AmericanChemicalSociety.com

I. Progress in hydrogen storage project
A. Nanogravimetric Evaluation of Hydrogen Uptake in Thin Film Storage Materials (2007-2008)

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. B. Electrical Study on Kinetics of Primary Hydrogen Spillover in Amorphous Carbon(2008-2009)

The reported spillover-enhanced hydrogen storage in metal-organic frameworks opens 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 has been obtained.

C. Hydrogen Spillover Enhanced Hydriding Kinetics of Palladium-Doped Lithium Nitride to Lithium Imide (2008-2009)

Hydrogen storage in complex metal hydrides often suffers from unsatisfied hydriding kinetics of the corresponding complex metals under moderated conditions, partly due to the kinetic barrier associated with the breaking of H-H bond. Therefore, doping catalysts for H-H bond breakage becomes a feasible strategy to improve the hydriding kinetics because hydrogen adatoms can efficiently spillover from catalyst to complex metals. To realize this strategy, we developed a unique method to uniformly dope catalytic metal in the storage complex metals via synthesis of the eutectic of the catalytic metal and the precursory storage metal. This method eliminates the use of support materials for catalysts, while still maintains the large surface area and uniformity of the catalysts. We demonstrated that Li₃NPd_0.03 with nanoscopic Pd uniformity can be prepared through nitridization of LiPd_0.01 eutectic. The resulted Li₃NPd_0.03 exhibits enhanced hydriding kinetics over pure Li₃N for reaction Li₃N + H₂ ↔ Li₂NH + LiH under moderate conditions. The activation barrier for the hydriding of Li₃NPd_0.03 was measured to be ~28 kJ/mol. DFT calculation reveals that the hydrogen adatom migration barrier from palladium to nitrogen atom is ~25 kJ/mol. These results indicate that the Pd-doped LiN₃ is hydrided by hydrogen adatom that migrate from the Pd catalyst and the kinetic barrier for such a hydriding process is mainly ascribed to the diffusion barrier of H adatom from palladium to nitrogen atom.

II. Progress solar energy conversion
New Nanoarchitectured Dye-sensitized Solar Cells for Enhanced Electron Transport (2008-2009)

Much attention has been directed towards the enhancement of electron transport in dye-sensitized solar cells (DSSC) using one-dimensional nanoarchitectured semiconductors. However, the improvement resulting from these ordered 1-D nanostructured electrodes is often offset or diminished by the deterioration in other device parameters intrinsically associated with the use of these 1-D nanostrucutres, such as the two-sided effect of the length of the nanowires impacting the series resistance and roughness factor. In this work, we mitigate this problem by allocating part of the roughness factor to the collecting anode instead of imparting all the roughness factors onto the semiconductor layer. A microscopically rough Zn microtip array is used as an anode on which ZnO nanotips are grown to serve as the semiconductor component in a DSSC. For the same surface roughness factor, our Zn-microtip|ZnO-nanotip DSSC exhibits an enhanced fill factor compared to planar anode supported ZnO nanowire-based DSSCs. In addition, the open circuit voltage of the Zn-microtip|ZnO-nanotip DSSC is also improved due to a favorable band shift at the Zn-ZnO interface, which raises the Fermi level of the semiconductor and consequently enlarges the energy gap between the quasi-Fermi level of ZnO and the redox species. Electrochemical impedance spectroscopic study reveals that the electron collection time is much shorter than the electron lifetime, suggesting that fast electron collection occurs in our device due to the significantly reduced electron collection distance along the short ZnO nanotips. The overall improvement demonstrates a new approach to enhance the efficiency of dye-sensitized solar cells.