With the rapid advancement of microelectronic devices and wearable technologies, on-chip energy-storage components must simultaneously achieve high energy density and compact form factors, making micro-supercapacitors (MSCs) a prime candidate. However, conventional planar electrode architectures suffer from low active-material loading and long ion-transport pathways. Two-dimensional Ti₃C₂Tₓ MXene materials, with their high specific surface area and outstanding electrochemical performance, are an ideal choice for fabricating high-performance MSCs; yet their 3D assembly faces three major challenges: first, the interlayer interactions among MXene nanosheets are limited to weak van der Waals forces and hydrogen bonds, lacking strong interfacial interactions and thus making it difficult to maintain a stable 3D architecture; second, existing fabrication methods rely on binders and other additives, which can occupy active sites and degrade device performance; and third, conventional 3D-printing techniques suffer from low resolution (typically >40 μm) and a low aspect ratio (generally <0.3), rendering them incapable of producing high-fidelity, complex 3D microstructures.

To address the aforementioned bottlenecks, a team from Carnegie Mellon University in the United States has developed an additive-free aerosol jet 3D printing (AJP) technique that successfully enables the 3D self-supporting assembly of MXene nanosheets. The relevant findings have been published in the journal Small.

Core Technical Solutions and Technological Innovation

(1) Preparation of Additive-Free MXene Ink

Ti₃C₂Tₓ MXene ink was prepared via a mild interlayer exfoliation method (MILD) without any additives, using only deionized water as the solvent. The process started with Ti₃AlC₂ MAX-phase powder as the precursor; LiF and concentrated hydrochloric acid were used for etching to remove the Al layer, followed by centrifugal purification to adjust the pH to approximately 6. Subsequently, probe-assisted ultrasonic exfoliation in an ice-water bath yielded a stable colloidal ink with a concentration of 15 mg/mL, which can be stored at room temperature for more than three weeks. Characterization by AFM and Raman spectroscopy revealed that the MXene nanosheets have a monolayer thickness of about 1.3 nm, high crystallinity, and intact surface functional groups. The hydrodynamic diameter is centered around 221 nm, meeting the requirements for aerosol printing.

(2) Mechanism and Key Innovations of Aerosol Jet Printing (AJP) 3D Printing

The core technical advantage of AJP lies in leveraging the real-time thickening effect of aerosol droplets and fluid-dynamic focusing to achieve support-free 3D assembly of MXene: the ink is ultrasonically atomized into micron-sized droplets, and during carrier-gas transport the solvent evaporates in real time, causing the ink to thicken; subsequently, the droplets are pneumatically focused by a sheath gas and ejected at high speed onto a preheated substrate (100°C), where they instantaneously solidify to form a stable 3D structure. This process requires no additives or supporting structures, thereby avoiding issues such as nozzle clogging and structural shrinkage. The optimal process parameters are: a delivery tube length of 50 cm, a carrier-gas flow rate of 30 sccm, and a sheath-gas flow rate of 55 sccm.

Key Research Findings

2D Printing Performance: High Precision and Versatility across Multiple Applications

AJP technology achieves a minimum line width of 24 μm in 2D printing, significantly surpassing conventional techniques such as inkjet printing (80 μm) and electronic jet printing (180 μm). It enables the printing of complex patterns—including school emblems, portraits, and helical electrodes—on a variety of substrates, including alumina and flexible polyimide, with continuous, uniform lines, sharp edges, and no color bleeding or coffee-ring effects.

3D Printing Performance: Achieving High Aspect Ratios and Complex Structures

Through process optimization, high-fidelity 3D microstructures have been successfully fabricated: a micropillar array with a diameter of approximately 63 μm and a height of up to 3,800 μm, achieving an aspect ratio as high as 60—far exceeding the state-of-the-art maximum value of less than 0.3. In addition, complex structures such as 3D microflowers, micropyramids, and vascular “pencil cacti” have been realized, with a structural shrinkage of less than 1%, a micropillar verticality error of ±0.5°, and oriented, layered stacking of MXene nanosheets within the structure, where the interlayer spacing matches the van der Waals gap, resulting in outstanding structural stability.

