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High Performance Supercapacitor Electrode Materials from Electrospun Carbon Nanofibers in-situ Activated by High Decomposition Temperature Polymer He Wang, Wenyu Wang, Hongjie Wang, Xin Jin, Haitao Niu, Hongxia Wang, Hua Zhou, and Tong Lin ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00083 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 3, 2018

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ACS Applied Energy Materials

High Performance Supercapacitor Electrode Materials from Electrospun Carbon Nanofibers in-situ Activated by High Decomposition Temperature Polymer He Wang,† Wenyu Wang,*,† Hongjie Wang,† Xin Jin,‡ Haitao Niu,§ Hongxia Wang, §Hua Zhou, §

and Tong Lin*§

†

School of Textiles, Tianjin Polytechnic University, NO. 399 Binshui West Street, Xiqing

District, Tianjin 300387, China ‡

School of Materials Science and Engineering, Tianjin Polytechnic University, NO. 399

Binshui West Street, Xiqing District, Tianjin 300387, China §

Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia

*

Corresponding authors’ emails: [email protected]; [email protected]

KEYWORDS: carbon nanofibers, electrospinning, activation, electrode, supercapacitor

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ABSTRACT: Most of previous works on in-situ activation of electrospun carbon nanofibers usually use a sacrificing polymer with a low decomposition temperature (e.g. less than 300 °C) as an activation agent, which leads to limited mesoporous structure and surface area in the carbon nanofibers. In this study, we have prepared carbon nanofibers by carbonization of electrospun polyacrylonitrile (PAN) in the presence of a high decomposition temperature polymer, polysulfone (PSF), which is homogeneously blended with PAN. The use of PSF as in-situ activation agent largely increased mesopore content, specific surface area, graphitization degree, inter-fiber connection, and conductivity in the carbon nanofibers. The PAN/PSF ratio showed an effect on these properties and 20% PSF within the precursor nanofibers (based on the polymer weight) had the best results. When using the PAN/PSF derived carbon nanofibers as an electrode material, the prepared supercapacitor showed a specific capacitance as high as 289 F/g at the scan rate of 10 mV/s and 257 F/g at the current density of 0.25 A/g. The device had excellent cycling stability (100% capacitance retention after 6000 cycles) and large energy capability (~36 Wh/kg). Carbon nanofibers activated with PSF may serve as a high performance electrode material for supercapacitor applications.


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INTRODUCTION Supercapacitor (SC) with high power density, excellent cycle stability and fast charge-discharge characteristics has attracted tremendous attention over recent years.1,2 Especially, the electrode materials for SC were widely developed because it is the key component deciding the capacitor performance. Among the electrode materials, such as carbon materials, transition metal oxides and conducting polymers, developed,3-5 carbon materials (e.g. activated carbon, carbon nanofiber, carbon aerogel, graphene and carbon nanotube) stand out owing to the high chemical/thermal stability, excellent cycle stability, and large current charge/discharge capability.6-10 Carbon nanofibers prepared from electrospun nanofibers (also called “electrospun CNFs” in some literatures) show great potential for applications as electrode materials not only for supercapacitor both also for batteries.11,12 Electrospun nanofibers from different polymeric materials have been used as precursors. For example, Kim et al.13-15 prepared CNFs from polyacrylonitrile (PAN), polybenzimidazole and poly (amic acid). Niu et al.16 prepared inter-bonded CNFs using polyvinylpyrrolidone (PVP)/polyacrylonitrile (PAN) side-by-side bicomponent nanofibers as precursor. They indicated that the inter-fiber connections can decrease the internal resistance and increase electrochemical capacitance. Cai et al.17 prepared inter-bonded CNFs by direct carbonization of cellulose acetate nanofibers which were partially hydrolyzed in KOH solution. Ma et al.18 fabricated microporous CNFs using the nanofibers electrospun from resole-type phenolic resin. By adjusting the KOH content in the spinning solution, the fiber diameter, microporous volume and specific area can be adjusted. Normally, carbon nanofibers prepared by the carbonization of electrospun PAN nanofibers show a dense structure. They need an activation treatment to form a porous surface before used for making supercapacitor electrodes. Several methods have been reported to increase surface area, the process of which was also referred to as activation treatment, such as 1) heating in a 3 ACS Paragon Plus Environment


