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 anadsorbent in improving gas storage capacity within a pressurized vessel. Specifically, the capsule techniqueis shown to be effective in directly measuring the total H2 contained within a pressurized vessel and can beused 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 sampleand was then subsequently evaluated for use with H2. The capsule data are in excellent agreement with thetotal storage capacities expected from calculations using equation of state information. In addition, H2 isothermdata from the sorption capsule are found to be within 1% of values obtained from a more sophisticateddifferential pressure adsorption unit (DPAU). Conditions for when the adsorbent aids or hinders storage arealso discussed in terms of the 2010 DOE H2 storage targets.

Introduction

Many researchers are currently committed to developingenabling technologies for the successful introduction of hydro-gen as an alternative fuel for both stationary and transportationapplications. One of the key technical hurdles to widespreaduse of hydrogen fuel cells is its ability to be stored at highdensities in a practical manner. To support the development ofadvanced hydrogen storage materials and processes, our labora-tory has designed and built two instruments: (1) a differentialpressure adsorption unit (DPAU), capable of accurately measur-ing H2 sorption isotherms up to∼2000 psia with as little as100 mg of sample,1,2 and (2) a sorption capsule, which is ideallysuited for rapidly screening candidate adsorbents and whichdirectly provides the total hydrogen loading in a vesselcontaining adsorbent. The latter is the subject of this Article.

If solid adsorbents are placed within a gas cylinder, theyoccupy a portion of the volumetric space. Despite this loss ofgas-phase volume, if the gas-solid interactions are sufficientlyfavorable, there is the potential to reversibly store more totalmolecules of adsorbate within this type of a system than withina conventional pressurized gas cylinder. Alternatively, one maybe able to store the same amount of H2 in a container containingadsorbent at lower pressures than in a pressurized emptycontainer, thereby yielding a storage system that is inherentlysafer (i.e., is at lower pressure) and that has less of a wallthickness requirement for the container. In turn, the reductionof wall thickness would lead to lower cost containers. Thesuccessful implementation of such an adsorbent-based storagesystem is centered on the development of adsorbent materialsthat have sufficient reversible H2 sorption characteristics.

Many experimental techniques have been developed tomeasure gas-solid equilibrium data based on knowledge of thetotal moles of adsorbate contained within a system and anexperimental assessment of the moles of the adsorbate residingin the gas phase by techniques such as IR spectroscopy,3 NMRspectroscopy,4 GC headspace analysis,5,6 and through simpleuse of a pressure transducer.7 The moles of gas adsorbed onthe solid phase, therefore, can be inferred by difference of thesetwo quantities.

In this work, we present our experimental methodology fora sorption capsule technique and provide benchmarking datato examine its effectiveness in measuring the total loading of agas within a pressurized vessel as well as the more difficultexperiment of evaluating a sorption isotherm, using a pressuretransducer to evaluate the amount of adsorbate in the gas phase.Experimental limitations will be discussed along with conditionsunder which the presence of the adsorbent is found to hinderthe total storage capacity.

Experimental Section

Materials. GX-31 Supercarbon was obtained from Amoco.All of the gases used were obtained from Airgas. The hydrogenused was Research Grade (99.9995%), the helium was ultrapureHe BIP PLUS (<20 ppb water,<10 ppb O2), and the methanewas ultrahigh purity (99.99%). Hydrogen and methane werefurther purified by passing the gases through an active metalpoint-of-use purifier (Matheson TriGas, model MN-12).

Sample Preparation.Samples for adsorption testing wereactivated by degassing the materials during a series of temper-ature ramps and isothermal soaks while under a dynamicvacuum. Typically,∼1.5-2.0 g of sample was loaded in anactivation cell within an argon glove box and attached to anASAP 2010 (Micromeritics). The samples were then heated ata rate of 10°C/min to 100°C and held at that temperature for30 min. The temperature was subsequently increased to 300°C at the same ramp rate and held there until a vacuum readingof less than 10-4 Torr was achieved.

After the activation, samples were transferred back into theargon glove box and weighed into a high-pressure sorptioncapsule cell for gas sorption studies. The activated sample wasthen removed from the glove box, connected to the sorptioncapsule apparatus, and outgassed at ambient temperature toremove the argon sorbed while loading the sample into thecapsule cell. Final weighings to determine sample weight wereperformed using a five-place analytical balance. Both theanalytical balance and the sorption capsule reside in a nitrogen-purged Lexan box to avoid complications associated with weightchanges based on condensation of humidity on the externalsurface of the sorption capsule.

