The Misconception of Mg²⁺ Insertion into Prussian Blue Analogue Structures from Aqueous Solution

Abstract

Prussian blue analogues (PBAs) are commonly believed to reversibly insert divalent ions, such as calcium and magnesium, rendering them as perspective cathode materials for aqueous magnesium‐ion batteries. In this study, the occurrence of Mg2+ insertion into nanosized PBA materials is shown to be a misconception and conclusive evidence is provided for the unfeasibility of this process for both cation‐rich and cation‐poor nickel, iron, and copper hexacyanoferrates. Based on structural, electrochemical, IR spectroscopy, and quartz crystal microbalance data, the charge compensation of PBA redox can be attributed to protons rather than to divalent ions in aqueous Mg2+ solution. The reversible insertion of protons involves complex lattice water rearrangements, whereas the presence of Mg2+ ion and Mg salt anion stabilizes the proton (de)insertion reaction through local pH effects and anion adsorption at the PBA surface. The obtained results draw attention to the design of proton‐based batteries operating in environmentally benign aqueous solutions with low acidity.

Resolving the Seeming Contradiction between the Superior Rate Capability of Prussian Blue Analogues and the Extremely Slow Ionic Diffusion

“…The studied PBAs follow different ion insertion mechanisms, as reflected in the shape of the cyclic voltammograms (CVs) in Figure (the potential interval in aqueous solution corresponds to the reversible extraction of one K + per Fe in MHCF). As previously reported, for K-rich K-FeHCF and K-NiHCF samples, a monoclinic-to-cubic phase transformation occurs upon K + deinsertion. ,− Phase transition potentials correspond to sharp maxima in the differential capacitance versus potential plots (Figure B,D). In this case, diffusion coefficients can be estimated by PITT and EIS methods in more narrow potential intervals compared to K-poor CuHCF and NiHCF materials, where a solid solution pathway is dominant and no lattice rearrangements take place upon Fe oxidation and/or reduction. , The single-phase nature of Fe redox in CuHCF and NiHCF is corroborated by a smooth change in the differential capacitance with potential (Figure A,C).…”

“…Despite the observed exceptionally low K + diffusivities in K-FeHCF, the material shows impressive rate capability with >70% of capacity retention at a 40C discharge rate when operating in a constant current mode (Figure C). Earlier, for the PBA materials used in this study, we reported similarly high capacity retention for operation in an aqueous of K + solutions . It is the superior rate capability of PBA materials that leads to the intuitive conclusion about high diffusivities of alkali ions in PBA structures, which is invalidated in this study (below we provide conclusive evidence that this contradiction is apparent).…”

“…Earlier, for the PBA materials used in this study, we reported similarly high capacity retention for operation in an aqueous of K + solutions. 34 It is the superior rate capability of PBA materials that leads to the intuitive conclusion about high diffusivities of alkali ions in PBA structures, 26 which is invalidated in this study (below we provide conclusive evidence that this contradiction is apparent).…”

“…The details of the crystal structure, thermal behavior, water content, and composition of these materials were recently reported in ref. . Details of the experimental methods are given in section S1 of the Supporting Information.…”

Understanding Electrochemical Intercalation of Al3+ Cation into the WO3 Electrochromic Electrode from Solid Electrolyte Interphase and Mass Changes

“…In the reverse process, the inserted species are de-intercalated from the WO 3 film during the oxidation bias (bleaching), resulting in a decrease of the electrode mass . By comparing the de-intercalation formal potentials of the two concentrations of 0.1 and 0.5 M Al­(ClO 4 ) 3 -PC solution, a shift of ∼30 mV could be observed based on the Nernst equation if Al 3+ is the charge-compensating ion. In contrast, a larger potential transition is obtained in the high-concentration solution, indicating that the counterions are also involved in the charge compensation process .…”