Intercalation into a Prussian Blue Derivative from Solutions Containing Two Species of Cations

Reversible mixed-ion intercalation in nonselective host structures has promising applications in desalination, mixed-ion batteries, wastewater treatment, and lithium recovery through electrochemical ion pumping. One class of host compound that possesses many of the requirements needed for such applications (cost effectiveness, fast ion kinetics, and stability in an aqueous medium) includes the Prussian blue derivatives. Herein, the fundamental process of intercalation of multiple cations is studied at the thermodynamic level by means of galvanostatic cycling. Nickel hexacyanoferrate is focused upon because of its stability and low potential for electrochemical process relative to other hexacyanoferrates. Various cations can be intercalated; large cations (K⁺ and NH₄⁺ ) are intercalated at higher potentials than those of smaller cations (Na⁺ ). When mixtures of cations are present in solution, the potential profile is not qualitatively altered with respect to single-salt solutions, but the potential of (de-)intercalation is shifted; a simple thermodynamic model is introducted that is able to predict the potential and distribution at which intercalation takes place.