More Is Different: Case Studies of How Chemical Complexity Influences Stability in High Entropy Oxides

High Entropy Oxides: Structure and Properties

Recent status and challenging perspective of high entropy oxides for chemical catalysis

High entropy oxides (HEOs) are solid state inorganic compounds in which entropy, rather than enthalpy, plays a dominant role in stabilizing a single-phase structure at high temperatures. This work has been motivated in part by prior studies of multicomponent alloys in which four or more cations occupy the same crystallographic site in equal proportions, known as high entropy alloys (HEAs), which have superior mechanical properties and high radiation tolerances due in part to high configurational entropy.[1] In the case of HEOs, the first example is the rock salt (Mg0.2Ni0.2Co0.2Cu0.2Zn0.2)O, which has generated a great deal of interest in this class of materials.[2,3] We have recently demonstrated that HEOs prepared by mechanochemical synthesis can be prepared in pure form, and may be useful for catalysis.[4,5] It has also been shown that HEOs are of interest for high ionic conductivity and electrochemical energy storage.[6,7] In this study, we have examined the electrochemical cycling of new high entropy rock salt phases versus lithium, and found an effect of composition on the cycling performance. Samples were prepared through high energy milling of starting binary oxides, which proceed to decompose and then reform pure compounds at high temperature. We have confirmed that entropy plays a role in this transformation for these rock salt HEOs, and that the precise composition has an impact on the temperature and kinetics of pure phase formation; investigations of the synthesis and subsequent decomposition have been conducted in our laboratory using high temperature in-situ X-ray diffraction on a Panalytical diffractometer equipped with an XRK900 stage. STEM/EDS studies on quenched ex-situ samples will be presented that show how elemental segregation occurs as a function of temperature. The results of this study will be highly impactful for the growing community of researchers investigating the design and synthesis of the new class of materials, the high entropy oxides.[1] Y. Lu, et. al., Sci. Rep. 4, 6200 (2014); Y. F. Ye, et. al., Mater. Today 19 (6), 349 (2016); Zhang, Y., et. al., Nature Commun. 6, 8736 (2015); [2] C. M. Rost, et. al., Nature Commun. 6, 8485 (2015); [3] B. Jiang, et. al., Probing the Local Site Disorder and Distortion in Pyrochlore High-Entropy Oxides. Journal of the American Chemical Society 2021, 143, (11), 4193-4204; [4] H. Chen, et al., Mechanochemical Synthesis of High Entropy Oxide Materials under Ambient Conditions: Dispersion of Catalysts via Entropy Maximization, ACS Materials Lett. 2019, 1, 1, 83–88; [5] H. Chen, et. al., Entropy-stabilized metal oxide solid solutions as CO oxidation catalysts with high-temperature stability, J. Mater. Chem. A 2018, 6, 11129-11133; [6] Q. Wang, et. al., Multi-anionic and -cationic compounds: new high entropy materials for advanced Li-ion batteries, Energy Environ. Sci., 2019, 12, 2433; [7] D. Berardan, et. al., Room temperature lithium superionic conductivity in high entropy oxides, J. Mater. Chem. A, 2016, 4, 9536.

The chemical complexity of single-phase multicationic oxides, commonly termed high entropy oxides (HEOs), enables the integration of conventionally incompatible metal cations into a single-crystalline phase. However, few studies have effectively leveraged the multicationic nature of HEOs for optimization of disparate physical and chemical properties. Here, we apply the HEO concept to design robust oxidation catalysts in which multicationic oxide composition is tailored to simultaneously achieve catalytic activity, oxygen storage capacity, and thermal stability. Unlike conventional catalysts, HEOs maintain single-phase structure, even at high temperature, and do not rely on the addition of expensive platinum group metals (PGM) to be active. The HEOs are synthesized through a facile, relatively low temperature (500 °C) sol-gel method, which avoids excessive sintering and catalyst deactivation. Nanostructured high entropy oxides with surface areas as high as 138 m2/g are produced, marking a significant structural improvement over previously reported HEOs. Each HEO contained Ce in varying concentrations, as well as four other metals among Al, Fe, La, Mn, Nd, Pr, Sm, Y, and Zr. All samples adopted a fluorite structure. First row transition metal cations were most effective at improving CO oxidation activity, but their incorporation reduced thermal stability. Rare earth cations were necessary to prevent thermal deactivation while maintaining activity. In sum, our work demonstrates the utility of entropy in complex oxide design and a low-energy synthetic route to produce nanostructured HEOs with cations selected for a cooperative effect toward robust performance in chemically and physically demanding applications.

