MXene Aerogel: Advanced Three-Dimensional Architectures For Multifunctional Applications In Energy, Environment, And Electronics

Molecular Composition And Structural Characteristics Of MXene Aerogel

MXene aerogels are constructed from MXene nanosheets—two-dimensional transition metal carbides, nitrides, or carbonitrides with the general formula M_n+1X_nT_x (where M = transition metal such as Ti, V, Nb; X = C or N; T_x = surface terminations including -OH, -O, -F) 4,13. The parent MAX phase (e.g., Ti₃AlC₂) undergoes selective etching of the A-layer (typically Al) using hydrofluoric acid (HF) or in-situ HF-generating etchants, yielding accordion-like multilayer MXene that can be delaminated into single- or few-layer nanosheets via sonication or intercalation 3,18. The resulting MXene dispersion exhibits colloidal stability due to electrostatic repulsion from negatively charged surface groups and abundant hydrophilic terminations (-OH, -F), enabling facile assembly into 3D architectures 16,19.

The 3D aerogel network is typically formed through directional freeze-casting or ambient gelation followed by freeze-drying or supercritical drying. Directional freezing in liquid nitrogen induces ice-templated assembly, creating vertically aligned porous channels with pore sizes ranging from nanometers to micrometers and porosity exceeding 90% 1,4,10. This hierarchical pore structure—comprising macropores (>50 nm), mesopores (2–50 nm), and micropores (<2 nm)—provides high specific surface area (often 50–200 m²/g depending on composition and processing) and facilitates rapid mass transport for ions, gases, or liquids 9,17. The MXene nanosheets within the aerogel framework are interconnected via van der Waals forces, hydrogen bonding, or covalent cross-linking with polymer or inorganic additives, forming a mechanically robust yet lightweight skeleton with densities as low as 5–50 mg/cm³ 1,6,10.

Key structural features include:

• Layered stacking and interlayer spacing: MXene sheets in aerogels retain interlayer distances of 1.0–1.5 nm (expandable to >2 nm with intercalants), enabling ion intercalation for supercapacitors 17.
• Surface functional groups: -OH and -O terminations dominate after mild etching, while -F groups persist under HF etching; these groups govern hydrophilicity, redox activity, and interfacial bonding with polymers or metal oxides 13,14.
• Electrical conductivity: Bulk MXene aerogels exhibit electrical conductivity in the range of 10²–10⁴ S/m, depending on MXene content, alignment, and inter-sheet contact resistance 6,10,16.

Precursors, Synthesis Routes, And Processing Parameters For MXene Aerogel

Etching And Delamination Of MXene Precursors

The synthesis begins with selective etching of MAX phase powders. Common protocols include:

• HF etching: Ti₃AlC₂ powder (5–10 g) is stirred in 40–48 wt% HF (50–100 mL) at 40–60 °C for 24–48 h, yielding multilayer Ti₃C₂T_x 3,18. The reaction is exothermic; temperature control is critical to prevent over-oxidation.
• In-situ HF generation: Mixing LiF and HCl (e.g., 1 g LiF in 20 mL 9 M HCl per 1 g MAX) at 35 °C for 24 h offers safer handling and comparable yield 1,4.
• Delamination: The etched product is washed with deionized water until pH >6, then sonicated (bath or probe, 100–400 W, 30–60 min) or intercalated with DMSO/TBAOH to exfoliate into single-layer nanosheets. Centrifugation (3500 rpm, 1 h) separates the colloidal supernatant (concentration 5–20 mg/mL) from unetched sediment 4,16.

Gelation And Assembly Strategies

MXene aerogels are formed via several routes:

