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Pseudocapacitance induced candle soot derived carbon for high energy density electrochemical supercapacitors: Non-aqueous approach
Journal of Energy Storage 27 (2020) 101114
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Pseudocapacitance induced candle soot derived carbon for high energy density electrochemical supercapacitors: Non-aqueous approach Darshna Potphode, Chandra Shekhar Sharma
T
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Creative & Advanced Research Based On Nanomaterials (CARBON) Laboratory, Department of Chemical Engineering, Indian Institute of Technology, Kandi, Hyderabad 502285, Telangana, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon Candle soot Ionic liquid Organic electrolyte Non-aqueous Supercapacitors
The commercialization aspect in electric double layer capacitors is highly dependent on source of carbon, its purity, and the specific capacitance with unique deliverable for proficient charge storage. Here, we report synthesis of interconnected mesoporous spherical carbon by direct flame synthesis using candle wax for symmetric supercapacitors with the non-aqueous approach. The heat treated candle soot (CST) at 450 °C results in a high purity carbon with the improved surface area of 608 m2 g−1 also the majority of pores in the range of 1–10 nm, which is ideal for charge storage in supercapacitors. The highest capacitance of 64 and 88.2 F g−1 at 0.1 A g−1 was obtained for 1 M TEABF4 in acetonitrile and EMIM BF4, respectively. A stability up to 82–85% can be achieved for 5,000 charge-discharge cycles. CST as a potent electrode material successfully delivered highest specific energy of 14 and 28 Wh kg−1 and specific power of 12.5 and 15 kW kg−1 in 1 M TEABF4 in ACN and EMIM BF4 electrolytes, respectively.
1. Introduction To satisfy ever-growing demand for energy a resourceful solution need more emphasis to provide prolific energy requirement. The effective storage of energy from sustainable sources can be a solution for necessity and massive requirements [1,2],. Supercapacitors are one of the promising energy storage technologies due to its enormous applications in various fields such as automotive, defense, toys, portable electronics, etc. [3]. Supercapacitors are also known as ultracapacitors due to its fast charge-discharge competencies. Based on the energy storage mechanism, supercapacitors are categorized into three subcategories [4]. First, electric double layer capacitors (EDLCs) which can store charges at the surface of the electrode materials by electrostatic interactions between electrode and electrolyte interface, generally high surface area carbon materials are being used in this category [5]. Second, pseudocapacitors where the charge storage takes place by surface redox reactions, which includes the electrode materials such as conducting polymers and some metal oxides with neutral electrolytes [6]. The third category involves hybrid supercapacitors (battery-type), which are composed of metal oxides/sulfides and the charge storage takes place with bulk utilization of electrode material by redox reactions [7]. Due to electrostatic charge storage in EDLCs, it possesses high power densities and longer charge-discharge cycling stability than the pseudocapacitors and hybrid capacitors, however, the energy density is ⁎
limited in case of EDLCs which hinders their commercialization [8]. Recent research focuses on improving the energy density of EDLCs without compromising on power density and electrochemical cycling stability. Several reports on carbon materials with high surface area such as biomass-derived carbon [9–11], organic precursors derived carbon [12], activated carbon [13], carbon nanotubes [14], graphene [15,16], and fullerenes [17] are reported as electrode materials for EDLCs [18]. Although carbon nanotubes, graphene, and fullerenes have higher electrical conductivity as compared to other forms of carbon, the specific capacitance obtain from those is less than the activated carbon because of lower electrochemical accessible surface area and lack of porosity in the material [12,19]. The electrochemical surface area of electrode material is the key parameter in charge storage more specifically in supercapacitors and can be achieved by KOH activation or various template assisted methods to improve the accessibility of electrolyte ions to the innermost part of the electrodes [20]. A lot of efforts has been made to improve upon these factors, for example, partial exfoliation of multiwalled carbon nanotube (MWCNTs) was reported by Sivaraman et al. [21] showed the tremendous increase in surface are of MWCNTs and hence the specific capacitance got increased to several orders. Exfoliation of graphite to form graphene oxide and reduced graphene oxide is another example of increasing specific charge storage thereby enhancing the electrochemical accessible surface area [22]. The
Corresponding author. E-mail address: [email protected] (C.S. Sharma).
https://doi.org/10.1016/j.est.2019.101114 Received 8 June 2019; Received in revised form 23 November 2019; Accepted 24 November 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Energy Storage 27 (2020) 101114
D. Potphode and C.S. Sharma
soot, the sample was heated in air at 450 °C for 2 h.
development of the high surface area and optimal porosity in the carbon is still on a focal point for researchers, to that end, organicinorganic precursors such as polymers, hydrocarbons, metal carbides, metal-organic frameworks, and template-assisted synthesis are being used to obtain ideal carbon structures for electrochemical charge storage [23,24]. But the above-mentioned techniques are expensive, complex, multi-step methods and also the carbon yield is very low. Direct synthesis of carbon from wax by wick supported combustion is a facile and promising method to obtain a pure form of carbon at the nano level. A simplistic and economical synthesis method for carbon nanomaterial still holds great prospect. Candle soot derived carbon is one of the ideal alternatives to get a pure form of carbon by direct flame combustion of wax. Due to its nano size particles and easy electrode fabrication method, candle soot derived carbon is being used in energy storage application. Zhang et al. reported core-shell MnO2@candle soot nanocomposite material for which the specific capacitance obtained was 309 F g−1 in aqueous 1 M Li2SO4 solution, for which bare candle soot showed 106 F g−1 at a current density of 1 A g−1 [25]. The nitric acid functionalized candle soot has been reported by Raj et al. showed a specific capacitance of 187 F g−1 at a current density of 0.15 A g−1 [26]. Being nanoparticles of size 20–50 nm, candle soot derived carbon has been utilized in making nanocomposites with polyaniline and MnO2, which are further explored for supercapacitor application with a highest specific capacitance of 140 F g−1 at the current density of 1 A g−1 [27]. However, these reports are limited to the aqueous-based supercapacitors with the maximum potential window of 1 V and hence candle soot derived carbon can be further explored for high energy density energy storage possibilities. The non-aqueous approach for carbon nanomaterial-based supercapacitors in combination with organic electrolytes and ionic liquids comes under the category of high energy density EDLCs. These non-aqueous approach offers a higher operating potential range from 2.5 to 4 V thereby enhancing the specific energy of the EDLCs. In this work, we report a single step synthesis of carbon soot from candle wax having a particle size of 20 −50 nm with optimal porosity in the material to achieve high specific energy supercapacitive performance. This is the first time report for candle soot derived carbon with the nonaqueous approach by means of 1 M tetraethylammonium tetrafluoroborate (TAEBF4) in acetonitrile (ACN) and pure 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIM BF4) and also renders the outstanding electrochemical charge storage performance. The detail electrochemical exploration of CST as an active material for supercapacitor application is highlighted in essence.
2.3. Electrode fabrication CST electrodes were fabricated by solution drop cast method, in detail, a mixture of active material (8 mg) and conducting carbon (1 mg) was dispersed in 2 mL of IPA for 10 min and then 5% PTFE solution added to the suspension to yield uniform dispersion of carbon in IPA with a binder. The ratio of active material: conducting carbon: binder was maintained at 80: 10: 10. The solution mixture then dropcasted over round cut carbon paper of 15 mm diameter (with the circular area of 1.766 cm2) and vacuum dried at 80 °C for 12 h. The total active mass loading of active material on each electrode was 2 mg. 2.4. Structural characterizations The structural and morphological investigation of CST was performed by field emission scanning electron microscopy (FESEM) with SUPRA 40 Zeiss India and transmission electron microscopy (TEM) with FEI Tecnai G2S-Twin operated at 200 kV. Powder X-ray diffraction (XRD) was performed by using a powder X-ray diffractometer with CuKα (λ = 1.5406 Å) radiation over a 2θ range of 7˚ to 80˚. The elemental composition of the material was determined by X-ray photoelectron spectroscopy (XPS) technique using the Omicron Nano Technology instrument. The structural features of the sample were studied by Raman analysis with Bruker Raman microscope spectrometer with an excitation wavelength of 532 nm. The Brunauer-Emmett-Teller (BET) surface area and pore size distribution measurement were carried out by Micromeritics ASAP 2020 analyzer. Fourier transform-infrared (FT-IR) was performed over Tensor 37, Bruker USA, KBr pellets were prepared by maintaining an equal amount of carbon ratio with respect to KBr (0.2:100 wt/wt). The hydrophobicity of the CS and CST samples were studied in triplicate using a sessile drop method with a goniometer (Rame-Hart, USA; Model: 290- F4). 2.5. Electrochemical measurements The coin cell assembly (CR 2032) of symmetric supercapacitors were fabricated in the glovebox with the help of crimper by sandwiching two symmetric CST electrodes between Glass microfiber filter separator. All electrochemical studies such as cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) measurement and electrochemical impedance spectroscopic (EIS) analysis were done using Biologic Potentiostat (VMP3). The electrochemical measurements using TAE BF4 in ACN was performed in the potential window of 0–2.5 V and for ionic liquid (EMIM BF4) the potential window of 0–3 V was maintained. The EIS measurement of supercapacitors was carried out in the frequency range of 1 MHz to 10 mHz at the cell potential of 1 V.