specific atmosphere e.g. steam12-15 or CO2,17 2) treatment with potassium hydroxide solution followed by heating treatment,18,19 3) adding inorganic filler, e.g. zinc chloride, zinc oxide, calcium carbonate into nanofibers through electrospinning followed by removal of the filler after carbonization,20-22 4) in-situ activation by blending a sacrificial polymer, e.g. PVP or polymethylmethacrylate (PMMA), with the precursor polymer and then heating treatment.3,16,23-25 The first two methods often lead to considerable loss of CNFs during heating treatment. For the 3rd method, inorganic oxides remain in CNFs, which need to be removed. In-situ activation is a simple method to prepare porous carbon nanofibers. It does not need an extra treatment process except for adding a sacrificial agent into the precursor. After carbonization, the sacrificial agent degrades in a lower carbon yield, leaving lots of tiny pores within the carbonized product. The process thus does not add processing cost. In previous studies, sacrificial polymers such as PMMA and PVP have been used for making porous carbon nanofibers. These polymers have a lower decomposition temperature than the stabilization temperature of PAN, typically below 300 °C.16,23 As a result, the premature degradation of these sacrificial polymers led to relatively low mesoporous content and graphitization degree, hence low electrical conductivity.3,23,25 However, in-situ activation using a high decomposition temperature polymer sacrificial agent has not been reported in research literature. In this study, we have for the first time prepared porous carbon nanofibers from PAN using polysulfone (PSF) as in-situ activation agent. PSF is a thermoplastic polymer with a decomposition temperature of 480 °C and high chemical/thermal stability.26-28 The addition of PSF was found to increase fiber-fiber interconnection, mesoporous structure and graphitization degree, but reduce fiber diameter. We found that the PAN/PSF derived carbon nanofibers was especially suitable for making supercapacitor as an electrode material. The SC devices made for the PAN/PSF derived carbon nanofibers showed high specific capacitance (289 F/g at the 4 ACS Paragon Plus Environment

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scan rate of 10 mV/s and 257 F/g at the current density of 0.25 A/g), excellent cycling stability (100% capacitance retention after 6000 cycles) and large energy capability (~36 Wh/kg).

RESULTS AND DISCUSSION Figure 1 schematically illustrates the process for making carbon nanofibers. The morphology of the nanofibers is shown in Figure 2. All the polymer nanofibers showed a straight and uniform fiber structure without bead. For the pure PAN fibers, the average diameter was 310 ± 130 nm. PAN/PSF fibers showed smaller diameter than the pure PAN fibers. With increasing the PSF composition in the blend, the average fiber diameter decreased. For the fibers containing 20% and 25% PSF, the diameter was 190 ± 145 nm and 230 ± 90 nm, respectively (see the Supporting Information, Figure S2a). The slightly reduced fiber diameter can be explained by that the addition of PSF increases solution conductivity, which increases charge density and the forces for stretching jet/fiber.29 Increasing the PSF content in the polymer solution slightly increased the solution conductivity (see the Supporting Information, Figure S2c).

Figure 1. Schematic illustration of carbon nanofiber preparation.

After stabilization treatment, all the stabilized nanofibers show a slightly curved morphology (see the Supporting Information, Figure S1). After carbonation treatment, the electrospun nanofibers converted into carbon nanofibers. Figure 2 also shows the SEM images of the 5 ACS Paragon Plus Environment


carbonized nanofibers. The CNFs have a curved morphology with slightly smaller diameter when compared with their precursor counterparts. CNF-0 had a diameter of 190 ± 60 nm. For CNF-20 and CNF-25, the fiber diameter was 115 ± 65 nm and 120 ± 90, respectively (see the Supporting Information, Figure S2b). With increasing the PSF composition in precursor fibers, the CNFs tended to have more interconnections. This can be attributed to the melt of PSF during the carbonization process. Figure 2 also shows the cross-sectional SEM images of the fractured CNF samples. Lots of particle domains can be observed on fiber surfaces. This phenomenon was similar to the previously reported carbon nanofibers.25 We also used HRSEM to observe CNFs. Indeed, some porous structure can be seen on the fibers (see the Supporting Information, Figure S3). The porous structures allow the CNFs to have a large specific surface area.


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Figure 2. SEM images of electrospun NFs, CNFs, corresponding the cross-sectional of CNFs and the diameter distribution of the electrospun nanofibers. (Scale bar 100 nm).

