Nitrogen-doped Ti3C2Tx MXene prepared by thermal decomposition of ammonium salts and its application in flexible quasi-solid-state supercapacitor

Two-dimensional (2D) Ti3C2Tx MXenes show great potential for application in flexible supercapacitors, due to their good hydrophilicity, metallic conductivity and excellent flexibility. Surface modification of the MXenes by heteroatom doping is a good strategy for adjusting the layer spacing and alleviateing various shortcomings. Herein, the ammonium salt decomposition method is shown to allow rapid nitrogen doping into Ti3C2Tx nanosheets at a low temperature of 350 °C. Compared to the raw Ti3C2Tx, the nitrogen-doped Ti3C2Tx (designated N-Ti3C2Tx) shows an increased layer spacing of 1.451 nm, nitrogen doping levels of 1.42 at% and a low number of residual fluorine functional groups. For the application as an electrode of supercapacitors, the N-Ti3C2Tx electrode shows an outstanding pseudocapacitance performance and mechanical flexibility, with a high specific capacitance of up to 449 F g−1 at 2 mV s−1, which is 1.4 times that of the raw Ti3C2Tx MXene (i.e., 321 F g−1). Furthermore, a quasi-solid-state symmetric supercapacitor assembled with a H2SO4-PVA gel electrolyte is shown to deliver an energy density of 9.57 Wh kg−1 at 250 W kg−1. The outstanding pseudocapacitance of the N-Ti3C2Tx is attributed to the positive effect of nitrogen doping on MXene, such as larger layer spacing and the increased number of surface active sites. The strategy presented herein also opens up a convenient and versatile approach for preparing high performance MXene Materials for energy conversion and storage.