Structure of Hydronium (H₃O⁺)/Chloride (Cl⁻) Contact Ion Pairs in Aqueous Hydrochloric Acid Solution: A Zundel-like Local Configuration

A comprehensive analysis of the H3O+ and H2O structure in the first solvation shell about Cl− in aqueous HCl solutions is reported from X-ray absorption fine structure (XAFS) measurements. Results show increasing degree of contact ion pairing between Cl− and H3O+ as the HCl concentration increases from 6.0 m, 10.0 m, and finally 16.1 m HCl (acid concentrations are expressed as molality or mole HCl/1000 g water). At the highest acid concentration there are on average, approximately 1.6 H3O+ ions and 4.2 H2O’s in the first shell about Cl−. The structure of the Cl−/H3O+ contact ion pair is distinctly different from that of the H2O structure about Cl−. The Cl−O bond length (2.98 Å) for Cl−/H3O+ is approximately 0.16 Å shorter than the Cl−/H2O bond. The bridging proton resides at an intermediate position between Cl and O at 1.60 Å from the Cl− and approximately 1.37 Å from the O of the H3O+. The bridging-proton structure of this contact ion pair, (Cl−H−OH2), is similar to the structure of the water Zundel ion, (H2O−H−OH2+). In both cases there is a shortened Cl−O or O−O bond, and the intervening proton bond distances are substantially longer than for the covalent bonds of either HCl or H2O. A detailed structural analysis of the aqueous chloride species, Cl−/(H2O)n, was also completed as part of this study in order to understand the relative importance of various XAFS photoelectron scattering paths. For aqueous Cl− the measured Cl−O and Cl−H distances of 3.14 Å and 2.23 Å, respectively, are in excellent agreement with earlier neutron and X-ray diffraction results. Overall, these results significantly improve our understanding of the interaction of H3O+ with Cl−. The results are of interest to fundamental physical chemistry and they have important consequences in biochemical, geochemical, and atmospheric processes

Hydrated Structure of Ag(I) Ion from Symmetry-Dependent, K- and L-Edge XAFS Multiple Scattering and Molecular Dynamics Simulations

Details of the first-shell water structure about Ag+ are reported from a corefinement of the K- and L2-edge multiple scattering signal in the X-ray absorption fine structure (XAFS) spectra. Detailed fits of the Ag K-edge data that include the contributions from multiple scattering processes in the hydrated ion structure cannot distinguish between models containing tetrahedral symmetry versus those containing collinear O−Ag−O bonds. However, we show that the multiple scattering oscillations at the L2-edges have distinctly different phase and amplitude functions than at the K-edge. These phase and amplitude functions depend not only on the symmetry of the multiple scattering paths but also on the nature of the final state electronic wave function probed by the dipole-allowed transition. Hence the multiple scattering portions of K- and L2-edge spectra provide independent measurements of the local symmetrynot a redundant measurement as is commonly believed. On the basis of the enhanced information content obtained by the simultaneous assessment of both the K- and L2-edges, we report that the hydrated Ag+ structure contains five or six water molecules in the first shell with a significant number of nearly collinear and 90° O−Ag−O bond angles. Finally, the K- and L2-edge spectra are used to benchmark the hydration structure that is generated from both DFT-based and classical molecular dynamics simulations. Simulated first-shell structures are compared to the experimental structures

Anomalous Thermal Expansion in the Square-Net Compounds <i>RE</i>₄<i>T</i>Ge₈ (<i>RE</i> = Yb, Gd; <i>T</i> = Cr–Ni, Ag)

The family of materials RE4TGe8 (RE = Yb, Gd; T = transition metal) exhibits directional zero thermal expansion (ZTE) via a process that is associated with the linking of planar square nets in the third dimension. The Ge square nets in these compounds exhibit commensurate long-range modulations similar to those observed in charge-density-wave compounds. The ZTE is manifested in the plane of the square nets from 10 to 300 K with negligible volume expansion below ∼160 K. The specific atomic arrangement in RE4TGe8 enables a Poisson-like mechanism that allows the structure to contract along one direction as it expands only slightly in the perpendicular direction

Depth-Dependent Understanding of Cathode Electrolyte Interphase (CEI) on the Layered Li-Ion Cathodes Operated at Extreme High Temperature

The high-temperature operation of Li-ion batteries is highly dependent on the stability of the cathode electrolyte interphase (CEI) formed during lithiation–delithiation reactions. However, knowledge on the nature of the CEI is limited and its stability under extreme temperatures is not well understood. Therefore, herein, we investigate a proof-of-concept study on stabilizing CEI on model LiNi0.33Mn0.33Co0.33O2 (NMC333) at an extreme operation condition of 100 °C using the thermally stable pyrrolidinium-based ionic liquid electrolyte. The electrochemical lithiation–delithiation reactions at 100 °C and the CEI evolution upon different cycling conditions are investigated. Further, the depth-dependent CEI chemistry was investigated using energy-tunable synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). The results reveal that the high-temperature operation accelerated the CEI formation compared to room temperature, and the surface of the interphase layer is rich in boron-based inorganic moieties than the deeper surface. Further, bulk-sensitive X-ray absorption spectroscopy (XAS) was used to investigate the transition-metal redox contributors during high-temperature electrochemical reactions; similar to room temperature, the Ni2+/4+ redox couple is the only charge-compensating redox couple during high-temperature operation. Finally, the physical nature of the conformal CEI on the cathode particles was visualized with high-resolution transmission electron microscopy, which confirms that the significant degradation of cathode particles without conformal CEI is due to the transformation of a layer-to-spinel formation at extreme temperature. In this study, understanding this high-temperature interfacial chemistry of NMC cathodes through advanced spectroscopy and microscopy will shed light on transforming the ambient-temperature Li-ion chemistry into high-temperature applications

