The cover art is attributed to an article entitled “Electrochemical Formation of Si Films by Co-deposition of Si and Zn in Molten KF–KCl–K₂SiF₆–ZnCl₂ Systems” by Wataru Moteki et al. Editor’s Choice of this issue is a systematic study by Dr. Nohira and co-workers on the electrochemical formation of crystalline Si films assisted by Zn co-deposition in molten salt bath.

Crystalline Si film is a paramount material to achieve durable Si-based solar cells. The authors developed here the fascinating method to create crystalline Si films with larger grain sizes. Upon ZnCl₂ addition to the base KF-KCl-K₂SiF₆ bath, liquid Si-Zn alloy with good wettability against graphite substrate was deposited, followed by the Si film deposition beneath the Si-Zn alloy layer. Subsequent removal of the upper Si-Zn layer resulted in the single Si films with larger grain size.

The cover art is attributed to an article entitled “Gold- and Silver-Nanoparticle-Assisted Etching of p-Si and n-Si: A Discussion of Etching Behavior Based on Polarization Curves” by Ayumu Matsumoto et al. which is selected as an Editor’s Choice of this issue because of its high rating by reviewers and editors.

Metal-assisted etching is a crucial method for making Si nanostructure; however, its mechanism with different metals has been clarified yet. In this article, the authors investigated the mechanism of etching assisted by Au- and Ag-nano­particles, which are known to cause different functions for remote etching (namely, general corrosion). The etching behavior of bare Si, metal deposited Si, and metal wires were also discussed based on their polarization curves.

In the tropical marine atmospheric environment of Zhanjiang, E690 steel was subjected to 7-, 15-, 30-, 90-, 180-, 270-, and 360-day exposure experiments. Electrochemical noise (EN) technology, combined with the weight loss method, corrosion morphology observation, X-ray diffraction analysis (XRD) and polarization curve testing, was used to study the corrosion behavior of E690 steel in a tropical marine atmospheric environment. The results showed that during the initial exposure period, the voltage and current noise fluctuation amplitudes of the EN electrode were large, the current standard deviation (S_I) and white noise level (W) were large, the corrosion current density was high, and the corrosion rate of E690 steel was high, mainly due to the small amount of corrosion products on the surface of the sample. Moreover, the voltage and current noise exhibited a nearly symmetrical distribution on both sides of the average value, accompanied by many transient peaks, indicating uniform corrosion of E690 steel. As the exposure time increased, the amplitude of the voltage and current noise fluctuations decreased, the current standard deviation (S_I) and white noise level (W) gradually decreased, the corrosion current density decreased, and the corrosion rate decreased and tended to stabilize. In the later stage of exposure to sunlight, pitting corrosion appeared on the surface of E690 steel, and the voltage noise exhibited high-frequency vibration without obvious transient peak characteristics. The current noise also showed high-frequency vibration, and the number of transient peaks significantly decreased. The corrosion type of E690 steel changed from uniform corrosion to pitting corrosion.

The use of streaming potential in chemical production with microfluidic devices offers a promising approach to reducing energy consumption. This study explored H₂ generation through the combined use of applied voltage and streaming potential in a microfluidic system. The microfluidic system with two platinum electrodes embedded in a microchannel was designed to explore the hydrogen evolution reaction (HER) and quantitative analysis was performed using the device with a liquid/gas loop system. The prepared microfluidic device demonstrated that the reduction current increased with flow rate when using an HCl aqueous solution. The results showed an increase in H₂ production efficiency by up to 23 % compared to a static state, which can be attributed to the streaming potential. Enhancing chemical production efficiency through streaming potentials could contribute to establishing an innovative chemical production process using microfluidic devices.

Crystalline silicon solar cells have become the leading technology in solar cell manufacturing. However, conventional solar cell manufacturing methods are energy-intensive and inefficient. We previously developed an electrodeposition method that improved crystalline Si film production but faced limitations in grain size, which is crucial for minimizing recombination losses in polycrystalline Si solar cells. In this study, the co-deposition of Si and Zn on a graphite substrate is investigated to obtain large grain-sized Si films. Electrochemical measurements and electrolysis are conducted at 923 K in molten KF–KCl–K₂SiF₆–ZnCl₂ systems. Reduction currents corresponding to the reduction of Zn(II) and Si(IV) ions are observed at approximately −2.6 V vs. Cl₂/Cl⁻ and −3.1 V, respectively in the cyclic voltammogram on a glassy carbon electrode. The optimal conditions for obtaining highly crystalline Si films are investigated by varying the ZnCl₂ concentration and current density. During co-deposition, a Si–Zn liquid alloy is deposited. Post-electrodeposition cross-sectional images reveal a Si layer on the graphite substrate with a Zn layer on top. After Zn removal, the Si films exhibit larger crystal sizes than those obtained without ZnCl₂, indicating deposition through liquid alloying with Zn.

