Three dimensional heteroatom-doped carbon composite film for flexible solid-state supercapacitors

Abstract

Three dimensional (3D) heteroatom-doped active carbon as a flexible supercapacitor electrode is explored with a starting material of silkworm fibers and low molecular weight phenol resin composite. The silk fiber offers a 3D interconnected network and high content of oxygen and nitrogen functionalities, leading to a unique hierarchical structure and a high performance of the electrode. The combination of phenolic resin with silk fibers increases mechanical robustness and specific capacitance of the composite. The porous carbon composite electrode shows a high gravimetric specific capacitance of 239 F g⁻¹ and areal specific capacitance of 637 mF cm⁻² at a current density of 0.57 A g⁻¹. The flexible solid-state supercapacitor assembled with two electrodes in Na₂SO₄/PVA gel electrolyte exhibits an energy density of 17.2 W h kg⁻¹ at a power density of 207 W kg⁻¹. The capacitance retention of 116% after 10 000 charge–discharge cycles over a voltage range of 1.6 V illustrates the excellent cyclic stability of the supercapacitors and indicates great potential applications for energy storage devices.

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Introduction

Electrochemical capacitors (ECs) are crucial energy storage devices because they can provide high power density and long cycle life.^1–3 Carbon materials represent the most common and important electrode candidates for ECs owing to their versatile availability, various microtextures, good stability, and environmental friendliness.^4–10 They have been widely employed as negative electrodes for supercapacitors with a large operating voltage, which is beneficial for enhancing energy density for ECs. From an economic point of view, activated carbons with high surface areas and controllable pore size distributions are especially attractive for ECs. They have been produced from various carbon-based precursors such as coal, pitch, resin, fibrous fabrics, shells, etc. The characteristics of activated carbons strongly depend on the carbon source, thermal treatment, activation process, microtexture, and the content of heteroatoms. The large capacitance exhibited by these electrodes has been demonstrated to arise from an appropriate match of pore size and distribution to the sizes of the ions in the electrolyte.^11–13

Additionally, carbon nanotubes (CNT) are of great interest due to high conductivity along a 1D direction, but a complicated synthesis process and high cost make them less competitive for practical usage. As a unique carbon nanomaterial, graphene, whose structure is a one-atom-thick planar sheet of sp²-bonded carbon atoms, has received increasing attention over the last few years. Many efforts have been devoted to fabricating graphene-based electrodes for supercapacitors.^14,15 Ultrathin graphenes are prone to stack and aggregate in assembling the electrode process, which may somehow dramatically reduce the capacitor's performance. Three dimensional (3D) graphene-based electrodes were designed to prevent the graphene layers from aggregating, using techniques including hydrothermal assembly of graphene oxide, chemical vapor deposition (CVD), and microwave plasma-enhanced chemical vapor deposition, etc.^16–18 However, the high cost of producing these electrodes still limits their wide economical applications.

Surface modification with heteroatoms, such as oxygen and nitrogen on a carbon electrode, is expected to improve the electrochemical performance of ECs. It is found that functional groups can provide carbon materials with acid/base character, and thus introduce pseudocapacitance by the Faradaic interaction between the electrolyte ions and a functional group. The presence of a functional group is proposed to be a more important feature than those such as a porous structure and BET surface area because it can significantly enhance capacitance performance.^19,20 Nitrogen functional groups are usually introduced into carbon materials by employing a nitrogen-containing compound as a nitrogen dopant.^21–26 One approach is post treatment by gaseous ammonia and melamine at a high temperature. Unfortunately, this approach is less controllable with the amount or distribution of the dopants. In comparison, using organic compounds such as polypyrrole and cyanamide as a nitrogen dopant to treat carbon materials by a carbonization process is a simple and controllable method.

