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) Nb2CTx, 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 Nb2CTx 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 Nb2CTx sheets and the as-formed disordered carbon.