Hydrated Mn(IV) oxide-exfoliated graphite composites for electrochemical capacitor

Electrochimica Acta 52 (2007) 3061–3066

Hydrated Mn(IV) oxide-exfoliated graphite composites for electrochemical capacitor Chuanyun Wan, Kazuhisa Azumi, Hidetaka Konno ∗ Laboratory of Advanced Materials Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan Received 27 June 2006; received in revised form 21 September 2006; accepted 21 September 2006 Available online 31 October 2006

Abstract Commercially available low cost exfoliated graphite (EG, nominal diameter 130 ␮m) was used as a conductive substrate for electrochemical capacitor of hydrated Mn(IV) oxide, MnO2 ·nH2 O. The MnO2 ·nH2 O–EG composites were prepared by addition of EG to potassium permanganate solution, followed by 1 h stirring and then slow addition of manganese(II) acetate solution. By this procedure submicrometer or smaller sized MnO2 ·nH2 O particles having mesopores of 6–12 nm in diameter were formed on the graphite sheets of EG. Although EG alone showed only about 2 F g−1 , the composites showed good rectangular cyclic voltammograms at 2–20 mV s−1 in 1 mol L−1 Na2 SO4 . The capacitance per net amount of MnO2 increased proportionally with EG content, that is, utilization ratio of MnO2 increased with EG content. The composites of MnO2 ·nH2 O and smaller diameter of EG (nominal diameter 45 ␮m) or artificial graphite powder (average diameter 3.7 ␮m) showed fairly good performance at 2 mV s−1 , but with increasing potential scan rate the rectangular shape was distorted and capacitance decreased drastically. The results implies that sheet-like structure is more effective than small particles as conductive materials, when the formation procedure of composite is the same. Large sized EG may be a promising conductive material for electrochemical capacitors. © 2006 Elsevier Ltd. All rights reserved. Keywords: Exfoliated graphite; Electrochemical capacitor; Hydrated Mn(IV) oxide

1. Introduction Electric double layer capacitor (EDLC) is one of the promising electric energy storage devices for future electric vehicles, and small size capacitors are already used for a variety of instruments. As is generally known, EDLC has several advantages compared with batteries but its low energy density is a shortcoming to be solved. For the last decade, electrochemical capacitor based on the redox reactions of transition metal oxides has been extensively investigated aiming for higher energy density. Among them hydrated or amorphous Mn(IV) oxide, MnO2 ·nH2 O, has been paid much attention by many researchers recently [1–19] because of its low cost, abundance and less harmful nature. The MnO2 for batteries are synthesized mainly by anodic deposition and it is also applied for the synthesis of MnO2 ·nH2 O for electrochemical capacitor [4,6,7,9,13,15,16,19]; chemical conversion deposition is also

∗

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0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.09.039

reported [14]. But more frequently MnO2 ·nH2 O is synthesized by the reduction of permanganate solution with Mn(II) species [1–3,8,18], reducing agents [5,10], or thermally [12], based on the formation method of colloidal MnO2 [20]. Since this material has poor electric conductivity, different measures have been devised to use as electrodes, such as either deposition on metals [2,13,17,19], graphite [4,7,9,14,15] and ITO [6] as thin films, or addition of carbon black [3,10–12,15], acetylene black [1,5,8] and carbon nanotube [16–19] as conductive materials. Though the reported values of specific capacitance for MnO2 ·nH2 O cannot be compared simply due to different measuring methods and bases of capacitance calculation, the tendency is toward higher specific capacity for the electrodes of metal or graphite plate loaded with very small amounts of MnO2 ·nH2 O [e.g. 2]. This type of electrode, however, is not expected to be useful for practical capacitors because of the low loading density of active material. The techniques to add conductive materials are more promising for practical devices, but so far the reported capacitances are mostly less than a quarter of the theoretical value, even with carbon nanotubes. We conceived from published works that platelet shape of carbon might be better additives than fine pow-

