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Microporous carbon materials by hydrogen treatment: The balance of porosity and graphitization upon the capacitive performance Wei Hu, Xiao Na Sun, Dong Xu, Zhenghui Xiao, and Xiang Ying Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 10, 2017

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Industrial & Engineering Chemistry Research

Microporous carbon materials by hydrogen treatment: The balance of porosity and graphitization upon the capacitive performance Wei Hu, Xiao Na Sun, Dong Xu, Zheng Hui Xiao*, and Xiang Ying Chen* School of Chemistry & Chemical Engineering, Anhui Key Laboratory of Controllable Chemistry Reaction & Material Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, P. R. China. * The corresponding authors. E-mail: [email protected]. Abstract The balance of porosity and graphitization towards carbon materials plays crucial role in determining the capacitive performance. In this work, this purpose has been successfully implemented by adjusting the carbonization temperature and hydrogen gas treatment. Oxygen containing functional groups have conspicuously reduced by the co-effects of hydrogen gas and high temperature on carbon materials. Besides, by treatment temperature at 800 °C with hydrogen, the electrochemical performances have greatly improved. The electrode treated by the temperature of 800 °C with hydrogen displays higher specific capacitances of 171 F g‒1 compared with that of bare carbon electrode of 145 F g‒1, owing to enlarged BET surface area, pore volume and the reduced resistance. At the same time, the electrode treated by the temperature of 800 °C with hydrogen exhibits a higher cycling stability of 96.3% of primary specific capacitances after 5000 cycles and energy density of 8.36 Wh kg‒1, respectively.

Keywords: Carbon materials; Hydrogen treatment; Surface property; Electrochemical performances; Supercapacitor.

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ACS Paragon Plus Environment


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1. Introduction Supercapacitor, as an important type of energy storage device, have gravitated intense attention and stimulated widely research over the past years owing to their long cycle lives, low costs and speedy charge and discharge processes.1-4 Nevertheless, compared with lithium-ion batteries, they generally displays the lower energy density.5, 6 According to the equation: E=1/2CV2, in which C is related to the total capacitances, meanwhile, V is corresponding to the operating potential of the cell. Therefore, the enhancements of total capacitances and broad operating potential window are significant for the supercapacitor to obtain high energy density.7-9 In terms of the promotion of total capacitances for supercapacitor, it can be classified into four parts as follows: surface areas, pore size, electrical conductivity, and additional redox capacitances.10 Specifically, in connection with the carbon materials, the improvement of specific capacitances can be realized through enlarging the specific surface areas and pore size distributions. Moreover, porosity is also a considerable factor of elevating the specific capacitances. As we know, the regulation of porosity is prepared by two methods: (1) in situ incorporating template including 1) hard template i.e., aluminium oxide membranes, zeolites and mesoporous silicas; 2) soft template i.e., F127 and PEO-PPO-PEO;11-13 (2) ex situ activation of the carbon materials by KOH or CO2 which can open occlusive pores and therefore enlarge pore size.14, 15 Nevertheless, both of them have lethal weaknesses for pollution and high expense. On the other hand, carbon materials with crystallization show advantages compared with amorphous carbon which are mainly owing to the high electronic conductivity, well-developed crystalline structure, satisfactory oxidation resistance and thermal stability.16, 17 Hence, how to increase graphitization degree applying for carbon materials is interesting, which mainly can be achieved by two aspects as follows. Firstly, incorporating metal catalysts as well as their oxides and salts into carbon precursors have been widely put into operation, i.e., Fe and Ni,18, 19 FeCl2 and FeSO4,20 NiCl2.21 However, metal catalysts and their oxides and salts have obvious disadvantages of high expense and pollution. Secondly, post-treatment by high temperature is also a useful method of largely enhancing the graphitization degree. For instance, Fuertes et al have reported the high temperature treatment on carbon precursor results in well ordered, graphitized carbon material.22 Besides, Yoon et al 2


