Suppressing Isomerization of Phosphine-Protected Au₉ Cluster by Bond Stiffening Induced by a Single Pd Atom Substitution

The fluxional nature of small gold clusters has been exemplified by reversible isomerization between [Au₉(PPh₃)₈]³⁺ with a crown motif (<b>Au</b>_<b>9</b>(C)) and that with a butterfly motif (<b>Au</b>_<b>9</b>(B)) induced by association and dissociation with compact counteranions (NO₃^–, Cl^–). However, structural isomerization was suppressed by substitution of the central Au atom of the Au₉ core in [Au₉(PPh₃)₈]³⁺ with a Pd atom: [PdAu₈­(PPh₃)₈]²⁺ with a crown motif (<b>PdAu</b>_<b>8</b>(C)) did not isomerize to that with a butterfly motif (<b>PdAu</b>_<b>8</b>(B)) upon association with the counteranions. Density functional theory calculation showed that the energy difference between <b>PdAu</b>_<b>8</b>(C) and <b>PdAu</b>_<b>8</b>(B) is comparable to that between <b>Au</b>_<b>9</b>(C) and <b>Au</b>_<b>9</b>(B), indicating that the relative stabilities of the isomers are not a direct cause for the suppression of isomerization. Temperature dependence of Debye–Waller factors obtained by X-ray absorption fine-structure analysis revealed that the intracluster bonds of <b>PdAu</b>_<b>8</b>(C) were stiffer than the corresponding bonds in <b>Au</b>_<b>9</b>(C). Natural bond orbital analysis suggested that the radial Pd–Au and lateral Au–Au bonds in <b>PdAu</b>_<b>8</b>(C) are stiffened due to the increase in the ionic nature and decrease in electrostatic repulsion between the surface Au atoms, respectively. We conclude that the formation of stiffer metal–metal bonds by Pd atom doping inhibits the isomerization from <b>PdAu</b>_<b>8</b>(C) to <b>PdAu</b>_<b>8</b>(B)

Revisiting Cationic Doping Impacts in Ni-Rich Cathodes

As a promising cathode material for high-energy-density Li-ion batteries, Ni-rich layered oxide cathode active materials deliver high specific capacity. However, their electrochemical performance degrades rapidly upon charge/discharge cycles probably due to electrochemical/thermochemical instabilities. While cationic doping in the transition-metal site has been regarded as an effective strategy to enhance the electrochemical performance, the true impact of cation doping is not well understood. To quantitatively assess the impact of cationic doping, in this work, the electrochemical performance and lattice oxygen stability of LiNi0.82Co0.18O2, isovalent Al3+-doped LiNi0.82Co0.15Al0.03O2, and high-valent Ti4+-doped LiNi0.82Co0.15Ti0.03O2 were investigated. Despite significant improvements in electrochemical performance by Al3+ and Ti4+ doping, it was revealed that these cation dopings had no discernible effect on the lattice oxygen stability. Such information suggests that the electrochemical enhancement by Al3+/Ti4+ doping is not attributed to the stabilization of lattice oxygen. This work highlights the importance of independent and quantitative experimental evaluations on kinetic electrochemical properties and thermodynamic stability of lattice oxygen to establish rational guidelines for doping strategy toward high-energy-density and reliable cathode-active materials

Disappearance of the Superionic Phase Transition in Sub‑5 nm Silver Iodide Nanoparticles

Bulk silver iodide (AgI) is known to show a phase transition from the poorly conducting β/γ-phases into the superionic conducting α-phase at 147 °C. Its transition temperature decreases with decreasing the size of AgI, and the α-phase exists stably at 37 °C in AgI nanoparticles with a diameter of 6.3 nm. In this Letter, we investigated the atomic configuration, the phase transition behavior, and the ionic conductivity of AgI nanoparticles with a diameter of 3.0 nm. The combination of pair distribution function (PDF) analysis and reverse Monte Carlo (RMC) modeling based on high-energy X-ray diffraction (XRD) revealed for the first time that they formed the β/γ-phases with atomic disorder. The results of extended X-ray absorption fine structure (EXAFS) analysis, differential scanning calorimetry (DSC), and AC impedance spectroscopy demonstrated that they did not exhibit the superionic phase transition and their ionic conductivity was lower than that of crystalline AgI. The disappearance of the superionic phase transition and low ionic conductivity in the very small AgI nanoparticles originates from their small size and disordered structure

Visualization of Inhomogeneous Reaction Distribution in the Model LiCoO₂ Composite Electrode of Lithium Ion Batteries

Two-dimensional X-ray absorption spectroscopy was carried out to observe the reaction distribution in a LiCoO₂ composite electrode from the shift of the peak top energy in Co K-edge X-ray absorption spectra. The influence of ionic transportation to the inhomogeneous reaction was evaluated by using the model electrode, which sandwiched the LiCoO₂ composite electrode between an aluminum foil and a polyimide ion blocking layer. When the model electrode was charged with the currents of 6, 9, and 12 mA cm^–2, the observed capacities were 51, 20, and 12 mAh g^–1 and the charged areas visualized from the shift of the peak top energy in Co K-edge X-ray absorption spectra were formed within ca. 700, 500, and 200 μm from the edge of the electrode, respectively. The observed reaction distribution indicated that the electrochemically active region decreases with increasing the current density because of the large potential loss of the electrochemical processes