2. Experimental 2.1. Materials 1-Ethyl-3-methylimidazolium tetrafluoroborate, acetonitrile (ACN), and Glass microfiber filters (Whatman, Grade GF/D) were purchased from Sigma-Aldrich and used as received. Carbon paper was purchased from Toray, Japan. Polytetrafluoroethylene (PTFE) suspension was obtained from Hindustan Fluorocarbons Ltd, India. Isopropyl alcohol (IPA) was purchased from SRL Pvt. Ltd. India. Acetylene black (Super P conductive, 99+% metal basis) was purchased from Alfa Aesar. Tetraethylammonium tetrafluoroborate (TEA BF4) salt was obtained from Tatva Chintan Pharma Chem Pvt. Ltd. India.
3. Results and discussion 3.1. Physico-chemical characterization The process of thermal treatment in air or in presence of oxygen to remove amorphous carbon and other carbonaceous impurities formed during synthesis has already been applied in the purification of various carbon nanomaterials [28]. Which also helps in improving the surface area by adding surface oxygen functionalities to the carbon. After successful synthesis of the bare candle soot (CS), the heat treatment was employed to yield high purity carbon and the sample was named as CST. The treatment in the air added the oxygen functionalities to the CS, Fig. 1 Shows the comparative FT-IR spectra of CS and CST (before and after heat treatment). The peaks at 800 and 2919 cm−1 for CeH bond present in the CS due to the presence of wax are minimized after heat treatment. The peak intensity for the C = C bond is increased as compared to CS sample which indicates impurities has been removed at
2.2. Synthesis of candle soot The wax candles were purchased from the local market of Sangareddy, Telangana-India and used as it is. The interconnected carbon nanoparticles of candle soot were synthesized by a single step flame combustion method. Carbon soot was collected over a glass substrate by placing above the flame tip. Further to remove unreacted wax and small particles of carbon impurities present in bare candle 2
Journal of Energy Storage 27 (2020) 101114
D. Potphode and C.S. Sharma
Fig. 3. A comparative XRD plot for CS and CST.
Fig. 1. Comparative FT-IR spectra of CS and CST, spectra recorded by maintaining an equal amount of carbon in KBr.
and ID/IG ratio of 1.12 indicates the presence of more defects in the carbon structure of CS. Whereas, Raman spectra of CST shows little sharp D band with lower intensity and improved ID/IG ratio of 1. As expected after heat treatment removal of dangling bonds and unreacted wax improves the carbon quality in CST. Furthermore, the G’ band at around ~2700 cm−1 is corresponding to two-phonon double resonance Raman scattering process which is characteristic band to confirm the layered structure in graphene [32]. The previous reports say, as the layers of carbon or graphene increases the G’ band shifts to the higher wavenumber value. The G’ band is the overtone of first-order D band therefore in some of the reports this band is labeled as 2D band or D’ band [33]. In this study, the broadening and shifting of G’ band to higher range of wavenumber confirms the disordered multilayered carbon in both the samples. The XRD plot for CS and CST is given in Fig. 3. Two major peaks at 2θ value of 25.7 and 43.8 are indexed to (002) and (100) plane of carbonaceous materials [34]. However shift in (002) plane from 26 towards the lower value of 2θ (25.7) shows the presence of small turbostratic carbon traces in the sample [35]. It is well known that the peak position of (002) plane identifies the spacing of aromatic ring layer in the graphitic carbon. The candle soot derived carbon also contains the graphitic structures or carbon crystallites which are confirmed by the presence of (002) plane at ~26° and 25.7° in both the
450 °C. However, the presence of sharp peaks at 1260 cm−1 and 1740 cm−1 indicates additional CeO functionalities after heat treatment and can be assigned to CeO and C=O bonds, respectively [26,29]. Therefore, the purity of carbon with additional surface functionality can be maintained over carbon structures by simple heat treatment in air. In addition to that, the heat treatment also reduced the hydrophobicity of CS which was confirmed by contact angle measurement with water. CS showed contact angle of 113.6 ± 0.2° which implies the hydrophobicity in the sample owing to its wax content. However, CST showed contact angle of 46.2 ± 0.5° which confirms the hydrophilicity in the sample after heat treatment. Comparative images for water contact angle measurement is given in Fig. S1 while the thermal stability is discussed in Fig. S2. Fig. 2(a) and (b) represents the Raman spectra of the CS and CST samples, respectively, with a highlight on characteristic disordered carbon structure. The G band present at around 1588 cm−1 due to E2g phonon vibrations of sp2 carbon atoms [30]. The peak present at 1348 cm−1 indicates the D band or defects present in the carbon materials. The broadening, shifting, and relative intensity of D band depends on the nature of impurities or dopant or functional groups present in the carbon matrix [31]. Therefore, the pure form of graphite does not exist D band in Raman spectra. The broaden G band (Fig. 2(a)) in the CS case
Fig. 2. (a) Raman spectra of CS, and (b) Raman spectra of CST. 3
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Fig. 4. (a) FESEM image, (b) low magnification TEM image, (C) and (d) are high magnification TEM images of CST.
obtained for CS is ~105 m2 g−1 which is very less for supercapacitive charge storage (Fig. S3). However, heat treated sample showed the specific surface area of 608 m2 g−1 which identifies the suitability of the material for better charge storage electrode material for EDLCs. Fig. 5(b) shows the pore size distribution obtained from the N2 adsorption-desorption isotherm. The pore size distribution is higher in the range of 1–10 nm; a small fraction of macroporous contribution in the range of 30–80 nm is due to the space between interconnecting carbon spherical. Based on previous reports, the optimal pore size matching with electrolyte ion or double the size of electrolyte ion are more likely to store apparent charges on the surfaces [37]. The inset of Fig. 5(b) confirms the pore volume density majorly in the mesoporous range. From the BET, TEM, and FESEM data it is confirmed that the CST sample contains a large fraction of mesoporous and small fraction of microporous density. The interconnecting carbon spherical in CST sample contributing for macrospores and mesopores will be accommodating for fast diffusion of electrolytic ions in the active sites of the electrodes. The carbon nanostructures reported for supercapacitor energy storage synthesized by flame combustion method are listed in a comparative Table 1. Fig. 6(a) and (b) represents XPS spectra of C1s and O1s clearly confirming the chemical composition of CST. After the de-convulsion of individual spectra of elements, the percent of carbon and oxygen was calculated and found to be 84.5% of carbon and 15.5% of oxygen. The core spectra of C1s shows three peaks at 284.2, 285.2, and 288.5 eV
samples (CS and CST). Along with (002) plane, the presence of weak band at 43.8° for (100) plane also confirms the graphitic structures in candle soot carbon. Based on the observations, the carbon crystallites present in the candle soot carbon are of intermediate structures of graphite and amorphous carbon or random layer lattice structures. According to literature, the shift in (002) plane to higher 2θ value indicates the increase in elemental carbon content, and shift in (002) plane to lower 2θ value indicates the increase in randomness in graphitic alignment [36]. The FESEM image shown in Fig. 4(a) clearly indicates the interconnectivity between spherical particles of CST. Further, Fig. 4 (b, c, and d) confirms the particle size of the CST sample is in the range of 20–50 nm. These nano size particles, as well as wellmaintained interconnectivity in carbon, is beneficial for optimum charge storage in EDLCs. Due to the nano size of the carbon, diffusion path length will be shorter and diffusion of electrolyte will be merely easy and effective at the electrode/electrolyte interface. Besides, the interconnectivity of CST will improve electrical conductivity by decreasing charge transfer resistance at the electrode, and hence supercapacitor performance can be improved at the electrode level. The specific surface area and porosity of the CST were analyzed in detail by N2 gas adsorption-desorption isotherm which is shown in Fig. 5. The type IV isotherm (Fig. 5(a)) indicates that the material contains mainly a mesoporous structure, additionally, small hysteresis loop present in the BET plot indicates the capillary condensation that occurs in the mesoporous materials. The BET specific surface area
4
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Fig. 5. (a) N2 adsorption-desorption isotherm for CST, (b) the pore size distribution and cumulative pore volume analysis for CST sample.
which corresponds to sp2 hybridized carbon (C=C), sp3 hybridized carbon (CeC), and OeC=O, respectively [41]. There are two peaks for oxygen (O1s- Fig. 6(b)) at 531.1 and 532.6 eV which corresponds to C=O and =CeO bonds, respectively [12]. In EDLCs, the sufficient oxygen content in the carbon materials helps for the better wettability of the electrodes. At the same time, the oxygen functionalities at the carbon surface involve in the pseudocapacitive charge storage to enhance the specific capacitance. A high degree of sp2 hybridized carbon in CST will be effective to maintain good electrical conductivity at the electrode.
