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Investigation of polyaniline films doped with Ni2+ as the electrode material for electrochemical supercapacitors
Accepted Manuscript Title: Investigation of polyaniline films doped with Ni2+ as the electrode material for electrochemical supercapacitors Authors: Hui Xu, Junling Li, Zhenjun Peng, Junxia Zhuang, Junlong Zhang PII: DOI: Reference:
S0013-4686(12)02006-3 doi:10.1016/j.electacta.2012.12.047 EA 19714
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
29-9-2012 10-12-2012 13-12-2012
Please cite this article as: H. Xu, J. Li, J. Zhuang, J. Zhang, Investigation of polyaniline films doped with Ni2+ as the electrode material for electrochemical supercapacitors, Electrochimica Acta (2010), doi:10.1016/j.electacta.2012.12.047 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation of polyaniline films doped with Ni2+ as the electrode material for electrochemical supercapacitors
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Hui Xu * , Junling Li, Zhenjun Peng, Junxia Zhuang, Junlong Zhang
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College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou 730050,
Highlights
Abstract
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2+ The PANI/Ni films was synthesized by CV on stainless steel substrates. 2+ The growth rate of PANI/Ni films was greatly increased. 2+ The PANI/Ni films show a larger specific capacitance and lower resistance.
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China
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Polyaniline (PANI) films doped with Ni2+ (PANI/Ni2+) were synthesized by cyclic
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voltammetry (CV) on stainless steel (SS) substrates in 0.2 mol/L aniline (An) and 0.5 mol/L sulfuric acid (H2SO4) electrolyte with various concentration of nickel sulfate (NiSO4). The
structure and morphology of PANI/Ni2+ films were characterized by Fourier transform infrared (FTIR), X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) techniques. The electrochemical properties of PANI/Ni2+ films were investigated by cyclic voltammetry, galvanostatic charge–discharge test and electrochemical impedance spectroscopy (EIS) in
*
Corresponding author. Tel.:+86-931-2975872; fax: +86-931-2975872 E-mail address: [email protected] (H. Xu), [email protected](J.L.Li).
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0.5mol/L H2SO4 electrolyte in three-electrode system. The results suggest that the growth rate of PANI was greatly increased, which may arise from the interactions between PANI chain and Ni2+. The PANI/Ni2+ films show a larger specific capacitance of 658.3 F/g at a current
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indicate that the PANI/Ni2+ films are promising material for supercapacitors.
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density of 5 mA/cm2 and lower resistance compared with the pure PANI film. The results
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Keywords: PANI film; Ni2+; supercapacitor; specific capacitance;
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1. Introduction
Electrochemical supercapacitors are a kind of energy storage devices between
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conventional capacitors and batteries, which have various applications in electric vehicles, cellular communication devices, portable computers and nanoelectronics
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[1–3]. Supercapacitors can be divided into two basic types based on the charge-storage mechanism: (1) electrochemical double-layer capacitors composed of carbon materials, in which the capacitance is derived from charge separation at electrode/electrolyte interface, and it is determined by the effective surface area and the dielectric constant of the electrolyte; (2) redox supercapacitors consisting of transition metal oxides and conducting polymers, in which the capacitance is generated from fast faradic reactions of the electrode material. Pseudocapacitance is produced from a bulk process whereas the double-layer capacitance is produced from a surface process [4,5]. Among all these materials investigated, conducting polymers
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are the most promising material as they possess redox pseudocapacitance in addition to double-layer capacitance. Conducting polymers offer the advantages of low production cost compared with noble metal oxides and high charge capacity
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compared with carbon materials [6]. In the case of conducting polymers, polyaniline
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(PANI) has attracted much attention and been intensively studied as electrode
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material owing to its unique proton doping mechanism [7,8], ease of synthesis, good environmental stability, controllable electrical conductivity and high specific
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capacitance [9-13].
PANI films are generally synthesized by different electrochemical methods in acid
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solutions containing aniline [14-16]. Usually the protonic acid dopants are inorganic acids (such as HCl, H2SO4, HClO4) [17–19] or organic acids (such as p-toluene
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sulfonic acid) [20]. The effect of anions on the properties of PANI films has been extensively investigated [21-23]. However, little attention has been given to the role
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of cations (other than protons) on the rate of PANI growth and on its electrochemical properties. Recently, PANI doped with metal ions has attracted much attention due to its potential practical applications such as redox-active catalyst [24], corrosion inhibitor [25], etc. Previous works [26,27] have demonstrated that alkaline cations, similarly to anions but to a smaller degree, affect the rate of PANI electropolymerization when it is performed by continuous potential cycling. On the contrary, there were rare studies with respect to PANI doped with transition metal salts as electrode material for supercapacitor. In this research, the PANI/Ni2+ films were synthesized in H2SO4 solution
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containing NiSO4 by cyclic voltammetry (CV) on low cost stainless steel (SS) substrates. The electrochemical properties of the PANI/Ni2+ films were investigated by cyclic voltammetry, galvanostatic charge–discharge test and electrochemical
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impedance spectroscopy (EIS) in three-electrode system. The structure and
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morphology of PANI/Ni2+ films were characterized by Fourier transform infrared
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(FTIR), X-ray diffraction (XRD) and Scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS)
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techniques, respectively.
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2. 1. Materials
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2. Experimental
Aniline was vacuum-distilled at 120°C and stored in refrigerator before use.
Analytical-grade reagents, ethanol, H2SO4 and NiSO4· 6H2O were used as received
without any pretreatment. Double-distilled water was used for the preparation of solutions.
2.2. Synthesis of the PANI/Ni2+ films
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All electropolymerization were carried out in a classic one-compartment cell using three-electrode configuration on CHI660B electrochemical work station. SS electrodes (type 201, 25×10×0.5mm) were used as working electrode (WE) for PANI
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deposition. The deposited area was 1cm2 (1×1cm) with other area insulated by a
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PTFE coating. SS electrodes (type 304, 25×15×0.5mm) and saturated calomel
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electrode (SCE) were used as the counter electrode (CE) and reference electrode (RE), respectively. SS electrodes were polished to a mirror surface successively with emery
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papers (600#,1500#, 2000#), rinsed with double-distilled water and immersed in ethanol ultrasonically for 10 min, then immersed in double-distilled water
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ultrasonically for 10 min, last air-dried for use.
The PANI/Ni2+ films were electrodeposited on SS substrates potentiodynamically
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in 0.2 mol/L aniline and 0.5 mol/L H2SO4 electrolyte with different concentration of
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0.2, 0.5, and 0.8 mol/L Ni2+. The deposition was carried out for two-steps: first
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between −0.2 and 1.2V vs. SCE for 2 cycles, then between −0.2 and 0.9V vs. SCE for
25 cycles at a scan rate of 50 mV/s. After deposition, the coated PANI/Ni2+ films were rinsed with 0.5 mol/L H2SO4 in order to remove soluble monomeric species, and then dried under vacuum for 12h at 60℃. The pure PANI film was prepared
following the above process in H2SO4 solution, without Ni2+, and a fine dark green
film was obtained.
2.3. Structure characterizations
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The samples used for structure characterizations were the powers scraped from PANI and PANI/Ni2+ films on SS substrates. The structure was characterized by FTIR (Nicolet, type210, America). FTIR spectrometer in the range of 4000–400cm-1. The
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XRD patterns were characterized by D/MAX-2400X X-ray diffractometer with CuKα
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radiation (λ = 0.154056 nm), employing a scanning rate of 10° min−1 in the 2θ range
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of 5–60°. The morphology of PANI and PANI/Ni2+ were characterized by SEM (JEOL JSM-6701F). Elemental analysis was performed by EDS (JEOL JSM-5600LV).
