Vapor Deposition Red Phosphorus to Prepare Nitrogen-Doped Ti3C2Tx MXenes Composites for Lithium-Ion Batteries

Engineering the highly efficient heterogeneous catalyst based on PdCu nanoalloy and nitrogen-doped Ti3C2Tx MXene for ethanol electrooxidation

Improving the electrocatalytic performance by modulating the surface and interface electronic structure of noble metals is still a research hotspot in electrocatalysis. Herein, we prepared the heterogeneous catalyst based on the well-dispersed PdCu nanoalloy and the N-doped Ti₃C₂T_x MXene support (PdCu/N-Ti₃C₂T_x) via in situ growth of PdCu nanoparticles on the fantastic N-Ti₃C₂T_x sheets. By exploring the electrocatalytic properties of ethanol oxidation reaction (EOR), the composition optimized Pd₁Cu₁/N-Ti₃C₂T_x delivers higher mass activity/specific activity/intrinsic activity (2200.7 mA mg_Pd^-1/13.1 mA cm^-2/2.2 s^-1), anti-poisoning ability and stability than those of Pd/N-Ti₃C₂T_x, Pd₁Cu₁/Ti₃C₂T_x and commercial Pd/C, which can be attributed to the modified surface electronic features of Pd by the participation of Cu atoms and N-Ti₃C₂T_x MXene, as well as the "metal-carrier" effect between the PdCu nanoalloy and N-Ti₃C₂T_x heterogeneous interface. Furthermore, the conductivity of N-Ti₃C₂T_x MXene can be improved by N-doping, and the abundant terminal groups (-O, -OH, -F and N) on the N-Ti₃C₂T_x surface can promote the electron exchange between PdCu and N-Ti₃C₂T_x. This work provides an effective strategy for engineering heterogeneous electrocatalysts for enhanced electrocatalytic EOR by adjusting the interfacial electronic structure of noble metals.

Design and synthesis of novel pomegranate-like TiN@MXene microspheres as efficient sulfur hosts for advanced lithium sulfur batteries†

Lithium–sulfur (Li–S) batteries have the characteristics of low cost, environmental protection, and high theoretical energy density, and have broad application prospects in the new generation of electronic products. However, there are some problems that seriously hinder the Li–S batteries from going from the laboratory to the factory, such as poor stability caused by the large volume expansion of sulfur during charging and discharging, sluggish kinetics of the electrochemical reaction resulting from the low conductivity of the active materials, and loss of active materials arising from the dissolution and diffusion of the intermediate product lithium polysulfides (LiPSs). In this paper, the two-dimensional layered material MXene and TiN are firstly combined by spray drying method to prepare pomegranate-like TiN@MXene microspheres with both adsorption capacity and catalytic effect on LiPSs conversion. The interconnected skeleton composed of MXene not only solves the problem of easy stacking of MXene sheets but also ensures the uniform distribution of sulfur. Without affecting the excellent characteristics of MXene itself, the overall conductivity of the composite electrode material is improved. The TiN hollow nanospheres are coated with MXene layers to form a shell, catalyzing the adsorption of LiPSs and accelerating the transformation of high-order LiPSs to Li₂S₂/Li₂S. As a result, the TiN@MXene cathode delivers a high initial discharge capacity of 1436 mA h g⁻¹ at 0.1C, excellent rate performance of 636 mA h g⁻¹ up to 3C, and an ultralong lifespan over 1000 cycles with a small capacity decay of 0.048% per cycle at the current density of 1.0C.

A novel pomegranate-like TiN@MXene microsphere was constructed by a facile spray drying method and synergistically enhanced the conversion of polysulfides in Li–S batteries.

Phosphorus-Based Materials for High-Performance Alkaline Metal Ion Batteries: Progress and Prospect

Alkaline metal-ion batteries (AIBs) such as lithium-ion batteries (LIBs), sodium-ion batteries (NIBs), and potassium-ion batteries (KIBs) are potential energy storage systems. Currently, although LIBs are widely used in consumer electronics and electric vehicles, the electrochemical performance, safety, and cost of current AIBs are still unable to meet the needs for many future applications, such as large-scale energy storage, due to the low theoretical capacity of cathode/anode materials, flammability of electrolytes and limited Li resources. It is thus imperative to develop new materials to improve the properties of AIBs. Several promising cathodes, anodes, and electrolytes have been developed and among the new battery materials, phosphorus-based (P-based) materials have shown great promise. For example, P and metal phosphide anodes have high theoretical capacity, resource abundance, and environmental friendliness boding well for future high-energy-density AIBs. Besides, phosphate cathode materials have the advantages of low cost, high safety, high voltage, and robust stability, and P-based materials like LiPF₆ and lithium phosphorus oxynitride are widely used electrolytes. In this paper, the latest development of P-based materials in AIBs, challenges, effective solutions, and new directions are discussed.

