Pseudocapacitive Mechanism of Manganese Oxide in 1-Ethyl-3

Jul 21, 2009 - (23, 24) A three-electrode electrochemical system was employed. .... electrode measured in P-HCOO IL with a potential sweep rate of 3 m...
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Pseudocapacitive Mechanism of Manganese Oxide in 1-Ethyl-3methylimidazolium Thiocyanate Ionic Liquid Electrolyte Studied Using X-ray Photoelectron Spectroscopy Jeng-Kuei Chang,† Ming-Tsung Lee,† Wen-Ta Tsai,† Ming-Jay Deng,‡ Hui-Fang Cheng,‡ and I-Wen Sun*,‡ †

Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan, and ‡ Department of Chemistry, National Cheng Kung University, Tainan, Taiwan Received April 6, 2009. Revised Manuscript Received July 7, 2009

The electrochemical behavior of anodically deposited manganese oxide was studied in pyrrolidinium formate (P-HCOO), 1-butyl-3-methylimidazolium hexafluorophosphate (BMI-PF6), and 1-ethyl-3-methylimidazolium thiocyanate (EMI-SCN) ionic liquids (ILs). The experimental data indicate that the Mn oxide electrode showed ideal pseudocapacitive performance in aprotic EMI-SCN IL. In a potential window of ∼1.5 V, the oxide specific capacitance, evaluated using cyclic voltammetry and chronopotentiometry, was about 55 F/g. The electrochemical energy storage reaction was examined using X-ray photoelectron spectroscopy (XPS). It was confirmed that the SCN- anions, instead of the EMIþ cations, were the primary working species that can become incorporated into the oxide and thus compensate the Mn3þ/Mn4þ valent state variation upon the charge-discharge process. According to the analytical results, a pseudocapacitive mechanism of Mn oxide in the SCN- based aprotic IL was proposed.

1. Introduction Because of environment issues and the depletion of fossil fuels, the development of alternative energy conversion/storage systems that can meet present day power demands has accelerated. Supercapacitors (or electrochemical capacitors) are energy-storage devices that have a greater power density and a longer cycle life than those of batteries and a higher energy density than that of conventional capacitors.1 Accordingly, they have been used for a wide range of applications, such as in hybrid electric vehicles, consumer electronics, medical electronics, and military missile systems. Supercapacitors can be classified, based on their operating mechanisms, into two categories: (i) double-layer capacitors, which are based on non-faradic charge separation at the electrode/electrolyte interface (high surface area carbon is the common electrode material), and (ii) pseudocapacitors, which are based on faradic redox reactions of electroactive materials, including conductive polymers and metal oxides. The charge storage mechanism of metal oxides in aqueous electrolytes has been proposed as follows:2-4 MO2 þnCþ þne - TMO2 -n ðOCÞn ðM denotes metalÞ

ð1Þ

where Cþ denotes proton or alkali metal cation (Liþ, Naþ, Kþ) in the electrolyte. Among the constituent components of a supercapacitor, electrolyte is a critical part that governs the overall performance. Common aqueous electrolytes, due to the intrinsic characteristics of water decomposition, have narrow potential windows *Corresponding author: Tel: þ886-6-2757575 ext. 65355. E-mail: iwsun@ mail.ncku.edu.tw.

(1) Conway, B. E. Electrochemical Supercapacitors; Kluwer-Plenum: New York, 1999. (2) Toupin, M.; Brousse, T.; Belanger, D. Chem. Mater. 2004, 16, 3184–3190. (3) Chang, J. K.; Lee, M. T.; Tsai, W. T. J. Power Sources 2007, 166, 590–594. (4) Nam, K. W.; Kim, M. G.; Kim, K. B. J. Phys. Chem. C 2007, 111, 749–758. (5) Pang, S. C.; Anderson, M. A.; Chapman, T. W. J. Electrochem. Soc. 2000, 147, 444–450.

