With the rapid advancement of microelectronic devices and wearable technologies, on-chip energy storage components must balance high energy density with compact form factors. Micro-supercapacitors (MSCs) have emerged as a leading candidate; however, conventional planar electrode architectures suffer from low active-material loading and long ion‑transport pathways. 2D Ti₃C₂Tₓ MXene materials, with their high specific surface area and excellent electrochemical performance, are an ideal choice for fabricating high‑performance MSCs. Nevertheless, their three-dimensional assembly faces three major challenges: first, MXene nanosheets are held together only by weak van der Waals forces and hydrogen bonds, lacking strong interfacial interactions and thus struggling to maintain a stable 3D architecture; second, existing approaches rely on binders and other additives, which can occupy active sites and degrade device performance; third, conventional 3D‑printing techniques exhibit low resolution (typically >40 μm) and a low aspect ratio (generally <0.3), making it impossible to fabricate 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, successfully achieving 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 the mild interlayer delamination method (MILD), with no additives throughout the process and using deionized water as the sole solvent. Starting from Ti₃AlC₂ MAX-phase powder, the Al layers were selectively etched away using LiF and concentrated hydrochloric acid; the resulting suspension was then purified by centrifugation to a pH of approximately 6. Following probe‑assisted ultrasonic exfoliation in an ice‑water bath, a stable colloidal ink with a concentration of 15 mg/mL was obtained, 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 well‑preserved surface functional groups. The hydrodynamic diameter is centered around 221 nm, meeting the requirements for aerosol printing.

(II) Mechanism and Key Innovations of Aerosol Jet Printing (AJP)

The core advantage of AJP technology lies in leveraging the real-time thickening effect of aerosol droplets and fluid‑dynamic focusing to achieve additive‑free 3D assembly of MXene: the ink is ultrasonically atomized into micron‑scale droplets, and during carrier‑gas transport, solvent evaporation causes the ink to thicken. Subsequently, the droplets are pneumatically focused by a sheath gas and ejected at high velocity onto a preheated substrate (100°C), where they solidify instantaneously to form a stable 3D structure. This process requires no additives or support 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 methods such as inkjet printing (80 μm) and electron-beam lithography (180 μm). It enables the fabrication of intricate patterns—such as school emblems, portraits, and spiral electrodes—on a variety of substrates, including alumina and flexible polyimide, with continuous, uniform lines and sharp edges, free from color bleeding and coffee-ring effects.

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

Through process optimization, high-fidelity 3D microstructures were 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 current state-of-the-art limit of less than 0.3. Additionally, complex architectures such as 3D microflowers, micro-pyramids, and vascular “pencil cacti” were realized, with a structural shrinkage of less than 1%, a micropillar verticality error of ±0.5°, and internally aligned, layered stacking of MXene nanosheets whose 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 process were assembled into MSCs (with an H₂SO₄/PVA hydrogel electrolyte), exhibiting electrochemical performance that significantly surpasses existing technologies: at a current density of 1.5 mA/cm², the device achieves an areal capacitance of 375 mF/cm² (equivalent to an electrode‑area 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 stands at 96.8%; the equivalent series resistance is only 14.3 Ω, and ion diffusion is highly efficient.

Image and Text Guide

Figure 1. Schematic illustration of the principles underlying the formation of three-dimensional architectures from two-dimensional MXene nanosheets. (a) A comparative chart of additive manufacturing (AM) routes for 3D microstructures, classifying techniques according to raw materials (metals, ceramics, polymers, and two-dimensional materials), initial phases (solid, liquid, dispersed phase), and binding mechanisms (strong primary forces versus weak secondary forces). The figure highlights the fundamental challenge of assembling two-dimensional materials into three-dimensional structures: the need to rely on relatively weak secondary interactions—such as van der Waals forces—for assembly, in contrast to the robust primary or secondary bonds that typically form in 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, thickening occurs in real time 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 produce a dense jet 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, rapidly losing their solvent 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, hydrogen bonding. These secondary interactions enable two-dimensional nanosheets to assemble into complex three-dimensional architectures with oriented stacking, without the need for auxiliary supports. (e) A comparison between 3D printing technologies and AJP in the context of additive-free fabrication of two-dimensional materials, demonstrating AJP’s capability to construct intricate microstructures in three dimensions at high resolution and scale. (f) A comparison of the characteristic dimensions of printed three-dimensional MXene nanosheet architectures with those achieved via two-photon polymerization, powder‑based methods, filament‑based approaches, and other techniques.

