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Exceptionally High Electric Double Layer Capacitances of Oligomeric Ionic Liquids Michio Matsumoto, Sunao Shimizu, Rina Sotoike, Masayoshi Watanabe, Yoshihiro Iwasa, and Takuzo Aida J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09156 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017
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Journal of the American Chemical Society is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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Exceptionally High Electric Double Layer Capacitances of Oligomeric Ionic Liquids Michio Matsumoto,†,¶ Sunao Shimizu,‡,¶ Rina Sotoike,§ Masayoshi Watanabe,§ Yoshihiro Iwasa,‡,#,* and Takuzo Aida†,‡,* †
Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 1138656, Japan
‡
RIKEN Center for Emergent Matter Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
§
Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan
#
Department of Applied Physics and Quantum Phase Electronics Center, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Supporting Information Placeholder
Electric double layer (EDL) capacitors are promising as next-generation energy accumulators if their capacitances and operation voltages are both high. However, only few electrolytes can simultaneously fulfil these two requisites. Here we report that an oligomeric ionic liquid such as IL4TFSI with four imidazolium ion units in its structure provides a wide electrochemical window of ~5.0 V, similar to monomeric ionic liquids. Furthermore, electrochemical impedance measurements using Au working electrodes demonstrated that IL4TFSI exhibits an exceptionally high EDL capacitance of ~66 µF/cm2, which is ~6 times as high as those of monomeric ionic liquids so far reported. We also found that an EDL-based field effect transistor (FET) using IL4TFSI as a gate dielectric material and SrTiO3 as a channel material displays a very sharp transfer curve with an enhanced carrier accumulation capability of ~64 µF/cm2, as determined by Hall-effect measurements. ABSTRACT:
An electric double layer (EDL),1 the thinnest capacitor, has attracted significant attention due to its potentially high capacitance (EDL capacitance), and has been considered a promising component for the next-generation energy storage devices.2 EDLs have also been used as gate dielectrics for field effect transistors (FETs),3 from which numerous electric fieldinduced phenomena such as superconductivity,3a–c ferromagnetism,3d and some other emerging iontronic devices3e,f were demonstrated. However, for expanding the scope of EDL-based devices, the charge accumulation ability of EDLs has to be further enhanced. The number of charges that can be accumulated in a capacitor (Q) is proportional to the capacitance (C) of a device and the applied voltage (V),
Figure 1. (a) Chemical structures of oligomeric ionic liquids IL4TFSI and IL2TFSI, and monomeric ionic liquids BMITFSI and DEMETFSI as references. (b) Schematic representations of the proposed structures of electric double layer (EDL) formed by oligomeric (left) and monomeric (right) ionic liquids, showing how the oligomeric ionic liquids exhibits the enhanced capacitance.
according to: Q = CV. Thus, a higher charge accumulation ability requires both a higher capacitance and a higher upperlimit operation voltage. For developing high-performance capacitors, a wide variety of electrodes such as nanostructured
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Figure 2. (a) Bode plots (f–q, f–|Z|) and (b) Nyquist plots (Z’–Z”) for IL4TFSI obtained by electrochemical impedance spectroscopy (EIS) at the bias potential of –0.5 V versus the open circuit voltage (OCV) with a voltage amplitude of 10 mV at 30 °C after different annealing times from 0 to 150 min. The symbols f, Z, q, Z’, and Z” denote frequency of the input signal, impedance, argument of Z, real part of Z, and imaginary part of Z, respectively. (c) Capacitances (C) of IL4TFSI (red), IL2TFSI (orange), BMITFSI (blue), and DEMETFSI (black) at different frequencies (f) at a fixed bias potential of –0.5 V versus OCV at 30 °C. (d) Capacitances (C) of IL4TFSI (red), IL2TFSI (orange), BMITFSI (blue), and DEMETFSI (black) at different bias potentials at a fixed frequency (f) of 1 Hz at 30 °C. The data acquisitions were done after 150-min annealing whenever the bias potential to the working electrode was changed.
