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Preparation and characterization of nanostructured Mn oxide by an ethanol-based precipitation method for pseudocapacitor applications
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Scripta Materialia 65 (2011) 448–451 www.elsevier.com/locate/scriptamat
Preparation and characterization of nanostructured Mn oxide by an ethanol-based precipitation method for pseudocapacitor applications Nam Dong Kim, Hyeong Jin Yun, In Kyu Song and Jongheop Yi⇑ World Class University (WCU) Program of Chemical Convergence for Energy and Environment (C2E2), School of Chemical and Biological Engineering, Institute of Chemical Engineering, Seoul National University (SNU), Seoul 151-742, Republic of Korea Received 10 April 2011; revised 25 May 2011; accepted 29 May 2011 Available online 6 June 2011
A precipitation method is proposed for the synthesis of nanostructured Mn oxide in the use of supercapacitor electrodes. Here, ethanol is used as a solvent, instead of water. The nanostructured Mn oxide synthesized has been characterized by various analytical methods. Experimental results reveal that the alkyl chain of ethanol plays an important role in forming the nanostructured Mn oxide. Importantly, this material has an amorphous structure as well as highly hydrous properties, resulting in a higher specific capacitance than crystalline Mn oxide. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Supercapacitor; Electrochemistry; Nanocrystalline materials; Amorphous manganese oxide
Nanostructured Mn oxide has attracted much attentions as an electrode material for various electric energy storage and conversion devices [1–3]. It is known to have an amorphous structure and a large accessible surface area [4–6]. Such characteristics are generally beneficial for the easy intercalation/deintercalation of ions in Mn oxide and this permits large amounts of electrolyte ions to accumulate, consequently resulting in an enhanced energy density [6]. In addition, its hydrous properties contribute to an increase in ionic conductivity, which can facilitate ion transport through an electrolyte to enhance electrochemical performance as a supercapacitor [7–9]. Due to its high pseudocapacitance effect, it has been extensively studied and considered to be an excellent electrode material for use in electrochemical capacitors. Wet chemical synthesis methods have been widely applied to prepare nanostructured Mn oxide. They mainly include reduction method using various organic or inorganic reducing agents [3,8–10], and a co-precipitation method using at least two Mn precursors with different oxidation states [11,12]. However, these methods are somewhat complicated and difficult to control. We report herein on a simple method to synthesize nanostructured Mn oxide by exploiting the effect of the solvent used during the precipitation. Generally, metal oxides can be prepared by adding a precipitating
⇑ Corresponding author. Tel.: +82 2 880 7438; e-mail: [email protected]
agent to an ionic solution of the metal [13]. This permits the properties of the metal precursor solution, such as pH, to be altered and allows the solution to overcome the nucleation threshold to form a precipitate. During the precipitation, many parameters can affect the properties of the resultant precipitates, including the type of precipitating agent, the temperature, the solvent and the pH [13]. Among these parameters, it is known that the type of solvent used can affect the textural properties and crystallinity of a precipitate [13]. Therefore, it is reasonable to assume that if a different solvent were used to prepare Mn oxide by a precipitation method, we could change the crystallinity of the resultant Mn oxide. To further investigate this issue, we introduce ethanol as a solvent during the precipitation instead of water, which is the most commonly used solvent. The synthetic mechanism and the physical/chemical properties of the resulting materials are investigated via various analytic methods and their electrochemical properties for use as a supercapacitor are evaluated. Mn oxide samples were prepared by a precipitation method using both deionized water and ethanol as the solvents. The metal precursor (MnCl22H2O, Sigma– Aldrich) was dissolved in each of the solvents. A 1 M NaOH (Sigma–Aldrich) solution was also prepared in both solvents. The metal salt and the NaOH solutions in the same solvent were mixed at pH 10, until a precipitate was formed. The resulting suspension was stirred to permit the preparation to age, and the precipitates were
