Polymer–Two-Dimensional Filler Composites for Hydrogen Barrier Materials: Transport Theory, Design Strategies, and Experimental Advances

Hydrogen energy is widely regarded as one of the most promising pathways toward sustainable, low-carbon energy systems. However, hydrogen permeation remains a critical challenges for pipelines, storage vessels, and other hydrogen-related equipment. These issues not only reduce structural reliability but also raise significant safety concerns during long-term service. To mitigate hydrogen permeation, various hydrogen barrier materials, including metals, ceramics, and polymers, have been extensively investigated. Recently, incorporating two-dimensional (2D) materials used as fillers into polymer matrices has emerged as an effective strategy for enhancing hydrogen barrier performance by increasing diffusion tortuosity and suppressing hydrogen transport pathways. As a result, layered silicates (e.g., montmorillonite, MMT), graphene-based materials, MXenes (e.g., Ti3C2Tx), molybdenum disulfide (MoS2), hexagonal boron nitride (h-BN), and layered double hydroxides (LDHs) have attracted increasing attention for advanced hydrogen barrier applications. Despite rapid progress in polymer/2D filler composite barrier materials, which can be implemented as surface coatings, bulk liners, or free-standing films, enhancement of barrier properties through the optimization of nanosheet alignment, interfacial stability, oxidation resistance as well as scalable fabrication and long-term durability remain major challenges. Moreover, a mechanistic understanding of hydrogen transport and the structure-property relationships that determine barrier performance remains also insufficient, limiting their practical employment in hydrogen-energy systems. Therefore, this review systematically summarizes recent advances in polymer/2D filler composite hydrogen barrier materials, with a particular focus on theoretical aspects related to gas permeability in 2D nanocomposites, hydrogen transport mechanisms, structure-property relationships, fabrication strategies, durability, and engineering applications. Finally, current challenges and future perspectives for next-generation multifunctional hydrogen barrier materials are discussed, aiming to provide guidance for the rational design and industrial implementation of advanced hydrogen barrier systems.