Elaboration and study of the capacitive properties of polyaniline composite �lms – mixed oxides, nickel-cobalt (Ni 0.6 Co 2.4 O 4 ) electrochemically synthesized on a platinum electrode

The design of electrode material for energy production and storage devices is becoming more and more timely in a world where energy is an essential pillar for development. This context urges us to develop and study the capacitive properties of polyaniline composite films – mixed oxide of Ni 0.6 Co 2.4 O 4 which can be used as electrode material. In the present study, the mixed oxides of Ni 0.6 Co 2.4 O 4 were synthesized by the sol gel method via propionic acid. Polyaniline – Ni 0.6 Co 2.4 O 4 composite films deposited on a platinum substrate (Pt/PANI - Ni 0.6 Co 2.4 O 4 ) were prepared by incorporating mixed nickel and cobalt oxide particles into the matrix of polyaniline. This process was realized by chronoamperometry by electropolymerization of the aniline monomer dissolved in a 1M HCl electrolyte with different contents of Ni 0.6 Co 2.4 O 4 oxides (0.5 g/L, 1 g/L).Physical and electrochemical characterizations were made with scanning electron microscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDS), UV-visible spectroscopy, and cyclic voltammetry. SEM, EDS and UV-visible analyzes confirmed the nature of the synthesized products and the incorporation of Ni 0.6 Co 2.4 O 4 in the composite films. The electrochemical performance measurements of the Pt/PANI-Ni 0.6 Co 2.4 O 4 film (1 g/L) give a specific capacity about 386.108 F.g -1 , which is much higher than that of the Pt/ PANI, 164.340 F.g -1 at 5 mV/s.


INTRODUCTION
Electronically conductive polymers are highly preferred in electrochemical applications such as sensors, redox capacitors, catalysis and batteries [1-4] due to their chemical structure, characterized by alternating single and double bonds ( −  − ) or another type of n-σ-π conjugation (alternating non-bonding doublet-single bond-double bond), energetically favorable to conduction and their high capacity [5].Among them, polyaniline occupies a prominent place owing to its conductivity which can reaches 104 s.cm -1 [6], its different oxidation states (leucoemeraldine, emeraldine base, emeraldine salt and pernigraniline) and its good specific capacity [7,8].In order to increase the conductivity, stability and specific capacity of conductive polymers, reinforcements are added to the reaction medium during polymerization to obtain composites.A composite material is, by definition, the association of at least two components of different natures and structures.It makes it possible to obtain a more efficient material than these components taken separately.Composites are generally made up of two parts:  reinforcements, generally taking the form of particles or fibers, providing most of the mechanical properties of the composite;  a matrix, in which the reinforcements are imprisoned, ensuring the cohesion of the assembly and the transfer of forces as well as other properties.
In this study, we were interested in a new generation of composites based on a porous organic matrix in electronically conductive polymer (polyaniline) in which is incorporated a new phase of nanometric dimensions consisting of particles of mixed nickel and cobalt oxides prepared by the sol-gel method.These oxides offer significant catalytic and electrocatalytic excess of propionic acid.To obtain the gel, the excess of propionic acid is evaporated by heating the mixture to 120°C in an oven.Liquid nitrogen is added to the gel to obtain a mixed propionate of cobalt and nickel in powder form.The latter is heated to 140°C to dehydrate it and, subsequently, heat treated at 350°C to obtain the spinel phase of the mixed oxides.

Synthesis of Pt/PANI and Pt/PANI-Ni0.6Co2.4O4 films by chronoamperometry:
The synthesis of the films was carried out using potentiostatic method (chronoamperometry), consisting of the deposit of polyaniline and polyaniline -Ni0.6Co2.4O4 films on the working electrode (platinum disc) at a constant potential.In this study, a potential of 0.8 V was used for a bias time of 400 seconds.The synthesis was carried out in a three electrode system containing 0.1 M aniline, 1M hydrochloric acid and 0, 0.5 and 1g/L Ni0.6Co2.4O4 mixed oxide.

RESULTS AND DISCUSSIONS
Interpretation of chronoamperometric curves: During the initial stage, the anodic current decreases this corresponds to the process of nucleation.It is also attributed to the formation of a uniform polyaniline film during the polymerization phase by radical coupling [18].In this step, the current is controlled by diffusion of the aniline.However, even after the initial step, ⁄ )(Figure 6) and of   = () (Figure 7) indicates that the redox reactions proceed rapidly at the electrodes [24,28] and reflects a prevalence of the diffusion phenomena on the absorption phenomena due to the correlation coefficients of the curves I p = f(v cobalt.An evolution of the anodic and cathodic currents is observed with the concentration of nickel and cobalt mixed oxide.And that the currents are higher in the composite PANI-Ni0.6Co2.4O4(1g.L -1 ) where the mixed oxide content is higher.These observations show that the charges are much freer in the films of PANI-Ni0.6Co2.4O4.This corroborates the results obtained by chronoamperometry.This improvement in conductivity is probably due to the contribution of cobalt and nickel ions from the mixed oxide.The values of the specific capacities of the synthesized films (Table 1) were obtained using equation 1.

