Probing the <i>In Situ</i> Pseudocapacitive Charge Storage in Ti₃C₂ MXene Thin Films with X-ray Reflectivity

MXenes are a large family of two-dimensional materials that are attractive for energy storage due to their high-rate charging capabilities as well as for electrochemical actuators, water purification, and many other technologies. Ion intercalation during electrochemically driven charge and discharge processes is the fundamental process associated with MXene functionality, which we have characterized using <i>in situ</i> and <i>operando</i> X-ray reflectivity (XRR). Experiments performed at the Advanced Photon Source at Argonne National Laboratory monitored the changes in the structure of a Ti₃C₂ MXene film on a platinum current collector as a function of static applied potential between 0.3 and -0.7 V vs Ag/AgCl in an aqueous 0.1 M Li₂SO₄ electrolyte. Negative potential sweeps lead to a contraction of 1.2 Å in the interlayer spacing and a loss of electron density between the layers, likely due to Li⁺ ion insertion and water removal. The change in lattice spacing includes a continuous variation vs potential as well as an additional discrete contraction that occurs near -0.35 V that has the characteristics of a first-order transition. The continuous change in the MXene interlayer spacing is associated with the capacitive charge, while the discrete change in structure correlated to the weak feature in the cyclic voltammogram at -0.35 V can be interpreted as either a pseudocapacitive charging process or a potential-dependent change in capacity.