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Mesoporous graphene nanoflakes for high performance supercapacitors with ionic liquid electrolyte
Microporous and Mesoporous Materials xxx (xxxx) xxx
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Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Mesoporous graphene nanoflakes for high performance supercapacitors with ionic liquid electrolyte Ekaterina A. Arkhipova a, *, Anton S. Ivanov a, Konstantin I. Maslakov a, Alexander V. Egorov a, Serguei V. Savilov a, b, Valery V. Lunin a, b a
Department of Chemistry, Lomonosov Moscow State University, 1–3 Leninskiye Gory, Moscow, 119991, Russia Department of Physical Chemistry, Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, 31 Leninsky Prospect, Moscow, 119991, Russia
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene nanoflakes Supercapacitor Ionic liquid Mesoporous materials
Mesoporous graphene nanoflakes (GNFs) of different thickness with a specific surface area of 660–1720 m2/g were produced by the template synthesis via hexane pyrolysis over MgO. The structure and morphology of GNFs were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy and low-temperature N2 physisorption. The electrochemical performance of GNF-based supercapacitors was tested in ionic liquid electrolyte (1.2 M NþEt4TFSI in CH3CN) by cyclic voltammetry and galvanostatic charge – discharge mea surements. The assembled symmetric supercapacitors with a wide voltage window of 3 V delivered specific capacitance of 112 F/g at 5 mV/s and 105 F/g at 0.5 A/g with a high rate capability of ~81% at a current density of 15 A/g. The energy density reached 32.8 Wh/kg corresponding to the power density of 0.7 kW/kg. Further more, even at a high power density of 19.9 kW/kg, GNFs still showed an energy density of 24.3 Wh/kg, which made them promising for application in supercapacitors.
1. Introduction
results in quite low capacitance of activated carbon-based electrodes: 140–330 F/g in aqueous media [16] and <100 F/g in non-aqueous systems [18]. In contrast to activated carbon the major part of the gra phene sheet surface is external and readily accessible to electrolyte [11]. Currently, graphite oxide (GO) is extensively applied for production of graphene-like structures. However, materials produced by GO reduction show poor electrical conductivity because of the structural defects in graphene sheets [19]. Moreover, van der Waals interaction between graphene sheets leads to their stacking, which decreases SSA [17]. Recently, the template synthesis of carbon materials from organic and inorganic precursors has been shown to be an effective approach for production of carbon nanomaterials with desirable textural character istics by controlling the synthesis parameters (synthesis temperature and time, precursor type, template agent, etc.) [20–22]. This method allows avoiding the challenges caused by the oxidation and reduction steps of the GO-based synthesis of a graphene material. In addition, using a template is an effective strategy to better control the shape and size of nanostructured carbon materials [23]. The template approach is easily scalable and doesn’t require the prolonged and laborious stage of the catalyst synthesis.
Carbon-based supercapacitors have been attracting interest in last decades due to their fast charge/discharge ability, good reversibility, long cycle lifetime, low maintenance, excellent stability and low cost, which makes them promising energy storage devices for electronics, automobiles and portable energy systems [1–7]. The main charge stor age mechanism of supercapacitors is based on the energy accumulation either by pure electrostatic adsorption of ions on the electro de/electrolyte interface without charge transfer reactions [1,8] or by fast Faradaic redox reactions on the electrode surface [9,10]. The pore structure of carbon-based materials affects the ion and electron transfer and is therefore a crucial factor influencing their spe cific capacitance and rate capability. The high specific surface area (SSA) is generally believed to provide high specific capacitance of carbon-based electrodes [1,11,12]. Activated carbon is traditionally used as an electrode material in supercapacitors. Depending on the activation process and type of precursor its SSA can achieve 3000 m2/g [13–16]. However, despite the high SSA of activated carbon its abun dant micro-pores are barely accessible for electrolyte ions [17], which
* Corresponding author. E-mail address: [email protected] (E.A. Arkhipova). https://doi.org/10.1016/j.micromeso.2019.109851 Received 8 August 2019; Received in revised form 8 October 2019; Accepted 28 October 2019 Available online 31 October 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Ekaterina A. Arkhipova, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109851
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The cell voltage is known to significantly affect the specific energy and power density of a supercapacitor. The use of aqueous electrolytes (H2SO4, KOH, etc.) is restricted by their electrochemical stability to less than 1 V cell voltage [24]. On the other hand, the organic media based on the room temperature ionic liquids (RTILs) allows increasing the operating voltage up to 5–6 V [25]. Moreover, RTILs have high thermal stability, negligible vapor pressure and high ionic conductivity [24,26]. For the RTIL electrolyte solution based on NþEt4BF4 a pore size of 1.2–1.3 nm seems to be the optimal pore size in the carbon electrode material that well balances the high volumetric capacitance and high rate performance in the electrolyte [1,27]. These micropores are elec trochemically accessible for electrolyte ions to form the electrical double layer. In this work we demonstrate the facile approach to template syn thesis of highly sp2-hybridized graphene nanoflakes (GNFs) that allows effectively controlling their pore distribution and SSA. Synthesized GNFs have been characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), low temperature N2 physisorption, cyclic voltammetry (CV) and galvanostatic charge – discharge measurements. GNFs show high supercapacitive performance in the 1.2 M tetraethylammonium bis(trifluoromethylsulfonyl)imide (NþEt4TFSI ) ionic liquid solution in acetonitrile with a wide electro chemical stability window of 3 V.