Mixed Valency in Yb₇Co₄InGe₁₂: A Novel Intermetallic Compound Stabilized in Liquid Indium

The quaternary compounds RE7Co4InGe12 (RE = Dy, Ho, Yb) were obtained from In flux reactions as thin silver needles. RE7Co4InGe12 crystallizes in the tetragonal P4/m space group under a new structure type which is characterized by columnar units forming three different types of channels with the RE atoms situated within these channels. Investigation of the Yb analog with magnetic susceptibility measurements, X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES) revealed that Yb7Co4InGe12 is a mixed-valence compound and that the relative Yb3+/Yb2+ ratio is slightly temperature-dependent. Additionally, resistivity measurements for Yb7Co4InGe12 exhibited negative magnetoresistance at low temperatures

Mixed Valency in Yb₇Co₄InGe₁₂: A Novel Intermetallic Compound Stabilized in Liquid Indium

The quaternary compounds RE7Co4InGe12 (RE = Dy, Ho, Yb) were obtained from In flux reactions as thin silver needles. RE7Co4InGe12 crystallizes in the tetragonal P4/m space group under a new structure type which is characterized by columnar units forming three different types of channels with the RE atoms situated within these channels. Investigation of the Yb analog with magnetic susceptibility measurements, X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES) revealed that Yb7Co4InGe12 is a mixed-valence compound and that the relative Yb3+/Yb2+ ratio is slightly temperature-dependent. Additionally, resistivity measurements for Yb7Co4InGe12 exhibited negative magnetoresistance at low temperatures

Amorphous TiO₂ Nanotube Anode for Rechargeable Sodium Ion Batteries

Sodium ion batteries are an attractive alternative to lithium ion batteries that alleviate problems with lithium availability and cost. Despite several studies of cathode materials for sodium ion batteries involving layered oxide materials, there are few low-voltage metal oxide anodes capable of operating sodium ion reversibly at room temperature. We have synthesized amorphous titanium dioxide nanotube (TiO₂NT) electrodes directly grown on current collectors without binders and additives to use as an anode for sodium ion batteries. We find that only amorphous large diameter nanotubes (>80 nm I.D.) can support electrochemical cycling with sodium ions. These electrodes maximize their capacity in operando and reach reversible capacity of 150 mAh/g in 15 cycles. We also demonstrate for the first time a full cell all-oxide Na ion battery using TiO₂NT anode coupled to a Na_1.0Li_0.2Ni_0.25Mn_0.75O_δ cathode at room temperature exhibiting good rate capability

Unveiling the Cerium(III)/(IV) Structures and Charge-Transfer Mechanism in Sulfuric Acid

The Ce3+/Ce4+ redox couple has a charge transfer (CT) with extreme asymmetry and a large shift in redox potential depending on electrolyte composition. The redox potential shift and CT behavior are difficult to understand because neither the cerium structures nor the CT mechanism are well understood, limiting efforts to improve the Ce3+/Ce4+ redox kinetics in applications such as energy storage. Herein, we identify the Ce3+ and Ce4+ structures and CT mechanism in sulfuric acid via extended X-ray absorption fine structure spectroscopy (EXAFS), kinetic measurements, and density functional theory (DFT) calculations. We show EXAFS evidence that confirms that Ce3+ is coordinated by nine water molecules and suggests that Ce4+ is complexed by water and three bisulfates in sulfuric acid. Despite the change in complexation within the first coordination shell between Ce3+ and Ce4+, we show that the kinetics are independent of the electrode, suggesting outer-sphere electron-transfer behavior. We identify a two-step mechanism where Ce4+ exchanges the bisulfate anions with water in a chemical step followed by a rate-determining electron transfer step that follows Marcus theory (MT). This mechanism is consistent with all experimentally observed structural and kinetic data. The asymmetry of the Ce3+/Ce4+ CT and the observed shift in the redox potential with acid is explained by the addition of the chemical step in the CT mechanism. The fitted parameters from this rate law qualitatively agree with DFT-predicted free energies and the reorganization energy. The combination of a two-step mechanism with MT should be considered for other metal ion CT reactions whose kinetics have not been appropriately described

Mixed Valency in Yb₇Co₄InGe₁₂: A Novel Intermetallic Compound Stabilized in Liquid Indium

The quaternary compounds RE7Co4InGe12 (RE = Dy, Ho, Yb) were obtained from In flux reactions as thin silver needles. RE7Co4InGe12 crystallizes in the tetragonal P4/m space group under a new structure type which is characterized by columnar units forming three different types of channels with the RE atoms situated within these channels. Investigation of the Yb analog with magnetic susceptibility measurements, X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES) revealed that Yb7Co4InGe12 is a mixed-valence compound and that the relative Yb3+/Yb2+ ratio is slightly temperature-dependent. Additionally, resistivity measurements for Yb7Co4InGe12 exhibited negative magnetoresistance at low temperatures

Mixed Valency in Yb₇Co₄InGe₁₂: A Novel Intermetallic Compound Stabilized in Liquid Indium

The quaternary compounds RE7Co4InGe12 (RE = Dy, Ho, Yb) were obtained from In flux reactions as thin silver needles. RE7Co4InGe12 crystallizes in the tetragonal P4/m space group under a new structure type which is characterized by columnar units forming three different types of channels with the RE atoms situated within these channels. Investigation of the Yb analog with magnetic susceptibility measurements, X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES) revealed that Yb7Co4InGe12 is a mixed-valence compound and that the relative Yb3+/Yb2+ ratio is slightly temperature-dependent. Additionally, resistivity measurements for Yb7Co4InGe12 exhibited negative magnetoresistance at low temperatures