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Article Cite This: ACS Omega 2019, 4, 195−202
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Chemical Vapor Deposition of Carbon Nanocoils ThreeDimensionally in Carbon Fiber Cloth for All-Carbon Supercapacitors Shin Hu,† Chi-Young Lee,‡ and Hsin-Tien Chiu*,† †
Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30010, ROC Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan 30013, ROC
‡
ACS Omega 2019.4:195-202. Downloaded from pubs.acs.org by 193.93.192.66 on 01/06/19. For personal use only.
S Supporting Information *
ABSTRACT: An Au/K bicatalyst-assisted chemical vapor deposition process using C2H2(g) to grow high-density carbon nanocoils (CNCs) uniformly on the fibers in carbon fiber cloth substrates three-dimensionally was developed. An as-deposited substrate (2.5 × 1.0 cm2) showed a high electrochemical active surface area (16.53 cm2), suggesting its potential usefulness as the electrode in electrochemical devices. The unique one-dimensional (1D) helical structure of the CNCs shortened the diffusion pathways of the ions in the electrolyte and generated efficient electron conduction routes so that the observed serial resistance Rs was low (3.7 Ω). By employing two-electrode systems, a liquid-state supercapacitor (SC) in H2SO4(aq) (1.0 M) and a solid-state SC with a polypropylene (PP) separator immersed in H2SO4(aq) (1.0 M)/polyvinylalcohol were assembled and investigated by using CNC-based electrodes. Both devices exhibited approximate rectangular shape profiles in the cyclic voltammetry measurements at various scan rates. The observations indicated their electric double-layer capacitive behaviors. From their galvanostatic charge/discharge curves, the specific capacitances of the liquid SC and the solid SC were measured to be approximately 137 and 163 F/g, respectively. In addition, the solid-state CNC-based SC possessed excellent energy density (15.3 W h/kg) and power density (510 W/kg). The light weight solid SC (0.1965 g, 2.5 × 1.0 cm2) was bendable up to 150° with most of the properties retained.
1. INTRODUCTION
materials include carbon nanotube (CNT), reduced graphene oxide, and carbon fiber (CF) composites.10−17 In addition to the carbon-based materials mentioned above, a unique class of one-dimensional (1D) carbon material known as carbon nanocoils (CNCs) has also been developed.18−22 Because of their unique morphology, they provide high specific surface areas. For example, a value 105 m2/g was reported for pristine CNCs.23 The result is much superior to those of similar electrospun carbon nanofibers, 10−30 m2/g. Also, it is comparable to the value of a CNT sample, 272 m2/g.24 The 1D structure of the CNCs not only offers high electrical conductivities 20−200 S/m but also allows more of the electrode material to penetrate deeply inside the electrolyte.25,26 The increased electrolyte/electrode contact reduces the interfacial resistance as well.27 The drawbacks of the traditional growth of CNCs by chemical vapor deposition
Portable and wearable devices, such as mobile phones, tablet computers, flexible displays, and biosensors, have flourished over the past decade.1−4 Thus, the development of energy storage devices is urgently required to meet these specific applications. Supercapacitors (SCs), an emerging class of energy storage devices, attract tremendous attention. They provide fast charge/discharge rates, high power densities, and long cycling life.5,6 Depending on different charge storage routes, SCs can be categorized into pseudocapacitors and electric double-layer capacitors (EDLCs). Pseudocapacitors store energy based on reversible Faradic redox reactions between the electrolyte and the electrode surface. On the other hand, EDLC devices store energy by charging and discharging the interface between the electrode surface and the electrolyte. Therefore, without chemical reactions, the EDLC devices exhibit extraordinary stability and reversibility in repeated charge/discharge cycles.7,8 Recently, a variety of all-carbon SCs are reported because of their thermal stability and chemical inertness in the environment.9,10 Here, the applied electrode © 2019 American Chemical Society
Received: August 29, 2018 Accepted: December 19, 2018 Published: January 4, 2019 195
DOI: 10.1021/acsomega.8b02215 ACS Omega 2019, 4, 195−202
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Scheme 1. Schematic Illustration of the Fabrication Process of CNCs-Based Electrodes
(CVD) include the needs of high growth temperatures and magnetic transition metal catalysts.21 Previously, we reported that by using Ag/K and Au/K cooperative bicatalysts, high-density CNCs were grown on the surfaces of Si and graphite foil substrates, respectively, by CVD at a relatively low temperature 723 K employing C2H2(g) as the precursor.28,29 The process provides possibility to grow nonmagnetic CNCs on substrates that may not sustain under high-temperature processing. To broaden the usefulness of the CNCs, we envision that growing CNCs by CVD threedimensionally inside the vacant space of a common CF woven cloth would provide a high surface area electrode for electrochemical devices. In order to achieve this goal, we employ our previous bicatalyst CVD process for the fabrication. It is further improved so that the nanoparticles (NPs) of both Au and K can be grown conformally deep inside on all fibers of the CF cloth (CFC). Here, we report our findings on the development of this new CVD process for the growth of CNCs inside the CFC substrate. We discover that the substrate can be applied as an electrochemical electrode for high-performance SC devices.
