IMPROVED CAPACITIVE BEHAVIOR OF MnO2 THIN

April 11, 2009 20:35 00047

Functional Materials Letters Vol. 2, No. 1 (2009) 13–18 © World Scientific Publishing Company

IMPROVED CAPACITIVE BEHAVIOR OF MnO2 THIN FILMS PREPARED BY ELECTRODEPOSITION ON THE PT SUBSTRATE WITH A MnOx BUFFER LAYER H. XIA, W. XIAO, M. O. LAI and L. LU∗ Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576

Funct. Mater. Lett. 2009.02:13-18. Downloaded from www.worldscientific.com by 201.59.201.82 on 03/11/13. For personal use only.

Received 5 January 2009; Revised 29 January 2009

Nanostructured MnO2 thin films were prepared on two types of substrates, Pt/Ti/SiO2 /Si (PT) and MnO x /Pt/Ti/SiO2 /Si (MnO x /PT), by the technique of cyclic-voltammetric electrodeposition. The MnOx buffer layer was deposited on the PT substrate by pulsed laser deposition (PLD). The as-deposited MnO2 thin films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS). The electrochemical properties of the thin film MnO2 electrodes were investigated using cyclic voltammetry (CV) in 1 M Na2 SO4 electrolyte. It was found that the adhesion between the MnO2 film and the Pt substrate was poor, resulting in cracks and peeling of the MnO2 film after deposition. However, the adhesion of the MnO2 film with the MnOx buffer layer was greatly improved, resulting in superior pseudocapacitive performance of the thin film electrodes. A specific capacitance of about 364 F/g of MnO2 thin films deposited on the MnOx buffer layer can be obtained at a scan rate of 10 mV/s in the voltage window between 0 and 0.9 V versus the Ag/AgCl reference electrode. The MnO2 thin film deposited on the MnOx /PT substrate exhibits good rate capability and excellent cycle performance, which makes it promising for supercapacitor application. Keywords: MnO2 ; electrodeposition; supercapacitor; thin film.

As alternative energy storage devices, supercapacitors have demonstrated some advantages, such as high power capability, extremely long cycle life and high efficiency, due to their fast and reversible charge storage mechanisms.1,2 Based on their charge storage mechanisms, supercapacitors can be classified into two types: (1) electrical double layer capacitors (EDLCs), where the capacitance arises from the charge separation at the electrode–electrolyte interface, and (2) Faradic pseudocapacitors, in which the capacitance arises from the redox reactions from the electrode material. In general, EDLCs using active carbon usually suffer from their limited specific capacitance in aqueous electrolyte. Pseudocapacitors using transitional metal oxides and conducting polymers are usually capable of delivering much higher specific capacitance than EDLCs, and therefore have been attracting considerable interest recently. Many transitional metal oxides (such as Ru, Mn, Fe, Co, Ni, Cr, In, Sn, Mo, and V) are considered as potential electrode materials for pseudocapacitors.3–12 Among them, excellent pseudocapacitve behavior with a large specific capacitance of 720 F/g has been obtained with ruthenium oxide. However, the high cost and toxicity of ruthenium oxide limit its commercial applications. Therefore, intensive research has been carried out to search for alternative, inexpensive electrode materials.

A lot of efforts have been devoted to the study of manganeseoxide-based materials, due to their low cost, environmental benignity, and good electrochemical properties. Since the capacitance of a supercapacitor is proportional to the surface area of the electrode, fabrication of nanostructured manganese oxide with porous structure can provide a high surface area, which is the key to achieving a high specific capacitance. There are mainly two methods for synthesizing nanostructured manganese oxide materials; the hydrothermal method13,14 and the electrodeposition method.15,16 For the latter, the process is fast and simple. The thin film preparation with electrodepostion can be divided into galvanostatic, potentiostatic, pulsed current, pulsed potential, and cyclic-voltammetric techniques, according to different styles of applied voltage or current. In this paper, the preparation of nanostructured MnO2 thin films on the Pt/Ti/SiO2 /Si (PT) and MnOx /Pt/Ti/SiO2 /Si (MnOx /PT) substrates by cyclic-voltammetric electrodeposition is reported. The MnOx buffer layer was deposited on the PT substrate to enhance the adhesion between the MnO2 film and the PT substrate. The as-deposited thin film electrodes were assembled in three-electrode cells to investigate their electrochemical properties. Nanostructured MnO2 thin films were electrodeposited on the PT and MnOx /PT substrates by cyclic-voltammetric electrodeposition. The MnOx buffer layer was deposited on

