Elaborate designed sandwich structural faradic material NPC/NiMn-LDH/MXene for enriched ion accessible transfer pathways in capacitive deionization

Layered double hydroxides (LDHs) have been recognized as prospective two-dimensional capacitive deionization (CDI) faradaic materials for Cl- capture, offering tunable interlayer distance, high anion exchange capacity, and reversible ion intercalation/deintercalation ability. However, the self-stacking tendency, poor electrical conductivity, and undesired stability of LDHs nanosheets severely impede ion migration and active sites accessibility, leading to unsatisfactory CDI performance. Herein, we present an elaborately designed three-dimensional sandwich structural composite N-doped porous carbon spheres/NiMn-layered double hydroxide/MXene (NPC/NiMn-LDH/MXene) to efficiently capture Cl- as the CDI anode. In this assembly, MXene not only ensures excellent electrical conductivity for rapid ion transfer but also serves as a robust substrate for the uniform dispersion of NiMn-LDH nanosheets. Moreover, NiMn-LDH effectively inhibits the agglomeration of MXene and provides substantial storage capacity for ion accommodation. The conductive intermedium NPC establishes interconnected conductive channels, enriching accessible charge transport pathways, resulting in an accelerated ion diffusion kinetics. Accordingly, the NPC/NiMn-LDH/MXene electrode demonstrated predominant specific capacitance (265 F g-1), lower ion transfer resistance, and an optimized Cl- diffusion coefficient (7.95 × 10-17 cm2 s−1). In an asymmetrical CDI cell assembly, the NPC/NiMn-LDH/MXene//NPC cell delivered outstanding Cl- removal capacity (43.5 mg g−1), swift salt removal rate (12.5 mg g−1 min−1), efficient energy utilization (0.183 kWh kg−1-NaCl), and remarkable cyclic adsorption/desorption stability (89.6 % retention rate). Additionally, density functional theory (DFT) calculations validated that the sandwich construction of NPC/NiMn-LDH/MXene facilitated interfacial charge transfer from NPC and MXene to NiMn-LDH, accelerated ion transfer rate, and reduced ion migration energy, resulting in accessible ion diffusion kinetics and smooth Cl- shuttle. This unique approach enriches ion transfer pathways by leveraging the distinctive structural characteristics of faradaic materials, providing valuable insights for further developments in CDI technology.