Distribution Of Oxidation States Research Articles

Binding mechanisms of trivalent chromium on colloidal phases from ultramafic systems: Insights from batch experiments and XAS analysis

Understanding the availability and mobility of chromium is a key factor in gaining insight into the fate of chromium and thus essential for assessing the risk of contamination and developing effective remediation strategies. In the natural environment, such as ultramafic systems, which are natural pools of chromium, chromium occurs mostly in the trivalent state (Cr(III)), which is known to have a high affinity for the solid phases such as particles and colloids. However, since Cr(VI) is more toxic, the vast majority of experimental studies focus on its reduction to Cr(III), which is often considered to form (hydr)oxide precipitates due to its poor solubility. Furthermore, while the oxidation state of Cr dictates its geochemical behavior, its interaction with particles and colloids is rarely controlled or monitored. Therefore, the interaction mechanisms between Cr(III) and particles or colloids require further attention. In this study, ten mineral phases (goethite, hematite, pyrolusite, kaolinite, montmorillonite, chlorite, serpentine, fuchsite, amphibole, and chromite) and one organic phase (humic acid), relevant to chromium bearing and carrier phases in ultramafic soils, were investigated to explore their interaction with Cr(III) species. These phases were first characterized by various techniques (including XRD, XRF, SEM, and BET). Adsorption and desorption experiments of Cr(III) on these phases were then carried out, and the distribution of Cr oxidation states in both the aqueous and solid phases was carefully monitored using UV–vis spectroscopy, geochemical speciation modeling, and Cr K-edge XAS spectroscopy. With the exception of pyrolusite, where partial oxidation of Cr(III) to Cr(VI) occurred, Cr(III) was shown to persist as a single oxidation state in all experiments. Desorption analyses showed significantly low desorbed rates, which further confirmed the prevalence of Cr(III). Experimental sorption isotherm data were interpreted using the Langmuir model and its linearized form, which provides important thermodynamic characteristics of adsorbents related to their structural and surface features. This study will provide better constraints for the understanding and prediction of Cr behavior and fate in the environment, especially in ultramafic systems.

Unveiling the Mn3+/ Mn4+ and Ni2+ potential as a high-capacity material for next-generation energy storage devices

In this study, we synthesized MnOx nanowires via hydrothermal methods and explored the impact of nickel (Ni) doping on their morphology and electrochemical properties. We investigated the structural and chemical changes induced by Ni incorporation by utilizing TEM and XPS analyses. Our results revealed that Ni doping influenced the distribution of manganese oxidation states, with 1.6 wt% Ni-doped nanowires exhibiting the optimized condition. Also, we observed a balance between the Mn3+/Mn4+ ratio with the Ni doping. Electrochemical characterization demonstrated enhanced capacity in Ni-MnOx nanowires at the 1.6 wt% Ni doping level. Galvanostatic charge-discharge measurements confirmed the superior performance of this level of Ni doping; moreover, the fabrication of asymmetric supercapacitor cells using these nanowires showed improved energy storage capabilities. The main results showed exceptionally high capacity (955.55 and 383.33 mAh g−1 at 1 and 20 A g−1, respectively) and an excellent rate capability. Cycling stability tests over 8500 cycles demonstrated excellent retention of capacity, underscoring the durability of the optimized Ni-MnOx nanowires, with a retention of 85 % of its initial capacity. Thus, this study emphasizes the significance of Ni doping in MnOx nanowires for enhancing electrochemical performance when the synthetic process is controlled, offering valuable insights for high-performance energy storage device development.

Plutonium Speciation and Oxidation State Distributions in the Presence of Citrate.

