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Abstract This is the first report of molybdenum carbide‐based electrocatalyst for sulfur‐based sodium‐metal batteries. MoC/Mo2C is in situ grown on nitrogen‐doped carbon nanotubes in parallel with formation of extensive nanoporosity. Sulfur impregnation (50 wt% S) results in unique triphasic architecture termed molybdenum carbide–porous carbon nanotubes host (MoC/Mo2C@PCNT–S). Quasi‐solid‐state phase transformation to Na2S is promoted in carbonate electrolyte, with in situ time‐resolved Raman, X‐ray photoelectron spectroscopy, and optical analyses demonstrating minimal soluble polysulfides. MoC/Mo2C@PCNT–S cathodes deliver among the most promising rate performance characteristics in the literature, achieving 987 mAh g−1at 1 A g−1, 818 mAh g−1at 3 A g−1, and 621 mAh g−1at 5 A g−1. The cells deliver superior cycling stability, retaining 650 mAh g−1after 1000 cycles at 1.5 A g−1, corresponding to 0.028% capacity decay per cycle. High mass loading cathodes (64 wt% S, 12.7 mg cm−2) also show cycling stability. Density functional theory demonstrates that formation energy of Na2Sx(1 ≤x ≤ 4) on surface of MoC/Mo2C is significantly lowered compared to analogous redox in liquid. Strong binding of Na2Sx(1 ≤x ≤ 4) on MoC/Mo2C surfaces results from charge transfer between the sulfur and Mo sites on carbides’ surface.
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Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes
Versatile chemical transformations of surface functional groups in two-dimensional transition-metal carbides (MXenes) open up a previously unexplored design space for this broad class of functional materials. We introduce a general strategy to install and remove surface groups by performing substitution and elimination reactions in molten inorganic salts. Successful synthesis of MXenes with oxygen, imido, sulfur, chlorine, selenium, bromine, and tellurium surface terminations, as well as bare MXenes (no surface termination), was demonstrated. These MXenes show distinctive structural and electronic properties. For example, the surface groups control interatomic distances in the MXene lattice, and Tin+1Cn(n= 1, 2) MXenes terminated with telluride (Te2−) ligands show a giant (>18%) in-plane lattice expansion compared with the unstrained titanium carbide lattice. The surface groups also control superconductivity of niobium carbide MXenes.
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- PAR ID:
- 10226642
- Publisher / Repository:
- American Association for the Advancement of Science (AAAS)
- Date Published:
- Journal Name:
- Science
- Volume:
- 369
- Issue:
- 6506
- ISSN:
- 0036-8075
- Page Range / eLocation ID:
- p. 979-983
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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