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Thursday, February 13, 2020

Using The Asphaltene From Coal as Supermolecular Carbon Precursor for the Fabrication of Carbon Nanosheets

ABSTRACT

Asphaltene supermolecules extracted from coal consist of highly condensed polyaromatic units and peripheral aliphatic chains, which is a natural source with high carbon content.

In this study, we demonstrate that the asphaltene can be used as an ideal supermolecular carbon precursor for the fabrication of carbon nanosheets by self-assembly via π−π and hydrogen bonding interactions with a sheet-structure-directing agent of graphene oxide. The overall thickness of the obtained asphaltene based carbon nanosheets can be tuned from 13 ± 3 to 41 ± 5 nm. These carbon nanosheets show an electrical conductivity of ca. 450 S m−1.

When they are used as electrode materials for supercapacitors, the carbon nanosheets demonstrate a specific capacitance of 163 F g−1 even at a current density of 30 A g−1 tested in a three-electrode system, due to high electrically conductive networks and short diffusive paths. The maximum specific gravimetric capacitance and surface area-normalized capacitance in two-electrode system are 191 F g−1 and 43 μF cm−2, respectively, indicating very high utilization of the available surface area. These results prove that asphaltene is a promising molecular precursor for the preparation of energy materials, further displaying an efficient route for staged conversion of coal that is abundant in nature.


1. INTRODUCTION

Carbon materials play a crucial role in electrochemical energy- storage systems due to their extraordinary physicochemical properties  such  as  high  surface  area,  good  electrical conductivity, and excellent thermal and chemical stability.1−4 In order to achieve good electrochemical performance, the structural design and synthesis of carbon materials in a controlled manner is thus of great interest and driving the exploration of new carbon precursors for both fundamental research and practical applications.5,6 The “classical” carbon precursors, e.g. coal, coconut shell, or other complex biomass, are naturally abundant, low-cost and easily accessible and have been widely used to prepare carbon materials through the pyrolysis process mostly followed by an activation step.7 However, these “classical” precursors have disadvantages of high impurity content and high polymerization degree which causes barriers to structural control at the molecular level and improvement of electrochemical performance.

The use of molecular precursors provides the opportunity to exert control over the mesoscopic morphology of the final carbon materials by self-assembly of block copolymers, sugars, phenolic resin oligomers, etc.8,9 In our previous work, poly(benzoxazine-co-resol) was developed based on phenols, aldehydes, and diamines to prepare porous carbon monoliths with multiple-length-scale porosity (macro-, meso-, and micro- pores).10,11 However, molecular precursors are often expensive or difficult to be synthesized.12 To obtain naturally abundant and accessible molecular precursors, directly extracting them from classical carbon sources would be a sustainable strategy, and meanwhile achieve an efficient utilization of raw materials.13 For example, coal macromolecule unit typically consists of more than two aromatic rings, which are coupled by “bridges” of aliphatic chains or heteroatoms.14,15 In addition to covalent bridges, there are a significant number of noncovalent bonds such as electrostatic interactions, hydrogen bonds, and π−π interactions between aromatic rings.16 By dissociating these abundant weak noncovalent bonds, polycyclic aromatic hydrocarbons can be selectively extracted from coal. Due to enriched  sp2-hybridized  carbon  species,  polycyclic aromatic hydrocarbons are considered to be a promising molecular precursor for highly graphitized porous carbon.9 This property makes it promising to prepare porous carbons with excellent electrical conductivity for use as electrode materials.17,18

In this report, an alkyl-decorated aromatic hydrocarbon derivative, so-called asphaltene, is extracted from coal liquefaction residues, which as the byproducts or wastes are required to be used as much as possible with the purpose of improving the coal utilization value. Although asphaltene has been used as a carbon precursor in some previous reports,19,20 it is the first time asphaltene is used to prepare carbon nanosheets, as a supermolecular precursor, through solution chemistry and self-assembly process. The asphaltene based carbon nanosheets (ACNs) used as electrode materials for supercapacitors  in  a  6  M  KOH  electrolyte  exhibit  a  high capacitance of 214 F g−1 in a three-electrode system and can retain 163 F g−1 even at 30 A g−1, due to high electrically conductive networks and short diffusive paths. In a symmetrical capacitor, the specific gravimetric capacitance (Cg) and surface area-normalized capacitance (Csa) of 191 F g−1 and 43 μF cm−2, respectively, can be achieved. The synthesis not only presents a staged conversion approach of coal but also provides a promising precursor to prepare carbon materials for energy applications.