to mass loading of active material. Further nano size carbon structure will have a smaller diffusion path length which is more beneficial to decrease internal resistance in the system. The slanted CV curves of CST in EMIM BF4 electrolyte (Fig. S5(a)) at higher scan rates are due to high electrolytic resistance associated with the viscous composition of pure ionic liquids compared to the diluted organic electrolytes. The higher scan rates lead to insufficient time for electrolyte ions to diffuse inside the mesopores of electrode materials which causes more resistance and hence overall resistance of the cell increases. Fig. 7(b) represents the rate performance of the CST in both the electrolytes at different scan rates, when scan rate increase from 5 to 100 mV s−1, CST retained ~74% and ~67% of specific capacitance in 1 M TEABF4 in ACN and EMIM BF4, respectively. The specific capacitance of CST was calculated from galvanostatic charge-discharge (GCD) analysis method. Fig. 7(c) represents GCD curves of CST at a current density of 0.5 A g−1 in both the electrolytes, current density applied in GCD analysis are based on the active mass of both the electrodes. The GCD curves of CST in 1 M TEABF4 in ACN and EMIM BF4 at different current densities are given in Fig. S4(b) and Fig. S5(b), respectively. Again the triangular shape of GCD curves confirms the EDLC type charge storage at electrodes. The gravimetric specific capacitance is calculated by using the formula given in Eq. (1) [7].
3.2. Electrochemical performance As the energy density is limited for the aqueous system in supercapacitors, therefore candle soot derived carbon was tested with two non-aqueous electrolytes such as 1 M TEABF4 in ACN and EMIM BF4. All the measurements were carried out in two electrode system in symmetric combinations. Fig. 7(a) shows the cyclic voltammograms of CST in both the electrolytes i.e. 1 M TEABF4 in ACN and ionic liquid electrolyte at a scan rate of 25 mV s−1. CV shows rectangular shape which can be related to the electric double layer formation at the electrode/electrolyte interface and confirms the absence of redox reactions at electrode [5]. There is a slight deviance of rectangular shape CV indicating the dominance of pseudocapacitance due to oxygen functionalities present on the carbon structures [38]. The comprehensive CV profile at different scan rates in both the electrolytes is given in Fig. S4(a) and S5(a). At higher scan rates, the rectangular shape of CV specifies the ease of charge storage in the carbon structure due to the presence of mesopores and viable for supercapacitive performance. The cation and anion present in the 1 M TEABF4 in ACN electrolyte in their solvated state with the size of 0.74 and 0.49 nm, respectively [3]. Hence the diffusion of these counterions from electrolyte to the respective electrode can be easily achievable with size less than 1 nm. Besides the nano size of carbon provides a huge accessible electrochemical surface area and hence adequate amount of charges can be stored with respect
CSP =
It mV
(1)
where, CSP is the specific capacitance for two electrode system, to find out the specific capacitance based on a single electrode the CSP value is multiplied by 4 [7]. The values reported in this paper is based on a single electrode. I is the current density or charge-discharge current, t is the discharge time, m is the total mass of the two electrodes, and V is the working potential window of the GCD analysis. Fig. 7(d) displays rate capability performance of CST in 1 M TEABF4 in ACN and EMIM BF4. The highest specific capacitance obtained for CST in 1 M TEABF4 in ACN is 64 F g−1 at a current density of 0.1 A g−1, it retained up to 40 F g−1 at a current density of 10 A g−1. The rate capability of CST in
Table 1 A comparison between carbon nanostructures obtained by flame combustion synthesis for supercapacitor application. Precursor Candle wax [25] Candle wax [26] Clarified Butter [38] Diesel [39] Thiophene [40]
Electrolyte
Specific capacitance −1
−1
@1 A g 107 F g 187 F g−1 @0.15 A g−1 102.2 F g−1 @20 mV s−1 68.5 F g−1 @0.8 A g−1 305 F g−1 @2 A g−1
6 M KOH 1 M H2SO4 1 M Na2SO4 0.5 M Na2SO4 1 M Na2SO4
5
Stability 98.5%(10,000 cycles) 98.3%(10,000 cycles) 95%(5000 cycles) 82%(5000 cycles) 95%(10,000 cycles)
Journal of Energy Storage 27 (2020) 101114
D. Potphode and C.S. Sharma
Fig. 6. XPS spectra of CST (a) C1s and (b) O1s.
supercapacitive performance evaluation. The impedance measurement was performed in the frequency range of 1 MHz to 0.01 Hz with sinus amplitude of 10 mV. In general, the Nyquist plot of EDLCs shows a semi-circle at high frequency with an angled line at the mid frequency and a vertical line at low frequency end. The Nyquist plot is shown in Fig. 8(a). The vertical line parallel to the imaginary axis evidently indicates the EDLC capacitive behavior of the fabricated cell [42]. The Xaxis intercept at high frequency region of the Nyquist plot shown in
1 M TEABF4 in ACN was maintained at 62.5% when current density increased from 0.1 to 10 A g−1 (Fig. 7(d)). However, CST in EMIM BF4 maintained the capacitance of ~51.2%. The good capacitance retention at higher current rates makes CST as a viable candidate for supercapacitive charge storage. The electrochemical impedance spectroscopy (EIS) was carried out to examine the frequency response on the capacitance of CST to investigate resistive and capacitive components for the practical
Fig. 7. Comparative electrochemical performance of CST in 1 M TEABF4 in ACN and EMIM BF4 electrolytes, (a) Cyclic voltammogram of CST at a scan rate of 25 mV s−1, (b) Rate performance plot for CST at different scan rates, (c) Galvanostatic charge-discharge at a current density of 0.5 A g−1, and (d) Rate capability: specific capacitance calculated from the charge-discharge curve at different current densities. 6
Journal of Energy Storage 27 (2020) 101114
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Fig. 8. (a) Impedance analysis of CST in 1 M TEABF4 in ACN and EMIM BF4, (b) and (c) are high frequency region of Nyquist plots for both the electrolytes, (d) Equivalent circuit used for fitting and the components involved are highlighted, Randles equivalent circuit: Rs-solution resistance, CPEconstant phase element associated with interfacial capacitance, RCTcharge transfer resistance, W0Warburg resistance, Cdl-mass capacitance.
little earlier (nearly after ~900th cycle) in the case of EMIM BF4 electrolyte. In general, EDLCs store the charges by electrostatic adsorption and desorption at the electrode surface, therefore the stability is expected to be immeasurable, but in actual practice, degradation does occur in all types of materials. Though the capacitance retention of CST is fairly good, more than 80% after 5000 charge-discharge cycling. In the case of CST, the reason for capacitance degradation could be addressed to the oxygen-containing surface functionalities of the CST. As the electrochemical performance of the CST carbon is greatly influenced by surface functionalities with pseudocapacitive charge storage, during the long term charge-discharge processes additional surface oxidation occurs on carbon surfaces due to electrolyte surrounding under the applied potentials, which results in a decrease in electrical conductivity and hence the capacitance. The surface functionalities in carbon also boost the self-discharge, therefore, the overall performance of the cell get affected. Over the several charge-discharge cycles, counter ions from electrolytes get trapped inside the small pores of carbon consequently causing pore clogging which reduces the active sites of the electrode materials and hence the charge storage gets hampered. As a result of side reactions at electrode with electrolyte ions and the reactions of electrolytic ions with impurities (parasitic reactions) causes capacitance degradation over the long term charge-discharge cycling in the carbon materials. To further evaluate the performance of the CST as a supercapacitor material in terms of specific energy (Es) and specific power (Ps), following Eqs. (2 and 3) [7] were used to calculate its Es and Ps, the results are displayed in Ragone plot in Fig. 10(a).
