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Pseudocapacitive behaviour of RuO2 in a proton exchange ionic liquid
Electrochemistry Communications 8 (2006) 1539–1543 www.elsevier.com/locate/elecom
Pseudocapacitive behaviour of RuO2 in a proton exchange ionic liquid Dominic Rochefort *, Anne-Laure Pont De´partement de chimie, Universite´ de Montre´al, CP6128 Succ. Centre-Ville, Montre´al, QC, Canada H3C 3J7 Received 8 June 2006; accepted 27 June 2006 Available online 8 August 2006
Abstract We studied the electrochemical behaviour of a thermally-prepared RuO2 electrode in a protic ionic liquid made from 2-methylpyridine and trifluoroacetic acid to determine if the current obtained on such electrode originates from double-layer charging or from pseudocapacitance. A thorough comparison of the electrochemical behavior in the ionic liquid and in aqueous electrolyte confirmed that pseudocapacitance is observed. The shape of the cyclic voltammograms obtained for the RuO2 electrode in the protic ionic liquid is similar to that obtained in H2SO4 and shows distinct redox peaks attributed to Faradaic reactions across the electrolytejelectrode interface, which give rise to pseudocapacitance. The specific capacitance of the electrode in the protic ionic liquid (83 F g1) is in the same order of magnitude to that obtained in water-based electrolyte but is ten times higher than that in an aprotic ionic liquid composed of 1-ethyl-3methylimidazolium tetrafluoroborate. Our observations lead to the conclusion that pseudocapacitance can occur on a ruthenium dioxide electrode in an ionic liquid. Our results could be of interest to develop new metal oxide electrochemical capacitors based on non-aqueous electrolytes. 2006 Elsevier B.V. All rights reserved. Keywords: Pseudocapacitance; Ruthenium dioxide; Ionic liquid; Coupled proton–electron transfer; Electrochemical capacitor
1. Introduction Pseudocapacitance arises on electrodes when the application of a potential induces Faradaic current from reactions such as electrosorption or from the oxidation/ reduction of electroactive materials [1]. Conducting metal oxides such as ruthenium dioxide exhibit pseudocapacitance via a coupled proton–electron transfer in aqueous solutions according to Eq. (1). The presence of protons required to observe pseudocapacitance restricted the study and use of metal oxides electrodes in aqueous electrolytes. Here we have studied the electrochemical behavior of a RuO2 electrode in a non-aqueous electrolyte composed of an ionic liquid which contains protons and we show that pseudocapacitance occurs in such system. RuO2 þ dHþ þ de $ RuO2d ðOHÞd *
ð1Þ
Corresponding author. Tel.: +1 514 343 6733; fax: +1 514 343 7586. E-mail address: [email protected] (D. Rochefort).
1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.06.032
In RuO2, the redox processes observed upon potential cycling and which involve the II, III, and IV oxidation states, yield high currents and the pseudocapacitance measured often exceeds that of the double-layer by a factor of ten [1–3]. This observation was used to demonstrate that while appreciable double-layer charging can occur on high surface area RuO2, the redox processes coupled with proton transfer giving rise to pseudocapacitance are predominant in the charging mechanism. The occurrence of proton insertion in the near surface region of RuO2 electrodes during potential cycling, demonstrated by quartz microbalance experiments [4], provided additional evidence that a coupled proton–electron transfer is the origin of the high capacitance of RuO2. Room temperature molten salts, which will be therein referred to by the term ionic liquids, have been the object of many studies in electrochemistry. The results of these studies can be found in recent reviews [5,6]. Ionic liquids are characterized by a wide electrochemical window of stability, a reasonable ionic conductivity (similar to most
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D. Rochefort, A.-L. Pont / Electrochemistry Communications 8 (2006) 1539–1543
non-aqueous electrolytes) and a very low vapor pressure. Their wide electrochemical window of stability in particular makes them very appealing electrolytes in capacitors because the maximum energy that can be stored in such devices depends on (DE)2 [7]. Details on this specific application can be found in the comprehensive review on ionic liquid electrolytes for carbon-based double-layer capacitors written by Ue [8]. Certainly the most frequently reported ionic liquids for that purpose are based on aprotic alkylimidazolium salts. Ionic liquids can also be formed by proton transfer, from a Brønsted acid to a Brønsted base [9]. The interactions between the resulting ionic species is responsible for the formation of a very stable liquid that is exclusively composed of ions while containing an appreciable proton concentration. These non-aqueous protic media could be of interest for charge storage in metal oxides since the use of ionic liquids could circumvent the problems imposed by the limited potential window of water-based electrolyte and instability of some metal oxides in aqueous solution of strong acids. Protic ionic liquids have recently been studied in fuel cells, as electrolytes in cell compartments for H2 oxidation and O2 reduction [10–14] and for impregnating proton diffusion membranes [15–18]. In both cases, the studies were motivated by the need to replace water as the proton conductor to increase the operating temperature. The idea exploited here is using protic ionic liquids as proton-rich electrolytes for conducting metal oxide electrodes. We provide the first evidence that pseudocapacitance arises on RuO2 electrode in a non-aqueous, ionic liquid, electrolyte based on 2-methylpyridine and trifluoroacetic acid. 2. Experimental 2.1. Electrode preparation
Fig. 1. Formation of the a-picoline:trifluoroacetic acid (P-TFA) protic ionic liquid with a 1:2 ratio.
ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (97%) was obtained commercially and was also heated under vacuum for 24 h before use. 2.3. Electrochemical measurements The electrochemical behavior of the RuO2 film electrode was studied in a half-cell using a standard three-electrode setup. The reference used in sulfuric acid electrolyte was 0 an aqueous Ag/AgCl (3 M NaCl, E0 = +0.197 V). For measurements in the ionic liquids, a silver wire quasi-reference electrode (QRE) was used. All electrochemical measurements were carried out under temperature-controlled conditions at (25.0 ± 0.5) C, under nitrogen, with a PAR2273 instrument. 3. Results and discussion 3.1. Electrochemistry in the protic ionic liquid electrolyte The electrochemical behavior of the P-TFA protic ionic liquid was firstly studied by cyclic voltammetry with a glassy carbon and a platinum electrode. To ensure minimal contamination of the ionic liquid by the water contained in air, the vacuum vessel in which the liquid was dried was transported and opened in a dry box filled with Ar. In order to establish the potential window of stability, the cyclic voltammograms in Fig. 2a were recorded in two distinct
The RuO2 film electrode that was used in our study was obtained by thermal decomposition in air (400 C for 2 h) of a thin layer of a 0.3 M aqueous solution of the salt RuCl3 painted on a 10 · 10 mm section of a Ti substrate. A sufficient amount of the electroactive material (3 mg) was obtained by repeating the procedure 8 times. The Ti substrate was covered with several layers of PTFE tape to expose only the area containing the RuO2 film. 2.2. Ionic liquids The protic ionic liquid was prepared by the slow addition of trifluoroacetic acid (TFA) to 2-methylpyridine (a-picoline, P) to reach a 2:1 molar composition. The PTFA ionic liquid was then heated under vacuum at 70 C for 48 h to remove water, yielding a clear to lightly brown colored liquid. Formation of ionic species (see Fig. 1) is evidenced at this point by the complete lack of smell of the liquid due to strong electrostatic interactions between the protonated a-picoline and the [(CF3CO2)2H] dimer as proposed by Angell and co-workers [9]. The aprotic
Fig. 2. (a) Cyclic voltammograms (m = 100 mV s1) obtained with a glassy carbon and a platinum working electrode in the P-TFA protic ionic liquid at RT in an Ar glove box. (b) Hydrogen adsorption/desorption and oxide formation/reduction occurring on the Pt surface in the ionic liquid.
D. Rochefort, A.-L. Pont / Electrochemistry Communications 8 (2006) 1539–1543
cycles, by sweeping the potential from a region where only double-layer current is observed, to a positive (or negative) value. The potential was reversed when a sudden current increase was observed, which is indicative of faradic processes occurring at the electrode surface at that point. Both electrodes exhibited reduction current which most likely corresponds to reduction of protons, present in a high concentration in the ionic liquid, to hydrogen. The overpotential for the proton reduction reaction is, as expected, much lower on platinum than on glassy carbon. Formation of bubbles, most likely H2(g), on the surface of the working electrode was observed concomitantly with the increase of cathodic current. The reactions occurring at the anodic limit of the curves, which could result from trifluoroacetic acid and/or 2-methylpyridine oxidation, are currently under study by DEMS. The inset (Fig. 2b) is an enlarged view of the cyclic voltammograms obtained with the Pt electrode when cycling the potential to different upper limits. The shape of the curves, recorded in the P-TFA ionic liquid, resembles that of the voltammograms for Pt in aqueous solutions. The usual regions, corresponding to hydrogen adsorption/desorption and to Pt surface oxide formation/reduction are found. An increase in the charge transferred to reduce the surface oxides is observed upon cycling to higher potential values. Surface oxide formation of the platinum electrode may arise from water hydrolysis (residual water is present at trace levels in the electrolyte) or from electrolyte oxidation. Position of oxide reduction peak (E = +0.25 V vs. AgQRE) gives a good indication of the potential of the silver wire QRE in the ionic liquid (E 0 = +0.33 V vs. Ag/AgCl). The voltammograms on Fig. 2b also show the features ascribed to hydrogen adsorption/desorption in the 0.25 to 0.0 V region. The lack of fine structure on the H-adsorption/desorption region of the CV indicates the presence of adsorbed (electrolyte) species. The fact that voltammograms of the Pt electrode in the protic ionic liquid presents this characteristic behavior of a Pt in a protic media is indicative that the protons are somewhat labile in the IL and that they might be able to diffuse to the active sites of the RuO2 electrode. 