Faraday Discussions最新文献

Multicomponent phase space is enormous and contains a vast number of complex new materials. Despite intensive investigation in the last decade and a half however, we are only slowly making progress towards understanding these new materials. This paper attempts to summarise some of the fundamental discoveries we have made about the geography of multicomponent phase space and the wide range of complex new materials that we have found within it. This paper discusses briefly the following topics: the size and shape of multicomponent phase space and the range of single- and multiple-phase fields that it contains; the (initially) surprising presence of many large near-ideal single-phase solid-solution phases, stabilised by a high configurational entropy of mixing; the extensive and wide-ranging variation of local nanostructure and associated mechanical and electronic lattice strain that permeates throughout high-entropy solid-solution phases; and some of the unusual, exciting and valuable properties that are then produced within multicomponent and high-entropy materials. Many of the results discussed have been obtained from the fcc Cantor alloys (based on the original Cantor alloy, equiatomic fcc CrMnFeCoNi) and the bcc Senkov alloys (based on the original Senkov alloy, equiatomic VNbMoTaW), two groups of multicomponent high-entropy single-phase materials that have been particularly widely studied. Similar behaviour is also found in other multicomponent high-entropy single-phase materials, though these have not been studied so intensively. In comparison with multicomponent high-entropy single-phase materials, rather little is known about multicomponent multiphase materials that have also not been studied so intensively.

Molten salt reactors (MSRs) expose structural materials to harsh conditions, such as elevated temperatures, corrosive fluoride salts, and substantial neutron irradiation. These factors contribute to intricate degradation processes, including radiation-induced defect development, void swelling, and corrosion. Refractory high-entropy alloys with a body-centered cubic structure provide noteworthy thermal stability and mechanical strength, making them excellent candidates for MSR application. This study explores the corrosion properties of NbTaMoW and NbTaMoWV in FLiBe molten salt via density functional theory and ab initio molecular dynamics simulations. Analyses of electronic structure, including density of states and crystal orbital Hamilton population, shed light on interfacial bonding and charge distribution. NbTaMoW shows minimal d-band shifts and weak fluorine interaction, indicating enhanced oxidation resistance. Adding vanadium to form NbTaMoWV further diminishes oxidative vulnerability and stabilizes the electronic structure at the salt interface, suggesting superior corrosion resistance in molten salt conditions.

Developing active and stable bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is essential for a wide range of applications of rechargeable air batteries, water electrolysers, and fuel cells. Here, we report that single-phase face-centred cubic structured PtPdFeCoNi high-entropy alloy (HEA) nanoparticles, synthesized via a facile colloidal synthesis approach, possess a good combination of activity and stability toward OER and ORR. Specifically, pristine PtPdFeCoNi HEA nanoparticles exhibit an overpotential of 306 mV at 10 mA cm^-2 for OER and a half-wave potential of 0.82 V versus RHE for ORR, with a narrow overvoltage (ΔE) of 0.71 V in alkaline media, outperforming commercial Pt/C and RuO₂ benchmark electrocatalysts. The OER and ORR activity of the HEA nanoparticles do not change significantly after prolonged electrochemical cycling (3000 cycles). Using X-ray photoelectron spectroscopy and transmission electron microscopy, we found no evident structural, morphological and compositional changes on the HEA nanoparticle surfaces after ORR cycling, explaining its high activity and stability. In contrast, after extended OER cycling, the PtPdFeCoNi nanoparticle surfaces transform into an amorphous layer embedded with Fe-, Co-, and Ni-rich oxyhydroxides, as well as Co-rich oxides, which likely promote activity. Additionally, the shell oxyhydroxide and oxide layer could prevent the continuous dissolution of Pt and Pd, providing long-term stability. Overall, this work underscores the importance of correlating morphological, structural, and compositional changes of HEA nanocatalysts with electrocatalytic performance, for understanding how individual elements behave toward bifunctional oxygen electrocatalysis.