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The XPS datas were obtained by a V.G. ESCA Laboratory 210 photoelectron spectrometer with a Mg Kα source, the spectra were acquired with a 30 eV pass
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2.4. Electrochemical tests
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energy.
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All electrochemical tests were performed on CHI660B electrochemical work
station in a three-electrode glass cell with PANI and PANI/Ni2+ films working
electrode, a platinum plate counter electrode, and a standard calomel reference electrode. The electrochemical properties were evaluated by cyclic voltammetry, galvanostatic charge–discharge test and electrochemical impedance spectroscopy techniques in 0.5mol/L H2SO4 aqueous solution. Cyclic voltammetry and charge–discharge test were performed in the potential window ranged from 0 to 0.7V vs. SCE. EIS measurements were recorded in the frequency range 105 to 10−2 Hz with
an excitation signal of 5mV.
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3. Results and discussion.
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3.1. FT-IR
Fig.1 shows the FT-IR spectra of the pure PANI film and PANI/Ni2+ film
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prepared in the solution with 0.8 mol/L Ni2+. The main characteristic peaks of
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PANI/Ni2+ film were assigned as follows: the bands at 1558 and 1469 cm−1 were assigned to the N=Q=N stretching (Q represents the quinoid unit) and the N-B-N
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stretching (B represents the benzenoid unit), respectively. The bands located at 1297 and 1232 cm−1 were ascribed to C-N stretching vibration of quinoid rings and
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benzenoid rings, respectively. While the bands at 1114 and 800 cm−1 reflected the C-H in-plane deformation and out-of-plane vibration, respectively. From Fig.1, we
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can see that some vibration frequencies of the PANI/Ni2+ film are shifted to lower wavenumbers compared with those of the PANI film. The red shift of the IR absorption peaks is referred to be a signature of the conversion of the quinoid rings to the benzenoid rings by proton-induced spin-unpairing mechanism [28], which was considered to be an indication of increasing degree of charge delocalization on the PANI backbone [29]. It implies that PANI doped with Ni2+ lead to an increase of charge delocalization on the PANI backbone compared to doping solely with H2SO4.
3.2. XRD studies
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Fig.2 depicts the X-ray diffraction patterns of the pure PANI film and PANI/Ni2+ film prepared in the solution with 0.8 mol/L Ni2+. The XRD pattern of PANI/Ni2+ film
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exhibited the broad and weak reflection in the range of 20–25°, which were the
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characteristic peak of amorphous PANI. However, the sharpness of the peak at 23°
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indicated that the PANI/Ni2+ film has lower crystalline in comparison with the pure
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PANI film.
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3.3. SEM characterization
The morphology of PANI film and PANI/Ni2+ film prepared in the solution with
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0.8 mol/L Ni2+ deposited on SS substrate characterized by SEM are shown in Fig.3. It
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is clear that both PANI and PANI/Ni2+ are in nanorods morphology with a coarse
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surface and porous. The nanorods just like an aggregation of nanoparticles which have dense parts covered with loose materials. The coarse surface and porous have positive effect on the specific surface area of PANI nanorods and the diffusion of counter-anions. However, the PANI/Ni2+ film takes on morphology of somewhat aggregation corresponded to the XRD patterns of PANI/Ni2+ film. This indicates that the presence of Ni2+ makes the growth rate of PANI greatly increased, which improve the density of nucleation. It probably because transition metal ions such as Ni2+ has
multiple doping positions and may bind to several nitrogen sites in a PANI chain or form inter-chain linkages among several adjacent PANI chains by coordination [30].
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3.4. EDS and XPS analyses
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Fig.4 gives the EDS of the PANI/Ni2+ film prepared in the solution with 0.8 mol/L
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Ni2+. From the EDS spectra, it demonstrates that the deposit is composed of C,N,O,S
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and Ni elements. The mass percent of the Ni element in PANI/Ni2+ film was 13.15%. We used XPS technique to further examine the composition of Ni element. Fig.5
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presentes the Ni 2p core levels in the PANI/Ni2+ film prepared in the solution with 0.8 mol/L Ni2+. The Ni 2p line shows four peaks, two associated to the 2p3/2 and 2p1/2
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final states and two correspond to their shake-up satellites. The main line of the 2p3/2 is located at 857.4eV of binding energy and its satellite at 862.4eV, in agreement with
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previous measurement from NiO [31]. The binding energy can be assigned to an Ni2+
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oxidation state. As we can see from Fig.4 and Fig.5, Ni2+ exactly insert the PANI film,
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which made the equal quantity of anions in the solution to be attracted into the PANI film, so its growth rate was greatly increased, and the electrochemical capacitances of PANI films were improved.
3.5. Cyclic voltammetric studies
To evaluate the effect of Ni2+ on the electrochemical characteristics of PANI film electrode for supercapacitors, current–potential response was employed in the potential range of 0 to 0.7V versus SCE. Fig.6 illustrates the CV curves of PANI and
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PANI/Ni2+ film electrodes at a scan rate of 5mV/s in 0.5mol/L H2SO4 electrolyte. The CV curves of PANI/Ni2+ films were similar to that of PANI film, and revealed a little deviation from a rectangular form, whereas some considerable differences were
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observed. The peak currents of the PANI/Ni2+ film electrodes were relatively higher
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than that of PANI film, implying that a larger specific capacitance for the PANI/Ni2+
film electrodes can be anticipated. This increase can be attributed to the redox process
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involved with Ni2+.
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In order to further research the oxidation and reduction potentials and the properties of the PANI/Ni2+ film electrodes, Fig. 7 shows the CV curves of the
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PANI/Ni2+ film prepared in the solution with 0.8 mol/L Ni2+ at scan rates of 5,10 and 20mV/s. In Fig. 7, three pairs of redox peaks (P1/P1′,P2/P2′ and P3/P3′) can be observed.
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They were attributed to the redox process of PANI. Peak P1/P1′ (0.19/0.02V) corresponds to the oxidization of leucoemeraldine (LE) to emeraldine base (EM).
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Peak P2/P2′ (0.48/0.41V) is attributed to formation of PANI charge carriers consisting of polaron (radical cation) and bipolaron (dication) forms delocalized on PANI chains [32]. Peak P3/P3′ (0.7/0.62V) is due to the transformation of emeraldine to pernigraniline base (PE) [33]. With the increase of a scan rate from 5 to 20mV/s, the redox peak current rapidly increased, indicating a good rate ability of the PANI/Ni2+ film electrodes in 0.5mol/L H2SO4 electrolyte.
3.6. Electrochemical impedance spectroscopy
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Fig.8 depicts the typical Nyquist plots pertaining to the PANI/Ni2+ film prepared in the solution with 0.8 mol/L Ni2+ at different potentials in 0.5mol/L H2SO4 electrolyte. All the plots consist of a distorted semicircle in the high frequency region
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and a straight line in the low frequency region. The high frequency intercept with the
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real axis corresponds to the solution resistance (Rsol) and the diameter of the
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semicircle provides the charge transfer resistance (Rct) of the PANI/Ni2+ film/electrolyte interface. The low frequency slightly tilted vertical line represents a
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limiting diffusion process in H2SO4 electrolyte, which was characteristic feature of pure capacitive behavior. From Fig. 8, we can see that when the potential values at
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about 0.1V, PANI is in a completely reduced state (leucoemeraldine, LE), the total resistance of LE is the largest; the structure of PANI changes to half-oxidized
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conducting emeraldine state (EM) at about 0.3-0.4V, the total resistance reaches the minimum; when the potential is shifted to about 0.6V, EM undergoes oxidation and
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produces pernigraniline (PE), the total resistance increases again. From the frequency (f*, hertz) corresponding to the maximum of imaginary
component (-Z′′) of the semicircle, the time constant (s, seconds) of the capacitor is calculated according to Eq.(1): =
1
2 f *
(1)
The value of τ obtained from the data of Fig.8 is approximately in the range 7.4×10-5-1.65×10-4 s. Low values of τ are preferred for electrochemical capacitors for fast charge-discharge characteristics [34]. Furthermore, Fig.9 shows the EIS of PANI and PANI/Ni2+ films to prove the
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capacitive performance at the open-circuit potential of 0.4V. The high-frequency resistance slightly decreased with the concentration of Ni2+ increased, nevertheless these resistances are lower compared with that of the pure PANI film. This is an
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expected result, When doped with Ni2+, the films shift toward more negtive resistance
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values owing to the lower ionic and electronic resistance of the PANI/Ni2+ film
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electrodes.