Apparatus. The basis of the sorption capsule technique liesin the ability to accurately quantify the amount of gas containedwithin a vessel by comparing the weight of the evacuated vessel

* To whom correspondence should be addressed. Tel.: (610) 481-7975. Fax: (610) 481-6578. E-mail: zielinjm@airproducts.com.

329Ind. Eng. Chem. Res.2007,46, 329-335

10.1021/ie060700y CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 12/06/2006

to the weight of the vessel when it contains pressurized gas.The difference in these two weights directly gives the amountof gas stored within the container at a known pressure,P, anda temperature,T.

The effect that a solid adsorbent has in either enhancing orhindering the amount of gas able to be stored can also be easilydiscerned by a similar procedure. First, a solid adsorbent isloaded into the sorption capsule and the system is evacuated.A comparison of the weight of the evacuated capsule, whichcontains solid, to the evacuated weight when empty yields thesample weight. When gas is introduced to the system containingadsorbent at the same pressure as in the empty cell experiment,P, and allowed to achieve equilibrium, gas resides both in thefree space and on the adsorbent. A comparison of the weightof the pressurized cell containing adsorbent with that of apressurized cell containing no solid reveals the enhancement(or hindrance) that the solid plays in terms of its gas storagecapability. As described, this experiment provides the totalamount of gas contained within a pressurized vessel containingsolid. It does not directly yield information regarding thepartitioning of the adsorbate between the solid phase and thegas phase.

It is interesting to note that neither the empty cell volume,the free space volume, nor the sample weight are needed toassess whether the presence of the solid improves or impedesgas sorption within the vessel by the method outlined. Thesequantities, however, are essential if a sorption isotherm isdesired. The explicit relationship between the extent of sorption,the volume occupied by the adsorbent, and the system free spaceis developed in the next section of this Article.

A schematic of the capsule apparatus is provided in Figure1. Two key components in the capsule experiment employedin our studies are the two sorption cells, which are hooked to agas handling system, and the analytical balance used forweighing them. The analytical balance employed was a SartoriusResearch Series MC1 balance, which has a weighing limit of210 g and a readability of 0.05 mg. The sorption cells wereconstructed out of T-316 stainless steel and were orbitallywelded to a 1/4” VCO male tube weld gland. The cells were∼7 cm3 in volume possessing an O.D. of 12.8 mm and an I.D.of 9.5 mm and were hydrostatic tested to 3200 psig. The shut-off valves on the cells, from Swagelock (SS-IRVCO4-SC11),are constructed out of stainless steel and have a maximumpressure rating of 6000 psia. The sorption cells were sized sothat the total weight of a cell, with a valve attached, was∼180g. This enables one to perform weighing measurements withthe five-place Sartorius balance even with an appreciable samplesize.

Performing weight measurements accurately to five decimalplaces can be challenging when one considers changes inlaboratory temperature and humidity levels. To circumvent theseproblems, the entire capsule apparatus is housed in a temper-ature-controlled Lexan box, which is nitrogen purged so thatthe relative humidity (RH) was maintained at a steady 10%.The box is also equipped with two rubber gloves so that thecells can be transferred from the gas manifold system to theanalytical balance without exposure to laboratory atmosphereor direct contact with human hands, which can contaminate theoutside of the sorption cells and alter their weight.

Theory

The number of moles of gas,n, which can be contained inan empty vessel of volume,VE, at pressure,P, and temperature,T, can be expressed as:

wherez is a compressibility factor that accounts for gas-phasenonidealities, andR is the universal gas constant.

If the vessel is filled with an adsorbent material and is againpressurized with an adsorbate gas, the total number of molesof gas within the container has contributions from both the gasphase as well as the solid phase such that:

whereVFS is the free space (gas-phase) volume,MS is the massof the solid adsorbent, andK is a partition coefficient, indicatingthe distribution of adsorbate between the gas phase and the solidphase, such that

Here,nADS is the Gibbsian excess moles adsorbed on the solid.In the case of a Langmuirian isotherm, the partition coefficientis given as:

The parametersm andb are termed the saturation capacity andadsorption coefficient, respectively. As indicated in eq 4, thepartition coefficient can be a function of pressure; however, inthe low-pressure limit,K ceases changing with pressure andequals the Henry’s constant,KH, which is also equal to theproductmb.

