Mechanochemical-Assisted Synthesis of High-Entropy Metal Nitride via a Soft Urea Strategy

An Unexpected Decrease in Vibrational Entropy of Multicomponent Rutile Oxides

High-entropy oxides (HEOs), featuring infinite chemical composition and exceptional physicochemical properties, are attracting much attention. The configurational entropy caused by a component disorder of HEOs is popularly believed to be the main driving force for thermal stability, while the role of vibrational entropy in the thermodynamic landscape has been neglected. In this study, we systematically investigated the vibrational entropy of multicomponent rutile oxides (including Fe0.5Ta0.5O2, Fe0.333Ti0.333Ta0.333O2, Fe0.25Ti0.25Ta0.25Sn0.25O2, and Fe0.21Ti0.21Ta0.21Sn0.21Ge0.16O2) by precise heat capacity measurements. It is found that vibrational entropy gradually decreases with increasing component disorder, beyond what one could expect from an equilibrium thermodynamics perspective. Moreover, all multicomponent rutile oxides exhibit a positive excess vibrational entropy at 298.15 K. Upon examinations of configuration disorder, size mismatch, phase transition, and polyhedral distortions, we demonstrate that the excess vibrational entropy plays a pivotal role in lowering the crystallization temperature of multicomponent rutile oxides. These findings represent the first experimental confirmation of the role of lattice vibrations in the thermodynamic landscape of rutile HEOs. In particular, vibrational entropy could serve as a novel descriptor to guide the predictive design of multicomponent oxide materials.

Recent advances and understanding of high-entropy materials for lithium-ion batteries

Lithium-ion batteries (LIBs) has extensively utilized in electric vehicles and portable electronics due to their high energy density and prolonged lifespan. However, the current commercial LIBs are plagued by relatively low energy density. High-entropy materials with multiple components have emerged as an efficient strategic approach for developing novel materials that effectively improve the overall performance of LIBs. This article provides a comprehensive review the recent advancements in rational design of innovative high-entropy materials for LIBs, as well as the exceptional lithium ion storage mechanism for high-entropy electrodes and considerable ionic conductivity for high-entropy electrolytes. This review also analyses the prominent effects of individual components on the high-entropy materials' exceptional capacity, considerable structural stability, rapid lithium ion diffusion, and excellent ionic conductivity. Furthermore, this review presents the synthesis methods and their influence on the morphology and properties of high-entropy materials. Ultimately, the remaining challenges and future research directions are outlined, aimed at developing more effective high-entropy materials and improving the overall electrochemical performance of LIBs.

High-entropy-based nano-materials for sustainable environmental applications

High entropy materials (HEMs), epitomized by high entropy alloys (HEAs), have sparked immense interest for a range of clean energy and environmental applications due to their remarkable structural versatility and adjustable characteristics. In the face of environmental challenges, HEMs have emerged as valuable tools for addressing issues ranging from wastewater remediation to energy conversion and storage. This review provides a comprehensive exploration of HEMs, spotlighting their catalytic capabilities in diverse redox reactions, such as carbon dioxide reduction to value-added products, degradation of organic pollutants, oxygen reduction, hydrogen evolution, and ammonia decomposition using electrocatalytic and photocatalytic pathways. Additionally, the review highlights HEMs as novel electrode nanomaterials, with the potential to enhance the performance of batteries and supercapacitors. Their unique features, including high capacitance, electrical conductivity, and thermal stability, make them valuable components for meeting crucial energy demands. Furthermore, the review examines challenges and opportunities in advancing HEMs, emphasizing the importance of understanding the underlying mechanisms governing their catalytic and electrochemical behaviors. Essential considerations for optimizing the HEM performance in catalysis and energy storage are outlined to guide future research. Moreover, to provide a comprehensive understanding of the current research landscape, a meticulous bibliometric analysis is presented, offering insights into the trends, focal points, and emerging directions within the realm of HEMs, particularly in addressing environmental concerns.

High-Throughput Designed and Laser-Etched NiFeCrVTi High-Entropy Alloys with High Catalytic Activities and Corrosion Resistance for Hydrogen Evolution in Seawater

Electrocatalytic hydrogen evolution from seawater through wind or solar energy is a cost-effective way to produce green hydrogen fuel. However, the lack of highly active and anti-corrosive electrocatalysts in seawater severely hinders the industrial application. Herein, a novel Ni1.1FeCr0.4V0.3Ti0.3 high-entropy alloy (HEA) is designed through high throughput computing and prepared via powder metallurgy with the surface treated by laser etching under different laser power. The laser-etched NiFeCrVTi high-entropy alloys exhibit a unique periodically ordered structure with multiple active centers and high porosity. The Ni-HEA-30 displays remarkable hydrogen evolution reaction (HER) performance with an overpotential of 55.9 mV and a Tafel slope of 47.3 mV dec-1 in seawater. Density functional theory (DFT) calculations are applied to identify the real active sites for HER on the HEA surface as the key factor for both proton and intermediate transformation, which also reveals that the Cr atom promotes the adsorption energy of water molecules, and the modulation of the electronic structure plays a crucial role in optimizing the hydrogen binding capabilities of the Ni atoms within the alloy. Additionally, the electrocatalyst displays high corrosion resistance in seawater, contributing to the good durability for hydrogen production. This work uncovers a new paradigm to develop novel electrocatalysts with superior reaction activity in seawater.