2.3. Electrochemical performance of GNFs The electrochemical performance of GNFs was evaluated in the twoelectrode symmetrical supercapacitor. The coating slurry was mixed from 80 wt% of GNFs, 10 wt% of polyvinylidene fluoride binder, and 10 wt% of Super P solution in N-methyl-2-pyrrolidone (NMP). Elec trodes were produced by spreading the obtained suspension with a spatula on a Ni-foam disk of 10 mm in diameter followed by drying at 120 � C for 72 h to remove NMP and moisture. Coin-cells were assembled in an argon filled glove box. 1.2 M NþEt4TFSI ionic liquid (>99.0%, Sigma Aldrich) solution in acetonitrile (99.8%, Sigma Aldrich) was used as an electrolyte. The concentration of the solution was chosen based on our previous results and corresponded to the highest conductivity of the NþEt4TFSI solution in acetonitrile [26]. Two identical electrodes separated face-to-face by a glass microfiber filter (Whatman) were impregnated with the electrolyte and assembled in a coin-cell using an MTI electric coin cell crimper. Cyclic voltammetry (CV) and galvano static charge – discharge (CD) measurements were carried out using a Biologic VSP 219 research-grade multichannel potentiostat/galvanostat (Bio-Logic Science Instruments SAS). The specific capacitance CCV (F/g) was calculated from the CV curve according to the following equation: R 2 ivdv CCV ¼ (1) μmΔV
2. Experimental
where i (A) and v (V) are, respectively, the current and the potential in a CV test, μ (V/s) is the scan rate, m (g) is the active mass in one electrode, ΔV (V) is the potential window. The specific capacitance CCD (F/g) was calculated from the galvanostatic charge – discharge curve as:
2.1. Synthesis of GNFs GNFs were synthesized by chemical vapor deposition according to the previously described technique using MgO as a template [28]. MgO was produced by mixing of the equimolar solutions of magnesium ni trate hexahydrate (98%, Sigma–Aldrich) and ammonium oxalate (�99.5%, Fluka). For GNF synthesis the MgO template was placed in the center of a horizontal Carbolite TZF 12/100/900 (d ¼ 55 mm) tube furnace and heated in a nitrogen flow up to 800 � C. Then hexane (99.9%, Reakhim) vapors used as a carbon precursor were passed over the MgO template (SBET ¼ 140 m2/g) in a nitrogen flow of 1000 ml/min for 15, 30, 60 and 90 min. After cooling to room temperature, the synthesized GNFs were boiled in 10 wt% hydrochloric acid to remove the template followed by washing in distilled water to neutral pH. Amorphous carbon impurities in GNFs were removed by annealing in air for 2 h at 350 � C. The yield of the synthesis was about 14% based on the weight of used hexane. The synthesized samples were named as GNFs-t, where t is synthesis time in minutes.
CCD ¼
4I mdV=dt
(2)
where m (g) is the active mass in both electrodes, I (A) is the discharge current and dV=dt is the slope of the discharge curve after ohmic drop (IR). In the Ragone plot, the specific energy density E(Wh/kg) and power density P (kW/kg) were calculated based on the following equations: E¼
CCD � ΔV 2 8 � 3:6
(3)
P¼
E t=3:6
(4)
The supercapacitors were also measured by electrochemical imped ance spectroscopy (EIS) in the frequency range from 400 kHz to 100 mHz at 10 mV amplitude.
2.2. Material characterization
3. Results and discussion
The morphology of the produced materials was investigated by TEM using a JEM 2100 F/Cs microscope operated at 200 kV and equipped with an aberration corrector (CEOS). The average lateral size and thickness of GNFs were calculated by analysis of at least 100 TEM im ages. Low temperature nitrogen physisorption measurements were performed on an Autosorb-1 gas sorption analyzer (Quantachrome, USA). Prior to measurement the sample was degassed in vacuum at 250 � C for 3 h. The BET SSA (SBET) was calculated at seven points of the adsorption isotherm in the p/p0 range of 0.15–0.3. The error in the calculation of SBET didn’t exceed 10%. Pore size distributions were calculated from the adsorption branches of isotherms by QSDFT method using the slit pore model for pore size <2 nm and the cylindrical pore model for pore size >2 nm. XPS spectra were acquired on a Kratos Axis DLD spectrometer with monochromatic AlKα radiation. The pass energy of the analyzer was 160 eV for survey spectra, and 40 eV for highresolution scans.
The effect of synthesis time on the morphology of GNFs was studied by TEM. GNFs replicate the shape of MgO particles used as a template and consist of graphene layers with bent edges (Fig. 1). This shape of GNF particles leads to the interconnected pore network in the synthe sized material. The thickness of GNFs increased with the synthesis time from 2.0 � 0.2 nm (GNFs-15) to 3.2 � 0.2 nm (GNFs-90), indicating the increase in the number of graphene layers in GNFs. At the same time no significant changes in the morphology and shape of GNFs were observed. Interestingly, the average lateral size of GNFs (10–50 nm) is significantly lower than the size of similar materials [29,30]. Never theless, the observed interconnected pore network in GNFs can provide favorable pathways for the penetration and migration of electrolyte ions. The chemical state of GNFs was studied by XPS. Only carbon and oxygen lines are observed in the survey spectra (Fig. S1 in Supplemen tary Materials). The oxygen content in GNFs (Table 1) decreases with increasing the synthesis time from 2.1 (GNFs-15) to 1.0 (GNFs-90) at. %. Small amount of oxygen probably results from the oxygen-containing 2
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Fig. 1. TEM images of GNFs-15 (a), GNFs-30 (b), GNFs-60 (c), and GNFs-90 (d).
