materials Article
The Influence of the Interlayer Distance on the Performance of Thermally Reduced Graphene Oxide Supercapacitors Jun-Hong Lin Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan;
[email protected] Received: 18 January 2018; Accepted: 5 February 2018; Published: 8 February 2018
Abstract: In this paper, cationic surfactant cetyltrimethylammonium bromide (CTAB) was employed to prevent the restack of the thermally reduce graphene oxide (TRG) sheets. A facile approach was demonstrated to effectively enlarge the interlayer distance of the TRG sheets through the ionic interaction between the intercalated CTAB and ionic liquids (ILs). The morphology of the composites and the interaction between the intercalated ionic species were systematically characterized by SEM, SAXS, XRD, TGA, and FTIR. In addition, the performance of the EDLC cells based on these TRG composites was evaluated. It was found that due to the increased interlayer distance (0.41 nm to 2.51 nm) that enlarges the accessible surface area for the IL electrolyte, the energy density of the cell can be significantly improved (23.1 Wh/kg to 62.5 Wh/kg). Keywords: ionic liquids; supercapacitors; reduced graphene oxide; cationic surfactant; interlayer distance
1. Introduction Owing to global warming and the depletion of fossil fuels, the demand on alternative energy storage and conversion devices is growing higher than ever. As an energy storage device, the supercapacitor has drawn much attention because it has the advantages of long cycle life, high power capability, wide range of operating temperature and being maintenance free [1,2]. Supercapacitors are known to store energy in electrical double layer capacitors (EDLCs) based on ion adsorption on the electrode/electrolyte interfaces. Following the energy density Equation (1), E = 1/2 CV2
(1)
Most works related to the performance of EDLC cells are focused on employing electrode materials with high specific surface area, appropriate electrolyte with wide electrochemical window, and tuning the electrode/electrolyte interface properties [1,2]. Various kinds of carbon-based conductive materials or compounds have been reported for EDLC electrodes. Among these materials, graphene is one of the promising electrode materials toward high-performance EDLC because of its unique 2-dimensional structure with high intrinsic electron mobility and large theoretical specific area (2630 m2 /g) [3]. There are several approaches to producing graphene such as chemical vapor deposition [4,5], electric arc discharge [6], epitaxial growth on the specific surface [7] and chemical oxidation-reduction methods [8,9], etc. However, challenges remain in the preparing and processing of this 2-dimensional material. Among these approaches, the chemical oxidation-reduction method has the potential for mass production since graphene oxide (GO) can be easily obtained by the oxidation of natural graphite [9,10]. During the oxidation process, the defects and oxygen groups are introduced to the basal plane of the graphene. Thus, GO has a unique amphiphile with a negatively charged hydrophilic group and
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a hydrophobic basal plane. Due to the attached oxygen groups, GO is an electrical insulator and can be Materials 2018, 11, x FOR PEER REVIEW 2 of 13 dispersed in water [3]. However, for EDLC applications, the removalbasal of these oxygen groups is required to convert GO charged hydrophilic group and a hydrophobic plane. Due to the attached oxygen groups,the GO to the highly conductive reduced graphene oxide (RGO) by a post reduction process. The hydrophobic is an electrical insulator and can be dispersed in water [3]. nature of the graphene would lead to thethe quick aggregation the RGO sheets; the RGO However, for EDLC applications, removal of these of oxygen groups is required to sheets convertwould the thus lose attraction as individual objects and further GO to their the highly conductive reduced graphene oxideprocessing (RGO) by of a the postcomposite reductionmaterials process. would The of the graphene would lead toto the quick the RGO sheets; the RGO be hydrophobic hindered [3].nature The use of surfactants is common keep theaggregation suspensionofstabilized in solution while sheets would thus lose their attraction as individual objects and further processing of the composite preventing the restack of the RGO sheets in solid [11–13]. Ionic surfactants are amphiphilic compounds materials be hinderedhead [3]. The use ofand surfactants is common to keepresidues the suspension stabilized made up of would ionic hydrophilic groups extended apolar, organic and hydrophobic in solution while preventing the restack the RGO sheets solid [11–13]. Ionic surfactants are tails. Thus, surfactants could interact withofRGO through theinresidual charged group. In addition, amphiphilic compounds made up of ionic hydrophilic head