Distinguishing Elements at the Sub-Nanometer Scale on the Surface of a High Entropy Alloy

Materials in crystalline form possess translational symmetry (TS) when the unit cell is repeated in real space with long- and short-range orders. The periodic potential in the crystal regulates the electron wave function and results in unique band structures, which further define the physical properties of the materials. Amorphous materials lack TS due to the randomization of distances and arrangements between atoms, causing the electron wave function to lack a well-defined momentum. High entropy materials provide another way to break the TS by randomizing the potential strength at periodic atomic sites. The local elemental distribution has a great impact on physical properties in high entropy materials. It is critical to distinguish elements at the sub-nanometer scale to uncover the correlations between the elemental distribution and the material properties. Here, the use of synchrotron X-ray scanning tunneling microscopy (SX-STM) with sub-nm scale resolution in identifying elements on a high entropy alloy (HEA) surface is demonstrated. By examining the elementally sensitive X-ray absorption spectra with an STM tip to enhance the spatial resolution, the elemental distribution on an HEA's surface at a sub-nm scale is extracted. These results open a pathway towards quantitatively understanding high entropy materials and their material properties.

High-Pressure-Field Induced Synthesis of Ultrafine-Sized High-Entropy Compounds with Excellent Sodium-Ion Storage

Emerging high entropy compounds (HECs) have attracted huge attention in electrochemical energy-related applications. The features of ultrafine size and carbon incorporation show great potential to boost the ion-storage kinetics of HECs. However, they are rarely reported because high-temperature calcination tends to result in larger crystallites, phase separation, and carbon reduction. Herein, using the NaCl self-assembly template method, by introducing a high-pressure field in the calcination process, the atom diffusion and phase separation are inhibited for the general formation of HECs, and the HEC aggregation is inhibited for obtaining ultrafine size. The general preparation of ultrafine-sized (<10 nm) HECs (nitrides, oxides, sulfides, and phosphates) anchored on porous carbon composites is realized. They are demonstrated by combining advanced characterization technologies with theoretical computations. Ultrafine-sized high entropy sulfides-MnFeCoCuSnMo/porous carbon (HES-MnFeCoCuSnMo/PC) as representative anodes exhibit excellent sodium-ion storage kinetics and capacities (a high rating capacity of 278 mAh g-1 at 10 A g-1 for full cell and a high cycling capacity of 281 mAh g-1 at 20 A g-1 after 6000 cycles for half cell) due to the combining advantages of high entropy effect, ultrafine size, and PC incorporation. Our work provides a new opportunity for designing and fabricating ultrafine-sized HECs.

High-Entropy Oxides for Rechargeable Batteries

High-entropy oxides (HEOs) have garnered significant attention within the realm of rechargeable batteries owing to their distinctive advantages, which encompass diverse structural attributes, customizable compositions, entropy-driven stabilization effects, and remarkable superionic conductivity. Despite the brilliance of HEOs in energy conversion and storage applications, there is still lacking a comprehensive review for both entry-level and experienced researchers, which succinctly encapsulates the present status and the challenges inherent to HEOs, spanning structural features, intrinsic properties, prevalent synthetic methodologies, and diversified applications in rechargeable batteries. Within this review, the endeavor is to distill the structural characteristics, ionic conductivity, and entropy stabilization effects, explore the practical applications of HEOs in the realm of rechargeable batteries (lithium-ion, sodium-ion, and lithium-sulfur batteries), including anode and cathode materials, electrolytes, and electrocatalysts. The review seeks to furnish an overview of the evolving landscape of HEOs-based cell component materials, shedding light on the progress made and the hurdles encountered, as well as serving as the guidance for HEOs compositions design and optimization strategy to enhance the reversible structural stability, electrical properties, and electrochemical performance of rechargeable batteries in the realm of energy storage and conversion.

Synthesis of high-entropy germanides and investigation of their formation process

High-entropy alloys and compounds have emerged as an attractive research area in part because of their distinctive solid-solution structure and multi-element compositions that provide near-limitless tailorability. A diverse array of reports describing high-entropy compounds, including carbides, nitrides, sulfides, oxides, fluorides, silicides, and borides, has resulted. Strikingly, exploration of high-entropy germanides (HEGs) has remained relatively limited. In this study, we present a detailed investigation into the synthesis of HEGs, specifically AuAgCuPdPtGe and FeCoNiCrVGe, via a rapid thermal annealing. The structural, compositional, and morphological characteristics of the synthesized HEGs were assessed using laboratory X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Complementing these post-synthesis analyses, we interrogated the formation and growth mechanisms using in situ heating XRD and TEM and determined that HEG formation involved initial decomposition of germanane (GeNSs) during the annealing, followed by gradual grain growth via atom diffusion at temperatures below 600 °C, and finally a rapid grain growth process at elevated temperatures.

High-Entropy Photothermal Materials

High-entropy (HE) materials, celebrated for their extraordinary chemical and physical properties, have garnered increasing attention for their broad applications across diverse disciplines. The expansive compositional range of these materials allows for nuanced tuning of their properties and innovative structural designs. Recent advances have been centered on their versatile photothermal conversion capabilities, effective across the full solar spectrum (300-2500 nm). The HE effect, coupled with hysteresis diffusion, imparts these materials with desirable thermal and chemical stability. These attributes position HE materials as a revolutionary alternative to traditional photothermal materials, signifying a transformative shift in photothermal technology. This review delivers a comprehensive summary of the current state of knowledge regarding HE photothermal materials, emphasizing the intricate relationship between their compositions, structures, light-absorbing mechanisms, and optical properties. Furthermore, the review outlines the notable advances in HE photothermal materials, emphasizing their contributions to areas, such as solar water evaporation, personal thermal management, solar thermoelectric generation, catalysis, and biomedical applications. The review culminates in presenting a roadmap that outlines prospective directions for future research in this burgeoning field, and also outlines fruitful ways to develop advanced HE photothermal materials and to expand their promising applications.

