Nitrogen-doped mesoporous graphene nanoflakes for high performance ionic liquid supercapacitors

Journal Pre-proof Nitrogen-doped mesoporous graphene nanoflakes for high performance ionic liquid supercapacitors Ekaterina A. Arkhipova, Anton S. Ivanov, Konstantin I. Maslakov, Serguei V. Savilov PII:

S0013-4686(20)30856-2

DOI:

https://doi.org/10.1016/j.electacta.2020.136463

Reference:

EA 136463

To appear in:

Electrochimica Acta

Received Date: 9 January 2020 Revised Date:

4 May 2020

Accepted Date: 4 May 2020

Please cite this article as: E.A. Arkhipova, A.S. Ivanov, K.I. Maslakov, S.V. Savilov, Nitrogen-doped mesoporous graphene nanoflakes for high performance ionic liquid supercapacitors, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2020.136463. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.

Author Statement Ekaterina A. Arkhipova: Conceptualization, Methodology, Formal analysis, Investigation, Writing - Original Draft, Writing - Review & Editing. Anton S. Ivanov: Conceptualization, Resources. Konstantin I. Maslakov: Formal analysis, Investigation, Writing - Review & Editing. Serguei V. Savilov: Project administration, Funding acquisition.

Nitrogen-doped mesoporous graphene nanoflakes for high performance ionic liquid supercapacitors Ekaterina A. Arkhipova*a, Anton S. Ivanova, Konstantin I. Maslakova and Serguei V. Savilova,b a

Department of Chemistry, Lomonosov Moscow State University, 1–3 Leninskiye Gory, Moscow, 119991, Russia

b

Department of Physical Chemistry, Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, 31 Leninsky Prospect, Moscow, 119991, Russia *

e-mail: [email protected]

Abstract Mesoporous nitrogen-doped graphene nanoflakes (N-GNFs) with high nitrogen doping level of 5.0 – 10.7 at.% were produced by a facile template CVD synthesis using the mixture of acetonitrile and benzene as a precursor. The pore structure, morphology and physicochemical properties of N-GNFs were characterized by TEM, XPS, Raman spectroscopy, and lowtemperature nitrogen physisorption. The electrochemical performance of N-GNFs was tested in electric double-layer capacitor (EDLC) with ionic liquid electrolyte (1.2 M N+Et4TFSI− solution in CH3CN) by cyclic voltammetry, galvanostatic charge-discharge measurements, and electrochemical impedance spectroscopy. With increasing the nitrogen content in N-GNFs their specific capacitance increased and achieved 167 F g−1 at a scan rate of 5 mV s−1. The maximum delivered energy density was 46.3 Wh kg−1, which corresponded to a power density of 0.74 kW kg−1. The high electrochemical performance of N-GNFs was attributed both to the pseudocapacitance of active nitrogen sites and to the developed mesoporosity. Different tetraalkylammonium bis(trifluoromethylsulfonyl)imide ionic liquids were used to evaluate the cation size effect on the stability and performance of N-GNFs-based EDLCs. In contrast to N+Et4TFSI− and N+Bu4TFSI− electrolytes, the cycling performance of N-GNFs in N+Me4TFSI− was limited because of the electrode degradation resulted from the intercalation of N+Me4 ions between graphene layers and their exfoliation. Keywords: Nitrogen-doped graphene; Supercapacitors; N-doping; Tetraalkylammonium bis(trifluoromethylsulfonyl)imide ionic liquids. 1

