High-entropy Materials Research Articles

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 towardsquantitativelyunderstanding high entropy materials and their material properties.

Simultaneous realization of high power factor and low thermal conductivity in medium-entropy La-doped (Sr1/3Ba1/3Ca1/3)1-xLaxTiO3 oxides with porous structure

Some studies have proven the effectiveness of high-entropy alloying in reducing lattice thermal conductivity; however, the synchronous degradation in carrier mobility limits the thermoelectric properties of high-entropy materials. Herein, we present a series of medium-entropy oxides (Sr1/3Ba1/3Ca1/3)1-xLaxTiO3 (x = 0.05, 0.1, 0.15) with different levels of La doping, breaking the trade-off between carrier and phonon transport. The medium-entropy design helps keep the carrier mobility at a moderate level while modulating the carrier concentration through La3+ doping, thereby yielding a peak power factor value of 1842 μW/(m⋅K2) at 773 K. Meanwhile, the incorporation of nano-micro-porous structure and medium entropy effect leads to a significant reduction in lattice thermal conductivity, reaching 2.3 W/(m⋅K) at 1073 K. Consequently, a ZTmax value of 0.67 is obtained for the MEP-10La sample at 1073 K, which is one of the state-of-the-art ZT values reported for the SrTiO3-based materials. The present work provides a viable approach for the development of thermoelectric materials with enhanced performance.

Clarifying the four core effects of high-entropy materials.

High-entropy materials emerged as a field of research in 2004, when the first research on high-entropy alloys was published. The scope was soon expanded from high-entropy alloys to medium-entropy alloys, as well as to ceramics, polymers and composite materials. A fundamental understanding on high-entropy materials was proposed in 2006 by the 'four core effects' - high-entropy, severe-lattice-distortion, sluggish-diffusion and cocktail effects - which are often used to describe and explain the mechanisms of various peculiar phenomena associated with high-entropy materials. Throughout the years, the effects have been examined rigorously, and their validity has been affirmed. This Perspective discusses the fundamental understanding of the four core effects in high-entropy materials and gives further insights to strengthen the understanding for these effects. All these clarifications are believed to be helpful in understanding low-to-high-entropy materials as well as to aid the design of materials when studying new compositions or pursuing their use in applications.

Machine Learning-Based Prediction of Stability in High-Entropy Nitride Ceramics

The field of materials science has experienced a transformative shift with the emergence of high-entropy materials (HEMs), which possess a unique combination of properties that traditional single-phase materials lack. Among these, high-entropy nitrides (HENs) stand out for their exceptional mechanical strength, thermal stability, and resistance to extreme environments, making them highly sought after for applications in aerospace, defense, and energy sectors. Central to the design of these materials is their entropy forming ability (EFA), a measure of a material’s propensity to form a single-phase, disordered structure. This study introduces the application of the sure independence screening and sparsifying operator (SISSO), a machine learning technique, to predict the EFA of HEN ceramics. By utilizing a rich dataset curated from theoretical computational data, SISSO has been trained to identify the most critical features contributing to EFA. The model’s strong interpretability allows for the extraction of complex mathematical expressions, providing deep insights into the material’s composition and its impact on EFA. The predictive performance of the SISSO model is meticulously validated against theoretical benchmarks and compared with other machine learning methodologies, demonstrating its superior accuracy and reliability. This research not only contributes to the growing body of knowledge on HEMs but also paves the way for the efficient discovery and development of new HEN materials with tailored properties for advanced technological applications.

High-Entropy Engineering in Thermoelectric Materials: A Review

Thermoelectric (TE) materials play a crucial role in converting energy between heat and electricity, essentially for environmentally friendly renewable energy conversion technologies aimed at addressing the global energy crisis. Significant advances in TE performance have been achieved over the past decades in various TE materials through key approaches, such as nanostructuring, band engineering, and high-entropy engineering. Among them, the design of high-entropy materials has recently emerged as a forefront strategy to achieve significantly low thermal conductivity, attributed to severe lattice distortion and microstructure effects, thereby enhancing the materials’ figure of merit (zT). This review reveals the progress of high-entropy TE materials developed in the past decade. It discusses high-entropy-driven structural stabilization to maintain favorable electrical transport properties, achieving low lattice thermal conductivity, and the impact of high entropy on mechanical properties. Furthermore, the review explores the theoretical development of high-entropy TE material and discusses potential strategies for future advancements in this field through interactions among experimental and theoretical studies.