BecauseVFS represents the difference between the empty cellvolume and the volume occupied by the solid adsorbent, onecan write:

where FS is the skeletal density of the solid adsorbent.Substitution of eq 5 into eq 2 yields a second, and somewhatmore insightful, expression for the total adsorbate containedwithin a pressurized vessel:

Figure 1. Schematic of the sorption capsule unit. The sample cells, gasmanifold, pressure transducer, and analytical balance are contained in atemperature-regulated (25°C), nitrogen-purged Lexan box. The nitrogenflow rate was regulated so that the relative humidity was maintained at∼10%.

n )PVE

zRT(1)

n )PVFS

zRT+ KPMS (2)

nADS ) KPMS (3)

K ) mb1 + bP

(4)

VFS ) VE -MS

FS(5)

n )PVE

zRT+ MSP[K - 1

FSzRT] (6)

330 Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007

The first term on the right-hand side of eq 6 represents thenumber of moles of gas that would be contained in an emptypressurized cell. The second term, therefore, represents the effectthe solid has on the total loading within the cell. Because thebracketed term is a difference, it can be either positive ornegative. To increase the amount of gas within the containercontaining solid above that amount contained in the emptycontainer, the sorption capacity (as indicated by the partitioncoefficient) must be sufficiently large to overcome the free spacevolume loss due to the physical presence of the solid. Thus, ifK > 1/FSzRT, the solid increases the total gas capacity; otherwisethe space that the solid occupies contains more gas moleculeswhen the container is empty.

To illustrate this point further, in Figure 2 we presentcalculated results for the weight of hydrogen contained withina 42.2 L volume, that is, the size of a commercial BX H2

cylinder, at 21°C and as a function of pressure, for the caseswhen the cylinder is empty or contains 66 lb of AmocoSupercarbon GX-31. The weight of GX-31 is assumed to bethe maximum loading based on a skeletal density of∼2 g/cm3

and a 0.71 g/cm3 packing density.8 The H2 weight contributiondue to adsorption onto GX-31 was calculated from a dual-siteLangmuir (DSL) isotherm correlation of variable-temperaturesorption data measured for this system.1,2 The DSL model isgiven as:

The model parameters for the H2-GX31 system are providedin Table 1. The bracketed term provides the temperature andpressure dependence of the partition coefficient,K.

The dotted line in Figure 2 indicates the grams of H2

contained in the empty vessel as a function of pressure. As onemight expect, the higher is the pressure, the higher is the H2

loading. For comparison, the specifications for a fresh BX H2

cylinder are 6000 psia at 21°C, which corresponds to∼1140g (2.5 lb) H2. Because a BX cylinder weighs∼300 lb, thestorage of H2 is 0.8% by weight (2.5 lb/302.5 lb) and 27 g H2/Lby volume. For comparison, the DOE 2010 storage targets are6 wt % and 45 g H2/L.9

The solid line, indicated as “a”, represents the total H2 loadingwithin the 42.2 L vessel when it contains 66 lb of GX31.Notably, at low pressures the presence of the adsorbent aidsthe storage capacity within the vessel, while at pressures above4700 psia the GX31 is expected to hinder hydrogen storage.The maximum improvement in storage capacity at a fixedpressure occurs at 1700 psia where the enhancement is∼71 g(0.16 lb). This is a minimal improvement over simply pressur-izing the empty container and is clearly not sufficient enhance-ment for a practical device.

An alternate way of viewing the sorption enhancement is toexamine the largest pressure savings one can obtain for aparticular gas loading. At 2000 psia, the total H2 capacity inthe empty cell is∼448 g. With GX31 loaded into the storagecell, the same total H2 capacity is achieved at 1665 psia, whichis a savings of 335 psia. Being able to store comparable H2

loadings at lower pressures implies savings in the wall thickness(and, consequently, weight) of the container used for storage.In addition, storage of a lower pressure gas is inherently saferthan storage of a high-pressure gas.

