Selective sodium(I) capture by olivine FePO4@C electrode in pseudocapacitive deionization: Mechanism and application

Selective extraction of target ions from aqueous solutions by capacitive deionization (CDI) has gradually become a highly efficient separation technology for water desalination. Herein, we present a carbon coated olivine phase iron phosphate (O-FePO4@C) composite as a pseudocapacitive electrode to explore the selective desalting behavior towards Na+, Mg2+, and Ca2+ ions, which also offers a new strategy to recycle wasted lithium iron phosphate batteries for sustainable development in energy field. Batch experiments reflects electrosorption capacity of 61.3 mg g−1 for Na+ at 1.2 V with O-FePO4@C as the electrode, and the separation factors of Na+ ion to Mg2+ and Ca2+ ions reached 1.52 and 1.91 in a 1:1:1 mixed solution, respectively. Cyclic voltammetry (CV) and X-ray diffraction (XRD) combined with density functional theory (DFT) simulation reveals that the electrosorption mechanism of O-FePO4@C is derived from ion intercalation for Na+ and Mg2+, and electrical double layers (EDLs) adsorption for Ca2+. Combining density of state (DOS) of electrons with spectroscopic analysis revealed that the intercalated ions can impact electron configuration and improve the conductivity of the electrode material. The selective desalination performance in natural groundwater shows superior cyclic stability of the O-FePO4@C electrode over 65 cycles without any obvious decline.