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Actinide 4f x-ray absorption in oxides
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Intrinsic Li Distribution in Layered Transition-Metal Oxides Using Low-Dose Scanning Transmission Electron Microscopy and Spectroscopy
Understanding Li distribution in layered lithium transition-metal oxide (LiTMO) cathodes in Li-ion batteries has been a major challenge at the atomic scale and nanoscale. Li is extremely difficult to study by transmission electron microscopy (TEM) because the high-energy electrons impart significant energy and cause massive migration. Here, we directly map the intrinsic spatial distribution and bonding of Li in LiNiO 2 -layered cathode materials using low-dose and low-loss electron energy loss spectroscopy (EELS). EELS spectra of the Li–K edge are measured simultaneously with O–K and Ni–L, M 3,2 edges from layered, cation-mixed, and rock-salt phases and directly matched with atomic-resolution scanning TEM images to correlate the changes in peak intensities and positions to the stoichiometry changes with continual loss of Li and O. Changes in the Li content in the LiNiO 2 particles as a function of electron beam dose are studied by sequential Li spectroscopic mapping. We show that the “intrinsic” Li distribution can be observed using a total dose of less than ~1.5 × 10 8 e – nm –2 at an accelerating voltage of 80 kV. The method of nanoscale mapping of Li distribution introduced in this study is applicable to high-Ni LiTMO cathode materials (>89% of Ni) as well as LiNiO 2 . Further study on the extra peaks of the Li–K edge reveals that the peak at ~59 eV is from the Li ions intercalated in between NiO 2 layers barely interacting with each other with less Li K shell electrons pulled to the L shell electrons in NiO 2 . Furthermore, the results shown here provide improved low-loss TEM characterization approaches that can be used to understand the intrinsic fundamental behaviors in Li-ion batteries.
Revealing the Conversion Mechanism of Transition Metal Oxide Electrodes during Lithiation from First-Principles
Transition metal oxides such as Co3O4 and NiO are of significant interest as conversion anode materials for lithium-ion batteries (LIBs), due to their remarkably high theoretical capacities and low cost. While many previous experiments have found that the charge/discharge reactions of Co3O4 and NiO can be highly reversible, detailed information about the mechanisms of these reactions, such as the origin of the voltage hysteresis (>1.0 V) between the charge/discharge cycles, is still poorly understood. In this work, we develop and utilize a new computational mechanistic approach that helps elucidate the hysteresis and nonequilibrium reaction pathways associated with these conversion materials. We apply this methodology to investigate a variety of lithiation reaction pathways of Co3O4 and NiO by systematically exploring the energetics of a large number of equilibrium and nonequilibrium LixCo3O4 (0 ≤ x ≤ 8) and LixNiO (0 ≤ x ≤ 2) structural configurations using first-principles calculations. The overall value of the voltages from our nonequilibrium pathway is in much better agreement with experimental lithiation than the calculated equilibrium voltage while the overall value of the latter reasonably agrees with experimental delithiation. Hence, we propose the charge and discharge processes proceed through equilibrium and nonequilibrium reaction paths, respectively, which contribute significantly to the experimentally observed voltage hysteresis in Co3O4 and NiO. Additionally, we find a low-energy, lithiated intermediate phase (Li3Co3O4) with an oxygen framework equal to that of the initial Co3O4 spinel phase. This intermediate phase represents the capacity threshold below which limited volume expansion and better reversibility can be realized and above which reactions lead to structural degradation and huge expansion.
Aqueous Synthesis of Technetium-Doped Titanium Dioxide by Direct Oxidation of Titanium Powder, a Precursor for Ceramic Nuclear Waste Forms
Technetium-99 (Tc) is a problematic fission product that complicates the long-term disposal of nuclear waste due to its long half-life, high fission yield, and the environmental mobility of pertechnetate, its stable form in aerobic environments. One approach to preventing Tc contamination is through incorporation into durable waste forms based on weathering-resistant minerals such as rutile (titanium dioxide). Here, the incorporation of technetium into titanium dioxide by means of simple, aqueous chemistry - direct oxidation of titanium powder in the presence of ammonium fluoride - is achieved. X-ray absorption fine structure spectroscopy and diffuse reflectance spectroscopy indicate that Tc(IV) replaces Ti(IV) within the structure. Rather than being incorporated as isolated Tc(IV) ions, Tc is present as pairs of edge-sharing Tc(IV) octahedra similar to molecular Tc(IV) complexes such as [(H 2 EDTA)TcIV](μ-O) 2 . Technetium-doped TiO 2 was suspended in deionized water under aerobic conditions, and the Tc leached under these conditions was followed for 8 months. The normalized release rate of Tc (LR Tc ) from the TiO 2 particles is low (3 × 10 -6 g m -2 d -1 ), which illustrates the potential utility of TiO 2 as waste form. However, the small size of the as-prepared TiO 2 nanoparticles results in an estimated retention of Tc of 10 4 years, which is only a fraction of the half-life of Tc (2.1 × 10 5 years).
