Probing the In Situ 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 in situ and operando 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 angstrom 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.

Automated summary

MXenes, a family of two-dimensional materials, are attractive for energy storage and other technologies due to their high-rate charging capabilities. In situ and operando X-ray reflectivity were used to characterize the ion intercalation process and structural changes in MXene films. Negative potential sweeps resulted in contraction of interlayer spacing and loss of electron density, likely due to Li+ ion insertion and water removal, with a continuous and discrete changes associated with capacitive charge and pseudocapacitive charging process, respectively.