Scalable Synthesis Methods for High‐Entropy Nanoparticles

High‐entropy materials (HEMs) represent a revolutionary class of materials that have garnered significant attention in the field of materials science due to their extraordinary properties in diverse fields of applications such as catalysis and electrochemistry. The past decade has witnessed a substantial increase in the study of these materials, exploring new synthesis routes and compositions. What began as the synthesis of high‐entropy alloys has expanded to encompass several classes of HEMs such as oxides, hydroxides, sulfides, nitrides, and carbides, among others. Several synthesis methods have been developed to produce these materials. This review therefore highlights the fundamental concepts of HEMs, including their core effects, with a major emphasis on their scalable synthesis routes. The advantages and drawbacks of these methods are also discussed. As HEMs transition from the lab to large‐scale production, there is a growing need for cost‐effective and scalable synthesis methods with high material yield suitable for a variety of applications like hydrogen storage, catalysis, batteries, supercapacitors, and fuel cells. Hence, this review serves as an introduction to scalable synthesis routes based on crystal structure, desired elements, synthesis times, and equipment costs.

Microwave construction of the 3D porous interconnected structure of NiCoCr medium-entropy alloy with ultra-high specific capacity for supercapacitors and electrocatalysis

The effective design of bifunctional electrodes with unique morphology and remarkable electrochemical properties for energy conversion and storage devices is a promising but challenging endeavor. Herein, self-supporting NiCoCr medium-entropy alloy (MEA) with a hierarchical porous network structure was in situ synthesized on nickel foam (NiCoCr/NF) by a simple and rapid microwave route. The optimal NiCoCr/NF presented an ultra-high specific capacity of 3011.8 C g−1 (1 A g−1) and outstanding charge/discharge sustainability (83.4% retention at 10 A g−1 over 50,000 cycles) due to its multi-component synergistic effect and hierarchical porous structure. Meanwhile, subtle lattice distortions and electron density redistribution in the NiCoCr MEA tuned electronic structures, resulting in a low overpotential and Tafel slope. Aqueous and solid-state asymmetric supercapacitors were fabricated by using NiCoCr/NF as a cathode and Bi/NF as an anode. The aqueous supercapacitor displayed an exceptional energy density of 314.4 Wh kg−1 at a power density of 800 W kg−1 with 81.8% retention at 10 A g−1 over 20,000 cycles. This study provides a new strategy for the efficient synthesis of electrode materials with interconnected network structures and paves the way for the broader application based on medium-entropy and high-entropy materials as promising candidates for future energy storage devices.

Rich phosphorus vacancies N-doped C high-entropy metal phosphides for efficient electrocatalytic water oxidation

High-entropy materials have received increasing attention due to their unique physicochemical characteristics and remarkable properties for oxygen evolution reaction (OER). The synthesis of high-entropy metal phosphides (HEMPs) has rarely been reported owing to the incompatibility between different metal ions and phosphorus. Herein, the phosphorus vacancies (Pv) enriched N-doped C HEMPs (N-C@CoNiMnCuFeP/NF) for OER were prepared by a facile ZIF-derived phosphide method. Abundant Pv and amorphous structures increase the active sites and modulate the electronic structure of electrocatalyst to improve the OER performance. Besides, the synergistic effect among the metallic elements is beneficial for enhancing the electrocatalytic activity. Accordingly, HEMP-450 shows outstanding catalytic performance with the overpotential of 199 mV and continuously reacts for 140 hours without performance degradation for OER at 100 mA cm−2. This work demonstrates a simple template strategy for reasonably preparing HEMPs electrocatalyst with rich Pv.

High-entropy MnCoCuZnAlSe2 nanosheet arrays for highly efficient alkaline oxygen evolution reaction

The oxygen evolution reaction is the primary obstacle in the electrochemical breakdown of water; whereas transition metal selenides exhibit remarkable OER activity, their stability is compromised by the oxidative dissolution of selenium. The high-entropy materials can provide a feasible strategy to solve this problem due to their multi-element synergy and entropy stabilization effect. Here, we designed novel high-entropy MnCoCuZnAlSe2 electrocatalysts with ultra-low overpotential (190 mV@10 mA cm−2) and exceptional endurance (500 h@10 mA cm−2), which outperform other selenide and IrO2 catalysts. The high entropy effect was experimentally confirmed to inhibit selenium dissolution and improve the stability of transition metal selenides. Calculations using Density Functional Theory suggest that cobalt is actually MnCoCuZnAlSe2's true active center. The extraordinary OER properties of high-entropy selenides offer great opportunities for the design of ideal catalysts for practical applications.

