Confinement-Induced Isosymmetric Metal-Insulator Transition in Ultrathin Epitaxial V2O3 Films

Dimensional confinement has shown to be an effective strategy to tune competing degrees of freedom in complex oxides. Here, we achieved atomic layered growth of trigonal vanadium sesquioxide (V2O3) by means of oxygen-assisted molecular beam epitaxy. This led to a series of high-quality epitaxial ultrathin V2O3 films down to unit cell thickness, enabling the study of the intrinsic electron correlations upon confinement. By electrical and optical measurements, we demonstrate a dimensional confinement-induced metal-insulator transition in these ultrathin films. We shed light on the Mott-Hubbard nature of this transition, revealing a vanishing quasiparticle weight as demonstrated by photoemission spectroscopy. Furthermore, we prove that dimensional confinement acts as an effective out-of-plane stress. This highlights the structural component of correlated oxides in a confined architecture, while opening an avenue to control both in-plane and out-of-plane lattice components by epitaxial strain and confinement, respectively.

Recent Development on Transition Metal Oxides-Based Core-Shell Structures for Boosted Energy Density Supercapacitors

In recent years, nanomaterials exploration and synthesis have played a crucial role in advancing energy storage research, particularly in supercapacitor development. Researchers have diversified materials, including metal oxides, chalcogenides, and composites, as well as carbon materials, to enhance energy and power density. Balancing energy density with electrochemical stability remains challenging, driving intensified efforts in advancing electrode materials. This review focuses on recent progress in designing and synthesizing core-shell materials tailored for supercapacitors. The core-shell architecture offers advantages such as increased surface area, redox active sites, electrical conductivity, ion diffusion kinetics, specific capacitance, and cyclability. The review explores the impact of core and shell materials, specifically transition metal oxides (TMOs), on supercapacitor electrochemical behavior. Metal oxide choices, such as cobalt oxide as a preferred core and manganese oxide as a shell, are discussed. The review also highlights characterization techniques for assessing structural, morphological, and electrochemical properties of core-shell materials. Overall, it provides a comprehensive overview of ongoing TMOs-based core-shell material research for supercapacitors, showcasing their potential to enhance energy storage for applications ranging from gadgets to electric vehicles. The review outlines existing challenges and future opportunities in evolving TMOs-based core-shell materials for supercapacitor advancements, holding promise for high-efficiency energy storage devices.

In Situ Design of a Nanostructured Interface between NiMo and CuO Derived from Metal-Organic Framework for Enhanced Hydrogen Evolution in Alkaline Solutions

Hydrogen shows great promise as a carbon-neutral energy carrier that can significantly mitigate global energy challenges, offering a sustainable solution. Exploring catalysts that are highly efficient, cost-effective, and stable for the hydrogen evolution reaction (HER) holds crucial importance. For this, metal-organic framework (MOF) materials have demonstrated extensive applicability as either a heterogeneous catalyst or catalyst precursor. Herein, a nanostructured interface between NiMo/CuO@C derived from Cu-MOF was designed and developed on nickel foam (NF) as a competent HER electrocatalyst in alkaline media. The catalyst exhibited a low overpotential of 85 mV at 10 mA cm-2 that rivals that of Pt/C (83 mV @ 10 mA cm-2). Moreover, the catalyst's durability was measured through chronopotentiometry at a constant current density of -30, -100, and -200 mA cm-2 for 50 h each in 1.0 M KOH. Such enhanced electrocatalytic performance could be ascribed to the presence of highly conductive C and Cu species, the facilitated electron transfer between the components because of the nanostructured interface, and abundant active sites as a result of multiple oxidation states. The existence of an ionized oxygen vacancy (Ov) signal was confirmed in all heat-treated samples through electron paramagnetic resonance (EPR) analysis. This revelation sheds light on the entrapment of electrons in various environments, primarily associated with the underlying defect structures, particularly vacancies. These trapped electrons play a crucial role in augmenting electron conductivity, thereby contributing to an elevated HER performance.

Electrochemical Sensors Based on Transition Metal Materials for Phenolic Compound Detection

Electrochemical sensors have been recognized as crucial tools for monitoring comprehensive chemical information, especially in the detection of a significant class of molecules known as phenolic compounds. These compounds can be present in water as hazardous analytes and trace contaminants, as well as in living organisms where they regulate their metabolism. The sensitive detection of phenolic compounds requires highly efficient and cost-effective electrocatalysts to enable the development of high-performance sensors. Therefore, this review focuses on the development of advanced materials with excellent catalytic activity as alternative electrocatalysts to conventional ones, with a specific emphasis on transition metal-based electrocatalysts for the detection of phenolic compounds. This research is particularly relevant in diverse sectors such as water quality, food safety, and healthcare.

Magnetic Properties and THz Emission from Co/CoO/Pt and Ni/NiO/Pt Trilayers

THz radiation emitted by ferromagnetic/non-magnetic bilayers is a new emergent field in ultra-fast spin physics phenomena with a lot of potential for technological applications in the terahertz (THz) region of the electromagnetic spectrum. The role of antiferromagnetic layers in the THz emission process is being heavily investigated at the moment. In this work, we fabricate trilayers in the form of Co/CoO/Pt and Ni/NiO/Pt with the aim of studying the magnetic properties and probing the role of very thin antiferromagnetic interlayers like NiO and CoO in transporting ultrafast spin current. First, we reveal the static magnetic properties of the samples by using temperature-dependent Squid magnetometry and then we quantify the dynamic properties with the help of ferromagnetic resonance spectroscopy. We show magnetization reversal that has large exchange bias values and we extract enhanced damping values for the trilayers. THz time-domain spectroscopy examines the influence of the antiferromagnetic interlayer in the THz emission, showing that the NiO interlayer in particular is able to transport spin current.

Carbon Nanosheets-Based Supercapacitor Materials: Recent Advances and Prospects

The need for inexpensive and ecologically sustainable energy storage technologies is rising rapidly along with the severity of the world's environmental challenges as well as with the rising demand for portable electronics and hybrid vehicles. Supercapacitors have drawn a lot of attentions lately in this regard because of their ultrahigh power density, outstanding electrochemical stability, and environmental friendliness. Due to various advantages, carbon materials are the choice of designer for developing commercial electrodes for various applications including devising supercapacitors. Two-dimensional (2D) carbon nanosheets (CNSs) with a large surface area and excellent electronic transport properties have fired up the interest of researchers due to their unique properties and potential applications in energy storage. Such engineered 2D porous CNS may significantly improve the energy storage performance of supercapacitor by enabling fast ion transport and charge transfer kinetics. This article summarizes the most recent and significant advances in the area of activated, porous, graphene-based various CNSs and their composites with a special focus on their use as supercapacitor electrodes. A succinct overview about their syntheses and key characterizations regarding their different structural aspects have been discussed. The present challenges and prospects in using CNS in supercapacitor applications are highlighted.

