La-doped CeO2/rGO hybrid with high oxygen vacancy to enhance phosphorus adsorption by capacitive deionization

Cerium oxide is one of the most attractive electrode materials in capacitive deionization (CDI) technology for selective phosphate adsorption due to the specific interaction. However, Ce3+/Ce4+ ratio and material conductivity of cerium oxide electrode determine the adsorption performance. Herein, a dual strategy was exploited with polyol-solvothermal reaction in the presence of graphite oxide and cerium salt followed by hydrothermal La-doping to obtain the three-dimensional (3D) assembly of La-doped CeO2 at the reduced graphene oxide (La-CeO2/rGO). The as-prepared La-CeO2/rGO hybrid exhibited interconnected structure with CeO2 uniformly distributed on the rGO support. La3+, by partially replacing Ce4+ in the lattice, was evenly fused into CeO2 nanosphere, enhancing the content of Ce3+ and oxygen vacancies and inhibiting lattice distortion. As a CDI positive electrode, the La0.15-CeO2/rGO with optimal La doping achieved the maximum theoretical adsorption capacity of 118.7 mg P/g with high separation factor (αSO42-P = 13.9), superior to the CeO2/rGO (67.8 mg P/g, αSO42-P = 10.1). In addition, the La-CeO2/rGO electrode is extremely robust, with excellent adsorption performance in wide pH range and good regeneration ability, compared to CeO2/rGO. This work proves that the synergy of trace La doping and 3D rGO support effectively improves the adsorption performance of La-CeO2/rGO electrodes and enhances the cycling stability of CDI devices. The corresponding adsorption mechanism was demonstrated.