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The recent discovery of nickelate superconductivity represents an important step toward understanding the four‐decade mastery of unconventional high‐temperature superconductivity. However, the synthesis of the infinite‐layer nickelate superconductors shows great challenges. Particularly, surface capping layers are usually unitized to facilitate the sample synthesis. This leads to an important question whether nickelate superconductors with d9 configuration and ultralow valence of Ni1+ are in metastable state and whether nickelate superconductivity can be robust? In this work, a series of redox cycling experiments are performed across the phase transition between perovskite Nd0.8Sr0.2NiO3 and infinite‐layer Nd0.8Sr0.2NiO2. The infinite‐layer Nd0.8Sr0.2NiO2 is quite robust in the redox environment and can survive the cycling experiments with unchanged crystallographic quality. However, as the cycling number goes on, the perovskite Nd0.8Sr0.2NiO3 shows structural degradation, suggesting stability of nickelate superconductivity is not restricted by the ultralow valence of Ni1+, but by the quality of its perovskite precursor. The observed robustness of infinite‐layer Nd0.8Sr0.2NiO2 up to ten redox cycles further indicates that if an ideal high‐quality perovskite precursor can be obtained, infinite‐layer nickelate superconductivity can be very stable and sustainable under environmental conditions. This work provides important implications for potential device applications for nickelate superconductors. [ABSTRACT FROM AUTHOR]

Many electronic and electrochemical devices rely on the exchange of light elements such as hydrogen and oxygen with the environment. Understanding and tailoring the device functionality requires accurate information about the concentration and chemical bonding of such species inside a solid, which is particularly difficult if several species are exchanged. In LaNiO3 thin films in situ transport experiments reveal a re‐entrant metal–insulator transition upon hydrogen exposure. The origin of this unusual behavior can be understood by combining information about the stoichiometry and chemical bonding of hydrogen and oxygen as determined by neutron reflectometry and x‐ray absorption spectroscopy, respectively. In addition to the metallic parent phase, an insulating phase with composition LaNiO2.65 and a re‐entrant metallic phase with composition LaNiO2.15(OH)0.5 are identified. They can be inter‐converted by redox reactions in different external environments. The methodology employed offers new insights into the mechanisms underlying the influence of hydrogen in functional devices. [ABSTRACT FROM AUTHOR]

This paper focuses on long‐term tests held during 1,000 h on different Membrane‐Electrode‐Interconnect Assemblies (MEIAs), consisting of an Fe‐Ni alloy interconnect and an electrochemical cell based on a Ce0.8Sm0.2O2–δ electrolyte, the double‐layer working electrode with a La2NiO4 + δ – Ce0.8Sm0.2O2–δ composite functional layer, and a LaNi0.6Fe0.4O3–δ current collector layer, and a platinum reference electrode (O2, LaNi0.6Fe0.4O3–δ|La2NiO4 + δ – Ce0.8Sm0.2O2–δ|Ce0.8Sm0.2O2–δ|Pt, O2). These tests were carried out on MEIAs with the Cr‐free 47ND alloy of the Fe–Ni system with and without surface modification at 850°C in air. The electrochemical performance of MEIAs was studied without polarization as well as under cathodic and anodic polarization with current density 0.5 A cm−2 by means of electrochemical impedance spectroscopy (EIS). The mechanism of the MEIA evolution during long‐term test is discussed. [ABSTRACT FROM AUTHOR]

The oxygen evolution reaction (OER) is a crucial process in electrochemical water splitting, a promising technology to renewably yield hydrogen gas from water. Designing and developing earth‐abundant, efficient, and stable OER electrocatalysts to replace the most widely used but scarce RuO2 and IrO2 are thus of critical interest. Recently, ABO3‐structured perovskite oxides, especially rare‐earth nickelates, are extensively studied for their potential use as OER electrocatalysts. In particular, the epitaxial synthesis of complex oxide thin films allows flexible and precise control over the materials so that their structure–stability–property relationships can be established. Using nickelate thin films as model systems, this review illustrates how epitaxial design allows researchers to test different hypotheses and proposed descriptors, as well as formulate new design principles. Following a brief introduction to the background of OER mechanisms, proposed activity descriptors, and synthesis methods, various epitaxial design strategies are surveyed including strain tuning, composition control, surface termination/orientation selection, defect engineering, and interface design. These have led to precise control over the atomic structures and electronic properties of nickelates which in turn determine their electrochemical performance. Finally, the remaining challenges and perspectives toward a deeper understanding and use of complex oxides as OER catalysts are discussed. [ABSTRACT FROM AUTHOR]

