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

Journal Article · Fri Dec 18 00:00:00 EST 2020 · ACS Applied Materials and Interfaces

DOI:https://doi.org/10.1021/acsami.0c17536· OSTI ID:1824958

Ganeshan, Karthik ^[1]; Shin, Yun Kyung ^[1]; Osti, Naresh C. ^[2]; Sun, Yangyunli ^[3]; Prenger, Kaitlyn ^[4]; Naguib, Michael ^[4]; Tyagi, Madhusudan ^[5]; Mamontov, Eugene ^[2]; Jiang, De-en ^[3]; van Duin, Adri C. T. ^[1]

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1. Pennsylvania State Univ., University Park, PA (United States)
2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
3. Univ. of California, Riverside, CA (United States)
4. Tulane Univ., New Orleans, LA (United States)
5. National Inst. of Standards and Technology (NIST), Gaithersburg, MD (United States); Univ. of Maryland, College Park, MD (United States)

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. Furthermore, 2D-TiO₂/MXene heterostructures are likely to exhibit a higher energy density but lower power density, especially with Na⁺ intercalation.

• Research Organization: Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: AC05-00OR22725
• OSTI ID: 1824958
• Journal Information: ACS Applied Materials and Interfaces, Journal Name: ACS Applied Materials and Interfaces Journal Issue: 52 Vol. 12; ISSN 1944-8244
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

25 ENERGY STORAGE
2D-TiO2
MXene
ReaxFF molecular dynamics
cations
confined electrolyte
energy storage materials
heterostructure
layered materials
molecules
two dimensional materials