Synthesis and electrochemical properties of Mn₃O₄ nanocrystals with controlled morphologies grown from compact ion layers

Abstract

Different morphologies of Mn₃O₄ nanocrystals including rod-like, plate-like, and round nanoparticles were successfully prepared by a simple one-step process combining electrodeposition and precipitation, directly growing on Ti substrates without using templates or surfactants. In our system, precursor concentration, alkaline medium (LiOH) as a precipitant and the reaction time play important roles in the microstructure evolution of the Mn₃O₄ nanocrystal morphologies. The shape-controlled Mn₃O₄ nanoplates exhibit a maximum specific capacitance of 211 F g⁻¹ at 2 mV s⁻¹ in 1 M Na₂SO₄ for ultracapacitors.

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Introduction

Throughout the 20th century and today, the need to address environmental challenges and the global demand in energy use for industrial, residential, and transportation requirements has increased to a great extent.¹ There is a rapid growth in demand for electrical energy as it plays a major role in industrialized societies, and even in transportation systems. Linked to the exploitation and development of sustainable and renewable energy resources, such as solar power, wind, water, geothermal energy and biofuel, the power and energy systems have a significant impact on modern societies.^1,2 Especially for the power and energy systems, batteries and capacitors are the most common alternative energy storage/conversion devices.³ Among these energy storage resources, electrochemical capacitors (ECs), also known as supercapacitors or ultracapacitors, have recently attracted much attention. Although supercapacitors have their attractive features such as high specific capacitance, longer shelf and cycle life, and great pulse charge–discharge properties⁴ compared with usual batteries, they may exhibit lower energy densities than batteries. Therefore, research on the ECs has recently focused on the improvement of the electrochemical properties for mobile power electronics and hybrid electric vehicles.^5–8

In recent years, a number of efforts and imagination have been employed to find new ways to achieve superior electrochemical performance and practical applications. Along the same line, improving the architecture of materials and designing electrode materials with better physicochemical properties are key objectives to challenge technological barriers connecting fabrication and optimization of electrode materials at the nanoscale.⁹ A fundamental goal of nanoscience and nanotechnology is control over the size and morphology of the nanomaterials and the physicochemical properties of matter.¹⁰ With the development of techniques for the controlled synthesis of nanomaterials, self-assembly, a “bottom–up” approach in which aggregations of nanoparticles act as building blocks, is being used to tailor novel functional materials on the nano- and micro-scale.^9–11 In addition, nanomaterials with morphological characteristics suitable for power sources, such as batteries and supercapacitors for mobile power electronics, hybrid electric vehicles, and electrical energy storage devices, are being investigated extensively.¹² It is generally recognized that the electrochemical performance of the electrode materials for supercapacitors depends on the morphological properties (i.e., size, shape, surface area, and architecture) of the materials.¹³

Recently, the metal oxide Mn₃O₄ (hausmannite), which has a number of polymorphs, has attracted considerable attention because of a variety of applications such as energy storage, ion exchange, and molecular adsorption.¹⁴ It can also act as a catalyst, a magnetic material and a sensor material for detecting combustible organic compounds.¹⁵ Moreover, Mn₃O₄ is known to be a potential candidate for applications in supercapacitor electrodes due to being the most stable oxide of manganese, nontoxic and cost-effective.¹⁶ A number of methods for synthesizing Mn₃O₄ nanocrystals of different morphologies have been reported. Many results suggest that the morphology of nanomaterials plays a critical role in determining their electrochemical performance.^17–20 These include solvothermal/hydrothermal methods^19–21 and template or surfactant-assisted methods.^17,18,22 However, these methods are complicated, expensive, and require additional steps to remove the templates or organic additives.²³

Herein, we report a simple method for the synthesis of Mn₃O₄ nanocrystals of different morphologies and examine their performance as electrode materials in supercapacitors. The method, which is a one-step process, combines precipitation and electrodeposition, allowing one to prepare rod-like, plate-like, and round Mn₃O₄ nanostructures at room temperature. Unlike conventional methods, this method does not involve complicated processes, surfactants, or templates. The two main advantages of this method are that it is simple and can be used with optimizing the synthesis parameters under easily controllable experimental conditions. It is expected that this method will lead to the design and fabrication of nanomaterials with controllable morphologies for use in energy storage systems. Furthermore, the method should also result in the development of nanomaterials that can be fabricated on a large scale and are cost effective and environmentally friendly.

