Selective Pseudocapacitive Deionization of Calcium Ions in Copper Hexacyanoferrate

In recent years, the capacitive deionization (CDI) technology has gradually become a promising technology for hard water treatment. Up to now, most of the work for water softening in CDI was severely limited by the inferior selectivity and electrosorption performances of carbon-based electrodes in spite of combining Ca²⁺-selective ion-exchange resin or membranes. Pseudocapacitive electrode materials that selectively interact with specific ions by Faradic redox reactions or ion (de)intercalation offer an alternative strategy for highly selective electrosorption of Ca²⁺ from water because of brilliant ion adsorption capacity. Here, we first used copper hexacyanoferrate (CuHCF) as a pseudocapacitive electrode to methodically study the selective pseudocapacitive deionization of Ca²⁺ over Na⁺ and Mg²⁺. Using the hybrid CDI cell consisting of a CuHCF cathode and an activated carbon anode without any ion-exchange membrane, the outstanding Ca²⁺ electrosorption capacity of 42.8 mg·g^-1 and superior selectivity &(Ca²⁺/Na⁺) of 3.05 at a molar ratio of 10:1 were obtained at 1.4 V, surpassing those of the reported carbon-based electrodes. Finally, electrochemical measurements and molecular dynamics (MD) simulations provided an in-depth understanding of the selective pseudocapacitive deionization of Ca²⁺ ions in a CuHCF electrode. Our study would be helpful for developing high-efficiency selective electrosorption of target charged ions by intrinsic properties of pseudocapacitive materials.