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 ultrasonic treatment induces exfoliation, yielding smaller‑sized 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‑ultrasonically treated Ti3C2Tx MXene, showing characteristic peaks corresponding to A1g (Ti, C, Tx) and A1g (C). (d) Dynamic light scattering (DLS) analysis of probe‑ultrasonically treated Ti3C2Tx MXene, providing an estimate of nanoplatelet size—approximately 221 nm—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 bearing a sketch of Andrew Carnegie. (h) Comparison of our technique’s resolution for two‑dimensional MXene patterning relative to other printing methods (inkjet, electronic inkjet, screen printing, extrusion, stamping, and pen writing).

Figure 3. Direct 3D printing of additive-free Ti3C2Tx MXene, featuring high aspect ratios 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. Different nozzle lengths (L) were used for printing, ranging from left to right: 30, 50, 100, and 200 cm. (b) Time evolution of aerosol droplet radii under varying droplet number densities, based on simulations discussed in Section S1. (c) Relationship between printed layer height and the number of printing passes. (d) CAD model of a 4×4 micropillar array (left) and an optical image of the corresponding 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, obtained 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 printed 3×3 micropillar array with a high aspect ratio of 60. (j) Various printing techniques—including inkjet, electron-beam jetting, screen printing, extrusion, and stamping—demonstrate AJ Printing’s capability to achieve high resolution and high aspect ratios in fabricating additive-free 2D nanomaterials, highlighting two key metrics attained thus far in the printing of additive-free 2D nanomaterials (MXene and graphene): “2D resolution” and “3D aspect ratio.” (k) Angle between printed overhanging features and the substrate, plotted as a function of the interlayer droplet-to-droplet offset. Corresponding optical images of inclined pillars are shown separately. Scale bar: 50 µm.

Figure 4. Complex 3D Ti3C2Tx MXene microarchitectures. (a) A CAD‑rendered model of a flower (top) and the corresponding colored 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 colored SEM image of the printed pyramid that matches the design (bottom). (c) A CAD model of a pencil cactus (left) and the colored SEM image of the printed pencil cactus that corresponds to the design (right). (d) A colored SEM image of the printed “forest,” demonstrating the controllability and reproducibility of our technique.

Figure 5. Structural characterization and performance evaluation of Ti3C2Tx MXene micro-supercapacitors with three-dimensional finger-like electrodes fabricated via AJ printing technology. (a) Schematic illustration of the fabrication process for the finger‑like micro-supercapacitors (MSCs). (b) Normalized cyclic voltammograms of MSC‑4 at various scan rates, expressed in terms of electrode capacitance. (c) Galvanostatic discharge curves of MSC‑4 at different current densities. (d) Electrode capacitance of MSCs with varying 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 prepared by AJ printing and those fabricated using other high‑resolution techniques—namely, imprinting, E‑jet printing, laser processing, plasma‑based methods, blade‑printing, inkjet printing, and screen printing—expressed as areal capacitance. (f) Ragone plots showing the energy density and power density of intercalated porous carbon nanomaterials obtained via AJ printing and other high‑resolution fabrication methods, including imprinting, E‑jet printing, laser‑plasma processing, blade‑printing, inkjet printing, and screen printing. (g) GCD curves of porous carbon nanomaterials at a current density of 20 mA·cm⁻², plotted over different numbers of charge–discharge cycles. (h) Cycling performance of the porous carbon nanomaterials prepared by AJ printing.

This study has overcome the bottleneck in 3D assembly of 2D materials, establishing a novel 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 far surpasses existing technologies, offering a viable solution for on‑chip energy storage in microelectronic devices. Meanwhile, the high resolution and multi‑material compatibility of AJP technology open new avenues for fabricating composite energy‑storage devices and multifunctional microsystems.

Future research directions include: developing multi-nozzle parallel printing systems to enhance efficiency; extending the approach to composite printing with MXene and other 2D materials; optimizing the process to fabricate under‑angle overhang structures; and exploring applications on flexible substrates to advance the development of wearable electronic devices. This work offers a new pathway for the engineering‑scale application of MXene and holds promise for driving technological innovation in areas such as micro‑energy storage and microrobotics.