Electrospinning of Carbon–Carbon Fiber Composites for High

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Electrospinning of carbon-carbon fiber composites for high-performance single coin-cell supercapacitors: Effects of carbon additives and electrolytes Phansiri Suktha, and Montree Sawangphruk Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02797 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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Industrial & Engineering Chemistry Research

Electrospinning of carbon-carbon fiber composites for high-performance single coin-cell supercapacitors: Effects of carbon additives and electrolytes 127x127mm (300 x 300 DPI)

ACS Paragon Plus Environment


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Electrospinning of carbon-carbon fiber composites for highperformance single coin-cell supercapacitors: Effects of carbon additives and electrolytes Phansiri Suktha,a and Montree Sawangphruka* a

Department of Chemical and Biomolecular Engineering, School of Energy

Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand *Corresponding author. Tel: +66(0)33-01-4251 Fax: +66(0)33-01-4445. E-mail address: [email protected] (M. Sawangphruk).

ABSTRACT

Carbon nanofibers incorporated with other carbon additives i.e. acetylene black (ACB), hollow carbon sphere (HCS) and reduced graphene oxide sheet (rGO) were successfully produced by an electrospinning process. The influence of carbon additives was evaluated through the electrochemical performances of carbon-based nanofiber supercapacitors. The carbon additives with high sp2 content not only enhance specific surface area but also improve electrical conductivity of the carbon nanofibers. A finely tuned 1 wt.% ACB loaded to the carbon nanofibers provides a specific surface area of 116 m2 g-1, a specific capacitance of 209 F g-1 at 2 mA a cell, a specific energy of 14.2 Wh kg-1, a maximum specific power of 8.3 kW kg-1, and


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>99% capacity retention over 10,000 cycles in 1 M H2SO4 aqueous electrolyte. The carbon fiber composites here may be practically used in supercapacitors.

KEYWORDS: Supercapacitors; Carbon fiber; Reduced graphene oxide; Carbon black; Energy storage

Introduction Carbon materials are widely used as electrode materials for electrochemical double layer capacitors (EDLCs) or supercapacitors due to their high specific surface area, excellent electrical conductivity, high thermal and chemical stability, and environment friendly. Among carbon materials, one-dimensional (1D) carbon fibers (CFs) have high aspect ratio, fast charge transport along the 1D fiber, and high porosity among adjacent fibers, which are good for high-performance supercapacitors. CFs can be prepared by a chemical vapor deposition (CVD) and spinning of carbon source. High purity of graphitic carbon fibers can be produced by CVD for which the decomposition of hydrocarbon molecules (e.g., methane, ethane, and carbon monoxide) takes places on the metal catalyst (e.g. iron, cobalt, nickel)1. However, this process has high production cost (US$50/kg)1. For the spinning process, the CFs are produced by the spinning of polymer precursor and then a thermal treatment of polymer fibers. There are many spinning techniques including wet, gel, melt, and dry spinning. Although the production cost of the spinning process is ca. US$20/kg1 lower than that of the CVD process, the diameter of CFs from traditional spinning is typically over 5 µm having very slow electron transport, which is not good for


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energy storage devices1. Among all spinning processes, an electrospinning technique is simple, efficient, and reproducible for producing CFs with a diameter less than 1 µm2-6.

CFs with large diamters are not widely used for high-performance supercapacitors since they show poor specific capacitance. This is because they have low specific area and poor charge

transport

leading

to

high

equivalent

series

resistance

(ESR)

of

the

supercapacitors7. It is well known that the charge trassport within carbon materials depends on the degree of graphitization. Of course, high carbonization or graphitization temperature (2200-3000 °C) of carbon materials including CFs can provide high graphitized content8. However, this process is rather expensive requiring very

much

energy. Alternatively, the addition of other conductive additives into CFs can enhance the graphitization of carbon fiber requiring much lower carbonization temperature9. As the low charge storage capacity of the CFs with large diameter (ca. 5 µm) limits their application for supercapacitors, in this work, we attempt to produce carbon/CF composites with small diamter (< 1 µm), which can have charge storage capacity due to their surface area and fast charge transport. Three carbon additives i.e., acetelene black (ACB), hollow carbon sphere (HCS), and reduced graphene oxide (rGO) were incorporated to CFs leading to carbon-carbon composites.

