Flexible structural engineering of PPy-NiCo-LDH@Mxene for improved capacitive deionization and efficient hard water softening process

Abstract Capacitive deionization (CDI), as a potential desalination technology, shows the prominent advantage in processing low-concentration brackish water. However, the traditional CDI electrodes suffer from the problems like the limited salt adsorption capacity and inferior practical application in selective ion separation process. Herein, the effective-designed three-dimensional Mxene-based composite PPy-NiCo-LDH@Mxene is constructed by integrating PPy, NiCo-LDH, and Mxene together through the hydrothermal method. Particularly, the excellent conductive PPy network can greatly alleviate the restacking of Mxene and NiCo-LDH flakes and provide effective charge diffusion and transfer channels. Moreover, the Mxene substrate can strengthen the structure stability of the composite and further reinforce the total electrical conductivity of the composite material. Meanwhile, the NiCo-LDH and Mxene provide rich ion intercalation sites, contributing to the high ion storage capacity and fast electrosorption kinetics. Consequently, the PPy-NiCo-LDH@Mxene achieves the significant specific capacitance (174 F g−1), superior charging and discharging property, and enhanced electrochemical cyclic stability. The asymmetric CDI cell (AC//PPy-NiCo-LDH@Mxene) displays the prominent salt adsorption capacity (31.5 mg g−1), rapid salt removal rate (4.7 mg g−1 min−1), fast electrosorption kinetics, and stable adsorption/desorption process. Furthermore, in the multi-salt solutions with different salt solution concentrations, the PPy-NiCo-LDH@Mxene electrode displays the superb adsorption selectivity to the Ca2+ and Mg2+ than Na+, which indicates its suitable application for hard water softening process. The effective structural design exhibits huge potential for applying effective electrode materials for fast CDI process and hard water softening.