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ders, such as carbon black, acetylene black and so on. For this purpose, exfoliated graphite (EG) is a promising material. We have developed the processes using EG to form various functional materials [21–24], including negative electrode materials for lithium ion batteries [23]. EG is produced industrially by rapid heating of the intercalation compounds of natural graphite at high temperatures, and it is very low cost material. It has bellows-like structure due to expansion of interlayer in graphite [cf. [25,26] for SEM images], and because of its structure and hydrophobicity, it has very high sorption capacity for oil, 60–85 g g−1 of EG [25,26]. As reported previously [26], sorption capacity strongly depends on the bulk density and pore volume but not on the specific surface area of EG (usually 50–60 m2 g−1 ). The sorption takes place in the gaps of EG and the spaces created by entangled EG, that is, high sorption capacity is owing to capillary action. Recently, EG was investigated as active material for solid-state electrochemical capacitors [27], though it is not directly connected to MnO2 ·nH2 O capacitors. In the present work, MnO2 ·nH2 O–EG composites were synthesized for the electrode material of electrochemical capacitor, considering that the formed composites might show better performance, if MnO2 ·nH2 O can be loaded as a layer or fine particles on the graphite sheets of EG, which are several tens to a few hundreds micrometers in diameter. As the surface of EG is hydrophobic, the starting materials and synthesis method of MnO2 ·nH2 O may affect properties of the final products. We selected the most common combination of potassium permanganate, KMnO4 , and manganese(II) acetate, Mn(OCOCH3 )2 , solutions and two routes were attempted: method (1) was addition in the order of KMnO4 , EG, then Mn(OCOCH3 )2 and method (2) Mn(OCOCH3 )2 , EG, then KMnO4 . After a series of systematic experiments, the composites formed by method (2) were found to have no advantage over those by method (1). Accordingly the results for the composites formed by method (1) are reported here.

ning electron microscopy at 5 kV (SEM; JEOL, JSM-6300F), nitrogen adsorption/desorption at 77 K (BEL Japan, BELSORPmini), and thermogravimetric analysis at 10 K min−1 in oxygen (TG; Seiko Instruments, TG/DTA6300). TG data were used to determine the amount of water, EG and Mn in the products, as described below. MnO2 ·nH2 O–EG composites were fabricated to working electrodes of electrochemical capacitors: 10 mass% of poly(tetrafluoroethylene) (PTFE) were added as a binder to the composite, and mixed using a small amount of acetone, then the mixture was pressed onto a titanium mesh current collector followed by drying at 50 ◦ C for about 10 h. The composites are electrically conductive by EG and so no other additive was used. The amount of composites on the electrode was approximately 10 mg cm−2 . All electrochemical measurements were done in a four-electrode setup: two platinum foils were used as the counter electrodes, and a saturated calomel electrode (SCE) as the reference electrode. The measurements were carried out in a deaerated 1 mol L−1 Na2 SO4 electrolyte without pH adjustment at room temperature after standing the electrode in the solution overnight. The capacitance was estimated mainly from cyclic voltammogram (CV) using the total amount of charge calculated by integrating CV. The CV was measured in a range of 0–1.0 V versus SCE at different potential scan rates of 2–20 mV s−1 , in which the electrodes were conditioned by cycling 10 times at 5 mV s−1 before measuring at different rates. Galvanostatic charge/discharge curves were also measured at 100–500 mA g−1 for selected samples. Specific capacitance was calculated with respect to the total mass of composite, but if needed be a net mass of MnO2 in the electrode was used.

2. Experimental

The thermogravimetric curve in pure oxygen for composite EG-24 in Table 1 is shown in Fig. 1, for example. The initial mass loss up to about 500 ◦ C was ascribed to dehydration. Above 500 ◦ C, EG burns and MnO2 looses oxygen to form Mn2 O3 , so that the second mass loss is the sum of EG and oxygen from MnO2 . The final small mass loss above 900 ◦ C is due to the conversion of Mn2 O3 to Mn3 O4 . These phases were confirmed by XRD. Thus, the composition of composites was estimated as shown in Table 1. EG or graphite content was rounded off to a whole number and put at the end of sample code. There is a possibility of contamination by a small amount of potassium ions [8] but they were neglected here. To examine the possible effect

Exfoliated graphite was commercially available one: a nominal diameter is released to be 130 ␮m but actually it is in a range of 100–300 ␮m by SEM observation [26]. The EG of 45 ␮m (referred to as EGS) and artificial graphite (SFG-6, TIMLEX(Lonza), average diameter 3.7 ␮m, specific surface area 14.9 m2 g−1 ) were also used for comparison. MnO2 ·nH2 O was synthesized by mixing the stoichiometric amounts of KMnO4 and Mn(II) acetate, similar to the early report [1]. The composites of MnO2 ·nH2 O–EG were prepared as follows:

3. Results and discussion 3.1. Characterization of composites

Addition of predetermined amounts of EG to 0.1 mol L−1 KMnO4 solution → 1 h stirring at ambient temperature → slow addition of 0.15 mol L−1 Mn(II) acetate solution → 5 h stirring at ambient temperature. Finally obtained brownish dark mixture was filtered, washed several times with distilled water, and then dried at 110 ◦ C for 24 h in air. The products were characterized using X-ray diffraction by Cu K␣ irradiation (XRD; Rigaku, RINT-Ultima+/PC), scan-

of potassium ions on the thermal analysis, EG-13 in Table 1 was heated in 6 mol L−1 HCl solution at 90 ◦ C for 2 h, washed, dried and weighed. The average EG content was 12.5 ± 1.2 mass%, which coincides with the value in Table 1 within the limits of experimental error. The number of hydrated water, n, is not the

