Two-dimensional Transition Metal Compounds Research Articles

Simultaneous derivatization and exfoliation of a multilayered Ti3C2Tx MXene into amorphous TiO2 nanosheets for stable K-ion storage.

Two-dimensional transition metal compounds (2D TMCs) have been widely reported in the fields of energy storage and conversion, especially in metal-ion storage. However, most of them are crystalline and lack active sites, and this brings about sluggish ion storage kinetics. In addition, TMCs are generally nonconductors or semiconductors, impeding fast electron transfer at high rates. Herein, we propose a facile one-step route to synthesize amorphous 2D TiO2 with a carbon coating (a-2D-TiO2@C) by simultaneous derivatization and exfoliation of a multilayered Ti3C2Tx MXene. The amorphous structure endows 2D TiO2 with abundant active sites for fast ion adsorption and diffusion, while the carbon coating can facilitate electron transport in an electrode. Owing to these intriguing structural and compositional synergies, a-2D-TiO2@C delivers good cycling stability with a long-term capacity retention of 86% after 2000 cycles at 1.0 A g-1 in K-ion storage. When paired with Prussian blue (KPB) cathodes, it exhibits a high full-cell capacity of 50.8 mA h g-1 at 100 mA g-1 after 140 cycles, which demonstrates its great potential in practical applications. This contribution exploits a new approach for the facile synthesis of a-2D-TMCs and their broad applications in energy storage and conversion.

MX Anti-MXenes from Non-van der Waals Bulks for Electrochemical Applications: The Merit of Metallicity and Active Basal Plane.

Two-dimensional transition-metal compounds (2DTMCs) are promising materials for electrochemical applications, but 2DTMCs with metallicity and active basal planes are rare. In this work, we proposed a simple and effective strategy to extract 2DTMCs from non-van der Waals bulk materials and established a material library of 79 2DTMCs, which we named as anti-MXenes since they are composed of one M atomic layer sandwiched by two X atomic layers. By means of density functional theory computations, 24 anti-MXenes were confirmed to be thermodynamically, dynamically, mechanically, and thermally stable. The metallicity and active basal plane endow these anti-MXenes with potential as excellent electrode materials, for example, as electrocatalysts for hydrogen evolution reactions (HER). Among the noble-metal free anti-MXenes with favorable H-binding, CuS can boost HER at the whole range of H coverages, while CoSi, FeB, CoB, and CoP show promise for HER at some specific H coverages. The active sites are the tetra-coordinating nonmetal atoms at the basal planes, thus rendering a very high density of active sites for these materials. CoB is also a promising anode material for lithium-ion batteries, showing low Li diffusion energy barriers, a very high capacity, and a suitable open circuit voltage. This work promotes the "computational exfoliation" of 2D materials from non-van der Waals bulks and exemplifies the applications of anti-MXenes in various electrochemical processes.

Doping regulation in transition metal compounds for electrocatalysis.

In electrocatalysis, doping regulation has been considered as an effective method to modulate the active sites of catalysts, providing a powerful means for creating a large variety of highly efficient catalysts for various reactions. Of particular interest, there has been growing research concerning the doping of two-dimensional transition-metal compounds (TMCs) to optimize their electrocatalytic performance. Despite the previous achievements, mechanistic insights of doping regulation in TMCs for electrocatalysis are still lacking. Herein, we provide a systematic overview of doping regulation in TMCs in terms of background, preparation, impacts on physicochemical properties, and typical applications including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO2 reduction reaction, and N2 reduction reaction. Notably, we bridge the understanding between the doping regulation of catalysts and their catalytic activities via focusing on the physicochemical properties of catalysts from the aspects of vacancy concentrations, phase transformation, surface wettability, electrical conductivity, electronic band structure, local charge distribution, tunable adsorption strength, and multiple adsorption configurations. We also discuss the existing challenges and future perspectives in this promising field.

Structural Engineering and Coupling of Two-Dimensional Transition Metal Compounds for Micro-Supercapacitor Electrodes.

The development of portable, wearable, and miniaturized integrated electronics has significantly promoted the immense desire for planar micro-supercapacitors (MSCs) among the extremely competitive energy storage devices. However, their energy density is still insufficient owing to the low electrochemical performance of conventional electrode materials. Compared with their bulk counterparts, the large specific surface area and fast ion transport with efficient intercalation of two-dimensional (2D) transition metal compounds have spurred the research platforms for their exploitation in the creation of high-performance MSCs. This Outlook presents a systematic summary of cutting-edge research on atomically thin, layered structures of transition metal dichalcogenides, MXenes, and transition metal oxides/hydroxides. Special emphasis is given to the rapid and durable storage of ions, benefiting from the low ion diffusion barriers of host interlayer spaces. Moreover, various strategies have been described to circumvent the structural damage due to the volume change and simultaneously evincing remarkable electronic properties.

Boosting the high-capacity with multi-active centers: A first-principles investigation of NiPS3 monolayer as an anode material

The specific capacity of lithium-ion batteries (LIBs) is one of the key challenge, which determines the performance in practical devices. Here, we explored that multi-active centers of electrode materials will significantly boost the Li storage capacity. Based on the comprehensive first-principles calculations of NiPS3 monolayer, our results show that Li atom can be strongly adsorbed on Ni and S atoms, which provides the possibility of multi-adsorption of Li atoms. Significantly, the lowest energy barrier of Li diffusing on NiPS3 monolayer is only 0.279 eV, benefiting the ultrafast charging/discharging process. Moreover, the estimated open circuit voltage is about 0.55 eV, implying NiPS3 monolayer as a suitable anode material for LIBs. Importantly, the calculated theoretical specific capacity of NiPS3 monolayer is about 608 (mA h)/g, which is almost the highest value among two-dimensional transition-metal compounds. Thus, our results clearly show that creating multi-active sites in transition-metal compounds has great promise for developing anode materials of high-performance LIBs.

Preparation, spectroscopic and magnetic characterization of a new series of two-dimensional transition metal compounds. The x-ray structure of poly-bis(thiocyanato- N)-bis-μ- [1,2,4] triazolo [1,5- a] pyrimidine- N1, N3)iron(II)

The synthesis of a series of polynuclear compounds of general formula [M(tp) 2(NCS) 2] n (M = Mn, Fe, Co, Ni and tp = [1,2,4]triazolo[1,5− a]pyrimidine, C 5H 4N 4) is described. The compounds are X-ray and IR isostructural and were further characterized by ligand field and magnetic susceptibility measurements. The structure of the iron compound was determined by X-ray diffraction methods. It crystallizes in the orthorhombic space group Aba2, with a=8.028(2), b=20.422(6), c=10.129(5) Å. The structure has been solved from diffractometer data by Patterson and Fourier methods and refined by full-matrtx least-squares to R = 0.040 for 706 observed reflections. The structure consists of octahedral iron complexes in which the metal atom is coordinated by two nitrogen atoms from two monodentate NCS groups and by four nitrogen atoms from four symmetry-related tp molecules; each organic ligand bridges two metal atoms through both non-bridgehead triazole nitrogen atoms, joining the octahedral complexes in a layered system where the Fe···Fe distance is 6.49 Å.