Substitutional Elements Research Articles

Ferroelectricity of Ce–Mn substituted ZnO thin films

In this study, we introduced Ce and Mn as new substitution elements to develop ferroelectricity in wurtzite-type ZnO thin films on Pt/Ti/SiO2/Si. By substituting (Ce,Mn) for Zn, we observed a decrease in the c/a ratio, which reached its minimum value of 1.582. The relative permittivity increased from 12 to 20 with increasing (Ce,Mn) concentration (2%–15%), while the dielectric loss remained low at about 0.01. The Zn(Ce,Mn)O thin films exhibited clear ferroelectric behavior with a remanent polarization of more than 80 μC cm−2 and a coercive field of about 2.5 MV cm−1.

Modelling bus-based substitution capabilities for metro systems using bipartite graphs

Abstract A disruption of metro services can have a negative impact in the performance of a city’s transportation system and hinder mobility needs of travellers. Investigating the vulnerability of metro systems is required for planning mitigation actions, such as bus substitution services. This study develops a model, which consists of a bipartite graph and its projection to represent the bus substitution capabilities for metro networks. The proposed methodology effectively identifies significant substitution elements (bus lines), evaluates the robustness of alternative options in terms of both connectedness and connectivity, and suggests effective strategies for enhancing bus line capacity to improve network robustness. By applying the methodology to a real-world metro network, valuable insights are gained regarding important bus lines and substitution robustness. Study findings suggest that approaches based on the weighted degree exhibit the greatest effectiveness when it comes to connectivity and the overall efficiency of the network. These findings can assist public transport operators in proactively managing disruptions and improving their services.

The impact of La/Y and Ni/Co substitutions on the gas-phase and electrochemical hydrogenation properties of the La3-xMgxNi9 alloys

This paper presents the results of investigations of the impact of elemental substitution on the hydrogenation properties of La3-xMgxNi9-based alloys, focusing on both gas-phase and electrochemical hydrogenation processes. Alloys derived from La3-xMgxNi9 with La → Y and Ni → Co substitutions were synthesized using powder metallurgy methods. All of them were crystallized with PuNi3 structure type. Gas-phase hydrogenation studies revealed that Y-free samples exhibited faster gas-phase hydrogenation rates compared to Y-substituted ones, whereas electrochemical performance measurements showed the opposite behaviour. Notably, the La1.5Y0.5MgNi6Co3 alloy demonstrated a discharge capacity of approximately 400 mAh/g, while the La1.5Y0.5MgNi7Co2 alloy exhibited good cyclic stability with a discharge capacity of 389.5 mAh/g and preserved approximately 84% of its capacity after 50 charge/discharge cycles at a current density of 50 mA/g. Y-containing samples demonstrated improved electrochemical characteristics, especially in terms of the HRD (high-rate dischargeability) parameter. The La1.5Y0.5MgNi9 alloy has shown ∼86% of its maximum discharge capacity at 1 A/g current density. This electrode with the highest HRD rate demonstrated highest diffusion coefficient (DH = 2.67·10−11 cm2/s), which also correlates with the highest value of exchange current density (I0 = 222 mA/g) and higher limit of the anodic current (IL=878 mA/g). This highlights the prospects of application of Y-containing alloys for areas where high-rate discharge capabilities are required for Ni-MH batteries.

Co/Mn ratio-regulated hexacyanoferrates as a long-life and high-rate cathode for aqueous Zn-ion batteries

