Ionic liquids (ILs) are defined as salts that have melting points lower than 100°C. Most are organic salts, and these may be designed and tailored to have suitable properties. ILs are recognized as a third group of solvents (and electrolytes), after water and organic solvents. They are characterized by their unique properties such as non-volatilities, high thermal stabilities, and high ionic conductivities. In this article, our work on the design and preparation of ILs is briefly reviewed. The concept of ionicity is proposed as a metric to characterize the basic nature (dissociativity) of ILs, which is affected by the Lewis acidity/basicity of cations/anions (i.e., coulombic interactions), the directionality of interactions between cations and anions, and van der Waals interactions between ions. The ionicity of ILs is dominated by a subtle balance between coulombic and van der Waals interactions, which clearly discriminates ILs from conventional high-temperature inorganic molten salts. Here, the design of protic ILs for fuel cell electrolytes, electron-transporting ILs for dye-sensitized solar cell electrolytes, and Li+-conducting solvate ILs for lithium battery electrolytes are discussed based on an understanding of the fundamental properties of ILs. Furthermore, the combination of ILs with polymers and colloidal particles affords intriguing quasi-solid materials (ion gels), and the ion transport in these gels is decoupled from the mechanical relaxation time of these materials, yielding new solid electrolytes. The possibility of temperature and photo-sensitive solubility dependence of polymers in ILs allows the creation of stimuli-responsive materials. Finally, protic ILs/protic salts are demonstrated to be good precursors for N-doped carbon materials.
“Abundance” is an important keyword in materials development. This is particularly the case for the energy storage sector, where materials themselves function as a storage host. The amount of materials is directly linked to the amount of energy stored in the device. In rechargeable batteries, transition metal elements are necessary to accommodate a large number of electrons/holes in a reversible redox reaction. Iron, as the fourth most abundant element in the earth’s crust, is an ideal redox center, but practical storage electrodes with Fe redox have long been the “holy grail” of the lithium-ion battery since its commercialization in 1991. In this review article, the history of replacing Co with Mn and/or Fe in lithium battery electrodes is briefly reviewed followed by recent technical achievements toward more sustainable batteries using Na+ as a guest ion, where the goal would be to discover a high voltage electrode material composed of Na and Fe without compromising the energy density. Further, our ultimate destination is set to high energy density aqueous lithium/sodium ion batteries, where the hydrate-melt electrolyte enables surprisingly high-voltage operation over 3 V. During materials identification and optimization, reaction mechanisms should be understood in a systematic way to provide a firm direction for strategic design. With this regards, important physicochemical properties of key materials will be introduced.
This article reviews our recent progress in the development of high-resolution scanning electrochemical microscopy (SECM) and its application to biological samples. SECM uses an ultramicroelectrode (UME) as a probe and a scanning mechanical stage for controlling the probe position. To improve the resolution of SECM, we have developed a fabrication method for pyrolytic carbon nanoelectrodes and a current feedback system for probe–sample distance control. The current feedback system effectively provides high-quality electrochemical and non-contact topography images because the current signal depends on the probe–sample distance. High-resolution SECM has overcome the limit of microscale imaging resolution and enabled the imaging of local regions within cells. In this study, we address four topics: nanoelectrode fabrication, current feedback probe–sample distance control systems, membrane protein imaging, and neurotransmitter detection.
In this comprehensive paper, we present an overview of our recent studies in reveal of influences of “atomic- and nano-scale structure” on catalytic and photocatalytic activity or preparation of functional materials for clean energy conversion systems such as water splitting, solar cells, photocatalysts and fuel cells. In the case of atomic scale structures, amorphous and atomic-scale roughed surface structure of RuO2 and TiO2 shows high catalytic activity for oxygen evolution reaction. On the other hand, in the case of nano-scale structures, the anodic porous TiO2 films controlled their crystallinity and pore size can be used as new applications such as flexible dye sensitized solar cells and size selective photocatalysts. The porous Al2O3 films can be used as a template for formation of highly durable platelet carbon nanofiber supports of cathode electrode for fuel cells.
Certain stoichiometric mixtures of salts and solvents (ligands) yield a salt complex. Salt complexes that are low melting and consist of discrete complex ions and their counter ions in the molten state can together form the essence of an ionic liquid (IL). This stable complex melt can now be categorized into a new subclass of ionic liquids, “solvate (or chelate) ILs.” In this paper, we describe the current criteria for this new family of ILs. Concentrated mixtures of lithium salts and oligoether solvents are useful models for these solvate ionic liquids; the effects of the ion-ion and ion-solvent interactions on their structure and properties were investigated to contrast with classical concentrated electrolyte solutions. The performance of solvate ILs as electrolytes for lithium-sulfur (Li-S) batteries is also reported. The dissolution of lithium polysulfides (i.e., reaction intermediates of the sulfur cathode) into the electrolyte, which is a serious issue for the practical application of Li-S cells, was greatly suppressed in the solvate ILs. Therefore, a stable charge-discharge with high Coulombic efficiency was achieved with the solvate IL electrolytes.
