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

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.