High-entropy energy materials: Challenges and new opportunities

The essential demand for functional materials enabling the realization of new energy technologies has triggered tremendous efforts in scientific and industrial research in recent years. Recently, high-entropy materials, with their unique structural characteristics, tailorable chemical composition and correspondingly tunable functional properties, have drawn increasing interest in the fields of environmental science and renewable energy technology. Herein, we provide a comprehensive review of this new class of materials in the energy field. We begin with discussions on the latest reports on the applications of high-entropy materials, including alloys, oxides and other entropy-stabilized compounds and composites, in various energy storage and conversion systems. In addition, we describe effective strategies for rationally designing high-entropy materials from computational techniques and experimental aspects. Based on this overview, we subsequently present the fundamental insights and give a summary of their potential advantages and remaining challenges, which will ideally provide researchers with some general guides and principles for the investigation and development of advanced high-entropy materials

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 conductivitycoupled with a negligible electronic contribution. Doped rare earth aluminatesare promising candidates for SOFC electrolytes due to their high ionicconductivity. However, they often suffer from p-type electronic conductivity atoperating temperatures above 500°C under oxidizing conditions caused bythe incorporation of oxygen into the lattice. High entropy materials are a newclass of materials conceptualized to be stable at higher temperatures due totheir high configurational entropy. Introducing this concept to rare earthaluminates can be a promising approach to stabilize the lattice by shifting thestoichiometric point of the oxides to higher oxygen activities, and thereby,reducing the p-type electronic conductivity in the relevant oxygen partialpressure range. In this study, the high entropy oxide (Gd,La,Nd,Pr,Sm)AlO3issynthesized and doped with Ca. The Ca-doped (Gd,La,Nd,Pr,Sm)AlO3compounds exhibit a higher ionic conductivity than most of thecorresponding Ca-doped rare earth aluminates accompanied by a reduction ofthe p-type electronic conductivity contribution typically observed underoxidizing conditions. In light of these findings, this study introduces highentropy aluminates as a promising candidate for SOFC electrolytes

Tailored Silicon/Carbon Compounds for Printed Li–Ion Anodes

Silicon (Si) has turned out to be a promising active material for next-generation lithium-ion battery anodes. Nevertheless, the issues known from Si as electrode material (pulverization effects, volume change etc.) are impeding the development of Si anodes to reach market maturity. In this study, we are investigating a possible application of Si anodes in low-power printed electronic applications. Tailored Si inks are produced and the impact of carbon coating on the printability and their electrochemical behavior as printed Si anodes is investigated. The printed Si anodes contain active material loadings that are practical for powering printed electronic devices, like electrolyte gated transistors, and are able to show high capacity retentions. A capacity of 1754 mAh/g$_{Si}$ is achieved for a printed Si anode after 100 cycles. Additionally, the direct applicability of the printed Si anodes is shown by successfully powering an ink-jet printed transistor

Synthesis of perovskite-type high-entropy oxides as potential candidates for oxygen evolution

High-entropy materials offer a wide range of possibilities for synthesizing new functional ceramics for different applications. Many synthesis methods have been explored to achieve a single-phase structure incorporating several different elements, yet a comparison between the synthesis methods is crucial to identify the new dimension such complex ceramics bring to material properties. As known for ceramic materials, the synthesis procedure usually has a significant influence on powder morphology, elemental distribution, particle size and powder processability. Properties that need to be tailored according to specific applications. Therefore, in this study perovskite-type high-entropy materials (Gd$_{0.2}$La$_{0.2–x}$Sr$_x$Nd$_{0.2}$Sm$_{0.2}$Y$_{0.2}$) (Co$_{0.2}$Cr$_{0.2}$Fe$_{0.2}$Mn$_{0.2}$Ni$_{0.2}$)O$_3$ (x = 0 and x = 0.2) are synthesized for the first time using mechanochemical synthesis and a modified Pechini method. The comparison of different syntheses allows, not only tailoring of the constituent elements of high-entropy materials, but also to optimize the synthesis method as needed to overcome limitations of conventional ceramics. To exploit the novel materials for a variety of energy applications, their catalytic activity for oxygen evolution reaction was characterized. This paves the way for their integration into, e.g., regenerative fuel cells and metal air batteries

High-entropy spinel-structure oxides as oxygen evolution reaction electrocatalyst

High-entropy oxides are an upcoming research topic due to their broad range of possible crystal structures and applications. In this work, we want to present the change in the catalytic properties when using different elements to create a high-entropy spinel. Therefore, we used the nebulized-spray pyrolysis to synthesize the high-entropy spinel (Mn$_{0.2}$Fe$_{0.2}$Ni$_{0.2}$Mg$_{0.2}$Zn$_{0.2}$)$_{3}$O$_{4}$ and later on exchanged the Mg or the Zn with elements with multiple possible oxidation states, in our example each with Cr or Co. The phase purity, morphology, microstructure and homogeneity were investigated by XRD, SEM and STEM-EDX. Their electrocatalytic performance and stability was measured via oxygen evolution reaction and cyclic voltammetry and compared to IrO$_{2}$, used as reference. The best performance of the synthesized materials was achieved by (Mn$_{0.2}$Fe$_{0.2}$Ni$_{0.2}$Mg$_{0.2}$Cr$_{0.2}$)$_{3}$O$_{4}$