electrodes with large surface areas2c,d have been investigated. However, much less attention has been focused on the development of better electrolytes. Electrolytes for EDL capacitors so far reported are classified into two categories depending on whether they can be used in aqueous or organic media.2c In general, the electrolytes that can be used in aqueous media can achieve very high EDL capacitances such as ~70 µF/cm2.2,4 However, the narrow electrochemical window of water (~1.2 V) lowers the upper-limit operation voltages for the corresponding EDL capacitors. In contrast, the electrolytes that can be used in organic media offer wider electrochemical windows (~2.7 V), so that the upper-limit operation voltages are higher. Nevertheless, the EDL capacitances attainable with such electrolytes are still low (~10 µF/cm2).2c,d Here we report oligomeric ionic liquids as a new class of electrolytes for EDL capacitors, which bear multiple ionic moieties in their structures, yet exhibit a liquid fluidity at room temperature. In 2004, we reported that imidazolium ionbased ionic liquids are excellent dispersants for pristine singlewalled carbon nanotubes and suggested the presence of a strong π–cation interaction between them.5 Recently, we also found that, under the application of microwaves, PF6 salts of imidazolium ion tetramer (IL4) and dimer (IL2) (Figure 1a) efficiently exfoliate graphite into single-layer graphene, in which a multivalently enhanced ‘ionic liquid/sp2 carbon network’ interaction is supposed to play a critical role.6 Being inspired by this multivalent effect, we envisioned that such oligomeric ionic liquids as electrolytes may facilitate the charge accumulation in EDL capacitors (Figure 1b). Although ionic liquids7 are known to provide wide electrochemical windows (~5.0 V),8 reported capacitances for ionic liquids are again nothing special. As for oligomeric ionic liquids, Feng et al. suggested the potential utility of an ionic liquid dimer for EDL capacitors by molecular dynamics
simulation with a preliminarily cyclic voltammetry (CV) analysis.9 However, no further studies have been done. We investigated the dielectric behaviors of tetrameric IL4TFSI and dimeric IL2TFSI together with BMITFSI and DEMETFSI as monomeric conventional references (Figures 1a, S3 and S4; TFSI = bis(trifluoromethanesulfonyl)imide),10 by electrochemical impedance spectroscopy (EIS) using Au (working), Ag/Ag+ in DEMETFSI (reference), and Pt (counter) electrodes. At first, we confirmed by means of linear sweep voltammetry (LSV) that the electrochemical windows of oligomeric IL4TFSI and IL2TFSI as well as monomeric BMITFSI and DEMETFSI are as wide as ~5.0 V (Figure S5). Next, we carried out EIS of IL4TFSI at 61 different frequencies (f) in a range of 0.1 Hz–100 kHz with a voltage amplitude of 10 mV. Figures 2a and 2b show Bode and Nyquist plots, respectively, for IL4TFSI at a bias potential of – 0.5 V versus open circuit voltage (OCV), where the data acquisitions were done every 30 min over a period of 150 min. As typically observed for the electrochemical impedance spectra of ionic electrolytes,11 the upper panel in Figure 2a displays a bimodal feature, indicating the presence of two different capacitative behaviors, i.e., EDL and bulk capacitors in lower and higher frequency regions, respectively. One may also notice that, in the lower frequency region, the plots change with the annealing time (see arrows), suggesting that EDL forms slowly. This was also the case with IL2TFSI but not so explicit with monomeric ionic liquids such as BMITFSI and DEMETFSI (Figure S6). Figure 2c shows frequency (f)-dependent capacitances observed with oligomeric IL4TFSI and IL2TFSI, and monomeric BMITFSI and DEMETFSI at a bias potential of –0.5 V (annealing time; 150 min). Here, the capacitance (C) is given by the following equation,11 where Z” represents the imaginary part of impedance:
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Figure 3. (a) Optical micrograph and (b) schematic representation of a FET device using crystalline SrTiO3 as a semiconducting channel material and an ionic liquid as a gate dielectric material. (c) Transfer curves and (d) sheet carrier densities (N) of the SrTiO3based FETs using IL4TFSI (red), IL2TFSI (orange), BMITFSI (blue), and DEMETFSI (black) as electrolytes at 27 °C under different reference electrode voltages (VR). ID denotes a source-drain current. The EDL capacitances (CEDL) were obtained from the slopes of the N–VR plots.
1 2𝜋𝑓𝑍" The observed capacitances were lower as the applied frequency f was higher (Figure 2c) due to the transition from electrode polarization (EDL capacitance) to dipole polarization (bulk capacitance).11 For monomeric BMITFSI and DEMETFSI, such a polarization transition did not occur explicitly until the f-value exceeded 104 Hz. In contrast, the oligomeric ionic liquids showed a polarization transition when the f -value exceeded ~102 and ~103 Hz for IL4TFSI and IL2TFSI, respectively. This is because IL4TFSI and IL2TFSI are more viscous than BMITFSI and DEMETFSI (Figure S3). Here, an even more important observation in Figure 2c is that oligomeric IL4TFSI and IL2TFSI in their low-frequency regions showed exceptionally high EDL capacitances that are much higher than other ionic liquids reported so far, including BMITFSI and DEMETFSI.8c Figure 2d shows how the EDL capacitances of IL4TFSI and IL2TFSI at f = 1 Hz change with the bias potential applied to the working electrode. In order to exclude a possible effect of their slow reorganization, the devices were annealed for 150 min whenever the bias potential was changed. Although the capacitances with BMITFSI and DEMETFSI did not show any significant dependency on the bias potential, those with oligomeric IL4TFSI and IL2TFSI dramatically increased by 8 and 3 times, respectively, when the bias potential versus OCV was changed from 0 V to –0.75 V. Note that such an abrupt increase did not appear when the bias potential was switched from negative to positive. 𝐶 =
