Synthesis and Electrochemical Capacitor Characterization of Novel Composite Materials with p-Type Conductive Polymer

Considering the importance of conductive polymer nanocomposite, the present paper attempts to create a method for increasing the conductivity of poly(o-aminophenol). Nanocomposite MnO2/poly(o-aminophenol) thin film was synthesized by using pulse potential electrodeposition technique on a graphite electrode. In this research, nanoparticles of MnO2 are used after synthesis to prepare polymer nanocomposites in one-step. Appending of MnO2 to polymer matrix increases the current. This current growth could be ascribed to the synergistic MnO2 nanostructure, which presents the superior surface area and smaller particle size that is increasingly acting sites. Morphology or samples composition was investigated by the scanning electron microscope and the UV-Vis method, which clearly indicate the formation of nanocomposites.The findings show that the capacitive behavior of MnO2poly(o-aminophenol) is superior to poly(o-aminophenol), especially at high potential high temperatures. The results showed that MnO2/poly(o-aminophenol) had a higher level of activity and the electron transfer capability was faster than pure polymer film. The dopedMnO2 polymer also has excellent cyclic performance and load discharge features. Additional electrochemical properties of these polymer composites were observed against pure polymer so that capacity of 645 Fg has been designated.


Introduction
Over the past years, the study of polymers has attracted much attention and has many advantages such as simple synthesis, low cost, high chemical stability, adjustable doping /doping properties, and high conductivity [1][2][3][4].Poly(oaminophenol) is one of the best conductivity polymers and it has been of great interest to researchers whereas it is more excellent electrical conductivity [5,6].They have a diversity of usages like microelectronic devices, capacitors, transistors, corrosion protection, and chemical sensors [7,8].Ehsani et al. [9] have fabricated POAP/1-octadecyl-3methylimidazolium bromide composite films as active electrodes supercapacitors and reported that the supercapacitor behavior of the composite film is due to the high active surface area, charge-transfer in the polymer chain, and synergistic efficacy amongst conductive polymer and [OMD] Br.In order to improve the performance of supercapacitors, many efforts have been done to incorporate metal oxides for polymerization, making composite electrode materials.Varaprasad et al. [10] have reported that metal-oxide polymer nanocomposite films are useful for the dielectric device and supportable energy harvesting applications.MnO 2 and its derivatives have attracted particular attention and are used in various applications such as catalysts, batteries, sensors, and batteries [11,12].Ozan Yanik and his colleagues [13] have been investigated magnetic conductive polymer/graphene nanocomposites for supercapacitors.They were able to achieve 255 Fg −1 specific capacitance values for it and they showed it has a healthy cell performance.Bahloul et al. [14] synthesized polypyrrole (PPy) deposited onto -MnO 2 particles.They concluded that PPy/MnO 2 composite has high specific capacitance of 141.6 F g −1 compared with 73.7 F g −1 for -MnO 2 .It indicates that the improvement of the coating is due to the electronic conductivity of the polymer and an increase of the BET surface area that raises to 125m 2 g −1 after coating, against 64 m 2 g −1 for MnO 2 .Hamid Mohammad Shiri and his colleague [15] have fabricated POAP/Dy 2 O 3 films to serve as the active electrode for electrochemical supercapacitor.They found the enhanced supercapacitor behavior observed for the composite film could be attributed to synergistic effect existing between Dy 2 O 3 nanoparticles and the conductive polymer.Sydulu Singu et al. [16] have reported that PPy-Ag nanoparticles nanocomposite exhibits the specific capacitance of 266.7 F g −1 and the power density of 3998 W kg −1 and excellent capacitance after 5000 charge-discharge cycles, which determines the potential electrode material for supercapacitor application.Poly-o-phenylenediamine/MnO2 materials were synthesized by the addition of cetyl trimethyl ammonium bromide and sodium dodecyl sulfate [17].Research results showed a high specific capacitance (262.2Fg −1 ) and a good electrochemical performance, making poly-o-phenylenediamine a promising material in supercapacitor application.However, a study on the synthesis of one-stage MnO 2 /poly(o-aminophenol) nanocomposite and its capacitive behavior has not been done.Poly(o-aminophenol) is a p-type conductive polymer [18] that exhibits its maximal electroactivity within the potential range -0.2 V < E < 0.5 V versus (SCE).One of the effective methods for increasing its life cycle is combination of polymer and nanomaterial for the preparation of nanocomposite coatings for various applications.Manganese dioxide is widely used for the cheap metal oxides supercapacitor applications due to abundant in nature, high porosity, highly reversible chemical adsorption/desorption, good electrochemical performance, and environment-friendly [19,20].It is believed that the MnO 2 electrode capacity is mainly due to reversible changes that involve the exchange of protons and/or cations with the electrolyte [21,22].In this research amount of specific capacity for pure MnO 2 is 203 F g −1 for MnO 2 .To improve electrical conductivity of the polymer and creation higher surface area, MnO 2 is mixed with poly(oaminophenol), which increases the specific capacity.
Therefore, we proposed a one-step easy procedure for the synthesis of MnO 2 doped poly(o-aminophenol), in which doping of MnO 2 was achieved simultaneously during the electrochemical polymerization of o-aminophenol.In this paper, after synthesis of nanocomposites, the use of the MnO 2 -poly(o-aminophenol) nanocomposite for capacitor applications with high performance is discussed.

