An acetate electrolyte for enhanced pseudocapacitve capacity in aqueous ammonium ion batteries.

Ammonium ion batteries are promising for energy storage with the merits of low cost, inherent security, environmental friendliness, and excellent electrochemical properties. Unfortunately, the lack of anode materials restricts their development. Herein, we utilized density functional theory calculations to explore the V2CTx MXene as a promising anode with a low working potential. V2CTx MXene demonstrates pseudocapacitive behavior for ammonium ion storage, delivering a high specific capacity of 115.9 mAh g-1 at 1 A g-1 and excellent capacity retention of 100% after 5000 cycles at 5 A g-1. In-situ electrochemical quartz crystal microbalance measurement verifies a two-step electrochemical process of this unique pseudocapacitive storage behavior in the ammonium acetate electrolyte. Theoretical simulation reveals reversible electron transfer reactions with [NH4+(HAc)3]···O coordination bonds, resulting in a superior ammonium ion storage capacity. The generality of this acetate ion enhancement effect is also confirmed in the MoS2-based ammonium-ion battery system. These findings open a new door to realizing high capacity on ammonium ion storage through acetate ion enhancement, breaking the capacity limitations of both Faradaic and non-Faradaic energy storage.

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.

Realizing Interfacial Electron/Hole Redistribution and Superhydrophilic Surface through Building Heterostructural 2 nm Co0.85 Se-NiSe Nanograins for Efficient Overall Water Splittings.

Electrochemical overall water splitting using renewable energy input is highly desirable for large-scale green hydrogen generation, but it is still challenged due to the lack of low-cost, durable, and highly efficient electrocatalysts. Herein, 1D nanowires composed of numerous 2 nm Co0.85 Se-NiSe nanograin heterojunctions as efficient precious metal-free bifunctional electrocatalyst are reported for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solution with the merits of high activity, durability, and low cost. The abundant microinterface among the ultrafine nanograins and the presence of lattice distortion around nanograin interface is found to create a superhydrophilic surface of the electrocatalyst, which significantly facilitate the fast diffusion of electrolytes and the release of the formed H2 and O2 from the catalyst surface. Furthermore, synergic effect between Co0.85 Se and NiSe grain on adjusting the electronic structure is revealed, which enhances electron mobility for fast electron transport during the HER/OER process. Owing to these merits, the rationally designed Co0.85 Se-NiSe heterostructures display efficient overall water splitting behavior with a low voltage of 1.54 V at 10 mA cm-2 and remarkable long-term durability for the investigated period of 50 h.

Surface Selenization Strategy for V2CTx MXene toward Superior Zn-Ion Storage.

MXenes are promising cathode materials for aqueous zinc-ion batteries (AZIBs) owing to their layered structure, metallic conductivity, and hydrophilicity. However, they suffer from low capacities unless they are subjected to electrochemically induced second phase formation, which is tedious, time-consuming, and uncontrollable. Here we propose a facile one-step surface selenization strategy for realizing advanced MXene-based nanohybrids. Through the selenization process, the surface metal atoms of MXenes are converted to transition metal selenides (TMSes) exhibiting high capacity and excellent structural stability, whereas the inner layers of MXenes are purposely retained. This strategy is applicable to various MXenes, as demonstrated by the successful construction of VSe2@V2CTx, TiSe2@Ti3C2Tx, and NbSe2@Nb2CTx. Typically, VSe2@V2CTx delivers high-rate capability (132.7 mA h g-1 at 2.0 A g-1), long-term cyclability (93.1% capacity retention after 600 cycles at 2.0 A g-1), and high capacitive contribution (85.7% at 2.0 mV s-1). Detailed experimental and simulation results reveal that the superior Zn-ion storage is attributed to the engaging integration of V2CTx and VSe2, which not only significantly improves the Zn-ion diffusion coefficient from 4.3 × 10-15 to 3.7 × 10-13 cm2 s-1 but also provides sufficient structural stability for long-term cycling. This study offers a facile approach for the development of high-performance MXene-based materials for advanced aqueous metal-ion batteries.

Ti3C2Tx nanosheet wrapped core-shell MnO2 nanorods @ hollow porous carbon as a multifunctional polysulfide mediator for improved Li-S batteries.

Lithium-sulfur (Li-S) batteries are regarded as potential next-generation energy storage systems due to their high theoretical energy densities. However, the dissolution of lithium polysulfides (LiPSs) upon cycling can result in severe capacity degradation. Achieving high rate capabilities with good cycling stability remains a huge obstacle for the practical implementation of Li-S batteries. Here we developed a novel, multifunctional, hierarchical structure by self-assembling core-shell MnO2 nanorods @ hollow porous carbon with 2D Ti3C2Tx nanosheets, labelled as MCT, as an efficient polysulfide mediator for Li-S cathodes. The integration of the polar MnO2 core and hollow porous carbon shell captures LiPSs two ways: physical confinement and chemisorption. The conductive Ti3C2Tx nanosheets construct a continuous and conductive network, which not only promotes charge transfer and ion diffusion but also boosts LiPS adsorption and conversion. Based on these merits, the MCT/S cathode delivers good rate capability (688 mA h g-1 at 2.0C) and outstanding long-term cyclability (0.044% capacity decay per cycle over 600 cycles at 2.0C).