inset gives the internal resistance of the circuit symbolized as RInt, comprised of the solution resistance (RS) also called as electrolytic resistance, resistance at the electrode/electrolyte interface (RCT), and contact resistance of electrode material to the current collector [43]. The fitted Nyquist plot is given in the Fig. S6. The solution resistance for CST in TEABF4 in ACN (Fig. 8(b)) was found to be 2.2 Ω, which is quite lower due to nano size electrode material and the identical pore size distribution at the nano level. The small semi-circle at high frequency end is because of the electric double layer formation at the electrode/ electrolyte interface. The equivalent series resistance is found to be 2.92 Ω, which reflects the combination of Ohmic resistance and charge transfer resistance. The mid frequency angled line shows the diffusion (Warburg-W0) resistance [7]. The vertical line at lower frequency indicates the mass capacitance (Cdl) of the cell, in the ideal case it is perfectly parallel to the imaginary axis, but its slight inclination towards real axis indicates the cumulative effect of mass capacitance and leakage resistance [44]. In the case of CST, at a lower frequency, the capacitive behavior is dominating due to adequate access of electrolyte ions to the innermost part of electrode materials. The good capacitive behavior with low internal resistance can be attributed to the nano size and mesopores structure of CST. Nyquist plot for CST in EMIM BF4 is displayed in Fig. 8(c) which depicts the high internal resistance associated with the cell in the presence of ionic liquid. As expected the charge transfer resistance is more in case of ionic liquid due to restricted mobility of electrolytic counterions at the electrode surface and also the difference in solvated cation and anion size in organic and ionic electrolytes. At lower frequency, the capacitive nature of electrode materials dominates over the resistive components because electrolyte ions get sufficient time to acquire whole electrochemical accessible electrode surface to store charges. Whereas at higher frequencies only surface charge storage takes place due to insufficient time for diffusion of electrolyte ions deeper inside the electrode material. The capacitive and resistive behavior at lower and higher frequency end is comparable with other carbon nanomaterials reported previously for EDLCs in organic and ionic liquid electrolytes [42,43]. The long term charge-discharge stability test was done at a current density of 0.5 A g−1 for both the electrolytes and plots are displayed in Fig. 9a) and (b), 84.6% and 82.2% of the initial specific capacitance was retained after continues 5000 charge-discharge cycles in case of organic and ionic liquid electrolytes, respectively. In case of TEABF4 in ACN electrolyte, a stable charge-discharge performance was maintained up to ~1300 cycles after that constant degradation observed for consecutive charge-discharge cycles. Whereas, the capacitance fade was
Es =
2 0.5 (CsP) Vmax 3·6
(2)
Ps =
ED tdischarge
(3)
Where Vmax is the operating potential window of the supercapacitor cell, tdischarge is the discharge time calculated from the highest potential value to zero. CST as an electrode material has the capability to deliver highest specific energy of 14 and 28 Wh kg−1 and the highest specific power of 12.5 and 15 kW kg−1 in 1 M TEABF4 in ACN and EMIM BF4 electrolytes, respectively. Besides, the candle soot derived carbon reported in aqueous electrolyte (1 M H2SO4) by Raj et al., showed specific energy of 4.82 Wh kg−1 for the bare candle soot and the improved specific energy of 16.63 Wh kg−1 for the functionalized candle soot [26]. Hence it is clear that the performance of the CST is better in non7
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Fig. 9. (a) Charge-discharge cycling stability of CST in 1 M TEABF4 in CAN, (b) Charge-discharge cycling stability of CST in EMIM BF4.
With a simplistic approach of synthesizing nanostructured carbon for energy storage, CST has proven its potential of being efficient carbonaceous electrode material for high energy density supercapacitor application. The nano-size spherical morphology of CST with significant contribution of sp2 hybridized carbon makes it more conductive and easy diffusion paths for electrolyte ions. The promising electrochemical performance of CST shows the great potential as an anode material for asymmetric and hybrid supercapacitors. This study opens the way forward for CST to be explored as a conductive additive in various redox active composite fabrication for high energy supercapacitors with improved capacitance and electrochemical stability for high-performance supercapacitors.
aqueous electrolytes more specifically in terms of energy density and power density. In addition, carbon materials such as carbon nanotubes, graphene, activated carbon (AC), polymer derived carbons with organic and ionic electrolyte have been compared in terms of electrochemical performance (data presented in Table 2). The energy density and power density obtained for CST in organic and ionic liquid is comparable with the carbon nanomaterials mentioned in Table 2. Sivaraman et al. reported multi-walled carbon nanotube (MWCNT) and unzipped multiwalled carbon nanotubes UN-MWCNT in organic electrolyte i.e. 1 M TEABF4 in propylene carbonate (PC) based supercapacitor cells with the highest specific energy of 20.5 and 35.8 Wh kg−1, respectively [45]. Yu et al. have reported single wall carbon nanotube (SWCNT) in an ionic liquid with nearly similar specific energy (107 Wh kg−1) like CST by maintaining a maximum potential window of 0–4 V [46]. Taking advantage of nano size with optimal pores structure, CST is a competent electrode material with the previously reported carbon nanomaterials. Indeed, the spherical morphology of CST with mesopores carbon structure demonstrated as an excellent candidate for high energy density supercapacitor electrode. Similarly, as shown in Fig. 10b) a CST based supercapacitor can light the light-emitting diode, which signifies the capability of the small supercapacitor for effectual charge storage giving high energy and delivering power to the electronic load for specific application.
4. Conclusions Simple high-performance candle soot derived carbon was synthesized by flame assisted combustion of wax, on further heat treatment high surface area nanosize carbon was obtained. CST exhibited spherical morphology with the particle size of 20–50 nm and a BET surface area of 608 m2 g−1. The symmetric supercapacitor coin cells were fabricated and electrochemical performance was evaluated for organic and ionic liquid electrolytes. CST as an active material for supercapacitor performed remarkably in both the electrolytes. The highest
Fig. 10. (a) Ragone plot: comparison of the specific energy and specific power of CST in both electrolytes, and (b) LED demonstration of the coin cell device in EMIM BF4. 8
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D. Potphode and C.S. Sharma
Table 2 Electrochemical performance of carbon materials in organic electrolytes and ionic liquids. Material
Electrolyte
Graphene [15] AC nanosheets [47] Pristine MWCNT [21] Un-MWCNT [21] SWCNT buckypaper [17] SWCNT [46] CST This work CST This work
BMIM TFSI 1 M TEABF4 1 M TEABF4 1 M TEABF4 EMIM BF4 EMIM BF4 EMIM BF4 1 M TEABF4
in ACN In PC in PC
in ACN
Potential window
Csp (F g−1)
Es (Wh kg−1)
Ps (kW kg−1)
Stability
0–3.0V 0–2.5V 0–2.5V 0–2.5V 0–4.0V 0–4.0V 0–3.0V 0–2.5V
108 134 23.6 42.8 95.3 203 88.4 64
32.3 30 20.5 35.8 55 107 28 14
82 78 Not reported Not reported Not reported 20 15 12.5
92% (1000 Cycles) 85% (10,000 cycles) No decay (2000 cycles) No decay (2000 cycles) Not reported Not reported 82.2% (5000 cycles) 84.6% (5000 cycles)
specific capacitance of 64 and 88.2 F g−1 was achieved with 1M TEABF4 in ACN and EMIM BF4 electrolytes, respectively. With organic electrolyte, 62.5% of capacitance was retained when current density increased from 0.1 A g−1 to 10 A g−1, while ionic liquid-based supercapacitor retained 51% of its original capacitance. It showed the highest specific energy of 28 and 14 Wh kg−1 with ionic liquid and organic electrolyte. The results from Ragone plot indicates that CST can deliver high energy without compromising on specific power. Impedance analysis showed the prominent capacitive nature of CST in both organic and ionic liquid electrolytes. In this way, CST exhibited potential electrochemical performance with additional pseudocapacitance, improved specific energy and high electrochemical stability.
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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. Acknowledgements The authors would like to acknowledge the financial support received from the Ministry of Human Resources Development and Department of Heavy Industries, Govt. of India under the IMPRINT scheme. Authors acknowledge Tatva Chintan Pharma Chem Pvt. Ltd. India for providing Tetraethylammonium tetrafluoroborate (TEA BF4) salt. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.est.2019.101114. References [1] X. Chen, C. Li, M. Grätzel, R. Kostecki, S.S. Mao, Nanomaterials for renewable energy production and storage, Chem. Soc. Rev. 41 (2012) 7909–7937, https://doi. org/10.1039/c2cs35230c. [2] J.R. Miller, P. Simon, Electrochemical capacitors for energy management, Sci. Mag. 321 (2008) 651–652, https://doi.org/10.1126/science.1158736. [3] E. Frackowiak, Supercapacitors based on carbon materials and ionic liquids, J. Braz. Chem. Soc. 17 (2006) 1074–1082, https://doi.org/10.1590/S010350532006000600003. [4] M. Mastragostino, F. Soavi, C. Arbizzani, Electrochemical supercapacitors, Adv. Lithium-Ion Batter. (2002) 481–505, https://doi.org/10.1007/0-306-47508-1_17. [5] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845, https://doi.org/10.1038/nmat2297. [6] N. Choudhary, C. Li, J. Moore, N. Nagaiah, L. Zhai, Y. Jung, J. Thomas, Asymmetric supercapacitor electrodes and devices, Adv. Mater.29 (n.d.) 1605336. doi:10.1002/ adma.201605336. [7] S. Zhang, N. Pan, Supercapacitors performance evaluation, Adv. Energy Mater. 5 (2015) 1–19, https://doi.org/10.1002/aenm.201401401. [8] H. Xie, S. Tang, J. Zhu, S. Vongehr, X. Meng, A high energy density asymmetric allsolid-state supercapacitor based on cobalt carbonate hydroxide nanowire covered N-doped graphene and porous graphene electrodes, J. Mater. Chem. A 3 (2015) 18505–18513, https://doi.org/10.1039/c5ta05129k. [9] T. Mitravinda, K. Nanaji, S. Anandan, A. Jyothirmayi, V.S.K. Chakravadhanula, C.S. Sharma, T.N. Rao, Facile synthesis of corn silk derived nanoporous carbon for an improved supercapacitor performance, J. Electrochem. Soc. 165 (2018)
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Pseudocapacitance induced candle soot derived carbon for high energy density electrochemical supercapacitors: Non-aqueous approach Darshna Potphode, Chandra Shekhar Sharma
T
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Creative & Advanced Research Based On Nanomaterials (CARBON) Laboratory, Department of Chemical Engineering, Indian Institute of Technology, Kandi, Hyderabad 502285, Telangana, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon Candle soot Ionic liquid Organic electrolyte Non-aqueous Supercapacitors
The commercialization aspect in electric double layer capacitors is highly dependent on source of carbon, its purity, and the specific capacitance with unique deliverable for proficient charge storage. Here, we report synthesis of interconnected mesoporous spherical carbon by direct flame synthesis using candle wax for symmetric supercapacitors with the non-aqueous approach. The heat treated candle soot (CST) at 450 °C results in a high purity carbon with the improved surface area of 608 m2 g−1 also the majority of pores in the range of 1–10 nm, which is ideal for charge storage in supercapacitors. The highest capacitance of 64 and 88.2 F g−1 at 0.1 A g−1 was obtained for 1 M TEABF4 in acetonitrile and EMIM BF4, respectively. A stability up to 82–85% can be achieved for 5,000 charge-discharge cycles. CST as a potent electrode material successfully delivered highest specific energy of 14 and 28 Wh kg−1 and specific power of 12.5 and 15 kW kg−1 in 1 M TEABF4 in ACN and EMIM BF4 electrolytes, respectively.