We explored the speciation and kinetics of the Pu(VI)-citrate and Pu(III)-citrate systems (pHm = 2.5-11.0, I = 0.1 M NaCl, T = 23 °C, O2(g) < 2 ppm) using ultraviolet-visible-near-infrared (UV-vis-NIR) spectrophotometry, solvent extraction, and PHREEQC modeling. Formation constants were determined for PuO2(HcitH)(aq) (log K°1,1 = 1.09 ± 0.05) and PuO2(HcitH)(citH)3- (log K°1,2 = -0.20 ± 0.07), and evidence for (PuO2)m(citH-k)n(OH)x2m(3+k)n-x was identified under alkaline conditions. Pu(VI) species were found to be less stable in the presence of citrate than in the absence of citrate (t ≤ 168 days); the rate of reduction increased with increasing pH. The direct reduction of Pu(VI) to Pu(IV) was required to fit experimental data in the presence of citrate but did not improve the fit for Pu in the absence of citrate. We also observed increased Pu(III) stability in the presence of citrate (t ≤ 293 days), with higher concentrations of Pu(III) favored at lower pH. Finally, we provide evidence of a radiolysis-driven mechanism for the citrate-mediated reduction of plutonium that involves electron transfer from the oxidative breakdown of citrate. Our work highlights the need to investigate the redox effect of organic ligands on plutonium oxidation states under repository-relevant conditions.

Interrogation of 3d Transition Bimetallic Nanocrystal Nucleation and Growth Using In Situ Electron Microscope and Synchrotron X-ray Techniques.

Understanding the nucleation and growth mechanism of 3d transition bimetallic nanocrystals (NCs) is crucial to developing NCs with tailored nanostructures and properties. However, it remains a significant challenge due to the complexity of 3d bimetallic NCs formation and their sensitivity to oxygen. Here, by combining in situ electron microscopy and synchrotron X-ray techniques, we elucidate the nucleation and growth pathways of Fe-Ni NCs. Interestingly, the formation of Fe-Ni NCs emerges from the assimilation of Fe into Ni clusters together with the reduction of Fe-Ni oxides. Subsequently, these NCs undergo solid-state phase transitions, resulting in two distinct solid solutions, ultimately dominated by γ-Fe3Ni2. Furthermore, we deconvolve the interplays between local coordination and electronic state concerning the growth temperature. We directly visualize the oxidation-state distributions of Fe and Ni at the nanoscale and investigate their changes. This work may reshape and enhance the understanding of nucleation and growth in atomic crystallization.

Probing the Interfacial Molecular Structure of a Co‐Prussian Blue In Situ

AbstractMolecular‐level insight into the interfacial composition of electrodes at the solid‐electrolyte and the solid‐electrode interface is essential to understanding the charge transfer processes, which are vital for electrochemical (EC) and photoelectrochemical (PEC) applications. However, spectroscopic access to both interfaces, particularly upon application of an external bias, remains a challenge. Here, in situ surface sensitive vibrational sum‐frequency generation (VSFG) spectroscopy is used for the first time to directly access the interfacial structure of a cobalt‐containing Prussian blue analog (Co‐PBA) in contact with the electrolyte and TiO2/Au surface. Structural and compositional changes of the Prussian blue layer during electrochemical oxidation are studied by monitoring the stretching vibration of the CN group. At open circuit potential, VSFG reveals a non‐homogeneous distribution of oxidation states of metal sites: FeIII–CN–CoII and FeII–CN–CoIII coordination motifs are dominantly observed at the Co‐PBA|TiO2 interface, while it is only the FeII–CN–CoII unit at the electrolyte interface. Upon increasing the potential applied to the electrode, the partial oxidation of FeII–CN–CoII to FeIII–CN–CoII is observed followed by its transformation to FeII–CN–CoIII via charge transfer and, finally, the formation of FeIII–CN–CoIII species at the interface with TiO2 and the electrolyte.

