Research

New ion-conductive mesophase materials (plastic and liquid crystals) and their structures

Mesophase observed between liquid and solid phases have been a matter of interest because of their interesting properties. Plastic and liquid crystals are typical mesophases and have many unknown properties. Plastic crystals consist of isotropically rotating molecules which behaves like spherical molecules and form expanded lattices with simple configurations. Liquid crystals have anisotropic structures based on intermolecular interactions. We are studying synthesis and characterization (structures, conductivity, and electrochemical behavior) of new ionic plastic and liquid crystals, targeting applications as ion-conductive materials. This study leads to safe and efficient batteries and capacitors in future.

Fig.1

References:
[1] R. Taniki, K. Matsumoto, R. Hagiwara, K. Hachiya, T. Morinaga, T. Sato, J. Phys. Chem. B, 117 (2013) 955-960.
[2] F. Xu, K. Matsumoto, R. Hagiwara, J. Phys. Chem. B, 116 (2012) 10106-10112.
[3] F. Xu, K. Matsumoto, R. Hagiwara, Dalton Trans., 41 (2012) 3494–3502.
[4] F. Xu, K. Matsumoto, R. Hagiwara, Chem. Eur. J., 16 (2010) 12970-12976.


Synthesis and characterization of fluorohydrogenate ionic liquids

One of our research topics is synthesis of new ionic liquids based on fluorohydrogenate anions (Fig. 1). Fluorohydrogenate ionic liquids exhibit high conductivities and low viscosities compared to usual ionic liquids [1,2]. Properties of ionic liquids are tunable by changing the counter ion and fluorohydrogenate anions form low melting salts with various cations, and the resulting ionic liquids have a wide range of applications.
Our approach to elucidate the mechanism of such high conductivities for fluorohydrogenate ionic liquids includes X-ray diffraction, NMR spectroscopy, and vibrational spectroscopy. Single crystal X-ray diffraction provides the structure of a crystal phase. For example, 1-ethyl-3-methylimdiazolium bifluoride (EMImFHF, melting point: 51oC) is a HF-deficient form of a room temperature ionic liquid, 1-ethyl-3-methylimdiazolium fluorohydrogenate (EMIm(FH)2.3F) and has a unique layered structure in the solid state [3]. The relationship between such a structure observed in the solid state and the structure in the liquid state is an interesting topic. Pulsed gradient spin echo NMR, which gives diffusion coefficients of chemical species in the liquid state, revealed that both the cation and anion in fluorohydrogenate ionic liquids migrate faster than the ions in usual ionic liquids [4]. Applications to electrochemical devices such as supercapacitors and fuel cells are described below.

Fig.1Fig.2

References:
[1] R. Hagiwara, K. Matsumoto, Y. Nakamori, T. Tsuda, Y. Ito, H. Matsumoto, K. Momota, J. Electrochem. Soc., 150 (2003) D195.
[2] K. Matsumoto, R. Hagiwara, Y. Ito, Electrochem. Solid State Lett., 7 (2004) E41.
[3] K. Matsumoto, T. Tsuda, R. Hagiwara, Y. Ito, O. Tamada, Solid State Sci., 4 (2002) 23.
[4] Y. Saito, K. Hirai, K. Matsumoto, R. Hagiwara, Y. Minamizaki, J. Phys. Chem. B, 109 (2005) 2942.


Unhumidified fuel cells operating at intermediate temperature using fluorohydrogenate ionic liquids

Operation of the polymer electrolyte fuel cell (PEFC) at intermediate temperatures (100-200ºC) has been recognized to provide many advantages. However, it is difficult to use the typical polymer electrolyte such as Nafion® at intermediate temperatures due to the loss of conductivity by drying-up as well as the deterioration of the membrane.
 From this background, we have proposed a fluorohydrogenate fuel cell (FHFC) operating at low to intermediate temperatures without humidification [1,2]. For the FHFC, fluorohydrogenate ionic liquids (FHILs) such as EMIm(FH)nF are employed as electrolytes. As shown in Fig.1, the FHFC operates not by proton conduction but by the conduction of fluorohydrogenate ion, (FH)nF–. Anode and cathode reactions, and overall reaction are expressed as:

Anode reaction: H2 + 6FHF- --> 4[(FH)2F-] + 2e (1)
Cathode reaction: 1/2O2 + 4[(FH)2F-] + 2e --> 6FHF- + H2O (2)
Overall reaction: H2 + 1/2O2 --> H2O (3)