An equivalent circuit consistent with the above behaviour is shown in Fig.10. In
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this circuit, Rsol is the solution resistance. Rct is represents as the charge transfer resistance of and PANI and PANI/Ni2+ films. Two constant-phase elements CPE1 and
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CPE2 are denoted as the double layer capacitance and ionic diffusion process resulting
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in Warburg behaviour in the electrode, respectively.
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3.7. Galvanostatic charge-discharge experiments
Fig. 11 gives the galvanostatic charge–discharge curves of PANI and PANI/Ni2+
film electrodes in the potential range of 0-0.7V at a current density of 5 mA/cm2 in
0.5 mol/L H2SO4 electrolyte. The specific capacitance (Cm) can be calculated according to Eq.(2): C I t Cm= = m Vm
(2)
where Cm is specific capacitance, I is charge/discharge current, △t is the discharge time, △V is the electrochemical window, and m denotes the mass of active material within the film electrodes. where m is controlled by the deposited charge-Qdep (C)
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m can be calculated as follows: m=
Q dep M zF
(3)
where m (grams) is the mass of deposited PANI, F is the Faraday constant, M is the
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molecular weight of aniline, and for the PANI reaction, z is 3 [35], The value of z= 3
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represents the achieved electronic number of aniline.
On the basis of the above equations, the Cm for PANI film is 565.2 F/ g and for
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PANI/Ni2+ films are 605.6, 621.0 and 658.3 F/g with concentration of 0.2, 0.5, and
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0.8 mol/L Ni2+, respectively. In comparison to the pure PANI film, the PANI/Ni2+ films show a higher specific capacitance, which may be attributed to the
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doping/dedoping process of Ni2+. It is known that Ni2+ can own coordination number less than or equal to 4. It means one Ni2+ may bind to more than one imine nitrogen
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sites in a PANI chain or form inter-chain linkages among several adjacent PANI chains by coordination, leading to a more effective conjugated network or matrix
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[36,37].
To further research the feasibility of PANI/Ni2+ film electrodes for supercapacitor,
Fig. 12 displays galvanostatic charge–discharge curves of the PANI/Ni2+ film
prepared in the solution with 0.8 mol/L Ni2+ at current densities of 5, 10 and 20 mA/cm2 in 0.5 mol/L H2SO4 electrolyte. The specific capacitances (in F/g) of the order are 658.3, 609.3 and 537.5, respectively. In Fig. 12, the voltage variation is nearly linear during both charging and discharging. Further, the anodic charging curves are symmetric to the corresponding cathodic discharging counterparts indicating that PANI/Ni2+ film electrode exhibit an ideally capacitive behavior. The
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specific capacitance decreased and IR drop rapidly increased of the PANI/Ni2+ film electrodes along with the increase of charge–discharge current densities, which was responsible to the internal resistance and polarization of the electrode [38]. So this
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kind of material is more suitable for supercapacitors in lower current density.
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In order to evaluate the stability of the film electrodes, the charge-discharge
cycling tests were conducted for 250 cycles. Fig. 13 shows the variation of specific
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capacitance with cycle number for PANI and PANI/Ni2+ film prepared in the solution
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with 0.8 mol/L Ni2+ at 5 mA/cm2. The results proposed that the cycleability of the PANI/Ni2+ film electrode was improved compared with the pure PANI film electrode.
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After 250 cycles, the specific capacitance of PANI/Ni2+ film electrode decreased for 35.29% a little less than that of the pure PANI film electrode 43.54%. The results
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indicated that Ni2+ is believed to improve the cycleability of the PANI film, which
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probably owing to the network or matrix of PANI chains formed by doped with Ni2+,
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allows for better accommodation of volume changes during the redox process [39]. However, considering that the stability of the film electrodes is not so good, it needs to be further researched.
4. Conclusions
The PANI/Ni2+ films were synthesized successfully by cyclic voltammetry on a stainless steel substrate in 0.2 mol/L aniline and 0.5 mol/L H2SO4 electrolyte with different concentration of Ni2+. It shows a larger specific capacitance of 658.3 F/g at a
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current density of 5 mA/cm2 and lower resistance compared with the pure PANI film. Owing to the presence of Ni2+, the PANI/Ni2+ films have an obvious improvement effect, which makes the PANI/Ni2+ films more active sites for faradaic reaction, better
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cyclic stability than the pure PANI film and facilitates the charge-transfer of the
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PANI/Ni2+ films.
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Acknowledgement
This work was supported by the National Natural Science Foundation of China
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References
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(No. 51062011).
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[1] S. Sarangapani, B.V. Tilak, C.P. Chen, J. Electrochem. Soc. 143 (1996) 3791–3799. [2] V. Gupta, N. Miura, Electrochim. Acta 52 (2006) 1721-1726. [3] K. Lota, V. Khomenko, E. Frackowiak, J. Phys. Chem. Solids 65 (2004) 295–301. [4] E. Frackowiak, F. Beguin, Carbon 39 (2001) 937-950. [5] B.E. Conway, W.G. Pell, J. Solid State Electron 7 (2003) 637-644. [6] Belanger D, Ren X, Davey J, Uribe F, Gottesfeld S (2000), J Electrochem Soc 147:2923. [7] N.S. Sariciftci, M. Bartonek, H. Kuzmany, H. Neugebauer, A. Neckel, Synth. Met 29 (1989) 193-202. [8] H. Reiss, Synth. Met 30 (1989) 257-263.
Page 15 of 24
[9] T.C. Girija, M.V. Sangaranarayanan, J. Power Sources 156 (2006) 705-711. [10] V. Gupta, N. Miura, Electrochem. Solid-State Lett. 8 (2005) A630-A632.
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[11] H.Y. Mi, X.G. Zhang, X.G.Ye , S.D.Yang, J. Power Sources 176 (2008). 403–409. [12] K.R. Prasad, N. Munichandraiah, J. Electrochem. Soc. 149 (2002) A1393-A1399.
cr
[13] H.Y. Mi, X.G. Zhang, S.D.Yang , X.G.Ye, J.M.Luo, Materials Chemistry and Physics 112
us
(2008) 127-131.
[14] K.S. Ryu, K.M. Kim, S.G. Kang, J. Joo, S.H. Chang, J. Power Sources 88 (2000) 197-201.
an
[15] H.H.Zhou, H.Chen, S.L.Luo, G.W.Lu, W.Z.Wei, Y.F.Kuang, J Solid State Electrochem 9 (2005) 574-580.
M
[16] T.C. Girija, M.V. Sangaranarayanan, Synthetic Metals 156 (2006) 244-250. [17] R.Y. Song, J.H. Park, S.R. Sivakkumar, S.H. Kim, J.M. Ko, D.Y. Rark, S.M. Jo, D.Y.Kim, J.
te
d
Power Sources 166 (2007) 297-301.
[18] H.L. Li, J.X. Wang, Q.X. Chu, Z. Wang, F.B. Zhang, S.C. Wang, J. Power Sources 190
Ac ce p
(2009) 578-586.
[19] J. Zhang, L.B. Kong, B.Wang, Y.C.Luo, L. Kang, Synthetic Metals 159 (2009) 260-266. [20] G.C. Xu, W. Wang, X.F. Qu, Y.S. Yin, L. Chu, B.L. He, H.K. Wu, J.R. Fang, Y.S. Bao, L. Liang, Eur. Polym. J. 45 (2009) 2701-2707.