Although the d‐band correlated rare‐earth nickelates (ReNiO3) exhibit broadly adjustable metal to insulator transitions (MIT) that enables emerging correlated electronic applications, it is yet difficult to regulate their associated critical temperature (TMIT) below 100 K. Herein, we extend the lower limit in TMIT of ReNiO3 down to 83 K while maintaining an abrupt switch in resistivity via partial La‐substitution of PrNiO3. The near edge X‐ray fine structure analysis and density function theory calculations indicate the strengthening in the metallic orbital configuration and reduction in the ground state band gap via La‐substitution of Re in NdNiO3 and PrNiO3. In contrast, analogous Ce substitution cannot reduce the TMIT of ReNiO3 owing to its valance variability toward +4, while La partial substitution of ReNiO3 with middle or heavy rare‐earth (e.g., Sm) easily disturb the co‐occupation of the Re‐site by the two rare‐earths as one dispersive phase. [ABSTRACT FROM AUTHOR]

Fundamental understanding and control of the electronic structure evolution in rare‐earth nickelates is a fascinating and meaningful issue, as well as being helpful to understand the mechanism of recently discovered superconductivity. Here the dimensionality effect on the ground electronic state in high‐quality (NdNiO3) m/(SrTiO3)1 superlattices is systematically studied through transport and soft X‐ray absorption spectroscopy. The metal‐to‐insulator transition temperature decreases with the thickness of the NdNiO3 slab decreasing from bulk to 7 unit cells, then increases gradually as m further reduces to 1 unit cell. Spectral evidence demonstrates that the stabilization of insulating phase can be attributed to the increase of the charge‐transfer energy between O 2p and Ni 3d bands. The prominent multiplet feature on the Ni L3 edge develops with the decrease of NdNiO3 slab thickness, suggesting the strengthening of the charge disproportionate state under the dimensional confinement. This work provides convincing evidence that dimensionality is an effective knob to modulate the charge‐transfer energy and thus the collective ground state in nickelates. [ABSTRACT FROM AUTHOR]

In this work, fundamental material properties of compounds in the system (La,Pr)2(Ni,Co)O4+δ as well as their performance as air electrodes in solid oxide electrolysis cells were investigated. Nickelates co‐doped with Pr and Co were characterized on a material basis by means of X‐ray diffraction and thermogravimetry. Conductivity and conductivity relaxation measurements were performed in order to obtain the electronic conductivity as well as the chemical surface exchange coefficient and the chemical diffusion coefficient of oxygen as a function of temperature and oxygen partial pressure. These parameters can be regarded as the most essential properties at the material level required to assess the suitability of mixed ionic‐electronic conducting ceramics for application as air electrode in solid oxide cells. The electrode performance of the materials was then tested on fuel electrode‐supported button cells at 800°C. The electrodes were applied by screen‐printing and the effect of varying the Pr‐content and Co‐content of the electrode powder was investigated. Cell tests were performed by means of current‐voltage measurements in electrolysis mode. While no significant impact of Pr‐doping on the investigated material properties was observed, the electrode performance of Pr‐containing materials was significantly better than for the Pr‐free compound, which has been discussed in detail. [ABSTRACT FROM AUTHOR]

Summary: This paper focuses on the long‐term tests for 1000 hours of different Membrane‐Electrode‐Interconnect Assemblies (MEIAs), consisting of a steel interconnect and an asymmetrical solid electrochemical cell based on the Ce0.8Sm0.2O2−δ electrolyte, the double‐layer working electrode with the La2NiO4+δ – Ce0.8Sm0.2O2−δ composite functional layer and the LaNi0.6Fe0.4O3−δ current collector layer, and the platinum reference electrode (O2, LaNi0.6Fe0.4O3−δ|La2NiO4+δ – Ce0.8Sm0.2O2−δ|Ce0.8Sm0.2O2−δ|Pt, O2). These tests were carried out on MEIAs with two types of Cr‐based stainless steels, Crofer 22 APU and 08X17T, with and without protective coatings at 850°C in air. The electrochemical performance of MEIAs was studied without polarization as well as under cathodic and anodic polarization (current density was 0.5 A cm−2) by means of electrochemical impedance spectroscopy (EIS) and the distribution of relaxation times method (DRT). It was found that the proposed protective coatings for the stainless steels inhibit chromium poisoning without polarization and under anodic polarization: for Crofer 22 APU the degradation of the polarization resistance after ~1100 hours of exposure was 0.35 ± 0.05 Ω cm2, and for 08X17T it was from 0.40 ± 0.05 Ω cm2. In the case of cathodic polarization, the degradation of the polarization resistance remained at a very high level, comparable to uncoated samples: for Crofer 22 APU and 08X17T with coatings, it was about 0.6 to 0.8 Ω cm2 after ~1100 hours. Post‐test analysis of the microstructure and chemical composition of the interconnect ‐ double‐layer electrode interface was carried out using scanning electron microscopy (SEM), wavelength dispersive X‐ray spectrometry (WDS), and respective image analysis of the obtained data. The influence of chromium poisoning on the degradation was considered for different stages of the electrode performance. It was demonstrated that chromium predominantly accumulates in the current collector layer in the case of anodic polarization and in the functional layer in the cases of open circuit and cathodic polarization. For the first time, a combined electrochemical‐chemical degradation mechanism of the La2NiO4+δ – Ce0.8Sm0.2O2−δ functional layer was discussed. Highlights: Long‐term tests of Membrane‐Electrode‐Inteconnect Assembles (MEIAs) were done.Chromium poisoning was detected on MEIAs with the interconnects without coating.Cathodic polarization terminates the protective properties of the coatings.Chromium poisoning decreases electrode activity to oxygen exchange and diffusivity.A combined electrochemical‐chemical degradation mechanism of the functional layer was revealed. [ABSTRACT FROM AUTHOR]

