much higher Cm of 130 F g-1. Nevertheless, a much lower capacitance retention of ACA as compared to that of CA (46 % v.s. 80%) was obtained since the mesopore size and pore volume of CA were decreased from 7.5 nm and 0.43 cm3 g-1 to 2 nm and 0.32 cm3 g-1 after activation, respectively. In addition, as far as we know, few efforts have been devoted to investigate the dependence of CV on the nanostructure, which becomes a major barrier for optimizing the performance of the NC in practical application. Based on the aforementioned facts, we conceive that a superior NC-based electrode material with high CV for practical use should exhibit the following characteristics: (1) relatively low porosity and surface area for guaranteeing a high coating density of the current collector; (2) a suitably developed pore structure beneficial to electrolyte diffusion/transfer for increasing the utilization of the limited surface area, that is, increasing the specific capacitance per surface area (CS) 22; (3) introducing heteroatom with pseudocapacitance for further increasing the CS and specific capacitance per pore volume (CPV) within the limited surface area and pore volume. Therefore, in the present work, we develop a series of nitrogen-doped NC with a 3D continuous mesoporous structure (NNCM) for supercapacitor application. The


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porosity represented by pore volume can be easily tailored within a wide range while the mesopore size is fixed by simply controlling the concentration of the PAN/DMF solution (Figure 1). As highlighted here, the 3D continuous mesoporous structure has been proposed to demonstrate high mass transfer efficiency

15, 22, 25

, especially under

the limited porosity; The nitrogen functional groups (NFGs) are believed to provide extra pseudocapacitance14, 20, 26, 27, 28; The tunable pore volume with the fixed pore size offers us the possibility to optimize the CV and Cm for different purposes: For pursuing light-weight devices, a high Cm should prove optimal, but for small and compact electric power sources, focusing on a high CV is undoubtedly more beneficial. High PAN concentration

High Carbon/ Silica ratio

Low porosity

TEOS H2O

Hydrolysis

PAN/DMF

HF

Gelation

Solution exchange

DMF

Excess HF

Drying Preoxidation/ Carbonization

Low PAN concentration

Low Carbon/ Silica ratio

Template removing

High porosity

Figure 1. Schematic diagram for the preparation of NNCMs with tunable porosity.

2. Experimental 2.1 Materials preparation First, a kind of silica gel template with 3D continuous skeleton was prepared via a


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simple sol-gel process: N, N-dimethylformamide (DMF), tetraethyl orthosilicate (TEOS), deionized water and hydrofluoric acid (HF, 40 wt%) were mixed and magnetically stirred continuously in a volume proportion of 5:2:1:0.2. After complete homogenization, the obtained mixture was gelated and aged at 90 oC for 3 days. Secondly, the obtained silica gel was transferred into a flask followed by adding polyacrylonitrile (PAN) /DMF solution with different concentrations. The mixture was oscillated with a rotational speed of 180 r min-1 at 30 oC for 1 day. After dried at 90 oC, the resulting samples were stabilized in a muffle furnace at 300 oC and then carbonized at 900 oC for 3 h under N2 atmosphere with a heating rate of 5 oC min-1. Thirdly, excessive HF solution was used to remove the silica from the obtained carbon/silica composites. The obtained PAN-based NNCMs were denoted as NNCM-x, where x represented the concentrations of the PAN solution (in g 10 mLDMF-1). 2.2 Materials characterization The X-ray diffraction (XRD) patterns were obtained using a diffractometer equipped with a Cu Ka source by D/MAX 2200 VPC equipment. Raman spectra were conducted on a Renishaw inVia Laser Micro-Raman spectrometer. X-ray photoelectron spectroscopy (XPS) measurement was carried out with an ESCALAB250 instrument. Elemental analysis (EA) was performed using a Vario EL CHNS Elemental Analyzer (Elementar Corporation, Germany). The morphology and nanostructure of the samples were observed by a scanning electron microscopy (SEM, JSM-6330F) and a transmission electron microscope (TEM, JEOL JEM-2010). A


