Polyvinylidene Fluoride-Based Carbon Supercapacitors: Notable

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Polyvinylidene fluoride-based carbon supercapacitors: Notable capacitive improvement of nanoporous carbon by the redox additive electrolyte of 4-(4-Nitrophenylazo)-1-naphthol Liang Xiao Cheng, Yan Qi Zhu, Xiang Ying Chen, and Zhongjie Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02490 • Publication Date (Web): 07 Oct 2015 Downloaded from http://pubs.acs.org on October 10, 2015

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Industrial & Engineering Chemistry Research

Polyvinylidene fluoride-based carbon supercapacitors: Notable capacitive improvement of nanoporous carbon by the redox additive electrolyte of 4-(4-Nitrophenylazo)-1-naphthol Liang Xiao Cheng1, Yan Qi Zhu1, Xiang Ying Chen1,*, and Zhong Jie Zhang2,** 1

School of Chemistry and Chemical Engineering, Anhui Key Laboratory of

Controllable Chemistry Reaction & Material Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, P. R. China. *Corresponding author. E-mail: [email protected]. 2

College of Chemistry & Chemical Engineering, Anhui Province Key Laboratory of

Environment-friendly Polymer Materials, Anhui University, Hefei 230039, Anhui, P. R. China. ** Also the corresponding author. E-mail: [email protected]

1

ACS Paragon Plus Environment


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Abstract The nanoporous graphitic carbon materials (NGCM) have been prepared by a synchronous carbonization and graphitization process, using waste polyvinylidene fluoride (PVDF) as carbon precursor, Ni(NO3)2·6H2O as graphitic catalyst. It reveals that the carbonization temperature plays a crucial role in determining the pore structures as well as their electrochemical performances. Increasing the carbonization temperature from 800 ºC to 1200 ºC, the corresponding porosity has slightly decreased, accompany with the increase of graphitization degree. Next, to further improve the electrochemical performance of the sample prepared at 800 ºC, a novel redox additive of 4-(4-Nitrophenylazo)-1-naphthol (NPN) with different amounts has been introduced in 2 mol L–1 KOH electrolyte. Therein, the specific capacitance by adding 4 mmol L–1 of NPN can reach 2.98 times higher than the pristine value. Apparently, the mixed electrolytes have largely enhanced the electrochemical performance, which is expected to be applied in field of high performance supercapacitors.

Keywords: Graphitic catalyst; Redox additive; Mixed electrolytes; Polyvinylidene fluoride; Supercapacitors.

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1. Introduction Due to high electronic conductivity, well-developed crystalline structure and thermal stability, nanoporous graphitic carbon materials (NGCM) show huge advantages over amorphous nanoporous carbon.1,2 Thus, the synthesis of NGCM with large and accessible surface area is of great interest in the practical application for supercapacitors.3 So far, various synthetic approaches have been attempted to produce NGCM with desirable graphitic structures.4-8 Conventional method requires high temperature treatment to form nanoporous carbon with well-developed graphitic order.9 For example, Yu et al. synthesized graphitized pitch-based carbon by using colloidal crystals as templates at 2500 ºC.5 The way at such a high temperature is of great energy consumption that can not be used massively. More effective approach used to prepare NGCM is catalytic graphitization with the aid of catalyst at relatively low temperature which can be further divided into two types: (1) First carbonization followed by graphitization. For example, Liu et al. prepared highly porous graphitic materials by a two-step method that activated carbon was firstly obtained via a chemical activation process and then further treated at 800 - 1000 ºC after impregnated in acetone solution of Ni(NO3)2;4 (2) Synchronous carbonization and graphitization.6,10 For instance, Cheng et al. reported the synthesis of 3D aperiodic hierarchical porous graphitic carbon by using Ni(OH)2/NiO as graphitic catalyst.6 The second method shows the merits of easier processing and better catalytic effect for producing graphitic porous carbon. Meanwhile, the processing of waste plastics has attracted enormous attention in recent years as the growth of environmental awareness.11,12 PVDF widely used in engineering plastics is one of the most important fluorinated polymers. However, because of the characteristic of hard-degradation and increase in consumption, the waste PVDF materials have become an increasingly serious problem for environment.13 Thereby, disposing waste PVDF materials when they come to the end of their life is an important subject for material scientists. Therein, converting it into porous carbon which can be used in the field of supercapacitors is an efficient way.

