Prof. Dr. Bruce A. Parkinson
Department of Chemistry and School of Energy Resources, University of Wyoming, Laramie, Wyoming 82071, USA
"Photoelectrochemical Energy Conversion on Earth and Mars"
New oxide semiconductors for the photoelectrolysis of water: Although photovoltaic cells excel at directly converting of solar energy to electricity, they do not directly produce stored energy or fuels that account for more than 75% of current energy use. Direct photoelectrolysis of water has the advantage of converting solar energy directly to hydrogen. Unfortunately no materials are currently known to efficiently photoelectrolyze water that are, efficient, inexpensive and stable under illumination in electrolytes for many years. Nanostructured semiconducting metal oxides could potentially fulfill these requirements, making them the most promising materials for solar water photoelectrolysis, however no known oxide semiconductor has all the required properties. We have developed a simple, high-throughput combinatorial approach to prepare and screen many multi-component metal oxides for water photoelectrolysis activity. Several promising compositions have been identified and we are in the process of optimizing and understanding the physical structure, electronic structure and catalytic ability of some of these new photocatalysts. In addition, due to the millions of possible combinations to be printed and screened, we have developed a distributed research project that uses simple and inexpensive printing and screening devices to enlist many undergraduate and high school student researchers into the search for the “Holy Grail” of materials. The Solar Hydrogen Activity research Kit or SHArK project has been distributed to over sixty sites.
Photoelectrochemistry on Mars: Chemical analysis of Martian soil in the north polar region by the Phoenix Mars Lander unexpectedly detected high concentrations of perchlorate ion (0.4-0.6 weight %) that accounted for ~60% of the anionic charge and exceeded chloride concentrations by factors of 4 to 8. Initially perchlorate formation on Earth was found to be due to an atmospheric chemical reaction (ozone oxidation of chloride aerosols) and was also proposed to explain perchlorate production on Mars. We showed that highly oxidizing valence band holes, produced by ultraviolet (UV) illumination of naturally occurring semiconducting minerals, such as the anatase and rutile, are capable of oxidizing chloride ion to perchlorate in aqueous solutions.
Our results can help explain the presence and accumulation of perchlorate in the polar Martian soil during Martian spring and fall where water is present without any contribution from atmospheric chemistry. In addition, our mechanism predicts that over millennia even small amounts of semiconducting oxide minerals could eventually convert almost all the chloride to kinetically stable perchlorate, explaining the disparate perchlorate to chloride ratios found on Mars compared to those in naturally occurring perchlorate-containing soils on Earth.
Geological features and clay minerals indicate that large amounts of water were present on early Mars, however where it all went is still a mystery. We demonstrate that photoelectrochemical water splitting with a natural heterojunction cell may have been operative in early Mars history resulting in an oxidation of the surface and the hydrogen escaping the weak Mars gravity.
Zeit & Ort
24.07.2018 | 17:15
Hörsaal Chemie, Takustr. 3, 14195 Berlin