Conducting polymers (CPs) possesses many advantages such as simple synthesis,low cost,high chemical stability,tunable doping/ de-doping properties,and high conductivity in a doped state ^{[1, 2, 3]}. In recent years,CPs based supercapacitors have attracted increasing attention due to their high charge storage capabilities ^{[4, 5, 6]}. For example,supercapacitors based on graphene-polyaniline derivative nanocomposite and MnO₂ embedded polypyrrole nanocomposites have been developed successfully ^{[7, 8]}. With the development of conducting copolymers composed of two or more monomer units ^{[9, 10, 11, 12]},it has aroused a great upsurge in investigation on the applications of conducting copolymers in the field of energy-storage ^{[13, 14]}. For example,Palaniappan et al. ^[13] synthesized a copolymer of aniline and pyrrole by inverted emulsion polymerization and applied the obtained copolymer for supercapacitors.

To improve the performance of CPs based supercapacitors,many efforts have been carried out to incorporate MnO₂ to CPs,forming CPs/MnO₂ composite electrode materials ^{[15, 16, 17, 18, 19, 20]}. However,the aforementioned incorporations were usually achieved via a two-step ^{[15, 16, 17, 18, 19]} or three-step ^[20] strategy,i.e.,MnO₂ and CPs were synthesized separately and then combined together,and this was a time-consuming and inconvenient operation.

Herein,we proposed a one-step strategy for the synthesis of MnO₂ doped poly(aniline-co-o-aminophenol) (PANOA),in which the doping of MnO₂ was achieved simultaneously during the chemical oxidative copolymerization of aniline and o-aminophenol using KMnO₄ as the oxidant. The influence of molar ratio (aniline/o-aminophenol) on the electrochemical activity of the copolymer was investigated in this work. The capacitive behaviors as well as the cyclic performance of the PANOA-MnO₂ electrode were also discussed.

All the chemicals were of analytical grade,and doubly distilled water was used in all runs. Aniline was distilled under reduced pressure before use,and other chemicals were used without further purification. The electrochemical measurements were performed on a CHI 660D electrochemical workstation in a conventional three-electrode system using the PANOA-MnO₂ as working,a platinum foil as counter,and a saturated calomel electrode (SCE) as reference electrode,respectively. The electrolyte for all the electrochemical experiments was 1 mol L^-1 H₂SO₄.

PANOA-MnO₂ was synthesized via chemical copolymerization of aniline and o-aminophenol using KMnO₄ as the oxidant. Aniline(0.2 mol L^-1) and a certain amount of o-aminophenol were dissolved in 50 mL of 1 mol L^-1 H₂SO₄ and stirred for 30 min. And then,50 mL of 0.2 mol L^-1 KMnO₄ was added dropwise to the solution placed in an ice-water bath. After the solution was stirred for 12 h,it was filtered and the precipitate was washed with ethanol and deionized water until the filtrate became colorless. Finally,the product was dried in a vacuum oven at 65 ℃ for 12 h and PANOA-MnO₂ was obtained. For the control experiments,the pure PANOA was synthesized by the same procedures using 0.2 mol L^-1 (NH₄)₂S₂O₈ as the oxidant instead of KMnO₄.

The PANOA-MnO₂ electrode was prepared by the following procedures: 20 mg of the PANOA-MnO₂ sample was first dispersed in 2 mL ethanol by sonication,and then 10 μL of this dispersion was dropped onto the surface of a glassy carbon electrode (GCE,φ = 3 mm) and dried at room temperature. Then 10 μL of nafion solution (5 wt%) was dropped onto the surface of the PANOA- MnO₂ to prevent the copolymer from detaching from the electrode surface during the electrochemical measurements. The PANOA electrode was prepared by the same procedures.

Fig. 1 shows the cyclic voltammograms (CV) and the chargedischarge curves of the PANOA synthesized at different molar ratios of aniline to o-aminophenol. As can be seen,when the molar ratio is set as 20:1,the area under the CV curve obtained at a potential scan rate of 10 mV s^-1 is the largest (Fig. 1A),indicating that the copolymer has the highest electrochemical activity at such a molar ratio. Accordingly,compared to other molar ratios,the PANOA synthesized at the molar ratio of 20:1 exhibits the highest specific capacitance at a current density of 0.5 A g^-1 (Fig. 1B). So the molar ratio of 20:1 is adopted throughout the following experiments.

Fig. 1

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Fig. 1.Cyclic voltammograms (A) and charge/discharge curves (B) of PANOA in 1 mol L-1 H2SO4 solution. PANOA was synthesized at different molar ratios of aniline to oaminophenol: without o-aminophenol (1), 40:1 (2), 20:1 (3), and 10:1 (4).

The FI-IR spectra of PANOA and PANOA-MnO₂ are shown in Fig. 2. For PANOA,the two peaks at 1587 and 1500 cm^-1 are attributed to the stretching vibrations of C5 5N and C5 5C,respectively. The peak at 1303 cm^-1 is assigned to the stretching vibration of C-N,and the peak at 1153 cm^-1 is caused by the stretching vibration of C-H ^[21]. For PANOA-MnO₂,a new characteristic Mn-O vibration peak at 525 cm^-1 is observed ^[22],suggesting the successful introduction of MnO₂ to the polymer. Fig. 3 shows the SEM images of PANOA and PANOA- MnO₂. As can be seen,the morphologies of PANOA and PANOA- MnO₂ are quite different from each other,indicating that the introduction of MnO₂ to PANOA changes the microstructure of the polymer.