with each other produces more compact material with larger contact area between GNFs, which results in the significant networking effects, such as ink-bottle pores. At higher synthesis times thicker and as a result more rigid GNFs are produced. The contact area between them reduces and pores become more accessible, which diminishes the networking effects. This conclusion is confirmed by the growth of the pore volume with the increase in the synthesis time from 15 to 60 min (Table 1). The decrease in the pore volume at higher synthesis times may be explained by the growth of the mass of each GNF without significant changes in its size and shape. It is worth noting that there is a small fraction of mi cropores in GNFs-15 and GNFs-30 that completely disappears in GNFs-60 and GNFs-90 (Fig. 2b). However, all GNF samples don’t contain very narrow pores (<1 nm) that are known to hinder the transport of large electrolyte ions deep into pores of the electrode material [32]. The electrochemical performance of GNF-based supercapacitors was evaluated using cyclic voltammetry and galvanostatic charge-discharge measurements. Fig. 3a and b shows the CV curves recorded at room temperature at different scan rates (5–1500 mV/s). Among all studied GNFs the CV profile of the supercapacitor with GNFs-15 based elec trodes (Fig. 3a) exhibits the highest current density, indicating the highest specific capacitance of this material. At low scan rates the vol tammograms are almost rectangular that is typical for capacitive behavior of the electrodes. Observed distortion of the voltammograms with increasing the scan rate is a well-known effect in electrochemical supercapacitors caused by the decrease in ion mobility in pores of car bon electrodes at high scan rates. Indeed, the specific capacitance of GNFs calculated by the integration of CV curve area decreases with scan rate (Fig. 3c). This trend reflects the growth of ion transport resistance in mesopores [32]. The capacitance of GNFs-15 achieves the highest value of 112 F/g at 5 mV/s while it doesn’t exceed 98 F/g for GNFs-30, 72 F/g
Table 1 XPS oxygen content and textural parameters of GNFs. Sample
OXPS, at.%
Vmicro, cm3/ga
Vmeso, cm3/ga
VDFT, cm3/ga
SDFT, m2/ga
SBET, m2/gb
GNFs15 GNFs30 GNFs60 GNFs90
2.1
0.15
2.38
2.53
1770
1720
1.3
0.05
2.82
2.87
1200
1230
1.2
0.00
3.14
3.14
870
900
1.0
0.00
2.34
2.34
630
660
a b
Pore volumes and SSA were calculated from DFT pore size distributions. SSA was calculated using the Brunauer-Emmett-Teller (BET) method.
groups bonded to surface defect sites of GNFs (vacancies, layer edges, etc.). Trace amount of MgO from the template may also contribute to the oxygen signal. The textural properties of GNFs were studied by low temperature N2 physisorption. The nitrogen adsorption–desorption isotherms (Fig. 2a) correspond to the combination of type II and type IV isotherms attrib uted to macro- and mesoporous materials, respectively [31]. The sharp step-down of the desorption branch of the GNFs-15 isotherm in the p/p0 range of 0.4–0.5 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]. With increasing the synthesis time the contribution from the cavitation-induced desorption decreases and completely disappears in the isotherm of GNFs-60. Probably, nanoflakes in GNFs-15 are not rigid because the number of graphene layers in these nanoparticles is low. Their deformation under interaction 3
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Fig. 2. Nitrogen adsorption–desorption isotherms (a) and normalized cumulative pore volume distributions (b) of GNFs.
for GNFs-60, and 46 F/g for GNFs-90. The similar trend was previously observed for starch-derived mesoporous carbon. M. Wu et al. showed that the main factor affecting the specific capacitance was mesoporosity of the sample and found that the increase in the fraction of mesopores led to the growth of capacitance [33]. D. Zhou et al. concluded that the high mesopore fraction shortened the diffusion distance normal to the plane of nanosheets and facilitated the smooth transport of ions at high rates [32]. At high scan rates the specific capacitance of GNFs-15 de creases faster than those of other GNFs and at 1500 mV/s it even be comes smaller than that of GNFs-30. This fact can be explained by the high resistance of electrolyte diffusion in ink-bottle pores and micro pores of GNFs-15 at high scan rates. The galvonostatic charge-discharge curves recorded at 1 A/g (Fig. 3d) demonstrate nearly triangular shape with small IR drops, which testifies the superior reversible electrochemical performance. At a cur rent density of 0.5 A/g the specific capacitance of GNFs-15 calculated from the discharge slope (105 F/g) is also higher than those for GNFs-30 (88 F/g), GNFs-60 (64 F/g), and GNFs-90 (37 F/g), which agrees with the CV data. The EIS results also confirm this trend. The low-frequency region of the Nyquist plot for the supercapacitor with GNFs-15 elec trodes (Fig. S2) demonstrates the steepest growth which is characteristic of better capacitive behavior [34]. Note that with increasing the current density up to 15 A/g the specific capacitance of GNFs-15 still remains as high as ~81% of its initial value at 0.5 A/g, while the specific capaci tance of GNFs-90 achieves only ~74% of the specific capacitance retention (Fig. 3e). This decrease in the specific capacitance with the increase in current density can be attributed to the growth of charge transfer resistance within micro/mesopores. The specific capacitance of GNFs-15 is higher or comparable to those reported in the literature for supercapacitors with ionic liquid electrolytes (Table 2). The electrochemical cycling stability of GNF-based supercapacitors was tested by charge-discharge measurements at 2 A/g (Fig. 3f). After 5000 cycles GNFs-15, GNFs-30, GNFs-60, and GNFs-90 show only the slight loss of capacitance of 11.8, 6.9, 6.5, and 5.4%, respectively. SBET of GNF materials decreased with synthesis time from 1720 m2/g (GNFs-15) to 660 m2/g (GNFs-90) (Table 1) as a result of the growth of the GNF thickness. The specific capacitance is known to be proportional to the specific surface area Sof the electrode/electrolyte interface and to electrolyte permittivity ε as C ¼ εdS [24] where d is the effective thickness
