ABSTRACT: Securing renewable energy sources is one of the most urgent challenges of our century. Fossil fuels are a limited resource and their use often has serious environmental impacts. As a result, there has been a major drive recently to advance science and technology for harvesting green and renewable energies using, for instance, the chemical energies of hydrogen and oxygen, photons, or even biomass. However, the “well to device” energy conversion efficiencies of these systems are currently still too low for them to be utilized for any practical applications, and their applicability is often hampered by inefficient catalyst materials.
The efficiency of oxygen reduction reaction (ORR) plays a key role in controlling over all performance of energy systems in many devices, for instance, fuel generation devices, metal-air batteries and fuel cells etc. One of the best examples of such kind is the Proton Exchange Membrane (PEM) fuel cell. In spite of its high-expected thermodynamic efficiency and environmentally friendly nature, its performance is limited severely by catalysts for ORRs that are still based on pure or alloyed Pt, which is quite expensive and provides efficiencies that are not high enough to compete with those provided by conventional fossil fuel energy systems from a commercial point of view.
This presentation shows that first principles density functional theory calculations are very useful for unveiling oxygen reduction reaction (ORR) mechanisms on a wide class of materials, including conventional nano-scale Pt and non-precious metallic catalysts of doped graphenes. As an example, considering the effect of both the geometry and concentration of dopant N atoms in bulk and edge N-Gr forms, we calculate the energies of a large number of model systems in order to cover possible N-Gr structures and determine the most stable N-Gr forms. In agreement with experiments, our calculations suggest that stable N-Gr forms are expected to have doping levels limited to less than approximately 30 at.% of N, above which the hexagonal graphene framework is broken. Remarkably, the ground state structures of bulk and edge N-Gr are found to differ depending on the doping level and poisoning of the edge bonds by H and O. Catalytic properties and possible ORR pathways for the stable N-Gr are estimated using Gibbs free energy diagrams, both with and without the effect of water solvation, to better understand the explicit effect of adsorbate binding energies. Our results indicate that nitrogen doping significantly alters the catalytic properties of pure graphene and that dilutely doped bulk N-Gr forms are the most active. Furthermore, it is shown that supporting or complexing the doped graphene with metallic elements can substantially modify the rate-determining step of ORR through the manipulation of electronic structures from an atomic scale.
Bio-Sketch: Byungchan Han, Ph.D. Materials Science and Engineering
Byungchan is currently the Assistant Professor of the Department of Energy Systems Engineering at DGIST of South Korea. He studied at Seoul National University at the Department of Nuclear Engineering as Bachelor and Mater degree programs. After that he moved to MIT for a Ph.D. degree at the Department of Material Science and Engineering aimed at first principles computational studies on renewable energy materials, such as fuel cells and Li-ion batteries. He continued the research work at Stanford University as a Postdoctoral Scholar. His expertise is to predict high functional energy materials through fundamental description of complicate chemical reactions and understanding of its mechanisms by discovering atomic level key parameters.
He got the best paper award from Korean electrochemical society, and was selected as ten most leading scientists and introduced at national newspaper of Korea.
He established a joint research center with Lawrence Berkeley National Laboratory for developing multi-purpose hybrid materials from systematic cooperation between theoretical prediction and experimental validation. He is currently the vice director of the center. He is also an active Review Editorial Board of Frontiers in Condensed Matter Physics.