To understand these phenomena, we propose the following hypothesis: When a negative bias potential is applied to the working electrode to allow the charge accumulation, the multiple cationic groups, covalently connected together in oligomeric IL4TFSI, electrostatically self-organize with a much smaller entropy loss than the case with monomeric ionic liquids. As a result, a well-ordered EDL12 possibly develops due to a multivalent electrostatic interaction and helps further
charge accumulation on the electrode surface (Figure 1b, left).8c In the case of using monomeric ionic liquids, such an electrostatic ordering is accompanied by a large entropic loss (Figure 1b, right) and therefore energetically unfavorable. This can also address why neither monomeric nor oligomeric ionic liquids showed any enhanced capacitance when a positive bias potential was applied. Obviously, this is because their counter anions, which are responsive to a positive bias potential, are monomeric without any multivalent effect. These intriguing results motivated us to investigate how our oligomeric ionic liquids behave as gating electrolytes for FETs (EDL transistors; Figures 3a and 3b). For the preparation of an EDL transistor, crystalline SrTiO3, an oxide semiconductor, was used as a channel material, and a Hall bar structure (Figure 3a) was patterned by a conventional photolithography technique.3a For measuring the transfer characteristic and Hall effect, we prepared two identical EDL transistor devices for each of IL4TFSI, IL2TFSI, BMITFSI, and DEMETFSI. A droplet of an ionic liquid was deposited on each FET device to cover its channel and gate electrode (Figures 3a and 3b). While a gate voltage VG was applied to the gate electrode, the reference electrode voltage VR, which is the actual voltage between the channel surface and electrolyte, was monitored at 300 K with a constant source–drain voltage VD of 100 mV.13 Note that the application of positive VR allows the channel to be negatively charged (Figure 1b). Figure 3c shows the transfer curves of four EDL transistor devices with different ionic liquids. In any case, an ID–VR profile typical of SrTiO3 (n-type)3a,13 was observed, where source–drain current ID was larger, as more positive VR was applied. Considering the operation mechanism of FET devices, this trend is consistent with our observations for the ionic liquids in the electrochemical impedance measurements (Figure 2d). Namely, in the EDL-based FET, positive-charged ionic liquid molecules assist the formation of a well-ordered EDL on the channel surface. Again, we must empathize that the FET devices with oligomeric IL4TFSI and IL2TFSI behaved much better than those with monomeric BMITFSI and DEMETFSI
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(Figure 3c), indicating a great advantage of the oligomeric structure in multivalent electrostatic interactions.
Author Contributions
The capacitor performances that were achieved with four ionic liquids in the FET devices (Figures 3a and 3b) were evaluated by means of the Hall effect measurements,11,14 which directly inform the number of carriers accumulated on the channel upon application of a gate voltage. As shown in Figure 3d, the sheet carrier densities N, evaluated by means of the Hall effect as a function of VR, unambiguously support our claim that the oligomeric ionic liquids enhance the carrier accumulation ability of the EDL transistor device much more than the monomeric ionic liquids. The EDL capacitances (CEDL) were obtained from the slopes of the N–VR plots in Figure 3d, which are given by N/e × VR, according to the conventional relationships of Q = N × e = CEDL × VR (e = elementary charge). The CEDL values observed with IL4TFSI, IL2TFSI, BMITFSI, and DEMETFSI were 64 µF/cm2, 48 µF/cm2, 10 µF/cm2, and 21 µF/cm2, respectively. It is now clear that oligomeric IL4TFSI and IL2TFSI are far better than monomeric BMITFSI and DEMETFSI in generating larger CEDL.
Notes No competing financial interests have been declared.
In conclusion, we demonstrated the outstanding behaviors of oligomeric ionic liquids IL4TFSI and IL2TFSI in EDL devices. Noteworthy is the exceptionally high EDL capacitance observed for tetracationic IL4TFSI, which is several times higher than those reported with organic electrolytes as well as monomeric ionic liquids and even comparable to EDL capacitances achieved with electrolytes usable in aqueous media. We also demonstrated that these oligomeric ionic liquids can be used as gating electrolytes for FETs to ensure efficient carrier accumulation on the surface of a metal oxide semiconductor (SrTiO3) with an excellent gating performance. In both cases, a multivalent electrostatic interaction, ensured by their oligomeric structures, plays a vital role. We must also emphasize the importance of the ether spacer in IL4TFSI and IL2TFSI for making these oligomers fluidic. In fact, we found that the use of alkylene spacers results in solid oligomeric products. Needless to say, further studies are necessary to facilitate the formation of EDLs by, e.g., lowering the viscosities of the media. Yet, considering the high affinities toward carbon-based materials,5,6 application of oligomeric ionic liquids to practical capacitor devices with carbon electrodes is a highly interesting subject worthy of indepth investigation. ASSOCIATED CONTENT
Supporting Information. Complete experimental procedures along with additional supporting data. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Authors
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¶
Both authors contributed equally.