1. Introduction Electric double-layer capacitors (EDLCs), also called supercapacitors, have been extensively studied in recent years. In contrast to Li-ion batteries, EDLCs demonstrate higher power density, long cycling lifetime, wide operating temperature range (−40°C to 70°C), low maintenance cost and low weight, which make them promising in many fields including electronics, hybrid electric vehicles, and power devices [1], [2]. Based on the charge-storage mechanism supercapacitors can be divided into two groups. In the first group the charge accumulation is achieved by pure electrostatic adsorption of electrolyte ions on the electrode surface without charge transfer reactions, while in the second one (named as pceudocapacitors) charge accumulation is based on the faradic charge transfer between electrode and electrolyte [3]. The operating potential of EDLCs is restricted by the electrochemical stability of the electrolyte. In contrast to commonly used aqueous electrolytes, ionic liquids (ILs) with an electrochemical window up to 4 – 5 V have attracted increasing interest as electrolytes of high energy density carbon-based EDLCs because of their high ionic conductivity, moderate viscosity, non-flammability, lower toxicity, low volatility, good thermal stability, and safety [4], [5]. The electrochemical performance of a carbon nanomaterial is mainly determined by its specific surface area, pore size distribution and surface chemistry. Depending on the activation technique the specific surface area of activated carbons (ACs) can reach up to 3000 m2 g−1 [6]. However, abundant micropores in the AC structure can hinder the migration of electrolyte in pores leading to a quite low capacitance of AC-based EDLCs [7]. In addition, with increasing the specific surface area, the conductivity of AC declines, resulting in poor rate performance and power density of EDLCs [8]. In contrast to ACs, aligned multi-walled carbon nanotubes (CNTs) show higher electronic and ionic conductivities. Nevertheless, because most of CNTs are bundled with each other due to the van der Waals forces, only the external tubes in the bundle are accessible for the formation of the electrical double layer [7]. Recently, graphene-based nanostructured materials have attracted interests as a promising electrode material for EDLCs because of their large specific surface area, high electrical conductivity, and chemical stability. Unfortunately, the strong van der Waals interaction between parallel graphene sheets can lead to their aggregation, which decreases the surface area accessible to electrolyte, that is, hinder the electron and ion transfer. To avoid these limitations, the porous structure of the graphene material should be tailored by varying the morphology of graphene sheets and introducing different curvatures to expand the space between graphene planes and facilitate their electrolyte accessibility [9]. In addition, incorporation of heteroatoms, such as nitrogen, boron, and sulfur, 2

can effectively modify the electronic structure of graphene layers and increase the number of active sites [10], [11]. Particularly, nitrogen incorporation has been reported to be an attractive way to improve the capacitance and performance of graphene-based electrodes by changing their electrical conductivity, electrochemical activity and by introducing Faraday redox reactions [12], [13], [14]. Pyrrolic and pyridinic nitrogen functionalities play a main role in enhancing the specific capacitance of N-doped carbon electrodes due to the pseudo-capacitive interactions, while graphitic-N could enhance the conductivity of the material [15]. In this work, we apply the template CVD synthesis to produce mesoporous nitrogendoped sp2-hybridized graphene nanoflakes (N-GNFs) with varying nitrogen content. The synthesized N-GNFs are analyzed by physicochemical methods and tested as electrodes in the EDLCs with different tetraalkylammonium bis(trifluoromethylsulfonyl)imide ionic liquid (IL) electrolytes. 2. Experimental 2.1. Synthesis of N-GNFs N-GNFs were synthesized by chemical vapor deposition from the CH3CN/C6H6 mixture over the MgO template according to the technique described in our previous work [16]. To vary nitrogen doping level the CH3CN/C6H6 mixtures of volume ratios of 1:9, 1:1, and 1:0 were used and the produced materials were respectively named as N-GNFs-0.1, N-GNFs-0.5, and N-GNFs1.0. MgO was produced by mixing the aqueous equimolar solutions of (NH4)2C2O4·H2O (≥ 99.5%, Fluka) and Mg(NO3)2·6H2O (98%, Sigma-Aldrich) with further thermal decomposition in air at 500 °C for 5 hours. The MgO template was placed in the center of a quartz reactor located in a horizontal tube furnace (Carbolite TZF 12/100/900) and heated up to 800 °C in N2 flow (200 mL min−1) for 1 hour. After that, the N2 flow was increased up to 1000 ml min−1 and switched to bubble through the CH3CN/C6H6 mixture for 30 minutes. Then, the reactor was cooled to room temperature in N2 flow (200 mL min−1). To remove metal impurities the asproduced N-GNFs were boiled in hydrochloric acid (10 wt. %) for 6 h and washed by distilled water. 2.2. Characterization of N-GNFs The morphology of N-GNFs was analyzed by transmission electron microscopy (TEM) using a JEM 2100 F/Cs microscope. X-ray photoelectron spectra (XPS) were acquired on an Axis Ultra DLD spectrometer (Kratos Analytical, UK) with a monochromatic AlKα source. The pass energies of the analyzer were 160 eV for survey spectra and 40 eV for high resolution 3

