Abstract
MXenes are a recently discovered class of two-dimensional materials that have shown great potential as electrodes in electrochemical energy storage devices. Despite their promise in this area, MXenes can still suffer limitations in the form of restricted ion accessibility between the closely spaced multistacked MXene layers causing low capacities and poor cycle life. Pillaring, where a secondary species is inserted between layers, has been used to increase interlayer spacings in clays with great success but has had limited application in MXenes. We report a new amine-assisted pillaring methodology that successfully intercalates silica-based pillars between Ti3C2 layers. Using this technique, the interlayer spacing can be controlled with the choice of amine and calcination temperature, up to a maximum of 3.2 nm, the largest interlayer spacing reported for an MXene. Another effect of the pillaring is a dramatic increase in surface area, achieving BET surface areas of 235 m2 g–1, a sixty-fold increase over the unpillared material and the highest reported for MXenes using an intercalation-based method. The intercalation mechanism was revealed by different characterization techniques, allowing the surface chemistry to be optimized for the pillaring process. The porous MXene was tested for Na-ion battery applications and showed superior capacity, rate capability and remarkable stability compared with those of the nonpillared materials, retaining 98.5% capacity between the 50th and 100th cycles. These results demonstrate the applicability and promise of pillaring techniques applied to MXenes providing a new approach to optimizing their properties for a range of applications, including energy storage, conversion, catalysis, and gas separations.
Highlights
MXenes are a recently discovered group of two-dimensional materials with the general formula Mn+1XnTx, where M is an early transition metal, X is carbon and/or nitrogen, and Tx represents surface functional groups, which are typically O, OH, F, or Cl.[1−3] MXenes have been employed as active materials in a variety of applications, such as batteries, supercapacitors, fuel cells, water desalination and purification, and catalysis, with great success.[4−6] This is due to their unique combination of properties such as high electrical and thermal conductivity, hydrophilic nature, and high chemical stability.[4,6,7]
This was calcined under argon at 300, 400, or 500 °C, which is shown by appending the calcination temperature onto the above sample names (e.g., Ti3C2-OHSi-400 refers to the Ti3C2-OH-Si material after calcining at 400 °C)
It was shown that the choice of amine copillar and calcination temperature could be used to control the interlayer separation from 1.7 to 4.2 nm, corresponding to a gallery height of 0.7 to 3.2 nm, the latter being a >10-fold increase over the unpillared material, the largest interlayer spacing reported to date for any MXene and one of the largest reported for any two-dimensional material
Summary
MXenes are a recently discovered group of two-dimensional materials with the general formula Mn+1XnTx, where M is an early transition metal, X is carbon and/or nitrogen, and Tx represents surface functional groups, which are typically O, OH, F, or Cl.[1−3] MXenes have been employed as active materials in a variety of applications, such as batteries, supercapacitors, fuel cells, water desalination and purification, and catalysis, with great success.[4−6] This is due to their unique combination of properties such as high electrical and thermal conductivity, hydrophilic nature, and high chemical stability.[4,6,7] As such, they combine the advantages of graphene (high conductivity and stability but hydrophobic) and graphene oxide (high stability, surface functional groups, and hydrophilic but lower conductivity) in one material. Like many other 2D materials, MXenes suffer from challenges associated with maintaining the properties and performance of the 2D nanosheets when combining them into stable 3D architectures. Common problems are comparatively poor performances when in multilayered form and a reduction in surface area due to the restacking of nanosheets over time.[8] In addition, the specific surface areas of MXenes (4−20 m2 g−1)[9−11] are significantly smaller than those of other 2D materials (300−2000 m2 g−1 for graphene/graphene oxide materials)[12−14] which limits their suitability for applications in energy storage, catalysis and gas capture, and storage. The reported specific surface areas are no greater than 98 m2 g−1.15 the delaminated nanosheets tend to restack over time, leading to reduced performance, for example, in a lithium-ion battery.[15]
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