Heterogeneous photocatalysis, as well as photoelectrochemical reaction, with semiconductor materials has been interpreted and discussed using so-called “band-structure model” as a leading principle since the 1970’s. However, this band-structure model shows only thermodynamics of photocatalytic reactions and no kinetic information is available. Furthermore, the model does not reflect the surface and size of photocatalyst materials, since band structure is of bulk, and thereby the photocatalyst materials are not identified. In this review, works by a group of the present author on physicochemical mechanistic studies on multielectron transfer kinetics and energy-resolved distribution of electron traps to go beyond the band-structure model is introduced.

Advanced electrode materials and tailored design of the electrified interface are essential for electrochemical processes that rely on electrode/electrolyte interfaces. In particular, electrochemical capacitors (supercapacitors) and electrocatalysts depend on surface confined reactions and thus high surface area nanomaterials are preferred. In this review, key advances in the development of conducting oxide nanosheets towards aqueous pseudocapacitors and fuel-cell related electrocatalysts will be highlighted, emphasizing results primarily from the authors’ laboratory. The synthesis of conductive nanosheets and its application towards pseudocapacitors and hybrid capacitors will be reviewed. The use of nanosheets as co-catalysts for Pt-based electrocatalysts as well as catalysts will be described. The morphology-property relation will be established for nanosheet electrochemistry, hopefully encouraging researchers in materials chemistry and electrochemistry to communicate and move forward together to stimulate enhancement in the important and expanding field of electrochemical energy storage and conversion.

Group I-III-VI₂ semiconductor-based quantum dots, such as AgInS₂, CuInS₂ and their solid solution with ZnS, have recently been reported to show intense photoluminescence, the wavelength of which is tunable in a wide range from visible to near-infrared (NIR) light regions by controlling their chemical composition and/or size. Due to their low toxicity and tunable optical properties, these multinary semiconductor quantum dots (QDs) have been intensively developed as alternative materials to conventional Cd- and Pb-based QDs for applications to photoluminescent devices, photocatalysts, quantum dot solar cells, and biological imaging materials. In this review, the recent progress in the photofunctionality of I-III-VI₂-based semiconductor QDs prepared by the colloidal method is outlined with foucus on two advances: (1) extension of the photoluminescence wavelength to NIR light for in vivo biological imaging and (2) improvement of photoenergy conversion properties by nanostructure control of QDs.

Reactive species generated by the oxidation and reduction of organic compounds are generally unstable. To utilize the reactive species as intermediates for organic synthesis and functional materials, it is crucial how to treat them. In this headline, electrochemical reactions utilizing stabilized reactive cations, which enabled benzylic C–H/aromatic C–H cross-coupling, aromatic C–H/C–H cross-coupling, and aromatic C–H amination are demonstrated. Furthermore, energy storage materials utilizing stabilized anions, which enabled to develop organic materials for lithium-ion batteries with high voltage of ca. 3 V and solvent-free redox flow batteries with high energy density, are demonstrated. These studies show the usefulness of stabilized reactive species.

Organic field-effect transistors (OFETs) are promising platforms for flexible and disposable sensors, because of their attractive properties such as mechanical stretchability, environmental friendliness, compact integration, and solution processability. Although physical sensors utilizing OFETs have been successfully demonstrated, the development of OFET-based chemical/bio-sensors is still in its infancy. In this regard, we have designed new enzyme-modified OFETs with electron mediators for selective analyte sensing. The fabricated OFET with an extended-gate electrode can reproducibly operate under ambient conditions. Importantly, the developed OFET successfully detected analytes such as nitrate, lactate, and biogenic amines in pseudo and real biological fluids. Thus, we believe that the proposed approach to develop OFET-based enzymatic sensors will open up new avenues for the realization of practical and flexible biosensing applications.

We investigated NaCuO₂ as a positive electrode material for sodium-ion energy storage batteries. Its charge/discharge properties in a Na/NaCuO₂ cell were evaluated in voltage ranges of 0.75–3.0, 1.7–4.2, and 0.75–4.2 V. It was found that the charge/discharge properties and cycling-induced new products depended on the voltage range. In the range of 0.75–4.2 V, NaCuO₂ showed a reversible capacity of 190 mAh/g after ten cycles, with an average voltage of 2.5 V versus Na/Na⁺. The results of an ex-situ XRD analysis of NaCuO₂ electrode at various charged/discharged stages revealed that NaCuO₂ is converted to NaCuO after the second cycle, and then, reversible structural changes occur between NaCuO and CuO.

