Janus mask prints hair-width MXene electrodes for high-energy micro-supercapacitors

Miniaturized electronics need power sources that can shrink with them. Micro-supercapacitors are attractive for wearable, flexible and on-chip systems because they can deliver power rapidly and survive repeated charging. Yet miniaturizing the interdigitated electrodes is not straightforward: finer features are harder to print at high throughput, and transferring delicate nanosheet patterns onto useful substrates can compromise their integrity.

In the International Journal of Extreme Manufacturing, Prof. Yang Li at Zhejiang University and his co-workers developed a dual-sided stencil printing technique that packs high-capacity energy storage into electrode tracks just 80 μm wide by giving a tungsten stencil two distinct physical personalities—a dual-sided "Janus mask" named after the two-faced Roman god.

MXenes are two-dimensional transition-metal carbides and nitrides that combine high electrical conductivity with electrochemically active surfaces, making them promising electrode materials for compact energy storage. Mask-defined vacuum filtration can assemble MXene nanosheets densely with little or no additive, but its resolution has been limited by fluid transport through narrow mask channels. As the channels become smaller, flow resistance rises and the ink can no longer fill them uniformly, producing discontinuities and missing features. Making the entire mask highly wettable improves ink entry but creates a second problem: the ink spreads sideways along the mask–membrane interface and blurs the pattern edges.

The team addressed this trade-off by giving the tungsten mask two different wetting behaviours. Selective oxygen-plasma treatment makes the top surface and internal microchannels strongly hydrophilic, reducing the water contact angle from about 94° to 15.1° and helping the aqueous MXene ink move through the narrow channels. The untreated underside remains hydrophobic, suppressing lateral spreading at the mask–membrane interface. In this way, the same mask simultaneously promotes through-channel transport and preserves sharp pattern boundaries.

With this Janus mask, the researchers printed Ti₃C₂T_x MXene interdigitated electrodes with finger widths down to 80 μm. At a scan rate of 10 mV·s^-1, reducing the finger width increased the areal capacitance from 2.53 to 4.57 mF·cm^-2 and the volumetric capacitance from 64.95 to 169.23 F·cm^-3. Using a LiTFSI-based polymer electrolyte with a 1.8 V operating window, the 80-μm device reached a volumetric energy density of 76.72 mWh·cm^-3, exceeding most previously reported MXene-based micro-supercapacitors.

The printed electrodes can also be moved from the filtration membrane to practical substrates by a capillary-assisted, stamp-free transfer process. A controlled rolling pressure of about 100 kPa brings the semi-wet pattern into conformal contact with the target surface before the membrane is peeled away. After transfer, the electrode surface roughness decreased from 156.8 nm to 7.8 nm, improving interfacial contact. The same printing concept was also demonstrated with Ti₂C MXene, MoS₂, graphene and carbon nanotubes, showing that the asymmetric-wettability strategy is not limited to one material system.

To demonstrate system-level use, four 80-μm-finger-width micro-supercapacitors were connected in a 2-series/2-parallel configuration on a flexible substrate to power a force sensor. The integrated device produced repeatable signals for more than 1,000 sensing cycles under a 10 N load and clearly tracked finger-joint bending. Separately, the flexible micro-supercapacitor retained approximately 100% of its capacitance after 500 bending cycles, supporting its use in mechanically compliant electronics.

Long-term stability remains an area for further improvement. After 4,000 charge-discharge cycles, the micro-supercapacitor retained 63.8% of its initial capacitance, and the integrated force sensor showed a gradual baseline drift during prolonged operation. These results identify cycling stability and long-term signal stability as important targets for future device and electrolyte optimization.

By using asymmetric wettability to separate ink transport from lateral spreading, the Janus mask pushes mask-defined filtration into the sub-100-μm regime without giving up the dense nanosheet assembly that benefits electrochemical performance. The strategy offers a route toward compact, transferable energy-storage components and may be extended to other solution-processed nanomaterials for integrated sensing and wearable electronics.

International Journal of Extreme Manufacturing (IJEM, IF: 25.1) is devoted to publishing articles of the highest quality and significance to pushing the limits of scales, precision, performance and environments in manufacturing.

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