Nanogravimetric Evaluation of Hydrogen Uptake in Thin Film Materials by A Quartz Crystal Microbalance
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1098-HH03-03
Nanogravimetric Evaluation of Hydrogen Uptake in Thin Film Materials by A Quartz Crystal Microbalance Tao Xu Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL, 60115 ABSTRACT To study the hydrogen storage materials in their thin film format provides a unique approach to investigate many interfacial phenomena associated with hydrogen storage materials. We demonstrate the application of a quartz crystal microbalance for nanogravimetric evaluation of thin film hydrogen storage materials up to 40 bars. The frequency shift of a quartz crystal coated with hydrogen absorbing materials is affected by the hydrogen mass uptake, the pressure and the viscosity of the gases, and the crystal surface roughness, of which the roughness contribution has no direct analytical expression. Through a control experiment on the same crystal in helium, the roughness contribution in hydrogen can be derived and the frequency shift due to the hydrogen mass uptake is obtained. INTRODUCTION A grand challenge of using hydrogen as primary energy carrier is to explore suitable onboard hydrogen storage materials [1]. Recent work indicates that catalytic doping can moderate the storage conditions for chemsorptive complex metal hydrides [2], and enhance the storage capability in physisorptive metal-organic-framework (MOF) [3]. Catalytic doping involves the hydrogen transfer at the interface between catalysts and storage materials, which has not been fully understood. An effective approach to study such hydrogen transfer at interface is to systematically constructing thin films of catalyst and storage materials into layer-layer structures. The remaining question is to measure the weight change of the thin film in pressurized H2, which is typical in the range of a few tens of ng/cm2, considering the film is a few tens of μg/cm2. 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. [3-6]. Piezoelectric quartz crystal microbalance (QCM) has been widely used to measure the mass change of thin film materials in vacuum due to its high mass sensitivity (< 1 ng/cm2) [7]. However, the application of QCM for gravimetric evaluation of H2 storage materials has not been explored due to the complicated crystal oscillation in pressurized fluids. We recently demonstrate that QCM can be used for nanogravimetric analysis on thin film hydrogen storage materials in H2 pressure up to 40 bar [8]. When placed in a pressurized fluid, the total frequency shift (Δf ) of the crystal is the sum of four factors, which can be express by Δf= Δfm + Δfp+ Δfη + Δfr, (Equation 1) where Δfm accounts for the effect of loaded mass on the crystal; Δfp represents the hydrostatic compression effect, resulting from the pressure of the fluid; Δfη, reflects the viscous effect, and Δfr is the additional contribution of the viscous fluid to the vibrating frequency of the crystal due to the surface roughness of the crystal [9, 10]. Δfm is directly prop
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