Impedance Monitoring of the Anodic Dissolution of PANi during the V 2 O 5 Deposition Step for the PANi/V 2 O 5 Composite Film Preparation

Transcription

1 / The Electrochemical Society Impedance Monitoring of the Anodic Dissolution of PANi during the V 2 O 5 Deposition Step for the PANi/V 2 O 5 Composite Film Preparation K.-I. Park a, H.-C. Dinh a, J.-S. Kim a, I.-H. Yeo b, and S.-i. Mho a * a Division of Energy Systems Research, Ajou University, Suwon , Korea b Department of Chemistry, Dongguk University, Seoul , Korea Vanadium pentoxide / polyaniline (V 2 O 5 /PANi) composite films were prepared by a two-step electrochemical method for the cathode material application in lithium batteries. As a first step the PANi film was potentiodynamically grown in an acid solution containing aniline monomer, and secondly vanadium oxide was oxidatively deposited on the polyaniline film in a temperature controlled VOSO 4 solution. The increased anodic current efficiency in the hot VOSO 4 solution results in high contents of V 2 O 5 in the composite films. The in-situ electrochemical impedance spectra for the second step of V 2 O 5 -deposition at the PANi-film electrodes were measured. The impedance increases as the oxidative deposition of V 2 O 5 proceeds. Polyaniline film is also oxidized during the second V 2 O 5 -deposition step in the high temperature acidic solutions, which result in the polyaniline being overoxidized to a less conductive form. Introduction The main advantages of layer-structured vanadium oxides as cathode materials in rechargeable lithium batteries include high theoretical specific capacities and structural reversibility with the lithium ion intercalation/deintercalation into lattices (1-7). Polyaniline (PANi) is unique among conducting polymers in that it can be easily produced with controlled morphological, structural, and electronic properties by chemical or electrochemical means (8-11). Polyaniline and vanadium oxide (V 2 O 5 ) composite materials as cathodes are of great interest to improve the lithium ion intercalation capacity and accessibility, mechanical flexibility, ion mobility and the conductivity for lithium rechargeable battery, because of the dual role of the conducting polymer in the composite acting as a binding and a conducting element (12-17). Since the PANi film can be prepared by electrochemical methods, vanadium oxide and PANi composite films that can be used for lithium batteries and flexible batteries in the future are electrochemically synthesized. In this work, we carried out a systematic study of the preparation of V 2 O 5 /PANi composite films. In order to evaluate the oxidative deposition of V 2 O 5 at the PANi covered electrode, the in-situ ac impedance technique was employed in the VOSO 4 solution. Experimental

2 180 Preparation of V 2 O 5 and polyaniline composite films All chemicals and materials were obtained from Sigma Aldrich chemical co. Aniline was used after distillation. All aqueous solutions were prepared with water deionized by a Millipore Milli-Q system (19.2 MΩ cm -1 ). The V 2 O 5 and polyaniline (PANi) composite film on a stainless-steel gauze (SUS mesh; 17mm dia.) substrate was prepared by two consecutive steps in a conventional three-electrode cell system. Firstly, the PANi film was grown by a potentiodynamic polymerization method involving scanning the potential repeatedly between -0.5 V and +1.0 V vs. a Ag/AgCl ref. electrode at a scan rate of 50 mv/s for 80 cycles in 1.0 M H 2 SO 4 aqueous solution containing M aniline monomer. Vanadium oxide was then deposited on this polymer film as a second step by a potentiostatic (or a galvanostatic) anodization method by applying a potential of +1.5 V in a 0.44 M VOSO 4 solution. The temperature of the VOSO 4 solution was varied from room temperature (RT; 23 ) to 60 in order to control the efficiency of the V 2 O 5 deposition in the composite films. With the constant anodization potential applied, the resulting anodization current increased with increasing solution temperature. The anodic current at the PANi covered electrode when applying V app = +1.5 V was measured and observed to vary from 21 ma to 43 ma, as the solution temperature increased from RT to 50. Characterization and measurements The in-situ ac impedance technique using a potentiostat (Zahner IM6) was employed to evaluate the oxidative deposition of V 2 O 5 at the PANi electrode. Impedance measurements were carried out by applying a 10 mv p-p ac signal in the frequency range of 0.1 Hz khz on the top of a dc voltage (+1.5 V) for the oxidative deposition of V 2 O 5. The impedance was measured every 10 minutes during the second anodic V 2 O 5 -deposition step on the PANi covered electrode in the VOSO 4 solution, in a three-electrode cell system with a Ag/AgCl (sat d KCl) reference electrode and Pt gauze counter electrode. The measured impedance data were numerically fit to an appropriate equivalent electrical circuit using the complex nonlinear least squares fitting procedure provided by Zahner Inc. The morphology of the resulting films was characterized by field-emission scanning electron microscopy (FE-SEM, JSM-6700F). Results and Discussion In this work, as an advanced cathode material for the lithium rechargeable battery, the V 2 O 5 and polyaniline composite (V 2 O 5 /PANi) films were electrochemically prepared by two consecutive steps in a conventional three-electrode cell system. Firstly, the PANi film was grown on a stainless steel gauze substrate by a potentiodynamic polymerization method scanning the potential between -0.5 V and +1.0 V in a solution containing aniline monomer. The electrochemical polymerization of aniline in a sulfuric acid solution gives fairly smooth-looking