The specific surface area and pore size distribution of CNFs were measured using the Brunauer-Emmett-Teller (BET) method (see the Supporting Information, Figure S4). All the samples had an adsorption isotherm of H4 type hysteresis loop, indicating the IV isotherm characteristic, a typical feature of mesoporous structure. In comparison with CNF-0, those from PAN/PSF blends showed increased adsorption capacity. The mesoporous structure and excellent adsorption capacity of CNFs facilitate the diffusion of ions and electrolyte, thus improving the charging/discharging rate, specific capacitance and cycle stability of the SCs.



The pore size distribution of CNFs was observed in Figure 3a. CNF-0 had a broad pore diameter distribution ranging from 1 nm to 100 nm. For the CNFs from PAN/PSF, the pore diameter was mainly on mesoporous scales (2-50 nm) (see the Supporting Information, Figure S5). The pore characteristics of CNF samples were listed in Table 1. The specific surface area and total pore volume of CNFs increased with increasing the PSF content in the precursor fibers, which were attributable to finer CNFs and the degradation of PSF during the carbonization. In addition, the mesoporous volume increased with increasing PSF content. Therefore, the addition of PSF to the nanofibers assists to the formation of mesoporous structure within CNFs. The chemical components of carbon nanofibers were examined by FTIR and XPS spectra. The PAN/PSF blends derived carbon nanofibers show similar vibration peak characteristics to the raw materials (see the Supporting Information, Figure S6). After carbonization, CNFs showed the disappearance of various functional groups such as –CN (2245 cm-1), C=O (1730 cm-1, 1666 cm-1) and –CH2 (1453 cm-1), suggesting that PAN nanofibers undergo cyclization of nitrile groups (-CN) during the stabilization process and form graphitized structure during carbonization. To verify the surface chemical composition, all the nanofiber samples were measured by X-ray photoelectron spectroscopy (XPS). As shown in Figure 3b, the CNFs mainly contain three elements, C, N and O. CNFs from PAN/PSF blends had relatively high C contents compared to CNF-0. The variation of element content is shown in the inset of Figure 3b. The CNF-20 had the highest carbon element content (95.6%). Nitrogen element in carbon shows important influence on the electrochemical performance of electrode material, resulting from excellent wettability of N-containing carbon in aqueous solution.30,31 For CNF-0, it had an N atomic content of 6.6%, which is higher than that of the CNFs from the PSF-containing precursors. It


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was noted that among all the CNF samples, CNF-20 had the lowest N content, as low as 2.7%, indicating the small effect of N on the performance of this CNFs. X-ray diffraction (XRD) reflects the crystallite property of CNFs. As shown in Figure 3c, the CNF samples have a strong diffraction peak at 2θ=22° and a weak diffraction peak at 2θ=16°, corresponding to the graphitic crystallite plane (002) and the PAN crystallite plane (100).32 The presence of (100) peak indicates incomplete carbonization. With increasing the PSF composition, the (100) peaks became weaker whereas the (002) peaks became stronger. When the PSF content increased from 20% to 25%, the (100) peak increased, and became stronger than that of CNF-10, CNF-15 and CNF-20. This result suggests that excessive PSF could diminish the formation of graphitic crystallite. The entire graphite crystalline thickness Lc (002) and the inter-layer distance d002 calculated are also listed in Table 1. The CNF-20 had the largest graphite crystallite thickness. The larger graphite crystallite thickness indicates higher electrical conductivity.33 Figure 3d shows the Raman spectra of the CNFs. All the samples showed two characteristic peaks at approximately 1585 cm-1 and 1350 cm-1 that corresponded to G and D bands. The G peak corresponded to the in-plane stretching vibration of sp2 hybridized carbon atom with perfect graphite crystal structure, while the D peak indicated the structural defect sites in the hexagonal framework of carbon and disorder-induced peak. The intensity ratio of the bands D and G (R=ID/IG) are used to characterize the graphitization degree of the CNFs. Lower R values indicate more sp2 hybridized carbon clusters in the samples. Table 1 shows the R values of CNFs. The CNF-0 showed the largest R value (1.26) among all the samples, and the R value decreased for the CNFs made of PAN/PSF. The lowest R value (0.84) was observed on CNF-20. The calculated in-plane graphitic crystallite size La values are also listed in Table 1. The CNFs from PAN/PSF had larger La value (see equation 3 in experimental section) than CNF-0. These results indicate that the presence of PSF improves the 9 ACS Paragon Plus Environment