Amorphous powder with the composition of high-Al substituted Li_1+xAl_xGe_2−x(PO₄)₃ (x = 1.0) [LAGP(x = 1.0)] were prepared by mechanical milling treatment of the starting materials. Crystalline compounds with LiGe₂(PO₄)₃-based NASICON-type structure were mainly formed by low-temperature heat treatment of amorphous LAGP(x = 1.0) at 450–750 °C. NASICON-type solid electrolytes prepared by the heat treatment of pelletized-type amorphous LAGP(x = 1.0) at 600–750 °C exhibited relatively high conductivities on the order of 10⁻⁵ S cm⁻¹ at room temperature, although they contained AlPO₄ and Li₉Al₃(P₂O₇)₃(PO₄)₂ compounds. The LAGP-type high-Al solid electrolytes would be a potential candidate as lithium-ion conductors for all-solid-state battery applications.

Rechargeable Zinc battery (RZB) is one of most promising candidates for large-scale energy storage facilities because of their advantages such as high energy, availability of safe aqueous electrolytes, and abundant zinc resources. Additives in aqueous electrolyte solutions are known to improve the charge-discharge cycle performance of RZBs, but further investigation of additives applicable to concentrated alkaline solutions is needed. In this study, starch, which is an environmentally friendly, safe and inexpensive polymer, was used as an additive. Raman spectra showed that starch was highly stable in alkaline solutions. The viscosity of the (4 mol dm⁻³ (= M) KOH + 0.3 M ZnO) electrolyte solution increased by 9.1-fold as 0.1 g mL⁻¹ starch was added. Cyclic voltammograms showed that the addition of starch significantly improved the reversibility of the Zn deposition/dissolution wave. A Zn/Zn symmetric cell with the (4 M KOH + 0.3 M ZnO) solution including 0.1 g mL⁻¹ starch exhibited sustained charge and discharge capabilities without short circuit for over 90 h due to the inhibition of dendrite growth. In-situ Raman spectra exhibited that the starch additive was adsorbed on the Ag electrode to inhibit the adsorption of zincate, and reduced the activity of free water during the Zn deposition.

K-ion batteries (KIBs) are regarded as a viable option for large-scale electrical energy storage devices on account of their abundant potassium resources. Although layered P3-type manganese oxides and their derivatives have been studied as positive electrodes for KIBs, most of them use organic solvent-based electrolytes. We recently reported the improved performance of P3-type K_xMnO₂ positive electrode for KIBs with a highly safe ionic liquid electrolyte. In the present study, iron, titanium, and aluminum were doped into potassium manganese oxide positive electrode, and their charge–discharge properties were evaluated in the above ionic liquid electrolyte. The doped materials exhibited enhanced rate capabilities and cycling performances owing to their reduced surface resistance. The Al-doped material demonstrated the best cycling performance and highest discharge capacity, reaching 64.5 mAh g⁻¹ with 92.8 % capacity retention at 100 mA g⁻¹ over 100 cycles, owing to its low surface resistance. In addition, the structural stabilities of the Fe-, Ti-, and Al-doped materials during the charge–discharge process were confirmed through reversible crystal structural evolution observed via ex-situ X-ray diffraction. Finally, a stable cycling performance was also demonstrated in a K-ion full cell composed of an Al-doped potassium manganese-oxide positive electrode and a graphite negative electrode.

Water-insoluble magnesium phosphate species, possibly magnesium ammonium phosphates (MAPs), are precipitated during the electrolysis of magnesium in aqueous solutions containing ammonium and phosphate ions. Electrolysis and subsequent precipitation have examined for magnesium electrode with pre-treatments, immersion in various biodegradable polymer solutions. Electrolysis at the constant potential of −1.4 V vs. Ag/AgCl occurs on magnesium electrodes in a phosphate-containing solution, providing precipitate on the magnesium electrode, consisting mainly of MAPs. The precipitate mass appears to be related linearly to the integrated charge, irrespective of the pre-treatment used. Pre-treatment of magnesium electrodes in poly(ethylene glycol) (PEG) and poly(vinyl alcohol) (PVA) exerted markedly negative and positive influences, respectively, on the electrolysis and the yield of the precipitate during electrolysis, attributable to changes in the interfacial resistance or surface morphology.

