Controllable synthesis of large-area free-standing amorphous carbon films and their potential application in supercapacitors

Abstract

A new and simple approach is proposed for the first time to fabricate free-standing amorphous carbon films (FS-ACFs) with controllable thickness by ambient pressure chemical vapor deposition (APCVD). The approach uses methane as the carbon source and Ni foil as the catalytic substrate, in which a large number of carbon atoms are trapped by controlling the experimental conditions during the APCVD growth process, and FS-ACFs are obtained by a simple corrosion of the Ni foil after APCVD growth. FS-ACFs having a uniform and continuous morphology with variable sizes over one hundred square centimeters and a controllable thickness of tens to hundreds of nanometers are synthesized by controlling the experimental conditions. Microstructure observations show that FS-ACFs have porous and transparent characteristics, which can be transferred or bent on different substrates, thus allowing for a variety of potential applications in electrochemistry. As a proof of concept, an electrochemical supercapacitor device directly assembled by using the FS-ACF exhibits an ultrashort time constant of 46 μs, a wide frequency range (∼kHz) for capacitive feature, and a good capacitance performance with an area specific capacitance of 0.28 mF cm⁻² at a scan rate of 50 mV s⁻¹. Furthermore, the FS-ACF-based supercapacitor shows a high power density with a maximum volumetric power density of 17.76 W cm⁻³.

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1. Introduction

Amorphous carbon film (ACF) has been an attractive material, which has been extensively studied in the last two decades because of its unique properties, such as low friction, biocompatibility, high melting point and tunable electrical properties.^1–6 ACF has exhibited great application in reducing the friction of automotive engines, biomedical implants, and as an electrode material in solar cells, lithium ion batteries and supercapacitors.^3,6–11 Several techniques have been developed to synthesize ACF, including pulsed laser deposition method,³ sputtering,^5,6 ion beam deposition,¹² filtered cathodic vacuum arc method,¹³ and high temperature carbonization.¹⁴ However, these methods are generally expensive involving the use of sophisticated reactors and complex steps. In addition, these methods are not suitable for the fabrication of free-standing ACF. Free-standing ACFs (FS-ACFs) with uniform and large areas provide many unique advantages, such as they can be easily transferred to any substrate of interest, they do not need any support substrate and they can be easily functionalized or targeted on a bilateral surface, for application in energy storage devices, filtration and biomedical engineering.¹⁵ However, there are few reports on such FS-ACFs.^15,16 Feng et al. illustrated that 0.1–3.0 μm thick FS-ACF with an ordered face-centered orthorhombic structure can be fabricated by a coating-carbonization-etching approach.¹⁵ Wang reported that FS-ACF with thicknesses of tens of micrometers can be fabricated by the carbonization of polymeric composite films.¹⁶ However, these synthesis methods still require high temperatures and multiple steps, and therefore are time-consuming and expensive for fabricating high-quality FS-ACFs with tunable shape and size.

Compared with the above techniques, chemical vapor deposition (CVD) is a simple, low-cost and easily controllable technique, which is commonly used in the fabrication of carbon films.^10,17–19 For example, large-area graphene can be fabricated by CVD on Ni foil.^17–19 The graphene growth process is a combination of equilibrium surface segregation and precipitation due to the saturation of a solid solution,²⁰ which strongly depends on the temperature and the carbon atom concentration on the surface of Ni foil.^20–24 Interestingly, several works have shown that hybrid carbon materials composed of amorphous carbon and graphene can be fabricated using a Ni foil/gauze by CVD.^10,25 The formation of the amorphous carbon during the CVD process indicates that the segregation/precipitation process of carbon atoms can be restricted by experimental conditions. Recently, we noticed that a large number of carbon atoms can be trapped in a Ni foil (called carbon containing Ni foil, CCNF for short) during the graphene CVD growth by controlling the experimental conditions (such as temperature, pressure, carbon source and cooling rate).

Inspired by this, we herein present a simple and versatile approach to fabricate large area FS-ACFs with controllable thickness by ambient pressure CVD (APCVD). The approach uses Ni foil as a catalytic substrate, in which a large number of carbon atoms are first dissolved and then trapped during APCVD process by controlling the experimental conditions. FS-ACFs are obtained by a simple corrosion of CCNF after APCVD growth. We have successfully synthesized FS-ACFs with variable sizes as large as about hundred square centimeters with controllable thickness from tens to hundreds of nanometers. FS-ACFs are easily transferred to arbitrary substrates, and are found to exhibit good compatibility with the substrates. Microstructure observation shows that FS-ACFs have a uniform, continuous and porous structure. Symmetry supercapacitors (SCs) based on these FS-ACFs exhibit a high rate capability, wide working frequency range and high power density.

2. Experimental

2.1 Chemicals and materials

Methane was used as the carbon source for the FS-ACF synthesis. Acetone (≥99.5%), acetic acid (≥99.5%), and ethanol (≥99.7%) were purchased from Sinopharm Chemical Reagent Co. Ltd. Nickel foils (99.6%, 30 μm thick) and silicon wafers were obtained from Hefei Kejing Materials Technology Co. Ltd. Argon (99.999%), methane (99.999%) and hydrogen (99.999%) were obtained from Wuhan Minghui Gas Technology Co. Ltd. Ferric chloride etchant was purchased from Sinopharm Chemical Reagent Co. Ltd. Deionized water was prepared by EASY-B Ultra-pure Water System for all the experiments.

2.2 Synthesis of FS-ACFs

Nickel foils (30 μm thick), used as the substrates for the synthesis of FS-ACFs, were cut into small pieces according to need. After cleaning with acetic acid for ten minutes, Ni foils were rinsed with deionized water. A schematic of the APCVD system used for the synthesis of FS-ACFs is shown in Fig. S1a.† The APCVD system consists of five components, including work gas (H₂, CH₄ and Ar), mass-flow controller (flowmeter), quartz tube furnace, regulator valve, and vacuum pump. The gas flow and the work pressure can be accurately controlled by the mass-flow controller and regulator valve. The Ni foil was placed in the middle of the quartz tube. In particular, for large-area FS-ACF synthesis, the Ni foil can be directly rolled up and stuffed into the quartz tube.

Fig. S1b† shows the synthesis process: steps, synthesis time and temperature. Before the growth of the FS-ACF, the prepared substrate was loaded into the horizontal quartz tube furnace and the system was pumped to the base pressure value of 10⁻³ Torr in 5 min, followed by the introduction of pure argon at 100 sccm (standard cubic centimeter per minute) for 10 min to create an inert atmosphere. The growing process of the FS-ACF can be briefly described in the following five steps: first, the temperature of the furnace was raised from room temperature to growth temperature (500–700 °C) in 20 min. The argon gas was allowed to flow at a constant rate (100 sccm) during the heating process. Second, the Ni foil was heated at the growth temperature for 10 min with 80 sccm hydrogen to remove surface oxides and impurities. Third, methane gas (CH₄) was introduced into the reactor tube at 80 sccm flow rate. The flow rate ratio of the mixture gas was CH₄ : H₂ = 80 : 8. To control the thickness of the FS-ACF, four flow-rate ratios were used in this work (CH₄ : H₂ = 150 : 8, 80 : 8, 8 : 8, and 80 : 80). The ambient pressure was obtained by carefully controlling the regulator valve. The process was completed in about 25 min. Then, the furnace was rapidly cooled to room temperature. To observe the effect of the cooling rate on the thickness of FS-ACF, two cooling rates were used in this work (3 and 10 °C min⁻¹). During the cooling process, a mixture of gases (CH₄ and H₂) remained. Finally, the Ni foil (or CCNF) was removed from the quartz tube after cooling to room temperature and dipped into a ferric chloride solution with a concentration of 0.8 mol L⁻¹. The FS-ACF appeared and floated on the surface of the solution after CCNF was completely dissolved. The FS-ACF was washed several times with ethanol to remove the residual ferric chloride.

2.3 Physical characterization

After FS-ACFs were transferred to a silicon wafer, the surface morphologies of FS-ACFs were observed on a field-emission scanning electron microscope (SEM) (FEI Sirion 200). The microstructures were characterized by transmission electron microscopy (TEM) (FEI Tecnai G2-20) and selected area electron diffraction (SAED). X-ray photoelectron spectroscopy (XPS) analysis was carried out on an AXIS-ULTRA DLD-600W spectrometer using an Al Kα X-ray radiation source. The Raman spectra were obtained by a HORIBA Jobin Yvon LabRAM Raman spectrometer using 514 nm laser excitation. X-ray diffraction (XRD) patterns were measured by an XRD instrument (X' Pert PRO) with Cu Kα radiation. Small-angle X-ray scattering (SAXS) measurements were obtained on a SAXSess mc2 small-angle X-ray scattering system (Anton Paar, Austria) using Cu Kα radiation. The thicknesses of FS-ACFs synthesized at different temperatures were measured by a step profiler (KLA-TENCOR P16+). Optical transmittance spectra of FS-ACFs on glass were analyzed by a PerkinElmer Lambda 35 UV-Visible spectrophotometer in the region from 370 to 1000 nm.

2.4 Electrochemical measurements

For electrochemical tests, FS-ACFs were transferred to Ni foils, and then were cut into two 1.0 × 1.0 cm² slices, which were used as electrodes for assembling SCs with a cellulose separator in a 6 M KOH aqueous electrolyte solution. The electrochemical performance of the SCs were tested by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy on a CHI 606B electrochemical analyzer system (Shanghai CHI Instruments Co.) under ambient conditions. The area specific capacitances of FS-ACF electrodes were calculated from the CV data according to the following equation: , where C represents capacitance contribution from electrodes, ν is the scan rate (V s⁻¹), ΔV is the potential window (V), I(V) is the current (A), and S is the effective area of the electrode material (cm²). The factor 2 is related to the normalization of two identical capacitors in series.¹¹ C of the device is also calculated from the galvanostatic charge–discharge (GCD) curves using the following equation: C = IΔt/ΔVS, where Δt/ΔV is the slope of the discharge curve. The Ragone plot is obtained based on the capacitance calculated from the discharge curves of GCD. The volumetric energy density is obtained from the formulae given as E_v = C_vV²/7200, where C_v is the volumetric capacitance, which can be given as C_v = C/h, where h is the thickness of FS-ACF in electrode, and E_v is the volumetric energy density (W h cm⁻³).²⁶ The volumetric power density can be obtained from P_v = 3600 × E_v/Δt, where P_v is the volumetric power density (W cm⁻³) and Δt is the discharge time.

3. Results and discussion

Schematics and corresponding images representing the fabrication process of FS-ACF are presented in Fig. 1. Fig. 1a shows a pure Ni foil before APCVD. It is well known that the Ni foil has a high carbon solubility and diffusivity. According to Lander's²¹ and Blakely's²⁴ theories, after hydrocarbon molecules decompose on a Ni surface, the carbon atoms will diffuse in Ni foil.^17,18 The solubility of carbon atom in Ni foil can be obtained as S_P = S₀ exp(−E_P/k_BT) (atoms per cm³), where S₀ is an entropic pre-factor, which depends on the concentration of carbon atom on the Ni surface and E_P is the energy of precipitation. Lander et al.²¹ have experimentally found S₀ = 5.33 × 10²² atoms per cm³ and E_P = 0.42 eV in the temperature range from 700 to 1300 °C. According to this theory, the solubility of carbon atom in Ni foil (S₁) is about 3.60 × 10²⁰ atoms per cm³ at 700 °C. Therefore, we estimate that the solubility of carbon atom in Ni is about 10¹⁹–10²⁰ atoms per cm³ in our experiments (500–700 °C). A schematic diagram of the hypothesis for carbon atoms dissolved in Ni foil is shown in Fig. 1b.

Fig. 1 Schematics of (a) Ni foil before APCVD growth, (b) CCNF after APCVD growth, and (c) FS-ACF after the complete corrosion of CCNF. Images of (d) the large area FS-ACF growing at 590 °C with the flow-rate ratio of CH₄ : H₂ = 80 : 8 floating on the surface of ferric chloride solution after the Ni foil was dissolved, (e) FS-ACF transferred on a transparent flexible plastic substrate.

Generally, the carbon atoms segregate onto a Ni surface to form graphene during a cooling process.^{17,18,20,22–24} However, it should be noted that the precipitation of carbon atoms from Ni foil may be limited by the following factors: (1) the rapid cooling may result in a quenching effect, in which the dissolved carbon atoms lose their mobility,¹⁷ (2) the diffusion capacity of carbon atom in Ni foil decreases as the temperature decreases,²¹ (3) the higher pressure and concentration of carbon source hinders the precipitation of carbon atoms in our experiments. In fact, we have not observed any graphene fragments, neither in the growth process nor in the follow-up corrosion process. Therefore, most of the carbon atoms are trapped in Ni foil at high work pressure and low temperature (CCNF, Fig. 1b). After CVD growth, the Ni foil is immersed into ferric chloride solution with the concentration of 0.8 mol L⁻¹. Several hours later, the FS-ACF with a uniform and transparent structure appears in the solution after the Ni foil is completely dissolved (Fig. 1c and d). The FS-ACF is washed several times with distilled water using a silicon wafer to remove the residual ferric chloride. The size of the FS-ACF is determined by the size of the pristine Ni foil for growth. Large-area FS-ACF can be grown by rolling up a Ni foil with a large area and stuffing it into the quartz tube. Fig. 1d shows a large area FS-ACF (about 9.5 × 10.5 cm²) floating on the solution after the Ni is dissolved. The FS-ACF can be easily transferred to arbitrary substrates, including a flexible substrate (Fig. 1e), and it reveals a good compatibility with the substrates.

The surface morphology and microstructure of the FS-ACF synthesized at 590 °C with the flow-rate ratio of CH₄ : H₂ = 80 : 8 were investigated by SEM and HRTEM. The top view SEM images (Fig. 2a and b) show that the FS-ACF has a uniform and continuous surface with a porous structure. Fig. 2c shows the cross sectional view SEM image of the FS-ACF. Clearly, the FS-ACF has a uniform and flat section structure, and the interface is clear and has a tight connection. The thickness of the FS-ACF is about 165 nm. The XRD profile of the FS-ACF is shown in Fig. 2d. A broad peak at 2θ = 23° clearly shows that the FS-ACF has disorder and amorphous characteristics. In addition, the SAXS method was used to estimate the pore diameter distribution and average pore diameter of the FS-ACF. The SAXS profile is depicted in Fig. 2e, and the pore size distribution is shown in Fig. 2f, which is calculated from the measured SAXS data using Irena.²⁷ The average pore diameter was about 49.7 nm. The pore diameter distribution ranged from 20 to 120 nm, mainly around 30 and 70 nm, which is in accordance with the SEM observation. Furthermore, the SAXS data shows that the pore size distribution is almost the same for several FS-ACFs grown at different temperatures (500–600 °C). We speculate that the pore size distribution mainly depends on the structure of the Ni foil (such as the distribution of grain boundary). Unfortunately, because of the ultra-light characteristic of the FS-ACF, nitrogen adsorption studies cannot be carried out for measuring the specific surface area and pore volume.

Fig. 2 (a) Top view SEM image of the FS-ACF synthesized at 590 °C with the flow-rate ratio of CH₄ : H₂ = 80 : 8; (b) a high magnification top view SEM image and (c) cross-sectional view SEM image of the FS-ACF; (d) XRD profile of the FS-ACF, (e) SAXS patterns of the FS-ACF, and (f) pore size distributions of the FS-ACF.

The low magnification TEM image (Fig. 3a) of FS-ACF produced at 590 °C with the flow-rate ratio of CH₄ : H₂ = 80 : 8 shows a continuous thin film with a uniform and flat surface, which is consistent with the observations from the SEM characterization. The SAED pattern of the carbon film exhibits a typical amorphous pattern (inset in Fig. 3a). The amorphous characteristic is further confirmed by the HRTEM image, which does not show any graphitic ordering (Fig. 3b). The Raman spectrum of the FS-ACF shows two intense peaks (Fig. 3c), which can be assigned to the D and G bands at 1400 and 1580 cm⁻¹, respectively. In general, the D peak is accounted for the breathing modes of rings or k-point phonons of A_1g symmetry, whereas the G peak is the Raman-active E_2g mode of graphite, which is caused by an in-plane bond stretching motion of sp²-hybridized C atoms.^28–30 The broadening of the D and G bands observed in the Raman spectra is characteristic of an amorphous nature due to the short range order,^29,30 which is consistent with the results of HRTEM and XRD. The intensity ratio (0.76) of the D and G band shows the sp²-hybridized dominated bond structure.³⁰ The broad Raman peak at 2900 cm⁻¹, which is defined as the D + G peak, can be related to the D-lines and G-lines by frequency summation.²⁸ Chemical element analysis of the FS-ACF on a Si substrate was determined by an XPS scan, and is shown in Fig. 3d. The result shows that the FS-ACF consists of C (97.6 at%), O (0.7 at%), Cl (0.8 at%) and Si (0.9 at%). The trace amounts of O, Cl and Si should come from the surface absorbents during the Ni foil etching and FS-ACF transferring process. From the above results, we conclude that the FS-ACF mostly constitutes C, and Ni component is completely etched.

Fig. 3 (a) TEM image of the FS-ACF synthesized at 590 °C with the flow-rate ratio of CH₄ : H₂ = 80 : 8 (inset is the SAED pattern, scale bar represents 500 nm), (b) high-resolution TEM image of the FS-ACF, (c) Raman spectra of the FS-ACF and (d) XPS scan of the FS-ACF.

The thickness of the FS-ACF can be easily controlled by the growth conditions. The effects of the growth temperature and flow-rate ratio (CH₄ : H₂) on the thickness of FS-ACF are shown in Fig. 4a. Clearly, the FS-ACF thickness increases with increasing growth temperature. The FS-ACF thickness increases from 96 to 300 nm with increasing growth temperature from 500 to 600 °C at a flow-rate ratio of 150 : 8; this growth can be attributed to the solubility of carbon atom in Ni, which increases with increasing growth temperature.^21,24 In addition to the growth temperature, the flow-rate ratio of CH₄ : H₂ strongly affects the thickness of FS-ACF. By decreasing the flow-rate ratio of CH₄/H₂ from 150/8 to 80/8 and 8/8 at a growth temperature of 600 °C, the FS-ACF thickness decreases from 300 to 200 and 75 nm, respectively. This is because the formation of active carbon species becomes weaker with the decreasing flow rate of CH₄,³¹ and this reduces the solubility of carbon atom in Ni foil, thus leading to a decrease in the thickness of FS-ACF.

Fig. 4 (a) Dependence of FS-ACF film thickness on the growth temperature and the flow ratio of the mixture gas, (b) optical transmission spectra of FS-ACFs synthesized at different temperatures with the flow-rate ratio of 80 : 8, from top to bottom, 500, 550, 570, 590, 600, 620 °C.

It should be noted that the Fig. 4a is divided into three regions by two dotted lines. Our experimental results show that the continuous FS-ACF can only be obtained in the region between the two dotted lines. In the bottom region, no FS-ACF or partial ACF fragments form because of the extremely low solubility of carbon atoms in Ni foil at low temperatures. The starting temperature for continuous FS-ACF growth increases with the decreasing flow-rate ratio of CH₄/H₂ because the solubility of carbon atoms strongly depends on the temperature and the flow-rate ratio of CH₄/H₂. On the other hand, graphene will grow on the Ni foil in the top region (Fig. 4a). The initial temperature for the graphene growth also increases with the decreasing flow-rate ratio of CH₄/H₂. This is because the solubility of carbon atoms in Ni foil at high temperatures is high, and thus part of the dissolved carbon atom may segregate onto the Ni surface to form graphene during the cooling process.^{17,18,20,22–24}

The effect of the cooling rate on the growth of the FS-ACF was also investigated, and is shown in Fig. S2.† It can be observed that the FS-ACF thickness increases with decreasing cooling rate. The FS-ACF thickness increases from 200 to 243 nm on decreasing the cooling rate from 10 to 3 °C min⁻¹ at a flow-rate ratio of 80 : 8 at the growth temperature of 600 °C, which suggests that the catalytic decomposition of CH₄ and dissolution of carbon atom in Ni continues even during the cooling process. The lower cooling rate leads to a high concentration of dissolved carbon atom in Ni, and thus the FS-ACF thickness increases. These results suggest that the growth temperature, flow-rate ratio of CH₄/H₂ and cooling rate greatly influence the growth of FS-ACF during APCVD process.

The optical transmission spectra of FS-ACFs synthesized at different temperatures are shown in Fig. 4b. Obviously, the optical transmittance of the FS-ACF increases with decreasing growth temperature (or film thickness). The FS-ACF synthesized at 500 °C has 65% optical transmittance over the spectral range from 370 to 1000 nm, and reaches nearly 90% over the spectral range from 750 to 1000 nm. The high optical transmittance further confirms that the FS-ACF has a porous structure.

The ultrathin structure, good compatibility with the substrates and porous characteristics made the FS-ACF suitable for their usage as electrode materials in energy storage devices (such as supercapacitors). To demonstrate the advantages of FS-ACF, two SCs were assembled using FS-ACFs with different thicknesses for comparison. The thicknesses of the two FS-ACFs were 123 and 243 nm (growth temperature: 550 and 600 °C, the flow-rate ratio CH₄ : H₂ = 80 : 8, cooling rate of 3 °C min⁻¹), respectively. Fig. 5a shows the cyclic voltammogram (CV) curves of FS-ACF electrode grown at 600 °C at scan rates ranging from 500 and 1000 mV s⁻¹. The CV curves of the FS-ACF electrode grown at 550 °C are shown in Fig. S3a.† The CV curves exhibit a typical rectangular shape, implying that an efficient electrical double-layer capacitive (EDLC) is established in the FS-ACF electrodes. The CV curves still retain the same shape at high scan rate (1000 mV s⁻¹), indicating an ideal electrochemical capacitive behavior with rapid diffusion and easy transportation of electrolyte ions in the FS-ACF electrodes.

Fig. 5 (a) Cyclic voltammogram (CV) curves of SC based on FS-ACF grown at 600 °C at different scan rates. (b) Area specific capacitance of FS-ACF electrodes at various scan rates. (c) Impedance phase angle vs. frequency. (d) Nyquist impedance plots of FS-ACF electrodes, and the inset shows the magnified portion of the Nyquist plots near the origin. (e) Galvanostatic charge–discharge (GCD) curves of the SC based on the FS-ACF grown at 600 °C at different current densities. (f) A Ragone plot showing volumetric energy and power densities of the FS-ACF-based SCs compared with different energy storage devices.

The area specific capacitances (C_s) of FS-ACF electrodes at different scan rates are calculated based on the CV curves, as shown in Fig. 5b. With increasing the scan rate from 50 to 1000 mV s⁻¹, the C_s decrease to about 60% for the two FS-ACF electrodes. This result suggests that the porous structure and good interface contact between the FS-ACF and Ni foil provide a basis for the fast migration of charges to form EDLC. The C increases with increasing ACF thickness (or growth temperature), indicating that the thicker ACF enhances the capacitor performance. The area specific capacitances for the two FS-ACF electrodes grown at 550 and 600 °C are 0.21 and 0.28 mF cm⁻², respectively, at the scan rate of 50 mV s⁻¹, which are comparable with those of carbon-based SCs reported by some groups.^32–35 The SC based on FS-ACF grown at 600 °C was tested for 2000 cycles at the scan rate of 500 mV s⁻¹, and it showed no visible capacitance loss (Fig. S3b†). Several CV curves exhibit almost the same shape (inset in Fig. S3b†), which reveals that the SC based on the FS-ACF has excellent long-term cyclability and superior cycling stability.

The dependence of the phase angle on the frequency is presented in Fig. 5c. The curves show capacitive behavior with phase angles of nearly −90° at low frequency and inductive behavior at high frequency.³⁵ Two SCs have similar shape in a significantly wide frequency range, and the variation is small between the two electrodes. This frequency range (∼6 kHz) is close to that of a vertically oriented graphene SC,³⁵ and larger than that of vertically oriented graphene (VG)-bridged nickel-foam SC³⁶ or laser-scribed graphene-based (LSG) SC.³² The phase angle reaches −45° (resistance and reactance have equal magnitudes at this phase angle³⁵) at the frequency of 25 and 22 kHz for the FS-ACF electrodes grown at 550 and 600 °C, respectively. The frequency at −45° phase angle provides a characteristic time constant τ₋₄₅ (1/f), which shows how fast the SC can be reversibly charged and discharged.^35,37 The τ₋₄₅ values are 40 and 46 μs for the FS-ACF electrodes grown at 550 and 600 °C, respectively, indicating that the FS-ACF SCs possess an ultrahigh-rate performance. At 120 Hz, the impedance phase angles of the capacitor are approximately −78° and −77° for the FS-ACF electrodes grown at 550 and 600 °C, respectively. These phase angles are larger than that of the activated carbon SC (∼0°),³⁵ the chemically converted graphene SC (∼−10°),³⁷ the VG-bridged nickel-foam SC (∼−15°)³⁶ and LSG SC (∼−25°),³² and is comparable with the vertically oriented graphene SC (∼−82°).³⁵

Fig. 5d shows the Nyquist plots of the FS-ACF electrodes grown at 550 and 600 °C. In the high frequency region (inset in Fig. 5d), the low intercepts at Z real axis are related to the equivalent series resistances (ESRs) of the devices. The ESRs of the two SCs are 0.32 and 0.37 Ω for the FS-ACF electrodes grown at 550 and 600 °C, respectively, indicating that FS-ACFs have low ESRs for fast ion diffusion/transport in electrodes. The high-frequency semicircles (the charge-transfer resistance or the contact resistance) are not present in the Nyquist plots, which further implies that FS-ACFs have fast ion transport and low ESR. In the low frequency region, the curves contain a line that intersects the real axis at about 90° angle, which indicates the pure capacitive behavior of the device.³⁵

Galvanostatic charge–discharge (GCD) curves of the SC based on the FS-ACF grown at 600 °C were measured at different current densities ranging from 0.02 to 0.50 mA cm⁻², and are shown in Fig. 5e. The GCD curves of the SC based on FS-ACF grown at 550 °C are shown in Fig. S3c.† The GCD curves exhibit a typical triangular shape, implying that the potential of charge or discharge linearly varies with time, which is indicative of good reversibility and high coulombic efficiency.³⁸ The discharge curves with linear characteristics demonstrate that the FS-ACF SC has an ideal EDLC behavior. No obvious IR drop is observed in Fig. 5e, which also indicates that the SC based on the FS-ACF has a low ESR. The area specific capacitances of the SCs based on the FS-ACF at different current densities calculated based on the GCD curves are shown in Fig. S3d.† With increasing the current density from 0.02 to 0.50 mA cm⁻², the C decreases to about 60% and 53% for the SCs based on the FS-ACF fabricated at 550 and 600 °C, respectively. The C of the SC increases with increasing ACF thickness (or growth temperature), which is consistent with the results of the CV measurement. However, the volumetric capacitance of the SC decreases with increasing FS-ACF thickness (or growth temperature) (Fig. S3e†). This is because the increase of FS-ACF thickness leads to a high electrolyte ion transport resistance.³⁹ The self-discharge behavior of the SC based on the FS-ACF was tested, and is shown in Fig. S3f.† The SCs were charged to 1.00 V with a constant voltage supply for about 4 h, and then allowed to undergo self-discharge for 15 h. For the SC based on the FS-ACF grown at 600 °C, the voltage rapidly decreased from 1.00 to 0.92 V in the first 1 h (Fig. S3f†) possibly because of charge redistribution, then the voltage slowly decreased from 0.92 to 0.80 V in the remaining 14 h possibly because of current leakage.

To demonstrate the overall performance of the FS-ACF SCs, a Ragone plot is shown in Fig. 5f comparing the performance of FS-ACF SCs with different energy storage devices, including the (MnO₂–PEDOT:PSS)/activated carbon (AC) SC,⁴⁰ the nano-engineered carbon films (CNC) SC,⁴¹ LSG SC,³² the 3 V/30 mF aluminum electrolytic capacitors and a 500 μAh lithium thin-film battery.³² The plot shows that the FS-ACF SCs delivered volumetric energy densities from 0.13 to 0.32 mW h cm⁻³, which are lower than that of the (MnO₂–PEDOT:PSS)/AC SC,⁴⁰ but larger than that of the aqueous LSG SC³² and the carbon nanocups (CNC) SC,⁴¹ and two-orders of magnitude higher than that of the aluminum electrolytic capacitor.³² Furthermore, the FS-ACF SC grown at 550 °C can obtain the highest power density of 17.76 W cm⁻³ at an energy density of 0.20 mW h cm⁻³, which is two-orders of magnitude higher than that of the (MnO₂–PEDOT:PSS)/AC SC⁴⁰ and the CNC SC,⁴¹ three-orders of magnitude higher than that of 500 μA h thin-film lithium battery, higher than that of the LSG EC, and comparable to that of the aluminum electrolytic capacitor.³² These results suggest that the SCs based on FS-ACFs have a high energy density and ultrahigh power density. The performance of the SC based on the FS-ACF grown at 550 °C is slightly better than that of the FS-ACF grown at 600 °C because the volumetric capacitance decreases with the increasing thickness of porous electrode (Fig. S3e†).³⁹

4. Conclusion

This work is the first report of synthesizing large area FS-ACFs with controllable thickness by APCVD using Ni as a catalyst, followed by a simple corrosion of Ni foil. The obtained FS-ACF has a porous and continuous morphology of over hundred square centimeters. The thickness of the FS-ACF, which depends on the amount of carbon atoms trapped in the Ni foil, is tuned from tens to hundreds of nanometers by controlling the experimental conditions (growth temperature, flow-rate ratio of CH₄ : H₂, and cooling rate). FS-ACFs can be directly transferred to arbitrary substrates, including flexible substrates, and they also reveal good compatibility with the substrates. Light transmission can be simply tuned by changing the synthesis temperature; this phenomenon is attributed to the thickness difference of FS-ACFs that are synthesized at different temperatures. Symmetry supercapacitors based on FS-ACFs have high rate capability, wide frequency range, and ultrahigh power density. This new method is simple, controllable, and scalable to industrial levels. We expect that this synthesis method opens up a new route for the application of FS-ACF in flexible energy storage/conversion devices and other functional nanodevices.

Acknowledgements

This work was supported by the NSAF (Grant no. U1230108) and the Fundamental Research Funds for the Central Universities (HUST: no. 2014NY010). The authors thank Haili Zhang, an engineer in the Center of Micro-Fabrication and Characterization (CMFC) of WNLO, for the support in Raman test. The authors thank the Analytical and Testing Center of Huazhong University of Science & Technology for FESEM, XRD, HRTEM and XPS testing.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1 showing experimental set-up and process used for the synthesis of FS-ACF (a) schematic of APCVD system, (b) the process used for synthesis of FS-ACF: steps, synthesis time and temperature; Fig. S2 showing film thickness of the FS-ACF dependence of the growth temperature and the cooling rate; Fig. S3 showing (a) CV curves, (b) cycling stability performance, and (c) GCD curves of SC based on FS-ACF grown at 550 °C, (d) area specific capacitance and (e) volumetric specific capacitance of the SCs based on FS-ACFs calculated from the GCD curves at various current densities. (f) Self-discharge profile in two SCs based on FS-ACFs. See DOI: 10.1039/c4ra11378k

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