One of the greatest challenges to accurate measurements of the interaction of hydrogen with storage materials it the fact that hydrogen is the smallest and lightest of all gas molecules. Measurement techniques that are considered standard in the investigation of other gases are significantly more challenging for hydrogen. Both volumetric and gravimetric measurements are common for gas uptake measurements such as BET where gases such as nitrogen and argon are measured at pressure generally below 1 bar.

With gravimetric measurements an important consideration is that nitrogen gas is 14 times heavier and Argon gas 20 times heavier than gaseous hydrogen. In other words, 14 to 20 times as much hydrogen must be adsorbed to a material to give the same weight change as the same molar adsorption of nitrogen or argon respectively. Or conversely, at least an order of magnitude higher sensitivity is required to accurately measure hydrogen uptake as most other gases. This is not the case for volumetric measurements where pressure change is directly proportional to moles of gas uptake independent of what the gas is.

Volumetric measurements are subject to errors caused by gas leaks or essentially a loss in the total accounting of all hydrogen present (mass balance). Hydrogen being the small molecule that it is exacerbates the absolute requirement for measurements to be made with as low a leak rate as possible. Clearly all seals between gas handling components should be leak free (generally metal-to-metal seals). But in addition, hydrogen will diffuse through all materials at some rate which is dependent on temperature and pressure. Teflon is an example of a materials that is particularly permeable to hydrogen, even some steels are not ideal for hydrogen applications (316L steel is most commonly used in instruments for hydrogen sorption measurements) and all steels have limitations with respect to high pressures and temperatures of hydrogen.

Impurities in either the hydrogen gas supply or the sample materials present some of the greatest issues for hydrogen capacity measurements. This is because most materials that are highly reactive or interactive with hydrogen will be the same with impurities (oxygen, water, nitrogen….). This is particularly true of high-surface-area materials. An important consideration is that the errors caused by impurities present themselves in very different ways depending on the measurement technique. Impurities in the gas that react with the sample generally show a large increase in mass in gravimetric measurements that is indistinguishable from hydrogen uptake and may indicate high hydrogen weight percent capacity. The opposite is true for volumetric measurements where impurities in the gas may deactivate the sample, reducing the apparent hydrogen uptake.

Impurities in the sample that cause un-intended hydrogen uptake (hydride-forming additives, water-forming oxides,…) would be expected to give invalid results in both gravimetric and volumetric techniques if not properly accounted for. Regardless, the formation of extraneous hydrogen compounds are often not reversible, therefore, a significant loss in reversible capacity (of reversible materials) with cycling would indicate the possibility if impurities in the sample.

Uncertainty in the skeletal density (gas displacement volume) of new materials creates errors in both gravimetric and volumetric measurements. In gravimetric measurements the error is directly proportional to the required buoyancy correction. In volumetric measurements error in the density of the sample is proportional to the total calibrated free gas volume. An example of these impact of uncertainty in a sample’s skeletal density is shown in below figure for 77K hydrogen absorption measurement on a 23 mg sample of activated carbon using a volumetric instrument with a total free gas volume of 1.78 ml. Given an underestimation of the sample skeletal density of 0.85 g/ml vs. actual 1.7 m/ml the resulting error in the measurements due to error in the free gas volume of the instrument (volumetric) or in the buoyancy correction for the sample are shown in the figure.

Error in capacity by gravimetric and volumetric measurements due to a 50% error in estimating the skeletal density of a sample (50% lower than actual). Sample 23 mg activated carbon at 77K.

These corrections are only for errors in sample volume and do not include temperature gradient effects on both buoyancy corrections (gravimetric) or apparent volume temperature corrections (volumetric).

It should also be noted that, in contrast to gravimetric measurements, the zero point calibrations of isotherm measurements is not effected by thermomolecular flow issues (buoyancy corrections) at low pressures.

Reference List:

• Blach, T.P., and McGray, E., “Sieverts apparatus and methodology for accurate determination of hydrogen uptake by light-atom hosts”, J. Alloys and Compounds, 446-447 (2007) p. 692-697.

• Massen, C.H., Robens, E., Poulis, J.A., and Gast, T., “Disturbances in weighing – Part I A survey of work presented at the preceding VMT conferences”, Thermochim. Acta, 82 (1984) p. 43.

• Ginoux, L. and Bonnctain, L., “Some problems about gas adsorption isotherm measurements by automated procedures in manometric devices”, In: F. Rodriguez-Reinoso, Rouquerol, J., Sing, K.S.W., and Unger K.K., (Eds.): “Characterization of Porous Solids Il” Elsevier, Amsterdam, (1991) p. 189.

• Fuller, E.L., Poulis, J., Czanderna, A.W., and Robens, E., “Volumetric and gravimetric methods of determining monolayer capacities”, Thermochim. Acta, 29 (1979) p. 315.