Controlling Structure and Stability in Ultrathin Ti 3 C 2 T x MXene Multilayers for Hydrogen Gas Barrier Applications

ABSTRACT Ti 3 C 2 T x MXene is a promising 2D nanofiller for gas barrier composites. However, controlling the composite's nanoscale structure and ensuring its stability in harsh thermal environments remain significant challenges. This study compares the multilayer properties, morphology, and hydrogen (H 2 ) gas barrier performance of two different MXene‐based layer‐by‐layer (LbL) assemblies, demonstrating the influence of polymer content and complementary positively charged assembling species. LbL assembly effectively promotes brick‐and‐mortar‐like multilayer ordering of MXene in nanoscale thin films, but the method requires positively charged species to electrostatically assemble with negatively charged MXene. Using either reduced graphene oxide (rGO) precomplexed with a polycation (rGO‐PDDA) or positively charged MXene (f‐MXene) functionalized with [3‐(2‐aminoethylamino)‐propyl] trimethoxysilane (AEAPTMS) as the complementary species, the two MXene‐based multilayers show strikingly different behaviors at elevated temperatures. Structural and chemical characterization shows that only the rGO‐PDDA/MXene multilayer maintains electrostatic bonding, leading to improved stability and performance in harsh conditions. In films as thin as 43 nm, the LbL multilayers retain their performance. The rGO‐PDDA/MXene multilayer exhibits low H 2 gas permeability (7.13 ± 0.09 × 10 −11 m 3 ·m·m −2 ·atm −1 ·day −1 ) even at 400°C. This study offers insightful guidelines for the rational design of MXene‐ or polymer‐based composite films for gas barrier applications in extreme temperatures.