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activated carbon cloth composite for supercapacitors
Journal of Energy Storage 20 (2018) 92–100
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Facile fabrication of polyaniline/molybdenum trioxide/activated carbon cloth composite for supercapacitors ⁎
Jingzhou Linga, Hanbo Zoua, , Wei Yangb, Wenshan Chena, Kangzhou Leib, Shengzhou Chenb, a b
T ⁎
School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou, China Guangzhou Key Laboratory for New Energy and Green Catalysis, Guangzhou University, Guangzhou, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyaniline Molybdenum trioxide Carbon cloth Capacitance Supercapacitor
A polyaniline/molybdenum trioxide/activated carbon cloth (PANI/MoO3/ACC) composite was successfully synthesized via facile hydrothermal and in-situ polymerization reactions. The morphology and microstructure of the PANI/MoO3/ACC composite were investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The electrochemical performance of the as-prepared electrode was evaluated by cyclic voltammetry, galvanostatic charge-discharge measurements, and electrochemical impedance spectroscopy. The coating PANI layer in PANI/ MoO3/ACC increases the electrochemical activity of MoO3 and accelerates the ion diffusion and electron transfer. Due to synergistic effects between the components, PANI/MoO3/ACC exhibits a specific capacitance of 1050 F g−1, significantly higher than that of MoO3/ACC (296.7 F g−1) and PANI/CC (500.0 F g−1) at a current density of 0.5 A g−1 in 1 M H2SO4 aqueous electrolyte, as well as good cycling stability for 2000 cycles with 71% retention. It also displays lower ion diffusion and charge transfer resistance, as evidenced by the electrochemical impedance data. PANI/MoO3/ACC exhibits excellent electrochemical performance and is thus a promising electrode material for supercapacitors.
1. Introduction Supercapacitors are potential energy storage devices and have been extensively studied owing to their excellent characteristics, such as high power density, fast charge-discharge rates and long cycle life [1,2]. They have been widely applied in hybrid electric vehicles, backup energy systems and mobile electronic devices, etc. [3]. It is worth noting that the electrochemical properties of electrode materials have a significant influence on the energy density of supercapacitors. The stored energy in the electrodes mainly comes from the charge separation at the interface between the electrode and electrolyte as well as from the fast Faradaic charge transfer on the electroactive material surface by a reversible redox reaction [4,5]. The former is known as the double layer capacitance and the latter is defined as the pseudocapacitance. Compared with the double layer capacitance, the pseudocapacitance is much higher due to the rapid and reversible redox reaction. Initially, noble metal oxides with high faradic capacitance and good conductivity, such as RuO2 [6], RhOx [7], etc., have been extensively studied. However, the practical application of these noble metals was limited by their scarcity and high price. Numerous reports then focused on replacing the precious metal oxides with cheap transition metal
⁎
oxides as electrodes due to the high theoretical specific capacitance, availability and strong redox characteristics of the latter [8]. For example, Zhang et al. synthesized hierarchical porous MnO2 nanoflasks by a rapid hydrothermal method without any templates and surfactants, and the maximum specific capacitance of the MnO2 nanosheet electrode reached 268 F g–1 [9]. Hollow Co3O4 octahedra synthesized via a facile one-step solvothermal route exhibited a charge storage capacity of 192 F g−1 [10]. Three types of NiO were prepared by hydrothermal synthesis, and the specific capacitance of the NiO nanocolumns (390 F g−1) was significantly higher than that of the nanoslices (176 F g−1) and nanoplates (285 F g−1) at a current density of 5 A g–1 [11]. Recently, MoO3 has great potential for application in supercapacitors [12,13], due to its particular 2D layered structure with alternately stacked layers bound by weak van der Waals forces along the [010] plane, which facilitates the intercalation of molecules and ions between layers [14]. This particular structure provides high power density for capacitor applications. Li et al. synthesized three different morphologies of MoO3 with different capacitances: nanoplates with 280 F g−1, nanowires with 110 F g−1 and nanorods with 30 F g-1 [15]. Moreover, Pujari et al. also reported that the hexagonal microrods displayed the energy density (7.33 Wh kg−1), power density
Corresponding authors. E-mail addresses: [email protected] (H. Zou), [email protected] (S. Chen).
https://doi.org/10.1016/j.est.2018.09.007 Received 20 July 2018; Received in revised form 11 September 2018; Accepted 11 September 2018 2352-152X/ © 2018 Published by Elsevier Ltd.
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(1200 W kg−1) and the highest specific capacitance (194 F g−1) [16]. However, the specific capacitances of MoO3 reported in previous literatures are not yet satisfactory for supercapacitor applications [4,17–20]. Although transition metal oxides exhibit high theoretical specific capacitance, the practical specific capacitance is relatively low resulting from poor conductivity and low available effective surfaces. Thus, one of the solutions is the use of smaller metal oxide particles designed to uniformly grow on conducting substrates or brackets. The open skeleton of the substrates provides mechanical support, collects electrons from the collector to the oxide particles, and facilitates the fast penetration of the electrolytes in the metal oxide particles [21]. Carbon cloth (CC) has attracted widespread attention in recent years as a current collector for supercapacitor electrodes due to its unique three-dimensional (3D) network structure, reasonable electrochemical stability, and high conductivity and flexibility. For instance, hierarchical WO3 nanofibers as supercapacitor electrodes synthesized on flexible CC by a facile hydrothermal route showed a high specific capacitance of 1716.92 m F cm−2 at a current density of 2 mA cm−2, where the specific capacitance dropped by 20.9% after 6000 cycles at a current density of 10 mA cm−2 [22]. In addition, conductive polymers can be designed as flexible electrodes due to their high energy storage, low cost, and easy preparation. Conductive polymers are able to form composites with carbon materials or metal oxides, which improves their electrochemical properties. In particular, conductive polymers decorated on CC exhibit improved specific capacitance and power capacity when used as composite electrodes. Horng et al. successfully fabricated polyaniline (PANI) nanowire/CC nanocomposites by electrochemical deposition, which exhibited an excellent capacitance of 1079 F g−1 at a specific energy of 100.9 Wh kg−1 and specific power of 12.1 kW kg−1 [23]. Polypyrrole synthesized via oxidative polymerization on flexible CC provided a wide working potential window, high specific capacitance of 603 F g–1, and excellent cycle stability at a discharge current density of 1 A g−1 [24]. A flexible CC/MoS2/PANI composite was prepared through a facile hydrothermal method followed by in-situ polymerization showing a specific capacitance of 972 F g−1 at a current density of 1 A g−1, significantly higher than that of CC/MoS2 (425 F g−1) and CC/PANI (440 F g−1) [25]. Therefore, metal oxide and conductive polymer composites with CC as the substrate are promising electrode materials for supercapacitors. Herein, we report the fabrication of a new composite consisting of PANI, MoO3, and activated CC (ACC) as an electrode material for supercapacitors. The prepared PANI/MoO3/ACC composite was expected to exhibit high specific capacitance and cycle stability. The morphology, microstructure, electrochemical properties, and capacitance of the as-prepared material were investigated in detail.
