Prussian blue (PB) is an important porous metal-cyanide framework structure (A x Fe 3+ [Fe II (CN) 6 ] y .nH 2 O) with exchangeable A cations. Along with its analogs, it shows promise for a range of important applications including as cathodes in secondary ion batteries. Indeed, PB cathodes have potential in widespread, low-cost energy storage applications, using abundant and affordable Na + or K + based aqueous electrolytes. In particular, K + ions can be readily accommodated by the open structure of PB, which allows for reversable intercalation processes that are challenging in layered transition metal oxide cathodes.