46374-G10
Spillover-Enhanced Hydrogen Storage
and Nanowires for Solar Thermoelectric Energy Conversion
I. Progress in hydrogen storage project
A. Nanogravimetric Evaluation of Hydrogen Uptake in Thin Film Storage Materials
Hydrogen is envisioned as the most attractive clean energy carrier. The search for high capacity hydrogen storage materials started long ago and gained new momentum due to the fossil fuel related problems. Recent work indicates that catalytic doping can moderate the storage conditions for chemsorptive complex metal hydrides, and enhance the storage capability in physisorptive metal-organic-framework (MOF). Catalytic doping involves the hydrogen transfer at the interface of catalysts and storage materials, which has not been fully understood. An effective approach to study such hydrogen transfer at interface is to systematically construct thin films of catalyst and storage materials into layer-layer structures. The rising question is how to measure the weight change in the thin film in pressurized H2. Considering the weight of film is in the range of a few tens of mg/cm2, a method to detect weight change of a few tens of ng/cm2 is desired. Current hydrogen storage measurement systems are designed for bulk materials and require at least a few tens of mg materials to assure the signal-to-noise. Piezoelectric quartz crystal microbalance (QCM) has been widely used to measure mass change of thin film materials in vacuum for its high mass sensitivity (< 1 ng/cm2). However, in pressurized gases, the frequency shift of a quartz crystal is very complicated and is affected by mass of absorbed hydrogen, the pressure and viscosity of H2, and the crystal surface roughness, of which the roughness contribution has no analytical expression. Through a control experiment on the same crystal in helium, we demonstrated that the roughness contribution in hydrogen can be derived and the frequency shift due to hydrogen uptake can be obtained. As an example, we obtained the Pd-H pressure-composition isotherm of a Pd thin film in pressurized hydrogen. This result has been published on Appl. Phys. Lett. (APL).
B. Electrical Study on Kinetics of Primary Hydrogen Spillover in Amorphous Carbon The reported spillover-enhanced hydrogen storage in metal-organic frameworks open a new door for physisorptive hydrogen storage. Hydrogen spillover arises in hydrogen catalyzed reactions on supported metal catalysts. Dihydrogen molecules dissociate on the metal catalyst. Some hydrogen atoms diffuse to the support and are said to spillover. To advance practical adsorbent development, the mechanism and kinetics of hydrogen spillover must be understood. However, in-situ studay on hydrogen spillover can only be realized a handful techniques including inelastic neutron scattering (INS) spectroscopy and temperature-programmed desorption (TPD). PI used a unique electrical method to study primary hydrogen spillover from metal nanocatalyst to carbon by monitoring the conductance of the carbon support. The electrical measurement has various advantages over INS or TPD for its accuracy, fast data acquisition and simplicity, such that kinetics of primary hydrogen spillover has been obtained. The result has been submitted to APL.
Free-standing nanowires are highly desired in many applications. For example, thermoelectric nanowires must not be in contact with each other so as to not to undo quantum confinement effects, which boost the figure of merit. However, it has been always a challenge to obtain free-standing nanowires in liquid-based electrochemical deposition using anodized alumina oxide (AAO) as template. This is because after the AAO template is removed by base, the embedded nanowires with a high aspect ratio normally collapse into an entangled mess due to the surface tension force exerted on the nanowires during the evaporation of the liquids. To avoid the liquid-gas interface, and thus to eliminate surface tension force during the evaporation of the liquids, supercritical CO2 drying is used. Unfortunately, this method is expensive and inconvenient due to the high pressurized CO2 used (100 bar). The pulling pressure causing the collapse of the nanowires is P=d/R, where d is the surface tension of the liquid and R is the radius of the curvature of the meniscus. Here, we developed a simple method to obtain free-standing nanowires through decreasing the polarity of mutual-soluble rinsing solvents. With the decreasing polarity, s decreases and consequently P decreases. The solvents used in sequence are water, isopropanol and hexane. The surface tension of hexane is 18 dynes/cm, in comparison to 72.8 dynes/cm for water. Therefore, its evaporation does not cause the collapse of the nanowires. A manuscript that describes the details of this new method is in preparation for submission.
