Prussian Blue finally transformed into octahedral structure after 300 years

For the first time in more than three centuries, Prussian Blue—long confined to its rigid cubic shape—has been transformed into an octahedral structure. A research team has successfully synthesized this new morphology by replacing water with a specialized solvent, glycerol, during the crystal growth process.

Their findings were recently published online in Advanced Functional Materials.

Accidentally discovered around the 1700s, Prussian Blue possesses a hollow three-dimensional framework that allows ions to move in and out with ease. These unique properties have enabled its use across diverse fields, from sodium-ion battery electrodes to radioactive cesium removal, catalysis, and environmental remediation.

However, until now, its morphology had been limited. When synthesized in water, the reaction proceeds too quickly, making it difficult to control particle growth and producing only cubic particles. This constraint has prevented scientists from exploring shape-dependent properties or unlocking new applications.

POSTECH researchers found the solution in the solvent. By using viscous glycerol instead of water, they were able to slow down crystal growth. In this glycerol medium, small cubic particles are initially nucleated, then repeatedly dissolved and recrystallized, self-assembling into octahedra structures. In effect, these tiny cubes stacked and transformed into gem-like eight-faced structures.

When tested as an electrode material in sodium-ion hybrid capacitors, the octahedral Prussian Blue demonstrated remarkable advantages. Its higher surface area enhanced electrochemical reactivity, while long-term charge,discharge cycling test confirmed stable performance. Simply changing the crystal shape resulted in significant performance improvements.

The research team included Prof. Changshin Jo (Department of Battery Engineering & Department of Chemical Engineering, POSTECH), Prof. Sangmin Lee (Department of Chemical Engineering, POSTECH), and Ph.D candidate Seunghye Jang (Department of Battery Engineering, POSTECH) as well as Dr. Carsten Korte of Forschungszentrum Jülich (Germany), who contributed to the structural analysis.

This study is the first to demonstrate that specific solvents can control both the growth rate and the orientation of Prussian Blue crystals. Beyond glycerol, the team anticipates that other organic solvents may enable the design of previously unprecedented crystal morphologies.

Prof. Jo stated, “The significance of this research lies not only in successfully creating a new morphology of Prussian Blue, but also in establishing the fundamental principles that enable us to observe and control its growth process. With the capability to design diverse morphologies, we anticipate a substantial expansion of applications, ranging from advanced energy storage systems to environmental purification technologies.”