The burgeoning sector of electric vehicles has significantly spurred the exploration of high-energy density and long-term cycling life electrode materials for advanced lithium-ion batteries (LIBs). Presently, the constrained capacity of commercialized graphite (LiC6: 372 mAh g–1) and Li4Ti5O12 (Li7Ti5O12: 175 mAh g–1) anodes falls short of meeting the heightened requirements for energy density in such applications. Transition metal oxides (TMOs, MxOy) with conversion reaction have been widely scrutinized as next-generation LIB anode materials owing to their resonable reversible capacity in comparison to conventional graphite anode. Unfortunately, the crystal structures of TMOs typically undergo severe degradation during rapid conversion/alloying reactions over continuous lithiation/delithiation process. This phenomenon leads to rapid capacity decay, poor reversibility, and consequently, a significant hindrance to their applicability as LIB anodes. In recent developments, high-entropy oxides (HEOs), analogous to high-entropy metallic alloys (HEMAs), have garnered considerable interest as an emerging class of solid solutions, involving a multitude of metalic cations in an equimolar ratio. Intriguingly, the high configurational entropy (Sconfig) present plays a crucial role in stabilizing their single-phase crystal structures. Pioneering this concept, Rost et al. introduced entropy-stabilization into TMOs, successfully creating the rocksalt HEO, (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O with the value of Sconfig ≥ 1.6R. Leveraging substantial compositional flexibility by modifying the stoichiometry and incorporating diverse cationic species, HEOs possess unforeseen and distinctive physicochemical properties. Consequently, they have been applied across diverse domains, encompassing catalysts, thermoelectrics, superionic conductors, and battery electrodes.The initial investigation into HEOs as LIB anode materials featured the rocksalt-type composition (Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)O. Sarkar et al. demonstrated its remarkably reversible Li-storage properties, showcasing stable cycling performance with reversible capacities ranging from 500 to 700 mAh g−1 at current density of 200 mA g−1 even after 300 cycles. Subsequently, Patra et al. introduced the single-phase spinel HEO (Fe0.2Cu0.2Ni0.2Cr0.2Mn0.2)3O4 as a LIB anode, achieving a specific capacity of 640 mAh g−1 at current density of 500 mA g−1 over 400 cycles. Impressively, the specific capacity remained at 596 mAh g−1 at a high current density of 2.0 A g−1 after 1200 cycles, retaining 86.2%. In contrast to conventional TMOs, entropy-stabilized HEOs possess the ability to maintain partial stability even in a fully lithiated state, acting as a matrix to accommodate conversion reactions and greatly improve reversible cycling stability. Nevertheless, rocksalt HEOs encounter challenges stemming from inadequate active components, thereby affecting the reversible capacity during cycling. In contrast, the spinel structure of HEOs facilitates ionic diffusion through three-dimensional pathways. Furthermore, the induction of oxygen vacancies by multivalent metallic cations at Wyckoff positions (tetrahedral and octahedral) in the spinel serves to augment ionic conduction. Therefore, the imperative further development of new spinel HEO anodes necessitates the rational design of active metallic cation components within HEOs. Concerning the enhancement of Li-storage properties in HEO anodes for LIBs, a comprehensive understanding of their Li storage mechanisms during Li-insertion/extraction is critically important.Herein, we present the development of a high-performance conversion-type anode comprising new multi-component HEOs with 5, 6, 7, and 8 cations for LIBs, synthesized through rapid techniques, specifically solution combustion synthesis (SCS). Additionally, we characterize their in-depth structural information using synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). With an increase in the number of cations, the HEO anodes exhibit improved Li-storage properties, including enhanced cycling performance and rate capability. To further evaluate the electrochemical performance of HEOs synthesized by different methods, we selected a specific composition based on its electrochemical performance and synthesized it using solvothermal synthesis. Subsequently, we assessed the electrochemical performance of these samples. Furthermore, we investigated the Li-storage mechanism of HEOs during lithiation/delithiation through ex-situ analytical techniques, encompassing XRD and XAS. The results collectively indicate that the highly reversible conversion reaction in cycled HEO anodes contributes to their outstanding Li-storage characteristics, facilitating stable cycling retention and fast rate capability. The findings of this study delve deeply into the highly reversible Li-storage in HEOs through conversion reactions, with potential implications extending to a broader class of HEO anodes, suggesting the promise of advanced LIBs exhibiting exceptional electrochemical performance.