Moreover, flexible electrodes based on three-dimensional (3D) films have been explored to meet a growing demand for lightweight, flexible, and highly efficient energy storage devices. It has been demonstrated that the development of 3D electrode materials is particularly important, because they possess many advantages such as hierarchical porosity, mechanical strength, high contacting efficiency, and ease of handing.^27,28 Thus, various types of three-dimensional carbon electrodes have been reported for use in ECs. Xu and co-workers explored 120 μm thick flexible 3D graphene hydrogel films using Au-coated polyimide substrate as current collector for fabrication of flexible solid-state supercapacitors.²⁹ This 3D graphene hydrogel exhibited exceptional mechanical robustness and a high specific capacitance. Wu et al. reported a 3D NiO/ultrathin derived graphene hybrid on Ni foam for a binder-free pseudocapacitor electrode by the nona-casting process and chemical bath deposition technique.³⁰ Yu et al. used polyester textiles as 3D porous substrate materials to fabricate 3D graphene/MnO₂ nanostructured textiles for high performance capacitors.³¹ Herein, we present a simple and convenient method to fabricate a 3D N- and O-doped carbon composite electrode from silkworm cocoons and low molecular weight phenolic resin as the carbon source. We explore the natural silk fiber interconnected network as a skeleton for preparing low-cost 3D carbon electrode materials. The high content of protein in silkworm cocoons supplies a nitrogen and oxygen functionalities enriched electrode. The combination of phenolic resin with a silk fibrous network framework is an important feature that is responsible for good flexibility and high capacitance. The flexible supercapacitor device assembled with two electrodes has a stable operational voltage of 1.6 V in Na₂SO₄/PVA gel electrolyte and a maximum energy density of 17.2 W h kg⁻¹.

Experimental

Synthesis of 3D N-doped active carbon composite

A three-dimensional N-doped carbon composite (NCC) was prepared in three steps. In the first step, a piece of silkworm cocoon was immersed in 0.2 M NaOH solution. After 20 h, the silkworm cocoon had swollen and was split into several pieces with small thickness. The silkworm cocoon was then washed with deionized water and dried in air at room temperature. The second step was to incorporate a low molecular weight phenolic resin onto the silkworm fiber network. The typical process is as followed in previous reports:^30,32 1.2 g phenol, 3.9 mL formaldehyde solution (37%), and 30 mL 0.1 M NaOH aqueous solution were added to 100 mL deionized water and stirred at 70 °C for 8 h. Pieces of silkworm cocoon were then placed in the solution, which was then transferred to a 100 mL Teflon-lined stainless autoclave and maintained at 130 °C for 16 h in an oven. After a hydrothermal reaction, the silkworm cocoons were removed from the vessel and washed thoroughly with distilled water and dried at 80 °C. Finally, they were carbonized in a tubular furnace at 350 °C for 1 h and further graphitized at 700 °C for 2 h under pure nitrogen atmosphere at heating rate of 10 °C min⁻¹. The N₂ flow rate was adjusted 5–10 mL min⁻¹. The carbonized product was termed NCC. To illustrate the advantage of the carbon composite NCC, pure silkworm cocoon derived carbon (named as SC) without low molecular weight phenolic resin and pure phenolic resin derived carbon (named as PC) without using silkworm cocoon as substrate were also prepared, respectively.

Materials characterization

The morphologies of the samples were investigated by field-emission scanning electron microscopy (FESEM, Carl Zeiss, Germany) and transmission electron microscopy (TEM, JEM2010-HR, 200 kv). Thermogravimetric analysis (DTA-TG, NETZSCH STA449F3, Germany) was carried out from room temperature to 1000 °C at a heating rate of 30 °C min⁻¹ under inert atmosphere with Ar. X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (PW3040/60, PANalytical B.V., Netherlands) employing monochromatized Cu Kα incident radiation. Raman spectroscopy was performed using a confocal laser microRaman spectrometer (HR800UV, Horiba Jobin Yvon, France) with the excitation wavelength at 633 nm. XPS spectra were recorded on a Thermo ECSALAB 250 electron spectrometer using Al Kα radiation (1486.6 eV). Nitrogen adsorption–desorption isotherms of the samples were obtained using a ASAP2020 instrument at −196 °C. Pore-size distributions obtained from the Dubinin-Astakhov and BJH (Barrett–Joyner–Halenda) methods were also added to confirm their reliability. Specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) equation. The single point adsorption total pore volume of pores was obtained at a relative pressure of P/P₀ = 0.97.