groups and extended apolar, organic the hydrophobic interactions between the aliphatic chains and basal plane play an important role in and hydrophobic tails. Thus,[11]. surfactants could interact with RGO through the residual theresidues stabilization of RGO sheets in water charged group. In addition, the hydrophobic interactions between thecells aliphatic chains and of basal On the other hand, ILs are favorable for high-performance EDLC not only because their plane play an important role in the stabilization of RGO sheets in water [11]. high electrochemical window but also due to their advantages of low volatility and high thermal On the other hand, ILs are favorable for high-performance EDLC cells not only because of their stability. Zhang et al. [12] employed a series of ionic surfactants to stabilize the GO sheets during high electrochemical window but also due to their advantages of low volatility and high thermal the reduction process. They found that the surfactants can be successfully intercalated in both GO stability. Zhang et al. [12] employed a series of ionic surfactants to stabilize the GO sheets during the and RGO to prevent the restack phenomena. In addition, their results indicate that for the same reduction process. They found that the surfactants can be successfully intercalated in both GO and surfactant intercalated electrodes, the aqueous electrolyte has a larger capacitance than the IL electrolyte. RGO to prevent the restack phenomena. In addition, their results indicate that for the same surfactant This might be electrodes, due to the the factaqueous that aqueous electrolytes usually have smaller ionsThis than intercalated electrolyte has a larger capacitance than thecomprising IL electrolyte. themight IL electrolytes. Thus, the aqueous solutions may take advantage of their small ion size that be due to the fact that aqueous electrolytes usually have smaller comprising ions than the IL facilitates the ion transport in small pores. In contrast, the average ion diameter of ILs, example, electrolytes. Thus, the aqueous solutions may take advantage of their small ionDsize thatfor facilitates forthe 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI) is Dfor ~0.7 nm [14], ion transport in small pores. In contrast, the average ion diameter D of ILs, example, which is usually larger than the reported interlayer distance of the surfactant intercalated RGO for 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI) is D ~ 0.7 nm [14], (~0.4 nm).is Thus, small interlayer distance might distance reduce the accessible surface area forRGO ILs in which usuallythe larger than the reported interlayer of the surfactant intercalated nm). Thus,Here, the small interlayer distance might reducegraphene the accessible for ILs in the the(~0.4 RGO sheets. we employed thermally reduced oxidesurface (TRG)area as the conductive RGO sheets. Here, employed reduced oxidestructure, (TRG) as high the conductive material material because it is we chemical freethermally and has few layersgraphene of graphene specific surface area, because it is chemical free and has few layers of graphene structure, high specific surface area, and and high electrical conductivity [15,16]. IL, EMI-TFSI is chosen because of its high conductivity and electrical conductivity [15,16]. IL, EMI-TFSI is chosen because of its high conductivity and low lowhigh viscosity [14]. viscosity [14]. As shown in Figure 1, the schematic depicts the proposed process for tuning the interlayer shown in Figure 1,sheets. the schematic depicts the proposed for tuning the interlayer distanceAsbetween graphene The cationic surfactant CTABprocess was applied to intercalate in the distance between graphene sheets. The cationic surfactant CTAB was applied to intercalate the TRG sheets forming the TRGC composite [13] and afterward the EMI-TFSI was added to in interact TRG sheets forming the TRGC composite [13] and afterward the EMI-TFSI was added to+interact − with with the TRGC during the filtration to obtain the TRGCE composite. Since both (EMI ) (TFSI ) and the TRGC during the filtration to obtain the TRGCE composite. Since both (EMI+) (TFSI−) and ionic ionic surfactant CTAB are ionic materials, the Coulombic force between the intercalated CTAB and the surfactant CTAB are ionic materials, the Coulombic force between the intercalated CTAB and the EMI-TFSI might result in the large ionic aggregates or micelles in between TRGCE sheets and hence EMI-TFSI might result in the large ionic aggregates or micelles in between TRGCE sheets and hence increase the interlayer distance. increase the interlayer distance.
Figure 1. The schematic depicts the EMI-TFSI Figure 1. The schematic depicts the EMI-TFSIILILincorporates incorporateswith withthe theintercalated intercalatedCTAB CTABsurfactant surfactant to enlarge the interlayer distance of the graphene sheets. to enlarge the interlayer distance of the graphene sheets.
