A high-entropy manganite in an ordered nanocomposite for long-term application in solid oxide cells

A Self-Assembled Multiphasic Thin Film as an Oxygen Electrode for Enhanced Durability in Reversible Solid Oxide Cells

The implementation of nanocomposite materials as electrode layers represents a potential turning point for next-generation of solid oxide cells in order to reduce the use of critical raw materials. However, the substitution of bulk electrode materials by thin films is still under debate especially due to the uncertainty about their performance and stability under operando conditions, which restricts their use in real applications. In this work, we propose a multiphase nanocomposite characterized by a highly disordered microstructure and high cationic intermixing as a result from thin-film self-assembly of a perovskite-based mixed ionic-electronic conductor (lanthanum strontium cobaltite) and a fluorite-based pure ionic conductor (samarium-doped ceria) as an oxygen electrode for reversible solid oxide cells. Electrochemical characterization shows remarkable oxygen reduction reaction (fuel cell mode) and oxygen evolution activity (electrolysis mode) in comparison with state-of-the-art bulk electrodes, combined with outstanding long-term stability at operational temperatures of 700 °C. The disordered nanostructure was implemented as a standalone oxygen electrode on commercial anode-supported cells, resulting in high electrical output in fuel cell and electrolysis mode for active layer thicknesses of only 200 nm (>95% decrease in critical raw materials with respect to conventional cathodes). The cell was operated for over 300 h in fuel cell mode displaying excellent stability. Our findings unlock the hidden potential of advanced thin-film technologies for obtaining high-performance disordered electrodes based on nanocomposite self-assembly combining long durability and minimized use of critical raw materials.

Exploring the properties of Zr2CO2/GaS van der Waals heterostructures for optoelectronic applications

We investigate the structural, electronic, and optical properties of eight possible Zr2CO2/GaS van der Waals (vdW) heterostructures using first-principles calculations based on a hybrid functional. These structures display favorable stability, indicated by matching crystal structures and negative formation energies. In all considered configurations, these heterostructures act as indirect band gap semiconductors with a type-II band alignment, allowing efficient electron-hole separation. Optical studies reveal their suitability for optoelectronic applications. Zr2CO2/GaS under 4% biaxial compressive strain meets the criteria for photocatalytic water splitting, suggesting their potential for electronic and optoelectronic devices in the visible spectrum. Our findings present prospects for advanced photocatalytic materials and optical devices.

A High-Entropy-Oxides-Based Memristor: Outstanding Resistive Switching Performance and Mechanisms in Atomic Structural Evolution

The application of high-entropy oxide (HEO) has attracted significant attention in recent years owing to their unique structural characteristics, such as excellent electrochemical properties and long-term cycling stability. However, the application of resistive random-access memory (RRAM) has not been extensively studied, and the switching mechanism of HEO-based RRAM has yet to be thoroughly investigated. In this study, HEO (Cr, Mn, Fe, Co, Ni)3 O4 with a spinel structure is epitaxially grown on a Nb:STO conductive substrate, and Pt metal is deposited as the top electrode. After the resistive-switching operation, some regions of the spinel structure are transformed into a rock-salt structure and analyzed using advanced transmission electron microscopy and scanning transmission electron microscopy. From the results of X-ray photoelectron spectroscopy and electron energy loss spectroscopy, only specific elements would change their valence state, which results in excellent resistive-switching properties with a high on/off ratio on the order of 105 , outstanding endurance (>4550 cycles), long retention time (>104 s), and high stability, which suggests that HEO is a promising RRAM material.

Tuning Ionic Conductivity in Fluorite Gd-Doped CeO2-Bixbyite RE2O3 (RE = Y and Sm) Multilayer Thin Films by Controlling Interfacial Strain

Interfacial strain in heteroepitaxial oxide thin films is a powerful tool for discovering properties and recognizing the potential of materials performance. Particularly, facilitating ion conduction by interfacial strain in oxide multilayer thin films has always been seen to be a highly promising route to this goal. However, the effect of interfacial strain on ion transport properties is still controversial due to the difficulty in deconvoluting the strain contribution from other interfacial phenomena, such as space charge effects. Here, we show that interfacial strain can effectively tune the ionic conductivity by successfully growing multilayer thin films composed of an ionic conductor Gd-doped CeO2 (GDC) and an insulator RE2O3 (RE = Y and Sm). In contrast to compressively strained GDC-Y2O3 multilayer films, tensile strained GDC-Sm2O3 multilayer films demonstrate the enhanced ionic conductivity of GDC, which is attributed to the increased concentration of oxygen vacancies. In addition, we demonstrate that increasing the number of interfaces has no impact on the further enhancement of the ionic conductivity in GDC-Sm2O3 multilayer films. Our findings demonstrate the unambiguous role of interfacial strain on ion conduction of oxides and provide insights into the rational design of fast ion conductors through interface engineering.

