(a) CuHCF has the Prussian Blue crystal structure, in which octahedrally coordinated transition metals such as Cu and Fe are linked by CN ligands, forming a face-centred cubic structure. Fe is sixfold carbon-coordinated, while Cu is sixfold nitrogen coordinated. Each of the eight subcells of the unit cell contains a large 'A site' that may be occupied by zeolitic water or hydrated alkali cations such as K+. Hydrated ions may readily pass between one A Site and the next through channels in the ‹100› directions. This three-dimensional network of A Sites and channels allows for rapid transport of K+ through the material without disturbance of the framework. Zeolitic water is omitted for clarity. (b) X-ray diffraction patterns of CuHCF and Prussian Blue. Bulk synthesis of CuHCF at room temperature by co-precipitation results in highly crystalline material. Synthesis of Prussian Blue under the same conditions results in rapid precipitation and low-crystallinity material. (c) TEM of CuHCF shows polydisperse 20–50 nm particles (Scale bar, 200 nm). An electron diffraction pattern could not be obtained because CuHCF is unstable under irradiation.