Experimental

Preparation of Mn₃O₄ electrode materials

The different shaped nanostructures of Mn₃O₄ were prepared using electrochemical precipitation from 100 mL aqueous solutions of 0.5, 1.0 M Mn(NO₃)₂·4H₂O, and 100 mL solution of 1.0 M LiOH as a precipitant at a potential of 0.9 V vs. an active carbon electrode with reaction time of 0.5, 1, 3, 6 and 12 h. The electrodeposition was conducted using a power supply (programmable DC, Human, Korea) at a constant cathodic current of 1 mA cm⁻², with a two-electrode cell consisting of pre-coated acetylene black as the cathode on a titanium plate working electrode (1.5 × 1.5 cm in area), a titanium plate of active carbon coating counter electrode. The deposited electrode was washed with deionized water several times by stirring the solution at 100 rpm until the pH was neutral in order to remove impurities. Then, the electrodes were dried at 80 °C for 12 h.

Characterization of Mn₃O₄ electrode materials

To address and confirm the characterization of Mn₃O₄ samples, the samples were collected by scratching the films gently from the Ti substrates. The crystallographic structure of the electrode materials were examined by X-ray diffraction (XRD, Rigaku D/Max 2500/PC, Japan) with Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV, 20 mA. The morphologies and particle sizes of the samples were observed using a field emission scanning electron microscope (FE-SEM, LA-950V2, Horiba, Japan). The structure analyses of the samples were performed with a high resolution-transmission electron microscope (HR-TEM, JEM 2000EX, JEOL, Japan). X-ray photoelectron spectroscopy (XPS) analysis was carried out with a PHI 5000 VeraProbe™ spectrometer using Al Kα radiation (hν = 1486.6 eV). The pore size and surface-area of the sample were analysed by the Barrett–Joyner–Halenda (BJH) method and the Brunauer–Emmett–Teller (BET) theory, respectively, using the N₂ adsorption/desorption data recorded with a BELSORP-miniII (BEL Japan, Inc., Japan) instrument at 77 K.

Electrochemical characterization

The typical three-electrode cell equipped with a working electrode, a platinum (Pt) foil counter electrode, and a saturated calomel electrode (SCE) reference electrode was used for measuring the electrochemical properties of the working electrode. The electrochemical measurements were carried out in 1 M Na₂SO₄ aqueous solution as the electrolyte. Cyclic voltammetry (CV) measurements were performed using a potentiostat/galvanostat (VSP, Bio-Logic, France), with a three-electrode system at scan rates of 2–100 mV s⁻¹ between 0.0 V and 0.9 V at room temperature.

Results and discussion

Based on the previous literature,^24–26 the Mn₃O₄ nanostructures of different morphologies were fabricated by the one-step procedure shown in Fig. 1. They were grown extending from the Helmholtz plane, which is formed at the electrode–electrolyte interface (i.e., the compact ion layer in the Helmholtz model).²⁷ The compact ion layer is divided into the inner and outer Helmholtz planes (IHP and OHP, respectively). The morphology-controlled Mn₃O₄ nanocrystals were formed in this region through the assistance of LiOH as a precipitant by varying the experimental conditions, such as the reaction time and precursor concentration. When the voltage was applied to the Ti substrate, manganese hydrated ions, which dissolved in deionized water, are initially located at the compact ion layer on the positive substrate through electrostatic attraction. At the same time, Mn²⁺ ions coordinate with OH⁻ ions by adding LiOH as a precipitant, resulting in the generation of the nuclei Mn(OH)₂ at the early stage of reaction. Then, manganese hydroxide dissolved quickly and partially oxidized into manganese oxyhydroxide by oxygen in the air, which is not stable in the system.²⁶ As the reaction proceeds, more LiOH leads to the increase in the concentration of OH⁻ ions in the system, which favors the formation of Mn₃O₄ nanocrystals^26,28,29 and furthers the self-assembly and Ostwald ripening process.³⁰ Also, the Pourbaix diagram³¹ (Fig. S1†) provides comprehensive information on the effects of proton activity, which explains possible reactions of the manganese oxide phases.^28,31,32 In the basic solution, the possible reaction could be expressed as follows:^24–26,33