scans. Raman spectra of N-GNFs were recorded using a LabRam HR800 UV (Horiba Jobin Yvon, Japan) microscope-spectrometer (5 mW argon laser excitation with 514.5 nm wavelength and 50× Olympus lens). The Raman spectrum was measured in five points of each sample and then the calculated parameters were averaged. Nitrogen physisorption isotherms were recorded on an Autosorb-1C/QMS analyzer (Quantachrome Inc., USA). The BET specific surface area (SBET) was calculated for seven points of the adsorption isotherm in the p/p0 range of 0.15–0.3. The error in the SBET calculation did not exceed 10%. 2.3. Electrochemical measurements To prepare EDLC electrodes, 80 wt.% of N-GNFs were thoroughly mixed with 10 wt.% of polyvinylidene fluoride (PVDF) binder and 10 wt.% of Super C65 carbon black dispersed in N-methyl-2-pyrrolidone (NMP). Then, the produced suspension was applied by spatula on a Nifoam (d = 10 mm) used as a current collector and dried for 72 hours at 120 °C to remove NMP impurities and moisture traces. Finally, the electrode was pressed by an MSK-HRP-1A electric rolling press (MTI Corporation) to improve the contact between N-GNFs and the current collector. 1.3 M N+Me4TFSI−, 1.2 M N+Et4TFSI− and 0.8 M N+Bu4TFSI− IL (salts >99.0%, Sigma Aldrich) solutions in acetonitrile (99.8%, Sigma Aldrich) were used as electrolytes of symmetric two-electrode supercapacitors. The concentrations of the electrolytes corresponded to the highest conductivities of their solutions in CH3CN [17]. Two electrodes separated face-to-face by a Whatman glass microfiber filter were impregnated with the electrolyte in an Ar filled glove box and assembled into a CR2032 coin-cell using an MTI electric coin cell crimper. Cyclic voltammetry (CV) and galvanostatic charge – discharge (GCD) measurements were carried out using a Biologic VSP 219 research-grade multichannel potentiostat/galvanostat (Bio-Logic Science Instruments SAS). Scan rates of 5, 10, 20, 50, 100 and 200 mV s−1 were used for CV tests, while current densities of 1, 2, 5, 7 and 10 A g−1 were applied for GCD. Alternatively, the specific capacitance

(F g−1) was calculated by the integration of the CV curve area according

to the following equation: =

(1)

∆

where v and i are the potential and current in the CV measurement, mass on one electrode, capacitance

(g) is the active carbon

(V s−1) is the scan rate, ∆ (V) is the operating window. The specific

(F g−1) was obtained from the

/

slope of the discharge curve after ohmic

drop (IR) as: 4

= where

(2)

/

(g) is the active carbon mass on both electrodes and

energy density

(Wh kg−1) and power density

(A) is the discharge current.The

(kW kg−1) were calculated based on Equations

3, 4: = =

×∆ !×".$ % /".$

(3) (4)

The Nyquist plots were recorded in the frequency range from 400 kHz to 0.1 Hz at a potential amplitude of 10 mV. All electrochemical tests were carried out at room temperature. 3. Results and discussion 3.1. Physical and morphological characterization of N-GNFs The TEM analysis reveals that synthesized N-GNF particles are of 20 – 30 nm in size and consist of a few imperfect graphene layers with bent edges (Fig. 1). N-doped nanoflakes replicate the shape of MgO template particles the same way as it was earlier observed for undoped GNFs [16]. At the same time, no changes in the shape and size of GNF particles are observed with increasing the nitrogen content in the precursor mixture.