graphitization degree of CNFs. The electrical conductivity results of CNFs are also listed in Table 1. The differential scanning calorimetry (DSC) curves of the polymer nanofibers in N2 atmosphere are shown in Figure 3e. The pure PAN nanofibers did not show melting peak, except for three exothermic peaks at approximately 285 °C, 530 °C and 820 °C. The sharp exothermic peak at 285 °C was originated from the stabilization process of PAN,34 while the other exothermic peaks corresponded to the carbonization process. In contrast, the pure PSF had a small melting peak at ~500 °C, and it tended to degraded at the same temperature with peaks at 700 °C and 900 °C. For PAN/PSF blends, the exothermic peak for stabilization did not change much. However, the carbonization peaks shafted to a certain extent, depending on the PSF composition. The carbonization peaks were identified based on the differential DSC curves (see the Supporting Information, Figure S7). The peak locations are also marked in Figure 3e. It was interesting to note that the samples NF-20 and NF-25 showed only one carbonization peak respectively at 620 °C and 550 °C, whereas the other samples show dual peaks. Such a single peak suggests that the carbonization completes in one step. Based on the DSC results, the thermal entropy for carbonization (∆Hcarbonization) was estimated (see the Supporting Information, Table S1). NF-20 and NF-25 showed larger ∆Hcarbonization than other NF samples. Although NF-25 has larger ∆Hcarbonization than NF-20, its main carbonization temperature was lower (550 °C). The difference in carbonization temperature could affect the carbon yield. Figure 3f shows the thermogravimetric analysis (TGA) curves of electrospun nanofiber samples (also see the differential TGA curves in Supporting Information Figure S8). The weight loss for electrospun PAN and PAN/PSF blend nanofibers all could be divided into three stages. They started losing weight at approximately 100 °C but degraded slowly. This weight loss resulted from the evaporation of moisture from the nanofibers. In the second stage (260 °C 10 ACS Paragon Plus Environment

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- 480 °C), the degradation was mainly stemmed from the cyclization PAN nitrile groups, and electrospun nanofibers degraded rapidly with increasing the temperature. In the range of 480 °C - 1000 °C, the stabilized PAN nanofibers converted to CNFs. The PSF composition showed an influence on carbon yield. With increasing the PSF content in the precursor fibers, the carbon yield increased initially until 20%, and then increased slightly with further increasing the PSF content. This result can be explained by the large carbonization temperature (620 °C) and ∆Hcarbonization. In contrast, pure PSF followed a different degradation route to PAN/PSF. It started degrading at 480 °C and lost weight up to 65% in the range of 480 - 600 °C. After carbonization, pure PAN (NF-0) and PSF showed carbon residue of 38.62% and 30.82%, respectively. Based on these data, one can calculate the theoretical residue of PAN/PSF blends using the equation: WT=YPANWPAN+YPSFWPSF, where Y is the residual mass percentage, and W is the mass percentage of the polymer in the PAN/PSF blends. The theoretical residues of NF-10, NF-15, NF-20 and NF-25 were 37.84%, 37.45%, 37.06% and 36.67%, respectively. In comparison, the actual residual values were 40.11%, 42.13%, 34.59% and 35.58%. NF-10 and NF-15 had an excessive residue than the theoretical value, while NF-20 and NF-25 had a lower carbon residual value (see the Supporting Information, Figure S9).



Figure 3. a) Pore diameter distribution curves of CNFs samples, b) XPS spectrum, c) XRD patterns and d) Raman spectrum of CNFs; e) DSC curves and f) TGA curves of PSF and electrospun nanofiber samples.

During carbonization, the PSF melt was proposed to undergo two possible processes, 1) directly decomposing at the original location within the PAN matrix, or 2) partially migrating out of the PAN matrix and accumulate at the fiber-fiber inter-section area. Because of the exothermic process, the decomposition of PSF might increase the local temperature, hence accelerating carbonization, and carbonization at a high temperature facilitates the graphitization process. On the other hand, carbonization at a higher temperature could increase carbon loss, favoring to form more pores. Therefore, the formations of mesoporous structure, graphitization and inter-fiber connection are all affected by the PAN:PSF ratio. It is reasonable to understand that the CNF-20 has the optimal PSF content leading to the most mesopores, higher graphitization degree with suitable fiber interconnection. To examine the electrochemical properties of the CNFs, we fabricated SC devices using CNFs as electrode materials and 6 M KOH aqueous solution as electrolytes. All the SC devices were 12 ACS Paragon Plus Environment

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measured using the cyclic voltammetry (CV) in the potential range of 0 to 1 V at a scan rate in the range of 10 mV/s – 100 mV/s. Each CV curve had a quasi-rectangular shape (Figure 4), indicating excellent electrochemical capacitive behavior and low internal resistance.

