Electrochemical capacitors (ECs) represent an emerging class of energy storage devices that have attracted increasing attention because of a number of important features including high power density, fast charge/discharge rate, and excellent cycle stability [1,2]. In ECs, the electrode material is one of the extremely important factors affecting the performance of ECs (energy, power density, safety, cycle life, etc) [3,4]. Carbon-based materials [5], metal oxides [6], conductive polymers [7] and their composites [8,9] have been used for producing ECs. High specific capacitance, flexibility and low cost are some advantages of conducting polymers, while they have disadvantages like shrinking nature during charge/discharge process. Based on its Mott-Schottky diagram, Polyaniline (PANI) is a p-type conductive polymer [10]. Because of excellent organic conductivity, biocompatibility and good environmental stability, PANI has been widely used in ECs and biosensors [11,12].

Many reports have been published on PANI and nano materials composites [13-17]. Increasing capacitance and stability is the goal of using PANI nano composite materials. There are two methods for PANI composite synthesis: chemical polymerization, and electropolymerization. In chemical method, the reaction product is always a powder. So, it has to be dried which leads to change the polymer structure [18,19]. Furthermore, in order to making electrode using the composite, binder is needed which in turn decreases its electrical and electrochemical performance.

Electropolymerisation of PANI has many advantages: not needing oxidants, film uniformity on the electrode, saving the use of binder, decreasing contact resistance between polymer and current collector [20-22]. Because of synergistic effect between two composite components, Dramatic improvements have been achieved in electrochemical properties of composites.

MnO₂ has been used as an active material in energy storage field. It has low cost, environmentally friendly nature, and various forms with different properties. So, It has been the subject of many researches as an electrode material for electrochemical capacitors [23-25]. In this paper we synthesized α-MnO₂ nano-wire which then was used to make PANI/NwMnO₂ in order to investigate the supercapacitive behavior and synergistic effect of these active materials together.

Fig. 1(a, b, c) presents the SEM graphs of PANI/NwMnO₂ composite electrode. As can be seen, MnO₂ nanowires have been coated on the surface of PANI filaments uniformly. The shape and structure of α- MnO₂ nanowire is shown in Fig. 1(d) It can be clearly seen that MnO₂ nanowires is actually composed with uniform shape.

X-ray diffraction was used to investigate the crystal structure of prepared materials. shows the XRD patterns of MnO₂ nanowires, PANI and PANI/NwMnO₂ electrodes. For MnO₂ nanowires, it can be seen clearly that all diffraction peaks can be exclusively indexed as the tetragonal α-MnO₂ (JCPDS 44-0141) and no other impurities were observed. XRD diagram of PANI/NwMnO₂ shows that by adding MnO₂ nanowires to the PANI matrix, its crystal structure was changed and some peaks in XRD pattern of PANI/NwMnO₂ were appeared.

Cyclic voltammetry technique was used to evaluate the electrochemical performance of electrodes. Fig. 3(a) present’s cyclic voltammograms of electropolymerised PANI and PANI/NwMnO₂ electrodes at 10^th cycle of electro polymerization process. It can be clearly seen that the electropolymerization charge of PANI/NwMnO₂ electrode is higher than that of PANI electrode and this has been considered as the contribution of faradic pseudo capacitance of MnO₂ nanowires and double layer behavior of PANI. In both cyclic voltammograms there are two redox peaks. The first one which is in negative potentials is related to the formation of free radicals in the polymerization of PANI and PANI/NwMnO₂ nano wire electrodes. The second one is due to oxidation of Polymer in both electrodes. Fig. 3(b) presents cyclic voltammograms of PANI and PANI/NwMnO₂ elec-trodes at the scan rate of 25 mV/s in 1 M H₂SO₄ solu-tion. PANI/NwMnO₂ electrode, a remarkable difference can be seen in the cyclic voltammogram loop areas. This shows that the electrochemical behavior of the composite electrode is distinctly improved after adding MnO₂ nanowires to PANI matrix. The charge storage in faradic process is achieved by electron transfer that leads to chemical changes in the electro active materials according to Faraday’s law related to the potential [28,29]. Furthermore, symmetrical and rectangular shape of composite cyclic voltammogram curve shows an ideal capacitive behavior for this electrode involving two types of capacitive performances: contributing the electric double-layer capacitance produced by PANI and pseudo capacitive behavior of MnO₂ nanowires. As shown in Fig. 3(b), a redox peak in potentials between 0.5 - 0.6 V were appeared that is attributed to PANI transition from emeraldine to pernigraniline form [30]. Specific capacitance (SC) of electrodes calculated from cyclic voltammogram curves according to following equation:

Where I is the current, m is the mass of reactive material and í is the potential scan rate. The SC of PANI and PANI/MnO₂ electrodes at the scan rate of 25 mV/s were calculated 190 and 456 F/g respectively.

Cyclic voltammograms of composite electrodes in 1M H₂SO₄ media in different scan rates is shown in a. As can be seen the excellent capacitive performance of dPANI/NwMnO₂ electrode was also verified based on these curves. With an increase of scan rate the current response increases that indicate an ideally capacitive behavior of PANI/NwMnO₂ electrode [30]. Furthermore the good rectangular shape of PANI/NwMnO₂ voltammograms is remained up to the scan rate of 100 mV/s. The deviation from rectangularity in cyclic voltammograms becomes obvious as scan rate increases. This phenomenon can be attributed to the electrolyte and film resistance, and this distortion is depending on scan rate. By increasing the sweep rate, active sites in electrode don’t have enough time to react with ions.