ACKNOWLEDGMENTS
We acknowledge a JSPS Grant-in-Aid for Specially Promoted Research on “Physically Perturbed Assembly for Tailoring HighPerformance Soft Materials with Controlled Macroscopic Structural Anisotropy” (25000005), a JSPS Grant-in-Aid for Specially Promoted Research on “Emergent Iontronics” (25000003), a Grant-in-Aid for Young Scientists (B) (JP26820298), and a Grant-in-Aid for Scientific Research on Innovative Areas (JP16H00923). We thank Dr. Takafumi Hatano of Nagoya University for discussions at the initial stages of this project. REFERENCES (1) (a) Parsons, R. Chem. Rev. 1990, 90, 813. (b) Kornyshev, A. A.; Spohr, E.; Vorotyntsev, M. A. Electrochemical Interfaces: At the Border line; Wiley: New York, 2002. (c) Schmickler, W.; Santos, E. Interfacial Electrochemistry, 2nd ed.; Springer: New York, 2010. (2) (a) Kötz, R.; Carlen, M. Electrochim. Acta 2000, 45, 2483. (b) Aricò, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.; van Schalkwijk, W. Nat. Mater. 2005, 4, 366. (c) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845. (d) Wang, G.; Zhang, L.; Zhang, J. Chem. Soc. Rev. 2012, 41, 797. (3) (a) Ueno, K.; Nakamura, S.; Shimotani, H.; Ohtomo, A.; Kimura, N.; Nojima, T.; Aoki, H.; Iwasa, Y.; Kawasaki, M. Nat. Mater. 2008, 7, 855. (b) Ueno, K.; Shimotani, H.; Yuan, H.; Ye, J.; Kawasaki, M.; Iwasa, Y. J. Phys. Soc. Japan 2014, 83, 32001. (c) Ueno, K.; Nakamura, S.; Shimotani, H.; Yuan, H. T.; Kimura, N.; Nojima, T.; Aoki, H.; Iwasa, Y.; Kawasaki, M. Nat. Nanotechnol. 2011, 6, 408. (d) Weisheit, M.; Fahler, S.; Marty, A.; Souche, Y.; Poinsignon, C.; Givord, D. Science 2007, 315, 349. (e) Zhang, Y. J.; Oka, T.; Suzuki, R.; Ye, J. T.; Iwasa, Y. Science 2014, 344, 725. (f) Bisri, S. Z.; Shimizu, S.; Nakano, M.; Iwasa, Y. Adv. Mater. 2017, 29, 1607054. (4) Pandolfo, A. G.; Hollenkamp, A. F. J. Power Sources 2006, 157, 11. (5) Fukushima, T.; Kosaka, A.; Ishimura, Y.; Yamamoto, T.; Takigawa, T.; Ishii, N.; Aida, T. Science 2003, 300, 2072. (6) Matsumoto, M.; Saito, Y.; Park, C.; Fukushima, T.; Aida, T. Nat. Chem. 2015, 7, 730. (7) (a) Hayes, R.; Warr, G. G.; Atkin, R. Chem. Rev. 2015, 115, 6357. (b) Perkin, S. Phys. Chem. Chem. Phys. 2012, 14, 5052. (c) Mezger, M.; Schröder, H.; Reichert, H.; Schramm, S.; Okasinski, J. S.; Schöder, S.; Honkimäki, V.; Deutsch, M.; Ocko, B. M.; Ralston, J.; Rohwerder, M.; Stratmann, M.; Dosch, H. Science 2008, 322, 424. (8) (a) Burt, R.; Birkett, G.; Zhao, X. S. Phys. Chem. Chem. Phys. 2014, 16, 6519. (b) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Nat. Mater. 2009, 8, 621. (c) Fedorov, M. V; Kornyshev, A. A. Chem. Rev. 2014, 114, 2978. (9) (a) Li, S.; Feng, G.; Cummings, P. T. J. Phys. Condens. Matter 2014, 26, 284106. (b) Li, S.; Van Aken, K. L.; McDonough, J. K.; Feng, G.; Gogotsi, Y.; Cummings, P. T. J. Phys. Chem. C 2014, 118, 3901. (10) Jin, H.; O’Hare, B.; Dong, J.; Arzhantsev, S.; Baker, G. a; Wishart, J. F.; Benesi, A. J.; Maroncelli, M. J. Phys. Chem. B 2008, 112, 81. (11) Yuan, H.; Shimotani, H.; Ye, J.; Yoon, S.; Aliah, H.; Tsukazaki, A.; Kawasaki, M.; Iwasa, Y. J. Am. Chem. Soc. 2010, 132, 18402. (12) Wen, R.; Rahn, B.; Magnussen, O. M. Angew. Chem. Int. Ed. 2015, 54, 6062. (13) Shimizu, S.; Ono, S.; Hatano, T.; Iwasa, Y.; Tokura, Y. Phys. Rev. B 2015, 92, 165304. (14) Shimotani, H.; Asanuma, H.; Tsukazaki, A.; Ohtomo, A.; Kawasaki, M.; Iwasa, Y. Appl. Phys. Lett. 2007, 91, 82106.