Among all kinds of cathodes for aqueous Zn-ion batteries, Mn-based hexacyanoferrates exhibit advantages of high-voltage and abundant resources. However, they suffer from inferior cycle-life and rate capability, which may be attributed to the structural instability and metal ion dissolution. In this work, a continuous element substitution (Mn replaced by Co) is employed to modify the electrochemical performance of Mn-based hexacyanoferrates. With the increasing ration of Co/Mn, the Mn, Co-based hexacyanoferrate (MnCoHCF) samples show unchanged crystalline phase with similar morphology and increasing Co dopant. Additionally, the initial specific capacity gradually decreases, but the electrochemical polarization is alleviated. Among the samples, MnCoHCF-4 (ratio of Co/Mn of 3:1) exhibits excellent cycling life (71.4% after 3000 cycles at 5 C) and rate performance (81.4 mA h g−1 at 10 C). In-situ techniques reveal the obvious metal ion dissolution with a low ratio of Co/Mn during the electrochemical process, resulting in poor structure stability and reaction kinetics of MnHCF electrode. However, When the ratio of Co/Mn is increased, the metal ion dissolution is effectively suppressed, and the MnCoHCF electrode shows improved structure stability and reaction kinetics. This work proposes an effective modification for hexacyanoferrates and other high-voltage Prussian blue analogues, which may promote the application of aqueous Zn-ion batteries.

Optimization of LaNi5 hydrogen storage properties by the combination of mechanical alloying and element substitution

LaNi5 is a commercial hydrogen storage alloy with great potential. But its performance still needs to be optimized to meet the standard proposed by the US Department of Energy. Element substitution is a very important method to optimize the performance of hydrogen storage alloys, especially suitable for AB5 alloys. As a novel method to produce nanocrystalline hydrogen storage alloys, mechanical alloying is also often applied to optimize the properties of hydrogen storage alloys.In this work, first of all, the simulation method based on first principles is applied to predict the performance of 8 elements substituting Ni respectively, and select the most suitable element to substitute Ni. Then the phase composition characterization, hydrogen storage performance tests and electrochemical performance tests were carried out. It was found that mechanical alloying can greatly improve the hydrogen storage performance of LaNi5, and different ball milling times can bring about different performance changes. After adding Cr to the ball-milled LaNi5, the properties of the formed LaNi4Cr were further improved.

Enhancing heat generation ability and magnetic properties of MgFe2O4 with Ni substitutes for biomedical applications

Herein, using the systematic experimental characterization and density functional theory calculations, the effects of Ni elemental substitution on the crystal structure, cation distribution, magnetic properties, and heat generation ability of Mg1–xNixFe2O4 (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) have been reported. We demonstrate that the presence of Ni can lead the lattice shrinkage, phase transition from partially to fully inverse spinel structure, and multi-domain structures in the samples. Moreover, Ni substitutional atoms make magnetically harder ferrite, associated with increasing magnetic inhomogeneity. Remarkably, anomalous enhancements of magnetic anisotropic and coercivity have been observed in Mg0.4Ni0.6Fe2O4. Heat generation ability (SAR) increases from 242 W g−1 for x = 0–391 W g−1 for x = 0.6 at an AC magnetic field of 370 kHz frequency. Thus, the magnetic hyperthermia of Mg0.4Ni0.6Fe2O4 sample on HeLa shows promising results that the viability of cells decreases to 25 % after 5 sessions. The present experimental and theoretical systematic studies provide novel avenue and paradigm for the development of ferrite nanoparticles with high heat generation ability and magnetic properties for biomedical applications.

Quantitative analysis of microstructure evolution, stress partitioning and thermodynamics in the dynamic transformation of Fe-14Ni alloy

Dynamic transformation (DT) of austenite (γ) to ferrite (α) in the hot deformation of various carbon steels was widely investigated. However, the nature of DT remains unclear due to the lack of quantitative analysis of stress partitioning between two phases and the uncertainty of local distribution of substitutional elements at the interface in multi-component carbon steels used in the previous studies. Therefore, in the present study, a binary Fe-Ni alloy with α + γ duplex microstructure in equilibrium was prepared and isothermally compressed in α + γ two-phase region to achieve a quantitative analysis of microstructure evolution, stress partitioning, and thermodynamics during DT. γ to α DT during isothermal compression and α to γ reverse transformation on isothermal annealing under unloaded condition after deformation were accompanied by Ni partitioning. The lattice strains during thermomechanical processing were obtained via in-situ neutron diffraction measurement, based on which the stress partitioning behavior between γ and α was discussed by using the generalized Hooke's law. A thermodynamic framework for the isothermal deformation in solids was established based on the basic laws of thermodynamics, and it was shown that the total Helmholtz free energy change in the deformable material during the isothermal process should be smaller than the work done to the deformable material. Under the present thermodynamic framework, the microstructure evolution in the isothermal compression of Fe-14Ni alloy was well explained by considering the changes in chemical free energy, plastic and elastic energies, and the work done to the material. In addition, the stabilization of the soft α phase in Fe-14Ni alloy by deformation was rationalized since the γ to α transformation decreased the total Helmholtz free energy by decreasing the elastic and dislocation energies.

