Direct Line-of-site Gas Desorption Study of LiBH 4 in Nanoporous Carbons: The Size Effect

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1 Direct Line-of-site Gas Desorption Study of LiBH 4 in Nanoporous Carbons: The Size Effect David Peaslee Center for Nanoscience and Department of Physics and Astronomy University of Missouri St. Louis Advised by E.H. Majzoub Thermodynamic and kinetic properties of metal hydrides may differ between bulk and nano-sized particles. By adjusting the size of these particles through confinement in nano-porous materials, properties including the hydrogen sorption rate and possibly decomposition temperature can be fine-tuned to meet engineering requirements for real-world systems. Studies of nano-structured carbon materials infiltrated with LiBH 4 reveal differences in the decomposition pathway that depend on the carbon nano-pore size and the surface chemistry. Results of gas desorption using a direct line-of-site residual gas analyzer mass spectrometer (RGA-MS) system to characterize gas desorption species, decomposition temperatures, and differences in reaction pathways, for a variety of LiBH 4 /carbon systems are presented here. This project is partially funded through the NASA/Missouri Space Grant Consortium. Additional funding was provided through the US Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE) through the hydrogen program, and the UMSL-Research Board. Introduction Metal hydrides are currently being studied to provide hydrogen for use in fuel cells for transportation applications. Hydrogen can be stored in chemical compounds at higher density and lower volume than liquid H 2 or compressed gas. Important characteristics of these compounds include the decomposition temperature and rates of release of the decomposition gases. Many metal hydrides with useful volumetric and gravimetric capacities have high decomposition temperatures, but when placed in nano-sized frameworks or templates the temperature may be reduced, owing to size effects at the nano scale or surface interaction effects with the framework. The goal of the project is to characterize desorption of the hydrogen (including byproduct gasses) and the decomposition of the metal hydrides. For example, bulk LiBH 4 contains about 18 weight % hydrogen. The ideal decomposition is illustrated in reaction 1: This substance would be an excellent hydrogen energy carrier except for the fact that it has a high decomposition temperature of around 380 C. The decomposition enthalpy for this reaction is ΔH = 68.9 kj/mol H 2. 1 The decomposition of LiBH 4 should undergo a low temperature decomposition following reaction 2 below. This decomposition has a lower hydrogen weight % of (2) (1)

2 Unfortunately, in the decomposition of LiBH 4, diborane gas has been observed 2. The diborane emission may be due to a pathway such as reaction 3 below. The release of this byproduct gas creates the problem of toxic gas emission as well as the release of active components that are essential for the materials recyclability (rehydriding). The experimental design of the RGA-MS provides a system that can identify and measure the gas emitted from a sample during the process of thermal decomposition. In our results diborane was confirmed in the bulk material s decomposition. The diborane release appears to follow a release of hydrogen, which could indicate that reaction (3) would follow reaction (2) from above. The figure below is a bargraph plot which is used to locate and identify gas species by their cracking pattern or mass spectrum. (3) Figure 1: RGA-MS Data of LiBH 4 at Temperature Ramp Rate of 240 C/hour It had been reported that the kinetics and perhaps thermodynamics of hydrides could be improved if confined in a nanostructured framework 3. This has increased interest in improving the properties of LiBH 4. In work by Vajo and co-workers it was shown that by infiltrating LiBH 4 into carbon aerogels (CA) there was an enhancement in the dehydrogenation kinetics and its reversibility 4. In light of this our group has developed a nano-porous carbon framework which has shown to reduce byproduct gas species such as diborane and reduce the hydrogen desorption temperature of borohydrides 5,6. Experimental Set Up

