Template Synthesis of Nitrogen-Doped Carbon Nanosheets for High

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Template Synthesis of Nitrogen-doped Carbon Nanosheets for High Performance Supercapacitors Improved by Redox Additives Wei Hu, Dong Xu, Xiao Na Sun, Zhenghui Xiao, Xiang Ying Chen, and Zhong Jie Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01189 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on August 31, 2017

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ACS Sustainable Chemistry & Engineering

Template

Synthesis

of

Nitrogen-doped

Carbon

Nanosheets

for

High

Performance Supercapacitors Improved by Redox Additives Wei Hu1, Dong Xu1, Xiao Na Sun1, Zheng Hui Xiao1, Xiang Ying Chen1*, and Zhong Jie Zhang2*

1

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. [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. * The corresponding author. E-mail: [email protected].

1

ACS Paragon Plus Environment


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Abstract In this work, nitrogen-doped sheet-like carbon materials have been synthesized by a template carbonization method using 1,5-diphenylcarbazide and MgCl2·6H2O as carbon source and template, respectively. It presents the carbon sample with amorphous characteristic as well as high surface area and large pore volume. More importantly, introducing the redox additives of 4-hydroxybenzoic acid (HBA), 3,4-dihydroxybenzoic acid (DHBA) and 3,4,5-trihydroxybenzoic acid (THBA) with functional hydroxyl groups into 1 mol L ‒ 1 H2SO4 has largely improved the capacitances as well as the energy density. As expected, supercapacitor with redox additive of HBA exhibits higher capacitances with an increase of 1.57 times compared with the conventional H2SO4 electrolyte. Besides, compared with the one without any redox additive, the redox additive of DHBA has a large improvement of capacitances and the resultant capacitances have been remarkably increased by 3.18 times. In addition, the redox reactions of HBA and DHBA are reversible, whilst that of THBA is irreversible. Moreover, HBA with a hydroxyl group can release/gain a proton/electron, and DHBA which owns a pair of hydroxyl groups can release two protons/electrons. What’s more, both of the redox processes of HBA and DHBA are controlled by diffusion mechanism.

Keywords:

Template;

Porous

carbon;

Redox

additive;

Supercapacitor.

2


Hydroxyl

group;

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Introduction As one of the most promising energy storage device, supercapacitor has drawn much attention due to its fast charge–discharge rates, low costs, long cycling lives and environmental-friendliness.1-3 There are three main types of supercapacitor distinguished from electrode active materials: metal oxide, conducting polymer and carbon based supercapacitor.4-7 Among them, carbon based supercapacitor has been the alternative one because of its advantages depending on the carbon materials involved.6 However, compared with batteries, carbon based supercapacitor displays lower energy density, which is related to the charge storage mechanism of electric double layer capacitor. Some literatures have been reported that energy density can be acquired according to the E=1/2CV2, where C is related to the total capacitances, at the same time, V is corresponding to the operational potential of the cell.8, 9 In order to promote energy density, adding additional capacitances has been regarded as a useful way to enhance the total capacitances.10 Interestingly, adding additional capacitances can be mainly classified into two aspects: 1) heteroatom doping such as nitrogen/sulfur doping; 2) introducing redox additives into electrolyte.11-13 The method of nitrogen-doped has been often reported while redox additives rarely mentioned till to now. As we know, the redox additives can be classified into two categories: inorganic and organic additives. However, the inorganic ones are limited quantitatively, which quite restrict the application areas. By contrast, the quantities of organic ones are much broader and more promising. Up to now, many redox additives have been employed to promote the electrochemical performances, such as p-phenylenediamine (PPD),14,

15

p-benzenediol (PB),16,

17

indigo carmine,18 methylene blue,19 rutin,20

which can enormously enhance the property of supercapacitor. Notably, most of these organic redox additives possess the functional groups of amino and/or hydroxyl which have redox reactions in alkaline/acidic electrolyte. Moreover, carboxyl functional groups are typical electron-withdrawing groups. Hence, it is interesting for researching the electrochemical behavior at the circumstances of co-existences of the

3



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hydroxyl and carboxyl functional groups. As a consequence, we employed the 4-hydroxybenzoic

acid

(HBA),

3,4-dihydroxybenzoic

acid

(DHBA)

and

3,4,5-trihydroxybenzoic acid (THBA) as redox additives owing to their characteristic functional groups of hydroxyl and carboxyl groups which can release/gain protons and electrons in electrolyte. In this work, we chose a simple but efficient protocol of salt template carbonization

method

for

compounding

porous

carbon

materials,

using

1,5-diphenylcarbazide and magnesium chloride hexahydrate as carbon source and template, respectively. Next, HBA/DHBA/THBA regarded as efficient redox additives were added into H2SO4 electrolyte to enlarge electrochemical performance of the supercapacitor.

