Publications

Reversible insertion of Mg 2+ into metal oxide frameworks stands as a critical factor for facilitating the operation of a Mg-ion battery boasting high energy density, imperative for advancing energy storage technologies. While functional Mg-ion batteries have been achieved in frameworks featuring soft anions like S 2– and Se 2– , they fall short of meeting the energy density benchmarks to rival current rechargeable lithium-ion batteries due to their low insertion potentials. This underscores the need to pinpoint an oxide-based cathode capable of operating at elevated potentials. A leading theory posits that the limited availability of oxide Mg-ion cathodes stems from the sluggish diffusion kinetics of Mg 2+ in oxides, attributed to robust electrostatic interactions between Mg2+ ions and oxide anions within the lattice. Consequently, it is hypothesized that rectifying these kinetic shortcomings could be achieved by tailoring an oxide framework to foster less stable Mg 2+ –O 2– coordination. Drawing upon theoretical computations and preliminary experimental findings, oxide spinels emerge as promising candidates for cathodes, possessing storage capacity, insertion potential, and cation mobility that align with the requisites for a secondary Mg-ion battery. However, spinels featuring a single redox metal, such as MgCr 2 O 4 or MgMn 2 O 4 , failed to exhibit adequately reversible Mg-ion intercalation at high redox potentials when paired with nonaqueous Mg-electrolytes. Consequently, a concerted effort in materials development was launched to conceive, synthesize, and assess a novel class of solid-solution oxide spinels designed to fulfill the requisite properties for a sustainable Mg-ion cathode. These materials were engineered by combining electrochemically active metals boasting stable redox potentials and charged states vis-a-vis the electrolyte, such as Mn 3+ , alongside a structural stabilization component, typically Cr 3+ . Additionally, common spinel structural defects known to compromise performance, such as antisite inversion, were managed to explore the relationship between structures and electrochemical overpotentials, thereby regulating overall hysteresis. Comprehensive structural characterization, encompassing both short- and long-range analyses in both ex-situ and in-situ settings, confirmed the nature of solid-solution and established a correlation between structural modifications and redox activity vis-a-vis electrochemical performance. Encouragingly, consistent and reproducible outcomes were noted, manifesting facile bulk Mg2+-ion activity sans phase transformations, thus bolstering energy storage capacity through reversible Mg 2+ intercalation, facilitated by an understanding of the variables governing the electrochemical performance of spinel oxide. Leveraging these insights, it becomes conceivable to engineer an oxide cathode material boasting many of the desired attributes of a Li-ion intercalation cathode, marking a significant milestone in the pursuit of high energy density Mg-ion batteries. This study delineates strategies for designing and developing novel spinel oxide intercalation materials tailored for high-energy Mg-ion battery cathodes, leveraging a blend of theoretical and experimental methodologies. We discuss the key factors dictating the kinetics of Mg2+ diffusion in spinel oxides, elucidating how material complexity aligns with the electrochemical Mg2+ activity in oxides, bolstered by supplementary characterization. The insights gleaned from fundamental inquiries into cation diffusion in oxide cathodes are poised to benefit chemists and materials scientists engaged in the advancement of rechargeable batteries. Related publication: B. Kwon et al., Accounts of Chemical Research, 2024, 57, 1, 1–9 Figure 1

A series of oligothiophene-perfluoro-9-heterofluorene donor-acceptor (DA) compounds was synthesized via a combination of nucleophilic aromatic substitution (S(N)Ar(F)) and palladium coupling reactions. These compounds are of interest as possible building blocks for materials with useful electron transport properties, since they possess relatively low LUMO energy levels of -3.3 to -3.6 eV (as determined by differential pulse voltammetry). The HOMO-LUMO energy gaps, as determined by UV-vis spectroscopy, range between 2.4 and 2.5 eV, and photoluminescence emission spectra reveal lambda(ems) values in the range of 480-600 nm (corresponding to yellow-orange emission). Dilute solution-state photoluminescence quantum yields were significantly lower than those of the pure acceptor heterofluorenes (0.02-0.38 for the DA compounds vs approximately 1 for the pure acceptors), and notable solvatochromism in the fluorescence suggests emission from a charge-separated state. Theoretical calculations show that HOMO-level electron density is more localized on the thiophene fragment, while the LUMO level electron density is mostly associated with the electron-deficient portion of the molecule. Photovoltaic (PV) devices based on DA/poly-3-hexylthiophene (P3HT) blends exhibit improved performance over P3HT-only devices, suggesting the ability of these DA compounds to transport electrons in the solid state.

ABSTRACT Major efforts in bioenergy research have focused on producing fuels that can directly replace petroleum-derived gasoline and diesel fuel through metabolic engineering of microbial fatty acid biosynthetic pathways. Typically, growth and pathway induction are conducted under aerobic conditions, but for operational efficiency in an industrial context, anaerobic culture conditions would be preferred to obviate the need to maintain specific dissolved oxygen concentrations and to maximize the proportion of reducing equivalents directed to biofuel biosynthesis rather than ATP production. A major concern with fermentative growth conditions is elevated NADH levels, which can adversely affect cell physiology. The purpose of this study was to identify homologs of Escherichia coli FabG, an essential reductase involved in fatty acid biosynthesis, that display a higher preference for NADH than for NADPH as a cofactor. Four potential NADH-dependent FabG variants were identified through bioinformatic analyses supported by crystallographic structure determination (1.3- to 2.0-Å resolution). In vitro assays of cofactor (NADH/NADPH) preference in the four variants showed up to ∼35-fold preference for NADH, which was observed with the Cupriavidus taiwanensis FabG variant. In addition, FabG homologs were overexpressed in fatty acid- and methyl ketone-overproducing E. coli host strains under anaerobic conditions, and the C. taiwanensis variant led to a 60% higher free fatty acid titer and 75% higher methyl ketone titer relative to the titers of the control strains. With further engineering, this work could serve as a starting point for establishing a microbial host strain for production of fatty acid-derived biofuels (e.g., methyl ketones) under anaerobic conditions.

We study the bifurcations of attracting cycles for a three-segment (bimodal) piecewise-linear continuous one-dimensional map. Exact formulas for the regions of periodicity of any rational rotation number (Arnold’s tongues) are obtained in the associated three-dimensional parameter space. It is shown that the destruction of any Arnold’s tongue is a result of a border-collision bifurcation, and is followed by the appearance of a cycle of intervals with the same rotation number, whose dynamics is determined by a skew tent map. Finally, for the interval cycle the merging bifurcation corresponds to a homoclinic bifurcation of some point cycle.

Lithium extraction from naturally occurring α-spodumene is hindered by poor lithium diffusivity, necessitating high-temperature phase transformation to a low-density β polymorph. Although β spodumene exhibits up to 5 orders of magnitude higher lithium-ion diffusivity, both phases have diffusion activation energies between 0.8 and 1 eV, indicating that polymorph density is not the controlling factor over diffusivity. We show that aluminum vacancies facilitate lithium-ion diffusion in α-spodumene by reducing the migration barrier from 2.4 to 0.9 eV. Bond valence site energy and nudged elastic band calculations show a new lithium local minimum site which promotes a one-dimensional percolation network by reducing the lithium intersite distance from 4.5 Å to 2.9 Å. However, aluminum vacancies are energetically unfavorable to percolate through the whole structure, resulting in very low net lithium diffusivity and highlighting the critical role of nonstoichiometric defects in facilitating lithium transport in rigid aluminosilicate structures.