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V2O5 xerogel electrochromic films
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Solid State Ionics 179 (2008) 1256 – 1262 www.elsevier.com/locate/ssi
Electrical and electrochemical characterization of poly (ethylene oxide)/V2O5 xerogel electrochromic films Aiping Jin a , Wen Chen a,⁎, Quanyao Zhu a , Ying Yang a , Victor L. Volkov b , Galina S. Zakharova b a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, and Institute of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China b Institute of Solid-State Chemistry, Ural Division, Russian Academy of Science, Pervomaiskaya 91, Yekaterinbur 620219, Russia Received 15 June 2007; received in revised form 9 January 2008; accepted 14 January 2008
Abstract Vanadium pentoxide xerogel films intercalated by poly (ethylene oxide), PEO, were prepared by direct intercalation method via a sol–gel route. The structural properties were investigated using XRD, HRTEM and FTIR analysis. The detailed studies of electrochromic process and their correlation with flatband potentials have been carried out by cyclic voltammograms and Mott–Schottky plots. The electrical properties along and across V–O layers at the heating and cooling process have been evaluated. These results revealed that the insertion of proper amount of PEO can improve the electrochemical properties of V2O5·nH2O films by increasing the electrical conductivity and decreasing the water vapor absorption. Moreover, Mott–Schottky plots verified the electrochromic process, which can be described using the double insertion and extraction mechanism of lithium ions and electrons. The electrochemical impedance spectra verified that PEO/V2O5 electrochromic films favored the kinetics of V4+/V5+ solid-state redox transitions. © 2008 Elsevier B.V. All rights reserved. Keywords: Vanadium pentoxide; Poly (ethylene oxide); Electrochromic films; Electrical conductivity; Electrochemical impedance spectroscopy
1. Introduction Electrochromic materials with semiconducting nature are capable of changing their optical properties under the action of an applied electric field or an electric current. The great interest on the study of electrochromic effect shown by thin films of transition metal oxides is due to their possible applications in smart windows and light shutters [1,2]. As a host material for a wide variety of metal cations, molecules and larger molecules [3], vanadium pentoxide (V2O5) xerogel with a very reaction layered structure and n-type semiconducting properties have drawn great attention over the past two decades. However, there are some disadvantages such as low conductivity, narrow color variation and bad cycle stability. The synergic effect reported in the literature reveals that the combined properties of organic– inorganic components in a unique material can produce interesting new properties, especially in electrochemistry and ⁎ Corresponding author. E-mail address: [email protected] (W. Chen). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.01.019
electrochromism [4]. Poly (ethylene oxide) (PEO) was chosen because of its ion conductance when it complexes with alkali– metal ions. Concerning the PEO/V2O5 layered system, previous studies have indicated that it leads to interesting lithium redox intercalation, photochemical, and electrical properties [5,6], but its electrochromic process and the contribution of interbedded water on electrical conductivity have not been reported. Preliminary results for PEO/V2O5 xerogel films have been reported earlier [7,8]. The nanocomposite films showed interesting multi-electrochromic behavior and improved Li+ ions inserted/extracted charge capacity and reversibility. In this article, our goal is to elucidate the reasons for the characteristics of PEO/V2O5 films with improving charge density and cyclic stability in comparison with pure V2O5 films. Moreover, the electrochromic mechanism of the films was discussed. The variable-temperature conductivity was evaluated along and across the V–O layers at the heating and cooling process. The cyclic voltammetry, Mott–Schottky plot and electrochemical impedance spectroscopy (EIS) were conducted to provide insights into the electrochromic process.
A. Jin et al. / Solid State Ionics 179 (2008) 1256–1262
2. Experimental The layered oxide V2O5 (99.5%) was used in these experiments. PEO was the commercial agent from Aldrich Chemical Company Incorporation with average molecular weight of 4 × 105 g/mol. The dark red V2O5 sol was prepared by the way of V2O5 melt quenched. Melting V2O5 powders (99.5%) at 1073 K in a ceramic crucible results in a molten liquid. When the molten liquid was quickly poured into deionized water, V2O5 sol was formed, filtered and laid motionless for a week. PEO solution was mixed with the V2O5 solution to form a mixed sol. The molar ratio of ethylene to V2O5 was x:1 (x = 0,0.5).The films were deposited onto ITO conducting glass substrates (b 20 Ω/□) by the dip-coating technique from the mixed sol after the mixed sol was laid motionless for two days. The films were subsequently dried in air at room temperature and then heated at 100 °C for 24 h in vacuum. The resulting coating for one dip was homogeneous without any visual cracking. The film thickness was measured with the sectional view of scanning electron micrographs (SEM) and found to be around 800 nm for (PEO) xV2O5·nH2O (x = 0,0.5) films. X-ray diffraction (XRD) experiments were performed on a X'Pert powder diffractometer (PANalytical, The Netherlands) with Cu–Kα radiation (λ = 1.5418 Å) and graphite monochromator. The diffraction data were recorded for 2θ between 2° and 60°. The samples, in the film form, were obtained on a glass plate. High-resolution transmission electron microscopy (HRTEM) image was obtained through an IEM-2100F microscope (JEOL, Japan) at accelerating voltage of 200 kV to further analyze the microstructure of the film. Fourier transform infrared (FTIR) absorption spectra were recorded on a Nicolet SXB-60 IR spectrometer as KBr pellets, the measuring wavenumber range is 400–4000 cm− 1. The conductivity of films was measured on an E7-12 digital meter at 1 MHz between 293 K and 393 K in air at a fixed relative humidity 11.9% maintained with