First-principles study of thermodynamic stability and mechanical properties of fifteen high-entropy quaternary metal disilicides

By combining first-principles density-functional calculations and thermodynamics, we investigated the thermodynamic stability and mechanical properties of 15 quaternary high-entropy metal disilicides composed of silicon and four of the six refractory transition metals Ti, Zr, Hf, V, Nb, and Ta. We constructed a three-dimensional diagram specified by two thermodynamic parameters (the mixing enthalpy and the ratio of the entropy term in the Gibbs free energy to enthalpy) and a structural parameter (the lattice size difference). The obtained diagram allows us to predict that, except for TiZrHfVSi8, the formation of all other fourteen single-phase metal disilicides is thermodynamically favorable. Our calculations show that, for the formation of each of the 14 metal disilicides, the driving force suppresses the resistance at temperatures well below the melting point, suggesting that it is feasible to synthesize these high-entropy materials. One of these (TiHfNbTaSi8) has already been experimentally realized. Furthermore, the values of the mechanical parameters and melting points of the predicted fourteen quaternary high-entropy metal disilicides are all greater than the corresponding average values of the four single-metal disilicides.

Charge Storage Mechanism in Electrospun Spinel-Structured High-Entropy (Mn0.2 Fe0.2 Co0.2 Ni0.2 Zn0.2 )3 O4 Oxide Nanofibers as Anode Material for Li-Ion Batteries

High-entropy oxides (HEOs) have emerged as promising anode materials for next-generation lithium-ion batteries (LIBs). Among them, spinel HEOs with vacant lattice sites allowing for lithium insertion and diffusion seem particularly attractive. In this work, electrospun oxygen-deficient (Mn,Fe,Co,Ni,Zn) HEO nanofibers are produced under environmentally friendly calcination conditions and evaluated as anode active material in LIBs. A thorough investigation of the material properties and Li+ storage mechanism is carried out by several analytical techniques, including ex situ synchrotron X-ray absorption spectroscopy. The lithiation process is elucidated in terms of lithium insertion, cation migration, and metal-forming conversion reaction. The process is not fully reversible and the reduction of cations to the metallic form is not complete. In particular, iron, cobalt, and nickel, initially present mainly as Fe3+ , Co3+ /Co2+ , and Ni2+ , undergo reduction to Fe0 , Co0 , and Ni0 to different extent (Fe < Co < Ni). Manganese undergoes partial reduction to Mn3+ /Mn2+ and, upon re-oxidation, does not revert to the pristine oxidation state (+4). Zn2+ cations do not electrochemically participate in the conversion reaction, but migrating from tetrahedral to octahedral positions, they facilitate Li-ion transport within lattice channels opened by their migration. Partially reversible crystal phase transitions are observed.

Defect Engineering of a High-Entropy Metallic Glass Surface for High-Performance Overall Water Splitting at Ampere-Level Current Densities

Platinum-based electrocatalysts possess high water electrolysis activity and are essential components for hydrogen evolution reaction (HER). A major challenge, however, is how to break the cost-efficiency trade-off. Here, a novel defect engineering strategy is presented to construct a nanoporous (FeCoNiB0.75 )97 Pt3 (atomic %) high-entropy metallic glass (HEMG) with a nanocrystalline surface structure that contains large amounts of lattice distortion and stacking faults to achieve excellent electrocatalytic performance using only 3 at% of Pt. The defect-rich HEMG achieves ultralow overpotentials at ampere-level current density of 1000 mA cm-2 for HER (104 mV) and oxygen evolution reaction (301 mV) under alkaline conditions, while retains a long-term durability exceeding 200 h at 100 mA cm-2 . Moreover, it only requires 81 and 122 mV to drive the current densities of 1000 and 100 mA cm-2 for HER under acidic and neutral conditions, respectively. Modelling results reveal that lattice distortion and stacking fault defects help to optimize atomic configuration and modulate electronic interaction, while the surface nanoporous architecture provides abundant active sites, thus synergistically contributing to the reduced energy barrier for water electrolysis. This defect engineering approach combined with a HEMG design strategy is expected to be widely applicable for development of high-performance alloy catalysts.

Understanding the Role of Entropy in High Entropy Oxides

The field of high entropy oxides (HEOs) flips traditional materials science paradigms on their head by seeking to understand what properties arise in the presence of profound configurational disorder. This disorder, which originates from multiple elements sharing a single lattice site, can take on a kaleidoscopic character due to the vast numbers of possible elemental combinations. High configurational disorder appears to imbue some HEOs with functional properties that far surpass their nondisordered analogs. While experimental discoveries abound, efforts to characterize the true magnitude of the configurational entropy and understand its role in stabilizing new phases and generating superior functional properties have lagged behind. Understanding the role of configurational disorder in existing HEOs is the crucial link to unlocking the rational design of new HEOs with targeted properties. In this Perspective, we attempt to establish a framework for articulating and beginning to address these questions in pursuit of a deeper understanding of the true role of entropy in HEOs.

