MXene molecular sieving membranes for highly efficient gas separation

Fabrication of high-performance MXene-based all-solid-state flexible microsupercapacitor based on a facile scratch method

MXenes have emerged as promising electrode materials for microsupercapacitors (MSCs) owing to their high volumetric and areal capacitances. In addition to the development of novel electrode materials, fabrication of interdigital electrodes is another key to realize high-performance MSCs. Herein, we demonstrate the patterning of few-layered Ti₃C₂T _x nanosheets on various substrates for MSCs by a facile, fast, and nearly zero-cost 'scratch' strategy. The fabricated Ti₃C₂T _x -based all-solid-state MSC achieves a high areal capacitance of 25.5 mF cm^-2, which benefits from the unique layered structure and high electrical conductivity of the electrode. The fabricated planar MSC also delivers good cycling stability and excellent flexibility. Moreover, our fabrication strategy can be readily extended to other composite films for MSCs and become potential micropower sources for miniaturized electronic devices.

Template-Free Synthesis of Two-Dimensional Fe/N Codoped Carbon Networks as Efficient Oxygen Reduction Reaction Electrocatalysts

A direct pyrolysis and template-free synthesis strategy is demonstrated to synthesize the two-dimensional (2-D) Fe/N codoped carbon networks by virtue of 2-D graphitic-carbon nitride (g-C₃N₄) intermediates derived from melamine. Because of the stabilization and steric hindrance of additional N ligands with bisnitrogen-containing groups (phenanthroline, phthalonitrile, and phenylenediamine), the thin graphitic-layered Fe/N codoped carbon materials have successfully inherited the 2-D morphology from the g-C₃N₄ intermediate after direct carbonization treatment. After the easy removal of inactive Fe particles, the resultant sample exhibits numerous well-dispersed Fe atoms embedded in the carbon layers with a hierarchical (meso- and micro-) porous structure. Owing to the high active site density and open porous structure, the thin graphitic-layered Fe/N codoped carbon electrocatalysts exhibit superior oxygen reduction reaction performance (a half-wave potential of 0.88 V and a kinetics current density of 3.8 mA cm^-2), even better than the commercial Pt/C catalysts (0.85 V and 1.6 mA cm^-2, respectively). The facile and effective synthesis strategy without template to build the graphene-like nanoarchitectures inherited from the 2-D intermediates will lead to a great development of 2-D carbon materials in various electrochemical applications.

Engineering Sub-Nanometer Channels in Two-Dimensional Materials for Membrane Gas Separation

Sub-nanochannels constructed by stacking two-dimensional (2D) nanosheets in parallel provide a unique molecular separation pathway with excellent size-sieving ability for membrane gas separation. Herein we review the progress in engineering these 2D channels for efficient gas separation including graphene, graphene oxide (GO), molybdenum disulfide (MoS₂), and MXene. Mixed matrix materials containing these 2D materials in polymers are also reviewed and compared with conventional polymers for gas separation.

2D Material Science: Defect Engineering by Particle Irradiation

Two-dimensional (2D) materials are at the heart of many novel devices due to their unique and often superior properties. For simplicity, 2D materials are often assumed to exist in their text-book form, i.e., as an ideal solid with no imperfections. However, defects are ubiquitous in macroscopic samples and play an important ⁻ if not imperative ⁻ role for the performance of any device. Thus, many independent studies have targeted the artificial introduction of defects into 2D materials by particle irradiation. In our view it would be beneficial to develop general defect engineering strategies for 2D materials based on a thorough understanding of the defect creation mechanisms, which may significantly vary from the ones relevant for 3D materials. This paper reviews the state-of-the-art in defect engineering of 2D materials by electron and ion irradiation with a clear focus on defect creation on the atomic scale and by individual impacts. Whenever possible we compile reported experimental data alongside corresponding theoretical studies. We show that, on the one hand, defect engineering by particle irradiation covers a wide range of defect types that can be fabricated with great precision in the most commonly investigated 2D materials. On the other hand, gaining a complete understanding still remains a challenge, that can be met by combining advanced theoretical methods and improved experimental set-ups, both of which only now begin to emerge. In conjunction with novel 2D materials, this challenge promises attractive future opportunities for researchers in this field.

