Reactive Particles for Efficient Solar Thermochemical Hydrogen Production

Ehrhart, Brian David

Graduate Thesis Or Dissertation

Reactive Particles for Efficient Solar Thermochemical Hydrogen Production Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/2v23vt558

Abstract

Hydrogen is very important and useful for chemical products, ammonia production, and as a transportation fuel. Future production will need to be done renewably and carbon-free to avoid further damage from climate change. Production of hydrogen directly from solar energy has the potential to be highly efficient. Some metal oxides can be thermally reduced in one reaction and then re-oxidized with steam to produce hydrogen in a two-step reduction/oxidation cycle.

A comprehensive solar-to-hydrogen (STH) efficiency model is developed for two-step solar thermochemical redox water splitting. Two redox materials are considered and compared in order to assess the impact of the rate of oxidation and the hydrogen productivity per cycle on STH efficiency. Near-isothermal redox processing is beneficial for materials with slower kinetics, especially with moderate to high gas heat recuperation. Gas heat recuperation is critical for high efficiency cycles, especially at conditions that lead to high steam and inert gas flowrates. Three methods for achieving low oxygen partial pressures for reduction are compared, and the effect of vacuum pump efficiency and inert gas/oxygen separation efficiency are quantified. Currently available vacuum pump technologies have very low thermodynamic efficiencies at low pressures and are unlikely to provide efficient hydrogen production relative to other oxygen partial pressure lowering technologies. A novel recycled inert gas sweep with high temperature separation is proposed and STH efficiency values are shown to vary significantly epending on the inert gas flowrate required. A high separation temperature for the recycled inert gas is beneficial, especially for cases of lower gas heat recuperation and increased inert gas flowrates.

Particles are an important aspect of many proposed solar thermochemical water splitting reactor concepts. These particles must maintain physical integrity and chemical performance at very high temperatures while moving around a system, which has the potential for highly efficiency continuous operation. Spray drying is used to produce iron aluminate (hercynite) particles using a pH-modified charge-stabilized sol. Nanoparticle suspensions are mixed and the pH is modified in order to induce partial flocculation. This procedure gives larger particles that are more spherical and structurally robust. Manganese oxide-based mixed metal oxide particles have been designed and tested for thermochemical energy storage. Al₂O₃, Fe₂O₃, and ZrO₂ are tested in Mn₂O₃-based spray-dried particles, and results are compared with thermodynamic predictions. A mixture of 2:1 Fe₂O₃:Mn₂O₃ formed iron manganese oxide spinel (FeMn₂O₄) on calcination, and demonstrated the highest thermochemical activity despite particle agglomeration and deformation. Conversely, zirconia was an inert support that does not react with manganese oxide. Differences in redox performance between materials with different Fe to Mn ratios have been attributed to ion diffusion and secondary phase formation.

Co-doped hercynite materials are examined for two-step solar thermochemical water splitting. Density functional theory (DFT+U) for iron and cobalt aluminate predict that electron density transfer on the creation of oxygen vacancies is almost exclusively to the atoms that are first-nearest-neighbor to the oxygen vacancy. Cases that have a single cobalt next to the oxygen vacancies tend to be very favorable in terms of both a lower oxygen vacancy formation energy and also a lower host structure energy, while sites with more than one Co next to the O-vacancy tend to be less favorable, due to cobalt being less favorable on the octahedral sites.

Cobalt appears to preferentially change oxidation state on formation of oxygen vacancies. Current XPS results seem to contradict the computational results, as the Co-doped hercynite sample does not show Co oxidation state changes while the un-doped hercynite shows Fe oxidation state changes. However, excess cobalt in the Co-doped sample makes oxygen vacancies less favorable, while samples with less cobalt should show more oxidation state change for Co. Undoped hercynite has demonstrated more hydrogen production capacity but slower reaction rates than the Co-doped hercynite. This could be due to kinetic/catalytic impacts of Co on the reaction mechanism, but also partially affects the thermodynamics of the reaction enthalpy change. This work suggests that lowering the amount of Co in the material or by introducing new dopants that can assume the octahedral site more easily will improve the hercynite cycle.

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