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For instance, Xu et al. prepared hierarchical porous carbon by directly heating the mixture of powdery PVDF and NaOH at 600 ºC, and the specific surface area and pore volume can reach values as high as 2711 m2 g–1 and 2.280 cm3 g–1, respectively.14 Of particularly note is that nitrogen-doped microporous carbon has been synthesized by the carbonization of the PVDF/melamine mixture without chemical activation, and the resulting specific capacitance can attain 310 F g–1 at 0.5 A g–1.15 Furthermore, many scientists have found that using redox additives (mediators) or electro-active materials can greatly improve the capacitance of supercapacitors.16-19 Based on the recent reports, electrolytes can be classified into three types: (1) Redox additive-liquid electrolytes; (2) Redox active liquid electrolytes; (3) Redox additive-polymer gel electrolytes.20 Compared with the latter two types, the former one shows advantages of good ionic mobility and high contact area, thereby it has been investigated extensively. For example, when adding hydroquinone in H2SO4 electrolyte of supercapacitors with carbon nanotubes or activated carbon, the specific capacitance of electrode was increased from ~320 F g–1 to 901 F g–1.21 Another example is that a redox-mediated organic electrolyte was prepared by adding p-phenylenediamine

into

lithium

perchlorate/acetonitrile

electrolyte

for the

application in activated carbon-based supercapacitors, and the electrochemical performance was also greatly enhanced.17 The excellent behaviors of these supercapacitors are attributed to the quick reversible Faradaic reactions of redox mediator (hydroquinone or p-phenylenediamine). On view of our previous work, searching a novel redox additive electrolyte that can produce Faradaic reactions in KOH solution evokes our attention. In this work, the NGCM were prepared by using waste PVDF materials as carbon precursor, Ni(NO3)2·6H2O as graphitic catalyst. Designating mass ratio of PVDF and Ni(NO3)2·6H2O as 2:1, different kinds of samples were obtained by adjusting carbonization temperature. Considering that NPN contains one phenol group and one nitro group which are expected to highly elevate the specific capacitance due to their electron/photon transfer, really leading to Faradic redox 4


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reaction and the resulting pseudocapacitance further enhance the overall capacitance. Thereby, we introduced the NPN as redox additive in KOH electrolyte, and the impacts of the dosage of NPN were also emphatically investigated. The electrochemical performance of all samples was studied by a three-electrode system in the 2 mol L–1 KOH. 2. Experiment PVDF powder and Ni(NO3)2·6H2O with the mass ratio of 2:1 were firstly mixed, and then transferred to a porcelain boat, flushed with Ar flow for 30 min, and further heated in a horizontal tube furnace up to 800 ºC at a rate of 5 ºC min–1 and maintained at this temperature for 2 h under Ar flow. The resultant product was immersed and ultrasonicated with dilute HCl solution to remove soluble/insoluble substances, subsequently washed with adequate deionized water. Finally, the sample was dried under vacuum at 120 °C for 12 h, giving rise to the C-800 sample. Just elevating the carbonization temperature to 1000 °C and 1200 °C, C-1000/1200 samples were obtained. To further promote the electrochemical behavior of the C-800 sample, we added different amounts of NPN additive as 2 and 4 mmol L–1 in the 2 mol L–1 KOH, noting as C-800-2/4. Characterizations and electrochemical measurements of our samples are detailedly depicted in Supporting Information. 3. Results and discussion The C-800/1000/1200 samples are obtained by synchronous carbonization and graphitization at 800 ~ 1200 ºC, as briefly depicted in Figure 1a, and the corresponding XRD patterns are also displayed in Figure 1b. The broad diffraction peak of C-800 sample centered at 22.6° reveals the low degree graphitization and disordered structures. When increasing the carbonization temperature to 1000 ºC, a peak at 26.5° emerges, suggesting the formation of partial graphitic structure. As the carbonization temperature continually elevated to 1200 ºC, the peak at 26.5° is

