Copper Electroactivity in Prussian Blue-Based Cathode Disclosed by Operando XAS

The electronic and structural evolution of copper hexacyanoferrate (CuHCF) cathode material was studied by operando X-ray absorption spectroscopy (XAS) simultaneously at both Fe and Cu K edges during a full galvanostatic cycle. The full set of XAS data collected during the electrochemical process was analyzed by a combined chemometric approach using the multicurve resolution analysis with the alternate least squares algorithm. Using this joint approach and by applying a simultaneous multiple-edge fitting procedure, it was possible to clarify the participation of both copper and iron centers to the redox processes and to analyze their local environment. The structural modifications occurring in CuHCF along with the redox processes are entirely reversible, with the steady multiplicity of Fe–C–N–Cu linear chains evidencing the structural stability of the material during cycling

Speciation of Gold Nanoparticles by Ex Situ Extended X‑ray Absorption Fine Structure and X‑ray Absorption Near Edge Structure

A combined X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) methodology is here presented on a series of partially and fully reduced Au^III samples. This allows monitoring the relative fraction of Au^III and Au⁰ in the studied samples, displaying a consistent and independent outcome. The strategy followed is based, for the first time, on two structural models that can be fitted simultaneously, and it evaluates the correlation among strongly correlated parameters such as coordination number and the Debye–Waller factor. The results of the present EXAFS and XANES approach can be extended to studies based on X-ray absorption spectroscopy experiments for the in situ monitoring of the formation of gold nanoclusters

Fluorinated Ether Based Electrolyte for High-Energy Lithium–Sulfur Batteries: Li⁺ Solvation Role Behind Reduced Polysulfide Solubility

By employing new electrolytes, the polysulfide shuttle phenomenon, one of the main problems of lithium–sulfur (Li–S) batteries, can be significantly reduced. Here we present excellent Coulombic efficiencies as well as adequate performance of high-energy Li–S cells by the use of a fluorinated ether (TFEE) based electrolyte at low electrolyte loading. The observed altered discharge profile was investigated both by electrochemical experiments and an especially tailored COSMO-RS computational approach, while the details of the discharge mechanism were elucidated by two <i>operando</i> techniques: XANES and UV–vis spectroscopy. A significant decrease of polysulfide solubility compared to tetraglyme is due to different Li⁺ solvation mode

Analytical Detection of Polysulfides in the Presence of Adsorption Additives by Operando X‑ray Absorption Spectroscopy

A mechanism for Li–S battery operation with a composite electrode and an adsorption additive obtained by using operando ultraviolet/visible (UV/vis) spectroscopy and X-ray absorption spectroscopy confirms the role of the adsorption additive and reflects the conversion mechanism of sulfur into Li₂S. Operando UV/vis spectroscopy shows a reversible appearance of the long-chain polysulfides in the separator in the fifth cycle, whereas the appearance of mid- and short-chain polysulfides suggests a polysulfide shuttle mechanism. By using a nonsulfur-containing electrolyte, a high-precision analysis of sulfur K-edge X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectra is possible. The XANES analysis shows that polysulfides reach the maximum concentration at the end of the high-voltage plateau, and the low-voltage plateau is characteristic of the polysulfides/Li₂S equilibrium. The relative amount of Li₂S increases linearly until the end of discharge and reaches a relative amount of 75%. This is confirmed by sulfur K-edge EXAFS analysis. Additionally, a quantitative analysis of EXAFS spectra measured during discharge evidences a decrease of the average S–S coordination number. This can be interpreted as a decrease of the chain length of polysulfides. EXAFS analysis showed that there are no specific interactions of the polysulfide species with the matrix or with other species in the electrolyte

Straightforward Synthesis of Gold Nanoparticles Supported on Commercial Silica-Polyethyleneimine Beads

Stable silica-supported gold nanoparticles (Au_NPs) suitable for catalysis applications were conveniently obtained in a straightforward, one-step synthesis by simply adding an aqueous solution of HAuCl₄ to commercial polyethyleneimine-functionalized silica beads (SiO₂-PEI) as the only reactant without any external reducing agent and/or conventional stabilizing moieties. Six different types of Au_NPs/(SiO₂-PEI) beads termed <b>Au</b>_{<b><i>x</i>–<i>y</i></b>}<b>h</b>, where <i>x</i> is the initial HAuCl₄ concentration (1, 5, or 10 mM) and <i>y</i> is the reaction time (1 or 24 h), were prepared and characterized by UV–vis diffuse reflectance spectroscopy, X-ray fluorescence, FE-SEM microscopy, and X-ray absorption spectroscopy. The SEM micrographs of <b>Au</b>_{<b><i>x</i>–<i>y</i></b>}<b>h</b> samples showed that the particle size distribution decreases with the increase of the starting gold concentration, i.e., 70–100 nm for <b>Au</b>_<b>1–</b>_{<b><i>x</i></b>}<b>h</b>, 40–70 nm for <b>Au</b>_{<b>5</b><b>–</b>}_{<b><i>x</i></b>}<b>h</b>, and <b>Au</b>_{<b>10</b><b>–</b>}_{<b><i>x</i></b>}<b>h</b>, whereas on passing from 1 to 24 h the aggregation phenomena overcome the nucleation ones, promoting the formation of bigger aggregates at the expense of small Au_NPs. The XAS analysis as a combination of XANES and EXAFS studies provided detailed structural information regarding the coordination geometry and oxidation state of the gold atoms present on the beads. Moreover, the catalytic activity of the modified silica beads in the reduction of 4-nitrophenol to 4-aminophenol by NaBH₄ was investigated and in one case the XAS analysis was repeated after recovery of the catalyst, demonstrating further reduction of the Au site to Au(0)

