Author

Yun Gan

Date of Award

Fall 2021

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Xingjian Xue

Abstract

The deteriorating climate is threatening our living habitat and arousing public awareness. To achieve the carbon neutral goal by the middle of 21st century, developing novel technologies to reduce CO2 emission or promote its capture become urgent. Among them, oxygen permeation membrane (OPM) and solid oxide cell (SOC) are two promising devices for clean energy conversion.

In chapter 1, the background of OPM and SOC are briefly introduced. Fundamental mechanism and application of phase inversion, the key process involved in fabrication of hollow fiber OPM and microtubular SOC are emphatically described. Mixed ionic and electronic conducting (MIEC) perovskites demonstrate advantages over Ni-cermet as fuel electrode for SOCs. However, SOCs are primarily electrolyte-supported planar designs in literature when MIEC perovskite fuel electrodes are employed, which are relatively easy to fabricate but usually have high electrolyte ohmic resistance. Perovskite fuel electrode-supported designs are rarely studied particularly for microtubular SOCs. In Chapter 2, (La0.3Sr0.7)0.9Ti0.9Ni0.1O3-δ-Sm0.2Ce0.8O1.9 (LSTN-SDC) electrode-supported microtubular cell LSTN-SDC/YSZ/(La0.8Sr0.2)0.95MnO3-δ (LSM) is fabricated and characterized. The LSTN-SDC microtubular substrate is prepared using an spinneret spinning in combination with modified phase inversion method, featuring radially well-aligned micro-channels open at the inner surface. A thin YSZ electrolyte and LSM electrode are then fabricated on the substrate. Upon reducing treatment, nickel nanoparticles are exsolved from LSTN grains and uniformly decorated onto grain surface. With CO/CO2 gas mixture as the fuel, the complicated electro-/chemical reactions are identified in the substrate electrode. The electrolysis process combines with surface catalytic process of nanostructured electrode substrate leads to highly efficient CO production with conversion efficiencies above 100%. The electrolysis also facilitates to regenerate surface catalytic functionality of electrode substrate. The redox stability advantages of the cell are demonstrated in both alternative reduction (CO)/oxidation (air) atmospheric conditions and reversible operating mode.

Due to high surface area to volume ratio, hollow fiber membranes have been extensively investigated for oxygen separation applications. The widely studied hollow fiber membranes are fabricated using spinneret process in combination with phase inversion method, followed by one-step high temperature sintering. The resultant membrane demonstrates multiple layers, where a relatively thick sponge-like layer is sandwiched by the radially aligned closed finger-liked pore layers on either side, and the shell and lumen sides are covered by relatively dense thin skin layers. To take advantage of finger-like pores for facile gas diffusion, hollow fibers with open finger-like pores are created using spinneret process with modified phase inversion method, built upon which thin film dense separation layer and porous catalyst layer are fabricated to form an asymmetric membrane. The asymmetric design can reduce sintering temperature for membrane fabrication and enhance conductivity for bulk charge transport. However, the mechanical strength of the hollow fiber membranes is usually not sufficient and high-cost material is used for the substrate. In Chapter 3, LSCF-ZnO composite hollow fiber is developed and optimized so that radially-aligned open microchannels are produced. Built upon the composite hollow fiber, thin film dense LSCF separation layer and porous LSCF catalyst layer are successfully fabricated. The performance and long-term stability of the membrane are systematically measured and characterized. Results indicate that ZnO addition to the hollow fiber substrate can not only decrease the sintering temperature for membrane fabrication but also significantly enhance the mechanical strength, robustness, durability, and stability. By replacing a considerable amount of high cost LSCF with low cost ZnO in the substrate, the capital cost of the membrane can also be significantly reduced.

Surface oxygen exchange is a critical reaction step for energy conversion and storage in OPM and SOC. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are typical operating modes of surface exchange process. To enhance ORR/OER properties, nanostructures are introduced into surface porous layer/electrode. Due to high operating temperatures, the synthesis routes for nanostructured devices at low temperatures are rarely suitable, and infiltration could be the only widely used technique. Recently, hydrothermal synthesis route was employed. Nevertheless, the ORR/OER mechanisms associated with such a surface is not clear and the stability is still a concern. In Chapter4, built upon oxygen separation membrane and symmetric cells, Co3O4 nanocluster-structured surfaces are directly fabricated using hydrothermal route. Electrochemical tests are conducted using the membrane and symmetric cells with two-electrode and three-electrode configurations. The ORR/OER surface exchange mechanisms are analyzed, and the effects of nanostructured surface are identified. A short-term stability (~ 100 h) is also conducted using a symmetric cell at 900 °C. While a slight degradation occurs over the course, both performance and surface microstructures are stabilized after a certain time of the stability test. This dissertation contributes to the fabrication and characterization of micro/nano-structured OPM and SOCs with decent stability.

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