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2-D Carbon Nanosheets for High Performance Supercapacitors: Large Scale Synthesis and Co-Doping with Nitrogen/Phosphorus Zhong Jie Zhang, Qian Cheng Zheng, Liang Sun, Dong Xu, and Xiang Ying Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03022 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017

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Industrial & Engineering Chemistry Research

2-D Carbon Nanosheets for High Performance Supercapacitors: Large Scale Synthesis and Co-Doping with Nitrogen/Phosphorus Zhong Jie Zhang1*, Qian Cheng Zheng1, Liang Sun1, Dong Xu2, Xiang Ying Chen2* 1

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

Environment-friendly Polymer Materials, Anhui University, Hefei 230601, Anhui, P. R. China. * The corresponding author. E-mail: [email protected]. 2

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. E-mail: [email protected].

1

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Abstract 2-D Carbon nanosheets co-doped with N/P species have been successfully synthesized by a template carbonization method couple with nitrogenization and phosphorylation process, using trisodium citrate dihydrate, melamine and NH4H2PO4 as C/N/P sources, respectively. Dopants of N/P species play crucial roles on the determination of carbon porosities and electrochemical performance; notably, increasing the P content can lead to the drop of BET surface area together with the correlative electrochemical performance. For instance, regulating the mass ratio of C, and N/P sources as 2:1 results in the maximum BET surface area of 1340 m2 g–1, while that of 1:2 has decreased to be only 47 m2 g–1. What’s up, the mass ratio of 1:1 exhibits superior electrochemical behaviors, whose maximum energy density can reach up to 13.3 Wh kg–1. The present synthesis method has provided an alternative avenue for producing N/P-carbon nanostructures with 2-D features, serving as excellent electrode materials for energy propagation and storage.

Keywords: Carbon nanosheets; Dopant; Nitrogen; Phosphorus; Supercapacitor.

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Introduction Doping carbon materials with heteroatoms commonly including N, P, B, S and O has been proved to be impactful for highly altering the character and performance of carbon-based devices such as supercapacitors.1 The most important and widely incorporated one is nitrogen (N) species, which can obviously result in the improved electronic conductivity, enlarged capacitance by surface faradaic reactions while remaining the rate capability and cycle stability almost invariable,2, 3 mainly because of their differences of electronic structures, electronegativities and radii.4 For instance, incorporating carbonaceous nanofibers (CNFs) with N dopant from polypyrrole has obtained larger capacitance of 202 F g–1, lower solution resistance of 0.14 Ω, and higher power density of 89.57 kW kg–1, which are largely improved compared with the blank without N doping.5 Subsequently, the doping-induced pseudo-capacitive effect for enhancing capacitance was extended to include other doping atoms such as phosphorus and boron even with a lower electronegativity than that of nitrogen. For example, Pdoping from H3PO4 has elevated the catalytic activity of carbon for the oxygen reduction reaction (ORR);6 remarkably, Xia et al. evinced that phosphate ion functionalization on Co3O4 nanosheets can significantly reduce the charge transfer resistance and increase the active reaction sites, resulting in greatly improved reactivity and pseudocapacitive performance (the capacitance increasing 7.98 times at 5 mV s–1).7 Given the above analysis, it is well revealed that doping carbon materials individually with N or P is fairly favorable of achieving better electrochemical and/or catalytic performance, and thus a lot of efforts have been focused on the design of materials with dual dopants of N and P (even with ternary components), basically derived from the cooperative/synergistic effect between them.8 Dai reported 3-D porous carbon networks codoped with N and P by pyrolysis of a supermolecular aggregate of self-assembled melamine, phytic acid, and graphene oxide, exhibiting high activities for both oxygen reduction and hydrogen evolution;9 moreover, the sources for N and P vary widely based on specific reaction conditions, mainly including cyanamide and phosphoric acid,10 polyacrylonitrile and H3PO4,11 melamine and H3PO4,12 dicyandiamide and H3PO4,13 (NH4)3PO4,14 polyaniline and H3PO4.15 Notwithstanding, large scale synthesis of 2-D carbon structures containing high 3



