3D Heterostructure Constructed by Few-Layered MXenes with a MoS2 Layer as the Shielding Shell for Excellent Hybrid Capacitive Deionization and Enhanced Structural Stability.

Abstract

Two-dimensional (2D) layered transition-metal carbides (MXenes) are attractive faradic materials for an efficient capacitive deionization (CDI) process owing to their high capacitance, excellent conductivity, and remarkable ion storage capacity. However, the easy restacking property and spontaneous oxidation in solution by the dissolved oxygen of MXenes greatly restrict their further application in the CDI domain. Herein, a three-dimensional (3D) heterostructure (MoS2@MXene) is rationally designed and constructed, integrating the collective advantages of MXene flakes and MoS2 nanosheets through the hydrothermal method. In such a design, the well-dispersed MXene flakes can effectively reduce the aggregation of MoS2 nanosheets, boost electrical conductivity, and provide efficient charge transfer paths. Furthermore, MoS2 nanosheets as the high-capacity interlayer spacer can prevent the self-restacking of MXene flakes and provide more active sites for ion intercalation. Meanwhile, the strong chemical interactions between MXene flakes and MoS2 nanosheets contribute to accelerating the charge transfer kinetics and enhancing structural stability. Consequently, the resulting MoS2@MXene heterostructure electrode possesses high specific capacitance (171.4 F g-1), fast charge transfer and permeation rate, abundant Na+ diffusion channels, and superior electrochemical stability. Moreover, the hybrid CDI cell (AC//MoS2@MXene) with AC as the anode and MoS2@MXene as the cathode delivers outstanding desalination capacity (35.6 mg g-1), rapid desalination rate (2.6 mg g-1 min-1), excellent charge efficiency (90.2%), and good cyclic stability (96% retention rate). Most importantly, the MoS2@MXene electrode can keep good structural integrity after the long-term repeated desalination process due to the effective shielding effect of the MoS2 layer to protect MXenes from being further oxidized. This work presents the flexible structural engineering to realize excellent ion transfer and storage process by constructing the 3D heterostructure.