2. EXPERIMENTAL SECTION

2.1. Chemicals and Materials

Tetrahydrofuran and 1,6- diaminohexane (99.0% AR) were supplied by Sinopharm Chemical Reagent Co., Ltd. Graphene oxide colloids used in this work were prepared following a modified Hummers method.21,22 The graphene oxide colloids were dispersed in deionized water to obtain a specified concentration and sonicated at a power of 100 W for 4 h before use (KQ- 100TDB, Kun Shan Ultrasonic Instruments Co., Ltd., China). The preparation procedure of asphaltene used in this study was described elsewhere.23,24 Briefly, the asphaltene was a product extracted from coal liquefaction residues by a Soxhlet extractor with n-hexane and toluene as the extraction solvents.

2.2. Sample Preparation

2.2.1. Oxidation of Asphaltene.

Asphaltene (2 g) was oxidized in 10 M HNO (100 mL) under reflux at 70 °C for 16 h. The resulting brown suspension was then filtered through a 0.22 μm microporous membrane and washed repeatedly with water until the decantate became neutral. The obtained asphaltene oxide was dried at 90 °C overnight.

2.2.2. Synthesis of Carbon Nanosheets

Typically, 0.2 g of asphaltene oxide was first dispersed in 4 mL of tetrahydrofuran under magnetic stirring at 25 °C. After completely dispersing, a certain amount of the graphene oxide water solution was added to the above asphaltene oxide-tetrahydrofuran dispersion and stirred for ca. 5 min. Subsequently, 0.6 mL of 1,6-diaminohexane water solution (0.1 g mL−1) was quickly injected into the solution. The reaction mixture was stirred at 25 °C for another 5−10 min. The homogeneous solution was then sealed and transferred to an oven at 90 °C. It gelled and solidified within 4 h. This gel was cured for an additional 20 h. The gel was washed with distilled water to remove the tetrahydrofuran solvent and then freeze-dried for 48 h. The obtained polymer prior to pyrolysis was denoted as ACN-P. The obtained product was pyrolyzed at 800 °C for 2 h under a nitrogen atmosphere to obtain carbon nanosheets. By varying the mass ratio of graphene oxide to asphaltene oxide, different ACNs were prepared (denoted ACN-x, where x represents the mass ratio of graphene oxide/asphaltene oxide). In all syntheses, the mass ratio of asphaltene oxide to 1,6- diaminohexane was set to 10/3. For comparison, the sample prepared without graphene oxide by the same procedure was denoted as ACN-0. Also a sample without asphaltene was synthesized in the same process.

2.3. Structure Characterization

Thermogravimetric (TG) analysis was performed in N2 from 100 to 800 °C with a heating rate of 10 °C min−1 on an STA449 F3 Jupiter thermogravimetric analyzer (NETZSCH). The mass spectra were acquired using a matrix assisted laser desorption/ionization-time-of-flight (MALDI-TOF) instrument (Waters MALDI Micro MX, USA). The samples were prepared by dispersing 0.5 mg of asphaltene oxide in 1 mL of tetrahydrofuran. 1 μL of dispersion was transferred to a stainless  steel  target  plate  and  taken  to  dryness  before measurements. UV−vis absorption spectra were obtained on a UV−vis spectrophotometer (TECHCOMP, UV-2300), and the wavelength range was 200−700 nm. Fourier transform infrared spectroscopy (FT-IR) was performed with a Nicolet 6700 (Thermo scientific Co., Ltd., USA) by averaging 64 scans in the 670−4000 cm−1 spectra range at 4 cm−1 resolution. The morphology of the samples was characterized by FE-SEM  (NOVA  NanoSEM  450)  and   TEM (FEI  Technai   F30). Nitrogen sorption isotherms were measured with a Micro-  meritics TriStar 3000 physisorption analyzer. The Brunauer− Emmett−Teller (BET) method was used to calculate the specific surface areas (SBET). The electrical conductivity was measured on a bulk sample by a four-point probe resistivity measurement instrument. Raman spectra were collected on a DXR Microscope Raman Spectrometer, using a 532 nm line of KIMMON laser.

