Journal of the Korean Physical Society, Vol. 66, No. 12, June 2015, pp. 1901∼1907

Synthesis and Supercapacitor Characteristics of Hydrothermally-deposited MnO2 Films and Chemically Co-deposited MnO2 -polyaniline Films on Stainless-steel Substrates Patin Tagsin Materials Science and Nanotechnology Program, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand

Pawinee Klangtakai, Viyada Harnchana, Samuk Pimanpang∗ and Vittaya Amornkitbamrung Materials Science and Nanotechnology Program, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand, Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand, Nanotec-KKU Center of Excellence on Advanced Nanomaterials for Energy Production and Storage, Khon Kaen 40002, Thailand, and The Integrated Nanotechnology Research Center, Khon Kaen University, Khon Kaen 40002, Thailand (Received 29 December 2014, in final form 8 April 2015) MnO2 films were hydrothermally grown directly onto stainless-steel substrates and used as supercapacitor electrodes. MnO2 with a cube-like structure was observed by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The measured sizes of the cubes were in the range of 270 − 820 nm. The specific capacitance (SC) of the 5-hour hydrothermallygrown MnO2 films was ∼20.2 F/g, which was slightly higher than that of the 3-hour films (∼13.8 F/g). The low values of the SC of the hydrothermally-deposited MnO2 films are attributed to their high series resistances of 1.73 − 2.06 Ω measured by using electrochemical impedance spectroscopy. However, the specific capacitance was greatly increased, up to ∼226 F/g, after a polyaniline polymer had been added into the MnO2 hydrothermal reaction, there by producing a composite of the MnO2 and the polyaniline polymer. This SC improvement was attributed to presence of two active materials (polyaniline and MnO2 ) and the reduction of the electrode series resistance to 0.93 Ω. PACS numbers: 81.07.-b, 82.47.Uv Keywords: Supercapacitor, Hydrothermal deposition, MnO2 , Polyaniline, Composited MnO2 -polyaniline DOI: 10.3938/jkps.66.1901

I. INTRODUCTION Portable devices are widely used and are growing in number. These portable devices require fast charging and high-energy density storage systems, which is now a driving force for the development of new energy storage systems. Supercapacitors or electrochemical capacitors are promising energy storage devices that can fulfill portable device needs. In addition, supercapacitors show potential functions in many areas of applications including hybrid and fully-electrical automobiles and industrial power and energy management. With such broad potential, materials with a high specific capacitance have been continuously developed. The specific capacitance (SC) strongly depends on the material’s properties, including the film’s morphology, crystal structure, particle ∗ E-mail:

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size, and electrical conductivity. Many materials, including RuO2 [1, 2], NiO [3], MnO2 [4–8], conducting polymers [9–11], carbon nanotubes [12] and graphene [13, 14], have been employed for manufacturing supercapacitor electrodes. Among these materials, MnO2 is one of the most promising substances because of its low cost, abundance, environmental friendliness and high SC of ∼1,300 F/g (theoretically predicted [4]). Additionally, MnO2 has a low charge potential, which could result in its having a high energy density compared to other transition-metal oxides [4]. Although, a MnO2 electrode has a high theoreticallypredicted specific capacitance, the reported values are much lower in range of 282 − 626 F/g [5–8]. The major factor contributing to the low SC values is the poor electrical conductivity of MnO2 (10−5 − 10−6 S/cm), which could diminish the electron’s transport rate and the utilization of the inside MnO2 material. A MnO2 thickness of approximately hundred nanometers has been observe

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to interact well with the electrolyte; i.e., an active material at a thickness exceeding hundred nanometers cannot store charge. Thus, for the best use of this active material, a MnO2 nanostructures should be employed to fabricate electrodes. MnO2 nanostructures can be prepared via various methods, such as electrochemical deposition, chemical deposition or hydrothermal deposition. Hydrothermal deposition is a simple technique that uses a moderate temperature and high pressure for film deposition. Many studies reported that hydrothermal treatment could be used to prepare MnO2 nanostructures. High specific capacitances have been reported to have been achieved from the combination of hydrothermallydeposited MnO2 nanoparticles with conductive materials (conducting metals, carbon black, conductive polymers) [5–8, 15]. However, not many studies report the direct growth of MnO2 films on conductive substrates by using hydrothermal processes and the electrochemical characteristics of those hydrothermal-prepared electrodes. In this work, cube-like MnO2 nanostructures were directly deposited onto stainless-steel substrates via a hydrothermal process. A significant increase in the SC up to 226 F/g was achieved by hybridizing MnO2 nanoparticles with polyaniline. To explain the SC improvement, the film’s properties (crystallinity, particle size, film’s impedance) are compared to those for pure MnO2 and polymer films.