Identification of the optimal site for oxygen adsorption in chemisorbed and oxidized states on pseudomorphic Fe/Ru(0001) surface: A density functional theory computational investigation

The oxidation state and charge distribution of FexOy binaries, including strained monolayers on transition metal surfaces, is a topic of significant interest. The p(2 × 2) and c(4 × 2) superstructures are two stable surface structures of chemisorbed oxygen on the pseudomorphic Fe/Ru(0001) surface that may coexist at 0.25 ML oxygen coverage. Density functional theory calculations were used to investigate the possibility of these two structures occurring on the surface. The calculations considered the effect of magnetic ordering on the choice of adsorption site for oxygen. Paramagnetic ordering of the Fe monolayer favors oxygen adsorption at the hcp site, while antiferromagnetic ordering favors oxygen adsorption at the fcc site at 0.25 ML oxygen coverage. Interestingly, in the case of antiferromagnetic ordering, although the adsorption of 1 ML coverage of oxygen for the oxidation reaction energetically prefers the hcp site, the chemisorbed structures at 0.25 ML coverage are found to prefer the fcc site on the pseudomorphic Fe/Ru(0001) surface. The DFT calculations suggest that both the p(2 × 2) and c(4 × 2) structures of oxygen are exothermic, indicating the possibility of coexistence on the pseudomorphic Fe/Ru(0001) surface and occurrence at room temperature. However, the use of the Hubbard potential parameter, which is used to correctly describe the electronic band structure of FeO and other Mott insulators, also revealed that the oxidation reaction of the pseudomorphic Fe monolayer is endothermic on the Ru(0001) surface.

In Situ Monitoring of Non-Thermal Plasma Cleaning of Surfactant Encapsulated Nanoparticles

Surfactants are widely used in the synthesis of nanoparticles, as they have a remarkable ability to direct their growth to obtain well-defined shapes and sizes. However, their post-synthesis removal is a challenge, and the methods used often result in morphological changes that defeat the purpose of the initial controlled growth. Moreover, after the removal of surfactants, the highly active surfaces of nanomaterials may undergo structural reconstruction by exposure to a different environment. Thus, ex situ characterization after air exposure may not reflect the effect of the cleaning methods. Here, combining X-ray photoelectron spectroscopy, in situ infrared reflection absorption spectroscopy, and environmental transmission electron microscopy measurements with CO probe experiments, we investigated different surfactant-removal methods to produce clean metallic Pt nanoparticles from surfactant-encapsulated ones. It was demonstrated that both ultraviolet-ozone (UV-ozone) treatment and room temperature O2 plasma treatment led to the formation of Pt oxides on the surface after the removal of the surfactant. On the other hand, when H2 was used for plasma treatment, both the Pt0 oxidation state and nanoparticle size distribution were preserved. In addition, H2 plasma treatment can reduce Pt oxides after O2-based treatments, resulting in metallic nanoparticles with clean surfaces. These findings provide a better understanding of the various options for surfactant removal from metal nanoparticles and point toward non-thermal plasmas as the best route if the integrity of the nanoparticle needs to be preserved.

Redox-Controlled Self-Assembly of Vanadium-Seamed Hexameric Pyrogallol[4]Arene Nanocapsules.

Here we report the controlled self-assembly of vanadium-seamed metal-organic nanocapsules with specific metal oxidation state distributions. Three supramolecular assemblies composed of the same numbers of components including 24 metal centers and six pyrogallol[4]arene ligands were constructed: a VIII24L6 capsule, a mixed-valence VIII18VIV6L6 capsule, and a VIV24L6 capsule. Crystallographic studies of the new capsules reveal their remarkable structural complexity and geometries, while marked differences in metal oxidation state distribution greatly affect the photoelectric conversion properties of these assemblies. This work therefore represents a significant step forward in the construction of intricate metal-organic architectures with tailored structure and functionality.

A Drive Towards Elemental Simplicity in Cathode Design by Controlling Grain Growth Behavior

