In-Situ-Activated N-Doped Mesoporous Carbon ... - ACS Publications

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In situ-activated N-doped mesoporous carbon from a protic salt and its performance in supercapacitors Tiago Correia Mendes, Changlong Xiao, Fengling Zhou, Haitao Li, Gregory P. Knowles, Matthias Hilder, Anthony Somers, Patrick C. Howlett, and Douglas R. MacFarlane ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11716 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 9, 2016

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ACS Applied Materials & Interfaces

In Situ-Activated N-Doped Mesoporous Carbon from a Protic Salt and its Performance in Supercapacitors Tiago C. Mendes,† Changlong Xiao,† Fengling Zhou,† Haitao Li,† Gregory P. Knowles,† Matthias Hilder,‡ Anthony Somers,‡ Patrick C. Howlett,‡ and Douglas R. MacFarlane, *,† †

School of Chemistry, Monash University, 3800 Melbourne, Victoria Australia

‡

ARC Centre of Excellence for Electromaterials Science (ACES), Institute for Frontier Materials

(IFM), Deakin University, Burwood, Victoria 3125, Australia

ACS Paragon Plus Environment

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ABSTRACT

Protic salts have been recently recognized to be an excellent carbon source to obtain highly ordered N-doped carbon without the need of tedious and time-consuming preparation steps that are usually involved in traditional polymer-based precursors. Herein, we report a direct copyrolysis of an easily synthesized protic salt (benzimidazole triflate) with calcium and sodium citrate at 850 °C to obtain N-doped mesoporous carbons from a single calcination procedure. It was found that sodium citrate plays a role in the final carbon porosity and acts as an in-situ activator. This results in a large surface area as high as 1738 m2/g with a homogeneous pore size distribution and a moderate nitrogen doping level of 3.1%. X-ray photoelectron spectroscopy (XPS) measurements revealed that graphitic and pyridinic groups are the main nitrogen species present in the material and their content depends on the amount of sodium citrate used during pyrolysis. Transmission electron microscopy (TEM) investigation showed that sodium citrate assists the formation of graphitic domains and many carbon nanosheets were observed. When applied as supercapacitor electrodes, a specific capacitance of 111 F/g in organic electrolyte was obtained and an excellent capacitance retention of 85.9% was observed at a current density of 10 A/g. At an operating voltage of 3.0 V, the device provided a maximum energy density of 35 Wh/Kg and a maximum power density of 12 kW/Kg.

Keywords: N-doped carbon, mesoporous carbon, protic salts, symmetric supercapacitors, one-step calcination, citrate salts.


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1. INTRODUCTION

Carbon materials have attracted massive attention in scientific literature recently due to their unique properties and wide application in energy storage devices. The combination of excellent chemical compatibility, good conductivity and low price of fabrication, certainly justifies the intensive application.1-3 The electrochemical double layer capacitor (EDLC or supercapacitor) is an electrochemical energy storage device that stores energy electrostatically through charge separation taking place at high surface area electrodes, thus allowing higher energy density than a traditional capacitor and higher power density than batteries, thanks to the fast adsorption/desorption energy storage mechanism. Notably, carbon-based electrodes with high surface area and fine-tuned structure are often present in such devices. Their long cycle life, the ability to operate in a wide temperature range and to deliver high power density, have made them a subject of intense investigation related to new synthesis approaches and their ultimate capacitive performance.4-6 Regarding the required porous structure of the carbon for supercapacitor application, mesoporous carbons show more attractive architecture when compared to activated microporous carbons; thanks to a large density of accessible active sites in mesoporous carbon, efficient diffusion of ions can take place due to the short pathway for ions moving through the pore structure, and enhanced mass transfer is usually obtained. To synthesize such mesoporous carbons, the template technique has proved to be a reliable method to obtain the desired structure, including high surface area, adequate pore size distribution and large pore volume.7-8 Currently, either soft or hard templates are widely used to create the structure; the most widely known are surfactants, block-copolymers, 3D silica, metal oxides and CaCO3.9-12


