Microwave solvothermal synthesis of mixed pine tree

J Electroceram DOI 10.1007/s10832-015-0002-1

Microwave solvothermal synthesis of mixed pine tree seed-like/disc-shaped microstructures of MnOx (x = 4/3 and 1) with high specific capacitance for electrochemical capacitors Jedsada Sodtipinta 1 & Hyun-Kyung Kim 3 & Suk-Woo Lee 3 & Siwaporn Meejoo Smith 1,2 & Pasit Pakawatpanurut 1,2 & Kwang-Bum Kim 3

Received: 19 February 2015 / Accepted: 19 October 2015 # Springer Science+Business Media New York 2015

Abstract The mixed pine tree seed-like/disc-shaped microstructures of MnOx (x = 4/3 and 1) were synthesized using a simple microwave-assisted solvothermal synthesis and heat treatment under N2 atmosphere (heating rate 5 °C/min), without the need for carbon composite. This new mixed morphology showed high specific capacitance, as measured by the cyclic voltammetry (182 F g−1 at 1 mV s−1) and by the constant galvanostatic charge-discharge cycling (195 F g−1 at 0.1 A g−1). During the cycling process the specific capacitance of the material increased, which was caused by the partial electrochemical oxidation of the major phase Mn3O4 (x = 4/3) to MnO2 in a neutral electrolyte of 1 M Na2SO4. According to the galvanostatic charge-discharge cycling tests at 1 A g−1, this material also showed good capacitance retention for over 1000 cycles. MnOx synthesized herein can thus be a good candidate for the electrode material for electrochemical capacitors.

Keywords Manganese oxides . Microwave-assisted solvothermal synthesis . Electrochemical capacitors * Pasit Pakawatpanurut [email protected] * Kwang-Bum Kim [email protected] 1

Department of Chemistry, Faculty of Science, Mahidol University, 272 Rama 6 Road, Ratchathewi, Bangkok 10400, Thailand

2

Center of Sustainable Energy and Green Materials and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Rama 6 Road, Ratchathewi, Bangkok 10400, Thailand

3

Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Seodaemoon-gu, Seoul 120-749, Republic of Korea

1 Introduction Electrochemical capacitors (ECs), or supercapacitors, represent an indispensible technology for energy storage devices for various applications, such as portable electronic devices, power tools, and electrical vehicles [1, 2]. Practical advantages of ECs include high power performance, long life cycle, and fast charge-discharge process. The energy storage mechanisms in ECs are generally based on charge accumulation process for electrical double layer capacitors (EDLCs, e.g., carbonaceous materials) and pseudocapacitance mechanism for pseudocapacitors (e.g., oxides of manganese and ruthenium, as well as other oxides and nitrides and carbides) [3, 4]. EDLCs, however, have low energy density. Therefore, pseudocapacitive materials have attracted considerable attention because of their fast and reversible redox reactions, which result in energy density 10–100 orders of magnitude higher than that of EDLCs [1, 5]. Among many reported metal oxide materials, manganese oxides have been extensively investigated for both ECs and Li-ion batteries due to their relatively high specific capacitance, low cost, high natural abundance, environmental friendliness, and high robustness toward aqueous solution processes [6]. However, manganese oxides have rarely been used as a lone capacitive material because of some unsolved drawbacks that include its low electrical conductivity (10−7 - 10−8 S cm−1), poor rate capability at high current density, and its tendency to aggregate during the preparation [7]. Thus, various composites of MnOx with other materials, particularly reduced graphene oxide (rGO), have often been the systems of choice in recent years. Table 1 and Table 2 give overviews of the electrochemical performances for a number of the reported Mn3O4 and Mn3O4/rGO composites, respectively. In this work, we demonstrated that it is possible to prepare MnOx powder (with x = 4/3, or Mn3O4, as a major phase and

hydrothermal

Mn3O4 (powder)

[6] [9] [10] [11]

– – – –

Stainless steel foil stainless steel granule Ni-foam

solvothermal

microwave solvothermal Ti-foil

hydrothermal

chemical reactions

Ni-foam

hydrothermal

Mn3O4/rGO (powder) Mn3O4/rGO (powder) Mn3O4 nanocubes/rGO (powder) Mn3O4/rGO (powder) MnOx (powder) pine tree seed-like/disc-shaped microstructure 85:10:5, 2 mg cm−2 70:20:10, 75:20:5, 3–5 mg cm−2 80:15:5, 3 mg cm−2 80:10:10, 1–2 mg cm−2