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(typically ∼1 V),5-7 which limit the capacitor cell voltage. Since both energy density and power density of a supercapacitor depend on the square of the cell voltage, a large potential stability window of the electrolyte is quite important. Although some organic solvents are stable over a relatively wide potential range, they pose serious health and safety hazards as they are volatile, flammable, and sometimes toxic. Therefore, finding a more suitable electrolyte to optimize the performance of supercapacitors is of great significance. Ionic liquids (ILs), characterized by intrinsic ionic conductivity, large electrochemical potential windows, excellent thermal stability, nonvolatility, nonflammability, and nontoxicity, have attracted enormous interest for various applications in synthesis, catalysis, analysis, separation, photoluminescence, and electrochemistry.8-12 Since their physical and chemical properties can be modified by selecting the appropriate cation/anion combination,13 ILs are considered “task specific” liquids.14 ILs have been used in energy storage devices.15-17 However, metal oxide supercapacitors that incorporate IL electrolytes have rarely been investigated. Rochefort et al.18 suggested that RuO2 can demonstrate (6) Toupin, M.; Brousse, T.; Belanger, D. Chem. Mater. 2002, 14, 3946–3952. (7) Hu, C. C.; Tsou, T. W. Electrochem. Commun. 2002, 4, 105–109. (8) Huang, J. F.; Luo, H.; Liang, C.; Sun, I. W.; Baker, G. A.; Dai, S. J. Am. Chem. Soc. 2005, 127, 12784–12785. (9) Endres, F.; Abedin, S. Z. E. Phys. Chem. Chem. Phys. 2006, 8, 2101–2116. (10) Earle, M. J.; Esperanca, J.; Gilea, M. A.; Lopes, J. N. C.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. Nature 2006, 439, 831–834. (11) Wasserscheid, P. Nature 2006, 439, 797. (12) Ohno, H. Electrochemical Aspects of Ionic Liquids; John Wiley & Sons: Hoboken, 2008. (13) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley-VCH: Weinheim, 2003. (14) Visser, A. E.; Swatlowski, R. P.; Reichert, W. M.; Mayton, R.; Sheff, S.; Wierzbicki, A.; Davis, J. H.; Rogers, R. D. Chem. Commun. 2001, 135–136. (15) Balducci, A.; Bardi, U.; Caporali, S.; Mastragostino, M.; Soavi, F. Electrochem. Commun. 2004, 6, 566–570. (16) Frackowiak, E.; Lota, G.; Pernak, J. Appl. Phys. Lett. 2005, 86, 164104. (17) Lazzari, M.; Mastragostino, M.; Soavi, F. Electrochem. Commun. 2007, 9, 1567–1572. (18) Rochefort, D.; Pont, A. L. Electrochem. Commun. 2006, 8, 1539–1543.

Published on Web 07/21/2009

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pseudocapacitive behavior only in protic IL (but not in aprotic IL). They proposed that protons in the IL play a crucial role in allowing for faradic redox reactions of the oxide (similar to eq 1); this conclusion may lead researchers to pursue protic IL electrolytes for use in metal oxide supercapacitors. However, we recently found that Mn oxide, a much cheaper alternative to RuO2 for supercapacitor applications,19-21 can show ideal pseudocapacitive performance in some aprotic IL without containing protons.22 In the absence of Hþ, Liþ, Naþ, or Kþ cations in the IL electrolytes, what would be the species that can be incorporated into the Mn oxide electrode, and what is the pseudocapacitive reaction mechanism? These fundamental issues are the key points to develop new kinds of IL electrolytes with better performance and thus should be further clarified. In addition, using aprotic IL electrolytes would enable large cell voltages and wide temperature operation ranges and also render less safety concerns of supercapacitors. In the present paper, we further investigate the electrochemical characteristics of Mn oxide in various cation/anion combinations of ILs, including a protic pyrrolidinium formate (P-HCOO) IL and two aprotic 1-butyl-3-methylimidazolium hexafluorophosphate (BMI-PF6) and 1-ethyl-3-methylimidazolium thiocyanate (EMI-SCN) ILs, and find out that the anions, instead of cations, are the key species that govern the electrode charge storage performance. Owing to the extremely low vapor pressure of ILs (definitely not for organic and aqueous electrolytes), the pseudocapacitive reaction mechanism of Mn oxide relevant to IL electrolytes was examined with X-ray photoelectron spectroscopy (XPS) in this study. This effective methodology clearly disclosed that the insertion/desertion depth of IL anions (into Mn oxide) was related to their geometric shape and size. Besides the supercapacitor applications, the results of this study could also provide a new concept for developing novel energy storage systems that involve metal oxide electrodes and ILs.

