X-ray Insights into Formation of -O Functional Groups on MXenes: Two-Step Dehydrogenation of Adsorbed Water

Ultraviolet Photothermal Radiation Customized Nitrogen Terminals of Ti3C2Tx MXene for High-Performance Supercapacitors

Two-dimensional MXenes have emerged as exceptional electrode materials for supercapacitors (SCs), making them highly attractive for energy storage and conversion applications. However, their electrochemical performance is strongly influenced by surface terminal groups and interlayer spacing. In this study, we introduce an ultraviolet(UV)-induced nitrogen-doping method that employs UV radiation to promote the thermal decomposition of ammonium oxalate while inducing nitrogen-doping. The resulting UV-induced nitrogen-doped Ti3C2Tx (I-Ti3C2Tx-N) exhibits a high N doping level of 1.46 at. % and an expanded lattice spacing of 1.43 nm. As a result, I-Ti3C2Tx-N demonstrates exceptional pseudocapacitance performance, achieving a remarkable specific capacitance of 466.28 F g-1, significantly exceeding that of raw Ti3C2Tx (367.96 F g-1). Furthermore, when integrated into a quasi-solid-state SC device, it delivers an impressive energy density of 12.58 Wh kg-1 at a power density of 250.00 W kg-1. The enhanced electrochemical performance of I-Ti3C2Tx-N is attributed to the effects of UV radiation, which introduces N terminal groups, eliminates detrimental -F and -OH terminals, and increases interlayer spacing. This study highlights a simple yet effective UV-induced nitrogen-doping method for modifying MXene materials, offering another way for optimizing their electrochemical properties in energy storage applications.

Interplay of Ultrafast Electron-Phonon and Electron-Electron Scattering in Ti3C2Tx MXenes: Ab Initio Quantum Dynamics

Nonthermal electrons are vital in solar energy and optoelectronics, yet their relaxation pathways are not fully understood. Ab initio quantum dynamics reveal that in Ti3C2O2 electron-phonon (e-ph) relaxation is faster than electron-electron (e-e) scattering due to strong coupling with the A1g phonon at 190 cm-1 and the presence of light C and O atoms. Nuclear quantum effects are minimal; vibrations influence e-e scattering only indirectly, and the A1g mode' zero-point energy is much lower than thermal energy at ambient conditions. Substituting O with heavier S in Ti3C2OS slows e-ph relaxation and enhances e-e scattering, making it a faster process. However, both channels proceed concurrently, challenging the e-e and e-ph time scale separation often used for metals. These results underscore the need for atomistic-level understanding of nonthermal electron dynamics, especially in light-element systems such as MXenes, and provide guidance for optimizing electronic relaxation in advanced optoelectronic materials and devices.

In situ oxidized Mo2CTx MXene film via electrochemical activation for smart electrochromic supercapacitors

Mo2CTx MXenes have great potential for multifunctional energy storage applications because of their outstanding electrical conductivity, superior cycling stability, and high optical transmittance. In this study, we fabricate Mo2CTx film electrodes (referred to as Mo2C) on fluorine-doped tin oxide (FTO) substrates using the layer-by-layer (LbL) self-assembly technique. To improve the energy-storage performance of Mo2CTx film electrodes, we develop a convenient electrochemical activation process to prepare in situ oxidized Mo2CTx/MoO3 film electrodes (referred to as EA-Mo2C). The Mo2CTx/MoO3 hybrid film benefits from the addition of MoO3, which introduces extra redox sites and enhances the charge-storage capacity. Furthermore, the unique layered structure of Mo2CTx significantly reduces the diffusion energy barrier for cations. The synergistic interaction between Mo2CTx and MoO3 results in superior electrochemical performance, and the EA-Mo2C displays a remarkable increase in areal specific capacitance, achieving 23.29 mF cm-2 at a current density of 1.5 mA cm-2, which is 518 % higher than that of Mo2C. The electrochromic supercapacitor, assembled using EA-Mo2C as the ion-storage layer and polyaniline (PANI) as the electrochromic layer, enables power visualization and quantitative display. In summary, this study utilizes in situ electrochemical activation to derive high-performance electrode materials, offering an innovative strategy for advancing MXene-based energy-storage materials.

Challenges and opportunities in 2D materials for high-performance aqueous ammonium ion batteries

Aqueous ammonium ion batteries (AAIBs) have attracted considerable attention due to their high safety and rapid diffusion kinetics. Unlike spherical metal ions, NH4 + forms hydrogen bonds with host materials, leading to a unique storage mechanism. A variety of electrode materials have been proposed for AAIBs, but their performance often falls short in terms of future energy storage needs. Hence, there is a critical need to design and develop advanced electrode materials for AAIBs. 2D materials, with their tunable interlayer spacing, remarkable interfacial chemistry and abundant surface functional groups, are an ideal choice for electrode materials for NH4 + storage. This review highlights the latest research on 2D electrode materials for AAIBs, providing insights into their working principles, NH4 + storage mechanisms and control strategies for designing high-performance AAIBs. Furthermore, a summary and future perspectives on 2D electrode materials in the development of AAIBs are provided, aiming to promote the advancement of high-performance AAIBs.

