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

Karthik Ganeshan; Yun Kyung Shin; Naresh C Osti; Yangyunli Sun; Kaitlyn Prenger; Michael Naguib; Madhusudan Tyagi; Eugene Mamontov; De-En Jiang; Adri C T van Duin

doi:10.1021/acsami.0c17536

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

ACS Appl Mater Interfaces. 2020 Dec 30;12(52):58378-58389. doi: 10.1021/acsami.0c17536. Epub 2020 Dec 18.

Authors

Affiliations

¹ Department of Mechanical Engineering, The Pennsylvania State University, University Park, State College, Pennsylvania 16802, United States.
² Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
³ Department of Chemistry, University of California, Riverside, California 92521, United States.
⁴ Department of Physics and Engineering Physics, Tulane University, New Orleans, Louisiana 70118, United States.
⁵ NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States.
⁶ Department of Materials Science, University of Maryland, College Park, Maryland 20742, United States.

PMID: 33337151
DOI: 10.1021/acsami.0c17536

Abstract

The synthesis of heterostructures of different two-dimensional (2D) materials offers an approach to combine advantages of different materials constituting the heterostructure and ultimately enhance their performance for applications such as electrochemical energy storage, achieving high energy, and high-power densities. Understanding the behavior of ions and solvents in confinement between these dissimilar layers is critical to understand their performance and control. Considering aqueous electrolytes, we explore the heterostructure of 2D lepidocrocite-type TiO₂ (2D-TiO₂) and hydroxylated or O-terminated Ti₃C₂ MXene using ReaxFF molecular dynamics simulations and elastic/quasielastic neutron scattering techniques. Simulating a bilayer water intercalation, we find that the extent of interlayer hydration is impacted most by the surface terminations on the MXene and is marginally affected by 2D-TiO₂. However, the introduction of 2D-TiO₂ decreases the water self-diffusion due to the notch sites (i.e., surface oxygen ridges) entrapping water molecules. Intercalating alkali cations into the heterostructures, we find that Li⁺ is predominantly adsorbed at the 2D-TiO₂ surface instead of the MXenes with the preferential occupation of the notch sites. In contrast, Na⁺ forms a planar solvation with water, while K⁺ is adsorbed both at the O-terminated MXene and 2D-TiO₂. This behavior is altered when OH-terminated MXene is involved-the repulsion from the protons on the MXene surface forces the K⁺ ions to be adsorbed exclusively to 2D-TiO₂, while Na⁺ retains some of its solvation in the water layer due to its smaller size. In OH-terminated MXenes, we see a consistent transfer of protons from the MXene surface toward 2D-TiO₂, implying a greater capacity to store protons in the heterostructures. Of the three cations simulated, Na⁺ hinders the proton migration the least and Li⁺ the most because of its position near the 2D-TiO₂ surface. Therefore, 2D-TiO₂/MXene heterostructures are likely to exhibit a higher energy density but lower power density, especially with Na⁺ intercalation.

Keywords: 2D-TiO2; MXene; ReaxFF molecular dynamics; confined electrolyte; energy storage materials; heterostructure.