Published papers

The dependence of the impedances of lithium phosphorous oxynitride (LiPON) thin film and solid electrolyte interphase (SEI) formed by decomposition of some electrolytes on the electrode potential was investigated by electrochemical impedance spectroscopy. A LiPON thin film was prepared on a Ni electrode by radio frequency magnetron sputtering of Li3PO4 under nitrogen atmosphere. The resistance of the LiPON thin film decreased with lowering the electrode potential in an ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide (BMPTFSA) containing 1M LiTFSA. The similar potential dependence of the impedance of the SEI formed in 1M LiTFSA/BMPTFSA was observed, suggesting that the Li+ carrier density in the LiPON thin film and SEI increased with lowering the electrode potential probably due to the doping of Li+ from the electrolyte into the thin Li+ conductors in order to compensate the negative charge on the electrode. On the other hand, the potential dependence of the SEI formed in LiTFSA-tetraglyme (G4) solvate ionic liquid was insignificant because of the high concentration of Li+ in the SEI and electrolyte. The resistance of the SEI formed in 1M LiClO4/EC (ethylene carbonate) + DMC (dimethyl carbonate) (1 : 1 vol%) did not depend on the electrode potential, suggesting the thin and highly Li+ conductive SEI is formed in the organic electrolyte.

The solid electrolyte interphase (SEI) between Li and ionic liquid electrolytes was investigated in 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)amide (MPPFSA) and 1-methyl-1-methoxyethylpyrrolidinium bis(fluorosulfonyl)amide (MOEMPFSA) with different LiFSA concentrations. The physicochemical properties and the coordination states of Li+ with FSA- with increasing the LiFSA concentration were similar between xLiFSA-MPPFSA and MOEMPFSA. On the other hand, the SEI resistance obtained by electrochemical impedance spectroscopy using a Li|Li symmetrical cell increased with the lapse of time in xLiFSA-MPPFSA more significantly than in xLiFSA-MOEMPFSA, suggesting that the SEI formed in xLiFSA-MOEMPFSA was more conductive than that in xLiFSA-MPPFSA. The SEI formed on Li was considered to be composed mainly of the ions in the electrolytes and their decomposed products by X-ray photoelectron spectroscopy. The cyclability of deposition and dissolution of Li was improved with increasing the LiFSA concentration in both xLiFSA-MPPFSA and MOEMPFSA. An increase in the conductivity of the SEI with increasing the LiFSA concentration may lead to the promotion of homogeneous growth of Li rather than dendritic growth of Li.

Electrochemical recovery of metallic Co was attempted from CoO in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing 0.5 M BMPCl. CoO was dissolved in 0.5 M BMPCl/BMPTFSA with a carbochlorination reagent, C2Cl6, to form [CoCl4]2- at 150 °C. After the removal of electroactive C2Cl6 and byproducts, the electrochemical reduction of [CoCl4]2- was found to be possible on a glassy carbon electrode in the ionic liquid. Co nanowires were deposited by electrochemical reduction of [CoCl4]2- in 0.5 M BMPCl/BMPTFSA under an external magnetic field, which promoted the cathodic reduction and affected the morphology of the deposits. The deposited Co nanowires were found to be composed of crystalline Co nanoparticles by transmission electron microscopy.

© 2018 The Electrochemical Society of Japan, All rights reserved. Electrochemical behavior of Sn was investigated on a GC electrode in an aprotic ionic liquid, 1-butyl-1- methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing BMPCl and SnCl2 at 298K. SnCl2 was found to dissolve as [SnCl4]2- by potentiometry. The formal potential of [SnCl4]2-/Sn was -1.67V vs. Ag|Ag(I) (-1.24V vs. ferrocene|ferrocenium), which was more negative than that in BMPTFSA without chloride ions. Electrodeposition of Sn was possible by reduction of [SnCl4]2- in the ionic liquid. The dissociation of [SnCl4]2- was suggested from the diffusion coefficient of [SnCl4]2-. Tubular deposits with an inner diameter of 1-1.5μm were obtained in addition to granular deposits by potentiostatic cathodic reduction at -2.0V.

© 2018 The Electrochemical Society of Japan, All rights reserved. Electrodeposition of cadmium (Cd) was investigated in a hydrophobic room-temperature ionic liquid, 1-butyl-1- methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) using CdCl2 as the source of Cd species in the presence of excess chloride ion. Raman spectroscopy and potentiometric measurement suggested formation of a cadmium tetrachlocomplex, [CdCl4]2-, in BMPCl/BMPTFSA. Cyclic voltammetry showed the possibility of electrochemical reduction from [CdCl4]2- to Cd(0) with interesting unusual electrochemical behavior, probably derived from the potential-dependent electric double layer structure typical to the ionic liquid. Electrodeposition of Cd was performed by potentiostatic electrolysis and the deposits were characterized by X-ray diffraction, energy dispersive X-ray analysis and scanning electron microscopy.