II. EXPERIMENTAL METHOD 1. Electrode Preparation

Four pieces of stainless-steel substrates (1.5 × 2 cm2 ) were placed at the bottom of a Teflon tube. Fifty (50) mL of a 1-mM manganese (II) acetate tetra hydrate (Mn(CH3 COO)2 ·4H2 O, Kanto Chemical Co., Japan) solution was poured into the Teflon tube. Then, the Teflon tube was placed inside a hydrothermal reactor (PARR, ACID DIGESTION VESSEL 125 mL). The hydrothermal reactor was heated to 180 ◦ C for 3 or 5 h. The coated films were annealed at 500 ◦ C for 2 h under an ambient atmosphere, and MnO2 electrodes were obtained. Four pieces of stainless steel (1.5 × 2 cm2 ) were placed at the bottom of the beaker. The aniline solution, 80 μl of aniline monomer (Panreac) in 20 ml of 0.5-M sulfuric acid (H2 SO4 ), was poured into the beaker. Then, 20 ml of 0.04-M ammonium persulfate solution, (NH4 )2 S2 O8 , was added to the aniline solution. This resulting solution was refrigerated (∼8 ◦ C) for 1 h to accomplish polyaniline polymerization. Polymer films were rinsed with distilled water to remove adsorbed monomers and unbounded particles. MnO2 powders from the 5-h hydrothermal suspension described above were collected and rinsed. Powders were annealed at 500 ◦ C for 2 h under an ambient atmosphere,

and MnO2 powders were obtained. Then, 0.1 g of MnO2 powder was added to 20 ml of the aniline solution (same concentration as in the previous paragraph). Four pieces of stainless steel (1.5 × 2 cm2 ) were immersed into this solution. Then, 20 ml of 0.04-M (NH4 )2 S2 O8 solution was added to the MnO2 -aniline solution. This new solution was refrigerated (∼8 ◦ C) for 1 h for film deposition onto the stainless-steel substrates. Films were rinsed with distilled water to remove adsorbed monomers and unbounded particles.

2. Characterizations

Scanning electron microscopy (SEM, JEOL, JSM7001F) was used to analyze the morphology of the resulting films. Their structures were characterized by using X-ray diffraction (XRD-6100, 2 kW, SHIMADZU, Japan). Particle structures and crystallinities were also characterized by using transmission electron microscopy (TEM) and selected area electron diffraction (SAED) (TECNAI G2 , The Netherlands), respectively. Cyclic voltammetry (CV, Gamry Instrument Reference 3000, USA) was conducted in a three-compartment cell by using Ag/AgCl as the reference electrode and Pt plating as the counter electrode in a 1-M Na2 SO4 electrolyte. Symmetric-electrode capacitors were assembled by fabricating electrodes with active areas of 1 × 1 cm2 . Parafilm was used as a spacer. The 1-M Na2 SO4 electrolyte filled the open space between two electrodes. Charge-discharge measurements (CD, Gamry Instrument Reference 3000, USA) were run on these symmetric-electrode cells at a constant current of 0.02 mA to estimate the electrode’s capacitance. Electrochemical impedance spectroscopy (EIS) was also done on these symmetric-electrode capacitors for measuring the cell’s impedance over a frequency range of 0.01 Hz to 100 kHz under a 10-mV AC signal. The weights of active materials were evaluated by coating a film on a small stainless-steel substrate (0.3 × 2 cm2 ) under the same conditions as in subsection 1 of the sections II and were measured with a seven-digit balance (Sartorius Ultramicro, Germany).

III. RESULTS AND DISCUSSION 1. Electrode Morphology

Figures 1(a) and (b) show SEM images of hydrothermally-deposited MnO2 films with 3-h and-5 h deposition durations, respectively. Cube-like particles were found on the surfaces of the stainless-steel substrates under both conditions. The average cube size of the particles grown for 5 h (∼655 nm) was slightly larger than that of those grown for 3 h (∼436 nm). This

Synthesis and Supercapacitor Characteristics of Hydrothermally-deposited MnO2 · · · – Patin Tagsin et al.