ACB and HCS are commonly used as conductive additives in supercapacitor and battery applications. Due to their excellent electrical conductivity and low cost, they are added into many materials (e.g. polymer and elastomer) for enhancing the electrical conductivity10-12. In the supercapacitor application, graphene film containning carbon


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black shows higher specific capacitance (136.6 F g-1 at 0.5 A g-1 in 6 M KOH) than that of pure graphene film (80 F g-1)11. To the best of our knowledge, ACB and HCS have not yet been incorporated into the as-electrospun carbon nanofiber for use in supercapacitors.

Previously,

porous

CFs

produced

by

the

electrospinning

process

of

polyacrylonitrile/poly(methyl methacrylate)/graphene exhibit 150 F g-1 at 1mA cm-2 in 6 M KOH when compared to pure CFs (110 F g-1 at 1 mA cm-2)13. CFs mixed with graphene nanosheets show 197 F g-1 at 1.25Ag-1 in 6 M KOH electrolyte14. CFs of PAN/graphene oxide/carbon nanotube show 120 F g-1 at 0.5Ag-1 in 0.5 M Na2SO415. CF/single-wall carbon nanotube composite exhibits 417 F g-1 at 0.5Ag-1 in 6 M KOH16. In this work, the CFs were produced by the electrospinning process with a following two-step thermal treatment and used as active materials for supercapacitors. To enhance the electrochemical performance of the CFs, carbon additives including ACB, HCS, and rGO were incorporated to the CFs. Interestingly, the ACB at 1 wt.% loading content finely tuned can significantly enhance the charge storage capacity of the CFs.

In addition, the ACB/CF

composite fibers here are an ideal electrode for metal current collector-free supercapacitors since they have high conductivity, high specific surface area, good corrosion resistance to acid electrolyte, controlled pore structure, and processability.

Results and discussion Morphologies and structures of carbon fiber composites The morphologies of the as-electrospun PAN composite nanofibers at different ACB contents (0, 1, 3, 5 wt.%) are shown in Figure 1. A diameter of PAN nanofibers is 500-600 nm (Figure 1a


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and b). Whilst, the diameters of ACB/PAN composite nanofibers loaded with ACB at 0.5, 1, 3, and 5 wt.% are ca. 900 nm, 900-1200 nm, 1-1.5 µm, and 1-2 µm, respectively. The larger diameters of ACB/PAN nanofibers are observed in Figure 1c-f due to the high viscosity of PAN solution when added with high ACB loading contents.


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Figure 1. FESEM images of (a) low- and (b) high-magnification of PAN fibers as well as (c) 0.5 wt.% ACB/PAN fibers, (d) 1 wt.% ACB/PAN fibers, (e) 3 wt.% ACB/PAN fibers, and (f) 5 wt.% ACB/PAN fibers.

After heat treatment, the diameter of CFs without ACB loading is decreased from 600 nm to ca. 400 nm (Figure 2a) because of the rearrangement of PAN chains to a cyclic structure so-called a ladder structure17 by the dehydrogenation and cyclization after stabilization and also the elimination of non-carbon atoms in the polymer after carbonization18. Figure 2b-d display the aggregated ACB along the carbon nanofibers causing the roughness on the surface. The diameters of ACB/CFs at 0.5, 1, 3, and 5 wt.% ACB are 700-800 nm, 800-900 nm, 0.8-1 µm, and 0.8-2 µm, respectively. A TEM image of the 1 wt.% ACB/CF composite is shown in Figure 2f. It clearly shows the incorporated ACB particles on the CF. Note, SEM and TEM images of ACB are shown in Figure S1a and b, respectively.


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Figure 2. SEM images of the carbonized samples: (a) carbon nanofibers (CFs), (b) 0.5 wt.% ACB/CFs, (c) 1 wt.%ACB/CFs, (d) 3 wt.% ACB/CFs, (e) 5 wt.%ACB/CFs, and (f) TEM image of 1 wt.%ACB/CFs (an inset lower magnification TEM image of CFs showing a diameter of CF).


