Structure and Dynamics of Aqueous Electrolytes Confined in 2D-TiO2/Ti3C2T2 MXene Heterostructures
Corpus note
The local PDF is an ACS galley proof; claims summarized from the article abstract. Galley/proof handling: Non-primary article PDF slugs (GitHub) (section D pattern).
Summary¶
Two-dimensional heterostructures that pair MXenes with oxide layers are candidates for electrochemical energy storage, but ion and solvent structure in nanometer confinement controls both capacity and rate capability. The study examines bilayer water intercalation between lepidocrocite-type two-dimensional TiO₂ and hydroxylated or oxygen-terminated Ti₃C₂Tₓ MXene using ReaxFF molecular dynamics, complemented by elastic and quasielastic neutron scattering experiments to probe dynamics. The central question is how heterointerface chemistry steers interlayer water and ion solvation when two dissimilar 2D components share the same electrolyte-filled gallery.
Methods¶
Reactive MD (ReaxFF). Atomistic stacks combine lepidocrocite-type 2D TiO₂ with Ti₃C₂Tₓ MXene and bilayer water in the gallery, varying MXene termination (O vs OH).
Electrolyte / ions. Li⁺, Na⁺, and K⁺ are compared for adsorption sites, solvation structure, and proton-transfer (including H migration from hydroxylated MXene toward TiO₂ where applicable).
Experiments. Elastic and quasielastic neutron scattering (ENS / QENS) on systems parallel to the simulated interfaces constrain dynamics of confined water and ions (time scales and mobility relative to bulk-like references as reported).
Analysis. Density profiles, coordination environments, water diffusivity, and proton shuttling are extracted as a function of termination and cation identity.
MD protocol (additional slots). LAMMPS-style ReaxFF MD on periodic heterostructure supercells with 1000s of atoms (exact counts in ACS Appl. Mater. Interfaces galley/SI). N/A — timestep, duration (ns), thermostat, and ensemble labels are not transcribed to this page. N/A — barostat / pressure control if production is NVT-like. N/A — external macroscopic electric field across the stack in the runs summarized in the abstract (local Coulomb and QEq only). N/A — metadynamics or replica exchange.
FF training (block 2). N/A — applies a CHOTi-class ReaxFF as described in the paper, not a new parameter fit in this work.
Static QM (block 3). N/A — not a DFT paper; neutron data are experimental benchmarks.
Findings¶
Termination control. MXene surface chemistry (especially O vs OH) dominates interlayer hydration; TiO₂ provides secondary templating (e.g., notch-like O ridges).
Water dynamics. TiO₂ lowers water self-diffusion versus MXene-only stacks because notch sites trap water.
Ion siting. Li⁺ binds preferentially at TiO₂ notches; Na⁺ shows more planar water coordination; K⁺ partitioning between MXene and TiO₂ depends on termination—OH-terminated MXene can repel K⁺ toward TiO₂ while Na⁺ solvation is partly preserved (size effects in the authors’ interpretation).
Protons. OH-rich surfaces favor net proton transfer toward TiO₂, increasing stored proton character in the heterostructure. Na⁺ impedes proton migration least and Li⁺ most (linked to Li⁺ proximity to TiO₂).
Performance trade-off. The inferred ion/water structure implies a capacity vs rate compromise (Na⁺ case called out for power response). Neutron data support both structural and dynamical interpretations of confined water/ion behavior beyond MD alone.
Limitations¶
Finite-size supercells and short trajectories may under-sample rare ion hops; reactive force-field errors accumulate for highly charged transition-metal environments.
Relevance to group¶
Joint ORNL–Penn State effort combining ReaxFF with neutron scattering on MXene–oxide electrolyte confinement, tightly aligned with electrochemical materials thrusts.
Citations and evidence anchors¶
- https://doi.org/10.1021/acsami.0c17536