Electrochemical test

1 M NaSO4 galvanostatic charge/ discharge (0.5 A g−1) 1 M NaSO4 galvanostatic charge/ discharge (0.1 A g−1) 1 M NaSO4 galvanostatic charge/ discharge (0.5 A g−1) 1 M NaSO4 galvanostatic charge/ discharge (0.1 A g−1) 1 M NaSO4 galvanostatic charge/ discharge (0.1 A g−1) CV (scan rate 1 mV s−1)

Electrolyte Current collector Composition of electrode solution (ratio by weight of active material: conducting carbon: binder, and mass loading)

Synthesis method

99 % (after 500 cycles at 5 A g−1) -

> 100 % (after 1000 This work cycles at 1 A g−1)

0 to 0.8

194.7 182.3

−0.2 to 0.8 171

[10]

[13]

[12] −0.9 to 0.0 131

−0.1 to 0.8 147

[8]

100 % (after 10,000 cycles at 5 A g−1) -

Specific Capacitance capacitance retention (F g−1) −0.1 to 0.7 121

Potential window (V)

Ref.

> 100 % (after 1000 This work cycles at 1 A g−1) -

[8]

Ref.

–

Specific Capacitance capacitance retention (F g−1)

−0.1 to 0.7 25 (0.5 A g−1) −0.2 to 0.8 55.1 (0.5 A g−1) −0.1 to 0.8 70 (0.2 A g−1) −0.2 to 0.8 85 (0.1 A g−1) 0 to 0.85 95 (0.05 A g−1) 0 to 0.8 194.7 182.3

Potential window (V)

Summary of the supercapacitive performances for the MnOx (Mn3O4 as a major phase) electrode reported in this work and the Mn3O4/rGO composite in the literature

Sample

Table 2

chemical bath synthesis microwave MnOx (powder) pine tree seed-like/ solvothermal disc-shaped microstructure

hydrothermal

Mn3O4 (powder)

Mn3O4 (powder)

hydrothermal

Mn3O4 (powder)

Electrochemical test

galvanostatic charge/ discharge 1 M Na2SO4 galvanostatic charge/ discharge 1 M Na2SO4 galvanostatic charge/ discharge 1 M Na2SO4 galvanostatic charge/ discharge Saturated Na2SO4 galvanostatic charge/ discharge 1 M NaSO4 galvanostatic charge/ discharge (0.1 A g−1) CV (scan rate 1 mV s−1)

1 M Na2SO4

Electrolyte Composition of electrode solution (ratio by weight of active material: conducting carbon: binder, and mass loading)

85:10:5, 2 mg cm−2 stainless foil 80:10:10, 3–5 mg cm−2 stainless steel 80:10:10, mesh Ni foam 80:15:5, 3 mg cm−2 Ni foam 85:10:5, Ti foil 80:10:10, 1–2 mg cm−2

hydrothermal

Mn3O4 (powder) Ni foam

Synthesis method Current collector

Summary of the supercapacitive performances for the MnOx (Mn3O4 as a major phase) electrode reported in this work and the Mn3O4 electrodes in the literature

Sample

Table 1

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x = 1, or MnO, as a minor phase) as a lone material for electrochemical capacitors, which can potentially simplify the production process. The preparation of MnOx was based on the microwave solvothermal method [14–16] using ethylene glycol (EG) [16, 17] because it is a polar solvent and a strong reducing agent that can dissolve and reduce polar inorganic materials to form the mesoporous metal oxide sphere [18]. Without any use of structure-directing template, surfactant, or composite with carbonaceous material, the resulting MnOx exhibited a novel mixed morphology that consists of pine tree seed-like and disc-shaped microstructures with high specific capacitance and good cycling performance.

24 h. The working electrode with a 1 × 1 cm2 active area contained ~1–2 mg of dried slurry. The electrochemical properties of the working electrodes were investigated in a 1 M Na2SO4 aqueous solution at room temperature using a three-electrode electrochemical cell, with platinum plate and saturated calomel electrode (SCE) as a counter and a reference electrode, respectively. The chargedischarge tests and cyclic voltammetry (CV) were performed using a potentiostat/galvanostat (VMP2, Princeton Applied Research) at different scan rates (1, 2, 5, 10, 20, 50, and 100 mV s−1) and different current densities (0.1, 0.2, 0.5, 1, 2, 5, and 10 A g−1) in the potential range of 0.0–0.8 V. Additionally, the CV curves and the galvanostatic charge-discharge curves were recorded after CV aging of 200 cycles at a scan rate of 30 mV s−1.