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have also been reported that carbon matrix treated by 2500 °C in argon demonstrated the high graphitization degree.23 However, high graphitization degree usually leads to reduction of porosity (i.e., surface areas and pore volume). Therefore, to explore another alternative approach to simultaneously improve the porosity and graphitization degree is challenging. To well modulate the pore structure and surface functional groups of the carbon materials, hydrogen treatment has been proved to be one facile but effective approach. For instance, Wu et al reported that treating carbon materials with hydrogen at 700 °C can lead to the apparent elevation of BET surface area from 60 to 235 m2 g–1, accompanied by the improvement of specific capacitance from 151 to 161 F g–1. 24 Herein, by hydrogen-assisted heat treatment, the pore structure and graphitization degree of carbon materials have been well manipulated. We emphatically studied the impacts of hydrogen treatment and high temperature carbonization upon the BET surface areas, pore volumes, pore sizes and graphitization degrees. Furthermore, the resultant electrochemical performances were measured in H2SO4 electrolyte in three and two-electrode system, respectively. 2. Experiment section 2.1 Typical synthesis and processing The activated carbon was purchased from Nanjing XFNANO Materials Tech Co., Ltd. The typical process was demonstrated as follows: the porous carbon powders were put into a porcelain boat, and then heated up to 800 °C at a rate of 4 ºC min–1 in nitrogen and kept for 120 minutes at the middle of the tubular furnace. After reducing the temperature to the room temperature, the C-blank sample was obtained. Similarly, the porous carbon powders were put into a porcelain boat, and then heated up to 800 °C & 1000 °C at a rate of 4 ºC min–1 in mixed gases (30% hydrogen gas and 70% nitrogen gas) and kept for 120/250 minutes, respectively. After reducing the temperature to the room temperature, the C-800 and C-1000 samples were obtained. The methods for detailed structural characterization and electrochemical 3



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measurement are presented in Supporting Information. 3. Results and discussion X-ray diffraction (XRD) was used to explore the phases and crystallinities of the acquired samples. Figure 1a presents the XRD patterns of the C-blank, C-800 and C-1000 samples. Notably, the XRD diffractions of the C-blank, C-800 and C-1000 samples exhibit no obvious peaks, implying the low graphitization degrees of the samples. Moreover, Raman technique was also employed for further studying the crystallographic structure of the acquired samples. As shown in Figure 1b, there are two distinct peaks located at 1334.8 and 1489.1 cm‒1, which are corresponding to D band and G band, respectively. Generally, the D band refers to the sp3 hybridized carbon. In other words, D band corresponds to the chaotic and defective structure of the samples, while the G band is related to graphitized degree.25,26 To further study the graphitization degree of porous carbon structure, the ID/IG is employed to evaluate the ratio of the intensity between D band and G band. As illustrated in Figure 1b, Raman spectra show the decrease of ID/IG ratios from the C-blank sample (1.10) to the C-800 (1.02) and the C-1000 (1.00) samples, suggesting the elevated graphitization degree with the rising temperature. Besides, the FESEM images of the samples are displayed in Figure S2, demonstrating their irregular morphologies with several micrometers in sizes.

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(a)

(b) D band

C-1000

Intensity (a.u.)

1334.8

Intensity (a.u.)

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


C-800

G band C-1000 ID/IG=1.00

1489.1

C-800 ID/IG=1.02

C-blank

C-blank

ID/IG=1.10

0

20 40 60 80 2 theta (deg.)

100

0

1000 2000 3000 -1 Wavenumber (cm )

Figure 1. The C-blank, C-800 and C-1000 samples: (a) XRD patterns; (b) Raman spectra. N2 adsorption-desorption isotherms, cumulative pore volume and pore size distribution curves of the C-blank, C-800 and C-1000 samples are shown in Figure 2. The N2 adsorption-desorption isotherms of the samples (Figure 2a) can be classified as type I, implying that the porosities of the samples mainly consist of micropores. Notably, as the temperature heats up to 800 °C, a wider of the knee of the isotherms takes place, which suggests an enlargement of the micropore size with the evaluated temperature in hydrogen gas.27 As illustrated in Figure 2b, c, d, it can be seen that micropore size distributions are concentrating in the scope of 0 ~ 2 nm, while extra pores also distributes continuously over a wider range. Besides, the textural properties of the C-blank, C-800 and C-1000 samples are compared in Table 1. The total pore volume (VT) of the samples increase initially and decrease afterwards, meanwhile, the VT of micropores of the samples decrease with elevated temperatures in hydrogen gas. Moreover, the VT, total BET surface areas (ST) and average pore width (dav) reach up 5



to the maximum in the C-800 sample. The decreasing surface areas and pore volume

0.4 0

1

2

3

4

5

-1 3

1.17 2.37

2 3 4 Pore Width (nm)

5

10 15 20 25 30 Pore Width (nm)

35

m icropores

0

m esopores

C-1000

1



isotherms; (b, c, d) Cumulative pore volume and pore size distribution curves.

