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The hydrogen storage in zeolite-like hexacyanometalates with different exchangeable alkali metals within the cavities was studied. The H 2 adsorption isotherms were recorded at 75 and 85 K in order to estimate the involved adsorption heats using the isosteric method. The electric field gradient within the porous framework favors the hydrogen adsorption in the materials under study but also could lead to kinetic effects for the pore filling. Such effects were particularly pronounced for sodium among the studied compositions: Zn 3 A 2 [Fe-(CN) 6 ] 2 (A ) Na + , K + , Rb + , Cs + ) and Zn 3 [Co(CN) 6 ] 2 . For Na + , a strong interaction with the H 2 molecule takes place, where appreciable kinetic effects even at 258 K are observed. For Zn 3 [Co(CN) 6 ] 2 (rhombohedral phase) where the cavities are free of exchangeable metal and, in consequence, have a weak electric field gradient on their surface, the largest hydrogen storage capacity, close to 12 H 2 molecules per cavity (1.82% by weight), was observed. The hydrogen adsorption in these materials involves adsorption heats in the 6-8.5 kJ/mol range, following the order K > Rb > Cs ≈ Zn 3 [Co(CN) 6 ] 2 . The porous framework of this family of materials is formed by ellipsoidal cavities communicated by elliptical windows. The alkali metals are sited close to the windows. The pore accessibility and pore volume were evaluated from CO 2 adsorption isotherms recorded at 273 K. The free volume was found to be accessible to the CO 2 molecule for all of the studied compositions. According to the obtained isotherms the stabilization of the CO 2 molecule within the pores is caused by the electrostatic interaction between the electric field gradient at the cavity and the adsorbate quadrupole moment. The estimated strength for the guest-host interaction and the accessible pore volume follow the order Na > K > Rb > Cs. The largest accessible pore volume was found for Zn 3 [Co(CN) 6 ] 2 , close to 8 CO 2 molecules per cavity (28% by weight), but with the weaker guest-host interaction. The materials under study were characterized from X-ray diffraction, thermo-gravimetric, infrared, and Mo ¨ssbauer data. The obtained results shed light on the role of the electric field gradient at the cavity for the hydrogen adsorption.

The rapid increasing of the population in combination with the emergence of new energy-consuming technologies has risen worldwide total energy consumption towards unprecedent values. Furthermore, fossil fuel reserves are running out very quickly and the polluting greenhouse gases emitted during their utilization need to be reduced. In this scenario, a few alternative energy sources have been proposed and, among these, proton exchange membrane (PEM) fuel cells are promising. Recently, polybenzimidazole-based polymers, featuring high chemical and thermal stability, in combination with fillers that can regulate the proton mobility, have attracted tremendous attention for their roles as PEMs in fuel cells. Recent advances in composite membranes based on polybenzimidazole (PBI) for high temperature PEM fuel cell applications are summarized and highlighted in this review. In addition, the challenges, future trends, and prospects of composite membranes based on PBI for solid electrolytes are also discussed.

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This work describes the preparation of mixed matrix membranes by casting from poly(1,4-phenylene ether−ether−sulfone) chloroform solutions containing dispersed zeolitic imidazolate frameworks. Diffusive studies of CO2 in the pristine poly(1,4-phenylene ether−ether−sulfone) membrane and composite membranes were performed at 6 bar and 298 K, using pulsed field gradient NMR techniques. The evolution of the heterogeneity of the diffusion environments as seen by NMR was monitored in terms of the diffusion time and a stretching parameter. The values of the self-diffusion coefficient increase with filler content, from 2.1 × 10−8 cm2 s−1 for pristine membranes to 9.3 × 10−8 cm2 s−1 for membranes with 30 wt % of filler. Sorption and permeation experiments carried out at different pressures were used to determine the dual-mode model parameters that describe the transport processes. Apparent diffusion coefficients of CO2 in the membranes were obtained from the time lag method, from parameters of the dual-mode model, and directly from the derivatives of the steady flux, expressed in terms of concentration and pressure, with respect to pressure. In general, the values of the apparent diffusion coefficients obtained by the three methods for pristine membranes are in rather good agreement with the self-diffusion coefficient obtained by the NMR technique. In composite membranes, the values of the self-diffusion coefficients are nearly 2 times those obtained using permeation and sorption experiments. The discrepancies between the values of the self-diffusion coefficients and the results obtained for the diffusion coefficient by other techniques are discussed. The filler contributes greatly to gas permeation by increasing the gas solubility in the composite membranes.