Structure and Dynamics of Aqueous Electrolytes Confined in 2D-TiO2/Ti3C2T2 MXene Heterostructures

Reversible Constrained Dissociation and Reassembly of MXene Films

Enabling materials to undergo reversible dynamic transformations akin to the behaviors of living organisms represents a critical challenge in the field of material assembly. The pursuit of such capabilities using conventional materials has largely been met with limited success. Herein, the discovery of reversible constrained dissociation and reconfiguration in MXene films, offering an effective solution to overcome this obstacle is reported. Specifically, MXene films permit rapid intercalation of water molecules between their distinctive layers, resulting in a significant expansion and exhibiting confined dissociation within constrained spaces. Meanwhile, the process of capillary compression driven by water evaporation reinstates the dissociated MXene film to its original compact state. Further, the adhesive properties emerging from the confined disassociation of MXene films can spontaneously induce fusion between separate films. Utilizing this attribute, complex structures of MXene films can be effortlessly foamed and interlayer porosity precisely controlled, using only water as the inducer. Additionally, a parallel phenomenon has been identified in graphene oxide films. This work not only provides fresh insights into the microscopic mechanisms of 2D materials such as MXene but also paves a transformative path for their macroscopic assembly applications in the future.

Covalent Benzenesulfonic Functionalization of a Graphene Nanopore for Enhanced and Selective Proton Transport

A fundamental understanding of proton transport through graphene nanopores, defects, and vacancies is essential for advancing two-dimensional proton exchange membranes (PEMs). This study employs ReaxFF molecular dynamics, metadynamics, and density functional theory to investigate the enhanced proton transport through a graphene nanopore. Covalently functionalizing the nanopore with a benzenesulfonic group yields consistent improvements in proton permeability, with a lower activation barrier (≈0.15 eV) and increased proton selectivity over sodium cations. The benzenesulfonic functionality acts as a dynamic proton shuttle, establishing a favorable hydrogen-bonding network and an efficient proton transport channel. The model reveals an optimal balance between proton permeability and selectivity, which is essential for effective proton exchange membranes. Notably, the benzenesulfonic-functionalized graphene nanopore system achieves a theoretically estimated proton diffusion coefficient comparable to or higher than the current state-of-the-art PEM, Nafion. Ergo, the benzenesulfonic functionalization of graphene nanopores, firmly holds promise for future graphene-based membrane development in energy conversion devices.

Low pressure reversibly driving colossal barocaloric effect in two-dimensional vdW alkylammonium halides

Plastic crystals as barocaloric materials exhibit the large entropy change rivalling freon, however, the limited pressure-sensitivity and large hysteresis of phase transition hinder the colossal barocaloric effect accomplished reversibly at low pressure. Here we report reversible colossal barocaloric effect at low pressure in two-dimensional van-der-Waals alkylammonium halides. Via introducing long carbon chains in ammonium halide plastic crystals, two-dimensional structure forms in (CH3-(CH2)n-1)2NH2X (X: halogen element) with weak interlayer van-der-Waals force, which dictates interlayer expansion as large as 13% and consequently volume change as much as 12% during phase transition. Such anisotropic expansion provides sufficient space for carbon chains to undergo dramatic conformation disordering, which induces colossal entropy change with large pressure-sensitivity and small hysteresis. The record reversible colossal barocaloric effect with entropy change ΔSr ~ 400 J kg-1 K-1 at 0.08 GPa and adiabatic temperature change ΔTr ~ 11 K at 0.1 GPa highlights the design of novel barocaloric materials by engineering the dimensionality of plastic crystals.

Fe3 C/nanocarbon-Enabled Lithium Dendrite Mitigation in Lithium-Sulfur batteries

Lithium dendrite-induced short circuits and material loss are two major obstacles to the commercialization of lithium-sulfur (Li-S) batteries. Here, a nanocarbon composite consisting of cotton-derived Fe3 C-encapsulated multiwalled carbon nanotubes (Fe3 C-MWCNTs) and graphene effectively traps polysulfides to suppress lithium dendrite growth is reported. Machine learning combined with molecular dynamics (MD) simulations unveils a new polysulfide-induced lithium dendrite formation mechanism: the migration of polysulfides away from the anode drags out lithium protrusions through localized lattice distortion of the lithium anode and traps lithium ions in the surrounding electrolyte, leading to lithium dendrite formation. The Li-S battery, constructed using the composite of cotton-derived Fe3 C-MWCNTs and graphene that serves as both the sulfur host and the anode interlayer, exhibits exceptional cycling stability, impressive capacity retention, and effective mitigation of lithium dendrite formation. The findings offer valuable strategies to prevent lithium dendrite formation and enhance understanding of lithium dendrite growth in Li-S batteries.