Fig. 1. TEM images of N-GNFs-0.1 (A), N-GNFs-0.5 (B) and N-GNFs-1.0 (C). The composition and chemical state of elements in N-GNFs were analyzed by X-ray photoelectron spectroscopy (XPS). Three peaks in the survey spectra at about 285, 400 and 532 eV (Figs.2a, S1 – S3 in Supplementary Materials) indicate the presence of C, N and O. The composition of the samples is summarized in Tables 1, S1 – S3. As expected, N-GNFs-1.0 produced from the precursor mixture with the highest CH3CN:C6H6 ratio shows the highest 5

nitrogen doping level of 10.7 at.%. The N1s spectra (Fig. 2b, S1 – S3) were fitted with eight components attributed to pyridinic N6 (398.2 eV) [18], [19], [20], pyrrolic N5 (399.4 eV) [20], pyridonic N-P (400.3 eV) [19], [21], graphitic (or quaternary) N-G (401.0 eV) [18], [19], [20], [22] nitrogen functionalities, and to different oxidized nitrogen species, such as pyridine N-oxide (402.5 eV), R−ONO (403.8 eV), R−NO2 (405.6 eV), and R−ONO2 (407.4 eV) [23], [18], [24], [25]. The N6, N5 and N-G components show the highest contributions to the N1s spectra while the fractions of oxidized nitrogen species are relatively small. The deconvolution procedure of C1s and O1s spectra is detailed in Supplementary Materials. Table 1. XPS elemental composition and content of nitrogen species in N-GNFs. Nitrogen species1/ at.%

Concentration/ at.% Sample

1

C

O

N

N6

N5

N-P

N-G

Pyridine N-oxide

N-GNFs-0.1

92.2

2.8

5.0

1.1

0.6

0.5

1.6

0.4

N-GNFs-0.5

89.5

2.7

7.8

2.0

1.1

0.8

2.3

0.5

N-GNFs-1.0

86.4

2.9

10.7

2.8

1.6

1.1

3.2

0.6

The content of highly oxidized nitrogen species is presented in Table S3.

Fig. 2. Survey (A) and N1s (B) XPS spectra of N-GNFs-0.1 and N-GNFs-1.0. The structural parameters of N-GNFs were analyzed by Raman spectroscopy. The Raman spectra in the range of 900 – 2000 cm−1 (Fig. 3) were fitted with four bands according to the previously reported model [26]. A Gaussian profile was applied for the D1, D3 and D4 bands, 6

while a pseudo-Voigt one for the G band. The positions of the D4, D1, D3 and G bands were fixed in the ranges of 1160 – 1200, 1340 – 1380, 1490 – 1510, and 1590 – 1620 cm−1, respectively. The G-band originates from the first-order scattering of the E2g vibration mode of the sp2-carbon domains and corresponds to the formation of well-graphitized carbon [27]. In contrast, the D1-band, commonly known as the breathing mode of the sp2-rings [28], appears when the symmetry of the graphite unit is broken by heteroatoms or edge atoms [26], [29]. Two additional D3 and D4 defect bands are assigned to polyene-/polyphenylene-type fragments of surface graphene layers and stacking defects, respectively [26], [30]. The Di to G area ratios are traditionally used to estimate the structural defectiveness of carbon nanomaterials. As expected, the SD1/SG ratio increases with the growth of nitrogen content in N-GNFs (Table 2) confirming the fact that nitrogen doping introduces defect sites and disorder into N-GNF layers. The SD1/SG ratio for N-GNFs-1.0 is more than 1.5 times higher than that for undoped GNFs [16]. At the same time, the SD3/SG and SD4/SG ratios remain almost constant with increasing the nitrogen content.

