To further improve the energy density and reliability of carbon based electrochemical capacitors, such as electric double-layer capacitors and lithium ion capacitors, it is necessary to increase both the electric double layer capacitance and stability of the porous carbon electrodes used in the capacitors. Not only the specific surface area and pore size, but also the pore shape and pore size distribution, and the analytical methods to properly evaluate the pore structure should be considered when improving the capacitance. In the case of the high stability to high voltage charging, it is important to design and control the three-dimensional structure of the electrode as well as the crystal structure and crystallinity of the carbon. In this review, some advanced examples of the recent research on these topics will be discussed.

Electrochemical capacitors are known for their high power density and cyclability, and various redox reactions can be utilized to improve their energy density. A great variety of redox materials have been investigated recently for use in electrochemical capacitors, not only transition metal oxides, which have been studied for many years. In this review, we provide a comprehensive explanation of redox materials for electrochemical capacitors. Manganese oxides and ruthenium oxides, which are typical metal oxides exhibiting pseudocapacitance, are first discussed. Nickel oxides used in hybrid capacitors are also covered. Various 0D and 2D nanomaterials are highlighted. Pseudo-capacitance using nanosized complex materials and metal-organic frameworks is also presented. Furthermore, electrolyte systems that exhibit redox characteristics for electrochemical capacitors are also reviewed.

Aqueous electrolytes containing sulfuric acid and organic electrolytes made by dissolving quaternary ammonium salts in propylene carbonate have long been used as electrolytes in supercapacitors. In the organic electrolyte system, acetonitrile was later also used as a solvent. This paper will describe more recent electrolytes for supercapacitors, including those in the research and development stage. The main electrolytes discussed here are novel-solvent-based electrolytes, highly concentrated electrolytes, deep eutectic electrolytes, polymer gel electrolytes, and solid-state electrolytes. The advantages as well as problems of these electrolytes are discussed, and the prospects of electrolytes for supercapacitors are presented.

A detailed understanding of the energy storage mechanism is essential to enhance the energy density of electrochemical capacitors. This necessity has led to the widespread use of in situ and operando measurements of electrochemical devices with laboratory X-ray scattering equipment and synchrotron X-rays. For electrochemical capacitors, it is crucial to obtain comprehensive information on the behavior of the electrolytes within the nanopores, the formation of the electric double-layer, and the structural changes in both the electrode and the porous carbon used as the electrode material. This review will present recent studies on electric double-layer capacitors, highlighting the insights gained from X-ray scattering measurements.

Possessing high electronic conductivity and the nature of chemical inertness, the Magnéli phase titanium oxide Ti₄O₇ is a promising material for various electrochemical applications. Herein, the Ti₄O₇ electrode in aqueous Li₂SO₄ electrolyte is characterized for its supercapacitor applications. The oxide electrode exhibits pseudocapacitive behavior over a wide potential range of ±1.0 V (vs. Ag/AgCl), showing a specific capacitance of 105 F g⁻¹, equivalent to 85 µF cm⁻²–oxide, along with outstanding high-rate performance and cycle stability (96 % capacitance retention after 5000 cycles). In situ X-ray absorption near-edge spectroscopy analysis on the Ti K-edge absorption reveals that the pseudocapacitance does not involve the redox reaction of the oxide electrode material. A pseudocapacitance mechanism attributed to the reversible redox reactions of the hydrogen and oxygen atoms adsorbed on the oxide surface is proposed.

Ag-Sn bimetallic alloys were synthesized via mechanical alloying using a ball-milling process as electrocatalysts for the carbon dioxide (CO₂) reduction reaction. Single-phase intermetallic compounds or solid solutions of bimetallic Ag-Sn alloy were successfully synthesized. The main reaction product for the CO₂ reduction reaction was formate over the synthesized Ag-Sn alloy catalysts, and the catalyst with an intermetallic phase exhibited the highest activity toward formate generation, especially at high current density. This study demonstrated that mechanical alloying is a potential approach for the development of CO₂ reduction reaction electrocatalysts.