Table 1. Characteristic surface areas and pore structures of the carbon samples.

C-blank C-800 C-1000

Pore Volume / cm3 g‒1

ST

Smicro

VT

Vmicro

1933 2195 1580

1732 1597 1270

0.95 1.14 0.80

0.72 0.66 0.53

dav / nm 2.04 2.08 1.97

Note: SBET = BET surface areas; ST = total BET surface areas; Smicro = the SBET of micropores; VT = total pore volume; Vmicro = the VT of micropores; dav = average pore

6


5

40

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2

-1

0

SBET / m2 g‒1

40

(d)

Figure 2. The C-blank, C-800, C-1000 samples: (a) N2 adsorption-desorption

Sample

0.2

5

0.0

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

40

1

0.4

-1


-1

0

-1

0.0

0.6

3

Pore Width (nm)

0.2

0.8

C-blank

2.34

2.35

0.6

C-800

1.0

m esopores

0.99 1.20

1.17 1.43

0.8

mesopores

Cummulative Pore Volume (cm g )

micropores

3


-1 3

1.0

0

0

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2

(c)

m icropores

Differential Pore Volume dV (cm nm g )

1.2

0.2 0.4 0.6 0.8 1.0 Relative Pressure (P / P0)

1.2

-1

0.0

(b) 0.97

-1 3


0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

-1


(a)

3

800 700 600 500 400 300 200 100 0


3 -1 Quantity Adsorbed (cm g STP)

of the C-1000 sample are mainly due to the collapse of carbon structure.14, 28


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width. Moreover, XPS technique was employed to gain a deeper investigation of the obtain samples on the chemical compositions, electronic states of various elements. The XPS total survey spectra of the samples are shown in Figure 3a, and a summary is listed in Table S1. As illustrated in Figure 3a, two obvious peaks can be observed in each XPS survey spectrum, corresponding to the C 1s and O 1s. Moreover, the C 1s spectra of the samples are shown in Figure S3a, b, c, and the four different peaks of the C-blank (284.7, 285.1, 286.5 and 290.6 eV), C-800 (284.6, 285.1, 286.6 and 289.3 eV) and C-1000 (284.6, 285.1, 286.6 and 290.8 eV) samples can be related to the sp3 hybridized carbon, sp3 hybridized carbon, C‒O and O=C‒O groups, respectively.29 Furthermore, the percentage of the sp3 hybridized carbon increases with the elevated temperatures in hydrogen gas, suggesting reduction of hydrogen gas at high temperature. Significantly, as shown in Figure 3b, the atom ratio of oxygen/carbon decreases from 13.10% to 8.70% with evaluated temperature, meaning that oxygen containing functional groups can be efficiently removed after the treatment of high temperature on the surface of the C-blank sample in hydrogen gas. Besides, three characteristic peaks located at binding energies of 532.8, 535.3 and 536.8 eV in O 1s spectrum of the C-blank sample (Figure 3d) can be corresponding to O-I (oxygen in carbonyl group, 16.6 at.%), O-II (other oxygen group, 31.8 at.%), and O-III (chemisorbed oxygen and/or water, 51.7 at.%) bound to the surface of carbon material.30,31 As shown in

Figure 3e, three characteristic peaks situated at the

binding energies of 533.5, 535.6 and 537.1 eV in the O 1s spectrum of the C-800 sample are related to O-II (52.3 at.%) and O-III (47.7 at.%). Meanwhile, the O 1s spectrum of the C-1000 sample is shown in Figure 3f. Two typical peaks located at 533.1 and 533.7 eV are corresponding to O-II (100 at.%). Moreover, as shown in Figure 3c, it is noted that the contents of O-I and O-III decrease even disappear with the rising temperature within hydrogen gas as well as the increase of the content of O-II, which may because of the co-effects between the reduction of hydrogen gas and temperature on the samples.