saturated solutions of appropriate salt LiCl. The samples were prepared by applying the gel to a nickel plate with nickel contacts rubbed into the film surface or platinum pressure contacts. The electrochemical experiments were carried out with an Autolab (EcoChemie) model PGSTAT30 (GPES/FRA) potentiostat/galvanostat interfaced to a computer. The conventional three-electrode arrangement was used consisting of an ITO supporting electrode, a platinum wire auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode, containing 0.5 M LiClO4 (Aldrich) in propylene carbonate (PC) (Aldrich)as the supporting electrolyte. The experiments were carried out in an inert atmosphere by bubbling N2 through the solution at room temperature. The Mott–Schottky plots were obtained from the potentiodynamic electrochemical impendence measurements using a 5 mV a.c. signal and a step rate of 5 mV for every 10 s at a potential range from −1.0 to 1.0 V vs SCE. The electrochemical impedance spectra were measured as a function of potential using an ac perturbation signal of 10 mV peak to peak (p/p), covering the 100 mHz– 10 kHz frequency range. The analyzed potentials were chosen
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from voltammetric behavior in the same solution. The experimental impedance spectra were interpreted on the basis of equivalent electrical circuits using a fitting software GPES/FRA version 4.9, Autolab-EcoChimie. 3. Results and discussion 3.1. Structural properties In Fig. 1, X-ray diffraction patterns were shown for V2O5 and (PEO)0.5·V2O5·nH2O in the film form. The result indicates that the lamellar structure of the V2O5 xerogel is preserved (001 basal reflection indices are largest), but the inter-planar space increased from 1.14 nm to 1.32 nm in agreement with many previous reports for intercalation of organic polymers into V2O5 [5,9]. The observed 0.18 nm net layer expansion represents the removal of one water molecules layer from the interlayer space followed by the insertion of PEO. The HRTEM image for (PEO)0.5·V2O5·nH2O xerogel film showed that the morphology of layered structures in which the interlayer spacing of the nanocomposite is 1–2 nm (Fig. 2), which agrees with the XRD results. The FTIR spectra of V2O5·nH2O and (PEO)0.5·V2O5·nH2O films are shown in Fig. 3. There are two large bands in 1610 and 3440 cm− 1 region corresponding to OH bending and OH–H stretching from water, respectively, which confirms the presence of water in the V2O5 xerogel. The V2O5·nH2O xerogel also exhibits three main vibration modes in the 400–1100 cm− 1 due to the V_O vibration at 1012 cm− 1, the V–O–V symmetric stretch at 516 cm− 1 and the V–O–V asymmetric stretch at 758 cm− 1. The absorption at 914 cm− 1 is associated to the presence of H2O–V bonds [10]. For (PEO)0.5·V2O5·nH2O film (Fig. 3b), the positions of the V–O–V vibration bands are 519 cm− 1 and 750 cm− 1, which are close to those found for the V2O5 xerogel. But the V_O vibration shifts from 1012 to 1002 cm− 1, which can be explained by a banding interaction between PEO and the V2O5 framework, most likely via H-bonds [5]. Other important point associated to the intimate contact between PEO and the V2O5
Fig. 1. X-ray diffraction patterns of films both (a) V2O5·nH2O xerogel and (b) (PEO)0.5V2O5·nH2O.
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Preliminary results of the electrochemical and electrochromic characterization for PEO/V2O5 xerogel films have been reported earlier [8]. The inserted and extracted charge densities (Qc and Qa) of (PEO)0.5V2O5·nH2O film for the 10th cycle are 17.9 and 18.1 mC/cm2, respectively, with a reversibility (defined as the ratio of Qa/Qc) of 98.9%, which are higher than that of V2O5·nH2O film (Qa and Qc is 9.60 and 9.80 mC/cm2 respectively, with a reversibility of 98.0%). Our value is also higher than the other polymer/V2O5 films. Nanoarchitectures of V2O5/Chitosan/PEO layer-by-layer films showed a capacity of
8 mC/cm2 [11]. Charge capacity can also be well matched with the best known ion storage films, such as TiO2–CeO2 films with a capacity of 18 mC/cm2 [12]. Moreover, (PEO)0.5V2O5·nH2O film has the reversible mutichromism with large contrast of 40% [8]. In order to further investigate the electrochromic process of (PEO) xV2O5·nH2O (x = 0,0.5) films, the cyclic voltammograms measured at 5 mV/s together with Mott–Schottky plots in steps of 5 mV at the second scan were shown in Figs. 4 and 5 respectively. Cyclic voltammograms of V2O5·nH2O and (PEO)0.5V2O5·nH2O films were similar, exhibiting reversible redox peaks from + 1.00 V to − 1.00 V (SCE). These peaks can be ascribed to the insertion and extraction of lithium ions ((PEO) xV2O5·nH2O + yLi + xe− ↔ Li y(PEO) xV2O5·nH2O) in two steps or can be due to two nonequivalent sites in the vanadium oxide matrix [13]. The redox couples of Epc1/Epa1 and Epc2/Epa2 for V2O5·nH2O film were − 0.19/0.00 and − 0.54/0.70 V respectively. For (PEO)0.5 V2O5·nH2O film, the redox couples of Epc1/Epa1 and Epc2/Epa2 were − 0.21/− 0.038 and − 0.56/0.64 V respectively. And the lower potential difference (ΔEp = Epc1 − Epa1 = − 0.19 V) and the higher current density in relation to V2O5·nH2O indicate the decrease of film resistance. The semiconducting properties of the films were analyzed and their related parameters have been calculated by using the Mott–Schottky studies [14]. The plots of 1 / C2 vs VSCE are nonlinear within a broad potential range as shown in Fig. 5, which indicated that the films showed the different semiconducting type at the different potential range. When the cathodic potential was more than − 0.80 V, the capacity increase observed in our experiments was related to surplus intercalation in very thin surface layer, thinner than the surface depletion layer [15]. When the applied potential was maintained from − 0.80 to 0.00 V, the films behaved like a p-type semiconductor (negative slope) and the acceptor injected the electrons. The measured capacitance increased until a maximum constant value between the ranges − 0.00 and 0.50 V due to the formation of Li y(PEO) xV2O5·nH2O [16]. When the applied potential was maintained from 0.50 to 1.00 V, the films worked
Fig. 3. The FTIR spectra of films both (a) V2O5·nH2O xerogel and (b) (PEO)0.5V2O5·nH2O.
