The cover art celebrates the 66th Special Feature entitled “Novel Aspects and Approaches to Experimental Methods for Electrochemistry,” published in Electrochemistry, Vol.90, No.10 (2022), which has been recognized as a Distinguished Downloaded Articles collection. The present commemorative cover art was created and published in April 2026.

This special feature originates from the long-standing Electrochemistry Workshop organized by the Kansai Branch of the Electrochemical Society of Japan, and represents a significant milestone in the dissemination of educational and practical knowledge in electrochemistry. As described in the editorial of this issue, the workshop materials—traditionally limited to participants—were transformed into openly accessible academic resources for the first time in its more than 50-year history, thereby expanding their value beyond temporal and geographical constraints.

The collection comprises comprehensive tutorial-style articles covering fundamental and advanced topics such as electrode potentials, electrochemical polarization, cyclic voltammetry, impedance spectroscopy, and in situ/operando techniques. These articles integrate theoretical understanding with practical measurement methodologies, reflecting the interdisciplinary and application-oriented nature of electrochemistry. The exceptionally high number of downloads across these articles demonstrates their broad impact and continued relevance to both academic researchers and industrial engineers.

The cover illustration symbolically represents key experimental concepts and analytical approaches in electrochemistry. Together with the photograph of the contributors involved in this special feature, the design emphasizes the collaborative effort of the electrochemical community and the transmission of knowledge across generations.

The Editorial Board outlines the history of publication by the Electrochemical Society of Japan as it celebrates its 90th anniversary. The history of the journal is told by the Society’s strong interest in publishing and its international development in line with current trends. The growing presence of the journal is described. This issue will also include the Comprehensive Papers by the winners of the ECSJ awards. The rationale for these awards will also be explained.

Research and development of Lithium Ion Battery (LIB) have been extensively performed based on material science and cell technology. In order to accelerate the development of LIB, a multi-scale researches have to be conducted under one roof. Here, several researches from nm scale to m scale on LIB were introduced to discuss an importance of multi-scale research. Then, the new platform for LIB development was proposed based on our several researches. (1) Interfacial analysis between cathode and electrolyte in LIB by using in-situ Fourier Transform Infrared method, (2) Preparation of LiFePO₄ (LFP) with carbon coating, (3) Interfacial analysis on lithium metal anode for solid electrolyte interphase (SEI), (4) Preparation of 3 dimensionally ordered macroporous separator and its application to lithium metal battery, (5) Single particle measurement for evaluation of composite electrodes, (6) Failure mode analysis of LFP/Graphite cell.

Boron-doped diamond (BDD) electrodes are next generation electrode materials and their electrochemical applications have been actively developed in recent years. They are expected to be useful electrode materials for improving the environment and for bio-medical applications. Here, examples of practical applications as electrochemical sensors, the development of in vivo real time measurements, and electrochemical organic synthesis using BDD electrodes are briefly introduced. In the second part, our recent work on the production of useful chemicals by means of the electrochemical reduction of CO₂ using BDD electrodes is described. The work has attracted particular attention for its potential contribution to carbon neutrality and carbon recycling.

Li salts and polar solvents form solvates, and certain solvates have low melting temperatures and remain in a liquid state at room temperature. Liquid-state solvates exhibit ionic conductivity and can be used as electrolytes in lithium batteries. The author and co-workers have systematically studied the interactions of Li⁺ ions with solvents and anions, Li⁺-coordination structures, thermal properties, transport properties, and electrochemical properties in molten-solvate electrolytes. In molten solvates, almost all solvent molecules are coordinated to Li⁺ ions, and uncoordinated (free) solvents are rare. Additionally, anions are involved in the coordination of the Li⁺ ions. The molten solvate electrolytes show non-flammability and negligible vapor pressure at room temperature because of the extremely low concentration (activity) of the free solvent, which can improve the thermal stability of Li batteries. The low activity of the free solvent results in a wide electrochemical window of the molten-solvate electrolytes, thereby suppressing undesired side reactions in Li batteries. The activity of the free solvent in the electrolytes significantly affects the electrochemical reaction processes, such as the reduction reaction of sulfur (S₈) in a Li–S battery and the oxygen reduction reaction (ORR) in a Li–air battery. The solubility of the reaction intermediates of the S₈ cathode and the ORR decreases with the decrease in solvent activity, which enables the highly efficient charge–discharge of Li–S and Li–air batteries. In molten solvates, Li⁺ ions diffuse and migrate by exchanging ligands (solvents and anions). Certain molten-solvate electrolytes show high Li⁺ ion transference numbers over 0.5, and these high transference numbers are useful in mitigating the concentration overpotential during the charging and discharging of Li batteries at high current densities.

