Multifunctional PVA/MXQDs hydrogels for integrated flexible strain sensing and solid-state energy storage systems

The development of multifunctional conductive hydrogels that integrate high electrochemical performance, mechanical robustness, and sensing capability is essential for next-generation wearable electronics. In this work, we report a novel conductive hydrogel based on MXene quantum dots (MXQDs), fabricated by crosslinking a poly(vinyl alcohol) (PVA) network with polyethyleneimine (PEI)-modified MXQDs through nitrogen-rich functional groups using a simple and scalable freeze-thaw method. Owing to its high ionic conductivity and mechanical flexibility, the hydrogel serves as an effective solid-state electrolyte in a symmetric supercapacitor assembled with polypyrrole-wrapped MXene electrodes prepared via interfacial polymerization. The resulting device exhibits a high specific capacitance of 165 F g^-1 and a notable energy density of 44.8 Wh kg^-1, together with good cycling stability, retaining 70% of its initial capacitance after 5000 charge-discharge cycles. Furthermore, the incorporation of copper nanowires significantly improves the electrical conductivity and stretchability of the hydrogel, yielding an elongation at break of up to 244%. When applied as a strain sensor, the hydrogel demonstrates excellent linearity, a wide operational strain range exceeding 60%, high durability over 300 loading-unloading cycles, and high strain sensitivity with a gauge factor of 3.50. In addition, the hydrogel-based sensor enables accurate monitoring of various human motions, including arm bending, finger motion, and throat movement. Collectively, these results highlight the strong potential of the proposed multifunctional hydrogel as an integrated platform for flexible energy storage and wearable sensing applications.