NO₃⁻ + H₂O + 2e⁻ → NO₂⁻ + 2OH⁻ (1)

Mn²⁺ + 2OH⁻ → Mn(OH)₂ (2)

6Mn²⁺ + 12OH⁻ + O₂ → 2Mn(OH)₂ + 4MnOOH + 2H₂O (3)

2Mn(OH)₂ + 4MnOOH → 2Mn₃O₄ + 4H₂O (4)

Fig. 1 Fabrication of different-shaped Mn₃O₄ nanocrystals.

The crystal structures of the synthesized samples were characterised using X-ray diffraction (XRD) analyses; the results are shown in Fig. 2(a). The patterns of the three representative samples could be indexed to the tetragonal phase of Mn₃O₄ (hausmannite) with the I4₁/amd space group (JCPDS Card no. 24-734); however, the peaks were wider owing to the small size of the nanoparticles. No characteristic peak attributable to any other phase was noticed in the XRD patterns, indicating the high purity of the samples. In addition, X-ray photoelectron spectroscopy (XPS)-based analyses were also performed to confirm the results of the XRD analyses. Fig. 2(b) shows that the characteristic peaks of the samples at around 640.7 and 642.8 eV correspond to the Mn 2p_3/2 binding energies; these can be attributed to MnO and MnO₂ for the bulk Mn₃O₄.³⁴ The splitting width of 11.7 eV between Mn 2p_3/2 and Mn 2p_1/2 levels of Mn₃O₄ is in accordance with the previous literature.^34,35 Moreover, the O 1s peak (Fig. 2(c)) at approximately 529.4 eV can be assigned to the lattice oxygen in Mn₃O₄ as well as surface defects/hydroxyl groups (M–OH (530.9 eV); H–O–H (532.5 eV)); these results were in agreement with those reported previously.^34–36 Thus, it was confirmed that Mn₃O₄ nanomaterials were successfully synthesized.

Fig. 2 (a) XRD patterns of the different-shaped Mn₃O₄ nanoparticles, (b) XPS spectra of the synthesized Mn₃O₄ samples for the Mn 2p region and (c) XPS spectra of the synthesised Mn₃O₄ samples for the O 1s region.

To elucidate the effects of the synthesis conditions (precursor concentration and reaction time) on the morphology of the nanocrystals, the various types of Mn₃O₄ nanocrystals were examined using SEM and TEM. Fig. 3(a) and (c) show the rod- and plate-like Mn₃O₄ nanocrystals, which were synthesized by varying the precursor concentration, while keeping the reaction time constant at 0.5 h. As can be seen in Fig. 3(a), rod-like Mn₃O₄ nanocrystals with a width of approximately 10 nm and length greater than 100 nm were formed when the precursor (Mn(NO₃)₂) solution concentration was relatively high (1 M). In contrast, when the precursor solution concentration was halved, the Mn₃O₄ nanocrystals formed were irregular and plate-like (Fig. 3(c)), having a length and width of several hundred nanometres and a thickness of approximately 10 nm. TEM images of the individual Mn₃O₄ nanostructures, shown in Fig. 3(b) and (d), confirm their rod- and plate-like morphology; the SEM images shown confirm the same too.