Metal-air secondary batteries have attracted attention as a next-generation energy storage system due to their high energy density and environmentally friendly characteristics. However, it is important to develop a catalyst that can rapidly promote both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). We have found that Co₃W₃C and Co₃Mo₃C composite metal carbides prepared by a wet method have high bifunctional electrode performance, but the mechanism of action has not been elucidated. In this study, we comprehensively evaluated the adsorption behavior of oxygen molecules and surface hydroxyl groups, the charge density difference, and the effects of surface structure and chemical composition on catalytic activity by combining DFT (density functional theory) calculations and instrumental analysis. In particular, we found that Co₃Mo₃C optimizes the adsorption of oxygen molecules by metal compositing, contributing to the improvement of electrode catalytic performance.

Interdigitated microarray electrodes were developed based on boron-doped amorphous carbon (B-a-C-IDA) that shows extremely higher overpotential for H₂ and O₂ evolution. Highly sensitive microanalysis of redox analytes with standard potentials higher than O₂ and H₂ evolution was achieved by B-a-C-IDA. The amplification of the oxidation current derived from redox cycling was observed at the generator electrode of B-a-C-IDA in the measurement with applying the reduction potential to the collector electrode (dual mode measurement). For Ce^3+/4+ with a standard potential of 1.6 V, the amperometric current was amplified 180-fold by applying potentials of 1.7 and 0.8 V to collector and generator electrodes in chronoamperometry (CA) measurement at dual mode. Theoretical detection limit (S/N = 3) for Ce³⁺ was 0.13 µM. It was two orders of magnitude better than that at B-a-C plate electrode.

The factors controlling amplification at B-a-C-IDA with varying gap values were investigated using redox analytes with different types of reactions, diffusion coefficients, and electron transfer rate constants. In addition, the amplification mechanism at B-a-C-IDA electrodes, the contributions of diffusion coefficients, and electron transfer rate constants to the amplification factor were clarified. The method to estimate the amplification factor with diffusion coefficients and electron transfer rate constants when using unmeasured redox analytes was established. The amplification factor when using radioactive analyte UO₂²⁺ at B-a-C-IDA with gap size of 2.0 µm was estimated to be 43.2. B-a-C-IDA is expected to enable highly sensitive electroanalysis (low detection limit 0.62 µM).

The development of hydrogel electrolytes is a crucial and urgent task to achieve reliable rechargeable alkaline zinc batteries. We have successfully prepared a starch-based hydrogel electrolyte membrane (SHEM) that holds a (4 mol dm⁻³ (= M) KOH + 0.3 M ZnO) aqueous solution. The prepared SHEM was so flexible that it could be stretched and bent freely, and showed high ionic conductivity comparable to that of the alkaline aqueous solution. A pouch-type Zn/Zn cell with SHEM did not short-circuit for more than 24 h, and uniform Zn deposition without dendritic growth was confirmed in scanning electron microscope images. In the charge-discharge cycle test of a pouch-type Zn/MnO₂ cell with SHEM, it was demonstrated that SHEM can fully function as an electrolyte for quasi-solid-state rechargeable alkaline zinc batteries.

In this study, a method for detecting and tracking hydrogen and oxygen bubbles during alkaline water electrolysis was developed using Faster R-CNN and DeepSORT. The images required for CNN training were automatically generated by a pseudo-bubble image generation algorithm specifically developed for the purpose of this study. The method was applied to the results of observations on alkaline water electrolysis obtained under various current densities and wire electrode diameters. Evaluation of detection performance using a confusion matrix showed that for the hydrogen evolution reaction (HER) at a current density of 1.0 A cm⁻² and a wire electrode diameter of 200 µm, the method achieved a precision of 1.00, recall of 0.840, and F₁ score of 0.940, indicating very high detection performance. For the oxygen evolution reaction (OER), bubbles were detected almost perfectly under all conditions, with all detection metrics exceeding 1.00. The proposed method was approximately 20000 times faster than humans. Bubble diameter distribution, total volume, total number, and Sauter mean diameter were obtained and quantitatively assessed, and the relationships between current density and electrode diameter for both HER and OER have been discussed. This method enables accurate, rapid, and automatic quantitative evaluation of visualization results from various alkaline water electrolysis observations, which were previously difficult and labor-intensive to perform manually.

