Open Access Nano Express

Controllable synthesis of MnO2/polyaniline nanocomposite and its electrochemical capacitive property

Fanhui Meng12, Xiuling Yan13, Ye Zhu2 and Pengchao Si1*

Author Affiliations

1 Key Laboratory for Liquid–solid Structural Evolution and Processing of Materials, Ministry of Education, School of Materials Science and Engineering, Shandong University, Jinan 250061, People's Republic of China

2 School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, People's Republic of China

3 School of Chemistry and Bioscience, Ili Normal University, Xinjiang 835000, People's Republic of China

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Nanoscale Research Letters 2013, 8:179  doi:10.1186/1556-276X-8-179


The electronic version of this article is the complete one and can be found online at: http://www.nanoscalereslett.com/content/8/1/179


Received:5 February 2013
Accepted:4 March 2013
Published:17 April 2013

© 2013 Meng et al.; licensee Springer.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Polyaniline (PANI) and MnO2/PANI composites are simply fabricated by one-step interfacial polymerization. The morphologies and components of MnO2/PANI composites are modulated by changing the pH of the solution. Formation procedure and capacitive property of the products are investigated by XRD, FTIR, TEM, and electrochemical techniques. We demonstrate that MnO2 as an intermedia material plays a key role in the formation of sample structures. The MnO2/PANI composites exhibit good cycling stability as well as a high capacitance close to 207 F g−1. Samples fabricated with the facile one-step method are also expected to be adopted in other field such as catalysis, lithium ion battery, and biosensor.

Keywords:
Interfacial synthesis; Polyaniline; MnO2; Composite; Electrochemical capacitor

Background

As a sort of classic conducting materials, polyaniline (PANI) possesses good conductivity with specific organic characters that metal cannot match, which has attracted a lot of attentions for its wide applications in capacitance, sensors, ultrafast nonvolatile memory devices, and chemical catalysis [1-5]. MnO2 has been widely studied as a promising environmentally benign transition metal oxide for sensor, catalyst, lithium battery, and electrochemical capacitor [6-9]. In the quest for superior performance, aniline has been polymerized in combining with other materials to obtain a promising performance, in which PANI/graphene, PANI/carbon nanotube, PANI/Au electrode, and others have been successfully synthesized [1,2,10]. Meanwhile, hybrid composites consisted of MnO2, and other materials have also been fabricated to improve their behaviors in battery or supercapacitor [11-15]. In particular, the structures of MnO2/PANI have been constructed with different methods, and the synergistic effect of MnO2 and PANI has been demonstrated in supercapacitor and catalysis toward H2O2 oxidation or organic dyes [16-20]. Considering the catalyst-size dependent reaction selectivity and agglomeration involved in nanostructures and specific nanoscale architecture, the big challenge for high-efficiency and outstanding features is still the controllable synthesis of uniform structures [21,22].

With respect to PANI synthesis, chemical and physical methods have been recommended [5,23-31], in which the facile interfacial polymerization is a highly flexible approach without any templates [3,23,24]. The oxidant and reducing agent are separated in the aqueous and organic solutions, while the redox reaction can occur at the interface. As far as the products are removed into the bulk solution, new polymerization can happen at the interface while secondary growth of PANI are prevented, in which both the shape and size of the products can be controlled. In addition, synthesis of MnO2 via reducing the compounds containing MnO4 and MnO42− has been extensively used due to its simpleness and low cost. During that procedure, the pH of KMnO4 solution plays a critical role in the intermediate oxidation state and finally the products:

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At a high pH, MnO2 is the main product while Mn2+ is the final product at a low pH.

Recently, due to the depleting of fossil fuels and the severe environmental problems caused by burning fossil fuels, supercapacitors with large-power density and long-time cycling have attracted attentions of many researchers [25,26]. As low-cost and easily obtained materials, the capacitive properties of MnO2, PANI, and MnO2/PANI composites have been widely studied [27-29].