of the double layer. Therefore, the increase in the SSA should propor tionally increase the capacitance C. However, the experimental dependence of C vs. SBET is not linear (Fig. S3). Similar dependence was also observed for supercapacitors based on aero/cryo/xerogel carbon electrodes with the PYR14TFSI ionic liquid [21]. Indeed, the effective ion size of NþEt4 is larger in acetonitrile solution (1.30 nm [39]) than in the pure ion liquid (0.67 nm [39]) because of the formation of the sol vated shell. According to the N2 physisorption results the GNFs-15 ma terial predominantly contains mesopores with the small contribution of micropores the size of which may be comparable with the size of elec trolyte ions. For these micropores the removal of the solvation sphere is needed to achieve the effective energy storage. In addition, the presence of ink-bottle pores can introduce diffusion limitations. P. Simon, Y. Gogotsi, and coworkers studied supercapacitors based on carbide-derived carbon with EMI–TFSI electrolyte and concluded that the pore size leading to the maximum double-layer capacitance was very close to the ion size [40]. On other hand, micropores can increase the ohmic resistance at high charge–discharge rates because of the higher diffusion losses and less effective ion adsorption [3]. Thus, the high SSA of the electrode material is necessary to achieve the large specific capacitance, while the morphology of pores determines the availability of the electrode surface to electrolyte ions. The Ragone plot (Fig. 3g) shows the relation between the energy and power densities of a supercapacitor. The sum of the active mass in both electrodes was used for the calculation. At 0.5 A/g GNFs-15 exhibit the highest energy density of 32.8 Wh/kg higher than that of GNFs-30 (27.3 Wh/kg), GNFs-60 (19.9 Wh/kg), and GNFs-90 (11.4 Wh/kg). Furthermore, even at high power density of 19.9 kW/kg GNFs-15 still deliver the energy density of 24.3 Wh/kg. These results highlight the high integrated power – energy properties of GNFs-15 in contrast to other GNF samples with lower SSA and less developed porosity. The energy and power characteristics of GNFs-based electrodes are also comparable with those of active carbon (19.2 Wh/kg at 19.0 kW/kg [41]), mesoporous carbon (47 Wh/kg at 13 kW/kg [21]), graphene (49 Wh/kg at 18 kW/kg [38]), carbon nanosheets (45.5 Wh/kg at 0.75 kW/kg [32], 56.1 Wh/kg at 93.1 kW/kg [42]), nitrogen and sulfur co-doped porous graphene aerogel (100.7 Wh/kg at 0.94 kW/kg [43]), and nitrogen-doped carbon nanosheets (51 Wh/kg at 0.362 kW/kg [44]) in ionic liquid electrolytes. 4
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Fig. 3. CV curves of supercapacitors with GNF electrodes recorded at 5 mV/s (a). CV curves of supercapacitors with GNFs-15 electrodes recorded at 5–1500 mV/s (b). Specific capacitances of GNFs at 5–1500 mV/s (c). Galvanostatic charge-discharge profiles of supercapacitors with GNF electrodes tested at 1 A/g (d). Specific capacitances of GNFs at 0.5–15 A/g (e). Specific capacitance of GNFs recorded at 2 A/g versus cycle number (f). Ragone plots (g).
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Table 2 Specific capacitance of carbon-based electrodes. Carbon nanomaterials
Ionic liquid
Specific capacitance, F/g
Reference
GNFs-15
1.2 M NþEt4TFSI
This work
Carbon microtubes/ carbon nanotubes Single-walled carbon nanotubes Activated carbon
1 M [EMI]BF4 aqueous solution [EMIM]BF4
112 F/g (5 mV/s) 105 F/g (0.5 A/g) 60 F/g (10 A/g)
[35]
95.3 F/g (1 mV/s)
[36]
7.9 F/g (1 mA) 94.1 F/g 84 F/g (1 A/g) 135 F/g (0.5 A/g) 140 (5 mV/s)
[18]
Activated carbon Modified graphene Disordered carbon
[OMA][TFSI] [BMIM][Cl] [EMIM]Ac [EMIM]BF4 PYR14TFSI
[37] [38] [21]
The results of our work demonstrate that GNFs are a promising material for supercapacitors. The improved electrochemical perfor mance (high power and energy densities) of GNF-based supercapacitors is provided by the combination of both the mesoporous structure of GNFs and wide operating voltage of the RTIL electrolyte. The developed mesoporosity of GNFs facilitates migration of electrolyte ions into pores of the electrode material accelerating the formation of the electrical double layer and increasing the specific capacitance. 4. Conclusions In summary, we demonstrated a facile approach which allowed producing GNFs with controllable porous characteristics (pore size dis tribution, SSA, total pore volume, etc.). Depending on the synthesis time the synthesized GNFs with interconnected mesopores showed high BET SSA of 1720–660 m2/g. The specific capacitance of GNFs was found to increase non-linearly with SBET with the maximum capacitance of 112 F/ g being observed for GNFs with the highest SBET (shortest synthesis time). Thus, the high SBET of GNFs provides their high capacitance while the mesoporous structure of this material ensures the availability of the internal surface to the ionic liquid-based electrolyte, which results in the high rate capability and integrated power – energy performance of GNFsupercapacitors. Undoubtedly, the obtained results demonstrate the possibility of the effective control and optimization of the mesoporous structure of GNFs for supercapacitor applications. Declaration of competing interest 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. Acknowledgments The reported study was funded by RFBR research project 16–29–06439. The authors acknowledge support from Lomonosov Moscow State University Program of Development for providing access to the TEM and XPS facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109851. References [1] E. Frackowiak, Q. Abbas, F. B� eguin, Carbon/carbon supercapacitors, J. Energy Chem. 22 (2013) 226–240, https://doi.org/10.1016/S2095-4956(13)60028-5. [2] T. Chen, L. Dai, Carbon nanomaterials for high-performance supercapacitors, Mater. Today 16 (2013) 272–280, https://doi.org/10.1016/j.mattod.2013.07.002.
