Ind. Eng. Chem. Res. 2007, 46, 329-335
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A Simple Technique for the Measurement of H2 Sorption Capacities John M. Zielinski,* Peter McKeon, and Michael F. Kimak Air Products and Chemicals, Inc., 7201 Hamilton BouleVard, Allentown, PennsylVania 18195
An accurate (and low-cost) experimental technique has been developed to screen the effectiveness of an adsorbent in improving gas storage capacity within a pressurized vessel. Specifically, the capsule technique is shown to be effective in directly measuring the total H2 contained within a pressurized vessel and can be used to evaluate the amount of gas in the free space and adsorbed on the solid, that is, a sorption isotherm. The capsule technique was benchmarked by measuring isotherm data for CH4 on an activated carbon sample and was then subsequently evaluated for use with H2. The capsule data are in excellent agreement with the total storage capacities expected from calculations using equation of state information. In addition, H2 isotherm data from the sorption capsule are found to be within 1% of values obtained from a more sophisticated differential pressure adsorption unit (DPAU). Conditions for when the adsorbent aids or hinders storage are also discussed in terms of the 2010 DOE H2 storage targets. Introduction Many researchers are currently committed to developing enabling technologies for the successful introduction of hydrogen as an alternative fuel for both stationary and transportation applications. One of the key technical hurdles to widespread use of hydrogen fuel cells is its ability to be stored at high densities in a practical manner. To support the development of advanced hydrogen storage materials and processes, our laboratory has designed and built two instruments: (1) a differential pressure adsorption unit (DPAU), capable of accurately measuring H2 sorption isotherms up to ∼2000 psia with as little as 100 mg of sample,1,2 and (2) a sorption capsule, which is ideally suited for rapidly screening candidate adsorbents and which directly provides the total hydrogen loading in a vessel containing adsorbent. The latter is the subject of this Article. If solid adsorbents are placed within a gas cylinder, they occupy a portion of the volumetric space. Despite this loss of gas-phase volume, if the gas-solid interactions are sufficiently favorable, there is the potential to reversibly store more total molecules of adsorbate within this type of a system than within a conventional pressurized gas cylinder. Alternatively, one may be able to store the same amount of H2 in a container containing adsorbent at lower pressures than in a pressurized empty container, thereby yielding a storage system that is inherently safer (i.e., is at lower pressure) and that has less of a wall thickness requirement for the container. In turn, the reduction of wall thickness would lead to lower cost containers. The successful implementation of such an adsorbent-based storage system is centered on the development of adsorbent materials that have sufficient reversible H2 sorption characteristics. Many experimental techniques have been developed to measure gas-solid equilibrium data based on knowledge of the total moles of adsorbate contained within a system and an experimental assessment of the moles of the adsorbate residing in the gas phase by techniques such as IR spectroscopy,3 NMR spectroscopy,4 GC headspace analysis,5,6 and through simple use of a pressure transducer.7 The moles of gas adsorbed on the solid phase, therefore, can be inferred by difference of these two quantities. * To whom correspondence should be addressed. Tel.: (610) 4817975. Fax: (610) 481-6578. E-mail:
[email protected].
In this work, we present our experimental methodology for a sorption capsule technique and provide benchmarking data to examine its effectiveness in measuring the total loading of a gas within a pressurized vessel as well as the more difficult experiment of evaluating a sorption isotherm, using a pressure transducer to evaluate the amount of adsorbate in the gas phase. Experimental limitations will be discussed along with conditions under which the presence of the adsorbent is found to hinder the total storage capacity. Experimental Section Materials. GX-31 Supercarbon was obtained from Amoco. All of the gases used were obtained from Airgas. The hydrogen used was Research Grade (99.9995%), the helium was ultrapure He BIP PLUS (
