Transition metal carbides and nitrides (MXenes), a family of two-dimensional (2D) inorganic compounds, are materials composed of a few atomic layers of transition metal carbides, nitrides, or carbonitrides. Ti3C2, the first 2D layered MXene, was isolated in 2011. This material, which is a layered bulk material analogous to graphite, was derived from its 3D phase, Ti3AlC2 MAX. Since then, material scientists have either determined or predicted the stable phases of >200 different MXenes based on combinations of various transition metals such as Ti, Mo, V, Cr, and their alloys with C and N. Extensive experimental and theoretical studies have shown their exciting potential for energy conversion and electrochemical storage. To this end, we comprehensively summarize the current advances in MXene research. We begin by reviewing the structure types and morphologies and their fabrication routes. The review then discusses the mechanical, electrical, optical, and electrochemical properties of MXenes. The focus then turns to their exciting potential in energy storage and conversion. Energy storage applications include electrodes in rechargeable lithium- and sodium-ion batteries, lithium-sulfur batteries, and supercapacitors. In terms of energy conversion, photocatalytic fuel production, such as hydrogen evolution from water splitting, and carbon dioxide reduction are presented. The potential of MXenes for the photocatalytic degradation of organic pollutants in water, such as dye waste, is also addressed, along with their promise as catalysts for ammonium synthesis from nitrogen. Finally, their application potential is summarized.

Applications of 2D MXenes in energy conversion and storage systems

2D MXenes: Electromagnetic property for microwave absorption and electromagnetic interference shielding

Recent advances in metal-organic framework membranes for water treatment: A review.

Traditional ways of producing drinking water from groundwater, water recycling and water conservation are not sufficient. Seawater desalination would close the gap but the main technology used is thermally driven multi-flash distillation, which is energy consuming and not sustainable. Stacking two-dimensional (2D) nanomaterials into lamellar membranes is a promising technique in the pursuit of both high selectivity and permeance in water desalination. However, 2D membranes tend to swell in water, and increasing their stability in aqueous solution is still challenging. Here, we report non-swelling, MXene membranes prepared by the intercalation of Al3+ ions. Swelling is prevented by strong interactions between Al3+ and oxygen functional groups terminating at the MXene surface. These membranes show excellent non-swelling stability in aqueous solutions up to 400 h and possess high rejection of NaCl (~89.5–99.6%) with fast water fluxes (~1.1–8.5 l m−2 h−1). Such membranes can be easily fabricated by simple filtration and ion-intercalating methods, which holds promise for their scalability. Two-dimensional lamellar membranes for water purification are promising but suffer from swelling that reduces their ion sieving performance in water. This study reports easy-to-fabricate, non-swelling MXene membranes prepared by the intercalation of Al3+ ions that could be scalable.

Effective ion sieving with Ti 3 C 2 T x MXene membranes for production of drinking water from seawater

2D transition metal carbides or nitrides, known as MXenes, are a new family of 2D materials with close to 30 members experimentally synthesized and dozens more theoretically investigated. Because o...

MXene/Polymer Membranes: Synthesis, Properties, and Emerging Applications

Ion transport is crucial for biological systems and membrane-based technology. Atomic-thick two-dimensional materials, especially graphene oxide (GO), have emerged as ideal building blocks for developing synthetic membranes for ion transport. However, the exclusion of small ions in a pressured filtration process remains a challenge for GO membranes. Here we report manipulation of membrane surface charge to control ion transport through GO membranes. The highly charged GO membrane surface repels high-valent co-ions owing to its high interaction energy barrier while concomitantly restraining permeation of electrostatically attracted low-valent counter-ions based on balancing overall solution charge. The deliberately regulated surface-charged GO membranes demonstrate remarkable enhancement of ion rejection with intrinsically high water permeance that exceeds the performance limits of state-of-the-art nanofiltration membranes. This facile and scalable surface charge control approach opens opportunities in selective ion transport for the fields of water transport, biomimetic ion channels and biosensors, ion batteries and energy conversions.

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Controllable ion transport by surface-charged graphene oxide membrane

2D MXene Nanofilms with Tunable Gas Transport Channels

Two-dimensional (2D) material membranes have shown great potential for molecular separation. However, it remains challenging to controllably tune 2D material membrane structures with ordered nanochannels without non-selective defects. Herein, for the first time, 2D Ti2CTx MXene membranes were designed and fabricated for efficient solvent dehydration via a new strategy. Specifically, positively charged hyperbranched polyethylenimine was intercalated between negatively charged MXene nanosheets to assemble regular stacking structures via electrostatic interactions, followed by interfacial polymerization with trimesoyl chloride to seal possible non-selective defects. This strategy realized integrated Ti2CTx membranes as thin as ∼100 nm with highly ordered 2D nanochannels, thereby exhibiting outstanding separation performance: the water content was enriched to >99 wt% in permeate from 10 wt% water/isopropanol mixtures. Compared with commonly studied Ti3C2Tx MXene membranes prepared by the same approach, the Ti2CTx membranes showed higher selectivity with similarly high flux. The membrane fabrication approach developed herein offers a powerful platform for the rational design of 2D material thin films for precise molecular separation and other applications.

Two-dimensional Ti2CTx MXene membranes with integrated and ordered nanochannels for efficient solvent dehydration

Ion transport is crucial for biological systems and membrane-based technologies from both fundamental and practical aspects. Unlike biological ion channels, realizing efficient ion sieving by using membranes with artificial ion channels remains an extremely challenging task. Inspired by biological ion channels with proper steric containment of target ions within affinitive binding sites along the selective filter, herein we design a system of biomimic two-dimensional (2D) ionic transport channels based on a graphene oxide (GO) membrane, where the ionic imidazole group tunes the appropriate physical confinement of 2D ionic transport channels to mimic the confined cavity structures of the biological selectivity filter, and the ionic sulfonic group creates a favorable chemical environment of 2D ionic transport channels to mimic the affinitive binding sites of the biological selectivity filter. As a result, the as-fabricated ionic GO membrane demonstrates an exceptional K+ transport rate of ∼1.36 mol m-2 h-1 and competitive K+/Mg2+ selectivity of ∼9.11, outperforming state-of-the-art counterparts. Moreover, the semiquantitative studies of ion transport through 2D ionic transport channels suggest that efficient ion sieving with the ionic GO membrane is achieved by the high diffusion and partition coefficients of hydrated monovalent ions, as well as the large energy barrier and limited potential gradient of hydrated divalent ions encountered.

Yufan Ji

Papers

Controllable ion transport by surface-charged graphene oxide membrane

2D MXene Nanofilms with Tunable Gas Transport Channels

Two-dimensional Ti2CTx MXene membranes with integrated and ordered nanochannels for efficient solvent dehydration

Designing Biomimic Two-Dimensional Ionic Transport Channels for Efficient Ion Sieving

Fabrication of ZIF-300 membrane and its application for efficient removal of heavy metal ions from wastewater