Establishing condensed matters at their thermodynamically metastable or unstable structures demonstrates merit in the adjustability of their electronic structures, benefiting the discovery of emerging new material functionalities and applications. Herein, a molten‐salt assisted heterogeneous nucleation approach is demonstrated to significantly improve the effectiveness in batch synthesis of metastable perovskites correlated oxides, such as rare‐earth nickelates (ReNiO3) with various rare‐earth compositions. In contrast to their conventional synthesis via solid state reactions, herein the metastable ReNiO3 is heterogeneously precipitated together with potassium chloride (KCl) within the liquid phase of KCl molten‐salt that effectively dissolves the Ni‐/Re‐ precursors and largely enhances their reaction homogeneity. With this solid base overcoming their synthesis metastabilities, the beyond conventional electronic transportation of ReNiO3 under high pressure is explored. It breaks the conventional thermodynamic equilibrium and triggers the formation of new electronic structures associated with unstable insulating and metallic SmNiO3 beyond already known manners. The unstable insulating SmNiO3 exhibits nontemperature‐dependent transportation character similar to saturation or bad metals but preserves 2–3 orders higher electronic conductivity, while a kinetic related hysteresis is observed in the temperature‐dependent transportations of the unstable metallic SmNiO3. These discoveries with nonequilibrium correlated materials are worthy to be explored further. [ABSTRACT FROM AUTHOR]

Lanthanum nickelates with Ruddlesden‐Popper structure (La2NiO4, La3Ni2O7, and La4Ni3O10) and perovskite structure (LaNiO3) have attracted considerable attention due to their potential applications such as solid oxide fuel cells. Currently, the ionic and electronic conduction properties of La3Ni2O7, La4Ni3O10, and LaNiO3 are not fully understood because it is quite difficult to prepare their dense bodies required for the characterization. The difficulty arises from their narrow thermodynamic stable temperature and oxygen partial pressure ranges. In this study, we successfully obtained dense bodies of single‐phase La3Ni2O7, La4Ni3O10, and LaNiO3 via a post‐sintering oxidation process. First, dense pellets composed of fine‐grain precursors La2NiO4 and NiO (~0.5 μm) were prepared by nitrate freeze‐drying technique and low‐temperature sintering at 1150°C‐1225°C. Then they were converted into almost single‐phase La3Ni2O7, La4Ni3O10, and LaNiO3 by high‐temperature oxidation. La3Ni2O7 and La4Ni3O10 were obtained under an oxygen partial pressure pO2 of 1 bar at 1275°C and 1200°C‐1250°C, respectively, while LaNiO3 was obtained under pO2 of 392 bar at 1250°C using hot isostatic pressing. The relative densities of the pellets exceeded 90%. With regard to their phase stability, decomposition was not detected at 600°C‐1100°C in air for at least 100 hour despite their thermodynamic instability. [ABSTRACT FROM AUTHOR]

After decades of research, superconductivity is finally found in nickel‐based analogs of superconducting cuprates, with infinite‐layer (IL) structure. These results are so far restricted to thin films in the case of IL‐nickelates. Therefore, the nature of the interface with the substrate, and how it couples with the thin film properties is still an open question. Here, using scanning transmission electron microscopy (STEM)‐ electron energy loss spectroscopy (EELS), a novel p‐type interface defined by SrO termination with the SrTiO3 substrate is observed in superconducting (SC) IL‐praseodymium nickelate samples. Its interfacial charge and polarity are compared with the previously reported n‐type interface characterized by TiO2 termination. In combination with ab‐initio calculations, it is found that the influence of the interface on the electronic structure is local and does not extend beyond 2–3 unit cells into the thin film. This decouples the direct influence of the interface in driving the superconductivity, and indicates that the IL‐nickelate thin films do not have a universal interface model. Insights into the spatial hole‐distribution in SC samples, provided by monochromated EELS and total reflection‐hard X‐ray photoemission spectroscopy, suggest that this particular distribution might be directly influencing superconductivity. [ABSTRACT FROM AUTHOR]