Fig. 3. Raman spectra of N-GNFs. Textural parameters of N-GNFs were analyzed by low temperature nitrogen physosorption. According to the IUPAC classification [31], the physisorption isotherms of the N-GNF samples (Fig. S4) are a combination of type II and IV(a) typical for macro- and mesoporous materials, respectively. The desorption branches of the N-GNFs-0.1 and N-GNFs0.5 isotherms demonstrate a sharp step-down in the p/p0 range of 0.4–0.5, which indicates the cavitation-induced desorption in the ink-bottle pores with the neck diameter smaller than a critical size (about 5 – 6 nm for nitrogen at 77 K) [31]. For N-GNFs-1.0 this step-down is not observed, indicating more accessible pores in this material. The specific surface areas of N-GNF are lower than that of undoped GNFs and decrease with increasing the nitrogen content (Table 7

2). This decrease was earlier attributed to the higher polarity of the nitrogen doped GNFs, which increased the contact area between individual nanoflakes [32]. Table 2. Raman band ratios and textural parameters of N-GNFs. Sample

SD1/SG

SD3/SG

SD4/SG

Smicro/ m2 g−1 a

SBET / m2 g−1 b

GNFs [16] c

1.11 ± 0.15

0.44 ± 0.06

0.24 ± 0.03

0

1230

N-GNFs-0.1

1.31 ± 0.05

0.38 ± 0.08

0.35 ± 0.09

50

1070

N-GNFs-0.5

1.61 ± 0.08

0.39 ± 0.04

0.29 ± 0.08

80

930

N-GNFs-1.0

1.73 ± 0.15

0.31 ± 0.12

0.31 ± 0.09

30

730

a

Micropore specific surface area calculated by the t-plot method. BET specific surface area. c undoped GNFs produced from C6H14 under the same CVD conditions. b

3.2. Electrochemical performance of N-GNFs 3.2.1. Effect of nitrogen doping The electrochemical performance of N-GNFs-based electrodes was studied by cyclic voltammetry and galvanostatic charge-discharge measurements in the symmetrical two-electrode coin-cell supercapacitor using the 1.2 M N+Et4TFSI− solution as an electrolyte. The CV curve of N-GNFs-1.0 at a 5 mV s−1 scan rate exhibits the highest current densities among all the samples (Fig. 4a), which confirms the highest specific capacitance of this material. Even at a high scan rate of 200 mV s−1 all the CV profiles retain near rectangular shapes (Figs. 4b) indicating the fast formation of the electrical double layer and the excellent capacitive behavior of N-GNFs-based electrodes. Minor distortions of the voltammograms observed at higher scan rates are typical for EDLCs and originate from insufficient time available for ion diffusion and adsorption inside the smallest inner pores [33]. The highest specific capacitances of 167 F g−1 for N-GNFs-1.0, 140 F g−1 for N-GNFs-0.5 and 119 F g−1 for N-GNFs-0.1 are achieved at a scan rate of 5 mV s−1 (Fig. 4c) and exceed that for undoped GNFs (105 F g-1 [34]). The specific capacitance (Cdl) of the EDLC is proportional to the surface area of the electrode/electrolyte interface (S) and permittivity of the electrolyte (&) as

'

= &(/ [35]. Therefore, the increase in the SBET should

lead to increased specific capacitance. Such growth was indeed observed for non-doped GNFs [34] but N-GNFs show the opposite trend: the specific capacitance of N-GNFs-1.0 with the lowest SBET is the highest. Thus, the changes in the electronic structure of GNFs under nitrogen doping significantly contribute to the specific capacitance of N-GNF based electrodes. 8

