Rechargeable Battery Based on Substituted LS Polyanilines

by
M.K. Ram^*, M. Adami^*, M. Sartore^*,
M Salerno^*, S. Paddeu^‡ and C. Nicolini ^‡,+

^*Polo Nazionale Bioelettronica,
Parco Scientifico-Technologie dell'Elba - 57030,
Via Roma -28, Marciana (LI) Italy
^‡Elba Foundation, Via A. Moro 15, 57033,
Marciana Marina (LI), Italy and
⁺Institute of Biophysics, University of Genoa,
Via Giotto 2, 16153, GE. Sestri Ponente, Italy

Abstract

Ultra thin films of polyaniline (PANI), poly(o-toluidine) (POT), poly(o-anisidine) (POAS) and poly(ethoxy aniline) (PEOA) were engineered by Langmuir Schaefer (LS) technique. It was shown that the effect of substituent groups plays a prominent role for the fabrication of ultra thin films. In this regards, Langmuir isotherm of the PANI, POT, POAS and PEOA were investigated at aqueous subphase of pH 1, where doping during the monolayer formation appeared as an essential step for the high quality of Langmuir film. The area per unit molecule was shown to increase by increment of the substituent groups in polyanilines. The deposited LS films of such polyanilines were characterized by cyclic voltammetry. The electrical properties of polyanilines were studied depositing on interdigited electrode. The uniformity in each deposition was verified by atomic force microscopy (AFM). In this communication, the main objective is to report on the existing link between optimization of the above properties and the charging/discharging properties of substituted polyanilines LS films towards the optimal manufacturing of a rechargeable battery based on this class of conducting polymer.

1. Introduction

Polyanilines have received great attention due to their environmental stability, ease in preparation, exciting electrochemical, optical and electrical properties [1-3]. Polyanilines have also been postulated as a potential candidate for numerous electronic applications such as electrochromic displays, rechargeable batteries, microelectronics devices, biosensors, protective coating and chemical sensors [4, 5]. For many device application, it is desirable to have polyanilines in a thin films structure, preferably with known thickness and molecular packing [5]. Langmuir-Blodgett (LB) films offers the unique control over the film architecture, thickness and molecular orientation [6, 7]. Recently, LB films of polyaniline have been obtained by exploiting the solubility of polyaniline in N-methyl 2-pyrrolidinone (NMP)/CHCl₃ mixture [8-10]. It is to be noted that even if polyaniline is not a typical amphiphilic molecule, still stable monolayer of polyaniline have been obtained and subsequently transferred as LB films [11]. The stability of monolayer has been seen improving for the substituted polyanilines [12, 13]. The effect of substituents group (-CH₃, -OCH₃ or -OC₂H₅ etc.) in monomer or polymeric chain of polyanilines appears to enhance such potentiality, by displaying at the same time a significant increase in electronic localisation with simultaneous decrease in conductivity but an excellent solubility in a number of organic solvents. The solubility of polyanilines enables to assemble such conducting polymers into ultrathin films at the molecular level with high degree of order [14-17]. However, the comparative studies of the fabrication for various types of polyanilines LS films is not shown in literature.

The influence of different subphase condition on the pressure (P) -Area (A) isotherm of polyanilines plays an important role for the stable Langmuir monolayer [18, 19, 20]. In our earlier investigation, we had shown the effect of subphase (maintaining at different pH value) on the orientation of poly(o-anisidine) (POAS) molecules at air/water interface [18]. POAS showed a decrease in area/molecule when Langmuir monolayer was fabricated at pH ranging from 6.4 to 1. In particular, we focused our attention on the effect of the polymer doping upon the Langmuir monolayer and LB film fabrication for various polyanilines.

With these consideration, we used various types of polyanilines, i.e. polyaniline (PANI), poly(o-anisidine) (POAS), poly(o-toluidine) (POT) and poly(o-ethoxy aniline) (PEOA). The effect of substituents on polyanilines can be observed in the fabrication of Langmuir monolayers at different aqueous subphases. The deposited LB films were examined using cyclic voltammetry in HCl media in order to assess the redox characteristics of various polyanilines at an identical condition. The surface morphology as well as the uniformity of polyanilines LS films were investigated by Atomic Force Microscopy. The current (I) -voltage (V) characteristics of polyanilines were measured deposited on interdigited electrodes. The charging and discharging effects of polyanilines were studied for their application in rechargeable batteries.