In this work, we utilize the above mechanism to deliberately synthesize a series of MnO2/PANI composites with controllable morphology and uniform size by means of the interfacial polymerization and adjusting the pH of solutions. In the synthesis, monomer aniline and KMnO4 are used as reducing agent in organic solution and oxidant in aqueous solution, respectively. PANI and MnO2/PANI are prone to diffusing into the aqueous phase because they are hydrophilic in the doped salt forms [3,23,24]. In the composite, PANI is expected to allow uniform MnO2 particle dispersion and convenient electron transfer. In the present study, the formation mechanism and the electrochemical capacitive performance of the composites have been investigated.

Methods

Preparation of MnO2/PANI

Aniline was firstly distilled under reduced pressure. Then, 0.6 mL of aniline was dissolved in benzene (10 mL) solution, while 0.2 g of KMnO4 was dissolved in the solution (20 mL) with 1 M ClO4 as the doping anion (we used HClO4 as the source of ClO4). The organic solution was added into aqueous solutions slowly, and the mixture was kept overnight until the reactions conducted completely. The products were then washed with ultrapure water and centrifuged twice to remove residual benzene and KMnO4. Finally, the products were dried in the air for the latter use.

Preparation of the electrode

The composites were mixed with acetylene black (15 wt.%) and dispersed in 0.5 mL of anhydrous ethanol solution by sonication for 5 min. The mixtures were then cast onto a polished glassy carbon electrode and fasten with 2 μL of nafion ethanol solution (1% V/V). The electrodes were dried in the air for latter testing.

Characterization

The morphology of the sample was characterized by scanning electron microscopy (SEM, JSM-6700 F, JEOL Ltd., Akishima-shi, Japan) at an accelerating voltage of 10 kV. Transmission electron microscope (TEM) micrographs are taken with a JEOL2100 TEM (JEOL Ltd., Akishima-shi, Japan) operating at 200 kV. X-ray diffraction (XRD) patterns were collected using X-ray powder diffraction (XRD, Bruker D8 Advance X-ray diffractometer, Bruker AXS, Inc., Madison, WI, USA; Cu Kα radiation λ=1.5418 Å) at a scan rate of 0.02 s−1. Fourier transform infrared spectroscopy (FTIR) analyses were carried out using a Vertex 70 FTIR spectrophotometer (Bruker AXS, Inc., Madison, WI, USA).

A CHI 760C electrochemical workstation (CHI Instruments, Austin, TX, USA) was used to collect electrochemical data. All electrochemical experiments were conducted in a three-electrode cell, in which a 1.5×1.5 cm2 Pt plate was used as the counter electrode and a saturated calomel electrode was selected as the reference electrode.

Results and discussion

The schematic of MnO2/PANI fabrication procedure is shown in Figure 1. The reaction commences at the interface of the two solutions immediately as the aniline solution is carefully spread onto the aqueous solution of KMnO4. The interfacial polymerization does not terminate until KMnO4 or aniline is consumed completely. The products diffuse into the aqueous solution spontaneously due to the doping procedure of the polymers and hydrophilic property of hydrate MnO2. The color of the products in different solutions (a to e: 1, 0.5, 0.2, 0.1, and 0 M HClO4, respectively, as shown in the inset of Figure 1) turns from green to brown. This color evolvement is attributed to the different components of composites accompanying with the change of PANI-doping degree. The SEM and TEM images, FTIR spectra, and XRD patterns were employed to investigate the components and the formation of the products.

thumbnailFigure 1. The schematic of the synthesis procedure and the morphologies of MnO2/PANI composites at different HClO4concentrations.

PANI nanofibers form at the interface between the organic and 1 MHClO4 aqueous solution. As observed in the SEM image (Figure 2), the diameter and length of the nanofibers are around 100 to 200 nm and over 1 μm, respectively. Additionally, it reveals that the nanofibers are twisted and networks are formed by random interconnection, which agrees with the previous reports [3,23,24]. To indicate the evolvement of the samples' morphologies with the changing of acid concentrations, the TEM images of MnO2/PANI fabricated at different acid concentrations are collected in Figure 3. As shown in Figure 3A, PANI nanowires synthesized in 1 M HClO4 solution is consistent with the SEM result in Figure 2. When the interfacial polymerization is carried out using 0.5 M HClO4 (Figure 3B), the conventional nanowire almost disappears. On the contrary, interconnected agglomerating chains appear. In addition, a number of hollow spheres can be observed. Interestingly, when the acid concentration decreases to 0.2 M (Figure 3C), a larger portion of hollow spheres is observed. However, the portion of hollow spheres is decreasing with the decrease of the acid concentrations in the range of 0.1 and 0 M HClO4 (shown in Figure 3D,E,F). In this way, we can modulate the sample structures easily by adjusting the pH of the aqueous solution.