Experiment All the chemicals were purchased from Sinopharm Chemical Reagent (Shanghai) Co. Ltd, and used without further purification. Synthesis Procedure of Porous Carbon Materials The process of synthesizing procedure of porous carbon materials was displayed as follows: Firstly, 1,5-diphenylcarbazide and magnesium chloride hexahydrate (MgCl2·6H2O) were mixed with a mass ratio of 1:3 and then pulverized into powders. The powders were put into a porcelain boat, and then heated up to 800 °C at a rate of 4 ºC min‒1 surrounded by N2 flow and kept it for 2 h. After reducing the temperature to the room temperature, the carbonized products were pulverized into powders and dissolved in 1 mol L–1 HCl solution to remove inclusions. Then, the products were washed by deionized water and end up till the colature becoming neutral. Finally, the products were dried overnight at 110 °C. Eventually, the C-blank sample was obtained. Preparation Procedure of Mixed Electrolytes A series of mixed electrolytes have been obtained via adjusting the molar

4


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quantities of HBA/DHBA/THBA. Concretely, the mixed electrolytes were prepared by introducing different molar masses (0.001, 0.0015 and 0.002 mol) of HBA/DHBA/THBA into 100 mL H2SO4 aqueous electrolyte (1 mol L–1), in which the concentrations of HBA/DHBA/THBA were 0.01, 0.015 and 0.02 mol L–1, respectively. And the obtained HBA/DHBA/THBA-based redox-active electrolytes were marked as C-HBA-10/15/20, C-DHBA-10/15/20 and C-THBA-10/15/20, which have been shown in Table 1, respectively. Table 1. Summary of mixed electrolytes by adding HBA/DHBA/THBA/mHBA into 1 mol L‒1 H2SO4 solution, respectively. HBA

DHBA

THBA

mHBA

(mmol L−1)

(mmol L−1)

(mmol L−1)

(mmol L‒1)

C-blank

/

/

/

/

C-HBA-10

10

/

/

/

C-HBA-15

15

/

/

/

C-HBA-20

20

/

/

/

C-DHBA-10

/

10

/

/

C-DHBA-15

/

15

/

/

C-DHBA-20

/

20

/

/

C-THBA-10

/

/

10

/

C-THBA-15

/

/

15

/

C-THBA-20

/

/

20

/

C-mHBA-20

/

/

/

20

Sample

Characterizations and Measurement Techniques The structure characterization and measurement techniques are given in Supporting Information.

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Parameters Calculations The detailed calculation equations employed in this work are illustrated in Supporting Information section.

Results and Discussion Physicochemical Characterization Figure 1a demonstrates the XRD pattern of the C-blank sample. And the XRD diffraction peak is located at 21.2° in the spectrogram. Besides, XRD diffraction peak is lower than 2 theta standard value of graphite (26.6°), which reflects an increase in the d-spacing and the unit cell size, corresponding to the amorphous feature.21, 22 Moreover, Raman technique was also employed to further study the crystallographic structure of the C-blank sample. Figure 1b exhibits the Raman spectrum of the C-blank sample at the wavenumber range of 0 ~ 4000 cm−1, and it can be seen that the D band and G band center at ~ 1353.0 and ~ 1587.1 cm−1, respectively. Generally, D band refers to the sp3 hybridized carbon. In other words, the D band corresponds to the chaotic and defective structure in the carbon sample, while the G band is related to graphitized degree. To further study the graphitization degree of porous carbon material, the ID / IG was employed to calculate the intensity ratio between D band and G band. As shown in Figure 1b, the ID / IG ratio is of 1.06, indeed revealing low graphitization degree. Besides, from the result of Figure 1b, it can be also seen that there is an inconspicuous peak situated at ~ 2706.0 cm−1, which corresponds to the 2D band, which manifests that the C-blank sample possesses the low graphitization degree.23-25 The surface characteristics of the obtained materials were analyzed by nitrogen adsorption-desorption. The textural properties of the C-blank sample is demonstrated in Table 2. Figure 1c and Figure 1d show a type IV adsorption-desorption isotherm of the C-blank sample with a hysteresis loop, implying that the samples exhibit a

6


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volume of the C-blank sample are 607 m2 g‒1 and 1.32 cm3 g‒1, respectively. The pore size distribution curve shows the peaks centered at 1.82 and 4.30 nm and an average pore diameter of 8.69 nm. The existence of multiple pore structure is beneficial of obtaining prominent electrochemical performance, for micropore and mesopore play important roles in charge storages and fast ion diffusions, respectively.27-29

(a)

o

21.2

1000

C-blank

400 Intensity(cps)

Intensity (cps)

500

300 200 100

(b)

ID / IG=1.06

D band 1353.0

800

G band 1587.1

600

2D band

400 200

0

SBET= 607 m2 g-1

800

VT = 1.32 cm3 g-1

600

dav = 8.69 nm

400 200 0 0.0

0.2 0.4 0.6 0.8 Relative Pressure (P / P0)

1.0

0

100

0.8 0.6

0

1000 2000 3000 -1 Wavenumber / cm

4000

0.15

(d)

0.12

dav=8.69 nm

0.09

0.4

0.06

0.2

0.03

17.4

80

9.02

(c)

40 60 2 theta(deg.)

