J. Phys. Chem. B 2005, 109, 20207-20214
20207
Hydrothermal Synthesis and Pseudocapacitance Properties of MnO2 Nanostructures V. Subramanian,† Hongwei Zhu,† Robert Vajtai,‡ P. M. Ajayan,‡ and Bingqing Wei*,† Department of Electrical and Computer Engineering and Center for Computation and Technology, Louisiana State UniVersity, Baton Rouge, Louisiana 70803, and Rensselaer Nanotechnology Center and Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180 ReceiVed: August 4, 2005; In Final Form: September 8, 2005
The effect of varying the hydrothermal time to synthesize manganese oxide (MnO2) nanostructures was investigated along with their influence on structural, morphological, compositional, and electrochemical properties in supercapacitor electrode materials. XRD and TEM studies showed that the MnO2 prepared in shorter hydrothermal dwell time was a mixture of amorphous and nanocrystalline particles, and there was an evolution of crystallinity of the nanostructures as the dwell time increased from 1 to 18 h. Interestingly, SEM, TEM, and HRTEM revealed a variety of structures ranging from nanostructured surface with a distinct platelike morphology to nanorods depending upon the hydrothermal reaction time employed during the preparation of the manganese oxide: increasing the amount of individual nanorods in the materials prepared with longer hydrothermal reaction time. The surface area of the synthesized nanomaterials varied from 100 to 150 m2/g. Electrochemical properties were evaluated using cyclic voltammetry (CV) and galvanostatic charge-discharge studies, and the capacitance values were in the range 72-168 F/g depending upon synthesis conditions. The formation mechanism of the nanorods and their impact on the specific capacitance were discussed in detail.
Introduction In recent years, due to environmental issues and depleting fossil fuels, interest in the development of alternative energy storage/conversion devices with high power and energy densities catering the present day demands has increased to a greater extent. Electrochemical capacitors or supercapacitors (SCs) have gained enormous attention owing to their potential applications ranging from mobile devices to electric vehicles (EV).1 SCs are broadly classified into two categories, electrical double layer capacitors (EDLCs) and pseudocapacitors depending on the nature of charge storage mechanism. EDLCs exhibit a nonfaradic reaction with accumulation of charges at the electrodeelectrolyte interfaces, while pseudocapacitors show faradic redox reactions. Different types of carbonaceous material ranging from amorphous carbons to carbon nanotubes have been used as electrode materials in EDLCs.2-5 In the case of pseudocapacitors, various noble and transition metal oxides such as RuO2, IrO2, NiO, CoOx, SnO2, and MnO2 were used as electrode materials.6-10 Of all the transition metal oxides studied as pseudocapacitor materials, hydrated RuO2 has been found to be most promising. However, the high cost of RuO2 has prompted the research community to focus on other transition metal oxides such as MnO2, NiO, and so forth, mainly because of the involved cost-effectiveness. In addition, hydrated RuO2 shows excellent performance mostly only in a highly acidic electrolyte such as sulfuric acid. The basic idea behind the choice of highly acidic electrolyte solutions for SC applications mainly relates to the fast charge and discharge leading to a high power density. This is because protons have better access not only to * To whom correspondence should be addressed. Email:
[email protected]. † Louisiana State University. ‡ Rensselaer Polytechnic Institute.
