Synergistic effects in high-performance Ni(OH)2@CNT heterojunction for improved salt removal in capacitive deionization

Capacitive deionization (CDI) is a promising water treatment technology that offers several advantages, including simple operation, low operational cost, environmental sustainability, and low power consumption. However, the performance of CDI systems is primarily constrained by the properties of the electrodes used. To improve the ion adsorption capacity and energy utilization efficiency of conventional carbon-based materials, the development of advanced electrode materials is of critical importance. In this study, a heterojunction structure composed of Ni(OH)₂ nanosheets and carbon nanotubes (Ni(OH)₂@CNT) was successfully synthesized using a straightforward solvothermal method. Electrochemical characterization confirmed that the Ni(OH)₂@CNT hybrid exhibited excellent capacitive Na⁺ storage performance, rapid Na⁺ ion diffusion kinetics, and long-term charge-discharge cycling stability. When combined with activated carbon to fabricate a CDI device, the composite electrode achieved a high adsorption capacity of 43 mg/g and a charge efficiency of 90.2 % in a 500 mg/L NaCl solution under a working voltage of 1.2 V. Moreover, the electrode demonstrated superior energy utilization efficiency and regeneration cycling stability. Through investigation of the Na⁺ adsorption mechanism, it was determined that the efficient removal of Na⁺ ions resulted from the synergistic effect between Ni(OH)₂ and CNT, involving intercalation adsorption by Ni(OH)₂ and electric double-layer adsorption by CNT. The high-performance Ni(OH)₂@CNT heterojunction developed in this study offers a promising technical solution for advancing capacitive desalination technology.