3 The experimental apparatus is composed of four main components: the residual gas analyzer (RGA), the low temperature stage quartz crystal microbalance (QCM), the high temperature heating stage, and two vacuum chambers separated by a perforation which allows a direct line-of-site of the sample to the RGA. The RGA is a quadrupole filtered ion detector with an electron multiplier for detection of low ion current. The output of the RGA (ion signal) is converted into partial pressures of ion ranges. The partial pressures (in Torr) are presented as a function of the mass-to-charge ratio m/z. The RGA has an accurate pressure range of to 10-5 Torr in the lower sample chamber. The high temperature stage ranges from room temperature (23 C) to 500 C with an adjustable ramp rate. The vacuum chambers and the small flow hole separate the RGA from the sample stages. This is to prevent rapid contamination of the RGA head. Figure 2 shows a schematic of the RGA-MS system. Since this system is a differentially pumped vacuum chamber, the pressures reported are proportional to the rate of change of the pressure, but are reported as instantaneous chamber pressures. Ion Gauge Gas Analyzer Turbopump 80 l/s Variable Ar Leak & Electro Valve 90 Micron Flow Hole with Line of Sight Gate Valve Rough Pump Argon Filled Glove Bag QCM and High Temp Sample stages Ion Gauge Turbopump 300 l/s Rough Pump Figure 2: The Schematics for the RGA/QCM Thermodynamics System Preparation of a Nano-Porous Carbon Framework Nanoporous carbon was prepared by using phenolic resins as carbon precursors and amphipilic triblock copolymers as soft templates. The resins were themopolymerized, and resulting powders were carbonized at 900 C under a flow of argon gas. The final product is a well ordered hexagonally packed set of cylindrical nanopores, denoted NPC. The Brunauer- Emmett-Teller (BET) method was used to calculate the specific surface areas (SBET) using adsorption data in a relative pressure range from 0.05 to 0.2 bars. The total pore volume was estimated from the adsorbed amount at a relative pressure P/P 0 of 0.98 based on the Barrett-

4 Joyner-Halenda (BJH) calculation model. The morphology of the nanoporous carbon can be seen in Figure 3 and was investigated with use of a Philips EM430 high-resolution transmission electron microscope (TEM). It was found that the pore size distribution could be tuned by adding a small amount of oxygen/nitrogen to the gas flow in the final carbonization process. With this tuning method, the effect of pore size on borohydride decomposition could be studied. Two NPC are used in this study. They have estimated pore sizes of 2 and 4 nm respectively. To contrast with larger pore sizes we have used carbon Aerogels (CA) prepared by T. Baumann of Lawrence Livermore National Laboratory. These two Aerogels have pore sizes of 9 and 15 nm respectively. Figure 3: TEM images for nano-porous carbon of columnar pores packed in a hexagonal geometry, viewed from the directions parallel (a) and perpendicular (b) to the pore channels. Results of LiBH 4 Infiltrated into the Nano-Porous Carbon The analysis of the LiBH 4 melt infiltrated into the NPC show that the borohydride may become amorphous. This analysis is done using differential scanning calorimetry (DSC) and x-ray diffraction (XRD) data. LiBH 4 is a crystalline structure and that structure is absent in the XRD data after being melted into the NPC. Similarly, the DSC data shows a lower temperature crystal phase transition in the bulk material, but no transition is present in the nano-confined material. It is also apparent that the decomposition starts at around 285 C (the melting temperature) in NPC as opposed to 385 C in the bulk material. Conversely with the unordered carbon aerogel there is an apparent phase transition in the DSC data, which indicates that the LiBH 4 is in a crystalline phase. The RGA mass spectrum data shows similar results. The measurements mentioned above take place at atmospheric pressures, so it is important to note that the RGA measurements are performed in a high vacuum, and may follow a different decomposition pathway due to the pressure. In Figure 4, the RGA data show that the decomposition temperature is reduced to around 300 C in all of the nano-confined LiBH 4. This data is shown normalized according to

5 the mass of the hydride in the sample, and gives a good approximation of the rate of gas pressure increase per mg of hydride. An interesting result is that the production of diborane now precedes the production of the hydrogen at this temperature. The data for the bulk material in Figure 4 indicate that the initial hydrogen release precedes the diborane production, while there is a concomitant release of diborane in some of the nano-confined samples, and indicates a possible change in reaction pathway for all the nano-confined materials. As can be seen in Figure 4, the diborane production is reduced with decreasing pore size. With the 2nm NPC the production of diborane is eliminated from the RGA signal. Figure 4: B2H6 Gas Emission is Reduced with Pore Size It has been suggested that the diborane produced can react with the remaining LiBH 4 and produce a closoborane Li 2 (B 12 H 12 ) in reaction 4 7. This is an undesired reaction as this will confine more hydrogen in the system and may impair the recyclability of the material. While closoboranes were not detected in the RGA data elimination of diborane from the reaction pathway would ensure this reaction will not take place. 2 (4) Other Complex Borohydrides in Nano-Porous Carbon Li 4 BN 3 H 10 has a very high hydrogen content (11.9 wt.%). This hydride has a lower decomposition temperature than LiBH 4 but in bulk form releases a wide variety of gasses,