An Overview of Electrochemical Sensors Based on Transition Metal Carbides and Oxides: Synthesis and Applications

Sensors play vital roles in industry and healthcare due to the significance of controlling the presence of different substances in industrial processes, human organs, and the environment. Electrochemical sensors have gained more attention recently than conventional sensors, including optical fibers, chromatography devices, and chemiresistors, due to their better versatility, higher sensitivity and selectivity, and lower complexity. Herein, we review transition metal carbides (TMCs) and transition metal oxides (TMOs) as outstanding materials for electrochemical sensors. We navigate through the fabrication processes of TMCs and TMOs and reveal the relationships among their synthesis processes, morphological structures, and sensing performance. The state-of-the-art biological, gas, and hydrogen peroxide electrochemical sensors based on TMCs and TMOs are reviewed, and potential challenges in the field are suggested. This review can help others to understand recent advancements in electrochemical sensors based on transition metal oxides and carbides.

Effect of Interdiffusion and Crystallization on Threshold Switching Characteristics of Nb/Nb2O5/Pt Memristors

The resistive switching response of two terminal metal/oxide/metal devices depends on the stoichiometry of the oxide film, and this is commonly controlled by using a reactive metal electrode to reduce the oxide layer. Here, we investigate compositional and structural changes induced in Nb/Nb2O5 bilayers by thermal annealing at temperatures in the range of 573-973 K and its effect on the volatile threshold switching characteristics of Nb/Nb2O5/Pt devices. Changes in the stoichiometry of the Nb and Nb2O5 films are determined by Rutherford backscattering spectrometry and energy-dispersive X-ray (EDX) mapping of sample cross sections, while the structure of the films is determined by X-ray diffraction, Raman spectroscopy, and transmission electron microscopy (TEM). Such analysis shows that the composition of the Nb and Nb2O5 layers is homogenized by interdiffusion at temperatures less than the crystallization temperature (i.e., >773 K) but that this effectively ceases once the films crystallize. This is explained by comparison with the predictions of a simple diffusion model which shows that the compositional changes are dominated by oxygen diffusion in the amorphous oxide, which is much faster than that in the crystalline phases. We further show that these compositional and structural changes have a significant effect on the electroforming and threshold switching characteristics of the devices, the most significant being a marked increase in their reliability and endurance after crystallization of the oxide films. Finally, we examine the effect of annealing on the quasistatic negative differential resistance characteristics and oscillator dynamics of devices and use a lumped element model to show that this is dominated by changes in the device capacitance resulting from interdiffusion.

Recent Advances on F-Doped Layered Transition Metal Oxides for Sodium Ion Batteries

With the development of social economy, using lithium-ion batteries in energy storage in industries such as large-scale electrochemical energy storage systems will cause lithium resources to no longer meet demand. As such, sodium ion batteries have become one of the effective alternatives to LIBs. Many attempts have been carried out by researchers to achieve this, among which F-doping is widely used to enhance the electrochemical performance of SIBs. In this paper, we reviewed several types of transition metal oxide cathode materials, and found their electrochemical properties were significantly improved by F-doping. Moreover, the modification mechanism of F-doping has also been summed up. Therefore, the application and commercialization of SIBs in the future is summarized in the ending of the review.

Current Trends and Promising Electrode Materials in Micro-Supercapacitor Printing

The development of scientific and technological foundations for the creation of high-performance energy storage devices is becoming increasingly important due to the rapid development of microelectronics, including flexible and wearable microelectronics. Supercapacitors are indispensable devices for the power supply of systems requiring high power, high charging-discharging rates, cyclic stability, and long service life and a wide range of operating temperatures (from -40 to 70 °C). The use of printing technologies gives an opportunity to move the production of such devices to a new level due to the possibility of the automated formation of micro-supercapacitors (including flexible, stretchable, wearable) with the required type of geometric implementation, to reduce time and labour costs for their creation, and to expand the prospects of their commercialization and widespread use. Within the framework of this review, we have focused on the consideration of the key commonly used supercapacitor electrode materials and highlighted examples of their successful printing in the process of assembling miniature energy storage devices.

Octahedral/Tetrahedral Vacancies in Fe3 O4 as K-Storage Sites: A Case of Anti-Spinel Structure Material Serving as High-Performance Anodes for PIBs

Potassium-ion batteries (PIBs) have attracted more and more attention as viable alternatives to lithium-ion batteries (LIBs) due to the deficiency and uneven distribution of lithium resources. However, it is shown that potassium storage in some compounds through reaction or intercalation mechanisms cannot effectively improve the capacity and stability of anodes for PIBs. The unique anti-spinel structure of magnetite (Fe3 O4 ) is densely packed with thirty-two O atoms to form a face-centered cubic (fcc) unit cell with tetrahedral/octahedral vacancies in the O-closed packing structure, which can serve as K+ storage sites according to the density functional theory (DFT) calculation results. In this work, carbon-coated Fe3 O4 @C nanoparticles are prepared as high-performance anodes for PIBs, which exhibit high reversible capacity (638 mAh g-1 at 0.05 A g-1 ) and hyper stable cycling performance at ultrahigh current density (150 mAh g-1 after 9000 cycles at 10 A g-1 ). In situ XRD, ex-situ Fe K-edge XAFS, and DFT calculations confirm the storage of K+ in tetrahedral/octahedral vacancies.

Magnetic Anisotropy and Planar Topological Hall Effect in SrMnxIr1-xO3Films

Strong Coulomb repulsion and spin-orbit coupling are known to give rise to exotic physical phenomena in transition metal oxides. Here, we report magnetic and transport characteristics of (001) oriented epitaxial SrMnxIr1-xO3 thin films, having both 3d and 5d elements on the perovskite B sites. With the increase of Mn concentration, perpendicular magnetic anisotropy decreases gradually in accompany with the magnetic easy axis tilting away from the out-of-plane [001] direction. X-ray absorption spectroscopy reveals that Mn eg electrons preferentially occupy the "d" _(〖"3z" 〗^"2" "-" "r" ^"2" ) orbital, which produces the observed perpendicular magnetic anisotropy in the framework of spin-orbital coupling. A planar topological Hall effect appears in SrMnxIr1-xO3 films with x about 0.30 when the magnetic field is applied along the current, which is a result of the noncoplanar spin structure due to the competition among the perpendicular magnetic anisotropy, the magnetic exchange interaction and the Zeeman energy. These results provide an example to show the subtle balance among complex competitions in materials with both strong correlation and spin-orbit coupling.

Metal Oxides Nanomaterials and Nanocomposite-Based Electrochemical Sensors for Healthcare Applications

Wide-ranging research efforts have been directed to prioritize scientific and technological inventions for healthcare monitoring. In recent years, the effective utilization of functional nanomaterials in various electroanalytical measurements realized a rapid, sensitive, and selective detection and monitoring of a wide range of biomarkers in body fluids. Owing to good biocompatibility, high organic capturing ability, strong electrocatalytic activity, and high robustness, transition metal oxide-derived nanocomposites have led to enhancements in sensing performances. The aim of the present review is to describe key advancements of transition metal oxide nanomaterials and nanocomposites-based electrochemical sensors, along with current challenges and prospects towards the development of a highly durable and reliable detection of biomarkers. Moreover, the preparation of nanomaterials, electrode fabrication, sensing mechanism, electrode-bio interface, and performance of metal oxides nanomaterials and nanocomposite-based sensor platforms will be described.