2. RESULTS AND DISCUSSION 2.1. Growth of CNCs. A process was developed to deposit Au NPs and K(l) catalysts conformally on CFC substrates so that CNCs would grow three-dimensionally inside the substrates via a vapor−liquid−solid (VLS) type reaction pathway.28−30 To a cleaned CFC, a thin Au layer was grown on both sides by e-gun physical vapor deposition (PVD) (Figure S1 in the Supporting Information). In the later high temperature steps, the Au layer was transformed into Au NPs. To increase the density of Au NPs inside the cloth, it was further immersed in HAuCl4(aq) followed by H2(g) reduction at 443 K. Then, the CFs of the substrate were coated uniformly with K(l) evaporated at 443 K. C2H2(g) and H2(g) were introduced to grow CNCs via the Au/K bicatalyst-assisted VLS mechanism. The overall fabrication process is displayed in Scheme 1. The substrate with the as-deposited brown product was washed and dried to offer the CNC-based electrode (see Figure S2 and Experimental Parameter Optimization in the Supporting Information). 2.2. Characterizations of CNCs. The as-fabricated electrode was characterized by scanning electron microscopy (SEM) studies (Figure 1). The image in Figure 1A reveals the
Figure 1. SEM studies of the CNCs grown on a CF substrate. (A) Low-magnification top-view SEM image of the as-fabricated sample, (B) high-magnification image of the CNCs (inset: detailed morphology of a CNC), and (C) low- and (D) high-magnification images from the cross-sectional edge of the substrate showing lots of CNCs. (E) High-magnification image of an individual CNC and (F) EDS data from the tip.
successful 3D growth of high-density CNCs on CFC. As shown in Figure 1B, CNCs with diameters of 100−300 nm and lengths of tens of micrometers were grown. In the inset of Figure 1B, a high-magnification image exhibits some creases on the CNC surface. The cross-sectional images of the as-grown CNCs on the edge of the CFC substrate sample are shown in Figure 1C,D. The result confirms that the Au NPs and K(l) catalysts were effectively deposited on all surface of fibers. On a particular CNC shown in Figure 1E, the energy dispersive spectroscopic (EDS) signals from both Au and K are observed 196
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Figure 2. TEM studies of a typical braidlike CNC. (A) Image of the tip of the CNC and (insets) SAED patterns from the squared regions, (B) HR image of the tip, and (C) EDS mapping from the red squared region in (A).
