Synchrotron Structural Characterization of Electrochemically Synthesized Hexacyanoferrates Containing K+: A Revisited Analysis of Electrochemical Redox

Abstract

The presumably soluble KFe+3[Fe2+(CN)6] structure of electrochemically synthesized hexacyanoferrate materials (Prussian Blue) containing K+ ions was determined for the first time in this study. Prior to drawing conclusions from a structural analysis, the main goal was to make a precise analysis of the inferred soluble structure, that is, KFe+3[Fe2+(CN)6], which is frequently referred to in the literature as the final stable electrochemically synthesized structure. Indeed, a successful X-ray powder diffraction experiment using X-ray synchrotron radiation was made of a powder placed in a 0.5 mm diameter borosilicate glass capillary, which was obtained by removing sixty 90 nm thin films from the substrates on which they were prepared. However, the conclusions were highly unexpected, because the structure showed that the [Fe(CN)6] group was absent from ∼25% of the structure, invalidating the previously presumed soluble KFe+3[Fe2+(CN)6] structure. This information led to the conclusion that the real structure of Prussian Blue electrochemically synthesized after the stabilization process is Fe4[Fe(CN)6]3·mH2O containing a certain fraction of inserted K+ ions. In fact, based on an electrogravimetric analysis (Gimenez-Romero et al., J. Phys. Chem. B 2006, 110, 2715 and 19352) complemented by the Fourier maps, it is possible to affirm that the K+ was part of the water crystalline substructure. Therefore, the interplay mechanism was reexamined considering more precisely the role played by the water crystalline substructure and the K+ alkali metal ion. As a final conclusion, it is proposed that the most precise way to represent the structure of electrochemically synthesized and stabilized hexacyanoferrate materials is Fe3+4[Fe2+(CN)6]3·[K+h·OH−h·mH2O]. The importance of this result is that the widespread use of the terms soluble and insoluble in the electrochemical literature could be reconsidered. Indeed, only one type of structure is insoluble, and that is Fe4[Fe(CN)6]3·mH2O; hence, the use of the terms soluble and insoluble is inappropriate from a structural point of view. The result of the presence of the [Fe(CN)6] vacancy group is that the water substructure cannot be ignored in the ionic interplay mechanism which controls the intercalation and redox process, as was previously confirmed by electrogravimetric analyses (Gimenez-Romero et al., J. Phys. Chem. B 2006, 110, 2715; Garcia-Jareno et al., Electrochim. Acta 1998, 44, 395; Kulesza, Inorg. Chem. 1990, 29, 2395).