- Email: [email protected]
Concurrent electropolymerization of aniline and electrochemical deposition of tungsten oxide for supercapacitor
Journal of Power Sources 342 (2017) 980e989
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Concurrent electropolymerization of aniline and electrochemical deposition of tungsten oxide for supercapacitor Jin-Wang Geng, Yin-Jian Ye, Di Guo**, Xiao-Xia Liu* Department of Chemistry, Northeastern University, Shenyang, 110819, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
PANI-WOx composites were fabricated through concurrent electrochemical deposition. Influences of aniline to WOx precursor ratio on composite fabrication were investigated. Cyclic stability and energy density of were improved PANI-WOx significantly. Assembled model supercapacitor displayed good stability and high energy density.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 October 2016 Received in revised form 18 December 2016 Accepted 5 January 2017
Polyaniline-tungsten oxide composite films (PANI-WOx) were prepared through concurrent electropolymerization of aniline and electrochemical deposition of tungsten oxide on partial exfoliated graphite (Ex-GF) for pseudocapacitive materials. The influence of aniline to WOx precursor ratio on pseudocapacitive properties of the afforded PANI-WOx/Ex-GF composite was investigated. PW-2:1/Ex-GF made from the solution containing aniline and WOx precursor in 2:1 ratio displayed a high specific capacitance (408 F g1/408 mF cm2 at 1 A g1/1 mA cm2) in a wide charge storage potential window of 0.6e0.7 V vs. SCE, leading to a high energy density of 95.8 Wh kg1 at 650 W kg1. Due to the synergistic effect between WOx and PANI, the composite showed much improved cyclic stability (91.6% capacitance retention after 5000 galvanostatic chargeedischarge cycles) compared to similarly prepared PANI/Ex-GF (69.1% capacitance after 5000 chargeedischarge cycles). The assembled symmetric model supercapacitor, by using PW-2:1/Ex-GF as both of the electrodes, also displayed good stability and high energy density, demonstrating that the PANI-WOx composite is promising electrode material for high-performance supercapacitor. © 2017 Elsevier B.V. All rights reserved.
Keywords: Concurrent electrochemical deposition Polyaniline Tungsten oxide Supercapacitor
1. Introduction Pseudocapacitors have drawn considerable attention in recent
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D. Guo), [email protected] (X.-X. Liu). http://dx.doi.org/10.1016/j.jpowsour.2017.01.029 0378-7753/© 2017 Elsevier B.V. All rights reserved.
years as they can usually store more charges than double-layered capacitors through fast and reversible surface redox reactions [1e3]. Electrode material is the most crucial factor to determine the electrochemical behaviors of pseudocapacitors. Polyaniline (PANI) is one of the promising pseudocapacitive materials due to its facile synthesis, high electrochemical activity, good flexibility and low density compared to metal oxides [4,5]. However, PANI suffers from poor cyclic stability due to volumetric swelling and shrinking and
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
subsequent mechanical degradation during the doping and dedoping of ions in chargeedischarge process [4,6e8]. Composites formation with inorganic materials is a possible solution to this challenge. For example, in the nanocomposite of SnO2@PANI, the SnO2 nanoparticles which incorporated in the polymer nanofibers played the role of a backbone to stabilize the PANI matrix during charge and discharge, leading to improved cyclic stability of the composite [9]. The NiCo2O4 nanorod array functioned as strain buffer for the polymer in the fabricated core-shell structured NiCo2O4@PANI [10]. Cyclic stability of PANI was reported to be enhanced by anchoring on MoS2 sheets and their composites, as well as TiN nanotube, which acted as mechanical support for the polymer [7,8,11]. Guo and coworkers demonstrated the improved cyclic stability of PANI/WO3 nanocomposite which was ascribed to the chemical bonding between the polymer matrix and WO3 particles [12]. Moreover, charge storage potential window of PANI based composites can be enlarged through the incorporation of inorganic oxides which possessing electroactivity in negative potentials, including WO3, V2O5 and MoOx [12e16]. This will in turn lead to increased energy density E for the electrode material as E is proportional to the square of the potential window U (E ¼ 1/2CsU2, where Cs is the specific capacitance). WO3 is mostly investigated among the many inorganic materials owing to its many advantages including low cost, good electrochemical stability, high conductivity and environmental friendliness [12,13,17]. WO3 displays multiple oxidation states due to the intercalation of electrons and protons to form hydrogen tungsten bronzes (HxWO3), enabling the oxide a good candidate to composite with PANI for electrode materials with enlarged charge storage potential window [14,18,19]. In addition, PANI is also electron and proton conductor, enabling synergistic effect between the two components [12,13,20]. After reduced, WO3 can work as a “proton sea” to keep high protonation level for PANI in the composite, realizing high conductivity of the polymer. The high conductivity of reduced WO3 can also compensate the low conductivity of de-doped PANI when it is reduced, offering possible solution for the large electrode polarization and poor rate capability. Several preparation routes have been reported for the syntheses of composites of conducting polymer and metal oxides. Among these, electrochemical deposition of composites has many advantageous compared to its rivals such as physical infiltration, mechanical mixing, and in situ chemical formation. Through electrochemical deposition, the composites can be directly formed as a thin film with high mechanical stability, which strongly adhered to the underlying current collector. Moreover, the film thickness, porosity and morphologies can be precisely controlled by electrochemical procedures [12,13,21]. A PANI/WO3 composite film was prepared through in-situ electropolymerization of aniline on WO3 sol-gel film. Chemical interactions were established between the polymer matrix and WO3 particles, which play a vital role for the improved chargeedischarge durability of the composite [12]. WO3/PANI hybrid films were prepared through potentiodynamic deposition of PANI from aniline on anodized nanoporous WO3 [13]. Due to the combined electroactivity of WO3 and PANI, the hybrid film displayed enlarged pseudocapacitive contribution potential window of 1.2 V from 0.5e0.7 V vs. Ag/AgCl. The PANI in the hybrid film exhibited a specific capacitance of 350 F g1 while the hybrid retained 93.4% of its initial capacitance after 200 chargeedischarge cycles. Electrochemical syntheses of WO3/PANI composite films were also demonstrated by our group through concurrent electrodeposition of WO3 and electropolymerization of aniline [14]. The composite displayed good pseudocapacitive performances in a widened potential range of 1.2 V. However, the energy density of the composite is needed to be further improved as the specific capacitance (168 F g1) is very limited.
981
Carbon-based, especially graphene based, current collectors open up new opportunities to improve charge storage performances of pseudocapacitive materials due to their high electrical conductivity, excellent mechanical property and good stability [22,23]. In this work, concurrent deposition of WOx and electropolymerization of aniline were conducted on a current collector of partial exfoliated graphite foil (Ex-GF) to afford PANI-WOx/Ex-GF. The obtained pseudocapacitive electrode exhibited a large charge storage potential window of 1.3 V from 0.6e0.7 V vs. SCE. The influence of the ratio of aniline to WOx precursor in the electrochemical deposition solution on pseudocapacitive properties of the obtained composite was investigated. The composite of PW-2:1/ExGF, made from the solution with a 2:1 ratio of aniline and WOx precursor, displayed a high composite based specific capacitance of 408 F g1 (408 mF cm2) at a discharging current density of 1 A g1 (based on PANI-WOx mass loading of ~1 mg cm2). Its energy density was as high as 95.8 Wh kg1 at 650 W kg1, thanks to the wide potential window and the high specific capacitance. The composite also demonstrated much improved cyclic stability compare to similar deposited PANI on Ex-GF (69.1% capacitance retention after 5000 chargeedischarge cycles), it can retain 91.6% of its capacitance after 5000 galvanostatic chargeedischarge cycles. 2. Experimental 2.1. Materials Aniline of chemical purity was distilled prior to use, all of the other chemicals were of analytical grade and used as received. Monohydrate sodium tungstate (Na2WO4$H2O), aniline, hydrogen peroxide (30%, H2O2) and sulfuric acid (H2SO4) were purchased from Sinopharm Chemical Reagent Co., Ltd. Graphite foil was bought from SGL group (Germany). A piece of graphite foil (1 1.5 cm2) and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively in the threeelectrode cell. 2.2. Partial exfoliation of graphite foil Partial exfoliation of graphite foil was conducted by two electrochemical steps according to our previous report [22]. The graphite foil (1 1 cm2) was firstly potential dynamically scanned from 0.5 to 1.8 V at 20 mV s1 in 0.5 M K2CO3 aqueous solution for six cycles. Subsequently, advanced cyclic voltammetry was conducted in 1 M KNO3 from 0.9e1.9 V at 20 mV s1 for ten cycles, the potential was kept at 1.9 V for 5 s in each cycle. The obtained partial exfoliated graphite foil (Ex-GF) was washed with ethanol and deionized water to remove residuals. 2.3. Concurrent electropolymerization of aniline and electrochemical deposition of WOx on partial exfoliated graphite Concurrent electropolymerization of aniline and electrochemical deposition of WOx were conducted on Ex-GF to deposite PANI-WOx by potential dynamic scans between 0.6 and 0.9 V vs. SCE at a scan rate of 100 mV s1. The deposition solutions were prepared by sodium tungstate (Na2WO4, 0.01 M), H2SO4 (0.5 M) and H2O2 (0.04 M) with subsequent adding of aniline (0.01, 0.02 and 0.03 M), in which the ratio of aniline to WOx precursor was 1:1, 2:1 and 3:1, respectively. The afforded composites through 14, 12 and 10 potential dynamic scans (to control the mass loadings of the composites to be 1.00 ± 0.05 mg cm2) were denoted as PW-1:1/ Ex-GF, PW-2:1/Ex-GF and PW-3:1/Ex-GF, respectively and were washed with deionized water and dried for further use. PANI/Ex-GF and WOx/Ex-GF were similarly prepared for
982
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