3 181 polyaniline films (ca. 90 μm-thick). V 2 O 5 was then deposited on the PANi film as a second anodization step by a potentiostatic method in VOSO 4 solution, in order to complete the formation of the composite film of V 2 O 5 /PANi. Temperature of the VOSO 4 solution was varied from RT to 60, in order to control the V 2 O 5 contents in the V 2 O 5 /PANi composite films. The amount of V 2 O 5 deposited increases as the temperature is increased. Polyaniline film is also oxidatively dissolved away during the second V 2 O 5 -deposition step in the high temperature acidic solutions, result in changing the solution color to brownish. The anodic dissolution of PANi at high currents is a well known phenomenon [8-10]. The increase in the anodic current with increasing solution temperature resulted in an increase in the amount of V 2 O 5 in the composites, even though the oxidative dissolution of PANi occurs along with the deposition of V 2 O 5. The weights of the V 2 O 5 /PANi composite films after applying the same anodic charge (Q = 67 coulombs) were 28 mg, 45 mg, and 65 mg at 23, 35, and 50, respectively. The large amount of composites formed after the passage of an equal amount of charge through the PANi electrodes indicates the increased current efficiency at the larger anodic current. The amount of V 2 O 5 in the V 2 O 5 /PANi composite films can be measured by thermal gravimetric analysis (TGA), because polyaniline is totally decomposed at ca. 400 o C. The weight above 400 o C in the TGA diagrams indicates the amount of V 2 O 5 in the composite films. The composite films contain a certain amount of water, because they were prepared in aqueous solution and they have to be dried well before being used as film cathodes for Li batteries. The composition of the V 2 O 5 /PANi films strongly depends on the preparation conditions such as the solution temperature and the anodization current. The V 2 O 5 contents in the V 2 O 5 /PANi composite films increased as the temperature of the VOSO 4 solution increased after applying 67 coulombs, viz. 42%, 65%, and 82% of V 2 O 5 at 23 o C, 35 o C and 50 o C, respectively. Measurement of the electrochemical impedance spectra The in-situ electrochemical impedance spectra for the oxidative deposition of V 2 O 5 at the PANi-film electrodes in the VOSO 4 solutions at different temperatures were measured with an ac perturbation on top of the anodization dc potential (+1.5 V). The plots in the complex-plane (Nyquist plots) of the impedance data are shown in Figure 1. As the anodic deposition of V 2 O 5 at the PANi film electrodes proceeds with time, the impedance increases as indicated by the arrow in Figure 1a. In the solutions at elevated temperatures, the absolute values of the impedance are small. Nevertheless, the electrochemical impedances increase as the oxidative deposition of V 2 O 5 proceeds, as shown in Figure 1b. The increase in the impedance can be ascribed to the decrease in the conductivity of PANi, as well as to the deposition of semiconductive V 2 O 5 on the PANi electrode. When the ac impedance of the PANi film itself in acidic (H 2 SO 4 ) solution was monitored, the electrochemical impedance of the PANi film also increased with time when the same anodic potential was applied (Figure 1c). This confirms that the PANi film is turned into a less conductive form by oxidation.