1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2011.05.034
N. D. Kim et al. / Scripta Materialia 65 (2011) 448–451
then isolated on a filter and successively washed with deionized water. The filtered powders were dried in a drying oven at 80 °C for 12 h and then annealed at various temperatures for 3 h. The resulting Mn oxides are denoted as Mn-H-P-X and Mn-E-P-X, respectively, where X indicates the annealing temperature at each stage of sample preparation (‘‘as-syn’’ indicates the dried sample that did not undergo the annealing process). Since the Mn ion solution is acidic, the metal hydroxides can be obtained by adding a precipitating agent, such as sodium hydroxide or ammonia, to increase the pH of the solution [13]. When sodium hydroxide is dissolved in water, it is nearly completely ionized and exists as Na+ and OH, resulting in an increase in the pH of the solution. As the pH of the solution increases, hydroxyl anions react with metal ions to form a solid precipitate via either olation or oxolation [14]. However, the dissociation of sodium hydroxide would be different in ethanol than in water, as follows [15]: ROH þ NaOH ! RO Naþ þ H2 O The acidity of water and ethanol is about 7, as indicated by their pKa values of 15.7 and 16, respectively [15]. Thus, sodium ethoxide and sodium hydroxide are in an equilibrium state that is closely balanced. Also, the ethoxy anion (RO) can be coordinated with Mn ions, similar to a hydroxyl anion in water. Therefore, a precipitate prepared in ethanol would contain many alkyl chains, which originate from the ethoxy anions. This can be confirmed by analyzing Fourier transform infrared (FT-IR) spectra for the as-synthesized precipitates prepared in both water and ethanol (Fig. 1a). Compared to the spectra of Mn-H-P-as-syn, numerous intense characteristic peaks can be seen in the spectra of Mn-E-P-as-syn; 2970 and 2880 cm1, C–H stretching vibration [16]; 1410–1335 and 1650–1560 cm1, CO 2 stretching vibrations; 1050 cm1, a primary alcohol, ethanol [17]. As we have previously assumed, ethoxy anions can be coordinated with Mn ions during the formation of precipitates in ethanol. All of these characteristic peaks of Mn-E-P-as-syn reveal the existence of organic species in the precipitates, which originate from the ethanol. On the other hand, the spectra of Mn-H-P-as-syn mainly show large amounts of H–O–H (3450 cm1), suggesting that hydroxyl ions are dominantly incorporated during the precipitation process. Also, the strong response of the Mn–O bending vibration (ca. 800 cm1) implies that it has a more crystallized structure than Mn-E-P-as-syn, as will be discussed later. The thermal properties of each of the precipitates prepared in water and ethanol were investigated by thermogravimetric analysis (TGA; Fig. 1b). In the case of MnH-P-as-syn, no significant weight change was observed until 600 °C, indicating that it is thermally stable. This is because large amounts of surface or structural water were removed during the drying process at 80 °C. Therefore, it would not be expected to undergo significant structural changes during the heat treatment. However, in the case of Mn-E-P-as-syn, continuous weight loss was detected over the wide range of temperatures. This is due to both the removal of the adsorbed and lattice water [9,18] and the decomposition of the organic
449
