Analysis of optimal conditions for adsorptive hydrogen storage in microporous solids

Abstract

There is much current interest in the storage of hydrogen in porous materials for mobile energy applications. Despite significant hydrogen storage capacities having been observed recently for some synthesised materials, the identification of optimal operating conditions (pressure and temperature) is perhaps an even more important consideration from an engineering and applied science perspective. There will be pressure and temperature limits for effective use of an adsorptive storage system, because the adsorbent will always displace a volume in the storage container, and so at very high pressures the amount of hydrogen stored at a given temperature will be greater for a container with no adsorbent. In order for an adsorbent to be used there has to be some gain in the amount of the hydrogen stored to compensate for the cost and mass of the solid. We present a methodology by which the pressure and temperature ranges where it is advantageous to use adsorptive storage can be easily identified and the real gain of using such systems in terms of the absolute amount of hydrogen stored can be quantified. Using a well-characterised commercial activated carbon as an example system, we modelled high pressure hydrogen sorption isotherms and identified the operating conditions for which there is a significant increase in storage capacity from using an adsorbent as opposed to storage in the same volume via compression of hydrogen at the same temperature. A novel comparison of the density enhancement in the micropores with respect to the bulk hydrogen gas, as well as the influence of incorporating different amounts of adsorbent into a high pressure storage container is also presented.