Figure 4. Cyclic voltammetry (CV) curves of a) CNF-0, b) CNF-10, c) CNF-15, d) CNF-20, e) CNF-25 from 10 mV/s to 100mV/s.

Figure 5a shows the CV curves of CNF electrodes at a scan rate of 10 mV/s. CNF-0 had the smallest rectangular area corresponding to low capacitance among the devices. The CV rectangular area for the CNFs made of PAN/PSF increased with increasing the PSF content in the precursor fibers. The CV specific capacitance (see equation 4 in experimental section) was calculated as listed in Table 1. The device made of CNF-20 had the highest specific capacitance of 289 F/g. Figure 5b shows the rate capability change when the scan rate changed from 10 mV/s to 100 mV/s. The CV specific capacitance retention of 67%, 71%, 69%, 70% and 71% resulted respectively for CNF-0, CNF-10, CNF-15, CNF-20, and CNF-25 (see the Supporting Information, Table S2). Specific capacitance with a small reduction indicates high cycle stability. 13 ACS Paragon Plus Environment


Figure 5. a) Cyclic voltammetry (CV) curves of CNFs samples, b) rate capability curves of CNFs from 10 mV/s to 100mV/s, c) Ragone plots of CNF samples, d) rate capability curves of CNFs from 0.25 A/g to 1.5 A/g, e) relationship between power density and energy density of the SC devices, and f) Nyquist plots of CNF samples.

The galvanostatic charge-discharge (GCD) is an important method to evaluate electrochemical performance of SCs. All the GCD curves have symmetric triangular shapes with small voltage drop at a current density of 0.25 A/g - 1.5 A/g, indicating excellent electrochemical capacitive behavior (Figure 6). Figure 5c shows that CNF-0 electrode has the shortest discharge time, indicating the smallest specific capacitance. The electrode made of the PAN/PSF-carbonized CNFs shown largely increased specific capacitance. Among them, the electrode from CNF-20 had the highest specific capacitance of 257 F/g. The calculated GCD specific capacitance (see equation 5 in experimental section) was listed in Table 1. The rate capability curves from 0.25 A/g to 1.5 A/g are shown in Figure 5d. As listed in Tab. S2, the retention rate of GCD specific capacitance was 86% - 94%. Such a high rate capability result indicates that CNFs possess good charge/discharge stability in SCs. 14 ACS Paragon Plus Environment

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Figure 6. Galvanostatic charge-discharge (GCD) curves of a) CNF-0, b) CNF-10, c) CNF-15, d) CNF-20, e) CNF-25 from 0.25 A/g to 1.5 A/g.

To further evaluate cycle performance of CNF electrodes, the SC devices were charged and discharged repeatedly for 6000 cycles by GCD technique at the current density of 1.0 A/g. After 6000 cycles of charge and discharge, the specific capacitance retention rate is almost 100% (see the Supporting Information, Figure S10). Power density and energy density are two important parameters determining the ultimate performance of SCs. Figure 5e shows the relationship between power density and energy density of the SC devices. The calculated power density and energy density results are listed in Table 1 (specific results in the Supporting Information, Table S3). The SC device has the highest energy density of 36 Wh/kg at a lower power density of 125 W/kg. The electrochemical impedance spectroscopy (EIS) was tested to analyze charge and ion transport process of CNF electrodes in SCs. Figure 5f shows the EIS spectra with the Nyquist plot in the frequency range of 10 mHz - 10 kHz at the open circuit voltage with an AC amplitude of 5 mV. At high frequency region, the semicircle reflects the charge transfer 15 ACS Paragon Plus Environment


resistance of CNF electrode materials. SC from CNF-0 had the largest semicircle, indicating the largest resistance for charge transport between the electrode and electrolyte. For the devices made of PAN/PSF carbonized CNF electrodes, the semicircle decreased clearly, suggesting the lower charge transfer resistance. The intercept at the real axis also showed that CNF electrodes prepared from PAN/PSF had smaller equivalent series resistance (Rs). At low frequency region, the linear part represents the ion transfer resistance of CNF electrode material. SC from CNF-0 exhibited an inclined linear relationship corresponding to poor capacitive performance. For the devices made of PAN/PSF carbonized nanofibers, the straight line moved toward the imaginary part (-Z2) when increasing the PSF content in the precursor fibers, indicating the excellent capacitive performance. The porous structure and physical properties of the CNFs were also compared with those prepared using a lower decomposition temperature polymer, i.e. PMMA3,24,25 and PVP,16 as the