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Figure 3 shows the morphology of the as-electrospun PAN composites with HCS and rGO at 1 wt.%. Figure 3a shows the node on the PAN fibers with a diameter range of 11.3 µm. Whilst, Figure 3b shows the rGO sheets on the PAN fibers with a diameter range of 1-3 µm. After heat treatment, it clearly shows the aggregated HCS particles attached on the CF surface with the diameters of 900-1000 nm (see Figure 3c). The rGO/CFs in Figure 3d have the diameter of 0.9-1.2 µm and the rGO sheets can be clearly observed on the CF surface. TEM images of CF composites with HCS and rGO are shown in Figure 3e and f, respectively. This confirmed that HCS and rGO are embedded onto the CFs. Note, SEM and TEM images of HCS and rGO are shown in Figure S1e-f.


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Figure 3. SEM images of PAN fiber composites with (a) 1wt.% HCS and (b) 1 wt.% rGO and SEM images of carbon fiber composites with (c) 1 wt.% HCS and (d) 1 wt.% rGO as well as TEM images of carbon fiber composites with (e) 1 wt.% HCS and (f) 1 wt.% rGO.


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The crystalline structures of the as-prepared CFs are shown in Figure 4a. The XRD patterns of carbon-based nanofibers exhibit the characteristic peaks at 26° and 44°, which are a (002) plane and overlapping (100) and (101) planes, respectively. Note, two seperated peaks of (100) and (101) planes can be observed in graphitic carbon after carbonized or graphitized at high temperature for which the (100) and (101) planes can be observed at ca. 42.5° and 44.5°, respectively19. In this work, an overlapping peak of (100) and (101) planes is found at 43.5° since the materials were carbonized at low temperature (1000 °C)4, 20.Table 1 listed the calculated spacing value for which the d002 value of the as-electrospun CFs is 3.626 Å, which is wider than 3.354 Å of graphite21. This is because of a weak interaction between each carbon layer in the CFs. The d002 value decreases when increasing the ACB content due to the strong interaction between ACB and PAN leading to higher orientation of PAN chians during heat treatment4. Not only does the interlayer spacing decrease, but also the stack height (Lc) increases. This is because the embedded ACB in PAN fibers affects the nucleation of carbon crystal growth and assists the orientation of PAN chains during carbonization4. Increasing the stack height (Lc) exhibits higher crystallinity of carbon composite nanofibers20. Note, the Lc value of CFs is 10.112 Å. Note, the XRD patterns of carbon addtives are shown in Figure S2. Raman spectroscopy is one of the most well-known techniques to study the graphitic structure

of

carbon-based

nanofibers.

The

Raman

spectra

of

carbon-carbon

nanocomposites are shown in Figure 4b. The characteristic peaks include G band around 1600 cm-1, which indicates the sp2 carbon graphitic structure and D band around 1300 cm1

due to the disorder sp3 carbon structure. A ratio of D band to G band (ID/IG) is the value


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used to explain the degree of graphitization of carbon fiber. The calculated lateral crystallite size (La)4 follows an equation (3)22. The ID/IG and La of carbon-based nanofibers are listed in Table 1. In Table 1, it can be seen that the ID/IG slightly decreases while the La increases with increasing of ACB contents. A degree of graphitization directly relates to the electrical conductivity since the mechanism of electrical conduction is due to the electrons moving in the graphene-like layers23. This result indicats that adding ACB to CFs can enhance the degree of graphitization of CFs, implying high electrical conductivity of CF/ACB composites. To further study the effect of additives, the XRD patterns of 1wt.%ACB/CFs, 1wt.%HCS/CFs, and 1wt.%rGO/CFs as compared to that of CFs in Figure 4a show the characteristic peaks at 26° (002 plane) and 44° ((100) and (101) planes). The interplanar spacing (d002) and the stack height (Lc) are shown in Table 1. The stack height (Lc) of 1wt.%HCS/CFs is 10.850 Å, which is slightly increased from CF (10.112 Å) but is lesser than 1wt.%ACB/CF (11.404 Å) and 1wt.%rGO/CF (11.216 Å). The Raman spectra of composites are shown in Figure 4b. Their characteristic peaks of D and G bands appear at ca. 1300 and 1590 cm-1, respectively. After the deconvolution, an A band at around 1500 cm−1 is also found, which refers to the amorphous or sp3-like carbon structure24. The fitted curves of D, G, and A bands are shown in Figure S3 and S4. The ID/IG, lateral crystallite size (La), and amorphous carbon content are listed in Table 1. The ID/IG of all samples slightly changes but the amorphous carbon content slightly decreases with increasing of ACB contents indicating higher graphitized content. The amorphous carbon contents of 1wt.%HCS/CF and 1wt.%rGO/CF are ca. 8-9% which are similar to that of