7



O-1 532.8

O-2 535.3

O 1s

528 530 532 534 536 538 540 542 Binding Energy / eV

O-2 533.5


8


0 00

00 -8

(f)

C-800

O 1s

O-2 535.6

C -1

C O-3 537.1

Intensity (a.u.)

O-3 536.8

C

C -b

(e)

C-blank Intensity (a.u.)

Intensity (a.u.)

(d)

la

200 400 600 800 1000 1200 1400 Binding Energy / eV

16.6

52.347.7

31.8

-b

0

00 0

2

51.7

nk

4

100

la

Atomic ratio / %

6

O-1 O-2 O-3

(c)

-1

1 C-

8

100 90 80 70 60 50 40 30 20 10 0

C

bl C800 C000

10

-8 00

a

nk

12

C

O 1s

(b)

nk

C 1s

14 O/C (%)

(a)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 0 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

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C-1000

O-2 533.1

O-2 533.7

O 1s


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Figure 3. The C-blank, C-800 and C-1000 samples: (a) XPS survey; (b) O/C ratios; (c) diagram for the content of three types oxygen.; (d, e, f) O 1s spectra.

On the basis of the structural analysis of the carbon material mentioned above, we further proposed the schematic illustration, vividly depicted in Figure 4. Figure 4a shows the decrease of ID/IG ratio from the C-blank sample to the C-800 and C-1000 samples, indicating the better graphitization degree with the rising temperature in hydrogen gas. Besides, some literatures have been reported that high temperature treatment can enhance the graphitization degree and lead to a significant reduction in the oxygen containing functional groups, BET surface areas and porosity,22,23,32 while the C-800 sample shows the largest BET surface areas in hydrogen gas. Hence, we indeed believe that hydrogen gas plays an important role in the process of high temperature treatment of the samples. The reduction of hydrogen gas may change the inner structures and thereby enlarge the porosity of the carbon samples. And the treatment temperature of 800 °C is the most suitable condition in the experiment, exhibiting the largest BET surface areas which are mainly owing to the dominant position of the reduction of hydrogen gas leading to the enlarged porosity, nevertheless, the major effect of graphitization on the C-1000 sample results in diminished porosity instead. Moreover, as shown in Figure 4b, the C-800 sample demonstrates larger area of cyclic voltammetry curve than that of the C-blank sample, while the C-1000 sample reveals the smaller one. It implies that the C-800 sample possesses the higher capacitive performance thanks to the excellent surface property.

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Figure 4. (a) ID/IG ratio of the C-blank, C-800 and C-1000 samples; (b) Illustration of the relationships between oxygen containing functional groups and CV curves.

To further illustrate the influences between porosity and graphitization degree of the C-blank, C-800 and C-1000 samples, electrochemical behaviors were characterized by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) in a three-electrode configuration. Figure 5a shows the cyclic voltammetry curves of the C-blank, C-800 and C-1000 samples at the scan rate of 50 mV s‒1. The C-800 and C-1000 samples show more rectangular shapes than that of the C-blank sample, which belongs to the characteristic of an electric double-layer capacitor (EDLC).33 Moreover, the CV curve of the C-blank sample immensely deviated from rectangular shape, probably caused by the oxygen containing functional groups on the surface of electrode.34 Besides, the CV curves of the samples show no obvious peak, indicating no or few pseudo-capacitances obtained from redox reaction. Besides, the integral areas of the C-800 sample are larger than that of the C-blank sample, while the C-1000 sample shows the opposite circumstance. It may be owing to the increasing BET surface areas as well as porosity of the C-800 sample, while the opposite conditions of the C-1000 sample, which is related to the BET results shown in Table 1.