2.3. Preparation of MoO3/ACC MoO3/ACC was synthesized by a facile hydrothermal method. First, 0.68 g ammonium molybdate was dissolved in 50 mL distilled water under ultrasonic agitation and then adjusted to pH 2 with 5 wt.% HNO3. The 3 cm × 3 cm CC was immersed in the above solution and ultrasonically agitated for 20 min to remove the air from the cloth and ensure it was completely wet. The resulting CC and solution were transferred to a Teflon-lined autoclave and heated to 180 °C for 20 h. Once the autoclave had cooled down to room temperature, the CC was removed and carefully washed several times with distilled water and anhydrous ethanol to remove impurities, and then dried under vacuum at 60 °C for 12 h. Finally, the CC was placed in a tube furnace and calcined at 450 °C at a heating rate of 5 °C min−1 under Ar atmosphere for 3 h. 2.4. Synthesis of PANI/MoO3/ACC A layer of highly conductive polyaniline (PANI) was grown on the surface of MoO3/ACC using a typical in situ oxidative polymerization method [26]. The specific steps were as follows: first, 0.47 g aniline was dissolved in 50 mL of 1 M HCl under ultrasonication treatment for 5 min; then, MoO3/ACC was added to the above solution, which was labeled solution A. Second, 1.14 g APS was solubilized with 50 mL 1 M HCl to obtain a solution labeled solution B. Ultrasonic treatment of solution A facilitated the full contact of aniline with MoO3/ACC. Solution A and MoO3/ACC were vigorously stirred in a cryostat, and solution B, containing the oxidizing agent, was slowly added dropwise to A and allowed to react at 0 °C for 24 h. Third, the CC was removed and washed several times with distilled water and anhydrous ethanol, respectively, and finally dried in a vacuum oven at 60 °C for 12 h to afford the PANI/MoO3/ACC composite. For comparison purposes, pure MoO3 was prepared by a hydrothermal method under the same conditions. Using the same oxidative polymerization process, pure PANI and PANI/ CC samples were also prepared. 2.5. Structural characterization The morphology and microstructure of the as-prepared materials were examined by scanning electron microscopy (SEM, JEOL JSM7001 F, Japan), powder X-ray diffraction (XRD, PANalytical PW3040/ 60, Netherlands) with a monochromatic Cu Kα radiation source (λ = 0.15406 nm), transmission emission microscopy (TEM, Tecnai G2 F20), X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi spectrometer) with monochromatized Al Kα X-rays (1486.6 eV) at 150 W, and Fourier transform infrared spectroscopy (FTIR, PerkinElmer Spectrum 100 FT-IR Spectrometer).
2. Material and methods
2.6. Electrochemical measurements
2.1. Materials
Samples of PANI/CC, MoO3/ACC, and PANI/MoO3/ACC with a size of 1 cm × 1 cm were directly used as test electrodes. To prepare the pure MoO3 and PANI electrodes, the active material, carbon black, and a polyvinylidene fluoride (PVDF) binder (weight ratio of 7.5:1.5:1.0) were evenly dispersed by ultrasonic agitation in ethanol to form a slurry. The slurry was then uniformly pressed on a 1 cm × 1 cm stainless steel net at a pressure of 20 MPa and finally dried in a vacuum drying oven at 60 °C for 6 h. It has been reported that CC can be activated by a hydrothermal treatment [27]. To investigate the effect of the CC substrate, the blank CC electrodes namely unactivated and activated carbon cloth (ACC) were used for comparison with the other electrodes. In a typical test, the mass of the prepared blank CC (1 cm × 1 cm) after activation was 14.6 mg. The masses of the active materials of the PANI/ CC, MoO3/ACC, and PANI/MoO3/ACC electrodes was 2.3 mg, 6.8 and 3.2 mg, respectively. The pure MoO3 and pure PANI electrodes without a CC substrate weighed 1.5 mg and 1.9 mg, respectively.
All chemicals were of analytical grade and used without further purification. (NH4)6Mo7O24·4H2O and aniline were purchased from Tianjin Damao Chemical Reagent Factory. Ammonium persulfate (APS) was purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd., and conductive CC was obtained from Shanghai Chuxi Industrial Co., Ltd.
2.2. Pretreatment of the CC The commercial CC was first immersed in concentrated hydrochloric acid (HCl, 37 wt%) and then cleaned by ultrasonic agitation for 0.5 h at 25 °C. Then, the CC was washed with distilled water several times, followed by successive ultrasonication with acetone and ethanol for 0.5 h and finally dried at 60 °C for 12 h under vacuum. 93
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MoO3 is coated with PANI with a thickness of about 20∼30 nm. The HRTEM image in Fig. 2b indicates the interplanar distances for the MoO3 [211] and [040] lattice planes of 0.173 and 0.346 nm, respectively. Furthermore, no clear crystal lattice regions on the edges of the sample may be attributed to the amorphous PANI, which is consistent with the image in Fig. 2a. The selected area electron diffraction (SAED) pattern of PANI/MoO3 in Fig. 2c displays a clear set of lattice structures corresponding to several crystal planes of orthorhombic MoO3 (already labeled in Fig. 2c) [4]. In addition, the elemental mapping of the PANI/ MoO3/ACC composite shown in Fig. 2e,h reveals that C, Mo, N, and O are uniformly distributed on the CC. The abundant carbon species correspond to the CC fibers themselves, and the Mo and O elements are identified as MoO3, indicating its uniform growth on the surface of CC by hydrothermal synthesis. The N elements can be attributed to the PANI obtained by the oxidation–polymerization reaction on the CC. To confirm the nature of the product, the as-prepared materials were characterized by X-ray diffraction, as shown in Fig. 3. Several characteristic peaks centered at 12.8°, 23.3°, 25.7°, 27.3°, 33.8°, 39.0° and 49.3° are observed, corresponding to the [020], [110], [040], [021], [111], [060] and [002] of planes of MoO3 (PDF#76-1003), respectively. The XRD pattern of pure PANI exhibits three broad peaks at 14.8°, 20.3° and 25.1°, indicating the amorphous nature of the polymer [28]. The XRD patterns of the pristine CC and ACC both exhibit two broad peaks at 16.7–31.5° and 40.5–46.8°, corresponding to the amorphous CC [29]. Similarly, the characteristic peaks of CC and PANI are present in the PANI/CC curve, proving that amorphous PANI was successfully deposited on the CC substrate. Furthermore, the major XRD peaks of the PANI/MoO3/ACC composite are consistent with those of the [020], [110], [040], [021], [111], [060] and [002] planes of pure MoO3. In addition, the weak and broad peaks of the CC and PANI affect the intensity ratio of the molybdenum oxide peaks for the [100], [040] and [021] planes. FTIR spectroscopy provides details of the band characteristics of the polymeric and inorganic components of hybrid nanocomposites. The IR spectra of the as-prepared nanocomposites are shown in Fig. 4 in the wavenumber ranges from 450 to 1800 cm−1, with no obvious characteristic peaks outside this range. The powder IR spectra revealed that PANI presents six distinct characteristic peaks arising from chemical bonds or groups in different chemical environments. The peaks at
The electrochemical tests were performed on a PMC1000 Electrochemical Workstation from the AMETEK Company. The as-prepared electrodes were used as the working electrode in a three-electrode system with 1 M aqueous H2SO4 as the electrolyte, Pt as the counter electrode, and Ag/AgCl (in saturated KCl) as the reference electrode. Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements were carried out at different current densities (the current densities are relative to the mass of the active materials) in the voltage window range of -0.2–0.8 V and 0–0.6 V, respectively. The specific capacitance (Cm) was calculated from the charge–discharge curves using the following equation [16]:
Cm = I Δt / mΔV
(1)
where I (A) is the constant discharge current, Δt (s) is the discharge time, m (g) is the weight of the active material on the working electrode, and ΔV (V) is the voltage window. The electrochemical impedance spectra (EIS) were measured in the frequency range from 100 kHz to 0.01 Hz with an alternating current amplitude of 5 mV. 3. Results and discussion 3.1. Structural properties characterization of all samples The smooth surface and 3D skeleton structure of the CC are clearly seen in the SEM image in Fig. 1a. The micron-sized rods composed of MoO3 nanoparticles (Fig. 1b) are uniformly loaded on the CC in the MoO3/ACC composite. As shown in Fig. 1c, the surface of the CC was decorated with PANI, and the magnified image in the inset clearly shows that the nano-PANI roughens the smooth surface of the carbon cloth. Moreover, the uniform PANI nanoarrays on the PANI/CC surface forms a 2D layered structure, which increases the PANI surface area and is benefit to improve the conductivity compared to the blank CC. In Fig. 1d, the surface of the MoO3 microrods is coarser and thicker in comparison with the raw MoO3 rods in Fig. 1b. As a result, the MoO3 surface of the PANI/MoO3/ACC composite was successfully coated with a layer of polyaniline (as displayed in Scheme 1). To further confirm that the surface of MoO3 was coated with PANI, PANI/MoO3 was removed from the PANI/MoO3/ACC surface by sonication and subjected to TEM analysis. As seen in Fig. 2a, the rod-shaped
Fig. 1. SEM images of (a) blank CC, (b) MoO3/ACC, (c) PANI/CC and (d) PANI/MoO3/ACC at different magnifications. The insets in (a), (c) and (d) are enlarged images of the regions indicated by a circle in the corresponding image. 94
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Scheme 1. Schematic illustration of the synthesis of the PANI/MoO3/ACC.