1.82

1000

20

4.30

0

Cummulative Pore Volume (cm3 g-1)

3 -1 Quantity Adsorbed (cm g STP)

property of mesopores.26 The Brunauer-Emmett-Teller (BET) surface area and pore

0.0

0.00 0

10 20 30 Pore Width (nm)

40

Figure 1. The C-blank sample: (a) XRD pattern; (b) Raman spectrum (c) N2 adsorption-desorption isotherm; (d) Cumulative pore volume and pore size distribution curves.

Differential Pore Volume dV (cm3 nm-1 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


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Table 2. Characteristic surface areas and pore structures of the C-blank sample.

SBET / m2 g‒1

Pore Volume / cm3 g‒1

Sample

C-blank

dav / nm ST

Smicro

VT

Vmicro

607

45

1.32

0.015

8.69

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 width.

The morphology of the C-blank sample was investigated by scanning electron microscopy (FESEM) and transmission electron microscope (HRTEM). The FESEM image of the C-blank sample (Figure 2a) shows a curly surface, where ultrathin layers can be observed in the whole sample. Besides, as shown in the inset of Figure 2a, we can gain a high productivity (2.69 g products come from 6.00 g 1,5-diphenylcarbazide). HRTEM characterization further confirms the result mentioned above. As presented in Figure 2b and Figure 2c, the C-blank sample displays an obviously wrinkled and 2D sheet-like structure. Besides, in Figure 2d, HRTEM image of the C-blank sample implies the amorphous and porous feature, which is further demonstrated by the dim diffraction rings of the SAED pattern (the inset of Figure 2d), corresponding to the XRD and Raman results mentioned above.

8


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Figure 2. The C-blank sample: (a) FESEM image and mass of the product from each experiment (the inset of Figure 2a); (b, c, d) HTREM images as well as the SAED pattern and enlarged HRTEM image (the inset of Figure 2d). On the basis of the structural analysis of the carbon material mentioned above, we further proposed the schematic illustration for producing amorphous 2D carbon structures when utilizing 1,5-diphenylcarbazide and MgCl2·6H2O as carbon/nitrogen precursor and template, which is vividly depicted in Figure 3. 1,5-diphenylcarbazide could decompose into carbon matrix together with some gases at the elevated carbonization temperature. In particular, the fresh produced gases containing NH3, N2 etc. would treat with carbon matrix, primarily giving rise to the occurrence of nitrogen-doped carbon materials. This kind of organic molecule jointly composed of carbon and nitrogen species also have been widely employed for producing nitrogen-doped carbon materials in recent years.30, 31 Noteworthily, both of them have increased the nitrogen incorporation during the carbonization process.

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Interestingly, we also employed commercially available MgCl2·6H2O as efficient template for largely forming pore structures within carbon materials. As we know, the melting point of MgCl2·6H2O is 117 °C, it thereby decomposes into MgO substance as well as some gases at the elevated temperature up to 415 °C under N2 flow.32, 33 In particular, as clearly shown in Figure 3, the freshly produced MgO has been well confirmed by the XRD tool. Obviously, the MgO substance can serve as hard template for formation mesopores, while the other gases as soft template for producing micropores. In other words, the porous inorganic salt of MgCl2·6H2O actually acts as multiple template, which is evidently favorable for the preparation of hierarchical pore structure of various kind of carbon materials.33

Figure 3. Schematic illustration for the production process of porous carbon materials, using MgCl2·6H2O as template. Moreover, XPS technique was employed to gain a deeper investigation of the obtained samples on chemical compositions and electronic states of various elements. As we know, the species of nitrogen functional groups contain pyridine nitrogen, pyridone/pyrrolic nitrogen, and quaternary nitrogen.34 Meanwhile, the types of oxygen functional groups consist of quinines, ketones and aldehydes type group, ethers and phenols type group and chemisorbed oxygen and/or water bonded to the carbon surface.35, 36 respectively. Besides, Figure 4a demonstrates that the C/O/N contents are quantitatively calculated to be 78.81 at.%, 12.73 at.% and 8.46 at.%,

10


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respectively. Obviously, as shown in Figure 4b, N 1s spectrum can be divided into three peaks located at binding energies of 398.3, 400.1 and 401.4 eV, which are related to pyridine nitrogen (N-6, ~16.38 at.%), pyrrolic (N-5, ~25.69 at.%) and quaternary (N-Q, ~57.93 at.%),1, 31, 35-37 respectively. Hulicova reported that nitrogen situated at the edge of graphene layers could produce an additional capacitive effect.38 Hence, the high content of nitrogen can have a contribution with electrochemical performance of carbon materials. Besides, Figure 4c displays that three peaks of the O 1s spectrum are located at 531.4, 532.3 and 533.2 eV, which are associated with O-1, (~41.23 at%), O-2, (~30.98 at%) and O-3, (~27.79 at%) bonded to the carbon surface, respectively.35, 36, 39, 40 Moreover, C 1s spectrum exhibits four peaks in Figure 4d: carbon atoms connected with each other by hybridization of sp2 in rings at 284.7 eV (C-1), carbon atoms sp3 combined with each other in rings at 285.5 eV (C-2), carbon atoms singly bound to oxygen in phenol at 286.5 eV (C-3) and carbon atoms sp2 linked to oxygen or carbon atoms sp3 linked to nitrogen at 287.5 eV (C-4),35, 41 respectively. At the same time, CV curve of C-blank sample is shown in Figure S2. It can be seen that a pair of broad, reversible humps, confirming the co-effects of electrical double-layer capacitances and additional capacitances at the presence of nitrogen and oxygen moieties on carbon materials. In general, rich nitrogen and oxygen functional groups can not only considerably contribute to additional capacitances, but also improve the wettability of carbon materials in aqueous electrolyte.36-40

