Takaaki Tomai

Metal-free aqueous redox capacitor via proton rocking-chair system in an organic-based couple

Safe and inexpensive energy storage devices with long cycle lifetimes and high power and energy densities are mandatory for the development of electrical power grids that connect with renewable energy sources. In this study, we demonstrated metal-free aqueous redox capacitors using couples comprising low-molecular-weight organic compounds. In addition to the electric double layer formation, proton insertion/extraction reactions between a couple consisting of inexpensive quinones/hydroquinones contributed to the energy storage. This energy storage mechanism, in which protons are shuttled back and forth between two electrodes upon charge and discharge, can be regarded as a proton rocking-chair system. The fabricated capacitor showed a large capacity (>20u2005Wh/kg), even in the applied potential range between 0–1u2005V, and high power capability (>5u2005A/g). The support of the organic compounds in nanoporous carbon facilitated the efficient use of the organic compounds with a lifetime of thousands of cycles.

Controlling the shape of LiCoPO4 nanocrystals by supercritical fluid process for enhanced energy storage properties

Lithium-ion batteries offer promising opportunities for novel energy storage systems and future application in hybrid electric vehicles or electric vehicles. Cathode materials with high energy density are required for practical application. Herein, high-voltage LiCoPO4 cathode materials with different shapes and well-developed facets such as nanorods and nanoplates with exposed {010} facets have been synthesized by a one-pot supercritical fluid (SCF) processing. The effect of different amines and their roles on the morphology-control has been investigated in detail. It was found that amine having long alkyl chain such as hexamethylenediamine played important roles to manipulate the shape of the nanocrystals by selective adsorption on the specific {010} facets. More importantly, the nanorods and nanoplates showed better electrochemical performance than that of nanoparticles which was attributed to their unique crystallographic orientation with short Li ion diffusion path. The present study emphasizes the importance of crystallographic orientation in improving the electrochemical performance of the high voltage LiCoPO4 cathode materials for Li-ion batteries.

Supercritical fluid assisted synthesis of N-doped graphene nanosheets and their capacitance behavior in ionic liquid and aqueous electrolytes

N-doped graphene nanosheets (N-doped GNS) were obtained by a single step supercritical fluid assisted reaction of N-containing organic compounds with graphene oxide (GO) solution. A N-doped GNS sample shows capacitances of 280 F g−1 in aqueous 1 M H2SO4 (0.9 V) and 104 F g−1 in ionic liquid EMI-TFSA (3.6 V). A NE-GNS electrode shows energy densities of 8 W h kg−1 and 40 W h kg−1 in 1 M H2SO4 and EMI-TFSA, respectively. The nature of chemical bonding and the amount of N doping in the graphene samples were estimated by XPS spectroscopy. The amount of N-doping varies with the nature of the N-containing organic compounds and the supercapacitance behaviour depends on the amount of N-doping as well as the nature of N-doping in the graphene. TEM, FE-SEM images and Raman spectroscopic characterization reveals the presence of few-layer N-doped GNS. FT-IR spectra exhibit the presence of various functional groups on N-doped GNS. XRD diffraction analysis showed the weakly stacked N-doped GNS due to the N-doping and the presence of N-containing functional groups on N-doped GNS. The cyclic voltammetry studies showed the capacitance behaviour of N-doped GNS electrodes at a high potential window of 4.25 V in ionic liquids. The charge–discharge profile showed the stable charge–discharge behaviour of the N-doped GNS electrodes.

Supercritical fluid methods for synthesizing cathode materials towards lithium ion battery applications

Lithium ion battery materials improve the current technology to store electrical energy for the creation of green environment. The synthesis of lithium ion battery materials is crucial for energy applications in mobile electronic devices to plug-in hybrid electric vehicles. This review summarizes the recent progress made to synthesize lithium ion battery materials via supercritical fluid methods, and particularly, its application towards the synthesis of layered transition metal oxides, spinel structured cathodes, lithium metal phosphates, lithium metal silicates and lithium metal fluorophosphates. The structure, particle size, morphology and electrochemical properties of cathode materials are discussed. From the perspective of material synthesis, supercritical fluid methods are economical and have several advantages such as phase purity, morphology control and size tuning down to 5 nm, which would significantly impact the performance of lithium ion batteries.