Fig. 3 SEM and TEM images of the different-shaped Mn₃O₄ nanocrystals: (a and b) rod-like, (c and d) plate-like and (e and f) round nanoparticles.

Further, the TEM (Fig. 3(b) and (d)) and HRTEM images (Fig. 4(a) and (b)) indicated that changes in the precursor concentration results in the selective suppression of the surfaces of the crystallites and adjustments in their anisotropic growth rate along the [001] direction. The HRTEM image in Fig. 4(a) shows the lattice fringes, which had separation distances of 0.31 nm and 0.49 nm; these correspond to the {112} and {101} planes of tetragonal Mn₃O₄, respectively,³⁷ and suggest that the growth plane of the rod-like Mn₃O₄ nanocrystals is one of the longitudinal {112} planes. Particularly, we found evidence of spherical droplets at the tips of rod-like Mn₃O₄ in Fig. 4(a) (circle). This observation is good evidence of a “solution–liquid–solid (SLS)” growth mechanism³⁸ for the rod-like Mn₃O₄. Fig. 4(b) shows an HRTEM image of the Mn₃O₄ nanoplates. The interplanar distances are approximately 0.27 and 0.49 nm; these correspond to the {103} and {101} planes of tetragonal Mn₃O₄, respectively, and indicate that the growth planes of the nanoplates are one of the longitudinal {103} planes and one of the lateral {101} planes. The fast Fourier transform (FFT) patterns of the nanocrystals (insets in Fig. 4(a) and (b)) were also consistent with these results. It is known that the preferential growth direction of micro/nanocrystals is determined by the minimization of anisotropic surface free energy, which can be related to the exposed facet. For tetragonal hausmannite-Mn₃O₄, it has been well-documented that the (001) facet possesses the lowest surface energy and it is the most commonly exposed facet.^38,39

Fig. 4 TEM images with corresponding FFT patterns and SAED patterns of (a) rod-like, (b) plate-like and (c) round-like Mn₃O₄ nanocrystals.

The reaction time also played an important role in determining the morphology of the Mn₃O₄ nanocrystals. Interestingly, as shown in Fig. 3(e) and (f), when the reaction time was increased to 12 h, round Mn₃O₄ structures were obtained for precursor solution concentrations of 1 M and 0.5 M. SEM (Fig. 3(e)) and HRTEM images (Fig. 3(f) and 4(c)) show that the round Mn₃O₄ nanoparticles had a diameter of approximately 100 nm. Although there were differences in the grain sizes of the particles synthesized using the two different concentrations, their shapes were similar. The fringe spacings of 0.49 and 0.30 nm correspond to the {101} and {112} planes, respectively, of tetragonal Mn₃O₄. Mn₃O₄ nanoparticles were polycrystalline structures.

From the above studies, the morphological evolution of different-shaped Mn₃O₄ nanostructures can be proposed using the schematic illustration and corresponding SEM images shown in Fig. 5 and S2.† Also, we suggest that the evolution process of different-shaped Mn₃O₄ could be divided into two stages. In the initial stage including nucleation and anisotropic growth stage, rod-like and plate-like Mn₃O₄ were formed by different synthetic conditions. In our system, precursor concentration, alkaline medium (LiOH) as a precipitant and the reaction time seem to play crucial factors. In the case of rod-like Mn₃O₄, relatively a high precursor concentration of Mn²⁺ leads to the preferential growth of rod-like Mn₃O₄ nanostructures along the [001] direction at the initial stage. Similar results were also reported for the formation of Mn₃O₄ nanorods by controlling solvent composition and the use of different hydroxides.^{29,33,40–42} On the other hand, in the case of plate-like Mn₃O₄, the relative abundance of LiOH plays a key role in the formation of the plate-like Mn₃O₄, which determines the chemical reaction rate.

Fig. 5 Schematic illustration of the morphological evolution of the different-shaped Mn₃O₄ nanocrystals.