(a)

C1s

200.0k

O1s

150.0k 100.0k

5.5k

80 60

78.81

5.0k

40 20 0

C1s

8.46

12.73

N1s

O1s

N1s

50.0k Survey

0.0 0

300

600

Intensity (cps)

250.0k

Atomic (%)

Intensity (cps)

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


4.5k 4.0k

N1s NQ N5

401.1 eV

400.0 eV

N6 398.4 eV

3.5k 3.0k 2.5k

900

1200

1500

396

Binding energy (eV)

(b)

399 402 405 Binding energy (eV)

11


408


25.0k

(c)

O1s

O1

Intensity (cps)

Intensity (cps)

1 2 3 4 5 6 7 10.0k 8 9 10 8.0k 11 12 6.0k 13 14 15 4.0k 16 17 2.0k 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

O2

531.4 eV

532.3 eV

O3

533.2 eV

(d)

Page 12 of 33

C1 284.7 eV

C1s

20.0k 15.0k

C2 285.5 eV

10.0k

C3 286.5 eV

5.0k

C4 287.5 eV

0.0 528

531 534 537 Binding energy (eV)

282

540

285 288 Binding energy (eV)

291

Figure 4. The C-blank sample: (a) XPS survey (the inset is the summary of carbon/nitrogen/oxygen contents); (b) N 1s spectrum; (c) O 1s spectrum; (d) C 1s spectrum.

In order to further study the electrochemical performance of the C-blank sample, we employed series measurements of three-electrode system using the conventional 1 mol L–1 H2SO4 aqueous solution as electrolyte and the corresponding results are shown in Figure S3. Figure S3a illustrates the cyclic voltammetry (CV) curves at different scan rates from 5 to 100 mV s−1 as well as the potential window ranges from 0 to 1 V. The CV curves show the rectangular shapes in 1 mol L‒1 H2SO4 electrolyte, and their capacitances primarily come from electrical double layer capacitor (EDLC).41 Besides, the galvanostatic charge-discharge (GCD) is also employed to evaluate the electrochemical capacitances of materials. The GCD profiles of the C-blank sample at the current densities of 1 ~ 10 A g−1 shown in Figure S3b demonstrated shapes of almost isosceles triangle, implying the feature of electrical double layer capacitor. In addition, specific capacitances of the C-blank sample are displayed in Figure S3c. It is observable that the specific capacitances of the C-blank sample are 114 F g−1 at 1 A g−1 and still remains 75 F g−1 at current density of 10 A g−1. Besides, cycling stability regarded as a significant factor has also been employed to distinguish the practicability of supercapacitor, which was evaluated by the 5000 times of charge/discharge processes at 10 A g‒1 and the results are displayed in Figure S3d. The retention of specific capacitances of the C-blank sample can still remain

12


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94.9% after 5000 charging/discharging cycles, indicating its excellent cycling stability. Secondly, the HBA/DHBA/THBA as redox additives were added into 1 mol L‒1 H2SO4 electrolyte in order to promote the electrochemical performance of supercapacitor and the CV and GCD curves of HBA/DHBA/THBA with different concentrations are displayed in Figure S4-6. As shown in Figure S4a, c, e, CV curves of the C-HBA-10/15/20 samples were measured in three-electrode system at different scan rates from 5 to 100 mV s−1. It is obvious that the CV curves exhibit distorted rectangular shapes with the protruding broad in 1 mol L‒1 H2SO4 electrolyte and a pair of distinct peaks centers at 0.35 V and 0.50 V, respectively. In addition, GCD curves at the current densities of 1 ~ 10 A g−1 are exhibited in Figure S4b, d, f. It can be clearly observed that the discharge time of the supercapacitor based on the redox additive of HBA is longer with the increased concentration of HBA in H2SO4, that is to say, adding HBA with the concentration of 20 mmol L‒1 is the most appropriate condition to improve the specific capacitances comparing with those of 10 and 15 mmol L‒1. And the conclusion is also applied for DHBA and THBA which are shown in Figure S5-6. Besides, it demonstrates the obvious charge/discharge potential platforms, confirming the presence of Faradaic reactions derived from the gain or loss of electrons and protons. In addition, DHBA regarded as redox additive was added into 1 mol L‒1 H2SO4 electrolyte and the corresponding CV and GCD curves at different concentrations are demonstrated in Figure S5. As shown in Figure S5a, c, e, CV curves of the C-DHBA-10/15/20 samples were measured in three-electrode system at different scan rates from 5 to 100 mV s−1. Clearly, the C-DHBA-10/15/20 samples reveal two pairs of obvious and reversible redox peaks, which are attributed to the redox reactions occurred at the electrolyte/electrode interface. Besides, GCD curves at the current densities of 1 ~ 10 A g−1 are exhibited in Figure S5b, d, f. Two obvious charge/discharge potential platforms are observed, well according with the reversible redox peaks of CV curves. In addition, the THBA was also used as redox additive comparing with HBA and DHBA. In Figure S6a, c, e, CV curves of the C-THBA-10/15/20 samples were measured in three-electrode system at different scan rates from 5 to 100 mV s−1. It is obvious that the CV curves exhibit distorted rectangular shapes with the protruding