The supporting electrolyte is an essential component of electrochemical reactions. Although there have been many reports on the influence of the type of electrolyte and its concentration on reaction efficiency in electrosynthesis, very few reports have systematically discussed the reasons for such effect. In several reaction systems, we have found that the coordination of anions from the supporting electrolyte to cationic organic species generated in electrochemical oxidation dramatically changes the reaction efficiency. In this comprehensive paper, we review these case studies, generalize the findings learned from them, and provide guidelines for strategic electrolyte design.

Renewable energy resources and rechargeable batteries are key to establishing a carbon-neutral society. Lithium-ion batteries (LIBs) have been widely used in portable electronic devices for the past 30 years. However, the further spread of large-scale batteries is essential in the household and industrial sectors, which drives the research and development of technologies beyond LIBs. Since ionic liquids are safe and confer unique physicochemical properties, several next-generation batteries utilizing ionic liquid electrolytes have been researched. Sodium-ion and potassium-ion batteries show promise in overcoming the potential problems of LIBs related to the uneven distribution of lithium and cobalt resources. Fluoride-shuttle batteries deliver significantly higher theoretical energy densities compared to current LIBs. Nevertheless, many issues remain unresolved for the practical application of these batteries. This comprehensive paper provides several research topics on next-generation rechargeable batteries utilizing ionic liquids and various charge carriers, unveiling their novelty, the issues to be solved, and future research directions.

Design, fabrication and characterization of semiconducting materials including metal oxides and sulfides are the most important to use them as functional materials such as catalysts, photocatalysts, electrodes and the others, though there seems to be no general principles to get the materials of the highest performance. This may be because there are so many structural parameters to be optimized while we do not know even how many parameters are there to be considered. In this review, the trial of the author’s research team for the development of functional semiconducting materials focusing on the control of their structure, especially the shape of particulate materials; materials introduced in this paper are bismuth tungstate, titanium(IV) oxide, solid solution of zinc sulfide and silver indium sulfide, and composites of cysteine and medal-metal ions.

The Kansai Branch of the Electrochemical Society of Japan publishes a collection of papers in Electrochemistry, which serve as a commentary to the 51st Electrochemistry Workshop. This attempt is motivated by the fact that the domestic seminars are now widely publicized through the on-demand event triggered by COVID-19. This preface consists of the significance of the publication and an introduction of the lecturers as a part of special future for “Novel Aspects and Approaches to Experimental Methods for Electrochemistry.” in this issue of Electrochemistry.

Electrochemistry deals with the interrelationship between electrical and chemical energy. Various potentials appear in electrochemistry and pertain to one another in practical cells. Understanding the electrode potential is an important step in acquiring basic knowledge of electrochemistry and extending it to specific applications. This comprehensive paper outlines the fundamentals and related subjects of electrode potentials, including electrochemical cells and liquid junction potentials. Aqueous solution systems are ideal for connecting the theoretical background of electrode potentials to practical electrochemical measurements. Accordingly, the basic electrode chemistry in aqueous systems is described in this paper, as well as several advanced concepts introduced in recent studies.

This comprehensive paper, Electrode Potentials Part 2, is a continuation of Electrode Potentials Part 1: Fundamentals and Aqueous Systems. Determining the electrode potential is crucial for understanding the nature of the electrochemical properties of materials or systems; however, an accurate evaluation of the potential of a target electrode has always been a challenge. The electrode potential can be used to predict the reaction mechanisms in electrochemistry and can be directly applied to the study of electrochemical applications. This paper introduces the methodologies and strategies for measuring electrode potentials in nonaqueous and solid-state electrolytes, including organic solvent electrolytes, ionic liquid electrolytes, and oxide and sulfide solid electrolytes. Experimental details are described for basic to state-of-the-art strategies, focusing on practical methods and know-how.