Performance of Micro-Supercapacitors: Record-Breaking Electrochemical Performance

3D interdigitated electrodes fabricated via the AJP method were assembled into MSCs using an H₂SO₄/PVA hydrogel electrolyte, exhibiting electrochemical performance that significantly surpasses state-of-the-art technologies: at a current density of 1.5 mA/cm², the device achieves a specific capacitance of 375 mF/cm² (equivalent electrode-specific capacitance of 1500 mF/cm²); at a power density of 0.40 mW/cm², the energy density reaches 11.04 μWh/cm², and even when the power density is increased to 5.42 mW/cm², the energy density remains at 7.53 μWh/cm²; after a tenfold increase in current density, the capacitance retention rate is 83.7%, and after 5,000 charge–discharge cycles, the capacitance retention rate is 96.8%; the equivalent series resistance is only 14.3 Ω, demonstrating highly efficient ion diffusion.

Illustrated Guide

Figure 1 presents a schematic illustrating the formation of three-dimensional architectures from two-dimensional MXene nanosheets. (a) A comparative chart of additive manufacturing (AM) routes for 3D microstructures, categorizing the techniques based on the feedstock material (metals, ceramics, polymers, and two-dimensional materials), the initial phase (solid, liquid, or dispersed phase), and the bonding mechanisms (strong primary forces versus weak secondary forces). This chart highlights the fundamental challenge in assembling two-dimensional materials into three-dimensional structures: the need to rely on weaker secondary interactions, such as van der Waals forces, for assembly—contrasting with the robust primary or secondary bonds that typically form in other bulk materials during conventional additive manufacturing. (b) The AJP process used to fabricate three-dimensional microstructures. (c) An enlarged view of an aerosol droplet containing two-dimensional nanosheets. During transport, the droplet undergoes real-time thickening within the delivery tube, reducing the solvent content of the ink. The thickened ink is then conveyed to the nozzle, where it is focused by a sheath gas to form a highly concentrated ink stream with a higher solid content than the original MXene ink. (d) Aerosol droplets containing two-dimensional materials are deposited from the nozzle in a single-layer fashion and rapidly lose their solvents through evaporation. This dried layer serves as the foundation for the next deposition, with microscale surface tension facilitating the stacking process. The resulting three-dimensional structure is held together by van der Waals forces and, where applicable, by hydrogen bonds. These secondary interactions enable two-dimensional nanosheets to assemble into complex, directionally stacked three-dimensional architectures without the need for support structures. (e) A comparison of the product-development processes for additive manufacturing using 3D printing versus AJP for additive fabrication of two-dimensional materials, demonstrating AJP’s capability to construct intricate microstructures in three-dimensional space at high resolution and high fidelity. (f) A comparison of the feature sizes of the printed three-dimensional MXene nanosheet structures with those achieved via two-photon polymerization, powder-based methods, and filament-based approaches.

Figure 2. Preparation and characterization of Ti3C2Tx MXene nanoink, along with demonstration of its AJP performance and design flexibility through the fabrication of planar micro-patterns of varying sizes and complexities. (a) Schematic illustrating the ink preparation steps: Ti3AlC2 MAX phase is etched with hydrofluoric acid to obtain Ti3C2Tx MXene nanosheets, which are then purified; probe-assisted ultrasonication is used to exfoliate the MXene, yielding smaller-diameter Ti3C2Tx MXene flakes; the concentration of Ti3C2Tx MXene is adjusted to suit aerosol jet printing. (b) AFM image of as-prepared Ti3C2Tx MXene (top) and the corresponding height profile along the scan line (bottom). (c) Raman spectrum of probe-sonicated Ti3C2Tx MXene, showing characteristic peaks corresponding to A1g (Ti, C, Tx) and A1g (C). (d) Dynamic light scattering (DLS) characterization of probe-sonicated Ti3C2Tx MXene, providing an estimate of the nanoflake size—approximately 221 nm in hydrodynamic diameter—within the recommended maximum effective aerosolization size of <500 nm. (e) Optical image of a digitally designed pattern (the Carnegie Mellon University logo) printed on an alumina substrate, featuring an MXene pattern from the University of Melbourne. (f) Optical image of MXene printed in the form of the Carnegie Mellon University logo on a flexible polymer substrate (Kapton film). (g) Optical image of Ti3C2Tx MXene printed with a sketch of Andrew Carnegie. (h) Comparison of the resolution of our technique for two-dimensional MXene patterning with that of other printing methods, including inkjet, electronic inkjet, screen printing, extrusion, stamping, and pen writing.