Lattice engineering of high-entropy olivine-type lithium metal phosphate as high-voltage cathodes

Engineering of high-entropy cathode materials for lithium-ion batteries has been actively pursued owing to the outstanding conductivity of high-entropy materials benefited from the maximum entropy and unique antisite disordering structure. Olivine lithium metal phosphates such as LiMnPO4 and LiNiPO4 feature high working voltages but low capacities due to their insulation nature. In this work, the synthesis of the high-entropy lithium metal phosphate materials (HELMPs) is realized by combining mechanochemistry with a calcination method. By regulating lattice of HELMPs, the high-entropy Li(Mn0.35Fe0.35Co0.1Mg0.1Ca0.1)PO4 reveals three typical high-voltage plateaus in charge–discharge curves corresponding to the redox of Fe, Mn, and Co in the voltage range of 2.0–4.9 V vs Li+/Li, and a much higher initial capacity than LiMnPO4 (104 vs 15 mAh g−1).

Preparation of Novel Layered High Entropy Bismuth-Based Materials and their Photocatalytic Degradation Mechanism.

We prepared BiOCl, BiO(ClBr), BiO(ClBrI), and BiO[ClBrI(CO3)0.5] materials using a simple coprecipitation method. It was found that adjusting the number of anions in the anion layer was conducive to adjusting the band structure of BiOX and could effectively promote the migration and separation of photogenerated carriers, thus improving the photocatalytic activity. We first selected methyl orange (MO) as the study pollutant and compared it with BiOCl, BiO(ClBr), and BiO(ClBrI). The first-order kinetic constants of MO degradation by BiO[ClBrI(CO3)0.5] increased by 90.3, 33.9, and 3.1 times, respectively. The photocatalytic degradation rate of methylene blue by BiO[ClBrI(CO3)0.5] was 89.5%, indicating the excellent photocatalytic performance of BiO[ClBrI(CO3)0.5]. The stability of BiO[ClBrI(CO3)0.5] was demonstrated through cyclic experiments and XRD analysis before and after the reaction. The photocatalytic degradation of MO by BiO[ClBrI(CO3)0.5] showed that h+ and 1O2 were the main active oxidizing species and •O2- was the secondary active substance. Overall, our work provides new ideas for the synthesis and degradation of organic pollutants by using two-dimensional anionic high-entropy materials.

High-entropy layered double hydroxide catalyst decorated with Ag nanoparticles for highly efficient water oxidation

Highly efficient and stable catalysts for oxygen evolution reaction (OER) are urgently needed for water splitting. Here, a novel FeNiMnCuAl LDH catalyst decorated with Ag nanoparticles for OER was fabricated by the hydrothermal and chemical reduction method. The heterostructure catalyst exhibits a low over potential of 208 mV to achieve a current density of 10 mA cm−2 and a low Tafel slope of 60 mV dec−1. In addition, it has an excellent long-term stability for more than 50 h at a large current density of ∼100 mA cm−2. By combining the experiment and theoretical calculation, it shows that the increased conductivity and optimized energy barrier are responsible for the boosted intrinsic activity. This work not only provides a promising electrocatalyst but also broadens the application field of high-entropy materials.

Controlled synthesis of nanoparticles of high-etropy materials. Optimization of traditional and creation of innovation strategies

In the last decade, the diversity of high-entropy materials (HEMs) has increased sharply, including due to the expansion of research into the field of amorphous, nano- and heterostructures. Interest in nanoscale HEMs is primarily associated with their potential application in various fields, such as renewable and green energy, catalysis, hydrogen storage, surface protection and others. The development of nanotechnology has made it possible to develop an innovative design of nanoscale HEMs with fundamentally new structures with unique physical and chemical properties. Problems of controlled synthesis with precisely specified parameters of chemical composition, microstructure and morphology are solved. At the same time, traditional technologies such as fast pyrolysis, mechanical alloying, magnetron sputtering, electrochemical synthesis, etc. are being modernized. Along with this, innovative synthesis technologies have appeared, such as carbothermic shock, the method of controlled hydrogen spillover. The review discusses various methods for the synthesis of nanoscale HEMs that have been developed in the last few 6–7 years for various applications. Some of them are modernization of traditional methods for producing HEM or nano-sized materials, while another group of techniques represents innovative solutions stimulated and inspired by the HEM phenomenon.