High‐temperature characterization of oxygen‐deficient Sr‐rich (La1–xSrx)2NiO4–δ (x = 0.5–0.8) solid solutions under mildly reducing conditions with p(O2) ≈ 5 × 10–5 atm was performed by employing structural and thermal analysis, TEM, and electrical conductivity measurements. Oxygen losses from the crystal lattice on reduction were found to result in a reversible transition from the tetragonal (I4/mmm) to the orthorhombic (Immm) structure and shrinkage of the crystal lattice for the compositions with x > 0.5. TEM and thermogravimetric analysis evidenced slow kinetics of the structural transition. The increase in oxygen deficiency under reducing conditions is accompanied by localization of the electronic charge carriers, a drop of the p‐type electronic conductivity, and a transition from metallic‐like to semiconducting behavior. The extent of changes in oxygen nonstoichiometry, unit‐cell dimensions, average Ni oxidation state, electron–hole concentration, and electronic conductivity on reduction is interrelated with the strontium content. The results suggest that the electrical conductivity of (La1–xSrx)2NiO4–δ ceramics depends mainly on the average Ni oxidation state. [ABSTRACT FROM AUTHOR]

Infinite layer (IL) nickelates provide a new route beyond copper oxides to address outstanding questions in the field of unconventional superconductivity. However, their synthesis poses considerable challenges, largely hindering experimental research on this new class of oxide superconductors. That synthesis is achieved in a two‐step process that yields the most thermodynamically stable perovskite phase first, then the IL phase by topotactic reduction, the quality of the starting phase playing a crucial role. Here, a reliable synthesis of superconducting IL nickelate films is reported after successive topochemical reductions of a parent perovskite phase with nearly optimal stoichiometry. Careful analysis of the transport properties of the incompletely reduced films reveals an improvement in the strange metal behavior of their normal state resistivity over subsequent topochemical reductions, offering insight into the reduction process. [ABSTRACT FROM AUTHOR]

The rich phase diagrams and peculiar physical properties of rare earth perovskite nickelates (RNiO3) have recently attracted much attention. Their electronic structures are highly sensitive to carrier density and bandwidth due to Mott physics. Here, the electrochemically driven giant resistive switching in Pt/RNiO3/Nb-SrTiO3 heterostructures is reported. Systematic investigation confirms that oxygen vacancies migration modifies the interfacial barrier at the RNiO3/Nb-SrTiO3 interface and causes the resistive switching behavior. An ON/OFF ratio of about 105 at room temperature is observed, which can be modulated by controlling the oxygen vacancies during sample fabrication or by varying the rare earth element in RNiO3. The findings provide an important step forward toward the development of multifunctional electronic devices based on perovskite nickelates. [ABSTRACT FROM AUTHOR]

Recently, superconductivity at high temperatures is observed in bulk La3Ni2O7−δ under high pressure. However, the attainment of high‐purity La3Ni2O7−δ single crystals remains a formidable challenge. Here, the crystal structure and physical properties of single crystals of Sr‐doped La3Ni2O7 synthesized at high pressure (20 GPa) and high temperature (1400 °C) are reported. Through single crystal X‐ray diffraction, it is shown that high‐pressure‐synthesized paramagnetic Sr‐doped La3Ni2O7 crystallizes in an orthorhombic structure with Ni─O─Ni bond angles of 173.4(2)° out‐of‐plane and 175.0(2)°and 176.7(2)°in plane. The substitution of Sr alters in band filling and the ratio of Ni2+/Ni3+ in Sr‐doped La3Ni2O7, aligning them with those of "La3Ni2O7.05", thereby leading to significant modifications in properties under high pressure relative to the unsubstituted parent phase. At ambient pressure, Sr‐doped La3Ni2O7 exhibits insulating properties, and the conductivity increases as pressure goes up to 10 GPa. However, upon further increasing pressure beyond 10.7 GPa, Sr‐doped La3Ni2O7 transits back from a metal‐like behavior to an insulator. The insulator–metal–insulator trend under high pressure dramatically differs from the behavior of the parent compound La3Ni2O7−δ, despite their similar behavior in the low‐pressure regime. These experimental results underscore the considerable challenge in achieving superconductivity in nickelates. [ABSTRACT FROM AUTHOR]

The discovery of superconductivity in hole‐doped infinite‐layer nickelates has fueled intense research to identify the critical factor responsible for high‐Tc superconductivity. Magnetism and superconductivity are closely entangled, and elucidating the magnetic interactions in hole‐doped nickelates is critical for understanding the pairing mechanism. Here, these calculations based on the generalized Bloch theorem (GBT) and magnetic force theorem (MFT) consistently reveal that hole doping stabilizes an incommensurate (IC) spin state and increases the IC wave vector continuously, in a way strikingly similar to hole‐doped cuprates. Going further, a nonlinear Heisenberg model including first‐neighbor and third‐neighbor in‐plane magnetic interactions is developed. The analytical solutions successfully reproduce GBT and MFT results and reveal that the competition between the two magnetic interactions is the decisive factor for the IC magnetic transition. Eventually, by analyzing the doping‐controlled spin splitting of dx2−y2${d}_{{x}^2 - {y}^2}$ band and orbital‐contributed exchange interactions, direct links between hole doping, magnetization, exchange constants, and magnetic order are established. This discovery of the IC spin state, new understanding of its electronic origin, and establishment of direct connection with the paring dx2−y2${d}_{{x}^2 - {y}^2}$ electrons radically change the current understanding of the magnetic properties in hole‐doped NdNiO2 and open new perspectives for the superconducting mechanism in nickelates superconductors. [ABSTRACT FROM AUTHOR]