High-entropy alloy nanoparticles (HEA NPs) constitute an interesting material class with high potential as heterogeneous catalysts due to their exceptional compositional and structural tunability and the complex interplay of different element-specific surface sites. Laser ablation in liquids (LAL) is a kinetically controlled synthesis method that allows the generation of colloidal HEA NPs. With CrMnFeCoNi-NPs, a facile control of the NP phase structure, switching between crystalline and amorphous via applied laser pulse duration, has been previously reported, attributed to the different particle solidification times and metalloidic carbon incorporation pathways. However, neither the replacement of the oxygen-affine Mn by the sp²-carbon coupling element Cu, nor the transferability of the pulsed laser fabrication process from bulk target to micropowder feedstock processing, has been studied. In the present work, we use scanning transmission electron microscopy, equipped with energy-dispersive X-ray spectroscopy (STEM-EDX), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and X-ray diffraction (XRD) to demonstrate the transferability of internal phase structure tunability to the CrFeCoNiCu alloy and confirm ns- and ps-pulsed LAL yielding amorphous and crystalline HEA NPs, respectively, with diameters of 10-40 nm. Furthermore, we examine the generation of CrMnFeCoNi and CrFeCoNiCu nanoparticles by scalable, fully continuous ns-pulsed microparticle laser fragmentation in liquid (MP-LFL) using a high-power UV-laser and find the emergence of amorphous phase structures only in the Cu-containing nanoparticles, a phenomenon we attribute to copper-catalyzed carbon incorporation into the HEA NPs. These studies are complemented by a detailed characterization of the surface electrochemistry of the HEA NPs via alkaline cyclic voltammetry (CV) and elemental compositions in surface-near volumes, quantified by X-ray photoelectron spectroscopy (XPS). We elucidate that primarily the chemical composition (Mn vs. Cu) and, only to a lower extent, the phase structure (amorphous vs. crystalline) determine the surface potential, electrochemical stability upon multiple CV cycling, and surface element distribution of the particles. Finally, the activity of the HEA NPs in the oxygen evolution reaction (OER) is evaluated via linear sweep voltammetry (LSV), where we find amorphous CrMnFeCoNi HEA NPs to be more active (lower overpotential, higher current density) than their crystalline counterparts, motivating future application-focused work and transfer to other material systems and relevant reactions.

High-entropy oxides (HEOs), as a subclass of high-entropy materials (HEMs), offer a versatile platform for catalysis by leveraging entropy-stabilized solid solutions with tunable compositions, lattice structures, and electronic properties. While exsolution-dissolution of metal species in crystalline HEOs has emerged as a promising strategy for reversible active sites regeneration, the dynamic behaviour of HEOs possessing amorphous nature remains under-explored, particularly the difference with crystalline counterparts. In this work, we systematically investigate the architecture-dependent exsolution-dissolution behavior of HEOs by comparing a crystalline-phase HEO (c-HEO) and an amorphous-phase HEO (a-HEO), both comprising Ni, Mg, Cu, Zn, and Co as principal metal elements. Using a combination of in situ variable-temperature X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron microscopy, and in situ CO diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS), the structural evolution of the two HEO phases under redox conditions was elucidated. Both materials exhibit reversible exsolution of metallic species or alloys in reducing environments, followed by re-incorporation into the host lattice upon oxidation. Remarkably, the a-HEO demonstrates more facile and dynamic self-healing behavior, with alloy exsolution and dissolution occurring under milder conditions because of its enhanced reducibility and structural disorder. This study provides critical insights into the design of next-generation regenerable catalysts based on amorphous HEOs, highlighting the role of phase structure in governing reversible metal-site formation dynamics and catalytic performance.

