Revealing the Pseudo-Intercalation Charge Storage Mechanism of MXenes in Acidic Electrolyte

Since the discovery of Ti3C2Tx in 2011, the family of two-dimensional transition metal carbides, carbonitrides, and nitrides (collectively known as MXenes) has quickly attracted the attention of those developing energy storage applications such as electrodes for supercapacitors with acidic aqueous electrolytes. The excellent performance of these MXenes is attributed to a pseudocapacitive energy storage mechanism, based on the non-rectangular shape of cyclic voltammetry curves and changes in the titanium oxidation state detected by in situ X-ray absorption spectroscopy. However, the pseudocapacitive mechanism is not well understood and no dimensional changes due to proton insertion have been reported. In this work, in situ X-ray diffraction and density functional theory are used to investigate the charge storage mechanism of Ti3C2Tx in 1 m H2SO4. Results reveal that a 0.5 angstrom expansion and shrinkage of the c-lattice parameter of Ti3C2Tx occur during cycling in a 0.9 V voltage window, showing that the charge storage mechanism is intercalation pseudocapacitance with implication for MXene use in energy storage and electrochemical actuators.