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Surface Area and Pore Size Analyzers (ASAP 2460, Micromeritics, USA) was used to obtain the nitrogen adsorption-desorption isotherms. BET surface area (SBET) was calculated based on Brunauer-Emmett-Teller (BET) method. The total pore volume (Vtotal) was estimated from the adsorbed quantity at a relative pressure (P/P0) of c.a. 0.997. Micropore volume (Vmic), micropore surface area (Smic), mesopore volume (Vmes), mesopore surface area (Smes), and mesopore size distribution of the samples were analyzed by t-plot theory and BJH (Barrett-Joyner-Halendar) theory, respectively. 2.3 Electrochemical measurements The electrodes consisting of NNCM, poly (vinylidene difluoride) (PVDF) and carbon black (CB) were used as working and counter electrode. To prepare the electrode, NNCM was mixed with CB and PVDF at a mass ratio of 8:1:1. N-methyl pyrrolidone was used as the solvent of PVDF. The obtained paste was pressed onto the current collector of titanium foil uniformly under 10 Mpa and dried in vacuum at 120 oC for 12 h. The thickness of the mixture coated on the current collector was measured from the SEM images of the electrode after coating. The weight of the mixture paste varied between 2-5 mg (equivalent to 1.6-4.0 mg of the active material) with a thickness of about 40 µm (Table S1 in the Supplementary Information). The electrochemical performance was measured in 1 M H2SO4 electrolyte using a sandwich-type coin supercapacitor. Cyclic voltammetry (CV) at a scan rate of 200 mV s-1 in the testing window of 0-1 V and electrochemical impedance spectroscopy (excitation signal: 5 mV and frequency range: 0.001-100,000 Hz) were carried out


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using a CHI660E electrochemical workstation. Galvanostatic charge-discharge (GCD) tests were executed by a Neware Battery Program-control Test System (CT30008W) at different current densities from 0.1 to 20 A g-1 at 0-1 V. The Cm (in F g-1) of NNCMs was calculated by the formula of =

×∆ ∆

×

×

, where I represented

the discharge current; △t represented the discharge time; △U was the discharge voltage; m1 and m2 were the mass of the positive and negative active electrode materials. CS (in µF cm-2) and CPV (in F cm-3) were obtained by dividing the Cm by the

SBET ( =

) and Vtotal ( =

), respectively. CV (in F cm-3) was achieved by

multiplying the Cm by the coating density ( = × ), where represented the coating density (The coating parameters and calculation of were shown in Table S1 in the Supplementary Information).

3. Results and discussion NNCM-0.3 NNCM-0.5 NNCM-1 NNCM-1.5

(B)

(A) Intensity (a.u.)

NNCM-0.3 Intensity (a.u.)

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NNCM-0.5

NNCM-1

ID/IG=0.84 ID/IG=0.86 ID/IG=0.86 ID/IG=0.88

NNCM-1.5 10

20

30

40

50

60

2 theta degree

70

80

600

900

1200 1500 1800 -1 Wavenumber (cm )

2100

2400

Figure 2. (A) XRD patterns and (B) Raman spectra of the NNCMs. Figure 2A shows the XRD patterns of the NNCM samples. According to the JCPDS Card No. 04-0850, two peaks observed at 22 o and 44 o (2θ) correspond to the (002) and (101) diffraction of hexagonal graphite

29, 30, 31, 32


. The d-spacings between

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(002) planes of all NNCMs are found to be about 3.9-4.0 Å, much larger than that of graphite (3.4 Å), suggesting the characteristics of amorphous carbon with a low degree of graphitization. Raman spectra of the NNCMs in Figure 2B depict two peaks around 1350 cm-1 (D-band) and 1590 cm-1 (G-band). The D-band is attributed to the disordered structures of carbon, while the G-band is associated with the phonon mode with E2g symmetry of graphite

30, 31

. The relatively high intensity ratio of D/G bands

(ID/IG) confirms the low graphitization degree of the NNCMs, i.e., abundant structural defects and pores. SEM and TEM images are presented below to directly demonstrate the porous structure of NNCMs (Figure 3).

Figure 3. SEM and TEM images of (A, E) NNCM-0.3, (B, F) NNCM-0.5, (C, G) NNCM-1 and (D, H) NNCM-1.5. As vividly seen from Figure 3, all the NNCM samples give a 3D continuous carbon nanoframework and numerous continuous mesopores. Obviously, PAN concentration plays an important role in tailoring the nanomorphology and porosity of the obtained NNCMs. As increasing the PAN concentration, the nanoframework becomes more


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and more compact and thus the porosity is decreased sharply. This can be explained by the different PAN/silica ratio under different PAN soaking concentration. Briefly, a lower PAN concentration will result in a lower PAN/silica ratio of the precursor composite, leading to a lower carbon proportion of the resulting carbon/silica

1400

0.06

(A) Pore Volume (cm g )

1200

NNCM-0.3 NNCM-0.5 NNCM-1 NNCM-1.5

1000


800

3

3

(B)

0.05

-1

-1

STP)

composite. After template removing, a larger porosity can be obtained (Figure 1).

