The cover art is attributed to an article entitled “Functionalization and Applications of Molten Salts and Ionic Liquids” commemorated for the Society Award of Electrochemical Society of Japan (Takei Award) by Professor Rika Hagiwara. The author has long been studying properties and functionalization of various molten salts and ionic liquids, mainly fluorine‐containing species. In this article, a synthesis process of 1‐ethyl‐3‐methylimidazolium fluorohydrogenate which has the highest ionic conductivity among the ionic liquids reported so far, and development of various electrochemical devices such as fuel cells are reviewed.

The author and co-workers developed 1-ethyl-3-methylimidazolium fluorohydrogenate (fluorohydrogenate anion: (FH)_nF⁻), which had the ionic conductivity of 100 mS cm⁻¹, the highest among those of ionic liquids reported to date, and clarified its physical properties and structures. We also applied an ionic liquid consisting of cesium fluorohydrogenate to an electrolytic bath for the production of fluorine gas, F₂, by electrolysis of metal fluorides dissolved in it. We proposed a fuel cell using a fluorohydrogenate ionic liquid. In this system, the fluorohydrogenate anion acts through a unique mechanism transporting hydrogen and charge, regarded as a completely new type of fuel cell that can operate even at medium temperatures (>80 °C) under non-humidified conditions. We investigated the properties of binary and ternary mixed salt systems of alkali metal salts systematically using bis(trifluoromethylsulfonyl)amide (TFSA) and bis(fluoromethylsulfonyl)amide (FSA) as anions, which had high electrochemical stability and low melting points, and succeeded in developing a series of ionic liquids that enabled deposition of metallic lithium and sodium. We also fabricated a sodium secondary battery by combining a metallic sodium anode and a NaCrO₂ cathode to demonstrate excellent charge/discharge and cycle performance. We also succeeded in developing an ionic liquid with a low-melting-point (61 °C), NaFSA-KFSA binary system, and reported high-performance operation of Na-Sn alloy-based anodes and NaCrO₂ cathodes. We created an FSA-based inorganic-organic hybrid ionic liquids with wide temperature ranges and high lithium and sodium ionic conductivities. Taking advantage of the heat resistance of the ionic liquid, we achieved ultra-fast charge/discharge by operating it at 90 °C. Furthermore, we produced a 27 Ah prismatic battery using a hard carbon negative electrode and a NaCrO₂ positive electrode, and achieved weight and volume energy densities of 75 Wh kg⁻¹ and 125 Wh L⁻¹, respectively. The power density of this battery was 225 W kg⁻¹ and capacity retention rate at 500 cycles of charge/discharge was 87 %.

Enzyme electrodes, which integrate enzymatic and electrode reactions, are essential components in electrochemical biosensors and biofuel cells. Specific materials are necessary to achieve efficient electron transfer (ET) between the enzyme and electrode in these devices. This review focuses on several key developments of materials in the field of enzyme electrode. First, it explores the development of redox mediators, which facilitate ET between the enzyme and electrode. Next, the co-immobilization of enzymes and redox mediators on electrode surfaces for enhancing their stability and efficiency is discussed. The use of porous carbon materials, which provide a large surface area for enzyme immobilization and thus improve the bioelectrocatalytic performance, is then reviewed. Finally, the influence of concentrated salts on the enzyme electrode reactions, potentially affecting their interactions among enzyme, mediator, and water, is explored. By examining these advancements, the review highlights the progress and future potential in the development of high-performance enzyme electrodes for various applications.

In biological metabolism, the conversion of materials and energy involving hydrogen, carbon, and oxygen occurs efficiently under ambient temperature and pressure conditions. For instance, in photosynthesis, the H₂O oxidation and the NADP⁺ reduction make a pair, ultimately transforming CO₂ into organic substances. In oxygenic respiration, the oxidation of organic substances is coupled with the O₂ reduction, providing the energy necessary to sustain life processes. Hydrogen-oxidizing bacteria synthesize organic substances from CO₂, utilizing H₂ as the reducing agent. Understanding the nature of these material transformations in biological processes, which are based on redox reactions, is valuable for advancing electrochemical technologies, including fuel cells, H₂O electrolysis, CO₂ electrolysis, air batteries, and artificial photosynthesis. The author has extensively conducted fundamental researches on electrode reactions, including H₂ evolution, H₂ oxidation, O₂ reduction, H₂O oxidation, CO₂ reduction, and oxidation of organic substances, based on these basic concepts. The individual studies cover a broad spectrum of fields. While these studies are interdependent for the author, it is difficult to systematically compile these interdisciplinary research results into a single article. Therefore, the focus of this article is put on presenting the results of fundamental researches on lithium-air batteries, which is a part of the electrode reaction research that the author has been conducting.