A Mg-Doped High-Nickel Layered Oxide Cathode Enabling Safer, High-Energy-Density Li-Ion Batteries
High-nickel layered oxide cathodes with a Ni content of > 90% show substantial potential for next-generation lithium-ion batteries (LIBs) due to their high capacity and lower cost. However, they are plagued by rapid capacity decay and poor thermal stability, which hamper their practical viability. Here we present Li 0.98 Mg 0.02 Ni 0.94 Co 0.06 O 2 (NC-Mg) with 2% Mg doping, aiming to provide a strategic guideline for solving the issues. The Mg 2+ ions occupy the lithium layer and are pro-posed to act as pillar ions, which substantially enhances the structural reversibility and reduces the anisotropic lattice distortion upon cycling, thereby greatly improving the electrochemical and thermal stability of NC-Mg compared to the undoped LiNi 0.94 Co 0.06 O 2 (NC). Specifically, NC-Mg delivers 214 mA h g -1 with a capacity retention of 80.1% after 500 cycles in pouch-type full cells, much higher than the retention of NC (56.3%). A discharge capacity of 158 mA h g -1 at 10C rate demonstrates its remarkable rate capability. Additionally, the Mg doping significantly elevates the exothermic peak tem-perature of NC-Mg to 211 °C, in sharp contrast to 177 °C for NC, highlighting the improved thermal stability of NC-Mg. Collectively, the superior performance of NC-Mg demonstrates a feasible alternative strategy for developing safer, high-energy-density LIBs.
Local and Bulk Probe of Vanadium-Substituted α-Manganese Oxide (α-KxVyMn8-yO16) Lithium Electrochemistry
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Enhanced Oxide Reduction by Hydrogen at Cuprous Oxide–Copper Interfaces near Ascending Step Edges
We report that understanding the dynamic processes involved in the interaction of hydrogen with oxides is of fundamental importance in catalysis. This paper probes the reduction of Cu 2 O-‘29’ surfaces by hydrogen at room temperature combining in situ ambient pressure scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. Reduction of the atomic layer thin Cu 2 O film is observed to be preferentially initiated2 at step edges and terrace defects, where perfect Cu 2 O(111) terraces are found to be stable towards hydrogenation under the same conditions. After a long induction period, regions of partially reduced Cu 2 O-‘29’ and metallic Cu co-exist before the surface is fully reduced to Cu(111). The reduction rate strongly depends on the nature of nearby Cu step edges. We propose a mechanism for the reduction of Cu 2 O-‘29’ by hydrogen where free copper atoms from ascending metallic step edges facilitate the formation of active ensembles for H 2 dissociation and transfer H to the edges of Cu2O regions
Wide Band Gap CuGa(S,Se) 2 Thin Films on Transparent Conductive Fluorinated Tin Oxide Substrates as Photocathode Candidates for Tandem Water Splitting Devices
Theaim of this report was to explore the potential of CuGa(S,Se)2 thin films as wide-E G top cell absorbers for photoelectrochemical (PEC) water splitting. A synthesis was developed on fluorinated tin oxide (FTO) photocathodes by converting copper-rich co-evaporated CuGaSe2 into CuGa(S,Se)2 via a post-deposition annealing. We found it necessary to first anneal CuGaSe2 at low-temperature in sulfur then at high-temperature in nitrogen to preserve the transparency and conductivity of the FTO. Using this two-step synthesis, we fabricated a 1.72eV CuGa(S,Se) 2 photocathode with a saturation current density and photocurrent onset potential of 10mA/cm 2 and -0.20V vs. RHE, respectively. Yet we found that the PEC performance and sub-E G transmittance, worsened with increasing copper content. Using flatband potential measurements and the Gerischer model, we show that divergences in PEC performance of CuGa(S,Se) 2 photocathodes can be explained by differences in conduction band minimums and Fermi levels. We also explain that sub-E G transmittance is likely hampered by a defect band 100-400 meV below E C . Additional external quantum efficiency measurements of a high-efficiency 1.1 eV Cu(In,Ga)Se 2 photovoltaic driver, while shaded by the CuGa(S,Se) 2 photocathode, yielded a short-circuit current density of 4.14 mA/cm 2 revealing that CuGa(S,Se) 2 reflects promise as a top cell for PEC water splitting.