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Recently, it has been reported that the presence of heteroatoms such as nitrogen, boron, phosphorous and sulfur in the carbon framework can significantly improve the electrochemical properties of energy storage devices. In particular, the presence of nitrogen atoms in hexagonal carbon rings improves the conductivity, basicity and oxidation stability of the carbons through conjugation between the nitrogen lone pair and the π system of the carbon lattice.13 Owing to the aforementioned benefits of doped carbons, nitrogen-doped mesoporous carbons (NDMCs) are being applied in different fields of research, including fuel cells, catalysis, adsorption and Li-ion batteries. Moreover, NDMCs are excellent materials for supercapacitors since the presence of nitrogen atoms introduces pseudocapacitance and enhances the surface wettability of the carbon, while the mesoporosity helps to maintain high rate capability.14-16 This wettability improvement arises from the increased polarity of the carbon surface due to different nitrogen functionalities within the carbon framework.17-19 Generally, nitrogen doping is carried out either by in-situ pyrolysis of N-containing compounds or post-treatment of carbonaceous materials by using a flow of ammonia gas under heating or by mixing and heating carbon with melamine/urea. In situ pyrolysis allows a uniform distribution of nitrogen throughout the carbon structure, while post-treatment may result in formation of some morphological defects and undesirable pore blocking.20-21 To introduce mesopores, a suitable template is usually pyrolyzed together with a carbon source and the desired porosity is obtained after removal of the sacrificial template by using caustic or acidic media.22-24 Unfortunately, NDMCs obtained by hard templates involve tedious steps such as carbon precursor syntheses, template preparation, infiltration of the template with the precursor, cross-linking reactions via calcination of the precursor and, finally, the removal of template. On the other hand, soft templates are more facile, because the organic sacrificial agent that acts as a template is removed during the


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carbonization process.25-26 However, soft templates suffer from issues such as slow polymerization, complex evaporation-induced self-assembly and use of toxic formaldehyde. Therefore, a method that involves direct pyrolysis of an easily synthesizable nitrogen-containing carbon source and does not require impregnation of a template is urgently required to minimize the high costs involved in mesoporous carbon syntheses. Most nitrogen-rich compounds are thermally unstable during the calcination step and may undergo total mass loss under the harsh carbonization conditions that are usually employed. Moreover, it is well known that final carbon properties such as nitrogen content, graphitic structure, surface area and conductivity strongly depend on the nature of the carbon source.27-28 Currently, the most popular precursors are polymers such as polypyrrole, polyaniline and polyacrilonitrile, but issues like poor solubility, complicated synthesis and metal contaminant from the catalyst have made the use of such polymers difficult for these purposes.29-31 Antonietti et al. have shown that N-doped carbons can be obtained from a direct pyrolysis of the aprotic ionic liquid 1-ethyl-3-methylimidazolium dicyanamide [EMIM][DCA]; this method resulted in a high nitrogen doping level of 11.4% and produced a reliable electroactive material for the oxygen reduction reaction.32 Despite the easy synthesis of N-doped carbon from [EMIM][DCA], ionic liquids based on such anions are relatively expensive and difficult to synthesize; this limits the use of this material as a source for large-scale, practical applications. Recently, Watanabe et al. have demonstrated that protic ionic liquids/salts (PILs/PSs), synthesized via a straightforward neutralization between amines and strong acids, resulted in carbons with high nitrogen content (up to 11%) after pyrolysis, and the final structure and physicochemical properties were strongly correlated to the starting amine structures, acids and calcination temperature.33 It was also found that some carbons obtained from PILs/PSs showed high degree of graphitization,


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high surface area and excellent conductivity (even higher than graphite). This approach opened up a new window in the N-doped carbon synthesis because of the immense diversity of amines readily available. In this way, the final carbon properties become designable by judicious choice of raw materials and operational parameters. Further studies with the novel carbon source pphenylenediamine bisulfate [pPDA][2HSO4], produced mesoporous carbon by a facile, direct synthesis by pyrolyzing the carbon source at 600-1000 ⁰C; however, since the final porosity was not high (487 m2/g at 700 ⁰C), an additional chemical activation step (calcination) was necessary to enhance the porosity for its final application.34 General, protic salt-derived carbons show either microporous structure or relatively low porosity after a single calcination step. It seems that a template technique involving a one-step calcination is necessary to overcome such issues regarding adequate porosity, pore size distribution and cost-effective synthesis. Seeking a low cost template and facile synthesis of high-porosity mesoporous carbons, Inagaki et al. first investigated the use of magnesium citrate, which acted as both carbon source and hard template.35 Formation of magnesium oxide nanoparticles, uniformly distributed over bulk carbon, resulted in a composite of carbon/MgO after pyrolysis at 900°C. After removing the oxide particles, a mesoporous carbon with an average pore size of 5.0 nm was obtained. It was reported elsewhere that calcium citrate also forms metal oxide/carbonate during calcination, leaving mesopores in the carbon structure after template removal, producing a surface area as high as 1446 m2/g, a total pore volume of 2.3 cm3/g and an average pore size of 6.7 nm.36. In a recent report, Yu et al. performed a co-pyrolysis of magnesium citrate with potassium citrate followed by NH3 treatment; as a result, hierarchical N-doped mesoporous carbon nanosheets were formed due to the ability of potassium ions to react with the carbon framework at high temperatures.37 Such structures allowed the authors to obtain superior performance in symmetric supercapacitors (a


