2D Transition Metal Carbides (MXenes) for Carbon Capture

Abstract

charge storage, [7,8] electromagnetic interference shielding, [9] filtering, [10] and a range of additional applications. [7] MXenes constitute a large and growing family of 2D materials [11,12] that are obtained from the laminated M n+1 AX n (MAX) phases (M is a transition metal, A is a group A element-mostly groups 13 and 14-and X is C and/or N) [13] by chemical etching of the atomically thin A element layers that separate sheets of M n+1 X n . As the A element is removed, the MXene surfaces are immediately functionalized by surface terminating species, T x . [6,14] Hence, the proper MXene formula is M n+1 X n T x . Accordingly, the MXene properties can be tuned through structure, intrinsic composition, and surface terminations. The structure is inherited from the parent MAX phase (hexagonal, space group P6 3 /mmc), but compositional tuning displays an extraordinary toolbox for property tuning through MXenes based on single M and X elements, as well as alloys on both M and X. [12,15] In addition, there are reports on MXenes forming out-of-plane [16] and in-plane [17] double-M elemental ordering, as well as vacancy-ordered structures. [18,19] Manipulation of the surface terminations constitutes the final and most powerful variable for property tuning. [20] Despite several theoretical investigations, [21][22][23] noninherent terminations have remained experimentally unexplored. Currently, the MXene preparation dictates that T x is inherent to the etchant and predominantly a combination of O and F, where OH has also been considered as a minor [24] or even negligible contribution. [25] In the area of carbon capture (CC), MXenes are predicted to be highly efficient for capturing CO 2 , enabling capture of 2-8 mol CO 2 kg −1 . [20,21] However, the MXene surfaces were assumed to be termination free, an experimentally unrealistic starting point, given the current wet-chemical preparation routes for MXenes. To unlock the MXene potential for noninherent terminations or adsorption of other molecules, such as CO 2 , we have subjected the archetype Ti 3 C 2 T x MXene to a novel approach. Using in situ environmental transmission electron microscopy (ETEM), single Ti 3 C 2 T x sheets were subjected to an initial high-temperature treatment to desorb F, [25] and a subsequent H 2 exposure to remove the persistent O from the surfaces. The thereafter termination-depleted MXene was subsequently exposed to CO 2 gas, resulting in the first MXene to be terminated by a noninherent molecule. Additionally, termination-depleted MXene surfaces were exposed to N 2 gas after which no N adsorption was observed, Global warming caused by burning of fossil fuels is indisputably one of mankind's greatest challenges in the 21st century. To reduce the everincreasing CO 2 emissions released into the atmosphere, dry solid adsorbents with large surface-to-volume ratio such as carbonaceous materials, zeolites, and metal-organic frameworks have emerged as promising material candidates for capturing CO 2 . However, challenges remain because of limited CO...

Recent advances in MXenes: From fundamentals to applications

“…MXenes have also been shown to be potential gas sensors due to the direct charge transfer mechanism [94]. Recent experiments have observed that T 3 C 2 exhibits affinity towards CO 2 capture selectivity over N 2 [148]. Furthermore, T 3 C 2 sensors can successfully detect gasses such as ethanol, methanol, acetone, and ammonia at room temperature [149].…”

Element Replacement Approach by Reaction with Lewis Acidic Molten Salts to Synthesize Nanolaminated MAX Phases and MXenes

“…Theses 2D derivatives show great promise for applications as battery electrodes, supercapacitors, electromagnetic absorbing and shielding coating, catalyst and carbon capture. [8][9][10][11][12][13][14][15] So far, about 80 ternary MAX phases have been experimentally synthesized, with more continuously being studied, often guided by theory. 16,17 However, MAX phases with the A-site elements of late transition metal (e.g., Fe, Ni, Zn, and Pt), which are expected to exhibit diverse functional properties (e.g., magnetism and catalysis), are difficult to synthesize.…”

Ti3C2Tx MXene-graphene composite films for wearable strain sensors featured with high sensitivity and large range of linear response

“…MXenes represent a new class of two-dimensional early transition metal carbide or carbonitride (e.g., Ti 3 C 2 T x , Ti 2 CT x , Ti 3 CNT x (T stands for the terminal groups of MXenes, x denotes the number of T) with high metallic conductivity, good oxidation resistance and excellent mechanical properties [33][34][35][36], which have set off a research boom in energy storage [37][38][39][40][41], electromagnetic shielding [42,43], catalytic fields [44,45], and strain sensing [46][47][48][49][50][51]. However, although MXenes are considered as two-dimensional materials, in the process of typical chemical etching, due to the destructive effect of the strong etchant (hydrofluoric acid) on the structure of MXenes, there are many irregular particles generated while the materials being delaminated into two-dimensional sheets [47].…”