(321c) Hydrogen Storage by Adsorption Onto Different Activated Carbons
Hydrogen is considered as an ideal energy medium for replacing fossil fuels such as oil and coal. However, due to its low density, one of the main obstacles to the widespread use of hydrogen in the energy sector is an efficient storage technology. A promising technology is adsorption on activated carbons which could enable the storage of a high density of hydrogen at much lower pressures than compression and higher temperatures than liquefaction. The objective of this work was the investigation, in a 250 mL reactor, of the hydrogen uptake in various carbonaceous materials having different shapes (granule, powder, fiber and clothe). The temperature was fixed either at 77 or 298 K and the pressure was increased up to 3 MPa. The activated carbons were chosen to represent a large range in surface areas and microporous volumes. At 298K, due to the weak adsorption capacities of the tested materials, the storage capacity was found to be a linear function of the pressure. At 77K, as shown in figure below representing typical data obtained with granular activated carbon having a specific area of 2150 m2/g, the storage capacity was found to be the sum of two phenomena: (i) adsorption of hydrogen on solid surface; (ii) compression in the void space that varies linearly with the pressure: [βP]). The same behavior was observed for every activated carbon samples. The adsorption of hydrogen on solid surface is obtained for pressures lower than 1 MPa. Beyond this pressure value, the storage capacity is mainly due to the compression of the hydrogen in the void space inversely related to the bulk density of the material. Adsorption was modeled with the Langmuir model [αbP/(1+bP)] where α parameter represents the maximum storage capacity due to the adsorption properties of the activated carbons. This parameter is correlated to the specific surface area of the material (figure 2). Globally, the results indicate that activated carbons having a specific surface area higher than 2000 m2/g permit to reach the 6.5wt% gravimetric density target of DEO at 77K and moderate pressures. However, the low bulk density of these materials (lower than 300 kg/m3) represents a major limitation. The obtained volumetric densities was far from the DOE target (<20 kg/m3). The challenge now consists in compressing carbon materials to increase the bulk density without losing their adsorption properties.
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