As indicated in the literature,^22,33 excessive LiOH can decrease Mn²⁺ concentration in the system and is easily adsorbed on the surface of nanocrystals. Subsequently it slows down the growth rate of two planes of the nanocrystals through selective adsorption on the {001} crystal planes and consequently plate-like Mn₃O₄ were formed. In the growth stage, each rod-like and plate-like Mn₃O₄ nanostructures were aggregated and self-assembled through the resolution and reconstruction process as the reaction time increased. Interestingly, in the case of high precursor concentration, Mn₃O₄ nanorods gradually aggregated, self-assembled and finally turned into whisker-like nanostructures. On the other hand, with relatively low precursor concentration (relative to the abundance of LiOH in the system), irregular Mn₃O₄ nanoplates grew perpendicular to the surface and well-developed Mn₃O₄ nanopetals were formed as the reaction time increased. And then, the Mn₃O₄ nanopetals transformed into flower-like Mn₃O₄ nanostructures. Therefore, it is reasonable to conclude that a relatively high growth rate favors the formation of rod-like Mn₃O₄ nanostructures, while a relatively low growth rate favors the formation of plate-like Mn₃O₄ nanostructures. In the second stage, when the reaction time was further increased to 12 h, round-like Mn₃O₄ nanostructures were finally formed from rod-like and plate-like Mn₃O₄ nanostructures. In consequence, the formation of rod-like and plate-like Mn₃O₄ nanostructures comes from Mn₃O₄ nanocrystals controlled by precursor concentration and the alkaline medium (LiOH) as a precipitant at the initial stage including nucleation and anisotropic growth. As suggested by Wulff’s theorem,⁴³ in the case of round-like Mn₃O₄, the shape of a crystal is determined by the minimum surface energy of each of its facets. This means that the dissolution of crystals may also occur preferentially at the high-energy facets. Therefore, the corners and tips of the rod- and plate-shaped Mn₃O₄ nanocrystals were smoothed in the case of the long reaction time, since the edges had been rounded.

The existence of the round Mn₃O₄ nanocrystals can also be explained as follows. Mn₃O₄ nanocrystals of diverse shapes form in the initial stage of the synthesis reaction, and as the reaction time is increased, the Mn₃O₄ nanoparticles become round with smoother edges owing to the Ostwald ripening process.^30,37 As a result, the transformation of the rod- and plate-like nanostructures into round ones was predominantly determined by selective adsorption onto specific crystal planes from the precursor solutions containing Mn in different concentrations and the balance between the kinetic and thermodynamic growth stages, which were related to the synthesis conditions (precursor concentration, alkaline medium (LiOH) and the reaction time).

To further understand the morphological features of the various types of Mn₃O₄ samples, N₂ adsorption/desorption measurements were conducted at 77 K. The resulting Brunauer–Emmett–Teller (BET) isotherms and the Barrett–Joyner–Halenda (BJH) pore size distribution curves are shown in Fig. S3.† The BET surface area measurement is crucial for investigating physical and chemical properties of materials because a material’s physical dimensions can influence its performance characteristics including electrochemical properties, reactivity and dissolution rate.⁴⁴ The BET surface area measurement indicates that the specific area of plate-like Mn₃O₄ (67.1 m² g⁻¹) is larger than those of rod-like Mn₃O₄ (61.9 m² g⁻¹) and round Mn₃O₄ (33.5 m² g⁻¹). The high specific surface area of nanostructures can be expected to facilitate the efficient transport of electrons and ions with more active sites in supercapacitor electrode applications, leading to a high electrochemical capacity. In general, it is well-accepted that the electrochemical properties of supercapacitors strongly rely on the morphology of electrode materials with high specific surface area.^44,45