comparison in the absence of WOx precursor and aniline through 25 and 8 potential dynamic scans, respectively to deposite the polymer and the oxide with a mass loadings of 1.00 ± 0.05 mg cm2. The loading of the materials was measured by the weight difference of the electrode (vacuum dried at room temperature) before and after electrochemical deposition, using Sartorius BT 25 S semi-microbalance with an accuracy of 0.01 mg. 2.4. Supercapacitor assembly Symmetric supercapacitor PW-2:1/Ex-GF//PW-2:1/Ex-GF was assembled by using PW-2:1/Ex-GF as both of positive and negative electrodes and 0.5 M H2SO4 as electrolyte. The area of the electrode was 1.0 1.0 cm2 and the total mass loading of the composite in the two electrodes was 2.0 mg. 2.5. Material characterization Surface morphologies and element distribution were investigated by scanning electron microscope and energy dispersive X-ray spectroscopy (SEM and EDX, Ultra Plus, Carl Zeiss, Germany). The structure and composition of the materials were characterized by X-ray diffraction (XRD, D8 Advance, Bruker, Germany), Fourier transform infrared spectroscopy (FT-IR, A Spectrum One, PerkinElmer, USA) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific Escalab, USA). A multichannel electrochemical analyzer (VMP3, Bio-Logic-Science Instruments, France) was used to study the electrochemical properties of the electrodes in a three-electrode cell containing 0.5 M H2SO4 aqueous solution and the model supercapacitor in a two-electrode configuration. Electrochemical impedance spectra (EIS) were measured on an electrochemical workstation (CHI660C, Chenhua, Shanghai) in the frequency range of 0.01 Hze100 kHz with an amplitude of 10 mV. 3. Results and discussion 3.1. Concurrent electropolymerization of aniline and electrochemical deposition of WOx Concurrent electropolymerization of aniline and electrochemical deposition of WOx was first conducted in the solution prepared by sodium tungstate (Na2WO4, 0.01 M), H2SO4 (0.5 M) and H2O2 (0.04 M) with subsequent adding of aniline (0.02 M), in which the ratio of aniline to WOx precursor was 2:1. To study the structure and composition of PANI-WOx composite, XRD and FT-IR
measurements were conducted for the obtained PW-2:1/Ex-GF, as well as similarly prepared WOx/Ex-GF and PANI/Ex-GF (Fig. 1). The WOx may be amorphous in both of PW-2:1/Ex-GF and WOx/ExGF, demonstrated by the XRD patterns in Fig. 1a. Except the diffractions of the Ex-GF substrate, there is no other diffraction can be detected in the XRD patterns of PW-2:1/Ex-GF and WOx/Ex-GF. FT-IR spectra of PW-2:1/Ex-GF, PANI/Ex-GF and WOx/Ex-GF are displayed in Fig. 1b. Typical vibrations of PANI can be seen in the spectra of PW-2:1/Ex-GF and PANI/Ex-GF. The characteristic C]C stretching vibrations of quinoid and benzenoid ring are at 1572, 1489 cm1 for PW-2:1/Ex-GF and 1567 and 1482 cm1 for PANI/ExGF [24]. The CeN stretching of the secondary amine is at 1305 and 1300 cm1, respectively for PW-2:1/Ex-GF and PANI/Ex-GF [25,26]. The CeH in plane bending vibrations of CeH for PW-2:1/Ex-GF and PANI/Ex-GF, which is characteristic of the protonated and conductive PANI, are at 1144 and 1133 cm1, respectively [4]. The peaks at 801 and 795 cm1 in the spectra of PW-2:1/Ex-GF and PANI/Ex-GF are corresponding to CeH out-of-plane bending vibration of the polymer [27]. Characteristic vibrations of WOx can also be seen in the spectrum of PW-2:1/Ex-GF. The asymmetric stretching mode of terminal W]O is at 895 and 910 cm1 for PW-2:1/Ex-GF and WOx/ Ex-GF, respectively [12]. The peaks at 658 cm1 for PW-2:1/Ex-GF and at 678 cm1 for WOx/Ex-GF are attributed to WOW stretching [28]. The W-O related vibration, displayed at 1062 cm1 in the spectrum of WOx/Ex-GF, is overlapped with the characteristic vibration of protonated PANI (1144 cm1) in the spectrum of PW2:1/Ex-GF [29,30]. The vibration at 1615 cm1 in both of the spectra of PW-2:1/Ex-GF and WOx/Ex-GF are due to structure water in WOx [30]. As can be seen, the PANI related vibrations are blue shifted in the composite, while the WOx related vibrations are red shifted, demonstrating the synergistic effect between the two components. PW-2:1/Ex-GF, PANI/Ex-GF and WOx/Ex-GF were further studied by XPS. The peaks of W element from WOx can be clearly seen in the wide range survey spectra of PW-2:1/Ex-GF and WOx/Ex-GF, while the peak of N element from PANI is in those of PW-2:1/Ex-GF and WOx/Ex-GF (Fig. S1). The W 4f core level spectra of PW-2:1/ExGF and WOx/Ex-GF are shown in Fig. 2a and b. Both of the spin-orbit split doublet peaks can be disassembled into four synthetic peaks. The synthetic peaks at 35.59 and 37.64 eV, as well as 35.89 and 37.70 eV in the spectra of PW-2:1/Ex-GF and WOx/Ex-GF, respectively correspond to W5þ. While the peaks located at 35.89 and 38.0 eV in the spectrum of PW-2:1/Ex-GF, as well as those at 36.20 and 38.28 eV in the spectrum of WOx/Ex-GF are related to W6þ, indicating that the tungsten existed in mixed oxidation states [31].
Fig. 1. (a) XRD patterns collected for PW-2:1/Ex-GF, WOx/Ex-GF and Ex-GF; (b) FT-IR spectra of PW-2:1/Ex-GF, PANI/Ex-GF and WOx/Ex-GF.
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
983
Fig. 2. W 4f XPS core level spectra of (a) PW-2:1/Ex-GF and (b) WOx/Ex-GF; N 1s XPS core level spectra of (c) PW-2:1/Ex-GF and (d) PANI/Ex-GF.
The W6þ percentage is determined to be 79.2% for WOx/Ex-GF, which increased to 81.6% for the composite of PW-2:1/Ex-GF film. This may be due to the partial oxidation of the oxide by PANI during the concurrent electrochemical deposition [17]. The N 1s core level spectra of PW-2:1/Ex-GF and PANI/Ex-GF are given in Fig. 2c and d. The spectra can be deconvolved into three Gaussian peaks at 399.7, 400.4, 401.5 eV for PW-2:1/Ex-GF and 399.6, 400.4, 401.7 eV for PANI/Ex-GF, which attributed to benzenoid amine (-NH-), protonated benzenoid amine (-NHþ-) and protonated quinonoid imine (] Nþe), respectively [32]. The protonation level of PANI in PW-2:1/ Ex-GF and PANI/Ex-GF can be calculated to be 49.0% and 42.8%, respectively. The polymer in PW-2:1/Ex-GF displayed higher protonation level, thanks to the “proton sea” provided by WOx in the composite. This is helpful to improve the electronic conductivity of PANI. Morphologies of PW-2:1/Ex-GF, PANI/Ex-GF and WOx/Ex-GF were investigated by SEM and shown in Fig. 3, together with those of the Ex-GF substrate. As can be seen, the PW-2:1/Ex-GF composite tightly and uniformly covered on surfaces of graphene sheets in Ex-GF (Fig. 3b). This is clearly contrasted with similarly prepared PANI (Fig. 3c), thanks to the strong adherence of WOx to substrate [12,33]. In WOx/Ex-GF, the oxide existed as comparatively large particles (Fig. 3d), which would hinder the contact of the inner electroactive materials with electrolyte. The uniform distribution of PW-2:1/Ex-GF composite on Ex-GF was also supported by SEM element mapping (Fig. 4bef). As can be seen, elements of N (from PANI chains), S (from sulphate anions which were doped into PANI), W and O (from tungsten oxide), as well as carbon (from Ex-GF) can be detected over the whole area of PW-2:1/Ex-GF displayed in
Fig. 4a, suggesting the presence of a homogeneous PANI-WOx composite film on Ex-GF. To compare the surface area of the electrodes, PW-2:1/Ex-GF, WOx/Ex-GF and PANI/Ex-GF were immersed in 8 mg L1 methylene blue (MB) aqueous solutions, respectively [34,35]. UVevis absorption spectra were collected after 24 h for the original MB solution and those absorbed by PW-2:1/Ex-GF, WOx/ Ex-GF and PANI/Ex-GF (Fig. S2). As can be seen, the intensity of the characteristic absorption peak of MB (665 nm) decreased after absorption by the electrodes, suggesting that some of the dye molecules were adsorbed. The absorption intensity of MB solution absorbed by PW-2:1/Ex-GF is lower than WOx/Ex-GF and PANI/ExGF. This indicates that PW-2:1/Ex-GF can absorb more dye molecules, implying that it has larger surface area than WOx/Ex-GF and PANI/Ex-GF. To investigate the electrochemical performances of PW-2:1/ExGF, PANI/Ex-GF and WOx/Ex-GF, cyclic voltammetry (CV) and galvanostatic chargeedischarge experiments were conducted in a three-electrode cell containing 0.5 M H2SO4 aqueous electrolyte, with a SCE and graphite foil as the reference and counter electrodes, respectively. The CV profiles are shown in Fig. 5a. As can be seen, the composite displayed combined electroactivities of PANI and WOx, including the typical redox pair of PANI at 0.21/0.01 V (A/A0 ) and the characteristic WOx-related redox pair at 0.14/0.21 V (B/ B0 ). The first one corresponds to the electrochemical exchange between leucoemeraldine and emeraldine states of PANI, while the second one is due to the reversible intercalation/deintercalation of Hþ ions into/out of the WOx host [36,37]. To investigate the influence of Hþ intercalation/deintercalation on crystallinity of PW-2:1/ Ex-GF, XRD measurements were conducted on the samples of PW-
984
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
Fig. 3. SEM images of (a) Ex-GF, (b) PW-2:1/Ex-GF, (c) PANI/Ex-GF and (d) WOx/Ex-GF.
Fig. 4. (a) SEM image of PW-2:1/Ex-GF and corresponding SEM elemental mapping images of (b) N, (c) S, (d) W, (e) O and (f) C.
2:1/Ex-GF which were galvanostatically discharged to 0.6 V and charged to 0.7 V, respectively at low current density of 0.2 mA cm2 (Fig. S3). Similar to the as prepared PW-2:1/Ex-GF (Fig. 1a), no other diffraction can be detected in the XRD patterns of the discharged and charged samples, except those of the Ex-GF substrate, implying that the tungstun oxide may be also amorphous through Hþ intercalation and deintercalation. In addition, keeping at 0.6 V for 10 min after slowly discharged also has no influence of the crystallinity of PW-2:1/Ex-GF based on the XRD measurement (Fig. S3). Galvanostatic chargeedischarge profiles of PW-2:1/Ex-GF, PANI/ Ex-GF and WOx/Ex-GF at current densities of 1e10 A g1 are in Fig. S4aec. The charge and discharge profiles of the three electrodes are quite symmetric, demonstrating their good pseudocapacitive behaviors. The PW-2:1/Ex-GF electrode displayed longer discharge time than those of PANI/Ex-GF and WOx/Ex-GF, showing its higher charge storage capacity. The specific capacitance Cs (F g1) and areal capacitance Ca (mF cm2) of the material can be calculated according to Eqs. (1) and (2):
Cs ¼ IDt/mDU
(1)
Ca ¼ IDt/SDU
(2)
Where I and Dt are discharge current (A) and time (s), respectively, DU and m are discharge potential window (V) and mass loading of the electroactive material (g), respectively, S is the electrode area (cm2). The calculated specific capacitances at different currents are in Fig. 5b. PW-2:1/Ex-GF displayed higher specific capacitance than PANI/Ex-GF and WOx/Ex-GF at all of the current densities. For example, at 1 A g1, the specific capacitance of PW-2:1/Ex-GF is 408 F g1 (408 mF cm2), while those of PANI/ExGF and WOx/Ex-GF are 242.8 F g1 (242.8 mF cm2) and 150.4 F g1 (150.4 mF cm2), respectively. To evaluate capacitance contribution of the Ex-GF substrate, charge-discharge experiment was conducted on a piece of Ex-GF (Fig. S5). It displayed an areal capacitance (Ca) of 90.1 mF cm2 at 1 mA cm2, which is much lower than that of PW-2:1/Ex-GF (408 mF cm2). Considering that the contact
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
985
Fig. 5. Electrochemical performances of PW-2:1/Ex-GF, PANI/Ex-GF and WOx/Ex-GF collected in a 0.5 M H2SO4 solution. (a) CV profile collected at a scan rate of 50 mV s1. (b) Specific capacitances at different currents; (c) Nyquist plots with the inset showing the magnified high-frequency region; (d) Capacitance retention in 5000 galvanostatic chargeedischarge cycles measured at a current density of 10 A g1.