Paralyzed membrane: Current-driven synthesis of a metal-organic framework with sharpened propene/propane separation

Metal-organic framework (MOF) membranes show great promise for propene/propane separation, yet a sharp molecular sieving has not been achieved due to their inherent linker mobility. Here, zeolitic imidazolate framework ZIF-8-type membranes with suppressed linker mobility are prepared by a fast current-driven synthesis (FCDS) strategy within 20 min, showing sharpened molecular sieving for propene/propane separation with a separation factor above 300. During membrane synthesis, the direct current promotes the metal ions and ligands to assemble into inborn-distorted and stiffer frameworks with ZIF-8_Cm (a newly discovered polymorph of ZIF-8) accounting for 60 to 70% of the membrane composition. Molecular dynamics simulations further verify that ZIF-8_Cm is superior to ZIF-8_I 4 ¯ 3 m (the common cubic phase) for propene/propane separation. FCDS holds great potential to produce high-quality, ultrathin MOF membranes on a large scale.

Modified MXene/Holey Graphene Films for Advanced Supercapacitor Electrodes with Superior Energy Storage

MXene films are attractive for advanced supercapacitor electrodes requiring high volumetric energy density due to their high redox capacitance combined with extremely high packing density. However, the self-restacking of MXene flakes unavoidably decreases the volumetric performance, mass loading, and rate capability. Herein, a simple strategy is developed to prepare a flexible and free-standing modified MXene/holey graphene film by filtration of the alkalized MXene and holey graphene oxide dispersions, followed by a mild annealing treatment. After terminal groups (-F/-OH) are removed, the increased proportion of Ti atoms enables more pseudocapacitive reaction. Meanwhile, the embedded holey graphene effectively prevents the self-restacking of MXene and forms a high nanopore connectivity network, which is able to immensely accelerate the ion transport and shorten transport pathways for both ion and electron. When applied as electrode materials for supercapacitors, it can deliver an ultrahigh volumetric capacitance (1445 F cm-3) at 2 mV s-1, excellent rate capability, and high mass loading. In addition, the assembled symmetric supercapacitor demonstrates a fantastic volumetric energy density (38.6 Wh L-1), which is the highest value reported for MXene-based electrodes in aqueous electrolytes. This work opens a new avenue for the further exploration of MXene materials in energy storage devices.

Gas Flow at the Ultra-nanoscale: Universal Predictive Model and Validation in Nanochannels of Ångstrom-Level Resolution

Gas transport across nanoscale pores is determinant in molecular exchange in living organisms as well as in a broad spectrum of technologies. Here, we report an unprecedented theoretical and experimental analysis of gas transport in a consistent set of confining nanochannels ranging in size from the ultra-nanoscale to the sub-microscale. A generally applicable theoretical approach quantitatively predicting confined gas flow in the Knudsen and transition regime was developed. Unlike current theories, specifically designed for very simple channel geometries, our approach can be applied to virtually all geometries, for which the probability distribution of path lengths for particle-interface collisions can be computed, either analytically or by numerical simulations. To generate a much needed benchmark experimental model, we manufactured extremely reproducible membranes with two-dimensional nanochannels. Channel sizes ranged from 2.5 to 250 nm, and angstrom level of size control and interface tolerances were achieved using leading-edge nanofabrication techniques. We then measured gas flow in the Knudsen number range from 0.2 to 20. Excellent agreement between theoretical predictions and experimental data was found, demonstrating the validity and potential of our approach.

Controlled Crumpling of Two-Dimensional Titanium Carbide (MXene) for Highly Stretchable, Bendable, Efficient Supercapacitors

Two-dimensional MXene materials have demonstrated attractive electrical and electrochemical properties in energy storage applications. Adding stretchability to MXene remains challenging due to its high mechanical stiffness and weak intersheet interaction, so the assembling techniques for mechanically stable MXene architectures require further development. We report a simple fabrication by harnessing the interfacial instability to generate higher dimensional MXene nanocoatings capable of programmed crumpling/unfolding. A sequential patterning approach enabled the design of sequence-dependent MXene textures across multiple length scales, which were utilized for controllable wetting surfaces and high-areal-capacitance electrodes. We next transferred the crumpled MXene nanocoating onto an elastomer to fabricate an MXene/elastomer electrode with high stretchability. The accordion-like MXene can be reversibly folded/unfolded and still preserve efficient specific capacitances. We further fabricated asymmetric MXene supercapacitors, and the devices demonstrated efficient electrochemical performance and large deformability (180° bendability, 100% stretchability). Our texturing techniques can be applied to large MXene families for designing stretchable architectures in wearable electronics.