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contents of N/P as well as the correlative capacitive performance for supercapacitors are still stringent and interesting for us. Herein, we demonstrate a straightforward template carbonization method synchronously coupled with nitrogenization and phosphorylation process, in which trisodium citrate dihydrate, melamine, NH4H2PO4 serve as carbon, nitrogen and phosphorus source, respectively. The effect of mass ratio between them upon carbon structures was deeply investigated; furthermore, the resulting electrochemical performance as supercapacitor electrode materials was studied in a three-electrode system and two-electrode system, respectively. Experimental section In this work, we presented a solid state carbonization approach to synthesizing carbon materials incorporating with high N and P species, and the brief schematic illustration is shown in Figure 1. The trisodium citrate dihydrate actually acts as carbon source, but also the template coming from the sodium carbonate etc in situ produced. Clearly, these templates are quite beneficial for forming various kinds of pores within the carbon matrix. Besides, melamine was chosen for N sources (the thermal decomposition products of cyanide, N2 etc), primarily owing to its high content of N (66.7 wt %);16 another creative issue is the usage of NH4H2PO4, and, as a matter of fact, it serves as both N and P sources (the decomposition products of NH3, H3PO4), which is relatively environmentally friendly in contrast to direct utilization of H3PO4 (more corrosive, higher volatile) usually reported in the literatures.10-15 Hence, these starting materials before carbonization can be treated only by simply grinding in a solid state circumstance, and carbon materials co-doped with N and P can be obtained. As for the detailed existence forms of N and P within the carbon materials, it will be thoroughly discussed in the following section.

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Figure 1. Schematic illustration for producing N,P-carbon materials. Synthesis procedure of N/P co-doped carbon materials In a typical process, trisodium citrate dihydrate (C6H5Na3O7·2H2O), melamine, and ammonium dihydrogen phosphate (NH4H2PO4) were first evenly milled in a mortar about 30 min and then put on a porcelain boat. After introducing into N2 flow for 30 min, the mixture was further heated up to 800 °C at a rate of 4 °C min−1 and maintained for 2h under N2 flow in a horizontal tube furnace. After that, to absolutely remove the impurities, the resultant products were washed in diluted hydrochloric acid (1 mol L–1) under ultrasonic agitation. Subsequently, the products were flushed by utilizing abundant deionized water till the pH of the filtrate to be 7. Finally, the filtered product was dried in vacuum at 110 °C for 12 h and then the carbon nanosheets are obtained. And also, the synthesis details for the carbon samples herein are listed in Table 1. Table 1. Synthesis details for the carbon samples when keeping the carbonization temperature of 800 °C. samples

C6H5Na3O7·2H2O/g

melamine/g

NH4H2PO4/g

Carbon-N/P-2:1

10

2.5

2.5

Carbon-N/P-1:1

10

5

5

Carbon-N/P-1:2

5

5

5

5



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Electrochemical measurements conducted in a three-electrode system Typically, 80 wt % the carbon sample, 15 wt % graphite and 5 wt % polytetrafluoroethylene (PTFE) were mixed in ethanol. The mixed slurry was coated onto platinum net (area of 1 cm2) to prepare the working electrode, and the electrode was dried at 110 °C in an oven for 12 h. The three electrode system was executed in the prepared electrolytes with a counter electrode of platinum foil (6 cm2) and a reference electrode of saturated calomel electrode (SCE). All texts were carried out on a CHI 760E (Chen Hua Instruments Co. Ltd., Shanghai). The electrochemical performances of the samples were evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) techniques. The EIS measurements were carried out in the frequency range from 100 kHz to 0.01 Hz at open circuit potential with an ac perturbation of 5 mV. Specific capacitances derived from galvanostatic tests can be calculated from the equation:

c

Idt mdU

(1)

where C (F g−1) is the specific capacitance; I (A) is the discharge current; t (s) is the discharge time; U (V) is the potential; and m (g) is the mass of active materials loaded in working electrode. Electrochemical measurements conducted in a two-electrode system In a two-electrode cell, a glassy paper separator was sandwiched between two electrodes, and each electrode contains a mixture of 80 wt% the carbon sample (~ 3 mg), 15 wt % graphite and 5 wt % polytetrafluoroethylene (PTFE) binder. Graphene paper serves as the current collector. Specific capacitances derived from galvanostatic tests can be calculated from the equation:

C

4I (dU dt ) m

6


(2)

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where I (A) is the constant current; m (g) is the total mass of active materials loaded in the two working electrode; U is the voltage window (V); C (F g−1) is the specific capacitance. Specific energy density (E) and specific power density (P) derived from galvanostatic tests can be calculated from the equations:

E

P

I V d t M

E t

(3)

(4)

E is the energy density;I is the current and M is the total active mass of both electrodes; ∫Vdt is the galvanostatic discharge current area;P is the power density and t is the discharge time.

Results and discussion XRD technique was first employed to test the phase, crystallinity, and purity of the Carbon-N/P-1:2, Carbon-N/P-1:1, Carbon-N/P-2:1 samples, and the patterns are displayed in Figure 2a. In the 2 theta scope from 5 to 90°, all these carbon samples exhibit broad but low intensity diffraction peaks centered at 21.3 or 24.2°, which obviously indicates their low crystallinity (i.e., low degree of graphitization), identified as amorphous carbon widely reported. Besides, apart from the carbon substance, no other matters are easily discerned from XRD patterns, revealing their high purities in composition. Next, Raman tool was adopted to further examine the crystallinity status of these carbon materials since Raman spectrum is particularly sensitive to the microstructure of the carbon.17 Figure 2b indicates the contrast Raman spectra of the Carbon-N/P1:2, Carbon-N/P-1:1, Carbon-N/P-2:1 samples when designating the Raman scope from 500 to 3500 cm‒1, and all of them have exhibited two strong but discontinuous peaks located at 1355.8 and 1590.2 cm–1, respectively. The former one usually is ascribed as D band, disordered graphitic lattice (A1g symmetry) and the latter as G band, graphitic lattice (E2g symmetry).18 And yet, in contrast to that of standard 7



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graphite, there exists an obvious frequency shift of 5 ~ 10 cm–1 toward higher wavenumber, which is probably incurred by their nanoscale sizes and/or amorphous features.19 Moreover, the ratio (ID/IG) of the intensities of D band-to-G band was also calculated as 1.14, 1.11, and 1.09, respectively,20 in response to the cases of CarbonN/P-1:2, Carbon-N/P-1:1, Carbon-N/P-2:1, to some extent revealing the decrease of graphitization degree of carbon materials by enlarging the N/P doping. 21 It has been inferred that heteroatoms (B, N, P, S etc) doping could break the hexagonal symmetry of graphite, inducing a higher D band intensity in the Raman spectrum.22,23 The porosity of carbon materials is another crucial factor that influences the electrochemical performance of supercapacitors, and it was measured by means of N2 adsorption-desorption technique. The relevant isotherms of the Carbon-N/P-1:2, Carbon-N/P-1:1, Carbon-N/P-2:1 samples are shown in Figure 2c in the pressure scope of 0 ~ 1.0. For the case of the Carbon-N/P-2:1 sample, its isotherm assigned to the combination of type I and IV is primarily composed of three stages, i.e., a steep increase of volume adsorbed at low pressure close to 0, suggesting the presence of abundant micropores within carbon matrix; an overt hysteresis loop at the pressure range of 0.45~0.90, indicative of the existence of mesopores; while the third sharp stage scoping from 0.90 ~ 1.0 commonly implies the presence of macropores.24 It is thereby discerned that the Carbon-N/P-2:1 sample possesses a hierarchical pore structure, which fairly benefits for energy propagation and storage. Analogously, the Carbon-N/P-1:2, Carbon-N/P-1:1 samples also present the isotherms with the similar tendency, except for the apparent drop of volumes adsorbed, signifying the adjustment of BET surface areas when altering the N/P contents, as revealed in Table 2. As a result, the Carbon-N/P-2:1 sample displays the maximum BET surface area of 1340 m2 g–1, but that of the Carbon-N/P-1:2 sample has decreased to be only 47 m2 g–1. Furthermore, Figure 2d shows the comparative pore size distribution curves, and all of them have exhibited hierarchical features with main pore distribution ranging from 1 ~ 7 nm (the boundary line of 2 nm), well consistent with the predication from isotherms.