Overview of Approaches to Increase the Electrochemical Activity of Conventional Perovskite Air Electrodes

The progressive research trends in the development of low-cost, commercially competitive solid oxide fuel cells with reduced operating temperatures are closely linked to the search for new functional materials as well as technologies to improve the properties of established materials traditionally used in high-temperature devices. Significant efforts are being made to improve air electrodes, which significantly contribute to the degradation of cell performance due to low oxygen reduction reaction kinetics at reduced temperatures. The present review summarizes the basic information on the methods to improve the electrochemical performance of conventional air electrodes with perovskite structure, such as lanthanum strontium manganite (LSM) and lanthanum strontium cobaltite ferrite (LSCF), to make them suitable for application in second generation electrochemical cells operating at medium and low temperatures. In addition, the information presented in this review may serve as a background for further implementation of developed electrode modification technologies involving novel, recently investigated electrode materials.

Colossal Ionic Conductivity in Interphase Strain-Engineered Nanocomposite Films

Owing to their wide application in oxide-based electrochemical and energy devices, ion conductors have attracted considerable attention. However, the ionic conductivity of the developed systems is still too low to satisfy the low-temperature application. In this study, by developing the emergent interphase strain engineering method, we achieve a colossal ionic conductivity in SrZrO₃-xMgO nanocomposite films, which is over one order of magnitude higher than that of the currently widely used yttria-stabilized zirconia below 673 K. Atomic-scale electron microscopy studies ascribe this superior ionic conductivity to the periodically well-aligned SrZrO₃ and MgO nanopillars that feature coherent interfaces. Wherein, a tensile strain as large as +1.7% is introduced into SrZrO₃, expanding the c-lattice and distorting the oxygen octahedra to decrease the oxygen migration energy. Combining with theoretical assessments, we clarify the strain-dependent oxygen migration path and energy and unravel the mechanisms for strain-tuned ionic conductivity. This study provides a new scope for the property improvement of wide-range ion conductors by strain engineering.

A High-Entropy Oxide as High-Activity Electrocatalyst for Water Oxidation

High-entropy materials are an emerging pathway in the development of high-activity (electro)catalysts because of the inherent tunability and coexistence of multiple potential active sites, which may lead to earth-abundant catalyst materials for energy-efficient electrochemical energy storage. In this report, we identify how the multication composition in high-entropy perovskite oxides (HEO) contributes to high catalytic activity for the oxygen evolution reaction (OER), i.e., the key kinetically limiting half-reaction in several electrochemical energy conversion technologies, including green hydrogen generation. We compare the activity of the (001) facet of LaCr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2O_3-δ with the parent compounds (single B-site in the ABO₃ perovskite). While the single B-site perovskites roughly follow the expected volcano-type activity trends, the HEO clearly outperforms all of its parent compounds with 17 to 680 times higher currents at a fixed overpotential. As all samples were grown as an epitaxial layer, our results indicate an intrinsic composition-function relationship, avoiding the effects of complex geometries or unknown surface composition. In-depth X-ray photoemission studies reveal a synergistic effect of simultaneous oxidation and reduction of different transition metal cations during the adsorption of reaction intermediates. The surprisingly high OER activity demonstrates that HEOs are a highly attractive, earth-abundant material class for high-activity OER electrocatalysts, possibly allowing the activity to be fine-tuned beyond the scaling limits of mono- or bimetallic oxides.

Room-Temperature Ferroelectricity of Paraelectric Oxides Tailored by Nano-Engineering

Inducing clear ferroelectricity in the quantum paraelectric SrTiO₃ is important for triggering methods to discover hidden phases in condensed matter physics. Several methods such as isotope substitution and freestanding membranes could introduce ferroelectricity in SrTiO₃ toward nonvolatile memory applications. However, the stable transformation from quantum paraelectric SrTiO₃ to ferroelectricity SrTiO₃ at room temperature still remains challenging. Here, we used multiple nano-engineering in (SrTiO₃)_0.65/(CeO₂)_0.35 films to achieve an emergent room-temperature ferroelectricity. It is shown that the CeO₂ nanocolumns impose large out-of-plane strains and induce Sr/O deficiency in the SrTiO₃ matrix to form a clear tetragonal structure, which leads to an apparent room-temperature ferroelectric polarization up to 2.5 μC/cm². In collaboration with density functional theory calculations, it is proposed that the compressive strains combined with elemental deficiency give rise to local redistribution of charge density and orbital order, which induce emergent tetragonality of the strained SrTiO₃. Our work thus paves a pathway for architecting functional systems in perovskite oxides using a multiple nano-design.