2 Experimental MnO x was prepared as follows. First, 620 mg of Mn(CH3COO)2·4H2O (Aldrich, USA) was dissolved in 60 mL ethylene glycol (Aldrich, USA) under a stirring condition for 30 min at room temperature. Then, 60 mL of the solution mixture was loaded into a sealed Teflon vessel (100 mL) and was irradiated at 200 °C for 20 min at 400 W power using a MARS-5 microwave reactor (CEM Corporation, USA). The resulting product was centrifuged at 7500 rpm and washed three times in absolute ethanol (95 %, Samchun Chemicals, South Korea), then dried at 90 °C for 12 h in a vacuum oven. Finally, the assynthesize product was heated to 400 °C with heating rate 5 °C min−1 and was kept constant at this temperature for 3 h under N2 atmosphere. The structure and morphology of the as-prepared products was determined by powder X-ray diffractometer (XRD; Rigaku, CuKα, 40 kV, 20 mA) from 5° to 80°, Fourier transform infrared (FT-IR; Bruker RFS 100/S) spectrophotometer using KBr pellet in the range of 4000–400 cm−1, fieldemission scanning electron microscope (FE-SEM; JEOL JSM-7001F Oxford Instrument), and high resolution transmission electron microscope (HRTEM; Philips CM200, 200 kV). X-ray photoelectron spectrophotometer (XPS; Thermo Electron Corporation ESCA Lab 250, 15 kV, 150 W) was used for the chemical state identification and elemental analyses of the products. Nitrogen adsorption-desorption experiment was carried out at 77 K using Autosorb-c1 analyzer (Quantachrome Instruments). The surface area was estimated using the BET equation, while pore-size distributions were calculated using the BJH method. Each working electrode consisted of a mixture of 80 wt% active material (MnOx), 10 wt% carbon black, and 10 wt% polyvinylidene fluoride (PVDF) dissolved in N-methylpyrrolidone (NMP) as a binder. The slurry mixture was coated on a titanium foil (99.7 % purity, Aldrich), and the coated foil was then dried at 90 °C for

Fig. 1 (a) FT-IR spectrum and (b) X-ray diffraction pattern of MnOx

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3 Results and discussion From the FT-IR spectrum of the as-prepared product shown in Fig. 1(a), the broad peak at 3444 cm−1 could be assigned to an overlapping of the O-H stretching vibration (ν3 and ν1) of H2O molecules and that of OH–groups in the lattice. The band position of 1627 cm−1 was assigned to the O-H bending vibration (ν2), indicating the presence of hydroxyl groups in the asprepared sample. The peak at 1379 cm−1 was assigned to the CH stretching from ethylene glycol in the sample. The two sharp peaks at about 606 cm−1 and 488 cm−1 arise from the metaloxygen stretching vibrational modes (νMn-O and νMn-O-Mn), which represent the characteristic vibrational peaks for Mn3O4 [19]. The crystal structure of the as-synthesized MnOx was determined by XRD. From Fig. 1(b), the XRD pattern of the MnOx sample shows typical diffraction peaks that correspond to the major phase of hausmannite Mn3O4 (JCPDS card No. 24–0734) [19, 20] coexisting with the minor phase of MnO (JCPDS card No. 07–0230).

Fig. 2 (a) FE-SEM images, (b) EDS spectrum, (c) FE-SEM-EDS elemental mapping, and (d-g) HR-TEM images of MnOx

According to an FE-SEM image (Fig. 2(a)), as-synthesized MnOx consisted of two different morphologies: pine tree seedlike (red circle, Fig. 2(a), inset) and disc-shaped structures (yellow circle). The size of both pine tree seed-like and discshaped structures were found to be in the range of 3–5 μm. Elemental analyses were performed using energy dispersive X-ray spectroscopy (EDS). The EDS spectrum (Fig. 2(b)) and elemental mapping (Fig. 2(c)) indicated the presence of Mn and O constituents, uniformly dispersed in the powder sample. The HR-TEM image in Fig. 2(d) also revealed a mix of pine tree seed-like and disc-shaped morphologies (Fig. 2(d), inset). The microstructure of pine tree seed-like assemblies are likely constructed from the aggregation of the disc-shape microplates. Furthermore, the pine tree seed-like and disc-shaped microplates (the red dotted areas in Figs. 2(d) and 2(e)) are composed of nanoparticles with a particle size of ~3–5 nm (the dotted yellow areas in Figs. 2(e-g)). According to the previous study of the polyol synthesis of hausmannite Mn3O4 nanoparticles [21], the growth mechanism for the formation of