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1355.8

1590.2

Carbon-N/P-1:2

Carbon-N/P-1:1

Carbon-N/P-2:1 Carbon-N/P-1:1 Carbon-N/P-1:2

(b)

Intensity (a.u.)

Intensity (a.u.)

(a)

Carbon-N/P-2:1

0

24.2

o

30 45 60 2 theta (deg.)

75

500

90

1000 1500 2000 2500 3000 3500 -1 Raman shift / cm

0.30 0.20

3

600

0.25

200 0

0.010

0.005

5.35

400

0.15


(d)

2.62

-1

800

-1

(c)

1.03 1.67 1.36

0.35 Carbon-N/P-2:1 Carbon-N/P-1:1 Carbon-N/P-1:2

dV/dD / cm g nm

3

-1

1000

15

o

3.81

21.3

Volume Adsorbed / cm g STP

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


0.000

0.10

0

2

4

6

8

10

0.05 0.00

0.0

0.2 0.4 0.6 0.8 Relative Pressure / P/P0

1.0

0

1

2

3 4 5 6 7 Pore Width / nm

Figure 2. The Carbon-N/P-1:2, Carbon-N/P-1:1, Carbon-N/P-2:1 samples: (a) XRD patterns; (b) Raman spectra; (c) N2 adsorption-desorption isotherms; (d) Pore size distribution curves.

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Table 2. Summary of the pore structures of the carbon materials. samples

BET surface area

Total pore volume

Pore width

/ m2 g‒1

/ cm3 g‒1

Carbon-N/P-2:1

1340

1.27

3.13

Carbon-N/P-1:1

672

0.93

2.97

Carbon-N/P-1:2

47

0.35

3.93

/ nm

The elemental composition, chemical/electronic state of the elements that exist within carbon materials were measured by XPS technique and the corresponding results are shown in Figure 3. Figure 3a indicates the XPS survey of the CarbonN/P-1:2, Carbon-N/P-1:1, Carbon-N/P-2:1 samples with the binding energy scoping from 0 to 1400 eV. Apparently, all of three samples exhibit high content of C1s (~ 284.5 eV), N1s (~ 399.6 eV), P2p (~ 133.2 eV) and O1s (~ 529.4 eV); especially, the Carbon-N/P-1:2 sample shows additional distinct P content with the binding energy below 200 eV. As a result, the atomic contents of C, N, P and O from XPS analysis are summarized in Table 3. The three N contents are quite high beyond 11%, and along with the increase of mass ratio of melamine/NH4H2PO4 and trisodium citrate dehydrate, the data also slightly augments. But, regarding the case of P content, the discrepancy between them varies greatly; the Carbon-N/P-1:2 sample delivers high P content of 8.84%, which is much larger than that of the other two carbon samples,

thus

reveal

the

rule

that

adjusting

the

mass

ratio

of

carbon/nitrogen/phosphorus sources plays crucial role in the determination of final carbon products. To be specific, Figure 3b indicates the typical C1s spectra of the Carbon-N/P1:2, Carbon-N/P-1:1, Carbon-N/P-2:1 samples when ranging the binding energy from 283 to 290 eV, which on the whole can be deconvoluted into 4 peaks with the help of XPSPEAK software. The C1 peaks located at 284.5 eV can be indexed as sp2 C═C bond; while the C2 ones at 285.4 eV as sp3 C─C/C─P, and/or C═N bond. The C3 at 286.3 eV are ascribed to be C─O/C─N; and the C4 with minor intensity at 287.6 eV are due to the contribution of C═O bond.9, 25, 26 In addition, the N 1s spectra from 396 to 406 eV towards the three carbon samples are given in Figure 3c, which are approximatively fitted 4 peaks with different binding energies. The N1 peaks at 10


