Large-scale wet-spinning of highly electroconductive MXene fibers

Two-dimensional nanosheets are attractive building blocks due to their electronic, chemical, physical, and mechanical properties, high surface area, and versatile surface chemistries. While macroscopic assembly of 2D materials into one-dimensional fibers via wet-spinning has been successful for graphene-based systems, realizing pure MXene fibers has been challenging. Ti3C2Tx MXene exhibits intrinsically high electrical conductivity in films, but in fiber form prior approaches relied on composites (e.g., with rGO, CNTs, or polymers) that limited conductivity. The key challenges include weak self-supporting organization due to poor interlayer interactions of relatively small MXene sheets and difficulty achieving sufficiently high, stable dispersion concentrations. This study aims to develop a continuously controlled, additive/binder-free wet-spinning process to assemble pure Ti3C2Tx MXene into flexible, meter-long fibers with high electrical conductivity, leveraging liquid-crystalline dispersions and ammonium-ion-induced coagulation.

The paper surveys 2D materials such as graphene, h-BN, g-C3N4, TMDs, BP, and TMOs and highlights wet-spinning as a versatile route for continuous fiber production from liquid-crystalline colloidal dispersions. Prior work achieved macroscopic graphene oxide and graphene fibers with desirable mechanical flexibility and textile integration. For MXenes, prior fiber fabrication used wet-spinning or electrospinning of MXene/polymer blends or MXene/rGO hybrids, but the resulting conductivities (e.g., 72–290 S cm−1 for MXene/graphene hybrids, 26 S cm−1 for CNT fibers, 1489 S cm−1 for MXene/PEDOT:PSS) fell short of leveraging pure MXene’s film conductivity (up to 9880 S cm−1), indicating a gap in achieving high-conductivity, pure MXene fibers. The authors also reference rheological criteria (G′/G″ ranges) for spinnability in liquid-crystalline dispersions as established for GO and anticipated for MXenes.

Synthesis of MXene sheets: Ti3C2Tx MXene was produced by selective etching of Al from Ti3AlC2 (MAX phase) using LiF and HCl (2 g LiF dissolved in 40 mL 9 M HCl, stirred 30 min at 35 °C; 2 g Ti3AlC2 added and stirred under Ar for 24 h). The product was washed via repeated centrifugation with deionized water until pH ~6 to remove acid and obtain a stable aqueous dispersion; the dispersion was purified and concentrated, then stored at −5 °C. Characterization of sheets: SEM visualized MAX and exfoliated MXene; AFM indicated monolayer height 1.35–1.81 nm (double layer 3.31–3.72 nm); TEM/HR-TEM showed crystalline lattice fringes (0.26 nm, (100) plane) and SAED hexagonal symmetry; XRD and elemental analysis confirmed Al removal; XPS deconvolution (C1s, O1s, Ti2p) identified surface terminations (C–Ti–T, C–Ti–(OH), C–Ti–Ox). Zeta potential vs pH indicated increasingly negative charge with increasing pH. UV–vis absorbance scaled linearly with concentration. Conductive AFM confirmed high sheet conductivity. Rheology and dispersion preparation: Concentrated dispersions up to 25 mg mL−1 were prepared, forming viscous, lyotropic liquid-crystalline (LC) inks (viscosity ~3.87×10^3 Pa·s at high concentration), showing birefringence between crossed polarizers. Steady shear measurements showed shear-thinning; shear stress decreased initially then increased with shear rate, indicating shear-induced alignment. Spinnability was assessed via G′/G″ at 0.02 Hz: dispersions ≤12 mg mL−1 were not stably spinnable (e.g., G′/G″ 13.33 at 5 mg mL−1; 6.64 at 12 mg mL−1), while ≥15 mg mL−1 (G′/G″ 5.29) enabled fiber formation. Wet-spinning process: MXene LC ink (typically 25 mg mL−1) was loaded in a syringe and extruded through a 210 µm nozzle at 7 mL h−1 into a coagulation bath containing NH4+ ions (coagulant: 50 g NH4Cl + 20 mL NH4OH in 1000 mL DI water). Ammonium ions induced gelation; without NH4+ no gel fibers formed. The nascent gel fibers were transferred via rollers to a water washing bath and then air-dried for 24 h to yield continuous, meter-long pure MXene fibers aligned along the axis. Fibers were wound on a bobbin for continuous production. Characterization of fibers: SEM examined overall, cross-section, and side-section morphologies, revealing compact lamellar stacking and surface ruggedness due to drying/shrinkage. Electrical conductivity of single fibers was measured in a dry chamber using a four-point probe (electrode spacing 0.4 mm); conductivity calculated using ρ = π d^2 R / (4 L), with diameter from SEM cross-sections. Mechanical properties were tested by tensile testing (gauge length 25 mm, crosshead speed 2.5 mm min−1) to derive strength and modulus. Demonstrations included powering an LED and transmitting audio signals in earphones using MXene fibers as wires.

