NiCu bimetallic metal–organic framework to improve the desalination performance of capacitive deionization

Due to its energy-saving and environment-friendly process without secondary pollutant, capacitive deionization (CDI), which separates the charged ion species from solution by applying external electric energy, provides an alternative desalination technology. Even though carbon-based electrodes have mostly been used so far, they suffer from limited salt removal performance because of insufficient ion uptake capacity and stringent regeneration conditions, which hinder the system from obtaining higher performance and scale-up. The development of a transition metal-based electrode has attracted much attention for emerging electrochemical applications, such as capacitive deionization, due to its unique electrochemical properties. However, synthesizing metal–organic frameworks (MOFs) in aqueous solution with high crystalline structure has been a challenging task. In this study, we used a simple hydrothermal technique to create NiCu–FA in aqueous solution to synthesize crystalline MOF structure. The newly developed NiCu–FA demonstrated high salt adsorption capacity, high salt adsorption rate, and outstanding CDI cycle stability, emphasizing the importance of high surface area with salt-activated transition metal. The high adsorption capacity of 10.55 mg/g could be achieved, which is over 76 % increase, compared to that of the pristine AC electrode 5.98 mg/g. Such enhanced desalination performance results from the synergistic contribution of electric double layer (EDL) capacitance from AC and pseudocapacitive behavior from NiCu–FA, which was confirmed by electrochemical measurements. We believe that our approach can offer a promising candidate for practical CDI application with high desalination performance and high salinity feed source.