MXenes as new materials for energy storage/delivery and

environmental applications

Byung-Moon Juna†, Sewoon Kima†, Jiyong Heob, Chang Min Parkc, Namguk Herb, Min Jangd,

Yi Huange, Jonghun Hanb*, Yeomin Yoona**

aDepartment of Civil and Environmental Engineering, University of South Carolina, Columbia,

300 Main Street, SC 29208, USA aDepartment of Civil and Environmental Engineering, Korea bArmy Academy at Young-cheon, 495 Hogook-ro, Kokyungmeon, Young-Cheon, Gyeongbuk

38900, Republic of Korea

cDepartment of Environmental Engineering, Kyungpook National University, 80 Daehak-ro,

Buk-gu, Daegu 41566, Republic of KoreadDepartment of Environmental Engineering, Kwangwoon University, 447-1 Wolgye-Dong

Nowon-Gu, Seoul, Republic of KoreaeSchool of Engineering, The University of Edinburgh, Colin Maclaurin Road

Edinburgh EH9 3DW, Scotland, United Kingdom

*Corresponding author: phone: +82-54-330-4761; fax: +82-54-335-5790;

e-mail: phoenixvov@nate.com

**Corresponding author. Tel.: +1- 803-777-8952; Fax: +1-803-777-0670;

e-mail: yoony@cec.sc.edu

†Byung-Moon Jun and Sewoon Kim co-equally contribute to this work.

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Abstract

Energy and environmental issues presently attract a great deal of scientific attention.

Recently, two-dimensional MXenes and MXene-based nanomaterials have attracted increasing

interest because of their unique properties (e.g., remarkable safety, a very large interlayer

spacing, environmental flexibility, a large surface area, and thermal conductivity). We present a

comprehensive review of recent studies on energy and environmental applications of MXene and

MXene-based nanomaterials, including energy conversion and storage, adsorption, membrane,

and photocatalysis, and antimicrobial. Future research needs are discussed briefly with current

challenges that must be overcome before we completely understand the extraordinary properties

of MXene and MXene-based nanomaterials.

Key words: MXenes; Energy; Environment; Applications; Review

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1. Introduction

Energy and environmental issues are presently at the forefront of global attention . Heavy-

duty, energy conversion/storage devices are required both for electric vehicles and stationary

batteries . Current energy conversion/storage methods/devices feature water-splitting, batteries,

and supercapacitors, but further development is essential because: (i) it is difficult to achieve

contemporaneous high energy conversion and power density; and, (ii) rechargeable devices are

expensive and restricted in terms of use . Two-dimensional (2D) materials including

graphene/graphene-based nanocomposites , metal-organic frameworks , and MXenes have

attracted increasing attention because of their unique physicochemical properties.

Graphene/graphene-based nanocomposites may possibly serve as next-generation energy devices

, but MXenes are very attractive in this context, being very safe, and exhibiting large interlayer

spacing, environmental flexibility, and outstanding biocompatibility . In addition, such electrodes

are able not only to store large quantities of charge per unit volume but also to discharge rather

rapidly . Various nanomaterials have found applications in water/wastewater treatment and

environmental sensing . For example, graphenes adsorb heavy metals and organic contaminants

from water and wastewater. Table 1 summarizes the removal of selected heavy metals and

organic contaminants by graphene-based nanoadsorbents. Recently, metal-organic frameworks

(i.e., inorganic-organic hybrid composites) have been widely used as adsorbents removing both

inorganic and organic contaminants . However, MXenes may be better in these contexts, because

of their exceptional physicochemical properties (a large surface area, excellent stability, a high

melting point, outstanding oxidation resistance, extraordinary electrical/thermal conductivity,

and hydrophilicity) . MXenes and MXene-based materials have been used as innovative

adsorbents and as next-generation membranes removing environmental contaminants .

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A few recent reviews on the use of MXenes used for energy conversion and storage have

appeared ; these differ from our present review in that we include all recent developments. In

addition, to the best of our knowledge, no comprehensive review of the environmental

applications of MXenes has appeared. Thus, it is useful to explore how MXenes and MXene-

based materials technologically enhance environmental applications. Our review is structured as

follows: (i) fabrication and properties of MXenes; (ii) the use of MXenes for energy conversion

and storage; and, (iii) the adsorption, membrane, photocatalytic, and antimicrobial properties of

MXenes. We also herald future research on MXenes for various applications.

2. Fabrication and properties of MXenes

2.1. Fabrication of MXenes

Several recent studies have comprehensively reviewed the current fabrication status of

MXenes . In brief, in 2011, multilayered MXenes (Ti3C2Tx, a new family of 2D materials)

produced by etching an A layer from a MAX phase of Ti3AlC2, were first described by

researchers at Drexel University . MAX phases consist of hexagonally layered, ternary transition

metal carbides, carbonitrides, and nitrides of the general formula Mn+1AXn, where M stands for

an early transition metal (e.g., Cr, Nb, Ti, V, and etc.), A stands for an element from groups (e.g.,

Al, As, Cd, P, and etc.), X stands for carbon and/or nitrogen, and n = 1, 2, or 3 . The term

‘MXene’ was coined to distinguish this new family of 2D materials from graphene, and applies

to both the original MAX phases and MXenes fabricated from them . Multilayered MXene flakes

were created via wet etching with hydrofluoric acid (HF) , HCl-LiF , or HCl-NaF , yielding

various MXenes including Cr2TiC2 , Mo2Ti2C3 , Mo2TiC2 , Mo2ScC2 , Mo2C , (Nb0.8Zr0.2)4C3 ,

(Nb0.8Ti0.2)4C3 , Nb2C , Nb4C3 , Ti3CN , TiNbC , Ta4C3 , Ti4N3 , Ti2C , Ti3C2 , (Ti0.5Nb0.5)2C ,

(V0.5Cr0.5)3C2 , and V2C,25 Zr3C2 .

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Fig. 1a shows the historical timeline of MXene fabrication: (i) in 2011, the first multilayered

MXene (Ti3C2Tx) was developed ; (ii) in 2012, various MXenes including Ta4C3Tx, Ti2CTx,

Ti3CNTx, (TiNb)2CTx, and (V,Cr)3C2Tx appeared ; (iii) in 2013, single-layer MXenes were

isolated via intercalation and delamination with organic compounds ; (iv) in 2014, the use of in

situ HF etchants including HCl-LiF or NH4HF2 to produce MXenes was introduced; (v) in 2015,

large-scale delamination of various MXenes employing an amine-assisted method or

tetrabutylammonium hydroxide was presented ; (vi) in 2016, delamination of large single flakes

of Ti3C2Tx (<2 μm in thickness) (without sonication) by modifying the HCl-LiF-etching method

was reported ; and, (vii) in 2017, synthesis of an MXene with ordered divacancies (Mo1.3CTx)

from Mo2/3Sc1/3)2Al was described . Of the various MXenes (approximately 80 members)

produced by selective etching of atomic layers from different MAX phases, Ti3C2Tx is one of the

most commonly studied. Fig. 1b shows the synthesis of Ti3C2Tx using various etching methods

employing HF or in situ HF-etching . HF at three different concentrations (5, 10, and 30% by

weight) was applied for various times (5, 18, and 24 h); all conditions adequately fabricated

Ti3C2Tx. However, in the in situ method, hazardous HF was used indirectly (as HCl-LiF or

NH4HF2) over an etching time of 24 h. Employing either HF or NH4HF2, delamination is

achieved by addition of large organic molecules dissolved in dimethyl sulfoxide or

tetramethylammonium hydroxide, followed by sonication. With HCl-LiF, a clay-like Ti3C2Tx

intercalated with Li+ can be delaminated with or without sonication (thus, via minimally

intensive layer delamination), depending on the etchant concentrations (HCl and LiF) .

