K+ Intercalation of NH4HF2-Exfoliated Ti3C2 MXene as Binder-Free Electrodes with High Electrochemical Capacitance

As a 2D material, Ti 3 C 2 (MXene) has been recently used as electrodes for electrochemical capacitors. Herein, a two-step procedure is used to obtain a highly opened layer structure for Ti 3 C 2 , which differs from the methods used in the literature for synthesizing exfoliated MXene. First, the mild NH 4 HF 2 etching agent is used to prepare the regular Ti 3 C 2 MXene. Second, KOH is added into the obtained solution for K + to intercalate the MXene. The K + intercalation is found to greatly increase the (002) crystal planar spacing of the Ti 3 C 2 from 10.8 to 12.4 Å. More importantly, this K + -intercalated MXene exhibits a highly opened layer structure, whereas the regular MXene only has a partially opened layer structure. Consequently, the former is expected to allow much more active sites exposed to liquid electrolyte, and also facilitates remarkably higher ion and electron transport efficiency. To investigate their electrochemical capacitance properties, the regular and K + -intercalated Ti 3 C 2 MXenes are used to fabricate binder-free electrodes on nickel foams by an electrophoretic deposition method. The K + -intercalated MXene-based electrode is found to have twice higher specific capacitance than the regular MXene. Furthermore, the K + -intercalated Ti 3 C 2 MXene-based electrode exhibits an excellent cyclic stability.