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dramatically enhanced. This is largely due to the apparent increase in graphitization degree, also implying that the graphitic microcrystallines arrange more orderly at higher temperature. Same condition also occurs in previous reports.4,22 Meanwhile, the G band, shown in the Figure 1c apparently becomes stronger as carbonization temperature increased, and the IG/ID values calculated in their Raman curves range from 0.96 to 2.27. As is well-known, the D band is ascribed to disordered graphitic lattice (A1g symmetry), whereas the G band as ideal graphitic lattice (E2g symmetry).23 Thus, the trend of the IG/ID values suggests that the graphitization degree increase obviously as the reacting temperature elevated to 1200 ºC, complying with the XRD results. Meanwhile, here we detect a noteworthy 2D peak around at 2683.7 cm−1 in the sample heated to above 1000 ºC, increasing intensity as the heating temperature increases,24 which validate once again that our method to prepare NGCM is efficient. In conclusion, the C-800 sample behaves characteristic of amorphous carbon, while the C-1000/1200 samples represent typical graphitic structures. Apart from the above-mentioned characterization, HRTEM technique was used to veritably depict the structures of the carbon materials, just as shown in Figure. S1. It can be clearly observed that the lattice fringes become apparent and diffraction rings turn into sharp and clear, as increasing carbonization temperature, which suggests the increased graphitization degree. XPS results including surveys as well as C1s and O1s spectra are also displayed in Figure. S2, and all of which are composed of carbon and oxygen elements. In details, the content of carbon increases from 87.71% to 93.45% when elevating the temperature, according to the Table S1, which is largely owing to the loss of oxygen-containing functional groups and volatile carbon at higher temperature.25

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(b)

26.5

G band

(c) Intensity (a.u.)

2D

Intensity (a.u.)

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43.0 C-1200

22.6

C-1000 C-800

D band IG/ID=2.27

C-1200

IG/ID=1.04

C-1000 C-800

IG/ID=0.96

500

10 20 30 40 50 60 70 80 90 2 theta (deg.)

1000 1500 2000 2500 3000 3500 -1 Wavenumber (cm )

Figure 1. C-800/1000/1200 samples: (a) Schematic illustration; (b) XRD patterns; (c) Raman spectra.

The growth of the graphitic structure would have a great influence on the porosity features of the samples. N2 adsorption-desorption isotherms, as well as pore size distribution curves are shown in Figure 2. Apparently, all isotherms are ascribed to be type-IV according to IUPAC classification on porosity. In detail, a sharply increase can be observed at low relative pressure, and the next slow rise of isotherms appear at medium relative pressure with distinct hysteresis loops.26 Characteristic surface areas and pore structures of the C-800/1000/1200 samples are summarized in Table. 1. When carbonization temperature is at 800 ºC, the sample exhibits large BET surface area of 1048.9 m2 g–1, pore volume of 1.03 cm3 g–1. But it decreases to 817.1 m2 g– and 0.73 cm3 g–1 as the carbonization temperature increased to 1200 ºC. 7



Obviously, this phenomenon is largely due to the collapse of the porosities at high temperature, and similar situation also happens in many researches.27-29 Pore size distribution curves are plotted by using the NLDFT method in which assumes a slit pore geometry for the micropores and a cylindrical pore geometry for the mesopores.30 As can be seen, all samples show pore size distribution mainly in the

-1

3

-1

VTotal = 1.03 cm g

100 0

1.0

3

(c)

3

500 C-1000

400 300

2

-1

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-1

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SBET = 989.4 m g

100

VTotal = 0.90 cm g

1.0

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-1


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SBET = 1048.9 m g

200

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C-800

-1

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Quantity Adsorbed (cm g STP)

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3

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(a)

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3


region of 0 ~ 6nm, suggesting the existence of abundant micropore/mesopores.


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2

-1

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0

VTotal = 0.73 cm g

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Figure 2. C-800/1000/1200 samples: (a, c, e) N2 adsorption-desorption isotherms; (b, d, f) Cumulative pore volumes, pore-size distribution curves.

Table 1. Characteristic surface areas and pore structures of the C-800/1000/1200 samples. BET surface (m2 g-1) Sample

Total pore volume

Micropore volume

Average pore width

Total

Smicro

Sext

(cm3 g-1)

(cm3 g-1)

(nm)

C-800

1048.9

308.0

740.9

1.03

0.26

3.94

C-1000

989.4

350.8

638.6

0.90

0.16

4.11

C-1200

817.1

105.3

711.8

0.73

0.08

5.64

Notes: 1. Smicro represents the micropore area calculated by t-plot; 2. Sext represents the external surface area calculated by t-plot.

