Pseudocapacitive Storage in Nanolayered Ti₂NT_xmxene Using Mg-Ion Electrolyte

Electrochemical supercapacitors are hybrids of a capacitor and battery that rely on materials capable of storing charges via faradaic redox reactions or 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 viapseudocapacitive mechanisms. Nitride MXenes such as Ti2NTxare 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 Ti2NTxnitride MXene. Successful formation of nanolayered Ti2NTxMXene is characterized by XRD, SEM, and N2physisorption analyses. The identity of the surface terminating groups Txare assigned to primarily O and/or OH based on Raman, FTIR, and STEM-EELS. When tested in various electrolytes, the nanolayered Ti2NTxMXene exhibits pronounced reversible redox peaks and high areal capacitances (~1350 uF cm–2 in 1 M MgSO4aqueous electrolyte) well exceeding that expected from a double-layer charge storage (~50 uF cm–2) indicating that charge is stored in the material viaa pseudocapacitive mechanism. We report a trend in the capacitance as a function of cation as follows: Mg2+> Al3+> H+> Li+> Na+> K+, that matches theoretical predictions. Remarkably, nanolayered Ti2NTxMXene 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 owing to the two-electron reaction and the unique multilayer adsorption behavior of the Mg2+cation on the Ti2NTxMXene. These findings identify Ti2NTxMXene as a new pseudocapacitive material that possesses high capacitance and wide working voltage in a safe and environmentally friendly Mg-ion electrolyte.