According to the XPS data, the contents of N5 and N6 nitrogen species are the highest in NGNFs-1.0 while the contents of N-G ones in N-GNFs-0.5 and N-GNFs-1.0 are close and higher than that in N-GNFs-0.1. Earlier Bandosz et al. [15] concluded that the pseudo-capacitive interactions took place on negatively charged pyrrolic and pyridinic nitrogen species, while the positive charge on quaternary-N and pyridinic-N-oxide improved the electron transfer through pores of the electrode material. In addition, nitrogen incorporation into GNFs creates the high density of active sites, which promotes capacitance performance of GNFs and decreases interfacial resistance [36]. The triangular shape of the galvanostatic charge-discharge plots recorded at 0.5 A g−1 and the small IR drops in these plots (Fig. 4d) testify to the double layer capacitive behavior of the N-GNF based electrodes. Thus, the remarkable capacitive performance of N-GNFs-1.0 can result from the large number of nitrogen active sites and the optimal pore structure of the electrode that facilitate transport pathways for N+Et4TFSI− ions. Indeed, according to N2 physisorption data N-GNFs are predominantly formed by mesopores whose size exceeds the effective N+Et4 size both in acetonitrile solution (1.30 nm [37]) and in pure IL (0.67 nm [37]). The specific capacitance of N-GNFs calculated from the slope of galvanostatic discharge curves at 0.5 – 10 A g−1 is shown in Fig. 4 d, e. At a low current density of 0.5 A g−1 the specific capacitance of N-GNFs-1.0 (152 F g−1) is higher than those of N-GNFs0.5 (132 F g−1) and N-GNFs-0.1 (116 F g−1), which agrees with the CV data (Table 3). With increasing the charge-discharge current density up to 10 A g−1 N-GNFs-1.0 still exhibits a high specific capacitance of 128 F g−1. The decrease in the specific capacitances with increasing the current densities is caused by the fact that the charge-discharge time becomes too short to allow electrolyte ions migrating deep into pores of the electrode material [38]. N-GNF based EDLCs were measured by electrochemical impedance spectroscopy (EIS) in the frequency range from 0.1 to 400 kHz. The Nyquist plots (Fig. 4g) display the similar behavior for all the N-GNF electrodes. The high-frequency intersection of the Nyquist plot with the real axis corresponds to the combined resistance (Rs) that includes the contribution from the intrinsic resistance of the electrode material, electrolyte resistance and contact resistance between the electrode material and the current collector [39], [40]. The Rs values for N-GNFs1.0, N-GNFs-0.5 and N-GNFs-0.1 were found to be 1.5, 1.7 and 2 Ohms, respectively. The smallest semi-circle diameter indicates the highest charge transfer rate of N-GNFs-1.0 among the tested materials, which can be attributed to the highest concentration of N6 nitrogen species in N-GNFs-1.0. A lone electron pair of these nitrogen species facilitates the adsorption of ionic liquid ions [41]. In addition, higher slope of the straight line at the low frequency region of Nyquist plot for N-GNFs-1.0 confirms better capacitive behavior of this material. 9

Fig. 4. Electrochemical characteristics of EDLCs with N-GNF electrodes: A – CV curves recorded at 5 mV/s (a); B – CV curves for N-GNFs-1.0 electrodes recorded at 5 – 200 mV/s; C – specific capacitances at 5 – 200 mV/s; D – galvanostatic charge-discharge profiles recorded at 0.5 A/g; E – galvanostatic charge-discharge profiles for N-GNFs-1.0 electrodes 10