Electrochemical behaviors of the C-800/1000/1200 samples were measured in a three-electrode system using 2 mol L−1 KOH as electrolyte. Galvanostatic charge-discharge (abbr. GCD) measurements of the C-800/1000/1200 samples were carried out at the current density of 2.0 A g–1, as given in Figure 3a, which exhibit triangular shape, a typical GCD symbol for EDLC. As a result, Figure 3b depicts the specific capacitances calculated from GCD curves at different current densities.

9



Basically, the order of the specific capacitance is C-800 > C-1000 > C-1200, which is consistent with the BET surface areas. It suggests that high temperature can increase graphitization degree, but it also leads to the collapse of the porosities at same time, thus resulting in the decrease of the specific capacitance.31 On the other hand, the specific capacitance retention of the C-800 sample is 60.5% from 2.0 A g–1 to 20.0 A g–1, while that of the C-1200 sample can attain 71.9%. It indicates that the rate capability is elevated at higher temperature, which is due to the more ordered graphitic microcrystallines at higher temperature.3,4 Figure 3c depicts the comparative cyclic voltammetry (abbr. CV) in a voltage window of -1.0 ~ 0.0 V. At the designated scan rate of 100 mV s–1, the CV curves of C-800/1000/1200 samples are close to rectangular shapes, indicating that the energy storages predominately derive from the contribution of electric double layer capacitance (EDLC). In addition, the specific capacitances calculated from CV curves at different scan rates are shown in Figure 3d, and the C-800 sample expresses the optimum electrochemical performance, complying with the results of GCD.

(a)

-0.2

-1

C-800 C-1000 C-1200

Specific capacitance / F g

0.0

Potential / V

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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-0.4 -0.6

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2Ag -0.8 -1.0 0

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180 160 140 120 100 80 60 40 20 0

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60.5% 69.9% 71.9% C-800 C-1000 C-1200 2

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100 mV s

10 5

C-800 C-1000 C-1200

0 -5 -10 -15 -1.0

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-0.6 -0.4 Potential / V

-0.2

(d)

280 240 200 160 120

C-800 C-1000 C-1200

80 40 0

0.0

0

50 100 150 -1 Scan rate / mV s

200

Figure 3. C-800, C-1000, C-1200 samples measured in a three-electrode system using 2 mol L−1 KOH as electrolyte: (a) GCD curves at a current density of 2.0 A g−1; (b) Specific capacitances calculated from GCD curves; (c) CV curves at a scan rate of 100 mV s−1; (d) Specific capacitances calculated from CV curves.

For NGCM, long term cycling durability is an important aspect to be considered for practical application. Just as shown in the Figure 4a, the retention of specific capacitances of the C-800/1000/1200 samples can still reach 96.9%, 98.9%, and 99.0% even after charging-discharging for 5000 cycles. These values are larger than those of we previously reported,25,32 which suggest that the increased graphitization degree not only improve the rate capability of samples, but also enhance the long term cycling durability. Figure 4b shows the CV comparison of C-800/1000/1200 samples before/after 5000 cycles. The approximate superposition of the cycles also indicates all samples have superior long term cycling durability.

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2Ag

96.9%

-1

200 (a) 180 160 140 120 100 80 60 40 20 0 0

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-0.8 -1.0 -0.4 -0.6 -0.2 0.0 tial / V Poten -15 (b) -10 -5 0 5 10 15 C-1200 20 25 C-1000

Ag Current density /


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


98.9% 99.0% C-800 C-1000 C-1200 1000 2000 3000 4000 5000 Cycle number

C-800

Figure 4. C-800/1000/1200 samples measured in a three-electrode system using 2 mol L−1 KOH as electrolyte: (a) Cycling stability measured at 2 A g−1; (b) CV curves before/after 5000 cycles cycling stability measured at 100 mV s-1.

