Published papers

Metal–organic frameworks (MOFs) comprise coordination bonds and have attracted attention for electrochemical applications. However, MOFs are usually structurally weak in aqueous solutions, especially in acidic aqueous solutions, owing to their coordination bonds, making their application in charge-storage devices challenging. In the current work, we demonstrate a redox-active MOF (RAMOF) that is structurally stable and achieves reversible charge storage with almost the theoretical capacity even in acidic aqueous electrolytes owing to its strong Zr–O bonds and the large coordination number. In addition, the RAMOF exhibits high durability (> 98% after 100 cycles) and high Coulombic efficiency (99.9%) owing to its high crystallinity and proton conductivity. An aqueous MOF–air rechargeable battery is fabricated and exhibits high durability and high Coulombic efficiency. Furthermore, the material recycling of the RAMOF based on its coordination bonds is demonstrated. Therefore, we conceptually prove the application and advantages of RAMOFs in aqueous environments.

The formation potentials of the solid-electrolyte interphase (SEI) on a platinum and glassy carbon electrode in two amide-type ionic liquids, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA), containing some metal salts were investigated by cyclic voltammetry in the presence of ferrocene (Fc). It is considered that the SEI with the thickness of less than a few nm can be observed by monitoring the shift in the peak potential of Fc+/Fc. The SEIs began to form at ∼2.4 V vs. Li|Li(I) in the presence of lithium salts because of a lowering of the lowest unoccupied molecular orbital level of the anion by the interaction with lithium ion. In the case of BMPTFSA containing Ni(TFSA)2, the SEI began to form at -0.7 V vs. Ag|Ag(I), suggesting that the deposition of Ni may be hindered by the SEI formation.

Electrodeposition of nickel was investigated in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide (BMPTFSA) containing Ni(TFSA)2 under a external magnetic field. The onset potential of cathodic reduction of Ni(II) with a magnetic field was more positive than that without the magnet field. Ni nanowires were deposited by potentiostatic cathodic reduction with the magnetic field, indicating the magnetic field influence not only the electrode kinetics of the magnetic species but also the morphology of the ferromagnetic metal.

The electrode reactions of Ag(I)/Ag and ferrocenium/ferrocene (Fc+/Fc) were investigated in an ionic liquid, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA). The potential of Ag(I)/Ag depended on the logarithm of the concentration of Ag(I), as predicted by the Nernst equation, indicating the Ag(I)/Ag can be used as a reference electrode reaction in BMPFSA. The reversible electrode reaction of Fc+/Fc was observed in BMPFSA by cyclic voltammetry. The donor number of BMPFSA was estimated to be 13 from the difference in the formal potentials of Ag(I)/Ag and Fc+/Fc, indicating the coordination ability of FSA- was slightly stronger than that of bis(trifluoromethylsulfonyl)amide (TFSA-). The diffusion coefficients (D) of Fc and Fc+ were (5.7 ± 0.7) and (3.3 ± 0.2) × 10-7 cm2 s-1, respectively. The ratio of D of Fc+ against that of Fc was smaller than those in TFSA--type ionic liquids, reflecting the higher charge density of FSA-. The standard rate constant (k 0) of Fc+/Fc was estimated to be (5.4 ± 1.1) × 10-3 cm s-1. The apparent activation energy for k 0 was close to the activation energy for D, suggesting the electrode reaction of Fc+/Fc can be regarded as the outer sphere electron transfer reaction with a very small reorganization energy.

The formation of solid-electrolyte interphase (SEI) on a glassy carbon (GC) electrode in 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing Li+, Na+, K+, and Ni2+ was investigated using the redox reaction of ferrocene (Fc). The anodic peak potential (E pa) for the oxidation of Fc changed after holding the electrode at -1.4 to -1.5 V vs Ag|Ag(I) in BMPFSA in the presence of the alkali metal ions. The decomposition of FSA- was confirmed by X-ray photoelectron spectroscopy (XPS) on a GC electrode held at -1.5 V vs Ag|Ag(I) for 6 h in BMPFSA containing the alkali metal ions. The change in E pa may also suggest the formation of homogeneous SEI in the FSA--type ionic liquid. Moreover, E pa changed after holding the electrode at the potential more negative than -0.9 V vs Ag|Ag(I) in 50 mM Ni(TFSA)2/BMPTFSA while the decomposition of TFSA- was not confirmed by XPS on the electrode held at -1.1 V vs Ag|Ag(I) for 6 h, suggesting the bonds except C-F (e.g., S-C) were dissociated to form the SEI. Since the deposition potential of Ni in BMPTFSA was more negative than the SEI formation potential, the deposition of Ni may be inhibited by the SEI.

The formation and aging of the solid electrolyte interphase (SEI) formed on a Li and Cu electrode in two bis(fluorosulfonyl)amide (FSA-)-type ionic liquids (ILs), 1-methyl-1-butylpyrrolidinium bis(fluorosulfonyl)amide and 1-methyl-1-methoxyethylpyrrolidinium bis(fluorosulfonyl)amide, containing a high concentration of LiFSA were investigated using electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and scanning electron microscopy. The intensity of peaks corresponding to the decomposition of the electrolyte in XPS spectra changed with time. The thicknesses of the SEI formed on a Cu electrode in both ILs were estimated to be 20-100 nm by TEM observation. The resistance of SEI increased with the lapse of time although the change in the thickness of the SEI was small, suggesting the aging of the SEI in the ILs was considered to depend on the composition of the SEI rather than the thickness of the SEI. The Li deposits shrank during dissolution. The Li anode became sparse and porous after deposition and dissolution cycles, probably due to the inhomogeneous reaction through the aged SEI. The SEI formation on the surface of the porous deposits was considered to lead to an increase in the polarization and the isolation of the deposits, i.e., the formation of “dead lithium”.