Sodium-ion battery technologies are emerging as an alternative to lithium-ion batteries by utilizing the earth abundant, widely distributed, and low-cost sodium resources. Among different types of sodium ion cathodes, transition metal based layered oxide cathodes have a superior edge due to their comparatively higher energy density, lower cost, environment friendly raw materials, and similar synthesis chemistry as the already commercialized lithium-ion battery cathode materials. They also enable the incorporation of iron, an abundant, low-cost, and environmentally benign element, replacing the toxic and expensive cobalt.In such layered oxide-based cathodes, transition metals exhibit heterogenous elemental and oxidation state distribution at different depths of the particles (e.g., from the surface to bulk) which can influence the stability, mechanical integrity, and electrochemical performance of these particles to a significant extent. The inherent local structural and electronic state heterogeneity developed in these materials during synthesis can modulate the charge distribution heterogeneity and consequent phase transitions during cycling, ultimately controlling the battery performance.In this work, a sodium transition metal layered oxide cathode is synthesized using a co-precipitation method followed by high temperature calcination under different calcination durations. Calcining the material at different conditions lead to the formation of particles with different morphologies and grain growth behavior. Using electron microscopy and Xray diffraction techniques, the grain growth behavior at different stages of calcination is studied. This will guide the design of particles with better morphologies and grain orientation which demonstrate improved electrochemical performance. Transition metal distribution and oxidation state change at different depths of the particles are probed using synchrotron based Xray absorption spectroscopy and three-dimensional Transmission Xray microscopy. The particles are probed at the surface, subsurface and deep into the bulk, which show heterogeneity in the distribution of transition metal oxidation state and provide insight into the phase transitions during calcination. This study can pave the way for better understanding of phase evolution during synthesis and improved design of cathode particle morphology and composition by identifying and eliminating the detrimental phases occurring during different stages of calcination.

Chemical Profiles of the Oxides on Tantalum in State of the Art Superconducting Circuits.

Over the past decades, superconducting qubits have emerged as one of the leading hardware platforms for realizing a quantum processor. Consequently, researchers have made significant effort to understand the loss channels that limit the coherence times of superconducting qubits. A major source of loss has been attributed to two level systems that are present at the material interfaces. It is recently shown that replacing the metal in the capacitor of a transmon with tantalum yields record relaxation and coherence times for superconducting qubits, motivating a detailed study of the tantalum surface. In this work, the chemical profile of the surface of tantalum films grown on c-plane sapphire using variable energy X-ray photoelectron spectroscopy (VEXPS) is studied. The different oxidation states of tantalum that are present in the native oxide resulting from exposure to air are identified, and their distribution through the depth of the film is measured. Furthermore, it is shown how the volume and depth distribution of these tantalum oxidation states can be altered by various chemical treatments. Correlating these measurements with detailed measurements of quantum devices may elucidate the underlying microscopic sources of loss.

Unusual Mn oxidation state distribution in the vicinity of the tensile-strained interface between CaMnO3−δ and La0.7Ca0.3MnO3 layers

Oxide perovskite materials with heterointerfaces are important structures with applications such as electronic devices. The functionality of these materials depends on many factors, such as the charge, structure, and presence of defects at the interface. Thus, understanding the properties of interfaces and their effects on material function is important in the design and optimization of functional materials. In this study, the interplay among the Mn oxidation state distribution, the presence of oxygen vacancies (VOs), and the structure of the interface is investigated in the heterointerface between CaMnO3−δ and La0.7Ca0.3MnO3 layers by using electron energy loss spectroscopy combined with scanning transmission electron microscopy. Unlike the expectation that the Mn oxidation state distribution is controlled by the distribution of cations intermixing at the interface, it is dominantly influenced by the presence of VOs when the substrate gives tensile stress to it. As a result, the tensile-strained heterointerface shows an anomalously sharp reduction in the Mn oxidation state at the interface. This result suggests that VOs and strain are two essential ingredients to consider for the understanding of oxidation state distribution at interfaces. This study provides insights into the nature of various oxide heterointerfaces.