Materials and Methods
. .Materials.The chemicals were prepared from Merck firm as the analytical grade.Electrochemical tests were carried out using a system three-electrode cellby a -Autolab potentiostat/galvanostat and a frequency response analyzer (Metrohm, Model 12/30/302, and The Netherlands).Graphite (G) with a diameter of 0.3 cm was applied as a worker electrode and platinum foil as an auxiliary electrode, while Ag /AgCl was operated as a reference electrode.The morphologies of the electrosynthesized conductive polymer obtained were investigated by field emission scanning electron microanalyzer model KYKY-EM3200.1.

. . Synthesis of MnO 2 -Poly(o-aminophenol).
MnO 2 nanoparticles were synthesized by reduction of 0.1 M KMnO 4 aqueous solutions with ethanol by ultrasonic method.Poly(oaminophenol) electrodeposited on the cleaned graphite disk electrode by pulse potential electrodeposition technique from an aqueous solution containing 0.02M o-aminophenol, 0.04M sodium dodecyl sulfate, and 0.6 M perchloric acid with the pulse current of 8 mA/cm 2 , pulse time and relaxation time of 0.5 s.At the start of work, the worker electrode was polished and then rinsed with ethanol and water, respectively.The MnO 2 -poly (o-aminophenol) electrode was prepared like the method of graphite, poly(o-aminophenol) electrode, but with an addition of 1% MnO 2 .The MnO 2 1% were scattered into a solution of 5 mM SDS for thirty minutes to acquire a uniform suspension.Sodium dodecyl sulfate is used as suspension additive MnO 2 particles and increase the firmness and electroactivity of the prepared films.The surface areas of the bare and modified electrode were gained by cyclic voltammetry method in 1 mM K 3 Fe (CN) 6 solution as a probe at various scan rates.The surface area for a reversible process can be computed according to the slope of as (1) [23]:

Results and Discussion
where A is the electroactive surface area (cm −2 ), while n is the amount of electrons participated in the redox reaction.It is obvious that the redox peak current growths linearly with enhance scan rate and the redox peak currents of G/MnO 2 -poly(o-aminophenol) significantly was strong.Figure 4 shows a linear dependence of anodic current for G/MnO 2 -poly(o-aminophenol) electrode on the ] 1/2 with a correlation coefficient of 0.9909.
Figure 5 shows cyclic voltammogram cycles of the bare and obtained electrodes (pure poly (o-aminophenol), MnO 2 -poly(o-aminophenol)).As seen, the pure poly(oaminophenol) film shows a pair of the wide peak of about -0.1 to +0.5 V, which is associate to the redox reaction of the phenoxazine.The redox peak currents of the composite film were stronger than both electrodes.The composite shows a higher background current in the potential sweep, and there exists faradic currents, because of the existence of MnO 2 .
Figure 6 shows CV curves of G/poly(o-aminophenol) and G/MnO 2 -poly(o-aminophenol) electrodes have been depicted in different scan rates in 0.6 M HClO4 media.As seen, two pairs of peaks become visible on the chart in CV of both electrodes.Based on CV results, with increasing scan rates, the current response of both films increases, but the current signal is low in G/poly(o-aminophenol).Clearly, with the increase in scan rate gradually, the shape of cyclic voltammograms does not deform strongly.It is clear that, the change at the peak potential of MnO 2 -poly(o-aminophenol) film is smaller than that for poly(o-aminophenol).These results confirm that it has a better electrochemical reversibility and an ideal capacity compared to poly(o-aminophenol).
Figure 7 shows the I-V characteristics of bare, MnO 2poly(o-aminophenol) composite, and poly(o-aminophenol) electrodes [24].The specimen resistance was measured using two-probe, where the I-V curves were obtained by scanning the voltage from -2 to 2 V with a step of 0.06 V. I-V curves display that the resistance of MnO 2 -poly(o-aminophenol) composite has decreased compared to poly(o-aminophenol) and bare electrodes.These data show the electro-catalytic effects of MnO 2 -POAP towards oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), in comparison to pristine POAP.
The ability of electron transfer of different electrodes was studied by using electrochemical impedance spectroscopy at the open-circuit potential for investigation of a polymer matrix.Figure 8 shows the Nyquist impedance curve of the electrodes using a 1 mM redox [Fe(CN) 6 ] −3/−4 (1 : 1) probe with 0.5 M KCl solution.It shows that both electrodes exhibit a single semicircle at higher frequency region and a linear part in the lower frequency region.As predicted, the electron transfer (Ret), which is related to the faradic reactions in the MnO 2 -poly(o-aminophenol) electrode, is less than pure polymer.This result shows that the presence of MnO 2 nanoparticles in the polymer matrix increases the conductivity of the composite electrode.It causes chargetransfer resistance decrease with MnO 2 nanoparticles.This phenomenon may be attributed to the large specific surface and faster transfer of composite electrons.It is clear that the vertical curve of MnO 2 -poly(o-aminophenol) has the large slope that is close to the imaginary impedance axis.It shows MnO 2 -poly(o-aminophenol) has the high conductivity or low internal resistance.This means that the results of both experiments consist of the impedance and cyclic voltammetry tests confirm each other.As shown in the diagram, both electrodes have Warburg (ZW) resistance impedance in the low-frequency region, is a result of the dependence of the ion emission/transfer frequency from an electrolyte to the electrode surface [25].The Nyquist design data is placed in an equivalent circuit, shown in Figure 8, indicating the a bulk solution resistance Rs, a charge-transfer resistance R ct, a constant phase element for simulating the doublelayer capacitance (Cdl), and a Warburg (ZW) resistance impedance.
Considering the higher voltages in the cyclic voltammogram of MnO 2 -poly(o-aminophenol), a larger capacity for MnO 2 -poly(o-aminophenol) is predictable, and the MnO 2poly(o-aminophenol) composite acts as a supercapacitor with a high specific capacitance.The specific capacitance of the samples can be estimated from the cyclic voltammetry curves based on this equation [26]: which is in this equation I identify the average cathodic current, s specifies the applied potential sweep rate, and m is the mass of the active electrode.An electrode is weighing, before and after the deposition process, which yields the amount of the active material or mass of film.

Conclusions
In summary, G/MnO 2 -poly(o-aminophenol) electrode was made by electropolymerization of ortho-aminophenol monomer and of MnO 2 .Optimum conditions for the providing of modified electrode have been obtained.With  the presentation of MnO 2 , the as-prepared MnO 2 -poly(oaminophenol) exhibits more capacitive behaviors than poly(o-aminophenol), especially at the high potential scan rate.In addition, the cyclic performance of the MnO 2 -poly(o-aminophenol) electrode is superior to poly(oaminophenol).Better electrochemical capacitor property was observed for these metal-oxide polymer composites against pure polymer.In addition, the results revealed that the MnO 2 /poly(o-aminophenol) had higher electric conductivity, less resistance, and faster charge-transfer than the pure polymer film.Scanning electron micrographs and UV-Vis spectrum were employed for characterization of the produced composites.The composite represented excellent electrochemical capacitor property.

Figure 1
Figure 1 exhibits the SEM images of poly(o-aminophenol) and MnO 2 -poly(o-aminophenol).As can be observed, the morphologies of poly(o-aminophenol) and MnO 2 -poly(oaminophenol) are completely different from each other, indicating that the presence of MnO 2 into poly(o-aminophenol)

Figure 2
displays UV-Vis spectrum prepared films of the poly(o-aminophenol) and MnO 2 -poly(o-aminophenol).It is observed that the shapes of UV spectra of nanocomposite and polymer are similar, but the poly(o-aminophenol) sample showed strong absorption band in the UV region.As observed by adding MnO 2 , the severity of the peaks decreased because of the interaction between MnO 2 and poly(o-aminophenol), demonstrating of the MnO 2 presence in the composite film.When MnO 2 nanoparticles are dispersed in the polymer matrix, significant changes are observed in the absorption spectra, which indicate the interaction of metal-oxide nanoparticles with the polymeric chain.These results confirm the formation of MnO 2 -poly(oaminophenol) nanocomposite.

Table 1 :
Comparison of specific capacitances for different composites.