The cover art is attributed to an article entitled “Development of Photocatalysts for Artificial Photosynthesis Aiming at Carbon Neutrality” commemorated for the Society Award of Electrochemical Society of Japan (Takei Award) by Professor Akihiko Kudo. The authors liken chess pieces to the roles of photoelectrochemical catalysts. The three chess pieces hint at different “winning lines”: a straight thrust, a leap, and long-range coordination suggesting suitable crystal engineering for development of photocatalysts and photoelectrochemical cells with band engineering. In this light, they point to two strands of the work. One concerns material systems that combine visible-range absorption with efficient hole transport by creating a “new valence band,” exemplified by BiVO₄ (Bi 6s) and SnNb₂O₆ (Sn 5s). The other concerns design approaches that steer photoexcited carriers in one direction through cascaded band alignment, including Z-scheme architectures such as the (CuGa)_0.5ZnS₂–BiVO₄ couple for CO₂ reduction with O₂ evolution. Taken together, these motifs may be read as evoking a variety of “moves”: robust performance in single-phase materials and defect/doping control; the opening of new pathways through valence-band re-design; and, further, cascaded band alignment realized through solid solutions, heterojunctions, and electron mediators.

The cover art is attributed to an article entitled “Non-faradaic Impedimetric Biosensing with Open Bipolar Electrode Platform” by Arisa Tochigi et al. as an Editor’s Choice. In this work, the authors successfully demonstrated a flexible impedimetric biosensor based on the bipolar phenomenon using an open bipolar electrode (oBPE). oBPEs offer biosensing capabilities with a simple structure and wireless design, enabling compact and adaptable sensor configurations. They are particularly well-suited for non-faradaic impedance measurements, allowing label-free detection of biomolecular interactions without the need for redox reagents or complex signal amplification, supporting sensitive, cost-effective, and user-friendly biochemical analysis.

A library of hundreds of metal oxide and metal sulfide photocatalysts has been constructed based on our original strategies of photocatalyst design that were metal cation doping, new valence band formation, and making a solid solution. Various single particulate and Z-scheme photocatalyst systems that are active in visible-light water splitting have been developed. By the doping strategy, SrTiO₃:Rh,Sb and SrTiO₃:Ir,Sb,Al of a single particulate metal oxide photocatalyst were developed for water splitting working under visible light irradiation. BiVO₄ was developed for water oxidation to form O₂ under visible light irradiation by the new valence band formation with Bi6s orbitals. Several types of a Z-scheme system have been constructed by using SrTiO₃:Rh of a H₂-evolving photocatalyst, BiVO₄ of an O₂ evolving photocatalyst, and a suitable electron mediator such as Fe^3+/2+, Co(bpy)₃^3+/2+, reduced graphene oxide (RGO), and poly-3,4-ethylenedioxythiophene (PEDOT). A CuGaS₂-ZnS solid solution photocatalyst was active for sacrificial H₂ evolution, and it was able to be applied to a Z-scheme photocatalyst with BiVO₄ of an O₂-evolving photocatalyst for water splitting without any sacrificial reagents. Novel cocatalysts such as Ag and Rh-Ru were found for photocatalytic CO₂ reduction using water as an electron donor. Ag/NaTaO₃:Ba and Rh-Ru/NaTaO₃:Sr photocatalysts gave 90 % and 10 % of an electron-based selectivity for the CO₂ reduction to form CO and CH₄ accompanied with stoichiometric O₂ evolution even in an aqueous solution under UV irradiation, respectively. A Z-scheme photocatalyst consisting of CuGaS₂-ZnS and BiVO₄ was active for the CO₂ reduction to form CO with 12 % of the selectivity under visible light irradiation in an aqueous suspension system.

In recent years, the unique characteristics and utility of bipolar electrochemistry have garnered considerable attention, leading to applications across diverse electrochemical fields. This comprehensive paper summarizes recent developments in electricity-free electrochemical reactions utilizing streaming potential-driven bipolar electrodes. Representative examples are presented, including the optimization of conditions for generating large streaming potentials, applications to the electropolymerization of aromatic monomers, and amine detection methodologies based on electrochemiluminescence techniques.

