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43431-G10
Fundamental Investigations of Carbon-Metal Composites for Catalyzed Hydrogen Storage
The vision of a hydrogen economy offers improved energy efficiency, decreased fuel imports, decreased emissions, and a diversified energy portfolio. Development of a hydrogen economy will require advances in methods by which to produce, store, transport, and distribute hydrogen in an economically viable manner. Although hydrogen has the highest energy density per mass, it has one of the lowest energy densities per volume. Well established methods for high-pressure compression or cryogenic liquefaction increase volumetric hydrogen density, but at a high energy penalty. These methods do not satisfactorily address safety concerns, and thermodynamic limitations indicate that they will not approach the DOE storage targets of 6 wt% for the year 2010 and 9 wt% for 2015. Incorporating hydrogen into a solid state storage matrix provides an alternative with an array of possibilities, yet no existing material meets goals established to enable a 300-mile vehicle range with consideration of practical operating conditions [2]. Incremental advances in existing materials will not likely meet these targets. The idea of storing hydrogen in carbon nanotubes or nanofibers has led to a great deal of hype, theoretical discussions, debate, and experimental controversy. Early—and often extraordinary—reports in carbon adsorbents have been disputed, and largely discredited. It is now established that residual, or introduced, catalysts may activate carbon nanomaterials for hydrogen adsorption.
We have been working in developing novel carbon-metal combinations with the ACS-PRF grant. The metal-carbon materials would store hydrogen through the process of hydrogen spillover, in which the metal activates the carbon service for hydrogen adsorption and temperatures that would otherwise be too high for appreciable hydrogen uptake. Surprisingly little is known about the details of catalytic hydrogen spillover at high pressure that are applicable to hydrogen storage applications and the corresponding effects of pressure on carbon nanostructures. With only small amounts of the metal as a catalyst, designing a material that meets the 2010 goals (6 wt%) would require a 1:1 H:C ratio. The carbon structure must be optimized such that each carbon participates in adsorption and any potential for induced physisorption is realized.
This year, our focus has turned to using carbide derived carbons (CDCs) as a carbon support for hydrogen spillover. CDCs, recently developed by Gogotsi et al, have unique characteristics such as tunable pore size, light weight, and high purity. The pore size tunability provides the opportunity to improve both the gravimetric and volumetric uptake of hydrogen; which is the focus of our research. By manipulating the synthesis conditions, we are developing synthesis routes that improve dramatically the total pore volume and allow for the introduction of metals for our carbon-metal materials. To better understand and improve the synthesis process and mechanism, we employ Raman spectroscopy, high resolution transmission electron microscope (HRTEM), electron energy loss spectroscopy (EELs) and surface area and porosity analyzer to acquire structural and morphological information. The high pressure hydrogen uptake measurements are done on a custom-made Sievert's apparatus that was supported, in part, on this project. To date, we have synthesized multiple CDCs with variable pore size and metal content, and introduced an additional activation step to increase porosity and optimize the pore structure for hydrogen adsorption. Tests of the undoped material at low temperatures and high pressures and the metal-doped material at moderate temperatures are underway.
As a part of an ACS-PRF supplement for Summer Research Fellowships (SRF), Prof. Hye-Young Kim of Southeastern Louisiana University (a primarily undergraduate institution) was a visiting scholar during Summer 2007. Prof. Kim explored modeling of the undoped CDC structure with two theoretical models: density functional theory (DFT) and grand canonical monte-carlo simulation with Fynnman-Hipp correction (GCMS-FH). In addition, Henry's law which works consistently at very low pressure was used to check the accuracy of the low pressure data of the two models. Furthermore, the results from all the theoretical data were verified with the experimental data using nitrogen and hydrogen as dosing gases. The models will serve as a springboard for additional modeling when metals are introduced into the system.
The ACS-PRF has allowed Lueking to finish developing and building a state-of-the-art adsorption laboratory and establish key research connections. Dr. Kofi Adu was supported as a post-doctoral fellow for 4 months in 2007, and has benefited in tackling all these issues for this new form of carbon material, especially, in developing the test equations and working model for the Sievert's system. Prof. Hye-Young Kim, Adu, and Lueking benefited from collaborations with Prof. Milton Cole, Prof. Karl Johnson, and Dr. Jacek Jagiello who participated in the modeling aspect of the project.
