The research work presented in this memorandum deals with the synthesis of advanced activated carbon materials in order to use them for hydrogen storage application. A total number of 90 samples are investigated, comprising activated carbon fibers (ACFs ), activated carbon nanofibers (ACNFs), as well as activated carbon monoliths from different synthetic precursors (ACFs and PVDC-based). After a broad introduction, the experimental characterization methods are explained which include: Textural analysis via sub-atmospheric adsorption isotherms ofN2 at 77 K and of CO2 at 273 K, thermogravimetric analysis (TGA), differential thermal analysis (DTA), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), different techniques for material density measurements, as well as the use of a number of hydrogen adsorption devices which permit measurements under different temperature-pressure conditions (ranging from high precision sub-atmospheric measurements to high pressure, as well as from cryogenic to room temperature ). For material synthesis, different methods are applied: Physical activation with CO2, chemical activation with hydroxides (KOH and NaOH), as well as monolith synthesis. For ACFs, different activation conditions are investigated which develop appropriate adsorbent characteristics for H2 adsorption. Here, aside from the nature and amount of the activating agent, also the carbonization temperature of the carbon fiber precursor plays a key role. A scale-up ACF production is performed, yielding in an output one order of magnitude higher than for laboratory scale synthesis. For a selection of ACFs H2 adsorption is measured at 298 K and 20 MPa and 77 K and 4 MPa. The total storage capacity is introduced which, apart from the adsorbed phase, also accounts for the compressed gas in the void space of the adsorbent. This quantity only depends on measurable sample characteristics (H2 adsorption amount, as well as bulk and real densities) and permits to estimate how much gas can be stored in a confined tank volume. Especially at 298 K the adsorbent density has a high impact on the total H2 storage capacity. Therefore, the density of the ACFs is increased by synthesizing monoliths from them. Furthermore, another class of high-density monoliths is investigated, whose adsorption characteristics are improved by CO2 activation. On a volumetric basis these materials outperform adsorbents with very high surface areas like Maxsorb or MOF-21 O, and achieve high total storage capacities up to 18.1 g r1. For carbon nanofiber activation, CO2 turns out to result in better bespoke porosity for H2 adsorption than other activating agents like hydroxides or steam. Their density is improved by monolith synthesis. Especially interesting H2 adsorption results are obtained for the nanofibers at 298 K and 20 MPa, where a hydrogen uptake of 1.2 wt.% is measured, which is more than would be expected from their porosity. finally, the system storage capacity and the dimensionless quantity K are introduced which allow to determine if a given storage tank device can be improved by filling it with an adsorbent. The calculations are done for a number of state-of-the-art tanks and using different materials from this study. The results reveal that in volumetric terms the system storage capacities can be easily improved. On a gravimetric basis, it is expected that the storage capacities could be improved for optimized tank devices.
| Identifer | oai:union.ndltd.org:ua.es/oai:rua.ua.es:10045/146581 |
| Date | 26 November 2013 |
| Creators | Kunowsky, Mirko |
| Contributors | Linares-Solano, Angel, Marco Lozar, Juan Pablo, Universidad de Alicante. Departamento de Química Inorgánica |
| Publisher | Universidad de Alicante |
| Source Sets | Universidad de Alicante |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/doctoralThesis |
| Rights | Licencia Creative Commons Reconocimiento-NoComercial-SinObraDerivada 4.0, info:eu-repo/semantics/openAccess |
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