Co-precipitation synthesis control for sodium ion adsorption capacity and cycle life of copper hexacyanoferrate electrodes in battery electrode deionization

Prussian blue analogues are being explored as electrode materials for electrochemical desalination of saline water in battery-type electrode deionization systems and hybrid capacitive deionization systems due to their open framework crystal structure that provides selective adsorption of multiple cations, high theoretical sodium adsorption capacities, and low costs. However, poor electronic conductivity and instability (dissolution) prevents the use of these materials for long-term desalination applications. To understand how synthesis conditions might improve the properties of copper hexacyanoferrate (CuHCF) powders relative to sodium ion adsorption capacity and cycle life, the co-precipitation process was investigated using multiple common synthesis strategies that included addition of chelators and sodium salts as well as different concentrations and oxidation states of precursors. Smaller crystallite sizes (<35 nm) and lower structural water contents increased the initial sodium removal capacity to 53.2 mAh/g from 40.4 mAh/g (control), but also reduced cycling stability in long-term operation (20–55% retention over 100 cycles). The trade-off in stability is thought to be a consequence of structural water facilitating ion diffusion within the material. Adding chelators as precursors led to a highly reversible Cu^II/Cu^I redox couple that increased the stability of the Fe^III/Fe^II redox couple in BDI cycling performance (79.4% retention over 100 cycles).