Thermodynamics of high entropy oxides

High entropy oxides (HEOs), in which multication occupation of a single crystallographic site plays an important role in the properties, have become relevant in energy storage [1,2], catalysis [3.4], and many more areas. In a subset of these compounds the entropy, rather than enthalpy, plays a dominant role in stabilizing a single-phase structure at high temperatures. In other cases, the multication occupation merely contributes to stability and properties, but the entropy remains dominant in the stability. The field originated with high entropy metal alloys (HEAs)[5], and then expanded to oxides, borides, sulfides, and more. In the case of HEOs, the first example is the rock salt (Mg0.2Ni0.2Co0.2Cu0.2Zn0.2)O, which has generated a great deal of interest in this class of materials.[6,7] It has been shown that HEOs are of interest for high ionic conductivity and electrochemical energy storage. We have examined the electrochemical performance of new high entropy elecrolytes and found an effect of composition on the cycling performance. Samples were prepared through sol-gel routes and high energy milling of starting binary oxides. We have investigated the synthesis using high temperature in-situ X-ray diffraction on a Panalytical diffractometer equipped with an XRK900 stage. STEM/EDS studies on ex-situ samples will be presented that show elemental distribution, with Raman and EIS measurements providing information on ionic diffusion. The results of this study will be highly impactful for the growing community of researchers investigating the design and synthesis of the new class of materials, the high entropy oxides.[1] Q. Wang, et. al., Multi-anionic and -cationic compounds: new high entropy materials for advanced Li-ion batteries, Energy Environ. Sci., 2019, 12, 2433; [2] D. Berardan, et. al., Room temperature lithium superionic conductivity in high entropy oxides, J. Mater. Chem. A, 2016, 4, 9536.; [3] H. Chen, et al., Mechanochemical Synthesis of High Entropy Oxide Materials under Ambient Conditions: Dispersion of Catalysts via Entropy Maximization, ACS Materials Lett. 2019, 1, 1, 83–88; [4] H. Chen, et. al., Entropy-stabilized metal oxide solid solutions as CO oxidation catalysts with high-temperature stability, J. Mater. Chem. A 2018, 6, 11129-11133; [5] Y. Lu, et. al., Sci. Rep. 4, 6200 (2014); Y. F. Ye, et. al., Mater. Today 19 (6), 349 (2016); Zhang, Y., et. al., Nature Commun. 6, 8736 (2015); [6] C. M. Rost, et. al., Nature Commun. 6, 8485 (2015); [7] B. Jiang, et. al., Probing the Local Site Disorder and Distortion in Pyrochlore High-Entropy Oxides. Journal of the American Chemical Society 2021, 143, (11), 4193-4204

Micro-structural and magnetic analysis of spinel high entropy oxides synthesized by two-step pressureless sintering provides insight into high entropy ceramics

In this work, a relatively new class of materials, rareearth (RE)based high entropy oxides (HEO) are discussed in terms of the evolutionof the oxygen vacant sites (Ov) content in their structureas the composition changes from binary to HEO using both experimentaland computational tools; the composition of HEO under focus is theCeLaPrSmGdO due to the importance of ceria-related (fluorite) materialsto catalysis. To unveil key features of quinary HEO structure, ceria-basedbinary CePrO and CeLaO compositions as well as SiO2, thelatter as representative nonreducible oxide, were used and comparedas supports for Ru (6 wt % loading). The role of the Ov in the HEO is highlighted for the ammonia production with particularemphasis on the N2 dissociation step (N2(ads) → Nads) over a HEO; the latter step is consideredthe rate controlling one in the ammonia production. Density functionaltheory (DFT) calculations and 18O2 transientisotopic experiments were used to probe the energy of formation, thepopulation, and the easiness of formation for the Ov at650 and 800 °C, whereas Synchrotron EXAFS, Raman, EPR, and XPSprobed the Ce–O chemical environment at different length scales.In particular, it was found that the particular HEO composition easesthe Ov formation in bulk, in medium (Raman), and in short(localized) order (EPR); more Ov population was found onthe surface of the HEO compared to the binary reference oxide (CePrO).Additionally, HEO gives rise to smaller and less sharp faceted Ruparticles, yet in stronger interaction with the HEO support and abundanceof Ru–O–Ce entities (Raman and XPS). Ammonia productionreaction at 400 °C and in the 10–50 bar range was performedover Ru/HEO, Ru/CePrO, Ru/CeLaO, and Ru/SiO2 catalysts;the Ru/HEO had superior performance at 10 bar compared to the restof catalysts. The best performing Ru/HEO catalyst was activated underdifferent temperatures (650 vs 800 °C) so to adjust the Ov population with the lower temperature maintaining betterperformance for the catalyst. DFT calculations showed that the HEOactive site for N adsorption involves the Ov site adjacentto the adsorption event.