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Fig. 1. SEM images of (a) 3h-MnO2 , (b) 5h-MnO2 , (c) MnO2 -polyaniline and (d) polyaniline films.

Fig. 3. (Color online) TEM image of (a) 3h-MnO2 , (b) 5h-MnO2 , (c) composited MnO2 -polyaniline and (d) polyaniline films. SAED patterns of (e) 3h-MnO2 , (f) 5h-MnO2 , (g) MnO2 -polyaniline and (h) polyaniline films. EDX spectra of (i) 3h-MnO2 , (j) 5h-MnO2 , (k) MnO2 -polyaniline and (l) polyaniline films.

Fig. 2. (Color online) XRD patterns of stainless steel, 3h-MnO2 , 5h-MnO2 , MnO2 powder, polyaniline and MnO2 polyaniline.

could be attributed to the longer growth period of the 5-h hydrothermally-deposited MnO2 (5h-MnO2 ) film. Various MnO2 nanostructures, nanorods, nanosheets, nanoflakes, nanowires and flower like forms, were also reported with the use of the hydrothermal technique [16–20]. Crystallites of MnO2 cubes were analyzed by using X-ray diffraction (XRD). The diffraction spectra are shown in Fig. 2. The XRD peaks at 35◦ , 40◦ and 55.46◦ of 5h-MnO2 were assigned to the (310), (020) and (221) planes of the orthorhombic MnO2 (JCPDF 82-2169). This implies the formation of MnO2 layers on the stainless-steel surfaces. However, MnO2 peaks were not clearly detected in the 3h-grown MnO2 (3h-MnO2 ) film. This could be due to the small amount of MnO2 deposited on the substrate. The deposited particles were further examined by using TEM, and the resulting micrographs are shown in Fig. 3. Facet particles were

observed with particle sizes of 416 nm and 540 nm for the 3h-grown and 5h-grown particles, respectively. SAED patterns of the 3h-grown (Fig. 3(e)) and the 5hgrown (Fig. 3(f)) particles also suggest the orthorhombic MnO2 structure (JCPDF 82-2169). The difference in the visible diffraction planes in the two SAED patterns is the effect of the direction of orientation for the crystals or the zone axis. In the 3h-MnO2 particles, the reflection from the (002) planes lie along the [111] zone axis whereas in the 5h-MnO2 , the (200), (301) and (501) planes lie along the [002] zone axis. EDX spectra of the hydrothermally deposited MnO2 particles (Figs. 3(i) and (j)) showed that the main elements were Mn and O. Experimental results confirm the formation of MnO2 nanoparticles on the stainless-steel substrate through the hydrothermal process. MnO2 -polyaniline composite and pure polyaniline films were also prepared. Figure 1(c) displays the surface morphology of the composite MnO2 -polyaniline film. Agglomerated MnO2 particles were observed on the surface of the composite film. Unlike the pure polyaniline film (Fig. 1(d)) where small nanowires (∼180 nm) were

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The morphology difference can be ascribed to the capability of MnO2 particles to grow in all directions within the solution. However, in the case of the hydrothermal MnO2 film on the substrate, MnO2 particles could only grow in the direction of the surface. The sizes of the collected MnO2 powders after annealing in an Ar atmosphere (Fig. 4(a)) were ∼333 nm. This was slightly smaller than that of the composite MnO2 -polyaniline particles (∼428 nm), implying a polymer coating on the MnO2 particles. The EDX spectrum of the MnO2 polyaniline film, Fig. 3(k), contains strong Mn and S peaks whereas no Mn peak was found in the polyaniline film (Fig. 3(l)). This suggests successful compositing of the MnO2 and polyaniline film onto the stainless-steel surface.