Elucidation of the Transport Properties of Calcium-Doped High Entropy Rare Earth Aluminates for Solid Oxide Fuel Cell Applications.

Solid oxide fuel cells (SOFCs) are paving the way to clean energy conversion, relying on efficient oxygen-ion conductors with high ionic conductivity coupled with a negligible electronic contribution. Doped rare earth aluminates are promising candidates for SOFC electrolytes due to their high ionic conductivity. However, they often suffer from p-type electronic conductivity at operating temperatures above 500°C under oxidizing conditions caused by the incorporation of oxygen into the lattice. High entropy materials are a new class of materials conceptualized to be stable at higher temperatures due to their high configurational entropy. Introducing this concept to rare earth aluminates can be a promising approach to stabilize the lattice by shifting the stoichiometric point of the oxides to higher oxygen activities, and thereby, reducing the p-type electronic conductivity in the relevant oxygen partial pressure range. In this study, the high entropy oxide (Gd,La,Nd,Pr,Sm)AlO3 is synthesized and doped with Ca. The Ca-doped (Gd,La,Nd,Pr,Sm)AlO3 compounds exhibit a higher ionic conductivity than most of the corresponding Ca-doped rare earth aluminates accompanied by a reduction of the p-type electronic conductivity contribution typically observed under oxidizing conditions. In light of these findings, this study introduces high entropy aluminates as a promising candidate for SOFC electrolytes.

Water-Stable High-Entropy Metal-Organic Framework Nanosheets for Photocatalytic Hydrogen Production.

Metal-organic frameworks (MOFs) have emerged as promising platforms for photocatalytic hydrogen evolution reaction (HER) due to their fascinating physiochemical properties. Rationally engineering the compositions and structures of MOFs can provide abundant opportunities for their optimization. In recent years, high-entropy materials (HEMs) have demonstrated great potential in the energy and environment fields. However, there is still no report on the development of high-entropy MOFs (HE-MOFs) for photocatalytic HER in aqueous solution. Herein, the authors report the synthesis of a novel p-type HE-MOFssingle crystal (HE-MOF-SC) and the corresponding HE-MOFsnanosheets (HE-MOF-NS) capable of realizing visible-light-driven photocatalytic HER. Both HE-MOF-SC and HE-MOF-NS exhibit higher photocatalytic HER activity than all the single-metal MOFs, which are supposed to be ascribed to the interplay between the different metal nodes in the HE-MOFsthat enables more efficient charge transfer. Moreover, impressively, the HE-MOF-NS demonstrates much higher photocatalytic activity than the HE-MOF-SC due to its thin thickness and enhanced surface area. At optimum conditions, the rate of H2 evolution on the HE-MOF-NS is ≈13.24 mmol h-1 g-1, which is among the highest values reported for water-stable MOF photocatalysts. This work highlights the importance of developing advanced high-entropy materials toward enhanced photocatalysis.

High-temperature oxidation and diffusion studies on selected Al–Cr–Fe–Ni high-entropy alloys for potential application in thermal barrier coatings

In this work, the properties of two high-entropy alloys (HEAs) Al25Cr20Fe20Ni35 and Al20Cr20Fe20Ni40 (at.%) were compared for potential application as bond coats in thermal barrier coatings (TBCs). For this reason, both their oxidation resistance under isothermal and thermal shock conditions, as well as diffusion phenomena occurring at the substrate-HEA diffusion couple interface, were investigated. It was determined that both alloys demonstrate good phase stability and oxidation resistance during prolonged exposure to air atmosphere at 1000°C. In the initial 100 h period, selective aluminum oxidation is responsible for the protective scale growth on both materials under both isothermal and thermal shock conditions. However, only the Al20Cr20Fe20Ni40 high entropy alloy maintains a single Al2O3 layer for over 1000 h of oxidation with cyclic temperature changes. On the other hand, diffusion studies indicate that mainly iron and chromium travel between the ferritic steel substrate and the HEAs. Aluminum diffusion is seemingly limited by the unexpected formation of a thin aluminum nitride layer at the interface between the materials. From all these results it can be concluded that the Al20Cr20Fe20Ni40 alloy shows more promise for application in TBCs based on high entropy materials than Al25Cr20Fe20Ni35.