Structural Flexibility and Role of Vicinal 2-Thienyl Rings in 2,3-Dicyano-5,6-di(2-thienyl)-1,4-pyrazine, [(CN)₂Th₂Pyz], Its Palladium(II) Complex [(CN)₂Th₂Pyz(PdCl₂)₂], and the Related Pentametallic Pyrazinoporphyrazines [(PdCl₂)₄Th₈TPyzPzM] (M = Mg^II(H₂O), Zn^II)

The solid state and solution structure of 2,3-dicyano-5,6-di(2-thienyl)-1,4-pyrazine, [(CN)₂Th₂Pyz], and its Pd^II derivative, [(CN)₂Th₂Pyz(PdCl₂)₂]·H₂O, formed by reaction of [(CN)₂Th₂Pyz] with [(C₆H₅CN)₂PdCl₂] were characterized by X-ray, UV–visible, ¹H and ¹³C NMR, and extended X-ray absorption fine structure (EXAFS) spectral measurements. The X-ray crystal structure of [(CN)₂Th₂Pyz] shows the presence of one thienyl ring positioned orthogonal to the rest of the molecule, with the two vicinal thienyl rings lying orthogonal to each other in a rare arrangement. NMR studies of [(CN)₂Th₂Pyz] in the solid state and in solutions of dimethylformamide or dimethyl sulfoxide confirm a nonequivalence of the thienyl rings in the solid state and also in solution. EXAFS results indicate that two distinct Pd^II coordination sites are formed at the di(2-thienyl)pyrazino moiety of [(CN)₂Th₂Pyz(PdCl₂)₂]·H₂O, with identical Pd–N_pyz (2.03(3) Å) and Pd–Cl (2.36(3) Å) bond lengths but with different Pd–S1 (2.25(4) Å) and Pd–S2 (3.21(5) Å) bond distances in an overall asymmetric molecular framework. Density functional theory (DFT) and time-dependent DFT (TDDFT) theoretical studies also provide information about the structure and spectral behavior of the precursor and its metalated Pd^II derivative. ¹H/¹³C NMR and UV–visible spectral measurements were also carried out on two heteropentametallic porphyrazine macrocycles which were prepared by a reaction of PdCl₂ with [Th₈TPyzPzM] where Th₈TPyzPz = tetrakis-2,3-[5,6-di-(2-thienyl)-pyrazino]porphyrazinato dianion and M = Mg^II(H₂O) or Zn^II. Spectroscopic data on the newly synthesized [(PdCl₂)₄Th₈TPyzPzM] compounds suggest that the binding of PdCl₂ involves coordination sites of the type S_2(th)PdCl₂ with the two thienyl rings of each di(2-thienyl)pyrazino fragment bound to Pd^II in an equivalent manner (“th-th” coordination). This is similar to what was found for the corresponding octapyridinated analogues (“py-py” coordination)

Unexpected Chain of Redox Events in Co-Based Prussian Blue Analogues

The electronic structure of electrode materials for metal-ion batteries has a great impact on their charge compensation mechanism and, consequently, electrochemical behavior. In this paper, we report on the cobalt doping in the potassium manganese hexacyanoferrate positive electrode material for potassium-ion batteries, resulting in the formation of a system of K2−δCoxMn1–x[Fe(CN)6] compounds with x = 0...1 and provide their comprehensive characterization including crystal structure evolution and charge compensation mechanisms upon K de/intercalation. Synthesized by a coprecipitation method, K2−δCoxMn1–x[Fe(CN)6] forms two series of solid solutions with monoclinic (Co-poor) and cubic (Co-rich) structures. According to energy-dispersive X-ray analysis, the K content diminishes with increasing x value. Electrochemical properties of electrode materials based on K2−δCoxMn1–x[Fe(CN)6] in K-metal half cells are also strongly dependent on Co doping regarding both specific capacity and redox potential. Attempts to interpret the results led to an unexpected conclusion that cobalt has influence on iron and manganese redox potentials, forming the following oxidation sequence: Co2+/3+, Mn2+/3+, and Fe2+/3+ in K2−δCoxMn1–x[Fe(CN)6], which is inverse to that of Co-free K2−δMn[Fe(CN)6] (Fe2+/3+, Mn2+/3+), as validated by ex situ, operando X-ray absorption spectroscopy, and 57Fe Mössbauer spectroscopy

Localized Symmetry Breaking for Tuning Thermal Expansion in ScF₃ Nanoscale Frameworks

The local symmetry, beyond the averaged crystallographic structure, tends to bring unusual performances. Negative thermal expansion is a peculiar physical property of solids. Here, we report the delicate design of the localized symmetry breaking to achieve controllable thermal expansion in ScF₃ nanoscale frameworks. Intriguingly, an isotropic zero thermal expansion is concurrently engineered by localized symmetry breaking, with a remarkably low coefficient of thermal expansion of about +4.0 × 10^–8/K up to 675 K. This mechanism is investigated by the joint analysis of atomic pair distribution function of synchrotron X-ray total scattering and extended X-ray absorption fine structure spectra. A localized rhombohedral distortion presumably plays a critical role in stiffening ScF₃ nanoscale frameworks and concomitantly suppressing transverse thermal vibrations of fluorine atoms. This physical scenario is also theoretically corroborated by the extinction of phonon modes with negative Grüneisen parameters in rhombohedral ScF₃. The present work opens an untraditional chemical modification route to achieve controllable thermal expansion by breaking local symmetries in materials