1. Directional freeze-casting: MXene dispersion (10–30 mg/mL) is poured into a mold with a metal base in contact with liquid nitrogen (-196 °C), inducing unidirectional ice growth and MXene sheet alignment perpendicular to the freezing front 1,4,10. Freezing time is 2–4 h at -10 to -20 °C, followed by freeze-drying at -56 to -60 °C for 36–48 h under <10 Pa 1,4.
2. Polymer-assisted gelation: Mixing MXene with polymers (e.g., polyvinyl alcohol, chitosan, sodium alginate, polyacrylamide) and cross-linkers (e.g., glutaraldehyde, Ca²⁺, borate ions) forms hydrogels that are subsequently freeze-dried 1,3,7,13,15. For example, MXene/dopamine-grafted hyaluronic acid (15–60 mg) with sodium tetraborate (12–48 mg) in 3 mL MXene dispersion (10 mg/mL) gels within 30 min and is freeze-dried to yield aerogels with compressive modulus ~0.5–2 MPa 1.
3. In-situ polymerization: Acrylamide monomer (4 g) is added to MXene/TEMPO-oxidized cellulose nanofiber suspension and photopolymerized under xenon lamp irradiation, forming MXene-polyacrylamide aerogels with dual photothermal and photocatalytic functions 13.
4. Hybrid composite assembly: MXene is co-assembled with graphene oxide (GO), carbon nanotubes (CNTs), silicon carbide nanowires (SiC_nw), or metal oxides (e.g., BiOBr, TiN) via mixing, reduction (e.g., hydrazine, ascorbic acid), and freeze-drying 2,5,6,16. For instance, MXene/GO aerogels are prepared by mixing MXene and GO dispersions (mass ratio 1:1 to 4:1), adding modifiers (e.g., ethylenediamine), and freeze-drying; subsequent thermal reduction at 800 °C in Ar/NH₃ converts GO to graphene and generates in-situ TiN coatings on MXene, enhancing oxidation resistance 2,6.

Critical Processing Parameters

• MXene concentration: 5–30 mg/mL; higher concentrations yield denser aerogels with improved mechanical strength but reduced porosity 1,4,17.
• Freezing rate: Rapid freezing (liquid nitrogen contact) produces smaller ice crystals and finer pores; slow freezing (e.g., -20 °C freezer) creates larger, more aligned channels 10,15.
• Cross-linker ratio: For polymer-MXene aerogels, cross-linker (e.g., Ca²⁺, borate) to polymer mass ratio of 0.2–0.8 optimizes gel strength and porosity 1,7,15.
• Drying method: Freeze-drying preserves pore structure; supercritical CO₂ drying further minimizes shrinkage but adds cost 4,12.
• Post-treatment: Thermal annealing (200–800 °C in inert atmosphere) improves crystallinity and conductivity; oxidation in air (300–400 °C) followed by reduction in Ar/NH₃ (600–800 °C) generates protective TiN layers, significantly enhancing thermal stability (up to 600 °C in air) 2.

Physical, Chemical, And Functional Properties Of MXene Aerogel

Mechanical Properties And Compressibility

MXene aerogels exhibit remarkable mechanical resilience, particularly when reinforced with polymers or carbon nanomaterials. Pure MXene aerogels typically show compressive strain recovery of 60–70% after 100 cycles at 50% strain 9,16. Polymer-modified variants (e.g., MXene/polyurethane, MXene/cellulose nanocrystals) achieve >85% recovery even after 100 cycles at 0 °C, 80 °C, and 150 °C, with compressive modulus of 0.5–2.0 MPa and ultimate compressive strength of 50–150 kPa at 80% strain 1,17. The elastic modulus scales with MXene content (typically 0.1–2.0 GPa for composites with 10–85 wt% MXene) and is influenced by the ratio of flexible (polymer) to rigid (MXene) segments 1,17. Core-sheath MXene/graphene fiber aerogels demonstrate tensile strength up to 10 MPa and elongation at break of 15–25%, suitable for flexible wearable devices 6.

Electrical Conductivity And Electromagnetic Shielding

MXene aerogels are among the most conductive aerogel materials, with bulk electrical conductivity ranging from 10² to 10⁴ S/m depending on MXene alignment, inter-sheet contact, and filler content 6,10,16. Directionally frozen MXene aerogels with vertically aligned channels exhibit anisotropic conductivity (parallel to channels: 10³–10⁴ S/m; perpendicular: 10²–10³ S/m) 10. This high conductivity translates to exceptional electromagnetic interference (EMI) shielding effectiveness (SE): pure MXene aerogels achieve SE of 30–50 dB in the X-band (8.2–12.4 GHz), while MXene/graphene or MXene/CNT composites reach 50–70 dB, sufficient for commercial and military applications (>40 dB blocks >99.99% of EM radiation) 5,10,16. The shielding mechanism is dominated by absorption (SEA) rather than reflection (SER), with SEA/SE ratios >0.7, attributed to multiple internal reflections within the porous network and ohmic loss from conductive MXene sheets 10,16. MXene/chitosan aerogels doped with phase-change materials (e.g., polyethylene glycol) exhibit SE of 40 dB and thermal conductivity of 1.446 W/(m·K), enabling dual thermal management and EMI shielding for battery enclosures 10.