[21] P. Nunziante and G. Pistoia, Electrochim. Acta 34 (1989) 223. [22] C.M.A. Brett, A. M. C. F. Oliveira Brett, J. L. C. Pereira and C. Rebelo, J. Appl. Electrochem. 23 (1993) 332. [23] V. Gupta, N. Miura, Mater. Lett. 60 (2006) 1466-1469. [24] S. Palaniappan, A. John, C.A. Amarnath, V.J. Rao, J. Mol. Catal. A. Chem. 218 (2004)
Page 16 of 24
47-53. [25] S. Sathiyanarayanan, C. Jeyaprabha, G. Venkatachari, Mater. Chem. Phys. 107 (2008)
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350-355. [26] M.J. Aguirre, B. A. Retamal, M. S. Ureta-Zafiartu, J. H.Zagal, R. C6rdova, R. Schrebler and
cr
S. R. Biaggio, J. Electroanal. Chem. 328 (1992) 349.
us
[27] S. R. Maggio, C. L. F. Oliveira, M. J. Aguirre, J. H. Zagal, J. Applied Electrochemistry 24 (1994) 1059-1065.
an
[28] S. Tao, B. Hong, Z. Kerong, Spectrochim. Acta Part A 66 (2007) 1364-1368. [29] Y.H. Kim, C. Foster, J. Chiangm, A.J. Heeger, Synth. Met 29 (1989) 285-290.
M
[30] C.M. Yang, C.Y. Chen, Synth. Met 153 (2005) 133-136.
[31] B.A. Reguig, M. Regragui, M. Morsli, A. Khelil, M. Addou, J.C. Berne` de, Solar Energy
te
d
Mater. Solar Cells 90 (2006) 1381-1392.
[32] T. Trung, T.H. Trung, C.S. Ha, Electrochim. Acta 51 (2002) 984-990.
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[33] Y.G. Wang, H.Q. Li, Y.Y. Xia, Adv. Mater. 18 (2006) 2619-2623. [34] A. F. Burke, J. Power Sources, 91 (2000) 17. [35] Stilwell DE, Park SM, J Electrochem Soc 135 (1988) 2254. [36] S. Tao, B. Hong, Z. Kerong, Spectrochim. Acta Part A 66 (2007) 1364-1368. [37] X.X. Li, Electrochim. Acta 54 (2009) 5634-5639. [38] Y.G. Wang, Y.Y. Xia, J. Electrochem. Soc 153 (2006) A450–A454. [39] H. Zhang, G.P. Cao, Z.Y. Wang, Y.S. Yang, Z.J. Shi, Z.N. Gu, Electrochem. Commun. 10 (2008) 1056-1059.
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ip t cr
800 798
1110
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1558 1469 1297 1232 1114
3440
a
3432
%Transmittance
b
1560 1460 1300 1240
2+ b:PANI/Ni a:PANI
4000
3500
3000
2500
2000
1500
500
an
Wavenumbers/cm-1
1000
d
M
2+ 2+ Fig. 1. FT-IR spectra of PANI film and PANI/Ni film prepared in the solution with 0.8 mol/L Ni
te
2+ b:PANI/Ni a:PANI
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Intensity /cps
b
a
0
10
20
30
40
50
60
2Theta / deg.
2+ 2+ Fig. 2. XRD patterns of PANI film and PANI/Ni film prepared in the solution with 0.8 mol/L Ni
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ip t cr us an M d 2+
2+ (c,d) prepared in the solution with 0.8 mol/L Ni
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Fig. 3. SEM images of PANI (a,b) and PANI/Ni
Fig. 4. EDS of the PANI/Ni
2+
2+ film prepared in the solution with 0.8 mol/L Ni
19
Page 19 of 24
2p3/2
Ni 2p
857.4eV 862.4eV
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cr
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Intensity /a.u.
2p1/2 880.1eV 875.3eV
895 890 885 880 875 870 865 860 855 850 845 840
an
Bonding Energy/eV
M
2+ 2+ Fig. 5. XPS Ni 2p in the PANI/Ni film prepared in the solution with 0.8 mol/L Ni
0.010
a
0.002 0.000 -0.002
te
b
0.004
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Current/A
0.006
d
d c
0.008
a:PANI 2+ b:PANI/0.2Ni 2+ c:PANI/0.5Ni 2+ d:PANI/0.8Ni
-0.004 -0.006 -0.008
-0.010 -0.1
0.0
Fig. 6. CV curves of PANI and PANI/Ni
0.1
2+
0.2
0.3
0.4
0.5
0.6
0.7
0.8
E vs.SCE/V
films at a scan rate of 5mV/s in 0.5mol/L H2SO4
20
Page 20 of 24
0.03
b
P1
0.01
P3
P2
a
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0.02
Current/A
c
a:5mV/s b:10mV/s c:20mV/s
0.00 P2'
P1'
P3'
cr
-0.01
-0.03 -0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
an
Potential/V vs.SCE
us
-0.02
M
2+ 2+ Fig. 7. CV curves of PANI/Ni film prepared in the solution with 0.8 mol/L Ni in 0.5mol/L H2SO4 at different scan rates
16
12 b
te
8
c
6
Ac ce p
-Z"/ohm
10
d
14
a:0.1V b:0.3V c:0.4V d:0.6V
d a
4 2 0
Fig. 8. EIS of PANI/Ni
2+
0
2
4
6
8
10
12
14
16
Z'/ohm
film prepared in the solution with 0.8 mol/L Ni
2+
at different applied voltages
21
Page 21 of 24
20 18 16
c
d
a:PANI 2+ b:PANI/0.2Ni 2+ c:PANI/0.5Ni 2+ d:PANI/0.8Ni
a
b
14
ip t
-Z"/ohm
12 10 8
cr
6 4
0
0
2
4
6
8
10
12
2+
16
18
20
films at the open-circuit potential of 0.4V
Ac ce p
te
d
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Fig. 9. EIS of PANI and PANI/Ni
14
an
Z'/ohm
us
2
Fig. 10. Equivalent circuit for the simulation of EIS spectra of PANI and PANI/Ni
2+
films.
22
Page 22 of 24
0.8 a:PANI 2+ b:PANI/0.2Ni 2+ c:PANI/0.5Ni 2+ d:PANI/0.8Ni
a b c d
0.7
ip t
0.5 0.4 0.3
cr
Potential/V vs.SCE
0.6
0.2
0.0
0
50
100
150
200
250
300
350
400
an
Time/sec
us
0.1
0.8 c
0.4 0.3
2
a
a:5mA/cm 2 b:10mA/cm 2 c:20mA/cm
te
0.5
b
d
0.6
Ac ce p
Potemtial/V vs.SCE
0.7
M
2+ 2 Fig. 11. Charge-discharge curves of PANI and PANI/Ni films at a current density of 5 mA/cm
0.2 0.1 0.0
0
50
Fig. 12. Charge-discharge curves of PANI/Ni
2+
100
150
200
250
300
350
400
Time/sec film prepared in the solution with 0.8 mol/L Ni
2+
at various current densities.