species, which was confirmed by FT-IR analysis. After the continuous weight loss up to a temperature of ca. 500 °C, another sharp weight loss was observed. This is caused by the release of oxygen from Mn oxide [1,18]. The structural characteristics of Mn-H-P-100 and Mn-E-P-100 were confirmed by X-ray diffraction (XRD) analysis (Fig. 2a). The diffraction patterns of Mn-H-P-100 showed well-crystallized Mn3O4 characteristics (PCPDF 80-0382). It is noteworthy that the crystalline structure of Mn-H-P is almost the in proceeding from Mn-H-P-as-syn to Mn-H-P-400, as highly crystallized Mn3O4 (Fig. S1). This indicates that the crystallization of Mn-H-P largely occurs during the drying step of 80 °C, as assumed from the TGA data. On the other hand, Mn-E-P-100 showed characteristics of amorphous Mn oxide with broad reflections at ca. 37° and 66° [2,6]. The reason for this difference can be explained from the XPS analysis for C1s (Fig. 2b). Compared to Mn-H-P100, a strong characteristic binding signal is seen for Mn-E-P-100 at ca. 283.4 eV. This is attributed to the bonding characteristic between carbon and the metal species [19,20] – in this study, the C–Mn binding state. Considering the above analytical results, the reason for the emergence of amorphous Mn oxide can be explained as follows. Large amounts of organic species included in the precipitate of Mn-E-P-as-syn starts to decompose as the heat treatment temperature increases. As a large proportion of the heat energy is used in the oxidation and removal of the organic species, the crystallization of Mn oxide would start slowly. Therefore, extensive crystallization of Mn oxide would not occur, and its amorphous characteristics would be maintained, until the organic species are sufficiently decomposed. As shown in Fig. S2, the amorphous structure of Mn-E-P is retained until the heat treatment temperature reaches 300 °C. Above the temperature of 400 °C, it shows the well-crystallized structure of Mn2O3 (PCPDF 41-1442). Morphological differences between Mn-H-P-100 and Mn-E-P-100 can be observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses (Figs. S3 and 3, respectively). Large sized (50–100 nm) Mn oxide particles can be found in the images of Mn-H-P-100 (Figs. S3a, and 3a and b). In the case of Mn-E-P-100, however, metal oxide nanoparticles with a particular shape are hardly seen (Figs. S3b, and 3c and d). Moreover, a careful examination of the SEM image of Mn-E-P-100 revealed that its structure is composed of aggregates of very small nanoparticles with sizes of a few nanometers, indicating that it has nanostructured properties. The high-resolution (HR) TEM image and selected area electron diffraction (SAED) pattern of Mn-H-P-100 clearly show that it has a highly crystallized structure with an inter-spacing distance of 0.27 nm, corresponding to the d(103) of Mn3O4 (PCPDF 80-0382). On the other hand, the HRTEM image and corresponding SAED pattern reveal the amorphous characteristics of Mn-E-P-100, implying it has loose structural characteristics. The chemical states of Mn and O in both the annealed samples were investigated by X-ray photoelectron spectroscopy (XPS) analysis (Fig. S4), and the results of deconvolution are summarized in Table 1. It is known that the valence state of Mn can be determined
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N. D. Kim et al. / Scripta Materialia 65 (2011) 448–451
Figure 1. (a) FT-IR spectra and (b) TGA curve for Mn-H-P-as-syn and Mn-E-P-as-syn.
Figure 2. (a) XRD patterns of Mn-H-P-100 and Mn-H-P-100 (d, Mn3O4; j, amorphous Mn oxide); (b) XPS spectra for the C1s core level of Mn-HP-100 and Mn-E-P-100. Table 1. XPS analysisof Mn-H-P-100 and Mn-E-P-100. Sample
Mn3s a
a b
Figure 3. TEM images of (a and b) Mn-H-P-100 and (c and d) Mn-EP-100.
by measuring the multiplet splitting width of the Mn3s XPS peaks [4,8]. The splitting width of Mn-H-P-100 was found to be 5.52. This value closely matches that of a standard sample of Mn3O4 (5.50), which is consistent with the results of XRD analysis. Thus, the oxidation state of Mn-H-P-100 was determined to be between Mn(a) and Mn(b). In the case of Mn-E-P100, the splitting width of the Mn3s spectra was found to be 5.30, which is between those of Mn2O3 (5.41) and MnO2 (4.78). Therefore, oxidation state of Mn-E-
O1s a
b
BE (1) (eV)
BE (2) (eV)
DE (eV)
BEa (eV)
Species
Peak area (%)
Mn-H-P-100
83.54
89.06
5.52
532.26 531.00 529.90
H–O–H M–O–H M–O–M
51.96 16.23 31.81
Mn-E-P-100
83.95
89.25
5.30
532.16 531.04 529.92
H–O–H M–O–H M–O–M
36.68 44.74 18.58
BE, binding energy. DE = BE(2) BE(1).
