doi: 10.1149/1.2388243
2007, Volume 10, Issue 1, Pages A9-A12.
Electrochem. Solid-State Lett.
Jeng-Kuei Chang, Wen-Ta Tsai, Po-Yu Chen, Chiung-Hui Huang, Fu-Hwa Yeh and I-Wen Sun
Liquid and Its Application for Supercapacitors
Butylmethylpyrrolidinium Bis(trifluoromethylsulfony)imide Ionic
Preparation of Manganese Thin Film in Room-Temperature
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Preparation of Manganese Thin Film in Room-Temperature
Butylmethylpyrrolidinium Bis(trifluoromethylsulfony)imide
Ionic Liquid and Its Application for Supercapacitors
Jeng-Kuei Chang,a,
*
,zWen-Ta Tsai,a,
*
Po-Yu Chen,b,*
Chiung-Hui Huang,a Fu-Hwa Yeh,cand I-Wen Sunc,*
a
Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan b
Faculty of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung, Taiwan
c
Department of Chemistry, National Cheng Kung University, Tainan, Taiwan
The butylmethylpyrrolidinium bis共trifluoromethylsulfony兲imide 共BMP-NTf2兲 ionic liquid was found to have a very wide potential window of 4.5 V, making it capable of being a medium for electrodepositing metallic Mn thin film. By using an electrochemical process, block manganese counter electrode was dissolved in the ionic liquid to generate divalent Mn, which was subsequently cathodically deposited on the nickel substrate. The current efficiency of this process was confirmed to be extremely high共close to 100%兲. The Mn thin film can be oxidized to manganese oxide by cyclic voltammetry in Na2SO4 electrolyte. The obtained manganese oxide exhibited an excellent pseudo-capacitive performance and possessed a satisfactory specific capacitance of 265 F/g at a moderate cyclic voltammetric sweep rate of 25 mV/s.
© 2006 The Electrochemical Society. 关DOI: 10.1149/1.2388243兴 All rights reserved.
Manuscript submitted June 20, 2006; revised manuscript received August 14, 2006. Available electronically November 13, 2006.
Electrochemical supercapacitors are charge-storage devices with greater power density and longer cycle life than batteries, and they possess higher energy density compared to conventional capacitors.1 They have attracted much attention in many fields such as hybrid power sources, peak-power sources, backup-power storage, light-weight electronic fuses, and starting power of fuel cells, etc.2,3The literature4,5has reported that pseudo-capacitance of amorphous hy-drous ruthenium oxide arises from the fast, reversible faradaic redox reaction that occurs near an electrode surface was higher than 700 F/g. Although excellent supercapacitive performance of the ox-ide was demonstrated, the high cost has limited its commercial ap-plications. Manganese oxide, which is naturally abundant and much cheaper than ruthenium oxide, was also found to possess pseudo-capacitive characteristics. Therefore, it was considered to be the most promising novel electrode material for supercapacitors. Re-cently, various kinds of processes, including thermal decomposition,6 co-precipitation,7 sol-gel process,8-10 physical va-por deposition,11 and anodic deposition,12,13etc., have been devel-oped to prepare manganese oxide with this demanding electrochemi-cal property. It was confirmed that the preparation methods and/or conditions significantly affected the material characteristics of the manganese oxides, and, consequently, their corresponding pseudo-capacitive behavior. Therefore, searching for a more favorable fab-rication process is worthy of further investigations.
Broughton et al.14have demonstrated that manganese oxide with excellent pseudo-capacitive performance can be prepared by anodi-cally oxidizing a metallic manganese thin film created by physical vapor deposition共PVD兲. The literature indicates that transformation of manganese oxide from its metallic state may be a feasible route to produce supercapacitor electrodes with higher performance. As compared to the PVD process, electrodeposition is a more conve-nient and cheaper method to produce metallic thin film. Moreover, the morphology, grain size, chemical state, and thickness of the deposit can be easily controlled by varying the electrochemical pa-rameters such as potential, current density, bath composition, and temperature. An optimum Mn thin-film electrode for the superca-pacitor application, therefore, could be achieved by electrodeposi-tion. However, because the reduction potential of Mn is lower than that of hydrogen, electrodeposition of Mn in aqueous solution is very difficult to control due to serious hydrogen evolution. Aprotic room-temperature ionic liquids owing to their proton free
character-istics have been successfully proposed by Endres et al.15-17to elec-trodeposit highly active metals such as Al, Ta, and Si, etc. Never-theless, preparation of metallic Mn in ionic liquid is still a challenge and was rarely reported in the literature,18 hence we attempted to study its practicability in BMP-NTf2ionic liquid in this
investiga-tion. Although electrochemical oxidation of bulk transition metals was discussed by Trasatti,19herein we propose the oxidation behav-ior of the nanocrystalline Mn thin film with special surface morphol-ogy which was electrodeposited in the BMP-NTf2ionic liquid.