in-situ activation agent (see the Supporting Information, Table S4). In comparison with those using PMMA, the mesoporous content for our CNFs is higher. The presence of PSF in PAN reduces the fiber diameter but increases the mesoporous content and surface area of the electrospun nanofibrous membranes. Based on these results, the formation mechanism for the highly-mesoporous carbon fibers was proposed below. During the stabilization of PAN, the PSF in the PAN matrix does not change because of the higher melt point and decomposition temperature than the stabilization temperature. During further carbonization process, PSF gets melted and then expends its volume. This causes some of the polymer migrates off the matrix, connecting between the fibers. Since PSF has a much lower carbon yield than PAN, its carbonization in PAN matrix results in voids, therefore a mesoporous structure. PSF melting before carbonization could be a reason to form a highly mesoporous structure after carbonization. 16 ACS Paragon Plus Environment

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It has been established that the storage of energy in double layer is mainly dependent on electrode/electrolyte interface performance.35 Mesoporous structure allows effective access of electrolyte to the electrode surface, and larger accessible surface offers larger double layer interface, which improves capacitator performances. Another reason for the improved SC performance is originated from the effect of PSF on carbon nanofiber formation. XRD and Raman results indicate that the presence of PSF in PAN increases the graphitization degree of the resulting CNFs. This can be explained by that the decomposition of PSF increases local temperature, which assists in the formation of oriented carbon structure. However, higher PSF composition could accelerate the decomposition reactions and increase carbon yield, hence decreasing mesoporous structure and SC performance. As a result of this two oppose trends, CNF-20 had high conductivity. In addition, when PSF is added to PAN nanofibers, the resulting CNFs show increased inter-fiber connection. The close connection is beneficial to the conduction of electrons, decreasing internal resistance.

CONCLUSIONS We have proven that in-situ activation of carbon nanofibers through adding PSF to the precursor PAN fibers can considerably increase the mesoporous content, graphitization degree, fiber-fiber interconnection and electrical conductivity, but reduce fiber diameter. These features largely improve the application performances of the carbon nanofibers as electrode materials for supercapacitors. In the optimal condition, the supercapacitor devices made of the nanofiber electrode show a specific capacitance as high as 289 F/g, excellent cycling stability (100% capacitance retention after 6000 cycles) and larger energy density (~36 Wh/kg). In-situ activation of carbon nanofibers through adding a high decomposition temperature polymer to



the precursor PAN nanofibers may offer a novel strategy to prepare high performance electrode materials for energy devices.

EXPERIMENTAL SECTION

Materials. Polyacrylonitrile (PAN, Mw=150000) from Spectrum Co., Ltd, Shanghai, polysulfone (PSF, Mw=56000) from Solvay Chemical Co., Ltd, N,N-dimethylformamide (DMF) from Kemiou Chemical Reagent Co., Ltd, Tianjin, potassium hydroxide (KOH, purity≥85%) from Fengchuan Chemical Reagent Science and Technology Co., Ltd, Tianjin were used as received. Deionized water was used in all experiment process.

Preparation of electrospun nanofibers and carbon nanofibers. PAN and PSF were dissolved separately in DMF to form homogeneous solutions. The two solutions were mixed together to form PAN/PSF solution mixtures. The overall concentration of PAN+PSF in the solution was 10%, and content of PSF based on the weight of the PAN/PSF blend was set at 10%, 15%, 20% and 25%. For electrospinning, a polymer solution was loaded to a purpose made electrospinning apparatus. During electrospinning, applied voltage, flow rate and spinning distance were controlled at 20 kV, 1.0 ml/h, and 18 cm. The as prepared nanofibers were collected on the collector covered with aluminum foil. The as-spun nanofibers were stabilized by heating at a rate of 2 °C/min to 260 °C and then maintained at 260 °C in air for 1 hour. The stabilized nanofibers were heated (heating rate of 5 °C/min). To prepare high performance carbon nanofibers, three carbonization temperatures, i.e. 800 °C, 1000 °C and 1200 °C (in nitrogen for 2 hours), were initially tested. The sample prepared at 1000 °C showed the best performance (see the result in Supporting Information). Therefore, carbonization temperature of 1000 °C was chosen for the systemic study. All the CNFs were treated without heat activation treatment. For comparison, pure PSF and PAN solutions were also prepared. In this paper, the polymer nanofibers were marked as NF-x 18 ACS Paragon Plus Environment

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where x is the content of PSF in the nanofibers. Similarly, the carbon nanofibers were marked as CNF-x, and x indicated the PSF content in the precursor fibers. For example, the carbon nanofiber prepared from pure PAN was marked as CNF-0, while the carbon nanofiber prepared from PAN/PSF nanofiber containing 20% PSF was marked as CNF-20.