Iron and Manganese Oxidation States, Bonding Environments, and Mobility in the Mining-Impacted Sediments of Coeur d’Alene Lake, Idaho: Core Experiments

The mobility of a metal in mining-impacted sediments is determined by the environmental conditions that influence the metal’s oxidation state and bonding environment. Coeur d’Alene Lake, USA, has been impacted by legacy mining practices that allowed the hydrologic transport of mining waste to the lakebed, resulting in substantial amounts of redox-sensitive Fe and Mn along with Ag, As, Cd, Cu, Hg, Pb, Sb, and Zn. Future lake conditions may include algal blooms and additional algal detritus at the sediment–water interface, which may alter Fe and Mn forms that can influence their, and other metal(loid)s, mobility during seasonal anoxia. Cores of the lakebed sediments were exposed to anoxic and anoxic + algal detritus conditions for 8 weeks. Sediment samples were collected biweekly for analysis of Fe and Mn oxidation states and bonding environments by synchrotron-based X-ray absorption spectroscopy. Over the 8-week period and at a location 12.5 cm deep in the sediments, anoxic and anoxic + algae conditions produced limited changes in Fe and Mn oxidation states and bonding environments. At a location 2.5 cm below the sediment–water interface, the anoxic condition promoted a relatively stable environment in which Fe and Mn oxidation states and bonding environments did not vary greatly during the experiment. At the 2.5 cm depth, the anoxic + algae condition substantially altered the Mn oxidation state distribution and bonding environment, but this condition did not strongly influence the Fe oxidation state distribution or bonding environment. The anoxic + algae condition increased the presence of Mn3+, produced Mn4+ at select times, altered the Mn bonding environment, and temporarily increased the release of Mn into porewater. The algae influence on sediment and porewater Mn likely occurred because of the increased formation of organo-Mn complexes produced during algae-enhanced enzymatic processes. The lack of influence of algal detritus on sediment and porewater Fe and the formation of soluble organo-Mn complexes may limit the potential increase in the mobility of other metal(loid)s with future lake conditions.

Atomic-Scale Insights into Morphological, Structural, and Compositional Evolution of CoOOH during Oxygen Evolution Reaction

Co-based electrocatalysts are an emerging class of materials for the oxygen evolution reaction (OER). These materials undergo dynamic surface changes, converting to an active Co oxyhydroxide (CoOOH) layer during OER. A better understanding of the structural, morphological, and elemental evolution of CoOOH formed during OER is, therefore, crucial for the optimization of Co-based electrocatalysts. Herein, we propose an innovative multimodal method, which combines X-ray photoelectron spectroscopy, electron backscatter diffraction, high-resolution transmission electron microscopy, and atom probe tomography with electrochemical testing to investigate the temporal evolution of the oxidation state, thickness, morphology, and elemental distribution of the CoOOH layer grown on Co(0001) during OER. We reveal that the oxyhydroxide layer with the highest OER activity is a 5−6 nm thick, strongly hydrated film resembling a β-CoOOH(0001) structure and consisting of stacks of nanocrystals; which explains the X-ray amorphous characteristics of active species formed on Co-based electrocatalysts observed by operando X-ray-based techniques. The large interfacial area at these hydrated β-CoOOH(0001) nanocrystals enables efficient mass transport of reacting hydroxyl ions to catalytically active sites, and thus, high OER rates. As OER proceeds, the well-hydrated β-CoOOH(0001) nanocrystals grow into a monolithic crystalline β-CoOOH(0001) film. This goes along with a decrease in water content and electrochemically accessible catalyst area, resulting in slightly decreased OER currents. Overall, our study unprecedentedly unveils that in situ generated thin β-CoOOH(0001) layer undergoes dynamic morphological and elemental changes along with (de)incorporation of water molecules and hydroxyl groups during OER, which in turn alters OER performance. We demonstrate the strength of our multimodal characterization approach when seeking mechanistic insights into the role of structural and compositional evolution of hydrous oxides in activity during electrocatalytic reactions.

Distribution of Mn Oxidation States in Grassland Soils and Their Relationships with Soil Pores.

