ABSTRACT
Hydrogen storage issues have been universally investigated in order to satisfy the goals for a hydrogen economy. Carbon aerogels (CAs) are regarded as one of the most promising candidates for hydrogen storage at cryogenic temperature (77 K) because they have ultrafine cell/pore sizes, continuous porosity and high surface areas. This chapter discusses the synthesis and characterization of various CAs. CAs were prepared from the sol-gel polymerization of resorcinol with furfural followed by carbonization and activation. The effect of pH values on microstructures of CAs were studied using acetic acid and potassium hydrate catalysts. Furthermore, an efficient and simple synthesis method was employed to prepare cobalt-doped CAs. The chemical reaction mechanism and optimum synthesis conditions were further investigated by Fourier Transform Infrared Spectroscopy and thermoanalyses with a focus on the sol-gel process. The CAs were investigated with respect to their microstructures, using small angle X-ray scattering and nitrogen adsorption measurements
at 77 K. Hydrogen storage properties were investigated at room temperature and liquid nitrogen temperature at pressures up to 6.5 MPa. 3.1 IntroductionOwing to the dramatic environmental impact and limited supply of fossil fuels, the search for alternative clean fuels is becoming increasingly important. Hydrogen has been identified as a future clean energy carrier [1]. Hydrogen-powered fuel cells are developing rapidly because they are more efficient than internal combustion engines and have only water as an emission. However, use of hydrogen as an energy carrier involves solving many problems that relate to its production, storage, transportation, and safety. Numerous efforts are being undertaken to develop efficient hydrogen storage media that comply with the U.S. DOE targets. These objectives fix a target of 2 kWh/kg (6 wt.%) and $4/kWh for 2010, and 3 kWh/ kg (9 wt.%) and $2/kWh for 2015 [2]. For practical applications, materials should demonstrate (a) high hydrogen capacity (>6 wt.%) (b) fast and reversible hydrogen sorption at temperatures <150°C and reasonable pressures (<2 MPa), and (c) should be light and environmentally friendly [3, 4]. Unfortunately, hydrogen energy densities acceptable to the automobile industry in the short term are presently achieved reversibly only with high pressure (>35 MPa) gas cylinders. Promising solid state materials for hydrogen storage include metal hydrides, complex hydrides, chemical storage, and carbon-based porous absorbents.Porous carbon materials, e.g., carbon aerogels (CAs), cryogels, aerogels, and xerogels, have ultrafine cell/pore sizes, continuous porosity and high surface areas. Among porous carbon materials, CA materials are one of the novel carbon-based porous materials with many fascinating properties [5, 6]. They are typically prepared from the sol-gel polymerization of resorcinol and formaldehyde and dried through supercritical extraction of the reaction solvent [6]. As for nanoporous absorbents, the enhancement of hydrogen storage capacity can be achieved by increasing surface area. In addition, the surface should be continuous and open, which not only enhances the hydrogen storage capacity, but also increases the hydrogen sorption kinetics. With the assumption that the structure of the adsorbed hydrogen is close-packed face-centered cubic, the minimum surface
area required for the adsorption of 1 mol of hydrogen is 85.917 m2[7]. Accordingly, based on the surface area of CAs, the storage capacity of hydrogen at ~6 wt.% adsorbed on CAs is about 2578 m2/g, which can typically be achieved at cryogenic temperatures. The hydrogen storage capacity per unit surface area of CA is similar to that for activated carbons, i.e., 1 wt.% for every 500 m2/g of surface area, i.e., Chahine rule [5, 8]. Reversible hydrogen uptake on these carbon materials has been consistently reported to be approximately proportional to surface area and micropore volume [9, 10]. The best linear correlation is usually obtained when relating hydrogen adsorption capacity to micropore volume [11], reflecting the fact that physisorption (and consequent hydrogen storage) is dominated by pores having a diameter of sub-nanometer range [12]. However, the physisorption is brought about by weak van der Waals forces. In the absence of relatively strong polarizing centers, the interaction between the adsorbent and nonpolar hydrogen molecules relies on dispersion forces, which are weak; typically of the order of 4-8 kJ/mol. Hence, significant hydrogen adsorption often takes place only at cryogenic temperatures [13]. Thermodynamic constraints for hydrogen storage by physisorption were analyzed recently and results indicated that the carbons are not practical for hydrogen storage at ambient temperature [14].