Fig. 4. Cyclic voltammograms for the second and the 5th cycle of V2O5·nH2O and (PEO)0.5V2O5·nH2O films, ν = 5 mV s− 1.
Fig. 2. HRTEM image of (PEO)0.5V2O5·nH2O xerogel film.
xerogel is the disappearance of the band at 914 cm− 1. In addition, the two large bands intensities in 1610 and 3440 cm− 1 regions are obviously decreasing. These facts can be attributed to the expulsion of partial water molecules coordinated to the vanadium ions by the inserted PEO. 3.2. Electrochemical characterization of semiconductor/electrolyte interface
A. Jin et al. / Solid State Ionics 179 (2008) 1256–1262
Fig. 5. Mott–Schottky plots of V2O5·nH2O (●) and (PEO)0.5V2O5·nH2O (○) films after the second cycle in a step of 5 mV, the insets show the Mott–Schottky plots for the positive potentials, I and II show the two different slope line sections.
as an n-type semiconductor (positive slope) and the donor extracted the electrons. Therefore, the electrochromic process can be described using the double insertion and extraction mechanism of lithium ions and electrons. Moreover, two different slope line sections (I and II) are both were observed at the applied potential between −0.80 and − 0.00 V, and 0.50 and 1.00 V as shown in Fig. 5. We can get the flatband potentials Efb from the intercept and they were +0.74, + 0.50, − 0.058, −0.50 V and + 0.74,+ 0.53, − 0.037, − 0.33 for V2O5· nH2O and (PEO)0.5V2O5· nH2O films, respectively. These Efb values are corresponding to the starting potentials of the electrochromism (yellow–green–blue) [8], which is consistent with the redox peaks in cyclic voltammograms. This interesting feature can be explained by the existence of two different kinds of acceptor and donor levels in the forbidden bands respectively. The lower slope (I) can be interpreted by deeper acceptor and donor levels which are not ionized within the bulk [17].
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between vanadium ions in different charge states. The proper amount of PEO reduced the V2O5·nH2O framework with increasing the amount of reduced V4+ ions and raised the electrical conductivity. It is well known that the decline of cycle stability for V2O5·nH2O film partly attributes to the water absorption of the films. So another point that must be considered is the difference in water content that can affect the conductivity. The variabletemperature conductance of (PEO) xV2O5·nH2O (x = 0,0.5) films during the heating and cooling process was investigated. The lower amount of water molecules in the nanocomposite ((PEO)0.5V2O5·nH2O) can contribute to the decrease in the ionic conductivity, as observed in Fig. 6. The most interesting results were the different conductivity at the heating and cooling process. The ionic conductivity of (PEO) xV2O5·nH2O (x = 0,0.5) films for cooling curves was higher than that in the heating process. This behavior is due to partial dehydration during heating and water vapor reversible absorption during cooling as shown in Fig. 6 (across curves). But the ionic and electronic conductivity of (PEO)0.5V2O5·nH2O films were closer to that in the heating curves compared to that of the pure V2O5·nH2O films as shown in Fig. 6. The above results indicate that the proper amount of PEO was inserted into the
3.3. Electrical conductivity As can be seen from the cyclic voltammograms results, the intercalation of PEO improved the electrochemical response of the V2O5·nH2O host material, such as charge density, cyclic reversibility and stability, which will mainly depend on the mobility of both ions and carriers and the reversible interbedded water molecules. The effect of intercalation of PEO on the conductivity of V2O5·nH2O is shown in Fig. 6. As discussed previously [8,18], (PEO) xV2O5·nH2O (x = 0,0.5) films are highly anisotropic and mixed conductivities. Among the conductivity across the V–O layers is mainly the ionic conduction governed by protons diffusion, which occurs through an ordered array of hydrogen-bonded water molecules. The conductivity along the V–O layers comprises ionic and electronic contributions and the electrical conductivity of (PEO)xV2O5·nH2O films is mainly due to the hopping transport of small polarons
Fig. 6. Variable-temperature conductance of along and across the V–O layers at the heating and cooling process for (a) V2O5·nH2O film, and (b) (PEO)0.5V2O5·nH2O films.
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layers of V2O5 and substituted the partial water molecules, decreasing the water content with expanding interlayer distance. And, according to FTIR spectra (Fig. 3) studies there is a decrease of water content in the (PEO)0.5V2O5·nH2O in comparison with V2O5·nH2O. This can be beneficial to the transfer of Li+ ions and decrease the water vapor reversible absorption of V2O5·nH2O films, therefore, improve the cyclic reversibility and stability of PEO/V2O5 thin films.
3.4. Electrochemical impedance spectroscopy The electrochemical process was also monitored using impedance spectroscopy. Fig. 7 showed Nyquist diagrams obtained for V2O5·nH2O and (PEO)0.5V2O5·nH2O films subjected to dc potentials of 1.0 V, 0.3 V, 0.0 V, − 0.3 V, − 0.5 V and − 0.7 V and their corresponding equivalent circuits were shown in Fig. 8.