2.4.   Electrochemical   Measurements

A  6  M KOH aqueous solution was used as the electrolyte. The working electrode was prepared by mixing the active materials, PTFE and carbon black (mass ratio 80:10:10) in 7 mL ethanol, followed by ultrasonication for 20 min. A slurry of the mixture was rolled into a film, cut into a plate and dried at 150 °C for 6 h, followed by placing it on a nickel foam current collector. The electrode mass loading is 5−6 mg cm−1 and the thickness of electrode is ∼27 μm.

The capacitive performance was tested on an electrochemical workstation (CH Instruments Inc., Shanghai, China, CHI660D). Cyclic voltammetry (CV), galvanostatic charge− discharge cycling (GC), and electrochemical impedance spectroscopy (EIS) measurements were carried out at room
temperature with a conventional two-electrode electrochemical setup, in which the active material served as the working electrode and a Pt plate and Hg/HgO oxide were used as the counter electrode and reference electrode, respectively. The EIS spectra were recorded in the range from 10 mHz to 0.1 MHz with a signal amplitude of 5 mV. All of the electrochemical measurements were carried out at room temperature.

Specific capacitance (C) under three-electrode system was calculated from the discharge curve based on the following equation:
where I (A g−1) is the discharge current density based on the mass of active material, Δt (s) is the discharge time, and ΔV (V) is the potential window from the end of the internal resistance (IR) drop to the end of a discharge process.

A full cell was assembled with two symmetrical electrodes. The stability measurement of the full cell was carried out with an Arbin BT2000 multichannel college station (Arbin Instruments USA). The specific capacitance (C2‑electrode) and full cell capacitance (Ccell) were calculated from the discharge process after 20 cycles’ activation, according to the equation below:

where I (A g−1) is the discharge current based on the total mass of active material on anode and cathode and the definitions of Δt (s) and ΔV (V) are as same as those in eq 1. The Warburg element Z0 can be given by
where Zw is the bounded Warburg impedance, j is the imaginary unit (j = √−1), and ω the angular frequency (ϖ = 2πf, f being the frequency). Based on Z0, the diffusion coefficient (D) is calculated by
where R is gas constant (J mol−1 K−1), T is the absolute temperature (K), n is the valency of the ion, F (C mol−1) is the Faraday constant, A (cm2) is the area of electrode, and Cs (mol L−1) is the concentration of the electrolyte on the surface of electrode. Cs can be replaced by the bulk concentration of the electrolyte because of the reversible formation process of electric double layer. The energy density (E) was obtained from the capacitance of supercapacitors:
where V (V) is the potential window. The power density (P) of a full cell was determined by
where Δt (s) is the discharge time. To measure the leakage current at the working potential, the supercapacitors were first charged to 1 V and then the potential was kept at 1 V for 2 h, while the current flowing through the supercapacitors was recorded. For self-discharge test, the supercapacitor was first charged to 1 V, which was followed by a period of several hours at open circuit when the dependence of voltage on time was recorded.

3. RESULTS AND DISCUSSION

3.1. Preparation Principle and Procedure of the Asphaltene-based Carbon Nanosheets

A typical procedure is presented in Scheme 1. The asphaltene was first extracted and subsequently oxidized in a pretreatment step. The carbon nanosheets were prepared by using asphaltene as a carbon precursor, a small amount of graphene oxide as a sheet- structure-directing agent, and 1,6-diaminohexane as a bridging agent. The molecular precursors reorganize into a designed carbon nanostructure through a self-assembly process and followed by a pyrolysis step. The Tyndall effect was observed when asphaltene oxide was dispersed in tetrahydrofuran (Figure S1a), as well as graphene oxide in  water  (Figure  S1b), indicating that both dispersions are in a colloidal state. In a tetrahydrofuran and water cosolvent, asphaltene oxide, graphene  oxide,  and  1,6-diaminohexane  can  accomplish  a two-dimensional  “bottom-up”  self-assembly  via  π−π and hydrogen bonding interactions.25 The synthesis follows the conversion from noncovalent bonding to covalent bonding. An in situ polycondensation was ultimately achieved between the amino group of 1,6-diaminohexane and the carboxylic acid groups of asphaltene oxide and graphene oxide. After condensation and subsequent pyrolysis, carbon nanosheets were obtained.