Fabrication of symmetric supercapacitor device and evaluation

Electrochemical experiments were conducted by using a CHI660e potentiostat in a three-electrode cell with a platinum plate as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode. The working electrode was prepared by fixing the electrode materials (pure SC, PC, and composite NCC) onto carbon cloth with the amount of carbon colloid, respectively. Carbon cloth was used as the working electrode's current collector. Electrochemical capacitive performances of the electrodes were studied by cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) with 1 M Na₂SO₄ electrolyte. EIS spectra were measured with a frequency range of 100 kHz to 10 mHz and the potential amplitude of 5 mV. Symmetric supercapacitors were assembled by using NCC as the two electrodes (without need of any additive), carbon cloth as current collector, and Na₂SO₄/PVA gel as the electrolyte as well as separator, which is similar to our previous reports.³³ The working area of two electrodes was 1.2 × 1.2 cm² and total mass loading of the two electrodes was 4.0 mg.

Results and discussion

Characterization of 3D NCC composite film

Silkworm fiber is one of the most abundant polymers in nature. It is composed of silk fibroin and sericin, which contains over 97% of protein. The regularly interconnected fibers construct a hierarchical three-dimensional (3D) network structure for silkworm that is lightweight and has good toughness. We explored this natural silk fiber as a skeleton for preparing a flexible 3D novel carbon based electrode for supercapacitors. The typical SEM image of the final product of NCC film, as shown in Fig. 1a, presents 3D interconnected frameworks with many opened macropores due to cross linking of the silk-carbon fibers. This indicated the fibrous network structure was well maintained and not destroyed after the carbonization process. A low-magnification cross-section SEM image in Fig. 1b shows the NCC film has loose three-dimensional porous structures with the thickness about 0.5 mm. Further high magnification of the SEM image shows numerous spherical particles, with an average diameter of around 0.5 μm, are randomly located on the NCC network (Fig. 1c), these particles are beneficial for reducing the size of large pores and increasing the density and mechanical strength of the film. An optical graph of the NCC foam under about an 80° bending angle is shown in the inset of Fig. 1c, indicating the flexible nature of the film. The TEM image presented as Fig. 1d reveals that NCC film possesses numerous mesopores with several nanometer pore-size (e.g., 10 nm and 15 nm). These mesopores may be derived from the thermal treatment process in which small molecule gases were released and left spaces in the film. Fig. 1e and f also show the coexistence of amorphous carbon, graphite crystal with clear lattice-resolved fingers (as illustrated in the red circle in Fig. 1e), and few-layer graphene sheets. The lattice spacing between adjacent graphene planes was measured as 0.34 nm in Fig. 1f, corresponding to the (020) planes of graphene.

Fig. 1 Surface morphologies of NCC carbon film: (a) low magnification SEM image of topside and (b) cross section, (c) high magnification SEM image of the topside and the optical graph of flexible NCC foam under about 80° bending angle shown in the inset, and (d–f) TEM images of NCC film.