3.2. Behavior of the RuO2 film electrode The RuO2 film electrode was used to record cyclic voltammograms in 0.1 M H2SO4 and in the P-TFA (1:2 stoichiometry) ionic liquid. The voltammograms obtained at a 10 mV s1 scan rate are presented in Fig. 3. Both curves exhibit a rectangular shape characteristic of the pseudocapacitance in RuO2. The peaks corresponding to oxidation and reduction of the different Ru oxidation states in RuO2, according to Eq. (1), are clearly visible on the voltammograms obtained both in aqueous and in the protic ionic liquid. These are typical features that appear on voltammograms for RuO2 with a relatively high degree of crystallinity such as that annealed at temperatures above 300 C [19]. The presence of these peaks is therefore a very strong indication that redox processes occur on the RuO2
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Fig. 3. Cyclic voltammograms (m = 10 mV s1) obtained with the thermally-prepared RuO2 working electrode in 0.1 H2SO4 (gray) and in the 2-methylpyridine:trifluoroacetic acid (1:2) ionic liquid (black).
electrode in the ionic liquid which demonstrate the pseudocapacitive behavior of RuO2 in the molten salt electrolyte. While the position of these peaks in the aqueous solution corresponds to those already previously [19,20], the peaks of the curve in the ionic liquid seems to be shifted to more positive potential values on the anodic scan and to more negative values on the cathodic scan, which could indicate slow processes. An appreciation of the rapidity of proton access in and out of the RuO2 electrode surface can also be made upon potential reversal (at Ek = +2 V). The curves on Fig. 2 show that the current response of the electrode upon changing the potential sweep direction is much slower in the ionic liquid than in the aqueous electrolyte. While protons from the ionic liquid are undoubtedly available to an appreciable fraction of the electrode material, their diffusion to the active sites is slow. Proton conduction in protic ionic liquids results from a combination of decoupled (Grotthus) and vehicle-type mechanisms [13,21]. It appears clearly that in the P-TFA ionic liquid under study here the protons transport occurs predominantly via a vehicle-type mechanism and that their diffusion to the RuO2 redox sites could be hindered by the relatively high viscosity of the electrolyte (g = 25.1 mPa s) [9]. Another noticeable feature of the voltammograms is the presence of a small peak (indicated by the arrows) which corresponds to hydrogen insertion in the RuO2 lattice [22,23]. This observation is another indication that the protons from the ionic liquid can be exchanged with the RuO2, in a manner similar to the well-established mechanism taking place in aqueous solutions. 3.3. Pseudocapacitance In order to demonstrate further that pseudocapacitance is responsible for the current observed in Fig. 3, we have calculated the total charge stored in the RuO2 electrode
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peaks indicates that Faradaic current does not contribute to a large proportion of the current measured in the aprotic electrolyte. Secondly, the currents obtained are much smaller in the aprotic EMI-BF4 ionic liquid than in the protic P-TFA. The specific capacitance of the RuO2 electrode in the EMI-BF4 was calculated to be 6.5 F g1 by measuring the current (i = 3.9 · 104 A) around 0.5 V, and using the relation C = i/m Æ m, where m is the mass of RuO2 and m is the scan rate (0.02 V s1). In comparison, the specific capacitance obtained with the same RuO2 electrode is more than ten times higher (83 F g1) in the protic ionic liquid. 4. Conclusions
Fig. 4. (a) Cyclic voltammograms obtained at different scan rates with the thermally-prepared RuO2 working electrode in the P-TFA ionic liquid. (b) Plot showing the dependence of 1/q* with m1/2, as calculated from the anodic half of the voltammograms in (a).