Polarization measurement is one of the major electrochemical methods used by electrochemists. The changes in current/potential with time at constant potential/current are investigated. The outcomes of these observations can be used to plot a current–potential curve. Therefore, it is important to understand the relationship between the three parameters: electrode potential, current, and time. In this paper, we described the fundamentals of the polarization, especially the current–potential curve (Butler–Volmer equation) and mass-transfer. In addition, the concept of polarization in corrosion reactions is explored.

In this paper, examples of polarization measurements with a battery, electric double-layer capacitor, and fuel cell are discussed. The analysis of the polarization of a real cell and analytical methodology are described. In battery section, the rate capability is examined in detail and the origin of the different rate properties is discussed. Charge/discharge tests and self-discharge measurements are performed in the electric double-layer capacitor. Detailed measurement conditions and data analysis methods are introduced in the fuel cell section. The contents indicated in each section are very useful for interpreting the obtained experimental data.

“Cyclic voltammetry” is one of the most common electrochemical techniques, not only for electrochemists but also for researchers in other fields. However, the principles of cyclic voltammetry are very complex; beginners and even experts may struggle with their application. Therefore, we explain the fundamental principles in Part 1 and introduce the practical issues associated with recent trends such as fuel cells, capacitors, and sensors in Part 2. In this comprehensive paper, we focus on the electrochemical reversibility in cyclic voltammograms classified as reversible, quasi-reversible, and irreversible processes, which are useful for obtaining information on the reaction rates of electrodes. We also explain the relevant basic principles, experimental setup, and ideas of background current.

This paper “Part 2: Surface Adsorption, Electric Double Layer, and Diffusion Layer” is the applicational part and is a continuation of “Part 1: Fundamentals” (https://doi.org/10.5796/electrochemistry.22-66082). This part explains the principles of cyclic voltammetry on adsorption, electric double layer, and diffusion layer with microelectrodes. Moreover, recent trends, such as polymer electrolyte fuel cells (PEFCs), electric double layer capacitors (EDLCs), and sensors, are highlighted. The relationship between the electrochemical surface area (ECSA) and electric quantity estimated on the basis of cyclic voltammograms is explained in the chapter on adsorption, where the adsorption of hydrogen and carbon monoxide on Pt is discussed. The following chapter introduces the pseudo-capacitance of activated carbon, which exhibits characteristic capacitor-like behavior, with Faradaic current flow on the surface. Finally, an explanation concerning the effect of diffusion-limiting currents on two types of microelectrodes and a practical case of enzyme sensing using a microelectrode is included.

Electrochemical impedance spectroscopy (EIS) enables the examination of the electrochemical nature of electrodes and electrochemical cells by applying an alternating voltage (or current) and measuring the resulting current (or voltage). The resistance and capacitance components of the electrode can be evaluated by applying an AC voltage and changing the frequency. In particular, analysis using the equivalent circuit can determine important parameters related to the electrochemical reaction of the electrode, such as the charge transfer resistance, electric double-layer capacitance, and Warburg impedance. Moreover, the internal resistance of the cell can be divided into resistances caused by the positive electrode, negative electrode, and electrolyte. Because of these advantages, EIS is a powerful technique used for basic research, such as in identifying the rate-determining step of an electrochemical reaction, and also for applied research, such as characterizing electrochemical devices (e.g., batteries and capacitors). In this paper, the concept of impedance, which represents the relationship between the AC voltage and current, is first explained; then, the AC characteristics of various circuit elements used in equivalent circuits, which are essential for understanding EIS, are described. Finally, treatments of more complex circuits based on transmission-line models (TLMs), which are used to represent equivalent circuits of porous electrodes, are presented. Analyses based on TLMs are the foundation for understanding electrodes for practical applications because porous electrodes are usually used in electrochemical devices.