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Moreover, GCD curves of the C-blank, C-800 and C-1000 samples are shown in Figure 5b. No obvious plateau can be observed in the GCD curves, which well accords to the CV in Figure 5a. In addition, the specific capacitances of the samples are illustrated in Figure 5c. It is obvious seen that the specific capacitances of the C-800 sample are the largest one among the samples. And the corresponding specific capacitances of the C-blank, C-800 and C-1000 samples reach up to 145, 171 and 107 F g‒1 at 1 A g‒1, respectively. Besides, cycling stability regarded as a significant factor has also been employed to distinguish the practicability of supercapacitor, which was evaluated by the continuous 5000 times of charge/discharge processes at the current density of 10 A g‒1 and the results are displayed in Figure 5d. As shown in Figure 5d, the capacitances retentions of all the samples decrease with the increasing cycles, which mainly results from the higher charge transfer resistance.35 Besides, the C-800 and C-1000 samples show higher cycling stabilities (96.3% and 98.4% of primary specific capacitances

8

(a)

1.0

50 mV s

-1

4 0 -4 C-blank C-800 C-1000

-8

Potential (vs. SCE) / V

-1

after 5000 cycles, respectively) than that of the C-blank sample (92.3%).

Current density / A 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


(b) C-blank C-800 C-1000

0.8 0.6 0.4 0.2 1Ag

-1

0.0

-12 0.0

0.2 0.4 0.6 0.8 Potential (vs. SCE) / V

1.0

0

11


100

200 Time / s

300

400

120


100 80 60 40 20

10

0

8

2

6

4

2

4 6 8 -1 Current density / A g

0

10

0

200 180 160 140 120 100 80 60 40 20 0

Specific capacitance / F g

-1


(c)

-1

180 160 140 120 100 80 60 40 20 0


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

-1


(d)

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96.3% 92.3% 98.4%

C-1000 C-blank C-800

0

1000 2000 3000 4000 5000 Cycle number

Figure 5. The C-blank, C-800 and C-1000 samples measured in three-electrode system: (a) CV curves at 50 mV s‒1; (b) GCD curves at 1 A g‒1; (c) specific capacitances calculated from GCD curves; (d) cycling stability measured at 10 A g‒1 before/after 5000 cycles.

It is generally recognized that the two-electrode system possesses extremely important practical significance on estimating the electrochemical performances of supercapacitor. Hence, in order to further investigate the performances of supercapacitor using the samples as electrode, the two-electrode configuration was applied for measurements and the corresponding results were provided in Figure 6. As shown in Figure 6a, the quasi-rectangular shape is exhibited in the CV curve, showing almost mirror images with respect to the zero-current line, which is characteristic of an electric double layer capacitor. Moreover, the integral areas of the C-800 sample are larger than those of the C-blank and C-1000 samples, respectively. That is to say, the C-800 sample carried out in 1 mol L‒1 H2SO4 electrolyte has higher capacitances compared with the C-blank and C-1000 samples, respectively. Besides, the C-1000 sample shows a more rectangular shape than those of the C-blank and C-800 samples, indicating the less oxygen containing functional groups attach to the surface of the C-1000 sample, which is corresponding to O 1s spectrum. Moreover, GCD curves of the C-blank, C-800 and C-1000 samples are shown in Figure 6b.

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Apparently, there is no obvious plateau in GCD curves and on the basis of GCD curves, we accurately calculated the specific capacitances of the C-blank, C-800 and C-1000 samples. As shown in Figure 6c, it can be obviously seen that the specific capacitances of the C-800 sample is the largest one among the samples. As described in Figure 6d, the C-800 and C-1000 samples show lower attenuation ratios (6.3% and 2.5%, respectively) of the available specific capacitances derived from the GCD curves after 5000 cycles than that of the C-blank sample (19.3%).

(a)


1.0

0 -2 -1

0.2

0.4 0.6 Potential / V

0.8

0.4 0.2

1.0


40 30 20 10 2 4 6 8 Current density / A g-1

10

0

20 40 60 80 100 120 140 160 Time / s

70 60

(d) 93.7%

50 40

80.7%

30

97.5%

20

C-1000 C-blank C-800

10 0

0


-20


(c)

50

0

0.6

-1

0.0

60

-1

1Ag

0.0

50 mV s

-4

70

(b)

0.8

2

Potential / V

-1

4

Current density / A g

Specific capacitance / F g-1

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


0

1000 2000 3000 4000 5000 Cycle number

Figure 6. The C-blank, C-800 and C-1000 samples measured in two-electrode system: (a) CV curves at 50 mV s‒1; (b) GCD curves at 1A g‒1; (c) specific capacitances calculated from GCD curves; (d) cycling stabilities measured at 10 A g‒1 before/after 5000 cycles. 13