Effect of pore density on gas permeation through nanoporous graphene membranes

Pore density is an important factor dictating gas separations through one-atom-thin nanoporous membranes, but how it influences the gas permeation is not fully understood. Here we use molecular dynamics (MD) simulations to investigate gas permeation through nanoporous graphene membranes with the same pore size (3.0 Å × 3.8 Å in dimensions) but varying pore densities (from 0.01 to 1.28 nm-2). We find that higher pore density leads to higher permeation per unit area of membrane for both CO2 and He, but the rate of the increase decreases greatly for CO2 at high pore densities. As a result, the per-pore permeance decreases for CO2 but remains relatively constant for He with the pore density, leading to a dramatic change in CO2/He selectivity. By separating the total flux into direct flux and surface flux, we find that He permeation is dominated by direct flux and hence the per-pore permeation rate is roughly constant with the pore density. In contrast, CO2 permeation is dominated by surface flux and the overall decreasing trend of the per-pore permeation rate of CO2 with the pore density can be explained by the decreasing per-pore coverage of CO2 on the feed side with the pore density. Our work now provides a complete picture of the pore-density dependence of gas permeation through one-atom-thin nanoporous membranes.

Two-Dimensional Oxides: Recent Progress in Nanosheets

Two-dimensional (2D) materials have been widely investigated for the last few years, introducing nanosheets and ultrathin films. The often superior electrical, optical and mechanical properties in contrast to their three-dimensional (3D) bulk counterparts offer a promising field of opportunities. Especially new research fields for already existing and novel applications are opened by downsizing and improving the materials at the same time. Some of the most promising application fields are namely supercapacitors, electrochromic devices, (bio-) chemical sensors, photovoltaic devices, thermoelectrics, (photo-) catalysts and membranes. The role of oxides in this field of materials deserves a closer look due to their availability, durability and further advantages. Here, recent progress in oxidic nanosheets is highlighted and the benefit of 2D oxides for applications discussed in-depth. Therefore, different synthesis techniques and microstructures are compared more closely.

Polypyrrole–MXene coated textile-based flexible energy storage device

Recently, more and more researchers have devoted their efforts to developing flexible electrochemical energy storage devices to meet the development of portable and wearable electronics. Among them, supercapacitors (SCs) have been widely studied due to their high specific capacitance and power density. However, most flexible SCs often use traditional carbon materials and transition metal oxides as electrode materials. In this paper, we used an easy and low-cost way to fabricate a flexible supercapacitor based on a new type of two-dimensional material, transition metal carbides, nitrides, or carbonitrides (MXenes). By taking full advantage of the hydrophilicity and metal conductivity of MXene nanosheets, an extremely simple “dipping and drying” method was used to achieve conductive textile electrodes with a specific capacitance of 182.70 F g⁻¹, which is higher than reported for carbon nanotubes (CNTs) and active carbon. To further improve the capacitive performance of the MXene-based electrode and avoid the poor oxygen oxidation of MXene, polypyrrole (PPy) was electrochemically deposited on the surface of MXene textiles, thus producing a PPy–MXene coated textile electrode with a specific capacitance of 343.20 F g⁻¹. In addition, a symmetrical solid-state supercapacitor based on MXene–PPy textiles was assembled, which achieved an energy density of 1.30 mW h g⁻¹ (power density = 41.1 mW g⁻¹). This work introduces a new type of MXene-based textile SC, which provides a promising candidate for flexible and wearable energy storage devices.

A kind of smart and flexible SCs based on MXene-based textile have been fabricated by a “dipping and drying” method.