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capacitance of 128 F/g and maximum energy density of 32.4 Wh/kg). However, achieving nitrogen-doped carbon was only possible after a second calcination step followed by heat treatment under ammonia. In this paper, inspired by the versatility offered by PILs/PSs and the facile application of citrate salts to obtain mesoporosity, we directly synthesized N-doped mesoporous carbon by using the protic salt benzimidazolium triflate ([BIm][TfO]) as carbon source and calcium citrate (CaCit) to give the hard template. In addition, sodium citrate (NaCit) was added to the above system to enhance the final surface area of the carbons. The specific surface area strongly depended on the amount of NaCit, which acts as “in situ” activator, and enhances the final capacitance. Full devices (symmetric-coin cells) using carbons obtained from this method were tested in organic as well as neat ionic liquid electrolytes and good rate capability was obtained. The striking feature of our method is that only a single calcination is necessary to obtain multi-atom doped, mesoporous carbons; no further activation process or heating is required, ensuring a low cost technique, tuneable porosity and excellent capacitive performance.

2. EXPERIMENTAL SECTION

2.1 Carbon materials preparation and physical characterization. Benzimidazole (C7H6N2, 98%), ethanol (C2H5OH), calcium citrate tetrahydrate ([O2CCH2C(OH)(CO2)CH2CO2]2Ca3.4H2O, 99%), sodium citrate dihydrate [(HOC(COONa)(CH2COONa)2.2H2O, 99%] and triflic acid (CF3SO3H, 99%) were obtained from Sigma Aldrich Company. All chemicals were analytical grade and were used as-received without further purification. Ultra-pure water (Millipore, 18 MΩ cm) was used throughout the study.


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First, the protic salt [BIm][TfO] was synthesized by a proton transfer reaction (see supporting information). Afterwards, the final carbon source with template was prepared by grinding the asobtained [BIm][TfO] with calcium citrate in a mass ratio of 1:1 (a ratio of 1:2 was also attempted, but yielded poorer porosity). Sodium citrate at different ratios was added to the above mixture in a mortar and ground until a homogeneous mixture was obtained. Table 1 lists the compositions of the four samples described in detail here. For simplification, the samples were named NDMC-X, where X is 10 times the ratio of NaCit/[BIm][TfO]. Carbon materials were prepared by heating the carbon source and citrate salts in a crucible (alumina) under N2 atmosphere using a tube furnace; the nitrogen flow was 100 mL/min. The heating rate was kept at 5°C/min and the final temperature was 850°C; this temperature was chosen as a compromise between nitrogen retention (which requires lower temperatures) and generation of porosity (which requires higher temperatures). After the furnace reached the final temperature, it was held for an additional 2 h and then was cooled down naturally. The resulting composites (carbon/Ca-Na oxides) were ground and added to hydrochloric acid solution (1 mol/L) under agitation for 12 h. After dissolution of the oxides, the carbon was filtered and washed several times with pure water until a neutral liquid was achieved. The as-obtained N-doped mesoporous carbons (NDMC-X) were collected and dried at 110°C for 24 h.


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Table 1. Composition of samples using different amounts of sodium citratea.

a

sample

[BIm][TfO] (g)

CaCit (g)

NaCit (g)

NaCit/[BIm][TfO]

NDMC-0

2.0

2.0

-

-

NDMC-4

2.0

2.0

0.8

0.4

NDMC-8

2.0

2.0

1.6

0.8

NDMC-10

2.0

2.0

2.0

1.0

higher concentration of NaCit led to corrosion of the crucible, and were not pursued further.