23
Page 23 of 24
700 a:PANI 2+ b:PANI/Ni
600
ip t
550 500 b
450 400 350
cr
Specific capacitance/ F/g
650
a
250
0
50
100
150
200
250
an
Cycle number
us
300
2+
film prepared in the solution with 0.8
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te
d
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Fig. 13. Variation of specific capacitance with cycle number for PANI film and PANI/Ni 2+ 2 mol/L Ni at 5 mA/cm
24
Page 24 of 24
S0013-4686(12)02006-3 doi:10.1016/j.electacta.2012.12.047 EA 19714
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
29-9-2012 10-12-2012 13-12-2012
Please cite this article as: H. Xu, J. Li, J. Zhuang, J. Zhang, Investigation of polyaniline films doped with Ni2+ as the electrode material for electrochemical supercapacitors, Electrochimica Acta (2010), doi:10.1016/j.electacta.2012.12.047 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation of polyaniline films doped with Ni2+ as the electrode material for electrochemical supercapacitors
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Hui Xu * , Junling Li, Zhenjun Peng, Junxia Zhuang, Junlong Zhang
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College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou 730050,
Highlights
Abstract
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M
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2+ The PANI/Ni films was synthesized by CV on stainless steel substrates. 2+ The growth rate of PANI/Ni films was greatly increased. 2+ The PANI/Ni films show a larger specific capacitance and lower resistance.
us
China
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Polyaniline (PANI) films doped with Ni2+ (PANI/Ni2+) were synthesized by cyclic
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voltammetry (CV) on stainless steel (SS) substrates in 0.2 mol/L aniline (An) and 0.5 mol/L sulfuric acid (H2SO4) electrolyte with various concentration of nickel sulfate (NiSO4). The
structure and morphology of PANI/Ni2+ films were characterized by Fourier transform infrared (FTIR), X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) techniques. The electrochemical properties of PANI/Ni2+ films were investigated by cyclic voltammetry, galvanostatic charge–discharge test and electrochemical impedance spectroscopy (EIS) in
*
Corresponding author. Tel.:+86-931-2975872; fax: +86-931-2975872 E-mail address: [email protected] (H. Xu), [email protected](J.L.Li).
Page 1 of 24
0.5mol/L H2SO4 electrolyte in three-electrode system. The results suggest that the growth rate of PANI was greatly increased, which may arise from the interactions between PANI chain and Ni2+. The PANI/Ni2+ films show a larger specific capacitance of 658.3 F/g at a current
cr
indicate that the PANI/Ni2+ films are promising material for supercapacitors.
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density of 5 mA/cm2 and lower resistance compared with the pure PANI film. The results
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Keywords: PANI film; Ni2+; supercapacitor; specific capacitance;
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1. Introduction
Electrochemical supercapacitors are a kind of energy storage devices between
te
d
conventional capacitors and batteries, which have various applications in electric vehicles, cellular communication devices, portable computers and nanoelectronics
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[1–3]. Supercapacitors can be divided into two basic types based on the charge-storage mechanism: (1) electrochemical double-layer capacitors composed of carbon materials, in which the capacitance is derived from charge separation at electrode/electrolyte interface, and it is determined by the effective surface area and the dielectric constant of the electrolyte; (2) redox supercapacitors consisting of transition metal oxides and conducting polymers, in which the capacitance is generated from fast faradic reactions of the electrode material. Pseudocapacitance is produced from a bulk process whereas the double-layer capacitance is produced from a surface process [4,5]. Among all these materials investigated, conducting polymers
Page 2 of 24
are the most promising material as they possess redox pseudocapacitance in addition to double-layer capacitance. Conducting polymers offer the advantages of low production cost compared with noble metal oxides and high charge capacity
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compared with carbon materials [6]. In the case of conducting polymers, polyaniline
cr
(PANI) has attracted much attention and been intensively studied as electrode
us
material owing to its unique proton doping mechanism [7,8], ease of synthesis, good environmental stability, controllable electrical conductivity and high specific
an
capacitance [9-13].
PANI films are generally synthesized by different electrochemical methods in acid
M
solutions containing aniline [14-16]. Usually the protonic acid dopants are inorganic acids (such as HCl, H2SO4, HClO4) [17–19] or organic acids (such as p-toluene
te
d
sulfonic acid) [20]. The effect of anions on the properties of PANI films has been extensively investigated [21-23]. However, little attention has been given to the role
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of cations (other than protons) on the rate of PANI growth and on its electrochemical properties. Recently, PANI doped with metal ions has attracted much attention due to its potential practical applications such as redox-active catalyst [24], corrosion inhibitor [25], etc. Previous works [26,27] have demonstrated that alkaline cations, similarly to anions but to a smaller degree, affect the rate of PANI electropolymerization when it is performed by continuous potential cycling. On the contrary, there were rare studies with respect to PANI doped with transition metal salts as electrode material for supercapacitor. In this research, the PANI/Ni2+ films were synthesized in H2SO4 solution
Page 3 of 24
containing NiSO4 by cyclic voltammetry (CV) on low cost stainless steel (SS) substrates. The electrochemical properties of the PANI/Ni2+ films were investigated by cyclic voltammetry, galvanostatic charge–discharge test and electrochemical
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impedance spectroscopy (EIS) in three-electrode system. The structure and
cr
morphology of PANI/Ni2+ films were characterized by Fourier transform infrared
us
(FTIR), X-ray diffraction (XRD) and Scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS)
an
techniques, respectively.
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2. 1. Materials
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2. Experimental
Aniline was vacuum-distilled at 120°C and stored in refrigerator before use.
Analytical-grade reagents, ethanol, H2SO4 and NiSO4· 6H2O were used as received
without any pretreatment. Double-distilled water was used for the preparation of solutions.
2.2. Synthesis of the PANI/Ni2+ films
Page 4 of 24
All electropolymerization were carried out in a classic one-compartment cell using three-electrode configuration on CHI660B electrochemical work station. SS electrodes (type 201, 25×10×0.5mm) were used as working electrode (WE) for PANI
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deposition. The deposited area was 1cm2 (1×1cm) with other area insulated by a
cr
PTFE coating. SS electrodes (type 304, 25×15×0.5mm) and saturated calomel
us
electrode (SCE) were used as the counter electrode (CE) and reference electrode (RE), respectively. SS electrodes were polished to a mirror surface successively with emery
an
papers (600#,1500#, 2000#), rinsed with double-distilled water and immersed in ethanol ultrasonically for 10 min, then immersed in double-distilled water
M
ultrasonically for 10 min, last air-dried for use.
The PANI/Ni2+ films were electrodeposited on SS substrates potentiodynamically
d
in 0.2 mol/L aniline and 0.5 mol/L H2SO4 electrolyte with different concentration of
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0.2, 0.5, and 0.8 mol/L Ni2+. The deposition was carried out for two-steps: first
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between −0.2 and 1.2V vs. SCE for 2 cycles, then between −0.2 and 0.9V vs. SCE for
25 cycles at a scan rate of 50 mV/s. After deposition, the coated PANI/Ni2+ films were rinsed with 0.5 mol/L H2SO4 in order to remove soluble monomeric species, and then dried under vacuum for 12h at 60℃. The pure PANI film was prepared
following the above process in H2SO4 solution, without Ni2+, and a fine dark green
film was obtained.
2.3. Structure characterizations
Page 5 of 24
The samples used for structure characterizations were the powers scraped from PANI and PANI/Ni2+ films on SS substrates. The structure was characterized by FTIR (Nicolet, type210, America). FTIR spectrometer in the range of 4000–400cm-1. The
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XRD patterns were characterized by D/MAX-2400X X-ray diffractometer with CuKα
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radiation (λ = 0.154056 nm), employing a scanning rate of 10° min−1 in the 2θ range
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of 5–60°. The morphology of PANI and PANI/Ni2+ were characterized by SEM (JEOL JSM-6701F). Elemental analysis was performed by EDS (JEOL JSM-5600LV).
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The XPS datas were obtained by a V.G. ESCA Laboratory 210 photoelectron spectrometer with a Mg Kα source, the spectra were acquired with a 30 eV pass
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2.4. Electrochemical tests
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energy.
