القسم: قسم العلوم الطبيعية
الجهة البحثية: الجامعة اللبنانية الامريكية
عنوان البحث المنشو ر: Local Pressure of Supercritical Adsorbed Hydrogen in Nanopores
سنة النشر: 2018
ملخص البحث المنشور:
Hydrogen transport and storage traditionally rely on physical-based methods such as compression, liquefaction, and cryo-compression. However, a promising alternative for vehicular applications is hydrogen storage in materials through physisorption or chemisorption. In chemisorption, metal hydrides absorb hydrogen through various bonds, but this process is non-reversible and necessitates recycling. On the other hand, physisorption involves weak van der Waals forces attracting hydrogen to sorbents like activated carbon, zeolites, and metal-organic frameworks. While these materials show potential for reversible hydrogen storage, their capacities at ambient temperatures are constrained by relatively low binding energies, typically in the range of 4–8 kJ/mol. The US Department of Energy (DOE) has set ambitious storage targets, but as of now, no material meets the technical system target. It's crucial to distinguish between material and system storage capacities, where engineering components' mass and volume contribute to falling short of DOE targets. To enhance hydrogen storage capacities and meet DOE goals, adsorbent materials must exhibit a high specific surface area and an optimal binding energy of 15 kJ/mol. Our previous research focused on modifying surface chemistry and pore size distribution to optimize binding energy. Determining hydrogen's binding energy is challenging, involving conversion from experimental excess adsorption to absolute adsorption using the adsorbed film volume. In this work, we present a method utilizing a single supercritical hydrogen adsorption isotherm based on the Ono-Kondo model for slit-shaped pores. This approach extracts both the average binding energy of hydrogen on activated carbon at low coverage (9.2 kJ/mol) and the overall average binding energy (4.4 kJ/mol) using the lattice gas model and chi-square minimization algorithm. Additionally, the method provides information on the film density at maximum capacity (0.10 g/mL) and local pore pressure (1500 bar) without requiring conversion between gravimetric excess adsorption and absolute adsorption. While the pressure in the gas phase is between 0 and 200 bar, hydrogen molecules are subject to a pressure of 1500 bar in the adsorbed phase. This pressure represents the combined average normal and tangential pressure on the carbon surface. Phases confined in nanospaces exhibit a behavior that is different from that of the bulk phase. Such differences arise from reduced dimensionality and from the van der Waals interaction of the adsorbate molecules with the walls of the porous materials. Our research offers a valuable method for determining key properties of adsorbed hydrogen films, addressing challenges in accurately assessing binding energy, film density, and local pressure.
رابط البحث المنشور :
https://www.mdpi.com/1996-1944/11/11/2235/htm