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1wt.%ACB/CF. In another word, loading ACB to CFs can improve the crystallinity of the composites leading to high electrical conductivity or fast chargr transport.

Nitrogen adsorption/desorption measurement was used to characterize the specific surface area and the porosity of the as-electrospun carbon-based nanofibers. Nitrogen adsorption/desorption isotherms of all samples in Figure 4c show type IV (IUPAC)

25

.

The specific surface area was calculated by using the BET equation. Figure 4d displays the pore size distribution in the mesopore range from Barrett-Joyner-Halenda (BJH) analysis. The specific surface area and average pore radius are summarized in Table 2. It is clearly seen that carbon nanofibers incorpolated with ACB, HCS, and rGO have higher specific surface area when compared with that of the bare CFs. Whilst, the average pore radius is decreased indicating that carbon additives can enhance the contact area of carbon composite nanofiber14. Especially, the 1 wt.%ACB/CF exhibits the highest specific surface area of 116.1 m2 g-1 and the lowest average pore radius (18.53 Å) when compared with other samples (see Table 2).


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Figure 4. (a) XRD patterns and (b) Raman spectra, (c) Nitrogen adsorption/desorption isotherms of CFs, ACB/CFs, HCS/CFs, and rGO/CFs and (d) pore size distribution curves of CF and 1wt.%ACB/CFs. Table 1. Crystalline parameters of carbon-based nanofibers with ACB contents.

Samples

Diametera d002 (nm) (Å)

Lc (Å)

ID/IG

La (Å)

%Amorphous

CF

400

3.626

10.112

0.941

201.9

10.69

0.5wt%.ACB/CF 700-800

3.610

10.165

0.933

203.4

10.38

1 wt%.ACB/CF

800-900

3.550

11.404

0.936

202.9

8.13

3 wt%.ACB/CF

800-1000

3.577

11.413

0.938

202.4

7.71

5 wt%.ACB/CF

800-2000

3.564

12.769

0.933

203.6

6.74


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1wt%.HCS/CF

900-1000

3.566

10.850

0.967

196.4

8.57

1wt%.rGO/CF

900-1200

3.564

11.216

0.931

203.9

9.05

a

The average diameter of composite fibers was determined by FE-SEM.

Table 2. Specific surface area and average pore radius of carbon-based nanofibers incorporated with ACB, HCS, and rGO at 1 wt%. Samples

Specific surface area (m2 g-1)

Average pore radius (Å)

CF

16.4

55.70

1wt.%ACB/CF

116.1

18.53

1wt.%HCS/CF

45.0

35.30

1wt.%rGO/CF

30.3

34.12

Effect of carbon additives to electrochemical properties of CF-based supercapacitors The electrochemical performances of carbon-based nanofiber composites with ACB, HCS, and rGO at 1 wt.% loading content are shown in Figure 5. The CV curves of 1wt.%HCS/CFs and 1wt.%rGO/CFs

are shown in Figure 5a for which the

electrochemical behaviour is similar to those of 1wt.%ACB/CFs and CFs. Although the capacitive current is a major contribution to the specific capacitance, the faradaic current stemming from the oxygen-containing groups on the carbon surfaces is clearly observed from the broad reversible oxidation and reduction peaks. This plays a significant role to the capacitance of the supercapacitors25-27. Note, cycling the potentials (0-2 V) to the carbon-based supercapacitors in 1 M H2SO4 (aq) can also generate the oxygen-containing