Also included in Figure 2 is a solid line (“b”), which indicatesthe total expected storage capacity of the system if the adsorbentisotherm were tripled. Clearly, this increase in the H2 isothermhas a profound impact on the extra amount of H2 that can bestored at a fixed pressure and the pressure savings at a fixedloading. For this scenario, the energy density at 6000 psia is 45g H2/L, which meets the DOE 2010 volumetric storage target.This type of increase in the sorption isotherm is consideredachievable by many and is precisely the reason that materialscientists are striving to develop improved materials for H2

storage.If the same BX cylinder were used as the storage container,

however, the gravimetric capacity would still only be 1.1 wt%, because the 66 lb of adsorbent must be added to the 300 lbweight of the cylinder to fairly consider the system storagecapability. Clearly, developing storage devices that are light-weight and high-strength is critical to the H2 storage effort.

Results and Discussion

Total Capacity Measurements (Empty Cell).The sorptioncapsule is ideally suited for performing measurements of thetotal loading of a gas within a pressurized container. Initially,an evacuated cell is attached to the gas handling system (GHS)and evacuated until a pressure of 10-2 Torr is reached. Theshut-off valve on the evacuated cell is then closed, and the cellis removed from the GHS and is weighed on the analyticalbalance. This procedure is used whether the sample cell is emptyor contains an adsorbent material. The cell is then reattachedto the gas handling system, the valve is opened, and the systemis pressurized with the gas of interest. Once equilibrium isestablished, as indicated by the pressure transducer on the GHS,the valve is once again shut off, and the cell is detached fromthe GHS and weighed on the analytical balance. This proceduredirectly measures the weight of the gas introduced into thesample chamber.

To test our experimental protocol for weighing gas, thevolume of the manifold,Vm (indicated in Figure 1), wasevaluated by performing pressure expansions using a calibrated

Figure 2. Variable pressure hydrogen loading calculations for a BX H2

cylinder volume (42.2 L) at 21°C. The dotted line indicates the H2 containedin the system when there is no adsorbent in the cell (eq 1). The solid line(a) illustrates the H2 capacity when 66 lb of Amoco Supercarbon GX31 isloaded into the cell (eq 2). The solid line (b) represents the total H2 loadingin the system if the adsorbent capacity were tripled (eq 2). The calculationshave assumed that the skeletal density,FS, is 2.0 g/cm3 and a packing densityof 0.71 g/cm3.

Table 1. DSL Parameters for the System H2-GX31

m1 4.486 mmol/gm2 4.733 mmol/gbo 4.198× 10-6 psia-1

Qb 2584.9 cal/moldo 2.459× 10-5 psia-1

Qd 1550.9 cal/mol

nADS ) [ m1b0 exp(Qb

RT)1 + b0 exp(Qb

RT)P+

m2d0 exp(Qd

RT)1 + d0 exp(Qd

RT)P]PMS (7)

Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007331

standard volume. Our manifold volume was determined to be9.6 cm3. The volumes of our empty sample cells,VE, weresubsequently evaluated by performing pressure expansions fromthe manifold volume. The two sample cells employed in thisinvestigation had volumes of 6.8 and 7.1 cm3, respectively.

The two empty sample cells were then pressurized withhelium and hydrogen, respectively, at isothermal conditions.Because the system pressure, temperature, and volume withineach sample cell were known, the weight of gas within thesample cell could be calculated from an equation of state(EOS).10-12 In Figure 3, we provide a comparison between theweight predicted by the equation of state and our experimentaldata. The data are found to agree with the EOS to within∼1%error.

Total Capacity and Isotherm Measurements (AdsorbentPresent). In traditional volumetric sorption studies, heliumexpansions are performed to assess the free space within asample cell. Because the manifold volume and individual cellvolumes of the sorption capsule unit have been calibrated, thistraditional method can be applied to assess the free space oncean adsorbent is loaded into the system. There are two alternativesby which the free space volume can be determined in thesorption capsule experiment.

In the first method, the cell can be pressurized with heliumto a known pressure at a known temperature, and the cell canbe weighed to gauge the mass of helium within the cell. Thishelium is presumed to reside exclusively in the gas phase,because helium is generally considered a non-adsorbing gas.Because pressure and temperature are known, the helium densitycan be determined from an equation of state. Consequently, thecell free space volume can be determined by dividing the massof helium measured by the helium gas density.

The second method requires knowledge of the skeletal densityof the adsorbent,FS. If a known mass of solid,MS, is loaded inthe sample cell, the free space volume can be calculated directlyfrom eq 5. All three of these techniques have been employedto estimate the free space volume using quartz as the “adsorbent”and have been found to agree to within∼1%.

