Co-Catalyzed In Situ Growth of CNT-Bridged TiO/TiN on MXene-Derived Frameworks for High-Performance Flexible Symmetric Supercapacitors

Abstract Constructing conductive heterostructures through transition-metal-assisted growth is an effective strategy for improving MXene-derived electrode materials. Herein, Co ions were introduced into Ti3C2Tx (MXene)-derived precursors, and Co/TiO/TiN/CNTs (TCo) was obtained through a two-step annealing process. The Co ion catalyzed the in situ growth of carbon nanotubes (CNTs) on the MXene-derived TiO/TiN-containing framework, and the resulting TCo was further hybridized with MXene to construct a Co/TiO/TiN/CNTs/MXene (MTCo) heterostructure electrode material. Microstructure image (TEM image) results revealed Co nanoparticles located at the CNT surface, indicating their catalytic role in CNT growth. This process generated a heterogenous composite structure with a TiO/TiN-containing framework serving as the core and CNTs as the coating. By further optimizing the amount of Co(NO3)2 added and the content of TCo, the microstructure and electrochemical performance of the composite electrode were controlled. The optimized sample, denoted as MTCo-40, which delivered the Co(NO3)2 loading, was 500 milligrams, and the TCo content was 40%. It delivered a high areal capacitance of 1993.87 mF cm−2 at 2 mA cm−2. The electrode retained 89.1% of its initial capacitance after 2000 charge−discharge cycles. The flexible symmetric supercapacitor assembled from MTCo-40 also exhibited excellent electrochemical performance. The device delivered an areal capacitance of 1498.24 mF cm−2, an energy density of 351.67 μWh cm−2, and 81.86% initial capacitance retained after 2,000 charge−discharge cycles. The CV curves of the device showed no significant changes after being bent at different angles. This indicates that the device has good flexibility. At the same time, DFT simulations helped support the charge distribution and electronic conduction in this heterostructure. This study utilizes transition metal ion doping to catalyze the growth of CNTs for the preparation of MXene derivatives; it can provide an effective strategy for researchers to further boost the MXene-based FSCs, as it can contribute to the development of high-performance devices.