In principle, the FHFC needs no humidification since the fluorohydrogenate ion conduction does not require the presence of water. In fact, we have already confirmed that the FHFC using a liquid EMIm(FH)1.3F electrolyte is operable at 100ºC without humidification [1]. We have also succeeded in preparing composite membranes formed of EMIm(FH)nF and polymers and achieved the power density of 20 mW cm-2 in the single cell test at 120ºC under no humidification [2].
Recently, a new FHIL, N-ethyl-N-methylpyrrolidinium fluorohydrogenate (EMPyr(FH)nF) has been investigated to improve the high temperature performance of FHFC. We fabricated composite membranes consisting of 2-hydroxyethyl methacrylate (HEMA) and EMPyr(FH)1.7F and tested the membranes using a single cell. Fig. 2 shows typical I-V and I-P curves for the FHFC. So far, the maximum power density of 200 mW cm-2 has been achieved.
Currently, we are tackling the clarification of the mechanisms for oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) on Pt electrode in FHILs. We are also working on the improvement of single cell performance by developing new composite membranes.

Fig.1図2

Reference:
[1] R. Hagiwara, T. Nohira, K. Matsumoto, Y. Tamba, Electrochem. Solid State Lett., 8, A231 (2005).
[2] J. S. Lee, T. Nohira, R. Hagiwara, J. Power Sources, 171, 535 (2007).


Analysis of characteristics for the EDLCs using fluorohydrogenate ionic liquids

Electric double layer capacitors (EDLCs) are energy devices which can charge or discharge as conventional batteries. Electric double layer capacitors have characteristics of high power density, long cycle life and short charging time, and have received much attention as application for assist batteries of hybrid electric vehicle. However, low energy density of EDLCs compared to that of secondary batteries is the drawback.
Figure 1 shows the principle of the EDLCs, which are constructed by two electrodes, a electrolyte, and a separator. In a charging process, cations and anions in the electrolyte are attracted to the surface of each electrode by applying voltage between two electrodes. In a discharging process, both cations and anions are desorbed from the surface of electrodes. This charge-discharge reaction of EDLCs does not involve a faradic reaction, but physical adsorption of cations and anions. This leads to the characteristic of EDLCs for efficient mechanism of charge-discharge system. In EDLCs, activated carbon is often used as the electrode material due to their high surface area.
Our laboratory examines fluorohydrogenate ionic liquids as electrolytes for the EDLCs to increase energy density of them. Figure 2 shows a voltage dependence of capacitance for the EDLCs using 6 different electrolytes. As figure 2 show, EDLCs using FHILs exhibit higher capacitance than other electrolytes. This suggests that a cationic structure of FHILs is suitable for the activated carbons. On the other hand, the change of capacitance for series of cations derived from not only the size of cations, but also charge distribution or changes of structures for adsorbed species by clustering of ions.
  Fluorohydrogenate ionic liquids are operable at wide temperature, we examine low temperature performances of EDLCs using FHILs.

Fig.1Fig.2

 

Syntheses and properties of novel ionic liquids based on fluoroanions

     Ionic liquids are liquids composed of anions and cations, which often exhibit nonflammability and low vapor pressure. These unique properties make them attractive alternative materials in various fields of chemistry and electrochemistry.
Fluorohydrogenanate ionic liquids above act as good precursors to synthesize new and pure ionic liquids (Scheme1) based on transition metals including fluoro-niobate, tantalate, and uranate. Systematic studies through ionic liquids with different fluorocomplex anions reveal the relationship between the size of the anion and their physical properties.
Our recent attempt to prepare ionic liquids based on PO2F2? that has a size between BF4? and PF6? resulted in the new ionic liquids with high conductivity and polarity.

反応スキーム1図1

References:
[1] K. Matsumoto, R. Hagiwara, R. Yoshida, Y. Ito, Z. Mazej, P. Benkic, B. Zemva, O. Tamada, H. Yoshino, S. Matsubara,Dalton Trans. 2004, 144-149.
[2] T. Kanatani, K. Matsumoto, R. Hagiwara, Eur. J. Inorg. Chem. 2010, 1049-1055.
[3] T. Kanatani, R. Ueno, K. Matsumoto, T. Nohira, R. Hagiwara, J. Fluorine Chem., 130, (2009) 979-984.
[4] K. Matsumoto, R. Hagiwara, Inorg. Chem. 48 (2009) 7350-7358.

