Heterostructure of NiCoAl-layered double hydroxide nanosheet arrays assembled on MXene coupled with CNT as conductive bridge for enhanced capacitive deionization

Layered double hydroxide (LDH) is considered as an attractive capacitive deionization (CDI) faradic material for anion (such as Cl−) capture due to its excellent anion storage capacity, highly reversible anion (de)insertion and superior chemical stability. Nevertheless, the self-stacking property and poor conductivity of LDH lead to undesirable CDI performance. Herein, the CNT@NiCoAl-LDH@MXene heterostructure with three-dimensional (3D) interconnected network was rationally designed with MXene as the conductive substrate for the uniform and vertical growth of NiCoAl-LDH nanosheet arrays and CNT as the conductive bridge for fast electron transfer. Strong chemical bonding interactions were formed at the interface of this heterostructure, creating tighter and more stable interconnections. Moreover, the specific surface area of this unique arrays structure is 4.28 and 8.62 times that of NiCoAl-LDH and MXene, respectively, effectively suppressing the aggregation of layered materials and providing more exposed active sites. Therefore, the obtained CNT@NiCoAl-LDH@MXene electrode exhibits prominent specific capacitance (179 F g−1), decreased equivalent series and transfer resistance, increased ion diffusion rate and splendid electrochemical stability (capacitance retention rate of 107.5 % after 1500 GCD cycles). Consequently, as employed for Cl− intercalation anode coupled with AC cathode for CDI, the CNT@NiCoAl-LDH@MXene//AC cell exhibited excellent Cl− removal capacity (79.5 mg g−1), higher charge efficiency (88.4 %), lower energy consumption (0.62 kWh kg−1-NaCl) and exceptional desalination capacity retention rate (96.4 % after 40 adsorption/desorption cycles). This work provides an excellent anode material selection for the enhancement of CDI performance and valuable insights for the design of 3D interconnected network heterostructure.