Manganese (Mn) is known to be an active contributor to processing and cycling of soil organic carbon (C), yet the exact mechanisms behind its interactions with C are poorly understood. Plant diversity in terrestrial ecosystems drives feedback links between plant C inputs and soil pores, where the latter, in turn, impact the redox environment and Mn. This study examined associations between soil pores (>36 μm Ø) and Mn within intact soils from two grassland ecosystems, after their >6-year implementation in a replicated field experiment. We used μ-XRF imaging and XANES spectroscopy to explore spatial distribution patterns of Mn oxidation states, combined with X-ray computed microtomography and 2D zymography. A high plant diversity system (restored prairie) increased soil C and modified spatial distribution patterns of soil pores as compared to a single species system (monoculture switchgrass). In switchgrass, the abundance of oxidized and reduced Mn oxidation states varied with distance from pores consistently with anticipated O2 diffusion, while in the soil from restored prairie, the spatial patterns suggested that biological activity played a greater role in influencing Mn distributions. Based on the findings, we propose a hypothesis that Mn transformations promote C gains in soils of high plant diversity grasslands.

Oxygen-Atom Defect Formation in Polyoxovanadate Clusters via Proton-Coupled Electron Transfer.

The uptake of hydrogen atoms (H-atoms) into reducible metal oxides has implications in catalysis and energy storage. However, outside of computational modeling, it is difficult to obtain insight into the physicochemical factors that govern H-atom uptake at the atomic level. Here, we describe oxygen-atom vacancy formation in a series of hexavanadate assemblies via proton-coupled electron transfer, presenting a novel pathway for the formation of defect sites at the surface of redox-active metal oxides. Kinetic investigations reveal that H-atom transfer to the metal oxide surface occurs through concerted proton–electron transfer, resulting in the formation of a transient VIII–OH2 moiety that, upon displacement of the water ligand with an acetonitrile molecule, forms the oxygen-deficient polyoxovanadate-alkoxide cluster. Oxidation state distribution of the cluster core dictates the affinity of surface oxido ligands for H-atoms, mirroring the behavior of reducible metal oxide nanocrystals. Ultimately, atomistic insights from this work provide new design criteria for predictive proton-coupled electron-transfer reactivity of terminal M=O moieties at the surface of nanoscopic metal oxides.

Approach to the Characterization of Monolithic Catalysts Based on La Perovskite-like Oxides and Their Application for VOC Oxidation under Simulated Indoor Environment Conditions

Catalysts are very important in controlling the pollutant emissions and are used for hundreds of chemical processes. Currently, noble metal-based catalysts are being replaced for other kinds of materials. In this study, three lanthanum-based perovskite-like oxides were synthesized (LaCo, LaCoMn, and LaMn) by the glycine-combustion method. The powder catalysts obtained were supported onto cordierite ceramic monoliths using an optimized washcoating methodology to obtain the subsequent monolithic catalysts (LaCo-S, LaCoMn-S, and LaMn-S). Sample characterization confirmed the formation of the perovskite-like phase in the powder materials as well as the presence of the perovskite phase after supporting it onto the monolithic structure. The XPS analysis showed a general decrease in lattice oxygen species for monolithic catalysts, mainly caused by the colloidal silica used as a binder agent during the washcoating process. Additionally, some variations in the oxidation state distribution for elements in Co-containing systems suggest a stronger interaction between cordierite and such catalysts. The catalytic activity results indicated that powder and monolithic catalysts were active for single-component VOC oxidation in the following order: 2-propanol &gt; n-hexane ≅ mixture &gt; toluene, and there was no evidence of loss of catalytic activity after supporting the catalysts. However, LaMn-S had a better catalytic performance for all VOC tested under dry conditions, achieving oxidation temperatures between 230–420 °C. The oxidation efficiency for the VOC mixture was strongly affected by the presence of moisture linking the oxidation efficiency at wet conditions to the VOC chemical nature. Additionally, for higher VOC concentrations, the catalyst efficiency decreased due to the limited number of active sites.