of Ex-GF with electrolyte will be significantly blocked when its surfaces were tightly covered by the dense PANI-WOx film (Fig. 3b) and charges are mainly stored at the surface or sub-surface of the electrode in supercapacitors, the capacitance contribution of Ex-GF substrate in PW-2:1/Ex-GF is negligible [38,39]. The energy density E (Wh kg1) and power density P (kW kg1) of the material can be calculated by Eq. (3) and Eq. (4): E ¼ 1000CsDU2/2 3600
(3)
P ¼ 3600E/1000Dt
(4)
Where Cs, DU, and Dt are the specific capacitance (F g1), discharge potential window (V) and time (s), respectively. PW-2:1/ Ex-GF displayed an extra high energy density of 95.8 Wh kg1 at the power density of 650 W kg1 which is higher than those of PANI/Ex-GF (57.0 Wh kg1) and WOx/Ex-GF (35.3 Wh kg1), thanks to its wide potential window and high specific capacitance. This is essentially higher than or comparable to those of other PANI based composites, for example the nanocomposite of SnO2@PANI (~47 Wh kg1 at ~500 W kg1) [9], the ternary composite of PANI, MoO3 and graphene nanoplatelets (99.58 Wh kg1 at 550.4 W kg1) [40], NiCo2O4@PANI with NiCo2O4 nanorod array as the conductive scaffold for the polymer (PANI based energy density of ~81.77 Wh kg1 at 399.3 W kg1) [10] and ternary composite of graphene, ZrO2 and PANI with PANI nanofiber vertically grown on graphene/ZrO2 composite sheets (104.76 Wh kg1 at 118 W kg1 for the sample casted on galssy carbon electrode) [41]. Our previous electrodeposited WO3/PANI composite only displayed an energy
density of 33.6 Wh kg1 at 300 W kg1 [14]. The much improved energy storage behaviors of PW-2:1/Ex-GF can be ascribed to the facilitated electron transportation supported by the Ex-GF substrate and the easy ion penetration in the active material due to its open structure. Moreover, the composite in PW-2:1/Ex-GF electrode showed good rate capability with a high specific capacitance of 275 F g1 (275 mF cm2) even at a larger current density of 10 A g1 (10 mA cm2). Its good charge storage kinetics was also demonstrated by cyclic voltammetry (Fig. S4d). The CV profiles of PW-2:1/ Ex-GF from 50 to 200 mV s1 showed similar shape without distortion, and the current density increases along with the increase of scan rate. PW-2:1/Ex-GF can retain 67.4% of its capacitance when the current density increased 10 times from 1 to 10 A g1, while PANI/Ex-GF and WOx/Ex-GF can retain 51.4% and 51.7% of their capacitance. The improved pseudocapacitive properties of PW-2:1/Ex-GF can be ascribed to its closely attachment on surfaces of partial exfoliated graphene sheets in Ex-GF. The formed open structure established avenues for ion transportation and so ensured the effect contact of the electroactive material with electrolyte for charge storage. Fast electron transfer through the graphene sheets seamlessly connected to the graphite substrate can further promote rate capability of the material. EIS measurements were also conducted and the Nyquist plots of PW-2:1/Ex-GF, PANI/Ex-GF and WOx/Ex-GF are shown in Fig. 5c. In the low frequency domain, PW-2:1/Ex-GF showed steeper line than PANI/Ex-GF and WOx/Ex-GF, demonstrating its better pseudocapacitive behavior [4,10]. All of PW-2:1/Ex-GF, PANI/Ex-GF and WOx/ Ex-GF displayed semicircles in the high frequency domain of the
986
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
plots (inset of Fig. 5c). Charge transfer resistance (Rct) can be deduced from the diameter of the semicircle [42]. PW-2:1/Ex-GF displayed lower Rct (0.08 U) than PANI/Ex-GF (0.11 U) and WOx/ Ex-GF (0.18 U). Moreover, PW-2:1/Ex-GF possessed the lowest combined series resistance (Rs) obtained from crossing of the highfrequency domain end and the real component axis (inset of Fig. 5c), suggesting its improved electrical conductivity [43]. The EIS results are consistent with the better rate capability of PW-2:1/ExGF. Cyclic stability of the electrodes was studied by prolonged galvanostatic chargeedischarge experiments at 10 A g1 in a 0.5 M H2SO4 aqueous electrolyte (Fig. 5d). After 5000 chargeedischarge cycles, PANI/Ex-GF and WOx/Ex-GF can retain 69.1% and 89.2% of their initial capacitances, respectively. In contrast, 91.6% of the initial capacitance of PW-2:1/Ex-GF can be retained, showing its superior cyclic stability, especially compared to PANI. The improved stability can be ascribed to the synergistic effect of the two components, including the mechanical support of the inorganic oxide particles for the polymer [9,12]. The stability of PW2:1/Ex-GF is better than or comparable to other PANI based composites which were stabilized by metal oxides or other inorganic compounds. For example, the SnO2@PANI with SnO2 nanoparticle backbone retained 60% of its capacitance after 3000 chargeedischarge cycles at 1 A g1 and didn't show capacitance decay after 10,000 cycles at a high current density of 15 A g1 [9]. The NiCo2O4@PANI with NiCo2O4 nanorod array functioned as strain buffer for the polymer retained 91% of its capacitance after 3000 galvanostatic chargeedischarge cycles [10]. After 20 cycles of in advanced cyclic voltammetric scans, the composite of MoS2/ PANI@C with a ~3 nm carbon shell exhibited a capacity retention of 80% after 10,000 scans at 100 mV s1 [7]. The composite of MoS2/ RGO@PANI retained over 82.5% of the starting capacitance after 3000 galvanostatic chargeedischarge loops [8]. The coaxial PANI/ TiN/PANI nanotube arrays showed a capacitance retention of 83% after 3000 cycles [11]. To study the effect of long-term cycling, XPS, FT-IR and XRD measurements were conducted on the PW-2:1/Ex-GF sample galvanostatically charged-discharged at 10 A g1 for 5000 cycles, while its surface morphologies were investigated by SEM (Fig. S6). Signals of W and O elements from WOx, as well as those of C and N from PANI can be detected in the XPS survey spectrum of the cycled PW-2:1/Ex-GF sample (Fig. S6a), showing the existence of both components. Based on the W 4f XPS core level spectrum (Fig. S6b), the W5þ percentage increased to 30.2% from 18.4% for the as prepared sample (Fig. 2a), implying that more protons were intercalated in the oxide due to repeated discharging to 0.6 V. This is also supported by the new vibration peak at 1392 cm1 in the FT-IR spectrum of the cycled sample (Fig. S6d), which can be ascribe to H-O-W [44]. After prolonged cycling, the protonation level of PANI in PW-2:1/Ex-GF (Fig. S6c) was increased to 56.7% from 49.0% for the as prepared sample (Fig. 2c). This indicates that the polymer was more doped due to the repeated charging, coincided with the 1 appearance of the strong vibration of the doped SO24 at 1216 cm (Fig. S6d) [45]. No additional diffraction can be detected in the XRD pattern of the cycled sample (Fig. S6e), showing that the long-term cycling didn't induce crystallinity change of the composite. As can be seen from the SEM image (Fig. S6f), the PANI-WOx film still tightly covered on graphene sheets in Ex-GF without significant morphology change after long-term cycling. 3.2. Influence of aniline to oxide precursor ratio on properties of the deposited composites To study the influence of aniline to oxide precursor ratio, PANIWOx composites were electrochemical deposited on Ex-GF in
solutions with aniline to oxide precursor ratio of 1:1, 2:1 and 3:1 to afford PW-1:1/Ex-GF, PW-2:1/Ex-GF and PW-3:1/Ex-GF. Surface morphologies of the composites were investigated by SEM and shown in Fig. 6. In PW-1:1/Ex-GF afforded from low aniline to WOx precusor ratio, the composite also intimately covered on surfaces of graphene sheets in Ex-GF, however, with many small aggregates. Many nonuniformly distributed aggregates can be seen on PW-3:1/Ex-GF, which may be due to the increased ratio of the polymer in the composite. The independent growth of PANI was also observed in SnO2@PANI with high polymer content [9]. To determine the amount ratio of PANI to WOx in the obtained PANI-WOx/Ex-GF composite films, energy dispersive X-ray spectrum (EDX) analyses of those three composites were collected (Fig. 7a). In the EDX spectra, the S signal came from SO24 ions doped in the PANI backbone, while the W signal originated from WOx. Assuming that PANI in all the three samples have similar doping concentrations, the amount of SO24 should be directly proportional to the amount of PANI. Thus, the S/W peak intensity ratio represents the amount ratio between PANI and WOx. As shown in Fig. 7b, the S/W increases with the increase of aniline concentration in electrodeposition solution. The inset of Fig. 7c compares the CV profiles of PW-1:1/Ex-GF, PW-2:1/Ex-GF and PW-3:1/Ex-GF. PW-2:1/Ex-GF displayed higher peak currents on its CV profile than PW-1:1/Ex-GF and PW-3:1/ExGF, indicating its higher electroactivity. It also showed longer discharge time (Fig. 7c). The specific capacitances were calculated by Eq. (1) and shown in Fig. 7d. As can be seen, all of the composite films displayed larger specific capacitance than similarly prepared PANI/Ex-GF and WOx/Ex-GF, in which PW-2:1/Ex-GF demonstrated the largest specific capacitance. The lower specific capacitances of PW-1:1/Ex-GF and PW-3:1/Ex-GF, compared with PW-2:1/Ex-GF, can be ascribed to the composite aggregation, which may block the effective contact of the composite with electrolyte. In addition, the comparatively larger proportion of the oxide in PW-1:1/Ex-GF, which has lower specific capacitance than the polymer, is another disadvantage for the material to display high specific capacitance. 3.3. Application of the composite in supercapacitor To investigate the application of the PANI-WOx composite in supercapacitor, a model symmetric supercapacitor (SSC) of PW-2:1/ Ex-GF//PW-2:1/Ex-GF was assembled by using two pieces of PW2:1/Ex-GF electrode as the anode and cathode, respectively and 0.5 M H2SO4 as electrolyte. Cyclic voltammetry and constant current chargeedischarge experiments were conducted with an operating voltage of 1.3 V. The device exhibited CV profiles without obvious distortion at scan rates up to 200 mV s1 (Fig. 8a), demonstrating a good electrochemical capacitive behavior and fast chargeedischarge properties for power device. Fig. 8b shows chargeedischarge profiles of the SSC at different current densities. The nearly symmetric triangle shape of chargeedischarge profiles demonstrated the good reversibility of the chargeedischarge processes. The specific capacitance C (F g1) of the model supercapacitor can be calculated according to Eq. (5): C ¼ IDt/V(ma þ mc)
(5)
where I and Dt are chargeedischarge current (A) and time (s), V is the operating voltage (V), ma and mc are mass loadings of the composite in anode and cathode (g), respectively. The specific capacitance of PW-2:1/Ex-GF//PW-2:1/Ex-GF supercapacitor was calculated to be 102 F g1 (102 mF cm2) at a current density of 1 A g1. When the chargedischarge current density increases 10 times from 1 to 10 A g1, the SSC can retain 50.8% of its capacitance (Fig. 8c).
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
987
Fig. 6. SEM images of PANI-WOx composites obtained in solutions with different aniline to WOx precusor ratios.
Fig. 7. (a) EDX spectra of PANI-WOx/Ex-GF composite films; (b) Peak intensity ratio of S/W from PANI-WOx/Ex-GF composite films made in solutions with different aniline to oxide precursor ratios; (c) Galvanostatic discharge profiles of PANI-WOx/Ex-GF composite films at a current density of 1 A g1 from 0.6 to 0.7 V vs. SCE in a 0.5 M H2SO4 solution, inset: CV curves at a scan rate of 50 mV s1; (d) Specific capacitances of PANI-WOx/Ex-GF composite films, together with those of PANI/Ex-GF and WOx/Ex-GF.