upon cycling in the ionic liquid and in H2SO4. To do so, we recorded cyclic voltammograms at various scan rates ranging from 1 to 250 mV s1 in both electrolytes (Fig. 4a). The increase in anodic and cathodic peak potential values with the scan rate for a given redox process is indicative of difficult electron transfer processes originating from the slow diffusion of protons to the electrode material (see above). The voltammetric charges (q*) for each curve was obtained by integration between 0.45 and 1.45 V of the anodic half of the curves. The reciprocal values of q* were then plotted against m1/2 (Fig. 4b) to calculate the total charge by extrapolation to m ! 0 [24]. The total charge obtained that way gives a value of 248 mC. Taking into account the 1 V potential window used for the integration and the mass of RuO2 material deposited on the electrode, the specific capacitance obtained is 83 F g1. In comparison, a total charge of 114 mC (or a specific capacitance of 38 F g1) was obtained in the 0.1 M H2SO4 with the same electrode, using the same calculation procedure. The only explanation we can provide for the moment about the higher value in the P-TFA is that a higher proton concentration is found in the ionic liquid than in 0.1 M H2SO4. These specific capacitance values are very small compared to those obtained for hydrous ruthenium dioxide, but they are similar to the values already reported for cristalline materials prepared at such high temperatures [3,19]. We have also looked at the electrochemical behaviour of the RuO2 electrode in the widely studied aprotic ionic liquid 1-ethyl-3-methylimidazolium tetrafluroroborate (EMI-BF4). The cyclic voltammograms obtained (not presented) show that in an aprotic ionic liquid, no pseudocapacitance occurs. Firstly, the curves obtained are very smooth and they lack the peaks attributed to the different redox couples of RuO2 that are present with H2SO4 and the protic ionic liquid P-TFA (Fig. 3). The absence of these
We have studied the electrochemical behaviour of a thermally-prepared RuO2 in a proton exchange room temperature molten salt composed of 2-methylpyridine and trifluoroacetic acid in order to demonstrate that pseudocapacitance can occur by a coupled proton–electron transfer in non-aqueous electrolytes. The protic ionic liquid used has properties similar to those of common aprotic ionic liquids based on imidazolium salts. The cyclic voltammograms of the RuO2 electrode in the protic ionic liquid show redox peaks that are ascribed to the Faradaic reactions involving protons at the near surface regions of a RuO2 electrode with a relatively high degree of cristallinity. These redox peaks, indicative of pseudocapacitance, are also seen in an aqueous electrolyte but were absent on the voltammograms recorded in an aprotic ionic liquid (EMI-BF4) where only double-layer capacitance is observed. The current intensities that were measured and the specific capacitance that was calculated in the protic ionic liquid are too high to be explained only by doublelayer charging. All these observations provide sufficient evidence to demonstrate that pseudocapacitance occurs on RuO2 in the protic ionic liquid in a manner similar to that in aqueous solutions. The high viscosity and slow proton transfer limit however the charging rate that can be obtained in the system described here. We are currently studying the pseudocapacitance of RuO2 and other conductive metal oxides in protic ionic liquids with higher conductivities. Acknowledgements We thank Be´atrice Garcia for her valuable help and the Universite´ de Montre´al for financial assistance through start-up funds. References [1] B.E. Conway, Electrochemical Supercapacitors (see chapters 10 and 11 in particular), Kluwer, Plenum, New York, 1999. [2] S. Hadzi-Jordanov, H.A. Kozlowska, M. Vukovic, B.E. Conway, J. Electrochem. Soc. 125 (1978) 1473. [3] J.P. Zheng, T.R. Jow, J. Electrochem. Soc. 142 (1995) L6.
D. Rochefort, A.-L. Pont / Electrochemistry Communications 8 (2006) 1539–1543 [4] I.D. Raistrick, in: J. McHardy, F. Ludwig (Eds.), The Electrochemistry of Semiconductors and Electronics – Processes and Devices, William Andrew Publishing/Noyes, Park Ridge, NJ, 1992. [5] M.C. Buzzeo, R.G. Evans, R.G. Compton, ChemPhysChem 5 (2004) 1107. [6] H. Ohno, Electrochemical Aspects of Ionic Liquids, John Wiley & Sons, Hoboken, NJ, 2005. [7] R. Kotz, M. Carlen, Electrochim. Acta 45 (2000) 2483. [8] M. Ue, in: H. Ohno (Ed.), Electrochemical Aspects of Ionic Liquids, Jonh Wiley & Sons, Hoboken, NJ, 2005. [9] M. Yoshizawa, W. Xu, C.A. Angell, J. Am. Chem. Soc. 125 (2003) 15411. [10] R.F. de Souza, J.C. Padilha, R.S. Goncalves, J. Dupont, Electrochem. Commun. 5 (2003) 728. [11] R. Hagiwara, T. Nohira, K. Matsumoto, Y. Tamba, Electrochem. Solid-State Lett. 8 (2005) A231. [12] H. Matsuoka, H. Nakamoto, M. Susan, M. Watanabe, Electrochim. Acta 50 (2005) 4015.
