The role of vacancies in the hydrogen storage properties of Prussian blue analogues

The porosity and hydrogen storage properties of the dehydrated Prussian blue type solids Ga[Co(CN)6], Fe4[Fe(CN)6]3, M2[Fe(CN)6] (M = Mn, Co, Ni, Cu), and Co3[Co(CN)5]2 are reported and compared to those of M3[Co(CN)6]2 (M = Mn, Fe, Co, Ni, Cu, Zn). Nitrogen sorption measurements suggest partial framework collapse for M2[Fe(CN)6] (M = Co, Ni) and Co3[Co(CN)5]2, and complete collapse for Mn2[Fe(CN)6]. Hydrogen sorption isotherms measured at 77 K reveal a correlation between uptake capacity and the concentration of framework vacancies, with Langmuir–Freundlich fits predicting saturation values of 1.4 wt.% for Ga[Co(CN)6], 1.6 wt.% for Fe4[Fe(CN)6]3, 2.1 wt.% for Cu3[Co(CN)6]2, and 2.3 wt.% for Cu2[Fe(CN)6]. Enthalpies of H2 adsorption were calculated from isotherms measured at 77 and 87 K. Importantly, the values obtained for compounds with framework vacancies are not significantly greater than for the fully-occupied framework of Ga[Co(CN)6] (6.3–6.9 kJ/mol). This suggests that the exposed metal coordination sites in these materials do not dominate the hydrogen binding interaction.