Person: Cox, Casandra R
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Cox
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Casandra R
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Cox, Casandra R
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Publication Earth-abundant water-splitting catalysts coupled to silicon solar cells for solar-to-fuels conversion(2014-10-21) Cox, Casandra R; Nocera, Daniel; Nocera, Daniel; Aspuru-Guzik, Alan; Buonassisi, TonioDirect solar-to-fuels conversion can be achieved by coupling semiconductors with water-splitting catalysts. A 10% or higher solar to fuels conversion is minimally necessary for the realization of a robust future technology. Many water-splitting devices have been proposed but due to expensive designs and/or materials, none have demonstrated the necessary efficiency at low-cost that is a requisite for large-scale implementation. In this thesis, a modular approach is used to couple water-splitting catalysts with crystalline silicon (c-Si) photovoltaics, with ultimate goal of demonstrating a stand-alone and direct solar-to-fuels water-splitting device comprising all non-precious, technology ready, materials. Since the oxygen evolution reaction is the key efficiency-limiting step for water-splitting, we first focus on directly interfacing oxygen evolution catalysts with c-Si photovoltaics. Due to the instability of silicon under oxidizing conditions, a protective interface between the PV and OER catalyst is required. This coupling of catalyst to Si semiconductor thus requires optimization of two interfaces: the silicon|protective layer interface; and, the protective layer|catalyst interface. A modular approach allows for the independent optimization and analysis of these two interfaces. A stand-alone water-splitting device based on c-Si is created by connecting multiple single junction c-Si solar cells in series. Steady-state equivalent circuit analysis allows for a targeted solar-to-fuels efficiency to be designed within a predictive framework for a series-connected c-Si solar cells and earth-abundant water-splitting catalysts operating at neutral pH. Guided by simulation and modeling, a completely modular, stand-alone water-splitting device possessing a 10% SFE is demonstrated. Importantly, the modular approach enables facile characterization and trouble-shooting for each component of the solar water-splitting device. Finally, as direct solar water-splitting is far from a mature technology, alternative concepts are presented for the future design and integration of solar water-splitting devices based on all earth-abundant materials.Publication Room temperature stable COx-free H2 production from methanol with magnesium oxide nanophotocatalysts(American Association for the Advancement of Science (AAAS), 2016) Liu, Z.; Yin, Z.; Cox, Casandra R; Bosman, M.; Qian, X.; Li, N.; Zhao, H.; Du, Y.; Li, J.; Nocera, DanielMethanol, which contains 12.6 weight percent hydrogen, is a good hydrogen storage medium because it is a liquid at room temperature. However, by releasing the hydrogen, undesirable CO and/or CO2 byproducts are formed during catalytic fuel reforming. We show that alkaline earth metal oxides, in our case MgO nanocrystals, exhibit stable photocatalytic activity for CO/CO2-free H2 production from liquid methanol at room temperature. The performance of MgO nanocrystals toward methanol dehydrogenation increases with time and approaches ~320 μmol g−1 hour−1 after a 2-day photocatalytic reaction. The COx-free H2 production is attributed to methanol photodecomposition to formaldehyde, photocatalyzed by surface electronic states of unique monodispersed, porous MgO nanocrystals, which were synthesized with a novel facile colloidal chemical strategy. An oxygen plasma treatment allows for the removal of organic surfactants, producing MgO nanocrystals that are well dispersible in methanol.