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)], Fe-4[Fe(CN)(6)](3), M-2[Fe(CN)(6)] (M = Mn, Co, Ni, Cu), and CO3[Co(CN)(5)](2) are reported and compared to those of M-3[Co(CN)(6)](2) (M = Mn, Fe, Co, Ni, Cu, Zn). Nitrogen sorption measurements suggest partial framework collapse for M-2[Fe(CN)(6)] (M = Co, Ni) and CO3[Co(CN)(5)](2), and complete collapse for Mn-2[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 Fe-4[Fe(CN)(6)](3), 2.1 wt.% for Cu-3[Co(CN)(6)](2), and 2.3 wt.% for Cu-2[Fe(CN)(6)]. Enthalpies of H-2 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. (c) 2006 Elsevier B.V. All rights reserved.