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All electrochemical tests were performed on CHI660B electrochemical work
station in a three-electrode glass cell with PANI and PANI/Ni2+ films working
electrode, a platinum plate counter electrode, and a standard calomel reference electrode. The electrochemical properties were evaluated by cyclic voltammetry, galvanostatic charge–discharge test and electrochemical impedance spectroscopy techniques in 0.5mol/L H2SO4 aqueous solution. Cyclic voltammetry and charge–discharge test were performed in the potential window ranged from 0 to 0.7V vs. SCE. EIS measurements were recorded in the frequency range 105 to 10−2 Hz with
an excitation signal of 5mV.
Page 6 of 24
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3. Results and discussion.
cr
3.1. FT-IR
Fig.1 shows the FT-IR spectra of the pure PANI film and PANI/Ni2+ film
us
prepared in the solution with 0.8 mol/L Ni2+. The main characteristic peaks of
an
PANI/Ni2+ film were assigned as follows: the bands at 1558 and 1469 cm−1 were assigned to the N=Q=N stretching (Q represents the quinoid unit) and the N-B-N
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stretching (B represents the benzenoid unit), respectively. The bands located at 1297 and 1232 cm−1 were ascribed to C-N stretching vibration of quinoid rings and
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d
benzenoid rings, respectively. While the bands at 1114 and 800 cm−1 reflected the C-H in-plane deformation and out-of-plane vibration, respectively. From Fig.1, we
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can see that some vibration frequencies of the PANI/Ni2+ film are shifted to lower wavenumbers compared with those of the PANI film. The red shift of the IR absorption peaks is referred to be a signature of the conversion of the quinoid rings to the benzenoid rings by proton-induced spin-unpairing mechanism [28], which was considered to be an indication of increasing degree of charge delocalization on the PANI backbone [29]. It implies that PANI doped with Ni2+ lead to an increase of charge delocalization on the PANI backbone compared to doping solely with H2SO4.
3.2. XRD studies
Page 7 of 24
Fig.2 depicts the X-ray diffraction patterns of the pure PANI film and PANI/Ni2+ film prepared in the solution with 0.8 mol/L Ni2+. The XRD pattern of PANI/Ni2+ film
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exhibited the broad and weak reflection in the range of 20–25°, which were the
cr
characteristic peak of amorphous PANI. However, the sharpness of the peak at 23°
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indicated that the PANI/Ni2+ film has lower crystalline in comparison with the pure
an
PANI film.
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3.3. SEM characterization
The morphology of PANI film and PANI/Ni2+ film prepared in the solution with
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0.8 mol/L Ni2+ deposited on SS substrate characterized by SEM are shown in Fig.3. It
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is clear that both PANI and PANI/Ni2+ are in nanorods morphology with a coarse
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surface and porous. The nanorods just like an aggregation of nanoparticles which have dense parts covered with loose materials. The coarse surface and porous have positive effect on the specific surface area of PANI nanorods and the diffusion of counter-anions. However, the PANI/Ni2+ film takes on morphology of somewhat aggregation corresponded to the XRD patterns of PANI/Ni2+ film. This indicates that the presence of Ni2+ makes the growth rate of PANI greatly increased, which improve the density of nucleation. It probably because transition metal ions such as Ni2+ has
multiple doping positions and may bind to several nitrogen sites in a PANI chain or form inter-chain linkages among several adjacent PANI chains by coordination [30].
Page 8 of 24
3.4. EDS and XPS analyses
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Fig.4 gives the EDS of the PANI/Ni2+ film prepared in the solution with 0.8 mol/L
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Ni2+. From the EDS spectra, it demonstrates that the deposit is composed of C,N,O,S
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and Ni elements. The mass percent of the Ni element in PANI/Ni2+ film was 13.15%. We used XPS technique to further examine the composition of Ni element. Fig.5
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presentes the Ni 2p core levels in the PANI/Ni2+ film prepared in the solution with 0.8 mol/L Ni2+. The Ni 2p line shows four peaks, two associated to the 2p3/2 and 2p1/2
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final states and two correspond to their shake-up satellites. The main line of the 2p3/2 is located at 857.4eV of binding energy and its satellite at 862.4eV, in agreement with
d
previous measurement from NiO [31]. The binding energy can be assigned to an Ni2+
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oxidation state. As we can see from Fig.4 and Fig.5, Ni2+ exactly insert the PANI film,
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which made the equal quantity of anions in the solution to be attracted into the PANI film, so its growth rate was greatly increased, and the electrochemical capacitances of PANI films were improved.
3.5. Cyclic voltammetric studies
To evaluate the effect of Ni2+ on the electrochemical characteristics of PANI film electrode for supercapacitors, current–potential response was employed in the potential range of 0 to 0.7V versus SCE. Fig.6 illustrates the CV curves of PANI and
Page 9 of 24
PANI/Ni2+ film electrodes at a scan rate of 5mV/s in 0.5mol/L H2SO4 electrolyte. The CV curves of PANI/Ni2+ films were similar to that of PANI film, and revealed a little deviation from a rectangular form, whereas some considerable differences were
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observed. The peak currents of the PANI/Ni2+ film electrodes were relatively higher
cr
than that of PANI film, implying that a larger specific capacitance for the PANI/Ni2+
film electrodes can be anticipated. This increase can be attributed to the redox process
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involved with Ni2+.
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In order to further research the oxidation and reduction potentials and the properties of the PANI/Ni2+ film electrodes, Fig. 7 shows the CV curves of the
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PANI/Ni2+ film prepared in the solution with 0.8 mol/L Ni2+ at scan rates of 5,10 and 20mV/s. In Fig. 7, three pairs of redox peaks (P1/P1′,P2/P2′ and P3/P3′) can be observed.
te
d
They were attributed to the redox process of PANI. Peak P1/P1′ (0.19/0.02V) corresponds to the oxidization of leucoemeraldine (LE) to emeraldine base (EM).
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Peak P2/P2′ (0.48/0.41V) is attributed to formation of PANI charge carriers consisting of polaron (radical cation) and bipolaron (dication) forms delocalized on PANI chains [32]. Peak P3/P3′ (0.7/0.62V) is due to the transformation of emeraldine to pernigraniline base (PE) [33]. With the increase of a scan rate from 5 to 20mV/s, the redox peak current rapidly increased, indicating a good rate ability of the PANI/Ni2+ film electrodes in 0.5mol/L H2SO4 electrolyte.
3.6. Electrochemical impedance spectroscopy
Page 10 of 24
Fig.8 depicts the typical Nyquist plots pertaining to the PANI/Ni2+ film prepared in the solution with 0.8 mol/L Ni2+ at different potentials in 0.5mol/L H2SO4 electrolyte. All the plots consist of a distorted semicircle in the high frequency region
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and a straight line in the low frequency region. The high frequency intercept with the
cr
real axis corresponds to the solution resistance (Rsol) and the diameter of the
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semicircle provides the charge transfer resistance (Rct) of the PANI/Ni2+ film/electrolyte interface. The low frequency slightly tilted vertical line represents a
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limiting diffusion process in H2SO4 electrolyte, which was characteristic feature of pure capacitive behavior. From Fig. 8, we can see that when the potential values at
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about 0.1V, PANI is in a completely reduced state (leucoemeraldine, LE), the total resistance of LE is the largest; the structure of PANI changes to half-oxidized
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d
conducting emeraldine state (EM) at about 0.3-0.4V, the total resistance reaches the minimum; when the potential is shifted to about 0.6V, EM undergoes oxidation and
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produces pernigraniline (PE), the total resistance increases again. From the frequency (f*, hertz) corresponding to the maximum of imaginary
component (-Z′′) of the semicircle, the time constant (s, seconds) of the capacitor is calculated according to Eq.(1): =
1
2 f *
(1)
The value of τ obtained from the data of Fig.8 is approximately in the range 7.4×10-5-1.65×10-4 s. Low values of τ are preferred for electrochemical capacitors for fast charge-discharge characteristics [34]. Furthermore, Fig.9 shows the EIS of PANI and PANI/Ni2+ films to prove the
Page 11 of 24
capacitive performance at the open-circuit potential of 0.4V. The high-frequency resistance slightly decreased with the concentration of Ni2+ increased, nevertheless these resistances are lower compared with that of the pure PANI film. This is an
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expected result, When doped with Ni2+, the films shift toward more negtive resistance
cr
values owing to the lower ionic and electronic resistance of the PANI/Ni2+ film
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electrodes.