1. Introduction To satisfy ever-growing demand for energy a resourceful solution need more emphasis to provide prolific energy requirement. The effective storage of energy from sustainable sources can be a solution for necessity and massive requirements [1,2],. Supercapacitors are one of the promising energy storage technologies due to its enormous applications in various fields such as automotive, defense, toys, portable electronics, etc. [3]. Supercapacitors are also known as ultracapacitors due to its fast charge-discharge competencies. Based on the energy storage mechanism, supercapacitors are categorized into three subcategories [4]. First, electric double layer capacitors (EDLCs) which can store charges at the surface of the electrode materials by electrostatic interactions between electrode and electrolyte interface, generally high surface area carbon materials are being used in this category [5]. Second, pseudocapacitors where the charge storage takes place by surface redox reactions, which includes the electrode materials such as conducting polymers and some metal oxides with neutral electrolytes [6]. The third category involves hybrid supercapacitors (battery-type), which are composed of metal oxides/sulfides and the charge storage takes place with bulk utilization of electrode material by redox reactions [7]. Due to electrostatic charge storage in EDLCs, it possesses high power densities and longer charge-discharge cycling stability than the pseudocapacitors and hybrid capacitors, however, the energy density is ⁎
limited in case of EDLCs which hinders their commercialization [8]. Recent research focuses on improving the energy density of EDLCs without compromising on power density and electrochemical cycling stability. Several reports on carbon materials with high surface area such as biomass-derived carbon [9–11], organic precursors derived carbon [12], activated carbon [13], carbon nanotubes [14], graphene [15,16], and fullerenes [17] are reported as electrode materials for EDLCs [18]. Although carbon nanotubes, graphene, and fullerenes have higher electrical conductivity as compared to other forms of carbon, the specific capacitance obtain from those is less than the activated carbon because of lower electrochemical accessible surface area and lack of porosity in the material [12,19]. The electrochemical surface area of electrode material is the key parameter in charge storage more specifically in supercapacitors and can be achieved by KOH activation or various template assisted methods to improve the accessibility of electrolyte ions to the innermost part of the electrodes [20]. A lot of efforts has been made to improve upon these factors, for example, partial exfoliation of multiwalled carbon nanotube (MWCNTs) was reported by Sivaraman et al. [21] showed the tremendous increase in surface are of MWCNTs and hence the specific capacitance got increased to several orders. Exfoliation of graphite to form graphene oxide and reduced graphene oxide is another example of increasing specific charge storage thereby enhancing the electrochemical accessible surface area [22]. The
Corresponding author. E-mail address: [email protected] (C.S. Sharma).
https://doi.org/10.1016/j.est.2019.101114 Received 8 June 2019; Received in revised form 23 November 2019; Accepted 24 November 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Energy Storage 27 (2020) 101114
D. Potphode and C.S. Sharma
soot, the sample was heated in air at 450 °C for 2 h.
development of the high surface area and optimal porosity in the carbon is still on a focal point for researchers, to that end, organicinorganic precursors such as polymers, hydrocarbons, metal carbides, metal-organic frameworks, and template-assisted synthesis are being used to obtain ideal carbon structures for electrochemical charge storage [23,24]. But the above-mentioned techniques are expensive, complex, multi-step methods and also the carbon yield is very low. Direct synthesis of carbon from wax by wick supported combustion is a facile and promising method to obtain a pure form of carbon at the nano level. A simplistic and economical synthesis method for carbon nanomaterial still holds great prospect. Candle soot derived carbon is one of the ideal alternatives to get a pure form of carbon by direct flame combustion of wax. Due to its nano size particles and easy electrode fabrication method, candle soot derived carbon is being used in energy storage application. Zhang et al. reported core-shell MnO2@candle soot nanocomposite material for which the specific capacitance obtained was 309 F g−1 in aqueous 1 M Li2SO4 solution, for which bare candle soot showed 106 F g−1 at a current density of 1 A g−1 [25]. The nitric acid functionalized candle soot has been reported by Raj et al. showed a specific capacitance of 187 F g−1 at a current density of 0.15 A g−1 [26]. Being nanoparticles of size 20–50 nm, candle soot derived carbon has been utilized in making nanocomposites with polyaniline and MnO2, which are further explored for supercapacitor application with a highest specific capacitance of 140 F g−1 at the current density of 1 A g−1 [27]. However, these reports are limited to the aqueous-based supercapacitors with the maximum potential window of 1 V and hence candle soot derived carbon can be further explored for high energy density energy storage possibilities. The non-aqueous approach for carbon nanomaterial-based supercapacitors in combination with organic electrolytes and ionic liquids comes under the category of high energy density EDLCs. These non-aqueous approach offers a higher operating potential range from 2.5 to 4 V thereby enhancing the specific energy of the EDLCs. In this work, we report a single step synthesis of carbon soot from candle wax having a particle size of 20 −50 nm with optimal porosity in the material to achieve high specific energy supercapacitive performance. This is the first time report for candle soot derived carbon with the nonaqueous approach by means of 1 M tetraethylammonium tetrafluoroborate (TAEBF4) in acetonitrile (ACN) and pure 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIM BF4) and also renders the outstanding electrochemical charge storage performance. The detail electrochemical exploration of CST as an active material for supercapacitor application is highlighted in essence.
2.3. Electrode fabrication CST electrodes were fabricated by solution drop cast method, in detail, a mixture of active material (8 mg) and conducting carbon (1 mg) was dispersed in 2 mL of IPA for 10 min and then 5% PTFE solution added to the suspension to yield uniform dispersion of carbon in IPA with a binder. The ratio of active material: conducting carbon: binder was maintained at 80: 10: 10. The solution mixture then dropcasted over round cut carbon paper of 15 mm diameter (with the circular area of 1.766 cm2) and vacuum dried at 80 °C for 12 h. The total active mass loading of active material on each electrode was 2 mg. 2.4. Structural characterizations The structural and morphological investigation of CST was performed by field emission scanning electron microscopy (FESEM) with SUPRA 40 Zeiss India and transmission electron microscopy (TEM) with FEI Tecnai G2S-Twin operated at 200 kV. Powder X-ray diffraction (XRD) was performed by using a powder X-ray diffractometer with CuKα (λ = 1.5406 Å) radiation over a 2θ range of 7˚ to 80˚. The elemental composition of the material was determined by X-ray photoelectron spectroscopy (XPS) technique using the Omicron Nano Technology instrument. The structural features of the sample were studied by Raman analysis with Bruker Raman microscope spectrometer with an excitation wavelength of 532 nm. The Brunauer-Emmett-Teller (BET) surface area and pore size distribution measurement were carried out by Micromeritics ASAP 2020 analyzer. Fourier transform-infrared (FT-IR) was performed over Tensor 37, Bruker USA, KBr pellets were prepared by maintaining an equal amount of carbon ratio with respect to KBr (0.2:100 wt/wt). The hydrophobicity of the CS and CST samples were studied in triplicate using a sessile drop method with a goniometer (Rame-Hart, USA; Model: 290- F4). 2.5. Electrochemical measurements The coin cell assembly (CR 2032) of symmetric supercapacitors were fabricated in the glovebox with the help of crimper by sandwiching two symmetric CST electrodes between Glass microfiber filter separator. All electrochemical studies such as cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) measurement and electrochemical impedance spectroscopic (EIS) analysis were done using Biologic Potentiostat (VMP3). The electrochemical measurements using TAE BF4 in ACN was performed in the potential window of 0–2.5 V and for ionic liquid (EMIM BF4) the potential window of 0–3 V was maintained. The EIS measurement of supercapacitors was carried out in the frequency range of 1 MHz to 10 mHz at the cell potential of 1 V.