Figure 3. Direct 3D printing of additive-free Ti3C2Tx MXene, featuring a high aspect ratio and overhanging features (without sacrificial support structures), enables the fabrication of complex 3D microstructures through the ink-thickening effect. (a) Optical image of MXene lines printed on an alumina substrate. Printing was carried out using different nozzle lengths (L): 30, 50, 100, and 200 cm, from left to right. (b) Time evolution of aerosol droplet radius under different droplet number densities, based on the simulations discussed in Section S1. (c) Relationship between the height of the printed layer and the number of printing passes. (d) CAD model of a 4 × 4 micropillar array (left) and the corresponding optical image of the printed micropillar array (right). (e) SEM image of the micropillar array. (f) SEM image of the internal microstructure of the micropillars shown in Figure 3e after focused ion beam (FIB) milling. (g) Small-angle X-ray scattering (SAXS) analysis reveals the uniform orientation of stacked MXene nanosheets within the three-dimensional microstructure. (h) XRD pattern of printed Ti3C2Tx MXene. (i) Optical image of a 3 × 3 micropillar array with a high aspect ratio of 60. (j) Various printing techniques—including inkjet, electron-beam jet, screen printing, extrusion, and stamping—demonstrate AJ Printing’s capability for high-resolution, high-aspect-ratio fabrication of additive-free 2D nanomaterials, achieving two key performance metrics to date in the printing of additive-free 2D nanomaterials (MXene and graphene): “2D resolution” and “3D aspect ratio.” (k) The angle between the printed overhang and the substrate as a function of the droplet-to-droplet offset between adjacent layers. Corresponding optical images of the printed inclined pillars are shown. Scale bar: 50 µm.

Figure 4. Complex 3D Ti3C2Tx MXene microarchitectures. (a) A CAD-rendered model of a flower (top) and the corresponding color SEM image of the printed flower (bottom). This structure integrates 1D rods (stems) with 3D curved surfaces (petals) into a single architecture. (b) A CAD-rendered model of a pyramid (top) and the color SEM image of the printed pyramid corresponding to the design (bottom). (c) A CAD model of a pencil cactus (left) and the color SEM image of the printed pencil cactus corresponding to the design (right). (d) A color SEM image of a printed “forest,” demonstrating the controllability and reproducibility of our fabrication technique.

Figure 5. Structural characterization and performance evaluation of Ti3C2Tx MXene micro-supercapacitors with three-dimensional finger-like electrodes fabricated via AJ printing. (a) Schematic illustration of the fabrication process for the finger-like micro-supercapacitors (MSCs). (b) Normalized cyclic voltammograms of MSC-4 at different scan rates, normalized by electrode capacitance. (c) Galvanostatic discharge curves of MSC-4 at various current densities. (d) Electrode capacitance of MSCs with different electrode heights at a current density of 4 mA·cm−2. The linear fit (red line) indicates a linear relationship between capacitance and electrode height. (e) Comparison of device capacitance among finger-like MSCs fabricated by AJ printing and those prepared using other high-resolution methods: imprinting, E-jet printing, laser processing, plasma-assisted fabrication, blade coating, inkjet printing, and screen printing, expressed as areal capacitance. (f) Ragone plots showing the energy density and power density of interdigitated porous carbon nanomaterials obtained via AJ printing and other high-resolution fabrication techniques, including imprinting, E-jet printing, laser–plasma processing, blade coating, inkjet printing, and screen printing. (g) GCD curves of porous carbon nanomaterials at different numbers of charge–discharge cycles under a current density of 20 mA·cm−2. (h) Cyclic stability of the porous carbon nanomaterials fabricated by AJ printing.

This study breaks through the bottleneck in 3D assembly of 2D materials, establishing a new additive-free, support-free, high-fidelity assembly paradigm. For the first time, it has realized a self-supporting 3D MXene structure with an aspect ratio of 60, and the resulting MSCs exhibit performance that significantly surpasses existing technologies, thereby providing a solution for on-chip energy storage in microelectronic devices. Meanwhile, the high resolution and multi-material compatibility of AJP technology open up new avenues for the fabrication of composite energy-storage devices and multifunctional microsystems.

Future research directions include: developing multi-nozzle parallel printing systems to enhance efficiency; extending applications to the composite printing of MXene with other two-dimensional materials; optimizing fabrication processes to enable lower-angled overhang structures; and exploring flexible substrate applications to advance the development of wearable electronic devices. This research offers new avenues for the engineering application of MXene and holds promise for driving technological innovation in areas such as microenergy storage and microrobotics.