Achieving Environmental Stability of MXenes in Air, Water and Electrolytes

MXenes display extraordinary electrical, optical, chemical, and electrochemical properties. There is a perception though that MXenes are unstable and degrade quickly in ambient environment, limiting potential applications and requiring specific storage conditions to last for a long time. However, significant developments in MXenes’ synthesis, processing, and understanding of its chemistry led to dramatic increases in their environmental stability. Herein, we analyze delaminated Ti₃C₂T<i>_x</i> MXene flakes in solution and on a substrate, electrodes in acidic electrolyte as well as free-standing films aged up to a decade. Structural, chemical and morphological characterization along with electronic conductivity measurements reveal the effect, or lack thereof, of prolonged storage under ambient conditions. Up to 90% conductivity was retained after 5 years of storage by the films dried after synthesis. Further, we show that decrease in electronic conductivity over time is largely caused by uptake of water by the hydrophilic surfaces of MXenes, and its effect can be, at least partially, reversed by vacuum drying at elevated temperature. MXene supercapacitor electrodes performed well over 500,000 cycles at 20 mV/s. The effects of Ti₃C₂T<i>_x</i> stoichiometry, surface chemistry, defects, intercalants, interlayer spacing and storage condition on oxidation and hydrolysis are discussed. While no systematic data is available for other MXenes, we demonstrate that the same principles are applicable to V₂C, Nb₄C₃ and other 2D carbides.