Quantity Adsorbed (cm g

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

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600 400 200

0.04 0.03 0.02 0.01 0.00

0 0.0

0.2

0.4

0.6

0.8

0

1.0

20

40

60

80

100

120

140

Pore width (nm)

Realative presure (P/P0)

Figure 4. (A) N2 adsorption-desorption isotherms and (B) BJH mesopore size distribution curves of the NNCMs. Table 1 Pore structure parameters of the NNCMs.

Sample

SBET

Smic

Smes

Vtotal

Vmic

Vmes

(m2 g-1)

(m2 g-1)

(m2 g-1)

(cm3 g-1)

(cm3 g-1)

(cm3 g-1)

NNCM-0.3

663

245

378

2.11

0.11

2.04

NNCM-0.5

593

226

313

1.70

0.10

1.61

NNCM-1

489

221

273

1.22

0.10

1.14

NNCM-1.5

391

174

181

0.90

0.08

0.83

Likewise, nitrogen adsorption-desorption tests were performed to confirm the controllable nanostructure of the obtained NNCMs quantitatively. All the isotherms in Figure 4A show obvious uptakes at high relative pressure (P/P0>0.9), indicative of a typical mesoporous structure. The increasing adsorbed quantity from NNCM-1.5 to NNCM-0.3 confirms the increasing porosity with decreasing the PAN soaking


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concentration, which is consistent with the SEM and TEM observations. According to the isotherms, BJH pore size distribution curves and pore structure parameters are calculated and the results are shown in Figure 4B and Table 1, respectively. Without doubt, Vtotal and SBET are decreased sharply as we raise the soaking concentration of PAN/DMF solution. This is primarily derived from the sharply decreasing Vmes (Table 1). Interestingly, despite the wide range of the pore volume, the mesopore size of all samples is fixed around 30 nm because of the same radial dimension of silica templates. This kind of 3D mesoporous morphology and high mesopore ratio will endow the obtained NNCMs with a smooth and easy ion transfer/diffusion capability, thus optimizing the surface area utilization. Additionally, the fixed mesopore size coupled with the controllable pore volume imparts to us the possibility to investigate the supercapacitive dependence on the porosity and thus obtains the optimal nanostructure of the NNCM for different application purposes. Therefore, CV, EIS and GCD tests of the NNCM samples were executed by using coin-type supercapacitors. Generally, ion diffusion/transfer rate within a nanoporous carbon structure can be estimated by the rectangle degree of the CV curves 22. The higher the rectangle degree, the faster is the ion diffusion/transfer rate, particularly at high scan rates. As shown in Figure 5A, all of the NNCM samples have good rectangular-shaped CV curves at a high scan rate of 200 mV s-1, indicative of a superior ion diffusion/transfer capability in such a 3D continuous mesoporous structure. This can be further supported by the


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Nyquist plots in Figure 5B. It is well known that the diameter of the semicircle in the high frequency region reflects the polarization resistance or charge transfer resistance (Rp/Rct) 30, 31. All the samples show relatively low Rp/Rct of 0.7-1.5 Ω. In addition, the 3D continuous carbon nanoframework also endows the NNCMs with high conductivity, as evidenced from their low equivalent series resistance (ESR) of c.a. 0.25 Ω, which can be obtained from the initial intersection between the curve and Z’ axis in Nyquist plots 30, 31.

(A)

20

-1

Current density (A g )

30

10 0 -10


-20 -30 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Voltage (V) 40

1.0

(B)

(C)


0.8

Voltage (V)

30

- Z'' (Ohm)

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20


10

0.6 0.4 0.2 0.0

0 0

1

2

3

4

5

6

0

500

Z' (Ohm)

1000

1500

2000

Time (s)

Figure 5. (A) CV curves at a scan rate of 200 mV s-1, (B) Nyquist plots and (C) GCD curves at a current density of 0.1 A g-1 of the NNCMs. Figure 5C gives the GCD curves of the NNCMs at the current density of 0.1 A g-1. Apparently, the discharge time decreases with the same sequence of SBET


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(NNCM-1.5

Nitrogen-Doped Mesoporous Carbons for Supercapacitor Electrodes

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