2. Experimental 2.1. Materials 1-Ethyl-3-methylimidazolium tetrafluoroborate, acetonitrile (ACN), and Glass microfiber filters (Whatman, Grade GF/D) were purchased from Sigma-Aldrich and used as received. Carbon paper was purchased from Toray, Japan. Polytetrafluoroethylene (PTFE) suspension was obtained from Hindustan Fluorocarbons Ltd, India. Isopropyl alcohol (IPA) was purchased from SRL Pvt. Ltd. India. Acetylene black (Super P conductive, 99+% metal basis) was purchased from Alfa Aesar. Tetraethylammonium tetrafluoroborate (TEA BF4) salt was obtained from Tatva Chintan Pharma Chem Pvt. Ltd. India.
3. Results and discussion 3.1. Physico-chemical characterization The process of thermal treatment in air or in presence of oxygen to remove amorphous carbon and other carbonaceous impurities formed during synthesis has already been applied in the purification of various carbon nanomaterials [28]. Which also helps in improving the surface area by adding surface oxygen functionalities to the carbon. After successful synthesis of the bare candle soot (CS), the heat treatment was employed to yield high purity carbon and the sample was named as CST. The treatment in the air added the oxygen functionalities to the CS, Fig. 1 Shows the comparative FT-IR spectra of CS and CST (before and after heat treatment). The peaks at 800 and 2919 cm−1 for CeH bond present in the CS due to the presence of wax are minimized after heat treatment. The peak intensity for the C = C bond is increased as compared to CS sample which indicates impurities has been removed at
2.2. Synthesis of candle soot The wax candles were purchased from the local market of Sangareddy, Telangana-India and used as it is. The interconnected carbon nanoparticles of candle soot were synthesized by a single step flame combustion method. Carbon soot was collected over a glass substrate by placing above the flame tip. Further to remove unreacted wax and small particles of carbon impurities present in bare candle 2
Journal of Energy Storage 27 (2020) 101114
D. Potphode and C.S. Sharma
Fig. 3. A comparative XRD plot for CS and CST.
Fig. 1. Comparative FT-IR spectra of CS and CST, spectra recorded by maintaining an equal amount of carbon in KBr.
and ID/IG ratio of 1.12 indicates the presence of more defects in the carbon structure of CS. Whereas, Raman spectra of CST shows little sharp D band with lower intensity and improved ID/IG ratio of 1. As expected after heat treatment removal of dangling bonds and unreacted wax improves the carbon quality in CST. Furthermore, the G’ band at around ~2700 cm−1 is corresponding to two-phonon double resonance Raman scattering process which is characteristic band to confirm the layered structure in graphene [32]. The previous reports say, as the layers of carbon or graphene increases the G’ band shifts to the higher wavenumber value. The G’ band is the overtone of first-order D band therefore in some of the reports this band is labeled as 2D band or D’ band [33]. In this study, the broadening and shifting of G’ band to higher range of wavenumber confirms the disordered multilayered carbon in both the samples. The XRD plot for CS and CST is given in Fig. 3. Two major peaks at 2θ value of 25.7 and 43.8 are indexed to (002) and (100) plane of carbonaceous materials [34]. However shift in (002) plane from 26 towards the lower value of 2θ (25.7) shows the presence of small turbostratic carbon traces in the sample [35]. It is well known that the peak position of (002) plane identifies the spacing of aromatic ring layer in the graphitic carbon. The candle soot derived carbon also contains the graphitic structures or carbon crystallites which are confirmed by the presence of (002) plane at ~26° and 25.7° in both the
450 °C. However, the presence of sharp peaks at 1260 cm−1 and 1740 cm−1 indicates additional CeO functionalities after heat treatment and can be assigned to CeO and C=O bonds, respectively [26,29]. Therefore, the purity of carbon with additional surface functionality can be maintained over carbon structures by simple heat treatment in air. In addition to that, the heat treatment also reduced the hydrophobicity of CS which was confirmed by contact angle measurement with water. CS showed contact angle of 113.6 ± 0.2° which implies the hydrophobicity in the sample owing to its wax content. However, CST showed contact angle of 46.2 ± 0.5° which confirms the hydrophilicity in the sample after heat treatment. Comparative images for water contact angle measurement is given in Fig. S1 while the thermal stability is discussed in Fig. S2. Fig. 2(a) and (b) represents the Raman spectra of the CS and CST samples, respectively, with a highlight on characteristic disordered carbon structure. The G band present at around 1588 cm−1 due to E2g phonon vibrations of sp2 carbon atoms [30]. The peak present at 1348 cm−1 indicates the D band or defects present in the carbon materials. The broadening, shifting, and relative intensity of D band depends on the nature of impurities or dopant or functional groups present in the carbon matrix [31]. Therefore, the pure form of graphite does not exist D band in Raman spectra. The broaden G band (Fig. 2(a)) in the CS case
Fig. 2. (a) Raman spectra of CS, and (b) Raman spectra of CST. 3
Journal of Energy Storage 27 (2020) 101114
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Fig. 4. (a) FESEM image, (b) low magnification TEM image, (C) and (d) are high magnification TEM images of CST.
obtained for CS is ~105 m2 g−1 which is very less for supercapacitive charge storage (Fig. S3). However, heat treated sample showed the specific surface area of 608 m2 g−1 which identifies the suitability of the material for better charge storage electrode material for EDLCs. Fig. 5(b) shows the pore size distribution obtained from the N2 adsorption-desorption isotherm. The pore size distribution is higher in the range of 1–10 nm; a small fraction of macroporous contribution in the range of 30–80 nm is due to the space between interconnecting carbon spherical. Based on previous reports, the optimal pore size matching with electrolyte ion or double the size of electrolyte ion are more likely to store apparent charges on the surfaces [37]. The inset of Fig. 5(b) confirms the pore volume density majorly in the mesoporous range. From the BET, TEM, and FESEM data it is confirmed that the CST sample contains a large fraction of mesoporous and small fraction of microporous density. The interconnecting carbon spherical in CST sample contributing for macrospores and mesopores will be accommodating for fast diffusion of electrolytic ions in the active sites of the electrodes. The carbon nanostructures reported for supercapacitor energy storage synthesized by flame combustion method are listed in a comparative Table 1. Fig. 6(a) and (b) represents XPS spectra of C1s and O1s clearly confirming the chemical composition of CST. After the de-convulsion of individual spectra of elements, the percent of carbon and oxygen was calculated and found to be 84.5% of carbon and 15.5% of oxygen. The core spectra of C1s shows three peaks at 284.2, 285.2, and 288.5 eV
samples (CS and CST). Along with (002) plane, the presence of weak band at 43.8° for (100) plane also confirms the graphitic structures in candle soot carbon. Based on the observations, the carbon crystallites present in the candle soot carbon are of intermediate structures of graphite and amorphous carbon or random layer lattice structures. According to literature, the shift in (002) plane to higher 2θ value indicates the increase in elemental carbon content, and shift in (002) plane to lower 2θ value indicates the increase in randomness in graphitic alignment [36]. The FESEM image shown in Fig. 4(a) clearly indicates the interconnectivity between spherical particles of CST. Further, Fig. 4 (b, c, and d) confirms the particle size of the CST sample is in the range of 20–50 nm. These nano size particles, as well as wellmaintained interconnectivity in carbon, is beneficial for optimum charge storage in EDLCs. Due to the nano size of the carbon, diffusion path length will be shorter and diffusion of electrolyte will be merely easy and effective at the electrode/electrolyte interface. Besides, the interconnectivity of CST will improve electrical conductivity by decreasing charge transfer resistance at the electrode, and hence supercapacitor performance can be improved at the electrode level. The specific surface area and porosity of the CST were analyzed in detail by N2 gas adsorption-desorption isotherm which is shown in Fig. 5. The type IV isotherm (Fig. 5(a)) indicates that the material contains mainly a mesoporous structure, additionally, small hysteresis loop present in the BET plot indicates the capillary condensation that occurs in the mesoporous materials. The BET specific surface area
4
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Fig. 5. (a) N2 adsorption-desorption isotherm for CST, (b) the pore size distribution and cumulative pore volume analysis for CST sample.
which corresponds to sp2 hybridized carbon (C=C), sp3 hybridized carbon (CeC), and OeC=O, respectively [41]. There are two peaks for oxygen (O1s- Fig. 6(b)) at 531.1 and 532.6 eV which corresponds to C=O and =CeO bonds, respectively [12]. In EDLCs, the sufficient oxygen content in the carbon materials helps for the better wettability of the electrodes. At the same time, the oxygen functionalities at the carbon surface involve in the pseudocapacitive charge storage to enhance the specific capacitance. A high degree of sp2 hybridized carbon in CST will be effective to maintain good electrical conductivity at the electrode.