Fig. 2. (a) TEM image of a single PANI/MoO3 nanorod. (b) HRTEM image of PANI/MoO3 showing interplanar distances along the [211] and [040] directions. (c) Corresponding SAED pattern of PANI/MoO3. (d) SEM image of the PANI/MoO3/ACC composite. (e–h) Element mappings of the area indicated in panel (d). 95
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aforementioned peaks shift to 232.8 and 236.0 eV, respectively, where such 0.2 eV offset results from the interaction between PANI and MoO3. The peak intensity in the MoO3/ACC spectrum is higher than that in the PANI/MoO3/ACC spectrum, indicating that the experimental molybdenum content in MoO3/ACC is higher than that in PANI/MoO3/ ACC. Upon deconvolution of the N 1s high-resolution scan of the N 1s electrons, three peaks appeared at 399.1, 399.6 and 400.9 eV (Fig. 5b), corresponding to the quinoid imine (eN]), benzenoid amine (eNHe) and positively charged nitrogen (eN+e), respectively, demonstrating that PANI was distinctly loaded in PANI/MoO3/ACC composite [34,35]. The TEM images and XRD, FTIR and XPS spectra discussed above confirm that the PANI/MoO3 composite was successfully prepared on the surface of the CC. 3.2. Capacitive performance characterization of all samples To explore the effect of the ACC substrate on the PANI/MoO3/ACC performance, CV and GCD measurements were conducted at a scan rate of 10 mV s–1 and current density of 2 A g–1, respectively. The blank CC sample was evaluated under the same conditions for comparison purposes, as shown in Fig. 6. The current (A g–1) in Fig. 6a was normalized to the mass of the whole electrode. It can be clearly seen that both the CC and ACC curves display the typical rectangular shape indicative of an electrical double-layer capacitive behavior. Moreover, the peaks of the CV curve of PANI/MoO3/ACC, caused by the redox reactions of MoO3 and PANI, imply the Faradic capacitive behavior. Compared to Fig. 7a, two peak potentials approximately located at +0.18 V and +0.5 V can be clearly seen and may be attributed to the redox reactions of Mo4+, Mo5+, and Mo6+ [4,33]. The three oxidation peaks observed in the +0.25 V to +0.65 V range (Fig. 7a, curve e) are attributed to the multivalent transitions of PANI [26]. The electrochemical capacitance is commonly considered to be proportional to the area of the closed CV curve at the same potential range. Therefore, PANI/MoO3/ACC exhibits remarkable capacitance, which is significantly higher than that of ACC and CC. To accurately calculate the specific capacitance of the measured electrodes, the GCD curves in Fig. 6b were used, and the capacitive performance was evaluated using Eq. (1). The specific capacitances of ACC and CC were only 37.0 F g–1 and 10.8 F g–1, respectively. Remarkably, when normalized to the mass of the materials involved in the redox reaction, the specific capacitance of PANI/MoO3/ACC reached 923.3 F g–1 at a current density of 2 A g–1. This result indicates that the main specific capacitance arises from the active materials. To further study the electrochemical performance of the PANI/ MoO3/ACC composite, the prepared electrodes were measured by CV and GCD measurements in an aqueous electrolyte of 1.0 M H2SO4 in a three-electrode system, as shown in Fig. 7. The CC, pure MoO3, pure PANI, MoO3/ACC and PANI/CC electrodes were tested under the same conditions. According to Fig. 7a, the specific capacitance of the PANI/ MoO3/ACC, PNAI/CC, pure PANI, MoO3/ACC, pure MoO3 and blank CC electrodes decreases in that order. The GCD curves in the potential range of 0–0.6 V at a current density of 2 A g–1 are shown in Fig. 7b. The PANI/CC and pristine PANI electrodes exhibit a specific capacitance of 500.0 F g–1 and 372.7 F g–1, respectively. The PANI/MoO3/ACC electrode presents the largest specific capacitance of 923.3 F g–1, which are more than 3 times larger than that of MoO3/ACC (296.7 F g–1) and markedly higher than that of MoO3 (102.7 F g–1). Obviously, the specific capacitance of the electrode with the flexible substrate CC is much higher than that without the substrate, which is attributed to the 3D skeleton and good conductivity of the CC allowing for more efficient electron transfer at the ordered arrays of MoO3 and PANI. Furthermore, the conductive PANI nanopolymers on the MoO3 surface provide a shorter transmission path for the electrons and the electrolyte ions. The CV curves of the PANI/MoO3/ACC electrode exhibit a similar shape at different potential scan rates from 2 to 25 mV s–1, as shown in Fig. 7c. The GCD curves of the PANI/MoO3/ACC electrode in a 1.0 M H2SO4 aqueous electrolyte at different current densities were shown in Fig. 7d.
Fig. 3. XRD patterns of blank CC, ACC, pure MoO3, pure PANI, MoO3/ACC, PANI/CC and PANI/MoO3/ACC.
Fig. 4. FTIR spectra of pure MoO3, pure PANI, MoO3/ACC, PANI/CC and PANI/ MoO3/ACC.
1625 cm−1 and 1522 cm−1 are attributed to the quinone structure and the vibration of the C]C bond of the benzenoid ring, respectively [30]. The absorption peaks at 1366 cm−1 and 1269 cm−1 are ascribed to the stretching vibration of the C–N and C]N bonds, respectively [31]. The in-plane and the out-of-plane bending vibrations of the CeH bonds are observed at 1192 cm−1 and 851 cm−1, respectively. The PANI/MoO3/ ACC and PANI/CC spectra exhibit the characteristic peaks of PANI, confirming that PANI was successfully loaded on the CC. The spectrum of pure MoO3 exhibits three major significant vibrational peaks in the 400–1000 cm−1 range: the Mo]O stretching vibration of the terminal oxygen and the symmetric and asymmetric stretching vibration of the bridging oxygen in MoeOeMo, which are located at 955, 826 and 753 cm−1, respectively [32]. Similarly, the MoO3/ACC spectrum shows the characteristic peaks of MoO3. In addition, the PANI/MoO3/ACC spectrum exhibits the characteristic peaks of PANI as well as that of MoO3, thus confirming the presence of both components in the PANI/ MoO3/ACC composite. To further study the surface elemental composition and valence of the PANI/MoO3/ACC composite, XPS spectra were acquired, as shown in Fig. 5. The Mo 3d spectrum of MoO3/ACC (Fig. 5a) revealed two contributions resulting from the spin-orbit splitting, Mo 3d5/2 and Mo 3d3/2, respectively located at 233.0 and 236.2 eV, which are assigned to the characteristic peaks of MoO3 in the composite [33]. By comparison, in the Mo 3d spectrum of the PANI/MoO3/ACC composite, the 96
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Fig. 5. (a) Mo 3d XPS survey spectra of the as-prepared MoO3/ACC (I curve) and PANI/MoO3/ACC (II curve) composites. (b) N 1 s core level region of the asprepared PANI/MoO3/ACC composite. Black curves correspond to the raw data and colored curves indicate the deconvolution fits.