Metal-assisted etching (metal-assisted chemical etching) is increasingly recognized as a crucial method for fabricating silicon (Si) nanostructures. This process involves the dissolution of Si both directly beneath metal catalysts (local etching) and at locations away from them (remote etching). We previously investigated general corrosion, a kind of remote etching, in metal-particle-assisted etching of moderately-doped n-Si and explained its mechanism in platinum-assisted etching where a sponge-like porous layer was observable on the top surface of Si, which dissolves spontaneously. However, general corrosion caused by gold (Au)-assisted etching has not been explained yet, as the soluble layer was not clearly observed. It is also difficult to explain why general corrosion was suppressed in silver (Ag)-assisted etching under the conditions we examined. In this work, we estimated the depth of general corrosion caused by Au- and Ag-nanoparticle-assisted etching of moderately-doped p-Si and n-Si based on the etched structures and mass loss of substrate. We also discussed etching behavior based on polarization curves of bare Si, metal-deposited Si, and metal wires. This work provides insights into the underlying mechanism of remote etching and proposes a potential strategy to mitigate it.

The electrochemical oxidation of ammonia (AOR) is a critical reaction for energy conversion and environmental remediation applications. However, the efficiency of AOR is significantly hindered by catalyst poisoning caused by adsorbed atomic nitrogen species (N_,ad), which are generated during the reaction. In this study, a novel composite catalyst consisting of iridium clusters loaded onto tungsten disulfide (Ir/WS₂) was synthesized and applied to AOR. Morphological and structural analyses confirmed the successful loading of metallic Ir clusters with an average diameter of 1.5 nm on WS₂ nanobelts. Electrochemical measurements demonstrated that the Ir/WS₂ catalyst exhibited superior activity and durability for AOR compared to conventional Ir clusters loaded onto activated carbon. This enhancement is attributed to the stable adsorption of NHx intermediate species on WS₂, which suppresses N_,ad formation via complete dehydrogenation and facilitates the dimerization of NHx intermediates. These findings highlight the potential of WS₂-based composite materials for the development of efficient and durable AOR catalysts.

Development of F⁻-containing electrolytes with high electrochemical oxidation stability is crucial for electrochemical fluorination. Although various combinations of fluoride salts and solvents have been investigated for preparing the electrolytes, there have been few studies on eutectic systems using fluoride salts and solid hydrogen bond donors, especially amide compounds. In this study, fluoride-based eutectic electrolytes (F-EEs) composed of tetramethylammonium fluoride ([TMA]F) and N-methyltrifluoroacetamide (MTFAA) as an amide compound are developed, and their physicochemical properties are evaluated including comparison with that composed of [TMA]F and N-methylacetamide (MAA). The F-EEs which are in liquid state at room temperature are successfully obtained for [TMA]F·x[MTFAA] with x = 8.0 and 4.0 owing to melting-point depression by hydrogen bonds between N–H and F⁻. The F-EEs with MTFAA and MAA exhibits the favorable ion transport properties because they have only one N–H bond per molecule and hydrogen bonds of N–H···F⁻ are moderate. The F-EEs with MTFAA exhibit superior electrochemical oxidation stability due to the CF₃ group compared to that with MAA and will be promising as electrolytes for electrochemical fluorination applications.

An efficient red phosphorescent electrogenerated chemiluminescence (ECL) cell was successfully designed by utilizing a cyclometalated iridium(III) complex and a carbazolyl dicyanobenzene derivative. The ECL solution, which contained bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) (Ir(piq)₂(acac)) as a luminescent material and 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as a redox mediator, was prepared and injected in a microfluidic ECL cell. The cell, which consisted of the 5-µm-thick microchannels sandwiched between transparent electrode pairs with the prepared solution, exhibited a saturated red emission with Commission Internationale de l'Éclairage (CIE) coordinates of (0.68, 0.31), a maximum luminance of 98.7 cd m⁻², and a maximum current efficiency of 1.63 cd A⁻¹. Photophysical and electrochemical studies were also carried out to support our experimental results.

To numerically evaluate the effect of residual stress on the performance of proton-conducting ceramic fuel cells (PCFCs), in this study, the impedance of a hydrated 20 mol% yttrium-doped barium zirconate (BZY20) proton conductor is measured under isostatic pressures up to 180 MPa near room temperature. A piston cylinder apparatus is used, with boron nitride (BN) powder as the pressure medium. The bulk resistance of the material decreases as the pressure increases, with a decrement ratio from −0.6 to −2.3 % per 60 MPa variation in pressure. As the proton concentration is constant at this temperature, this finding indicates that the proton conductivity and proton diffusivity increases under compressive stress. For various solid electrolytes, including other BZYs, this is the first experimental observation of an increase in diffusivity under compressive stress. The BZY20 exhibits an activation volume, the index describing the effect of pressure effect on ionic conductivity, in the range of −0.23–0.99 cm³ mol⁻¹. To date, negative activation volumes have not been reported. However, their absolute values, that is, the sensitivities to pressure, are smaller than those of other solid electrolytes. In particular, BZY10 powder and BZY20 thin film exhibit significantly high positive activation volumes in the range of 4.0–14.3 cm³ mol⁻¹ at higher temperatures and pressures. As the activation volume is determined by the dominant mechanism of ionic conduction, these results suggest that the dominant mechanism changes with temperature and pressure. This study reports the first instance of negative activation volume, which provides fundamental knowledge for evaluating the effects of stress near room temperature and low pressure. However, the impact of the negative activation volume for the performance of PCFCs is may be limited to a specific temperature and pressure ranges.