The redox reaction of tris(acetylacetonato)iron(III) (Fe(acac)3) on a glassy carbon electrode was investigated in 1- butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA). A one-electron transfer redox reaction of [Fe(acac)3]0/- showed the relatively negative potential of -1.4V vs. Ag|Ag(I). The diffusion coefficient of Fe(acac)3 estimated by chronoamperometry was (1.4 + 0.1) × 10-7cm2 s-1. The redox properties examined in this study suggested that the redox reaction of [Fe(acac)3]0/- can be utilized in the negative half-cell reaction of redox flow batteries.

The in-situ Fourier transform infrared spectroscopy of molten eutectic LiCl-KCl-CsCl was conducted in order to identify the dissolved ions in the molten salt electrolytes. The transflectance spectrum of the melts could be easily obtained at temperatures higher than 350°C using the commercially available diffuse reflectance optical system and an air-tight chamber with a built-in heater. The sharp absorption peak attributable to the stretching vibration of OH- was observed in the melt containing LiOH, indicating the availability of identification of dissolved species in the melts. The intensities of transflection possibly assignable to N-H bonds in imide (NH2-) and amide (NH2 -) anions in the melt containing Li3N changed during supplying H2 gas because of the progress of the protonation reactions of these anions to form NH3. The in-situ analysis of the dissolved ions in the melts under the reaction conditions by infrared spectroscopy with the diffuse reflectance optical system can be a simple and powerful tool for understanding the reaction mechanism.

© 2020 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited. The electrode reaction of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was investigated in a solvate ionic liquid composed of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) and tetraglyme (G4), [Li(G4)]TFSA. A quasi-reversible redox reaction of the oxoammonium (TEMPO+) and TEMPO was observed with a formal potential of 0.19 V vs ferrocene|ferrocenium. The diffusion coefficient of TEMPO+ (2.1 × 10-8 cm2 s-1) was smaller than that of neutral TEMPO (1.3 × 10-7 cm2 s-1), reflecting the coulombic interaction between monovalent TEMPO+ and TFSA- in the solvate ionic liquid. The standard rate constant for TEMPO+/TEMPO redox reaction was estimated to be 1.9 × 10-4 cm s-1 at 25 C by electrochemical impedance spectroscopy. The solubility of TEMPO in [Li(G4)]TFSA was found to be 4.4 M at 25 C.

Electrodeposition of Co was investigated in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA), under an external magnetic field. Neodymium magnets were used as a source of the magnetic field, which was parallel to the ionic current during the electrodeposition of Co on a glassy carbon electrode. Potentiostatic cathodic reduction applying -1.6 and -2.0 V vs Ag|Ag(I) under the magnetic field in BMPTFSA containing Co(TFSA)2 at 25 C gave nanowire-shaped deposits on the electrode surface. The deposits were found to be composed of Co, which was confirmed by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. However, no diffraction peak was observed in the deposits by X-ray diffraction. The nanowires were further examined by transmission electron microscopy. The lattice fringe corresponding to (111) plane of Co was found in the deposits, indicating the nanowires were composed of metallic Co nanoparticles. Therefore, crystalline Co nanoparticles were considered to be formed in the presence of the magnetic field.

Aluminum species forming in Lewis acidic chloroaluminate and bis(trifluoromethylsulfonyl)amide (TFSA-) mixed ionic liquids having 1-butyl-3-methylimidazolium (BMI+) was investigated by potentiometry and Raman spectroscopy. A decrease in the concentration of [Al2Cl7]- and an increase in that of [AlCl4]- with addition of BMITFSA into acidic BMICl-AlCl3 were observed by Raman spectroscopy. The redox potential of Al(III)/Al in BMICl-AlCl3 (1:2 in molar ratio) was found to be -0.68V vs. an Ag|Ag(I) reference electrode, which was corresponding to -0.25V vs. the ferrocene/ferrocenium couple at 25°C. The potentiometry of an Al electrode in the mixed ionic liquids with different compositions using the Ag|Ag(I) reference electrode suggested formation of a mixed ligand aluminum complex, Al(TFSA)Cl2, indicating the instability of [Al2Cl7]- against TFSA-.

Electrochemical formation of selenium (Se) nanoparticles in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing selenium tetrachloride (SeCl4) was investigated in the presence of excess chloride anion on a glassy carbon electrode. Electrochemical reduction of [SeCl6]2- resulted in deposition of Se on the electrode surface. The deposited Se was further reduced electrochemically at the more negative potential region to form [Se2]2-, which was suggested to exist by ultraviolet-visible spectroscopy. Se nanoparticles dispersed in the ionic liquid were formed by the proportionation reaction between [SeCl6]2- and [Se2]2- in the ionic liquid. The Se nanoparticles were characterized by energy dispersive X-ray spectroscopy and transmission electron microscopy.

The cathodic reduction of nitrogen (N2) gas in molten chloride was investigated using a porous nickel (Ni) electrode. A LiCl-KCl-CsCl melt containing cathodically formed nitride ion (N3-) was sampled from an electrolytic bath and analyzed quantitatively by ion chromatography. The current efficiency of the cathodic reaction calculated from the ratio of quantity of the produced N3- was approximately 78% on average. The main reason why the measured efficiency did not reach 100% is considered as the amount of N3- increased more in the pores of the gas electrode than in the bulk of the electrolytic bath.

