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

Area of Science:

• Materials Science
• Electrochemistry
• Inorganic Chemistry

Background:

• Prussian blue analogues (PBAs) are explored as cathode materials for aqueous magnesium-ion batteries.
• The reversible insertion of divalent ions, like Mg²⁺, is a widely assumed mechanism in PBAs.

Purpose of the Study:

• To investigate the mechanism of ion insertion in Prussian blue analogues in aqueous Mg²⁺ solutions.
• To provide conclusive evidence regarding the feasibility of Mg²⁺ insertion into PBAs.

Main Methods:

• Structural analysis
• Electrochemical measurements
• Infrared (IR) spectroscopy
• Quartz crystal microbalance (QCM) analysis

Main Results:

• Mg²⁺ insertion into nanosized PBAs (nickel, iron, copper hexacyanoferrates) is demonstrated to be infeasible.
• Charge compensation in PBAs is attributed to proton insertion, not Mg²⁺.
• Proton insertion involves lattice water rearrangements, stabilized by Mg²⁺ and anions via pH effects and adsorption.

Conclusions:

• The widely held belief of Mg²⁺ insertion in PBAs is a misconception.
• Proton insertion is the dominant charge compensation mechanism in aqueous Mg²⁺ solutions.
• Findings support the development of proton-based batteries in aqueous electrolytes.