Chloride-Free Phosphoric-Acid Processing of Ti ₃ C ₂ MXene for Humidity Stability and High Conductivity: Mechanistic Insights from ¹ H/ ⁷ Li HETCOR NMR and DFT-GIPAW

MXenes are layered conductive materials whose outstanding electronic properties are often compromised under humid conditions. Conventional synthesis typically employs hydrochloric acid (HCl), which causes environmental concerns. This study aims to develop a sustainable, high-performance MXene with improved humidity stability, environmental stability, and long operational lifetime. In this study, we succeeded in synthesizing delaminated phosphoric acid-processed MXene (P-MXene) without using HCl, a conventional etching aid. P-MXene obtained in our research simultaneously shows high electronic conductivity up to 15,000 S/cm and high humidity stability, with a conductivity retention rate of more than 90% at room temperature and 100% relative humidity for 2 weeks. The XRD and XPS measurements revealed that interlayer distances and surface functional groups strongly affect the humidity stability. High fluorine occupancy and short interlayer distance contribute to the inhibition of water adsorption and stabilize the nonintercalated structures. The combination of the solid-state 1 H and 7 Li, heteronuclear correlation NMR measurements, TOF-SIMS analysis, and DFT, the gauge-including projector augmented wave method calculations revealed the relationship between the environments of interlayer lithium and humidity, which is attributed to the interaction of the surface functional groups and intercalators. The SEM and film density measurements explain that low porosity and high density are the key factors for high electronic conductivity because they realize successive electronic pathways. Our results provide guidance for the design of MXenes with high environmental stability, long-term life, and high electronic performance and expand their applicability to real-world applications.