Ap-plication of the oxide electrode in a supercapacitor was also evaluated.
Experimental
The BMP-NTf2 ionic liquid was prepared and purified
princi-pally according to the method described in the literature.20The ionic liquid was transferred to a nitrogen-filled glove box and then elimi-nated trace water by an electrolysis process. Cyclic voltammetric 共CV兲 behavior of a glassy carbon electrode was measured in this solution. A Pt wire was used as the counter electrode. The reference electrode was a Pt wire placed in a separated fritted glass tube con-taining BMP-NTf2 ionic liquid with the ferrocene/ferrocenium
couple关Fc/Fc+, +0.55 V vs a standard hydrogen electrode共SHE兲兴
as the internal reference standard. Afterward, manganese ion was introduced into the ionic liquid by anodically oxidizing a manganese block共Alfa Aesar, 99.9%兲 to 0.05 M. A sufficient positive potential of 0.5 V共vs Fc/Fc+ couple兲 was applied on the manganese block, and a Pt wire was used as a counter electrode. To prevent undesired impurity created during the process from polluting the ionic liquid, the Pt counter electrode was separated in a glass tube with a porous ceramic tip.
Manganese thin films were fabricated by cathodic deposition in the above-mentioned ionic liquid at 50°C. The electrochemical deposition was accomplished with an Autolab potentiostat/ galvanostat controlled by GPES software. A three-electrode electro-chemical cell was adopted for the experiment. Nickel foil 共with 1 cm2area and⬃120 m thick兲 was etched in an 85°C 1 M H
2SO4
solution, then washed with pure water in an ultrasonic bath, and finally used as the working electrode. The reference electrode was the same as previously described. In addition, the counter electrode was a manganese block, which can compensate the consumption of manganese ion in the ionic liquid during the electrochemical depo-sition process. The deposited manganese was dissolved by 2 M ni-tric acid, and the amount was determined by an atomic absorption spectroscope共AAS, SOLAAR M6兲.
The electrochemical property of the manganese thin film was *Electrochemical Society Active Member.
z
examined by CV in 0.1 M Na2SO4aqueous solution at 25°C. The counter and reference electrodes were a platinum sheet and a satu-rated calomel electrode共SCE兲, respectively. An EG&G M263 was used to perform the test in the range of 0–0.9 V共vs SCE兲 with a sweep rate of 25 mV/s. Surface morphologies of the deposited man-ganese thin films before and after the CV experiment were observed using a scanning electron microscope共SEM, Philip XL-40FEG兲. An auxiliary X-ray energy dispersive spectroscope共EDS兲 was also car-ried out to examine the chemical composition. Pseudocapacitive per-formance of the prepared oxide was evaluated by chronopotentiom-etry共CP兲 with an applied current of 0.5 mA.
Results and Discussion
To explore the electrochemical potential window of the pure BMP-NTf2ionic liquid, CV behavior of an inert glassy carbon
elec-trode was measured in this solution. The voltammogram determined at a potential sweep rate of 50 mV/s is shown in Fig. 1a. The result indicated that the ionic liquid possessed a very wide potential win-dow stretching from −2.5 to 2 V 共vs Fc/Fc+ couple兲. Within the
potential range the ionic liquid was considered to be stable, and no redox reaction occurred, suggesting it could be a promising solvent for electrodepositing very reactive elements. Manganese cations were thereafter introduced into the ionic liquid by anodic dissolution of a manganese block. For understanding the number of charges carried by the Mn cations which were produced by anodic dissolu-tion, the weight change of the manganese block was measured after passing a given amount of anodic charge. The data collected during several controlled-potential coulometry experiments 共not shown here兲 revealed that the calculated number of electrons transferred during the anodic dissolution of manganese were all in the range of 1.99–2.02. Clearly, divalent manganese cations were the anodized products existing in the BMP-NTf2ionic liquid.
The Mn共II兲 contained BMP-NTf2ionic liquid was used to
per-form electrochemical deposition of manganese at 50°C. The work-ing electrode in this experiment was a nickel substrate. Curve a in Fig. 1b represents the voltammetric response of this electrochemical system in which the potential was initially swept to negative 共ca-thodic兲 direction. Apparently, Mn共II兲 in the ionic liquid can be re-duced to its metal state at a sufficiently negative potential, beginning at around −1.75 V共vs Fc/Fc+ couple兲. In addition, an obvious
ca-thodic current loop occurred during the reverse sweep indicating that the nucleation process happened during the cathodic deposition of manganese. This nucleation mechanism usually happens along with the electrodeposition of metals on foreign substrates. A corre-sponding anodic peak, located at about −0.43 V, was also observed. The result revealed that the deposited manganese solid film was dissolved during the subsequent anodic sweep. However, it was found that the total charges integrated from the cathodic region were 60% higher than those of the anodic region. The fact pointed out that
only a certain portion of Mn deposit could be oxidized after it had been formed. Mn共II兲 needed Tf2N−anions to form complex ions to
be dissolved in the ionic liquid. This process may be too slow to respond to the high potential scan rate共50 mV/s兲, and consequently resulted in the partial dissolution of Mn. The voltammogram of an identical nickel substrate in the pure BMP-NTf2ionic liquid without
Mn共II兲 was also superimposed as curve b in this figure. The com-parison result indicated that the nickel electrode was inert in this potential range and was capable of being a suitable substrate.
