Electrochemical coupling of redox enzyme reactions, called bioelectrocatalysis, has been attracting great attention over the last four decades. It has become an important technology that can be applied to a wide range of bioelectrochemical devices including biosensors, biofuel cells, and bioreactors. This article presents an overview of the basic concepts of steady-state catalytic waves of mediated- and direct electron transfer (DET)-type bioelectrocatalysis. Several equations that can be used for the analysis of steady-state waves are introduced. The analysis may provide important thermodynamic and kinetic parameters that can be used not only for performance evaluation of the devices but also for fundamental research on the enzymes. Important progress made on how to tune electrode surfaces and enzymes for DET-type reactions are presented. Applications to bioelectrochemical devices are also summarized with emphasis on the achievements recorded in our research group.

Rechargeable batteries are capable of storing electric energy on the basis of pairing electrochemical redox reactions to realize sustainable energy society in our future. Since lithium-ion batteries with the highest specific energy among all the practical batteries were commercialized in 1991, many studies on lithium insertion materials and their electrochemical characterization have been reported to achieve even higher energy density, longer cycle life, and safer lithium-ion battery technologies. It is quite fortunate that the author had an opportunity to contribute to the research and development of lithium battery materials since 1997. In particular, studies on the influence of dissolved metallic ions like Mn²⁺, Co²⁺, Ni²⁺, Na⁺, and K⁺ ions in electrolyte solution on graphite negative electrodes in lithium-ion batteries motivated the author to extend the research scope to electrochemical sodium insertion chemistry. Furthermore, the author’s research experiences as a postdoctoral fellow in Dr. Delmas’ group in FY 2003 and a remarkable oral presentation on alpha-NaFeO₂ electrode properties given by Professor Okada’s group in 2004 provided motivations and opened up new avenue toward the successful demonstration of non-aqueous sodium-ion batteries later in the career. Since 2009, the author’s research group has successfully demonstrated 3-volt class charge and discharge of a sodium-ion battery of a NaNi_1/2Mn_1/2O₂ // hard carbon cell and a brand-new potassium-ion battery of a K₂Mn[Fe(CN)₆] // graphite cell. The systematic studies of three different alkali-metal insertion systems synergistically induce deeper understanding and faster development of new materials for the next-generation rechargeable batteries.

Metal nanoparticles (NPs) that exhibit bright colors have been used as coloring agents of glass artifacts since before Christ. After the origin of the coloration, localized surface plasmon resonance (LSPR), was elucidated, their application has been expanded widely. This account focuses on electrochemical and photoelectrochemical applications of the plasmonic metal and compound NPs. Because LSPR is based on collective oscillation of free electrons in a NP in resonance with electric field oscillation of light, the resonant wavelength can be tuned electrochemically by transferring electrons to or from the NP. On this basis, we developed a new class of plasmonic sensors without gratings, and near infrared electrochromic smart windows. Meanwhile, resonant NPs exhibit charge separation at the interface with a semiconductor such as TiO₂. We studied this phenomenon, plasmon-induced charge separation (PICS), in particular about mechanisms including behavior of energetic carriers (hot holes and electrons), by using a wide variety of plasmonic NPs with different morphologies and compositions. Knowledge thus obtained was exploited for nanofabrication and development of near infrared PICS devices.

Polymer electrolyte fuel cells are attractive electrochemical devices for constructing a hydrogen energy society. The actual fuel cells consist of polymer membrane, electrocatalysts, gas diffusion layers, bipolar plates, and end plates. From the viewpoint of saving resources, herein we have described three kinds of research have been conducted. The first involved reducing the amount of anode Pt electrocatalyst. The second involved development of reaction selectivity in the electrocatalyst for a direct methanol fuel cell because the bipolar plates can be removed from the direct methanol fuel cell system. The third involved improvement of corrosion resistance of metallic bipolar plates using ferric stainless steel.

K-ion battery is a potential candidate as a next-generation battery with a high energy density, long cycle life, and high safety. To commercialize the battery, the improvement of safety is absolutely essential. We apply a nonflammable ionic-liquid electrolyte to a Sn₄P₃ electrode as negative-electrode for K-ion battery. The electrode achieves the excellent cycling performance with a discharge capacity of 365 mA h g⁻¹ over 100 cycles in the ionic-liquid electrolyte. Rate capability in the ionic-liquid electrolyte is almost the same as that in the organic-liquid electrolyte.

The antioxidative activity of the extracts from amaranth flower and leaves was characterized using 1,1-diphenyl-2-picrylhydrazyl (DPPH) method and cyclic voltammetry. It was found that the extracts prepared with amaranth flower (AM-F) showed an improved antioxidative activity compared to the extracts prepared with amaranth leaves (AM-L) using the conventional DPPH method. Furthermore, the antioxidative activities of AM-F and AM-L were evaluated using cyclic voltammetry as an electrochemical method. AM-F exhibited an improved electrochemical oxidation for active oxygen and a fast removal of active oxygen compared to AM-L. In addition, the CV analysis to evaluate antioxidant activity was found to be more accurate, compared to DPPH method.

This study explains the charge/discharge mechanism transformation from two-phase to a solid-solution reaction at over 4.5 V vs. Li/Li⁺ for monoclinic Li₃V₂(PO₄)₃ (LVP). An electrochemical characterization that combines galvanostatic cycling and intermittent titration technique confirms that this mechanism transformation occurs in LVP not only for the discharge reaction (lithiation) as previously reported but also for the charge reaction (delithiation), starting gradually within the initial 10 cycles. A similar type of electrochemical characterization of Li₃V_1.5Al_0.5(PO₄)₃ (LVAP) indicates that transition metal doping (25 at.% of Al³⁺) accelerates such mechanism transformation completed within an initial cycling. Additional cycling operation at a lower potential (below 4.3 V vs. Li/Li⁺) shows that the transformed state of LVP is metastable, as plateau recovery can be observed within 100 cycles, while LVAP maintains its solid-solution reaction over 1,000 cycles. In situ X-ray absorption spectroscopy (XAS) anal