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A quinone-impregnation process using scCO2 yielded nearly twice the specific capacity and higher rate properties than that with liquid sonication.

Glycine was applied as the leachant for the hydrothermal leaching of lithium-ion battery (LIB) cathode materials, LiCoO2 and LiNiO2, at 90–180 °C for 5–90 min. LiCoO2 was completely leached at 180 ...

Hard carbon (HC) is the most promising candidate for sodium-ion battery anode materials. Several material properties such as intensity ratio of the Raman spectrum, lateral size of HC crystallite (La ), and interlayer distance (d002 ) have been discussed as factors affecting anode performance. However, these factors do not reflect the bulk property of the Na+ intercalation reaction directly, since Raman analysis has high surface sensitivity and La and d002 provide only one-dimensional crystalline information. Herein, it was proposed that the crystallite interlayer area (Ai ) defined using La , d002 , and stacking height (Lc ) governs Na+ intercalation behavior of various HCs. It was revealed that various wood-derived HCs exhibited the similar total capacity of approximately 250 mAh g-1 , whereas the Na+ intercalation capacity (Ci ) was proportional to Ai with the correlation coefficient of R2 =0.94. The evaluation factor of Ai was also adaptable to previous reports and strongly correlated with their Ci , indicating that Ai is more widely adaptable than the conventional evaluation methods.

The first run of continuous hydrothermal leaching of lithium-ion battery cathode materials, LiCoO2, was performed using citric acid as the leachant at 200 °C. The flow system was specially designed and customized. Prior to the hydrothermal leaching experiments, a three-layer model was used to predict the flow state in this flow system, and a cold flow test using two kinds of flow lines was performed to determine the conditions in the preliminary experiments. Finally, the pulp density of the slurry and the flow rate were set to 10 g L−1 and 30 ml min−1, respectively, for the continuous hydrothermal leaching experiments. At 60 min after the start of slurry feeding, the leaching efficiency of Li and Co reached 81.3% and 92.7%, respectively, and can continue to increase with the extension of time. The successful run indicated that the process of hydrothermal leaching is feasible and promising to be applied in practice. Meanwhile, a problem of acid corrosion caused by the use of citric acid during this process was revealed and is expected to be resolved using inner coating materials with high acid corrosion resistance or organic acids with low or no acid corrosion as the leachant.

Molybdenum disulfide (MoS2), an attractive material for energy conversion devices, is known to exhibit varying properties depending on the number of layers and the phase structure. In this study, we developed a supercritical hydrothermal process that allows the controllable synthesis of MoS2 nanosheets with various structural and morphological characteristics. Detailed characterization of the synthesized materials confirmed that the number of layers in the MoS2 nanosheets could be controlled by varying the organic reducing agent under supercritical hydrothermal conditions. In addition, the MoS2 phase could be controlled kinetically by varying the reaction time with ascorbic acid as the reducing agent. Because water and elemental sulfur were respectively used as the solvent and sulfur source and as the reaction time was minimal, the developed hydrothermal process represents a facile processing method for different types of MoS2 nanosheets.

Inexpensive, high-performing, and environmentally friendly energy storage devices are required for smart grids that efficiently utilize renewable energy. Energy storage devices consisting of organic active materials are promising because organic materials, especially quinones, are ubiquitous and usually do not require harsh conditions for synthesis, releasing less CO2 during mass production. Although fundamental research-scale aqueous quinone-based organic supercapacitors have shown excellent energy storage performance, no practical research has been conducted. We aimed to develop a practical-scale aqueous-quinone-based organic supercapacitor. By connecting 12 cells of size 10 cm × 10 cm × 0.5 cm each in series, we fabricated a high-voltage (> 6 V) aqueous organic supercapacitor that can charge a smartphone at a 1 C rate. This is the first step in commercializing aqueous organic supercapacitors that could solve environmental problems, such as high CO2 emissions, air pollution by toxic metals, and limited electricity generation by renewable resources.

