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

Nanosized Effect on Electronic/Local Structures and Specific Lithium-Ion Insertion Property in TiO₂–B Nanowires Analyzed by X-ray Absorption Spectroscopy

TiO2–B nanowires were prepared by hydrothermal reaction, and the nanosized effect on lithium-ion insertion was investigated by using X-ray absorption spectroscopy (XAS). On the basis of the results of O K-edge X-ray absorption near-edge structure (XANES) of TiO2–B with various particle sizes, it was suggested that the surface of TiO2–B has local and electronic structures being different from the bulk, and the band energy of surface of TiO2–B was lower than that of the bulk. The band vended structure, which is called the space charge layer (SCL), makes lithium-ion insertion in TiO2–B smooth because the local structure of the SCL is maintained during the lithium-ion insertion, which is shown by Ti K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. It is suggested that control of the SCL, that is the nanosized effect, is a new design concept for achieving higher rate capability

Quantitating the Lattice Strain Dependence of Monolayer Pt Shell Activity toward Oxygen Reduction

Lattice strain of Pt-based catalysts reflecting d-band status is the decisive factor of their catalytic activity toward oxygen reduction reaction (ORR). For the newly arisen monolayer Pt system, however, no general strategy to isolate the lattice strain has been achieved due to the short-range ordering structure of monolayer Pt shells on different facets of core nanoparticles. Herein, based on the extended X-ray absorption fine structure of monolayer Pt atoms on various single crystal facets, we propose an effective methodology for evaluating the lattice strain of monolayer Pt shells on core nanoparticles. The quantitative lattice strain establishes a direct correlation to monolayer Pt shell ORR activity

Pore Development in Carbonized Hemoglobin by Concurrently Generated MgO Template for Activity Enhancement as Fuel Cell Cathode Catalyst

Various carbon materials with a characteristic morphology and pore structure have been produced using template methods in which a carbon-template composite is once formed and the characteristic features derived from the template are generated after the template removal. In this study, hemoglobin, which is a natural compound that could be abundantly and inexpensively obtained, was used as the carbon material source to produce a carbonaceous noble-metal-free fuel cell cathode catalyst. Magnesium oxide was used as the template concurrently generated with the hemoglobin carbonization from magnesium acetate mixed with hemoglobin as the starting material mixture to enable pore development for improving the activity of the carbonized hemoglobin for the cathodic oxygen reduction. After removal of the MgO template, the substantially developed pores were generated in the carbonized hemoglobin with an amorphous structure observed by total-electron-yield X-ray absorption. The extended X-ray absorption fine structure at the Fe-K edge indicated that Fe was coordinated with four nitrogen atoms (Fe–N4 moiety) in the carbonized hemoglobin. The oxygen reduction activity of the carbonized hemoglobin evaluated using rotating disk electrodes was dependent on the pore structure. The highly developed pores led to an improved activity

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

X-ray Absorption Spectroscopic Study on La_0.6Sr_0.4CoO_3−δ Cathode Materials Related with Oxygen Vacancy Formation

The electronic structural changes of La0.6Sr0.4CoO3−δ cathodes with oxygen vacancy formation by reducing oxygen partial pressures, p(O2)'s were investigated in detail using X-ray absorption spectroscopy to understand metallic-like electronic conduction mechanism. The oxygen nonstoichiometry of La0.6Sr0.4CoO3−δ was controlled by annealing the samples under various p(O2)'s and quenched to room temperature. Co K-edge X-ray absorption near edge structure (XANES) spectra revealed that the Co average valence decreased with decreasing p(O2), which was also confirmed by iodometric titration. The Co L-edge XANES spectra were hardly changed with changing p(O2)'s. Meanwhile, the peak area of the O K-edge XANES spectra strongly depended on p(O2). This result revealed the strong hybridization between the O 2p and Co 3d states. It was concluded the introduction of oxygen vacancies narrowed the hybridized orbital of O 2p and Co 3d states, resulted in a decrease in the mobility as well as the concentration of electron holes with decreasing p(O2)