T.A.: [email protected] Y.I.: [email protected]
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Exceptionally High Electric Double Layer Capacitances of Oligomeric Ionic Liquids Michio Matsumoto, Sunao Shimizu, Rina Sotoike, Masayoshi Watanabe, Yoshihiro Iwasa, and Takuzo Aida J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09156 • Publication Date (Web): 11 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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Exceptionally High Electric Double Layer Capacitances of Oligomeric Ionic Liquids Michio Matsumoto,†,¶ Sunao Shimizu,‡,¶ Rina Sotoike,§ Masayoshi Watanabe,§ Yoshihiro Iwasa,‡,#,* and Takuzo Aida†,‡,* †
Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 1138656, Japan
‡
RIKEN Center for Emergent Matter Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
§
Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan
#
Department of Applied Physics and Quantum Phase Electronics Center, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Supporting Information Placeholder
Electric double layer (EDL) capacitors are promising as next-generation energy accumulators if their capacitances and operation voltages are both high. However, only few electrolytes can simultaneously fulfil these two requisites. Here we report that an oligomeric ionic liquid such as IL4TFSI with four imidazolium ion units in its structure provides a wide electrochemical window of ~5.0 V, similar to monomeric ionic liquids. Furthermore, electrochemical impedance measurements using Au working electrodes demonstrated that IL4TFSI exhibits an exceptionally high EDL capacitance of ~66 µF/cm2, which is ~6 times as high as those of monomeric ionic liquids so far reported. We also found that an EDL-based field effect transistor (FET) using IL4TFSI as a gate dielectric material and SrTiO3 as a channel material displays a very sharp transfer curve with an enhanced carrier accumulation capability of ~64 µF/cm2, as determined by Hall-effect measurements. ABSTRACT:
An electric double layer (EDL),1 the thinnest capacitor, has attracted significant attention due to its potentially high capacitance (EDL capacitance), and has been considered a promising component for the next-generation energy storage devices.2 EDLs have also been used as gate dielectrics for field effect transistors (FETs),3 from which numerous electric fieldinduced phenomena such as superconductivity,3a–c ferromagnetism,3d and some other emerging iontronic devices3e,f were demonstrated. However, for expanding the scope of EDL-based devices, the charge accumulation ability of EDLs has to be further enhanced. The number of charges that can be accumulated in a capacitor (Q) is proportional to the capacitance (C) of a device and the applied voltage (V),
Figure 1. (a) Chemical structures of oligomeric ionic liquids IL4TFSI and IL2TFSI, and monomeric ionic liquids BMITFSI and DEMETFSI as references. (b) Schematic representations of the proposed structures of electric double layer (EDL) formed by oligomeric (left) and monomeric (right) ionic liquids, showing how the oligomeric ionic liquids exhibits the enhanced capacitance.
according to: Q = CV. Thus, a higher charge accumulation ability requires both a higher capacitance and a higher upperlimit operation voltage. For developing high-performance capacitors, a wide variety of electrodes such as nanostructured
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Figure 2. (a) Bode plots (f–q, f–|Z|) and (b) Nyquist plots (Z’–Z”) for IL4TFSI obtained by electrochemical impedance spectroscopy (EIS) at the bias potential of –0.5 V versus the open circuit voltage (OCV) with a voltage amplitude of 10 mV at 30 °C after different annealing times from 0 to 150 min. The symbols f, Z, q, Z’, and Z” denote frequency of the input signal, impedance, argument of Z, real part of Z, and imaginary part of Z, respectively. (c) Capacitances (C) of IL4TFSI (red), IL2TFSI (orange), BMITFSI (blue), and DEMETFSI (black) at different frequencies (f) at a fixed bias potential of –0.5 V versus OCV at 30 °C. (d) Capacitances (C) of IL4TFSI (red), IL2TFSI (orange), BMITFSI (blue), and DEMETFSI (black) at different bias potentials at a fixed frequency (f) of 1 Hz at 30 °C. The data acquisitions were done after 150-min annealing whenever the bias potential to the working electrode was changed.