2. Electrochemical Properties

Fig. 4. (Color online) (a) TEM image, (b) SAED patterns and (c) EDX spectra of hydrothermally MnO2 powders.

detected. The XRD spectra of the polymer and the composite films in Fig. 2 reveal no new peaks compared with the stainless-steel spectrum. This is attributed to the thin film layer coated on the substrate for both films, and the films did not fully cover the stainless-steel surface as seen in SEM images (Fig. 1). The TEM image of the composite MnO2 -polyaniline film in Fig. 3(c) shows a particle size of ∼428 nm, and its SAED pattern in Fig. 3(g) shows a spot pattern corresponding to orthorhombic MnO2 . Worth mentioning is that the structure of the hydrothermal MnO2 powders collected from the hydrothermal solution in Fig. 4 was not a cube-like structure as seen in the 3h-MnO2 and the 5h-MnO2 films.

To examine the storage characteristics of these films, we performed cyclic voltammetry (CV) and chargedischarge measurements. CV profiles of the 3h-MnO2 (Fig. 5(a)) and the 5h-MnO2 films (Fig. 5(b)) were rectangular-like at low scan rates (5 and 10 mV/s). As the scan rate was increased, the CV curves were found to deviate from a rectangular shape. This could result from the electrolyte diffusion resistance to diffuse into the film [21]. In the case of polymer-based films, the CV curves of the MnO2 -polyaniline (Fig. 5(c)) and the polyaniline (Fig. 5(d)) films also formed rectangular-like shapes at only 5 mV/s. The CV curves of the polymerbased films greatly deviated from rectangular shape at a 100-mV/s scan rate. This was likely constrained by the slow charge-storage rate of the polymer. To compare the film’s specific capacitances, we plotted the current density curves of all electrodes together in Fig. 6. We observed that the current density of the 3h-MnO2 film was close to that of the 5h-MnO2 film. This means that these two MnO2 films had comparable SC values. The SC values, estimated from the CV curves at a scan rate of 5 mV/s, were about 25.1 F/g and 19.0 F/g for the 3h-MnO2 and the 5h-MnO2 films, respectively. Interestingly, the current densities of the composite and the polymer films were pronouncedly larger than that of the pure MnO2 films, as seen in Fig. 6. This implies that the polymer-based films had SC values superior to those of the hydrothermal MnO2 films. The SC values were also determined by using charge-discharge (CD) measurements on the symmetricelectrode cell, CEelectrolyteCE, at a constant current (I) of 0.02 mA, and its value was estimated according to C=

2IΔt , mΔV

(1)

where Δt is the discharge time, ΔV is the discharge potential and m is the active material’s weight. The SC values of the 3h-MnO2 and the 5h-MnO2 films shown in

Synthesis and Supercapacitor Characteristics of Hydrothermally-deposited MnO2 · · · – Patin Tagsin et al.

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Fig. 5. (Color online) CV curves of (a) 3h-MnO2 , (b) 5h-MnO2 , (c) MnO2 -polyaniline and (d) polyaniline electrodes.

Fig. 6. (Color online) CV curves of 3h-MnO2 , 5h-MnO2 , MnO2 -polyaniline and polyaniline electrodes at a scan rate of 10 mV/s.

Fig. 7(a) were only 13.8 and 20.2 F/g, respectively. The specific capacitances of the composite MnO2 -polyaniline and the pure polyaniline films were greater than those of the 3h-MnO2 and the 5h-MnO2 films. The composite MnO2 -polyaniline film had the highest specific capacitance of ∼226 F/g at the 1st discharge, which agreed well

with the highest CV current density. The pure polyaniline film also produced a promising SC value of 191 F/g, which was close to the values of 62 − 195 F/g reported in the literature [9,10]. To clarify the factors causing the varying SC values among these electrodes, we conducted electrochemical impedance spectroscopy (EIS) measurements on the symmetric-electrode (CEelectrolyteCE) cell, and the impedance spectra are shown in Fig. 8(a). The MnO2 film had high series resistances (the intercept point on the x-axis) of 1.73 − 2.06 Ω. The series resistance of the MnO2 films is generated from the low electrical conductivity of MnO2 and/or the poor connection of MnO2 nanoparticles to the substrate. According to some previous work [4–8], published SC values of 282 − 626 F/g corresponded to series resistances of only 0.5 − 0.7 Ω. The low series resistances in those work were achieved by the addition of conductive materials (carbon) to the MnO2 particles, but in our case, no conductive material was added into the MnO2 films. On the other hand, the series resistances of the polymer and the composite MnO2 -polymer electrodes were very low (0.90 − 0.92 Ω) compared to those of the MnO2 films. This explains the elevation of the SC values for composite electrodes. Although the incorporation of MnO2 powders into a poly-