Observing the Structural Evolution of Quasi-Monolayer Pt Shell on Pd Core in the Electrocatalytic Oxygen-Reduction Reaction

The use of a quasi-monolayer Pt shell (Ptqms) on a Pd core (Pdc) can reach cost and activity targets for the electrocatalytic oxygen-reduction reaction (ORR). The structure of PdcPtqms in the ORR will vary; however, direct observation of this issue is scarce. Here, during cyclic staircase voltammetry (ranging from 0.5 to 1.15 VRHE) in 0.1 M O2-saturated HClO4, the structure of PdcPtqms was monitored by in situ X-ray absorption spectroscopy. The qualitative and quantitative structural information clearly exhibits a complete picture that Ptqms will directly restructure to form Pt clusters and holes, while Pdc almost remains stable. These findings identify the initial structural evolution of PdcPtqms in the ORR, highlighting the importance of protecting Pdc in the development of high-performance PdcPtqms electrocatalysts

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

State of the Active Site in La_1–<i>x</i>Sr_<i>x</i>CoO_3−δ Under Oxygen Evolution Reaction Investigated by Total-Reflection Fluorescence X‑Ray Absorption Spectroscopy

Recent developments in hydrogen energy devices have furthered the research on sustainable hydrogen production methods. Among these, the water splitting process has been considered a promising hydrogen production method, particularly, in alkaline media. The lack of information on the reaction active sites under the conditions of the oxygen evolution reaction (OER) hinders establishing guidelines for catalyst development. In the case of powder catalysts, many operando techniques also measure bulk information, and therefore, extracting information on the reaction active sites is challenging. Accordingly, film electrodes were used in this study, and the electrochemical performance and reaction kinetics of perovskite-type La1–xSrxCoO3−δ films as OER catalysts synthesized by pulsed laser deposition were investigated. By combining ex situ X-ray absorption spectroscopy (XAS) and operando total-reflection fluorescence X-ray absorption spectroscopy (TRF-XAS), we succeeded in observing a significant oxidation state change on the surface of La0.6Sr0.4CoO3−δ, which indicated that the active surface sites were formed upon applying the OER potential. This surface reconstruction resulted in numerous active sites at the reaction interface, thereby enhancing the OER activity. This study provides definitive evidence for the surface reconstruction of OER catalysts, which enhances the fundamental understanding of OER catalyst behaviors, and can inspire the design of active OER catalysts by suitable surface modulation

Quantitative Elucidation of the Non-Equilibrium Phase Transition in LiFePO₄ via the Intermediate Phase

Phase-transition route according to compositional change strongly affects the reaction kinetics within materials in energy devices such as lithium-ion batteries. The promising electrode material of LiFePO4 exhibits a high rate performance due to the crucial but controversial act of the metastable intermediate phase in addition to the end members of the two phases at room temperature. Here, we investigated the electrochemical and crystal structural behavior of the intermediate phase in LixFePO4 to be thermodynamically stable at elevated temperature. The current-induced intermediate phase was detected by electrochemical measurements as well as operando X-ray diffraction using a molten salt electrolyte at 230 °C and shows hysteretic charge/discharge characteristics. The nucleation of the intermediate phase occurs at its composition of x = 0.64–0.65 in the independent reaction direction. Both the composition of the intermediate phase and the total composition of LixFePO4 are equal on the charge and not on the discharge. This discrepancy produces the unexpected results that the sequential phase transition via the intermediate phase as a single phase proceeds on the charge, but the three phases coexist in a whole reaction on the discharge. The charge process is a kinetically favorable direction for a current-induced phase transition being responsible for the intermediate phase. This phase-transition mechanism could be deduced to the actual environment. The formation of the intermediate phase is important, as its further stabilization leads to an extension of a single-phase reaction, realizing high-rate electrode materials