thumbnailFigure 2. SEM images of PANI synthesized by interfacial polymerization at 1 M HClO4.

thumbnailFigure 3. TEM images of MnO2/PANI composites synthesized at different acid concentrations. (A) 1, (B) 0.5, (C) 0.2, (D) 0.1, (E) 0.05, and (F) 0 M HClO4.

An explanation in the procedure of composite fabrication is proposed in our work. Firstly, aniline monomers are polymerized only at the interface of the organic and aqueous phases, so that hydrophilic nanofibers can be separated from the interface and diffuse into the aqueous solution, which prevent the secondary growth and provide space for new nanofiber growing. Additionally, MnO2, as an oxidative regent for PANI polymerization, is used as sacrificial materials in forming various PANI structures [31,32]. According to the change of the morphologies (nanofibers, hollow spheres, and solid particles), it is reasonable to assume that the appearance of the intermediate of MnO2 is a critical role in the formation of hollow spheres. As illustrated in Equations 1 and 2, for the low-acid concentration (0.5, 0.2, and 0.1 M), there is not enough H+ at the interface to resolve the intermediate of MnO2 because of the rapid H+ consumption in the reaction (Equation 2). In the meantime, the resolution of MnO2 restarts while the composite removes from the interface. The consequential reducing reaction of MnO2 follows Equation 3 [33]:

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In the acid solution of lower concentrations (0.1 and 0 M HClO4), MnO2 appears both at the interface and the bulk solution, which caused a little portion of or no hollow spheres to obtain. In our study, it is thought that large amount of MnO2/PANI composites can be obtained at low-acid concentration, and the MnO2 nanoparticles are wrapped by PANI.

FTIR spectra (Figure 4) were measured to identify the component of the different samples. Among peaks assigned to PANI, the characteristic peaks around 1,580 and 1,497 cm−1 relate to the stretching vibration of quinoid (−N=(C6H4)=N-) ring and benzenoid (−(C6H4)-) ring, respectively. Another main band at 1,303 cm−1 can be assigned to the stretching of C-N in -NH-(C6H4)-NH-. The bands appeared at 1,143 cm−1 and 829 cm−1 which correspond to the stretching of C-H in-plane and C-H out-of-plane bendings. In addition, the bands of N-H (PANI) and O-H (H2O) at 3,230 and 3,400 cm−1, respectively, are observed. As noticed, the band near 3,400 cm−1 (O-H) is becoming intense with the decrease of the acid concentration, which is attributed to the appearance of hydrate MnO2. The above conclusion is proved by the annealing experiments: the band at 3,400 cm−1 (O-H) of hydrate MnO2 vanished after 500°C heat treatment (Additional file 1: Figure S1). The band near 1,303 cm−1 is becoming weaker from curves g to a in Figure 4, which suggests that the doping degree of PANI is changing with the acid concentration. The characteristic bands of curves a, b, and c in Figure 4 shifted right compared with the others, which is ascribed to the effect of MnO2 on PANI. It demonstrates that some special interaction exists between MnO2 and PANI.

thumbnailFigure 4. FTIR spectra of the as-prepared samples. Curves a to g: MnO2/PANI fabricated in 0, 0.02, 0.05, 0.1, 0.2, 0.5, and 1 M HClO4, respectively.

Additional file 1: Figure S1. FTIR spectra of MnO2/PANI fabricated in 0.1 M NaOH, 0 HClO4, 0.02 M. Figure S2. FTIR spectra of polyaniline (curve a) and the composites after heat treatment (curves b to f): MnO2/PANI fabricated in 0.1 M NaOH, and 0, 0.02, 0.05, and 0.1 M HClO4. Figure S3. CV curves of the composites before and after 100 cycles stability tests in 0.1 M HClO4 solution at 50 mV s−1, (A-D) samples fabricated in 1, 0.05, and 0.02 M HClO4, and 0.1 M NaOH and (E) MnO2 obtained by heating MnO2/PANI composite fabricated in 0.02 M HClO4.