The porous structure of the NCC film was confirmed by nitrogen adsorption and desorption measurements. The isotherm, as shown in Fig. 2a, reveals the hysteresis loop in the high relative pressure region (P/P₀ > 0.4) which is indicative of an interconnected mesopores system and slit pore geometry for micropores.¹⁰ It seems that the larger hysteresis between adsorption and desorption of nitrogen indicates the better capacitive properties.²¹ The BET surface area of the NCC sample is about 499 m² g⁻¹, which is higher than values for carbon nanotubes (200–400 m² g⁻¹), but lower than values for graphene and active carbon materials (∼2000 m² g⁻¹), and comparable to those reported 3D carbon materials such as 3D-GFs (295 m² g⁻¹) and flexible graphene/PANI (12.7 m² g⁻¹).^34,35 From the pore size distribution curve shown in the inset of Fig. 2a, it is evident that micro–mesopores less than 4 nm are dominant in the NCC sample. The adsorption average pore width by BET is 2.0 nm, which could be accessible electrochemically for an aqueous solution. The total pore volume is ∼0.24 cm³ g⁻¹, ranging from the pore size of 1.7 to 300 nm, assuming the hierarchical nanopore structure of NCC sample, which appear to be reasonable with regard to the TEM results. This hierarchical pore structure is effective for charge transfer and may lead to better electrochemical performance for energy devices.

Fig. 2 (a) N₂ adsorption–desorption isotherms and pore size distribution of NCC foam shown inset. (b) XRD pattern of NCC in the low-angle region and in the wide region shown inset.

XRD measurements were conducted on as-prepared NCC film. The XRD patterns of the film showed diffraction peaks in the low-angle region in Fig. 2b, suggesting the presence of a high density of micropores, which is consistent with the observations from TEM and BET results. The wide region of XRD patterns is represented in the inset of Fig. 2b. It is clearly shown that the NCC film has remarkable diffraction peaks, similar to the characteristic diffraction peaks of carbon (JCPDS no. 00-046-0945), indicating that the NCC film possesses a well crystallized structure which may arise from the carbonization and rearrangement of its regular protein fibular structures during thermal treatment.

Raman spectroscopy is a well-developed tool for identification and structure determination of graphite and graphene based species. Fig. 3a shows the Raman spectra measured on NCC film. Three bands are clearly observed. The G band occurred at 1592 cm⁻¹, D band at 1331 cm⁻¹, and 2D at 2700 cm⁻¹, respectively. The G band can be assigned to the E_2g vibrational mode of sp²-hybrid carbon, which reflects the symmetric and crystallized structures of graphite/graphene based material, indicating the presence of graphite/graphene structures in NCC film. The D band at 1331 cm⁻¹ is associated with vibrations of carbon atoms with dangling bonds in plane terminals of disordered graphene and 2D originates from so-called double resonance processes.³⁰ The high D peak is indicative of a few defects or functional groups in NCC film. The intensity ratio of G band versus D band (I_G/I_D) is about 1.0, suggesting the disordered or functionalities enriched graphite coexists with graphitic materials.²⁴ The broad and lower peak of 2D relative to the G peak reveals a more disordered or amorphous graphite in NCC film.

Fig. 3 (a) Raman spectra of NCC. (b) XPS survey spectra of NCC. (c and d) High resolution XPS spectra of deconvoluted C1s and N1s peaks.

Surface measurement by XPS was carried out for NCC to verify the chemical composition of the film. The full XPS spectrum of NCC, as shown in Fig. 3b, reveals signals from C, O, and N elements, indicating the presence of oxygen and nitrogen functionalities. XPS analyses results showed that the composition ratio of N/C is about 3.35% and the O/C is about 11.7%. The involvement of nitrogen and oxygen can dramatically improve the wettability of NCC in the electrolyte solution and converted NCC from hydrophobic to hydrophilic in nature. The hydrophilic and wettability nature, together with 3D architecture, can provide an excellent electrolyte reservoir for the NCC electrode and a short transport distance for electrolyte ions, and in turn increase the charge-transfer efficiency of the electrode while creating a large capacitance. The C1s spectrum of NCC is shown in Fig. 3c. It is fitted with four components, which are located at 284.7 eV for C–C (or C=C), 285.8 eV for C–N, 288.6 eV for C–O, and 290.6 eV for C=O, respectively.²⁸ The nitrogen doped in NCC is also confirmed by the high-resolution XPS spectrum of the N1s peak as shown in Fig. 3d. It can be fitted by four component peaks, namely, graphitic N (396.1 eV), pyrrolic N (N-5, 399.5 eV), pyridinic N (N-6, 398.6 eV), and quaternary N (N-Q, 400.8 eV), respectively.^16–18 Previous reports suggested that the presence of the N-Q bonded to three C atoms in the central or valley position of a graphene layer can enhance the conductivity of carbon materials. Otherwise, the presence of N-5 and N-6 bonded to two C atoms in five- and six-membered rings at the edge of graphene layer can introduce pseudocapacitive Faradaic reactions and thus increase the capacitance performance of the electrode.