Modifying Ti3C2 MXene with NH4+ as an excellent anode material for improving the performance of microbial fuel cells

Poor anode performance is one of the main bottlenecks in the development of microbial fuel cells (MFCs) for practical applications. Multilayered Ti₃C₂ MXene (m-MXene) is an alternative anode modification material because of its high specific surface area and electrical conductivity. However, the multilayered structure, negatively charged surface, and electropositivity of m-MXene could limit its modification effects. In this work, we used a solution-phase flocculation method (ammonium ion method) to restack and aggregate MXene nanosheets as an anode modification material (n-MXene). The n-MXene-modified anode had a higher specific surface area, surface hydrophilicity and surface electropositivity than the m-MXene-modified anode. The n-MXene-modified anode obtained a maximum current density of 2.1 A m^-2, which was 31.2% and 61.5% higher than that of the m-MXene-modified anode (1.6 A m^-2) and bare carbon fiber cloth anode (1.3 A m^-2). This improved anode performance was attributed to both the decrease in the charge transfer resistance and diffusion resistance, which were related to the increased quantity of biomass and microbial nanowire (or pili)-shaped filaments on the electrode surface. This work provides a simple and cost-effective approach to prepare MXene nanosheets for the modification of MFC anodes.

Ti3C2 MXene with pillared structure for hybrid magnesium-lithium batteries cathode material with long cycle life and high rate capability

Cationic surfactants (CS) pillared Ti₃C₂ composites (Ti₃C₂/CS) were prepared by a facile electrostatic assembly method, which have large interlayer spacing and slight N-doping. In hybrid magnesium-lithium batteries (HMLBs), the Ti₃C₂/CS composites exhibit excellent performance by utilizing both Li⁺ and Mg²⁺ as charge carriers. Among these composites, the Ti₃C₂/CTAB (CTC) electrode displays a reversible capacity of 115.9 and 60 mAh g^-1 in APC/LiCl (APCL) and APC electrolytes at 0.1 A g^-1, and it also exhibits excellent high rate performance and ultralong cycle performance. It is verified that CS is vital to significantly improve the diffusion kinetics of Mg²⁺ on the electrode surface. The CS can act as the conductive "bridge" which connects different Ti₃C₂ layers and the interlayer pillar which expands the interlayer distance. In addition, the N element in CS is effective in neutralizing electronegativity and enhancing electrical conductivity for the CTC electrode. The electrode design strategy can adapt to the synthesis of cathode materials with high rate capability in HMLBs.

Delamination and Engineered Interlayers of Ti3C2 MXenes using Phosphorous Vapor toward Flame-Retardant Epoxy Nanocomposites

As recently created inorganic nanosheet materials, more and more light has been shed on MXenes, which have emerged as a hotspot of intensive investigations. The simple exfoliation method for MXenes attracts numerous studies to pay efforts on. Compared with the extensive research about ultrasonication and mechanical milling, gas-assisted exfoliation has never been carried out for MXenes. Meanwhile, MXene-based nanocomposites are always prepared after exfoliation step by step. In this work, a facile way to fabricate a few-layered Ti₃C₂ MXene delaminated using phosphorous vapor evolved from commercial red phosphorous (RP) is put forward. The vapor deposits on the surface of Ti₃C₂ and also partially intercalates into the interlayers to obtain a novel two-dimensional RP/Ti₃C₂ nanocomposite directly. The P element strongly connects with the substrate by a covalent bond that improves the safety problems for RP during storage and usage. Due to the versatile feature of MXenes, the nanocomposite has the potential to be applied in a variety of fields. Herein, it is employed as a flame retardant for epoxide resin and effectively reduces fire disaster. The one-step exfoliation plus nanocomposite fabrication provides a more feasible way for the practical application of MXenes.

Hetero-MXenes: Theory, Synthesis, and Emerging Applications

Since their discovery in 2011, MXenes (abbreviation for transition metal carbides, nitrides, and carbonitrides) have emerged as a rising star in the family of 2D materials owing to their unique properties. Although the primary research interest is still focused on pristine MXenes and their composites, much attention has in recent years been paid also to MXenes with diverse compositions. To this end, this work offers a comprehensive overview of the progress on compositional engineering of MXenes in terms of doping and substituting from theoretical predictions to experimental investigations. Synthesis and properties are briefly introduced for pristine MXenes and then reviewed for hetero-MXenes. Theoretical calculations regarding the doping/substituting at M, X, and T sites in MXenes and the role of vacancies are summarized. After discussing the synthesis of hetero-MXenes with metal/nonmetal (N, S, P) elements by in situ and ex situ strategies, the focus turns to their emerging applications in various fields such as energy storage, electrocatalysts, and sensors. Finally, challenges and prospects of hetero-MXenes are addressed. It is anticipated that this review will be beneficial to bridge the gap between predictions and experiments as well as to guide the future design of hetero-MXenes with high performance.

