New insights into synergistic ion exchange and electrochemistry reaction mechanism in bismuth oxybromide reduction process

Bismuth oxybromide (BiOBr) demonstrates considerable potential for electrochemical bromide extraction. However, the applied reduction potential governs the reduction state of BiOBr through a synergistic multi-step process, thereby directly modulating its selective Br^- separation performance. Hence, fundamental elucidation of this reduction mechanism is imperative for advancing the practical application efficacy. The synergistic mechanism of interlayer anion-cation exchange and electrochemical reaction during the reduction process was verified by designing characterizations and density functional theory (DFT) calculations on distinct cut-off reduction potentials. Furthermore, tracking reduction intermediates and Br^- deintercalation kinetics elucidated a dual potential-concentration driving mechanism. At low potentials, potential and concentration driving forces act as primary motivators alternately, whereas reactions are dominated principally by potential driving forces at high potentials. Meanwhile, confirmation was obtained for the critical role of BiOBr layer retention in Br^-/Cl^- separation, with a selectivity of 4.10 achieved at low applied potential. These mechanistic insights are vital for advancing Br^- separation performance in practical BiOBr applications.