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Due to the ordered and metallic-like property, conducting polymers possess particular crystallinity and orientation. As shown in the XRD patterns in Figure 5A, there are no identified peaks appeared for the products synthesized in low-acid concentrations (curves a to e: 0.1 M NaOH, and 0, 0.02, 0.05, and 0.1 M HClO4, respectively), which indicates the products are amorphous. For the products obtained at 0.2 (curve f), 0.5 (curve g), and 1 M HClO4 (curve h), two intense XRD peaks 2θ≈20 and 25° are observed corresponding to pure PANI according to previous literature [2]. All above results confirm that the crystallized PANI can be formed at higher acid concentrations in this work.

thumbnailFigure 5. XRD patterns of the samples. (A) The XRD patterns of the composites, curves a to h: MnO2/PANI fabricated in 0.1 M NaOH and 0, 0.02, 0.05, 0.1, 0.2, 0.5, and 1 M HClO4, respectively. (B) XRD pattern of the samples, curves a to d: annealed MnO2/PANI fabricated in 0, 0.02, 0.05, and 0.1 M HClO4, respectively.

To further analyze the components at different acid concentrations, the samples were treated at 500°C (at which MnOx is crystallizing and PANI will be burned). The products obtained at 1, 0.5, and 0.2 M HClO4 were burned out with no solids left, which indicates that there is no MnO2 generating at such acid concentrations. Contrary to higher acid concentration, the solid residue of the products obtained at 0.1, 0.05, 0.02, and 0 M HClO4 turned black. The FTIR spectra of the heat-treated composites fabricated in 0.1, 0.05, 0.02, and 0 M HClO4 were measured (Additional file 1: Figure S2). The characteristic FTIR spectra bands of PANI vanish after heat treatment, which confirms that PANI has been pyrolyzed after heat treatment.

The XRD patterns of the samples after heat treatment are shown in Figure 5B. The XRD patterns of the composite obtained in 0 (curve a) and 0.02 M HClO4 (curve b) can be indexed to α-MnO2 crystal structures [34]. Meanwhile, different XRD peaks are observed in Figure 5B (curves c and d), indicating the heat-treated product obtained in 0.1 M HClO4 is Mn2O3 and the heat-treated product obtained in 0.05 M HClO4 are MnO2 and Mn2O3. The results show that for as-prepared samples, Mn2O3 phase is increasing with acid concentration. It is reported that the phase of manganese oxides is changing with temperature, and MnO2 may transform to suboxide Mn2O3 at 500°C to 900°C [33,35-38]. The reductive matters such as CH3OH, CH4, and CO were studied as reductions for the phase transforming of MnO2 to Mn2O3, and the mechanism was also suggested [34,39]. Therefore, we assume that the reductive matters generated during PANI decomposition procedure assists the transformation of MnO2 to Mn2O3. Additionally, the aggravating degree of phase transforming of the heat-treated samples could be attributed to the increasing proportion of PANI in the composites. All the above results indicate that the MnO2 generated in the polymerization of PANI process at low-acid concentration has a great effect on the formation of the hollow structure at higher acid concentrations as an intermediate.

In this work, the electrochemical performance of the composite was evaluated. The capacitance of MnO2 is generated by the charge transferring among multivalent Mn element (Mn2+, Mn3+, Mn4+, and Mn6+) [35], while PANI endures doping/dedoping companying with the redox process of PANI:

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Cyclic voltammetry (CV) curves of the composites are shown in Figure 6A. CV curves of as-prepared PANI nanofibers/MnO2 crystallines are comparable with pure PANI and MnO2, respectively. The rectangle-like shape of CV curve suggests that MnO2/PANI fabricated in 0.02 M HClO4 has an ideal capacitive characterization. Additionally, the rectangle-like shape potential region of MnO2/PANI (curve c) is relatively larger compared with that of the crystallized MnO2 (curve e) and PANI (curve a). The capacitance CCP can be estimated according to the equation: CCP = (Qa + Qc)/(2 × ΔV), where Qa, Qc, and ΔV are indicative of the anodic and cathodic charges of CV and the potential region of CV, respectively. The capacitances of the samples in curves a to e are 80, 45, 207, 143, and 46 F g-1, respectively. The capacitance of MnO2/PANI (curve c) is larger than that of PANI (curve a) and MnO2 (curve e). The extended ideal capacitive potential region and larger capacitance of MnO2/PANI composite are possibly due to the synergistic effect between the core of MnO2 and the shell of PANI [32,35,40].

