Hydrogen adsorption on microporous materials at ambient temperatures and pressures up to 50 MPa

High surface area microporous adsorbents are often proposed as potential hydrogen storage materials, although typically at 77 K and less than 5 MPa. In this study, we focus on conditions more suitable for automotive applications by investigating the storage capacities of microporous materials at 298 K and at pressures up to 50 MPa. In an effort to derive trends within and across material classes, we examined a wide range of materials with varying microstructures including the activated carbons AX-21, KUA-5, and MSC-30; a zeolite templated carbon; a hypercrosslinked polymer; and the Metal Organic Frameworks MOF-177, IRMOF-20, MIL-53, ZIF-8, and Cu-3(btc)(2). The peak excess adsorption of these materials ranged from 0.8-1.8 wt.%, although many did not reach their maximum capacity even at high pressures. However, the total volumetric storage gains over compressed hydrogen gas were quite low and, in many cases, negative. In addressing ambient temperature adsorption at significantly higher pressures than previously reported, our data confirms and extends the range of validity of several existing DFT calculations. Furthermore, our data suggest that, for both activated carbons and MOFs, factors other than specific surface area govern ambient temperature adsorption capacity. Contrary to some reports, the high fractions of sub-nanometer pores in some of the investigated MOFs did not appear to enhance the excess adsorption even at high pressures. For on-board applications with ambient temperature storage, significant enhancements to the attractive force at the materials' surface are required, beyond merely increasing specific surface area, or for MOFs, tuning of pore sizes.