There is an urgent need to develop renewable sources of energy and use existing resources in an efficient manner. In this study, in order to improve the utilization of unused biomass and develop green processes and sustainable technologies for energy production and storage, unused Douglas fir sawdust (SD) was transformed into catalysts for the oxygen reduction reaction. Fe and N were doped into SD during hydrothermal carbonization, and the N- and Fe-doped wood-derived carbon (Fe/N/SD) was carbonized in a nitrogen atmosphere. After the catalyst had been calcined at 800°C, its showed the highest current density (−5.86 mAcm −2 at 0.5 V versus reversible hydrogen electrode or RHE) and E onset value (0.913 V versus RHE). Furthermore, its current density was higher than that of Pt/C (20 wt% Pt) (−5.66 mA cm −2 @0.5 V versus RHE). Finally, after 50 000 s, the current density of sample Fe/N/SD (2 : 10 : 10) remained at 79.3% of the initial value. Thus, the synthesized catalysts, which can be produced readily at a low cost, are suitable for use in various types of energy generation and storage devices, such as fuel cells and air batteries. This article is part of the theme issue ‘Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 2)’.

Molybdenum disulfide (MoS2), an attractive material for energy conversion devices, is known to exhibit varying properties depending on the number of layers and the phase structure. In this study, we developed a supercritical hydrothermal process that allows the controllable synthesis of MoS2 nanosheets with various structural and morphological characteristics. Detailed characterization of the synthesized materials confirmed that the number of layers in the MoS2 nanosheets could be controlled by varying the organic reducing agent under supercritical hydrothermal conditions. In addition, the MoS2 phase could be controlled kinetically by varying the reaction time with ascorbic acid as the reducing agent. Because water and elemental sulfur were respectively used as the solvent and sulfur source and as the reaction time was minimal, the developed hydrothermal process represents a facile processing method for different types of MoS2 nanosheets.

Structural defects, including point defects, dislocation and planar defects, significantly affect the physical and chemical properties of low-dimensional materials, such as layered compounds. In particular, inversion domain boundary is an intrinsic defect surrounded by a 60° grain boundary, which significantly influences electronic transport properties. We study atomic structures of the inversion domain grain boundaries (IDBs) in layered transition metal dichalcogenides (MoSe2 and MoS2) obtained by an exfoliation method, based on the aberration-corrected scanning transmission electron microscopy observation and density functional theory (DFT) calculation. The atomic-scale observation shows that the grain boundaries consist of two different types of 4-fold ring point shared and 8-fold ring edge shared chains. The results of DFT calculations indicate that the inversion domain grain boundary behaves as a metallic one-dimensional chain embedded in the semiconducting MoSe2 matrix with the occurrence of a new state within the band gap.

The electrocatalytic hydrogen evolution reaction (HER) is known to be an important technique for efficient hydrogen production, and the development of a highly active electrocatalyst with lower overpotential is a key issue. In this study, we demonstrated the rapid, one-pot synthesis of highly active molybdenum dichalcogenide/reduced graphene oxide (Mo(S,Se)/rGO) electrocatalysts from cost-effective and environmentally friendly substances using reductive supercritical conditions. We found that edge-rich MoS2 is synthesized under a high-density ethanol condition and successfully controlled the composition of molybdenum dichalcogenide (MoS2–xSex) nanoflowers at the atomic level by varying the ratio of chalcogen sources. The electrocatalytic HER activity increased with the formation of an edges-rich structure and a solid solution of MoS2 and MoSe2, and by complexing with graphene. The synthesized MoS0.9Se1.1/graphene showed excellent HER properties that can be categorized into the highest-activity group of th...