thumbnailFigure 6. CV and charge–discharge curves of the composites. (A) CV curves of the as-prepared samples in 0.1 M HClO4solution at 50 mV s−1, curves a to d: MnO2/PANI fabricated in 1, 0.05, and 0.02 M HClO4, and 0.1 M NaOH, respectively. Curve e: 500°C-treated MnO2/PANI fabricated in 0.02 M HClO4. (B) Charge–discharge curves of the MnO2/PANI composite in 0.1 M HClO4 solution at different current densities. (C) First 20 cycles of charge–discharge curves for the MnO2/PANI composite at the current density of 1 mA cm−2 (D) Dependence of capacitance of the MnO2/PANI composite on the charge–discharge cycles at the current density of 1 mA cm−2.

The charge–discharge curves of MnO2/PANI fabricated in 0.02 M HClO4 were measured at various current densities (shown in Figure 6B). The E-t plots show symmetry, which indicate the reversible charge–discharge process of the MnO2/PANI composite. The specific capacitance of the sample can be calculated via the equation: CCP = i/|dE/dt|, where |dE/dt| is estimated from the slope of the discharging curves. The capacitance of the composite at 2, 1, 0.5, 0.3, and 0.2 mA cm−2 achieves 159, 161, 170, 174, and 168 F g−1, respectively. Additionally, the discrepancy of the largest composite capacitance values estimated from discharging and CV curves is lower than 20%, which suggests the high credibility of both techniques.

The stabilities of the samples were tested with 100 CV scan cycles (Additional file 1: Figure S3). After 100 cycles, the CV curves of PANI change obviously and the capacitances decreased largely (Additional file 1: Figure S3 A, B). However, with the increase of MnO2, the CV curves change a little and even no capacitance decrease is observed (as shown in Additional file 1: Figure S3 C,D,E). Compared with PANI samples obtained at higher acid concentration, MnO2/PANI nanocomposites possess noticeable capacitive stability. To investigate the long-term stability of as-prepared MnO2/PANI nanocomposites, the charge–discharge test of 1,000 cycles was conducted at 1 mA cm−2 in 0.1 M HClO4. As shown in Figure 6C (first 20 cycles are shown for clearly observation), the E-t plots are symmetric in shape and have almost no change during the long-term test. From Figure 6D, it can be seen that the discrepancy of capacitance of MnO2/PANI during 2,000-cycle test is lower than 5%, and there is no evident capacitance decrease after 1,000 cycles. The stability of the MnO2/PANI composite is thought due to the protection of the shield-surrounded PANI and uniform dispersion of MnO2 particles, whereby avoiding severe particles conglomeration involved in the charge–discharge process [35,36]. The facile synthesis and ideal electrochemical capacitive performance will probably give the composites a promising prospect in the application of supercapacitors.

Conclusions

A series of samples including MnO2/PANI composites and PANI nanofibers were successfully synthesized by the facile interfacial polymerization. Controllable and uniform synthesis of the composites has been realized by simply adjusting the pH of the solution. The effect of acid concentration and the related mechanism of the formation of the products are investigated. We demonstrate that the intermediate of MnO2 plays a key role in forming the hollow structures of PANI. The capacitance of the composite achieves 207 F g−1, and the results suggest that the MnO2/PANI composites show superior performance over pure PANI or MnO2.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

FM carried out the total experiment and wrote the manuscript. XY participated in the detection of the SEM and TEM. YZ participated in the data analysis. PS participated in the design of the experiment and performed the data analysis. All authors read and approved the final manuscript.

Acknowledgements

This work was supported by the National Basic Research Program of China (2012CB932800) and the National Science Foundation of China (51171092, 20906045, 90923011). The authors also thank the Shandong University for their financial support (nos.31370056431211, 31370070614018, and 31370056431211).

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