The electrochemical properties of the Mn₃O₄ nanomaterials were evaluated through cyclic voltammetry (CV) (Fig. 6(a)–(c)) for potentials of 0.0–0.9 V (vs. the standard calomel electrode (SCE)) at various scan rates. The shapes of the CV curves revealed that the capacitance behavior of the nanomaterials was that of a pseudocapacitor based on redox mechanisms rather than that of an electric double-layer capacitor, whose CV curves are rectangular.²⁷ The average specific capacitances of the rod-like, plate-like, and round Mn₃O₄ samples were 171, 211, and 61 F g⁻¹, respectively, at a scan rate of 2 mV s⁻¹. At a scan rate of 100 mV s⁻¹, the average specific capacitances of the rod-like, plate-like, and round Mn₃O₄ nanoparticles (Fig. 6(d)) were 103, 125, and 34 F g⁻¹, respectively. To estimate the specific capacitance (SC) of the samples as supercapacitor electrodes, galvanostatic charge–discharge experiments were performed at a current density of 1 A g⁻¹ (Fig. 6(e)). Galvanostatic discharge is the accepted and more reliable method for determining the specific capacitance compared to CV.^46,47 The SC values of 162.5 (rod-like Mn₃O₄), 202 (plate-like Mn₃O₄) and 57.5 F g⁻¹ (round Mn₃O₄) were obtained at a current density of 1 A g⁻¹, respectively. These differences in the specific capacitances can also be attributed to the differences in the morphologies of the Mn₃O₄ nanostructures. For plate-like Mn₃O₄, thermodynamically, an individual plate with two main exposed {001} planes is still unstable. Thus, Mn₃O₄ nanoplates tend to grow perpendicularly to the surface planes. Subsequently, nanoplates transformed into nanopetals. Simultaneously, the nanoplates possess a large quantity of edges and a larger fraction of the relatively high-energy {101} crystal plane than rod- or round-like Mn₃O₄ nanocrystals. It is well-known that the exposed high-energy facets with a high density of atomic steps, kinks and edges usually show enhanced reactivity and high selectivity superior to that of low-energy facets.^15,38,39 In comparison with rod- and round-like Mn₃O₄ electrodes, the enhanced pseudo-capacitive behavior of the plate-like Mn₃O₄ electrode might be a result of the higher reactivity and selectivity due to more exposed high-energy facets. The cycling performances of the as-prepared rod-like, plate-like and round Mn₃O₄ electrodes are shown in Fig. 6(f). Interestingly, the capacitance retention was found to be 81.1% after 1000 cycles for the round-shaped Mn₃O₄ electrode, while those for the rod-like and plate-like Mn₃O₄ electrodes were 76.9% and 69%, respectively. The decline in capacitance might have been caused by phase transitions and structural degradations (see Fig. S4†) during the discharge/charge cycling processes.^16,48

Fig. 6 Electrochemical performance of different shaped Mn₃O₄. (a–c) CV curves, (d) specific capacitances of the rod-like, plate-like, and round Mn₃O₄ nanocrystals for different scan rates, (e) galvanostatic charge/discharge curves, and (f) specific capacitance versus cycle number at a galvanostatic charge–discharge current density of 1 A g⁻¹.

Conclusions

In summary, we have demonstrated a simple method for the synthesis of various shaped Mn₃O₄ nanostructures, with optimization of the synthesis parameters under easily controllable experimental conditions, directly grown on a Ti substrate using a simple method that combined electrodeposition and precipitation. The method does not require templates or surfactants. Also, we have investigated that the morphologies of the synthesized Mn₃O₄ nanostructures were determined by the concentration of the precursor solution, alkaline medium (LiOH) as a precipitant and the reaction time. In addition, it is worth noting that not only the formation of rod-like and plate-like Mn₃O₄ nanostructures but also microstructure evolution of Mn₃O₄ come from Mn₃O₄ nanocrystals governed by precursor concentration and the alkaline medium (LiOH) as a precipitant in the initial stage including nucleation and anisotropic growth through the time-dependant experiment. As-synthesized plate-like Mn₃O₄ nanostructures exhibited a specific capacitance of 211 F g⁻¹ at 2 mV s⁻¹. This capacitive behavior of the electrode can be attributed to the morphological features of the plate-like Mn₃O₄ nanostructures. The approach in this study can be used to guide the design and fabrication of nanomaterials with controllable morphologies for use in energy storage systems.

