Enhancement of Ti₃C₂ MXene Pseudocapacitance after Urea Intercalation Studied by Soft X-ray Absorption Spectroscopy

MXenes have shown outstanding properties due to their highly active hydrophilic surfaces coupled with high metallic conductivity. Many applications rely on the intercalation between Ti3C2Tx (Tx describes the -OH, -F and -O- surface terminations) flakes by ions or molecules, which in turn might alter the Ti3C2Tx surface chemistry and electrochemical properties. In this work, we show that the capacitance, rate capability, and charge carrier kinetics in Ti3C2Tx MXene electrodes are remarkably enhanced after urea intercalation (u-Ti3C2Tx). In particular, the areal capacitance increased to 1100 mF/cm2, which is 56% higher than that of pristine Ti3C2Tx electrodes. We attribute this dramatic improvement to changes in the Ti3C2Tx surface chemistry upon urea intercalation. The oxidation state and the oxygen bonding of individual Ti3C2Tx flakes before and after urea intercalation are probed by soft X-ray absorption spectroscopy (XAS) at the Ti L- and O K-edges with 30 nm spatial resolution in vacuum. After urea intercalation, a higher Ti oxidation state is observed across the entire flake compared to pristine Ti3C2Tx. Additionally, in situ XAS of u-Ti3C2Tx aqueous dispersions reveal a higher Ti oxidation similar to dry samples, while for pristine Ti3C2Tx the Ti atoms are significantly reduced in water compared to dry samples.