Materials by Design Principles in Artificial Photosynthesis:  Discovery and Synergistic Integration of Light Absorbers, Electrocatalysts and Membranes for a Complete, Stable, Efficient, and Safe Solar Fuels Generator

Speaker: Nate Lewis, Professor, California Institute of Technology
Host: Energy Graduate Group
Date: 11/8/2019
Time: 2:30pm to 3:30pm
Location: 198 Youg Hall, UC Davis

Abstract: We are developing an artificial photosynthetic system that will utilize sunlight and water as the inputs and produce hydrogen and oxygen as the outputs.  We are taking a modular, parallel development approach in which three distinct primary components-the photoanode, the photocathode, and the product-separating but ion-conducting membrane-are fabricated and optimized separately before assembly into a complete water-splitting system. The design principles incorporate two separate, photosensitive semiconductor/liquid junctions that will collectively generate the 1.7-1.9 V at open circuit necessary to support both the oxidation of H2O (or OH-) and the reduction of H+ (or H2O). The photoanode and photocathode will consist of rod-like semiconductor components, with attached heterogeneous multi-electron transfer catalysts, which are needed to drive the oxidation or reduction reactions at low overpotentials.  The high aspect-ratio semiconductor rod electrode architecture allows for the use of low cost, earth abundant materials without sacrificing energy conversion efficiency due to the orthogonalization of light absorption and charge-carrier collection.  Additionally, the high surface-area design of the rod-based semiconductor array electrode inherently lowers the flux of charge carriers over the rod array surface relative to the projected geometric surface of the photoelectrode, thus lowering the photocurrent density at the solid/liquid junction and thereby relaxing the demands on the activity (and cost) of any electrocatalysts.  A flexible composite polymer film will allow for electron and ion conduction between the photoanode and photocathode while simultaneously preventing mixing of the gaseous products.  Separate polymeric materials will be used to make electrical contact between the anode and cathode, and also to provide structural support.  Interspersed patches of an ion conducting polymer will maintain charge balance between the two half-cells.

Bio: Nathan S. Lewis, Ph.D., is the George L. Argyros Professor of Chemistry at the California Institute of Technology where he has been a faculty member since 1988.  Lewis is best known for developing artificial photosynthesis technology that enables sustainable production of hydrogen fuel using sunlight, water and carbon dioxide as well as an “electronic nose” for artificial olfaction.  From 2009 to 2019 he served as editor-in-chief of Energy and Environmental Science, a journal focusing on sustainable energy research, published by the Royal Society of Chemistry.  He is the recipient of the Princeton Environmental Award and the American Chemical Society Award in Pure Chemistry.  In 2019, he is the recipient of the Europe Section Heinz Gerischer Award for his outstanding contribution to the science of semiconductor electrochemistry and photoelectrochemistry.  In 2017, Lewis was elected to the National Academy of Inventors.  He holds approximately 70 U.S. and foreign patents.  Lewis has authored more than 500 papers and mentored more than 100 graduate students and postdoctoral researchers.