2. Experimental Section 2.1. Mn Oxide Preparation. Mn oxide was anodically deposited from 0.25 M Mn(CH3COO)2 aqueous plating solution at room temperature; this preparation process was already established in the previous work.23,24 A three-electrode electrochemical system was employed. A Ni coupon with a thickness of 120 μm and an exposed area of 1 cm2 was etched in 2 M HCl solution at 80 °C, washed with pure water in an ultrasonic bath, and then used as the working electrode after drying. In addition, a platinum sheet and a saturated calomel electrode (SCE) were assembled as the counter electrode and reference electrode, respectively. The anodic deposition was performed under a constant potential of 0.8 V (vs SCE) to give a total passed charge of 0.4 C/cm2. The typical mass of the deposited oxide, measured using a microbalance with an accuracy of 0.01 mg, was ∼0.3 mg/cm2 (with a thickness of about 1 μm). The surface morphology of the electrode was examined using a scanning electron microscope (SEM, Philip XL-40 FEG). The oxide microstructure was observed with a transmission electron microscope (TEM, JEOL 3010). A camera length of 120 cm was adopted for the electron diffraction analysis. The Brunauer-Emmett-Teller (BET) specific surface area of the deposited Mn oxide, detached from the Ni substrate, was also measured. The nitrogen sorption isotherm was obtained using a Micromeritics Gemini 2360 automated gas sorption analyzer.

2.2. Ionic Liquid Syntheses. The protic P-HCOO IL was prepared according to the procedure reported in the literature.25 A frequently used aprotic IL electrolyte, namely BMI-PF6 IL, was also synthesized.26 In addition, a low-viscosity and high-conductivity EMI-SCN aprotic IL, which has been used for solar cell applications,27 was prepared and purified following a previously published method.28 The water contents of the three ILs, measured with a Karl Fisher titrator, were ∼100 ppm. 2.3. Electrochemical Measurements. The electrochemical properties of Mn oxide electrodes in the three ILs were studied using cyclic voltammetry (CV) and chronopotentiometry (CP) at 25 °C in a nitrogen-purified glovebox (Vacuum Atmospheres Co.), where both the moisture and oxygen contents were maintained below 1 ppm. The CV potential sweep rate was 3 mV/s. The applied current for the CP measurement was 0.1 mA/cm2. The reference electrode was a platinum wire placed in a fritted

(19) Fischer, A. E.; Pettigrew, K. A.; Rolison, D. R.; Stroud, R. M.; Long, J. W. Nano Lett. 2007, 7, 281–286. (20) Liu, R.; Lee, S. B. J. Am. Chem. Soc. 2008, 130, 2942–2943. (21) Hu, C. C.; Wu, Y. T.; Chang, K. H. Chem. Mater. 2008, 20, 2890–2894. (22) Chang, J. K.; Lee, M. T.; Cheng, C. W.; Tsai, W. T.; Deng, M. J.; Sun, I. W. Electrochem. Solid-State Lett. 2009, 12, A19–A22. (23) Chang, J. K.; Tsai, W. T. J. Electrochem. Soc. 2003, 150, A1333–A1338. (24) Chang, J. K.; Chen, Y. L.; Tsai, W. T. J. Power Sources 2004, 135, 344–353.

(25) Anouti, M.; Caravanier, M. C.; Dridi, Y.; Galiano, H.; Lemordant, D. J. Phys. Chem. B 2008, 112, 13335–13343. (26) Schr€oder, U.; Wadhawan, J. D.; Compton, R. G.; Marken, F.; Suarez, P. A. Z.; Consorti, C. S.; de Souza, R. F.; Dupont, J. New J. Chem. 2000, 24, 1009–1015. (27) Mazille, F.; Fei, Z.; Kuang, D.; Zhao, D.; Zakeeruddin, S. M.; Gr€atzel, M.; Dyson, P. J. Inorg. Chem. 2006, 45, 1585–1590. (28) Pringle, J. M.; Golding, J.; Forsyth, C. M.; Deacon, G. B.; Forsyth, M.; MacFarlance, D. R. J. Mater. Chem. 2002, 12, 3475–3480.