The energy density E (Wh kg1) and power density P (kW kg1) of PW-2:1/Ex-GF//PW-2:1/Ex-GF can be calculated by Eq. (6) and Eq. (7): E ¼ 1000CV2/2 3600
(6)
P ¼ 3600E/1000Dt
(7)
Where C, V, and Dt are the specific capacitance (F g1), operating voltage (V) and discharge time (s), respectively. The calculated results are shown in Fig. S7. PW-2:1/Ex-GF//PW-2:1/Ex-GF displayed a high energy density of 24 Wh kg1 at the power density of 1.29 kW kg1 and still maintained an energy density of 12 Wh kg1 at the high power density of 14.4 kW kg1. Long-term stability is a very important concern of supercapacitors. The stability of PW-2:1/Ex-GF//PW-2:1/Ex-GF was
investigated by constant current chargedischarge at 2 A g1 (Fig. 8d). The SSC can retain 87.5% of its capacitance after 10,000 galvanostatic chargedischarge cycles, exhibiting its excellent cycling stability. The PW-2:1/Ex-GF//PW-2:1/Ex-GF displayed improved pseudocapacitive behaviours, especially the high stability, compared to other supercapacitors with PANI composites as the electrode, including the SSC assembled by using MoS2/PANI hybrid electrode in which the PANI nanowire arrays were vertically aligned on surfaces of 3D tubular MoS2 (79% capacitance retention after 6000 cycles, 124 F g1 at 1 A g1) [4], PANI-RuO2 core-shell nanofiber-based SSC (~88% capacitance retention after 10,000 cycles, 11.5 Wh kg1 at 4.4 kW kg1 and 10 Wh kg1 at 42.2 kW kg1) [46] and the SSC assembled by using the composite of PANI, cellulose fiber, exfoliated graphite and silver nanoparticles as the electrodes (no capacitance decay after 2000 cycles) [47].
988
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
Fig. 8. Electrochemical performances of PW-2:1/Ex-GF//PW-2:1/Ex-GF SSC device. (a) CV profiles collected at various scan rates; (b) Galvanostatic chargeedischarge curves collected at various current densities; (c) Specific capacitance of the SSC at different current densities; (d) Capacitance retention of the SSC in 10,000 galvanostatic chargeedischarge cycles at 2 A g1, inset: profiles of the first and the 10,000th chargeedischarge cycles.
4. Conclusions In summary, a facile electro-codeposition method was conducted in this work through concurrent electropolymerization of aniline and electrodeposition of WOx on partial exfoliated graphite to prepare high performance pseudocapacitive electrode of PANIWOx/Ex-GF. Due to the open structure constructed by the intimate coating of the composite on surfaces of partial exfoliated graphene sheets in Ex-GF, PANI-WOx/Ex-GF electrode exhibited a high specific capacitance of 408 F g1 (408 mF cm2) at 1 A g1 (1 mA cm2). The composite also displayed a wide charge storage potential window of 1.3 V, ensuring its high energy density of 95.8 Wh kg1 at the power density of 650 W kg1. The cyclic stability of PANIWOx/Ex-GF was much improved compared to similarly prepared PANI/Ex-GF. The composite can retain 91.6% of its capacitance after 5000 galvanostatic chargedischarge cycles, while the polymer lost 30.9% of its capacitance. The excellent stability can be ascribed to the synergistic effect of the two components, including the mechanical support of the inorganic oxide particles for the polymer, as well as the volume change buffer provided by the Ex-GF substrate. The assembled symmetric model supercapacitor PW2:1/Ex-GF//PW-2:1/Ex-GF also displayed good stability with 87.5% capacitance retention after 10,000 galvanostatic chargedischarge cycles. The concurrent deposition is a good way to induce interactions among different components for synergistic effects and provides a facile method to fabricate organic-inorganic hybrid film. Acknowledgements We gratefully acknowledge financial supports from the National
Natural Science Foundation of China (Grant No. 21673035, 21273029 and 51604067) and China Postdoctoral Science Foundation (Grant no. 2015M580228). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.01.029. References [1] N. Ogihara, Y. Ozawa, O. Hiruta, J. Mater. Chem. A 4 (2016) 3398e3405. , E. Morallon, D. Cazorlaamoro s, J. Mater. [2] S. Leyvagarcía, D. Lozanocastello Chem. A 4 (2016) 4570e4579. [3] B.N. Choi, W.W. Chun, A. Qian, S.J. Lee, C.H. Chung, Nanoscale 7 (2015) 18561e18569. [4] L.J. Ren, G.N. Zhang, Z. Yan, L.P. Kang, H. Xu, F. Shi, Z.B. Lei, Z.H. Liu, ACS Appl. Mater. Interfaces 7 (2015) 28294e28302. [5] L.F. Lai, H.P. Yang, L. Wang, B.K. Teh, J.Q. Zhong, H. Chou, L.W. Chen, W. Chen, Z.X. Shen, R.S. Ruoff, J.Y. Lin, ACS Nano 6 (2012) 5941e5951. [6] C. Pan, H.T. Gu, L. Dong, J. Power Sources 303 (2016) 175e181. [7] C. Yang, Z.X. Chen, I. Shakir, Y.X. Xu, H.B. Lu, Nano Res. 9 (2016) 951e962. [8] X. Li, C.F. Zhang, S. Xin, Z.C. Yang, Y.T. Li, D.W. Zhang, P. Yao, ACS Appl. Mater. Interfaces 8 (2016) 21373e21380. [9] L. Wang, L. Chen, B. Yan, C.G. Wang, F. Zhu, X.F. Jiang, Y.M. Chao, G. Yang, J. Mater. Chem. A 2 (2014) 8334e8341. [10] N. Jabeen, Q.Y. Xia, M. Yang, H. Xia, ACS Appl. Mater. Interfaces 8 (2016) 6093e6100. [11] X. Peng, K.F. Huo, J.J. Fu, X.M. Zhang, B. Gao, P.K. Chu, Chem. Commun. 49 (2013) 10172e10174. [12] H.G. Wei, X.R. Yan, S.J. Wu, Z.P. Luo, S.Y. Wei, Z.H. Guo, J. Phys. Chem. C 116 (2012) 25052e25064. [13] G.F. Samu, K. Pencz, C. Jan aky, K. Rajeshwar, J. Solid State Electrochem 19 (2015) 2741e2751. [14] B.X. Zou, Y. Liang, X.X. Liu, D. Diamond, K.T. Lau, J. Power Sources 196 (2011) 4842e4848.
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989 [15] M.H. Bai, T.Y. Liu, F. Luan, Y. Li, X.X. Liu, J. Mater. Chem. A 2 (2014) 10882e10888. [16] X.X. Liu, L.J. Bian, L. Zhang, L.J. Zhang, J. Solid State Electrochem 11 (2007) 1279e1286. [17] M.J. Qiu, P. Sun, L.X. Shen, K. Wang, S.Q. Song, X. Yu, S.Z. Tan, C.X. Zhao, W.J. Mai, J. Mater. Chem. A 4 (2016) 7266e7273. [18] T. Pauporte, J. Electrochem. Soc. 149 (2002) C539eC545. [19] P.J. Kulesza, L.R. Faulkner, J. Am. Chem. Soc. 110 (2002) 4905e4913. [20] C. Jan aky, N.R.D. Tacconi, W. Chanmanee, K. Rajeshwar, J. Phys. Chem. C 116 (2012) 4234e4242. [21] C. Jan aky, K. Rajeshwar, Prog. Polym. Sci. 43 (2015) 96e135. [22] Y. Song, D.Y. Feng, T.Y. Liu, Y. Li, X.X. Liu, Nanoscale 7 (2015) 3581e3587. [23] S. Chabi, C. Peng, D. Hu, Y.Q. Zhu, Adv. Mater 26 (2014) 2440e2445. [24] B.L. Liang, Z.Y. Qin, J.Y. Zhao, Y. Zhang, Z. Zhou, Y.Q. Lu, J. Mater. Chem. A 2 (2014) 2129e2135. [25] Y.L. Guo, T. Wang, F.H. Chen, X.M. Sun, X.F. Li, Z.Z. Yu, P.B. Wan, X.D. Chen, Nanoscale 8 (2016) 12073e12080. [26] J.J. Xu, K. Wang, S.Z. Zu, B.H. Han, Z.X. Wei, ACS Nano 4 (2010) 5019e5026. [27] G.H. Xu, N. Wang, J.Y. Wei, L.L. Lv, J.N. Zhang, Z.M. Chen, Q. Xu, Ind. Eng. Chem. Res. 51 (2012) 14390e14398. [28] H.G. Wei, D.W. Ding, X.R. Yan, J. Guo, L. Shao, H.R. Chen, L.Y. Sun, H.A. Colorado, S.Y. Wei, Z.H. Guo, Electrochim. Acta 132 (2014) 58e66. [29] R. Yuksel, C. Durucan, H.E. Unalan, J. Alloys Compd. 658 (2016) 183e189. [30] B.X. Zou, X.X. Liu, D. Diamond, K.T. Lau, Electrochim. Acta 55 (2010) 3915e3920. [31] J. Wang, Z. Wang, C.J. Liu, ACS Appl. Mater. Interfaces 6 (2014) 12860e12867. [32] M.G. Han, S.S. Im, Polymer 41 (2000) 3253e3262.
989
[33] S. Balaji, Y. Djaoued, A. Albert, R. Brüning, N. Beaudoin, J. Robichaud, J. Mater. Chem. 21 (2011) 3940e3948. [34] G.M. Wang, H.Y. Wang, X.H. Lu, Y.C. Ling, M.H. Yu, T. Zhai, Y.X. Tong, Y. Li, Adv. Mater 26 (2014) 2676e2682. [35] D.Y. Feng, Y. Song, Z.H. Huang, X.X. Xu, X.X. Liu, J. Power Sources 324 (2016) 788e797. [36] Y.Y. Tian, S. Cong, W.M. Su, H.Y. Chen, Q.W. Li, F.X. Geng, Z.G. Zhao, Nano Lett. 14 (2014) 2150e2156. [37] W.M. Sun, M.T. Yeung, A.T. Lech, C.W. Lin, C. Lee, T.Q. Li, X.F. Duan, J. Zhou, R.B. Kaner, Nano Lett. 15 (2015) 4834e4838. [38] J. Yan, E. Khoo, A. Sumboja, P.S. Lee, ACS nano 4 (2010) 4247e4255. [39] J.H. Kim, K.H. Lee, L.J. Overzet, G.S. Lee, Nano Lett. 11 (2011) 2611e2617. [40] A.K. Das, S.K. Karan, B.B. Khatua, Electrochim. Acta 180 (2015) 1e15. [41] S. Giri, D. Ghosh, C.K. Das, Adv. Funct. Mater 24 (2014) 1312e1324. [42] G.J. He, J.M. Li, W.Y. Li, B. Li, N. Noor, K.B. Xu, J.Q. Hu, I.P. Parkin, J. Mater. Chem. A 3 (2015) 14272e14278. [43] S.N. Guo, Y. Zhu, Y.Y. Yan, Y.L. Min, J.C. Fan, Q.J. Xu, H. Yun, J. Power Sources 316 (2016) 176e182. [44] A. Georg, D. Schweiger, W. Graf, V. Wittwer, Sol. Energy Mat. Sol. C 70 (2002) 437e446. [45] Y. Gao, J. Wu, W. Zhang, Y. Tan, T. Tang, S. Wang, B. Tang, J.C. Zhao, Ceram. Int. 40 (2014) 8925e8929. [46] C. Xia, W. Chen, X.B. Wang, M.N. Hedhili, N.N. Wei, H.N. Alshareef, Adv. Energy Mater 5 (2015) 1401805. [47] A. Khosrozadeh, M.A. Darabi, M. Xing, Q. Wang, ACS Appl. Mater. Interfaces 8 (2016) 11379e11389.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Concurrent electropolymerization of aniline and electrochemical deposition of tungsten oxide for supercapacitor Jin-Wang Geng, Yin-Jian Ye, Di Guo**, Xiao-Xia Liu* Department of Chemistry, Northeastern University, Shenyang, 110819, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
PANI-WOx composites were fabricated through concurrent electrochemical deposition. Influences of aniline to WOx precursor ratio on composite fabrication were investigated. Cyclic stability and energy density of were improved PANI-WOx significantly. Assembled model supercapacitor displayed good stability and high energy density.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 October 2016 Received in revised form 18 December 2016 Accepted 5 January 2017
Polyaniline-tungsten oxide composite films (PANI-WOx) were prepared through concurrent electropolymerization of aniline and electrochemical deposition of tungsten oxide on partial exfoliated graphite (Ex-GF) for pseudocapacitive materials. The influence of aniline to WOx precursor ratio on pseudocapacitive properties of the afforded PANI-WOx/Ex-GF composite was investigated. PW-2:1/Ex-GF made from the solution containing aniline and WOx precursor in 2:1 ratio displayed a high specific capacitance (408 F g1/408 mF cm2 at 1 A g1/1 mA cm2) in a wide charge storage potential window of 0.6e0.7 V vs. SCE, leading to a high energy density of 95.8 Wh kg1 at 650 W kg1. Due to the synergistic effect between WOx and PANI, the composite showed much improved cyclic stability (91.6% capacitance retention after 5000 galvanostatic chargeedischarge cycles) compared to similarly prepared PANI/Ex-GF (69.1% capacitance after 5000 chargeedischarge cycles). The assembled symmetric model supercapacitor, by using PW-2:1/Ex-GF as both of the electrodes, also displayed good stability and high energy density, demonstrating that the PANI-WOx composite is promising electrode material for high-performance supercapacitor. © 2017 Elsevier B.V. All rights reserved.