X-ray diffraction (XRD), small angle X-ray scattering (SAXS), Fourier-transform infrared X-ray diffraction (XRD), small angle X-ray scattering (SAXS), Fourier-transform infrared spectroscopy (FTIR), and thermal gravimetric analysis (TGA) were employed to characterize the spectroscopy (FTIR), and thermal gravimetric analysis (TGA) were employed to characterize the interlayer distance, the bond vibration and the compositions of these electrode materials. In addition, interlayer distance, the bond vibration and the compositions of these electrode materials. In addition, a VersaSTAT 4 potentiostat was employed to characterize the electrical performance of these two-electrode EDLC cells. It was found that the vibration mode of the CTAB head group was shifted
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a VersaSTAT 4 potentiostat was employed to characterize the electrical performance of these two-electrode EDLC cells. It was found that the vibration mode of the CTAB head group was shifted due to the2018, interaction implying the Coulombic or ion exchange interactions between Materials 11, x FOR with PEER EMI-TFSI, REVIEW 3 of 13the CTAB and EMI-TFSI [17]. Furthermore, the X-ray results revealed an interlayer distance of 0.41 nm as due toofthe interaction with EMI-TFSI, implying the Coulombic or ion exchange interactions between a result CTAB intercalation in between the TRGC sheets. In addition, the interlayer distance of the the CTAB and EMI-TFSI [17]. Furthermore, the X-ray results revealed an interlayer distance of 0.41 TRGCE sheets was further enlarged to 2.51 nm suggesting the formation of CTAB/EMI-TFSI aggregates as a result CTAB intercalation in between the TRGCand sheets. In addition, interlayer distance or nm micelles. Theofsubstantial increase in both capacitance energy densitythe of the TRGCE cell may of the TRGCE sheets was further enlarged to 2.51 nm suggesting the formation of CTAB/EMI-TFSI mainly be attributed to the larger interlayer distance that leads to more accessible surface area for the aggregates or micelles. The substantial increase in both capacitance and energy density of the TRGCE IL electrolytes. cell may mainly be attributed to the larger interlayer distance that leads to more accessible surface for the IL electrolytes. 2. area Experimental Procedures The thermally reduced graphene oxide (TRG) was purchased from GIBusiness company, 2. Experimental Procedures Taiwan and was synthesized from natural graphite by a modified Hummers method [8] followed The thermally reduced graphene oxide (TRG) was purchased from GIBusiness company, by a thermal treatment at elevated temperature. CTAB intercalated TRG termed as TRGC was prepared Taiwan and was synthesized from natural graphite by a modified Hummers method [8] followed by in athe presence of CTAB surfactant in water. Normally, TRGC electrodes were obtained by dispersing thermal treatment at elevated temperature. CTAB intercalated TRG termed as TRGC was prepared 10 in mg TRG powder in 30 mL of 0.1 M CTAB solutionTRGC with electrodes the aid of were ultra-sonication vigorous theofpresence of CTAB surfactant in water. Normally, obtained byand dispersing stirring 12 h. powder After that, the TRGC deposited andaid washed with D.I. water a Celgard 10 mgfor of TRG in 30 mL of 0.1solution M CTABwas solution with the of ultra-sonication andon vigorous 3500 separator (Celgard LLC, Charlotte, NC, USA) by a vacuum filter. Further, the TRGC electrodes stirring for 12 h. After that, the TRGC solution was deposited and washed with D.I. water on a were rinsed3500 with 15 mL of(Celgard 0.2 M ofLLC, EMI-TFSI ethanol filtration obtainthe theTRGC TRGCE Celgard separator Charlotte, NC,solution USA) byduring a vacuum filter. to Further, electrodes. comparison, the15TRG also prepared by dispersing 10filtration mg of TRG in 20 mL electrodesFor were rinsed with mL electrodes of 0.2 M ofwere EMI-TFSI ethanol solution during to obtain of the 20 wt % ethanol solution the same and filtration The TRGCE electrodes. For with comparison, thedispersion TRG electrodes were alsoprocess. prepared bydeposited dispersingelectrode 10 mg 2 of% was on aof1 20 cmwt 304 stainless steel current collector for EDLC cell EDLC of flipped TRG inover 20 mL ethanol solution with the same dispersion andassembly. filtration The process. 