Direct-ink-write 3D printing of flexible all-solid-state micro-supercapacitor based on MXene-hydroxylated nanocellulose-carbon nanotubes

Micro-supercapacitors (MSCs) face significant limitations due to low energy density despite their high power density and long cycle life. In this study, single-layer Ti₃C₂T_x nanosheets were employed to fabricate a MXene-hydroxylated nanocellulose-carbon nanotube (MHC) composite ink, which was used to fabricate high-energy flexible MSCs via direct ink writing three-dimensional (3D) printing technology. The introduction of the rheological modifier hydroxylated nanocellulose (HNC) not only constructs interlayer spacers to inhibit nanosheet restacking but also optimizes the rheological properties and 3D printability of the composite ink. Meanwhile, the synergistic effect of carbon nanotubes (CNTs) as conductive agents enhances interlayer electron transport and electrochemical performance. Benefiting from the rational design of the ink and printing process, the fabricated MSCs exhibit high-precision structures (electrode width of 250 μm and electrode area of 0.2625 cm²) and outstanding energy storage properties, achieving an areal capacitance of 543 mF·cm⁻², an energy density of 27.15 μWh·cm⁻², and a power density of 6 mW·cm⁻², significantly surpassing previously reported MXene-based MSCs. Moreover, the flexible all-solid-state MSCs demonstrate excellent performance stability under mechanical bending, series/parallel module integration, and long-term cycling tests, providing a customizable energy storage solution for flexible wearable microelectronic systems.