While the multiple Mottronic and electronic phase transitions as recently discovered in nickelates (e.g., ReNiO3) open up a new paradigm in correlated electronic applications, these applications are largely impeded by the intrinsic material metastability of the perovskite nickelates. Herein, the study demonstrates the analogous multiple electronic phase transition properties in the thermodynamically stable NiO, compared to ReNiO3, from both perspectives of band gap regulation and orbital filling regulation. The adjustment in band gap of NiO with t2g6eg2 orbital configuration is achieved via establishing biaxial tensile or compressive interfacial strains that increase or reduce the material resistivity, respectively. The relaxor ferroelectricity of 0.7Pb(Mg2/3Nb1/3)O3‐0.3PbTiO3 (PMNPT) further enables an electric field adjustable resistance switch (ΔR/R) within NiO/PMNPT heterostructure with higher performances (e.g., ΔR/R of 82% upon a bias voltage of 20 V) than the reported oxides/PMNPT heterostructure. Furthermore, the magnitude in resistance switch of the tensile strained NiO via hydrogenation associated Mottronic process reaches ≈1011 that exceeds the previously reported ones. This study highlights the higher material stability and easier growth of NiO, compared to ReNiO3, with analogous multiple Mottronic and electronic phase transition properties that pave the way to its practical applications in correlated electronics. [ABSTRACT FROM AUTHOR]

The Ni‐catalysed cross‐coupling of aryl ethers is a powerful method to forge new C−C and C−heteroatom bonds. However, the inert C(sp2)−O bond means that a canonical mechanism that relies on the oxidative addition of the aryl ether to a Ni0 centre is thermodynamically and kinetically unfavourable, which suggests that alternative mechanisms may be involved. Here, we provide spectroscopic and structural insights into the anionic pathway, which relies on the formation of electron‐rich hetero‐bimetallic nickelates by adding organometallic nucleophiles to a Ni0 centre. Assessing the rich co‐complexation chemistry between Ni(COD)2 and PhLi has led to the structures and solution‐state chemistry of a diverse family of catalytically competent lithium nickelates being unveiled. In addition, we demonstrate dramatic solvent and donor effects, which suggest that the cooperative activation of the aryl ether substrate by Ni0‐ate complexes plays a key role in the catalytic cycle. [ABSTRACT FROM AUTHOR]

Mixed ionic electronic conductors (MIECs) oxides are used as electrode materials for solid oxide cell (SOC) application, as they combine high electronic conductivity as well as high oxygen diffusivity and oxygen surface exchange coefficients. The ionic transport properties can be directly determined thanks to the isotopic exchange depth profiling (IEDP) method. To date, the reported measurements have been performed at ambient pressure and below. However, for a higher efficiency of hydrogen production at the system level, it is envisaged to operate the cell between 10 and 60 bar. To characterize the MIEC oxides properties in such conditions, an innovative setup able to operate up to a total pressure of 50 bar and 900°C has been developed. The main goal of this study was to compare the behavior of two types of reference materials: the oxygen deficient La‐Sr‐Fe‐Co perovskites, and the overstoichiometric lanthanide nickelates Ln2NiO4+δ (Ln = La, Pr, Nd). Diffusion and surface exchange coefficients obtained under 6.3 bar of oxygen are measured and their evolution discussed in light of the change in oxygen stoichiometries. This analysis allows better understanding of the dependency of the surface exchange coefficient with the oxygen partial pressure. [ABSTRACT FROM AUTHOR]

Perovskite‐type rare earth nickelates based memristor have recently attracted extensive attention in the field of novel storage computing due to their special electronic structure and exotic physical properties. However, there is still a shortage of memristors with ultra‐high stability performance, which will provide a solid foundation for future neural network computing with high accuracy recognition rates. Here, a GdNiO3‐based interfacial memristor is presented, which possesses ultra‐high stable performance, such as electroforming‐free, low device‐to‐device variation, reliable cyclic switching, high on/off ratio (≈104) and stable pulse modulation of conduction. Combined with the comprehensive microstructure results, this behavior is ascribed to the interface Schottky barrier variation caused by the 1D oxygen vacancy channel conduction according to the transmission electron microscopy results. In particular, based on the device's stable pulse modulation plasticity performance, the study also succeeds in achieving highly accurate neural firing pattern recognition up to ≈99.75% accuracy and monitoring of pattern transitions by implementing a reservoir computing system based on the device. This research advances the progress of nickelates in novel storage computing and paves the way for future efficient memristor‐based reservoir computing systems to handle more complex temporal tasks. [ABSTRACT FROM AUTHOR]