Solid-State Electrochemical Thermal Transistors with Strontium Cobaltite-Strontium Ferrite Solid Solutions as the Active Layers

Thermal transistors have potential as thermal management devices because they can electrically control the thermal conductivity (κ) of the active layer. Recently, we realized solid-state electrochemical thermal transistors by utilizing the electrochemical redox reaction of SrCoO_y (2 ≤ y ≤ 3). However, the guiding principle to improve the on/off κ ratio has yet to be clarified because the κ modulation mechanism is unclear. This study systematically modulates κ of SrCo_1-xFe_xO_y (0 ≤ x ≤ 1, 2 ≤ y ≤ 3) solid solutions used as the active layers in solid-state electrochemical thermal transistors. When y = 3, the lattice κ of SrCo_1-xFe_xO_y is ∼2.8 W m^-1 K^-1 and insensitive to x. When x = 0 and y = 3, κ increases to ∼3.8 W m^-1 K^-1 due to the contribution of the electron κ. When y = 2, κ slightly depends on the ordered atomic arrangement. Materials that are high electrical conductors with highly ordered lattices when the transistor is on but are electrical insulators with disordered lattices when the transistor is off should be well-suited for the active layers of solid-state electrochemical thermal transistors.

Enhanced OER Performance and Dynamic Transition of Surface Reconstruction in LaNiO3 Thin Films with Nanoparticles Decoration

In an electrocatalytic process, the cognition of the active phase in a catalyst has been regarded as one of the most vital issues, which not only boosts the fundamental understanding of the reaction procedure but also guides the engineering and design for further promising catalysts. Here, based on the oxygen evolution reaction (OER), the stepwise evolution of the dominant active phase is demonstrated in the LaNiO₃ (LNO) catalyst once the single-crystal thin film is decorated by LNO nanoparticles. It is found that the OER performance can be dramatically improved by this decoration, and the catalytic current density at 1.65 V can be enhanced by ≈1000% via ≈10⁹ cm^-2 nanoparticle adhesion after extracting the contribution of surface enlargement. Most importantly, a transition of the active phase from LNO to NiOOH via surface reconstruction with the density of LNO nanoparticles is demonstrated. Several mechanisms in terms of this active phase transition are discussed involving lattice orientation-induced change of the surface energy profile, the lattice oxygen participation, and the A/B-site ions leaching during OER cycles. This study suggests that the active phases in transition metal-based OER catalysts can transform with morphology, which should be corresponding to distinct engineering strategies.

Multicomponent Metal Oxide- and Metal Hydroxide-Based Electrocatalysts for Alkaline Water Splitting

Developing cost-effective, highly catalytic active, and stable electrocatalysts in alkaline electrolytes is important for the development of highly efficient anion-exchange membrane water electrolysis (AEMWE). To this end, metal oxides/hydroxides have attracted wide research interest for efficient electrocatalysts in water splitting owing to their abundance and tunable electronic properties. It is very challenging to achieve an efficient overall catalytic performance based on single metal oxide/hydroxide-based electrocatalysts due to low charge mobilities and limited stability. This review is mainly focused on the advanced strategies to synthesize the multicomponent metal oxide/hydroxide-based materials that include nanostructure engineering, heterointerface engineering, single-atom catalysts, and chemical modification. The state of the art of metal oxide/hydroxide-based heterostructures with various architectures is extensively discussed. Finally, this review provides the fundamental challenges and perspectives regarding the potential future direction of multicomponent metal oxide/hydroxide-based electrocatalysts.

Local Electric Field Induced by Atomic-Level Donor-Acceptor Couple of O Vacancies and Mn Atoms Enables Efficient Hybrid Capacitive Deionization

Transition metal oxides suffer from slow salt removal rate (SRR) due to inferior ions diffusion ability in hybrid capacitive deionization (HCDI). Local electric field (LEF) can efficiently improve the ions diffusion kinetics in thin electrodes for electrochemical energy storage. Nevertheless, it is still a challenge to facilitate the ions diffusion in bulk electrodes with high loading mass for HCDI. Herein, this work delicately constructs a LEF via engineering atomic-level donor (O vacancies)-acceptor (Mn atoms) couples, which significantly facilitates the ions diffusion and then enables a high-performance HCDI. The LEF boosts an extended accelerated ions diffusion channel at the particle surface and interparticle space, resulting in both remarkably enhanced SRR and salt removal capacity. Convincingly, the theoretical calculations demonstrate that electron-enriched Mn atoms center coupled with an electron-depleted O vacancies center is formed due to the electron back-donation from O vacancies to adjacent Mn centers. The resulted LEF efficiently reduce the ions diffusion energy barrier. This work sheds light on the effect of atomic-level LEF on improving ions diffusion kinetics at high loading mass application and paves the way for the design of transition metal oxides toward high-performance HCDI applications.

In Situ Reconstructed Mo-doped Amorphous FeOOH Boosts the Oxygen Evolution Reaction

Developing a fast and highly active oxygen evolution reaction (OER) catalyst to change energy kinetics technology is essential for making clean energy. Herein, we prepare three-dimensional (3D) hollow Mo-doped amorphous FeOOH (Mo-FeOOH) based on the precatalyst MoS₂ /FeC₂ O₄ via in situ reconstruction strategy. Mo-FeOOH exhibits promising OER performance. Specifically, it has an overpotential of 285 mV and a durability of 15 h at 10 mA cm^-2 . Characterizations indicate that Mo was included inside the FeOOH lattice, and it not only modifies the electronic energy levels of FeOOH but also effectively raises the inherent activity of FeOOH for OER. Additionally, in situ Raman analysis indicates that FeC₂ O₄ gradually transforms into the FeOOH active site throughout the OER process. This study provides ideas for designing in situ reconstruction strategies to prepare heteroatom doping catalysts for high electrochemical activity.

Breaking the Electronic Conductivity Bottleneck of Manganese Oxide Family for High-Power Fluorinated Graphite Composite Cathode by Ligand-Field High-Dimensional Constraining Strategy

Primary lithium fluorinated graphite (Li/CF_x ) batteries with superior energy density are an indispensable energy supply for multiple fields but suffer from sluggish reaction kinetics of the CF_x cathode. Designing composite cathodes emerges as a solution to this problem. Despite the optimal composite component for CF_x , the manganese oxide family represented by MnO₂ is still faced with an intrinsic electronic conductivity bottleneck, which severely limits the power density of the composite cathode. Here, a cation-induced high-dimensional constraining strategy from the perspective of ligand-field stacking structure topological design, which breaks the molecular orbital hybridization of pristine semiconductive oxides to transform them into the high-conductivity metallic state while competitively maintaining structural stability, is proposed. Through first-principles phase diagram calculations, mixed-valent Mn₅ O₈ ( Mn 2 2 + Mn 3 4 + O 8 ${\rm{Mn}}_2^{2 + }{\rm{Mn}}_3^{4 + }{{\rm{O}}_8}$ ) is explored as an ideal high-dimensional constraining material with satisfied conductivity and large-scale production feasibility. Experiments demonstrate that the as-proposed CF_x  @ Mn₅ O₈ composite cathode achieves 2.36 times the power density (11399 W kg^-1 ) of pristine CF_x and a higher CF_x conversion ratio (86%). Such a high-dimensional field-constraining strategy is rooted in the established four-quadrant electronic structure tuning framework, which fundamentally changes the orbital symmetry under the ligand field to overcome the common conductivity challenge of wide transition metal oxide materials.

