Controllable synthesis of MnO2/polyaniline nanocomposite and its electrochemical capacitive property
© Meng et al.; licensee Springer. 2013
Received: 5 February 2013
Accepted: 4 March 2013
Published: 17 April 2013
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.
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].
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.
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.
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
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.
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 . 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.
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: C CP = 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.
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.
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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