∫ i(E)dE E2 E1
: Voltammetric load obtained by integration of the CV curve.
m: mass of the film deposited on the working electrode.
The results show that the specific capacities of the Pt/PANI film are lower than those of the Pt/PANI-Ni0.6Co2.4O4films.And that the capacity increases with the mixed oxide content, the values with 1 g.L -1 are higher than those with 0.5g.L -1 .At 5 mV/s, the specific capacitance of the Pt/PANI electrode is 164.340F.g -1 , it increases to 386.108 F.g -1 following the incorporation of 1 g.L -1 of Ni0.6Co2.4O4.A significant increase in the specific capacity is then Nevertheless, it should be noted that, among the three films, that of PANI-Ni0.6Co2.4 O4 (1g/L) offers the best specific capacities.This capacitance gain due to the presence of mixed oxide particles confirms that the latter allow a more efficient transport of electrons and ions within the surface of the composite electrode and improve the pseudo capacitance of the polyaniline.

CONCLUSION
The composite films of PANI-Ni0.6Co2.4O4were prepared by the electrochemical method via chronoamperometry on a platinum substrate.The characterization of the films by SEM, EDS and UV-vis allowed to verify the nature of our products.The UV-visible spectra show a more intense polaronic band of the composite film favoring electronic conduction compared to the pure polyaniline film.Tests to estimate the specific capacitance of Pt/PANI-Ni0.6Co2.4O4 films by cyclic voltammetry revealed that these films would be good materials for energy storage devices such as supercapacitors.Indeed, A significant increase in specific capacity is noted following the incorporation of Ni0.6Co2.4O4particles.The specific capacitances are worth 164.340F.g -1 and 386.108F.g -1 respectively for Pt/PANI and Pt / PANI-Ni0.6Co2.4O4electrode (1g.L -1 ).This is due to the beneficial contribution of the electronic conduction of the nickel and cobalt mixed oxide particles.

2 ⁄ 2 ⁄
the anode current continues to increase with the electrolysis time.This stage corresponds to two-dimensional growth progressive nucleation and one-dimensional growth of progressive nucleation[18].In addition, the current/time slope is greater in the presence of incorporated mixed oxide particles and increases with the mixed oxide content.This suggests that the polyaniline -Ni0.6Co2.4O4composite film has greater capacitance and conductivity than polyaniline alone.Similar curves were obtained by G. Zotti et al [19] and by A. Sayah [5].Scanning Electron Microscopy and Energy Dispersive Spectroscopy: Figure 2 shows the scanning electron microscopy (SEM) image of Ni0.6Co2.4O4oxide and the energy dispersion spectrum (EDS) obtained using a Hitachi model S-520 device under a voltage of 5.0 kV.The SEM microphotograph shows micronic aggregates of poorly dispersed sizes with absence of platelets.It reveals a good dispersion of the prepared mixed oxide powders.The EDS shows characteristic peaks of the main elements of the mixed oxide prepared according to increasing atomic number, namely oxygen, cobalt and nickel.Figures 3.a and 3.c show the morphology of our films.In both cases the structure of the films obtained is in the form of interconnected aggregates.Compared to pure PANI (Fig. 3.a), the composite PANI -Ni0.6Co2.4O4(Fig. 3.c) appears to exhibit a coarser structure.This observed change shows that the cobalt and nickel oxide particles have been integrated into the PANI via adequate interconnections.The EDS spectra in Figures 3.b and 3.d confirm the presence of cobalt and nickel mixed oxide particles in the polyaniline matrix.Characteristic peaks of polyaniline main elements as carbon and nitrogen are observed in figure 3.b and those of carbon, nitrogen, oxygen, nickel and cobalt characteristic of the composite PANI-Ni0.6Co2.4O4 in figure 3.d.The chlorine represented appearing in the spectra is from the acid used during electrosynthesis.UV-visible spectral analyzes: The spectrum of pure PANI shows two absorption bands at 380-420 nm and 680-950 nm (Figure 4.a) characteristic of the emeraldine salt form of PANI.The band between 380-420 nm is a combination of the π-π* transitions of the benzenoid rings (~350 nm) and the polaron band (~430 nm) and the absorption band observed between 680 -22].Similar bands were observed with a slight shift towards the highest wavelength in the case of the PANI-Ni0.6Co2.4O4composite film (figure4.b)but the absorption intensity band in the visible domain of the composite film is greater than that of the pure PANI.This may be attributable to the strong interaction between the mixed oxide particles and the polymer chain which seems to promote conjugation within the composite film.Electroactivity of Pt/PANI-Ni0.6Co2.4O4 film: The electrochemical response (electroactivity) of the film obtained was analyzed by cyclic voltammetry in an 1M HCl aqueous solution.The study was conducted in a potential range between -0.2 and 1 V/Ag/AgCl with scan rates of 5 mV/s, 10 mV/s, 15 mV/s, 20 mV/s, 25 mV/s and 30mV/s.The voltammograms obtained (figure5) show that the synthesized film is electroactive in the aqueous medium (HCl, 1M).This is highlighted by the appearance of three peaks which reflect the oxidation and reduction of the different forms of polyaniline.They also allow to highlight a very interesting property of polyaniline, namely the doping and dedoping phenomenon which means its ability to pass from the reduced state to the oxidized state and vice versa during a voltammetry cycle.In our case, the films are doped with Cl -chloride anions.One can see that the intensity of these peaks increases with the scanning rate.In fact; during rapid scanning stage, the diffusion layer cannot extend far from the surface of the electrode, thus facilitating a higher flow of electrolyte to the electrode and leading to a greater value of the electric current, unlike in the case of low sweep speeds where the large width of the diffusion layer reduces considerably the flow of electrolyte towards the working electrode and therefore a decrease in current[23].We also observe a shift of the anodic and cathodic peaks towards the increasing and decreasing potentials respectively with the increase in the scanning rate.This shifting of the peaks indicates an enhanced of the electrochemical bias at high scan rates [24-26].Kinetics of electrochemical systems in the composite electrode: To study the electrochemical kinetics of the electroactive system and its reversibility, the values of the peaks currents (Ip) and potentials (Ep) of the oxidation and reduction processes noted on the voltammograms of figure 5 have been plotted to obtain the curves Ip as a function of the square root of the scan rate (v 1 ) and as a function of the scan rate (v) (Figures 6 and 7).The current intensities of the peaks vary with the scanning rate (v) according to a the type relation   = . 1 [27, 28].The linearity of the curves   = ( 1 2