Fig. 7. Nyquist plots of electrochemical impedance spectra for V2O5·nH2O (●) and (PEO)0.5V2O5·nH2O (○) films in 0.5 M LiClO4/PC, recorded at several applied potentials: (a) 1.0 V, (b) 0.3 V, (c) 0.0 V (d) −0.3 V, (e) − 0.5 V and (f) − 0.7 V.
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Fig. 8. Equivalent circuits at (a) +1.0 V and (b) the potential range from +0.3 V to − 0.7 V for (PEO) xV2O5·nH2O (x = 0, 0.5) films.
At 1.0 V (Fig. 7a), the Nyquist plots showed a single chargetransfer step and the equivalent circuits obtained at this potential correspond to a single modified Randles scheme containing also a constant phase element (CPE), expressed as R(Rct1Q1) shown in Fig. 8a. Here V2O5·nH2O and (PEO)0.5V2O5·nH2O films are oxidized and mainly composed of V5+ centers, resulting in a higher resistance. At the potential range from 0.3 to − 0.7 V, the impedance spectra showed the following characteristics at different potential values: a depressed arc in the high-frequency range followed by a line inclined at a constant angle to the real axis in the low-frequency range, and a second straight line with different angular coefficient in the lower frequency range. The kinetics passes from charge transfer controlled at high frequencies to diffusion controlled at lower frequencies. Following this argument, the high-frequency behavior is probably related to the charge-transfer reaction of solid-state V4+/V5+ transitions at the electrolyte/electrode interface. The tilted line, with an angular coefficient close to 45°, in the lowfrequency range is attributable to Warburg impedance associated with lithium diffusion through the vanadium pentoxide matrix. The second line at lower frequencies can be attributed to finite length effects and related to the accumulation of lithium ions at the center of the oxide particle. The equivalent circuit, which best fits the experimental data in this potential range can be expressed as R (Q2[Rct2(RdQ3)])) shown in Fig. 8b. R represents the sum of ohmic resistance of the solution and electrode, Rct is the charge-transfer resistance of Faradaic processes occurring at the oxide/solution interface, and Rd is the ionic resistance arising from the diffusion of lithium ions. Q1, Q2 and Q3 are the constant phase elements (CPEs). Among them, the more important parameter of Rct-value is related to V2O5 electroreduction that occurs during Li+ intercalation into the V2O5 lamellar structure. During lithiation, Li+ ions are incorporated into the oxide and at the same time V5+ sites are reduced to V4+ by donating an electron. The Rct behavior as a function of applied potential is very similar in both systems, which is possible to verify the electronic conduction coming from V2O5. Rct-value was about 1000, 10, 5, 8, 45 and 100 Ω cm− 2 for V2O5 at + 1.0, +0.3, 0.0, − 0.3, − 0.5 and
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− 0.7 V, respectively. The conductivity increased when V2O5 was reduced at 0.0 V, which is attributed to an increase in the number of V4+ sites. As more charge was injected into the V2O5 film, the conductivity decreased, due to the excess of V4+ sites over V5+ sites [11], which was coincident with the Mott– Schottky plots. But this excess occurred at a higher potential than in the cyclic voltammogram because in the impedance measurements stationary-state conditions were used [11]. Rctvalues were much lower in (PEO)0.5V2O5·nH2O film compared to the matrix, suggesting that PEO intercalation into the V2O5 lamellar structure favors the kinetics of V4+/V5+ solid-state redox transitions. Moreover, these results are in good agreement with a decrease of ΔEp observed in the redox process of the cyclic voltammetry under the same experimental and the higher charge capacity of (PEO)0.5V2O5·nH2O. One also observes smaller values for the imaginary part of the impedance for (PEO)0.5V2O5·nH2O compared to the parent material at any given frequency. The kinetics at 50 Hz passes from charge transfer controlled at the positive potentials(+ 1.0 V and +0.3 V) to diffusion controlled at negative potentials (0.0 V, − 0.3 V, − 0.5 V and − 0.7 V). These results associate an improvement in the kinetics of the charge-transfer reaction into the film, as a consequence of the substitute of partial bound water molecules by PEO and a higher interlamellar distance, which indicates fast insertion of lithium ions. Another feature is the straight line at low frequencies in Fig. 7f, which has a slope of ca. 45° with x-axis, indicating the semi-infinite diffusion process of lithium ions. In this case, the diffusion coefficient of Li+ in the film can be calculated by Randles–Sevcik equation[19], and it is about 7.0 × 10− 10 cm2/s at − 0.7 V, which lies between 10− 8 and 10− 17 cm2/s [20]. The mobility of Li+ decreases with increasing concentration of Li+ into the film due to Li+–Li+ interactions in the host structure [21]. 4. Conclusions PEO/V2O5 xerogel film was verified a stabilization of the electrochemical process compared with the matrix alone. The insertion of proper amount of PEO can improve the electrical conductivity and decrease the water vapor absorption of V2O5. Cyclic voltammogram and Mott–Schottky plots showed that the electrochromic process for PEO/V2O5 film can be described using the double insertion and extraction mechanism of lithium ions and electrons with two different kinds of acceptor and donor levels in the forbidden bands. The charge-transfer resistance found to be lower than those observed in V2O5·nH2O film, suggesting that PEO intercalation into the V2O5 lamellar structure favors the kinetics of V4+/V5+ solidstate redox transitions. Acknowledgments This work was financially supported by the International Cooperation Technology Project of Wuhan City, PR China (050418), the Function for Innovation Research Team of Hubei Province (No. 2005ABC004) and Program for Changjiang
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Solid State Ionics 179 (2008) 1256 – 1262 www.elsevier.com/locate/ssi
