Flexible Nb 4 C 3 T x Film with Large Interlayer Spacing for High‐Performance Supercapacitors

MXenes derived from 413 MAX phases are rarely studied but they have the potential to have superior electrical and mechanical properties thanks to a thicker monolayer (four layers of transition metal and three layers of carbon or nitrogen). In this paper, Nb 4 C 3 T x MXene nanosheets are delaminated and freestanding film with 1.77 nm interlayer spacing is obtained, which is larger than that of most previous MXenes. When Nb 4 C 3 T x freestanding films are tested as supercapacitors electrodes, Nb 4 C 3 T x shows high volumetric capacitance, 1075, 687, and 506 F cm −3 in 1 m H 2 SO 4 , 1 m KOH, and 1 m MgSO 4 , respectively, at the scan rate of 5 mV s −1 . An in situ X‐ray diffraction technique is used to study the structural changes during the electrochemical charging in 1 m H 2 SO 4 and 1 m MgSO 4 . There is almost no change in the 21 Å interlayer spacing during the cycling, because the space between the MXene layers is sufficient to accommodate the insertion and deinsertion of cations. This can lead to stable performance of Nb 4 C 3 T x MXene energy storage devices.