Laser-Induced MXene-Functionalized Graphene Nanoarchitectonics-Based Microsupercapacitor for Health Monitoring Application

Microsupercapacitors (micro-SCs) with mechanical flexibility have the potential to complement or even replace microbatteries in the portable electronics sector, particularly for portable biomonitoring devices. The real-time biomonitoring of the human body's physical status using lightweight, flexible, and wearable micro-SCs is important to consider, but the main limitation is, however, the low energy density of micro-SCs as compared to microbatteries. Here using a temporally and spatially controlled picosecond pulsed laser, we developed high-energy-density micro-SCs integrated with a force sensing device to monitor a human body's radial artery pulses. The photochemically synthesized spherical laser-induced MXene (Ti3C2Tx)-derived oxide nanoparticles uniformly attached to laser-induced graphene (LIG) act as active electrode materials for micro-SCs. The molecular dynamics simulations and detailed spectroscopic analysis reveal the synergistic interfacial interaction mechanism of Ti-O-C covalent bonding between MXene and LIG. The incorporation of MXene nanosheets improves the graphene sheet alignment and ion transport while minimizing self-restacking. Furthermore, the micro-SCs based on a nano-MXene-LIG hybrid demonstrate high mechanical flexibility, durability, ultrahigh energy density (21.16 × 10-3 mWh cm-2), and excellent capacitance (∼100 mF cm-2 @ 10 mV s-1) with long cycle life (91% retention after 10 000 cycles). Such a single-step roll-to-roll highly reproducible manufacturing technique using a picosecond pulsed laser to induce MXene-derived spherical oxide nanoparticles (size of quantum dots) attached uniformly to laser-induced graphene for biomedical device fabrication is expected to find a wide range of applications.

Charge equilibration model with shielded long-range Coulomb for reactive molecular dynamics simulations

Atomic description of electrochemical systems requires reactive interaction potential to explicitly describe the chemistry between atoms and molecules and the evolving charge distribution and polarization effects. Calculating Coulomb electrostatic interactions and polarization effects requires a better estimate of the partial charge distribution in molecular systems. However, models such as reactive force fields and charge equilibration (QEq) include Coulomb interactions up to a short-distance cutoff for better computational speeds. Ignoring long-distance electrostatic interaction affects the ability to describe electrochemistry in large systems. We studied the long-range Coulomb effects among charged particles and extended the QEq method to include long-range effects. By this extension, we anticipate a proper account of Coulomb interactions in reactive molecular dynamics simulations. We validate the approach by computing charges on a series of metal-organic frameworks and some simple systems. Results are compared to regular QEq and quantum mechanics calculations. The study shows slightly overestimated charge values in the regular QEq approach. Moreover, our method was combined with Ewald summation to compute forces and evaluate the long-range effects of simple capacitor configurations. There were noticeable differences between the calculated charges with/without long-range Coulomb interactions. The difference, which may have originated from the long-range influence on the capacitor ions, makes the Ewald method a better descriptor of Coulomb electrostatics for charged electrodes. The approach explored in this study enabled the atomic description of electrochemical systems with realistic electrolyte thickness while accounting for the electrostatic effects of charged electrodes throughout the dielectric layer in devices like batteries and emerging solid-state memory.

Effect of Electrode/Electrolyte Coupling on Birnessite (δ-MnO2) Mechanical Response and Degradation

Understanding the deformation of energy storage electrodes at a local scale and its correlation to electrochemical performance is crucial for designing effective electrode architectures. In this work, the effect of electrolyte cation and electrode morphology on birnessite (δ-MnO₂) deformation during charge storage in aqueous electrolytes was investigated using a mechanical cyclic voltammetry approach via operando atomic force microscopy (AFM) and molecular dynamics (MD) simulation. In both K₂SO₄ and Li₂SO₄ electrolytes, the δ-MnO₂ host electrode underwent expansion during cation intercalation, but with different potential dependencies. When intercalating Li⁺, the δ-MnO₂ electrode presents a nonlinear correlation between electrode deformation and electrode height, which is morphologically dependent. These results suggest that the stronger cation-birnessite interaction is the reason for higher local stress heterogeneity when cycling in Li₂SO₄ electrolyte, which might be the origin of the pronounced electrode degradation in this electrolyte.

Reactive Molecular Dynamics Simulation on Degradation of Tetracycline Antibiotics Treated by Cold Atmospheric Plasmas