Rational Design of Pillared SnS/Ti3C2Tx MXene for Superior Lithium-Ion Storage

MXenes have been widely explored in energy storage because of their extraordinary properties; however, the majority of research on their application was staged at multilayered MXenes or assisted by carbon materials. Scientifically speaking, the two most distinctive properties of MXenes are usually neglected, composed of large interlayer spacing and abundant surface chemistry, which distinguish MXenes from other two-dimensional materials. Herein, few-layered MXene (f-MXene) nanosheet powders can be easily prepared according to the modified solution-phase flocculation method, completely avoiding the restacking phenomenon of f-MXene nanosheets in preparation and oxidation issues during the storage process. Via further employing the solvothermal reaction and annealing treatment, we successfully constructed pillared SnS/Ti₃C₂T_x composites decorated with in situ formed TiO₂ nanoparticles. In the composites, MXenes can play the role of a conductive network, a buffer matrix for volume expansion of SnS, while the active SnS nanoplates can fully deliver the advantage of high capacity and further induce interlayer engineering of Ti₃C₂T_x during cycling. As a result, the pillared SnS/Ti₃C₂T_x MXene composites exhibit obvious improvement in electrochemical performance. Interestingly, there is an apparent enhancement of capacity at succedent cycling, which can be ascribed to the "pillar effect" of Ti₃C₂T_x MXenes. The efforts and attempts made in this work can further broaden the development of pillared MXene composites.

Rational Design of Two-Dimensional Transition Metal Carbide/Nitride (MXene) Hybrids and Nanocomposites for Catalytic Energy Storage and Conversion

Electro-, photo-, and photoelectrocatalysis play a critical role toward the realization of a sustainable energy economy. They facilitate numerous redox reactions in energy storage and conversion systems, enabling the production of chemical feedstock and clean fuels from abundant resources like water, carbon dioxide, and nitrogen. One major obstacle for their large-scale implementation is the scarcity of cost-effective, durable, and efficient catalysts. A family of two-dimensional transition metal carbides, nitrides, and carbonitrides (MXenes) has recently emerged as promising earth-abundant candidates for large-area catalytic energy storage and conversion due to their unique properties of hydrophilicity, high metallic conductivity, and ease of production by solution processing. To take full advantage of these desirable properties, MXenes have been combined with other materials to form MXene hybrids with significantly enhanced catalytic performances beyond the sum of their individual components. MXene hybridization tunes the electronic structure toward optimal binding of redox active species to improve intrinsic activity while increasing the density and accessibility of active sites. This review outlines recent strategies in the design of MXene hybrids for industrially relevant electrocatalytic, photocatalytic, and photoelectrocatalytic applications such as water splitting, metal-air/sulfur batteries, carbon dioxide reduction, and nitrogen reduction. By clarifying the roles of individual material components in the MXene hybrids, we provide design strategies to synergistically couple MXenes with associated materials for highly efficient and durable catalytic applications. We conclude by highlighting key gaps in the current understanding of MXene hybrids to guide future MXene hybrid designs in catalytic energy storage and conversion applications.

Fast and Universal Solution-Phase Flocculation Strategy for Scalable Synthesis of Various Few-Layered MXene Powders

MXenes have gained great attention in various fields because of their fascinating properties; however, the preparation of few-layered MXene powders is still limited by serious restacking of MXene nanosheets. Herein, for the first time, we have demonstrated an effective ammonium ion route to fundamentally address restacking and aggregation of the MXene nanosheets, using a solution-phase flocculation method (NH⁴⁺ method and modified NH⁴⁺ method) for large-scale preparation of few-layered Ti₃C₂T_x MXene powders in large quantities. The as-prepared few-layered MXene nanosheet powders show large size in the ab plane without the restacking phenomenon even at scanning electron microscopy measurements of 400× magnification, demonstrating the effectiveness of the proposed method. The method is also suitable for large-scale synthesis of other few-layered MXene powders, including Nb₄C₃T_x, V₂CT_x, Nb₂CT_x, etc., providing a general approach for the preparation of various few-layered MXene nanosheet powders, which represents a significant result for the development of MXenes.

Recent advances in MXenes and their composites in lithium/sodium batteries from the viewpoints of components and interlayer engineering

An up-to-date review about MXenes based on their distinguishing properties, namely, large interlayer spacing and rich surface chemistry.