to mass loading of active material. Further nano size carbon structure will have a smaller diffusion path length which is more beneficial to decrease internal resistance in the system. The slanted CV curves of CST in EMIM BF4 electrolyte (Fig. S5(a)) at higher scan rates are due to high electrolytic resistance associated with the viscous composition of pure ionic liquids compared to the diluted organic electrolytes. The higher scan rates lead to insufficient time for electrolyte ions to diffuse inside the mesopores of electrode materials which causes more resistance and hence overall resistance of the cell increases. Fig. 7(b) represents the rate performance of the CST in both the electrolytes at different scan rates, when scan rate increase from 5 to 100 mV s−1, CST retained ~74% and ~67% of specific capacitance in 1 M TEABF4 in ACN and EMIM BF4, respectively. The specific capacitance of CST was calculated from galvanostatic charge-discharge (GCD) analysis method. Fig. 7(c) represents GCD curves of CST at a current density of 0.5 A g−1 in both the electrolytes, current density applied in GCD analysis are based on the active mass of both the electrodes. The GCD curves of CST in 1 M TEABF4 in ACN and EMIM BF4 at different current densities are given in Fig. S4(b) and Fig. S5(b), respectively. Again the triangular shape of GCD curves confirms the EDLC type charge storage at electrodes. The gravimetric specific capacitance is calculated by using the formula given in Eq. (1) [7].
3.2. Electrochemical performance As the energy density is limited for the aqueous system in supercapacitors, therefore candle soot derived carbon was tested with two non-aqueous electrolytes such as 1 M TEABF4 in ACN and EMIM BF4. All the measurements were carried out in two electrode system in symmetric combinations. Fig. 7(a) shows the cyclic voltammograms of CST in both the electrolytes i.e. 1 M TEABF4 in ACN and ionic liquid electrolyte at a scan rate of 25 mV s−1. CV shows rectangular shape which can be related to the electric double layer formation at the electrode/electrolyte interface and confirms the absence of redox reactions at electrode [5]. There is a slight deviance of rectangular shape CV indicating the dominance of pseudocapacitance due to oxygen functionalities present on the carbon structures [38]. The comprehensive CV profile at different scan rates in both the electrolytes is given in Fig. S4(a) and S5(a). At higher scan rates, the rectangular shape of CV specifies the ease of charge storage in the carbon structure due to the presence of mesopores and viable for supercapacitive performance. The cation and anion present in the 1 M TEABF4 in ACN electrolyte in their solvated state with the size of 0.74 and 0.49 nm, respectively [3]. Hence the diffusion of these counterions from electrolyte to the respective electrode can be easily achievable with size less than 1 nm. Besides the nano size of carbon provides a huge accessible electrochemical surface area and hence adequate amount of charges can be stored with respect
CSP =
It mV
(1)
where, CSP is the specific capacitance for two electrode system, to find out the specific capacitance based on a single electrode the CSP value is multiplied by 4 [7]. The values reported in this paper is based on a single electrode. I is the current density or charge-discharge current, t is the discharge time, m is the total mass of the two electrodes, and V is the working potential window of the GCD analysis. Fig. 7(d) displays rate capability performance of CST in 1 M TEABF4 in ACN and EMIM BF4. The highest specific capacitance obtained for CST in 1 M TEABF4 in ACN is 64 F g−1 at a current density of 0.1 A g−1, it retained up to 40 F g−1 at a current density of 10 A g−1. The rate capability of CST in
Table 1 A comparison between carbon nanostructures obtained by flame combustion synthesis for supercapacitor application. Precursor Candle wax [25] Candle wax [26] Clarified Butter [38] Diesel [39] Thiophene [40]
Electrolyte
Specific capacitance −1
−1
@1 A g 107 F g 187 F g−1 @0.15 A g−1 102.2 F g−1 @20 mV s−1 68.5 F g−1 @0.8 A g−1 305 F g−1 @2 A g−1
6 M KOH 1 M H2SO4 1 M Na2SO4 0.5 M Na2SO4 1 M Na2SO4
5
Stability 98.5%(10,000 cycles) 98.3%(10,000 cycles) 95%(5000 cycles) 82%(5000 cycles) 95%(10,000 cycles)
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Fig. 6. XPS spectra of CST (a) C1s and (b) O1s.
supercapacitive performance evaluation. The impedance measurement was performed in the frequency range of 1 MHz to 0.01 Hz with sinus amplitude of 10 mV. In general, the Nyquist plot of EDLCs shows a semi-circle at high frequency with an angled line at the mid frequency and a vertical line at low frequency end. The Nyquist plot is shown in Fig. 8(a). The vertical line parallel to the imaginary axis evidently indicates the EDLC capacitive behavior of the fabricated cell [42]. The Xaxis intercept at high frequency region of the Nyquist plot shown in
1 M TEABF4 in ACN was maintained at 62.5% when current density increased from 0.1 to 10 A g−1 (Fig. 7(d)). However, CST in EMIM BF4 maintained the capacitance of ~51.2%. The good capacitance retention at higher current rates makes CST as a viable candidate for supercapacitive charge storage. The electrochemical impedance spectroscopy (EIS) was carried out to examine the frequency response on the capacitance of CST to investigate resistive and capacitive components for the practical
Fig. 7. Comparative electrochemical performance of CST in 1 M TEABF4 in ACN and EMIM BF4 electrolytes, (a) Cyclic voltammogram of CST at a scan rate of 25 mV s−1, (b) Rate performance plot for CST at different scan rates, (c) Galvanostatic charge-discharge at a current density of 0.5 A g−1, and (d) Rate capability: specific capacitance calculated from the charge-discharge curve at different current densities. 6
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Fig. 8. (a) Impedance analysis of CST in 1 M TEABF4 in ACN and EMIM BF4, (b) and (c) are high frequency region of Nyquist plots for both the electrolytes, (d) Equivalent circuit used for fitting and the components involved are highlighted, Randles equivalent circuit: Rs-solution resistance, CPEconstant phase element associated with interfacial capacitance, RCTcharge transfer resistance, W0Warburg resistance, Cdl-mass capacitance.
little earlier (nearly after ~900th cycle) in the case of EMIM BF4 electrolyte. In general, EDLCs store the charges by electrostatic adsorption and desorption at the electrode surface, therefore the stability is expected to be immeasurable, but in actual practice, degradation does occur in all types of materials. Though the capacitance retention of CST is fairly good, more than 80% after 5000 charge-discharge cycling. In the case of CST, the reason for capacitance degradation could be addressed to the oxygen-containing surface functionalities of the CST. As the electrochemical performance of the CST carbon is greatly influenced by surface functionalities with pseudocapacitive charge storage, during the long term charge-discharge processes additional surface oxidation occurs on carbon surfaces due to electrolyte surrounding under the applied potentials, which results in a decrease in electrical conductivity and hence the capacitance. The surface functionalities in carbon also boost the self-discharge, therefore, the overall performance of the cell get affected. Over the several charge-discharge cycles, counter ions from electrolytes get trapped inside the small pores of carbon consequently causing pore clogging which reduces the active sites of the electrode materials and hence the charge storage gets hampered. As a result of side reactions at electrode with electrolyte ions and the reactions of electrolytic ions with impurities (parasitic reactions) causes capacitance degradation over the long term charge-discharge cycling in the carbon materials. To further evaluate the performance of the CST as a supercapacitor material in terms of specific energy (Es) and specific power (Ps), following Eqs. (2 and 3) [7] were used to calculate its Es and Ps, the results are displayed in Ragone plot in Fig. 10(a).