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015R1A2A2A03006633). This work was also supported by the Energy Efficiency & Resources program of the Korea Institute of Energy Technology Evaluation Planning (KETEP), and was granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20122010100140). This work was also noted by the Energy Efficiency & Resources program of the Korea Institute of Energy Technology Evaluation Planning (KETEP), and was granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20152020105770).

Notes and references

1. Electrochemical Technologies for Energy Storage and Conversion, ed. R.-S. Li, L. Zhang, X. Sun, H. Liu and J. Zhang, Wiley-VCH, Weinheim, 2012, pp. 1–17.
2. Nanomaterials for Energy Storage Applications, ed. H. S. Nalwa, American Scientific Publishers, 2009, pp. 1–4.
3. Linden’s Handbook of Batteries, ed. D. Linden and T. B. Reddy, McGraw-Hill, New York, 4th edn, 2011, ch. 2, pp. 2.1–2.37.
4. V. S. Bagotsky, Fundamentals of electrochemistry, Wiley-VCH, New York, 2006, pp. 369–375.
5. Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, H. Dong, X. Li and L. Zhang, Int. J. Hydrogen Energy, 2009, 34, 4889.
6. S. Yamazaki, K. Obata, Y. Okuhama, Y. Matsuda, M. Yamagata and M. Ishikawa, J. Power Sources, 2010, 195, 1753.
7. X. Dong, W. Shen, J. Gu, L. Xiong, Y. Zhu, H. Li and J. Shi, J. Phys. Chem. B, 2006, 110, 6015.
8. J. Chang, M. Park, D. Ham, S. B. Ogale, R. S. Mane and S.-H. Han, Electrochim. Acta, 2008, 53, 5016.
9. G. Schmid, Nanoparticles: From Theory to Application, Wiley-VCH, Weinheim, 2004, pp. 1–3.
10. J. Park, J. Joo, S. G. Kwon, Y. Jang and T. Hyeon, Angew. Chem., Int. Ed., 2007, 46, 4630.
11. C. M. Lieber, MRS Bull., 2003, 28, 486.
12. J. Jiang, Y. Y. Li, X. T. Huang, C. Z. Yuan and X. W. Lou, Adv. Mater., 2012, 24, 5166.
13. C. Largeot, C. Portet, J. Chmiola, P. Taberna, Y. Gogotsi and P. Simon, J. Am. Chem. Soc., 2008, 130, 2730.
14. P. Li, C. Nan, Z. Wei, J. Lu, Q. Peng and Y. Li, Chem. Mater., 2010, 22, 4232.
15. L. C. Zhang, Q. Zhou, Z. H. Liu, X. D. Hou, Y. B. Li and L. Yi, Chem. Mater., 2009, 21, 5066.
16. J. Jiang and A. Kucernak, Electrochim. Acta, 2002, 47, 2381.
17. Y. Dai, K. Wang and J. Xie, Appl. Phys. Lett., 2007, 90, 104102.
18. D. P. Dubal, D. S. Dhawale, R. R. Salunkhe, S. M. Pawar, V. J. Fulari and C. D. Lokhande, J. Alloys Compd., 2009, 484, 218.
19. H. Jiang, T. Zhao, C. Yan, J. Ma and C. Z. Li, Nanoscale, 2010, 2, 2195.
20. M. Fang, X. Tan, M. Liu, S. Kang, X. Hu and L. Zhang, CrystEngComm, 2011, 13, 4915.
21. I. Djerdj, D. Arcon, Z. Jaglicic and M. Niederberger, J. Phys. Chem. C, 2007, 111, 3614.
22. M. Liu and H. C. Zeng, Cryst. Growth Des., 2012, 12, 5561.
23. X. T. Du, Y. Yang, J. Liu, B. Liu, J. B. Liu, C. Zhong and W. B. Hu, Electrochim. Acta, 2013, 111, 562.
24. D. M. MacArthur, J. Electrochem. Soc., 1970, 117, 422.