6 most notably ammonia (NH 3 ) and diborane. As can be seen in Figure 5, nano-confinement in 4nm NPC decreases the production ammonia and nitrogen derivatives, and can be seen to eliminate diborane production. The effect on the decomposition pathway is not as easily determined here, but seems to indicate that since the N x H x production is reduced the hydrogen is saved for release at a later temperature. Figure 5: Nano-porous carbons have a similar effect on the complicated hydride Li 4 BN 3 H 10 Research Efforts in Progress The majority of the work so far has been to characterize the effect of nano-confinement on the decomposition of complex hydrides. We continue to build a library of the general decomposition products of different hydrides, in bulk as well as in nano-confined environments. Future studies on reaction pathway optimization will continue. Conclusion Hydrogen is an important energy carrier and the materials used to store hydrogen often involve complicated, multi-step decomposition processes. By incorporating these materials in nano-scale frameworks one may attempt to alter the decomposition temperatures, and reaction processes. We have shown that the production of B 2 H 6 gas can be reduced and essentially eliminated in the decomposition of LiBH 4 if confined in a nanoporous framework. This suggests that tunable reaction pathways are possible for decomposing complex hydrides, and that hydrogen storage in borohydrides may be possible without the loss of active boron. The purpose of the apparatus described above is to characterize the reaction processes. More importantly, the ultimate goal of the research is to help develop efficient hydrogen storage materials that can release hydrogen at temperatures that are compatible with PEM fuel cells, and with lower enthalpies of absorption and desorption.

7 Acknowledgments All nanoporous carbons as well as the carbons characterizations were prepared by Dr. Xengfeng Liu. Zak Jost took and analyzed the TEM images of the NPC. All of the carbon Aerogels were prepared by T. Baumann of Lawrence Livermore National Laboratory, Livermore, CA. Dr. Eric Majzoub directs this work as part of the Majzoub Research Group. This project is partially funded through the NASA/Missouri Space Grant Consortium. Additional funding was provided through the US Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE) through the hydrogen program, and the UMSL- Research Board. References 1. Mosegaard, L. et al., Intermediate phases observed during decomposition of LiBH4, Journal of Alloys and Compounds. Proceedings of the International Symposium on Metal-Hydrogen Systems, Fundamentals and Applications (MH2006) : , (2007). 2. Kostka, J., Lohstroh, W., Fichtner, M., and Hahn, H., Diborane Release from LiBH4/Silica-Gel Mixtures and the Effect of Additives, J. Phys. Chem. C, 37, , (2007). 3. Cahen, S., Eymery, J. B., Janot, R., Tarascon, J. M., Improvement of the LiBH4 hydrogen desorption by inclusion into mesoporous carbons, J. Power Sources, 189, , (2009). 4. Gross, A. F., Vajo, J. J., Van Atta, S. L., Olson, G. L, Enhanced Hydrogen Storage Kinetics of LiBH4 in Nanoporous Carbon Scaffolds, J. Phys. Chem. C, 14, , (2008). 5. Liu, X., Peaslee, D., Jost, C. Z., Majzoub, E. H., Controlling the Decomposition Pathway of LiBH4 via Confinement in Highly Ordered Nanoporous Carbon, J. Phys. Chem. C, 33, , (2010). 6. Liu, X., Peaslee, D., Jost, C. Z., Baumann, T. F., Majzoub, E. H., Systematic Pore- Size Effects of Nanoconfinement of LiBH4: Elimination of Diborane Release and Tunable Behavior for Hydrogen Storage Applications, Chem. Mater., 5, , (2011). 7. Friedrichs, O., Remhof, A., Hwang, S.-J., Züttel, A., Role of Li2B12H12 for the Formation and Decomposition of LiBH4, Chem. Mater., 10, , (2010).

8 Biography David Peaslee is a California native attending the University of Missouri St. Louis. He graduated with a B.S. in Physics and a B.A. in Mathematics in 2007 from UMSL. He earned his M.S. in physics in 2008 and is presently a PhD student working as a research assistant with Dr. Eric Majzoub s material science research group.