In order to establish clear strategies for the design of electrocatalysts with high activity and high durability, fuel cell reactions have been analyzed by multilateral techniques. The use of monodisperse Pt or Pt-alloy nanocatalysts with uniform composition, which were highly dispersed over the whole surface of the carbon support by the nanocapsule method, contributed greatly in clarifying the effects of various properties (particle size, composition, and surface structure, etc.) towards the activity and durability for the anode and cathode in polymer electrolyte fuel cells (PEFCs). New catalyst layers have been developed in order to make the electrocatalysts work effectively specifically under low humidity. Several important concepts have been demonstrated for developing high-performance oxygen and hydrogen electrodes with high durability for a reversible solid oxide cell (R-SOC), which is expected to be a reciprocal direct energy converter between hydrogen and electricity with high efficiency.

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Semiconductor nanoparticles of several nanometers in size that exhibit the quantum size effect have recently been called “quantum dots”. Their unique physicochemical properties, which are different from those of bulk material or molecules, have attracted much attention for various applications, because of the controllability and designability of electronic, optical, and photochemical properties by changing the size, shape, and chemical composition of particles. Many efforts have contributed to the development of high-quality nanoparticles via solution phase syntheses. In this review, the key parameters for controlling the physicochemical properties of semiconductor nanoparticles are discussed. Furthermore recent progress in the preparation of multinary nanoparticles with no highly toxic elements, such as I-III-VI₂ semiconductors and their solid solutions, is outlined on the basis of our research results. The performance of devices fabricated with these semiconductor nanoparticles, that is, the photoluminescence property, photocatalytic activity and conversion efficiency of solar cell, is tunable depending on both the particle morphology and chemical composition.

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This paper reviews my recent investigations on fluorescent semiconductor nanoparticles based on the observation of photoluminescence quenching caused by photoinduced electron transfer to surrounding molecules. Semiconductor nanoparticles having photoluminescence at room temperature were quenched intentionally by the addition of redox species. The magnitude of quenching was greatly varied according to the interaction between the nanoparticles and quenchers, redox potential of quenchers, and the states of surface ligands protecting the semiconductor nanoparticles. The study began from the basic investigations on factors determining the magnitude of quenching and several material sensing systems were proposed as output of research. The phenomenon was then used to investigate the condition of surface ligands by using the strong distance dependence of the photoinduced electron transfer efficiency. Quantitative analyses of quenching revealed the differences in protection ability between the types of ligands. In the last part of this paper, I mention the possibility of semiconductor nanoparticles as practical fluorescence materials. The precise control of the distance between the nanoparticles realized emission from condensed nanoparticles with photoluminescence quantum yield over 80%.

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It is well known and accepted that oxygen nonstoichiometry, the deviation of oxygen content from the stoichiometric composition, significantly affects electrochemical properties of functional oxides. Therefore, it is important to understand the defect formation mechanism and the influences of defect species on electrochemical properties. In this paper, layered perovskite type La₂NiO₄-based oxides were investigated as a model system. Oxygen nonstoichiometry was evaluated by thermogravimetry and coulometric titration and analyzed based on defect chemistry. To understand the governing factor for interstitial oxygen formation, thermochemical deformation and electronic structural variation were evaluated by in-situ X-ray diffraction measurement and soft X-ray absorption spectroscopy. While La₂NiO_4+δ can deform flexibly to accept interstitial oxygen, more than 90% of interstitial sites are not occupied under equilibrium state. This strongly indicates the restriction due to crystal structure is not dominant for interstitial oxygen formation. Ni L-edge and O K-edge spectra reveal that lattice oxygen works as a sink/source of electronic carrier for the interstitial oxygen formation. These analyses indicate that the interstitial oxygen formation in La₂NiO_4+δ is mainly limited by the electronic structural restriction. This hypothesis is supported by the fact that electron doping increases the equilibrium concentration of interstitial oxygen.

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The nature of electrolyte solutions is dominated by the three factors of Li salts, solvents, and their mixing ratios (salt concentrations). Conventionally, the selections of Li salts and solvents have been considered of prime importance for Li-ion battery electrolytes, while the salt concentrations have been always fixed to approximately 1 mol dm⁻³ based on maximized ionic conductivities. Recently, however, the salt concentrations are increasingly recognized as a key to developing new functionalities for battery electrolytes in the wake of various unusual interfacial/bulk properties discovered in superconcentrated (highly concentrated) electrolytes. For example, highly concentrated electrolytes i) passivate effectively negative electrodes, ii) facilitate rapid Li⁺ intercalation reactions, iii) show high oxidative stabilities, iv) prevent the corrosion of Al current collectors, and v) suppress the dissolution of transition metals from positive electrodes, all of which are beneficial for battery applications. This article discusses those unique functionalities of highly concentrated electrolytes from the viewpoint of their ion-solvent and ion-ion coordination structures.

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This paper summarizes our previous studies on the application of metal/polymer electrolyte membrane composites to three kinds of electrochemical devices: polymer actuators, direct polymer electrolyte fuel cells, and air electrodes for use in metal-air batteries. Au composites were successfully prepared in a wet condition and their use as actuator devices showed rapid bending toward the anode in response to an applied voltage of 2.0 V in water without gas evolution. Direct polymer electrolyte fuel cells have also been reported as typical examples of the application of a polymer electrolyte membrane. These are represented by direct ascorbic acid fuel cells that use carbon anodes without exhausting any harmful chemicals, and by the technical advantages of an anion-exchange membrane alternative to the conventional cation-exchange membrane in direct fuel cells with ethanol or glucose fuels. Finally, a reversible air electrode for the oxygen reduction and evolution reaction was integrated with an anion-exchange membrane to diminish the influence of atmospheric carbon dioxide on the oxygen reaction.

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Recently, the development of fabrication technology has simplified the construction of micro/nano electrical-mechanical systems (MEMS/NEMS). For example, bottom-up techniques allowing the deposition of atoms and molecules to fabricate nanostructures have been studied. Molecular self-assembly using biomolecules has become an important process for the fabrication of nanostructures through bottom-up techniques. Of the various biomolecules, DNA has several advantages such as ease of synthesis, and well-developed cut-and-paste techniques that are suitable for nanostructure fabrication. Also, analytical methods that enable detection of the activity of living cells and biomolecules are critical for biomedical sensing applications because these substrates are the basis of life. MEMS/NEMS technologies enable the micro/nanometer-scale construction of many types of devices and thus hold great potential for biotechnology and biomedical sensing applications. Consequently, numerous studies have been performed using cell-based micro-devices based on MEMS/NEMS technologies. BioMEMS/NEMS devices will become powerful tools for the measurement of biological and biomedical functions.

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Cubic Li₂SnS₃ with NaCl structure and the solid solution with Li₃NbS₄ were developed by mechanochemical synthesis. Cubic Li₂SnS₃ is a metastable phase, and the previously reported monoclinic Li₂SnS₃ is a stable phase. Cubic Li₂SnS₃ acts as an ionic conductor, and the solid solution of Li₃NbS₄-Li₂SnS₃ can be used as an electrode active material in lithium secondary batteries. Li_2.933Nb_0.8Sn_0.267S₄ and Li_2.857Nb_0.571Sn_0.571S₄ shows large reversible capacities of 318 and 286 mAh g⁻¹, respectively. The capacity of these materials depends less on the niobium content than on the electrical conductivity. The reversible capacity in the 2-V region is mainly attributable to redox reactions involving rather sulfur than tin.

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