Besides, Nyquist plots of the C-blank, C-800 and C-1000 samples were measured to investigate the chemical and physical processes occurring in electrode. As demonstrated in Figure 7a, it can be observed that the C-800 sample possesses a small span of the single semi-circle implying the lower interfacial charge transfer resistance

and

a

shorter

Warburg-type

line

indicating

the

faster

ionic

transport/diffusion in H2SO4, which accelerates the efficient access of the electrolyte ions to the surface of carbon material.36 Besides, the C-1000 sample shows a more vertical shape at low frequency which indicates an ideal capacitor.37 Moreover, in terms of the practical application, the energy density and power density have been regarded as vital and significant elements of supercapacitor, and Ragone plots of the samples have been displayed in Figure 7b. It can be clearly observed that the C-800 sample possesses the larger energy density of 8.36 Wh kg‒1 as well as the power density of 500 W kg‒1, which is compared with those of the C-blank (7.31 Wh kg‒1 at 500 W kg‒1) and C-1000 (4.82 Wh kg‒1 at 500 W kg‒1) samples. In addition, it is remarkable that the energy density of the C-800 sample is larger than those other

(a)


30 25 20 15 10 5 0

0 0

5

10

1 15 20 25 Z' / ohm

2 30

100

Energy density / Wh kg

35

-1

electrodes of supercapacitor in the literatures.38-41

Z'' / ohm

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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(b) 10

36s C-blank C-800 C-1000

Ref 40

18s 3.6s 1.8s

Ref 41 Ref 39 Ref 38

1

0.1

35

6min

1h

100

1k -1 10k Power density / W kg

Figure 7. The C-blank, C-800 and C-1000 samples: (a) Nyquist plots; (b) Ragone plots of specific energy versus specific power for the C-blank, C-800 and C-1000 samples using the 1 mol L−1 H2SO4 electrolytes.

14


0.36s

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In addition, to have a visual comparison of the samples in electrochemical performance, the Radar chart is exhibited in Figure 8. It is known that radar chart is a graphical method of displaying multivariate data which is represented on axes starting from the same point and each direction of axes represents an electrochemical characteristic of supercapacitor. Moreover, the areas of closed curve indicate the electrochemical performance. As demonstrated in Figure 8, it can be obviously seen that the areas of the C-800 sample are larger than those of the C-blank and C-1000 samples. Therefore, it reveals that the C-800 sample exhibits better electrochemical performances by hydrogen gas treatment at 800 °C of carbon matrix.

Maximum specific capacitance / F g-1 171

98.4%

Energy73.5% efficiency / % 1 A g-1

Cycling ability C-blank C-800 C-1000 8.36

Maximum energy density / Wh kg-1 Figure 8. Radar chart of maximum specific capacitances, cycling abilities, maximum energy densities and energy efficiencies of the C-blank, C-800 and C-1000 samples.

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4. Conclusions In this work, we have primarily investigated the effects of hydrogen gas and high temperature on carbon materials as well as their electrochemical performance by meaning of physical characterizations and electrochemical tests, respectively. Thus, considering the experiment results above, we can draw the conclusions as follow: 1. The properties of the carbon materials’ surface (i.e. BET surface areas, average pore width and pore volume) were increased by the hydrogen gas treatment at 800 °C on the bare carbon matrix, which was used as the electrode of supercapacitor. 2. With the increasing of the temperature surrounded by hydrogen gas, the samples show the evaluated graphitization degree of carbon material as well as the decreasing oxygen containing functional groups instead. 3. The carbon materials treated by hydrogen gas at 800 °C demonstrate a better electrochemical activity and therefore improve the electrochemical performance of supercapacitor. In summary, the effects of hydrogen gas and apposite temperature on carbon material not only improve the graphitization degree and surface properties, but enhance the electrochemical performance as well. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21101052).

Supporting Information Structure characterizations, electrochemical measurements of the samples regarded as electrode materials, electrochemical measurements conducted in a two-electrode system, device and schematic of the two-electrode system, C 1s spectra of samples. The Supporting Information is available free of charge on the ACS Publications website.

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