To benchmark the sorption capsule technique using a realadsorbent, CH4 and H2 adsorption experiments at 25°C wereperformed using Amoco Supercarbon GX31 as the solidadsorbent. For the CH4 experiments, 1.6235 g of GX31 wasloaded into the sample chamber, the free space volume wasanalyzed by helium expansions, and the system was subse-quently pressurized with methane and allowed to equilibrate.The total amount of methane contained in the system wasdetermined by closing the valve on the sample chamber and

weighing the entire sorption capsule. The capsule could thenbe reattached to the system and exposed to methane at a newpressure. In this way, the total loading of methane could bedetermined as a function of pressure. The free space volumedoes not need to be known to determine the total amount ofmethane contained within the system. This result is providedin Figure 4.

AlthoughVFS is not needed to determine the total amount ofmethane in the system, it is needed if a sorption isotherm isdesired. Included in Figure 4 is the calculated amount ofmethane in the gas phase of the sorption capsule using anequation of state and knowledge ofP, T, andVFS. By dividingthe mass of methane on the solid adsorbent by the mass of solid,one can determine the sorption capacity (mmol/g), that is,surface excess at the correspondingP andT.

The methane isotherm resulting from the measurementsshown in Figure 4 is provided in Figure 5 along with a literatureisotherm from a very sensitive differential pressure adsorptionunit (DPAU).1 Five repeat points were measured at 250 psia toillustrate the reproducibility of the sorption capsule method. Theagreement between the two experimental data sets is excellentand highlights the capability of the relatively inexpensivesorption capsule unit.

One point that needs to be highlighted is the magnitude ofthe y-axis in Figure 4. The mass of methane weighed in theseexperiments is on the order of 100 mg, which is easilyaccomplished with a five-place analytical balance that has areadability of 0.05 mg. For the methane/GX31 system, onaverage∼60% of the total methane mass within the system is

Figure 3. Comparison of the experimentally measured total mass of helium(O) and hydrogen (b) contained in pressurized cells at 25°C to that expectedfrom an equation of state analysis based on knowledge of the pressure,temperature, and system volume.

Figure 4. Comparison of the experimentally measured total mass ofmethane (O) at 25 °C in a system containing 1.6235 g of AmocoSupercarbon GX31. The solid line indicates the contribution of gas-phasemethane to the total methane weight in the system.

Figure 5. Isotherm data at 25°C for methane adsorption on AmocoSupercarbon GX31 measured by the sorption capsule technique (b) andby a differential pressure adsorption unit (O). Five repeat runs wereperformed at 250 psia to illustrate the reproducibility of the sorption capsulemethod.

332 Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007

found to reside on the solid phase. In this case, it is relativelysimple to evaluate both the isotherm data and the total methaneloading.

Because methane is 8 times heavier than hydrogen, conduct-ing weight measurements with methane is appreciably easierthan with hydrogen. To assess the utility of the capsule techniqueto perform hydrogen measurements, an experimental investiga-tion comparable to that of CH4 was performed using hydrogenas the adsorbate and employing a fresh 1.7726 g sample ofGX31. The total H2 loading in the system at 25°C was measuredby the procedure outlined previously, and the results areprovided in Figure 6. For comparison, the amount of hydrogenresiding in the gas phase of the system is also provided as afunction of pressure. In this experiment,∼80% of the total gasweight within the system is due to gas-phase hydrogen and themagnitude of the total H2 weight contained within the systemis approximately an order of magnitude lower than that for CH4.

Subtracting out the gas-phase contribution from the totalhydrogen loading data in Figure 6 enables one to establish ahydrogen isotherm. In Figure 7, isotherm data from the capsuleunit based on the experimental data shown in Figure 6 arecompared to literature data for the H2-GX31 system measuredby a traditional gravimetric sorption technique2,13 and a dif-ferential pressure adsorption unit (DPAU).1 In addition, dataare included for a second, completely independent, capsuleinvestigation using a 1.6787 g sample. The agreement betweenthe capsule data and the literature data is outstanding andhighlights both the accuracy and the repeatability of ourexperimental protocols.

