Capacity Fading Mechanism of the Commercial 18650 LiFePO₄‑Based Lithium-Ion Batteries: An in Situ Time-Resolved High-Energy Synchrotron XRD Study

In situ high-energy synchrotron XRD studies were carried out on commercial 18650 LiFePO₄ cells at different cycles to track and investigate the dynamic, chemical, and structural changes in the course of long-term cycling to elucidate the capacity fading mechanism. The results indicate that the crystalline structural deterioration of the LiFePO₄ cathode and the graphite anode is unlikely to happen before capacity fades below 80% of the initial capacity. Rather, the loss of the active lithium source is the primary cause for the capacity fade, which leads to the appearance of inactive FePO₄ that is proportional to the absence of the lithium source. Our in situ HESXRD studies further show that the lithium-ion insertion and deinsertion behavior of LiFePO₄ continuously changed with cycling. For a fresh cell, the LiFePO₄ experienced a dual-phase solid-solution behavior, whereas with increasing cycle numbers, the dynamic change, which is characteristic of the continuous decay of solid solution behavior, is obvious. The unpredicted dynamic change may result from the morphology evolution of LiFePO₄ particles and the loss of the lithium source, which may be the cause of the decreased rate capability of LiFePO₄ cells after long-term cycling

Highly Active Pt₃Pb and Core–Shell Pt₃Pb–Pt Electrocatalysts for Formic Acid Oxidation

Formic acid is a promising chemical fuel for fuel cell applications. However, due to the dominance of the indirect reaction pathway and strong poisoning effects, the development of direct formic acid fuel cells has been impeded by the low activity of existing electrocatalysts at desirable operating voltage. We report the first synthesis of Pt₃Pb nanocrystals through solution phase synthesis and show they are highly efficient formic acid oxidation electrocatalysts. The activity can be further improved by manipulating the Pt₃Pb–Pt core–shell structure. Combined experimental and theoretical studies suggest that the high activity from Pt₃Pb and the Pt–Pb core–shell nanocrystals results from the elimination of CO poisoning and decreased barriers for the dehydrogenation steps. Therefore, the Pt₃Pb and Pt–Pb core–shell nanocrystals can improve the performance of direct formic acid fuel cells at desired operating voltage to enable their practical application

Adsorbate-Induced Structural Changes in 1–3 nm Platinum Nanoparticles

We investigated changes in the Pt–Pt bond distance, particle size, crystallinity, and coordination of Pt nanoparticles as a function of particle size (1–3 nm) and adsorbate (H₂, CO) using synchrotron radiation pair distribution function (PDF) and X-ray absorption spectroscopy (XAS) measurements. The ∼1 nm Pt nanoparticles showed a Pt–Pt bond distance contraction of ∼1.4%. The adsorption of H₂ and CO at room temperature relaxed the Pt–Pt bond distance contraction to a value close to that of bulk fcc Pt. The adsorption of H₂ improved the crystallinity of the small Pt nanoparticles. However, CO adsorption generated a more disordered fcc structure for the 1–3 nm Pt nanoparticles compared to the H₂ adsorption Pt nanoparticles. <i>In situ</i> XANES measurements revealed that this disorder results from the electron back-donation of the Pt nanoparticles to CO, leading to a higher degree of rehybridization of the metal orbitals in the Pt-adsorbate system

Correlating Size and Composition-Dependent Effects with Magnetic, Mössbauer, and Pair Distribution Function Measurements in a Family of Catalytically Active Ferrite Nanoparticles

The magnetic spinel ferrites, MFe₂O₄ (wherein “M” = a divalent metal ion such as but not limited to Mn, Co, Zn, and Ni), represent a unique class of magnetic materials in which the rational introduction of different “M”s can yield correspondingly unique and interesting magnetic behaviors. Herein we present a generalized hydrothermal method for the synthesis of single-crystalline ferrite nanoparticles with M = Mg, Fe, Co, Ni, Cu, and Zn, respectively, which can be systematically and efficaciously produced simply by changing the metal precursor. Our protocol can moreover lead to reproducible size control by judicious selection of various surfactants. As such, we have probed the effects of both (i) size and (ii) chemical composition upon the magnetic properties of these nanomaterials using complementary magnetometry and Mössbauer spectroscopy techniques. The structure of the samples was confirmed by atomic pair distribution function analysis of X-ray and electron powder diffraction data as a function of particle size. These materials retain the bulk spinel structure to the smallest size (i.e., 3 nm). In addition, we have explored the catalytic potential of our ferrites as both (a) magnetically recoverable photocatalysts and (b) biological catalysts and noted that many of our as-prepared ferrite systems evinced intrinsically higher activities as compared with their iron oxide analogues