Magnesium rechargeable batteries (MRBs) are ones of promising low-cost, safe, and high-energy post-lithium-ion batteries. However, critical issues still lie both in electrolytes and cathodes, despite 20 years since the first report of prototype MRB. Difficulty in the utilization of high-energy oxide-type cathode materials derives from their low ion-diffusion and electrical conductivity, leading to low reversible capacity. Herein, various nanotechnologies for spinel oxide cathode materials and cathode/electrolyte interface are displayed to operate the MRB, even at room temperature conditions. Surface-modified ultraporous spinel nanoparticles exhibit electrochemical magnesium intercalation, achieving the theoretical capacity. In addition, the decomposition reaction of the ether-based electrolyte at the highly reactive cathode surface is suppressed by the Mg²⁺-conductive coating layer. These cathode material designs will open up new technologies for the practical application of room-temperature MRB.

An electrolyte is an important component of rechargeable batteries. The Debye–Hückel theory is applicable to liquid electrolytes with salt concentrations up to approximately 0.01 mol/L. Thus, electrolyte solutions with approximately 1 mol/L salt concentration are considered “highly concentrated” from the perspective of the classical solution theory. However, electrochemical devices use them at these concentrations, because their ionic conductivity reaches the maximum level across the entire concentration range. Although a salt concentration of 1 mol/L is considered very high, recent studies have indicated that the use of further “super-concentrated” electrolytes, typically those with concentrations exceeding 3 mol/L and also known as “molten solvate” or “solvent-in-salt” can exhibit unique reaction behaviors in various electrochemical systems, including Li-ion batteries. This review examines the characteristics of such “super-concentrated” electrolytes.

The lithium superionic conductor Li₁₀GeP₂S₁₂ (LGPS) and its crystal structure analogues can be applied as solid electrolytes for all-solid-state lithium-ion batteries due to their exceptionally high ionic conductivities, which exceed 10 mS cm⁻¹ at room temperature. This paper reviews the author’s publications into LGPS and its derivatives. Initially, its unique crystal structure is described with a particular focus on the ionic conduction mechanism. Subsequently, the syntheses of various structural analogues of LGPS are summarized, focusing on the phase diagram containing LGPS, as well as highlighting Li_9.54[Si_0.6Ge_0.4]_1.74P_1.44S_11.1Br_0.3O_0.6, which was discovered during high-entropy material design. Finally, the development and understanding of all-solid-state batteries that incorporate the developed solid electrolytes are described. This review is notable due to the importance of discovering novel solid electrolytes for further clarification of the superionic conduction mechanism and for improving battery performances.

The coupling between bipolar electrochemistry and electrochemiluminescence (ECL) technologies has resulted in a fascinating field of the academic research, because the advantages of both techniques are often emphasized in their combined form. For instance, the light emission resulting from an electrochemical reaction typical of the ECL process can be exploited for reporting redox events occurring on a wireless bipolar electrode (BPE). After introducing the concept of bipolar electrochemiluminescence, this comprehensive paper summarizes the most recent research results of the author in the aforementioned research area at Tokyo Institute of Technology during her multi-year stay as overseas researcher. These results include the development of an analytical tool based on ECL imaging to map the potential distribution on different types of bipolar electrochemical cells, and the characterization of different conducting polymer materials based on their ability to promote the ECL reaction. Such findings are assessed in relation to the current literature scenario, whereas future perspectives of this fascinating field of research are also discussed at the end of this manuscript.

The role of electrolyte is very important in improving the performance of batteries, such as higher energy density and higher power. This comprehensive paper summarizes two main topics: research on the redox resistance of electrolyte solutions, which is essential for high energy density, and research on improving the reaction rate of active materials, which is essential for high power. With regard to the redox resistance, the concentration effect and local pH in the vicinity of the electrode were focused on. As for the improvement of the active material reaction rate, a novel method for analyzing the current-potential curve using the concept of electrocatalytic reaction was proposed. The availability of the conductometric titration for analyzing complexes in the electrolyte was also presented. These fundamental studies, based on electrochemical theory and methods from a thermodynamic and kinetic point of view, are important for the design of battery electrolytes and for the interpretation of the phenomena.

