Protein Salting Out Observed at an Air−Water Interface

A protein salting-out process is directly observed at an air−water interface. By using time-resolved X-ray specular reflectivity and off-specular diffuse scattering, we identified several key stages in the adsorption of hen egg white lysozyme in a concentrated NaCl solution, (1) adsorption-induced unfolding, (2) monolayer formation with unfolded proteins, (3) protein refolding, and (4) island formation with the refolded proteins. Stages 3 and 4 are not observed either at the isoelectric point or in the salt-free solution, suggesting that they are induced by screening of the positive charges in the lysozyme by chloride ions. It is considered that the hydrated salt ions act to minimize the water-accessible surface area of the protein, not only enhancing protein dehydration (stages 1 and 2) but also assisting in protein refolding and association (stages 3 and 4). These results provide insight into the early stages of protein crystal nucleation

Elucidating the Driving Force of Relaxation of Reaction Distribution in LiCoO₂ and LiFePO₄ Electrodes Using X‑ray Absorption Spectroscopy

The reaction distribution in the composite electrodes used in lithium-ion batteries greatly affects battery performances, including rate capability and safety. In this study, the generation of the reaction distribution and its relaxation in cross sections of LiCoO₂ and LiFePO₄ composite electrodes were analyzed using microbeam X-ray absorption spectroscopy. The reaction distribution immediately after delithiation could be observed clearly with different oxidation states of the transition metal (i.e., different concentrations of lithium ions). The distribution in the Li_1–<i>x</i>CoO₂ electrodes disappeared, whereas that in the Li_1–<i>x</i>FePO₄ electrodes remained unchanged even after 15 h of relaxation. After comparing the potential profile of both types of electrodes, it is suggested that the potential difference between the more delithiated area and the less delithiated area in the composite electrode is the primary driving force for the relaxation

Driving Force Behind Adsorption-Induced Protein Unfolding: A Time-Resolved X-ray Reflectivity Study on Lysozyme Adsorbed at an Air/Water Interface

Time-resolved X-ray reflectivity measurements for lysozyme (LSZ) adsorbed at an air/water interface were performed to study the mechanism of adsorption-induced protein unfolding. The time dependence of the density profile at the air/water interface revealed that the molecular conformation changed significantly during adsorption. Taking into account previous work using Fourier transform infrared (FTIR) spectroscopy, we propose that the LSZ molecules initially adsorbed on the air/water interface have a flat unfolded structure, forming antiparallel β-sheets as a result of hydrophobic interactions with the gas phase. In contrast, as adsorption continues, a second layer forms in which the molecules have a very loose structure having random coils as a result of hydrophilic interactions with the hydrophilic groups that protrude from the first layer

Direct Observation of a Metastable Crystal Phase of Li_<i>x</i>FePO₄ under Electrochemical Phase Transition

The phase transition between LiFePO4 and FePO4 during nonequilibrium battery operation was tracked in real time using time-resolved X-ray diffraction. In conjunction with increasing current density, a metastable crystal phase appears in addition to the thermodynamically stable LiFePO4 and FePO4 phases. The metastable phase gradually diminishes under open-circuit conditions following electrochemical cycling. We propose a phase transition path that passes through the metastable phase and posit the new phase’s role in decreasing the nucleation energy, accounting for the excellent rate capability of LiFePO4. This study is the first to report the measurement of a metastable crystal phase during the electrochemical phase transition of LixFePO4

Transient Phase Change in Two Phase Reaction between LiFePO₄ and FePO₄ under Battery Operation

Transient states of phase transition in LiFePO₄/FePO₄ for lithium ion battery positive electrodes are investigated by time-resolved measurements. To directly detect changes in electronic and crystal structures under battery operation, <i>in situ</i> time-resolved X-ray absorption and diffraction measurements are performed, respectively. The phase fraction change estimated by the iron valence change is similar to the electrochemically expected change. The transient change of lattice constant during two phase reaction is clearly observed by the time-resolved X-ray diffraction measurement. The nonequilibrium lithium extraction behavior deviates from the thermodynamic diagram of the two phase system, resulting in continuous phase transition during electrochemical reactions

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

Dynamic Behavior at the Interface between Lithium Cobalt Oxide and an Organic Electrolyte Monitored by Neutron Reflectivity Measurements

Clarification of the interaction between the electrode and the electrolyte is crucial for further improvement of the performance of lithium-ion batteries. We have investigated the structural change at the interface between the surface of a 104-oriented epitaxial thin film of LiCoO₂ (LiCoO₂(104)), which is one of the stable surfaces of LiCoO₂, and an electrolyte prepared using a carbonate solvent (1 M LiClO₄ in ethylene carbonate and dimethyl carbonate) by <i>in situ</i> neutron reflectivity measurements. Owing to the decomposition of the organic solvent, a new interface layer was formed after contact of LiCoO₂(104) with the electrolyte. The composition and thickness of the interface layer changed during Li⁺ extraction/insertion. During Li⁺ extraction, the thickness of the interface layer increased and the addition of an inorganic species is suggested. The thickness of the interface layer decreased during Li⁺ insertion. We discuss the relationship between battery performance and the dynamic behavior at the interface