11956 DOI: 10.1021/la9012119

Figure 1. (a) SEM and (b) TEM micrographs of the anodically deposited Mn oxide. (c) Electron diffraction pattern, examined using TEM, taken from the oxide shown in (b).

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Figure 2. Cyclic voltammograms of (a) an inert glassy carbon electrode and (b) the Mn oxide electrode measured in P-HCOO IL with a potential sweep rate of 3 mV/s.

Figure 4. Cyclic voltammograms of (a) an inert glassy carbon electrode and (b) the Mn oxide electrode measured in EMI-SCN IL with a potential sweep rate of 3 mV/s. (c) Chronopotentiogram for five charge-discharge cycles of the Mn oxide electrode measured in EMI-SCN IL at an applied current density of (0.1 mA/cm2. Figure 3. Cyclic voltammograms of (a) an inert glassy carbon electrode and (b) the Mn oxide electrode measured in BMI-PF6 IL with a potential sweep rate of 3 mV/s. glass tube containing butylmethylpyrrolidinium bis(trifluoromethylsulfony)imide IL that had a ferrocene/ferrocenium couple (Fc/Fcþ = 50/50 mol %, showing a potential of þ0.55 V vs SHE). The counter electrode was a spiral platinum wire, which was directly immersed in the bulk electrolyte. The applied potential and current were regulated using an AUTOLAB potentiostat. 2.4. Chemical State Analyses. An X-ray photoelectron spectrometer (PHI 5000 Versa-Probe) was used to probe the chemical states of EMI-SCN IL and the Mn oxide electrodes. Prior to XPS analyses, the oxide electrodes underwent 10 CV scanning cycles and then were held at certain potentials of interest for 1 h. Afterward, the electrodes were cleaned with methanol, dried in an inert nitrogen gas atmosphere, and finally delivered to the XPS prelocked chamber using a transfer vessel, which was filled with high-purity nitrogen to prevent the samples from being exposed to air. Monochromatic Al KR radiation (1486.6 eV) was used as the X-ray source. The pressure in the main chamber was below 1  10-9 Torr during the analyses.

3. Results and Discussion Figure 1a shows the surface morphology, examined using SEM, of the electrodeposited Mn oxide; a fibrous morphology was observed. The cracks observed in the micrograph can be attributed to the shrinkage of the deposit during drying and/or vacuuming. Figure 1b is a typical TEM bright-field image of the Mn oxide, which was detached from the substrate. This photo clearly reveals that the oxide is composed of numerous interweaving nanowhiskers, which have a diameter of ∼5 nm and a Langmuir 2009, 25(19), 11955–11960

length up to hundreds of nanometers. TEM cross-section observation also confirmed that the whisker-like microstructure was continuous through the depth of the film. The crystallinity of the oxide was analyzed using an electron diffraction technique; the obtained data are shown in Figure 1c. The three major signals in the diffraction pattern can be identified to be associated with the (211), (301), and (002) crystal planes of R-MnO2 (JCPDS 44-0141), as marked in the figure. However, the faint diffraction rings suggest that the deposited Mn oxide is nanocrystalline and/ or nonstoichiometric in nature. Parts a and b of Figure 2 show the cyclic voltammograms, measured in P-HCOO IL, of an inert glassy carbon electrode and the Mn oxide electrode, respectively. Although a wide electrochemical window (-1.9 to þ0.2 V) of the IL was found (see Figure 2a), Mn oxide was quite unstable in this electrolyte; after a few CV scans, the oxide was partially dissolved. As seen in Figure 2b, an irreversible cathodic reaction, most likely associated with the dissolution of Mn oxide, occurred when the potential was negatively scanned from the open-circuit potential, i.e., ∼0 V, which is already close to the upper potential limit of the IL. Therefore, there was no capacitive behavior of the oxide can be recognized in this environment. Rochefort et al.18,29 have suggested that, in protic IL, RuO2 can show ideal pseudocapacitive properties; however, our study indicates that the P-HCOO IL is definitely not a candidate electrolyte for Mn oxide supercapacitors. The chemical compatibility between the electrode material and the IL electrolyte employed is an important issue, which deserves further investigation. (29) Mayrand-Provencher, L.; Rochefort, D. J. Phys. Chem. C 2009, 113, 1632– 1639.