Capacitive performance of 3D NCC composite film

To investigate the electrochemical performance of the composite NCC, cyclic voltammetric scans were conducted on pure SC, PC, and the composite NCC electrodes in 1 M Na₂SO₄ at a scan rate of 20 mV s⁻¹ (Fig. 4a). The CV curve of NCC is more rectangular than those of pure SC and PC, indicating the better capacitive behavior of the NCC. The NCC composite film displays pseudocapacitive behavior in a comparatively larger potential range from −1.0 V to 1.0 V vs. SCE than that of the pure PC electrode. The wide potential range for NCC is associated with more heteroatom functionalities which involve the electrochemical reactions and give rise to higher over-potentials for gas evolution on the electrode, which is beneficial to an increase of energy density for supercapacitors.³⁶ From the CV curves, both SC and NCC films display much higher current densities than that of PC film, suggesting that the nitrogen functional groups in these films play important roles in improving the pseudocapacitive performance. It came to be recognized that incorporation of nitrogen is favored for charge mobility in a carbon matrix by introducing electron-donor characteristics, which facilitates the redox reaction involving nitrogen or neighbouring functional groups.¹⁸ The NCC exhibits highest current density in CV curves, which means the specific capacitance is increased by incorporation of SC with PC carbon material. This should be related to the hierarchical porous structure and larger specific surface area of the composite NCC film, which may provide more opportunity for the reactive centers on the film to contact with electrolyte and thus facilitate the charge transfer in the bulk of the film. Likewise, the galvanostatic charge–discharge measurements by CP were carried out on these films (Fig. 4b). As can be observed, the NCC composite displays the longest discharge time and the highest specific capacitance among these films. The specific capacitance of NCC is calculated to be 239 F g⁻¹, which increases by 31% as compared with pure SC (182 F g⁻¹) and increases by 81% for pure PC (131 F g⁻¹).

Fig. 4 (a) Cyclic voltammograms of pure SC, PC, and composite NCC electrodes in 1 M Na₂SO₄ at a scan rate of 20 mV s⁻¹. (b) Galvanostatic charging/discharging curves of pure SC, PC, and NCC electrodes at 1.4 mA cm⁻² (c) Nyquist plots for pure SC, PC, and composite NCC electrodes measured at 0 V DC potential in the frequency range of 100 kHz to 10 mHz.

Frequency responses in the range of 100 kHz to 10 mHz yield the Nyquist plots for these films shown in Fig. 4c. The distorted semicircle in the high frequency region and nearly vertical linear curve in the low frequency region indicate the typical capacitive behavior of these electrodes. The RC semicircle is attributed to the process at the electrode–electrolyte interface, which is expected to be the capacitance (C) in parallel with the charge-transfer resistance (R_ct) due to charge exchange of electroactive functionality groups such as C–N, C–OH, –COOH, etc. at the interface.³⁹ The R_ct for NCC is slightly higher than those of the others, suggesting the charge-transfer resistance increases due to combination of the two carbon materials. The R_s exhibits 0.44 Ω at high frequency of 100 kHz and R_ct of 12 Ω for NCC, which are much lower than those reported values for flexible (or textile) electrodes using cellulose or polyester fibers as supports.^31,40 The NCC electrode displays the smallest value of Z′′ and the highest capacitance of ∼0.455 F at the low frequency of 10 mHz, as calculated by the equation of C = 1/(2πfZ′′), where f is the frequency and Z′′ is the corresponding imaginary impedance,⁴¹ which is in agreement with CV and CP results.