Electrochemical impedance spectroscopy (EIS) is widely used for the analysis of various electrochemical devices, as it can quantitatively evaluate the main kinetic parameters related to electrochemical phenomena by analysis using equivalent circuits. This paper describes practical applications of EIS, along with EIS measurement and analysis methods for solid electrolytes, Li-ion batteries (LIBs), and electric double-layer capacitors (EDLCs). In all applications, it is necessary to properly measure the impedance data for an adequate equivalent circuit analysis. Therefore, after presenting the backgrounds of EIS applications in the Section 1 (Introduction), the experimental cautions in the measurements are discussed in detail in Sections 2–4. Section 2 (“EIS for Solid Electrolytes”) presents practical examples of measurements for accurate data, as the EIS analysis of solid electrolytes requires impedance data in the high-frequency range above 1 MHz. Section 3 (“EIS for Lithium-Ion Batteries”) describes a method of separating the internal resistance into the resistances of the positive and negative electrodes and electrolyte resistance, as the output power capabilities of LIBs are frequently evaluated based on an internal resistance. In particular, a symmetrical cell technique enabling measurements of the impedance data only for the positive or negative electrode is demonstrated. As described in Section 4 (“EIS for Electric Double-Layer Capacitors”), the excessive and unwanted impedances arising from instruments and cells must be suppressed as much as possible for appropriately measuring the correct EIS of EDLCs, because the resistance of EDLCs is very small. Therefore, the experimental setup that should be considered in EIS measurements for EDLCs leading to disturbed impedance data is discussed, along with the effects of this scenario on the impedance data. Finally, we summarize our conclusions in Section 5 (Summary).

Spectroscopic and microscopic techniques are complementary to electrochemical studies because electrochemical data consists of current, voltage and time, and has no direct information concerning the chemical structure of active species. Hence electrochemical in situ/operando spectroscopy and microscopy become powerful tools for identification of the electrochemically active species during the electrochemical reactions. The present comprehensive paper provides the fundamental theory, cell design concepts, and measurement tips of various spectroscopic and microscopic techniques. X-ray absorption spectroscopy, infrared spectroscopy, surface plasmon resonance, Raman spectroscopy, confocal microscopy and electron microscopy are covered in the present paper. The introduced cell design becomes the critical part for chemists and materials scientists to start these measurements. Several advanced techniques from recent studies are also introduced.

In situ/operando techniques for electrochemical systems are useful for understanding the electrochemical reactions, as we presented in Part 1. Here we present a series of in situ/operando techniques for battery applications. Now the in situ/operando techniques presented in this paper has become powerful tools for the development of advanced battery systems such as Li-ion batteries, solid-state batteries, and other beyond Li-ion batteries. In the present paper we introduce the in situ/operando cell design of each measurement technique and discuss how we apply each technique for in the advanced battery materials development.

Methods for measuring and analyzing the electrical conductivity of electrolyte solutions are reviewed. The accuracy of conductivity measurements, which have been around for more than 100 years, depends on the physical properties related to ionic conductivity, especially the concentration of the reference material, the electrical signal at the time of measurement, and the temperature, each of which has been redefined each time according to changes in definitions and standards. This article summarizes elemental factors in electrical conductivity measurement, including the concentration and traceability of potassium chloride as a reference material, the history of the bridge method and types of measuring cells, and important notes and recent topics regarding the concept of temperature dependence.

We have established a method for measuring the zeta potential generated at the interface between a nonaqueous electrolyte solution utilized in LiClO₄/propylene carbonate (PC) electrolyte and lithium cobalt oxide (LiCoO₂) by the streaming potential method. Since the surface potential of the metal oxide dispersed in the aprotic nonaqueous solvent contains only a very small amount of water-based potential-determining ions such as H⁺ and OH⁻, the potential is determined by the adsorption of the solvated electrolyte itself. Unlike aqueous systems with potential-determining ions that exhibit specific adsorption, it took a very long time until the equilibrium state of the ion distribution near the solid surface was reached and the potential stabilized, with a time constant that amounted to about 5 minutes. Therefore, a detailed analysis of the change over time of the potential after the pressure setting showed that the predictive potential showed a change over time with almost a single relaxation having certain time constant. The measurement time of the streaming potential was corresponded to about the time constant, and the resulting zeta potential showed an anomalous concentration dependence as a maximum around 1.0 mol L⁻¹ PC and a minimum at 1.5 mol L⁻¹ PC for the concentration of each solution.