∗[email protected]

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the PT substrate by pulsed laser deposition (PLD) using a Mn metal target for 10 min at an O2 pressure of 200 mTorr and a substrate temperature of 600◦C. The PT substrate was then used as the working electrode and a Pt foil was used as the counter electrode for the electrodeposition. An electrolyte solution composed of 1 M Na2 SO4 and 0.1 M Mn(CH3 COO)2 and a reference electrode of Ag/AgCl were used for the electrodeposition of MnO2 thin films. Electrodeposition was carried out by a cyclic-voltammetric scan between 0.2 and 0.6 V versus Ag/AgCl at a scan rate of 20 mV/s for 20 min. The structure and crystallinity of thin films were characterized using a Shimadzu XRD-6000 X-ray diffractometer with Cu Kα radiation. The surface morphology and roughness of the thin films were characterized using a Hitachi S-4100 field emission scanning electron microscope. The samples were also examined with X-ray photoelectron spectroscopy (XPS) using a VG ESCALAB MKII spectrometer with an Al X-ray source (1486.6 eV). An analyzer with a pass energy of 20 eV was adopted and the C 1s peak at 284.6 eV from adventitious carbon was used as an internal reference. The weights of the MnO2 thin films were investigated using a microbalance with an accuracy of 0.01 mg. All electrochemical measurements were conducted using a Solarton 1287 cell test system combined with a Solatron 1260 frequency response analyzer. The electrochemical properties of the as-deposited MnO2 thin films were characterized by cyclic voltammetry (CV) in 1 M Na2 SO4 electrolyte at room temperature. The three-electrode test cell was composed of a MnO2 thin film as the working electrode, a high surface carbon rod as the counter electrode, and an Ag/AgCl reference electrode. CV measurements were performed on the test cells in the voltage window between 0 and 0.9 V at different scan rates from 10 to 200 mV/s. Electrochemical impedance spectroscopy (EIS) of the test cell was investigated at open-circuit potential in the frequency range from 10 kHz to 10 mHz. It was found that the MnO2 film directly deposited on the PT substrate easily formed cracks and peeled off from the substrate when dried in air after deposition, as shown in Fig. 1(a). Although the adhesion between the as-deposited film and the substrate could be improved by increasing the roughness of the substrate such as using stainless steel or Ni foils with rough surfaces, this was not effective, while the active material (thin film) was observed to be constantly lost during the electrochemical cycling. To enhance the adhesion, a MnOx buffer layer was introduced before electrodeposition of the MnO2 film using PLD. It was found that the electrodeposited MnO2 thin film had good adhesion with the MnOx buffer layer without peeling even after cycling, as shown in Fig. 1(b). Figure 2 shows the XRD spectra of the PT substrate, the MnOx buffer layer grown on the PT substrate, and the MnO2 thin film electrodeposited on the MnOx /PT substrate. It can be seen that the MnOx buffer layer is a mixture of Mn2 O3

Fig. 1. Images of electrodeposited MnO2 films on (a) a PT substrate and (b) a MnO x /PT substrate.

Fig. 2. XRD spectra of (a) a PT substrate, (b) a MnOx buffer layer grown on a PT substrate, and (c) a MnO2 film electrodeposited on a MnOx /PT substrate.

