(3bi) Atomistic-Level Investigation of Efficient Energy Conversion and Storage

Lim, D. H. - Presenter, University of Michigan

A first-principles approach based on density functional theory (DFT) calculations coupled with thermodynamics greatly facilitates finding solutions to energy-related challenges, providing knowledge of atomic structures, reaction mechanisms, and electronic properties. My research interests focus on atomic and molecular-level investigations of efficient energy conversion and storage using approaches based upon quantum mechanics and thermodynamics. I am specifically interested in understanding and enhancing the electrochemical CO2 conversion to hydrocarbon-based fuels, the oxygen reduction reaction (ORR) in polymer electrolyte membrane (PEM) fuel cells, and the phase stability of metal complex hydrides for hydrogen storage.

The main challenge for advancing CO2 reduction is to improve the energy efficiency. Although copper has been widely accepted as a promising metal for the reduction of CO2 to hydrocarbon fuels, its conversion is primarily hindered by the key step of CO protonation since CO binds only weakly to copper. This large kinetic barrier may be overcome by utilizing nanostructures of copper on graphene supports due to changes associated with nanoparticle size that may enhance the surface activity of copper for improved stabilization of the intermediates of CO2 reduction. Furthermore, graphene support may not only prevent the sintering of copper nanoparticles, but may also provide a balance in the intermediate binding strength that may allow for enhanced catalyst turnover.

The ORR is one of the central focuses amidst ongoing studies of electrode reactions in polymer electrolyte membrane (PEM) fuel cells due to the slow kinetics taking place at the cathode electrode. To improve ORR kinetics, Pt and Pt alloy nanocatalysts supported on graphene, graphene nanoplatelets, and nanoscale graphite have been recently utilized. These supports reduce the high cost of the precious metal, improve the kinetics, and increase the durability of Pt support materials. Despite efforts on experimental investigations of the ORR mechanisms, few theoretical studies of the ORR mechanisms on graphene-supported Pt nanoparticles have been conducted. We provide details of the mechanisms of ORR on defective graphene supported-Pt nanoparticles.

Complex hydrides including alanates ([AlH4]) have recently gained attention as alternative hydrogen storage materials. Many of these materials have been known to release H2 upon contact with water; however, the hydrolysis reactions are highly irreversible, a process known as “one-pass” hydrogen storage. Nanostructuring and nanocatalysis have been accepted as promising methods to overcome the irreversible hydrogenation process. Predicting which phases may be more stable as a function of nanoparticle size may contribute to nanostructuring complex hydrides for hydrogen storage. We provide phase stability diagrams of the nanoparticle of Mg(AlH4)2 as a function of particle size and temperature using DFT calculations, cluster expansion, and Monte Carlo simulations.

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