Materials Science and Engineering Seminar Series: Jonah Erlebacher
Friday, February 11, 2011
1:00 p.m.-2:00 p.m.
Room 2108, Chemical and Nuclear Engineering Bldg.
Dealloyed Nanoporous Metals: A Tortured Path from Ancient Material to High-Performance Fuel Cell Electrocatalyst
Department of Materials Science and Engineering and
Department of Chemical and Biomolecular Engineering
Johns Hopkins University
Nanoporous metals, with pore and ligament sizes of nearly atomic scale, can be formed by the selective dissolution of a component(s) from a binary or even multicomponent alloy. This process, known as dealloying, has a long history stretching back to the ancient Americas where copper/gold artifacts were surface enriched in gold via chemical dealloying in some sort of mineral salt coating. The history passes through the problem of dezincification of brasses identified in the Civil War period and through the fundamental physics of element mixing in alloys in the 20th century; indeed, dealloying remains a major problem in corrosion today. In this talk, I will first discuss the recent trends that see dealloying not as a corrosion problem, but as a new fabrication tool to make nanostructured porous materials with a wide variety of compositional and structural architectures. As such, nanoporous metals are finding applications in (bio)sensing, micromechanics, and catalysis. This last application is of particular interest to me. We have recently used what weve learned in dealloying and porosity evolution to develop a new class of highly effective catalysts for the oxygen reduction reaction (ORR), the primary bottleneck in fuel cell catalysis. The ORR accounts for approximately 80% of the losses in proton exchange membrane fuel cells due to the slow kinetics of a complex reaction involving oxygen, four protons, and four electrons. Our catalysts are based on a nanoporous NiPt material that is impregnated with a hydrophobic, protic ionic liquid that chemically and geometrically confines oxygen to chemically reactive pore surfaces, and expels water from them. These effects bias the reaction toward completion. With our composite catalyst we have attained mass activities nearly an order of magnitude higher than that of typical carbon-support platinum nanoparticles, and this innovation allows us to take advantage of the high intrinsic catalytic activity of Pt-based materials but use significantly less precious metal.