Fig. 6. (a) CV curves of the ACC, CC and PANI/MoO3/ACC electrodes at a scan rate of 10 mV s–1. (b) GCD curves at a current density of 2 A g–1.
Notably, the specific capacitance calculated by the discharge time of the GCD curves only slightly decreases from 1060.1 F g–1 at a current density of 0.5 A g–1 to 923.3 F g–1 at a current density of 2 A g–1. The specific capacitance of PANI/MoO3/ACC remains at 593.8 F g–1 despite increasing the current density to 10.0 A g–1. The excellent capacitive performance of the PANI/MoO3/ACC electrode can be attributed to the fast and efficient charge transport of the 3D CC framework and electron transfer of the high-conductivity PANI (Scheme 2), which support the efficiency of the electrochemical reactions. The relationship between the specific capacitance and the current density for the different prepared electrodes was displayed in Fig. 7e, where the PANI/MoO3/ACC electrode presents the highest specific capacitance. An increase in the current density usually requires fast ions migration to the surface of the electrode material for charge storage. Hence, the construction of an effective morphology for ions migration is of utmost importance. Our results confirm that the 3D CC framework with uniform PANI arrays allows the PANI/MoO3/ACC electrode to store energy more efficiently under high current densities. The electrochemical stability of the different electrodes was also investigated by GCD measurements at a current density of 2 A g–1 in the potential range from 0 to 0.6 V for 2000 cycles, as shown in Fig. 7f. With the increasing number of cycles increases to 1200, the specific capacitance of the PANI/CC electrode gradually decreased from an initial value of 500.0 F g–1 to 353.3 F g–1 and remained at ∼351 F g–1
during the following cycles. Despite the similarity of their curves, the specific capacitance of pure PANI is generally lower than that of PANI/ CC, with a value of 219.8 F g–1 after 2000 cycles. Surprisingly, the specific capacitance of the MoO3/ACC electrode was maintained at about 270 F g–1 after 600 cycles and only decreased by about 30 F g–1 from the initial value of 296.7 F g–1, indicating the good cyclic stability of the composite materials. In contrast, the pure MoO3 electrode displayed the lowest specific capacitance and the worst stability, which was almost zero after 1000 cycles. The initial specific capacitance of the PANI/MoO3/ACC composite is 923.3 F g–1, which quickly decreased to 779.5 F g–1 after 200 cycles. However, in the subsequent 900 cycles, the specific capacitance was maintained at ∼770 F g–1 and finally slowly decreased to 656.1 F g–1 in the 2000th cycle. As shown in Fig. 8a, the initial drop of 15.8% may be due to the separation of PANI and MoO3 from the CC during the redox reactions accompanied by frequent phase transitions, resulting in a loss of active material mass from the PANI/ MoO3/CC electrode and consequent loss of the specific capacitance [19]. The final 12.4% drop in the specific capacitance may be connected to the oxidation of PANI and the swelling and shrinking of the PANI nanostructures caused by the intercalation-deintercalation of electrons and electrolyte ions, which causes PANI to detach from the electrode [26]. To further investigate the kinetic properties of the MoO3/ACC, PANI/CC and PANI/MoO3/ACC electrodes, the AC impedance was 97
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Fig. 7. (a) CV curves of the different electrodes at a scan rate of 10 mV s–1. (b) GCD curves at a current density of 2 A g–1. (c) CV curves of the PANI/MoO3/ACC electrode at different scan rates. (d) GCD curves of the PANI/MoO3/ACC electrode at different current densities. (e) Specific capacitance of the different electrodes as a function of the current density. (f) Cycling stability of the different electrodes at a current density of 2 A g–1. The electrochemical tests were performed in 1.0 M H2SO4.
and electrode [38]. The three electrodes present Rs values that are very similar. At low frequencies, the slope of the straight line represents the Warburg impedance caused by ionic diffusion from the electrolyte to the electrode surface [39]. The Warburg resistance (W) of the PANI/ MoO3/ACC (0.806 Ω) is almost equal to that of PANI/CC (0.794 Ω) at the same frequency of 0.464 Hz. Significantly, it is less than half of the value for MoO3/ACC (1.713 Ω), which demonstrates that the PANI layer can accelerate the ion movement to form an electrical double layer and decrease the ion diffusive resistance. The EIS results show that the PANI/MoO3/ACC composite possesses lower charge transfer and diffusive resistances, which overall improve the capacitive performance. 4. Conclusions In summary, a facile strategy for the synthesis of a PANI/MoO3/ACC composite based on hydrothermal synthesis and in-situ oxidation polymerization is presented in this work. The PANI/MoO3/ACC composite, with a unique 3D network, high conductivity and efficient Faraday reactions, exhibits a high specific capacitance of 1050 F g–1 and long cycle stability. Rod-like MoO3 structures coated with a PANI layer were deposited on CC, as confirmed by SEM, TEM and other techniques. The results of electrochemical tests revealed that the capacitive behavior and rate capability of the PANI/MoO3/ACC electrode are better than those of PANI/CC and MoO3/ACC electrodes, indicating a synergistic effect between PANI, MoO3 and ACC that efficiently increases the specific capacitance and cycle stability. The PANI/MoO3/ACC composite was found to exhibit lower charge transfer and ion diffusion resistances by EIS measurements. This study provides a strategy to develop promising ternary composites containing flexible CC, metal oxides, and polymers for supercapacitor applications.
Scheme 2. Schematic of the charge/discharge processes for the PANI/MoO3/ ACC nanocomposite.
measured within the frequency range from 10 kHz to 0.01 Hz at an open circuit potential. The Nyquist plots with a single semicircle in the high frequency region and a linear spike in the low frequency region [36], as shown in Fig. 8b, were fitted using the Zview software with an equivalent circuit model. The typical small semicircle in the high frequency region, which is related to the Faradaic charge transfer resistance (Rct) of the electrode material, is associated with the charge transfer path between the electrode material and the electrolyte and with the electrical conductivity [37]. Calculated from the fitted Nyquist plots, the Rct values of MoO3/ACC (2.543 Ω), PANI/MoO3/ACC electrode (1.824 Ω) and PANI/CC (1.725 Ω) decrease sequentially. This indicates that the highly conductive PANI decorated on the surface of CC, which evidently enhances the charge transportation. The real axis intercept of the equivalent series resistance (Rs) is a combination of the intrinsic resistance of the electrode, the bulk resistance of the electrolyte and the contact resistance at the interface between the electrolyte
Acknowledgments This work was supported by the National Natural Science Foundation of China [grant number 21776051]; the Guangzhou 98
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Fig. 8. (a) Cycle stability of the PANI/MoO3/ACC composite at a current density of 2 A g–1. (b) Nyquist plots from the EIS measurement of the MoO3/ACC, PANI/CC and PANI/MoO3/ACC electrodes. Inset: equivalent circuit used for the Nyquist plots.
Education Bureau [grant number 1201541563], Department of Science and Technology of Guangdong Province [grant number 2017B090917002]; and Department of Education of Guangdong Province [grant number 2016KQNCX125].