Electrical and electrochemical characterization of poly (ethylene oxide)/V2O5 xerogel electrochromic films Aiping Jin a , Wen Chen a,⁎, Quanyao Zhu a , Ying Yang a , Victor L. Volkov b , Galina S. Zakharova b a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, and Institute of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China b Institute of Solid-State Chemistry, Ural Division, Russian Academy of Science, Pervomaiskaya 91, Yekaterinbur 620219, Russia Received 15 June 2007; received in revised form 9 January 2008; accepted 14 January 2008
Abstract Vanadium pentoxide xerogel films intercalated by poly (ethylene oxide), PEO, were prepared by direct intercalation method via a sol–gel route. The structural properties were investigated using XRD, HRTEM and FTIR analysis. The detailed studies of electrochromic process and their correlation with flatband potentials have been carried out by cyclic voltammograms and Mott–Schottky plots. The electrical properties along and across V–O layers at the heating and cooling process have been evaluated. These results revealed that the insertion of proper amount of PEO can improve the electrochemical properties of V2O5·nH2O films by increasing the electrical conductivity and decreasing the water vapor absorption. Moreover, Mott–Schottky plots verified the electrochromic process, which can be described using the double insertion and extraction mechanism of lithium ions and electrons. The electrochemical impedance spectra verified that PEO/V2O5 electrochromic films favored the kinetics of V4+/V5+ solid-state redox transitions. © 2008 Elsevier B.V. All rights reserved. Keywords: Vanadium pentoxide; Poly (ethylene oxide); Electrochromic films; Electrical conductivity; Electrochemical impedance spectroscopy
1. Introduction Electrochromic materials with semiconducting nature are capable of changing their optical properties under the action of an applied electric field or an electric current. The great interest on the study of electrochromic effect shown by thin films of transition metal oxides is due to their possible applications in smart windows and light shutters [1,2]. As a host material for a wide variety of metal cations, molecules and larger molecules [3], vanadium pentoxide (V2O5) xerogel with a very reaction layered structure and n-type semiconducting properties have drawn great attention over the past two decades. However, there are some disadvantages such as low conductivity, narrow color variation and bad cycle stability. The synergic effect reported in the literature reveals that the combined properties of organic– inorganic components in a unique material can produce interesting new properties, especially in electrochemistry and ⁎ Corresponding author. E-mail address: [email protected] (W. Chen). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.01.019
electrochromism [4]. Poly (ethylene oxide) (PEO) was chosen because of its ion conductance when it complexes with alkali– metal ions. Concerning the PEO/V2O5 layered system, previous studies have indicated that it leads to interesting lithium redox intercalation, photochemical, and electrical properties [5,6], but its electrochromic process and the contribution of interbedded water on electrical conductivity have not been reported. Preliminary results for PEO/V2O5 xerogel films have been reported earlier [7,8]. The nanocomposite films showed interesting multi-electrochromic behavior and improved Li+ ions inserted/extracted charge capacity and reversibility. In this article, our goal is to elucidate the reasons for the characteristics of PEO/V2O5 films with improving charge density and cyclic stability in comparison with pure V2O5 films. Moreover, the electrochromic mechanism of the films was discussed. The variable-temperature conductivity was evaluated along and across the V–O layers at the heating and cooling process. The cyclic voltammetry, Mott–Schottky plot and electrochemical impedance spectroscopy (EIS) were conducted to provide insights into the electrochromic process.
A. Jin et al. / Solid State Ionics 179 (2008) 1256–1262
2. Experimental The layered oxide V2O5 (99.5%) was used in these experiments. PEO was the commercial agent from Aldrich Chemical Company Incorporation with average molecular weight of 4 × 105 g/mol. The dark red V2O5 sol was prepared by the way of V2O5 melt quenched. Melting V2O5 powders (99.5%) at 1073 K in a ceramic crucible results in a molten liquid. When the molten liquid was quickly poured into deionized water, V2O5 sol was formed, filtered and laid motionless for a week. PEO solution was mixed with the V2O5 solution to form a mixed sol. The molar ratio of ethylene to V2O5 was x:1 (x = 0,0.5).The films were deposited onto ITO conducting glass substrates (b 20 Ω/□) by the dip-coating technique from the mixed sol after the mixed sol was laid motionless for two days. The films were subsequently dried in air at room temperature and then heated at 100 °C for 24 h in vacuum. The resulting coating for one dip was homogeneous without any visual cracking. The film thickness was measured with the sectional view of scanning electron micrographs (SEM) and found to be around 800 nm for (PEO) xV2O5·nH2O (x = 0,0.5) films. X-ray diffraction (XRD) experiments were performed on a X'Pert powder diffractometer (PANalytical, The Netherlands) with Cu–Kα radiation (λ = 1.5418 Å) and graphite monochromator. The diffraction data were recorded for 2θ between 2° and 60°. The samples, in the film form, were obtained on a glass plate. High-resolution transmission electron microscopy (HRTEM) image was obtained through an IEM-2100F microscope (JEOL, Japan) at accelerating voltage of 200 kV to further analyze the microstructure of the film. Fourier transform infrared (FTIR) absorption spectra were recorded on a Nicolet SXB-60 IR spectrometer as KBr pellets, the measuring wavenumber range is 400–4000 cm− 1. The conductivity of films was measured on an E7-12 digital meter at 1 MHz between 293 K and 393 K in air at a fixed relative humidity 11.9% maintained with saturated solutions of appropriate salt LiCl. The samples were prepared by applying the gel to a nickel plate with nickel contacts rubbed into the film