For completeness, we note that for each of the data setspresented in Figure 7, helium adsorption effects were assumednegligible. In the volumetric measurements, this assumption isintroduced in the free space analysis, while in the gravimetricmethod this occurs in the bouyancy analysis. For a truly rigorousisotherm analysis, the effects of helium sorption should beaddressed. Procedures for this have been outlined previously.14

The agreement between our Capsule data and the gravimetricand volumetric results presented, however, will not change dueto compensations due to helium sorption effects, because all ofthe data will be equally shifted by this correction factor.

To gauge the limitation of the capsule technique in itsapplication to screening hydrogen storage materials, we considerthe weighing capabilities of the analytical balance employed.The Sartorius analytical balance used in these experiments hasa readability of 0.05 mg, meaning that the fifth decimal placeis either 0 or 5. To best capitalize on the measurement capabilityof this unit, a calibration standard was maintained within thesorption capsule chamber and was weighed prior to eachexperimental measurement to within 0.05 mg. Because thiscalibration standard could be weighed accurately, and allexperiments were performed within a nitrogen-purged Lexanbox with a constant relative humidity, the total hydrogen loadingwithin the cell could be measured to(0.05 mg.

Measuring a hydrogen sorption isotherm is appreciably morechallenging than measuring the total hydrogen loading becauseit requires distinguishing how the hydrogen contained in thesystem is partitioned between the gas phase and the solid phase.To investigate the limits of the capsule technique in itsapplication to measuring hydrogen isotherms, we draw attentionto the data in Figure 6, particularly the difference between thetotal hydrogen loading data (represented by circles) and the gas-phase contribution (represented by the solid line). As pressuredecreases, the total gas-phase contribution, as a percentage ofthe total hydrogen content, increases. Thus, the lower is thepressure, the more difficult it is to measure a point on theisotherm accurately.

There are several predominant considerations to measuringH2 isotherms with the capsule technique. These include: (1)the sorption capacity, (2) sample size, that is, adsorbent mass,and (3) the bulk density. The bulk density limits the amount ofadsorbent that can be physically loaded into the sample cell.Maximizing the amount of solid employed or, conversely,minimizing the free space volume is clearly a main objective.For the GX31 experiments, the packing density achieved was0.24 g/cm3, which is relatively low. This value could have beenincreased by grinding the material but was not attempted in thisinvestigation.

On the basis of the experiments with GX31, we estimate that2 mg of H2 uptake by the adsorbent was required, while using1.6787 g of this material, to effectively characterize the H2

partitioning between the adsorbent and gas phases. To illustratethis experimental sensitivity limit, in Figure 8 we haveconstructed a plot of sorption capacity versus sample mass.Figure 8 indicates that an adsorbent mass smaller than 1.6787g can be used if a new material being examined has a sorptioncapacity higher than that of GX31. Alternatively, capacity valueslower than 1.19 mg/g ()2 mg/1.6787 g) can be measured, iflarger sample sizes are employed.

Included in Figure 8 are two isotherm data points measuredat 255 and 1484 psia, respectively. These data points revealthat isotherm measurements at higher pressures are far removedfrom the sensitivity limit of the capsule technique, while at lowpressures the sensitivity limit is approached. The total hydrogen

Figure 6. Comparison of the experimentally measured total mass ofhydrogen (O) at 25 °C in a system containing 1.7726 g of AmocoSupercarbon GX31. The solid line indicates the contribution of gas-phasehydrogen to the total hydrogen weight in the system.

Figure 7. Comparison of 25°C hydrogen adsorption isotherms on AmocoSupercarbon (GX-31) measured on the sorption capsule unit with 1.7726 g(4) and 1.6787 g samples (2), on the DPAU with a 0.580 g sample (O),and on a Rubotherm gravimetric sorption balance with a 1.014 g sample(+).

Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007333

contained within the system, however, can be easily measuredat either pressure.

Last, as a word of caution, care should be taken if subambienttemperature measurements are undertaken with the capsule unitfrom the standpoint of both condensation on the cell, which issimply an operational problem, and overpressurization uponwarming the cell back to room temperature, which is a moreimportant safety consideration. For example, if a cell werepressurized to 100 atm at 77 K and subsequently warmed toroom temperature, the pressure within the cell would climb to∼390 atm. This estimate does not account for any contributionfrom gas desorbing from a contained adsorbent, which wouldtend to increase the pressure further. Because some piping andvalves may not be able to withstand this elevated pressure, wedo not recommend performing any measurements with thecapsule technique at subambient temperatures.