First-principles calculation of Si–C–N structures with metallicity and high hardness

Stable hard and conductive ceramics under ambient conditions are vital for many industrial applications. However, maintaining high hardness while achieving semiconducting-metallic transition in Si3N4 materials is greatly challenging both experimentally and theoretically. Herein, two hexagonal Si–C–N structures, denoted SiC6N7-1 and SiC6N7-2, were constructed using α-Si3N4 as a matrix model. Calculations of elastic constants and phonon spectra showed stable structures under ambient pressure. Both SiC6N7-1 and SiC6N7-2 can possibly be synthesized at the pressures attainable by large-volume press technology. Compared to a-Si3N4, both Si–C–N ternary compounds achieved a semiconductive-metallic transition owing to the elemental substitution. Property calculations showed conductive Si–C–N compounds with high hardness values of 52 GPa for SiC6N7-1 and 31 GPa for SiC6N7-2. Therefore, the introduction of C atoms tuned the intrinsic properties of both structures, achieving high hardness along with good electrical conductivity. Overall, these findings look promising for future design and synthesis paths of novel conductive ceramics with prospective applications.

Improved hydrogen ab-/desorption performance of Ti–Cr based alloys via dual-effect of oxide reduction and element substitution by minor Al additive

Hysteresis of hydrogen storage alloys essentially hinders the compressibility and operation efficiency of metal hydride hydrogen compressors. In this work, minor Al additive was added into Ti–Cr based hydrogen compression alloys through induction levitation melting, and the apparent hysteresis was successfully ameliorated. The XRD and SEM results indicate homogeneous element distribution of a main C14 Laves phase of Ti0.96Zr0.06Cr1.0Mn0.6Fe0.4Alx (x = 0, 0.01, 0.02, 0.04, 0.08) alloys, and vanishing of a minor oxide phase in case the Al additive is introduced. As Al stoichiometry increases in the alloys, the hydrogenation plateau decreases obviously but the dehydrogenation plateau hardly changes. Meanwhile, the hydrogen capacity ascends initially but descends slightly. Additionally, no severe performance degradation is induced. The mechanism for overall performance improvement can be concluded as dual functions of Al additive: one can be oxide reduction of other elements during melting determined by XPS and ICP, and the other can be partial Al substitution for the B-side elements, leading to relieved deformation energy surrounding the vacant Al-containing interstitials. As to hydrogenation kinetics, the alloys exhibit accelerated hydrogenation rate with increased Al stoichiometry due to decreased activation energy and hydrogenation plateau. Furthermore, the first-principles calculation results indicate random Al substitution for the B-side atoms, and anisotropic expansion of the unit cell can be attributed to Al substitution at 6h sites. This work reveals a simple and effective way to enhance hydrogen storage performance of Ti–Cr based alloys.

Decaniobate: The Fruit Fly of Niobium Polyoxometalate Chemistry.