In rare earth nickelates, the metal‐to‐insulator transition observed as a function of temperature can be described using an electronic and a structural order parameter. The electronic one is linked to the electronic disproportionation observed below the transition temperature and the structural one characterizes the breathing mode that develops in the low temperature phase. Here SmNiO3/NdNiO3 superlattices are grown with insulating LaAlO3 spacer layers to study the unusual coupling of metal‐to‐ insulator transitions observed at SmNiO3/NdNiO3 interfaces and determine the role of the two order parameters. Temperature‐dependent transport measurements reveal that a single unit cell of LaAlO3 inserted between SmNiO3 and NdNiO3 leads to a complete decoupling of the metal‐to‐insulator transitions suggesting that the order parameter controlling the coupling is the electronic one. [ABSTRACT FROM AUTHOR]

Advancing the understanding of lithum nickelate complexes, here we report a family of homoleptic organonickelate complexes obtained by reacting Ni(COD)2 and lithium aryl‐acetylides in the presence of the bidentate donor TMEDA. These compounds represent rare examples of low‐valent transition‐metals supported solely by organolithium ligands. Whilst the solid‐state structures indicate a hexagonal planar geometry around Ni0 with Ni−Li bonds, bonding analysis via QTAIM, NCI, NBO and ELI methods reveals that the Ni−Li interactions are repulsive in nature, characterising these complexes as tri‐coordinated. London dispersion forces between TMEDA and the organic substituents on nickel are found to play a crucial role in the stabilisation and thus isolation of these complexes. Preliminary reactivity studies demonstrate that the homoleptic lithium nickelates undergo stoichiometric cross‐coupling with PhI to give dinickel clusters containing both anionic acetylide and neutral alkyne ligands. [ABSTRACT FROM AUTHOR]

Resistive switching effects offer new opportunities in the field of conventional memories as well as in the booming area of neuromorphic computing. Here the authors demonstrate memristive switching effects produced by a redox‐driven oxygen exchange in tunnel junctions based on NdNiO3, a strongly correlated electron system characterized by the presence of a metal‐to‐insulator transition (MIT). Strikingly, a strong interplay exists between the MIT and the redox mechanism, which on the one hand modifies the MIT itself, and on the other hand radically affects the tunnel resistance switching and the resistance states' lifetime. That results in a very unique temperature behavior and endows the junctions with multiple degrees of freedom. The obtained results bring up fundamental questions on the interplay between electronic correlations and the creation and mobility of oxygen vacancies in nickelates, opening a new avenue toward mimicking neuromorphic functions by exploiting the electric‐field control of correlated states. [ABSTRACT FROM AUTHOR]

Topotactic transition is a structural phase change in a matrix crystal lattice mediated by the ordered loss/gain and rearrangement of atoms, leading to unusual coordination environments and metal atoms with rare valent states. As early as in 1990s, low temperature hydride reduction is utilized to realize the topotactic transition. Since then, topological transformations are developed via multiple approaches. Especially, the recent discovery of the Ni‐based superconductivity in infinite‐layer nickelates has greatly boosted the topotactic transition mean to synthesizing new oxides for exploring exotic functional properties. In this review, a detailed and generalized introduction is provided to oxygen‐related topotactic transition. The main body of the review includes four parts: the structure‐facilitated effects, the mechanism of the topotactic transition, some examples of topotactic transition methods adopted in different metal oxides (V, Mn, Fe, Co, Ni) and the related applications. This study is to provide timely and thorough strategies to successfully realize topotactic transitions for researchers who are eager to create new oxide phases or new oxide materials with desired functions. [ABSTRACT FROM AUTHOR]

Oxide interfaces provide a fertile ground for new forms of magnetic textures that arise from different symmetry constraints, proximity effects, and charge transfer. Here, a room‐temperature spin glass state stabilized in heterostructures of the ferromagnetic metallic oxide La2/3Sr1/3MnO3 (LSMO) and the layered Lan+1NinO3n+1 (L‐LNO) is reported. Electron energy loss spectra and depth‐profile X‐ray photoelectron spectroscopy reveal the interfacial electron transfer from Mn3+ to localized Ni3+ states. Density functional theory calculations indicate that charge transfer drives the enhancement of ferromagnetic exchange interaction in the nickelate, leading to the formation of local magnetic order. The room temperature spin‐glass state in this artificially engineered LSMO/L‐LNO bilayer structure affords opportunities for designing emergent topological spin textures and spintronic devices. [ABSTRACT FROM AUTHOR]

Perovskite oxides have been explored as promising electrocatalysts for the oxygen evolution reaction (OER), while a lack of understanding of key factors impacting the catalytic activity restricts their further design and development. Here, for the first time, the contributions of oxygen vacancy (VO) and orbital occupancy of B‐site cations to the catalytic activity of NdNiO3 films are systematically investigated. It is found that OER activity follows a typical volcano‐shaped dependence on the oxygen pressure. In the range of 0.2–10 Pa, proper concentration of VO can provide a moderate bonding strength with intermediate hydroxyl OH* and the increased ratio of Ni3+/Ni2+ provides a more favorable occupancy of eg orbital for the catalytic activity; while in the range of 10–60 Pa, insufficient concentration of VO leads to an enhanced strength of hybridization between Ni 3d and O 2p band and thus deteriorated catalytic activity. The superior OER catalytic performance can be only achieved with both appropriate concentration of VO and the ratio of B‐site metal cations with different valences. [ABSTRACT FROM AUTHOR]