 

Structural analysis of fluoro-compounds

 Many compounds have a crystalline phase where atoms are three-dimensionally ordered. One of our research interests is to determine the crystal structure and molecular geometries using X-ray diffraction and vibrational spectroscopy. In particular, we mainly deal with fluorine-containing compounds because fluorine has the smallest size in halogens with the largest electronegativity and often forms unique crystal structures. Our targets range from touchy and unstable fluorides to frozen ionic liquids. The following examples are coordination environment around Li+ in Li[BF4] (right) and Mo2O4F62- in [BPy][MoO2F3] (BPy=1-butylpyridinium). Other examples of crystal structures from our group are summarized in Structure Archives.

反応スキーム1 図1

References:
[1] K. Matsumoto, R. Hagiwara, Z. Mazej, E. Goreshnik, B. Zemva, J. Phys. Chem. B 110 (2006) 2138.
[2] T. Kanatani, K. Matsumoto, R. Hagiwara, Eur. J. Inorg. Chem. 2010, 1049.

Physicochemical properties of novel molten salts and their applications as electrolytes

Electrolytes are the essential materials for electrochemical devices such as batteries. Electrolytes possessing superior properties can provide high safety, durability and energy density for batteries. In recent years, molten salts, especially, room temperature molten salts, attract much attention as novel electrolytes. Generally, molten salts have excellent features such as non-flammability, negligible volatility and wide electrochemical window. However, room temperature molten salts have a weak point that organic cations are not stable against the alkali metal deposition. This is one of the reasons that have hindered the practical application of room temperature molten slats to lithium secondary batteries.
In these circumstances, we focused on molten salts consisting of five kinds of alkali metal cations (Li+, Na+, K+, Rb+, Cs+) and three kinds of perfluorosulfonylamide anions (FSA-, TFSA-, BETA-). They are abbreviated MFSA, MTFSA, MBETA (M = Li, Na, K, Rb, Cs), respectively. For these single salts and their mixtures, we investigated physicochemical properties such as melting temperatures, decomposition temperatures, viscosities, ionic conductivities and electrochemical windows. On the basis of these data, we evaluated the new molten salts as electrolytes and discussed application to electrochemical devices.
As an example, a ternary phase diagram of the (Li,K,Cs)TFSA system is shown in Fig. 1. The melting temperatures of the mixtures are much lower than those of constituent single salts. Considering the ionic conductivities and cathode limit reactions, it was found that several MTFSA mixtures and MFSA mixtures are promising electrolytes for lithium secondary battery and sodium secondary battery.

Fig.1Fig.2


Application of molten salts (ionic liquids) as electrolytes of lithium rechargeable batteries

Rechargeable lithium batteries have been widely used as high performance power sources for portable electronic devices owing to their high energy densities and cycle abilities. In recent years, the rechargeable lithium batteries have also been highly expected to be used for electric vehicles (EVs), hybrid electric vehicles (HEVs) and storage of surplus electricity. However, the current rechargeable lithium batteries have safety problems as symbolized by the recent ignition accidents of laptop computers and mobile phones. One of the key technologies to improve the safety of the batteries is the development of nonvolatile and nonflammable electrolytes.
In our laboratory, molten salts (ionic liquids) are focused as the electrolyte of lithium rechargeable batteries. Generally, molten salts have negligible volatility, nonflammability, high thermal and electrochemical stabilities. In this study, (Li,K,Cs)TFSA, whose melting temperature is 419 K, is used as the electrolyte [1,2]. The structure of MTFSA is showed in Fig. 1.
Practically, positive electrodes, negative electrodes, electrolytes, along with the test cells were prepared and charge-discharge cycling tests were performed at 443 K. After the test, XRD and SEM measurement were conducted to evaluate the behavior of cathode active material. TG and DSC measurement were also conducted to evaluate the thermal stability of electrolytes. Fig. 2 shows charge-discharge curves of this battery. It shows high cycle ability even at high C-rate, which discharge end in 1 hour. It has been clarified that there are a lot of advantages such as high power density owing to the high operation temperature [3]. In the near future, this battery is expected to be used as the batteries for EVs and storage of surplus electricity such as solar energy and wind power energy.