Surface Reactions and Catalytic Activities for Small Alcohols over LaMnO3(100) and La0.7Sr0.3MnO3(100): Dehydrogenation, Dehydration, and Oxidation
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One- or Two-Electron Water Oxidation, Hydroxyl Radical, or H 2 O 2 Evolution
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Dynamic Control over Aqueous Poly(butadiene- b -ethylene oxide) Self-Assembly through Olefin Metathesis
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Spin-Selective Oxygen Evolution Reaction in Chiral Iron Oxide Nanoparticles: Synergistic Impact of Inherent Magnetic Moment and Chirality
Electron spin polarization is identified as a promising avenue for enhancing the oxygen evolution reaction (OER), which is the bottleneck that limits the energy efficiency of water-splitting. Here, in this work, we report that both ferrimagnetic (f-Fe 3 O 4 ) and superparamagnetic iron oxide (s-Fe 3 O 4 ) catalysts can exhibit external magnetic field (Hext)-induced OER enhancement, and the activity is proportional to their intrinsic magnetic moment. Additionally, the chirality-induced spin selectivity (CISS) effect was utilized in synergy with Hext to get a maximum enhancement of up to 89% improvement in current density (at 1.8 V vs RHE) with a low onset potential of 270 mV in s-Fe 3 O 4 catalysts. Spin polarization and the resultant spin selectivity suppress the production of H 2 O 2 and promote the formation of ground state triplet O 2 during the OER. Furthermore, the design of chiral s-Fe 3 O 4 with synergistic spin potential effect demonstrates a high spin polarization of ∼42%, as measured using conductive atomic force microscopy (c-AFM).
Synthesis of [60]- and [70]Fullerene-Fused Tetrahydroquinoxaline Derivatives by Oxidative [4 + 2] Cycloaddition with Unusual Reactivity and Regioselectivity
The oxidative [4 + 2] reaction of o-phenylenediamine-derived disulfonamides with fullerene C 60 and C 70 is reported, in which electron-deficient reactants showed high reactivity. The reaction of C 70 exhibited unusual regioselectivity, yielding a [5,6]-adduct as the major product, which was characterized by 1 H, 13 C NMR and single-crystal X-ray diffraction. Here, DFT calculations revealed the reaction is an inverse-electron-demand Diels–Alder (IEDDA) reaction, and the [5,6]-adduct of C 70 is a kinetic product.
Glassy Carbon Substrate Oxidation Effects on Electrode Stability for Oxygen Evolution Reaction Catalysis Stability Benchmarking
Employing benchmarking metrics to capture the activity and stability of electrocatalysts for the oxygen evolution reaction (OER) in acid is a critical practice that enables meaningful comparison of catalyst material candidates reported throughout the literature. In this work, we find that ubiquitously used glassy carbon electrode substrates oxidize under typical OER operating conditions, forming a pacified, electrically insulating, and oxygen-rich surface layer that causes drastic loss of current density over the course of extended chronoamperometric stability tests at an anodic potential of 1.7 VRHE. We show that the experimentally observed stability of glassy carbon-based electrodes is approximately two orders of magnitude lower than that expected solely from dissolution-based catalyst intrinsic stability of Ir-based catalysts. We additionally find that glassy carbon-based electrode stability measured by chronoamperometric holds is greatly impacted by catalyst loading, with high catalyst loadings improving the stability of the overall electrode via a protective effect on the glassy carbon substrate. Altogether, our investigation highlights that glassy carbon is not electrochemically inert under OER conditions on the timescale of common stability tests, which can cause electrodes to exhibit performance losses that do not reflect the intrinsic stability of the actual catalyst material being investigated. In light of our findings, we underscore the usefulness of metrics, such as the S-number, to reflect intrinsic catalyst material stability.