the surface of the electrode but also to the interior of the electrode than larger alkali ions such as K+ or Na+.6 Hence, the chemisorption of the H+ in an acidic-hydrated oxide electrode system is exceptionally fast, leading to a promising pseudocapacitor material.6 However, the main disadvantage of using a highly acidic electrolyte is the dissolution of metal oxide over a period of cycling time. This leads to SC showing a faster fading in capacitance with respect to cycling. Hence, alternative materials which are much cheaper and more promising in a neutral electrolyte system such as Na2SO4, KCl, LiCl, and so forth have been investigated in recent years.11-13 Of the various non-noble metals or transition metal oxides studied, MnO2 enjoys a place of pride because of its lower cost and environmentally benign nature. Beyond these advantageous properties, MnO2 is very promising in a neutral electrolyte system.11-17 Manganese oxides as pseudocapacitor electrode materials were synthesized using different techniques such as simple reduction, coprecipitaion, sol-gel, thermal decomposition, and so forth.11-17 Also, various thin film electrodes of MnO2 were synthesized via electrochemical and chemical routes.18,19 Kim and Papov prepared MnO2 and Pb,Ni-mixed MnO2 by the reduction of KMnO4 with Mn/Ni/Pb acetate solutions.11 Jeong and Manthiram reported the synthesis of MnO2 by the reduction of KMnO4 using potassium borohydride, sodium dithionate, and sodium hypophosphite.12 Lee et al. synthesized MnO2 by a simple thermal decomposition of finely ground KMnO4 powder at different temperatures ranging 300-1000 °C.13 Simple precipitation of MnO2 was achieved by mixing aqueous solutions of KMnO4 and MnSO4.14 Precipitation of MnO2 from aqueous solutions of Mn acetate and KMnO4 was also reported.20 Hydrothermal synthesis has been an interesting technique to prepare materials with different nanoarchitectures such as
10.1021/jp0543330 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/12/2005
20208 J. Phys. Chem. B, Vol. 109, No. 43, 2005 nanowires, nanorods, nanobelts, nanourchins, and so forth.21,22 The main advantages of the hydrothermal technique over other soft chemical routes are the abilities to control the nanostructures ranging from nanoparticles to nanorods or nanourchins to nanotubes by properly choosing the temperature or time of the reaction or the active fill level in the pressure vessel or solvent used for the reaction without any major structure-directing agents or templates. Each structure has its own merits when used in a potential application such as photocatalyst, electrodes in rechargeable batteries, supercapacitors, electrocatalysts for fuel cells, and so forth. The nature of the nanomaterial formed by the hydrothermal route depends on various factors such as solvent used, temperature and time of the reaction, effective fill level in the pressure vessel, and so forth. The factors mentioned above can be judicially chosen to get the desired architecture. Apart from the aforementioned advantages, the starting material for the reaction needs not be soluble as in the case of other soft or wet chemical methods such as sol-gel or coprecipitation. MnO2 has been prepared using hydrothermal techniques in different nano phases such as rods, urchins, wires, and so forth.21,22 In this paper, we report the synthesis of various nanostructures of MnO2 by a hydrothermal route under mild conditions. The change in the nanoarchitectures was achieved by simply tuning the hydrothermal reaction time. The corresponding variation in morphology, surface properties, and electrochemical properties as supercapacitor electrodes were studied elaborately and discussed in detail. Experimental Section Synthesis of MnO2 was carried out hydrothermally starting with aqueous solutions of MnSO4‚H2O and KMnO4 following the procedure reported by Wang and Li22 with little modification. The well-mixed aqueous solutions of KMnO4 and hydrated MnSO4 were transferred to a Teflon-lined pressure vessel (PARR Instruments, U.S.A.) and loaded into an oven preheated to 140 °C. The dwell time for the reaction has been varied from 1 to 18 h in order to optimize the material for electrochemical applications. The pressure vessel was allowed to cool to room temperature naturally after the dwell time at 140 °C. The precipitate formed was filtered and washed with distilled water until all the unreacted materials were removed. The washing was done until the pH of the washed water was 7. The precipitated MnO2 was dried at 100 °C in air. The same amounts of the starting materials were left in a beaker overnight for the formation of amorphous MnO2 precipitate in order to see the structural evolution of the MnO2 nanorods from room temperature to the time-dependent hydrothermal treatment. A schematic representation of the structural evolution of nanorods, as well as the corresponding transmission electron microscopy (TEM) images, is shown in Figure 1. The phase purity of synthesized materials was studied using X-ray diffraction (XRD) (Siemens X-ray Diffractometer, Germany). The particle morphology and structural properties of the prepared MnO2 were further elucidated by scanning electron microscopy (SEM) (Hitachi S 3600N, Japan) and high-resolution transmission electron microscopy (HRTEM) (JEOL 2010, Japan) studies. Energy-dispersive X-ray (EDX) was used to confirm the composition of the formed oxide and the presence of other metal ions such as potassium. The surface properties of the synthesized oxides were studied in detail by X-ray photoelectron spectroscopy (XPS) (Kratos AXIS 165 XPS/SAM, U.S.A.). The surface area of the synthesized materials was studied using Brunauer-Emmett-Teller (BET) measurements (Quantachrome Instruments, Model NOVA 2000 Series, U.S.A.).