P-100 was determined to be between Mn(b) and Mn(v). In O1s spectra, there are three different oxygen states, corresponding to a water molecule (H–O–H, 532 eV), a metal hydroxide (M–O–H, 531 eV), and a metal oxide (M–O–M, 530 eV) [8,9]. As shown in Table 1, the oxygen state of Mn-H-P-100 was mainly composed of a metal oxide (M–O–M) with a small fraction of a metal hydroxide (M–O–H). Compared to Mn-H-P100, Mn-E-P-100 contained a relatively high content of metal hydroxide oxygen species, indicating that large amounts of hydrous Mn oxide were retained in the sample, even after the heat treatment. The electrochemical activities of the synthesized materials were evaluated for supercapacitor operation in an aqueous electrolyte solution and the results of the capacitance–voltage (CV) measurements obtained in 1 M Na2SO4 electrolyte solution are shown in Figure 4a. From a comparison of the CV area, which is directly related to charge capacitance, Mn-E-P-100 shows greater electrochemical activity than Mn-H-P-100 for the sup-
N. D. Kim et al. / Scripta Materialia 65 (2011) 448–451
451
Figure 4. (a) Cyclic voltammograms and (b) the galvanostatic charge/discharge curve for Mn-H-P-100 and Mn-E-P-100. (c) Variation of the specific capacitance as a function of the number of cycles for Mn- E-P-100 (h) and Mn-H-P-100 (s).
ercapacitor operation. The specific capacitance calculated from the galvanostatic charge/discharge measurements at a discharging current of 2 mA is ca. 122.1 and 83.6 F g1 for Mn-E-P-100 and Mn-H-P-100, respectively (Fig. 4b). This high capacitance of Mn-E-P-100 compared to Mn-H-P-100 originates from both its amorphous characteristics and its highly hydrous characteristics. It is well known that, compared to the highly crystalline Mn oxide, the amorphous-structured Mn oxide has a high ionic conductivity for electrolyte ions and a short diffusion pathway into the bulk of materials, resulting in an enhanced capacitance [3,5,8–10,21]. Moreover, it is also well know that considerable amounts of lattice water are essential to achieve a high capacitance value, because it confers a high ionic conductivity on the electrode materials [7,9]. In this study, the highly hydrous property of Mn-E-P-100 can also permit electrolyte ions to easily penetrate into the bulk regions of the metal oxide species. Therefore, Mn-E-P-100, which has a high ionic conductivity and a lower diffusive resistance because of its nanostructured amorphous characteristics, would be highly desirable for use as an electrode material in a supercapacitor. Mn-E-P-100 also shows stable capacitance retention during the long-term cycling operation, as well as Mn-H-P-100, with its crystalline structure (95%), maintaining 94% of its initial capacitance after 300 test cycles (Fig. 4c). In conclusion, Mn oxide with highly desirable structural properties was prepared by a simple and straightforward precipitation method using ethanol as the solvent, and its electrochemical activities were evaluated for supercapacitor operation. The findings indicate that the alkyl chain of the ethanol used in the precipitation of the Mn oxide plays an important role in the formation of nanostructured Mn oxide. The presence of an amorphous structure facilitated the electrochemical accessibility of electrolyte ions and reduced the charge transfer resistance, resulting in an enhanced capacitance. Since precipitation is a widely used method for the preparation of active metal materials, we believe this method could be easily applied to the preparation of materials with structural properties that are favorable for supercapacitors. This work was supported by the WCU (World Class University) Program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (R31-10013).