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Journal of Energy Storage journal homepage: www.elsevier.com/locate/est
Facile fabrication of polyaniline/molybdenum trioxide/activated carbon cloth composite for supercapacitors ⁎
Jingzhou Linga, Hanbo Zoua, , Wei Yangb, Wenshan Chena, Kangzhou Leib, Shengzhou Chenb, a b
T ⁎
School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou, China Guangzhou Key Laboratory for New Energy and Green Catalysis, Guangzhou University, Guangzhou, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Polyaniline Molybdenum trioxide Carbon cloth Capacitance Supercapacitor
A polyaniline/molybdenum trioxide/activated carbon cloth (PANI/MoO3/ACC) composite was successfully synthesized via facile hydrothermal and in-situ polymerization reactions. The morphology and microstructure of the PANI/MoO3/ACC composite were investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The electrochemical performance of the as-prepared electrode was evaluated by cyclic voltammetry, galvanostatic charge-discharge measurements, and electrochemical impedance spectroscopy. The coating PANI layer in PANI/ MoO3/ACC increases the electrochemical activity of MoO3 and accelerates the ion diffusion and electron transfer. Due to synergistic effects between the components, PANI/MoO3/ACC exhibits a specific capacitance of 1050 F g−1, significantly higher than that of MoO3/ACC (296.7 F g−1) and PANI/CC (500.0 F g−1) at a current density of 0.5 A g−1 in 1 M H2SO4 aqueous electrolyte, as well as good cycling stability for 2000 cycles with 71% retention. It also displays lower ion diffusion and charge transfer resistance, as evidenced by the electrochemical impedance data. PANI/MoO3/ACC exhibits excellent electrochemical performance and is thus a promising electrode material for supercapacitors.
1. Introduction Supercapacitors are potential energy storage devices and have been extensively studied owing to their excellent characteristics, such as high power density, fast charge-discharge rates and long cycle life [1,2]. They have been widely applied in hybrid electric vehicles, backup energy systems and mobile electronic devices, etc. [3]. It is worth noting that the electrochemical properties of electrode materials have a significant influence on the energy density of supercapacitors. The stored energy in the electrodes mainly comes from the charge separation at the interface between the electrode and electrolyte as well as from the fast Faradaic charge transfer on the electroactive material surface by a reversible redox reaction [4,5]. The former is known as the double layer capacitance and the latter is defined as the pseudocapacitance. Compared with the double layer capacitance, the pseudocapacitance is much higher due to the rapid and reversible redox reaction. Initially, noble metal oxides with high faradic capacitance and good conductivity, such as RuO2 [6], RhOx [7], etc., have been extensively studied. However, the practical application of these noble metals was limited by their scarcity and high price. Numerous reports then focused on replacing the precious metal oxides with cheap transition metal
⁎
oxides as electrodes due to the high theoretical specific capacitance, availability and strong redox characteristics of the latter [8]. For example, Zhang et al. synthesized hierarchical porous MnO2 nanoflasks by a rapid hydrothermal method without any templates and surfactants, and the maximum specific capacitance of the MnO2 nanosheet electrode reached 268 F g–1 [9]. Hollow Co3O4 octahedra synthesized via a facile one-step solvothermal route exhibited a charge storage capacity of 192 F g−1 [10]. Three types of NiO were prepared by hydrothermal synthesis, and the specific capacitance of the NiO nanocolumns (390 F g−1) was significantly higher than that of the nanoslices (176 F g−1) and nanoplates (285 F g−1) at a current density of 5 A g–1 [11]. Recently, MoO3 has great potential for application in supercapacitors [12,13], due to its particular 2D layered structure with alternately stacked layers bound by weak van der Waals forces along the [010] plane, which facilitates the intercalation of molecules and ions between layers [14]. This particular structure provides high power density for capacitor applications. Li et al. synthesized three different morphologies of MoO3 with different capacitances: nanoplates with 280 F g−1, nanowires with 110 F g−1 and nanorods with 30 F g-1 [15]. Moreover, Pujari et al. also reported that the hexagonal microrods displayed the energy density (7.33 Wh kg−1), power density
Corresponding authors. E-mail addresses: [email protected] (H. Zou), [email protected] (S. Chen).
https://doi.org/10.1016/j.est.2018.09.007 Received 20 July 2018; Received in revised form 11 September 2018; Accepted 11 September 2018 2352-152X/ © 2018 Published by Elsevier Ltd.
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(1200 W kg−1) and the highest specific capacitance (194 F g−1) [16]. However, the specific capacitances of MoO3 reported in previous literatures are not yet satisfactory for supercapacitor applications [4,17–20]. Although transition metal oxides exhibit high theoretical specific capacitance, the practical specific capacitance is relatively low resulting from poor conductivity and low available effective surfaces. Thus, one of the solutions is the use of smaller metal oxide particles designed to uniformly grow on conducting substrates or brackets. The open skeleton of the substrates provides mechanical support, collects electrons from the collector to the oxide particles, and facilitates the fast penetration of the electrolytes in the metal oxide particles [21]. Carbon cloth (CC) has attracted widespread attention in recent years as a current collector for supercapacitor electrodes due to its unique three-dimensional (3D) network structure, reasonable electrochemical stability, and high conductivity and flexibility. For instance, hierarchical WO3 nanofibers as supercapacitor electrodes synthesized on flexible CC by a facile hydrothermal route showed a high specific capacitance of 1716.92 m F cm−2 at a current density of 2 mA cm−2, where the specific capacitance dropped by 20.9% after 6000 cycles at a current density of 10 mA cm−2 [22]. In addition, conductive polymers can be designed as flexible electrodes due to their high energy storage, low cost, and easy preparation. Conductive polymers are able to form composites with carbon materials or metal oxides, which improves their electrochemical properties. In particular, conductive polymers decorated on CC exhibit improved specific capacitance and power capacity when used as composite electrodes. Horng et al. successfully fabricated polyaniline (PANI) nanowire/CC nanocomposites by electrochemical deposition, which exhibited an excellent capacitance of 1079 F g−1 at a specific energy of 100.9 Wh kg−1 and specific power of 12.1 kW kg−1 [23]. Polypyrrole synthesized via oxidative polymerization on flexible CC provided a wide working potential window, high specific capacitance of 603 F g–1, and excellent cycle stability at a discharge current density of 1 A g−1 [24]. A flexible CC/MoS2/PANI composite was prepared through a facile hydrothermal method followed by in-situ polymerization showing a specific capacitance of 972 F g−1 at a current density of 1 A g−1, significantly higher than that of CC/MoS2 (425 F g−1) and CC/PANI (440 F g−1) [25]. Therefore, metal oxide and conductive polymer composites with CC as the substrate are promising electrode materials for supercapacitors. Herein, we report the fabrication of a new composite consisting of PANI, MoO3, and activated CC (ACC) as an electrode material for supercapacitors. The prepared PANI/MoO3/ACC composite was expected to exhibit high specific capacitance and cycle stability. The morphology, microstructure, electrochemical properties, and capacitance of the as-prepared material were investigated in detail.