2.2. Properties of MXenes

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The structural, mechanical, electronic, surface, optical, magnetic, and transport properties of

MXenes are significantly influenced by the MXene precursor used , the etchant , the etching

protocol, the intercalation method , and sonication frequency . Fig. 2 shows top and side views

of the pristine MXene M2X (Fig. 2a), Model M2X-1 with two functional groups on top of hollow

site A (Fig. 2b), and Model M2X-2 with two functional groups on top of hollow site B (Fig. 2c) .

As transition metal ions have co-ordination numbers of six, it is assumed that transition metals in

MXenes form six chemical bonds to X atoms and surface chemical groups, causing production

of M2XO2, M2XF2, and M2X(OH)2 . In addition to Models M2X-1 and M2X-2, two other

configurations are possible; in Model M2X-3, two functional groups are on top of a single hollow

site and, in Model M2X-4, one functional group is on top of hollow site A and the other on top of

hollow site B . When sufficient transition metal electrons are donated to both X and attached

functional groups, Model M2X-2 is the most solid configuration .

In theory, mechanical tensile stress is influenced by F, OH, and O functionalization in

Tin+1Cn; Ti2CO2 tolerates biaxial and uniaxial tensions much higher than are acceptable to

graphene, because surface functionalization reduces atomic layer breakdown, enhancing

mechanical elasticity . For example, the exceptional solidification afforded by oxygen

functionalization is presumably attributable to substantial charge transfer between inner Ti–C

and outer Ti-C bonds . Chemical exfoliation of MAX phases is very commonly used when

synthesizing MXenes, but mechanical exfoliation is also possible . General bonding in the ab

plane is stronger than in the c direction, presumably because C33<C11 in terms of the elastic

constants of some MAX phases . Both MAX phases and pristine MXenes are metallic, but some

MXenes exhibit semiconductive properties via surface functionalization . Several theoretical

studies found that the MXenes were both topologically trivial metals/semiconductors and

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topologically non-trivial semimetals/semiconductors, depending on the flexibility of relativistic

spin-orbit coupling .

Only a few experimental studies have evaluated MXene optical properties using Ti3C2Tx thin

films . A thin film based on a MAX phase (i.e., Ti3AlC2) exhibited relatively low transmittance

(30%) , but visible light (550 nm) transmittance rose to approximately 80% for Ti3C2Tx and to

90% for intercalated Ti3C2Tx/NH4HF2 (Ti3C2.3O1.2F0.7N0.2) of various Ti3C2Tx thicknesses . In

general, both pristine and most functionalized MXenes would be expected to be nonmagnetic

because of solid covalent bonding between the transition metal, the X element, and attached

functional groups . However, even strong covalent bonds can be modified by external strain,

triggering magnetism based on the discharge of d electrons . In fact, most pristine MXenes

become naturally magnetic. For example, near-semi-metallic ferromagnetism was evident in

pristine Ti2N and Ti2C of formula units 1.9 and 1.0 mB, respectively . A few theoretical studies

have explored electronic transport in MXenes . When comparing pristine and functionalized

Ti3C2, the type of surface functionization greatly affected electronic transport. For example, the

current for any given bias voltage of Ti3C2F2 was fourfold that of pristine Ti3C2 .

3. MXenes for energy applications

3.1. Energy conversion

Chemical energy is directly converted into electrical power by fuel cells . In general, the

common electrochemical processes are oxygen reduction, oxygen evolution, and hydrogen

evolution, used in water-splitting, regenerative fuel cells, and metal-air batteries, respectively .

During oxygen reduction, H2O or H2O2 is produced by reaction of oxygen with electrons and

protons at the cathodes of metal-air batteries and fuel cells . The lack of effective oxygen

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reduction catalysts has limited commercialization of metal-air batteries . Recently, Xue et al.

reported superb oxygen reduction by Mn3O4 nanocomposites combined with Ti3C2 MXenes

produced by the simple method shown in Fig. 3a; this is the first example of a non-noble metal-

MXene composite catalyst mediating oxygen reduction . The Mn3O4 nanoparticles are evenly

distributed on MXenes in the nanocomposite structure (Fig. 3b). This is very important; highly

conductive MXene layers permit direct charge transfer, greatly improving electrode performance

. Rotating disk evaluation showed that the Mn3O4-MXene nanocomposites had the same initial

potential (0.89 V) as Pt/C but a greater positive potential than Mn3O4-acetylene black (0.80 V)

(Fig. 3c) . Both the positive initial potential and high current density of Mn3O4-MXene

nanocomposites contribute to the outstanding oxygen reduction capacity. Also, the high-level

electron transfer of Mn3O4-MXene nanocomposites reflects a superior catalytic capacity. The

composites were also more stable than Mn3O4-acetylene black (Fig. 3d), presumably because

highly conductive MXene enhanced both the conductivity and stability of Mn3O4-MXene

nanocomposites . In particular, wide-open, terminal metal sites (Ti) afforded considerably higher

redox reactivity than carbon nanomaterials .

Electrochemical oxygen evolution is critical in terms of renewable energy applications

(metal-air batteries and water-splitting ), but it is difficult to overcome the significant

overpotential and sluggish kinetics of proton-coupled charge transfer; effective electrocatalysts

are lacking . Yu et al. synthesized a novel, non-precious metal electrocatalyst exhibiting

outstanding structural strength, and excellent interfacial junctions and electrical properties, by

layering double hydroxides synergistically combined with a highly conductive MXene (FeNi-

layered double hydroxides/Ti3C2-Mxene) . This catalyst exhibited a lower overpotential (240

mV) than FeNi-layered double hydroxides/2Ti3C2-Mxene (270 mV) and FeNi-layered double

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hydroxides/0.5Ti3C2-Mxene (298 mV). The poor catalytic behavior of both of the latter catalysts

is probably attributable to the relatively small content of the active FeNi/double hydroxide phase

in the former catalyst and the relatively large content of the poorly conductive FeNi/double

hydroxide phase in the latter catalyst; MXene support was inadequate . In another study, a new

catalyst (a 2D metal-organic framework combined with the MXene cobalt 1,4-

benzenedicarboxylate-Ti3C2Tx) afforded good electrocatalytic oxygen evolution, attributable to

the porous structure, fast mass/ion transport, and the high proportion of exposed, active metal

centers . The nanocomposite afforded a current density of 10 mA cm -2 at a potential of 1.64 V,

similar to that of a common IrO2-based catalyst , and superior to those of earlier transition metal-

based catalysts (oxides and sulfides) . In general, although both fuel cells and metal-air batteries

are commonly used for energy storage, fuel cells require optimization of oxygen reduction and

metal-air batteries require both efficient oxygen reduction and oxygen evolution during

discharging and charging, respectively .

Hydrogen has been considered an ideal clean, recyclable energy source . In general, most

electrocatalysts for hydrogen evolution are based on Pt but, recently, many alternatives such as

borides, metal sulfides, carbides, nitrides, and phosphides have been discovered . However, the

electrocatalytic capacities require improvement in terms of electrical conductivity, active site

reactivity, and numbers of active sites . Li et al. fabricated ultrathin MXene nanosheets rich in

fluorine groups, which synergistically improved electrocatalytic performance by enhancing the

reactivities of active sites and the numbers of such sites . Ti2CTx-rich fluorine nanocomposites

exhibit very effective catalytic activity, attributable to the small onset overpotential (75 mV) and

the high, exchange current density (0.41 mA cm-2). Experimental findings combined with density

functional theory indicate that fluorine groups not only enhance proton adsorption kinetics but

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also reduce charge transfer resistance. Therefore, both active site reactivity and electrode kinetics

improve, increasing electrocatalytic capacity .