Nanocrystalline CoMnFeNiGa high entropy alloys (HEAs) were successfully synthesized and characterized across different length scales. Compositionally homogeneous single-phase FCC HEA micropowder particles with a nanocrystalline structure (∼8 nm) were produced by short-term (190 min) high energy ball milling (HEBM). These powders were subsequently used as precursors for fabricating dense HEA bulk by spark plasma sintering (SPS) and HEA nanoparticles (NPs) by laser fragmentation in liquids (LFL)-both synthesis routes are not achievable by direct processing of elemental powder blends. We show that the single-phase FCC CoMnFeNiGa HEA micropowder partially transforms into a BCC phase upon consolidation by SPS at 1073 K. As a result, the HEA bulk consists of a mixture of FCC and BCC phases. In addition, Mn-rich BCC precipitates (10-50 nm) were formed in both HEA phases. The LFL of HEA micropowder leads to a formation of HEA NPs with two morphologies (spheres and quasi-2D platelets with 5-10 nm thickness and 40-150 nm lengths) with FCC, BCC, and hexagonal structures (birnessite-type layered δ-MnO₂ structure). All three nanocrystalline CoMnFeNiGa HEAs exhibit soft ferromagnetic behavior at RT with a saturation magnetization (M_s) of 19.5-33.5 A m² kg^-1 for the micropowder and NPs, while the M_s of HEA bulk is 2-4 times larger (88.8 A m² kg^-1). A short thermal treatment (1000 K, 30 s) significantly enhanced M_s and increased the Curie temperature of the micropowder to 105.6 A m² kg^-1 and 785 K, of the NPs to 46.9 A m² kg^-1 and 850 K, and of the bulk material to 106 A m² kg^-1 and 793 K. The coercivity increased threefold to 1.8 kA m^-1 only in NPs. Structure-property relationships in CoMnFeNiGa HEAs are herein systematically compared across all length scales, demonstrating that magnetic behavior can be effectively tuned by nanoscale structural control and rapid thermal treatment.

Interstitial doping is a common approach to improve the mechanical or functional properties of high-entropy alloys (HEAs); their stability is usually predicted by a specific single descriptor. Herein, we consider six types of microstructure-based descriptor, seven types of electronic-structure-based local-environment descriptor and their combinations to predict the stability of the C- or N-doped VNbMoTaWTiAl_0.5 (BCC) HEA, mainly using density functional theory (DFT) calculations. A machine-learning interatomic potential and Monte Carlo simulations were employed to verify the short-range order in the HEA. The microstructure-based descriptors include the composition of the first-, second-, and third-nearest neighbour shells (1NN, 2NN and 3NN), OctaDist distortion parameters (ζ, Δ, Σ, Θ), the Voronoi volume (V_Voronoi) of the dopant, and the volume change of the unit cell after doping (ΔV_cell); the electronic-structure-based local-environment descriptors include the local potential (LP), the electrostatic potential (EP), the charge density (CHG), the electron localization function (ELF) at the vacant doping site, the d-band center (ε_d), the mean electronegativity (EN) of the 1NN shell around the dopant, and the Bader charge of the C or N dopants. For a single descriptor, the best correlation between the descriptor and the doping energy (indication of HEA stability) is found for 1NN with coefficient of determination (Q²) values of ∼51 or ∼61% obtained using the LOOCV (leave-one-out cross-validation) approach for C or N doping, respectively. After adding volume descriptor(s) into the linear regression model with the 1NN descriptor, Q² increases to 72 and 76% for C and N doping, respectively. After further adding the electronic-structure-based EP descriptor, Q² further increases to 75 and 80% for C and N doping, respectively, despite the poor correlation using a single volume descriptor. This study quantitatively combined and compared the independent contributions of different types of local-environment descriptors to the stability of the C- or N-doped HEA, demonstrating the importance of considering both key microstructure-based and electronic-structure-based local-environment descriptors using the regression models to achieve more accurate correlation of dopant stability in HEA; these combined approaches could be further applied to other materials systems, research fields and applications.

Multielemental alloys (MEAs) based on Earth-abundant 3d transition metals hold significant promise as low-cost electrocatalysts for the oxygen evolution reaction (OER), but their long-term stability under oxidative conditions remains a major challenge. In this study, we investigate the effect of palladium incorporation on the electrochemical performance and structural durability of FeCoNiCu MEA nanoparticles. Building upon our previous findings that trace Pd addition significantly enhances catalyst durability, an accelerated durability test (ADT) performed at 100 mA cm^-2 reveals that the degradation rate (0.356 mV h^-1) decreased dramatically to approximately 1/350th that of Pd-free FeCoNiCu (125 mV h^-1). In this study, we systematically synthesized a series of Pd-FeCoNiCu alloys with Pd contents ranging from 0.177 to 1.97 at%. Advanced characterization techniques including inductively coupled plasma optical emission spectroscopy (ICP-OES), electron microscopy, synchrotron-based spectroscopy, and electrochemical measurements, were employed to elucidate the correlation between composition, structure, and performance. Our findings reveal a highly non-linear dependence of catalyst performance on Pd content: an optimal range (0.336-0.389 at%) enables long-range d-d/sp orbital hybridization that delocalizes the local density of states (LDOS) of surrounding 3d metals, thereby suppressing oxidative dissolution. In contrast, higher Pd concentrations lead to Pd-Pd interactions, localize electronic perturbation, and accelerate degradation. This volcano-type correlation between Pd content and durability, highlights a general strategy for engineering catalyst longevity via minimal noble-metal doping and spatially cooperative electronic modulation.