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graphene nanoflakes: synthesis, oxidation, and hydrothermal N-doping, Appl. Surf. Sci. 439 (2018) 371–373, https://doi.org/10.1016/j.apsusc.2018.01.059. S. Meenakshi, S. Jancy Sophia, K. Pandian, High surface graphene nanoflakes as sensitive sensing platform for simultaneous electrochemical detection of metronidazole and chloramphenicol, Mater. Sci. Eng. C 90 (2018) 407–419, https://doi.org/10.1016/j.msec.2018.04.064. S. Meenakshi, G. Kaladevi, K. Pandian, P. Wilson, Cobalt phthalocyanine tagged graphene nanoflakes for enhanced electrocatalytic detection of N-acetylcysteine by amperometry method, Ionics 24 (2018) 2807–2819, https://doi.org/10.1007/ s11581-017-2410-5. M. Thommes, K. Kaneko, A.V. Neimark, J.P. Olivier, F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing, Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report), Pure Appl. Chem. 87 (2015) 1051–1069, https://doi.org/10.1515/pac-2014-1117. D. Zhou, H. Wang, N. Mao, Y. Chen, Y. Zhou, T. Yin, H. Xie, W. Liu, S. Chen, X. Wang, High energy supercapacitors based on interconnected porous carbon nanosheets with ionic liquid electrolyte, Microporous Mesoporous Mater. 241 (2017) 202–209, https://doi.org/10.1016/j.micromeso.2017.01.001. M. Wu, P. Ai, M. Tan, B. Jiang, Y. Li, J. Zheng, W. Wu, Z. Li, Q. Zhang, X. He, Synthesis of starch-derived mesoporous carbon for electric double layer capacitor, Chem. Eng. J. 245 (2014) 166–172, https://doi.org/10.1016/j.cej.2014.02.023. L. Sun, C. Tian, M. Li, X. Meng, L. Wang, R. Wang, J. Yin, H. Fu, From coconut shell to porous graphene-like nanosheets for high-power supercapacitors, J. Mater. Chem. A. 1 (2013) 6462, https://doi.org/10.1039/c3ta10897j. Y. Zhou, J. Jin, L. Chen, Y. Zhu, B. Xu, Open-ended carbon microtubes/carbon nanotubes for high-performance supercapacitors, Mater. Lett. 241 (2019) 80–83, https://doi.org/10.1016/j.matlet.2019.01.052. C. Zheng, W. Qian, Y. Yu, F. Wei, Ionic liquid coated single-walled carbon nanotube buckypaper as supercapacitor electrode, Particuology 11 (2013) 409–414, https://doi.org/10.1016/j.partic.2012.07.013.
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Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Mesoporous graphene nanoflakes for high performance supercapacitors with ionic liquid electrolyte Ekaterina A. Arkhipova a, *, Anton S. Ivanov a, Konstantin I. Maslakov a, Alexander V. Egorov a, Serguei V. Savilov a, b, Valery V. Lunin a, b a
Department of Chemistry, Lomonosov Moscow State University, 1–3 Leninskiye Gory, Moscow, 119991, Russia Department of Physical Chemistry, Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, 31 Leninsky Prospect, Moscow, 119991, Russia
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene nanoflakes Supercapacitor Ionic liquid Mesoporous materials
Mesoporous graphene nanoflakes (GNFs) of different thickness with a specific surface area of 660–1720 m2/g were produced by the template synthesis via hexane pyrolysis over MgO. The structure and morphology of GNFs were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy and low-temperature N2 physisorption. The electrochemical performance of GNF-based supercapacitors was tested in ionic liquid electrolyte (1.2 M NþEt4TFSI in CH3CN) by cyclic voltammetry and galvanostatic charge – discharge mea surements. The assembled symmetric supercapacitors with a wide voltage window of 3 V delivered specific capacitance of 112 F/g at 5 mV/s and 105 F/g at 0.5 A/g with a high rate capability of ~81% at a current density of 15 A/g. The energy density reached 32.8 Wh/kg corresponding to the power density of 0.7 kW/kg. Further more, even at a high power density of 19.9 kW/kg, GNFs still showed an energy density of 24.3 Wh/kg, which made them promising for application in supercapacitors.
1. Introduction
results in quite low capacitance of activated carbon-based electrodes: 140–330 F/g in aqueous media [16] and <100 F/g in non-aqueous systems [18]. In contrast to activated carbon the major part of the gra phene sheet surface is external and readily accessible to electrolyte [11]. Currently, graphite oxide (GO) is extensively applied for production of graphene-like structures. However, materials produced by GO reduction show poor electrical conductivity because of the structural defects in graphene sheets [19]. Moreover, van der Waals interaction between graphene sheets leads to their stacking, which decreases SSA [17]. Recently, the template synthesis of carbon materials from organic and inorganic precursors has been shown to be an effective approach for production of carbon nanomaterials with desirable textural character istics by controlling the synthesis parameters (synthesis temperature and time, precursor type, template agent, etc.) [20–22]. This method allows avoiding the challenges caused by the oxidation and reduction steps of the GO-based synthesis of a graphene material. In addition, using a template is an effective strategy to better control the shape and size of nanostructured carbon materials [23]. The template approach is easily scalable and doesn’t require the prolonged and laborious stage of the catalyst synthesis.
Carbon-based supercapacitors have been attracting interest in last decades due to their fast charge/discharge ability, good reversibility, long cycle lifetime, low maintenance, excellent stability and low cost, which makes them promising energy storage devices for electronics, automobiles and portable energy systems [1–7]. The main charge stor age mechanism of supercapacitors is based on the energy accumulation either by pure electrostatic adsorption of ions on the electro de/electrolyte interface without charge transfer reactions [1,8] or by fast Faradaic redox reactions on the electrode surface [9,10]. The pore structure of carbon-based materials affects the ion and electron transfer and is therefore a crucial factor influencing their spe cific capacitance and rate capability. The high specific surface area (SSA) is generally believed to provide high specific capacitance of carbon-based electrodes [1,11,12]. Activated carbon is traditionally used as an electrode material in supercapacitors. Depending on the activation process and type of precursor its SSA can achieve 3000 m2/g [13–16]. However, despite the high SSA of activated carbon its abun dant micro-pores are barely accessible for electrolyte ions [17], which
* Corresponding author. E-mail address: [email protected] (E.A. Arkhipova). https://doi.org/10.1016/j.micromeso.2019.109851 Received 8 August 2019; Received in revised form 8 October 2019; Accepted 28 October 2019 Available online 31 October 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Ekaterina A. Arkhipova, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109851
E.A. Arkhipova et al.
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The cell voltage is known to significantly affect the specific energy and power density of a supercapacitor. The use of aqueous electrolytes (H2SO4, KOH, etc.) is restricted by their electrochemical stability to less than 1 V cell voltage [24]. On the other hand, the organic media based on the room temperature ionic liquids (RTILs) allows increasing the operating voltage up to 5–6 V [25]. Moreover, RTILs have high thermal stability, negligible vapor pressure and high ionic conductivity [24,26]. For the RTIL electrolyte solution based on NþEt4BF4 a pore size of 1.2–1.3 nm seems to be the optimal pore size in the carbon electrode material that well balances the high volumetric capacitance and high rate performance in the electrolyte [1,27]. These micropores are elec trochemically accessible for electrolyte ions to form the electrical double layer. In this work we demonstrate the facile approach to template syn thesis of highly sp2-hybridized graphene nanoflakes (GNFs) that allows effectively controlling their pore distribution and SSA. Synthesized GNFs have been characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), low temperature N2 physisorption, cyclic voltammetry (CV) and galvanostatic charge – discharge measurements. GNFs show high supercapacitive performance in the 1.2 M tetraethylammonium bis(trifluoromethylsulfonyl)imide (NþEt4TFSI ) ionic liquid solution in acetonitrile with a wide electro chemical stability window of 3 V.
