Elucidating Structure-Composition-Property Relationships of Ni-Based Prussian Blue Analogues for Electrochemical Seawater Desalination

Nickel hexacyanoferrate (NiHCF), a type of Prussian blue analogue (PBA), has recently emerged as one of the most promising Na-storage electrodes for use in electrochemical desalination. Previous studies have revealed that NiHCF can be prepared with both cubic and rhombohedral symmetries depending on the oxidation state of Fe (Fe-II vs Fe-III) and the related A-site occupancy. However, our understanding of the effects of the lattice-type of the as-prepared samples on their electrochemical performances, structural transitions that occur during sodiation/desodiation, cyclability, and rate capabilities is presently lacking. Additionally, the optimum structural and compositional features required to prepare high-performing NiHCF electrodes have not yet been clearly established. In this work, we report the synthesis of two sets of cubic and rhombohedral NiHCF samples with different particle sizes, crystallinities, and compositions. Using these samples, we systematically elucidated the structure-composition-property relationships of NiHCF to develop rational design principles to prepare high-performing PBAs. Our results show that high crystallinity, a low number of Fe(CN)(6) vacancies, and a large unit cell size to allow for consistent structural changes during cycling are critical factors to produce NiHCF with a high capacity, good cycling stability, and good rate capabilities, and these factors are considerably affected by the synthesis conditions. One of the samples prepared in this study with optimum structural features demonstrates the best performance and stability among any PBA electrode tested in neutral saline solutions to date.