Porous MXenes: Synthesis, structures, and applications

[Display omitted] •A new classification method of Porous MXenes based on their formation routes is proposed.•Synthetic strategies for porous MXenes with different structures are summarized and the formation mechanisms are discussed.•Promising applications of porous MXenes in energy storage, EMI shielding, piezoresistive sensors, and cancer therapy are outlined.•The roles of pores in related applications are represented and the detailed composition-structure-property relationships are explored.•Perspectives on the future opportunities and challenges of porous MXene-based materials are provided. Due to highly tunable metallic compositions and surface functional groups, MXenes have attracted significant interests for a wide range of applications, such as energy storage, electromagnetic interference shielding, sensors, and biomedicine. With the introduction of porous structures, which have unique advantages in tuning the conductivity and dielectric constant, adjusting the ion/guest and even electromagnetic wave transport, and also directing the loading and distribution of other functional materials, porous MXenes hold great potential in profoundly enhancing their properties. We have surveyed rapidly increasing efforts in the design and synthesis of porous MXenes with advantageous structures for diverse applications, especially in the last three years. Here we classify Porous MXenes into four categories according to their formation routes, including (i) assembly by MXenes, (ii) depositing or inserting MXenes into porous substrates, (iii) loading or coating functional porous materials on the surface of MXenes, and (iv) creating in-plane pores within MXenes. Then we summarize the primary synthetic methods for each kind of porous MXenes and discuss their applications for pseudo-capacitors, lithium/sodium batteries, lithium-sulfur batteries, electromagnetic interference shielding and adsorption, piezoresistive sensors, and cancer therapy. A particular emphasis is on the formation mechanisms of different porous structures and the detailed composition-structure-property relationships in related applications. We lastly conclude with a brief perspective on future opportunities and challenges.