2.3. Electrochemical performance of GNFs The electrochemical performance of GNFs was evaluated in the twoelectrode symmetrical supercapacitor. The coating slurry was mixed from 80 wt% of GNFs, 10 wt% of polyvinylidene fluoride binder, and 10 wt% of Super P solution in N-methyl-2-pyrrolidone (NMP). Elec trodes were produced by spreading the obtained suspension with a spatula on a Ni-foam disk of 10 mm in diameter followed by drying at 120 � C for 72 h to remove NMP and moisture. Coin-cells were assembled in an argon filled glove box. 1.2 M NþEt4TFSI ionic liquid (>99.0%, Sigma Aldrich) solution in acetonitrile (99.8%, Sigma Aldrich) was used as an electrolyte. The concentration of the solution was chosen based on our previous results and corresponded to the highest conductivity of the NþEt4TFSI solution in acetonitrile [26]. Two identical electrodes separated face-to-face by a glass microfiber filter (Whatman) were impregnated with the electrolyte and assembled in a coin-cell using an MTI electric coin cell crimper. Cyclic voltammetry (CV) and galvano static charge – discharge (CD) measurements were carried out using a Biologic VSP 219 research-grade multichannel potentiostat/galvanostat (Bio-Logic Science Instruments SAS). The specific capacitance CCV (F/g) was calculated from the CV curve according to the following equation: R 2 ivdv CCV ¼ (1) μmΔV
2. Experimental
where i (A) and v (V) are, respectively, the current and the potential in a CV test, μ (V/s) is the scan rate, m (g) is the active mass in one electrode, ΔV (V) is the potential window. The specific capacitance CCD (F/g) was calculated from the galvanostatic charge – discharge curve as:
2.1. Synthesis of GNFs GNFs were synthesized by chemical vapor deposition according to the previously described technique using MgO as a template [28]. MgO was produced by mixing of the equimolar solutions of magnesium ni trate hexahydrate (98%, Sigma–Aldrich) and ammonium oxalate (�99.5%, Fluka). For GNF synthesis the MgO template was placed in the center of a horizontal Carbolite TZF 12/100/900 (d ¼ 55 mm) tube furnace and heated in a nitrogen flow up to 800 � C. Then hexane (99.9%, Reakhim) vapors used as a carbon precursor were passed over the MgO template (SBET ¼ 140 m2/g) in a nitrogen flow of 1000 ml/min for 15, 30, 60 and 90 min. After cooling to room temperature, the synthesized GNFs were boiled in 10 wt% hydrochloric acid to remove the template followed by washing in distilled water to neutral pH. Amorphous carbon impurities in GNFs were removed by annealing in air for 2 h at 350 � C. The yield of the synthesis was about 14% based on the weight of used hexane. The synthesized samples were named as GNFs-t, where t is synthesis time in minutes.
CCD ¼
4I mdV=dt
(2)
where m (g) is the active mass in both electrodes, I (A) is the discharge current and dV=dt is the slope of the discharge curve after ohmic drop (IR). In the Ragone plot, the specific energy density E(Wh/kg) and power density P (kW/kg) were calculated based on the following equations: E¼
CCD � ΔV 2 8 � 3:6
(3)
P¼
E t=3:6
(4)
The supercapacitors were also measured by electrochemical imped ance spectroscopy (EIS) in the frequency range from 400 kHz to 100 mHz at 10 mV amplitude.
2.2. Material characterization
3. Results and discussion
The morphology of the produced materials was investigated by TEM using a JEM 2100 F/Cs microscope operated at 200 kV and equipped with an aberration corrector (CEOS). The average lateral size and thickness of GNFs were calculated by analysis of at least 100 TEM im ages. Low temperature nitrogen physisorption measurements were performed on an Autosorb-1 gas sorption analyzer (Quantachrome, USA). Prior to measurement the sample was degassed in vacuum at 250 � C for 3 h. The BET SSA (SBET) was calculated at seven points of the adsorption isotherm in the p/p0 range of 0.15–0.3. The error in the calculation of SBET didn’t exceed 10%. Pore size distributions were calculated from the adsorption branches of isotherms by QSDFT method using the slit pore model for pore size <2 nm and the cylindrical pore model for pore size >2 nm. XPS spectra were acquired on a Kratos Axis DLD spectrometer with monochromatic AlKα radiation. The pass energy of the analyzer was 160 eV for survey spectra, and 40 eV for highresolution scans.
The effect of synthesis time on the morphology of GNFs was studied by TEM. GNFs replicate the shape of MgO particles used as a template and consist of graphene layers with bent edges (Fig. 1). This shape of GNF particles leads to the interconnected pore network in the synthe sized material. The thickness of GNFs increased with the synthesis time from 2.0 � 0.2 nm (GNFs-15) to 3.2 � 0.2 nm (GNFs-90), indicating the increase in the number of graphene layers in GNFs. At the same time no significant changes in the morphology and shape of GNFs were observed. Interestingly, the average lateral size of GNFs (10–50 nm) is significantly lower than the size of similar materials [29,30]. Never theless, the observed interconnected pore network in GNFs can provide favorable pathways for the penetration and migration of electrolyte ions. The chemical state of GNFs was studied by XPS. Only carbon and oxygen lines are observed in the survey spectra (Fig. S1 in Supplemen tary Materials). The oxygen content in GNFs (Table 1) decreases with increasing the synthesis time from 2.1 (GNFs-15) to 1.0 (GNFs-90) at. %. Small amount of oxygen probably results from the oxygen-containing 2
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Fig. 1. TEM images of GNFs-15 (a), GNFs-30 (b), GNFs-60 (c), and GNFs-90 (d).