Energy storage technology is indispensable, and Li-ion batteries are currently widely used as power sources for portable electronics and automobile applications. The Li-ion battery market for electric vehicles has expanded rapidly over the past decade. High-energy Ni-rich electrode materials, such as LiNi_0.8Co_0.1Mn_0.1O₂, and cost-effective Fe-based materials, such as LiFePO₄, are commonly employed as positive electrode materials for electric vehicles. In this article, recent research progress and design strategies for positive electrode materials are summarized. Defect engineering has proven effective in enhancing electrode reversibility, enabling the development of high-performance, pure Ni-based layered materials. Furthermore, a Co-/Ni-free Mn-based Li insertion material with a domain structure and optimum surface area has been successfully synthesized. The electrode reversibility of cation-disordered rocksalt oxides containing V and Ti ions is also significantly improved through the structural defect enrichment and increased surface area. Highly concentrated electrolyte solutions, which possess high oxidation tolerance and reduced solubility for transition metal ions, further contribute to enhanced reversibility of these electrode materials. Similarly, the electrode reversibility of Mn-based Na insertion materials is greatly improved by using highly concentrated electrolyte solutions, due to the suppression of Mn dissolution. The practical feasibility of these Li/Na insertion materials is also discussed based on these findings.

There is a concept called Ecofitting technology. It is a concept that making industrial processes more environmentally friendly leads to a transformation into a more economical industrial process as a result. I have developed several new chemical processes based on this research concept. One of them is the radical vapor reactor (RVR), which is already in practical use (commercially available) as a process device. I found and developed a reaction (called the oxygen P/L reaction or RVR reaction) that can generate high concentrations of hydroxyl radicals and singlet oxygen, which are highly oxidizing among reactive oxygen species, and developed a device that can expose objects to the high-concentration reactive oxygen atmosphere generated by the reaction. I have then put it into practical use as a reactive oxygen species generation and exposure device that can be used in various chemical industrial processes. In this paper, the author, who discovered the RVR reaction, developed the RVR device, and further demonstrated various applications of the RVR, outlines the research and development of the RVR reaction (device design and commercialization), and examples of applications of the commercialized RVR device.

Direct electron transfer (DET)-type bioelectrocatalysis, in which enzymatic reactions proceed without the involvement of redox mediators, is a powerful approach for investigating intrinsic enzymatic features relevant to the development of advanced biomimetic systems. However, because numerous DET-capable enzymes are associated with cellular membranes, determining their full three-dimensional conformations is challenging when relying solely on conventional X-ray crystallography. The advent of cryo-electron microscopy coupled with single-particle reconstruction techniques has recently made it feasible to quickly resolve their structures. A notable interdisciplinary advancement termed “structural bioelectrochemistry” has emerged, integrating structural biology with electrochemical analysis to accelerate functional determination through enzyme modification and electrochemical probing. This review highlights recent fundamental research on DET-type enzymes and summarizes their bioelectrochemical applications, such as biosensors, biofuel cells, and bioreactors.

In energy storage and conversion applications, high performance can be achieved by maximizing nanomaterial utilization—ideally approaching 100 %. This study comprehensively investigates the preparation of layered nanomaterials and the effects of nanoconfinement on the performance of energy storage and conversion devices. Specifically, K_yIrO_x and K_yPtO_x layered materials were successfully synthesized via solid-state reactions, thereby expanding the library of oxide-layered compounds. Additionally, a free-standing nanosheet structure (2.5D) was fabricated using a macroporous carbon template. Quinone-based aromatic compounds were adsorbed into activated carbon micropores smaller than 1 nm, yielding an adsorption-controlled system. A model electrode study employing reduced graphite oxide demonstrated that the redox reversibility of redox-active aromatic compounds was improved with decreasing interlayer distance. This finding suggests that the redox reversibility can be tuned effectively by controlling the nanoscale spacing in electrode materials.

A key challenge in advancing solid-state Li batteries is the accurate probing of internal electrochemical reactions under operating conditions. Operando scanning transmission electron microscopy coupled with electron energy-loss spectroscopy (STEM-EELS) is a powerful technique that enables the real-time, real-space observation of ionic diffusion, phase transitions, and changes in the electronic states of transition metals and oxygen during charging and discharging. In Ni-rich layered cathodes, operando STEM-EELS reveals non-uniform delithiation and phase evolution at the nanoscale, highlighting the critical role of two-dimensional Li-ion diffusion and grain architecture in polycrystalline particles. Meanwhile, in lithium titanate anodes, the study uncovers contrasting diffusion behaviors during Li insertion and extraction, including the formation of core-shell structures and preferential surface diffusion, both of which significantly impact the overall rate performance. These findings collectively indicate that battery electrodes undergo highly non-uniform reactions during electrochemical cycling, with intricate gradients in the Li content, transition-metal valences, and oxygen redox states dynamically evolving. These gradients are influenced by factors such as the crystalline orientation, grain boundaries, surfaces, and particle morphology. By providing nanoscale insights into these fundamental processes, operando STEM-EELS provides valuable guidance for optimizing material design, refining electrode architecture, and enhancing the overall battery performance.

