Platinum-Based Electrocatalysts for the Oxygen-Reduction Reaction: Determining the Role of Pure Electronic Charge Transfer in Electrocatalysis

In the oxygen-reduction reaction (ORR), electronic charge transfer (ECT) derived from alloy components and support materials generates noticeable impact on the electrocatalytic activity of Pt. However, generally, ECT will not individually occur; thus, its role remains controversial. Here, using different amount of Au to decorate Pt nanoparticles, ECT from Au to Pt is isolated to correlate with the ORR activity of Pt. The linear correlation, where pure ECT (assessed by Pt 5d orbital vacancy) depresses the adsorption of oxygenated species to enhance the ORR activity, predicts that the maximum activity enhancement should be smaller than 200%. These findings highlight that the ECT effect in the ORR is weaker than the previously reported size, facet, or strain effects, which establishes a basis for understanding exceptional ORR electrocatalysis and developing efficient Pt-based electrocatalysts

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

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

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

Hidden Two-Step Phase Transition and Competing Reaction Pathways in LiFePO₄

LiFePO₄ is a well-known electrode material that is capable of high-rate charging and discharging despite a strong phase-separation tendency of the lithium-rich and poor end-member phases. X-ray diffraction measurements (XRD) with high time-resolution are conducted under battery operation conditions to reveal the phase-transition mechanism of LiFePO₄ that leads to the high rate capability. We here propose a hidden two-step phase transition of LiFePO₄ via a metastable phase. The existence of the metastable phase, not just a member of a transient solid solution, is evidenced by the <i>operando</i> XRD measurements. Our two-step phase-transition model explains the behavior of LiFePO₄ under the battery operation conditions. It also explains asymmetric behavior during the charging and discharging at high rates and low temperatures, as well as apparent single-step two-phase reaction between the end members at low rates at room temperature. This model also suggests underlying, rate-dependent electrochemical processes that result from a competing disproportion reaction of the metastable phase

Relationship between Phase Transition Involving Cationic Exchange and Charge–Discharge Rate in Li₂FeSiO₄

Li₂FeSiO₄ is considered a promising cathode material for the next-generation Li-ion battery systems owing to its high theoretical capacity and low cost. Li₂FeSiO₄ exhibits complex polymorphism and undergoes significant phase transformations during charge and discharge reaction. To elucidate the phase transformation mechanism, crystal structural changes during charge and discharge processes of Li₂FeSiO₄ at different rates were investigated by X-ray diffraction measurements. The C/50 rate of lithium extraction upon initial cycling leads to a complete transformation from a monoclinic Li₂FeSiO₄ to a thermodynamically stable orthorhombic LiFeSiO₄, concomitant with the occurrence of significant Li/Fe antisite mixing. The C/10 rate of lithium extraction and insertion, however, leads to retention of the parent Li₂FeSiO₄ (with the monoclinic structure as a metastable phase) with little cationic mixing. Here, we experimentally show the presence of metastable and stable LiFeSiO₄ polymorphic phases caused by lithium extraction and insertion

Crystal Structural Changes and Charge Compensation Mechanism during Two Lithium Extraction/Insertion between Li₂FeSiO₄ and FeSiO₄

Li₂FeSiO₄ is a promising cathode material for lithium ion batteries because of its theoretically high capacity if two lithium ions can be extracted/inserted per formula unit; however, the extraction/insertion of two lithium ions from Li₂FeSiO₄ remains a challenge. Herein, we successfully synthesized carbon-coated Li₂FeSiO₄ nanoparticles which exhibit a capacity commensurate to a reversible two-lithium extraction/insertion at elevated temperature. This study investigates the mechanism underlying a two lithium ion extraction/insertion in Li₂FeSiO₄ using synchrotron X-ray absorption spectroscopy and X-ray diffraction. Our results reveal that the contribution of the Fe-3<i>d</i> band is dominant for the first lithium extraction process from Li₂FeSiO₄ to LiFeSiO₄. During the second lithium extraction process from LiFeSiO₄ to FeSiO₄, however, ligand holes are formed in the O-2<i>p</i> band rather than further oxidation of Fe³⁺. Structural analyses further reveal a phase transformation between Li₂FeSiO₄ and LiFeSiO₄, while a single-phase behavior is observed for Li_2–<i>x</i>FeSiO₄ (1.0 ≤ <i>x</i> ≤ 2.0). Together with a tentatively refined crystal structure of the FeSiO₄ phase (<i>x</i> = 2.0), we discuss the charge compensation mechanism during two lithium extraction/insertion in Li₂FeSiO₄

Layered Perovskite Oxide: A Reversible Air Electrode for Oxygen Evolution/Reduction in Rechargeable Metal-Air Batteries

For the development of a rechargeable metal-air battery, which is expected to become one of the most widely used batteries in the future, slow kinetics of discharging and charging reactions at the air electrode, i.e., oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), respectively, are the most critical problems. Here we report that Ruddlesden–Popper-type layered perovskite, RP-LaSr₃Fe₃O₁₀ (<i>n</i> = 3), functions as a reversible air electrode catalyst for both ORR and OER at an equilibrium potential of 1.23 V with almost no overpotentials. The function of RP-LaSr₃Fe₃O₁₀ as an ORR catalyst was confirmed by using an alkaline fuel cell composed of Pd/LaSr₃Fe₃O_{10–2<i>x</i>}(OH)_2<i>x</i>·H₂O/RP-LaSr₃Fe₃O₁₀ as an open circuit voltage (OCV) of 1.23 V was obtained. RP-LaSr₃Fe₃O₁₀ also catalyzed OER at an equilibrium potential of 1.23 V with almost no overpotentials. Reversible ORR and OER are achieved because of the easily removable oxygen present in RP-LaSr₃Fe₃O₁₀. Thus, RP-LaSr₃Fe₃O₁₀ minimizes efficiency losses caused by reactions during charging and discharging at the air electrode and can be considered to be the ORR/OER electrocatalyst for rechargeable metal-air batteries

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

Nanoscale Observation of the Electronic and Local Structures of LiCoO₂ Thin Film Electrode by Depth-Resolved X-ray Absorption Spectroscopy

The electronic and local structural changes during electrochemical delithiation processes occurring at the electrode/electrolyte interface of LiCoO₂ thin film electrode prepared by pulsed laser deposition were clarified by using depth-resolved X-ray absorption spectroscopy technique. We successfully obtained detailed microstructural information around the Li_1–<i>x</i>CoO₂ electrode surface with a depth resolution of ca. 3 nm using a spectro-electrochemical cell. Our results revealed a remarkable increase in the local distortions at the Li_1–<i>x</i>CoO₂ surface after charging, and the distortions extended to the bulk of Li_1–<i>x</i>CoO₂. Such unprecedented local distortions might be attributable to the marked deterioration of the electrodes