Hydrogen production from water via different types of photocatalysis has attracted a great

deal of research attention . Of the many semiconductor-based photocatalysts, including C3N4,

TiO2, CoO, and MoS2 , Nb2O5 exhibits exceptional chemical and photochemical properties . Su et

al. showed that Nb2O5-C-Nb2C (an MXene) split water to yield a very high hydrogen production

rate of 7.81 mol h-1 gcat-1 (fourfold that of pristine Nb2O5) , presumably because of increased

photoinduced carrier transport and enhancement of photogenerated electron-hole pair departure.

The Nb2C surface accepts and transports electrons from Nb2O5. An et al. explored the synergistic

effects of Ti3C2 MXene and Pt co-catalysts, combined with g-C3N4; photocatalytic hydrogen

evolution was most effective . Ti3C2/Pt/g-C3N4 nanocomposites exhibited high-level hydrogen

production (5.1 mmol h-1 g-1) attributable to an increase in reactive sites and enhanced electrical

conductivity during photoreduction, which was 3–4-fold better than afforded by Ti3C2/g-C3N4

and Pt/g-C3N4. The greater activity of the Ti3C2/Pt/g-C3N4 photocatalyst was attributed to

enhanced separation of photogenerated charge carriers and effective electron transfer from the g-

C3N4 conduction band to Ti3C2 and Pt .

3.2. Energy storage

3.2.1. Supercapacitors

A supercapacitor is an energy storage device intermediate between batteries and conventional

capacitors; the power density is greater than that of a battery and the energy density greater than

that of a conventional capacitor . Supercapacitor energy storage is of three types: (i) a rapidly

reversible redox reaction (e.g., using hydroxides or transition metal oxides); (ii) reversible

chemical adsorption/desorption (e.g., oxygen adsorption onto Au or Pt); and, (iii) reversible

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electrochemical doping/de-doping (e.g., using a polypyrrole or polyaniline) . One conventional

supercapacitor is a double-layer capacitor used for energy storage; a double electric interface lies

between the electrolyte used for charge storage and the electrode-active material . Such

capacitors exhibit outstanding strengths because both charge and discharge processes are

principally physical in nature, but the capacities remain relatively small . Pseudo-capacitors (i.e.,

asymmetric supercapacitors) use oxidation-reduction or reversible electrochemical

adsorption/desorption for energy storage , allowing deposition of materials on various surfaces

and sub-surfaces .

2D titanium carbide has often been used to form supercapacitor electrodes, affording good

electrical conductivity, a high melting point, good hydrophilicity, and high-level, electrochemical

surface action . Ti3C2Tx-reduced graphene oxides (rGOs) were used to fabricate asymmetric

flexible micro-supercapacitors, greatly improving ion transport in devices with interdigitated

electrodes . A Ti3C2Tx-rGO asymmetric micro-supercapacitor exhibited an energy density of 8.6

mW h cm-3 at a power density of 0.2 W cm-3 under a 1-V voltage window; 97% of the initial

capacitance remained after 10,000 cycles. Fig. 4a shows the preparation of asymmetrical,

flexible Ti3C2Tx-rGO nanocomposites . Fu et al. reported a large areal capacitance (314 F cm-3)

of Ti3C2Tx, single walled, carbon nanotubes that self-assembled into nanocomposite electrodes;

95% of the initial capacitance remained after 10,000 cycles at a mass loading of 1.7 mg cm-2 . A

high areal capacitance (610 F cm-3) was evident even at this high mass-loading rate. Fig. 4b

shows the preparation of a Ti3C2Tx, single walled, carbon nanotube, self-assembling composite

film . Xia et al. synthesized BiOCl nanosheets immobilized on the surfaces of Ti3C2Tx MXene

nanocomposite flakes via simple, cost-effective, chemical bath deposition . Fig. 4c shows

nanocomposite preparation. A nanocomposite electrode exhibited excellent volumetric specific

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capacitances of 397 F cm-3 at 1 A g-1 and 228 F cm-3 at 15 A g-1. In addition, a very high energy

density (15.2 Wh kg-1) at a power density of 567 W kg-1 was evidenced by a symmetrical

supercapacitor assembled using BiOCl nanosheets/immobilized Ti3C2Tx MXene

nanocomposites; the performance was significantly better than that of an earlier symmetric

supercapacitor created using Ti3C2Tx MXene nanocomposites . The former supercapacitor

exhibited cycling capability; the volumetric capacitance fell by only 5% after 5,000 cycles at 10

A g-1 .

3.2.2. Batteries

Batteries are essential energy-storage devices; improvements are critical because of demands

posed by consumer electronics and electric vehicles . Supercapacitors afford large power

densities, short recharging times, and long operation times, but remain expensive and of low

efficiency . Unlike lead-acid batteries, Ni-Cd, Ni-metal-hydride, and Li batteries are inexpensive,

exhibit high energy densities, are extremely stable, have high cycle lives, exhibit good electronic

conductivities, and do not self-discharge . In the time since Li batteries were developed in the

1960s, graphite has been the most widely employed anodic material, with a specific capacity of

372 mA h g-1 . Although all of Ni, K, Ca, Mg, and Al batteries have been studied , the

development of new materials for Li batteries is essential. In this context, MXenes are valuable

candidates, exhibiting high electrical conductivity, rapid molecule and ion transport, low

operating voltages, and high storage capacities . Naguib et al. were the first to explore the

potential of MXenes as new anodic materials for Li batteries; an exfoliated 2D Ti2C MXene was

fabricated by etching Al from Ti2AlC . The new material exhibited a significantly greater surface

area (approximately 10-fold) compared to that of the MAX phase (Ti2AlC), and also a five-fold

greater specific capacity (225 mA h g-1) than Ti2AlC, attributable to the open structure, larger

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surface area, relatively weak bonds between MX layers on the Ti2C surface, and larger interlayer

gaps between exfoliated Ti2C sheets .

2D heterostructures combining MoS2 with conductive supports have been used for energy

storage . Chen et al. fabricated 2D MoS2-MXene heterostructures with metallic properties, as

revealed by computational analyses ; the structures exhibited large, reversible capacities of

approximately 550 mA h g-1 at a current density of 50 mA g-1, presumably because the MXene

(Ti3C2Tx) improved Li and Li2S adsorption by stabilizing heterostructures during intercalation

and conversion . Organic molecules are often used to prevent layer restacking, thus retaining the

interlayer spaces . Glycine-MXene hybrids have been used to enhance such spacing and to form

possible Ti-N bonds, improving charge storage and cycling . Glycine-Ti3C2Tx nanocomposite

electrodes exhibited a capacitance (325 F g-1) at 10 mV s-1 about twice that of pristine Ti3C2Tx

electrodes, presumably attributable to the lower ion diffusion-resistance of the former

nanocomposite, associated with enhanced interlayer spacing. The electrodes were extremely

stable and, indeed, capacitance improved somewhat after 20,000 cycles as the interlayer spacing

increased further . Huang et al. fabricated sandwich-like Na0.23TiO2 nanobelt/Ti3C2 MXene

nanocomposites for high-drain Li/Na batteries; these exhibited a significant increase in capacity

(from 47 mA h g-1 to 138 mA h g-1) when the current density decreased from 3 to 0.1 A g-1 . Good

charge retention (approximately 90%) was evident after 180 cycles at 0.1 A g-1; after 4,000

cycles, a discharge capacity of 56 mA h g-1 was achieved at a high current density (2 A g-1).

