Design high-entropy carbide ceramics from machine learning

High-entropy ceramics (HECs) have shown great application potential under demanding conditions, such as high stresses and temperatures. However, the immense phase space poses great challenges for the rational design of new high-performance HECs. In this work, we develop machine-learning (ML) models to discover high-entropy ceramic carbides (HECCs). Built upon attributes of HECCs and their constituent precursors, our ML models demonstrate a high prediction accuracy (0.982). Using the well-trained ML models, we evaluate the single-phase probability of 90 HECCs that are not experimentally reported so far. Several of these predictions are validated by our experiments. We further establish the phase diagrams for non-equiatomic HECCs spanning the whole composition space by which the single-phase regime can be easily identified. Our ML models can predict both equiatomic and non-equiatomic HECs based solely on the chemical descriptors of constituent transition-metal-carbide precursors, which paves the way for the high-throughput design of HECCs with superior properties.

Phase controlled synthesis of transition metal carbide nanocrystals by ultrafast flash Joule heating

Nanoscale carbides enhance ultra-strong ceramics and show activity as high-performance catalysts. Traditional lengthy carburization methods for carbide syntheses usually result in coked surface, large particle size, and uncontrolled phase. Here, a flash Joule heating process is developed for ultrafast synthesis of carbide nanocrystals within 1 s. Various interstitial transition metal carbides (TiC, ZrC, HfC, VC, NbC, TaC, Cr2C3, MoC, and W2C) and covalent carbides (B4C and SiC) are produced using low-cost precursors. By controlling pulse voltages, phase-pure molybdenum carbides including β-Mo2C and metastable α-MoC1-x and η-MoC1-x are selectively synthesized, demonstrating the excellent phase engineering ability of the flash Joule heating by broadly tunable energy input that can exceed 3000 K coupled with kinetically controlled ultrafast cooling (>104 K s−1). Theoretical calculation reveals carbon vacancies as the driving factor for topotactic transition of carbide phases. The phase-dependent hydrogen evolution capability of molybdenum carbides is investigated with β-Mo2C showing the best performance.

In situ investigation of water on MXene interfaces

A continuum of water populations can exist in nanoscale layered materials, which impacts transport phenomena relevant for separation, adsorption, and charge storage processes. Quantification and direct interrogation of water structure and organization are important in order to design materials with molecular-level control for emerging energy and water applications. Through combining molecular simulations with ambient-pressure X-ray photoelectron spectroscopy, X-ray diffraction, and diffuse reflectance infrared Fourier transform spectroscopy, we directly probe hydration mechanisms at confined and nonconfined regions in nanolayered transition-metal carbide materials. Hydrophobic (K+) cations decrease water mobility within the confined interlayer and accelerate water removal at nonconfined surfaces. Hydrophilic cations (Li+) increase water mobility within the confined interlayer and decrease water-removal rates at nonconfined surfaces. Solutes, rather than the surface terminating groups, are shown to be more impactful on the kinetics of water adsorption and desorption. Calculations from grand canonical molecular dynamics demonstrate that hydrophilic cations (Li+) actively aid in water adsorption at MXene interfaces. In contrast, hydrophobic cations (K+) weakly interact with water, leading to higher degrees of water ordering (orientation) and faster removal at elevated temperatures.

Synthesis of Metastable Ternary Pd-W and Pd-Mo Transition Metal Carbide Nanomaterials

Research and catalytic testing of platinum group transition metal carbides have been extremely limited due to a lack of reliable, simple synthetic approaches. Powder samples have been reported to phase separately above 1%, and only thin-film samples have been reported to have appreciable amounts of precious metal doping. Herein, we demonstrated, through the simple co-precipitation of Pd and W or Mo precursors and their subsequent annealing, the possibility to readily form ternary carbide powders. During the investigation of the Pd-W ternary system, we discovered a new hexagonal phase, (PdW)2C, which represents the first non-cubic Pd ternary carbide. Additionally, the solubility of Pd in the Pd-W-C and Pd-Mo-C systems was increased to 24 and 32%, respectively. As a potential application, these new materials show an enhanced activity for the methanol oxidation reaction (MOR) compared to industrial Pd/C.

Electromagnetic Interference Shielding of 2D Transition Metal Carbide (MXene)/Metal Ion Composites

In this work, Ti3C2, which has a loosely packed accordion-like structure in transition metal carbide (MXene) form, is fabricated and adsorbed by three metal ions (Fe3+/Co2+/Ni2+). The electromagnetic interference (EMI) shielding performance of Ti3C2 and Ti3C2:Fe3+/Co2+/Ni2+ films is researched in detail, demonstrating that the EMI shielding effectiveness can be improved by adsorbing by Fe3+/Co2+/Ni2+ ions because the metal ion adsorbing can improve the absorption efficiency via electromagnetic wave scattering. The studied Ti3C2:Fe3+/Co2+/Ni2+ films can be used as good EMI shielding materials for communications, electronics, military, and other applications.

MXene for aqueous zinc-based energy storage devices

MXene, a class of 2D transition metal carbide/nitride materials, has attracted widespread attention since its first discovery in 2011. Due to its high electronic conductivity, large specific surface area, good mechanical stability, and adjustable surface functional groups, MXene-based nanomaterials have shown great potential in energy storage devices. Meanwhile, zinc-based aqueous energy storage devices became a hotspot recently in energy storage field on account of their high security and low cost. In this review, the research progress on the preparation routes, preserving method, related structure and properties of MXene is first summarized. Followed by is an introduction of the recent state-of-the-art development of MXene-based electrodes for zinc-based aqueous energy storage devices, including zinc ion batteries (ZIBs), zinc-air batteries (ZABs), and zinc-halide batteries (ZHBs). Finally, the major bottleneck and perspectives for MXene-based nanomaterials in zinc-based aqueous energy storage devices are pointed out.

Elucidating the Role and Interplay of Nickel and Molybdenum Carbide Particles Supported on Zeolite Y in Methane-Steam Reforming

Methane steam reforming (MSR) reaction is a mature industrial process that has been applied for large-scale hydrogen production. Here, we report the synthesis and characterization, reaction kinetics, and deactivation mechanism of a series of catalysts with metallic nickel (Ni) clusters and molybdenum carbide (Mo2C) particles supported on zeolite Y (Ni-Mo2C/FAU) in MSR reaction at 850 oC. Despite low Ni loading less than 2.4 wt%, MSR on Ni-Mo2C/FAU exhibits high activity and stability, yet deactivation of Ni-FAU (the sample without Mo2C) is significant. Further investigations elucidate that the catalyst deactivation is caused by Ni particle sintering via Ostwald ripening instead of coking, and steam induces hydroxylated Ni surface that accelerates sintering. Moreover, encapsulated Mo2C boosts the activity and stability of Ni on zeolite Y by enhancing CH4 activation rather than activating H2O. The interplays among Mo2C and Ni particles dynamically balance the carbon formation and consumption rates, and inhibit Ni sintering. This study enables insights into an alternative design principle of transition metal carbide – Ni catalysts with high activity and stability for effective MSR by tuning the compositional, structural, and interfacial factors.