Ultrathin MnO2 nanosheets/carbon nanotube heterostructures via electrostatic self-assembly for high-efficiency capacitive deionization

Abstract

The development of high-performance electrode materials is crucial for enhancing the performance of capacitive deionization (CDI) and advancing its practical application. Among various candidates, manganese dioxide (MnO₂) is regarded as a particularly promising electrode material owing to its high theoretical specific capacitance, excellent redox activity, diverse crystal structures, and non-toxicity. However, its practical application is severely limited by insufficient active sites, poor conductivity and structural stability. In this study, we successfully prepared a CNTs@MnO₂ composite material with one-dimensional/two-dimensional (1D/2D) heterostructure via a simple and efficient electrostatic self-assembly method. Specifically, the 2D MnO₂ nanosheets provide abundant active sites and larger interlayer spacing, significantly enhancing ion storage capacity and transport kinetics. Additionally, the ultrathin 2D structure helps to accommodate the volume expansion of MnO₂ during repeated charge-discharge, thereby improving structural stability. The introduction of CNTs not only improves the electrical conductivity of the MnO₂ nanosheets, but also offers mechanical support to suppress their restacking, improves the charge transfer efficiency and optimizes the ion diffusion pathways. Furthermore, the electrostatic self-assembly provides the strong interface bonding force between the two materials, promoting a synergistic enhancement effect. Benefiting from the 1D/2D heterostructure design, the CNTs@MnO₂ composite material exhibited a outstanding desalination capacity of 51.8 mg g⁻¹ and retained 90.92 % after 50 cycles. Moreover, the ion storage mechanism of the CNTs@MnO₂ was revealed through systematic ex situ characterization techniques. This work provides a promising strategy for the rational design and development of MnO₂-based electrode materials for CDI.