Tiny Pores, Big Stories: Perforating MXenes for Through-Sheet Transport in Energy and Environmental Applications

Conspectus Transport remains a central bottleneck limiting the practical performance of MXene electrodes and membranes. Although MXenes possess metallic conductivity, hydrophilicity, and rich surface chemistry, their two-dimensional (2D) layered architecture imposes barriers to ion and molecular diffusion. During assembly into electrodes and membranes, colloidal MXene nanosheets readily restack and force transport mainly through thin interlayer galleries. The as-formed slit channels lengthen diffusion pathways leading to sluggish kinetics. These limitations become increasingly important in compact, high-loading MXene structures, where long and tortuous transport pathways can further reduce active-site utilization and rate capability. This Account argues that in-plane pore engineering transforms MXenes from predominantly lamellar-type materials into through-sheet transport platforms. Introducing nanopores into the MXene basal planes creates transverse pathways that directly connect neighboring interlayer galleries, thereby shortening diffusion distances and increasing edge-site and basal-plane accessibility. Holey MXenes therefore represent more than simply porous MXenes; they are transport-engineered 2D materials in which nanoscale perforation provides an additional direction for the movement of ions and molecules while retaining the intrinsic advantages of the MXene framework. This concept is particularly relevant to densely assembled electrodes and membranes, where in-plane pathways enable rapid mass transport. We summarize our efforts toward the synthesis and structural engineering of holey MXenes, including oxidative perforation of MXene hydrogels, microwave-assisted ultrafast etching, controlled H2O2-mediated perforation, and radical-intensified selective etching (RISE) for the direct conversion of MAX phases into holey MXenes. Across these approaches, we identify a common oxidation-driven pathway involving localized oxidation, pore nucleation, and pore propagation. Importantly, controlled oxidation can introduce nanoholes while largely preserving the conductive MXene framework, whereas excessive oxidation leads to pore enlargement, conductivity loss, and structural degradation. We further demonstrate how through-sheet transport translates into improved performance in supercapacitors, batteries, ion capacitors, capacitive deionization, membrane separation, and water purification. In electrochemical systems, nanoholes facilitate ion access to otherwise less accessible regions of the nanosheets while increasing the density of exposed edge sites. In membranes, they provide transverse pathways that can shorten molecular transport distances across compact layered structures. These examples establish in-plane perforation as a versatile approach for coupling enhanced transport with MXene-specific electronic and surface-chemical functionality. At the same time, practical implementation requires precise and reproducible control over pore size, density, geometry, and edge chemistry, together with improved chemical stability, shelf life, and scalable processing. In-plane perforation also exposes additional reactive sites that can accelerate oxidation, creating an inherent trade-off between transport enhancement and structural stability. Establishing quantitative structure–transport–property relationships will therefore be essential for rational optimization. Overall, we anticipate that through-sheet transport will serve as a useful design principle for layered 2D materials, analogous to how hierarchical porosity transformed porous carbons, and accelerate their transformative role in sustainable energy and environmental technologies.