Scheme 1. Illustration of Preparation Principle and Procedure of Asphaltene Derived Carbon Nanosheets

The π−π interaction between graphene oxide and asphaltene oxide can be demonstrated by UV−vis absorption as shown in Figure S2. The graphene oxide curve exhibits a broad peak at 231 nm and a shoulder peak at 290 nm, corresponding to π−π* electron transition of aromatic C−C bond and n-π* electron transition of C=O bonds, respectively.26,27 The absorption peaks gradually shift from 231 to 222 nm with adding asphaltene oxide into graphene oxide dispersion, resulting from the increased π conjugation between graphene oxide and asphaltene oxide molecules. The π−π interaction could assist the formation of the composite of graphene oxide and asphaltene oxide in the subsequent polymeric process.28,29 According to previous research, asphaltene is 0.1−1 nm in size,23,30 and the sizes of graphene oxide sheets are in the range of 500−3000 nm.31 The large size difference between asphaltene oxides and graphene oxide allows asphaltene oxide molecules to assemble on the graphene oxide substrate. The molecular weights of asphaltene oxides were evaluated based on a MALDI mass spectrum. The asphaltene oxide in this experiment has a widely polydisperse molecular weight distribution of m/z 500−1000 as shown in Figure S3.

The condensation reaction between asphaltene oxide and graphene oxide was evidenced by FT-IR. Figure 1a shows FTIR spectra of graphene oxide, asphaltene oxide, obtained polymer (denoted as ACN-P) and ACN after pyrolysis. The FT-IR spectrum of ACN-P shows peaks at 2927 and 1346 cm−1 for aliphatic C−H, which are also observed for asphaltene oxide. The formation of an amide covalent bond is proved by the movement of the C=O electron cloud from 1730 to 1700 cm−1.21 After pyrolysis, ACN shows peaks at 3400, 1560, and 1172 cm−1 for O−H stretching and C=N and C−N vibrations, respectively, indicating nitrogen/oxygen containing functional groups. This is coincident with element analysis results of ACN (C ≈ 91.5%, H ≈ 0.84%, O ≈ 6.66%, N ≈ 1.0%). UV−vis absorption and FT-IR results indicate that the supermolecular asphaltene oxide is capable of entering, through reactive groups, into further polymerization, thereby contributing as the molecular precursor to final polymer. Additionally, one sample without asphaltene was synthesized to demonstrate the reaction between graphene oxide and diamine. As shown in Figure S4 the FT-IR spectrum, the peak observed at 1184 cm−1 can be assigned to C−N vibrations.21,32,33

Figure 1. (a) FT-IR spectra of asphaltene oxide, graphene oxide, the obtained ACN-P, and ACN after pyrolysis and (b) TG curves under N2 atmosphere of asphaltene oxide, graphene oxide, and the obtained ACN-P.

To evaluate the pyrolysis behavior and the yield of precursors, TG tests of asphaltene oxide, graphene oxide and ACN-P were measured from 100 to 800 °C under N2 atmosphere and the TG curves were shown in Figure 1b. For asphaltene oxide, a decomposition of oxygenated carbon species mainly occurred below 500 °C. Destruction of molecules appears at 400−550 °C, involving dealkylation of aliphatic chains and dehydrogenation of naphthenic rings.34 For graphene oxide, a large mass loss around 200 °C is attributed to pyrolysis of the labile oxygen-containing functional groups, yielding CO, CO2 and H2O. Up to 200 °C, the sluggish mass loss corresponds to the removal of more stable oxygen functionalities.21,35 The TG curve of ACN-P is found to  decay smoothly from 200 °C to the final temperature, which is consistent with the pyrolysis behavior of polymeric precur- sors.36,37 The residual solid of ACN-P at 800 °C under N2 atmosphere is 63 wt %, which comfirms the polymerization and cross-linking between asphaltene oxide, graphene oxide, and diaminohexane. The high yield suggests that asphaltene can serve as a promising carbon precursor with high yield.

3.2. Structural Characteristics of the Asphaltene- Based  Carbon  Nanosheets

By varying the mass ratios of graphene oxide/asphaltene oxide, a series of ACNs samples with different thickness were synthesized and correspondingly denoted ACN-x, where x represents the mass ratio of graphene oxide/asphaltene oxide. As shown in Figure 2, the obtained ACNs were composed of thin carbon nanosheets with average thicknesses from 13 ± 3 to 41 ± 5 nm. The thickness gradually increases as the mass ratios of graphene oxide/asphaltene oxide decrease from 0.1 to 0.01. Once the mass ratio of graphene oxide/asphaltene oxide decreases to 0.005, the carbon units of ACN-0.005 form a honeycomb framework with macropores as shown in Figure S5a,b. The sample of ACN-0 was prepared in the absence of graphene oxide. It shows a coalesced structure with large particles size of 5−10 μm as shown in Figure S5c,d. The results reveal the sheet-structure-directing function of graphene oxide in the formation of sheet structure and the mass ratio  of  graphene  oxide/asphaltene  oxide  has  a significant impact on the morphology of the carbon products.