2.3. Preparation of MoO3/ACC MoO3/ACC was synthesized by a facile hydrothermal method. First, 0.68 g ammonium molybdate was dissolved in 50 mL distilled water under ultrasonic agitation and then adjusted to pH 2 with 5 wt.% HNO3. The 3 cm × 3 cm CC was immersed in the above solution and ultrasonically agitated for 20 min to remove the air from the cloth and ensure it was completely wet. The resulting CC and solution were transferred to a Teflon-lined autoclave and heated to 180 °C for 20 h. Once the autoclave had cooled down to room temperature, the CC was removed and carefully washed several times with distilled water and anhydrous ethanol to remove impurities, and then dried under vacuum at 60 °C for 12 h. Finally, the CC was placed in a tube furnace and calcined at 450 °C at a heating rate of 5 °C min−1 under Ar atmosphere for 3 h. 2.4. Synthesis of PANI/MoO3/ACC A layer of highly conductive polyaniline (PANI) was grown on the surface of MoO3/ACC using a typical in situ oxidative polymerization method [26]. The specific steps were as follows: first, 0.47 g aniline was dissolved in 50 mL of 1 M HCl under ultrasonication treatment for 5 min; then, MoO3/ACC was added to the above solution, which was labeled solution A. Second, 1.14 g APS was solubilized with 50 mL 1 M HCl to obtain a solution labeled solution B. Ultrasonic treatment of solution A facilitated the full contact of aniline with MoO3/ACC. Solution A and MoO3/ACC were vigorously stirred in a cryostat, and solution B, containing the oxidizing agent, was slowly added dropwise to A and allowed to react at 0 °C for 24 h. Third, the CC was removed and washed several times with distilled water and anhydrous ethanol, respectively, and finally dried in a vacuum oven at 60 °C for 12 h to afford the PANI/MoO3/ACC composite. For comparison purposes, pure MoO3 was prepared by a hydrothermal method under the same conditions. Using the same oxidative polymerization process, pure PANI and PANI/ CC samples were also prepared. 2.5. Structural characterization The morphology and microstructure of the as-prepared materials were examined by scanning electron microscopy (SEM, JEOL JSM7001 F, Japan), powder X-ray diffraction (XRD, PANalytical PW3040/ 60, Netherlands) with a monochromatic Cu Kα radiation source (λ = 0.15406 nm), transmission emission microscopy (TEM, Tecnai G2 F20), X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi spectrometer) with monochromatized Al Kα X-rays (1486.6 eV) at 150 W, and Fourier transform infrared spectroscopy (FTIR, PerkinElmer Spectrum 100 FT-IR Spectrometer).
2. Material and methods
2.6. Electrochemical measurements
2.1. Materials
Samples of PANI/CC, MoO3/ACC, and PANI/MoO3/ACC with a size of 1 cm × 1 cm were directly used as test electrodes. To prepare the pure MoO3 and PANI electrodes, the active material, carbon black, and a polyvinylidene fluoride (PVDF) binder (weight ratio of 7.5:1.5:1.0) were evenly dispersed by ultrasonic agitation in ethanol to form a slurry. The slurry was then uniformly pressed on a 1 cm × 1 cm stainless steel net at a pressure of 20 MPa and finally dried in a vacuum drying oven at 60 °C for 6 h. It has been reported that CC can be activated by a hydrothermal treatment [27]. To investigate the effect of the CC substrate, the blank CC electrodes namely unactivated and activated carbon cloth (ACC) were used for comparison with the other electrodes. In a typical test, the mass of the prepared blank CC (1 cm × 1 cm) after activation was 14.6 mg. The masses of the active materials of the PANI/ CC, MoO3/ACC, and PANI/MoO3/ACC electrodes was 2.3 mg, 6.8 and 3.2 mg, respectively. The pure MoO3 and pure PANI electrodes without a CC substrate weighed 1.5 mg and 1.9 mg, respectively.
All chemicals were of analytical grade and used without further purification. (NH4)6Mo7O24·4H2O and aniline were purchased from Tianjin Damao Chemical Reagent Factory. Ammonium persulfate (APS) was purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd., and conductive CC was obtained from Shanghai Chuxi Industrial Co., Ltd.
2.2. Pretreatment of the CC The commercial CC was first immersed in concentrated hydrochloric acid (HCl, 37 wt%) and then cleaned by ultrasonic agitation for 0.5 h at 25 °C. Then, the CC was washed with distilled water several times, followed by successive ultrasonication with acetone and ethanol for 0.5 h and finally dried at 60 °C for 12 h under vacuum. 93
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MoO3 is coated with PANI with a thickness of about 20∼30 nm. The HRTEM image in Fig. 2b indicates the interplanar distances for the MoO3 [211] and [040] lattice planes of 0.173 and 0.346 nm, respectively. Furthermore, no clear crystal lattice regions on the edges of the sample may be attributed to the amorphous PANI, which is consistent with the image in Fig. 2a. The selected area electron diffraction (SAED) pattern of PANI/MoO3 in Fig. 2c displays a clear set of lattice structures corresponding to several crystal planes of orthorhombic MoO3 (already labeled in Fig. 2c) [4]. In addition, the elemental mapping of the PANI/ MoO3/ACC composite shown in Fig. 2e,h reveals that C, Mo, N, and O are uniformly distributed on the CC. The abundant carbon species correspond to the CC fibers themselves, and the Mo and O elements are identified as MoO3, indicating its uniform growth on the surface of CC by hydrothermal synthesis. The N elements can be attributed to the PANI obtained by the oxidation–polymerization reaction on the CC. To confirm the nature of the product, the as-prepared materials were characterized by X-ray diffraction, as shown in Fig. 3. Several characteristic peaks centered at 12.8°, 23.3°, 25.7°, 27.3°, 33.8°, 39.0° and 49.3° are observed, corresponding to the [020], [110], [040], [021], [111], [060] and [002] of planes of MoO3 (PDF#76-1003), respectively. The XRD pattern of pure PANI exhibits three broad peaks at 14.8°, 20.3° and 25.1°, indicating the amorphous nature of the polymer [28]. The XRD patterns of the pristine CC and ACC both exhibit two broad peaks at 16.7–31.5° and 40.5–46.8°, corresponding to the amorphous CC [29]. Similarly, the characteristic peaks of CC and PANI are present in the PANI/CC curve, proving that amorphous PANI was successfully deposited on the CC substrate. Furthermore, the major XRD peaks of the PANI/MoO3/ACC composite are consistent with those of the [020], [110], [040], [021], [111], [060] and [002] planes of pure MoO3. In addition, the weak and broad peaks of the CC and PANI affect the intensity ratio of the molybdenum oxide peaks for the [100], [040] and [021] planes. FTIR spectroscopy provides details of the band characteristics of the polymeric and inorganic components of hybrid nanocomposites. The IR spectra of the as-prepared nanocomposites are shown in Fig. 4 in the wavenumber ranges from 450 to 1800 cm−1, with no obvious characteristic peaks outside this range. The powder IR spectra revealed that PANI presents six distinct characteristic peaks arising from chemical bonds or groups in different chemical environments. The peaks at
The electrochemical tests were performed on a PMC1000 Electrochemical Workstation from the AMETEK Company. The as-prepared electrodes were used as the working electrode in a three-electrode system with 1 M aqueous H2SO4 as the electrolyte, Pt as the counter electrode, and Ag/AgCl (in saturated KCl) as the reference electrode. Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements were carried out at different current densities (the current densities are relative to the mass of the active materials) in the voltage window range of -0.2–0.8 V and 0–0.6 V, respectively. The specific capacitance (Cm) was calculated from the charge–discharge curves using the following equation [16]:
Cm = I Δt / mΔV
(1)
where I (A) is the constant discharge current, Δt (s) is the discharge time, m (g) is the weight of the active material on the working electrode, and ΔV (V) is the voltage window. The electrochemical impedance spectra (EIS) were measured in the frequency range from 100 kHz to 0.01 Hz with an alternating current amplitude of 5 mV. 3. Results and discussion 3.1. Structural properties characterization of all samples The smooth surface and 3D skeleton structure of the CC are clearly seen in the SEM image in Fig. 1a. The micron-sized rods composed of MoO3 nanoparticles (Fig. 1b) are uniformly loaded on the CC in the MoO3/ACC composite. As shown in Fig. 1c, the surface of the CC was decorated with PANI, and the magnified image in the inset clearly shows that the nano-PANI roughens the smooth surface of the carbon cloth. Moreover, the uniform PANI nanoarrays on the PANI/CC surface forms a 2D layered structure, which increases the PANI surface area and is benefit to improve the conductivity compared to the blank CC. In Fig. 1d, the surface of the MoO3 microrods is coarser and thicker in comparison with the raw MoO3 rods in Fig. 1b. As a result, the MoO3 surface of the PANI/MoO3/ACC composite was successfully coated with a layer of polyaniline (as displayed in Scheme 1). To further confirm that the surface of MoO3 was coated with PANI, PANI/MoO3 was removed from the PANI/MoO3/ACC surface by sonication and subjected to TEM analysis. As seen in Fig. 2a, the rod-shaped
Fig. 1. SEM images of (a) blank CC, (b) MoO3/ACC, (c) PANI/CC and (d) PANI/MoO3/ACC at different magnifications. The insets in (a), (c) and (d) are enlarged images of the regions indicated by a circle in the corresponding image. 94
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Scheme 1. Schematic illustration of the synthesis of the PANI/MoO3/ACC.
