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) analysis suggests that such mechanism transformation of LVP occurs through a change in the coordination environment of vanadium (from an octahedron to a distorted, possibly tetrahedral environment) evidenced by changes in pre-edge peaks in the V K-edge spectra. Such a change in the coordination environment is smoothed and stabilized by the introduced Al³⁺, possibly due to an enhancement of Li⁺ diffusivity in the LVP crystals.

The catalytic activity of H₃PMo₁₂O₄₀ (PMO) was enhanced by a chemical reduction treatment (CRT) and a subsequent electrochemical reduction treatment (ERT) with the hydrogen pump method for use in the hydrogen oxidation reaction in fuel cells, and the underlying mechanism was discussed. Both CRT and ERT with the hydrogen pump method can eliminate the suspicion that the improved catalytic activity was only due to precipitation of Pt in the vicinity of the catalyst. Catalyst of excellent quality was produced by keeping the anode containing PMO at a negative potential vs. SHE in acid for a given period during ERT. The activity per weight of the PMO catalyst after ERT is about 20% that of Pt catalyst (20 wt.% Pt/Vulcan XC72), and the numerical value of activity will change further in the future depending on experimental conditions. The new catalyst works well even in strong acids. Furthermore, a Pt free H₂-O₂ fuel cell using PMO and H₅PMo₁₀V₂O₄₀ as the anode and cathode, respectively, was constructed.

It is known that the deterioration of LiNi_0.5Co_0.2Mn_0.3O₂ is suppressed by inhibiting direct contact between the cathodes and the electrolyte by surface coating. In order to evaluate the influence of the electrode/electrolyte interface degradation, it is necessary to eliminate the influence of particle cracking. In this study, LiNi_0.5Co_0.2Mn_0.3O₂ particles with a small size (500 nm to 1 µm) without crack formation after charge-discharge cycling were synthesized by a spray pyrolysis method. Hard X-ray photoelectron spectroscopy was used to investigate the structural changes of the cathode. It was revealed that the cathode coated with lithium boron oxide (LBO) by an antisolvent precipitation method had high durability against the surface structure changes by the reduction of transition metal ions. The formation and dissolution of NiO occurred in the uncoated sample during cycling, but the formation of NiO was suppressed in the LBO-coated sample. It was considered that the structural changes of the active material surface during cycling led to an increase in surface resistance of the uncoated sample, which is the main reason for the capacity fading of the spray pyrolyzed LiNi_0.5Co_0.2Mn_0.3O₂ cathode particles.

Manganese-containing salen-type complexes of (R,R)-(−)-N,N′-Bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride (MnSl) were examined as a novel soluble catalyst in nonaqueous electrolyte solutions for lithium air secondary batteries (LABs). The LAB cells with MnSl exhibited a larger first discharge capacity and better cycle performance (893 mAh g⁻¹, 738 mAh g⁻¹ up to 10 cycles) than those without MnSl. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) conducted during the discharge/charge cycle showed deposition and decomposition of the discharge product, Li₂O₂, on the surface of the air electrode. Cyclic voltammetry results suggested that MnSl promotes the oxygen reduction reaction and oxygen evolution reaction because of its high reactivity with O₂.

The degradation of charging/discharging capacities in the rate-performance test of lithium iron phosphate (LFP) cathodes with different loading amounts of an active material on both sides of a current collector (i.e., “unbalanced” LFP/LFP cathodes) in a laminated cell (typically composed of anode/separator/unbalanced cathodes/separator/anode) was not observed actually at low C-rates (e.g., 0.1 C). However, the rate-performance data obtained at high C-rates (e.g., >5 C) indicated that the imbalance of the loading amounts of an active cathode material on both sides of an Al current collector causes a significant capacity degradation. We have found that it is possible to prevent the capacity degradation observed at high C-rates by holing the unbalanced LFP/LFP cathodes in a micrometer-sized grid-patterned way (the percentages of the holed area are typically several %) using a pico-second pulsed laser: The non-holed unbalanced LFP/LFP cathodes exhibited a considerable capacity degradation at C-rates which are, for example, larger than 5 C, while the holed ones showed no degradation in capacity even at high C-rates (e.g., 5–20 C). Forming micrometer-sized grid-patterned holes in the LFP/LFP cathodes leads to an improved capacity and high-rate performance of their charging/discharging processes.