2. Experimental Section

2.1 Synthesis of POAS conducting polymer

The monomer aniline, ortho-toluidine, ortho-anisidine and ortho-ethoxy aniline, oxidizing agents and various reagents were obtained from Sigma for the synthesis of various polyanilines. Polyanilines were chemically synthesized by oxidative polymerization of monomer using ammonium perdisulphate [(NH₄)₂S₂O₈)] under controlled conditions [19, 21, 22]. 0.215, 0.215, 0.219 and 0.220 M of distilled aniline, toluidine, o-anisidine and o-ethoxy aniline were used separately for the synthesis of polyaniline (PANI), poly(o-toluidine) (POT), poly(o-anisidine) (POAS) and poly(o-ethoxy aniline) (PEOA), respectively. Each monomer was added slowly in a 200 ml solution of HCl containing 0.05M (11.5 gm) ammonium perdisulphate precooled to 4°C in an ice bath. The reaction was continued for 12 hours. The dark green precipitate of each polyanilines recovered from the reaction vessel, was filtered and washed by using 1M HCl to remove the oxidant and oligomers. This precipitate was subsequently washed by deionized water (for several times), methanol and diethyl ether to remove the low molecular weight polymers as well as oligomers (violet in colour). The emeraldine form of each polyanilines was heated at 100°C. The green powder thus obtained was the emeraldine salt (ES). Such ES of each polyanilines was subsequently treated by using aqueous ammonia for 24 hours. Further, each emeraldine base (EB) form of each polyanilines was washed in distilled water and acetone for several times and then, dried for 6 hours at a temperature of 100°C. It should be noted that poly(o-anisidine) and the poly(o-ethoxy aniline) were washed by using acetone during the synthesis, which is very similar to the preparation of polyaniline because of the removal of low molecular weight polymers and oligomers [19]. It was found that POT, POAS and PEOA were soluble in chloroform. The powder thus obtained was emeraldine base (dark blue in colour) of each polyanilines. The precipitation of POT occurred in one or two days, while POAS and PEOA did not show any precipitation for a prolonged periods.

2.2 Formation of polyanilines LS film.

The polyaniline solution for the fabrication of LS films was prepared by using stabilizer such as N-methyl pyrrolidinione (NMP) but the subsituted polyanilines were simply dissolved in chloroform. A stock solution was prepared by dissolving 5 mg of polyaniline in 2 ml of NMP and 20 ml of CHCl₃ for immediate use. The resulting solution was filtered with solvent resistant filter (0.5l ). The 0.2 mg/l solution of each POT, POAS and PEOA polyanilines were made in CHCl₃ and 100 -150 µl of solution was spread onto two types of aqueous subphases, i.e., pH 1 using HCl and deionised water. Fig.1 shows the general structure of polyanilines conducting polymer, where y = 1/2, 1 & 0 give rise to emeraldine, leucoemeraldine and pernigraniline form, respectively. Fig.1 also shows the doped form of each polyanilines. After the isotherms were recorded, it appeared that the film formed at pH 1 showed higher collapse pressure. So, pH 1 was separately used as subphase for the deposition of LS films for various polyanilines. LS films were formed in LB trough, with 240mm x 100 mm in size and 300 ml in volume (MDT corp., Russia) having a compression speed 1.667 mm/sec. Different number of monolayers were transferred onto glass, platinum, glass indium-tin-oxide plates and at interdigited electrode substrates (containing chromium electrodes, which were cleaned with ethanol and chloroform previously) by Langmuir-Schaefer technique, respectively. The stability of the Langmuir monolayers of each polyanilines was verified at various pressure and later, 20 mN/m, 22 mN/m, 25 mN/m and 18 mN/m surface pressure was maintained for the deposition of PANI, POT, POAS and PEOA LS films.