Field emission Scanning electron microscopy (FEG-SEM) was performed using a JEOL JSM 7001-F microscope. Transmission electron microscopy (TEM) was run using FEI Tecnai G2 T20 with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out using an Axis ultra spectrometer. N2 adsorption/desorption was measured by a Tristar II volumetric adsorption analyzer at 77 K (Micromeritics). Before the measurements, the samples were degassed under vacuum (p < 10−5mbar) at 373 K for 5 hours. The Brunauer– Emmett–Teller (BET) equation was used to calculate the specific surface area from adsorption data. The total volume of pores was calculated from the amount of nitrogen adsorbed at P/P0= 0.99. The pore size distributions were calculated by analyzing the adsorption branch of the N2 from sorption isotherm using the Barret–Joyner–Halenda (BJH) procedure. The micropore specific surface area (Smicro) and pore volume (Vmicro) were calculated via the t-plot method. Raman spectra were obtained by using Renishaw Raman spectrometer (514 nm) and XRD measurements were carried out on a Bruker D8 diffractometer with monochromatic Cu Kα radiation (λ = 0.1541 nm).

2.2 Electrochemical measurements. Electrodes were fabricated from the different mesoporous carbons using the slurry coating method and applied onto aluminum foil current collectors (refer


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to the supporting information). In a glovebox under argon atmosphere, two equivalent electrodes were separated by a glass fiber separator (Whatman GF/F soaked in either 1 M TEABF4/AN or EMIMBF4) and assembled in a coin cell (CR-2032 type, Japan). Electrochemical characterization was performed on a multi-channel potentiostat (VMP-2 Princeton applied research) using a twoelectrode setup. Gravimetric (or specific) capacitances (C, F/g) were calculated using equation (1) for cyclic voltammetry data and equation (2) for the galvanostatic charge-discharge method:

1

𝐶𝐶(𝐹𝐹/𝑔𝑔) = 𝑚𝑚𝑚𝑚(𝑉𝑉

𝑏𝑏 −𝑉𝑉𝑎𝑎 )

2𝐼𝐼𝐼𝐼𝐼𝐼

𝐶𝐶𝑠𝑠𝑠𝑠 (𝐹𝐹/𝑔𝑔) = 𝑚𝑚𝑚𝑚𝑚𝑚

𝑉𝑉

𝑏𝑏 ∫𝑉𝑉 𝐼𝐼𝐼𝐼𝐼𝐼

(1)

𝑎𝑎

(2)

where I is the discharge current (A), Δt is the discharge time (s), m is the active carbon mass in a single electrode (g), ν is the scan rate (V/s) and ΔV is the potential difference with time. To construct the Ragone plot, equations (3) and (4) were used to calculate both specific energy density (Wh/Kg) and power density (W/Kg), respectively:

1

1000

𝐸𝐸 = 8 𝐶𝐶𝑠𝑠𝑠𝑠 (𝛥𝛥𝛥𝛥)2 . 3600 𝐸𝐸

𝑃𝑃 = 𝛥𝛥𝛥𝛥 . 3600 𝑑𝑑

(3) (4)

where 𝐶𝐶𝑠𝑠𝑠𝑠 is the specific capacitance (F/g), ΔV is the operating potential (Vmax – IR drop) and Δtd is the discharge time (s).


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3. RESULTS AND DISCUSSION

Unlike most amines, which undergo thermal degradation above 400 °C, protic salts obtained by neutralization with strong acids (e.g. sulfuric and triflic acid) are able to resist harsh conditions; if the amine used possesses stable aromatic groups, bonds such as C=C can cross-link and polymerize at high temperatures.28 When [BIm][TfO] is pyrolyzed at 800-1000°C, a cross-linking reaction and polymerization take place and a graphite-like powder is obtained. This method allows use of numerous combination of amines and acids that can result in different material structures. Herein, we used benzimidazole due to its benzyl group attached to the imidazolium moiety, which in turn results in both high yield and nitrogen content, as reported elsewhere.33 When [BIm][TfO] and CaCit are co-pyrolyzed at 850 °C, besides polymerization, CaO particles are formed and act as a hard template, leaving a mesoporous carbon structure after oxide removal. Subsequently, we decided to introduce NaCit to tailor the carbon surface area; as well as changes in porosity, we found intrinsic structure alterations related to the nitrogen-bonding environment in the carbon framework and the formation of graphitic domains.