A supercritical ethanol (scEtOH) environment, in which sulfur is highly soluble, can lead to safety improvements and cost reductions in the sulfurization process. In this study, the feasibility of the sulfurization process in scEtOH for the preparation of Cu2ZnSnS4 (CZTS) thin films at low temperature was verified with the purpose of creating a sustainable and cost-effective process for fabricating metal-sulfide solar cells. We found that to promote atomic diffusion of sulfur and achieve sulfurization at a low temperature, the presence of defects in the amorphous oxide thin film was preferable. Moreover, in thin film fabricated by sulfurization under ethanol, the crystal size was strongly affected by ethanol density. The grain size increased up to about 1 μm as the ethanol density increased, and grain growth was remarkable, particularly in the high-density conditions of over 3.0 mol/L. Finally, we fabricated a crystalline CZTS thin film, which exhibited structural and optical properties comparable to those of a film fabricated using conventional vapor-phase sulfurization. For the prepared disordered-kesterite CZTS thin film with a Cu-poor and Zn-rich composition, photoluminescence measurements confirmed that the donor or acceptor defects engage in the emission of about 1.24 eV at 5 K, and UV–Vis measurement revealed a bandgap of 1.38 eV at room temperature.

Top-down graphene production via exfoliation from graphite produces a mass of graphene with structural variation in terms of the number of layers, sheet size, edge type, and defect density. All of these characteristics affect its electronic structure. To develop useful applications of graphene, structural separation of graphene is necessary. In this study, we investigate the adsorption behavior of different types of graphene fragments using a multicolumn gel chromatography system with a view to developing an efficient method for separating high-quality graphene. The graphene was dispersed in an aqueous sodium dodecyl sulfate (SDS) surfactant solution and flown through allyl-dextran-based gel columns connected in series. In the chromatographic operation, we observed that the small-sized or oxidized graphene fragments tended to bind to the gel and the relatively large-sized graphene with a low oxygen content eluted from the gel column. In this system, the adsorbed SDS molecules on the graphitic surface prevented graphitic materials from binding to the gel and the oxygen functional groups on the graphene oxide or at the abundant edge of small-sized graphene hindered SDS adsorption. We hypothesize that the reduced SDS adsorption density results in the preferential adsorption of small-sized or oxidized graphene fragments on the gel. This type of chromatographic separation is a cost-effective and scalable method for sorting nanomaterials. The structural separation of graphene based on the adsorption priority found in this study will improve the quality of graphene nanosheets on an industrial scale.

The ultrathin two-dimensional nanosheets of layered transition-metal dichalcogenides (TMDs) have attracted great interest as an important class of materials for fundamental research and technological applications. Solution-phase processes are highly desirable to produce a large amount of TMD nanosheets for applications in energy conversion and energy storage such as catalysis, electronics, rechargeable batteries, and capacitors. Here, we report a rapid exfoliation by supercritical fluid processing for the production of MoS2 and MoSe2 nanosheets. Atomic-resolution high-angle annular dark-field imaging reveals high-quality exfoliated MoS2 and MoSe2 nanosheets with hexagonal structures, which retain their 2H stacking sequence. The obtained nanosheets were tested for their electrochemical performance in a hybrid Mg–Li-ion battery as a proof of functionality. The MoS2 and MoSe2 nanosheets exhibited the specific capacities of 81 and 55 mA h g–1, respectively, at a current rate of 20 mA g–1.

In this study, we fabricated a three-dimensional CuInS 2 solar-cell structure using supercritical fluid deposition and sulfurization, in which p-n junction is formed at the large surface of a nanoporous n-type TiO 2 film filled with a p-type CuInS 2 semiconductor to facilitate the efficient collection of photoinduced electrons and holes. Using the reductive reaction of organometallic precursors dissolved in supercritical fluid at the surface, we successfully and efficiently deposited Cu–In film inside the nanoporous film consisting of TiO 2 nanoparticles with a diameter of a few tens of nanometers. The CuInS 2 film formed following the sulfurization of the Cu–In film exhibited an ideal band gap (1.5 eV) and photoconductivity. The results obtained in this study expand the applicability of supercritical fluid technology to the fabrication of chalcogenide semiconductors inside nanostructured materials and will contribute to the future development of or improvement in various types of energy devices.

This study presents a simple one-pot synthesis method to achieve few-layered and defective Mo(S,Se)2 and (Mo,W)S2 by using supercritical water with organic reducing agents from simple and less-toxic precursors. This synthesis process is expected to be suitable for preparing other various kinds of TMD solid solutions.