Fig. 2. (a) TEM image of a single PANI/MoO3 nanorod. (b) HRTEM image of PANI/MoO3 showing interplanar distances along the [211] and [040] directions. (c) Corresponding SAED pattern of PANI/MoO3. (d) SEM image of the PANI/MoO3/ACC composite. (e–h) Element mappings of the area indicated in panel (d). 95
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aforementioned peaks shift to 232.8 and 236.0 eV, respectively, where such 0.2 eV offset results from the interaction between PANI and MoO3. The peak intensity in the MoO3/ACC spectrum is higher than that in the PANI/MoO3/ACC spectrum, indicating that the experimental molybdenum content in MoO3/ACC is higher than that in PANI/MoO3/ ACC. Upon deconvolution of the N 1s high-resolution scan of the N 1s electrons, three peaks appeared at 399.1, 399.6 and 400.9 eV (Fig. 5b), corresponding to the quinoid imine (eN]), benzenoid amine (eNHe) and positively charged nitrogen (eN+e), respectively, demonstrating that PANI was distinctly loaded in PANI/MoO3/ACC composite [34,35]. The TEM images and XRD, FTIR and XPS spectra discussed above confirm that the PANI/MoO3 composite was successfully prepared on the surface of the CC. 3.2. Capacitive performance characterization of all samples To explore the effect of the ACC substrate on the PANI/MoO3/ACC performance, CV and GCD measurements were conducted at a scan rate of 10 mV s–1 and current density of 2 A g–1, respectively. The blank CC sample was evaluated under the same conditions for comparison purposes, as shown in Fig. 6. The current (A g–1) in Fig. 6a was normalized to the mass of the whole electrode. It can be clearly seen that both the CC and ACC curves display the typical rectangular shape indicative of an electrical double-layer capacitive behavior. Moreover, the peaks of the CV curve of PANI/MoO3/ACC, caused by the redox reactions of MoO3 and PANI, imply the Faradic capacitive behavior. Compared to Fig. 7a, two peak potentials approximately located at +0.18 V and +0.5 V can be clearly seen and may be attributed to the redox reactions of Mo4+, Mo5+, and Mo6+ [4,33]. The three oxidation peaks observed in the +0.25 V to +0.65 V range (Fig. 7a, curve e) are attributed to the multivalent transitions of PANI [26]. The electrochemical capacitance is commonly considered to be proportional to the area of the closed CV curve at the same potential range. Therefore, PANI/MoO3/ACC exhibits remarkable capacitance, which is significantly higher than that of ACC and CC. To accurately calculate the specific capacitance of the measured electrodes, the GCD curves in Fig. 6b were used, and the capacitive performance was evaluated using Eq. (1). The specific capacitances of ACC and CC were only 37.0 F g–1 and 10.8 F g–1, respectively. Remarkably, when normalized to the mass of the materials involved in the redox reaction, the specific capacitance of PANI/MoO3/ACC reached 923.3 F g–1 at a current density of 2 A g–1. This result indicates that the main specific capacitance arises from the active materials. To further study the electrochemical performance of the PANI/ MoO3/ACC composite, the prepared electrodes were measured by CV and GCD measurements in an aqueous electrolyte of 1.0 M H2SO4 in a three-electrode system, as shown in Fig. 7. The CC, pure MoO3, pure PANI, MoO3/ACC and PANI/CC electrodes were tested under the same conditions. According to Fig. 7a, the specific capacitance of the PANI/ MoO3/ACC, PNAI/CC, pure PANI, MoO3/ACC, pure MoO3 and blank CC electrodes decreases in that order. The GCD curves in the potential range of 0–0.6 V at a current density of 2 A g–1 are shown in Fig. 7b. The PANI/CC and pristine PANI electrodes exhibit a specific capacitance of 500.0 F g–1 and 372.7 F g–1, respectively. The PANI/MoO3/ACC electrode presents the largest specific capacitance of 923.3 F g–1, which are more than 3 times larger than that of MoO3/ACC (296.7 F g–1) and markedly higher than that of MoO3 (102.7 F g–1). Obviously, the specific capacitance of the electrode with the flexible substrate CC is much higher than that without the substrate, which is attributed to the 3D skeleton and good conductivity of the CC allowing for more efficient electron transfer at the ordered arrays of MoO3 and PANI. Furthermore, the conductive PANI nanopolymers on the MoO3 surface provide a shorter transmission path for the electrons and the electrolyte ions. The CV curves of the PANI/MoO3/ACC electrode exhibit a similar shape at different potential scan rates from 2 to 25 mV s–1, as shown in Fig. 7c. The GCD curves of the PANI/MoO3/ACC electrode in a 1.0 M H2SO4 aqueous electrolyte at different current densities were shown in Fig. 7d.
Fig. 3. XRD patterns of blank CC, ACC, pure MoO3, pure PANI, MoO3/ACC, PANI/CC and PANI/MoO3/ACC.
Fig. 4. FTIR spectra of pure MoO3, pure PANI, MoO3/ACC, PANI/CC and PANI/ MoO3/ACC.
1625 cm−1 and 1522 cm−1 are attributed to the quinone structure and the vibration of the C]C bond of the benzenoid ring, respectively [30]. The absorption peaks at 1366 cm−1 and 1269 cm−1 are ascribed to the stretching vibration of the C–N and C]N bonds, respectively [31]. The in-plane and the out-of-plane bending vibrations of the CeH bonds are observed at 1192 cm−1 and 851 cm−1, respectively. The PANI/MoO3/ ACC and PANI/CC spectra exhibit the characteristic peaks of PANI, confirming that PANI was successfully loaded on the CC. The spectrum of pure MoO3 exhibits three major significant vibrational peaks in the 400–1000 cm−1 range: the Mo]O stretching vibration of the terminal oxygen and the symmetric and asymmetric stretching vibration of the bridging oxygen in MoeOeMo, which are located at 955, 826 and 753 cm−1, respectively [32]. Similarly, the MoO3/ACC spectrum shows the characteristic peaks of MoO3. In addition, the PANI/MoO3/ACC spectrum exhibits the characteristic peaks of PANI as well as that of MoO3, thus confirming the presence of both components in the PANI/ MoO3/ACC composite. To further study the surface elemental composition and valence of the PANI/MoO3/ACC composite, XPS spectra were acquired, as shown in Fig. 5. The Mo 3d spectrum of MoO3/ACC (Fig. 5a) revealed two contributions resulting from the spin-orbit splitting, Mo 3d5/2 and Mo 3d3/2, respectively located at 233.0 and 236.2 eV, which are assigned to the characteristic peaks of MoO3 in the composite [33]. By comparison, in the Mo 3d spectrum of the PANI/MoO3/ACC composite, the 96
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Fig. 5. (a) Mo 3d XPS survey spectra of the as-prepared MoO3/ACC (I curve) and PANI/MoO3/ACC (II curve) composites. (b) N 1 s core level region of the asprepared PANI/MoO3/ACC composite. Black curves correspond to the raw data and colored curves indicate the deconvolution fits.