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[13] A. Noda, M.A.B.H. Susan, K. Kudo, S. Mitsushima, K. Hayamizu, M. Watanabe, J. Phys. Chem. B 107 (2003) 4024. [14] M.A.B.H. Susan, A. Noda, S. Mitsushima, M. Watanabe, Chem. Commun. (2003) 938. [15] J.Z. Sun, L.R. Jordan, M. Forsyth, D.R. MacFarlane, Electrochim. Acta 46 (2001) 1703. [16] M.A.B.H. Susan, M. Yoo, H. Nakamoto, M. Watanabe, Chem. Lett. 32 (2003) 836. [17] M. Yamada, I. Honma, Electrochim. Acta 48 (2003) 2411. [18] M. Yamada, I. Honma, Electrochim. Acta 50 (2005) 2837. [19] J.P. Zheng, P.J. Cygan, T.R. Jow, J. Electrochem. Soc. 142 (1995) 2699. [20] S. Hadzi-Jordanov, H.A. Kozlowska, B.E. Conway, J. Electroanal. Chem. 60 (1975) 359. [21] W. Xu, C.A. Angell, Science 302 (2003) 422. [22] C. Chabanier, D. Guay, J. Electroanal. Chem. 570 (2004) 13. [23] D. Galizzioli, F. Tantardini, S. Trasatti, J. Appl. Electrochem. 4 (1974) 57. [24] S. Ardizzone, G. Fregonara, S. Trasatti, Electrochim. Acta 35 (1990) 263.
Pseudocapacitive behaviour of RuO2 in a proton exchange ionic liquid Dominic Rochefort *, Anne-Laure Pont De´partement de chimie, Universite´ de Montre´al, CP6128 Succ. Centre-Ville, Montre´al, QC, Canada H3C 3J7 Received 8 June 2006; accepted 27 June 2006 Available online 8 August 2006
Abstract We studied the electrochemical behaviour of a thermally-prepared RuO2 electrode in a protic ionic liquid made from 2-methylpyridine and trifluoroacetic acid to determine if the current obtained on such electrode originates from double-layer charging or from pseudocapacitance. A thorough comparison of the electrochemical behavior in the ionic liquid and in aqueous electrolyte confirmed that pseudocapacitance is observed. The shape of the cyclic voltammograms obtained for the RuO2 electrode in the protic ionic liquid is similar to that obtained in H2SO4 and shows distinct redox peaks attributed to Faradaic reactions across the electrolytejelectrode interface, which give rise to pseudocapacitance. The specific capacitance of the electrode in the protic ionic liquid (83 F g1) is in the same order of magnitude to that obtained in water-based electrolyte but is ten times higher than that in an aprotic ionic liquid composed of 1-ethyl-3methylimidazolium tetrafluoroborate. Our observations lead to the conclusion that pseudocapacitance can occur on a ruthenium dioxide electrode in an ionic liquid. Our results could be of interest to develop new metal oxide electrochemical capacitors based on non-aqueous electrolytes. 2006 Elsevier B.V. All rights reserved. Keywords: Pseudocapacitance; Ruthenium dioxide; Ionic liquid; Coupled proton–electron transfer; Electrochemical capacitor
1. Introduction Pseudocapacitance arises on electrodes when the application of a potential induces Faradaic current from reactions such as electrosorption or from the oxidation/ reduction of electroactive materials [1]. Conducting metal oxides such as ruthenium dioxide exhibit pseudocapacitance via a coupled proton–electron transfer in aqueous solutions according to Eq. (1). The presence of protons required to observe pseudocapacitance restricted the study and use of metal oxides electrodes in aqueous electrolytes. Here we have studied the electrochemical behavior of a RuO2 electrode in a non-aqueous electrolyte composed of an ionic liquid which contains protons and we show that pseudocapacitance occurs in such system. RuO2 þ dHþ þ de $ RuO2d ðOHÞd *
ð1Þ
Corresponding author. Tel.: +1 514 343 6733; fax: +1 514 343 7586. E-mail address: [email protected] (D. Rochefort).
