Solvent‐Induced Octahedral Self‐Assembly of Prussian Blue and Its Applications in Sodium‐Ion Capacitors

Abstract Controlling the morphology and exposed facets of nano/microparticles is crucial for enhancing material properties in electrochemical reactions, photoreactions, and biosensors. However, Prussian Blue (PB), a type of metal–organic framework (MOF), rapidly and stably forms a cubic structure, making shape control difficult and leaving its formation process unclear. Here, the octahedral formation of PB particles in glycerol is discovered, which differs from the cubic particles formed in water. Glycerol slows down the crystallization‐growth process of PB, allowing direct observation of a stepwise growth in which cubic seeds initially form and subsequently self‐assemble into octahedra. Molecular dynamics (MD) simulation suggests that glycerol molecules stabilize the (111) surfaces of PB crystals over the (100) planes, inducing the self‐assembly of PB particles into an octahedral shape. It is also observed that the water‐to‐glycerol ratio influences PB's surface charge and affects the assembly behavior of PB crystals, resulting in the formation of isolated nanocubes (edge length ≈200 nm), microcubes (edge length ≈1 µm), and octahedra (edge length ≈1 µm). As an active material for sodium‐ion capacitors, octahedron‐shaped PB, with its significantly higher surface area, exhibits outstanding cycling performance, surpassing cubic PB.