and Mn3 O4 . After the electrodeposition, except for the substrate and the MnOx buffer layer, the diffraction peaks can be attributed to MnO2 only. It is well known that MnO2 can exist in several crystallographic structural forms, namely α, β, γ, and λ structures.17 The diffraction peak from the electrodeposited MnO2 film in Fig. 2(c) can be ascribed to the (110) crystal plane of γ -MnO2 , which is usually produced by the electrodeposition method.4 Figures 3(a) and 3(b) show the low magnification scanning electron microscopy (FESEM) images of the MnO2 films electrodeposited on the MnOx /PT and PT substrates. It can be seen that the electrodeposited MnO2 film on the MnOx /PT substrate is very uniform, without any cracks or peeling. However, the MnO2 film electrodeposited directly on the PT substrate is full of cracks and a large part of it has peeled off from the substrate, indicating that the MnOx buffer layer can greatly enhance the adhesion between the MnO2 film and the PT substrate. Figures 3(c) and 3(d) show the high magnification FESEM images of the MnO2 films electrodeposited on the MnOx /PT and PT substrates. Both MnO2 films exhibit a porous microstructure and are basically composed

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Improved Capacitive Behavior of MnO2 Thin Films with a MnOx Buffer Layer 15

(a)

(b)

(c)

(d)

Fig. 3. (a) A low magnification FESEM image of the MnO2 film deposited on a MnO x /PT substrate, (b) a low magnification FESEM image of the MnO2 film deposited on a PT substrate, (c) a high magnification FESEM image of the MnO2 film deposited on a MnOx /PT substrate, and (d) a high magnification FESEM image of the MnO2 film deposited on a PT substrate.

of nanoflakes. The MnO2 film electrodeposited on the PT substrate has a denser structure, with closely packed small nanoflakes, similar to that observed by Chou et al., who electrodeposited the MnO2 film on a Ni sheet.4 The MnO2 film electrodeposited on the MnOx buffer layer has a more porous structure, with larger nanoflakes and pores. To investigate the chemical state of Mn cations and oxygen anions, the as-deposited MnO2 film was analyzed using XPS measurements. The Mn 2 p and oxygen 1s XPS spectra of the MnO2 film deposited on the MnOx /PT substrate are shown in Fig. 4. The Mn 2 p XPS spectrum can be best fitted with two peaks, one of which is located at 642.1 eV and another at 653.6 eV, corresponding to Mn 2 p3/2 and Mn 2 p1/3. The simulated curves are in good agreement with the original spectrum, revealing the oxidation state of Mn cations to be 4+. For the O 1s XPS spectrum, the original curve can be best fitted with three peaks, corresponding to different oxygen-containing species such as Mn–O–Mn, Mn–O– H, and H–O–H. The XPS results are in good agreement with those obtained by other investigations for the electrodeposited MnO2 films,4,18 which further indicates that the electrodeposited film consisted of MnO2 without appreciable impurity of other species. The CV behaviors of the different samples — the MnO2 film deposited on the PT substrate, the MnO x film deposited on the PT substrate, and the MnO2 film deposited on the MnOx /PT substrate — are shown in Fig. 5. The CV measurements were conducted in the 1 M Na2 SO4 electrolyte in the voltage window of 0–0.9 V with a scan rate of 50 mV/s. As shown in Fig. 5(a), the CV curves of the MnO2 film deposited on the PT substrate exhibit a very small current, indicating a

Fig. 4. XPS spectra of Mn 2p and O 1s from an as-deposited MnO2 film on the MnOx /PT substrate.

small charge storage capability. The current did not remain constant but clearly varied with the electrode potential, resulting in a considerably distorted rectangular CV shape. The poor capacitive performance of the MnO2 film deposited on the PT substrate can be attributed to the poor adhesion between the film and the PT substrate. As shown in Fig. 5(b), the CV curves of the MnOx film deposited on the PT substrate exhibit a small current but less distorted rectangular shape compared to that in Fig. 5(a). As observed from the XRD results, the MnOx film is composed of Mn2 O3 and Mn3 O4 oxides, which can also be used as electrode materials for supercapacitors.19 However, since the MnOx film deposited by PLD is very smooth and dense, this MnOx film can only deliver a very small capacitance due to its low oxidation state of Mn and limited surface area. It was observed that the CV curve of the MnOx film exhibited a large enclosed anodic portion but a small cathodic one, hence manifesting irreversible reactions of the oxide electrode. This phenomenon may be attributed to the anodic oxidation of the low oxidation state of Mn to the high oxidation state of Mn, which is often observed from the CV scan of Mn film.20 As shown in Fig. 5(c), the CV curve of the MnO2 film deposited on the MnOx /PT substrate exhibits a large current and symmetrical rectangular shape, indicating ideal capacitive behavior. The improved capacitive behavior of the MnO2

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(a)

Fig. 6. Comparison of Nyquist plots for two electrochemical cells: one uses the MnO2 film deposited on the MnOx /PT substrate, and the other uses the MnO2 film deposited on the PT substrate.