ConspectusPolyoxometalates (POMs, metals = V4/5+, Nb5+, Ta5+, Mo5/6+, and W5/6+) can be described as molecular metal oxides. The V, Mo, and W-POMs (classic POMs) exhibit rich structural diversity with interesting redox properties, acid catalysis, inorganic ligands, and colorimetric properties and behavior. Nb and Ta POMs, while structurally similar, are generally stable only in base and redox behavior is rare, and they are synthetically far less accessible. The V, Mo, and W-POMs have been studied for well over a century, Nb-POM chemistry has emerged in the last 20 years, and Ta-POM chemistry is yet to see consistent and significant advances. Early and current success in Nb-POM chemistry is owed mainly to hydrothermal synthesis, which is wholly unsatisfying, given the black box nature of this technique.For the last 5 years and as summarized in this Account, we have exploited decaniobate, [Nb10O28]6- (Nb10), as a foundation to perform room-temperature, nearly pH-neutral manipulations of Nb-POM solutions. Nb10, with a rare neutral self-buffering pH, responds to any interactions with electrolytes (specifically oxoanions and metal cations) by undergoing transformations, leading to new topologies. The ease of Nb10 transformation yielding new generations of Nb-POMs, akin to an inorganic analogue of biological model organisms such as the fruit fly, inspired the title of this Account. The common building unit born from the disassembly of Nb10 is [Nb7O20(OH, H2O)2](5-7)-, and the hydroxyl/aqua ligands provide reactivity for linking via condensation reactions, ligand exchange, heterometals, or oxoanions. We can coax these newly assembled Nb-POMs (detected by small-angle X-ray scattering, SAXS) to crystallize via the usual methods of vapor diffusion, salting out, and reduced temperature, and the single-crystal X-ray diffraction structures are valuable for understanding reaction mechanisms to fine-tune control and yield a landscape of topologies and compositions. Beyond providing an opportunity to comprehend and diversify POM chemistry, the reactivity of Nb10 yields highly soluble (i.e., >2 M Nb), nearly neutral aqueous solutions of niobium, ideal for the solution-phase deposition of thin films, demonstrated with LiNbO3, (Na,K)NbO3, Nb2O5, and heterometal-doped Nb2O5. The obtained films are cohesive and smooth, enabled by the tendency of these solutions to gel if simply evaporated quickly.Per our current endeavors, this gelation behavior provides an opportunity to develop new soft, flexible materials including inorganic networks, organic-inorganic networks, and porous solids and explore their material properties including base catalysis and sorption (i.e., CO2). Nb-POM (and Ta-POM) discovery and implementation of properties is far from complete. While heterometal (d and f element) substitution is easy with classic POMs, imparting a whole host of functions (tuned luminescence, catalysis, electroactivity, etc.), it remains a challenge with Nb-POMs due to pH incompatibility with most heterometals. This grand challenge that defies fundamental aqueous behavior of metal cations requires the creation of liquid mixtures that include polymer and/or ionic liquid components, and the creation of such reaction media can impact synthesis beyond POM chemistry. The goal of this Account is to describe the recent advances in Nb-POM chemistry, afforded by the Nb10 "fruit fly", and to also provide insight into the next large steps needed to advance Nb-POM chemistry.

Rational Atom Substitution to Obtain Efficient, Lead‐Free Photocatalytic Perovskites Assisted by Machine Learning and DFT Calculations

AbstractInorganic lead‐free halide perovskites, devoid of toxic or rare elements, have garnered considerable attention as photocatalysts for pollution control, CO2 reduction and hydrogen production. In the extensive perovskite design space, factors like substitution or doping level profoundly impact their performance. To address this complexity, a synergistic combination of machine learning models and theoretical calculations were used to efficiently screen substitution elements that enhanced the photoactivity of substituted Cs2AgBiBr6 perovskites. Machine learning models determined the importance of d10 orbitals, highlighting how substituent electron configuration affects electronic structure of Cs2AgBiBr6. Conspicuously, d10‐configured Zn2+ boosted the photoactivity of Cs2AgBiBr6. Experimental verification validated these model results, revealing a 13‐fold increase in photocatalytic toluene conversion compared to the unsubstituted counterpart. This enhancement resulted from the small charge carrier effective mass, as well as the creation of shallow trap states, shifting the conduction band minimum, introducing electron‐deficient Br, and altering the distance between the B‐site cations d band centre and the halide anions p band centre, a parameter tuneable through d10 configuration substituents. This study exemplifies the application of computational modelling in photocatalyst design and elucidating structure–property relationships. It underscores the potential of synergistic integration of calculations, modelling, and experimental analysis across various applications.

Rational Atom Substitution to Obtain Efficient, Lead-Free Photocatalytic Perovskites Assisted by Machine Learning and DFT Calculations.