2 deposited electrode was flipped over onina a1 sealed cm of 304 stainless steel with current collector EDLC cellThe was in the form of two-electrode package testing bag filled EMI-TFSI asfor electrolytes assembly. The 2. EDLC was inmeasurement the form of two-electrode sealed testing bag filled 4, as cell shown in Figure The cell electrical was carriedpackage out withinaapotentiostat (VersaSTAT with EMI-TFSI asResearch, electrolytes as Ridge, shown TN, in Figure The electrical measurement was carried with 2 ). Princeton Applied Oak USA),2.with the uncertainty of resistance (±0.1 out ohm/cm a potentiostat (VersaSTAT 4, Princeton Applied Research, Oak Ridge, TN, USA), with the uncertainty The uncertainty of the averaged capacitance and the averaged energy density are TRG. and TRGCE cells show a more rectangle capacitance response, indicating that they are closer to ideal Taking advantage of the high electrochemical window of EMI-TFSI, the high operation voltage capacitors. addition, the capacitance ofThe these cells shows a clear sequence of TRGCE (3.2 V) In brings the high energy density ofresponse these cells. galvanostatic charge/discharge for these two- > TRGCelectrode > TRG. cells is performed at various current densities. The discharge currents of TRG, TRGC, and Taking of the high electrochemical EMI-TFSI, the high operation voltage TRGCEadvantage cells are shown in Figure 9. As can be seen,window a voltage of drop at the beginning of the discharging assigned to the voltage loss current across the equivalent resistance.for The (3.2 V)process bringsisthe high energy density ofwhen thesethe cells. Theflow galvanostatic charge/discharge these specific capacitance Cs of the graphene composites can be extracted from the galvanostatic discharge two-electrode cells is performed at various current densities. The discharge currents of TRG, TRGC, curve following and TRGCE cells areEquation shown (4) in [30–32], Figure 9. As can be seen, a voltage drop at the beginning of the 4C the 4I∆tcurrent flow across the equivalent resistance. discharging process is assigned to the voltage loss when (4) C = = m m∆V The specific capacitance Cs of the graphene composites can be extracted from the galvanostatic discharge m is the total weight of the active materials on both electrodes, C is the capacitance of the cell, curve where following Equation (4) [30–32], 4I∆t∆V is the potential change (excluding the I is the constant current, ∆t is the discharging4C time, and = (4) CS = initial voltage drop) during the discharging. As in Figure 10, the capacitance response of the mshown m∆V decreases a function of the discharging current density. The decrease in capacitance capacitance at wherecell m is the totalasweight of the active materials on both electrodes, C is the ofhigh the cell, current density is significant especially for the TRG cell. This is because the measured high current I is the constant current, ∆t is the discharging time, and ∆V is the potential change (excluding the density is a result of massive ions accumulated on the electrode in a short period. Therefore, the ions initial voltage drop) during the discharging. As shown in Figure 10, the capacitance response of the cell decreases as a function of the discharging current density. The decrease in capacitance at high current density is significant especially for the TRG cell. This is because the measured high current density is a result of massive ions accumulated on the electrode in a short period. Therefore, the ions
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may not have sufficient time to diffuse in and out of the deep region of the pores and would tend may not have timeof tothe diffuse in and out of the deep region of the and would tendarea to and to accumulate onsufficient the surface electrodes, leading to the decease ofpores accessible surface accumulate on the surface of the electrodes, leading to the decease of accessible surface area and hence capacitance [31]. At the low current density region, the ions might have sufficient time for the hence capacitance [31]. At the low current density region, the ions might have sufficient time for the accessible surface area. Therefore, the value of the specific capacitance implies the accessible surface accessible surface area. Therefore, the value of the specific capacitance implies the accessible surface area at the specific current density. At the low current density of 1 A/g, the specific capacitances area at the specific current density. At the low current density of 1 A/g, the specific capacitances of of the andTRGCE TRGCE 67 and 141respectively. F/g, respectively. Thehas TRGCE has a capacitance theTRG, TRG, TRGC, TRGC, and areare 43, 43, 67 and 141 F/g, The TRGCE a capacitance which which is 2.1 folds higher than that of TRGC, suggesting the increase of accessible surface area for the is 2.1 folds higher than that of TRGC, suggesting the increase of accessible surface area for the TRGCE TRGCE electrodes. As expected, thecurrent high current region, the capacitances of TRGC, the TRG, electrodes. As expected, at the at high densitydensity region, the capacitances of the TRG, andTRGC, to to 15 15 F/gF/g (at 6(at A/g), 54 F/g 54 (at F/g 18 A/g) 75 F/gand (at 18 the and TRGCE TRGCEdropped dropped 6 A/g), (at and 18 A/g) 75A/g), F/g respectively. (at 18 A/g),Still, respectively. has the capacitance becausebecause of its large distance distance that facilitates the ion the Still,TRGCE the TRGCE hashighest the highest capacitance of itsinterlayer large interlayer that facilitates transport. ion transport.