Figure 2. SEM images of the carbon nanosheets: (a, b) ACN-0.01, (c, d) ACN-0.02, and (e, f) ACN-0.1. The thin carbon nanosheets have average thicknesses from 13 ± 3 to 41 ± 5 nm.

The high resolution transmission electron microscope (HR- TEM) image of ACN-0.02 shows the interconnected micro- pores and graphene ribbons (Figure 3a). The crystallites compose of only several graphene layers; thus, they are considered as graphene ribbons.38 Abundant nanosized graphene ribbons formed in the asphaltene derived carbon are not yet found in other resin or biomass derived porous carbons.39,40 The TEM image is based on a view from the surface of carbon nanosheet. We believe that the formation of graphene ribbon regions results from the large number of sp2 graphene-like carbon in asphaltene. The graphene-like structure can enhance the conductivity of the obtained carbon. The measured conductivity of ACN-0.02 is 450 S·m−1, and the conductivity values of the other samples are in the same level. It is higher than that of commercial activated carbon (100−300 S· m−1) but lower than the reported reduced graphene oxide (500−5000 S·m−1).41,42

Meanwhile, the Raman spectra of ACNs were collected and shown in Figure 3b. There are two broadening bands: the D mode peak centered on ca. 1345 cm−1 and the G band at ca. 1588 cm−1. The D peak resulting from phonon mode is broad and intense due to the reduction in size of the in-plane sp2 domain caused by extensive oxidation. The G peak originates from the first order scattering of the E2g phonon of sp2 carbon hybridization.43,44 The graphitization degree of carbon materials can be evaluated by the intensity ratio of Raman peaks denoted as ID/IG, as listed in Table 1. The ID/IG values of ACNs are generally in range of 1.95 to 2.54, suggesting a feature of amorphous carbons.45 The wide peak at ca. 2680 to 2750 cm−1 are denoted as 2D, related to sp2 carbon, which is in agreement with the graphene layers observed by the HR- TEM.46 This could be ascribed to the plane polyaromatic units in asphaltene raw,47,48 which is hardly observed for the other polymer or resin carbon precursors.

Figure 3. (a) TEM image of ACN-0.02. The graphene ribbons structure was noted with yellow box and enlarged in the inset image. (b) Raman spectra of the carbon nanosheets derived from asphaltene and (c) N2 sorption isotherms of ACNs. The isotherms of samples ACN-0.005, ACN-0.01, ACN-0, and ACN-0.02 are vertically offset by 120, 105, 80, and 35 cm3 g−1 STP, respectively.

Table 1. Structural Parameters of the  ACNs

The porosity of the ACNs was measured by using N2 sorption technique. As shown in Figure 3c, the N2 sorption isotherms of all samples are essentially of type I, indicating microporous characteristics with an average pore size below 2 nm. ACN-0.005 presents a specific surface area of 671 m2 g−1, while that of ACN-0 is only 491 m2 g−1 (Table 1). The added small amount of graphene oxide can restrain asphaltene oxide from the stacking and ensure the pores on asphaltene oxide derived carbon are accessible, which is supposed to explain the increased surface area of ACN-0.005.49 When the graphene oxide continues to increase, specific surface area and micro- porous volume of samples gradually decrease. The similar phenomenon was observed in our previous work.37,50 It is likely because that graphene oxide does not contribute surface area but only plays a structure-directing role.

Figure 4. (a) CV curves of ACN-0.02 at scan rates of 5, 10, 20, 50, and 100 mV s−1 and (b) GC curves of ACN-0.02 at current densities of 1, 2, 5, 10, 20, and 30 A g−1. Inset is the enlarged GC curves at current densities of 10, 20, and 30 A g−1.