High-Entropy Materials in SOFC Technology: Theoretical Foundations for Their Creation, Features of Synthesis, and Recent Achievements

In this review, recent achievements in the application of high-entropy alloys (HEAs) and high-entropy oxides (HEOs) in the technology of solid oxide fuel cells (SOFC) are discussed for the first time. The mechanisms of the stabilization of a high-entropy state in such materials, as well as the effect of structural and charge factors on the stability of the resulting homogeneous solid solution are performed. An introduction to the synthesis methods for HEAs and HEOs is given. The review highlights such advantages of high-entropy materials as high strength and the sluggish diffusion of components, which are promising for the use at the elevated temperatures, which are characteristic of SOFCs. Application of the medium- and high-entropy materials in the hydrocarbon-fueled SOFCs as protective layers for interconnectors and as anode components, caused by their high stability, are covered. High-entropy solid electrolytes are discussed in comparison with traditional electrolyte materials in terms of conductivity. High-entropy oxides are considered as prospective cathodes for SOFCs due to their superior electrochemical activity and long-term stability compared with the conventional perovskites. The present review also determines the prioritizing directions in the future development of high-entropy materials as electrolytes and electrodes for SOFCs operating in the intermediate and low temperature ranges.

Nanostructured La0.75Sr0.25Cr0.5Mn0.5O3-Ce0.8Sm0.2O2 Heterointerfaces as All-Ceramic Functional Layers for Solid Oxide Fuel Cell Applications

The use of nanostructured interfaces and advanced functional materials opens up a new playground in the field of solid oxide fuel cells. In this work, we present two all-ceramic thin-film heterostructures based on samarium-doped ceria and lanthanum strontium chromite manganite as promising functional layers for electrode application. The films were fabricated by pulsed laser deposition as bilayers or self-assembled intermixed nanocomposites. The microstructural characterization confirmed the formation of dense, well-differentiated, phases and highlighted the presence of strong cation intermixing in the case of the nanocomposite. The electrochemical properties─solid/gas reactivity and in-plane conductivity─are strongly improved for both heterostructures with respect to the single-phase constituents under anodic conditions (up to fivefold decrease of area-specific resistance and 3 orders of magnitude increase of in-plane conductivity with respect to reference single-phase materials). A remarkable electrochemical activity was also observed for the nanocomposite under an oxidizing atmosphere, with no significant decrease in performance after 400 h of thermal aging. This work shows how the implementation of nanostructuring strategies not only can be used to tune the properties of functional films but also results in a synergistic enhancement of the electrochemical performance, surpassing the parent materials and opening the field for the fabrication of high-performance nanostructured functional layers for application in solid oxide fuel cells and symmetric systems.

Nanoscaled LiMn2O4 for Extended Cycling Stability in the 3 V Plateau

Extending the potential window toward the 3 V plateau below the typically used range could boost the effective capacity of LiMn2O4 spinel cathodes. This usually leads to an "overdischarge" of the cathode, which can cause severe material damage due to manganese dissolution into the electrolyte and a critical volume expansion (induced by Jahn-Teller distortions). As those factors determine the stability and cycling lifetime for all-solid-state batteries, the operational window of LiMn2O4 is usually limited to 3.5-4.5 V versus Li/Li+ in common battery cells. However, it has been reported that nano-shaped particles and thin films can potentially mitigate these detrimental effects. We demonstrate here that porous LiMn2O4 thin-film cathodes with a certain level of off-stoichiometry show improved cycling stability for the extended cycling range of 2.0-4.5 V versus Li/Li+. We argue through operando spectroscopic ellipsometry that the origin of this stability lies in the surprisingly small volume change in the layer during lithiation.

Can Empirical Biplots Predict High Entropy Oxide Phases?

High entropy oxides are entropy-stabilised oxides that adopt specific disordered structures due to entropy stabilisation. They are a new class of materials that utilises the high-entropy concept first discovered in metallic alloys. They can have interesting properties due to the interactions at the electronic level and can be combined with other materials to make composite structures. The design of new meta-materials that utilise this concept to solve real-world problems may be a possibility but further understanding of how their phase stabilisation is required. In this work, biplots of the composition’s mean electronegativity are plotted against the electron-per-atom ratio of the compounds. The test dataset accuracy in the resulting biplots improves from 78% to 100% when using atomic-number-per-atom Z/a ratios as a biplot parameter. Phase stability maps were constructed using a Voronoi tessellation. This can be of use in determining stability at composite material interfaces.