An equivalent circuit consistent with the above behaviour is shown in Fig.10. In
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this circuit, Rsol is the solution resistance. Rct is represents as the charge transfer resistance of and PANI and PANI/Ni2+ films. Two constant-phase elements CPE1 and
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CPE2 are denoted as the double layer capacitance and ionic diffusion process resulting
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d
in Warburg behaviour in the electrode, respectively.
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3.7. Galvanostatic charge-discharge experiments
Fig. 11 gives the galvanostatic charge–discharge curves of PANI and PANI/Ni2+
film electrodes in the potential range of 0-0.7V at a current density of 5 mA/cm2 in
0.5 mol/L H2SO4 electrolyte. The specific capacitance (Cm) can be calculated according to Eq.(2): C I t Cm= = m Vm
(2)
where Cm is specific capacitance, I is charge/discharge current, △t is the discharge time, △V is the electrochemical window, and m denotes the mass of active material within the film electrodes. where m is controlled by the deposited charge-Qdep (C)
Page 12 of 24
m can be calculated as follows: m=
Q dep M zF
(3)
where m (grams) is the mass of deposited PANI, F is the Faraday constant, M is the
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molecular weight of aniline, and for the PANI reaction, z is 3 [35], The value of z= 3
cr
represents the achieved electronic number of aniline.
On the basis of the above equations, the Cm for PANI film is 565.2 F/ g and for
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PANI/Ni2+ films are 605.6, 621.0 and 658.3 F/g with concentration of 0.2, 0.5, and
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0.8 mol/L Ni2+, respectively. In comparison to the pure PANI film, the PANI/Ni2+ films show a higher specific capacitance, which may be attributed to the
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doping/dedoping process of Ni2+. It is known that Ni2+ can own coordination number less than or equal to 4. It means one Ni2+ may bind to more than one imine nitrogen
te
d
sites in a PANI chain or form inter-chain linkages among several adjacent PANI chains by coordination, leading to a more effective conjugated network or matrix
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[36,37].
To further research the feasibility of PANI/Ni2+ film electrodes for supercapacitor,
Fig. 12 displays galvanostatic charge–discharge curves of the PANI/Ni2+ film
prepared in the solution with 0.8 mol/L Ni2+ at current densities of 5, 10 and 20 mA/cm2 in 0.5 mol/L H2SO4 electrolyte. The specific capacitances (in F/g) of the order are 658.3, 609.3 and 537.5, respectively. In Fig. 12, the voltage variation is nearly linear during both charging and discharging. Further, the anodic charging curves are symmetric to the corresponding cathodic discharging counterparts indicating that PANI/Ni2+ film electrode exhibit an ideally capacitive behavior. The
Page 13 of 24
specific capacitance decreased and IR drop rapidly increased of the PANI/Ni2+ film electrodes along with the increase of charge–discharge current densities, which was responsible to the internal resistance and polarization of the electrode [38]. So this
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kind of material is more suitable for supercapacitors in lower current density.
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In order to evaluate the stability of the film electrodes, the charge-discharge
cycling tests were conducted for 250 cycles. Fig. 13 shows the variation of specific
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capacitance with cycle number for PANI and PANI/Ni2+ film prepared in the solution
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with 0.8 mol/L Ni2+ at 5 mA/cm2. The results proposed that the cycleability of the PANI/Ni2+ film electrode was improved compared with the pure PANI film electrode.
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After 250 cycles, the specific capacitance of PANI/Ni2+ film electrode decreased for 35.29% a little less than that of the pure PANI film electrode 43.54%. The results
d
indicated that Ni2+ is believed to improve the cycleability of the PANI film, which
te
probably owing to the network or matrix of PANI chains formed by doped with Ni2+,
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allows for better accommodation of volume changes during the redox process [39]. However, considering that the stability of the film electrodes is not so good, it needs to be further researched.
4. Conclusions
The PANI/Ni2+ films were synthesized successfully by cyclic voltammetry on a stainless steel substrate in 0.2 mol/L aniline and 0.5 mol/L H2SO4 electrolyte with different concentration of Ni2+. It shows a larger specific capacitance of 658.3 F/g at a
Page 14 of 24
current density of 5 mA/cm2 and lower resistance compared with the pure PANI film. Owing to the presence of Ni2+, the PANI/Ni2+ films have an obvious improvement effect, which makes the PANI/Ni2+ films more active sites for faradaic reaction, better
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cyclic stability than the pure PANI film and facilitates the charge-transfer of the
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PANI/Ni2+ films.
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Acknowledgement
This work was supported by the National Natural Science Foundation of China
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References
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(No. 51062011).
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[1] S. Sarangapani, B.V. Tilak, C.P. Chen, J. Electrochem. Soc. 143 (1996) 3791–3799. [2] V. Gupta, N. Miura, Electrochim. Acta 52 (2006) 1721-1726. [3] K. Lota, V. Khomenko, E. Frackowiak, J. Phys. Chem. Solids 65 (2004) 295–301. [4] E. Frackowiak, F. Beguin, Carbon 39 (2001) 937-950. [5] B.E. Conway, W.G. Pell, J. Solid State Electron 7 (2003) 637-644. [6] Belanger D, Ren X, Davey J, Uribe F, Gottesfeld S (2000), J Electrochem Soc 147:2923. [7] N.S. Sariciftci, M. Bartonek, H. Kuzmany, H. Neugebauer, A. Neckel, Synth. Met 29 (1989) 193-202. [8] H. Reiss, Synth. Met 30 (1989) 257-263.
Page 15 of 24
[9] T.C. Girija, M.V. Sangaranarayanan, J. Power Sources 156 (2006) 705-711. [10] V. Gupta, N. Miura, Electrochem. Solid-State Lett. 8 (2005) A630-A632.
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[11] H.Y. Mi, X.G. Zhang, X.G.Ye , S.D.Yang, J. Power Sources 176 (2008). 403–409. [12] K.R. Prasad, N. Munichandraiah, J. Electrochem. Soc. 149 (2002) A1393-A1399.
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[13] H.Y. Mi, X.G. Zhang, S.D.Yang , X.G.Ye, J.M.Luo, Materials Chemistry and Physics 112
us
(2008) 127-131.
[14] K.S. Ryu, K.M. Kim, S.G. Kang, J. Joo, S.H. Chang, J. Power Sources 88 (2000) 197-201.
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[15] H.H.Zhou, H.Chen, S.L.Luo, G.W.Lu, W.Z.Wei, Y.F.Kuang, J Solid State Electrochem 9 (2005) 574-580.
M
[16] T.C. Girija, M.V. Sangaranarayanan, Synthetic Metals 156 (2006) 244-250. [17] R.Y. Song, J.H. Park, S.R. Sivakkumar, S.H. Kim, J.M. Ko, D.Y. Rark, S.M. Jo, D.Y.Kim, J.
te
d
Power Sources 166 (2007) 297-301.
[18] H.L. Li, J.X. Wang, Q.X. Chu, Z. Wang, F.B. Zhang, S.C. Wang, J. Power Sources 190
Ac ce p
(2009) 578-586.
[19] J. Zhang, L.B. Kong, B.Wang, Y.C.Luo, L. Kang, Synthetic Metals 159 (2009) 260-266. [20] G.C. Xu, W. Wang, X.F. Qu, Y.S. Yin, L. Chu, B.L. He, H.K. Wu, J.R. Fang, Y.S. Bao, L. Liang, Eur. Polym. J. 45 (2009) 2701-2707.