recorded at 0.5 – 10 A/g; F – specific capacitances at 0.5 – 10 A/g; G –Nyquist plots; H – Ragone plots. Ragone plots for N-GNF based EDSLs respect to the active mass of both electrodes are presented in Fig. 4h. The highest energy density of 46.3 Wh kg−1 is delivered at a power density of 0.74 kW kg−1 for N-GNFs-1.0 and exceeds those for N-GNFs-0.5 (40.9 Wh kg−1) and NGNFs-0.1 (36.1 Wh kg−1). Furthermore, even at a high power density of 9.7 kW kg−1 N-GNFs1.0 displays the energy density of 36.1 Wh kg−1 that is higher than that for the undoped GNFsbased EDLC (32.8 Wh kg−1 [34]) making N-GNFs promising for application in high performance devices. 3.2.2. Effect of ionic liquid cation size on superpacitive performance The 1.3 M N+Me4TFSI−, 1.2 M N+Et4TFSI− and 0.8 M N+Bu4TFSI− IL solutions in acetonitrile were used as EDLC electrolytes to evaluate the effect of cation sizes on the supercapacitive performance of N-GNFs-1.0. A near rectangular shape of the CV curves typical for capacitive behavior is observed for the N+Et4TFSI− and N+Bu4TFSI− electrolytes (Fig. 5a) while the curve for N+Me4TFSI− shows the wide peak in the range of 0.7 – 1.6 V attributed to irreversible interactions. Based on the solvent-independent Robinson–Stokes model [42], the crystallographic radii of N+Bu4, N+Et4 and N+Me4 are 0.494, 0.400 and 0.347 nm, respectively. Smaller ions are known to better penetrate in narrow pores. At a scan rate of 5 mV s−1 the specific capacitance of N-GNFs-1.0 decreases from 179 to 142 F g−1 with increasing the cation size (Table 2). Mesopores in N-GNFs-1.0, because of their relatively large size, should be fully accessible for the electrolyte ions. Thus, in contrast to most microporous ACs the removal of solvation shell is not required to achieve the effective energy storage. The specific capacitances of N-GNFs-1.0 based EDLCs with different electrolytes calculated from the slope of discharge curves (see insert in Fig. 5b) are close to those obtained from the CV measurements (Table 3). In both cases the specific capacitance increases with decreasing the cation size. The similar dependence was previously observed for quaternary ammonium BF4 salt electrolytes for which the specific capacitance was proportional to the reciprocal radius of neat cation [43]. The Ragone plots for N-GNFs-1.0 based EDLCs with different electrolytes are compared in Fig. S5. The high energy density of 53.2 Wh kg−1 was observed for 1.3 M N+Me4TFSI− (Table 3). The electrochemical cycling stability of the EDLCs with N-GNFs-1.0 electrodes and different electrolytes was tested by charge-discharge measurements at 2 A g−1 (Fig. 5c). The capacitances of the EDLCs with 1.2 M N+Et4TFSI− and 0.8 M N+Bu4TFSI− electrolytes show a 11

monotonous decrease reaching about 87% of their initial values after 4000 cycles. In contrast, in the case of the 1.3 M N+Me4TFSI− electrolyte the significant growth of capacitance is observed before a drastic drop after about 400 cycles. The repeated stability test for the reassembled EDLC shows the same result (Fig. S6). This growth testifies to the increase in the active area of the electrode probably because of the intercalation of N+Me4 ions between graphene layers and their exfoliation, which results in a fluffy morphology of N-GNF particles. Indeed, the intercalation of N+Me4, N+Et4 and N+Bu4 ions into the graphite electrode and the exfoliation of graphene layers was earlier observed at a potential of −5 V [44]. Lower potential used in our work probably allows the intercalation of only the smallest N+Me4 ions. Exfoliation of the graphene layers by intercalated ions initially increases the specific surface area of the electrode and, therefore, its capacitance. But at some moment the migration of completely exfoliated graphene layers in the electrolyte short-circuits the EDLC leading to a drop in the capacitance. Thus, the use of N+Me4TFSI− containing electrolyte in EDLCs with graphene-based electrodes is limited by the short cycling lifetime despite the high initial specific capacitance of the electrodes. 1.2 M N+Et4TFSI− based electrolyte looks more attractive for these EDLCs because of the low ion diffusive resistivity, high electrochemical stability, and high conductivity [17].

Fig. 5. Electrochemical characteristics of EDLCs with N-GNFs-1.0 based electrodes and different electrolytes: A – CV specific capacitance versus scan rate; B – GCD specific 12

capacitance versus current density; C – specific capacitance versus cycle number recorded at 2 A g−1. Table 3. Electrochemical parameters of EDLCs with IL electrolyte.