Fig. 5a presents CV curves of the NCC film at various scan rates with potential range from −0.8 V to 0.8 V. The CV curves are close to rectangular shapes at scan rates between 10 and 50 mV s⁻¹, indicating a good capacitive behavior of the electrode. Consistently, the charge/discharge curves in Fig. 5b exhibit nearly symmetric plots at different current densities, showing good reversibility and high coulombic efficiency (ca. 98%). The specific capacitance value of the NCC electrode is 239 F g⁻¹ and the area specific capacitance is 637 mF cm⁻² at a current density of 0.57 A g⁻¹ (1.4 mA cm⁻²). The specific capacitance of our NCC electrode is comparable to those values recently reported for carbon-based electrodes which are listed in Table 1. When the current density increased from 0.57 to 5.7 A g⁻¹, about 53% of its initial value was retained, showing a good rate capability of NCC electrode.

Fig. 5 (a) Cyclic voltammograms of the NCC electrode at different scan rates in 1 M Na₂SO₄. (b) Galvanostatic charging/discharging curves of the NCC electrode measured with different current densities.

Table 1 Comparison of the specific capacitance of carbon based electrode in the literature

Materials	Specific capacitance	Area capacitance	Reference
N-doped graphene hydrogels	205 F g^−1 (5 mV^−1)		22
N-doped carbon nanofiber	202.0 F g^−1 (1.0 A g^−1)		23
N-doped ordered mesoporous carbons	230 F g^−1 (0.5 A g^−1)		8
N-doped carbon nanotube	190.8 F g^−1 (5 mV s^−1)		37
3D N-doped graphene–CNT networks	180 F g^−1 (0.5 g^−1)		28
3D graphene hydrogel	186 F g^−1 (1A g^−1)	372 mF cm^−2	29
3D CNF-RGO	207 F g^−1 (5 mV^−1)	158 mF cm^−2	38
3D N-doped active carbon	239 F g^−1 (0.57 A g^−1)	637 mF cm^−2	This work

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Flexible solid-state supercapacitor performance

To evaluate the performance of the NCC electrode for supercapacitors, a two-electrode symmetric supercapacitor cell was assembled with Na₂SO₄/PVA gel as the electrolyte and separator. The CV curve of the symmetric supercapacitor shows a voltage range up to 1.6 V within a potential scan rate between 10 and 50 mV s⁻¹ in Fig. 6a. The galvanostatic charge–discharge profiles of the supercapacitor at different current densities are shown in Fig. 6b. The charge–discharge curves are nearly symmetric linear, indicating the capacitive behavior of the supercapacitor. The specific capacitance of the capacitor was calculated from discharge curves with values of 48.6, 42.3, 30.1, and 23.5 F g⁻¹ obtained from current densities of 0.7, 1.4, 4.2, and 7.0 mA cm⁻², respectively. The energy density E_s and power density of the symmetric capacitor was calculated by the equation E_s = 1/2C_sU² and P_s = 3600E_s/t, where C_s is the specific capacitance of the total mass of the two electrodes, U is the operating voltage of the cell, and t is the discharge time. Fig. 6c shows the Ragone plot of the NCC//NCC symmetric supercapacitor device. The device achieved a maximum energy density of 17.2 W h kg⁻¹ at a power density of 207 W kg⁻¹. The energy density is comparable to other types of electrochemical capacitors, which suggests that this NCC electrode is a good candidate for ECs. Furthermore, to demonstrate the potential usefulness of the NCC electrode based supercapacitors, we used assembled supercapacitors unit to light up a red light-emitting-diode (LED, the lowest working voltage is about 1.6 V). Each unit has the same mass loading of NCC (about 2.0 mg for one electrode). As demonstrated in Fig. 6d, one supercapacitor unit (d1) and the flexible supercapacitor unit under a 50° bending angle (d2) can light up a light-emitting-diode, suggesting the flexible nature and potential applications of the as-prepared supercapacitors.