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j. scriptamat.2011.05.034. [1] S. Devaraj, N. Munichandraiah, J. Electrochem. Soc. 154 (2007) A80. [2] J.K. Chang, W.T. Tsai, J. Electrochem. Soc. 150 (2003) A1333. [3] S. Ching, J.A. Landrigan, M.L. Jorgensen, Chem. Mater. 7 (1995) 1604. [4] M. Chigane, M. Ishikawa, J. Electrochem. Soc. 147 (2000) 2246. [5] R.N. Reddy, R.G. Reddy, J. Power Sources 132 (2004) 315. [6] J.J. Xu, J. Yang, Electrochem. Commun. 5 (2003) 230. [7] C.C. Hu, C.C. Wang, J. Electrochem. Soc. 150 (2003) A1079. [8] M.T. Lee, J.K. Chang, Y.T. Hsieh, W.T. Tsai, J. Power Sources 185 (2008) 1550. [9] P. Ragupathy, D.H. Park, G. Campet, H.N. Vasan, S.J. Hwang, J.H. Choy, N. Munichandraiah, J. Phys. Chem. C 113 (2009) 6303. [10] Y.U. Jeong, A. Manthiram, J. Electrochem. Soc. 149 (2002) A1419. [11] D. Portehault, S. Cassaignon, E. Baudrin, J.P. Jolivet, J. Mater. Chem. 19 (2009) 2407. [12] M. Toupin, T. Brousse, D. Belanger, Chem. Mater. 16 (2004) 3184. [13] G. Ertl, H. Knoxinger, F. Schuth, J. Weitkamp, Handbook of Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2008. [14] A.F. Holleman, E. Wiberg, Inorganic Chemistry, Academic Press, San Diego, CA, 2001. [15] J.M. Hornback, Organic Chemistry, second ed., Brooks/ Cole, Pacific Grove, CA, 2005. [16] M.A. Garza-Navarro, A. Torres-Castro, D.I. Garci´aGutie´rrez, L. Ortiz-Rivera, Y.C. Wang, V.A. Gonza´lezGonza´lez, J. Phys. Chem. C 114 (2010) 17574. [17] G. Socrates, Infrared Characteristic Group Frequencies: Table and Charts, scond ed., Wiley, New York, 1994. [18] X. Xie, W. Liu, L. Zhao, C. Huang, J. Solid State Electrochem. 14 (2010) 1585. [19] G. Deniau, L. Azoulay, P. Je´gou, G. Le Chevallier, S. Palacin, Surf. Sci. 600 (2006) 675. ˚ . Persson, M. Lattemann, M. Stu¨ber, M. [20] E. Lewin, P.O.A Gorgoi, A. Sandell, C. Ziebert, F. Scha¨fers, W. Braun, J. Halbritter, S. Ulrich, W. Eberhardt, L. Hultman, H. Siegbahn, S. Svensson, U. Jansson, Surf. Coat. Technol. 202 (2008) 3563. [21] P. Ragupathy, H.N. Vasan, N. Munichandraiah, J. Electrochem. Soc. 155 (2008) A34.
Scripta Materialia 65 (2011) 448–451 www.elsevier.com/locate/scriptamat
Preparation and characterization of nanostructured Mn oxide by an ethanol-based precipitation method for pseudocapacitor applications Nam Dong Kim, Hyeong Jin Yun, In Kyu Song and Jongheop Yi⇑ World Class University (WCU) Program of Chemical Convergence for Energy and Environment (C2E2), School of Chemical and Biological Engineering, Institute of Chemical Engineering, Seoul National University (SNU), Seoul 151-742, Republic of Korea Received 10 April 2011; revised 25 May 2011; accepted 29 May 2011 Available online 6 June 2011
A precipitation method is proposed for the synthesis of nanostructured Mn oxide in the use of supercapacitor electrodes. Here, ethanol is used as a solvent, instead of water. The nanostructured Mn oxide synthesized has been characterized by various analytical methods. Experimental results reveal that the alkyl chain of ethanol plays an important role in forming the nanostructured Mn oxide. Importantly, this material has an amorphous structure as well as highly hydrous properties, resulting in a higher specific capacitance than crystalline Mn oxide. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Supercapacitor; Electrochemistry; Nanocrystalline materials; Amorphous manganese oxide
Nanostructured Mn oxide has attracted much attentions as an electrode material for various electric energy storage and conversion devices [1–3]. It is known to have an amorphous structure and a large accessible surface area [4–6]. Such characteristics are generally beneficial for the easy intercalation/deintercalation of ions in Mn oxide and this permits large amounts of electrolyte ions to accumulate, consequently resulting in an enhanced energy density [6]. In addition, its hydrous properties contribute to an increase in ionic conductivity, which can facilitate ion transport through an electrolyte to enhance electrochemical performance as a supercapacitor [7–9]. Due to its high pseudocapacitance effect, it has been extensively studied and considered to be an excellent electrode material for use in electrochemical capacitors. Wet chemical synthesis methods have been widely applied to prepare nanostructured Mn oxide. They mainly include reduction method using various organic or inorganic reducing agents [3,8–10], and a co-precipitation method using at least two Mn precursors with different oxidation states [11,12]. However, these methods are somewhat complicated and difficult to control. We report herein on a simple method to synthesize nanostructured Mn oxide by exploiting the effect of the solvent used during the precipitation. Generally, metal oxides can be prepared by adding a precipitating
⇑ Corresponding author. Tel.: +82 2 880 7438; e-mail: [email protected]
