Multi-year Study of Environmental Stability of Ti$_3$C$_2$T$_x$ MXene Films

Abstract: MXenes are a family of two-dimensional (2D) carbides and nitrides that display extraordinary electrical, optical, chemical, and electrochemical properties. There is a perception that MXenes are unstable and degrade quickly, limiting potential applications and requiring specific storage conditions to last for a long time. This was true for delaminated MXenes flakes in dilute dispersions prepared from defective precursors when MXene research was in its infancy. Since then, significant developments in MXene synthesis, processing, and understanding of its chemistry led to dramatic increases in environmental stability. Herein, we analyze Ti$_3$C$_2$T$_x$ free-standing films aged from 4 to 9 years through structural and morphological characterization along with electrical conductivity measurements to reveal the effect, or lack thereof, of prolonged storage under ambient conditions. Further, we show that the decrease in electronic conductivity over time is largely caused by the uptake of water by the hydrophilic surface chemistry of MXenes, which can be easily removed and its effect reversed by vacuum annealing.