Date of Award

2011

Document Type

Campus Access Dissertation

Department

Mechanical Engineering

First Advisor

Fanglin (Frank) Chen

Abstract

There has been an increasing interest in clean and renewable energy generation for highlighted energy and environmental concerns. Solid oxide cells (SOCs) have been considered as one of the promising technologies, since they can be operated efficiently both in electrolysis mode by generating hydrogen through steam electrolysis and fuel cell mode by electrochemically combining fuel with oxidant. The present work is devoted to performing a fundamental study of SOC in both fuel cell mode for power generation and electrolysis mode for fuel production. The objectives of the work are to achieve a fundamental understanding of the mechanism of this new technology, explore new approaches to fabricate SOC, increase its effectiveness and develop quality assurance guideline for implementing the new technology to industrial applications. The specific aspects have been addressed and studied in this work as follows:

Novel button SOCs fabrication methods,

Novel micro-tubular SOCs (MT-SOCs) fabrication methods,

Microstructure characterization of the button and micro-tubular SOCs,

Electrochemical properties and durability of the button and micro-tubular SOCs in electrolysis and fuel cell mode.

The research work on SOCs that can be operated reversibly for power generation and fuel production has been conducted in the following six projects:

(1) High performance solid oxide electrolysis cell (SOEC)

Fabrication of novel structured SOEC oxygen electrode with the conventional and commercial solid oxide fuel cell materials by screen-printing and infiltration fabrication methods. The microstructure, electrochemical properties and durability of SOECs has been investigated. It was found that the LSM infiltrated cell has an area specific resistance (ASR) of 0.20 ohm cm2 at 900 oC at open circuit voltage with 50% absolute humidity (AH), which is relatively lower than that of the cell with LSM-YSZ oxygen electrode made by a conventional mixing method. Electrolysis cell with LSM infiltrated oxygen electrode has demonstrated stable performance under electrolysis operation with 0.33 A/cm2 and 50 vol.% AH at 800 oC. (2) Advanced performance high temperature micro-tubular solid oxide fuel cell (MT-SOFC)

Phase-inversion, dip-coating, high temperature co-sintering process and impregnation method were used to fabricate micro-tubular solid oxide fuel cell. The micro-structure of the micro-tubular fuel cell will be investigated and the power output and thermal robustness has been evaluated. High performance and rapid start-up behavior have been achieved, indicates that the MT-SOFC developed in this work can be a promising technology for portable applications.

(3) Promising intermediate temperature micro-tubular solid oxide fuel cells for portable power supply applications

Maximum power densities of 0.5, 0.38 and 0.27 W/cm2 have been obtained using H2-15% H2O as fuel at 550, 600 and 650oC, respectively. Quick thermal cycles performed on the intermediate temperature MT-SOFC stability demonstrate that the cell has robust performance stability for portable applications.

(4) Micro-tubular solid oxide cell (MT-SOC) for steam electrolysis

The electrochemical properties of MT-SOC will be investigated in detail in electrolysis mode. The mechanism of the novel hydrogen electrode structure benefiting the cell performance will be demonstrated systematically. The high electrochemical performance of the MT-SOC in electrolysis mode indicates that MT-SOC can provide an efficient hydrogen generation process.

(5) Micro-tubular solid oxide cell (MT-SOC) for steam and CO2 co-electrolysis

\ The MT-SOC will be operated in co-electrolysis mode for steam and CO2, which will provide an efficient approach to generate syngas (H2+CO) without consuming fossil fuels. This can potentially provide an alternative superior approach for carbon sequestration which has been a critical issue facing the sustainability of our society. (6) Steam and CO2 co-electrolysis using solid oxide cells fabricated by freeze-drying tape-casting

Tri-layer scaffolds have been prepared by freeze-drying tape casting process and the electrode catalysts are obtained by infiltrating the porous electrode substrates. Button cells will be tested for co-electrolysis of steam and CO2. The mechanism and efficiency of steam and CO2 co-electrolysis will be systemically investigated.

In conclusion, SOCs have been fabricated with conventional materials and evaluated, but their performance has been found to be limited in either SOFC or SOEC mode. The cell performance has been significantly improved by employing an infiltrated LSM-YSZ electrode, due to dramatically decreased polarization resistance. However, mass transport limitation has been observed, particularly in electrolysis mode. By utilizing micro-tubular SOCs with novel hydrogen electrode produced via a phase inversion method, mass transport limitation has been mitigated. Finally, mass transport has been further improved by using cells with electrodes fabricated through a freeze-drying tape-casting method.

Rights

© 2011, Chenghao Yang

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