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Fig. 7. (Color online) (a) Specific capacitances of 3hMnO2 , 5h-MnO2 , MnO2 -polyaniline and polyaniline electrodes. (b) Charge-discharge curves at the 1st, 20th and 100th cycles of the composite MnO2 -polyaniline.

mer matrix can greatly increase the film’s specific capacitance, the SC values were not as stable as that of the hydrothermal MnO2 films, as observed in Fig. 7(a). The charge and the discharge times of the composite MnO2 polyaniline film, Fig. 7(b), significantly decreased with increasing number of charge-discharge cycles. After the 100th discharge cycles, the SC retention of the 3h-MnO2 , the 5h-MnO2 and the composite films were found to be 82.60%, 76.92% and ∼51.76%, respectively, but that of the polymer film was only 22.14%. The large degradation of the SC in the polymer-based films was likely due to polymer swelling, which minimizes the film’s contact with the substrate and the electron transport rate. Notice that the hybridizing of MnO2 nanoparticles with a polyaniline matrix can suppress the reduction of the capacitance. To deeply understand the reason for decreasing SC values, we conducted EIS measurements on the 100th CD cycle films. The EIS slope of the 100th CD cycle MnO2 -polyaniline film is smaller than that of the asprepared MnO2 -polyaniline film, as shown in Fig. 8(b). A reduction of the EIS slopes was also detected for the 3h-MnO2 , the 5h-MnO2 and the polyaniline films after

Fig. 8. (Color online) (a) Nyquist plots of 3h-MnO2 , 5h-MnO2 , MnO2 -polyaniline and polyaniline electrodes. (b) Nyquist plots of the as-prepared and the 100th CD cycle of the composite MnO2 -polyaniline. The inset in Fig. 8(b) represent the equivalent circuit of the electrode. Rs represents the series resistance, Rct represents the charge-transfer resistance, CPE represents the capacitance phase element and Ddif f represents the Warburg resistance.

100th cycles. The decreasing EIS slopes implies a deviation away from an ideal capacitor [14]. The EIS curves were fitted according to the equivalent-circuit in the inset of Fig. 8(b). The fitted result suggests that the main factor suppressing the SC value is a reduction in the charge-transfer resistance (Rct ) from 50.56 kΩ to 8.56 kΩ for the as-prepapred and the 100th CD cycle MnO2 polyaniline films, respectively. The small Rct would induce a large charge leakage during the charge-discharge operation, resulting in a low EIS slope. Recently, Yu et al. [22] observed that by coating a conductive polymer layer on the surfaces of MnO2 fibers high specific capacitance and long charge/discharge cycles could be achieved. This is due to polymer swelling at the surface while MnO2 film remained in contact with the substrate. Thus, if a thin polyaniline layer is chemically coated on the surfaces of hydrothermally-deposited MnO2 films,

Synthesis and Supercapacitor Characteristics of Hydrothermally-deposited MnO2 · · · – Patin Tagsin et al.

good film conductivity, good specific capacitance and high charge/discharge cycle stability should be attained. This hypothesis is under test, and the results will be reported in the future.

IV. CONCLUSION Crystalline MnO2 films were directly coated on stainless-steel substrates by using a hydrothermal method. MnO2 films deposited for 3 and 5 h showed specific capacitances of 13.8 F/g and 20.2 F/g, respectively. The specific capacitance was greatly increased to 226 F/g after hydrothermally-grown MnO2 nanoparticles had been incorporated into a polyaniline matrix. The increasing SC of the composite film was attributed to the presence of two pseudocapacitive materials and the reduction of the series resistance to 0.93 Ω. Additionally, the SC retention of the composite MnO2 -polyaniline film (51.76%) was higher than that of the pure polymer film (22.14%).

ACKNOWLEDGMENTS This work was supported by the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission, through the Advanced Functional Materials Cluster of Khon Kaen University, by NanotecKKU Excellence Center on Advanced Nanomaterials for Energy Production and Storage, by the Integrated Nanotechnology Research Center, Khon Kaen University, Thailand, by Toray Science Foundation (TTSF), and by the Thailand Research Fund (Contract No. TRG5780142).

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