Inorganic lead-free halide perovskites, devoid of toxic or rare elements, have garnered considerable attention as photocatalysts for pollution control, CO2 reduction and hydrogen production. In the extensive perovskite design space, factors like substitution or doping level profoundly impact their performance. To address this complexity, a synergistic combination of machine learning models and theoretical calculations were used to efficiently screen substitution elements that enhanced the photoactivity of substituted Cs2 AgBiBr6 perovskites. Machine learning models determined the importance of d10 orbitals, highlighting how substituent electron configuration affects electronic structure of Cs2 AgBiBr6 . Conspicuously, d10 -configured Zn2+ boosted the photoactivity of Cs2 AgBiBr6 . Experimental verification validated these model results, revealing a 13-fold increase in photocatalytic toluene conversion compared to the unsubstituted counterpart. This enhancement resulted from the small charge carrier effective mass, as well as the creation of shallow trap states, shifting the conduction band minimum, introducing electron-deficient Br, and altering the distance between the B-site cations d band centre and the halide anions p band centre, a parameter tuneable through d10 configuration substituents. This study exemplifies the application of computational modelling in photocatalyst design and elucidating structure-property relationships. It underscores the potential of synergistic integration of calculations, modelling, and experimental analysis across various applications.

In-doped Li7La3Zr2O12 nanofibers enhances electrochemical properties and conductivity of PEO-based composite electrolyte in all-solid-state lithium battery

Garnet type Li7La3Zr2O12 solid electrolyte exhibits excellent thermal stability and high ionic conductivity, which is an ideal solid electrolyte. However, it is found that the cubic phase is a high conductivity phase and the synthesis condition is harsh. Therefore, low temperature synthesis and partial element substitution are expected to prepare cubic phase LLZO. In this paper, the LLZO cubic phase structure is stabilized by electrospinning technology and high-priced cation substitution strategy, and the transportation efficiency of lithium ions is improved. The experimental results show that LLZO NFs inorganic filler with stable cubic phase structure is obtained when the molar amount of In3+ is 0.2. The In3+ with large ion radius replaces the Li+ inside the crystal, effectively increases the lithium vacancy concentration, and increases the Li+ transport path inside the crystal, and successfully doubles the ionic conductivity of the composite solid electrolyte at 60 °C (9.39 × 10−4 S/cm). The lithium ions migration number and electrochemical window were increased to 0.19 and 5.2 V respectively. With excellent cycle performance (60 °C), it can cycle 2000 cycles at a high rate of 0.5 C charge and discharge, and the Coulomb efficiency has been close to 100 %. Li|PL-LILZO NFs (0.2)|LFP all-solid-state lithium batteries can light small bulbs at room temperature and have practical application prospects. Therefore, in the composite electrolyte, the increase of the lithium ions pathway inside the active inorganic filler crystal will also improve the comprehensive performance of the composite electrolyte for preparation.

Comparison between Lithium Substitution and Doping on the Physical and Piezoelectric Properties of Lead-Free BCZT Ceramics

Defects generation in ceramics by element substitution or doping may improve their properties. However, different results will be obtained even though the same element is involved due to different mechanisms. Ceramics Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZT) is one of the potential candidates to replace lead-based material PZT. However, the conventional method requires high-temperature calcination and sintering process to synthesize this material. Thus, Lithium (Li) can act as the sintering aid to lower the temperature at the same time, the properties of piezoelectric can be enhanced. In this paper, the structural, physical, and piezoelectric performance of lead-free Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZT) ceramics with Lithium Li substitution and doping, synthesized by the solid-state reaction method were studied. The substitution of Li increases the density, ρ and piezoelectric coefficient, d33 of BCZT which are 4.075 g/cm3 and 304.6 pC/N, respectively. These results demonstrate that Li substitution is more beneficial to the piezoelectric properties than doping because it produces higher grain boundary resistance and activation energy and has lower conductivity than doping. This can provide a definite strategy during the chemical composition design and manufacture of BCZT ceramics.