A photonic crystal, which is a periodic structure on the scale of optical wavelength, can inhibit the propagation of light at particular wavelengths. Various optical devices have been demonstrated using the photonic crystals such as low-threshold lasers, light-emitting diodes, waveguide structures bending at acute angles, and single-mode optical fibers: However, there are few reports on the chemical application of photonic crystals. The author struggle to fabricate electrochemically functional photonic crystals using both bottom-up and top-down techniques, i.e., periodic structures composed of titanium dioxide, and examined the optical and electrochemical characteristics.
A novel nano-biointerface for the detection and control of neurons has been proposed as “an artificial synapse.” This article introduces two approaches that I have been focusing on. The first approach is the observation of a single functioning receptor protein by using atomic force microscopy (AFM). The receptor protein was purified, reconstituted into a lipid bilayer and observed in a physiological solution. The tetrameric and dimer-of-dimer structure of the receptor protein and the electrical signals from the protein proved that the protein was successfully handled in the process, which was designed to integrate it in an artificial post-synapse. The second approach is a neuronal affinity examination based on a single neuron that is undertaken by combining a scanning electron microscope (SEM) and a focused ion beam (FIB). Then, according to the information thus obtained, the neurons exhibited the potential to be guided by employing topographic features, namely nano-pillars, made of suitable materials. This approach enables neurons to grow close to the artificial post-synapse and to interact with the device. These approaches will improve the technologies needed to realize an artificial synapse, which will be a useful platform for neurons that will allow us to examine the mechanism of synaptic formation and that will be applied to pharmacology, drug discovery, and medical science.
By In-Operando cross-sectional observation of laminated Li-ion cells, morphology of deposited Li on surface of graphite anode and reaction distribution of anode was investigated. During 20C-CCCV charge, we observed 2 types of deposited Li as follows; Li whisker and Li dendrite under different conditions of constant voltage charging current, and found that Li dendrite decreased Li-ion concentration of electrolyte solution in anode. With electrochemical impedance spectroscopy, it was determined that Li dendrite increased ohmic resistance of a cell and charge transfer resistance of anode, while there was no affection by Li whisker. These results suggest that Li dendrite depresses the charge reaction of graphite anode, and Li whisker does not affect charge reaction.
Re60Ni40 alloy was electrodeposited galvanostatically from aqueous solution containing 34 mM ammonium perrhenate, 124 mM nickel sulfamate tetrahydrate and 343 mM citric acid in a three-electrode cell system at bath temperature of 70°C. The deposition mechanisms of the alloy were discussed by cyclic voltammetry and cathodic potentiodynamic tests. Microstructure and morphology of surface and fracture surface of the alloy were examined by ESEM. The phase and chemical composition were tested by XRD and EDS, respectively. The thermal stability of the alloy was estimated by DSC. The results show that electrodeposition of Re-Ni alloy is an anomalous deposition. Many cracks were present on the surface and fracture surface of the alloy. The fracture surface of the alloy exhibited both a fibrous structure and a laminar structure. The chemical composition distribution in the alloy was uniform. The alloy adhered well to the substrate with no evidence of delamination. Re60Ni40 alloy was composed of an amorphous structure, and the deposition rate was about 16.5 µm h−1. The average roughness Ra and Rq of the alloy surface were 123 ± 13 and 158 ± 17 nm, respectively. After DSC test, the amorphous Re60Ni40 alloy exhibited a good thermal stability from 25°C to 550°C.
NaTi2(PO4)3/carbon and NaTi2(PO4)3/graphite composites as anode materials for aqueous rechargeable Na–ion batteries were synthesized by modified Pechini method and pyrolysis treatment in the inert atmosphere. The structures and carbon contents of the samples were characterized by X-ray diffraction, Scanning Electron Microscopy and carbon–sulfur analyzer respectively. Electrochemical properties were evaluated by galvanostatic discharge/charge test and cyclic voltammetry using a three-electrode system. The first discharge specific capacity of NaTi2(PO4)3/carbon composites was 128.6 mAh g−1 and maintained 117.4 mAh g−1 after 50 cycles at a discharge rate of 2 C. The reversible capacity of the composites remained 66.2 mAh g−1 even at 20 C. NaTi2(PO4)3/carbon composites exhibited better electrochemical performance than NaTi2(PO4)3 and NaTi2(PO4)3/graphite samples.
The heat treatments of Ni-free SUS445 stainless steel under nitrogen and argon gas atmospheres were conducted at 1373–1473 K in the present study. The heat-treated samples were characterized by an X-ray diffraction (XRD), glow discharge optical emission spectroscopy (GD-OES), and scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDX). The corrosion behavior of the heat-treated samples was evaluated by linear sweep voltammetry (LSV) measurements in a 0.5 mol dm−3 H2SO4 solution. The results revealed that the nitrided stainless steel at 1373 K presents the best corrosion resistance and the AlN, expanded austenite γN, CrN and Cr2N phases appeared based on the XRD results. In contrast, the corrosion resistance of the heat-treated stainless steel under an argon atmosphere was decreased and there was no change in the microstructure.