In aqueous solution, the major problem encountered in elec-trodeposition of manganese is the very low current efficiency, result-ing from the serious hydrogen evolution. The formation of hydrogen gas during deposition would also cause the reduction in uniformity and density of the deposit. It is very difficult to prevent hydrogen formation because the reduction potential of manganese is much more negative than that of hydrogen evolution. However, as dis-cussed in the previous paragraph, the reduction potential of Mn共II兲 in the aprotic ionic liquid共−1.75 V兲 is still far away from the ca-thodic limit共−2.5 V兲 of the electrochemical window. Therefore, it is expected that the Mn deposition efficiency in this ionic liquid could be high. In this study, the deposition current efficiency was evalu-ated by measuring the amount of Mn deposit and comparing with the theoretical value共the deposition reaction of Mn2++ 2e−→ Mn
was assumed兲. Manganese was cathodically deposited on the nickel substrates by a constant potential of −1.8 V共vs Fc/Fc+ couple兲 to 0.2, 0.3, 0.4, and 0.5 Coulomb, respectively. Then, the deposits were totally dissolved in 2 M HNO3solution, and the amount of Mn was
analyzed by an atomic absorption spectroscope 共AAS, SOLAAR M6兲. The experimental results are summarized and listed in Table I. The deposition efficiencies were all higher than 96%, and even very close to 100% in some cases. As expected, the extremely high cur-rent efficiency demonstrated that the BMP-NTf2 ionic liquid is a
promising solvent for electrodeposition of manganese. Also included
Figure 1. 共a兲 Voltammogram of a glassy
carbon electrode in the pure BMP-NTf2 ionic liquid.共b兲 Voltammetric response of nickel substrates in the Mn共II兲 contained BMP-NTf2ionic liquid共curve a兲 and the pure BMP-NTf2ionic liquid共curve b兲.
Table I. Deposition time and current efficiency of preparing vari-ous amounts of manganese, which were controlled by total ca-thodic charge applied.
Deposition charge Deposition time Theoretical mass of Mn共g兲 Experimental mass of Mna共g兲 Efficiency 共%兲 0.2 770 56.93 55.79 98 0.3 1150 85.40 84.89 99 0.4 1610 113.87 109.31 96 0.5 2000 142.33 141.34 99
aThe data are measured by an atomic absorption spectroscope 共SOLAAR M6兲.
A10 Electrochemical and Solid-State Letters, 10共1兲 A9-A12 共2007兲 A10
in Table I are the deposition times for different Mn quantities, indi-cating the deposition rate was almost constant共4.3 g/min兲.
Accordingly, a novel and promising process to prepare manga-nese thin films was successfully proposed in this study. The BMP-NTf2ionic liquid worked only as an inert medium or solvent,
which was not consumed during the process and could be reusable. Free of waste disposing problem has made it an environmentally friendly scheme, and wide range potential applications could be ex-pected. Moreover, the significant advantage of this Mn thin film preparation process is the ease of electrochemically controlling the deposition rate and thickness by adjusting potential and total pass charge, respectively. Moreover, the roughness and morphology of the thin film were also found to substantially change with the elec-trodeposition conditions. Further investigation is already underway and will be published elsewhere. Actually, the mechanism of this wet electrochemical deposition process is very similar to that of the PVD process, which is a common method to prepare Mn thin films.11 The manganese block was consumed at the counter elec-trode 共like the target in the PVD system兲 and deposition on the desired substrate. Note, however, that the PVD process requires much more complicated and expensive equipment as compared to the electrochemical method. The yielding rate of the PVD process is also much lower. Most importantly, the conversion efficiency共Mn from the target to substrate兲 for PVD is low, because most Mn is deposited on the chamber wall. On the contrary, almost 100% effi-ciency can be obtained as the electrochemical deposition of manga-nese was performed in the BMP-NTf2ionic liquid.
