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 Ca 2+ -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 Ca 2+ 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 Ca 2+ over Na + and Mg 2+ . Using the hybrid CDI cell consisting of a CuHCF cathode and an activated carbon anode without any ion-exchange membrane, the outstanding Ca 2+ electrosorption capacity of 42.8 mg•g −1 and superior selectivity &(Ca 2+ /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 Ca 2+ 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.
Pseudocapacitor-type hybrid capacitive deionization (PHCDI) has been developed extensively for deionization, which enables to address the worldwide freshwater shortage. However, the exploitation of selective hardness ion removal in resourceful hard water via the intrinsic pseudocapacitive effect, rather than the ion-sieving or ion-swapping effect based on the electric double layer (EDL) of porous carbon, is basically blank and urgent. Herein, manganese spinel ferrite (MFO) nanospheres were successfully fabricated by one-step solvothermal synthesis and used as the cathode for PHCDI assembled with commercial activated carbon. The MFO electrode exhibited prominent capacities of 534.6 μmol g–1 (CaCl2) and 980.4 μmol g–1 (MgCl2), outperforming those of other materials ever reported in the literature. Fascinatingly, systematic investigation of binary and ternary ion solutions showed the high electro-affinity of hardness ions (Ca2+ and Mg2+) toward Na+, especially the leading affinity of Mg2+, in which the superhigh hardness selectivity of 34.76 was achieved in the ternary solution with a molar ratio of Na–Ca–Mg as 20:1:1. Unexpectedly, the ion-swapping trace in a multi-ion environment was also first detected in our pseudocapacitive-based electrode. The electrochemical response in unary and multiple electrolytes disclosed that the unique pseudocapacitive affinity based on the cation (de)intercalation-redox mechanism was from the synergistic effect of the relative redox potential, ionic radius, and valence, in which the redox potential was the dominant factor.
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