[21] P. Nunziante and G. Pistoia, Electrochim. Acta 34 (1989) 223. [22] C.M.A. Brett, A. M. C. F. Oliveira Brett, J. L. C. Pereira and C. Rebelo, J. Appl. Electrochem. 23 (1993) 332. [23] V. Gupta, N. Miura, Mater. Lett. 60 (2006) 1466-1469. [24] S. Palaniappan, A. John, C.A. Amarnath, V.J. Rao, J. Mol. Catal. A. Chem. 218 (2004)
Page 16 of 24
47-53. [25] S. Sathiyanarayanan, C. Jeyaprabha, G. Venkatachari, Mater. Chem. Phys. 107 (2008)
ip t
350-355. [26] M.J. Aguirre, B. A. Retamal, M. S. Ureta-Zafiartu, J. H.Zagal, R. C6rdova, R. Schrebler and
cr
S. R. Biaggio, J. Electroanal. Chem. 328 (1992) 349.
us
[27] S. R. Maggio, C. L. F. Oliveira, M. J. Aguirre, J. H. Zagal, J. Applied Electrochemistry 24 (1994) 1059-1065.
an
[28] S. Tao, B. Hong, Z. Kerong, Spectrochim. Acta Part A 66 (2007) 1364-1368. [29] Y.H. Kim, C. Foster, J. Chiangm, A.J. Heeger, Synth. Met 29 (1989) 285-290.
M
[30] C.M. Yang, C.Y. Chen, Synth. Met 153 (2005) 133-136.
[31] B.A. Reguig, M. Regragui, M. Morsli, A. Khelil, M. Addou, J.C. Berne` de, Solar Energy
te
d
Mater. Solar Cells 90 (2006) 1381-1392.
[32] T. Trung, T.H. Trung, C.S. Ha, Electrochim. Acta 51 (2002) 984-990.
Ac ce p
[33] Y.G. Wang, H.Q. Li, Y.Y. Xia, Adv. Mater. 18 (2006) 2619-2623. [34] A. F. Burke, J. Power Sources, 91 (2000) 17. [35] Stilwell DE, Park SM, J Electrochem Soc 135 (1988) 2254. [36] S. Tao, B. Hong, Z. Kerong, Spectrochim. Acta Part A 66 (2007) 1364-1368. [37] X.X. Li, Electrochim. Acta 54 (2009) 5634-5639. [38] Y.G. Wang, Y.Y. Xia, J. Electrochem. Soc 153 (2006) A450–A454. [39] H. Zhang, G.P. Cao, Z.Y. Wang, Y.S. Yang, Z.J. Shi, Z.N. Gu, Electrochem. Commun. 10 (2008) 1056-1059.
Page 17 of 24
ip t cr
800 798
1110
us
1558 1469 1297 1232 1114
3440
a
3432
%Transmittance
b
1560 1460 1300 1240
2+ b:PANI/Ni a:PANI
4000
3500
3000
2500
2000
1500
500
an
Wavenumbers/cm-1
1000
d
M
2+ 2+ Fig. 1. FT-IR spectra of PANI film and PANI/Ni film prepared in the solution with 0.8 mol/L Ni
te
2+ b:PANI/Ni a:PANI
Ac ce p
Intensity /cps
b
a
0
10
20
30
40
50
60
2Theta / deg.
2+ 2+ Fig. 2. XRD patterns of PANI film and PANI/Ni film prepared in the solution with 0.8 mol/L Ni
Page 18 of 24
ip t cr us an M d 2+
2+ (c,d) prepared in the solution with 0.8 mol/L Ni
Ac ce p
te
Fig. 3. SEM images of PANI (a,b) and PANI/Ni
Fig. 4. EDS of the PANI/Ni
2+
2+ film prepared in the solution with 0.8 mol/L Ni
19
Page 19 of 24
2p3/2
Ni 2p
857.4eV 862.4eV
us
cr
ip t
Intensity /a.u.
2p1/2 880.1eV 875.3eV
895 890 885 880 875 870 865 860 855 850 845 840
an
Bonding Energy/eV
M
2+ 2+ Fig. 5. XPS Ni 2p in the PANI/Ni film prepared in the solution with 0.8 mol/L Ni
0.010
a
0.002 0.000 -0.002
te
b
0.004
Ac ce p
Current/A
0.006
d
d c
0.008
a:PANI 2+ b:PANI/0.2Ni 2+ c:PANI/0.5Ni 2+ d:PANI/0.8Ni
-0.004 -0.006 -0.008
-0.010 -0.1
0.0
Fig. 6. CV curves of PANI and PANI/Ni
0.1
2+
0.2
0.3
0.4
0.5
0.6
0.7
0.8
E vs.SCE/V
films at a scan rate of 5mV/s in 0.5mol/L H2SO4
20
Page 20 of 24
0.03
b
P1
0.01
P3
P2
a
ip t
0.02
Current/A
c
a:5mV/s b:10mV/s c:20mV/s
0.00 P2'
P1'
P3'
cr
-0.01
-0.03 -0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
an
Potential/V vs.SCE
us
-0.02
M
2+ 2+ Fig. 7. CV curves of PANI/Ni film prepared in the solution with 0.8 mol/L Ni in 0.5mol/L H2SO4 at different scan rates
16
12 b
te
8
c
6
Ac ce p
-Z"/ohm
10
d
14
a:0.1V b:0.3V c:0.4V d:0.6V
d a
4 2 0
Fig. 8. EIS of PANI/Ni
2+
0
2
4
6
8
10
12
14
16
Z'/ohm
film prepared in the solution with 0.8 mol/L Ni
2+
at different applied voltages
21
Page 21 of 24
20 18 16
c
d
a:PANI 2+ b:PANI/0.2Ni 2+ c:PANI/0.5Ni 2+ d:PANI/0.8Ni
a
b
14
ip t
-Z"/ohm
12 10 8
cr
6 4
0
0
2
4
6
8
10
12
2+
16
18
20
films at the open-circuit potential of 0.4V
Ac ce p
te
d
M
Fig. 9. EIS of PANI and PANI/Ni
14
an
Z'/ohm
us
2
Fig. 10. Equivalent circuit for the simulation of EIS spectra of PANI and PANI/Ni
2+
films.
22
Page 22 of 24
0.8 a:PANI 2+ b:PANI/0.2Ni 2+ c:PANI/0.5Ni 2+ d:PANI/0.8Ni
a b c d
0.7
ip t
0.5 0.4 0.3
cr
Potential/V vs.SCE
0.6
0.2
0.0
0
50
100
150
200
250
300
350
400
an
Time/sec
us
0.1
0.8 c
0.4 0.3
2
a
a:5mA/cm 2 b:10mA/cm 2 c:20mA/cm
te
0.5
b
d
0.6
Ac ce p
Potemtial/V vs.SCE
0.7
M
2+ 2 Fig. 11. Charge-discharge curves of PANI and PANI/Ni films at a current density of 5 mA/cm
0.2 0.1 0.0
0
50
Fig. 12. Charge-discharge curves of PANI/Ni
2+
100
150
200
250
300
350
400
Time/sec film prepared in the solution with 0.8 mol/L Ni
2+
at various current densities.
23
Page 23 of 24
700 a:PANI 2+ b:PANI/Ni
600
ip t
550 500 b
450 400 350
cr
Specific capacitance/ F/g
650
a
250
0
50
100
150
200
250
an
Cycle number
us
300
2+
film prepared in the solution with 0.8
Ac ce p
te
d
M
Fig. 13. Variation of specific capacitance with cycle number for PANI film and PANI/Ni 2+ 2 mol/L Ni at 5 mA/cm
24
Page 24 of 24