Table 2 summarizes recent MAX phase/MXene materials; their synthetic methods,

morphologies, energy conversion and storage applications, and performances.

4. MXenes for environmental applications

4.1. Adsorption

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Many waterborne inorganic and organic contaminants occur worldwide ; effective

water/wastewater treatments are essential. Existing methods include

coagulation/flocculation/sedimentation, softening, addition of granular/powdered activated

carbon or carbon nanomaterials, oxidation (e.g., chlorination, ozonation, ultraviolet (UV) light

oxidation, and sonodegradation), biological degradation, and membrane filtration . Adsorption is

favored, being feasible, extensively applicable, and relatively low-cost . As is true of graphene-

based nanomaterials, MXenes can be intercalated with, and delaminated into, nanosheets;

various organics (e.g., urea and methylene blue ) and heavy metals (e.g., Ba , Cr , Hg , Cu , and

U ) naturally intercalate various MXenes, suggesting that MXenes may be valuable adsorbents; it

is thus necessary to understand the interactions between MXenes and contaminants. Aqueous

removal of contaminants by MXenes is affected by the nature of the contaminants (e.g.,

inorganic/heavy metals or organics, size/shape, functional group, hydrophilicity, and charge), the

properties of the adsorbent (e.g., surface area, functional group, hydrophilicity, and charge), and

water quality (e.g., adsorbate concentration, inorganic/organic issues, pH, and temperature).

The MXene Ti3C2Tx adsorbed the cationic dye methylene blue from water to the extent of

approximately 40 mg g-1 , thus lower than that of commercial activated carbons (up to 1,000 mg

g-1) , but comparable to that of materials having similar surface areas (pristine kaolinite

[approximately 15 mg g-1] and graphenes-Fe3O4 ). Thus, MXenes of various structures with

diverse surface chemistries may adsorb as well as do other 2D materials . Methylene blue

intercalation with MXenes is possible, given the behaviors of dyes with other layered materials

such as graphite oxide and clay nanoadsorbents . X-ray diffraction revealed that the c-lattice

constant of the MXene interlayer space (0.7–2 nm) was larger than the methylene blue molecule

(1.7 × 0.76 × 0.325 nm3 [l × w × h] ). The interlayer spacing of Ti3C2Tx was expanded by tuning

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the surface functional groups (from –F to –OH) . The adsorption capacity for methylene blue of

different functionalized MXenes decreased in the order NaOH-Ti3C2Tx (184 mg g-1)>LiOH-

Ti3C2Tx (119 mg g-1)>Ti3C2Tx (100 mg g-1)>KOH-Ti3C2Tx (74.2 mg g-1). The adsorption behavior

of alk-Ti3C2Tx followed the Langmuir model, indicating that alk-Ti3C2Tx exhibited homogeneous

adsorption surfaces and engaged in single-layer adsorption. X-ray diffraction indicated that the c-

lattice constants increased in alk-Ti3C2Tx MXenes, presumably via a “pillar effect” imparted by

surrounding alkali metal ions. Although exfoliation or splitting of alk-Ti3C2Tx MXenes caused by

increments in the c-lattice constants (30% for NaOH-Ti3C2Tx and LiOH-Ti3C2Tx, and 20% for

KOH-Ti3C2Tx) remain of concern, both NaOH and LiOH enlarge MXene interlayer spacing,

allowing facile intercalation of methylene blue molecules into the interlayer between alk-Ti 3C2Tx

sheets, parallel to their surfaces. However, dye adsorption is not entirely attributable to

intercalation of KOH-Ti3C2Tx and the increment in the c-lattice constant is relatively small .

MXenes adsorptively removed various heavy metals (Ba , Cr , Hg , Cu , and U ) from waters

of various qualities. 2D Ti3C2Tx nanosheets removed >99% of Ba2+ under optimized conditions;

the adsorption capacity was 9.3 mg g-1 . The pH was important in this context, affecting

adsorbent surface charge and both speciation and ionization . In general, maximum Ba2+

adsorption onto Ti3C2Tx nanosheets was evident at pH 6–7 because: (i) at acidic pH values,

MXenes with less-negative surfaces (zero charge pH=pH 2.7 ) exhibited lower affinities for

heavy metal actions; (ii) at neutral pH, the MXenes were negatively charged, modulated

principally by electrostatic attraction; and, (iii) at basic pH values, Ba(OH)2 precipitated, and was

thus removed from the bulk solution . Fig. 5 shows adsorption of Ba2+ onto MXene surfaces: (i)

adsorption occurred in intercalated and unstacked MXene interlayers ; and, (ii) adsorption was

facilitated by -OH, -F, and -O groups on MXene surfaces . Delaminated Ti3C2Tx afforded

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significantly more Cu2+ removal (40.9 mg g-1) than commercial activated carbon (15.1 mg g-1) .

The Cu2+ adsorption capacity of nanosheets increased from 15 to 40 mg g-1 as the pH rose from 2

to 5, but rapidly decreased at pH 6 (to 20 mg g-1), because: (i) at low pH, delaminated Ti3C2Tx

nanosheets were less negatively charged than at higher pH values, reducing electrostatic

attraction between nanosheets and Cu2+ and creating high-level competition between H+ and Cu2+

for adsorption sites ; and, (ii) at pH 5–6, adsorption was minimal because of Cu2+ precipitation as

Cu(OH)2, depending on the solution Cu2+concentration .

Although TiO2-based materials have been widely used as photocatalysts, their adsorption

capacities for both organic and inorganic compounds are rather low . Urchin-like rutile TiO2-C

combined with a Ti3C2(OH)0.8F1.2 MXene nanocomposite exhibited a high Cr(VI) adsorption

capacity (approximately 225 mg g-1), which was significantly greater than those of pristine

Ti3C2(OH)0.8F1.2 MXene (62 mg g-1) and TiO2-C (11 mg g-1) . Adsorption of Cr(VI) varied by the

solution pH: (i) at pH 1–6, adsorption increased slightly with increasing pH, presumably because

of mutual attraction between Cr(VI) ions and TiO2 in the rutile/MXene nanocomposite; under

acidic conditions, monovalent HCrO4- predominated, and sequestered protonated Ti-OH2

+ rather

weakly ; but, (ii) under basic conditions, strong electrostatic repulsion was in play between

Cr2O72- and deprotonated Ti-O-, significantly reducing Cr(VI) adsorption . In addition, high-level

-OH competition reduced Cr(VI) adsorption .

An MXene completely removed Ba2+ within 60 min; this is one of the fastest rates reported .

Perlite required >60 min to remove only 80% of Ba2+ . Ti3C2Tx nanosheets exhibited fast Ba2+

adsorption kinetics (approximately 35% Ba2+ removal within a few seconds), because many Ba2+-

binding sites were available given the multilayer nature of the MXene and complete saturation

of all available adsorption sites via chemical bonding with Ba2+. A pseudo-second-order model

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rather accurately described Cu2+ adsorption kinetics onto delaminated Ti3C2Tx nanosheets ,

possibly via capture by surface functional groups such as -F, -O, and -OH , the hydrophilic

nature of the MXene, and the fact that surface functional groups [C-Ti-Fx, C-Ti-(OH)x, and C-

Ti-Ox] engaged in strong electrostatic reactions via inner-sphere complex formation between

positively charged Cu2+and negatively charged terminal groups (-O and -OH) on the surfaces of

delaminated nanosheets .