metric analyses (TGA) shown in Figure S5. The mass loss of CFC is observed at ca. 850 K. On the other hand, the CNC electrode shows two mass drops at ca. 600 and 800 K. Consequently, the 15.8% mass-loss at 600 K is assigned to the vanishing of the less-ordered CNCs while the one at 800 K is attributed to the CFC. From this result, the mass loading of CNCs in the electrode (0.019 g, 2.5 × 1.0 cm2) is estimated to be 3.0 × 10−3 g. The thermal stability of the CNC electrode agrees with the structural difference between the CNCs and substrates discussed in the XRD and the Raman studies above.33 Previously, we developed CVD processes to grow CNCs from C2H2(g) on Si and graphite substrates using K(l) and NPs of Ag or Au as the catalysts.28,29 We anticipated that a similar route could be applied to grow CNCs on CFC too. However, by using the experimental steps shown in Figure S6 in the Supporting Information, we discovered that CNCs grew only as a thin layer on the top of the substrate surface, as shown in Figure S7. When a CFC with both sides coated with K(l) and Au NPs were placed vertically in the reactor, thin layers of CNCs were grown only on two surfaces of the substrate. No carbon nanostructures were observed inside the CFC (Figure S8). This observation is attributed to the absence of Au or K catalysts in the interior of the CFC. Another possibility is that the precursor molecules did not diffuse effectively inside the CFC. As displayed in Figure S9, K signals were detected on the surface of CFs in the cloth by EDS. This suggests that K atoms, one of the catalysts, covered all CF surfaces evenly. On the other hand, as shown by the SEM image and the EDS data in Figure S10, the PVD Au layer did not grow uniformly on every CF, especially the ones inside the cloth. This is due to the shadow effect from the PVD process.34−36 We conclude that high-density Au NPs must be deposited homogeneously on all fibers of the substrate to assist the uniform growth of CNCs inside the CFC. An improved process was developed to accomplish this goal. As shown in the steps depicted in Scheme 1, this was achieved by further impregnating the CFC substrates coated with a layer of PVD Au into a HAuCl4(aq) solution followed by thermal reduction in H2(g) at 443 K. As described in the Supporting Information, a set of optimized reaction conditions were determined (see Experimental Parameter Optimization, Figures S11−S16, and Table S1). Under this condition, sample 8, with the densest and longest CNCs on all CFs of the substrate, is selected as the CNCbased electrode for the SC studies. In Figure S17 in the Supporting Information, the cross-sectional SEM image and the EDS characterization of the interior of the processed CFC reveal the even growth of Au NPs by the improved process.
(Figure 1F). The role of the catalysts and the details of the growth mechanism will be discussed more below. A typical braidlike CNC, shown in Figure 2A, was characterized by transmission electron microscopy (TEM). It tightly spirals with a uniform coil diameter of ca. 80 nm and a pitch of ca. 60 nm. The selected area electron diffraction (SAED) pattern in the upper left inset of Figure 2A exhibits diffused diffraction rings corresponding to lattice spacing values 0.34, 0.22, and 0.13 nm. These are assigned to (002), (100), and (110) planes of less-ordered graphitic carbon (Joint Committee on Powder Diffraction Standards, JCPDS file no. 75-1621). A NP with a diameter ca. 40 nm is found at the tip of the coil. In the lower right inset of Figure 2A, a clear dot pattern from the NP is observed. By corresponding the dots to the (111), (200), and (11̅1̅) planes of a face-centered cubic structure, the lattice parameter a is estimated to be 0.41 nm (JCPDS file no. 04-0784). The high-resolution TEM image of the NP in Figure 2B shows well-defined fringes corresponding to lattice spacing values 0.23, 0.23, and 0.20 nm. They are assigned to (111), (11̅1̅), and (200) planes of Au. As displayed in Figure 2C, the EDS mapping of the red rectangle area in Figure 2A shows the signals from Au and C atoms. Clearly, the observation confirms that the whole Au NP is enclosed in a carbon matrix. The X-ray diffraction (XRD) patterns of a pristine CFC substrate and the as-fabricated CNC-based electrode are presented in Figure S3 in the Supporting Information. Both samples show patterns from a reported hexagonal carbon structure (JCPDS file no. 26-1076). They demonstrate characteristic 2θ peaks of (006) and other super-lattice planes of the unique carbon material. In addition, the pattern of the as-fabricated CNC-based electrode displays more peaks at 38.0°, 44.2°, 64.4°, and 77.4°. They correspond to (111), (200), (220) and (311) spacing of metallic Au (JCPDS file no. 04-0784), respectively. It is worth noting that the peak at 25.6° of the CNC-based electrode broadens significantly. This implies that in addition to the CFC substrate, the sample consists of a less-ordered carbon material. The observation agrees with the results from the TEM studies mentioned above. The Raman spectra of the samples are presented in Figure S4. Two signature peaks at 1350 cm−1 (D band) and 1580 cm−1 (G band) are observed for both carbon samples. In addition, the CFC substrate displays 2D band (G′ band) and D′ band. Although the ratio of the intensity of the D/G bands is higher for the CNC electrode, the broad D band and its lack of 2D band suggest that the CNCs are less-ordered carbon.10,31,32 This agrees with the TEM and the XRD observation. Both samples were investigated by thermogravi197
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Figure 3. Electrochemical performance of the liquid-state CNC-based two-electrode capacitor in H2SO4(aq) (1.0 M). (A) CV profiles, (B) GCD curves, (C) capacitance retention at various scan rates and current densities, and (D) long term cycling tests at 10.00 A/g and (insets) the first 10 and the last 10 cycles.
2.3. Electrochemical Performances of the CNC-Based Electrode. In general, the capacitance of an EDLC depends on the number of active sites for electrolyte ion storage. In other words, the EDLC capacitance is determined by the surface area provided by the geometrical structure of the device. Large specific surface area not only creates a great number of accessible sites but influences the diffusion of the ions in the electrolyte. By using the Randles−Sevcik equation, the electrochemical active surface area (EASA) can be estimated from cyclic voltammetry (CV) tests (see EASA Measurement in the Supporting Information) by using the three-electrode system.37 In Figure S18A, the CV plots of the CFC and samples 7 and 8 are displayed. By fixing the electrode area to 1.0 cm2, their EASAs are derived from the responses of Ip (voltammetry peak current) versus ν1/2 (square root of sweep rate) and plotted in Figure S18B. As a result, the EASAs of the CFC and samples 7 and 8 are calculated to be 5.84, 12.1, and 16.5 cm2, respectively. Clearly, the EASA of sample 8 is nearly 3 times as large as that of the original CFC. Thus, sample 8 is selected to be the CNC-based electrode for further investigations. The serial resistances Rs of the CFC and the CNC-based electrode are characterized to be 1.4 × 102 and 3.7 Ω, respectively, by electrochemical impedance spectroscopy (EIS) (see Figure S19 and EIS Results in the Supporting Information). After the growth of CNCs on the CFC, Rs lowers significantly. Galvanostatic charge/discharge (GCD) curves of the CFC and the CNC-based electrode are measured and displayed in Figure S22 in the Supporting Information. From the data, their capacitances are estimated to be 0.542 and 107 F/g, for the CFC and the CNC-based electrodes, respectively. The capacitance contribution from the substrate
is negligible. Obviously, the CNC-based electrode is a potential candidate for SC devices. 2.4. Electrochemical Tests of the Liquid-State CNCBased SC. The CNC-based electrodes were assembled into a symmetric capacitor by employing the two-electrode system with H2SO4(aq) (1.0 M) as the electrolyte. The results of the electrochemical tests are shown in Figure 3. Figure 3A presents the CV profiles of the CNC-based device at various scan rates. The approximately rectangular-shaped CV loops suggest that it is capacitive.10 It is interesting that shapes of the loops maintain undeformed even at scan rates as high as 80 mV/s and above. This indicates that the device possesses a rapid charge transfer character. As presented in Figure 3B, the GCD curves of the device display approximate triangle shapes in the potential range (0.00−1.00 V). The specific capacitance (Csp) at 1.00 A/g is calculated to be 137 F/g.38 The value confirms that the device is supercapacitive. On the basis of the approximate symmetric triangular shapes of the charge/ discharge profiles, the device is proposed to follow the electric double-layer charge storage mechanism.39 It is worth noting that at 1.00 A/g, a slight IR drop (0.0166 V) at 217.5 s indicates that the SC may possess a low internal resistance. This property is the result of the high electrical conductivity and ion diffusion ability of the device. Rs and the charge transfer resistance Rct of the CFC and the CNC-based SC devices are estimated to be 1.2 × 102 and 7.5 × 102 Ω as well as 6.5 and 4.9 Ω, respectively, by EIS (see Figures S20, S21A, Table S2, and EIS Results in the Supporting Information).40,41 Clearly, because of the growth of CNCs on the CNC-based SC, both Rs and Rct decrease considerably so that the performance is enhanced. The improvements are attributed to the increased electrical conductivity and interfacial areas for 198
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Figure 4. Electrochemical performance of the solid-state CNC-based two-electrode capacitor. (A) CV profiles, (B) GCD curves, (C) capacitance retention at various scan rates and current densities, and (D) long-term cycling tests at 5.00 A/g and (insets) the first and the last 10 cycles.
better charge transfer between the crisscrossing 1D CNC network and the deeply penetrated electrolyte. The leakage resistance (RL) of the CNC-based device is 526.0 Ω. The large difference between the double-layer capacitance CDL (1.3 × 10−1 F) and the pseudocapacitance CL (3.3 × 10−4 F) suggests that the device is fundamentally an EDLC. Figure 3C depicts plots of the capacitance retention capability (C/C0) versus the scan rate (mV/s) and the current density (A/g) of the SC. These exhibit a moderate performance of this device. Figure 3D displays the result of the cycling tests (charging/ discharging at 10.00 A/g). After 5000 cycles, 85.6% of the original capacitance is still retained. By comparing the first 10 cycles and the last 10 cycles shown in the inset of Figure 3D, we discover that the triangular-shaped profiles are maintained. This indicates that the CNC-based SC retains a high cycling stability. 2.5. Fabrication and Electrochemical Performance of the Symmetric All-Carbon Solid-State CNC-Based SC. A solid-state sandwich-type CNC-based symmetric electrode SC (2.5 × 1.0 cm2, 0.1965 g) with H2SO4/polyvinylalcohol (PVA) gel as the electrolyte is shown in Figure S23A,B in the Supporting Information. The capacitive behaviors of the device were examined by CV and GCD techniques within a potential window (0.00−1.00 V). As shown in Figure 4A, the CV curves are recorded at different scan rates (10−100 mV/s). At low scan rates, the CV loops display approximate rectangular shapes. With increasing scan rates, the shapes of the CV profiles alter slightly. In Figure 4B, the GCD profiles (1.00− 10.00 A/g) are displayed. As observed in the GCD curves, the profiles from the high current density show triangular shapes,
while the one measured at the low current density (1.00 A/g) deviates significantly. As discussed above, this is attributed to the long transporting distance for the ions passing from the surface of the internal CNCs to the separator. We speculate that due to the complicated interface between the gel electrolyte and the CNCs, the electrolyte may not cover all CNC surface. This would increase the internal resistance and cause the IR drops observed in the GCD curves. The specific capacitance of the device is determined from the CV profile integrations and the slopes of the discharge curves (see Electrochemical Measurements in the Supporting Information). From the CV profile integrations, the highest specific capacitance is calculated to be 143 F/g at 10 mV/s. On the basis of the slopes of the discharge curves, the specific capacitance is determined to be 163 F/g at 1.00 A/g. This value is close to the result reported for the liquid-state SC device. Rs, Rct, and RL of the solid-state device are derived to be 10.3, 41.5, and 6236.0 Ω, respectively, by EIS (see Figure S21B, Table S2, and EIS Results in the Supporting Information). These are compared to the data of the liquidstate SC device. While Rs does not increase much, Rct is about 8 times, and RL is about 12 times of the values of the liquidstate one. We attribute the change to the less efficient charge transfer at the interface between the gel and the electrode surface. The high RL suggests that the solid-state SC device is less prone to self-discharge than the liquid-state one. On the other hand, the values of CDL (1.5 × 10−1 F) and CL (1.5 × 10−4 F) are comparable to those of the liquid-state device. Overall, the solid-state SC maintains much of the performance of the liquid-state one. An ideal SC should possess not only 199
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Figure 5. (A) CV profiles of the solid-state SC at various bending angles (θ). (B) Calculated capacitance retention from (A) and (inset) photograph of the device at θ 150°.
high specific capacitance but also high energy and power densities. Here, this solid-state CNC-based SC displays an outstanding energy density 15.3 W h/kg and an excellent power density 510 W/kg. As shown in Table S3 in the Supporting Information, literature reports of the potential window, capacitance, energy density, and power density of other carbon-based SCs are compared to our all-carbon solidstate CNC-based SC. Clearly, the performance of our device is among one of the best values. In Figure 4C, the capacitance retention capability (C/C0) versus the scan rate (mV/s) and the current density (A/g) is plotted. Both types of measurements suggest that the capacitance retention capabilities decrease as the applied variables are increased. We speculate that these are the results of the relatively thick device geometry. In terms of long-term cycling stability, the CNCbased SC exhibits a superior cycling performance. As presented in Figure 4D, the device is examined by a constant charge/ discharge current density of 5.00 A/g for 5000 cycles. After the cycling, it retains 80.2% of the initial capacitance due to the EDLC behavior. The flexible device could be bended extremely (Figure S23C). For practical applications, the device needs to be stable after deformation. As shown in Figure 5A, the CV curves are measured at various bending angles (θ, 0°−150°). The CV loop at 150° does not show significant difference from the ones at the other angles. The capacitance retentions (C/C0) vs the bending angles are plotted in Figure 5B. It is clear that 97.4% of the initial capacitance is still retained even at the high θ 150° (see inset). It is interesting to see that at 60°, the capacitance retention exceeds 100%. We speculate that the bending action might squeeze the gel electrolyte into initially unoccupied nanovoids among CNCs and cause the apparent increased value. To demonstrate the possibility for potential applications, the CV profile (Figure 6) of a tandem device of the CNCbased SCs in a serial connection (inset) is presented. For this device, the CV profile indicates that its potential window can be extended at least to 2.00 V. This suggests that by using the tandem configuration, the SC could be employed to meet the requirements for more applications.
Figure 6. CV loops of single and tandem devices at scan rate 100 mV/s and (inset) a photograph of the tandem device.
provides increased surface areas but also shortens the diffusion pathways of the ions in the electrolyte. In addition, the intercrossing of the CNCs also generates efficient electron conduction routes. Thus, the electrode structure not only boosts fast charge transfer but also promotes efficient electron conduction. By employing the CNC-based electrodes, both liquid- and solid-state SCs with excellent electrochemical properties, within a working potential window 0.00−1.00 V, are fabricated. The liquid-state and the solid-state devices demonstrate outstanding capacitances 137 and 163 F/g, respectively. In addition, the solid-state CNC-based SC exhibits remarkable energy density (15.3 W h/kg) and power density (510 W/kg). The device demonstrates superior flexibility at the bending angle 150° without sacrifice its performance. In summary, the ultralight weight flexible solidstate CNC-based SC offers both high energy and power densities. We envisage that such devices will be promising candidates for eco-friendly wearable and flexible energy storage systems in the future.
