Thermodynamics of metals oxides for hydrogen production via thermochemical cycling
- Hydrogen has long been researched as a carbon-free energy carrier and an environmentally friendly replacement for fossil fuels in transportation and chemical industries. However, current large-scale hydrogen production takes place via steam reforming of fossil fuels, a process inherently coupled to large-scale carbon emissions. Two-step thermochemical water splitting, in which a metal oxide is first thermally reduced to emit oxygen and subsequently oxidized by water at a lower temperature, is a promising carbon-free method of producing hydrogen at scale. Currently, the extremely high operating temperatures required for the initial reduction step make this process incompatible with industry infrastructure. The selection of a metal oxide active material with appropriate thermodynamic properties is key to lowering the temperature of the thermal reduction step while also maximizing hydrogen yield and water-to-hydrogen conversion. This dissertation focuses on understanding the relationships between chemistry, phase behavior, thermodynamics, and water-splitting performance. First, I will identify the key thermodynamic properties that control hydrogen yield and water-to-hydrogen conversion in single-phase systems. This work extracted thermodynamic quantities from measurements of approximately 120 previously-studied perovskite materials and evaluated the trends observed. The mined thermochemistry data were then fit to various defect models to explain these trends for perovskites with Mn, Fe, and Co on the B-site. The data show that alkaline earth doping onto the perovskite A-site primarily controls the enthalpy and entropy of oxygen incorporation, while selection of the B-site transition metal controls both the enthalpy of oxygen incorporation and the disproportionation of electronic species. Numeric data have been compiled into a library that allows the estimation of oxygen-to-metal-ratio in perovskite oxides at arbitrary temperature and pressure, and are a powerful tool for rational materials design. Second, I will expand this thermodynamic framework to the case of thermochemical cycling in a two-phase, binary system. Although two-phase binary systems have potentially better hydrogen yield and conversion, in practice they are nearly-impossible to control via chemistry. Nevertheless, they provide an interesting thought experiment toward understanding thermodynamic design of phase transitions. Third, I will present surprising chemistry trends in ternary-ferrites. Decreasing the concentration of iron in traditionally studied spinel ferrites improved thermochemical cycling yield for fixed conversion. We demonstrated that the results can be explained by thermodynamics, and that the trends observed with chemistry are very different from perovskites. We end by comparing the single-phase, two-phase binary, and two-phase ternary systems, and summarizing the outlook for future materials exploration.
|Type of resource
|electronic resource; remote; computer; online resource
|1 online resource.
|Ahlborg, Nadia Lorraine
|Degree committee member
|Degree committee member
|Stanford University, Department of Materials Science and Engineering.
|Statement of responsibility
|Nadia Lorraine Ahlborg.
|Submitted to the Department of Materials Science and Engineering.
|Thesis Ph.D. Stanford University 2019.
- © 2019 by Nadia Lorraine Ahlborg
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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