13



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broad in 1 mol L‒1 H2SO4 which is mainly due to Faradaic reactions and notably the CV curves only have an obvious oxidation peak but no reduction peak exists. The oxidation peak is attributed to the oxidation process of phenolic hydroxyl group, nevertheless, the reaction is irreversible.42 Meanwhile, GCD curves at the current densities of 1 ~ 10 A g−1 are demonstrated in Figure S6b, d, f. We can find out that there is an obvious charge potential platform while the no potential platform of discharge curve exists. Based on the information mentioned above, we can conclude that the redox additives

of

HBA/DHBA/THBA

possess

possibilities

of

promoting

the

electrochemical performances of supercapacitor. In order to discuss influence of redox additives which possess different hydroxyl quantities on the electrochemical properties, we carried out comparison of HBA/DHBA/THBA exhibited in Figure 5. In detail, Figure 5a presents the CV curves of the C-blank sample and C-HBA/DHBA/THBA-20 samples at the scan rate of 10 mV s−1 in the potential range of 0 ~ 1 V. Notably, the C-DHBA-20 sample possesses the largest integral area among these samples, indicating the largest specific capacitances of the C-DHBA-20 sample. As seem from the GCD curves at 2 A g ‒ 1 shown in Figure 5b, the C-DHBA-20 sample has the longest discharge time as compared to other samples, implying the prominent discharge ability of the sample for electrode materials, which is well according with the results of CV measurements. In addition, specific capacitances of the C-blank sample and C-HBA/DHBA/THBA-20 samples at different current densities are displayed in Figure 5c. As a result, the specific capacitances of all samples decrease with the increase of current density, which mainly result from the increasing limitation of ion diffusion.43 What’s more, it is clearly shown that the specific capacitances of the C-HBA-20 sample are much higher than those of other samples. Besides, the cycling stabilities of the C-blank sample and C-HBA/DHBA-20 samples were further measured by 5000 times of charging/discharging at the current density of 10 A g–1, and the corresponding results are displayed in Figure 5d. Observably, the C-blank sample and the C-HBA/DHBA-20 samples possess noticeable electrochemical stability with high

14


Page 15 of 33

capacitances retentions (94.9%, 85.2% and 89.9% of primary specific capacitances after 5000 cycles, respectively). Moreover, the specific capacitances of the C-HBA/DHBA-20 samples have been compared with the specific capacitances of some other redox additives, which are listed in Table 3. As shown in Table 3, introducing DHBA into H2SO4 can achieve large capacitances up to 337 F g–1, which are about 3.2 folds than that of H2SO4. Notably, the present data are better than most of other redox additives for supercapacitors. With respect to the electrochemical impedance spectroscopy (EIS) of the C-blank, C-HBA-20 and C-DHBA-20 samples, they were also employed to investigate the chemical and physical processes occurring in electrode and demonstrated in Figure S10.

10

(a)

1.0 Potential (vs. SCE) / V

-1

15

-1

10 mV s

5 0 -5

C-blank C-HBA-20 C-DHBA-20 C-THBA-20

-10

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

Specific capacitance / F g

320 280 240 200 160 120 80 40 0

capacit an

Cu 10 9 rr 8 7 en td 6 5 en 4 si t y/ 3 A - 2 g 1

0.6 0.4 0.2 0.0 0

ce / F g -1

(c)

C-blank C-HBA-20 C-DHBA-20 C-THBA-20

(b)

0.8

1.0

-1

0.0

Specific

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


C-DHBA-20 C-HBA-20 C-THBA-20 C-blank

180 160 140 120 100 80 60 40 20 0

100

200 300 Time / s

400

500

(d) 89.9%

94.9%

C-blank C-HBA-20 C-DHBA-20

0

15


86.2% -1

10 A g

1000 2000 3000 4000 5000 Cycle number


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 33

Figure 5. The C-blank, C-HBA/DHBA/THBA-20 samples measured in a three-electrode system: (a) CV at 10 mV s‒1; (b) GCD at 2 A g‒1; (c) specific capacitances calculated from GCD curves; (d) cycling stability of C-blank, C-HBA/DHBA-20 samples measured at 10 A g‒1. Table 3. Comparison of the increased specific capacitances by introducing redox additives. Redox additive