Mechanochemical synthesis of novel rutile-type high entropy fluorides for electrocatalysis

Multicomponent rutile (P4$_{2}$/mnm) structured fluorides, containing 4 to 7 transition metals (Co, Cu, Mg, Ni, Zn, Mn, and Fe) in equiatomic ratios, were synthesized using a simple mechanochemical approach. The high entropy fluorides were characterized using different techniques, all of which indicate that the high entropy fluorides tend to crystallize into a homogeneously mixed solid solution and single-phase structure. These high entropy fluorides represent an additional class of high entropy ceramics, which have recently attracted attention especially due to the development of high entropy oxides. With the introduction of these novel high entropy fluorides, similar interest could be generated due to the variety of different applications for fluoride materials and the improvements the high entropy concept might bring. Here we present an in-depth characterization study and the potential application of high entropy fluorides as a catalyst for the oxygen evolution reaction, in which the high entropy fluorides do show increased performance compared to a state-of-the-art catalyst for the oxygen evolution reaction, IrO$_{2}$, despite eliminating noble metal constituents

Mechanochemical synthesis: route to novel rock-salt-structured high-entropy oxides and oxyfluorides

A facile mechanochemical reaction at ambient temperature was successfully applied to synthesize novel single-phase rock-salt-structured high-entropy oxides, containing five, six and seven metal elements in equiatomic amounts. This synthesis approach overcomes the limitations of the commonly known synthesis procedures, which would result in multiple-phase compounds. Redox-sensitive elements, such as Fe$^{2+}$ and Mn$^{2+}$, can now be considered. The corresponding single-phase Li-containing high-entropy oxyfluorides were obtained by introducing LiF into the lattice using the same strategy. All materials show single-phase rock-salt structures with lattice parameters depending on the incorporated ion sizes. Solid solution states result in high configurational entropies, and all elements appear homogenously distributed over the whole cationic and anionic sublattice. The straightforward synthesis technique, combined with utilized simple binary oxide precursors, paves the way for a multitude of novel high-entropy oxide and oxyfluoride compounds. The compounds were studied by means of X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray spectroscopy and Mössbauer spectroscopy

Spinel to Rock-Salt Transformation in High Entropy Oxides with Li Incorporation

High entropy oxides (HEOs) constitute a promising class of materials with possibly new and largely unexplored properties. The virtually infinite variety of compositions (multi-element approach) for a single-phase structure allows the tailoring of their physical properties and enables unprecedented materials design. Nevertheless, this level of versatility renders their characterization as well as the study of specific processes or reaction mechanisms challenging. In the present work, we report the structural and electrochemical behavior of different multi-cationic HEOs. Phase transformation from spinel to rock-salt was observed upon incorporation of monovalent Li+ ions, accompanied by partial oxidation of certain elements in the lattice. This transition was studied by X-ray diffraction, inductively coupled plasma-optical emission spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and attenuated total reflection infrared spectroscopy. In addition, the redox behavior was probed using cyclic voltammetry. Especially, the lithiated rock-salt structure HEOs were found to exhibit potential for usage as negative and positive electrode materials in rechargeable lithium-ion batteries

Synthesis of perovskite-type high-entropy oxides as potential candidates for oxygen evolution

High-entropy materials offer a wide range of possibilities for synthesizing new functional ceramics for different applications. Many synthesis methods have been explored to achieve a single-phase structure incorporating several different elements, yet a comparison between the synthesis methods is crucial to identify the new dimension such complex ceramics bring to material properties. As known for ceramic materials, the synthesis procedure usually has a significant influence on powder morphology, elemental distribution, particle size and powder processability. Properties that need to be tailored according to specific applications. Therefore, in this study perovskite-type high-entropy materials (Gd₀.₂La₀.₂₋ₓ SrₓNd₀.₂Sm₀.₂Y₀.₂) (Co₀.₂Cr₀.₂Fe₀.₂Mn₀.₂Ni₀.₂)O₃ (x = 0 and x = 0.2) are synthesized for the first time using mechanochemical synthesis and a modified Pechini method. The comparison of different syntheses allows, not only tailoring of the constituent elements of high-entropy materials, but also to optimize the synthesis method as needed to overcome limitations of conventional ceramics. To exploit the novel materials for a variety of energy applications, their catalytic activity for oxygen evolution reaction was characterized. This paves the way for their integration into, e.g., regenerative fuel cells and metal air batteries

High-entropy spinel-structure oxides as oxygen evolution reaction electrocatalyst

High-entropy oxides are an upcoming research topic due to their broad range of possible crystal structures and applications. In this work, we want to present the change in the catalytic properties when using different elements to create a high-entropy spinel. Therefore, we used the nebulized-spray pyrolysis to synthesize the high-entropy spinel (Mn₀.₂Fe₀.₂Ni₀.₂Mg₀.₂Zn₀.₂)₃O₄ and later on exchanged the Mg or the Zn with elements with multiple possible oxidation states, in our example each with Cr or Co. The phase purity, morphology, microstructure and homogeneity were investigated by XRD, SEM and STEM-EDX. Their electrocatalytic performance and stability was measured via oxygen evolution reaction and cyclic voltammetry and compared to IrO₂, used as reference. The best performance of the synthesized materials was achieved by (Mn₀.₂Fe₀.₂Ni₀.₂Mg₀.₂Cr₀.₂)₃O₄