Steady-state experiments are often conducted to understand complicated cases in chemistry, since the kinetics does not have a time valuable and allows simple modeling of the reactions. The reciprocal of the overall rate of sequential steady-state reactions is often given in the reciprocal sum formula: sum of the reciprocals of the rates of the hypothetical rate-limiting processes at the individual stages. In this paper, the reciprocal sum relationship is generalized for sequential multi-step steady-state reactions, and the importance and usefulness of the concept is shown by applying it to describe several typical steady-state systems in enzyme reactions and voltammetry using rotating disk- and ultramicro-electrodes.

Rotating disk voltammograms of electrocatalytic reactions were often analyzed on a model of the totally irreversible reaction. The problem with the conventional method is pointed out, and the validity of an analysis method on a model of the electrocatalytic reaction is demonstrated for oxygen-reduction reaction (ORR) as an example. Rotating disk voltammograms of ORRs sometimes show gradual change in the limiting current region called residual slope. The phenomenon has been explained on a random distribution model in which the catalytic sites communicate in long-range electron transfer with the electronic conductors that locate at distances (z), and are uniformly distributed with respect to z. Observed data of an ORR were well reproduced by non-linear least squares analysis on the random distribution model. The result of the analysis is briefly discussed.

Chemical reaction equilibrium was often introduced based on the kinetic assumption called the mass action law. In this paper, problems in the kinetic concept are pointed out and several equilibrium constants are derived on the concept of thermodynamic balance using chemical potential. Points to note in the pressure dependence of several equilibrium constants are emphasized. An important relationship between the reaction quotient and the entropy of mixing and the importance of sigmoidal characteristics of equilibrium are comprehensively introduced. Comments on steady-state current–potential curves in reversible multiple electron transfer reactions are also detailed.

The signal detection of quantitative ¹H nuclear magnetic resonance (¹H qNMR) for the amount of water in HCl and KOH aqueous solutions at low pH (pH −1.2) and high pH (pH 16) was discussed, and an “anomaly of decreasing ¹H NMR signal intensity” was observed. After adding significant amounts of acid (H⁺) or base (OH⁻) to the solution for pH regulation, the number of ¹H nuclei observed by ¹H NMR significantly decreased at pH ≤ 2 and ≥ 13. The mobility and the activity coefficient of some or all the water molecules hydrated to H₃O⁺ significantly decreased; hence, they could not be detected by ¹H qNMR. The remarkably strong electrostatic interactions of the solvent with H₃O⁺ and OH⁻ significantly reduce the activity coefficient of the solvent by restricting the solvent molecule to the vicinity of the ion. These results are comparable to those reported for vapor pressure measurements and can be attributed to a decrease in activity. H⁺ (H₃O⁺), which has a relatively small ionic radius, has a significant effect on the solvation structure and hydrogen-bond network owing to strong electrostatic interactions with the solvent. Subsequently, significant reduction in the ¹H NMR relaxation time of the water molecules and signal intensity, along with a low magnetic field shift in the ¹H NMR signal were also observed in the strongly acidic and strongly basic regions.

Oxygen reduction and evolution reactions (ORR and OER, respectively) of perovskite-type La_0.8Sr_0.2CoO₃ were characterized using two-dimensional model electrodes with different reaction planes (001), (110), and (111). Synthesized by pulsed laser deposition, these thin (30 nm) and flat (roughness < 1 nm) electrodes can reveal the reaction plane dependence of the ORR activity. From steady-state polarization measurements in KOH (aq.), the ORR activity was the highest on the (001) film during the first ORR/OER cycle, and it decreased significantly during the second cycle. In-situ synchrotron X-ray diffraction clarified crystal structure changes in the bulk and surface regions of La_0.8Sr_0.2CoO₃, and these changes are associated with forming oxygen defects during the initial electrochemical process. Furthermore, the La_0.8Sr_0.2CoO₃ surface partially decomposed upon reacting with the aqueous solution, as clarified by hard X-ray photoemission spectroscopy. Therefore, the interfacial structures formed in the electrochemical reaction field is important for enhancing ORR and OER activities.