Cs (F g-1) After Pristine adding

Current density

Increase fold

Ref.

rutin

66

120

2 A g–1

1.8

20

pyrocatechol

66

368

2 A g–1

5.6

20

pyrocatechol violet

254

483

2 A g–1

1.9

44

Fe3+/Fe2+

379

1062

2 A g–1

2.8

45

HBA

106

166

2 A g–1

1.6

Present work

DHBA

106

337

2 A g–1

3.2

Present work

Moreover, we also study the electrochemical characteristics and charge-storage kinetics of the C-HBA-20 and C-DHBA-20 samples in a three-electrode cell configuration. According to the CV curves at various scan rates, we can infer the relationships between the peak current (ip) and the scan rate (v), (redox reactions belong to diffusion-controlled or surface controlled).46 Hence, Figure 6a and Figure 6c exhibit the CV curves of the C-HBA-20 and C-DHBA-20 samples at scan rates from 5 to 50 mV s‒1, respectively. And the corresponding relationships between the peak current (ip) and the scan rate (v) are shown in Figure 6b and Figure 6d, respectively. As demonstrated in Figure 6b, linear relationships are observed between the peak current (ip) and the scan rate (v1/2) with R2O1 = 0.9945 and R2R1 = 0.9955 for O1 and R1 peaks, respectively, indicating the diffusion-limited redox reaction of HBA.47, 48 On the other hand, Figure 6d shows the peak current ip vs. v1/2 plot, giving a linear relationship with R2O2 = 0.9977 and R2R2 = 0.9980 for anodic and cathodic

16


Page 17 of 33

peaks in Figure 6c, respectively. Likewise, it indicates that the redox reaction of DHBA is diffusion-limited.

-1

5 mV s -1 10 mV s -1 20 mV s

-1 -1

50 mV s

R1

0.0

25 20 15 10 5 0 -5 -10 -15 -20 -25

O1


(a)

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

1.0

10 8 6 4 2 0 -2 -4 -6 -8 -10

-1

5 mV s -1 10 mV s -1 20 mV s

(c)

O2 -1

10 8 6 4 2 0 -2 -4 -6 -8 -10



-1


-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


-1

50 mV s R2

0.0

0.2

0.4

0.6

0.8

1.0

25 20 15 10 5 0 -5 -10 -15 -20 -25

(b)

2

R O1=0.9945

anode

2

R R1=0.9955 2

cathode

3 4 5 6 -1 1/2 (Scan rate / mV s )

(d)

2

R O2=0.9977

anode

cathode 2

R R2=0.9980

2

Potential (vs. SCE) / V

3 4 5 6 -1 1/2 (Scan rate / mV s )

Figure 6. The C-HBA-20 and C-DHBA-20 samples measured in a three-electrode system: (a, c) CV curves at different scan rates; (b, d) variation of anodic and cathodic peak current with scan rate. As the detailed information of results mentioned above, we can find out that certain dosage of redox additives (HBA, DHBA and THBA) can produce Faradaic capacitances at electrode and electrolyte interface. In consideration of the fact that HBA, DHBA and THBA can release protons and electrons in the H2SO4 electrolyte, resulting in increased total capacitances. Moreover, the reversible reaction

7

17


7


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

mechanisms of HBA and DHBA are illustrated in Figure 7I. As seen from the constitutional formula of HBA in Figure 7I(1), it contains functional group of phenolic hydroxyl located at position a, which can produce reversible redox reaction. That is to say, the hydroxyl group located at position a can be oxidized into aromatic ketone group by loss of a proton and an electron in positive scan. On the contrary, the aromatic ketone group can be reduced into phenolic hydroxyl group after getting proton and electron in the reverse scan. In addition, the constitutional formula of DHBA is also shown in Figure 7I(2), which contains two hydroxyl groups located at position a & b, respectively. During the reversible redox reaction, the hydroxyl groups situated at position a &b can be oxidized into quinone by loss of two protons and two electrons and in contrast, the quinone groups can be reduced into phenolic hydroxyl groups after receiving two protons and two electrons.49 And additional capacitances are caused by redox reactions, which result in a rising total capacitances. As seen from the CV curves shown in Figure 7II, the C-HBA-20 sample exhibits a pair of distinct peaks which is related to the reversible redox reaction of HBA. Moreover, the C-DHBA-20 sample delivers two pairs of obvious redox peaks, which is according to the redox reaction of DHBA. In addition, on the basis of the integral area of the C-blank sample, the C-HBA-20 sample demonstrates an extra part which stands for the capacitances produced by the redox reaction of phenolic hydroxyl group located in position a. Besides, in terms of the integral area of the C-HBA-20 sample, the C-DHBA-20 sample exhibits an additional area which is corresponding to the capacitances produced by the redox reaction of phenolic hydroxyl group situated at position b. What’s more, as shown in Figure 7III, the discharge time of the HBA/DHBA-20 sample is longer than the C-blank sample suggesting the contribution for capacitances caused by redox reactions of phenolic hydroxyl groups located at position a and b, respectively. As shown in Figure 7IV, the specific capacitances of the C-blank, C-HBA/DHBA-20 samples are 106 F g−1, 166 F g−1 and 337 F g−1, respectively. According to the C-blank sample, the specific capacitances of the