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3.1 Porosity properties of N-doped carbons. To obtain information about the porosity features of samples produced in this work, N2 sorption isotherms ranging from relative pressures (P/P0) of 0 - 1.0 were carried out to study the effect of NaCit on specific surface area of the carbons and their pore size distribution; this comparison can be seen in Figure 1. For NDMC-8 and NDMC-10 samples, an increased adsorbed volume is observed at P/P0 ~ 0; such a response indicates that some micropores are present, whilst the adsorption/desorption hysteresis from P/P0 ~ 0.4-0.8 is assigned to capillary condensation due to mesopores. The fact that N2 uptake can be seen at P/P0 ~ 1 indicates that some macropores are also present. All samples yielded curves comprising IUPAC type II and type IV isotherms, which characterizes such materials as mesoporous carbons.27 Furthermore, the decreased adsorbed volume at P/P0 ~ 1 in Figure 1a directly indicates that the more NaCit is added, the higher is the amount of micro/mesopores produced. Such behavior leads us to state that NaCit is increasing the surface area and decreasing the average pore size with the formation of new micro/mesopores (see table 2); this is plausible since NaCit acts as an etching/activator agent and releases large amounts of CO which helps to create new

a)

b)

Figure 1. (a) N2 sorption isotherms of NDMCs and (b) pore size distribution of NDMCs.


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micro/mesopores. The most likely reaction for generation of new micropores by etching the carbon is Na2O + C→ 2Na + CO.38-39

Table 2. Specific surface area, pore volume and average pore sizes of samples. Stotal

Vtotal

(m2/g)

(cm3/g) (m2/g) (m2/g) (cm3/g) (cm3/g) (nm)

NMDC-0

393

1.33

176

217

0.05

1.28

13.5

NMDC-4

700

1.41

246

454

0.06

1.35

8.6

NDMC-8

1200

1.90

554

646

0.08

1.82

6.1

NMDC-10 1738

1.75

730

1008

0.15

1.60

4.9

sample

Smicro

Smeso

Vmicro

Vmeso

Dp

Figure 1b displays the pore size distribution of the samples. Remarkably, a narrow pore distribution was achieved for NDMC-10 and the average pore size was calculated to be 4.9 nm, according to the BJH method. Such high specific surface area combined with the presence of micro/mesopores may create both high energy density sites and a fast ion diffusion through the pores.

3.2 Chemical structures of the carbons. To probe the chemistry of NDMCs and their nitrogen chemical states, XPS measurements were carried out, and the results for NDMC-10 and NDMC0 are shown in Figure 2. For all other samples, a summarized result is provided in table 3 (see figure S1 for XPS spectra of samples).


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In Figure 2a the XPS survey is shown for binding energies from 0 - 900 eV. The spectrum shows peaks assigned to C 1s, N 1s, O 1s, S 2p and F 1s, revealing moderate N-doping level of 3.1 at%. It is noteworthy that the analysis reveals no metal contaminants, which means the template was successfully removed during acidic washing. The appearance of sulfur and fluorine, even at low amounts (table 3) means that samples NDMC-8 and NDMC-10 should be considered tri-doped carbons (N/S/F tri-doped carbon). Interestingly, fluorine is inserted as a foreign heteroatom in the

a)

b)

c)

d)

Figure 2. (a) XPS spectra of NDMC-10: XPS survey; (b) N-high resolution scan; (c) C-high resolution scan and (d) N-high resolution scan for NDMC-0.


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Table 3. Chemical composition of samples obtained by XPS analysis. Sample

C (%)

O (%)

N (%)

S (%)

F (%)

Graphitic N (%)

Pyridinic N (%)

ID/IG

NDMC-0

88.3

8.67

2.18

0.84

-

68.5

31.5

1.00

NDMC-4

88.3

7.56

2.87

1.26

-

78.6

21.4

0.95

NDMC-8

89.4

7.16

2.25

0.85

0.29

80.1

19.9

0.97

NDMC-10

90.6

5.25

3.11

0.73

0.35

78.9

21.1

0.97

structure if higher amount of sodium citrate is used (>28.5%, NDMC-8). Sulfur is favored at lower concentrations. Possible electrochemical effects of these dopants are currently being investigated as part of a study of the feasibility of these materials in battery electrodes. In Figure 2b, the high resolution XPS spectra of N 1s was resolved into two strong peaks at 401.2 eV and 398.5 eV, corresponding to graphitic-quaternary nitrogen and pyridinic nitrogen configurations, respectively.40 Higher amounts of graphitic-N states than pyridinic-N were observed for all samples, but when calcination was performed with no NaCit (NDMC-0, Figure 2d), pyridinic-N reached a total composition of 31.5% (see table 3). It is clear that NaCit is not only acting as an activator, but structure modification is also gradually happening due to its ability to allow different elements to be accommodated into the carbon structure. As an example, the nitrogen content increases with NaCit addition (from 2.18% to 3.11%). Further, upon increasing the ratio of NaCit/[BIm][TfO], from NDMC-0 to NDMC-10, the carbon composition increased from 88.3% to 90.6% whilst oxygen decreased from 8.67% to 5.25%, respectively. The sodium citrate stimulated the formation of graphitic domains. This was further evidenced by Raman spectroscopy and TEM microscopy