Subramanian et al. The electrodes for evaluating the electrochemical properties of the synthesized MnO2 were fabricated by mixing the prepared MnO2 with 20 wt % carbon black (Black Pearl 2000, Cabot Corp., U.S.A.) and 5 wt % PVdF-HFP binder. A slurry of the above mixture was made using N-methyl-2-pyrrolidone (NMP) as a solvent which was subsequently brush-coated onto a Ni mesh. The mesh was dried at 110 °C in air for 1 h for the removal of the solvent. After drying, the coated mesh was uniaxially pressed to more competely adhere the electrode material with the current collector. Cyclic voltammetry (CV) studies were performed using a potentiostat/galvanostat (PGSTAT30, Autolab, EchoChemie, The Netherlands) in a three-electrode configuration with the Ni mesh coated with MnO2 as the working electrode, Pt wire as the counter, and saturated calomel electrode (SCE) as the reference. CV was done between -0.2 and 0.8 V in a 1 M Na2SO4 electrolyte at different scan rates. The specific capacitance was evaluated from the area of the charge and discharge curves of the CV plot. Galvanostatic charge-discharge experiments were performed in a similar setup as described above, with a specific current of 200 mA/g and between 0 and 1 V. The specific capacitance of the system has been evaluated using the formula
specific capacitance, C (F/g) ) i∆t/m∆V
(1)
where i is the current used for charge/discharge, ∆t is the time elapsed for the charge or discharge cycle, m is the mass of the active electrode, and ∆V is the voltage interval of the charge or discharge. Results and Discussions A schematic representation of the formation of the MnO2 nanorods is shown in Figure 1. The typical corresponding TEM images are shown on the lower part of the Figure 1. It can be clearly noticed that mixing hydrated MnSO4 and KMnO4 immediately leads to the formation of a precipitate of MnO2. The TEM picture shows the formation of flowerlike nanowhiskers of MnO2 at room temperature. When hydrothermally treated for 1 h at 140 °C, there has been an increase in the size of the individual whiskers which replicates the formation of a nanostructured surface with a distinct platelike morphology as observed by SEM (Figure 2). The largely agglomerated nanowhiskers show an increase in density and tend to loosen as the hydrothermal dwell time increases. A subsequent increase in the hydrothermal dwell time not only increases the size of the individual nanowhiskers but also leads to the formation of a rodlike structure in nanoscale as shown in the Figure 1, for example, the MnO2 nanorods prepared for 12 h. Thus, the formation of nanorods evolves from the nanowhiskers formed at room temperature. The hydrothermal treatment has led to the formation of rodlike architectures wherein the size of the individual nanorods varies with respect to the hydrothermal dwell time. The electron diffraction (ED) and HRTEM measurements performed for MnO2 prepared hydrothermally at 140 °C for 1 and 18 h are shown as an inset in the respective TEM pictures in Figure 1. The ED recorded for the sample prepared for 1 h shows initiation of the nucleation of nanocrystalline particles. However, the lattice fringes are not clearly evolved at 1 h of hydrothermal dwell time. In the case of the material prepared at 18 h, there has been a well-developed crystallinity as shown by the ED studies and well-supported by the HRTEM observations. The lattice fringes are explicitly clear with a d spacing of 4.8 Å for the (002) plane. There has been a strong correlation on the hydrothermal dwell time with respect to the
MnO2 Nanostructures
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Figure 1. Schematic representations and TEM images of formation of MnO2 nanorods under hydrothermal conditions. Inset of 1 h and 18 h TEM shows the ED and HRTEM of the MnO2 prepared at the respective hydrothermal time.