with each other produces more compact material with larger contact area between GNFs, which results in the significant networking effects, such as ink-bottle pores. At higher synthesis times thicker and as a result more rigid GNFs are produced. The contact area between them reduces and pores become more accessible, which diminishes the networking effects. This conclusion is confirmed by the growth of the pore volume with the increase in the synthesis time from 15 to 60 min (Table 1). The decrease in the pore volume at higher synthesis times may be explained by the growth of the mass of each GNF without significant changes in its size and shape. It is worth noting that there is a small fraction of mi cropores in GNFs-15 and GNFs-30 that completely disappears in GNFs-60 and GNFs-90 (Fig. 2b). However, all GNF samples don’t contain very narrow pores (<1 nm) that are known to hinder the transport of large electrolyte ions deep into pores of the electrode material [32]. The electrochemical performance of GNF-based supercapacitors was evaluated using cyclic voltammetry and galvanostatic charge-discharge measurements. Fig. 3a and b shows the CV curves recorded at room temperature at different scan rates (5–1500 mV/s). Among all studied GNFs the CV profile of the supercapacitor with GNFs-15 based elec trodes (Fig. 3a) exhibits the highest current density, indicating the highest specific capacitance of this material. At low scan rates the vol tammograms are almost rectangular that is typical for capacitive behavior of the electrodes. Observed distortion of the voltammograms with increasing the scan rate is a well-known effect in electrochemical supercapacitors caused by the decrease in ion mobility in pores of car bon electrodes at high scan rates. Indeed, the specific capacitance of GNFs calculated by the integration of CV curve area decreases with scan rate (Fig. 3c). This trend reflects the growth of ion transport resistance in mesopores [32]. The capacitance of GNFs-15 achieves the highest value of 112 F/g at 5 mV/s while it doesn’t exceed 98 F/g for GNFs-30, 72 F/g
Table 1 XPS oxygen content and textural parameters of GNFs. Sample
OXPS, at.%
Vmicro, cm3/ga
Vmeso, cm3/ga
VDFT, cm3/ga
SDFT, m2/ga
SBET, m2/gb
GNFs15 GNFs30 GNFs60 GNFs90
2.1
0.15
2.38
2.53
1770
1720
1.3
0.05
2.82
2.87
1200
1230
1.2
0.00
3.14
3.14
870
900
1.0
0.00
2.34
2.34
630
660
a b
Pore volumes and SSA were calculated from DFT pore size distributions. SSA was calculated using the Brunauer-Emmett-Teller (BET) method.
groups bonded to surface defect sites of GNFs (vacancies, layer edges, etc.). Trace amount of MgO from the template may also contribute to the oxygen signal. The textural properties of GNFs were studied by low temperature N2 physisorption. The nitrogen adsorption–desorption isotherms (Fig. 2a) correspond to the combination of type II and type IV isotherms attrib uted to macro- and mesoporous materials, respectively [31]. The sharp step-down of the desorption branch of the GNFs-15 isotherm in the p/p0 range of 0.4–0.5 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]. With increasing the synthesis time the contribution from the cavitation-induced desorption decreases and completely disappears in the isotherm of GNFs-60. Probably, nanoflakes in GNFs-15 are not rigid because the number of graphene layers in these nanoparticles is low. Their deformation under interaction 3
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Fig. 2. Nitrogen adsorption–desorption isotherms (a) and normalized cumulative pore volume distributions (b) of GNFs.
for GNFs-60, and 46 F/g for GNFs-90. The similar trend was previously observed for starch-derived mesoporous carbon. M. Wu et al. showed that the main factor affecting the specific capacitance was mesoporosity of the sample and found that the increase in the fraction of mesopores led to the growth of capacitance [33]. D. Zhou et al. concluded that the high mesopore fraction shortened the diffusion distance normal to the plane of nanosheets and facilitated the smooth transport of ions at high rates [32]. At high scan rates the specific capacitance of GNFs-15 de creases faster than those of other GNFs and at 1500 mV/s it even be comes smaller than that of GNFs-30. This fact can be explained by the high resistance of electrolyte diffusion in ink-bottle pores and micro pores of GNFs-15 at high scan rates. The galvonostatic charge-discharge curves recorded at 1 A/g (Fig. 3d) demonstrate nearly triangular shape with small IR drops, which testifies the superior reversible electrochemical performance. At a cur rent density of 0.5 A/g the specific capacitance of GNFs-15 calculated from the discharge slope (105 F/g) is also higher than those for GNFs-30 (88 F/g), GNFs-60 (64 F/g), and GNFs-90 (37 F/g), which agrees with the CV data. The EIS results also confirm this trend. The low-frequency region of the Nyquist plot for the supercapacitor with GNFs-15 elec trodes (Fig. S2) demonstrates the steepest growth which is characteristic of better capacitive behavior [34]. Note that with increasing the current density up to 15 A/g the specific capacitance of GNFs-15 still remains as high as ~81% of its initial value at 0.5 A/g, while the specific capaci tance of GNFs-90 achieves only ~74% of the specific capacitance retention (Fig. 3e). This decrease in the specific capacitance with the increase in current density can be attributed to the growth of charge transfer resistance within micro/mesopores. The specific capacitance of GNFs-15 is higher or comparable to those reported in the literature for supercapacitors with ionic liquid electrolytes (Table 2). The electrochemical cycling stability of GNF-based supercapacitors was tested by charge-discharge measurements at 2 A/g (Fig. 3f). After 5000 cycles GNFs-15, GNFs-30, GNFs-60, and GNFs-90 show only the slight loss of capacitance of 11.8, 6.9, 6.5, and 5.4%, respectively. SBET of GNF materials decreased with synthesis time from 1720 m2/g (GNFs-15) to 660 m2/g (GNFs-90) (Table 1) as a result of the growth of the GNF thickness. The specific capacitance is known to be proportional to the specific surface area Sof the electrode/electrolyte interface and to electrolyte permittivity ε as C ¼ εdS [24] where d is the effective thickness