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398.6 eV owe to the pyridine nitrogen (N-6); the N2 at 399.4 eV are attributed to pyridone/pyrrolic nitrogen (N-5(1), N-5(2)); the N3 at 400.8 eV contribute to quaternary nitrogen (N-Q);27 furthermore, as for the N4 at 401.6 eV, they might represent the N-P bonding.26 These nitrogen dopants with various existence forms also have been depicted in Figure 1. What’s more, Figure 3d demonstrates the typical P2p spectra with the binding energy of 130 ~ 137 eV. From the viewpoint of XPS intensity, the Carbon-N/P-2:1 sample exhibits the lowest feature without any prominent peaks while the other two samples almost show 3 fitted peaks. The P1 at 132.8 eV is due to P─C bond; the P2 at 133.5 eV to P─N bond; the P3 at 134.2 eV to P─O bond.11, 26 Other kinds of functionalities containing P also probably exist within the carbon such as H2PO4‒1, PO3‒1,28 considering the thermal decomposition of NH4H2PO4 at the elevated temperatures.

(a)

C1s

O1s

Carbon-N/P-1:2

Carbon-N/P-1:2

survey

Intensity (a.u.)

Intensity (a.u.)

C1s

(b)

N1s P2p Carbon-N/P-1:1

Carbon-N/P-1:1

Carbon-N/P-2:1

Carbon-N/P-2:1 C1 C2

0

200

400

282

600 800 1000 1200 1400 Binding energy (eV)

N1s

(c)

C3

C4

284 286 288 290 Binding energy (eV)

Intensity (a.u.)

Carbon-N/P-1:1

Carbon-N/P-1:2

Carbon-N/P-1:1

Carbon-N/P-2:1 N1 N2 N3

396

Carbon-N/P-2:1 P1 P2 P3

N4

398 400 402 404 Binding energy (eV)

292

P2p

(d)

Carbon-N/P-1:2

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


406

408

130

11


132 134 136 Binding energy (eV)

138


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Figure 3. The Carbon-N/P-1:2, Carbon-N/P-1:1, Carbon-N/P-2:1 samples: (a) XPS survey; (b) C1s; (c) N 1s; (d) P 2p. Table 3. The atomic contents of C, N, P and O from XPS analysis. samples

C/at%

N/at%

P/at%

O/at%

Carbon-N/P-2:1

80.33

11.45

0.13

8.10

Carbon-N/P-1:1

81.76

12.52

0.54

5.18

Carbon-N/P-1:2

83.02

14.13

8.84

4.01

To authentically describe the characteristics (mainly including morphology and size) of the present Carbon-N/P-1:2, Carbon-N/P-1:1, Carbon-N/P-2:1 samples, HRTEM technique was adopted and the corresponding results are displayed in Figure 4. Concretely, Figure 4a-b shows the representative HRTEM images with various magnifications, where there exist substantial 2-D carbon nanostructures, clearly possessing wrinkled features. And also, it is nearly several micrometers in width together with low contrast, which to some extent is analogous to graphene, looking from the exterior. On the other hand, as we know, elemental mapping allows us to in situ visualize the chemical landscape in samples with each element present shown as a different color. Resultantly, Figure 4c indicates the typical elemental mapping of C/N/P elements concerning the Carbon-N/P-2:1 sample. As a whole, it is readily inferred that the elements of C/N exhibit high contents but that of P element is low (conforming to the EDAX result in Figure 4j), which is also well consistent with the XPS results shown in the above Table 3. As for the other samples of Carbon-N/P-1:2, Carbon-N/P-1:1, similar HRTEM results also occur, as shown in Figure 4d-i. Even so, some difference between them still is easy to be found out, especially for the case of the Carbon-N/P-1:2 sample. Macroscopically, due to the enhancement of N/P contents, it has somewhat become compact in shapes, as revealed in Figure 4g-h. Moreover, the P content in Figure 4i turns into being enlarged compared with that of the other carbon samples, also in good accordance with the variation tendency in Figure 4j and Table 3.

