Recently, scientists at the Institute of Solid State Physics (ISSP) of the Hefei Institutes of Physical Science (HFIPS) developed pseudocapacitive electrode for electrosorption of calcium ions, which is an effective way with high absorption capacity, selectivity and stability for water softening.

Over 85% of accessible fresh water has been identified as hard water. Although water treatment technology for water softening-capacitive deionization (CDI) technique has aroused great concern, most of the electrode materials used in CDI are carbon-based materials, which lack high-efficiency selectivity of target ions.

In this research, the researchers displayed how they achieved highly selective electrosorption of Ca²⁺ in a mixed solution of Na⁺, Ca²⁺ and Mg²⁺ ions when taking advantage of the intercalation effect of Ca²⁺ ions and utilizing copper hexacyanoferrate (CuHCF), for the first time, as a pseudo-capacitive electrode.

In the process, the highest electrosorption capacity of 42.8 mg.g-1 for Ca²⁺ ions was obtained at optimal applied voltage 1.4V. Especially, the raising trend of selectivity coefficient (Ca²⁺ over Na⁺) with increasing molar ratio of Na:Ca could be observed in the mixed solution of Na⁺ and Ca²⁺ ions, and the maximum selectivity coefficient of 3.05 was achieved on the Na⁺-to-Ca²⁺ ratio of 10:1.

The researchers applied the first and second order kinetics and adsorption isotherm to analyze the pseudo-capacitive adsorption behavior of CuHCF electrode. By combining electrochemical characterization and molecular dynamics simulation technique, they clarified the intrinsic pseudocapacitive characteristics of CuHCF electrode for selective electrosorption of calcium ions.

This research work is of great significance for exploring CDI pseudocapacitive electrode materials with high efficiency and selective electrosorption of target ions and hard water softening by CDI technology.

Schematic diagram of CuHCF-based capacitive deionization technology and selective electrosorption of calcium ions. (Image by XU Yingsheng)