Improving electron-ion transportation in BiOI via Fe-doping for efficient I− extraction by photo-assisted electrochemically switched ion exchange

Abstract

Low electronic and ionic conductivity of intrinsic BiOI limit its application in iodide ion (I⁻) extraction via photo-assisted electrochemically switched ion exchange (P-ESIX). Herein, we prepared a Fe-doped BiOI (Fe-BiOI) photoelectric film electrode using a facile grinding method to enhance I⁻ extraction performance via P-ESIX. Physicochemical characterization combined with density functional theory (DFT) calculations reveal that Fe substitutes part of Bi in BiOI, resulting in stronger Fe-O-Bi bonds compared to Bi-O-Bi, which causes contraction of the BiOI lattice. Additionally, the d electron-rich Fe increases the density of delocalized electrons, enhancing the electronic conductivity of BiOI. DFT calculations also confirm that Fe-doping reduces the I⁻ migration energy barrier, thereby increasing ionic conductivity. As a result of improved electron–ion transportation, under optimized conditions with an Fe doping ratio of 0.5 wt% and an applied adsorption potential of 0.6 V (vs. Ag/AgCl), the Fe-BiOI film electrode demonstrated I⁻ adsorption capacities of 172 mg·g⁻¹ in the conventional ESIX process and 312 mg·g⁻¹ in the P-ESIX process. The photo-assisted efficiency reached 81.4 %, which is significantly higher than the 59.3 % efficiency of pristine BiOI. Moreover, the Fe-BiOI film electrode demonstrates high selectivity for I⁻ over competing anions, with separation factors of 4.28 for I⁻/F⁻, 4.72 for I⁻/Cl⁻, and 1.69 for I⁻/Br⁻, respectively. After 10 consecutive adsorption–desorption cycles, the I⁻ adsorption capacity retains 87.7 % of its initial value, indicating good regeneration performance and cycling stability. This research provides new insights into the design of photoelectric materials for iodine extraction from aqueous solutions.