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Figure 4. HRTEM images and elemental mappings: (a-c) Carbon-N/P-2:1; (d-f) Carbon-N/P-1:1; (g-i) Carbon-N/P-1:2 as well as the EDAX spectra (j). Next, electrochemical performance of the present Carbon-N/P-1:2, CarbonN/P-1:1, Carbon-N/P-2:1 samples were tested as electrode materials for supercapacitors. The resultant CV and GCD curves measured in a three-electrode system are shown in Figure S1. For convenience, Figure 5a indicates the contrast CV curves at 20 mV s–1 while maintaining the potential scope of -1.0 ~ 0 V. As far as the

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CV outline is concerned, those of the Carbon-N/P-1:1, Carbon-N/P-2:1 samples are in general close to rectangles, revealing the predominant contribution of EDLCs for them. Nevertheless, the CV profile towards the Carbon-N/P-1:2 sample seems to be more distorted, which are overtly derived from its high contents of N/P and low porosity mentioned before. Notably, something that needs to be pointed out in Fig. 5a is that the deviation concerning CV profiles from ideal ones might be primarily incurred by functionalities of N species within carbon matrix. The correlative pseudocapacitative behaviors are ascribed to the redox reactions involved between the electrolyte ions and these doped functionalities12,29 (e.g., the redox reaction between pyridone- and pyridinic-N and the redox reaction between N–O and pyridinic-N),27 quite strongly depending on the electrolyte and its pH conditions. And also, oxygen doping can also lead to the formation of pseudocapacitive behavior, just like that of N species. However, this conclusion is still contentious that requires deeper investigation.30 On the other hand, from the viewpoint of integral areas of CV curves, the order existent is as follows: Carbon-N/P-1:1>Carbon-N/P-2:1>Carbon-N/P-1:2. It is therefore inferred to us that the electrochemical performance regarding the present carbon samples strongly depends not only on the contents of N/P within carbon matrix but also on their BET surface areas and pore volumes. Similar varying pattern also occurs for the case of GCD results, as displayed in Figure 5b, in which the charging/discharging time for the Carbon-N/P-1:2 sample is also negligible in a comparative manner. Furthermore, the equivalent series resistance (ESR) of the Carbon-N/P-1:2 sample obtained from the X-intercept of the Nyquist plots in Figure 5c approaches to be 10Ω, much larger than that of the other carbon samples.31 As a consequence, in virtue of the CV, GCD and Nyquist results, it is certainly concluded that the CarbonN/P-1:1,

Carbon-N/P-2:1 samples

have delivered 14


superior electrochemical

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performance compared with the Carbon-N/P-1:2 sample, and thus the former ones will be investigated more detailed. Besides, Figure 5d shows the specific capacitances against different current densities for the cases of Carbon-N/P-1:1, Carbon-N/P-2:1, in which the Carbon-N/P-1:1 sample delivers the better performance than that of the Carbon-N/P-2:1 sample. For instance, their specific capacitances have reached up to 176 and 160 F g–1 at the current density of 1 A g–1, also indicating the higher rate capability of the Carbon-N/P-1:1 sample. In short, based on the electrochemical performance depicted in Figure 5, it can be readily concluded that the Carbon-N/P1:1 sample has performed superior capacitive behavior in a three-electrode system, mostly rooting in its proper balance between BET surface area (672 m2 g–1), total pore volume (0.93 cm3 g–1), N% content (12.52) and P% (0.54). Finally, as for the reason why the Carbon-N/P-1:1 sample has exhibited much better electrochemical performance including larger CV area, higher rate capability etc among the samples, some probably rational illustration are given as follows: the Carbon-N/P-2:1 sample possesses larger EDLCs contribution due to its larger porosity (SBET and VT), but the Carbon-N/P-1:1 sample can give rise to more pseudo-capacitive contribution that is derived from its higher N/P contents, and as a consequence, the total capacitive performance of the Carbon-N/P-1:1 sample has probably surpassed the other one. In other words, determination of capacitive performance depends on not only the porosity of carbon materials (for EDLCs) but also the heteroatoms incorporated (for pseudocapacitance).

