Structure of Porous Copper Prussian Blue Analogues: Nature of Their High H₂ Storage Capacity

Abstract

Within porous Prussian blue to copper the highest H2 storage capacity is observed. Such behavior finds explanation in the crystal structure for Cu3[M(CN)6]2 with M = Fe, Co, Ir. The crystal structure of Prussian blue analogues is usually solved and refined with a cubic unit cell in the Fm3̅m space group, which corresponds to a random vacancy distribution. However, a careful evaluation of X-ray diffraction powder patterns of copper-containing compositions reveals a deviation from that structural model. The crystal structure for the considered series of copper hexacyanometallates(III) was found to be also cubic but in the Pm3̅m space group related to a nonrandom vacancy distribution. To this model 50% of vacancies for the building block, [M(CN)6], corresponds, which is quite different from the value of 33.3% in the Fm3̅m structural model. Mössbauer spectra and high pressure H2 adsorption isotherms support the assignment of the Pm3̅m space group for the studied series of copper Prussian blue analogues. The implications of a nonrandom vacancy distribution on the physical properties of these materials are discussed.