Biosensors and biofuel cells are human-friendly and eco-friendly technologies that utilize enzymatic redox reactions. To operate in a solution flow environment, enzymes and electron mediators must be immobilized on the electrodes. The state of the electron mediator at an enzyme-immobilized electrode affects not only the electron transfer between the enzyme and the electrode but also the retention and orientation of the enzyme. Phenazine dyes, often used as electron mediators, can be easily immobilized on an electrode by electropolymerization. However, little is known about what kind of polymer films result from the conditions of electropolymerization. Here, we show that the electropolymerized film of methylene green (MG), a phenazine dye, changes from a uniformly distributed mesoscale structure to a nonuniformly dispersed state of spherical particles as polymerization progresses. Furthermore, we found that polyMG films with a uniform surface are suitable for enzyme-immobilized electrodes. More MG was deposited on the electrode under conditions of wide potential sweeps, slower scan rates, and more cycles of electrolysis. Uniform surfaces were observed in films with dissipation (ΔD), a measure of film softness as measured by EQCM-D, of less than approximately 7 × 10⁻⁶. These results indicate that the electrochemical polymerization conditions of MG can control the surface morphology of the polymerized film, as well as the properties of the enzyme-immobilized electrode. This finding could be applied to NAD-dependent and FAD-dependent glucose dehydrogenases, in which MG is known to function as a mediator, and could contribute to improving the current of biosensors and biofuel cells in which they are used.

Deep eutectic electrolytes (DEEs) are attracting increasing attention as liquid-state electrolytes for secondary batteries because they are potentially low cost, display low flammability, and are environmentally friendly. However, to date limited DEEs have been developed and explored for lithium-ion battery (LIB) applications, with most reports showing unsatisfactory capacity retention, a narrow potential window for battery operation, and an unstable solid electrolyte interphase (SEI) layer leading. Herein, we develop DEEs based on lithium bis(fluorosulfonyl)amide, LiFSA, and a series of urea derivatives as Li ion-conducting DEEs. Despite similar structures for the urea derivatives, i.e. methylated urea, we found that 1,3-dimethylurea (1,3-DMU) could form Li ion-concentrated DEEs across a wide range of LiFSA : 1,3-DMU ratio, while the LiFSA : urea DEE was liquid only in a limited range of molar ratios, i.e. LiFSA : urea close to 1 : 4 (mol/mol). By examining the electrolyte structure via Raman spectroscopy, we observed increased aggregation for DEE with higher LiFSA concentrations. We further confirmed non-flammability and electrochemical stability among the DEEs with potential windows ranging from ∼3.35 V for LiFSA : urea (1 : 4) to an impressive 6.42 V for LiFSA : 1,3-DMU (1 : 2) at a Pt foil electrode. During charge-discharge of Li₄Ti₅O₁₂ (LTO) electrodes, we observed good capacities and retention for the LiFSA : urea (1 : 4) and LiFSA : 1,3-DMU (1 : 2) DEEs. High Coulombic efficiencies (CEs) were achieved in the LiFSA : 1,3-DMU (1 : 2) DEE with its high LiFSA content that led to more substantial FSA-derived components in the SEI structures after cycling. We further tested positive electrode materials, including LiFePO₄ that showed excellent capacity retention and CEs near 100 % across 50 cycles. In all, we find that the dimethylurea-based DEEs show an opportunity for non-flammable and high-voltage Li batteries.

Benzo[d]thiazole-2-carbonitrile and the related derivatives are important compounds with applications in a variety of chemical fields. In this paper, electrochemically oxidative cyanation of benzothiazole and their derivatives was successfully achieved by using trimethylsilyl cyanide as a cyanide source in n-Bu₄NBF₄/DMF in the divided cell, in which the corresponding products were obtained in moderate yields. The extensive reaction optimization as well as scope and limitations were investigated in this paper.