Curvature enhanced NH2-MIL-53(Al) electrode for boosting ion diffusion and capacitive deionization defluorination†

Abstract

Traditional capacitive deionization (CDI) materials typically exhibit low fluorine adsorption capacity (FAC) due to limitations in the optimization of their specific surface area and chemical composition. A prospective strategy for efficient ion storage is modulating the local electric field strength (LEF) by changing the curvature. In this study, we developed a novel modulator-based curvature modulation method to prepare three different morphologies of NH₂-MIL-53(Al) electrode materials with similar specific surface areas but different curvatures, which were used to investigate the direct constitutive relationship between curvature and CDI performance. The results show that the urchin-like electrode (NCMOF-3) with high surface curvature has an ultra-high fluoride removal capacity (61.29 mg_NaF g_electrodes⁻¹), a fast fluoride removal rate (mg_NaF g_electrodes⁻¹ min⁻¹), and excellent charging/discharging cycle stability (10 000 cycles). CDI performance exceeds all previously reported MOF electrodes. Finally, in combination with the surface curvature/electric field model, we found that higher surface curvature may lead to higher concentration of ion distribution. The mechanism of action may be that high surface curvature enhances the local electric field enhancement (LEFE) effect of the electrode material, which in turn increases the ion storage capacity and diffusion rate during CDI. This study demonstrates firstly the potential effect of curvature on CDI performance by experimental design. More importantly, this study breaks the limitations of material design based on specific surface area and provides new design ideas for next-generation CDI materials based on curvature structure engineering.