Conversion-type active materials show promise for use in large-scale lithium-ion batteries by virtue of their low cost and large specific capacities. However, there are some challenges in the application of these materials to practical Li-ion batteries. To adapt the conversion-type active materials to the next-generation Li-ion batteries, it is necessary that we understand the conversion reaction in detail. In this review, the electrochemical properties of the iron-based conversion cathode are introduced, and their reaction mechanisms are described based mainly on our own experiments. Finally, we introduce a composite of LiF as an Li source and FeO as an anion accepter as a novel cathode system for the next-generation Li-ion batteries.

It is difficult to perform the electrodeposition of aluminum alloys from aqueous solutions. In this study, aluminum–nickel was electrodeposited from an electrolyte containing dimethyl sulfone, AlCl₃, and 0.0 mol%–1.0 mol% NiCl₂ at deposition potentials ranging from −1.0 to −5.0 V (Al/Al³⁺). The surface morphologies of the Al–Ni alloys depended on the NiCl₂ concentration and the deposition potential. The crystalline structure of the Al–Ni alloys also depended on the NiCl₂-concentration. Furthermore, the content of Ni in the Al–Ni alloys increased with increase in the NiCl₂ concentration.

The crystal structure, morphology, and galvanostatic cycling and rate performances of cobalt-substituted Li₂MnSiO₄/C compounds, Li₂Mn_1−xCo_xSiO₄/C (x = 0.25, 0.5, and 0.75), were evaluated and compared with those of Li₂MnSiO₄/C and Li₂CoSiO₄/C. Li₂Mn_1−xCo_xSiO₄/C (x = 0.25, 0.5, and 0.75) compositions comprising uniform nanosized primary particles and no impurities were successfully synthesized using a hydrothermal method, followed by carbon coating. In addition, Li₂MnSiO₄/C and Li₂CoSiO₄/C were synthesized for comparison. The synthesized Li₂Mn_1−xCo_xSiO₄/C (x = 0.25, 0.5, and 0.75) were solid solutions and were identified using an orthorhombic unit cell with Pmn2₁ space group symmetry. Although the capacity fades for Li₂Mn_1−xCo_xSiO₄/C were similar to those for Li₂MnSiO₄/C, the discharge capacity, average discharge voltage and rate capability of Li₂MnSiO₄/C improved when Co was substituted for Mn. Li₂Mn_0.25Co_0.75SiO₄/C exhibited the best electrochemical performance with first energy density of 659.7 Wh kg⁻¹ which was greater than that of LiMn₂O₄ (500 Wh kg⁻¹) and LiNi_1/3Co_1/3Mn_1/3O₂ (600 Wh kg⁻¹). The good electrochemical performance of Li₂Mn_0.25Co_0.75SiO₄/C is attributed to its lower charge transfer resistance relative to that of Li₂MnSiO₄/C.

Air electrodes with millimeter-order thickness for lithium air secondary batteries were prepared by loading electrode materials containing carbon and electrocatalyst into a single-layer nickel foam sheet or stacking three nickel-foam sheets loaded with the electrode materials. Discharge properties of the lithium air secondary battery cells incorporating these air electrodes were examined in 1 mol/l LiTFSA/TEGDME under a pure oxygen flow. The cell incorporating the air electrode with carbon loaded into the three-layer stack of nickel-foam sheets with total thickness of 3 mm showed a rather large discharge capacity of about 80 mAh/cm² compared to about 30 mAh/cm² for the cell with a carbon-only air electrode incorporating a single nickel-foam sheet with 3-mm thickness. The cell incorporating an air electrode loaded with Pt₁₀Ru₉₀ electrocatalyst could be cycled under the condition of large cut-off capacities of 10 mAh/cm² at current density of 0.2 mA/cm².