pore structure and surface area of the MnO2 nanostructures, which, in turn, has a direct effect on the electrochemical properties of the materials. The evolution of the nanostructurs of MnO2 prepared at different hydrothermal dwell time is clearly shown with SEM images in Figure 2. As can be seen from the pictures, there has been a strong correlation between the time and the resulting nanostructure. For the material prepared at room temperature, there have been largely agglomerated nanowhiskers of MnO2 which change to nanostructured platelike morphology on hydrothermal treatment for 1 h. Increasing the dwell time to 3 h and 6 h showed a corresponding change in the nanoarchitecture with few rods evolving in addition to the nanostructured platelike morphology. Further increasing the hydrothermal reaction time to 12 h and 18 h leads to formation of large amount of individual nanorods. All these morphological manipulations are the biggest advantage of the hydrothermal technique. The XRD patterns for the prepared MnO2 at different time are shown in Figure 3. The material prepared at room temperature showed an amorphous MnO2 having peaks evolving to an R-MnO2 structure, and the details are discussed elsewhere.23 In the case of MnO2 prepared at 140 °C for 1 h, the system showed evolution of the nucleation of the nanocrystalline nature of the MnO2 particles (also see inset of Figure 1) with peaks indexable in R-MnO2 structure. The broad peak features show
the mixture of amorphous and nanocrystalline nature of the sample because of the nanowhiskers of flowerlike architecture of MnO2. There is an increase in crystallinity with respect to the increase of the hydrothermal dwell time from 1 h to 18 h as evidenced by the appearance of sharper peaks (also, please see the inset of the TEM in Figure 1 for 18 h MnO2 showing ED and HRTEM). As can be seen from the figure, all the peaks for the sample prepared at a dwell time of 12 h and longer can be indexed to a pure tetragonal phase (space group I4/m (no. 87)) of R-MnO2 (JCPDS 44-0141). There has always been a strong correlation between the crystallinity and the surface area of the system on the electrochemical properties as discussed later. N2 adsorption-desorption studies were performed to determine the specific surface area of the MnO2 prepared at different hydrothermal reaction times. The BET surface area was found to be 100 m2/g for 1 h hydrothermally treated sample. Interestingly, an increase in the hydrothermal dwell time to 6 h and 12 h showed an increase in the surface area to 132 m2/g and 150 m2/g, respectively. Although XRD showed a more amorphous material at shorter dwell time than longer, the surface area is more dependent on the pore structure where the N2 adsorptiondesorption takes place. Here, as explained previously in the schematic model for the evolution of nanorods, the materials prepared with a shorter dwell time exhibit more agglomerated nanowhiskers of MnO2, which eventually leads to the nanorod
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Figure 2. SEM images of MnO2 prepared at different hydrothermal dwell times.
architecture when the dwell time is increased. This leads to the increase in pore volume associate with the system. However, the pore diameters for MnO2 prepared at different dwell times were in the mesoporous region with a value of 8 nm. XPS is an excellent technique to understand the oxidation state of the transition metal ion and the relative composition of the synthesized material. Typical XPS spectra for the MnO2 prepared for 1 h are shown in Figure 4. Similar spectra were observed for the MnO2 prepared for different dwell times. The Mn 3s, Mn 2p, and O 1s spectra were used to evaluate the oxidation state of the Mn in MnO2 prepared for different hydrothermal times. It has been well-established by many earlier studies that the Mn 3s will have a splitting and a doublet due to the parallel spin coupling of the 3s electron with the 3d electron during the photoelectron ejection.14 We have found the doublet for the Mn 3s as reported earlier. Most important, the separation between the peaks of the doublet will give an idea
of the oxidation state of the Mn ion. In the present study for all the reaction times, the difference of the Mn 3s spectra in the peak energy for the splitting corresponds to values between 4.7 and 4.8 eV. According to Toupin et al.14 and other researchers,15,16 when the oxidation state of Mn is +4, the binding energy difference is in the range 4.7-4.8 eV. This clearly shows that the MnO2 prepared by the hydrothermal technique has resulted in MnO2 phase, consistent with XRD results. Also, the Mn 2p3/2 and 2p1/2 in the Mn 2p spectra for MnO2 prepared at different dwell times of 1, 6, and 12 h were found to be 642 and 653.8 eV, 642.3 and 654.1 eV, and 642.1 and 654 eV, respectively. These values agree well with those reported for the MnO2 indicating that the oxidation state is +4 and further confirming the earlier claim. Similarly, the amount of potassium present in the synthesized MnO2 was quantified from K 2p spectra relative to the Mn 2p. It is interesting to note that the potassium content seems to
MnO2 Nanostructures
J. Phys. Chem. B, Vol. 109, No. 43, 2005 20211 TABLE 1: Summary of Structural, Surface/Morphological, and Electrochemical Properties of Nanostructured MnO2 Prepared Hydrothermally at 140 °C for Different Times synthesis time (h) 1 3 6
9
12 18
structure nanostructured surface with a distinct platelike morphology nanostructured surface with a distinct platelike morphology nanostructured surface with a distinct platelike morphology + nanorods nanostructured surface with a distinct platelike morphology + nanorods nanorods nanorods
surface area (m2/g)
specific capacitancea (F/g)
100
140 140
132
168
115
150
118 72
a Calculated from CV studies at a scan rate of 5 mV/s between -0.2 and 0.8 V in a 1 M Na2SO4 aqueous electrolyte.