Tailoring the oxidation state of metallic TiO through Ti3+/Ti2+ regulation for photocatalytic conversion of CO2 to C2H6

The regulation and stabilization of the oxidation state to promote the conversion of CO2 to C2 fuel still faces many challenges. Based on the principle of charge balance, we creatively propose a co-doping strategy to adjust the surface oxidation state distribution of metallic catalysts. A TiO-based photocatalyst co-doped with Zn and N was synthesized by ammonia assisted one-step calcination method, named ZN-TC. XPS characterization shows that Zn and N adjust the valence states of adjacent Ti elements respectively, so that the surface of TiO maintains a relatively stable Ti3+/Ti2+ ratio. Under visible light irradiation, the material can catalyze CO2 into CO (324.11 μmol·g−1·h−1) and C2H6 (10.27 μmol·g−1·h−1) in the liquid phase. The selectivity of C2H6 reached 14.45%. When irradiated with near-infrared light, ZN-TC shows 100% CO selectivity because the photon energy is not enough to support the catalytic hydrogenation of CO2. Theoretical calculations and experiments proved that Zn and N elements mainly act on the B-1 band to regulate the Ti valence state. In-situ DRIFTS and in-situ Raman tests confirmed the function of oxidation state adjustment to promote the C-C coupling on the catalyst surface to produce ethoxy groups, which ultimately led to the production of C2H6.

Structural Changes of Spinel MCo2O4 (M = Mn, Fe, Co, Ni, and Zn) Electrocatalysts during the Oxygen Evolution Reaction Investigated by In Situ X-ray Absorption Spectroscopy

A comparison of the electrochemical and physicochemical behavior of cobalt-based oxides with spinel structure MCo2O4 (M = Mn, Fe, Co, Ni, and Zn) was conducted to investigate the effect of the oxidation state and cation distribution in the spinel on the electrocatalytic activity of the oxygen evolution reaction (OER) in an alkaline solution. Various spinel MCo2O4 electrocatalysts were synthesized by a facile microwave-assisted synthesis and low-temperature annealing. The overpotential of these MCo2O4 electrocatalysts for the OER is comparable to the reported overpotentials of catalysts based on cobalt oxides. From the findings, the catalytic activity of OER decreases in the order of ZnCo2O4 > NiCo2O4 > FeCo2O4 > Co3O4 > MnCo2O4. It was revealed that the active sites are controlled by the balance of M3+/M2+ cation distribution in octahedral and tetrahedral sites and by the bond strength between M and oxygen atoms at the catalyst surface from the direct combination of in situ X-ray absorption fine structure (XAFS) spectroscopy with the electrochemical experiments to track the oxidation state and the structural changes of electrocatalysts before and after the exposure to the OER conditions. This study provides insights into the effects of cation distributions on the OER activity and demonstrates a promising method for determining the fundamental mechanism of cation-substituted cobalt oxides for OER.

Unusual Mn Oxidation State Distribution in the Vicinity of the Tensile-Strained Interface between Camno3 and La0.7ca0.3mno3 Layers

Unusual Mn Oxidation State Distribution in the Vicinity of the Tensile-Strained Interface between Camno3 and La0.7ca0.3mno3 Layers

Insights into lithium manganese oxide-water interfaces using machine learning potentials.

Unraveling the atomistic and the electronic structure of solid-liquid interfaces is the key to the designof new materials for many important applications, from heterogeneous catalysis to battery technology. Density functional theory (DFT) calculations can, in principle, provide a reliable description of such interfaces, but the high computational costs severely restrict the accessible time and length scales. Here, we report machine learning-driven simulations of various interfaces between water and lithium manganese oxide (LixMn2O4), an important electrode material in lithium ion batteries and a catalyst for the oxygen evolution reaction. We employ a high-dimensional neural network potential to compute the energies and forces several orders of magnitude faster than DFT without loss in accuracy. In addition, a high-dimensional neural network for spin prediction is utilized to analyze the electronic structure of the manganese ions. Combining these methods, a series of interfaces is investigated by large-scale molecular dynamics. The simulations allow us to gain insights into a variety of properties, such as the dissociation of water molecules, proton transfer processes, and hydrogen bonds, as well as the geometric and electronic structure of the solid surfaces, including the manganese oxidation state distribution, Jahn-Teller distortions, and electron hopping.