Charge–Discharge Behavior of Bismuth in a Liquid Electrolyte for Rechargeable Batteries Based on a Fluoride Shuttle

Rechargeable batteries based on fluoride transfer have attracted attention because of the possibility of achieving high energy densities surpassing those of current lithium-ion batteries. Although the batteries of this type, fluoride shuttle batteries (FSBs), have been developed using solid electrolytes, most operate at relatively high temperatures, greater than 423 K. In addition, in attempts to fabricate FSBs using liquid electrolytes, only the discharge reactions have been investigated, and they still suffer from serious issues of reversibility. In the present study, we have prepared a fluoride-conducting liquid electrolyte by dissolving an organic fluoride in a room-temperature ionic liquid, yielding a FSB electrolyte with a high fluoride concentration (0.35 mol dm^–3) and conductivity (2.5 mS cm^–1). By using this electrolyte, we have demonstrated a rechargeable FSB working at room temperature that is constructed from Bi|BiF₃ and PbF₂|Pb couples as the positive and negative electrodes, respectively

Fluorination/Defluorination Behavior of Y₂C in Fluoride-Ion Battery Anodes

Despite the high theoretical energy density of fluoride-ion batteries (FIBs), their practical applications are hindered by the large volume changes associated with the redox reactions (typically metal ↔ metal fluoride interconversions) of most of the corresponding anode materials. Consequently, FIB anode materials that react at low potentials with small expansion and shrinkage are desired. Inspired by the low theoretical volume change (8%) of the Y2C ↔ Y2CF2 interconversion, we herein evaluated Y2C as an FIB anode material and determined its initial discharge and charge capacities as 565 and 432 mAh g–1, respectively. The first fluorination was characterized by a capacity plateau equivalent to a two-electron reaction at −2 V vs Pb/PbF2. The first and second halves of this region corresponded to the Y2C → Y2CF2 intercalation reaction and Y2CF2 lattice expansion, respectively, whereas further fluorination led to a YF3-like structure. Y2CF2 formed at the end of the first plateau was reversibly defluorinated to Y2C upon charging. The reversible change in the shape of the C K-edge electron energy loss spectrum during charge–discharge indicated the contribution of carbon to the redox reaction. Thus, this paper presents, for the first time, an account of the reversible electrochemical intercalation of fluoride ions in FIB anode materials, paving the way for FIB commercialization

Hierarchically Porous Carbon Monoliths Comprising Ordered Mesoporous Nanorod Assemblies for High-Voltage Aqueous Supercapacitors

This report demonstrates a facile one-pot synthesis of hierarchically porous resorcinol-formaldehyde (RF) gels comprising mesoporous nanorod assemblies with two-dimensional (2D) hexagonal ordering by combining a supramolecular self-assembly strategy in the nanometer scale and phase separation in the micrometer scale. The tailored multilevel pore system in the polymer scaffolds can be preserved through carbonization and thermal activation, yielding the multimodal porous carbon and activated carbon (AC) monoliths. The thin columnar macroframeworks are beneficial for electrode materials due to the short mass diffusion length through small pores (micro- and mesopores). By employing the nanostructured AC monolith as a binder-free electrode for supercapacitors, we have also explored the capability of “water-in-salt” electrolytes, aiming at high-voltage aqueous supercapacitors. Despite that the carbon electrode surface is supposed to be covered with salt-derived decomposition products that hinder the water reduction, the effective surface area contributing to electric double-layer capacitance in 5 M bis­(trifluoromethane sulfonyl)­imide (LiTFSI) is found to be comparable to that in a conventional neutral aqueous electrolyte. The expanded stability potential window of the superconcentrated electrolyte allows for a 2.4 V-class aqueous AC/AC symmetric supercapacitor with good cycle performance

High-Level Doping of Nitrogen, Phosphorus, and Sulfur into Activated Carbon Monoliths and Their Electrochemical Capacitances

The present report demonstrates a new technique for doping heteroatoms (nitrogen, phosphorus, and sulfur) into carbon materials via a versatile post-treatment. The heat-treatment of carbon materials with a reagent, which is stable at ambient temperatures and evolves reactive gases on heating, in a vacuum-closed tube allows the introduction of various heteroatom-containing functional groups into a carbon matrix. In addition, the sequential doping reactions give rise to dual- and triple-heteroatom-doped carbons. The pore properties of the precursor carbon materials are preserved through each heteroatom doping process, which indicates that independent tuning of heteroatom doping and nanostructural morphology can be achieved in various carbon materials. The electrochemical investigation on the undoped and doped carbon monolithic electrodes applied to supercapacitors provides insights into the effects of heteroatom doping on electrochemical capacitance

Penetration of Platinum Complex Anions into Porous Silicon: Anomalous Behavior Caused by Surface-Induced Phase Transition

