Bismuth as a New Chloride-Storage Electrode Enabling the Construction of a Practical High Capacity Desalination Battery

Materials that can selectively store Na and Cl ions in the bulk of their structures and release these ions with good cycle stability can enable the construction of a high capacity, rechargeable desalination cell for use in seawater desalination. In this study, the ability of a nanocrystalline Bi foam electrode to serve as an efficient and high capacity Cl-storage electrode using its conversion to BiOCl was investigated. When Bi as a Cl-storage electrode was coupled with NaTi₂(PO₄)₃ as a Na-storage electrode, a new type of rechargeable desalination cell, which is charged during desalination and discharged during salination, was constructed. The resulting Bi-NaTi₂(PO₄)₃ cell was tested under various salination and desalination conditions to investigate advantages and potential limitations of using Bi as a Cl-storage electrode. Slow Cl^- release kinetics of BiOCl in neutral conditions and an imbalance in Cl and Na storage (i.e., Cl storage requires three electrons/Cl, while Na storage requires one electron/Na) were identified as possible drawbacks, but strategies to address these issues were developed. On the basis of these investigations, optimum desalination and salination conditions were identified where the Bi/NaTi₂(PO₄)₃ cell achieved a desalination/salination cycle at ±1 mA cm^-2 with a net potential input of only 0.20 V. The kinetics of Cl^- release from BiOCl was significantly improved by the use of an acidic solution, and therefore, a divided cell was used for the salination process. We believe that with further optimizations the Bi/BiOCl electrode will enable efficient and practical desalination applications.