Keywords: Concurrent electrochemical deposition Polyaniline Tungsten oxide Supercapacitor
1. Introduction Pseudocapacitors have drawn considerable attention in recent
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D. Guo), [email protected] (X.-X. Liu). http://dx.doi.org/10.1016/j.jpowsour.2017.01.029 0378-7753/© 2017 Elsevier B.V. All rights reserved.
years as they can usually store more charges than double-layered capacitors through fast and reversible surface redox reactions [1e3]. Electrode material is the most crucial factor to determine the electrochemical behaviors of pseudocapacitors. Polyaniline (PANI) is one of the promising pseudocapacitive materials due to its facile synthesis, high electrochemical activity, good flexibility and low density compared to metal oxides [4,5]. However, PANI suffers from poor cyclic stability due to volumetric swelling and shrinking and
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
subsequent mechanical degradation during the doping and dedoping of ions in chargeedischarge process [4,6e8]. Composites formation with inorganic materials is a possible solution to this challenge. For example, in the nanocomposite of SnO2@PANI, the SnO2 nanoparticles which incorporated in the polymer nanofibers played the role of a backbone to stabilize the PANI matrix during charge and discharge, leading to improved cyclic stability of the composite [9]. The NiCo2O4 nanorod array functioned as strain buffer for the polymer in the fabricated core-shell structured NiCo2O4@PANI [10]. Cyclic stability of PANI was reported to be enhanced by anchoring on MoS2 sheets and their composites, as well as TiN nanotube, which acted as mechanical support for the polymer [7,8,11]. Guo and coworkers demonstrated the improved cyclic stability of PANI/WO3 nanocomposite which was ascribed to the chemical bonding between the polymer matrix and WO3 particles [12]. Moreover, charge storage potential window of PANI based composites can be enlarged through the incorporation of inorganic oxides which possessing electroactivity in negative potentials, including WO3, V2O5 and MoOx [12e16]. This will in turn lead to increased energy density E for the electrode material as E is proportional to the square of the potential window U (E ¼ 1/2CsU2, where Cs is the specific capacitance). WO3 is mostly investigated among the many inorganic materials owing to its many advantages including low cost, good electrochemical stability, high conductivity and environmental friendliness [12,13,17]. WO3 displays multiple oxidation states due to the intercalation of electrons and protons to form hydrogen tungsten bronzes (HxWO3), enabling the oxide a good candidate to composite with PANI for electrode materials with enlarged charge storage potential window [14,18,19]. In addition, PANI is also electron and proton conductor, enabling synergistic effect between the two components [12,13,20]. After reduced, WO3 can work as a “proton sea” to keep high protonation level for PANI in the composite, realizing high conductivity of the polymer. The high conductivity of reduced WO3 can also compensate the low conductivity of de-doped PANI when it is reduced, offering possible solution for the large electrode polarization and poor rate capability. Several preparation routes have been reported for the syntheses of composites of conducting polymer and metal oxides. Among these, electrochemical deposition of composites has many advantageous compared to its rivals such as physical infiltration, mechanical mixing, and in situ chemical formation. Through electrochemical deposition, the composites can be directly formed as a thin film with high mechanical stability, which strongly adhered to the underlying current collector. Moreover, the film thickness, porosity and morphologies can be precisely controlled by electrochemical procedures [12,13,21]. A PANI/WO3 composite film was prepared through in-situ electropolymerization of aniline on WO3 sol-gel film. Chemical interactions were established between the polymer matrix and WO3 particles, which play a vital role for the improved chargeedischarge durability of the composite [12]. WO3/PANI hybrid films were prepared through potentiodynamic deposition of PANI from aniline on anodized nanoporous WO3 [13]. Due to the combined electroactivity of WO3 and PANI, the hybrid film displayed enlarged pseudocapacitive contribution potential window of 1.2 V from 0.5e0.7 V vs. Ag/AgCl. The PANI in the hybrid film exhibited a specific capacitance of 350 F g1 while the hybrid retained 93.4% of its initial capacitance after 200 chargeedischarge cycles. Electrochemical syntheses of WO3/PANI composite films were also demonstrated by our group through concurrent electrodeposition of WO3 and electropolymerization of aniline [14]. The composite displayed good pseudocapacitive performances in a widened potential range of 1.2 V. However, the energy density of the composite is needed to be further improved as the specific capacitance (168 F g1) is very limited.
981
Carbon-based, especially graphene based, current collectors open up new opportunities to improve charge storage performances of pseudocapacitive materials due to their high electrical conductivity, excellent mechanical property and good stability [22,23]. In this work, concurrent deposition of WOx and electropolymerization of aniline were conducted on a current collector of partial exfoliated graphite foil (Ex-GF) to afford PANI-WOx/Ex-GF. The obtained pseudocapacitive electrode exhibited a large charge storage potential window of 1.3 V from 0.6e0.7 V vs. SCE. The influence of the ratio of aniline to WOx precursor in the electrochemical deposition solution on pseudocapacitive properties of the obtained composite was investigated. The composite of PW-2:1/ExGF, made from the solution with a 2:1 ratio of aniline and WOx precursor, displayed a high composite based specific capacitance of 408 F g1 (408 mF cm2) at a discharging current density of 1 A g1 (based on PANI-WOx mass loading of ~1 mg cm2). Its energy density was as high as 95.8 Wh kg1 at 650 W kg1, thanks to the wide potential window and the high specific capacitance. The composite also demonstrated much improved cyclic stability compare to similar deposited PANI on Ex-GF (69.1% capacitance retention after 5000 chargeedischarge cycles), it can retain 91.6% of its capacitance after 5000 galvanostatic chargeedischarge cycles. 2. Experimental 2.1. Materials Aniline of chemical purity was distilled prior to use, all of the other chemicals were of analytical grade and used as received. Monohydrate sodium tungstate (Na2WO4$H2O), aniline, hydrogen peroxide (30%, H2O2) and sulfuric acid (H2SO4) were purchased from Sinopharm Chemical Reagent Co., Ltd. Graphite foil was bought from SGL group (Germany). A piece of graphite foil (1 1.5 cm2) and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively in the threeelectrode cell. 2.2. Partial exfoliation of graphite foil Partial exfoliation of graphite foil was conducted by two electrochemical steps according to our previous report [22]. The graphite foil (1 1 cm2) was firstly potential dynamically scanned from 0.5 to 1.8 V at 20 mV s1 in 0.5 M K2CO3 aqueous solution for six cycles. Subsequently, advanced cyclic voltammetry was conducted in 1 M KNO3 from 0.9e1.9 V at 20 mV s1 for ten cycles, the potential was kept at 1.9 V for 5 s in each cycle. The obtained partial exfoliated graphite foil (Ex-GF) was washed with ethanol and deionized water to remove residuals. 2.3. Concurrent electropolymerization of aniline and electrochemical deposition of WOx on partial exfoliated graphite Concurrent electropolymerization of aniline and electrochemical deposition of WOx were conducted on Ex-GF to deposite PANI-WOx by potential dynamic scans between 0.6 and 0.9 V vs. SCE at a scan rate of 100 mV s1. The deposition solutions were prepared by sodium tungstate (Na2WO4, 0.01 M), H2SO4 (0.5 M) and H2O2 (0.04 M) with subsequent adding of aniline (0.01, 0.02 and 0.03 M), in which the ratio of aniline to WOx precursor was 1:1, 2:1 and 3:1, respectively. The afforded composites through 14, 12 and 10 potential dynamic scans (to control the mass loadings of the composites to be 1.00 ± 0.05 mg cm2) were denoted as PW-1:1/ Ex-GF, PW-2:1/Ex-GF and PW-3:1/Ex-GF, respectively and were washed with deionized water and dried for further use. PANI/Ex-GF and WOx/Ex-GF were similarly prepared for
982
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
comparison in the absence of WOx precursor and aniline through 25 and 8 potential dynamic scans, respectively to deposite the polymer and the oxide with a mass loadings of 1.00 ± 0.05 mg cm2. The loading of the materials was measured by the weight difference of the electrode (vacuum dried at room temperature) before and after electrochemical deposition, using Sartorius BT 25 S semi-microbalance with an accuracy of 0.01 mg. 2.4. Supercapacitor assembly Symmetric supercapacitor PW-2:1/Ex-GF//PW-2:1/Ex-GF was assembled by using PW-2:1/Ex-GF as both of positive and negative electrodes and 0.5 M H2SO4 as electrolyte. The area of the electrode was 1.0 1.0 cm2 and the total mass loading of the composite in the two electrodes was 2.0 mg. 2.5. Material characterization Surface morphologies and element distribution were investigated by scanning electron microscope and energy dispersive X-ray spectroscopy (SEM and EDX, Ultra Plus, Carl Zeiss, Germany). The structure and composition of the materials were characterized by X-ray diffraction (XRD, D8 Advance, Bruker, Germany), Fourier transform infrared spectroscopy (FT-IR, A Spectrum One, PerkinElmer, USA) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific Escalab, USA). A multichannel electrochemical analyzer (VMP3, Bio-Logic-Science Instruments, France) was used to study the electrochemical properties of the electrodes in a three-electrode cell containing 0.5 M H2SO4 aqueous solution and the model supercapacitor in a two-electrode configuration. Electrochemical impedance spectra (EIS) were measured on an electrochemical workstation (CHI660C, Chenhua, Shanghai) in the frequency range of 0.01 Hze100 kHz with an amplitude of 10 mV. 3. Results and discussion 3.1. Concurrent electropolymerization of aniline and electrochemical deposition of WOx Concurrent electropolymerization of aniline and electrochemical deposition of WOx was first conducted in the solution prepared by sodium tungstate (Na2WO4, 0.01 M), H2SO4 (0.5 M) and H2O2 (0.04 M) with subsequent adding of aniline (0.02 M), in which the ratio of aniline to WOx precursor was 2:1. To study the structure and composition of PANI-WOx composite, XRD and FT-IR