Fig. 6. (a) CV curves of the ACC, CC and PANI/MoO3/ACC electrodes at a scan rate of 10 mV s–1. (b) GCD curves at a current density of 2 A g–1.
Notably, the specific capacitance calculated by the discharge time of the GCD curves only slightly decreases from 1060.1 F g–1 at a current density of 0.5 A g–1 to 923.3 F g–1 at a current density of 2 A g–1. The specific capacitance of PANI/MoO3/ACC remains at 593.8 F g–1 despite increasing the current density to 10.0 A g–1. The excellent capacitive performance of the PANI/MoO3/ACC electrode can be attributed to the fast and efficient charge transport of the 3D CC framework and electron transfer of the high-conductivity PANI (Scheme 2), which support the efficiency of the electrochemical reactions. The relationship between the specific capacitance and the current density for the different prepared electrodes was displayed in Fig. 7e, where the PANI/MoO3/ACC electrode presents the highest specific capacitance. An increase in the current density usually requires fast ions migration to the surface of the electrode material for charge storage. Hence, the construction of an effective morphology for ions migration is of utmost importance. Our results confirm that the 3D CC framework with uniform PANI arrays allows the PANI/MoO3/ACC electrode to store energy more efficiently under high current densities. The electrochemical stability of the different electrodes was also investigated by GCD measurements at a current density of 2 A g–1 in the potential range from 0 to 0.6 V for 2000 cycles, as shown in Fig. 7f. With the increasing number of cycles increases to 1200, the specific capacitance of the PANI/CC electrode gradually decreased from an initial value of 500.0 F g–1 to 353.3 F g–1 and remained at ∼351 F g–1
during the following cycles. Despite the similarity of their curves, the specific capacitance of pure PANI is generally lower than that of PANI/ CC, with a value of 219.8 F g–1 after 2000 cycles. Surprisingly, the specific capacitance of the MoO3/ACC electrode was maintained at about 270 F g–1 after 600 cycles and only decreased by about 30 F g–1 from the initial value of 296.7 F g–1, indicating the good cyclic stability of the composite materials. In contrast, the pure MoO3 electrode displayed the lowest specific capacitance and the worst stability, which was almost zero after 1000 cycles. The initial specific capacitance of the PANI/MoO3/ACC composite is 923.3 F g–1, which quickly decreased to 779.5 F g–1 after 200 cycles. However, in the subsequent 900 cycles, the specific capacitance was maintained at ∼770 F g–1 and finally slowly decreased to 656.1 F g–1 in the 2000th cycle. As shown in Fig. 8a, the initial drop of 15.8% may be due to the separation of PANI and MoO3 from the CC during the redox reactions accompanied by frequent phase transitions, resulting in a loss of active material mass from the PANI/ MoO3/CC electrode and consequent loss of the specific capacitance [19]. The final 12.4% drop in the specific capacitance may be connected to the oxidation of PANI and the swelling and shrinking of the PANI nanostructures caused by the intercalation-deintercalation of electrons and electrolyte ions, which causes PANI to detach from the electrode [26]. To further investigate the kinetic properties of the MoO3/ACC, PANI/CC and PANI/MoO3/ACC electrodes, the AC impedance was 97
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Fig. 7. (a) CV curves of the different electrodes at a scan rate of 10 mV s–1. (b) GCD curves at a current density of 2 A g–1. (c) CV curves of the PANI/MoO3/ACC electrode at different scan rates. (d) GCD curves of the PANI/MoO3/ACC electrode at different current densities. (e) Specific capacitance of the different electrodes as a function of the current density. (f) Cycling stability of the different electrodes at a current density of 2 A g–1. The electrochemical tests were performed in 1.0 M H2SO4.
and electrode [38]. The three electrodes present Rs values that are very similar. At low frequencies, the slope of the straight line represents the Warburg impedance caused by ionic diffusion from the electrolyte to the electrode surface [39]. The Warburg resistance (W) of the PANI/ MoO3/ACC (0.806 Ω) is almost equal to that of PANI/CC (0.794 Ω) at the same frequency of 0.464 Hz. Significantly, it is less than half of the value for MoO3/ACC (1.713 Ω), which demonstrates that the PANI layer can accelerate the ion movement to form an electrical double layer and decrease the ion diffusive resistance. The EIS results show that the PANI/MoO3/ACC composite possesses lower charge transfer and diffusive resistances, which overall improve the capacitive performance. 4. Conclusions In summary, a facile strategy for the synthesis of a PANI/MoO3/ACC composite based on hydrothermal synthesis and in-situ oxidation polymerization is presented in this work. The PANI/MoO3/ACC composite, with a unique 3D network, high conductivity and efficient Faraday reactions, exhibits a high specific capacitance of 1050 F g–1 and long cycle stability. Rod-like MoO3 structures coated with a PANI layer were deposited on CC, as confirmed by SEM, TEM and other techniques. The results of electrochemical tests revealed that the capacitive behavior and rate capability of the PANI/MoO3/ACC electrode are better than those of PANI/CC and MoO3/ACC electrodes, indicating a synergistic effect between PANI, MoO3 and ACC that efficiently increases the specific capacitance and cycle stability. The PANI/MoO3/ACC composite was found to exhibit lower charge transfer and ion diffusion resistances by EIS measurements. This study provides a strategy to develop promising ternary composites containing flexible CC, metal oxides, and polymers for supercapacitor applications.
Scheme 2. Schematic of the charge/discharge processes for the PANI/MoO3/ ACC nanocomposite.
measured within the frequency range from 10 kHz to 0.01 Hz at an open circuit potential. The Nyquist plots with a single semicircle in the high frequency region and a linear spike in the low frequency region [36], as shown in Fig. 8b, were fitted using the Zview software with an equivalent circuit model. The typical small semicircle in the high frequency region, which is related to the Faradaic charge transfer resistance (Rct) of the electrode material, is associated with the charge transfer path between the electrode material and the electrolyte and with the electrical conductivity [37]. Calculated from the fitted Nyquist plots, the Rct values of MoO3/ACC (2.543 Ω), PANI/MoO3/ACC electrode (1.824 Ω) and PANI/CC (1.725 Ω) decrease sequentially. This indicates that the highly conductive PANI decorated on the surface of CC, which evidently enhances the charge transportation. The real axis intercept of the equivalent series resistance (Rs) is a combination of the intrinsic resistance of the electrode, the bulk resistance of the electrolyte and the contact resistance at the interface between the electrolyte
Acknowledgments This work was supported by the National Natural Science Foundation of China [grant number 21776051]; the Guangzhou 98
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Fig. 8. (a) Cycle stability of the PANI/MoO3/ACC composite at a current density of 2 A g–1. (b) Nyquist plots from the EIS measurement of the MoO3/ACC, PANI/CC and PANI/MoO3/ACC electrodes. Inset: equivalent circuit used for the Nyquist plots.
Education Bureau [grant number 1201541563], Department of Science and Technology of Guangdong Province [grant number 2017B090917002]; and Department of Education of Guangdong Province [grant number 2016KQNCX125].
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