2.3 Atomic force microscopy

The Atomic Force Microscope used was home-built instrument (Polo Nazionale Bioelettronica) working contact mode [24]. It was operated in air, at constant deflection (i.e. vertical contact force) with a triangular shaped gold-coated Si₃N₄ microlever (commercially available Park Scientific Instrument chips). The tip of the microlever had standard aspect ratio (about 1:1) and the lever had nominal force constant of 0.03 N/m. The constant force set point was about 0.1 nN, while the images acquired were 128x128 pixels maps. During acquisition the row scanning frequency was 4 Hz, i.e. a physical tip-sample motion speed of 8-4-2 micron/sec in the 2-1-0.5 micron scan size images, respectively. All images are standard top-view topographic maps, where the brightness is proportional to the quote of the features over the sample surface, i.e. light means mountain, dark means valley. Some of the images present high features that were saturated in the post-processing redistribution of the available grey levels, because much higher than the most of the data points. Henceforth, it was possible to observe the finest structure of the samples. The images shown are representative of the samples, since images of the same looking appears in 4 different regions of the analyzed samples, positioned at the vertices of a square of side about 4 mm, centred at the specimen.

2.4 Electrical and Electrochemical measurements.

The electrical characterization was performed using an electrometer (from Keithley model 6517) as well as Potentiostat/Galvanostat (EG &G PARC model 163). The electrochemical measurements were made by Potentiostat/Galvanostat (from EG & G PARC model 163 with a software of M270). A standard three electrodes configuration was used, where LS films on glass ITO plate acted as a working electrode, platinum as a counter and Ag/AgCl as a reference electrode. The cyclic voltammograms were measured for 30 monolayers of PANI, POT, POAS and PEOA polyanilines films at 1M HCl. The charging and discharging experiments on such polyanilines films were performed at various concentration of 1M HCl. Further, the charging and discharging of polyanilines were performed using Al/LiClO₄+propylene carbonate(PC)/ polyanilines configuration.

3. Results and discussions

3.1 Pressure -area isotherm

Figure 2a shows the P-A isotherms of polyanilines in deionized water. The effect of substituents in polyanilines can be seen while they form the Langmuir monolayers. The stability of Langmuir monolayer is found to be associated to a high collapse pressure, a steep increase in the pressure in the condensed phase for PANI (curve 1), POT (curve 2) and POAS (curve 3), conducting polyanilines. It shows an increase of pressure in the condensed phase and the yielding (breaking) point for the pressure has been obtained at a different surface pressures for each polyanilines. The wrinkles could be observed for the pressure of 42, 40, 39 and 28 mN/m for PANI (curve 1), POT (curve 2), POAS (curve 3) and PEOA (curve 4), respectively. The PANI shows higher collapse pressure with respect to other class of polyanilines (as studied), which may be liked to the NMP (i.e., NMP is sparingly soluble in water). Nevertheless, the effect of substituents in polyanilines could be observed at air/water interface, which shows the large change in area per molecule at the condensed phase. For the estimation of molecular area at condensed phase, one repeating unit of each polyanilines have been taken into account as shown in Fig.1. The obtained area per molecule for PANI, POT, POAS and PEOA at air/water interface estimated using Fig.2a, were found to be respectively at around 26, 45, 55 and 62 Å² by extrapolating the pressure-area isotherm curves. The cross sectional area of the aniline repeat unit at air - water interface has already estimated to be 20 Å² [8]. These results show that substituted polyanilines molecules are not oriented and occupy larger area at air-water interface than the repeat unit for each polyanilines [25].

The stability of Langmuir films at air/water interface according to the subphase is another important issue for the better quality of films [4,5,8,18, 19, 20]. Figure 2b shows the pressure -area isotherms of each substituted polyanilines at the aqueous subphase maintained at pH 1 using HCl. The doping appears to be an important factor for the stability of the monolayers, which is probably associated with the ordering that is introduced in the polymer at air/water interface at pH 1. So an attempt has been made for the estimation of molecular area of substituted polyanilines at air-water interface by extrapolating the pressure -area isotherm curves shown in Fig.2b. The area obtained at 1 pH has been found to be about 20, 25, 21 and 45 Å² for PANI, POT, POAS and PEOA respectively. The area per molecule for PANI, POT, POAS supports the concepts that at pH 1, the emeraldine base form of polyanilines is changed to emeraldine salt [8]. The bigger value in the area per molecule, found for the repeat unit molecule of PEOA in curve 4 of Fig.2b, could be linked to the bulky group (-OC₂H₅) in each monomer of PEOA. It can also be speculated that -OC₂H₅ causes substituted polyaniline to form different structure to accommodate the side group and possibly not as unfolded similar to PANI, POT or POAS conducting polymer. The stability of the films was further checked at different pressure with compression speed of 1.67 mm/sec for each polyanilines. 20, 22, 25 and 18 mN/m surface pressures were found to be best values for the deposition pressure for PANI, POT, POAS and PEOA LS films. Such deposition pressures were also reported in literature [4, 8, 18].