4 182 R s = 0.5Ω, R p = 0.32Ω (1 st curve); 0.91Ω (100 min. 5 th curve) C = 31.3mF(1 st curve); 11mF(100 min. 5 th curve) R s = 0.48 Ω, R p = 0.31Ω (1 st curve); 0.52Ω (50 min. 4 th curve) C = 46.3mF(1 st curve); 28mF(50 min. 4 th curve) R s = 0.6Ω, R p = 5.8Ω (1 st curve); 13.3Ω (20 min. 3 rd curve) C = 2.7mF(1 st curve); 2.9mF(20 min. 3 rd curve) Figure 1. Nyquist plots of the impedance spectra of the PANi-film electrodes during the second oxidative V 2 O 5 deposition step (a) at RT and (b) at 60 in VOSO 4 solution. (c) The impedance spectra of the PANi-film electrode measured oxidizing in a 0.44 M H 2 SO 4 acid solution. (d) The simple equivalent circuit used to fit the impedance data. An appropriate description of the oxidation in terms of an equivalent circuit is shown in Figure 1d. In the equivalent circuit, R s is the solution resistance, R p is the anodic polarization (charge transport) resistance, and C is the capacitance at the electrode / electrolyte interface. Some component values of the fitted equivalent circuit are given in the Figure. It is clear that both the polarization resistance (R p ) and the capacitive reactance (X c =1/2πfC) increase as the V 2 O 5 and PANi composite film is formed during the second anodization process, as indicated in the Figure. The increase in the capacitive reactance (X c ) indicates the decrease in the capacitance and the decrease in the available surface area for the anodizing film electrodes. The anodic polarization resistance (R p ) corresponds to the sum of the oxidative deposition of V 2 O 5 and the oxidation/dissolution of PANi. The values of R p are smaller in the high temperature solution than in the low temperature one, indicating that the oxidation processes at the interface are facilitated at high temperature. The smaller R p in the high temperature solution is evidenced by the larger anodic current at high temperature. As the oxidation proceeds up to an anodic charge of ca. 134 coulombs, the value of R p increases from 0.32 Ω to 0.96 Ω (after oxidizing for 110 min.) in the room temperature solution, and from 0.31 Ω to 0.52 Ω in the 60 solution (after oxidizing for 50 min.). It is clear that the value of R p increases significantly as the oxidative V 2 O 5 -deposition proceeds at the PANi covered electrode, as

5 well as the oxidation of the PANi network. The values of the capacitive reactance (XC) also increase as the oxidation proceeds, which can be explained by the decrease in the available conductive surface area, resulting from the oxidative deposition of V2O5 and from the oxidation to a less conductive form of PANi and/or dissolution of the exposed PANi network. This phenomenon is well correlated with the SEM images shown in Fig. 2. Both the large anodic currents and the small values of Rp in the high temperature VOSO4 solutions indicate that the oxidation processes at the interface are facilitated at high temperature, resulting the extensive oxidation deposition of V2O5 from the solution and the oxidative dissolution of the polymer. Optimal conditions, such as the solution temperature and the anodization charge for the second V2O5 deposition step, are necessary to prepare the V2O5 and polyaniline composite (V2O5/PANi) films exhibiting excellent cathode characteristics of high capacity and cycling stability for Li batteries. Morphology and composition of the V2O5/PANi composite films The morphologies of the polyaniline film after the first step and the V2O5/PANi composite films during and after the second step are shown in Fig. 2. The polyaniline fibers with a diameter of ca. 100 nm are interconnected with each other to form a network structure (Fig. 2a). The nucleation and growth of V2O5 particles on the fibrillar network of the PANi film from the VOSO4 solution is clearly shown in Figures 2b and 2c. However, the morphology of the V2O5/PANi composite films prepared in the VOSO4 solutions at high temperature is quite different from that of the films prepared at RT. An obvious change of the PANi network is observed in addition to the deposition of the V2O5 particles on the PANi network (Figures 2d-2e). The diameter of the PANi fibers increased (fiber diameter of ca. 200 nm) and some part of the fibers exposed to the solution seemed to be dissolved away. The morphology of the final V2O5/PANi composite films after in the VOSO4 solution at different temperatures is quite different as shown in Figures 2d and 2e. The oxidative dissolution of PANi along with the deposition of V2O5 at high anodic current leaves the large pores in the composite film. 183