surface or platinum pressure contacts. The electrochemical experiments were carried out with an Autolab (EcoChemie) model PGSTAT30 (GPES/FRA) potentiostat/galvanostat interfaced to a computer. The conventional three-electrode arrangement was used consisting of an ITO supporting electrode, a platinum wire auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode, containing 0.5 M LiClO4 (Aldrich) in propylene carbonate (PC) (Aldrich)as the supporting electrolyte. The experiments were carried out in an inert atmosphere by bubbling N2 through the solution at room temperature. The Mott–Schottky plots were obtained from the potentiodynamic electrochemical impendence measurements using a 5 mV a.c. signal and a step rate of 5 mV for every 10 s at a potential range from −1.0 to 1.0 V vs SCE. The electrochemical impedance spectra were measured as a function of potential using an ac perturbation signal of 10 mV peak to peak (p/p), covering the 100 mHz– 10 kHz frequency range. The analyzed potentials were chosen
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from voltammetric behavior in the same solution. The experimental impedance spectra were interpreted on the basis of equivalent electrical circuits using a fitting software GPES/FRA version 4.9, Autolab-EcoChimie. 3. Results and discussion 3.1. Structural properties In Fig. 1, X-ray diffraction patterns were shown for V2O5 and (PEO)0.5·V2O5·nH2O in the film form. The result indicates that the lamellar structure of the V2O5 xerogel is preserved (001 basal reflection indices are largest), but the inter-planar space increased from 1.14 nm to 1.32 nm in agreement with many previous reports for intercalation of organic polymers into V2O5 [5,9]. The observed 0.18 nm net layer expansion represents the removal of one water molecules layer from the interlayer space followed by the insertion of PEO. The HRTEM image for (PEO)0.5·V2O5·nH2O xerogel film showed that the morphology of layered structures in which the interlayer spacing of the nanocomposite is 1–2 nm (Fig. 2), which agrees with the XRD results. The FTIR spectra of V2O5·nH2O and (PEO)0.5·V2O5·nH2O films are shown in Fig. 3. There are two large bands in 1610 and 3440 cm− 1 region corresponding to OH bending and OH–H stretching from water, respectively, which confirms the presence of water in the V2O5 xerogel. The V2O5·nH2O xerogel also exhibits three main vibration modes in the 400–1100 cm− 1 due to the V_O vibration at 1012 cm− 1, the V–O–V symmetric stretch at 516 cm− 1 and the V–O–V asymmetric stretch at 758 cm− 1. The absorption at 914 cm− 1 is associated to the presence of H2O–V bonds [10]. For (PEO)0.5·V2O5·nH2O film (Fig. 3b), the positions of the V–O–V vibration bands are 519 cm− 1 and 750 cm− 1, which are close to those found for the V2O5 xerogel. But the V_O vibration shifts from 1012 to 1002 cm− 1, which can be explained by a banding interaction between PEO and the V2O5 framework, most likely via H-bonds [5]. Other important point associated to the intimate contact between PEO and the V2O5
Fig. 1. X-ray diffraction patterns of films both (a) V2O5·nH2O xerogel and (b) (PEO)0.5V2O5·nH2O.
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Preliminary results of the electrochemical and electrochromic characterization for PEO/V2O5 xerogel films have been reported earlier [8]. The inserted and extracted charge densities (Qc and Qa) of (PEO)0.5V2O5·nH2O film for the 10th cycle are 17.9 and 18.1 mC/cm2, respectively, with a reversibility (defined as the ratio of Qa/Qc) of 98.9%, which are higher than that of V2O5·nH2O film (Qa and Qc is 9.60 and 9.80 mC/cm2 respectively, with a reversibility of 98.0%). Our value is also higher than the other polymer/V2O5 films. Nanoarchitectures of V2O5/Chitosan/PEO layer-by-layer films showed a capacity of
8 mC/cm2 [11]. Charge capacity can also be well matched with the best known ion storage films, such as TiO2–CeO2 films with a capacity of 18 mC/cm2 [12]. Moreover, (PEO)0.5V2O5·nH2O film has the reversible mutichromism with large contrast of 40% [8]. In order to further investigate the electrochromic process of (PEO) xV2O5·nH2O (x = 0,0.5) films, the cyclic voltammograms measured at 5 mV/s together with Mott–Schottky plots in steps of 5 mV at the second scan were shown in Figs. 4 and 5 respectively. Cyclic voltammograms of V2O5·nH2O and (PEO)0.5V2O5·nH2O films were similar, exhibiting reversible redox peaks from + 1.00 V to − 1.00 V (SCE). These peaks can be ascribed to the insertion and extraction of lithium ions ((PEO) xV2O5·nH2O + yLi + xe− ↔ Li y(PEO) xV2O5·nH2O) in two steps or can be due to two nonequivalent sites in the vanadium oxide matrix [13]. The redox couples of Epc1/Epa1 and Epc2/Epa2 for V2O5·nH2O film were − 0.19/0.00 and − 0.54/0.70 V respectively. For (PEO)0.5 V2O5·nH2O film, the redox couples of Epc1/Epa1 and Epc2/Epa2 were − 0.21/− 0.038 and − 0.56/0.64 V respectively. And the lower potential difference (ΔEp = Epc1 − Epa1 = − 0.19 V) and the higher current density in relation to V2O5·nH2O indicate the decrease of film resistance. The semiconducting properties of the films were analyzed and their related parameters have been calculated by using the Mott–Schottky studies [14]. The plots of 1 / C2 vs VSCE are nonlinear within a broad potential range as shown in Fig. 5, which indicated that the films showed the different semiconducting type at the different potential range. When the cathodic potential was more than − 0.80 V, the capacity increase observed in our experiments was related to surplus intercalation in very thin surface layer, thinner than the surface depletion layer [15]. When the applied potential was maintained from − 0.80 to 0.00 V, the films behaved like a p-type semiconductor (negative slope) and the acceptor injected the electrons. The measured capacitance increased until a maximum constant value between the ranges − 0.00 and 0.50 V due to the formation of Li y(PEO) xV2O5·nH2O [16]. When the applied potential was maintained from 0.50 to 1.00 V, the films worked
Fig. 3. The FTIR spectra of films both (a) V2O5·nH2O xerogel and (b) (PEO)0.5V2O5·nH2O.