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Table 1 Composition of prepared composites Sample code

EG-2 EG-13 EG-24 EGS-8 SFG-6–11 a b

Composition (mass%)

Specific surface area

In electrodea (mass%)

EG

MnO2 ·nH2 O

n

(m−2 g−1 )

EG

As MnO2

2.3 12.9 23.9 7.9 10.7b

97.7 87.1 76.1 92.1 89.3

0.83 0.66 0.65 0.65 0.81

258 255 237

2.1 11.6 21.5 7.1 9.6b

76.2 69.8 60.4 72.3 68.8

After assembling as an electrode by adding 10 mass% of PTFE as a binder. Artificial graphite powder.

same but in a reasonable range, considering the method of estimation. XRD patterns of MnO2 ·nH2 O–EG composites and MnO2 ·nH2 O prepared without adding EG (referred to as ‘pristine’ MnO2 ·nH2 O) were similar to those reported by several investigators [5,10,16] except that two sharp peaks assigned to EG, C 002 and C 004, were observed: weak and broad peaks were distinguished around 2θ = 38 and 67◦ , but corresponding crystalline compounds were not found in the powder diffraction index. There is no negative reason to accept that the formed MnO2 ·nH2 O is a mixture of poorly crystallized oxides and amorphous oxide. Nitrogen adsorption/desorption isotherms are shown in Fig. 2. The isotherm for EG-2 was very similar to ‘pristine’ MnO2 ·nH2 O, so that it was omitted in Fig. 2. The specific surface area of composites calculated from the isotherm is listed in Table 1. As the surface area of ‘pristine’ MnO2 ·nH2 O was 293 m2 g−1 and that of EG was about 60 m2 g−1 , most of the area is the contribution from the oxide. The isotherms resembles Type IV, suggesting the presence of mesopores. As shown in Fig. 3, peak pore volume is present in a range of 6–12 nm in diameter by BJH method [28] but the poresize distribution becomes broad with increasing amount of EG. SEM photographs of EG-24 are shown in Fig. 4. Graphite sheets of EG are clearly distinguished and small MnO2 ·nH2 O particles are deposited sparsely on the sheets or dispersed sepa-

Fig. 1. Thermogravimetric curve at 10 K min−1 in oxygen for EG-24 in Table 1.

Fig. 2. Nitrogen adsorption/desorption isotherms at 77 K. See Table 1 for sample codes.

rately: sizes are submicrometers or smaller. The original EG is hydrophobic, but the surface of EG is oxidized during the initial 1 h stirring in the KMnO4 solution [14], which makes the surface hydrophilic as confirmed by separate experiments, leading to the favorable results by the present procedure. The procedure used here is reported to form colloidal MnO2 [20].

Fig. 3. Calculated pore size distribution by BJH method using the data in Fig. 2. See Table 1 for sample codes.

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Fig. 4. SEM photographs of EG-24 (cf. Table 1) at different magnifications.

3.2. Electrochemical properties of composites

Fig. 5. Cyclic voltammograms at 2 mV s−1 . See Table 1 for sample codes.

CVs at 2 mV s−1 in 1 mol L−1 Na2 SO4 electrolyte are shown in Fig. 5. Specific capacitance of ‘pristine’ MnO2 ·nH2 O is very small, ca. 1 F g−1 , for lack of electric conductivity. The exfoliated graphite presents a rectangular shape but also gives a very low capacitance, less than 2 F g−1 , and the capacitive contribution by EG to the composites can be disregarded. The current is given per unit mass of composites in Fig. 5, so that the amount of active mass is smaller in order of EG-2 > EG-13 > EG-24 as summarized in Table 1. Rectangular characteristics of CV accomplished with increasing amounts of EG. The total capacitance per composite, which is proportional to the area of CV, does not change very much with increasing amount of EG as shown in Fig. 6(a) by open symbols, but the capacitance per net amount of MnO2 is almost proportional to EG content as indicated by filled symbols. It is obvious that utilization ratio of MnO2 increases with EG content and the capacitance for EG-24 is 220 F g(MnO2 )−1 . Similar effect of carbon black on Mn–Ni mixed oxide was reported, but good rectangular char-

Fig. 6. Summaries of the total capacitance calculated from cyclic voltammograms at 2 mV s−1 . Open symbols are calculated per amounts of composite in the electrode and filled ones per net amounts of MnO2 . See Table 1 for sample codes.