Conclusions

The sorption capsule technique, an accurate and relativelyinexpensive experimental method, has been successfully appliedto measure the total gas content within a storage volumecontaining adsorbent as a function of pressure and at a fixedtemperature (25°C). Both hydrogen and methane were em-ployed as the adsorbates. A comparison of these experimentaldata to the total adsorbate mass anticipated from equation ofstate calculations, based on the empty cell volume, has provento be an effective means of evaluating whether the presence ofthe solid aids gas storage and is a reasonable candidate as astorage material. Because a capsule assembly system can bedesigned to have multiple capsules operating simultaneously,this technique enables one to perform a high-throughputevaluation of candidate storage materials.

The gas-phase volume loss due to the physical volume ofthe sorbent and extent of adsorption based on the partitioncoefficient of the gas-solid pair have been shown to be twocompeting effects in determining whether a solid promotes orhinders gas storage within a container (see eq 6). If themagnitude ofK is greater than 1/FSzRT, then the presence ofthe solid increases the total gas capacity within the system;otherwise the space that the solid occupies contains more gasmolecules when the container is empty. Calculations based ona typical BX cylinder reveal that achieving the 2010 DOEvolumetric targets for H2 storage will be easier than attainingthe gravimetric goals.

Gas-solid isotherm data were successfully measured bydistinguishing how the total gas loading in the storage systemis divided between the gas phase and the adsorbent phase.Methane and hydrogen isotherm data were measured at 25°Cwith GX31. The hydrogen data were benchmarked againstgravimetric data measured with a Rubotherm balance13 andvolumetric data measured with a differential pressure adsorptionunit.1,2 An empirical error analysis based on our GX31 datahas revealed that∼2 mg of uptake on a 1.6787 g sample is theapproximate limiting weight uptake required to measure asorption isotherm accurately at a bulk packing density of 0.24g/cm3. For materials exhibiting sorption capacities less thanGX31, measurements can be made if a larger sample size isused. Likewise, smaller sample sizes can be employed if thesorption capacity is greater than that of GX31. Last, use ofsmaller sample weights also could be achieved by minimizingthe system free space volume, that is, maximizing bulk packingdensity, and creating smaller sample cells.

Acknowledgment

We are pleased to acknowledge funding for this workprovided by the U.S. Department of Energy’s Office of EnergyEfficiency and Renewable Energy within the Center of Excel-lence on Carbon-based Hydrogen Storage Materials, AwardNumber DE-FC36-05GO15074. In addition, we thank our APCIcolleagues, Drs. Charles Coe, Guido Pez, and Alan Cooper, fortheir support and for stimulating technical discussions. Last, wethank F. Dreisbach of Rubotherm Pra¨zisionsmesstechnik GmbHfor providing an H2 isotherm measured on a magnetic suspen-sion balance.

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(10) McCarty, R. D.; Hord, J.; Roder, H. M. Selected properties ofHydrogen (Engineering Design Data). NBS Monograph 168; U.S. Depart-ment of Commerce, February, 1981.

(11) Arp, V. D.; McCarty, R. D. Thermophysical Properties of Helium-4from 0.8 to 1500 K with Pressures to 2000 Mpa. NIST Technical Note1334; U.S. Department of Commerce, November, 1989.

Figure 8. Sensitivity limit for measuring isotherms with the capsuletechnique based on sorption data on GX31. The solid curve is based on alimiting capacity value of 1.19 mg/g on a 1.6787 g sample at a packingdensity of 0.24 g/cm3. The curve indicates that accurate isotherm data canbe measured with samples exhibiting a lower capacity, if larger samplesizes are used, and with smaller sample masses, if the adsorbent capacityis higher.

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(12) Younglove, B. A.; Ely, J. F. Thermophysical Properties of Fluids,II. Methane, Ethane, Propane, Isobutane, and Normal Butane.J. Phys. Chem.Ref. Data1987, 15, 577.

(13) Dreisbach, F. Measurement of H2 Adsorption on Activated Carbon.Rubotherm Pra¨zisionsmesstechnik GmbH, Bochum, Germany, 2004; privatecommunication.

(14) Sircar, S. Measurement of Gibbsian Surface Excess.AIChE J.2001,47, 1169-1175.

ReceiVed for reView June 2, 2006ReVised manuscript receiVed September 15, 2006

AcceptedOctober 11, 2006

IE060700Y

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