6 184 Figure 2. FESEM images of (a) PANi film potentiodynamically prepared at RT, (b) V 2 O 5 /PANi composite film prepared by applying an anodic current of 21 ma at RT for 10 min., (c) 21 ma at RT for 55 min., (d) PANi network part (d-1) & the whole surface (d-2) oxidized at 43 ma in 50 o C for 4 min., and (e) 43 ma at 50 o C for 8 min. The generally accepted structure of PANi includes benzoid (reduced form) units and quinoid (oxidized form) units [8-11]. The electrical properties of PANi are affected by many factors such as the doping and oxidation levels, film morphology and orientation, and the presence of ionic species. The most important conductive form of PANi is known as emeraldine, which is composed of approximately one quinoid units for every three benzoid units. The fully oxidized form of polyaniline, pernigraniline, consists of the benzoid amine (-NH- site) and the quinoid imine (=N- site) in approximately equal proportions in the polymer chain. Some schematic structures of emeraldine and pernigraniline of the PANi films are shown in Figure 3. Compared with the pristine PANi chain, the oxidized PANi containing more quinoid rings would have a comparatively straighter structure, resulting in thicker fibers with the more extensive stacking of the polymer chains. This speculation is supported by the change in the morphology of the PANi fiber during the second anodic deposition step in the high temperature solutions, as shown in Figures 1a and 1d. Figure 3. Chemical formula and some structures of emeraldine and pernigraniline of polyaniline (PANi).

7 185 Conclusions The V 2 O 5 /PANi composite film was prepared in order to have the synergistic effect of the conducting polymer and inorganic oxide composite for the cathode material for Li battery. The V 2 O 5 /PANi composite films can be prepared by two consecutive electrochemical steps; the conductive fibrillar PANi network was firstly grown by a potentiodynamic polymerization method in an acid solution containing aniline monomer. Vanadium oxide was anodically deposited on the PANi film as a second step in a VOSO 4 solution, whose temperature was varied in order to control the V 2 O 5 contents in the composite films. The deposition of electroactive V 2 O 5 increases with high current efficiency in the solution at high temperatures. The over-oxidation of PANi during the V 2 O 5 deposition process in the high temperature solutions was identified by the electrochemical impedance monitoring and morphological characterization. Acknowledgments This work was supported by the National Research Foundation of Korea (grant Nos and ). References 1. J. W. Fergus, J. Power Sources, 195, 939 (2010). 2. Y. Wang, G. Cao, Chem. Mater. 18, 2787 (2006). 3. B. Scrosati and J. Garche, J. Power Sources, 195, 2419 (2010). 4. M. S. Whittingham, Chem. Rev. 104, 4271 (2004). 5. A. S. Arico, P. Bruce, B. Scrosati, J.-M. Tarascon and W. Van Schalkwijk, Nature Mater., 4, 366 (2005). 6. H. P. Wong, B. C. Dave, F. Leroux, J. Harreld, B. Dunn and L. F. Nazar, J. Mater. Chem. 8, 1019 (1998). 7. F. Coustier, J. Hill, B. B. Owens, S. Passerini and W. H. Smyrl, J. Electrochem. Soc., 146, 1355 (1999). 8. I. Karatchevtseva, Z. Zhang, J. Hanna and V. Luca, Chem. Mater., 18, 4908 (2006). 9. F. Huguenin, M. Ferreira, V. Zucolotto, F. C. Nart, R. M. Torresi and O. N. Oliveira, Jr., Chem. Mater., 16, 2293 (2004). 10. W. -S. Huang, B. D. Humphrey and A. G. MacDiarmid, J. Chem. Soc. Faraday Trans., 82, 2385 (1986). 11. Y.-L. Gao, J. Liu and D.-H. Huang, Synthetic Metals, 159, 1450 (2009). 12. Y. Sato, T. Nomura, H. Tanaka and K. Kobayakawa, J. Electrochem. Soc., 138, L37 (1991). 13. D. L. da Silva, R. G. Delatorre, G. Pattanaik, G. Zangari, W. Figueiredo, R.-P. Blum, H. Niehus and A. A. Pasa, J. Electrochem. Soc., 155, E14 (2008). 14. A. Malinauskas, J. Malinauskiene and A. Ramanavicius, Nanotechnology, 16, R51 (2005). 15. S. Kuwabata, S. Masui, H. Tomiyori and H. Yoneyama, Electrochim. Acta, 46, 91 (2000).

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