Electrochemical and Chemical Stability of Iron Hexacyanoferrate as a Sodium-Storage Electrode for Desalination Applications

For many electrochemical desalination devices based on ion-storage electrodes, a Na-storage electrode is a key component. Therefore, the successful commercialization of these devices critically depends on the development of practical, efficient, and robust Na-storage electrodes that have a high Na-storage capacity. Iron hexacyanoferrate (FeHCF) has many advantages for use as a Na-storage electrode for desalination applications; it is composed of only abundant and inexpensive elements and can achieve one of the highest Na-storage capacities among all of the Prussian blue analogues that perform electrochemical Na-storage and/or -release reactions. Regardless of these advantages, FeHCF has not been comprehensively examined for desalination applications, and previous studies have shown conflicting results in terms of its stability in aqueous media. In this study, we systematically investigated the effects of potential and pH on the performance and stability of FeHCF electrodes operating in aqueous solutions containing 0.6 M NaCl mimicking the salinity of seawater. We compared two types of FeHCF electrodes, Na_xFeFe(CN)₆ and Na_xFe[Fe(CN)₆]_3/4 electrodes, under the same solution and operating conditions. Through this study, we identified the factors that are detrimental to the stability of each of the FeHCF electrodes and the conditions that enable their stable operation. The results presented in this study provide new insights into the chemical and electrochemical stability of FeHCF in aqueous media, which helps identify the optimal applications and operating conditions of FeHCF.