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Figure 5. (a) Mn 2p3/2 and (b) Mn 3s XPS spectra of the Mn oxide electrodes previously polarized at -0.3 and -1.8 V in EMI-SCN IL.

Aprotic IL electrolyte was also studied. Figure 3a demonstrates a very wide potential window of BMI-PF6 IL, extending from approximately -2.4 to þ0.8 V. Within this potential window, only tiny current, which was associated with electrolysis (or decomposition) of some electroactive impurity in the IL, on the double-layer charging background can be detected. The cyclic voltammograms of the Mn oxide electrode, with various upper and lower potential boundaries, in BMI-PF6 IL is shown in Figure 3b. The CV curves showed rectangular shapes and mirror-image features (symmetric anodic/cathodic area) in a range from -1.1 to þ0.2 V, before irreversible cathodic and anodic reactions of Mn oxide occurred. Inside the region of ∼1.3 V, the CV response current remained almost constant during forward and backward scans, but it immediately changed its flow direction when the potential was reversed. These characteristics indicate ideal capacitive behavior of the electrode. The specific capacitance (C) of Mn oxide can be calculated using the following equation: C ¼ Qm =ΔV

ð2Þ

where Qm is the specific voltammetric charge (based on weight) integrated from the CV curve and ΔV is the potential scanning range. The calculation result indicates that the oxide specific capacitance is ∼20 F/g in BMI-PF6 IL. Since the BET surface area of the deposited Mn oxide was measured to about 50 m2/g and the typical double-layer capacitance of an electrode in ILs is less than 10 μF/cm2,30 this specific capacitance value (20 F/g) is too high to be solely attributed to the surface double-layer charging/ discharging effect. A contribution from faradic redox reaction (i.e., pseudocapacitance) of Mn oxide is suspected. Figure 4a shows the cyclic voltammogram of an inert glassy carbon electrode recorded in EMI-SCN IL; the result indicates that the cathodic and anodic breakdown potentials of this IL are approximately -2.4 and þ0 V, respectively. Figure 4b exhibits the electrochemical behavior of the Mn oxide electrode measured in EMI-SCN IL (with various potential limits). The quasi-rectangular shape of the CV curves can be recognized in a potential region essentially stretching from -1.8 to -0.3 V. It was found that the CV enclosed area was much larger than that found in Figure 3b, indicating a better charge storage capability of Mn (30) Lewandowski, A.; Galinski, M. J. Phys. Chem. Solids 2004, 65, 281–286.

11958 DOI: 10.1021/la9012119

Figure 6. XPS N 1s spectrum of EMI-SCN IL. The inset shows the chemical structure of this IL.

oxide in EMI-SCN IL. The slightly oblique CV shape could be attributed to insertion/desertion resistance of charge carriers into/ from the Mn oxide structure (this reaction will be discussed later). The calculated specific capacitance of Mn oxide (according to eq 2) is as high as 55 F/g, which implies that the pseudocapacitive reaction of Mn oxide is most likely operative in this aprotic IL. Electrochemical properties of the oxide electrode were also evaluated by CP, in which applied charging (anodic) and discharging (cathodic) current densities were both set at 0.1 mA/cm2; Figure 4c shows the obtained chronopotentiogram of the five sequent charge-discharge cycles in EMI-SCN IL. As can be seen, the charging and discharging branches are nearly linear and symmetric, again indicating the ideal pseudocapacitive performance and great reversibility of Mn oxide in a potential range of 1.5 V, which is larger than that (∼1 V) typically found in traditional aqueous electrolytes.5-7 From the CP data, the oxide specific capacitance can also be evaluated according to the calculation C ¼ I=ðΔE=ΔtÞw

ð3Þ

where I is the applied current (0.1 mA), ΔE is the potential range (1.5 V  2), Δt is the total charging and discharging time, and w is the mass of the oxide. The measured specific capacitance of Mn oxide from Figure 4c is 53 F/g, which is pretty close to that derived from the CV data. While the literature reported that RuO2 Langmuir 2009, 25(19), 11955–11960

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Figure 7. XPS N 1s depth-profiling spectra taken from the Mn oxide electrodes previously polarized at (a) -0.3 V and (b) -1.8 V in EMISCN IL.

