Pseudocapacitive Storage in Nanolayered Ti2NTx MXene Using Mg-Ion Electrolyte

Electrochemical supercapacitors are hybrids of a capacitor and battery that rely on materials capable of storing charges via pseudocapacitive reactions in addition to conventional electrostatic double-layer charge storage. MXenes, a relatively new class of two-dimensional (2D) transition metal carbides and nitrides, are ideal candidates for supercapacitors due to their high electronic conductivity, high surface area, and ability to store charges via pseudocapacitive mechanisms. Nitride MXenes such as Ti2NTx are predicted to have higher pseudocapacitance than carbide MXenes but have not been explored experimentally. Here, we report on the synthesis, characterization, and pseudocapacitive charge storage mechanism in the Ti2NTx nitride MXene. Successful formation of nanolayered Ti2NTx MXene is characterized by XRD, SEM, and N-2 physisorption analyses. The identity of the surface terminating groups T-x are assigned to primarily O and/or OH based on Raman, FTIR, and STEM-EELS. When tested in various electrolytes, the nanolayered Ti2NTx MXene exhibits pronounced reversible redox peaks and high areal capacitances (similar to 1350 mu F cm(-2) in 1 M MgSO4 aqueous electrolyte) well exceeding that expected from a double-layer charge storage (similar to 50 mu F cm(-2)) showing that charge is stored in the Ti2NTx MXene via a pseudocapacitive mechanism. We report a trend in the capacitance as a function of the cation as follows: Mg2+ > Al3+ > H+ > Li+ > Na+ > K+, which matches theoretical predictions. Remarkably, nanolayered Ti2NTx MXene exhibits >200 F g(-1) capacitance over a 1.0 V range in the Mg-ion electrolyte, and the capacitance increases to 160% of its initial value after 1000 cycles because of the 2 e-process and the distinctive multilayer adsorption characteristic of the Mg2+ cation on the Ti2NTx MXene. These findings identify Ti2NTx MXene as a new pseudocapacitive material that possesses high capacitance and wide working voltage in a safe and environmentally friendly Mg-ion electrolyte.