To further improve the electrochemical performance of the C-800 sample, a three-electrode system using mixed electrolytes of 2 mol L−1 KOH with different amounts NPN of 2 and 4 mmol L–1, acting as redox additive were utilized for the electrochemical tests. These mixed electrolytes are marked as C-800-2 and C-800-4, respectively. Figure 5a. b. c display CV curves of the C-800, C-800-2/4 samples at different scan rates ranging from 20 to 200 mV s−1. Interestingly, the curves of the C-800-2/4 samples possessing four distinct redox peaks much differ from the C-800 sample, indicating that there exist Faradaic reactions from the redox additive of NPN. Furthermore, the four redox peaks show that there exist two oxidative reactions and two reductive reactions in one cycle, and the reaction mechanism will be illustrated in the following section. Specific capacitances from CV in Figure 5d certify that adding redox additive of NPN can indeed enhance electrochemical behavior notably. This is owing to the contribution of quick self-discharge reaction of the active additive,33 indicating that NPN is a good redox additive for improving electrochemical performance. In addition, the C-800-4 sample delivers better specific capacitance 12


st

1 th 5000

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than that of the C-800-2 sample. It can be easily explained that low concentration of the NPN (2 mmol L–1) is not able to supply enough electrons or/and ions for enhancing the conductivity of the new system.

-1 200 mV s 45 (b) -1

-1 (a) 200 mV s

30 20

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C-800-4

0 -20 -40 -1.0

-0.8

-0.6 -0.4 -0.2 Potential / V

0.0

540 480 420 360 300 240 180 120 60 0 -60

-0.8

-0.6 -0.4 -0.2 Potential / V

(d)

0

0.0

C-800 C-800-2 C-800-4

100 200 300 -1 Scan rate / mV s

Figure 5. The C-800, C-800-2/4 samples measured in a three-electrode system using mixed electrolytes (2 mol L−1 KOH with different amounts of NPN acting as redox additive): (a) CV curves of C-800 sample; (b) CV curves of C-800-2 sample; (c) CV curves of C-800-4 sample; (d) Specific capacitances calculated from CV curves.

The performance of GCD depicted in Figure 6 is a more important aspect for the

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practical application of our electrode materials. Complying with the redox peaks in CV curves, the GCD curves emerge two platforms in charging process and two platforms in discharging process, though the platforms are not obvious. What’s more, the platforms coming from C-800-4 sample is distinct than that of C-800-2, suggesting that more electrons or/and ions are provided in this concentration. Meanwhile, the order of the specific capacitance in Figure 6 is C-800-4 > C-800-2 > C-800, which is in accordance with the CV results. For example, at a fixed current density of 5.0 A g−1, the resultant specific capacitance of the C-800-2/4 samples reach up to 212.0 and 252.8 F g−1, while that of the C-800 sample is 130.6 F g−1. To conclude, NPN is an effective redox additive that can enhance electrochemical behavior notably, and higher amount of NPN (4 mmol L−1 in present work) s more favorable for improving the electrochemical performance. Meanwhile, the cycling durabilities of the C-800 and C-800-4 samples have been tested and given in Supporting Information section (Figure S3). The long term cycling durability of the C-800-4 sample exhibits retention of 83.3%, which is below that of C-800 sample (96.1%). But the specific capacitance of the C-800-4 sample is still considerably higher than that of the C-800 sample, even after charging-discharging for 5000 cycles. To date, some reports regarding redox additives are summarized in Table 2. We can clearly see that all additives have notably elevated the specific capacitances. In addition, the NPN we adopted enhances electrochemical performance more apparently, compared with other additives. In details, when adding NPN of 4 mmol L–1 in KOH electrolyte, the specific capacitance can reach 2.98 times higher than the pristine value, while those of the other additives are only 1.23 ~ 2.14. Simultaneously, we have also extended the application of NPN to C-1000/1200 samples, and the corresponding specific capacitances are depicted in Supporting Information section (Figure S4). It can be clearly seen that NPN also elevates specific capacitances of C-1000/1200 samples notably, indicating that the NPN we adopted presents universal effects.

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0.0 (a)

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20 30 Time / s

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-1.0 0

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6A g -1 8A g -1 10A g -1 20A g

-0.2

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6A g -1 8A g -1 10A g -1 20A g

-0.2 Potential / V

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-0.2

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420

20

(d)

360 300

40 Time / s

60

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C-800 C-800-2 C-800-4

240 180 120 60 0

120

4

8

12 16 20 24 -1 Current density / A g

Figure 6. The C-800, C-800-2/4 samples measured in a three-electrode system using mixed electrolytes (2 mol L−1 KOH with different amounts of NPN acting as redox additive): (a) GCD curves of C-800 sample; (b) GCD curves of C-800-2 sample; (c) GCD curves of C-800-4 sample; (d) Specific capacitances calculated from GCD curves.