Photothermal Conversion And Solar Evaporation

MXene's broad-spectrum optical absorption (UV to near-IR) and metallic electronic structure enable efficient photothermal conversion, with solar-to-thermal efficiency exceeding 90% under 1 sun (1 kW/m²) illumination 7,13,15. MXene aerogels designed for solar-driven water evaporation typically feature:

• Janus structure: Hydrophobic upper layer (e.g., silane-modified cellulose/MXene) and hydrophilic lower layer (cellulose aerogel) enable stable floating at the air-water interface, with the lower half submerged for continuous water supply and the upper half dry for heat localization 7,15.
• Vertical channels: Directionally frozen MXene/polyvinyl alcohol/alginate aerogels with aligned pores (10–50 μm diameter) facilitate rapid water transport (capillary rise rate ~5 mm/min) and vapor escape, achieving evaporation rates of 2.5–3.8 kg/(m²·h) under 1 sun, significantly exceeding the theoretical limit for bulk water heating (1.47 kg/(m²·h)) 15.
• Salt resistance: The vertical pore architecture and surface water film prevent salt crystallization; MXene aerogel evaporators maintain >90% of initial evaporation rate after 10 cycles in 20 wt% NaCl solution 15.
• Dual functionality: MXene/BiOBr aerogels combine photothermal evaporation with photocatalytic degradation of organic pollutants (e.g., methylene blue, phenol) under visible light, achieving >95% dye removal in 2 h while sustaining evaporation rates of 2.0–2.5 kg/(m²·h) 13.

Electrochemical Performance For Energy Storage

MXene aerogels serve as high-performance electrodes for supercapacitors and batteries due to their high surface area, rapid ion transport, and pseudocapacitive charge storage. Key metrics include:

• Specific capacitance: MXene/cellulose nanocrystal/polyurethane aerogels (85 wt% MXene) deliver gravimetric capacitance of 225 F/g at 2 mV/s in 1 M H₂SO₄, with areal capacitance of 450 mF/cm² at 1 mA/cm² 17. MXene/polypyrrole-Fe²⁺ aerogels achieve 180 F/g at 1 A/g with 92% retention after 5000 cycles 3.
• Rate capability: The open 3D network enables capacitance retention of >70% at 100 mV/s (50× scan rate increase), attributed to short ion diffusion paths (<10 μm) and high electronic conductivity 17.
• Cycling stability: Polymer cross-linking and TiN coating mitigate MXene oxidation, maintaining >85% capacitance after 10,000 charge-discharge cycles 2,17.

Adsorption And Catalytic Properties

MXene aerogels functionalized with polymers or metal oxides exhibit high adsorption capacity for heavy metals, phosphate, and organic pollutants:

• Vanadium ion adsorption: MXene/polyethyleneimine/orange peel powder aerogels achieve adsorption capacity of 120–150 mg V/g at pH 3–5, with >80% removal efficiency in simulated wastewater; the material is regenerable via acid washing 18.
• Phosphate removal: MXene-doped sodium alginate aerogel beads (MXene content 10–30 wt%) adsorb 80–100 mg P/g, with selectivity over sulfate and chloride due to surface complexation between Ti-O terminations and phosphate groups 14.
• Catalytic degradation: MXene/polypyrrole-Fe²⁺ aerogels as particle electrodes in three-dimensional electrocatalytic oxidation systems degrade 95% of phenol (100 mg/L) within 60 min at 10 mA/cm², with total organic carbon (TOC) removal >80% 3.

Thermal Stability And Oxidation Resistance

Pristine MXene aerogels oxidize in air above 300 °C, forming TiO₂ and losing conductivity