Flower-petal-like Nb2C MXene combined with MoS2 as bifunctional catalysts towards enhanced lithium-sulfur batteries and hydrogen evolution

MAX Phase (Nb4 AlC3 ) For Electrocatalysis Applications

In search for novel materials to replace noble metal-based electrocatalysts in electrochemical energy conversion and storage devices, special attention is given to a distinct class of materials, MAX phase that combines advantages of ceramic and metallic properties. Herein, Nb4 AlC3 MAX phase is prepared by a solid-state mixing reaction and characterized morphologically and structurally by transmission and scanning electron microscopy with energy-dispersive X-ray spectroscopy, nitrogen-sorption, X-ray diffraction analysis, X-ray photoelectron and Raman spectroscopy. Electrochemical performance of Nb4 AlC3 in terms of capacitance as well as for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is evaluated in different electrolytes. The specific capacitance Cs of 66.4, 55.0, and 46.0 F g-1 at 5 mV s-1 is determined for acidic, neutral and alkaline medium, respectively. Continuous cycling reveals high capacitance retention in three electrolyte media; moreover, increase of capacitance is observed in acidic and neutral media. The electrochemical impedance spectroscopy showed a low charge transfer resistance of 64.76 Ω cm2 that resulted in better performance for HER in acidic medium (Tafel slope of 60 mV dec-1 ). In alkaline media, the charge storage value in the double layer is 360 mF cm-2 (0.7 V versus reversible hydrogen electrode) and the best ORR performance of the Nb4 AlC3 is achieved in this medium (Tafel slope of 126 mV dec-1 ).

Endogenous Nb2CTx/Nb2O5 Schottky heterostructures for superior lithium-ion storage

Schottky heterostructures have significant advantages for exciting charge transfer kinetics at material interfaces. In this work, endogenous Nb2CTx/Nb2O5 Schottky heterostructures with a large active surface area were constructed using an in-situ architectural strategy. The semiconductor Nb2O5 has a low work function, and during the construction of Nb2CTx/Nb2O5 Schottky heterostructures, there was an interfacial electron transfer, which resulted in a built-in electric field. The electrochemical reaction kinetics of Nb2CTx/Nb2O5 Schottky heterostructures were enhanced due to the rapid transfer of charge driven by the electric field. The Nb2CTx/Nb2O5 Schottky heterostructures have a large active surface area, which contributes to excellent electrolyte diffusion kinetics. Therefore, Nb2CTx/Nb2O5 Schottky heterostructures have excellent lithium-ion storage capacity with 575 mAh/g after 200 cycles at 0.10 A/g, and 290 mAh/g after 1000 cycles at 2.00 A/g, without capacity fading. Furthermore, in-situ X-ray diffraction and ex-situ X-ray photoelectron spectroscopy analyses reveal the mechanisms for structure evolution and lithium-ion storage optimization of Nb2CTx/Nb2O5 Schottky heterostructures during the electrochemical reaction. The construction of Schottky heterostructures with excited charge transport kinetics provides a novel idea for optimizing the lithium-ion storage activity of MXenes materials.

Binary Transition-Metal Sulfides/MXene Synergistically Promote Polysulfide Adsorption and Conversion in Lithium-Sulfur Batteries

Currently, severe shuttle effects and sluggish conversion kinetics are the main obstacles to the advancement of lithium-sulfur (Li-S) batteries. Modification of the battery separator by a catalyst is a promising approach to tackle these problems, but simultaneously obtaining rich catalytic active sites, high conductivity, and remarkable stability remains a great challenge. Herein, a flower-like MXene/MoS2/SnS@C heterostructure as the functional intercalation of Li-S batteries was prepared for accelerating the synergistic adsorption-electrocatalysis of sulfur conversion. The MXene skeleton constructs a three-dimensional conductive network that anchors polysulfides and enhances charge transfer. Meanwhile, the MoS2/SnS has rich active sites for accelerating polysulfide conversion, leading to excellent electrochemical performances. A battery with MXene/MoS2/SnS@C displays an extraordinary capacity of 836.1 mAh g-1 over 200 cycles at 0.5C and demonstrates a remarkable cycling stability with a capacity attenuation of approximately 0.051% per cycle during 1000 cycles at 2C. When the sulfur loading reaches 5.1 mg cm-2, the capacity still maintains 722.4 mAh g-1 over 50 cycles. This research proposes a novel strategy to design stable catalysts for Li-S batteries with an extended lifespan.

A Heterostructured Gel Polymer Electrolyte Modified by MoS2 for High-Performance Lithium-Sulfur Batteries

In Li-S batteries, the shuttle effect of polysulfide lithium (LiPS) on the cathode side and the growth of lithium dendrites on the anode side are two major problems that lead to an insufficient cycle life. Herein, in light of the challenges brought on by the different chemical environments on both sides of Li-S batteries, a heterostructured poly(ethyl acrylate-co-ionic liquid) gel electrolyte with a single-sided electrocatalytic reduced graphene oxide/MoS2 coating (MoS2@rGO-GPE) was developed in order to assemble a high-performance Li-S battery with a self-supporting graphene sulfur cathode. In such a device architecture, there is multiposition suppression of the shuttle effect; that is, the confinement of the graphene foam, the catalysis of the MoS2 composite, and the capture of the gel polymer electrolyte. Our results show that the ionic conductivity of the heterostructured electrolyte is 1.98 mS cm-1, and the Li ion transference number reaches 0.81. The assembled lithium-sulfur battery displays a high initial discharge capacity of 1027 mAh g-1 at 0.1 C, superior cycle stability (80% capacity retention after 500 cycles), and excellent rate performance. This design strategy provides a valuable route for the development of high-performance lithium-sulfur batteries.

Oxygen-Terminated Nb2CO2 MXene with Interfacial Self-Assembled COF as a Bifunctional Catalyst for Durable Zinc-Air Batteries

The desirable air cathode in Zn-air batteries (ZABs) that can effectively balance oxygen evolution and oxygen reduction reactions not only needs to adjust the electronic structure of the catalyst but also needs a unique physical structure to cope with the complex gas-liquid environment. In this work, first-principles calculations were carried out to prove that oxygen-terminated Nb₂CO₂ MXene played an active role in enhancing the sluggish reaction of oxygen intermediates. Nb₂CO₂ MXene could also stimulate the spatial accumulation of discharge products, which was beneficial to improve the stability of secondary ZABs. Molecular dynamics simulation was used to show that the confinement effect of COF could effectively regulate the concentration of O₂ on the surface of Nb₂CO₂@COF, which was conducive to an efficient and durable reaction. COF-LZU1 was self-assembled on the interface of Nb₂CO₂ MXene (Nb₂CO₂@COF) for the first time. The Nb₂CO₂@COF electrode had excellent OER/ORR overpotentials with the potential difference (ΔE) of 0.79 V. When applied to the configuration of ZABs, Nb₂CO₂@COF showed a power density of 75 mW cm^-2 and favorable long-term charge/discharge stability, so it could be used as a potential candidate cathode for noble-metal-based catalysts. This idea of combining MXenes and COFs sheds some light on the design of ZABs.