1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.06.032
In RuO2, the redox processes observed upon potential cycling and which involve the II, III, and IV oxidation states, yield high currents and the pseudocapacitance measured often exceeds that of the double-layer by a factor of ten [1–3]. This observation was used to demonstrate that while appreciable double-layer charging can occur on high surface area RuO2, the redox processes coupled with proton transfer giving rise to pseudocapacitance are predominant in the charging mechanism. The occurrence of proton insertion in the near surface region of RuO2 electrodes during potential cycling, demonstrated by quartz microbalance experiments [4], provided additional evidence that a coupled proton–electron transfer is the origin of the high capacitance of RuO2. Room temperature molten salts, which will be therein referred to by the term ionic liquids, have been the object of many studies in electrochemistry. The results of these studies can be found in recent reviews [5,6]. Ionic liquids are characterized by a wide electrochemical window of stability, a reasonable ionic conductivity (similar to most
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non-aqueous electrolytes) and a very low vapor pressure. Their wide electrochemical window of stability in particular makes them very appealing electrolytes in capacitors because the maximum energy that can be stored in such devices depends on (DE)2 [7]. Details on this specific application can be found in the comprehensive review on ionic liquid electrolytes for carbon-based double-layer capacitors written by Ue [8]. Certainly the most frequently reported ionic liquids for that purpose are based on aprotic alkylimidazolium salts. Ionic liquids can also be formed by proton transfer, from a Brønsted acid to a Brønsted base [9]. The interactions between the resulting ionic species is responsible for the formation of a very stable liquid that is exclusively composed of ions while containing an appreciable proton concentration. These non-aqueous protic media could be of interest for charge storage in metal oxides since the use of ionic liquids could circumvent the problems imposed by the limited potential window of water-based electrolyte and instability of some metal oxides in aqueous solution of strong acids. Protic ionic liquids have recently been studied in fuel cells, as electrolytes in cell compartments for H2 oxidation and O2 reduction [10–14] and for impregnating proton diffusion membranes [15–18]. In both cases, the studies were motivated by the need to replace water as the proton conductor to increase the operating temperature. The idea exploited here is using protic ionic liquids as proton-rich electrolytes for conducting metal oxide electrodes. We provide the first evidence that pseudocapacitance arises on RuO2 electrode in a non-aqueous, ionic liquid, electrolyte based on 2-methylpyridine and trifluoroacetic acid. 2. Experimental 2.1. Electrode preparation
Fig. 1. Formation of the a-picoline:trifluoroacetic acid (P-TFA) protic ionic liquid with a 1:2 ratio.
ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (97%) was obtained commercially and was also heated under vacuum for 24 h before use. 2.3. Electrochemical measurements The electrochemical behavior of the RuO2 film electrode was studied in a half-cell using a standard three-electrode setup. The reference used in sulfuric acid electrolyte was 0 an aqueous Ag/AgCl (3 M NaCl, E0 = +0.197 V). For measurements in the ionic liquids, a silver wire quasi-reference electrode (QRE) was used. All electrochemical measurements were carried out under temperature-controlled conditions at (25.0 ± 0.5) C, under nitrogen, with a PAR2273 instrument. 3. Results and discussion 3.1. Electrochemistry in the protic ionic liquid electrolyte The electrochemical behavior of the P-TFA protic ionic liquid was firstly studied by cyclic voltammetry with a glassy carbon and a platinum electrode. To ensure minimal contamination of the ionic liquid by the water contained in air, the vacuum vessel in which the liquid was dried was transported and opened in a dry box filled with Ar. In order to establish the potential window of stability, the cyclic voltammograms in Fig. 2a were recorded in two distinct
The RuO2 film electrode that was used in our study was obtained by thermal decomposition in air (400 C for 2 h) of a thin layer of a 0.3 M aqueous solution of the salt RuCl3 painted on a 10 · 10 mm section of a Ti substrate. A sufficient amount of the electroactive material (3 mg) was obtained by repeating the procedure 8 times. The Ti substrate was covered with several layers of PTFE tape to expose only the area containing the RuO2 film. 2.2. Ionic liquids The protic ionic liquid was prepared by the slow addition of trifluoroacetic acid (TFA) to 2-methylpyridine (a-picoline, P) to reach a 2:1 molar composition. The PTFA ionic liquid was then heated under vacuum at 70 C for 48 h to remove water, yielding a clear to lightly brown colored liquid. Formation of ionic species (see Fig. 1) is evidenced at this point by the complete lack of smell of the liquid due to strong electrostatic interactions between the protonated a-picoline and the [(CF3CO2)2H] dimer as proposed by Angell and co-workers [9]. The aprotic
Fig. 2. (a) Cyclic voltammograms (m = 100 mV s1) obtained with a glassy carbon and a platinum working electrode in the P-TFA protic ionic liquid at RT in an Ar glove box. (b) Hydrogen adsorption/desorption and oxide formation/reduction occurring on the Pt surface in the ionic liquid.