inset gives the internal resistance of the circuit symbolized as RInt, comprised of the solution resistance (RS) also called as electrolytic resistance, resistance at the electrode/electrolyte interface (RCT), and contact resistance of electrode material to the current collector [43]. The fitted Nyquist plot is given in the Fig. S6. The solution resistance for CST in TEABF4 in ACN (Fig. 8(b)) was found to be 2.2 Ω, which is quite lower due to nano size electrode material and the identical pore size distribution at the nano level. The small semi-circle at high frequency end is because of the electric double layer formation at the electrode/ electrolyte interface. The equivalent series resistance is found to be 2.92 Ω, which reflects the combination of Ohmic resistance and charge transfer resistance. The mid frequency angled line shows the diffusion (Warburg-W0) resistance [7]. The vertical line at lower frequency indicates the mass capacitance (Cdl) of the cell, in the ideal case it is perfectly parallel to the imaginary axis, but its slight inclination towards real axis indicates the cumulative effect of mass capacitance and leakage resistance [44]. In the case of CST, at a lower frequency, the capacitive behavior is dominating due to adequate access of electrolyte ions to the innermost part of electrode materials. The good capacitive behavior with low internal resistance can be attributed to the nano size and mesopores structure of CST. Nyquist plot for CST in EMIM BF4 is displayed in Fig. 8(c) which depicts the high internal resistance associated with the cell in the presence of ionic liquid. As expected the charge transfer resistance is more in case of ionic liquid due to restricted mobility of electrolytic counterions at the electrode surface and also the difference in solvated cation and anion size in organic and ionic electrolytes. At lower frequency, the capacitive nature of electrode materials dominates over the resistive components because electrolyte ions get sufficient time to acquire whole electrochemical accessible electrode surface to store charges. Whereas at higher frequencies only surface charge storage takes place due to insufficient time for diffusion of electrolyte ions deeper inside the electrode material. The capacitive and resistive behavior at lower and higher frequency end is comparable with other carbon nanomaterials reported previously for EDLCs in organic and ionic liquid electrolytes [42,43]. The long term charge-discharge stability test was done at a current density of 0.5 A g−1 for both the electrolytes and plots are displayed in Fig. 9a) and (b), 84.6% and 82.2% of the initial specific capacitance was retained after continues 5000 charge-discharge cycles in case of organic and ionic liquid electrolytes, respectively. In case of TEABF4 in ACN electrolyte, a stable charge-discharge performance was maintained up to ~1300 cycles after that constant degradation observed for consecutive charge-discharge cycles. Whereas, the capacitance fade was
Es =
2 0.5 (CsP) Vmax 3·6
(2)
Ps =
ED tdischarge
(3)
Where Vmax is the operating potential window of the supercapacitor cell, tdischarge is the discharge time calculated from the highest potential value to zero. CST as an electrode material has the capability to deliver highest specific energy of 14 and 28 Wh kg−1 and the highest specific power of 12.5 and 15 kW kg−1 in 1 M TEABF4 in ACN and EMIM BF4 electrolytes, respectively. Besides, the candle soot derived carbon reported in aqueous electrolyte (1 M H2SO4) by Raj et al., showed specific energy of 4.82 Wh kg−1 for the bare candle soot and the improved specific energy of 16.63 Wh kg−1 for the functionalized candle soot [26]. Hence it is clear that the performance of the CST is better in non7
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Fig. 9. (a) Charge-discharge cycling stability of CST in 1 M TEABF4 in CAN, (b) Charge-discharge cycling stability of CST in EMIM BF4.
With a simplistic approach of synthesizing nanostructured carbon for energy storage, CST has proven its potential of being efficient carbonaceous electrode material for high energy density supercapacitor application. The nano-size spherical morphology of CST with significant contribution of sp2 hybridized carbon makes it more conductive and easy diffusion paths for electrolyte ions. The promising electrochemical performance of CST shows the great potential as an anode material for asymmetric and hybrid supercapacitors. This study opens the way forward for CST to be explored as a conductive additive in various redox active composite fabrication for high energy supercapacitors with improved capacitance and electrochemical stability for high-performance supercapacitors.
aqueous electrolytes more specifically in terms of energy density and power density. In addition, carbon materials such as carbon nanotubes, graphene, activated carbon (AC), polymer derived carbons with organic and ionic electrolyte have been compared in terms of electrochemical performance (data presented in Table 2). The energy density and power density obtained for CST in organic and ionic liquid is comparable with the carbon nanomaterials mentioned in Table 2. Sivaraman et al. reported multi-walled carbon nanotube (MWCNT) and unzipped multiwalled carbon nanotubes UN-MWCNT in organic electrolyte i.e. 1 M TEABF4 in propylene carbonate (PC) based supercapacitor cells with the highest specific energy of 20.5 and 35.8 Wh kg−1, respectively [45]. Yu et al. have reported single wall carbon nanotube (SWCNT) in an ionic liquid with nearly similar specific energy (107 Wh kg−1) like CST by maintaining a maximum potential window of 0–4 V [46]. Taking advantage of nano size with optimal pores structure, CST is a competent electrode material with the previously reported carbon nanomaterials. Indeed, the spherical morphology of CST with mesopores carbon structure demonstrated as an excellent candidate for high energy density supercapacitor electrode. Similarly, as shown in Fig. 10b) a CST based supercapacitor can light the light-emitting diode, which signifies the capability of the small supercapacitor for effectual charge storage giving high energy and delivering power to the electronic load for specific application.
4. Conclusions Simple high-performance candle soot derived carbon was synthesized by flame assisted combustion of wax, on further heat treatment high surface area nanosize carbon was obtained. CST exhibited spherical morphology with the particle size of 20–50 nm and a BET surface area of 608 m2 g−1. The symmetric supercapacitor coin cells were fabricated and electrochemical performance was evaluated for organic and ionic liquid electrolytes. CST as an active material for supercapacitor performed remarkably in both the electrolytes. The highest
Fig. 10. (a) Ragone plot: comparison of the specific energy and specific power of CST in both electrolytes, and (b) LED demonstration of the coin cell device in EMIM BF4. 8
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Table 2 Electrochemical performance of carbon materials in organic electrolytes and ionic liquids. Material
Electrolyte
Graphene [15] AC nanosheets [47] Pristine MWCNT [21] Un-MWCNT [21] SWCNT buckypaper [17] SWCNT [46] CST This work CST This work
BMIM TFSI 1 M TEABF4 1 M TEABF4 1 M TEABF4 EMIM BF4 EMIM BF4 EMIM BF4 1 M TEABF4
in ACN In PC in PC
in ACN
Potential window
Csp (F g−1)
Es (Wh kg−1)
Ps (kW kg−1)
Stability
0–3.0V 0–2.5V 0–2.5V 0–2.5V 0–4.0V 0–4.0V 0–3.0V 0–2.5V
108 134 23.6 42.8 95.3 203 88.4 64
32.3 30 20.5 35.8 55 107 28 14
82 78 Not reported Not reported Not reported 20 15 12.5
92% (1000 Cycles) 85% (10,000 cycles) No decay (2000 cycles) No decay (2000 cycles) Not reported Not reported 82.2% (5000 cycles) 84.6% (5000 cycles)
specific capacitance of 64 and 88.2 F g−1 was achieved with 1M TEABF4 in ACN and EMIM BF4 electrolytes, respectively. With organic electrolyte, 62.5% of capacitance was retained when current density increased from 0.1 A g−1 to 10 A g−1, while ionic liquid-based supercapacitor retained 51% of its original capacitance. It showed the highest specific energy of 28 and 14 Wh kg−1 with ionic liquid and organic electrolyte. The results from Ragone plot indicates that CST can deliver high energy without compromising on specific power. Impedance analysis showed the prominent capacitive nature of CST in both organic and ionic liquid electrolytes. In this way, CST exhibited potential electrochemical performance with additional pseudocapacitance, improved specific energy and high electrochemical stability.
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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. Acknowledgements The authors would like to acknowledge the financial support received from the Ministry of Human Resources Development and Department of Heavy Industries, Govt. of India under the IMPRINT scheme. Authors acknowledge Tatva Chintan Pharma Chem Pvt. Ltd. India for providing Tetraethylammonium tetrafluoroborate (TEA BF4) salt. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.est.2019.101114. References [1] X. Chen, C. Li, M. Grätzel, R. Kostecki, S.S. Mao, Nanomaterials for renewable energy production and storage, Chem. Soc. Rev. 41 (2012) 7909–7937, https://doi. org/10.1039/c2cs35230c. [2] J.R. Miller, P. Simon, Electrochemical capacitors for energy management, Sci. Mag. 321 (2008) 651–652, https://doi.org/10.1126/science.1158736. [3] E. Frackowiak, Supercapacitors based on carbon materials and ionic liquids, J. Braz. Chem. Soc. 17 (2006) 1074–1082, https://doi.org/10.1590/S010350532006000600003. [4] M. Mastragostino, F. Soavi, C. Arbizzani, Electrochemical supercapacitors, Adv. Lithium-Ion Batter. (2002) 481–505, https://doi.org/10.1007/0-306-47508-1_17. [5] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845, https://doi.org/10.1038/nmat2297. [6] N. Choudhary, C. Li, J. Moore, N. Nagaiah, L. Zhai, Y. Jung, J. Thomas, Asymmetric supercapacitor electrodes and devices, Adv. Mater.29 (n.d.) 1605336. doi:10.1002/ adma.201605336. [7] S. Zhang, N. Pan, Supercapacitors performance evaluation, Adv. Energy Mater. 5 (2015) 1–19, https://doi.org/10.1002/aenm.201401401. [8] H. Xie, S. Tang, J. Zhu, S. Vongehr, X. Meng, A high energy density asymmetric allsolid-state supercapacitor based on cobalt carbonate hydroxide nanowire covered N-doped graphene and porous graphene electrodes, J. Mater. Chem. A 3 (2015) 18505–18513, https://doi.org/10.1039/c5ta05129k. [9] T. Mitravinda, K. Nanaji, S. Anandan, A. Jyothirmayi, V.S.K. Chakravadhanula, C.S. Sharma, T.N. Rao, Facile synthesis of corn silk derived nanoporous carbon for an improved supercapacitor performance, J. Electrochem. Soc. 165 (2018)
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