agent to an ionic solution of the metal [13]. This permits the properties of the metal precursor solution, such as pH, to be altered and allows the solution to overcome the nucleation threshold to form a precipitate. During the precipitation, many parameters can affect the properties of the resultant precipitates, including the type of precipitating agent, the temperature, the solvent and the pH [13]. Among these parameters, it is known that the type of solvent used can affect the textural properties and crystallinity of a precipitate [13]. Therefore, it is reasonable to assume that if a different solvent were used to prepare Mn oxide by a precipitation method, we could change the crystallinity of the resultant Mn oxide. To further investigate this issue, we introduce ethanol as a solvent during the precipitation instead of water, which is the most commonly used solvent. The synthetic mechanism and the physical/chemical properties of the resulting materials are investigated via various analytic methods and their electrochemical properties for use as a supercapacitor are evaluated. Mn oxide samples were prepared by a precipitation method using both deionized water and ethanol as the solvents. The metal precursor (MnCl22H2O, Sigma– Aldrich) was dissolved in each of the solvents. A 1 M NaOH (Sigma–Aldrich) solution was also prepared in both solvents. The metal salt and the NaOH solutions in the same solvent were mixed at pH 10, until a precipitate was formed. The resulting suspension was stirred to permit the preparation to age, and the precipitates were
1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2011.05.034
N. D. Kim et al. / Scripta Materialia 65 (2011) 448–451
then isolated on a filter and successively washed with deionized water. The filtered powders were dried in a drying oven at 80 °C for 12 h and then annealed at various temperatures for 3 h. The resulting Mn oxides are denoted as Mn-H-P-X and Mn-E-P-X, respectively, where X indicates the annealing temperature at each stage of sample preparation (‘‘as-syn’’ indicates the dried sample that did not undergo the annealing process). Since the Mn ion solution is acidic, the metal hydroxides can be obtained by adding a precipitating agent, such as sodium hydroxide or ammonia, to increase the pH of the solution [13]. When sodium hydroxide is dissolved in water, it is nearly completely ionized and exists as Na+ and OH, resulting in an increase in the pH of the solution. As the pH of the solution increases, hydroxyl anions react with metal ions to form a solid precipitate via either olation or oxolation [14]. However, the dissociation of sodium hydroxide would be different in ethanol than in water, as follows [15]: ROH þ NaOH ! RO Naþ þ H2 O The acidity of water and ethanol is about 7, as indicated by their pKa values of 15.7 and 16, respectively [15]. Thus, sodium ethoxide and sodium hydroxide are in an equilibrium state that is closely balanced. Also, the ethoxy anion (RO) can be coordinated with Mn ions, similar to a hydroxyl anion in water. Therefore, a precipitate prepared in ethanol would contain many alkyl chains, which originate from the ethoxy anions. This can be confirmed by analyzing Fourier transform infrared (FT-IR) spectra for the as-synthesized precipitates prepared in both water and ethanol (Fig. 1a). Compared to the spectra of Mn-H-P-as-syn, numerous intense characteristic peaks can be seen in the spectra of Mn-E-P-as-syn; 2970 and 2880 cm1, C–H stretching vibration [16]; 1410–1335 and 1650–1560 cm1, CO 2 stretching vibrations; 1050 cm1, a primary alcohol, ethanol [17]. As we have previously assumed, ethoxy anions can be coordinated with Mn ions during the formation of precipitates in ethanol. All of these characteristic peaks of Mn-E-P-as-syn reveal the existence of organic species in the precipitates, which originate from the ethanol. On the other hand, the spectra of Mn-H-P-as-syn mainly show large amounts of H–O–H (3450 cm1), suggesting that hydroxyl ions are dominantly incorporated during the precipitation process. Also, the strong response of the Mn–O bending vibration (ca. 800 cm1) implies that it has a more crystallized structure than Mn-E-P-as-syn, as will be discussed later. The thermal properties of each of the precipitates prepared in water and ethanol were investigated by thermogravimetric analysis (TGA; Fig. 1b). In the case of MnH-P-as-syn, no significant weight change was observed until 600 °C, indicating that it is thermally stable. This is because large amounts of surface or structural water were removed during the drying process at 80 °C. Therefore, it would not be expected to undergo significant structural changes during the heat treatment. However, in the case of Mn-E-P-as-syn, continuous weight loss was detected over the wide range of temperatures. This is due to both the removal of the adsorbed and lattice water [9,18] and the decomposition of the organic