4. EXPERIMENTAL SECTION 4.1. Growth of CNCs. After several experimental attempts (see Experimental Parameter Optimization and sample 8 in Table S1 in the Supporting Information), an optimized set of growth conditions were determined and described below. To a cleaned CFC (6.5 cm × 6.5 cm, CeTech), a layer of Au was uniformly deposited on both sides by e-gun PVD (Figure S1 in the Supporting Information). The cloth was further cut into rectangles 2.5 × 1.0 cm2 and immersed into HAuCl4(aq) (1 wt %, Alfa Aesar 49.5%) for 10 min. The as-processed CF
3. CONCLUSIONS In conclusion, we have developed an Au/K bicatalyst-assisted CVD process using C2H2(g) to grow uniform high-density CNCs three-dimensionally on the fibers in the CFC substrate. The as-deposited substrate shows high EASA, suggesting its potential usefulness as the electrodes for electrochemical devices. The unique 1D helical structure of the CNCs not only 200
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substrates were loaded on a specially designed quartz device as shown in Figure S11 and placed in a hot-wall CVD reactor (Figure S12 in the Supporting Information). Under a flow of H2(g) (20 sccm), the HAuCl4 was reduced at 443 K for 30 min to form Au NPs. After the reactor was purged by Ar(g) (100 sccm) for 15 min, the quartz device was pushed downstream to 20 cm away from the center of the furnace. Then, KH powder (0.25 g, Sigma-Aldrich) was loaded on another quartz tube and placed at the center of the furnace. The KH powder was thermally decomposed at 623 K under Ar(g) flow (20 sccm, 1 atm) for 1 h to from K(l). K(l) was evaporated at 2.0 × 10−3 Torr, 443 K, for 30 min to deposit K(l) uniformly inside and out of the CFC substrate at 373 K. After all the K(l) was deposited, the quartz tube loader was removed under Ar(g) (100 sccm). Then, the substrate was placed at the center of the furnace and ramped under 1 atm of Ar(g) (20 sccm) to 723 K. C2H2(g) (11 sccm) and H2(g) (10 sccm) were introduced to deposit carbon for 1 h. After the reaction was completed, the furnace was cooled down to room temperature under 1 atm of Ar(g) (20 sccm). The substrate turned brown (Figure S2) and was washed by ethanol and DI water several times to remove potassium containing byproducts. After being dried in an oven at 333 K, the CNC-based electrode was obtained. 4.2. Instruments for Characterization. SEM images and EDS data of samples were obtained from a JEOL JSM-7401F operated at 10 keV. TEM images, SAED patterns, and EDS data were acquired on a JEOL JEM-ARM200FTH at 200 kV. XRD patterns were recorded by using a Bruker AXS D8 ADVANCE X-ray Diffractometer with Cu Kα 1 radiation. Raman spectra were taken on a HORIBA Jobin Yvon HR800 UV confocal micro-Raman spectrometer with a 632.8 nm He− Ne laser beam source. TGA data were recorded with a Netzsch STA 409 PC Luxx. 4.3. Fabrication of Solid-State SCs. A gel electrolyte was prepared by adding PVA powder (1.0 g, 88%, average Mw 88 000, Acros Organic) into H2SO4(aq) (10.0 mL, 1.0 M, J.T. Baker). The mixture was vigorously stirred at 358 K, until a clear solution was obtained. Two CNC-based electrodes (2.5 × 1.0 cm2) were immersed into the solution for 10 min, so that the electrolyte may penetrate effectively into the voids among the CNCs. A separator composed of polypropylene was sandwiched between the electrodes. After this device was dried at room temperature in air to solidify the gel electrolyte, the CNC-based solid-state symmetric SC (0.1965 g) was obtained. 4.4. Electrochemical Measurements. CV profiles, EIS data, and GCD curves were collected on a CHI 6081C electrochemical workstation. Both three- and two-electrode systems were employed to characterize performance of the electrodes. In a three-electrode system, the electric properties of the as-fabricated CNC-based electrodes were measured against a Pt wire counter electrode and an Ag/AgCl reference electrode in H2SO4 (1.0 M). The solution-state SC performance in H2SO4 (1.0 M) was examined with a two-electrode system by using the CNC electrode for both electrodes. The solid-state SC was characterized similarly.42 See more discussions in EASA Measurements, Electrochemical Measurements, and EIS Results in the Supporting Information.
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Photographs; SEM, EDS, XRD, Raman, TGA, CV, EIS, and GCD data; schematic illustrations; table of growth conditions; discussions; table of results of electrochemical measurements; and table of summary of performance of solid-state all-carbon SCs (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +886 3 5131514. ORCID
Hsin-Tien Chiu: 0000-0002-0564-3756 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
We thank the support from the Ministry of Science and Technology, National Chiao Tung University, and the Ministry of Education of Taiwan, the Republic of China.
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DOI: 10.1021/acsomega.8b02215 ACS Omega 2019, 4, 195−202
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