Overpotential-Induced Introduction of Oxygen Vacancy in La_0.67Sr_0.33MnO₃ Surface and Its Impact on Oxygen Reduction Reaction Catalytic Activity in Alkaline Solution

Oxygen reduction reaction (ORR) catalytic activity of La_0.67Sr_0.33MnO₃ epitaxial thin films was investigated in a KOH solution by using a rotating-disk electrode. We found that while the films exhibit ORR current, the current is not limited by oxygen transport resulting from the film electrode rotation and shows the large hysteresis against the potential sweep direction. This behavior is in stark contrast to the oxygen reduction reaction activity of an electrode ink made from LSMO bulk powder, whose ORR current is oxygen-transport limited. <i>In situ</i> synchrotron X-ray absorption spectroscopy also reveals that the valence state of Mn in the LSMO film surface is lowered under the reducing atmosphere caused by the overpotential. This indicates the overpotential-induced introduction of oxygen vacancies in the film surface. We also show that the ORR current of the LSMO films exposed to the reducing atmosphere is lowered than that of the original surface. These results indicate that the ORR catalytic activity of LSMO surfaces is strongly influenced by oxygen vacancies

Protracted Relaxation Dynamics of Lithium Heterogeneity in Solid-State Battery Electrodes

The lithium (Li) heterogeneity formed in the composite electrodes has a significant impact on the performance of solid-state batteries (SSBs). Whereas the influence of various factors on the Li heterogeneity, such as (dis)charge currents, ionic and/or electronic conductivity of the constituent materials, and interfacial charge transfer kinetics, is extensively studied, the influence of the relaxation on the Li heterogeneity in SSB electrodes is largely unexplored, despite its unignorable impact on the battery performance. Here, we performed a three-dimensional operando evaluation of the relaxation dynamics of the electrode-scale Li heterogeneity in a composite SSB electrode under open-circuit conditions after charging using the computed tomography combined with X-ray absorption near-edge structure spectroscopy (CT-XANES). In contrast to the electrode for the liquid-based Li-ion batteries, the Li heterogeneity formed in the composite SSB electrode during charging was not fully relaxed, even after a long open-circuit hold, leaving both higher and lower Li content regions. Such protracted relaxation dynamics in the composite SSB electrode may be due to the high interfacial resistance between active material particles as well as between active material and solid electrolyte particles and is potentially an essential issue for SSBs. This work demonstrated that our CT-XANES technique can three-dimensionally resolve the relaxation dynamics of Li heterogeneity within SSB electrodes, which has only been analyzed indirectly by conventional electrochemical methods such as electrochemical impedance spectroscopy. Our technique can be a valuable tool for identifying detrimental factors affecting the battery performance, ultimately contributing to the development of high-performance SSBs

Operando Time-Resolved X-ray Absorption Fine Structure Study for Surface Events on a Pt₃Co/C Cathode Catalyst in a Polymer Electrolyte Fuel Cell during Voltage-Operating Processes

The structural kinetics of surface events on a Pt₃Co/C cathode catalyst in a polymer electrolyte fuel cell (PEFC) was investigated by operando time-resolved X-ray absorption fine structure (XAFS) with a time resolution of 500 ms. The rate constants of electrochemical reactions, the changes in charge density on Pt, and the changes in the local coordination structures of the Pt₃Co alloy catalyst in the PEFC were successfully evaluated during fuel-cell voltage-operating processes. Significant time lags were observed between the electrochemical reactions and the structural changes in the Pt₃Co alloy catalyst. The rate constants of all the surface events on the Pt₃Co/C catalyst were significantly higher than those on the Pt/C catalyst, suggesting the advantageous behaviors (cell performance and catalyst durability) on the Pt₃Co alloy cathode catalyst

Data File 1: Atomic scattering factor of the ASTRO-H (Hitomi) SXT reflector around the gold’s L edges

Atomic Scattering Factors Originally published in Optics Express on 31 October 2016 (oe-24-22-25548

Dynamic Behavior of Rh Species in Rh/Al₂O₃ Model Catalyst during Three-Way Catalytic Reaction: An <i>Operando</i> X‑ray Absorption Spectroscopy Study

The dynamic behavior of Rh species in 1 wt% Rh/Al₂O₃ catalyst during the three-way catalytic reaction was examined using a micro gas chromatograph, a NO_<i>x</i> meter, a quadrupole mass spectrometer, and time-resolved quick X-ray absorption spectroscopy (XAS) measurements at a public beamline for XAS, BL01B1 at SPring-8, <i>operando</i>. The combined data suggest different surface rearrangement behavior, random reduction processes, and autocatalytic oxidation processes of Rh species when the gas is switched from a reductive to an oxidative atmosphere and vice versa. This study demonstrates an implementation of a powerful <i>operando</i> XAS system for heterogeneous catalytic reactions and its importance for understanding the dynamic behavior of active metal species of catalysts