The toluene-methylcyclohexane organic hydride has been expected to be a candidate of the hydrogen energy carrier system for the effective utilization of renewable energy. We have developed an electrocatalyst for the toluene electro-hydrogenation electrolyzer. However, hydrogen evolution as the side reaction decreases the current efficiency by inhibiting the toluene mass transfer. In this study, we demonstrated the effect of the toluene-methylcyclohexane chemical-hydrogenation by the loading of Pt nanoparticles in the carbon-paper as a porous flow-field. The loaded Pt in the flow-field functioned as the toluene hydrogenation catalyst with the generated hydrogen gas. This simultaneous functioning of chemical- and electro-hydrogenation in the flow-field and the catalyst layer was designed to enhance the overall apparent current efficiency. Based on the electrochemical measurement, the Pt-loaded carbon paper flow-field showed an outstanding enhancement of the current efficiency without a decrease in performance for the Pt loading from 0.5 to 0.02 mg cm⁻². The conversion from toluene to methylcyclohexane by a one-through operation of the electrolyzer achieved 92–96% by using the Pt-loaded carbon paper flow-field. Simultaneously, the cell voltage was 1.92 V at 0.4 A cm⁻² for a 0.5 mg_-Pt cm⁻² loaded carbon paper-used membrane cathode assembly based on linear sweep voltammetry.

Electrochemical detection of ethylenebis(dithiocarbamic acid) manganese zinc complex (Manzeb), a widely used insecticide, by reductive desorption from Au(111) and Au(100) has been studied. Dithiocarbamate groups containing adlayer were formed on gold single-crystal electrodes by immersing into a Manzeb aqueous solution. The electrochemical responses and the adsorption amounts were evaluated based on the reductive desorption of sulfur species from the gold surface in alkaline solution. The detection limit for Manzeb depended on the modification time and the crystallographic orientation of gold. The limits of detection were 500 nM and 100 nM when a Au(111) electrode and a Au(100) electrode were used, respectively. The surface-enhanced infrared absorption spectra of Manzeb on a Au thin film electrode in its monolayer state showed that Manzeb was adsorbed on the substrate via the dithiocarbamate moiety and the long axis of ethylenebis(dithiocarbamate) dianion was almost perpendicular to the surface.

The lifetime of a nickel-metal hydride battery power system (BPS) used in railway systems is investigated. Frequent acceleration and deceleration are performed in normal railway operation, so the narrow range of SOC of the corresponding BPS is discussed. Since the lifetime of the battery used in that BPS should be estimated in accordance with the operation pattern, the operation pattern was analyzed by Fourier transform, and battery lifetime is evaluated in the frequent discharge/charge with a current change every 6 seconds within a 3% depth of discharge (DOD) as the main pattern and the float charging at a constant voltage. Major factors of the battery lifetime are operation voltage and the internal cell temperature caused by an internal resistance. The lifetime of the battery do not depend on the operation pattern such as frequent discharge/charge and float charging. Furthermore, it was almost the same as the lifetime at rest potential in the open circuit state. Applying electrochemical reaction kinetics equations to this system, calculation results indicate a good correlation with the experimental data.

Green cleanup processes for adhered organic fouling on solid surfaces can be successfully performed using a radical vapor reactor (RVR). The RVR can produce large concentrations of reactive oxygen species (ROS, e.g. singlet oxygen (¹O₂) and hydroxyl radicals (•OH)) and can expose them to objective materials. The RVR finds excellent utility in the fields of sterilization and surface functionalization. In this study, RVR is employed in a green cleanup of solid surfaces fouled by an organic polymer and a protein. The RVR produced ROS and removed the adhered organic polymers and proteins from the solid surface. The mechanism of how ROS react with fouling molecules was also elucidated by surface analysis. The greatest advantage of this green RVR cleanup process is that it discharges only air and water. The ROS production and exposure by the RVR successfully cleaned the adhered organic polymer and protein at ambient temperature and pressure without any chemicals. This high-quality, low-cost cleaning technology, which does not require much time and produces no hazardous waste, makes a great contribution to the cleaning industry.

Organic photovoltaics are expected to be suitable for indoor use because they have a relatively high conversion efficiency even at a low irradiance when compared to crystalline silicon photovoltaics. However, the performance measurement procedures of such photovoltaics under indoor light have not been established. In this paper, the illuminance fluctuation of a commercially-available light device originating from an AC power source has been precisely investigated, and the methods to decrease the influence of this fluctuation on the J-V measurements of the photovoltaics were proposed. Furthermore, an LED light irradiation apparatus was developed for more reliable and reproducible J-V measurements of the photovoltaics. This apparatus was found to provide an excellent time stability of illuminance and in-plane uniformity, both of which satisfy the Class A solar simulator permitted by IEC 60904-9. J-V characteristics of the photovoltaics determined by using this apparatus was coincident with that determined by using a commercially-available LED light driven by an AC power source. Finally, it was confirmed that the conversion efficiency of a DSC is greater than that of a Si photovoltaic under the indoor light condition, the illuminance of which is several tens to hundreds lx.