We apply the hydroflux method as a potential cathode-coating process for solid-state batteries. The garnet-type Li-ion conductor Li_6.4La₃Zr_1.4Ta_0.6O₁₂ reacts with the precursor material of LiCoO₂. Water molecules in the molten alkaline hydroxide initiate Li⁺/H⁺ ion exchange and dissolution of the Zr⁴⁺ species. The NASICON-type lithium-ion conductor LATP also reacts with molten hydroxide owing to the formation of soluble species of Al(OH)₄⁻ under high pH conditions. Perovskite-type Li_0.33La_0.55TiO₃ is stable under the hydroflux condition because titanium and lanthanum do not form soluble species in alkaline solution. The chemical compatibility of the solid-state electrolyte is mostly estimated using Pourbaix diagram of each element in the system. The solid-state electrolyte containing only insoluble species in an alkaline solution is preferable for the hydroflux cathode-coating process.

A H₁₅P₃V₆Mo₁₈O₈₄ aqueous solution containing phosphovanadomolybdate anions such as [PV₃Mo₉O₄₀]⁶⁻ and [PV₂Mo₁₀O₄₀]⁵⁻ were applied to a catholyte of redox flow polymer electrolyte fuel cells (PEFCs). This system enables continuous power generation by the electrochemical reduction of the phosphovanadomolybdate anions over the carbon cathode and subsequent reoxidation of the reduced phosphovanadomolybdate anions in the cathode tank. Currently, investigations on H₁₅P₃V₆Mo₁₈O₈₄ aqueous solution have been limited to compositional analysis only, and there are no reports of its application to the catholyte. In this study, the performance of H₁₅P₃V₆Mo₁₈O₈₄ aqueous solution using as the catholyte in redox flow PEFCs was investigated. In addition, the pH of catholytes was increased, and effects on the performance were compared. Current–voltage measurements showed that increasing the pH (0.3 → 2.5) decreased the power density (42 mW cm⁻² → 17 mW cm⁻²). On the other hand, the reduction–reoxidation operation showed that increasing the pH increased the reoxidation rate of the phosphovanadomolybdate anions (0.5 mmol min⁻¹ → 2.2 mmol min⁻¹). This study is the first step toward the use of H₁₅P₃V₆Mo₁₈O₈₄ aqueous solution as a catholyte in redox flow PEFCs.

This study addresses the problems of low product purity, low current efficiency, and environmental pollution caused by the excessive use of selenium additives in the electrolytic manganese (Mn) industry. It investigates the effects of composite additives on reducing selenium (Se) usage while enhancing the quality and current efficiency of Mn products. The primary additive is an industrial minimum amount of selenious acid (H₂SeO₃), with sodium sulfite (Na₂SO₃) and sodium dodecyl sulfate (SDS) as auxiliary additives. This research explores their impact on the membrane electrowinning process. The results show that the use of composite additives significantly enhances cathodic polarization of the coating, thereby inhibiting the hydrogen evolution reaction (HER) and playing a crucial role in forming dense and smooth coatings. When Na₂SO₃ is utilized as an auxiliary additive, it achieves a higher current efficiency than SDS, resulting in better quality Mn deposition. With a composite additive ratio of H₂SeO₃ at 0.0175 g/L and Na₂SO₃ at 1.5 g/L, the cathodic and anodic current efficiencies reach 84.59 % and 80.13 %, respectively, with an acid increment of 23.88 g/L. Additionally, the voltage stands at 3.9 V, the cathodic energy consumption at 4498.02 kW h t⁻¹, and the anodic energy consumption at 3001.41 kW h t⁻¹. The Mn surface after electrolysis is smooth, with a metallic luster and uniform thickness, exhibiting α-type crystal morphology, while electrolytic manganese dioxide (EMD) appears as a powder, black in color, with uniform size and ε-type crystal morphology. Thus, the use of Na₂SO₃ effectively reduces Se content in composite additives, enhancing current efficiency and product quality, and provides theoretical guidance for the study of low-Se composite additives in the co-electrolysis process of EMD.