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groups i.e. -OH and -COOH28. From galvanostatic charge/discharge (GCD) curves at 2 mA (Figure 5b), the discharged time of 1wt.%ACB-CF is longer than those of other samples, resulting in the highest specific capacitance as shown in Figure 5c. This is because 1wt.%ACB-CF has higher specific surface area than 1wt.%HCS/CF, 1wt.%rGO/CF and CF. Also, 1wt.%ACB-CF has the smallest and most appropriate pore diameter (1.85 nm) for the adsorption of solvated electrolyte ions29 when compared with other samples prepared in this work. The Nyquist plots of the as-fabricated supercapacitors by electrochemical impedance spectroscopy (EIS) are shown in Figure 5d. At the low-frequency region, the 1 wt.%ACB/CF shows the vertical line closer to the Y-axis than CF, 1wt.%HCS/CF and 1wt.%rGO/CF leading to fast diffusion of the electrolytes into the electrode. At highfrequency region, it shows the smaller semicircle of CFs composited with ACB, HCS, and rGO when compared with the bare CFs. Note, the equivalent electrical circuit of CF and carbon-based nanofibers incorporated with ACB, HCS, and rGO at 1 wt% are shown in Figure S5e27. The ESR and charge transfer resistance (Rct) values of CF and its composites are listed in Table 3. The Ragone plots of the symmetric carbon-based nanofiber supercapacitors are shown in Figure 5e. The 1wt.%ACB/CF exhibits the highest specific energy and maximum specific power. The cycling stability of the 1wt.%ACB/CF supercapacitor is shown in Figure 5f. The results show the excellent stability >99% capacity retention over 10,000 cycles.


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Figure 5. (a) CVs, (b) GCDs, (c) specific capacitance vs. applied currents, (d) Nyquist plots, (e) Ragone plot of carbon-based nanofiber composite with ACB, HCS, and rGO at 1wt.%, and (f) cycling stability of the 1wt.%ACB/CF supercapacitor. Table 3. Equivalent series resistance (ESR)and charge transfer resistance (Rct) of carbonbased nanofibers incorporated with ACB, HCS, and rGO at 1 wt.%.


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Samples

ESR (Ω)

Rct (Ω)

CF

3.58

7.10

1wt.%ACB

3.10

3.30

1wt.%HCS

1.14

2.20

1wt.%rGO

2.50

2.00

Electrochemical properties of ACB/CF composites at different loading contents Figure 6 shows the electrochemical properties of the CF-based supercapacitors using 1 M H2SO4 electrolyte characterized by CV and GCD. A potential window of the cell is ca. 1.5 V indicating the charge storage mechanism of the CF-based EDLCs. It is clearly seen that 1 wt.% ACB in CFs (Figure 6a) provides the highest specific capacitance of 216.4 F g-1 at 20 mV s-1, which is higher than that of other samples. The specific capacitances of pure CFs, 0.5 wt.%ACB/CFs, 3 wt.%ACB/CFs, and 5 wt.%ACB/CFs are 123.8 F g-1, 132.8 F g-1, 123.7 F g-1 and 96.2 F g-1, respectively. The specific capacitances of 3wt.%ACB/CFs and 5wt.%ACB/CFs are rather low because of large composite fibers produced at these loading contents (see Table 1)30. The GCD results of the CF composites with ACB are shown in Figure 6b. A small iR drop is observed indicating a low ESR, which is rather good for high power application28. The discharged time of 1wt.%ACB/CFs is longer than that of other samples at the same applied current (2 mA). These results are in good agreement with the CV results. Thus, the stack height, lateral crystallite size, and degree of graphitization are increased by adding 1 wt.% ACB enhancing the electrical conductivity of CFs. Besides, the specific


