Excellent defluorination performance in defective CoNiO₂ based asymmetric flow capacitive deionization system

High concentration fluoride ions (F^-) in industrial fluorine-containing wastewater unavoidably impose tremendous hazards on environmental safety and public health. In comparison to traditional techniques, flow electrode capacitive deionization (FCDI) has attracted significant attention due to the advantages of easy operation, low operational cost, and lack of secondary pollution. However, FCDI with the same electrode materials, always accompanies with mismatched response to anions and cations adsorption, failing to achieve high adsorption capacity under moderate energy consumption. To solve the drawbacks, we first adopted defective metallic rock salts (CoNiO₂), being characteristic of a high rate of electron transition and ion diffusion, with carbon black (CB) as anodic materials (CoNiO₂/CB), coupled by Prussian blue derivatives (CoHCC) as cathodic electrode in asymmetric FCDI. As expected, the CoNiO₂/CB flow electrode demonstrates excellent performance in terms of F^- adsorption capacity (13.78 mg/g), low energy consumption (5.39 J/mg), and high stability when treating 1000 mg/L NaF solution under 1.2 V cell voltage. Further decreasing the initial NaF concentration (500 mg/L) or increasing cell voltage (1.6 V), the F^- removal and adsorption capacity are further improved, approaching 95 % and 19 mg/g, respectively. A combination of experimental characterizations and computational modeling suggests that excellent defluorination performance of CoNiO₂/CB is ascribed to F^- intercalation in oxygen defects of CoNiO₂. Thus, this work provides newly defective strategy for the improvements of defluorination performance in fluoridated wastewater cross asymmetric FCDI technique.