Figure 3. X-ray diffraction patterns of MnO2 synthesized hydrothermally at 140 °C for different times.
increase with the increase in the hydrothermal reaction time from 1 h to 12 h. The relative percentage of potassium with respect to manganese can be estimated from the K 2p and Mn 2p spectra. The K/Mn ratios for MnO2 prepared at 1, 6, and 12 h were 0.13, 0.15, and 0.16, respectively. This is probably due to the incorporation of potassium ion under the autogenous pressure condition into the MnO2 matrix as the dwell time is increased. The presence of a larger quantity of K+ inside the porous MnO2 matrix will have an appreciable effect on the electrochemical properties. Typical cyclic voltammograms recorded at a scan rate of 5 mV/s for MnO2 prepared at 140 °C for different time are shown in Figure 5. It can be seen from the figures that there has been an ideal capacitive behavior with the plots showing an almost rectangular profile. The area under the CV curve can be used to estimate the capacitance of the system. The variation of specific capacitance with respect to the hydrothermal dwell time is shown in Table 1. MnO2 prepared for 6 h showed the maximum capacitance with a value of 168 F/g. There has been an increase in capacitance from 140 F/g for the sample prepared at 1 h to 168 F/g for the sample prepared for 6 h, which then decreased to 118 F/g for the 12 h and 72 F/g for 18 h. Similar results have been observed in the case of temperature-dependent capacitance for MnO2 prepared at different temperatures.14 Also, the system prepared at 140 °C for 1 h is more amorphous, and there is a large agglomeration of the particles or the slablike morphology, which prevents the accessibility of the electrolyte for the electrochemical reactions. The increase in the hydrothermal dwell time to 3 h and 6 h loosens the agglomeration leading to better capacitance because of better electrolyte accessibility. The variation of specific capacitance with respect to dwell time is attributed to the increase in the crystallinity, specific surface area, coexistence of more metal ions such as K+, and the loss of the chemically and physically adsorbed water14-25 from MnO2. The cycling capability of the MnO2-6h electrode was studied galvanostatically at a constant current of 200 mA/g and between 0 and 1 V. Typical charge-discharge curves for the MnO2-6h
sample are shown in Figure 6. The specific capacitance was found to be 168 F/g which is very much comparable to that calculated from the CV studies. Further work on long cycle performance and impedance spectroscopy is in progress. It has to be mentioned here that, since the present electrode is a composite one with a significant amount of Black Pearl 2000 (BP2000), which is a high surface area material that can contribute to the total capacity of the material, the contribution has to be subtracted from the total specific capacitance. The values reported in this paper are after the elimination of the contribution from BP2000. The capacitance of BP2000 has been estimated by Kim and Popov to be 70 F/g11 in a 1 M Na2SO4 electrolyte. The specific capacitance of BP2000 in 1 M H2SO4 is 216 F/g, which is much lower than in a 1 M Na2SO4 electrolyte, which is mainly due to size considerations arising from the Na+ and H+ ions and the formation of the double layer at the electrode-electrolyte interface.11 Hence, it is clear that the ionic size in the electrolyte plays a critical role in the resultant capacitance of a system. There have been two mechanisms proposed for the charge storage in MnO2-based electrodes. The first one is based on the concept of intercalation of H+ or alkali metal cations such as Na+ in the electrode during reduction and deintercalation upon oxidation24
MnO2 + C+ + e- T MnOOC+
(2)
The second one is the adsorption of cations in the electrolyte on the MnO2 electrode13
(MnO2)surface + C+ + e- T (MnO2-C+)surface
(3)
In the present study, the redox process is mainly governed by the insertion and deinsertion of Na+ and or H+ from the electrolyte into the porous MnO2 matrix. The reason for the increase in the capacitance with the increase in the hydrothermal treatment time may be attributed to the increase in the surface area of the material from 100 to 132 m2/g when the reaction time is increased from 1 to 6 h. However, there has been an observed decrease in capacitance for the material prepared for 12 h, which has a higher surface area (150 m2/g) than that for 6 h. This has to be carefully considered, as many parameters
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Figure 4. Typical XPS spectra for MnO2 prepared at 140 °C for 1 h.
