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

Abstract

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 NaTi(PO) 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-NaTi(PO) 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/NaTi(PO) cell achieved a desalination/salination cycle at ±1 mA cm 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.

Redox‐Polysilsesquioxane Film as a New Chloride Storage Electrode for Desalination Batteries

“…Figure A shows the well‐defined quasi‐redox peaks with a formal potential value ( E 0″ = E p a + E p c /2) of 0.287 V and a potential difference of 0.97 V between the anodic and cathodic peak potentials versus normal calomal electrode (NCE) in the sea water. This potential difference is ≈13 times lesser than the recently reported Cl − ion storage material, that is, Bi electrode (Δ E p = 1.28 V), and it indicates that a better redox kinetics occur at the interface of [Fe(CN) 6 ] 4− ‐PSQ electrode/sea water than the sluggish reduction kinetics that occur during the conversion of BiOCl to Bi electrochemical reduction reaction at pH = 6.4 . Furthermore, this result suggests that our electrode materials can effectively utilize the anions present in the sea water even at a pH value of >7 without any adjustment.…”

“…Moreover, the surface area of the electrode (0.07 cm 2 ) used in our system is much smaller than the other reported systems (e.g., 2 cm 2 in Ag/NMO cell and 2.8 cm × 2.8 cm in BiOCl/NMO). Table S1, Supporting Information, shows the removal of surface coverage‐dependent Cl − ions from the sea water.…”

“…Currently, reverse osmosis (RO) and thermal distillation are the most advanced technologies that are used in the sea water desalination process . However, the energy consumed by these two processes is too high, which limits their use in developing countries . To overcome this, capacitive deionization (CDI) with different approaches, such as membrane CDI, flow‐through electrodes, flow electrodes, and hybrid CDI, and thermal dialysis have been introduced to operate with low energy consumption and low cost .…”

“…However, the use of silver has limitations due to its high cost and poor electrical conductivity of AgCl. In 2017, Nam and Choi and Chen et al demonstrated a new Cl − ion storage material using a Bi electrode for sea water desalination. However, they found two important drawbacks when they used a Cl − storage electrode in combination with NaTi 2 (PO 4 ) 3 as a Na storage material: 1) the slow Cl − release kinetics of BiOCl in neutral conditions, which was formed during the charging process and 2) an imbalance in Cl − and Na + storage (i.e., Cl storage requires three electrons/Cl − , while Na storage requires one electron/Na) .…”

“…Scheme illustrates the redox anion immobilization and its charge compensation process, which occurs during the electrochemical reactions. We have adopted this charge compensation concept for the removal of Cl − ions from the sea water, which is entirely different from the chemical covalent bond formation mechanism prevailing in the case of Ag and Bi electrodes toward the desalination process . This charge compensation mechanism slightly differs from the conducting polymer electrochemistry.…”