Fig. 2

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Fig. 2.FT-IR spectra of PANOA and PANOA–MnO2.

Fig. 3

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Fig. 3.SEM images of PANOA (A) and PANOA–MnO2 (B).

The CV of PANOA-MnO₂ and PANOA at varying scan rates are shown in Fig. 4. Two pairs of redox peaks appear on the CV of PANOA-MnO₂ (Fig. 4A) and PANOA (Fig. 4B). At low potential scan rates (10,30and50 mV s^-1),the peakcurrentsatPANOAare alittle higher than those at PANOA-MnO₂; however,at relatively high scan rates (70,90,110,130 and 150 mV s^-1),the peak currents at PANOA-MnO₂ are obviously higher than those at PANOA. This is because that MnO₂,the reduction product of KMnO₄,is doped into the PANOA during the chemical oxidative copolymerization process,which functions as the redox active catalyst of the copolymer,especially at high potential scan rates. Moreover,it is noteworthy that the anodic and the cathodic peak potentials shift to a more positive and negative direction,respectively,with the increase in the potential scan rate. For PANOA-MnO₂,the shift in the peak potential is smaller than that for PANOA,indicating that the PANOA-MnO₂ has a better electrochemical reversibility compared to PANOA. The specific capacitance of PANOA-MnO₂ and PANOA can be calculated from the data on the CV based on the following equation: C = (q_a + q_c)/2mΔV,where C is the specific capacitance of the electrode material (F g^-1),q_a + q_c is the sum anodic and cathodic voltammetric charges on positive and negative sweeps,m is the mass of active material,and DV is the potential range of the CV. Fig. 4C shows the relationship between the specific capacitance values and the potential scan rates. Although the specific capacitance of PANOA is a little higher than that of PANOA-MnO₂ at low scan rates (for example,470 vs. 459 F g^-1 at the scan rate of 10 mV s^-1),it decays quickly with the increase in the scan rate and only 268 F g^-1 is obtained at the scanrate of 150 mV s^-1. However,for PANOA-MnO₂,a specific capacitance as high as 335 F g^-1 is still remained at such a high potential scan rate. The results indicate that the capacitive behaviors of PANOA-MnO₂ are superior to PANOA.

Fig. 4

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Fig. 4.Cyclic voltammograms of PANOA–MnO2 (A) and PANOA (B) in 1 mol L-1 H2SO4 at different scan rates of 10, 30, 50, 70, 90, 110, 130, and 150 mV s-1. (C) Relationship between the specific capacitance of PANOA–MnO2 and PANOA and the potential scan rate.

The galvanostatic charge/discharge plots of PANOA-MnO₂ and PANOA are recorded at different current densities,as shown in Fig. 5. Compared to PANOA,the charge/discharge curves of PANOA-MnO₂ present a more symmetric shape,especially at a low current density of 0.5 A g^-1 (curve 1 in Fig. 5A and B),indicating good capacitive characteristics of the PANOA-MnO₂ ^[23]. The specific capacitance of PANOA and PANOA-MnO₂ can also be estimated from the charge/discharge curves according to the following equation: C = IΔt/mΔV,where I is the applied current,Δt is the total discharge time,ΔV is the potential change,and the definitions of C and m have mentioned above. The results show that the specific capacitance of PANOA decreases by 48.7% when the current density increases from 0.5 A g^-1 to 6 A g^-1; however,onlya 27.6% decrease in the specific capacitance for PANOA-MnO₂ is observed after the same variation in the current density,indicating good high-current charge/discharge characteristics of the asprepared PANOA-MnO₂ electrode.

Fig. 5

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Fig. 5.Charge/discharge curves of PANOA–MnO2 (A) and PANOA (B) in 1 mol L-1 H2SO4 at different current densities of 0.5 (1), 1 (2), 2 (3), 4 (4), and 6 A g-1 (5).

The cyclic performances of PANOA-MnO₂ and PANOA are also investigated and compared in this work. Fig. 6 shows the cycle life of PANOA-MnO₂ and PANOA in 1 mol L^-1 H₂SO₄ at a charge/ discharge current density of 2 A g^-1. As can be seen,after 600 cycles,the specific capacitance of PANOA remains about only 63% of its initial value; however,the specific capacitance of PANOA- MnO₂ still retains about 74% of the initial performance,indicating that as a supercapacitor electrode material,the cyclic performance of PANOA-MnO₂ is superior to PANOA. This enhancement in cyclic performance of PANOA-MnO₂ can also be attributed to the enhancement of the electrochemical reversibility of the copolymer via MnO₂ doping.

Fig. 6

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Fig. 6.Cyclic performances of PANOA–MnO2 and PANOA.

MnO₂ doped PANOA is synthesized facilely via a one-step process,in which the doping of MnO₂,the reduction product of KMnO₄,and the chemical oxidative copolymerization of aniline and o-aminophenol is achieved simultaneously. Due to the introduction of MnO₂,the as-prepared PANOA-MnO₂ exhibits better capacitive behaviors compared to PANOA,especially at high potential scan rate and high charge/discharge current density. Also,the cyclic performance of the PANOA-MnO₂ electrode is superior to PANOA.

The authors are grateful to the financial supports of National Natural Science Foundation of China (Nos. 21275023,21173183),Natural Science Foundation of Jiangsu Province (No. BK2012593),Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology (No. BM2012110) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).