3.2 Atomic force microscopy (AFM)

This technique was used to study the morphology of multilayer films prepared using different polyanilines LS films. Fig.3 shows AFM images of dimensions 1 x 1 µm of 15 monolayers for PANI, POT, POAS and PEOA films. The AFM images of this film reveal a fine grained structure for PANI, POT and POAS, whereas it shows the extremely fine grain size for PEOA. Moreover, these images show the uniformity in the polyanilines LS deposited films. It means that 15 monolayers of polyanilines are smooth, complete and continuous [26]. Table 1 summarizes the results of atomic force microscopic pictures of each polyanilines. The all forms of substituted polyanilines exhibit microscopic structure with small to little bigger grain size. The interesting features which could be noticed that PANI shows the bigger grain size linked to N-methyl pyrrolidinone, whereas the grain size decreases from POT to PEOA. The brighter intensity of the light can be visualised at some places in the above pictures, and related to the overlapping of some molecules during the drying process by using the flux of nitrogen. The size of the grains simply depends upon the nature of polyanilines molecules. It should be noted cautiously that though PEOA is a bigger molecule, still it shows the smaller grain compared to other studied polyanilines.

Table 1. The results of atomic force microscopy

Material	Grain Density (N/²)	Grain Lateral size , (nm)	The distribution of the Grains Height (nm)	Total grain scale corrugation (nm)	RMS Roughness (nm)
PANI	53	7320	5 - 15	57	6.9
POT	34	8224	5 - 25	65	10.2
POAS	27	6817	5 - 20	50	7.2
PEOA	19	8214	5 - 25	51	5.0

The numerical results shown in Table 1 are the representative of selected images.

3.3 Electrical characterisation

The industrial application of this class of ultra thin electro-conducting material is promising. So, the electrical conductivity of each polyanilines LS films was studied by depositing on the interdigited electrodes [18, 19]. Fig.4 shows the I-V characteristics of polyanilines films measured at a scan rate of 20 mV/sec. The current -voltage characteristics for PANI (curve 1), POT (curve 2) and POAS (curve 3) do not show the Ohmic behaviour. It can be related to the possible redox reaction of the interdigited electrodes with HCl during the preparation of LB films, which could also be related to some potential barrier with the degenerately doped conducting polyanilines. The only curve showing a different behaviour is that of PEOA (curve 4), with a linear relationship in I-V characteristics. The PANI LS films (curve 1) shows the higher magnitude of current for each measured potential. The magnitude of current shows minimum for the PEOA LS films. The decrease in the current magnitude in POT can be related to the decrease in interchain order which is related to conductivity value, whereas POAS leads to an increase in electronic localisation and attributes to the decrease in conductivity. Still the larger substituents group ethoxy in PEOA shows the other effect like charge localisation along the polymer chains which increases the orientation thus diminishing the current (or conductivity) value.

3.4 Electrochemical investigation

The electrochemistry of substituted polyanilines LS films has been investigated by cyclic voltammetry. The cyclic voltammogram of 30 layers of each polyanilines LS films prepared at pH 1 in 1 M HCl medium is shown in Figure 5 (curves a-d). The bias potential was swept from -0.3 to 1 V at a scan rate of 50 mV/sec. The shape of the cyclic voltammograms in Fig.5 of curves (a-d) shows the surface confined species as expected. The cyclic voltammetry (CVs) peaks are associated to the oxidation and reduction processes of polyanilines LS films. These CVs exhibits redox features characteristics of individual polyanilines. Table 2 shows the peak potential values of polyanilines as derived from Figure 5 (curves a-d). It means that there is a gradual decrease in redox peak potential as a function of substituents in polyaniline. The positive shift in the position of peak 1 is associated to the electron transfer, which implies that the substituent groups can induce some non-polar conformations that decreases the conjugation along the polymer backbone, which is responsible for the oxidation potential peak. In fact, the blue shift in the oxidized peak potential can be related to the decrease in the polarons/bipolarons or charged states as an increase of substituents in polyanilines. In addition the lower value of electrochemical reduction potential for POT (662 mV), POAS (637 mV) and PEOA (465 mV) of LS films can also be noticed in close comparison to the polyaniline films (at 780 mV) [27]. The change in oxidation potential value (shown in Table 2) is linked to the higher electronic density states due to the substitution in the aromatic ring, which facilitates the protonation and the oxidation of the amine group. The similar results were observed for the electrochemically grown polyanilines [27]. The middle peaks found in POT, POAS and PEOA are variably interpreted for the formation of quinoid like species, specifically speaking when the potential is brought to the higher values like 1 V. The other interesting aspect such as electrochromic effects of the films were also observed. The electrochromism was noted from yellow to green and later to violet as the potential was swept from -0.2 to 1.0 V for each polyanilines conducting polymer LS films.

