Enhanced Stability of Captured Sodium Ions in Bimetallic Aluminum Ferric Hexacyanide Synthesized at Low Temperature by Suppression of Phase Transition.

Prussian blue analogue compounds are widely recognized as the most promising cathode materials for large-scale capacitive desalination of brackish water in the future owing to their open three-dimensional framework structure and abundant active sites for metal ions. However, practical applications face challenges such as limited desalination capacity and short cycle life due to phase transitions induced by the insertion and extraction of sodium ions. In this study, theoretical calculations revealed that cubic phase aluminum iron hexacyanoferrate (AlFeHCF) does not undergo significant phase transitions during the sodium ion intercalation/extraction process. The coordination environment of aluminum is found to play a crucial role, providing valuable insights for the experimental direction. Building on these findings, we developed a preparation method for AlFeHCF aimed at removing sodium ions during the capacitive deionization (CDI) process. In this approach, aluminum partially substitutes for high-spin iron as a bimetallic component within iron hexacyanoferrate. Experimental validation demonstrates that this material exhibits exceptional performance when utilized in CDI cells, achieving a desalination capacity of up to 27.1 mg g-1 and exhibiting an extended cycle life with a capacity retention rate of 91.8% after 50 cycles. This innovative material shows great potential for advancing the development of CDI devices.