449
species, which was confirmed by FT-IR analysis. After the continuous weight loss up to a temperature of ca. 500 °C, another sharp weight loss was observed. This is caused by the release of oxygen from Mn oxide [1,18]. The structural characteristics of Mn-H-P-100 and Mn-E-P-100 were confirmed by X-ray diffraction (XRD) analysis (Fig. 2a). The diffraction patterns of Mn-H-P-100 showed well-crystallized Mn3O4 characteristics (PCPDF 80-0382). It is noteworthy that the crystalline structure of Mn-H-P is almost the in proceeding from Mn-H-P-as-syn to Mn-H-P-400, as highly crystallized Mn3O4 (Fig. S1). This indicates that the crystallization of Mn-H-P largely occurs during the drying step of 80 °C, as assumed from the TGA data. On the other hand, Mn-E-P-100 showed characteristics of amorphous Mn oxide with broad reflections at ca. 37° and 66° [2,6]. The reason for this difference can be explained from the XPS analysis for C1s (Fig. 2b). Compared to Mn-H-P100, a strong characteristic binding signal is seen for Mn-E-P-100 at ca. 283.4 eV. This is attributed to the bonding characteristic between carbon and the metal species [19,20] – in this study, the C–Mn binding state. Considering the above analytical results, the reason for the emergence of amorphous Mn oxide can be explained as follows. Large amounts of organic species included in the precipitate of Mn-E-P-as-syn starts to decompose as the heat treatment temperature increases. As a large proportion of the heat energy is used in the oxidation and removal of the organic species, the crystallization of Mn oxide would start slowly. Therefore, extensive crystallization of Mn oxide would not occur, and its amorphous characteristics would be maintained, until the organic species are sufficiently decomposed. As shown in Fig. S2, the amorphous structure of Mn-E-P is retained until the heat treatment temperature reaches 300 °C. Above the temperature of 400 °C, it shows the well-crystallized structure of Mn2O3 (PCPDF 41-1442). Morphological differences between Mn-H-P-100 and Mn-E-P-100 can be observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses (Figs. S3 and 3, respectively). Large sized (50–100 nm) Mn oxide particles can be found in the images of Mn-H-P-100 (Figs. S3a, and 3a and b). In the case of Mn-E-P-100, however, metal oxide nanoparticles with a particular shape are hardly seen (Figs. S3b, and 3c and d). Moreover, a careful examination of the SEM image of Mn-E-P-100 revealed that its structure is composed of aggregates of very small nanoparticles with sizes of a few nanometers, indicating that it has nanostructured properties. The high-resolution (HR) TEM image and selected area electron diffraction (SAED) pattern of Mn-H-P-100 clearly show that it has a highly crystallized structure with an inter-spacing distance of 0.27 nm, corresponding to the d(103) of Mn3O4 (PCPDF 80-0382). On the other hand, the HRTEM image and corresponding SAED pattern reveal the amorphous characteristics of Mn-E-P-100, implying it has loose structural characteristics. The chemical states of Mn and O in both the annealed samples were investigated by X-ray photoelectron spectroscopy (XPS) analysis (Fig. S4), and the results of deconvolution are summarized in Table 1. It is known that the valence state of Mn can be determined
450
N. D. Kim et al. / Scripta Materialia 65 (2011) 448–451
Figure 1. (a) FT-IR spectra and (b) TGA curve for Mn-H-P-as-syn and Mn-E-P-as-syn.
Figure 2. (a) XRD patterns of Mn-H-P-100 and Mn-H-P-100 (d, Mn3O4; j, amorphous Mn oxide); (b) XPS spectra for the C1s core level of Mn-HP-100 and Mn-E-P-100. Table 1. XPS analysisof Mn-H-P-100 and Mn-E-P-100. Sample
Mn3s a
a b
Figure 3. TEM images of (a and b) Mn-H-P-100 and (c and d) Mn-EP-100.