Thermodynamic parameters such as the standard entropy change (ΔS0), the standard enthalpy

change (ΔH0), and the standard free-energy change (ΔG0) yield in-depth information on internal

energy changes associated with adsorption. The Cu2+ adsorption capacity of delaminated Ti3C2Tx

nanosheets was examined at 298, 308, and 318 K (Cu0 = 25 mg L-1 at pH 5) . A slight increase in

maximum adsorption capacity (39.2 to 43.3 mg g-1) with increasing temperature suggested that

Cu2+ adsorption onto MXene nanosheets was endothermic. In addition, the positive ΔS0 (0.152 J

mol-1 K-1) and the increasingly negative ΔG0 (–22.0 to –25.1 kJ mol-1) recorded as temperature

increased indicated that adsorption was both spontaneous and occurred at high affinity; at higher

temperatures, adsorption was rapid after simple dehydration of Cu2+ . Shahzad et al. observed

similar thermodynamic trends when Hg(II) was adsorbed by magnetic Ti3C2Tx nanocomposites .

Relatively high adsorption levels of 54.2, 54.8, 55.4, and 56.6 mg g-1 were evident at 288, 298,

308, and 318 K, respectively; the ΔG0 values became increasingly negative (–27.0 to –34.9 kJ

mol-1) with increasing temperature, indicating strong, spontaneous, adsorbent-adsorbate

interactions.

It is assumed that ion adsorption affinities vary by the selectivities and chemical potentials of

adsorbents. Ba2+ removal by Ti3C2Tx nanosheets occurred in the presence of several other metal

ions including As, Ca, Cr, Pb, and Sr, added to evaluate their effects on Ba2+ adsorption . Ba2+

16

removal was highest (98%), followed by Ca/Pb/Sr (94%) and As (approximately 70%),

indicating that Ba2+ was most effectively removed by the MXene, possibly because (i) adsorption

was affected by ion electronegativity [As (2.2)>Cr (1.56)≥Pb (1.55)>Ca (1.04)>Sr (0.99)≥Ba

(0.97)] and material affinity ; and, (ii) surface hydroxyl groups of the MXene bound Ba2+ more

strongly than other metal ions . Table 3 shows the adsorptive removals of selected dyes and

heavy metals by MXenes.

4.2. Membrane

Membrane processes such as forward osmosis, reverse osmosis, nanofiltration, ultrafiltration,

and microfiltration have been commonly used to treat water and wastewater . Membrane

filtration affords several advantages (low-level/no chemical requirement, high removal

efficiency, and a small carbon footprint), but trade-offs are apparent between membrane

selectivity and permeability . In recent years, many efforts have been made to overcome this

limitation by developing membranes made of ultrathin 2D materials including graphene-based

nanomaterials [e.g., graphenes, graphene oxides (GOs), and rGOs] , metal-organic frameworks ,

covalent organic nanosheets , and MXenes . Compared to conventional materials, the 2D

materials allow fabrication of ultrathin coating/separating layers enabling rapid passage of

selected solutes, affording both excellent permeation and remarkable selectivity . In particular,

MXene-based membranes on different support materials have been used to separate waterborne

organic and inorganic components (Fig. 6).

Ding et al. fabricated a 2D, lamellar MXene membrane on an anodic aluminum oxide

support; the membrane exhibited extremely high water permeability (>1000 L m -2 h-1 bar-1) and

rapid removal (>90%) of Evans blue, cytochrome, and gold nanoparticles . The extent of bovine

serum albumin removal (>99%) and that of rhodamine B (85%) allowed estimation of membrane

17

pore size (2–5 nm). Therefore, size exclusion explained the removal of large solutes/particles

[Evans blue (1.2 nm × 3.1 nm), cytochrome (2.5 nm × 3.7 nm), and gold nanoparticles (5 nm)]

from water; a small molecule (<1 nm) such as K3[Fe(CN)6] (0.9 × 0.9 nm2) was not readily

removed (32% only). The water permeability of the membrane (1,120 L m-2 h-1 bar-1) was

approximately 10-fold higher than that of commercial polyethersulfone membranes with nominal

molecular weight cut-offs of 30 kDa (111 L m-2 h-1 bar-1) and 50 kDa (130 L m-2 h-1 bar-1), and the

MXene membrane afforded much better removal (32–>99%) than commercial membranes

(0.15–34%) of all solutes/particles tested . The enhanced water permeability is attributable to the

nanochannels and large interspaces associated with the loosely lamellar structure of the MXene

membrane. Also, a new 2D, MXene-polyethersulfone composite membrane exhibited relatively

small water fluxes (57.5 and 58.8 L m-2 h-1 bar-1) attributable to fouling, with high-level removal

of Congo red (93%) and gentian violet (80%); larger water fluxes (230, 316, and 218 L m -2 h-1

bar-1) were evident during low-level inorganic salt removal (23% of MgCl2, 13.2% of Na2SO4,

and 13.8% of NaCl) . The pristine polyethersulfone ultrafiltration membrane (nominal molecular

weight cut-off 10 kDa) did not effectively remove these inorganic salts (<5% of the totals). In

addition, the flux of the pristine membrane declined in the presence of Congo red and gentian

violet, indicating that the introduction of MXene nanoparticles into the membrane surface

reduced hydrophobicity and, thus, membrane fouling .

Compared to a 2D, MXene-polyethersulfone composite membrane , a MXene-Ag membrane

of average pore size 2.1 nm exhibited slightly better removal of inorganic salts (26, 41, and 50%

of NaCl, MgCl2, and AlCl3, respectively) at similar water fluxes (approximately 420 L m-2 h-1

bar-1) . In addition, rhodamine B (79.9%), methyl green (92.3%), and bovine serum albumin

(>99%) were effectively removed; however, both the extent of removal and flux were

18

significantly affected by membrane thickness. The optimum membrane thickness for rhodamine

B removal (79.9%) was 470 nm at a water flux of 387 L m-2 h-1 bar-1; the flux fell significantly to

234 and 125 L m-2 h-1 bar-1 at membrane thicknesses of 969 and 1,420 nm with a similar extent of

rhodamine B removal (81.6 and 82.8%). An MXene-Ag membrane fouled with methyl green and

bovine serum presented at 50 mg L-1 exhibited remarkable flux recovery (91–97%) compared to

that of the pristine MXene membrane (81–86%) , attributable to the enhanced hydrophilicity of

the MXene-Ag membrane with additional nanopores. The water contact angle decreased from 40

to 35° as the Ag nanoparticle level rose from 0 to 21%, confirming that Ag improved membrane

surface hydrophilicity . Fig. 7a shows a schematic of, and the mechanism of dye removal by, the

MXene-Ag composite membrane.

Graphene is impermeable , and many studies have used graphene-based membranes for water

treatment . Kang et al. fabricated a 90-nm-thick Ti3C2Tx-GO membrane on a porous support

(polycarbonate/nylon) that inhibited target solute movement through inter-edge defects or poorly

packed spaces . A membrane with an effective interlayer spacing of approximately 0.5 nm

exhibited high-level removal rates [68% for methyl red (neutral, 0.49 nm), 99% for methylene

blue (positive, 0.51 nm), 94% for Rose Bengal (negative, 0.59 nm), and 100% for Coomassie

brilliant blue (negative, 0.8 nm)]. However, the pristine Ti3C2Tx membrane exhibited lower

removal rates [40% (methyl red), 95% (methylene blue), 66% (Rose Bengal), and 95.4%

(Coomassie brilliant blue)]. The poor removal of methyl red is attributable to its relatively small

size and hydrodynamic diameter, and its neutral charge. However, Rose Bengal removal was less

effective than that of methylene blue although the former molecule is larger, presumably because

the negative charge of the former molecule triggered low-level electrostatic attraction to the

MXene surface . Ionic salts (NaCl, Na2SO4, MgSO4, and MgCl2) were very poorly removed

19

(<10%). Thus, the interlayer spaces of the swollen Ti3C2Tx-GO membrane were too large to

allow of complete salt ion removal, but the negatively charged membrane surface removed some

ions via electrostatic repulsion. Fig. 7b shows schematics of the steric exclusion of hydrated ions

and dyes by the Ti3C2Tx-GO membrane. Table 4 summarizes the removal of selected species by

MXene membranes.

