Title: Adsorption of Gases on Ti3C2Tx MXene: Implications from X-ray Photoelectron Spectroscopy

Abstract: One of the most explored MXenes is the Ti3C2Tx, where Tx is designated to inherently formed termination species. Among many applications, Ti3C2Tx is an excellent material for energy storage, energy converting, and CO2-capturing devices. However, active sites for adsorption and surface reactions on the Ti3C2Tx-surface are still open questions to explore, which have implications for preparation methods when to obtain correct and optimized surface requirements. Here we use X-ray photoelectron spectroscopy to study adsorption of common gas molecules such as H2, CO2, and H2O, which all might be present in energy storage, energy converting, and CO2-capturing devices based on 2D flakes of Ti3C2Tx. The study shows that H2O, with a strong bonding to the Ti-Ti bridge-sites, can be considered as a termination species. An H2O terminated Ti3C2Tx-surface restricts the CO2 adsorption to the Ti on-top sites and reduces the ability to store positive ions, such as Li+ and Na+. On the other hand, an H2O terminated Ti3C2Tx-surface shows the capability to split water. The study further shows that H2 has the ability to remove F at moderate temperatures. The results from this study have implications for correct selection of MXene preparations and the environment around the MXene in different implementations.