High-entropy alloys (HEA) are a distinct class of materials made up of multiple principal components (≥5) in near-equimolar ratios, resulting in extraordinary properties, including high catalytic activity, corrosion and oxidation resistance, and tunable magnetic properties. In nanoparticle form, these alloys are highly promising for a variety of advanced applications, such as catalysis, magnetic storage, and biomedical technology [Zoubi et al., Nano Energy, 2023, 110, 108362]. This study used an isolating-medium-assisted solid-state reaction to synthesise FeCoNiCuPt HEA nanoparticles with ultrafine NaCl particles as the isolating medium [Meng et al., Mater. Adv., 2024, 5, 719]. The nanoparticles were stabilised with a range of hydrophobic and hydrophilic capping agents, such as polyethylenimine, polyvinylpyrrolidone, stearic acid, octadecylamine, etc., introduced before or after the removal of the isolating medium. The formation of single-phase nanoparticles and the chemical composition of FeCoNiCuPt was validated using X-ray diffraction and energy-dispersive X-ray spectroscopy. Transmission electron microscopy and dynamic light scattering were used to determine particle sizes, effective capping agent thickness, and particle stability. The results highlight the successful synthesis of the FeCoNiCuPt nanoparticles, the effect of capping agents on the control of particle size, and the stability of capped-nanoparticle suspensions in water and organic solvents. The study emphasises the importance of selecting the appropriate capping agent to maintain nanoparticle stability and prevent agglomeration.

Electrocatalysis is critical for mitigating climate change by providing green energy solutions, e.g. for hydrogen production by electrolysis of water implying high catalytic activity not only for hydrogen evolution but also for oxygen evolution as the counter reaction. Moreover, reactions such as oxygen reduction and nitrate reduction are of high importance in fuel cells or for environmental remediation. This study focuses on the exploration of electrocatalysts in the enormous composition spaces encountered in multinary materials like high-entropy alloys in the form of compositionally complex solid solutions. These provide paradigm-changing design principles for new electrocatalysts based on their tuneable surface atom arrangements resulting from their multinary composition. However, to master the combinatorial explosion problem of polyelemental catalysts, efficient exploration approaches need to be adapted. For this purpose, we present a comprehensive strategy to compare the electrocatalytic activity for different reactions in alkaline media, namely the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), hydrogen evolution reaction (HER) and nitrate reduction reaction (NO_xRR) over large compositional spaces in three multinary systems: Cu-Pd-Pt-Ru, Ir-Pd-Pt-Ru and Ni-Pd-Pt-Ru. To generate the necessary large and multidimensional experimental dataset, thin-film materials libraries were synthesised and analysed using high-throughput characterisation methods. This allows for a comparative overview over correlations between composition and electrocatalytic activity, considering also relevant information on crystal structure and surface morphology. Similarities and differences, trends, maxima and minima in electrocatalytic activity are revealed and discussed. Main findings include that for the OER Ir₂₃Pd₃Pt₈Ru₆₆ exhibits the highest activity, exceeding any alloy of the other two systems by 51% (Ni-Pd-Pt-Ru) and 74% (Cu-Pd-Pt-Ru). For HER, Ir₃₆Pd₄Pt₄₈Ru₁₂ surpasses any of its elemental constituents by 26% and maxima in other systems by 5% (Ni-Pd-Pt-Ru) and 23% (Cu-Pd-Pt-Ru). For the NO_xRR, only a marginal increase of 4% was found between the most active measured alloy and the elemental constituent Cu. By comparing activity across systems, we demonstrate the tunability of electrochemical activity on compositionally complex solid solutions, achievable through variations in composition both within and across different material systems for four different reactions.