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MnOx microstructure in this work can be explained by the following sequence (see Fig. 3(a)): (i) acetate ions, ethylene glycol, and water molecules from the hydrate salts form a coordination sphere around the manganese ions, [MnII], and the partial oxidation of [MnII] to [MnIII] in the reducing polyol solution under air atmosphere can occur along with the solid phase precipitation of [MnIII]-O-O− peroxo end-on species; (ii) [MnII] precursors can react with [MnIII]-O-O−, producing μ-peroxo ligand bridging dinuclear complexes of [MnIII]-OO-[MnIII]; (iii) the homolytic cleavage of the dinuclear complexes ([MnIII]-O-O-[MnIII]) can take place via the protonation from water or polyol, generating the monomeric [MnIII(OH)] species; and (iv) because of the presence of water in the coordination sphere of the manganese species, the olation/oxolation reactions can happen between [MnIII(OH)] species and [MnII(OH)2] or [MnIII(OH)] species, resulting in an agglomerate manganese cluster. Different morphologies of the MnOx particles were observed depending on the degree of agglomeration of the forming microstructured particles; high agglomeration led to the pine tree seed-like morphology, while lower agglomeration led to well-separated disc-shaped particles (see Fig. 3(b)). The XPS survey spectrum (0–1400 eV) for the MnOx sample (Fig. 4(a)) shows peaks that can be attributed to Mn 2p, Mn 3 s, O 1 s, and C 1 s photoelectrons. The C

Fig. 3 (a) Schematic diagram of the proposed mechanism for the microstructural MnOx pine tree seed-like/disc-shaped microstructure and (b) simplified illustration of the formation process

1 s peak is from the carbon tape used in the XPS measurements. In the region of Mn 2p, the binding energies of Mn 2p3/2 and Mn 2p1/2 spin orbit peaks were centered at 641.6 and 653.4 eV, respectively, which give a larger energy separation (ΔE = 653.4–641.6 = 11.8 eV) compared to those of manganese oxides of a single oxidation state (e.g., ΔE = 11.7 eV for MnO2 [22]). This is due to a weak interaction between Mn at different ion sites [23] that are characteristics of the mixture of divalent and trivalent manganese systems [6, 8, 12, 24]. In addition, the mixed oxidation states of Mn (II), Mn (III), and small amount of Mn (IV) were detected by the XPS region of Mn 2p3/2. Interestingly, when resolved, the Mn 2p3/2 region reveals the presence of three Mn-based components (see Fig. 4(b)) [25, 26]. From Fig. 4(c), the energy separation of the Mn 3 s doublet peaks is 5.6 eV, which indicates a mixed oxidation state for the manganese in the MnO x sample—energy separations of 5.41 and 5.79 eV were previously reported for Mn2O3 and MnO, respectively [24, 27]. From the specific surface area and pore size distribution, the synthesized MnOx showed type II B adsorption isotherm with type H3 hysteresis loop according to the IUPAC classification, with specific surface area of 72.07 m2 g−1(Fig. 5(a)). The hysteresis loop covers a large range of P/

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Fig. 4 XPS spectra before CV scans of the electrode prepared using MnOx of the pine tree seed/plate-like microstructure: (a) survey scan, (b) Mn 2p fits, (c) Mn 3 s fits, (d) survey scan of the electrode after

1000 CV cycles at 10 mV s−1, (e) Mn 2p fits showing the mixed oxidation states of Mn2+, Mn3+, and a small amount of Mn4+, and (f) Mn 3 s fits

Po (ca. 0.4 to 1.0), indicating a substantial presence of porous texture. In addition, this type of isotherm suggests a structure with slit-shaped pores [28], which is consistent with the pine tree seed-like morphology observed in the sample. From the BJH pore size distribution, the MnOx samples showed an average pore diameter of ~7.37 nm (Fig. 5(b)).