(b)

(c) Fig. 5. CV curves for different thin film samples: (a) a MnO2 film deposited on the PT substrate, (b) a MnO x film deposited on the PT substrate, and (c) a MnO2 film deposited on the MnOx /PT substrate. (CV was measured between 0 and 0.9 V versus Ag/AgCl with a scan rate of 50 mV/s.)

film deposited on the MnOx /PT substrate can be attributed to the porous nanostructure of the MnO2 film and good adhesion between the film and the substrate. In order to further understand the electrochemical properties of the MnO2 film deposited on different substrates, EIS was performed on the test cells in the frequency range between 10 kHz to 10 mHz. Figure 6 compares the Nyquist plots of the MnO2 films deposited on the PT and MnOx /PT substrates. For both EIS spectra, at high frequencies, the intercepts at

the real part of Z correspond to the uncompensated electrical resistance (Rohm ). It can be seen that Rohm of the MnO2 film deposited on the MnOx /PT substrate is larger than that of the MnO2 film deposited on the PT substrate, which can be attributed to the MnOx buffer layer. The depressed semicircle in the high frequency range corresponds to charge transfer resistance of the electrode. It is obvious that the MnO2 film deposited on the MnOx /PT substrate has a much smaller charge transfer resistance than the film deposited on the PT substrate. Finally, in the low frequency region, a straight line nearly vertical to the realistic impedance axis is observed. The finite slope of the straight line indicates the diffusive resistance of the electrolyte in the electrode pores and the proton diffusion in the host materials.18 Normally, a higher slope means a lower diffusive resistance for cation intercalation/deintercalation. It is apparent that the MnO2 film deposited on the MnOx /PT substrate has a lower diffusive resistance due to the highly porous nanostructure. The EIS results confirm the improved capacitive performance of the MnO2 film deposited on the MnOx /PT substrate. The CV curves and specific capacitances of the MnO2 film deposited on the MnOx /PT substrate at different scan rates from 10 to 200 mV/s are displayed in Fig. 7. As shown in Fig. 7(a), the rectangular shape of the CV curves is maintained as the scan rate increases up to 200 mV/s, indicating the high reversibility and excellent reactivity of the MnO2 film. The specific capacitance of the MnO2 film, which was obtained by integrating the CV curves divided by the weight of the MnO2 film and the voltage window (0.9 V) versus the scan rate, is displayed in Fig. 7(b). As shown, the MnO2 film possesses a high specific capacitance of about 367 F/g at a slow scan rate of 10 mV/s. The specific capacitance of the MnO2 film decreases only slightly with the increase in the scan rate. An approximately 300 F/g specific capacitance can

April 11, 2009 20:35 00047

Improved Capacitive Behavior of MnO2 Thin Films with a MnOx Buffer Layer 17

(a)


(a)

(b)

(b)

Fig. 7. (a) CV curves of the MnO2 film deposited on the MnOx /PT substrate at different scan rates from 10 to 200 mV/s, and (b) the specific capacitance of the thin film electrode versus the scan rate.

Fig. 8. (a) CV curves of the MnO2 film deposited on the MnOx /PT substrate for 1000 cycles, and (b) the specific capacitance of the thin film electrode versus the cycle number.

be maintained at the highest scan rate of 200 mV/s. Compared with CV results from the MnO2 film deposited on the Ni substrate using the same method by Chou et al.,4 the MnO2 film deposited on the MnOx /PT substrate exhibits much a better rate capability. It is speculated that the better rate capability of the MnO2 film deposited on the MnOx /PT substrate is due to its porous nanostructure favoring fast kinetics of cation intercalation/deintercalation. To investigate the cycle performance of the MnO2 film deposited on the MnOx /PT substrate, 1000 cycles of CV curves were measured at a scan rate of 50 mV/s in the voltage window between 0 and 0.9 V. The CV curves and the plot of specific capacitance versus cycle number are shown in Figs. 8(a) and 8(b). It can be seen from Fig. 8(a) that the CV shape of the film becomes more rectangular with the cycling, indicating improved capacitance behavior. From Fig. 8(b), it can be seen that the specific capacitance of the film first decreases for the first 200 cycles and then starts to increase slowly with the cycling. A more than 300 F/g specific capacitance is maintained after 1000 cycles, while some