electrodes with large surface areas2c,d have been investigated. However, much less attention has been focused on the development of better electrolytes. Electrolytes for EDL capacitors so far reported are classified into two categories depending on whether they can be used in aqueous or organic media.2c In general, the electrolytes that can be used in aqueous media can achieve very high EDL capacitances such as ~70 µF/cm2.2,4 However, the narrow electrochemical window of water (~1.2 V) lowers the upper-limit operation voltages for the corresponding EDL capacitors. In contrast, the electrolytes that can be used in organic media offer wider electrochemical windows (~2.7 V), so that the upper-limit operation voltages are higher. Nevertheless, the EDL capacitances attainable with such electrolytes are still low (~10 µF/cm2).2c,d Here we report oligomeric ionic liquids as a new class of electrolytes for EDL capacitors, which bear multiple ionic moieties in their structures, yet exhibit a liquid fluidity at room temperature. In 2004, we reported that imidazolium ionbased ionic liquids are excellent dispersants for pristine singlewalled carbon nanotubes and suggested the presence of a strong π–cation interaction between them.5 Recently, we also found that, under the application of microwaves, PF6 salts of imidazolium ion tetramer (IL4) and dimer (IL2) (Figure 1a) efficiently exfoliate graphite into single-layer graphene, in which a multivalently enhanced ‘ionic liquid/sp2 carbon network’ interaction is supposed to play a critical role.6 Being inspired by this multivalent effect, we envisioned that such oligomeric ionic liquids as electrolytes may facilitate the charge accumulation in EDL capacitors (Figure 1b). Although ionic liquids7 are known to provide wide electrochemical windows (~5.0 V),8 reported capacitances for ionic liquids are again nothing special. As for oligomeric ionic liquids, Feng et al. suggested the potential utility of an ionic liquid dimer for EDL capacitors by molecular dynamics
simulation with a preliminarily cyclic voltammetry (CV) analysis.9 However, no further studies have been done. We investigated the dielectric behaviors of tetrameric IL4TFSI and dimeric IL2TFSI together with BMITFSI and DEMETFSI as monomeric conventional references (Figures 1a, S3 and S4; TFSI = bis(trifluoromethanesulfonyl)imide),10 by electrochemical impedance spectroscopy (EIS) using Au (working), Ag/Ag+ in DEMETFSI (reference), and Pt (counter) electrodes. At first, we confirmed by means of linear sweep voltammetry (LSV) that the electrochemical windows of oligomeric IL4TFSI and IL2TFSI as well as monomeric BMITFSI and DEMETFSI are as wide as ~5.0 V (Figure S5). Next, we carried out EIS of IL4TFSI at 61 different frequencies (f) in a range of 0.1 Hz–100 kHz with a voltage amplitude of 10 mV. Figures 2a and 2b show Bode and Nyquist plots, respectively, for IL4TFSI at a bias potential of – 0.5 V versus open circuit voltage (OCV), where the data acquisitions were done every 30 min over a period of 150 min. As typically observed for the electrochemical impedance spectra of ionic electrolytes,11 the upper panel in Figure 2a displays a bimodal feature, indicating the presence of two different capacitative behaviors, i.e., EDL and bulk capacitors in lower and higher frequency regions, respectively. One may also notice that, in the lower frequency region, the plots change with the annealing time (see arrows), suggesting that EDL forms slowly. This was also the case with IL2TFSI but not so explicit with monomeric ionic liquids such as BMITFSI and DEMETFSI (Figure S6). Figure 2c shows frequency (f)-dependent capacitances observed with oligomeric IL4TFSI and IL2TFSI, and monomeric BMITFSI and DEMETFSI at a bias potential of –0.5 V (annealing time; 150 min). Here, the capacitance (C) is given by the following equation,11 where Z” represents the imaginary part of impedance:
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Figure 3. (a) Optical micrograph and (b) schematic representation of a FET device using crystalline SrTiO3 as a semiconducting channel material and an ionic liquid as a gate dielectric material. (c) Transfer curves and (d) sheet carrier densities (N) of the SrTiO3based FETs using IL4TFSI (red), IL2TFSI (orange), BMITFSI (blue), and DEMETFSI (black) as electrolytes at 27 °C under different reference electrode voltages (VR). ID denotes a source-drain current. The EDL capacitances (CEDL) were obtained from the slopes of the N–VR plots.
1 2𝜋𝑓𝑍" The observed capacitances were lower as the applied frequency f was higher (Figure 2c) due to the transition from electrode polarization (EDL capacitance) to dipole polarization (bulk capacitance).11 For monomeric BMITFSI and DEMETFSI, such a polarization transition did not occur explicitly until the f-value exceeded 104 Hz. In contrast, the oligomeric ionic liquids showed a polarization transition when the f -value exceeded ~102 and ~103 Hz for IL4TFSI and IL2TFSI, respectively. This is because IL4TFSI and IL2TFSI are more viscous than BMITFSI and DEMETFSI (Figure S3). Here, an even more important observation in Figure 2c is that oligomeric IL4TFSI and IL2TFSI in their low-frequency regions showed exceptionally high EDL capacitances that are much higher than other ionic liquids reported so far, including BMITFSI and DEMETFSI.8c Figure 2d shows how the EDL capacitances of IL4TFSI and IL2TFSI at f = 1 Hz change with the bias potential applied to the working electrode. In order to exclude a possible effect of their slow reorganization, the devices were annealed for 150 min whenever the bias potential was changed. Although the capacitances with BMITFSI and DEMETFSI did not show any significant dependency on the bias potential, those with oligomeric IL4TFSI and IL2TFSI dramatically increased by 8 and 3 times, respectively, when the bias potential versus OCV was changed from 0 V to –0.75 V. Note that such an abrupt increase did not appear when the bias potential was switched from negative to positive. 𝐶 =