The electrochemical performance for the pine tree seedlike/disc-shaped microstructures of MnOx electrode was investigated using cyclic voltammetry and galvanostatic charge-discharge tests in 1 M Na2SO4 electrolyte solution in the potential window of 0.0–0.8 V. The current increases with the scan rate, showing a rectangular shape and a small redox

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Fig. 5 (a) Nitrogen adsorption-desorption isotherms and (b) BJH pore size distribution curve of MnOx

peak (see Fig. 6(a)). The specific capacitance of the assynthesized MnOx product can be estimated from the CV curves using Eq. 1 [6, 8]:

ZV c Q 1 I ðV ÞdV ; C¼ ¼ 2ΔV 2mν ðVc−VaÞ

ð1Þ

Va

where C is the specific capacitance (F g−1), Q is the charge obtained by integrating the area under the CV curves, m is the mass of active material in the electrode (g), ν is the potential scan rate (mV s−1), ΔV = Vc - Va is the potential range for each CV measurement (0.0–0.8 V, SCE), and I(V) is the response current (mA) on the CV curve. The synthesized Mn3O4 showed specific capacitances of 182, 174, 161, 146, 125, 88, and 58 F g−1 at the scan rates of 1, 2, 5, 10, 20, 50, and 100 mV s−1, respectively. The rectangular shape of CV curves at high scan rate (100 and 50 mV s−1) is indicative of a good rate capability of the material (Fig. 6(a)). On the specific capacitance values, it is interesting to note that Y. Dai et al. [29] and D.P. Dubal et al. [30] previously prepared the Mn3O4 thin films using spray pyrolysis technique and chemical bath deposition method with measured specific capacitances of 133 F g−1 and 183 F g−1, respectively, at 50 mV s−1. They also reported the surface oxidation of Mn3O4 to MnO2 during the CV cycling test. When compared at this scan rate, the MnOx powder reported herein yielded a lower specific capacitance value of 88 F g−1. However, it should be noted that the thin-film manganese oxide prepared by spray pyrolysis technique and chemical bath deposition normally gives a higher specific capacitance than that prepared by the sol-gel coating techniques, because the contact area at the interface between the electrode and the electrolyte solution is higher [29, 30]. Another reason that explains this disparity lies in the fact that the contact resistance between the active material and the current collector is lower in the thin films than in the powder manganese oxide. From the specific capacitances measured at different CV cycles, both the anodic and the cathodic peak currents

increased, particularly after 200 cycles (Fig. 6(b)). The CV currents gradually decreased after 400 cycles, and the CV shapes became more rectangular. However, the specific capacitance after 200 cycles increased when compared with the previous cycles. These behaviors could come from the surface oxidation of the as-prepared MnOx (with Mn3O4 as a major component) to the higher oxidation states of manganese oxide, in particular MnO2 (Mn4+), which is confirmed by the XPS spectra of the electrode after 1000 CV cycles (see Figs. 4(d),(e), and (f)). In Fig. 4(d), the survey spectrum shows the characteristic peaks for manganese oxide, such as Mn 2p, Mn 3 s, and O 1 s. The peaks of C 1 s, Na 1 s, and Na KLL come from the carbon tape of the XPS substrate and the aqueous 1 M Na2SO4 electrolyte solution. In order to examine whether the increase in the pseudocapacitive behavior could come from a change in the oxidation state of manganese oxide at the surface, the energy separations for the two peaks of Mn 2p (see Fig. 4(e)) and the doublet peaks of Mn 3 s (see Fig. 4(f)) are considered. For the Mn 2p region, the energy separation between Mn 2p3/2 and Mn 2p1/2 is 11.7 eV, and the Mn 2p3/2 fits reveal an increased amount of Mn3+ (642.1 eV), Mn4+ (643.5 eV), and Mn7+ (646.2 eV). In addition, the energy separation for the doublet peaks of Mn 3 s after CV cycling tests is 4.6 eV, which is smaller than that of the fresh electrode (5.6 eV). These results indicate the surface conversion of the major phase Mn3O4 (Mn2+, Mn3+) into MnO2 (Mn4+) during the cycling tests, which is consistent with other studies [9, 29, 30]. The electrochemical surface oxidation of the mixed pine tree seed-like/disc-shaped structure from MnOx to MnO2 under the CV cycling process at a scan rate of 10 mV s−1 for 1000 cycles in 1 M Na2SO4 can be represented by a complex irreversible process as follows [29, 30]. potential cycling MnOx ðpine tree seed‐like=disc‐shapedÞ → Naδ MnOx  nH2 O