1 2 ⁄
) (0.99968 in oxidation and -0.99789 in reduction) are closer to 1 than those of the curves IP = f(v) (0.9901 and -0.9976).That's mean the diffusion of electrolytic Cl -ions is a rate-controlling process [29 -32].A slope of absolute value 4.70 was found in reduction and a slope of 3.80 in oxidation.The ratio of the two slopes is equal to 1.2.This value, close to unity, shows a fairly good reversibility of the electrochemical system studied and the symmetrical nature of the redox peaks of the voltammograms taken at different scanning speeds confirms it [24, 33, 34].Capacitive properties: The characterization of the capacitive performance of the films obtained was analyzed by cyclic voltammetry in a 1M HCl aqueous medium and without any monomer and/or mixed oxide.Figures 8.a, 8.b and 8.c illustrate the cyclic voltammograms of Pt/PANI and Pt/PANI-Ni0.6Co2.4O4 (0.5 and 1g.L -1 ) composite films performed in a potential interval from -0.2 to 1V, relative to the Ag/AgCl electrode, and at different scanning rates (5, 10, 20, 25, 30 mVs -1 ).The analysis of the cyclic voltammetry curves of the Pt/PANI-Ni0.6Co2.4O4films with those of Pt/PANI shows an increase in the charge and discharge curves surfaces of Pt/PANI-Ni0.6Co2.4O4compared to those of Pt/PANI.In addition, the curves surfaces increase with the content of incorporated nickel-cobalt mixed oxide nickel noted.It can be deduced that the pseudo capacitance of the polyaniline and its porosity increase in the composite film.The porosity of the film facilitates the movement of charge carriers and the pseudocapacitive effect of polyaniline results from surface oxidationreduction reactions.Mixed oxides of nickel and cobalt (Ni0.6Co2.4O4)have exceptional electrochemical properties, due to their rich oxidation-reduction reactions involving nickel and cobalt ions and their synergistic effects [32].The immersion of these particles in the matrix of the polyaniline offers a more extensive contact interface between the polymer and the electrolyte [35-38], a larger active electrochemical surface, it is then obvious that the specific capacity increases with the film mixed oxide content.Similar results are reported in the literature, Sayah et al had a capacity increase from 176.29 F.g -1 to 305.57F.g -1 by incorporating 3mg of graphene into the FTO/PANI film with a slew rate of 5 mV/s [5].J. H Shendkar et al found an average capacity of 215 F.g -1 for the PANI /Co(OH)2 composite with a scan rate of 10 mVs -1 in a 1M NaOH medium [39].N. Harfouche obtained a raise of significant capacitance with the PANI-LiMn2O4 film on a platinum substrate attributed to the contribution of Li + ions released by LiMn2O4 [6].In figure 9, the specific capacities are represented according to the sweep rates speeds, All the capacities decrease with the increase of the sweep rate.Indeed, when with the scanning rate increases, the diffusion of Cl -ions decreases.The ions can only reach the outer surface of the electrode thus the active material inside is not fully involved in the electrochemical process, so the decrease in capacity [40].

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Figure 6 Current
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Figure 7 Current
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Figure 8 a
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