To understand these phenomena, we propose the following hypothesis: When a negative bias potential is applied to the working electrode to allow the charge accumulation, the multiple cationic groups, covalently connected together in oligomeric IL4TFSI, electrostatically self-organize with a much smaller entropy loss than the case with monomeric ionic liquids. As a result, a well-ordered EDL12 possibly develops due to a multivalent electrostatic interaction and helps further
charge accumulation on the electrode surface (Figure 1b, left).8c In the case of using monomeric ionic liquids, such an electrostatic ordering is accompanied by a large entropic loss (Figure 1b, right) and therefore energetically unfavorable. This can also address why neither monomeric nor oligomeric ionic liquids showed any enhanced capacitance when a positive bias potential was applied. Obviously, this is because their counter anions, which are responsive to a positive bias potential, are monomeric without any multivalent effect. These intriguing results motivated us to investigate how our oligomeric ionic liquids behave as gating electrolytes for FETs (EDL transistors; Figures 3a and 3b). For the preparation of an EDL transistor, crystalline SrTiO3, an oxide semiconductor, was used as a channel material, and a Hall bar structure (Figure 3a) was patterned by a conventional photolithography technique.3a For measuring the transfer characteristic and Hall effect, we prepared two identical EDL transistor devices for each of IL4TFSI, IL2TFSI, BMITFSI, and DEMETFSI. A droplet of an ionic liquid was deposited on each FET device to cover its channel and gate electrode (Figures 3a and 3b). While a gate voltage VG was applied to the gate electrode, the reference electrode voltage VR, which is the actual voltage between the channel surface and electrolyte, was monitored at 300 K with a constant source–drain voltage VD of 100 mV.13 Note that the application of positive VR allows the channel to be negatively charged (Figure 1b). Figure 3c shows the transfer curves of four EDL transistor devices with different ionic liquids. In any case, an ID–VR profile typical of SrTiO3 (n-type)3a,13 was observed, where source–drain current ID was larger, as more positive VR was applied. Considering the operation mechanism of FET devices, this trend is consistent with our observations for the ionic liquids in the electrochemical impedance measurements (Figure 2d). Namely, in the EDL-based FET, positive-charged ionic liquid molecules assist the formation of a well-ordered EDL on the channel surface. Again, we must empathize that the FET devices with oligomeric IL4TFSI and IL2TFSI behaved much better than those with monomeric BMITFSI and DEMETFSI
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(Figure 3c), indicating a great advantage of the oligomeric structure in multivalent electrostatic interactions.
Author Contributions
The capacitor performances that were achieved with four ionic liquids in the FET devices (Figures 3a and 3b) were evaluated by means of the Hall effect measurements,11,14 which directly inform the number of carriers accumulated on the channel upon application of a gate voltage. As shown in Figure 3d, the sheet carrier densities N, evaluated by means of the Hall effect as a function of VR, unambiguously support our claim that the oligomeric ionic liquids enhance the carrier accumulation ability of the EDL transistor device much more than the monomeric ionic liquids. The EDL capacitances (CEDL) were obtained from the slopes of the N–VR plots in Figure 3d, which are given by N/e × VR, according to the conventional relationships of Q = N × e = CEDL × VR (e = elementary charge). The CEDL values observed with IL4TFSI, IL2TFSI, BMITFSI, and DEMETFSI were 64 µF/cm2, 48 µF/cm2, 10 µF/cm2, and 21 µF/cm2, respectively. It is now clear that oligomeric IL4TFSI and IL2TFSI are far better than monomeric BMITFSI and DEMETFSI in generating larger CEDL.
Notes No competing financial interests have been declared.
In conclusion, we demonstrated the outstanding behaviors of oligomeric ionic liquids IL4TFSI and IL2TFSI in EDL devices. Noteworthy is the exceptionally high EDL capacitance observed for tetracationic IL4TFSI, which is several times higher than those reported with organic electrolytes as well as monomeric ionic liquids and even comparable to EDL capacitances achieved with electrolytes usable in aqueous media. We also demonstrated that these oligomeric ionic liquids can be used as gating electrolytes for FETs to ensure efficient carrier accumulation on the surface of a metal oxide semiconductor (SrTiO3) with an excellent gating performance. In both cases, a multivalent electrostatic interaction, ensured by their oligomeric structures, plays a vital role. We must also emphasize the importance of the ether spacer in IL4TFSI and IL2TFSI for making these oligomers fluidic. In fact, we found that the use of alkylene spacers results in solid oligomeric products. Needless to say, further studies are necessary to facilitate the formation of EDLs by, e.g., lowering the viscosities of the media. Yet, considering the high affinities toward carbon-based materials,5,6 application of oligomeric ionic liquids to practical capacitor devices with carbon electrodes is a highly interesting subject worthy of indepth investigation. ASSOCIATED CONTENT
Supporting Information. Complete experimental procedures along with additional supporting data. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Authors
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Both authors contributed equally.