C. Wan et al. / Electrochimica Acta 52 (2007) 3061–3066

Fig. 7. Cyclic voltammograms at different scan rates for EG-24 (cf. Table 1).

acteristics was attained by addition of 15 mass% of carbon black [11]. Increment of EG content, however, decreases the amount of active mass in the electrode, so that there is a limit. When the total capacitance of electrode is an issue, decent total capacity can be attained by addition of 2 mass% of EG in the present case (Fig. 6). The capacitance, however, decreased with increasing scan rate, 95.5 F g−1 at 5 mV s−1 and 70.7 F g−1 at 10 mV s−1 , while EG-13 and EG-24 did not show such large decreases. Contrary to expectations, specific surface area does not involve capacitance significantly as shown in Fig. 6(b). The total capacitance is slightly decreasing with increasing surface area. Decreases of capacitance per net amount of MnO2 are much more marked as shown by filled symbols. This is not attributed to pore size, since sufficiently large mesopores are present (Fig. 3). As the greater part of surface area is originated from MnO2 ·nH2 O, the results in Fig. 6(b) suggest that the contribution from electric double layer is minimal in these electrodes. The results that there was no correlation between

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Fig. 8. Effect of potential scan rate on the capacitance for electrodes with different conductive materials. Open symbols are calculated per amounts of composite in the electrode and filled ones per net amounts of MnO2 . See Table 1 for sample codes.

capacitance and the specific surface area of MnO2 ·nH2 O were reported also by other investigators [5,11]. CVs at different scan rates for EG-24 are shown in Fig. 7. Although the potential jump at lower and higher ends delays and the capacitance decreases with increasing scan rate, the rectangular characteristics are good up to 10 mV s−1 . EG of 45 ␮m in nominal diameter used for comparison was found to be not well expanded by SEM because of too small size of the natural graphite used. So far only one sample EGS-8 was tested (Table 1). Specific capacitance of EGS-8 at 2 mV s−1 is plotted in Fig. 6(a). Rectangular shape was kept up to 5 mV s−1 but distorted at 10 mV s−1 . The composite with artificial graphite powder, SFG-6–11, has similar composition with EG-13 (Table 1) and cyclic voltammogram at 2 mV s−1 was nearly the same with that of EG-13 (Fig. 5), but the shape was distorted even at 5 mV s−1 . The effect of potential scan rate on

Fig. 9. Galvanostatic charge/discharge behavior of EG-24 (cf. Table 1).

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the capacitance of electrodes with different conductive materials is summarized in Fig. 8. The amount of carbon in the electrode is not the same but it is possible to draw a comparison. Fig. 8 shows that small size of EG may not be a preferable factor and fine graphite powder also gives poor performance. It is evident that the increased amounts of EG effectively prevent the lowering of capacitance with raising scan rate (EG-24 in Fig. 8). These results implies that sheet-like structure is more effective than small particles as conductive materials, when the formation procedure of composite is the same. Fig. 9 shows the charge/discharge behavior of EG-24 in a potential range of 0–1.0 V versus SCE at different current densities. The curves are approximately symmetric and linear for both charge and discharge portions, except for small ir drops of less than 5  at all current densities. The averages of charge/discharge capacitances at 0.1 and 0.2 A g−1 were 124 F g(composite)−1 (205 F g(MnO2 )−1 ). The potential changes at 0.1 and 0.2 A g−1 in Fig. 9 are about 0.8 and 1.7 mV s−1 , respectively, but the capacitance is comparable to that by CV at 2 mV s−1 (Fig. 7). It is considered that the surface condition of electrode may be altered immediately after galvanostatic polarization, that is, under potential scanning condition current increases gradually during the initial few hundred seconds, whereas under galvanostatic condition the electrode is compelled to react at high rate from the beginning. In the charge/discharge measurements for Fig. 9, the initial conditioning of electrode (described in the experimental section) was not carried out. 4. Conclusion Commercially available low cost exfoliated graphite (nominal diameter 130 ␮m) was used as a conductive substrate for the first time for electrochemical capacitor of hydrated Mn(IV) oxide, MnO2 ·nH2 O. The most common combination of potassium permanganate, KMnO4 and manganese(II) acetate, Mn(OCOCH3 )2 , solutions were selected and two routes were attempted to form MnO2 ·nH2 O–EG composites: method (1) was addition in the order of KMnO4 , EG, then Mn(OCOCH3 )2 and method (2) Mn(OCOCH3 )2 , EG, then KMnO4 . After a series of experiments, the composites formed by method (2) were found to have no advantage over those by method (1). By method (1) submicrometer or smaller sized MnO2 ·nH2 O particles having mesopores of 6–12 nm in diameter were formed on the graphite sheets of EG. The composites showed good rectangular cyclic voltammograms at 2–10 mV s−1 in 1 mol L−1 Na2 SO4 . Although EG alone showed only about 2 F g−1 , the capacity per net amount of MnO2 increased almost proportionally with EG content, that is, utilization ratio of MnO2 increased with EG content. Experimental results suggested that specific surface area did not involve capacity, that is, the contribution