can show a specific pseudocapacitance of ∼40 F/g in optimum protic IL,29 this study shows that Mn oxide, a much cheaper electrode for supercapacitor applications, has an even better performance in EMI-SCN IL electrolyte. This finding would provide a possibility of developing aprotic types of electrolytes for use in oxide-based supercapacitors. The identical Mn oxide electrode showed a capacitance of 180 F/g within a potential range of 0.9 V in 0.1 M Na2SO4 aqueous solution. Clearly, the IL electrolyte is still far from optimum, further research is indeed required. XPS was used to examine the valent state of Mn oxide under various applied potentials in EMI-SCN IL. Figure 5a shows the Mn 2p3/2 spectra of the oxides previously polarized at -0.3 and -1.8 V, respectively; a chemical shift can be clearly recognized. Specifically, a higher applied potential led to a higher binding energy of the Mn 2p3/2 electron, suggesting that the Mn oxide was in a higher oxidation state.31 It is known that the valence of Mn can be more precisely identified by measuring the multiplet splitting width of two Mn 3s XPS peaks.32 Therefore, the spectra of this orbit were also acquired for the two oxide electrodes; the obtained data are shown in Figure 5b. According to the analytical data, the splitting width (ΔE) values for the -0.3 and -1.8 V polarized electrodes are 4.9 and 5.4 eV, respectively. The exchange interaction between the core level electron (3s) and the unpaired electrons in the valence band (3d) results in the peak separation of the Mn 3s spectrum upon photoelectron ejection.33,34 A lower valence of Mn leads to a wider splitting of the Mn 3s peaks. As reported by Chigane et al.,32 Mn2O3 has a ΔE of 5.41 eV while MnO2 has a ΔE of 4.78 eV. The comparison results indicate that while the -0.3 V polarized Mn oxide was essentially tetravalent, the -1.8 V polarized electrode mainly consisted of trivalent Mn. It was also found that the as-deposited Mn oxide had a ΔE of 5.1 eV, which represented a mixture of trivalence and tetravalence. Accordingly, a Mn3þ/Mn4þ faradic redox transition of Mn oxide in aprotic EMI-SCN IL, contributing to the pseudocapacitance (in Figure 4), was confirmed. (31) Hashemi, T.; Brinkman, A. W. J. Mater. Res. 1992, 7, 1278–1282. (32) Chigane, M.; Ishikawa, M. J. Electrochem. Soc. 2000, 147, 2246–2251. (33) Carver, J. C.; Schweitzer, G. K.; Carlson, T. A. J. Chem. Phys. 1972, 57, 973–982. (34) Oku, M.; Hirokawa, K.; Ikeda, S. J. Electron Spectrosc. Relat. Phenom. 1975, 7, 465–473.

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Since neither protons nor alkali cations (Liþ, Naþ, Kþ) exist in EMI-SCN IL, the pseudocapacitive reaction mechanism of Mn oxide must be different from that observed in aqueous electrolytes, as described in eq 1. To explore how the cations and anions react with Mn oxide, the chemical characteristics of the EMI-SCN electrolyte were first analyzed with XPS. The extremely low vapor pressure of IL makes it stable in the ultrahigh-vacuum XPS chamber.35,36 Figure 6 shows the acquired N 1s spectrum of EMI-SCN IL. Because of the different chemical environments of N atoms in the IL (i.e., NC and NC3 bonds), as shown in the inset, two N peaks with distinct binding energies can be found. While N1 in the SCN- anion have a binding energy at 397.6 eV, N2 in the EMIþ cation has a binding energy at 401.6 eV. It is also noted in this figure that the relative area under of the two peaks is approximately 1:2, which is consistent with the stoichiometric ratio of N1 and N2 in EMI-SCN IL. The distinct N signals from the EMIþ cation and the SCN- anion give us a great opportunity to understand what kinds of ions can become incorporated into Mn oxide during the redox process and participate in the pseudocapacitive mechanism. Figure 7a shows the N 1s depth-profiling spectra taken from the -0.3 V polarized (or anodically polarized) oxide electrode. The oxide surface was enriched with SCN- due to charge attraction. As compared to Figure 6, the binding energy of the N1 peak positively shifts to 398.7 eV while the N2 peak remains unchanged at 401.6 eV. The analytical results suggest that N1 in the SCN- anion bonded with Mn oxide and thus experienced a chemical state change; on the other hand, the EMIþ cation did not seem to have a significant interaction with the oxide. The depth profiling analysis was performed using Arþ sputtering. The removal rate per sputtering shot (3 s) was measured to be ∼5 nm for SiO2 film. Ten serial profiles of N 1s spectra are superimposed in Figure 7a. The signal from EMIþ was found to disappear after only one shot of Arþ sputtering, indicating that the cation was just adsorbed on the electrode surface. In contrast, since the SCN- anion penetrated into Mn oxide its peak intensity gradually decreased with depth. Figure 7b shows the spectra obtained from the -1.8 V polarized (or cathodically polarized) oxide. (35) Smith, E. F.; Garcia, I. J. V.; Briggs, D.; Licence, P. Chem. Commun. 2005, 45, 5633–5635. (36) Kolbeck, C.; Killian, M.; Maier, F.; Paape, N.; Wasserscheid, P.; Steinr€uck, H.-P. Langmuir 2008, 24, 9500–9507.