Table 2 The electrochemical parameters of polyanilines

Material	Oxidation potential (mV)	Reduction potential (mV)	Charging potential (V)	open circuit potential (V) measured after 48 hours in 1 M resistance
PANI	0.78, 320	130, 710	3.0	1.2
POT	662, 531.8, 304	627.4, 499.3, 165, 23.54	3.0	1.1
POAS	707, 506, 282,	680, 410, 144, -5.91	3.0	1.0
PEOA	563.4, 374.4, 100.5	401.1, 262	3.0	0.8

3.5 Charging and Discharging of polyanilines LS films

The charging and discharging on 30 monolayers containing polyanilines LS films were performed in 0.01M HCl as shown in Fig.6. Polyanilines can be charged completely nearly a time period of 15 minutes. Whereas, the discharging in HCl can be seen as soon as there is short circuit and the potential decreases sharply for the first instant but remains constant for a period of one hour as shown in Fig.6b. It could be seen that in the electrolytes containing aqueous media, it is difficult to handle such charging and discharging phenomena.

In fact, the amount of charge stored in PANI is more than the other class of substituted polyanilines. In the substituted polyanilines the substituents brings the hysteric hindrance which decreases the conductivity and charge storing capacity. So we decided to see the charging and discharging phenomenon in widely used electrolytes i.e. LiClO₄/PC. The films composed of thirty monolayers of the polyanilines deposited on platinum was used for the charging and discharging effect. It was first seen that the charging of the battery at 3.0 V showed the un-stability in the voltage value as shown in Fig.7a. The unstability in the storage of the charge could be explained due to the overoxidation of the polyaniline at 3.0V in LiClO₄/PC electrolytes. So, we waited for three days to see the remaining voltage in the battery, which was found to be nearly 1.0V. Later, the battery was charged at 1.0 V for fifteen minutes. The self discharge was tested for three days and an interesting phenomenon was observed. The voltage value was rising for certain amount of time till it came to a saturation value of one volt. The charging and discharging of the cell containing each polyanilines, aluminium and 1M LiClO₄/PC electrolytes were performed, since there is a reversibility of the electrode process in such electrolytes [28]. So, the doping charge is also enhanced and saturates with time as shown in Fig.7a. The discharging of the battery was performed by using a resistance of 1 M and Fig.7b showed the constant drop of voltage for a period of one day. The charging and discharging processes of PANI LS films were shown to be same for 10 cycles. Further, works are in progress to test the charging and discharging cycles of such battery.

4. Conclusions

Langmuir-Schaefer technique has been successfully applied in obtaining the ultrathin films of various polyanilines. The performed investigation emphasised the inclusion of HCl dopants in the polymeric backbone, which occurs during the film formation and indicating a different organization of polyanilines molecules at the air/water interface. The area per molecule occupied by molecules increases as a function of substituents in polyanilines either studied in deionised water or pH 1. Virtually all forms of polyanilines exhibit a microscopic structure formed from small, nanometer-scale grains or bundles, which falls in the range of 100-800 Å. The LS films of various polyanilines reveal stable cyclic voltammetric response. The PANI LS films was found to display better storage charge capability than the other substituted polyanilines. We are currently pursuing the optimization of the parameters in order to build polyanilines based batteries.

Acknowledgements

We are thankful to Dr. Victor Erokhin and Mr. P. Faraci for their interesting discussions during the preparation of the manuscript. Thanks are due to Mr. D. Nardeli and Mr. A. Sardi for their help in carrying out the experiments. Financial supports from EL. B.A. Foundation (CAP. 2102) and Polo Nazionale Bioelettronica are gratefully acknowledged.

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