Theoretical calculation assisted materials screening of BiOX (X = F, Cl, Br, I) for electrochemical absorption of cesium ions.

The electrochemical switching ion exchange (ESIX) technique has been widely used for the separation and recovery of radioactive cesium ions (Cs+) from wastewater. In this study, a series of BiOX (X = F, Cl, Br, I) materials were first evaluated for their absorption properties to Cs+ through density functional theory (DFT) calculations. The calculations predict that BiOBr has the best absorption performance among the four materials, BiOF, BiOCl, BiOBr, and BiOI, due to its high absorption energy and low ion migration energy barrier to Cs+. Simultaneously, the selectivity calculations revealed that BiOBr also showed the best selectivity for Cs+ compared with Li+ and Na+. Subsequently, four materials were prepared using the hydrothermal synthesis method and their electrochemical absorption performance was tested. The results showed that BiOBr has the highest electroactivity, and its absorption capacity was up to 16 mg Cs+/g BiOBr in a solution mixture of 50 ppm Li+, Na+, and Cs+. Based on our theoretical calculations and experiments, our findings provide prospective insights for predicting the electrochemical absorption performance of materials using first-principles calculations.

Theoretical and experimental investigations of BiOCl for electrochemical adsorption of cesium ions.

BiOCl was found to have excellent electrochemical adsorption properties for cesium ions (Cs+) in electrochemically switched ion exchange (ESIX). In this work, BiOCl nanosheets were synthesized by a hydrothermal method and used for electrochemical adsorption of Cs+. The experimental results showed that BiOCl exhibited higher electrochemical adsorption selectivity for Cs+ than Li+ and Na+. Quantum chemical calculations based on density functional theory (DFT) were first performed to compare the adsorption and migration mechanisms of three ions Li+, Na+, and Cs+ in BiOCl crystals. The calculation results revealed that the excellent electrochemical adsorption performance of BiOCl for Cs+ is due to the interaction of embedded Cs with Cl and Bi in BiOCl crystals. This makes it have a higher adsorption energy and a lower ion migration energy barrier due to the balance of interaction forces. In this work experimental and theoretical calculations were used to systematically analyze the adsorption and migration of three ions in BiOCl, which has important guiding significance for the design of highly-efficient electroactive materials for electrochemical adsorption of Cs+.

DFT calculations of the synergistic effect of λ-MnO2/graphene composites for electrochemical adsorption of lithium ions.

Recently, the composite of spinel-type manganese oxide (λ-MnO2)/graphene has drawn wide attention because of its good electrochemical adsorption selectivity for low concentrations of Li+ ions from lake brine or seawater to cope with the fast-rising demand of lithium resources. In this composite, the synergistic effect between the good selectivity of λ-MnO2 for Li+ ions and the excellent conductivity of graphene play an important role for the electrochemical adsorption of Li+ ions. In order to reveal the synergistic mechanism in the electronic conductivity, the ionic conductivity and the ion selectivity of the λ-MnO2/graphene composite, density functional theory (DFT) calculations combined with electrochemical adsorption experiments were carried out. The calculation results show that the enhanced electronic conductivity of the composite is due to the decrease of the band gap (Eg) in the λ-MnO2/graphene composite compared with pure λ-MnO2. Meanwhile, the graphene composited with λ-MnO2 decreased the diffusion energy barrier of Li+ ions in λ-MnO2. In addition, the competitive adsorption of Li+, Na+ and Mg2+ ions were investigated by the nudged elastic band (NEB) method and charge distribution analysis. The results show that Li+ ions in λ-MnO2 exist in their pure ion state and have the lowest diffusion energy barrier compared with Na+ and Mg2+. The results of the DFT calculations were validated by cyclic voltammetry, electrochemical impedance spectroscopy and electrochemical adsorption experiments.