Fig. 4. Cyclic voltammograms for the second and the 5th cycle of V2O5·nH2O and (PEO)0.5V2O5·nH2O films, ν = 5 mV s− 1.
Fig. 2. HRTEM image of (PEO)0.5V2O5·nH2O xerogel film.
xerogel is the disappearance of the band at 914 cm− 1. In addition, the two large bands intensities in 1610 and 3440 cm− 1 regions are obviously decreasing. These facts can be attributed to the expulsion of partial water molecules coordinated to the vanadium ions by the inserted PEO. 3.2. Electrochemical characterization of semiconductor/electrolyte interface
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Fig. 5. Mott–Schottky plots of V2O5·nH2O (●) and (PEO)0.5V2O5·nH2O (○) films after the second cycle in a step of 5 mV, the insets show the Mott–Schottky plots for the positive potentials, I and II show the two different slope line sections.
as an n-type semiconductor (positive slope) and the donor extracted the electrons. Therefore, the electrochromic process can be described using the double insertion and extraction mechanism of lithium ions and electrons. Moreover, two different slope line sections (I and II) are both were observed at the applied potential between −0.80 and − 0.00 V, and 0.50 and 1.00 V as shown in Fig. 5. We can get the flatband potentials Efb from the intercept and they were +0.74, + 0.50, − 0.058, −0.50 V and + 0.74,+ 0.53, − 0.037, − 0.33 for V2O5· nH2O and (PEO)0.5V2O5· nH2O films, respectively. These Efb values are corresponding to the starting potentials of the electrochromism (yellow–green–blue) [8], which is consistent with the redox peaks in cyclic voltammograms. This interesting feature can be explained by the existence of two different kinds of acceptor and donor levels in the forbidden bands respectively. The lower slope (I) can be interpreted by deeper acceptor and donor levels which are not ionized within the bulk [17].
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between vanadium ions in different charge states. The proper amount of PEO reduced the V2O5·nH2O framework with increasing the amount of reduced V4+ ions and raised the electrical conductivity. It is well known that the decline of cycle stability for V2O5·nH2O film partly attributes to the water absorption of the films. So another point that must be considered is the difference in water content that can affect the conductivity. The variabletemperature conductance of (PEO) xV2O5·nH2O (x = 0,0.5) films during the heating and cooling process was investigated. The lower amount of water molecules in the nanocomposite ((PEO)0.5V2O5·nH2O) can contribute to the decrease in the ionic conductivity, as observed in Fig. 6. The most interesting results were the different conductivity at the heating and cooling process. The ionic conductivity of (PEO) xV2O5·nH2O (x = 0,0.5) films for cooling curves was higher than that in the heating process. This behavior is due to partial dehydration during heating and water vapor reversible absorption during cooling as shown in Fig. 6 (across curves). But the ionic and electronic conductivity of (PEO)0.5V2O5·nH2O films were closer to that in the heating curves compared to that of the pure V2O5·nH2O films as shown in Fig. 6. The above results indicate that the proper amount of PEO was inserted into the
3.3. Electrical conductivity As can be seen from the cyclic voltammograms results, the intercalation of PEO improved the electrochemical response of the V2O5·nH2O host material, such as charge density, cyclic reversibility and stability, which will mainly depend on the mobility of both ions and carriers and the reversible interbedded water molecules. The effect of intercalation of PEO on the conductivity of V2O5·nH2O is shown in Fig. 6. As discussed previously [8,18], (PEO) xV2O5·nH2O (x = 0,0.5) films are highly anisotropic and mixed conductivities. Among the conductivity across the V–O layers is mainly the ionic conduction governed by protons diffusion, which occurs through an ordered array of hydrogen-bonded water molecules. The conductivity along the V–O layers comprises ionic and electronic contributions and the electrical conductivity of (PEO)xV2O5·nH2O films is mainly due to the hopping transport of small polarons
Fig. 6. Variable-temperature conductance of along and across the V–O layers at the heating and cooling process for (a) V2O5·nH2O film, and (b) (PEO)0.5V2O5·nH2O films.
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layers of V2O5 and substituted the partial water molecules, decreasing the water content with expanding interlayer distance. And, according to FTIR spectra (Fig. 3) studies there is a decrease of water content in the (PEO)0.5V2O5·nH2O in comparison with V2O5·nH2O. This can be beneficial to the transfer of Li+ ions and decrease the water vapor reversible absorption of V2O5·nH2O films, therefore, improve the cyclic reversibility and stability of PEO/V2O5 thin films.
3.4. Electrochemical impedance spectroscopy The electrochemical process was also monitored using impedance spectroscopy. Fig. 7 showed Nyquist diagrams obtained for V2O5·nH2O and (PEO)0.5V2O5·nH2O films subjected to dc potentials of 1.0 V, 0.3 V, 0.0 V, − 0.3 V, − 0.5 V and − 0.7 V and their corresponding equivalent circuits were shown in Fig. 8.