© 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. Electrochemical preparation of Co-Sm nanoparticles was conducted in an aprotic room temperature ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing Co(TFSA)2 and Sm(TFSA)3. The cyclic voltammetry on a glassy carbon (GC) electrode indicated the electrochemically generated Sm(II) reacted with Co(II) at 25 C. Potentiostatic cathodic reduction on a GC electrode in BMPTFSA containing 30 mM Co(TFSA)2 and 5 mM Sm(TFSA)3 at 25 C gave the deposits, which were found to be composed of Co and Sm by energy dispersive X-ray spectroscopy (EDX). The deposits were found to be the aggregates of SmCo7 nanoparticles by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The formation of SmCo7 nanoparticles dispersed in the ionic liquid was also confirmed by TEM. SmCo7 nanoparticles were considered to form by the disproportionation reaction of Sm(II) in the presence of elementary Co, which was formed by the reduction of Co(II) by Sm(II).

© 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. The electrochemical behavior of a chlorocomplex of nickel (Ni) was investigated in a Lewis basic and hydrophobic ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing Cl-. NiCl2 dissolved in BMPTFSA with addition of 0.5 M BMPCl. The dissolved species of Ni(II) in BMPTFSA was identified as [NiCl4]2- by UV-visible spectrometry and magnetic susceptibility balance. The cathodic and anodic currents assignable to deposition and dissolution of Ni, respectively, were observed in cyclic voltammetry. The granular deposits of Ni were obtained on a glassy carbon electrode at 25 and 100 °C. Ni nanoparticles were dispersed in the electrolyte by galvanostatic cathodic reduction. The initial stage of Ni deposition was found to be regarded as instantaneous. The diffusion coefficient of [NiCl4]2- was calculated to be 6.6 × 10-8 cm2 s-1

© 2018 The Electrochemical Society of Japan, All rights reserved. Deposition and dissolution of copper were investigated by electrochemical quartz crystal microbalance (EQCM) technique using a Pt-coated quartz crystal resonator in contact with a separator in order to simulate the configuration of practical batteries. The admittance analysis indicated the quartz crystal resonated even in contact with a glass separator impregnated with the electrolyte under pressure. The mass change estimated from the change in the resonance frequency during deposition and dissolution of Cu on a Pt-coated resonator in contact with the separator was linearly related to the passed electric charge and closed to that without the separator, suggesting the mass monitoring according to Saurbrey equation is possible in the presence of the separator on the electrode. The highly sensitive mass monitoring with an overtone resonance was also found to be possible in the presence of the separator using the third harmonic vibration mode.

© The Royal Society of Chemistry 2018. New electrochemical application using room-temperature ionic liquids (ILs) are introduced, such as lithium secondary batteries, electrochemical double layer capacitors, and novel types of electrical devices for sustainable and renewal energy society. ILs have so many combinations, owing to many cation/anion species. In this chapter, we introduce properties from fundamental (general and special physicochemical properties) to electrochemical applications of ILs. We also discuss importance of molecular design and application target of ILs.

© The Author(s) 2019. Electrochemical reaction of samarium (Sm) species was explored in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA), at 25 and 100°C. The cathodic and anodic current peaks corresponding to the reduction of Sm(III) and the oxidation of Sm(II), respectively, were observed in BMPTFSA containing Sm(TFSA)3 by cyclic voltammetry at 25°C. However, the anodic current peak corresponding to the oxidation of Sm(II) diminished significantly at lower scan rates when the temperature was elevated to 100°C, suggesting the disproportionation of Sm(II) at higher temperatures. Deposits were obtained on a glassy carbon electrode by potentiostatic cathodic reduction at -1.6 and -2.5 V in BMPTFSA containing Sm(TFSA)3 at 100°C. The deposits were found to contain Sm by energy dispersive X-ray spectroscopy (EDX), although no diffraction peak was observed by X-ray diffraction. The formation and dispersion of nanoparticles in the ionic liquid were also confirmed by transmission electron microscopy with and without exposure of the samples to air. The nanoparticles were considered to be metallic Sm, which was identified by electron diffraction and EDX. Sm nanoparticles were anticipated to be formed by the disproportionation reaction of Sm(II) to Sm(III) and Sm in the ionic liquid.

Anodic dissolution and electropolishing of Sn were investigated in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide. The rate of anodic dissolution was considered to be determined by the diffusion of anodically dissolved Sn(II) species. A large increase in the local viscosity during dissolution was observed in-situ by the impedance-type electrochemical quartz crystal microbalance, reflecting an increase in the local concentration of Sn(II) near the electrode. A shiny and smooth surface was obtained after anodic dissolution at 0.1 V vs Ag|Ag(I) with agitation. A decrease in the surface roughness estimated by confocal laser scanning microscopy suggested electropolishing of Sn was possible in the ionic liquid within the electrochemical potential window probably due to the formation of the viscous layer near the electrode.