Fig. 4(b) presents the calculated SCs of both electrodes at different scan rates. PANI/NwMnO₂ composite electrode shows SCs of 575 and 150 F/g at the scan rates of 2 and 200 mV/s, respectively, whereas at the same range these magnitudes for PANI electrode were calculated from 240 and 60 F/g. As can be seen, the capacitance of both electrodes decayed over the entire range of scan rate, but for PANI/NwMnO₂ electrode the slope of SC vs. scan rate was not only more than that of PANI electrode, but also less than it. This phenomenon can be resulted from the fact that both electrodes are porous and the presence of MnO₂ nano wires do not cause to block the porosities of PANI film. Therefore, different electrochemical behaviors of the electrodes could come from the synergistic effect between MnO₂ nanowires and PANI. Chemical methods of composite fabrication could motive to block some porosities of polymer film which cause to decay PANI super capacitive behavior. Therefore using electrochemical method, the whole pseudocapacitive behavior of PANI electrode could be saved.

Galvanostatic charge-discharge (CD) is one of the methods that have been used to study the capacitive properties of active materials[31]. Fig. 5(a) shows the CD behavior of PANI and PANI/NwMnO₂ electrodes in the potential range from 0.2 to 0.8 V at the current density of 3.6 A/g. In this range, a triangular shape can be seen indicating that PANI/NwMnO₂ composite electrode have a good Columbic efficiency and ideal capacitive behavior in order to use for supercapacitors. The charge and discharge phase of PANI/NwMnO₂ lasts longer than that of PANI electrode which shows it has more capacitance than PANI. In addition, Because of increasing conductivity, using MnO₂ nanowires cause to decrease the voltage drop (iR) which appeares in the curves during the change of current sign [31,32].

Fig. 5(b) presents the CD curves of PANI/ NwMnO₂ electrode at various specific currents of 1.8, 2, 2.4, 2.8, 3.6, 3.6, 4, 4.8, 6 and 7.2 A/g. As can be seen, the shapes of all curves are approximate isosceles triangles which are the characteristic of super capacitors. Here, SC was measured according to the charge/discharge curves, using Eq. (2).

With I being the applied current; -∆E/∆t, the slope of discharge curve after voltage drop at the beginning of each discharge process (ESR); and m is the mass of composite electrodes. The highest SC for composite electrode was obtained when the current density for CD process was 1.8 A/g. The SC of PANI/NwMnO₂ changes in the range of 677 to 340 F/g. It can be seen that enhancing the specific current due to the intercalation of ions at the surface of active materials in the electrode/electrolyte interface, the specific capacitance values decrease. On the other hand, at low specific currents, the specific capacitance increases because there would be enough time for insertion and deinsertion of ions at deeper porosities of the active materials in the electrode/electrolyte interface.

EIS technique was used for further studying the supercapacitive behavior of polymeric film and composite electrodes. The Nyquist plots of PANI and PANI/NwMnO₂ electrodes is shown in a. As illustrated, there is a semicircle in high frequencies related to the charge transfer resistance (R_ct) caused by the Faradic Reactions and the double-layer capacitance (C_dl) at the contact interface between electrode and electrolyte solution. The magnitude of R_ct in PANI/NwMnO₂ electrode was smaller than that of PANI electrode, because the addition of MnO₂ nanowires enhances the conductivity and improves charge transfer performance of PANI/NwMnO₂ electrode. Both electrodes exhibit a nearly linear branch in the low-frequency region. A transition to linearity at low frequencies exhibit an ideal capacitive behavior which was observed for PANI/NwMnO₂ electrode clearer than PANI one. The low frequency capacitance (C_lf) of each film was determined using Eq. (3):

where f and Z” are the frequency and imaginary component of impedance respectively.

It can be seen that PANI/NwMnO₂ electrode has more capacitance than PANI one. These results also confirmed the data obtained by cyclic voltammetry and galvanostatic charge-discharge techniques. One of the important features of supercapacitor electrodes is the stability of electrodes against consecutive or life cycles. Life cycles of both electrodes were investigated by cyclic voltametery at the scan rate of 50 mV/s for 900 cycles in 1M H₂SO₄ solution ( b). Synergistic effect of PANI and MnO₂ nanowires is illustrated in the figure. As can be seen after 900 cycles, PANI electrode losses it’s capacitance down to 70% while for PANI/NwMnO₂ electrode, the capacitance increases up to about 450^th cycle and then decreases with a gentle slope. Increasing the capacitance of composite electrode can be attributed to surface modification of MnO₂ nano wires in consecutive cycles. During the polymerization of PANI/MnO₂ composite, some parts of MnO₂ is covered by polymer chains which in turn are refreshed during next cycles resulted in increasing the electrode capacitance.

In summary, an electrochemical method to produce a polymer/metal oxide composite has been proposed. The morphology and the structure of the composite as well as pure PANI and MnO₂ nanowires were fully characterized using XRD and SEM analysis. Electrochemical studies showed excellent synergistic effect between PANI and MnO₂ nanowires resulted in increasing the capacitance and cycle stability of composite electrode against PANI one. Specific capacitance of 456 F/g was obtained at the scan rate of 25 mV/s for composite electrode against 190 F/g for PANI. Increasing the cycle ability of electrode is another advantage of using MnO₂ nanowires polymer matrix.