measurements were conducted for the obtained PW-2:1/Ex-GF, as well as similarly prepared WOx/Ex-GF and PANI/Ex-GF (Fig. 1). The WOx may be amorphous in both of PW-2:1/Ex-GF and WOx/ExGF, demonstrated by the XRD patterns in Fig. 1a. Except the diffractions of the Ex-GF substrate, there is no other diffraction can be detected in the XRD patterns of PW-2:1/Ex-GF and WOx/Ex-GF. FT-IR spectra of PW-2:1/Ex-GF, PANI/Ex-GF and WOx/Ex-GF are displayed in Fig. 1b. Typical vibrations of PANI can be seen in the spectra of PW-2:1/Ex-GF and PANI/Ex-GF. The characteristic C]C stretching vibrations of quinoid and benzenoid ring are at 1572, 1489 cm1 for PW-2:1/Ex-GF and 1567 and 1482 cm1 for PANI/ExGF [24]. The CeN stretching of the secondary amine is at 1305 and 1300 cm1, respectively for PW-2:1/Ex-GF and PANI/Ex-GF [25,26]. The CeH in plane bending vibrations of CeH for PW-2:1/Ex-GF and PANI/Ex-GF, which is characteristic of the protonated and conductive PANI, are at 1144 and 1133 cm1, respectively [4]. The peaks at 801 and 795 cm1 in the spectra of PW-2:1/Ex-GF and PANI/Ex-GF are corresponding to CeH out-of-plane bending vibration of the polymer [27]. Characteristic vibrations of WOx can also be seen in the spectrum of PW-2:1/Ex-GF. The asymmetric stretching mode of terminal W]O is at 895 and 910 cm1 for PW-2:1/Ex-GF and WOx/ Ex-GF, respectively [12]. The peaks at 658 cm1 for PW-2:1/Ex-GF and at 678 cm1 for WOx/Ex-GF are attributed to WOW stretching [28]. The W-O related vibration, displayed at 1062 cm1 in the spectrum of WOx/Ex-GF, is overlapped with the characteristic vibration of protonated PANI (1144 cm1) in the spectrum of PW2:1/Ex-GF [29,30]. The vibration at 1615 cm1 in both of the spectra of PW-2:1/Ex-GF and WOx/Ex-GF are due to structure water in WOx [30]. As can be seen, the PANI related vibrations are blue shifted in the composite, while the WOx related vibrations are red shifted, demonstrating the synergistic effect between the two components. PW-2:1/Ex-GF, PANI/Ex-GF and WOx/Ex-GF were further studied by XPS. The peaks of W element from WOx can be clearly seen in the wide range survey spectra of PW-2:1/Ex-GF and WOx/Ex-GF, while the peak of N element from PANI is in those of PW-2:1/Ex-GF and WOx/Ex-GF (Fig. S1). The W 4f core level spectra of PW-2:1/ExGF and WOx/Ex-GF are shown in Fig. 2a and b. Both of the spin-orbit split doublet peaks can be disassembled into four synthetic peaks. The synthetic peaks at 35.59 and 37.64 eV, as well as 35.89 and 37.70 eV in the spectra of PW-2:1/Ex-GF and WOx/Ex-GF, respectively correspond to W5þ. While the peaks located at 35.89 and 38.0 eV in the spectrum of PW-2:1/Ex-GF, as well as those at 36.20 and 38.28 eV in the spectrum of WOx/Ex-GF are related to W6þ, indicating that the tungsten existed in mixed oxidation states [31].
Fig. 1. (a) XRD patterns collected for PW-2:1/Ex-GF, WOx/Ex-GF and Ex-GF; (b) FT-IR spectra of PW-2:1/Ex-GF, PANI/Ex-GF and WOx/Ex-GF.
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
983
Fig. 2. W 4f XPS core level spectra of (a) PW-2:1/Ex-GF and (b) WOx/Ex-GF; N 1s XPS core level spectra of (c) PW-2:1/Ex-GF and (d) PANI/Ex-GF.
The W6þ percentage is determined to be 79.2% for WOx/Ex-GF, which increased to 81.6% for the composite of PW-2:1/Ex-GF film. This may be due to the partial oxidation of the oxide by PANI during the concurrent electrochemical deposition [17]. The N 1s core level spectra of PW-2:1/Ex-GF and PANI/Ex-GF are given in Fig. 2c and d. The spectra can be deconvolved into three Gaussian peaks at 399.7, 400.4, 401.5 eV for PW-2:1/Ex-GF and 399.6, 400.4, 401.7 eV for PANI/Ex-GF, which attributed to benzenoid amine (-NH-), protonated benzenoid amine (-NHþ-) and protonated quinonoid imine (] Nþe), respectively [32]. The protonation level of PANI in PW-2:1/ Ex-GF and PANI/Ex-GF can be calculated to be 49.0% and 42.8%, respectively. The polymer in PW-2:1/Ex-GF displayed higher protonation level, thanks to the “proton sea” provided by WOx in the composite. This is helpful to improve the electronic conductivity of PANI. Morphologies of PW-2:1/Ex-GF, PANI/Ex-GF and WOx/Ex-GF were investigated by SEM and shown in Fig. 3, together with those of the Ex-GF substrate. As can be seen, the PW-2:1/Ex-GF composite tightly and uniformly covered on surfaces of graphene sheets in Ex-GF (Fig. 3b). This is clearly contrasted with similarly prepared PANI (Fig. 3c), thanks to the strong adherence of WOx to substrate [12,33]. In WOx/Ex-GF, the oxide existed as comparatively large particles (Fig. 3d), which would hinder the contact of the inner electroactive materials with electrolyte. The uniform distribution of PW-2:1/Ex-GF composite on Ex-GF was also supported by SEM element mapping (Fig. 4bef). As can be seen, elements of N (from PANI chains), S (from sulphate anions which were doped into PANI), W and O (from tungsten oxide), as well as carbon (from Ex-GF) can be detected over the whole area of PW-2:1/Ex-GF displayed in
Fig. 4a, suggesting the presence of a homogeneous PANI-WOx composite film on Ex-GF. To compare the surface area of the electrodes, PW-2:1/Ex-GF, WOx/Ex-GF and PANI/Ex-GF were immersed in 8 mg L1 methylene blue (MB) aqueous solutions, respectively [34,35]. UVevis absorption spectra were collected after 24 h for the original MB solution and those absorbed by PW-2:1/Ex-GF, WOx/ Ex-GF and PANI/Ex-GF (Fig. S2). As can be seen, the intensity of the characteristic absorption peak of MB (665 nm) decreased after absorption by the electrodes, suggesting that some of the dye molecules were adsorbed. The absorption intensity of MB solution absorbed by PW-2:1/Ex-GF is lower than WOx/Ex-GF and PANI/ExGF. This indicates that PW-2:1/Ex-GF can absorb more dye molecules, implying that it has larger surface area than WOx/Ex-GF and PANI/Ex-GF. To investigate the electrochemical performances of PW-2:1/ExGF, PANI/Ex-GF and WOx/Ex-GF, cyclic voltammetry (CV) and galvanostatic chargeedischarge experiments were conducted in a three-electrode cell containing 0.5 M H2SO4 aqueous electrolyte, with a SCE and graphite foil as the reference and counter electrodes, respectively. The CV profiles are shown in Fig. 5a. As can be seen, the composite displayed combined electroactivities of PANI and WOx, including the typical redox pair of PANI at 0.21/0.01 V (A/A0 ) and the characteristic WOx-related redox pair at 0.14/0.21 V (B/ B0 ). The first one corresponds to the electrochemical exchange between leucoemeraldine and emeraldine states of PANI, while the second one is due to the reversible intercalation/deintercalation of Hþ ions into/out of the WOx host [36,37]. To investigate the influence of Hþ intercalation/deintercalation on crystallinity of PW-2:1/ Ex-GF, XRD measurements were conducted on the samples of PW-
984
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
Fig. 3. SEM images of (a) Ex-GF, (b) PW-2:1/Ex-GF, (c) PANI/Ex-GF and (d) WOx/Ex-GF.
Fig. 4. (a) SEM image of PW-2:1/Ex-GF and corresponding SEM elemental mapping images of (b) N, (c) S, (d) W, (e) O and (f) C.
2:1/Ex-GF which were galvanostatically discharged to 0.6 V and charged to 0.7 V, respectively at low current density of 0.2 mA cm2 (Fig. S3). Similar to the as prepared PW-2:1/Ex-GF (Fig. 1a), no other diffraction can be detected in the XRD patterns of the discharged and charged samples, except those of the Ex-GF substrate, implying that the tungstun oxide may be also amorphous through Hþ intercalation and deintercalation. In addition, keeping at 0.6 V for 10 min after slowly discharged also has no influence of the crystallinity of PW-2:1/Ex-GF based on the XRD measurement (Fig. S3). Galvanostatic chargeedischarge profiles of PW-2:1/Ex-GF, PANI/ Ex-GF and WOx/Ex-GF at current densities of 1e10 A g1 are in Fig. S4aec. The charge and discharge profiles of the three electrodes are quite symmetric, demonstrating their good pseudocapacitive behaviors. The PW-2:1/Ex-GF electrode displayed longer discharge time than those of PANI/Ex-GF and WOx/Ex-GF, showing its higher charge storage capacity. The specific capacitance Cs (F g1) and areal capacitance Ca (mF cm2) of the material can be calculated according to Eqs. (1) and (2):
Cs ¼ IDt/mDU
(1)
Ca ¼ IDt/SDU
(2)
Where I and Dt are discharge current (A) and time (s), respectively, DU and m are discharge potential window (V) and mass loading of the electroactive material (g), respectively, S is the electrode area (cm2). The calculated specific capacitances at different currents are in Fig. 5b. PW-2:1/Ex-GF displayed higher specific capacitance than PANI/Ex-GF and WOx/Ex-GF at all of the current densities. For example, at 1 A g1, the specific capacitance of PW-2:1/Ex-GF is 408 F g1 (408 mF cm2), while those of PANI/ExGF and WOx/Ex-GF are 242.8 F g1 (242.8 mF cm2) and 150.4 F g1 (150.4 mF cm2), respectively. To evaluate capacitance contribution of the Ex-GF substrate, charge-discharge experiment was conducted on a piece of Ex-GF (Fig. S5). It displayed an areal capacitance (Ca) of 90.1 mF cm2 at 1 mA cm2, which is much lower than that of PW-2:1/Ex-GF (408 mF cm2). Considering that the contact
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
985
Fig. 5. Electrochemical performances of PW-2:1/Ex-GF, PANI/Ex-GF and WOx/Ex-GF collected in a 0.5 M H2SO4 solution. (a) CV profile collected at a scan rate of 50 mV s1. (b) Specific capacitances at different currents; (c) Nyquist plots with the inset showing the magnified high-frequency region; (d) Capacitance retention in 5000 galvanostatic chargeedischarge cycles measured at a current density of 10 A g1.