18


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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-HBA-20 sample have an increase of 60 F g−1, which is related to the redox reaction of the phenolic hydroxyl group located in position a. Moreover, the specific capacitances of the C-DHBA-20 sample have a growth of 172 F g−1 compared with that of the C-HBA-20 sample, which is connected with the effects of phenolic hydroxyl group located in position b. However, it is noticeable that the additive of THBA is not the same with HBA and DHBA, for the irreversibility of the reaction. The electrochemical reaction mechanism of redox additive of THBA is displayed in Figure 7V. Each process gains/losses a proton and an electron, with no peak on the reverse scan which is well matched with the results of CV curves, indicating an irreversible process, which corresponds to the direct electrochemical oxidation of the THBA.42, 50 To further explore the contribution of phenolic hydroxyl groups to capacitances, we have also studied the 3-hydroxybenzoic acid (the C-mHBA-20 sample) in this work. As shown in Figure S7a, CV curve of the C-mHBA-20 sample was measured in three-electrode system at different scan rates from 5 to 100 mV s-1. It is obvious that the CV curves exhibit distorted rectangular shapes with the protruding broad as well as a pair of redox peaks. Besides, GCD curves at different current densities were also demonstrated in Figure S7b. The GCD curves exhibit obvious charge/discharge platforms, implying the redox reaction of the hydroxyl group connected with benzene ring. And it is well corresponding to the redox peaks of CV curves. In order to visually observe the contribution of phenolic hydroxyl groups to capacitance, the GCD curves and specific capacitances of C-mHBA/HBA/DHBA-20 samples were provided in Figure S7c and Figure S7d. As displayed in Figure S7d, the C-mHBA-20 sample delivers the maximal specific capacitances of 137 F g‒1 at the current density of 2 A g‒1 compared with those of the C-HBA/DHBA-20 samples (166 and 337 F g‒1, respectively). Noticeably, the specific capacitances of the C-DHBA-20 sample are larger than the total capacitances of the C-mHBA-20 and C-HBA-20 samples, which may be due to synergistic effects of the hydroxyl groups in position a & b.

19



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

Figure 7. (I) The electrochemical reaction mechanisms of redox additives of HBA/DHBA; the C-blank, C-HBA/DHBA-20 samples: (II) CV curves at 10 mV s‒1; (III) discharge curves at 2 A g‒1; (IV) specific capacitances calculated from discharge curves; (V) the electrochemical reaction mechanism of redox additive electrolyte of THBA. It is generally recognized that the two-electrode system plays an importantly practical role in estimating the electrochemical performance of supercapacitor. Hence, in order to further investigate the performance of supercapacitor using the HBA and DHBA as redox additives, the two-electrode configuration is applied for measurements and the corresponding results are provided in Figure 8. Figure 8a

20


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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


presents the CV curves of the C-blank, C-HBA-20 and C-DHBA-20 samples at the scan rate of 10 mV s–1 (the detailed results are exhibited in Figure S8). Clearly, the CV curve of the C-blank sample is the quasi-rectangular shape. Similarly, the absence of redox peaks derived from Faraday reaction also can be observed from the CV curves of C-HBA-20 and C-DHBA-20 samples. However, the integral area of the C-DHBA-20 sample is larger than those of the C-blank and C-HBA-20 samples, respectively. That is to say, the C-DHBA-20 sample carried out in 1 mol L‒1 H2SO4 electrolyte has higher capacitances compared with the C-blank and C-HBA-20 samples. Besides, GCD curves of the C-blank, C-HBA-20 and C-DHBA-20 samples are shown in Figure 8b. Obviously, the discharging time of the C-DHBA-20 sample is much longer than that of the C-blank and C-HBA-20 samples, which is corresponding to the results of CV curves. In addition, the specific capacitances of the C-blank, C-HBA-20 and C-DHBA-20 samples calculated from GCD curves are revealed in Figure 8c. Evidently, the C-DHBA-20 sample possesses the largest specific capacitances at the same current density and excellent rate capability among all samples. 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 under the current density of 10 A g‒1 and the results are displayed in Figure 8d. Apparently, the C-DHBA-20 sample displays the higher cycling stability (91% of primary specific capacitances after 5000 cycles) than that of the C-HBA-20 sample (87% of primary specific capacitances after 5000 cycles) (the CV curves of the samples before/after 5000 cycles at 20 mV s-1 are displayed in Figure S9). Hence, the C-DHBA-20 sample exhibits excellent electrochemical performances, which is vastly motivated to be employed in supercapacitor. However, there are obvious decreases of cycling stabilities for C-HBA-20 and C-DHBA-20 samples compared with the C-blank sample. The reduced specific capacitances are mainly due to the increasing aggregation of HBA and DHBA in pores of the electrode active material.51