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(refer to Figure 3). The deconvolution of the C 1s spectrum in Figure 2c exhibited peaks at 284.8 eV, 285.9 eV and 289.4 eV. The sharp peak at 284.8 eV corresponds to graphitic carbon (sp2hybridized) whilst peaks at 285.9 eV and 289.4 eV are indexed to C=N/ C-S and C-C=O/C=O.33

3.3 Morphological and structural properties. In Figure 3, the typical morphology and structure of NDMC-0 and NDMC-10 is shown by means of SEM and TEM. A well-developed porous structure can be seen for NDMC-0 (Figure 3a) and the sample shows a layered structure. When NaCit is used to etch carbon, a highly porous material is achieved (as confirmed by N2 adsorption) and the morphology changes to show a coral-like structure in case of NDMC-10 (Figure 3b). This interconnected macro/mesopores permit an easy insertion of ions from organic electrolytes to enter the bulk carbon, and the short channels created over the entire structure of NDMC-10 is b)

a)

d )

e)

c)

f)

Figure 3. SEM images of NDMC-0 (a) and NDMC-10 (b), NDMC-10 TEM images (c) and HRTEM (d), Raman spectra of NDMCs and XRD spectra of NDMC-0 and NDMC-10 (f).


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considered to facilitate the diffusion mass-transfer. TEM micrographs of NDMC-10 interestingly showed the presence of porous carbon nanosheets on the edge of particles (Figure 3c and Figure S2), suggesting that NaCit treatment increases the graphitic domains and creates a large amount of uniform porous sheets. High resolution (HR-TEM) of NDMC-10 sample was also displayed in Figure 3d; as we can see, NDMC-10 is composed of a disordered and rough structure, showing that carbons obtained in this work have an amorphous nature. Raman spectroscopy is a powerful method to elucidate the degree of graphitization in carbon materials. It is expected that the incorporation of heteroatoms (N, S, F) into carbon structure causes a distortion in carbon hexagonal lattice, creating structural defects and resulting in a disordered carbon with an increased D-band intensity.41 In Figure 3e, two peaks related to D and G bands were observed at 1342 cm-1 and 1590 cm-1, respectively. The G-band is ascribed to the in-plane vibration of sp2-hybridized graphitic carbon atoms, whereas the D-band is ascribed to out of plane vibration due to some disorder in structure. The intensity ratio of D-band to G-bands (ID/IG) has been universally used as an approach to estimate the graphitic degree of carbons. The addition of NaCit (Figure 2d) sharpened the G-band at 1590 cm-1. Moreover, carbon samples obtained with NaCit showed ID/IG ratios equal to 0.95, 0.97 and 0.97 for NDMC-4, NDMC-8 and NDMC-10, respectively, whereas NDMC-0 was 1.00 (table 3). This band sharpening and the slightly decreased ID/IG ratio are showing that NaCit is allowing the formation of graphitic domains, whilst NDMC-0 broad peaks are related to a more amorphous structure. Considering the Raman results of our samples, a partially graphitized-amorphous carbon could be obtained by adding NaCit, reinforcing the role of this salt in final carbon structures. Figure 3f compares the XRD patterns of sample NDMC-0 and NDMC-10, both samples showed two major peaks situated at 2θ =23.3° and


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43.7°, corresponding to graphite (002) and (101) plane, the exhibited broad peaks demonstrates the amorphous characteristics of the samples.42

3.4 Electrochemical measurements. To investigate the capacitive potential of NDMCs, electrochemical techniques including cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) measurements were applied. Initial

a) a)

b)

c)

d)

Figure 4. Cyclic voltammetry of different NDMCs at 50 mV/s (a) Comparison of electrolyte stabilities at different potentials with NDMC-10 (b) NDMC-10 charge-discharge curves using 1M TEABF4/AN at different current densities (c) and capacitance as a function of current density using NDMC-10 with 1M TEABF4/AN and EMIMBF4. Inset: cyclic stability over 5,000 cycles (d).