Li-doped transition metal high-entropy oxides (Li/TM-HEOs) as Li-Ion batteries cathodes: a first report on capacity fading and cycling stability

High-Entropy Oxides (HEOs) have gained significant attention for their wide range of compositions and potential applications across various sectors, including rechargeable batteries. This study explores the characterization of two distinct HEO systems as potential cathode materials for Lithium-ion batteries (LIBs). A series of rock salt structured HEOs with varying Li loadings (16Li/RS-HEO, 25Li/RS-HEO, 33Li/RS-HEO, and 41Li/RS-HEO) and a spinel-structured HEO with 16 mol% of Li loading (16Li/SP-HEO) were firstly synthesized through co-precipitation. Electrochemical analyses via cyclic voltammetry revealed stark differences in the behavior of these structures. The Li/SP-HEO sample displayed broad and strongly irreversible hysteresis cycles, while the Li/RS-HEO series manifested thin, narrow hysteresis cycles with single oxidation peaks between 0.5 V and 0.7 V. As the lithium content increases in the RS-HEO system, the cycling stability of the cell decreases, most likely due to the reduced ratio of transition metal cations to lithium ions. Although there was a noticeable decrease in capacity under higher current rates, the higher lithium loadings positively impact the cell capacity, albeit with notable capacity fading under higher current rates. Li-doped rock salt structured high entropy materials show potential for LIB cathodes in terms of high specific capacities; the observed stability issues at medium and high current densities indicate a rapid electrode degradation.

Remarkable energy-storage density together with efficiency of above 92% in high-entropy ferroelectric ceramics

Recently, the use of “entropy engineering” to form high-entropy ceramic dielectric materials is considered to be an effective means to break through the traditional doping which modified local structures. However, the low energy storage efficiency (η) of most high-entropy ceramics cannot match their excellent energy storage density (Wrec). This work is the first to combine scheelite structure (SmTaO4) with high-entropy perovskite structure ((NaBiBaSrCa)0.2TiO3). On the one hand, the superposition of multiple structures makes the inherent lattice distortion and sluggish diffusion of high-entropy materials more obvious, thus playing a role in refining grains, reducing leakage current density, and obtaining a high breakdown electric field (Eb). On the other hand, the introduction of donor elements inhibits the generation and migration of oxygen vacancies, reducing losses to improve η and Eb. Ultimately, the goal of obtaining a remarkable Wrec (5.6 J/cm3) and η (92.2%) at 445 kV/cm was successfully achieved. This work offers a practical approach to attain superior overall performances in high-entropy ceramics.

The effect of dual-sites high-entropy strategy on thermal conductivity of pyrochlore ceramics

The emergence of high-entropy materials has developed a new strategy for the design and optimization of ceramics with desired properties. In this work, the (Gd1/7La1/7Nd1/7Sm1/7Eu1/7Dy1/7Ho1/7)2(Ti1/4Zr1/4Sn1/4Hf1/4)2O7 pyrochlore ceramic with dual-sites high-entropy design has been successfully prepared by a solid-state reaction sintering method. The samples possess a single-phase of pyrochlore structure, and all introduced elements are distributed homogeneously at A and B sites, respectively. Based on the synergistic high-entropy design of A and B sites, the thermal conductivity value of high-entropy pyrochlore ceramic could be diminished to 1.37 W∙m−1∙K−1.

Ultrasound-Assisted Synthesis of High-Entropy Materials for Enhanced Oxygen Evolution Electrocatalysis

High-entropy materials (HEMs) play a significant role in the electrocatalytic oxygen evolution reaction (OER) due to their unique properties. However, there are still challenges in the preparation of HEMs for OER catalysts. In this study, the FeCoNiMnCr catalyst is synthesized for the first time using the ultrasonic hydrothermal-sintering technique and exhibits excellent performance for OER electrocatalysis. There is an optimal ultrasonic hydrothermal time and power for achieving the best OER performance. The results demonstrate that the performance of FeCoNiMnCr catalysts prepared through ultrasonic hydrothermal sintering (US-FeCoNiMnCr) is significantly improved compared with the traditional hydrothermal-sintering method. The US-FeCoNiMnCr catalyst exhibits an overpotential of 228 mV at the current density of 10 mA cm−2 and a Tafel slope as low as 45.39 mV dec−1 in an alkaline medium. Moreover, the US-FeCoNiMnCr catalyst demonstrates remarkable stability in electrocatalytic OER with a minimal potential increase observed even after 48 h. This work not only provides valuable insights into high-entropy material synthesis, but also presents a powerful electrocatalyst for water electrolysis.