We investigate the dynamics of the penetration of platinum complex anions into nanopores during platinum deposition within a nanoporous silicon electrode. The pore-wall surface is hydrophobic and the anions are large enough to behave like hydrophobic solutes. Some of the observations are anomalous in the sense that they cannot be understood in terms of the phenomenological theory for diffusion based on Fick’s law. For example, the penetration is faster when the pore diameter is smaller and the anion size is larger. The penetration rate remains unexpectedly fast even when the pores become deeper. The penetration can be made faster using large coexisting cations with sufficiently high hydrophobicity. We show that these results can be interpreted only by statistical mechanics of confined molecular liquids. When the manipulated variables (pore diameter, anion concentration, sizes of platinum complex anions and coexisting cations, etc.) are chosen so that the surface-induced phase transition (SIFT) can take place, the penetration is drastically accelerated. Under the condition with the SIFT occurrence, a strongly attractive, long-ranged effective surface–anion interaction comes into play, leading to the anomalous behavior. The experimental result is in qualitatively good accord with the theoretical argument. The outcome is of vital importance in controlling the mass transfer within nanoporous media and designing next-generation electrochemical devices

Hierarchically Porous Monoliths Based on N‑Doped Reduced Titanium Oxides and Their Electric and Electrochemical Properties

In this report, we demonstrate a novel synthesis method to obtain reduced titanium oxides with monolithic shape and with a well-defined hierarchically porous structure from the titanium-based network bridged with ethylenediamine. The hierarchically porous monoliths are fabricated by the nonhydrolytic sol–gel reaction accompanied by phase separation. This method allows a low-temperature crystallization into Ti₄O₇ and Ti₃O₅ at 800 and 900 °C, respectively, with N-doped carbon. These reduced titanium oxides are well-doped with N atoms even under argon atmosphere without NH₃, which accounts for the low-temperature reduction. The resultant monolithic materials possess controllable macropores and high specific surface area together with excellent electric conductivity up to 230 S cm^–1, indicating promise as a conductive substrate that can substitute carbon electrodes

Ultrafine Fiber Raman Probe with High Spatial Resolution and Fluorescence Noise Reduction

Considerable interest has been shown in fiber Raman probes as powerful tools for in situ biomedical diagnosis and monitoring processes in the materials industry. Miniaturization and high spatial resolution are required for less invasive measurements with accurate locations. In analysis of organs, widespread visible excitation light produces problematic fluorescence backgrounds. Here, we report an ultrafine fiber Raman probe that is thinner than the needle of a mosquito (labrum: 50–80 μm in diameter) with high spatial resolution (23 μm) and with a function of fluorescence background reduction. Due to the fineness and resolution, the distribution of ions in an electrolyte solution in narrow spaces could be measured. Backgrounds in spectra of liquid containing fluorescent impurity were reduced by using the probe. The probe has wide applicability for noninvasive in situ molecular diagnosis of organs and small devices

Effect of Calcination Conditions on Porous Reduced Titanium Oxides and Oxynitrides via a Preceramic Polymer Route

A preceramic polymer route from Ti-based inorganic–organic hybrid networks provides electroconductive N-doped reduced titanium oxides (Ti_<i>n</i>O_{2<i>n</i>–1}) and titanium oxynitrides (TiO_<i>x</i>N_<i>y</i>) with a monolithic shape as well as well-defined porous structures. This methodology demonstrates an advantageously lower temperature of the crystal phase transition compared to the reduction of TiO₂ by carbon or hydrogen. In this study, the effect of calcination conditions on various features of the products has been explored by adopting three different atmospheric conditions and varying the calcination temperature. The detailed crystallographic and elemental analyses disclose the distinguished difference in the phase transition behavior with respect to the calcination atmosphere. The correlation between the crystallization and nitridation behaviors, porous properties, and electric conductivities in the final products is discussed

Mechanism of Accelerated Zinc Electrodeposition in Confined Nanopores, Revealed by X‑ray Absorption Fine Structure Spectroscopy

Recent studies have revealed that electrochemistry at the nanometer scale is profoundly different from its conventional framework. We reported that the combination of a hydrophobic nanoporous electrode and low-charge-density metal ions resulted in a drastic acceleration in the electrodeposition reaction. In the present study, we analyzed Zn embedded in nanoporous silicon by X-ray absorption fine structure spectroscopy. As a precursor to Zn electrodeposition, Zn­(II) chelate was used under different pH conditions. The spectroscopy results clearly suggest that the accumulation of Zn­(II) chelate occurred at pH conditions where the Zn­(II) chelate had zero charge. The accumulation resulted in the promotion of Zn electrodeposition within confined nanopores. Based on this spectroscopic investigation, we propose a model for the accelerated electrodeposition of Zn in confined nanopores