reported data showed a continuing decrease in capacitance with cycling.15,19 It is believed that since the MnOx buffer layer is composed of Mn2 O3 and Mn3 O4 oxides, they might be slowly oxidized to MnO2 during the CV scan (0–0.9 V), leading to the increase in the specific capacitance of the electrode with cycling. In summary, porous nanostructured MnO2 thin films were prepared on PT and MnOx /PT substrates by the technique of cyclic-voltammetric electrodeposition. The MnOx buffer layer can greatly improve the adhesion between the MnO2 film and the PT substrate, avoiding cracks and peeling of the film. By using a MnOx buffer layer, the electrodeposited MnO2 film showed a more porous structure than the film directly deposited on the PT substrate, leading to a superior capacitive behavior. The MnO2 film deposited on the MnOx /PT substrate had high reversibility and reactivity, exhibiting a specific capacitance of about 376 F/g in the voltage window of 0–0.9 V at a scan rate of 10 mV/s. As confirmed by EIS, the MnO2 film deposited on the MnOx /PT substrate had a much smaller charge transfer resistance and

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18 H. Xia et al.

lower diffusive resistance, resulting in excellent rate capacity performance. Instead of capacitance fading, the MnO2 film deposited on the MnOx /PT substrate exhibited a slow capacitance increase with the cycling, which can be attributed to the slow oxidation of Mn in the MnOx buffer layer.

Acknowledgments This research is supported by the National University of Singapore and the Agency for Science, Technology and Research through the research grant R-265-000-292-305 (072 134 0051).


References 1. S. Sarangapani, B. V. Tilak and C. P. Chen, J. Electrochem. Soc. 143, 3791 (1996). 2. R. Kotz and M. Carlen, Electrochim. Acta 45, 2483 (2000). 3. Y. Z. Zheng, H. Y. Ding and M. L. Zhang, Thin Solid Films 516, 7381 (2008). 4. S. L. Chou, F. Y. Cheng and J. Chen, J. Power Sources 162, 727 (2006).

5. N. Nagarajan and I. Zhitomirsky, J. Appl. Electrochem. 36, 1399 (2006). 6. S. L. Chou et al., J. Power Sources 182, 359 (2008). 7. D. D. Zhao, W. J. Zhou and H. L. Li, Chem. Mater. 19, 3882 (2007). 8. G. Lota et al., Chem. Phys. Lett. 434, 73 (2007). 9. J. Chang et al., Electrochem. Solid-State Lett. 11, A9 (2008). 10. R. S. Mane et al., Curr. Appl. Phys. 9, 87 (2009). 11. C. L. Chen et al., Mater. Chem. Phys. 95, 84 (2006). 12. D. Choi, G. E. Blomgren and P. N. Kumta, Adv. Mater. 18, 1178 (2006). 13. M. W. Xu et al., J. Phys. Chem. C 111, 19141 (2007). 14. X. Wang and Y. D. Li, J. Am. Chem. Soc. 124, 2880 (2002). 15. K. R. Prasad and N. Miura, J. Power Sources 135, 354 (2004). 16. J. N. Broughton and M. J. Brett, Electrochim. Acta 50, 4814 (2005). 17. S. Devaraj and N. Munichandraiah, J. Phys. Chem. C 112, 4406 (2008). 18. W. F. Wei et al., J. Phys. Chem. C 112, 15075 (2008). 19. N. Nagarajan, H. Humadi and I. Zhitomirsky, Electrochim. Acta 51, 3039 (2006). 20. B. Djurfors et al., Acta Mater. 53, 957 (2005).

IMPROVED CAPACITIVE BEHAVIOR OF MnO2 THIN

IMPROVED CAPACITIVE BEHAVIOR OF MnO2 THIN

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