Fig. 7. Nyquist plots of electrochemical impedance spectra for V2O5·nH2O (●) and (PEO)0.5V2O5·nH2O (○) films in 0.5 M LiClO4/PC, recorded at several applied potentials: (a) 1.0 V, (b) 0.3 V, (c) 0.0 V (d) −0.3 V, (e) − 0.5 V and (f) − 0.7 V.
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Fig. 8. Equivalent circuits at (a) +1.0 V and (b) the potential range from +0.3 V to − 0.7 V for (PEO) xV2O5·nH2O (x = 0, 0.5) films.
At 1.0 V (Fig. 7a), the Nyquist plots showed a single chargetransfer step and the equivalent circuits obtained at this potential correspond to a single modified Randles scheme containing also a constant phase element (CPE), expressed as R(Rct1Q1) shown in Fig. 8a. Here V2O5·nH2O and (PEO)0.5V2O5·nH2O films are oxidized and mainly composed of V5+ centers, resulting in a higher resistance. At the potential range from 0.3 to − 0.7 V, the impedance spectra showed the following characteristics at different potential values: a depressed arc in the high-frequency range followed by a line inclined at a constant angle to the real axis in the low-frequency range, and a second straight line with different angular coefficient in the lower frequency range. The kinetics passes from charge transfer controlled at high frequencies to diffusion controlled at lower frequencies. Following this argument, the high-frequency behavior is probably related to the charge-transfer reaction of solid-state V4+/V5+ transitions at the electrolyte/electrode interface. The tilted line, with an angular coefficient close to 45°, in the lowfrequency range is attributable to Warburg impedance associated with lithium diffusion through the vanadium pentoxide matrix. The second line at lower frequencies can be attributed to finite length effects and related to the accumulation of lithium ions at the center of the oxide particle. The equivalent circuit, which best fits the experimental data in this potential range can be expressed as R (Q2[Rct2(RdQ3)])) shown in Fig. 8b. R represents the sum of ohmic resistance of the solution and electrode, Rct is the charge-transfer resistance of Faradaic processes occurring at the oxide/solution interface, and Rd is the ionic resistance arising from the diffusion of lithium ions. Q1, Q2 and Q3 are the constant phase elements (CPEs). Among them, the more important parameter of Rct-value is related to V2O5 electroreduction that occurs during Li+ intercalation into the V2O5 lamellar structure. During lithiation, Li+ ions are incorporated into the oxide and at the same time V5+ sites are reduced to V4+ by donating an electron. The Rct behavior as a function of applied potential is very similar in both systems, which is possible to verify the electronic conduction coming from V2O5. Rct-value was about 1000, 10, 5, 8, 45 and 100 Ω cm− 2 for V2O5 at + 1.0, +0.3, 0.0, − 0.3, − 0.5 and
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− 0.7 V, respectively. The conductivity increased when V2O5 was reduced at 0.0 V, which is attributed to an increase in the number of V4+ sites. As more charge was injected into the V2O5 film, the conductivity decreased, due to the excess of V4+ sites over V5+ sites [11], which was coincident with the Mott– Schottky plots. But this excess occurred at a higher potential than in the cyclic voltammogram because in the impedance measurements stationary-state conditions were used [11]. Rctvalues were much lower in (PEO)0.5V2O5·nH2O film compared to the matrix, suggesting that PEO intercalation into the V2O5 lamellar structure favors the kinetics of V4+/V5+ solid-state redox transitions. Moreover, these results are in good agreement with a decrease of ΔEp observed in the redox process of the cyclic voltammetry under the same experimental and the higher charge capacity of (PEO)0.5V2O5·nH2O. One also observes smaller values for the imaginary part of the impedance for (PEO)0.5V2O5·nH2O compared to the parent material at any given frequency. The kinetics at 50 Hz passes from charge transfer controlled at the positive potentials(+ 1.0 V and +0.3 V) to diffusion controlled at negative potentials (0.0 V, − 0.3 V, − 0.5 V and − 0.7 V). These results associate an improvement in the kinetics of the charge-transfer reaction into the film, as a consequence of the substitute of partial bound water molecules by PEO and a higher interlamellar distance, which indicates fast insertion of lithium ions. Another feature is the straight line at low frequencies in Fig. 7f, which has a slope of ca. 45° with x-axis, indicating the semi-infinite diffusion process of lithium ions. In this case, the diffusion coefficient of Li+ in the film can be calculated by Randles–Sevcik equation[19], and it is about 7.0 × 10− 10 cm2/s at − 0.7 V, which lies between 10− 8 and 10− 17 cm2/s [20]. The mobility of Li+ decreases with increasing concentration of Li+ into the film due to Li+–Li+ interactions in the host structure [21]. 4. Conclusions PEO/V2O5 xerogel film was verified a stabilization of the electrochemical process compared with the matrix alone. The insertion of proper amount of PEO can improve the electrical conductivity and decrease the water vapor absorption of V2O5. Cyclic voltammogram and Mott–Schottky plots showed that the electrochromic process for PEO/V2O5 film can be described using the double insertion and extraction mechanism of lithium ions and electrons with two different kinds of acceptor and donor levels in the forbidden bands. The charge-transfer resistance found to be lower than those observed in V2O5·nH2O film, suggesting that PEO intercalation into the V2O5 lamellar structure favors the kinetics of V4+/V5+ solidstate redox transitions. Acknowledgments This work was financially supported by the International Cooperation Technology Project of Wuhan City, PR China (050418), the Function for Innovation Research Team of Hubei Province (No. 2005ABC004) and Program for Changjiang
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