ACKNOWLEDGMENTS
We acknowledge a JSPS Grant-in-Aid for Specially Promoted Research on “Physically Perturbed Assembly for Tailoring HighPerformance Soft Materials with Controlled Macroscopic Structural Anisotropy” (25000005), a JSPS Grant-in-Aid for Specially Promoted Research on “Emergent Iontronics” (25000003), a Grant-in-Aid for Young Scientists (B) (JP26820298), and a Grant-in-Aid for Scientific Research on Innovative Areas (JP16H00923). We thank Dr. Takafumi Hatano of Nagoya University for discussions at the initial stages of this project. REFERENCES (1) (a) Parsons, R. Chem. Rev. 1990, 90, 813. (b) Kornyshev, A. A.; Spohr, E.; Vorotyntsev, M. A. Electrochemical Interfaces: At the Border line; Wiley: New York, 2002. (c) Schmickler, W.; Santos, E. Interfacial Electrochemistry, 2nd ed.; Springer: New York, 2010. (2) (a) Kötz, R.; Carlen, M. Electrochim. Acta 2000, 45, 2483. (b) Aricò, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.; van Schalkwijk, W. Nat. Mater. 2005, 4, 366. (c) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845. (d) Wang, G.; Zhang, L.; Zhang, J. Chem. Soc. Rev. 2012, 41, 797. (3) (a) Ueno, K.; Nakamura, S.; Shimotani, H.; Ohtomo, A.; Kimura, N.; Nojima, T.; Aoki, H.; Iwasa, Y.; Kawasaki, M. Nat. Mater. 2008, 7, 855. (b) Ueno, K.; Shimotani, H.; Yuan, H.; Ye, J.; Kawasaki, M.; Iwasa, Y. J. Phys. Soc. Japan 2014, 83, 32001. (c) Ueno, K.; Nakamura, S.; Shimotani, H.; Yuan, H. T.; Kimura, N.; Nojima, T.; Aoki, H.; Iwasa, Y.; Kawasaki, M. Nat. Nanotechnol. 2011, 6, 408. (d) Weisheit, M.; Fahler, S.; Marty, A.; Souche, Y.; Poinsignon, C.; Givord, D. Science 2007, 315, 349. (e) Zhang, Y. J.; Oka, T.; Suzuki, R.; Ye, J. T.; Iwasa, Y. Science 2014, 344, 725. (f) Bisri, S. Z.; Shimizu, S.; Nakano, M.; Iwasa, Y. Adv. Mater. 2017, 29, 1607054. (4) Pandolfo, A. G.; Hollenkamp, A. F. J. Power Sources 2006, 157, 11. (5) Fukushima, T.; Kosaka, A.; Ishimura, Y.; Yamamoto, T.; Takigawa, T.; Ishii, N.; Aida, T. Science 2003, 300, 2072. (6) Matsumoto, M.; Saito, Y.; Park, C.; Fukushima, T.; Aida, T. Nat. Chem. 2015, 7, 730. (7) (a) Hayes, R.; Warr, G. G.; Atkin, R. Chem. Rev. 2015, 115, 6357. (b) Perkin, S. Phys. Chem. Chem. Phys. 2012, 14, 5052. (c) Mezger, M.; Schröder, H.; Reichert, H.; Schramm, S.; Okasinski, J. S.; Schöder, S.; Honkimäki, V.; Deutsch, M.; Ocko, B. M.; Ralston, J.; Rohwerder, M.; Stratmann, M.; Dosch, H. Science 2008, 322, 424. (8) (a) Burt, R.; Birkett, G.; Zhao, X. S. Phys. Chem. Chem. Phys. 2014, 16, 6519. (b) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Nat. Mater. 2009, 8, 621. (c) Fedorov, M. V; Kornyshev, A. A. Chem. Rev. 2014, 114, 2978. (9) (a) Li, S.; Feng, G.; Cummings, P. T. J. Phys. Condens. Matter 2014, 26, 284106. (b) Li, S.; Van Aken, K. L.; McDonough, J. K.; Feng, G.; Gogotsi, Y.; Cummings, P. T. J. Phys. Chem. C 2014, 118, 3901. (10) Jin, H.; O’Hare, B.; Dong, J.; Arzhantsev, S.; Baker, G. a; Wishart, J. F.; Benesi, A. J.; Maroncelli, M. J. Phys. Chem. B 2008, 112, 81. (11) Yuan, H.; Shimotani, H.; Ye, J.; Yoon, S.; Aliah, H.; Tsukazaki, A.; Kawasaki, M.; Iwasa, Y. J. Am. Chem. Soc. 2010, 132, 18402. (12) Wen, R.; Rahn, B.; Magnussen, O. M. Angew. Chem. Int. Ed. 2015, 54, 6062. (13) Shimizu, S.; Ono, S.; Hatano, T.; Iwasa, Y.; Tokura, Y. Phys. Rev. B 2015, 92, 165304. (14) Shimotani, H.; Asanuma, H.; Tsukazaki, A.; Ohtomo, A.; Kawasaki, M.; Iwasa, Y. Appl. Phys. Lett. 2007, 91, 82106.
T.A.: [email protected] Y.I.: [email protected]
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