Phase formation capability and compositional design of β-phase multiple rare-earth principal component disilicates

A key strategy to design environmental barrier coatings focuses on doping multiple rare-earth principal components into β-type rare-earth disilicates (RE₂Si₂O₇) to achieve versatile property optimization. However, controlling the phase formation capability of (nRE_xi)₂Si₂O₇ remains a crucial challenge, due to the complex polymorphic phase competitions and evolutions led by different RE³⁺ combination. Herein, by fabricating twenty-one model (RE^I_0.25RE^II_0.25RE^III_0.25RE^IV_0.25)₂Si₂O₇ compounds, we find that their formation capability can be evaluated by the ability to accommodate configurational randomness of multiple RE³⁺ cations in β-type lattice while preventing the β-to-γ polymorphic transformation. The phase formation and stabilization are controlled by the average RE³⁺ radius and the deviations of different RE³⁺ combinations. Subsequently, based on high-throughput density-functional-theory calculations, we propose that the configurational entropy of mixing is a reliable descriptor to predict the phase formation of β-type (nRE_xi)₂Si₂O₇. The results may accelerate the design of (nRE_xi)₂Si₂O₇ materials with tailored compositions and controlled polymorphic phases.

Defect modeling and control in structurally and compositionally complex materials

Conventional computational approaches for modeling defects face difficulties when applied to complex materials, mainly due to the vast configurational space of defects. In this Perspective, we discuss the challenges in calculating defect properties in complex materials, review recent advances in computational techniques and showcase new mechanistic insights developed from these methods. We further discuss the remaining challenges in improving the accuracy and efficiency of defect modeling in complex materials, and provide an outlook on potential research directions.

Rational Design and General Synthesis of High-Entropy Metallic Ammonium Phosphate Superstructures Assembled by Nanosheets

Three-dimensional (3D) superstructure nanomaterials with special morphologies and novel properties have attracted considerable attention in the fields of optics, catalysis, and energy storage. The introduction of high entropy into ammonium phosphate (NPO·nH₂O) has not yet attracted much attention in the field of energy storage materials. Herein, we systematically synthesize a series of 3D superstructures of NPOs·nH₂O ranging from unitary, binary, ternary, and quaternary to high-entropy by a simple chemical precipitation method. These materials have similar morphology, crystallinity, and synthesis routes, which eliminates the performance difference caused by the interference of physical properties. Subsequently, cobalt-nickel ammonium phosphate (Co_xNi_y-NPO·nH₂O) powders with different cobalt-nickel molar ratios were synthesized to predict the promoting effect of mixed transition metals in supercapacitors. It is found that the Co_xNi_y-NPO·nH₂O 3D superstructures with a Co/Ni ratio of 1:1 show the best electrochemical performance for energy storage. The aqueous device shows a high energy density of 36.18 W h kg^-1 at a power density of 0.71 kW kg^-1, and when the power density is 0.65 kW kg^-1, the energy density of the solid-state device is 13.83 W h kg^-1. The work displays a facile method for the fabrication of 3D superstructures assembled by 2D nanosheets that can be applied in energy storage.

Composition-Tuned (MoWV)Se2 Ternary Alloy Nanosheets as Excellent Hydrogen Evolution Reaction Electrocatalysts

Ternary alloying of transition metal dichalcogenides (TMDs) has the potential for altering the electronic structure of materials to suit electrochemical applications. Herein, we synthesized (MoWV)Se₂ nanosheets at various compositions via a colloidal reaction. The mole fraction of V atoms (x_V) was successfully increased up to 0.8, producing a metallic phase that is highly durable against hydration. Furthermore, we synthesized (MoW)Se₂ nanosheets over the entire composition range. The atomic mixing of the ternary alloys is more random than that of the constitutional binary alloys, as supported by first-principles calculations. Compared to binary alloying, ternary alloying more effectively enhanced the electrocatalytic activity for acidic hydrogen evolution reaction (HER). The HER performance increased upon increasing x_V to 0.44, and thereafter, it declined at higher x_V primarily owing to surface oxidation. The analysis of Gibbs free energy for H adsorption revealed that ternary alloying strongly activates the basal plane for the HER. VSe₂ contains numerous sites favorable for H adsorption, facilitating the composition-dependent HER. These results provide a pioneering strategy for designing multicomponent TMD catalysts that maximize the advantages of each component.

Physicochemical Properties of High-Entropy Oxides

The development of industry has triggered an increasingly severe demand for new functional materials. In recent years, researches on high-entropy oxides (HEOs) are more comprehensive and in-depth, and their fascinating properties are gradually known to the public. The unique elemental synergistic effect and lattice distortion endow the high-entropy family with various untapped potential, and wide application fields and outstanding performance of HEOs make them candidates for future materials. In this review, the concept, structure, and synthesis of HEOs are firstly highlighted. Secondly, a variety of excellent properties and applications in the fields of mechanics, electrics, thermotics, optics and magnetics are summarized. This work provides a comprehensive overview about HEOs, facilitating the development of modern functionalities of the high-entropy family.

Structural Framework-Guided Universal Design of High-Entropy Compounds for Efficient Energy Catalysis

High-entropy compounds with extraordinary properties due to the synergistic effect of multiple components have exhibited great potential and attracted extensive attention in various fields, including physics, mechanical property analysis, and energy storage. Achieving universal stability and synthesis of high-entropy compounds with a wide range of components and structures continues to be difficult due to the high complexity of multicomponent mixing. Here, we propose a design strategy with high generality for realizing the stability and synthesis of high-entropy compounds that one metal site like the framework in the compound structures with bimetallic sites stabilizes another site to accommodate different elements. Several typical metal compounds with bimetallic sites, including perovskite hydroxides, layered double hydroxide, spinel sulfide, perovskite fluoride, and spinel oxides, have been synthesized into high-entropy compounds. High-entropy perovskite hydroxides (HEPHs) as representative compounds have been synthesized with a highly wide range of components even a septenary component and exhibit great oxygen evolution activity. Our work provides a design platform to develop more high-entropy compound systems with promising development potential for electrocatalysts.