• Stable, aggregate-free aqueous MXene dispersions at high concentration (up to 25 mg mL−1) exhibiting lyotropic liquid-crystalline behavior and high viscosity (~3.87×10^3 Pa·s at 25 mg mL−1).
• Exfoliated Ti3C2Tx monolayers with lateral size distribution (average lateral length ~2.26 ± 0.95 µm; average area reported ~5.11 µm2) and monolayer thickness 1.35–1.81 nm; high in-plane electrical conductivity confirmed by C-AFM.
• Surface terminations (O, F, OH) verified by XPS; zeta potential becomes more negative with increasing pH, supporting electrostatic stabilization in water.
• Spinnability correlates with viscoelastic ratio G′/G″ at 0.02 Hz; successful fiber formation for concentrations ≥15 mg mL−1 (G′/G″ ≤ ~6.36 threshold), whereas lower concentrations fail due to weak gel strength.
• Ammonium ions (NH4+) are essential coagulants for gelation and continuous fiber formation; no fibers form without NH4+.
• Achieved continuous, meter-long, additive/binder-free, pure MXene fibers with compact lamellar cross-sections.
• Very high electrical conductivity of fibers: 7713 S cm−1, outperforming prior MXene/graphene hybrid fibers (72–290 S cm−1) by ~27–107× and MXene/PEDOT:PSS fibers (1490 S cm−1) by ~5×, and exceeding many graphene fibers (~12–220× higher). Conductivity is ~3.2× higher than reported macroscopic MXene films.
• Demonstrated practical electrical applications: powering a white LED and functioning as earphone wires to transmit audio signals.
• Fibers exhibit high flexibility and favorable mechanical properties; Ashby plot indicates superior combination of conductivity and Young’s modulus versus prior MXene hybrid and graphene fibers.

The study addresses the core challenge of fabricating pure, self-supporting MXene fibers by leveraging highly concentrated, lyotropic liquid-crystalline dispersions and tuning viscoelastic properties to meet wet-spinning criteria. Introducing NH4+ in the coagulation bath induces rapid gelation and alignment of Ti3C2Tx sheets, enabling continuous meter-long fibers without polymeric binders or secondary fillers. The resulting compact lamellar architecture and axial alignment translate the intrinsic nanoscale conductivity of MXene into macroscopic fibers, achieving 7713 S cm−1—well above prior MXene-composite fibers and even reported MXene films. This demonstrates that careful control of sheet size, dispersion stability, rheology (G′/G″), and coagulation chemistry can realize high-performance, all-MXene fibers. The successful demonstrations as electrical wires (LED and earphones) underscore the fibers’ capability for integration in flexible and wearable electronics, where high conductivity and mechanical compliance are critical.

The work presents a continuous, large-scale wet-spinning strategy to fabricate additive/binder-free, pure Ti3C2Tx MXene fibers from stable, lyotropic LC dispersions. Through ammonium-ion-induced coagulation and process optimization, the authors obtain meter-long fibers with ultrahigh electrical conductivity (7713 S cm−1), surpassing prior MXene hybrid fibers and many graphene fibers, and demonstrating practical electrical wiring applications. This scalable approach unlocks the translation of MXene’s nanoscale properties to macroscopic, flexible fibers, paving the way for next-generation portable and wearable electronic devices. Future work can extend the method to other MXene compositions, optimize mechanical performance, and integrate fibers into complex textile architectures and multifunctional devices.

• Conductivity of MXene fibers is highly sensitive to atmospheric humidity; measurements were performed in a dry chamber, indicating environmental dependence that may affect practical deployment.
• Successful spinning requires high dispersion concentrations (≥15 mg mL−1) and specific rheological windows (G′/G″ ≤ ~6.36 at 0.02 Hz); lower concentrations exhibit insufficient gel strength.
• Fiber morphology exhibits surface ruggedness from drying/shrinkage; drying conditions may influence uniformity and mechanical properties.
• Coagulation relies on ammonium ions; process may need adaptation for other environments or for biocompatibility constraints.