Cationic segregation of Ca2Mn3O8 enabling high selectivity for fluoride ions through capacitive deionization

Transition metal oxides (TMOs) have great application potential in capacitive deionization (CDI), which can selectively extract ions from water with high charge efficiency and low contamination. Previous studies have found that cationic segregation of TMOs has adverse effects on their electrochemical performance, but its effect on CDI remains unclear. Herein, we demonstrate for the first time that Ca 2+ segregation provides more sites for F − adsorption by synthesizing Ca 2 Mn 3 O 8 for CDI selective defluorination . The Ca 2 Mn 3 O 8 material has a graded porous structure , low charge transfer resistance , high specific capacitance (255.64 F/g, 1 mV/s, 0.5 M NaF solution) and good charge/discharge reversibility (about 100 % charge specific capacity retention after 50 cycles). Moreover, Ca 2 Mn 3 O 8 has a high specific adsorption capacity (SAC) for NaF (1419.07 μmol/g) and excellent cyclic stability (capacity retention rate is 96.69 % after 100 cycles). The Ca 2+ segregation from lattice to surface provides more sites for F − adsorption and greatly shortens ion transport paths. Density functional theory (DFT) calculation further suggests that the adsorption energy of Ca 2 Mn 3 O 8 for F − is much greater than that of Cl − . As a result, the removal rate of F − by Ca 2 Mn 3 O 8 in mixed solutions is up to 20 times higher than that of Cl − .