of the double layer. Therefore, the increase in the SSA should propor tionally increase the capacitance C. However, the experimental dependence of C vs. SBET is not linear (Fig. S3). Similar dependence was also observed for supercapacitors based on aero/cryo/xerogel carbon electrodes with the PYR14TFSI ionic liquid [21]. Indeed, the effective ion size of NþEt4 is larger in acetonitrile solution (1.30 nm [39]) than in the pure ion liquid (0.67 nm [39]) because of the formation of the sol vated shell. According to the N2 physisorption results the GNFs-15 ma terial predominantly contains mesopores with the small contribution of micropores the size of which may be comparable with the size of elec trolyte ions. For these micropores the removal of the solvation sphere is needed to achieve the effective energy storage. In addition, the presence of ink-bottle pores can introduce diffusion limitations. P. Simon, Y. Gogotsi, and coworkers studied supercapacitors based on carbide-derived carbon with EMI–TFSI electrolyte and concluded that the pore size leading to the maximum double-layer capacitance was very close to the ion size [40]. On other hand, micropores can increase the ohmic resistance at high charge–discharge rates because of the higher diffusion losses and less effective ion adsorption [3]. Thus, the high SSA of the electrode material is necessary to achieve the large specific capacitance, while the morphology of pores determines the availability of the electrode surface to electrolyte ions. The Ragone plot (Fig. 3g) shows the relation between the energy and power densities of a supercapacitor. The sum of the active mass in both electrodes was used for the calculation. At 0.5 A/g GNFs-15 exhibit the highest energy density of 32.8 Wh/kg higher than that of GNFs-30 (27.3 Wh/kg), GNFs-60 (19.9 Wh/kg), and GNFs-90 (11.4 Wh/kg). Furthermore, even at high power density of 19.9 kW/kg GNFs-15 still deliver the energy density of 24.3 Wh/kg. These results highlight the high integrated power – energy properties of GNFs-15 in contrast to other GNF samples with lower SSA and less developed porosity. The energy and power characteristics of GNFs-based electrodes are also comparable with those of active carbon (19.2 Wh/kg at 19.0 kW/kg [41]), mesoporous carbon (47 Wh/kg at 13 kW/kg [21]), graphene (49 Wh/kg at 18 kW/kg [38]), carbon nanosheets (45.5 Wh/kg at 0.75 kW/kg [32], 56.1 Wh/kg at 93.1 kW/kg [42]), nitrogen and sulfur co-doped porous graphene aerogel (100.7 Wh/kg at 0.94 kW/kg [43]), and nitrogen-doped carbon nanosheets (51 Wh/kg at 0.362 kW/kg [44]) in ionic liquid electrolytes. 4
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Fig. 3. CV curves of supercapacitors with GNF electrodes recorded at 5 mV/s (a). CV curves of supercapacitors with GNFs-15 electrodes recorded at 5–1500 mV/s (b). Specific capacitances of GNFs at 5–1500 mV/s (c). Galvanostatic charge-discharge profiles of supercapacitors with GNF electrodes tested at 1 A/g (d). Specific capacitances of GNFs at 0.5–15 A/g (e). Specific capacitance of GNFs recorded at 2 A/g versus cycle number (f). Ragone plots (g).
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Table 2 Specific capacitance of carbon-based electrodes. Carbon nanomaterials
Ionic liquid
Specific capacitance, F/g
Reference
GNFs-15
1.2 M NþEt4TFSI
This work
Carbon microtubes/ carbon nanotubes Single-walled carbon nanotubes Activated carbon
1 M [EMI]BF4 aqueous solution [EMIM]BF4
112 F/g (5 mV/s) 105 F/g (0.5 A/g) 60 F/g (10 A/g)
[35]
95.3 F/g (1 mV/s)
[36]
7.9 F/g (1 mA) 94.1 F/g 84 F/g (1 A/g) 135 F/g (0.5 A/g) 140 (5 mV/s)
[18]
Activated carbon Modified graphene Disordered carbon
[OMA][TFSI] [BMIM][Cl] [EMIM]Ac [EMIM]BF4 PYR14TFSI
[37] [38] [21]
The results of our work demonstrate that GNFs are a promising material for supercapacitors. The improved electrochemical perfor mance (high power and energy densities) of GNF-based supercapacitors is provided by the combination of both the mesoporous structure of GNFs and wide operating voltage of the RTIL electrolyte. The developed mesoporosity of GNFs facilitates migration of electrolyte ions into pores of the electrode material accelerating the formation of the electrical double layer and increasing the specific capacitance. 4. Conclusions In summary, we demonstrated a facile approach which allowed producing GNFs with controllable porous characteristics (pore size dis tribution, SSA, total pore volume, etc.). Depending on the synthesis time the synthesized GNFs with interconnected mesopores showed high BET SSA of 1720–660 m2/g. The specific capacitance of GNFs was found to increase non-linearly with SBET with the maximum capacitance of 112 F/ g being observed for GNFs with the highest SBET (shortest synthesis time). Thus, the high SBET of GNFs provides their high capacitance while the mesoporous structure of this material ensures the availability of the internal surface to the ionic liquid-based electrolyte, which results in the high rate capability and integrated power – energy performance of GNFsupercapacitors. Undoubtedly, the obtained results demonstrate the possibility of the effective control and optimization of the mesoporous structure of GNFs for supercapacitor applications. Declaration of competing interest 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. Acknowledgments The reported study was funded by RFBR research project 16–29–06439. The authors acknowledge support from Lomonosov Moscow State University Program of Development for providing access to the TEM and XPS facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109851. References [1] E. Frackowiak, Q. Abbas, F. B� eguin, Carbon/carbon supercapacitors, J. Energy Chem. 22 (2013) 226–240, https://doi.org/10.1016/S2095-4956(13)60028-5. [2] T. Chen, L. Dai, Carbon nanomaterials for high-performance supercapacitors, Mater. Today 16 (2013) 272–280, https://doi.org/10.1016/j.mattod.2013.07.002.
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