A safe etching route to synthesize highly crystalline Nb2CTx MXene for high performance asymmetric supercapacitor applications

We present a novel strategy to prepare Nb2CTx by etching Nb2AlC powders with a mixed solution of lithium fluoride and hydrochloric acid. The Nb2CTx exhibits a layered structure with excellent crystalline degree and structural order. Meanwhile, the electrochemical performance of Nb2CTx in aqueous electrolyte was examined for the first time. The Nb2CTx shows good electrochemical capacitance performance, and the addition of carbon nanotubes (CNT) as a conductive agent can significantly improve the rate performance of the electrode. This may provide an idea to improve the performance for the M2C-type MXenes electrodes with lower conductivity than Ti3C2Tx. The asymmetric supercapacitor was assembled by using Nb2CTx/CNT as negative electrode and activated carbon as positive electrode, which can deliver a high energy density of 154.1 μWh/cm2 and a maximum power density of 74843.1 μW/cm2 under the high mass loading. The power density is higher than most of the reported MXene-based supercapacitors.