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Figure 8. Cyclic voltammetry (CV) curves of (a) TRG, (b) TRGC and (c) TRGCE cells at the various Figure 8. Cyclic voltammetry (CV) curves of (a) TRG, (b) TRGC and (c) TRGCE cells at the various voltage scan rates. (d) the capacitance response of the TRG, TRGC and TRGCE cells. voltage scan rates. (d) the capacitance response of the TRG, TRGC and TRGCE cells.
The energy densities are obtained by the following Equation (5) [30–32],
The energy densities are obtained by the following Equation (5) [30–32], 2 2 E = 1/2 CV = 1/8 Cs V
(5)
Moreover, the power density is deduced toCs Equation (6) [31,32], E = 1/2 according CV2 = 1/8 V2 E
P= Moreover, the power density is deduced according to Equation (6) [31,32], ∆t
(5) (6)
Ragone plot reveals the energy density of the TRG, TRGC and TRGCE cells as a function of E = (Figure 9), the large voltage drop and the small (6) power density as shown in Figure 11. As can bePseen ∆t capacitance of the TRG cell result in its small energy density of 15.3 Wh/kg at 1 A/g. In contrast, both TRGC and TRGCE have a relatively small voltage 1 A/g.and However, thecells TRGCE an energy Ragone plot reveals the energy density of the drop TRG,atTRGC TRGCE as ahas function of power density of 62.5 Wh/kg, which is 2.7 folds higher than that of TRGC (23.1 Wh/kg). The capacitance of density as shown in Figure 11. As can be seen (Figure 9), the large voltage drop and the small capacitance the TRGCE is 2.1 folds higher than that of the TRGC, suggesting that the increase of energy density of the TRG cell result in its small energy density of 15.3 Wh/kg at 1 A/g. In contrast, both TRGC and is mainly attributed to the increased capacitance. On the other hand, the energy density of the TRG TRGCE have a relatively small voltage drop at 1 A/g. However, the TRGCE has an energy density drops quickly with the current density as a result of its small capacitance and large initial voltage
of 62.5 Wh/kg, which is 2.7 folds higher than that of TRGC (23.1 Wh/kg). The capacitance of the TRGCE is 2.1 folds higher than that of the TRGC, suggesting that the increase of energy density is mainly attributed to the increased capacitance. On the other hand, the energy density of the TRG drops
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Materials x FOR PEER REVIEW of 13 and quickly with 2018, the 11, current density as a result of its small capacitance and large initial voltage 10 drop that limited its energy density of 0.3 Wh/kg at 6 A/g. Further, at a high discharge current of drop and that limited its energy density of 0.3 Wh/kg at 6 A/g. Further, at a high discharge current18ofA/g, drop and that limited its energy density of 0.3 Wh/kg at 6 A/g. Further, at a high discharge current of it is found that (12.6 Wh/kg) still hasstill a larger energy density than the TRGC Wh/kg). 18 A/g, it isTRGCE found that TRGCE (12.6 Wh/kg) has a larger energy density than the TRGC(10.4 (10.4 Wh/kg).