In situ trace elements and sulfur isotope analyses of layered sphalerite as a record of ore-forming processes in the world-class Jinding sediment-hosted Zn-Pb Ore Deposit, China

The Jinding deposit is well-known to contain high concentrations of metals Zn, Pb, Cd, Tl, Ag, etc. in the world. The associated critical metals and their substitution mechanisms were wildly concerned. This study investigated and compared sphalerites using a combination of textural, trace element, and sulfur isotopic analyses to give insights into element substitution mechanisms and possible sources of ore-forming fluids and to constrain processes involved in ore genesis of the world-class Jinding sediment-hosted Zn-Pb deposit. LA-ICP-MS analyses reveal chemical zoning in the sphalerites together with the coupling of two main groups of trace elements substitution mechanisms: one is Fe + Ge + Mn + Pb + As + Tl and vacancies to substitute Zn, and the other is Cd + Cu + Ag + Sb to substitute Zn. The incorporation of trace elements is facilitated by the replacement of Zn by Cd and Fe. These two group elements have distinct color oscillatory zonation and are intertwined in the trace element LA-ICP-MS element maps. The sulfur isotopic values show high isotope fractionation in the sphalerite from −28.4 to −0.5 ‰, suggesting a bacteriogenic sulfur origin at Jinding. Sphalerite sample has observed that sulfur isotope is correlated with Fe concentration and anti-correlated with Cd concentration. These variations in isotope fractionation and trace element disequilibrium partitioning were interpreted as the effects of temperature and pH with less influence from sulfur activity. These variability zones of sphalerite alternately enriched in Cd- and Fe-related elements suggested that ore fluids may flow from metal-fertile sedimentary/basement sequences during their ascent, and colloform sphalerite was likely formed by a high degree of supercooling, over-saturation, and rapid mixing between bacteriogenic sulfur and metal-rich in spaces of collapse dome. Based on our observations, we suggested that these variability zones of layered sphalerite alternately enriched in Cd and Fe would likely provide useful insight for a potential economic Zn-Pb and associated critical metals mineral exploration deposit.

Gd3 TeBO9 : A Rare-Earth Borate with Significant Magnetocaloric Effect.

Magnetic refrigeration technology based on Gd-based paramagnets is expected to be applied to refrigeration in extremely low temperatures, thereby reducing the consumption of liquid helium. Here, we obtained a compound, Gd3 TeBO9 with high Gd3+ concentration through element substitution. The Gd3+ concentration in this compound is as high as 2.4×1024 ions/kg, which is 33 % higher than the commercial Gd3 Ga5 O12 (GGG), and further magnetic tests show that Gd3 TeBO9 has a large magnetic entropy change (57.44 J/(kg K) and 408 mJ/(cm3 K) at 2.6 K and 7 T), which is 1.5 times that of GGG, implying the possibility of its further development as an potential magnetocaloric material.

Theoretical investigation of the structural stability, electronic and optical properties of the double perovskite Cs2ZrX6 (X=Cl, Br, I)

In recent years, in order to solve the energy issue researchers have performed much investigations, among which inorganic metal halide chalcogenides have received extensive attention in the field of materials, energy and other fields for their excellent photovoltaic properties. However, the traditional halide chalcogenide has two deadly deficiencies, poor thermal stability and the presence of toxic lead elements, which prevent its applications. Compared with ABX3-type halide chalcogenides, double lead-free perovskites usually have the advantages of better thermal stability and less toxicity, which makes it easier to be commercialized. In addition, elemental substitution methods can be employed to modulate their energy bands and optical properties. Here, the structural stability, electronic and optical properties, and thermodynamic properties of cesium zirconium halide double perovskites Cs2ZrX6 (X = Cl, Br, I) are systematically investigated based on density functional theory (DFT) to predict their applications in the photovoltaic and optoelectronic industries. The computational results show that these cesium zirconium halide double perovskites have very good structural stability as well as special optical properties such as strong light absorption, large dielectric constant, and low reflectivity. Moreover, all of them belong to direct bandgap semiconductors and possess small effective masses, which indicates that the double perovskites could be potential candidates for future optoelectronic devices and applications.

