Available Hydrogen Adsorption Facilities (in Lueking Laboratories)
by Prof. A.D. Lueking

Lueking has previously developed a high-pressure volumetric adsorption apparatus designed to minimize potential artifacts associated with volumetric measurements, including: leakage, inaccuracy in pressure readings, differences in the relative volumes of the sample and the reference chambers, non-ideal gas corrections, and temperature effects associated with adsorption amplified at high pressure.¹ Lueking's previous design relied primarily on desorption measurements, but a capability for varying the system volumes allowed for increased accuracy in both adsorption and desorption measurements. In collaboration with Prof. Peter Eklund and Dr. David Narehood of the Carbon Center of Excellence, an updated design which incorporates a differential pressure transducer, controlled temperature chambers, and automated values will be installed by October 2005. Two units are schedule to be built, with Eklund’s funded by DOE and Lueking’s funded through Penn State start-up funds. These updates further increase the accuracy and precision of high-pressure volumetric measurements: A differential pressure transducer monitors the pressure difference arising due to adsorption between identical sample and ballast volumes, thus further minimizing problems of accuracy of pressure readings and non-ideal gas corrections. Use of a Sievert's apparatus with a differential pressure transducer was reported to give reproducible hydrogen uptakes of +/- 0.1% for 100 mg of sorbent.²

The volumetric system will be used in conjunction with an existing high-pressure gravimetric units (IGA-003) capable of hydrogen storage measurements up to 20 bar and temperatures between 77 and 773 K, or alternatively at 1 bar at temperatures up to 1273 K. The IGA will be used for initial screening tests of materials, when precise temperature control and activation are needed, and for independent verification of the volumetric measurements. Temperature and pressure corrections in the sample mass are made through the use of standardized helium densitometry measurements. The gravimetric measurements have the advantage that they allow precise temperature control and cycling, smaller sample size, and allow for rapid screening of candidate storage materials. Materials showing promise at 20 bar will be tested on the volumetric system at pressures up to 100 bar. Currently, we have performed both sensitivity tests and calibration checks of the accuracy of the IGA measurements. Using error propagation software, the precision of the IGA measurements for 100 mg at 20 bar is estimated to be + 0.01 wt%. This corresponds to 0.3% relative error for a sample with 5% uptake, 1.4% relative error for a sample with 1% uptake, and 4.7% relative error for a sample with 0.03% uptake. This error analysis takes into account uncertainties in temperature, pressure, initial mass, and sample density (and therefore buoyancy corrections). Standards with both palladium and magnesium hydrides have shown the IGA measurements are accurate with the IGA capable of reproducing standard PdH_0.6 and MgH₂ isotherms. Further, we have incorporated a mass spectrometer (led by Dr. D. Fonseca) to the IGA outlet which allows us to confirm that the gravimetric measurements are due to hydrogen uptake rather than the possible contaminants.

Rates and Energetics of Adsorption and Desorption. A key issue in the development of hydrogen storage materials lies in the adsorption/desorption kinetics. The adsorption rate will be tracked for both the gravimetric and volumetric measurements. The automated gravimetric analysis on the IGA allows for rapid screening of rates at multiple pressures and temperatures for both adsorption and desorption. Typically, static adsorption measurements on the IGA allow pressure and temperature control within +/- 4 mbar and +/- 1 °C. Adsorption energies will be quantified by calculating the isosteric heat of adsorption with multiple temperature adsorption isotherms and the desorption activation energy will be estimated with TPD.

REFERENCES
1.  Lueking, A.D.; Yang, R.T. Hydrogen Storage in Carbon Nanotubes: Residual Metal Content and Pretreatment Temperature. AIChE Journal, 49, 1556, (2003).

2.  Browning, D. J.; Gerrard, M. L.; Lakeman, J. B.; Mellor, I. M.; Mortimer, R. J.; Turpin, M. C. Nano Lett. 2002, 2, 201.