High-efficiency oxygen evolution reaction: Effect of phosphorus doped on the surface reconstruction of high-entropy spinel oxides

High entropy spinel oxides (CrFeMnNiCox)3O4 (x = 2, 3, 4) nanoparticles as anode material towards electrochemical properties

A novel type of oxide material, high entropy oxide (Mn0.2Fe0.2Co0.2Ni0.2Cu0.2)3O4 (MFO) composites with spinel structure were successfully synthesized by a simple solution combustion in this paper, and it was first applied to the degradation of antibiotic organic pollutants in water by photo-Fenton. SEM and BET characterization showed that the composite was porous and had a large specific surface area. XPS results showed that Fe, Mn, Cu, Co and Ni all participated in the redox reaction of the catalytic process. The redox pairs of Mn2+/Mn3+, Cu+/Cu2+, Co2+/Co3+, Ni2+/Ni3+ can accelerate the Fe2+/Fe3+ redox cycling in MFO to activate H2O2 and produce more reactive oxygen species. The catalytic performance of MFO composite was investigated using tetracycline hydrochloride (TC-HCl) as a model pollutant. The results displayed that the degradation rate of TC-HCl by MFO was 92.9% when the initial pH was 4, the dose of H2O2 was 50 mM, and the irradiation time was 60 min. The high entropy oxide MFO composites could build up an internal electric field, which restrains electron–hole recombination, improves the transfer of photogenerated charge carriers and maximize photocatalytic property. In addition, the free radical capture experiment determined that the main active species of the degradation reaction were e−, •O2 − and •OH. The synergistic effect of the five components in the high entropy oxide strengthens lattice distortion and defects, increases oxygen vacancies, and thus has enhanced catalytic effect for TC-HCl degradation. This work shows that high entropy oxides have excellent catalytic performance for tetracycline organic pollutants, and it is speculated that high entropy oxides have good application prospects in the field of advanced oxidation technology for the degradation of organic pollutants.

Comprehensive investigation of crystallographic, spin-electronic and magnetic structure of [formula omitted]: Unraveling the suppression of configuration entropy in high entropy oxides

SummaryAlthough it has been shown that configurational entropy can improve the structural stability in transition metal oxides (TMOs), little is known about the oxidation state of transition metals under random mixing of alloys. Such information is essential in understanding the chemical reactivity and properties of TMOs stabilized by configurational entropy. Herein, utilizing electron energy loss spectroscopy (EELS) technique in an aberration-corrected scanning transmission electron microscope (STEM), we systematically studied the oxidation state of binary (Mn, Fe)3O4, ternary (Mn, Fe, Ni)3O4, and quinary (Mn, Fe, Ni, Cu, Zn)3O4 solid solution polyelemental transition metal oxides (SSP-TMOs) nanoparticles. Our findings show that the random mixing of multiple elements in the form of solid solution phase not only promotes the entropy stabilization but also results in stable oxidation state in transition metals spanning from binary to quinary transition metal oxide nanoparticles.

High entropy oxides are a class of materials distinguished by the use of configurational entropy to drive material synthesis. These materials are being examined for their exciting physiochemical properties and hold promise in numerous fields, such as chemical sensing, electronics, and catalysis. Patterning and integration of high entropy materials into devices and platforms can be difficult due to their thermal sensitivity and incompatibility with many conventional thermally-based processing techniques. In this work, we present a laser-based technique, laser-induced thermal voxels, that combines the synthesis and patterning of high entropy oxides into a single process step, thereby allowing patterning of high entropy materials directly onto substrates. As a proof-of-concept, we target the synthesis and patterning of a well-characterized rock salt-phase high entropy oxide, (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O, as well as a spinel-phase high entropy oxide, (Mg0.2Ni0.2Co0.2Cu0.2Zn0.2)Cr2O4. We show through electron microscopy and x-ray analysis that the materials created are atomically homogenous and are primarily of the rock salt or spinel phase. These findings show the efficacy of laser induced thermal voxel processing for the synthesis and patterning of high entropy materials and enable new routes for integration of high entropy materials within microscale platform and devices.