The adoption of green hydrogen economy is an indispensable necessity in the current global scenario of environment and energy security. In this endeavor, ammonia is poised to play a key vector of hydrogen to mitigate the challenges arising from transportation, storage and safety. Besides containing a high volumetric and gravimetric hydrogen density, NH3 decomposition into H2 for onsite utilization as a distributed energy source is devoid of greenhouse gases production. In this endeavor, significant technological advancements have been made for in situ production of H2 from NH3 decomposition and the use of NH3 in fuel cell devices to produce electricity. The ammonia decomposition methods to produce H2 mainly involve thermocatalytic, oxidative, electrocatalytic and photocatalytic, among which the catalyst assisted thermal cracking of NH3 has been widely investigated. The research progress in electrolysis of NH3 has been notable in the last couple of years and provides a low‐cost alternative to produce H2 at room temperature. In the area of device development, solid oxide fuel cells (SOFC) have witnessed rapid development in the performances and stability, as ammonia is completely decomposed into H2 and N2 at high operating temperature above ~700 °C. [ABSTRACT FROM AUTHOR]

Ni–Fe‐based oxides are among the most promising catalysts developed to date for the bottleneck oxygen evolution reaction (OER) in water electrolysis. However, understanding and mastering the synergy of Ni and Fe remain challenging. Herein, we report that the synergy between Ni and Fe can be tailored by crystal dimensionality of Ni, Fe‐contained Ruddlesden–Popper (RP)‐type perovskites (La0.125Sr0.875)n+1(Ni0.25Fe0.75)nO3n+1 (n = 1, 2, 3), where the material with n = 3 shows the best OER performance in alkaline media. Soft X‐ray absorption spectroscopy spectra before and after OER reveal that the material with n = 3 shows enhanced Ni/Fe–O covalency to boost the electron transfer as compared to those with n = 1 and n = 2. Further experimental investigations demonstrate that the Fe ion is the active site and the Ni ion is the stable site in this system, where such unique synergy reaches the optimum at n = 3. Besides, as n increases, the proportion of unstable rock‐salt layers accordingly decreases and the leaching of ions (especially Sr2+) into the electrolyte is suppressed, which induces a decrease in the leaching of active Fe ions, ultimately leading to enhanced stability. This work provides a new avenue for rational catalyst design through the dimensional strategy. [ABSTRACT FROM AUTHOR]

Mixed transition-metal oxides (MTMOs), including stannates, ferrites, cobaltates, and nickelates, have attracted increased attention in the application of high performance lithium-ion batteries. Compared with traditional metal oxides, MTMOs exhibit enormous potential as electrode materials in lithium-ion batteries originating from higher reversible capacity, better structural stability, and high electronic conductivity. Recent advancements in the rational design of novel MTMO micro/nanostructures for lithium-ion battery anodes are summarized and their energy storage mechanism is compared to transition-metal oxide anodes. In particular, the significant effects of the MTMO morphology, micro/nanostructure, and crystallinity on battery performance are highlighted. Furthermore, the future trends and prospects, as well as potential problems, are presented to further develop advanced MTMO anodes for more promising and large-scale commercial applications of lithium-ion batteries. [ABSTRACT FROM AUTHOR]

Protonic solid oxide electrolysis cells (P‐SOECs) operating at intermediate temperatures, which have low costs, low environmental impact, and high theoretical electrolysis efficiency, are considered promising next‐generation energy conversion devices for green hydrogen production. However, the developments and applications of P‐SOECs are restricted by numerous material‐ and interface‐related issues, including carrier mismatch between the anode and electrolyte, current leakage in the electrolyte, poor interfacial contact, and chemical stability. Over the past few decades, considerable attempts have been made to address these issues by improving the properties of P‐SOECs. This review comprehensively explores the recent advances in the mechanisms governing steam electrolysis in P‐SOECs, optimization strategies, specially designed components, electrochemical performance, and durability. In particular, given that the lack of suitable anode materials has significantly impeded P‐SOEC development, the relationships between the transferred carriers and the cell performance, reaction models, and surface decoration approaches are meticulously probed. Finally, the challenges hindering P‐SOEC development are discussed and recommendations for future research directions, including theoretical calculations and simulations, structural modification approaches, and large‐scale single‐cell fabrication, are proposed to stimulate research on P‐SOECs and thereby realize efficient electricity‐to‐hydrogen conversion. [ABSTRACT FROM AUTHOR]

