Nanostructured dimagnesium manganese oxide (Spinel): Control of size, shape and their magnetic and electro catalytic properties

Electrolyte Reactivity on the MgV2O4 Cathode Surface

Predictive understanding of the molecular interaction of electrolyte ions and solvent molecules and their chemical reactivity on electrodes has been a major challenge but is essential for addressing instabilities and surface passivation that occur at the electrode-electrolyte interface of multivalent magnesium batteries. In this work, the isolated intrinsic reactivities of prominent chemical species present in magnesium bis(trifluoromethanesulfonimide) (Mg(TFSI)2) in diglyme (G2) electrolytes, including ionic (TFSI-, [Mg(TFSI)]+, [Mg(TFSI):G2]+, and [Mg(TFSI):2G2]+) as well as neutral molecules (G2) on a well-defined magnesium vanadate cathode (MgV2O4) surface, have been studied using a combination of first-principles calculations and multimodal spectroscopy analysis. Our calculations show that nonsolvated [Mg(TFSI)]+ is the strongest adsorbing species on the MgV2O4 surface compared with all other ions while partially solvated [Mg(TFSI):G2]+ is the most reactive species. The cleavage of C-S bonds in TFSI- to form CF3- is predicted to be the most desired pathway for all ionic species, which is followed by the cleavage of C-O bonds of G2 to yield CH3+ or OCH3- species. The strong stabilization and electron transfer between ionic electrolyte species and MgV2O4 is found to significantly favor these decomposition reactions on the surface compared with intrinsic gas-phase dissociation. Experimentally, we used state-of-the-art ion soft landing to selectively deposit mass-selected TFSI-, [Mg(TFSI):G2]+, and [Mg(TFSI):2G2]+ on a MgV2O4 thin film to form a well-defined electrolyte-MgV2O4 interface. Analysis of the soft-landed interface using X-ray photoelectron, X-ray absorption near-edge structure, electron energy-loss spectroscopies, as well as transmission electron microscopy confirmed the presence of decomposition species (e.g., MgFx, carbonates) and the higher amount of MgFx with [Mg(TFSI):G2]+ formed in the interfacial region, which corroborates the theoretical observation. Overall, these results indicate that Mg2+ desolvation results in electrolyte decomposition facilitated by surface adsorption, charge transfer, and the formation of passivating fluorides on the MgV2O4 cathode surface. This work provides the first evidence of the primary mechanisms leading to electrolyte decomposition at high-voltage oxide surfaces in multivalent batteries and suggests that the design of new, anodically stable electrolytes must target systems that facilitate cation desolvation.

Towards hard-magnetic behavior of CoFe2O4 nanoparticles: a detailed study of crystalline and electronic structures, and magnetic properties

We have used the coprecipitation and mechanical-milling methods to fabricate CoFe2O4 nanoparticles with an average crystallite size (d) varying from 81 to ∼12 nm when changing the milling time (t m) up to 180 min. X-ray diffraction and Raman-scattering studies have proved the samples crystalizing in the spinel structure. Both the lattice constant and residual strain tend to increase when t m(d) increases (decreases). The analysis of magnetization data has revealed a change in the coercivity (H c) towards the hard-magnetic properties. Specifically, the maximum H c is about 2.2 kOe when t m = 10 min corresponding to d ≈ 29 nm; beyond this t m(d) value, H c gradually decreases. Meanwhile, the increase of t m always reduces the saturation magnetization (M s) from ∼69 emu g-1 for t m = 0 to 35 emu g-1 for t m = 180 min. The results collected as analyzing X-ray absorption data have indicated a mixed valence state of Fe2+,3+ and Co2+ ions. We think that the migration and redistribution of these cations between the tetrahedral and octahedral sites together with lattice distortions and defects induced by the milling process have impacted the magnetic properties of the CoFe2O4 nanoparticles.

Synthesis, Electronic Structure, and Electrochemical Properties of the Cubic Mg2MnO4 Spinel with Porous-Spongy Structure

Mg2MnO4 nanoparticles with cubic spinel structure were synthesized by the sol-gel method using polyvinyl alcohol (PVA) as a chelating agent. X-ray powder diffraction, infrared spectrum (IR), scanning electron microscope (SEM), and transmission electron microscope (TEM) were used to characterize the crystalline phase and particle size of as-synthesized nanoparticles. The electronic structure of Mg2MnO4 spinel was studied by X-ray photoelectron spectroscopy (XPS). The results showed that pure cubic Mg2MnO4 spinel nanoparticles were obtained when the annealing temperature was 500-700 °C. The samples had a porous-spongy structure assembled by nanoparticles. XPS studies indicated that Mg2MnO4 nanoparticles were mixed spinel structures and the degree of cation inversion decreased with increasing annealing temperature. Furthermore, the performance of Mg2MnO4 as lithium anode material was studied. The results showed that Mg2MnO4 samples had good cycle stability except for the slight decay in the capacity at 50 cycles. The coulombic efficiency (ratio of discharge and charge capacity) in most cycles was near 100%. The sample annealed at 600 °C exhibited good electrochemical properties, the first discharge capacity was 771.5 mAh/g, and the capacity remained 340 mAh/g after 100 cycles. The effect of calcination temperature on the charge-discharge performance of the samples was studied and discussed.

Unveiling new stable manganese based photoanode materials via theoretical high-throughput screening and experiments

With the increasing energy demand, developing renewable fuel production strategies such as photoelectrocatalytic hydrogen production is critical to mitigating the global climate change. In this work, we experimentally validate a new stable and photoactive material, Mg₂MnO₄, from the exhaustive theoretical exploration of the chemical space of X (=Mg and Ca), Mn and O.

Design of diverse nanostructures by hydrothermal and microemulsion routes for electrochemical water splitting

Hydrothermal and microemulsion methods are low temperature methods used to obtain nanostructures of definite morphologies, sizes, facet termination and other structural features which result in the corresponding unique response to chemical, electrochemical or photochemical stimuli. An efficient catalyst to electrochemically split water to produce hydrogen and oxygen is of scientific, economic and societal relevance, especially due to the abundance of the starting material, water, and due to the product hydrogen, which is an ideal fuel, due to its highest mass density and clean combustion in air. In this review we focus on the hydrogen evolution reaction, HER, and the oxygen evolution reaction, OER, activity of the electrocatalysts produced by hydrothermal or microemulsion methods. The variation in electrochemical response resulting from the unique shape, composition and nano-architecture is discussed. Broadly, the catalysts were categorized as binary and ternary metal alloys as well as metal chalcogenides and oxides. This compilation would aid in the design of more effective water splitting electrocatalysts as well as in the selection of appropriate candidates for advanced mechanistic studies.

A systematic study of 25Mg NMR in paramagnetic transition metal oxides: applications to Mg-ion battery materials

Solid-state ²⁵Mg paramagnetic nuclear magnetic resonance spectra were studied both experimentally and with density functional theory calculations.

Oxide-based nanostructures for photocatalytic and electrocatalytic applications

The enormous efforts on the design of efficient oxide-based materials towards photocatalysis & electrocatalysis have been highlighted in this article with emphasis on their size, structure & morphology.