The physicochemical and electrochemical properties of an ionic liquid consisting of the dichlorocuprate anion, [CuCl2]-, were investigated. The equimolar mixture of CuCl and 1-butyl-1-methylpyrrolidinium chloride (BMPCl) was found to yield an ionic liquid at 298 K composed predominantly of BMP+ and [CuCl2]-. Fine deposits of metallic Cu were obtained on a glassy carbon electrode by potentiostatic and galvanostatic reduction of [CuCl2]- in CuCl-BMPCl (50.0-50.0 mol%). The overpotential for Cu nucleation was larger on a glassy carbon electrode than on a Pt electrode. The electrochemical deposition and dissolution of Cu were analyzed using an electrochemical quartz crystal microbalance. The local viscosity and density of the electrolyte near the electrode/electrolyte interface increased during both deposition and dissolution of Cu, probably reflecting the shift in the electrolyte composition to the local basic and acidic conditions, respectively.

The formation of solid-electrolyte interphase (SEI) on a Pt electrode was investigated in ethylene carbonate (EC) and diethyl carbonate (DEC) containing LiPF6 and lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) using the redox probe method. The formation and properties of the SEI were examined in the electrolytes containing ferrocene as a redox probe. Inhomogeneous and rigid SEI was found to form in 1 M (= mol dm−3) LiPF6/EC+DEC, probably due to the conversion of decomposition products to LiF by the reaction with HF derived from LiPF6. On the other hand, the formation of homogeneous and soluble (or dispersible) SEI was suggested in 1 M LiTFSA/EC+DEC.

The formation and breakdown of the solid electrolyte interphase (SEI) on a Pt electrode were investigated in 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA) containing a high concentration of LiFSA using a ferrocene/ferrocenium couple as a redox probe. The formation of the SEI was confirmed at the potential more negative than −1.1 V vs. Ag|Ag(I). However, the SEI was found to disappear at the open circuit potential. The SEI was suggested to form by the dissolution and/or dispersion of the decomposition products in the electrolyte.

The electrode reactions of copper species were investigated in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl) amide (BMPFSA). The anodic current assignable to the dissolution of Cu to form monovalent Cu(I) complex was observed at −0.42 V vs. Ag|Ag(I). A pair of anodic and cathodic current peaks attributable to Cu(II)/Cu(I) was observed at 0.5 V, suggesting that a 4 V class redox flow battery combined with a Li metal anode is expected using the redox reaction of Cu(II)/Cu(I) in BMPFSA as the cathode reaction. The electrodeposition of Cu and the quasi-reversible redox reaction of Cu(II)/Cu(I) were affected by the electrode materials.

© 2018 Electrochemical Society Inc.All rights reserved. The redox reaction of 2, 2, 6, 6-tetramethylpiperidine-l-oxyl (TEMPO) has been investigated in a solvate ionic liquid composed of lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) and tetraglyme (G4). The one-electron transfer and quasi-reversible redox reaction of TEMPO+/TEMPO was observed in the stable electrochemical potential window of the solvate ionic liquid. The formal potential of TEMPO'S/TEMPO was 0.19 V vs. Ferrocene/ferrocenium. The diffusion coefficients of TEMPO and TEMPO + at 298 K were 1.8 × 10 -7 and 7.6 × 10 -8 cm 2 s -1 , respectively, reflecting the coulombic interaction between the charged species with the anion of the solvate ionic liquid. The standard rate constant for the redox reaction of TEMPO + /TEMPO at 298 K was 1.9 × 10 -4 cm s -1 . Charge-discharge of a lithium secondary cell using a lithium-ion conductive glass-ceramics (LICGC) as a diaphragm and [Li(G4)]TFSA containing TEMPO as a catholyte was found to be possible with the average discharge voltage of 3.4 V.

© The Electrochemical Society The electrochemical behavior of halogenocomplexes of Ag was investigated in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) in the presence of the corresponding halogen anion. Silver halides, AgX (X- = Cl-, Br-, and I-) were found to be soluble in BMPTFSA containing 0.5 M BMPX by forming [AgX3]2-. The cathodic reduction of [AgX3]2- to metallic Ag was possible within the electrochemical window of BMPTFSA. The reduction potentials were in the order of [AgCl3]2- > [AgBr3]2- > [AgI3]2-. The morphology of Ag deposits depended on the reduction potential. The diffusion coefficients of [AgCl3]2-, [AgBr3]2-, and [AgI3]2- were 2.5, 2.0, and 1.6 × 10-7 cm2 s-1, respectively. Formation of Ag nanoparticles dispersed in the ionic liquids was confirmed after potentiostatic cathodic reduction at -2.5 V using transmission electron microscopy.

The formation potentials of solid-electrolyte interphase (SEI) were investigated in 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing LiFSA and LiTFSA, respectively, by monitoring the redox reaction of ferrocene with high sensitivity. The SEI was found to affect the electrode kinetics of the outersphere electron transfer reaction. The SEIs with the thickness of less than a few nm formed in LiFSA/BMPFSA and LiTFSA/BMPTFSA at 2.4 and 2.5 V vs Li|Li(I), respectively. The cathodic decomposition of FSA- and TFSA- was considered to be promoted by the interaction with Li+ based on the ab-initio calculation.