Inspired by the brain, neuromorphic computing seeks to improve data‐centric applications in terms of power consumption and device footprint. The processing efficiency can be attributed to reconfigurable connections coupled with complex dynamics and efficient data sensing and signal processing. With field‐effect tunability, the three‐terminal thin‐film transistor is initially explored as a switch. Recently, transistors with added gates and modes have been fashioned as multi‐terminal devices capable of interconnecting complex networks and transducing multiple inputs. Here, the advances in both the semiconducting channel material and the configurational design of multi‐terminal devices are reviewed. The use of strong coupling and dynamical material properties in novel ion‐based capacitors, mixed‐ionic‐electronic semiconductors, low‐dimensional confined systems, stimuli‐responsive materials and redox‐active semiconductors has enabled new neuromorphic functionalities. These include improving reconfigurability using the regulatory ability of homeostasis, dendritic integration of sensory signals and the synaptic competition of resources. The transistor devices are also highly relevant in circuits that improve the efficiency of sensory transduction and signal processing; stimuli‐responsive semiconductors that adapt to the sensory inputs can be employed in the transistors and highly efficient sensorimotor connections between sensors and actuators can be emulated. These highly efficient, low‐complexity analog devices are crucial design elements for next‐generation neuromorphic systems. [ABSTRACT FROM AUTHOR]

In the global quest to renounce from fossil fuels, a large demand for the renewable production of hydrogen via water electrolysis exists. In this context, the solid oxide electrolyzer (SOE) is an interesting technology due to its high efficiency resulting from elevated operating temperatures of up to 900 °C. Physical modeling plays a vital role in the development of SOEs, as it lowers experimental costs and provides insight where measurements reach limits. A main challenge for modeling SOEs is the multitude of physical effects, occurring and interacting on various spatial and temporal scales. This requires assumptions and simplifications, particularly when increasing scope and dimensions of a model. In this review, we discuss the different approaches currently available in literature. [ABSTRACT FROM AUTHOR]

The high‐temperature solid oxide fuel cells (SOFCs) are the most efficient and green conversion technology for electricity generation from hydrogen‐based fuel as compared to conventional thermal power plants. Many efforts have been made to reduce the high operating temperature (>800 °C) to intermediate/low operating temperature (400 °C

State‐of‐the‐art perovskite‐based electrocatalysts for zinc−air batteries (ZABs) may include one or more Critical Raw Materials (CRMs) like Co, Ru, and rare earth elements, for oxygen reduction/evolution reactions. Under the warnings described by the CRMs Act in Europe, future research in this field must limit the use of these elements, find more sustainable formulations, and better design/integration strategies to trade off costs, supply, and performances. In this benchmarking study, three main perovskites, i. e., LaNiO3‐δ (LNO), LaMnO3+δ (LMO), and LaMn0.5Ni0.5O3 (LMNO), are studied as quasi‐CRM‐free benchmarks and integrated with a novel gel electrolyte from a natural biopolymer in primary and secondary ZABs. By comparing literature performances based on the number of total/active site CRMs for the first time, these reference perovskites showed Zn utilizations >90 % in primary ZABs, with LMO and LMNO approaching 94 % at 2 mA/cm2, the highest result in the field. In secondary ZABs, LMO demonstrated limited cyclability (62 cycles), while LMNO had a moderate number of cycles (117 cycles) and the highest round‐trip efficiency (54.3 %), while LNO presented the lowest charge potential (2.04 V) and longer cyclability (455 cycles). This work demonstrates encouraging performance with fewer CRMs and proper integration in more sustainable ZABs, opening the way to future CRM‐free perovskites. [ABSTRACT FROM AUTHOR]

A breakthrough in the development of air electrodes is critical to minimizing the performance deterioration of reversible protonic ceramic electrochemical cells (R‐PCECs), which have demonstrated the potential to be the most promising electrochemical device for energy storage and conversion. Here, a Co‐doped Ruddlesden‐Popper (RP) perovskite with a nominal formula of Pr2Ni0.6Co0.4O4‐δ is reported, which is self‐configured into a composite of Pr4Ni1.8Co1.2O10‐δ (PNCO, 89.57 wt.%) and Pr6O11 (10.43 wt.%), suggested by the X‐ray diffraction refinement. The composite electrodes exhibit improved electrochemical activity with an area‐specific resistance of 0.33 Ω cm2 at 600 °C after being treated with wet air, due primarily to the raised surface exchange, bulk diffusion capabilities, and the increasing amount of Pr6O11 with catalytic activity. The R‐PCECs with the composite electrodes achieve a maximum power density of 1.32 W cm−2 and a current density of 3.09 A cm−2 at 1.3 V with acceptable Faradaic efficiencies at 650 °C. Furthermore, the composite electrodes show benign operational durability in fuel cell mode (‐0.5 A cm−2 for 167 h) and electrolysis mode (+0.5 A cm−2 for 176 h), and promising cycling stability (+/‐0.5 A cm−2) of 124 h at an interval of 2 h. [ABSTRACT FROM AUTHOR]