Properties of adsorbed hydrogen and methane films on nanoporous solids
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The environmental impacts and costs of fossil fuels necessitate the development of clean, renewable fuel sources for vehicular applications. Hydrogen based systems, with water as their byproduct, have zero carbon emissions, which mitigates the negative effects of using conventional fossil fuels. Further, hydrogen can be produced from renewable energy sources, such as renewable electrolysis and biohydrogen. When produced from such methods, hydrogen is a renewable fuel. The main drawback of hydrogen as a fuel is its low energy density at ambient pressures and temperatures. Hydrogen has a mass energy density three times that of gasoline, but occupies more than 30 times the volume. Therefore, it is necessary to increase the volumetric energy density of hydrogen before it can be considered as a practical option. Conventional storage methods for hydrogen include compression and liquefaction. In order to yield a sufficient deliverable storage capacity, these methods require high pressures or cryogenic temperatures. Compressed gas systems require tanks with massive walls which reduce spatial and mass efficiency and thus, vehicle performance. These systems are geometrically constrained due to their high pressure, making them difficult to integrate into the vehicle. Due to these constraints, conventional storage methods are insufficient at increasing the energy density of hydrogen to compete with that of fossil fuels. To bridge this gap, it is necessary to develop a low-pressure, high-capacity storage technology in order to address the temperature, pressure, weight, and volume constraints present in the conventional storage methods. To achieve this, we investigate the storage capacity of nanoporous solids, which are capable of densifying a high volume of hydrogen on their surfaces through the process of adsorption. Several factors affect the adsorptive capacity of these materials, such as specific surface area, pore geometry, and the strength of the adsorption potential. The strength of the adsorption potential often cited as a figure of merit for the adsorptive capacity of new materials and is commonly estimated through the Clausius-Clapeyron relation between two adsorption isotherms. However, this method requires an assumption of the adsorbed film volume, which poses as the primary source of error. From supercritical hydrogen isotherms from 77 - 473 Kelvin, we propose a method to measure the volumes, densities, and thicknesses of the adsorbed film. This method will lead to more accurate isosteric heat calculations, which is an important factor to consider when designing storage tanks. Furthermore, we investigated the correlation between the isosteric heat of adsorption, surface chemistry, and pore size distribution with an adsorbed film. In most of the samples the saturated film density was approximately 100 g/L across a large range of temperatures. The specific volumes of the adsorbed film scaled with specific surface area and binding energies. The saturated, adsorbed film density approaches 100 g/L for all adsorbent types at 77 K. The saturated, adsorbed film thickness was between 3.1 – 3.2 Å for hydrogen on most sorbent materials. In the future, we intend to investigate changes in these parameters of the adsorbed film with increasing temperature as well as the affects that these changes may have on the estimated values of isosteric heat. Improved estimates of isosteric heats of adsorption will assist in optimizing the thermal properties of on-board storage tanks.
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