Synthesis and Charge Storage Properties of Hierarchical Niobium Pentoxide/Carbon/Niobium Carbide (MXene) Hybrid Materials

Orthorhombic niobium pentoxide (T-Nb₂O₅) offers high capacitance and fast charging-discharging rate capabilities when used as an electrode material for Li-ion capacitors. A homogeneous distribution of T-Nb₂O₅ nanoparticles in a highly conductive matrix represents a promising approach to maximize its energy and power densities. Here we report a one-step CO₂ oxidation of two-dimensional (2D) Nb₂CT_x, a member of the MXenes family of 2D transition metal carbides, which leads to a hierarchical hybrid material with T-Nb₂O₅ nanoparticles uniformly supported on the surface of Nb₂CT_x sheets with disordered carbon. The oxidation temperature, duration, and CO₂ flow rate determine the T-Nb₂O₅ crystallite size as well as the structure, composition, and the charge storage properties of the hybrid material. Fifty micrometer thick electrodes of the hybrid material exhibit high capacitance (330 C g^-1 and 660 mF cm^-2 at a charge-discharge time of 4 min) and good cycling performance in a nonaqueous lithium electrolyte. The charge storage kinetics are dominated by a surface-controlled process. The observed electrochemical performance is attributed to the intrinsic pseudocapacitive response and excellent energy storage capability of T-Nb₂O₅ coupled with the fast charge transfer pathways provided by the conductive 2D Nb₂CT_x sheets and the as-formed disordered carbon.