All-solid-state thin-film batteries were fabricated without annealing for potential application in flexible devices. The amorphous cathode active material Li_xM_yPO_z (M = Ni, Mn, Co, Cu, Ag) was developed as the key material. Notably, the capacity for M = Ni and Mn exceeded 300 mAh g⁻¹ at room temperature. Oxygen K-edge spectra were acquired using X-ray absorption spectroscopy (XAS), and the anion redox reaction associated with Li extraction was confirmed. Although the cathode material is amorphous, it locally contains PO₄ tetrahedra and MO₆ octahedra. As these two units approach each other and interact more strongly through delithiation, the covalency of oxygen increases. M-3d-O-2p-P-3d hybridization contributes to the charge compensation process in delithiation. In addition, the battery durability was improved. The volume change of the electrochemically plated Li was effectively controlled by covering the anode current collector with a layer of UV-curable resin. Furthermore, a 25 nm barrier layer sputtered onto the cover layer provided an excellent moisture barrier. As a result, the cycling performance was significantly improved.

Recently, high-speed voltage measurement systems have been developed to detect micro-internal short circuits in conventional lithium-ion batteries. The envelope voltage measurements were performed at a sampling rate of 1 MHz. Characteristic voltage behavior attributable to a micro-internal short circuit was detected. This study provides an important basis for anomaly detection to improve the safety of lithium-ion secondary batteries.

All-solid-state lithium metal batteries are considered next-generation lithium-ion batteries that surpass the limits of conventional lithium-ion batteries owing to their ability to simultaneously reduce the risk of flammability and achieve high energy densities. Theoretically, a zero-excess Li configuration, in which the negative electrode comprises only a current collector, can achieve the highest energy density. However, low Coulombic efficiency and performance degradation caused by non-uniform plating are inevitable in this configuration. In contrast, thin-film Li metal anodes are advantageous owing to their potential to maintain the excess amount as low as possible and minimize the reduction in energy density. In this study, we report on the characteristics of thin-film Li metal anodes prepared by vacuum deposition in all-solid-state batteries. The thickness of the deposited Li metal anodes was 5 µm for the ultra-thin (UT-Li) and 23 µm for the thin (T-Li). The T-Li exhibited higher active Li utilization (98 %) and a more stable cycle life of over 2000 h, whereas the critical current density was high for UT-Li (2.5 mA cm⁻²). The differences in these properties can be explained by the microstructure of the vacuum-deposited Li metal anode surface and the mechanical properties of the anode. Domain size and crystal orientation are important factors, as they can be controlled by the vacuum deposition technique used in industrial production, making them useful for the practical application of Li metal anodes.

Biomass (corn cob) derived porous carbon with a very high surface area for supercapacitor application is reported by carbonization with a non-toxic mixed salt of NaCl and KCl as the reaction medium. The corn cob was subjected to molten salt carbonization in a molten NaCl-KCl system (mass ratio of 1 : 5) at 800 °C, porous carbon material (C-1-5) with a high specific surface area (1054.36 m² g⁻¹) and a three-dimensional honeycomb network structure was obtained. The electrode material exhibits a specific capacitance of 222.5 F g⁻¹ in a three-electrode system at a current density of 0.1 A g⁻¹. The assembled symmetric supercapacitor achieves a specific capacitance of 76.47 F g⁻¹ at a scan rate of 5 mV s⁻¹. The energy density is 6.22 Wh kg⁻¹ at a power density of 50 W kg⁻¹, and the capacitance retention reaches 96.30 % after 2000 cycles. This green and simple technology is expected to become an effective and scalable method for producing porous carbon from biomass for electrochemical energy storage applications.

We propose an impedimetric biosensor using the bipolar phenomenon with an open bipolar electrode (oBPE). The oBPE platform offers a structurally simple and highly flexible design. The unique electrochemical biosensor consists of a BPE, driving electrodes for detection, and an electrolytic cell as its components. We employed non-faradaic electrochemical impedance spectroscopy to detect biomolecular events occurring on the BPE surface. The biosensor showed a 45 % increase in impedance at 2 kHz after bovine serum albumin adsorption onto the BPE surface. Additionally, the significance of solution resistance, which was uniquely defined by the distance between driving electrodes and a BPE, was demonstrated through changes in impedance. We used the detection system as an immunosensor to detect the binding of C-reactive protein (CRP) to antibody-modified BPE, based on changes in impedance. The biosensor detected the binding of CRP to antibody without the need for redox-active reagents or complex signal amplification steps. The biosensor showed a linear response to the concentration of CRP in the range from 0.001 to 1 µg/mL, comparable to that of conventional three-electrode systems. These findings demonstrate the potential of non-faradaic impedimetric biosensors with an oBPE for sensitive detection of biomolecular events in biochemical analysis.