15



(a)

4

Potential (vs. SCE) / V

Current density / A g

-1

6 Carbon-N/P-1:2 Carbon-N/P-1:1 Carbon-N/P-2:1

2 0 -2

0.08

-4

0.04 0.00 -0.04

-6

20 mV s

-1

-0.08 -0.12

-1.0

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

0.0


(b) -0.2 0.0

-0.4

-0.2 -0.4 -0.6

-0.6

-0.8 -1.0 0.0 0.5 1.0 1.5 2.0 2.5

-0.8 -1

1Ag

-1.0

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

-8

0.0

0

100

200

300 400 Time / s

500

600

-1

80

Specific capacitance / F g

(c) 60 40

''

-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

Page 16 of 25

20


0 0

20

40 ' Z / ohm

60

80

180 (d) 150

Carbon-N/P-1:1 Carbon-N/P-2:1

120 90 60 30 0 1

2

3 4 5 6 7 8 9 10 -1 Current density / A g

Figure 5. Electrochemical performance of the Carbon-N/P-1:2, Carbon-N/P-1:1, Carbon-N/P-2:1 samples tested in a three-electrode system: (a) CV curves; (b) GCD curves; (c) Nyquist plots; (d) Specific capacitance versus current density.

To more precisely measure the electrochemical performances of the CarbonN/P-1:1, Carbon-N/P-2:1 samples, a two-electrode system was put to use at room temperature. Figure 6 a-b shows the typical CV and GCD curves obtained in a twoelectrode system, quite similar to those in a three-electrode system. Other than that, the specific capacitances (Cs) have contrastively reduced in present two-electrode system, 16


Page 17 of 25

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


as usually reported for supercapacitors.32, 33 For example, the Cs data for Carbon-N/P1:1, Carbon-N/P-2:1 samples are of 96 and 81 F g–1, respectively. And furthermore, the other important factor greatly influencing the practical application of supercapacitors is the cycling stability. For present cases, consecutive 5000 cycles were designated as the measurement criterion and the results are given in Figure 6d. Both of the Carbon-N/P-1:1, Carbon-N/P-2:1 samples have exhibited superior cycling

stabilities

as

87.3%

and

91.0%,

respectively,

after

long

term

charging/discharging for 5000 times. Besides, Ragone plot is conventionally used for performance comparison of various energy-storing devices. On such a chart, the values of specific energy (in Wh kg–1) are plotted versus specific power (in W kg–1). To be specific, the energy density describes how much energy is available, while the power density shows how quickly that energy can be delivered.34, 35 As a consequence, Ragone plot is yet another pivotal factor for supercapacitor application. Herein, the energy densities for the CarbonN/P-1:1, Carbon-N/P-2:1 samples can reach up to 13.3 and 11.2 Wh kg–1, respectively, when achieved at the power density of 500 W kg–1, as displayed in Figure 6e. Obviously, the present data are comparable even superior to the reported cases, such as redox-mediated gel polymer electrolyte (PVA-H2SO4-P-benzenediol) and activated carbon electrodes (11.3 Wh kg–1),36 MnO2/CNTs composite (13.3 Wh kg–1),37 carbonaceous nanofibers (CNFs) coated with polypyrrole (6.7 Wh kg–1),5 N/O/P decorated porous carbons derived from shrimp shells (5.2 Wh kg–1).38 Finally, as is well known, a radar chart is a graphical method of displaying multivariate data in the form of a two-dimensional chart of three or more quantitative variables represented on axes starting from the same point. Hence, we presented the Radar chart of maximum specific capacitance, rate capability, cycling ability, maximum energy density and energy efficiency concerning the Carbon-N/P-1:1, 17



Carbon-N/P-2:1 samples. Resultantly, the integral area of closed curve for the Carbon-N/P-1:1sample is in the mass larger than that of the Carbon-N/P-2:1 sample,

4

-1

1.0

20 mV s

(a)

Potential / V

Current density / A g

-1

further evincing the better electrochemical performance of the former one.