Fig. 6 (a) Cyclic voltammograms of assembled symmetric ECs based on two NCC electrodes at different scan rates. (b) Galvanostatic charging/discharging curves of NCC//NCC symmetric ECs measured with different current densities. (c) Ragone plot of energy density versus power density for our NCC ECs and others reported in literatures (ref. 22, 27, 42 and 43) and (d) photographs of the red LEDs powered by the one EC (1) and EC under a 50° bending angle (2).

The cyclic stability of the NCC//NCC supercapacitor device was investigated by constant charge–discharge for 10 000 cycles with a voltage range of 1.6 V at a current density of 5 mA cm⁻². As shown in Fig. 7a, the specific capacitance of the device increased with the charge/discharge time, especially in the first 2000 cycles. After 10 000 cycles, it retained about 116% capacitance, showing the excellent stability of the device. A slight increase of the capacitance during the cycling test is mainly attributed to the increased effective interfacial area between electrode and electrolyte with the prolonged charge/discharge time.⁴⁴ It is suggested that the foam structure of the electrode material is favored for stabilizing the device for long term running and offers a better opportunity for applications.¹⁷

Fig. 7 (a) Cyclic stability test for the NCC//NCC supercapacitor over a potential range of 1.6 V for 10 000 cycles. (b) Nyquist plots for the NCC//NCC supercapacitor measured at 0 V DC potential before and after the 10 000th cycle; the insets show the expansion of impedance spectra and corresponding equivalent circuit.

The cyclic stability of the capacitor was also evaluated in terms of the frequency response of the impedance (Fig. 7b). The Nyquist plots collected at 1st and 10 000th cycles show nearly identical linear characteristics in any frequency region indicating the electrochemical stability of the materials. A decrease of the imaginary axis of Z′′ value after 10 000 cycles implies an increase of the capacitance, which is in agreement with the CP results. The R_ct in the RC semicircle is changed from 8.0 to 8.6 Ω over 10 000 cycles, suggesting the charge transfer resistance of the device increases with charge/discharge time. Meanwhile, the series resistance of R_s decreases from 6.8 to 6.0 Ω, due to increased effective interfacial area and accessibility of high-area materials to the electrolyte. It is worth noting that a pure capacitor should exhibit a phase angle of 90° at all frequencies. The phase angle for our supercapacitor approaches 70° at low frequencies, indicating the overall capacitance arises from both double-layer capacitance (C_dl) and pseudocapacitance (C_φ). The pseudocapacitance should be associated with some participation of the electrochemical surface reactions involving nitrogen or oxygen functional groups on the carbon composite NCC film.

Conclusions

We have demonstrated a three-dimensional heteroatom doped active carbon (NCC) as a flexible supercapacitor electrode with nitrogen-containing silkworm fiber and low molecular weight phenol resin composite as the carbon source. The unique hierarchical porous structure and high content of oxygen and nitrogen functionalities lead to several desirable natures for the electrode such as flexibility, wettability, and high capacitive performance. The incorporation of low molecular weight phenol resin with silk fibers increased the specific capacitance and improved the mechanical strength of the electrode. The NCC electrode-based symmetric supercapacitor cell exhibited excellent stability at an operating voltage of 1.6 V with a maximum energy density of 17.2 W h kg⁻¹, which is expected for a practical packaged device. The energy and power density of the NCC electrode can be further improved by following with a KOH activation process. It is noteworthy that this 3D silk carbon-based active carbon may be applied to design a hierarchical electrode with other electrochemical materials (i.e. graphene, metal oxide, and conducting polymer).

Acknowledgements

We acknowledge Liaoning Key Laboratory of Functional Textile Materials and Dalian Institute of Chemical Physics for the technical support of SEM, XRD, TEM and BET measurements. We acknowledge financial support by National Natural Science Foundation of China (project number: 51343002) and Liaoning Province new ph. D start up fund (project number: 20131039).

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