All-solid-state lithium–sulfur batteries are excellent candidates for next-generation batteries due to their high energy density, safety, and affordability. However, the current fabrication process faces challenges related to processing steps and the high cost of raw materials. In this study, we provide a highly effective and affordable one-step method for fabricating sulfur-positive composite electrodes in a short processing time using low-cost materials. High-performance positive electrodes were prepared by mechanical milling using sulfur, high-surface-area carbon, P₂S₅, and lithium oxoacid salt or oxide such as Li₂CO₃, or Li₂O for 2 h. The resulting sulfur-positive composite electrodes showed capacities of over 1600 and 1200 mAh g⁻¹(S) (7.6 and 5.7 mAh cm⁻²) at 0.25 and 3.2 mA cm⁻² at 25 °C, respectively. The high performance is attributed to the simultaneous production of an ionic conductive material through the reaction of P₂S₅ with lithium oxoacid salts or oxide and the formation of a sulfur composite. A full battery cell comprising this positive electrode and Li-Si alloy negative electrode exhibits an energy density of 700 Wh kg⁻¹ (based on positive and negative composite electrodes) and an area capacity of 8.4 mAh cm⁻² at 0.25 mA cm⁻² and 25 °C.

Polymer electrolyte fuel cells (PEFCs) have attracted significant attention as clean energy conversion systems. Pt nanoparticles are commonly used as cathode catalysts in PEFCs due to their high activity for the oxygen reduction reaction (ORR). However, Pt is rare and expensive; therefore, its mass activity needs to be improved for the dissemination of fuel cells. Recently, ionic liquid (IL) layers have been used to enhance the ORR catalytic activity on Pt surfaces via the suppression of OH adsorption. In this paper, we discuss the optimum structure of phosphonium ILs, which have high thermal stability, favourable ionic conductivity, and hydrophobicity, for Pt/C-solid catalysts with ionic liquid layers (SCILLs). Using the microelectrode method, we found that the oxygen solubility depends on the molar volume of the IL, in accordance with regular solution theory, and that the diffusion coefficient approximately follows the Einstein–Stokes equation. This indicates that the resistance encountered by a solute O₂ molecule is affected more by the microscopic viscosity generated locally by the cationic structure rather than the macroscopic viscosity of the bulk IL. In addition to the reduction in OH poisoning achieved by hydrophobic ILs, some physicochemical properties that lead to high mass and specific activity for the ORR on Pt/C-SCILLs, such as the oxygen solubility, H⁺ activity, and conductivity, were clarified.

A voltabsorptmetric study of redox properties of methyl viologen in an optically transparent thin-layer electrode cell was performed. The obtained UV-visible absorption spectra were analyzed using a higher-order derivative conversion (HODC) method. The superimposed absorption bands assigned to the radical cation of the reduced form (MV^+·) and its dimer ((MV^+·)₂) were resolved using the HODC technique. The HODC method was shown to be effective for the analysis of the electrode reaction of the compound with viologen unit.

Piroxicam is a common painkiller and anti-inflammatory drug. Precise monitoring of piroxicam levels can mitigate side effects, but current methods are often slow and complicated. An easily fabricated and highly sensitive voltammetric sensor for piroxicam was developed by drop-coating a composite of multi-walled carbon nanotubes (MWCNTs) and Nafion onto the electrode surface. The morphological and electrochemical characteristics of the Nafion-MWCNTs composite film were analyzed to reveal the synergistic effects of Nafion and MWCNTs. The composite film was then applied to modify a glassy carbon electrode (GCE), which served as a voltammetric sensor for the detection of piroxicam. When compared to previous reports, the sensor demonstrated efficient electrochemical oxidation of piroxicam with remarkable sensitivity, reproducibility, and stability. Under optimized conditions, the Nafion/MWCNTs/GCE exhibited a linear voltammetric response for piroxicam in the concentration range of 5 × 10⁻⁹ to 8 × 10⁻⁶ mol L⁻¹ and 8 × 10⁻⁶ to 6 × 10⁻⁵ mol L⁻¹ respectively, with a detection limit of 4 × 10⁻⁹ mol L⁻¹. The sensor was successfully applied to detect piroxicam in real tablet samples, providing satisfactory recovery values. Furthermore, the composite film was applied to the surface of a platinum microelectrode, resulting in a micro-sensor for piroxicam detection. With the typical small size and the radial diffusion field, the micro-sensor of piroxicam exhibited increased sensitivity and possessed unique properties. Particularly, fast and accurate response across various piroxicam concentrations was achieved without the necessity of additional forced-convection for preconcentration, demonstrating its great potential for in vivo determination of piroxicam.