Structure and magnetism of electrospun porous high-entropy (Cr1/5Mn1/5Fe1/5Co1/5Ni1/5)3O4, (Cr1/5Mn1/5Fe1/5Co1/5Zn1/5)3O4 and (Cr1/5Mn1/5Fe1/5Ni1/5Zn1/5)3O4 spinel oxide nanofibers

High-entropy oxide nanofibers, based on equimolar (Cr,Mn,Fe,Co,Ni), (Cr,Mn,Fe,Co,Zn) and (Cr,Mn,Fe,Ni,Zn) combinations, were prepared by electrospinning followed by calcination. The obtained hollow nanofibers exhibited a porous structure consisting of interconnected nearly strain-free (Cr_1/5Mn_1/5Fe_1/5Co_1/5Ni_1/5)₃O₄, (Cr_1/5Mn_1/5Fe_1/5Co_1/5Zn_1/5)₃O₄ and (Cr_1/5Mn_1/5Fe_1/5Ni_1/5Zn_1/5)₃O₄ single crystals with a pure Fd3̄m spinel structure. Oxidation state of the cations at the nanofiber surface was assessed by X-ray photoelectron spectroscopy and cation distributions were proposed satisfying electroneutrality and optimizing octahedral stabilization. The magnetic data are consistent with a distribution of cations that satisfies the energetic preferences for octahedral vs. tetrahedral sites and is random only within the octahedral and tetrahedral sublattices. The nanofibers are ferrimagnets with relatively low critical temperature more similar to cubic chromites and manganites than to ferrites. Replacing the magnetic cations Co or Ni with non-magnetic Zn lowers the critical temperature from 374 K (Cr,Mn,Fe,Co,Ni) to 233 and 105 K for (Cr,Mn,Fe,Ni,Zn) and (Cr,Mn,Fe,Co,Zn), respectively. The latter nanofibers additionally have a low temperature transition to a reentrant spin-glass-like state.

Visualizing the Formation of High-Entropy Fluorite Oxides from an Amorphous Precursor at Atomic Resolution

High-entropy oxides (HEOs) have a large tuning space in composition and crystal structures, offering the possibility for improved material properties in applications including catalysis, energy storage, and thermal barrier coatings. Understanding the nucleation and growth mechanisms of HEOs at the atomic scale is critical to the design of their structure and functions but remains challenging. Herein, we visualize the entire formation process of a high-entropy fluorite oxide from a polymeric precursor using atomic resolution in situ gas-phase scanning transmission electron microscopy. The results show a four-stage formation mechanism, including nucleation during the oxidation of a polymeric precursor below 400 °C, diffusive grain growth below 900 °C, liquid-phase-assisted compositional homogenization under a "state of supercooling" at 900 °C, and entropy-driven recrystallization and stabilization at higher temperatures. The atomistic insights are critical for the rational synthesis of HEOs with controlled grain sizes and morphologies and thus the related properties.

Discovery of High-Entropy Oxide Electrocatalysts: From Thin-Film Material Libraries to Particles

Discovery of new high-entropy electrocatalysts requires testing of hundreds to thousands of possible compositions, which can be addressed most efficiently by high-throughput experimentation on thin-film material libraries. Since the conditions for high-throughput measurements ("screening") differ from more standardized methods, it is frequently a concern whether the findings from screening can be transferred to the commonly used particulate catalysts. We demonstrate the successful transfer of results from thin-film material libraries to particles of Cantor alloy oxide (Co-Cr-Fe-Mn-Ni)₃O₄. The chemical compositions of the libraries, all single-phase spinels, cover a wide compositional range of (Cr_8.1-28.0Mn_11.6-28.4Fe_10.6-39.0Co_11.4-36.7Ni_13.5-31.4)_37.7±0.6O_62.3±0.6, with composition-dependent lattice constant values ranging from 0.826 to 0.851 nm. Electrochemical screening of the libraries for the oxygen evolution reaction (OER) identifies (Cr_24.6±1.4Mn_15.7±2.0Fe_16.9±1.8Co_26.1±1.9Ni_16.6±1.7)_37.8±0.8O_62.2±1.2 as the most active composition, exhibiting an overpotential of 0.36 V at a current density of 1 mA cm^-2. This "hit" in the library was subsequently synthesized in the form of particles with the same composition and crystal structure using an aerosol-based synthesis strategy. The similar OER activity of the most active thin-film composition and the derived catalyst particles validates the proposed approach of accelerated discovery of novel catalysts by screening of thin-film libraries.

MoSe2 -VSe2 -NbSe2 Ternary Alloy Nanosheets to Boost Electrocatalytic Hydrogen Evolution Reaction

Alloying of transition metal dichalcogenides (TMDs) is a pioneering method for engineering electronic structures with expanded applications. In this study, MoSe₂ -VSe₂ -NbSe₂ ternary alloy nanosheets are synthesized via a colloidal reaction. The composition is successfully tuned over a wide range to adjust the 2H-1T phase transition. The alloy nanosheets consist of miscible atomic structures at all compositions, which is distinct from immiscible binary alloys. Compared to each binary alloy, the ternary alloys display higher electrocatalytic activity toward the hydrogen evolution reaction (HER) in an acidic electrolyte. The HER performance exhibits a volcano-type composition dependence, which is correlated with the experimental d-band center (ε_d ). Spin-polarized density functional theory (DFT) calculations consistently predict the homogenous atomic distributions. The Gibbs free energy of H adsorption (ΔG_H* ) and the activation barrier (E_a ) support that miscible ternary alloying greatly enhances the HER performance.