such as pore structure, pore volume, and chemical composition have to be taken into account when discussing the electrochemical properties. Similar results of systems showing lower capacitance for a larger surface area material have been reported in the literature.12,16,17 Two important parameters have to be considered in present study, the surface area and the effective potassium concentration in the MnO2 matrix. It has been reported that an increase in specific surface area will lead to an increase in the specific capacitance and also a decrease in the molecular weight will lead to a same result. Here, there has been an increase in the specific surface area and also a corresponding increase in the molecular weight because of the presence of more K+ in the
MnO2 matrix when the reaction time is increased from 1 to 12 h as we discussed earlier. The increase in surface area in the case of the 12 h sample does not prove that effective because longer hydrothermal time leads to a system with more crystallinity but less chemisorbed water as well. Also, an increase in the presence of the K+ ion inside the MnO2 matrix will have an impeding effect for the diffusion of the Na+ ion from the electrolyte, which is the predominant reaction for the redox process. This is because the ionic radius of K+ is much bigger than that of Na+ (K+ ) 1.33 Å and Na+ ) 0.95 Å), which will obviously have an effect on the diffusion of Na+ ions. The increase of the potassium ion content is a result of incorporation of K+ from the KMnO4 during the hydrothermal reaction. So,
MnO2 Nanostructures
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Figure 5. Cyclic voltammograms of MnO2 nanostructures prepared at different dwell times recorded at a scan rate of 5 mV/s in a 1 M Na2SO4 aqueous electrolyte.
it is clear that the surface area is not the only factor that determines the electrochemical properties. Other parameters such as coexisting ions and their impact must also have to be considered. The structural, surface/morphological, and electrochemical properties of the prepared nanostructured MnO2 samples for different synthesis times are summarized in Table 1. From the present study, it is conclusive that the MnO2 prepared for 6 h has an optimum surface area and K+ content and hence showed the best electrochemical properties.
The effect of scan rate was performed by varying the scan rate of CV as 1, 2, 5, 10, 25, and 50 mV/s on the MnO2 sample prepared for 6 h. As the scan rate increases, the CV profile deviates from the ideal capacitive behavior. This is mainly because the redox reactions depend on the insertion-deinsertion of the alkali ion or protons from the electrolyte as explained earlier in eqs 2 and 3. At slower scan rates, the diffusion of ions from the electrolyte can gain access to almost all available pores of the electrode, leading to a complete insertion reaction
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Subramanian et al. nanostructures prepared hydrothermally at 140 °C for 6 h showed the best performance in the present study with a specific capacitance of 168 F/g when cycled at a constant current of 200 mA/g. Increasing the scan rate in CV studies revealed a decrease in specific capacitance. Further work is in progress to see the effect of long cycle performance on the electrode structure and the associated structural and impedance changes. Acknowledgment. The authors gratefully acknowledge financial support from Louisiana Board of Regents under the award number LEQSF(2005-08)-RD-B-05 and National Science Foundation under the NSF award number DMI-0457555. R.V. and P.M.A. are grateful to Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF award number DMR-0117792.