21


(a)

-1

1.0

-1


-1

10 mV s


0.2

80

0.8

40 20 0

2

3

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

-1

1Ag

0.4 0.2 0.0 0

60

10


(b)

0.6

1.0

C-bank C-HBA-20 C-DHBA-20

Page 22 of 33

0.8

-1

(c)

100

0.4 0.6 Potential / V



4 3 2 1 0 -1 -2 -3

0.0


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


40

40

80 120 Time / s

(d)

160

91% 87%

30 20 C-blank C-HBA-20 C-DHBA-20

10 0

0

97%

1000 2000 3000 4000 5000 Cycle number

system: (a) CV curves at 10 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. In terms of the practical application, energy density and power density have been regarded as vital and significant elements of the supercapacitor, and the Ragone plots of the C-blank, C-HBA-20 and C-DHBA-20 samples have been displayed in Figure 9. As demonstrated in Figure9, the C-HBA/DHBA-20 samples possess larger energy density of 10.5/14.7 Wh kg ‒ 1 with the power density of 1.0 KW kg ‒ 1 compared with that of the C-blank sample (5.92 Wh kg‒1 at 1.0 KW kg‒1). And the

22


-1

10 A g

Figure 8. The C-blank, C-HBA/DHBA-20 samples measured in a two-electrode

200

Page 23 of 33

results are better than some other supercapacitor based on the mixed electrolyte in literatures.52-55

1h

10

6min

36s


-1

100 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


Ref 53

18s

3.6s 1.8s

Ref 54 Ref 52

Ref 55

0.36s

1

0.1

100

1k 10k -1 Power density / W kg

Figure 9. Ragone plots of specific energy versus specific power for the C-blank, C-HBA/DHBA-20 samples. Conclusion In this work, we displayed a simple and efficient approach for producing hierarchical carbon materials. Besides, HBA, DHBA and THBA were employed as effective redox additives to modulate the capacitive performance. Thus, considering the above experimental results, we can draw as following conclusions: 1) Template carbonization method is quite simple but effective for producing hierarchical carbon structures with high porosities, which are expected to deliver superior capacitive performance. Especially, the MgCl2·6H2O is commercially available and inexpensive, making it possible for large scale production of templated carbon materials. It thereby decomposes into MgO

23



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 24 of 33

substance as well as some gases at the elevated temperature, in which MgO substance can serve as hard template for formation mesopores, and the other gases as soft template for producing micropores. In a words, the porous inorganic salt of MgCl2·6H2O actually acts as multiple template, which is evidently favorable for the preparation of hierarchical pore structure of various kind of carbon materials. 2) Compared

with

supercapacitor

the

with

supercapacitor

with

H2SO4-HBA/DHBA

traditional electrolyte

electrolyte, possesses

the

higher

capacitances as well as energy density. The redox additives of HBA and DHBA are chosen mainly because of their functional hydroxyl groups. In terms of HBA and DHBA, the specific capacitances are proportional to the number of hydroxyl groups embedded in the benzene ring. Besides, the redox reactions of HBA and DHBA introduced into H2SO4 are reversible. 3) Compared with the redox reactions of HBA and DHBA, the redox reaction of THBA substance added into H2SO4 is irreversible. Moreover, each process gains and losses a proton and an electron leading to higher capacitances than traditional electrolyte and lower capacitances compared with those of HBA and DHBA, respectively.

In summary, the effects of the different quantities of phenolic hydroxyl groups have been explicated in details for modulating performance of supercapacitor, which could act as useful manual for the future research about phenolic hydroxyl groups in the application of supercapacitor. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21101052). Dr. Zhong Jie Zhang 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

24


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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


Province (KJ2016A039) and Startup Foundation for Doctors of Anhui University (J01003211). Supporting Information The methods for detailed structural characterization and electrochemical measurement, Schematic representation of the two-electrode system. The CV and GCD curves of the C-blank, C-HBA/DHBA-10/15/20 samples measured in three-electrode system. The CV and GCD curves of the C-mHBA-20 sample at different scan rates and current densities, respectively. GCD curves and specific capacitances of the C-mHBA/HBA/DHBA-20 samples. The CV and GCD curves of the C-blank, C-HBA/DHBA-20 samples measured in two-electrode system. The CV curves of the samples before/ after 5000 cycles at 20 mV s-1 in two-electrode system. Electrochemical impedance spectroscopy of C-blank, C-HBA-20 and C-DHBA-20 samples as well as the detailed analysis of impedance plots. The Supporting Information is available free of charge on the ACS Publications website.

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For Table of Contents Only Template Synthesis of Nitrogen-doped Carbon Nanosheets for High Performance Supercapacitors Improved by Redox Additives Wei Hu, Dong Xu, Xiao Na Sun, Zheng Hui Xiao, and Xiang Ying Chen*

The 2D carbon regarded as sustainable materials exhibit excellent electrochemical performance at the circumstance of introducing redox additive.

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Template Synthesis of Nitrogen-Doped Carbon Nanosheets for High

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