The utilization of boron doped diamond (BDD) as an electrode material for the electrochemical reduction of CO₂ has been studied in recent decades. Its stability and ability to suppress hydrogen evolution makes it an attractive choice for the electrochemical reduction of CO₂. It has been confirmed that, when using a bare BDD electrode, very high selectivity and productivity can be achieved in the production of formic acid. Moreover, by modifying the surface of a BDD electrode with copper (Cu) particles, we have been able to produce compounds with a high number of carbon atoms. In this article, we summarize the results of our work on the electrochemical reduction of CO₂ using BDD electrodes, with the specific aim of the producing compounds with a high number of carbon atoms.

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.

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.

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.