Structure-Dependent Solvent and Ion Intercalation in Reduced and Oxidized Nickel Hexacyanoferrates

Abstract

Water and alkali cation and intercalation, configuration, and dynamics in various nickel hexacyanoferrate (NiHCF) structures were studied using molecular dynamics (MD) techniques and compared to experimental results. We examined three different solid structures common to the transition-metal hexacyanoferrate family, namely, structural analogs to "soluble" and "insoluble" Prussian blue, and a third more highly defective solid. For MD simulations, water was represented by an extended simple point-charge model, and all other atomic interactions were represented by a universal force field. Atomic charges on alkali cations and solid matrix atoms were calculated using the charge equilibration method; these compared favorably to charges derived from ab initio quantum calculations. In most cases, water intercalation into the various NiHCF structures reduced the total potential energy of the solid. The minimum energy state (the details of which depended on the presumed solid structure, specific intercalated cation, and whether the solid was oxidized or reduced) represented a balance between beneficial coulombic interactions and detrimental steric interactions. The largest difference in hydration state between reduced and oxidized NiHCF occurred in the Ni analog of insoluble Prussian blue where as many as 2.7 waters were expelled for each intercalated into the structure. Experimental results in the literature have reported about three waters per We also experimentally measured the ratio of intercalated in oxidized and reduced NiHCF matrices grown as thin films by electrodeposition. These results showed that the cation intercalation behavior was most consistent with a structural analog of insoluble Prussian blue. © 2002 The Electrochemical Society. All rights reserved.