Modulating oxygen vacancies in MXene/MoO_3-x smart fiber by defect engineering for ultrahigh volumetric energy density supercapacitors and wearable SERS sensors

MXene fibers are expected to accelerate the sustainable development of intelligent era as multi-functional smart fibers. However, MXene fibers have serious obstacles when used for multi-functional applications. Here, we fabricated a multi-functional MXene fiber by incorporating MoO_3-x nanobelts with abundant oxygen vacancies via wet spinning and defect engineering methods. Benefiting from the presence of abundant oxygen vacancies, the produced MXene/MoO_3-x fibers not only provide more active sites to interact with electrolyte ions, but also substantially augment the rate capacities of fibers due to the enlarged distances between MXene flakes. Thus, the assembled fiber-based supercapacitor (SCs) realized high capacitance of 869.1F cm⁻³ at 0.5 A cm⁻³, good volumetric energy density of 77.3 mWh cm⁻³ at a power density of 204.8 mW cm⁻³ and long-term cycling stability along with 86.4 % capacity retention after 5000 cycles. Moreover, due to the narrowed bandgap of MoO_3-x produced by defect engineering enhanced the charge transfer between the fiber and molecules, MXene/MoO_3-x fiber also demonstrated a low limit of detection and reliable biomarkers detection in artificial sweat. These findings provide the potential application of MXene-based fibers in multi-functional devices.