Sputter-Deposited High Entropy Alloy Thin Film Electrocatalyst for Enhanced Oxygen Evolution Reaction Performance

Thin film catalysts, giving a different morphology, provide a significant advantage over catalyst particles for the gas evolution reaction. Taking the advantages of sputter deposition, a high entropy alloy (HEA) thin film electrocatalyst is hereby reported for the oxygen evolution reaction (OER). The catalyst characteristics are investigated not only in its as-deposited state, but also during and after the OER. For comparison, unary, binary, ternary, and quaternary thin film catalysts are prepared and characterized. The surface electronic structure modification due to the addition of a metal is studied experimentally and theoretically using density functional theory calculation. It is demonstrated that sputtered FeNiMoCrAl HEA thin film exhibits OER performance superior to all the reported HEA catalysts with robust electrocatalytic activity having a low overpotential of 220 mV at 10 mA cm^-2 , and excellent electrochemical stability at different constant current densities of 10 and 100 mA cm^-2 for 50 h. Furthermore, the microstructure transformation is investigated during the OER, which is important for the understanding of the OER mechanism provided by HEA electrocatalyst. Such a finding will contribute to future catalyst design.

Rapid liquid phase-assisted ultrahigh-temperature sintering of high-entropy ceramic composites

High-entropy ceramics and their composites display high mechanical strength and attractive high-temperature stabilities. However, properties like strong covalent bond character and low self-diffusion coefficients make them difficult to get sintered, limiting their mass popularity. Here, we present a rapid liquid phase-assisted ultrahigh-temperature sintering strategy and use high-entropy metal diboride/boron carbide composite as a proof of concept. We use a carbon-based heater to fast-heat the composite to around 3000 K, and a small fraction of eutectic liquid was formed at the interface between high-entropy metal diborides and boron carbide. A crystalline dodecaboride intergranular phase was generated upon cooling to ameliorate the adhesion between the components. The as-sintered composite presents a high hardness of 36.4 GPa at a load of 0.49 N and 24.4 GPa at a load of 9.8 N. This liquid phase-assisted rapid ultrahigh-temperature strategy can be widely applicable for other ultrahigh-temperature ceramics as well.

High-Entropy Spinel Oxides Produced via Sol-Gel and Electrospinning and Their Evaluation as Anodes in Li-Ion Batteries

In the last few years, high-entropy oxides (HEOs), a new class of single-phase solid solution materials, have attracted growing interest in both academic research and industry for their great potential in a broad range of applications. This work investigates the possibility of producing pure single-phase HEOs with spinel structure (HESOs) under milder conditions (shorter heat treatments at lower temperatures) than standard solid-state techniques, thus reducing the environmental impact. For this purpose, a large set of HESOs was prepared via sol-gel and electrospinning (by using two different polymers). Ten different equimolar combinations of five metals were considered, and the influence of the synthesis method and conditions on the microstructure, morphology and crystalline phase purity of the produced HESOs was investigated by a combination of characterization techniques. On the other hand, the presence of specific metals, such as copper, lead to the formation of minority secondary phase(s). Finally, two representative pure single-phase HESOs were preliminarily evaluated as active anode materials in lithium-ion batteries and possible strategies to enhance their rate capability and cyclability were proposed and successfully implemented. The approaches introduced here can be extensively applied for the optimization of HEO properties targeting different applications.

Bottom-up synthesis of 2D layered high-entropy transition metal hydroxides

Low-dimensional high-entropy materials, such as nanoparticles and two-dimensional (2D) layers, have great potential for catalysis and energy applications. However, it is still challenging to synthesize 2D layered high-entropy materials through a bottom-up soft chemistry method, due to the difficulty of mixing and assembling multiple elements in 2D layers. Here, we report a simple polyol process for the synthesis of a series of 2D layered high-entropy transition metal (Co, Cr, Fe, Mn, Ni, and Zn) hydroxides (HEHs), involving the hydrolysis and inorganic polymerization of metal-containing species in ethylene glycol media. The as-synthesized HEHs demonstrate 2D layered structures with interlayer distances ranging from 0.860 to 0.987 nm and homogeneous elemental distribution of designed equimolar stoichiometry in the layers. These 2D HEHs exhibit a low overpotential of 275 mV at 10 mA cm-2 in a 0.1 M KOH electrolyte for the oxygen evolution reaction. Superparamagnetic spinel-type high-entropy nanoparticles can also be obtained by annealing these HEHs. Our polyol approach creates opportunities for synthesizing low-dimensional high-entropy materials with promising properties and applications.

Direct observation of elemental fluctuation and oxygen octahedral distortion-dependent charge distribution in high entropy oxides

The enhanced compositional flexibility to incorporate multiple-principal cations in high entropy oxides (HEOs) offers the opportunity to expand boundaries for accessible compositions and unconventional properties in oxides. Attractive functionalities have been reported in some bulk HEOs, which are attributed to the long-range compositional homogeneity, lattice distortion, and local chemical bonding characteristics in materials. However, the intricate details of local composition fluctuation, metal-oxygen bond distortion and covalency are difficult to visualize experimentally, especially on the atomic scale. Here, we study the atomic structure-chemical bonding-property correlations in a series of perovskite-HEOs utilizing the recently developed four-dimensional scanning transmission electron microscopy techniques which enables to determine the structure, chemical bonding, electric field, and charge density on the atomic scale. The existence of compositional fluctuations along with significant composition-dependent distortion of metal-oxygen bonds is observed. Consequently, distinct variations of metal-oxygen bonding covalency are shown by the real-space charge-density distribution maps with sub-ångström resolution. The observed atomic features not only provide a realistic picture of the local physico-chemistry of chemically complex HEOs but can also be directly correlated to their distinctive magneto-electronic properties.