D. Rochefort, A.-L. Pont / Electrochemistry Communications 8 (2006) 1539–1543
cycles, by sweeping the potential from a region where only double-layer current is observed, to a positive (or negative) value. The potential was reversed when a sudden current increase was observed, which is indicative of faradic processes occurring at the electrode surface at that point. Both electrodes exhibited reduction current which most likely corresponds to reduction of protons, present in a high concentration in the ionic liquid, to hydrogen. The overpotential for the proton reduction reaction is, as expected, much lower on platinum than on glassy carbon. Formation of bubbles, most likely H2(g), on the surface of the working electrode was observed concomitantly with the increase of cathodic current. The reactions occurring at the anodic limit of the curves, which could result from trifluoroacetic acid and/or 2-methylpyridine oxidation, are currently under study by DEMS. The inset (Fig. 2b) is an enlarged view of the cyclic voltammograms obtained with the Pt electrode when cycling the potential to different upper limits. The shape of the curves, recorded in the P-TFA ionic liquid, resembles that of the voltammograms for Pt in aqueous solutions. The usual regions, corresponding to hydrogen adsorption/desorption and to Pt surface oxide formation/reduction are found. An increase in the charge transferred to reduce the surface oxides is observed upon cycling to higher potential values. Surface oxide formation of the platinum electrode may arise from water hydrolysis (residual water is present at trace levels in the electrolyte) or from electrolyte oxidation. Position of oxide reduction peak (E = +0.25 V vs. AgQRE) gives a good indication of the potential of the silver wire QRE in the ionic liquid (E 0 = +0.33 V vs. Ag/AgCl). The voltammograms on Fig. 2b also show the features ascribed to hydrogen adsorption/desorption in the 0.25 to 0.0 V region. The lack of fine structure on the H-adsorption/desorption region of the CV indicates the presence of adsorbed (electrolyte) species. The fact that voltammograms of the Pt electrode in the protic ionic liquid presents this characteristic behavior of a Pt in a protic media is indicative that the protons are somewhat labile in the IL and that they might be able to diffuse to the active sites of the RuO2 electrode. 3.2. Behavior of the RuO2 film electrode The RuO2 film electrode was used to record cyclic voltammograms in 0.1 M H2SO4 and in the P-TFA (1:2 stoichiometry) ionic liquid. The voltammograms obtained at a 10 mV s1 scan rate are presented in Fig. 3. Both curves exhibit a rectangular shape characteristic of the pseudocapacitance in RuO2. The peaks corresponding to oxidation and reduction of the different Ru oxidation states in RuO2, according to Eq. (1), are clearly visible on the voltammograms obtained both in aqueous and in the protic ionic liquid. These are typical features that appear on voltammograms for RuO2 with a relatively high degree of crystallinity such as that annealed at temperatures above 300 C [19]. The presence of these peaks is therefore a very strong indication that redox processes occur on the RuO2
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Fig. 3. Cyclic voltammograms (m = 10 mV s1) obtained with the thermally-prepared RuO2 working electrode in 0.1 H2SO4 (gray) and in the 2-methylpyridine:trifluoroacetic acid (1:2) ionic liquid (black).
electrode in the ionic liquid which demonstrate the pseudocapacitive behavior of RuO2 in the molten salt electrolyte. While the position of these peaks in the aqueous solution corresponds to those already previously [19,20], the peaks of the curve in the ionic liquid seems to be shifted to more positive potential values on the anodic scan and to more negative values on the cathodic scan, which could indicate slow processes. An appreciation of the rapidity of proton access in and out of the RuO2 electrode surface can also be made upon potential reversal (at Ek = +2 V). The curves on Fig. 2 show that the current response of the electrode upon changing the potential sweep direction is much slower in the ionic liquid than in the aqueous electrolyte. While protons from the ionic liquid are undoubtedly available to an appreciable fraction of the electrode material, their diffusion to the active sites is slow. Proton conduction in protic ionic liquids results from a combination of decoupled (Grotthus) and vehicle-type mechanisms [13,21]. It appears clearly that in the P-TFA ionic liquid under study here the protons transport occurs predominantly via a vehicle-type mechanism and that their diffusion to the RuO2 redox sites could be hindered by the relatively high viscosity of the electrolyte (g = 25.1 mPa s) [9]. Another noticeable feature of the voltammograms is the presence of a small peak (indicated by the arrows) which corresponds to hydrogen insertion in the RuO2 lattice [22,23]. This observation is another indication that the protons from the ionic liquid can be exchanged with the RuO2, in a manner similar to the well-established mechanism taking place in aqueous solutions. 3.3. Pseudocapacitance In order to demonstrate further that pseudocapacitance is responsible for the current observed in Fig. 3, we have calculated the total charge stored in the RuO2 electrode
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peaks indicates that Faradaic current does not contribute to a large proportion of the current measured in the aprotic electrolyte. Secondly, the currents obtained are much smaller in the aprotic EMI-BF4 ionic liquid than in the protic P-TFA. The specific capacitance of the RuO2 electrode in the EMI-BF4 was calculated to be 6.5 F g1 by measuring the current (i = 3.9 · 104 A) around 0.5 V, and using the relation C = i/m Æ m, where m is the mass of RuO2 and m is the scan rate (0.02 V s1). In comparison, the specific capacitance obtained with the same RuO2 electrode is more than ten times higher (83 F g1) in the protic ionic liquid. 4. Conclusions
Fig. 4. (a) Cyclic voltammograms obtained at different scan rates with the thermally-prepared RuO2 working electrode in the P-TFA ionic liquid. (b) Plot showing the dependence of 1/q* with m1/2, as calculated from the anodic half of the voltammograms in (a).