The electrochemical property of the manganese thin film, depos-ited at −1.8 V共vs Fc/Fc+couple兲 for the total pass charge of 0.3 C, was examined by CV in 0.1 M Na2SO4aqueous solution. Figure 2a shows its typical first-cycle voltammogram with a potential sweep rate of 25 mV/s. A very broad and irreversible anodic peak was observed. The result indicated that the Mn metallic thin film was oxidized in the electrolyte. According to the Pourbaix diagram,21
Mn could be electrochemically transferred to Mn2+, Mn3O4, Mn2O3,
and MnO2with increasing the anodic potential in this neutral aque-ous solution. The subsequent CV cycles of the electrode were su-perimposed in Fig. 2b. Although the further oxidation reaction was still visible, the electrochemical behavior of the electrode gradually became stable. After ten cycles of CV sweep, a fully anodized man-ganese oxide was formed and the electrode demonstrated a steady capacitor-like behavior. The final CV curve was close to rectangular shapes and exhibited mirror-image characteristics, suggesting its ex-cellent reversibility and ideal pseudo-capacitive property. The fully anodized manganese oxide was dissolved in 2 M HNO3 solution and the amount of Mn was quantified by AAS. The Mn concentra-tion in the Na2SO4testing solution was also analyzed in the same manner. The analytical data revealed that about 40共±5兲% of the original manganese was dissolved into Na2SO4electrolyte共to form Mn2+兲 during the CV cycling, while the 60 共±5兲% remained on the
substrate and anodically transferred to manganese oxide. Djurfors et al.22had reported that Mn3O4was formed as the metallic Mn was
electrochemically oxidized in Na2SO4electrolyte. Assumed that the electrochemical active material was Mn3O4and took account of the
40% originally deposited Mn dissolution, the specific capacitance of the oxide was as high as 265 F/g measured at a CV sweep rate of 25 mV/s. Calculation of oxide capacitance from the CV curve fol-lowed the method described previously.13This satisfactory capacity is at least 40% higher than that of the manganese oxide prepared by anodic deposition共based on the same testing condition兲, as reported in our previous study.13
Figures 3a and b show the SEM surface morphologies of the deposited manganese thin films before and after the CV oxidation process. The as-deposited Mn film was composed of spherical par-ticles with the diameter of⬃400 nm. However, close examination further revealed that each particle consisted of numerous sub-grains with a few nanometers in the size. After the oxidation process, the
Figure 2. CVs of the manganese thin film
in 25°C 1 M Na2SO4 aqueous solution. 共a兲 The first cycle and 共b兲 the subsequent 2–10 cycles. The potential sweep rate is 25 mV/s.
Figure 3. Surface morphologies of the
electrochemically deposited manganese thin film共a兲 before and 共b兲 after the oxi-dation process.
particles were found to lump together as shown in Fig. 3b, due to the volume expansion of oxide formation. Moreover, the fiberlike mi-crostructure, which is typical for the anodically deposited manga-nese oxide23,24was also clearly observed on the surface. The cracks recognized in this figure were suggested to have been caused by shrinkage stress during drying. Chemical composition analysis of the manganese oxide was also performed by EDS, and the Mn/O atomic ratio of 43/57 was found. The experimental data implied that the Mn metallic thin film could transform to Mn3O4after the CV oxidation process. This result coincided well with the literature.22
Galvanostatic charge-discharge performance of the fully anod-ized manganese oxide electrode was examined. Figure 4 shows its chronopotentiogram of initial four charge-discharge cycles in 0.1 M Na2SO4electrolyte with an applied current of 0.5 mA. Good
sym-metry and near-linear slope in both charge and discharge branches again supported that the oxide electrode was electrochemically re-versible and possessed excellent pseudo-capacitive characteristics. After 500 charge-discharge cycles, about 90% capacitance of the oxide electrode can be retained which was 10% higher than that of the anodically deposited manganese oxide reported in our previous paper.25The experimental result clearly revealed the great electro-chemical stability of the manganese oxide prepared in this study.
Conclusions
A promising electrochemical deposition process was successfully proposed to prepare metallic manganese thin films in the
room-temperature BMP-NTf2ionic liquid. An extremely high current
ef-ficiency and an almost constant deposition rate would make the process very precise and easily controlled. The prepared Mn thin film can be anodized in Na2SO4electrolyte and transformed to
man-ganese oxide. The manman-ganese oxide exhibited excellent pseudo-capacitive performance and provided the capacitance of 265 F/g at a moderate CV sweep rate of 25 mV/s. Moreover,⬃90% of initial capacitance can be retained after 500 charge-discharge cycles. A further study to improve the oxide electrochemical property has been started and will be reported in the near future. Optimizations of the deposition and anodization conditions are attempted to control the film morphology and chemical state, which were considered to be the critical issues as pseudo-capacitive performance of the oxide was concerned.
Acknowledgment
The authors thank the National Science Council of the Republic of China for financially supporting this research under contract no. NSC 94-2216-E-006-020.
National Cheng Kung University assisted in meeting the publication costs of this article.
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A12 Electrochemical and Solid-State Letters, 10共1兲 A9-A12 共2007兲 A12