2 Carbon-N/P-1:1 Carbon-N/P-2:1

0 -2

(b)

0.8 0.6 0.4 0.2


0.0 0.0

0.2

0.4

0.6

0.8

1.0

0

Potential / V

-1

(c)

2Ag

80

-1

3Ag

60

-1

100

1Ag


-1

4Ag

40

-1

5Ag

-1

6Ag

-1

7Ag

20

-1

8Ag

-1

9Ag

-1

10Ag

-1

0 0

50

100 150 200 Cycle number

50

100

150

200

250

Time / s

250

120 100 (d) 80 60 Carbon-N/P-1:1 40 20 5000 0 0 20 40 60 80 100 91.0%


-1

-1

1A g

-4


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

Page 18 of 25

87.3%

5000 Carbon-N/P-2:1

Cycle number

300

18


Page 19 of 25

6min

(e)

-1

Energy density / Wh kg

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


2.4min

14.4s

13.3

3.6s

10

1

0.36s [31] [32] [5] [33]


0.1 100

1k -1 Power density / W kg

10k

Figure 6. Electrochemical performance of the Carbon-N/P-1:1, Carbon-N/P-2:1 samples tested in a two-electrode system: (a) CV curves; (b) GCD curves; (c) Specific capacitance versus current density; (d) Cycling stability; (e) Ragone plots; (f) Radar chart.

Conclusions In summary, a simple but efficient template carbonization method assisted with synchronous nitrogenization and phosphorylation process has been implemented for large scale production of 2-D carbon nanosheets doped with N/P species, using trisodium citrate dihydrate (C6H5Na3O7·2H2O), melamine and NH4H2PO4 as C/N/P sources, respectively. The effect of mass ratio of these raw materials upon carbon structures and electrochemical performances were investigated in depth. The advantages/merits existing in present work can be conclusively categorized as follows: 1. Simply carbonizing the mixture of commercially available trisodium citrate dihydrate, melamine and NH4H2PO4 can result in the formation of 2-D carbon nanostructures, and moreover the doping content of N/P strongly depends on the mass ratio of carbon and N/P sources. 19



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Page 20 of 25

2. Dopants of N/P species have important roles in the determination of carbon porosities and electrochemical performance for supercapacitor application; in particular, increasing the P content can lead to the drop of BET surface area together with the correlative electrochemical performance. 3. The present Carbon-N/P-1:1 sample exhibits superior electrochemical behaviors, whose maximum energy density can reach up to 13.3 Wh kg–1. In brief, the present template carbonization method coupled with nitrogenization and phosphorylation process is expected to be utilized for producing N/P-carbon materials especially with 2-D nanostructures in a large scale manner. Acknowledgments This work was financially supported by the financial supports from National Natural Science Foundation of China (51602003)，Natural Science Foundation of Anhui Province (1508085QE104), University Scientific Research Project from Department of Education of Anhui Province (KJ2016A039) and Startup Foundation for Doctors of Anhui University (J01003211). Supporting Information The methods for detailed structural characterization and electrochemical measurement in three/two-electrode systems, electrochemical performance of the Carbon-N/P-1:2, Carbon-N/P-1:1, Carbon-N/P-2:1 samples tested in three/twoelectrode systems and the 1st and 5000th GCD curves towards the Carbon-N/P-1:1, Carbon-N/P-2:1 samples tested in a two-electrode system. References (1) Lin, T. Q.; Chen, I. W.; Liu, F. X.; Yang, C. Y.; Bi, H.; Xu, F. F.; Huang, F. Q. Nitrogen-doped

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