ð2Þ

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Fig. 6 EC behaviors of the pine tree seed-like/disc-shaped microstructures of MnOx in 1 M Na2SO4 electrolyte solution: (a) CV curves at different scan rates, (b) CV cycle tests at 10 mV s−1 of scan rate, (c) galvanostatic chargedischarge curve at different current densities, (d) specific capacitance vs.

cycle number at different current densities, (e) galvanostatic chargedischarge at constant current density of 0.1 A g−1, and (f) percentage of the capacitance retention vs. cycle number, along with galvanostatic chargedischarge at constant current density of 1 A g−1 (inset)

The Faradaic pseudocapacitive charge storage mechanism (reversible process) of the latter manganese oxide in the neutral aqueous electrolyte solution can be explained via insertiondeinsertion of alkali ions/protons as follows [29, 30]:

From the galvanostatic charge-discharge curves for MnOx at different current densities of 0.1, 0.2, 0.5, 1, 2, 5, and 10 A g−1 (Figs. 6(c) and (d)), the specific capacitance obtained at low current density was higher than that measured at high current density. This could be due to a good interaction between the ions and the electrode. The

Naδ MnOx  nH2 O þ yHþ þ zNaþ ðy þ zÞe− ↔Naδþz MnOx  nH2 O

ð3Þ

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specific capacitance of the as-synthesized can be calculated as follows: C¼

I  Δt ; ΔV  m

ð4Þ

where C is the specific capacitance (F g−1), m is the total mass of active material on the electrode (g), Δt is the discharge time, and ΔV is the potential window (0.0–0.8 V). The specific capacitance values at current densities of 0.1, 0.2, 0.5, 1, 2, 5, and 10 A g−1 are 195, 189, 178, 162, 145, 87, and 27 F g−1, respectively. The small IR drop was found at high current density of 1 A g−1 (Fig. 6(c)), which suggests a good contact between the active material (as-synthesized MnOx) and the current collector (Ti foil). When compared at the same current densities, the specific capacitance of MnOx, with Mn3O4 as the major phase, is higher than the previously reported values for pure Mn3O4 (see Table 1), e.g., Li et al. (70 F g−1 at 0.2 A g−1) [9], Wu et al. (55 F g−1 at 1 A g−1) [6], Lee et al. (25 F g−1 at 0.5 A g−1) [8], Fan et al. (85 F g−1 at 0.1 A g−1) [10], and Jiangying et al. (95 F g−1 at 0.05 A g−1) [31]. According to the galvanostatic charge-discharge curve for MnOx at current density 0.1 A g−1 shown in Fig. 6(e), the existence of a typical symmetrical shape suggests a reaction with minimal Faradaic behavior coupled with contribution from the pseudocapacitive and double layer processes. A small IR drop at the onset of the cycle indicates a small equivalent series resistance of the MnOx electrode. At a current density of 1 A g−1 (see Fig. 6(f)), the capacitance retention was slightly over 100 %, and the specific capacitance increased in each cycling. These observed behaviors are likely due to the observed shift in the surface oxidation states of manganese, between MnOx and MnO2 [11].

Acknowledgments The authors would like to thank Dr. Sang-Hoon Park, Mr. Myeong-Seong Kim, Mr. Chang-Wook Lee, and Mr. Jun-Hui Jeong at Yonsei University for their help and advices. This work was financially supported by the University Staff Development Consortium, the Office of the Higher Education Commission, Thailand (JS), the Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University (SMS and PP), and the National Research Laboratory of Energy Conversion & Storage Materials, Yonsei University, Seoul, South Korea (HKK, SWL, and KBK).

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

4 Conclusion Interesting morphology of the pine tree seed-like/disc-shaped microstructures of MnOx, with Mn3O4 as the major component, was prepared using the microwave solvothermal of Mn(CH3COO)2.4H2O dissolved in ethylene glycol, without using chemical template, surfactant, or compositing with carbonaceous materials. According to the CV and the galvanostatic charge-discharge experiments, the specific capacitance progressively increased during the cycling test. This behavior can be explained by the fact that MnOx could be partially transformed via surface oxidation into a semiconductor MnO2 over the cycles—an explanation confirmed by the XPS analysis. Overall, the synthesized manganese oxide showed high specific capacitances of 195 F g−1 at 0.1 A g−1 and 182 F g−1 at 1 mV s−1. This material also showed high cycle stability, with the capacitance retention slightly over 100 % after 1000 cycles.

18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

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