Fundamental understanding of local atomic features of high entropy oxides (HEOs) is limited due to their compositional complexity. Here the authors show composition fluctuation induced local oxygen octahedral distortion, chemical bond covalency and correlated properties in a series of HEOs.

Acoustic Emission Monitoring of High-Entropy Oxyfluoride Rock-Salt Cathodes during Battery Operation

High-entropy materials with tailorable properties are receiving increasing interest for energy applications. Among them, (disordered) rock-salt oxyfluorides hold promise as next-generation cathodes for use in secondary batteries. Here, we study the degradation behavior of a high-entropy oxyfluoride cathode material in lithium cells in situ via acoustic emission (AE) monitoring. The AE signals allow acoustic events to be correlated with different processes occurring during battery operation. The initial cycle proved to be the most acoustically active due to significant chemo-mechanical degradation and gas evolution, depending on the voltage window. Irrespective of the cutoff voltage on charge, the formation and propagation of cracks in the electrode was found to be the primary source of acoustic activity. Taken together, the findings help advance our understanding of the conditions that affect the cycling performance and provide a foundation for future investigations on the topic.

High-Entropy Alloys for Electrocatalysis: Design, Characterization, and Applications

High-entropy alloys (HEAs) are expected to function well as electrocatalytic materials, owing to their widely adjustable composition and unique physical and chemical properties. Recently, HEA catalysts are extensively studied in the field of electrocatalysis; this motivated the authors to investigate the relationship between the structure and composition of HEAs and their electrocatalytic performance. In this review, the latest advances in HEA electrocatalysts are systematically summarized, with special focus on nitrogen fixation, the carbon cycle, water splitting, and fuel cells; in addition, by combining this with the characterization and analysis of HEA microstructures, rational design strategies for optimizing HEA electrocatalysts, including controllable preparation, component regulation, strain engineering, defect engineering, and theoretical prediction are proposed. Moreover, the existing issues and future trends of HEAs are predicted, which will help further develop these high-entropy materials.

Engineering the oxygen vacancies of rocksalt-type high-entropy oxides for enhanced electrocatalysis

High-entropy oxides (HEOs), a class of compounds that include five or more elemental species, have gained increasing attraction for their capability of optimizing the target properties. To date, even though some high-entropy oxides have been successfully prepared, their applications still need to be explored. In the present study, a lithium-manipulation strategy for constructing transition metal oxides (LTM) via a modified solid-state method was investigated. The as-synthesized LTM contained six highly dispersed metal species (Li, Fe, Co, Ni, Cu, Zn) and demonstrated a rocksalt-type structure. Besides, with the introduction of Li, more oxygen vacancies were produced which was also accompanied by shrinking of the lattice constant. When the molar ratio of Li was equal to the other TM cations (LTM16.7), the electrical conductivity was greatly enhanced by a factor of 10 times. Moreover, LTM16.7 achieved the best HER (η = 207 mV at 10 mA cm^-2) and OER performances (η = 347 mV at 10 mA cm^-2) with elevated electrical conductivity. To facilitate further design of this new kind of materials, we also conducted DFT calculations and elemental alternation experiments, which showed that Fe acted as electrocatalytic sites in this HEOs system. This Li-incorporation strategy opens a new way to understand and modify defect-related HEOs.

Metal nitride-based nanostructures for electrochemical and photocatalytic hydrogen production

The over-dependence on fossil fuels is one of the critical issues to be addressed for combating greenhouse gas emissions. Hydrogen, one of the promising alternatives to fossil fuels, is renewable, carbon-free, and non-polluting gas. The complete utilization of hydrogen in every sector ranging from small to large scale could hugely benefit in mitigating climate change. One of the key aspects of the hydrogen sector is its production via cost-effective and safe ways. Electrolysis and photocatalysis are well-known processes for hydrogen production and their efficiency relies on electrocatalysts, which are generally noble metals. The usage of noble metals as catalysts makes these processes costly and their scarcity is also a limiting factor. Metal nitrides and their porous counterparts have drawn considerable attention from researchers due to their good promise for hydrogen production. Their properties such as active metal centres, nitrogen functionalities, and porous features such as surface area, pore-volume, and tunable pore size could play an important role in electrochemical and photocatalytic hydrogen production. This review focuses on the recent developments in metal nitrides from their synthesis methods point of view. Much attention is given to the emergence of new synthesis techniques, methods, and processes of synthesizing the metal nitride nanostructures. The applications of electrochemical and photocatalytic hydrogen production are summarized. Overall, this review will provide useful information to researchers working in the field of metal nitrides and their application for hydrogen production.

Operando acoustic emission monitoring of degradation processes in lithium-ion batteries with a high-entropy oxide anode

In recent years, high-entropy oxides are receiving increasing attention for electrochemical energy-storage applications. Among them, the rocksalt (Co_0.2Cu_0.2Mg_0.2Ni_0.2Zn_0.2)O (HEO) has been shown to be a promising high-capacity anode material. Because high-entropy oxides constitute a new class of electrode materials, systematic understanding of their behavior during ion insertion and extraction is yet to be established. Here, we probe the conversion-type HEO material in lithium half-cells by acoustic emission (AE) monitoring. Especially the clustering of AE signals allows for correlations of acoustic events with various processes. The initial cycle was found to be the most acoustically active because of solid-electrolyte interphase formation and chemo-mechanical degradation. In the subsequent cycles, AE was mainly detected during delithiation, a finding we attribute to the progressive crack formation and propagation. Overall, the data confirm that the AE technology as a non-destructive operando technique holds promise for gaining insight into the degradation processes occurring in battery cells during cycling.