of Ex-GF with electrolyte will be significantly blocked when its surfaces were tightly covered by the dense PANI-WOx film (Fig. 3b) and charges are mainly stored at the surface or sub-surface of the electrode in supercapacitors, the capacitance contribution of Ex-GF substrate in PW-2:1/Ex-GF is negligible [38,39]. The energy density E (Wh kg1) and power density P (kW kg1) of the material can be calculated by Eq. (3) and Eq. (4): E ¼ 1000CsDU2/2 3600
(3)
P ¼ 3600E/1000Dt
(4)
Where Cs, DU, and Dt are the specific capacitance (F g1), discharge potential window (V) and time (s), respectively. PW-2:1/ Ex-GF displayed an extra high energy density of 95.8 Wh kg1 at the power density of 650 W kg1 which is higher than those of PANI/Ex-GF (57.0 Wh kg1) and WOx/Ex-GF (35.3 Wh kg1), thanks to its wide potential window and high specific capacitance. This is essentially higher than or comparable to those of other PANI based composites, for example the nanocomposite of SnO2@PANI (~47 Wh kg1 at ~500 W kg1) [9], the ternary composite of PANI, MoO3 and graphene nanoplatelets (99.58 Wh kg1 at 550.4 W kg1) [40], NiCo2O4@PANI with NiCo2O4 nanorod array as the conductive scaffold for the polymer (PANI based energy density of ~81.77 Wh kg1 at 399.3 W kg1) [10] and ternary composite of graphene, ZrO2 and PANI with PANI nanofiber vertically grown on graphene/ZrO2 composite sheets (104.76 Wh kg1 at 118 W kg1 for the sample casted on galssy carbon electrode) [41]. Our previous electrodeposited WO3/PANI composite only displayed an energy
density of 33.6 Wh kg1 at 300 W kg1 [14]. The much improved energy storage behaviors of PW-2:1/Ex-GF can be ascribed to the facilitated electron transportation supported by the Ex-GF substrate and the easy ion penetration in the active material due to its open structure. Moreover, the composite in PW-2:1/Ex-GF electrode showed good rate capability with a high specific capacitance of 275 F g1 (275 mF cm2) even at a larger current density of 10 A g1 (10 mA cm2). Its good charge storage kinetics was also demonstrated by cyclic voltammetry (Fig. S4d). The CV profiles of PW-2:1/ Ex-GF from 50 to 200 mV s1 showed similar shape without distortion, and the current density increases along with the increase of scan rate. PW-2:1/Ex-GF can retain 67.4% of its capacitance when the current density increased 10 times from 1 to 10 A g1, while PANI/Ex-GF and WOx/Ex-GF can retain 51.4% and 51.7% of their capacitance. The improved pseudocapacitive properties of PW-2:1/Ex-GF can be ascribed to its closely attachment on surfaces of partial exfoliated graphene sheets in Ex-GF. The formed open structure established avenues for ion transportation and so ensured the effect contact of the electroactive material with electrolyte for charge storage. Fast electron transfer through the graphene sheets seamlessly connected to the graphite substrate can further promote rate capability of the material. EIS measurements were also conducted and the Nyquist plots of PW-2:1/Ex-GF, PANI/Ex-GF and WOx/Ex-GF are shown in Fig. 5c. In the low frequency domain, PW-2:1/Ex-GF showed steeper line than PANI/Ex-GF and WOx/Ex-GF, demonstrating its better pseudocapacitive behavior [4,10]. All of PW-2:1/Ex-GF, PANI/Ex-GF and WOx/ Ex-GF displayed semicircles in the high frequency domain of the
986
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
plots (inset of Fig. 5c). Charge transfer resistance (Rct) can be deduced from the diameter of the semicircle [42]. PW-2:1/Ex-GF displayed lower Rct (0.08 U) than PANI/Ex-GF (0.11 U) and WOx/ Ex-GF (0.18 U). Moreover, PW-2:1/Ex-GF possessed the lowest combined series resistance (Rs) obtained from crossing of the highfrequency domain end and the real component axis (inset of Fig. 5c), suggesting its improved electrical conductivity [43]. The EIS results are consistent with the better rate capability of PW-2:1/ExGF. Cyclic stability of the electrodes was studied by prolonged galvanostatic chargeedischarge experiments at 10 A g1 in a 0.5 M H2SO4 aqueous electrolyte (Fig. 5d). After 5000 chargeedischarge cycles, PANI/Ex-GF and WOx/Ex-GF can retain 69.1% and 89.2% of their initial capacitances, respectively. In contrast, 91.6% of the initial capacitance of PW-2:1/Ex-GF can be retained, showing its superior cyclic stability, especially compared to PANI. The improved stability can be ascribed to the synergistic effect of the two components, including the mechanical support of the inorganic oxide particles for the polymer [9,12]. The stability of PW2:1/Ex-GF is better than or comparable to other PANI based composites which were stabilized by metal oxides or other inorganic compounds. For example, the SnO2@PANI with SnO2 nanoparticle backbone retained 60% of its capacitance after 3000 chargeedischarge cycles at 1 A g1 and didn't show capacitance decay after 10,000 cycles at a high current density of 15 A g1 [9]. The NiCo2O4@PANI with NiCo2O4 nanorod array functioned as strain buffer for the polymer retained 91% of its capacitance after 3000 galvanostatic chargeedischarge cycles [10]. After 20 cycles of in advanced cyclic voltammetric scans, the composite of MoS2/ PANI@C with a ~3 nm carbon shell exhibited a capacity retention of 80% after 10,000 scans at 100 mV s1 [7]. The composite of MoS2/ RGO@PANI retained over 82.5% of the starting capacitance after 3000 galvanostatic chargeedischarge loops [8]. The coaxial PANI/ TiN/PANI nanotube arrays showed a capacitance retention of 83% after 3000 cycles [11]. To study the effect of long-term cycling, XPS, FT-IR and XRD measurements were conducted on the PW-2:1/Ex-GF sample galvanostatically charged-discharged at 10 A g1 for 5000 cycles, while its surface morphologies were investigated by SEM (Fig. S6). Signals of W and O elements from WOx, as well as those of C and N from PANI can be detected in the XPS survey spectrum of the cycled PW-2:1/Ex-GF sample (Fig. S6a), showing the existence of both components. Based on the W 4f XPS core level spectrum (Fig. S6b), the W5þ percentage increased to 30.2% from 18.4% for the as prepared sample (Fig. 2a), implying that more protons were intercalated in the oxide due to repeated discharging to 0.6 V. This is also supported by the new vibration peak at 1392 cm1 in the FT-IR spectrum of the cycled sample (Fig. S6d), which can be ascribe to H-O-W [44]. After prolonged cycling, the protonation level of PANI in PW-2:1/Ex-GF (Fig. S6c) was increased to 56.7% from 49.0% for the as prepared sample (Fig. 2c). This indicates that the polymer was more doped due to the repeated charging, coincided with the 1 appearance of the strong vibration of the doped SO24 at 1216 cm (Fig. S6d) [45]. No additional diffraction can be detected in the XRD pattern of the cycled sample (Fig. S6e), showing that the long-term cycling didn't induce crystallinity change of the composite. As can be seen from the SEM image (Fig. S6f), the PANI-WOx film still tightly covered on graphene sheets in Ex-GF without significant morphology change after long-term cycling. 3.2. Influence of aniline to oxide precursor ratio on properties of the deposited composites To study the influence of aniline to oxide precursor ratio, PANIWOx composites were electrochemical deposited on Ex-GF in
solutions with aniline to oxide precursor ratio of 1:1, 2:1 and 3:1 to afford PW-1:1/Ex-GF, PW-2:1/Ex-GF and PW-3:1/Ex-GF. Surface morphologies of the composites were investigated by SEM and shown in Fig. 6. In PW-1:1/Ex-GF afforded from low aniline to WOx precusor ratio, the composite also intimately covered on surfaces of graphene sheets in Ex-GF, however, with many small aggregates. Many nonuniformly distributed aggregates can be seen on PW-3:1/Ex-GF, which may be due to the increased ratio of the polymer in the composite. The independent growth of PANI was also observed in SnO2@PANI with high polymer content [9]. To determine the amount ratio of PANI to WOx in the obtained PANI-WOx/Ex-GF composite films, energy dispersive X-ray spectrum (EDX) analyses of those three composites were collected (Fig. 7a). In the EDX spectra, the S signal came from SO24 ions doped in the PANI backbone, while the W signal originated from WOx. Assuming that PANI in all the three samples have similar doping concentrations, the amount of SO24 should be directly proportional to the amount of PANI. Thus, the S/W peak intensity ratio represents the amount ratio between PANI and WOx. As shown in Fig. 7b, the S/W increases with the increase of aniline concentration in electrodeposition solution. The inset of Fig. 7c compares the CV profiles of PW-1:1/Ex-GF, PW-2:1/Ex-GF and PW-3:1/Ex-GF. PW-2:1/Ex-GF displayed higher peak currents on its CV profile than PW-1:1/Ex-GF and PW-3:1/ExGF, indicating its higher electroactivity. It also showed longer discharge time (Fig. 7c). The specific capacitances were calculated by Eq. (1) and shown in Fig. 7d. As can be seen, all of the composite films displayed larger specific capacitance than similarly prepared PANI/Ex-GF and WOx/Ex-GF, in which PW-2:1/Ex-GF demonstrated the largest specific capacitance. The lower specific capacitances of PW-1:1/Ex-GF and PW-3:1/Ex-GF, compared with PW-2:1/Ex-GF, can be ascribed to the composite aggregation, which may block the effective contact of the composite with electrolyte. In addition, the comparatively larger proportion of the oxide in PW-1:1/Ex-GF, which has lower specific capacitance than the polymer, is another disadvantage for the material to display high specific capacitance. 3.3. Application of the composite in supercapacitor To investigate the application of the PANI-WOx composite in supercapacitor, a model symmetric supercapacitor (SSC) of PW-2:1/ Ex-GF//PW-2:1/Ex-GF was assembled by using two pieces of PW2:1/Ex-GF electrode as the anode and cathode, respectively and 0.5 M H2SO4 as electrolyte. Cyclic voltammetry and constant current chargeedischarge experiments were conducted with an operating voltage of 1.3 V. The device exhibited CV profiles without obvious distortion at scan rates up to 200 mV s1 (Fig. 8a), demonstrating a good electrochemical capacitive behavior and fast chargeedischarge properties for power device. Fig. 8b shows chargeedischarge profiles of the SSC at different current densities. The nearly symmetric triangle shape of chargeedischarge profiles demonstrated the good reversibility of the chargeedischarge processes. The specific capacitance C (F g1) of the model supercapacitor can be calculated according to Eq. (5): C ¼ IDt/V(ma þ mc)
(5)
where I and Dt are chargeedischarge current (A) and time (s), V is the operating voltage (V), ma and mc are mass loadings of the composite in anode and cathode (g), respectively. The specific capacitance of PW-2:1/Ex-GF//PW-2:1/Ex-GF supercapacitor was calculated to be 102 F g1 (102 mF cm2) at a current density of 1 A g1. When the chargedischarge current density increases 10 times from 1 to 10 A g1, the SSC can retain 50.8% of its capacitance (Fig. 8c).
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
987
Fig. 6. SEM images of PANI-WOx composites obtained in solutions with different aniline to WOx precusor ratios.
Fig. 7. (a) EDX spectra of PANI-WOx/Ex-GF composite films; (b) Peak intensity ratio of S/W from PANI-WOx/Ex-GF composite films made in solutions with different aniline to oxide precursor ratios; (c) Galvanostatic discharge profiles of PANI-WOx/Ex-GF composite films at a current density of 1 A g1 from 0.6 to 0.7 V vs. SCE in a 0.5 M H2SO4 solution, inset: CV curves at a scan rate of 50 mV s1; (d) Specific capacitances of PANI-WOx/Ex-GF composite films, together with those of PANI/Ex-GF and WOx/Ex-GF.