Gas-Temperature-Dependent Characteristics of Cryo-Dielectric Barrier Discharge Plasma under Atmospheric Pressure

In the present work, cryo-dielectric barrier discharge (DBD) plasma was generated continuously below room temperature down to liquid nitrogen temperature (296 to 78 K) under atmospheric pressure using parallel indium–tin-oxide (ITO)-coated electrodes. Gas-temperature-dependent optical emission spectroscopy (OES) measurements and discharge pattern observation of cryo-DBD plasma were performed. In the case of helium gas, the discharge mode of cryo-DBD plasma changed from filamentary mode to glow mode as the gas temperature decreased. When introducing a small amount of nitrogen in helium gas, the filamentary discharge mode persisted upon decreasing the gas temperature, although the discharge pattern changed from concentric rings (296 K) to a hexagon-like pattern (78 K).

Antisite defects in LiCoPO4 nanocrystals synthesized via a supercritical fluid process

LiCoPO4 nanocrystals are synthesized via a supercritical fluid process at 380 °C for 1 h and we visualize Li/Co antisite defects along two crystallographic directions using annular dark and bright field scanning transmission electron microscopy (STEM). The antisite defects are observed with bright/dark contrast produced by Co atoms in Li columns in both HAADF (high angle annular dark field) and ABF (annular bright field) images viewed along [010] directions. Interestingly, few antisite defects are observed along [101] directions with weak contrast and P atoms are observed near to Co atoms.

Exfoliated MoS2 and MoSe2 Nanosheets by a Supercritical Fluid Process for a Hybrid Mg–Li-Ion Battery

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.

Controllable bandgap of Cu2ZnSn(S,Se)4 thin films via simultaneous supercritical fluid chalcogenization

Supercritical ethanol (scEtOH), with its high solubility and reducibility, can be used as a medium to fabricate chalcogenide semiconductors from stable solid chalcogen sources. We fabricated Cu2ZnSn(S,Se)4 films via chalcogenization of Cu–Zn–Sn oxide precursor films using scEtOH to dissolve SeO2 and elemental sulfur (S8). The S/Se molar ratio and the bandgap of Cu2ZnSn(S,Se)4 films were controlled by changing the input ratio of selenium source (SeO2) to sulfur source (S8). Analysis indicated that high-density atomic chalcogens are the dominant reactive species. This process may contribute to the development of non-vacuum fabrication methods for chalcogenide-semiconductor solar cells.

Synthesis of ZrO2 nanoparticles for liquid scintillators used in the detection of neutrinoless double beta decay

We synthesized liquid scintillators incorporating ZrO2 nanoparticles for application in neutrinoless double beta decay experiments. ZrO2 nanoparticles of less than 10xa0nm in size were synthesized with sub- and supercritical hydrothermal methods. The Zr concentrations in the liquid scintillators were determined to be up to 1.4xa0wt% with inductively coupled plasma analysis, and the liquid scintillators were transparent to scintillation. These results indicate that these methods are applicable for the preparation of liquid scintillators for neutrinoless double beta decay experiments.

High-energy-density electrochemical flow capacitors containing quinone derivatives impregnated in nanoporous carbon beads

Electrochemical flow capacitors (EFCs) are promising energy storage devices for smart grids due to their combination of the high power capabilities of electrochemical capacitors and the scalability of redox flow batteries. However, the limited energy densities of EFCs are their drawback. In this study, we demonstrate a promising approach to capacitance enhancement in EFCs, based on the impregnation of redox-active quinone derivatives in nanoporous carbon for flowable slurry electrolytes. Moreover, we employed two different quinone derivatives with different redox potentials as anodic and cathodic electrolytes, facilitating a proton-shuttle mechanism. The resulting slurry electrolytes exhibited good flowability, high power capability and an energy density of approximately 20 W h kg−1, 2.5 times higher than that of the unmodified carbon slurry. Charge–discharge measurements in an intermittent flow system confirmed that the modified slurry electrolytes were suitable for use in EFC systems. This approach, based on the use of solid-state redox-active organics, should allow extraordinary capacitance enhancement in flow-type energy storage systems such as redox flow batteries and EFCs.