from electric double layer was minimal with the electrodes prepared in the present work. The composites of MnO2 ·nH2 O and smaller diameter of EG (nominal diameter 45 ␮m) or artificial graphite powder (average diameter 3.7 ␮m) showed fairly good performance at 2 mV s−1 , but with increasing potential scan rate the rectangular shape was distorted and capacitance decreased drastically. The results implies that sheet-like structure is more effective than small particles as conductive materials, when the formation procedure of composite is the same. Large sized EG may be a promising conductive material for electrochemical capacitors. Acknowledgement A part of the present work was supported by the Grant-in-Aid for Scientific Research (B) from JSPS (no. 18350102). References [1] H.Y. Lee, J.B. Goodenough, J. Solid State Chem. 144 (1999) 220. [2] S.C. Pang, M.A. Anderson, T.W. Chapman, J. Electrochem. Soc. 147 (2000) 444. [3] H.Y. Lee, S.W. Kim, H.Y. Lee, Electrochem. Solid-State Lett. 4 (2001) A19. [4] C.C. Hu, T.W. Tsou, Electrochem. Commun. 4 (2002) 105. [5] Y.U. Jeoung, A. Manthiram, J. Electrochem. Soc. 149 (2002) A1419. [6] J.H. Jiang, A. Kucernak, Electrochim. Acta 47 (2002) 2381. [7] C.C. Hu, T.W. Tsou, Electrochim. Acta 47 (2002) 3523. [8] M. Toupin, T. Brousse, D. B´elanger, Chem. Mater. 14 (2002) 3946. [9] C.C. Hu, T.W. Tsou, J. Power Sources 115 (2003) 179. [10] R.N. Reddy, R.G. Reddy, J. Power Source 124 (2003) 330. [11] H. Kim, B.N. Popov, J. Electrochem. Soc. 150 (2003) D56. [12] D.J. Jones, E. Wortham, J. Rozi´ere, F. Favier, J.L. Pascal, L. Monconduit, J. Phys. Chem. Solids 65 (2004) 235. [13] M.S. Wu, P.C.J. Chiang, Electrochem. Solid-State Lett. 7 (2004) A123. [14] M. Wu, G.A. Snook, G.Z. Chen, D.J. Fray, Electrochem. Commun. 6 (2004) 499. [15] J.K. Chang, C.T. Lin, W.T. Tsai, Electrochem. Commun. 6 (2004) 666. [16] Y.T. Wu, C.C. Hu, J. Electrochem. Soc. 151 (2005) A2066. [17] E. Raymundo-Pi˜nero, V. Khomenko, E. Frackowiak, F. B´eguin, J. Electrochem. Soc. 152 (2005) A229. [18] G.X. Wang, B.L. Zhang, Z.L. Yu, M.Z. Qu, Solid State Ionics 176 (2005) 1169. [19] C.Y. Lee, H.M. Tsai, H.J. Chuang, S.Y. Li, P. Lin, T.Y. Tseng, J. Electrochem. Soc. 152 (2005) A716. [20] W. Stumm, J.J. Morgan, J. Colloid Sci. 19 (1964) 347. [21] H. Konno, T. Kinomura, H. Habazaki, M. Aramata, Carbon 42 (2004) 737. [22] H. Konno, T. Kinomura, H. Habazaki, M. Aramata, Surf. Coatings Technol. 194 (2005) 24. [23] H. Konno, T. Morishita, S. Sato, H. Habazaki, M. Inagaki, Carbon 43 (2005) 1111. [24] H. Konno, D. Abe, H. Habazaki, Tanso (221) (2006) 8. [25] M. Toyoda, M. Inagaki, Carbon 38 (2000) 199. [26] M. Toyoda, K. Moriya, J. Aizawa, H. Konno, M. Inagaki, Desalination 128 (2000) 205. [27] S. Mitra, S. Sampath, Electrochem. Solid State Lett. 7 (2004) A264. [28] E.P. Barrett, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373.