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electrochemical characterization of the NDMC-10 electrodes involved cyclic voltammetry using a 3-electrode setup to assess the performance of the single electrode in both organic and ionic liquid electrolytes; the results are provided in Figure S4. A typical rectangular shape was obtained from the CV measurements, indicating simple capacitive behavior with no significant indication of faradaic processes. We then focused here on evaluating the performance of our NDMCs in a symmetrical coin cell configuration that provides a realistic estimate of the packed configuration relevant to commercial electrochemical double layer capacitors, following the best practice methods for performance evaluation of supercapacitors.43-44 To study the difference in electrochemical response of the carbons, cyclic voltammograms involving each cell are compared in Figure 4a. A quasi-ideal rectangular shape was observed for all samples, indicating good electrochemical double layer capacitive behavior. Additionally, the contribution to the capacitance as we add more NaCit during pyrolysis is quite clear. Initially, NDMC-0 shows a very low capacitance of 16 F/g, followed by NDMC-4 and NDMC-8 with a specific capacitance of 44 F/g and 62 F/g, respectively. However, when the NaCit/[BIm][TfO] ratio is equal to 1 (NDMC-10), a specific capacitance as high as 117 F/g was achieved. This capacitance improvement can be ascribed to both the high surface area (1738 m2/g) and the enhanced porous structure, where the micropores help to provide access to a large internal surface area, whilst homogenous mesopores around 4.9 nm and some random macropores provide an efficient ion diffusion pathway. Besides, when NaCit is absent, graphitic domains are considered to be low whilst NDMC-10 showed formation of carbon nanosheets. In addition, sodium citrate increases the nitrogen content in the carbon structure, resulting in an improved wettability. Since the energy of a capacitor depends on the square of voltage (eq. 3), if the cell can work at enlarged potential limits, a significant improvement in energy and power density will be obtained.


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As an attempt to increase the energy density of our supercapacitor, the operating voltage using the non polar solvent electrolyte 1M TEABF4/AN was investigated at 3.0V. In parallel, [EMIM][BF4] ionic liquid was used to evaluate the response of more concentrated electrolytes; these results are exhibited in Figure. 4(b,d). Figure 4b shows that at 3.0V the cell was stable and no current increase that could be assigned to electrochemical decomposition was observed. Moreover, the rectangular shape of the CV curve was maintained, highlighting its good efficiency and the capacitive behavior close to ideal. The specific capacitance of NDMC-10 in organic electrolyte at 3.0V was calculated to be 119 F/g; this value is 1.9% higher than the capacitance obtained at 2.5V, suggesting that the extended operating voltage did not diminish the cell performance. A cell operating at 3.0V with [EMIM][BF4] showed similar capacitance (103 F/g); such close results prove that NDMC-10 offers good compatibility with more viscous hydrophilic electrolytes, suggesting that different ionic liquids can probably be used to exploit an even higher operating voltage than 3V. Galvanostatic charge-discharge cycling was also used to validate the capacitance values obtained by CV and to analyze the rate capability by increasing the current density from 0.5 A/g to 10 A/g (Figure 4c,d and S3). Only a slight deviation from the ideal isosceles triangle shape (pure double layer behavior) was achieved at 0.5 and 1.0 A/g and a symmetric shape was observed at all current densities. The capacitance obtained at 0.5 A/g was 111 F/g whilst at 10 A/g the specific capacitance was 96 F/g, indicating good capacitance retention of 86%. These results are in good agreement with the capacitance values obtained from the CV plots in Figure 4b. Furthermore, at 20 A/g, the cell is still able to show a substantial specific capacitance of 78 F/g; the ability to maintain a good performance even at very high current densities is due to the combination of micro/meso/macropores in NDMCs, as hypothesized.


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To investigate the long-term cyclic stability of the devices, NDMC-10 cells with both electrolytes were charged and discharged at 10 A/g for 5,000 cycles. As we can see as inset in Figure 4d, the stability for NDMC-10 in TEABF4/AN after long cycling resulted in a capacitance retention of 92% whereas for EMIMBF4 a capacitance retention of 84% was observed. These results mean that TEABF4/AN is more stable with extended cycling than EMIMBF4. Nonetheless, both electrolytes are considered suitable and the full cells have a long cycle life, even at high operating current densities. Figure 5a compares the Nyquist plot of NMDC-10 in both organic and ionic liquid electrolytes measured at open circuit potential of 10 mV amplitude with a frequency range of 100,000-0.010 Hz. The spectra show the typical behavior of an electric double layer capacitor with a semi-circle at high-mid frequency and a 45° Warburg region followed by a nearly vertical straight line at midlow frequency range, exhibiting a very good capacitive behavior.45 The Warburg region is ascribed to the resistance of ions diffusing through the pores of NDMCs. Notably, according to the inset in Figure 4a, the 45° segment is longer for the EMIMBF4 electrolyte, suggesting a higher diffusion resistance. This is attributed to the bigger EMIM cation ions in comparison to TEA that may be suffering from the so-called “ion sieving effect” when diffusing through small micropores.46-47 Considering that micropores are present to some extent in NDMCs, an increased Warburg impedance is expected for EMIMBF4. Such a response is in good agreement with the CV curves