The abuse of tetracycline antibiotics (TCs) has caused serious environmental pollution and risks to public health. Degradation of TCs by cold atmospheric plasmas (CAPs) is a high efficiency, low energy consumption and environmentally friendly method. In this study, a reactive molecular dynamics (MD) simulation is applied to study the interactions of reactive oxygen species (ROS) produced in CAPs and TCs (including tetracycline (TC), oxytetracycline (OTC), chlortetracycline (CTC) and demeclocycline (DMC)). As revealed by the simulation data at the atomic level, the main reaction sites on TCs are the C2 acylamino, the C4 dimethylamine, the C6 methyl group, the C8 site on the benzene ring and the C12a tertiary alcohol. The interaction between ROS and TCs is usually initiated by H-abstraction, followed by the breaking and formation of the crucial chemical bonds, such as the breaking of C-C bonds, C-N bonds and C-O bonds and the formation of C=C bonds and C=O bonds. Due to the different structures of TCs, when the ROS impact OTC, CTC and DMC, some specific reactions are observed, including carbonylation at the C5 site, dechlorination at the C7 site and carbonylation at the C6 site, respectively. Some degradation products obtained from the simulation data have been observed in the experimental measurements. In addition, the dose effects of CAP on TCs by adjusting the number of ROS in the simulation box are also investigated and are consistent with experimental observation. This study explains in detail the interaction mechanisms of degradation of TCs treated by CAPs with the final products after degradation, provides theoretical support for the experimental observation, then suggests optimization to further improve the efficiency of degradation of TCs by CAPs in applications.

Oxidation and hydrogenation of monolayer MoS2 with compositing agent under environmental exposure: The ReaxFF Mo/Ti/Au/O/S/H force field development and applications

An atomistic modeling tool is essential to an in-depth understanding upon surface reactions of transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2), with the presence of compositing agents, including Ti and Au, under different environmental exposures. We report a new ReaxFF reactive force field parameter set for Mo, Ti, Au, O, S, and H interactions. We apply the force field in a series of molecular dynamics (MD) simulations to unravel the impact of the Ti dopant on the oxidation/hydrogenation behaviors of MoS2 surface. The simulation results reveal that, in the absence of Ti clusters, the MoS2 surface is ruptured and oxidized at elevated temperatures through a process of adsorption followed by dissociation of the O2 molecules on the MoS2 surface during the temperature ramp. When the MoS2 surface is exposed to H2O molecules, surface hydrogenation is most favored, followed by oxidation, then hydroxylation. The introduction of Ti clusters to the systems mitigates the oxidation/hydrogenation of MoS2 at a low or intermediate temperature by capturing the O2/H2O molecules and locking the O/H-related radicals inside the clusters. However, OH− and H3O+ are emitted from the Ti clusters in the H2O environment as temperature rises, and the accelerating hydrogenation of MoS2 is consequently observed at an ultra-high temperature. These findings indicate an important but complex role of Ti dopants in mitigating the oxidation and hydrogenation of MoS2 under different environmental exposures. The possible mechanisms of oxidation and hydrogenation revealed by MD simulations can give an insight to the design of oxidation resistant TMDs and can be useful to the optical, electronic, magnetic, catalytic, and energy harvesting industries.

Importance of Nuclear Quantum Effects on Aqueous Electrolyte Transport under Confinement in Ti3C2 MXenes

Protons display a high chemical activity and strongly affect the charge storage capability in confined interlayer spaces of two-dimensional (2D) materials. As such, an accurate representation of proton dynamics under confinement is important for understanding and predicting charge storage dynamics in these materials. While often ignored in atomistic-scale simulations, nuclear quantum effects (NQEs), e.g., tunneling, can be significant under confinement even at room temperature. Using the thermostatted ring polymer molecular dynamics implementation of path integral molecular dynamics (PIMD) in conjunction with the ReaxFF force field, density functional tight binding (DFTB), and NequIP neural network potential simulations, we investigate the role of NQEs on proton and water transport in bulk water and aqueous electrolytes under confinement in Ti₃C₂ MXenes. Although overall NQEs are relatively small, especially in bulk, we find that they can alter both quantitative values and qualitative trends on both proton transport and water self-diffusion under confinement relative to classical MD predictions. Therefore, our results suggest the need for NQEs to be considered to simulate aqueous systems under confinement for both qualitative and quantitative accuracy.

Wafer-Scale Lateral Self-Assembly of Mosaic Ti3C2Tx MXene Monolayer Films

Bottom-up assembly of two-dimensional (2D) materials into macroscale morphologies with emergent properties requires control of the material surroundings, so that energetically favorable conditions direct the assembly process. MXenes, a class of recently developed 2D materials, have found new applications in areas such as electrochemical energy storage, nanoscale electronics, sensors, and biosensors. In this paper, we present a lateral self-assembly method for wafer-scale deposition of a mosaic-type 2D MXene flake monolayer that spontaneously orders at the interface between two immiscible solvents. ReaxFF molecular dynamics simulations elucidate the interactions of a MXene flake with the solvents and its stability at the liquid/liquid interface, the prerequisite for MXene flakes self-assembly at the interface. Moreover, facile transfer of this monolayer onto a flat substrate (Si, glass) results in high-coverage monolayer films with uniform thickness and homogeneous optical properties. Multiscale characterization of the resulting films reveals the mosaic structure and sheds light on the electronic properties of the films, which exhibit good electrical conductivity over cm-scale areas.