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capacitance is decreased with increasing of the applied currents due to the limit of electrolyte diffustion (see Figure 6c). The Nyquist plots of the symmetric supercapacitors of the ACB/CFs at different ACB loading contents are shown in Figure 6d for which the X-axis is the real part and the Y-axis is the imaginary part. At low frequency, the straight line indicates the fast diffusion of electrolytes into the electrode. The 1 wt.% ACB/CF supercapacitor has nearly vertical Nyquist plot to the Y-axis indicating the ideal EDLC behavior than other samples31. At high frequency, the X-intercept means the ESR, which includes the resistance of electrolyte/separator, the intrinsic resistance of active material, the contact resistance between the active material and the conductive material, and the contact resistance between the electrode films and the current collectors and the semicircle means the charge transfer resistance (Rct). The inset image of Figure 6d displays the ESR values of both symmetric of CF and 1wt.%ACB/CF supercapacitors around 3 Ω while Rct value of 1wt.%ACB/CF is 3.05 Ω, which is ca. 2-fold smaller than that of the pure CF supercapacitor. This indicates the lower Rct of ACB/CF.


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Figure 6. (a) CVs and (b) GCDs, (c) specific capacitance vs. applied currents, and (d) Nyquist plots of ACB-CFs at different ACB loading contents. Conclusions The carbon/carbon composite nanofibers are produced by an electrospinning process with a twostep heat treatment process. The as-carbonized carbon-based nanofibers were used as the active materials in supercapacitors. Effects of carbon additives and contents were evaluated toward their electrochemical performances. Among the additives used, ACB shows a superior physicochemical property providing a high specific surface area of 116 m2 g-1 and high degree of graphitization or high electrical conductivity.

The ACB additive content affects the

electrochemical performance for which 1 wt.% of ACB finely tuned shows higher specific capacitance than other contents. In addition, 1 wt.%ACB in CFs shows higher electrochemical


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performance than HCS and rGO at the same content. The symmetric supercapacitor of 1 wt.%ACB/CF in 1 M H2SO4 shows the specific capacitance, specific energy, and maximum specific power of 209 F g-1 at 2 mA, 14.2 W h kg-1, 8.3 kW kg-1, and 100% capacity retention over 10,000 cycles, respectively. The charge storage mechanisms rely on both electrochemical double layered capacitance and pseudocapacitance. This could lead to the development of carbon fiber composites for use in supercapacitors since the production of the carbon fibers is commercially established in the large scale, which is not yet for graphene and carbon nanotubes. Experimental Electrospinning process of carbon nanofibers 1-g polyacrylonitrile (PAN, Mw=150,000, Aldrich) was dissolved in 10-ml dimethylformamide (DMF, Qrec 99.8%min) at 50 °C under a stirring condition. ACB (IRPC Co., Ltd., Thailand), hollow carbon sphere (HCS, EC300J from Lion Specialty) and reduced graphene oxide (rGO) synthesized from the modified Hummer’s method 32-34 were used as carbon conductive additives. For example, in the case of PAN-ACB CFs, 10 mg of ACB was mixed into a PAN solution and stirred for 24 h. A distance between the tip of the mixed solution in syringe and the aluminium foil collector is about 10 cm. As-electrospun fiber mat was then dried at an ambient temperature for 24 h. Finally, the mat was heated at 280°C at 2 °C min-1 for 2 h in air and carbonized at 1000 °C at 5°C min-1 for 1 h in N2 atmosphere. Besides, the ACB/PAN nanofibers were studied by varying ACB contents (0.01 g for 1 wt.%, 0.03 g for 3 wt.%, and 0.05 g for 5 wt.%). Fabrication of aqueous-based supercapacitor devices The active material/polyvinylidene fluoride adhesive binder (PVDF, MW=534,000) at a ratio of 9:1 by wt. was dispersed in N-methyl pyrrolidone (NMP, Qrec 99.5%) by sonication for 30 min.


