Incorporating magnesium and calcium cations in porous organic frameworks for high-capacity hydrogen storageElectronic supplementary information (ESI) available: The quantum chemistry optimized structures and chemical potential of hydrogen are given in a supporting file. See DOI: 10.1039/c0fd90025g

We propose incorporating a bi-functional group consisting of magnesium or calcium cations and a 1,2,4,5-benzenetetroxide anion (C6H2O44−) in porous materials to enhance the hydrogen storage capacity. The C6H2O4M2bi-functional group is highly stable and polarized, and each group provides 18 (M = Mg) or 22 (M = Ca) binding sites for hydrogen molecules with an average binding energy of ca.10 kJ mol−1per hydrogen molecule based on RIMP2/TZVPP calculations. Two porous materials (PAF-Mg or PAF-Ca) constructed with the bi-functional groups show remarkable improvement in hydrogen uptakes at normal ambient conditions. At 233 K and 10 MPa, the predicted gravimetric uptakes are 6.8 and 6.4 wt% for PAF-Mg and PAF-Ca respectively. This work reveals that fabricating materials with large numbers of binding sites and relatively low binding energies is a promising approach to achieve high capacity for on-board storage of hydrogen. [ABSTRACT FROM AUTHOR]