4.3. Photocatalytic and antimicrobial applications

When two cationic dyes (methylene blue and acid blue 80) were subjected to UV for 5 h in

the presence of Ti3C2Tx, degradation was very rapid (81 and 62%, respectively) , but acid blue 80

was not degraded at all and methylene blue to only 18% over 20 h in the dark, presumably

because UV triggered formation of TiH4O4 and/or TiO2 on the Ti3C2Tx surface, enhancing

photocatalysis . Although the photocatalytic mechanisms in play require further study, it is

possible that, as is true for graphene-titania hybrids, MXene-supported TiO2 composites may

find applications in catalysis, environmental remediation, energy conversion, and energy

storage . Li et al. showed that photocatalytic degradation of methylene blue was enhanced under

UV in the presence of titania-carbon nanocomposites derived from 2D Ti2CTx . Fig. 8a shows a

schematic of high-energy ball-milling of Ti2CTx to form titania-carbon. Such nanocomposites

have valuable photocatalytic applications; carbon efficiently improved TiO2 photocatalytic

capacity by inhibiting recombination of photoexcited e- and h+ pairs . Furthermore, such

recombination was inhibited by O–Ti–C bond formation in the nanosheets; these bonds

transported photo-excited electrons from the conduction band of TiO2 to the nanosheets. In

particular, disordered, uniform, graphitic carbon nanosheets bearing immobilized TiO2

nanoparticles serve as highly conductive electron acceptors . In another study, Ti3C2-TiO2-CuO

nanocomposites were fabricated via decomposition of mixtures of 2D MXene (Ti3C2) (Fig. 8b) .

20

The nanocomposites exhibited excellent photocatalytic degradation of methyl orange (>99% in

80 min); the photoelectron separation vacancies of CuO and Ti3C2 were enhanced by UV,

explaining photocatalysis by these nanocomposites (Fig. 8c).

The control of microbial activity is often very challenging; surfaces are exposed for long

periods to complex media rich in bacteria and nutrients . In particular, biofouling of membranes

used in wastewater treatment plants is a major issue in that membrane efficiency is seriously

compromised . Surface nanomaterials inhibit bacterial growth via nano-bio interaction . Pandey

et al. found that an MXene-Ag (21%) membrane inhibited E. coli much more than did pristine

MXene and control polyvinylidene difluoride membranes . In addition, the Ag nanoparticle

levels in MXenes correlated positively with growth inhibition, presumably because both Ag and

TiO2 particles formed on the membrane surfaces. TiO2 on the MXene surface enhanced

antimicrobial behavior, particularly under UV . Ag nanoparticles are commonly used as

antimicrobial agents . Rasool et al. showed that a Ti3C2Tx membrane on a polyvinylidene

difluoride support inhibited Bacillus subtilis and E. coli growth (>99%) much more effectively

than did the control polyvinylidene difluoride membrane (73% for B. subtilis and 67% for E.

coli) . In another study, the antibacterial activity of a Ti3C2Tx MXene against Gram-negative E.

coli and Gram-positive B. subtilis was explored. A MAX dispersion exhibited very low

antimicrobial rates (14% for E. coli and 18% for B. subtilis). However, when bacteria were

exposed to a colloidal solution of delaminated Ti3C2Tx MXene, the growth inhibition rates were

98 and 97%, respectively. Similar trends were evident in previous studies; several graphene- and

cellulose-based nanomaterials exhibited stronger antimicrobial activities against Gram-positive

than against Gram-negative bacteria, presumably reflecting bacterial cell-wall differences .

Gram-negative and Gram-positive membranes are both negatively charged but differ in terms of

21

the isoelectric point (approximately pH 4–5 and pH 7, respectively) . Thus, at pH 7, E. coli with

a relatively more negative charge could resist the Ti3C2Tx MXene more so than B. subtilis .

5. Conclusions and outlook

We have reviewed progress in the use of MXenes and MXene-based materials for energy and

environmental applications. Over the last few years, substantial progress has been made. The

materials show great promise in terms of energy conversion and storage, being very safe, and

exhibiting very large interlayer spaces, environmental flexibility, and outstanding

biocompatibility. In addition, the materials may play useful roles in environmental processes.

The electrochemical performance of MXene-based materials is enhanced by increasing the

specific surface area, electrical conductivity, hydrophilicity, and stability. MXene-based

nanomaterials are promising alternatives to traditional adsorbents (e.g., granular/powdered

activated carbon) and other nanomaterials (e.g., graphene-based nanomaterials) widely used to

treat water and wastewater.

While current energy conversion and storage methods/devices have been developed, further

development of high-performance MXene-based nanomaterials is critical for the following

reasons: (i) it is difficult to contemporaneously achieve high-level energy conversion with

adequate energy storage; and (ii) existing power supplies for rechargeable devices are expensive

and suboptimal. In particular, appropriate spacing of MXene-based layers is essential, obviating

the need for restacking, maintaining a high surface area, and allowing target molecules and ions

to intercalate. MXene-based nanomaterials must be cost-competitive. Also, the environmental

implications of MXene-based nanomaterials must be considered. Comprehensive

ecotoxicological assessments and life-cycle analyses of MXene-based nanomaterials are

imperative, but MXene-based nanomaterials remain very promising.

22

Acknowledgements

This research was also supported by the Korea Ministry of Environment, ‘GAIA Project,

2018XXXXX’. This research was also supported by Basic Science Research Program through

the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-

2017R1D1A1B04033506).

23

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Table 1. Summary of removal of selected heavy metals and organic species by graphene-based nanoadsorbents.

Adsorbent Contaminant Co (mg L-1) Experimental condition

qm (mg g-1) Main finding Ref.

Graphenes Sb 3 pH 3-11temp. = 303 Ksynthetic water

8.1 The pseudo-second-order kinetic shows the best correlation for the adsorption, suggesting that the process was governed by the chemical process.

GOs Pb 1-60 pH 5.6single/mixed solutionsynthetic water

227.2 Once used as filter media in sand columns, GO efficiently removed Pb from water. In the mixed solution system, while Pb showed good sorption performance, the GOs-sand columns still effectively reduced both contaminants.

rGOs Cd 0.2-10 pH 2-12temp. = 298 Ksynthetic water

0.499-6.32 The oxygen-containing groups might be first occupied by Cd over 1-naphthol and the straight H-bonding could be replaced by a moderately strong cation– interaction between Cd and 1-naphthol.

rGOs-iron oxide Pb 10-15 pH 5temp. = 303 Ksynthetic water

455 Adsorption ability of Pb(II) on rGOs-iron oxides is considerably lower than that of Pb(II) on GOs-iron oxides, owing to the difference between GOs-iron oxides and rGOs-iron oxides.

Graphenes Oilstoluene chloroform

0-100 Spongy graphene synthetic seawater

86,000 The results exhibited some excellent features of spongy graphene: quick and efficient absorption of organic solvents and oils, high restoration of absorbates, recyclability, and long-life of the sorbent material.