The lithium/vanadium tetra sulfide (Li|VS₄) battery can be considered a promising next-generation battery because of its high theoretical energy density, and it is expected to overcome the problems inherent in Li-S batteries. However, the charge/discharge cycle degradation of the Li|VS₄ battery strongly depends on the choice of the organic solvent. We investigated the equilibrium potentials of the decomposition reactions involving the VS₄ electrode and organic solvent molecules using the density functional and classical solution theories. We first modelled the decomposition reactions between VS₄ and different organic solvent molecules, such as ethylene, dimethyl, and propylene carbonates (EC, DMC, and PC). Next, we calculated the change in the Gibbs free energy of the decomposition reaction by assuming a thermodynamic cycle and estimated the equilibrium potential vs. Li/Li⁺. From the equilibrium potential, the overpotential of the DMC against the potential plateau of the Li|VS₄ battery is negative and shows the lowest value in the considered solvents. This result suggests that the battery cycle with DMC deteriorates more quickly than that with EC and PC. This suggestion explains the experimental tendency of battery cycle degradation and will be a useful guide for improving the electrochemical performance of Li|VS₄ batteries.

The dissolution of lithium polysulfide into electrolytes is a major limitation of Li-S batteries that hinders their practical applications. Lithium polysulfide dissolution is suppressed by using sulfur-complexed microporous-carbon cathodes in lithium–sulfur batteries. This study reports the use of a localized high-concentration electrolyte (LHCE), in which a sulfolane (SL) high-concentration electrolyte is diluted with hydrofluoroether (HFE), for the strong suppression of lithium polysulfide elution. A previously reported sulfolane electrolyte in the solvated state exhibits a short cycle life of eight cycles, whereas the sulfur-containing microporous-carbon cathode with a sulfolane-based electrolyte prepared in the ratio of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) : SL : HFE = (0.572–0.953) : 2.00 : 1.26 (mol) exhibits a discharge capacity of more than 550 mAh g⁻¹, even after 50 charge–discharge cycles. Increasing the LiTFSI ratio decreases the response current during discharge, as indicated by cyclic voltammetry. A sulfur cathode with microporous carbon (AZC) shows equivalent performance on decreasing the amount of solvated LiTFSI. This study could guide the design and development of electrolytes for sulfur-containing microporous-carbon cathodes.

As a way to explore methods to capture and transform CO₂ into useful carbon materials, the electrochemical mechanism of carbonate ion reduction in molten Li₂CO₃–K₂CO₃ eutectic mixtures is studied by cyclic voltammetry, chronopotentiometry, and chronocoulometry. Evidence indicates CO₃²⁻ to undergo reduction to produce solid carbon before the alkali metal ions Li⁺ and K⁺. The electrode reactions consist of a diffusion-controlled, totally irreversible process comprising a one-step electron transfer. Notably, the described electrochemical process produces carbon sheets, submicron carbon tubes, or submicron carbon particles, depending on the applied cathodic overpotential. When O²⁻ ions are released as part of the electrochemical reduction of CO₃²⁻, some of these ions immediately combine with Li⁺ to form insoluble Li₂O, which are disadvantageous processes from the standpoint of CO₂ capture in the cathodic region. Results suggest thus that a type of molten salt electrolyte in which alkali metal oxides are highly soluble should be employed for effective molten salt CO₂ capture and electrochemical transformation.

Lithium metal batteries (LMB)-laminate-type full cells were prepared by combining a high-concentrated electrolytes lithium bis(fluorosulfonyl)imide (LiFSI)/ethylene carbonate (EC) and the 3-Dimensionally Ordered Macroporous (3DOM) Polyimide (PI) separator, and the cycle characteristics were compared with the full cell using the conventional polypropylene (PP) + polyethylene (PE) separator, which is widely used in lithium ion batteries. The capacity of the laminate-type cell using the PP + PE separator decreased to half of the initial capacity after 50 cycles, while the cell using the 3DOM PI separator kept its initial capacity and showed stable performance even after 50 cycles.

We also investigated pore size effect of the 3DOM separators on cycle characteristics. The cell using 2500 nm pore size 3DOM separator showed quickly decreasing capacity after the first cycle, while those cells using 300 nm and 1000 nm pore size 3DOM separator maintained their initial capacities and showed highly reversible charge-discharge performances.

It was revealed from those results that the highly concentrated electrolytes are available to use with the 3DOM separators and the cycle characteristics of the LMB full cell strongly depends on the pore size. Both the separators with too large or too small pores resulted in short-circuit with lithium dendrites.