3.3. Electrochemical Performance of the Asphaltene- Derived Carbon Nanosheets

Encouraged by the attractive structural characteristics discussed above, ACNs are expected to achieve advanced performances as electrode materials for supercapacitors. The capacitive behavior of ACNs was first investigated in 6 M KOH electrolyte under a three-electrode system. Figure 4 show the cyclic voltammetry (CV) curves and galvanostatic  charge−discharge  cycling  (GC)  curves  of  a representative  sample ACN-0.02  at different scan  rates  and current densities. The other CV and GC curves of the rest ACNs samples are shown in Figure S6−S9. Specific capacitance of ACNs at various current densities ranging from 1 to 30 A g−1 are shown in Figure S10.

For the representative sample ACN-0.02, CV curve at 100 mV·s−1 (Figure 4a) still retains a quasi-rectangular shape, indicating a fast charge transport and ion diffusion. As GC curves shown in Figure 4b, the charge−discharge profiles have quasi-linear shape indicating a capacitve storage and good charge propagation. The Coulombic efficiency of ACN-0.02 is 97−100%. From the Figure 4b inset, the IR drops are 0.032, 0.067, and 0.15 Ω, for 10, 20, and 30 A g−1, respectively, indicating good conductivity of this sample. A slight deviation from a straight line in GC discharge curve at a low current density of 1 A g−1 demonstrates some Faradaic processes (Figure 4b) and the pseudocapacitance is estimated as 93 F g−1, which could be ascribed to nitrogen/oxygen doping and hydrogen storage as certified in many other reports.51−53 The other sheet-structure ACNs samples show similar capacitive behavior except for the sample ACN-0 with the coalesced structure (Figure S6−S9). When the current density increases to 30 A g−1, all thin carbon nanosheets remain high capacitance retention ratios of 69% 76%, and 75% for ACN-0.01, ACN-0.02, and ACN-0.1, respectively. For the representative sample ACN-0.02, the Cg remains 163 F g−1 even at 30 A g−1, highlighting an excellent suitability for high-rate operation. As provided in Table S1, a comparison indicates that the ACN-0.02 is superior to commercial carbons and comparable to many reported carbon nanosheet electrodes at high rates. The thin carbon nanosheets exhibit both high specific capacitance and good rate performance, due to their proper sheet thickness and good electrical conductivity. This suggests that ions can not only approach the spaces of carbon nanosheets but also diffuse fast during charge and discharge processes.

Figure 5. Capacitive performance of ACN-0.02 in a two-electrode system: (a) CV curves at a scan rates of 5, 10, 20, 50, 100, and 200 mV s−1 and (b) GC curves at current densities of 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, and 30 A g−1. Inset is the enlarged GC curves at current densities of 5, 10, 20, and 30 A g−1. (c) The capacitance retention of the supercapacitor in 15 000 cycles’ charge−discharge at a current density of 2 A g−1. Inset is the comparison of GC curves in the first ten cycles and last ten cycles of the long-term cycling process.

The electrochemical performance of ACN-0.02 has also been evaluated in a symmetrical two-electrode system. Measured in a potential range of 0−1 V, the CV curves remain quasi- rectangular shapes even at 200 mV s−1, as shown in Figure 5a. Based on GC curves in Figure 5b, the specific capacitance of ACN-0.02 is 191 F g−1 at 0.1 A g−1, which is well retained at 125 F g−1 when the current rate is increased to 30 A g−1. Based on the mass loading (∼5 mg cm−2) and thickness (∼27 μm) of one electrode, the carbon nanosheet electrode exhibits a density of 1.85 g cm−3. This electrode could show potentially a volumetric capacitance of 84 F cm−3, which is superior to that of most low-density carbon materials.54 The calculated Csa of ACN-0.02 is 43 μF cm−2 at a current density of 0.1 A g−1, which is much higher than the theoretical EDL capacitance (15−25 μF cm−2),55,56 implying an efficient utilization of surface area and a contribution of heteroatom doping. The value of Csa is also comparable to that of B or N doping graphene-based composites (38 and 22 uF cm−2, respec- tively),57,58 hierarchical porous carbon fiber (38 uF cm−2),59 and graphitic carbon fiber based composites (59 uF cm−2).60