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a)

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b)

Figure 5. (a) EIS spectra at OCP of NDMC-10 with both organic and ionic liquid electrolytes and (b) Ragone plots comparing NDMC-10 in 1M TEABF4/AN and other works in literature.

when both electrolytes are compared (Figure 4b). Besides, EMIMBF4 showed higher equivalent series resistance (ESR), which can be estimated by the interception of curve to the real axis at high frequency region; both electrolytes showed ESR on the range of 0.9-1.9Ω but EMIMBF4, due to its lower conductivity, showed a small increase in ESR. The charge transfer resistance (semi-circle width) follows the same trend and exhibits a slightly higher value for EMIMBF4. These results obtained from EIS support the evaluation in terms of performance obtained from the CV and charge-discharge techniques where the 1M TEABF4/AN electrolyte showed better response under different conditions. In Figure 5b we also display the relationship between energy density and power density of cells on the well-known Ragone plot. According to the plot, a very similar response was obtained for both electrolytes, but higher energy density was achieved with 1M TEABF4/AN due to its higher capacitance, especially at intermediate current densities. Therefore, an energy density of 35 Wh/kg at a power density of 411 W/kg was reached with 1M TEABF4/AN. At power density as high as 12 kW/kg, a substantial energy density of 21 Wh/kg was delivered. The high performance of


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NDMC-10 outperforms many porous carbon materials reported in recent literature as shown in Figure 5b. Graphene-based carbons are not included in Fig 5b as typically graphene requires many steps to prepare and sometimes activation with KOH which further increases the cost; on the other hand, the key point of this work is high performance from a one-step calcination without need for further activation processes.

CONCLUSION

In summary, we have synthesized N-doped mesoporous carbon (NDMCs) from a novel one-step calcination procedure by co-pyrolyzing the protic salt Benzimidazolium triflate ([BIm][TfO]) with calcium and sodium citrate at different mass ratios. It was demonstrated that sodium citrate is responsible to achieve a high surface area due to its ability to etch the material during calcination and final structures are strongly related to its amount added to the mixture. When the ternary mixture is pyrolyzed with a mass ratio of 1:1:1, a highly mesoporous material is obtained, showing a capacitance of 111 F/g in organic electrolyte with an excellent capacitance retention of 86% at a current density of 10 A/g. Even after 5,000 cycles at 10 A/g, the device retained 92% of its initial capacitance. The superior performance achieved in this work is attributed to the homogenous mesopores and the formation of carbon nanosheets, with graphitic and pyridinic-nitrogens successfully provided by the protic salt, which in turn is easily synthesized through a simple neutralization procedure. Therefore, considering that nitrogen doping and mesoporous structure are obtained by a single calcination of easily obtained materials, NDMCs produced in this work are considered excellent materials for future application in supercapacitors and other electrochemical devices.


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ASSOCIATED CONTENT Supporting information Synthesis procedure of the protic salt [BIm][TfO]. XPS high resolution scans for different NDMC’s, HR-TEM micrographs of NDMC-10, charge-discharge rates of NDMC-10 using the ionic liquid EMIMBF4 and capacitance measurements using the 3-electrode setup. “This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author (D. R. M) Email: [email protected]. Telephone: +61 3 9905 4540

ACKNOWLEDGMENTS The authors acknowledge the Brazilian Science without borders program and the National Council for Scientific and Technological Development (no. 206866/2014-3). DRM is grateful to the Australian Research Council for support under the Laureate Fellowship Scheme.


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TABLE OF CONTENTS

“Mesoporous N-doped carbons prepared from simple protic salts show high performance in supercapacitors with both organic and ionic liquid electrolytes.”


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In-Situ-Activated N-Doped Mesoporous Carbon ... - ACS Publications

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