by measuring the multiplet splitting width of the Mn3s XPS peaks [4,8]. The splitting width of Mn-H-P-100 was found to be 5.52. This value closely matches that of a standard sample of Mn3O4 (5.50), which is consistent with the results of XRD analysis. Thus, the oxidation state of Mn-H-P-100 was determined to be between Mn(a) and Mn(b). In the case of Mn-E-P100, the splitting width of the Mn3s spectra was found to be 5.30, which is between those of Mn2O3 (5.41) and MnO2 (4.78). Therefore, oxidation state of Mn-E-
O1s a
b
BE (1) (eV)
BE (2) (eV)
DE (eV)
BEa (eV)
Species
Peak area (%)
Mn-H-P-100
83.54
89.06
5.52
532.26 531.00 529.90
H–O–H M–O–H M–O–M
51.96 16.23 31.81
Mn-E-P-100
83.95
89.25
5.30
532.16 531.04 529.92
H–O–H M–O–H M–O–M
36.68 44.74 18.58
BE, binding energy. DE = BE(2) BE(1).
P-100 was determined to be between Mn(b) and Mn(v). In O1s spectra, there are three different oxygen states, corresponding to a water molecule (H–O–H, 532 eV), a metal hydroxide (M–O–H, 531 eV), and a metal oxide (M–O–M, 530 eV) [8,9]. As shown in Table 1, the oxygen state of Mn-H-P-100 was mainly composed of a metal oxide (M–O–M) with a small fraction of a metal hydroxide (M–O–H). Compared to Mn-H-P100, Mn-E-P-100 contained a relatively high content of metal hydroxide oxygen species, indicating that large amounts of hydrous Mn oxide were retained in the sample, even after the heat treatment. The electrochemical activities of the synthesized materials were evaluated for supercapacitor operation in an aqueous electrolyte solution and the results of the capacitance–voltage (CV) measurements obtained in 1 M Na2SO4 electrolyte solution are shown in Figure 4a. From a comparison of the CV area, which is directly related to charge capacitance, Mn-E-P-100 shows greater electrochemical activity than Mn-H-P-100 for the sup-
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Figure 4. (a) Cyclic voltammograms and (b) the galvanostatic charge/discharge curve for Mn-H-P-100 and Mn-E-P-100. (c) Variation of the specific capacitance as a function of the number of cycles for Mn- E-P-100 (h) and Mn-H-P-100 (s).
ercapacitor operation. The specific capacitance calculated from the galvanostatic charge/discharge measurements at a discharging current of 2 mA is ca. 122.1 and 83.6 F g1 for Mn-E-P-100 and Mn-H-P-100, respectively (Fig. 4b). This high capacitance of Mn-E-P-100 compared to Mn-H-P-100 originates from both its amorphous characteristics and its highly hydrous characteristics. It is well known that, compared to the highly crystalline Mn oxide, the amorphous-structured Mn oxide has a high ionic conductivity for electrolyte ions and a short diffusion pathway into the bulk of materials, resulting in an enhanced capacitance [3,5,8–10,21]. Moreover, it is also well know that considerable amounts of lattice water are essential to achieve a high capacitance value, because it confers a high ionic conductivity on the electrode materials [7,9]. In this study, the highly hydrous property of Mn-E-P-100 can also permit electrolyte ions to easily penetrate into the bulk regions of the metal oxide species. Therefore, Mn-E-P-100, which has a high ionic conductivity and a lower diffusive resistance because of its nanostructured amorphous characteristics, would be highly desirable for use as an electrode material in a supercapacitor. Mn-E-P-100 also shows stable capacitance retention during the long-term cycling operation, as well as Mn-H-P-100, with its crystalline structure (95%), maintaining 94% of its initial capacitance after 300 test cycles (Fig. 4c). In conclusion, Mn oxide with highly desirable structural properties was prepared by a simple and straightforward precipitation method using ethanol as the solvent, and its electrochemical activities were evaluated for supercapacitor operation. The findings indicate that the alkyl chain of the ethanol used in the precipitation of the Mn oxide plays an important role in the formation of nanostructured Mn oxide. The presence of an amorphous structure facilitated the electrochemical accessibility of electrolyte ions and reduced the charge transfer resistance, resulting in an enhanced capacitance. Since precipitation is a widely used method for the preparation of active metal materials, we believe this method could be easily applied to the preparation of materials with structural properties that are favorable for supercapacitors. This work was supported by the WCU (World Class University) Program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (R31-10013).
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