Figure 6. Typical charge-discharge cycling for MnO2 prepared at 140 °C for 6 h recorded at a constant current of 200 mA/g.
and hence a reduction process, and the reverse happens during the deinsertion process as well. But, when the scan rate is increased, the effective interaction between the ions and the electrode is greatly reduced; hence, there is a reduction in capacitance. The decrease in capacitance with respect to the increase in the scan rate, which is predominantly due to the slow diffusion of Na+ ions into the pores of MnO2, i.e., when the scan rate is higher the effective utilization for the redox reaction has been limited only to the outer surface of MnO2 electrode. The outer charge and the total charge have been calculated by following the earlier works,14-17 and it has been found that the total charge is 190 C/g and the outer charge is 83 C/g. The difference between the outer charge, qT*, and qo* gives the inner voltammetric charge qi*, which is the inaccessible active surface area for storage when the scan rate is high.26 This indicates that the effective charge when the scan rate is high arises not from the entire electrode but instead the outer surface near the electrode-electrolyte interface. The main reason for such a behavior is that the higher sweep rate prevents the accessibility of Na+ ions to all the pores of the electrode. Conclusions MnO2 with different nanoarchitectures has been successfully synthesized by varying the hydrothermal reaction time at 140 °C, and the pseudocapacitance properties of hydrothermally prepared MnO2 nanostructures were studied for the first time. The XRD, TEM, and BET studies showed an amorphous phase at shorter dwell time, and increasing the time led to an increase of crystalline nanorods as well as of the surface area. MnO2
References and Notes (1) Conway, B. E. Electrochemical Capacitors: Scientific Fundamentals and Technological Applications; Kluwer Academic/Plenum: New York, 1999. (2) Frackowiak, E.; Be´guin, F. Carbon 2001, 39, 937. (3) Endo, M.; Maeda, T.; Takeda, T.; Kim, Y. J.; Koshiba, K.; Hara, H.; Dresselhaus, M. S. J. Electrochem. Soc. 2001, 148, A910. (4) Taberna, P. L.; Simon, P.; Fauvarque, J. F. J. Electrochem. Soc. 2003, 150, A292. (5) Bonhomme, F.; Lassegues, J. C.; Servant, L. J. Electrochem. Soc. 2001, 148, E450. (6) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699. (7) Liu, K. C.; Anderson, M. A. J. Electrochem. Soc. 1996, 143, 124. (8) Yoon, Y. S.; Cho, W. I.; Lim, J. H.; Choi, D. J. J. Power Sources 2001, 101, 126. (9) Conway, B. E.; Briss, V.; Wojtowicz J. Power Sources 1997, 66, 1. (10) Lin, C.; Ritter, J. A.; Popov, B. N. J. Electrochem. Soc. 1998, 145, 4097. (11) Kim, H.; Popov, B. N. J. Electrochem. Soc. 2003, 150, D56. (12) Jeong, Y. U.; Manthiram, A. J. Electrochem. Soc. 2002, 149, A1419. (13) Lee, H. Y.; Goodenough, J. B. J. Solid State Chem. 1999, 144, 220. (14) Toupin, M.; Brousse, T.; Belanger, D. Chem. Mater. 2002, 14, 3946. (15) Toupin, M.; Brousse, T.; Belanger, D. Chem. Mater. 2004, 16, 3184. (16) Reddy, R. N.; Reddy, R. G. J. Power Sources 2003, 124, 330. (17) Reddy, R. N.; Reddy, R. G. J. Power Sources 2004, 132, 315. (18) Pang, S. C.; Anderson, M. A. J. Mater. Res. 2000, 15, 2096. (19) Hu, C. C.; Tsou, T. W. Electrochem. Comm. 2002, 4, 105. (20) Lee, H. Y.; Mannivanan, V.; Goodenough, J. B. C. R. Acad. Sci. Paris 1999, t.2, series II c, 565. (21) Du, G. H.; Yuan, Z. Y.; Van Tendeloo, G. Appl. Phys. Lett. 2005, 86, 063113. (22) Wang, X.; Li, Y. Chem. Commun. 2002, 764. (23) Subramanian, V.; Wei, B. Q. In preparation. (24) Pang, S. C.; Anderson, M. A.; Chapman, T. W. J. Electrochem. Soc. 2000, 147, 444. (25) Bruke, A. J. Power Sources 2000, 91, 37. (26) Soudan, P.; Gaudet, J.; Guay, D.; Belanger, D.; Schulz, R. Chem. Mater. 2002, 14, 1210.