Impact of Graphene Monolayer on the Performance of Non-Conventional Silicon Heterojunction Solar Cells with MoOx Hole-Selective Contact

In this work, a new design of transparent conductive electrode based on a graphene monolayer is evaluated. This hybrid electrode is incorporated into non-standard, high-efficiency crystalline silicon solar cells, where the conventional emitter is replaced by a MoO_x selective contact. The device characterization reveals a clear electrical improvement when the graphene monolayer is placed as part of the electrode. The current-voltage characteristic of the solar cell with graphene shows an improved FF and V_oc provided by the front electrode modification. Improved conductance values up to 5.5 mS are achieved for the graphene-based electrode, in comparison with 3 mS for bare ITO. In addition, the device efficiency improves by around 1.6% when graphene is incorporated on top. These results so far open the possibility of noticeably improving the contact technology of non-conventional photovoltaic technologies and further enhancing their performance.

Challenges and Opportunities of Transition Metal Oxides as Electrocatalysts

As a sustainable energy technology, electrocatalytic energy conversion and storage has become increasingly prominent. The oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), nitrogen reduction reaction (NRR), and carbon dioxide reduction reaction (CO₂ RR) are the key steps in the industrial applications of energy conversion and storage. Compared to the widely used precious metal catalysts, less-noble transition metal oxides (TMOs) and TMO-like materials have attracted broad attention as electrocatalysts in the above reactions. In this concept, we summarize the challenges and opportunities of some typical TMOs in electrocatalysis, and modification strategies of TMOs as electrocatalysts are discussed.

Semi-Embedding Zn-Co3O4 Derived from Hybrid ZIFs into Wood-Derived Carbon for High-Performance Supercapacitors

Transition metal oxides (TMOs) can provide high theoretical capacitance due to the change of multiple valence states of transition metals. However, their intrinsic drawbacks, including poor electrical conductivity, lower energy density, and huge volume expansion, will result in the pulverization of electrode materials and restricted electrochemical kinetics, thus leading to poor rate capability and rapid capacity fading. Composite electrodes based on transition metal oxides and carbon-based materials are considered to be promising candidates for overcoming these limitations. Herein, we reported a preparation method of hybrid ZIFs derived Zn-doped Co₃O₄/carbon (Zn-Co₃O₄/C-230) particles semi-embedded in wood-derived carbon skeleton for integrated electrodes. A large specific surface area, excellent conductivity, and electrochemical stability provide a larger electrochemical activity and potential window for the electrode. Prepared Zn-Co₃O₄@CW-230 electrode (0.6 mm thick) displays ultrahigh area specific capacitances of 7.83 and 6.46 F cm^-2 at the current densities of 5 and 30 mA cm^-2, respectively. Moreover, a symmetric supercapacitor assembled by two identical Zn-Co₃O₄@CW-230 electrodes delivers a superior area-specific capacitance of 2.61 F cm^-2 at the current densities of 5 mA cm^-2 and great energy densities of 0.36 mWh cm^-2 (6.0 mWh cm^-3) at 2.5 mW cm^-2, while maintaining 97.3% of initial capacitance over 10,000 cycles. It notably outperforms those of most carbon-based metal oxides, endowing the Zn-Co₃O₄@CW-230 with extensive prospects for practical application.

Soot Combustion over Cu-Co Spinel Catalysts: The Intrinsic Effects of Precursors on Catalytic Activity

In this work, a series of CuCo₂O₄-x (x = N, A and C) catalysts were synthesized using different metal salt precursors by urea hydrothermal method for catalytic soot combustion. The effect of CuCo₂O₄-x catalysts on soot conversion and CO₂ selectivity in both loose and tight contact mode was investigated. The CuCo₂O₄-N catalyst exhibited outstanding catalytic activity with the characteristic temperatures (T₁₀, T₅₀ and T₉₀) of 451 °C, 520 °C and 558 °C, respectively, while the CO₂ selectivity reached 98.8% during the reaction. With the addition of NO, the soot combustion was further accelerated over all catalysts. Compared with the loose contact mode, the soot conversion was improved in the tight contact mode. The CuCo₂O₄-N catalysts showed better textural properties compared to the CuCo₂O₄-A and CuCo₂O₄-C, such as higher specific surface areas and pore volumes. The XRD results confirmed that the formation of a CuCo₂O₄ crystal phase in all catalysts. However, the CuO crystal phase only presented in CuCo₂O₄-N and CuCo₂O₄-A. The relative contents of Cu²⁺, Co³⁺ and O_ads on the surface of CuCo₂O₄-x (x = N, A and C) catalysts were analyzed by XPS. The CuCo₂O₄-N catalyst displayed the highest relative content of Cu²⁺, Co³⁺ and O_ads. The activity of catalytic soot combustion showed a good correlation with the order of the relative contents of Cu²⁺, Co³⁺ and O_ads. Additionally, the CuCo₂O₄-N catalyst exhibited lower reduction temperature compared to the CuCo₂O₄-A and CuCo₂O₄-C. The cycle tests clarified that the copper-cobalt spinel catalyst obtained good stability. In addition, based on the Mars-van Krevelen mechanism, the process of catalytic soot combustion was described combined with the electron transfer process and the role of oxygen species over CuCo₂O₄ spinel catalysts.

Bifunctional OER/NRR Catalysts Based on a Thin-Layered Co3O4-x/GO Sandwich Structure

Due to ample low-coordinated surface atoms, a distorted lattice endows thin-layered transition metal oxides with a flexible electronic structure, making them the ideal candidates for overall ammonia synthesis. This work proposes a novel and facile method for the controllable synthesis of thin-layered Co₃O₄ catalysts with graphene as a conductive matrix to further enhance the overall N₂ fixation performance. X-ray photoelectron spectroscopy (XPS) and synchrotron radiation X-ray absorption spectroscopy (XAS) demonstrate that the sandwiched Co₃O_4-x/GO catalysts enable exposure of more coordination unsaturated active sites, resulting in numerous oxygen vacancies. With a higher conductivity and distorted crystalline structure, excellent electrochemical NRR activity is realized with a NH₃ production rate of 5.19 mmol g^-1 h^-1 and a Faradaic efficiency of 10.68% at -0.4 V vs reversible hydrogen electrode (RHE). The density functional theory (DFT) calculation demonstrates that introducing oxygen vacancies in thin-layered cobalt oxides could result in an increased density of states (DOS) near the Fermi level, which would accelerate the NRR rate-determining step. Charge transfer could be accelerated through a weak Co 3d-N 2p σ hybrid bond with a lower energy level. No obvious performance decay could be found after six cycles. Furthermore, the sandwiched Co₃O_4-x/GO catalyst exhibits a low overpotential of 280 mV@10 mA cm^-2 and an outstanding durability for the anode OER, even better than those of the benchmark RuO₂. Such an inexpensive sandwiched transition metal oxide catalyst shows great potential in the field of overall N₂ fixation.

