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 Ti₂NT_x 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 Ti₂NT_x nitride MXene. Successful formation of nanolayered Ti₂NT_x MXene is characterized by XRD, SEM, and N₂ 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 Ti₂NT_x MXene exhibits pronounced reversible redox peaks and high areal capacitances (∼1350 μF cm^-2 in 1 M MgSO₄ aqueous electrolyte) well exceeding that expected from a double-layer charge storage (∼50 μF cm^-2) showing that charge is stored in the Ti₂NT_x 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 Ti₂NT_x 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 Ti₂NT_x MXene. These findings identify Ti₂NT_x MXene as a new pseudocapacitive material that possesses high capacitance and wide working voltage in a safe and environmentally friendly Mg-ion electrolyte.