|About the Book|
The use of hydrogen as an energy carrier suitable to replace gasoline and other fossil fuels has been proposed as a way to sustainably fuel our civilization. Hydrogen has a gravimetric specific energy of 120 kJ/g, which is far higher than those ofMoreThe use of hydrogen as an energy carrier suitable to replace gasoline and other fossil fuels has been proposed as a way to sustainably fuel our civilization. Hydrogen has a gravimetric specific energy of 120 kJ/g, which is far higher than those of gasoline, coal, or methane. Unfortunately, it has a very low volumetric energy density of only about 10 kJ/L at standard temperature and pressure. High pressures or liquefaction can be used to increase the density of hydrogen, but liquefaction is expensive and inefficient while high pressure storage requires heavy fuel tanks and presents challenges with regard to safety. One approach to solving this problem is the development of safe, compact, and high capacity storage systems which adsorb hydrogen for fueling and release it when needed. The Department of Energy has set a target of development of systems which can store and release 6 wt% of hydrogen near room temperature by 2010. Because the adsorption of hydrogen requires a nearly ideal gas to be stored in a condensed form, the enthalpy of adsorption is the critical parameter for determining the temperature at which reversible hydrogen storage takes place. The majority of high surface area materials exhibit enthalpies of hydrogen adsorption which are between -4 and -7 kJ/mol. This is sufficient for hydrogen storage at cryogenic temperatures but enthalpies between -15 and -20 kJ/mol are required for operation at room temperature. In this work, a series of nanostructured polymers is developed and their application to the problem of hydrogen storage examined.-Poly(divinylbenzene-co-chloromethylstyrene) was synthesized via suspension polymerization. The resulting beads were swollen in dichloroethane and crosslinked using Friedel-Crafts acylation catalyzed by FeCl3. The resulting sponge-like materials exhibit specific surface areas of up to 2000 m 2/g as calculated by applying the Brunauer-Emmett-Teller (BET) equation to nitrogen adsorption isotherms. These materials reversibly adsorb up to 3.8 wt% excess H2 at 4.5 MPa and a temperature of 77K. The total hydrogen adsorption to the best of these nanoporous polystyrenes is 5.4 wt% at 8.0 MPa and 77K. The enthalpy of adsorption of the first few molecules of hydrogen to this material is -6.6 kJ/mol and decreases monotonically with coverage.-A similar strategy for the formation of sponge-like, nanoporous materials was applied to the synthesis of polyanilines with large surface areas. Polyaniline base was swollen in dimethylformamide and crosslinked via n-alkylation of the amine. The resulting nanoporous polyanilines exhibit BET surface areas of up to 630 m2/g and hydrogen storage capacities of up to 2.2 wt% at 3 MPa and a temperature of 77K. The highest enthalpy of hydrogen adsorption observed in these systems is 9.3 kJ/mol.-Polyaniline and diaminobenzene were crosslinked with polyhalogenated arenes such as diiodobenzene and tribromobenzene. The resulting materials resemble networks of aromatic rings linked by trivalent nitrogen atoms. While these materials exhibit smaller surface areas and hydrogen storage capacities than alkyl-crosslinked polyanilines, they possess pores which are large enough for adsorption of hydrogen, but too small for adsorption of other small gas molecules such as nitrogen. At low coverages, the enthalpy of hydrogen adsorption within these small pores reaches as high as -19 kJ/mol. This places them within the thermodynamic range desired for room temperature hydrogen storage.-To confirm that networks of aromatic rings linked by single-atom, trivalent crosslinkers can contain pores which adsorb hydrogen but not nitrogen, polypyrrole was crosslinked with a series of crosslinking groups, including boron. The resulting materials exhibit similar size-selective adsorption properties and large enthalpies of hydrogen adsorption.