Mott Insulator Ca2RuO4 under External Electric Field

We have investigated the structural, electronic and magnetic properties of the Mott insulator Ca₂RuO₄ under the application of a static external electric field in two regimes: bulk systems at small fields and thin films at large electric fields. Ca₂RuO₄ presents S- and L-Pbca phases with short and long c lattice constants and with large and small band gaps, respectively. Using density functional perturbation theory, we have calculated the Born effective charges as response functions. Once we break the inversion symmetry by off-centering the Ru atoms, we calculate the piezoelectric properties of the system that suggest an elongation of the system under an electric field. Finally, we investigated a four-unit cell slab in larger electric fields, and we found insulator-metal transitions induced by the electric field. By looking at the local density of states, we have found that the gap gets closed on surface layers while the rest of the sample is insulating. Correlated to the electric-field-driven gap closure, there is an increase in the lattice constant c. Regarding the magnetic properties, we have identified two phase transitions in the magnetic moments with one surface that gets completely demagnetized at the largest field investigated. In all cases, the static electric field increases the lattice constant c and reduces the band gap of Ca₂RuO₄, playing a role in the competition between the L-phase and the S-phase.

Anomalous dielectric response in the triple perovskite ruthenate Ba3BiRu2O9

We have investigated the magnetic, dielectric and thermal properties of theSRu= 1 magnetic dimer Ba₃BiRu₂O₉, which is known to exhibit a spin-gap opening in conjunction with a first-order magneto-elastic phase transition at ∼175 K. Above the spin-gap temperature, the temperature dependence dielectric constant shows a peak like feature with pronounced frequency dependence. The critical slowing down behavior of this frequency dispersion suggests that a ferroelectric relaxor like electrical glassy state exists above the spin-gap opening temperature regime. The extermination of frequency dispersion-right at the magneto-elastic phase transition, is suggestive of a strong coupling between the lattice and charge-spin degrees of freedom in this triple perovskite system.

Persona of Transition Metal Ions in Solids: A Statistical Learning on Local Structures of Transition Metal Oxides

The local structure of a transition metal (TM) ion is a function of cation elements and valence states. More than that, in this work, by employing a trove of first-principles data of TM oxides, the local structures of TM cations are statistically analyzed to extract detailed information about cation site preference, bond length, site structural distortion, and cation magnetization. It is found that cation radius alone poorly describes the local structure of a transition metal oxide, while the statistics of coordination number as well as the TMO bond length distribution, especially that of the 3d TMs, can provide comprehensive knowledge for understanding the behavior of TM elements. Based on these statistics, the interplay of site distortion due to the Jahn-Teller effect, cation site similarity, and a new set of ionic radii are all obtained to chart the "persona" of transition metal ions in solids.

Three-Dimensional Monolithically Self-Grown Metal Oxide Highly Dense Nanonetworks as Free-Standing High-Capacity Anodes for Lithium-Ion Batteries

Transition metal oxides (TMOs) have been widely studied as potential next-generation anode materials, owing to their high theoretical gravimetric capacity. However, to date, these anodes syntheses are plagued with time-consuming preparation processes, two-dimensional electrode fabrication, binder requirements, and short operational cycling lives. Here, we present a scalable single-step reagentless process for the synthesis of highly dense Mn₃O₄-based nanonetwork anodes based on a simple thermal treatment transformation of low-grade steel substrates. The monolithic solid-state chemical self-transformation of the steel substrate results in a highly dense forest of Mn₃O₄ nanowires, which transforms the electrochemically inactive steel substrate into an electrochemically highly active anode. The proposed method, beyond greatly improving the current TMO performance, surpasses state-of-the-art commercial silicon anodes in terms of capacity and stability. The three-dimensional self-standing anode exhibits remarkably high capacities (>1500 mA h/g), a stable cycle life (>650 cycles), high Coulombic efficiencies (>99.5%), fast rate performance (>1.5 C), and high areal capacities (>2.5 mA h/cm²). This novel experimental paradigm acts as a milestone for next-generation anode materials in lithium-ion batteries, and pioneers a universal method to transform different kinds of widely available, low-cost, steel substrates into electrochemically active, free-standing anodes and allows for the massive reduction of anode production complexity and costs.

Frustration, strain and phase co-existence in the mixed valent hexagonal iridate Ba3NaIr2O9

Using detailed synchrotron diffraction, magnetization, thermodynamic and transport measurements, we investigate the relationship between the mixed valence of Ir, lattice strain and the resultant structural and magnetic ground states in the geometrically frustrated triple perovskite iridate Ba₃NaIr₂O₉. We observe a complex interplay between lattice strain and structural phase co-existence, which is typically not observed in this family of compounds. The low temperature magnetic ground state is characterized by the absence of long-range magnetic order, and points towards the condensation of a cluster glass state from an extended regime of short range magnetic correlations.

A Facile Strategy To Construct Au@VxO2x+1 Nanoflowers as a Multicolor Electrochromic Material for Adaptive Camouflage

Transition metal oxides (TMOs) are promising inorganic electrochromic materials (ECMs) that can be widely used in electronic displays and adaptive camouflage. However, there are still huge challenges for TMOs to simultaneously achieve multicolor transformation capability and good cycling stability. Herein, we assemble Au-modified (0.01 wt %) V_xO_2x+1 (x > 2) nanoflowers (Au@V_xO_2x+1 NFs) composed of two-dimensional porous nanosheets containing two valences states of vanadium (V⁴⁺ and V⁵⁺). The Au@V_xO_2x+1 NFs exhibits outstanding electrochromic performance with five reversible color transformations (orange, yellow, green, gray, and blue) at a voltage less than 1.5 V and excellent cycling stability (2000 cycles without significant decay). To the best of our knowledge, this is the first time that a single vanadium oxide ECM, rather than a device, realizes five color changes. This work provides a feasible way for the efficient preparation of multicolor electrochromic TMOs. The newly developed Au@V_xO_2x+1 NFs demonstrate the potential application in adaptive camouflage.

Concentration Gradient Induced Delithiation Failure of MoO3 for Li-Ion Batteries

Electric vehicle manufacturers worldwide are demanding superior lithium-ion batteries, with high energy and power densities, compared to gasoline engines. Although conversion-type metal oxides are promising candidates for high-capacity anodes, low initial Coulombic efficiency (ICE) and poor capacity retention have hindered research on their applications. In this study, the ICE of conversion-type MoO₃ is investigated, with a particular focus on the delithiation failure. A computational modeling predicts the concentration gradient of Li⁺ in MoO₃ particles. The highly delithiated outer region of the particle forms a layer with low electronic conductivity, which impedes further delithiation. A comparative study using various sizes of MoO₃ particles demonstrated that the electrode failure during delithiation is governed by the concentration gradient and the subsequent formation of a resistive shell. The proposed failure mechanism provides critical guidance for the development of conversion-type anode materials with improved electrochemical reversibility.

Polyaniline coating enables electronic structure engineering in Fe3O4to promote alkaline oxygen evolution reaction

The electronic structure of active sites is of importance for catalysts to achieve an optimized interaction with the intermediates. In this study, a unique organic-inorganic hybrid oxygen evolution reaction electrocatalyst composed of electrochemically inactive conducting polyaniline (PANI) and non-precious Fe-based oxide Fe₃O₄is presented. PANI molecules werein situloaded on Fe₃O₄nanoparticles through an efficient and simple process under mild conditions. The electronic structure of Fe₃O₄was modulated by creating a strong interaction with PANI molecules, leading to enhanced activity and stability of the catalyst to achieve 10 mA cm^-2geometrical current density at overpotential of 265 mV in 1 M aqueous KOH solution. This work demonstrates that a highly efficient electrocatalyst can be achieved by molecular modification and provides a novel strategy for the optimization of the inactive non-precious catalysts.

