High capacity rocking-chair capacitive deionization using highly crystalline sodium cobalt hexacyanoferrate (NaCoHCF) electrodes

Prussian blue analogue (PBA) electrodes are widely used cation-selective electrodes for electrochemical desalination technologies due to their high specific capacity rates and fast kinetic properties. Despite the fact that previous studies of PBAs for electrochemical desalination have shown remarkable desalination capacity levels, they remain insufficient if used to desalt highly concentrated salt water such as seawater. Here, we applied highly crystalline sodium cobalt hexacyanoferrate (NaCoHCF) electrodes, a type of PBA that can utilize two redox active sites, to a rocking-chair capacitive deionization (RCDI) process. The specific capacity of the NaCoHCF electrode using two redox active sites was 88 mAh g-1 (active material: 110 mAh g-1), confirmed to be 1.5 times higher than that of PBA electrode that use one redox active site. As a result of desalination tests, this system achieved a high desalination capacity of 123 mg g-1 (active material: 154 mg g-1) with 88% ion removal in a 500 mM NaCl solution. The results of this study present a considerable increase in the desalination capacity through the introduction of NaCoHCF electrodes that utilize two redox active sites in the RCDI system.