Ternary Ag₈SnSe₆ quantum dots (QDs) were synthesized via a heating-up method in which the reaction of corresponding metal acetates and selenourea was carried out at 250 °C in oleylamine containing 1-dodecanethiol (DDT) as a capping ligand. The obtained QDs were spherical particles with a cubic Ag₈SnSe₆ crystal structure. The size of Ag₈SnSe₆ QDs decreased from 7.8 to 4.9 nm as the amount of DDT in the reaction mixture was increased from 0 to 0.23 mmol. The absorption spectra of obtained QDs were broad, and the wavelength of the absorption onset was blue-shifted from ca. 1600 nm to ca. 1300 nm with an increase in the amount of DDT added. The energy gap determined from Tauc plots of the absorption spectra increased from 0.82 eV to 1.10 eV with a decrease in the QD size from 7.8 to 4.9 nm. Photoelectrochemical measurements revealed that Ag₈SnSe₆ QDs immobilized on ITO electrodes generated photocurrents under light irradiation. The action spectra of photocurrents roughly matched the corresponding absorption spectra, indicating that Ag₈SnSe₆ QDs photoexcited with near-IR light generated photocurrents. The onset potentials of photocurrent generation were located at 0.20–0.27 V vs. Ag/AgCl for the QDs. This suggested that intragap states, acting as trap sites for photogenerated carriers, were located above the valence band maximum (VBM) level of QDs and mediated the electron and hole transfers to ITO electrodes, generating anodic and cathodic photocurrents, respectively.

The phase behavior, Na⁺ coordination structure, Na⁺ conduction and transport properties, and battery performance of liquid electrolytes comprising sodium (fluorosulfonyl) (trifluoromethylsulfonyl) amide (NaFTA) and ethylene carbonate (EC) are investigated. A highly concentrated electrolyte with a molar ratio of EC/NaFTA = 1.5 is shown to exhibit a stable liquid state at room temperature. It is revealed that Na⁺ in the concentrated electrolyte coordinates—not only with EC but also with anions—to satisfy stable coordination numbers. From this specific coordination structure, dynamic ligand exchange of Na⁺ is shown to be enhanced, and [Na | NaFTA+1.5EC | Na_0.44MnO₂] cells are shown to achieve stable cycling performance by constant charge-discharge tests at 303 K.

Highly concentrated electrolytes have been studied to prevent the leaching diffusion of lithium (Li) polysulfides from sulfur (S) cathodes in Li–S batteries. Additionally, high-concentration electrolytes suppress the growth of Li dendrites at Li anodes. In this study, an ether-based high-concentration electrolyte was developed, enabling a microporous activated carbon–S composite cathode to achieve near-theoretical capacity performance and a long cycle life. The reference electrolyte was a high-concentration solution of Li bis(fluorosulfonyl)imide (LiFSI) in 1,2-dimethoxyethane (DME), known for its suitability for the stable dissolution and deposition of Li. However, its high viscosity impeded full penetration into the activated carbon–S composite cathode. To enhance the reversibility of the activated carbon–S composite cathode, we optimized the LiFSI-based electrolyte by adding hydrofluoroether (HFE) to reduce the viscosity and adjusting the LiFSI concentration to prevent the dissolution of Li polysulfides. Furthermore, the addition of Li difluoro(oxalate)borate (LiDFOB) to this electrolyte stabilized the cycling performance for over 100 cycles. When applied to a 1-Ah-class pouch cell, the electrolyte achieved an energy density of greater than 300 Wh kg⁻¹.

The extraction of health factors (HFs) is a crucial step in estimating the state of health (SOH) of lithium-ion batteries using data-driven methods. In this study, five feature indices were extracted from the constant voltage stage of the constant current-constant voltage (CC-CV) charging curve. The correlation between these feature indices and SOH exceeds 80 %, reaching up to 98 %. Unlike traditional lithium-ion battery data processing methods such as incremental capacity analysis (ICA) or probability density function (PDF), this strategy avoids complex data preprocessing. Through variance inflation factor (VIF) analysis, the slope index was identified as a key health factor. When combined with data-driven algorithms, it successfully estimated the SOH of lithium-ion batteries composed of three different material types under different proportions of training sets. The results show that with a 20 % training set proportion, the LSTM algorithm achieved an RMSE of less than 1.3 % and a mean absolute error (MAE) of no more than 1.1 %. Additionally, the health factor extraction strategy proved to be robust across different time and current parameter ranges, with RMSE fluctuations within 0.35 %.