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Table 2. Comparison of the speciﬁc capacitances by adding different redox additives. Cs (F g–1) Redox additive Pristine

After adding redox additive

Increase times

Current density

Ref.

Lignosulfonates

145.1

178.2

1.23

0.1 A g–1

34

K4Fe(CN)6

226.2

317.1

1.40

2 A g–1

35

m-Phenylenediamine

36.43

78.01

2.14

0.5 A g–1

36

KI

135.93

236.90

1.74

0.8 A g–1

37

NPN

144.5

430.0

2.98

5 A g–1

This work

According to the previous reports, nirtro group generates nirtroso group after obtaining two electrons,38 and phenolic group turns into ketone group after losing one electron.39,40 Thereby, considering that NPN contains one phenol group and one nitro group, the mechanism of redox-reaction may be concluded as depicted in Figure 7. For the convenience of description, we denote A, B, C, and D as O2NArOH, O2NarO, ONArO, and ONArOH, respectively. Thus, the mechanism can be detailedly depicted as follows: Process 1:

O 2 NArOH → O 2 NArO + H + + e − O 2 NArOH + 2H + + 2e − → ONArOH + H 2O

Process 2:

O 2 NArO + 2H + + 2e − ⇔ ONArO + H 2O ONArO + H + + e − ⇔ ONArOH

At the beginning of the scanning, the redox additive of O2NarOH we adopted starts to turn into O2NarO after losing one electron or ONArOH after gaining two electrons (process 1),38-40 and once the O2NarO or ONArOH is generated, the process 2 is also begin to react. The process 2 contains two equations, which both are reversible reactions. In charging stage, the O2NarOH transforms into ONArOH in process 1,38 and in process 2, the O2NarO firstly generates ONArO after gaining two electrons, and then the 16


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ONArO continually generates ONArOH after achieving one electron. In discharging stage, the product of process 1 is O2NarO, while the process 2 is just opposite to the charging stage. When O2NarOH turns into O2NarO and ONArOH thoroughly, the process 1 is completed, only process 2 is left.

Figure 7. Electrochemical reaction mechanism of redox additive electrolyte of NPN in mixed electrolytes.

4. Conclusions In present work, a simple but effective synchronous carbonization and graphitization method has been demonstrated to produce NGCM sample, in which PVDF waste and Ni(NO3)2·6H2O serve as carbon precursor and graphitic catalyst, respectively. The as-prepared C-800/1000/1200 samples have exhibited superior electrochemical performances, especially delivering high rate capability and long term cycling durability. To further enhance the electrochemical performance of C-800 sample, we have introduced NPN into the conventional 2 mol L−1 KOH electrolyte. In summary, the scientific advantages for this new system are summarized as follows:

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(1) Waste PVDF materials have been successfully converted into NGCM in an efficient way, and the present synthesis method can be extended to dispose other kinds of wastes, thus decreasing the resultant environmental pollution and boosting our world more green and harmonious; (2) The NPN is effective for the enhancement of the specific capacitance, and the NPN concentration has a crucial influence on electrochemical behavior. For example, the specific capacitances can reach 2.21, 2.98 times higher than the pristine value when adding NPN of 2, or 4 mmol L–1; (3) The pore structures can be readily moderated by adjusting the carbonization temperature. For example, the specific surface area and pore volume decrease from 1048.9 to 817.7 m2 g–1 and 1.03 to 0.73 cm3 g–1, respectively, as carbonization temperature increases from 800 to 1200 ºC; (4) The NPN is low cost, commercially available and can be easily operated at ambient condition, which is quite promising for practical application.

Notes The authors declare no competing financial interest.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21101052). Dr. Zhong Jie Zhang thanks the financial support from Anhui Provincial Natural Science Foundation (1508085QE104).

Supporting Information Electrochemical measurements employed in this work; XPS survey of the

C-800/1000/1200 samples as well as the HRTEM images in different magnifications ; Cycling stabilities of the C-800/800-4 samples; Specific capacitances before/after adding 4 mmol L–1 NPN in 2 mol L−1 KOH of C-1000/1200 sample.

18


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FOR TABLE OF CONTENTS ONLY 297x420mm (300 x 300 DPI)


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