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The mixture was spray-coated on a carbon paper substrate27 with a diameter of 1.56 cm and a mass loading around 2 mg per an electrode. The electrode was dried in an oven at 60°C for 24 h. The cellulose separator (Whatman no.5) soaked in an aqueous electrolyte (1 M H2SO4) was inserted between the two as-coated electrodes and assembled in a coin case (2016 size). The symmetric supercapacitors were fabricated by a hydraulic compressing machine at 800 psi. Characterization of the materials The morphologies of as-prepared materials were characterized by field-emission scanning electron microscopy (FE-SEM, JEOL JSM 7600F) and transmission electron microscopy (TEM, Hitachi HT7700). The crystalline structure was characterized by X-ray diffraction (XRD, PHILIPS, X'Pert-MPD 40 kV 35 mA, Cu Kα 1.54056 Å) with a 2Ө range between 10-80°, a step size of 0.02 °s-1. The graphitic structure was analysed by Raman microscope (SENTERRA, Bruker). The specific surface area was characterized by the Brunauer-Emmett-Teller

(BET)

method

(Autosorb

1

MP,

Quantachrome).

Electrochemical properties were measured by cyclic voltammetry (CV) at scan rates of 20-100 mV s-1 and electrochemical impedance spectroscopy (EIS) at a potential of 10 mV, frequencies of 1 mHz-100 kHz with the Autolab type III equipment. The galvanostatic charge/discharge was tested with the NEWARE battery tester at currents of 2-8 mA.

Calculations From the XRD result, the peak at the (002) plane can be used to calculate the interplanar spacing (d002) by Bragg’equation (1) and the stack height (Lc) by Scherrer’s equation (2)35;


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݀(଴଴ଶ) =

ఒ

ଶ௦௜௡௾(బబమ)

(1)

where λ is a wavelength of X-ray, d is a crystal plane separation, and Ө is an angle between the incident and the reflected beam; ‫ܮ‬௖ =

଴.ଽఒ

(2)

ఉ௖௢௦௾

where β is a full width at half maximum at (002) plane. From the Raman result, a ratio of D band to G band (ID/IG) is the value used to explain the degree of the graphitization of carbon fiber. This value can be used to calculate the lateral crystallite size (La) 4 by an equation (3)22; La (nm) =

560 E4l

ቀ ID ቁ I

-1

(3)

G

where an excitation laser energy (El) is equal to 2.33 eV (a wavelength of 532 nm). In addition, the specific capacitance of the supercapacitor electrodes from the CV is calculated by the eq. (4). The specific capacitance from the GCD as a function of the current is calculated by eq. (5); ொ

‫ܥ‬௦௣,஼௏ = 4 (௱௏∙௠)

(4)

where Csp,CV is a specific capacitance (F g-1) from CV, Q is an average charge in the discharged process (Coulomb, C), and m is the total mass of active material (g); ‫ܥ‬௦௣,ீ஼஽ = 4 ∆V x m I x ∆t

(5)

where Csp is a specific capacitance (F g-1) from GCD, I is a current (A), ∆t is a discharging time (s), ∆V is a potential window (V), and m is a total mass of active material (g).


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The specific energy and maximum specific power can be calculated by the equations (6) and (7), respectively27; 1

Specific Energy (E) = 2 Ccell ∆V2 V2

Maximum Specific Power (Pmax ) = 4R 0

(6) (7)

cell

where Ccell is the cell capacitance, V0 is an initial voltage of the cell, and Rcell is the cell’s resistance.

Supporting Information The supporting information consists FESEM and TEM images of carbon additives, XRD of carbon additives, the fitted RAMAN spectra of carbon-carbon composites, the fitted electrochemical impedance spectroscopy, and the specific capacitance calculated from cyclic voltammograms. Acknowledgments This work was financially supported by Thailand Research Fund and Vidyasirimedhi Institute of Science and Technology (RSA5880043). Instrumental support from the Frontier Research Center (FRC) at VISTEC is also acknowledged. Author information Affiliations Department of Chemical and Biomolecular Engineering, School of Energy Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand Phansiri Suktha, Montree Sawangphruk


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Contributions P. S. synthesized and characterized the materials and performed the electrochemical experiment and discussed the results. M. S. designed and directed the work, discussed the results, and wrote the manuscript.

Competing interests The authors declare no competing financial interests.

Corresponding author Correspondence to Montree Sawangphruk; E-mail: [email protected]

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Electrospinning of Carbon–Carbon Fiber Composites for High

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