GOs 17-estradiol 0.05-4 pH 3-12temp. = 298-338 Kvarious background ions

129-149 The large adsorption affinity of GOs for 17-estradiol could be mostly owing to hydrogen bonds and– interactions. The adsorption capacity was insignificantly influenced by the solution pH.

rGOs Sulfapyridinesulfathiazole

0-75 pH 1-12temp. = 298 Ksynthetic WW

35-191(sulfapyridine)34.9-245(sulfathiazole)

The uncharged species showed strong adsorption affinity and high contributions to the general adsorption of sulfonamides, which attributed to the great hydrophobicity and -electron accepting capability.

GOs-Fe3O4 Tetracyclineantibiotics

50 pH 3-10, various ionic strengthroom temp.synthetic water

35.5-45.0 It is extraordinary that the ionic strength and the pH of solution nearly had no effect on the adsorption process, which makes GOs-Fe3O4 to be the possible adsorbents in the application of removing TCs from environment water samples.

GOs-Fe3O4-SiO2

p-nitrophenol 100-800 pH 3-9, various ionic

strengthtemp. = 298-318 K

1,142-1,549 The Fe3O4@mSiO2/GO hybrid composites might be readilyseparated from solutions through an external magnetic force.

33

Co = initial concentration; qm = maximum adsorption capacity; GOs = graphene oxideds; rGOs = reduced graphene oxides; temp. = temperature.

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Table 2. MXenes: Summary of material composition, synthetic method, morphology, application, and performance in energystorage and conversion modified from .

Material composition Synthesis method Morphology Application Performance Ref.

MAX phase MXene-hybrid

Mo2Ga2C Mo2CTx HF etching Nanosheets Hydrogen evolution reaction

609 mV at 10 mA cm-2

Ti3AlC2 Ti3C2-C3N4 HF etching Nanosheets Oxygen evolution reaction

210 mV at 10 mA cm-2

Ti3AlC2 Ti3C2Tx HCl–LiF Clay Supercapacitors 900 F cm-3 at 2 mV s-1

Ti3AlC2 Ti3C2Tx-EG HF etching Nanosheets Supercapacitors 33 F cm-2 at 2 mV s-1

Ti3AlC2 Ti3C2Tx HF etching Paper Supercapacitors 340 F cm-3 at 1 A g-1

Ti3AlC2 Ti3C2Tx-CNT HF etching Film Li-ion batteries 1250 mA h g-1 at 0.1CTi2AlC Ti2COx HF etching Particles Li-ion batteries 225 mA h g-1 at 1CTi3AlC2 Ti3C2Tx HF etching Film Supercapacitors 528 F cm-3 at 2 mV s-1

Ti3AlC2 Ti3C2Tx HF etching Nanosheets Supercapacitors 517 F g-1at 1 A g-1

Hf3Al4C6 Hf3C2Tz HF etching Nanosheets Li-ion batteries Na-ion batteries

1567 mA h cm-3 at 200 mA g-1

504 mA h cm-3 at 200 mA g-1

V2AlC V2C HCl–NaF Nanosheets Li-ion batteries 467 mA h g-1 at 50 mA g-1

Mo2TiAlC2 Mo2TiC2 HF etching Paper Li-ion batteries 176 mA h g-1 at 1CNb2AlC3 Nb2C3Tx HF etching Nanosheets Li-ion batteries 222 mA h g-1 at 4C Ti3AlC2 Ti3C2 HF etching Paper Li-ion batteries 410 mA h g-1 at 1CTi3AlC2 Ti3C2Tx NH4F-hydrothermal Nanosheets Supercapacitors 141 F g-1 at 2 A g-1

Mo2Ga2C Mo2TiCx HCl–LiF Paper Supercapacitors Li-ion batteries

196 F g-1 at 2 mV s-1 560 mA h g-1 at 0.4 A g-1

Nb2AlC Nb2CTx-CNT HF etching Paper Li-ion batteries 400 mA h g-1 at 0.5CTi2AlC Ti2COx HF etching NA Li-ion batteries 65 mA h g-1 at 10CV2AlC V2CTx HF etching Nanosheets Li-ion batteries 260 mA h g-1 at 1CV2AlC NaV2CTx-HC HF etching Nanosheets Na-ion batteries 100 F g-1 at 0.2 mV s-1

Ti2AlC Ti2CTx HF etching Nanosheets Na-ion hybrid capacitor

90 mA h g-1 at 1C

Ti3AlC2 Ti3C2Tx HF etching Nanosheets Na-ion batteries K-ion batteries

370 mA h g-1 at 0.2 mV s-1 260 mA h g-1 at 0.2 mV s-1

NA Sulphur-Ti2C NA Nanosheets Li–S batteries 1200 mA h g-1 at 5CTi3AlC2 Ti3C2Tx-polypyrrole HCl–LiF Paper Supercapacitors 416 F g-1 at 5 mV s-1

Ti3AlC2 Ti3C2Tx HF etching Nanosheets Na-ion batteries 270 mA h g-1 at 20 mA g-1

Ti3AlC2 d-Ti3C2Tx-glycine HCl–LiF Film Li-ion batteries 324 F g-1 at 1 V s-1

Ti3AlC2 Ti3C2Tx-rGO HCl–LiF Nanosheets Supercapacitors 8.6 mW h cm-3 at 0.2 W cm-3 Ti3AlC2 Ti3C2Tx-CNT HCl–LiF Nanosheets Supercapacitors 314 F cm-3 at 1.7 mg cm-2

Ti3AlC2 Na0.23TiO2-Ti3C2 HF etching Sandwich Li/Na-ion batteries 673 mA h g-1 at 3C

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Ti3AlC2 Ti3C2Tx HF etching Nanosheets Supercapacitors 2.8 mW h cm-3 at 0.225 W cm-3

Ti3AlC2 Ti3C2-rGO HF etching Film Li-ion batteries 336 mA h g-1 at 0.05 A g-1

Ti3SiC2 Ti3C2Tx-CMK5 HF etching Nanosheets Li-ion batteries 749 mA h g-1 at 1CTi3AlC2 Li4Ti5O12-Ti3C2Tx HF etching Nanosheets Li-ion batteries 116 mA h g-1 at 10 A g-1

Ti3AlC2 Ti3C2Tx-SnS2 HF etching Sandwich Na-ion batteries 882 mA h g-1 at 0.1 A g-1

Ti3AlC2 BiOCl-Ti3C2Tx HF etching Nanosheets Supercapacitors 397 F cm-3 at 1 A g-1

Ti3AlC2 Ti3C2Tx-CNT HF etching Nanosheets Li-ion batteries 489 mA h g-1 at 0.05 A g-1

Ti3AlCN Ti3C2Tx HCl–LiF Nanosheets Supercapacitors 61 mF cm-2 at 5 A cm-2

Ti3AlC2 SnS-Ti3C2Tx HF etching Nanosheets Na-ion batteries 413 mA h g-1 at 0.1 A g-1

HF = hydrofluoric acid; EG = exploited graphene; CNT = carbon nanotube; HC = hard carbon; NA = not available; rGO = reduced graphene oxide; CMK5 = mesoporous carbon;

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Table 3. Summary of adsorptive removal of selected species by MXenes.MXene Species Co

(mg L-1) Experimental condition

qm

(mg g-1) Main finding Ref.

Ti3C2Tx Methylene blue 50 Kinetic = 8 htemp. = 293 Ksynthetic water

38.9 The adsorption ability of Ti3C2Tx for methylene blue is somewhat lower than commonly used commercial activated carbons, but similar with other materials.