Long-term cycling performance of ACN-0.02 supercapacitor was studied at a constant current density of 2 A g−1. As shown in Figure 5c, no capacitance decay is observed after consecutive 15 000 cycles. The specific capacitance gradually increases in the first 500 cycles. The initial increase is probably attributed to an activation process of the electrode material, i.e. gradual wetting of the electrolyte deep inside the electrode material. A similar phenomenon was also observed in other carbon electrodes.61,62 When the cycles went up to 5000 cycles, the abnormal increased capacitive effects may be attributed to the reduction of oxygen groups. A similar phenomenon was also observed in the other carbon electrode.63,64 In Figure 5c, inset, the GC curves in the first ten cycles and last ten cycles are both almost isosceles triangles, which illustrates the enhanced ideal capacitive characteristic during the cycling process. These results denote that highly reversible electrostatic adsorption and desorption of electrolyte ions occur on the electrode surface, indicating an infinite lifetime for the repetitive charge− discharge cycling. This suggests that the asphaltene derived carbon nanosheet possesses superior electrochemical stability with long cycle life, making it a promising candidate for long- term energy storage devices.

Figure 6. (a) Nyquist plots of ACNs in two-electrode system. Inset is the electric equivalent circuit model of these samples. (b) The enlarged high frequency range of the samples synthesized by adding graphene oxide and (c) leakage current curves and (d) self-discharge curves of the supercapacitors charged at 1 V for an aqueous electrolyte, and kept for 2 h.

The kinetic feature of the ion diffusion in nanosheet was further investigated in two-electrode system by using electro- chemical impedance spectroscopy (EIS). Figure 6a presents the Nyquist plots and electric equivalent circuit model of the ACNs. Except for ACN-0, all samples exhibit a straight line approaching 90° in the low frequency, representing an ideal capacitive behavior of the electrode. The enlarged plots of these samples in the high-frequency are shown in Figure 6b. The series resistor (Rs) represents the sum resistance of the electrodes, electrolyte, and separator. The Rs values of ACN supercapacitors are in range of 0.63 to 1.19 Ω. The Rs in three- electrode system can represent the real conductivity of the carbon materials. As shown in Figure S11, Rs decreases from 0.61 to 0.27 Ω with the graphene content of samples increases.

Table 2 shows the fitting value of charge transfer resistance (Rct), Warburg element (Z0), and diffusion coefficient (D) of oxide. Due to the short ion diffusion paths and good conductivity, the nanosheets samples show not only lower Rct but also higher diffusion coefficient compared to ACN-0 and ACN-0.005. Z0 presents the diffusion impedance for 1D linear diffusion. The Z0 and D of the ions in porous electrode were calculated based on eqs 4 and 5 in the Experimental Section. As listed in Table 2, with the thickness of ACNs decreasing from 41  ± 5  nm  (ACN-0.01)  to  13  ± 3  nm  (ACN-0.1),  the D increases successively from 1.18 × 106 to 5.13 × 106 cm2 s−1, implying an accelerated ion diffusion rate. This coincides with the relationship between thickness and rate capability as discussed earlier.

Table 2. Fitting Values of Rct, Z0, and D in two-electrode EIS of ACNs

For practical applications, leakage current and self-discharge characteristic of electrode materials are indispensable factors to consider. The charging currents (Figure 6c) reached to stable values of 0.0068 mA g−1 at the end of the time. As shown in Figure 6d, the voltage decays and keeps at 0.60 V after 2 h, which means a self-discharge rate of 40%. The results demonstrate   that  the  supercapacitor   exhibits   low leakage current and self-discharge characteristics. The supercapacitor achieves a maximum power density of 22.0 kW kg−1, as the Range plot shown in Figure S12, indicating high power performance and rate capability of this carbon nanosheet.

4. CONCLUSION

Using asphaltene as a carbon-rich molecular precursor, carbon nanosheets with controllable thicknesses have been prepared. This synthesis strategy demonstrates not only a novel carbon precursor for the preparation of a carbon nanosheet but also an upgrade utilization of naturally abundant coal. Excellent capacitive performance was achieved by using this carbon nanosheet as a supercapacitor electrode material. The unprecedented surface area-normalized capacitance and rate performance can be attributed to a short ion diffusion path and good electrical conductivity. Ion/charge transfer kinetics improved by the superior microstructure leads to an efficient utilization of the surface area. Carbon nanosheets with various thicknesses serve as a typical model for research on ion diffusion kinetics. These materials also have scope for uses in alternative applications such as catalyst supports, water purification, gas adsorption, and separation.

Source: Wen-Hui Qu,† Yu-Bo Guo,† Wen-Zhong Shen,*,‡ and Wen-Cui Li*,†
†State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China
‡State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Science, Taiyuan, 030001, China

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