Self-regeneration of supported transition metals by a high entropy-driven principle

The sintering of Supported Transition Metal Catalysts (STMCs) is a core issue during high temperature catalysis. Perovskite oxides as host matrix for STMCs are proven to be sintering-resistance, leading to a family of self-regenerative materials. However, none other design principles for self-regenerative catalysts were put forward since 2002, which cannot satisfy diverse catalytic processes. Herein, inspired by the principle of high entropy-stabilized structure, a concept whether entropy driving force could promote the self-regeneration process is proposed. To verify it, a high entropy cubic Zr0.5(NiFeCuMnCo)0.5Ox is constructed as a host model, and interestingly in situ reversible exsolution-dissolution of supported metallic species are observed in multi redox cycles. Notably, in situ exsolved transition metals from high entropy Zr0.5(NiFeCuMnCo)0.5Ox support, whose entropic contribution (TΔSconfig = T⋆12.7 J mol-1 K-1) is predominant in ∆G, affording ultrahigh thermal stability in long-term CO2 hydrogenation (400 °C, >500 h). Current theory may inspire more STWCs with excellent sintering-resistance performance.

High-Entropy Atomic Layers of Transition-Metal Carbides (MXenes)

High-entropy materials (HEMs) have great potential for energy storage and conversion due to their diverse compositions, and unexpected physical and chemical features. However, high-entropy atomic layers with fully exposed active sites are difficult to synthesize since their phases are easily segregated. Here, it is demonstrated that high-entropy atomic layers of transition-metal carbide (HE-MXene) can be produced via the selective etching of novel high-entropy MAX (also termed M_{n +1} AX_n (n = 1, 2, 3), where M represents an early transition-metal element, A is an element mainly from groups 13-16, and X stands for C and/or N) phase (HE-MAX) (Ti_1/5 V_1/5 Zr_1/5 Nb_1/5 Ta_1/5 )₂ AlC, in which the five transition-metal species are homogeneously dispersed into one MX slab due to their solid-solution feature, giving rise to a stable transition-metal carbide in the atomic layers owing to the high molar configurational entropy and correspondingly low Gibbs free energy. Additionally, the resultant high-entropy MXene with distinct lattice distortions leads to high mechanical strain into the atomic layers. Moreover, the mechanical strain can efficiently guide the nucleation and uniform growth of dendrite-free lithium on HE-MXene, achieving a long cycling stability of up to 1200 h and good deep stripping-plating levels of up to 20 mAh cm^-2 .

Phase-Property Diagrams for Multicomponent Oxide Systems toward Materials Libraries

Exploring the vast compositional space offered by multicomponent systems or high entropy materials using the traditional route of materials discovery, one experiment at a time, is prohibitive in terms of cost and required time. Consequently, the development of high-throughput experimental methods, aided by machine learning and theoretical predictions will facilitate the search for multicomponent materials in their compositional variety. In this study, high entropy oxides are fabricated and characterized using automated high-throughput techniques. For intuitive visualization, a graphical phase-property diagram correlating the crystal structure, the chemical composition, and the band gap are introduced. Interpretable machine learning models are trained for automated data analysis and to speed up data comprehension. The establishment of materials libraries of multicomponent systems correlated with their properties (as in the present work), together with machine learning-based data analysis and theoretical approaches are opening pathways toward virtual development of novel materials for both functional and structural applications.

Defect-Rich High-Entropy Oxide Nanosheets for Efficient 5-Hydroxymethylfurfural Electrooxidation

High-entropy oxides (HEOs), a new concept of entropy stabilization, exhibit unique structures and fascinating properties, and are thus important class of materials with significant technological potential. However, the conventional high-temperature synthesis techniques tend to afford micron-scale HEOs with low surface area, and the catalytic activity of available HEOs is still far from satisfactory because of their limited exposed active sites and poor intrinsic activity. Here we report a low-temperature plasma strategy for preparing defect-rich HEOs nanosheets with high surface area, and for the first time employ them for 5-hydroxymethylfurfural (HMF) electrooxidation. Owing to the nanosheets structure, abundant oxygen vacancies, and high surface area, the quinary (FeCrCoNiCu)₃ O₄ nanosheets deliver improved activity for HMF oxidation with lower onset potential and faster kinetics, outperforming that of HEOs prepared by high-temperature method. Our method opens new opportunities for synthesizing nanostructured HEOs with great potential applications.

Recent Development of Flexible and Stretchable Supercapacitors Using Transition Metal Compounds as Electrode Materials

Flexible and stretchable supercapacitors (FS-SCs) are promising energy storage devices for wearable electronics due to their versatile flexibility/stretchability, long cycle life, high power density, and safety. Transition metal compounds (TMCs) can deliver a high capacitance and energy density when applied as pseudocapacitive or battery-like electrode materials owing to their large theoretical capacitance and faradaic charge-storage mechanism. The recent development of TMCs (metal oxides/hydroxides, phosphides, sulfides, nitrides, and selenides) as electrode materials for FS-SCs are discussed here. First, fundamental energy-storage mechanisms of distinct TMCs, various flexible and stretchable substrates, and electrolytes for FS-SCs are presented. Then, the electrochemical performance and features of TMC-based electrodes for FS-SCs are categorically analyzed. The gravimetric, areal, and volumetric energy density of SC using TMC electrodes are summarized in Ragone plots. More importantly, several recent design strategies for achieving high-performance TMC-based electrodes are highlighted, including material composition, current collector design, nanostructure design, doping/intercalation, defect engineering, phase control, valence tuning, and surface coating. Integrated systems that combine wearable electronics with FS-SCs are introduced. Finally, a summary and outlook on TMCs as electrodes for FS-SCs are provided.