Characterizations. The conductivity of polymer solution was measured by using METTLER TOLEDO conductivity meter. The surface morphology of nanofibers was observed on scanning electronic microscope (SEM, Gemini SEM500). High resolution transmission electron microscopy (HRTEM, JEM-2100F) was used to observe carbon nanofibers. Chemical structures were verified using Fourier transformed infrared spectroscopy (FTIR, Nicolet iS50, the wavenumber of 3000-500 cm-1). The graphitization degree of CNFs was measured on X-ray diffraction (XRD, D8 DISCOVER). The crystallite size along the c axis (Lc) and the inter-layer distance (d) were calculated based on the XRD data using the Scherrer36 and Bragg equations,37 respectively.

=

=

(1) (2)

where k is the shape factor (0.89), λ (0.15406 nm) is the wavelength of the X-rays, and β is the full width at half the maximum intensity (FWHM) of the (002) peak at 2θ of approximately 22°, n=1. Raman spectra were collected on a Raman spectrometer (Raman, XploRA PLUS) with a 514 nm excitation laser. The in-plane graphitic crystallite size (La) was calculated by the equation:38

=

(3)

C is a function of laser wavelength. Here C (514) = 4.4 nm. R=ID/IG and ID, IG are the intensity of D peak (1350 cm-1) and G peak (1585 cm-1). The thermogravimetric data were obtained on STA449F3 analyzer at a scanning rate of 10 °C/min in nitrogen atmosphere. Thermal



performances were measured on a differential scanning calorimeter (DSC, DSC 200F3) at a heating rate of 10 °C/min in nitrogen atmosphere. The surface chemical composition was examined by X-ray photoelectron spectroscopy (XPS, K-alpha) equipped with a 165 mm hemispherical electron energy analyzer. BET surface area was determined using the nitrogen adsorption method on Autosorb-iQ-C. The electrical conductivity of CNFs was measured using a semiconductor powder resistivity tester (ST-2722).

Fabrication of SC device and electrochemical measurement. CNFs were crushed by agate mortar. Then the crushed CNFs, carbon black, and polytetrafluoroethylene (PTFE) emulsion were mixed in ethanol at a mass ratio of 80:10:10 to obtain a slurry. The slurry was subsequently pressed on a circular nickel foam current collector (diameter: 1.2 cm; loading: ~2 mg) and dried at 80 °C overnight. The SC device was fabricated using two symmetric electrodes and a piece of PP/PE composite separator material. All SC devices were tested in two-electrode models. The cyclic voltammetry (CV) of the SC devices were measured by using an electrochemical workstation (Zahner, Zennium 2.0+CIMPS-2) between 0 and 1.0 V for a 6 M KOH aqueous electrolyte by varying the scan rate from 10 to 100 mV/s. The CV specific capacitances were calculated using following equation.

= ××∆

(4)

Where Cs (F/g) is the specific capacitance of SC at the scan rate from 10 to 100 mV/s, I (A) is the response current, ν (mV/s) is the scan rate, ∆V (V) is the potential change and M is the total mass of active materials on the two electrodes of the SC device. The charge/discharge performances at a current density of 0.25 A/g - 1.5 A/g and the 6000-cycling performances at the current density of 1.0 A/g of SC were tested using a cell test system (LANHE, CT2001A). The electrochemical impedance spectroscopy (EIS) measurements of the devices were carried out over the frequency range of 10 mHz to 10 kHz at the open circuit voltage with an AC amplitude of 5 mV. Power density (P) (W/kg) and energy 20 ACS Paragon Plus Environment

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density (E) (Wh/kg) were the key parameters of the SC, and they were calculated using following equations. ×∆

= ×∆

(5)

!

= × × ∆

(6)

" = ∆

(7)

where Cm (F/g) is the specific capacitance of SCs, ∆V (V) is the potential change within the discharge time ∆t (s) and M is the total mass of active materials in the two electrodes.