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that for SCN-. Presumably, the linear SCN-, rather than the octahedral PF6-, is more favorable to be inserted into Mn oxide. Since the pseudocapacitive reaction of Mn oxide seems to occur in more restricted regions (near the electrode surface) in BMI-PF6 IL, the lower measured specific capacitance (20 F/g vs 55 F/g) can be explained. Different physicochemical properties, including the geometric shape, of the constituent ions in ILs lead to the different interactions with Mn oxide and thus affect the obtained pseudocapacitive performance. The search for a more suitable working ion in the IL electrolyte that can further improve the charge storage properties of Mn oxide is already in progress.

4. Conclusions

Figure 8. XPS P 2p depth-profiling spectra taken from the Mn oxide electrode previously polarized at þ0.2 V in BMI-PF6 IL.

Although the surface had a high EMIþ content (also located at 401.6 eV), this cation cannot really enter into (and react with) the oxide and was easily removed upon Arþ sputtering. As also seen in this figure, the intensity from SCN- was much lower than that found in Figure 7a because this anion was favorably extracted from the oxide under the cathodic applied potential. The experimental results indicate that the linear SCN- cation, instead of the large EMIþ anion with an imidazolium ring, is the working species that can be incorporated into Mn oxide, compensate the Mn valent state variation, and thus contribute to the pseudocapacitance. Figure 8 shows the P 2p depth-profiling spectra taken from the Mn oxide electrode previously polarized in BMI-PF6 IL at the above-mentioned anodic limit potential of þ0.2 V. Clearly, the peak intensity is weaker as compared to that shown in Figure 7a, indicating that the PF6- concentration on the electrode surface is lower. This result could be attributed to the weak bonding between Mn oxide and PF6-, which was easily washed out during the methanol cleaning process. Figure 8 also reveals that the penetration depth of PF6- is only about one-third of

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The ideal pseudocapacitive performance of Mn oxide was found in a nonaqueous and aprotic EMI-SCN IL electrolyte; within a potential range of ∼1.5 V, a specific capacitance of 55 F/g and a great charge-discharge reversibility of the oxide electrode were obtained. XPS was used to examine the charge storage mechanism of Mn oxide in this IL. The analytical results indicate that the linear-shape SCN-, instead of the EMIþ with a large imidazolium ring, is the working ion that can become incorporated into the oxide and charge compensate the Mn3þ/Mn4þ redox transition during the charge-discharge process, thus contributing to the pseudocapacitance. Accordingly, the pseudocapacitive reaction mechanism can be proposed as follows: MnO2 -x ðSCNÞ2x þ2xe - TMnO2 -x þ2xSCN - ð0exe0:5Þ It was also found that the penetration depth of PF6- in Mn oxide was much shallower than that of SCN-; this can be attributed to the different geometric structures and different physicochemical properties of the two anions. As a result, a much lower oxide specific capacitance, i.e., ∼20 F/g, was obtained in BMI-PF6 IL. This paper has demonstrated an effective approach to probe the electrochemical reaction mechanism of metal oxide in IL electrolytes. Acknowledgment. The authors thank the National Science Council of the Republic of China for financially supporting this research under Contract 98-ET-E-006-009-ET.

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