25. G. H. A. Therese and P. V. Kamath, J. Appl. Electrochem., 1998, 28, 539.
26. T. Yousefi, A. N. Golikand, M. H. Mashhadizadeh and M. Aghazadeh, J. Taiwan Inst. Chem. Eng., 2012, 43, 614.
27. B. E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, New York, 1999.
28. V. G. Kumar, D. Aurbuch and A. Gedanken, Ultrason. Sonochem., 2003, 10, 17.
29. T. R. Bastami and M. H. Entezari, Ultrason. Sonochem., 2012, 19, 560.
30. W. Ostwald, Z. Phys. Chem., 1897, 22, 289.
31. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solution, National Association of Corrosion Engineers, Houston, 2nd English edn, 1974, pp. 286–293.
32. F. Zhou, A. Izgorodin, R. K. Hocking, L. Spiccia and D. R. MacFarlane, Adv. Energy Mater., 2012, 2, 1013.
33. L. Balan, C. M. Ghimbeu, L. Vidal and C. Vix-Guterl, Green Chem., 2013, 15, 2191.
34. A. Moses Ezhil Raj, S. Grace Victoria, V. Bena Jothy, C. Ravidhas, J. Wollschläger, M. Suendorf, M. Neumann, M. Jayachandran and C. Sanjeeviraja, Appl. Surf. Sci., 2010, 256, 2920.
35. D. P. Dubal and R. Holze, J. Power Sources, 2013, 238, 274.
36. Z. P. Feng, G. R. Li, J. H. Zhong, Z. L. Wang, Y. N. Ou and Y. X. Tong, Electrochem. Commun., 2009, 11, 706.
37. W. Z. Wang, C. K. Xu, G. H. Wang, Y. K. Liu and C. L. Zheng, Adv. Mater., 2002, 14, 837.
38. V. Bayer, R. Podloucky, C. Franchini, F. Allegretti, B. Xu, G. Parteder, M. G. Ramsey, S. Surnev and F. P. Netzer, Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76, 165428.
39. W. Xiao, J. S. Chen and X. W. Lou, CrystEngComm, 2011, 13, 5685.
40. H. Huang, Q. Yu, X. Peng and Z. Ye, Chem. Commun., 2011, 47, 12831.
41. Z. W. Chen, J. K. L. Lai and C. H. Shek, Appl. Phys. Lett., 2005, 86, 181911.
42. A. Giri, N. Goswami, M. Pal, M. T. Z. Myint, S. Al-Harthi, A. Singha, B. Ghosh, J. Dutta and S. K. Pal, J. Mater. Chem. C, 2013, 1, 1885.
43. G. Wulff, Z. Kristallogr. Mineral., 1901, 34, 449.
44. C. Chen, G. Ding, D. Zhang, Z. Jiao, M. Wu, C.-H. Shek, C. M. Lawrence Wu, J. K. L. Lai and Z. Chen, Nanoscale, 2012, 4, 2590.
45. G. Wang, L. Zhang and J. Zhang, Chem. Soc. Rev., 2012, 41, 797.
46. M. D. Stoller and R. S. Ruoff, Energy Environ. Sci., 2010, 3, 1294.
47. C. Ye, Z. M. Lin and S. Z. Hui, J. Electrochem. Soc., 2005, 152, A1272.
48. D. P. Dubal, D. S. Dhawale, R. R. Salunkhe and C. D. Lokhande, J. Electrochem. Soc., 2010, 157, A812.

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Footnote

† Electronic supplementary information (ESI) available: The Pourbaix diagram showing the potential–pH equilibrium for the manganese–water system, SEM images of the morphological evolution of Mn₃O₄ nanocrystals, BET analysis and SEM images of the Mn₃O₄ electrode after the charging–discharging (after 1000 cycles) experiments. See DOI: 10.1039/c5ra28110e

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