Oxygen reduction reaction (ORR) in aqueous zinc chloride solution results in an electrodeposition of highly crystallized zinc oxide (ZnO) thin films. Due to the mechanistic complexity of the ORR and the experimental difficulties associated with the precipitation of ZnO to change the electrode surface, kinetic analysis of the ORR during the electrodeposition of ZnO has not been achieved. In this study, we have employed a rotating ring-disk electrode (RRDE) and followed Nabae’s approach to tackle this goal. While full 4-electron ORR is predominant at the surface of ZnO, as long as the electrolyte solution is free of Zn²⁺, 2-electron ORR down to hydrogen peroxide (H₂O₂) becomes the main path during the electrodeposition of ZnO, although, even slowly, further reduction of H₂O₂ and direct 4-electron ORR do operate. Having pre-determined the kinetic parameters for the H₂O₂RR, we successfully came up with Koutecky-Levich expression of elemental steps of the ORR along with the determination of their kinetic parameters. Thus proposed comprehensive and universal model is able to describe quantitative contributions of individual steps of the ORR to the electrochemical precipitation of metal oxide thin films under given overpotential and diffusion flux.

It was found that diphenylacetylene was synthesized by using electrochemical reduction with Mg electrodes, coiled with Cu wires. The electrochemical reaction of tetrachloroethylene and bromobenzene in LiClO₄/THF afforded diphenylacetylene in up to 38 % yield. The reaction took place in the absence of Pd catalyst. The scope and limitations, and investigation of reaction mechanism were also studied.

We report the temperature dependence of the average structures of 0.4Li₂MnO₃–0.6LiMn_1/3Ni_1/3Co_1/3O₂ during the 5th discharge process, where the structures are characterized via neutron and synchrotron X-ray diffraction analyses. In the cycle tests at room temperature (25 °C) and high temperature (60 °C), the 0.4Li₂MnO₃–0.6LiMn_1/3Ni_1/3Co_1/3O₂ electrode delivered a high capacity of over 280 mAh/g in the 5th discharge process within the voltage range from 2.5 V to 4.8 V vs. Li/Li⁺ at the high temperature, which was greater than about 250 mAh/g at room temperature. However, the cycle characteristics during high-temperature operation were inferior to those during room temperature operation. To clarify the cause of the change in the electrode characteristics, electrodes were prepared in the pristine state, the 5th charge state, and the 5th discharge state and their average structural change was evaluated by Rietveld analysis. The Rietveld analysis showed that the distortion of parameter for the M–O₆ (M = transition metal) octahedra was larger for the samples cycled at 60 °C. A valence evaluation was also performed on the basis of the peak shifts in the Mn, Ni, and Co X-ray absorption near edge structure spectra. The results suggested that the temperature change for Mn greatly affected the distortion, especially at 60 °C. The aforementioned results revealed that changing the operating temperature led to a large change in the structure of 0.4Li₂MnO₃–0.6LiMn_1/3Ni_1/3Co_1/3O₂, mainly related to Mn, which might affect battery performance.

This paper developed a one-dimensional multiphase model to analyze the cold start behavior of fuel cell, focusing on the effects of various parameters such as loading rates, maximum loading current densities, and coolant flow rates. The findings reveal that the loading rate significantly influences the ice volume fraction and the duration required for cold start, while exerting a lesser effect on voltage and the voltage variation. Cold start fail at maximum loading current densities below 0.4 A cm⁻² due to a heating rate insufficient to outpace the rate of ice formation. However, as the maximum loading current density surpasses this threshold, successful start-ups are achieved, although the benefits to voltage, start-up time, and voltage variation diminish with increasing current density. The coolant flow rate primarily impacts the temperature distribution’s uniformity across individual cells, with negligible effects on overall cold start performance. Utilizing Gaussian process regression models for training and prediction reveals that elevating the loading rate and maximum loading current density can effectively mitigate ice formation. The relationship between these variables and performance metrics such as voltage, start-up time (The time required from the loading start to the successful start-up), and voltage variation is characterized by an initial increase followed by a subsequent decrease as the loading rate climbs. Conversely, the influence of the maximum loading current density on these metrics is comparatively modest.