upon cycling in the ionic liquid and in H2SO4. To do so, we recorded cyclic voltammograms at various scan rates ranging from 1 to 250 mV s1 in both electrolytes (Fig. 4a). The increase in anodic and cathodic peak potential values with the scan rate for a given redox process is indicative of difficult electron transfer processes originating from the slow diffusion of protons to the electrode material (see above). The voltammetric charges (q*) for each curve was obtained by integration between 0.45 and 1.45 V of the anodic half of the curves. The reciprocal values of q* were then plotted against m1/2 (Fig. 4b) to calculate the total charge by extrapolation to m ! 0 [24]. The total charge obtained that way gives a value of 248 mC. Taking into account the 1 V potential window used for the integration and the mass of RuO2 material deposited on the electrode, the specific capacitance obtained is 83 F g1. In comparison, a total charge of 114 mC (or a specific capacitance of 38 F g1) was obtained in the 0.1 M H2SO4 with the same electrode, using the same calculation procedure. The only explanation we can provide for the moment about the higher value in the P-TFA is that a higher proton concentration is found in the ionic liquid than in 0.1 M H2SO4. These specific capacitance values are very small compared to those obtained for hydrous ruthenium dioxide, but they are similar to the values already reported for cristalline materials prepared at such high temperatures [3,19]. We have also looked at the electrochemical behaviour of the RuO2 electrode in the widely studied aprotic ionic liquid 1-ethyl-3-methylimidazolium tetrafluroroborate (EMI-BF4). The cyclic voltammograms obtained (not presented) show that in an aprotic ionic liquid, no pseudocapacitance occurs. Firstly, the curves obtained are very smooth and they lack the peaks attributed to the different redox couples of RuO2 that are present with H2SO4 and the protic ionic liquid P-TFA (Fig. 3). The absence of these
We have studied the electrochemical behaviour of a thermally-prepared RuO2 in a proton exchange room temperature molten salt composed of 2-methylpyridine and trifluoroacetic acid in order to demonstrate that pseudocapacitance can occur by a coupled proton–electron transfer in non-aqueous electrolytes. The protic ionic liquid used has properties similar to those of common aprotic ionic liquids based on imidazolium salts. The cyclic voltammograms of the RuO2 electrode in the protic ionic liquid show redox peaks that are ascribed to the Faradaic reactions involving protons at the near surface regions of a RuO2 electrode with a relatively high degree of cristallinity. These redox peaks, indicative of pseudocapacitance, are also seen in an aqueous electrolyte but were absent on the voltammograms recorded in an aprotic ionic liquid (EMI-BF4) where only double-layer capacitance is observed. The current intensities that were measured and the specific capacitance that was calculated in the protic ionic liquid are too high to be explained only by doublelayer charging. All these observations provide sufficient evidence to demonstrate that pseudocapacitance occurs on RuO2 in the protic ionic liquid in a manner similar to that in aqueous solutions. The high viscosity and slow proton transfer limit however the charging rate that can be obtained in the system described here. We are currently studying the pseudocapacitance of RuO2 and other conductive metal oxides in protic ionic liquids with higher conductivities. Acknowledgements We thank Be´atrice Garcia for her valuable help and the Universite´ de Montre´al for financial assistance through start-up funds. References [1] B.E. Conway, Electrochemical Supercapacitors (see chapters 10 and 11 in particular), Kluwer, Plenum, New York, 1999. [2] S. Hadzi-Jordanov, H.A. Kozlowska, M. Vukovic, B.E. Conway, J. Electrochem. Soc. 125 (1978) 1473. [3] J.P. Zheng, T.R. Jow, J. Electrochem. Soc. 142 (1995) L6.
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