Fabrication of porous Ti 3 C 2 T x MXene films by in situ foaming strategy for unparalleled high-rate energy storage.

MXene film electrodes for supercapacitors, assembled with MXene nanosheets as structure units, inevitably suffer from self-restacking, which severely restrains ion transport kinetics and results in insufficient active site utilization and poor rate capability. Herein, an in situ foaming strategy employing the accumulated gas generated during the etching of sacrificial templates, combined with the confinement between the MXene film, has been proposed to fabricate pores between stacked MXene sheets and along the vertical direction of the film. The surface chemistry of the foamed film was further modified by alkali washing treatment to replace hydrophobic -F groups with hydrophilic -OH groups to improve electrolyte penetration and introduce more active sites. The resultant porous MXene film electrode exhibits a greatly improved specific capacitance (354.1 F g -1 at 5 mV s -1 ) and unparalleled rate capability (with a capacitance retention of 96.1% at 2000 mV s -1 ). This work demonstrates a facile but efficient method for the construction of porous MXene films, simultaneously providing insights into the promising applications of MXenes.