Utilization of Interfacial Charge Storage toward Ultra-high Capacity: Li2SO4 Sealed Micron Sized Iron Oxides as Anode for Lithium Batteries

The interfacial charge storage is derived from spin-polarized electrons stored on the surface of iron metal nanoparticles, and reasonable utilization can achieve a capacity far beyond the traditional conversion mechanism. Generally, iron oxide is easy to crack, pulverize, and fall off due to its poor conductivity and large volume change during cycling, and causes serious side reactions with the electrolyte. Herein, this pulverization phenomenon was intentionally utilized to in situ form nano-sized iron particles and create a large number of Fe/Li₂O interfaces. Specifically, a Li⁺ conductor like Li₂SO₄ was utilized to seal micron sized iron oxides and also work as an aggregation barrier. Thus, the in situ formed nanoparticles were separated from the electrolyte and could provide huge capacity through interfacial charge storage. Therefore, the specific capacity of this unique composite continues to rise upon activation cycling and finally reaches 1708 mA h g^-1, which is more than twice its theoretical capacity based on the conversion mechanism. The gradually increasing interfacial charge storage capacity was also directly confirmed by X-ray photoelectron spectroscopy tests. This novel strategy provides new opportunities for the design and commercialization of advanced energy storage systems.

Hierarchical coral-like MnCo2O4.5@Co-Ni LDH composites on Ni foam as promising electrodes for high-performance supercapacitor

Transition metal oxides are generally designed as hybrid nanostructures with high performance for supercapacitors by enjoying the advantages of various electroactive materials. In this paper, a convenient and efficient route had been proposed to prepare hierarchical coral-like MnCo₂O_4.5@Co-Ni LDH composites on Ni foam, in which MnCo₂O_4.5nanowires were enlaced with ultrathin Co-Ni layered double hydroxides nanosheets to achieve high capacity electrodes for supercapacitors. Due to the synergistic effect of shell Co-Ni LDH and core MnCo₂O_4.5, the outstanding electrochemical performance in three-electrode configuration was triggered (high area capacitance of 5.08 F cm^-2at 3 mA cm^-2and excellent rate capability of maintaining 61.69% at 20 mA cm^-2), which is superior to those of MnCo₂O_4.5, Co-Ni LDH and other metal oxides based composites reported. Meanwhile, the as-prepared hierarchical MnCo₂O_4.5@Co-Ni LDH electrode delivered improved electrical conductivity than that of pristine MnCo₂O_4.5. Furthermore, the as-constructed asymmetric supercapacitor using MnCo₂O_4.5@Co-Ni LDH as positive and activated carbon as negative electrode presented a rather high energy density of 220μWh cm^-2at 2400μW cm^-2and extraordinary cycling durability with the 100.0% capacitance retention over 8000 cycles at 20 mA cm^-2, demonstrating the best electrochemical performance compared to other asymmetric supercapacitors using metal oxides based composites as positive electrode material. It can be expected that the obtained MnCo₂O_4.5@Co-Ni LDH could be used as the high performance and cost-effective electrode in supercapacitors.

Disclosing the Origin of Transition Metal Oxides as Peroxidase (and Catalase) Mimetics

Since Fe3O4 was reported to mimic horseradish peroxidase (HRP) with comparable activity (2007), countless peroxidase nanozymes have been developed for a wide range of applications from biological detection assays to disease diagnosis and biomedicine development. However, researchers have recently argued that Fe3O4 has no peroxidase activity because surface Fe(III) cannot oxidize tetramethylbenzidine (TMB) in the absence of H2O2 (cf. HRP). This motivated us to investigate the origin of transition metal oxides as peroxidase mimetics. The redox between their surface Mn+ (oxidation) and H2O2 (reduction) was found to be the key step generating OH radicals, which oxidize not only TMB for color change but other H2O2 to produce HO2 radicals for Mn+ regeneration. This mechanism involving free OH and HO2 radicals is distinct from that of HRP with a radical retained on the Fe-porphyrin ring. Most importantly, it also explains the origin of their catalase-like activity (i.e., the decomposition of H2O2 into H2O and O2). Because the production of OH radicals is the rate-limiting step, the poor activity of Fe3O4 can be attributed to the slow redox of Fe(II) with H2O2, which is two orders of magnitude slower than the most active Cu(I) among common transition metal oxides. We further tested glutathione (GSH) detection on the basis of its peroxidase-like activity to highlight the importance of understanding the mechanism when selecting materials with high performance.

Neuromorphic learning with Mott insulator NiO

Habituation and sensitization (nonassociative learning) are among the most fundamental forms of learning and memory behavior present in organisms that enable adaptation and learning in dynamic environments. Emulating such features of intelligence found in nature in the solid state can serve as inspiration for algorithmic simulations in artificial neural networks and potential use in neuromorphic computing. Here, we demonstrate nonassociative learning with a prototypical Mott insulator, nickel oxide (NiO), under a variety of external stimuli at and above room temperature. Similar to biological species such as Aplysia, habituation and sensitization of NiO possess time-dependent plasticity relying on both strength and time interval between stimuli. A combination of experimental approaches and first-principles calculations reveals that such learning behavior of NiO results from dynamic modulation of its defect and electronic structure. An artificial neural network model inspired by such nonassociative learning is simulated to show advantages for an unsupervised clustering task in accuracy and reducing catastrophic interference, which could help mitigate the stability-plasticity dilemma. Mott insulators can therefore serve as building blocks to examine learning behavior noted in biology and inspire new learning algorithms for artificial intelligence.

Unraveling the Synergy of Chemical Hydroxylation and the Physical Heterointerface upon Improving the Hydrogen Evolution Kinetics

Efficient transition metal oxide electrocatalysts for the alkaline hydrogen evolution reaction (HER) have received intensive attention to energy conversion but are limited by their sluggish water dissociation and unfavorable hydrogen migration and coupling. Herein, systematic density functional theory (DFT) predicts that on representative NiO, the hydroxylation (OH^-) and heterointerface coupled with metallic Cu can respectively reduce the energy barrier of water dissociation and facilitate hydrogen spillover. Motivated by theoretical predictions, we subtly designed a delicate strategy to realize the electrochemical OH^- modification in KOH with moderate concentration (HO_M-NiO) and to channel rapid hydrogen spillover at the heterointerface of HO_M-NiO and Cu, ensuring an enhanced HER kinetic. This HO_M-NiO/Cu is systematically investigated by in situ XAS and electrochemical simulations, verifying its extraordinary merits for HER including the enhanced water dissociation, alleviated oxophilicity that is advantageous for consecutive adsorptions of water, and accelerated hydrogen spillover, thereby exhibiting superb HER activity with 33 and 310 mV overpotentials at the current densities of 10 and 1000 mA cm^-2 in 1.0 M KOH, outperforming the Pt/C. This study might provide a reasonable strategy for the functionalized design of superior electrocatalysts.