Ti3C2Tx

Ti3C2Tx-LiOHTi3C2Tx-NaOHTi3C2Tx-KOH

Methylene blue 50 pH 6-9temp. = 298 Ksynthetic water

99.911918474.2

Adsorption degree alk-Ti3C2Tx follows the Langmuir model, which indicates that alk- i3C2Tx have homogeneous adsorption surfaces and only can have one adsorbate layer.

Ti3C2Tx Ba 1-55 pH 2-13temp. = 298 Ksynthetic water

9.3 The maximum adsorption capacity (9.3 mg g-1 at Ba = 55 mg L-1) is higher than the adsorption capacity of other adsorbents such as activated carbons and carbon nanotubes.

Ti3C2Tx Cu 25 pH 2-6temp. = 298 -318 Ksynthetic water

86.5 The adsorption capacity of delaminated-Ti3C2Tx was approximately 3 times higher than that of a commercially available activated carbon.

Ti3C2Tx U 100 pH 5temp. = 293 Ksynthetic water

214 The adsorption of U(VI) in hydrated versus dry Ti3C2Tx was significantly enhanced, primarily due to the more flexible nature and much larger interlayer space of hydrated Ti3C2Tx.

Magnetic Ti3C2Tx Hg 25-1,000 pH 2-9temp. = 288 -318 Ksynthetic water

1,128 The maximum experimental adsorption capacity for Hg(II) by magnetic Ti3C2Tx was significantly greater than that previously reported for 2D nano-materials and nanocomposites.

Ti3C2(OH)0.8F1.2 Cr 10 pH 3-6temp. = 298Ksynthetic water

62 The chemical exfoliation of MXene generally takes place in solutions containing F ions, and then the surface of MXene is covered by F groups.

Co = initial concentration; qm = maximum adsorption capacity; temp. = temperature.

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Table 4. Summary of removal of selected species by MXene membranes.MXene Species Experimental

conditionSupport layer Thickness

(nm)Pure water flux Key removal (%) Ref.

Ti3C2Tx Rhodamine BEvans blueCytochrome

Dead-end/cross-flow Co =10-20 mg L-1

Anodic aluminum oxide

200-1,000 1084 L m-2 h-1 bar-1

85 (rhodamine B)90 (Evans blue)97 (cytochrome)

Ti3C2Tx Congo redgentian violetMgCl2

Na2SO4

NaCl

Dead-endCo =100 – 1,000 mg L-1

Polyethersulfone NA 405 L m-2 h-1 bar-1 92.3 (Congo red)80.3 (gentian violet)2.3 (MgCl2)13.2 (Na2SO4) 13.8 (NaCl)

Ti3C2Tx Rhodamine Bmethyl greenbovine serum albumin

Dead-endCo =50-100 mg L-1

Polyvinylidene difluoride

470 ~420 L m-2 h-1 bar-1 79.9 (rhodamine B)92.3 (methyl green)>99 (bovine serum albumin)

Ti3C2Tx E. coliB. subtilis

Co =~2,700 CFU mL-1 Polyvinylidene difluoride

1,200 37.4 L m-2 h-1 bar-1 >99 (E. coli)>99 (B. subtilis)

Ti3C2Tx-Ag Rhodamine Bmethyl greenbovine serum albumin

Dead-endCo =50-100 mg L-1

Polyvinylidene difluoride

470 ~420 L m-2 h-1 bar-1 79.9 (rhodamine B)92.3 (methyl green)>99 (bovine serum albumin)

Ti3C2Tx-graphene oxide

Brilliant bluerose Bengalmethylene bluemethylene redMgSO4

NaCl

Dead-endCo =10 mg L-1

Polycarbonate and nylon

20-90 ~25 L m-2 h-1 bar-1 95.4 (brilliant blue)94.6 (rose Bengal)40 (methylene blue)5 (MgSO4)<1 (NaCl)

Co = organic initial concentration; NA = not available.

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(a)

(b) Synthesis of Ti3C2Tx MXene

Fig. 1. (a) Timeline of MXenes: from Ti3C2 discovery to ordered divacancies and (b) general map for Ti3C2Tx fabrication from Ti3AlC2 (reprinted with permission).

39

(a) (b) (c)

Fig. 2. Top and side views of (a) pristine MXene M2X. Top and side views of (b) Model M2X-1 and (c) Model M2X-2 for functionalized MXene. A and B indicate different types of hollow sites in (a). M, X, and T denote transition metal (green), C/N (blue), and attached chemical groups such as F, O, and OH (red), respectively (reprinted with permission).

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Ti3C2 MXene

80oC 10 h

Mn2+

Mn nucleusMn3O4

(a) (b)

(c)

Voltage (V vs. RHE)

(d)

J (m

A cm

-2)

J (m

A cm

-2)

Time (hour)

Mn3O4/ABMn3O4/MxenePt/C

Mn3O4/MxeneMn3O4/AB

Fig. 3. (a) Schematic of preparation of Mn3O4-MXene nanocomposites, (b) SEM image of the Mn3O4-MXene, (c) linear sweep voltammetry curves of various nanocomposite catalysts, and (d) stability test of two different nanocomposites (reprinted with permission).

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2D Ti3C2Tx 2D rGO SO42- H+

CTAB-grafted SCNTs Pure Ti3C2Tx sheets

(a) (b)CTA+

(c)Bi compound MXene sheet BiOCl nanosheet

Adding Mxene in the ionized Bi solution

BiOCl nanosheets growing in the solution and on MXene surfaces

Multi-layer BiOCl nanosheets absorbed on MXene surfaces

Fig. 4. Schematic of preparation of (a) asymmetric flexible Ti3C2Tx-rGO nanocomposites , (b) Ti3C2Tx-single walled carbon nanotubes self-assembled composite film , and (c) bismuth oxychloride nanosheets-immobilised Ti3C2Tx MXene material via a facile and cost-effective chemical bath deposition method (reprinted with permission).

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Functional groups (-OH, -F, -O) CTib

0.7-2 nm

c

Ti C

Fig. 5. Schematic of adsorption mechanism of Ba on surface of MXene layers (reprinted with permission).

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(a)

(c)

(b)

HOC Ti

Anodic aluminum oxide (AAO)

SonicationHF etching

CAlTi

Ti3AlC2(MAX)

HCltreatment

Ti3C2Tx

Mxene membrane

Graphene oxides

Ti3C2Tx

Fig. 6. Fabrication schematic of (a) MXene membrane supported on anodic aluminum oxide , (b) MXene membrane supported on polyvinylidene fluoride (PVDF) , and (c) MXene-GO membrane supported on polycarbonate (reprinted with permission).

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Ti3C2Tx

GO

Intercalation

Ions: Low rejection

0.96 nm

0.5nm

Dyes: High rejection

H2O Rhodamine B Methylene green Bovine serum albumin Ag nanoparticles

Bacteria Ti3C2Tx

(a)

(b)

Fig. 7. (a) Schematic structure and mechanisms of removal of the MXene-Ag composite membrane and (b) schematic diagrams for the steric exclusion mechanisms for hydrated ions and dye molecules on the Ti3C2Tx-GO membrane (reprinted with permission).

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(a)

(b)

(c)

T3C2

High-energy ball milling

Ti C Agate ball TiO2

Cu(NO3)2

H2O

CuO TiO2

Methylene orange

CO2

TiO2

CuO

Cu(NO3)2 Decomposition

MXene

Fig. 8. (a) Schematic of high-energy ball milling of Ti2CTx to form titania-carbon , (b) fabrication procedure of the Ti3C2-TiO2-CuO nanocomposites, and (c) the schematic illustration of a potential photocatalytic mechanism of the Ti3C2-TiO2-CuO nanocomposites (reprinted with permission).

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