Metal ions in ternary LIB cathode materials (LiCoO2/LiNiO2/LiMn2O4) were leached into hydrothermal water with citric acid at 60–90 °C. The leaching efficiency increased with temperature and time. Mn ion was completely leached at 60 °C for 5 min, and at 90 °C for 20 min, the leaching efficiencies of Li, Ni, and Co arrived at 81 %, 73 %, and 100 %, respectively. Hydrothermal leaching has a higher rate than the traditional method and avoids the pollutions caused by high-concentration acids and reductants like H2O2. To improve the Ni leaching efficiency, hydrothermal leaching of binary LIB cathode materials (LiCoO2/LiNiO2) with citric acid was performed at 90–120 °C. At above 100 °C for 20 min, the Ni leaching efficiency reached to 90 %. A shrinking unreacted-core model was adopted to predict reaction behavior, and the activated energy for leaching each metal was calculated. Additionally, a reasonable correlation of the rate constants of Co leaching at different pulp densities was attempted.

Single- to few-layers MoSe2 nanosheets were obtained from bulk MoSe2 by a supercritical fluid exfoliation. High angle annular dark field (HAADF) imaging showed the presence of a great variety of vacancies and edge defects within the exfoliated MoSe2 nanosheets. Thanks to these defects, the exfoliated MoSe2 nanosheets showed attractive catalytic performance and robustness for the hydrogen evolution reaction in water. The best electrode made of these nanosheets required 300 mV overpotential to generate a catalytic current of 10 mA/cm2 and showed a Tafel slop of 90 mV/s. The catalytic performance was further enhanced by twice when the MoSe2 nanosheets catalyst was conditioned at a mild oxidative potential, e.g.+0.73 V vs. RHE. The mild oxidation treatment was proposed to create novel Se-vacancy and Se2-vacancy while an oxidation treatment at higher potential, e.g. +1.23 V vs. RHE, completely oxidized MoSe2 into inactive MoO3. As a consequence, the later caused a rapid degradation of catalytic performance.

Structural defects, including point defects, dislocation and planar defects, significantly affect the physical and chemical properties of low-dimensional materials, such as layered compounds. In particular, inversion domain boundary is an intrinsic defect surrounded by a 60° grain boundary, which significantly influences electronic transport properties. We study atomic structures of the inversion domain grain boundaries (IDBs) in layered transition metal dichalcogenides (MoSe

In this study, we achieved the conversions of Cu–In and Cu–Zn–Sn oxide precursor films to CuInSe 2 and Cu 2 ZnSnSe 4 films, respectively, using SeO 2 as a selenium source in supercritical ethanol (EtOH). EtOH enables sufficient supply of SeO 2 to reaction field due to its high solubility. Moreover, EtOH in high temperature condition reacts with SeO 2 to form diethylselenide, Et 2 Se, and elemental selenium, and shows self-oxidation to release hydrogen. These features of EtOH will contribute to the selenization for the fabrication of the metal-selenide film facilitates the fabrication of metal-selenide film with ideal band gap, even when both precursor film and selenium source are oxides. Supercritical-alcohol-assisted selenization will contribute to the development of the cost-effective non-vacuum fabrication process for metal-selenide in optical devices, such as solar cells.

The ultrathin two-dimensional nanosheets of layered transition-metal dichalcogenides (TMDs) have attracted great interest as an important class of materials for fundamental research and technological applications. Solution-phase processes are highly desirable to produce a large amount of TMD nanosheets for applications in energy conversion and energy storage such as catalysis, electronics, rechargeable batteries, and capacitors. Here, we report a rapid exfoliation by supercritical fluid processing for the production of MoS2 and MoSe2 nanosheets. Atomic-resolution high-angle annular dark-field imaging reveals high-quality exfoliated MoS2 and MoSe2 nanosheets with hexagonal structures, which retain their 2H stacking sequence. The obtained nanosheets were tested for their electrochemical performance in a hybrid Mg–Li-ion battery as a proof of functionality. The MoS2 and MoSe2 nanosheets exhibited the specific capacities of 81 and 55 mA h g–1, respectively, at a current rate of 20 mA g–1.