1.4 Properties of Physisorption Materials
1.4.2 State-of-the-Art: Physisorption materials
Pressure
Absolute Uptake Excess Uptake Gas Density
Figure 1.8: Idealized adsorption curves showing the relationship between absolute adsorption, excess adsorption, and gas density and typical lineshapes for the three types. Curves are based on the Langmuir isotherm and the ideal gas equation of state.
within measurements. These values typically lie between 5 kJ/mol and 20 kJ/mol, representing the relatively weak binding in these materials.58
or artificial in origin, and research into the benefits to the properties of the final carbon material resulting from various starting materials, particularly polymers such as polyether ether ketone59 (PEEK), is ongoing.
For most carbon materials, the resulting pyrolyzed material is then “activated” to produce the high surface areas for adsorption. Several processes for performing this operation exist, and many proprietary methods are claimed by commercial producers. Two common laboratory techniques for activation are steam explosion, which rapidly flashes superheated water introduced into the carbon structure to physically rend the structure, and carbon dioxide maceration, which used high temperature reaction of the carbon surface with CO2 to produce gaseous CO, causing chemical erosion of the surface. These processes are capable of producing carbon materials with surface areas up to∼3000 m2/g, but consume a large portion of the starting material in the process. These high surface-area materials, such as AX-21 and MSC-30, are used commonly as a standard for comparison of new physisorption materials.60, 61
Recently, efforts to increase the surface area of carbon materials beyond activated carbons and to control the microporosity have led to templated carbon materials. These materials contain carbon in regular nanostructures that provide very high surface areas, and still largely obey Chahine’s rule.
Investigations into carbon nanotubes and nanotube bundles quickly led to more advanced regular nanostructured materials63, 65, 64 and carbon aerogels.66 The latest materials in this area are based on zeolite frameworks.60, 62 Pyrolysis of polymers within the channels of a zeolite leads to a carbon negative of the zeolite, which can be extracted by dissolving the zeolite material in hydrofluoric acid.
This produces a high surface area carbon with very uniform micropores and low skeletal density, and has shown similar uptake to the high surface area MSC-30 with an improved adsorption profile.
Unfortunately, the limits to surface area modification are being approached for pure carbon materials. Research efforts in this area relative to hydrogen storage have been increasingly directed toward engineering applications and reduction of the cost associated with producing these materials.
Carbon nanoscaffolds and aerogels are also used frequently to improve the kinetics of absorption materials, and the interactions of the scaffold with these materials is under investigation.67–69
Metal Decorated, Doped, and Intercalated Carbons—The low adsorption enthalpies for
hydrogen in carbon-based physisorption materials have led to continuous efforts to dope or otherwise transform these materials without adding significant weight to the structure. The most common technique used is to incorporate a small fraction of adatoms or inclusions, which have strong bulk interactions with hydrogen gas, into the material’s structure. These dopants may then increase the overall interaction potential of the material.
Initial doping strategies used platinum or palladium nanocluster incorporations onto carbon structures, to produce a material similar to supported-metal catalysts.51, 71 Because of the high cost of precious metals, later studies have focused on the use of 3d transition metals, particularly tita- nium,70to produce similar effects. The metallic clusters in these materials were unfortunately found to act independently in most cases, with the metal clusters maintaining their own surface coverage of chemisorbed hydrogen atoms at stronger energies than the remaining carbon structure. Several groups have reported spillover in such materials, where dissociated hydrogen is transferred from metallic clusters to the carbon structure, providing higher surface area coverage of strongly bound hydrogen atoms.72, 73 These interactions have been observed at high temperature for platinum and ruthenium catalysts on carbon supporting structures through neutron spectroscopy,74and enhanced adsorption capacity at room temperature has been demonstrated. However, these enhancements appear to be a minor effect, and spillover processes are unlikely to produce increases in adsorption enthalpies that are significant for storage applications.
Additional techniques involve direct doping of the carbon structure, rather than surface modifica- tions. Boron and nitrogen doping of the graphitic layers have both shown to increase the adsorption capacity of activated carbons.75–77 However, whether the increase is due primarily to modification of the surface potential or to increased accessible surface area due to bowing of graphitic layers remains unknown. Intercalation of alkali metals into graphitic structures have also been used, and shown to both modify the interlayer spacing in the structure and increase the hydrogen adsorption enthalpy.52 These materials, however, are difficult to produce and tend to produce alkali metal hydrides after a few cycles.
Relatively few studies continue to examine these methods for improving the binding enthalpy in hydrogen storage applications, but continue to be relevant for chemical catalysis applications.
Studies are largely directed at examination of metal substitutions and reduction of metal size and increase in metal dispersion to reduce the amount of metal loading necessary to produce desired effects.
Metal-Organic Frameworks—Metal organic frameworks (MOFs) have received a large amount
of attention in physisorptive hydrogen storage research, both for the very high specific surface areas available in these materials and for large variability in the possible frameworks.78 These crystalline materials, composed of metal atoms linked together with organic molecules in a three-dimensional framework, contain uniform pores and structures which allow for greater control over the adsorption capacity and enthalpy. MOFs have consistently shown meaningful adsorption capacities at room temperature, making them promising candidates for further development.79–81
Hundreds of MOFs have been identified, with varying pore sizes and chemical makeup, and these variations are being actively pursued for gas adsorption applications. Overall, trends in hydrogen binding show an increase in hydrogen adsorption capacity with smaller pore sizes or higher specific surface area, in accordance with Chahine’s rule. Incorporation of mid-row transition metals and alkaline earth metals at the metal site have also been shown to increase binding enthalpies, increasing hydrogen storage at high temperatures.82 Examination of various chemistries and binding motifs are still ongoing. Besides basic materials work, research into improvements in the scale-up and large scale production of MOFs are needed before these materials can be implemented outside of the laboratory.