Graphene faces the same problem, but its self-assembly into a 3D structures can effectively solve these problems. [5,6] The gelation of graphene oxide (GO) is the mostly used method to prepare a 3D assembled structure, and this process is triggered by the surface chemistry changes of GO NSs in solution. However, the gelation of the other 2D materials, such as transition metal dichalcogenides (TMDs), transition metal oxides (TMOs), and h-BN, [7][8][9][10] is difficult to achieve because of the fewer functional groups on their surface and their poor dispersion in an aqueous solution.Different from the above 2D materials, MXenes prepared by selective etching contain a number of functional groups on their surfaces and edges (F − , O, and OH) [11] show good dispersion in aqueous solution and have the potential to build 3D MXene-based assemblies by solution-based techniques. With the GO as the linker, 3D graphene/MXene hybrid monoliths have been assembled in solution with a low temperature heating. [12,13] In the assembly processes, the laminated MXene domains interacted with the π-conjugated GO sheets which then guided the formation of the 3D structure. The use of surfactants or polymers as the linkers to realize side-by-side joining of MXene NSs also has the potential to build 3D MXene-based monoliths. [14][15][16] However, all these strategies involve a complex process in which the MXenes are easily oxidized due to their high chemical activity. In addition, the above assembly process is different from the GO assembly that relies on self-gelation and has many advantages for achieving structure control by simply changing the solvent or the drying process. [17,18] Until now, the gelation of MXene in a similar manner to that of GO has been difficult to realize.Here, we report the fast gelation of MXene in an aqueous solution initiated by divalent metal ions. In a typical process, Fe 2+ ions are introduced in the MXene solution to destroy the electrostatic repulsive forces between the NSs and link them together, forming a stable 3D MXene hydrogel, similar to the GO hydrogel and its gelation process. [19] The use of metal ions as the joining sites enhances the interlinking of MXene NSs to form a 3D network due to their strong binding energy with the OH groups on the MXene surface. As a result, the formed hydrogel effectively suppresses the restacking of MXene NSs Gelation is an effective way to realize the self-assembly of nanomaterials into different macrostructures, and in a typical use, the gelation of graphene oxide (GO) produces various graphene-based carbon materials with different applications. However, the gelation of MXenes, another important type of 2D materials that have different surface chemistry from GO, is difficult to achieve. Here, the first gelation of MXenes in an aqueous dispersion that is initiated by divalent metal ions is reported, where the strong interaction between these ions and OH groups on the MXene surface plays a key role. Typically, Fe 2+ ions are introduced in the MXene dispersion whic...

Assembly of 2D MXene sheets into a 3D macroscopic architecture is highly desirable to overcome the severe restacking problem of 2D MXene sheets and develop MXene-based functional materials. However, unlike graphene, 3D MXene macroassembly directly from the individual 2D sheets is hard to achieve for the intrinsic property of MXene. Here a new gelation method is reported to prepare a 3D structured hydrogel from 2D MXene sheets that is assisted by graphene oxide and a suitable reductant. As a supercapacitor electrode, the hydrogel delivers a superb capacitance up to 370 F g −1 at 5 A g −1 , and more promisingly, demonstrates an exceptionally high rate performance with the capacitance of 165 F g −1 even at 1000 A g −1 . Moreover, using controllable drying processes, MXene hydrogels are transformed into different monoliths with structures ranging from a loosely organized porous aerogel to a dense solid. As a result, a 3D porous MXene aerogel shows excellent adsorption capacity to simultaneously remove various classes of organic liquids and heavy metal ions while the dense solid has excellent mechanical performance with a high Young's modulus and hardness.