Preparation of layered structure MnO₂/CNTs composites for high-performance salt removal by hybrid capacitive deionization

Capacity deionization (CDI), which combines capacitive carbon electrodes and redox-active electrodes, has emerged as a promising method for water desalination. It enables higher ion removal capacity than CDI-containing carbonaceous electrodes. Our work aimed to synthesize layered structure δ-MnO₂/CNTs composites via the sol-gel method, examining their suitability for hybrid capacitive deionization (HCDI). The XRD results showed a layered structure birnessite for all composites with a distance interlayer over 7.0 Å, while the SEM images confirmed the intercopration of MnO₂ particles and CNT fibers. The electrochemical evaluations evidenced the role of CNTs in enhancing the charge transfer and ionic diffusion in composites. Composite δ-MnO₂/CNTs presented a specific capacitance of 160 F g⁻¹ at a charge-discharge rate of 1 A g^-1 upon 1600 cycles. In salt removal performance, the asymmetric cell AC||δ-MnO₂/CNTs-1 showed the highest salt adsorption capacity (SAC) of 30.2 mg g⁻¹ and fast average salt adsorption rate (ASAR) of 3 mg g⁻¹ s⁻¹ under a constant applied potential 1.4 V in 200 ppm NaCl, rendering it a promising candidate for efficient and sustainable desalination technology.