Publications

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Despite rapid progress, solar thermochemistry remains high risk; improvements in both active materials and reactor systems are needed. This claim is supported by studies conducted both prior to and as part of this project. Materials offer a particular large opportunity space as, until recently, very little effort apart from basic thermodynamic analysis was extended towards understanding this most fundamental component of a metal oxide thermochemical cycle. Without this knowledge, system design was hampered, but more importantly, advances in these crucial materials were rare and resulted more from intuition rather than detailed insight. As a result, only two basic families of potentially viable solid materials have been widely considered, each of which has significant challenges. Recent efforts towards applying an increased level of scientific rigor to the study of thermochemical materials have provided a much needed framework and insights toward developing the next generation of highly improved thermochemically active materials. The primary goal of this project was to apply this hard-won knowledge to rapidly advance the field of thermochemistry to produce a material within 2 years that is capable of yielding CO from CO2 at a 12.5 % reactor efficiency. Three principal approaches spanning a range of risk and potential rewards were pursued: modification of known materials, structuring known materials, and identifying/developing new materials for the application. A newly developed best-of-class material produces more fuel (9x more H2, 6x more CO) under milder conditions than the previous state of the art. Analyses of thermochemical reactor and system efficiencies and economics were performed and a new hybrid concept was reported. The larger case for solar fuels was also further refined and documented.

The oxidation in air of high-purity Al foil was studied as a function of temperature using Thermogravimetric Analysis with Differential Scanning Calorimetry (TGA/DSC). The rate and/or extent of oxidation was found to be a non-linear function of the temperature. Between 650 and 750 ÀC very little oxidation took place; at 850 ÀC oxidation occurred after an induction period, while at 950 ÀC oxidation occurred without an induction period. At oxidation temperatures between 1050 and 1150 ÀC rapid passivation of the surface of the aluminum foil occurred, while at 1250 ÀC and above, an initial rapid mass increase was observed, followed by a more gradual increase in mass. The initial rapid increase was accompanied by a significant exotherm. Cross-sections of oxidized specimens were characterized by scanning electron microscopy (SEM); the observed alumina skin thicknesses correlated qualitatively with the observed mass increases.

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Ferrites are promising materials for enabling solar-thermochemical cycles. Such cycles utilize solar-thermal energy to reduce the metal oxide, which is then re-oxidized by H2O or CO2, producing H2 or CO, respectively. Mixing ferrites with zirconia or yttria-stabilized zirconia (YSZ) greatly improves their cyclabilities. In order to understand this system, we have studied the behavior of iron oxide/8YSZ (8 mol-% Y2O3 in ZrO2) using in situ X-ray diffraction and thermogravimetric analyses at temperatures up to 1500 °C and under controlled atmosphere. The solubility of iron oxide in 8YSZ measured by XRD at room temperature was 9.4 mol-% Fe. The solubility increased to at least 10.4 mol-% Fe when heated between 800 and 1000 °C under inert atmosphere. Furthermore iron was found to migrate in and out of the 8YSZ phase as the temperature and oxidation state of the iron changed. In samples containing >9.4 mol-% Fe, stepwise heating to 1400 °C under helium caused reduction of Fe2O3 to Fe3O4 to FeO. Exposure of the FeO-containing material to CO2 at 1100 °C re-oxidized FeO to Fe3O4 with evolution of CO. Thermogravimetric analysis during thermochemical cycling of materials with a range of iron contents showed that samples with mostly dissolved iron utilized a greater proportion of the iron atoms present than did samples possessing a greater fraction of un-dissolved iron oxides.© 2012 JCPDS-ICDD.

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