Fig.1Fig.2

References:
[1] K. Kubota, T. Nohira, T. Goto, R. Hagiwara, Journal of Chemical & Engineering Data, 53, 2144 (2008).
[2] A. Watarai, K. Kubota, M. Yamagata, T. Goto, T. Nohira, R. Hagiwara, K. Ui, N. Kumagai, Journal of Power Sources, 183, 724 (2008).
[3] T. Fujimori, T. Goto, T. Nohira, R. Hagiwara, K. Ui, N. Kumagai, Abstract of the 50th battery symposium in Japan, p.168 (2009).

An application of molten NaTFSA-CsTFSA to sodium secondary batteries

We are studying sodium secondary batteries as well as lithium secondary batteries. Maybe, very few people have heard the word “sodium secondary battery”. Now, what is the reason to study such a battery?
Element abundances in earth’s crust and seawater, and standard redox potentials of lithium and sodium are shown in Table 1 [1]. Lithium has very high energy density because of the lowest standard redox potential among all metal and low atomic weight. However, the reserve of lithium is not very abundant and the cost of lithium is relatively high. Currently, much effort has been given on the scale-up of lithium secondary batteries aiming at electric vehicles and storage of electricity from solar energy and wind energy. When the demand of lithium is largely increased, the stable supply of lithium batteries will face a difficulty. On the other hand, sodium is one of the most abundant elements which is naturally found in the form of NaCl. Since the standard redox potential of sodium is reasonably low, sodium secondary batteries are expected as large-scale batteries with high performance and low price.
Na/S (NAS) battery is a famous sodium secondary battery which has been commercially used for load leveling [2]. One of the problems for this battery is the high operation temperature (about 300oC) necessary to attain a sufficient ionic conductivity of ceramic electrolyte. Although sodium secondary batteries operable at room temperature have been studied, their performances are not high enough.
In these circumstances, we have developed binary molten NaTFSA-CsTFSA as a new electrolyte for sodium secondary batteries working at intermediate temperature [3]. The phase diagram of this system is shown in Fig. 1. The eutectic point of this melt is about 130oC, suggesting that various kinds of materials can be used for sodium secondary battery with simple heating equipment.
Currently, we are investigating the charge-discharge property of sodium secondary batteries using binary molten NaTFSA-CsTFSA.

Table1Fig.1

References:
[1] The Chemical Society of Japan, “The Chemical Handbook Fundamental Edition”, (2004). (in Japanese)
[2] J. L. Sudworth, J. Power Sources, 11 (1984) 143.
[3] R. Hagiwara, K. Tamaki, K. Kubota, T. Goto, T. Nohira, J. Chem. Eng. Data, 53(2) (2008) 355.

Development of new production methods of solar-grade silicon: Electrolytic reduction of SiO2 in molten salt

We are studying new production methods of solar-grade silicon. As the raw material, we focus on high purity SiO2, which can be produced at relatively low cost. High purity silicon is expected to be produced by electrolytic reduction of the high purity SiO2 in molten salt such as CaCl2 at 850 oC.
Very recently, as a result of the sharp increase in production of silicon solar cells, a shortage of solar-grade silicon became a serious problem. Solar-grade silicon is currently produced by high-cost methods similar to the production of semiconductor grade silicon. Therefore, new and low-cost methods are required for the production of solar-grade silicon. Although it is very difficult to remove some impurities like boron and phosphorus from silicon, they can be eliminated easily in the form of SiO2. Thus, if the purified SiO2 is reduced to silicon without contamination, especially boron and phosphorus, solar-grade silicon is expected to be produced with low-cost. We have found that solid SiO2 can be electrochemically reduced to silicon in molten CaCl2. Now, we are trying to further decrease the impurities in the produced silicon, to increase the reduction rate and to scale-up the apparatus.

Fig.1

Reference:
[1] T. Nohira, K. Yasuda and Y. Ito, Nature materials, 2 (2003) 397. 
[2] K. Yasuda, T. Nohira, K. Amezawa, Y. H. Ogata and Y. Ito, J. Electrochem. Soc., 152 (2005) D69.
[3] K. Yasuda, T. Nohira, K. Takahashi, R. Hagiwara and Y. H. Ogata, J. Electrochem. Soc., 152 (2005) D232.
[4] K. Yasuda, T. Nohira, R. Hagiwara and Y. H. Ogata, J. Electrochem. Soc., 154 (2007) E95.
[5] K. Yasuda, T. Nohira, R. Hagiwara and Y. H. Ogata, Electrochimica Acta, 53 (2007) 106.