The energy density E (Wh kg1) and power density P (kW kg1) of PW-2:1/Ex-GF//PW-2:1/Ex-GF can be calculated by Eq. (6) and Eq. (7): E ¼ 1000CV2/2 3600
(6)
P ¼ 3600E/1000Dt
(7)
Where C, V, and Dt are the specific capacitance (F g1), operating voltage (V) and discharge time (s), respectively. The calculated results are shown in Fig. S7. PW-2:1/Ex-GF//PW-2:1/Ex-GF displayed a high energy density of 24 Wh kg1 at the power density of 1.29 kW kg1 and still maintained an energy density of 12 Wh kg1 at the high power density of 14.4 kW kg1. Long-term stability is a very important concern of supercapacitors. The stability of PW-2:1/Ex-GF//PW-2:1/Ex-GF was
investigated by constant current chargedischarge at 2 A g1 (Fig. 8d). The SSC can retain 87.5% of its capacitance after 10,000 galvanostatic chargedischarge cycles, exhibiting its excellent cycling stability. The PW-2:1/Ex-GF//PW-2:1/Ex-GF displayed improved pseudocapacitive behaviours, especially the high stability, compared to other supercapacitors with PANI composites as the electrode, including the SSC assembled by using MoS2/PANI hybrid electrode in which the PANI nanowire arrays were vertically aligned on surfaces of 3D tubular MoS2 (79% capacitance retention after 6000 cycles, 124 F g1 at 1 A g1) [4], PANI-RuO2 core-shell nanofiber-based SSC (~88% capacitance retention after 10,000 cycles, 11.5 Wh kg1 at 4.4 kW kg1 and 10 Wh kg1 at 42.2 kW kg1) [46] and the SSC assembled by using the composite of PANI, cellulose fiber, exfoliated graphite and silver nanoparticles as the electrodes (no capacitance decay after 2000 cycles) [47].
988
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989
Fig. 8. Electrochemical performances of PW-2:1/Ex-GF//PW-2:1/Ex-GF SSC device. (a) CV profiles collected at various scan rates; (b) Galvanostatic chargeedischarge curves collected at various current densities; (c) Specific capacitance of the SSC at different current densities; (d) Capacitance retention of the SSC in 10,000 galvanostatic chargeedischarge cycles at 2 A g1, inset: profiles of the first and the 10,000th chargeedischarge cycles.
4. Conclusions In summary, a facile electro-codeposition method was conducted in this work through concurrent electropolymerization of aniline and electrodeposition of WOx on partial exfoliated graphite to prepare high performance pseudocapacitive electrode of PANIWOx/Ex-GF. Due to the open structure constructed by the intimate coating of the composite on surfaces of partial exfoliated graphene sheets in Ex-GF, PANI-WOx/Ex-GF electrode exhibited a high specific capacitance of 408 F g1 (408 mF cm2) at 1 A g1 (1 mA cm2). The composite also displayed a wide charge storage potential window of 1.3 V, ensuring its high energy density of 95.8 Wh kg1 at the power density of 650 W kg1. The cyclic stability of PANIWOx/Ex-GF was much improved compared to similarly prepared PANI/Ex-GF. The composite can retain 91.6% of its capacitance after 5000 galvanostatic chargedischarge cycles, while the polymer lost 30.9% of its capacitance. The excellent stability can be ascribed to the synergistic effect of the two components, including the mechanical support of the inorganic oxide particles for the polymer, as well as the volume change buffer provided by the Ex-GF substrate. The assembled symmetric model supercapacitor PW2:1/Ex-GF//PW-2:1/Ex-GF also displayed good stability with 87.5% capacitance retention after 10,000 galvanostatic chargedischarge cycles. The concurrent deposition is a good way to induce interactions among different components for synergistic effects and provides a facile method to fabricate organic-inorganic hybrid film. Acknowledgements We gratefully acknowledge financial supports from the National
Natural Science Foundation of China (Grant No. 21673035, 21273029 and 51604067) and China Postdoctoral Science Foundation (Grant no. 2015M580228). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.01.029. References [1] N. Ogihara, Y. Ozawa, O. Hiruta, J. Mater. Chem. A 4 (2016) 3398e3405. , E. Morallon, D. Cazorlaamoro s, J. Mater. [2] S. Leyvagarcía, D. Lozanocastello Chem. A 4 (2016) 4570e4579. [3] B.N. Choi, W.W. Chun, A. Qian, S.J. Lee, C.H. Chung, Nanoscale 7 (2015) 18561e18569. [4] L.J. Ren, G.N. Zhang, Z. Yan, L.P. Kang, H. Xu, F. Shi, Z.B. Lei, Z.H. Liu, ACS Appl. Mater. Interfaces 7 (2015) 28294e28302. [5] L.F. Lai, H.P. Yang, L. Wang, B.K. Teh, J.Q. Zhong, H. Chou, L.W. Chen, W. Chen, Z.X. Shen, R.S. Ruoff, J.Y. Lin, ACS Nano 6 (2012) 5941e5951. [6] C. Pan, H.T. Gu, L. Dong, J. Power Sources 303 (2016) 175e181. [7] C. Yang, Z.X. Chen, I. Shakir, Y.X. Xu, H.B. Lu, Nano Res. 9 (2016) 951e962. [8] X. Li, C.F. Zhang, S. Xin, Z.C. Yang, Y.T. Li, D.W. Zhang, P. Yao, ACS Appl. Mater. Interfaces 8 (2016) 21373e21380. [9] L. Wang, L. Chen, B. Yan, C.G. Wang, F. Zhu, X.F. Jiang, Y.M. Chao, G. Yang, J. Mater. Chem. A 2 (2014) 8334e8341. [10] N. Jabeen, Q.Y. Xia, M. Yang, H. Xia, ACS Appl. Mater. Interfaces 8 (2016) 6093e6100. [11] X. Peng, K.F. Huo, J.J. Fu, X.M. Zhang, B. Gao, P.K. Chu, Chem. Commun. 49 (2013) 10172e10174. [12] H.G. Wei, X.R. Yan, S.J. Wu, Z.P. Luo, S.Y. Wei, Z.H. Guo, J. Phys. Chem. C 116 (2012) 25052e25064. [13] G.F. Samu, K. Pencz, C. Jan aky, K. Rajeshwar, J. Solid State Electrochem 19 (2015) 2741e2751. [14] B.X. Zou, Y. Liang, X.X. Liu, D. Diamond, K.T. Lau, J. Power Sources 196 (2011) 4842e4848.
J.-W. Geng et al. / Journal of Power Sources 342 (2017) 980e989 [15] M.H. Bai, T.Y. Liu, F. Luan, Y. Li, X.X. Liu, J. Mater. Chem. A 2 (2014) 10882e10888. [16] X.X. Liu, L.J. Bian, L. Zhang, L.J. Zhang, J. Solid State Electrochem 11 (2007) 1279e1286. [17] M.J. Qiu, P. Sun, L.X. Shen, K. Wang, S.Q. Song, X. Yu, S.Z. Tan, C.X. Zhao, W.J. Mai, J. Mater. Chem. A 4 (2016) 7266e7273. [18] T. Pauporte, J. Electrochem. Soc. 149 (2002) C539eC545. [19] P.J. Kulesza, L.R. Faulkner, J. Am. Chem. Soc. 110 (2002) 4905e4913. [20] C. Jan aky, N.R.D. Tacconi, W. Chanmanee, K. Rajeshwar, J. Phys. Chem. C 116 (2012) 4234e4242. [21] C. Jan aky, K. Rajeshwar, Prog. Polym. Sci. 43 (2015) 96e135. [22] Y. Song, D.Y. Feng, T.Y. Liu, Y. Li, X.X. Liu, Nanoscale 7 (2015) 3581e3587. [23] S. Chabi, C. Peng, D. Hu, Y.Q. Zhu, Adv. Mater 26 (2014) 2440e2445. [24] B.L. Liang, Z.Y. Qin, J.Y. Zhao, Y. Zhang, Z. Zhou, Y.Q. Lu, J. Mater. Chem. A 2 (2014) 2129e2135. [25] Y.L. Guo, T. Wang, F.H. Chen, X.M. Sun, X.F. Li, Z.Z. Yu, P.B. Wan, X.D. Chen, Nanoscale 8 (2016) 12073e12080. [26] J.J. Xu, K. Wang, S.Z. Zu, B.H. Han, Z.X. Wei, ACS Nano 4 (2010) 5019e5026. [27] G.H. Xu, N. Wang, J.Y. Wei, L.L. Lv, J.N. Zhang, Z.M. Chen, Q. Xu, Ind. Eng. Chem. Res. 51 (2012) 14390e14398. [28] H.G. Wei, D.W. Ding, X.R. Yan, J. Guo, L. Shao, H.R. Chen, L.Y. Sun, H.A. Colorado, S.Y. Wei, Z.H. Guo, Electrochim. Acta 132 (2014) 58e66. [29] R. Yuksel, C. Durucan, H.E. Unalan, J. Alloys Compd. 658 (2016) 183e189. [30] B.X. Zou, X.X. Liu, D. Diamond, K.T. Lau, Electrochim. Acta 55 (2010) 3915e3920. [31] J. Wang, Z. Wang, C.J. Liu, ACS Appl. Mater. Interfaces 6 (2014) 12860e12867. [32] M.G. Han, S.S. Im, Polymer 41 (2000) 3253e3262.
989
[33] S. Balaji, Y. Djaoued, A. Albert, R. Brüning, N. Beaudoin, J. Robichaud, J. Mater. Chem. 21 (2011) 3940e3948. [34] G.M. Wang, H.Y. Wang, X.H. Lu, Y.C. Ling, M.H. Yu, T. Zhai, Y.X. Tong, Y. Li, Adv. Mater 26 (2014) 2676e2682. [35] D.Y. Feng, Y. Song, Z.H. Huang, X.X. Xu, X.X. Liu, J. Power Sources 324 (2016) 788e797. [36] Y.Y. Tian, S. Cong, W.M. Su, H.Y. Chen, Q.W. Li, F.X. Geng, Z.G. Zhao, Nano Lett. 14 (2014) 2150e2156. [37] W.M. Sun, M.T. Yeung, A.T. Lech, C.W. Lin, C. Lee, T.Q. Li, X.F. Duan, J. Zhou, R.B. Kaner, Nano Lett. 15 (2015) 4834e4838. [38] J. Yan, E. Khoo, A. Sumboja, P.S. Lee, ACS nano 4 (2010) 4247e4255. [39] J.H. Kim, K.H. Lee, L.J. Overzet, G.S. Lee, Nano Lett. 11 (2011) 2611e2617. [40] A.K. Das, S.K. Karan, B.B. Khatua, Electrochim. Acta 180 (2015) 1e15. [41] S. Giri, D. Ghosh, C.K. Das, Adv. Funct. Mater 24 (2014) 1312e1324. [42] G.J. He, J.M. Li, W.Y. Li, B. Li, N. Noor, K.B. Xu, J.Q. Hu, I.P. Parkin, J. Mater. Chem. A 3 (2015) 14272e14278. [43] S.N. Guo, Y. Zhu, Y.Y. Yan, Y.L. Min, J.C. Fan, Q.J. Xu, H. Yun, J. Power Sources 316 (2016) 176e182. [44] A. Georg, D. Schweiger, W. Graf, V. Wittwer, Sol. Energy Mat. Sol. C 70 (2002) 437e446. [45] Y. Gao, J. Wu, W. Zhang, Y. Tan, T. Tang, S. Wang, B. Tang, J.C. Zhao, Ceram. Int. 40 (2014) 8925e8929. [46] C. Xia, W. Chen, X.B. Wang, M.N. Hedhili, N.N. Wei, H.N. Alshareef, Adv. Energy Mater 5 (2015) 1401805. [47] A. Khosrozadeh, M.A. Darabi, M. Xing, Q. Wang, ACS Appl. Mater. Interfaces 8 (2016) 11379e11389.