Structure and Electron Engineering for Nitrate Electrocatalysis to Ammonia: Identification and Modification of Active Sites in Spinel Oxides.

Cobalt spinel oxides, which consist of tetrahedral site (AO4) and octahedral site (BO6), are a potential group of transition metal oxides (TMO) for electrocatalytic nitrate reduction reactions to ammonia (NRA). Identifying the true active site in spinel oxides is crucial to designing advanced catalysts. This work reveals that the CoO6 site is the dominant site for NRA through the site substitution strategy. The suitable electronic configuration of Co at the octahedral site leads to a stronger interaction between the Co d-orbital and the O p-orbital in O-containing intermediates, resulting in a high-efficiency nitrate-to-ammonia reduction. Furthermore, the substitution of metallic elements at the AO4 site can affect the charge density at the BO6 site via the structure of A-O-B. Thereafter, Ni and Cu are introduced to replace the tetrahedral site in spinel oxides and optimize the electronic structure of CoO6. As a result, NiCo2O4 exhibits the best activity for NRA with an outstanding yield of NH3 (15.49mgcm-2h-1) and FE (99.89%). This study introduces a novel paradigm for identifying the active site and proposes an approach for constructing high-efficiency electrocatalysts for NRA.

Self-assembled microflower-like NiCo2X4 (X = O, S, Se) as electrodes for asymmetric supercapacitors

The substitution of chalcogen elements has a notable influence on the microstructure and electrochemical properties of transition metal oxides. However, the underlying mechanism necessitates clarification. In this investigation, self-assembled microflower-like NiCo2X4 (X = O, S, Se) structures were successfully synthesized, and their microstructures and electrochemical performance were systematically examined using both theoretical and experimental approaches. Theoretical analysis demonstrated that the replacement of chalcogen elements significantly enhances the energy band structure of the material, thus effectively improving its electrochemical performance. Experimental results revealed that, in comparison to NiCo2O4 and NiCo2S4, NiCo2Se4 displayed remarkable electrochemical performance, with a specific capacitance of 1899 F g–1 at a current density of 1 A g–1 and a promising rate capability (with a capacitance retention increase of up to 71.9% when the current density was elevated from 1 A g–1 to 10 A g–1). Additionally, the asymmetric device (NiCo2Se4//AC), composed of NiCo2Se4 as the positive electrode and activated carbon (AC) as the negative electrode, exhibited an energy density of 42.44 Wh kg–1 at a power density of 1600 W kg–1. As a result, this study offers novel insights into the fabrication of transition metal compounds for energy storage applications.

Iron substitution enabled lattice oxygen oxidation and cation leaching for promoting surface reconstruction in electrocatalytic oxygen evolution

The low-cost transition metal oxides have drawn widespread interest as alternatives to noble metal-based electrocatalysts for oxygen evolution reaction (OER). Transition metal oxides usually undergo surface reconstruction during electrochemical reaction to form the actual active species. However, in-depth understanding and regulating of the surface reconstruction of active phases for oxides in OER remains an onerous challenge. Herein, we report a simple Fe element substitution strategy to facilitate the surface reconstruction of spinel oxide NiCr2O4 to generate active (oxy)hydroxides. The activated Fe-doped NiCr2O4 (Act-Fe-NCO) exhibits a lower OER overpotential of 259 mV at 10 mA cm−2 than activated NiCr2O4 (Act-NCO, 428 mV), and shows excellent stability for 120 h. The electrochemically activated CV measurement and nanostructure characterizations reveal that Fe substitution could promote the consumption of lattice oxygen during electrochemical activation to induce the leaching of soluble Cr cations, thereby facilitating the reconstruction of remaining Ni cations on the surface into (oxy)hydroxide active species. Moreover, theoretical calculations further demonstrate that the O 2p band center of NiCr2O4 moves towards the Fermi level due to Fe substitution, thus promoting lattice oxygen oxidation and providing greater structural flexibility for surface reconstruction. This work shows a promising way to regulate the surface reconstruction kinetics and OER electrocatalytic activity of transition metal oxides.