Potential(V) (V) Potential
18 A/g, it is found that TRGCE (12.6 Wh/kg) still has a larger energy density than the TRGC (10.4 Wh/kg).
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Figure 9. The discharge current vs. time for the TRG, TRGC and TRGCE cells at various discharging
The discharge current time theTRG, TRG,TRGC TRGC and at at various discharging FigureFigure 9. The9.discharge current vs.vs. time forforthe and TRGCE TRGCEcells cells various discharging current densities. current densities. current densities.
Figure 10. The capacitance density vs. current density of the TRG, TRGC and TRGCE cells.
Figure 10. The capacitance density currentdensity density of of the and TRGCE cells. Figure 10. The capacitance density vs.vs.current theTRG, TRG,TRGC TRGC and TRGCE cells.
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Figure The Ragoneplot plotof ofthe the TRG, TRG, TRGC cells. Figure 11. 11. The Ragone TRGCand andTRGCE TRGCE cells.
4. Conclusions 4. Conclusions In summary, we investigate the influence of the interlayer distance on the performance of the
In summary, we investigate the influence of the interlayer distance on the performance of the TRG based EDLC cells. The intercalation of CTAB in TRGC sheets results in an interlayer distance of TRG based EDLC intercalation CTAB in TRGC sheetsAresults in an interlayer distance of 0.41 nm, whichcells. is notThe sufficiently large forofthe EMI-TFSI ionic liquid. facile approach is demonstrated 0.41 nm, whichenlarge is not sufficiently large for the EMI-TFSI ionic liquid. A facile approach is demonstrated to further the interlayer distance between TRGCE sheets by the ionic interaction between the to further enlarge the interlayer distanceItbetween sheets by the interaction intercalated CTAB and the EMI-TFSI. is found TRGCE that for the TRGCE, theionic vibration mode ofbetween the intercalated CTAB CTAB was due to theIt ionic interaction the TRGCE, additionalthe EMI-TFSI. In addition, the intercalated andshifted the EMI-TFSI. is found that with for the vibration mode of the the X-ray data was reveal that the interlayer distance of the TRGCE is enlarged to 2.51 nm. TheIn results intercalated CTAB shifted due to the ionic interaction with the additional EMI-TFSI. addition, abovedata suggest the formation of large ionic aggregates or micelles TRGCE sheets. the X-ray reveal that the interlayer distance of the TRGCEinisbetween enlarged to 2.51 nm. Further, The results TRGCEthe cellformation has the highest energy of 62.5or Wh/kg, which is 2.7 folds higher than thatFurther, of abovethe suggest of large ionicdensity aggregates micelles in between TRGCE sheets. the TRGC cell (23.1 Wh/kg). The TRGCE cell (141 F/g) has a capacitance which is 2.1 folds higher than the TRGCE cell has the highest energy density of 62.5 Wh/kg, which is 2.7 folds higher than that of that of the TRGC cell (67 F/g), suggesting that the increase in energy density of the TRGCE cell is the TRGC cell (23.1 Wh/kg). The TRGCE cell (141 F/g) has a capacitance which is 2.1 folds higher mainly attributed to the increased interlayer distance that facilitates the ion transport through pores than that the increases TRGC cell F/g), suggesting increase in electrolyte. energy density of the TRGCE cell is and of hence the(67 accessible surface areathat for the the ionic liquid mainly attributed to the increased interlayer distance that facilitates the ion transport through pores Acknowledgments: The authors acknowledge the financial support of the electrolyte. Ministry of Science and Technology and hence increases the accessible surface area for the ionic liquid (MOST 106-2221-E-151-037).
Acknowledgments: The authors acknowledge theand financial support of the Ministry of Science and Technology Author Contributions: Jun-Hong Lin conceived designed the experiments, performed the experiments and (MOST 106-2221-E-151-037). wrote the paper; the authors read and approved the final manuscript.
Author Contributions: Jun-Hong Lin conceived and designed the experiments, performed the experiments and Conflicts of Interest: The author declares no conflict of interest. The funding sponsors had no role in the design wrote the paper; the authors read and approved the final manuscript. of the study, in the collection, analyses, or interpretation of data; in the writing of the manuscript; and in the
Conflicts of Interest: The decision to publish theauthor results.declares no conflict of interest. The funding sponsors had no role in the design of the study, in the collection, analyses, or interpretation of data; in the writing of the manuscript; and in the decision to publish the results. References 1.
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