Oxidic 2D Materials

The possibility of producing stable thin films, only a few atomic layers thick, from a variety of materials beyond graphene has led to two-dimensional (2D) materials being studied intensively in recent years. By reducing the layer thickness and approaching the crystallographic monolayer limit, a variety of unexpected and technologically relevant property phenomena were observed, which also depend on the subsequent arrangement and possible combination of individual layers to form heterostructures. These properties can be specifically used for the development of multifunctional devices, meeting the requirements of the advancing miniaturization of modern manufacturing technologies and the associated need to stabilize physical states even below critical layer thicknesses of conventional materials in the fields of electronics, magnetism and energy conversion. Differences in the structure of potential two-dimensional materials result in decisive influences on possible growth methods and possibilities for subsequent transfer of the thin films. In this review, we focus on recent advances in the rapidly growing field of two-dimensional materials, highlighting those with oxidic crystal structure like perovskites, garnets and spinels. In addition to a selection of well-established growth techniques and approaches for thin film transfer, we evaluate in detail their application potential as free-standing monolayers, bilayers and multilayers in a wide range of advanced technological applications. Finally, we provide suggestions for future developments of this promising research field in consideration of current challenges regarding scalability and structural stability of ultra-thin films.

Electric Field Control of the Magnetic Weyl Fermion in an Epitaxial SrRuO3 (111) Thin Film

The magnetic Weyl fermion originates from the time reversal symmetry (TRS)-breaking in magnetic crystalline structures, where the topology and magnetism entangle with each other. Therefore, the magnetic Weyl fermion is expected to be effectively tuned by the magnetic field and electrical field, which holds promise for future topologically protected electronics. However, the electrical field control of the magnetic Weyl fermion has rarely been reported, which is prevented by the limited number of identified magnetic Weyl solids. Here, the electric field control of the magnetic Weyl fermion is demonstrated in an epitaxial SrRuO₃ (111) thin film. The magnetic Weyl fermion in the SrRuO₃ films is indicated by the chiral anomaly induced magnetotransport, and is verified by the observed Weyl nodes in the electronic structures characterized by the angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. Through the ionic-liquid gating experiment, the effective manipulation of the Weyl fermion by electric field is demonstrated, in terms of the sign-change of the ordinary Hall effect, the nonmonotonic tuning of the anomalous Hall effect, and the observation of the linear magnetoresistance under proper gating voltages. The work may stimulate the searching and tuning of Weyl fermions in other magnetic materials, which are promising in energy-efficient electronics.

Atomic Resolution Electron Microscopy: A Key Tool for Understanding the Activity of Nano-Oxides for Biomedical Applications

Transition metal oxides constitute one of the most fruitful sources of materials with continuously increasing potential applications prompted by the expectations derived from the reduction of the particle size. The recent advances in transmission electron microscopy, because of the development of lenses, have made it possible to reach atomic resolution, which can provide answers regarding the performance of the transition metal nano-oxides. This critical information is related not only to the ability to study their microstructural characteristics but also their local composition and the oxidation state of the transition metal. Exploring these features is a well-known task in nano-oxides for energy and electronic technologies, but they are not so commonly used for elucidating the activity of these oxides for biomedical applications. Nevertheless, the identification at the atomic level of a certain dopant or the unambiguous determination of the oxidation state of a transition metal in a nano-oxide can be important questions to be answered in a certain biomedical application. In this work, we provide several examples in transition metal nano-oxides to show how atomic-resolution electron microscopy can be a key tool for its understanding.

Construction of rGO-Encapsulated Co3 O4 -CoFe2 O4 Composites with a Double-Buffer Structure for High-Performance Lithium Storage

Transition metal oxides (TMOs) are promising anode materials for next-generation lithium-ion batteries (LIBs). Nevertheless, their poor electronic and ionic conductivity as well as huge volume change leads to low capacity release and rapid capacity decay. Herein, a reduced graphene oxide (rGO)-encapsulated TMOs strategy is developed to address the above problems. The Co₃ O₄ -CoFe₂ O₄ @rGO composites with rGO sheets-encapsulated Co₃ O₄ -CoFe₂ O₄ microcubes are successfully constructed through a simple metal-organic frameworks precursor route, in which Co[Fe(CN)₅ NO] microcubes are in situ coated by graphene oxide sheets, followed by a two-step calcination process. As anode material of LIBs, Co₃ O₄ -CoFe₂ O₄ @rGO exhibits remarkable reversible capacity (1393 mAh g^-1 at 0.2 A g^-1 after 300 cycles), outstanding long-term cycling stability (701 mAh g^-1 at 2.0 A g^-1 after 500 cycles), and excellent rate capability (420 mAh g^-1 at 4.0 A g^-1 ). The superior lithium storage performance can be attributed to the unique double-buffer structure, in which the outer flexible rGO shells can prevent the structure collapse of the electrode and improve its conductivity, while the hierarchical porous cores of Co₃ O₄ -CoFe₂ O₄ microcubes can buffer the volume expansion. This work provides a general and straightforward strategy for the construction of novel rGO-encapsulated bimetal oxides for energy storage and conversion application.

Evaluation of optical band gaps and dopant state energies in transition metal oxides using oxidation-state constrained density functional theory

We report the use of oxidation-state constrained density functional theory (OS-CDFT) to calculate the optical band gaps of transition metal oxides and dopant state energies of different doped anatase. OS-CDFT was used to control electron transfer from the valence band maximum of the transition metal system to the conduction band minimum or to the dopant state in order to calculate the band gap or the dopant state energies respectively. The calculation of the dopant state energies also allows identification of the transition responsible for the reduced band gap of the doped system in ambiguous cases. We applied this approach to the band gap calculation in TiO₂anatase and rutile, vanadium pentoxide (V₂O₅), chromium(III) oxide (Cr₂O₃), manganese(IV) oxide (MnO₂), ferric oxide (Fe₂O₃), ferrous oxide (FeO) and cobalt(II) oxide (CoO). The dopant state energies calculations were carried out in the V-, Cr-, Mn-, and Fe-doped anatase.

Single-Step Direct Laser Writing of Multimetal Oxygen Evolution Catalysts from Liquid Precursors

We investigate a laser direct-write method to synthesize and deposit metastable, mixed transition metal oxides and evaluate their performance as oxygen evolution reaction catalysts. This laser processing method enabled the rapid synthesis of diverse heterogeneous alloy and oxide catalysts directly from cost-effective solution precursors, including catalysts with a high density of nanocrystalline metal alloy inclusions within an amorphous oxide matrix. The nanoscale heterogeneous structures of the synthesized catalysts were consistent with reactive force-field Monte Carlo calculations. By evaluating the impact of varying transition metal oxide composition ratios, we created a stable Fe_0.63Co_0.19Ni_0.18O_x/C catalyst with a Tafel slope of 38.23 mV dec^-1 and overpotential of 247 mV, a performance similar to that of IrO₂. Synthesized Fe_0.63Co_0.19Ni_0.18O_x/C and Fe_0.14Co_0.46Ni_0.40O_x/C catalysts were experimentally compared in terms of catalytic performance and structural characteristics to determine that higher iron content and a less crystalline structure in the secondary matrix decrease the charge transfer resistance and thus is beneficial for electrocatalytic activity. This conclusion is supported by density-functional theory calculations showing distorted active sites in ternary metal catalysts are key for lowering overpotentials for the oxygen evolution reaction.