ASSOCIATED CONTENT

Supporting Information SEM images (Fig. S1), effect of PSF content on diameter and solution conductivity (Fig. S2), HRTEM images (Fig. S3), nitrogen adsorption isotherms (Fig. S4), pore diameter distribution (Fig. S5), FTIR spectra (Fig. S6), change rate curves-DSC (Fig. S7), differential TGA curves (Fig. S8), theoretical and actual residue percentage-TGA (Fig. S9), cycle stability (Fig. S10), effect of different carbonization temperature on the performances of carbon nanofibers (Fig. S11), thermal properties results (Tab. S1), specific capacitance results (Tab. S2), power/energy density results (Tab. S3), comparison of electrode performances (Tab. S4 and Tab. S5), comparison of material and electrode performances of carbon nanofibers at different carbonization temperature.

AUTHOR INFORMATION

Corresponding Authors *

Emails: [email protected]; [email protected]

ORCID Tong Lin: 0000-0002-1003-0671 21 ACS Paragon Plus Environment


Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51103101, 51573136), the China Postdoctoral Science Foundation (Nos. 2011M500525, 20110490785), the National scholarship fund of China (No. 2011812002), the Natural Science Foundation of Tianjin (Nos.12JCYBJC17800, 16JCTPJC45100), Science and Technology Plans of Tianjin (Nos.15PTSYJC00230, 15PTSYJC00240 and 15PTSYJC00250), and Australian Research Council Industrial Transformation Research Hub project (ARC IH140100018).

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Table 1. Pore and crystalline characteristics, conductivity and capacitance CNFs

a)

XRD

Pore Characteristics SBETa) (m2/g)

Vtotb) (cm3/g)

Raman

Vmesoc) (cm3/g)

Vmeso/Vtot (%)

d (Å)

Lc (nm)

La (nm)

R=ID/IG

Capacitance Conductivity

Carbon yield

(S/cm)

(%)

CV d) (F/g)

GCD e) (F/g)

Power density

Energy density

W/kg

Wh/kg

CNF-0

96

0.14

0.11

78.6

3.93

0.98

3.49

1.26

6.64

38.62

72

62

125

9

CNF-10

356

0.30

0.25

83.3

3.95

1.04

3.86

1.14

7.96

40.11

165

147

125

20

CNF-15

542

0.48

0.42

88.0

3.99

1.05

3.93

1.12

8.14

42.13

202

179

125

25

CNF-20

687

0.52

0.47

90.4

3.95

1.09

5.24

0.84

13.85

34.59

289

257

125

36

CNF-25

651

0.46

0.41

89.1

3.96

0.95

4.40

1.00

10.94

35.58

269

236

125

33

b)

c)

d)

SBET: specific surface area by BET; Vtot: total pore volume, measured at P/P0=0.995; Vmeso: mesopore volume by DFT; Measured by cyclic voltammetry (CV) at 10 mV/s;

e)

Measured by galvanostatic charge/discharge (GCD) at 0.25.

27

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Figure 1. Schematic illustration of carbon nanofiber preparation. 32x10mm (300 x 300 DPI)



Figure 2. SEM images of electrospun NFs, CNFs, corresponding the cross-sectional of CNFs and the diameter distribution of the electrospun nanofibers. (Scale bar 100 nm). 94x89mm (300 x 300 DPI)


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Figure 3. a) Pore diameter distribution curves of CNFs samples, b) XPS spectrum, c) XRD patterns and d) Raman spectrum of CNFs; e) DSC curves and f) TGA curves of PSF and electrospun nanofiber samples. 53x28mm (300 x 300 DPI)



Figure 4. Cyclic voltammetry (CV) curves of a) CNF-0, b) CNF-10, c) CNF-15, d) CNF-20, e) CNF-25 from 10 mV/s to 100mV/s. 53x28mm (300 x 300 DPI)


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Figure 5. a) Cyclic voltammetry (CV) curves of CNFs samples, b) rate capability curves of CNFs from 10 mV/s to 100mV/s, c) Ragone plots of CNF samples, d) rate capability curves of CNFs from 0.25 A/g to 1.5 A/g, e) relationship between power density and energy density of the SC devices, and f) Nyquist plots of CNF samples.



Figure 6. Galvanostatic charge-discharge (GCD) curves of a) CNF-0, b) CNF-10, c) CNF-15, d) CNF-20, e) CNF-25 from 0.25 A/g to 1.5 A/g. 51x26mm (300 x 300 DPI)


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TOC


High Performance Supercapacitor Electrode Materials from

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