Ultrabroadband Optical Properties of 2D Titanium Carbide MXene

MXenes have rapidly ascended as a prominent class of two-dimensional (2D) materials, renowned for their distinctive optical and electrical properties. Despite extensive exploration of MXenes' optical properties, existing studies predominantly focus on the near-infrared (NIR) to the ultraviolet spectral range, leaving the mid-infrared (mid-IR) range relatively uncharted. In this study, we conducted a comprehensive characterization of the intrinsic optical properties of Ti3C2Tx MXene across an ultrabroadband spectral range, spanning from mid-IR (28 meV) to vacuum ultraviolet (VUV, 6.4 eV). For this purpose, Ti3C2Tx MXene films of varying thicknesses were coated on quartz substrates, resulting in two distinct categories: thin film samples with thicknesses below 50 nm and bulk-like samples with thicknesses exceeding 500 nm. Using spectroscopic ellipsometry, we analyzed the optical properties of films of various thicknesses and extracted detailed information on their dielectric functions. Our findings reveal resonances in the mid-IR to VUV range. Employing the Lorentz-Drude model to examine these resonances has uncovered the optical resistivity of MXene films and led to the identification of multiple plasmonic modes active in the visible to NIR range, as well as broad band-to-band transition-like resonances in the mid-IR range. This ultrabroadband optical versatility of Ti3C2Tx MXene is anticipated to bring about a wide range of thermal and optical applications.

Manipulating d-Band Center of Nickel by Single-Iodine-Atom Strategy for Boosted Alkaline Hydrogen Evolution Reaction

Ni-based electrocatalysts have been predicted as highly potential candidates for hydrogen evolution reaction (HER); however, their applicability is hindered by an unfavorable d-band energy level (Ed). Moreover, precise d-band structural engineering of Ni-based materials is deterred by appropriative synthesis methods and experimental characterization. Herein, we meticulously synthesize a special single-iodine-atom structure (I-Ni@C) and characterize the Ed manipulation via resonant inelastic X-ray scattering (RIXS) spectroscopy to fill this gap. The complex catalytic mechanism has been elucidated via synchrotron radiation-based multitechniques (SRMS) including X-ray absorption fine structure (XAFS), in situ synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy, and near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). In particular, RIXS is innovatively applied to reveal the precise regulation of Ni Ed of I-Ni@C. Consequently, the role of such single-iodine-atom strategy is confirmed to not only facilitate the moderate Ed of the Ni site for balancing the adsorption/desorption capacities of key intermediates but also act as a bridge to enhance the electronic interaction between Ni and the carbon shell for forming a localized polarized electric field conducive to H2O dissociation. As a result, I-Ni@C exhibits an enhanced alkaline hydrogen evolution performance with an overpotential of 78 mV at 10 mA/cm2 and superior stability, surpassing the majority of the reported Ni-based catalysts. Overall, this study has managed to successfully tailor the d-band center of materials from the SRMS perspective, which has crucial implications for nanotechnology, chemistry, catalysis, and other fields.

Self-Thermal Management in Filtered Selenium-Terminated MXene Films for Flexible Safe Batteries

Li-ion batteries with superior interior thermal management are crucial to prevent thermal runaway and ensure safe, long-lasting operation at high temperatures or during rapid discharging and charging. Typically, such thermal management is achieved by focusing on the separator and electrolyte. Here, the study introduces a Se-terminated MXene free-standing electrode with exceptional electrical conductivity and low infrared emissivity, synergistically combining high-rate capacity with reduced heat radiation for safe, large, and fast Li+ storage. This is achieved through a one-step organic Lewis acid-assisted gas-phase reaction and vacuum filtration. The Se-terminated Nb2Se2C outperformed conventional disordered O/OH/F-terminated materials, enhancing Li+-storage capacity by ≈1.5 times in the fifth cycle (221 mAh·g-1 at 1 A·g-1) and improving mid-infrared adsorption with low thermal radiation. These benefits result from its superior electrical conductivity, excellent structural stability, and high permittivity in the infrared region. Calculations further reveal that increased permittivity and conductivity along the z-direction can reduce heat radiation from electrodes. This work highlights the potential of surface groups-terminated layered material-based free-standing flexible electrodes with self-thermal management ability for safe, fast energy storage.

High-Throughput Screening of Sulfur Reduction Reaction Catalysts Utilizing Electronic Fingerprint Similarity

The catalytic performance is determined by the electronic structure near the Fermi level. This study presents an effective and simple screening descriptor, i.e., the one-dimensional density of states (1D-DOS) fingerprint similarity, to identify potential catalysts for the sulfur reduction reaction (SRR) in lithium-sulfur batteries. The Δ1D-DOS in relation to the benchmark W2CS2 was calculated. This method effectively distinguishes and identifies 30 potential candidates for the SRR from 420 types of MXenes. Further analysis of the Gibbs free energy profiles reveals that MXene candidates exhibit promising thermodynamic properties for SRR, with the protocol achieving an accuracy rate exceeding 93%. Based on the crystal orbital Hamilton population (COHP) and differential charge analysis, it is confirmed that the Δ1D-DOS could effectively differentiate the interaction between MXenes and lithium polysulfide (LiPS) intermediates. This study underscores the importance of the electronic fingerprint in catalytic performance and thus may pave a new way for future high-throughput material screening for energy storage applications.