Transition Metal Oxide Electrode Materials for Supercapacitors: A Review of Recent Developments

In the past decades, the energy consumption of nonrenewable fossil fuels has been increasing, which severely threatens human life. Thus, it is very urgent to develop renewable and reliable energy storage devices with features of environmental harmlessness and low cost. High power density, excellent cycle stability, and a fast charge/discharge process make supercapacitors a promising energy device. However, the energy density of supercapacitors is still less than that of ordinary batteries. As is known to all, the electrochemical performance of supercapacitors is largely dependent on electrode materials. In this review, we firstly introduced six typical transition metal oxides (TMOs) for supercapacitor electrodes, including RuO₂, Co₃O₄, MnO₂, ZnO, XCo₂O₄ (X = Mn, Cu, Ni), and AMoO₄ (A = Co, Mn, Ni, Zn). Secondly, the problems of these TMOs in practical application are presented and the corresponding feasible solutions are clarified. Then, we summarize the latest developments of the six TMOs for supercapacitor electrodes. Finally, we discuss the developing trend of supercapacitors and give some recommendations for the future of supercapacitors.

Facilitating the Deprotonation of OH to O through Fe4+ -Induced States in Perovskite LaNiO3 Enables a Fast Oxygen Evolution Reaction

Aliovalent doping is widely adopted to tune the electronic structure of transition-metal oxides for design of low-cost, active electrocatalysts. Here, using single-crystalline thin films as model electrocatalysts, the structure-activity relationship of Fe states doping in perovskite LaNiO₃ for oxygen evolution reaction (OER) is studied. Fe⁴⁺ state is found to be crucial for enhancing the OER activity of LaNiO₃ , dramatically increasing the activity by six times, while Fe³⁺ has negligible effect. Spectroscopic studies and DFT calculations indicate Fe⁴⁺ states enhance the degree of Ni/Fe 3d and O 2p hybridization, and meanwhile produce down-shift of the unoccupied density of states towards lower energies. Such electronic features reduce the energy barrier for interfacial electron transfer for water oxidization by 0.2 eV. Further theoretical calculations and H/D isotope experiments reveal the electronic states associated with Fe⁴⁺ -O^2- -Ni³⁺ configuration accelerate the deprotonation of *OH to *O (rate-determining step), and thus facilitate fast OER kinetics.

Nickel-Rich Layered Cathode Materials for Lithium-Ion Batteries

Nickel-rich layered transition metal oxides are considered as promising cathode candidates to construct next-generation lithium-ion batteries to satisfy the demands of electrical vehicles, because of the high energy density, low cost, and environment friendliness. However, some problems related to rate capability, structure stability, and safety still hamper their commercial application. In this Review, beginning with the relationships between the physicochemical properties and electrochemical performance, the underlying mechanisms of the capacity/voltage fade and the unstable structure of Ni-rich cathodes are deeply analyzed. Furthermore, the recent research progress of Ni-rich oxide cathode materials through element doping, surface modification, and structure tuning are summarized. Finally, this review concludes by discussing new insights to expand the field of Ni-rich oxides and promote practical applications.

Regulating Crystal Structure and Atomic Arrangement in Single-Component Metal Oxides through Electrochemical Conversion for Efficient Overall Water Splitting

Single-component transition-metal oxide (TMO: FeO_x, NiO_x, or CoO_x) nanosheets grown on nickel foam (NF) were electrochemically optimized with Li ion (Na ion)-induced conversion reaction for bifunctional electrocatalysis. The optimum FeO_x/NF-Li electrocatalyst exhibits low overpotentials of 239 mV for hydrogen evolution reaction and 276 mV for oxygen evolution reaction at a current density of 100 mA cm^-2. A two-electrode water splitting cell using FeO_x/NF-Li as both anode and cathode requires only 1.60 V to achieve a current density of 10 mA cm^-2. The impressive water splitting performance of the FeO_x/NF-Li electrode is revealed to be attributed to Li-induced electrochemical conversion, which alters the crystal structure, creating more active sites for electrocatalytic reactions, as well as introduces O vacancies increasing the electron density and the intrinsic conductivity. More importantly, the atomic arrangement is regulated from tetrahedral Fe(Td) to octahedral Fe(Oh) coordination, which acts as catalytically active sites with reduced Gibbs free energy for the rate-determining steps. This electrochemical conversion reaction can be extended to other TMOs (i.e., NiO_x/NF and CoO_x/NF) for promoted electrocatalytic water splitting performances. This study provides an in-depth understanding on the nature of atomic and electronic structure evolution to promote the electrocatalytic activity.

Direct Bandgap-like Strong Photoluminescence from Twisted Multilayer MoS2 Grown on SrTiO3

While direct bandgap monolayer 2D transition metal dichalcogenides (TMDs) have emerged as an important optoelectronic material due to strong light-matter interactions, their multilayer counterparts exhibit an indirect bandgap resulting in poor photon emission quantum yield. We report strong direct bandgap-like photoluminescence at ∼1.9 eV from multilayer MoS₂ grown on SrTiO₃, whose intensity is significantly higher than that observed in multilayer MoS₂/SiO₂. Using high-resolution electron microscopy we observe interlayer twist and >8% increase in the van der Waals gap, which leads to weaker interlayer coupling. This affects the evolution of the band structure in multilayer MoS₂ as probed by transient absorption spectroscopy, causing higher photo carrier recombination at the direct gap. Our results provide a platform that could enable multilayer TMDs for robust optical device applications.

Regulating the Coordination of Co sites in Co3 O4 /MnO2 Compounding for Facilitated Oxygen Reduction Reaction

Binary transition metal oxides as a promising oxygen reduction reaction (ORR) catalyst have received significant attention. However, their exact reaction mechanisms are often too complex to be discussed. Herein, novel Co-Mn composites with a well-defined nanostructure were developed for understanding the role of each component. The growth pattern of cobalt oxide and the effects of the coordination environment of Co sites during growth on the overall activity were investigated. Based on experimental and density functional theory studies, it was found that the decaying coordination number directly affected the expression of crystal planes of cobalt oxide, which further had a great influence upon limiting current density of Co-Mn catalysts. The cuboid-Co/Mn catalyst exhibited outstanding limiting current density and showed good stability, related to more highly active (110) planes exposed in Co₃ O₄ . These provided many references for the preparation of related nonprecious catalysts in various domains.

Facile One-Step Hydrothermal Synthesis of the rGO@Ni3V2O8 Interconnected Hollow Microspheres Composite for Lithium-Ion Batteries

Low-cost, vanadium-based mixed metal oxides mostly have a layered crystal structure with excellent kinetics for lithium-ion batteries, providing high energy density. The existence of multiple oxidation states and the coordination chemistry of vanadium require cost-effective, robust techniques to synthesize the scaling up of their morphology and surface properties. Hydrothermal synthesis is one of the most suitable techniques to achieve pure phase and multiple morphologies under various conditions of temperature and pressure. We attained a simple one-step hydrothermal approach to synthesize the reduced graphene oxide coated Nickel Vanadate (rGO@Ni₃V₂O₈) composite with interconnected hollow microspheres. The self-assembly route produced microspheres, which were interconnected under hydrothermal treatment. Cyclic performance determined the initial discharge/charge capacities of 1209.76/839.85 mAh g⁻¹ at the current density of 200 mA g⁻¹ with a columbic efficiency of 69.42%, which improved to 99.64% after 100 cycles. High electrochemical performance was observed due to high surface area, the porous nature of the interconnected hollow microspheres, and rGO induction. These properties increased the contact area between electrode and electrolyte, the active surface of the electrodes, and enhanced electrolyte penetration, which improved Li-ion diffusivity and electronic conductivity.