Analysis of Hydrogen Adsorption in Microporous Adsorbents at Room Temperature and High Pressures
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Analysis of Hydrogen Adsorption in Microporous Adsorbents at Room Temperature and High Pressures Tyler G. Voskuilen1 and Timothée L. Pourpoint2 1
School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907, USA
2
School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47907, USA
ABSTRACT An experimental study of hydrogen adsorption in a variety of high-surface area adsorbent materials has been conducted at room temperature and pressures up to 500 bar on high surface area activated carbons, zeolite templated carbons (ZTC), and metal organic frameworks (MOFs). For all materials, excess hydrogen adsorption isotherms were measured up to 500 bar and have been analyzed in terms of the BET surface area and pore size distribution. The materials were also evaluated for their increase in hydrogen storage density over compressed gas. It was determined that, due to the lower excess adsorption and skeletal densities for the microstructured materials, MOF-177 and ZTC have worse storage densities than compressed gas at most pressures, even when assuming a bed compaction factor of two, while the activated carbons offer marginal increases in storage density over the pressure range investigated. INTRODUCTION Adsorbent materials adsorb hydrogen gas via a physisorption mechanism. This storage mechanism relies on short-range, weak, Van der Waals interactions between the adsorbate and the substrate. The potential energy associated with the interaction of hydrogen with a specific surface site can be expressed either as a macroscopic thermodynamic quantity, the adsorption enthalpy, or as a chemical potential for which typical values are on the order of tens of meV. Because of such values, one expects to see significant hydrogen adsorption only at low temperatures (77 K and lower) or high pressures (300 bar and higher). To date, few hydrogen sorption isotherms have been reported for adsorbent materials at high pressures. Most experiments and conclusions have been drawn from low temperature measurements. Building upon recent experimental results [1-2], hydrogen adsorbent materials have been evaluated at room temperature and pressures up to 500 bar. The primary classes of adsorbent materials that have been studied in the literature are activated carbons and metal organic frameworks (MOFs). These materials are typically characterized at 77 K and relatively low pressures (less than 50 bar) where gravimetric hydrogen adsorption can reach its peak. However, recent studies at room temperature have reported gravimetric capacities of around 1.5 to 2 wt.% at pressures ranging from 300 to 700 bar for a variety of activated carbons, including AX-21, MSC30, KUA1, and KUA5. [1-3]. Modeling of the limits of physisorption on carbon in slit-like pores at room temperature has been reported by several authors. Alcaniz-Monge et al. calculated a theoretical maximum excess adsorption of 3.5 wt.% for microporous activated carbon at room temperature [4]. Züttel et al. calculated a theoretical maximum uptake of 3.3 wt.% f
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