Electrode material

Current density/ A g−1

Specific capacitance/ F g−1

Energy density/ Wh kg−1

Reference

0.5

175 (0.5 A g−1) 179 (5 mV s-1)

53.2

This work

−

46.3

This work

Electrolyte

+

−

N-GNFs-1.0

1.3 M N Me4TFSI

N-GNFs-1.0

+

1.2 M N Et4TFSI

0.5

152 (0.5 A g−1) 167 (5 mV s-1)

N-GNFs-1.0

0.8 M N+Bu4TFSI−

0.5

133 (0.5 A g−1) 142 (5 mV s-1)

38.6

This work

N-GNFs-0.5

1.2 M N+Et4TFSI−

0.5

132 (0.5 A g−1) 140 (5 mV s-1)

40.9

This work

N-GNFs-0.1

+

−

1.2 M N Et4TFSI

0.5

116 (0.5 A g−1) 119 (5 mV s-1)

36.1

This work

GNFs

1.2M N+Et4TFSI−

0.5

105 (0.5 A g−1) 112 (5 mV s-1)

32.8

[34]

Carbon nanosheets

[EMIM]BF4

1

147

45.5

[45]

N,S co-doped graphene hydrogel

[EMIM]BF4

1

203

100.7

[40]

N- graphene hydrogel

[BMIM]PF6

0.5

194.4

92.5

[46]

Sponge-like graphene

[BMPY]TFSI

0.1

68

21.4

[47]

Activated carbon

[EMIM]Ac

1

84 – 122 (at 21 – 120 °C)

4.3 – 7.4

[48]

N,S co-doped graphene

[EMIM]BF4

1

169.4

84.5s

[49]

N,O co-doped carbon

[EMIM]BF4

0.5

201

111

[41]

N-graphene

[EMIM]BF4

0.1

117

36

[50]

N- doped carbon nanofoam

[EMIM]BF4

0.25

89 – 204

26.6 – 63.4

[51]

Our results suggest that the formation of nitrogen active sites in mesoporous graphene nanoflakes under N-doping both introduces the pseudo-capacitance and facilitates the adsorption 13

of electrolyte ions reducing charge transfer resistance. In addition, the abundant mesopores provide the effective transport pathways for ions deep into pores improving the formation of the electrical double-layer, which in combination with the high operating potential of 1.2 M N+Et4TFSI− IL increases the electrochemical performance of the N-GNFs-based EDLC (Table 3). 4. Conclusions N-doped mesoporous graphene nanoflakes with high nitrogen doping level up to 10.7 at. % were produced by template CVD synthesis. Though the nitrogen doping decreases the specific surface area of graphene nanoflakes their specific capacitance in the EDLC with IL electrolyte increases and reaches 167 F g−1 at 5 mV s−1. The N-GNFs-1.0-based EDLC delivers the energy density of 46.3 Wh kg−1 at 0.74 kW kg−1 and 36.5 Wh kg−1 even at a high power density of 9.8 kW kg−1. The high electrochemical performance of N-GNFs in EDLCs results from the combination of several factors. Firstly, mesoporous structure of N-GNFs facilitates the charge transfer and migration of electrolyte ions. Secondly, the nitrogen doping leads to the formation of N-active surface species (N-5 and N-6) responsible for the pseudo-capacitance contribution, while N-G species can enhance the conductivity of N-GNFs improving the electron transfer during charge – discharge processes. The N-GNF based electrodes show high cycling stability in the N+Et4TFSI− based IL electrolyte in contrast to N+Me4TFSI− based one, in which the cycling life is limited by cathode degradation as a result of the electrochemical intercalation of N+Me4 ions between graphene layers and their exfoliation. Undoubtedly, the obtained results demonstrate that the high nitrogen doping level and optimal mesoporous structure of graphenebased materials are the key factors, determining their electrochemical performance in EDLC.

Acknowledgments The reported study was funded by the Russian Science Foundation (Project #18–13– 00217). The authors acknowledge support from Lomonosov Moscow State University Program of Development for providing access to the XPS and TEM facilities. We gratefully thank Dr. S.V. Maksimov and Dr. O.Y. Isaikina for TEM and Raman spectroscopy measurements.

14

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19

Highlights •

EDLC electrodes from mesoporous nitrogen-doped graphene nanoflakes

•

High specific capacitance in [N+R4]TFSI− based electrolytes (up to 167 F g-1)

•

Significant contribution from pseudo-capacitance

•

High cycling stability in N+Et4TFSI− and N+Bu4TFSI− based electrolytes

•

Degradation in N+Me4TFSI− because of the cation intercalation

Declaration of interests x The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: